STUDY of CARBON AEROGEL SUPPORTED Fe CATALYSTS for BIOMASS GASIFICATION GAS CLEANING

UNIVERSIDAD DE CONCEPCIÓN DEPARTAMENTO DE INGENIERÍA QUÍMICA

STUDY OF CARBON AEROGEL SUPPORTED Fe CATALYSTS FOR BIOMASS GASIFICATION GAS CLEANING

POR

OSCAR GÓMEZ CÁPIRO

Tesis presentada a la Facultad de Ingeniería de la Universidad de Concepción para optar el grado de Doctor en Ciencias de la Ingeniería con mención en Ingeniería Química

Tutor: Prof. Romel Mario Jiménez Concepción, PhD. Departamento de Ingeniería Química Universidad de Concepción

Co-tutor: Prof. Luis Ernesto Arteaga Pérez, PhD. Departamento de Ingeniería en Madera Universidad del Bío-Bío

Concepción, Chile. Junio de 2020

Se autoriza la reporducción total o parcial , con fines académicos , por cualquier medio o procedimiento, incluyendo la cita bibliográfica del documento.

Advisor: Prof. Romel Mario Jiménez Concepción, PhD. Chemical Engineering Department University of Concepción

Co-advisor: Prof. Luis Ernesto Arteaga Pérez, PhD. Department of Wood Engineering University of Bío-Bío

Examination Prof. Ximena García Carmona, PhD. Committee Chemical Engineering Department University of Concepción

Prof. Fernando Mariño, PhD. ITHES University of Buenos Aires; CONICET

Dedicatoria

A mi abuela Dora, Doña Cedora del Rosario Muñoz y Anoceto…

Agradeciemientos

Agradecimientos Quisiera aprovechar para agradecer a algunas de las personas que han hecho posible que se realizara el trabajo que se detalla más adelante. Comenzaré por dar gracias al profesor Luis Ernesto Arteaga Pérez que no solo me guío académicamente, sino que me ofreció apoyo en todo, abriendo su casa y su familia para recibirme en Chile, por eso debo agradecer también a su esposa Yannay Casas Ledón. Quiero dar gracias al profesor Romel Mario Jiménez Concepción quien guió mi trabajo académico y mi formación como doctor. Ambos no solo fueron tutores, sino soportes en el cambio que implicó dejar mi país. Mi gratitud también es para mis amigos en Cuba, quienes con un simple mensaje alegraban los días a pesar de la distancia, especialmente a Osvaldo de la Fuente, Dailenys Garcia y Freddy Santo, Andrés Feitó y muy especialmente a Yenisleidys Rodríguez, Eduardo González y Osdeny Chaviano. Chile también me dio nuevos amigos a los que quiero agradecer su presencia y paciencia para soportarme, esto incluye a Karen Aravena, Álvaro Iglesia, Isidora Ortega, Paulina Melo, José Luis Daroch, Diego Villagrán y Carla Riffo. Además, a todos los compañeros de doctorado con los que he compartido desde el 2015, en especial a los que me recibieron en mis inicios, Nabin Kumar, Norberto Abreu, Raydel Manrique, Yaine Beltrán y su familia, y a mi buen amigo Luis Pino. Corresponde hacer un agradecimiento especial para Bryan Gómez y su esposa María Amalia Salazar que pasaron a ser como mi familia aquí y a todos los que conocí gracias a ellos. Son unos amigos excepcionales que nos hacían sentir en casa tanto a mí como a mi esposa. A lo largo del desarrollo del doctorado fueron llegando otros que poco a poco se hicieron imprescindibles, pude y puedo contar con ellos para todo y siempre estaré agradecido de la feliz coincidencia de encontrar a Jessica Borges, Daviel Gómez, Maray Ortega y Anamary Pompa en este camino. Dentro del marco del trabajo, muchos contribuyeron en los experimentos y la caracterización de materiales. No sería posible presentar los resultados finales de esta investigación sin la colaboración de Aaron Delgado, Adrian Hinckle, Nicolás Grob y Kimberley Matschuk. También apoyaron el trabajo Mónica Uribe en el GEA y los encargados del Centro de Microscopía Electrónica. Requieren un reconocimiento especial todas las personas con las que coincidí en el Laboratorio de Carbono y Catálisis (CarboCat) y en la Unidad de Desarrollo Tecnológico, en especial Mauricio Flores, Héctor Grandón, Robert Irribarra, Manuel Morales,

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Agradeciemientos

Patricia Olivera y Cristina Segura; además de mis coterráneos Daniel Travieso y Gustavo Cabrera. Igualmente doy gracias a los profesores que me impartieron clases en diferentes departamentos de la Universidad de Concepción y en especial del Departamento de Ingeniería Química que a través de clases o consejos y comentarios sobre este trabajo en diferentes instancias hicieron posible mejorar todos los aspectos de la investigación. Mi agradecimiento incluye, por supuesto, al personal administrativo de dicho departamento, especialmente a Erika Carrasco, Carol Soto y Héctor Fierro. Agradezco la formación recibida que permitió el honor de ser aceptado en el programa de doctorado. Todos mis maestros y profesores a todos los niveles aportaron algo sobre lo que me apoyé para llegar hasta aquí, especialmente a Aleida Rodríguez, mi profesora de secundaria que me inició en los caminos del estudio las ciencias. Son las familias las que sustentan todo el trabajo que pueda hacerse como estudiante y en este caso venir hasta Chile requirió el sacrificio de la familia de mi esposa que tuvo que verla partir, por eso les estoy eternamente agradecido a sus padres Teresita González y José Antonio González, a su hermano Adriamny González y a toda su familia. Agradezco a mi familia, a mi tía Mercedes Gómez, a mi abuela Cedora Muñoz, a quien va dedicada esta tesis, y sobre todo a mi madre Claribel Cápiro y mi padre Oscar Gómez que forjaron este camino para mí, aunque me haya llevado lejos de ellos. Todo lo que implicó dejar mi país, mis amigos y mi familia también lo sufrió mi esposa Amaidy González. Sin mi compañera de viaje, sin su apoyo incondicional, su amor y cuidado, no existiría tesis ni me hubiese graduado. Fue todo, desde correctora de trabajos, consejera, sustento, hasta enfermera en los momentos más difíciles. A ella le debo esto y nunca le podré agradecer lo suficiente.

Content

List of Contents i. Introduction...... xiv ii. Technical Problem: ...... 5 iii. Hypothesis: ...... 5 iv. General objective: ...... 5 v. Specific objectives: ...... 5 1 Chapter 1: Background...... 6 1.1. Chile's energy situation...... 6 1.2. Lignocellulosic biomass for energy production. Thermochemical conversion of biomass. Applications...... 6 1.2.1. Liquefaction...... 8 1.2.2. Torrefaction...... 8 1.2.3. Pyrolysis...... 9 1.2.4. Combustion...... 9 1.2.5. Gasification...... 10 1.3. Distributed generation...... 14 1.4. Problems related to biomass gasification gases...... 15 1.5. Gas cleaning...... 17 1.5.1. Physical and mechanical removal...... 18 1.5.2. Thermal cracking...... 18 1.5.3. Catalytic cracking and reforming...... 19 1.5.4. Iron as active phase in catalytic tars removal...... 22 1.6. Chars as support...... 24 1.6.1. Carbon aerogels...... 25 1.6.2. Carbon aerogels as support...... 26 1.7. Kinetic aspects...... 27 1.7.1. Toluene decomposition...... 27 1.7.2. Ammonia decomposition...... 32 1.8. Research questions...... 33

Content

2 Chapter 2: Carbon Aerogel-Supported Iron for Gasification Gas Cleaning: Ammonia Adsorption...... 35 2.1. Introduction...... 36 2.2. Materials and Methods...... 42 2.2.1. Microfibers Treatment...... 42 2.2.2. Carbon Aerogel Preparation. Carbonization...... 43 2.2.3. Catalysts Preparation...... 43 2.2.4. Compositional Analysis...... 44

2.2.5. N2 Adsorption-Desorption at 77K...... 44 2.2.6. Thermal Resistance...... 44 2.2.7. X-Ray Diffraction (XRD)...... 45 2.2.8. Raman Spectroscopy Analysis...... 46 2.2.9. Transmission Electron Microscopy (TEM)...... 47 2.2.10. Ammonia Adsorption Experiment...... 47 2.3. Results and Discussion...... 50 2.3.1. Characterization of as-Prepared and pre-Treated Cellulose Microfibers...... 50 2.3.1.1. As-Prepared MFCs Composition...... 50 2.3.1.2. Cellulose Crystalline Structure...... 51 2.3.2. Carbon Aerogel Preparation...... 53 2.3.3. Catalyst Synthesis and Characterization...... 61 2.3.3.1. Superficial and Composition Characteristics...... 61 2.3.3.2. Catalyst XRD Patterns...... 62 2.3.3.3. TEM ...... 63 2.3.4. Adsorption Experiments ...... 64 2.3.4.1. Ammonia Adsorption over CAG 90...... 64 2.3.4.2. Ammonia Adsorption over Catalyst...... 68 3 Chapter 3: Catalytic decomposition of tars over Fe/CAG...... 71 3.1. Introduction...... 72

Content

3.2. Materials and methods...... 73 3.2.1. Microfibrils Treatment. Support obtaining...... 73 3.2.2. Catalysts preparation...... 74 3.2.3. X-ray diffraction (XRD)...... 74 3.2.4. Transmission Electron Microscopy (TEM)...... 75 3.2.5. Scanning Electron Microscopy-Energy Dispersive X-ray spectroscopy (SEM-EDX)...... 75 3.2.7. Naphthalene decomposition experiments...... 76 3.3. Results and discussion...... 78 3.3.1. Compositional analysis...... 78 3.3.2. CAG surface characterization...... 78 3.3.3. Particle size distribution and estimated metallic surface. . 80 3.3.4. Experiments in differential reactor...... 82 3.3.4.1. Toluene decomposition...... 82 3.3.4.2. Effect of reaction temperature...... 87 3.3.4.3. Effect of reaction conditions over Fe/CAG stability. .. 89 3.3.5. Experiments at BENCH scale...... 92 3.3.5.1. Effect of reaction temperature...... 92 3.3.5.2. Effect of the reaction condition over Fe/CAG-ps characteristics...... 94 3.4. Partial Conclusions...... 98 4 Chapter 4: Carbothermic reduction of carbon aerogel- supported Fe during catalytic tar decomposition...... 100 4.1. Introduction...... 101 4.2. Materials and methods...... 103 4.2.1. Support precursor and obtaining...... 103 4.2.2. Toluene decomposition activity tests...... 103 4.2.3. X-ray diffraction (XRD)...... 104 4.2.4. Scanning Electron Microscopy-Energy Dispersive X-ray spectroscopy (SEM-EDX)...... 104

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Content

4.2.5. Carbothermic reduction verification...... 105 4.3. Results and discussion...... 105 4.3.1. Compositional analysis...... 105 4.3.2. Catalytic activity test...... 105 4.3.3. XRD of spent catalysts...... 108 4.3.4. Thermogravimetric analysis...... 110 4.3.5. Partial Conclusions...... 111 5. Conclusions...... 112 6. Acknowledgments...... 113 7. References...... 114 8. Annexes...... 137

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Content

List of Tables Table 1.1 Thermochemical processes for biomass energy use...... 7 Table 1.2 Gasification gas composition and Lower Heat Value for different conditions...... 10 Table 1.3 Gasification plant status reported by IEA (IEA 2020)...... 12 Table 1.4 Efficiencies for different power generation systems according to their capacity, adapted from (Bridgwater 1995)...... 14 Table 1.5 Contaminant content in gases at the exit of different gasifiers and requirements for use in different technologies (Asadullah 2014b). 15 Table 1.6 Operating temperatures and efficiency of different gasification gas cleaning technologies (Zwart 2009)...... 17 Table 1.7 Tar content in gasification gases after thermal cracking cleaning...... 19 Table 1.8 Conversions of tar cracking for different catalysts...... 21 Table 1.9 Activation energy for tars decomposition in different conditions...... 30 Table 2.1 Effect of temperature, dwell time, and heating rate on carbonized cellulose-based precursors reported in the literature...... 40 Table 2.2 Cellulose microfiber (MFC) compositional analysis...... 51 Table 2.3 Crystalline index (Eq. 2.1a,b), d-spacing (Eq. 2.3), and crystal size (Eq. 2.4)...... 52 Table 2.4 Elemental analysis of selected carbon aerogels (CAG 90, CAG 100, and CAG 110)...... 53 Table 2.5 Comparison between pre-treated and pure MFC, carbonized at the same conditions...... 54 Table 2.6 Graphite crystallite dimensions (from X-ray diffraction (XRD) analysis) and bands D and G areas ratio (from Raman spectroscopy analysis)...... 61 Table 2.7 XRD crystallite size (Equation (1)) for Fe crystal plane...... 63 Table 2.8 Ammonia adsorbed over CAG 90 (mg NH3/g CAG 90)...... 65 Table 2.9 Langmuir and Freundlich models’ parameters and statistical adjustment...... 67 Table 2.10 Thermodynamics parameters for ammonia adsorption over CAG 90...... 67 Table 2.11 Ammonia adsorbed over the catalyst (mg NH3/g catalyst).. 68 Table 3.1 Experiments carried out with benzene: naphthalene flowrate ratio of 2.45:1. (Catalyst: ~1g of Fe/CAG, space velocity 940-960 ml/min g catalyst)...... 77

Content

Table 3.2 C/Fe rates in spent catalysts at different space velocity...... 83 Table 3.3 C/Fe ratio in spent catalysts at different toluene concentration in the fed...... 91 Table A1 MFC carbonization experimental conditions and CAGs textural properties...... 137 Table A2 XRD planes identifications in spent Fe/CAG...... 137 Table A3 Particle size estimated by XRD patterns for three different Fe phases in toluene decomposition spent catalysts...... 138 Table A4 XRD planes identifications in spent Fe/CAG-ps...... 138

Content

List of Figures Figure 1.1 Comparison of existing gasification plants between August 2016 (image a) and January 2020 (image b) (IEA 2016, 2020)...... 12 Figure 1.2 Effects of unwanted compounds in a combustion engine (Brandin et al. 2011)...... 17 Figure 1.3 Dependence of coke formation with temperature. Adapted from (Trimm 1977)...... 21 Figure 1.4 The most common functional groups on carbonaceous materials surface...... 22 Figure 1.5 Gibbs free energy for the carbothermic reduction of iron oxides. Adapted from: (Hoekstra et al. 2016)...... 24 Figure 1.6 Product distribution obtained in the conversion of glucose to aromatics according to the catalyst average pore diameter (Jae et al. 2011)...... 27 Figure 1.7 Approximate composition of gasification gases at the outlet of a gasifier (Abu El-Rub, Bramer, and Brem 2004)...... 28 Figure 1.8 Interaction of a ringed compound (phenol) with active sites on a carbon surface (from left to right: hydroxyl group, carboxylic acid, amine.) (Shen 2015) ...... 30 Figure 2.1. Normalized X-ray diffraction (XRD) patterns of cellulose microfiber (MFC) and pre-treated impregnated MFC...... 52 Figure 2.2 Effect of dwell time and temperature on morphology of carbon aerogels (CAGs). (a) Specific surface area, (b) pore volume. Heating rate: 10 °C/min...... 55 Figure 2.3 N2 adsorption-desorption isotherms at 77 K and pore size distribution for CAG 90...... 56 Figure 2.4 DTG for thermal resistance in the air with heating rate: a-10 °C/min; airflow: 80 mL/min. b- Comparison between thermal resistance indicators in chars’ samples (b)...... 58 Figure 2.5 Normalized XRD pattern for CAG 80, 90, 90 0%, 110, and commercial activated carbon Pittsburgh...... 59 Figure 2.6 Normalized Raman spectra for CAG 80, 90, 90 0%, 110, and commercial activated carbon Pittsburgh...... 60 Figure 2.7 XRD patterns of Fe/CAG 90 catalyst...... 62 Figure 2.8 The TEM image of Fe/CAG 90...... 63 Figure 2.9 Particle size distribution for Fe/CAG 90...... 64

Content

Figure 2.10 Coverage of ammonia over CAG 90 and model representation. Freundlich model presented by dashed lines and Langmuir model by solid lines...... 66 Figure 2.11 Ammonia Coverage over Fe surface...... 69 Figure 2.12 Variation of Enthalpy of ammonia adsorption with coverage of ammonia over the metal surface...... 69 Figure 3.1 N2 adsorption-desorption isotherms at 77K and pore size distribution on a- Fe/CAG and b-Fe/CAG-ps...... 79 Figure 3.2 Transmission electron microscopy image of carbon aerogel iron supported. a- Fe/CAG and b- Fe/CAG-ps...... 81 Figure 3.3 Particle size distribution obtained from TEM images analysis. a- Fe/CAG and b- Fe/CAG-ps...... 81 Figure 3.4 Toluene conversion and product variation with space velocity change. Stars-toluene conversion, circles-concentration of aliphatic product...... 82 Figure 3.5 Tars model decomposition. a- Toluene (1979 ppm) decomposition over Fe/CAG catalyst. b-Benzene (1175 ppm) decomposition over Fe/CAG catalyst. Temperature, 600°C. Space velocity, 350 ml·min-1g-1...... 85 Figure 3.6 Arrhenius plot for toluene decomposition. Temperature values: 575, 600, and 625°C. The space velocity was 700 ml·min-1·g-1. Catalyst mass of 0.1g. Note that the y-axis is logarithmic. For TOF calculation the metal exposed surface was as follow: Fe/CAG has 0.03894 mmol Fes/g catalyst and Fe/CAG-ps has 0.03789 mmol Fes/g of the catalyst, according to the method described previously in section 2.2.10 ...... 88 Figure 3.7 Normalized XRD patterns of fresh and spent catalysts during toluene decomposition. Each pattern is identified with the corresponding spent catalyst condition at the right /catalyst, temperature, toluene concentration in the feed. All experiments were carried out at 700 ml·min- 1 -1 ·gcat , except for spent catalyst marked with SV 1051, which were -1 -1 performed at 1051 ml·min ·gcat ...... 90 Figure 3.8 Percentage of Fe0 remaining in the spent catalyst at different conditions. Each experiment is denoted with the toluene concentration in -1 -1 the feed. All experiments were carried out at 700 ml·min ·gcat , except for spent catalyst marked with 875 SV, which was performed at 875 -1 -1 ml·min ·gcat ...... 91 Figure 3.9 Naphthalene decomposition on Fe/CAG-ps. a- different temperature. b- with co-fed of different gases at 565°C. Both at 960 ml ×

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Content min-1× g-1 space velocity, 3130 ppmv of benzene, 660 ppmv of naphthalene...... 93 Figure 3.10 Arrhenius plot for naphthalene. Temperature values: 565, 620, and 665°C...... 94 Figure 3.11 XRD pattern of fresh and used catalysts during naphthalene decomposition...... 96 Figure 3.12 Schematic representation of the molecules of a- benzene, b- toluene, and c- naphthalene...... 98 Figure 4.1 Toluene conversion and C/Fe ratio for fresh and spent catalysts after reaction at 500, 600, and 700°C. Space velocity=350 ml·min-1·g-1. Toluene concentration in the feed: 1979 ppm. The estimated C deposition mass is declared over each C/Fe ratio marker...... 107 Figure 4.2 Screenshot of SEM-EDX results. (a- fresh catalyst, b- spent catalyst at 600°C) ...... 108 Figure 4.3 XRD patterns of fresh and spent catalysts at different temperatures (reflections planes in Table A2). Peak marked as ‘quartz’ corresponds to quartz wool used to hold the catalyst...... 109 Figure 4.4 Thermogravimetric analysis coupled to Mass spectrometry of spent catalyst at 600°C. Temperature program, DTG, and mass 44 signal are shown for a heating rate of 2°C/min under 50 ml/min He flows. .. 111

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Abstract

Abstract Gasification gas cleaning is the cornerstone for gasification as a technology to be used commercially. Among the variants used to eliminate pollutants that hinder the use of gases (internal combustion engines, turbines, or as a raw material for synthesis), current studies promote those that turn pollutants into inert or energy-usable compounds. The studies focus on the elimination of tars, the presence of these compounds is what greater restrictions impose on the use of gasification gases. The use of supported catalysts is the alternative that has shown the best results with the use of various metals as the active phase. Among these, Fe stands out for being of low cost and environmental impact as well as having the highest reported activity for the breakdown of C-C bonds. The choice of the support plays a fundamental role in the performance of the catalysts used, highlighting the carbons of vegetable origin with a low concentration of oxygenated groups, since they favor the formation of coke. The report presented below presents the results of the study of a Fe catalyst supported on carbon aerogel obtained from cellulose aerogel by carbonization, with pretreatments and conditions manipulated to maximize the mass yield and thermal resistance of the support. Both powder and pellet catalysts were prepared for testing at different scales. Characterization of the support, fresh and used catalyst, included adsorption-desorption of N2, XRD, SEM-EDX, TEM, Raman, thermal resistance, and ICP-OES. The interaction of these solids with ammonia was evaluated to determine the thermodynamic parameters that govern the adsorption process of this contaminant on the surface of the solid. For the case of the decomposition of tars, the decomposition of benzene, toluene, and/or naphthalene was studied at different scales and conditions of temperature, feed concentration, and space velocity to propose a decomposition mechanism and evaluate the stability of the catalyst. The results show that a support with high thermal resistance was obtained manipulating the carbonization conditions to achieve defined morphologies. The presence of Fe as an active phase favored the adsorption of ammonia in the solid and the thermodynamic parameters show that the process went from physisorption on the support to chemisorption on the Fe. The decomposition studies of tars show that the

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Abstract rupture of the aromatic rings is the rate-determining step and that the deactivation of the catalyst is caused by carbon accumulation and active phase oxidation with a strong influence of temperature and the presence of permanent gases CO and H2.

Resumen

Resumen La limpieza de los gases de gasificación es la piedra angular que permitirá usar esta tecnología comercialmente. Entre las variantes utilizadas para eliminar los contaminantes que dificultan el uso de los gases (motores de combustión interna, turbinas o como materia prima para la síntesis), los estudios actuales promueven las que descomponen los contaminantes en compuestos inertes o de valor agregado. Los estudios se centran en la eliminación de los alquitranes, la presencia de estos compuestos impone las mayores restricciones al uso de los gases de gasificación. El uso de catalizadores es la alternativa que ha dado mejores resultados con el uso de varios metales como fase activa. Entre ellos, el Fe se destaca por tener bajo costo e impacto ambiental, así como por tener la mayor actividad reportada para la ruptura de los enlaces C-C. La elección del soporte juega un papel fundamental en el rendimiento de los catalizadores utilizados, destacando los carbones de origen vegetal con baja concentración de grupos oxigenados, ya que favorecen la formación de coque. En el informe que se presenta a continuación se exponen los resultados del estudio de un catalizador de Fe soportado sobre aerogel de carbono obtenido a partir de aerogel de celulosa por carbonización, con pretratamientos y condiciones manipuladas para maximizar el rendimiento de masa y la resistencia térmica del soporte. Se prepararon catalizadores en polvo y en pellets para su ensayo a diferentes escalas. La caracterización del soporte, catalizador fresco y usado, incluyó la adsorción-desorción de N2, XRD, SEM-EDX, TEM, Raman, resistencia térmica e ICP-OES. Se evaluó la interacción de estos sólidos con el amoníaco para determinar los parámetros termodinámicos que rigen el proceso de adsorción de este contaminante en cada una de las superficies. Para el caso de la descomposición de los alquitranes, se estudiaron el benceno, el tolueno y/o el naftaleno como modelos a diferentes escalas y condiciones de temperatura, concentración de alimentación y velocidad espacial para proponer un mecanismo de reacción y evaluar la estabilidad de los catalizadores. Los resultados muestran que se obtuvo un soporte con alta resistencia térmica manipulando las condiciones de carbonización para lograr morfologías definidas. La presencia de Fe como fase activa favoreció la adsorción de

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Resumen amoníaco en el sólido y los parámetros termodinámicos muestran que el proceso pasó de la fisisorción en el soporte a la quimisorción en el Fe. Los estudios de descomposición de los alquitranes muestran que la ruptura de los anillos aromáticos es el paso que determina la velocidad y que la desactivación del catalizador es causada por la acumulación de carbono y la oxidación de la fase activa con una fuerte influencia de la temperatura y la presencia de gases permanentes CO e H2.

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Introduction

i. Introduction.

The reality of the depletion of fossil fuels together with the environmental problems associated with their daily use drives the need to look for alternative sources of energy. In this sense, several variants have been identified and are being applied (wind power, solar and geothermal energy, biomass use), which have been validated by a high number of investigations (Dresselhaus and Thomas 2001). There is a consensus that the conversion of the energy matrix currently established will be possible only through the integration of these sources and not by the individual effect of each one. This assertion is supported by the fact that the availability and feasibility of exploitation are functions that depend on the geo-economic characteristics of each region. In the specific case of Chile, the availability of forest biomass stands out, among others, with an area that can be used for energy purposes of 6.5 million hectares (CONAF 2020). Only the management of these resources generates waste equivalents to 7500 GWh/y, using conventional biomass-based technologies (Berg et al. 2013). However, the highest installed capacity of power generation from renewables in Chile comes from wind power and solar energy with ca. 80% of all non-conventional energy resources (Chile 2019). Biomass use has been relegated to a secondary place, which has a major share in the household heating sector. Nevertheless, in the endeavor of transforming the actual energy matrix into a more sustainable one, it is impossible to relegate the biomass to a second plane, not only due to its potentialities as a fuel but also as a source for new building blocks for chemical synthesis.

Within the technologies for exploiting biomass as an energy source, thermochemical routes are the most used, highlighting gasification as one of the most efficient, compared even with traditional combustion or co- combustion (Mínguez et al. 2013). Gases produced by biomass gasification contain H2, CO, and CO2 and can be used in the synthesis of chemical products or directly as fuels in internal combustion engines (Sikarwar et al. 2016). The distributed cogeneration is one of the alternatives that use these gases with great advantages in terms of actual

Introduction

implementation possibilities, power supply, and a chemical feedstock CO2 neutral (Dong et al. 2013). A recurring obstacle in this way has been the heterogeneity of the composition of the gases obtained and their high level of unwanted compounds. This heterogeneity is influenced by the origin and composition of the biomass and by the gasification operational conditions (temperature, oxidizing agent, and presence of catalysts). Regardless of these factors, the gas mixture contains substances such as tars, ammonia, sulfurized and chlorinated compounds in quantities that hinder their use in technologies such as turbines or the synthesis of chemicals with high marketable interest (Dong et al. 2013). Tars are defined as all hydrocarbons with a molecular weight higher than benzene (Maniatis and Beenackers 2000); they are prone to condense into equipment and pipes downstream of the gasifier causing obstructions and blockages in moving parts. Moreover, they are unacceptable for using the gas in the synthesis of new chemicals and it decreases the potential energy of the produced gases (Dayton 2002). Eliminating these compounds represents the highest investment and operation costs in the gasification process (Dong et al. 2013). Other compounds such as ammonia, HCl, and sulfides -although they are formed to a lesser extent- make gas use difficult by poisoning commercial catalysts or transforming into substances with toxicological potential (ammonium chloride, NOx). The most commonly used routes to remove such contaminants are thermal cracking, mechanical removal, wet or dry reforming, physical removal, and catalytic cracking (Zwart 2009).

All gasification gas cleaning alternatives tend to decrease their energy power, except reforming (wet or dry) and catalytic cracking. The reforming involves an auxiliary service for supplying steam or CO2 and, in most cases, the use of a catalyst. Catalytic cracking is then presented as the option of the greatest technical-economic appeal. The main challenges for the implementation of catalysts in the cleaning of gasification gases at the commercial level are the durability of the catalysts, knowing the mechanism by which those reactions occur, and the results escalation to real gasification conditions.

Introduction

The most commonly used catalysts are minerals in their natural form (olivines, dolomites), carbons produced by the gasification itself, and noble or transition metals (e.g. iron) supported on organic and inorganic matrixes (Asadullah 2014a). In general, the catalytic activity is variable and usually suffers poisoning or coking, and/or does not resist the operating conditions for a lifetime that justifies their use (Asadullah 2014a). In the case of the support, the common matrices (alumina, silica) are quickly covered by coking, depending on the acidity of the support. Carbons are resistant to the poisoning, but they are consumed in reactions with gasification gases as CO2 and/or steam; also at high temperatures, metal clusters tend to agglomerate and lose their structure, due to mechanical loads inside the reactor. Novels carbonaceous materials have been used as support and mass catalysts, among them, carbon nanocomposites, activated carbons to resist reaction conditions, and carbon aerogels (CAGs) (Maldonado-Hódar 2013; Park et al. 2013; Lam and Luong 2014). The (CAGs) form a separate group within the carbons because they have shown a greater manipulability of their characteristics (specific surface area, average pore diameter, surface functionality) than conventional activated carbons. Moreover, their thermal and mechanical properties can be tailored to get high resistance to extreme conditions (Arteaga-Pérez, Gómez-Cápiro, et al. 2017). CAGs are currently obtained by pyrolysis of aerogels of resorcinol and other synthetic resins as well as microfibrilated cellulose (Maldonado-Hódar 2011; Moreno-Castilla and Maldonado-Hódar 2005; Park et al. 2013; Arteaga-Pérez, Gómez Cápiro, et al. 2017). Cellulose pyrolysis for this purpose is the technology that generates less waste while it comes from a renewable source and has been tested in different reactions as catalytic support. Carbon aerogel obtained from cellulose precursor as support has been used in Proton-exchange membrane fuel cell applications with results similar to commercial products thanks to the ease of physical-chemical properties modifications that carbon aerogel has (Guilminot et al. 2007). Other applications for carbon aerogel found in the literature are related to catalytic upgrading of pyrolysis products using Ni as an active phase, which favors the selectivity to more valuable fractions such as phenolics (Arteaga-Pérez, Jiménez, et al. 2018).

Introduction

Experimental studies of tar decomposition are divided into two groups, those that use models of tars (toluene, benzene, naphthalene, mixes, etc.) and those that use biomass gasification gases directly. These can be found at lab, bench, or pilot scale. Kimura et al. (Kimura et al. 2006) developed lab-scale experiments with biomass gasification gases produced to feed a reactor with a catalyst bed downstream of the gasifier, results allow compare the removal efficiency of different catalysts and set the best reaction conditions. However, it was impossible to obtain any kinetic information for the process due to the heterogeneity of the gasification gases. This scheme of experiments also allows evaluating the temperature effects, metal loading, and promoters on the distribution of the products (Abedi and Dalai 2019). Kinetic details are usually investigated using tar models at lab-scale. It is for those reasons that is preferable to study the kinetics of tars decomposition using models of tars at lab scale.

A way to explain the process is through (or by means) schemes of reaction, which present routes to different products of the most probable reactions without establishing kinetics steps for each of them. Experimental studies that allow establishing these schemes also allow estimating apparent kinetic parameters for each reaction (Jess 1996). Despite the variety of mechanisms proposed, it is generally accepted that the reaction steps after tars adsorption include the breakdown of C-C bonds between functional groups and aromatics rings, followed by ring dehydrogenations and subsequent breakdown (Oemar et al. 2014). Some authors propose that similar catalysts, like carbon-based materials, present a similar reaction mechanism even though tars are different (Fuentes-Cano et al. 2013). Another way to encompass higher groups of tars is the mathematical modeling to represent reactions (Faúndez, García, and Gordon 2001), although without establishing a mechanism. However, heterogeneity of tars along with the variety of reactions that occur simultaneously during their decomposition always makes difficult to propose reactions mechanisms. Literature reports dissimilar alternatives without a consensus as to a specific type of catalyst or mechanism.

Introduction

It would be necessary to design a catalyst that meets the requirements of the process, combining the benefits of the materials used so far. Study its interaction with models of contaminants on biomass gasification gas alone and with other typical gasification gases at lab scale and propose a reaction mechanism together with a reaction model and the thermodynamics parameters of the process. This allows us to pose the following problem:

ii. Technical Problem: The presence of tars and ammonia in biomass gasification gases hinders the use of this gas for energy and chemical synthesis applications, downstream of the gasifier. For its solution and based on the background, which will be discussed later, the following hypotheses are proposed.

iii. Hypothesis: The physicochemical properties of carbon aerogels justify their use as catalyst supports for the decomposition of tars and ammonia. The catalytic decomposition of tars and ammonia on carbon aerogel- supported Fe will facilitate the direct use of biomass gasification gases in internal combustion engines and turbines.

iv. General objective:  To study the catalytic cracking of tars and ammonia on a carbon aerogel-supported Fe catalyst.

v. Specific objectives: 1-. To evaluate the synthesis conditions of carbon aerogels for their use as support for catalytic decomposition of tars and ammonia. 2-. To evaluate the effect of the concentration and nature of tars on the activity, selectivity, and stability of the catalyst during the decomposition of tars. 3-. To understand the decomposition mechanism of tars in the presence of the main gasification gases (CO, H2)

Chapter 1: Background

1 Chapter 1: Background. 1.1. Chile's energy situation. In Chile, the lack of local energy sources is latent, according to the Energy Agenda published by the Chilean Ministry of Energy in 2014 (Ministerio de Energía 2014), in that year the country imported 60% of its primary energy. The same source mentioned, suggest a series of measures to reverse the energy deficiency focusing attention on renewable sources. These include wind, solar, small-scale hydropower (<20MW), and the use of biomass, all framed within the concept of non-conventional renewable energy (NCRE). According to governmental reports, from 2013 to 2019 the use of solar energy to produce electricity passed from zero to 10% of installed capacity, which together with wind power, sum the 80% of NCREs input into the national electric network. Distributed generation is a concept that arises precisely as a solution to the need to take advantage of energy resources, such as NCRE, near its location on a small or medium scale, reducing distribution costs and allowing reaching places far from the main electricity grid (Bayod-ru 2009). Biomass utilization is less than 1.8 % of supply into the Chilean electric system (Chile 2019; Ministerio de Energía 2016). In contrast, Chile has among its most abundant resources, with a planted area of native forest, usable for energy purposes, of 6.5 million hectares (CONAF 2020). The management and use of these forest resources in industries generate wastes equivalent to 24.6% of the total mass harvested. The energy use of waste is equivalent to 7500 GWh/y considering biomass use in conventional generation technologies (Berg et al. 2013).

1.2. Lignocellulosic biomass for energy production. Thermochemical conversion of biomass. Applications. Lignocellulosic biomass has been a source of energy for humanity since the discovery of fire. There is currently a marked interest in its energy use and as a raw material for the chemical industry, driven by the depletion of fossil fuels, its renewable character, and its neutrality in the CO2 generation cycle. There are two main ways to perform the biomass transformation into energy and/or fuels: biochemistry (acid, basic, and

Chapter 1: Background

enzymatic hydrolysis) as the process "organosolv" and thermochemical (liquefaction, torrefaction, pyrolysis, combustion, and gasification). The biochemical pathway comprises extremely long reaction times and complex separation processes of the products obtained. Furthermore, the energy and products from biomass in thermochemical processes (Table 1.1) can be obtained faster than from biological and/or biochemical way. In some cases, it allows obtaining energy directly, although it may also require complex processes of separation before obtaining the desired products. The moisture content of biomass limits its use in most of the thermochemical processes (combustion, torrefaction, pyrolysis, gasification) due to the energy losses associated with drying. Among the thermochemical transformations, the most widely used are combustion and gasification, both with the main purpose of generating heat (Basu 2013).

Table 1.1 Thermochemical processes for biomass energy use. Process Temperature Pressure Main goal (°C) (MPa) Liquefaction 250-300 5-20 Obtain liquid products (resins, a raw material for refining, easy transportation fuel) Torrefaction 200-300 0.1 Raise biomass energy content

Pyrolysis 300-600 0.1-0.5 Liquid products are lighter than in liquefaction. Combustion 700-1400 ≥ 0.1 Energy

Gasification 500-1300 ≥ 0.1 Gaseous products (CO, H2, CH4,

CO2) and energy.

Chapter 1: Background

Thermochemical processes are classified according to the reaction medium and the range of temperatures and residence times in which the transformation itself occurs.

1.2.1. Liquefaction. In this operation, the biomass is transformed into liquid fuels through the contact of the material with water and a catalyst at moderate temperatures (up to 330 ºC) and high pressures (above 12 MPa). This causes the breakdown of lignin and cellulose polymers, replacing oxygen atoms with hydrogen, thus obtaining a mixture of hydrocarbons with energy applications and a source of raw materials for chemical synthesis. Although satisfactory tests have been carried out with urban, livestock, and agricultural wastes it has not been implemented on a large scale. The main burdens for scaling up liquefaction are the high costs associated with the installation and operation of equipment for high working pressures (Basu 2013).

1.2.2. Torrefaction. This operation has aroused interest in obtaining charcoal for co- combustion in coal-consuming plants and as a cleaner fuel for domestic use. It allows efficient and clean combustion, as well as lower transportation costs of biomass, once roasted. These advantages are given by the characteristics of the product obtained: greater carbon composition and lower moisture content, that is, a product with higher energy density than untreated biomass. Torrefacted biomass has an energy density between 4 and 5 MWh/m3 while natural biomass with 10% humidity only has between 0.7 and 1.7 MWh/m3 (Barnó 2009). In essence, the operation is heating under an oxygen-free or oxygen-depleted atmosphere, whereby hemicellulose decomposes, and changes in the structure of cellulose and lignin occur. These changes mean the rupture of C-O and C-H bonds, so water is mostly released in the form of steam along with carbon oxides and traces of hydrocarbons. The use of these exhaust gases as fuel for preheating the feedstock allows the process to be done more energy efficient. Temperature control is essential for the quality and efficiency of the process; the C:O and C:H ratios are directly proportional to it and the

Chapter 1: Background carbon composition determines the caloric power of the product obtained. On the other hand, the process goes from being endothermic to slightly exothermic from 280 °C. The loss of water mentioned above makes the product obtained hydrophobic and inert to decomposition by microorganisms, which facilitates its storage for long periods (Basu 2013).

1.2.3. Pyrolysis. It is a process of partial and controlled oxidation (in the absence of oxygen) at moderate temperatures that allows obtaining a combination of solid fuels (char), liquids (hydrocarbons), and gaseous fuels (poor gas). The composition of the final product is determined by the rate of heating: higher values allow obtain liquid products (bio-oil), while slower heating rates allow obtaining higher yields of gaseous and solid products. The gas mixture is mainly composed of carbon oxides, methane, water vapor, and aromatic hydrocarbons. The liquid is rich in phenolic and other oxygenated organic compounds, together with approximately 20% water. Although the compositions depend on the type of biomass, the energy content of the poor gas varies between 8 and 15 MJ/m3 while the liquid product ranges around 19-21 MJ/m3 on a dry basis and the char around 30 MJ/m3 (Basu 2013).

1.2.4. Combustion. It is the oldest process for obtaining energy by subjecting the biomass to heating in the presence of oxygen to generate CO2 and H2O. The other operations of the thermochemical transformation of the biomass generally allow obtaining products that will be a raw material for combustion. The energy efficiency is approximately 90% of the potential energy of lignocellulosic biomass, which ranges between 9 and 20 MJ/kg when burned directly (Basu 2013). It is common to use the operation for steam generation in boilers in combined cycles and for household heating. It can have a negative environmental impact mainly due to emissions of particulates and carbon monoxide associated with the operational inefficiencies.

Chapter 1: Background

1.2.5. Gasification. Biomass gasification is an alternative that is implemented in several countries, with technologies available in the market, using the resulting gases for different purposes (Sikarwar et al. 2016; S. Zhang et al. 2016). It consists in subjecting the biomass to relatively high temperatures in a slightly oxidizing atmosphere, to obtain a gas with high H2 and CO concentration (synthesis gas). It has broad advantages about the operations mentioned above, especially with combustion, among others, Basu (Basu 2013) mentions: - The possibility of obtaining steam, electricity, and synthesis gas in a single plant. - A gaseous fuel has a greater range of applications than solid fuel. - The transport and distribution of gaseous fuel is cheaper than the transport of solid fuel. - Gasification plants produce significantly low amounts of major air

pollutants like SO2, NOx, and particulates. Total water consumption in a gasification-based power plant is much lower than that in a conventional power plant. To perform gasification, three agents are mainly used: oxygen, steam, or air. The choice of the gasification agent and the amount in which the gasifier is fed determine the composition of the gasification gases. Oxygen causes a greater presence of carbon oxides; the dioxide composition increases as the stoichiometric ratio O:C increases, this produces a hydrogen-poor gas. The excess of oxygen can turn the operation into combustion, producing a gas without caloric power. The use of steam generates a hydrogen-rich gas with a high H:C ratio. The use of air dilutes gasification gases in N2 reducing their caloric power as shown in Table 1.2.

Table 1.2 Gasification gas composition and Lower Heat Value for different conditions. Gasification agent Parameter Oxyge Reference s Steam Air n

Chapter 1: Background

(Saxena et al. 2008; Sikarwar et al. 30- H (%) 26-40 10-44 2017; Dincer 2012; Lv et al. 2007; 2 62.5 Burhenne et al. 2013) (Lv et al. 2007; Burhenne et al. 2013; 20- 22.2- Patil et al. 2011; Geyer, Argent, and CO (%) 17-48 42.6 29.5 Walawender 1987; Demirbas, Karshoglu, and Ayas 1996) (Lv et al. 2007; Geyer, Argent, and 22.3- 27- CO (%) 10-36 Walawender 1987; Demirbas, 2 32 34.6 Karshoglu, and Ayas 1996) (Lv et al. 2007; Geyer, Argent, and CH4 (%) 3.3-6 6-15.8 0.5-8.2 Walawender 1987; Demirbas, Karshoglu, and Ayas 1996) 2.7- (Burhenne et al. 2013; Lv et al. 2007; Tars 4-15b 0.01-48b 7.8a Pinto et al. 2017) (X. Li et al. 2001; X. T. Li et al. 2004; Heat Mathieu and Dubuisson 2002; Value (MJ 12-28 10-18 4-7 Rapagnà et al. 2000; R. Zhang et al. Nm-3) 2004; García, Alarcón, and Gordon 1999) a g/ kg biomass, wet basis b (g/Nm3)

The application of this technology is limited by three fundamental problems: dosing the biomass to the gasifier, achieving a homogeneous distribution of the gasification agent in the cross-section of the gasifier and the presence of compounds like tars and ammonia in the produced gas; the latter is the one that most limits its use. Even so, there are several gasification-to-energy plants at different construction stages and its numbers have an increase in the last few years (See Figure 1.1). Table 1.3 shows a list of plants located in Figure 1.1b.

Chapter 1: Background

Figure 1.1 Comparison of existing gasification plants between August 2016 (image a) and January 2020 (image b) (IEA 2016, 2020).

Table 1.3 Gasification plant status reported by IEA (IEA 2020). Number of Country Status plants Operational 38 Under 3 Germany construction Non-operational 6 No status 1 Operational 17 Italy No status 1 Finland Operational 14 Operational 10 Austria Non-operational 5 Switzerland Operational 7

Chapter 1: Background

Non-operational 3 Operational 5 Under 1 Sweden construction Planned 2 Non-operational 5 Operational 4 Canada Under 1 construction Operational 4 United Under 1 Kingdom construction Non-operational 1 Operational 4 United States Planned 3 Non-operational 3 Operational 3 Denmark Non-operational 3 Japan Operational 2 Operational 2 Netherlands Planned 1 Non-operational 5 Operational 1 France Under 2 construction Operational 1 Turkey Non-operational 1 Indonesia Operational 1 Ireland Operational 1 Thailand Operational 1 New Zealand No status 1

Chapter 1: Background

1.3. Distributed generation. Electricity distribution implies energy losses proportional to the grid long (Nouni, Mullick, and Kandpal 2009). Distributed generation arises as a solution to the need to take advantage of energy resources close to its location on a small or medium scale, reducing distribution costs and allowing to reach places farther from the main electricity network (Bayod- ru 2009). It allows the consumer to generate energy using nearby sources and meet their need for energy consumption, in a more advanced state it allows the consumer to deliver and sell the excess energy generated by becoming a generator. The utilization of biomass gasification gases in combustion engines coupled to electrical generators (Pepermans et al. 2005), as part of distributed generation, is an economic solution to the energy deficit (S. Zhang et al. 2016). The efficiency of this alternative always beats combustions coupled to the steam cycle in installations with capacities between 5 to 100 MWh (Table 1.4); the integrated gasification to the combined cycle (IGCC) reaches around 1.65 times the efficiency of conventional installations (Bridgwater 1995). Gasification systems are available in the market by companies such as Spanner Re² GmbH and Inerco, the last one has offices in Chile (Spanner-Re2 2016; Inerco 2020). The objectives of the aforementioned “Agenda Energética” include promoting self-consumption of energy, the replacement of diesel in generators coupled to combustion engines, and the electrification of rural areas. All these milestones can be fulfilled within the concept of distributed generation, with the implementation of biomass gasifiers to power combustion engines.

Table 1.4 Efficiencies for different power generation systems according to their capacity, adapted from (Bridgwater 1995). Efficiency (%) Capacity Combustion+steam Gasifier+gas (MWe) IGCC cycle engine 20 23 27 37 40 25 30 44

Chapter 1: Background

60 26 30 46 80 28 30 47 100 29 29 48

1.4. Problems related to biomass gasification gases. The composition of the gases at the exit of the gasifier is heterogeneous and varies with different kind of biomass used, their humidity at the moment of being gasified, the gasification conditions (gasification agent, temperature, residence time) and the type of gasifier used (Ruiz et al. 2013). Consequently, their composition never matches the ranges that can be used in internal combustion engines, gas turbines, and synthesis processes (Table 1.5).

Table 1.5 Contaminant content in gases at the exit of different gasifiers and requirements for use in different technologies (Asadullah 2014b). Types of gasifiers Gasification gases uses. Gas FT Unwanted Upstrea Fluidized IC Downdraft turbin synthesi compound m bed engine e s. 10000- 1500- Tars (mg/Nm 3) 10-6000 <100 <5 <0.01 150000 10000 Microparticles 100- 7000- 100-3000 <50 <20 0 mg/kg 8000 12000 Sulfur components - - - - <1 <1

(H2S, CS2) ppm Nitrogen components - - - - - <20

(NH3, HCN) ppb

The biggest problems are associated with the presence of tars, considering as such any organic compound with a higher molecular weight than benzene (78.11 g/mol) (Nordgreen, Liliedahl, and Sjöström 2006). Within

Chapter 1: Background the biomass gasification process, tars are formed as an unwanted product from the decomposition of cellulose and are characterized by structures generally formed by aromatic rings where it is possible to find heteroatoms as nitrogen and sulfur, depending on the composition of the biomass used for gasification (Zwart 2009). Tars usually condense at relatively high temperatures, around 300°C (Nordgreen, Liliedahl, and Sjöström 2006), which causes blockages and fouling in combustion (Figure 1.2), coking in catalysts and decreases the energy content of the gas by sequestering a part of the hydrogen and carbon available in the biomass (Dayton 2002; Zwart 2009). Other compounds such as NH3 are also present in a smaller concentration. However, they can react with chlorinated compounds generating, for example, NH4Cl. This salt solidifies at temperatures below 280°C, causing the blockage in moving parts of equipment and obstruction of pipes (Zwart 2009). In comparison, combustion can also generate NOx, these gases are strongly regulated in the pollutant emission standards (Min et al. 2014), therefore it has no advantages over gasification. For the commercial application of gasification technology, it is essential to design a pre-treatment of cleaning gases that allows a better energy use of biomass via gasification. This treatment should be able to reduce the content of impurities such as NH3,

H2S, HCl, particulate matter, metals, and tars (Asadullah 2014b) to the levels required by the gas technologies or technologies in which the gasification gas will be used (Table 1.5).

Chapter 1: Background

Figure 1.2 Effects of unwanted compounds in a combustion engine (Brandin et al. 2011).

1.5. Gas cleaning. Gas cleaning (Table 1.6) is classified in the literature in four forms as the most commonly used (Zwart 2009). Mechanical and physical cleaning include filters, water traps, rotary separators, and electrostatic precipitators. Besides, thermal cracking and catalysts are used to break down the heaviest substances such as tars into lighter compounds, allowing the use of gasification gases in a wider range of applications.

Table 1.6 Operating temperatures and efficiency of different gasification gas cleaning technologies (Zwart 2009). Work temperature (° % Tar Technology C) Removal Sand filter 10-20 50-97 Washed 50-100 50-90 Electrostatic 40-60 0-60 precipitator Filters 130 0-50

Chapter 1: Background

Fixed bed absorber 130 50 Hot catalysis 500-900 > 95

1.5.1. Physical and mechanical removal. In these operations, the removal of entrained solid particles, part of the tars, and soluble compounds is achieved if the technology involves water in contact with the gases. It may be at the outlet temperature of the gasifier gases or require prior cooling and maybe with or without the presence of water. Cooling the gases for cleaning does not necessarily mean the use of its energy in the process since its composition prevents the use of conventional technologies for heat exchange, which leads to an important exergy loss (Min, Asadullah, et al. 2011). The presence of water facilitates the removal of ammonia, sulfur and nitrogen oxides, hydrochloric, hydrogen sulfide, and cyanic acids, also of one part the tars with polar characteristics and condenses the non-polar a floating phase (Asadullah 2014a). This generates a highly contaminated aqueous stream from which non-soluble tars can be recovered by decantation, for use as heavy fuel in boilers and other burners (Brandin et al. 2011). Parallel to this, an aqueous effluent is generated in a proportion of approximately 11 to 1 with the current heavy tars. It transfers the problem of a gaseous phase that leaves the gasifiers to a liquid phase that requires treatment before leaving the plant, raising operating costs, and limiting the use of this cleaning technology. The removal, by eliminating the pollutants, decreases the caloric power of the gas, losing part of the hydrogen and carbon that make up the structure of the tars and other unwanted compounds.

1.5.2. Thermal cracking. Another way of removing the tars is the thermal cracking process, which is effective above 1000°C. The application of this alternative requires equipment with a special design (Huber, Iborra, and Corma 2006) and implies a significant decrease in the caloric power of the output current, approximately 3.5% for every 100°C that increases the temperature above 1000°C (Asadullah 2014b). The energy losses added to the costs of equipment with special alloys to meet the demands of the process make this alternative of cleaning one of the least economically attractive. Even

Chapter 1: Background so, it has been implemented on a pilot scale (Table 1.7) with a few favorable results in terms of tar levels at the exit of the process.

Table 1.7 Tar content in gasification gases after thermal cracking cleaning. Cracking Gasifier feed Tars content Temperature Reference (kg/h) (mg/Nm3 ) (°C) (Burhenne et al. 12 1000 <50 2013) 30 950 450 (Sulc et al. 2012) (Dogru et al. 5.4 1000-1200 3000 2002) 18.7 950 4800 (Patil et al. 2011)

1.5.3. Catalytic cracking and reforming. The catalytic cracking can decompose simultaneously both, tars (Table 1.8), ammonia and other undesired compounds, being a technical option and economically advantageous (Abu El-Rub, Bramer, and Brem 2004; Huber, Iborra, and Corma 2006). This solution is energy efficient as long as the reaction is carried out at a temperature in the order of the gasifier outlet temperatures. It can be developed with the addition of the catalyst to the gasification chamber (in-situ) or immediately after the operation, (ex-situ). The in-situ option causes greater coking and agglomeration of the catalyst (Dayton 2002), therefore, to put a catalytic bed after the gasifier is considered the most advantageous option, but it is limited by the need to find a catalyst to withstand the requirements of the cleaning operation as a whole (Asadullah 2014a). Favorable results have been reported using iron-rich minerals, alkali metal, alkaline earth oxides, dolomites, and olivines as catalysts, although they are rapidly deactivated by agglomeration, coke deposition or sintering associated with reaction conditions (Min, Asadullah, et al. 2011; David Sutton, Kelleher, and Ross 2001). Among the transition metals commonly used in supported

Chapter 1: Background catalysts, nickel (Jun Han and Heejoon 2009; D. Wang, Yuan, and Ji 2011; Sikarwar et al. 2016) and iron (Stevens 2001; J. Yu et al. 2006; Sikarwar et al. 2016) has shown the best results in terms of selectivity and activity for the decomposition of tars and ammonia. However, they are also deactivated by the deposition of coke or sintering, both decreasing their activity relatively quickly. In general, the decomposition of tars and ammonia is highly favored by catalysts with acid surfaces (Bhandari, Kumar, and Huhnke 2013). The aromatic compounds are adsorbed thanks to the π electrons not shared in the ring(s) and ammonia by the pair of non- shared electrons on nitrogen atoms. Since there is such a strong interaction with adsorbates, acid surfaces favor the formation of coke, mainly in Lewis acid sites (Shen 2015) where tar cracking starts (Cumming and Wojciechowski 1996). In this way, remaining chains and rings are adsorbed irreversibly on active sites, encapsulating the clusters under a shell of carbonaceous material (Trimm 1977). This explains why catalysts on acidic supports such as alumina deactivate rapidly. However, coke formation is multifactorial, depends, for example, on the reaction temperature, the crystalline phase of the metal, and the presence of oxidizing agents. (O2, CO2, H2O). Temperature rise favors the cracking of tars and with this the formation of coke (Figure 1.3) (Yan Zhang et al. 2010); and the presence of oxidizing agents allows the reaction of carbon oxidation, forming the respective oxides that pass into the gas phase, with an order of activity O2 > H2O> CO2 (Trimm 1977). On crystalline phases, nickel, for example, favors coking by light compounds in the order (110)> (111)> (100) planes and by heavy compounds in the order (111)> (110)> (100) indicating that the reaction is sensitive to the structure (Trimm 1977). Coke can be formed at low temperatures, by polymers and copolymers whose structure depends strictly on the reactants. On the other hand, at high temperatures, the coke is formed by polyaromatics and its composition is practically independent of the reactants (Guisnet and Magnoux 2001). In general, carbon deposition can be quantified and qualified by gravimetric and oxidation methods by temperature- programmed treatments (Querini and Fung 1994).

Chapter 1: Background

Table 1.8 Conversions of tar cracking for different catalysts. Temperature % Catalyst Reference (°C) Removal (De Andrés, Narros, and Dolomite 850 76 Rodríguez 2011) (De Andrés, Narros, and Olivine 850 50 Rodríguez 2011) Ni + MnOx 550-650 100 (Koike et al. 2013) /Al2O3 (C. Li, Hirabayashi, and Ni/Ca12 Al14O33 500-800 100 Suzuki 2009) (Lamacz, Krzton, and Ni/CeZrO2 500-900 > 95 Djéga-Mariadassou 2011) (Min, Yimsiri, et al. Fe/Char 500-850 95 2011) Fe/Char 900 97 (Dong et al. 2013) Fe/Olivine 825 91 (Virginie et al. 2010)

Figure 1.3 Dependence of coke formation with temperature. Adapted from (Trimm 1977).

Carbon-based supports have demonstrated to promote the decomposition of tars and inhibit the formation of coke (Min, Yimsiri, et al. 2011; L.

Chapter 1: Background

Wang et al. 2011). Its promoter action is probably because the surface of the carbons is heterogeneous in terms of functional groups allowing reactants adsorption (Figure 1.4). Not all of these functional groups are acidic sites, so coking does not occur so commonly compared with the homogeneous surface.

Figure 1.4 The most common functional groups on carbonaceous materials surface.

The wet (D. Li et al. 2015; Kaewpanha et al. 2015) and dry (D. Sutton, Parle, and Ross 2002) reforming are cleanup alternatives that also require the use of catalysts to achieve the removal of contaminants required by combustions engines and other technologies. In the absence of catalyst, the reforming requires higher temperatures (Min, Yimsiri, et al. 2011), which favor the coke formation. In addition to this, both (wet and dry) need auxiliary steam or CO2 supply service, thus the operation becomes more expensive compared to catalytic cracking.

1.5.4. Iron as active phase in catalytic tars removal. Several reasons made iron one of the principal choices as active phase for tar removal: activity, availability, low pollution potential, price, coke formation resistance, and capability to auto-regenerate itself during the reaction. Metallic iron is considered the most active material for the breakdown of carbon-carbon bonds. Therefore, its study for cracking aromatic compounds such as tars can be considered the basis for designing active and stable catalysts for upgrading gasification gases (Virginie et al. 2010). Fe is also highly active for ammonia decomposition (Stevens 2001), which is an added value for treating complex gases such as those

Chapter 1: Background derived from biomass gasification. Additionally, iron is much more abundant and environmentally manageable than Ni (Shen et al. 2014) and its price is an additional advantage over noble metals and nickel (Nordgreen et al. 2012). During tars decomposition, one of the most difficult obstacles to overcome is carbon deposition, which encapsulates metals clusters. This gives rise to one of the main advantages of Fe to be chosen for tars decomposition since its clusters are resistant to coke deposition (Theofanidis et al. 2015). Furthermore, iron can stay in a reduced phase if is supported on chars or activated carbons; also, carbothermic reduction of iron oxides is possible at temperatures above 700°C (Figure 1.5). A comparison between Fe and Ni catalysts over the same support at lab scale to clean real gasification gases found that Ni catalyst shows higher tars decomposition activity at lower temperatures, but this difference decreased with temperature doing Fe more attractive due to economical evaluations (Min, Yimsiri, et al. 2011). The apparent activation energy reported in these works does not show a significant difference. The values were 57 kJ/mol for Ni and 60 kJ/mol for Fe.

During tars decomposition (under real gasification gases), the oxidation of the metallic Fe depends thermodynamically on the CO2/CO ratio and on the temperature at which CO2 can oxidize the Fe; keeping a low ratio

(PCO2/PCO<0.74) the metal does not suffer oxidation (Nordgreen, Liliedahl, and Sjöström 2006). However, Fe oxides are also reported as active catalysts in the decomposition of tars, but different iron oxides have different activities (Duman, Uddin, and Yanik 2014), although, in all cases, lower than metallic iron. Keep reaction conditions in a range to make spontaneous the carbothermic reduction is an interesting way to avoid activity loss for this cause. However, in the literature are a few reports studying both reactions simultaneously. In addition to oxidation, other causes of activity loss and /or deactivation of Fe catalysts in this type of reactions are sulfur poisoning, if it is present in the gas mixture in some form (H2S, SOx), water oxidation that causes magnetite formation (Fe3O4), hydrothermal sintering and coking (Duvenhage, Espinoza, and Coville 1994).

Chapter 1: Background

Figure 1.5 Gibbs free energy for the carbothermic reduction of iron oxides. Adapted from: (Hoekstra et al. 2016).

1.6. Chars as support. Chars are obtained by carbonization of carbon-rich precursors, generally lignocellulosic materials or synthetic resins. They have the ability to adsorb hydrocarbons in amounts that exceed several times their weight, usually have a high specific area and their surface functionality and morphology can be manipulated (Bhandari, Kumar, and Huhnke 2013). Chars have proven to prevent sintering when used as supports for transition metal catalysts (i.e. Fe) during gasification gas cleaning reactions (Min, Yimsiri, et al. 2011). However, chars are not thermally stable, particularly at high temperatures in slightly oxidizing atmospheres such as gasification gases. Moreover, chars’ physicochemical characteristics depend on the type of biomass used as precursor and preparation conditions (temperature, heating rate, residence time, carbonization atmosphere). Biomass heterogeneity makes difficult to obtain reproducibility in its production, which could hinder the control of catalytic properties if they are used as supports. Many works in the literature reported the use of chars as catalyst support (see Table 1.8). Char supported Fe catalysts have been used to diminish the tars concentration on real gasification gases at lab scale; the tars conversion was around 97% obtaining a gas with a higher concentration of H2 and CO compared to the untreated gas (Dong et al. 2013). Some authors made a comparison

Chapter 1: Background between char and other materials as olivine and dolomite. They report tar conversion values for char as the catalyst, of 81.6%, almost twice the olivine, and near to dolomite (Abu El-Rub, Bramer, and Brem 2008). Similar works studied the influence of metal loading on char catalytic activity using several transition elements (Co, Ni, Cu, Zn); the results analyze the conversion raised and the distribution of the products for each material, proving that all metallic load enhanced the tars decomposition activity for chars (Jiangze Han et al. 2014). Other works report biomass char-supported Fe catalyst to decompose toluene (used as tar model), the results show a 47% decrease in apparent energy activation compared with Fe-free biomass char and lower benzene selectivity during toluene decomposition (Kastner, Mani, and Juneja 2015). The literature reports a majority of articles that analyze the conditions for the total conversion of tars. It is no different when chars are used as catalysts or catalyst supports, given the heterogeneity of biochars, which makes it difficult for the kinetic study of this kind of reaction. They do not report the stability of chars catalyst under reaction conditions or how to improve their resistance to the oxidizing conditions of the gasification gases.

1.6.1. Carbon aerogels. Aerogel is understood as a gel from which the liquid was replaced by a gas without modifying the structure of the solid (Kistler 1931). This is achieved by controlling the drying conditions and freeze-drying has demonstrated to be effective for producing aerogels from different sources. In the last decades, CAG’s have been used in several research applications and obtained from various sources. One of the most studied uses is as adsorbents. Lai et al. (Lai et al. 2016) prepared an aerogel capable to adsorb up to 40 times its weight. Fairén et al. (Fairén-Jiménez, Carrasco-Marín, and Moreno-Castilla 2007) studied the adsorption capacity of benzene, toluene, and xylene from an airflow. With 768 ppm of aromatic, carbon aerogels reached 380 mg of aromatic per g, evidencing the high adsorption capacity of these materials. The electric conductivity is another interesting property of CAG’s giving the possibility to be used as capacitors with characteristics according to their morphology, which can be manipulated during carbonization (Yang et al. 2016). This

Chapter 1: Background possibility to manipulate the pore size and specific surface allows to obtain

CAGs to H2 storage, a critical issue associated with the utilization of this fuel source in the transportation sector (Biener et al. 2011). As catalyst support, CAGs has been studied on several reactions including isomerization, combustion of aromatics, synthesis of organic compounds, and nanofilaments, cathode catalysts in Proton-exchange membrane fuel cell, tars decomposition, etc. (Moreno-Castilla and Maldonado-Hódar 2005; Guilminot et al. 2007; Arteaga-Pérez, Gómez Cápiro, et al. 2017; Arteaga-Pérez, Jiménez, et al. 2018).

CAGs can be synthesized generating an organic gel that is dried via freeze- drying followed by carbonization of the resultant solid. The organic gel can be obtained through organic synthesis (Guilminot et al., 2007; Jia et al., 2016; Maldonado-Hódar, Moreno-Castilla and Pérez-Cadenas, 2004). All those methods require hazardous substances, which make more attractive the use of cellulose-based raw materials, as cellulose has high carbon content and it is renewable, chemically stable, no toxic, and globally abundant (Jazaeri and Tsuzuki 2013; Meng et al. 2015).

1.6.2. Carbon aerogels as support. The pyrolysis of cellulose aerogels allows obtaining carbon aerogels (CAG), which are considered the best candidates among carbonaceous supports due to their typical physical-chemical properties that can be tailored by manipulating the carbonization conditions (Maldonado-Hódar, Moreno-Castilla, and Pérez-Cadenas 2004; Moreno-Castilla and Maldonado-Hódar 2005). This is of extreme importance, mainly because the average pore diameter and its distribution can determine the composition of the reaction products (Figure 1.6) (Jae et al. 2011). Regardless of the synthesis method, CAGs presents several advantages as catalyst supports, allowing to get a uniform distribution of metal particles at the surface (Moreno-Castilla and Maldonado-Hódar 2005); because of their nanostructure, they show mechanical strength (Qian et al. 2002) and high thermal resistance (Aegerter et al. 2012). However, reported mass yield of about 15% (Kulenkampff Schrewe 2015) is far from the theoretical 44.4 % of carbon in cellulose.

Chapter 1: Background

Figure 1.6 Product distribution obtained in the conversion of glucose to aromatics according to the catalyst average pore diameter (Jae et al. 2011).

Different strategies have been reported to increase mass yield: Impregnation of raw material with ammonium salts (George and Susott 1971), carbonization under acid atmosphere (HCl) (Ishida et al. 2004), and modification of the carbonization conditions (Tang and Bacon 1964; Brunner and Roberts 1980). In particular, the impregnation with ammonium salts (i.e., ammonium sulfate) modifies the pyrolysis mechanism that is normally observed for biomass (Arteaga-Pérez, Gómez-Cápiro, et al. 2017). It favors dehydration at lower temperatures and stabilizes the carbonaceous structure (Branca and Blasi 2008), which results in a lower mass loss, allows obtaining a CAG with higher thermal resistance than untreated samples.

1.7. Kinetic aspects. 1.7.1. Toluene decomposition. For catalytic studies of tars removal (catalytic cracking, reforming wet or dry) benzene, toluene and naphthalene are used as models, being typical compounds one and two rings in gasification gases (D. Li et al. 2015). The

Chapter 1: Background reason for the tendency to use toluene is clearly shown in a study by El- rub (Abu El-Rub, Bramer, and Brem 2004) where it publishes an approximate composition of gasification gases (Figure 1.7). It is stated that toluene can be dissociatively adsorbed on transition metals in two * * ways: by separating the ring from the methyl group (C7H8 + 2 = C6H6 + * * CH2 ) or by donating the hydrogen from the methyl group (C7H8 + 2 = * * C7H7 + H ); these radicals dehydrogenize or react with H2O, H2, CO2 to form the reaction products (Oemar et al. 2014). Thermal decomposition of toluene (without catalyst) begins to be spontaneous above 800°C, while naphthalene and benzene require higher temperatures (Jess 1996), the equipment and energy requirements for these conditions are demanding and make the use of catalysts virtually mandatory.

Figure 1.7 Approximate composition of gasification gases at the outlet of a gasifier (Abu El-Rub, Bramer, and Brem 2004).

The kinetic aspects of tars decomposition have been studied and reported under gasification gas conditions. The tars catalytic cracking reaction has been studied using carbons and/or calcium oxides, with phenol as model tar (Guo et al. 2016b). It was observed that the activation energy for H2 formation is lower than for CO formation assuming first order in the reaction for phenol. In later studies (Guo et al. 2016a) the same authors used gangue as catalyst from mineral coal and γ-alumina to eliminate the same tar model and reported activation energies of 55.9 and 47.5 kJ/mol

Chapter 1: Background

for H2 production, and 79.1 and 54.3 kJ/mol for the production of CO, similar to what happens with iron as catalyst. This suggests that catalytic cracking of tars will increase the concentration of H more than CO in the treated gasification gases. Both studies mentioned differences in reaction mechanism between catalysts; however, they do not propose a reaction sequence to explain those differences. Min et al (Min, Yimsiri, et al. 2011) studied the activity of carbons supported iron and nickel catalysts to clean real gasification gases comparing the results with the same reaction over metal unloaded carbon. The authors proposed that the mechanisms in the char catalysts with and without metal are similar, which evidences the char participation in the reaction. In this sense, they proposed that tars adsorb on the carbonaceous surface while metal sites favor the dissociation of the other molecules (CO, CO2, H2), leading to the formation of radicals that - by spillover-, react with hydrocarbons on the char surface (Figure 1.8). The authors themselves note that since they work with real gasification gases, they cannot deepen the reaction mechanism given the complexity of the mixture of tars that comes out of the gasification. As can be seen, it is difficult to find reports of tar decomposition mechanisms, either based on experimental results in labs or simulation environments, which allow establishing what happens during this reaction.

The activation energies reported in the literature for the decomposition of tars vary with the support, the metal, and the tar model molecule used (Table 1.9). For toluene decomposition, Fe shows lower apparent activation energy than Ni. The variation and uncertainty in apparent energy activation are clear, evidencing the need for more in-depth studies to define the kinetic parameters that govern the decomposition of tars.

Chapter 1: Background

Figure 1.8 Interaction of a ringed compound (phenol) with active sites on a carbon surface (from left to right: hydroxyl group, carboxylic acid, amine.) (Shen 2015)

Table 1.9 Activation energy for tars decomposition in different conditions. Eapp Catalyst Reaction Type Tar model Reference (kJ/mol) Benzene 443 (Jess No catalyst S. reforming Naphthalene 350 1996) Toluene 247 Real (Aznar et Commercial nickel Decomposition gasification 58 al. 1998) gases (Devi, Ptasinski, Ni/Olivine S. reforming Naphthalene 213 and Janssen 2005) (Oemar et La Sr Ni Fe O S. reforming Toluene 109 0.8 0.2 0.8 0.2 3 al. 2014) (Oh et al. Ni/Ru-Mn/Al O S. reforming Toluene 45.8 2 3 2016)

Chapter 1: Background

Ni/perovskite (Takise et S. reforming Toluene 78.9 (LSAO) al. 2019) (Yoon, Dolomite S. reforming Toluene 57 Choi, and Lee 2010) (Mani, Kastner, Pine bark biochar S. reforming Toluene 90.6 and Juneja 2013) (Fuentes- Toluene 75 Coconut biochar S. reforming Cano et Naphthalene 72 al. 2013) (Arteaga- Pérez, Fe/activated carbon Decomposition Toluene 98 Jiménez, et al. 2018) (Kastner, Mani, and Fe/Biochar S. reforming Toluene 48.4 Juneja 2015) Real (Cahyono Fe ore Decomposition gasification 44.8 et al. gases 2018)

When toluene is used as tar model, several possible reactions are depending on the rest of the gases that are included in the mixture, some are shown in the following equations (Equations R1.1-R1.5) under the assumption of having in the mixture other common gases present in gasification (e.g., CO2, H2, and CO).

퐶7퐻8 + 7퐶푂2 → 14퐶푂 + 4퐻2 (푅 1.1)

퐶7퐻8 + 퐻2 → 퐶6퐻6 + 퐶퐻4 (푅 1.2)

Chapter 1: Background

퐶푂 + 3퐻2 ↔ 퐻2푂 + 퐶퐻4 (푅 1.3)

퐶7퐻8 + 7퐻2푂 → 7퐶푂 + 11퐻2 (푅 1.4)

퐶7퐻8 + 14퐻2푂 → 7퐶푂2 + 18퐻2 (푅 1.5)

As mentioned before, there is also no consensus regarding the elemental steps followed by the decomposition of toluene. Some studies– based on in situ infrared spectroscopy – propose that the toluene would adsorb on the metal site followed by the cracking of the molecule into adsorbed methyl and adsorbed benzene and the reaction surface intermediates (i.e.

C6H6, CH2, C2H2) have much lower concentrations than the total exposed sites (Oemar et al. 2014). Also, products such as CO, CO2, and H2 quickly desorb. On the contrary, in previous work (Jess 1996) suggested a break in a C-H bond from the methyl group as the next step after adsorption. In any case, the aromatic ring break occurs after the attack to the functional group in tars with these characteristics such as toluene or phenol, which is one of the few conclusions similar in all kinetics studies of tars decomposition.

1.7.2. Ammonia decomposition. In the case of the ammonia decomposition, iron has higher activity among transition metals (Liang et al. 2000). The explanation is probably because reactants and products are adsorbed in different sites, specifically ammonia molecules are adsorbed on top sites while NH2 and H are adsorbed on bridge and hollow sites, respectively, on the iron surface

(Otero et al. 2016). It has been proposed that NH3 decomposition on iron has a first-order dependence on ammonia partial pressure and is inhibited by hydrogen partial pressure and to a lesser extent by nitrogen partial pressure; the data is properly represented by a Langmuir-Hinshelwood type model (Love and Emmett 1941). It has also been reported activation energy values in the order of 87 kJ/mol on iron, for first-order kinetics for ammonia partial pressure (Arabczyk and Zamlynny 1999). The reaction mechanism considers the subsequent dehydrogenation of the molecule

Chapter 1: Background after its adsorption. However, the rate-determining step is different depending on the catalyst. For Ni catalysts, the dehydrogenation is reported as limiting. For Ru, the N2 desorption is the RDS (Takahashi and Fujitani 2016). The difference in the mechanism is caused by the adsorption site on the metal surface. It has been probed by first-principles studies, that ammonia can be adsorbed in bridge, hollow or top sites and this depends on the metal crystallographic plane and changing the reaction pathway, which means that ammonia decomposition is a structure sensitive reaction (Duan et al. 2012). Similar studies confirm that Fe cluster has higher activity as their size is smaller, additionally, the first dehydrogenation has been proposed as the rate-determining step (G. Lanzani and Laasonen 2010). However, other results contradict the latter using theoretical calculations (DFT) to study the effect of the size of iron clusters on ammonia adsorption and decomposition, suggesting that higher clusters size favors the first dehydrogenation step and causes a rise in the adsorption energy (Otero et al. 2016). Experimental studies are not common for the reaction system iron-ammonia to quantify the energy involved in these processes and discern from simulations results.

1.8. Research questions. In general, there are two major problems in the catalytic cleaning of gasification gases, which were approached in the following chapters: (i) reaction mechanisms, and (ii) the characteristic thermodynamic parameters for describing the decomposition reactions, and the stability of supports and catalysts, under the studied reaction conditions. There is no consensus about the reaction mechanism of tars decomposition during gasification gas cleaning. Also, the literature does not show enough data about the parameters that govern the interaction of other pollutants, such as ammonia, with the catalysts used for gasification gas cleaning. The carbonaceous supports are proposed and studied because of their resistance to coke deposition. Literature reports how to manipulate chars’ morphological characteristics. However, other problems as their thermal stability, how to increase the mass yield during carbonization, or how to avoid the excessive formation of oxygenated groups receive far less attention.

Chapter 1: Background

The results showed below, propose a way to design carbonaceous supports with characteristics to be used as catalysts support for gasification gas cleaning. Besides, a pre-treatment with ammonium sulfate is applied to the precursor to increase thermal resistance and final mass yield of the char obtaining during the carbonization process. A carbonaceous material produced through this method was chosen as support for iron as the active phase. The interactions of both materials (support and iron supported) were studied to determine the thermodynamic parameters of the process. Tars decomposition over the catalyst was investigated to propose a reaction mechanism of different aromatics compounds and to determine the thermodynamic parameters of the reaction. In parallel, the stability of the catalyst during the tars decomposition was analyzed to associate it with the iron deactivation under reaction conditions.

The following sections are presented in a format similar to that of a research article, due to most of the results were published or are submitted to scientific journals. The Chapters are independent; however, they follow a logic corresponding to fulfill the thesis objectives and to demonstrate the research hypothesis.

Chapter 2: Carbon Aerogel-Supported Iron for Gasification Gas Cleaning: Ammonia Adsorption.

2 Chapter 2: Carbon Aerogel-Supported Iron for Gasification Gas Cleaning: Ammonia Adsorption.

Biomass gasification is a promising way to obtain “green energy”, but the gas composition makes it unsuitable for use in traditional technologies (i.e., IC engine). Gas purification over iron catalysts is an attractive alternative. Cellulose-based carbon aerogels (CAGs) have shown suitable physical-chemical properties for use as catalyst supports. In this work, the iron catalyst is supported on CAG made from cellulose microfibers.

Microfibers were impregnated with (NH4)2SO4 until reach 5% w/w to increase the mass yield. Carbonization was evaluated at different heating rates (10-20°C/min), maximum temperatures (800-1100°C), and dwell times (0-2h) to generate CAGs. Resulting chars were characterized by N2 adsorption, X-ray diffraction (XRD), and Raman spectroscopy. The CAG with better properties (larger specific surface, higher thermal resistance) was impregnated with (Fe(NO3)3·9H2O via incipient wetness and treated with H2 at 700°C during 2h with 2°C/min of heating rate. The catalyst was characterized by transmission electron microscopy (TEM), XRD, N2 adsorption at 77K, and inductively coupled plasma optical emission spectrometry (ICP-OES). Ammonia adsorption was studied over CAG and catalyst to estimate the thermodynamic parameters. The impregnation with ((NH4)2SO4 improves the thermal resistance of the CAG obtained from carbonization. The catalyst exhibits higher adsorption capacity than CAG (without metal), indicating chemical interaction between ammonia and metals.

Chapter redrafted after: Gómez-Cápiro, O., A. Hinkle, A.M. Delgado, C. Fernández, R. Jiménez, and L.E. Arteaga-Pérez. 2018. “Carbon Aerogel-Supported Nickel and Iron for Gasification Gas Cleaning. Part I: Ammonia Adsorption.” Catalysts 8 (9). https://doi.org/10.3390/catal8090347.

Chapter 2: Carbon Aerogel-Supported Iron for Gasification Gas Cleaning: Ammonia Adsorption.

2.1. Introduction. Biomass is abundant and easy to harvest but faces difficulties associated with its distribution logistics (collection, transportation, and distribution). Furthermore, biomass, especially wood, has lower energy density (2–3 GJ/m3) than other common fuels such as ethanol (23 GJ/m3) and gasoline (35 GJ/m3)(Abu El-Rub, Bramer, and Brem 2004). One of the most traditional and promising technologies for converting solid biomass into energy, fuels, and commodities is gasification (Vamvuka et al. 2012). The composition of gasification gases (CO2, H2, CO, CH4) can vary depending on gasifying agents (i.e., steam, air, oxygen), gasification temperature, reactors design, as well as feedstock composition. For example, Molino et al. (Molino et al. 2018) have reported that Pine sawdust gasification results in a syngas composition with a high content of CO (35–43%) and H2 (21–

39%), while CH4 (6–10%) and CO2 (18–20%) appear in lower concentrations. However, changing biomass type by α-cellulose the balance is quite different with less CO (6.5–11.2%), H2 (13.5–18.5%), and

CH4 (2.2–3.7%) and increasing CO2 (26.3–27.7%). Regardless of the gasification conditions, the syngas is contaminated with traces of undesirable species such as ammonia (NH3) and sulfur compounds (H2S,

SOx), with condensable polyaromatics known as tar and with solid particles(Dong et al. 2013). These contaminants are not allowed for most technologies used downstream the gasifier such as IC engines, turbines, FT-synthesis reactors, etc. Tar elimination is the higher investment and operation cost in gasification; thus, the removal of tar is of paramount importance to reduce the overall cost of biomass gasification (Dong et al. 2013). However, ammonia, even at low concentrations, is inadmissible for F-T synthesis processes and may limit the use of combustion gases due to the generation of NOx (Min et al. 2014). Physical cleaning pathways eliminate these compounds employing filters, water traps, rotary separators, electrostatic precipitators, etc. (Zwart 2009). All of these technologies waste the potential energy present in the contaminants (especially tar) (Min, Asadullah, et al. 2011) and transfer the contaminants to another effluent (liquid or solid), which also requires treatment before disposition, thus elevating the operation costs (Brandin et al. 2011). A promising way to eliminate tar and ammonia is via thermal or catalytic

Chapter 2: Carbon Aerogel-Supported Iron for Gasification Gas Cleaning: Ammonia Adsorption. cracking/reforming, which could also contribute to increasing the concentration of COx and/or H2 in the syngas (Asadullah 2014b; Huber, Iborra, and Corma 2006; Abu El-Rub, Bramer, and Brem 2004). Thermal cracking depends on temperature and begins to be effective over 1000 °C, so it requires special equipment, which elevates the installation costs (Huber, Iborra, and Corma 2006). Above this temperature, each 100 °C increase implies a caloric power loss of 3.5% (Asadullah 2014b). Wet and dry reforming processes do not require high temperatures but depending on gasification conditions they could require an auxiliary service to supply steam or CO2, respectively. Furthermore, both technologies require catalysts to reduce the activation energies of specific reactions (Min, Asadullah, et al. 2011; D. Sutton, Parle, and Ross 2002; D. Li et al. 2015; Kaewpanha et al. 2015). Catalytic cracking works at the same temperature as reforming, but without the need of an auxiliary service, making it the most economically attractive technology. Oxides of alkaline and alkaline- earth metals, dolomites, and olivine have been reported as active for cleaning gasification gases. Nevertheless, all of them are deactivated by agglomeration, coke deposition, and/or sintering at the reaction conditions (Min, Asadullah, et al. 2011; David Sutton, Kelleher, and Ross 2001). Iron is used in this kind of reaction because it is considered the metal with the highest activity for breaking carbon-carbon bonds (Virginie et al. 2010). It is also active for ammonia decomposition (Stevens 2001). One of the main drawbacks of using metal catalysts to clean gasification gases is the coke formation, which is influenced by the operational conditions, and the catalyst and support properties (Min, Asadullah, et al. 2011).

The most used supports (e.g., alumina) act as reaction promoters, but they have a strong interaction with hydrocarbons so they are affected by coke formation, mostly on acid sites (Shen 2015). On the other hand, carbonaceous materials have proven to have a high affinity and adsorption selectivity to hydrocarbon compounds (Xu, Hamilton, and Ghosh 2009) such as tars. Apart from an easily tailorable porous structure and surface chemistry, carbon presents other advantages as catalyst support: metals at the surface can be easily reduced; the structure is resistant to acids and bases and is stable at high temperatures if proper treatment is used; the

Chapter 2: Carbon Aerogel-Supported Iron for Gasification Gas Cleaning: Ammonia Adsorption. active phase can be easily recovered; conventional carbon supports usually have lower costs than other conventional supports, such as silica (R.A. van Santen et al. 2000; Serp and Machado 2015); and higher resistance to surface coke formation (Shen 2015). Nevertheless, carbon supports also present some disadvantages, such as their low reproducibility, since different carbonization batches of the same raw material can contain varying ash amounts.

In particular, carbon aerogels (CAG) solve the problem of reproducibility because they are obtained from standardized raw materials, mostly by carbonization of cellulose microfibers (MFCs) (Meng et al. 2015) or aerogels obtained via organic synthesis (Pekala 1989). CAGs can be synthesized using a sol-gel process, generating an organic gel that is dried via freeze-drying. The dry gel is pyrolyzed (carbonized) to obtain the corresponding CAG. Cellulose-based organic gels are of particular interest, as cellulose has high carbon content and it is renewable, chemically stable, and globally abundant (Jazaeri and Tsuzuki 2013). Alternative synthesis methods, using different raw materials, include: sol- gel systems based on the crosslinking of cellulose acetate with polyfunctional isocyanate in acetone (Guilminot et al. 2007); sol-gel polymerization of linear phenolic resin and hexamethylenetetramine in alcoholic solutions (Jia et al. 2016); sol-gel polymerization reaction of resorcinol and formaldehyde in water (Maldonado-Hódar, Moreno- Castilla, and Pérez-Cadenas 2004). All of them correspond to organic synthesis with hazardous substances, which make more attractive the use of cellulose-based raw materials. Regardless of the synthesis method, CAGs presents several advantages as catalyst supports, compared to common carbon materials. They allow a uniform distribution of metal particles at the surface, and the metal dispersion is stable upon heating treatments (Moreno-Castilla and Maldonado-Hódar 2005); because of their nanostructure, they show higher thermal resistance (Aegerter et al. 2012) and mechanical strength (Qian et al. 2002). In addition to catalyst supports, CAGs can be used as adsorbents, electrodes, and supercapacitors for secondary batteries, hydrogen storage, and desalination (Biener et al. 2011; E. Hu et al. 2016; Maleki 2016).

Chapter 2: Carbon Aerogel-Supported Iron for Gasification Gas Cleaning: Ammonia Adsorption.

Morphological and structural properties of CAGs from cellulose can be easily manipulated by controlling the carbonization process (Table 1). According to Jazaeri et al., temperature and heating rate are key parameters in the carbonization (Jazaeri and Tsuzuki 2013). Their experiments indicated that long holding times (above 17 h) and slow heating rates (below 2 °C/min) help to preserve desired fibrous morphology and acceptable mass yield, although reported mass yields are far from the theoretical 44.4 % of carbon in cellulose (about 15% (Kulenkampff Schrewe 2015)). Different strategies have been reported to increase mass yield: Impregnation of raw material with ammonium salts (George and Susott 1971), carbonization under acid atmosphere (HCl) to promote fast dehydration and avoid carbon losses (Ishida et al. 2004), and modification of the carbonization conditions (Tang and Bacon 1964; Brunner and Roberts 1980). In particular, the impregnation with ammonium salts (i.e., ammonium sulfate) modifies the pyrolysis mechanism that is normally observed for biomass. It favors dehydration at lower temperatures and stabilizes the carbonaceous structure (Branca and Blasi 2008), which results in a lower mass loss.

Usually, higher carbonization temperatures lead to carbons with higher specific surface area, up to the point (different for each raw material) where the structure collapses as a result of the cross-linking of carbon atoms, which reduces the space between them (Marsh and Rodríguez- Reinoso 2006). Nevertheless, Table 2.1 shows that for CAGs, the temperature has a lower effect on the carbon surface area when lower heating rates and shorter dwell times are used, probably because the energy-to-time ratio is not enough to remove high amounts of solid mass to create more space between atoms and, consequently, higher porosity. Several authors have reported the use of CAGs as catalyst supports for the conversion of organic compounds that can be classified as tar. Maldonado-Hódar used CAGs-supported Pt catalysts to study the adsorption and decomposition of toluene, xylene, and acetone, finding that even at high temperatures, the support retains part of these compounds (Maldonado-Hódar 2011). Ábrahám et al. published a study about the acetic acid hydroconversion reaction over Mo/CAGs catalysts;

Chapter 2: Carbon Aerogel-Supported Iron for Gasification Gas Cleaning: Ammonia Adsorption. their results show that the reaction pathways and product distributions are controlled by the accessibility of carbon surface, as well as by the amount and shape of Mo particles (Ábrahám et al. 2014). Hai Woong Park et al. published an extensive work on lignin model decomposition over Pd/CAGs catalysts, showing the need of modifying the catalyst surface to reach the required performances (Park et al. 2011, 2013). These researches show that CAGs, used as catalysts support, have a direct influence on activity and selectivity in tar decomposition reactions.

Table 2.1 Effect of temperature, dwell time, and heating rate on carbonized cellulose-based precursors reported in the literature. Researcher Conditions Results

T Dwell Heating Rate SBET rpore (°C) Time (h) r (°C/min) (m2/g) (nm) Brunner (Brunner 460 - 70 5.2– - and Roberts 1980) 370a 900 - 0.03 610 - 11 525 - 70 470 - Meng (Meng et al. 700 0.25 5 550 > 4 2015) 950 100 ~ 5 Yu (M. Yu, Li, 950 2 5 742 - and Wang 2016) Xie(Xie et al. 400 8 2.5 2.6 45.5 2009) 500 5.2 46 700 438 45 1,000 449 44 Grzyb (Grzyb et 1,000 1 4 117 4.5–7 al. 2010) 1,100 165 4–7.5 a first value: N2 atmosphere carbonization, second value: CO2 atmosphere carbonization.

Chapter 2: Carbon Aerogel-Supported Iron for Gasification Gas Cleaning: Ammonia Adsorption.

The use of carbon-supported catalysts for ammonia decomposition is not common. Research has mostly focused on ammonia adsorption over carbon and carbon-supported metals as a solution to emission control. Rodrigues et al. studied the influence of initial ammonia concentration and carbon bed temperature during the ammonia adsorption on activated carbon (Rodrigues et al. 2007). They found a direct dependence of adsorption capacity of ammonia over carbon with the amount of ammonia in the fluid phase, contrary to rise temperature dependence. Bandosz and Petit used carbon from different sources and different metal impregnation to evaluate strength and amount of ammonia adsorbed over carbons surface and its interaction with metals particles (Bandosz and Petit 2009); they concluded that in unmodified carbon, porosity controls the adsorption process. More detailed studies on metal-ammonia interaction have been published using theoretical ways, in which the effect of support is not considered. Results suggest that the adsorption of ammonia on transition metal atoms occurs in bridge, hollow, and top positions, through the unpaired electrons in nitrogen (Duan et al. 2012; Otero et al. 2016; Torrente-Murciano, Hill, and Bell 2017). Their results suggest a chemical interaction between ammonia and metals supported by calculated thermodynamic parameters.

CAG-based catalysts can be prepared using organics and inorganics precursors. Smirnova et al. impregnated CAG with Co- mesotetramethoxy-phenylporphine in tetrahydrofuran (Smirnova et al.

2009) and Hu et al. impregnated mesoporous CAG with Cu(NO3)2 3H2O (E. Hu et al. 2016). Most synthesis methods entail the addition of the active metal to the support via incipient wetness impregnation (Maldonado-Hódar, Moreno-Castilla, and Pérez-Cadenas 2004; Wojcieszak et al. 2006), followed by drying for 4 to 12 h, and calcination (Xiong et al. 2010) and/or reduction for up to 7 h, at temperatures of maximum 400 to 600 °C, with slow heating rates.

In general, experimental studies show enhancement of adsorption capacity with carbon surface modification and metal impregnation independent of

Chapter 2: Carbon Aerogel-Supported Iron for Gasification Gas Cleaning: Ammonia Adsorption. catalysts preparation methods. Thermodynamic parameters are defined by theoretical studies. Experimental data of thermodynamics parameters of ammonia adsorption over metallic phases over carbon are not determined. In the present work, the adsorption of ammonia on an iron catalyst supported on cellulose-based CAG is studied, as a first step to propose a kinetic mechanism of gasification gas cleaning reactions. An experimental design was made for the synthesis of supports and catalysts used in the study of ammonia and tar decomposition reactions. The present report shows the first part related to support preparation and ammonia adsorption. The support (CAG) was produced from freeze-dried MFCs, which were carbonized at different carbonization temperatures (900, 1,000, and 1,100 °C), heating rates (10 and 20 °C/min), and dwell times (0, 1, and 2 h). As the support will be used for gasification gas catalytic upgrading, the lowest carbonization temperature was restricted at 900°C, to guarantee the stability of support during operation, typically between 700°C and 900°C (Asadullah 2014a). An impregnation of MFCs with ammonium sulfate was carried out to enhance the thermal resistance and mass yield of char in carbonization processes. The catalyst was characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and inductively coupled plasma optical emission spectrometry (ICP-OES). Finally, the effect of temperature and initial ammonia concentration on the adsorption of ammonia over synthesized catalysts was studied.

2.2. Materials and Methods. 2.2.1. Microfibers Treatment. The University of Maine (USA) provided the freeze-dried microfibers used for CAGs preparation. The MFCs were prepared from a gel produced from a Bleached Kraft Pulp at 3 wt.% of solids. The gels were freeze-dried according to the procedure reported by Demers (Spender et al. 2012) and known as ice segregation induced self-assembling (ISISA). The MFCs were treated with a flame retardant, according to a procedure reported elsewhere (ABNT 8112 1968). The freeze-dried MFCs, previously ground, was conformed as disk-shaped pellets in a Parr press (Parr Instrument Company, Moline, IL, USA). The pellets were impregnated

Chapter 2: Carbon Aerogel-Supported Iron for Gasification Gas Cleaning: Ammonia Adsorption. via incipient wetness with an appropriate amount of ammonium sulfate

((NH4)2SO4, 99.5 % purity, Merck) to get 5 wt.% (dry basis) of salt in the pellets. Impregnated samples were dried at 40°C for 20h, and the impregnation effectivity was confirmed by gravimetric analysis.

2.2.2. Carbon Aerogel Preparation. Carbonization. Carbonization was carried out in a split Thermo Scientific Lindberg/Blue M tube furnace. Approximately 7.4 g of pre-treated MFC pellets (36 pellets) were stacked in aluminum-oxide trays, which were placed parallel to the direction of N2 gas flow (Air Liquide, 99.999% purity, Coronel, Chile) of 20 mL/min/g of sample. The furnace was heated up to the selected temperature (900 °C, 1,000 °C, or 1,100 °C), at a constant heating rate of 10 °C/min and maintained at that temperature for different dwell times (0, 1, or 2 h). Resulting samples were denoted as CAG XY, where X is the number of hundreds of Celsius degrees at which the carbonization was carried out, and Y is the number of hours of dwell time. For example, the sample treated at 900 °C with a dwell time of 1h was denoted as “CAG 91”, and the sample treated at 1,100 °C with 0h of dwell time was denoted as “CAG 110”. An additional sample was prepared from the as-prepared MFC, at 900 °C, and 0h of dwell time, and it was denoted as “CAG 90– 0%”.

2.2.3. Catalysts Preparation. Catalysts with a metal loading of 10 wt.% were prepared via incipient wetness impregnation. Iron nitrate (Fe(NO3)3·9H2O, >99% purity, Merck) was used as metal precursors. The selected CAG support was ground with an agate mortar (<80 meshes). Afterward, the corresponding nitrate aqueous solution was slowly added to the CAG support, in the appropriate quantities to reach 10 wt.% of metal in the final catalyst, according to the pore volume determined by N2 adsorption-desorption (see Section 2.6). Iron loading samples were dried at 105 °C for 4 h, and ground again. The solid was reduced for 2 h at 700 °C, under 40 mL/min of hydrogen, using a heating rate of 2 °C/min.

Chapter 2: Carbon Aerogel-Supported Iron for Gasification Gas Cleaning: Ammonia Adsorption.

2.2.4. Compositional Analysis. Inductively coupled plasma optical emission spectrometry (ICP-OES) (Pekin-Elmer, Waltham, MA, USA) analysis was performed to determine the actual iron content on the catalysts. MFC and CAG samples were also characterized using the standard practice for elemental analysis (Poletto, Ornaghi Júnior, and Zattera 2014), and a Leco True Spec analyzer (LECO Argentina S.A., Buenos Aires, Argentina). The main and trace inorganic elements were quantified by using a PerkinElmer Optima 7000 DV ICP- OES series instrument (Pekin-Elmer, Waltham, MA, USA).

2.2.5. N2 Adsorption-Desorption at 77K.

N2 adsorption was performed to estimate the specific surface area, by the Brunauer-Emmet-Teller (BET) model and pore volume. Barret-Joyner- Halenda (BJH) pore size distribution was determined using desorption data. Both isotherms were recorded in a Micromeritics Gemini VII 2390t device (Micromeritics, Communications Dr Norcross, GA, USA), for MFCs, CAGs, and catalysts. Before tests, 0.2–0.5 g of samples were properly degasified, at 105 °C for MFC and 150 °C for other solids, under a continuous pure N2 flow, during 24 h, as recommended by De Lange et al., (De Lange et al. 2014).

2.2.6. Thermal Resistance. A Netzsch TGA thermobalance (model STA 409 PC) (Netzsch, Selb, Germany) was used to compare CAGs thermal resistances in the oxidizing atmosphere. Samples (~20 mg) were heated from room temperature up to 1,000 °C, at 10 °C/min, under a constant airflow of 80 mL/min (Air Liquide, 99.999 % purity, Coronel, Chile). Different CAG samples were analyzed together with a commercial activated carbon (Pittsburgh) used here as a patron to compare characteristics. Furthermore, the thermal resistance of the CAG selected for supporting Fe was compared with another CAG sample prepared at the same conditions, but without treatment with ammonium sulfate.

Chapter 2: Carbon Aerogel-Supported Iron for Gasification Gas Cleaning: Ammonia Adsorption.

2.2.7. X-Ray Diffraction (XRD).

XRD analysis of as-prepared and (NH4)2SO4-treated MFCs, CAGs, and catalyst were performed to evaluate the crystallinity, crystallite size, and polymorphy of the samples. The same commercial activated carbon (Pittsburgh) was analyzed as a reference to compare with the CAGs. The XRD patterns were recorded on a Bruker AXS model D4 Endeavor diffractometer (Bruker AXS GmgH, Karlsruhe, Germany), using monochromatic CuKα radiation (λ = 0.15418). The signal was generated at 40 kV and 20 mA. The intensities were measured in the range 5° < 2ϴ < 60° for MFCs, and 5° < 2ϴ < 90 °C for CAGs and the catalyst, with a step size of 0.02° and scans at 1 s/step. The crystalline index for MFC was determined by the well-known empirical method proposed by Segal (Segal et al. 1958) (Equation (Eq. 2.1a)) and further corrected by Herman’s equation (Equation (Eq. 2.1b)).

(퐼 − 퐼 ) 퐶푟퐼 = 200 푎푚 (퐸푞. 2.1푎) 퐼200

(퐴퐶푟푦푠푡) 퐶푟퐼 = (퐸푞. 2.1푏) 퐴푇표푡푎푙 where I_200 and I_am are the intensities in the plane (200) and amorphous phase, respectively, A_Cryst is the total area of crystalline bands, and A_Total is the total area of the XRD pattern.

The Z-function of (Wada and Okano 2001) (Eq. 2.2)) was used to determine the crystal structure (Iα or Iβ) based on d-spacing. This discriminant analysis states that for Z < 0, the most probable structure is monoclinic (Iβ), and on the contrary, if Z > 0, triclinic (Iα) structure prevails.

푍 = 1693푑1 − 902푑2 − 549 (퐸푞. 2.2)

Chapter 2: Carbon Aerogel-Supported Iron for Gasification Gas Cleaning: Ammonia Adsorption. where d1 is the d-spacing of the plane (1͞10) and d2 is the d-spacing of the (110) peak, and they are determined according to Bragg’s correlation (Eq. 2.3):

푛휆 = 2푑푠푖푛(휃) (퐸푞. 2.3) where n is the order of reflection, d the interplanar spacing, and θ is the angle of incidence. Z > 0 indicate cellulose type Iα (triclinic crystallite structure) and Z < 0 indicate cellulose type Iβ (monoclinic crystallite structure).

The crystallite apparent size was calculated using the Scherrer’s equation for all solids (Eq. 2.4)).

퐾휆 퐿 = (퐸푞. 2.4) 훽cos (휃) where K is a constant equal to 0.94 for all cellulose and catalyst samples. For char, K is equal to 1.84 in (010) plane and 0.89 in (002) plane to calculate the graphitic crystallite dimensions La and Lc [68], λ is the wavelength (in nm), β is the full width at half maximum intensity (FWHM) (in rad), and θ is the plane angle.

2.2.8. Raman Spectroscopy Analysis. Raman spectra were obtained in a LabRAM HR Evolution Raman spectrometer (HORIBA Scientific) (Horiba Ltd., KyotoJapan) with an excitation laser wavelength of 633 nm, in a range of 50–4,000 cm−1. The obtained spectra were decomposed according to the peak/band assignment proposed by X. Li et al. (Xiaojiang Li, Hayashi, and Li 2006), using OriginPro 8 software (OriginLab, Northampton, MA, USA). Raman spectra reveal CAGs surface chemical structure, especially oxygenated groups; its presence in alpha position increases the adsorption capacity of chars due to acid-base interactions with ammonia (Gonçalves et al. 2011). To compare all Raman spectra, a ratio between D (1,300 cm−1) and G (1,590 cm−1) bands area was used, in this case, a higher ratio is associated

Chapter 2: Carbon Aerogel-Supported Iron for Gasification Gas Cleaning: Ammonia Adsorption. with better thermal stability because D bands are associated with graphite, more thermally stable than G bands associated with disordered carbon (Kordouli et al. 2017). It is expected the absence of graphitization in the CAGs. Activated carbon Pittsburgh was analyzed using Raman spectroscopy to compare with CAGs, just like in thermal resistance and XRD analyses.

2.2.9. Transmission Electron Microscopy (TEM). The metal cluster sizes on the catalyst were observed by transmission electron microscopy in a JEOL JEM 1200 EXII device (JEOL Ltd., Peabody, MA.USA), with voltage 120 kV. Each sample was suspended in a solution of ethanol-water (50–50 wt.%), supported in a copper grill, and covered by a carbon layer. The size distribution and mean cluster sizes for each catalyst were estimated after measuring more than 300 metal particles in 10 images.

2.2.10. Ammonia Adsorption Experiment. Ammonia adsorption was studied over selected support and the catalyst at

50, 100, and 150 °C, using N2 as the carrier gas. The total flow was kept unchanged to guarantee the same residence time in all experiments. The adsorption tests were carried out in a system consisting of a furnace (Omega Eng, CRFC-312/240-C-A, Stamford, CT, USA), into which a quartz reactor was placed with 0.4 g of CAG or catalyst (particle size ≤ 53.3 μm). Mass flow controllers (Aalborg series GFC, WReichman,

Chile) were used to control the ammonia (Air Liquide, at 1,000 ppm, N2 balance) and N2 (Air Liquide, 99.999% purity, Coronel, Chile) flows to achieve the required ammonia concentration, i.e., 135, 193, 232, 331, and 390 ppm for CAG support and 135, 193, 232 ppm for CAG-supported Fe catalyst. An electrochemical-type detector (Dräger X-am 7000, Drägerwerk AG & Co. KGaA, Lübeck, Germany) was used to measure ammonia concentration in the gas phase (Schematic representation in Annexes, Scheme B1). The detector was calibrated before each measurement, as suggested (Bandosz and Petit 2009; Le Leuch and Bandosz 2007). Before each experiment, the CAGs samples were in situ degassed for 4h at 150 °C in N2 flow. Catalyst samples were heated up to

Chapter 2: Carbon Aerogel-Supported Iron for Gasification Gas Cleaning: Ammonia Adsorption.

400 °C using a heating rate of 2 °C/min, under constant H2 flow (Air Liquide, 99.999% purity, Coronel, Chile). This treatment was performed to eliminate adsorbed water and oxide formed over the catalyst surface during the manipulation of reduced samples. Furthermore, this was done to avoid the presence of adsorbed oxygen that can modify the interaction metal-ammonia drastically (Netzer and Madey 1982). Each temperature- ammonia concentration point corresponds to a certain quantity of ammonia adsorbed, which was calculated by integration of breakthrough curve, just as reported in the literature for similar ammonia adsorption studies (Rodrigues et al. 2007; Gonçalves et al. 2011; Mangun et al. 1999). In this work, the experiments were stopped after reaching the equilibrium and the breakthrough curves were numerically integrated. The calculated quantity was normalized per gram of solid. To determine the quantity of ammonia adsorbed on the metal surface, the corresponding quantity adsorbed on CAG was estimated from the metals/CAG specific surface ratio as it is explained later.

The data were treated with Langmuir (Eq. 2.5)) and Freundlich (Eq. 2.6)) models to obtain equilibrium constant. Thermodynamic parameters were calculated with equilibrium constant using van Hoff linearized equation (Eq. 2.7)). Freundlich model was taken as proposed by Vannice (Vannice 2005).

퐾푃 휃 = 푟 (퐸푞. 2.5) (1 + 퐾푃푟) where ϴ is the coverage, K is the equilibrium constant, and Pr is the relative pressure assuming ideal gas behavior.

푅푇 푄 휃 = 퐾표푃푟 푎푑 (퐸푞. 2.6) where ϴ is the coverage, Ko is defined by Vannice (Vannice 2005) in Equation (Eq. 2.6a), R is the universal gas constant (J/mol K), T is the temperature (K), and Qad is the heat of adsorption (J/mol).

Chapter 2: Carbon Aerogel-Supported Iron for Gasification Gas Cleaning: Ammonia Adsorption.

Δ푆° 퐾표 = 푒 푅 (퐸푞. 2.6푎) where ΔS° is the entropy (J/mol K) change associated with the adsorption process in standard conditions.

훥푆 ΔH 푙푛퐾 = ln ( 푎푑) + ( 푎푑) (퐸푞. 2.7) 푅 푅푇

where ΔSads (J/mol K) is the entropy change for the adsorption process, and ΔHads (J/mol) is the enthalpy change associated with the adsorption process.

Total surface coverage was required to model the adsorption data obtained for each sample (CAG selected as support, supported iron). In this work, coverage obtained from adsorption experiments performed at 0°C and 490 ppm of ammonia were considered as the reference for total coverage. These experiments were carried out following the same procedure described before but placing the U-quartz reactor inside an ice bath. Defined as θ (Eq. 2.8), coverage was calculated as follows:

푁퐻3 푎푡 푇푖; 푃푟푖 휃 = ln ( 푎푑 ) (퐸푞. 2.8) 푁퐻3푎푑 푎푡 푇 = 0°퐶; 푃푟 = 0.00049 푝푝푚

where Ti and Pri are the temperatures and the relative pressure in experimental condition “i”.

The metal surface for the catalyst was calculated by considering the particle size distribution obtained from TEM images. The distributions were extrapolated to reach one gram of pure metal at the surface, using the Solver tool. The objective function (Eq. 2.9) included a volume calculated from density (at room temperature) of each metal. The restriction (Eq. 2.10) was kept constant the ratio between particles counts for each particle size range of the distribution by selecting one range as a reference. The surface area (Eq 2.11) was estimated assuming that particles were hemispheres with the plane of the circumference in contact

Chapter 2: Carbon Aerogel-Supported Iron for Gasification Gas Cleaning: Ammonia Adsorption. with the support. Thus, each metal particle has only the convex area of the hemisphere exposed.

푛

∑ 푉푖 = 푉푟 (퐸푞. 2.9) 푖

where Vi is the volume for each metallic particle and Vr is the volume of a gram of pure metal.

푁 푖 = 푐표푛푠푡푎푛푡 (퐸푞. 2.10) 푁푗 where Ni is a number of particles established in the counts made from

TEM images in a specific size range “i” and Nj is a number of particles established in the counts made from TEM images in the reference range j.

푛

퐴푡 = ∑ 퐴푖 (퐸푞. 2.11) 푖

where At is the exposed surface per gram of metal according to previous assuming and Ai is defined by(Eq. 2.11a):

2 퐴푖 = 2휋푟 (퐸푞. 2.11푎)

Where 푟 is the radii of the average particles of each size range. The result is a metallic surface-exposed value per gram of metal; the latter was calculated according to real metal content obtained by ICP-OES.

2.3. Results and Discussion. 2.3.1. Characterization of as-Prepared and pre-Treated Cellulose Microfibers. 2.3.1.1. As-Prepared MFCs Composition. The elemental and ICP-OES analyses of as-prepared MFCs are shown in Table 2.2. The molecular formula estimated with the mass composition was quite similar to the pure cellulose (experimental: C5.6H10.6O5.3N0.04;

Chapter 2: Carbon Aerogel-Supported Iron for Gasification Gas Cleaning: Ammonia Adsorption. theoretical: C6H10O5). It is well known that alkaline and alkaline earth metals have significant influence, as catalysts or precursors, in carbonization processes (Bridgwater and Boocock 2006). However, the low content of inorganics detected in MFCs by ICP-OES allows discarding their effect on the carbonization process.

Table 2.2 Cellulose microfiber (MFC) compositional analysis. Ultimate a (wt.%) C H N Ob 41.2 6.52 0.21 52.07 Sugars c Glucose (%) Xylose (%) Extractive (%) MFC 81.9 14.2 3.9 Inorganics Ash Na Ca Fe Mg K Si (%) (mg/g) (mg/g) (mg/g) (mg/g) (mg/g) (mg/g) <0.01 0.77 0.912 0.112 0.273 0.651 0.005 a Standard deviations (r) = %C ± 0.15, %H ± 0.008, %N ± 0.04, %Ash ± 0.008. b Oxygen was calculated by difference from C, H, N. c According to NREL/TP-510-42618.

2.3.1.2. Cellulose Crystalline Structure.

The crystalline structure of raw as-prepared and (NH4)2SO4-treated MFCs were studied by X-ray diffraction. No significant differences were observed between both XRD patterns (Figure 2.1), with the main reflections appearing at the same 2theta positions.

Similar crystal parameters were calculated from the XRD analysis (Table 2.3) of as-prepared and pre-treated MFC materials, which also match the results reported for other celluloses (Poletto, Ornaghi Júnior, and Zattera 2014). Peaks positions can be assigned to the presence of (100), (010), and (110) crystallographic planes, for cellulose Iα, and (1͞10), (110), and (200) planes, for cellulose Iβ (Poletto, Pistor, and J. 2013). Negative Z-values, −27 and −72 for as-prepared and pre-treated MFC, respectively, indicate that both celluloses are predominantly of type Iβ, with a monoclinic

Chapter 2: Carbon Aerogel-Supported Iron for Gasification Gas Cleaning: Ammonia Adsorption. structure. Accordingly, the signals appearing at 2ϴ of about 14.8°, 16.5°, and 22.2° are assigned to (1͞10), (110), and (200) planes.

Figure 2.1. Normalized X-ray diffraction (XRD) patterns of cellulose microfiber (MFC) and pre-treated impregnated MFC.

Table 2.3 Crystalline index (Eq. 2.1a,b), d-spacing (Eq. 2.3), and crystal size (Eq. 2.4). Segal Herman d-Spacing (nm) Crystallite Size CrI CrI (%) (nm) (%) (1͞10) (110) (002) (1͞10) (110) (002) as- prepared 79 74.2 0.6 0.544 0.395 3.6 2.7 3.9 MFC pre- treated 80.3 76.8 0.57 0.539 0.395 4.7 2.0 4.0 MFC

Chapter 2: Carbon Aerogel-Supported Iron for Gasification Gas Cleaning: Ammonia Adsorption.

As shown in Table 2.3, crystallite sizes change after treatment with the ammonium salt, probably caused by partial dissolution of crystallites during impregnation. The CrI values calculated using Herman’s equation (Eq. 1b)) were slightly lower than the values of Seagal’s index. This result supports the well-known fact that Segal’s index tends to overestimate the crystallinity. Based on results reported by Kim et al. (Kim, Eom, and Wada 2010), suggesting that cellulose with higher crystallite size has higher thermal stability, pre-treated MFC samples are expected to have higher thermal stability that can also be expressed as a higher mass yield in the carbonization process.

2.3.2. Carbon Aerogel Preparation. All MFC samples, as-prepared and pre-treated with an ammonium-based solution, were carbonized at different temperatures and times, under N2 flow. Initially, nine CAG samples were prepared and three of them (those prepared without a dwell time after reaching the pyrolysis temperature) were selected for elemental analysis (Table 2.4). In all the samples, the carbon content exceeds 90 wt.%, which is a characteristic feature of carbon aerogels (Standard 472:1999). Furthermore, the sulfur and nitrogen contents are very low, indicating that all S-species from the ammonium salt were removed during the drying and carbonization processes.

Table 2.4 Elemental analysis of selected carbon aerogels (CAG 90, CAG 100, and CAG 110). Sample Carbon Nitrogen Hydrogen Oxygen Sulfur Ash wt.% wt.% wt.% wt.% * wt.% wt.% CAG 90 91.20 1.7 0.8 ~5.3 BDL <1 CAG 100 92.50 2.0 0.9 ~3.6 BDL <1 CAG 110 90.70 1.3 0.6 ~6.4 BDL <1 * calculated by difference.

Chapter 2: Carbon Aerogel-Supported Iron for Gasification Gas Cleaning: Ammonia Adsorption.

The textural properties of CAGs materials were correlated to the different carbonization conditions used in the synthesis, aiming to identify the conditions leading to CAG materials with the best properties as catalyst support for tar decomposition. Particularly, surface area and pore volume measured by N2-physisorption were considered in the first screening. A full table with all the carbonization conditions and carbon material properties is provided in the annexes (Table A1).

Results from N2 physisorption analysis at 77 K are shown in Figure 2.2a,b.

As observed, the specific surface area (SBET) and pore volume decrease with the increase in temperature and dwell time. According to Marsh and Rodriguez-Reinoso (Marsh and Rodríguez-Reinoso 2006), this behavior is common in carbonization. Depending on the carbon precursor, the specific surface area reaches a maximum at a certain temperature, due to the release of small molecules (i.e., water and methanol) allowing the free space that constitutes the porosity. Further increases in temperature cause loss of heteroatoms and cross-linking of carbon atoms, which reduces the space between them and essentially closes off porosity. For MFC as precursor material, results (shown below) indicate that the maximum specific surface area of carbon is reached at a temperature lower than 900 °C. An additional sample was prepared at 800 °C (CAG 80), while the other synthesis conditions were the same as for CAG 90. The textural characterization for this sample is included in Table 2.5; following the same trend, and in agreement with the results from Marsh and Rodriguez- Reinoso (Marsh and Rodríguez-Reinoso 2006), the specific surface was even higher, although other textural properties did not significantly change, compared to the samples prepared at higher temperatures.

Table 2.5 Comparison between pre-treated and pure MFC, carbonized at the same conditions. CAG Mass Specific Surface Pore Volume Pore Size Yield (m2/g) (cm3/g) (nm) 80 28.0 464 0.20 10.2

Chapter 2: Carbon Aerogel-Supported Iron for Gasification Gas Cleaning: Ammonia Adsorption.

90 27.7 370 0.19 11.0 90–0% 13.9 327 0.17 10.2

Figure 2.2 Effect of dwell time and temperature on morphology of carbon aerogels (CAGs). (a) Specific surface area, (b) pore volume. Heating rate: 10 °C/min.

Adsorption-desorption N2 isotherms and pore size distribution of CAG 90 are shown in Figure 2.3. The isotherms can be classified as type IV, according to IUPAC classification. This shape is associated with

Chapter 2: Carbon Aerogel-Supported Iron for Gasification Gas Cleaning: Ammonia Adsorption. mesoporosity and condensation inside the pores (Balbuena and Gubbins 1992). Mesoporosity is an advantageous characteristic for catalyst supports used in the decomposition of aromatic compounds, since the kinetic diameter of typical tar molecules is in the range 6.9–10.7 Å (Choudhary, Nayak, and Choudhary 1997).

Figure 2.3 N2 adsorption-desorption isotherms at 77 K and pore size distribution for CAG 90.

Another key parameter in CAG production via MFC carbonization is the mass yield, defined as the quantity of carbon remaining in the CAG times that contained in the MFCs. Several authors argue that the low carbon yields of cellulose-derived CAGs are a restriction for the process economy. MFCs used in this work were pre-treated with ammonia-based salts, which has been reported as an alternative to increasing the carbon yield (Branca and Blasi 2008). Regardless of the carbonization temperature or time, the mass yield of all materials, pre-treated with 5 wt.% (NH4)2SO4, ranged from 26 to 28%. For the carbonization of as- prepared MFC, without any treatment, the yield is reduced to almost half (CAG 90–0%, Table 3.5). These values are in line with those reported by Kulenkampff (Kulenkampff Schrewe 2015) and Arteaga et al. (Arteaga-

Chapter 2: Carbon Aerogel-Supported Iron for Gasification Gas Cleaning: Ammonia Adsorption.

Pérez, Gómez-Cápiro, et al. 2017). When the heating ramp used in carbonization is increased to 20 °C/min, the increase in mass yield achieved by sulfate impregnation is 23–69% lower. This behavior matches with findings previously reported, showing that higher heating rates cause greater mass losses, as observed in Table 2.1. Besides its effect on yield, the impregnation with (NH4)2SO4 has no major effects on the textural properties of CAGs.

Thermogravimetric analysis was used to compare the thermal resistance of CAG samples (Figure 2.4a). Note that CAG 110 has its maximum mass loss at a lower temperature than CAG 90, however, the mass-loss rate (%/min) is lower than CAG 90 at the same conditions. Starting from 350 °C until the temperature of maximum mass loss, DTG increases almost linearly with the temperature, with a “slope” (rate of mass loss per °C). Plots can be compared by this “slope” or by the temperature at which the maximum mass loss rate occurs (Figure 2.4b). In any case, CAG 90–0% has the lowest thermal resistance, confirming that ammonium sulfate pre- treatment has a positive effect on this property. For CAGs prepared from pre-treated MFC, the thermal resistance increases with the carbonization temperature, CAG 110 > CAG 90 > CAG 80. Activated carbon Pittsburgh has the highest resistance among all samples, probably due to an activation process carried out at high temperatures and for long times, resulting in a thermally stable structure.

To complement the thermal resistance results, CAGs and activated carbon were analyzed by XRD and Raman spectroscopy, respectively. The results from those analyses were also used to study the crystallite and chemical structure of all the materials.

Chapter 2: Carbon Aerogel-Supported Iron for Gasification Gas Cleaning: Ammonia Adsorption.

Figure 2.4 DTG for thermal resistance in the air with heating rate: a-10 °C/min; airflow: 80 mL/min. b- Comparison between thermal resistance indicators in chars’ samples (b).

The main reflections in the XRD patterns of the CAGs (Figure 2.5) agree with those reported in the literature for other cellulose-derived chars (Jazaeri and Tsuzuki 2013). None of the CAGs prepared here reach graphitization, which occurs above 2000 °C and leads to a singular reflection at 2ϴ = 26.5° (Marsh and Rodríguez-Reinoso 2006; Z. Q. Li et al. 2007). In contrast, all CAG samples showed two wide reflections, at 23.4° and 44.4°, assigned to the (002) and (010) planes of the amorphous carbon structure. Both peaks are characteristic of disordered chars and

Chapter 2: Carbon Aerogel-Supported Iron for Gasification Gas Cleaning: Ammonia Adsorption. have been reported in earlier investigations published by Biscoe et al. (Biscoe and Warren 1942). All CAGs patterns show a shoulder at around 15°, which is more pronounced for CAG 80, having almost the same intensity as the signal assigned to the (002) carbon plane. This specific signal could be related to the (1͞10) and/or (110) planes of cellulose remaining after carbonization.

Figure 2.5 Normalized XRD pattern for CAG 80, 90, 90 0%, 110, and commercial activated carbon Pittsburgh.

Raman spectra of the synthesized CAG materials are shown in Figure 2.6. In the analysis of ordered carbonaceous materials like graphite, D, and G bands usually refer to Defect and Graphite structures, respectively. However, X-ray diffraction (XRD) spectra of CAGs did not show any characteristic signal of graphitization (2ϴ = 26.5°), so D and G bands do not belong to defects and graphite structures, respectively (Xiaojiang Li, Hayashi, and Li 2006). D band might represent C–C bonds between aromatic rings and aromatics with no less than 6 rings. On the other hand, G bands can be attributed to the aromatic ring breathing rather than the 2 −1 E 2g fundamental vibration of graphite. The band at 2,700 cm is a result of the second-order phonon related to the disorder in the stacking of

Chapter 2: Carbon Aerogel-Supported Iron for Gasification Gas Cleaning: Ammonia Adsorption. carbon layers (Viswanathan, Neel, and Varadarajan 2009). Heavy aromatic compounds confer higher thermal resistance to any char.

Figure 2.6 Normalized Raman spectra for CAG 80, 90, 90 0%, 110, and commercial activated carbon Pittsburgh.

Assuming that bands' areas are proportional to the concentration of their associated compounds, a higher ratio between D (1,300 cm−1) and G (1,590 cm−1) areas can be interpreted as a higher thermal resistance. In the same way, Lc (from (002) plane) and La (from (010) plane) crystallite dimensions (Eq. 2.4) have a direct proportionality with thermal resistance. As expected, the highest values of La, Lc and AD/AC ratio (shown in Table 2.6) were obtained for the commercial activated carbon Pittsburgh, since it has the highest thermal resistance. This fact helps to confirm that the higher ordered carbons are prone to have a higher thermal resistance, which is in line with the effect of ammonium salt on the carbonization mechanism, as shown in a previous work (Arteaga-Pérez, Gómez-Cápiro, et al. 2017). However, only La value follows the same relative order of

Chapter 2: Carbon Aerogel-Supported Iron for Gasification Gas Cleaning: Ammonia Adsorption.

CAGs materials as in terms of thermal resistance. Lc and AD/AG values for CAG 80 are out of trend, which could be associated with a different structure or the presence of some non-carbonized fractions of cellulose (in agreement with the signal at 15° detected in XRD patterns).

Based on the above discussion on the textural properties, elemental analysis, and thermal resistance of the CAG materials, the CAG 90 sample, prepared by carbonization of (NH4)2SO4-treated MFCs at 900 °C and 0 h holding time, was selected as the material gathering the best properties as catalyst support for gasification gas cleaning applications. Therefore, the ammonia adsorption study presented in the upcoming sections was performed using CAG 90-supported catalysts.

Table 2.6 Graphite crystallite dimensions (from X-ray diffraction (XRD) analysis) and bands D and G areas ratio (from Raman spectroscopy analysis). Sample Dimensions (nm) AD/AG Lc La CAG 80 0.67 3.36 1.73 CAG 90 1.03 3.17 3.05 CAG 90 0 1.02 2.90 2.92 CAG 110 0.97 3.27 2.99 AC Pittsburgh 1.28 3.78 3.23

2.3.3. Catalyst Synthesis and Characterization. 2.3.3.1. Superficial and Composition Characteristics. Synthetized catalyst has been denoted as Fe/CAG 90. Specific surface area 2 (SBET) of catalysts is 288 m /g, close to 22% lower than the area of the corresponding support (370 m2/g), presumably due to pore blockage during metal impregnation (Wojcieszak et al. 2006). The metal loading calculated by theoretical mass balance (10 wt.%), agreed to that measured by ICP-OES analysis: 9.4 of wt.% Fe in Fe/CAG 90. This metal content

Chapter 2: Carbon Aerogel-Supported Iron for Gasification Gas Cleaning: Ammonia Adsorption. was used along with the particle size distribution to estimate the surface metal dispersion.

2.3.3.2. Catalyst XRD Patterns. XRD pattern for the catalyst is shown in Figure 2.7. Peaks at 44.6°, 65°, and 82.2° are characteristic of metallic iron and correspond to (110), (220), and (211) planes, respectively (Lin et al. 2013). The absence of peaks at 30°, 35.6°, and 38.5°, corresponding to FeO, Fe2O3, and Fe3O4 (Shen et al. 2014), suggests that iron oxides were not formed or were fully removed in the reduction treatment. Weak reflections around 2ϴ = 23° are assigned to the amorphous phase of CAG 90 support.

Figure 2.7 XRD patterns of Fe/CAG 90 catalyst.

Sizes of metal crystallites were calculated from Scherrer’s, and the results are presented in Table 2.7 together with particle sizes estimated from transmission electron microscopy images (TEM).

Chapter 2: Carbon Aerogel-Supported Iron for Gasification Gas Cleaning: Ammonia Adsorption.

Table 2.7 XRD crystallite size (Equation (1)) for Fe crystal plane. Catalyst Crystallite Size (nm) XRD Crystallite Size (nm) (110) TEM Fe/CAG 14.3 24.7 90

2.3.3.3. TEM Transmission electron microscopy was used to obtain the size distribution of metal particles. Figure 2.8 shows a TEM image and Figure 2.9 presents the frequency histograms of particle sizes, obtained from the measurement of more than 300 particles on each catalyst. These distributions together with the metal content were used to estimate the exposed metal surface. The spherical shape of particles is characteristic of the predominant metallic phase, as was shown in the XRD patterns. The average size of metal particles is shown in Table 2.7.

Figure 2.8 The TEM image of Fe/CAG 90.

Chapter 2: Carbon Aerogel-Supported Iron for Gasification Gas Cleaning: Ammonia Adsorption.

Figure 2.9 Particle size distribution for Fe/CAG 90.

2.3.4. Adsorption Experiments 2.3.4.1. Ammonia Adsorption over CAG 90. Quantities of ammonia adsorbed on CAG 90 are shown in Table 2.8, as milligrams of NH3 per gram of adsorbent, taking as a reference the quantity adsorbed (2.13 mg NH3/g CAG) at the conditions selected as saturation (0 °C and 494 ppm) once reached equilibrium. For an inlet concentration of 390 ppm, the quantity of ammonia adsorbed ranges from

0.19 to 0.45 mg NH3/g CAG 90, for temperatures in the range 50–150 °C. Similar results on adsorbed quantities have been already reported. Zheng et al. (Zheng et al. 2016) reported 1mg of NH3 adsorbed per gram of char, at 23 °C and an inlet ammonia concentration of 500 ppm. Rodrigues et al. (Rodrigues et al. 2007) worked at an inlet ammonia concentration of 600 ppm and reported 0.63, 0.31, and 0.22 mg NH3/g char, at 40, 80, and 120

°C, respectively. NH3 can be adsorbed on (and interact with) the surface through various mechanisms, roughly categorized into non-specific van der Waals force, dissolution in water film adsorbed on the surface, and Brønsted acid-base interactions (mainly carboxylic acid sites via ammonium ions)(Zheng et al. 2016). Char surface can be modified to enhance its adsorption capacity, thus promoting ammonia adsorption mechanisms, but this is not the aim of the present work. No water was

Chapter 2: Carbon Aerogel-Supported Iron for Gasification Gas Cleaning: Ammonia Adsorption. supplied to the system and the presence of oxygenated groups in alpha positions was discarded by Raman spectroscopy analysis, where the signal at 1700cm−1, assigned to C=O groups (Keown et al. 2007), was not found. Consequently, non-specific van der Waals forces are expected to prevail in ammonia adsorption on CAG 90 surface.

Table 2.8 Ammonia adsorbed over CAG 90 (mg NH3/g CAG 90). mg NH3/g CAG 90 Ammonia Relative 50 °C 100 °C 150 °C Pressure 0.000135 0.22 ± 1.84 × 0.07 ± 3.84 × 0.02 ± 4.47 × 10−3 10−4 10−5 0.000193 0.35 ± 1.29 × 0.15 ± 1.61 × 0.06 ± 5.89 × 10−3 10−4 10−6 0.000232 0.39 ± 1.14 × 0.22 ± 1.09 × 0.04 ± 1.62 × 10−3 10−3 10−4 0.000332 0.41 ± 3.09 × 0.19 ± 1.77 × 0.10 ± 7.24 × 10−3 10−3 10−4 0.000390 0.45 ± 6.77 × 0.28 ± 8.25 × 0.19 ± 6.54 × 10−4 10−5 10−4

Figure 2.10 shows the resulting isotherms at 50, 100, and 150 °C. Langmuir and Freundlich, models do not perfectly represent the adsorption behavior. Freundlich’s model overestimates NH3 coverage at low pressures, but it is a good fit for higher coverages. On the contrary, the Langmuir model better fits low ammonia coverages. High temperatures exhibit the highest errors, which is in line with that already reported by Saha and Deng (Saha and Deng 2010) for the study of ammonia adsorption over alumina.

Chapter 2: Carbon Aerogel-Supported Iron for Gasification Gas Cleaning: Ammonia Adsorption.

Figure 2.10 Coverage of ammonia over CAG 90 and model representation. Freundlich model presented by dashed lines and Langmuir model by solid lines.

Models and thermodynamic parameters shown in Tables 2.9 and 2.10, respectively, were regressed using adsorption data. The values obtained for entropies and enthalpies using the Freundlich model are markedly higher than for Langmuir’s, given that the former is better suited for the higher values of the quantity loaded, it was expected that the thermodynamic parameters were the highest between the two models. The Freundlich adjustment was worse than that obtained by the Langmuir model, which reflects the error associated with the thermodynamic parameters. In a recent paper, Rezaei et al. (Rezaei et al. 2017) reported similar values of adsorption enthalpy over metal oxide nanoparticles suggesting that NH3 was physisorbed on metallic clusters. According to Langmuir’s assumptions, this model did not fit with physisorption. However, at low pressures as all experiments were performed, the assumptions of Langmuir are satisfied and the model is capable to represent the physisorption process.

Chapter 2: Carbon Aerogel-Supported Iron for Gasification Gas Cleaning: Ammonia Adsorption.

Table 2.9 Langmuir and Freundlich models’ parameters and statistical adjustment. Langmuir Model’ Freundlich Model’ Parameters Parameters 2 2 Temperature K R Ko Qad R °C 50 909.09 0.75 3.2 × 10−5 7516.9 0.92 100 322.58 0.81 0.56 2469.8 0.61 150 94.33 0.86 0.91 2169.9 0.59

Table 2.10 Thermodynamics parameters for ammonia adsorption over CAG 90. Model ΔH (kJ/mol) ΔS (J/mol K) Langmuir −20.7 ± 0.509 −7.61 ± 0.43 Freundlich −30.1 ± 3.9 −45.11 ± 7.7

Domingo-García et al. (Domingo-García et al. 2002) reported adsorption enthalpies between −61.5 and −122 kJ/mol, for carbonaceous materials with chemical modification. On the other hand, Saha et al. (Saha and Deng 2010) got similar values to this work for alumina without modification (−35.6 to −15 kJ/mol), where the adsorption mechanism was the interaction by non-specific van der Waals forces as it is postulated that happens in this case. It is worth noticing that, under real gasification conditions, the adsorption process could be affected by the presence of other species (e.g., CO2, CO, CH4, H2, and tars), which might change the above-reported values. In this regard Vasiliev et al. (Vasiliev et al. 2006), studied the adsorption of CH4, H2, and NH3 over activated carbon fibers and demonstrated that hydrogen is only adsorbed at cryogenic temperatures while the adsorption capacity for NH3 was nearly twice that of CH4 at 40°C. These results suggest that the affinity of carbon for NH3 was higher than for the other gases; although the study did not include the simultaneous fed of those gases, which do not allow verifying if there is

Chapter 2: Carbon Aerogel-Supported Iron for Gasification Gas Cleaning: Ammonia Adsorption. any competition for adsorption sites. An extra issue that could involve working with gasification gas, is the presence of steam which could lead to an adsorption mechanism ruled by the dissolution in water film adsorbed on the surface. Nevertheless, this will happen if the carbon is used at temperatures below water dew point.

2.3.4.2. Ammonia Adsorption over Catalyst. Table 2.11 shows the quantity of ammonia adsorbed by the catalyst.

Table 2.11 Ammonia adsorbed over the catalyst (mg NH3/g catalyst). Ammonia Temperature (°C) Relative Pressure 50 100 150 0.46 ± 4.63 0.26 ± 1.33 0.08 ± 3.90 0.000135 × 10−5 × 10−3 × 10−4 0.60 ± 2.10 0.31 ± 1.32 0.17 ± 2.30 0.000193 × 10−3 × 10−3 × 10−4 0.74 ± 6.56 0.54 ± 4.33 0.19 ± 9.59 0.000232 × 10−3 × 10−4 × 10−4

A positive effect of metal clusters on the adsorption process was observed by comparing the adsorbed quantities to those obtained with the support. Some authors (Duan et al. 2012; Otero et al. 2016) have used DFT methods to study ammonia adsorption over the Fe surface. They have demonstrated that ammonia is preferentially adsorbed on on-top sites rather than a bridge or hollow sites. The binding with the iron surface occurs through the lone electron pair in N atom. On-top sites have electron deficiencies, which explains their strong interaction with the electron-rich N atoms (G. Lanzani and Laasonen 2010). Thus, adsorption over the metallic surface has a configuration with H atoms pointing outwards. The normalized value of ammonia adsorbed on the metal surface (Figure 2.11), calculated by subtracting what is adsorbed on the support and considering the available metal surface (estimated by Equation (2.11)), showed that the Fe surface can adsorb 1.05 mg NH3/g Fe.

Chapter 2: Carbon Aerogel-Supported Iron for Gasification Gas Cleaning: Ammonia Adsorption.

Figure 2.11 Ammonia Coverage over Fe surface.

Figure 2.12 Variation of Enthalpy of ammonia adsorption with coverage of ammonia over the metal surface.

Enthalpy values estimated by extrapolation of experimental results are in line with the theoretical estimations Otero et al. (Otero et al. 2016), their work show values of enthalpy of ammonia adsorption between −92.6 and

Chapter 2: Carbon Aerogel-Supported Iron for Gasification Gas Cleaning: Ammonia Adsorption.

−153.4 kJ/mole in nanoclusters with 16 and 190 Fe atoms respectively, all for 25% of coverage. The experimental determinations shown in Figure 2.12, exhibited an enthalpy of −191.3 kJ/mole for the same coverage, which is in the same order of magnitude. An important conclusion from Otero’s calculations is that the adsorption energies increase with the size of the nanoclusters, which could be the cause for the slight differences in the enthalpy estimations.

2.4. Partial Conclusions - The carbonization of the cellulose reaches twice the mass yield during carbonization when it is pre-treated with (NH4)2SO4. Similarly, the thermal resistance of the chars obtained from pre-treated cellulose is higher than when the pre-treatment was omitted. - Ammonia adsorption over char behave according to Langmuir and Freundlich models and shows enthalpies between 20 and 30 kJ/mol, which correspond to physisorption. - The presence of Fe sites on carbon surface enhances the adsorption of ammonia; which according to the value of enthalpies proceeds via a chemical adsorption mechanism.

Chapter 3: Catalytic decomposition of tars over Fe/CAG.

3 Chapter 3: Catalytic decomposition of tars over Fe/CAG.

Gasification is the most efficient way to transform biomass in energy and chemical feedstock. Unwanted compounds generate in the process prevent the use of the gases in most of the current technologies (turbines, synthesis). Tar removal from gasification gases represents the higher limitation and its elimination is the determinant step to guarantee the operational feasibility of gasification-to-chemicals/energy systems. This study aimed to develop novel carbon-supported Fe catalysts for the decomposition of toluene, naphthalene, and benzene as models of tars. Effects of reaction temperature (565 < T < 665 ºC) and catalyst stability on the activity were assessed by considering a simulated gasification gas at lab and BENCH scales, respectively. The fresh catalyst (before the reaction) and support were characterized by X-ray diffraction (XRD), N2 physisorption, transmission electron microscopy (TEM), inductively coupled plasma optical emission spectrometry (ICP-OES), scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDX) and compositional analyses. Spent catalysts were characterized using XRD and SEM-EDX as well. Results from activity tests allow proposing a tar decomposition mechanism and estimating apparent activation energies. Spent catalysts characterization evidenced deactivation by carbon deposition and active phase oxidation simultaneously. The experiments show naphthalene as the most recalcitrant tar between all tars models studied. The results help to establish catalytic gasification gas cleaning as a viable alternative to use biomass gasification as energy and chemical feedstocks supply.

Chapter 3: Catalytic decomposition of tars over Fe/CAG.

3.1. Introduction. Tars presence in biomass-derived gasification gases and its elimination, removal or conversion into valuable products, are the main concerns in the endeavor to establish the biomass to energy/chemicals route via gasification (C. Li and Suzuki 2010). Indeed, gasification is one of the most promising ways to process the biomass to obtain energy (Molino et al. 2018), and tuning the gasifier operation parameters allows producing a syngas (Valderrama Rios et al. 2018). However, tars are unacceptable for technologies using gasification gases as feedstock, even in small concentrations (in the order of the ppm). The literature reports several ways to remove the tars, most of which require post-treatment (Zwart 2009). The challenge is to transform the tars so that their internal energy can be harnessed. In this sense, the catalytic cracking of these molecules is the most used alternative to turn tars into useful compounds, but the results obtained to date are still far from its commercial application (You et al. 2018; Arregi et al. 2018; J. Hu et al. 2019).

Therefore, it is necessary to understand the mechanism of decomposition of tars and to obtain models that represent the process to be able to design gasification gas upgrading technologies at a commercial level. The literature reports several studies that point to this objective. Gai et. al. (Gai et al. 2015) studied the thermal decomposition of toluene (used as model tar) and found low conversions below 650°C and an increment in the activation energy for product formation in the following order

C3H8

Chapter 3: Catalytic decomposition of tars over Fe/CAG.

(Liu et al. 2017). These gases have a marked influence in reaction mechanisms and strong interactions with active phases (Melander, Latsa, and Laasonen 2013; Giorgio Lanzani et al. 2009). Metals, in general, are needed to break the aromatic ring present in tars, which is the most difficult step in these kinds of reactions; followed by radicals-ring bonds and C-H bonds in the radicals (only in compound with radicals as toluene) (Zhou et al. 2017). The presence of radicals facilitates the tars break down. Studies on toluene decomposition demonstrate that chemisorption on metallic sites trough the ring promotes the breakage of methyl from the benzene ring as a subsequent step after adsorption (Youhua Zhang 2018). Besides the above-mentioned studies, which mainly focusses on the first reaction steps, no reports exist on how the mechanism proceeds to the formation of lower molecular weight compounds. Exist few reports on whether this mechanism is valid for tars (toluene, naphthalene, and benzene as models) decomposition over Fe/CAG. Therefore, the present work aims to understand the mechanism for tars decomposition over the CAG-supported Fe catalyst. The reaction was studied at two scales and with different tar models. Firstly, the experiments consisted of breaking down toluene to study the activity of the Fe/CAG at lab-scale to obtain some insights into the reaction mechanism and kinetics. Secondly, the same catalyst was used to treat a gas containing a tars mixture

(naphthalene and benzene) alone and with CO or H2 co-fed. The characterization before and after the reaction help to explain the process quantifying the relative depositions of carbon in the catalyst. Iron was supported on a carbon aerogel produce from microfibrils of cellulose carbonization in a process described in the previous chapter.

3.2. Materials and methods. 3.2.1. Microfibrils Treatment. Support obtaining. The precursor used and the pretreatment before carbonization are described in section 2.2.1. Carbonization was carried out according to conditions chosen after the results discussed in section 2.3.2. Finally, the compositional analysis and the morphological characterization were obtained as described in sections 2.2.4 and 2.2.5, respectively.

Chapter 3: Catalytic decomposition of tars over Fe/CAG.

3.2.2. Catalysts preparation. For toluene decomposition, catalysts with a Fe loading of 10 wt.% were prepared via incipient wetness impregnation. Iron nitrate

(Fe(NO3)3·9H2O, >99% purity, Merk) was used as a metal precursor. The support was ground with an agate mortar (<80 meshes). Afterward, the corresponding nitrate aqueous solution was slowly added to the CAG support, in the appropriate quantities to reach 10 wt.% of metal in the final catalyst, according to the pore volume determined by N2 adsorption- desorption (see section 2.5). Naphthalene decomposition was carried out with pellets shaped catalysts. The same iron nitrate solution was used to submerge the pellets for 15 minutes at room conditions. Iron loaded samples were dried at 105 °C for 4 h. The resultant precursor was reduced for 2 h at 700 °C, under 40 mL/min of H2, using a heating rate of 2°C/min to reach the final temperature before each tars decomposition experiment. Solids obtained were denoted as Fe/CAG and Fe/CAG-ps, the last one referred to pellets shaped.

3.2.3. X-ray diffraction (XRD). XRD analysis of catalysts before and after used was performed to evaluate the crystallite size of the samples. The XRD patterns were recorded on a Bruker AXS model D4 Endeavor diffractometer, using monochromatic CuKα radiation (λ=0.15418). The signal was generated at 40 kV and 20 mA. The intensities were measured in the range 5°<2ϴ<90°C for CAGs, with a step size of 0.02° and scans at one s/step. The crystallite apparent size was calculated using the Scherrer’s equation for all solids (Eq. 3.1).

푲흀 푳 = (푬풒. ퟑ. ퟏ) 휷퐜퐨퐬 (휽) Where K is a constant equal to 0.94 for all catalyst samples, λ is the wavelength (in nm), β is the full width at half-maximum intensity (FWHM) (in rad), and θ is the plane angle.

Chapter 3: Catalytic decomposition of tars over Fe/CAG.

3.2.4. Transmission Electron Microscopy (TEM). The metal particle sizes for the catalyst was observed by transmission electron microscopy in a JEOL JEM 1200 EXII device, with voltage 120 kV. The sample was suspended in a solution of ethanol-water (50-50 wt.%), supported in a copper grill, and covered by a carbon layer. The size distribution and mean cluster sizes were estimated after measuring more than 300 metal particles in 10 images.

3.2.5. Scanning Electron Microscopy-Energy Dispersive X-ray spectroscopy (SEM-EDX). Carbon deposition over the catalyst was correlated by comparing the C/Fe ratio before and after the reaction. The relative amounts of C, H, N, O, and Fe elements were estimated using SEM-EDX in a LEO 1420VP microscope using 300 X magnification for fresh and used (during toluene decomposition) Fe/CAG. Similar elements were recorded in a TESCAN VEGA3 SBU EASYPROBE device for fresh and used (during naphthalene decomposition) Fe/CAG-ps. The change in the C/Fe ratio was attributable to the C deposition or consumption during the reaction, assuming negligible or no Fe loss in the process.

3.2.6. Toluene decomposition experiments. Activity tests of toluene decomposition were carried out in UDT facilities in Coronel, Chile. All activity tests were performed with 100 mg of Fe/CAG catalyst. The catalyst (fine powder) was loaded into a U-shape quartz reactor (10 mm inside diameter) placed inside a tubular furnace (Omega Eng, CRFC-312/240-C-A, Stamford, CT, USA). The toluene (>99% purity, Merck) was fed by a syringe pump (Cole-Parmer GmbH, Wertheim, Germany) to a custom-made evaporator where a He flow (Air Liquide, 99.999% purity, Coronel, Chile) circulate with a specific rate according to the required concentration between 900 and 1979 ppm of toluene. A mass flow controller (Kofloc, model 8500, Kyoto, Japan) is used to control the He flow. Both He and toluene fed are oxygen-free. All pipelines were heated at 150°C using heating tapes (Omega Eng, Stamford, CT, USA), and a reactor by-pass was installed to measure the feed concentration before the reaction. Reactants and products

Chapter 3: Catalytic decomposition of tars over Fe/CAG. concentrations composition were analyzed in a Clarus 580 GC (Perkin Elmer, Chile) equipped with an Elite-5 (Perkin-Elmer, Chile) and Carboxen-1000 (Supelco Analytical, Germany) columns coupled to FID and TCD detectors, respectively (Schematic representation in Annexes, Scheme B2). An external heater with an independent control system set up to the same temperature that pipelines heat the injection valve. Firstly, different space velocities at constant concentration and temperature were studied. Secondly, the feed concentrations and reaction temperatures were varied at a constant space velocity, to estimate the kinetic parameters. To identify the reaction products, samples taken after the reactor using a 150 cm3 stainless steel cylinder with two exits equipped with needle valves, were injected in a gas chromatograph (GC-2010 Plus, Shimadzu) equipped with an Elite-5 (Perkin-Elmer, Chile) and coupled to a single quadrupole mass spectrometry detector (QP 2010 Ultra, Shimadzu). The samples were taken when the higher conversion of toluene was achieved to have higher product concentrations.

The conversion of toluene (x) was calculated according to equation 3.2 (Eq. 3.2) assuming constant volume and pressure. ° 푓 푃푝푇표푙푢푒푛푒 − 푃푝푇표푙푢푒푛푒 푥 = ° (퐸푞. 3.2) 푃푝푇표푙푢푒푛푒

° 푓 Where 푃푝푇표푙푢푒푛푒 and 푃푝푇표푙푢푒푛푒 are the partial pressure of toluene on the feed and the reactor exhaust, respectively. To estimate the kinetic constant, first-order reaction on toluene partial pressure was assumed, a similar assumption was taken for toluene and naphthalene decomposition by Fuentes-Cano et al. (Fuentes-Cano et al. 2013).

3.2.7. Naphthalene decomposition experiments. Naphthalene decomposition was studied at the Bench scale in the Fraunhofer-Institut für Umwelt-, Sicherheits- und Energietechnik (UMSICHT) facilities in Oberhausen, Germany. A quartz reactor (500 mm length, 18 mm inner diameter) placed within a split furnace was used.

Chapter 3: Catalytic decomposition of tars over Fe/CAG.

In a typical experiment, the reactor was loaded with 1 g of catalyst (Fe/CAG-ps), using a small steel basket. The naphthalene and benzene mixture was prepared by varying the benzene:naphthalene flowrates ratio before fed the evaporator. The experiments (Table 3.1) were performed at

2.45:1 (mL/min)bzn/(mL/min)nph, except for 1 experiment with pure benzene at 565°C. CO and H2 were fed to the reactor to emulate the real gasification gases hence those are the most common components at the gasifier exit (Lv et al. 2007; Burhenne et al. 2013; Sikarwar et al. 2017). These experiments allowed evaluating the stability and activity of the catalyst to decompose tars in the presence of other compounds that compete for the active sites. The tars were dosed with a dual syringe pump (MDSP3f, MMT GmbH, Siegen, Germany) connected to an evaporator (custom-made design, manufactured by Integrated lab Solutions GmbH, Berlin, Germany) via a 1/16” polytetrafluoroethylene (PTFE) tube. The gasses are fed to the evaporator by digital mass flow controllers (M + W Instruments GmbH, Allershausen, Germany). Gasses-tars mixture leaving the evaporator are mixed in a chamber, to homogenize the composition before the reaction. The reactor is heated with a three-zone vertical split- tube furnace (Horst GmbH, Lorsch, Germany) (Schematic representation in Annexes, Scheme B3).

Table 3.1 Experiments carried out with benzene: naphthalene flowrate ratio of 2.45:1. (Catalyst: ~1g of Fe/CAG, space velocity 940-960 ml/min g catalyst). Temperature N° Gases1 (v/v) (°C) 1 565 Ar (balance) 2 620 Ar (balance) 3 660 Ar (balance) 4 565 10% CO, Ar (balance)

5 565 10% H2, Ar (balance)

6 565 10%CO + 10% H2, Ar (balance)

Chapter 3: Catalytic decomposition of tars over Fe/CAG.

1 CO and H2 purity >99.5% and Ar >99.999%, Linde gas, Munich, Germany.

To ensure a homogenous temperature and a good mixing of the gas components, the bottom half (about 200 mm) of the quartz glass tube was filled with SiC bulk material. The quasi-continuous gas analysis was done using an online quadrupole mass spectrometer (MS) with electron ionization (GAM 200, InProcess Instruments GmbH, Bremen, Germany). Gas composition was measured before and after reaction by switching the position of a multiport valve (Valco Instruments Co. Inc., Houston, TX, USA). The conversion of naphthalene and kinetic constant were calculated with the same assumptions and equation (Eq. 3.2) than for toluene decomposition.

Tars studied here (benzene, toluene, and naphthalene) not suffer thermal decomposition (homogeneous reaction) in the temperature range of work with contact time values of 0.5s according to previous reports(Jess 1996).

3.3. Results and discussion. 3.3.1. Compositional analysis. To establish the CAG composition, elemental analysis of the support was performed in a Leco True Specter. Carbon composition was 91.2% wt., which is characteristic of carbons aerogels (Standard 472:1999). Nitrogen and hydrogen are present in 1.7 and 0.8 % wt., respectively. Ashes below 1% were quantified and sulfur is not detectable. The oxygen is estimated by the difference in 5.3 % wt., approximately. MFCs ICP-OES analysis for trace metals does not evidence the presence of any element in a concentration capable to cause a significant change in catalytic activity.

3.3.2. CAG surface characterization. Adsorption-desorption tests are the most common experiments to study the supports and catalysts morphology. Figure 3.1 (a, b) shows the N2 adsorption-desorption isotherms for support and Fe/CAG and Fe/CAG-ps. The specific surface (by BET model) and the average pore size obtained for the support were 370 m2/g and 11 nm, respectively. Normally, after

Chapter 3: Catalytic decomposition of tars over Fe/CAG. active phase impregnation supports loss part of their specific surface. Following this tendency, the Fe/CAG catalyst haves 249 m2/g with an average pore size of 10 nm while Fe/CAG-ps haves 304 m2/g with a similar average pore size. The shapes of the support isotherm and the hysteresis loop founded are quite common in biochars.

Figure 3.1 N2 adsorption-desorption isotherms at 77K and pore size distribution on a- Fe/CAG and b-Fe/CAG-ps.

Chapter 3: Catalytic decomposition of tars over Fe/CAG.

Previous reports relate these results to restricted diffusion of N2 or irreversible pore deformation by the sorbate. Liquid displacement and sedimentation measurements cause swelling, that is, the internal matrix expands in response to the presence of the N2 (Braida et al. 2003). Iron presence modifies CAG pore morphology evidenced by the behavior of the hysteresis classified as type H4 for the catalysts, which are typical for zeolites and micro-mesoporous carbons (Thommes et al. 2015). These results allow confirming that the catalysts and support have textural properties favorable for tars diffusions. The high surface could lead to a good distribution of metallic clusters.

3.3.3. Particle size distribution and estimated metallic surface. Literature reports that particle sizes larger than 10 nm have no differences related to the effects of surface atom coordination on specific reaction rates (TOF) (Rutger A. Van Santen 2009). Fe/CAG has an average particle size of 22.2±0.73nm, while Fe/CAG-ps have 11.9±0.21nm, according to the results gathered from TEM images (Figure 3.2). These results allowed assuming a similar structure on catalysts surfaces. Exposed Fe (moles of Fe in cluster surface/grams of Fe impregnated) were estimated for each distribution (Figure 3.3). According to ICP-OES results, Fe/CAG has 10.49 wt. % of metal and Fe/CAG-ps has 7.38 wt. %. This implies that the Fe dispersion on Fe/CAG and Fe/CAG-ps surfaces are 6.6% and 9.2%, respectively.

Chapter 3: Catalytic decomposition of tars over Fe/CAG.

a b

Figure 3.2 Transmission electron microscopy image of carbon aerogel iron supported. a- Fe/CAG and b- Fe/CAG-ps.

Figure 3.3 Particle size distribution obtained from TEM images analysis. a- Fe/CAG and b- Fe/CAG-ps.

According to the metal content, dispersion of the active phase (in the same order for Fe/CAG and Fe/CAG-ps) and textural properties the effect of these intrinsic catalytic properties are considered similar for both catalysts. Therefore, the differences in the activity results for both catalysts (reported in the following sections) are attributable only to the chemical reactions.

Chapter 3: Catalytic decomposition of tars over Fe/CAG.

3.3.4. Experiments in differential reactor. 3.3.4.1. Toluene decomposition. The effect of external mass transport limitations on the kinetic data gathered for toluene decomposition over Fe/CAG was discarded by applying the Mears criterion, the results are far below 0.15 (2.06·10-9). Additionally, the Weizs-Prater criterion resulted far below 0.3 (1.78·10-3), which rule out internal mass transfer limitations (Fogler 2008).

The influence of space velocity on catalytic activity to decompose toluene and on product distribution was evaluated (Figure 3.4). The reaction temperature was set at 600°C, the concentration of the feed was 1979 ppm and the conversion was calculated after reaching the steady-state condition, assumed as such once 3 hours or more have passed without significant variation in the reactant concentration. The products detected by the GC-FID were identified as propylene and benzene (confirmation was performed ex-situ by GC/MS). When the space velocity increases, toluene conversion increased reaching a maximum near to 700 ml/min×g. Higher values of space velocities cause the reaction to not be fully verified as confirmed by decreasing the concentration of propylene on the product gases causing a rise in carbon deposition in catalyst surface.

Figure 3.4 Toluene conversion and product variation with space velocity change. Stars-toluene conversion, circles-concentration of aliphatic product.

Chapter 3: Catalytic decomposition of tars over Fe/CAG.

Methane and other alkanes are almost negligible in the product gases even on reaction systems with H2 in the feed (Oemar et al. 2014) and poly- unsaturated as acetylene are very unstable at high temperatures. Benzene was detected only at the beginning of each reaction. The spent catalysts were analyzed by SEM-EDX to verify and quantify the carbon deposition produced during the reaction as well (Table 3.2). The mass of Fe in the catalytic bed was assumed constant during the reaction, thus, a rise in C/Fe ratio implies higher carbon deposition. According to the results, the largest space velocity causes a decrease in C/Fe ratio, which suggests that carbon is being removed from catalyst surface as soot (Fitzer et al. 1995) due to higher turbulence, attrition. These lead to indirect prevention of carbon deposition and, thus a lower C/Fe ratio was measured via SEM-EDX analysis.

Table 3.2 C/Fe rates in spent catalysts at different space velocity. Operation’s space velocity (ml/min g C/Fe catalyst) 0 (Fresh catalyst) 5.03 700 5.41 864 6.39 955 5.93

Figure 3.5a shows an example of an experiment at 600°C and 1979 ppm of toluene in the feed. It is observed that benzene concentration initially increases and then decreases as toluene is converted at the beginning of reaction; finally, after 5h of the reaction benzene was undetectable. This result suggests that toluene and benzene are involved in a reversible step, therefore, a test replacing toluene by benzene in the feed was done to gain insight into the reaction mechanisms on Fe/CAG catalysts. The comparison between the results of identical experiments with toluene or benzene in the feed allowed to establish the nature of the elemental steps involved in the toluene dissociation. Then, when benzene is fed (Figure 3.5b), the toluene formation is observed at the beginning, suggesting that

Chapter 3: Catalytic decomposition of tars over Fe/CAG.

the reversible step (Eq. S2) is occurring. The CH2 group formed after ring breaking (steps Eq. S3 and Eq. S4) may react with benzene to form back toluene (step Eq. S2). Toluene is undetectable around 5h after the test beginning. In steady-state conditions over Fe/CAG catalyst, only propylene was detected and carbon deposition was verified after SEM analysis of the spent catalyst. Moreover, the total conversion of benzene, opposite to what happens when toluene is fed (Figure 3.5a), suggests that toluene-to-benzene step (Eq. S2) is kinetically relevant, and as each tar is detected in the first moments of both decompositions, it can be inferred that the step is reversibly equilibrated.

The following steps imply the aromatic rings break into segments formed by two carbon as proposed by Oemar et al. (Oemar et al. 2014). During the process, two active Fe sites are required according to Eq. S3. Without any oxidizing or reducing agent to take carbon atoms, the subsequent steps imply carbon deposition (Eq. S4) as was confirmed by SEM-EDX analysis. Intermediate CH2* generates the aliphatic chain desorbed from catalyst surface as propylene as the only product detected in GC-MS analysis. However, is not possible to discard the production of ethylene or another similar unsaturated compound. Steps Eq. S5 and Eq. S6 represents the formation of these products. Results published by Gai et al., report a higher probability to obtain propylene than ethylene, and even lower to obtain methane and hydrogen as was discussed above (Gai et al. 2015).

Chapter 3: Catalytic decomposition of tars over Fe/CAG.

Figure 3.5 Tars model decomposition. a- Toluene (1979 ppm) decomposition over Fe/CAG catalyst. b-Benzene (1175 ppm) decomposition over Fe/CAG catalyst. Temperature, 600°C. Space velocity, 350 ml·min-1g-1.

On the other hand, steps Eq. S1-Eq. S6 does not explain the loss in toluene conversion represented in Figure 3.5 neither the rise in carbon deposition evidenced in Table 3.2. The causes of both experimental results could be the same. While reactant-catalyst contact time is shorter (higher space velocities), fewer toluene molecules finish the catalytic cycle, leaving traces of aromatic rings on the catalyst surface, which form polyaromatic hydrocarbons (PAHs) in a secondary reaction, quantified as carbon deposition and responsible for catalyst partial deactivation.

Chapter 3: Catalytic decomposition of tars over Fe/CAG.

Korus et. al. have seen the PAHs formation at similar temperatures from benzyl and phenyl radicals formed from toluene dissociation in the catalyst surface (Korus et al. 2017), the other reaction product in the mentioned study was benzene. The main difference with this work is the presence of a metallic active phase, where the breakdown of benzyl radicals occurs after toluene dissociation, hindering PAHs formation at larger reactant-catalyst contact time (lower space velocities). Other works studied the decomposition of ethylbenzene over differents chars; with a loss of catalytic activity such that the conversion reached values corresponding to the thermal decomposition (Hervy et al. 2018). Since the thermal conversion of more common tars in gasification gases (benzene, toluene, and naphthalene) began above 800°C (Jess 1996), is possible to affirm that the catalyst deactivation is never total because the operating temperature is below 625°C in all toluene decomposition experiments reported here. The conversion reached suggests that the activity shown by the solids studied in this work should be attributed to Fe clusters.

To close the catalytic cycle represented in the proposed mechanism only the elementary steps corresponding to the desorption remain, which are established in Eq.S7-Eq.10. Since not full deactivation of the catalyst was verified, it is assumed that carbon accumulation occurs in a site not related to toluene decomposition.

∗ ∗ 퐶7퐻8 + 퐶7퐻8 (퐸푞. 푆1) ∗ ∗ ∗ ∗ 퐶7퐻8 + 퐶6퐻6 + 퐶퐻2 (퐸푞. 푆2) ∗ ∗ ∗ ∗ 퐶6퐻6 + ⟶ 퐶2퐻2 + 퐶4퐻4 (퐸푞. 푆3) ∗ ∗ ∗ ∗ 퐶4퐻4 + ⟶ 퐶2퐻2 + 퐶2퐻2 (퐸푞. 푆4) ∗ ∗ ∗ ∗ 퐶2퐻2 + ⟶ 퐶퐻2 + 퐶 (퐸푞. 푆5) ∗ ∗ ∗ 2퐶퐻2 ⟶ 퐶2퐻4 + (퐸푞. 푆6) ∗ ∗ ∗ ∗ 퐶2퐻4 + 퐶퐻2 ⟶ 퐶3퐻6 + (퐸푞. 푆7) ∗ ∗ 퐶6퐻6 퐶6퐻6 + (퐸푞. 푆8) ∗ ∗ 퐶3퐻6 퐶3퐻6 + (퐸푞. 푆9) 퐶∗ ⟶ 퐶 + ∗ (퐸푞. 푆10)

Chapter 3: Catalytic decomposition of tars over Fe/CAG.

A simplified model (based on LHHW) is proposed to describe the kinetics of the process including the effects of reactant and intermediate interaction on the surface:

푘 퐾 퐾 푝 3 1 2 푡표푙푢푒푛푒 ( ) 푇푂퐹 = 2 퐸푞. 3.3 (1 + ∑ 퐾푖푝푖)

Where k3 is the forward kinetic constant in the third step (Eq. 3.3); K1 is the equilibrium constant in the toluene adsorption (Eq. S1); K2 is the equilibrium constant for the dissociation of adsorbed toluene; ptoluene is the toluene partial pressure in the feed; Ki is the equilibrium constant related to each reaction intermediate in catalyst surface and pi their corresponding partial pressure. These intermediates were not identified during the present work.

According to the proposed model, Equation 3.4 (Eq. 3.4) can express the apparent activation energy (Eapp) as a relation between the activation energy of the RDS and the heat of adsorption for reactant and intermediates:

퐸푎푝푝 = 퐸3 − 푄푡표푙푢푒푛푒 − 푄2 + 푄푖 (퐸푞. 3.4)

Where E3 is the activation energy of the rate-determining step; in this case,

Eq. S3; Qtoluene is the adsorption heat corresponding to toluene adsorption in this case, Eq. S1; Q2 is the heat associated with the equilibrated toluene dissociation and Qi represent the adsorption heat of all reaction intermediates in catalyst surface.

3.3.4.2. Effect of reaction temperature. Figure 3.6 shows the Arrhenius plot where a slope change is evident. Mass transfer limitations were discarded, as explained above. Experiments at different space velocities had the same behavior confirming the absence of mass transfer limitation. The change in the slope can be explained using

Chapter 3: Catalytic decomposition of tars over Fe/CAG.

Equation 3.4. At higher temperatures, the coverage of intermediates in the surface is lower, which is reflected in a diminish on Qi value. Toluene adsorption and dissociation heats (Qtoluene and Q2) have higher modular values than the activation energy of the rate-determining step (E3) and turn negative the apparent activation energy (Eapp), that is the slope of the Arrhenius plot. The combined effect of higher temperature and lower toluene concentration in the feed causes a stronger fall in the coverage of intermediates and, consequently, a stronger fall in the slope, just what happens when the lowest concentration of toluene (990 ppm) is fed according to Figure 3.6. This behavior is similar to a previously described for a benzene hydrogenation reaction over iron were a Langmuir- Hinshelwood mechanism was proposed (Vannice 2005).

Figure 3.6 Arrhenius plot for toluene decomposition. Temperature values: 575, 600, and 625°C. The space velocity was 700 ml·min-1·g-1. Catalyst mass of 0.1g. Note that the y-axis is logarithmic. For TOF calculation the metal exposed surface was as follow: Fe/CAG has 0.03894 mmol Fes/g catalyst and Fe/CAG-ps has 0.03789 mmol Fes/g of the catalyst, according to the method described previously in section 2.2.10

Chapter 3: Catalytic decomposition of tars over Fe/CAG.

The values at 2 lower temperatures were used to obtain a straight line and the corresponding slope, apparent activation energy values between 135 and 173 kJ/mol were estimated for the range of concentrations studied in the feed. Previous works report the apparent activation energy of 36 and 40 kJ/mol for a rate-determining step of the toluene activation on the toluene decomposition reaction on a perovskite catalyst (Mukai, Tochiya, et al. 2013). Values of 44.8 kJ/mol were reported using iron ore as a catalyst to decompose toluene and coal tars mixed (Cahyono et al. 2018). On the other hand, 109 kJ/mol was reported, in the same order of magnitude of values obtained here (Oemar et al. 2014). The difference between previous reports and the results shown here can be explained by the presence of the deactivation process by the joint action of carbon deposits and metal oxidation. Furthermore, it is necessary to emphasize that values obtained in the present work are for apparent energy activation using only 2 points in the Arrhenius plot, while other activation energies from works cited here are an approximation for rate-determining steps.

3.3.4.3. Effect of reaction conditions over Fe/CAG stability. Post-reaction characterization of the catalysts using XRD was made to verify changes in the crystalline phases during the reaction. It is well known that the iron metallic phase is active for C-C bond breaking, but the oxide phase has a lower activity (Virginie et al. 2010). The XRD patterns (Figure 3.7) of spent catalysts show oxidized phase as identified according to Table A2 (see Annexes), which suggests that the catalytic activity decreases due to Fe oxidation as the reaction proceeds. The only way to get oxygen for iron oxidation is from support complexes, where 5.3% w/w is oxygen according to the elemental composition. Fe oxidation implies the absence of H2 available as a reducing agent. Therefore, from these results, it can be inferred that no H2 is formed from toluene decomposition, which supports its exclusion from the mechanism previously proposed. There are no significant differences in crystallite sizes of Fe and oxides clusters capable to explain the magnitude of deactivation (Table A3). Figure 3.8 shows the percentage of the XRD peaks area corresponding to metallic Fe which were calculated as the ratio peaks(Fe0)/Total pattern area; it is observed that the amount of active

Chapter 3: Catalytic decomposition of tars over Fe/CAG. phase (metallic Fe) rises with temperature, while the space velocity shows a minor effect on Feº concentration. These results allow discarding sintering and oxidation as main deactivation causes at higher temperatures. It is important to mention that the FeO:Fe3O4 ratio is still almost constant at 3:2, and independent of the reaction conditions, which indicates that the kind of iron oxides haves no influences on the activity loss.

Figure 3.7 Normalized XRD patterns of fresh and spent catalysts during toluene decomposition. Each pattern is identified with the corresponding spent catalyst condition at the right /catalyst, temperature, toluene concentration in the feed. All experiments were carried out at 700 ml·min- 1 -1 ·gcat , except for spent catalyst marked with SV 1051, which were -1 -1 performed at 1051 ml·min ·gcat .

Table 3.3 shows the C/Fe ratio on spent catalysts. The amount of carbon increased at higher temperatures in all conditions of the toluene concentration fed and space velocity studied. Previous literature reports demonstrated that PAHs molecular weight increased with the reaction temperature (Trimm 1977; Jess 1996). This increment in the PAHs

Chapter 3: Catalytic decomposition of tars over Fe/CAG. molecular weight is related to the occurrence of polymerization reactions, which necessarily leads to carbon deposition in the catalyst surface. The later was demonstrated here by the C/Fe ratios calculated from SEM-EDX data. The influence of carbon deposition on the catalytic performance of Fe/CAG for tars decomposition is further discussed in Chapter 4.

Figure 3.8 Percentage of Fe0 remaining in the spent catalyst at different conditions. Each experiment is denoted with the toluene concentration in -1 -1 the feed. All experiments were carried out at 700 ml·min ·gcat , except for spent catalyst marked with 875 SV, which was performed at 875 -1 -1 ml·min ·gcat .

Table 3.3 C/Fe ratio in spent catalysts at different toluene concentration in the fed. Catalyst C/Fe ratio Fe/CAG 575-990 7.14 Fe/CAG 600-990 5.13 Fe/CAG 625-990 5.58

Chapter 3: Catalytic decomposition of tars over Fe/CAG.

Fe/CAG 575-990 SV 875 6.84 Fe/CAG 600-990 SV 875 6.72 Fe/CAG 625-990 SV 875 7.22 Fe/CAG 625-1485 5.97 Fe/CAG 625-1979 8.71

3.3.5. Experiments at BENCH scale. 3.3.5.1. Effect of reaction temperature. Naphthalene is considered to be the most difficult tar to be decomposed (Devi, Ptasinski, and Janssen 2005). As was described above, the catalyst used for BENCH scale experiments is quite similar to that used for toluene decomposition, the support is the same but it was shaped as pellets to avoid pressure drop in the catalytic bed. The experiment carried out in UMSICHT facilities shows conversions of naphthalene (Figure 3.9a). However, benzene concentration in the product gas is higher than in the fed, which suggests that it could be produced from the breakdown of naphthalene into monoaromatics.

Chapter 3: Catalytic decomposition of tars over Fe/CAG.

Figure 3.9 Naphthalene decomposition on Fe/CAG-ps. a- different temperature. b- with co-fed of different gases at 565°C. Both at 960 ml × min-1× g-1 space velocity, 3130 ppmv of benzene, 660 ppmv of naphthalene.

The later was confirmed by an experiment carried out at 565°C with pure benzene in the feed and Ar as balance. The benzene was converted by around 8%, which confirms that the increase in benzene concentration registered before is due to naphthalene decomposition.

A dynamic experiment allows evidencing that conversion diminishes as the reaction proceeds, particularly at higher temperatures (665°C) were Fe oxidation is favored. Moreover, the presence of common gasification gases such as CO or H2 (Figure 3.9b) causes an increase in naphthalene conversion and has a different influence on catalyst structure. While CO causes a strong oxidation process, H2 avoids it (Figure 3.9b). The effect of CO increasing naphthalene conversion is higher than deactivation caused by the Fe oxidation when this gas is co-fed, indicating a change in the reaction mechanism. Although, the difference between XRD patterns of both spent catalysts is not reflected in a difference in naphthalene conversion.

Chapter 3: Catalytic decomposition of tars over Fe/CAG.

Figure 3.10 Arrhenius plot for naphthalene. Temperature values: 565, 620, and 665°C.

Figure 3.10 shows the Arrhenius plot for the naphthalene decomposition over Fe/CAG-ps; the apparent activation energy for this process resulted in 57.2 kJ/mol, this value was estimated using the results obtained at two lower temperatures, which implies a limitation to the calculation. The literature report values of 72 kJ/mol for the steam reforming of naphthalene over activated carbon (Fuentes-Cano et al. 2013). Compared with the apparent energy determined for toluene, which is affected by the deactivation, naphthalene shows values 3 times lower. The spent catalysts characterization allows elucidating the differences between what happened during the tar decomposition reaction with Fe/CAG and Fe/CAG-ps.

3.3.5.2. Effect of the reaction condition over Fe/CAG-ps characteristics. Oxide formation on metal clusters was also verified, according to the effect that this change could exert on the activity results (See section 3.3.4.3). All the XRD patterns for the spent catalysts allow confirming the formation of oxide phases (Figure 3.11 and Table A4). The loss in the activity here is attributed to the change in the metal oxidation degree. Fe

Chapter 3: Catalytic decomposition of tars over Fe/CAG. oxides are reported like active in tars decomposition, breaking bond different to C-C type, but always with lower activity than Fe0 and with different activity depending on oxide nature (Duman, Uddin, and Yanik 2014). The oxides formations depend on O availability, while is higher, the oxidation state change from FeO to Fe3O4 and γ-Fe2O3 (Kazeminezhad and Mosivand 2014).

The catalysts used in experiments with CO in the feed show a higher oxidation degree after being used. The CO favors the Fe oxidation that’s why the XRD patterns for those spent catalyst show almost exclusively γ-

Fe2O3 and Fe3C phases. Contrariwise, for spent catalysts collected after the reactions co-fed H2, the patterns do not show oxides signals, being quite similar to the fresh catalyst. The presence of H2 favor the metal reduction reactions, keeping the more active Fe0 during the catalytic cycle, so, iron oxide presence implies no H2 in the products. Interestingly, tars co-fed with CO and H2 together cause the same effect than CO co-fed pure. CO dominates the interaction with the active phase surface. The simultaneous formation of iron oxides and carbides suggests that CO is adsorbed dissociatively. Oxides are reported to be inactive in the tars decomposition, or less active (Virginie et al. 2010). On the other hand, carbides are active in reactions involving hydrocarbons such as Fischer- Tropsch (Ordomsky et al. 2016; Horáček 2020). Catalyst activity increases when CO is co-fed, even with the loss of Fe0 active sites. This suggests that iron carbides are responsible for this increase. If this is the case, their activity is much higher than that of metallic Fe.

Chapter 3: Catalytic decomposition of tars over Fe/CAG.

Figure 3.11 XRD pattern of fresh and used catalysts during naphthalene decomposition.

The experiments are shown here suggest differences between reaction mechanisms for toluene and naphthalene decomposition. While benzene produced after toluene decomposition is consumed totally in the reaction (intermediary), suggesting free active sites presence for the reaction, benzene produced from naphthalene decomposition leaves the reactor as a product. According to that reported by (Mukai, Murai, et al. 2013), Naphthalene should be adsorbed in the surface by one ring in planar form and after that, it decomposes. According to the results of the experiments with pure benzene in the feed, both catalysts are capable to break the aromatic ring, due to the presence of Fe0 sites. It is possible to assume that one of the naphthalene rings is broken in contact with the surface-active sites (Eq. S12), while the second ring is released contributing to an increase in the concentration of benzene in the product gas (Eq. S20). Since no radicals, as methyl in the toluene molecule, are present in naphthalene structure to trigger the decomposition, the break of the aromatic ring is markedly slower during naphthalene decomposition.

Chapter 3: Catalytic decomposition of tars over Fe/CAG.

However, each naphthalene molecule produces only one of these groups * from C2H4 resulting from the molecule break (Eq. S12). This break occurs in the ring adsorbed in the surface through the bonds C1-C8a and C4-C4a

(Figure 3.12), the C8a and C4a take the HC1 and HC4 to form the released benzene molecule, leaving in the surface the C2 and C3 with their corresponding H, together with C1 and C4. These ring fragments are * capable to form only one CH2 group following the sequence Eq. S13→Eq. S14→Eq. S17, occupying an active site until at least 2 other naphthalene molecules react in neighboring active sites to form two other

CH2* groups, to finally form aliphatic chains (Eq. S19). On the other hand, the conversion Toluene and benzene leads to the formation of three CH2*, which leave the surface as propylene. Thus, naphthalene decomposition becomes more difficult than toluene and benzene because it requires the adsorption of three molecules to liberate the occupied sites, while monoaromatic molecules only required one molecule adsorbed to form aliphatic products and let the catalytic cycle go on.

a 1 b 6 2 1 6 2 3 5 5 3 4 4

Chapter 3: Catalytic decomposition of tars over Fe/CAG.

c 8 1 7 8a 2

6 3 4a 5 4

Figure 3.12 Schematic representation of the molecules of a- benzene, b- toluene, and c- naphthalene.

The following mechanism resumes the elemental steps above proposed. ∗ ∗ 퐶10퐻8 + 퐶10퐻8 (퐸푞. 푆11) ∗ ∗ ∗ ∗ 퐶10퐻8 + ⟶ 퐶6퐻6 + 퐶4퐻2 (퐸푞. 푆12) ∗ ∗ ∗ ∗ 퐶4퐻2 + ⟶ 퐶3퐻2 + 퐶 (퐸푞. 푆13) ∗ ∗ ∗ ∗ 퐶3퐻2 + ⟶ 퐶2퐻2 + 퐶 (퐸푞. 푆14) ∗ ∗ ∗ ∗ 퐶6퐻6 + ⟶ 퐶2퐻2 + 퐶4퐻4 (퐸푞. 푆15) ∗ ∗ ∗ ∗ 퐶4퐻4 + ⟶ 퐶2퐻2 + 퐶2퐻2 (퐸푞. 푆16) ∗ ∗ ∗ ∗ 퐶2퐻2 + ⟶ 퐶퐻2 + 퐶 (퐸푞. 푆17) ∗ ∗ ∗ 2퐶퐻2 ⟶ 퐶2퐻4 + (퐸푞. 푆18) ∗ ∗ ∗ ∗ 퐶2퐻4 + 퐶퐻2 ⟶ 퐶3퐻6 + (퐸푞. 푆19) ∗ ∗ 퐶6퐻6 퐶6퐻6 + (퐸푞. 푆20) ∗ ∗ 퐶3퐻6 퐶3퐻6 + (퐸푞. 푆21) 퐶∗ ⟶ 퐶 + ∗ (퐸푞. 푆22)

3.4. Partial Conclusions. - The oxygen available in the support oxidizes iron catalyst during the reaction; however, XRD characterization does not show differences to explain the drop in the activity at higher temperature levels. - Benzene and toluene decomposition over Fe/CAG shows strong differences: while benzene is totally consumed, toluene conversion is low

Chapter 3: Catalytic decomposition of tars over Fe/CAG.

(40%). When the reaction is studied using pure benzene in the fed; toluene is detected in the products and vice versa, suggesting that the elementary step governing the reaction is reversibly balanced. Furthermore, the break of the aromatic ring during benzene decomposition must generate *CH2 groups capable to form toluene with the adsorbed benzene leaving in the surface one C atom for each methyl group generated. - The generation of propylene as reaction product suggests the presence of surface *CH2 groups as part of tars decomposition and ring breakdown. - Naphthalene decomposition over Fe/CAG-ps produces benzene as the main product, which is demonstrated by a rise of benzene concentration. The amount of C deposited in the catalyst surface for each

*CH2 is higher, causing higher deactivation and preventing benzene decomposition after its formation during naphthalene break. - The apparent activation energy of toluene shows a reversal at higher temperatures caused by a decrease of the reaction intermediates on surfaces making the modular values of toluene adsorption and dissociation higher than the RDS activation energy. - The incorporation of typical gases produced during gasification,

CO and H2, into the reactor generate an increment in naphthalene conversion; however, CO promotes oxidation of the active phase at a level higher than observed during decomposition of pure tars, forming iron oxides and carbides; suggesting that a new active phase appears with higher activity than metallic Fe.

Chapter 4: Carbothermic reduction of carbon aerogel-supported Fe during catalytic tar decomposition.

3. Chapter 4: Carbothermic reduction of carbon aerogel-supported Fe during catalytic tar decomposition.

The iron-catalyzed tars decomposition is studied by inspecting changes in catalytic activity and its correlation to the metal oxidation numbers. A carbon aerogel-supported iron (Fe/CAG) catalyst was used to decompose toluene (model tar) between 500 ºC and 700ºC. The catalytic decomposition was studied in a conventional reaction system and, the catalyst was characterized prior and after the reaction, to understand the effect of the structural changes in the catalytic activity. Results demonstrated that the iron oxidation state and coke deposition strongly depends on the reaction temperature. The XRD diffraction patterns confirmed that at T<700ºC several Fei+ species are present, while at 700 ºC the Fe0 prevails, suggesting a reconstruction of the metal clusters on the surface. The ratio Fe/C determined by elemental mapping (EDS), as well as, the thermogravimetric analysis coupled to mass spectrometry (TGA-MS) of spent catalysts, confirmed that surface reconstruction is related to the carbothermic reduction of the metal oxides.

Chapter redrafted after: Gómez-Cápiro, O., R. Jiménez and L.E. Arteaga-Pérez. 2020. “Carbothermic reduction of carbon aerogel-supported Fe during catalytic the catalytic decomposition of toluene.” Submitted to Catalysis Today. Manuscript Number CATTOD-D-20-00244.

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Chapter 4: Carbothermic reduction of carbon aerogel-supported Fe during catalytic tar decomposition.

4.1. Introduction. The catalytic elimination of tars from gasification gas is one of the principal ways to make these mixes of gases useful for energy generation and chemical synthesis (Abu El-Rub, Bramer, and Brem 2004; Asadullah 2014b; Korus et al. 2017). The literature contains numerous papers on this catalytic process using traditional approaches based on performance indicators such as conversion, selectivity, etc. However, the change in the properties of catalysts at reaction conditions affects the whole process and is often ignored. Ideally, the catalyst must be characterized under reaction conditions (in operando) (Ertl et al. 2008); however, the characterization of fresh and used catalysts could help to shed light on possible changes occurring during the reaction when in operando characterization is not possible. Morphological characterization through adsorption-desorption techniques and/or electronic microscopy (Nestler et al. 2016) and thermogravimetric analysis (Korus et al. 2017) are the most common methods used to understand what happened during the reaction. For example, Courson et. al. (Courson et al. 2000) found neither sintering nor coke formation on the Ni/olivine catalyst using X-ray diffraction (XRD) and elemental analysis characterization after tars decomposition reactions and suggested that the catalyst undergoes an aging behavior during the testing. Świerczyński et al. performed post-reaction catalyst characterization with transmission electron microscopy (TEM) and Mössbauer spectroscopy (Świerczyński et al. 2007), they witnessed the formation of Ni-Fe alloy in a Ni/olivine material after being used for tars steam reforming. In fact, (Świerczyński et al. 2007) state that these bimetallic Ni-Fe sites inhibit carbon formation, leading to promising results in terms of activity and stability of the catalyst under reaction conditions.

The changes observed in the spent catalysts can be caused by a variety of factors e.g.: the main reaction (Guo et al. 2016b; Korus et al. 2017); poisoning caused by contaminants or products formed during reactions (Appari et al. 2014); active phase-support interaction (X. K. Li et al. 2005; Kong et al. 2011); sintering (Ochoa et al. 2018) and/or secondary reactions (i.e.: coking) (Jagtap, Kale, and Gokarn 1992; H. Wang et al. 2018; Chen

101

Chapter 4: Carbothermic reduction of carbon aerogel-supported Fe during catalytic tar decomposition. et al. 2019). Changes in the oxidation state during the reaction are another important issue to take into account. Metallic catalysts, normally, are active in a single oxidation state (oxide or metallic phase, depending on metal and reaction). Literature reports that an iron catalyst for tars decomposition must have oxidation number 0. However, oxygenated species present in support and/or fed can change the Fe0 to higher oxidation states. Accordingly, thermodynamic studies of metal oxidation reactions could help to find a better condition to prevent, decrease, or even reverse the oxidation influence. According to Hoekstra (Hoekstra et al. 2016), iron carbothermic reduction (CTR) is expontaneous near to 700°C. To the best of our knowledge, CTR of Fe metallic sites has not been studied as a phenomenon interfering with the catalytic process during tars conversion. Indeed, carbonaceous support and use of transition metals like Fe have become more common thanks to a renewed interest in biomass as raw material (Kastner, Mani, and Juneja 2015; Shen et al. 2014; Hervy et al. 2018). In two previous papers, Arteaga et al. (Arteaga-Pérez, Delgado, et al. 2018) and Gómez-Cápiro et al. (Gómez-Cápiro et al. 2018), the activity of carbon aerogel-supported transition metals (Ni, Fe) as a catalyst for treating biomass-derived gasification gases was demonstrated. In those studies, the authors witnessed a reduction in the catalytic activity which was attributed to the formation of surface carbon with a higher effect at the lower temperatures (<800ºC). Nevertheless, no characterization of the spent catalyst was reported, thus there is no evidence for discarding that these results could also be related to changes of the Fe oxidation state. The C-reduction sites for Fe2+ or Fe3+ could be provided by (i) the carbonaceous support (in carbon-supported Fe catalysts), (ii) the carbon species formed as intermediate products of the reactions or by (iii) the coke deposited in the surface(Jiang et al., 2018; Raj et al., 2019).

The aim of the next sections is to elucidate the effect of CTR on the performance of carbon aerogel (CAG) supported Fe catalyst (Fe/CAG) during the toluene decomposition reaction. The fresh and used Fe/CAG were analyzed by XRD, scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDX), and thermogravimetric analysis coupled to mass spectrometry (TGA-MS) to explain the relationship between

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Chapter 4: Carbothermic reduction of carbon aerogel-supported Fe during catalytic tar decomposition. activity and changes occurring in metallic sites. As support, carbon aerogel obtained from cellulose aerogel was used. Carbothermic reduction was corroborated, allowed explaining changes in conversion during toluene decomposition. Changes in oxidation states during the reaction were explaining as well.

4.2. Materials and methods. 4.2.1. Support precursor and obtaining. The precursor used and the pretreatment before carbonization are described in section 2.2.1. Carbonization was carried out according to conditions chosen after the results discussed in section 2.3.2. Catalysts preparation was described in section 2.2.3. Finally, the morphological characterization was obtained as described in section 2.2.5.

4.2.2. Toluene decomposition activity tests. Activity tests for toluene (tar model) decomposition were performed in a fixed bed reactor connected in-line to a gas chromatograph. In a typical experiment, 200 mg of catalyst was placed into a U-shape tubular quartz reactor located inside a furnace. Toluene (>99% purity, Merk) was fed by a syringe pump (Cole-Parmer GmbH, Wertheim, Germany) at 0.04 mL/h to a custom-made evaporator (180ºC) where the reactant is vaporized and diluted in He (Air Liquide, 99.999% purity, Coronel, Chile) to achieve a specific partial pressure. All pipelines were heated using heating tapes (Omega Eng, Stamford, CT, USA) to avoid cold spots and condensation. Gas composition was recorded prior (reactor by-pass) and after reaction in a Clarus 580 GC (Perkin-Elmer, Chile) equipped with Elite-5 (Perkin- Elmer, Chile) and Carboxen-1000 (Supelco Analytical, Germany) columns coupled with FID and TCD detectors, respectively; the injection valve is heated the same way as pipelines. The reaction temperatures value were 500, 600 and 700°C and the conversion of toluene (x) was calculated according to equation 1 (Eq. 4.1)

° 푓 푃푝푇표푙푢푒푛푒 − 푃푝푇표푙푢푒푛푒 푥 = ° (퐸푞. 4.1) 푃푝푇표푙푢푒푛푒

103

Chapter 4: Carbothermic reduction of carbon aerogel-supported Fe during catalytic tar decomposition.

° 푓 Where 푃푝푇표푙푢푒푛푒 and 푃푝푇표푙푢푒푛푒 are the partial pressure of toluene on the feed and the reactor exhaust, respectively.

As a reaction product, aliphatic hydrocarbons and carbon were expected according to the reaction (R. 4.1):

퐶7퐻8(푔) → 퐶푛퐻푚(푔) + 퐶(푠) (푅. 4.1) Where n=(1, 2, 3) and m=(2, 4, 6) are most probable values. Since aliphatic hydrocarbons can react between them in the gas phase (Morrison and Boyd 1987), C deposition quantification on the spent catalyst is a most reasonable way to follow the changes of reaction products related to different temperature levels. C deposition analysis is described below (section 4.2.7).

4.2.3. X-ray diffraction (XRD). XRD patterns of catalysts before and after reaction were recorded to evaluate the crystalline structure of the samples and detect the presence or not of iron oxides in spent catalysts. The XRD patterns were recorded in a Bruker AXS model D4 Endeavor diffractometer (Bruker AXS GmgH, Karlsruhe, Germany), using monochromatic CuKα radiation (λ=0.15418). The device generates a signal at 40 kV and 20 mA. The intensities were measured in the range 5°<2ϴ<90°C for CAGs, with a step size of 0.02° and scans at one s/step.

4.2.4. Scanning Electron Microscopy-Energy Dispersive X-ray spectroscopy (SEM-EDX). C deposition over the catalyst correlated to the C/Fe ratio before and after the reaction assuming Fe content quantified by the ICP-OES test remained constant. C/Fe ratio comparison between spent and fresh catalysts shows if there was carbon consumption related to CTR. The relative amounts of each element were estimated using SEM-EDX in a LEO 1420VP microscope using 300 X magnification and following C, H, N, O, and Fe elements. The change in the C/Fe was attributed to the C deposition or support consumption during the reaction.

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Chapter 4: Carbothermic reduction of carbon aerogel-supported Fe during catalytic tar decomposition.

4.2.5. Carbothermic reduction verification. Thermogravimetric analysis of a Fe/CAG sample after the reaction was performed in a Netzsch TGA thermobalance (model STA 409 PC) (Netzsch, Selb, Germany). In a typical experiment, 40 mg of spent catalyst was placed in a crucible and the weight variation in inert gas was recorded with the temperature varying from ambient temperature up to 700ºC at 2°C/min and under a constant flow of He (50 mL/min) (Air Liquide, 99.999% purity, Coronel, Chile). The composition of products exiting the thermobalance was recorded with a mass spectrometer (MS) (QMS 403C Aëolos, Netzsch) (Netzsch, Selb, Germany) in SIR mode to monitor the evolution of CO and CO2.

4.3. Results and discussion. 4.3.1. Compositional analysis. CAG elemental composition (C, 91.2%, N, 1.7%, H, 0.8% and O, 5.3%) is similar to that established by (Standard 472:1999) for carbon aerogels and confirmed the presence of oxygen in the support. The ash content was below 1% and sulfur was not detectable. The ICP-OES results from CAG and Fe/CAG confirm the absence of inorganics in the CAG and a 10.45% wt. of Fe in the Fe/CAG. This composition is in line with the quantity estimated during IWI and the determinations of EDX (section 3.3.2).

4.3.2. Catalytic activity test. Three catalytic activity tests were performed to correlate reaction data with catalyst characterization to verify the evidence of CTR during tars decomposition. For such experiments, the reactant mixture was fed at a constant space velocity (350 mL/min/g) and toluene concentration (1979 ppm). All measurements were recorded after reaching the steady-state. Experiments were performed at 500, 600, and 700°C; the absence of external mass transfer limitations was confirmed by the Mears criterion, which was far below 0.15 (2.06·10-9). Also, the Weizs-Prater criterion was far below 1 (1.78·10-3), which rules out internal mass transfer limitations (Fogler 2008).

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Chapter 4: Carbothermic reduction of carbon aerogel-supported Fe during catalytic tar decomposition.

The conversion achieved in each experiment shows the typical sigmoid behavior with temperature increase. However, C deposition measurement with SEM-EDX shows a fall at 700°C (Figure 4.1). It is observed that the C/Fe ratio increased by 11% for Fe/CAG (500ºC) and 58% for Fe/CAG (600ºC) as compared to the fresh catalyst. This increment was attributable to the C content considering that Fe mass remained unchanged during experiments. With the catalyst mass in all the experiments (0.2g), and maintaining the assumption that the Fe content does not change, the quantities of C were estimated. The values are shown on the markers of the C/Fe ratio in Figure 4.1. Moreover, the 23% reduction in the C/Fe ratio for Fe/CAG (700ºC) about the catalyst tested at 600ºC could be associated with the CTR of FexOy species with a C consumption of, at least, 0.019g.

In this sense, (Hoekstra et al. 2016) suggested that FexOy is spontaneously reduced in the presence of C at 700ºC, according to the following stoichiometric reaction R. 4.2 and R. 4.3:

퐹푒3푂4(푠) + 퐶(푠) → 3 퐹푒푂(푠) + 퐶푂(푔) (푅. 4.2)

0 퐹푒푂(푠) + 퐶(푠) → 3 퐹푒 (푠) + 퐶푂(푔) (푅. 4.3)

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Chapter 4: Carbothermic reduction of carbon aerogel-supported Fe during catalytic tar decomposition.

Figure 3.1 Toluene conversion and C/Fe ratio for fresh and spent catalysts after reaction at 500, 600, and 700°C. Space velocity=350 ml·min-1·g-1. Toluene concentration in the feed: 1979 ppm. The estimated C deposition mass is declared over each C/Fe ratio marker.

The carbon source for the carbothermic reaction is not clear at this point, but there are three possibilities: the support; carbon from coke formation during a parallel coking reaction and/or carbon from the tars decomposition reaction formed as co-product. Coke formation was analyzed by Guisnet et. al. (Guisnet and Magnoux 2001). The authors proposed that the coke formation is a secondary reaction over acid sites on the active phase and/or support, which deactivates most catalysts as the reaction progresses. However, catalytic activity remained unchanged while the toluene decomposition was taking place, which suggests that no coke formation over metal sites was occurring. Thus, it can be assumed that C deposition occurs on the support and is due to its formation as a reaction product, not by coking.

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a b

Figure 3.2 Screenshot of SEM-EDX results. (a- fresh catalyst, b- spent catalyst at 600°C)

The carbon involved in the CTR should be available for the iron oxide, thus it probably is the carbon formed during the tars decomposition as suggested by Jess (Jess 1996) in free oxygen feed and confirmed here by the SEM-EDX determinations. Carbon from the CAG is less available because of the high cross-linking of the matrix (Gómez-Cápiro et al. 2018). Figure 4.2 (a and b) shows the screenshots of SEM-EDX results where fresh and spent catalyst at 600°C are compared, the rise in C signal is observable in Figure 2b, the other signals match with ultimate analysis and iron loading during catalyst preparation.

4.3.3. XRD of spent catalysts. A widely accepted fact is that the Fe in metallic phase is active for the C- C bond scission, but when this metal is in oxide form this activity is almost null (Virginie et al. 2010). XRD patterns of fresh and spent catalysts from the reaction at 500, 600 and 700 ºC (Figure 4.3 and Table A2), show the presence of oxidized phases in catalysts used at 500 and 600°C, while for

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Chapter 4: Carbothermic reduction of carbon aerogel-supported Fe during catalytic tar decomposition. fresh and 700ºC spent catalyst these reflections were absent or considerably weak, respectively.

Figure 3.3 XRD patterns of fresh and spent catalysts at different temperatures (reflections planes in Table A2). Peak marked as ‘quartz’ corresponds to quartz wool used to hold the catalyst.

The low intensity of oxides signal for Fe/CAG 700°C pattern significate that almost all iron sites are in the most active phase for tars decomposition, unlike catalysts used at lower temperatures, where it is the opposite. The effectivity of the pre-treatment of the fresh catalyst with H2 was confirmed by the sharp signal in 44.6° reflections, corresponding to (110) planes of Fe0, in a f.c.c structure. Nevertheless, the intensity of this signal was steeply reduced for Fe/CAG (500ºC) and Fe/CAG (600ºC), for which new reflections at 30º, 35.4º, 43°, 56.9° and 62.5º corresponding to 0 FexOy (1

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Chapter 4: Carbothermic reduction of carbon aerogel-supported Fe during catalytic tar decomposition. practically completely by CTR in a parallel reaction indicates that a similar oxidation process occurred at all three temperatures.

4.3.4. Thermogravimetric analysis. Spent catalyst at 500°C (containing Fe oxides) was heated according to the condition described in section 4.2.8. Since the test simulates the conditions at which the tars decomposition occurs at 700°C, the detection of carbon oxides in the MS at this temperature in the absence of toluene is strong evidence of CTR. No COx signal must appear below this temperature. Figure 4.4 shows the change in CO2 (m/z=44). According to reactions R. 4.2 and R. 4.3, the CO concentration should increase, however, 10 minutes after reaching 700°C a peak appears in the CO2 signal in MS plot (Figure 5.4). This suggests that the Boudouard reaction (R.4.4) is carried out in the gas phase, in addition to tars decomposition and CTR. At temperatures above 300°C, CO rapidly produces CO2 in a reversible reaction (Ertl et al. 2008). When iron oxide reduction becomes spontaneous at T > 695°C, CO concentration increases fast in the gas phase and must be consumed immediately to form CO2 because no CO signal is detected on MS detector. Independently of this reaction sequence, the observation of carbon oxides confirms that CTR is possible in the catalysts studied in this work to decompound tars and explain why the XRD pattern of spent catalysts at 700°C does not show iron oxides. Thermogravimetric derivative (DTG) shows a weight loss synchronized with a CO2 formation, which confirms that the O atoms involved in CTR are released from the solid. During the heating process, mass 44 signal increases; however, this increase is not synchronized with a significant weight change as can be seen on the DTG plot, which implies that no reaction has occurred at this point.

2퐶푂(푔) ⇄ 퐶푂2(푔) + 퐶(푠) (푅. 4.4)

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Chapter 4: Carbothermic reduction of carbon aerogel-supported Fe during catalytic tar decomposition.

Figure 3.4 Thermogravimetric analysis coupled to Mass spectrometry of spent catalyst at 600°C. Temperature program, DTG, and mass 44 signal are shown for a heating rate of 2°C/min under 50 ml/min He flows.

4.3.5. Partial Conclusions. - The comparison of the oxidation states between the spent catalysts during toluene decomposition at 500°C, 600°C, and 700°C shows the following results: The Fe0 was the main sites in the spent catalysts at 700°C, while a higher presence of Fe2+ and Fe3+ on spent catalysts al lower temperatures was confirmed by the XRD. These results suggest the absence of H2 in the system, which allows discarding it as a reaction product. - The drop in carbon deposition on the spent catalyst at 700°C (measured by EDX) suggests the consumption of C during the process via carbothermic reduction. This was confirmed by the calculation of thermodynamic parameters (e.g., equilibrium constants) and is in line with that previously reported. - Carbothermic reduction was confirmed by a TGA-MS experiment under He atmosphere were carbon oxides were detected as CO2 (m/z = 44) when the system reaches 700°C, coinciding with the temperature where carbothermic reduction become spontaneous.

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5. Conclusions. The analysis of the carbonization conditions of the cellulose microfiber allowed establishing the conditions to achieve a wide range of specific surfaces and pore volumes, not only those required for the study of gasification gas cleanliness. The effect of the conditions change on the morphology was in decreasing order temperature>heating rate>dwell time. The impregnation of the Fe on the selected CAG increases the capacity and nature of the adsorption compared to the pure support. Its decomposition also increases significantly on the surface of the metallic particles. The same applies to the decomposition capacity of tars, which only began when the iron was impregnated on the CAG. The activity of the catalyst concerning the nature of the tar follows the decreasing order naphthalene>toluene>benzene. The deactivation processes are mainly by oxidation of the active phase and deposition of carbon. While the oxidation depends firstly on the co-firing compounds and secondly on the reaction temperature; the carbon deposition is governed by the conversion as it is generated as a reaction product. The increase of the temperature can trigger parallel reactions that decrease the deactivation processes as was shown in the case of the carbothermal reduction verified above 700°C.

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Acknowledgments

6. Funding support. The author would like to thank to CONICYT-PFCHA/National Ph.D. /2016-21160609; Beca REDOC-2015 and Programa de Asistencia a Eventos form Dirección de Postgrado de la Universidad de Concepción; FONDECYT project 11150148, BMBF project 20150029; Proyecto Apoyo a Centros de Investigación de Excelencia AFB/ 170007. 2017- 2021 of the Technological Development Unit of the Universidad de Concepción; Carbon and Catalysis Laboratory (CarboCat), Department of Chemical Engineering, Faculty of Engineering, University of Concepcion; Fraunhofer UMSICHT, Institute for Environmental, Safety and Energy Technology, Germany. Also, to Douglas Bousfield of the University of Maine for providing us with MFC.

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8. Annexes. Table A1. MFC carbonization experimental conditions and CAGs textural properties. Heating Temperature Dwell Yield S Dp Vp Code rate Bet (°C) time (h) (%) (m2/g) (nm) (cm3/g) (°C/min) 90 900 0 10 27.6 370 10.2 0.21 90-2 900 0 20 24.6 590 10.2 0.24 91 900 1 10 27.5 214 12 0.152 92 900 2 10 27 121 8.8 0.045 100 1000 0 10 27.7 273 13 0.11 100-2 1000 0 20 24.5 74 10.4 0.044 101 1000 1 10 26.9 162 11 0.068 102 1000 2 10 27.5 40 8.8 0.016 110 1100 0 10 26.9 76 10 0.032 110-2 1100 0 20 17.8 797 11 0.39 111 1100 1 10 27.5 21 10.4 0.0097 112 1100 2 10 26.4 13 9.2 0.0073 80 800 0 10 28 460 10.2 0.19 90-0% 900 0 10 13.9 327 11 0.36

Table A2. XRD planes identifications in spent Fe/CAG. Signal’s Metal Angle Plane References Phase (°) 30 (220) Fe3O4 (Cheng et al. 2010; Shen et al. 2014) 35.4 (311) FeO (Cheng et al. 2010; Shen et al. 2014) 43 (400) Fe3O4 (Cheng et al. 2010; Shen et al. 2014) 44.6 (110) Fe0 (Lin et al. 2013) 54 (422) Fe3O4 (Cheng et al. 2010; Shen et al. 2014) 56.9 (511) Fe3O4 (Cheng et al. 2010; Shen et al. 2014) 62.5 (440) FeO (Cheng et al. 2010; Shen et al. 2014) 65 (220) Fe0 (Lin et al. 2013) 82.2 (211) Fe0 (Lin et al. 2013)

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Table A3. Particle size estimated by XRD patterns for three different Fe phases in toluene decomposition spent catalysts. Pattern area FeO Fe0 Fe O Catalyst 3 4 Fe0 Fex+ (311) (110) (511) (%) (%) Fe/CAG fresh - 27.1 - ~99 ~1 Fe/CAG 575°C-990ppm 19.7 31.7 15.2 38.3 61.7 Fe/CAG 600°C-990ppm 21.2 29.6 17.3 51.7 48.3 Fe/CAG 625°C-990ppm 17.4 30.2 14.9 61.5 38.5 Fe/CAG 575°C-1485ppm 20.0 29.6 16.4 57 43 Fe/CAG 600°C-1485ppm 18.7 29.6 17.3 62.9 37.1 Fe/CAG 625°C-1485ppm 17.0 29.3 20.0 75.5 24.5 Fe/CAG 575°C-990ppm SV 875 21.9 30.8 14.8 54.3 45.7 Fe/CAG 625°C-990ppm SV 875 20.5 30.8 14.3 77.3 22.7

Table A4. XRD planes identifications in spent Fe/CAG-ps. Sign al’s Plane Metal Phase References Angl e (°) 37.6 (222) Fe3O4 (Kazeminezhad and Mosivand 2014) 39.7 (109) γ-Fe2O3 (Kazeminezhad and Mosivand 2014) 40.6 (119) γ-Fe2O3 (Kazeminezhad and Mosivand 2014) 43 (400) Fe3O4 (Cheng et al. 2010; Shen et al. 2014) 43.7 (202) α-Fe2O3 (Kazeminezhad and Mosivand 2014) 44.56 (102) Fe3C (Yan et al. 2013) 44.6 (110) Fe0 (Lin et al. 2013) 45.8 (330) γ-Fe2O3 (Kazeminezhad and Mosivand 2014) 49.12 (112) Fe3C (Yan et al. 2013) 50 (421) γ-Fe2O3 (Kazeminezhad and Mosivand 2014) 51.7 (00 12) γ-Fe2O3 (Kazeminezhad and Mosivand 2014) 54 (422) Fe3O4 (Cheng et al. 2010; Shen et al. 2014) 58 (21 12) γ-Fe2O3 (Kazeminezhad and Mosivand 2014) 61.8 (440) γ-Fe2O3 (Cheng et al. 2010; Shen et al. 2014) 65 (220) Fe0 (Lin et al. 2013) 70.7 (620) Fe3O4 (Kazeminezhad and Mosivand 2014) 78.6 (133) Fe3C (Yan et al. 2013) 82.2 (211) Fe0 (Lin et al. 2013)

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Scheme B1. System for ammonia adsorption experiments.

MFC

MFC Adsorbent

TC Furnance

Temperature controller

10% NH3 Range: 50-150°C N2 H2 90% N2

Dräger X-am 7000 detector

Scheme B2. System for toluene-benzene decomposition experiments.

Temperature controller 150°C Temperature TC controller 150°C Toluene TC or Benzene Evaporator

Catalysts powder TC MFC

MFC Furnance

Temperature controller Range: 500-700°C H2 He Exhaust

Valve FID

GC Clarus TCD Oven 580

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Scheme B3. System for naphthalene-benzene decomposition experiments.

Temperature controller 150°C Temperature TC controller 150°C Naphthalene TC + Benzene Mixing Evaporator chamber Catalysts MFC (pellets) TC

MFC Furnance

Temperature controller Range: 565-665°C Exhaust He CO H2

Mass spectrometer (MS) with electron ionization (GAM 200)

140