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ARCHITECTURE AaltoUniversity Aalto University School of Chemical Technology SCIENCE + Department of Biotechnology and Chemical Technology TECHNOLOGY www.aalto.fi CROSSOVER DOCTORAL DOCTORAL DISSERTATIONS DISSERTATIONS

2015 Aalto University publication series DOCTORAL DISSERTATIONS 214/2015

The effect of H2S on oxidation properties of ZrO2-based biomass gasiﬁcation gas clean-up catalysts

Inkeri Kauppi

A doctoral dissertation completed for the degree of Doctor of Science (Technology) to be defended, with the permission of the Aalto University School of Chemical Technology, at a public examination held at the lecture hall Ke2 of the school on 15th of January 2016.

Aalto University School of Chemical Technology Department of Biotechnology and Chemical Technology Industrial Chemistry Supervising professor Prof. Juha Lehtonen

Thesis advisor Dr. Sc. Jaana Kanervo

Preliminary examiners Prof. Dr. Hab. Maria Ziolek Adam Mickiewicz University, Poland

Prof. Dr. JH (Harry) Bitter Wageningen University, The Netherlands

Opponents Prof. Lars J. Pettersson Kungliga tekniska högskolan (KTH, Royal Institute of Technology), Sweden

Aalto University publication series DOCTORAL DISSERTATIONS 214/2015

ISBN 978-952-60-6583-0 (printed) ISBN 978-952-60-6584-7 (pdf) ISSN-L 1799-4934 ISSN 1799-4934 (printed) ISSN 1799-4942 (pdf) http://urn.fi/URN:ISBN:978-952-60-6584-7

Unigrafia Oy Helsinki 2015

Finland

Abstract Aalto University, P.O. Box 11000, FI-00076 Aalto www.aalto.fi

Author Eeva Inkeri Kauppi Name of the doctoral dissertation The effect of H2S on oxidation properties of ZrO2-based biomass gasiﬁcation gas clean-up catalysts Publisher School of Chemical Technology Unit Department of Biotechnology and Chemical Technology Series Aalto University publication series DOCTORAL DISSERTATIONS 214/2015 Field of research Industrial Chemistry Manuscript submitted 14 August 2015 Date of the defence 15 January 2016 Permission to publish granted (date) 28 October 2015 Language English Monograph Article dissertation (summary + original articles)

Abstract Biomass gasification gas contains impurities which have to be removed before the gas can be utilized in e.g. energy or liquid fuels production. ZrO2-based catalysts can be used to oxidize tar impurities when oxygen is added to the gas. These catalysts have proven activity even when H2S is present. Moreover, an improving effect of H2S on ZrO2 oxidation activity has been observed at temperatures of 600 and 700 °C. Therefore, the reactivities of unsulfided and sulfided ZrO2-based catalysts were studied in order to understand the enhancing effect of sulfur on oxidation activity during gasification gas clean-up. Different adsorption modes of H2S on the ZrO2-based catalysts were established. Molecular adsorption occurred on all the studied catalysts at room temperature. The molecularly adsorbed H2S species are weakly bound and thus are not likely to be present on the surface at the high temperatures of gasification gas clean-up. Therefore, they cannot be the source of the enhanced reactivity, either. Dissociative adsorption of H2S was suggested on cation-anion pairs or by titration of terminal hydroxyl groups on ZrO2 and doped ZrO2. However, the terminal SH groups formed in these surface processes cannot contribute to the observed oxidation activity improvement on ZrO2, as revealed by density functional theory (DFT) calculations. Stable sulfur species formed on the surface of ZrO2 during adsorption of H2S at elevated temperatures (at 100 °C and above). H2S reacts with the surface probably via replacement of surface lattice oxygen at specific defect sites. The amount of sulfur deposited on the surface in these high temperature processes was found to correlate with enhanced oxidation activity compared to unsulfided ZrO2. This indicates that H2S at elevated temperatures produces surface sulfur which improves the redox properties of ZrO2. Based on DFT calculations, sulfur species in the lattice (multicoordinated SH or S) at specific sites can cause enhanced reactivity of lattice oxygen, but so far the exact structure of this site remains unknown. The interaction of H2S with ZrO2 is limited to specific surface sites, comprising not more than approximately 11 % of the surface. Studies on toluene oxidation (a model compound for tar) also indicated that tar oxidation occurs on specific sites where the intermediate species form. Based on observations, it is proposed that the oxidation improvement by sulfur occurs on these specific sites where sulfur improves the reactivity of surface lattice oxygen. The sulfur- tolerance of ZrO2 is thus originated by the limited number of sites capable of binding sulfur.

Keywords zirconia, hydrogen sulfide, tar oxidation, gasification gas clean-up ISBN (printed) 978-952-60-6583-0 ISBN (pdf) 978-952-60-6584-7 ISSN-L 1799-4934 ISSN (printed) 1799-4934 ISSN (pdf) 1799-4942 Location of publisher Helsinki Location of printing Helsinki Year 2015 Pages 57 urn http://urn.fi/URN:ISBN:978-952-60-6584-7

Tiivistelmä Aalto-yliopisto, PL 11000, 00076 Aalto www.aalto.fi

Tekijä Eeva Inkeri Kauppi Väitöskirjan nimi Rikkivedyn vaikutus biomassan kaasutuskaasun puhdistuksessa käytettävien ZrO2 - katalyyttien hapetusominaisuuksiin Julkaisija Kemian tekniikan korkeakoulu Yksikkö Biotekniikan ja kemian tekniikan laitos Sarja Aalto University publication series DOCTORAL DISSERTATIONS 214/2015 Tutkimusala Teknillinen kemia Käsikirjoituksen pvm 14.08.2015 Väitöspäivä 15.01.2016 Julkaisuluvan myöntämispäivä 28.10.2015 Kieli Englanti Monografia Yhdistelmäväitöskirja (yhteenveto-osa + erillisartikkelit)

Tiivistelmä Biomassan kaasutuksessa syntyvän tuotekaasun sisältämät epäpuhtaudet täytyy poistaa ennen kuin kaasua voidaan käyttää esimerkiksi energiantuotannossa tai nestemäisten polttoaineiden valmistukseen. Epäpuhtautena olevat tervat voidaan hapettaa ZrO2- katalyyteillä hapen läsnä ollessa, jopa kaasun sisältäessä rikkivetyä (H2S). Aikaisemmin on todettu, että H2S:llä on ZrO2:n hapetusaktiivisuutta parantava vaikutus lämpötiloissa 600 ja 700 °C. Tämän vuoksi tässä työssä tutkittiin rikittämättömien ja rikitettyjen ZrO2-katalyyttien reaktiivisuutta. Tavoitteena oli löytää syyt H2S:n hapetusaktiivisuutta parantavaan vaikutukseen biomassan kaasutuskaasun puhdistuksessa. H2S adsorboitui molekuläärisesti kaikille katalyyteille huoneen lämpötilassa. Molekuläärisesti adsorboituneet muodot ovat sitoutuneet heikosti eivätkä ole läsnä katalyytillä edellä mainituissa korkeissa lämpötiloissa. Siksi molekuläärisesti adsorboitunut H2S ei ole syy havaittuun hapetusaktiivisuuden paranemiseen. Dissosiatiivinen H2S:n adsorptio tapahtui joko ZrO2:n kationi-anioni pareille tai titraten terminaalisia OH-ryhmiä muodostaen terminaalisia SH-ryhmiä. Terminaaliset SH-ryhmätkään eivät vaikuttaneet ZrO2-pinnan hapen reaktiivisuuteen, mikä havaittiin laskennallisesti. Yli 100 °C:ssa muodostui H2S:n adsorboituessa ZrO2:n pinnalle vahvasti sitoutuneita rikin muotoja, jotka ovat luultavimmin sitoutuneet hilaan syrjäyttäen pinnan happea erityispaikoilla. Tällä tavalla adsorboituneen rikin määrä korreloi lisääntyneen reaktiivisen pintahapen määrään verrattuna puhtaan ZrO2:n pinnan hapen reaktiivisuuteen viitaten siihen, että pinnan hapetus-pelkistys ominaisuudet paranivat. Laskennallisten tulosten perusteella ZrO2:n hilaan liittyneet rikin muodot (multikoordinoitunut SH tai S) voivat parantaa hilahapen reaktiivisuutta erityispaikoilla, kuitenkin näiden paikkojen tarkka luonne on vielä epäselvä. H2S:n vuorovaikutus ZrO2:n kanssa on rajoittunut tietyille erityispaikoille, jotka peittävät enintään 11 % pinnasta. Tutkimukset tervan malliaineena käytetyn tolueenin adsorptiosta ja hapetuksesta osoittivat myös, että tervojen hapetus tapahtuu erityispaikoilla, joille muodostuu reaktiivisia hapetuksen välituotteita. Oletettavasti rikin vaikutuksesta tapahtuva hapetusaktiivisuuden paraneminen tapahtuu näillä erityispaikoilla, joilla rikin läsnäolo parantaa hapetusreaktioon osallistuvan pintahapen reaktiivisuutta. ZrO2-katalyyttien hyvä rikin sietokyky perustuu osin rikkiyhdisteiden rajalliseen reaktiivisuuteen pinnan kanssa.

Avainsanat zirkonia, rikkivety, tervojen hapetus, kaasutuskaasun puhdistus ISBN (painettu) 978-952-60-6583-0 ISBN (pdf) 978-952-60-6584-7 ISSN-L 1799-4934 ISSN (painettu) 1799-4934 ISSN (pdf) 1799-4942 Julkaisupaikka Helsinki Painopaikka Helsinki Vuosi 2015 Sivumäärä 57 urn http://urn.ﬁ/URN:ISBN:978-952-60-6584-7

Acknowledgements

The work for this thesis was conducted at Aalto University School of Chemical Technology, former Helsinki University of Technology, within the Research Group Industrial Chemistry. The work was made possible financially by Gradu- ate School of Chemical Engineering (GSCE) funded by Academy of Finland. I would also like to thank Finnish Foundation for Technology Promotion and Al- fred Kordelin foundation for financial support. I am indebted to my supervisors, Professors Outi Krause and Juha Lehtonen, for their support and for giving me the possibility to carry out this thesis work. I also want to thank the people who acted as my supervisors, Dr. Sanna Airaksinen, Dr. Juha Linnekoski, and Dr. Jaana Kanervo for their guidance and help. I specifically want to thank Dr. Jaana Kanervo for insightful discussions and support during the work, as well as Professor Outi Krause for her guidance. I am most grateful for Professor Leon Lefferts’ contribution to the progress of the thesis and the related publications, as well as for many hours of discussions that pushed the work forward. I thank Dr. Ella Rönkkönen for support and in- spiration and most of all for her kind friendship throughout the years. I would also like to sincerely thank my co-author Ms. Tiia Viinikainen for her energy and positivity and help with research. Moreover, I would like to express my gratitude to my co-authors Dr. Sanna Airaksinen, Dr. Satu Korhonen, and Dr. Karoliina Honkala for their contributions. I thank Dr. Maija Honkela for her help. My warm thanks also to the laboratory personnel in the Research Group Industrial Chemistry for support and creating a relaxed working atmosphere. During the thesis work I have had the opportunity to pursue research abroad and I am grateful to my home university for making that possible. During 6 months in 2010 the work for this thesis was carried out in the Instituto de Catálisis y Petroleoquimica, CSIC, Madrid, Spain. I warmly thank Professor Dr. Miguel Bañares for his support, guidance and teaching. As co-authors I thank him and Dr. Søren Rasmussen for co-operation and discussions. Professor Dr. Pedro Ávila at CSIC is also thanked for co-operation. I sincerely thank Dr. Ri- cardo López Medina for his kind and professional help during experiments as well as everyone working in the Catalytic Spectroscopy Laboratory for their help and support, and most of all their friendship. The FTIR experiments were performed during summer 2013 in the University of Caen, in the Catalysis and Spectrochemistry Laboratory (LCS) under supervision of Dr. Laetitia Oliviero, whose expert professional guidance is gratefully acknowledged. Director of Re- search, Dr. Francoise Maugé and Professor Dr. Marco Daturi are also thanked

1 for facilitating the visit to LCS. Moreover, I thank Dr. Jianjun Chen for assis- tance in experimental work. I thank Ms. Nathalie Perrier for help in practicali- ties in Caen. I sincerely acknowledge both laboratories for providing me a space to carry out the research, and for the warm atmosphere which made me feel welcome. Finally, I kindly thank all my friends and family members for all the love and joy they bring into my life. Thank you for support and your patience throughout the years.

Helsinki, 26 November 2015 Eeva Inkeri Kauppi

2 Contents

Acknowledgements...... 1 List of abbreviations and symbols...... 5 List of publications...... 7 Author’s contribution ...... 8 1. Introduction...... 9 1.1 Biomass gasification and gas clean-up...... 9

1.2 Oxidation of tars over ZrO 2-based catalysts...... 10

1.3 Properties and surface sites of ZrO 2 ...... 11

1.4 Effect of H2S on catalysis...... 12 1.5 Scope...... 13 2. Experimental...... 15 2.1 Temperature-programmed techniques ...... 15 2.1.1 Temperature-programmed sulfidation...... 15 2.1.2 Temperature-programmed desorption ...... 16 2.1.3 Temperature-programmed surface reaction of methanol...16

2.1.4 Temperature-programmed reduction with H2 and CO...... 16 2.1.5 Temperature-programmed experiments with toluene...... 16 2.2 Infrared spectroscopy of solid catalysts...... 17 2.2.1 Characterization of the surface sites by MeOH-DRIFTS.....17 2.2.2 Toluene adsorption and oxidation under DRIFTS...... 17

2.2.3 Transmission IR experiments on sulfided ZrO 2...... 18 2.3 Computational work ...... 19 3. Results and discussion...... 21

3.1 Surface properties of ZrO 2-based catalysts and effect of H2S..21

3.1.1 Methanol adsorption on ZrO2-based catalysts ...... 21

3.1.2 H2S adsorption modes and catalysts’ reactivity with MeOH22

3.1.3 MeOH surface species on ZrO 2 after H2S adsorption ...... 26

3.2 The effect of H2S on reactivity of surface oxygen on ZrO2...... 29

3.2.1 Interaction of H2S with ZrO 2...... 29

3 3.2.2 Stability of adsorbed H2S...... 30

3.2.3 Reduction of unsulfided and sulfided ZrO 2 by CO...... 31

3.3 Redox properties of ZrO2 and the effect of H2S studied on a molecular level ...... 34 3.3.1 Calculations performed to study the improving effect of adsorbed H2S on reactivity of surface lattice oxygen...... 34

3.3.2 ZrO2 as a redox catalyst and defect sites acting in relevant catalysis ...... 36

3.4 Toluene adsorption and oxidation studies on ZrO2-based biomass gasification gas clean-up catalysts...... 37 3.4.1 Surface species formed upon toluene adsorption and oxidation ...... 37 3.4.2 Gas-phase products from toluene adsorption and oxidation38

3.4.3 Possible toluene oxidation mechanism and sites involved on ZrO2 ...... 41 4. Conclusions...... 43 References...... 47

4 List of abbreviations and symbols

a.u. arbitrary unit

DRIFTS Diffuse Reflectance Infrared Fourier Transform Spectrum

FT Fischer-Tropsch synthesis

FTIR Fourier Transform Infrared

CO-TPR Temperature-programmed reduction with CO

CPOM catalytic partial oxidation of methane c.u.s. coordinatively unsaturated site

DFT density functional theory

DME dimethyl ether

DMS dimethyl sulfide

H2-TPR Temperature-programmed reduction with H2

TPA Temperature-programmed adsorption

TPD Temperature-programmed desorption

TPO Temperature-programmed oxidation

TPS Temperature-programmed sulfidation t-OH terminal hydroxyl group m-OH multicoordinated hydroxyl group t-SH terminal sulfide group m-SH multicoordinated sulfide groups

MeOH-TPSRTemperature-programmed surface reaction with methanol

ML monolayer (coverage)

MS mass spectrometer/spectrometry

SNG synthetic natural gas

į bending mode (in spectroscopy)

5 ǌ as asymmetric stretching mode (in spectroscopy)

ǌ s symmetric stretching mode (in spectroscopy)

WGS water-gas-shift

6 List of publications

This doctoral dissertation consists of a summary and of the following publications which are referred to in the text by their numerals

I. Kauppi, Eeva Inkeri; Rönkkönen, Ella Hanne; Airaksinen Sanna; Rasmus- sen, Søren; Bañares, Miguel; Krause, Outi. 2012. Influence of H2S on ZrO 2- based gasification gas clean-up catalysts: MeOH temperature-programmed reaction study. Elsevier. Applied Catalysis B: Environmental, 111 - 112, pages 605 - 613. 0926-3373. 10.1016/j.apcatb.2011.11.013.

II. Kauppi, Eeva Inkeri; Kanervo, Jaana; Lehtonen, Juha; Lefferts, Leon. 2015. Interaction of H2S with ZrO 2 and its influence on reactivity of surface oxygen. Elsevier. Applied Catalysis B: Environmental, 164, pages 360 – 370. 10.1016/j.apcatb.2014.09.042.

III. Kauppi, Eeva Inkeri; Honkala, Karoliina; Krause, Outi; Kanervo, Jaana; Lefferts, Leon. 2015. ZrO2 acting as a redox catalyst. Accepted for publication in Topics in Catalysis 09/2015.

IV.Viinikainen, Tiia; Kauppi, Eeva Inkeri; Korhonen, Satu; Lefferts, Leon; Kanervo, Jaana; Lehtonen, Juha. Molecular level insights to the interaction of toluene with ZrO2-based biomass gasification gas clean-up catalysts. Elsevier. Applied Catalysis B: Environmental, 142 – 143, pages 769 – 779. 0926-3373. 10.1016/j.apcatb.2013.06.008.

7 Author’s contribution

Publication I: Influence of H2S on ZrO 2-based gasification gas clean-up catalysts: MeOH temperature-programmed reaction study

The author planned the research with the help of the co-authors, carried out the experiments except the MeOH-DRIFTS experiments, and interpreted the results and wrote the manuscript with the help of the co-authors.

Publication II: Interactionof H2S with ZrO 2 and its influence on reactivity of surface oxygen

The author planned, carried out the experiments and interpreted the results together with the co-authors and wrote the first as well as the final version of the manuscript.

Publication III: ZrO2 acting as a redox catalyst

This review article consists of material that was made available to the author, including unpublished DFT results provided by Karoliina Honkala, and partly the author’s own previous work. The author contributed considerably to the selection of the data and wrote the first version of the manuscript as well as participated in finalizing the text and conclusions.

Publication IV: Molecular level insights to the interaction of toluene with ZrO2-based biomass gasification gas clean-up catalysts

The author planned the experiments together with the other main author, carried out temperature-programmed experiments, carried out partly the DRIFTS-experiments, interpreted results and wrote the manuscript together with the other main author. This work has two main authors.

8 1. Introduction

1.1 Biomass gasification and gas clean-up

Among the thermochemical processes (combustion, pyrolysis, gasification) which convert biomass into value-added products, gasification has the highest efficiency. Gasification converts biomass through partial oxidation into synthesis gas which is a mixture of mainly CO and H2, but CO2, H2O and CH4 and other light hydrocarbons are also present (Torres et al., 2007). The purified product can be targeted at liquid fuel production by Fischer-Tropsch (FT) synthesis, methanol synthesis, dimethyl ether production, synthetic natural gas (SNG) or

H2 production (Kurkela et al., 2008). The gas can also be fed to gas turbines or fuel cells to generate electricity. The quality of the producer gas has to be adjusted to meet the end-use speci- fications. Generally, heavier purification measures have to be taken for synthesis purposes. The impurity tars, i.e. polyaromatic hydrocarbons having a molecular mass greater than benzene, present the main challenge for the utilization of the produced gas due to their tendency to plug downstream processing equipment. Therefore, their content has to be reduced. Nitrogen and sulfur compounds are also present in small quantities, depending on the fuel used (Juuti- lainen et al., 2006). The H2/CO-ratio of the gas is determined by the gasifying agent used (air, oxygen or steam) and is adjusted by the water-gas-shift reaction (WGS, CO + H2O ļ CO2 +H2). A schematic representation of a biomass gasification process is shown in Figure 1.

Figure 1. Biomass gasification process (adapted from Kurkela et al., 2008).

An effective gas-cleaning step is key for a feasible biomass gasification process and must be added if the formation of tars cannot be avoided in the gasifier. Particulate dust and tar removal technologies can be divided into two categories, mainly referred to as primary and secondary methods (Göransson et al., 2011, Wang et al., 2008). Primary methods comprise treatments during gasification and secondary methods gas-cleaning after gasification, such as hot-gas clean- up. Studies and reviews are widely available on different hot-gas clean-up tech-

9 Introduction nologies, including mechanical methods, tar reforming, and catalytic conversion of tars to less harmful compounds. The choice of technology depends on many factors, from the quality of the raw materials to the end-use of the gas. Catalytic hot-gas cleaning with a catalyst-coated monolith is a competitive way to convert tars into less harmful compounds (Torres et al., 2007, Simell et al., 1996). A gas clean-up catalyst must be resistant to high temperatures and sulfur tolerant, and still have high selectivity in tar oxidation, so that oxidations of CO and H2 are avoided. The studied catalysts are, for example, metal catalysts such as nickel, precious metal catalysts such as Rh and Pt, alkali metals, and dolo- mites (Sutton et al., 2001, Rönkkönen et al., 2011, Simell et al., 1996). ZrO 2 catalysts are active and selective in tar and ammonia oxidation at 600-900 °C when oxygen is added to the gas (Juutilainen et al., 2006), even when H2S is present

(Rönkkönen et al., 2009). Oxidation over a ZrO2-based catalyst is often used together with tar reforming over metal catalysts.

1.2 Oxidation of tars over ZrO2-based catalysts

ZrO2 is a versatile material with high thermal stability, which has proven activity towards tar oxidation when oxygen is present (Juutilainen et al., 2006, Simell and Kurkela, 2004, Rönkkönen, 2014). It can be used as a catalyst or a catalyst support. The activities of ZrO 2 and doped ZrO 2 catalysts (Y 2O3-ZrO2,

SiO 2-ZrO2 and La2O3-ZrO2) have been investigated previously, and their surface properties were related to their tar oxidation activity during gas clean-up (Rönkkönen, 2014, Viinikainen et al., 2009). The main tar decomposition reaction over ZrO2 catalysts has been proposed (Juutilainen et al., 2006) to be a two-step oxidation, presented for naphthalene (a model compound for tar) in Equations 1 and 2 (Juutilainen et al., 2006, Rönkkönen et al., 2009b):

C H +7 O 10 CO+ 4 HO (1) CO + O CO (2)

Since gasification gas is a complex mixture of gases, the reaction network contains several competitive and consecutive reactions (Rönkkönen et al., 2009b). The individual gas components affect tar decomposition activity, e.g. water has an inhibiting effect on the conversion of naphthalene (Rönkkönen et al., 2009b).

H2S, which will always be present when real gasification gas feeds are processed, has a specific effect on the performanceof ZrO2 catalysts which will be discussed later. In the gas clean-up application, the oxygen is typically nearly completely con- sumed towards the end of the monolith, which results in changes in the reaction conditions along the monolith. At the inlet, where oxygen is still available, tar is oxidized to CO and CO 2. On the other hand, near the outlet of the monolith, steam and dry reforming reactions also contribute, as do hydrocracking and carbon formation (Rönkkönen et al., 2009b, Viinikainen et al., 2009). Understand-

10 Introduction ing of e.g. WGS, reforming, and oxidation catalysis in general is required to obtain a complete picture of ZrO2 performance during gas clean-up. Moreover, studies using individual impurity components are needed in order to determine the tar decomposition mechanism (relevant surface species formed and their reactions) and the sites involved.

1.3 Properties and surface sites of ZrO2

The oxidizing-reducing properties of ZrO 2 are established (Nakano et al., 1979), however, the oxidation ability of pure ZrO 2 is limited. It is a stable oxide which has been claimed to lose surface oxygen only after thermal treatment under a high vacuum at above 700 °C (Daturi et al., 1998). Nevertheless, ZrO2 can be used as an oxidation catalyst in some high-temperature applications (Zhu, 2005). ZrO2 also has proven activity in e.g. reforming (Kaila, 2008), partial oxidation of methane (Zhu, 2005), and WGS (Graf et al., 2009, Kouva et al., 2014). It has been proposed that tar oxidation on ZrO2-based catalysts during gasification gas clean-up proceeds via the Mars-van Krevelen mechanism (Rönkkönen, 2014). According to this mechanism, organic molecules are oxidized in redox cycles by surface lattice oxygen (Mars and van Krevelen, 1954). Oxidation via reaction with surface lattice oxygen creates vacancies, which are replenished by gas-phase oxygen (Mars and van Krevelen, 1954, Busca et al., 1996a). Surface lattice oxygen is present on ZrO2 ascoordinatively unsaturated (c.u.s.) oxygen and multicoordinated hydroxyl groups (m-OH), which exhibit different reactivity depending on the site/coordination. Surfaces of crystals are created when they are cut along a plane, generating atoms with different chemical envi- ronments. The surfaces are either flat or more open with steps and kinks, i.e. the minority sites. (Niemantsverdriet, 2007). Hydroxyl groups are generated on ZrO2 upon adsorption of water and are present as terminal OH (t-OH) or m-OH. It is possible that OH groups also participate in redox cycles, however, their role in oxidation is not completely clear.

The properties of ZrO2 can be changed by adding dopants, such as Y 2O3 or CeO2, to improve e.g. oxygen mobility on the surface and induce vacancy generation (Zhu et al., 2005a, Dutta et al., 2006). Improved ability to generate vacancies assists oxidation reactions on the oxide catalyst. Dopants also affect the surface area, which is low ion ZrO2. In general, the surface area of ZrO2 is increased by dopant addition. A change in the crystal structure from monoclinic to cubic or tetragonal or a mixture also occurs when adding dopants (Viinikainen et al.,

2009). It has been observed that doping with La2O3-ZrO2 generates a catalyst with the highest activity towards tar decomposition in gasification gas clean-up application when H2S is present in the gas (Rönkkönen, 2014).

11 Introduction

1.4 Effect of H2S on catalysis

Sulfur compounds, most importantly H2S and COS, are present in the producer gas. In fact, the amount of H2S may even be as high as 500 ppm, depending on the feed stock used (Torres et al., 2007). H2S is known to poison especially metal catalysts even at extremely low gas-phase concentrations. Sulfur binds strongly on metal surfaces and the effects are often irreversible, leaving no chance of regeneration (Bartholomew et al., 1982). Thus, sulfur tolerance of the catalyst is essential during gasification gas clean-up. Interestingly, on ZrO2- based catalysts, enhancement of naphthalene and ammonia oxidation in the presence of H2S has been noted during gas-cleaning experiments on synthetic biomass gasification gas. The effect was not observed on SiO2-ZrO2 (Rönkkönen et al., 2009a).

Beneficial effects of H2S on catalyst activity have been reported at least by (Jackson et al., 1991, Ziolek et al., 1995, Erdöhelyi et al., 2004, Laosiripojana et al., 2010, Roushanafshar et al., 2012, Sugioka et al., 1989, Vincent et al., 2011, Stenberg et al., 1982) on various catalysts, including mainly metal oxides. The proposed reasons for the observed effects vary and may be linked with phenomena occurring on the surface of the catalyst or in the gas phase. For example,

H2S has been reported to act as a gaseous promotor in the water-gas shift reaction, where H2S offers a pathway for CO to react via the formation of COS further to the products (Stenberg et al., 1982). On the other hand, adsorbed sulfur may cause surface modifications which change catalyst activity or selectivity. Different mechanisms have been suggested for the promoting effect of adsorbed H2S on the catalyst. For example, the reaction between CO2 and CH4 over

Rh/TiO2 and Rh/SiO2 catalysts was reported to be enhanced by selective poisoning of active sites (Erdöhelyi et al., 2004). Jackson et al. (Jackson et al., 1991) suggested that sulfur may modify the catalyst surface, changing the reaction mechanism. Sulfur-containing surface complexes, such as sulfate (SOx) species, have known effects in promoting the acidity of the catalyst and enhancing mainly isomerization and condensation reactions (Sohn and Kim, 1989, Vera et al., 2002). Moreover, even weak interaction (physical adsorption) with H2S and the catalyst surface has been found to affect catalyst properties (Maugé et al., 2002).

It is not commonly known what kinds of interactions between H2S and ZrO2 benefit oxidation reactions. The study by Rönkkönen et al. (Rönkkönen et al.,

2009a) suggested that a strongly adsorbed form of H2S affects the performance of ZrO2-based catalysts, since the effect was also seen after H2S was removed from the stream. This may also indicate a surface reaction of H2S with the catalyst. In general, H2S can interact with ZrO2 via three pathways: i) molecular adsorption on Zr4+ cations or interacting with hydroxyl groups, ii) dissociative adsorption on Zr4+-O2- anion-cation pairs or replacing a t-OH group, or iii) exchange of sulfur with lattice oxygen (Ziolek et al. 1995, Travert et al., 2002).

12 Introduction

1.5 Scope

The main aim of this work was to clarify the reasons for the observed enhance- m ent effect of H 2S on tar oxidation activity on ZrO2-based gasification gas clean- up catalysts. With regard to this aim, the target was to elucidate the effect of H2S on catalysts’ surface properties and generally the oxidizing/reducing properties on three ZrO2-based catalysts. Studies using methanol as a probe also looked into why H2S improves the activities of ZrO2 and Y 2O3-ZrO2, whereas no effect can be found on SiO2-ZrO2. Since tar oxidation has been suggested to proceed via reaction with surface lattice oxygen, it was thought that adsorbed H2S improves the reactivity of surface lattice oxygen or induces the generation of new sites able to participate in redox cycles. Therefore, sulfur interactions with ZrO2 surface were thoroughly examined. Temperature-programmed (TP) methods were utilized to obtain information on H2S adsorption pathways and the stability of adsorbed H2S on

ZrO2. The aim of the TP studies was primarily to obtain knowledge about how adsorbed H2S changes the reactivity of ZrO2 towards oxidation reactions. With regard to this, a density functional theory (DFT) study was also conducted to find out the most stable adsorbed H2S species on the ZrO 2 surface and to determine how they modify the reactivity of surface oxygen on specific sites. Gaining more insightful knowledge about the effect of H2S requires a better understanding of the tar oxidation mechanism on ZrO2. Studies on toluene (used as a tar model compound) adsorption and oxidation were targeted to give an estimation of the tar decomposition mechanism and site. Moreover, studies were aimed at discovering the relevant surface species formed from toluene. The role of defect sites and OH chemistry is fundamental in redox catalysis on ZrO2. Relevant to this work, the redox behavior of ZrO2 and the sites involved were thus briefly reviewed, which served the aim of understanding the behavior of

ZrO2 in gasification gas clean-up application and the effect of H2S on oxidation catalysis.

2. Experimental

Full experimental details are given in PublicationsI-IV. Brief descriptions of the experimental techniques used are given here.

2.1 Temperature-programmed techniques

Temperature-programmed (TP) methods are techniques monitoring a chemical reaction while the temperature of the solid catalyst sample is increased linearly in time (Niemantsverdriet, 2007). A schematic presentation of the experimental setup used in the studies is given in Figure 2. The reactor, which is usually a U- tube, is placed in a controllable oven and packed with the catalyst. The feed gases are specified by the experimental procedure used.

Figure 2. A schematic presentation of the TP technique used in the studies (adapted from Nie- mantsverdriet, 2007).

Relevant to this thesis, the specific qualitative TP techniques described in the following sections were used. A mass spectrometer was always applied for the analysis of gas-phase products.

2.1.1 T emperature-programmed sulfidation

Temperature-programmed sulfidation (TPS), where the sample is heated under an H2S atmosphere, is often used to describe the activation treatment in which oxidic catalysts that are active in their sulfided state are transformed into metal sulfides (e.g. alumina-supported molybdenum)(Niemantsverdriet, 2007). In this work (Publication II), the technique was used to study transformations occurring in the ZrO2 structure under mild sulfiding conditions. It is known that ZrO2 does not transform into ZrS2 (Clearfield, 1958); however, H2S adsorption occurs. Therefore, even though the bulk sulfide does not form, the term sulfided

15 Experimental

ZrO2 is used in this work when referring to ZrO2 treated with H2S. In the study

(Publication II), H2S(in N2, 500 ppm) was adsorbed on ZrO2 isothermally at 30 °C (1 h) and via TPS with four different end temperatures of 100, 200, 300 and 400 °C, aiming to affect the amount (and nature) of the H2S adsorbed species on the surface.

2.1.2 T emperature-programmed desorption

Temperature-programmed desorption (TPD) provides information on the binding states of adsorbed elements (intermediates and products) on the catalyst surface. As the sample with adsorbate is heated, the energy transferred to the adsorbed species will cause it to desorb, giving an observable peak maximum (Tmax)ofthe desorption temperature. In the publications regarding this thesis, TPD was used to study the thermal stability of adsorbed species and to probe the adsorption modes of H2S and toluene on the studied catalysts. TPD was performed after isothermal H2S adsorption at 40 °C and after H2S adsorption at 30-400 °C (respectively in Publications

I and II) to study the interaction of H2S with the catalyst surface and the stability of adsorbed species. Toluene-TPD was performed on ZrO 2, Y 2O3-ZrO2,and SiO2-

ZrO2 to study the decomposition of toluene adsorbed species (Publication IV). Accurate initial temperatures as well as the ramp rates are given in the relevant publications.

2.1.3 T emperature-programmed surface reaction of methanol

Methanol is a well-established surface probe (Badlani and Wachs, 2001, Tati- bouët, 1997). The selectivity pattern of gas-phase products is used to character- ize catalyst properties: dimethyl ether (DME) for acid sites, formaldehyde for redox, and CO 2 for basic sites (Badlani and Wachs, 2001, Tatibouët, 1997). The temperature-programmed surface reaction of methanol (MeOH-TPSR) from 100 to 700 °C under Ar flow was done for the unsulfided and sulfided (at 100 °C,

30 vol-% H2Sin H2)ZrO2, Y 2O3-ZrO2, and SiO2-ZrO2 samples to study the effect of H2S addition on catalyst properties. H2S was adsorbed isothermally at 100 °C on the samples. The study is presented in Publication I.

2.1.4 T emperature-programmed reduction with H2 and CO

Temperature-programmed reductions with H2 and CO (H2-and CO-TPR) were applied in studies on sulfided ZrO 2 in order to reveal how sulfur modifies the reducibility of the ZrO2 surface (Publication II). Both experiments were started at 30 °C and carried out until an end temperature of 600 °C with a ramp rate of 10 °C/min. H2 (4 vol-%) and (CO 5 vol-%) were fed during the ramp and at 600 °C for o.5 h.

2.1.5 T emperature-programmed experiments with toluene

TP gas-phase analysis was applied to study the gas-phase products during adsorption and oxidation of toluene adsorbed species in detail (Publication IV).

16 Experimental

Toluene temperature-programmed adsorption (TPA) and temperature-programmed oxidation (TPO) from 30 to 600 °C (10 °C/min) was done for ZrO2, Y 2O3-ZrO2, and SiO2-ZrO2 in the same test cycle. In this way, the adsorption could be monitored as well as the oxidation of all the deposited surface species during the ramp up to 600 °C.

2.2 Infrared spectroscopy of solid catalysts

In studies relevant to this thesis, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) combined with mass spectrometry was used

(Publications I and IV). Sulfided ZrO2 was also characterized in transmission mode. As a technique, DRIFTS is only qualitative, whereas the transmission technique can also yield the quantification of surface species. However, sample preparation for DRIFTS is easier than for transmission-type experiments (Ar- maroli et al., 2004). Catalyst surface sites can be characterized by in situ DRIFTS using specific probe molecules. Furthermore, the technique allows the formation of surface species (may be reaction intermediates) under relevant reaction conditions to be monitored. However, the most reactive species are usually either detected in very small amounts or not detected at all, and thus spectators rather than intermediates are detected under reaction conditions (Busca, 1996b). The DRIFTS equipment consisted of a Nicolet Nexus FTIR spectrometer and a Spectra-Tech high-temperature reaction chamber with ZnSe windows. Gas- phase products were monitored using an on-line mass spectrometer (Omnistar, Pfeiffer Vacuum). The spectrum of an aluminum mirror measured under nitrogen flow (50 cm3/min) was used as the background.

2.2.1 Characterization of the surface sites by MeOH-DRIFTS

When using MeOH as a probe, the catalyst surface can be characterized based on characteristic vibrations of the adsorbed methoxy type species on the catalyst surface derived from MeOH which show up in the IR spectra. The technique was applied when characterizing the surface ofZrO2 and the differently doped oxides

Y 2O3-ZrO2 and SiO2-ZrO2 at 100-500 °C (Publication I).

2.2.2 Toluene adsorption and oxidation under DRIFTS

Toluene adsorption and transformations with temperature as well as with oxygen were measured on ZrO2, Y 2O3-ZrO2, and SiO2-ZrO2 in four different kinds of DRIFTS experiments. TPD, TPA, TPO and TPSR-type experiments were conducted with the aim to find the relevant surface species formed from toluene in the absence and presence of gas-phase oxygen, and the results were linked with the results from the corresponding TP experiments. The details can be found in Publication IV.

17 Experimental

2.2.3 T ransmission IR experiments on sulfided ZrO2

The IR setup (Figure 3) used in this work has three parts: the evacuation system, IR cell, and spectrometer. The IR cell pressure can be evacuated down to 10-3 kPa. A controlled amount of probe molecules can then be directed to the cell. The spectrometer is a Fourier Transform IR spectrometer (Nexus) from Ni- colet with a MCT (Mercury Cadmium Telluride) detector. The spectra were collected using 256 scans with a resolution of 4 cm -1 .

Figure 3. Scheme of FTIR setup.

The IR cell used was a low-temperature glass IR cell (Figure 4). In this cell, the samples can be treated at high temperature (atmospheric pressure) with e.g. flowing H2S. A small volume (2.23 cm3) of gas can be introduced in calibrated doses into the IR cell. The outlet at the upper part and the inlet at the basis of the cell allow the pellet to be under a gas flow.

18 Experimental

Figure 4. Low-temperature IR cell with flow system.

The powder ZrO2 sample was pressed (3 tons) into the form of a pellet (area of 2 cm2, mass in the range of 35-45 mg). Afterwards, the activation of the sample was carried out in situ in the IR cell. The pellet was activated under approx.

4 kPa of O2 from room tem pera tu re ( R T) to a pprox . 500 °C ( hea ting 1 0 °C /min). The high temperature wasmaintained for two hours. Evacuation was performed for 1 h and continued while the sample was cooled down to room temperature. H2S was adsorbed on ZrO 2 at temperatures RT -> 400 °C -> RT. A mixture containing 500 ppm H2S in nitrogen was used.

2.3 Computational work

Adsorption of H2S was studied on a molecular level by applying density functional theory (DFT) calculations. Calculations were performed on edge and corner sites, which were modelled as representative minority sites on monoclinic

ZrO2. Computational details can be found in Publication III.

3. Results and discussion

3.1 Surface properties of ZrO2-based catalysts and effect of H2S

3.1.1 Methanol adsorption on ZrO2-based catalysts

Surface species formed during methanol dissociative adsorption and decomposition as a function of temperature on the ZrO2, Y 2O3-ZrO2, and SiO2-ZrO2 were studied with in situ DRIFTS in Publication I. Methanol dissociation on the ZrO2 surface may occur by 1) adsorption on surface hydroxyl groups with formation of methoxy groups and water, or 2) on c.u.s. Zr4+-O2- pairs to form methoxy-and hydroxyl groups (Fisher and Bell, 1999).

Figure 5 shows spectra measured for the ZrO2 catalyst during the MeOH- DRIFTS experiment until 500 °C. The presented spectra were measured under nitrogen flow to minimize the effect of gas-phase methanol (the peak positions were similar under methanol). The corresponding spectra are not shown for the doped oxides, for brevity, but can be seen in Publication I.

Figure 5. In situ DRIFT spectra on pure ZrO2 during temperature-programmed experiment with methanol (Publication I).

Characterization with MeOH showed differences between the three studied catalysts, which were mainly seen in the transformations of methoxy groups with temperature. Hydroxyl groups characteristic to ZrO 2 and the mixed oxides

21 Results and discussion

(at 3800-3600 cm-1 ) were titrated during MeOH adsorption at 100 °C and re- stored with temperature, which can be seen on pure ZrO 2 in Figure 5. This suggests that dissociative adsorption took place, where hydroxyl groups were sub- stituted by methoxy groups from methanol. In addition, hydrogen-bonded hydroxyls appeared during adsorption (broad band centered at ca. 3400 cm–1). Adsorption of methanol resulted in the appearance of peaks at 2920 and 2815 cm-1 (Fig. 5), which are attributed to methoxy species on ZrO 2 (Zr–OCH3) (Korhonen et al., 2007). The peak at 1165 cm-1 is also due to methoxy species,

PRUHVSHFLILFDOO\WRǌ 2&+3) of on-top methoxy species(Korhonen et al., 2007). Similar formation of methoxies was observed on Y2O3-ZrO2.Two types of methoxies were seen on the SiO2-ZrO2 sample at 2927, 2830, 1443, and 1150 cm–1, (Zr–OCH3) and at 2953, 2850, and 1461 cm–1 (Si–OCH3) (Fisher and Bell, 1999). The new peaks appearing above 250 °C at 1573, 1380 and 1370 cm–1 (Fig. 5) DUHGXHWRǌas &22 į &+ DQGǌs(COO) vibrations of formates (Korhonen et al., 2007)7KHIRUPDWHǌ &H) peak at 2870 cm–1 appeared at 275 °C. Methoxies reacted to formates in a similar manner on ZrO2 and Y 2O3-ZrO2.On the SiO2-

ZrO2 formates appeared at above 250 °C, but the peak intensities were lower than on the other samples. MoreoverQRIRUPDWHǌ &+ VWUHWFKZDVGHWHFWHGDW approximately 2880-2870 cm –1. This indicates that the formation of formates was not as significant on SiO 2-ZrO2 as on ZrO2 and Y 2O3-ZrO2.Italsoseems that the methoxy species on Si-O phase do not evolve to form formates. Formates are formed on the surface via reaction with surface lattice oxygen (Busca et al., 1987). Based on methanol reactivity to formate on the different catalysts, it is suggested that the number of reacting sites of surface lattice oxygen is increased on ZrO 2 and Y 2O3-ZrO2 compared to that on SiO2-ZrO2. More- over, the oxygen contained on the SiO2-ZrO2 phase does not seem to be reactive and it is suggested that oxygen vacancies cannot form on the silica phase. This may be due to the strong covalent nature of oxygen bonded to silicon atoms, which was noted in a study by del Monte et al. (del Monte et al., 2000). MeOH-DRIFTS characterization suggested different kind of distribution of dopants on the mixed oxides. Two phases exist on SiO2-ZrO2, whereas Y2O3 seems more homogeneously distributed in the ZrO 2 matrix.

3.1.2 H2S adsorption modes and catalysts’ reactivity with MeOH

H2S retention on ZrO2 and doped ZrO 2

H2S-TPD was conductedfrom 40 to 750 °C on ZrO2, Y 2O3-ZrO2, and SiO2-ZrO2

(Publication I) to probe the adsorption modes of H2S with catalyst surfaces. Fig- ure 6 presents desorbed H2S from the studied catalysts after adsorption at 40 °C. The high-temperature region is not shown, since no desorption of H2S was detected after ~300 °C.

22 Results and discussion

Figure 6. H2S-TPD results (m/z=34) of ZrO2, Y2O3-ZrO2, and SiO2-ZrO2 catalysts. Heating rate 10 °C/min (Publication I).

H2S-TPD profiles (Figure 6) show one maximum for ZrO 2 and Y 2O3-ZrO2 (at 100 and 115 °C, respectively), whereas the SiO2-ZrO2 clearly shows two maxima

(at 115 and 210 °C). This suggests that additional H2S adsorption sites exist on SiO 2-ZrO2 compared to ZrO2 and Y 2O3-ZrO2. Furthermore, the intensity of the low-temperature peak decreased in the order Y 2O3-ZrO2 > ZrO2 > SiO2-ZrO2.

The latter peak on the SiO2-ZrO2 may be connected to H2S adsorbed on an in- terphase (between the SiO2 and ZrO2 phases on the catalyst), since the MeOH- DRIFTS characterization revealed separate phases on this catalyst. Adsorption on the SiO2 phase is unlikely, since H2S-TPD tests on pure SiO 2 showed that it does not adsorb H2S. It is realized that the observed phenomena occur on the surface and bulk sulfides do not form; however, the term sulfided ZrO 2 catalyst will be used for brevity.

MeOH product distribution on the calcined and sulfided ZrO 2 catalysts

The changes in the MeOH-TPSR product distribution on ZrO2, Y 2O3-ZrO2, and SiO 2-ZrO2 were compared in order to determine the effect of H2S adsorption on catalyst acidic and basic properties, i.e. c.u.s. cationic Zr4+ as well as anionic O2- sites, and different kinds of hydroxyl groups possessing Brønstedt acidity or ba- sicity. Figure 7 shows the DME generation on the calcined and sulfided samples.

23 Results and discussion

Figure 7. DME generation (MS signal,m/z= 45) in the MeOH-TPSR experiment on calcined and sulfided ZrO2, Y2O3-ZrO2, and SiO2-ZrO2 (Publication I).

24 Results and discussion

Figure 7 shows that the desorption temperature of DME is lower for ZrO2 than for the doped oxides (350 °C on ZrO 2, 480 and 370 °C on Y 2O3-ZrO2 and SiO2- ZrO2 respectively). Moreover, the amount of DME generated was higher on the doped oxides than on the pure ZrO 2. DME is generated by the condensation of two methoxy groups on the catalyst surface (Bianchi et al., 1995), and selectivity to DME generally describes the pure dehydration ability of the catalyst, i.e. its Lewis acidity (Tatibouët, 1997). Therefore, the result suggests the following order in Lewis acidity: ZrO 2 << Y 2O3-ZrO2 < SiO2-ZrO2, which is consistent with studies by Viinikainen et al. who probed the catalysts’ acidity with NH3 (Viini- kainen et al., 2009).

H2S adsorption decreased the DME peak intensities on all catalysts (Figure 7). This indicates that H2S adsorption reduced the amount of DME produced, i.e. the amount of Lewis acid sites decreased after sulfidation. With ZrO 2 and SiO2- ZrO2, the temperature of the peak maximum also shifted upwards, suggesting that sulfur on the surface affected the nature of the Lewis acid sites on those catalysts by making them stronger. According to Travert et al. (Travert et al.,

2002), H2S adsorbs partly on the Lewis acid sites (Zr4+)on ZrO2, which is in line with the decreased amount of DME produced on sulfided catalysts.

The formation of CO2 is an indicator of the MeOH oxidation process (Tati- bouët et al., 1997). CO2 was produced through a wide temperature range on all the catalysts (not shown). Sulfidation increased CO 2 production on the ZrO2 catalyst, whereas a decrease was observed on the sulfided SiO 2-ZrO2 catalyst. This is probably because H2S adsorbs on the SiO 2-ZrO2 in a different manner, possibly by deactivating the O2- sites. On the other hand, H2S adsorbed on the ZrO2 surface could enhance its basic properties or oxidation ability. In addition to normal methanol derived products, dimethyl sulfide (DMS) generation was observed on the sulfided catalysts (presented in Figure 8). The

Tmax for DMS was higher than for the DME generation, at 490-530 °C. In addition, a shoulder was seen on the SiO 2-ZrO2 at 390 °C.No H2S was seen to desorb during the MeOH-TPSR tests, indicating that the species from pre-adsorbed H2S were not replaced by methanol during adsorption. Thus, H2S adsorbed at 100 °C was strongly bound to the surface.

25 Results and discussion

Figure 8. DMS generation (MS signal, m/z=47) on sulfided ZrO2, Y2O3-ZrO2, and SiO2-ZrO2 during MeOH-TPSR experiments (Publication I).

In the study (Publication I), it was speculated that sulfur exists on the surface in a form incorporated in the lattice of ZrO2. DMS formation was suggested to occur via condensation of CH3S, where S is in the lattice, and CH3O surface species. The different behavior of SiO2-ZrO2 compared to ZrO2 and Y 2O3-ZrO2 was noted by the two maxima for DMS. SiO 2-ZrO2 is also the catalyst, the activity of which is not promoted by the presence of H2S during gasification gas clean-up. The nature of the sulfur adsorbed is therefore different on the studied catalysts. It is possible that SiO 2-ZrO2 behaves differently because of the covalent nature of oxygen bonding to Si-ions, as noted during the MeOH-TPSR experiments under DRIFTS.

3.1.3 MeOH surface species on ZrO2 after H2S adsorption

Methanol adsorption was compared on unsulfided and sulfided ZrO2 to clarify the pathway of MeOH to DMS on the sulfided ZrO2. DMS was observed earlier in Publication I to be produced from MeOH on the sulfided surfaces, indicating a possible condensation reaction between either –CH3Sor–CH3O surface species. IR probing with MeOH on the sulfided surface could reveal more about sulfur adsorption modes on ZrO 2. Methanol adsorption was performed on calcined ZrO2 as well as on ZrO2 treated with H2S (from room temperature to approx. 400 °C, 500 ppm H2S) by adding calibrated doses of methanol to the catalyst under a vacuum until an equilibrium was reached. H2S (500 ppm) was adsorbed on ZrO 2 at room temperature up to 400 °C and the catalyst was also cooled down in H2S flow. The C-H stretching region (approximately 3200 to 2800 cm-1 ) and the region showing C-O vibrations for calcined ZrO 2 are shown in Figure 9. Typical bands IRUǑs(CH3) vibrations of methoxy species are observed at 2927 and 2815 cm -1 .

The band observed at 1162 cm-1 is also attributed to methoxy, to the OCH3 vi- bration. These peaks correspond to those observed under DRIFTS at 100 °C in

26 Results and discussion

Publication I. In addition to these bands, bands at 1087, 1057 and 1033 cm-1 appear at increasing methanol concentrations in the cell. The additional band at 1087 cm-1 can be attributed to bridging methoxy species on ZrO2 over Zr4+ at room temperature (Finocchio et al., 1999), which was not detected at 100 °C under DRIFTS. The bands at 1087 and 1033 cm-1 disappear upon evacuation (not shown), and thus the latter can possibly be attributed to molecularly adsorbed methanol. In addition, it seems that the bridging species are not stable under a vacuum.

Figure 9. MeOH adsorption on calcined ZrO2 at room temperature. Application of MeOH consecutively until reaching equilibrium pressure in the FTIR cell.

Figure 10 shows the results of a methanol adsorption experiment under a vacuum at room temperature after H2S adsorption on ZrO2. Additional peaks in the –CH3 stretching region appeared at 2947 and 2843 cm-1 compared to when methanol was adsorbed on the unsulfided catalyst. It can also be seen that the sulfided sample lacks the band at 1087 cm -1 indicative of bridging methoxies on Zr4+ cations (Finocchio et al., 1999), so these species are not formed on the sulfided ZrO 2 surface. It is possible that H2S-derived surface species occupy the sites needed for their formation. It has been observed earlier, that H2S adsorption on ZrO2 titrates t-OH groups and creates new, acidic, m-OH groups through dissociation on Zr4+-O2- pairs (Tarvert et al., 2002).

27 Results and discussion

Figure 10. MeOH adsorption on ZrO2 at room temperature after H2S (500 ppm) adsorption at 30- 400-30 °C. Application of MeOH consecutively until reaching equilibrium pressure in the FTIR cell.

Figure 11 shows the spectral regions shown in Figure 10 for sulfided ZrO 2 under methanol and after evacuation of the cell. The intensities of the peaks at 2945 and 2843 cm-1 decrease significantly and the peak at 1033 cm -1 vanishes under a vacuum.

Figure 11. FTIR spectra of sulfided ZrO2 (H2S adsorption at 30-400-30 °C) under methanol equilibrium pressure and under a vacuum.

Ziolek et al. (Ziolek et al., 1994) REVHUYHG WKHIROORZLQJEDQGVGXHWR Ǒ &+3) afterthe admission of CH3SH on the ZrO 2 catalyst: 2980, 2924, 2845, 1437, 1314 cm-1 . The band at 2845 cm-1 is close to the one observed in Figure 10 on the sulfided sample; however, corresponding bands for 1437 and 1314 cm -1 could not be observed (not shown). Therefore, no methanol-derived –CH3S surface species could be evidenced on the sulfided ZrO 2 at room temperature. It is possible that the additional peaks at 2945 and 2843 cm-1 on the sulfided surface

28 Results and discussion correspond to molecularly adsorbed methanol species, since they are not stable under a vacuum.

3.2 The effect of H2S on reactivity of surface oxygen on ZrO2

3.2.1 Interaction of H2SwithZrO2

The H2S adsorption studies were continued in Publication II on pure ZrO 2 in order to find out more specifically the interactions with H2S and the catalyst, and also the effect of elevated temperatures. The possibility to use surface characterization tehcniques in the study was considered, but finally it seems that their applicability is very limited as explained in Publication II. TPS studies from 30 °C until 100, 200, 300 and 400 °C were conducted (samples designated as TPS100 °C, TPS200 °C, TPS300 °C and TPS400 °C respectively). H2S adsorption was also performed isothermally at 30 °C (sample H2Sads 30 °C). Table 1 shows the amounts of H2S(ʅmol) adsorbed during the experiments and the estimated respective monolayer (ML) coverages.

Table 1. &DOFXODWHGDPRXQWV ȝPRO RIDGVRUEHGDQGGHVRUEHG+2S during adsorption at 30 °C and TPS runs at varied end temperatures on ZrO2. Value in parenthes es indicates the ML am ount (% of surface coverage) (Publication II).

H2Sads30°C TPS100°C TPS200°C TPS300°C TPS400°C at 30 °C 3.5 (5.8) 3.6 (6) 3.5 (6) 3.5 (6) 3.5 (5.9) desorption during - 1.7 (2.9) 1.7 (2.8) 1.7 (2.9) 1.7 (2.8) ramp activated adsorption - 0.4 (0.7) 1.4 (2.3) 2.6 (4.3) 3.2 (5.4) adsorption during - 1.2 (2) 1.8 (3) 1.6 (2.8) 1.6 (2.6) cooling sum 3.5 (5.8) 3.5 (5.9) 4.7 (8.6) 6.0 (10.1) 6.5 (11)

The amount of H2S adsorbed at 30 °C was reproducible, 3.5 ʅmol, which corresponds to about 6 % of a ML. Approximately half of this amount was desorbed in the beginning of the temperature ramp and can thus be attributed to phy- sisorbed H2S, which is relatively easily desorbed. A similar amount of H2S was adsorbed during cooling, suggesting that the process is reversible. The study in

Publication II suggested that the other half of the adsorbed H2S at 30 °C was dissociated titrating the t-OH groups on the ZrO2 surface. It was suggested that dissociation of H2S at low temperature occurs on t-OH groups and Zr4+-O2- pairs generating t-SH species, as presented in Equations 3 and 4:

t-OH (s) + HS (g) H2O (g) + t-SH (s) (3)

O2- (s) + HS (g) m-OH (s) + t-SH (s) (4) where (g) refers to a species in the gas phase and (s) to a species on the surface. The amount of adsorbed H2S species during the temperature ramp increased when increasing the final temperature of the TPS (from 0.4 to 3.2 ʅm ol). There- fore, adsorption of H2S was activated with temperature. Sulfidation capacity was probably saturating at around 400 °C. Furthermore, water was generated

29 Results and discussion

during the adsorption of H2S at elevated temperatures at approximately 170 and 280 °C (shown in Publication II), indicating surface reaction. It was therefore suggested that upon H2S adsorption, surface oxygen was replaced by sulfur at two positions. The amount (a few % of a ML, Table 1) indicates minority sites. The reaction is described by the following equation (Equation 5) (Ziolek et al., 1995):

HS (g) + O2- (s) H2O (g) + S2- (s) (5)

If H2S replaces m-OH on the surface, m-SH is formed, which also seems possible. It was thus suggested that activated adsorption on the surface creates lattice sulfur through the replacement of O with S at specific sites.

H2S was not seen to react to form SO2, which was observed e.g. on TiO2 (Erdöhelyi et al., 2004). Evidently, lattice oxygen cannot be extracted from ZrO2 by H2S via formation of SO2.

3.2.2 Stability of adsorbed H2S

TPD and H2-TPR from 30 to 600 °C was performed after TPS400 °C in order to discover the stability of sulfur surface species formed and the effect of adsorbed H2S on the stability of surface oxygen (m-OH and surface lattice oxygen). Ac- cording to the mass balances, only approximately 30 % of the deposited H2S could be removed by heating in inert during TPD and 50 % by H2 during H2- TPR (both experiments were performed from 30 to 600 °C). Therefore, the adsorbed species were partly irreversibly adsorbed on the surface. The sulfur species that were shown to desorb during TPD could be identified as molecular H2S that desorbed immediately after heating was started, and some H2S also desorbed at approx. 300 °C, which was probably due to the desorption of some activatedly adsorbed species. During H2-TPR, sulfur surface species could be removed from the catalyst in a process generating H2S and water at approx. 320 °C (with simultaneous consumption of H2). This was suggested to be due to reduction of SH and OH species on the surface by dissociated

H2. Water desorption from unsulfided and sulfided (via TPS400 °C) ZrO2 was shown to differ during TPD. The water peak responsible for dehydroxylation, where surface OH groups react to form water (Bianchi et al., 1995), was significantly smaller on the sulfided compared to the unsulfided ZrO2. Therefore it was concluded that H2S adsorption occurred dissociatively, titrating at least t-OH groups on the ZrO2 surface. Water desorption during H2-TPR was increased by sulfidation. It was noted that the unsulfided surface did not show any H2 consumption (which is in line with ZrO 2 being very hardly reducible), whereas during H2-TPR on the sulfided ZrO2, a significant H2 consumption peak was detected (at approx. 320 °C). Thus, the reducibility of the sulfided ZrO2 was enhanced compared to that of unsulfided oxide.

30 Results and discussion

3.2.3 Reduction of unsulfided and sulfided ZrO2 by CO

CO-TPR was performed on unsulfided and differently sulfided ZrO 2 (via TPS at 100, 200, 300 and 400 °C) to study the reactivity of surface oxygen with CO to CO2 as a measure of the reducibility of the surface. Other possible products, such as H2, H2S, H2O, H2, COS and some other sulfur compounds, were also monitored. Figure 12 shows the CO2 generation on unsulfided and differently sulfided ZrO2 during CO-TPR. The amounts of CO2 generated (ʅmol) are presented in Table 2. An additional maximum for CO2 is seen at 190 °C on all the sulfided catalysts and its intensity increases with the increasing end temperature of the

TPS (and therefore increasing amount of activatedly adsorbed H2S species). On the other hand, H2S adsorption isothermally at 30 °C and via TPS100°C produced additional high-temperature CO2 maxima (at 500 and 440 °C, respectively). The high-temperature peak for CO2 at above 500 °C is common for all catalysts. H2 is produced concurrently with CO2 at these temperatures. Therefore these peaks are due to the reductive decomposition of surface formate species.

Figure 12. CO2 production during CO-TPR (5 vol-% CO) on unsulfided and differently sulfided ZrO2 (Publication II).

31 Results and discussion

Table 2. Amount of CO2 JHQHUDWHG ȝPRO GXULQJ&2-TPR on unsulfided and differently sulfided catalysts. The value in parentheses indicates the relative quantity of oxygen removed from the ZrO2 surface (% of a ML) (Publication II).

CO unsul- 2 H S TPS TPS TPS TPS ȝPRO fided 2 ads30°C 100°C 200°C 300°C 400°C ~200 °C 0 (0) 0.3 (0.3) 0.9 (0.7) 1.6 (1.4) 2.6 (2.2) 2.9 (2.4) ~500 °C 2.7 (2.3) 4.0 (3.4) 3.7 (3.1) 2.2 (1.9) 2.1 (1.8) 2.1 (1.8) tot 2.7 (2.3) 4.3 (3.7) 4.9 (4.1) 5.5 (4.6) 6.1 (5.1) 6.2 (5.2)

CO2 can be formed via the decomposition of surface carbonate, which is formed via the reaction of CO and surface lattice oxygen (Hertl et al., 1989,

Kondo et al., 1988). The additional CO2 during CO-TPR at 500 and 440 °C after isothermal H2S adsorption at 30 °C and after TPS100 °C may thus be caused by the reaction of CO with surface lattice oxygen. Adsorbed H2S probably modified the reactivity of this oxygen species, since it was not detected on the unsulfided surface. The relatively high desorption temperature indicates strong adsorption on the surface, and therefore it is suggested that the formed CO 2 is retained on the surface as monodentate carbonates, which are known to be the most stable form of CO2 on the surface (Viinikainen et al., 2009, Bachiller-Baeza et al., 2002).

The amount of CO2 produced at approx. 200 °C on the samples sulfided at elevated temperatures (Table 2) increased with the increasing amount of activatedly adsorbed H2S (Table 1). A linear correlation between the amount of activatedly adsorbed H2S (ML) and the amount of extracted surface oxygen by CO (ML) is presented in Figure 13.

Figure 13. Relation between the amount of extracted surface oxygen vs. the amount of H2S adsorbed at elevated temperature during TPS (Publication II).

32 Results and discussion

It was suggested that the activatedly adsorbed H2S species are very strongly bound on the surface, replacing O2- with S2- at low coordination sites. In addition, the more the sulfidation temperature is increased, the more lattice sulfur is created, which increasingly destabilizes the oxygen in the surface and en- hances the reactivity towards CO oxidation. The results of H2-TPR on sulfided

ZrO2 also indicated that the reactivity of surface oxygen was enhanced by the presence of sulfur on the surface. The low temperature of CO 2 desorption indicates that, as the sulfidation temperature is increased, the sites able to bind carbonates become occupied with sulfur-derived species. The CO-TPR results can be correlated with the observed enhancement in naphthalene oxidation activity in the presence of H2S. Naphthalene oxidation is thought to involve surface oxygen, as the mechanism has been suggested to fol- low the Mars-van Krevelen mechanism (Bampenrat et al., 2008). Thus, it is possible that the enhancement effect is connected with increased reactivity of lattice oxygen affected by adsorbed sulfur on the catalyst surface in the H2S-containing environment. The form of sulfur must be strongly adsorbed.

Effect of adsorbed water in CO-TPR experiments The effect of adding water on the sulfided catalyst was studied to rule out the possible role of water-derived species, maybe accumulating during the TPS experiment, in the oberved improved oxidation performance of the catalyst. An experiment was performed, where the sulfided catalyst (TPS400 °C)was treated with H2O - conta ining flow a nd then C O -TPR wa s perform ed. I t w as seen tha t the low-temperature peak for CO2 at ~200 °C, indicating enhanced CO oxidation activity, was not affected by the added water (not shown).

In another experiment, CO-TPR was performed after TPS300 °C and consecutive TPD until 300 °C, to rule out effects regarding adsorbed water and to study the stability of the sulfur species in view of activity enhancement. The CO-TPR results are shown in Figure 14.

Figure 14. CO2 production (MS signal m/z=44) during CO-TPR (from 30 to 600 °C) after TPS300 °C and consecutive TPD until 300 °C.

33 Results and discussion

The results in Figure 14 show production of CO2 at ~200 °C, which was also observed earlier in the case when TPD was not performed (Figure 12). After the weakly held sulfur species had been desorbed during TPD, the intensity of the low-temperature CO2 peak was also reduced. However, a sharp CO2 peak was detected at approx. 330 °C without H2 production. It is suggested that the CO2 adsorption sites were partly recovered during TPD and thus we see CO2 desorption at higher temperature. The high-temperature process at above 500 °C is not affected. It can be said that the water-derived species do not play a clear role in the enhancement of activity on the sulfided surface, or at least they are not responsible for the origin of the effect. The effect can be connected to the increased reactivity of surface lattice oxygen and, moreover, the most stable sulfur species on the surface are the cause of the enhanced reactivity.

3.3 Redox properties of ZrO2 and the effect of H2S studied on a molecular level

3.3.1 Calculations performed to study the improving effect of adsorbed H2S on reactivity of surface lattice oxygen

A DFT study was conducted to find an explanation on a molecular level for the observed enhancement of surface lattice oxygen reactivity in Publication II, and oxidation activity improvement by H2S (Rönkkönen et al., 2009). Based on the

TP study in Publication II, the sites involved in H2S adsorption are minority sites, such as c.u.s. Zr and O sites at edges, kinks, corners or steps. It was also suggested that the enhancement of the reactivity of surface lattice oxygen occurs by sulfur adsorption on these sites. Thus the calculations were performed on edge and corner models as representative minority sites. t-OH and m-OH species were added to the models to mimic the adsorbed water which is most likely present on the surface under reaction conditions. Figure 15 presents the models used.

34 Results and discussion

Figure 15. Presentation of the edge model (A and B, front and side view, respectively) and corner model (C and D, front and side view, respectively) accommodating SH species.Zr atoms are indicated in grey, oxygen in red and sulfur in yellow. Figure by Karoliina Honkala (Publication III).

Molecular adsorption of H2S on the Zr atom was calculated to be exothermic with an energy of -14.5 kJ/mol. This suggests weak adsorption and it is likely that molecularly adsorbed H2S is not present on the surface at elevated temperatures. Therefore it was concluded that molecularly adsorbed H2S is not responsible for the observed enhancement of surface oxygen reactivity. Dissociative adsorption of H2S was considered on t-OH groups and on an oxygen vacancy at the hydroxylated edge. H2S can replace t-OH groups via reaction 3 (shown in Section 3.2.1). On a hydroxylated edge containing no vacant sites, the calculated reaction enthalpy was +28 kJ/mol, suggesting that the process is not favorable. On the other hand, a process where H2S adsorbs on an oxygen vacant site on the hydroxylated edge forming t-SH and a new m-OH has a reaction enthalpy of -69.5 kJ/mol (strongly exothermic). This demonstrates that the energetics of dissociation is dependent on the nature of the site. The reactivity of surface oxygen was studied using hydrogen as a reducing agent to oxidize surface O to H2O. The effect of t-SH species (on the edge model) on the reactivity of surface oxygen was calculated and practically no effect was found on the neighboring oxygen atoms. Therefore it was concluded that t-SH species also cannot be the reason for the observed enhancement in oxygen reactivity on ZrO2. A situation where multicoordinated SH (m-SH) is present was considered using a corner model (Figure 15). The energy difference for removing the neighboring m-OH and t-OH species was calculated and compared to a situation

35 Results and discussion where sulfur was not present. The results revealed a decrease in the reduction enthalpies of 36 and 31 kJ/mol for m-OH and t-OH, respectively. It was demonstrated by DFT calculations that the interactions of molecularly adsorbed H2S or t-SH species are too weak to be able to increase the reactivity of surface oxygen on ZrO2. The calculations suggested that m-SH species are able to enhance the reactivity; however, the observations cannot be explained quantitatively by the achieved improvement. The nature of the relevant site might be modified from the used model.

The formation of bulk ZrS2 is not favored by thermodynamics; however, there are sites on the surface able to bind H2S and apparently to form stable Zr-S lattice structures. These sites must therefore possess significant difference in their nature compared to the bulk of the oxide or even the majority of the surface sites.

Limited interaction with H2S is also a benefit for the catalyst, since the sulfur tolerance is guaranteed when the oxide phase is unaffected for the major part. According to Anderson et al. (Anderson et al., 1997) partial substitution of O with S in the ZrO2 lattice leads to stabilization of the ZrO2 structure to the cubic phase so that the volume of the unit cell increases. In this structure, the Zr-O bond length is expected to increase, thus increasing the ionicity of the bond (An- derson et al., 1997). It is possible that the observed increase in reactivity of surface O could also be attributed to these factors.

3.3.2 ZrO2 as a redox catalyst and defect sites acting in relevantcatalysis

ZrO2 is a hardly reducible oxide, on which the reactivity of surface oxygen participating in oxidation reactions has been found to be limited to specific sites (minority sites, such as edges, corners, and steps) (Zhu et al., 2005a). It has proven, but low, activity towardscatalytic partial oxidation of methane (CPOM), as shown by the studies reviewed in Publication III. A surface-confined redox mechanism has been postulated on ZrO2-based catalysts by Zhu et al. (Zhu et al., 2005a), where surface oxygen at defect sites participates in oxidizing methane to CO and H2 according to the Mars-van Krevelen mechanism (Mars and van Krevelen, 1954, Zhu et al., 2005a).

C PO M w a s concluded to proceed on fla t terra ces on Y 2O3-ZrO2, where intrinsic oxygen vacancies (Zr-Ŀ-Y) are operating. Structural defects present on ZrO2 at corners, edges, or kinks seem not to be responsible for CPOM at high (approx. 900 °C) temperatures (Zhu et al., 2005b). CO has been demonstrated to become oxidized by surface OH groups through mechanisms presented by Graf et al. (Graf et al., 2009) (WGS mechanism). CO reacts with t-OH groups to surface formate, which is reductively decomposed to

CO2 and H2 with m-OH. This occurs on pure ZrO 2 at high temperature (approx. 500 °C) (Kouva et al., 2014), but the reaction can be aided by the presence of metal on the catalyst (Graf et al. 2009). Water present in the reaction media is responsible for the regeneration of surface hydroxyl groups (Graf et al., 2009, Azzam et al., 2007). Azzam et al. demonstrated that lattice oxygen could be removed from ZrO2 with CO via either the reaction of surface hydroxyls or its reaction with lattice oxygen. When ZrO2 was regenerated with H2O, a pathway involving hydroxyls

36 Results and discussion

became possible, whereas when N2O was used as an oxidant, oxidation of CO to

CO2 occurred,involving surface lattice oxygen at defect sites (Azzam et al. 2007). Furthermore, studies (Azzam et al. 2007, Zhu et al.2005a) have clearly pointed out that defect sites are involved in redox catalysis involving hydroxyl groups on ZrO2 (Azzam et al., 2007, Zhu et al., 2005a, Graf et al., 2009). WGS is relevant to gasification gas clean-up, since the gases participating in WKHUHD FWLRQD UHSUHVHQWWKHUHD QG: * 6 RUUHY HUVLEOH: * 6Ʃ* D W7 & for CO +H2O ļ CO2 + H2 (Roine, 2007)) is likely to occur to some extent.

Whether the shift is feasible depends on the H2/CO-ratio of the producer gas and the desired ratio of the cleaned gas, but anyway it most likely plays a role. It was demonstrated by Azzam et al. (Azzam et al., 2007) that CO could be oxidized to CO 2 on ZrO2, which is simply an undesired reaction considering gasification gas clean-up, since it lowers the heating value of the gas; the same applies for H2 oxidation. However, in gasification clean-up conditions, water is present and the surface is hydrated, so oxidation by surface lattice oxygen should be hindered. It is questionable whether OH groups on ZrO 2 are involved in the oxidation of tar molecules, but it seems possible. That means that reforming reactions could play a role in tar decomposition.

3.4 Toluene adsorption and oxidation studies on ZrO2-based biomass gasification gas clean-up catalysts

Toluene adsorption and oxidation was studied using DRIFTS and TP gas- phaseanalysis. The experiments performed in the complementary experimental setups were toluene-TPD, toluene-TPA and TPO of deposited species from toluene adsorption. Toluene was also co-fed with oxygen during temperature rise with DRIFTS (and TP, not shown) to study the surface species formed.

3.4.1 Surface species formed upon toluene adsorption and oxidation

Toluene-TPD under DRIFTS revealed adsorbed surface species at 30 °C and their stability with increasing temperature until 600 °C. Figure 16 shows vibrations on all the studied catalysts in the spectral region 3200-2600 cm -1 , where e.g. the C-H stretches occur. The peak at 3030 cm-1 belongs to the C-H stretching mode of the aromatic ring of toluene. The methyl group of toluene is vibrating at 2926 cm -1 ǌ&+ (Hernández-Alonso et al., 2011).

37 Results and discussion

Figure 16. DRIFTS spectra in the region 3200-2600 cm-1 collected during toluene-TPD experiments on ZrO2, Y2O3-ZrO2, and SiO2-ZrO2 (Publication IV).

Toluene-TPA experiments under DRIFTS revealed adsorbed benzoate species (peaks at ca. 1510 and 1410 cm-1 , not shown) on the studied catalysts after toluene adsorption above 300 °C. This surface species forms from toluene by abstraction of hydrogen from the methyl group and thereafter benzoate is formed with two surface oxygen species. Thus, the C-H bonds in the methyl group of toluene can dissociate and the species can be further oxidized to surface benzoate. In addition to benzoate species some carbonates were present on the surface after TPA was performed until 600 °C. In the toluene-TPO experiments performed under DRIFTS, the toluene deposited at 600 °C was oxidized in the temperature range 30-600 °C. The species present on the surface were equal to those after TPA. In addition, a peak appearing at ca. 1600 cm-1 was observed, probably attributed to carbonaceous deposits from toluene adsorption at high temperature. These species are oxidized at above 300 °C. The benzoate species were evidently more stable, since they van- ished at temperatures above 400 °C on Y 2O3-ZrO2, and above 500 °C from the spectra of ZrO 2 and SiO2-ZrO2. Toluene oxidation was carried out with oxygen in the gasphase under DRIFTS (and in TP equipment). In the experiment, toluene and oxygen were co-fed and a temperature ramp was applied from 30 to 600 °C. During the experiment,the appearance of new peaks at ca. 2960 and 2880 cm-1 , which were assigned to C– H stretching vibrations of benzyl species, was noted at the same time as toluene started to convert to oxidation products consuming oxygen, i.e. at above 300 °C. The oxidation products formed were CO 2, CO, H2O, and H2.

3.4.2 Gas-phase products from toluene adsorption and oxidation

Molecularly adsorbed toluene desorbed from the catalysts immediately after heating was started, giving maxima in the TPD gas-phase analysis on ZrO2,

38 Results and discussion

Y 2O3-ZrO2, and SiO2-ZrO2 at 70, 80, and 88 °C, respectively (not shown). Tolu- ene adsorption capacities on different catalysts could also be compared based on the TP gas phase experiments and the integrated values of adsorbed toluene. The adsorbed amount was always less than 4 % of a ML, and toluene adsorbed weakly on the surfaces of the studied ZrO 2-based catalysts at 30 °C. The amount of toluene desorbing from the catalyst decreased in the order SiO 2-ZrO2 > Y2O3- ZrO2 > ZrO2. TPA gas-phase experiments from 30 to 600 °C revealed the formation of gas- phase products during adsorption of toluene. The formation of benzene, CO2, H2 and H2O is presented in Figure 17.

39 Results and discussion

Figure 17. MS responses for benzene (m/z= 78), CO2 (m/z=44), H2 (m/z=2) and H2O (m/z=18) during toluene-TPA experiments (Publication IV).

It can be seen that benzene and CO2 were formed concurrently at high temperature (at 525 °C on ZrO2 and Y 2O3-ZrO2, and at 530 on SiO2-ZrO2, Figure 17).

The molar amounts for CO2 and benzene at high temperature were equivalent by estimation (approx. 0.1 ʅmol/m2 for Y 2O3-ZrO2). It is suggested that these products were formed into the gas phase from surface benzoate species detected during toluene-TPA DRIFTS experiments, which decomposes to benzene and CO2 in the gas phase and reduces the surface. The fraction of surface oxygen taking part in this process is ~0.8 %, suggesting minority sites.

40 Results and discussion

TPO experiments carried out after TPA showed mainly the formation of CO2,

CO, H2O, H2 and consumption of O2. The oxidation of carbonaceous surface species took place between 200 and 400 °C. The amount of carbonaceous deposits formed during adsorption at high temperatures was notable, as suggested by the intense evolution of CO 2 and CO. Benzoate species were suggested to be very stable with oxygen, based on the combined analysis of TPA under DRIFTS and gas-phase experiments. Therefore, it can be interpreted that benzoate is not the active intermediate in the toluene oxidation reaction, but is rather a spectator species. The TPO analyses also revealed that the nature of the carbonaceous deposits was different on the pure and doped ZrO 2. The deposits were more heter- ogeneous on the doped oxides.

3.4.3 Possible toluene oxidation mechanism and sites involved on ZrO2

Surface benzoate species could decompose to yield CO 2 and benzene, as suggested by their appearance in DRIFTS at temperatures below the temperature at which CO2 and benzene were detected in the gas phase during the TP experiments. The amount of oxygen atomsin the CO2 formed supposedly from surface benzoate corresponds to approx. 0.8 % of a ML of surface oxygen. So, a minor fraction of the surface is likely to participate, indicating that minority sites like edges and corners are involved (similar to the findings with other redox reactions and H2S adsorption in Publication II). The high stability of surface benzo- ates indicates that they are more likely spectator species rather than reaction intermediates in toluene oxidation. When toluene was fed together with oxygen to the catalyst and a temperature ramp was applied, surface benzyl species appeared simultaneously when the conversion of toluene and oxygen started. Therefore, the benzyl species was concluded to be the active intermediate in toluene oxidation reaction over ZrO2 and the doped ZrO 2 catalysts. Benzyl species form by the abstraction of one hydrogen atom from the methyl group of toluene and adsorb on the surface, possibly via the methylene (–CH2) group. The suggested different pathways for toluene interacting with ZrO 2-based gasification gas clean-up catalysts are shown in Figure 18. The mechanism is not conclusive, especially on how the benzene ring is oxidized; however, new knowledge was gained about the adsorption mechanisms of toluene and possible intermediate species during toluene oxidation on ZrO 2.

41 Results and discussion

Figure 18. Adsorption and oxidation of toluene over ZrO2-based gasification gas clean-up catalysts; a schematic presentation (Publication IV).

Defect sites are responsible for the adsorption and oxidation of tar compounds, as suggested by the calculated values of adsorbed toluene (molecular) and gas- phase products (fractions of a ML). This is in line with studies by Zhu et al. (Zhu et al., 2005a),who revealed that oxidation reactionsoccur atdefect sites on ZrO2, as reviewed earlier. The participation of OH groups during oxidation is possible and could also play a role in toluene decomposition. The studies reviewing WGS on ZrO2 prove that surface OH groups at defect sites are active in converting CO to CO2 and hydrogen. It is plausible that they could also be involved in oxidizing intermediates from adsorbed tar molecules. The effect of sulfur on toluene adsorption and oxidation can be estimated based on the studies conducted. If surface oxygen is exchanged for surface sulfur on the minority sites, where species derived from toluene supposedly also adsorb, the formation of benzoate could be hindered, since it requires the presence of two adjacent surface oxygen atoms. On the other hand, it is more difficult to estimate the effect of surface sulfur species on the formation and reactions of benzyl species. It is still under question as to how the tar molecules become oxidized by the surface lattice oxygen (the breaking of the benzene ring and further oxidation). It seems that the improvement mechanism with sulfur is related to these steps rather than the adsorption step, since the studies indicated that the initial adsorption of toluene on surface oxygen ions results in strongly adsorbed species not participating in oxidation. Moreover, the c.u.s. O 2- sites are more electro- negative than the suggested S2- and therefore it is not expected that tar molecules would interact more favorably with sulfur-incorporated sites. It is suggested that the adsorbed form of sulfur increases the reactivity of surface oxygen at specific sites, which results in enhanced reaction rates of adsorbed intermediates from tar molecules with surface lattice oxygen.

42 4. Conclusions

The effect of H2S on ZrO 2-based catalysts was investigated in tar oxidation as a part of biomass gasification gas clean-up. The positive effect of H2S on tar oxidation activity had been observed earlier on the ZrO2 and Y 2O3-ZrO2 catalysts; how ev er, no effect w a s observ ed on SiO 2-ZrO2. The reactivities of unsulfided and sulfided ZrO 2-based catalysts were examined to understand the improvement in oxidation activity by sulfur during gasification gas clean-up. Moreover, H2S adsorption modes on ZrO2 were thoroughly examined.

Molecular and dissociative adsorption modes of H2S were established on ZrO2 and doped ZrO 2 based on temperature-programmed studies. On ZrO 2, the dissociative adsorption was suggested to yield t-SH species on the surface, elimi- nate t-OH species and possibly form new m-OH sites. The dissociatively adsorbed species were found to be strongly adsorbed even at low temperature (30 °C), whereas molecularly adsorbed H2S was weakly held. H2S was found to react with ZrO2 to form H2O at elevated temperatures, producing activatedly adsorbed sulfur species. Those species are likely incorporated in the surface via the replacement of surface lattice oxygen (m-OH and c.u.s. O2-) at specific sites, i.e. minority sites such as edges, steps, and corners. They were also concluded to be strongly held. The amount of sulfur deposited on the surface in the high- temperature processes was found to correlate with the amount of surface oxygen removed, which was revealed by CO-TPR on differently sulfided ZrO 2. This indicates that at elevated temperatures H2S produces surface sulfur which improves the oxidation properties of ZrO2 on minority sites. The reactivity of surface lattice oxygen was thus concluded to be enhanced by adsorbed H2S. In addition, CO 2 adsorption sites were blocked on the sulfided surface.

Studies employing DFT calculations ruled out molecularly adsorbed H2S and t-SH species formed by dissociative adsorption as possible species to enhance the reactivity of surface oxygen on ZrO2. The molecular adsorption of H2S was confirmed to be weak by DFT calculations, as already suggested by TP experiments. In fact, the molecularly adsorbed species are not even likely present on the surface at the high temperatures of gasification gas clean-up. The effect of adsorbed H2S on the reactivity of surface oxygen was tested using edge and corner models as representative minority sites. An increase in the reactivity of m- OH and t-OH was found when m-SH was present on the corner model. Based on the calculations, it is concluded that the reactivity of surface lattice oxygen or t-OH is enhanced at very specific sites when m-SH is present, i.e. sulfur is

43 Conclusions incorporated into the lattice on a minority site. The exact nature of the site is probably different from the model used, since quantitatively the calculation results do not resolve the observations. Stable sulfur surface species are likewise formed on doped oxides, since adsorbed H2S was not reversed by adsorbing methanol at 100 °C as indicated by MeOH-TPSR studies. Moreover, those stable species were likely incorporated in the lattice based on dimethyl sulfide generation from methanol on the sulfided catalysts at high temperature (500 °C). The reactivity of the deposited sulfur with methanol differed between ZrO2, Y2O3-ZrO2, and SiO2-ZrO2,and it was concluded that the reactivity of sulfur is reduced on the SiO2-ZrO2. Differences in the surface properties of the catalysts were also identified. Doping with SiO2 was concluded to produce a catalyst with two distinct phases on ZrO 2,with SiO2 forming islands on the ZrO2 phase. On the other hand, doping with Y 2O3 produced a well-distributed matrix. The reactivity of surface lattice oxygen was suggested to be lower on the SiO2-ZrO2 catalyst compared to the ZrO2 and Y 2O3- ZrO2 catalysts. Vacancy formation is possibly more limited and cannot be enhanced by sulfur because of the covalent bonding to Si-atoms. SiO 2-ZrO2 was also concluded to be the most acidic catalyst. H2S adsorbed on the Lewis acidic sites, i.e. the Zr4+ cationic sites, on all the catalysts. Thus, dissociative adsorption of H2S on Zr4+-O2- pairs was suggested to also occur on the doped oxides. On

ZrO2, CO2 was produced from methanol more favorably on the sulfided catalyst, whereas the effect was opposite on SiO 2-ZrO2. No clear effect was found on

Y 2O3-ZrO2. Based on the TP studies, the adsorption of H2S occurs on similar sites on Y 2O3-ZrO2 as on pure ZrO 2, whereas there are additional sites on SiO2- ZrO2 to adsorb H2S. Therefore, the activity improvement effect during gasification gas clean-up is suggested to be originated by sulfur also interacting with the lattice on Y 2O3-ZrO2. The oxidation properties of SiO 2-ZrO2 cannot be improved by sulfur, probably because of the covalent nature of the bonds formed. The interaction of H2S with ZrO 2 is concluded to be limited to specific sites, comprising not more than approximately 11 % of a ML, based on the amount of adsorbed H2S. Strong adsorption found on a very limited amount of surface sites suggests that those surface sites must possess a significant difference in their nature. It seems that H2S does not reduce the catalyst but forms sulfur species on a limited amount of sites. The limited reactivity could be an advantage regarding applications with H2S content in streams, since sulfur poisoning does not occur at least with the relatively low concentrations considered here. The sulfur tolerance of ZrO2 is originated by the low degree of sulfidation. It is concluded that ZrO2-based catalysts are the optimal choice for applications with some H2S content, offering possibilities for the simplification of related processes when H2S does not necessarily have to be removed from the stream.

The surface of ZrO2 exhibits limited reducibility; however, part of the surface oxygen is able to participate in the redox cycles. Redox reactions occur at defect sites and hydroxyl groups could also be involved in oxidations. Therefore, hydroxyl groups probably also play a role during gasification gas clean-up, where water is always present. The reactivities of both the m-OH and t-OH groups

44 Conclusions were suggested to be enhanced by sulfur present on the surface (as m-SH species) and hence the observed apparent enhancement in tar oxidation activity could also be connected to the increased reactivity of OH groups on ZrO2. Tar decomposition could involve reactions where hydrocarbons react with hydroxyl groups on the surface generating CO and H2 to the gas phase. However, studies on the oxidation of tar molecules (transient and/or steady-state) in the presence of H2S but without the presence of other gases in gasification gas mixtures are needed. Toluene was used as a model compound for tar when studying its interaction with ZrO2, Y 2O3-ZrO2, and SiO2-ZrO2. The studies indicated that the active intermediate in the toluene oxidation is not benzoate, the formation of which requires surface oxygen, but the benzyl species which are formed by abstraction of the hydrogen atom from the methyl group of toluene. The molecular adsorption of toluene was found to be weak. Toluene adsorption and oxidation sites were concluded to be minority sites on ZrO2, similarly to H2S adsorption sites. This is also in line with the observations on the redox ability of ZrO 2, which has been resolved to be limited to minority sites. It is unclear how the stable bonds in the benzene ring of tar molecules are broken when the molecule is oxidized on the surface. However, it is claimed that the reaction intermediates formed during tar oxidation are oxidized to CO or

CO2 via a reaction with the surface lattice oxygen in a Mars-van Krevelen type reaction. Interaction with surface oxygen atoms during initial adsorption of toluene on ZrO2 seems not to be the preferred pathway in toluene oxidation, since the formed species are strongly adsorbed. The oxidation activity improvement mechanism with sulfur is therefore suggested to be related to the oxidation of the intermediate species, rather than the adsorption of tar molecules. This work contributes to understanding how adsorbed H2S modifies the oxi- dative properties of ZrO2-based catalysts towards enhanced performance in tar oxidation during gasification gas clean-up. It is concluded that the improving effect of H2S must be originated by strongly adsorbed sulfur species present in the lattice at minority sites, where the intermediate species from tar molecules also form. Oxidation improvement by sulfur occurs on these specific sites where sulfur improves the reactivity of surface lattice oxygen.

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50 Department of Biotechnology and Chemical Technology Aalto- Kauppi Inkeri DD The effect of H S on