Effect of Biofuel Impurities on the Diesel Oxidation Catalyst

Liene KIENKAS

Degree Project

Supervisors: PhD student. J. GRANESTRAND PhD, development engineer R. SUÁREZ PARÍS Examiner: L. J. PETTERSSON

Stockholm – 2017 ANOTĀCIJA DĪZEĻDEGVIELAS OKSIDĒŠANAS KATALIZATORS, KATALIZATORA SAINDĒŠANA, FOSFORS, NĀTRIJS, KALCIJS, MATERIĀLU RAKSTUROŠANA, AKTIVITĀTES TESTĒŠANA Literatūras apskatā ir izskaidrota dīzeļdegvielas oksidēšanas katalizatora uzbūve un tā aktīvās fāzes struktūra. Ir aprakstīta katalizatora deaktivēšana saindēšanas dēļ, detalizētāk aprakstot fosfora, nātrija, kalcija un sēra mijiedarbību ar dīzeļdegvielas oksidēšanas katalizatoru. Ir apkopotas saindēšanas simulēšanas, aktivitātes testēšanas un materiālu raksturošanas metodes, kas tiek izmantotas katalizatoru pētījumos.

Metodiskā daļā tiek aprakstīta jauna PtPd/Al2O3 katalizatora sagatavošana ar slapjo impregnēšanas metodi. Ir aprakstīta metode katalizatora saindēšanai ar fosforu, kalciju un nātriju un monolītu pārklājumu uznešanas tehnika. Tiek izklāstītas materiālu raksturošanas metodes (BET, ICP-OES, CO hemisorbcija, TPR, SEM-EDS, TEM-EDS, XRD) un to nozīme sagatavotā materiāla raksturošanā. Šī darba daļa iekļauj arī aktivitātes testēšanas reaktora sagatavošanu, metodes izstrādi un reakcijām izmantoto apstākļu aprakstu. Eksperimentālajā daļā tiek salīdzinātas jauna un saindētu katalizatoru materiālu īpašības un to veiktspēja aktivitātes testos. Elementu sastāvs materiālos ir noteikts ar ICP- OES monolītu paraugiem pirms un pēc sēra iedarbības. Saindēšanas laikā notikušās īpatnējās virsmas laukuma un cēlmetālu dispersijas izmaiņas ir noteiktas un salīdzinātas. Izmantojot temperatūras programmētu reducēšanu ar ūdeņradi, ir izstrādāta metode inžu-substrāta un inžu-aktīvās fāzes mijiedarbību pētīšanai un izskaidrošanai. Aktīvās fāzes morfoloģija un nanodaļiņu izmēri ir noteikti ar TEM. Inžu koncentrācijas un izkliede katalizatora substrātā ir noteikta ar SEM. Aktīvās fāzes kristāliskās struktūras ir analizētas ar XRD. Ir veikti laboratorijas mēroga katalizatora aktivitātes testi. Slāpekļa (II) oksīda, oglekļa monoksīda un propilēna oksidēšanas reakcijas ir veiktas simulētā izplūdes gāzes plūsmā, atsevišķās reakcijās un pēc sēra iedarbības. 16 ‘konversijas pakāpes – temperatūras’ līknes ir iegūtas un analizētas katrai reakcijai. Jauna un saindēta katalizatora veiktspēja katrā reakcijā ir analizēta un izmaiņas skaidrotas, salīdzinot materiālu īpašības. Darbā izmantotas KTH Karaliskā Tehnoloģiju Institūta atbalstītās elektroniskās datu bāzes, internets un Scania ierobežotas pieejamības protokoli. Apskatītie literatūras avoti ir angļu un izdoti no 1922. līdz 2017. g. Maģistra darbs uzrakstīts angļu valodā, satur 93 lpp, 43 attēlus, 20 tabulas, 5 vienādojumus un 5 pielikumus, darbā izmantoti 99 literatūras avoti.

ANOTATION DIESEL OXIDATION CATALYST, POISONING, PHOSPHORUS, SODIUM, CALCIUM, MATERIAL CHARACTERIZATION, ACTIVITY TESTING A literature review covers the diesel oxidation catalyst design and its active phase structure. It describes catalyst deactivation due to poisoning. Phosphorus, sodium, calcium and sulphur interactions with the diesel oxidation catalyst are explained in detail. The review summarizes poisoning simulation, activity testing and material characterization techniques that have been developed over the years.

The materials and methods section includes PtPd/Al2O3 catalyst preparation by incipient wetness impregnation. Catalyst poisoning with phosphorus, sodium and calcium in a liquid phase and subsequent monolith coating is described. Material characterization techniques (BET, ICP-OES, CO chemisorption, TPR, SEM-EDS, TEM-EDS, XRD) and their significance in the study are presented. Furthermore, this section covers catalyst activity testing equipment, development of testing method and the final set-up for activity tests. Fresh and poisoned catalyst materials and their performance are compared in the experimental section. The final material elemental composition is determined by ICP-OES for coated monolith samples before and after sulphur poisoning. The BET area and the precious metal dispersion changes during poisoning are determined and compared. Moreover, the temperature programmed reduction method is developed and poisonous species interactions with the support material and the precious metals explained. Active phase morphology, the size of particles and crystalline structure are characterized by TEM and XRD. Poisonous species distribution and loadings are determined by SEM. Laboratory scale activity tests are performed for fresh and poisoned catalysts. Nitric oxide, carbon monoxide and propylene oxidation is carried out in simulated exhaust, as single reactions and after sulphur treatment. 16 conversion versus temperature curves are obtained for each reaction. Fresh and poisoned catalyst performance is analysed in each reaction and explained by material characterization results. The literature was gathered from KTH Royal Institute of Technology supported electronic databases, internet resources and Scania internal reports. Reviewed literature sources are in English and have been published from 1922 to 2017. The Master thesis is written in English. It contains 93 pages, 43 figures, 20 tables, 5 equations, 5 appendices and 99 literature references.

ABBREVIATIONS DOC Diesel oxidation catalyst

PtPd/Al2O3 γ-alumina supported bimetallic platinum and palladium oxidation catalyst

Pt/Al2O3 γ-alumina supported monometallic platinum oxidation catalyst

Pd/Al2O3 γ-alumina supported monometallic palladium oxidation catalyst TX Temperature for X % conversion of reactants F-Cat Fresh γ-alumina supported bimetallic platinum and palladium oxidation catalyst P-Cat Phosphorus poisoned γ-alumina supported bimetallic platinum and palladium oxidation catalyst Na-Cat Sodium poisoned γ-alumina supported bimetallic platinum and palladium oxidation catalyst Ca-Cat Calcium poisoned γ-alumina supported bimetallic platinum and palladium oxidation catalyst F-Al Fresh γ-alumina powder after the temperature treatment P-Al Phosphorus poisoned γ-alumina powder Na-Al Sodium poisoned γ-alumina powder Ca-Al Calcium poisoned γ-alumina powder Combo Activity test with fully simulated exhaust gas NO-Single Activity test with simulated exhaust gas excluding carbon monoxide and propylene CO-Single Activity test with simulated exhaust gas excluding nitric oxide and propylene C3H6-Single Activity test with simulated exhaust gas excluding carbon monoxide and nitric oxide Combo-T Activity test with fully simulated exhaust gas after aging

Combo-SO2 Activity test with fully simulated exhaust gas after sulphur poisoning NO-F-Combo Nitric oxide oxidation with F-Cat in Combo CO-F-Combo Carbon monoxide oxidation with F-Cat in Combo C3H6-F-Combo Propylene oxidation with F-Cat in Combo NO-P-Combo Nitric oxide oxidation with P-Cat in Combo CO-P-Combo Carbon monoxide oxidation with P-Cat in Combo C3H6-P-Combo Propylene oxidation with P-Cat in Combo NO-Na-Combo Nitric oxide oxidation with Na-Cat in Combo CO-Na-Combo Carbon monoxide oxidation with Na-Cat in Combo C3H6-Na-Combo Propylene oxidation with Na-Cat in Combo NO-Ca-Combo Nitric oxide oxidation with Ca-Cat in Combo CO-Ca-Combo Carbon monoxide oxidation with Ca-Cat in Combo C3H6-Ca-Combo Propylene oxidation with Ca-Cat in Combo experiment NO-F-Single Nitric oxide oxidation with F-Cat in NO-Single CO-F-Single Carbon monoxide oxidation with F-Cat in CO-Single C3H6-F-Single Propylene oxidation with F-Cat in C3H6-Single

NO-P-Single Nitric oxide oxidation with P-Cat in NO-Single CO-P-Single Carbon monoxide oxidation with P-Cat in CO-Single C3H6-P-Single Propylene oxidation with P-Cat in C3H6-Single NO-Na-Single Nitric oxide oxidation with Na-Cat in NO-Single CO-Na-Single Carbon monoxide oxidation with Na-Cat in CO-Single C3H6-Na-Single Propylene oxidation with Na-Cat in C3H6-Single NO-Ca-Single Nitric oxide oxidation with Ca-Cat in NO-Single CO-Ca-Single Carbon monoxide oxidation with Ca-Cat in CO-Single C3H6-Ca-Single Propylene oxidation with Ca-Cat in C3H6-Single NO-F-Combo-T Nitric oxide oxidation with F-Cat in Combo-T CO-F-Combo-T Carbon monoxide oxidation with F-Cat in Combo-T C3H6-F-Combo-T Propylene oxidation with F-Cat in Combo-T NO-P-Combo-T Nitric oxide oxidation with P-Cat in Combo-T CO-P-Combo-T Carbon monoxide oxidation with P-Cat in Combo-T C3H6-P-Combo-T Propylene oxidation with P-Cat in Combo-T NO-Na-Combo-T Nitric oxide oxidation with Na-Cat in Combo-T CO-Na-Combo-T Carbon monoxide oxidation with Na-Cat in Combo-T C3H6-Na-Combo-T Propylene oxidation with Na-Cat in Combo-T NO-Ca-Combo-T Nitric oxide oxidation with Ca-Cat in Combo-T CO-Ca-Combo-T Carbon monoxide oxidation with Ca-Cat in Combo-T C3H6-Ca-Combo-T Propylene oxidation with Ca-Cat in Combo-T NO-F-Combo-SO2 Nitric oxide oxidation with F-Cat in Combo-SO2 CO-F-Combo-SO2 Carbon monoxide oxidation with F-Cat in Combo-SO2 C3H6-F-Combo-SO2 Propylene oxidation with F-Cat in Combo-SO2 NO-P-Combo-SO2 Nitric oxide oxidation with P-Cat in Combo-SO2 CO-P-Combo-SO2 Carbon monoxide oxidation with P-Cat in Combo-SO2 C3H6-P-Combo-SO2 Propylene oxidation with P-Cat in Combo-SO2 NO-Na-Combo-SO2 Nitric oxide oxidation with Na-Cat in Combo-SO2 CO-Na-Combo-SO2 Carbon monoxide oxidation with Na-Cat in Combo-SO2 C3H6-Na-Combo-SO2 Propylene oxidation with Na-Cat in Combo-SO2 NO-Ca-Combo-SO2 Nitric oxide oxidation with Ca-Cat in Combo-SO2 CO-Ca-Combo-SO2 Carbon monoxide oxidation with Ca-Cat in Combo-SO2 C3H6-Ca-Combo-SO2 Propylene oxidation with Ca-Cat in Combo-SO2 NO-F-trial-Combo Nitric oxide oxidation with F-Cat trial sample in Combo CO-F-trial-Combo Carbon monoxide oxidation with F-Cat trial sample in Combo C3H6-F-trial-Combo Propylene oxidation with F-Cat trial sample in Combo GHSV or SV Gas hourly space velocity

TABLE OF CONTENTS

INTRODUCTION ______9

LITERATURE REVIEW ______10

1. DIESEL OXIDATION CATALYST ______10

1.1. Diesel engine exhaust system Euro VI and regulations ______10

1.2. Significance of the diesel oxidation catalyst ______11

1.3. Design of the diesel oxidation catalyst used in a heavy-duty truck ______11

1.4. The active phase of the diesel oxidation catalyst ______12

1.5. Bimetallic Pt and Pd active phase in the diesel oxidation catalyst formulation _ 13

1.6. Alumina supported Pt-Pd bimetallic catalyst preparation by the incipient wetness impregnation method ______15

2. OXIDATION REACTIONS OVER THE DIESEL OXIDATION CATALYST ___ 16

2.1. Significance of hydrocarbons, carbon monoxide and nitric oxide oxidation reactions ______16

2.2. Light-off temperature ______16

2.3. Oxidation reaction kinetics ______17

2.4. Competitiveness between oxidation reactions ______18

3. POISONING OF THE DIESEL OXIDATION CATALYST ______20

3.1. General deactivation mechanisms of heterogeneous catalysts ______20

3.2. General mechanisms of the poisoning – background studies ______20

3.3. Sources of inorganic poisons ______22

3.4. Poisoning simulation of the diesel oxidation catalyst ______23

3.5. Catalyst characterization techniques ______25

3.6. Characterization of inorganic poisons over the diesel oxidation catalyst ______26

3.7. Laboratory scale activity testing equipment ______29

MATERIALS AND METHODS ______30

4. CATALYST PREPARATION AND POISONING______30

4.1. Catalyst preparation ______30 6

4.2. Catalyst poisoning ______31

5. MONOLITH COATING ______33

5.1. Parameters and treatment of monolithic micro-cores ______33

5.2. Dip-coating procedure ______33

5.3. Dip-coating procedure trial tests ______34

6. MATERIAL CHARACTERIZATION ______36

6.1. Inductively coupled plasma optical emission spectrometry (ICP-OES) ______36

6.2. BET surface area, pore size and pore distribution ______36

6.3. CO chemisorption ______36

6.4. Temperature Programmed Reduction (TPR) ______37

6.5. Transmission electron microscopy (TEM-EDS) and scanning electron microscopy (SEM-EDS) ______39

6.6. X-Ray Powder Diffraction (XRD) ______39

7. CATALYST ACTIVITY TESTING ______40

7.1. Activity testing reactor set-up and preparation for experiments ______40

7.2. Experimental set-up for the activity testing ______42

7.3. The light-off curves for oxidation reactions obtained in the activity testing experiments ______44

RESULTS AND DISCUSSION ______46

8. MATERIAL CHARCTERIZATION ______46

8.1. ICP-OES Elemental analysis ______46

8.2. BET Specific surface area ______47

8.3. BJH pore size distribution ______48

8.4. Precious metal dispersion and average particle size (CO chemisorption) ______49

8.5. SEM-EDS Results ______50

8.6. TEM-EDS Results ______53

8.7. Temperature programmed reduction (TPR) ______57

8.8. Powder X-Ray Diffraction ______59 7

9. CATALYST ACTIVITY TESTING ______62

9.1. CO, NO and C3H6 oxidation reaction competitiveness ______62

9.2. NO oxidation reaction ______64

9.3. CO oxidation reaction ______69

9.4. C3H6 oxidation reaction ______76

CONCLUSIONS ______81

SECINĀJUMI ______82

RECOMMENDATIONS FOR FUTURE STUDIES ______83

REFERENCES ______84

APPENDICES ______94

INTRODUCTION

Scania provides sustainable transport systems powered by bioethanol, biogas, biodiesel along with hybrid and conventional solutions. Today Scania offers the largest variety of engines operating on alternative fuels in the market. The number of the alternative fuel operated vehicles sold in 2016 increased by 40 % [1]. Nevertheless, one of the alternative fuels – biodiesel - is a source of inorganic contaminants. These impurities can detrimentally affect the diesel truck after-treatment system that is responsible for harmful emission abatement. As a consequence, better understanding of the alternative fuel impact on the after-treatment system is necessary for further development of a sustainable transportation system. This thesis is focused on the diesel oxidation catalyst (DOC) that is one of the major components in the diesel truck after-treatment system. Catalyst performance due to chemical deactivation of biodiesel derived inorganic contaminants (P, Na and Ca) is determined and analysed. The study covers PtPd/Al2O3 DOC preparation and poisoning by the incipient wetness impregnation method, monolith dip-coating, fresh and poisoned catalyst characterization (BET, CO chemisorption, TPR, ICP-OES, TEM-EDS, SEM-EDS, XRD). Catalyst activity tests in a laboratory scale activity testing rig are performed to study carbon monoxide, nitric oxide and propylene oxidation reactions before and after the poisoning.

Sulphur effect on the catalyst activity is determined after the gas-phase poisoning with SO2. Objective of the study: To assess the effect of inorganic poisons (sodium (Na), calcium (Ca) and phosphorus (P)) on the diesel oxidation catalyst material properties and performance. Tasks:

1. Prepare PtPd/Al2O3 diesel oxidation catalyst by the incipient wetness impregnation method. 2. Poison the fresh catalyst with P, Na and Ca by the wet impregnation method. 3. Characterize the fresh and poisoned catalyst by the BET, CO chemisorption, TPR, ICP-OES, TEM-EDS, SEM-EDS, XRD techniques. 4. Perform catalyst dip-coating on the cordierite monolith micro-cores. 5. Test the fresh and poisoned catalyst activity in the activity testing rig. 6. Couple material characterization and activity testing results to explain the catalyst poisoning phenomenon.

LITERATURE REVIEW

The literature review covers the diesel engine exhaust after-treatment system characterization; diesel oxidation catalyst significance in a heavy duty truck; the DOC design and experimental preparation; oxidation reactions over the DOC – their significance, kinetics and competitive behaviour; poisoning of the DOC - origin of the inorganic poisons and their interactions with the catalyst; aging simulation of the DOC and activity testing of the catalyst.

1. DIESEL OXIDATION CATALYST

1.1. Diesel engine exhaust system Euro VI and regulations

Nowadays heavy-duty trucks use diesel engines for power generation. Better fuel efficiency and expected longer life-length over the conventional gasoline engine have steered the compression ignition engine development [2]. The lean fuel combustion process in the diesel engine differs significantly from the stoichiometric gasoline combustion. Therefore, harmful emissions from both combustion processes have to be treated in different ways. Emissions from the diesel engine form a 3-phase system. Particulate matter (PM) emissions or particulates are total solids and liquids. It consists of unburnt fuel, lubricating oils (HC – hydrocarbons), inorganic oxides, sulphates and soot (dry carbon). Lean combustion results in gas-phase emissions of inorganic oxides – nitrogen oxides (NOx), carbon monoxide (CO), carbon dioxide (CO2) and water steam (H2O) [2]. Since 1980 the European Union has been developing and revising new limits for the atmospheric pollutants from the diesel cars known as Euro 1, 2, 3 etc. Since the 1st of January, 2014 the Euro 6 emission standard is applicable for all new-made diesel trucks. Emission limits in a steady-state driving cycle have been reduced to 0.40 g NOx/km, 0.01 g PM /km,

1.5 g CO /km and 0.13 g HC /km. It corresponds to 80 % and 50 % reduction of NOx and PM emissions as compared to Euro 5 (valid from 2009). In addition, a particulate number (PN) limit (8.0x1011 particles/km) is included in the Euro 6 regulation and the allowed limit is decreased by 99% since 2009 [3]. Relatively complex diesel engine exhaust after-treatment systems have been developed to reach the Euro 6 emission standards. Scania trucks use the emission abatement system schematically showed in Figure 1. Step by step process is realised before the harmful emissions are released in to the atmosphere. After the lean combustion process unburnt hydrocarbons (HC), CO and NOx are oxidized over the diesel oxidation catalyst (DOC). The

PM is trapped in the diesel particulate filter (DPF) that is located right after the DOC. The regeneration of the DPF is used to increase the temperature above 600 °C and to burn the organic PM. NO2 is further reduced to N2 over the selective catalytic reduction catalyst (SCR) by a urea-water solution (32.5%) (commercially known as AdBlue). The ammonia slip catalyst (ASC) is responsible for trapping any extra ammonia released over the SCR catalyst.

The remaining emissions, mostly consisting of nitrogen (N2), carbon dioxide (CO2) and water (H2O), are released in the atmosphere after this after-treatment sequence [2, 4, 5].

Figure 1. Euro VI diesel engine exhaust after-treatment system. VGT – variable geometry turbocharger, EGR – exhaust gas recirculation, XPI – extra high pressure injection, ΔP – pressure drop (for other abbreviations see Paragraph 1.1.) [6].

1.2. Significance of the diesel oxidation catalyst

The DOC is the first catalyst in the diesel engine exhaust after-treatment train. Primarily it is designed for oxidation of low-temperature incomplete combustion products.

Carbon monoxide and unburnt hydrocarbons are oxidised to relatively harmless CO2 and

H2O. NOx oxidation to NO2 is carried out to allow its further reduction to N2 over the SCR catalyst. Secondarily, the DOC is also responsible for the DPF regeneration by either active hydrocarbon oxidation (direct fuel injection in the DOC) or passive regeneration by NO2 (formed over the DOC). During this process high temperature exotherms (usually over 500 °C) are generated over the DOC to further burn the organic part of PM on the DPF [2, 4, 7, 8].

1.3. Design of the diesel oxidation catalyst used in a heavy-duty truck

Monolithic diesel oxidation catalysts have been used over the last decade in the automotive industry (see Figure 2). The monolith has a ‘honeycomb’ structure with a defined number of cells per square inch (usually 400 cells per square inch (cpsi)) [9, 10]. High-

11 temperature resistant ceramic materials like cordierite and mullite are used for the monolith substrate formulation [5]. Monolith bricks are washcoated with a high surface area support. The support material has two main functions: 1) to provide a large surface area for the noble metals (high dispersion of the precious metal sites – catalyst active phase) and 2) to make the catalyst resistant to sintering at high temperatures [4]. These functions are ensured by high surface area supports like gamma alumina (γ-Al2O3), silica (SiO2), zeolites, zirconia or combinations of these materials. Different properties of each support material determine its application in the DOC formulation. For example, zeolite supports can store hydrocarbons during the cold- start operation when the exhaust temperature is not high enough for oxidation and thus reduce cold-start emissions. However, γ-Al2O3 has an excellent thermal stability and it is the most widely used support material for the DOC formulations [5, 7].

‘’Honeycomb’’ monolith

NO, HC, CO NO2, H2O, CO2

Metallic canning Washcoat - High Defined number surface area of cells per support with the square inch precious metal (cpsi) active sites

Figure 2. Design of the DOC.

The DOC for a heavy-duty truck has size and shape limitations. As a general rule the volume of the DOC monolith equals the volume of the engine with a diameter to length ratio ~ 1.4. The diameter of the DOC usually varies between 17.78 and 25.40 cm and the corresponding length varies from 12.7 to 17.18 cm. Gas hourly space velocities (SV) vary between 20’000 and 250’000 h-1 throughout a monolith of this size depending on the duty cycle of the heavy-duty vehicle [2]. The final DOC formulation is canned in a stainless steel can and placed in the exhaust train right after the engine [5].

1.4. The active phase of the diesel oxidation catalyst

Reactions over the DOC are catalysed by the active phase that consists of the precious metals. The most commonly used precious metals are platinum (Pt), palladium (Pd)

12 and combinations of both (PtPd) [7]. The quantity of the noble metals in the catalyst formulation is usually between 50 and 90 g/ft3. It is required to reduce the size of the noble metal particles as small as possible since only the surface metal atoms have their chemical activity [4]. Pt has been commonly used in the DOC formulations due to its activity in NO, CO and hydrocarbon low-temperature oxidation reactions [11, 12]. On the other hand, Pd has been reported sufficiently active only in hydrocarbon oxidation reactions [13]. Unfortunately,

Pt and Pd nanoparticles are also good catalysts for SO2 oxidation. Harmful atmospheric pollutant SO3 formation is more pronounced over Pd than Pt [2, 14]. In some catalyst formulations vanadium pentoxide has been added to suppress SO2 oxidation to SO3 [15]. Figure 3 illustrates typical TEM images of Pt particles from a new DOC and from an aged DOC inlet. It has been observed that the active phase over the fresh catalyst surface is highly dispersed. Nevertheless, thermal aging caused particle sintering and considerable size increase at the DOC inlet [16].

Figure 3. Highly dispersed Pt particles on the a) fresh DOC; b) aged DOC inlet [16].

1.5. Bimetallic Pt and Pd active phase in the diesel oxidation catalyst formulation

Several reasons have been reported for bimetallic catalyst use in the automotive industry. Pd addition to Pt-only catalyst formulations has shown decelerated Pt particle sintering, thus enhancing catalytic performance over time at elevated temperatures [12, 17, 18]. Besides sintering, reduced formation of oxidised forms of platinum during aging has been reported after Pd incorporation in Pd catalyst formulations [19]. Furthermore, some authors have shown that Pt addition to Pd-only catalyst

13 formulations increased sulphur tolerance of the catalyst [20]. As compared to pure Pd catalysts, addition of Pt has promoted PdO formation during operation, thus, prevented catalyst aging [21]. The active phase chemical composition in Pt-Pd bimetallic catalyst formulations depends on the catalyst synthesis conditions. In oxidative calcination atmosphere the active phase has been reported to consist of Pt and Pd bimetallic clusters (usually depositing in a ratio close to 1:1), along with PdO and metallic Pt and Pd particles [13] [18]. The catalyst synthesis conditions have been reported responsible for different catalyst activities. Auvray et al. have observed that NO oxidation activity was improved for a sequentially impregnated PtPd catalyst as compared to a co-impregnated material [19]. Different ratios of Pt to Pd have been experimentally investigated to find the optimum between cost, light-off temperatures, NO oxidation properties, thermal stability and other parameters. Kim et al. [22] have tested hydrocarbon, carbon monoxide and nitric oxide oxidation activity in a simulated exhaust for various commercially available DOCs. The precious metal ratios in their experiments were varied from Pt to Pd weight ratios 1:0 to 0:1. For hydrocarbon simulation they tested both light hydrocarbons (propylene and propane) and heavy hydrocarbons (dodecane and xylene). Moreover, activity measurements were performed also after thermal aging for 72 hours at 750 °C with 10 % water in air. The obtained results indicated that the Pd content in the Pt catalyst was related to thermal stability,

NO2 formation and hydrocarbon and CO oxidation, as presented in Figure 4. It was concluded that relatively high Pt content in the catalyst was required to satisfy catalytic performance and costs (according to the average price in 2011). The same Pt content – light- off temperature relationship for hydrocarbon and propylene oxidation has been recently reported by Hazlett et al. [23].

Figure 4. Pt to Pd ratio effect on Pt-based DOC. PGM – Platinum group metals; Light-

off temperature – temperature for 50 % conversion of CO and C3H6 [22].

1.6. Alumina supported Pt-Pd bimetallic catalyst preparation by the incipient wetness impregnation method

Incipient wetness impregnation is the most common method of catalyst active phase deposition on a pre-dried support. Prior to the impregnation, precursor salts are dissolved in a solvent volume that is equal or slightly less than the pore volume of the support material. The precursor salt solution is added drop-wise to the support allowing capillary forces to induce the solution absorption into the pore structure of the support. More than one impregnation step can be used, then the total volume of the solution should be equal to the pore volume of the support for one batch multiplied by the number of impregnations steps. Solvent is removed by drying in between the impregnation steps and after the whole process. By using this method a precise amount of metals is deposited on the support pore structure [5, 24]. The incipient wetness impregnation method has been used in a great amount of studies for alumina-supported monometallic and bimetallic Pt and Pd oxidation catalyst preparation, including [23, 25 - 29]. Co-impregnation is simultaneous Pt and Pd deposition on the support and it has been widely reported in the literature. Different inorganic salts have been used as precursors of the precious metals. The most commonly used Pt precursors are platinum nitrate

(Pt(NO3)2) [12, 21, 25], tetraamineplatinum (II) nitrate (Pt(NH3)4(NO3)2) [23], chloroplatinic acid (H2PtCl6) [26, 30] and hydrogen hexahydroxyplatinate H2Pt(OH)6 [18].

Whereas Pd precursors are palladium nitrate (Pd(NO3)2) [12, 21, 23, 25], palladium chloride

(PdCl2) [30] and tetraaminepalladium nitrate (Pd(NH3)4(NO3)2) [18]. Various drying and calcination procedures can follow the impregnation. Most of the authors have reported the drying in air at 100 °C to 120 °C for 12 to 24 hours [18, 23, 30]. The calcination temperatures varied according to the final use of the material and the desired experimental results. Usually catalyst samples were calcined in air at 300 °C to 1200 °C for 1 to 4 hours [12, 18, 21, 23, 25, 26]. Nevertheless, some authors have reported calcination in reducing atmosphere. Castillo et al. [30] calcined catalyst samples in hydrogen flow for 2 hours at 400 °C. Moreover, Morlang et al. [18] carried out reduction in flowing mixture of

H2 (5 vol.%) in N2 for 2 hours at 500 °C after the calcination in air. It is clear from the previous studies that there is no straightforward approach for the support pre-treatment, the choice of the precursor salts, catalyst calcination temperatures etc. Therefore, the method has to be experimentally validated according to the desired use of the final material. 15

2. OXIDATION REACTIONS OVER THE DIESEL OXIDATION CATALYST

2.1. Significance of hydrocarbons, carbon monoxide and nitric oxide oxidation reactions

The DOC is responsible for the abatement of different unburnt organics - usually odour-bearing aldehydes, benzo-pyrenes, ketones and butadienes. Furthermore, it also takes care of carbon monoxide (CO) – harmful pollutant that is incomplete oxidation product derived from the fuel combustion. Oxidation of high concentrations of nitric oxide is carried out to ensure its further removal in the subsequent after-treatment train [2, 8]. During the vehicle application the unburnt organics and carbon monoxide are almost fully oxidised to CO2 and H2O. Starting from temperatures 170 °C to 200 °C conversion rises to over 90% within a temperature interval of 20°C to 30°C. The start of the reaction is dependent on the catalyst formulation, exhaust gas composition (length and structure of the hydrocarbon chains) and space velocity of the exhaust gases [4, 5]. It has been approximated that particle mass can be reduced by 15 to 30 % over the diesel oxidation catalyst during hydrocarbon oxidation reaction over the noble metals [4].

Relatively high NO2 content of the NOx composition is important for the DOC downstream components – the DPF and the SCR. Untreated exhaust is composed of NO2 to

NO ratio approximately 1:10. It is necessary to reach a ratio of 1:1 of NO2 to NOx in the

DOC for successful reduction of NO2 in the SCR unit. Depending on the catalyst coating material and the exhaust gas composition the maximum ratio of NO2 to NOx in the vehicle application is reached in between 180°C and 230°C [4].

2.2. Light-off temperature

Oxidation reactions over the DOC start at a specific temperature limit – light-off temperature [4]. According to Bartholomew and Farrauto light-off temperature has been defined as the region in the temperature versus conversion curve (light-off curve) in which conversion increases rapidly with an increasing temperature. At this temperature particular conversion is reached and the reaction ‘ignites’ [5]. Figure 5 shows an example of the light- off curves. These light-off curves have been obtained for hydrocarbon oxidation reaction at the inlet, in the middle and at the outlet for a Phosphorus poisoned catalyst, dynamometer aged (for 120 hours) and fresh three-way catalyst (TWC). T30, T50, T80, T90 correspond

16 to the temperatures of 30%, 50%, 80% and 90% conversion of the reactants, respectively [31].

T90

T80

T50

T30

Figure 5. Example of the light-off curves for HC oxidation reaction over a TWC [31].

2.3. Oxidation reaction kinetics

The suggestion by Voltz et al. [32] that oxidation of carbon monoxide, hydrocarbons (usually studied as propylene) and nitric oxide over platinum catalysts follows a Langmuir- Hinshelwood dual site mechanism dates back to 1973. Nevertheless, several authors [33 - 36] have widely used this theory also nowadays to model the DOC. The preliminary kinetic model has described oxidation over the DOC as a three reaction system from which carbon monoxide oxidation (Equation 1) and propylene oxidation (Equation 2) are irreversible reactions, but nitric oxide oxidation is an equilibrium reaction (Equation 3) [32]. 1 (1) 퐶푂 + 푂 → 퐶푂 2 2 2 9 (2) 퐶 퐻 + 푂 → 3 퐶푂 + 3 퐻 푂 3 6 2 2 2 2 1 (3) 푁푂 + 푂 ⇄ 푁푂 2 2 2 Nevertheless, this three reaction system (Equations 1 to 3) has not considered hydrocarbon oxidation by NO (Equation 4) and oxidation of hydrogen (Equation 5). These reactions were successfully implemented in the DOC kinetic modelling by Pandya et al [35]. 7 (4) 퐶 퐻 + 2푁푂 + 푂 → 3 퐶푂 + 3 퐻 푂 + 푁 3 6 2 2 2 2 2

1 (5) 퐻 + 푂 → 퐻 푂 2 2 2 2

CO oxidation is a relatively simple catalyzed reaction. During this reaction CO is almost fully oxidised to the less harmful oxide - CO2 [37]. Also hydrocarbon oxidation occurs easily in an excess of oxygen which is the case for the lean diesel exhaust [35]. NO oxidation is an exothermic equilibrium reaction. At low temperatures (< 250°C) the equilibrium is on the NO2 side (product). When temperature is increased (above 450°C), the equilibrium shifts to the reactant side and NO formation is thermodynamically favoured [4]. Moreover, it has been reported that rates of NO, CO and hydrocarbon oxidation reactions increased with increasing oxygen concentrations [32, 38] and they were inhibited by increasing either NO, CO or propylene [23, 32]. At last, hydrogen oxidation has been observed occurring easily at room temperatures with no CO presence. Whenever CO was present hydrogen oxidation started simultaneously with CO oxidation and reached 50 % conversion at approximately the same time [39].

2.4. Competitiveness between oxidation reactions

CO, HC and NO oxidation reactions have been described as ‘self-poisoning’ or ‘self- inhibiting’ due competitive adsorption with oxygen on the precious metal sites. As a consequence they cause each other’s inhibition [32]. Self-inhibition of carbon monoxide has been most widely explained. It has been reported that CO adsorption (CO chemisorption) on the catalyst active sites inhibited the oxygen adsorption on the active metal sites and the overall CO oxidation rate. This phenomenon decreased with increasing temperature and no significant inhibition has been observed above 370-425°C [7, 32]. Moreover, CO has been reported to adsorb more strongly on the active sites than hydrocarbons, thus delaying hydrocarbon oxidation when CO and hydrocarbons were present simultaneously [40]. CO and hydrocarbon presence in the exhaust gas has shown to significantly inhibit the overall NO oxidation activity [41]. This phenomenon has been explained by the competitive adsorption on the active sites as well as reduction of NO2 by hydrocarbons [42]. Therefore, the light-off was shifted to higher temperatures. It has been observed that after the start phase of the hydrocarbon and CO oxidation reactions, NO2 was observed in the outlet stream and the inhibition effect was diminished [11, 40].

The inhibition effect between these three oxidation reactions has been observed in a large number of DOC studies, including kinetic modelling of the DOC [23, 35]. Figure 6.a demonstrates propylene oxidation reaction improvement when more oxygen was introduced in the inlet gas stream and self-inhibition due to competitive adsorption reduced. As a consequence the propylene light-off curve was shifted to lower temperatures [38]. Figure

6.b presents typical inhibiting behaviour between C3H6 and NO oxidation reactions over the

PtPd/Al2O3 catalyst. It was observed that an increase of the propylene concentration from 0 to 1000 ppm shifted the NO light-off to considerably higher temperatures [42]. Figure 6.c illustrates carbon monoxide oxidation inhibition due to competitiveness between NO and

C3H6 for the adsorption on the active sites of a Pt/Al2O3 catalyst. Nitric oxide addition to the inlet gas stream resulted in 17 °C increase of the light-off temperature for 50 % conversion with further increase when C3H6 and NO2 were introduced in the inlet gas stream. Finally, Figure 6.d shows propylene oxidation inhibition due to the presence of CO, NO and dodecane in the reaction gas mixture. It was observed that the light-off was slightly shifted to higher temperatures when longer chain hydrocarbons were present. Further inhibition occurred by sequential introduction of CO and NO in the feed [40].

a b

c d

Figure 6. a) C3H6 light-off curve for PtPd/Al2O3 catalyst. Inlet gases - 5 % H2O, 150 ppm

C3H6, 0.075 or 10 % O2 balanced with N2 [38]. b) NO and C3H6 light-off curves for PtPd

(1:2)/Al2O3 catalyst. Inlet gases - 200 ppm NO, 10% O2, 5 % H2O and 0, 100, 200, 800 or

1000 ppm C3H6 balanced with N2 [42]. c) CO and d) C3H6 light off curves for Pt/Al2O3

catalyst. Inlet gases- 3240 ppm CO or/and 1080 ppm C3H6, or/and 200 ppm NO, or/and

200 (or 100) ppm NO2, 10 % O2, 5 % H2O, 5 % CO2 balanced with N2 [40]. 19

3. POISONING OF THE DIESEL OXIDATION CATALYST

3.1. General deactivation mechanisms of heterogeneous catalysts

Heterogeneous catalysts can experience serious deactivation due to mechanical, thermal or chemical factors. Catalyst deactivation is defined as ‘the loss of catalyst activity and/or selectivity over time of its performance’. Deactivation mechanisms of heterogeneous catalysts are listed and explained in Table 1 [15]. Table 1 Deactivation mechanisms of heterogeneous catalysts [15] Mechanism Type Description Chemisorption of the poisonous species on Poisoning (selective) the catalyst active sites or washcoat Catalyst reaction with gas phase Vapour formation Chemical components and volatile product formation Vapour, support or promoter reaction with Vapour-solid and solid- catalytic phase and formation of an inactive solid reactions phase Fouling (non-selective Fluid phase components physical deposition poisoning) [2] on the catalyst surface and into pores Mechanical Loss of specific surface area of the catalyst (Physical) Attrition due to abrasion and mechanical-induced crushing Thermal degradation and Loss of catalyst active sites surface area and Thermal sintering support area due to thermal stresses.

This study focuses on the deactivation of the DOC due to selective and non-selective poisoning phenomenon.

3.2. General mechanisms of the poisoning – background studies

Catalyst deactivation can result from contamination that is present in the feed stream of reactants. This deactivation mechanism is called poisoning. Exhaust gas can be contaminated due to impurities in the fuel or due to additives in the lubrication oil. Impurities can detrimentally poison the DOC [2, 5]. Two general mechanisms of the poisoning have been proposed:

Selective poisoning – chemical process; Non-selective poisoning – physical process [2] .

Selective poisoning

It is a direct chemical reaction of the poisoning species with the catalyst active sites or the support material that results in a loss of catalyst activity. Selective poisons are deposited on the active sites and thus distributed along the length of the catalyst and throughout the washcoat [2, 15]. Irreversible selective poisoning leads to permanent catalyst deactivation. In this process poisons react with the catalyst active sites and form an inactive alloy, thus catalyst activity cannot be restored. On the other hand, deactivation due to reversible selective poisoning is temporary. Processes such as a heat treatment, poison removal from the feed stream or washing can induce poisonous species desorption from the active sites [2, 15]. The catalyst light-off properties are shifted to higher temperatures when selective poisoning occurs. Since the activation energy of the non-poisoned active metal sites do not change, their functionality remains as before. Thus, the conversion versus temperature curves for poisoned catalysts will be parallel to the fresh ones with a shift to higher light-off temperatures. In Figure 7 curve ‘fresh catalyst’ and the parallel curve ‘loss of active sites’ represent the selective poisoning phenomenon [2]. However, the DOC carrier material alumina (γ-Al2O3) can also react with sulphur dioxide (SO2 ) and in the presence of water aluminium sulphate (Al2(SO4)3) is formed. This process results in pore blockage of the washcoat and limits pore diffusion. The activation energy of the catalytic reaction increases and temperature versus conversion curves shift to higher temperatures with a lower slope. In Figure 7 it is represented as ‘pore diffusion’ curve [2, 15].

Non-selective poisoning

It is a physical deposition of the contamination on the catalyst active sites and in the washcoat pores (sometimes referred as fouling or masking). This adsorption process results in an increased resistance to bulk-mass transfer. Non-selective contaminants accumulate at the inlet of the monolith (first 1 to 3 cm of the bed) and on the top surface of the washcoat. The penetration depth is usually no more than 30 µm [2, 15]. Concentration versus temperature profiles due to the deactivation by non-selective poisoning results in a decrease of the maximum conversion of the reactants and the light-off

21 temperature shifts to higher temperatures. In Figure 7 the curve ‘masking’ shows the temperature versus conversion profile for the non-selective poisoning [2].

Figure 7. Idealized temperature vs. conversion profiles for different poisoning mechanisms [2].

3.3. Sources of inorganic poisons

Biofuels (biodiesel)

Biodiesel is a mixture of fatty acid methyl esters produced from vegetable oils (soybean oil, waste vegetable oil) or animal fats. Combustion of biodiesel in a cylinder has been reported to have the same efficiency as conventional diesel fuel combustion. Moreover, it has been determined that biodiesel burns with lower soot and sulphur emissions. From an environmental point of view biodiesel has been accepted as non-toxic and biodegradable [43, 44]. The production process involves transesterification reaction between triglycerides in oils and fats and simple alcohols like methanol. Three types of catalysts have been commonly used in the biodiesel synthesis – lipase, acid and alkaline catalysts [44]. Strong alkaline compounds such as sodium methoxide, sodium hydroxide and potassium hydroxide have been widely used as liquid catalysts in industrial biodiesel plants. Moreover, different Ca and Mg activated oxide powders have been tested as heterogeneous catalysts. As a consequence, catalyst residues as well as other inorganic impurities (Phosphorus and water, sulphur) derived from oils and fats have been detected in the biodiesel after the purification step [43]. The standard EN 14214 “Automotive fuels - Fatty acid methyl esters (FAME) for diesel engines - Requirements and test methods” is a common European standard for

22 biodiesel. It limits the total allowed sodium and potassium (Na + K) content, total calcium and magnesium content (Ca + Mg), phosphorus content as well as water and sulphur content among other organic and inorganic compounds in the biofuel. Both Na + K and Ca + Mg limits are set to 5 ppm (5 mg per 1 kg of the fuel), the phosphorus limit is 10 ppm, the water limit is 500 ppm and the sulphur content cannot exceed 10 ppm [44, 45]. It has been estimated that a DOC with a volume of 3.2 dm3 can accumulate around 12 wt.% of K and Na deposits after 240 000 km run with a 6 dm3 engine. This calculation has been performed by assuming the maximum limit of potassium and sodium in the fuel according to the EN 14214 [10].

Engine oils

The primary function of engine oils is to reduce friction between moving metal surfaces. The engine oils for heavy-duty trucks are composed of approximately 75 – 85 % of a base oil and 15 – 25 % of additives. The base oil can be a mineral oil (petroleum base oil) or synthetic oil (mixture of several chemical compounds) – most common nowadays. The additives are antiwear, anticorrosion, detergent, antioxidant compounds, viscosity modifiers and other substances. Additives is a source of non-sulphur and sulphur impurities that can contaminate the DOC. Zinc dialkyldithiophosphate (ZDDP) is a multifunctional additive (antiwear, anticorrosion and antioxidation properties) and a source of phosphorus and sulphur. Detergent additives are sources of magnesium and calcium contamination [46, 47].

3.4. Poisoning simulation of the diesel oxidation catalyst

Poisoning of the DOC has been studied by different methods. The poisoning can be carried out in vehicle-driving conditions as well as through an accelerated aging process. During accelerated aging the poisonous species are introduced in the fuel, engine oil, exhaust gas or in the catalyst formulation at an accelerated rate. The main differences between methods are the phase of the contaminant and the scale of poisoning. The different simulation methods and references to the studies where the methods have been reported are summarised in Table 2.

Table 2 Poisoning simulation methods Phase of the Scale of the Deposition of Method Description contaminant method poisons Coating of the Wet Impregnation with Laboratory poisoned slurry impregnation Liquid poisonous solution of -scale on the micro-core [10, 48] the catalyst washcoat monolith walls Contamination Laboratory deposition by Aerosol -scale [14, evaporation of the deposition in the 31, 49] poisonous salt gas-flow reactor solution Gas-phase Gas Contamination contamination Engine- deposition by oil/fuel- Through small- bench scale doped, oil/fuel-borne scale engine and [50-55] poisonous species or the exhaust after- modification of the treatment system oil injection process

Laboratory scale poisoning methods are the most cost-effective ways to analyse and study diesel oxidation catalyst poisoning. Uniform contamination deposition along the length of the catalyst and throughout the washcoat has been easily obtained by the wet impregnation method. Concentration of the poison did not depend on any transport limitations that occurred in the gas-phase poisoning. This method is easy to perform, but it does not mimic the poisoning process in the exhaust after-treatment system [10, 48]. Gas-phase contamination has been reported to be more similar to the poisoning species deposition, concentration and distribution in a real exhaust after-treatment system [9]. Moreover, Eschrich et al. [56] compared laboratory scale (liquid and gas phase) aging, vehicle aging and engine bench aging of a DOC. They have concluded that poisoning in laboratory scale was not fully representative of field aging of the DOC, but correlations were noteworthy. Therefore, further investigations of laboratory scale aging and comparison with field-aging studies are required to understand the differences and develop a method which is both realistic and time efficient.

Wet impregnation method for catalyst poisoning

The wet impregnation method has not been frequently reported in recent studies of the DOC aging. Most likely it is due to the methods ‘unrealistic’ way of poisoning. Cavataio et al. [10] have used wet impregnation to poison fresh DOC monoliths with 1 wt.% to 3 wt.% sodium and potassium. They used Na and K nitrates as precursor salts and determined the DOC wet-incipient capacity prior to the impregnation. The precursor salt solution was prepared according to the poisonous species target concentrations on the monolith. Impregnation of the monolith was realised in a crucible by doping both ends of the monolith followed by drying at 150 °C for 1 hour. Further, poisoned monolith samples were calcined in a flow reactor system by a gas consisting of 14 % O2, 5 % CO2, 5 % H2O and balanced with N2. Samples were heated to 800 °C calcination temperature with a rate of 1 °C/min. Matam et al. [48] have also used wet impregnation to poison the catalyst powder with phosphorus. Nevertheless, they also performed the activity testing for powdered samples.

They impregnated the fresh 2 wt.% Pt/Al2O3 catalyst powder with ammonium phosphate

((NH4)3PO4) solution and dried the catalyst overnight at 120 °C. The drying step was followed by aging in air or in a simulated exhaust flow (3,000 ppm CO, 250 ppm NO, 1,200 ppm C3H6, 300 ppm C3H8, 11 vol.% O2, 25 vol.% CO2 in N2 with a flow rate of 200 ml/min) at 800 °C for 5 hours. Both studies have obtained results that more or less correlate with vehicle aged catalysts (analysed in more detail in the results and discussion paragraph). Therefore, this method is used in the present paper to study DOC poisoning phenomenon.

3.5. Catalyst characterization techniques

Different material characterization techniques have been used to determine the catalyst properties before and after the poisoning process. The fresh and poisoned material characterization has been realised for monolith as well as for powdered samples depending on the desired results. The most widely used characterization techniques and results obtained by these techniques are listed in the Table 3 [5].

Table 3 Fresh and poisoned catalyst characterization techniques [5] Technique Results Active surface area of the precious metals CO chemisorption (dispersion of the active phase) BET (Brunauer–Emmett–Teller) specific Specific surface area, pore size and surface area determination distribution Microstructure analysis of the catalyst TEM (Transmission Electron Microscopy) material, size, shape and size distribution of the precious metals and poisons ICP-OES (Inductively Coupled Plasma Elemental composition of the catalyst bulk Optical Emission Spectroscopy). XRF (X-ray fluorescence spectroscopy) Elemental composition of the catalyst bulk Chemical phase composition of the XRD (X-ray diffraction) catalyst and characterization of the bulk crystal structure Quantitative measurement of lateral and in EPMA (Electronic Probe Microanalysis) depth distribution (in the order of 1 mm) of the poisonous species TPR (Temperature programmed Studies of the oxidation state of the reduction) catalyst SEM-EDS (Scanning Electron Topography imaging of the catalyst Microscopy with Energy Dispersive X-ray surface coupled with the elemental Spectroscopy) analysis

3.6. Characterization of inorganic poisons over the diesel oxidation catalyst

Sodium Sodium distribution along the length of the catalyst has been reported controversially in previous studies. The highest Na concentration in fuel-borne catalyst poisoning studies was observed at the inlet of Pt/Al2O3 DOC with a gradual decrease along the length of the catalyst [53]. Nevertheless, other studies have indicated uniform distribution throughout the catalyst from the inlet to the outlet [50, 51] . Evenly distributed Na particles throughout the

26 washcoat surface (see Figure 8.a) have been observed by Lance et al. [50] using the TEM- EDS mappings. No sodium segregation on the active Pt sites has been observed, proving that Na does not poison the DOC. On the other hand, EPMA analysis of a Na poisoned Pd-rich DOC has shown large amount of Na diffused throughout the primary washcoat layer and penetrating the underlying layer (cordierite). This study has concluded that Na is volatile at temperatures typical for the exhaust gas and can easily penetrate the cordierite washcoat [52]. Calcium Calcium distribution along the length of the catalyst has been reported to be dependent on the accelerated aging method. The concentration of Ca species has a gradual decrease along the length of the DOC starting from the inlet to the outlet [57, 58]. However, uniform Ca distribution throughout the catalyst was determined in extra-oil added engine- bench studies [54]. Gas-phase aging of calcium has showed very low accumulation (less than 0.1 wt.% [49] and 0.2 wt.% [54]) of the Ca species [9, 59]. Calcium has been found on the catalyst surface in the form of calcium phosphate Ca3(PO4)2 [58, 60], calcium sulphate CaSO4 [58] and calcium carbonate CaCO3 [49]. Phosphorus The highest phosphorus concentration has been reported at the inlet of the DOC (first 10 µm down the DOC [41]) with a gradual drop along the length of the catalyst [9, 31, 61] (see Figure 8.b). Some studies have reported phosphorus penetration into the washcoat [9, 31, 56], with penetration depth up to 10 [9] to 40 µm [62]. On the other hand, Kärkkäinen et al. [41] have observed the maximum accumulation of phosphorus compounds on the surface of the Pt/Al2O3 and PtPd/Al2O3 DOCs. The accumulation of phosphorus has been more pronounced on Pt/Al2O3 than on PtPd/Al2O3 DOC. Various chemical forms of phosphorus components have been reported in the literature. In most of the studies phosphorus has been found in the form of amorphous aluminium phosphate AlPO4 on the DOC surface [41, 62]. In field-aging studies phosphorus has also been found in 3-5 µm thick over-layer on the washcoat. The over-layer was a contamination deposit from lubricating oils and fuel additives [59, 60]. The contamination deposit mostly consisted of zinc phosphate Zn3(PO4)2 [59, 60], calcium phosphate Ca3(PO4)2, magnesium phosphate Mg3(PO4)2 [60]. Furthermore, the study of TWC phosphorus poisoning has reported the same inorganic salt formation [31].

Figure 8. a) Elemental map of Pt (red) ad Na (blue) from the inlet of aged DOC [50]. b) EDS mapping of a cross-section from an inlet DOC sample; red – phosphorus, green – zinc [59]. Sulphur Sulphur has been found on the catalyst washcoat surface as well as penetrated through it [9, 59]. Some authors have reported uniform sulphur distribution throughout the DOC [62], but others have indicated steep concentration gradients from the inlet to the outlet [9]. A majority of the studies have found sulphur on the DOC in the form of sulphate [9, 14, 49, 63].

Formation of aluminium sulphate Al2(SO4)3 and further pore blockage have been reported as the major causes of deactivation of the DOC [9, 49]. Al2(SO4)3 is formed due to the SO2 reaction with the alumina washcoat at high temperatures. At elevated temperatures

SO2 desorbs from the precious metal sites and penetrates the washcoat. This process can result in very high sulphur loadings on the field-aged catalysts without dramatic loss of its oxidation performance. A model mechanism (see Figure 9) for sulphur mobility over the DOC has been proposed with kinetic studies strengthening the model. Kinetic studies compared the apparent activation energy before and after SO2 deposition on field-aged catalysts. The apparent activation energy was doubled and a dramatic loss in activity observed. Nevertheless, the activity was partly regenerated after the temperature treatment. It indicated direct SO2 interactions with the precious metal sites and its mobility over the DOC. Authors suggested that it was not due to physical blockage of the active sites. It would have resulted in activity loss with constant apparent activation energy [64].

Figure 9. Sulphur mobility over the DOC [64].

3.7. Laboratory scale activity testing equipment

Poisoned catalyst performance has been measured as CO, C3H6 and NO light-off temperature changes during activity testing of the catalyst. The use of a fixed bed reactor for catalyst activity testing has been reported in numerous studies [9, 10, 14, 31, 41, 48, 49, 51, 56, 61, 65] . Activity testing has been carried out with simulated exhaust gas flow through the catalyst monolith micro-core samples. The temperature was ramped to initiate the oxidation reactions over the catalyst and the concentration of the outlet gases recorded with a spectrometer. Simulated exhaust gas composition, temperature ramp range and gas space velocities have been varied from study to study. Commonly reported gases to simulate the diesel exhaust were CO, NO, C3H6 (or other short chain hydrocarbons), O2, H2O and CO2 balanced with inert gas like N2 or Ar. For example, Galisteo et al. [65] have studied vehicle- aged catalyst poisoning. They used a simulated exhaust composed of 10 % CO2, 10 % H2O,

6 % O2, 300 ppm NO, 300 ppm C3H6, 400 ppm CO and 133 ppm H2 balanced with argon. The gases had a space velocity of 100 000 h-1 and the temperature was ramped from 100 °C to 500 °C with a rate of 5 °C/min. Furthermore, Eschrich et al. [56] have compared different poisoning methods of the DOC. In their experiments they used a gas composition of 15 %

O2, 5 % H2O, 500 ppm CO, 200 ppm C3H6, 500 ppm NO balanced with nitrogen at a space velocity of 120 000 h-1. The monolith sample was heated from 100 °C to 400 °C. Some studies have tried to investigate the poisonous species effect separately on NO,

CO or C3H6 reactions. For instance, Wienberga et al. [9] have excluded CO oxidation reaction from their vehicle-aged and fresh catalyst comparison study. They introduced -1 simulated exhaust flow at 30 000 h consisting of 200 ppm NO, 1050 ppm C3H6:C3H8

(2.9:1), 8 % O2, 8% H2O balanced with N2 and ramped the temperature from 100 °C to 300 °C with a rate of 2 °C/min. Furthermore, Matam et al. [48] have studied the phosphorus effect on a liquid-phase poisoned DOC and considered only the NO oxidation reaction.

Therefore, they used 400 ppm NO, 10 % O2 balanced with N2 at a space velocity of 92 840 1/h. There have been rarely similar methods for activity testing measurement reported in the literature. Therefore, it can be concluded that the method depends on the available equipment and its characteristics as well as the desired experimental results. Previous studies have come up with different opinions of the effect of inorganic species on the DOC activity. This literature is addressed in more detail in the results and discussion paragraph.

MATERIALS AND METHODS

4. CATALYST PREPARATION AND POISONING

4.1.Catalyst preparation

γ-alumina supported platinum (Pt) and palladium (Pd) diesel oxidation catalysts

(PtPd/γ-Al2O3) were prepared by the incipient wetness impregnation method. Catalysts were prepared in one batch considering the methods’ wide use in industry, the availability of the precious metal reactants and price. Prior to the incipient wetness impregnation of the γ-alumina support (specification of γ-alumina in Table 4) a precursor salt solution was prepared. Platinum and palladium nitrates were used as the precious metal precursor salts (specification of the used solutions in Table 5). The concentration of both metals in the solution was calculated according to the target metal loading on the washcoat that was equal to 1.2 wt.%, with a Pt and Pd ratio 3:1. Precursor salt solutions were weighed in on a balance (Mettler Toledo PB 303-S/FACT Delta Range, d = 10 mg, e = 10 mg, Max/Min weight = 310/0.02 g) and diluted with deionized water (deionized by Millipore Direct Q3 deioniser, electrical resistivity 16 MΩ·s) up to the total solution volume for impregnation. For two-step impregnation the total solution volume was twice the determined pore volume of the alumina support. Table 4 Specification of γ-alumina Alumina Chemical Specific Pore Name Provider crystalline formula surface area volume phase 2 3 Puralox NGa 150 SASOL γ-Al2O3 γ 150 m /g 0.45 cm /g

Table 5 Specification of the precious metal salt solutions Solution Chemical Solution concentration w/w % Molar mass, Name Provider formula composition (weight metal to g/mol weight solution) Platinum Pt(NO3)4 15 % w/w 443.1 (IV) nitrate Alfa Water Palladium Aesar solution Pd(NO3)2 8.5 % w/w 230.4 (II) nitrate

The impregnation process was realised in two steps. Prior to the first-step impregnation γ-alumina powder (see Figure 10.a) was dried for 24 hours at 110 °C. Afterwards powder was weighed on a balance and placed in a crucible. The precursor salt solution was added dropwise on the dry γ-alumina powder. Each deposition (see Figure 10.b) was followed by a waiting time to allow salt solution drops to completely absorb in the alumina pores. Afterwards, the solution drops were homogeneously crushed with spatula and the powder surface was flattened by hand-induced vibrations. New depositions of the precursor salts were carried out on the flattened powder surface in the same way as described above until the first half of the solution was impregnated. The first impregnation step was followed by catalyst powder drying for three hours at 110 °C. The same sequence of dropwise impregnation sub-steps was performed for the other half of the solution. After the second impregnation step, the impregnated powder was crushed with a pestle in a mortar to obtain a homogeneously distributed active phase. The last step was followed by 3-hour drying at 110 °C (heating rate 5°C/min) in air and calcination in air at 500 °C (heating rate 5°C/min) (see Figure 10.c). The fresh catalyst sample was marked as F-Cat.

a b c

Figure 10. a) Alumina powder after 24-hour drying at 110°C; b) Dropwise impregnation sub-step of the precursor salt solution; c) Calcined fresh catalyst powder.

4.2. Catalyst poisoning

Catalyst poisoning was carried out by the incipient wetness impregnation method of the poisoning salt solutions. Unlike catalyst preparation, poisoning was performed only in one impregnation step by using a poisoning salt solution volume equal to the pore volume of the specific fresh catalyst batch. The fresh catalyst was poisoned separately with phosphorus (P), sodium (Na) and calcium (Ca) in accordance to the target value of poison. The target values were equal to 1 wt.%, 0.74 wt.% and 1.3 wt.% of phosphorus, sodium and

31 calcium (mass of poison/total washcoat mass), respectively. These values correspond to a poison loading that equals to 0.32 mmol of poison per 1 gram of catalyst. Phosphorus, sodium and calcium poisoned catalyst samples were marked as P-Cat, Na-Cat, Ca-Cat, respectively. Ammonium hydrogen phosphate, sodium nitrate and calcium nitrate tetrahydrate were used as poisonous species precursor salts (technical specification in Table 6). Precursor salts were weighed on a balance (Mettler Toledo PB 303-S/FACT Delta Range, d = 10 mg, e = 10 mg, Max/Min weight = 310/0.02 g) and diluted with deionized water (deionized by Millipore Direct Q3 deioniser, electrical resistivity 16 MΩ·s) up to the total solution volume for impregnation. Dropwise impregnation sub-steps of every poison were carried out in the same way as for the fresh catalyst. Impregnation was followed by crushing with a pestle in a mortar to obtain homogeneously distributed poisonous species. Poisoned catalyst samples were afterwards dried in air for 3 hours at 110 °C (heating rate 5 °C/min) and calcined in air at 500 °C for 4 hours (heating rate 5 °C/min). Both fresh and poisoned catalyst samples were subjected to the same temperature treatment. Table 6 Specification of the precursor salts for poisoning Molar Chemical Melting Name Provider Purity, % mass, formula point, °C g/mol Ammonium 155 Alfa Aesar (NH4)2HPO4 98 132.1 phosphate (decomposition) Sodium Alfa Aesar NaNO3 99 308 85.0 nitrate Calcium Sigma- 44 nitrate Ca(NO3)2·4H2O 99 236.2 Aldrich (decomposition) tetrahydrate

5. MONOLITH COATING

5.1. Parameters and treatment of monolithic micro-cores

Monolithic cordierite honeycomb substrates (extracted from vehicle-scale monolith) were used for preparation of the fresh and poisoned catalysts. Parameters of used substrate are listed in Table 7. Prior to the coating procedure cordierite monolithic cores were treated in air at 500 °C (heating rate 10 °C/min) for 2 hours to remove any dust and grease. Table 7 Parameters of the cordierite cores Cells per square inch (cpsi) 400 Cells per monolith 177 Diameter of the core, mm 20 Length of the core, mm 30 Size of one cell, mm2 1

5.2. Dip-coating procedure

Monolith coating was carried out from fresh and poisoned catalyst ethanol (96%, Solveco) slurry. A similar technique has been used by Persson et al. [21]. Powdered catalyst samples and ethanol were weighed on a balance to prepare a slurry of 18 wt.% of catalyst in ethanol. The slurry was ball-milled for 24-hours and afterwards poured into beaker with magnetic mixer and stirred continuously to avoid sedimentation during coating. The dip-coating procedure was performed in 4 steps. One step of coating procedure is schematically shown in Figure 11.

N2 flow through the monolith cells

Primary monolith dipping in Monolith rotation Monolith removal the slurry of 180° and from the slurry and secondary dipping solvent evaporation by flushing with N2

Figure 11. One step of the monolith dip-coating procedure.

A primary monolith dipping was performed by immersing the monolith in the slurry. During this immersion the slurry filled the monolith cells through capillary forces. After the monolith rotation by 180°, the monolith was immersed in the slurry for the second time. This secondary dipping was followed by ethanol evaporation and channel unplugging with flowing N2 gently through the channels. Afterwards, the monolith was dried in air at 110 °C for 45 minutes and weighed. For every monolith this procedure was repeated 4 times to reach a washcoat loading of approximately 20 wt.% of the total monolith mass. The dip-coating procedure was followed by calcination in air at 500 °C for 4 hours (heating rate 5 °C/min). Coated and uncoated monolith cores are showed in Figure 12.

a a

b b

Figure 12. a) Coated monolith core; b) Uncoated cordierite monolith.

5.3. Dip-coating procedure trial tests

Coating quality and technique depends on the operator and has to be adjusted in order to obtain appropriate coating. Therefore, two ethanol slurry concentrations (18 wt.% and 19 wt.% of catalyst in the slurry) were tested as a trial. This was done to obtain visually homogeneous and uniform coating (see Figure 13).

a.1 a.1

b.1 b.1

a.2 a.2

b.2 b.2 Figure 13. Positive (1) and negative (2) image of coating with a) 18 wt.% and b) 19 wt.%.

Cores were also cut and examined to observe visual differences inside channels between coating with 18 wt.% and 19 wt.% slurry. The coating with 19 wt.% slurry was inhomogeneous on monolith walls and in channels and it easily cracked after the calcination. On the contrary, coating with 18 wt.% slurry was homogeneous with no cracks or clogging of channels. It was decided to use 18 wt.% ethanol slurry for the monolithic catalyst preparation.

6. MATERIAL CHARACTERIZATION

6.1. Inductively coupled plasma optical emission spectrometry (ICP-OES)

Inductively coupled plasma optical emission spectrometry (ICP-OES) was used to determine the elemental composition of the fresh and poisoned catalyst samples. ICP-OES was performed for coated monolith samples before the activity testing and after the sulphur treatment to measure the deposited sulphur content on the samples. Pt, Pd, P, Na, Ca and S content in mass percent of the total monolith mass was determined for each sample. The analyses were performed externally at RISE Research Institutes of Sweden AB and no further information about equipment is available.

6.2. BET surface area, pore size and pore distribution

BET (Brunauer–Emmett–Teller) surface area of the washcoat and BJH (Barrett- Joyner-Halenda) pore size distribution were determined using nitrogen (77 K) as adsorptive in a Micromeritics ASAP 2000. Approximately 0.4 grams of the fresh and poisoned powder- like catalyst samples were degassed for 8 to 10 hours at 250 °C until stable vacuum in the sample holder was achieved. After degassing the samples were weighted again to determine the actual sample mass. Sample analysis followed the degassing procedure.

6.3. CO chemisorption

Precious metal dispersion was determined for the fresh and poisoned catalyst samples by carbon monoxide (CO) chemisorption in a Micromeritics ASAP 2020. Approximately 0.2 grams of powdered catalyst samples were degassed for 2 to 3 hours at 200 °C until stable vacuum was achieved. The actual mass of the sample used in the calculations was determined after the sample analysis. The analyses were performed twice for every sample by using method Table 8. Table 8 CO chemisorption analysis method Gas in the Task Temperature, Heating Time, Task name sample number °C rate, °C/min min holder 1 Evacuation He 250 10.0 240 2 Leak test - 120 10.0 - 3 Evacuation He 120 10.0 30

Table 8 (continuation) Gas in the Task Temperature, Heating Time, Task name sample number °C rate, °C/min min holder

4 Flow H2 120 10.0 10

5 Flow H2 400 5.0 120 6 Evacuation - 400 10.0 30 7 Evacuation - 35 10.0 30 8 Leak test - 35 10.0 - 9 Evacuation - 35 10.0 30 10 Analysis CO 35 10.0 -

The method included 10 steps. During the first three steps catalyst experienced flushing with He to remove any water from the sample. A leak test was carried out in between the first and third step to ensure a stable vacuum in the sample holder. In case of the leak test failure, it was reported to an operator after analysis. He flushing was followed by heating in reducing atmosphere (H2) to remove any oxygen from the catalyst. Next steps ensured and tested stable vacuum in the sample holder. The last step of this sequence was the actual chemisorption with CO. Results were given as either precious metal dispersion – percent of active metal available for interactions with CO molecules or as a consumption of CO per gram of the sample (μmol/g sample). The former expression uses manually entered active phase composition and theoretical stoichiometry CO : precious metals. Therefore, the latter as direct measurement is more representative.

6.4. Temperature Programmed Reduction (TPR)

Testing method

Temperature programmed reduction (TPR) with hydrogen (H2) of the fresh and poisoned catalyst samples was studied in a Micromeritics AutoChem 2910. The fresh catalyst powder (F-Cat) was used to develop a testing method prior to the actual testing. The method development was started by gathering literature from TPR studies for similar catalysts (see Appendix 1) [19, 25, 66 - 74]. Several experimental set-ups were tried and the optimal one chosen (see Appendix 1 for the repeatability of the test method).

Approximately 0.5 grams of the F-Cat, P-Cat, Na-Cat and Ca-Cat powders were used for the reduction. Prior to the reduction the catalyst was pre-treated in 50 cm3/min pure helium (He) at 200 °C for 90 minutes (heating rate 10 °C/min). Afterwards the sample was cooled down to ambient temperature in 50 cm3/min He flow. The gas flow was changed to 3 5 vol.% H2 in argon (Ar) with a flow rate 30 cm /min when ambient temperature was reached. The temperature during reduction was increased with 5 °C/min from ambient up to 800 °C and held at the maximum temperature for 30 minutes. The temperature ramp was followed by cooling to ambient temperature in 50 cm3/min He flow. Water was condensed before the resulting gas from the reduction passed a thermal conductivity detector (TCD). Sample temperature was plotted versus the TCD signal.

Study of precious metals – poisons interactions and alumina support – poisons interactions

Pure γ-alumina support poisoning with P, Na and Ca was carried out in order to distinguish between the precious metal and support interactions with poisons. Support poisoning was performed by the same technique as the catalyst poisoning described in Paragraph 4.2. The target poison loading in the support was equal to the poison loading in the catalyst - 1 wt.% P, 0.74 wt.% Na and 1.3 wt.% Ca. Prior to support poisoning γ-alumina was dried for 24 hours at 110 °C followed by calcination at 500 °C for 4 hours to obtain the same temperature treatment as for the fresh catalyst (see Appendix 1 for the temperature treatment effect on the reduction profile). After impregnation, samples were dried for 3 hours at 110 °C (heating rate 5 °C/min) and calcined for 4 hours at 500 °C (heating rate 5 °C/min). The same temperature treatment was carried out for pure γ-alumina powder prior to the TPR. The samples were marked as F-Al, P-Al, Na-Al and Ca-Al, representing the fresh alumina powder and phosphorus, sodium and calcium poisoned alumina powders, respectively.

6.5. Transmission electron microscopy (TEM-EDS) and scanning electron microscopy (SEM-EDS)

Morphology of the washcoat, precious metal morphology and distribution and poisonous species distribution in the powder-like catalyst samples (F-Cat, P-Cat, Na-Cat and Ca-Cat) were studied by scanning electron microscopy (SEM, JEOL JSM-7000F, 20 kV). A transmission electron microscope (TEM, JEOL JEM-2100F, accelerating voltage range 200 kV) was used to study the morphology and elemental distribution on the nanoscale. Prior to the TEM studies powdered samples were grinded with ethanol, transferred on Lacey carbon film with copper grid and dried under quartz lamp. Both microscopes were equipped with an energy X-ray dispersive detector (EDS). The TEM-EDS and SEM-EDS analyses were performed externally at Stockholm University.

6.6. X-Ray Powder Diffraction (XRD)

Powdered catalyst samples F-Cat, P-Cat, Na-Cat, Ca-Cat and F-Al were studied by X-ray powder diffraction using a D500 (Siemens Kristalloflex) diffractometer working with Cu Kα (λ = 1.5418 Å) monochromatic radiation. The diffraction patterns were obtained in a step-by-step scanning mode from 10 to 90 ° 2θ at 0.02 ° 2θ steps with a dwell time of 6 seconds. The support phase and crystalline precious metal phases were identified by using ICDD (The International Centre of Diffraction Data) database.

7. CATALYST ACTIVITY TESTING

7.1. Activity testing reactor set-up and preparation for experiments

Performance of the catalyst samples F-Cat, P-Cat, Na-Cat and Ca-Cat was studied in nitric oxide (NO), carbon monoxide (CO) and propylene (C3H6) (hydrocarbon model gas) oxidation reactions. Reactions were performed in an atmospheric pressure heated tubular quartz flow reactor coupled with a Fourier transform infrared spectrometer for gas analysis (MKS 2030 FTIR MultiGas Analyser). As a result temperature versus conversion curves were obtained and analysed for the oxidation reactions. The flow reactor is schematically shown in Figure 14. In order to simulate exhaust gas flow oxygen (O2), nitric oxide (NO), carbon monoxide (CO), carbon dioxide (CO2), propylene (C3H6), nitrogen (N2) and water steam (H2O) were mixed before entering the flow reactor. Hydrogen (H2) was used for the monolith pre-treatment and was also added to the gas system. O2 as technical air, pure H2, CO2 and N2 were obtained from in-house gas pipes.

Gas bottles of 3 % NO in N2, 10 % CO in N2 and liquid C3H6 were installed and used for the other gases. Steam was generated by a Controlled Evaporator Mixer (CEM) (Bronkhorst High-Tech). Gas flows were controlled with mass flow controllers (Bronkhorst High-Tech EL-Flow Select). Gas leak tests for all the connections and fittings in the reactor system was performed with a leak-test solution (Swagelok Snoop Liquid Leak Detector, working temperature range -2 °C to 93 °C) prior to the experiments. The gas-flow reactor was placed in a high temperature furnace (Carbolite 1.5 kW). Moreover, the reactor pre-heating system consisted of 3 heating tapes set to 60 °C. The FTIR pre-heating system consisted of 3 heating tapes in a row starting from the reactor outlet set to 177 °C, 150 °C and 200 °C in order to reach 191 °C in the FTIR test gas cell. The temperature for the heating tapes was controlled by thermocouples (type K) that were assembled prior to the testing (T1-3 pre-heating and T4-6 FTIR pre-heating in Figure 14). The temperature was also measured for the monolith sample with type K thermocouples at the inlet and outlet of the monolith (T1 inlet and T2 outlet in Figure 14). A LabVIEW programme was developed to log the temperature for these two thermocouples. Linearization of the furnace was carried out to obtain linear heating rate at desired gas space velocities (see Appendix 2 for linearization effect on the stability of heating). It was followed by calibration of the thermocouples with an inserted trial monolith. The temperature of thermocouples and in the furnace were monitored and compared during heating ramps at different gas (air) space velocities. 40

Linearization of the FTIR Multigas 2030 analyser, system health-check and empty reactor test runs with calibration gas bottles of known-concentration were performed to confirm the FTIR signal accuracy. Afterwards the flow meter readings were harmonised with the FTIR readings for the test gases – NO, CO, C3H6, CO2 and steam for the desired testing concentrations (see Paragraph 7.2.).

It was challenging to get appropriate pressure of C3H6 through the reactor when CO and NO were not present in the gas stream. As a result of this problem, the signal of propylene in the FTIR was weak and unstable. Thus, extra N2 lines were added to the system to obtain a higher pressure of propylene through the reactor and stable FTIR signal response. Trial tests with fresh catalyst monoliths were run to confirm the FTIR, furnace, thermocouples and flow meter calibration/harmonisation results in the desired experimental setup (see Paragraph 7.2.). Only after the trial experiments, the first test monolith was inserted in the reactor for being tested. Appendix 2 incudes the developed instructions for the rig operation, trial test results and the FTIR - flow meter calibration/harmonisation results.

Figure 14. Gas flow reactor for activity testing

7.2. Experimental set-up for the activity testing

Pre-treatment of the catalyst cores

A pre-treatment procedure was carried out prior to the catalyst activity testing. During the pre-treatment the catalyst temperature was increased from ambient up to 500 °C -1 with a heating rate of 5 °C/min in N2 flow at a space velocity of 10 000 h . Volumetric flows of gases at particular space velocities were calculated according to the catalyst monolith dimensions listed in Table 7. When 500 °C was reached on the monolith the N2 gas was changed to 5 vol.% H2 in N2 at the same space velocity. The hydrogen treatment at 500 °C was carried out for 2 hours. After the hydrogen treatment the gas flow was changed to technical air and the space velocity was increased to 80 000 h-1. Thus, the furnace temperature was increased to maintain 500 °C on the catalyst monolith. The air treatment at 500 °C was performed for 2 hours followed by reactor cooling to ambient temperature in air with a space velocity of 80 000 h-1.

Test set-ups for the activity testing

Catalyst activity for the monolithic F-Cat, P-Cat, Na-Cat and Ca-Cat samples was tested for NO, CO and C3H6 oxidation reaction at 6 different conditions: 1. Fully simulated diesel engine exhaust gas. The composition of the test gas was fully simulated exhaust gas. This experiment was marked as Combo (see Table 9).

2. NO oxidation reaction.

The composition of the test gas was simulated exhaust excluding CO and C3H6. This experiment was marked as NO-Single (see Table 9). 3. CO oxidation reaction.

The composition of the test gas was simulated exhaust gas excluding NO and C3H6. This experiment was marked as CO-Single (see Table 9).

4. C3H6 oxidation reaction. The composition of the test gas was simulated exhaust gas excluding NO and CO. This experiment was marked as C3H6-Single (see Table 9).

5. Fully simulated diesel engine exhaust gas after aging. The monoliths were exposed to 4 temperature ramps (50 °C to 400 °C) prior to this experiment. The composition of the test gas was the same as in the Combo experiment. This experiment was marked as Combo-T (see Table 9).

6. Fully simulated diesel engine exhaust gas after the catalyst monolith aging

with sulphur dioxide (SO2).

Monolith aging with SO2 was performed in a sulphur poisoning rig (the reactor set- up is not described in this thesis report). Monolith samples were exposed to the temperature ramp from ambient to 280 °C. The heating was performed for 30 minutes. When 280 °C was reached, samples were treated with a gas flow consisting of 100 ppm SO2, 10 vol.% O2, 5 -1 vol.% H2O and balance N2 for 8 hours at a space velocity of 20 000 h .

After the SO2 treatment, the samples were tested in the activity testing rig. The composition of the test gas was simulated exhaust gas from the diesel fuel combustion. This experiment was marked as Combo-SO2 (see Table 9). Table 9 Test gas compositions for the activity testing

Experiment Combo NO-Single Component Combo-T CO-Single Combo-SO2 C3H6-Single

Oxygen O2 10 vol.% 10 vol.%

Carbon dioxide CO2 5 vol.% 5 vol.% Nitric oxide NO 1000 ppm Carbon monoxide CO 100 ppm 150 ppm for the test gas, others 0 ppm

Propylene C3H6 100 ppm

Water steam H2O 5 vol.% 5 vol.%

Nitrogen N2 balance balance

During the activity testing experiments, the temperature of the catalyst monolith was increased from 50 °C to 400 °C with a heating rate of 5 °C/min. The volumetric gas flow through the reactor was equivalent to a space velocity of 80 000 h-1.

Before the temperature ramp, stable FTIR readings for NO, CO and C3H6 were recorded at 50 °C for 45 to 60 minutes. The average values of these readings were used as an initial concentration of each gas. During each temperature ramp, FTIR signals for NO,

NO2, CO and C3H6 were recorded as a function of the monolith inlet temperature (T1inlet) every second. Afterwards the data were analysed with MS Excel and light-off curves were obtained for every experiment.

In between the temperature ramps, the catalyst samples were cooled to 50 °C in air by using a space velocity of 80 000 h-1 with no addition of a cooling system. Whenever it was required to leave the reactor overnight at the room temperature, the -1 gas flow of N2 was kept at 10 000 h through the monolith.

7.3. The light-off curves for oxidation reactions obtained in the activity testing experiments

24 reactions with different catalysts were performed in the activity testing rig in the order showed in Table 10. The symbol F, P, Na and Ca represents the fresh (F-Cat), phosphorus (P-Cat), sodium (Na-Cat) and calcium (Ca-Cat) poisoned catalyst, respectively, that was used in the experiment.

16 light-off curves were obtained for each (NO, CO and C3H6) oxidation reaction.

Besides the light-off curves, NO2 yield was determined whenever NO was initially present in the gas stream. The obtained temperature versus conversion curves are summarised in Table 10.

Table 10

Temperature versus conversion curves obtained in the activity testing experiments

Light-off curves Nr. NO oxidation CO oxidation C3H6 oxidation

1 NO-F-Combo CO-F-Combo C3H6-F-Combo

2 NO-P-Combo CO-P-Combo C3H6-P-Combo

3 NO-Na-Combo CO-Na-Combo C3H6-Na-Combo

4 NO-Ca-Combo CO-Ca-Combo C3H6-Ca-Combo

5 NO-F-Single - -

6 - CO-F-Single -

7 - - C3H6-F-Single

8 NO-P-Single - -

9 - CO-P-Single -

10 - - C3H6-P-Single

11 NO-Na-Single - -

12 - CO-Na-Single -

13 - - C3H6-Na-Single

14 NO-Ca-Single - -

15 - CO-Ca-Single -

16 - - C3H6-Ca-Single

17 NO-F-Combo-T CO-F-Combo-T C3H6-F-Combo-T

18 NO-P-Combo-T CO-P-Combo-T C3H6-P-Combo-T

19 NO-Na-Combo-T CO-Na-Combo-T C3H6-Na-Combo-T

20 NO-Ca-Combo-T CO-Ca-Combo-T C3H6-Ca-Combo-T

21 NO-F-Combo-SO2 CO-F-Combo- SO2 C3H6-F-Combo- SO2

22 NO-P-Combo- SO2 CO-P-Combo- SO2 C3H6-P-Combo- SO2

23 NO-Na-Combo- SO2 CO-Na-Combo- SO2 C3H6-Na-Combo- SO2

24 NO-Ca-Combo- SO2 CO-Ca-Combo- SO2 C3H6-Ca-Combo- SO2

The colours in the Table 10 represent the catalyst used for each reaction:

Fresh catalyst (F-Cat)

P-poisoned catalyst (P-Cat)

Na-poisoned catalyst (Na-Cat)

Ca-poisoned catalyst (Ca-Cat)

RESULTS AND DISCUSSION

8. MATERIAL CHARCTERIZATION

8.1. ICP-OES Elemental analysis

ICP-OES results for the catalyst monolith samples are shown in Table 11. These values are calculated in accordance to the catalyst washcoat mass, raw data is presented in Appendix 3. Table 11 ICP-OES Elemental analysis of the catalyst samples

Target loading of inorganic Pd, Pt, Na, Ca, P, S, Sample species (of wt. wt. wt. wt. wt. wt. name

Pt to Pd ratio total % % % % % %

washcoat Pt and Pd ratio

Pt loading and Pd metal

Target mass)

Total Pt and Pd loading, wt.% PtTotal and Pd loading,

Target F-Cat - 0.26 0.72 0.98 2.75 - - - 5.95 1.2 P-Cat 1 wt.% P 0.27 0.76 1.03 2.84 - - 0.97 5.07 wt. 3 Na-Cat 0.74 wt.% Na 0.27 0.76 1.03 2.83 0.75 - - 6.83 % Ca-Cat 1.3 wt.% Ca 0.26 0.72 0.98 2.82 - 1.15 - 6.65 wt.% - mass percent of the total washcoat mass

The precious metal loading for all of samples is approximately 1.0 wt.% from the total washcoat mass with Pt:Pd ratio 3:1. Raw ICP-OES results revealed elevated levels of Na and Ca for all of the coated monolith samples. Therefore, it was assumed that Ca and Na impurities were present in the cordierite substrate in concentrations equal to the values that were determined for samples without Na and Ca contamination. The actual Ca and Na loadings in the washcoat were calculated by taking in account cordierite mass and washcoat mass for every sample (calculations presented in Appendix 3). It is calculated that the Na and Ca content in the washcoat is around 0.75 wt.% and 1.15 wt.%, respectively. Moreover, these calculations are in a close agreement by SEM-EDS for powdered washcoat samples (see Paragraph 8.4.). The Na and Ca loadings around 0.58 wt.% and 1.2 wt.%, respectively, were determined by EDS spot measurements. The

Phosphorus content in the P-Cat sample is around 1 wt.%. It is also confirmed by SEM-EDS measurements (see Paragraph 8.4.). The sulphur loadings for all of samples are in between 5 and 7 wt.% from the washcoat. Therefore, it indicates that the sulphur treatment does not equally affect all samples (see Appendix 4 for an explanation). The sulphur loading in catalyst samples can be ordered, starting with the lowest concentration, as follows P-Cat < F-Cat < Ca-Cat < Na- Cat. As a consequence, the expected deactivation should follow the same order. Nevertheless, the catalyst activity testing results from CO, NO and C3H6 oxidation reactions are not in agreement with this order (see Paragraph 9).

8.2. BET Specific surface area

Specific surface area measurements are shown in Table 12. Table 12 Specific surface area of the catalyst samples measured by BET Decrease in the BET Sample Total pore BET specific surface area, m2/g area as compared to the 3 name volume, cm /g fresh catalyst, % F-Cat 146 - 0.46

P-Cat 137 7 0.44

Na-Cat 145 1 0.46

Ca-Cat 142 3 0.45

The results indicate minor changes in the specific surface area for Na-Cat and Ca- Cat as compared to the F-Cat. Cavataio et al. [10] have also reported no changes in the specific surface area for a PtPd/Al2O3 catalyst that was impregnated with 1 wt.% Na. On the contrary to the present study, Kolli et al. [49] have determined a decrease by 11 % when

Pt/Al2O3 catalyst was poisoned with Ca in the gas phase (0.3 wt.% Ca in the catalyst formulation). It can be speculated that the contamination technique may affect surface area changes for Ca poisoned catalysts. A specific surface area decrease by 7 % was observed when the fresh catalyst was poisoned with phosphorus (P-Cat). These results are in agreement with literature data where the surface area decrease was explained by P deposition in the washcoat pores [48] and formation of an aluminium phosphate (AlPO4) [41, 58, 75]. Moreover, gas-phase aging with phosphorus has shown that the BET area decrease is proportional to the phosphorus loading

47 on the catalyst. The authors of this study have claimed a 13 % and 26% decrease for

PtPd/Al2O3 catalysts poisoned by 1.7 wt.% and 3.5 wt.% Phosphorus, respectively [41]. These observations are in agreement with the BET results from the present study, where lower P loadings are used and, as a consequence, a lower decrease in the surface area is obtained.

8.3. BJH pore size distribution

BJH pore size distribution of the catalyst samples is presented in Figure 15. The pore size distribution measurements show that the majority of the catalyst pores are in the size range from 6 to 35 nm. Nevertheless, some differences can be observed between fresh and poisoned catalyst samples. The pore size distribution of the F-Cat, P-Cat and Ca-Cat reveals a large contribution of pores in the size of 9 and 17 nm. Additionally, P- Cat and Ca-Cat indicate formation of pores of 12 nm. The pore size distribution of the Na- Cat sample differs from the other samples. Pores in the size of 9 nm take up the same volume as in the other catalysts. Nevertheless, the formation of pores of 12 nm is more pronounced. Additionally, the Na-Cat sample has a lower volume of 17 nm pores and a higher volume is taken up by pores of about 22 nm.

BJH pore size distribution

0.08 /g

3 0.07 0.06 0.05 0.04 0.03 0.02 0.01 0 Incremental pore cm volume, Incremental 0 10 20 30 40 50 60 70 Average pore diameter, nm F-Cat P-Cat Na-Cat Ca-Cat

Figure 15. Pore size distribution.

8.4. Precious metal dispersion and average particle size (CO chemisorption)

The results from CO chemisorption measurements are arranged in Table 13. The precious metal dispersion for the F-Cat is 29 % which corresponds to 22 µmol CO uptake per gram of sample. A decrease of around 24 % is measured for the Na and P poisoned samples and around 15 % for the Ca poisoned sample. This results in 22 % dispersion of Pt and Pd for the P-Cat and Na-Cat and 25 % dispersion for the Ca-Cat. Average Pt and Pd crystallite size according to CO chemisorption results is 4 nm for the fresh catalyst and 5 nm for poisoned catalysts. Nevertheless, these are software- calculated values. Assumptions considering ideal spherical particles, geometry of the CO molecule, stoichiometry ‘CO : precious metals’ have been taken into account. Therefore, particle size determination with CO chemisorption is not as accurate as microscopy observations. Moreover, it is also possible to distinguish between Pt and Pd particles in TEM-EDS analysis. Table 13 CO chemisorption results

repeat repeat

- Sample

repeat analysis,repeat

name -

sample

catalyst, % catalyst,

analysis, % analysis,

analysis, nm analysis,

dispersion,

µmol/g sample

Crystallite size, nm Crystallite size,

Average Pt and PdAverage

compared to the fresh to the fresh compared

Dispersion decrease as as decrease Dispersion

Crystallite size Crystallite size

Pt and Pd dispersion, %

CO uptake, µmol/gCO sample uptake,

Pt and Pd dispersion

CO uptake CO uptake

Average crystallite size, crystallite nm Average

Average CO uptake, µmol/gAverage F-Cat 28 30 29 - 20.5 22.5 22 4 4 4 P-Cat 23 20 22 24 17.4 15.2 16 5 6 5 Na-Cat 23 21 22 24 17.3 15.5 16 5 5 5 Ca-Cat 25 24 25 15 18.5 18.0 18 5 5 5

Previous studies have indicated that precious metal dispersion is dependent on the catalyst preparation method and calcination temperatures. A metal dispersion of 21.47 % and 5.3 nm average particle size were determined for an alumina supported Pt-Pd (1:5) catalyst with a precious metal loading 2.4 wt.%. The catalyst was prepared by incipient wetness impregnation method and calcined in air at 500 °C for 2 hours [76]. Therefore, the 49 determined F-Cat dispersion in the present study is reasonable, considering the relatively similar catalyst preparation method and temperature treatment. Nevertheless, two times lower loading of the precious metals are used in the present study, therefore, exact comparison is not relevant.

8.5. SEM-EDS Results

SEM imaging gives a general idea of washcoat morphology and precious metal distribution. Washcoat morphology remains unchanged for the fresh and poisoned catalyst samples. Figure 16 shows a SEM image of the fresh catalyst with 4.5 x 104 magnification. Bright dots indicate evenly distributed precious metal sites.

Figure 16. F-Cat SEM image with 100 nm resolution.

The loading of poisons is determined for P-Cat, Na-Cat and Ca-Cat in resolution of 20 μm by using 12 EDS spot measurements. Every spot measurement and average values are listed in Table 14. Furthermore, Figures 17 - 19 show SEM images and the overall spectrum (Spectrum 1) of the spot measurement grid. The spot measurement is marked as (x,y). X corresponds to column and y to row in the grid starting from the left upper corner. As an example, spot (1,1) is marked as red square in Figure 17 and spot (4,3) as blue square. It can be concluded that poisonous species are uniformly distributed over the washcoat. Moreover, mean values from the EDS spot measurements are in close agreement with the elemental composition determined by ICP-OES (see Paragraph 8.1.).

Table 14 Poisonous species loading according to EDS spot measurements P-Cat Na-Cat Ca-Cat Spot Phosphorus, wt.% Sodium, wt.% Calcium, wt.% Spectrum 1 1.11 0.58 1.11 (1,1) 1.38 0.44 1.13 (2,1) 1.4 0.39 1.08

Table 14 (continuation)

Spot Phosphorus, wt.% Sodium, wt.% Calcium, wt.% (3,1) 1.09 0.47 1.09 (4,1) 1.27 0.61 1.04 (1,2) 1.39 0.51 1.43 (2,2) 1.28 0.43 1.22 (3,2) 1.12 0.64 1.5 (4,2) 1.13 0.66 1.41 (1,3) 0.93 0.65 0.88 (2,3) 1.17 0.66 0.64 (3,3) 0.97 0.75 0.62 (4,3) 1.28 0.75 0.75 Mean, wt.% 1.19 0.58 1.07 STDEV 0.15 0.12 0.29

Figure 17. P-Cat SEM EDS spot measurement grid SEM image (left) and EDS spectrum (right).

Figure 18. Na-Cat EDS spot measurement grid SEM image (left) and EDS spectrum (right)

Figure 19. Ca-Cat EDS spot measurement grid SEM image (left) and EDS spectrum (right)

Na-Cat EDS mappings

SEM-EDS mappings was performed to confirm even distribution of Na, because during TEM studies (see Paragraph 8.6.) it was challenging to observe Na. EDS mapping of sodium is presented in Figure 20.b with the corresponding SEM image in Figure 20.a. Bright dots in Figure 20.b represent elemental sodium measured by EDS. According to the mapping sodium is evenly distributed over the washcoat. The bright spot in Figure 20.a is a Pd-Pt cluster with precious metal ratio 3:1. According to EDS measurements, PdO is formed. EDS mapping of the precious metals in the Na-Cat is shown in Figure 21. A large precious metal cluster is shown in Figure 21.a and its elemental mapping in Figure 21.b. The blue area in the centre indicates Pd and the red spots all over the sample represent Pt. Therefore, a Pd rich core with Pt segregated on the surface is detected. Lance et al. [50] have concluded that Na was evenly distributed throughout the alumina washcoat with no segregation on the Pt active sites during engine-aging. Therefore, they concluded that Na did not poison to a great extent the DOC. The elemental mappings draw the same conclusion in the present study.

a b

Figure 20. Sodium mapping in the Na-Cat.

a b

Figure 21. The precious metal elemental mapping in the Na-Cat.

8.6. TEM-EDS Results

F-Cat

TEM-EDS imaging indicates a crystalline γ-alumina washcoat with very well dispersed precious metal particles. Two types of precious metal sites are observed for the fresh catalyst sample. First, small (1 to 2 nm) Pt particles that are evenly distributed all over the crystalline alumina washcoat and agglomerates in the size of 4 - 5 nm (marked green in Figure 22.b). Second, large (around 10 to 20 nm in size) clusters consisting of Pt and Pd in a ratio close to 1:1 (marked red in Figure 22 a and b).

a b

Figure 22. The fresh catalyst (F-Cat) precious metal morphology

Therefore, TEM studies indicate that Pt and Pd, when synthesised in a ratio 3:1, form bimetallic clusters with a Pt:Pd ratio close to 1:1. The rest of the Pt deposits as small metallic particles. This trend is in close agreement with Hazlett et al. [23]. They observed formation of Pt-Pd particles in a ratio 1:1 after synthesis of Pt:Pd in a ratio 1:3. The rest of the Pd were deposited as small PdO particles. On the other hand, a synthesis of Pt:Pd in a ratio of 3:1 resulted in bimetallic particle formation with a Pt:Pd ratio 3:2. They also observed particles in the size of 2 to 5 nm together with large agglomerates for Pt-Pd bimetallic catalysts. Bimetallic particle composition of ratio 1:1 has been reported in the study by Gremminger et al. [77] where they synthesised catalysts with a Pt:Pd ratio of 1:5. They found small active phase nanoparticles in the size of 5 nm along with large Pt-Pd clusters (50 nm) with a precious metal ratio 1:1. On the other hand, Martin et al. [76] determined only monometallic Pt and Pd with a particle size around 5 nm for PtPd (5:1) catalysts. Furthermore, Honkanen et al. [75] in TEM studies observed PdO oxide formation in Pt rich bimetallic PtPd catalysts (4:1). Nevertheless, they observed the size of bimetallic particles smaller than 5 nm .

P-Cat

Phosphorus is mostly localized on Pd rich sites forming large (marked red in Figure 23.b) clusters. According to EDS measurements these particles consist of Pd and Pt in a ratio 5:1 and P content is 0.9 wt.%. On the other hand, phosphorus is also incorporated in Pt-Pd- P clusters where the Pt:Pd ratio is 1:1 and the P content approximately 1.4 wt.% (marked red in Figure 23.a). Nevertheless, small Pt particles (1 – 2 nm particles and agglomerates up to 54

5 nm) and larger Pt-Pd clusters as well as pure alumina washcoat are observed in the same size and morphology as for the F-Cat sample. Phosphorus interaction with small Pt particles is not confirmed by EDS analysis of Pt particle spots (marked green in Figure 23.a). This spot measurement indicates P (around 1 wt.%) in the bulk washcoat with no segregation on Pt particles. Other spot measurements of bulk washcoat indicated phosphorus, but not in large amounts. Therefore, it suggests possible formation of AlPO4. Therefore, it can be concluded that phosphorus is not uniformly distributed over the alumina washcoat. This observation is not in agreement with SEM-EDS studies that indicated evenly distributed phosphorus. Nevertheless, SEM-EDS spot measurements were performed at much lower resolution.

a b

Figure 23. The precious metal morphology of the P-Cat.

According to previous studies slightly larger particles have been observed for phosphorus poisoned Pt catalysts in laboratory scale [75, 78] and for field scale poisoned DOCs [59]. Furthermore, it has been concluded from TEM that phosphorus presence in the catalyst formulation smoothens edges of cuboctahedron Pt particles resulting in more spherically shaped active phase. These morphology changes have been reported responsible for the loss in catalyst activity [59, 75, 78]. Phosphorus content in the catalyst samples was 7.5 wt.% [78] and 1.7 wt.% [75] as determined by ICP-OES. It is not possible to observe the previously described P effect in the present study. The author suggests that it is due to a lower loading of P.

Na-Cat

Amorphous alumina washcoat structures are observed for the sodium poisoned catalyst. The precious metal morphology is observed with the same crystalline sizes and composition as for the F-Cat sample. Due to low loadings it was hard to determine Na distribution by EDS measurements. Therefore, a SEM-EDS mappings at higher resolution than SEM-EDS spot measurements were performed for the Na-Cat sample (see Paragraph 8.5.) to observe the Na distribution.

Ca-Cat

TEM-EDS studies reveal evenly distributed Ca over the washcoat (see Figure 24). Nevertheless, it is also deposited together with Pt and Pd metallic particles. Some of the imaging indicated small black dots that according to the EDS measurements were metallic Ca particles. Nevertheless, the overall precious metal morphology is the same as for the fresh sample.

Black dots – probability of Ca metallic particles

Pt-Pd (1:1) clusters with deposited Ca

Site of Pt and Ca with no sign of Pd

Pt particles, Pt-Pd clusters and Ca

Figure 24. Calcium and active phase morphology in the Ca-Cat sample.

8.7. Temperature programmed reduction (TPR)

Poisoned γ-alumina TPR

Temperature programmed reduction profiles for the fresh and poisoned alumina samples as well as for F-Cat are shown in Figure 25. Reduction of the support over the entire temperature range can be observed for F-Al. Two remarkable peaks are in the temperature ranges 290 °C – 390 °C and 590 °C - 690 °C . The fresh catalyst reduction profile is very similar to the F-Al profile, but the peaks are even less pronounced. This can suggest overlapping of the alumina reduction peaks with the peaks responsible for bimetallic active phase reduction around 300 °C [28, 79 - 81]. A noticeable difference from the alumina profile is PdHx formation peak located at 67 °C. According to previous studies, PdO is easily reduced to metallic Pd already at room temperatures. At 67 °C Pd reacts with H2 and PdHx formation is observed from TPR profiles [25, 28, 70]. P-Al and Na-Al reduction behaviour is similar to the one of the fresh alumina sample. This indicates no interactions between Phosphorus, sodium and the alumina support. Nevertheless, interactions between Ca and alumina cause significant profile transformations. The reduction peaks are more pronounced for Ca-Al as compared to F-Al.

TPR of poisoned alumina and fresh catalyst TCD signal, a.u. signal, TCD

0 100 200 300 400 500 600 700 800

Temperature, °C F-Al Na-Al Ca-Al P-Al F-Cat Figure 25. TPR profiles for poisoned alumina samples.

Fresh and poisoned catalyst TPR

Figure 26 presents temperature programmed reduction profiles for the fresh and poisoned catalyst samples. It can be observed that the intensity of PdHx formation peak at 67 °C decreases when poisonous species are present. This is not in agreement with the CO chemisorption results and literature where decreased active phase dispersion was explained by sharpening of the PdHx peak [28, 67]. The intense peak at 290 °C – 390 °C in Ca-Al is shifted to lower temperatures for the Ca-Cat sample and the maximum is more pronounced at 290 °C. The same trend can be observed for Na-Cat and P-Cat. For the Na-Cat sample the maximum is more pronounced at around 310 °C. The maximum for P-Cat is at the lowest temperature (280 °C) as compared to Na-Cat, Ca-Cat and F-Cat. Nevertheless, for the fresh catalyst the peak maximum is around 320 °C.

TPR of the fresh and poisoned catalyst samples TCD signal, a.u. signal, TCD

0 100 200 300 400 500 600 700 800 Temperature, °C Ca-Cat Na-Cat P-Cat F-Cat Figure 26. TPR profiles for the fresh and poisoned catalyst samples

According to previous studies, the peak around 300 °C (and in some studies also around 460 °C [66]) is a result from two reduction events. Some studies have suggested reduction of PdO species that were strongly bonded to alumina support [72, 70]. Others indicated reduction of Pd particles along with the co-impregnated Pt particles [28, 79 - 81]. On the other hand, there are speculations that above 350 °C no metal particle reduction occurs and the TCD signal increases only due to support reduction [74]. Nevertheless, the

58 authors of previous studies agree that peaks above 550 °C - 600 °C are only bulk alumina reduction [68, 82]. The author of this study speculates that the intensification of the peak around 300 °C for the poisoned catalyst samples is due to modifications in the precious metal – support interactions. These modifications might be induced by the catalyst poisoning method. Weaker interactions between the alumina support and bimetallic Pt-Pd particles would result in an easier reduction process at lower temperatures. As a consequence, the TCD signal is increased at lower temperatures. This might explain the higher activity of P-Cat and Na-Cat as compared to the F-Cat in the oxidation reactions (see Paragraph 9).

8.8. Powder X-Ray Diffraction

Figure 27 shows the F-Al XRD pattern. All the peaks in 2θ range 20 to 90 ° are corresponding to crystalline γ-alumina. Sharp peaks at 2θ 45.5° and 67.1° have been previously attributed as typical gamma phase indicators [83]. The XRD peak at 2θ 14° is recognised as aluminium silicate that might be present in the alumina powder as impurity.

Figure 27. XRD pattern for fresh γ-alumina powder F-Al.

The fresh alumina XRD pattern is very similar to the fresh catalyst (see Figure 28) and poisoned catalyst XRD patterns (presented in Appendix 5).

Figure 28. XRD pattern for the F-Cat. Due to low loadings and high dispersion of the precious metals no characteristic peaks were observed for the active phase. Therefore, the F-Al pattern was normalised and subtracted from the normalised F-Cat, P-Cat, Na-Cat and Ca-Cat XRD patterns in order to investigate Pt and Pd phases. The resulting XRD patterns are shown in Figure 29. The PdO peak at 2θ 34° is obvious for all of the samples. The presence of Pd in the form of PdO is confirmed by its reduction peak at 67 °C in the TPR studies. Moreover, palladium oxide form was detected at 2θ 34° for bimetallic Pt-Pd catalyst formulations in other studies [84, 85]. Other minor differences from the pure support pattern can be observed at 2θ 47° and 68°. The author of the present study speculates that these are not single peaks, but they represent bimetallic Pt and Pd alloys highly dispersed on the washcoat. This speculation is based on the overlapping of γ-alumina and metallic Pt, and Pd peaks in the XRD patterns at these 2θ values. Due to the fact that Pt and Pd peaks are slightly shifted to higher 2θ values, support subtraction from the catalyst samples induce negative intensities in the XRD pattern. Moreover, Castillo et al. [30] have reported the presence of bimetallic Pt-Pd alloys at the same 2θ values as in the present study. They investigated PtPd/Al2O3 catalysts with a precious metal loading of 1 wt.%.

Figure 29. Fresh and poisoned catalyst XRD patterns.

9. CATALYST ACTIVITY TESTING

9.1. CO, NO and C3H6 oxidation reaction competitiveness

The F-Cat light-off curves for NO-Single, CO-Single, C3H6-Single and Combo experiments are presented in Figure 30. They are used to confirm CO, NO, and C3H6 competitiveness for the active sites when gases are present simultaneously. Competitiveness has been largely reported in the literature [11, 40, 42, 77, 86 - 88].

CO-F-Combo

CO-F-Single C3H6-F-Combo

C3H6-F-Single

NO-F-Single

NO-F-Combo

Figure 30. F-Cat light-off curves for CO-F-Combo, CO-F-Single, C3H6-F-Combo, C3H6- F-Single, NO-F-Combo and NO-F-Single reactions.

Reactions over the F-Cat start with CO oxidation, followed by propylene and NO oxidation. CO oxidation in CO-F-Single reaction ignites at very low temperatures and reaches 10 % conversion at 79 °C (see Figure 31). Due to competitiveness for the active sites during the Combo reaction, 10 % conversion in CO-F-Combo reaction is reached at 97 °C. The subsequent propylene oxidation reaches 10 % conversion at 108 °C and 163 °C in C3H6- F-Single and C3H6-F-Combo reactions, respectively. Moreover, 50 ppm lower concentration of CO and C3H6 is used in Combo experiment. It strengthens the inhibition effect, because at lower concentrations light-off should be earlier as compared to higher concentrations. At last, 10 % conversion of NO is reached at 190 °C and 242 °C in NO-F- Single and NO-F-Combo reactions, respectively. Nevertheless, higher NO concentrations are used in Single experiment and, therefore, inhibition should be somewhat affected also by this factor.

The difference between light-off temperatures in the Single and Combo experiments increases at higher conversions. This observation is valid for both CO and C3H6 oxidation reactions. Moreover, propylene oxidation fails to reach 99 % conversion during Combo experiments. A maximum conversion of 47 % and 28 % is reached at 380 °C for NO in NO- F-Single and NO-F-Combo experiments, respectively. It indicates an almost 50 % decrease of the maximum NO conversion due to inhibition of CO and C3H6.

Δ84 Δ91 Δ79

Δ72 Δ45 Δ65 Δ39 Δ55 Δ34 Δ25 Δ18

Figure 31. F-Cat light-off temperatures for CO and C3H6 oxidation reactions in Single and Combo reactions. Numbers in bars show exact light-off temperatures; numbers above

Combo reaction bars show difference between the light-off temperatures for CO and C3H6 oxidation reaction in Single and Combo experiments.

Fresh DOC light-off performance has been reported to be dependent on the precious metal loadings, Pt:Pd ratios and inlet gas concentrations. Previous studies have also argued loss in the oxidation activity with increasing inlet concentrations of NO, CO and

C3H6 [87 - 90]. Therefore, direct comparison between exact light-off temperatures in the present study and in previous studies is not reasonable due to the use of different catalyst formulations, preparation techniques, inlet gas concentrations. The same competitiveness trend was, as expected, observed for all oxidation reactions in Combo, Combo-T, Combo-SO2 experiments with the fresh and poisoned catalysts. Therefore, the subsequent paragraphs focus only on particular oxidation reactions on fresh and poisoned catalysts.

9.2. NO oxidation reaction

Combo experiments

NO oxidation, independent on the poisonous species, ignites at temperatures slightly above 150 °C (see Figure 32). The maximum conversion is 28 %, 33 %, 30% and 27% for F-Cat, P-Cat, Na-Cat and Ca-Cat, respectively. It is reached around 380 °C with all catalysts. Further temperature increase leads to thermodynamically favoured reverse reaction and NO conversion decreases.

Figure 32. NO light-off curves for the fresh and poisoned catalyst Combo experiments.

It can be observed that the fresh catalyst poisoning with P and Na slightly improves oxidation performance. The maximum conversion is increased by 5 % and 2 % for P-Cat and Na-Cat, respectively. On the other hand, Ca improves oxidation at lower temperatures, but at higher temperatures the performance is slightly lower. Previous studies have suggested that NO oxidation can be promoted by larger Pt particles. It has been explained by the fact that small Pt particles are easily oxidised. As a consequence, lower dispersion of the active phase promotes NO oxidation activity [91 - 93]. Improved NO oxidation activity after Na poisoning has also been reported in previous studies [50, 51]. Lance et al. [50] have discussed an optimum dispersion of Pt for NO oxidation. They speculated that lower dispersion increases oxidation activity until there are too few active sites for a successful reaction and the activity decreases.

On the contrary, Cavataio et al. [10] have observed a decrease in maximum NO2 by 50 % after Na poisoning by the wet impregnation method (1 wt.%). Nevertheless, they have

64 not reported whether the Na loading corresponds to the whole monolith or only to the washcoat. Moreover, comparison between the active phase dispersion for fresh and poisoned catalysts was not present in their study. Nevertheless, no effect of Na sodium on NO oxidation has also been reported. An engine-bench with Pd-rich DOC study observed no changes in NO2 formation after fuel doping with 14 ppm Na [52]. Several studies have indicated serious deactivation of NO oxidation due to P poisoning. Nevertheless, usually much higher phosphorus loadings were used, that resulted in a significant decrease in the specific surface area [31, 41, 48, 61]. At last, the author of the present study suggests that the active phase dispersion for P-Cat, Na-Cat and Ca-Cat is low enough to improve the NO oxidation as compared to the F-Cat. Moreover, these results are in agreement with speculations from TPR studies (see Paragraph 8.7.). This statement is valid for NO oxidation in both Single and Combo-T experiments.

Single experiments

Due to the absence of CO and C3H6 in the inlet gas stream NO light-off temperatures are shifted to lower temperatures as compared to the Combo experiment (see Figure 33). Furthermore, much higher conversions are reached over all catalysts. NO oxidation reaction starts at temperatures slightly above 100 °C. It reaches its maximum conversion of 50 % (380 °C), 54 % (350 °C), 50 % (370 °C) and 47 % (380 °C) for F-Cat, P-Cat, Na-Cat and Ca-Cat, respectively. Poisonous species show the same effect on NO oxidation activity as in Combo experiment. P-Cat shows the highest NO conversion over the whole temperature range. Meanwhile Na-Cat and Ca-Cat exhibit better performance for conversions below 40 % as compared to the F-Cat. Conversions above 40 % are reached approximately at the same temperatures for F-Cat, Na-Cat and Ca-Cat.

Figure 33. NO light-off curves for the fresh and poisoned catalyst Single experiments.

Combo-T experiments

The fresh and poisoned catalyst NO oxidation activity is improved after the temperature treatment as compared to the Combo experiment (see Table 15). The F-Cat, P- Cat, Na-Cat and Ca-Cat T25 are shifted to 52 °C, 19 °C, 34 °C and 52 °C lower temperatures as compared to the same catalysts in NO-Combo experiment.

Table 15

NO light-off temperatures T10 and T25 in Combo and Combo-T experiments

F-Cat P-Cat Na-Cat Ca-Cat

T T T T

- -

- - - -

- -

F P

Ca Ca

Na Na Light-off - -

Combo Combo

- -

temperature,°C F P

NO NO

- -

Combo Combo

NO NO

Combo Combo Combo Combo

NO NO T10 242 223 220 213 232 213 232 226 T25 349 297 297 278 325 291 349 297 ΔT10 -19 -7 -19 -6 ΔT25 -52 -19 -34 -52 ΔT10 (25) - Difference between T10 and T25 values in NO-Combo and NO-Combo-T experiments

A maximum conversion of 37 % over P-Cat, 33% over F-Cat and Na-Cat and 31 % over Ca-Cat is reached at temperatures around 370 °C (see Figure 34). The maximum NO conversions reached in Combo-T experiments are higher than in Combo experiments.

Figure 34. NO light-off curves for the fresh and poisoned catalyst Combo-T experiments.

Relatively lower poisonous species effect can be observed from the curves as compared to the first Combo experiment. Up to 15 % conversion all curves are within 10 °C difference. On the other hand, a positive P effect is obvious above 20 % conversion. The difference between F-Cat and P - Cat T25 is almost 20 °C. Improved oxidation activity after the temperature treatment of catalysts suggest Pt and Pd particle sintering and decrease in dispersion of the active phase. Moreover, sintering due to temperature treatment has been observed in numerous studies [9, 58 – 60, 94]. Therefore, it can be hypothesised that after the temperature treatment the active phase dispersion was low enough to improve NO oxidation performance in the present study.

Combo-SO2 experiments

Figure 35 shows NO light-off curves from Combo-SO2 experiments. Light-off curve for NO-F-Combo experiment is added to show the activity decay as compared to the fresh catalyst. Light-off curves for all catalysts are shifted to higher temperatures. Reaction ignites at 220 °C and it is around 70 °C higher than that for the fresh catalyst in the Combo experiment. Furthermore, the maximum NO conversion is less than 20 % for all catalysts. Therefore, it is obvious that sulphur poisoning has greater impact on the DOC activity as compared to the P, Na and Ca at the studied poisoned levels. Nevertheless, the catalyst deactivation does not correlate with the sulphur loadings in the samples (Paragraph 8.1).

This indicates interactions between P, Na, Ca and SO2 that are further explained in the following paragraphs.

Figure 35. NO light-off curves for the fresh and poisoned catalyst in Combo-SO2.

Oxidation over the F-Cat, P-Cat and Na-Cat exhibit very similar behaviour with T10 and T15 differences of less than 10 °C (see Table 16). Nevertheless, the fresh catalyst shows the best performance and Ca-Cat experiences the greatest loss in the NO oxidation performance. There is an increase of T10 and T15 of around 30 - 40 °C when compared to the other catalysts. This is not in agreement with Kolli et al. [49] who reported Ca as inhibitor of sulphur poisoning. Table 16 Comparison between NO light-off temperatures T10 and T15 in Combo and

Combo-SO2 experiments F-Cat P-Cat Na-Cat Ca-Cat

(

SO2

)] )]

SO2 )]

–

SO2 SO2

SO2

Combo

Combo Light-off -

- -

Combo Combo

- -

Combo

F P temperature,°C Combo

Combo

Combo)

- -

Combo)

- Combo)

NO NO

Combo

Combo Combo

T[(F

ΔT[(F

T10 242 295 220 300 58 232 300 58 232 335 93 T15 270 335 245 345 75 261 345 75 265 383 113 ΔT10 53 80 68 103 ΔT15 65 100 84 118 ΔT10, ΔT15 - Difference between T10, T15 values in Combo and Combo-SO2 experiments

ΔT[(F-Combo) – (P, Na, Ca) -Combo-SO2)] – Difference between light-off temperatures in NO-F-Combo experiment and NO-P, Na, Ca-Combo-SO2 experiments

According to the data presented in Table 16 (ΔT[(F-Combo) – (P, Na, Ca-Combo- SO2)] columns) sulphur poisoning of the fresh catalyst has similar effect on NO oxidation as combined P+SO2 and Na+SO2 effect. Nevertheless, the highest S loading is for Na-Cat, but the lowest for P-Cat. F-Cat, P-Cat and Na-Cat light-off temperatures for 10 % and 15 % conversion are shifted to 53 °C – 58 °C and 65 °C – 75 °C higher temperatures, respectively.

On the other hand, poisoning with Ca+SO2 shows the most considerable deactivation as compared to the fresh catalyst in NO-F-Combo experiment. The light-off temperature increases by 93 °C and 113 °C for 10 % and 15 % conversion, respectively. Poisonous species interactions can be observed when ΔT10, ΔT15 data for each catalyst are compared to ΔT[(F-Combo) – (P, Na, Ca) -Combo-SO2)] values (see Table 16). The F-Cat shows the lowest increase in the light-off temperatures T10 and T15 as compared to the first NO-F-Combo experiment. As a consequence, it experiences the lowest deactivation due to sulphur poisoning. Na-Cat is influenced more than F-Cat, resulting in 84 °C higher light-off temperature for 15 % conversion as compared to the NO-Na-Combo. The strongest interactions are observed for P-Cat and Ca-Cat when 15 % conversion is reached at more than 100 °C higher temperatures as compared to NO-P-Combo and NO-Ca-

Combo. Therefore, it can be speculated that phosphorus and calcium interact with SO2, resulting in higher deactivation as compared to the fresh and sodium poisoned catalysts. As previously mentioned in the literature review, sulphur poisoning results in significant activity decay. Li et al. [64] performed DOC treatment with 5 ppm SO2 at 200 °C for 1 hour. It resulted in a decrease of the maximum NO conversion by 30 % and lower conversions were reached at 50 °C higher temperatures. They used and engine-aged DOC with Pt:Pd=5:1 and a total precious metal loading of 28 g/ft3.

9.3. CO oxidation reaction

Combo experiments

CO oxidation starts immediately after the start of the temperature ramp. 50 % conversion with all catalysts is reached in the temperature range 123 °C – 141 °C (see Figure 36). T50 is reached at 134 °C, 123 °C, 120 °C, and 141 °C for F-Cat, P-Cat, Na-Cat and Ca- Cat, respectively. Up to 60 % conversion the Ca-Cat shows the lowest and P-Cat the highest CO oxidation performance. Above 60 % F-Cat, P-Cat and Ca-Cat oxidation performance is similar and the sodium poisoned sample has the highest activity. Nevertheless, there is less

69 than 30 °C difference between all 4 curves. Therefore, CO oxidation seems to be largely unalterable (or even improved) at high conversions with P-Cat, Na-Cat and Ca-Cat.

Figure 36. CO light-off curves for the fresh and poisoned catalyst Combo experiments.

P, Na and Ca impact on DOC CO oxidation has been controversially reported in the literature. An engine-bench study of a Pd-rich DOC has claimed sodium deactivation effect on T80 and T90 at the outlet when the fuel was doped with 14 ppm Na [52]. Furthermore,

Cavataio et al. [10] showed a T50 increase of around 50 °C when PtPd/Al2O3 catalyst was impregnated with Na. The deactivation was more pronounced at higher conversions and higher Na loadings. Nevertheless, they used much higher CO concentrations (2500 ppm) and doubled Na loadings in their experiments as compared to the present study. This could explain the no deactivation effect due to Na poisoning in the present study. No effect of calcium on CO oxidation has been reported after gas-phase aging by Kolli et al. [49] and engine bench aging by Williams et al. [52]. Though, Kolli et al. [49] tested very low Ca loadings (less than 0.1 wt.%). More than 10 times higher Ca loadings are used in the present study and as a consequence a loss in performance at low conversions could be observed. Phosphorus poisoning in the gas phase (1.7 wt.%) has showed slightly decreased CO oxidation activity for T50 (8 °C decrease of light-off temperature) and only 3 °C increase of T90 [41]. This confirms the findings in the present study.

Single experiments As presented in Figure 37, CO oxidation ignites at very low temperatures. It reaches

50 % conversion around 100 °C with all catalysts. Absence of NO and C3H6 allows shifting of light-off curves to even lower temperatures as compared to the curves in Figure 36 for the CO-Combo experiments.

Figure 37. CO light-off curves for the fresh and poisoned catalyst Single experiments.

Due to the strong fluctuations of conversion at low temperatures it is hard to indicate exact light-off temperatures, especially for poisoned catalysts. Nevertheless, it is clear from Figure 37, that the oxidation reaction over Na-Cat shows the best performance at high conversions. High conversions for poisoned catalysts are reached at approximately the same temperatures. For example, 97 % conversion over P-Cat, Na-Cat and Ca-Cat is reached at around 175 °C. The light-off over the F-Cat is shifted to higher temperatures as compared to the poisoned catalysts at high conversions (above 60 %). 97 % conversion is reached at 200 °C. An oscillatory behaviour of the CO oxidation reaction has been observed in some Pt and Pd oxidation catalyst studies [41, 88, 95, 96]. Usually it has been observed in light-off curves, but not explained in detail [41, 88]. Nevertheless, Lynch et al. [95, 96] explained it by changes in the reaction rate controlling step. They reported that the rate controlling step changes from oxygen adsorption to surface reaction that is followed by a violent reaction between both gases. Therefore, these processes induced oscillations in CO conversion around 20 % at constant temperatures.

Moreover, the CO oxidation reaction has been characterized as structure sensitive and dependent on the physical properties on the catalyst surface and the active sites [7]. McCarthy et al. [97] have observed that CO oxidation rate was higher for larger Pt particles at low CO conversions. It can be speculated that lower dispersion of Na-Cat, Ca-Cat and P-Cat as compared to F-Cat and interactions between poisonous species and active sites (as observed by TPR) in the present study improve CO oxidation activity over the whole conversion range. The effect is more pronounced at higher conversions.

Combo-T experiments

Figure 38 and Table 17 represent Combo-T experiment light-off curves and temperatures for the fresh and poisoned catalysts.

Figure 38. CO light-off curves for the fresh and poisoned catalyst Combo-T experiments.

Combo-T experiments for CO oxidation reveal improved activity for Ca and Na poisoned samples. Light-off shifts to approximately 10 °C lower temperatures as compared to the F-Cat, Na-Cat and Ca-Cat in the respective Combo-T experiments. On the other hand, P-Cat activity in the Combo-T experiment is significantly decreased as compared to the CO- P-Combo. Deactivation of the P-Cat is more pronounced at low temperatures (T10 is shifted to 36 °C higher temperature, but T90 only to 16 °C higher temperature). Nevertheless, at conversions higher than 60 % also P-Cat shows better CO oxidation performance than CO- F-Combo-T.

These observations indicate that the active phase morphology has changed after the temperature treatment. Causing an improve in activity for Na-Cat and Ca-Cat and a loss in activity for F-Cat and P-Cat. Table 17 Comparison between CO light-off temperatures in Combo and Combo-T experiments

F-Cat P-Cat Na-Cat Ca-Cat

T T

- -

Combo Light-off Combo

Combo Combo

Combo

- -

Combo Combo

- -

temperature,°C F P

- -

F P

- -

CO CO

T10 97 103 76 112 91 82 104 95 T30 117 124 107 132 107 109 125 107 T50 134 143 123 143 120 119 141 130 T80 181 197 171 190 156 144 178 168 T90 214 225 200 216 180 170 200 193 ΔT10 6 36 -9 -9 ΔT30 7 25 2 -18 ΔT50 9 20 -1 -11 ΔT80 16 19 -12 -10 ΔT90 11 16 -10 -7 ΔT10 (30,50,80,90) - Difference between T10,30,50,80 and T90 values in CO- Combo and CO-Combo-T experiments

Combo-SO2 experiments

The fresh and poisoned catalyst CO light-off performance after sulphur treatment is presented in Figure 39.

Figure 39. CO light-off curves for the fresh and poisoned catalyst Combo-SO2 experiments. The same deactivation trend as for NO oxidation can be observed. F-Cat, Na-Cat and P-Cat light-off curves are very close to each other within the entire temperature range. P-Cat shows slightly better light off performance up to 70 % conversion. T50 for P-Cat is 5 °C lower as compared to F-Cat and Na-Cat. On the other hand, Na-Cat reaches 80 % and 90 % conversion at 13 °C and 25 °C lower temperatures, respectively, than F-Cat and P-Cat. Moreover, Ca-Cat shows a severe deactivation. The light-off curve shifts to 20 °C - 40 °C higher temperatures as compared to the fresh catalyst in CO-F-Combo-SO2. Nevertheless, results do not correlate with the sulphur loadings in samples, indicating interactions between poisons.

Table 18 is used to analyse interactions of poisonous species. The same conclusions as for NO oxidation reaction can be drawn. Combined P+SO2 and Na+SO2 effect is similar to sulphur poisoned F-Cat. It results in 67 °C higher T50 for F-Cat and Na-Cat and 61 °C higher T50 for P-Cat as compared to CO-F-Combo. On the other hand, Ca+SO2 deactivation effect is more pronounced which indicates, that calcium and sulphur interactions may occur. It results in 91 °C higher T50 as compared to CO-F-Combo.

Table 18

Comparison between CO light-off temperatures Combo and Combo-SO2 experiments F-Cat P-Cat Na-Cat Ca-Cat

(

SO2

–

)] )] )]

SO2 SO2

–

2 2 2

- -

SO SO SO

- - -

Combo Light-off Combo

Combo Combo

- -

Combo

temperature,°C F P

Combo Combo

Combo)

- -

Combo)

F P

- -

CO CO Combo Combo Combo

T[(F

CO CO

ΔT[(F

Δ T10 97 158 76 149 52 91 158 61 104 169 72 T30 117 176 107 174 57 107 179 62 125 197 80 T50 134 201 123 195 61 120 201 67 141 225 91 T80 181 253 171 253 72 156 240 59 178 291 110 T90 214 284 200 286 72 180 265 51 200 319 105 ΔT10 61 73 67 65 ΔT30 59 67 72 72 ΔT50 67 72 81 84 ΔT80 72 82 84 113 ΔT90 70 86 85 119 ΔT10(30,50,80,90)- Difference between T10,30,50,80 and T90 values in Combo and Combo-SO2 experiments

ΔT[(F-Combo) – (P (Na, Ca) -Combo-SO2)] – Difference between light-off temperatures in CO-F-Combo experiment and CO-P,Na,Ca-Combo-SO2 experiments

On the other hand, Table 18 also indicates that F-Cat is less influenced by SO2 treatment than other catalysts in their Combo experiments. T90 is reached at 70 °C higher temperatures for F-Cat as compared to CO-F-Combo. Nevertheless, 90 % conversion over P-Cat and Na-Cat is reached at more than 85° C higher temperatures than in CO-P-Combo and Co-Na-Combo, respectively. Activity of the Ca poisoned catalyst is influenced the most. T90 is reached at 119 °C higher temperatures as compared to CO-Ca-Combo. Deactivation of CO oxidation due to sulphur poisoning has been reported quite straightforward in previous studies. Kärkkäinen et al. [14] have reported a 32 °C increase of

T50 for CO oxidation over PtPd/Al2O3 when the sulphur content was 1.9 wt.%. Kolli et al. [49] have observed around 20 °C increase of T50 for CO oxidation over a 4 wt.%

Pt/Al2O3 catalyst with a sulphur content of 1.6 wt.%. Nevertheless, they have also shown Ca inhibition of the sulphur poisoning which is not in agreement with the results in the present study.

9.4. C3H6 oxidation reaction

Combo experiments

Figure 40 shows that P, Na and Ca does not considerably affect the DOC C3H6 oxidation performance. Propylene oxidation reaction ignites slightly before 150 °C and reaches approximately 97 % conversion at 370 °C with all catalysts. Nevertheless, approximately 10 °C shift to lower temperatures can be observed for P-Cat in the conversion range from 10 to 80 %.

Figure 40. C3H6 light-off curves for the fresh and poisoned catalyst Combo experiments.

The effect of inorganic impurities on the hydrocarbon oxidation has been studied by different aging methods. Calcium has been reported with no effect on C3H6 oxidation after laboratory scale gas-phase aging [49] and engine bench aging [52]. Brookshear et al. [51] have reported that Na has negligible effect on ethylene conversion after engine-bench aging with fuel-doped Na (5000 ppm). Other engine-bench aging study used 14 ppm of Na in the fuel and observed deactivation only for 90 % conversion [52]. The former study used Pt-rich DOC [51] and the latter - Pd-rich DOC [52]. Further, Cavataio et al. [10] have indicated a T50 increase of 50 °C for PtPd DOC after poisoning with Na (1 wt.%) by wet impregnation. Thus, activity testing results for Na poisoned catalysts have been shown to be dependent on catalyst formulation, an aging method and Na loadings. Nevertheless, phosphorus poisoning

(1.7wt.%) has shown a slight increase of T50 and 50 °C increase of T90 for C3H6 oxidation. Further activity decay was observed for higher P loadings [41]. Results in the present study indicate that P and Na loadings are too low to observe any deactivation effect. With low poisonous species loadings there might be still enough 76 sites for oxidation. Thus, the oxidation behaviour with low propylene concentrations remains unchanged with low loadings of P, Na and Ca. Furthermore, the author of the present study speculates that the deactivation effect in the former studies might have been observed due to the use of higher (1500-1800 ppm [10], 500 ppm [61], 300 ppm [41, 51]) hydrocarbon inlet concentrations. It has been reported that higher concentrations shift the light-off to higher temperatures [23]. More active sites are required to oxidise higher hydrocarbon concentrations.

Single experiments

P, Na and Ca show the same impact on the fresh catalyst activity in propylene oxidation in Single and Combo experiments (see Figure 41).

Figure 41. C3H6 light-off curves for the fresh and poisoned catalyst Single experiments. Propylene light-off over all catalysts is shifted to lower temperatures (around 50 °C decrease) due to the no inhibition induced by NO and CO presence. P-Cat in Single experiments shows slightly better light-off characteristics starting from 20 % conversion.

Combo-T experiment

Temperature treatment has very low impact on the catalyst light-off performance (see Figure 42).

Figure 42. C3H6 light-off curves for the fresh and poisoned catalyst Combo-T experiments.

The light-off curves of F-Cat, P-Cat, Na-Cat and Ca-cat are overlapping and no significant difference between fresh and poisoned catalysts can be observed. The differences between catalysts in the Combo and Combo-T experiments are negligible (see Table 19) (less than 10 °C). Therefore, it can be concluded that the temperature treatment of the fresh and poisoned catalysts has no impact on propylene oxidation.

Table 19

Comparison between C3H6 light-off temperatures in Combo and Combo-T experiments

F-Cat P-Cat Na-Cat Ca-Cat

- -

- - - -

T T T T

- - - -

F F P P

Light-off Ca Ca

Na Na

- - - -

- - temperature, - -

°C Combo Combo Combo Combo

C3H6 C3H6 C3H6 C3H6

Combo Combo Combo Combo

C3H6 C3H6

C3H6 C3H6 T10 163 160 154 151 160 155 154 154 T30 190 190 180 180 189 187 184 180 T50 220 222 206 211 217 213 214 214 T80 287 296 271 278 277 273 276 276 T90 325 329 304 306 303 303 311 319 ΔT10 -3 -3 -5 0

Table 19 (continuation)

F-Cat P-Cat Na-Cat Ca-Cat

- -

- - - -

T T T T

- - - -

F F P P

Light-off Ca Ca

Na Na

- - - -

- - temperature, - -

°C Combo Combo Combo Combo

C3H6 C3H6 C3H6 C3H6

Combo Combo Combo Combo

C3H6 C3H6

C3H6 C3H6 ΔT30 0 0 -2 -4 ΔT50 2 5 -4 0 ΔT80 9 7 -4 0 ΔT90 4 2 0 8 ΔT10 (30,50,80,90) - Difference between T10,30,50,80 and T90 values in C3H6-Combo and C3H6-Combo-T experiments

Combo-SO2 experiments

The sulphur treatment of the catalysts shows a noticeable deactivation in propylene oxidation (see Figure 43). Nevertheless, the deactivation is less pronounced than for NO and

CO oxidation reactions after SO2 treatment.

Figure 43. C3H6 light-off curves for the fresh and poisoned catalyst Combo-SO2 experiments. The fresh catalyst shows the greatest resistance to sulphur poisoning according to Table 20. The F-Cat light-off temperatures increase approximately 20 °C to 30 °C within the whole conversion range as compared to the C3H6-F-Combo experiment (see Table 20). The increase of light-off temperatures for P-Cat and Na-Cat is slightly higher than for the F-Cat when Combo and Combo-SO2 experiments are compared with the same catalysts. T50 for the F-Cat, P-Cat and Na-Cat increases by 28 °C, 47 °C and 38 °C, in comparison to C3H6-

F-Combo, C3H6-Na-Combo and C3H6-Ca-Combo, respectively. Ca-Cat shows the most considerable deactivation (50 °C - 80 °C increase of the light-off temperatures) as compared to its performance in C3H6-Ca-Combo experiment.

It can be concluded from the data in Table 20 that P+SO2, Na+SO2 and Ca+SO2 poisoning effect on the fresh catalyst (comparison to C3H6-F-Combo) is lower than the SO2 effect on the P, Na and Ca poisoned catalysts alone (comparison to C3H6-(P, Na, Ca)- Combo). This is in agreement with observations for CO and NO oxidation reactions. The results are in agreement with previous studies where different hydrocarbon oxidation activity was tested after various SO2 treatments and severe deactivation was observed [9, 14, 64, 98].

Table 20

Comparison between C3H6 light-off temperatures in Combo and Combo-SO2 experiments

F-Cat P-Cat Na-Cat Ca-Cat

(

SO2

(

SO2 SO2

- -

–

)] )] )]

–

2 2 2

Combo

SO SO SO

Combo

Combo Combo

- - - Light-off -

- -

Combo

F P

Combo Combo

- - SO2

Na temperature,°C - -

Combo) -

F P

Combo)

- -

Combo Combo Combo

C3H6 C3H6

C3H6

T[(F

ΔT[(F

C3H6 C3H6

C3H6 T10 163 184 154 181 18 160 186 23 154 200 37 T30 190 212 180 217 27 189 220 30 184 241 51 T50 220 248 206 253 33 217 255 35 214 278 58 T80 287 319 271 318 31 277 311 24 276 354 67 T90 325 348 304 348 23 303 338 13 311 379 54 ΔT10 21 27 26 46 ΔT30 22 37 31 57 ΔT50 28 47 38 64 ΔT80 32 47 34 78 ΔT90 23 44 35 68 ΔT10(30,50,80,90)- Difference between T10,30,50,80 and T90 values in Combo and Combo-SO2 experiments

ΔT[(F-Combo) – (P, Na, Ca) -Combo-SO2)] – Difference between light-off temperatures in C3H6-F-Combo experiment and C3H6-P,Na,Ca-Combo-SO2 experiments

CONCLUSIONS

1. 1 wt.% PtPd (3:1)/Al2O3 diesel oxidation catalyst material properties and performance are somewhat affected by poisoning with washcoat loadings of 1.0 wt.% phosphorus, 0.7 wt.% sodium and 1.1 wt.% calcium. 2. Catalyst poisoning with phosphorous, sodium and calcium affects slightly more nitric oxide and carbon monoxide oxidation than propylene oxidation. 3. The precious metal dispersion decreases around 24 % after poisoning with phosphorous and sodium. Larger active phase particles could be responsible for a minor nitric oxide and carbon monoxide light-off improvement for these catalysts.

4. Sulphur poisoning (5.9 wt.%) considerably affects 1 wt.% PtPd (3:1)/Al2O3 catalyst activity. The light-off is shifted to 60 °C – 70 °C higher temperatures in nitric oxide and carbon monoxide oxidation. In propylene oxidation it is shifted to 20 °C – 30 °C higher temperatures. 5. The catalyst activity after sulphur poisoning does not correlate with the sulphur loading in phosphorus, sodium and calcium poisoned catalysts. This would indicate poisonous species interactions.

SECINĀJUMI

1. 1 wt.% PtPd (3:1)/Al2O3 dīzeļdegvielas oksidēšanas katalizatora materiāla īpašības un veiktspēju nelielā mērā ietekmē substrāta saindēšana ar 1.0 wt.% fosforu, 0.5 wt.% nātriju un 1.1 wt.% kalciju. 2. Katalizatora saindēšana ar fosforu, nātriju un kalciju vairāk ietekmē slāpekļa monoksīda un oglekļa monoksīda oksidēšanas reakcijas kā propilēna oksidēšanu. 3. Cēlmetālu dispersija samazinās par 24 % pēc jauna katalizatora saindēšanas ar fosforu un nātriju. Lielākas aktīvās fazes daļiņas varētu izskaidrot uzlabotas slāpekļa un oglekļa monoksīdu oksidēšanas īpašības ar šiem katalizatoriem.

4. Saindēšana ar sēru (5.9 wt.%) nozīmīgi ietekmē 1 wt.% PtPd (3:1)/Al2O3 dīzeļdegvielas oksidēšanas katalizatora aktivitāti. Slāpekļa un oglekļa monoksīda reakcijas norise tiek novērota 60 °C – 70 °C augstākās temperatūrās, bet propilēna oksidēšana 20 °C – 30 °C augstākās temperatūrās. 5. Katalizatoru aktivitātei pēc saindēšanas ar sēru nav korelācijas ar sēra daudzumu paraugos. Tas norāda uz katalizatoru inžu mijiedarbību.

RECOMMENDATIONS FOR FUTURE STUDIES

1. Poisoning of DOC by incipient wetness impregnation with 2 to 5 times (according to field-aged catalyst elemental analyses) higher loadings of Phosphorus, sodium and calcium followed by activity testing in the rig. 2. Poisoning of DOC in the gas-phase with the same poison loadings. Comparative activity tests in the rig can show differences between poisoning in gas and liquid phases. 3. Poisoning of DOC by model fuel with soluble Na, P and Ca salts with limits according to EN 14214 and simultaneous testing in the rig.

4. Simultaneous SO2 treatment and activity testing in the rig. 5. Activity tests with different washcoat loadings and inlet gas concentrations. 6. Catalyst treatment with simulated exhaust flow prior to activity tests. 7. Different temperature treatments for monolith cores prior to activity testing to observe poison-induced sintering. 8. TEM, BET and CO chemisorption analyses for monolithic samples before and after activity tests. 9. Linearization of the furnace at lower temperatures to reduce fluctuations during carbon monoxide and propylene oxidation. 10. Longer and thinner monolith inlet and outlet thermocouples (to ensure a proper location inside the monolith channel). 11. Exchange of the rig’s support from wooden to metallic surface as well as insulation of heating tapes from wires would prevent fire. 12. Automation of the activity testing process to ensure exactly the same experimental conditions for every run.

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APPENDICES