Hydrocarbon and ammonia chemistry on noble metal surfaces, catalysis on a molecular scale

The work described in this thesis concerns hydrocarbon and ammonia oxidation on single crystal surfaces of Pt, Rh and Ir. In Chapter3 the oxidation of n-butane and propene on a stepped Pt surface is discussed. In Chapter 4 benzene decomposition and oxidation on Ir(111) is discussed, and in Chapter 5 the adsorption of methane and oxygen on Rh(100) is studied. The oxidation of ammonia was studied on two different surfaces, i.e. Pt(410) and Ir(110). It was found that ammonia dissociates on Ir(110), but not on Pt(410). Radiation can induce NH3 dissociation, and the NHx chemistry could be studied in this way, even on Pt(410). The presence of Oad enhances NH3 dissociation on Pt surfaces, The reaction between NH3 and Oad already occurs around 150 K. NO formation was also found on this surface, already at low temperature. On Ir(110) the formation of NO was also found, but only at higher temperatures. A model is proposed in which the different selectivity found for Ir and Pt catalysts during ammonia oxidation is explained.LEI Universiteit Leidenthe research was financially supported by the Technology Foundation STW, applied science division of NWO and the technology programme of the Ministry of Economic Affairs, under project number UPC-5037Surfca

Understanding FTS selectivity: the crucial role of surface hydrogen

Monomeric forms of carbon play a central role in the synthesis of long chain hydrocarbons via the Fischer-Tropsch synthesis (FTS). We explored the chemistry of C1Hxad species on the close-packed surface of cobalt. Our findings on this simple model catalyst highlight the important role of surface hydrogen and vacant sites for product selectivity. We furthermore find that COad affects hydrogen in multiple ways. It limits the adsorption capacity for Had, lowers its adsorption energy and inhibits dissociative H2 adsorption. We discuss how these findings, extrapolated to pressures and temperatures used in applied FTS, can provide insights into the correlation between partial pressure of reactants and product selectivity. By combining the C1Hx stability differences found in the present work with literature reports of the reactivity of C1Hx species measured by steady state isotope transient kinetic analysis, we aim to shed light on the nature of the atomic carbon reservoir found in these studies

CO as a promoting spectator species of CxHy conversions relevant for Fischer-Tropsch chain growth on cobalt: evidence from temperature-programmed reaction and reflection absorption infrared spectroscopy

Cobalt-catalyzed low temperature Fischer-Tropsch synthesis is a prime example of an industrially relevant reaction in which CxHy intermediates involved in chain growth react in the presence of a large quantity of COad. In this study, we use a Co(0001) single-crystal model catalyst to investigate how CO, adsorbed alongside CxHy adsorbates affects their reactivity. Temperature-programmed reaction spectroscopy was used to determine the hydrogen content of the CxHy intermediates formed at different temperatures, and infrared absorption spectroscopy was used to obtain more specific information on the chemical identity of the various reaction intermediates formed. Ethene, propene, and but-1-ene precursors decompose below 200 K. The 1-alkyne adsorbate is identified as a major product, and some alkylidyne species form as well when the initial alkene coverage is high. The surface hydrogen atoms produced in the low temperature decomposition step start leaving the surface >300 K. When an alkyne/Had-covered surface is heated in the presence of CO, the alkyne adsorbates are hydrogenated to the corresponding alkylidyne at temperatures <250 K. This finding shows that CxHy surface species react differently in the presence of COad, a notion of general importance for catalytic reactions where both CO and CxHy species are present. In the context of Fischer-Tropsch synthesis, the observed CO-induced reaction is of specific importance for the alkylidyne chain growth mechanism. In this reaction, scheme hydrocarbon chains grow via coupling of CHad with a (Cn) alkylidyne adsorbate to produce the (Cn+1) alkyne. A subsequent hydrogenation of the alkyne product to the corresponding alkylidyne is required for further growth. The present work shows that this specific reaction is promoted by the presence of CO. This suggests that the influence of CO spectators on the stability of CxHy surface intermediates is beneficial for efficient chain growth

Cobalt-Fischer-Tropsch catalyst regeneration: the crucial role of the Kirkendall effect for cobalt redispersion

Redispersion of cobalt is a key process during Fischer–Tropsch catalyst regeneration. Using model catalysts we show that redispersion is a two step process. Oxidation of supported metallic cobalt nanoparticles produces hollow oxide particles by the Kirkendall effect; reduction leads to break-up of these hollow oxide shells, forming multiple metallic particles. This mechanism is to a large extent independent of the support

Effect of aldehyde and Carboxyl functionalities on the surface chemistry of biomass-derived molecules

The adsorption and decomposition of acetaldehyde and acetic acid were studied on Rh(100) to gain insight into the interaction of aldehyde and carboxyl groups of biomass-derived molecules with the surface. Temperature-programmed reaction spectroscopy (TPRS) was used to monitor gaseous reaction products, whereas Reflection absorption infrared spectroscopy (RAIRS) was used to determine the nature of surface intermediates and reaction paths. The role of adsorbate interactions in oxygenate decomposition chemistry was also investigated by varying the surface coverage. Acetaldehyde adsorbs in an η2(C, O) configuration for all coverages, where the carbonyl group binds to the surface via the C and O atoms. Decomposition occurs below room temperature (180-280 K) via C-H and C-C bond breaking, which releases CO, H, and CHx species on the surface. At low coverage, CHx dehydrogenation dominates and surface carbon is produced alongside H2 and CO. At high coverage, about 60% of the CHx hydrogenates to form methane, whereas only 40% of the CHx decomposes further to surface carbon. Acetic acid adsorbs dissociatively on the Rh(100) surface via O-H bond scission, forming a mixture of mono- and bidentate acetate. The decomposition of acetate proceeds via two different pathways: (i) deoxygenation via C-O and C-C bond scissions and (ii) decarboxylation via C-C bond scission. At low coverage, the decarboxylation pathway dominates, a process that occurs at slightly above room temperature (280-360 K) and produces CO2 and CHx, where the latter decomposes further to surface carbon and H2. At high coverage, both decarboxylation and deoxygenation occur, slightly, above room temperature (280-360 K). The resulting O adatoms produced in the deoxygenation path react with surface hydrogen or CO to form water and CO2, respectively. The CHx species dehydrogenate to surface carbon for all coverages. Our findings suggest that oxygenates with a C=O functionality and an alkyl end react on the Rh(100) surface to produce synthesis gas and small hydrocarbons whereas CO2 and synthesis gas are produced when oxygenates with a COOH functionality and an alkyl end react with the Rh(100) surface. For both cases, carbon accumulation occurs on the surface

Promoter segregation in Pt and Ru promoted cobalt model catalysts during oxidation–reduction treatments

A flat model approach was used to study segregation phenomena in Pt and Ru-promoted cobalt catalysts prepared by co-impregnation. In the calcined state the cobalt (Co3O4) and promoter phases are well mixed and in the oxidic form (PtO2 and RuOx). Upon reduction the promoter alloys with the metallic cobalt and it is evenly distributed over the cobalt particles. During reoxidation of the metallic system Pt and Ru segregate: Kirkendall voids of Co3O4 are formed, and the promoter is left inside the hollow oxide shell. Reduction of the reoxidized state results in re-alloying of the promoter with the cobalt phase, but the promoter concentration in the surface region of the sample is lower than after reduction of the calcined catalyst. This is explained by incomplete remixing, a result of the segregation in the re-oxidized state. The results suggest that break-up of the Pt-containing hollow oxide shells leads to uneven distribution of the promoter over the cobalt phase, forming both particles with high promoter concentration and promoter-free particles. Our approach provides a simple method to generate encapsulated noble metal particles, which are potentially interesting as sinter-resistant catalysts

Cobalt-Fischer-Tropsch catalyst regeneration: the crucial role of the Kirkendall effect for cobalt redispersion

Redispersion of cobalt is a key process during Fischer–Tropsch catalyst regeneration. Using model catalysts we show that redispersion is a two step process. Oxidation of supported metallic cobalt nanoparticles produces hollow oxide particles by the Kirkendall effect; reduction leads to break-up of these hollow oxide shells, forming multiple metallic particles. This mechanism is to a large extent independent of the support

Promoter segregation in Pt and Ru promoted cobalt model catalysts during oxidation–reduction treatments

A flat model approach was used to study segregation phenomena in Pt and Ru-promoted cobalt catalysts prepared by co-impregnation. In the calcined state the cobalt (Co3O4) and promoter phases are well mixed and in the oxidic form (PtO2 and RuOx). Upon reduction the promoter alloys with the metallic cobalt and it is evenly distributed over the cobalt particles. During reoxidation of the metallic system Pt and Ru segregate: Kirkendall voids of Co3O4 are formed, and the promoter is left inside the hollow oxide shell. Reduction of the reoxidized state results in re-alloying of the promoter with the cobalt phase, but the promoter concentration in the surface region of the sample is lower than after reduction of the calcined catalyst. This is explained by incomplete remixing, a result of the segregation in the re-oxidized state. The results suggest that break-up of the Pt-containing hollow oxide shells leads to uneven distribution of the promoter over the cobalt phase, forming both particles with high promoter concentration and promoter-free particles. Our approach provides a simple method to generate encapsulated noble metal particles, which are potentially interesting as sinter-resistant catalysts

Oxygen adsorption and water formation on Co(0001)

Oxygen adsorption and removal on flat and defective Co(0001) surfaces have been investigated experimentally using scanning tunneling microscopy, temperature-programmed and isothermal reduction, synchrotron X-ray photoemission spectroscopy, and work function measurements under ultrahigh vacuum conditions and H2/CO pressures in the 10-5 mbar regime. Exposure of the Co(0001) to O2(g) at 250 K leads to the formation of p(2 × 2) islands with a local coverage of 0.25 ML. Oxygen adsorption continues beyond 0.25 ML, reaching a saturation point of ∼0.39 ML Oad, without forming cobalt oxide. Chemisorbed oxygen adlayers can be reduced on both flat and defective Co(0001) surfaces by heating in the presence of ∼2.3 × 10-5 mbar H2(g). The onset of the oxygen removal as water during temperature-programmed reduction experiments (1 K s-1) is at around 450 K on flat Co(0001) and 550 K on defective Co(0001). By evaluation of isothermal reduction experiments using a kinetic model, the activation energy for water formation is found to be ∼129 ± 7 kJ/mol for the flat Co(0001) and ∼136 ± 7 kJ/mol for the defective Co(0001). Adsorbed oxygen cannot be reduced by CO(g) on flat and defective Co(0001) using CO pressures up to 1 × 10-5 mbar and temperatures up to 630 K

Ammonia adsorption and decomposition on Co(0001) in relation to Fischer-Tropsch synthesis

In order to fundamentally understand cobalt catalyst deactivation in Fischer-Tropsch synthesis (FTS) due to parts per million levels of NH3 in the synthesis gas, the adsorption and decomposition of NH3 on Co(0001) are investigated experimentally under ultrahigh vacuum (UHV) conditions and theoretically using density functional theory (DFT) calculations. NH3 desorbs intact from the surface, between 100 and 270 K. In agreement with this, DFT calculations show that the activation barrier for NH3 decomposition, 105 kJ/mol, is higher than the adsorption energy of NH3, 59 kJ/mol. Neither COad nor Had block the adsorption of NH3. Instead, CO and NH3 form a stable coadsorbed layer. Preadsorbed ammonia negatively affects dissociative H2 adsorption. Electron-induced dissociation produces NHx species on the surface at low temperature. The order of stability is NH(+2 Had) > N(+3 Had) > NH2(+ Had) > NH3. N and NH lower the quantity of CO that can be accommodated on the surface but do not affect the adsorption energy significantly. For FTS, we conclude that (i) NH3 adsorption on cobalt is not inhibited by the other FTS reactants and thus parts per million levels of NH3 can already be detrimental, (ii) due to their high stability, NHx species are most likely responsible for catalyst deactivation