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Catalyst passivation through carbon poisoning is a common and costly problem as it reduces the lifetime and performance of the catalyst. Adding oxygen to the feed stream could reduce poisoning but may also affect the activity negatively. We have studied the dehydrogenation, decomposition, and desorption of naphthalene co-adsorbed with oxygen on Ni(111) by combining temperature-programmed desorption (TPD), sum frequency generation spectroscopy (SFG), photoelectron spectroscopy (PES), and density functional theory (DFT). Chemisorbed oxygen reduces the sticking of naphthalene and shifts H2 production and desorption to higher temperatures by blocking active Ni sites. Oxygen increases the production of CO and reduces carbon residues on the surface. Chemisorbed oxygen is readily removed when naphthalene is decomposed. Oxide passivates the surface and reduces the sticking coefficient. But it also increases the production of CO dramatically and reduces the carbon residues. Ni2O3 is more active than NiO., QC 20240322

Catalytic reactions on solid surfaces have been studied under Ultra-high vacuum (UHV) conditions for decades. These studies provide crucial information for catalytic reactions, such as surface structures, adsorption sites, and reaction mechanisms. However, industrial catalysis operates under high gas pressure to increase the reaction rate, so the knowledge we learn from the previous UHV studies may not be able to directly transfer to the industry. This difference is referred to as the “pressure gap”, and it represents the difficulties that scientists encounter when attempting to investigate and comprehend catalytic reactions at high pressure. To address this issue, in situ/operando techniques and instruments have been developed to study reactions at pressures closer to real-world applications.The present thesis aims to showcase the new instrument, Near-Ambient Pressure Velocity Map Imaging (NAP-VMI), and its applications to molecular spectroscopy and surface science at near-ambient pressures. This instrument features a velocity map imaging (VMI) setup with redesigned ion optics and uses differential pumping to achieve a working pressure of 10−3 mabr. It allows time-resolved measurements at microsecond time scales using the pump-probe technique with a pulsed molecular beam and a pulsed laser. The performance is validated using N2O photodissociation and N2 surface scattering. CO oxidation on Pd(110) and Pd(100) surfaces is studied at elevated oxygen pressure (1×10−5 mbar) where the surfaces reconstruct.The results show the suppression of CO2 production in oxygen rich environments for both surfaces. The difference in kinetics and dynamics behavior between the two surfaces also suggests that surface structures and adsorption sites are important in the reaction mechanisms. These findings highlight the importance of surface structure in catalytic reactions and pave the way for more effective catalysts to be designed by tailoring surface properties and reaction conditions., Katalytiska reaktioner på fasta ytor har studerats under ultrahögt vakuum (UHV) i årtionden. Dessa studier ger avgörande information för katalytiska reaktioner, såsom ytstrukturer, adsorptionsställen och reaktionsmekanismer. Industriell katalys arbetar dock under högt gastryck för att öka reaktionshastigheten, så kunskapen vi lär oss från de tidigare UHV-studierna kanske inte direkt kan överföras till industrin. Denna skillnad kallas “tryckgapet” och den representerar de svårigheter som forskare möter när de försöker undersöka och förstå katalytiska reaktioner vid högt tryck. För att lösa detta problem har in situ/operando metoder och instrument utvecklats för att studera reaktioner vid tryck som ligger närmare verkliga tillämpningar. Den föreliggande avhandlingen syftar till att visa upp det nya instrumentet, nära-omgivande tryckhastighetskarta-avbildning (NAP-VMI) och dess tillämpningar för molekylär spektroskopi och ytvetenskap vid nära-omgivande tryck. Detta instrument har en hastighetskarta-avbildning (VMI) med omdesignadjonoptik och använder differentialpumpning för att uppnå ett arbetstryckpå 10−3 mabr. Den tillåter tidsupplösta mätningar på mikrosekunders tidsskalormed hjälp av pump-probe-teknik med pulsmolekylär stråle och pulslaser. Prestandan valideras med hjälp av N2O fotodissociation och N2 ytspridning. CO oxidation på Pd(110) och Pd(100) ytan studeras vid förhöjt syretryck (1 × 10−5 mbar) där ytorna rekonstrueras. Resultaten visar undertryckandet av CO2 produktion i syrerika miljöer för båda ytorna. Skillnaden i kinetik och dynamikbeteende mellan de två ytorna tyder också på att ytstrukturer och adsorptionsställen är viktiga i reaktionsmekanismerna. Dessa fynd framhäver vikten av ytstruktur i katalytiska reaktioner och banar väg för mer effektiva katalysatorer som kan utformas genom att skräddarsy ytegenskaper och reaktionsförhållanden., QC 20240117

There are several difficulties when experimentally determined reaction mechanisms are applied from model systems to real catalysis. Besides the infamous pressure and material gaps, it is sometimes necessary to consider impurities in the real reactant feedstock that can act as promoters or catalyst poisons and alter the reaction path. In this study, the effect of sulfur on the dehydrogenation of naphthalene on Ni(111) is investigated by using X-ray photoelectron spectroscopy and scanning tunneling microscopy. Sulfur induces a (5√3 × 2) surface reconstruction, as previously reported in the literature. The sulfur does not have a strong effect on the dehydrogenation temperature of naphthalene. However, the presence of sulfur leads to a preferred formation of carbidic over graphitic carbon and a strong inhibition of carbon diffusion into the nickel bulk, which is one of the steps of destructive whisker carbon formation described in the catalysis literature.

There are several difficulties when experimentally determined reaction mechanisms are applied from model systems to real catalysis. Besides the infamous pressure and material gaps, it is sometimes necessary to consider impurities in the real reactant feedstock that can act as promoters or catalyst poisons and alter the reaction path. In this study, the effect of sulfur on the dehydrogenation of naphthalene on Ni(111) is investigated by using X-ray photoelectron spectroscopy and scanning tunneling microscopy. Sulfur induces a (5 root 3 x 2) surface reconstruction, as previously reported in the literature. The sulfur does not have a strong effect on the dehydrogenation temperature of naphthalene. However, the presence of sulfur leads to a preferred formation of carbidic over graphitic carbon and a strong inhibition of carbon diffusion into the nickel bulk, which is one of the steps of destructive whisker carbon formation described in the catalysis literature., QC 20240216

Time-resolved kinetics of CO oxidation on Pd(100) are studied using near-ambient pressure velocity map imaging (NAP-VMI) with a pulsed molecular beam. We observed two types of bimodel activities to CO oxidation at different oxygen exposures. To explain this behavior, we have developed two kinetic models with two different surface configurations. Three reaction channels are discovered on Pd(100) under oxygen rich environment, representing CO oxidation on different metastable surfaces. We assign those reaction channels to: CO oxidation (1) on a pristine metal surface, (2) on an epitaxial multilayer PdO(101), and (3) on a domain boundary between Pd(100) and (√5 × √5) single layer surface oxide. All reaction channels can be described in the Langmuir–Hinshelwood mechanism. This is a direct evidence of the coexistence of multiple surface activities to the CO oxidation on the Pd(100) surface., QC 20240122

We extend the use of our recently developed Near-Ambient Pressure Velocity Map Imaging (NAP-VMI) technique to study the kinetics and dynamics of catalytic reactions in the pressure gap. As an example, we show that NAP-VMI combined with molecular beam surface scattering allows the direct measurement of time- and velocity-resolved kinetics of the scattering and oxidation of CO on the Pd(110) surface with oxygen pressures at the surface up to 1× 10−5 mbar, where different metastable surface structures form. Our results show that the c(2×4) oxide structure formed at low O2 pressure is highly active for CO oxidation. The velocity distribution of the CO2 products shows the presence of two reaction channels, which we attributeto reactions starting from two distinct but rapidly interconverting CO binding sites. The effective CO oxidationreaction activation energy is Er = (1.0 ± 0.13) eV. The CO2 production is suppressed at higher O2 pressure due to the number of antiphase domain boundaries increases, and the missing row sites are filled by O–atoms at O2 pressures approaching 1× 10−6 mbar. Filling of these sites by O–atoms reduces the CO surface lifetime, meaningthe surface oxide is inactive for CO oxidation. We briefly outline further developments planned for the NAP-VMI and its application to other types of experiments., QC 20240118