Identifying the Role of Brønsted and Lewis Acid Sites in the Diels-Alder Cycloaddition of 2,5-DMF and Ethylene

The role of Lewis and Brønsted acid sites in the Diels-Alder cycloaddition (DAC) of ethylene to 2,5-dimethylfuran (2,5-DMF) to p-xylene was investigated. Amorphous silica catalysts containing Al3+ (ASA), Ga3+ (ASG), and In3+ (ASI) were prepared via homogeneous deposition-precipitation. Silica modified with Zr4+ (ASZ) was prepared by impregnation. Their acidic properties were characterized by various IR and NMR spectroscopic techniques. Measurements using pyridine as a probe molecule highlighted the presence of mostly Lewis acid sites (LAS) in all materials. Using CO as a probe, in contrast, demonstrated the existence of Brønsted acid sites (BAS) in ASA and ASG, which were nearly absent in ASI and ASZ. Differences in basic strength can explain the contrast in results observed between the two probe molecules. The highest p-xylene yield (~20%) in the DAC reaction, could be achieved with ASA and ASG. The lack of BAS in ASI and ASZ resulted in inferior performance in the DAC, with p-xylene yields below 5%. These results indicate the importance of BAS for the DAC reaction. Several other heterogeneous and homogeneous catalysts were explored for the DAC reaction to show the generality of our conclusion that BAS play a critical role in obtaining p-xylene from 2,5-DMF and ethylene

Computational study of CO2 methanation on Ru/CeO2 model surfaces:On the impact of Ru doping in CeO2

The Sabatier reaction (CO 2 + H 2 → CH 4 + H 2O) can contribute to renewable energy storage by converting green H 2 with waste CO 2 into CH 4. Highly dispersed Ru on CeO 2 represents an active catalyst for the CO 2 methanation. Here, we investigated the support effect by considering a single atom of Ru and a small Ru cluster on CeO 2 (Ru 6/CeO 2). The influence of doping CeO 2 with Ru was investigated as well (Ru 6/RuCe x-1O 2x-1). Density functional theory was used to compute the reaction energy diagrams. A single Ru atom on CeO 2 can only break one of the C-O bonds in adsorbed CO 2, making it only active in the reverse water-gas shift reaction. In contrast, Ru 6 clusters on stoichiometric and Ru-doped CeO 2 are active methanation catalysts. CO is the main reaction intermediate formed via a COOH surface intermediate. Compared to an extended Ru(11-21) surface containing step-edge sites where direct C-O bond dissociation is facile, C-O dissociation proceeds via H-assisted pathways (CO → HCO → CH) on Ru 6/CeO 2 and Ru 6/RuCe x-1O 2x-1. A higher CO 2 methanation rate is predicted for Ru 6/RuCe x-1O 2x-1. Electronic structure analysis clarifies that the lower activation energy for HCO dissociation on Ru 6/RuCe x-1O 2x-1 is caused by stronger electron-electron repulsion due to its closer proximity to Ru. Strong H 2 adsorption on small Ru clusters explains the higher CO 2 methanation activity of Ru clusters on CeO 2 compared to a Ru step-edge surface, representative of Ru nanoparticles, where the H coverage is low due to stronger competition with adsorbed CO.</p

Unravelling CO Activation on Flat and Stepped Co Surfaces: A Molecular Orbital Analysis

Structure sensitivity in heterogeneous catalysis dictates the overall activity and selectivity of a catalyst whose origins lie in the atomic configurations of the active sites. We explored the influence of the active site geometry on the dissociation activity of CO by investigating the electronic structure of CO adsorbed on 12 different Co sites and correlating its electronic structure features to the corresponding C-O dissociation barrier. By including the electronic structure analyses of CO adsorbed on step-edge sites, we expand upon the current models that primarily pertain to flat sites. The most important descriptors for activation of the C-O bond are the decrease in electron density in CO’s 1π orbital , the occupation of 2π anti-bonding orbitals and the redistribution of electrons in the 3σ orbital. The enhanced weakening of the C-O bond that occurs when CO adsorbs on sites with a step-edge motif as compared to flat sites is caused by a distancing of the 1π orbital with respect to Co. This distancing reduces the electron-electron repulsion with the Co d-band. These results deepen our understanding of the electronic phenomena that enable the breaking of a molecular bond on a metal surface.</p

Electrochemical interfaces during CO₂ reduction on copper electrodes

Copper has received significant attention for decades as electrode material for the electrochemical reduction of carbon dioxide (CO2RR) because of its capability to form multi-carbon products (C2+). However, despite substantial research, CO2RR with Cu-based electrocatalysts has yet to be commercialized. Understanding the physical and chemical changes of the catalyst surface and the dynamics of the electrochemical interface during CO2RR is key to improve the activity and selectivity. This review article focuses on recent studies that provide important insights of the surfaces and interfaces during reduction using ex-situ, in-situ and operando characterization techniques.</p

Al Promotion of In2O3 for CO2 Hydrogenation to Methanol

In2O3 is a promising catalyst for the hydrogenation of CO2 to methanol, relevant to renewable energy storage in chemicals. Herein, we investigated the promoting role of Al on In2O3 using flame spray pyrolysis to prepare a series of In2O3−Al2O3 samples in a single step (0−20 mol % Al). Al promoted the methanol yield, with an optimum being observed at an Al content of 5 mol %. Extensive characterization showed that Al can dope into the In2O3 lattice (maximum ∼ 1.2 mol %), leading to the formation of more oxygen vacancies involved in CO2 adsorption and methanol formation. The rest of Al is present as small Al2O3 domains at the In2O3 surface, blocking the active sites for CO2 hydrogenation and contributing to higher CO selectivity. At higher Al content (≥10 mol %Al), the particle size of In2O3 decreases due to the stabilizing effect ofAl2O3. Nevertheless, these smaller particles are prone to sintering duringCO2 hydrogenation since they appear to be more easily reduced. These findings show subtle effects of a structural promoter such asAl on the reducibility and texture of In2O3 as a CO2 hydrogenation catalyst

Ni and ZrO₂ promotion of In₂O₃ for CO₂ hydrogenation to methanol

Transition metals, such as Ni, Pd, and Pt, and ZrO2 are known as efficient promoters in M-In2O3-ZrO2 catalysts for CO2 hydrogenation to methanol. Herein, we systematically investigated the role of Ni and ZrO2 promoters by preparing ternary NiO-In2O3-ZrO2 catalysts and binary counterparts by flame spray pyrolysis. The highest methanol rate was obtained for the Ni(6 wt%)-In2O3(31 wt%)-ZrO2(63 wt%) composition. DRIFTS-SSITKA shows that formate is the key intermediate in the hydrogenation of CO2 to methanol. Kinetic analysis shows the competition between methanol and CO formation. The rate-limiting step in methanol formation is likely the hydrogenation of surface methoxy species. Ni and ZrO2 play different promoting roles without showing synergy with respect to each other. Ni promotes hydrogenation of surface formate and methoxy species, while ZrO2 maintains a high In2O3 dispersion, the smaller In2O3 size likely stabilizing formate and other intermediates during their conversion to methanol.</p

Pressure dependence and mechanism of Mn promotion of silica-supported Co catalyst in the Fischer-Tropsch reaction

The mechanism of Mn promotion of a silica-supported Co catalyst in the Fischer-Tropsch reaction has been studied at varying pressures up to 20 bar. IR spectroscopy in combination with DFT calculations suggest adsorbed CO is activated by reaction with an oxygen vacancy in the MnO, which covers the Co surface. This leads to a higher activity, higher CHx coverage and thus higher C5+ and lower CH4 selectivity. Increasing the pressure magnifies the selectivity differences. However, above around 4 bar, the effect of Mn on the selectivities is reversed and the C5+ selectivity is decreased by Mn addition. This is tentatively attributed to Mn promoting the C-O bond dissociation but not the chain growth. Formed monomers have to migrate to stepped sites for chain growth on the Co surface. Whilst this is migration is not impeded by co-adsorbates at low pressure, migration could be hindered by especially the high CO coverage at high pressure

Tuning stability of titania-supported Fischer-Tropsch catalysts:Impact of surface area and noble metal promotion

Cobalt oxidation is a relevant deactivation pathway of titania-supported cobalt catalysts used in Fischer-Tropsch synthesis (FTS). To work towards more stable catalysts, we studied the effect of the surface area of the titania support and noble metal promotion on cobalt oxidation under simulated high conversion conditions. Mössbauer spectroscopy was used to follow the evolution of cobalt during reduction and FTS operation as a function of the steam pressure. The reduction of the oxidic cobalt precursor becomes more difficult due to stronger metal-support interactions when the titania surface area is increased. The reducibility was so low for cobalt on GP350 titania (surface area 283 m2/g) that the catalytical activity was negligible. Although cobalt was more difficult to reduce on P90 titania (94 m2/g) than on commonly used P25 titania (50 m2/g), the Co/P90 catalyst showed increased resistance against cobalt sintering and higher FTS performance than Co/P25. The addition of platinum to Co/P90 led to a higher reduction degree of cobalt and a higher cobalt dispersion, representing a catalyst with promising performance at relatively low steam pressure. Nevertheless, the stronger cobalt-titania interactions result in more extensive deactivation at high steam pressure due to oxidation.</p

Isomorphously Substituted [Fe,Al]ZSM-5 Catalysts for Methane Dehydroaromatization

Dehydroaromatization of methane (MDA) under non-oxidative conditions is a promising reaction for direct valorization of natural gas and biogas. Typically, Fe-modified ZSM-5 catalysts display low aromatic productivity and high coke selectivity in the MDA reaction. Herein, we show the benefit of starting from isomorphously substituted Fe-sites in [Fe,Al]ZSM-5 zeolites prepared by direct hydrothermal synthesis. Upon calcination, these samples contain predominantly isolated Fe3+ species, either atomically dispersed within the zeolite framework or anchored at exchange sites inside zeolite channels. In terms of the integral hydrocarbon productivity, [Fe,Al]ZSM-5 catalysts outperform Fe/ZSM-5, prepared by impregnation, as well as Mo/ZSM-5 catalysts with the same Si/Al ratio and molar metal loading. Operando X-ray absorption spectroscopy coupled with mass spectrometry (XANES-MS) demonstrates that the initial tetrahedral Fe3+ within the zeolite framework or at exchange sites are transformed into octahedral extraframework Fe2+ active sites during the MDA reaction and form small Fe2O3 clusters during oxidative regeneration. Combining activity measurements and operando thermogravimetry shows that the duration of the induction period, related to the formation of active hydrocarbon pool intermediates, strongly depends on the Fe dispersion and loading and can be used as a suitable descriptor for the MDA activity of [Fe,Al]ZSM-5. The shorter induction period of [Fe,Al]ZSM-5 in comparison to impregnated Fe/ZSM-5 can be linked to the higher methane conversion rate over highly dispersed Fe-sites and faster formation of active hydrocarbon pool intermediates.</p

Structure sensitivity of silver-catalyzed ethylene epoxidation

The influence of particle size (20-200 nm) of Ag/α-Al 2O 3 catalysts for epoxidation of ethylene to ethylene oxide (EO) under industrial conditions was investigated. Small silver particles up to 40 nm are predominantly monocrystalline and show a decreasing weight-normalized reaction rate with increasing particle size. Particles larger than 50 nm consist of multiple silver crystallites with a much smaller domain size between 25 and 30 nm. For these polycrystalline silver particles, the weight-normalized reaction rate is independent of particle size. The ethylene conversion rate normalized to the external surface area increases when the silver particles become larger. We attribute this to a specific role of the grain boundaries between silver crystallites in supplying oxygen atoms to the external surface. Oxygen is likely activated at defects of an otherwise low-reactivity silver surface (for oxygen adsorption) followed by diffusion along grain boundaries, dissolution in the bulk, and diffusion to the external surface, where oxygen atoms react with ethylene. The reaction rate normalized to the surface area of the first outer shell of crystallites making up silver particles is independent of size for polycrystalline particles. A higher reaction pressure benefits ethylene oxidation rate and EO selectivity due to a higher oxygen coverage. Adding chlorine further improves the EO selectivity through modification of the active surface. The same particle size dependences are observed at 1 bar and at 20 bar without and with chlorine. The main finding of our work is that for large enough particles the ethylene oxidation rate normalized to the silver weight is independent of size. In addition to the size-independent weight-based activity, the preference for larger particles in industrial catalysts can be attributed to the high silver loadings used to obtain larger silver particles. The resulting high coverage of the α-Al 2O 3 support with silver decreases undesired consecutive reactions of EO on its hydroxyl groups. </p