Pathways for H₂ Activation on (Ni)-MoS₂ Catalysts

The activation of H₂ and H₂S on (Ni)­MoS₂/Al₂O₃ leads to the formation of SH groups with acid character able to protonate 2,6-dimethylpyridine. The variation in concentrations of SH groups induced by H₂ and H₂S adsorption shows that both molecules dissociate on coordinatively unsaturated cations and neighboring S^2–. In the studied materials, one sulfur vacancy and four SH groups per 10 metal atoms exist at the active edges of MoS₂ under the conditions studied. H₂–D₂ exchange studies show that Ni increases the concentration of active surface hydrogen by up to 30% at the optimum Ni loading, by increasing the concentration of H₂ and H₂S chemisorption sites

Carbon–Carbon Bond Scission Pathways in the Deoxygenation of Fatty Acids on Transition-Metal Sulfides

The mechanism of the deoxygenation of fatty acids on transition-metal sulfides was determined on the basis of kinetic data obtained with fatty acids, their reaction intermediates (aldehyde and alcohol), and reactants of restricted reactivity (adamantanyl-substituted carboxylic acids). Deoxygenation on MoS₂ proceeds exclusively via hydrogenolysis to aldehyde, followed by hydrogenation to the corresponding alcohol, consecutive dehydration to the olefin, and hydrogenation to the alkane. In contrast, the selectivity on Ni-MoS₂ and on Ni₃S₂ substantially shifts toward carbon oxide elimination routes: i.e., direct production of C_<i>n</i>–1 olefins and alkanes. The carbon losses occur by decarbonylation of a ketene intermediate, which forms only on sites associated with Ni. The rate determining steps are the cleavage of the C–C bond and the removal of oxygen from the surface below and above, respectively, 2.5 MPa of H₂. The different reaction pathways catalyzed by MoS₂ and Ni-MoS₂ are attributed to a preferred deprotonation of a surface acyl intermediate formed upon the adsorption of the fatty acid on Ni-MoS₂. The shift in mechanism is concluded to originate from the higher basicity of sulfur induced by nickel

Mechanistic Pathways for Methylcyclohexane Hydrogenolysis over Supported Ir Catalysts

H/D isotope effects on methylcyclohexane hydrogenolysis over Ir/Al₂O₃ catalysts were examined by combining measured rates with theoretical estimates provided by partition function based analyses. Normal H/D isotope effects (<i>r</i>_H/<i>r</i>_D > 1) were observed for endocyclic and exocyclic C–C bond hydrogenolysis. Hydrogenolysis is concluded to occur via stepwise dehydrogenation followed by cleavage of the C–C bond and subsequent hydrogenation of the cleaved entities. The so-called “multiplet” mechanism (i.e., the C–C bond of a flat-lying physisorbed cyclic molecule is cleaved upon the attack of a coadsorbed H atom) is unequivocally excluded. For ring-opening, either C–C bond cleavage or C–H­(D) bond reformation may be rate-determining, due to their indistinguishable isotope effects under the studied conditions. C–H­(D) bond dissociation does not control the rate of C–C bond hydrogenolysis. For the exocyclic cleavage of the methyl group, a higher degree of unsaturation of the surface intermediate and the potential impact of mobile H atoms on large Ir particles are noted

Deoxygenation of Palmitic Acid on Unsupported Transition-Metal Phosphides

Highly active bulk transition-metal phosphides (WP, MoP, and Ni₂P) were synthesized for the catalytic hydrodeoxygenation of palmitic acid, hexadecanol, hexadecanal, and microalgae oil. The specific activities positively correlated with the concentration of exposed metal sites, although the relative rates changed with temperature due to activation energies varying from 57 kJ mol^–1 for MoP to 142 kJ mol^–1 for WP. The reduction of the fatty acid to the aldehyde occurs through a Langmuir–Hinshelwood mechanism, where the rate-determining step is the addition of the second H to the hydrocarbon. On WP, the conversion of palmitic acid proceeds via R-CH₂COOH → R-CH₂CHO → R-CH₂CH₂OH → R-CHCH₂ → R-CH₂CH₃ (hydrodeoxygenation). Decarbonylation of the intermediate aldehyde (R-CH₂COOH → R-CH₂CHO → R-CH₃) was an important pathway on MoP and Ni₂P. Conversion via dehydration to a ketene, followed by its decarbonylation, occurred only on Ni₂P. The rates of alcohol dehydration (R-CH₂CH₂OH → R-CHCH₂) correlate with the concentrations of Lewis acid sites of the phosphides

Overcoming the Rate-Limiting Reaction during Photoreforming of Sugar Aldoses for H₂‑Generation

Photoreforming of sugars on metalloaded semiconductors is an attractive process for H₂-generation. However, the reaction proceeds typically with rapidly decreasing rates. We identified that this decrease is due to kinetic constraints rather than to catalyst deactivation. Thus, the nature of the rate-limiting reaction was elucidated by investigation of the reaction pathways and oxidation mechanisms during photoreforming of sugar aldoses on TiO₂ decorated with Rh, Pd, or Pt. Using selective isotope labeling it is shown that ring opening of aldoses via direct hole transfer to the chemisorbed oxygenates yields primary formate esters. Under pH-neutral and acidic conditions, formates convert to the consecutive aldose intermediate through light-driven, redox-neutral hydrolysis. The slow kinetics of this step, which requires interaction with negative and positive photogenerated charges, leads to blocking of active sites at the photoanode and enhanced electron–hole recombination. Stable H₂-evolution and sugar conversion over time is achieved through alkalinization of the aqueous-phase due to fast OH^–-induced hydrolytic cleavage of formate intermediates

Effects of the Support on the Performance and Promotion of (Ni)MoS₂ Catalysts for Simultaneous Hydrodenitrogenation and Hydrodesulfurization

MoS₂ and Ni-promoted MoS₂ catalysts supported on γ-Al₂O₃, siliceous SBA-15, and Zr- and Ti-modified SBA-15 were explored for the simultaneous hydrodesulfurization (HDS) of dibenzothiophene (DBT) and hydrodenitrogenation (HDN) of <i>o</i>-propylaniline (OPA). In all cases, OPA reacted preferentially via initial hydrogenation, and DBT was converted through direct sulfur removal. HDN and HDS activities of MoS₂ catalysts are determined by the dispersion of the sulfide phase. Ni promotion increased its dispersion and activity for DBT HDS and also increased the rate of HDN via enhancing the rate of hydrogenation. On nonpromoted MoS₂ catalysts, HDS was strongly inhibited by NH₃, and the addition of Ni dramatically reduced this inhibiting effect. The conclusion is that HDS is proportional to the concentration of Mo and Ni on the edges of sulfide particles. In contrast, the direct hydrodenitrogenation of OPA occurs only on accessible Mo cations and, hence, decreases with increasing Ni substitution. The nature of the support influences the dispersion of the nonpromoted catalysts as well as the decoration degree of Ni on the edges of the Ni–Mo–S phase. Furthermore, the acidity of the support influences the acidity of the supported sulfide phase, which may play an important role in HDN

Kinetic Coupling of Water Splitting and Photoreforming on SrTiO₃‑Based Photocatalysts

Coupling the proton reduction of overall water splitting with oxidation of oxygenated hydrocarbons (photoreforming) on Al-doped SrTiO₃ decorated with cocatalysts enables efficient photocatalytic H₂ generation along with oxygenate conversion, while decreasing the accumulation of harmful byproducts such as formaldehyde. Net H₂ evolution rates result from the contributions of the individual rates of water oxidation, oxygenate oxidation, and the back-reaction of H₂ and O₂ to water. The latter reaction is suppressed by a RhCrO_<i>x</i> cocatalyst or by high concentrations of oxygenates in the case of Rh cocatalyst, whereas the rates of organic oxidation depend on their molecular structure. In the absence of the back-reaction to water, the H₂ evolution rates are independent of the oxygenate type and concentration because the rates of water splitting compensate the variations in the rates of oxygenate conversion. Under such conditions of suppressed back-reaction, the selectivities to water and oxygenate oxidation, both occurring with the same quantum efficiencies, depend on the oxygenate type and concentration. The dominant pathways for organic transformations are ascribed to the action of intermediates generated at the semiconductor during water oxidation and O₂ evolution. On a semiconductor without cocatalyst, the O₂ produced during overall water splitting is reductively activated to participate in oxidation of organics without consuming evolved H₂

Enabling Overall Water Splitting on Photocatalysts by CO-Covered Noble Metal Co-catalysts

Photocatalytic overall water splitting requires co-catalysts that efficiently promote the generation of H₂ but do not catalyze its reverse oxidation. We demonstrate that CO chemisorbed on metal co-catalysts (Rh, Pt, Pd) suppresses the back reaction while maintaining the rate of H₂ evolution. On Rh/GaN:ZnO, the highest H₂ production rates were obtained with 4–40 mbar of CO, the back reaction remaining suppressed below 7 mbar of O₂. The O₂ and H₂ evolution rates compete with CO oxidation and the back reaction. The rates of all reactions increased with increasing photon absorption. However, due to different dependencies on the rate of charge carrier generation, the selectivities for O₂ and H₂ formation increased in comparison to CO oxidation and the back reaction with increasing photon flux and/or quantum efficiency. Under optimum conditions, the impact of CO to prevent the back reaction is identical to that of a Cr₂O₃ layer covering the active metal particle

Influence of the Molecular Structure on the Electrocatalytic Hydrogenation of Carbonyl Groups and H₂ Evolution on Pd

We investigated the electrocatalytic hydrogenation (ECH) of model aldehydes and ketones over carbon-supported Pd in the aqueous phase. We propose reaction mechanisms based on kinetic measurements and on spectroscopic and electrochemical characterization of the working catalyst. The reaction rates of ECH and of the H2 evolution reaction (HER) vary with the applied electric potential following trends that strongly depend on the organic substrate. The intrinsic rates of hydrogenation and H2 evolution are influenced, in opposing ways, by the sorption of the reacting organic substrate. Strong interactions, that is, higher standard free energies of adsorption of the organic compound, induce high hydrogenation rates. The fast hydrogenation kinetics produces a hydrogen-depleted environment that kinetically hinders the HER and the bulk phase transition of Pd to a H-rich bulk Pd hydride, which is triggered by the applied potential in the absence of reacting organic compounds. As a consequence of strong organic–metal interactions, hydrogenation dominates at low overpotential. However, the coverages of organic substrates on the metal surface decrease, and the rates of H2 evolution surpass those of hydrogenation with increasingly negative electric potential. We determined the range of electric potential favoring hydrogenation on Pd and quantitatively deconvoluted the effects of the sorption of the organic compound, and of the rates of proton-coupled electron transfers, on the kinetics of both ECH and HER. The results indicate that electrocatalysis offers hydrogenation pathways for polar molecules which are different and, in some cases, faster than those dominating in the absence of an external electric potential

Impact of the Environment of BEA-Type Zeolites for Sorption of Water and Cyclohexanol

The (mutual) interactions of water and cyclohexanol with the pore walls and functional groups of Brønsted acidic zeolites of the BEA type (H-BEA) have been investigated. Upon reaction with Brønsted acid sites, water forms hydrated hydroxonium ions limited in size by the sorption free energy, creating in this way domains occupied by water. Organic molecules, such as cyclohexanol, occupy the remaining unoccupied volume. The pore size of the zeolite H-BEA stabilizes hydrated hydroxonium ions (H+(H2O)10) that are two H2O molecules larger than those formed in the smaller pore zeolite H-ZSM-5. Increasing the density of hydroxonium ions by increasing the concentration of aluminum in the zeolite gradually leads to less negative standard free energy of adsorbed cyclohexanol. The increasing proximity of positive charges of the hydroxonium ions induces a higher excess chemical potential of the sorbed molecule, which is manifested in a weakened interaction strength with the zeolite pores