Understanding the Role of Metal-Modified Mo(110) Bimetallic Surfaces for C–O/CO and C–C Bond Scission in C3 Oxygenates

Bond scission of C–O/CO, C–C, and C–H from oxygenates on Mo(110), Ni/Mo(110), and Co/Mo(110) has been investigated via density functional theory (DFT) calculations, temperature-programmed desorption (TPD), and high-resolution electron energy loss spectroscopy (HREELS). Propanal and 1-propanol are used as probe molecules for biomass-derived oxygenates due to their relatively high vapor pressures, allowing their easy introduction into UHV systems. DFT results predict that the binding energy trend of propanal and 1-propanol is Mo(110) > Co/Mo(110) > Ni/Mo(110), which suggests that binding energies are reduced by the modification of Mo(110) with Ni and Co admetals. TPD and HREELS results show that bond scission activity and selectivity can be tuned upon admetal modification of Mo(110). For both molecules, Mo(110) shows a highly selective deoxygenation pathway toward C–O/CO bond scission to produce propene, while bimetallic surfaces instead exhibit a higher activity for C–C and C–H bond scission. Among the three surfaces, Ni modification leads to the highest selectivity for decarbonylation to produce ethylene and Co modification results in the highest selectivity for reforming to produce syngas

Comparison of Methodologies of Activation Barrier Measurements for Reactions with Deactivation

Methodologies of activation barrier measurements for reactions with deactivation were theoretically analyzed. Reforming of ethane with CO₂ was introduced as an example for reactions with deactivation to experimentally evaluate these methodologies. Both the theoretical and experimental results showed that due to catalyst deactivation, the conventional method would inevitably lead to a much lower activation barrier, compared to the intrinsic value, even though heat and mass transport limitations were excluded. In this work, an optimal method was identified in order to provide a reliable and efficient activation barrier measurement for reactions with deactivation

Grand Canonical Quantum Mechanical Study of the Effect of the Electrode Potential on N‑Heterocyclic Carbene Adsorption on Au Surfaces

Increasing interest has been focused on using N-heterocyclic carbenes (NHCs) as surface ligands to replace thiols in the preparation of self-assembled monolayers (SAMs) on gold due to their larger adsorption energies. However, one of the drawbacks of these NHC-based SAMs is that they are unstable under electrochemically reducing conditions. In this study, grand canonic quantum mechanics (GC-QM) were used to study the effect of the electrode potential (<i>U</i>) on the adsorption of NHC on Au(111). The NHC adsorption energies were significantly weaker (∼0.92 eV) under constant <i>U</i> conditions compared to those under constant charge conditions, demonstrating the importance of using GC-QM for studying electrochemical systems. Consistent with experiments, the results from our calculations indicated that the adsorption energy decreased as <i>U</i> became more negative but increased as <i>U</i> became more positive. These results were rationalized using the frontier orbital theory. Importantly, based on the same analysis, when NHCs or their analogues with a smaller gap between the singlet ground and triplet first excited states (Δ<i>E</i>_S–T < 1.1 eV) were employed as molecular anchors, the adsorption energy was much less affected by <i>U</i>. The same results were obtained for other common SAM substrates (i.e., Ag(111), Cu(111), and Pt(111)). Therefore, based on our GC-QM calculations, we propose that the key to developing a stable NHC-based SAM under electrochemical reducing conditions is to focus on NHCs or their analogues as surface ligands with small Δ<i>E</i>_S‑T’s

Reaction Pathways and Intermediates in Selective Ring Opening of Biomass-Derived Heterocyclic Compounds by Iridium

While the catalytic hydrogenolysis of biomass-derived aromatic cyclic compounds to functionalized long chain alcohols and polyols has been known for decades, the factors that control the selectivity remain either unknown or controversial. Previous reports have hypothesized full ring saturation of the aromatic ring is necessary prior to hydrogenolysis. Contradictorily, recent studies have shown hydrogenolysis occurs prior to the saturation of the conjugated bonds. Furthermore, it has been assumed the functional groups present are fully reduced prior to hydrogenolysis; however, this has not been shown a priori. In order to resolve these controversies, we combine density functional theory and high-resolution electron energy loss spectroscopy (HREELS) to probe the catalytic hydrogenolysis of saturated and unsaturated heterocyclic molecules (furan, furfural, furfuryl alcohol, and tetrahydrofurfuryl alcohol) on iridium. Our results reveal that full saturation of the aromatic ring is not only unnecessary but leads to slower kinetics and differing selectivities. In contrast to previous studies, we show selective partial ring saturation can enhance the kinetics of the hydrogenolysis process. Reduction/oxidation of the functional group leads to a change in the electronegativity, resulting in a change in selectivity. These results provide important mechanistic insights allowing for further improvement of catalysts for the effective transformations of biomass-derived oxygenates to value-added products

Quantum Mechanical Study of N‑Heterocyclic Carbene Adsorption on Au Surfaces

There is increasing interest in using N-heterocyclic carbenes (NHCs) as surface ligands to stabilize transition-metal nanoparticles (NPs) and to replace thiols for the preparation of self-assembled monolayers (SAMs) on gold surfaces. This type of surface decoration is advantageous because it leads to improved catalytic activity of NPs and increased stability of SAM, as shown by recent experiments. In this work, we used quantum mechanics combined with periodic surface models to study the adsorption of NHCs on the Au(111) surface. We found that NHCs prefer to bind to the top site with adsorption energies (Δ<i>E</i>s) varying from 1.69 to 2.34 eV, depending on the type of NHC, and the inclusion of solvents in the calculations leads to insignificant variation in the calculated Δ<i>E</i>s. Three types of NHCs were found to bind to Au(111) more tightly and therefore should be better stabilizers than those commonly used. Importantly, by analyzing electronic structures using the Bader charge and energy decomposition analysis, we find that during adsorption NHC acts as an electron donor, transferring its electron density from the lone pair orbital at the carbene center to the empty d orbital of Au with negligible π-back-donation. This binding pattern is very different from that of CO, a ligand commonly used in organometallics, where both interactions are equally important. This leads to the identification of the protonation energies of NHCs as a descriptor for predicting Δ<i>E</i>s, providing a convenient method for computational high-throughput screening for better NHC-type surface ligands

Reaction Pathways and Intermediates in Selective Ring Opening of Biomass-Derived Heterocyclic Compounds by Iridium

While the catalytic hydrogenolysis of biomass-derived aromatic cyclic compounds to functionalized long chain alcohols and polyols has been known for decades, the factors that control the selectivity remain either unknown or controversial. Previous reports have hypothesized full ring saturation of the aromatic ring is necessary prior to hydrogenolysis. Contradictorily, recent studies have shown hydrogenolysis occurs prior to the saturation of the conjugated bonds. Furthermore, it has been assumed the functional groups present are fully reduced prior to hydrogenolysis; however, this has not been shown a priori. In order to resolve these controversies, we combine density functional theory and high-resolution electron energy loss spectroscopy (HREELS) to probe the catalytic hydrogenolysis of saturated and unsaturated heterocyclic molecules (furan, furfural, furfuryl alcohol, and tetrahydrofurfuryl alcohol) on iridium. Our results reveal that full saturation of the aromatic ring is not only unnecessary but leads to slower kinetics and differing selectivities. In contrast to previous studies, we show selective partial ring saturation can enhance the kinetics of the hydrogenolysis process. Reduction/oxidation of the functional group leads to a change in the electronegativity, resulting in a change in selectivity. These results provide important mechanistic insights allowing for further improvement of catalysts for the effective transformations of biomass-derived oxygenates to value-added products

A New Class of Electrocatalysts for Hydrogen Production from Water Electrolysis: Metal Monolayers Supported on Low-Cost Transition Metal Carbides

This work explores the opportunity to substantially reduce the cost of hydrogen evolution reaction (HER) catalysts by supporting monolayer (ML) amounts of precious metals on transition metal carbide substrates. The metal component includes platinum (Pt), palladium (Pd), and gold (Au); the low-cost carbide substrate includes tungsten carbides (WC and W₂C) and molybdenum carbide (Mo₂C). As a platform for these studies, single-phase carbide thin films with well-characterized surfaces have been synthesized, allowing for a direct comparison of the intrinsic HER activity of bare and Pt-modified carbide surfaces. It is found that WC and W₂C are both excellent cathode support materials for ML Pt, exhibiting HER activities that are comparable to bulk Pt while displaying stable HER activity during chronopotentiometric HER measurements. The findings of excellent stability and HER activity of the ML Pt–WC and Pt–W₂C surfaces may be explained by the similar bulk electronic properties of tungsten carbides to Pt, as is supported by density functional theory calculations. These results are further extended to other metal overlayers (Pd and Au) and supports (Mo₂C), which demonstrate that the metal ML-supported transition metal carbide surfaces exhibit HER activity that is consistent with the well-known volcano relationship between activity and hydrogen binding energy. This work highlights the potential of using carbide materials to reduce the costs of hydrogen production from water electrolysis by serving as stable, low-cost supports for ML amounts of precious metals

Optimizing Binding Energies of Key Intermediates for CO₂ Hydrogenation to Methanol over Oxide-Supported Copper

Rational optimization of catalytic performance has been one of the major challenges in catalysis. Here we report a bottom-up study on the ability of TiO₂ and ZrO₂ to optimize the CO₂ conversion to methanol on Cu, using combined density functional theory (DFT) calculations, kinetic Monte Carlo (KMC) simulations, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements, and steady-state flow reactor tests. The theoretical results from DFT and KMC agree with in situ DRIFTS measurements, showing that both TiO₂ and ZrO₂ help to promote methanol synthesis on Cu via carboxyl intermediates and the reverse water–gas-shift (RWGS) pathway; the formate intermediates, on the other hand, likely act as a spectator eventually. The origin of the superior promoting effect of ZrO₂ is associated with the fine-tuning capability of reduced Zr³⁺ at the interface, being able to bind the key reaction intermediates, e.g. *CO₂, *CO, *HCO, and *H₂CO, moderately to facilitate methanol formation. This study demonstrates the importance of synergy between theory and experiments to elucidate the complex reaction mechanisms of CO₂ hydrogenation for the realization of a better catalyst by design

Porous MS₂/MO₂ (M = W, Mo) Nanorods as Efficient Hydrogen Evolution Reaction Catalysts

Highly efficient and stable electrochemical catalysts of porous one-dimensional (1D) MS₂/MO₂ (M = W, Mo) nanorods have been developed for the hydrogen evolution reaction (HER). The materials are synthesized via a “low-temperature conversion” method. The as-prepared catalysts exhibit numerous advantages: abundant amount of exposed MS₂ edges, high conductivity, and enhanced mass transport. The MS₂/MO₂ nanorods show efficient HER activity

Ordered Mesoporous Metal Carbides with Enhanced Anisole Hydrodeoxygenation Selectivity

Mesoporous metal carbides are of particular interest as catalysts for a variety of reactions because of their high surface areas, porous networks, nanosized walls, and unique electronic structures. Here, two ordered mesoporous metal carbides, Mo₂C and W₂C, were synthesized using a nanocasting approach coupled with a simultaneous decomposition/carburization process under a continuous methane flow. The as-synthesized mesoporous Mo₂C and W₂C have three dimensionally ordered porous structures, large surface areas (70–90 m² g^–1), and crystalline walls. In vapor-phase anisole hydrodeoxygenation (HDO) reactions, they exhibited turnover frequencies of approximately 9 and 2 × 10^–4 mol mol_CO^–1 s^–1, respectively, at relatively low reaction temperatures (423–443 K) and ambient hydrogen pressures. Notably, the ordered mesoporous W₂C catalyst showed a greater than 96% benzene selectivity in anisole HDO, the highest benzene selectivity reported to date