Tuning the MoS₂ Edge-Site Activity for Hydrogen Evolution via Support Interactions

The hydrogen evolution reaction (HER) on supported MoS₂ catalysts is investigated using periodic density functional theory, employing the new BEEF-vdW functional that explicitly takes long-range van der Waals (vdW) forces into account. We find that the support interactions involving vdW forces leads to significant changes in the hydrogen binding energy, resulting in several orders of magnitude difference in HER activity. It is generally seen for the Mo-edge that strong adhesion of the catalyst onto the support leads to weakening in the hydrogen binding. This presents a way to optimally tune the hydrogen binding on MoS₂ and explains the lower than expected exchange current densities of supported MoS₂ in electrochemical H₂ evolution studies

How Doped MoS₂ Breaks Transition-Metal Scaling Relations for CO₂ Electrochemical Reduction

Linear scaling relationships between the adsorption energies of CO₂ reduction intermediates pose a fundamental limitation to the catalytic efficiency of transition-metal catalysts. Significant improvements in CO₂ reduction activity beyond transition metals require the stabilization of key intermediates, COOH* and CHO* or COH*, independent of CO*. Using density functional theory (DFT) calculations, we show that the doped sulfur edge of MoS₂ satisfies this requirement by binding CO* significantly weaker than COOH*, CHO*, and COH*, relative to transition-metal surfaces. The structural basis for the scaling of doped sulfur edge of MoS₂ is due to CO* binding on the metallic site (doping metal) and COOH*, CHO*, and COH* on the covalent site (sulfur). Linear scaling relations still exist if all the intermediates bind to the same site, but the combined effect of the two binding sites results in an overall deviation from transition-metal scaling lines. This principle can be applied to other metal/<i>p</i>-block materials. We rationalize the weak binding of CO* on the sulfur site with distortion/interaction and charge density difference analyses

Understanding the Reactivity of Layered Transition-Metal Sulfides: A Single Electronic Descriptor for Structure and Adsorption

Density functional theory is used to investigate the adsorption and structural properties of layered transition-metal sulfide (TMS) catalysts. We considered both the (101̅0) M-edge and (1̅010) S-edge terminations for a wide range of pure and doped TMSs, determined their sulfur coverage under realistic operating conditions (i.e, steady-state structures), and calculated an extensive set of chemisorption energies for several important reactions. On the basis of these results, we show that the d-band center, ε_d, of the edge-most metal site at 0 ML sulfur coverage is a general electronic descriptor for both structure and adsorption energies, which are known to describe catalytic activity. A negative linear correlation between adsorbate–S binding and S–metal binding allows ε_d to describe the adsorption of species on both metal and sulfur sites. Our results provide a significant simplification in the understanding of structure–activity relationships in TMSs and provides guidelines for the rational design and large-scale screening of these catalysts for various processes

Automated Discovery and Construction of Surface Phase Diagrams Using Machine Learning

Surface phase diagrams are necessary for understanding surface chemistry in electrochemical catalysis, where a range of adsorbates and coverages exist at varying applied potentials. These diagrams are typically constructed using intuition, which risks missing complex coverages and configurations at potentials of interest. More accurate cluster expansion methods are often difficult to implement quickly for new surfaces. We adopt a machine learning approach to rectify both issues. Using a Gaussian process regression model, the free energy of all possible adsorbate coverages for surfaces is predicted for a finite number of adsorption sites. Our result demonstrates a rational, simple, and systematic approach for generating accurate free-energy diagrams with reduced computational resources. The Pourbaix diagram for the IrO₂(110) surface (with nine coverages from fully hydrogenated to fully oxygenated surfaces) is reconstructed using just 20 electronic structure relaxations, compared to approximately 90 using typical search methods. Similar efficiency is demonstrated for the MoS₂ surface

Predicting Promoter-Induced Bond Activation on Solid Catalysts Using Elementary Bond Orders

In this Letter, we examine bond activation induced by nonmetal surface promoters in the context of dehydrogenation reactions. We use C–H bond activation in methane dehydrogenation on transition metals as an example to understand the origin of the promoting or poisoning effect of nonmetals. The electronic structure of the surface and the bond order of the promoter are found to establish all trends in bond activation. On the basis of these results, we develop a predictive model that successfully describes the energetics of C–H, O–H, and N–H bond activation across a range of reactions. For a given reaction step, a single data point determines whether a nonmetal will promote bond activation or poison the surface and by how much. We show how our model leads to general insights that can be directly used to predict bond activation energetics on transition metal sulfides and oxides, which can be perceived as promoted surfaces. These results can then be directly used in studies on full catalytic pathways

Scaling Relations for Adsorption Energies on Doped Molybdenum Phosphide Surfaces

Molybdenum phosphide (MoP), a well-documented catalyst for applications ranging from hydrotreating reactions to electrochemical hydrogen evolution, has yet to be mapped from a more fundamental perspective, particularly in the context of transition-metal scaling relations. In this work, we use periodic density functional theory to extend linear scaling arguments to doped MoP surfaces and understand the behavior of the phosphorus active site. The derived linear relationships for hydrogenated C, N, and O species on a variety of doped surfaces suggest that phosphorus experiences a shift in preferred bond order depending on the degree of hydrogen substitution on the adsorbate molecule. This shift in phosphorus hybridization, dependent on the bond order of the adsorbate to the surface, can result in selective bond weakening or strengthening of chemically similar species. We discuss how this behavior deviates from transition-metal, sulfide, carbide, and nitride scaling relations, and we discuss potential applications in the context of electrochemical reduction reactions

Computational Design of Active Site Structures with Improved Transition-State Scaling for Ammonia Synthesis

The Haber–Bosch process for the reduction of atmospheric nitrogen to ammonia is one of the most optimized heterogeneous catalytic reactions, but there are aspects of the industrial process that remain less than ideal. It has been shown that the activity of metal catalysts is limited by a Brønsted–Evans–Polanyi (BEP) scaling relationship between the reaction and transition-state energies for N₂ dissociation, leading to a negligible production rate at ambient conditions and a modest rate under harsh conditions. In this study, we use density functional theory (DFT) calculations in conjunction with mean-field microkinetic modeling to study the rate of NH₃ synthesis on model active sites that require the singly coordinated dissociative adsorption of N atoms onto transition metal atoms. Our results demonstrate that this ”on-top” binding of nitrogen exhibits significantly improved scaling behavior, which can be rationalized in terms of transition-state geometries and leads to considerably higher predicted activity. While synthesis of these model systems is likely challenging, the stabilization of such an active site could enable thermochemical ammonia synthesis under more benign conditions

Chemical and Phase Evolution of Amorphous Molybdenum Sulfide Catalysts for Electrochemical Hydrogen Production

Amorphous MoS_<i>x</i> is a highly active, earth-abundant catalyst for the electrochemical hydrogen evolution reaction. Previous studies have revealed that this material initially has a composition of MoS₃, but after electrochemical activation, the surface is reduced to form an active phase resembling MoS₂ in composition and chemical state. However, structural changes in the MoS_<i>x</i> catalyst and the mechanism of the activation process remain poorly understood. In this study, we employ transmission electron microscopy (TEM) to image amorphous MoS_<i>x</i> catalysts activated under two hydrogen-rich conditions: <i>ex situ</i> in an electrochemical cell and <i>in situ</i> in an environmental TEM. For the first time, we directly observe the formation of crystalline domains in the MoS_<i>x</i> catalyst after both activation procedures as well as spatially localized changes in the chemical state detected <i>via</i> electron energy loss spectroscopy. Using density functional theory calculations, we investigate the mechanisms for this phase transformation and find that the presence of hydrogen is critical for enabling the restructuring process. Our results suggest that the surface of the amorphous MoS_<i>x</i> catalyst is dynamic: while the initial catalyst activation forms the primary active surface of amorphous MoS₂, continued transformation to the crystalline phase during electrochemical operation could contribute to catalyst deactivation. These results have important implications for the application of this highly active electrocatalyst for sustainable H₂ generation

Chemical and Phase Evolution of Amorphous Molybdenum Sulfide Catalysts for Electrochemical Hydrogen Production

Amorphous MoS_<i>x</i> is a highly active, earth-abundant catalyst for the electrochemical hydrogen evolution reaction. Previous studies have revealed that this material initially has a composition of MoS₃, but after electrochemical activation, the surface is reduced to form an active phase resembling MoS₂ in composition and chemical state. However, structural changes in the MoS_<i>x</i> catalyst and the mechanism of the activation process remain poorly understood. In this study, we employ transmission electron microscopy (TEM) to image amorphous MoS_<i>x</i> catalysts activated under two hydrogen-rich conditions: <i>ex situ</i> in an electrochemical cell and <i>in situ</i> in an environmental TEM. For the first time, we directly observe the formation of crystalline domains in the MoS_<i>x</i> catalyst after both activation procedures as well as spatially localized changes in the chemical state detected <i>via</i> electron energy loss spectroscopy. Using density functional theory calculations, we investigate the mechanisms for this phase transformation and find that the presence of hydrogen is critical for enabling the restructuring process. Our results suggest that the surface of the amorphous MoS_<i>x</i> catalyst is dynamic: while the initial catalyst activation forms the primary active surface of amorphous MoS₂, continued transformation to the crystalline phase during electrochemical operation could contribute to catalyst deactivation. These results have important implications for the application of this highly active electrocatalyst for sustainable H₂ generation

The Predominance of Hydrogen Evolution on Transition Metal Sulfides and Phosphides under CO₂ Reduction Conditions: An Experimental and Theoretical Study

A combination of experiment and theory has been used to understand the relationship between the hydrogen evolution reaction (HER) and CO₂ reduction (CO₂R) on transition metal phosphide and transition metal sulfide catalysts. Although multifunctional active sites in these materials could potentially improve their CO₂R activity relative to pure transition metal electrocatalysts, under aqueous testing conditions, these materials showed a high selectivity for the HER relative to CO₂R. Computational results supported these findings, indicating that a limitation of the metal phosphide catalysts is that the HER is favored thermodynamically over CO₂R. On Ni-MoS₂, a limitation is the kinetic barrier for the proton–electron transfer to *CO. These theoretical and experimental results demonstrate that selective CO₂R requires electrocatalysts that possess both favorable thermodynamic pathways and surmountable kinetic barriers