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Active Site Engineering via Optimizing the Heterogeneous Support Structure for Single-Atom Catalysis
Supported single-atom catalysts show a large range of activities and selectivities that depend on the local environment of the catalytic sites. Here a theory-based optimization strategy is presented that is based on a density functional theory determination of the transition states and intermediates for a low-dimensional coordinate representation of the heterogeneity of the active sites. The approach is applied to a vanadium catalyst on an amorphous SiO 2 support that involves a large kinetic network described using a full chemistry model. Without assuming a priori scaling relations or mechanism reduction, the optimal state of heterogeneity is found to lie at atomic configurations where the activation energies for two distinct key chemical processes are equal. It is found a posteriori that the behavior of the system is consistent with linear free energy scaling relations in the randomness parameters. The energetic span theory proves quite useful in reducing the full chemistry model to a small number of key reactions. The use of a nonlinear optimization algorithm in combination with energetic span theory provides significant simplification in treating disordered systems.
Assessing Exchange-Correlation Functionals for Heterogeneous Catalysis of Nitrogen Species
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Solvation Effects for Oxygen Evolution Reaction Catalysis on IrO 2 (110)
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Density functional analysis of fluorite-structured (Ce, Zr)O 2 /CeO 2 interfaces [Density functional analysis of fluorite-structured (Ce, Zr)O 2 /CeO 2 interfaces: Implications for catalysis and energy applications]
The structures and properties of Ce 1–x Zr x O 2 (x = 0–1) solid solutions, selected Ce 1–x Zr x O 2 surfaces, and Ce 1–x Zr x O 2 /CeO 2 interfaces were computed within the framework of density functional theory corrected for strong electron correlation (DFT+ U ). The calculated Debye temperature increases steadily with Zr content in (Ce, Zr)O 2 phases, indicating a significant rise in microhardness from CeO 2 to ZrO 2 , without appreciable loss in ductility as the interfacial stoichiometry changes. Surface energy calculations for the low-index CeO 2 (111) and (110) surfaces show limited sensitivity to strong 4f-electron correlation. The fracture energy of Ce 1–x Zr x O 2 (111)/CeO 2 (111) increases markedly with Zr content, with a significant decrease in energy for thicker Ce 1–x Zr x O 2 films. These findings suggest the crucial role of Zr acting as a binder at the Ce 1–x Zr x O 2 /CeO 2 interfaces, due to the more covalent character of Zr–O bonds compared to Ce–O. Finally, the impact of surface relaxation upon interface cracking was assessed and found to reach a maximum for Ce 0.25 Zr 0.75 O 2 /CeO 2 interfaces.
First-Principles Study of Alkoxides Adsorbed on Au(111) and Au(110) Surfaces: Assessing the Roles of Noncovalent Interactions and Molecular Structures in Catalysis
Microscopic understanding of molecular adsorption on catalytic surfaces is crucial for controlling the activity and selectivity of chemical reactions. However, for complex molecules, the adsorption process is very system-specific and there is a clear need to elaborate systematic understanding of important factors that determine catalytic functionality. In this study, we investigate the binding of eight molecules, including seven alkoxides and one carboxylate, on the Au(111) and Au(110) surfaces. Our density-functional theory calculations including long-range van der Waals interactions demonstrate the significant role of these “weak” noncovalent forces on the adsorption structures, energetics, and relative adsorbate stabilities. Interestingly, the binding energy trends are insensitive to the surface structure. Instead, the adsorption stability depends strongly on the structural and chemical characteristics of the molecules: linear vs branching configurations, number of unsaturated C–C bonds, bidentate adsorption, and the presence of electronegative atoms. Finally, our calculations help establish the influence of individual and collective chemical factors that determine the catalytic selectivity of alkoxides.
Water Oxidation Catalysis via Size-Selected Iridium Clusters
Here, the detailed mechanism and efficacy of four electron electrochemical water oxidation depend critically upon the detailed atomic structure of each catalytic site, which are numerous and diverse in most metal oxides anodes. In order to limit the diversity of sites, arrays of discrete iridium clusters with identical metal atom number (Ir 2 , Ir 4 , or Ir 8 ) were deposited in submonolayer coverage on conductive oxide supports, and the electrochemical properties and activity of each was evaluated. Exceptional electroactivity for the oxygen evolving reaction (OER) was observed for all cluster samples in acidic electrolyte. Reproducible cluster-size-dependent trends in redox behavior were also resolved. First-principles computational models of the individual discrete-size clusters allow correlation of catalytic-site structure and multiplicity with redox behavior.
Role of Undercoordinated Sites for the Catalysis in Confined Spaces Formed by Two-Dimensional Material Overlayers
Adding a two-dimensional (2D) overlayer on a metal surface is a promising route for activating reactants confined in the interfacial space. However, an atomistic understanding of the role played by undercoordinated sites of the 2D overlayer in the activation of molecules in this nanoscaled confined space is yet to be developed. In this paper, we study CO dissociation as a prototypical reaction to investigate CO activation in the confined space enclosed by Rh(111) and a monolayer of hexagonal boron nitride (h-BN). The effect of the space size (i.e., the distance between h-BN and the metal surface), the type of undercoordinated sites, and the size of the defect are explicitly studied by density functional theory with dispersion correction. The following temperature-programmed X-ray photoelectron spectroscopy measurement suggests that a small portion of the CO dissociated during the desorption, leaving the residual atomic oxygen incorporated into the h-BN lattice, which validates the theoretical prediction. Finally, the combination of theory and experiment calls for further attention to be paid to the role of undercoordinated sites in the 2D overlayers in confined systems forming potential new catalytic environments.
Tuning Proton Transfer Thermodynamics in SARS-CoV-2 Main Protease: Implications for Catalysis and Inhibitor Design
The catalytic reaction in SARS-CoV-2 main protease is activated by a proton transfer (PT) from Cys145 to His41. The same PT is likely also required for the covalent binding of some inhibitors. Here we use a multiscale computational approach to investigate the PT thermodynamics in the apo enzyme and in complex with two potent inhibitors, N3 and the α-ketoamide 13b. We show that with the inhibitors the free energy cost to reach the charge-separated state of the active-site dyad is lower, with N3 inducing the most significant reduction. We also show that a few key sites (including specific water molecules) significantly enhance or reduce the thermodynamic feasibility of the PT reaction, with selective desolvation of the active site playing a crucial role. The approach presented is a cost-effective procedure to identify the enzyme regions that control the activation of the catalytic reaction and is thus also useful to guide the design of inhibitors.
Bridging Structural and Chemical Insights: Integrating In Situ Electron Microscopy and X-ray Spectroscopy for Catalysis Research
Environmental transmission electron microscopy probes the local structure, composition, and chemistry of materials under gas environments, while ambient-pressure X-ray photoelectron spectroscopy provides ensemble chemical and electronic structure information in gaseous conditions. Both techniques utilize similar differential pumping schemes to mitigate electron scattering by the gas phase, allowing for unique opportunities to correlate gas–surface interactions across comparable pressure ranges. Their integration has advanced the understanding of various catalytic reactions, including the water–gas-shift reaction, CO oxidation, and surface passivation dynamics. In conclusion, this Mini-Review discusses their methodological advancements, challenges, and potential for further integration with other in situ techniques to address complex catalytic phenomena and guide catalyst design.
Tracking Rh Atoms in Zeolite HY: First Steps of Metal Cluster Formation and Influence of Metal Nuclearity on Catalysis of Ethylene Hydrogenation and Ethylene Dimerization
The initial steps of rhodium cluster formation from zeolite-supported mononuclear Rh(C2H4)2 complexes in H2 at 373 K and 1 bar were investigated by infrared and extended X-ray absorption fine structure spectroscopies and scanning transmission electron microscopy (STEM). The data show that ethylene ligands on the rhodium react with H2 to give supported rhodium hydrides and trigger the formation of rhodium clusters. STEM provided the first images of the smallest rhodium clusters (Rh2) and their further conversion into larger clusters. The samples were investigated in a plug-flow reactor as catalysts for the conversion of ethylene + H2 in a molar ratio of 4:1 at 1 bar and 298 K, with the results showing how the changes in catalyst structure affect the activity and selectivity; the rhodium clusters are more active for hydrogenation of ethylene than the single-site complexes, which are more selective for dimerization of ethylene to give butenes
Resolving the Iron Phthalocyanine Redox Transitions for ORR Catalysis in Aqueous Media
Metal macrocycles are among the most important catalytic systems in electrocatalysis and biocatalysis owing to their rich redox chemistry. Precise understanding of the redox behavior of metal macrocycles in operando is essential for fundamental studies and practical applications of this catalytic system. Here we present electrochemical data for the representative iron phthalocyanine (FePc) in both aqueous and nonaqueous media coupled with in situ Raman and X-ray absorption analyses to challenge the traditional notion of the redox transition of FePc at the low potential end in aqueous media by showing that it arises from the redox transition of the ring. Our data unequivocally demonstrate that the electron is shuttled to the Pc ring via the Fe(II)/Fe(I) redox center. The Fe(II)/Fe(I) redox transition of FePc in aqueous media is indiscernible by normal spectroscopic methods owing to the lack of a suitable axial ligand to stabilize the Fe(I) state.