Partial Oxidation of Methanol on MoO₃ (010): A DFT and Microkinetic Study

Methanol oxidation is employed as a probe reaction to evaluate the catalytic properties of the (010) facets of molybdenum trioxide (MoO₃), a reducible oxide that exhibits a rich interplay of catalytic chemistry and structural transformations. The reaction mechanism is investigated with a combination of electronic structure calculations, using the BEEF-vdW and HSE06 functionals, and mean-field microkinetic modeling. Considered pathways include vacancy formation and oxidation, monomolecular dehydrogenation of methanol on reduced and nonreduced surfaces, bimolecular reactions between dehydrogenated intermediates, and precursor steps for hydrogen molybdenum phase (H_<i>y</i>MoO_3–<i>x</i>) formation. Methanol dissociation begins with C–H or O–H scission, with the O–H route found to be kinetically and thermodynamically preferred. Dehydrogenation of CH₂O* to CHO* is slow in comparison to desorption, leading to complete selectivity toward CH₂O. C–H scission of CH₃O* and recombination of dissociated OH* to form H₂O* are kinetically significant steps exhibiting positive degrees of rate control, while oxidation of the reduced surface through adsorbed O₂ has a negative degree of rate control. The energetics of the latter elementary step are somewhat sensitive to the choice of density functional, and although this does not affect the predicted reaction orders, the overall rate may change. To estimate the impact of the surface oxidation state on the kinetics, the external pressure of oxygen is varied in the microkinetic model, and the reaction rate is found to follow a volcano-like dependency, with the optimum rate located where surface oxidation neither promotes nor inhibits the overall rate. The methodology demonstrated in this study should be more broadly applicable to modeling catalytic kinetics on reducible oxide single-crystal surfaces

Scaling Relationships for Molecular Adsorption and Dissociation in Lewis Acid Zeolites

Linear scaling relationships between adsorption energies of selected C₁ and C₂ oxygenates, as well as sulfur-containing compounds, are developed for Lewis acid-substituted zeolites in the chabazite framework. The relationships are shown to hold for dissociative adsorption of molecules cleaving similar classes of bonds, including O–H scission in CH₃OH and H₂O, C–H scission in CH₃OH and CH₄, S–H activation in CH₃SH and H₂S, C–H cleavage in CH₂O and CH₃CHO, and C–H activation in CH₂ and CHOH, where charge is conserved in the zeolite framework. These correlations are explained in terms of bond order conservation arguments that have been previously developed for metal and simple oxide surfaces, and extension of the scaling relationships to Brønsted acid sites is briefly discussed. For adsorbates with nonzero net spin, use of a high accuracy hybrid functional is shown to improve the accuracy of the scaling relationships, while such accuracy does not substantially affect the results for adsorption complexes with zero net spin. Finally, two representative applications of the scaling relationships are explored, including evaluation of the competition between C–H and O–H bond activation in methanol and ethanol and prediction of Lewis acid site opening energetics by dissociative adsorption reactions

Trends in Selective Hydrogen Peroxide Production on Transition Metal Surfaces from First Principles

We present a comprehensive, density functional theory based analysis of the direct synthesis of hydrogen peroxide, H₂O₂, on 12 transition metal surfaces. We determine the full thermodynamics and selected kinetics of the reaction network on these metals, and we analyze these energetics with simple, microkinetically motivated rate theories to assess the activity and selectivity of hydrogen peroxide production on the surfaces of interest. By further exploiting Brønsted–Evans–Polanyi relationships and scaling relationships between the binding energies of different adsorbates, we express the results in the form of a two-dimensional contour volcano plot, with the activity and selectivity being determined as functions of two independent descriptors: the atomic hydrogen and oxygen adsorption free energies. We identify both a region of maximum predicted catalytic activity, which is near Pt and Pd in descriptor space, and a region of selective hydrogen peroxide production, which includes Au. The optimal catalysts represent a compromise between activity and selectivity and are predicted to fall approximately between Au and Pd in descriptor space, providing a compact explanation for the experimentally known performance of Au–Pd alloys for hydrogen peroxide synthesis, and suggesting a target for future computational screening efforts to identify improved direct hydrogen peroxide synthesis catalysts. Related methods of combining activity and selectivity analysis into a single volcano plot may be applicable to, and useful for, other aqueous phase heterogeneous catalytic reactions where selectivity is a key catalytic criterion

First-Principles Study of Structure Sensitivity of Ethylene Glycol Conversion on Platinum

Periodic density functional theory (DFT) calculations are used to investigate the structure sensitivity of ethylene glycol (EG) decomposition on terraced and stepped platinum surfaces, including Pt(111) and Pt(211). On both surfaces, the binding energies of lightly dehydrogenated intermediates resulting from C–H bond scission in EG are typically stronger than the binding energies of intermediates associated with O–H bond breaking, and the corresponding kinetic trends generally track the thermochemical results. C–C and C–O bond cleavage have significantly higher barriers than dehydrogenation until relatively late in the dehydrogenation reaction network, and the transition state energies associated with C–C bond scission decrease almost monotonically with increasing levels of EG dehydrogenation, whereas the transition state energies of C–O bond breaking first decrease and then increase slightly. The most favorable reaction pathways for EG decomposition on Pt(111) and Pt(211) are very similar, with CO and H₂ as the main predicted products. However, Pt(211) shows substantially stronger binding of intermediates than does Pt(111). These results imply that platinum catalysts for EG conversion are likely to be relatively structure-sensitive in terms of activity but less sensitive in terms of selectivity. The results also demonstrate that linear relationships for prediction of both binding energies of dehydrogenated intermediates and barriers of elementary steps, which have been previously derived on close-packed terraces, are also found on steps, providing an important extension of these scaling and correlation principles to defected geometries. These relationships could, in turn, be used to accelerate the computational analysis of related complex reaction networks on undercoordinated surface features

Thermodynamic Stability of Low- and High-Index Spinel LiMn₂O₄ Surface Terminations

Density functional theory calculations are performed within the generalized gradient approximation (GGA+<i>U</i>) to determine stable terminations of both low- and high-index spinel LiMn₂O₄ (LMO) surfaces. A grand canonical thermodynamic approach is employed, permitting a direct comparison of off-stoichiometric surfaces with previously reported stoichiometric surface terminations at various environmental conditions. Within this formalism, we have identified trends in the structure of the low-index surfaces as a function of the Li and O chemical potentials. The results suggest that, under a range of chemical potentials for which bulk LMO is stable, Li/O and Li-rich (111) surface terminations are favored, neither of which adopts an inverse spinel structure in the subsurface region. This thermodynamic analysis is extended to identify stable structures for certain high-index surfaces, including (311), (331), (511), and (531), which constitute simple models for steps or defects that may be present on real LMO particles. The low- and high-index results are combined to determine the relative stability of each surface facet under a range of environmental conditions. The relative surface energies are further employed to predict LMO particle shapes through a Wulff construction approach, which suggests that LMO particles will adopt either an octahedron or a truncated octahedron shape at conditions in which LMO is thermodynamically stable. These results are in agreement with the experimental observations of LMO particle shapes

Adsorption Energy Correlations at the Metal–Support Boundary

The emergence of supercell density functional theory (DFT) over the past few decades has enabled great progress in rational catalyst design for extended surfaces of transition metals. However, insights from such metal-only models may not translate directly to metal nanoparticles dispersed on high surface area supporting phases, where metal and support may both participate in catalysis. To quantify these differences, we investigate the adsorption behavior of common catalytic intermediates at the boundaries of late transition metals supported on MgO(100). We show that the oxide can either strengthen or weaken adsorption at the metal–oxide boundary, depending on the metal–adsorbate combination. Using a thermodynamic cycle, we trace the origins of these stabilization/destabilization effects to a combination of multiple structural and electronic perturbations, including strain and ligand effects, geometric reorientation, and charging of the adsorbate. These perturbations in some cases result in departures from the linear scaling relations developed on metal-only models. Computational screening studies based on typical scaling relations may thus miss potential catalyst materials where bifunctional gains are possible

Electrochemical Surface Stress Development during CO and NO Oxidation on Pt

The adsorbate-induced surface stress during the electrochemical oxidation of CO and NO on Pt is studied with in situ surface stress measurements and density functional theory (DFT) calculations. The changes in the surface stress response, Δstress, demonstrate the interplay between the adsorbed species during the oxidation process, which is determined by the coverage and the nature of the adsorbates. The oxidation of adsorbed CO, CO_ads, shows a nonlinear surface stress response in both acidic and alkaline electrolytes, with the greatest tensile Δstress observed in the beginning of the oxidation where the CO coverage is the highest. Once a significant amount of CO is removed, OH starts to populate the surface and the Δstress becomes compressive. This surface stress development profilethe nonlinear stress development at high CO_ads coverages and the inflection point due to coadsorption of CO and OHis further interrogated by DFT calculations. While a tensile to compressive switch in Δstress is observed during CO oxidation, the oxidation of another strongly bound diatomic adsorbate, NO_ads, shows a continuous compressive Δstress. DFT calculations show that this behavior is attributed to the adsorption of the oxidation product, NO₃^–, which induces a similar magnitude of compressive Δstress compared to that of NO_ads. Hence, the compressive Δstress from the oxide and hydroxide on the surface governs the surface stress response

Electronic Structure of Lithium Peroxide Clusters and Relevance to Lithium–Air Batteries

The prospect of Li–air­(oxygen) batteries has generated much interest because of the possibility of extending the range of electric vehicles due to their potentially high gravimetric density. The exact morphology of the lithium peroxide formed during discharge has not been determined yet, but the growth likely involves nanoparticles and possibly agglomerates of nanoparticles. In this article, we report on density functional calculations of stoichiometric lithium peroxide clusters that provide evidence for the stabilization of high spin states relative to the closed shell state in the clusters. The density functional calculations indicate that a triplet state is favored over a closed shell singlet state for a dimer, trimer, and tetramer of lithium peroxide, whereas in the lithium peroxide monomer, the closed shell singlet is strongly favored. Density functional calculations on a much larger cluster, (Li₂O₂)₁₆, also indicate that it similarly has a high spin state with four unpaired electrons located on the surface. These results have been confirmed by higher level G4 theory calculations that indicate that the singlet and triplet states of the dimer are nearly equal in energy and that the triplet state is more stable than the singlet for clusters larger than the dimer. The high spin states of the clusters are characterized by O–O moieties protruding from the surface, which have superoxide-like characteristics in terms of bond distances and spin. The existence of these superoxide-like surface structures on stoichiometric lithium peroxide clusters may have implications for the electrochemistry of formation and decomposition of lithium peroxide in Li–air batteries including electronic conductivity and charge overpotentials

Towards First Principles-Based Prediction of Highly Accurate Electrochemical Pourbaix Diagrams

Electrochemical potential/pH (Pourbaix) diagrams underpin many aqueous electrochemical processes and are central to the identification of stable phases of metals for processes ranging from electrocatalysis to corrosion. Even though standard DFT calculations are potentially powerful tools for the prediction of such diagrams, inherent errors in the description of transition metal (hydroxy)­oxides, together with neglect of van der Waals interactions, have limited the reliability of such predictions for even the simplest pure metal bulk compounds, and corresponding predictions for more complex alloy or surface structures are even more challenging. In the present work, through synergistic use of a Hubbard <i>U</i> correction, a state-of-the-art dispersion correction, and a water-based bulk reference state for the calculations, these errors are systematically corrected. The approach describes the weak binding that occurs between hydroxyl-containing functional groups in certain compounds in Pourbaix diagrams, corrects for self-interaction errors in transition metal compounds, and reduces residual errors on oxygen atoms by preserving a consistent oxidation state between the reference state, water, and the relevant bulk phases. The strong performance is illustrated on a series of bulk transition metal (Mn, Fe, Co, and Ni) hydroxides, oxyhydroxides, binary, and ternary oxides, where the corresponding thermodynamics of redox and (de)­hydration are described with standard errors of 0.04 eV per reaction formula unit. The approach further preserves accurate descriptions of the overall thermodynamics of electrochemically relevant bulk reactions, such as water formation, which is an essential condition for facilitating accurate analysis of reaction energies for electrochemical processes on surfaces. The overall generality and transferability of the scheme suggests that it may find useful application in the construction of a broad array of electrochemical phase diagrams, including both bulk Pourbaix diagrams and surface phase diagrams of interest for corrosion and electrocatalysis

Highly Stable Bimetallic AuIr/TiO₂ Catalyst: Physical Origins of the Intrinsic High Stability against Sintering

It has been a long-lived research topic in the field of heterogeneous catalysts to find a way of stabilizing supported gold catalyst against sintering. Herein, we report highly stable AuIr bimetallic nanoparticles on TiO₂ synthesized by sequential deposition-precipitation. To reveal the physical origin of the high stability of AuIr/TiO₂, we used aberration-corrected scanning transmission electron microscopy (STEM), STEM-tomography, and density functional theory (DFT) calculations. Three-dimensional structures of AuIr/TiO₂ obtained by STEM-tomography indicate that AuIr nanoparticles on TiO₂ have intrinsically lower free energy and less driving force for sintering than Au nanoparticles. DFT calculations on segregation behavior of AuIr slabs on TiO₂ showed that the presence of Ir near the TiO₂ surface increases the adhesion energy of the bimetallic slabs to the TiO₂ and the attractive interactions between Ir and TiO₂ lead to higher stability of AuIr nanoparticles as compared to Au nanoparticles