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Pt−Fe intermetallic compound (IMC) catalysts, including Pt3Fe, PtFe and PtFe3, are shown to have advantageous catalytic properties similar to those reported for single‐atom alloy catalysts. Suppresion of the Pt ensembles responsible for hydrogenolysis results in high olefin selectivity during propane dehydrogenation. In situ resonant inelastic X‐ray scattering (RIXS) results and density functional theory calculations show that these changes are associated with a decrease in the average energy of the filled 5d states of Pt in the Pt−Fe intermetallic compound structures compared to monometallic Pt, accompanied by an increase in the average energy of the unfilled valence bands. The decrease in energy of the filled Pt 5d orbitals increases with increasing Fe content in the IMC, i.e. PtFe3>PtFe>Pt3Fe. These results demonstrate that by altering the stoichiometry of IMC catalysts it is possible to control both the ensemble size and electronic properties of active sites, which affords another mechanism for tuning catalytic properties, in addition to changing promoter metals. The present study demonstrates the potential of ordered intermetallic compounds as an alternative to traditional solid‐solution single‐atom alloys to serve as catalysts with well‐defined and uniform active sites. [ABSTRACT FROM AUTHOR]

It is important to utilize glycerol, the main by-product of biodiesel, to manufacture value-added chemicals such as 1,3-dihydroxyacetone (DHA). In the present work, the performance of five different catalysts (Pt-Bi/AC, Pt-Bi/ZSM-5, Pt/MCM-41, Pt-Bi/MCM-41, and Pt/Bi-doped-MCM-41) was investigated experimentally, where Pt-Bi/MCM-41 was found to exhibit the highest DHA yield. To better understand the experimental results and to obtain insight into the reaction mechanism, density functional theory (DFT) computations were conducted to provide energy barriers of elementary steps. Both experimental and calculated results show that for high DHA selectivity, Bi should be located in an adatom-like configuration Pt, rather than inside Pt. A favorable pathway and catalytic cycle of DHA formation were proposed based on the DFT results. A cooperative effect, between Pt as the primary component and Bi as a promoter, was identified for DHA formation. Both experimental and theoretical considerations demonstrate that Pt-Bi is efficient to convert glycerol to DHA selectively. © 2016 American Institute of Chemical Engineers AIChE J, 63: 705-715, 2017 [ABSTRACT FROM AUTHOR]

Determining the atomic-scale structure of oxygenated species at water/Pt(111) interfaces under electrochemical oxygen reduction reaction (ORR) conditions has been a significant, long-term challenge for both experimentalists and theorists. Numerous techniques have elucidated important information about this system, but significant uncertainties relating to the structure and hydration state of adsorbed oxygenated species remain. To resolve some of these questions, an approach based on careful calibration of Density Functional Theory (DFT)-determined energetics, as well as detailed simulation of X-ray Photoelectron Spectroscopy (XPS) signatures, is developed. The combined energetic and XPS analysis of various oxygenated species demonstrates that non-hydrated OH, which has been suggested, by analysis of O 1s core-level binding data from state-of-the-art XPS studies, to be an abundant surface species at water/Pt(111) interfaces during ORR, is in fact unlikely to exist at potentials relevant to ORR, although it might be found in other conditions. The OH is more likely to be present in a fully hydrated state, and the experimentally observed features can be assigned to other oxygen arrangements. This insight about the nature of OH binding at water-Pt interfaces has implications not only for the general understanding of electrified water/Pt interfaces, but also for the design of ORR catalysts with improved performance on the basis of first principles calculations. [ABSTRACT FROM AUTHOR]

We report density functional theory (DFT) results for five competing water–gas shift (WGS) pathways on identical Pd and Pt(1 1 1) surface models. We develop analytical kinetic models to compare intrinsic catalytic activity under conditions relevant to low-temperature WGS. The baseline model predicts that WGS proceeds preferentially through a carboxyl intermediate, that water dissociation is rate limiting, and that the intrinsic rate on Pt is greater than on Pd. Absolute rates are unphysically low because of unphysically high CO coverage. Systematically decreasing the CO binding energy in keeping with its known coverage dependence yields more realistic coverages and absolute rates, shifts the rate limiting step to carboxyl formation, and retains the same ordering of activity. Predicted rate orders and activation energies are not consistent with those observed for supported Pt and Pd catalysts, suggesting that even on nominally inert supports, the support plays an active role in WGS activity. [ABSTRACT FROM AUTHOR]

Abstract: Electronic structure calculations based on Density Functional Theory (DFT) are increasingly used to probe reaction mechanisms on catalyst surfaces. These calculations provide information about geometries, stabilities, and reactivities of adsorbed species on various surfaces. Conducting microkinetic analysis in conjunction with DFT calculations forms a powerful combination of methodologies to allow quantitative information to be derived about catalytic reactions at the molecular level. Specifically, whereas the microkinetic approach does not make any a priori assumptions about which steps may be rate controlling or which species may be abundant on the surface, there is usually not sufficient information to extract values of all kinetic parameters from experimental data. In contrast, results from DFT calculations alone cannot be used to assess the relative rates of various pathways, because these rates depend on reaction conditions. Instead, the results from DFT calculations form initial estimates of parameters for the microkinetic model, and the microkinetic model is used to predict rates of elementary steps and surface coverages under various reaction conditions, thereby forming a bridge with reaction kinetics data and surface spectroscopic measurements. In this paper, we outline some of the concepts useful for understanding DFT calculations and for using the results of these calculations in reaction modeling. We also introduce key concepts in microkinetic analysis, and we illustrate how microkinetic models are used to probe reaction pathways, reaction orders, and surface coverages for the decomposition of methanol on platinum. [Copyright &y& Elsevier]

Density functional theory calculations are combined with kinetic measurements of ethanol dehydration to diethyl ether to identify the relative catalytic contributions of structurally distinct speciations of Sn sites in zeolite Beta frameworks. The structural complexities of the Beta framework require nonstandard techniques for entropy and energy calculations, including consideration of anharmonic effects in vibrational modes, employment of quasi-harmonic densities of states methods to evaluate entropies, and use of hybrid density functionals to evaluate binding energies. Calculated energies and entropies are used to construct a microkinetic model that is iteratively refined to identify all kinetically and thermodynamically sensitive reaction steps and intermediates which are subsequently treated with the higher-level methods. The rate and equilibrium constants obtained from this tiered approach agree well with measured reaction orders in ethanol and water. Site balances provide evidence for the interconversion of Sn sites between “closed” configurations that are tetra-coordinated to the framework, open configurations formed by hydrolysis that are tri-coordinated to the framework and contain a hydroxyl ligand and proximal silanol group (“hydroxy-open”), and open tri-coordinate configurations formed by reaction with ethanol, yielding an ethoxy ligand and proximal silanol group (“ethoxy-open”). Closed, hydroxy-open, and ethoxy-open Sn sites adsorb ethanol via distinct modes and react via distinct pathways, with the consequence that prevalent dehydration pathways depend on the speciation of Sn sites under reaction conditions. The kinetic modeling indicates that, under the conditions studied (404 K, 0.5–35 kPa ethanol, 0.1–50 kPa water), bimolecular dehydration on the closed Sn site is the sole kinetically-relevant step, and ethanol, ethanol-ethanol dimers, and ethanol-water dimers are the most abundant surface intermediates. These results highlight the importance of considering the distribution of heteroatom coordination modes to zeolite frameworks under reaction conditions, as well as pathways for their interconversion in the presence of reacting molecules, to obtain more robust mechanistic and kinetic interpretations of catalytic pathways in Lewis acid zeolites. [ABSTRACT FROM AUTHOR]

Changes in surface chemistry and morphology of Re–Pt surfaces synthesized by ultra-high vacuum chemical vapor deposition (UHV-CVD) of Re on Pt(111) were studied by a combination of experiment and density functional theory (DFT) modeling. A Re oxide formed following exposure of the as-deposited Re to 1 × 10 − 6 mbar oxygen at 600–673 K. Subsequent annealing at 973 K resulted in oxygen desorption and a decrease in Re coverage, as calculated by XPS and as observed by STM. This observation was explained by DFT calculations which showed that a clean Pt surface slab with subsurface Re is thermodynamically more favorable than Pt(111) with Re on the surface. DFT calculations also predicted weaker O and CO binding on this surface compared to both monometallic Pt and Re, and HREELS and temperature desorption measurements suggested that O binds weakly to the Pt skin surface, with oxygen on the Pt skin desorbing from this surface following annealing at 373 K. Trends in adsorption energies were consistent with DFT calculated d-band centers of surface atoms for model Pt–Re structures. Comparison of HREELS data and STM images with DFT calculated vibrational frequencies have been used to understand the structure of rhenium oxide on Pt(111). [ABSTRACT FROM AUTHOR]

Abstract: Finding the solids that are the best catalysts for a given reaction is a daunting task due to the large number of combinations and structures of multicomponent surfaces. In addition, it is not only the reaction rate that needs to be optimized; the selectivity, durability, and cost must also be taken into account. Here we propose a computational screening approach and apply it to design a new metal alloy catalyst for the methanation reaction (CO+3H2 → CH4 + H2O). [Copyright &y& Elsevier]