Adsorption and dissociation of CO on body-centered cubic transition metals and alloys: Effect of coverage and scaling relations

The adsorption and dissociation of CO have been calculated on the (100) surfaces of the body-centered cubic transition metals Fe, Mo, Cr, and W and the alloys Fe3Mo and Fe3Cr using density functional theory for two CO coverages, 0.25 and 0.5 ML. A complete analysis of the vibrational frequencies was performed to check whether the calculated structures are stable geometries or transition-state structures. For coverages up to 0.25 ML, carbon monoxide adsorbs molecularly onto all four metals at fourfold hollow sites with tilting angles with respect to the surface normal of 47°, 57°, 57°, and 58° and adsorption energies of -1.53, -2.64, -3.03, and -3.01 eV for Fe, Mo, Cr, and W, respectively. The calculated CO stretching frequencies at this coverage are 1211, 1062, 1037, and 926 cm-1. At higher coverages, CO adsorption does not exhibit significant changes in both adsorption energy and tilting angle on all four metals but leads to blue shifts of the CO frequency for Fe and Cr and red shifts for Mo and W. Furthermore, scaling relations apply to a bent CO species at a surface coverage of 0.25 ML of CO on all four transition metals as well as the metal alloys Fe3Mo and Fe3Cr, in the sense that the heat of adsorption of CO and the activation energy of CO dissociation scale linearly with the heat of adsorption of the carbon as well as both dissociation products. © 2009 American Chemical Society

Density functional theory study of CO adsorption and dissociation on molybdenum(100)

The adsorption of CO on Mo(100) has been calculated for several adsorption states at four surface coverages using density functional theory (DFT). Dissociation of CO on Mo(100) has been investigated for two surface coverages: 0.25 and 0.5 monolayer (ML). A full analysis of the vibrational frequencies of CO was performed, to determine whether structures are stable adsorption states or transition states. Results show that CO adsorbs molecularly on the Mo(100) surfaces up to coverages of 0.5 ML at 4-fold hollow sites with the molecular axis tilted away from the surface normal by 55-57 and dissociates easily with activation energies ranging from 0.45 to 0.56 eV, leading to energy gains of -1.71 and -0.59 eV at 0.25 and 0.5 ML, after dissociation, respectively. The adsorption energy of the CO molecule at 0.25 ML is -2.64 eV with a C-O stretching vibration of 1062 cm-1. Increasing the CO surface concentration leads to a lower C-O stretching frequency of 958 cm-1, which is remarkable, and it is in conflict with the Blyholder model and previous experimental observations for CO on transition-metal surfaces. Furthermore, calculations reveal that reported CO desorption peaks in literature, thought to be due to recombination of carbon and oxygen, are more likely due to molecular desorption of CO at the 4-fold hollow position with a tilted geometry. This conclusion is supported by the low recombination energies calculated (one-third of that described in literature)

Testing the pairwise additive potential approximation using DFT: coadsorption of CO and N on Rh(100)

The interaction between adsorbates is a key issue in surface science, because these interactions can influence strongly the properties of chemisorbed species with consequences for the thermodn. and kinetics of surface processes. The simplest representation of adsorbate-adsorbate interactions is based on the assumption that all interactions are pairwise additive. This approach has been satisfactorily used in the modeling of temp.-programmed desorption (TPD) spectra using both continuum and Monte Carlo methods. However, the energies estd. within the pairwise approxn. have never been compared to the energies calcd. using d. functional theory (DFT) methods. We demonstrate that the pairwise additive potential approxn. is indeed a good representation of the adsorbate-adsorbate interactions, and that we do not need to include three-body interactions or higher-order terms to est. the perturbation of the adsorption energy of an adsorbate by the presence of other coadsorbates. Moreover, we show for the first time how DFT can be used to explain the desorption features that one finds in TPD expts., thus linking the TPD desorption features with actual microscopic configurations. [on SciFinder (R)

Effect of the Polymeric Stabilizer in the Aqueous Phase Fischer-Tropsch Synthesis Catalyzed by Colloidal Cobalt Nanocatalysts

A series of small and well defined cobalt nanoparticles were synthesized by the chemical reduction of cobalt salts in water using NaBH4 as a reducing agent and using various polymeric stabilizers. The obtained nanocatalysts of similar mean diameters (ca. 2.6 nm) were fully characterized and tested in the aqueous phase Fischer-Tropsch Synthesis (AFTS). Interestingly, the nature and structure of the stabilizers used during the synthesis of the CoNPs affected the reduction degree of cobalt and the B-doping of these NPs and consequently, influenced the performance of these nanocatalysts in AFTS

Automated Multiscale Universal Simulation Environment

Multiscale techniques should allow for the integration of detailed atomistic information on materials and reactions to predict the catalytic performance of full-scale reactors. Although many attempts have been presented in the literature, difficulties still appear. These challenges are grouped into two main groups: catalytic complexity and differences between time and length scales of chemical and transport phenomena. Here, we introduce AMUSE (Automated Multiscale Universal Simulation Environment), which allows for building a seamless Multiscale modeling workflow. Starting from Density Functional Theory (DFT) data and automated analysis of the reaction networks through graph theory, microkinetic modeling is integrated into a standard open-source Computational Fluid Dynamics (CFD) code. We present technologically relevant case studies to demonstrate the capabilities of AMUSE by applying it to the CO2 hydrogenation on In2O3-based catalysts and isopropanol dehydrogenation on two Co facets

Impact of hybrid CO2-CO feeds on methanol synthesis over In2O3-based catalysts

Catalysts for CO2-to-methanol are typically evaluated in a single-pass regime using pure CO2streams. In a practical process however, CO shall be present as a feed impurity or as a recycled byproduct. Herein, the sensitivity to CO was evaluated on In2O3 catalysts in bulk, supported, or metal-promoted forms, using cycle experiments with variable CO2 and CO contents at H2/(CO + CO2) = 4. The methanol productivity was decreased (−20-−40 %) on all catalysts except In2O3/monoclinic-ZrO2, the activity of which was boosted by 10 %. In-depth characterization of the catalysts uncovered controlled formation of oxygen vacancies and resistance to sintering as the main reasons for the activation of the latter and an interplay of CO/H2O-induced sintering and CO inhibition as the origin of performance loss. Focusing on the most representative systems, operation protocols were explored to maximize their methanol yield. We emphasize that assessment with hybrid CO2-CO feeds is key for the design of industrially-viable catalysts for sustainable methanol production.ISSN:1873-3883ISSN:0926-337

Microfabrication Enables Quantification of Interfacial Activity in Thermal Catalysis

A myriad of heterogeneous catalysts comprises multiple phases that need to be precisely structured to exert their maximal contribution to performance through electronic and structural interactions at their peripheries. In view of the nanometric, tridimensional, and anisotropic nature of these materials, a quantification of the interface and the impact of catalytic sites located there on the global performance is a highly challenging task. Consequently, the true origin of catalysis often remains subject of debate even for widely studied materials. Herein, an integrated strategy based on microfabricated catalysts and a custom-designed reactor is introduced for determining interfacial contributions upon catalytic activity assessment under process-relevant conditions, which can be easily implemented in the common catalysis research infrastructure and will accelerate the rational design of multicomponent heterogeneous catalysts for diverse applications. The method is validated by studying the high-pressure continuous-flow hydrogenation of CO and CO2 over Cu–ZnO catalysts, revealing linear correlations between the methanol formation rate and the interface between the metal and the oxide. Characterization of fresh and used materials points to the model catalyst preparation as the current challenge of the methodology that can be addressed through further development of nanotechnological tools.ISSN:2366-960

Ruthenium silica nanoreactors with varied metal–wall distance for efficient control of hydrocarbon distribution in Fischer–Tropsch synthesis

International audienc

Nanostructure of nickel-promoted indium oxide catalysts drives selectivity in CO2 hydrogenation

Metal promotion in heterogeneous catalysis requires nanoscale-precision architectures to attain maximized and durable benefits. Herein, we unravel the complex interplay between nanostructure and product selectivity of nickel-promoted In2O3 in CO2 hydrogenation to methanol through in-depth characterization, theoretical simulations, and kinetic analyses. Up to 10 wt.% nickel, InNi3 patches are formed on the oxide surface, which cannot activate CO2 but boost methanol production supplying neutral hydrogen species. Since protons and hydrides generated on In2O3 drive methanol synthesis rather than the reverse water-gas shift but radicals foster both reactions, nickel-lean catalysts featuring nanometric alloy layers provide a favorable balance between charged and neutral hydrogen species. For nickel contents >10 wt.%, extended InNi3 structures favor CO production and metallic nickel additionally present produces some methane. This study marks a step ahead towards green methanol synthesis and uncovers chemistry aspects of nickel that shall spark inspiration for other catalytic applications.ISSN:2041-172