The identification of the most important descriptors that drive the activation CO2 on transition-metal (TM) catalysts is a crucial step toward the conversion of CO2 into value-added chemicals; however, our atomistic understanding is far from satisfactory. Thus, aiming at the potential use of TM clusters in the conversion of CO2, we report density functional theory calculations of CO2, CO, H2O, and H2 adsorption on TM13 clusters (TM = Fe, Co, Ni, and Cu). Among the descriptors to evaluate the activation of the studied molecules, we found that the bond lengths increase, angles decrease, and their energetic variations upon the adsorption are the most important ones. From the structural response in anionic gas-phase molecules, the charge transfer toward CO2 and CO is pointed as relevant in their activation, and our results and analyses suggest that the adsorption on 3d TM13 clusters promote this charge donation process, decreasing in the order Fe13 > Co13 > Ni13 > Cu13. For CO2 and CO on Cu13, the activation was observed for highest energy configurations, indicating that is necessarily an additional driving force to occur the molecular activation on this material. Also, energetic parameters, adsorption energy, and interaction energy indicated that the strength of the adsorption is not necessarily proportional to the activation; it is difficult to point out these parameters as descriptors. Our results also provide interesting insights about steps of the CO2 reduction mechanism within the context of the modified Fischer–Tropsch synthesis.
Transition-metal (TM) nanoparticles supported on oxides or carbon black have attracted much attention as potential catalysts for ethanol steam reforming reactions for hydrogen production. To improve the performance of nanocatalysts, a fundamental understanding of the interaction mechanism between water and ethanol with finite TM particles is required. In this article, we employed first-principles density functional theory with van der Waals (vdW) corrections to investigate the interaction of ethanol and water with TM13 clusters, where TM = Ni, Cu, Pd, Ag, Pt, and Au. We found that both water and ethanol bind via the anionic O atom to onefold TM sites, while at higher-energy structures, ethanol binds also via the H atom from the CH2 group to the TM sites, which can play an important role at real catalysts. The putative global minimum TM13 configurations are only slightly affected upon the adsorption of water or ethanol; however, for few systems, the compact higher-energy icosahedron structure changes its configuration upon ethanol or water adsorption. That is, those configurations are only shallow local minimums in the phase space. Except few deviations, we found similar trends for the magnitude of the adsorption energies of water and ethanol, that is, Ni13 > Pt13 > Pd13 and Cu13 > Au13 > Ag13, which is enhanced by the addition of the vdW correction (i.e., from 4% to 62%); however, the trend is the same. We found that the magnitude of the adsorption energy increases by shifting the center of gravity of the d-states toward the highest occupied molecular orbital. On the basis of the Mulliken and Hirshfeld charge analysis, as well as electron density differences, we identified the location of the charge redistribution and a tiny charge transfer (from 0.01 e to 0.19 e) from the molecules to the TM13 clusters. Our vibrational analysis indicates the red shifts in the OH modes upon binding of both water and ethanol molecules to the TM13 clusters, suggesting a weakening of the O-H bonding.
Metal-oxide clusters, (MO 2 ) n , have been widely studied along the years by experimental and theoretical techniques, however, our atomistic knowledge is still far from satisfactory for systems such as ZrO 2 and CeO 2 , which play a crucial role in nanocatalysis. Thus, with the aim to improve our atomistic understanding of the physical and chemical properties of the metal-oxide clusters as a function of size, n, we performed a systematic ab initio density functional theory study of the (MO 2 ) n clusters, where M = Ti, Zr, or Ce and n = 1−15. In this work, the trial atomic configurations were obtained by a tree-growth (TG) scheme combined with the Euclidean similarity distance (ESD) algorithm. Using the (TiO 2 ) n clusters, we validated the TG-ESD algorithm, which found the same putative global minimum configurations (pGMCs) reported in the literature for most of the (TiO 2 ) n systems, and in a few cases, there are lower energy configurations than previous data. From our analyses, the structural parameters of the (MO 2 ) n clusters show an asymptotic behavior toward the values obtained from the nonoptimized bulk fragments, and hence, the differences between the asymptotic (MO 2 ) n values and the bulk values are due to the surface and relaxation effects. We found a very similar increase in the binding energy with increased n for both systems, in particular for large n values, which is associated with an increase in the coordination of the core atoms toward the bulk values, whereas the magnitude of the binding energy is largely determined by the ionic contribution due to the charge transfer among the cation and oxygen atoms. From the relative stability function, the most stable clusters are (TiO 2 ) 6 pGMC , (ZrO 2 ) 8 pGMC , and (CeO 2 ) 10 pGMC . As expected, from the density of states, we found discrete energy levels for smaller n, which form the valence and conduction bands separated by an energy gap for large n values, and hence, the evolution of the highest occupied molecular orbital−lowest unoccupied molecular orbital energy separation was obtained for the studied metal oxides.
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