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1.
Phys Chem Chem Phys ; 22(15): 8067-8076, 2020 Apr 15.
Artigo em Inglês | MEDLINE | ID: mdl-32239075

RESUMO

Understanding the formation of nano-interfaces between metallic clusters and nanoscale metal-oxides is an important step towards using such systems for catalytic applications. Thus, in this work, we employ density functional theory calculations to study the TMn-(ZrO2)13 interactions, for TM = Fe, Co, Ni, or Cu, and n = 1, 4, and 8. We found a general trend for adsorption and interaction energies (ad/int) for all cluster sizes, with . In terms of size effects, both adsorption and interaction (frozen adsorbed structures) energies become stronger with increasing cluster sizes due to the increase in the number of TM atoms in direct contact with the (ZrO2)13 nanocluster. The structural and electronic properties change for each TMn/(ZrO2)13 system, indicating that these properties could be tuned through variables like the TM species, cluster size and morphology (isomers with different structures). The results also indicate that, from the studied TMs, Ni (Cu) should form the smallest (largest) clusters when supported on the (ZrO2)13 nanoclusters. These and other results discussed here help understand the formation of the nano-interface in the TM-ZrO2 systems, which can be useful in the development of new catalysts.

2.
J Chem Phys ; 151(21): 214301, 2019 Dec 07.
Artigo em Inglês | MEDLINE | ID: mdl-31822101

RESUMO

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.

3.
Phys Chem Chem Phys ; 21(48): 26637-26646, 2019 Dec 11.
Artigo em Inglês | MEDLINE | ID: mdl-31774074

RESUMO

Mixed CeO2-ZrO2 nanoclusters have the potential to play a crucial role in nanocatalysis, however, the atomistic understanding of those nanoclusters is far from satisfactory. In this work, we report a density functional theory investigation combined with Spearman rank correlation analysis of the energetic, structural and electronic properties of mixed CenZr15-nO30 nanoclusters as a function of the composition (n = 0, 1,…,14, 15). For instance, we found a negative excess energy for all putative global minimum CenZr15-nO30 configurations with a minimum at about n = 6 (i.e., nearly 40% Ce), in which both the oxygen anion surroundings and cation radii play a crucial role in the stability and distribution of the chemical species. We found a strong energetic preference of Zr4+ cations to occupy larger coordination number sites, i.e., the nanocluster core region, while the Ce4+ cations are located near vacuum exposed O-rich regions. As expected, we obtained an almost linear decrease of the average bond lengths by replacing Ce4+ by Zr4+ cations in the (ZrO2)15 nanoclusters towards the formation of mixed CenZr15-nO30 nanoclusters, which resulted in a shift towards higher vibrational frequencies. Besides, we also observed that the relative stability of the mixed oxides is directly correlated with the increase (decrease) of the Zr d-state (Ce f-state) contribution to the highest occupied molecular orbital with the increase of the Zr content, hence driving the gap energy towards higher values.

4.
J Phys Chem A ; 120(24): 4231-40, 2016 Jun 23.
Artigo em Inglês | MEDLINE | ID: mdl-27269477

RESUMO

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.

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