Hartree-Fock-Slater Model Cluster Calculations. II. Hydrogen Chemisorption on Transition Metal Surfaces
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Self-consistent Hartree-Fock-Slater model cluster calculations are carried out in order to investigate the electronic structure of hydrogen chemisorption on transition metal surfaces. Orbital energies and results of Mulliken's population analysis are presented for H·Ni5, H·Ni9, H·Pd9 and H·W9 clusters. The level structures of the clusters are also given by density-of-states (DOS) curves and compared with the experimental UPS and EELS spectra. Hydrogen chemisorption gives rise to bonding levels near the bottom of the valence band and antibonding ones on the top. The energy difference between them in H·Ni9 is in good agreement with that obtained by EELS. It is concluded that not only localized d electrons but also sp electrons play important roles in hydrogen-metal bondings. Details of hydrogen-metal bond formation, electronic charge transfer and their differences with adsorbate metals are also discussed.Keywords:
Antibonding molecular orbital
Chemisorption
Mulliken population analysis
Antibonding molecular orbital
Chemisorption
Mulliken population analysis
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In this paper, we present the results of electronic structure, ab initio calculations performed on ReO3, WO3, and the stoichiometric tungsten bronze NaWO3. We examine the relation between the structural and the electronic properties of the three materials and comment on the solid state chemistry governing the interaction between the transition metal and its oxygen ligands. We show that off-center displacements of the W ion in WO3 are driven by the onset of covalent interactions with the nearest oxygen, while the metallic materials ReO3 and NaWO3 are stable when cubic. In the latter case, antibonding contributions due to the occupation of the conduction band oppose the deformation. The different behavior is justified by examining the band structure of the compounds. The effect of the different number of valence electrons and of the different nature of the transition metal on the electronic distribution in the solid are analyzed. Finally, by comparing the mechanical properties of the three oxides, we show that the antibonding conduction electron makes ReO3 very rigid and can suggest an explanation for the pressure-induced phase transition observed for this material.
Antibonding molecular orbital
Valence electron
Hypervalent molecule
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The interaction between a co-catalyst and photocatalyst usually induces spontaneous free-electron transfer between them, but the effect and regulation of the transfer direction on the hydrogen-adsorption energy of the active sites have not received attention. Herein, to steer the free-electron transfer in a favorable direction for weakening S-Hads bonds of sulfur-rich MoS2+x , an electron-reversal strategy is proposed for the first time. The core-shell Au@MoS2+x cocatalyst was constructed on TiO2 to optimize the antibonding-orbital occupancy. Research results reveal that the embedded Au can reverse the electron transfer to MoS2+x to generate electron-rich S(2+δ)- active sites, thus increasing the antibonding-orbital occupancy of S-Hads in the Au@MoS2+x cocatalyst. Consequently, the increase in the antibonding-orbital occupancy effectively destabilizes the H 1s-p antibonding orbital and weakens the S-Hads bond, realizing the expedited desorption of Hads to rapidly generate a lot of visible H2 bubbles. This work delves deep into the latent effect of the photocatalyst carrier on cocatalytic activity.
Antibonding molecular orbital
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Calculations including electron correlation have been performed for methyl adsorbed on nickel clusters mimicking the Ni(111) surface. The chemisorption energies are evaluated following a recently developed scheme, in which the cluster is prepared for bonding. In this way cluster dependent oscillations of the chemisorption energies are largely eliminated. By also using calculated vibrational frequencies of the adsorbed methyl an almost certain assignment of the preferred chemisorption site is obtained. Methyl is found to adsorb in the threefold hollow site with a chemisorption energy in the range 50–55 kcal/mol. The origin of the soft C–H frequencies observed experimentally is a charge transfer from the metal into the C–H antibonding orbitals. The only weak sign of a direct metal–carbon–hydrogen interaction in the calculations is that the C–H frequency is slightly lower for an eclipsed compared to a staggered orientation of methyl in the threefold hollow. The present results are compared to previous experimental and theoretical results for methyl adsorbed on metal surfaces.
Antibonding molecular orbital
Chemisorption
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A bond-by-bond partitioning of the electron energy within the p(2 × 2)-CO/Pd(111) and p(2 × 2)-NO/Pd(111) chemisorption systems has been used to demonstrate a preference for CO and NO chemisorption in hollow sites. The changes in bonding within the adsorbate and within the surface that accompany formation of the chemisorption bond are quantified using Hamilton population analysis: a partitioning of the electron energy among the atoms and bonds. In this way, the preference for CO and NO chemisorption in hollow sites is seen to result from the inability of the increased reduction in bonding within both the adsorbate and surface to counter the increase in surface−adsorbate bonding with increasing adsorbate coordination. By comparison with CO chemisorption, the chemisorption of NO is characterized by stronger surface−adsorbate bonding on all sites; principally the result of increased mixing between the NO(2π) orbitals and the surface d band. Increased mixing between the NO(2π) orbitals and the surface d band, in turn, results in increased back-donation to the NO(2π) orbitals on all sites and, correspondingly, a greater degree of bond weakening within NO on all sites. The increase in 2π-d mixing on chemisorbing NO does not, however, result in increased Pd−Pd bond weakening. Instead, increased 2π-d mixing on chemisorbing NO serves to depopulate a greater number of those surface states contributing to d−d antibonding interactions within Pd−Pd bonds about chemisorbed NO. In this way, the analysis of CO and NO chemisorption presented provides new insight into the mechanism by which chemisorbed CO and NO perturb the electronic structure of the surface and, potentially, influence the chemisorption of neighboring adsorbates at higher coverages. Detailed analysis of the adsorbate orbital contributions to Pd−CO and Pd−NO bonding also reveals that both the adsorbate σ and π orbitals mix primarily with the surface s and p bands. Within the context of the molecular orbital picture of CO and NO chemisorption presented, interaction of the adsorbate orbitals with the d band acts only to perturb the more substantial interaction between the adsorbate orbitals and the surface s and p bands. In this way, the molecular orbital picture of CO and NO chemisorption presented serves to validate the d-band model of chemisorption devised previously by Hammer et al.
Chemisorption
Antibonding molecular orbital
Bond energy
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It is shown that the consequence of filling both a bonding molecular orbital (MO) and its antibonding counterpart leads to a total orbital energy greater than that of the separated atoms. The resulting antibonding effect can be buffered if the antibonding MO mixes with higher empty MOs of the same symmetry. These considerations explain why Be2 has a weak covalent bond, much stronger than in He2. The antibonding effect also helps to explain the weakness of the F-F, O-O, and N-N single bonds. It is also useful in dealing with the stereochemistry of dn transition metal ions (n > 7); the favored coordination geometries are those that minimize the antibonding effect, or which allow its effective buffering.
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The adsorption of NO on spinel-type CuCr2O4 (100) surface has been studied within the framework of density functional theory (DFT). Four possible on-top sites designated as surface Cu and Cr,surface Osuf and subsurface Osub were considered. The results show that Cu site and Cr site adsorptions through N-down are energetically active with adsorption energies 98.1 kJ·mol-1 and 92.9 kJ·mol-1,respectively. For active sites,Cu site and Cr site,N-down adsorption is preferred than the O-down one and the latter is considered to be physisorption. The red shifts for N-O bond toke place at the two orientations. The Mulliken population analysis indicates NO molecules lose electrons. NO 2π antibonding orbitals get electrons for adsorptions at Cu site and Cr site by means of density of states analysis. We also further discussed the bonding mechanism for NO adsorptions at active Cu site and Cr site of CuCr2O4 (100) surface.
Antibonding molecular orbital
Physisorption
Mulliken population analysis
Density of states
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The electronic structure and bond lengths in AICN, AINC, PCN, and PNC have been investigated using near Hartree-Fock wave functions. A detailed analysis of the Mulliken population analysis and comparison of the results with several other XCN and XNC molecules is presented.
Mulliken population analysis
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Antibonding molecular orbital
Chemisorption
Density of states
Charge density
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Abstract The interaction between a co‐catalyst and photocatalyst usually induces spontaneous free‐electron transfer between them, but the effect and regulation of the transfer direction on the hydrogen‐adsorption energy of the active sites have not received attention. Herein, to steer the free‐electron transfer in a favorable direction for weakening S−H ads bonds of sulfur‐rich MoS 2+ x , an electron‐reversal strategy is proposed for the first time. The core–shell Au@MoS 2+ x cocatalyst was constructed on TiO 2 to optimize the antibonding‐orbital occupancy. Research results reveal that the embedded Au can reverse the electron transfer to MoS 2+ x to generate electron‐rich S (2+δ)− active sites, thus increasing the antibonding‐orbital occupancy of S−H ads in the Au@MoS 2+ x cocatalyst. Consequently, the increase in the antibonding‐orbital occupancy effectively destabilizes the H 1s‐p antibonding orbital and weakens the S−H ads bond, realizing the expedited desorption of H ads to rapidly generate a lot of visible H 2 bubbles. This work delves deep into the latent effect of the photocatalyst carrier on cocatalytic activity.
Antibonding molecular orbital
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