Density functional theory investigation of structure, stability, and glycerol/hydrogen adsorption on Cu, CuZn, and CuZnO clusters

Abstract

Extensive density functional theory calculations are performed to analyze the structure and activity of Cu and Cu Zn/Cu ZnO clusters containing up to 10 Cu/Zn atoms. The minimum-energy structures of Cu Zn and Cu ZnO clusters are found by doping minimum-energy pure Cu clusters with Zn atom(s) and ZnO molecule(s), respectively, followed by energy minimization of the resultant clusters. Odd-even alteration in properties that determine cluster stability/activity is observed with cluster size, which may be attributed to the presence/absence of unpaired electrons. The difference in behavior between Zn/ZnO doping can be interpreted in terms of charge transfer between atoms. Charge transfers from Zn to Cu in the Cu Zn clusters and from Cu and Zn atoms to O atom in Cu-ZnO clusters, which implies that the Cu atom acts as an electron acceptor in the Cu Zn clusters but not in the Cu ZnO clusters. Finally, the adsorption energies of glycerol and hydrogen on Cu Zn/Cu ZnO clusters are computed in the context of the use of Cu Zn/Cu ZnO catalysts in glycerol hydrogenolysis. Glycerol adsorption is generally found to be more energetically favorable than hydrogen adsorption. Dual-site glycerol adsorption is also observed in some of the planar clusters. Fundamental insights obtained in this study can be useful in the design of Cu Zn/Cu ZnO catalysts. K E Y W O R D S catalysis, copper-zinc clusters, DFT, glycerol adsorption, mixed oxides | INTRODUCTIONAlloys of more than one metal have been of interest to mankind since prehistoric days as they provide the means to attain desired synergistic behavior using a proper combination of constituents. With recent advances in material synthesis and nanotechnology, newer applications of such materials have emerged. For instance, in the area of catalysis, a large variety of bimetallic catalysts has been reported that often exhibit enhanced properties such as stability, selectivity to desired products, resistance to poisoning/deactivation, etc. compared to single-metal catalysts formed by their respective constituents. [1][2][3] Besides, the addition of a second, cheaper metal is also often an economical choice when dealing with expensive metals. Such catalysts are of tremendous interest in emerging areas of hydrogen production, [4,5] CO x utilization, [6] water treatment, [7] biomass conversion, [8] etc.The experimental design of bimetallic catalysts using the trial and error method is a costly and time-consuming affair, which may benefit from theoretical knowledge of metal-metal interactions and charge transfers in a bimetallic system. Although detailed quantum chemistry calculations of bulk catalysts containing thousands of atoms are far from feasible with current computational capabilities, two plausible alternatives exist.

Identification of the Active Sites in the Dehydrogenation of Methanol on Pt/Al2O3 Catalysts

“…The four-atom size is chosen to optimize computational tractability and chemical accuracy. Prior literature − indicates that binding energies, reaction energetics, and catalytic species geometries on four-atom metal clusters are only minorly different than the analogous values calculated on 6–10 metal-atom clusters; indeed, the binding energy of CO* (of −2.41 eV; see Table ) calculated in this work is within the range of values calculated by Heyden and co-workers (spanning from −0.76 to −2.74 eV) on a supported Pt 8 /TiO 2 cluster …”

Study of the Glycerol Hydrogenolysis Reaction on Cu, Cu–Zn, and Cu–ZnO Clusters

“…Minimum energy 8-atom clusters, Cu 8 , Cu 7 Zn, and Cu 7 ZnO, are taken from our previous study. 25 This cluster size is selected because of the existence of a three-dimensional (3D) structure and availability of several types of coordination sites. In Section 3.2 , we discuss the study of the adsorption of the reactants and products of these reactions.…”

“…Optimized geometries are obtained by DFT calculations performed using the B3LYP 27 functional combined with the double-ζ-type LanL2DZ 28 − 30 basis set, which has been validated and used successfully in our previous study. 25 Minimum energy geometries of catalyst clusters ( Figure 2 ), reactants and products, reactants and products adsorbed on catalyst clusters, and transition states of reactions are obtained using optimization calculations. Doublet spin multiplicity is chosen for Cu 3 , Cu 7 Zn, and Cu 7 ZnO clusters having one unpaired electron, and singlet spin multiplicity is chosen for the Cu 8 cluster having no unpaired electrons.…”

“…The adsorption energy of a species (“adsorbate”) on a catalyst cluster is computed by the following equation, as used in our previous study. 25 where E cl·ad is the energy of the cluster with the adsorbate (reactant, transition states, products) and E cl and E ad are the individual energies of the cluster and adsorbate, respectively. The reaction rate constants ( k ) are calculated using transition state theory 32 , 33 as where k B is the Boltzmann constant, h is the Planck constant, R is the gas constant, and Δ G ≠ is the Gibbs free energy of activation at temperature T = 298.15 K. Δ G ≠ is calculated as the difference between the Gibbs free energy of the transition state and the reactant(s).…”

“…This study is the extension of our previous study, 25 wherein we performed DFT calculations to study the structure and stability of Cu, Cu–Zn, and Cu–ZnO clusters and adsorption of glycerol and hydrogen on each and every possible coordination of these clusters. In this study, we shift our focus to the hydrogenolysis reaction and first establish that the hydrogenolysis to PDO indeed follows the formation of an acetol intermediate when using a Cu 3 cluster, unlike a noncatalyzed system where ethylene glycol formation is preferred.…”