Authored by Shenna M Shearin*
Abstract
In this study the various adsorption modes of binding of copper on SiO2 and TiO2 surfaces were investigated by advanced computational techniques. The central objective of this study was to develop a working model of metal-oxide surface-mediated copper clusters, since such catalytic matrixes have a wide-range of applications in the Methanol Steam Reform process. The structural models of the copper clusters ranging from n=2 to n=20 were created using the Birmingham Cluster genetic algorithm (BCGA) coupled with the Gupta potential based on the physiochemical parameters published by Cleri and Rosato [1]. Optimization of the copper clusters was performed using Density Functional Theory (DFT) with PBE XC functional of Pbe0 and LANL2DZ basis set of NWChem package. Adsorption binding of the Cu clusters on SiO2 and TiO2 surfaces were performed using periodic Density Functional Theory (DFT) and PBE XC functional of the Quantum Expresso package. to investigate the binding free energy, the most optimal mode of binding, and the key adsorption interactions of Cu atoms on SiO2 and TiO2 surfaces.
Keywords: Adsorption; Copper; Metal-oxide; Density functional theory; Methanol steam reform
Introduction
Depletion of petroleum based fossils fuels provoked the pursuit for generating alternative safe and environmentally clean fuels over the past decade. Hydrogen gas (H2) has long been regarded as a promising alternative to fossil fuels. For instance, fuel cells polymer electrolyte membrane fuel cells (PEMFC) are one of the primary sources as high efficiency energy converting devices. In such processes, producing safe storage of explosive hydrogen gas and circumventing the opportunities of leakage remains a challenging task. Compared to gaseous hydrogen gas storage, on-board hydrogen production by reforming liquid hydrogen carriers will be more promising for future commercialization. Steam reforming of alcoholic sources such as methanol, ethanol or glycerol have been investigated comprehensively for their potential to be converted to hydrogen. Methanol steam reforming (MSR) is considered one of the most favorable chemical processes for on-the-fly hydrogen production for several reasons [2,3]. (i) Methanol is in liquid-state at ambient condition; (ii) it has high H-to-C atom ratio; (iii) requirement of relatively low temperatures (200-400 °C) for activation of methanol and (iv) methanol is sulfur-free and can be easily produced from biomass. Various catalysts have been developed for MSR reactions.
In particular, copper (Cu) based catalysts have demonstrated the ability to produce gas with high hydrogen (H2) concentration and high selectivity for carbon dioxide (CO2) [2,3]. Metal oxide substrates exhibit a strong effect on the catalytic efficiency. Copper supported catalysts exhibited a higher selectivity for hydrogen and a lower selectivity for CO2 on silica (SiO2) than on a titania (TiO2) at 250 °C [2,3]. SiO2 has been used as a structural support matrix as it stabilizes copper particles and enhances the activity in Methanol Steam Reforming (MSR) reactions [2,3]. Additionally, experimen- tal studies reveal Cu-SiO2 catalysts exhibits a higher selectivity for hydrogen and a lower selectivity for carbon monoxide (CO) than that of Cu in the framework of TiO2 [2,3]. Understanding the nature of these reactions on an atomic level by experimental techniques is difficult due to the electronic effect of metal-oxide interactions. Furthermore, quantum mechanical methods are powerful tools that contribute useful information about the microscopic aspects of metal-oxide interactions [4-19]. In a study by Pacchioni and coworkers, Density Functional Theory (DFT) calculations were used to predict the interaction of isolated Cu atoms and small Cun clusters (2 ≥ n ≤ 5) on metal-oxide support which resulted due to the electrostatic interactions between the Cu metal and the support of a two-coordinated oxygen of the SiO2 [4]. It was also demonstrated that isolated Cu atoms deposited on TiO2 shows a preference to transfer their valence electron to the bridging oxygen sites of TiO2 with formation of a strong bond while the interaction with the Ti sites are weak [4].
The adsorption behavior and binding interactions of Cu clusters on SiO2 and TiO2 supports are difficult to elucidate via experimental methods. This study is useful for gaining an in-depth understanding of the adsorption behavior of small Cun clusters on SiO2 and TiO2 supports and the thermodynamic properties of cluster- oxide interactions for producing optimal yields of hydrogen in the MSR process.
Methods and Design
Construction of Cun clusters
The Birmingham Cluster Genetic Algorithm (BCGA) combined with Gupta potential [20] and DFT using XC functional of Pbe0 and LANL2DZ basis set of NWChem package were employed to construct and optimize the Cu clusters [22,20,5,6]. The inter-atomic interactions provided by the Gupta potential, a many-body potential based on the Friedel’s tight-binding model [20], was used to account for the repulsive and attractive terms. In this model, the configurational energy of a cluster is written as the summation of all atoms of attractive and repulsive energy components in equation (1);
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