Density functional theory study of dry reforming of methane on nickel and nickel based bimetallic catalysts

Abstract

Dry reforming of methane (DRM) reaction utilizes CO2, a major greenhouse gas to convert natural gas (mainly composed of methane) to synthesis gas, an important feedstock which could further be converted into valuable chemicals and cleaner fuel. This reaction presents a clear incentive in terms of its positive impact on the global environment and thus it has gained increasing attention in the last couple of decades. The superior catalytic activity of Nickel based catalysts and also their relatively lower costs make them the most promising catalyst for this reaction. However, these catalysts also deactivate rapidly owing to their high susceptibility to coke formation and filamentous carbon deposition. This severe catalyst deactivation is one of the major drawbacks that is obstructing the widespread commercialization of DRM. Several alternative catalysts have been explored for this reaction, including noble metals such as Rh and Ru. Even though these metals are found to be more reactive, as well as more resistant to carbon deposition, their high cost generally hinders their use [1]. One of the techniques that has been proposed to suppress the coke formation on the nickel surface is the substitution of single foreign transition metal atom which could modify the electronic structure [2]. In the current work, we present our exhaustive work on the solid state density functional theory (DFT) model results to study the wide network of elementary reactions comprising the DRM reaction on various facets of pure nickel catalyst, such as Ni(111) and Ni(100) surfaces. Calculations were performed using rev-PBE as exchange-correlation functional within the generalized gradient approximation (GGA) as implemented in the software VASP. Adsorption energies were calculated for all the DRM reaction intermediate species and then subsequently the activation barriers were calculated for all the elementary reactions in the DRM cycle. The catalytic activity of these pure nickel surfaces in terms of DRM reaction rate are then compared to the rates obtained on the single overlayer deposited nickel (X/Ni) surfaces where X is a transition metal such as Cu, Rh and Pd. Electronic structure analysis of various pure as well as overlayer modified surfaces is performed in terms of the d-band theory of catalysis. Our results indicate that the catalyst stability is greatly improved by a transition metal overlayer deposition on nickel surface. The carbon adsorption energy on a catalyst surface could be a good thermodynamic descriptor for estimating the coking tendency of this particular surface. One of the ways proposed in the literature to improve the coking resistance of a catalyst is to weaken the carbon adsorption energy. From a thermodynamic point of view, the decrease in the carbon adsorption energy on a catalyst surface would lower the surface coverage of carbon. This leads to a lower affinity for carbon deposition and an improvement of the coke reasistance of the catalyst. Our initial results indicate that the most stable adsorption energies of carbon on pure nickel surfaces are of the order of − 7.5 and − 8.96 eV respectively for the (111) AND (100) surfaces. Whereas, for the copper modified (111) and (100), the similar adsorption energies are of the order of − 5.62 and − 7.12 eV. This clearly shows that the copper modified nickel surface show an improved resistance for coking tendency and have lower affinity for carbon deposition.