A First Principles Based Microkinetics Model of Dry Reforming of Methane over Nickel-Based Bimetallic Catalytic Systems

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 from the scientific community lately. 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 are also subjected to rapid deactivation 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 Pt. 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]. Many approaches have been proposed in the literature to tackle the issue of rapid coking of the catalysts. One of the techniques among these is to modify the electronic structure of the host catalytic system by its doping with foreign transition metal atoms [2].

In the current work, we present our exhaustive work on the solid state density functional theory (DFT) model results to study theoretically the wide network of elementary reactions comprising the DRM reaction on facets of pure nickel catalyst such as Ni(111) surface. 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 various transition metal doped bimetallic systems such as Cu/Ni, Fe/Ni and Co/Ni systems. Electronic structure analysis of various catalytic surfaces is performed in terms of the d-band theory of catalysis.

We then proceed to construct a comprehensive microkinetic model of DRM reaction mechanism consisting of more than 30 elementary reactions on each of the above-mentioned bimetallic catalysts. Our results help to improve our mechanistic understanding of DRM reaction on bimetallic catalysts. They indicate that the catalyst stability is greatly improved by a transition metal doped bimetallic nickel based surfaces. Furthermore, our DFT results for the predicted performance of the Ni-Cu system have been confirmed experimentally under typical DRM reaction conditions.

References:

1. Pakhare, D. and J. Spivey, A review of dry (CO2) reforming of methane over noble metal catalysts. Chemical Society Reviews, 2014. 43(22): p. 7813-7837
2. Fan, C.; Zhu, Y.; Xu, Y.; Zhou, Y.; Zhou, X. and Chen, D; Origin of synergistic effect over Ni-based bimetallic surfaces: A density functional theory study. The Journal of Chemical Physics, 2012. 137: 014703