A DFT study of elementary reaction steps of dry reforming of methane catalyzed by Ni: Explaining the difference between Ni particles supported on CeO2 and MnO[formula omitted]-doped CeO2

Abstract

With the aid of DFT calculations, we investigate the elementary reaction steps of dry reforming of methane (DRM) on Ni(111) and NiO(100) as simple models of metallic and oxidized Ni catalysts. The reaction-path calculations reveal that DRM is feasible on metallic Ni at elevated temperatures. However, a notable problem with metallic Ni catalysts is the coke formation because the activation barrier for the C* formation is not considerably higher than those of the competing reactions that lead to the DRM products. In contrast, NiO does not encounter issues with coke formation, but it is not an effective catalyst due to too high activation energies and slow surface diffusion of H*. We also explain the experimentally observed difference between the DRM catalysts consisting of Ni particles supported on undoped and MnOx-doped CeO2 supports (designated as Ni/CeO2 and Ni/MnxCeO2, respectively). Specifically, we explain the absence of the 2020 cm−1 vibrational peak on the Ni/MnxCeO2 catalyst. Calculations univocally attribute the experimentally observed 2020 cm−1 peak to CO adsorbed on a top site of metallic Ni because all other sites and involved species display considerably different frequencies. The CO stretching frequency increases as Ni oxidizes, and on NiO(100), it is similar to the vibration of CO on the CeO2(111) support, about 2100 cm−1. Current results thus provide a sound explanation of why Ni/MnxCeO2 is a superior DRM catalyst to Ni/CeO2. In particular, the presence of the 2020 cm−1 peak on the Ni/CeO2 catalyst signals that Ni particles are sufficiently metallic and thus susceptible to carbon poisoning. In contrast, the absence of the 2020 cm−1 peak on the Ni/MnxCeO2 catalyst indicates that Ni particles are oxidized, i.e., the Ni oxidation is low enough to allow the DRM reaction but high enough to reduce the catalyst’s carbon poisoning.