Benzene adsorption on Mo 2C: A theoretical and experimental study

Abstract

Continuous gas-phase benzene hydrogenation at 363 K and atmospheric pressure was studied using bulk molybdenum carbide as a catalyst. It was observed that benzene conversion to cyclohexane was initially 100%, dropping to zero after four hours of reaction. After deactivation the catalyst was submitted to a heating program under helium flow from room temperature (RT) up to 1273 K. On-line mass spectrometry showed desorption of benzene and hydrogen suggesting that the deactivation occurred due to strong absorption of benzene on the molybdenum carbide surface. Density Functional Theory (DFT) calculations support this hypothesis showing the adsorption energy of benzene on the molybdenum carbide varies from −377 kJ mol −1 to −636 kJ mol −1 depending on the surface and on the crystal phase. Based on the experimental and theoretical results, a reaction scheme is proposed with the following steps: (i) after synthesis, the molybdenum carbide surface is completely covered by chemisorbed hydrogen; (ii) when the reaction begins, benzene molecules react with the chemisorbed hydrogen via an Eley-Riedeal mechanism, producing cyclohexane and vacant sites on the surface; (iii) other benzene molecules strongly and irreversibly adsorb to these vacant sites, poisoning the surface and thus leading to deactivation of the catalyst.