Reactive adsorption of thiophene on Ni/ZnO: Role of hydrogen pretreatment and nature of the rate determining step

Abstract

Reactive adsorption of thiophene on reduced and unreduced NiO/ZnO adsorbents was studied by thermal gravimetric analysis and by sulfidation in a fixed bed reactor at 330–375°C and 10–40mbar of thiophene in hydrogen. The adsorbents (12wt% Ni) were prepared by co-precipitation of corresponding nitrates with sodium carbonate followed by calcination at 400°C. We have found that such solids can react with thiophene without any prior reduction. Metallic Ni, indispensable for thiophene decomposition, is formed in this case in situ upon the contact with thiophene/H2 reaction mixture. The reduction of NiO/ZnO in H2 (360°C, 6h) results in the formation of Ni–Zn alloyed particles (as attested by XRD data) and leads to a decrease of the sulfidation rate in comparison with the unreduced sample. Concerning the mechanism of the reaction, we found that H2S is absent in the gas phase during sulfidation in a fixed bed reactor for both reduced and unreduced solids, showing that all produced H2S is rapidly absorbed by ZnO. This observation points out that catalytic thiophene decomposition on Ni is the rate determining process under used conditions. Existence of two stages in the reactive adsorption, characterized by different rates and activation energies, is explained by the change of the rate determining step of thiophene decomposition from desulfurization reaction itself in the first stage to thiophene transport in the second one. Based on these findings we attributed the lower reaction rate of the reduced sample to a lower activity and/or accessibility of metallic Ni in such adsorbents.