Mechanism of sulfur poisoning by H2S and SO2 of nickel and cobalt based catalysts for dry reforming of methane.

Abstract

Nickel catalysts employed in the production of syngas throughout CO2 reforming of CH4 can be poisoned and deactivated by the presence of H2S or SO2 found in the feedstock. It is necessary to understand the poisoning mechanism to develop more resistant catalysts. The effect of sulfur poisoning by H2S and SO2 on the mechanism of carbon dioxide reforming of methane was studied for Ni/AlMgOx, Ni-Co/AlMgOx and Co/AlMgOx catalysts prepared by coprecipitation and impregnation methods. The method employed for mechanism study was Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS). DRIFTS made possible to observe that the mechanism of carbon dioxide reforming of methane reaction involved: 1) methane adsorption and possibly dissociation into C, CHx and H2; 2) carbon dioxide adsorption and dissociation into CO; 3) formation of carbonates that could occur after CO2 attaches to an oxygen atom or after CO binds to two oxygen atoms present in the catalyst surface. It was found a relation ship between the concentration of carbonates and adsorbed CO.

The presence of H2S in reacting gasses caused a decrease in the intensity of the methane and CO2 absorption bands on the nickel (Ni)-monometallic catalysts, suggesting that H2S blocks the nickel active sites. Also, H2S caused a decrease in the OH- species, suggesting that NiS was formed. The band at 2077 cm-1, attributed to linear carbonyl, was replaced by two bands located at 2071 and 2051 cm-1, which are assigned to carbonyl sulfide produced by the reaction H2S + CO2 ⇌ COS + H2O on the catalyst surface. When SO2 interacted with the catalysts, a band at about 1358 cm-1 was observed, which is assigned to sulfate species. SO2 also caused a decline in the concentration of carbonates and adsorbed CO. This study allowed a better understanding of the poisoning mechanism by H2S and SO2.