Promoting effect of 2000 ppm H2S on the dry reforming reaction of CH4 over pure CeO2, and in situ observation of the behavior of sulfur during the reaction

•Stable activity of pure CeO2 was demonstrated in dry reforming under 2000 ppm H2S.•No impurity phases and carbon deposition appeared after the reaction.•Catalytic activity was enhanced as the concentration of H2S increased.•DRIFT and S K-edge XANES visualized fast redox cycle of sulfur on CeO2.•The fast redox cycle of sulfur accelerated re-oxidation of CeO2-x by CO2.