DFT combined with XPS to investigate sulfur removal and sulfur-fixation mechanisms of copper oxide during coal pyrolysis and semi-coke combustion

In this study, Yangquan (YQ) high-sulfur coal was selected to investigate the sulfur-fixation effect of copper oxide (CuO) on H2S and SO2 and its catalytic effect on the decomposition of thiophene by pyrolysis connected with gas chromatography (Py-GC) and XPS. The mechanisms of sulfur-fixation and thiophene decomposition catalyzed by CuO were investigated by density functional theory (DFT) calculations. After optimization, the stable adsorption configurations of thiophene, H2S and SO2 on different CuO (1 1 1) surfaces can be obtained. By Py-GC analysis, H2S release of YQ coal significantly decreases after adding CuO, meanwhile Cu2S obviously appears in XPS spectra, suggesting CuO possesses the strong sulfur fixation effect on H2S. This is very consistent with calculated results about stable absorption of H2S and its easier dissociated reactions whether on the perfect or enriched - oxygen surfaces. However, CuO possesses the weak ability for break SO2 by GC analysis and DFT calculation. But the enriched oxygen can promote CuO to absorb and decompose SO2. Thiophene is much easier theoretically to decompose under oxygen condition or on the enriched- oxygen CuO (1 1 1) than under Ar atmosphere. This is very consistent with the obvious increasing of thiophene content in XPS spectra of 12% CuO-YQ char and SO2 concentration during different CuO-YQ char combustion. Thus, these experimental results are very in consistence with the DFT theoretical calculations in this study.