Catalytic activities of cobalt, nickel and copper ferrospinels for sulfuric acid decomposition: The high temperature step in thesulfur based thermochemical water splitting cycles

摘要

The catalytic decomposition of sulfuric acid is the most endothermic step of the sulfur based water splitting thermochemical cycles, which are promising technologies for large scale hydrogen production in future. In the present study the catalytic activities of three ferrospinels Afe_2O_4 (A = Co, Ni, Cu) were evaluated for high temperature sulfuric acid decomposition reaction. Catalyst characterization by Mossbauer spectroscopy confirmed the occupancy of octahedral and tetrahedral sites by Fe~(3+) ions in all three inverse spinels. The temperature programmed reduction studies revealed that the reducibility of Fe~(3+) was greatly enhanced in CuFe_2O_4 as compared to other ferrites. Copper ferrite was found to be the most active catalyst for the reaction with ~78% conversion at 800 °C. Presence of sulfate species on the spent catalysts was revealed by an ex situ analysis of the spent catalyst samples by FTIR, SEM and evolved gas analysis (EGA). FTIR spectra of all the three spent catalyst samples exhibit four prominent peaks in the region 950-1200 cm~(-1), which is an indicative of C_(2v) symmetry and bidentate sulfate coordination. A plausible mechanism for the sulfuric acid decomposition over spinel ferrites was proposed via the metal sulfate formation and then decomposition followed with an oxygen evolution step with the sulfate decomposition being the rate determining step at higher temperatures. EGA showed evolution of SO_2 as a decomposition product from existed sulfate of spent catalysts at high temperatures, with the rate of SO_2 evolution following the order: CuFe_2O_4 > NiFe_2O_4 > CoFe_2O_4. The enhanced rate of decomposition of the sulfates of copper ferrite can be attributed to the higher electronegativity of Cu~(2+) as compared to Ni~(2+)and Co~(2+), which renders the S-O bond in the mixed metal sulfate weaker than others and thus more susceptible to dissociation. The lower thermal stability of sulfate and better reducibility are responsible for the improved catalytic properties of copper ferrite among the three ferrospinels investigated for sulfuric acid decomposition.