Over recent decades there has been an increase in the study of catalysts for the carbon dioxide (dry) reforming of methane (DRM). The ability for the DRM reaction to consume two of the most destructive greenhouse gases, methane and carbon dioxide, for the production of synthesis gas (an intermediary for the production of synthetic diesel and methanol) makes it a highly desirable process for future research. Additionally, with the depletion of natural gas, the DRM has the ability to utilise sustainable biogas as a CH4 source without CO2 removal. Although highly desirable from an environmental perspective, the implementation of DRM catalysts on an industrial scale is limited by cost and stability. Reduction-oxidation-reduction (ROR) pre-treatment has been suggested as strategy for enhancing the activity and stability of supported metal catalysts by reducing active metal deposit size and altering metal support interaction, although it is yet to be studied for the DRM.
The impact of ROR pre-treatment on Ni-SiO2 catalysts for the DRM was studied at a variety of Ni loadings (5, 7.5 and 10% wt.). Samples were reduced in 50% H2/Ar at 350oC for 10 h, oxidised in air at 300oC for 2 h and subsequently re-reduced in 50% H2/Ar at 275oC for 10 h. X-ray diffraction, transmission electron microscopy and hydrogen temperature programmed reduction demonstrated that ROR pre-treatment decreased the average Ni size and altered the interaction between the Ni and SiO2, which significantly impacted the catalytic performance. ROR pre-treatment resulted in an increase in CH4 conversion, most significantly for the 10% Ni-SiO2 samples (from 57% to 69% at 800°C) with comparable carbon formation between the reduced and ROR pre-treated samples. At lower temperatures (600°C) the reduced samples deactivated, due to carbon build-up, after 3-4 h reaction time. On the other hand, the ROR samples maintained stable conversion after 6h over a 12h time frame. Additionally, for all samples (500-800°C), ROR pre-treatment resulted in a more stoichiometric H2/CO ratio. ROR pre-treatment provides a simple and inexpensive means of improving DRM catalysts, which can be pre-treated in-situ in reaction conditions with the use of air and hydrogen.