Selective hydrogen production from formic acid decomposition over Mo carbides supported on carbon materials

Abstract

The decomposition of formic acid to obtain hydrogen has been studied using molybdenum carbides supported on activated carbon and two high surface area graphites, H₂₀₀ (200 m² g⁻¹) and H₄₀₀ (400 m² g⁻¹). Particular attention is paid to the effect of Mo loading. The catalysts were prepared in situ using a mixture of CH₄ and H₂ at a temperature of up to 700 °C. Under these conditions, carburization was mostly complete. We observed that the support influenced the Mo_xC phase obtained so that it seems that the ratio of defective carbon influences the phase. However, for these materials the C/Mo ratio did not influence the obtained crystal phase. Characterization by XRD showed that while the β-Mo₂C phase was obtained over activated carbon and over H₂₀₀, in contrast, MoO_xC_y was obtained over H₄₀₀. These catalysts reached 100% conversion on formic acid decomposition at temperatures in the range of 190–250 °C and were also highly selective under these mild conditions, with values for CO₂ selectivity in the range of 85.0–96.5%. The best results were achieved over a 10 wt% Mo loading on activated carbon that reached 96.5% selectivity to H₂. Also, changes in the molybdenum phases were observed on the spent catalyst. Some redox transformations during reaction were responsible for the transformation of β-Mo₂C into oxycarbide MoO_xC_y. In summary, the results of the catalytic performance indicated that the β-Mo₂C phase was more active, selective and stable than MoO_xC_y under the studied conditions.