Ceria-based catalysts for low-temperature dry reforming of methane

The fast depletion of fossil fuels and its environmental concerns are pushing forward the new perspective of the development of clean and renewable energy. The increasing CO2 emission in the world is challenging the humankind's harmony habitant. The utilisation of CO2 from both anthropogenic and nature source is becoming an indispensable need. Catalytic dry reforming of methane (DRM) is a highly endothermic process, typically operated at temperatures as low as 800 °C, that converts two greenhouse gases (GHGs) into an important chemical feedstock - syngas, a mixture of H2 and CO. The conventional catalysts based on Ni, Co, or noble metals alone are not sufficient to provide good catalytic performance, as a result of carbon formation and fast catalyst deactivation. These problems are challenging the wide applications in industries. Many attempts have been made to minimise carbon deposition and to improve DRM's reactivity especially at low temperatures, in spite of the impregnation of mono-metals or bi-metals onto different supports. In this study, CeO2-ZrO2 (CZ) mixed oxides were selected as catalysts and/or supports to initiate the low-temperature DRM reactivity owing to its outstanding redox properties present in the system. It is primarily ascribed to lattice oxygen defects induced by the incorporation of zirconium, offering active sites for CO2 activation. As these structural properties of ceria-zirconia solid solutions (CexZr1-xO2) are inextricably intertwined with the ratio of zirconium, optimising the ratio of Ce/Zr is undoubtedly one of the best ways to screen out the best CZ supports. It is found that when x>0.5, it is more likely to maintain the cubic fluorite structure of CeO2, which is the fundamental source of oxygen and storage capacity (OSC). This study reported the influence of oxygen exchanging capability on catalytic activity of dry reforming of methane (DRM) over a small loading amount of Ru (0.10 wt%) onto ceria-zirconia (CZ). The as-prepared samples were characterised by a range of techniques, including N2 adsorption-desorption, XRD, TPR, and Raman spectroscopy. It was found that the reducibility of CZ sample was significantly increased in the presence of Ru, and it was primarily due to the increased degree of structural defects by the promotional effect of Ru. This effect in turn increased the oxygen mobility and further promoted the catalytic activity of CZ for DRM in the temperature range of 600 °C - 850 °C. Density Functional Theory (DFT) calculations confirmed the increased oxygen mobility from the increased structural defects. Therefore, high conversions of CH4 and CO2 were observed at 850 °C, ca. 100% and 96.8%, as well as a near stoichiometric H2/CO ratio of 1.06 with the very small amount of Ru (0.10 wt%) was introduced. Low-temperature dry reforming of methane (DRM) is highly attractive due to the potential increased thermal efficiency and commercial perspective. The development of an effective and robust catalyst is becoming indispensable to acquire high catalytic activities, strong coke-resistance, and long-term stability. Herein, a structure-activity relationship of DRM catalysts was investigated when a very small amount of Ru (0.10 wt%) was impregnated onto ceria-zirconia prepared by two different methods. High catalytic conversions of CH4 and CO2 at low temperature of 600 °C were achieved with 44.0% and 57.4%, respectively, on 0.1Ru1CZ5U-180. A significant reduction in the onset reaction temperature was observed from TPR results due to the increased number of exchangeable oxygens in the presence of Ru. N2 adsorption-desorption results then correlated with the larger specific surface area and the total pore volume to the improvement of the lattice oxygen exchanging process, thus giving rise to the number of lattice oxygens. No considerable carbon deposits were noted during the 2 h testing experiment. The further study of SEM revealed the formation of different morphologies under various preparation conditions, thereby paving the way to develop a structure-activity relationship for DRM. Carbon formation is always a major challenge in dry reforming of methane (DRM) reaction. In this study, the hydrothermally prepared ceria-zirconia support impregnated with 0.1 wt% Ru was used to analyse the carbon profile of the samples after long-term stability testing of DRM reaction. N2 adsorption-desorption and XRD were applied to characterise the textural properties of fresh and used samples after reaction. H2-TPR was used to estimate the change of the reducibility of the samples before and after DRM reaction. TGA-TPO-MS and Raman spectroscopy were collectively employed for carbon analysis. It was found that low temperature (600 °C) produced two types of carbon deposits, namely, amorphous carbon and crystalline carbon, while high temperature (850 °C) only formed one type of carbon - amorphous carbon. Since the amorphous carbon would not cause any catalyst deactivation, the catalytic activity under both reforming conditions remained constant without any significant drop. This work has contributed to the engineered DRM catalysts for low temperature operation. By doing so, it could pave the way for commercialisation. On the other hand, the feasibility of low-temperature DRM could provide a way to separate hydrogen produced during DRM by commercial membranes. In this way, the operational costs would be decreased accordingly.