Unravelling the role of light in the photo-assisted reduction of CO2 to CH4 using transition metal catalysts

Abstract

In recent years, there has been an increased interest in utilising light and heat from the sun to produce solar fuels. Photo-assisted conversion of CO2 to CH4 is an effective and straightforward approach to produce solar fuels. The role of light, coupled with heat, for the methanation of carbon dioxide was investigated using different transition metal catalysts.

The synergistic effect of heat and light was probed for the methanation of CO2 using Co, Cu and Ni supported on CeO2 and Al2O3. The role of light was explored using different wavelengths and intensities. Illuminating Co/CeO2 and Ni/CeO2 catalysts with blue light (450-460 nm) improved catalytic activity and selectivity. This improvement was attributed to the direct photoactivation of reaction intermediates on the catalyst surface by photogenerated hot electrons and the presence of intrinsic oxygen vacancies on CeO2. The absence of considerable light enhancement for Co/Al2O3 validated the importance of surface oxygen vacancies in accessing light enhancement.

Based on the findings from the Co-based catalysis study, a detailed understanding of the reaction system was then developed by probing the role of surface basicity under light for CO2 methanation. The modification of commercial TiO2 with different loadings of La was investigated for a Co/La-TiO2-based catalyst. It was shown that homogeneously dispersing lanthanum on the surface of the TiO2 support boosted the catalytic performance of Co deposits under both non-illuminated (dark) and illuminated conditions. It was revealed that La promotion increased the surface basicity which facilitated CO2 adsorption and activation and allowed access to light enhancement.

Subsequently, to gain further insights into the support properties which enable light enhancement during CO2 methanation, a Co/Al2O3 catalyst was modified with La and Pd via a double flame spray pyrolysis synthesis approach. La addition to the Al2O3 support played a crucial role, introducing actives site for CO2 adsorption and its potential transformation into an intermediate product which is more responsive under visible light illumination. In accompaniment to its plasmonic behavior, Pd metal addition to the Co/La-Al2O3 system facilitated H2 activation and provided further enhancement to the activity and CH4 yield.

Overall, the research has demonstrated that through the rational design of transition metals loaded on a suitably active support the benefits of visible light illumination can be harnessed and can reduce the activation energy for thermal catalytic reactions.