Role of support in photothermal carbon dioxide hydrogenation catalysed by Ni/CexTiyO2

Nickel was supported on varied ratios of ceria-titania mixed oxides (Ni/CexTiyO2) to evaluate the role the support plays in photothermal carbon dioxide hydrogenation to produce methane. In a batch photothermal reactor system, Ni/CeO2 achieved the highest conversion rate, reaching a conversion of 93% in approximately 60-90 min. To decouple the influence of light and heat, the CO2 hydrogenation was examined in an in-house designed photothermal reactor, whereby heat can be applied externally. Decoupling experiments revealed that heat from the thermalisation by light was the main driving force for the reaction. In addition, the conversion and temperature profile of the different catalysts revealed that the catalyst performance was governed by catalyst reducibility. H2-TPR analyses showed that the Ni became more readily reducible with increasing CeO2 content, suggesting that the oxide plays a role in activating the Ni. The reduction temperature of the nickel catalyst (following a reduction and passivation process) was below 200 °C, which meant that the inherent heating temperature of the photothermal reactor was sufficient to initiate Ni/CexTiyO2 catalyst activity. The exothermic methanation reaction was then able to heat the system further, ultimately reaching a temperature of 285 °C. The ancillary rise in temperature promotes further nickel reduction and methane formation, leading to a “snow-ball” effect. The findings demonstrate that, to achieve a “snow-ball” effect in a photothermal system, designing a catalyst which is easy to reduce, active for CO2 hydrogenation, and capable of converting light to heat for its initial activation is critical.