Engineering nickel on metal oxide catalysts: Photothermal carbon dioxide hydrogenation to methane

Abstract

The anthropogenic emission of carbon dioxide has led to detrimental environmental impacts. One method to mitigate emissions is by carbon dioxide hydrogenation to produce methane. As one of the fuels currently heavily relied on in society, methane production is a preferable process. In order to promote sustainability, photothermal conditions are practical as they solely require solar energy for light to heat conversion. The area of photothermal methanation has not been extensively studied and therefore there are limits in the understanding of the reaction and suitable catalyst design.

This work probes the design of nickel catalysts for the photothermal methanation of carbon dioxide. Nickel was selected as the active metal due to its high conversion under conventional thermal conditions, as well as its economic value. Catalyst supports comprising varied metal oxides were studied to alter the catalyst characteristics and understand their impact on the methanation reaction.

With ceria and titania, changing the support composition provided an understanding on how the ease of catalyst reduction governed the photothermal carbon dioxide methanation. As catalyst reduction occurs in situ, partial reduction lead to catalyst activation and therefore methane formation. The exothermic nature of methanation aided catalyst reduction and product formation. The findings led to subsequent probing of those characteristics important in the catalyst support, including a semiconductor with oxygen vacancies (ceria), a semiconductor (titania), and an insulator support (alumina). The different catalyst support properties highlighted the importance of oxygen vacancies. The role of ceria oxygen vacancies was further studied by the addition of silica to suppress their impact. While the oxygen vacancies were inhibited at a 9:1 ceria to silica ratio, the catalytic activity was similar to those of nickel/ceria, highlighting the importance of oxygen vacancy proximity to the nickel sites. As nickel manipulation has been identified to result in a catalytic performance comparable to a catalyst with oxygen vacancies, plasma treatment was utilised to induce defects on nickel/silica and nickel/titania catalysts for enhancing photothermal methanation activity.