Operando-DRIFTS Investigations for Identifying the Active Phase of RuO2 and IrO2 in the CO and CH4 Oxidation Reaction

Abstract

Catalytic processes are widely used in industry from synthesis to waste treatment. Hence, the development of new catalysts has profound impact on these processes bearing the potential to increase productivity and efficiency. The catalysts studied in this work are RuO2 and IrO2. Those two were chosen as previous studies have shown that, despite their similar crystal structure and proximity on the periodic table, they exhibit somewhat different activity in the oxidation reactions of CO and CH4. A comparison of similarities and differences allows to draw conclusions about the workings of both materials. As catalysts, especially those consisting of reducible oxides, are prone to change under reaction conditions, it is necessary to perform the catalytic studies under operando conditions. For this purpose, an operando DRIFTS reactor was constructed that allows the conversion to be measured in conjunction with IR spectra revealing the surface species present on the catalyst. In order to gather information about the state of the catalyst CO was utilized as a probe molecule. To establish the complex relations of band position and shape to surface state of the catalyst, Ru and Ir were prepared in their fully oxidic and metallic form as well as a partially reduced oxide using TiO2 as a support. By using this approach, in conjunction with ex situ XPS, band patterns could be established that were used to distinguish the oxidation state of the catalyst. Correlating the conversion curves to the CO bands observed, the most active phase as well as reaction mechanisms could be deduced. In case of CO oxidation both RuO2 and IrO2 revealed an improved activity for the partially reduced samples with respect to their fully oxidized counterparts. For RuO2 the surface is known to undergo phase separation into oxidized and reduced areas and CO band positions could be assigned to these phases with CO in the boundary region between these phases showing the highest activity. It could also be shown that the surface of RuO2 undergoes some reduction even under oxidizing reaction conditions. As this is not observed for IrO2 and the partially reduced catalysts were more active, this may explain the superior activity of RuO2 in the CO oxidation reaction. For IrO2 it was also found that it is significantly more active towards CH4 oxidation in its partially reduced than its fully oxidized form. However, under reducing conditions the surface was depleted of oxygen, thus hampering the activity at high temperatures. Hence, IrO2 was most active in its partially reduced form under oxidizing conditions. Here the partially reduced IrO2 will slowly oxidize, subsequently losing its activity advantage. Furthermore, it could be shown that the methane oxidation on partially reduced IrO2 proceeds through a formiate-like (HyCO) intermediate, as opposed to the formate-like (HyCO2) intermediate found for fully oxidized IrO2 in previous studies.