Preparation of supported In2O3/Pd nanocatalysts using natural pollen as bio-templates for CO2 hydrogenation to methanol: Effect of acid-etching on template

The conversion of CO2 into value-added chemicals is an effective way to mitigate the environmental problems caused by huge anthropogenic CO2 emissions. Although In2O3/Pd catalyst possesses excellent catalytic performance in CO2 hydrogenation to methanol, the formation of Pd-In alloy on the catalyst surface greatly reduces the yield of methanol. Herein, we adopt a natural biomaterial (viz., rape pollen) as a template to synthesize hierarchically structured bio-In2O3 and bio-In2O3/Pd catalysts. In particular, we found that the pretreatment of pollen using hydrochloric acid (HCl) played a critical role in determining catalytic performance. A series of bio-In2O3 were prepared by using the pollen templates with different etching degrees, and then the bio-In2O3-x/Pd catalysts (x denoted as the amount of HCl, mL) were prepared using an impregnation-reduction method. It was surprising that the CO2 conversion of bio-In2O3–15/Pd catalyst was 8.2% with methanol selectivity of 74.7%, which was much better than the bio-In2O3–0/Pd (2.6% and 54.6%, respectively). The dramatic enhancement was due to that the acid-etched pollen template owns a hollow cage-like structure and abundant surface functional groups (-NH2 and -COOH), which is beneficial to the growth of In2O3 with a high amount of surface oxygen vacancies. Also, the bio-In2O3/Pd catalyst (prepared with untreated pollen template) was more prone to form Pd-In alloys as a result of the strong interaction between Pd with In2O3. In contrast, the In2O3 spatial distribution of bio-In2O3–15/Pd was more uniform with an appropriate interaction of Pd with In2O3 that inhibited the formation of Pd-In alloys. Even Pd loaded on commercial In2O3 exhibited inferior performance (3.4% CO2 conversion and 81% methanol selectivity) to bio-In2O3–15/Pd catalyst. Accordingly, the present work offers a green and efficient method to prepare supported In2O3/Pd catalysts by using the acid-etched pollen as a template.