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 CO 2 into value-added chemicals is an effective way to mitigate the environmental problems caused by huge anthropogenic CO 2 emissions. Although In 2 O 3 /Pd catalyst possesses excellent catalytic performance in CO 2 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-In 2 O 3 and bio-In 2 O 3 /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-In 2 O 3 were prepared by using the pollen templates with different etching degrees, and then the bio-In 2 O 3 -x/Pd catalysts (x denoted as the amount of HCl, mL) were prepared using an impregnation-reduction method. It was surprising that the CO 2 conversion of bio-In 2 O 3 –15/Pd catalyst was 8.2% with methanol selectivity of 74.7%, which was much better than the bio-In 2 O 3 –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 (-NH 2 and -COOH), which is beneficial to the growth of In 2 O 3 with a high amount of surface oxygen vacancies. Also, the bio-In 2 O 3 /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 In 2 O 3 . In contrast, the In 2 O 3 spatial distribution of bio-In 2 O 3 –15/Pd was more uniform with an appropriate interaction of Pd with In 2 O 3 that inhibited the formation of Pd-In alloys. Even Pd loaded on commercial In 2 O 3 exhibited inferior performance (3.4% CO 2 conversion and 81% methanol selectivity) to bio-In 2 O 3 –15/Pd catalyst. Accordingly, the present work offers a green and efficient method to prepare supported In 2 O 3 /Pd catalysts by using the acid-etched pollen as a template.