Tuning Pd–In<sub>2</sub>O<sub>3</sub> Interaction and CO<sub>2</sub> Hydrogenation Activity for Methanol Synthesis via In<sub>2</sub>O<sub>3</sub> Crystal Phase Engineering
Yan Shao, Jun Wan, Xiaoxia Ou, Cui Quan, Ningbo Gao, Xin Wang, Feng Zeng, Rongsheng Cai, Xiaolei Fan, Huanhao Chen
Abstract
Engineering Pd–In 2 O 3 interaction is key to developing catalysts with the desired CO 2 hydrogenation activity toward methanol synthesis. Here, the crystalline phase of In 2 O 3 nanospheres was tuned by changing the calcination temperature, which was found to affect the Pd–In 2 O 3 interaction and thus the supported Pd states and CO 2 hydrogenation performance of the prepared Pd/In 2 O 3 - a catalysts (where a refers to the calcination temperature for preparing In 2 O 3 ). The fresh Pd/In 2 O 3 - a catalysts show varied initial activities, and after the induction period, their performance stabilized though being different. During the 100 h catalysis, catalyst microstructures changed, showing Pd aggregation and Pd–In alloying, which was related to the nature of the crystalline phase of In 2 O 3 . The hexagonal ( h -In 2 O 3 ) phase in Pd/In 2 O 3 -400 possesses concentrated surface OH groups and limited mobility. The relatively poor mobility limits Pd–In alloying, which possibly suppresses the hydrogen spillover effect, causing low CO 2 conversion (8%) and methanol selectivity (45%) under steady-state conditions at 5 MPa and 300 °C. Conversely, the cubic In 2 O 3 ( c -In 2 O 3 ) phase promotes Pd–In alloying and modifies Pd–In 2 O 3 interaction during the reaction. The activity data show that Pd/In 2 O 3 -600 with the mixed phases of In 2 O 3 ( h / c -In 2 O 3 ) demonstrated appropriate Pd–In 2 O 3 interaction, leading to the Pd core InO x shell structure with the comparatively best methanol selectivity of about 65% at steady state. Conversely, Pd/In 2 O 3 -800 with the pure cubic In 2 O 3 ( c -In 2 O 3 ) phase and a relatively low specific surface area of 16 m 2 g –1 encourages the sintering of Pd and thereby the formation of homogeneous Pd–In alloys, having a moderate methanol selectivity of about 50%. These findings highlight the importance of the In 2 O 3 crystal phase engineering in the catalytic CO 2 hydrogenation over Pd/In 2 O 3 catalysts and the dynamics of Pd–In interactions, which affect the methanol yield.