Fabrication of Pd/In2O3 Nanocatalysts Derived from MIL-68(In) Loaded with Molecular Metalloporphyrin (TCPP(Pd)) Toward CO2 Hydrogenation to Methanol

Methanol synthesis from CO2 hydrogenation with H-2 produced from renewable energy has emerged as a promising method for carbon neutrality. The supported Pd/In2O3 catalyst has attracted great attention due to its superior activity and methanol selectivity, but the formation of the In-Pd bimetallic phase upon over-reduction would lead to quick catalyst deactivation. In this work, we elucidate the reduction behavior of Pd/In2O3 catalysts by using TCPP(Pd)@MIL-68(In) as precursors. During catalyst fabrication, metalloporphyrins (viz., TCPP(Pd)) served as both a capping agent for the growth of MIL-68(In) and a shuttle for transporting the Pd2+, which enhanced the dispersion of Pd-0 species on In2O3-x during the calcination and reduction treatments and prevented the formation of In-Pd bimetallic phase by over-reduction. With a low Pd loading of 0.53 wt %, the resultant Pd/In2O3 catalyst exhibited a maximum methanol space-time yield of 81.1 gMeOH h(-1) gPd(-1) with a CO2 conversion of 8.0% and a methanol selectivity of 81% over 50 h on stream (295 degrees C, 3.0 MPa, 19,200 mL gcat(-1) h(-1)). In contrast, the comparative Pd/In2O3 catalyst prepared by the impregnation of PdCl2 in MIL-68(In) showed poor activity and stability due to the formation of InPd/In2O3-x surface structures. In addition, we found a strong connection between the reduced degree of In2O3 and the catalytic performance of the supported Pd/In2O3 catalysts by integrating catalyst characterization results with density functional theory (DFT) calculations. Interestingly, the surface In/O ratio detected by XPS can reflect information about both metal-support interaction and the amount of oxygen vacancy, which is highly related to the catalytic activity. The DFT calculation also shows that the Pd/In2O3 catalyst has excellent thermodynamic selectivity for the CH3OH product. This work provides an alternative synthetic strategy for Pd/In2O3 nanocatalysts and sheds light on the deactivation mechanism of the supported catalysts.