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

Abstract

Methanol synthesis from CO2 hydrogenation with H2 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 Pd0 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 °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.