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CO2 hydrogenation to methanol offers a promising route for valorizing captured CO2 and contributes to the decarbonization of the chemical industry. A key challenge lies in designing active, selective, and stable catalysts. In2O3-based catalysts are highly active and selective for the hydrogenation of CO2 to methanol. However, unsupported In2O3 suffers from deactivation via overreduction to molten metallic In (In0) and subsequent amorphization. [1] Strategies such as the use of adequate supports, as well as the addition of a second metal, notably Pd, provide a means to tune the performance of In2O3-based catalysts (enhancing methanol yield and stability). However, the nature of active sites in the resulting complex Pd-In2O3-based catalysts is highly debated: some study claims that Pd and In2O3 interfaces are the active sites, while other study argues that the alloy formed under the reaction also plays a role. [2-3]
Here, we aim to gain insight into structure-activity relationships in Pd-In catalyst under CO2 hydrogenation conditions. To this end, we developed a model InPd nanoparticle catalyst via a colloidal approach and subsequent deposition on a high surface area amorphous SiO2 (InPd@SiO2). To ensure the formation of a single-phase InPd intermetallic structure, the catalyst was pretreated under a reductive treatment (10% H2/N2, 600°C). However, carbonaceous residues originating from the synthesis may persist on the catalyst surface. To remove these residues and assess their influence on catalytic performance, an oxidative (5% O2/N2, 600°C) pretreatment was applied. The oxidative pretreatment proved more effective at eliminating surface contaminants, resulting in a catalyst with superior activity compared In2O3@SiO2 and Pd@SiO2.
To probe the active phase, we conducted operando XRD-PDF and XAS studies under CO2 hydrogenation (260°C, 20bar, H2/CO2/N2=3:1:1) and after the reductive and oxidative pretreatments. These studies revealed that InPd intermetallic crystal structure is formed under reductive pretreatment and remains stable under CO2 hydrogenation conditions. Under oxidative treatment, the intermetallic structure transforms into In2O3, PdO and Pd. Remarkably, under the CO2 hydrogenation reaction conditions, the intermetallic InPd nanoparticles reform readily, preserving the average crystallite sizes. Operando XAS confirms that both Pd and In are in their reduced states under the reaction condition and reveals a charge transfer between the metals. While CO2 can oxidize In in InPd@SiO2, no detectable In oxidation was observed under CO2 hydrogenation.
These findings demonstrate that the intermetallic PdIn phase modifies the electronic structures of both In and Pd, distinct from those of otherwise inactive metallic In0 or Pd0 species, resulting in a highly active and stable catalyst for CO2 hydrogenation to methanol. This study also showcases that operando XRD–PDF–XAS on model catalyst can help decipher structure-activity relationships in complex catalytic systems.
[1] Tsoukalou et al., J. Am. Chem. Soc. 2019, 141, 13497−13505
[2] Potter et al., Angew. Chem. Int. Ed 2023, e202312645.
[3] Araújo et al., Angew. Chem. Int. Ed 2023, e202306563.
