Structure and Role of a Ga-Promoter in Ni-Based Catalysts for the Selective Hydrogenation of CO2 to Methanol

Nora K Zimmerli; Lukas Rochlitz; Stefano Checchia; Christoph R Müller; Christophe Copéret; Paula M Abdala

doi:10.1021/jacsau.3c00677

Structure and Role of a Ga-Promoter in Ni-Based Catalysts for the Selective Hydrogenation of CO₂ to Methanol

JACS Au. 2024 Jan 3;4(1):237-252. doi: 10.1021/jacsau.3c00677. eCollection 2024 Jan 22.

Authors

Nora K Zimmerli¹, Lukas Rochlitz², Stefano Checchia³, Christoph R Müller¹, Christophe Copéret², Paula M Abdala¹

Affiliations

¹ Department of Mechanical and Process Engineering, ETH Zürich, Leonhardstrasse 21, CH 8092 Zürich, Switzerland.
² Department of Chemistry and Applied Biosciences, ETH Zürich, Vladimir-Prelog-Weg 2, CH 8093 Zürich, Switzerland.
³ ESRF - The European Synchrotron, 71 Avenue des Martyrs, 38000 Grenoble, France.

Abstract

Supported, bimetallic catalysts have shown great promise for the selective hydrogenation of CO₂ to methanol. In this study, we decipher the catalytically active structure of Ni-Ga-based catalysts. To this end, model Ni-Ga-based catalysts, with varying Ni:Ga ratios, were prepared by a surface organometallic chemistry approach. In situ differential pair distribution function (d-PDF) analysis revealed that catalyst activation in H₂ leads to the formation of nanoparticles based on a Ni-Ga face-centered cubic (fcc) alloy along with a small quantity of GaO_x. Structure refinements of the d-PDF data enabled us to determine the amount of both alloyed Ga and GaO_x species. In situ X-ray absorption spectroscopy experiments confirmed the presence of alloyed Ga and GaO_x and indicated that alloying with Ga affects the electronic structure of metallic Ni (viz., Ni^δ-). Both the Ni:Ga ratio in the alloy and the quantity of GaO_x are found to minimize methanation and to determine the methanol formation rate and the resulting methanol selectivity. The highest formation rate and methanol selectivity are found for a Ni-Ga alloy having a Ni:Ga ratio of ∼75:25 along with a small quantity of oxidized Ga species (0.14 mol_Ni^-1). Furthermore, operando infrared spectroscopy experiments indicate that GaO_x species play a role in the stabilization of formate surface intermediates, which are subsequently further hydrogenated to methoxy species and ultimately to methanol. Notably, operando XAS shows that alloying between Ni and Ga is maintained under reaction conditions and is key to attaining a high methanol selectivity (by minimizing CO and CH₄ formation), while oxidized Ga species enhance the methanol formation rate.