Bimetallic Ru–Mo Phosphide Catalysts for the Hydrogenation of CO₂ to Methanol

Metal phosphides have been investigated as promising catalysts for many hydrogenation reactions, including CO₂ reduction. Due to the vast compositional space available to discover active and selective transition metal phosphide catalysts for energy-related reactions, we report a variety of Mo-based and Ru-based phosphide catalysts for the hydrogenation of CO₂ to methanol in 1,4-dioxane (200 °C, 1 MPa CO₂, and 3 MPa H₂). We determined that from the monometallic catalysts studied (MoP, Mo₃P, RuP, and Ru₂P), MoP and Mo₃P displayed higher methanol production rates than RuP or Ru₂P. However, with the addition of Ru to form bimetallic Ru_xMo_(2–x)P (x = 0.8, 1.0, 1.2), the methanol production rate per CO titrated site increased by 3-fold, in comparison to MoP. The combination of X-ray photoelectronic spectroscopy (XPS), density functional theory (DFT), CO₂ temperature-programmed desorption (TPD), and hydrogenation experiments of reaction intermediates provided evidence that the combination of Ru and Mo in the bimetallic catalyst provides a favorable interaction with CO₂ through electronic effects to promote hydrogenation toward methanol. Lastly, recycling experiments were performed with Ru₁Mo₁P, which showed stable methanol production rates for three consecutive reactions. Overall, this paper showcases the promotional effect associated with bimetallic phosphide catalysts for CO₂ hydrogenation to methanol and provides new directions for catalyst discovery with other metal compositions.