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 degrees C, 1 MPa CO2, and 3 MPa H-2). 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 RuxMo(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.