Mechanistic Study of Formic Acid Decomposition over Ru(0001) and PX-Ru(0001): Effects of Phosphorus on C-H and C-O Bond Rupture

Transition metal phosphide (TMP) catalysts are selective and active toward C-O bond rupture in hydrodeoxygenation reactions; however, the manner in which C-O bond rupture mechanisms and intrinsic energy barriers differ between transition metals and TMP is not well understood. In this study, we characterize the chemical and structural properties of phosphorus (P) modified Ru(0001) surface using Auger electron spectroscopy, low-energy electron diffraction, and temperature-programmed desorption (TPD) of CO and NH3. The decomposition pathways for formic acid and the differences between the associated barriers were studied using temperature-programmed reaction (TPR) and reactive molecular beam scattering (RMBS) of DCOOH on pristine and P-modified Ru(0001) surfaces. TPR and TPD results suggest that P atoms introduce an electronic (and perhaps a geometric) effect that decreases the extent of electron exchange between Ru atoms and adsorbates, which decreases desorption energies and increases barriers for C-O and C-H/D bond rupture. RMBS of DCOOH (measured from 500 to 800 K) shows the addition of P atoms enhances C-O bond over C-D bond rupture. These findings provide useful information for the rational design of TMP catalysts to enhance C-O bond rupture, which will increase biomass conversion efficiency to platform chemicals.