(641d) Mechanistic Study of C-O  Bond Rupture on Ruthenium Phosphide Surface: Transient and Steady State Measurements

Transition metal phosphide (TMP) catalysts are selective and active towards C-O bond rupture during oxygenate hydrodeoxygenation (HDO), however, the manner in which C-O bond rupture mechanisms and intrinsic barriers differ between transition metals and TMP are not well understood. Here, we present results from the decomposition of formic acid (e.g., DCOOH, a model oxygenate) on pristine and P-modified Ru(0001) surfaces using a combination of surface science methods. DCOOH adsorbs on Ru(0001) and forms a formate intermediate, which decomposes either by dehydration, initiated by C-O bond rupture followed by C-H bond rupture to form CO, or by dehydrogenation through a direct C-H bond rupture to form CO₂. The P_x-Ru(0001) surfaces (x is the coverage of P-atoms in monolayer equivalents) are prepared by dosing PH₃ gas onto Ru(0001) followed by a 1200 K flash anneal. The composition and structure of the resulting surfaces are characterized by Auger electron spectroscopy and low-energy electron diffraction. P_x-Ru(0001) surfaces are chemically characterized using CO and NH₃ temperature programmed desorption (TPD). TPD results demonstrate that the addition of P-atoms to Ru(0001) decreases the binding energy of CO by 12 ± 2 kJ mol^-1 at θ < 0.33 ML and that of NH₃ by 11 ± 2 kJ mol^-1 at θ < 0.1 ML compared with Ru(0001). These TPD data suggest that P-atoms introduce an electronic effect which decreases the extent of electron exchange between Ru(0001) surface atoms and adsorbates, which is consistent with density function theory (DFT) calculations for Ni₂P that show net charge transfer from Ni atoms to P-atoms.¹ Temperature programmed reaction (TPR) and reactive molecular beam scattering (RMBS) of DCOOH were used to determine how barriers and selectivities for C-O bond rupture differ under transient and steady-state conditions on Ru(0001) and P_x-Ru(0001) surfaces. TPR spectra of DCOOH show that formate intermediates decompose at higher temperatures on P_0.43-Ru(0001) and have activation barriers of 14 ± 2 kJ mol^-1 greater than on Ru(0001). These differences suggest that the electronic effect of P-atom decreases the tendency for electrons to back donate from Ru atoms to anti-bonding orbitals of the formate intermediate, and thus, increases activation energies. P-atom addition may, however, also enhance the stability of formate intermediates by geometric effects. TPR spectra of DCOOH show that the ratio of CO to CO₂ produced decreases as P to Ru increases, which suggests that C-O bond rupture is affected by the addition of P-atoms more than C-H bond rupture. RMBS of DCOOH on Ru(0001) and P_0.43-Ru(0001) surfaces support this finding and show that activation energies of C-O bond rupture and C-H bond rupture (measured from 500 - 700 K) are greater on P_0.43-Ru(0001) by 28 ± 5 kJ mol^-1 and 11 ± 7 kJ mol^-1, respectively, than on Ru(0001). Steady-state CO to CO₂ratios, measured as a function of temperature, are consistent with those determined by TPR of DCOOH, and together, these data demonstrate that the addition of P-atoms to Ru(0001) favors dehydration pathway at temperature above ~ 475 K compared to Ru(0001). In summary, P-atom addition to Ru(0001) increases the activation energy of C-O bond rupture and alters the metal surface-adsorbate interaction via electronic and/or geometric effects that stabilize formate intermediates. These findings may provide useful information for the rational design of TMP catalysts by enhancing C-O bond rupture, which will increase bio-mass conversion efficiency to platform chemicals.

(1) Liu, P.; Rodriguez, J. A.; Asakura, T.; Gomes, J.; Nakamura, K. J. Phys. Chem. B 2005, 109, 4575.