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

Authors: 
Chang, S. A., University of Illinois Urbana Champaign
Flaherty, D., University of Illinois, Urbana-Champaign
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 Px-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. Px-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 Px-Ru(0001) surfaces. TPR spectra of DCOOH show that formate intermediates decompose at higher temperatures on P0.43-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 P0.43-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 P0.43-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.

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