Transition metal phosphide (TMP) catalysts are selective towards C-O bond rupture during hydrodeoxygenation reactions used to upgrade bio-oil. However, the manner in which bond rupture mechanisms and intrinsic barriers (i.e., C-H, C-C, and C-O bond) differ between transition metals and TMP catalysts are not well understood. In this study, a phosphorus (P) modified Ru(0001) surface is created using sequences of PH₃ adsorption and annealing treatments followed by Auger electron spectroscopy to determine the P:Ru ratio. Synthesized P_0.4-Ru(0001) surfaces have a (r7xr7) low energy electron diffraction pattern and appear to resemble the (111) facet of bulk Ru₂P materials. The results from temperature programmed desorption of CO and NH₃ demonstrate that the addition of P atoms to Ru(0001) decreases the binding energy of CO and NH₃ by ~12 kJ mol^-1 compared to Ru, suggesting that P atoms decrease the extent of electron exchange between Ru surfaces and adsorbates (i.e., CO and NH_­3).

Results from temperature programmed reaction (TPR) of C₁-C₄ carboxylic acid decomposition on Ru(0001) and P_0.4-Ru(0001) surfaces indicate that both P atoms and the length of alkyl substituents on carboxylic acids (i.e., R = H, CH₃, CH₂CH₃, and CH₂CH₂CH₃) alter the intrinsic activation energy (E_a) of bond ruptures. On both surfaces, TPR and reactive molecular beam scattering (RMBS) results are consistent with carboxylic acid decomposition mechanism, that involves the reaction of carboxylate intermediates to form alkyl surface species with either CO (by C-O bond rupture followed by C-H/C-C bond rupture) or CO₂ (by direct C-H/C-C bond rupture). The addition of P atoms to Ru(0001) increases E_a values for the rupture of all bonds (i.e., C-O, C-H and C-C bonds) by 5-50 kJ mol^-1 and increases also the ratio of CO to CO₂ production (in the case of formic acid and acetic acid decomposition). In addition, P atoms weaken the linear correlation that exists between E_a for C-C and C-H bond rupture and the homolytic bond dissociation energies (BDE) of the involved bonds (e.g., R-COOH), thereby decreasing the strength of the correlation from near parity on Ru(0001) (i.e., slope m = 1) to moderate changes with BDE on P_0.4-Ru(0001) (i.e., slope m = 0.2). The RMBS results from formic acid in the presence of P atoms show a higher production of CO than CO₂, which reflects the catalytic consequences of the differences between the C-H and C-O bond rupture energy barriers on P_0.40-Ru(0001) and those for Ru(0001). Collectively, these results suggest that P atoms alters the production selectivity of CO and CO₂ through a greater increase in the energy barriers of C-O bond relative to C-H/C-C bond rupture.