Quantitative analysis of temperature programmed reaction (TPR) spectra of formic acid, acetic acid, propionic acid, and butyric acid decomposition on Ru(0001) and phosphorus (P)-modified Ru(0001) surfaces show that both the length of carboxylic acids alkyl substituents (i.e., R=H, CH3, CH2CH3, and CH2CH2CH3) and the presence of P atoms alter the intrinsic activation energy for bond rupture. Inversion analysis of TPR spectra reveal that the intrinsic activation energy barriers on Ru(0001) for C-H bond cleavage in formate is 94 kJ mol-1, while C-C bond cleavage (decarboxylation) barriers for acetate, propionate, and butyrate are 67, 49, and 55 kJ mol-1, respectively. The activation energies to decarboxylate all four of these species correlate linearly with differences between the homolytic dissociation energies of the relevant bonds (e.g., H-COOH, CH3-COOH, CH3CH2-COOH, and CH3CH2CH2-COOH), which suggests that longer alkyl substituents (e.g., propyl and butyl) stabilize bond rupture transition states by donating electron density to the R-COOH bond via inductive effects. Simultaneously, longer alkyl chains also promote self-stabilizing lateral interactions between carboxylates via van der Waals forces that increase the barriers for R-COOH bond rupture slightly (by ∼1-5 kJ mol-1) at high surface coverages. The addition of P atoms to Ru(0001) increases the intrinsic activation energies for the rupture of all bonds (i.e., C-O, C-H, and C-C bonds), specifically, by 5-50 kJ mol-1 for C-H and C-C bonds. P atoms change the Ru(0001) surface likely via an electronic effect by decreasing the extent of electron back donation from Ru atoms to the antibonding orbitals of the carboxylate intermediate. These results provide useful information for transition metal phosphide catalyst design and carboxylic acid alkyl substituent selection to tailor selectivity toward C-O, C-H, and C-C bond rupture.
|Original language||English (US)|
|Journal||Journal of Vacuum Science and Technology A: Vacuum, Surfaces and Films|
|State||Published - Sep 1 2017|
ASJC Scopus subject areas
- Condensed Matter Physics
- Surfaces and Interfaces
- Surfaces, Coatings and Films