Adsorption, surface reactions and hydrodeoxygenation of acetic acid on platinum and nickel catalysts

Acetic acid adsorption and surface reactions were studied on Pt(111) and Ni(110) with infrared reflection–absorption spectroscopy (IRAS) and temperature programmed desorption as well as computationally on the same well-defined single-crystal surfaces with density functional theory calculations. Additional calculations were performed for Ni(111) for comparison with Pt(111). At high acetic acid exposures at the dosing temperature of 90 K, acetic acid forms a chemisorbed layer covered by a physisorbed multilayer. The chemisorbed layer at 90 K for both Pt and Ni contains a mixture of molecularly adsorbed acetic acid as well as acetate. The physisorbed multilayer desorbs at 157–172 K. At 200 K, while a mixture of molecular acetic acid and acetate remains on Pt(111), acetic acid almost completely decomposes into acetate and, furthermore, produces some CO on Ni(110). Due to the almost complete decomposition of acetic acid on Ni at 200 K, only a small fraction of the chemisorbed acetic acid desorbs molecularly at 220 K. Unlike on Ni(110), most of the chemisorbed acetic acid desorbs molecularly from Pt(111) at a similar temperature of 218 K. Recombinative desorption of acetic acid is observed as a broad peak at 345 K for Pt(111) and as a tail peak at 220–300 K for Ni(110). At 450 K, the decomposition of acetic acid is almost complete on both metals. In contrast with acetic acid surface reactions where Ni is equally or even more reactive than Pt, a 1.5 wt % Ni/SiO₂ catalyst is less active than a 5 wt % Pt/SiO₂ catalyst in vapor-phase acetic acid hydrodeoxygenation at 473 and 573 K. Hydrogen temperature programmed reduction measurements for calcined catalysts show that Ni requires a higher temperature of 622 K for its reduction than 564 K for Pt. Therefore, the lower catalytic activity of Ni in hydrodeoxygenation can be attributed to the lower reducibility of Ni and not the differences in acetic acid adsorption or surface reactions.