Catalytic Deoxygenation of Octanoic Acid over Supported Palladium: Effects of Particle Size and Alloying with Gold

Abstract

Catalytic deoxygenation of octanoic acid (OA) to n-heptane was investigated over silica-supported Pd and PdAu catalysts at 260 °C and 1 atm in a fixed-bed microreactor. Pd/SiO2 catalysts were prepared by incipient wetness (IW) and ion exchange (IE). Bimetallic catalysts were prepared using an IE procedure that is known to produce supported PdAu nanoparticles. The Pd nanoparticles (7.5 nm average size) in the Pd/SiO2 (IW) catalyst exhibit well-defined (100) and (111) facets, as evidenced by high-resolution electron microscopy (HREM) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) of adsorbed CO. As expected, the smaller nanoparticles (1.5 nm average size) in the Pd/SiO2 (IE) catalyst display strong linear and bridging CO DRIFTS bands. The PdAu/SiO2 (1/1 atomic ratio) catalyst contains 5 nm alloy nanoparticles with Pd-rich surfaces, as evidenced by HREM with energy-dispersive X-ray (EDX) analysis and in situ EXAFS spectroscopy. DRIFTS thermal desorption experiments demonstrated that alloying with Au reduced the CO adsorption energy on surface Pd sites. The Pd/SiO2 (IE) catalyst initially exhibited OA decarboxylation and decarbonylation activity but lost decarboxylation activity rapidly with time on stream (TOS). In contrast, the Pd/SiO2 (IW) catalyst had only decarbonylation activity, deactivated less rapidly with TOS, and could be regenerated by heating in H2 to remove OA residues. Alloying Pd with Au was found to improve catalyst stability without significantly affecting decarbonylation activity, as evidenced by the equivalent OA turnover frequencies of the Pd/SiO2 (IW) and PdAu/SiO2 (2/3) catalysts. The geometric and electronic effects of alloying reduce the CO adsorption energy and mitigate self-poisoning by OA and related species.