Infrared Study of the Absorption of Formic Acid on Clean and Ca-promoted Pd/SiO2 Catalysts

Abstract

The adsorption and decomposition of formic acid on a highly dispersed supported Pd/SiO2 catalyst (2 wt.% Pd) prepared via ion exchange (IE) of [Pd(NH3)4]2+ in alkaline solution, together with two Ca-promoted preparations (Ca/Pd=2 at./at.) where calcium was added either to the prereduced Pd crystallites or to the diammine palladium complex, were studied by FTIR at 298–653 K. On the support, HCOOH is mostly adsorbed molecularly at room temperature, with partial dimerization, condensation and extensive hydrogen bonding, but readily decomposes, on the Pd crystallites of Pd/SiO2, via decarbonylation, to give CO multicoordinated to the metal surface, and water. With heating, formic acid decomposition is accompanied by some water gas shift as well, while CO reacts to give methyl (methane) and methoxy. Calcium promotion to both the prereduced Pd and its diammine complex precursor, enhanced HCOOH decomposition onto the catalyst surface, even at 298 K. Together with sorbed HCOOH and chemisorbed CO, mono- and bidentate formates were observed on these materials, owing to the incorporation of well-dispersed CaOxHy. These formates were readily decomposed by atomic hydrogen produced by decarbonylation/WGS of formic acid on Pd. At increasing temperatures, some carbonates (polydentate and simple) were formed, but hardly any methane was detected. On Ca–Pd/SiO2 with calcium added to prereduced Pd metal particles the extension and/or onset of all these processes was more straightforward than on the promoted Ca–Pd/SiO2 where calcium was added to diammine palladium instead, most likely owing to the combined impact of a higher dispersion of the Pd crystallites on the former preparation, and calcium oxyhydroxide decoration (CaOxHy) of the metal particles on the latter, which hamper H-spillover from them.