Dynamic Surface Processes of Nanostructured Pd₂Ga Catalysts Derived from Hydrotalcite-Like Precursors

The stability of the surface termination of intermetallic Pd₂Ga nanoparticles and its effect on the hydrogenation of acetylene was investigated. For this purpose, a precursor synthesis approach was applied to synthesize supported intermetallic Pd₂Ga nanoparticles. A series of Pd-substituted MgGa-hydrotalcite (HT)-like compounds with different Pd loading was prepared by coprecipitation and studied in terms of loading, phase formation, stability and catalytic performance in the selective hydrogenation of acetylene. Higher Pd loadings than 1 mol % revealed an incomplete incorporation of Pd into the HT lattice, as evidenced by XANES and TPR measurements. Upon thermal reduction in hydrogen, Pd₂Ga nanoparticles were obtained with particle sizes varying with the Pd loading, from 2 nm to 6 nm. The formation of intermetallic Pd₂Ga nanoparticles led to a change of the CO adsorption properties as was evidenced by IR spectroscopy. Dynamic changes of the surface were noticed at longer exposure times to CO and higher coverage at room temperature as a first indication of surface instability. These were ascribed to the decomposition into a Ga-depleted Pd phase and Ga₂O₃, which is a process that was suppressed at liquid nitrogen temperature. The reduction of the Pd precursor at 473 K is not sufficient to form the Pd₂Ga phase and yielded a poorly selective catalyst (26% selectivity to ethylene) in the semihydrogenation of acetylene. In accordance with the well-known selectivity-promoting effect of a second metal, the selectivity was increased to 80% after reduction at 773 K due to a change from the elemental to the intermetallic state of palladium in our catalysts. Interestingly, if air contact was avoided after reduction, the conversion slowly rose from initially 22% to 94% with time on stream. This effect is interpreted in the light of chemical response of Pd and Pd₂Ga to the chemical potential of the reactive atmosphere. Conversely to previous interpretations, we attribute the initial low active state to the clean intermetallic surface, while the increase in conversion is related to the surface decomposition of the Pd₂Ga particles.