Structure and activity of supported bimetallic NiPd nanoparticles: influence of preparation method on CO2 reduction

Bimetallic Ni-Pd and monometallic reference catalysts were prepared by decomposing organometallic precursors, Ni(cod)(2) and Pd-2(dba)(3), leading to nanoparticles with sizes ranging from 3 to 6 nm. Two different synthesis procedures were followed: i) solution synthesis using capping ligand (hexadecylamine) followed by impregnation of pre-formed nanoparticles on SiO2, called Sol-immobilization (SI); and 2) direct precursor decomposition onto SiO2, without stabilizer, called Direct Decomposition (DD). Samples prepared by SI procedure are alloyed bimetallic nanoparticles, whereas samples obtained by DD one show phase segregation. Interestingly, DD samples show better activity for CO2 hydrogenation into CO (reverse water-gas shift reaction - RWGS) than SI ones. The best compromise between activity for CO2 activation (at lower temperature) and CO selectivity was achieved with Ni DD and NiPd DD catalysts. Moreover, the addition of palladium increased the concentration of surface undercoordinated sites, which chemisorb CO weakly, thus improving activity and selectivity, in opposition to other samples that chemisorb CO strongly, in multibond configuration. In the presence of Pd, different decomposition rates drives the formation of smaller and more active Ni clusters. The knowledge acquired here on the influence of synthesis conditions on the catalytic properties of Ni-Pd catalysts should guide us to better catalysts for CO2 transformations into valuable products.