Strategies to tune the selectivity of gold for glycerol electro-oxidation
Thia, Larissa Yi Ping
Date of Issue2016-08-19
Interdisciplinary Graduate School
The research work presented in this thesis aims to develop a unified strategy to tune the selectivity of carbon supported Au nanoparticles towards different products in the glycerol electro-oxidation pathway, under alkaline conditions. Enhanced selectivity towards C3 products, glycerate and tartronate, could be achieved by simple electro-deposition of Cu onto Au nanoparticles. Our initial studies showed that the most selective catalysts were obtained when Cu electro-deposition occurred at -0.1 V and +0.015 V for 30 min. Enhancement in C3 selectivity is attributed to the presence of an Au+ species that was generated as a result of electron transfer between Au and electro-deposited Cu2O. Furthermore, Au+ content tripled when Cu electro-deposition took place at +0.015 V for 90 min. As a result, C3 selectivity obtained by the resulting catalyst doubled with respect to pure carbon supported Au nanoparticles. However, beyond these optimal conditions, little to no Au+ species were observed and C3 selectivity decreased. Electro-deposition of Ni onto Au nanoparticles also gave rise to enhanced C3 selectivity. The most selective catalysts were obtained when Ni electro-deposition occurred at -0.3 V for 40 min. Au+ species was similarly identified post Ni electro-deposition and it was generated due to electron transfer between Au and NiOOH species. When Ni electro-deposition occurred at more negative potentials, the resultant thick Ni surface layer partially shielded Au from the reaction medium thus reduced access of glycerol molecules to the Au active sites. As such, C3 selectivity of the Ni-Au/CB catalysts prepared under these conditions were relatively similar to that of pure Au/CB. Any possible synergistic effect between Au and Ni was thus nullified by the thick Ni surface coverage. Lastly, we prepared residual Ag containing porous Au structures which were electrochemically active for glycerol electro-oxidation and highly selective towards C-C bond breaking products, glycolate and formate. Porous Au catalysts were prepared by etching alloyed AuAg sheets in concentrated nitric acid. However, etching does not completely remove all traces of Ag, hence small amounts are still present. Low temperature annealing of the porous Au structures enhanced selectivity and product conversion. Annealing caused atomic rearrangement on the catalyst surface such that all atoms present necessarily participated in oxidizing glycerol. In this way, glycerol molecules were more likely evenly adsorbed onto the metal surface and this gave rise to the higher product conversion.