Improving selectivity of gold nanoparticles for glycerol electro-oxidation via interaction with non-noble metals
Thia, Larissa Yi Ping
Date of Issue2017-04-14
Interdisciplinary Graduate School (IGS)
Residues and Resource Reclamation Centre
This thesis begins by examining the current state of the art for glycerol electro-oxidation. Extensive research has been carried out to determine the effect of size, morphology, shape, support, experimental conditions and catalyst preparation methods on the catalytic performance of both Au and Pt based catalysts. While the effect of non-noble metal promoters has been relatively well studied for Pt group catalysts, the same cannot be said for Au based catalysts. Thus, the research work presented in this thesis aims to demonstrate how the selectivity of carbon supported Au nanoparticles for glycerol electro-oxidation can be improved via interaction of Au/C with non-noble metals. Enhanced selectivity towards C3 products, glycerate and tartronate, was achieved by simple electro-deposition of Cu onto Au nanoparticles. 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 was attributed to the presence of an Au+ species that was generated via electron transfer between Au and electro-deposited Cu2O. C3 selectivity was maximized at double that of pure carbon supported Au nanoparticles when Cu electro-deposition took place at +0.015 V for 90 min. This result is attributed to the doubling of Au+ content that takes place at this experimental condition. 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, residual Ag containing porous Au structures were prepared and tested for glycerol electro-oxidation. These catalysts were highly active and selective towards C-C bond breaking products, glycolate and formate. Porous Au catalysts were obtained 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. Additionally, low temperature annealing enhanced selectivity and product conversion due to the increase in mixed Au-Ag sites after annealing.