Fundamentals of heterogeneous gas phase catalysis on flame-made nanoparticles

: influence of metal oxide supports on energy-related applications

Abstract

This Ph.D. thesis focuses on the development of flame-made nanoparticles for a range of energy-related applications, namely, the preferential oxidation of CO (COPROX), CO2 methanation (Sabatier reaction), and solid oxygen ion conductors. Fundamental advancement is made throughout the thesis in elucidating the underlying heterogeneous gas-solid interactions taking place in each of the applications. First, the two-nozzle flame spray synthesis of a novel Pt-FeOx-CeO2 PROX catalyst is reported, where the Pt-FeOx was sprayed independently from the CeO2 component. Besides controlling the spatial deposition of Pt, the two-nozzle synthesis provided the control over the sintering of the FeOx and CeO2 components. The exclusive deposition of Pt on FeOx promoted the low temperature reducibility of the support through the occurrence of H-spillover. The high content of thereby formed support oxygen vacancies around the Pt deposits weakened the CO-Pt chemisorption. This enabled O2 to dissociate on Pt at low temperatures causing an increased CO conversion but decreased CO2 selectivity at elevated temperatures. The manipulation of CO chemisorption strength through the support reducibility was further extended by designing a range of metal oxide (ZnO, MnOx, CeO2, Al2O3)-supported Ru catalyst for the methanation of CO2. Below 400 °C the formation of CH4 was favored and the CO2 conversion followed the order Ru-CeO2 < Al2O3 < MnOx < ZnO. Based on the CO2 activation energies and reaction orders it was shown that enhanced support reducibility decreased the Ru surface coverage with CO while strengthening the C-O bond. Above 400 °C all samples approached the thermodynamic limit of the system with elevated CO formation according to the reverse water-gas shift reaction. By capitalizing on the high temperature and rapid quenching of flame spray pyrolysis, and the ease in introducing small amount of Ti4+ or Mn2+ dopants, the solid oxygen ion conductor δ-Bi2O3 was obtained. The dopants essentially stabilized the δ-Bi2O3 phase at room temperature, which otherwise transformed to the metastable β-Bi2O3. The δ-phase showed enhanced bulk reducibility compared to β-Bi2O3, illustrating its potential for catalytic applications particularly those requiring lattice oxygen transport. The thesis concludes with the direct simulation Monte Carlo (DSMC) of gas mass transport in highly porous beds as readily formed from the flame spray synthesis. Unlike the analytical solution or the dusty gas model, the DSMC allowed the simulation of diffusion without knowledge about the Knudsen number, porosity of the bed and the pore tortuosity. As such, DSMC was applied to a realistic non-isotropic, three-dimensional porous bed, as assisted by a new binary collision model, to improve accuracy to the nanoscale.

Date of Award	2 Oct 2015
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Wey Yang TEOH (Supervisor)