The electrochemical and electrocatalytic behaviour of gold nanoparticles
Thesis DisciplineChemical Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Despite the commonly held belief that gold is unreactive, in 1986, gold was shown to be active toward the electrocatalytic oxidation of CO and hydrochlorination of ethyne. Since then, research into additional reactions catalysed by gold has been conducted on bulk gold, gold nanoparticle and gold-alloyed catalysts. In this thesis, a study of the electrochemical behaviour of gold nanoparticles and their electrocatalytic promotion of the glycerol oxidation and CO2 reduction reactions has been undertaken. Gold nanoparticles between 0.8 nm and 23.9 nm were produced and the size and surface area of each batch of nanoparticles was characterised. Models based on a number- weighted particle size distribution with a mean particle size of 10.0 nm showed that if the same distribution was weighted according to the particle volume, surface area or specific surface area, the mean particle size would be 10.8, 10.4 and 9.0 nm, respectively. Electrochemical surface area characterization was performed by cyclic voltammetry and measuring the charge associated with the gold oxide reduction peak. Quantitatively measuring this charge for an electrode containing 3.2 nm gold particles showed a gradient of 2.52 mC· V-1 for bulk gold electrodes and 1.62 mC· V-1. These gradients were only applicable for the electrodes that were being tested as a change in the electrode surface area will lead to a different amount of gold oxide reduction and hence a different charge capacity of the electrode. As gold dissolution occurs across the same potential range as gold oxide formation, the inherent stability of the electrode is linked to the time spent in this region of a CV scan. It was found that 50 CV scans in 0.5 M H2SO4 will artificially age 3.2 nm nanoparticles. This is seen by an electrode that had an initial gold oxide reduction charge of 969 µC· cmAu-2 for CV scans with a maximum potential of 1.85 V vs. RHE, whereas by the end of the 50th CV scan, the gold oxide charge was recorded as 29 μC· cmAu-2, a reduction of 97%. CV scans with a maximum potential of 1.55 V vs. RHE reached a maximum gold oxide reduction charge of 115 μC· cmAu-2 however, this value decayed to 89 μC· cmAu-2 by the end of the 50th CV scan, a reduction of 23%. The maximum useable depth of the catalytic layer was determined to be 26.6 µm, after which the full availability of the layer is uncertain. The Nafion content of the layer was found to prevent all gold oxide formation at 10 wt% due to insufficient ionic transfer through the layer, and caused rapid ageing of the sample at 47 wt%. Intermediate loadings of 20 wt% and 33 wt% measured a smaller maximum gold oxide reduction charge than the 47 wt% electrode, however, they exhibited comparatively slow ageing, suggesting between 20 wt% and 33 wt% are the preferred targets for the Nafion content of a catalytic layer. Gold nanoparticles, ranging in size from 0.8 nm clusters up to 4.5 nm nanoparticles were synthesised by a variety of different methods and supported on carbon black. An accelerated ageing process was applied to a catalytic layer containing these nanoparticles by performing 100 CV scans to establish the long-term stability of the nanoparticles in an electrochemical cell. Particle growth was observed via TEM imaging such that the average iii number-weighted particle size after CV scanning ranged from 4.5 nm to 8.2 nm. The gold content of the electrolyte indicated 3% – 6% of the gold in the electrode was lost to the electrolyte during the ageing process. The results of this accelerated ageing investigation were published in Electrochimica Acta . The gold surface plasmon resonance feature was investigated by performing CV scanning of 23.9 nm gold particles while simultaneously recording the UV-Vis spectrum. Analysis of the SPR absorption intensity showed how an absorption-based spectroscopic CV scan could be produced for electrodes in 0.5 M H2SO4 while a similar analysis on the SPR peak wavelength produced a wavelength-based spectroscopic CV scan for electrodes in 1.0 M KOH. These analysis techniques were then applied to an electrode performing glycerol oxidation in a 1.0 M KOH + 0.1 M glycerol solution. The wavelength-based spectroscopic CV scan was able to show a promotion of the reduction of gold oxide by a shift in the gold oxide reduction feature by +0.09 ± 0.01 V, relative to the corresponding spectroscopic CV scan in 1.0 M KOH. A small, but the similar promotion of gold oxide formation was seen via a shift in the gold oxide formation feature by -0.02 ± 0.01 V. The spectroscopic observations of electrochemical processes are a new technique that allows for the monitoring of the underlying gold behaviour during catalytic reactions, typically unable to be observed in electrochemical measurements. The electrocatalytic reduction of CO2 was performed on 3.2 nm gold nanoparticles supported by carbon black in a CO2 saturated solution of 0.2 M KHCO3. GC and HPLC measurements confirmed that CO and H2 were the major products of all reactions on both bulk gold and nanoparticle-based electrodes. It was found that a bulk gold electrode, while the rate of CO production began at 2.45 µmol· min-1, by the end of a four-hour galvanostatic charging experiment, this rate had dropped to 0.59 µmol· min-1. Comparatively, the gold nanoparticles had less than 30% of the electrochemical surface area of the bulk gold, yet retained a constant CO production rate of 1.10 µmol· min-1 throughout the four-hour experiment. Catalytic gold loading experiments showed that ~1.5 wt% gold electrodes (98.5% carbon) produced CO at similar rates to 7.5 wt% and 15.1 wt% gold electrodes. Layer thickness experiments showed that as the layer thickness decreased, both the normalised and the overall CO production rates increased. This effect has been attributed to the carbon support playing an active role in the hydrogen evolution reaction and with the removal of carbon from the electrodes, the gold was able to reduce more CO2.