The electrochemical oxidation of hydrogen peroxide on nickel electrodes in phosphate buffer solutions : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Chemistry at Massey University, Palmerston North, New Zealand

The electrochemical oxidation of hydrogen peroxide was studied at nickel electrodes in phosphate buffer solutions. This reaction is of interest because of its possible use in the construction of devices for the electrochemical detection of hydrogen peroxide. The devices developed could be advantageous in many industrial and medical processes. Using the electrochemical technique, staircase potentiometry, the activity of the nickel electrode in catalysing hydrogen peroxide oxidation was evaluated over a range of bulk hydrogen peroxide concentrations, rotation rates, electrode potentials, temperatures, buffer concentrations and pH. A mechanism was developed to account for the observed activity. This was based on a previous model developed for the electrochemical oxidation of hydrogen peroxide at platinum electrodes [1-6], The mechanism involved H2O2 interaction with binding sites on the surface of the electrode. These were initially identified as a nickel phase oxide, Ni(OH)2. Later, the involvement of buffer phosphate species HPO4- was identified. Hydrogen peroxide is adsorbed onto the binding site to form the complex NiBs H2O2. This complex then undergoes an internal charge transfer to form a reduced nickel site, liberating the products water and oxygen. The binding site regenerates electrochemically to give rise to an amperometric signal with the release of protons. A side reaction was proposed which involved an interaction between the binding sites and dioxygen. This interaction competitively inhibited the binding of H2O2. A rate equation was derived to account for all the surface sites involved in the proposed reaction mechanism. The kinetic, equilibrium and thermodynamic constants of the resulting model were optimised by a SIMPLEX procedure. These constants were in turn used in conjunction with the rate equation to produce synthetic responses, which were then compared to the observed steady-state response. A satisfactory fit was found over the entire range of conditions studied. This supported the proposition that the mechanism was appropriate. The equilibrium constants were found to be potential invariant, with K1= 4.43 × 10-3 and K4 = 0.360 m3 mol-1 at 20°C. The former, K1, was exothermic, with ΔH = -28.32 kJ K-1 between 5 and 25°C, and became significantly more exothermic, with ΔH = -198.33 kJ K-1 between 25 and 35°C. In contrast, K4 was slightly endothermic, with ΔH = -16.5 kJ K-1 over the temperature range. One rate constant could be approximated to be potential invariant, k3N = 7.99 × 10-4 mol m-2s-1 at 20°C. Whereas, the other, k2N, varied with potential. Both rate constants were endothermic with pseudo-activation energies for k3N being 24.3 kJ mol-1 and for k2N ranging between 130-80 kJ mol-1 (depending on electrode potential). An optimum pH region for the study of H2O2 oxidation at nickel was found to be between pH 4 and 9. Above and below these bounds competitive reactions occurred that were not attributable to hydrogen peroxide and insignificant rates of reaction for electrochemical measurement were found. The phosphate species HPO4-2 was identified as being involved in the oxidative mechanism. The nature of this involvement was complex, with HPO4-2 both inhibiting and facilitating H2O2 oxidation, depending on surface concentration. To accommodate this, the proposed mechanism was further modified to include this involvement. It was proposed that HPO4-2 was required to form the H2O2 binding site from a nickel precursor site on the electrode surface. However, the complexation of a second HPO4-2 to this site would inhibit H2O2 binding. The work presented in this thesis represents a fundamental study into the electrochemical behaviour of hydrogen peroxide at nickel electrodes. This behaviour has been clearly identified over a range of temperatures, hydrodynamic conditions, buffer compositions and concentrations. This enabled a new and comprehensive mechanism, for the oxidation of hydrogen peroxide at nickel electrode, to be develope

Numerical Simulations of Transverse Compression and Densification in Wood

Numerical modeling, such as finite element analysis (FEA), of complex structures and complex materials is a useful tool for stress analysis and for failure modeling. Although FEA of wood as an anisotropic continuum is used, numerical modeling of realistic wood structures, including details of wood anatomy and variations in structure within specimens, has been beyond the capabilities of FEA and other methods. In contrast, the recently derived material point method (MPM) has features that make it amenable to analysis of realistic wood structures. To demonstrate the capabilities of MPM, simulations were done for wood in transverse compression. Some advantages of MPM are that it is easy to discretize micrographs of wood specimens into a numerical model, it can handle large deformations, it can model elastic-plastic cell-wall properties, and it automatically accounts for contact between cell walls. MPM simulations were run for softwood and hardwood loaded in either radial or tangential compression. The simulations reproduced many features of wood compression, gave insight into effects of wood anatomy on compression, and may be the first numerical calculations of realistic wood structures extended through to full densification without numerical difficulties

Microcracking, microcrack-induced delamination, and longitudinal splitting of advanced composite structures

A combined analytical and experimental study was conducted to analyze microcracking, microcrack-induced delamination, and longitudinal splitting in polymer matrix composites. Strain energy release rates, calculated by a variational analysis, were used in a failure criterion to predict microcracking. Predictions and test results were compared for static, fatigue, and cyclic thermal loading. The longitudinal splitting analysis accounted for the effects of fiber bridging. Test data are analyzed and compared for longitudinal splitting and delamination under mixed-mode loading. This study emphasizes the importance of using fracture mechanics analyses to understand the complex failure processes that govern composite strength and life

DNA sequence of the mouse H-2Dd transplantation antigen gene

The inbred BALB/c mouse has three transplantation antigens, H2-Kd, H2-Ld, and H2-Dd. We present the complete nucleotide sequence of the H2-Dd gene as well as 777 residues of previously unpublished H-2Dd protein sequence. These data complete the sequences of all the BALB/c transplantation antigen genes and permit detailed comparison with each other and with their counterparts from the inbred C57BL/10 mouse. Transplantation antigens may differ from one another by as much as 5%-15% of their amino acid sequence for the external domains. These extensive differences may arise by gene conversion. The H-2D region of the BALB/c mouse encodes the H2-Dd and the H2-Ld genes. Serologic data suggest that at least two additional transplantation antigen molecules, H2-Rd and H2-Md, are encoded in the H-2D region of the major compatibility complex. Paradoxically, gene cloning studies have only identified the H2-Dd and the H2-Ld genes in the H-2D region. A complete DNA sequence of the H2-Dd gene shows that a variety of alternative splice sites exist throughout the gene, which may lead to additional gene products and may explain the multiplicity of H-2D-encoded polypeptides

The initiation, propagation, and effect of matrix microcracks in cross-ply and related laminates

Recently, a variational mechanics approach was used to determine the thermoelastic stress state in cracked laminates. Described here is a generalization of the variational mechanics techniques to handle other cross-ply laminates, related laminates, and to account for delaminations emanating from microcrack tips. Microcracking experiments on Hercules 3501-6/AS4 carbon fiber/epoxy laminates show a staggered cracking pattern. These results can be explained by the variational mechanics analysis. The analysis of delaminations emanating from microcrack tips has resulted in predictions about the structural and material variables controlling competition between microcracking and delamination failure modes

Fracture Toughness of Wood and Wood Composites during Crack Propagation

Mode I fracture toughness as a function of crack length of medium-density fiberboard (MDF), particleboard (PB), and Douglas-fir (DF) was measured using a new energy-based method. PB and MDF are examples of composites that develop fiber bridging during crack propagation, which causes their toughness to increase with crack length. Longitudinal cracks in DF also displayed fiber-bridging behavior, but only when the crack plane was normal to the tangential direction. MDF and PB experiments were performed for both in-plane and out-of-plane cracks. The toughness of the former was much higher than the latter. The in-plane crack toughness of MDF was higher than PB, but its out-of-plane toughness was lower. PB made using a new soy-based resin had an in-plane toughness similar to commercial PB but an out-of-plane toughness three times higher. Out-of-plane crack propagation is suggested as an improved method for measuring internal bond (IB) properties. When the fracture method was compared with conventional IB tests, both methods showed that the soy PB was better but the fracture method provided a clearer distinction

Measurement, impact and mitigation of heatwaves

Heatwaves are frequently dismissed as uncomfortable seasonal events which are of little consequence. Agreement on a definition has evaded this natural hazard, obscuring lessons from historical losses. This work establishes a robust innovative definition which has transformed management of Australia’s most dangerous natural hazard. The Excess Heat Factor (EHF) is a 3-day heatwave index which combines long- and short-term daily temperature anomalies to produce a sensitive signal to noise signature which is proportional to impact. A statistically robust percentile-based temperature-only index, the EHF measures locally significant heatwave intensity. Intensity is normalised using points over threshold (POT) from extreme value theory to measure and map heatwave severity. City, regional and national epidemiological studies have used EHF to measure heatwave vulnerability by location across Australia. Local, national and international health, emergency services and meteorological authorities now operate within a common framework for the delivery of coordinated heatwave services based on EHF. The Excess Heat Factor is introduced and evaluated for its effectiveness as a heatwave intensity and severity index. The utility of EHF in monitoring and forecasting heatwaves is investigated, as is its effectiveness in predicting impact. This study underpinned the general utility of EHF as a heatwave hazard for health outcomes. Further investigations have found it an effective tool in understanding the impact of severe and extreme heatwaves on infrastructure and utilities. Many collaborative epidemiological studies have now utilised EHF to study the impact of heatwaves, including a national Reducing Illness and Lives Lost for Heatwaves (RILLH) project which has developed heatwave vulnerability data for Australia at local (SA2), regional, major cities and by individual morbidity. We have found EHF to work as an effective warning index across Australia’s diverse mid-latitude and subtropical climate zones. It is also effective in the tropics, particularly in unusually dry environments or in humidity conditions normal for each location. Unusually humid heatwaves have been examined and are not always warned by the current operational temperature-only EHF index. Application of EHF across historical and future climate scenarios, and for multi-day and seasonal predictions is developed in support of a comprehensive heatwave service framework that allows for coordinated heatwave warnings and targeted services.Thesis (Ph.D.) -- University of Adelaide, School of Biological Sciences, 202