Physico-Chemical nanomaterials science

Oxidation of dihydrogen molecules by composite catalysts based on highly dispersed nano-scale precious metal particles

Klymchuk V. Yu.1, Boldyrieva O. Yu.1, Lisnyak V. V.1
1 Faculty of Chemistry, Taras Shevchenko National University of Kyiv, Volodymyrska Street, 64/13, Kyiv 01601, Ukraine.
E-mail: ob@univ.kiev.ua
Here, we report on the catalytic and adsorption properties of new metal nanocatalysts, which are highly dispersed nano-scale precious metal particles, mainly platinum, palladium, and gold, supported on selected oxides and porous activated carbon materials. The mild reduction of precious metal chlorides and the partial destruction of the surface complexes support the dispersing of nano-scale precious metal particles. The high specific surface area of the supports maintains a large surface-area-to-volume ratio of nanocatalysts. We compared their catalytic efficiency and showed the effect of support and particle size. By analysis of the results of thermal desorption experiments with mass spectrometric detection of the desorbed species and by the photoluminescent method and fluorescent spectrometry analytic methods of determination of OH and HO2, we independently proved these states on the surface of oxidation catalysts, in the reaction of dihydrogen recombination with oxygen. The thermal desorption spectra have confirmed the presence of weakly bound forms of water and H3O+ species. Their ability to form during catalytic act correlates with the existence of temperature hysteresis in the conversion of dihydrogen. The recombination is the first-order reaction, and the Eley-Rideal model fits precisely the experimental data. The data obtained are explained by the reactions that have occurred in its highly active state through the heterogeneous-homogeneous mechanism. The reason for thermal hysteresis is the effect of the local overheating on the surface of highly dispersed nano-scale precious metal particles acting as adsorbent and catalysts. Effective oxidation catalysis is reached at room temperatures over low content of highly dispersed nano-scale precious metal particles. The proposed nanocatalysts are effective to be used for remediation of dihydrogen in the mixture with the excess of oxygen. Such catalysts are required in converters that can recombine a gas mixture of air oxygen and leaked hydrogen. The latter is sourced from the emissions from tanks feeding the power engines or fuel cells. The dispersed metals’ particles are stable enough under the operating conditions and maintain a steady performance over 200 cycles of reaction.