Supported platinum catalysts for nitrogen oxide sensors

The aim of this study was to select, to test and to optimise supported Pt catalysts for nitrogen oxide sensors application in order to improve their selectivity. A series of catalysts based on platinum dispersed onto different supports: SiO2, γ-Al2O3, Al2O3-SiO2, ZrO2, TiO2, CeO2 and YSZ (formulae: (ZrO2)0.92(Y2O3)0.08) were prepared and tested for NO oxidation. The effect of the nature of the support, and of the Pt dispersion and loading on the catalytic activity has been examined under operating conditions similar to those encountered by NOx sensors, i.e. traces of NO in air. Catalytic activity and TPD measurements have clearly shown the prevailing role of the support. Silica is the most adequate support because it weakly adsorbed NO and did not store NO2 as nitrates, then favouring the reaction between NO molecules coming from the support and oxygen adsorbed on Pt. The effect of the Pt particles sizes was also important and our results have shown that larger particles exhibit the highest activity. Finally, the most effective catalyst was found to be Pt/SiO2 containing 2 wt.% of platinum and presenting a low Pt dispersion. This kind of catalyst enables to fix a determined value of the NO/NO2 ratio near the sensing layer of the sensor by reaching the thermodynamic equilibrium over the whole working temperature range.