Efficient exhaust gas cleaning from NO x (NO and NO 2 ) by absorption and adsorption based methods requires the oxidation of NO. The application of non-thermal plasma is considered as a promising oxidation method but the oxidation of NO by direct plasma remains limited due to the back-reaction of NO 2 to NO mediated by O radicals in plasma. Indirect NO oxidation by plasma produced ozone allows to circumvent the back-reaction and further oxidize NO 2 to N 2 O 5 but the slow reaction rate for the latter process limits the efficiency of this process. Present paper gives an overview of the role of metal-oxide catalysts in the improvement of oxidation efficiency for both direct and indirect plasma oxidation of NO x . The plasma produced active oxygen species (O, O 3 ) were shown to play an important role in the reactions taking place on the catalyst surfaces while the exact mechanism and extent of the effect were different for direct and indirect oxidation. In the case of direct plasma oxidation, both short and long lifetime oxygen species could reach the catalyst and participate in the oxidation of NO to NO 2 . The back-reaction in the plasma phase remained still important factor and limited the effect of catalyst. In the case of indirect oxidation, only ozone could reach the catalyst surface and improve the oxidation of NO 2 to N 2 O 5 . The effect of catalyst at different experimental conditions was quantitatively described with the aid of simple global chemical kinetic models derived for the NO x oxidation either by plasma or ozone. The models allowed to compare the effect of different catalysts and to analyze the limitations for the efficiency improvement by catalyst.
The present study was devoted to the investigation of adsorption of nitrogen oxides on TiO, with the focus on the effect of NO concentration, composition, and flow rate. The inlet NO with a concentration of 200-800 ppm in pure N was mixed with ozone, produced from pure oxygen, and directed to a reactor with catalytic TiO powder. The oxidation of NO by ozone allowed to prepare mixtures with variable concentrations of NO and NO or NO and NO which were adsorbed on the catalyst surface during the oxidation phase and were desorbed when only NO was flowing through the reactor. Diffuse reflectance infrared Fourier transform spectroscopy studies showed NO as the main adsorbed nitrogen oxide specimen on the surface. The amount of adsorbed nitrogen oxide species increased with the increasing fraction of NO in the gas phase and was inversely proportional with the gas-phase NO concentration. An important finding was the abrupt increase in the nitrogen oxide adsorption capacity of TiO when the inlet concentration of ozone became sufficiently large to oxidize NO to NO. On the basis of the results, a model of the surface processes is proposed, involving the production of NO and NO on the surface of TiO.
NO x represents an important group of pollutants that are formed in fuel combustion. As these pollutants cause significant environmental problems, removal of NO x from exhaust gases is necessary. The present study investigated the influence of metal oxide powders on the removal of NO x by oxidation with ozone. The aim of the study was to compare the catalytic effect of TiO 2 , Al 2 O 3 , and Fe 2 O 3 and to investigate the dependence of the effective rate constant on temperature. NO (400 ppm) and a variable concentration of ozone in a mixture of N 2 and O 2 was directed through the catalyst chamber and heated to 60−140 °C. The addition of metal oxides resulted in a significant increase in the efficiency of the oxidation of NO to N 2 O 5 . Fe 2 O 3 had the largest effect with a maximum of an approximately 3-fold increase in the effective rate constant at 100 °C. At the same time Fe 2 O 3 had the lowest NO x adsorption capacity. In the case of all metal oxides, oxidation of NO to N 2 O 5 caused an abrupt increase in adsorption of NO x .
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