Comparison of the Catalytic Activity between Au and Pt for CO Oxidation within the Range of the Concentration of the Order from ppm to % for Indoor Air Purification
Hiroaki Sakurai, Masatake Haruta, Susumu Tsubota,
Gold catalysts have long been reported to show especially high activity for CO oxidation. The reaction, when evaluated by using flow reactor, progresses enough in the range of the temperature between 200K and 273K for the oxidation of 1% concentration of CO in synthetic air. However, there is little report about the result of applying the reaction to the CO removal in actual air. It is not also clear about whole features of the gold catalysis compared with other catalysts such as such platinum group metals and Hopcalite for the wide range of CO concentration from the ppm level to % order. Because from tens to hundreds ppm level of CO in a close indoor space, which was originated in smoking etc., becomes a problem in recent years, the clarification of the catalysis of the gold in the above-mentioned real air, which contain CO2, water vapor and trace amount of VOC, is a pressing need.
In this study, using the batch reactor equipped with CO and CO2 gas sensor (Fig.1), CO oxidation over Au catalysts in real air was investigated and compared with Pt catalysts. Au(5wt%)/Fe2O3 was prepared by coprecipitation method, Au(3wt%)/TiO2 and Pt(3wt%)/TiO2 were by deposition-precipitation, and Pt(5wt%)/SiO2 was by impregnation method. Gold catalysts were calcined in air at 673K, and platinum catalysts were reduced under H2 flow at 673K. All the catalysts were stocked in vials with screw cap immediately before use for the reaction. After the setting of catalyst sample in the reactor, different amount of CO was injected into reactor, and small sample vessel was opened to start reaction after the stabilization of CO concentration.
Figure 3 shows the time course of CO concentration over each catalyst. Although the shape of curves were different depending on the initial CO concentration and the kind of catalyst, all the curves of log [CO] during initial 10 min was well fitted by straight line, which mean the first order dependence on [CO]. In Fig.3, these fitting lines are shown in dotted lines with extension to the range of time after 10 min. The calculated rate constants from each fitting curves were plotted in Fig.2. The rate constant decreased with the increase of initial CO concentration in general. The reaction was almost stopped over Pt/TiO2 and Pt/SiO2 for initial CO concentration higher than 1000ppm, due to the strong adsorption of CO on Pt metal surface, while gold catalysts were active for the all range of concentration. The gap from the fitting curves after 10 min was observed in the case of initial concentration higher than 100ppm. The direction of the gap (the upper side or the lower side) was different depending on the case and the interpretation is difficult at the present stage. It is thought the result of the participation of both factors of the difference of rate constant with CO concentration and the deactivation with time through the accumulation of poisoning species like carbonates.
These results clearly showed that Au/Fe2O3 and Au/TiO2 are potentially useful for the removal of CO in closed space for from low to high concentration, however, deactivation is a serious problem. The method for the relaxation of deactivation is now under investigation.
gold catalyst, platinum catalyst, CO oxidation, concentration dependence, indoor air purification, Deactivation