Preferential CO oxidation over supported noble metal catalysts

We conducted a comparative study for the preferential CO oxidation (PROX) over supported noble metal catalysts. The CO chemisorption in the absence and presence of H2, the O2 chemisorption, the temperature-programmed oxidation (TPO), the temperature-programmed reduction (TPR) after O2 chemisorption and the transmission electron microscopy (TEM) were conducted to characterize the catalysts. Commercial catalysts such as 1 wt.% Pt/γ-Al2O3, 0.5 wt.% Ru/γ-Al2O3, 0.5 wt.% Rh/γ-Al2O3, 5 wt.% Pt/γ-Al2O3, 5 wt.% Ru/γ-Al2O3, and 5 wt.% Rh/γ-Al2O3 were utilized. Among them, 5 wt.% Ru/γ-Al2O3 showed the highest PROX activity. This catalyst can be considered to be promising because it can reduce the high inlet CO concentration to be less than 10 ppm at low temperatures where the reverse water–gas shift reaction can be minimized. No detectable amount of chemisorbed O2 was measured at 373 K over 1 wt.% Pt/γ-Al2O3, 0.5 wt.% Rh/γ-Al2O3 and 5 wt.% Pt/γ-Al2O3. The moderate degree of CO and O2 chemisorption at reaction temperatures appeared to be essential for the optimum PROX activity. Although 0.5 wt.% Ru/γ-Al2O3 and 5 wt.% Ru/γ-Al2O3 have the similar particle size distribution based on TEM analysis, the latter catalyst with smaller amount of chemisorbed CO and O2 per Ru metal showed much better PROX activity. Based on TPO and TPR result, 0.5 wt.% Ru/γ-Al2O3 can be oxidized at lower temperatures but can be reduced at higher temperatures compared with 5 wt.% Ru/γ-Al2O3. Therefore, the easily reducible Ru species can give us the better PROX activity over a wide reaction temperature for Ru catalysts.