H2 oxidation and preferential CO oxidation over Au: New approaches
H2 oxidation and preferential CO oxidation (CO-PROX) were studied over Au/Al2O3-catalysts with gold content 0.2–1 wt.% in a flow reactor of quartz.It is established that the reactions of H2 oxidation and CO-PROX can be performed in a mode of the catalyst surface ignition (CSI). If temperature/gas flow rate are varied within certain limits, this mode can be maintained. Oxygen is the key component in CSI mode in the case of H2-rich gases; its residual content is below 70 ppm. Residual content of CO in CSI mode depends on temperature, gradually decreasing upon a decrease in temperature in the “hot spot” of the reaction zone.In oxidation reactions conducted in CSI mode, the “hot spot” of the reaction zone is situated at the exit from the catalyst bed in two cases: when the catalysts are of low Au content, or when the temperature is lowered. This is a sign of catalyst deactivation at the entrance to the catalyst bed.Self-activation of the catalyst is found in both presence and absence of CO as a spontaneous overheat of the catalyst over time with “hot spot” drifting from the exit of the catalyst bed to the entrance.It is proposed that both deactivation and self-activation are caused by rival processes on the surface of gold: strong binding of oxygen and oxygen removal by desorption or consumption in exothermic oxidation processes. Thus, the positive effect of hydrogen on CO oxidation can be connected both with a change in ratio between different valent forms of gold and the conjugation of oxidation reactions.
Graphical abstract.Figure optionsDownload full-size imageDownload as PowerPoint slideResearch highlights▶ H2 oxidation and CO-PROX (∼1% O2) can be performed in the mode of Au-catalyst surface ignition. ▶ Transfer into catalyst surface ignition starts at the exit from the catalyst bed with height ∼1.5 cm. ▶ Key factors of CO-PROX: exothermic oxidation reactions CO and H2, the character of interaction between Au and oxygen of reaction media. ▶ Self-activation of Au-catalysts is discovered for H2-rich gases with or without CO.
Journal: Applied Catalysis B: Environmental - Volume 103, Issues 1–2, 14 March 2011, Pages 39–47