Effects of noble metal-doping on Cu/ZnO/Al2O3 catalysts for water–gas shift reaction: Catalyst preparation by adopting “memory effect” of hydrotalcite Original Research Article

Trace amounts of noble metal were doped on Cu/ZnO/Al2O3 catalysts by adopting “memory effect” of hydrotalcite. Ternary Cu/ZnO/Al2O3 (molar ratio Cu/Zn = 1/1, Al content 0–25 mol%) catalysts were prepared by co-precipitation (cp) from metal nitrates; a mixture of hydrotalcite (Cu,Zn)6Al2(OH)16CO3·4H2O, and aurichalcite (Cu,Zn)5(CO3)2(OH)6, was formed at pH 9 with NaOH as the pH controller. Finally the sample, cp-Cu/Zn/Al(45/45/10) containing mainly aurichalcite together with a small amount of hydrotalcite, was selected as the precursor for the noble metal-doping. Noble metal-doping was conducted by dipping the precursor calcined at 300 °C in aqueous solutions of the noble metal nitrates; hydrotalcite was reconstituted by the “memory effect” and simultaneously noble metals were incorporated. The noble metal-doped samples were calcined at 300 °C and tested for the water–gas shift (WGS) reaction. Among the noble metals, Pt was the most effective for stabilizing the catalytic activity although some deactivation due to Cu sintering took place. An intrinsic promoting effect of Pt was clearly observed by evaluating the turnover frequency of the catalyst. Aurichalcite was indispensable for producing active Cu/ZnO sites, whereas a small amount of hydrotalcite was effective for improving the sustainability of the catalyst by the surface modification. It is likely that hydrogen-spillover from trace Pt to active Cu metal not only enhanced the activity via the reduction–oxidation cycle between Cu0 and Cu+ but also stabilized the active Cu metal species against oxidative sintering during the reaction.