Identification of CuO Species in High Surface Area CuO−CeO₂Catalysts and Their Catalytic Activities for CO Oxidation

Abstract

Nanosized CuO-CeO 2 catalysts with high surface area (>90 m 2 g -1 ) were prepared by a modified citrate sol-gel method with incorporation of N 2 thermal treatment. CO temperature-programmed reduction results indicated that there are three CuO species in the catalyst, namely, the finely dispersed CuO, the bulk CuO, and the Cu 2+ in the CeO 2 lattice. Using CO oxidation as a model reaction, catalytic activity of each species was evaluated. It was found that the finely dispersed CuO species had the highest activity (183.3 mmol CO g Cu -1 h -1 ) and the bulk CuO had medium activity (100.4 mmol CO g Cu -1 h -1 ), while the Cu 2+ in the CeO 2 lattice had the lowest activity (21.3 mmol CO g Cu -1 h -1 ). Furthermore, the Cu 2+ species in the CeO 2 lattice could migrate to the catalyst surface to form finely dispersed CuO under high-temperature calcination or under reaction conditions, which could consequently enhance the catalytic activity.

CO–PROX reactions on copper cerium oxide catalysts prepared by melt infiltration

“…From these data, it is clear that the high catalytic activity in CO oxidation is due to the presence of highly dispersed CuO particles on the ceria support. Such conclusion is close to the results reported by Luo et al [24]. The bulk CuO crystallites in CuCe(8/1)/SHS are more active than the CuO crystallites of CuO/SHS due to the presence of dispersed ceria particles on their surface.…”

“…The samples with a relatively low Cu content contain two types of Cu species [24], namely (i) Cu in a solid solution in ceria and (ii) highly dispersed CuO species. The proportion and size of the latter will increase with the Cu content up to the point where obvious bulk CuO is observed in the patterns (CuCe(1/1)/SHS).…”

“…In CuCe(1/4)/SHS, the first reduction peak at 180 • C is due to finely dispersed CuO and the second one around 220-230 • C should have two origins, namely Cu ions in solid solution and larger CuO particles interacting with the ceria. The use of nitric acid to selectively dissolve bulk and small CuO has been reported [6,24,38]. To facilitate the identification of Cu species, CuCe(1/4)/SHS, CuCe(1/4)/SHS-IMG and CuCe(1/8)/SHS were treated with HNO 3 .…”

Characterization of Active Sites/Entities and Redox/Catalytic Correlations in Copper-Ceria-Based Catalysts for Preferential Oxidation of CO in H2-Rich Streams

“…In order to attempt to rationalize this behavior, the catalysts of the Ce-Tb series have been examined by means of CO-TPR tests [48]. In this sense, recent work by Luo et al [32] provides an interesting rationalization of redox/catalytic correlations in catalysts combining copper and cerium oxides; on the basis of classical CO-TPR results, they could differentiate between the reducibilities of the three types of oxidized copper entities generically proposed to be present in this type of catalyst (in decreasing order of reducibility or CO oxidation activity): finely dispersed CuO; bulk-like CuO; and, finally, Cu 2+ in the ceria lattice, likely substitutionally. Nevertheless, such an investigation has been undertaken on a series of catalysts combining in all cases copper oxide and ceria while, as previously mentioned, changes in the support nature can modify such redox or catalytic properties [46].…”

“…The particular ability of copper-ceria catalysts for the CO-PROX or related processes has been essentially attributed to the synergistic redox properties exhibited upon formation of copper oxide-ceria interfacial sites [4,6,17,18,[25][26][27][28][29][30][31][32][33][34][35]. In this sense, generally speaking, the properties of copper oxide entities for CO oxidation promotion depend strongly on their dispersion degree and/or related degree of interaction with ceria [18,25,26,36,37].…”