Highly Dispersed Copper Oxide Clusters as Active Species in Copper-Ceria Catalyst for Preferential Oxidation of Carbon Monoxide

Abstract

Copper-ceria is one of the very active catalysts for the preferential oxidation of carbon monoxide (CO-PROX) reaction, which is also a typical system in which the complexity of copper chemistry is clearly exhibited. In the present manuscript, copper−ceria catalysts with different Cu contents up to 20 wt % supported on CeO 2 nanorods were synthesized by a deposition−precipitation (DP) method. The as-prepared samples were characterized by various structural and textural detections including X-ray diffraction (XRD), Vis-Raman, transmission electron microscopy (TEM), ex situ/in situ X-ray absorption fine structure (XAFS), and temperatureprogrammed reduction by hydrogen (H 2 -TPR). It has been confirmed that the highly dispersed copper oxide (CuO x ) clusters, as well as the strong interaction of Cu-[O x ]-Ce structure, were the main copper species deposited onto the ceria surface. No separated copper phase was detected for both preoxidized and prereduced samples with the Cu contents up to 10 wt %. The fresh copper−ceria catalysts were pretreated in either O 2 -or H 2 -atmosphere and then tested for the CO-PROX reaction at a space velocity (SV) of 60 000 mL. The prereduced 5 and 10 wt % Cu samples exhibited excellent catalytic performance with high CO conversions (>50%, up to 100%) and O 2 selectivities (>60%, up to 100%) within a wide temperature window of 80−140 °C. The in situ XAFS technique was carried out to monitor the structural evolution on the copper−ceria catalysts during the PROX experiments. The X-ray absorption near edge spectra (XANES) profiles, by the aid of linear combination analysis, identified the oxidized Cu(II) were the dominant copper species in both O 2 -and H 2 -pretreated samples after CO-PROX at 80 °C. Furthermore, the extended X-ray absorption fine structure (EXAFS) fitting results, together with the corresponding H 2 -TPR data distinctly determined that the highly dispersed CuO x (x = 0.2−0.5) cluster, other than the Cu−[O x ]−Ce (x = 0.7−3.2) structure, were the crucial active species for the studied CO-PROX reaction.

Improved CO-PROX Performance of CuO/CeO2 Catalysts by Using Nanometric Ceria as Support

“…In Table 2, the H2 uptake evaluated from the integration of TPR curves is reported. As reported in the literature, ceria reduction starts at temperatures higher than 350 • C [52,59,60]; however, a small copper amount supported on ceria significantly modifies the TPR pattern, showing two to four reduction peaks at temperatures much lower than those obtained on pure ceria [9,19,22,43,52,61,62].…”

“…This finding is in agreement with the behaviors of other catalysts reported in the literature. For instance, Wang et al [61] showed a significant improvement of the catalytic activity by increasing the Cu content from 2 to 5 wt %, while a further increase to 10 wt % showed a lower effect. Jampa et al [46] and Gu et al [33] reported the catalytic performance of CuO/CeO 2 catalysts with different copper contents.…”

Fabrication of CeO2–MOx (M = Cu, Co, Ni) composite yolk–shell nanospheres with enhanced catalytic properties for CO oxidation

“…So far, a remarkable process has been developed for the synthesis of CeO 2 -based composite oxides, including CeO 2 –CuO x [13], CeO 2 –ZnCo 2 O 4 [14], CeO 2 –CoO x [15], CeO 2 –MnO x [16], CeO 2 –ZnO [17], CeO 2 –Fe 2 O 3 [18], and CeO 2 –ZrO 2 systems [19]. Taking the CeO 2 –CuO catalyst as a typical example, the improved catalytic activity is closely related to the synergistic interaction between copper and ceria, which promotes the exchange of charges between Ce 4+ /Ce 3+ and Cu 2+ /Cu + and leads to faster oxidation and reduction than that of the corresponding independent forms.…”

“…Taking the CeO 2 –CuO catalyst as a typical example, the improved catalytic activity is closely related to the synergistic interaction between copper and ceria, which promotes the exchange of charges between Ce 4+ /Ce 3+ and Cu 2+ /Cu + and leads to faster oxidation and reduction than that of the corresponding independent forms. The formation of highly-dispersed copper species promotes the adsorption of CO molecules, while the presence of oxygen vacancies provided by CeO 2 can in turn create active oxygen in the oxidation reactions [12–13]. Therefore, the creation of two-phase interfaces as numerous as possible and, thus, the facilitation of synergistic interaction between two components are necessary to optimize the catalytic performances.…”

“…The unique structure and texture of CeO 2 -based catalysts is also associated with high activity and stability in the catalytic reaction. For instance, rod-like CeO 2 –CuO catalysts with highly dispersed copper oxide clusters as active species had been reported to exhibit superior activity toward CO oxidation in contrast with commonly used CeO 2 /CuO composite catalysts [13]. Consequently, the construction of ceria-based composite oxides with pore features, hollow structure or/and hierarchical architecture, which possess excellent redox properties and abundant oxygen vacancies, will be favorable for the enhancement of catalytic activity toward CO oxidation.…”