Shape-controlled Cu_xCe_1-xO₂ nanorods catalyst and the active components functioned in selective oxidation of CO in hydrogen-rich stream

Highlights

• Shape-controlled Cu_xCe_1-xO₂ nanorods catalyst was successfully synthesized.

• Copper dopant enriches the lattice distortion and oxygen mobility of ceria.

• Highly dispersed CuO_x determines the low-temperature CO oxidation activity.

• Strongly bound Cu-[O_x]-Ce affects the high-temperature CO oxidation activity.

Abstract

A novel shape-controlled Cu_xCe_1-xO₂ nanorods catalyst is synthesized with a simple coprecipitation method at room temperature and Cu(I) chloride as copper sources for the first time. Various analysis methods (XRD Rietveld Refinement, N₂ adsorption-desorption, dynamic OSC, XPS, HRTEM, H₂-TPR and In situ DRIFTs) and catalytic activity test techniques are used to investigate the structural properties and the catalytic performance of the catalysts in CO-PROX. Introducing copper dopant into ceria nanorods has positive effect on enriching the lattice distortion, oxygen vacancy concentration and the lattice oxygen mobility of ceria support. The incorporation of copper ions generates two main types of active copper species that play different roles in CO-PROX. Specifically, the highly dispersed CuO_x species determine the low-temperature CO oxidation activity, while strongly bound Cu-[O_x]-Ce species at the copper-ceria interface regulates the high-temperature CO oxidation activity of the catalyst. Appropriate copper doping (7–11 wt %) is in favor of forming sufficient CuO_x species highly dispersed on the surface of ceria nanorods and accelerating the oxygen mobility, as well as strengthening the copper-ceria interaction in the catalyst, which significantly improves its catalytic activity and broadens the temperature window in CO-PROX.

Introduction

In recent years, environmental pollution becomes increasingly serious, and new energy vehicles based on fuel cells have received more and more attention. Proton exchange membrane fuel cells (PEMFCs) show some unique characteristics, such as high energy density, low operating temperature and high mobility, making them the most ideal power source for fuel cell vehicles [[1], [2], [3]]. However, the anode of PEMFCs is usually Pt-based catalyst, which is extremely sensitive to CO in hydrogen fuel, and even a trace amount of CO would cause it to be poisoned [4,5]. The typical industrial technology of producing hydrogen fuel usually adopts the liquid organic fuel reforming technology, however, the produced hydrogen still hardly meets the application requirements even the water gas shift (WGS) reaction is applied [6,7]. Among various deep purification methods, the selective oxidation of CO (CO-PROX) is the most effective and economical means for removing CO from hydrogen-rich gas to below 100 ppm [8,9].

At present, the most popular catalysts for CO-PROX are mainly involved with the supported precious metal catalyst and the copper-ceria based catalyst. Generally speaking, Au- and Pt-based catalysts exhibit very high oxidation activity for CO at low temperature range, however, the high cost and poor selectivity for CO oxidation at high temperature limit the large-scale application of precious metal catalysts in CO-PROX [[10], [11], [12], [13], [14], [15]]. Copper-ceria catalyst is another kind of the most researched catalytic system, which perfectly balanced the cost and the catalytic performance (CO oxidation activity and selectivity) in CO-PROX and is regarded as a promising alternative for the precious metal catalysts with bright application prospect [[16], [17], [18], [19], [20], [21], [22]]. The superior catalytic property of copper-ceria catalyst is commonly attributed to the synergetic interaction between copper and ceria species [[23], [24], [25], [26]]. The rapid reversible conversion between Ce⁴⁺ and Ce³⁺ effectively and mutually promotes the redox process between Cu²⁺ and Cu⁺ and the oxidation reaction of CO.

In order to further optimize the catalytic performance of copper-ceria catalyst in CO-PROX, researchers have made much effort to develop the synthesize strategies to maximize the synergetic effect between copper and ceria [[27], [28], [29], [30], [31], [32], [33], [34], [35]]. For example, one widely used method is to increase the concentration of highly dispersed copper oxide in copper-ceria catalysts [29,31]. Ding et al. prepared a highly active copper-ceria catalyst with the surface area of 247 m² g⁻¹ using porous organic polymer-templated method, in which the concentration of highly dispersed copper oxide is up to 30 at. % [29]. In addition, doping transition metals is an effective means to boost the synergetic interaction in copper-ceria catalyst. Recently, Luo et al. studied the doping effect of Fe over Cu/Ce-Based Catalyst and revealed the nature of the active species for CO-PROX. They found that Fe incorporated into CeO₂ lattice favors to form Fe–O–Ce structure and generate more oxygen vacancies, which not only strengthen the copper-ceria interaction to form more Cu⁺ but also promote the release of subsurface lattice oxygen to supply more reactive oxygen [27,28]. Moreover, Regulating the space structure of the active components, such as forming core-shell structure, is another potent method to maximize the active interfacial sites and promote the charge transfer [30,32]. Xu et al. synthetized core-shell CuO_x-CeO₂catalysts by introducing citric acid and hexamethylenetetramine ((CH₂)₆N₄) to instruct CuO_x only nucleate on the ceria surface. The core-shell structure ensures the intimate contact between copper and ceria and the catalyst shows high conversion and broad operating temperature window (95–235 °C) for CO-PROX reaction [32]. On the other hand, the cubic CeO₂ nanocrystal has three low-index crystal planes ({111}, {110}, and {100}). The different stability and defect formation energy of the three crystal planes varies the surface structure and composition of ceria nanocrystal and adjusts the concentration and structure of oxygen vacancy [[36], [37], [38]]. Therefore, controlling the morphology of ceria with specific crystal faces exposed is found to be an efficient way to regulate the synergetic interaction between copper and ceria [33,34,39]. In our previous study [39], we found that CuO/CeO₂-rod catalyst exhibits much better catalytic performance with lower T_50% and broader temperature window for CO-PROX compared with CuO/CeO₂-plate and CuO/CeO₂-cube, as a consequence of the exposed crystal facet, abundant oxygen vacancies and strong interaction between copper and ceria nanorods. And based on this result, we further optimized the copper loading amount and discovered the doping effect of transitional metals on the supported copper-ceria nanorods catalyst [[40], [41], [42]]. The results demonstrated that the copper-ceria interaction could be strengthened to the maximum degree with appropriate copper loading on ceria nanorod support [42] and the incorporation of specific transitional metals (such as manganese and titanium) to the ceria lattice [41]. On the other hand, Ciston et al. [43] studied the structural and electronic properties of Ce_0.8Cu_0.2O₂ and CuO/CeO₂ using synchrotron-based time-resolved in situ XRD and XAS, and they discovered that strong interaction between copper and the ceria matrix was produced in the reduced Ce_0.8Cu_0.2O₂ solid solution, which has not been seen for a simple blend of the oxides (CuO/CeO₂). Therefore, we are quite curious about the structure properties and catalytic activity of copper-ceria catalyst if we could embed copper ions into the crystal lattice of ceria with well-controlled morphology. Moreover, considering the close correlation between the concentration of Cu⁺-CO and CO oxidation activity of the copper-ceria catalysts [26], we prospect to synthesis the copper-ceria catalyst by means of CuCl as copper sources on the purpose to enrich the concentration of Cu⁺ species. Therefore, in the present work, we prepared a novel shape-controlled Cu_xCe_1-xO₂ nanorods catalyst by means of a simple coprecipitation method at room temperature and CuCl as copper sources for the first time. Multiple characterization techniques were applied to study the structural properties and the catalytic performance of the catalysts in CO-PROX. Interesting results have been discovered about the specific active components in Cu_xCe_1-xO₂ nanorods catalyst that are functioned differently in CO-PROX at different temperature range.