The influence of Mn-doped CeO2 on the activity of CuO/CeO2 in CO oxidation and NO + CO model reaction
Graphical abstract
Introduction
Given that the worsening air pollution and global warming threats are harmful to human health and the environment across the world, it has been a worldwide topic to build a low carbon society [1], [2]. CO oxidation is a prototype reaction in heterogeneous catalysis and has been paid huge attention for many years, and two main reasons should be considered. Technologically, CO oxidation is a major reaction in vehicle exhaust control [3], CO2 lasers [4], and other industrial processes [5]; theoretically, CO oxidation is a relatively simple reaction and it is significant for a model system to study the mechanism of heterogeneous catalysis process [6]. On the other hand, nitrogen oxides (NOx) are one of the main air pollutant particularly correlated to photochemical smog, acid rain, and ozone depletion [7], [8], [9], [10]. Chinese legislation of NOx emissions from both stationary and mobile sources is increasingly rigorous [8]. Over the past several years, noble metals supported on reducible or irreducible supports are used for catalysis and unique and obvious activities can be presented no matter for CO oxidation [11], [12], [13] or for NO + CO reaction [14], [15]. However, the high prices limit their wide use. In recent years, tremendous attention has been paid to base metals [16], [17], especially to copper oxide [18], [19], [20].
Ceria has been widely used in energy, environmental, material, and catalysis fields in recent decades due to its outstanding redox ability and high oxygen storage/release capacity (OSC) which is associated with the formation of oxygen vacancies and the Ce4+/Ce3+ redox couple [2], [10], [20], [21]. On the other hand, however, the practical application of pure ceria is highly discouraged due to its poor thermal stability and low specific surface area. It is common knowledge that the introduction of foreign metal cations into the ceria system will improve its redox behavior and catalytic activity obviously due to the formation of more oxygen vacancies [22], [23], [24]. In CuO–CeO2 binary oxides, a strong catalytic synergy interaction between copper oxide and ceria has ever been found [25], [26]. It has also been found that MnOx with various labile oxygen plays an important role in the catalytic reaction [27]. MnOx–CeO2 mixed oxides possess much higher catalytic activity than pure MnOx or CeO2,which is due to a favorable synergistic interaction between manganese oxide and ceria. For example, Machida et al. [28] found that the redox of Mn ions with simultaneous oxygen equilibration with the gas phase should play an important role in facilitating the oxidative adsorption of NO. Li et al. [29] reported that appropriate doping of Mn into CuOCeO2 catalyst was conducive to the formation of a more stable solid solution with a larger specific surface area and smaller particle size, and the redox properties of the catalysts were also increased, which also promoted the selective oxidation performance of CO in hydrogen-rich streams. Xu et al. [30] found that many reasons resulted in the best catalytic activity and the widest reaction window, for MnOx–CeO2/10% WO3–ZrO2 catalyst, one of which was attributed to the appropriate surface atomic ratio of Mn: Ce. Kinetic studies also showed that the incorporation of Mn into CeO2 enhanced catalytic activities [31], [32].
Although MnOx is widely regarded as dopant and used in many catalytic reactions [27], [28], [29], [30], [31], [32], the doping amount of MnOx is relatively large. Researches [20], [33] have suggested that a small amount of dopant may cause obvious change on the characteristic of the materials. However, as far as we know, relative studies are few for small amount of MnOx (which is regarded as dopant). Furthermore, copper oxide supported on MnOx–CeO2 mixed oxides used for CO oxidation or NO + CO reaction is scarcely reported. And the study of CO oxidation and NO + CO reaction to which they are simultaneously applied is vacant. The modified storage capacity of CeO2 by MnOx may also improve the activity of supported Cu species for above reactions. Therefore, the influence of Mn-doped CeO2 on the activity of CuO/CeO2 is necessary to be studied.
In this work, a series of MnOx-doped CeO2 with different Ce/Mn molar ratios which were 20: 1, 10: 1, 5: 1, 5: 2, 5: 3, 5: 4, and pure CeO2 supported by CuO were synthesized by inverse co-precipitation method, respectively, and the obtained samples were characterized using N2-physisorption (BET), XRD, LRS, TEM, EDS-Mapping, ICP-AES, XPS, H2-TPR, O2-TPD, in situ DRIFTS, CO oxidation, and NO + CO model reaction. This study is mainly focused on: (i) exploring the influence of MnOx with different molar ratio doping into CeO2 on texture, structure, chemical composition, surface state, redox property, and activity of CO oxidation and NO + CO model reaction over CuO supported on MnOx-doped CeO2 catalysts; (ii) investigating the interaction of CO and/or NO with CuO supported on MnOx-doped CeO2 catalysts via in situ DRIFTS technique to understand the nature of CO oxidation and NO + CO model reaction.
Section snippets
Catalyst preparation
MnOx-doped CeO2 supports with different Ce/Mn molar ratios i.e., 20: 1, 10: 1, 5: 1, 5: 2, 5: 3 and 5: 4 were synthesized by inverse co-precipitation method, respectively. Mn(NO3)2 (50% solution) and Ce(NO3)3·6H2O were dissolved in deionized water, then excess ammonia solution (a precipitating agent) was then added into the solution, drop by drop, with vigorously stirring until pH = 10.0. After stirring for 3 h and aging for 24 h, then the mixture solution filtered and washed with deionized water
Catalytic performance of the catalysts (CO oxidation and NO + CO reaction)
The catalytic performances of these samples in CO oxidation reaction and of fresh and thermal treated samples at 100 °C are shown in Fig. 1(a) and (b), respectively. For Fig. 1(a), it is obvious that pure CeO2is almost no activity in this temperature window, by contrast, when the molar ratio of Ce to Mn reaches 5: 2, its catalytic activity is improved to a certain extent, suggesting that appropriate doping of MnOxinto CeO2is contributive to the enhancement of catalytic performance in CO
Conclusions
This work studies the effect of different Ce/Mn molar ratios supported by CuO on the texture, structure, chemical composition, surface state, redox property, and activity of CO oxidation and NO + CO model reaction. Several major conclusions can be obtained as follows:
- (1)
Appropriate doping MnOx into the lattice of CeO2 increases the specific surface area, which is conducive to the dispersion of CuO, and enhances the catalytic activity and thermal stability.
- (2)
Cu/CeMn-10: 1 catalyst exhibits the best
Notes
The authors declare no competing financial interest.
Acknowledgments
The financial supports from National Basic Research Program of China (973 program, No. 2012CB21500203), the National Nature Science Foundation of China (No. 21507014), the China Postdoctoral Science Foundation (No. 2014M550451), the Nature Science Foundation of Guangxi Province (No. 2014GXNSFBA118036), the Project of Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology (No. 2014K005) are gratefully acknowledged.
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