ArticleStable CuO/La2Sn2O7 catalysts for soot combustion: Study on the monolayer dispersion behavior of CuO over a La2Sn2O7 pyrochlore support
Graphical abstract
Active surface O2– and O22– sites determine the soot combustion activity on CuO/La2Sn2O7. The best catalyst can be obtained by covering La2Sn2O7 surface with a CuO monolayer.
Introduction
Compared with gasoline engines, diesel engines have generally higher fuel efficiencies and lower costs [1, 2]. However, the soot particulate matter (PM) released from diesel engines has a severe impact on the environment and human health [3, 4, 5]. Trapping the soot PM in the exhaust using a diesel particulate filter (DPF) honeycomb, which is usually coated with an oxidation catalyst, has been shown to be an effective way to reduce pollution and has been applied in vehicles [6, 7, 8]. Presently, noble metals are most commonly used catalysts for soot oxidation, but their wide application is restricted by their limited availability and high cost. Therefore, the development of low cost and efficient catalysts is highly desirable. To date, catalysts based on alkaline metal oxides [9, 10], transition metal oxides [4, 7, 11], Ce-Zr solid solutions [12, 13], perovskite oxides [14, 15], and pyrochlore oxides [16, 17] have been studied. Nevertheless, achieving a feasible soot combustion catalyst without using noble metals remains a challenge [18].
CuO-based materials are active catalysts used for many oxidation processes because of the highly reactive Cu–O bonds [19, 20]. A typical example is the famous Hopcalite catalyst, which is composed of CuO and manganese oxides, and catalyzes complete CO oxidation at room temperature [21]. Over the past few years, CuO supported on materials such as Al2O3 [22], CeO2 [23], perovskites [24], Ce-Zr-O solid solutions [25], and ZrO2-TiO2 composite oxides [26] have been investigated as soot combustion catalysts, and considerable catalytic activity has been observed. Meng et al. [27] prepared monolithic lawn-like pure CuO nanorod arrays for soot combustion and found that the catalysts display higher activity than typical CuO nanoparticles. To understand the structure–reactivity relationship of CuO for soot combustion, we previously prepared a series of CuO samples using various Cu precursors and precipitants [28], and we found that the redox properties, morphology, and texture significantly affect the soot oxidation activity. For example, a CuO sample synthesized with a Cu(NO3)2 precursor and Na2CO3 precipitant showed the best soot oxidation performance of all the tested samples, mainly because of the formation of abundant active surface superoxide (O2–) and peroxide (O22–) anions. However, pure CuO suffers from low thermal stability, which hinders its practical use. To obtain more applicable catalysts, CuO was supported on a stable SnO2 support [29]. Although the catalytic stability of CuO can be improved by using a support (as in CuO/SnO2), the catalytic activity is decreased. Therefore, more appropriate supports are required to obtain practical CuO-based catalysts.
A2B2O7 pyrochlore compounds are important functional materials that have been applied in gas sensors [30], ferroelectric materials [31], and catalysts [32, 33]. In particular, these compounds possess excellent thermal stability and excellent oxygen anion conductivity, which are vital features of redox catalysts for exothermic processes [34, 35]. Moreover, in the unit cell of pyrochlore compounds, eight inherent 8a oxygen vacancies are present, which results in excellent lattice oxygen mobility, especially when the radii of the A- and B-site ions are similar [35, 36]. As a consequence, active O2– and O22– anions at the surface are generated, which is favorable for many redox reactions, as demonstrated in our previous publications [35, 37]. In particular, we have shown that supporting Pd onto a high-surface-area La2Sn2O7 pyrochlore results in catalysts with better activities than those of Pd/Al2O3 and Pd/SnO2 [38]. Therefore, pyrochlore compounds with suitable chemical compositions and surface areas might be good candidates as supports for CuO for use as soot combustion catalysts.
Monolayer dispersion is an important theory established by Xie and co-workers [39] in the late 1970s, and it is particularly useful in the design of supported metal oxide catalysts for various reactions. Xie and co-workers [29, 39] discovered that metal oxides can spontaneously disperse onto the surface of many supports to generate a monolayer or sub-monolayer, and the monolayer capacity can be quantified by extrapolation of data obtained from X-ray diffractometry (XRD), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS) measurements. For many reactions, an evident monolayer dispersion threshold effect is observed. That is, a catalyst containing the active components at the monolayer dispersion capacity limit often exhibits the best activity and selectivity [29, 39]. As pointed out by Wachs et al. [40, 41], in the monolayer dispersion state, interfacial oxygen bonds (M–O–S) between the supported metal oxides and supports are generated, and these species show unique catalytic activity for many reactions. Over the past decades, although metal oxide dispersions on individual component supports, such as Al2O3, SiO2, ZrO2, and TiO2, have been extensively explored, the use of composite oxides to support active components to prepare catalysts has been investigated little.
Inspired by these previous investigations and aiming to obtain more applicable soot combustion catalysts, we used a La2Sn2O7 pyrochlore compound as the support for CuO to prepare a series of soot combustion catalysts. In addition, to understand the dispersion behavior of the supported metal oxide on the composite oxide support, the interfacial interaction between CuO and the La2Sn2O7 support was thoroughly investigated by various characterization techniques and density functional theory (DFT) calculations. We found that there is a significant monolayer threshold effect. Moreover, the supported CuO preferentially interacts with the B-site Sn cations in the support instead of the A-site La cations. We found that the CuO/La2Sn2O7 catalyst with a CuO loading close to the monolayer dispersion capacity showed the best performance for soot oxidation of all samples because of the formation of a high quantity of electrophilic oxygen sites at the catalyst surface.
Section snippets
Preparation of La2Sn2O7 pyrochlore support
A La2Sn2O7 pyrochlore support was prepared using a hydrothermal method. The chemical reagents used in the preparation were analytical grade to ensure the purity of the obtained samples. First, 0.5 mol L–1 aqueous La(NO3)3 and Na2SnO3 solutions were prepared separately. Then, the two solutions were mixed together in equal volumes (150 mL) in a Teflon liner with 500-mL capacity. On mixing, a white sodium stannate precipitate was formed. On adding nitric acid solution (30%) dropwise, the
N2 sorption results
The textural features of the CuO/La2Sn2O7 catalysts with different CuO loadings were analyzed by N2 sorption. Fig. S1(a) shows that the catalysts, together with the La2Sn2O7 support, exhibit type-IV isotherms and H3-type hysteresis loops. This indicates that the CuO/La2Sn2O7 catalysts possess a mesoporous structure, which is due to the accumulation of holes in the sample particles. Fig. S1(b) shows that the CuO/La2Sn2O7 catalysts have pore size distribution profiles similar to that of La2Sn2O7,
Conclusions
To understand the dispersion behavior of a metal oxide on a composite oxide support and with the aim of developing catalysts for soot oxidation, CuO/La2Sn2O7 catalysts with different CuO loadings were prepared on the basis of monolayer dispersion theory. Using various characterization techniques and DFT calculations, the surface and bulk structure properties of the samples were thoroughly explored.
- (1)
As for single component metal oxide supports, a spontaneous monolayer dispersion of CuO on the La2
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Published 5 March 2021
This work was supported by the National Natural Science Foundation of China (21962009, 21567016, 21666020), the National Key Research and Development Program of China (2016YFC0209302), the Natural Science Foundation of Jiangxi Province (20181ACB20005, 20171BAB213013, 20181BAB203017), the Key Laboratory Foundation of Jiangxi Province for Environment and Energy Catalysis (20181BCD40004).