Article pubs.acs.org/Langmuir
Designing CuOx Nanoparticle-Decorated CeO2 Nanocubes for Catalytic Soot Oxidation: Role of the Nanointerface in the Catalytic Performance of Heterostructured Nanomaterials Putla Sudarsanam,† Brendan Hillary,† Baithy Mallesham,‡ Bolla Govinda Rao,‡ Mohamad Hassan Amin,† Ayman Nafady,§,∥ Ali M. Alsalme,§ B. Mahipal Reddy,‡ and Suresh K. Bhargava*,† †
Centre for Advanced Materials and Industrial Chemistry (CAMIC), School of Applied Sciences, RMIT University, Melbourne, Victoria 3001, Australia ‡ Inorganic and Physical Chemistry Division, Council of Scientific and Industrial Research (CSIR)−Indian Institute of Chemical Technology, Uppal Road, Hyderabad, Telangana 500 007, India § Chemistry Department, College of Science, King Saud University, Riyadh 12372, Saudi Arabia ∥ Chemistry Department, Faculty of Science, Sohag University, Sohag 11432, Egypt S Supporting Information *
ABSTRACT: This work investigates the structure−activity properties of CuOx-decorated CeO2 nanocubes with a meticulous scrutiny on the role of the CuOx/CeO2 nanointerface in the catalytic oxidation of diesel soot, a critical environmental problem all over the world. For this, a systematic characterization of the materials has been undertaken using transmission electron microscopy (TEM), transmission electron microscopy−energydispersive X-ray spectroscopy (TEM−EDS), high-angle annular dark-field− scanning transmission electron microscopy (HAADF−STEM), scanning transmission electron microscopy−electron energy loss spectroscopy (STEM−EELS), X-ray diffraction (XRD), Raman, N2 adsorption−desorption, and X-ray photoelectron spectroscopy (XPS) techniques. The TEM images show the formation of nanosized CeO2 cubes (∼25 nm) and CuOx nanoparticles (∼8.5 nm). The TEM−EDS elemental mapping images reveal the uniform decoration of CuOx nanoparticles on CeO2 nanocubes. The XPS and Raman studies show that the decoration of CuOx on CeO2 nanocubes leads to improved structural defects, such as higher concentrations of Ce3+ ions and abundant oxygen vacancies. It was found that CuOx-decorated CeO2 nanocubes efficiently catalyze soot oxidation at a much lower temperature (T50 = 646 K, temperature at which 50% soot conversion is achieved) compared to that of pristine CeO2 nanocubes (T50 = 725 K) under tight contact conditions. Similarly, a huge 91 K difference in the T50 values of CuOx/CeO2 (T50 = 744 K) and pristine CeO2 (T50 = 835 K) was found in the loose-contact soot oxidation studies. The superior catalytic performance of CuOxdecorated CeO2 nanocubes is mainly attributed to the improved redox efficiency of CeO2 at the nanointerface sites of CuOx− CeO2, as evidenced by Ce M5,4 EELS analysis, supported by XRD, Raman, and XPS studies, a clear proof for the role of nanointerfaces in the performance of heterostructured nanocatalysts. providing a higher catalytic efficiency.5,6 In addition, by tuning the morphology of nano-CeO2, an enhancement in its catalytic efficiency can be expected because of morphology-dependent properties.1,3,6−8 For example, cubic-shaped nano-CeO2 with well-defined {100} facets shows a better catalytic performance compared to randomly shaped CeO2 nanoparticles, which preferentially expose the {111} facets.1 A key reason is that less activation energy is needed for the formation of oxygen vacancies and for the transformation of Ce4+ to Ce3+ over the
1. INTRODUCTION Ceria (CeO2), a dynamic rare earth metal oxide, plays a key role in the promotion of various heterogeneous catalytic processes, such as diesel soot oxidation, CO oxidation, combustion of volatile organic compounds, water-gas shift reaction, oxidative coupling of amines, and oxidation of alcohols.1−4 The unique ability of CeO2 to shift between Ce4+ and Ce3+ and the ensuing effect in the creation of oxygen vacancy defects are found to be key factors for its application in heterogeneous catalysis. It is a known fact that the particle size of CeO2 shows a noticeable effect on the modification of redox and catalytic properties of CeO2. For example, nanosized ceria exhibits a superior surface/volume ratio and improved surface redox properties compared to those of bulk ceria, thus © XXXX American Chemical Society
Received: December 15, 2015 Revised: February 16, 2016
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the nanointerface sites of CuOx−CeO2 may play a key role in the promotion of soot oxidation. Therefore, much attention has been paid to elucidate the role of the nanointerface in the catalytic performance of CuOx nanoparticle-decorated CeO2 cubes for the oxidation of diesel soot.
{100} surfaces of CeO2 nanocubes than on the {111} surfaces of CeO2 nanoparticles. Despite its size- and shape-dependent properties, the wide applicability of CeO2 is still limited in heterogeneous catalysis as a result of its insignificant catalytic activities.9−11 It has been demonstrated that the addition of transition metals to CeO2 nanocubes leads to the formation of synergistic interfaces that can combine the unique properties of the respective components and, hence, improved catalytic performances.12−14 Particularly, the combination of copper oxides (CuOx) and CeO2 is of paramount research interest as a result of their cooperative redox and catalytic activities.15−17 The activity of CuOx/CeO2 catalysts is related to the high degree of CuOx dispersion, its strong interaction with ceria, and facile interplay between Cu1+/2+ and Ce3+/4+ redox couples, resulting in the generation of abundant oxygen vacancies essential for most catalytic applications. It is therefore highly possible that the integration of CuOx nanoparticles with CeO2 nanocubes may lead to unusual properties and exceptional activities. For example, Gamarra et al. reported that the existence of a strong interaction between copper oxide and the {100} faces of ceria nanocubes plays a favorable role in the catalytic performance of CuO/CeO2 materials for preferential oxidation of CO.18 This observation could be mainly due to the existence of the synergistic effect at the nanointerface sites of CuOx/CeO2 nanocubes.19 To understand the role of nanointerfaces in heterogeneous catalysis, a systematic investigation of the structure−activity relationships of CuOx-decorated CeO2 nanocubes is needed. This could provide new strategies toward the rational design of novel heterostructured nanocatalysts with advanced properties and remarkable catalytic behaviors. Therefore, the present work has been undertaken against the above background. In this work, we developed CeO 2 nanocubes and CuOx-decorated CeO2 nanocubes using feasible solution-based methodologies. A range of advanced analytical techniques, such as transmission electron microscopy (TEM), transmission electron microscopy−energy-dispersive X-ray spectroscopy (TEM−EDS), high-angle annular dark-field− scanning transmission electron microscopy (HAADF− STEM), scanning transmission electron microscopy−electron energy loss spectroscopy (STEM−EELS), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Brunauer− Emmett−Teller (BET) surface area, Barrett−Joyner−Halenda (BJH) analysis, and Raman spectroscopy, have been used to systematically characterize the physical, chemical, morphological, and surface electronic properties of the developed materials. Because CeO2 is a commercially used catalyst component in automotive exhaust purification, the catalytic efficiency of CuOx-decorated CeO2 nanocubes was tested for the oxidation of diesel soot particulates. Soot particulates, released from diesel engines, cause serious health problems, including irritation of the eyes, vomiting, heartburn, bronchitis, lung cancer, or even premature death.20−24 Thus, the abatement of diesel soot is an urgent task. Diesel particulate filters (DPFs) are used commercially for the abatement of soot emissions, in which the collected soot particles on a DPF can be oxidized by oxygen at a relatively high temperature (>873 K).24 However, the temperature of diesel exhaust engines is typically in the range of 473−773 K. In this context, the use of a promising oxidation catalyst is an efficient solution to lowering the temperature required for soot combustion in diesel engines, which will eventually improve the efficiencies of the DPF and diesel engines as well. It is believed that the synergistic effect at
2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. The CeO2 nanocubes were synthesized using a template-free alkaline hydrothermal method. Briefly, the desired amount of Ce(NO3)3·6H2O [Aldrich, analytical reagent (AR) grade] was dissolved in Milli-Q water under mild stirring conditions. An aqueous NaOH solution (60 mL, 6 M) was added to the above solution, and stirring was continued for 30 min at room temperature. The reaction mixture was placed in a Teflon-lined autoclave, and the hydrothermal treatment was performed at 453 K for 24 h. After cooling to room temperature, the sample was collected, centrifuged several times with Milli-Q water, and oven-dried at 373 K for 12 h. Finally, the sample was calcined at 773 K for 4 h in air with a heating ramp of 5 K/min. Cu (10 wt %) with respect to Ce was loaded onto CeO2 nanocubes using a wet impregnation method. In a typical procedure, the estimated amount of Cu(NO3)2·6H2O (Aldrich, AR grade) was dissolved in Milli-Q water while stirring. Finely powdered CeO2 was then added to the above suspension. The excess water was evaporated on a hot plate at ∼373 K while stirring. The obtained sample was oven-dried at 373 for 12 h and finally calcined at 773 K for 4 h in air with a heating ramp of 5 K/min. 2.2. Catalyst Characterization. The TEM analysis was performed using a JEOL 2100F operating at 80 kV accelerating voltage. The TEM was equipped with a Gatan Orius SC1000 charge-coupled device (CCD) camera for standard imaging, and an EELS spectrometer (Gatan GIF Tridium) and an EDS spectrometer (Oxford XMax80T) were used for elemental mapping in STEM mode. A Rigaku diffractometer with Cu Kα radiation (1.540 Å) was used for the powder XRD studies of the samples. The data were recorded in the 2θ range of 10−80° with a step size of 0.02° and a step time of 2.4 s. The lattice parameter of CeO2 was calculated with the help of a standard cubic indexation method using the intensity of the most prominent (111) peak of CeO2. Raman spectroscopy analysis of the samples was carried out using a PerkinElmer-Raman Station 400F spectrometer equipped with a liquid N2-cooled CCD detector and a confocal microscope. For the analysis, a 350 mW near-infrared 785 nm laser was used. N2 adsorption−desorption analysis was performed using a Micromeritics ASAP 2020 instrument at 77 K. Prior to analysis, the materials were degassed under slow vacuum for 30 min at ambient temperature, followed by fast-mode degassing at 423 K for 12 h to remove the adsorbed residual moisture on the catalyst surface. The BET surface area of the samples was estimated from the desorption data. Pore volume and pore size of the materials were determined using the BJH method applied to the desorption leg of the isotherms. The XPS studies of the samples were performed using a Thermo K-Alpha XPS equipped with an Al Kα radiation (1486.6 eV) X-ray source at a pressure lower than 10−7 Torr and an electron take-off angle (angle between electron emission direction and surface plane) of 90°.25,26 A survey scan was performed using a pass energy of 200 eV to determine possible contaminants. A high-resolution scan was performed with a pass energy of 50 eV in the regions of Ce 3d, Cu 2p, and O 1s under the same irradiation time. The binding energies of the samples were charge-corrected with respect to the adventitious carbon (C 1s) peak at 284.6 eV. All binding energies estimated were within a precision of ±0.2 eV. 2.3. Activity Measurements. Soot oxidation experiments were conducted using a thermogravimetric analyzer (TGA, Mettler Toledo, TGA/SDTA851e) under both tight and loose contact conditions.20−23 The TGA method is widely used for soot oxidation because of its simplicity and versatile performance.27 The soot and catalyst with a weight ratio of 1:4 were ground in a mortar for 10 min to obtain tight contact mixtures. The same weight ratio of soot and catalyst was mixed B
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Langmuir with a spatula for 2 min to obtain loose contact mixtures. The experiments consisted of heating the catalyst−soot mixtures at a rate of 10 K/min from ambient temperature to 1273 K under a 100 mL/min flow of air. The soot used in this study was Printex-U provided by Degussa. This soot was proven to be an appropriate model for the soot oxidation reaction. Each test was repeated 3 times to ensure the reproducibility of the results, and the results in terms of the temperature at which 50% soot conversion is achieved (T50) were within ±5 K.
using XRD, Raman, XPS, and STEM−EELS techniques and the results are well-discussed in the following paragraphs. The powder XRD patterns of CeO2 nanocubes and CuOxdecorated CeO2 nanocubes are shown in Figure 2A. The
3. RESULTS AND DISCUSSION 3.1. Morphology and Physicochemical Properties. The TEM studies were carried out to obtain a clear picture about the morphologies of CeO2 and CuOx as well as to determine their particle size. Figure 1 shows TEM images of CeO2 and Figure 2. (A) Powder XRD patterns and (B) Raman spectra of CeO2 nanocubes and CuOx-decorated CeO2 nanocubes.
obtained XRD peaks at 2θ values of 28.28°, 32.82°, 47.18°, 56.02°, 58.78°, 69.25°, 76.47°, and 78.92° can be assigned to fluorite-structured CeO2.28,29 In addition, the CuOx/CeO2 catalyst exhibits two weak XRD peaks at about 35.5° and 38.8°, which can be attributed to a monoclinic CuO phase.30 The enlarged portion of the XRD patterns reveals that the peaks of cerium oxide are shifted to some extent in CuOxdecorated CeO2 cubes compared to those of pure CeO2 cubes (Figure S2 of the Supporting Information). There are two probable reasons for shifting of cerium oxide peaks in the CuOx/CeO2 sample: (1) doping of a few Cu ions into the ceria lattice by replacing larger Ce4+ (0.097 nm) ions with smaller sized Cu ions (Cu2+ = 0.073 nm, and/or Cu+ = 0.077 nm), resulting in a ceria lattice contraction,31−33 and (2) conversion of Ce4+ to Ce3+, which is evidenced from XPS studies (Figure 4), leading to an expansion of the ceria lattice as a result of the higher ionic radius of Ce3+ (0.143 nm) than that of Ce4+ (0.097 nm). The estimated ceria lattice parameters reveal that the CuOx/CeO2 cubes have a smaller lattice parameter compared to that of pure CeO2 cubes (Table 1). This observation suggests that a few Cu ions are incorporated into the CeO2 lattice and, hence, a ceria lattice contraction (Figure S2 of the Supporting Information and Table 1). Because XPS is a surface characterization analysis, the estimated Ce3+ ions are mostly present on the surface of the Cu/CeO2 sample; thus, the effect of surface Ce3+ ions on the ceria lattice may be negligible. Raman spectra of CeO2 nanocubes and CuOx-decorated CeO2 nanocubes are presented in Figure 2B. A strong peak at ∼463 cm−1 was found for both materials. This band corresponds to the F2g Raman vibration mode of fluorite CeO2, in line with the XRD results (Figure 2A).34−36 In addition, two weak bands were found at about 295 and 604 cm−1 for the CuOx/CeO2 material. The band at 295 cm−1 reveals the presence of monoclinic CuO in the CuOx/CeO2 sample, in good agreement with XRD results (Figure 2A). Another band observed at ∼604 cm−1 indicates the presence of oxygen vacancy defects (Ov band) in the CeO2 lattice.37,38 In contrast, no visible O v band was found in pure CeO2 nanocubes. This observation indicates the beneficial effect of CuOx addition toward the enhancement of oxygen vacancy defects (Ov) in CeO2 nanocubes. The estimated wavenumber difference between F2g and Ov bands was ∼137 cm−1, indicating that oxygen vacancies in the CuOx−CeO2 catalyst are generated from intrinsic defects, i.e., from the conversion of
Figure 1. (A and B) TEM images of CeO2 nanocubes, (C and D) TEM images of CuOx-decorated CeO2 nanocubes, and (E) HAADF− STEM image of CuOx-decorated CeO2 nanocubes and corresponding EDS elemental mapping images.
CuOx/CeO2 materials. The TEM images of pure CeO2 confirm the formation of cubic-shaped CeO2 particles (panels A and B of Figure 1 and Figure S1 of the Supporting Information). The average particle size of CeO2 nanocubes is found to be ∼25 ± 5 nm. The lattice fringes in TEM images of CeO2 nanocubes are clearly visible, with a d spacing of ∼0.27 nm (panels A and B of Figure 1). This d spacing corresponds to the distance between the adjacent (100) planes of fluorite-structured CeO2. The presence of CeO2 nanocubes can also be clearly seen in the TEM images of the CuOx/CeO2 sample (panels C and D of Figure 1). The lattice d spacing in the CuOx/CeO2 sample was found to be ∼0.232 nm, which can be assigned to (111) crystal planes of CuO (panels C and D of Figure 1). The estimated average particle size of CuOx was ∼8.5 ± 1 nm. The HAADF− STEM image and corresponding EDS elemental mapping images of the CuOx/CeO2 sample are presented in Figure 1E. Interestingly, most of the copper species are deposited along the edges of the CeO2 nanocubes. This interesting decoration may lead to a strong interaction between CuOx and CeO2 along the edges of the CeO2 nanocube, which could assist with achieving improved redox and catalytic properties of CeO2. To understand this, the materials are thoroughly characterized C
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Table 1. Lattice Parameter (LP), Specific Surface Area (SSA), Particle Size (D), Pore Size (P), and Pore Volume (V) of CeO2 Nanocubes and CuOx-Decorated CeO2 Nanocubes
a
sample
LP (nm)a
SSA (m2/g)b
D (nm)c
P (nm)d
V (cm3/g)d
CeO2 CuOx/CeO2
0.5468 0.5398
30 36
25 ± 5 (CeO2) 8.5 ± 1 (CuOx)
14.041 22.082
0.165 0.143
XRD studies. bBET analysis. cTEM analysis. dBJH studies.
Ce4+ to Ce3+.39 Another interesting observation that can be noticed from the enlarged Raman spectra is that the addition of CuOx to CeO2 leads to a shifting of the F2g band toward lower wavenumbers and its broadening (Figure S3 of the Supporting Information). It has been shown that the incorporation of various metal ions (e.g., Fe, La, Zr, Sm, etc.) leads to perturbations in the CeO2 lattice as a result of differences in the ionic radii of Ce4+ and dopant ions and, thus, variations in the frequency of the Ce−O bond and its F2g band in doped CeO2 catalysts compared to that of pure CeO2.21,23,40 Therefore, the incorporation of Cu ions into the CeO2 lattice, as evidenced by XRD studies (Figure 2A), is the key reason for shifting of the F2g band and its broadening in the CuOx/CeO2 sample. The N2 adsorption−desorption isotherms of CeO2 nanocubes and CuOx-decorated CeO2 nanocubes are shown in Figure 3A. Both materials exhibit type IV isotherms with H1-
Figure 4. Ce 3d XPS spectra of CeO2 nanocubes and CuOx-decorated CeO2 nanocubes.
obvious that the CeO2 and CuOx/CeO2 materials exhibit both Ce4+ and Ce3+ ions, indicating the redox nature of the materials. The strength of Ce3+ in the CeO2 nanocubes and CuOxdecorated CeO2 nanocubes was determined from the ratio of integrated Ce3+ peaks to the total Ce (Ce3+ + Ce4+) peaks, as shown in eq 1. Ce3 + (%) = Ce3 +/(Ce3 + + Ce 4 +)
(1)
It was found that CuOx-decorated CeO2 nanocubes exhibit higher percentages of Ce3+ ions (16.62%) compared to those of pure CeO2 nanocubes (10.98%). It is a well-known fact that the concentration of Ce3+ is directly related to the amount of oxygen vacancies because oxygen vacancies are formed via the transformation of Ce4+ to Ce3+. It is therefore clear that the CuOx/CeO2 material possesses large amounts of oxygen vacancies, in good agreement with Raman results (Figure 2B). The presence of ample oxygen vacancies and higher numbers of Ce3+ ions are highly beneficial for CeO2-mediated catalytic reactions, including diesel soot oxidation.21 It is therefore concluded that the incorporation of Cu ions into CeO2 is the key reason for improved oxygen vacancies and Ce3+ ions in CuOx-decorated CeO2 nanocubes, as evidenced by XRD (Figure 2A), Raman (Figure 2B), and XPS (Figure 4) studies. However, it must be mentioned here that the conversion of Ce4+ to Ce3+ is also possible under the vacuum conditions employed during XPS studies.44 Therefore, the actual concentration of Ce3+ in CeO2 cubes and CuOxdecorated CeO2 cubes may be different from the concentration estimated by XPS studies. The deconvoluted Cu 2p spectrum of CuOx/CeO2 reveals the existence of different Cu ions (Figure S4 of the Supporting Information). The peak noticed at ∼932.46 eV can be assigned to Cu+ species.25,26,45 A small peak is observed at ∼934.15 eV, which indicates the presence of Cu2+ species. The satellite peaks observed at 940.4 and 943.2 eV can be assigned to the presence of CuO. It is therefore clear that the CuOx/CeO2 material contains both Cu+ and Cu2+ ions. On the other hand, two peaks were found in the O 1s XP spectra of CeO2 and CuOx/CeO2 samples (Figure S5 of the Supporting Information). The observed lower binding energy band can be assigned
Figure 3. (A) N2 adsorption−desorption isotherms and (B) pore size distribution profiles of CeO2 nanocubes and CuOx-decorated CeO2 nanocubes.
type hysteresis, indicating the mesoporous nature of the materials.40,41 Bimodal pore size distribution was found in both CeO2 and CuOx/CeO2 materials with average pore diameters of ∼14.041 and 22.082 nm, respectively (Figure 3B and Table 1). In addition, the obtained pore volumes for CeO2 and CuOx/CeO2 materials are ∼0.165 and 0.143 cm3/g, respectively (Table 1). It was found that the CuOx/CeO2 material exhibits a higher BET surface area compared to that of pure CeO2 (Table 1). This observation indicates a high dispersion of CuOx species on the surface of CeO2 nanocubes. Because heterogeneous catalytic reactions take place on the catalyst surface, XPS analysis was further performed to understand the surface electronic features of the materials. Figure 4 shows deconvoluted Ce 3d XPS spectra of CeO2 nanocubes and CuOx-decorated CeO2 nanocubes. As shown in Figure 4A, the peaks labeled by v (882.29 eV), v// (888.73 eV), v/// (898.09 eV), u (900.67 eV), u// (907.67 eV), and u/// (916.68 eV) indicate Ce4+ ions, whereas the peaks labeled by v/ (884.79 eV) and u/ (903.29 eV) indicate Ce3+ ions.42,43 It was D
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soot oxidation in the absence of a catalyst was also performed (panels A and B of Figure 5). The achieved T10 and T50 values under uncatalyzed conditions are significantly higher than those obtained with pristine CeO2 nanocubes and CuOx-decorated CeO2 nanocubes. This observation indicates the necessity of a catalyst to efficiently remove soot particulates at lower temperatures. As shown in Figure 5B, the estimated T10 and T50 values for CuOx-decorated CeO2 nanocubes are ∼626 and 646 K, which are of 54 and 79 K lower with respect to pristine CeO2 nanocubes, respectively. This observation evidently reveals the promotional role of CuOx species on the catalytic performance of CeO2 nanocubes for the oxidation of soot. Figure 6 shows the soot oxidation results obtained under loose contact conditions as a function of the temperature. Both CeO2 and CuOx/CeO2 catalysts showed a positive soot conversion trend with the increase of the temperature: soot oxidation starts in the range of ∼550 K and is completed at the higher temperatures (∼935 K). The achieved T10 (and T50) values for the CuOx-decorated CeO2 nanocubes and pristine CeO2 nanocubes were ∼615 (and 744) and 681 K (and 835 K), respectively. These values are quite higher than those obtained under tight contact conditions (Figure 5). It is interesting to notice here that a low T50 value is obtained for CuOx/CeO2 nanocubes under even loose contact conditions with a huge temperature difference of 91 and 135 K compared to pure CeO 2 nanocubes and uncatalyzed conditions, respectively. This remarkable observation reveals the significance of the present study for low-temperature soot oxidation. 3.3. Structure−Activity Relationships. It has been shown that the redox efficiency of CeO2 plays a prominent role in the oxidation of diesel soot.20,22 The active oxygen species generated by filling the oxygen vacancies of CeO2 by gaseous phase oxygen are responsible for the oxidation of soot particulates (eq 2). In this context, the interface of ceria− soot plays a favorable role for a rapid transfer of these surface oxygen species over carbon soot, thus leading to improved soot oxidation rates
to lattice oxygen in CeO2 and/or CuOx, while the band at higher binding energy indicates the adsorbed hydroxyl groups on the catalyst surface.40,43 The relative percentages of lattice oxygen (and adsorbed oxygen) are found to be ∼52 (and 48) and 44% (and 56%) in CeO2 cubes and CuOx-decorated CeO2 cubes, respectively. 3.2. Catalytic Soot Oxidation Studies. Catalytic soot oxidation is a three-phase boundary reaction among a solid catalyst, a solid reactant (soot particles), and a gaseous reactant (air).46 The efficiency of a catalyst is highly dependent upon the contact mode between the soot particles and the catalyst. Quite good results were found for soot oxidation under the conditions of tight contact between the soot and catalyst. This is because there are greater numbers of contact points between the soot and catalyst under tight contact conditions, resulting in higher soot removal efficiencies.47 However, investigating soot oxidation under loose contact of soot and catalyst provides a deep understanding into the function of the catalysts under practical conditions. Therefore, in this study, soot oxidation was carried out under both tight (Figure 5) and loose (Figure 6) contact conditions.
Figure 5. (A) Soot conversion (%) versus temperature (K) under tight contact conditions and (B) estimated T10 and T50 (K) values for CeO2 nanocubes, CuOx-decorated CeO2 nanocubes, and without a catalyst (bare soot).
Ce3 + −□ + O2 → Ce 4 +−O2−
(2)
O2−
where □ is an oxygen vacancy and is active oxygen. Therefore, the concentration of active oxygen species generated on the ceria surface and consequent soot oxidation rate is directly related to the concentration of oxygen vacancies and Ce3+ ions. The Raman (Figure 2B) and XPS (Figure 4) studies reveal that abundant oxygen vacancies and higher numbers of Ce3+ ions were found for CuOx-decorated CeO2 nanocubes compared to those for CeO2 nanocubes. It is therefore highly possible that higher numbers of active oxygen species can be generated on the surface of CuOx-decorated CeO2 nanocubes. Hence, improved conversions of soot particulates were found for the CuO x -decorated CeO 2 nanocubes compared to those for CeO2 nanocubes in both tight (Figure 5) and loose (Figure 6) contact conditions. This remarkable redox and catalytic efficiency might be due to synergistic nanointerfaces of CuOx-decorated CeO2 nanocubes. Thus, understanding the redox properties of cerium oxide at the nanointerface sites of CuOx−CeO2 could provide direct evidence for the observed superior redox and catalytic performance of CuOx-decorated CeO2 nanocubes. EELS analysis in STEM mode is a useful technique to estimate the redox properties of cerium oxide at the nanoscale range. Figure 7 shows the dark-field STEM images and corresponding Ce M5,4 EELS spectra of pristine CeO 2
Figure 6. (A) Soot conversion (%) versus temperature (K) under loose contact conditions and (B) estimated T10 and T50 (K) values for CeO2 nanocubes and CuOx-decorated CeO2 nanocubes.
Figure 5 shows the soot conversion as a function of the temperature over CeO2 nanocubes and CuOx-decorated CeO2 nanocubes. It can be noted from Figure 5A that soot conversion increases with the increase of the reaction temperature: soot oxidation starts in the range of 470−480 K for CuOx-decorated CeO2 nanocubes and is completed at a temperature of ∼805−810 K for pristine CeO2 nanocubes. To clearly compare the catalytic efficiency of the materials, we estimated the T10 and T50 values, temperatures at which 10 and 50% of soot conversions were achieved, respectively. The obtained results are presented in Figure 5B. For comparison, E
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reaction of soot oxidation.50,51 It can be noted from Figure 3B that CeO2 and Cu/CeO2 catalysts show three types of pores in the ranges of 4−10, 11−15, and 16−40 nm, with a major percentage of 16−40 nm sized pores. It was clear from Figure 3B that the Cu/CeO2 catalyst has a larger fraction of 16−40 nm sized pores compared to that of pure CeO2. Hence, soot particles could easily enter and diffuse into the pores of the Cu/ CeO2 catalyst and then flexibly access the active sites of the catalyst, thus improving soot oxidation rates (Figures 5 and 6).
4. CONCLUSION In summary, promising CuOx-decorated CeO2 nanocubes along with CeO2 nanocubes were developed and their catalytic efficiency was tested for the oxidation of diesel soot. It was found that CuOx-decorated CeO2 nanocubes exhibit a higher catalytic performance for soot oxidation compared to that of pristine CeO2 nanocubes under both “loose” and “tight” soot− catalyst conditions. A number of analytical techniques have been used to elucidate the key factors accountable for the superior catalytic performance of CuOx-decorated CeO2 nanocubes. The presence of higher numbers of Ce3+ ions and abundant oxygen vacancies, especially at the nanointerface of CuOx−CeO2, as evidenced by XPS, Raman, and STEM−EELS studies, is responsible for the achieved catalytic performance of CuOx−CeO2 heterostructured nanocatalysts.
Figure 7. HAADF−STEM images of (A) CeO2 and (B) CuOx−CeO2 samples and (C and D) corresponding Ce M5,4 EELS spectra.
nanocubes and CuOx-decorated CeO2 nanocubes. The M4 and M5 lines of Ce indicate the transitions of 3d core electrons to unoccupied states of p- and f-like symmetries. The oxidation state of cerium can be estimated by measuring variations in the M4 and M5 white lines.48,49 The Ce M5 edge of CeO2 nanocubes was shifted to a lower energy (by 1 eV) after the addition of Cu, indicating high concentrations of Ce3+ ions in the CuOx/CeO2 sample. In addition, the intensity ratio of M5 and M4 lines of the materials was estimated to know the variation in the oxidation state of cerium: a higher ratio of M5/ M4 indicates a higher concentration of Ce3+ ions. The estimated M5/M4 ratio of CeO2 and CuOx/CeO2 samples was ∼0.486 and 0.740, respectively. This observation reveals that CuOxdecorated CeO2 nanocubes exhibit larger amounts of Ce3+ ions at the nanointerface sites of CuOx−CeO2 compared to those of pristine CeO2 nanocubes, in agreement with Raman (Figure 2B) and XPS (Figure 4) studies. These Ce3+ ions can promote the formation of active oxygen species at the interface of CuOx−CeO2, which can participate in the oxidation of soot, as shown in Figure 8. It can therefore be concluded that the
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b04590. TEM, XRD, Raman, Cu 2p XPS, and O 1s XPS of catalysts (PDF)
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AUTHOR INFORMATION
Corresponding Author
*Telephone: +61-3-9925-2330. E-mail: suresh.bhargava@rmit. edu.au. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank Dr. Matthew Field and Edwin Mayes, RMIT University, for their immense help with technical assistance for characterizations. The authors duly acknowledge the RMIT Microscopy and Microanalysis Facility (RMMF) for providing access to instruments used in this study. The authors extend their sincere appreciation to the Deanship of Scientific Research at King Saud University for its funding of the Prolific Research Group (PRG-1437-30) to Professor Ayman Nafady.
Figure 8. Schematic diagram for the oxidation of diesel soot over CuOx-decorated CeO2 nanocubes.
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improved redox properties at the nanointerface sites of CuOx− CeO2 are a key reason for the outstanding catalytic efficiency of CuOx-decorated CeO2 nanocubes in the oxidation of diesel soot, a clear proof for the role of nanointerfaces in the performance of heterostructured nanocatalysts. In addition to redox properties, the pore size of the catalysts plays a key role in the oxidation of soot. Usually, the size of the soot particles is >20 nm; therefore, the pore size of the catalysts should be above 20 nm to capture the soot particles and to accelerate the
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