Pei
Zhao
,
Feng
Qin
,
Zhen
Huang
,
Chao
Sun
,
Wei
Shen
and
Hualong
Xu
*
Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials and Laboratory of Advanced Materials, Collaborative Innovation Center of Chemistry for Energy Materials, Fudan University, Shanghai 200433, P. R. China. E-mail: shuhl@fudan.edu.cn; Fax: +86 21 65641740; Tel: +86 21 65642401
First published on 22nd November 2017
Oxygen vacancies and metal–support synergistic effects in heterogeneous catalysis play a decisive role in catalytic efficiency. In this work, NiO supported on ceria nanorods (Ni/CeO2-NR) and ceria nanocubes (Ni/CeO2-NC) catalysts exhibit strong morphology-dependent oxygen vacancies content and NiO–CeO2 synergistic actions for N2O decomposition. Ni/CeO2-NR catalysts possess higher amounts of oxygen vacancies than Ni/CeO2-NC, while Ni/CeO2-NC samples display stronger metal–support synergistic actions to anchor more surface NiO clusters and boundary Ni–O–Ce nanostructures. Catalytic activity tests show that Ni/CeO2-NC catalysts exhibit a superior efficiency to Ni/CeO2-NR catalysts. The outstanding catalytic activities of Ni/CeO2-NC catalysts are related to a larger number of surface NiO clusters and boundary Ni–O–Ce nanostructures, which are much more active than oxygen vacancies for N2O decomposition. Furthermore, the boundary Ni–O–Ce is found to be a more reactive site than surface NiO clusters. This study presents a new strategy to design high-efficiency supported metal catalysts through controlling oxygen vacancies and metal–support synergistic effects by morphology-dependent synthesis.
In recent years, catalytic decomposition of N2O has been explored intensively over a variety of catalysts, such as noble metals,8,9 mixed metal oxides,10,11 perovskites,12 and hexaaluminate catalysts.13 Among numerous promising catalysts, mixed oxides catalysts have been proved to be excellent catalysts with prominent activity and economy. In particular, ceria-based metal oxide catalysts were believed to be satisfactory catalysts for N2O decomposition,14–16 due to their outstanding redox ability and their excellent metal–ceria boundary properties. As reported in previous literature, boundary properties, determined by synergistic effects between metal oxides and their support, have great impacts on catalytic properties.17–19 The synergism between metal oxides and support can not only alter the dispersion and reducibility of active sites, but also prevent metal oxides from sintering, and thus increase catalytic activity. For example, according to reported work,20 synergistic effects between CeO2 and Ni/MCM-22 can suppress sintering of Ni nanoparticles and the formation of coke. Our group also found the synergy of CuO and CeO2 can promote stability and reducibility of the active Cu1 site.14
Besides a synergism effect, the content of oxygen vacancies is also proved to be important for catalytic performance of ceria-based catalysts. Up to now, many researchers have investigated the role of oxygen vacancies on catalytic efficiency of ceria-based catalysts, where the oxygen vacancies concentration can be tailored by morphology control21–23 and cations doping.24,25 Wu et al. fabricated various CeO2-supported VOx species catalysts with controllable CeO2 morphology for oxidative dehydrogenation of isobutene and they related catalytic activity to the oxygen vacancy concentration of the CeO2 support.22 Similarly, a correlation between the catalytic activity of VOx active sites and oxygen vacancy density of CeO2 was revealed in oxidative dehydrogenation of methanol.23 In addition, loading of VOx species on CeO2 support promoted formation of oxygen vacancies, which enhanced catalytic performance of VOx/CeO2 catalysts, confirming the contribution of cations doping to oxygen storage capacity (OSC). Similar research conducted by Huang's group also confirmed that increased oxygen vacancy concentration in NbOx/CeO2 catalysts played a decisive role in promoting oxidative dehydrogenation of propane.26
Since adsorbed oxygen species generated by N2O decomposition process will inhibit catalysts' activity and stability, improvements of oxygen storage capacity and mobility are needed for N2O decomposition. Among the various ceria-based metal oxides catalysts, Ni-doped CeO2 catalyst is an excellent candidate for this reaction owing to nickel's doping for improving oxygen storage capacity and raising catalytic ability.5,27 Our group has studied Ni–Ce mixed oxides with various Ni/Ce molar ratios for N2O decomposition and found that Ni–Ce catalysts presented a noteworthy behaviour for N2O destruction.28 Despite various evidences illustrating the dominant role of oxygen vacancy and synergistic effects in a ceria-based catalyst for many other catalytic reactions, the substantial role of oxygen vacancies and synergistic effects in well-defined Ni–Ce still remains less comprehended in N2O decomposition.
On the basis of the fact that morphology-dependent oxygen vacancies and synergism action have great importance to catalytic reactions, in this study, CeO2 nanorods and nanocubes supported NiO catalysts were prepared, over which the impact of oxygen vacancies and synergistic effects between the nickel oxide and CeO2 system for N2O decomposition were evaluated.
K = Aexp(−Ea/RT) |
Fig. 1 TEM and HRTEM images of CeO2-NR (a and b), and CeO2-NC (d and e); the schematic illustrations for CeO2-NR (c), and CeO2-NC (f). |
Fig. 2 shows the morphology of CeO2-NR and CeO2-NC after loading of Ni species. Results indicate that the catalysts maintained their original shapes. For the 8% Ni/CeO2-NR in Fig. 2a–c, the nickel species can be spotted on the surfaces of nanorods in a disorderly distribution. TEM (HRTEM) images in Fig. 2b and c display the nearly spherical nickel species (ca. 5–8 nm in diameters cycled in Fig. 2b and c) with exposed NiO {200} planes (Fig. 2c). For 8% Ni/CeO2-NC catalysts (Fig. 2d–f), in contrast, most of the nickel species are preferentially decorated along the corner of the CeO2 nanocubes and the particle size of spherical nickel species is around 5–10 nm, displaying NiO {200} lattice fringes with a spacing of 0.21 nm in Fig. 2f. This impressive corner deposition may give rise to the strong synergistic effects between NiO and CeO2 support and in turn, enhance physicochemical properties of Ni/CeO2-NC catalysts. Similar observations were obtained for CuOx and MnOx-dispersed CeO2 nanocubes.33,34 In their work, most of the copper species and manganese oxide were deposited along the edges of the CeO2 nanocubes, which intriguing distribution contributed to catalytic properties. Meanwhile, ICP-AES results (Table 1) for nickel contents in Ni/CeO2 with different morphologies and their corresponding STEM-EDS elemental mapping (Fig. S1†) prove the successful impregnation of Ni in CeO2 nanocrystalline.
Samples | Nia (wt%) | Surface areab (m2 g−1) | CeO2 crystallite sizec (nm) | Nickel species sized (nm) | Lattice parametere (nm) |
---|---|---|---|---|---|
a Values determined by ICP-AES. b The specific surface area is calculated using the BET model. c Lattice parameter is obtained by XRD studies. d Lattice parameter is obtained by XRD studies. e Lattice parameter is obtained by XRD studies. | |||||
CeO2-NR | — | 135.0 | 9.2 | — | 0.5426 |
2% Ni/CeO2-NR | 2.28 | 106.0 | 8.0 | — | 0.5421 |
4% Ni/CeO2-NR | 3.96 | 95.2 | 8.3 | — | 0.5418 |
6% Ni/CeO2-NR | 6.15 | 85.6 | 7.7 | 14.0 | 0.5424 |
8% Ni/CeO2-NR | 7.80 | 90.8 | 7.9 | 17.4 | 0.5424 |
10% Ni/CeO2-NR | 9.51 | 86.1 | 7.8 | 17.5 | 0.5424 |
CeO2-NC | — | 38.3 | 19.4 | — | 0.5424 |
2% Ni/CeO2-NC | 1.95 | 34.9 | 18.9 | — | 0.5417 |
4% Ni/CeO2-NC | 3.86 | 34.7 | 19.3 | — | 0.5422 |
6% Ni/CeO2-NC | 5.68 | 31.4 | 18.1 | 12.3 | 0.5422 |
8% Ni/CeO2-NC | 7.66 | 30.4 | 19.4 | 14.8 | 0.5422 |
10% Ni/CeO2-NC | 9.39 | 29.6 | 18.7 | 16.2 | 0.5422 |
Fig. 3 The XRD patterns of the (A) x% Ni/CeO2-NR, and (B) x% Ni/CeO2-NC catalysts (x denotes as weight percentage of nickel loading value). |
On the other hand, in the case of high nickel loading from 6% to 10%, besides the characteristic peaks of fluorite-structures of ceria, three weak XRD peaks located at 37.2, 43.2 and 62.8° (2θ) can be attributed to crystalline NiO species and these peaks correspond to (111), (200), and (220) planes, respectively.35,36 However, below a nickel loading of 6%, neither distinct diffraction peaks of NiO nor of Ni are observed, implying a high dispersion of nickel species or relatively low metal contents. Additionally, in spite of identical diffraction peak positions of CeO2-NR and CeO2-NC, the broader diffraction peaks for CeO2-NR suggests a smaller crystallite size. The mean crystallite size of ceria was determined by Scherrer's equation based on the (111) plane of CeO2. With regard to the crystallite size of NiO in Ni/CeO2, their average particle sizes were estimated by line broadening of the (200) line of NiO at 2θ = 43.3° (also utilizing the Scherrer's equation). The calculated results are listed in Table 1.
As displayed in Table 1, the mean crystallite size of CeO2 with two morphologies is nearly the same after the addition of various amounts of nickel nanocrystalline. What's more, the nickel species on CeO2-NR catalysts have a relatively larger size than that on CeO2-NC counterparts at the same metal loading. The NiO crystallite sizes are estimated to be 14.0, 17.4, and 17.5 nm for 6% Ni/CeO2-NR, 8% Ni/CeO2-NR, and 10% Ni/CeO2-NR, respectively. Similarly, the NiO crystallite sizes of Ni/CeO2-NC counterparts increase from 12.3 to 16.2 nm, which is related to agglomerates of nickel species with an increase of nickel loading. It is noteworthy that the estimated similar NiO crystallite size in two types of nickel–ceria can provide corroborative evidence for the exclusion of NiO size effect on catalytic properties.
In addition, the lattice parameter of the Ni/CeO2-NR (Table 1) decreases from 2% to 4% of Ni loading and increased abruptly at 6%, remaining constant till a 10% Ni loading. This interesting trend can be attributed to the limit of solubility for a Ce ↔ Ni exchange within the range of 10–12% (mol) Ni addition (equal to 3.7–4.5 wt%).37 According to a previous report,38 the contraction of the CeO2 lattice parameter upon Ni incorporation was caused by the substitution of Ce4+ ions (rCe4+ = 0.094 nm) by smaller Ni2+ ions (rNi2+ = 0.072 nm) to form a Ni–O–Ce bond in the range of 2% to 4% loading. This result is consistent with Vegard's rule. Nevertheless, beyond the 4% doping, increasing Ni loading leads an increase of lattice parameters. This unusual phenomenon can be attributed to the additional incorporation of Ni in an interstitial position upon a 4% Ni impregnation. This is because both substitution and interstitial point defects can change unit cell parameters, and the latter can generate the lattice expansion.39 Therefore, we can deduce that the maximum Ni substitution for Ce4+ occurs at 4% Ni doping for CeO2-NR, and then the excess Ni ions will occupy the interstitial sites, thus the enhancement of lattice parameters takes place at 6% Ni doping. In the case of Ni/CeO2-NC, the nickel substitution limit is 2% metal loading. For this reason, the lattice parameter of ceria with 4% Ni doping suddenly increases. To that end, the different nickel solubility limits and doping behaviour of nickel in ceria may induce the substantial differences in oxygen vacancies for Ni/CeO2-NR and Ni/CeO2-NC.40
The surface areas of CeO2-NR and CeO2-NC supports are 135.0 and 38.3 m2 g−1, respectively. After nickel deposition, the specific surface areas of Ni/CeO2-NR and Ni/CeO2-NC decrease to different extents. In addition, the Ni/CeO2-NR catalysts with various nickel content show surface areas almost three times higher than the Ni/CeO2-NC catalysts. Even with the much higher surface areas of Ni/CeO2-NR, it does not dominate the excellent catalytic properties.
Fig. 4 Visible Raman spectra of various (A) Ni/CeO2-NR, (B) Ni/CeO2-NC, and (C) the corresponding ID/IF2g values for various Ni/CeO2 samples. |
However, the Raman profiles for ceria in Fig. 4 show a red shift to 457 cm−1, and this discrepancy is suspected to cause the diversity of ceria particle sizes.
Generally, the relative intensity ratio of ID/IF2g is widely accepted as an indicator for the concentration of Ov. On the basis of the calculated information in Fig. 4C, the ID/IF2g values of ceria follows the sequence: CeO2-NR (0.053) > CeO2-NC (0.024), implying a richer Ov density in the CeO2-NR nanostructure. To understand the distinct impact of nickel species on Ov for ceria supports with different morphologies, the Raman spectra for various Ni/CeO2-NR and Ni/CeO2-NC were also recorded in Fig. 4A and B. After the addition of nickel species, the Raman profiles of all the nickel-containing samples were nearly the same as the ceria supports except for the peak shift to a lower wave number. The F2g band of all the Ni/CeO2-NR and Ni/CeO2-NC shifts to a range of 441–447 and 440–450 cm−1, respectively, which indicated that Ni2+ can substitute for Ce4+ to form the Ce–O–Ni bond. This observation is consistent with the XRD results. Fig. 4C shows the ID/IF2g ratio as a function of nickel loading for Ni–CeO2 catalysts. For CeO2-NR and CeO2-NC catalysts, the ID/IF2g ratio increases with the nickel loading, verifying the vital role of nickel doping for yielding Ov. Meanwhile, the intensity ratio increases along with the increase of nickel loading from 2% to 6%, and then basically remains unchanged from 6% to 10% Ni/CeO2-NR. This interesting trend may be associated with the substitution and interstitial point defects limit. As analysed by XRD, the maximum Ni substitution for Ce4+ occurs at 4% Ni doping for CeO2-NR to form substitution defects and then the additional nickel species serve as interstitial point defects, which both significantly enhance Ov concentration. From this inference, it is expected that the 10% Ni loading will have the highest Ov concentration. However, that does not happen as desired. Therefore, one has to think that this abnormal result can be caused by the limit value of interstitial point defects, which occurs at 6% Ni loading. Similarly, the Raman curve for Ni/CeO2-NC catalysts shows an ultimate Ov density at 4% nickel impregnation. What's more, in Fig. 4C, it is evident that the Ni/CeO2-NR catalysts present a higher Ov concentration than that of Ni/CeO2-NC, which implies that the ceria morphology has a significant effect on the Ov amounts.
To further quantitate the amount of Ov, the OSC tests were measured (shown in Table 2). In Table 2, the O2 consumption for Ni/CeO2 catalysts is displayed, where the Ni/CeO2-NR and Ni/CeO2-NC catalysts exhibit a marked increase for OSC values compared with ceria supports. The Ni/CeO2-NR offers a larger OSC value than that of Ni/CeO2-NC at all nickel contents, which is consistent with the Raman results. According to Raman and XRD results, both substitution and interstitial point defects, which are caused by Ni incorporation into ceria, can contribute to the creation of Ov. Apparently, the maximum value of substitution and interstitial point defects for Ni/CeO2 with two morphologies are detected to take place at different nickel loadings, resulting in the diversity of Ov for each nickel–ceria sample.
T max/(°C) | OSC (μmol [O2] gcat−1) | Ni dispersion (%) | Reaction ratea (10−6 mol s−1 gcat−1) | TOFNib (s−1) | E a (kJ mol) | ||
---|---|---|---|---|---|---|---|
γ peak | δ peak | ||||||
a The reaction rate was defined as the mole number of N2O produced per gram of catalyst per second at 325 °C. b The TOF value was calculated at 325 °C. | |||||||
CeO2-NR | — | — | 41.00 | — | 8.43 | — | 141.6 |
2% Ni/CeO2-NR | 322 | 353 | 152.9 | 39.7 | 11.41 | 6.35 × 10−2 | 136.3 |
4% Ni/CeO2-NR | 320 | 352 | 346.5 | 19.9 | 16.03 | 8.54 × 10−2 | 133.5 |
6% Ni/CeO2-NR | 324 | 360 | 571.7 | 12.0 | 18.68 | 11.27 × 10−2 | 130.8 |
8% Ni/CeO2-NR | 325 | 361 | 725.5 | 9.9 | 43.87 | 24.06 × 10−2 | 119.0 |
10% Ni/CeO2-NR | 325 | 366 | 874.6 | 6.1 | 21.41 | 15.27 × 10−2 | 122.4 |
CeO2-NC | — | — | 25.36 | — | 5.89 | — | 160.7 |
2% Ni/CeO2-NC | 315 | 373 | 136.4 | 17.1 | 26.81 | 32.11 × 10−2 | 128.1 |
4% Ni/CeO2-NC | 306 | 372 | 277.3 | 13.3 | 30.50 | 34.94 × 10−2 | 122.2 |
6% Ni/CeO2-NC | 314 | 370 | 447.5 | 9.5 | 49.99 | 36.18 × 10−2 | 111.1 |
8% Ni/CeO2-NC | 316 | 376 | 648.8 | 6.8 | 62.50 | 50.69 × 10−2 | 107.7 |
10% Ni/CeO2-NC | 312 | 375 | 798.7 | 3.8 | 44.46 | 47.88 × 10−2 | 112.8 |
After impregnation of Ni, the reduction behaviour of ceria alters dramatically in the low temperature zone, although the reduction ability of bulk oxygen (ε peak in Fig. 5A and B) remains unchanged. For the H2-TPR profiles of the Ni/CeO2 samples (Fig. 5), there are five main reduction peaks (α, β, γ, δ, and ε). The first two peaks α and β (starting at 133 °C and 136 °C, while reaching maximum in the region of 241–260 °C and 254–261 °C for Ni/CeO2-NR and Ni/CeO2-NC, separately) are ascribed to adsorbed oxygen species caused by the formation of Ni–O–Ce structures.46 Because once the Ni2+ is incorporated into a CeO2 lattice to generate a solid solution, the charge unbalance and lattice distortion will generate increased oxygen vacancies on the surface. In that case, surface oxygen can oxidize H2 at low temperatures. Similarly, the β peak's position in Ni/CeO2-NC continuously decrease from 260 to 241 °C with the increase of nickel content; however, the reduction temperature of Ni/CeO2-NR nearly remains constant.
As reported in the literature,39,47 the γ peaks can be linked to the reduction of the well-dispersed NiO phase interacting strongly with the CeO2 support (called boundary Ni–O–Ce), whereas the δ peak can be attributed to the reduction of the phase separated surface NiO clusters on CeO2. In addition, with careful observations for Ni/CeO2-NR and Ni/CeO2-NC profiles, the peak area ratio of γ(γ/γ + δ) continuously increases with the nickel content, as shown in Table S1.† These results suggest most nickel species can exist in the form of a well-dispersed Ni–O–Ce structure. What's more, the Tmax of γ and δ peak (Table 2) of Ni/CeO2-NC catalysts present a lower reduction temperature than that of Ni/CeO2-NR, implicating the synergistic effects between nickel species and CeO2-NC.48
In short, the reducible NiO species exist in two different forms, i.e., NiO clusters and the Ni–O–Ce structure. Because the subtle variations in surface structures of the support may change the redox properties of catalysts,49 the difference in synergism between metal oxides and support may be due to different nickel bonding ability with ceria. This essential disparate synergistic action can play an influential role for heterogeneous catalysis.
Fig. 6 DRS-UV-vis spectra of (A) CeO2-NR and x% Ni/CeO2-NR, and (B) CeO2-NC and x% Ni/CeO2-NC catalysts (x denotes as weight percentage of nickel loading value). |
In contrast, in Fig. 6A for Ni/CeO2-NR with a nickel loading from 2% to 10%, all the catalysts show an extra distinctive band at around 720 nm except for the characteristic bands at 260 and 352 nm of ceria. The blue shift for Ce4+ ← O2− charge transfer at 343 nm is detected in nickel-containing catalysts, and the lattice distortions in ceria from nickel dopant can account for this shift. According to a literature report,53 the band at about 720 nm can be related to octahedral Ni2+. What's more, the intensity of octahedrally coordinated Ni2+ gets stronger with increasing nickel loading, which further verifies that the generation of bulk NiOx species can derive from octahedrally coordinated Ni2+.54 For the DRS bands of CeO2-NC (Fig. 6B), similar spectra to CeO2-NR are observed and a blue shift appears in the DR spectra since the coordination number of Ce4+ increases or the crystallite size changed.52
Although the DRS profiles of all the Ni/CeO2-NC (shown in Fig. 6B) resemble those for Ni/CeO2-NR, the additional peak at 430 nm is also assigned to the charge transfer of octahedral Ni2+.53 For DRS UV-vis results displayed in Fig. 6A, no noticeable peaks are in this region, which manifests the diversity in nickel environment. In the comparison of surface nickel species on two different type of ceria, i.e., CeO2-NR and CeO2-NC, we took the 8% Ni/CeO2 catalyst for example (in Fig. S2†). The weaker signals at 430 nm and 720 nm indicate a lower content of octahedrally coordinated Ni2+ in the 8% Ni/CeO2-NR structure. The surface content of nickel species was further analysed by the following XPS analysis.
Fig. 7 The core-level XPS spectra of all the CeO2 and Ni/CeO2 catalyst Ce 3d (A), O1s (B) and Ni 2p (C). |
Samples | Surface content of Ni (wt%) | Ce3+/Ce3+ + Ce4+ (at%) | Oads/Oads + Olatt (at%) |
---|---|---|---|
CeO2-NR | — | 19.50 | 29.86 |
2% Ni/CeO2-NR | 1.36 | 24.93 | 35.19 |
4% Ni/CeO2-NR | 2.35 | 25.69 | 37.50 |
6% Ni/CeO2-NR | 1.28 | 26.94 | 37.69 |
8% Ni/CeO2-NR | 3.79 | 26.30 | 38.18 |
10% Ni/CeO2-NR | 4.95 | 26.16 | 37.83 |
CeO2-NC | — | 16.48 | 27.14 |
2% Ni/CeO2-NC | 5.28 | 18.70 | 29.96 |
4% Ni/CeO2-NC | 7.14 | 19.98 | 30.33 |
6% Ni/CeO2-NC | 12.61 | 18.52 | 31.56 |
8% Ni/CeO2-NC | 16.49 | 19.72 | 31.71 |
10% Ni/CeO2-NC | 22.47 | 19.74 | 31.32 |
In the case of nickel species, the Ni 2p photoelectron spectrum is shown in Fig. 7C. The peaks at ca. 855 and 874 eV for Ni/CeO2-NC samples can be attributed to the Ni 2p3/2 and Ni 2p1/2 signals, respectively, fitting with the characteristic states of NiO species.58 Another weak broad peak located at 862 eV can be associated with a shakeup satellite peak corresponding to Ni2O3 species,59 where the electron transfer between the nickel species and ceria occurs along the Ni–CeO2 boundary, reflecting strong synergistic effects between Ni–CeO2. In contrast, Ni/CeO2-NR exhibits much weaker Ni 2p and shakeup satellite peaks. Further analysis for Ni content from XPS date (see Table 3) indicates that surface Ni content in Ni/CeO2-NC is higher than its theoretical value, while the Ni/CeO2-NR offers a lower Ni content, which corresponds with the UV-vis results. This phenomenon confirms that nickel species preferably exist on the CeO2-NC surface, and are inclined to incorporate into the subsurface/bulk phase of CeO2-NR support. These results can be interpreted by the better nickel solubility ability in the CeO2-NR, which agreed with the XRD results.
Fig. 8 DRIFT spectra after flowing 2% N2O in He at 325 °C over various catalysts for 60 min (A), and followed by He purge at 325 °C for 30 min (B). |
In order to gain a better comparison for the catalytic efficiency of the materials, TOF values with respect to the exposed Ni atom (TOFNi) and the reaction rate (r) at 325 °C for the series of Ni/CeO2 catalysts were calculated (plotted in Fig. 11c and listed in Table 2). As clearly presented in Table 2, the reaction rate of CeO2-NR is higher than that of CeO2-NC, where r value for CeO2-NR and CeO2-NC is 8.43 × 10−6 mol s−1 gcat−1 and 5.89 × 10−6 mol s−1 gcat−1, respectively. However, compared with Ni/CeO2-NC catalysts, nickel–ceria with rod structures instead provide a lower r value. The TOFNi values of N2O over x% Ni/CeO2-NC are higher than that of x% Ni/CeO2-NR, demonstrating the superiority of Ni/CeO2-NC to Ni/CeO2-NR. Moreover, it can be seen that the TOFNi values for x% Ni/CeO2 catalysts (x = 2, 4, 6, 8) reach a maximum at 8% Ni loading, namely, nickel–ceria with moderate nickel concentration possess the highest activity for N2O decomposition. Nevertheless, excessive deposited Ni may form large NiO clusters, which in turn inhibits the efficiency of nickel–ceria materials. Therefore, based on these data, it can be concluded that the catalytic activities of nickel–ceria catalysts are not only associated with the catalyst's morphology, but also with existence of a nickel species. The apparent activation energies (Ea) for these Ni/CeO2 catalysts are calculated from their Arrhenius plots (Fig. 11e and f), which are listed in Table 2. For the x% Ni/CeO2-NC catalysts, the apparent activation energies are lower than the values of x% Ni/CeO2-NR catalysts. As a result, the catalytic activities of x% Ni/CeO2-NC catalysts are higher than that of x% Ni/CeO2-NR catalysts.
Table 4 lists a comparison of catalytic N2O decomposition results over Ni/CeO2-NC catalysts with other reported ceria-based catalysts. By comparison, the T50 of 8% Ni/CeO2-NC catalyst is lower than other ceria-based materials except for the RhOx/Ce0.9Pr0.1O2 reported by Bueno-López's group.66 Although Rh-based catalysts have much lower T50 temperatures, the lower cost of mixed oxides could remedy their inferior performance. Therefore, the Ni/CeO2-NC is a superior catalyst for N2O decomposition.
Catalysts (metal loading) | Reaction conditions during catalytic test | T 50 (°C) | Ref. |
---|---|---|---|
a Half conversion temperature (T50). | |||
Co3O4/CeO2 (10 wt%) | GHSV = 30000 h−1, [N2O] = 1000 ppm | 410 | 16 |
CuO/CeO2 (10 wt%) | GHSV = 45000 h−1, [N2O] = 2500 ppm | 380 | 61 |
CuO–CeO2 (20 wt%) | GHSV = 40000 h−1, [N2O] = 1000 ppm | 462 | 62 |
Fe2O3/CeO2 (40 mol% of Cu) | GHSV = 23800 h−1, [N2O] = 4500 ppm | 470 | 63 |
CuO/CeO2 (40 mol% of Cu) | GHSV = 45000 h−1, [N2O] = 2500 ppm | 440 | 64 |
Ir/Al2O3 + CeO2 (0.5 wt%) | GHSV = 40000 h−1, [N2O] = 1000 ppm | 480 | 65 |
RhOx/Ce0.9Pr0.1O2 (1 wt%) | GHSV = 42000 h−1, [N2O] = 1000 ppm | 242 | 66 |
Ni/CeO2-NC (8 wt%) | GHSV = 19000 h−1, [N2O] = 26000 ppm | 350 | Present work |
The impact of O2 in the reactant feed on deN2O performance was investigated over the 8% Ni/CeO2-NC and 8% Ni/CeO2-NR samples and the results were depicted in Fig. 12. For comparison, we also investigated the catalytic behaviour over 8% Ni/γ-Al2O3. As shown in Fig. 12, the catalytic activities over the studied catalysts take the following order: 8% Ni/CeO2-NC > 8% Ni/CeO2-NR > 8% Ni/γ-Al2O3 in the absence of O2. However, under O2 reaction conditions, the deN2O activities of all the catalysts suffer a decrease to various degrees, as indicated by the upward shift of T50. It can be seen from the resulting N2O conversion profiles that the upward shifts of T50 are up to 15 °C, 6 °C, and 28 °C for 8% Ni/CeO2-NC, 8% Ni/CeO2-NR, and 8% Ni/γ-Al2O3, respectively. These results indicate the Ni/CeO2-NC and Ni/CeO2-NR catalysts can be a better candidate for N2O destruction with or without the presence of O2 in comparison with Ni/γ-Al2O3.
Fig. 12 Conversion curves for N2O decomposition reaction over 8% Ni/CeO2-NR, 8% Ni/CeO2-NC, and 8% Ni/γ-Al2O3 in the absence and presence of O2 in the feed stream. |
In terms of the structure–activity correlation between catalytic activity and OSC, it is apparent that the TOFNi values of N2O for Ni/CeO2-NC are notably higher than the date from Ni/CeO2-NR, despite the less Vo on the Ni/CeO2-NC nanostructures. This essential information hints the fact that apart from the Vo, there ought to be other vital factors in improving catalyst efficiency. Indeed, numerous factors are closely relevant to catalytic activity, such as the existence of forms of active components, BET surface areas, and particle size.4,16,71 However, in our work, BET surface areas and NiO particle size does not dominate the catalytic efficiency. Based on the claim from the aforementioned catalytic data in Fig. 11, both surface NiO clusters and boundary Ni–O–Ce contribute a momentous portion to the raised deN2O performance of Ni/CeO2-NC catalysts.
Taking account of the verdict from Raman spectra, OSC measurements, H2-TPR and XPS data, although the Ni/CeO2-NR has more Vo as well as bulk Ce1−xNixO2 solid solution, the CeO2-NC occupies stronger synergistic actions to anchor more surface NiO and Ni–O–Ce structure on the surface. The results can be successfully verified by H2-TPR and XPS. Based on the catalytic data depicted in Fig. 11, the surface NiO clusters and boundary Ni–O–Ce, the main active sites, are more active than Vo (serving as secondary active site) for the deN2O reaction. Therefore, nickel–ceria nanocubes, offering less Vo but possessing stronger synergistic actions to hold more NiO clusters and boundary Ni–O–Ce structure, exhibit superior performance to nickel–ceria nanorods. Accordingly, the concentration of Vo and the degree of synergistic actions, deciding the distribution of surface NiO and boundary Ni–O–Ce, are vital factors for catalytic efficiency.
In order to explore the impact of NiO clusters and boundary Ni–Ce–O on catalytic efficiency, we calculated the TOF value of surface NiO clusters and boundary Ni–Ce–O, respectively, and the results are listed in Table S1.† It can be seen that the TOF values of boundary Ni–Ce–O are larger than those of NiO clusters over all the catalysts. This result indicates that the boundary Ni–Ce–O is the more reactive site for deN2O performance. As shown in Table S1,† when the nickel loading increases from 2% to 6%, the Ni2+ can incorporate into the ceria lattice to engender the bulk Ce1−xNixO2 solid solution to generate Vo, along with the formation of a small amount of surface NiO and boundary Ni–O–Ce nanostructures. Meanwhile, the TOF of NiO and boundary Ni–Ce–O increases. From 6% to 8% loading, the TOF of surface NiO and boundary Ni–O–Ce increase dramatically and an excess of 10% loading leads to a slight decrease of TOF, which is induced by coverage of active sites. These proofs indicate that catalytic performance is closely related to the Ni loading. Taking into account all the results, we propose the following tentative mechanism of N2O decomposition over Ni/CeO2 catalysts:
Ce3+–V0 + ONN → N2 + Ce4+–O− | (1) |
Ce4+–O− + Ce4+–O– → O2 + 2Ce3+–V0 | (2) |
Ce4+–O− + ONN → N2 + O2 + Ce3+ –V0 | (3) |
Ni2+–ONN → Ni2+–ONN | (4) |
Ni2+–ONN → Ni3+–O− + N2 | (5) |
2Ni3+−O− → 2Ni2+ + O2 | (6) |
Ni3+–O− + ONN → N2 + O2 + Ni2+ | (7) |
Ni3+–O− + Ce3+–V0 → Ni2+ + Ce4+–O− | (8) |
In the case of CeO2-NR and CeO2-NC supports, the participation of Vo in the N2O decomposition reaction can follow steps (1)–(3). While for the Ni/CeO2-NR and Ni/CeO2-NC, N2O molecules adsorb on Ni2+ active sites (step (4)) and then the Ni3+–O− species form (step (5)). The chemisorption of N2O on catalysts can be confirmed by the in situ DRIFTs results. Regeneration of active Ni2+ sites can occur by oxygen recombination step (step (6)) or by reaction of Ni3+–O− complex with nitrous oxide (step (7)). Taking the H2-TPR and XPS results into account, since synergetic effects between NiO and CeO2, and high lattice oxygen mobility originating from the reducible ceria occurs, the Ce3+ can reduce Ni3+–O− to Ni2+, and in turn regenerate the catalyst active site (step (8)). The formed Ce4+–O− can desorb oxygen molecules and regenerate Ce3+ by step (2) or (3). In summary, both Vo and surface NiO as well as boundary Ni–O–Ce can be involved in the proposed mechanism. Therefore, by comparison with CeO2-NC supports, the higher catalytic efficiency of CeO2-NR can be attributed to a larger amount of Vo on the material. Considering the synergetic effects between NiO and CeO2 support, boundary, Ni–O–Ce can be a superior active site to NiO clusters, which can be confirmed by the higher TOF value of boundary Ni–O–Ce. According to the proposed reaction mechanism, the higher the amount of boundary Ni–O–Ce is, the better catalytic efficiency a material possesses. Therefore, the Ni/CeO2-NC catalysts with higher concentrations of boundary Ni–O–Ce have higher catalytic activity.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7cy02301d |
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