ARTICLE pubs.acs.org/JPCC
Distinguished Roles with Various Vanadium Loadings Of CoCr2 xVxO4 (x = 0 0.20) for Methane Combustion Jinghuan Chen,† Wenbo Shi,† Shijian Yang,† Hamidreza Arandiyan,† and Junhua Li*,†,‡ † ‡
School of Environment, Tsinghua University, Beijing 100084, China State Key Joint Laboratory of Environment Simulation and Pollution Control (SKLESPC), Beijing 100084, China
bS Supporting Information ABSTRACT: A series of vanadium-doped CoCr2 xVxO4 (x = 0 0.20) catalysts were prepared via the citric acid method and investigated for methane combustion. A significant improvement of the catalytic activity was observed over a CoCr1.95V0.05O4 catalyst, and the catalytic activity reached 90% of methane conversion at 438 °C, which was 76 °C lower than that of the undoped catalyst. Vanadium doping caused disorder in the spinel structure and various changes in physicochemical properties. The higher concentration of suprafacial, weakly chemisorbed oxygen and the active cobalt species, especially Co3+, could play a major role in the oxidation reaction. Results of XRD, Raman, and XPS suggested that excess vanadium probably formed separate catalytically inactive VOx species, which would explain the decline of catalytic activity for samples with a higher substitution degree.
1. INTRODUCTION With the soaring gasoline prices and the increasingly strict emission limits, natural gas engines are widely applied to vehicles nowadays.1 In addition to economic and political reasons, the use of compressed natural gas (CNG) for automotive applications offers significant environmental advantages over gasoline and diesel. However, the natural gas vehicles' (NGVs) advantages are partially balanced against the emission of unburned methane.2 As one of the greenhouse gases, methane is recognized to contribute more to the global atmosphere warming than CO2 at equivalent emission rates, all the more since its lifetime is quite long.1,3 Therefore, the abatement of unconverted methane exhausted from CNG vehicles becomes rather important.2 Catalytic combustion of methane is a promising technology that can convert unreacted methane into carbon dioxide at a relatively low temperature and has extensively been studied over the past decade. Noble metal-based catalysts, especially Pd-based ones, can exhibit outstanding catalytic behavior at low temperatures.2,4 6 Nevertheless, catalysts based on transition metal oxides, such as Mn, Cu, Cr, Fe, and Co oxides,7 9 become appealing due to their lower cost and relatively abundant resources. Among these catalysts, spinel-type oxides have attracted much attention for many years,1,10,11 and among the spineltype-oxides catalysts, CoCr2O4 was found to be very active for the oxidation of hydrocarbons,10,11 as well as for the catalytic removal of NOx12 and diesel soot.13 As it is known, doping of metal-oxide can result in a material which has significantly different properties than the original. The above has been exploited in several investigations so as to synthesize better combustion catalysts.7,14 17 On the other hand, there are few r 2011 American Chemical Society
reports from the literature concerning vanadium-containing catalysts used in the catalytic combustion of methane. Thus, vanadium was chosen as the dopant to understand the effect of vanadium on the catalytic activity of cobalt chromite. In this work, vanadium-substituted CoCr2O4 catalysts, of a general formula CoCr2 xVxO4 (x = 0 0.20), were prepared by the citric acid method, and then the catalytic activities were evaluated toward methane combustion. The bulk structure was determined by XRD and Raman, and the surface composition has been analyzed using XPS. After that, the oxygen desorption behavior was studied by O2-TPD, and the reducibility was studied by H2-TPR.
2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. The CoCr2 xVxO4 (x = 0 0.20) catalysts were prepared by the so-called citric acid method.18,19 Stoichiometric amounts of high-purity of cobalt nitrate, chromium nitrate, and ammonium metavanadate were dissolved in deionized water. After constant stirring at 60 °C for 15 min, a little excess of citric acid (metal/citric acid molar ratio =1/1.2) was added. This solution was stirred at 60 °C for 1 h and then evaporated in a rotary evaporator at 90 °C to produce viscous syrup, which was transferred to an oven kept at 200 °C for 1 h to decompose the citric acid. The solid product of decomposition was ground and calcined at 700 °C for 4 h. The available powders Received: March 30, 2011 Revised: August 3, 2011 Published: August 04, 2011 17400
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The Journal of Physical Chemistry C were then ready for the methane oxidation activity measurements after being crushed and sieved. 2.2. Catalyst Characterization. The X-ray diffraction (XRD) measurements were carried out with a Rigaku D/max-2500 X-ray diffractometer using Ni filtered Cu KR radiation. The applied current and voltage were 200 mA and 45 kV, respectively. During the analysis, the samples were scanned from 10 to 90° at a rate of 6 °/min. Raman spectra of the samples were recorded on a Renishaw RM 2000 instrument. A 514.5 nm Ar+ laser source at a power of 4.7 mW was used at a scanning range of 100 1200 nm. Brunauer Emmett Teller (BET) surface areas and pore parameters of the samples were measured by nitrogen physisorption at liquid nitrogen temperature (77 K) on a Micromeritics ASAP 2010 micropore size analyzer. X-ray photoelectron spectroscopy (XPS) data were obtained with a PHI-5300/ESCA electron spectrometer with Mg and Al radiation at a pressure lower than 10 7 Pa. The binding energies were referenced to the C 1s line at 284.8 eV from adventitious carbon. All XPS curves were fitted with mixed Gaussian Lorentzian line shape functions after nonlinear Shirley background subtraction. Peak areas were normalized by using appropriate atomic sensitivity factors. Temperature-programmed desorption of oxygen (O2-TPD) was studied in the same system. The catalyst (100 mg) was loaded into a quartz reactor and prior to the measurement, the sample was heated in a 5% O2/He flow (30 mL/min) at 600 °C and remained at that temperature for 1 h. After cooling in O2 to the room temperature, the flowing gas was then switched to helium (20 mL/min) to eliminate the physically adsorbed oxygen. Temperature was then raised at a rate of 10 °C/min from the room temperature to 1000 °C, under the same helium flow. Oxygen in the reactor outlet was detected using a quadrupolar mass spectrometer. A TCD was used for the quantification of the oxygen. The amount of O2 desorbed from the catalyst was quantified by calibrating the peak area against that of a standard O2 pulse (50.0 μL). Temperature-programmed reduction of hydrogen (H2-TPR) was carried out with Micromeritics ChemiSorb 2720 using approximately 100 mg of samples. The sample was pretreated in a 5% O2/He flow (50 mL/min) at 500 °C for 1 h and then cooled down to the room temperature in the same atmosphere. After being purged in a N2 flow (50 mL/min) for 30 min, the treated sample was reduced in a flow of 10% H2/Ar (50 mL/min) at a ramping rate of 10 °C/min. The effluent was monitored with a thermal conductivity detector. The thermal conductivity response was calibrated against the reduction of a known AgO powder sample. 2.3. Catalytic Activity Measurement. The methane combustion activities were carried out in a fixed-bed quartz reactor (i.d. 8 mm) located inside a vertical furnace and fed from the top using 250 mg of catalyst. Operating conditions were as follows: 2000 ppm CH4, 10 vol. % O2, and N2 as the balance gas, with a total flow rate of 150 mL/min, corresponding to a GHSV about 36,000 mL 3 h 1 3 g 1. The range of test temperature was 250 to 700 °C. The concentrations of CH4 in inlet and outlet gases were measured by an online gas chromatograph (Agilent 7890A) equipped with FID and TCD detectors. The activities were evaluated in terms of the CH4 conversion defined as (cin cout)/ cin 100%, cin and cout being the methane concentration corresponding to the inlet and outlet, respectively. Activity data were collected at every measurement point after stabilizing for 30 min.
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Figure 1. XRD patterns of the CoCr2 xVxO4 (x = 0 0.20) catalysts: (a) x = 0, (b) x = 0.05, (c) x = 0.10, (d) x = 0.12, (e) x = 0.15, and (f) x = 0.20. Inset: stepwise XRD pattern for sample f in the range of 20° to 30°.
3. RESULTS 3.1. XRD and Specific Surface Area. The XRD spectra of the CoCr2 xVxO4 (x = 0 0.20) catalysts are shown in Figure 1. According to JCPDS 22-1084, the CoCr2 xVxO4 samples were of cubic spinel-type structure. All of the diffraction peaks could be well indexed, as indicated in Figure 1. However, for x = 0.20, when the data were collected at 0.02° with a counting time of 1 s per step in the 20 30° range, new diffraction peaks appeared at about 21.7°, 24.3°, and 26.2° (Figure 1: inset). The peaks at 21.7° and 26.2° might belong to V2O5 (JCPDS 41-1426), and the peak at 24.3° could be attributed to V2O3 (JCPDS 71-0344). No such peaks can be observed for x e 0.15. The observations demonstrated that vanadium might exist as vanadium oxides for samples with a higher substitution level. In addition, the crystalline sizes of CoCr2 xVxO4 samples were calculated from the line broadening of the peak according to the (440) reflections using the Scherrer equation, and the results are summarized in Table 1. The mean crystalline sizes of the CoCr2 xVxO4 were in the range of 20 30 nm. The specific surface areas (BET) of the CoCr2 xVxO4 (x = 0 0.20) catalysts are also presented in Table 1. Results indicated that BET surface areas increased first and then decreased with the increment of vanadium content. The CoCr1.88V0.12O4 catalyst got the largest surface area, while its mean crystalline size was the smallest among these catalysts. 3.2. Raman Spectroscopy. Taking into account the results obtained by XRD analysis, it can be concluded that CoCr2O4 has a cubic spinel structure with a space group of Fd3m. There are five Raman active modes (one A1g, one Eg, and three F2g) of the spinel phase as suggested by group theory.20 As shown in Figure 2, the Raman spectrum of CoCr2O4 showed bands at 190, 448, 510, 530, 634, and 675 cm 1. The bands at 675 and 448 cm 1 can be ascribed to the A1g and Eg modes of the spinel CoCr2O4, respectively. The Raman-active bands at 190, 510, and 634 cm 1 corresponded to F2g modes of the spinel phase. Whereas, the band near 530 cm 1 was an unexpected mode. The origin of this mode remained unclear; it could be Raman-active due to the cation disorder that induced a breakdown of the translation symmetry. Similar phenomena have been reported for ZnCr2O421,22 and MgAl2O4.23 17401
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Table 1. BET Surface Area, Mean Crystallite Size, and Reaction Rate of the CoCr2 xVxO4 Catalysts reaction rate a surface area
mean crystallite
8
(10
sample
2
(m /g)
size (nm)
x = 0.00
18.9
30.0
0.59
3.14
x = 0.05 x = 0.10
17.5 23.1
22.0 25.8
0.84 0.80
4.82 3.47
x = 0.12
27.4
18.8
0.46
1.67
x = 0.15
11.8
28.8
0.04
0.32
x = 0.20
10.7
26.7
0.01
0.14
(μmol 3 g
1
1
3 s ) mol 3 m
2
1 3s )
Determined at 450 °C, reaction conditions: 2000 ppm CH4, 10 vol. % O2, and N2 as the balance gas, with a total flow rate of 150 mL/min, corresponding to a GHSV of about 36,000 mL 3 h 1 3 g 1. a
Figure 2. Raman spectra of the CoCr2 xVxO4 (x = 0 0.20) catalysts: (a) x = 0, (b) x = 0.05, (c) x = 0.10, (d) x = 0.12, (e) x = 0.15, and (f) x = 0.20.
For vanadium-doped samples, new additional bands were observed in the 750 850 cm 1 region, which can be attributed to the stretching vibrations of the VO4 tetrahedron.24,25 As the vanadium amount further increased (x g 0.15), a broad band maxima at 916 cm 1 started emerging. The bands may be ascribed to surface VOx species.26 In addition, a clear shift toward higher frequency was observed for the 750 850 cm 1 band with increasing vanadium content. This could be due to local distortions caused by vanadium substitution in chromite, as reported in the literature.24,25,27 In another interesting observation, the intensity of Raman bands at 190, 510, and 680 cm 1 of the CoCr2O4 catalyst were distinctly lowered compared with those of vanadium-doped catalysts. It may indicate the change of electronic properties of the catalysts, as has been reported by Marinkovic Stanojevic et al.21 over ZnCr2O4. 3.3. Surface Analysis: XPS. To obtain detailed information on the surface composition and local atomic environments of the CoCr2 xVxO4 catalysts, XPS spectra of CoCr2-xVxO4 with x = 0, 0.05, 0.10, and 0.20 were carried out. A wide scan of all samples did not show any lines other than the excepted ones, indicating the absence of surface impurities. The experimental and fitted
O 1s, V 2p, Cr 2p, and Co 2p XPS spectra are displayed in Figure 3, and the surface composition and atomic ratios are compiled in Table 2. As shown in Figure 3A, the intense main oxygen peaks have shoulders at lower binding energies, which can be resolved into two components with peaks around 530.4 ( 0.1 and 528.6 ( 0.2 eV,1,28,29 respectively. The two peaks of O 1s are distinguished as Oads and Olat, while Oads has a binding energy about 1.8 eV higher than that of Olat. The Oads peaks can be associated with the adsorption of oxygen species at the surface,1,28,30 while the low binding energy of Olat could be ascribed to the lattice oxygen.3,30 The area ratio of Oads/Olat could be a yardstick to measure the proportion of the two different oxygen species; thus, a higher area ratio indicates a larger amount of vacant oxygen.1,31 It can be seen from Table 2 that CoCr1.95V0.05O4 has a comparatively higher amount of surface oxygen than that of others and the largest oxygen vacancies concentration. Figure 3B presents the V 2p3/2 core-level spectra and fitted curves of CoCr1.95V0.05O4, CoCr1.90V0.10O4, and CoCr1.80V0.20O4. All the V 2p3/2 peaks have a highly asymmetric shape with high and low binding energy shoulders that point to a multicomponent nature of the V 2p3/2 peaks. The V 2p3/2 peaks were resolved into three components: V3+ (515.6 ( 0.2 eV), V4+ (516.7 ( 0.1 eV), and V5+ (517.5 ( 0.2 eV).32 34 The quantitative data given in Table 2 indicated that about 85% of surface vanadium species in the CoCr1.95V0.05O4 catalyst was in the +3 oxidation state, while the relative V3+ ratios at the surface were 66% and 47% over CoCr1.90V0.10O4 and CoCr1.80V0.20O4, respectively. Therefore, the surface vanadium tended to exist in higher oxidation state (+4 or +5) at the surface with the increment of vanadium amount. The Cr 2p3/2 peaks (Figure 3C) show a clear asymmetry and can be resolved into two peaks with maxima at 575.8 ( 0.2 and 577.0 ( 0.3 eV. The peaks were assigned to Cr3+ and Cr6+, respectively.10,12,35 The surface atomic ratios of Cr6+/Crtot are summarized in Table 3. The relative percentage of Cr6+ at the surface increased along with the increase of vanadium. Rida et al.,36 Sloczynski et al.,12 and Kim et al.10 also reported the copresence of Cr3+ and Cr6+ at the surface of the chromite catalysts. The Co 2p spectra of the four CoCr2 xVxO4 samples (Figure 3D) consisted of two main bands (i.e., 2p3/2 at about 780.4 eV and 2p1/2 at about 796.0 eV) with the spin orbit splitting of about 15.6 eV, as reported in the literature.12,37,38 Because of the small difference and band broadening caused by multiplet splitting, the position of the Co 2p band alone proved to be unreliable for detecting the presence of Co2+ and/or Co3+. A spectral feature that would distinguish between Co2+ and Co3+ was that the main Co2+ peaks presented a satellite, whereas Co3+ peaks did not.30,39 For spectra of the CoCr2 xVxO4 samples, Co 2p3/2 and Co 2p1/2 peaks both exhibited a satellite at higher binding energy by about 6 eV, indicating the presence of Co2+. As the vanadium content increased, the intensity ratio of the 2p3/2 satellite to the relevant main peak, Isat./Imain, decreased initially and then increased, indicating the amount of Co3+ increased first and then decreased.39 Therefore, the surface Co3+ ratios in CoCr1.95V0.05O4 was higher than those of other catalysts. In addition, the binding energy of vanadium-doped catalysts shifted toward higher binding energy, indicating the change of the chemical environment around cobalt ions when vanadium was introduced into the spinel structure. According to the XPS results presented in Table 2, for all the samples investigated, the surface oxygen content was higher than 17402
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Figure 3. Experimental and fitted (A) O 1s, (B) V 2p3/2, (C) Cr 2p, and (D) Co 2p photoelectron spectra of the CoCr2 xVxO4 catalysts with x = 0, 0.05, 0.10, and 0.20.
Table 2. Surface Compositions of the CoCr2 xVxO4 Catalysts Based on XPS Results surface composition x
O
0.00
0.69 (0.57) a
0.05
0.64 (0.57)
0.02 (0.01)
0.10
0.63 (0.57)
0.20
0.66 (0.57)
atomic ratio Cr6+/Crtot
Isat./Imainb
0.17
0.22
0.20
0.20
0.66
0.30
0.24
0.47
0.31
0.33
V3+/Vtot
Cr
Co
Oads/Olat
0.23 (0.29)
0.09 (0.14)
2.40
0.22 (0.28)
0.12 (0.14)
4.62
0.85
0.02 (0.01)
0.22 (0.27)
0.13 (0.14)
2.54
0.08 (0.03)
0.17 (0.26)
0.11 (0.14)
1.62
V
a
The data in parentheses are estimated according to the nominal bulk compositions. b Isat./Imain ratios were based on the Co 2p3/2 levels of the four samples.
the nominal bulk values, suggesting that there was excess oxygen at the surface. Meanwhile, it was found that the surface vanadium content was double, or even more than, that of the corresponding theoretical values, indicating a strong vanadium enrichment of the surface. In addition, there was less chromium and cobalt at the surface compared with
the nominal bulk composition. In view of the surface atomic ratios, the vanadium and chromium both tended to exist in a higher oxidation state as the vanadium amount increased, while the cobalt had the tendency to present in a lower oxidation state except for CoCr1.95V0.05O4. The results may indicate that charge compensating might occur when 17403
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Table 3. Amounts of O2 Desorbed from the CoCr2 xVxO4 Catalysts during the O2-TPD Experiments onset temperature (°C)
amount of oxygen
numbers of
desorbed (μmol/gcat) monolayers adsorbed a
b
sample
R-O2
β-O2
R-O2
β-O2
R-O2
β-O2
x = 0.00 x = 0.05
228 145
691 678
43.9 92.6
71.0 106.2
0.58 1.32
0.94 1.52
x = 0.10
145
606
46.0
25.3
0.50
0.27
x = 0.12
225
622
16.2
30.7
0.15
0.28
x = 0.15
464
709
12.0
60.7
0.25
1.29
x = 0.20
458
740
10.1
34.2
0.24
0.80
a
Calculated by deconvolution from the TCD signal. b Calculated by assuming 4 μmol/m2 of oxygen per monolayer.
Figure 4. O2-TPD profiles of the CoCr2 xVxO4 (x = 0 0.20) catalysts: (a) x = 0, (b) x = 0.05, (c) x = 0.10, (d) x = 0.12, (e) x = 0.15, and (f) x = 0.20.
vanadium with variable valence was introduced into the spinel structure, which was in accord with the Raman results. 3.4. Oxygen Desorption Behavior. Figure 4 shows the O2TPD profiles of the CoCr2 xVxO4 catalysts. It was observed that for the CoCr2O4 sample, there was one broad peak in the 250 700 °C range (with shoulders) and one intense peak maximum at 824 °C, corresponding to an O2 desorption of 43.9 and 71.0 μmol/gcat, respectively. Similar results have been reported by Fino et al.11,13 over spinel-type oxides. For the CoCr2 xVxO4 catalysts with x e 0.12, there were clear oxygen desorption peaks at 271, 400, 746, and 935 °C for x = 0.05; at 250, 409, 679, and 920 °C for x = 0.10; and at 301, 450, 700, and 920 °C for x = 0.12. The amount of O2 desorbed was 92.6, 46.0, and 16.2 μmol/gcat for the former two peaks at low temperature, and 106.2, 25.3, and 30.7 μmol/gcat for the latter two peaks at relatively high temperature for x = 0.05, 0.10, and 0.12, respectively. As for x = 0.15 and 0.20, there was one broad oxygen desorption peak in the 450 700 °C range and one intense desorption centered at about 860 °C. The amount of oxygen desorbed for the first peak was 12.0 and 10.1 μmol/gcat, and for the second peak it was 60.7 and 34.2 μmol/gcat over CoCr1.85V0.15O4
and CoCr1.80V0.20O4, respectively. The amounts of oxygen desorbed was measured by deconvolution of the O2 desorption curves using Gaussian peak shapes in a computer peak-fitting routine. The initial temperature was also determined by peakfitting analysis, and the data are presented in Table 3. In the O2-TPD profiles, the signals recorded below about 700 °C are generally denoted as R-oxygen species, and they correspond to oxygen species adsorbed on the surface oxygen vacancies, whereas those recorded at a higher temperature (above ∼700 °C) are known as β-oxygen species related to lattice oxygen, which causes generation of oxygen vacancies and reduced cations.31,40,41 Generally speaking, the amount of R-O2 can be considered as a measure of the oxygen vacancies, while the amount as well as the onset temperature of β-O2 desorbed reflects the lattice oxygen mobility or activity.29,31,40 The desorption behavior of R-oxygen was strongly dependent on the vanadium substitution level. As shown in Figure 4, an obvious shift of the oxygen desorption peaks toward lower temperatures was observed for x = 0.05 and 0.10. For instance, the onset temperature of the first signal for CoCr1.95V0.05O4 and CoCr1.90V0.10O4 was about 145 °C, which was much lower than those of CoCr2O4 (225 °C) and other catalysts. Nevertheless, the onset temperature of oxygen desorption signals increased as the substitution level further increased (x g 0.12). It was found that below 450 °C there was no obvious oxygen desorption for x = 0.15 and 0.20. The onset temperature of oxygen desorption signals and the amount of oxygen species were summarized in Table 3. On the basis of the amount of the R-oxygen desorbed (Table 3), we can deduce that the oxygen vacancy density of the CoCr2 xVxO4 catalysts decreased in the order of CoCr1.95V0.05O4 . CoCr1.90V0.10O4 > CoCr2O4 > CoCr1.88V0.12O4 > CoCr1.85V0.15O4 > CoCr1.80V0.20O4. Furthermore, the amount of β-oxygen desorbed of CoCr1.95V0.05O4 was much higher, while the onset desorption temperatures of CoCr1.90V0.10O4 and CoCr1.88V0.12O4 were relatively lower than those of other catalysts. In terms of the monolayer (assuming that one monolayer of oxygen is equal to 4 μmol/m2), the amount of R-oxygen desorbed from each of our samples was found to be less than or close to that required for monolayer coverage, confirming that the oxygen species dwelled on the catalyst surface.31,40 3.5. Reducibility. TPR experiments were performed over the CoCr2 xVxO4 (x = 0 0.20) catalysts to investigate their reduction behavior. Figure 5 shows the H2-TPR profiles of these catalysts. The H2-TPR profile of the CoCr2O4 exhibited four reduction peaks at 250 (with a shoulder), 455, 646, and 955 °C. According to the literature,42,43 the peak centered at 646 °C (PH2-III) can be ascribed to the reduction of Co2+ to metallic cobalt, and the peak (PH2-IV) in the range of 800 1000 °C is attributed to the reduction of Cr3+ to low-oxidation state. However, there is no agreement on the assignment of reduction stages of PH2-I (peak at around 250 °C) and PH2-II (peak appeared at 455 °C). Him et al. asserted that the TPR peak around 200 °C may be due to a simple chemisorption of hydrogen on the coordinatively unsaturated Cr3+ species, whereas Sloczynski et al.12 and Wang et al.44 assigned the hydrogen consumption peak PH2-I to the reduction of chromium species at the surface. In view of the restrictions of test conditions of Him et al.10 and the results reported by Grzybowska et al.,45 we tentatively attributed the reduction peak PH2-I and PH2-II to the reduction of Cr6+ to Cr3+ at surface and chemisorptions of hydrogen, respectively. For x e 0.12, the four hydrogen consumption peaks could still be observed in the TPR profiles but 17404
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Figure 5. H2-TPR profiles of the CoCr2 xVxO4 (x = 0 0.20) catalysts: (a) x = 0, (b) x = 0.05, (c) x = 0.10, (d) x = 0.12, (e) x = 0.15, and (f) x = 0.20.
changed in positions. There were no obvious reduction peaks which could be ascribed to PH2-I for x = 0.15 and 0.20. It may be due to the fact that the peak PH2-I shifted to higher temperatures and overlapped with peak PH2-II. In addition, we should mention that there were no separate vanadium reduction peaks in the TPR profiles of V-containing samples because the reduction of chromium and vanadium species takes place in similar temperature ranges.42,46 For the purpose of this work, state identification of the reduction intermediate is not the key point. Partial vanadium doping significantly modified the reducibility of the CoCr2 xVxO4 catalysts. The peak temperatures as well as the onset temperatures shifted to lower temperatures initially then moved back to higher temperatures with the increase of x (Figure 5). For instance, the peak temperature of the first reduction signal followed a sequence of CoCr1.95V0.05O4 (215 °C) < CoCr1.90V0.10O4 (246 °C) < CoCr2O4 (250 °C) < CoCr1.88V0.12O4 (264 °C) < CoCr1.85V0.15O4 (478 °C) < CoCr1.80V0.20O4 (515 °C). This result may indicate that the appropriate amount of vanadium would enhance the reducibility of the CoCr2 xVxO4 catalysts. However, the reduction of the CoCr2 xVxO4 could be retarded with a higher substitution level (x g 0.12). The hydrogen consumption of the CoCr2 xVxO4 catalysts was measured by a TCD signal. The total hydrogen consumption was found to range between 8.33 and 9.14 mmol/gcat, and the H2 consumption of CoCr1.95V0.05O4 was slightly higher than those of other samples. It may indicate that vanadium substitution has little influence on the total hydrogen consumption. 3.6. Catalytic Activity for Methane Combustion. The catalytic activity in the methane combustion reaction was measured as a function of the reaction temperature; the results are shown in Figure 6. The catalytic test results indicated that, in the temperature range investigated, all the catalysts showed 100% selectivity toward CO2, and no CO was observed during the reaction. As shown in Figure 6A, the light-off temperature (T50, the temperature required for 50% methane conversion) and the 90% conversion temperature (T90) over the CoCr2O4 catalyst were 420 and 514 °C, respectively. Partial substitution of chromium with appropriate amount of vanadium enhanced the catalytic performance toward methane combustion. For instance, the T50 and T90 were 388 and 438 °C over CoCr1.95V0.05O4, 404
Figure 6. CH4 conversion (A) and reaction rate (B) as a function of reaction temperature over the CoCr2 xVxO4 (x = 0 0.20) catalysts. Reaction condition: 2000 ppm CH4, 10 vol. % O2, and N2 as the balance gas, with a total flow rate of 150 mL/min, corresponding to a GHSV of about 36,000 mL 3 h 1 3 g 1.
and 450 °C over CoCr1.90V0.10O4, respectively. Therefore, the 90% conversion temperature dropped by 76 and 64 °C for x = 0.05 and 0.10, respectively, compared with that of the CoCr2O4 catalyst. However, a higher vanadium substitution level decreased the catalytic activity. The T50 was 448, 590, and 632 °C for CoCr2 xVxO4 at x = 0.12, 0.15, and 0.20, respectively, which was 28, 170, and 212 °C higher than that of the CoCr2O4 catalyst. The lowest methane conversion was found over the CoCr1.80V0.20O4 catalyst, which did not reach 70% methane conversion even at 700 °C. In summary, if T90 was set as the standard of activity, the descending order of activity for all the observed catalysts was as follows: CoCr1.95V0.05O4 > CoCr1.90V0.10O4 > CoCr2O4 > CoCr1.88V0.12O4 > CoCr1.85V0.15O4 > CoCr1.80V0.20O4. For the sake of making better comparison, the methane consumption rates over the catalysts are shown in Figure 6B. In addition, the data for reaction rates based on the unit mass and unit specific surface area at 450 °C are summarized in Table 1. These results further confirm the superiority of the CoCr1.95V0.05O4 catalyst among the CoCr2 xVxO4 catalysts. However, the reaction conditions, such as space velocity and reactant/ oxygen molar ratio, have an influence on the catalytic activity.31 17405
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The Journal of Physical Chemistry C This makes it difficult to compare the catalytic performance of a material on the basis of reactant conversion or temperature for its complete conversion under different conditions. Additionally, the catalytic activity of CoCr1.75V0.25O4 was given in the Supporting Information. As expected, further vanadium doping significantly decreased the activity, and the methane conversion at 700 °C over CoCr1.75V0.25O4 was as low as 21%.
4. DISCUSSION 4.1. Effect of Vanadium on the Structure, Composition, and Catalytic Activity. Spinel oxides belong to a class of
complex oxides with the chemical formula of AB2O4 in which A-site ions are generally divalent cations occupying tetrahedral sites, and B-site ions are trivalent cations in octahedral sites.47 Doping of spinel oxides with foreign metal ions can cause disorder in the spinel structure and then give rise to various changes in physicochemical properties. As revealed by XRD results, the CoCr2 xVxO4 samples were of cubic spinel-type structure. Although weak diffraction lines attributed to the VOx species were observed in an enlarged XRD pattern of the CoCr1.80V0.20O4 sample, we cannot confirm the presence of vanadium oxides because XRD analysis is not a sensitive quantitative method. However, based on the Raman spectra, one can clearly see the emerging of a new band ascribed to VOx species for samples with x = 0.15 and 0.20. This result further supported such a suggestion. It should be mentioned that the XRD pattern did not show any sign for the presence of crystalline VOx in CoCr1.85V0.15O4. This is a case that indicates the advantage of using different techniques for the determination of structures of vanadium species. A similar phenomenon has been reported by Gu et al.48 Considering that catalysis is a surface phenomenon, the results of the oxide surface composition from XPS are very important. As shown in Table 2, it was found that in the catalysts investigated the vanadium content was double, or even more than, that of the theoretical values, indicating strong vanadium enrichment of the surface. Therefore, based on the results above, it can be deduced that not all of the vanadium was incorporated into spinel lattice, particularly for samples with higher substitution levels, and there was a strong preference of vanadium atoms toward catalyst surfaces. However, since the amount of vanadium in the CoCr2 xVxO4 catalysts was so little (less than 2.86 at. %), we cannot detect vanadium oxides by TEM technique (Figure S2 of the Supporting Information). It seems that the catalytic performance strongly depends on the vanadium substitution degree. Appropriate vanadium doping (x = 0.05) would enhance the catalytic activity, while the activity decreased with further vanadium content. Thus, the descending order for methane conversion was CoCr1.95V0.05O4 > CoCr1.90V0.10O4 > CoCr2O4 > CoCr1.88V0.12O4 > CoCr1.85V0.15O4 > CoCr1.80V0.20O4 within the whole reaction temperature range. In contrast, the catalytic activity for methane combustion over several substituted perovskite oxides, e.g., La1-xSrxCoO3,19 La1-xCexCoO3,49 and La1-xSrxMnO3,50 was at a maximum for x = 0.1 0.2, and it decreased for higher x. However, as far as we know, few literatures have been reported on the catalytic activity over doped spinel oxides. In spinel oxides, the relationship between the occupation of the metal ions in the lattice and the catalytic performance is rather complex. Although many researchers asserted that octahedral (B) sites would be more effective than tetrahedral (A) sites,51,52 both A-site and B-site ions may play a role in the oxidation
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reaction.51 Therefore, in the present study, cobalt, chromium, and vanadium ions may be all active. As shown in Figure S4 of the Supporting Information, cobalt oxide was much more active than CrOx and VOx in the methane oxidation reaction. Although this result was not enough to warrant the fact that cobalt ions were more important than other metal ions, it provided us with some insight that cobalt ions were more active regardless of the occupation in the spinel lattice. According to the Co 2p XPS results shown in Table 2, the surface Co3+ component in CoCr1.95V0.05O4 is larger than that of other samples, and the Co3+ decreased with further increments of the vanadium content. In addition, octahedral coordinated Co3+ is assumed to be the active site for the oxidation reaction in a wide range of oxides.51,53,54 Therefore, it is reasonable to deduce that the active Co3+ species could play an important role in the oxidation reaction over the materials studied. More interesting results can be found if we correlate the composition with the catalytic activity. The catalytic activity of the CoCr2 xVxO4 catalysts seems to reach a maximum just before the vanadium doping was about to reach supersaturation and tend to segregate; thus, leading to an increase of the surface area, as observed. Regarding the XRD, Raman, and XPS results stated above, the excess surface vanadium species formed separate catalytically inactive VOx species (Figure S4 of the Supporting Information), which would explain the drastic decline in catalytic activity for samples with a higher degree of vanadium substitution. 4.2. Oxygen Species and Their Relationship with the Catalytic Activity. As clearly shown in Raman spectra (Figure 2), the intensity of the spinel-characteristic peaks of the vanadiumdoped catalysts decreased significantly, indicating that the presence of vanadium in the spinel lattice deformed the structure. It has been reported that the deformation would increase oxygen vacancy concentration, which facilitates oxygen mobility.1,15 From the O 1s results (Table 2), it can be seen that CoCr1.95V0.05O4 had a comparatively higher amount of surface oxygen than others and the largest vacant oxygen. The oxygen vacancies in the catalysts would promote the adsorption and dissociation of oxygen molecules and then facilitate the oxidation reactions.29 As mentioned above, the oxygen vacancies play an important role in oxidation reactions since they are responsible for the adsorption of gas phase oxygen and facilitate the diffusion of lattice oxygen from the bulk to the surface.29,55 Both causes would lead to the enhancement of the catalytic activity.55,56 In the elucidation of the predominant role of the oxygen vacancies in the catalytic activity, studies of O2-TPD can be a valuable tool. The O2-TPD experiments showed two kinds of desorbed oxygen species: R-oxygen (or suprafacial oxygen) desorbed at a low temperature range and β-oxygen (or intrafacial oxygen) desorbed at relatively high temperature. The R-oxygen was attributed to oxygen species adsorbed on the surface oxygen vacancies. Thus, on the basis of the intensity of an R-oxygen desorption signal, one can roughly estimate the amount of oxygen species at the surface oxygen vacancies.29 As summarized in Table 3, the amount of R-oxygen desorbed followed the sequence of CoCr1.95V0.05O4 . CoCr1.90V0.10O4 > CoCr2O4 > CoCr1.88V0.12O4 > CoCr1.85V0.15O4 > CoCr1.80V0.20O4, coinciding with the sequence of their catalytic performance. In addition, we should note that the order of temperature at which R-oxygen is desorbed is generally consistent with that of the catalytic performance. Therefore, the amount of R-oxygen desorbed and the temperature at which oxygen is desorbed appear to be the key 17406
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The Journal of Physical Chemistry C factors determining the catalytic activity, and it seems that for methane combustion over the studied catalysts, the abundance of R-oxygen plays a more important role than the temperature at which the oxygen species desorbs. Similar results have been reported by Bialobok et al.16 and Deng et al.29 The oxygen desorption signal (above ∼700 °C) is called βoxygen, and it is generally ascribed to lattice oxygen. The presence of β-oxygen and the value of the onset temperature can be considered as a qualitative index of catalyst reducibility and of oxygen mobility within the bulk.57 A lower onset temperature of the β-oxygen signal indicates an easy reducibility of the transient metal ions and hence a quick oxygen transport through the spinel lattice. Both these parameters are fundamental to describe catalytic activity. As marked in Figure 4, the onset temperature of the β-oxygen followed a sequence of CoCr1.90V0.10O4 < CoCr1.88V0.12O4 < CoCr1.95V0.05O4 < CoCr2O4 < CoCr1.85V0.15O4 < CoCr1.80V0.20O4. In consideration of the reducibility of metal ions, we should note the reducibility of the CoCr2 xVxO4 catalysts revealed by H2-TPR. As far as low-temperature reduction by hydrogen is concerned, reducibility of the CoCr2 xVxO4 catalysts decreased according to the sequence of CoCr1.95V0.05O4 > CoCr1.90V0.10O4 > CoCr2O4 > CoCr1.88V0.12O4 > CoCr1.85V0.15O4 > CoCr1.80V0.20O4. Therefore, the CoCr1.95V0.05O4 and CoCr1.90V0.10O4 were much more easily reduced than other catalysts. The results indicated that at high temperature the lattice oxygen in the bulk of CoCr1.95V0.05O4 and CoCr1.90V0.10O4 was much more mobile than that in the bulk of other catalysts. In other words, the diffusion of lattice oxygen from the bulk to the surface of CoCr1.95V0.05O4 and CoCr1.90V0.10O4 became facile at high temperatures. As demonstrated by Ferri et al.19 and Deng et al.,29 the rate of methane oxidation boosts when bulk oxygen becomes available since methane reacts with the oxygen coming from the bulk at high temperature via the intrafacial mechanism. Although the two mechanisms, intrafacial and suprafacial processes, proposed by Voorhoeve et al.,58 have been widely accepted for oxidation reactions over perovskites,16,29,55 the same behavior was observed for spinels.11,13 Thus, the βoxygen species could have an important role under reaction conditions, especially for the CoCr2 xVxO4 samples with higher vanadium substitution levels. By comparing the reducibility revealed by the onset temperature of β-oxygen, one can see that the sequence of the onset temperature of β-oxygen was nearly the same as that of the reducibility, except for CoCr1.90V0.10O4 and CoCr1.88V0.12O4. If we take the specific surface area into account, we would find that the surface area would play a certain role since the surface areas of the two catalysts were much larger than those of other catalysts (Table 1). As stated above, the close parallel between catalytic activity for methane oxidation and the extent of surface oxygen adsorption indicates that adsorbed oxygen is the dominant oxygen species participating in this reaction. In addition, it should be mentioned that the β-oxygen as related to the reducibility of the catalysts may also play a certain role in the oxidation reaction.
5. CONCLUSIONS CoCr2 xVxO4 (x = 0 0.20) catalysts were prepared via the citric acid method. The catalytic activity followed a descending order of CoCr1.95V0.05O4 > CoCr1.90V0.10O4 > CoCr2O4 > CoCr1.88V0.12O4 . CoCr1.85V0.15O4 > CoCr1.80V0.20O4. The best result has been attained with CoCr1.95V0.05O4 catalyst, and the catalytic activity reached 90% of methane conversion at
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438 °C, which was 76 °C lower than that of the undoped CoCr2O4 catalyst. Doped in an appropriate amount, vanadium causes disorder in the spinel structure of cobalt chromites. This distortion favors oxygen mobility and thus promotes the catalytic performance of vanadium-doped catalysts. The higher concentration of suprafacial, weakly chemisorbed oxygen and the active surface cobalt species, especially Co3+, could play a major role in the oxidation reaction. Results of XRD, Raman, and XPS measurements suggest that the excess vanadium formed separate catalytically inactive VOx species, which would explain the decline in catalytic activity for the samples with higher substitution degrees.
’ ASSOCIATED CONTENT
bS
Supporting Information. Catalytic performance of CoCr1.75V0.25O4, TEM images of CoCr1.80V0.20O4, and preparation, characterization, and activity results of pure oxides of vanadium, chromium, and cobalt. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Phone: +86 10 62771093. Fax: +86 10 62771093. E-mail:
[email protected].
’ ACKNOWLEDGMENT Financial support by the National High-Tech Research and Development (863) Program of China (Grant No. 2009AA064806) and special funding from the State Key Joint Laboratory of Environment Simulation and Pollution Control (10Z01ESPCT) is gratefully acknowledged. ’ REFERENCES (1) Li, J.; Liang, X.; Xu, S.; Hao, J. Appl. Catal., B 2009, 90, 307. (2) Gelin, P.; Primet, M. Appl. Catal., B 2002, 39, 1. (3) Li, J.; Fu, H.; Fu, L.; Hao, J. Environ. Sci. Technol. 2006, 40, 6455. (4) Farrauto, R. J.; Hobson, M. C.; Kennelly, T.; Waterman, E. M. Appl. Catal., A 1992, 81, 227. (5) Ramirez-Lopez, R.; Elizalde-Martinez, I.; Balderas-Tapia, L. Catal. Today 2010, 150, 358. (6) Matam, S. K.; Aguirre, M. H.; Weidenkaff, A.; Ferri, D. J. Phys. Chem. C 2010, 114, 9439. (7) Choudhary, V. R.; Uphade, B. S.; Pataskar, S. G. Appl. Catal., A 2002, 227, 29. (8) Kirchnerova, J.; Klvana, D. Catal. Lett. 2000, 67, 175. (9) Lahousse, C.; Bernier, A.; Grange, P.; Delmon, B.; Papaefthimiou, P.; Ioannides, T.; Verykios, X. J. Catal. 1998, 178, 214. (10) Kim, D.-C.; Ihm, S.-K. Environ. Sci. Technol. 2001, 35, 222. (11) Fino, D.; Russo, N.; Saracco, G.; Specchia, V. Catal. Today 2006, 117, 559. (12) Sloczynski, J.; Janas, J.; Machej, T.; Rynkowski, J.; Stoch, J. Appl. Catal., B 2000, 24, 45. (13) Fino, D.; Russo, N.; Saracco, G.; Specchia, V. J. Catal. 2006, 242, 38. (14) Zhang-Steenwinkel, Y.; Beckers, J.; Bliek, A. Appl. Catal., A 2002, 235, 79. (15) Bueno-Loez, A.; Krishna, K.; Makkee, M.; Moulijn, J. A. J. Catal. 2005, 230, 237. (16) Bialobok, B.; Trawczynski, J.; Mista, W.; Zawadzki, M. Appl. Catal., B 2007, 72, 395. 17407
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