J. Phys. Chem. 1993,97, 12870-12875
12870
Oxide/Support Interaction and Surface Reconstruction in the NazWO&iOz System Zhi-ChengJiang,' Chang-Jiang Yu, Xue-Ping Fang, Shu-Ben Li, and Hong-Li Wang Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China Received: June 22, 1993; In Final Form: August 25, 1993'
The surface dispersion state and oxide/support interaction in the MnOx-Na2W04/Si0~ catalyst system for oxidative coupling of methane have been studied by XRD (X-ray diffraction), XPS (X-ray photoelectron spectroscopy), and PASCA (positron annihilation spectroscopy for chemical analysis). It was observed that Mn existed as MnzO3-free crystallites and Na2W04 was dispersed on the silica support preferentially. A close-packed monolayer of Na2W04 was formed at a loading of 4 wt 7%. Strong interaction between Na2W04 and silica is indicated in the region of 0-1.5 wt % Na2W04, while a weaker interaction is exhibited in the region 1.5-4.0 wt 7% Na2W04. A phase transformation of Si02 from silica gel to cristobalite upon calcination at 1023-1 123 K took place as the consequence of this strong interaction. Reconstruction of surface W 0 4tetrahedral units into a structure containing a W-0 and three W-0-Si surface bonds was demonstrated. These species are believed to be relevant to the oxidative coupling of methane. A possible model for the structure of the new surface cluster compound is proposed.
Introduction
A large amount of work has been done on the search for a good catalyst for oxidative coupling of methane (OCM) as a new and promising route to produce ethylene and other chemically important materials from natural Among the many catalyst systems published so far to this end, a novel type of catalyst comprised of Na2W04 supported on silica with the promotion of Mn oxide has been developed in this laboratory. This catalyst was found to give a good yield of 23.9% C2 hydrocarbons under the reaction conditions of 800 OC, CH4/air = 1, and GHSV (gas hourly space velocity) of 36 000-l; its catalytic activity had no conspicuous decline in a run of 30 h on ~ t r e a m . ~ Although the catalytic performance for such systems has been extensively studied, the surface chemistry of these catalysts has not been sufficiently explored. However, it is well-known that the oxygen species on the surface of the catalysts used in OCM plays a vital role in this reaction. For instance, the 0- species present in Li+O- centers formed by the substitution of Li for Mg ion has been identified as the active species for the Li/MgO system.4-s 02-and 022-, which are likely associated with the oxygen vacancy present in La2O3, have been proposed to be the surface oxygen species responsible for the OCM taking place on La203-containing catalytic systems, based on XPS and LRS (laser-Raman spectroscopy)studies, respective1yqH On the other hand, the surfacelatticeoxygenin the LiNiOZcatalyst was thought to be the active species in this system.9JO Thus different species of oxygen appear to be involved in different systems used in OCM. In this paper we will first examine the dispersion of the active components, namely Mn oxide and Na2W04,on the silica support mainly through XRD, XPS, and PASCA measurements. Strong interaction between Na2W04and the Si02 support is indicated. Then the oxygen species on the surface of such a system will be studied by the XPS technique. The formation of W-0-Si and W=O surface bonds appears to be justified. Finally, a scheme for the surface structure of the Na2W04/Si02catalyst system which may play a key role in methane coupling will be proposed on the basis of the experimental findings. Experimental Section
Catalyst samples were prepared by slurry mixing methods from stoichiometric solutions of Mn(N03)~and Na2W04and silica
* To whom correspondence should be addressed.
Abstract published in Aduance ACS Absrracrs, October 15, 1993.
0022-365419312097-12870$04.00/0
gel. After drying, the catalysts were calcined at 1023-1 123 K for 8 hand, then, crushed into granules (mesh 20-60). Catalytic runs were made on a conventional fixed bed microflow reactor under atmospheric pressure. Characterization of the catalysts was carried out by surface area measurement, XRD, XPS, and PASCA. XRD analyses were carried out on a D/MAX-RB X-ray diffraction spectrometer (Cu K a radiation, 50 kV, 100 mA). XPS measurements were done on a PHI-550 ESCA/AES spectrometer using Mg K a radiation (1253.6 eV, 320 W) with CMA pass energy = 50 eV. The samples for XPS measurements were pressed into self-supportingdisks (cross section 12 X 1 mm) and pretreated in a prechamber at 400 OC, 1 V Torr and subsequently in the ultrahigh vacuum (UHV) chamber at 700 OC with a base pressure of 5 X Torr. This was done to remove the surface hydroxyls and other species adsorbed on the surfaceof the samples. This treatment would not affect the lattice oxygen in this system because it was found in earlier work that the lattice oxygen in this catalyst system desorbs at 750 OC.3 The characteristic peaks of the surface elements were recorded at room temperature with an on-linecomputer in multichannel mode. It took about 1 h for each data acquisition in order to ensure the quality of the X-ray photoelectron spectra obtained with high signal to noise ratio. The binding energy (&) values were calibrated by taking both the C l s peak (Eb= 284.6 eV) of the contaminated carbon and the Si2p peak (& = 103.4 eV) of the silica support of the catalyst. Computer curve-fitting of the 01s peaks was done with Gaussian function simulation and the reliability of the curve-fitting results was demonstrated by agreement with the high-resolution 01s spectrum recorded with CMA pass energy = 15 eV for a duration of more than 3 h as shown in Figure 6b. The positron annihilation technique has been introduced into surface chemistry studies in recent years and is known as positron annihilation spectroscopy for chemical analysis (PASCA).IlJ2 It has been shown that 0-Ps, one of the bound states of an electron and a positron (e-e+) may react chemically with the active sites on a catalyst surface by the so-called quenching reactions, and its reaction rate (A) and lifetime ( T ) mainly dependon theelectron density at the sites. Therefore, 0-Ps acts as a chemical probe atom. In this work, 22Na was used as the positron radiation source for the PASCA measurements. The lifetime spectra obtained were resolved into three components TI,T2, and T3, respectively. The corresponding intensities 11,12, and I3 were 0 1993 American Chemical Society
The Journal of Physical Chemistry, Vol. 97,No. 49, 1993 12871
Na2W04/Si02 Catalyst System
TABLE I: Catalytic Properties of W-Mn/SiOl Catalysts. catalyst composition Si02 1.9 wt % Mn/SiOz 5 wt 4% NazWO4/SiOz 1.9 wt 7% Mn-5 wt %
c2
CH4 conversion (4%)
selectivity
0.6 16.4 12.3 36.8
90.1 34.3 14.3 64.9
(%)
yield (5%) C2-/c0 0.5 5.6 9.1 23.9
0 0.8 0.9 2.1
NazWO4/SiOz T = 1073 K; CH4:Oz:Nz = 3:1:2.5; flow rate = 36 OOO mL-g-l.h-l.
0
20
10
30
60
50
60
20 Figure 2. X-ray diffraction patterns for various samples: (a) Si02; (b) 1.9 wt % Mn/SiOz; (c) 1.9 wt % Mn-2 wt % NazWO4/SiOz; (d) 1.9 wt % Mn-5 wt % NazWO4/SiOz (X, cristobalite; 0, NazW04; A,Mn203). lo4 Content (ut%) Figure 1. Dependence of methane conversion on NazWO4 loading for 1.9 wt 4% Mn-NazWO@O2 catalysts.
also obtained. T1 was on the order of 0.1 ns, attributed to the annihilation of the free positrons and 0-Ps formation in the bulk of the catalysts. T2 was found to be 0.42-0.45 ns, attributed to the annihilation of the 0-Ps in the bulk. The long-lifetime component, T3, was attributed to the annihilation of 0-Ps on the catalyst surface. It is the latter, T3, which is highly sensitive to the chemical environment and structure of the surface,12that we are most interested in. The three intensity values ZI,12. and 4 correspond to the rate of production of the three components with different lifetimes, respectively. 0-Ps formation rate (I) and lifetime (T) were recorded in the conventional fast-fast coincident mode.
Results
Catalytic Activity. Catalytic reactions were performed on samples of silica, Mn oxide supported on silica, and Na2W04 supported on silica promoted with different amounts of Mn oxide. Results are shown in Table I. It is seen from Table I that the Na2WO4 catalysts supported on Si02 and promoted with Mn oxide possess much better catalytic properties than the individual components. While keeping the amount of Mn oxide constant at 1.9 wt % and varying the amount of the main component of the catalyst system, namely Na2W04,a maximum in catalytic performance was observed at 4 wt % Na2W04. Higher loadings of Na2W04did not increase further the yield of C2 hydrocarbons as seen from Figure 1. This suggests that the catalytic performance of the Mn-Na2W04/SiOz system may be closely related to thestateof theNa2W04dispersedonthesurfaceof thecatalyst. XRD Analysis. The silica support used in this work was amorphous as shown by the XRD pattern given in Figure 2a. It kept its amorphous state when 1.9 wt % Mn oxide was present was shown in Figure 2b, but no crystalline form of Mn oxide was observed. When Na2W04 was added to Si02 in increasing amounts, the XRD pattern for cristobalite with a preferential orientation of (1 11) surface started to appear when the Na2W04 loading was 1 wt % and its intensity increased with the amount of Na2W04 added (Figure 2c). The amorphous silica was completely transformed into cristobalite with the preferred orientation when the Na2W04 loading reached 4 wt %. At this point, the surface area of the catalyst decreased dramatically from the original 140 m2/g for the amorphous silica to only 7.4
'iL MvOyNa2W@/Si 02 Mnz1.9 w t %
.-c W
r
d?
1.5
b
10
1 Na2WOk 2 3 Content 4 5 (wt%) 6 7
Figure 3. Dependence of 0-Ps lifetime (T3)on the NaZW04 content in 1.9 wt % Mn oxide-NazWO4/SiOz catalysts: X, 0-Pslifetime for silica glass (ref 11).
m2/g for the samples of 4 wt % Na2W04/Si02. Meanwhile, the crystalline form of Na2W04 started to appear when the loading exceeded 4 wt %. When Mn and Na2W04 were both introduced into the silica gel, e.g. in the sample of 1.9 wt % Mn-x wt % Na2W04/Si02, the same phase transformations for Na2W04 and Si02 were observed as noted above, but the XRD peaks for crystallites of MnzO3 appeared in the spectra when the Na2W04 loading was as low as 1 wt % (Figure 2c shows the result for the sample of 1.9 wt % Mn-2 wt % Na2WO4/SiOz). PASCA Measurements. The formation of 0-Ps (4)and its lifetime (T3) on the surface changed markedly with the loading of Na2W04 while keeping the Mn content constant at 1.9 wt % as shown in Figures 3 and 4. It is seen from Figure 3 that the 0-Ps lifetime (T3) on the surface initially decreased rapidly with the loading of Na2W04, then more slowly after 1.5 wt % Na2W04, and leveled off when the loading of Na2W04 exceeded 4 wt %. The corresponding formation rate (Z3) changed with the Na2W04 loading in a more complex manner. It first increased sharply with the Na2W04 content until 1.5 wt % was reached, then it decreased with the further addition of Na2W04 until the loading reached 4 wt % Na2W04, above which it leveled off as shown in Figure 4. XPS Data. XPS measurements were made in two different aspects. One was to examine the dispersion of NazW04 on the surface of Si02. Figure 5 is the plot of the XPS peak intensity ratio of W4f/Si2p versus NazW04 loading for the catalysts containing 1.9 wt % Mn oxide but different loading of Na2W04. It is seen that the peak intensity ratio of W4f/SiZp increases linearly with the Na2W04 loading in the range of 0-4 wt % Na2W04,then a steeper linear segment with higher slope was observed for catalysts with Na2W04 loading higher than 4 wt 7% as shown in the figure.
Jiang et al.
12872 The Journal of Physical Chemistry, Vol. 97, No. 49. 1993
- 4
0
1
2
3
4
5
6
7
N a 2 ~ 0 4Content (wt%)
Figure4. Dependence of 0-Ps formation rate (13)on the NazW04 content in 1.9 wt W Mn-Na2WOdSiOz catalysts.
0
1
2
3
4
5
6
NqWO4 Content (wt%)
Figure 5. XPS peak intensity ratio I w 4 f / l ~ i 2versus ~ Na2WO4 loading in 1.9 wt % Mn-NazWO4/SiOZ catalysts.
1
OlS xps
1
..I a
z
3
U
..'.. ! 530.1 **
533
531
529
BINDING ENERGY (eV1 Figure 7. (a) 0 1 s XPS spectra of 5 wt 5% NazWO4/SiOz catalyst. (b) 0 1 s XFS spectra of 5 wt W NazWO4/SiOl catalyst washed in boiling
water.
.
.531.6 ' I 530.6 c
535
PE=15 e V
533.2
..I
BINDING ENERGY (eV)
Figure 6. (a) 0 1 s XPS spectra of 1.9 wt 9%Mn-5 wt % NazW04/SiO~
catalyst (recorded with CMA pass energy = 50 eV): dot line, original data;dash line, curvefitting peaks,solid line, envelope of the curve fitting peaks (same in the subsequent figures). (b) 0 1 s XFS spectra of 1.9 wt W Mn-5 wt 9% Na2W04/SiOz catalyst (recorded with CMA pass energy = 15 eV). The other aspect of the XPS measurements was to study the oxygen species present on the surface of the catalyst in which both Mn oxide and Na2W04 were present. Figure 6 gives the XPS 0 1 s spectrum for the sample of 1.9 wt % Mn-5 wt % Na2WO4/SiO2 catalyst. It is seen upon deconvolutionof the spectrum that, in addition to the three lattice oxygens ascribed to the three components present (Si02, Mn2O3, and Na2W04) with binding energy of 533.2,530.1, and 530.6 eV, respectively, a new species Of Surfaceoxygen with a binding energy of E b = 53 1.6eV emerges from the spectrum. A high-resolutionXPS spectrumof the sample is shown in Figure 6b for comparison. A separate spectrum of XPS 0 1 s for the sample of 5 wt % Na2W04 was taken under identical experimental conditions to
examine the possibility that this new species of surface oxygen may be related to the Na2W04 dispersed on the Si02 support. This XPS 0 1 s spectrum of 5 wt % Na2WO,/Si02 is shown in Figure 7a. Upon careful examination of the spectrum, it can be seen that besides the peaks ascribed to the lattice oxygens of Na2W04 and Si02, there are two additional peaks which are centered at 53 1.6 and 530.3 eV, respectively. The former was the one found earlier in the sample 1.9 wt 5% Mn-5 wt % NazWO4/SiO2. The latter, being close to the binding energy of the lattice oxygen in Mn2O3, could not be singled out for the sample 1.9 wt % Mn-5 wt % Na2W04/Si02. In order to further identify these species of oxygen present on the surface of 5 wt % Na2WOJSi02, a spectrum was taken for the sample of 5 wt % Na2W04/Si02 after washing and drying. This treatment would remove any free NatW04 which was present on the catalyst sample. The resulting spectrum is shown in Figure 7b. It is seen that indeed, the 0 1 s peak for Na2W04 (Eb = 530.6 eV) was not found in this spectrum. However, the peak for the lattice oxygen of Si02 (& = 533.2 eV) and the two new species of surface oxygen (& = 53 1.6 and 530.3 ev) still remained, The relevant data for the binding energies of the different species of oxygen involved are given in Table 11. Discussion Dispersion of Ma oxide and Na2W04. Mn oxide, when supported on silica, has been found to be active in the oxidative conversion of emthane to higher hydrocarbons.13J4 The activity of the Mn oxide/Si02 system was ascribed by the authors to be related to a unique Mn oxide-silica support interaction and the newly formed surface M n S i - O compound. For the 1.9 wt % Mn oxide/Si02 sample used in this work, we found no new X-ray diffraction pattern in comparison with pure SiO2supportas shown in parts a and b of Figure 2, respectively. This suggests that the silica support still kept its amorphous state despite the hightemperature calcination. The Mn oxide appeared to be highly
The Journal of Physical Chemistry, Vol. 97, No. 49, 1993 12873
Na~WO4/Si02Catalyst System
TABLE II: XPS Binding Energy of W-Mo/Si@ Catalysts' Si02 Na2WO4 sample 1
01s 533.2
Si2p
W4f7/2
Nals
103.4, 102.4
35.2 35.8
1072.4 1072.8
103.4, 102.4
35.6
1073.2
103.4, 102.4
36.2
1073.0
103.4
530.6
533.2.531.6 530.8,530.1
sample2 sample3
533.2.531.6 530.6,530.3 533.2,531.6 530.3
The binding energy (&) value was calibrated by taking both Cls peak (Eb 284.6 ev) of contaminated carbon and si2p pcak (& = 103.4 eV) of silica support as the references: sample 1, 1.9 wt IMn-5 wt % NazW04/SiOz; sample 2, 5 wt 5% NazWO,/SiOz; sample 3, washed 5 wt % NazWO4/SiOz.
dispersed on silica, suggesting the presence of a strong interaction with the support. However, for the case of the catalyst samples in which Mn and Na2W04 were both present, crystalline Mn2O3 was found to exist as shown in parts c and d of Figure 2. Thus Na2W04 appears to have even higher affinity to Si02 than the Mn species. It is the Na2W04, not Mn species, that disperseson the silica surface when both are present. This is supported by the XPS data shown in Figure 5. It is seen from this figure that the peak intensity ratio I(W4f)/I(Si2p) increased linearly with the loading of Na2WO4 up to 4 wt % in the catalyst system of 1.9 wt % Mn-NazW04. A steeper line with a higher slope was obtained when the Na2W04 loading exceeded 4 wt %. This is consistent with the idea that Na2W04 is dispersed on the silica up to 4 wt 9% loading at which a close-packed monolayer of dispersion is formed. Above this threshold value of Na2W04 loading, the photoelectron emission from the silica substrate would thus be attenuated by the close-packed dispersed phase layer of Na2W04, and a line with a higher slope would result as shown in Figure 5. The excess of Na2W04 would then crystallize out as a separate phase as detected by the XRD pattern shown in Figure 2d. Interaction between the Components on the Catalyst Surface. The dispersion state of Na2W04 on the surface of the catalyst was further revealed by the PASCA measurements. It is seen from Figure 3 that the 0-Ps lifetime on the surface ofthesecatalysts decreases with theNa2W04loading in therange o f 0 4 wt 9% Na2W04but staysconstantwhen theNa2W0410ading exceeded 4 wt 9%. This is in full agreement with the results derived from XRD and XPS data and once again confirms that the dispersed-phaselayer of Na2W04 is completed at the threshold mentioned above. However, owing to the sensitivityofthe PASCA technique to electron density and structure of the solid, within the range of 04wt % Na2W04, two regions could be differentiated. For pure silica support, 0-Ps formation rate is very low and its lifetime (2.6 ns) is much longer than that of Si02 glass (T3 = 1.5 nsll). That is due to the fact that the structure of silica gel, having a large quantity of vacancies, is much looser than that of glass and its electron density is relatively deficient, so that the probability of the incident positron capturing an electron to form 0-Ps is low. Furthermore, 0-Ps has hydrogen-like structure and tends to exist in a solid medium with looser structure and has less chance for annihilation reaction in an electron-deficient environment," so that the 0-Ps lifetime in silica gel is much longer. When Na2WO4 is loaded on the silica carrier, the lifetimeof 0-Ps (T3)drops steeply and the production rate (Z3) increases sharply with the increase of NazW04 loading which initiates the transformation of silica gel into cristobalite at 1 wt % Na2W04 and increases the electron density of the system as seen from Figures 3 and 4, respectively. This is what is to be expected and suggests a strong interaction between the Na2W04 and the Si02 support, perhaps site-selective. While in the range of 1S-4.0 wt % Na2W04, the lifetime of 0-Ps still drops but in a more moderate manner, suggesting a weaker interaction between Na2W04 and
Si02, perhaps site nonselective. Meanwhile, the production rate of 0-Ps drops dramatically with the Na2WO4 loading, a natural consequence of the drastic decrease of surface area when the Si02 support transforms from silica gel to cristobalite which completes at 4 wt 9% Na2WO4. The low value of 0-Ps lifetime (T3 = 1.4 ns) for the system with Na2W04 loading higher than 4 wt % is compatible with the more compact structure of the crystalline cristobalite than the silica gel used initially. The interaction between theNa2W04 and thesilica supportapparently initiates the transformation of the silica support from an amorphous state of silica gel to a crystalline compound of cristobalite. The appearance of crystalline cristobalite upon calcination at a temperature of 1023-1 123 K is quite remarkable because such a phase transformation is not expected at this temperature under normal conditions. It is known that quartz transforms into tridymite at 870 OC and tridymite transforms into cristobalite at 1470 OC.15 However, it is possible that the calcination of Na2WO4 at 1023-1 123 K may produce tungsten ions with low oxidation numbers in the tetrahedral units of W04.16 These units may assist or even take part in the phase transformationfromsilicagel tocristobaliteat thecalcinationtemperature of 1023-1 123 K by the inclusion of Na+ in the interstitial holes of cri~tobalite.~~ The presence of M 3 +ions may give the similar effect but in a less efficient manner. Cristobalite was also found to be present along with quartz and Mn silicates in the systems which contain silica, Mn oxide, and Na studied by Sofranko and his co-workers.14 This cristobalite structure of the silica support appears to be crucial in the generation of active centers for OCM in such systems as to be discussed further later in this paper. Fornution of Surface W - o S i W.Following the phase transformation due to the incorporation of tungsten ions of low oxidation state and possibly M 3 + as well into the silica lattice and the simultaneousinclusion of Na+ in the resultant cristobalite crystal, a surface restructuring in the system of Mn-NazW04/ Si02 seems possible. We have found from the XPS 0 1 s spectrum for 1.9 wt 9% Mn-5 wt % Na2W04/Si02 shown in Figure 6 that in addition to the peaks ascribed to the three lattice oxygens of the components involved (Si02, Mn2O3, and Na2W04),a peak due to a new species of surface oxygen with a binding energy of 531.6 eV emerges from the spectrum. Although this binding energy falls in the range usually found for the hydroxyls, this peak could not be attributed to a surface hydroxyl or absorbed oxygen because the sample had been treated in UHV at 700 OC prior to its XPS spectrum being taken. Coupled with the experimental fact observed earlier that in the system of 1.9 wt 9% Mn-5 wt % NazW04/SiO2 in which Na2W04 was dispersed on the surface of silica presumably molecularly up to a loading of 4 wt %and that Mn2O3 was separated into a crystalline phase, this species of oxygen could well be a species of surface oxygen brought about by the strong interaction between Na2W04 and silica. From the XPS spectrum shown in Figure 7 for a sample of 5 wt 9% NazWO,/SiOz before and after the crystallites of soluble Na2W04 were removed by treating in boiling water, two new peaks with the binding energies of 531.6 and 530.3 eV emerged and the lattice oxygen of Na2W04 at 530.6 eV was absent. The new peak of 530.3 eV was not observed for the sample of 1.9 wt 9% Mn-5 wt 9% Na~WO4/Si02because of interference by the lattice oxygen in Mn2O3 with the binding energy at 530.1 eV. In addition, the binding energy of w4f7/2 was observed to shift upward from 35.2 eV for Na2W04 to 36.2 eV for the new surface compound. Moreover, a shoulder peak (& = 102.4 ev) was observed at the lower binding energy side of the Si2p peak (& = 103.4 eV) for silica. By putting all of this informationtogether, a new surface compound composed of W, 0,and Si and insoluble in water was formed on the surface of 5 wt % Na~W04/Si02as the result of strong interaction between Na2W04 and silica. This statement is applicable to the sample of 1.9 wt % Mn-5 wt %
Jiang et al.
12874 The Journal of Physical Chemistry, Vol. 97, No. 49, 1993
0 B
V
3 8 -w 0
-Si
Figure8. Surfacecluster structureof Na2WO4/Si02catalyst: (a) W-0 tetrahedron; (b) a surfacestructureunitof cristobalitc; (c) W-0 tetrahedra
on Si02.
Na2W04/Si02because the same observations with regard to the emergence of new peaks and shift in the binding energies were observed. The argument for the formation of a new surface compound was further strengthened by the previously published LRS data taken on the Mn-Na2W04/Si02 catalyst.3 In that study, three new Raman vibrational modes of 783,842, and 1073 cm-l were observed for the 5 wt % Na2WO,/SiOz catalyst, in addition to the vibrational modes for silica and the four fundamental modes at 928 cm-l (AI), 312 cm-l (E), 813 cm-l ( F A and 373 cm-l (F2) for crystalline Na2W04. In conjunction with the above XPS findings, these three new modes are believed to be responsible for the formation of the new surface W-0-Si compound. The Td symmetry of the W 0 4 tetrahedron was lowered as the result of its attachment to the plane of Si02tetrahedra. The peaks at 783 and 842 cm-1 may be assigned to the vibrational modes of the W-0-Si species and the 1073-cm-1 peak may be attributed to a terminal W - 0 bond present in the distorted W 0 4tetrahedron by analogy with W-0-W and W = O bonds reported in the literature,17 but W-0-Si bonds would be expected in our case. Since the same Raman spectrum was obtained on the Mnpromoted Na2WO,/Si02 catalyst,3 Mn2O3 was thought to have no Raman vibrational band in this wavenumber region and there was no strong interaction between free Mn2O3 and silica support, which is consistent with the above discussions. Surface Structure of the Catalyst 5 wt % NalW04/Si02. Having established the formation of a new surface bond of W-0Si on the catalyst 5 wt % Na2W04/Si02, we are now in a position to propose a scheme for the surface structure of such a catalyst system. From the XPS measurements, two species of surface oxygen with binding energies of 53 1.6 and 530.3 eV, respectively, are present. Furthermore, the peak intensity ratio for these two new oxygen species I(531.6 eV)/I(530.3 eV) is calculated from Figure 6 to be 3:l. And that for the lattice oxygen of silica support (533.2 eV) to the species with binding energy of 53 1.6 eV is also calculated from Figure 6 to be 3.6: 1. We known from the XRD analysis that the silica support in the 5 wt % NazWO4/SiO2 system has transformed from its amorphous state into cristobalite with a preferential orientation of (1 1 1) surface upon calcination presumably brought about by the strong interaction between Na2W04 and silica. The surface structure of cristobalite with this preferred orientation can be expressed as the extension of the structural unit shown in Figure 8b,l5 in which six Si02 tetrahedra, linked to each other in a horizontal plane, form a circular ring terminated by three upward corner oxygens. On the other hand, the structure of Na2W04 may be described as a regular tetrahedron consisting of four close-packed corner oxygens and a central W6+ion as shown in Figure 8a. Na+ ions are labile speciesand need not be taken into account when surface structure is being considered.
Based on the above information, a possible model for the surface cluster structureof Na2W04/Si02catalyst is presented in Figure 8c. In this model, the W 0 4 tetrahedron occupies the central 3-fold site among three upward corner oxygen atoms on the Si02(1 11) surface and undergoes a restructuring upon attaching to the surface. A W atom is bonded with three Si atoms through three bridge oxygens, producing three W U S i bonds. There is one W - 0 left in the upward direction. Thus, the new oxygen species with & = 531.6 eV may be reasonably attributed to the bridge oxygen, and the other one with Eb = 530.3 eV may be assigned to the terminal 02-in the W - 0 double bond. With this structural arrangement, the ratio of the former to the latter is 3:1, in agreement with the intensity ratio obtained in the XPS spectra. Moreover, according to this model, for each reconstructed W U S i surface bond, there are three original Si-0 bonds in a S i 4 tetrahedron for the new surface cluster compound. The experimentallyobserved intensity ratio for the latter to the former was 3.6: 1 instead of the expected 3: 1. This higher value may be attributed to the contribution by the Si+ bonds in the bulk. Again according to this model, the oxidation state of the W ion in the surface compound should be W5+instead of W6+. This is supported by the XPS binding energy shift of the W4f7p peak for Na2WO4 in comparison to that of the 5 wt % Na2WO4/SiO2 catalyst before and after washing given in Table 11. This 0.6-1 .O eV shift in the W4f712 binding energy is consistent with the formation of W5+.16J8 The stoichiometryfor the surface compound on theNazWO4/SiOzcatalyst thus would beSi3WOg.5. It might be produced by the surface reaction of Na,W04
+ 3Si0,
-
750450 'C
WSi,O,,,
+ 2Na' + 3/40,
The above considerationsshould also be applicable to the Mn promoted Na2W04/SiO~system. Apparently, this new surface compound is in correspondence to the site-selective part of dispersed Na2W04 with a loading of less than 1.5 wt % on the surface of silica support. It is suggested that the system discussed above may be regarded as an example of the 'surface coordination chemi~try".'~ Sofranko and co-workers have suggested that the species of manganese silicate, or manganese oxide perturbed by a strong Mn oxide-support interaction, might be responsible for the selectiveoxidation of methane.13J4 But the above results indicate that, in the case of Mn-Na2W04/SiO2 catalysts, the species responsible for the selectiveoxidation may be the surface tungsten silicate produced by a strong interaction between Na2W04 and Si02 support. It is interesting to note that, with this interaction, W6+ can be transformed into Ws+ in the Na2WO,/SiOz system and the mobility of the labile Na+ species would be enhanced at high temperature, which leaves vacancies on the surface to facilitate the methane adsorption. Thus, one could expect that this exchange process between the high/low valency tungsten ions would be brought into action during the OCM reaction at high temperature (800 "C), and, accompanied by the lattice oxygen depletion in the reaction with methane and regeneration by the adsorption of gaseous oxygen, an effective redox cycle could be established in the catalytic system. A redox mechanism involvinglattice oxygen ions and dissociativelyadsorbed methane has been proposed by Otsukag and Lambert,lo respectively, on LiNIO2 catalysts for oxidative coupling of methane. A similar catalytic selective oxidation may be, very likely, effective in the NazW04/SiO2 catalyst system. The role of Mn oxide in the catalysts studied is likely to enhance the gas phase oxygen/lattice oxygen exchange and lattice oxygen transport, but it does not seem to manifest itself in this room-temperaturecharacterization. Its synergetic effect may only become evident at the high temperature practiced in the OCM reaction. Apparently,further study on the surface oxygen species and the surface structure of Mn-Na2W04/SiOz catalyst under the high-temperature con-
Na2WOd/SiOz Catalyst System dition is desirable and necessary, in order to gain some insight into the surface chemistry involved in the activation of methane. The high-temperatureXRD and XPS as well as high-temperaturequenching EPR studies on the system, currently being undertaken in this laboratory, have provided further evidence to support our considerationspresented above and will be discussed in subsequent articles.
Conclusion For the catalyst system of 1.9 wt 9% Mn-Na2W04/SiO~ used in the oxidative coupling of methane, Na2W04 was found to disperse on the surface of silica in preference to Mn oxide as a close-packed dispersed layer at 4 wt 9% Na2WO4. Strong interaction between Na2W04 and Si02 brings about phase transformation from silica gel to cristobalite upon calcination at 1023-1 123 K. The formation of a surface cluster compound as the result of surface restructuring of W 0 4 tetrahedron on the surface of Si02 with a stoichiometryof SiBWOs.5 is proposed, in which three surface bonds of W-0-Si and a double bond of W - 0 are present. This surfacecluster compound may constitute the active center of oxidative coupling of methane on this catalyst. Acknowledgment. We gratefully acknowledge the National Natural Science Foundation of China (NSFC) and the State Key Laboratory for Oxo Synthesis and Selective Oxidation (OSSO) for financial support of this work. We also thank Luo X. H. and Cong Q.Z. for the assistance with PASCA and XRD measurements, respectively.
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