Diethyl oxalate catalysts

Abstract

A highly effective catalyst for the preparation of diethyl oxalate using carbon monoxide using Pd/-Al.sub.2O.sub.3 and CeO.sub.2 as a promoter. High conversion rates with greatly extended catalyst life is achieved with the CeO.sub.2-enhanced Pd catalysts. The catalysts can be used for the production of high-value diethyl oxalate, and eventually ethylene glycol, from coal-derived syngas.

Claims

1. A highly effective catalyst for the preparation of diethyl oxalate using carbon monoxide, comprising: an active component; a carrier; and a promoter agent, wherein: the active component is palladium present in the form of particles and the particles have an average surface area within the range of 2 m.sup.2/g and 12 m.sup.2/g; the carrier is -alumina: the promoter agent is CeO.sub.2 the loading of CeO.sub.2 within 0.15 and 0.25 percent by weight; and the catalyst demonstrates a conversion rate of at least about 62 percent.

2. The catalyst of claim 1, wherein the loading of active component is within the range of 0.1 and 1 percent by weight.

3. The catalyst of claim 2, wherein the loading of CeO.sub.2 is 0.2 percent by weight.

4. The catalyst of claim 1, wherein the active component particles have an average size within the range of 2 and 80 nm and an average surface area within the range of 3 m.sup.2/g and 8 m.sup.2/g.

5. The catalyst of claim 1 wherein the carbon monoxide is from coal-derived syngas.

6. A method of improving the conversion rate of carbon monoxide to diethyl oxalate by at least fifty percent by catalysts comprising: an active component; a carrier; and a promoter agent, wherein: the active component is palladium present in the form of particles with an average surface area within the range of 3 m.sup.2/g and 8 m.sup.2/g; the carrier is -alumina; and the promoter agent is CeO.sub.2, the loading of CeO.sub.2 within 0.07 and 0.6 percent by weight.

7. The method of claim 6, wherein the active component is present at about 0.8 to 0.9 percent by weight.

8. The method of claim 7, wherein the active component particles have a size of between about 9 and 17 nm.

9. The method of claim 8, wherein the carbon monoxide is from coal derived syngas.

10. The method of claim 4, wherein the active component particles have a size of between about 9 and 17 nm.

11. The method of claim 10, wherein the active component is present between about 0.7 and 1 percent by weight.

12. The method of claim 11, wherein the active component is present at about 0.8 percent by weight and the promoter is present at about 0.2 percent by weight.

13. A method for producing diethyl oxalate comprising: providing carbon monoxide; providing ethyl nitrite; providing a catalyst, the catalyst comprising: a palladium active agent, wherein: the palladium is present in the form of particles; the particles have an average surface area between 2 m.sup.2/g; and 12 m.sup.2/g and the palladium loading is between 0.8 and 0.9% wt.; a CeO.sub.2 promoter, the CeO.sub.2 loading being between 0.15 and 0.25% wt.; and an -alumina carrier; and reacting the carbon monoxide and ethyl nitrite in the presence of the catalyst, wherein the reaction has a carbon monoxide conversion rate greater than about 62% and an ethyl nitrite conversion rate of about 92%.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic diagram of the apparatus used in the conversion of ethylene nitrite to diethyl oxalate.

(2) FIG. 2 shows the TEM results (a) and the size distribution of the Pd/-Al.sub.2O.sub.3 catalyst (b) and the TEM results (c) and size distribution of the PdCeO.sub.2/-Al.sub.2O.sub.3 catalyst (d).

(3) FIG. 3 shows the Pd 3d XPS spectra of catalysts Pd/-Al.sub.2O.sub.3(a) before and (b) after reaction and PdCeO.sub.2/-Al.sub.2O.sub.3(c) before and (d) after reaction.

(4) FIG. 4(a) is a chart of the conversion of CO (blue lines) and EN (red lines) of CO oxidative coupling to DEO with different catalysts within 72 h; FIG. 4(b) DEO selectivity of CO oxidative coupling to DEO with different catalysts within 72 h.

(5) FIG. 5 are graphs of the in situ FTIR spectra for the CO oxidative coupling to DEO reaction with Pd/-Al.sub.2O.sub.3(a) and PdCeO.sub.2/-Al.sub.2O.sub.3(b).

(6) FIG. 6(a) is a graph of the conversion of CO (blue lines) and EN (red lines) of CO oxidative coupling to DEO at different reaction temperatures (reaction conditions: 3.5 g of PdCeO.sub.2/-Al.sub.2O.sub.3 catalyst, 1200 h-1 of gas hourly space velocity; reactant's volume ratio CO/EN is 1.2, 0.1 Mpa), and FIG. 6(b) is a chart of the DEO selectivity of CO oxidative coupling to DEO with different reaction temperatures.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(7) As used herein, a promoter or enhancer is a chemical that is added to a catalyst to improve the performance of the catalyst in catalyzing a chemical reaction.

(8) The Pd catalysts promoted by CeO.sub.2 of the present invention had optimal performance characteristics when used at a Pd loading of between 0.1 wt % and 1.2 wt %, more preferably between 0.4 wt % and 1.4 wt %, and even more preferably between 0.7 wt % and 1.0 wt %.

(9) The Pd catalysts promoted by CeO.sub.2 of the present invention had optimal performance characteristics when used at a CeO.sub.2 loading of between 0.02 wt % and 1 wt %, more preferably between 0.07 wt % and 0.6 wt %, and even more preferably between 0.15 wt % and 0.25 wt %.

(10) The Pd catalysts promoted by CeO.sub.2 of the present invention had optimal performance characteristics when the Pd particles had an average size of between 2 nm and 80 nm, more preferably between 5 nm and 40 nm, and even more preferably between 9 nm and 17 nm.

(11) The Pd catalysts promoted by CeO.sub.2 of the present invention had optimal performance characteristics when the Pd particles had an average surface area of between 1 m.sup.2/g and 20 m.sup.2/g, more preferably between 2 m.sup.2/g and 12 m.sup.2/g, and even more preferably between 3 m.sup.2/g and 8 m.sup.2/g.

(12) The Pd catalysts promoted by CeO.sub.2 of the present invention maintained a high conversion rate for a time between 2 and 100 times longer, more preferably between 10 and 70 times longer, and even more preferably between 30 and 55 times longer, than Pd catalysts that had not been promoted with CeO.sub.2.

(13) Where ranges are used in this disclosure, the end points only of the ranges are stated so as to avoid having to set out at length and describe each and every value included in the range. Any appropriate intermediate value and range between the recited endpoints can be selected. By way of example, if a range of between 0.1 and 1.0 is recited, all intermediate values (e.g., 0.2, 0.3, 6.3, 0.815 and so forth) are included as are all intermediate ranges (e.g., 0.2-0.5, 0.54-0.913, and so forth).

Example 1

(14) Materials

(15) Palladium (II) chloride (PdCl.sub.2, ReagentPlus, 99%), potassium hexachloropalladale (IV) (K.sub.2PdCl.sub.6), potassium chloride (KCl), L-ascorbic acid, aluminum oxide (fused, powder, a-phase, 325 mesh), ammonium cerium nitrate (CeH.sub.8N.sub.8O.sub.18) and polyvinylpyrrolidone (PVP, Mw: 40 K) were purchased from Sigma-Aldrich and used as received without further purification.

(16) Preparation of Catalysts

(17) Pd/-Al.sub.2O.sub.3:

(18) In a typical synthesis, K.sub.2PdC.sub.16 (0.25 mmol). PdCb (025 mmol), KCl (7.5 mmol), L-ascorbic acid (1 mmol) and PVP (10 mmol) were dissolved in 100 mL of deionized water. The mixed solution was refluxed at 100 C. for 3 h. Al.sub.2O.sub.3 (5 g) were added in the solution and the mixture was stirred at 60 C. until all the solvents evaporated. The product was washed with ethanol/water and ethanol for several times and vacuum dried at 60 C. overnight.

(19) CeO.sub.2/-Al.sub.2O.sub.3:

(20) Ammonium cerium nitrate (0.05 mmol) was dissolved into 10 mL of deionized water. Al.sub.2O.sub.3 (5 g) were added in the solution and the mixture was stirred at 60 C. until all the solvents evaporated. The mixture was dried at 100 C. overnight and then calcined at 500 C. for 5 h to get CeO.sub.2/-Al.sub.2O.sub.3.

(21) PdCeO.sub.2/-Al.sub.2O.sub.3 Catalysts:

(22) 025 mmol K.sub.2PdCl.sub.6, 0.25 mmol PdCl.sub.2, KCl (7.5 mmol), L-ascorbic acid (1 mmol), and PVP (10 mmol) were dissolved in 100 mL of deionized water, followed by refluxing at 100 C. for 3 hrs. Then CeO.sub.2/-Al.sub.2O.sub.3 (5 g) was added to the solution and the mixture was stirred at 60 C. until the complete evaporation of all the solvents. The resulting mixture was washed with ethanol/water and then ethanol for several times, and vacuum dried at 60 C. for 12 h.

(23) Characterization of Catalysts

(24) The surface areas of the catalysts were measured using nitrogen physisorption by standard Brunauer-Emmet-Teller (BET) analysis (MicromeriticsTriStar 3000 V 6.04 A). The surface areas were calculated with TriStar II 3000 software.

(25) X-ray photoelectron spectroscopy (XPS) was measured with a Physical Electronics ESCA 5800 spectrometer which is equipped with a monochromatic Al Ka X-ray source (E=1486.6 eV). The scanning step and working pressure were 0.1 eV and 210.sup.9 mbar, respectively. Binding energies were calibrated to C1s peak at 284.5 eV.

(26) X-ray diffraction (XRD) analyses were performed on a Rigaku Smartlab X-ray diffraction system. A Cu K radiation source (=1.392 ) working at 40 kV and 40 mA was used in the tests. The range of 28 measurements was between 20 to 80 with 0.02 steps.

(27) The morphology and the particle size as well as the dispersion of the catalysts were studied by a scanning electron microscope (SEM-EDS, FEI Quanta FEG MK2; Oxford Instruments America, Model #51-XMX0005) and a transmission electron microscopy. (TEM, FEI, Tecnai G2 F20 S-Twin 200 kV). Samples for TEM observations were prepared by dispersing the catalysts in ethanol and drying one drop of the solution on copper grids. The particle size distribution was calculated by using Image J software.

(28) In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements were performed on a Nicolet iS50 FT-IR spectrometer (Thermo Scientific, Waltham, Mass., USA) equipped with a DRIFTS system with reaction chamber (Praying Mantis model, Barrick Scientific Products, Pleasantville, N.Y., USA) employing ZnSe windows. After loading 20 L of powder samples, the cell temperature was raised to 140 C. and allowed to thermally equilibrate for ten minutes, after which the gaseous reactants were introduced at constant flow with pressure inside the reaction chamber maintained at 2 atm. The spectra are a composite of 16 scans recorded at a resolution of 4 cm.sup.1.

(29) Synthesis

(30) The measurement of catalytic activity was performed in a fix-bed continuous flow reactor. The reactor configuration is shown schematically in FIG. 1. The catalyst (3.5 g, 2 mL) was sandwiched with sands in as stainless steel tube reactor with an inner diameter of 1.2 cm. The reactant gases (18% CO, 15% CH.sub.3CH.sub.2ONO, and N.sub.2 balance) were passed through the reactor at a gas hourly space velocity (GHSV) of 600-3000 h.sup.1. The DEO production was carried out at a temperature of 100-160 C. and under a pressure of 0.1 MPa. A thermocouple was inserted into the catalyst bed to detect the reaction temperature. Therefore, the temperature reported here is the catalyst bed temperature. A SRI 8610C gas chromatography (GC) fitted with a capillary column (Restek MXT-1 60 m0.53 mm5 m) and a molecular sieves column (MS 13) was used to analyze the tail gases. Gas chromatography/mass spectroscopy (GC-MS) for liquid products were performed with an Agilent Technology 7890A GC system with a split-splitless injector (model 7863 Auto-sampler Injector) fitted with a 5957C VLMSD MS system using a capillary column (HP-5MS, 30 m0.250 mm0.25 m). When the GC analysis results show that the reaction is in steady state, the sample cylinder vessel is emptied and the start time of the reaction is recorded. Samples from the sample cylinder vessel are collected in every 6 hours and the liquid products were collected and analyzed by offline GC-MS.

(31) Results and Discussion

(32) Characteristics of Catalysts

(33) The textural characteristics of PdCeO.sub.2/-Al.sub.2O.sub.3 catalyst were investigated by TEM, STEM and SEM (FIG. 2). TEM images presented in FIGS. 2a and 2c clearly indicate that the Pd nanoparticles of PdCeO.sub.2/-Al.sub.2O.sub.3 are highly dispersed on the -Al.sub.2O.sub.3 while the dispersion of Pd nanoparticles of Pd/-Al.sub.2O.sub.3 is poor. Moreover, the results in FIGS. 2b and 2d show that the average Pd nanoparticles size of the PdCeO.sub.2/-Al.sub.2O.sub.3 catalyst is 13.2 nm which is smaller than that of the Pd/-Al.sub.2O.sub.3 catalyst (17.3 nm). The Pd nanoparticles size distribution of the PdCeO.sub.2/-Al.sub.2O.sub.3 catalyst is narrower than that of the Pd/-Al.sub.2O.sub.3 catalyst in the meanwhile. CeO.sub.2 was difficult to detect by TEM maybe due to its low loading concentration. However, the red circles in Fig. S 4a indicate the dispersion of CeO.sub.2 on the -Al.sub.2O.sub.3 support, which is confirmed by energy dispersive X-ray (EDX) spectra. The dispersion of CeO.sub.2 particles was not as good as Pd particles. They were aggregated into large nanoparticles with the diameters around 200 nm. In summary of the results from TEM and SEM, it can be concluded that the promoter CeO.sub.2 not only promotes the dispersion of Pd on the support, but also decreases the nanoparticle size of Pd.

(34) The two catalysts, Pd/-Al.sub.2O.sub.3 and PdCeO.sub.2/-Al.sub.2O.sub.3, were detected with XPS (Pd 3d) before and after the reaction with CO and EN at 140 C. (FIG. 3). Although there were small differences between Pd/-Al.sub.2O.sub.3 and PdCeO.sub.2/-Al.sub.2O.sub.3, the obtained Pd 3d.sub.3/2 and Pd 3d.sub.5/2 values for both Pd(0) and Pd(II) were consistent with the published literatures. [32-34] In FIGS. 3a and 3c, both the Pd 3d.sub.5/2 and Pd 3d.sub.3/2 of catalysts Pd/-Al.sub.2O.sub.3 and PdCeO.sub.2/-Al.sub.2O.sub.3 are around 335 and 340 eV, respectively, which indicates that the oxidation state of Pd in the catalysts is Pd(0). However, after reaction, two new peaks appeared in both the two catalysts (FIGS. 3c and 3d), which are assigned to Pd(II), [34] indicating that some Pd(0) in the two catalysts was oxidized to Pd(II) by methyl nitrite to form an intermediate. CH30-Pd(II)-OCH3. [13] The peaks area of the Pd(II) in FIG. 3d is much bigger than the peaks area in FIG. 3b, which indicates that more intermediate were generated on the surface of PdCeO.sub.2/-Al.sub.2O.sub.3 catalyst, and therefore PdCeO.sub.2/-Al.sub.2O.sub.3 may have higher catalytic activity with the addition of CeO.sub.2. Furthermore, the percentage of the Pd on both catalysts was calculated using the peaks area of the XPS, the PdCeO.sub.2/-Al.sub.2O.sub.3 catalyst showed higher Pd concentration (0.92%) than that of the Pd/-Al.sub.2O.sub.3 catalyst (0.81%), which strongly suggests that the promoter CeO.sub.2 can also enhance the Pd loading concentration on the support. XPS was also detected to further confirm the XPS results. However, no detectable CeO.sub.2 or Pd peak was found maybe due to their low concentrations and the high dispersion of Pd.[28]

(35) Factors Affecting the Conversion of CO/EN to DEO

(36) Addition of CeO.sub.2

(37) The catalytic performances of the two catalysts were evaluated under the same conditions. With the addition of CeO.sub.2, the conversion of CO and EN was increased from 39% to 65% and 64% to 92%, respectively (FIG. 4a). There is 50% more of conversion for both of the reactants. The STY of DEO with PdCeO.sub.2/-Al.sub.2O.sub.3 was also greatly increased, which is 60% higher than that of Pd/-Al.sub.2O.sub.3 at 140 C. (Table 1).

(38) TABLE-US-00001 TABLE 1 CO oxidative coupling to DEO with different catalysts.sup.a Pd Ce content content Conversion.sup.b Selectivity STY Catalysts (wt %) (wt %) (%) (%) (gL.sup.1h.sup.1) Pd/-Al.sub.2O.sub.3 0.8 39 95 195 PdCeO.sub.2/ 0.8 0.15 65 93 318 -Al.sub.2O.sub.3 CeO.sub.2/-Al.sub.2O.sub.3 0.2 .sup.aReaction conditions: 3.5 g of catalyst, 1200 h.sup.1 of gas hourly space velocity (GHSV), reactants' volume ratio CO/EN is 1.2 0.1 Mpa, 140 C. .sup.bConversion of CO

(39) In the meanwhile, the selectivity of DEO with these two catalysts was almost the same (around 92%). Since there was no catalytic activity found for the catalyst CeO.sub.2/-Al.sub.2O.sub.3, the CeO.sub.2 plays an important role as a promoter and the interaction of CeO.sub.2 with Pd was responsible for the high activity and selectivity in CO oxidative coupling to DEO. Most of all, the catalytic activity of catalyst PdCeO.sub.2/-Al.sub.2O.sub.3 can be maintained for at least 72 h (FIG. 4b), which lays a good foundation for long-term stability.

(40) FIG. 5 illustrate the in situ DR-FTIR spectra for the reaction of CO with ethyl nitrite to DEO. The band at 1774 cm.sup.1 is attributed to the CO stretching vibrations of the DEO product. It is important to note that the intensity of the band at 1774 cm.sup.1 in the spectrum of FIG. 5b is stronger than that in the spectrum of FIG. 5a and from the integration results of the two peaks, the peak area of PdCeO.sub.2/-Al.sub.2O.sub.3 catalyst is 20% bigger than that of the catalyst without CeO.sub.2, which demonstrates the superior catalytic activity of PdCeO.sub.2/-Al.sub.2O.sub.3 relative to Pd/-Al.sub.2O.sub.3, consistent with the results of catalytic activity evaluation and TEM, XPS results.

(41) Temperature

(42) FIG. 6a shows the effect of temperature on both EN and CO conversion where PdCeO.sub.2/-Al.sub.2O.sub.3 was used as the catalyst for CO oxidative coupling to DEO reaction. The conversion of EN and CO became higher with the temperature increasing, especially when the temperature increased from 120 C. to 140 C. Both EN and CO conversion increased 20% accordingly. However, with the temperature increasing, the selectivity of DEO had almost no change except when the temperature reached to 180 C. Both the CO conversion and DEO selectivity decreased at 180 C. due to the decomposition of the EN. The PdCeO.sub.2/-Al.sub.2O.sub.3 catalyst can be optimized to achieve a lower reaction temperature with high DEO selectivity.

(43) Conclusion

(44) In summary, a low Pd loading Pd/-Al.sub.2O.sub.3 nanocatalyst with 0.8% Pd (wt %) loading and the average Pd size of 13.2 nm was synthesized for CO oxidative coupling to DEO. After the introduction of 0.2 wt % CeO.sub.2, PdCeO.sub.2/-Al.sub.2O.sub.3 catalyst showed remarkably higher catalytic activity compared with the catalyst without CeO.sub.2. The CO conversion was increased SO % more (from 39% to 62%) with the DEO selectivity higher than 90% when the CeO.sub.2 was used as a promoter and, importantly, the high activity and selectivity could be maintained up to 72 h without visible decrease. TEM results showed clearly that CeO.sub.2 not only improved the dispersion of palladium on the surface of the support but also decreased the palladium size as well, thus resulted in the excellent catalytic activity. In consideration of the facile synthesis and low Pd loading of this catalyst as well as the insecurity factors of methyl nitrite, this highly efficient and stable nanocatalyst may have a promising industrial application, especially in the US, of the coal to ethylene glycol.

(45) The foregoing exemplary descriptions and the illustrative preferred embodiments of the present invention have been explained in the figures and tables herein and attached to this application and have been described in detail, with varying modifications and alternative embodiments being taught. While the invention has been shown, described and illustrated herein, it should be understood by those skilled in the art that equivalent changes in form and detail may be made therein without departing from the spirit and scope of the invention, and that the scope of the present invention is to be limited only as precluded by prior art. Moreover, the invention as described and disclosed herein may be suitably practiced in the absence of the specific elements which are described and disclosed herein.

REFERENCES

(46) [1] W. Kotowski, J. Freiberg, W. Spisak, S. Zamorow skabiemacik, Przemysl Chemiczny, 68 (1989) 73-76. [2] D. F. Othmer, M. S. Thakar, Industrial and Engineering Chemistry, 50 (1958) 1235-1244. [3] T. J. Zhao, D. Chen, Y. C. Dai, W. K. Yuan, A. Holmen, Industrial & Engineering Chemistry Research, 43 (2004) 4595-4601. [4] E. Jin, Y. Zhang, L. He, H. G. Harris, B. Teng, M. Fan. Applied Catalysis A: General, 476 (2014) 158-174. [5] F. Zhang, D. Xu, Y. Wang, X. Guo, L. Xu, M. Fan, Applied Energy, 130 (2014) 1-6. [6] S. N. Naik, V. V. Goud, P. K. Rout, A. K. Dalai, Renewable & Sustainable Energy Reviews, 14 (2010) 578-597. [7] A. Kumar, D. D. Jones, M. A. Hanna, Energies, 2 (2009) 556-581. [8] M. E. Dry, Abstracts of Papers of the American Chemical Society, 219 (2000) U254-U254. [9] H. Y. Song, R. H. Jin, M. R. Kang, J. Chen. Chinese Journal of Catalysis, 34 (2013) 1035-1050. (10] Q. L. Chen, W. M. Yang, J. W. Teng, Chinese Journal of Catalysis, 34 (2013) 217-224. [11] J. S. Bae, L. S. Hwang, Y. J. Kweon, Y. C. Choi, S. J. Park, H. J. Kim, H. Jung, C. Han, Korean Journal of Chemical Engineering, 29 (2012) 868-875. [12] F. X. Li, L. S. Fan, Energy & Environmental Science, 1 (2008) 248-267. (13] Z. N. Xu, J. Sun, C. S. Lin, X. M. Jiang, Q. S. Chen, S. Y. Peng, M. S. Wang, G. C. Guo, Acs Catalysis, 3 (2013) 118-122. (14] F. D. Meng, G. X. Xu, R. Q. Guo, H. F. Yan, M. Q. Chen, Chemical Engineering and Processing, 43 (2004) 785-790. (15] B. Sadeghi, S. Ghamami, Chemical Engineering Communications 200 (2013) 178-184. (16] C. W. Jiang, Z. W. Zheng, Y. P. Zhu, Z. H. Luo, Chemical Engineering Research & Design, 90 (2012) 915-925. (17] X. C. Gao, Y. J. Zhao, S. P. Wang, Y. L. Yin, B. W. Wang, X. B. Ma, Chemical Engineering Science, 66 (2011) 3513-3522. (18] Z. H. Gao, C. Q. Hu, Z. H. Li, F. He, G. H. Xu, Chinese Journal of Catalysis, 25 (2004) 205-209. (19] Z. H. Gao, Q. Wu, F. He, Z. H. Li, G. H. Xu, Chinese Journal of Catalysis, 23 (2002) 95-98. (20] Q. Wu, Z. H. Gao, F. He, Z. H. Li, G. H. Xu, Chinese Journal of Catalysis, 24 (2003) 289-293. [21] G-H. Xu, Y. C. Li, Z. H. Li, H. J. Wang, Industrial & Engineering Chemistry Research, 34 (1995) 2371-2378. [22] Q. Lin, X. G. Zhao, W. Bi, W. D. Xiao, Chinese Journal of Catalysis, 27 (2006) 911-915. [23] S. Y. Peng, Z. N. Xu, Q. S. Chen, Y. M. Chen, J. Sun, Z. Q. Wang, M. S. Wang, G. C. Guo, Chemical Communications, 49 (2013) 5718-5720. [24] X. Gao, Y. P. Zhu, Z. H. Luo, Chemical Engineering Science, 66 (2011) 6028-6038. [25] Z. H. Gao, Z. C. Liu, F. He, G. H. Xu, Journal of Molecular Catalysis a-Chemical, 235 (2005) 143-149. [26] Q. Lin. Y. Ji, Z. D. Jiang, W. D. Xiao, Industrial & Engineering Chemistry Research, 46 (2007) 7950-7954. [27] Y. Yamamoto, T. Matsuzaki, S. Tanaka, K. Nishihira, K. Ohdan, A. Nakamura, Y. Okamoto, Journal of the Chemical Society-Faraday Transactions, 93 (1997) 3721-3727. [28] X. G. Zhao, Q. Lin, W. D. Xiao, Applied Catalysis a-General, 284 (2005) 253-257. [29] G. L. Zhuo, X. Z. Jiang, Chinese Journal of Catalysis, 24 (2003) 509-512. [30] F. D. Meng, G. H. Xu, Q. R. Guo, Journal of Molecular Catalysis a-Chemical, 201 (2003) 283-288. [31] Z. H. Li, Y. Song, P. Du, X. B. Ma, B. W. Wang, G. H. Xu, Reaction Kinetics and Catalysis Letters, 73 (2001) 135-142. [32] A. Tressaud, S. K. hairoun, H. Touhara, N. Watanabe, Zeitschrift Fur Anorganische Und Allgemeine Chemie, 541 (1986) 291-299. [33] C. J. Jenks, S. L. Chang, J. W. Anderegg, P. A. Thiel, D. W. Lynch. Physical Review B, 54 (1996) 6301-6306. [34] W. E. Moddeman, W. C. Bowling, D. C. Carter, D. R. Grove, Surface and Interface Analysis, 11 (1988) 317-326.