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In situ Raman spectroscopy studies on the methanol oxidation over silver surface
Applied Surface Science 120 Ž1997. 99–105
In situ Raman spectroscopy studies on the methanol oxidation over silver surface Jinhai Wang a , Xinhua Xu a , Jingfa Deng a
a,)
, Yuanyan Liao b, Bifeng Hong
b
Surface Chemistry Laboratory, Department of Chemistry, Fudan UniÕersity, 220 Handan Road, Shanghai 200433, China b State Key Laboratory for Physical Chemistry of Solid Surface, Xiamen UniÕersity, Xiamen 361005, China Received 10 March 1997; accepted 23 May 1997
Abstract In situ surface Raman spectroscopy has been employed to investigate the oxidation of methanol on silver. The intermediates at various temperatures were identified. According to the results, the reaction pathways in the practical process were suggested and compared with the conclusion obtained under UHV condition previously. They showed a remarkably close agreement except the temperature at which the surface intermediates exist. All these results proved that the mechanism of methanol oxidation obtained under UHV condition is effective under the industrial conditions. q 1997 Elsevier Science B.V.
1. Introduction The reactions of methanol on metal surfaces are of interest in surface science w1–18x. On silver surface, in terms of its actual application in the manufacture of formaldehyde w19x, much of interest has been obtained. On the single crystalline AgŽ110. surface, Wachs and Madix w2x examined the reaction of methanol with oxygen below 180 K and observed that adsorbed atomic oxygen assisted the adsorption of methanol to yield water and methoxide species ŽCH 3 Oa .. Formaldehyde was produced upon decomposition of methoxide species along with H 2 and CH 3 OH. Felter et al. employed XPS and UPS to study methanol oxidation on AgŽ111. w3x. When active oxygen was initially on the surface, they observed the surface intermediates including formal)
Corresponding author. Tel.: q86-21-65492222; fax: q86-2165341642; e-mail: [email protected].
dehyde and methoxide by employing XPS and the kinetics of oxidation reaction can be described by Langmuir–Hinshelwood mechanism. Gellman and his co-workers w7,20x also found that the preadsorbed atomic oxygen on AgŽ110. could react with methanol to form methoxide. On a practical catalyst, Bazilio w6x studied the reaction kinetics by IR and observed that CH 3 Oa is an important intermediate. In addition, Bao and Deng investigated the surface chemistry of commercial electrolytic silver intensively w4,5,21x. By TPRS w4x and HREELS w5x, they revealed that on the electrolytic silver surface with preadsorbed oxygen, the interaction between methanol with preadsorbed atomic oxygen species led to the breaking of an OH bond and the formation of adsorbed methoxide species at 180 K. Subsequent heating resulted in the decomposition of methoxide species in different reaction channels. Although these results have suggested that the oxidation of methanol occurred through a methoxide
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J. Wang et al.r Applied Surface Science 120 (1997) 99–105
intermediate, it is not unambiguous to conclude the configuration of the intermediates and the reaction pathway in a practical reaction yet. Because the practical process is carried out at a much higher temperature Ž873–973 K. and a much higher partial pressure of reactant, which is far beyond the condition of the studies mentioned above. In order to settle the speculations of this reaction directly, it would be particularly valuable to carry out the studies of reaction intermediates with in situ technology under practical conditions. In this case, the in situ Raman spectroscopy is a suitable tool because it can provide information about the characteristic vibrational modes of surface adsorbed intermediates. In this paper, we report the results of in situ Raman spectroscopy studies of methanol oxidation on a commercial electrolytic silver catalyst under practical conditions. The results presented below can be correlated with those obtained from the UHV condition.
bubbling pure nitrogen through reagent-grade liquid ethanol at 608C under atmospheric pressure.
3. Results 3.1. Raman spectra for oxygen species under practical condition In our previous work w22x, the oxygen species adsorbed on electrolytic silver has been studied by in situ Raman spectroscopy. We found that under ambient pressure the adsorption behavior of oxygen is more complex than that under UHV conditions and the oxygen species formed are more stable. As shown in Fig. 1a, the exposure of oxygen to silver surface causes four Raman bands at 463, 602, about 800 and 1054 cmy1 . These bands correspond to subsurface oxygen species, peroxide oxygen species, atomically adsorbed oxygen species and superoxide oxygen species, respectively. When the temperature is raised, the peroxide and superoxide oxygen species diminish
2. Experimental As described in our previous papers w22,23x, the Raman spectra were obtained on a Jobin Yvon U1000 laser Raman spectrometer with a specially designed Raman sample cell for in situ catalysis studies. The sample was put in a quartz tube surrounded with an electric furnace. Its temperature was controlled by a temperature-programmer and measured by a Ni–CrrNi–Si thermocouple inserted in the sample. The experimental conditions are: l 0 s 514.5 nm, laser power: 150 mW, resolution: 2 cmy1 and time constant: 0.2 s. The electrolytic silver of 99.999% purity Žemployed as industrial catalyst. was obtained by means of triple electrolytic refining of silver w4,24x. The grain size was 40–60 mesh. The silver sample was first heated to 973 K and kept at this temperature for 30 min in a mixed oxygenrnitrogen ŽO 2rN2 s 1r4. flow. Then the oxygen was cut off and the sample was kept in pure N2 flow at 973 K for another 30 min and finally cooled to room temperature. By these treatments, no obvious vibrational bands were shown in the Raman spectra of the sample. The oxygenrnitrogen flow mentioned above was employed as an oxygen source and methanol was carried onto the silver surface by
Fig. 1. Raman spectra of adsorbed oxygen on electrolytic silver at various temperatures: Ža. at room temperature Ž ; 300 K., Žb. at 473 K, Žc. at 673 K and Žd. at 873 K.
J. Wang et al.r Applied Surface Science 120 (1997) 99–105
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gradually ŽFig. 1b and c.. When the sample is heated to 873 K, there are still two Raman bands at 463 and about 800 cmy1 ŽFig. 1d and Fig. 2a.. This result indicates that the subsurface oxygen species and the atomically adsorbed oxygen species exist on the silver surface at 873 K w22x. When a pure N2 flow saturated with methanol steam is passed through the silver sample at 873 K, only the Raman band for subsurface oxygen species remains ŽFig. 2b.. The another band for adsorbed atomic oxygen species at about 800 cmy1 , diminishes completely. It means that the atomic oxygen species is consumed in the reaction with methanol. Thus, we can deduce that the atomic oxygen species is the active surface one in the practical process. This is in agreement with the previous results w2,4,5x. When an O 2rCH 3 OH mixture flow is passed through the silver surface, the band at about 800 cmy1 appears again ŽFig. 2c.. This result indicates that the gaseous oxygen flow can supply the consumption of atomic adsorbed oxygen in the reaction. Fig. 2. Raman spectra of adsorbed oxygen on electrolytic silver at 873 K under various conditions: Ža. in pure N2 flow, Žb. after CH 3 OH is passed through and Žc. in O 2 rCH 3 OH flow.
3.2. Raman spectra for methanol interaction with adsorbed oxygen The silver sample is exposed in an O 2 flow, swept by a N2 flow, exposed in a N2rCH 3 OH flow and swept by a N2 flow sequentially. The Raman spectra of heating silver samples to various temperatures are shown in Figs. 3–5. In Fig. 3, the bands at 460, 602, 794 and 1054 cmy1 are attributed to various adsorbed oxygen species formed at room temperature Ž; 300 K. w22x. On the other hand, new vibrational bands at 683, 972, 1319, 1482, 1738, 2083, 2846 and 3105 cmy1 are present due to the interaction between methanol and adsorbed atomic oxygen species. In order to assign these vibrational bands, the data of the methoxide species ŽI. and formate species ŽII. are given in Tables 1 and 2, respectively.
Fig. 3. Raman spectra of the interaction between methanol and oxygen at room temperature Ž ; 300 K..
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Fig. 4. Raman spectra of the interaction between methanol and oxygen at 473 K.
Fig. 5. Raman spectra of the interaction between methanol and oxygen at 673 K.
Relying upon the agreement of the vibrational frequencies with those in Tables 1 and 2, the bands can be interpreted as follows: n ŽC–O. for ŽI.: 972 cmy1 , nsŽOCO. for ŽII.: 1319 cmy1 , d ŽC–H. for both: 1482 cmy1 , nas ŽOCO. for ŽII.: 1738 cmy1 and n ŽC– H. for both: 2846 cmy1 . The band at 2083 cmy1 is ascribed to the vibration of CO adsorbed on the surface. The band at 3105 cmy1 corresponds to the stretching of a ‘softened’ OH bond of adsorbed hydroxide on silver, which agrees with the HREELS assignment reported by Wu for single crystalline
AgŽ111. w37x and Bao for electrolytic silver w5x. The band at 683 cmy1 is difficult to identify, since both d ŽOCO. and d ŽOH. are available in the range 600 to 900 cmy1 . Thus, in Fig. 3 we can see that at room temperature, both methoxide species ŽI. and formate species ŽII. are present on silver surface. And since the intensity of n ŽC–O. is much higher than that of n ŽOCO., the dominant configuration of the surface intermediate is the methoxide species. The Raman spectrum at 673 K ŽFig. 5. is quite different. The band at 972 cmy1 corresponding to
Table 1 Vibrational frequencies Žin cmy1 . and mode assignments for methoxide species on various metal surfaces Mode
PtŽ111. w10x
PdŽ100. w25x
RhŽ100. w26x
RuŽ100. w27x
FeŽ100. w28x
NiŽ110. w29,30x
CuŽ100. w9x
ZnŽ0001. w31x
AgŽ110. w20x
naŽCH. nsŽCH. d ŽCH. r ŽCH. n ŽCO. n ŽM–OMe.
2910 2910 1430 — 1000 370
2905 2905 1455 — 997 —
2960 2890 1440 1115 1005 400
2955 2810 1435 1140 1005 325
2895 2895 1430 — 1020 405
2910 2790 1440 1150 1030 400
2910 2830 1450 — 1010 290
2995 2885 1485 — 1030 510
2915 2915 1450 1150 1040 330
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Table 2 Vibrational frequencies Žin cmy1 . and mode assignments for formate species on various surfaces and for HCOOy in sodium salt Mode
AgŽ111.r 400 K w32x
CuŽ100.r 400 K w14x
CuŽ110.r 400 K w33x
AgŽ110.r 200 K w34x
HCOOyr Ag w35x
HCOOy, sodium salt w36x
Electrolytic silverr473 K
n ŽCH. naŽOCO. nsŽOCO. d ŽCH. d ŽOCO. n ŽM–O.
2970, 2840 1680 1312 1488 750 —
2910, 2840 1460 1330 — 772 340
2940, 2840 1560 1360 1377 760 —
2900 1640 1340 — 770 280
2840, 2920 1550 1340 — 775 290
2841 1567 1360 1377 760
2857 1657 1306 1411 nr nr
n ŽC–O. for methoxide, disappears completely. While bands corresponding to ns and nas of the O–C–O mode remains at 1294 and 1629 cmy1 . Meanwhile, the n ŽC–H. at 2913 cmy1 and d ŽC–H. at 1483 cmy1 also exist. The disappearance of the stretching of C–O and the existence of the characteristic stretching of the O–C–O mode shows evidence that at 673 K, the dominant configuration of the surface intermediate is the formate species ŽII.. Comparing Fig. 4 with Figs. 3 and 5, we can see that both n ŽC–O. and n ŽOCO. obviously existed at 473 K. So two types of intermediates, methoxide and formate species, are available on a silver surface at this temperature. By heating the sample to 873 K, there are only two Raman bands at 468 and 2081 cmy1 on the surface, which are assigned to subsurface oxygen species and adsorbed carbon monoxide, respectively. Therefore, we can conclude that in the temperature range from 300 to 873 K, the configuration of methanol adsorbed on oxygen-covered silver changes gradually. The dominant configuration is the methoxide species ŽI. at room temperature and it becomes a formate species ŽII. at 673 K. Atomic oxygen was found to be highly reactive towards methanol. Chemisorption of methanol on the silver surface with preadsorbed oxygen caused instantaneous OH bond breaking and the formation of methoxide: CH 3 OH a ™ CH 3 Oa q OH a This reaction is proven by the detection of CH 3 Oa and the ‘softened’ OH strength mode for hydroxide in Raman spectra. Further heating of the sample leads to the decomposition of CH 3 Oa and the formation of HCHO. In the meanwhile, another intermedi-
ate, formate species, becomes the dominant intermediate on the surface. Sexton and Madix w34x had studied the oxidation of HCHO and HCOOH on AgŽ110. and proved that the decomposition of formate species gave off CO 2 , which is the major by-product of methanol oxidation. Hence, the following reaction should be proposed carrying on the surface: CH 3 Oa ™ HCHOa q H a HCHOa ™ HCHOg HCHOa q Oa ™ HCOOa q H a HCOOa ™ CO 2 g q H a The HCHOa cannot be detected directly on the silver surface, maybe its desorption and the oxidation is too rapid. 3.3. Raman spectrum of silÕer surface in oxygen r methanol mixture flow Fig. 6 shows the Raman spectrum of silver in a mixed flow of oxygen and methanol at 873 K. It gives some interesting features. The band at 990 cmy1 , which has been diminished completely above 673 K ŽFig. 5., appears again. The existence of a stretching vibration of a C–O bond shows that there is methoxide species ŽI. on the silver surface. The co-existence of bands at 1367r1672 cmy1 , corresponding to the symmetric and asymmetric stretching mode of O–C–O, gives evidence that the formate species ŽII. also adsorbed on the silver surface at 873 K. The vibrational bands at 1402 cmy1 are attributed to the bending mode of C–H. It should be noticed that besides the stretching vibration of C–H
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proceed simultaneously in the practical process. The continuous reaction also results in the existence of HCHOa on the surface.
4. Discussion The oxidation of methanol on silver surface has been investigated intensively by TPRS w2,4x, XPS, UPS w3x and HREELS w5x. All these results revealed that very little methanol chemisorbed on the oxygen-free silver surface and the ability of the silver surface to dissociatively chemisorb methanol was greatly enhanced by surface oxygen and resulted in the formation of methoxide. Further decomposition of methoxide gave off HCHO. If there is excess atomic oxygen species on the surface, HCHO will oxidize additionally to CO 2 through a formate intermediate. The reaction pathway can be summarized schematically as follows: Fig. 6. Raman spectra of the interaction between methanol and oxygen at 873 K in the methanolroxygen flow.
at 2860 cmy1 , a new band at 2702 cmy1 appears. According to the difference of the stretching mode of C–H in alcohol and in aldehyde w38x, the C–H vibration of aldehyde has two bands at 2900–2700 cmy1 but usually one of them appears at 2720 cmy1 . Therefore, it could be suggested that there are a few adsorbed formaldehyde on the surface. According to the work by Bhattacharya et al. w17x, the configuration of adsorbed formaldehyde on Pd has a weaker bond between the carbon atom and palladium atom rather than a Pd–O bond. This point with metal– carbon bonds is also suggested by Wu w37x for adsorbed formaldehyde species on AgŽ110. surface. Thus, this adsorbed formaldehyde species is not very stable and this is one of the reasons why under static conditions we cannot detect it. The existence of methoxide and formaldehyde on the surface under practical conditions is interesting because they disappeared at 873 K in the case of static experiment mentioned in Section 3.2. This indicates that the ambient reactant can supply the consumption of surface intermediate in the reaction. The reaction pathway suggested in Section 3.2 should
The mechanism we proposed from the proceeding data obtained by in situ Raman spectroscopy is essentially in agreement with that proposed earlier from TPRS and HREELS, except for the different temperature range. The results in Figs. 3–5 show no evidence of HCHOa , however, and suggested that the desorption of HCHO and the reaction HCHOa q Oa ™ HCOOa q H a occur very rapidly under these conditions. The decomposition of CH 3 Oa is correlated with the gradual disappearance of the n ŽC–O. in Figs. 3–5. The formate species is the most stable intermediate and its characteristic n ŽOCO. vibration diminishes at 873 K. A summary of the interaction process may now be given. Oxygen adsorbs on the silver surface to form an atomic oxygen species. Further exposure of
J. Wang et al.r Applied Surface Science 120 (1997) 99–105
CH 3 OH results in the OH bond breaking and the formation of methoxide. The methoxide decomposes to formaldehyde subsequently and some of the formaldehyde reacts with the excess atomic oxygen species to generate formate intermediate. The annealing of the sample results in the decomposition of methoxide gradually. Only the formate species exists on the surface at 673 K. Further raising the temperature causes the formate species to decompose to CO 2 . Finally, the reaction under the practical condition is discussed. By the in situ Raman spectrum, we can observe CH 3 Oa , HCOOa as well as trace HCHOa . Obviously, all these reactions occur simultaneously under practical conditions. At an ambient partial pressure of oxygen and methanol, the desorption and the formation of surface intermediates reaches an equilibrium. Thus, we can capture all of the intermediates at such a high temperature. The results in the present work show a remarkably close agreement to those obtained under UHV condition except for the stability of the intermediates. This is due to the great effect of the ambient pressure on the adsorption behavior. These results also prove that the mechanism of methanol oxidation obtained under UHV condition is also effective under industrial conditions.
Acknowledgements This work was supported by grants from the State Key Laboratory for Physical Chemistry of the Solid Surface at Xiamen University and National Natural Sciences Foundation of China.
References w1x M.A. Barteau, R.J. Madix, in: D.A. King, P. Woodruff ŽEds.., The Chemical Physics of Solid Surface and Heterogeneous Catalysis, vol. 4, Elsevier, Amsterdam, 1982, p. 95. w2x I.E. Wachs, R.J. Madix, Surf. Sci. 76 Ž1978. 531. w3x T.E. Telter, W.H. Weinberg, G.Y. Lastushkina, P.A. Zudan, G.K. Boreskov, J. Hrbek, Appl. Surf. Sci. 16 Ž1983. 351.
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w4x X.H. Bao, J.F. Deng, J. Catal. 99 Ž1986. 391. w5x X.H. Bao, J.F. Deng, R.S. Zhai, D.Z. Wang, X.X. Guo, Catal. Lett. 4 Ž1990. 25. w6x C.A. Bazilio, W.J. Thomas, U. Ullah, K.E. Hayes, Proc. R. Soc. London A 399 Ž1985. 181. w7x R. Zhang, A.J. Gellman, J. Phys. Chem. 95 Ž1991. 7433. w8x I.E. Wachs, R.J. Madix, J. Catal. 53 Ž1978. 208. w9x B.A. Sexton, Surf. Sci. 88 Ž1979. 299. w10x B.A. Sexton, Surf. Sci. 102 Ž1981. 271. w11x B.A. Sexton, K.P. Rendulic, A.E. Hughes, Surf. Sci. 121 Ž1982. 181. w12x S.L. Miles, S.L. Bernasek, J.L. Gland, J. Electron Spectrosc. Rel. Phen. 29 Ž1983. 239. w13x J. Hrbek, R.A. de Paola, F.M. Hoffmann, J. Chem. Phys. 81 Ž1984. 15. w14x B.A. Sexton, A.E. Hughes, N.R. Avery, Surf. Sci. 155 Ž1985. 366. w15x J. Hrbek, R.A. de Paola, F.M. Hoffmann, Surf. Sci. 166 Ž1986. 361. w16x J.L. Davis, M.A. Barteau, Surf. Sci. 187 Ž1987. 387. w17x A.K. Bhattacharya, M.A. Chesters, M.E. Pemble, N. Sheppard, Surf. Sci. 206 Ž1988. L845. w18x P.D.A. Pudney, M. Bowker, R.W. Joyner, Surf. Sci. 251r252 Ž1991. 1106. w19x Ullman Encyclopedia of Industrial Chemistry, vol. 21, Verlag Chemie, Weinheim, 1982, p. 311 w20x Q. Dai, A.J. Gellman, Surf. Sci. 257 Ž1991. 103. w21x X.H. Bao, S.Z. Dong, J.F. Deng, Surf. Sci. 199 Ž1988. 493. w22x J.F. Deng, X.H. Xu, J.H. Wang, Catal. Lett. 32 Ž1995. 159. w23x Y. Liao, P. Hong, in: Proc. 7th Chinese Light Scatt. Conf., Yichang, 1983, p. 218. w24x J.F. Deng, Y.F. Xiang, L.Y. Ye, X.Z. Zhu, J. Catal. 4 Ž1983. 70, Žin Chinese.. w25x S.W. Jorgensen, R.J. Madix, Surf. Sci. 183 Ž1987. 27. w26x J.E. Parmeter, X.D. Jiang, D.W. Goodman, Surf. Sci. 240 Ž1990. 85. w27x J. Hrbek, R.A. de Paola, F. Hoffmann, J. Chem. Phys. 81 Ž1984. 2818. w28x J.P. Lu, M. Albert, S.L. Bernasek, Surf. Sci. 218 Ž1989. 1. w29x S.R. Barre, J.A. Stroscio, W. Ho, Surf. Sci. 150 Ž1985. 399. w30x S.R. Barre, J.A. Stroscio, W. Ho, Surf. Sci. 155 Ž1985. L281. w31x P. Sen, C.N.R. Rao, Surf. Sci. 72 Ž1986. 269. w32x B.A. Sexton, Surf. Sci. 88 Ž1979. 319. w33x E.M. Stuve, R.J. Madix, Surf. Sci. 119 Ž1982. 279. w34x B.A. Sexton, R.J. Madix, Surf. Sci. 105 Ž1981. 177. w35x A.J. McQuillan, C.G. Pope, Chem. Phys. Lett. 71 Ž1980. 349. w36x K. Ho, H.J. Bernstein, Can. J. Chem. 34 Ž1956. 170. w37x K. Wu, Ph.D. Thesis, Dalian Institute of Chem. Phys., Chinese Academy of Science, 1991. w38x L.J. Bellamy, The Infrared Spectra of Complex Molecules, Chapman and Hall, London, 1975, p. 151.
In situ Raman spectroscopy studies on the methanol oxidation over silver surface Jinhai Wang a , Xinhua Xu a , Jingfa Deng a
a,)
, Yuanyan Liao b, Bifeng Hong
b
Surface Chemistry Laboratory, Department of Chemistry, Fudan UniÕersity, 220 Handan Road, Shanghai 200433, China b State Key Laboratory for Physical Chemistry of Solid Surface, Xiamen UniÕersity, Xiamen 361005, China Received 10 March 1997; accepted 23 May 1997
Abstract In situ surface Raman spectroscopy has been employed to investigate the oxidation of methanol on silver. The intermediates at various temperatures were identified. According to the results, the reaction pathways in the practical process were suggested and compared with the conclusion obtained under UHV condition previously. They showed a remarkably close agreement except the temperature at which the surface intermediates exist. All these results proved that the mechanism of methanol oxidation obtained under UHV condition is effective under the industrial conditions. q 1997 Elsevier Science B.V.
1. Introduction The reactions of methanol on metal surfaces are of interest in surface science w1–18x. On silver surface, in terms of its actual application in the manufacture of formaldehyde w19x, much of interest has been obtained. On the single crystalline AgŽ110. surface, Wachs and Madix w2x examined the reaction of methanol with oxygen below 180 K and observed that adsorbed atomic oxygen assisted the adsorption of methanol to yield water and methoxide species ŽCH 3 Oa .. Formaldehyde was produced upon decomposition of methoxide species along with H 2 and CH 3 OH. Felter et al. employed XPS and UPS to study methanol oxidation on AgŽ111. w3x. When active oxygen was initially on the surface, they observed the surface intermediates including formal)
Corresponding author. Tel.: q86-21-65492222; fax: q86-2165341642; e-mail: [email protected].
dehyde and methoxide by employing XPS and the kinetics of oxidation reaction can be described by Langmuir–Hinshelwood mechanism. Gellman and his co-workers w7,20x also found that the preadsorbed atomic oxygen on AgŽ110. could react with methanol to form methoxide. On a practical catalyst, Bazilio w6x studied the reaction kinetics by IR and observed that CH 3 Oa is an important intermediate. In addition, Bao and Deng investigated the surface chemistry of commercial electrolytic silver intensively w4,5,21x. By TPRS w4x and HREELS w5x, they revealed that on the electrolytic silver surface with preadsorbed oxygen, the interaction between methanol with preadsorbed atomic oxygen species led to the breaking of an OH bond and the formation of adsorbed methoxide species at 180 K. Subsequent heating resulted in the decomposition of methoxide species in different reaction channels. Although these results have suggested that the oxidation of methanol occurred through a methoxide
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intermediate, it is not unambiguous to conclude the configuration of the intermediates and the reaction pathway in a practical reaction yet. Because the practical process is carried out at a much higher temperature Ž873–973 K. and a much higher partial pressure of reactant, which is far beyond the condition of the studies mentioned above. In order to settle the speculations of this reaction directly, it would be particularly valuable to carry out the studies of reaction intermediates with in situ technology under practical conditions. In this case, the in situ Raman spectroscopy is a suitable tool because it can provide information about the characteristic vibrational modes of surface adsorbed intermediates. In this paper, we report the results of in situ Raman spectroscopy studies of methanol oxidation on a commercial electrolytic silver catalyst under practical conditions. The results presented below can be correlated with those obtained from the UHV condition.
bubbling pure nitrogen through reagent-grade liquid ethanol at 608C under atmospheric pressure.
3. Results 3.1. Raman spectra for oxygen species under practical condition In our previous work w22x, the oxygen species adsorbed on electrolytic silver has been studied by in situ Raman spectroscopy. We found that under ambient pressure the adsorption behavior of oxygen is more complex than that under UHV conditions and the oxygen species formed are more stable. As shown in Fig. 1a, the exposure of oxygen to silver surface causes four Raman bands at 463, 602, about 800 and 1054 cmy1 . These bands correspond to subsurface oxygen species, peroxide oxygen species, atomically adsorbed oxygen species and superoxide oxygen species, respectively. When the temperature is raised, the peroxide and superoxide oxygen species diminish
2. Experimental As described in our previous papers w22,23x, the Raman spectra were obtained on a Jobin Yvon U1000 laser Raman spectrometer with a specially designed Raman sample cell for in situ catalysis studies. The sample was put in a quartz tube surrounded with an electric furnace. Its temperature was controlled by a temperature-programmer and measured by a Ni–CrrNi–Si thermocouple inserted in the sample. The experimental conditions are: l 0 s 514.5 nm, laser power: 150 mW, resolution: 2 cmy1 and time constant: 0.2 s. The electrolytic silver of 99.999% purity Žemployed as industrial catalyst. was obtained by means of triple electrolytic refining of silver w4,24x. The grain size was 40–60 mesh. The silver sample was first heated to 973 K and kept at this temperature for 30 min in a mixed oxygenrnitrogen ŽO 2rN2 s 1r4. flow. Then the oxygen was cut off and the sample was kept in pure N2 flow at 973 K for another 30 min and finally cooled to room temperature. By these treatments, no obvious vibrational bands were shown in the Raman spectra of the sample. The oxygenrnitrogen flow mentioned above was employed as an oxygen source and methanol was carried onto the silver surface by
Fig. 1. Raman spectra of adsorbed oxygen on electrolytic silver at various temperatures: Ža. at room temperature Ž ; 300 K., Žb. at 473 K, Žc. at 673 K and Žd. at 873 K.
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gradually ŽFig. 1b and c.. When the sample is heated to 873 K, there are still two Raman bands at 463 and about 800 cmy1 ŽFig. 1d and Fig. 2a.. This result indicates that the subsurface oxygen species and the atomically adsorbed oxygen species exist on the silver surface at 873 K w22x. When a pure N2 flow saturated with methanol steam is passed through the silver sample at 873 K, only the Raman band for subsurface oxygen species remains ŽFig. 2b.. The another band for adsorbed atomic oxygen species at about 800 cmy1 , diminishes completely. It means that the atomic oxygen species is consumed in the reaction with methanol. Thus, we can deduce that the atomic oxygen species is the active surface one in the practical process. This is in agreement with the previous results w2,4,5x. When an O 2rCH 3 OH mixture flow is passed through the silver surface, the band at about 800 cmy1 appears again ŽFig. 2c.. This result indicates that the gaseous oxygen flow can supply the consumption of atomic adsorbed oxygen in the reaction. Fig. 2. Raman spectra of adsorbed oxygen on electrolytic silver at 873 K under various conditions: Ža. in pure N2 flow, Žb. after CH 3 OH is passed through and Žc. in O 2 rCH 3 OH flow.
3.2. Raman spectra for methanol interaction with adsorbed oxygen The silver sample is exposed in an O 2 flow, swept by a N2 flow, exposed in a N2rCH 3 OH flow and swept by a N2 flow sequentially. The Raman spectra of heating silver samples to various temperatures are shown in Figs. 3–5. In Fig. 3, the bands at 460, 602, 794 and 1054 cmy1 are attributed to various adsorbed oxygen species formed at room temperature Ž; 300 K. w22x. On the other hand, new vibrational bands at 683, 972, 1319, 1482, 1738, 2083, 2846 and 3105 cmy1 are present due to the interaction between methanol and adsorbed atomic oxygen species. In order to assign these vibrational bands, the data of the methoxide species ŽI. and formate species ŽII. are given in Tables 1 and 2, respectively.
Fig. 3. Raman spectra of the interaction between methanol and oxygen at room temperature Ž ; 300 K..
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Fig. 4. Raman spectra of the interaction between methanol and oxygen at 473 K.
Fig. 5. Raman spectra of the interaction between methanol and oxygen at 673 K.
Relying upon the agreement of the vibrational frequencies with those in Tables 1 and 2, the bands can be interpreted as follows: n ŽC–O. for ŽI.: 972 cmy1 , nsŽOCO. for ŽII.: 1319 cmy1 , d ŽC–H. for both: 1482 cmy1 , nas ŽOCO. for ŽII.: 1738 cmy1 and n ŽC– H. for both: 2846 cmy1 . The band at 2083 cmy1 is ascribed to the vibration of CO adsorbed on the surface. The band at 3105 cmy1 corresponds to the stretching of a ‘softened’ OH bond of adsorbed hydroxide on silver, which agrees with the HREELS assignment reported by Wu for single crystalline
AgŽ111. w37x and Bao for electrolytic silver w5x. The band at 683 cmy1 is difficult to identify, since both d ŽOCO. and d ŽOH. are available in the range 600 to 900 cmy1 . Thus, in Fig. 3 we can see that at room temperature, both methoxide species ŽI. and formate species ŽII. are present on silver surface. And since the intensity of n ŽC–O. is much higher than that of n ŽOCO., the dominant configuration of the surface intermediate is the methoxide species. The Raman spectrum at 673 K ŽFig. 5. is quite different. The band at 972 cmy1 corresponding to
Table 1 Vibrational frequencies Žin cmy1 . and mode assignments for methoxide species on various metal surfaces Mode
PtŽ111. w10x
PdŽ100. w25x
RhŽ100. w26x
RuŽ100. w27x
FeŽ100. w28x
NiŽ110. w29,30x
CuŽ100. w9x
ZnŽ0001. w31x
AgŽ110. w20x
naŽCH. nsŽCH. d ŽCH. r ŽCH. n ŽCO. n ŽM–OMe.
2910 2910 1430 — 1000 370
2905 2905 1455 — 997 —
2960 2890 1440 1115 1005 400
2955 2810 1435 1140 1005 325
2895 2895 1430 — 1020 405
2910 2790 1440 1150 1030 400
2910 2830 1450 — 1010 290
2995 2885 1485 — 1030 510
2915 2915 1450 1150 1040 330
J. Wang et al.r Applied Surface Science 120 (1997) 99–105
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Table 2 Vibrational frequencies Žin cmy1 . and mode assignments for formate species on various surfaces and for HCOOy in sodium salt Mode
AgŽ111.r 400 K w32x
CuŽ100.r 400 K w14x
CuŽ110.r 400 K w33x
AgŽ110.r 200 K w34x
HCOOyr Ag w35x
HCOOy, sodium salt w36x
Electrolytic silverr473 K
n ŽCH. naŽOCO. nsŽOCO. d ŽCH. d ŽOCO. n ŽM–O.
2970, 2840 1680 1312 1488 750 —
2910, 2840 1460 1330 — 772 340
2940, 2840 1560 1360 1377 760 —
2900 1640 1340 — 770 280
2840, 2920 1550 1340 — 775 290
2841 1567 1360 1377 760
2857 1657 1306 1411 nr nr
n ŽC–O. for methoxide, disappears completely. While bands corresponding to ns and nas of the O–C–O mode remains at 1294 and 1629 cmy1 . Meanwhile, the n ŽC–H. at 2913 cmy1 and d ŽC–H. at 1483 cmy1 also exist. The disappearance of the stretching of C–O and the existence of the characteristic stretching of the O–C–O mode shows evidence that at 673 K, the dominant configuration of the surface intermediate is the formate species ŽII.. Comparing Fig. 4 with Figs. 3 and 5, we can see that both n ŽC–O. and n ŽOCO. obviously existed at 473 K. So two types of intermediates, methoxide and formate species, are available on a silver surface at this temperature. By heating the sample to 873 K, there are only two Raman bands at 468 and 2081 cmy1 on the surface, which are assigned to subsurface oxygen species and adsorbed carbon monoxide, respectively. Therefore, we can conclude that in the temperature range from 300 to 873 K, the configuration of methanol adsorbed on oxygen-covered silver changes gradually. The dominant configuration is the methoxide species ŽI. at room temperature and it becomes a formate species ŽII. at 673 K. Atomic oxygen was found to be highly reactive towards methanol. Chemisorption of methanol on the silver surface with preadsorbed oxygen caused instantaneous OH bond breaking and the formation of methoxide: CH 3 OH a ™ CH 3 Oa q OH a This reaction is proven by the detection of CH 3 Oa and the ‘softened’ OH strength mode for hydroxide in Raman spectra. Further heating of the sample leads to the decomposition of CH 3 Oa and the formation of HCHO. In the meanwhile, another intermedi-
ate, formate species, becomes the dominant intermediate on the surface. Sexton and Madix w34x had studied the oxidation of HCHO and HCOOH on AgŽ110. and proved that the decomposition of formate species gave off CO 2 , which is the major by-product of methanol oxidation. Hence, the following reaction should be proposed carrying on the surface: CH 3 Oa ™ HCHOa q H a HCHOa ™ HCHOg HCHOa q Oa ™ HCOOa q H a HCOOa ™ CO 2 g q H a The HCHOa cannot be detected directly on the silver surface, maybe its desorption and the oxidation is too rapid. 3.3. Raman spectrum of silÕer surface in oxygen r methanol mixture flow Fig. 6 shows the Raman spectrum of silver in a mixed flow of oxygen and methanol at 873 K. It gives some interesting features. The band at 990 cmy1 , which has been diminished completely above 673 K ŽFig. 5., appears again. The existence of a stretching vibration of a C–O bond shows that there is methoxide species ŽI. on the silver surface. The co-existence of bands at 1367r1672 cmy1 , corresponding to the symmetric and asymmetric stretching mode of O–C–O, gives evidence that the formate species ŽII. also adsorbed on the silver surface at 873 K. The vibrational bands at 1402 cmy1 are attributed to the bending mode of C–H. It should be noticed that besides the stretching vibration of C–H
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proceed simultaneously in the practical process. The continuous reaction also results in the existence of HCHOa on the surface.
4. Discussion The oxidation of methanol on silver surface has been investigated intensively by TPRS w2,4x, XPS, UPS w3x and HREELS w5x. All these results revealed that very little methanol chemisorbed on the oxygen-free silver surface and the ability of the silver surface to dissociatively chemisorb methanol was greatly enhanced by surface oxygen and resulted in the formation of methoxide. Further decomposition of methoxide gave off HCHO. If there is excess atomic oxygen species on the surface, HCHO will oxidize additionally to CO 2 through a formate intermediate. The reaction pathway can be summarized schematically as follows: Fig. 6. Raman spectra of the interaction between methanol and oxygen at 873 K in the methanolroxygen flow.
at 2860 cmy1 , a new band at 2702 cmy1 appears. According to the difference of the stretching mode of C–H in alcohol and in aldehyde w38x, the C–H vibration of aldehyde has two bands at 2900–2700 cmy1 but usually one of them appears at 2720 cmy1 . Therefore, it could be suggested that there are a few adsorbed formaldehyde on the surface. According to the work by Bhattacharya et al. w17x, the configuration of adsorbed formaldehyde on Pd has a weaker bond between the carbon atom and palladium atom rather than a Pd–O bond. This point with metal– carbon bonds is also suggested by Wu w37x for adsorbed formaldehyde species on AgŽ110. surface. Thus, this adsorbed formaldehyde species is not very stable and this is one of the reasons why under static conditions we cannot detect it. The existence of methoxide and formaldehyde on the surface under practical conditions is interesting because they disappeared at 873 K in the case of static experiment mentioned in Section 3.2. This indicates that the ambient reactant can supply the consumption of surface intermediate in the reaction. The reaction pathway suggested in Section 3.2 should
The mechanism we proposed from the proceeding data obtained by in situ Raman spectroscopy is essentially in agreement with that proposed earlier from TPRS and HREELS, except for the different temperature range. The results in Figs. 3–5 show no evidence of HCHOa , however, and suggested that the desorption of HCHO and the reaction HCHOa q Oa ™ HCOOa q H a occur very rapidly under these conditions. The decomposition of CH 3 Oa is correlated with the gradual disappearance of the n ŽC–O. in Figs. 3–5. The formate species is the most stable intermediate and its characteristic n ŽOCO. vibration diminishes at 873 K. A summary of the interaction process may now be given. Oxygen adsorbs on the silver surface to form an atomic oxygen species. Further exposure of
J. Wang et al.r Applied Surface Science 120 (1997) 99–105
CH 3 OH results in the OH bond breaking and the formation of methoxide. The methoxide decomposes to formaldehyde subsequently and some of the formaldehyde reacts with the excess atomic oxygen species to generate formate intermediate. The annealing of the sample results in the decomposition of methoxide gradually. Only the formate species exists on the surface at 673 K. Further raising the temperature causes the formate species to decompose to CO 2 . Finally, the reaction under the practical condition is discussed. By the in situ Raman spectrum, we can observe CH 3 Oa , HCOOa as well as trace HCHOa . Obviously, all these reactions occur simultaneously under practical conditions. At an ambient partial pressure of oxygen and methanol, the desorption and the formation of surface intermediates reaches an equilibrium. Thus, we can capture all of the intermediates at such a high temperature. The results in the present work show a remarkably close agreement to those obtained under UHV condition except for the stability of the intermediates. This is due to the great effect of the ambient pressure on the adsorption behavior. These results also prove that the mechanism of methanol oxidation obtained under UHV condition is also effective under industrial conditions.
Acknowledgements This work was supported by grants from the State Key Laboratory for Physical Chemistry of the Solid Surface at Xiamen University and National Natural Sciences Foundation of China.
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