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Structure and reactivity of thin-film oxides and metals
Applied Surface Science 142 Ž1999. 99–105
Structure and reactivity of thin-film oxides and metals C.M. Friend ) , K.T. Queeney, D.A. Chen HarÕard UniÕersity, Department of Chemistry, 12 Oxford St. Cambridge, MA 02138 USA
Abstract The structure and reactivity of thin-film oxides of molybdenum and Co metal supported on oxidized MoŽ110. is discussed. Reactions of interest in heterogeneous oxidation catalysis, in particular hydrocarbon oxidation is the focus of the work. A combination of electron energy loss, infrared, and X-ray photoelectron is used to characterize the structures of the oxides and Co films. Oxidation conditions are used to control the nature of the oxygen coordination sites available as well as the thickness and morphology of the oxide. Accordingly, the reactivity of specific types of oxygen coordination sites was investigated. In the case of the Co overlayers, thermal treatment was used as a means of varying the structure and morphology of the metal supported on the oxidized Mo. Oxygen bound to MoŽ110. in low-symmetry, high-coordination sites was found to play an important role in the hydrocarbon oxidation process. For example, gaseous methyl radicals selectively add to oxygen in these sites, but not to terminal oxygen. In the microscopic reverse of methyl radical oxidation, vacancies at high-coordination sites are necessary for methanol reaction to methoxy to occur. The site-specific oxidation chemistry is modeled in selected cases using first-principles electronic structure calculations. The reactions of alcohols on various Co thin films were also investigated. The selectivity for alcohol reaction is altered by electronic and structural modification of the film. The reactions of ethanol and methanol were used to illustrate these principles. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Oxidation; Catalysts; Methanol reaction
1. Introduction One of the most important problems in heterogeneous catalysis is the partial oxidation of alkanes, especially methane, to oxygenates such as alcohols. The catalytic partial oxidation of alkanes poses a particular challenge because they are very unreactive and do not readily form strong bonds to other species. As a result, high temperatures are generally required for C–H bond activation—the rate-limiting step in alkane reaction. Unfortunately, the high temperatures also favor nonselective secondary reactions, such as )
Corresponding author. Tel.: q1-617-495-4052; Fax: q1-617496-8410; E-mail: [email protected]
combustion. An effective catalyst must therefore balance the desire to achieve high conversion rates with the goal of high selectivity for partial oxidation. In recent years, a commercial catalytic process for methane conversion to formaldehyde based on molybdenum trioxide has been patented w1x. As a result, there has been considerable interest in understanding what controls selectivity and activity in methane oxidation catalyzed by MoO 3-based materials w2–8x. Studies of working catalysts have shown that the activity and selectivity for formaldehyde production is dependent on the nature of the oxygen species present w3,9x. For example, higher selectivity for formaldehyde formation on catalysts with a larger
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C.M. Friend et al.r Applied Surface Science 142 (1999) 99–105
fraction of exposed MoO 3 Ž010. planes, which contain Mo5O moieties, was reported w10x. These results suggest that selectivity and activity can be manipulated by controlling the material properties of the oxide catalyst. However, due to the complex nature of the catalyst, it is not possible to determine how these factors affect the elementary steps in methane oxidation. The goal of our work is to gain an understanding of how different types of reaction sites affect selectivity and activity in model systems. Model thin-films of oxides and metals have been targeted for these studies because the type of coordination sites populated on the surface can be systematically varied in ways that may not be possible in bulk systems. In addition, it is possible to use surface science techniques to define the geometric and electronic structure of the thin films and to identify reactive intermediates w11–13x.
tion temperature. The types of sites populated are determined using vibrational Žhigh-resolution electron energy loss. spectroscopy ŽFig. 1. w20,21x. Oxygen is confined to the surface and only occupies multiple coordination Žlong-bridge and lowsymmetry three-fold. sites when the surface is oxidized at low temperatures and low O 2 pressures
2. Experimental Experiments were performed in two separate ultrahigh vacuum chambers, which have been described previously w14–16x, each with base pressures of - 1 = 10y1 0 Torr. Both chambers are equipped with a computer-controlled UTI mass spectrometer for temperature-programmed reaction experiments and low-energy electron ŽLEED. optics for characterizing the Co overlayers. Furthermore, one chamber has a computer-controlled high-resolution energy loss spectrometer ŽLK2000. and the other has an infrared spectrometer ŽNicolet 800.. Sample cleaning and preparation procedures are described elsewhere w15,17x. Detailed descriptions of the experimental procedures are given in our previous papers w16–20x.
3. Results 3.1. Preparation and characterization of oxidized Mo surfaces The reactivity of specific types of oxygen can be addressed by using model thin-film oxides because the population of different oxygen moieties can be controlled by variation of the oxygen flux and oxida-
Fig. 1. Electron energy loss spectra showing the characteristic vibrational frequencies for different oxygen coordination sites. The top spectrum is for a chemisorbed oxygen layer prepared by low-temperature oxidation. Oxygen is bound to high-coordination Žlong-bridge and quasi-three-fold. sites in the chemisorbed layer. Vibrational spectra for thin-film oxides without Žmiddle. and with Žbottom. terminal oxygen are shown.
C.M. Friend et al.r Applied Surface Science 142 (1999) 99–105
Ž; 1 = 10y9 Torr.. The long-bridge site is identified on the basis of the symmetric Õ ŽMo–O. mode at 546 cmy1 , while the lower-symmetry three-fold site is signified by vibrations at 369 cmy1 Ž d ŽM–O.. and 605 cmy1 Ž Õ ŽMo–O.. ŽFig. 1. w21x. Under these conditions, no oxygen dissolves into the Mo lattice. Thin-film oxides, defined as systems where oxygen dissolves into the lattice, can be formed via oxygen transport into the bulk induced by high-temperature oxidation Ž1200 K.. Subsurface oxygen is signified by a vibration in the range of 745–782 cmy1 , in the electron energy loss spectrum ŽFig. 1. w21x, a range generally associated with lattice oxygen in oxides w22–24x. A major advantage of using these thin-film oxides is that they can be prepared both with and without Mo5O w20x, enabling us to systematically investigate the effect of these terminal sites on chemical reactivity. The Mo5O sites are readily identified on the basis of the characteristic Õ ŽMo5O. peak near 1000 cmy1 in the electron energy loss spectrum ŽFig. 1. w21–24x.
101
Fig. 2. Electron energy loss spectra demonstrating the formation of methoxy from methyl radical addition to oxygen bound in low-symmetry, high-coordination sites on MoŽ110.. The top spectrum was obtained after exposure of methyl radicals to MoŽ110. containing ; 0.75 ML of oxygen bound in high-coordination, low-symmetry sites corresponding to the top spectrum of Fig. 1. The bottom spectrum provides a reference spectrum for methoxy formed from methanol reaction on the same surface. In both cases, the adsorption temperature was 100 K.
3.2. Reaction mechanisms on oxidized Mo The conversion and selectivity of the methane oxidation process is dependent on both the rates of C–H bond breaking and C–O bond formation. The first and rate-determining step in the oxidation process is C–H bond dissociation of methane, yielding methyl radicals w3,4,25,26x. In this work, we have specifically investigated the C–O bond formation step, which is important in determining the selectiÕity for methane activation. Our overall goal is to relate specific elementary steps to particular active sites on the thin-film oxides. Our studies demonstrate that oxygen in high-coordination sites readily reacts with methyl radicals to form methoxy. Methoxy forms following exposure of MoŽ110. containing only oxygen in high-coordination sites to gaseous methyl radicals at a surface temperature of 100 K w20x. Methoxy is readily identified in the low coverage limit on the basis of its characteristic Õ ŽC–O. at 967 cmy1 in the electron energy loss spectrum as well as the other vibrations of methoxy, d ŽCH 3 . at 1450 cmy1 and Õ ŽCH 3 . at 2980 cmy1 ŽFig. 2.. The assignment of the peak at 967 cmy1 to the Õ ŽC–O. is confirmed by the iso-
topic shift of y30 cmy1 observed for methyl reaction with an 18 O-labelled surface. Our investigations of the reactions of methanol on oxidized MoŽ110. provide further insight into the methyl oxidation step w20,27,28x. Under the conditions of our experiments, methoxy formed from methanol primarily decomposes to liberate gaseous methyl radicals and surface oxygen—the microscopic reverse of methyl radical addition to surface oxygen. Methyl radical production has been welldocumented using both mass spectrometric w29,30x and infrared measurements w31x. Methyl radical evolution from methoxy occurs for a wide range of oxygen coverages, including the thin-film oxides containing terminal oxygen. A single, common methoxy species was detected using infrared spectroscopy for all oxygen coverages investigated w28x. Unfortunately, infrared spectroscopy does not provide information about the symmetry of the methoxy coordination site. However, theoretical studies indicate that methoxy preferentially binds to high-coordination sites on MoŽ110. w32x.
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Our recent work also illustrates that Õacant highcoordination sites are necessary for methanol to react with oxidized Mo w27x. This result is in agreement with experimental w33,34x and theoretical w35–37x studies which conclude that methoxy preferentially binds to high-coordination sites on metal surfaces, including MoŽ110. w32x and with our observation that only highly coordinated oxygen is deposited from the H 3 C–O bond dissociation w27x. Importantly, the number of vacancies at high-coordination sites were varied on thin-film oxides that do not contain terminal oxygen ŽMo5O. in order to isolate their effect w27x. While our studies of methyl radicals and methanol clearly demonstrate that oxygen in high-coordination sites is involved in the C–O bond formation step, it is also important to understand the dehydrogenation of methoxy, since these ensuing dehydrogenation steps control selectivity. Formaldehyde production was observed with vastly different selectivities from the reaction of methanol on oxygen-covered MoŽ110. w27,30x and MoŽ112. w38x, indicating that surface structure is important in determining selectivity. In these cases, the oxidation state of Mo is extremely low, since only chemisorbed oxygen Ž uo - 1 ML. is present under reaction conditions. Extremely high selectivity Ž; 90%. for formaldehyde production from methanol reaction was reported for MoŽ112. containing chemisorbed oxygen w38x. Studies of MoO 3-based catalysts also indicate that the oxide structure affects selectivity w3,5–7,39x. 3.3. Methanol reaction on Co thin films Bimetallic materials play an exceedingly important role in heterogeneous catalysis because of the potential for combining desirable properties of both metals and of the possibility for synergy. The central theme in our work on mixed metal systems has been to understand how to manipulate their electronic and geometric structure and thereby affect reactivity and selectivity. The reactions of methanol provide an excellent test case for the study of bimetallic systems because it has been investigated in detail on a wide range of transition metal surfaces and it is a desirable product of the partial oxidation of methane. Although many of these commercial processes employ Co-containing
multimetallic catalysts, the superior properties of multimetallic over single-metal catalysts are not well-understood w40–42x. We have investigated the reactivity of methanol on Co overlayers deposited on MoŽ110. in order to develop a fundamental understanding of oxygenate chemistry on bimetallic surfaces. We are specifically interested in determining whether C–O bond retention is favored on Co overlayers and whether there is any special reactivity associated with mixed Co–Mo sites. The Co-on-MoŽ110. system is ideal for these investigations, since it has been studied in detail previously w17,43–45x. At coverages below 1.0 monolayer ŽML., the Co atoms are believed to adopt the Mo lattice structure so that a pseudomorphic ˚ by 3.15 layer with a Co–Co lattice spacing of 2.73 A ˚ is formed. As the Co coverage is increased to 1.3 A ML, the Co atoms compress such that the Co–Co lattice spacing is decreased to that of bulk Co Ž2.51 ˚ .. A The chemistry of methanol on the continuous Co overlayers is similar to the reactivity on bulk midtransition metal surfaces ŽFig. 3.. On the 1.3 ML Co overlayer, approximately 46% of the methanol reacts via C–O bond retention, yielding gaseous CO at 430 K. The evolution of CO is generally consistent with the intermediate Co–O bond strength Ž; 90 kcalrmol. and is similar to the reactions on other metals with intermediate metal–O strengths, such as FeŽ110. w46x, FeŽ100. w47x, RhŽ111. w48,49x, PdŽ111. w50x, NiŽ100. w51x, NiŽ110. w52x, NiŽ111. w53x, and PtŽ111. w54x. Decomposition to atomic carbon and oxygen also occurs and is partially attributed to the dissociation of adsorbed CO. Electron energy loss and temperature-programmed reaction studies show that methanol desorption competes with O–H bond breaking at ; 200 K and that adsorbed CO is first formed from dehydrogenation of methoxy around 300 K ŽFig. 4.. Furthermore, there is no special reactivity associated with the mixed Co–Mo sites present at low Co coverages, since no new products are formed. There is also a periodic trend in the ability of different surfaces to promote O–H bond breaking correlated with metal–O bond strengths. Specifically, the temperature required for O–H bond dissociation on mid-transition metal surfaces, including the Co overlayers, is higher than on MoŽ110.. For
C.M. Friend et al.r Applied Surface Science 142 (1999) 99–105
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reaction results in ; 54% decomposition to atomic carbon and oxygen. However, based on the amount of decomposition for CO itself on the 1.3 ML Co overlayer, approximately 63% of the atomic carbon and oxygen is attributed to the direct decomposition of methoxy. A possible explanation is that the Co overlayer leaves some Mo sites exposed, since Mo is known to induce C–O bond dissociation. On FeŽ110. w46x, C–O bond scission in methanol reaction accounts for ; 25% of the total reactivity, while the major products are CO and H 2 , but in this case, C–O bond scission can be completely attributed to the dissociation of adsorbed CO. C–O bond scission is also a minor reaction pathway for methanol reaction on FeŽ100. w47,51x and RuŽ001. w56x. Notably, there is no evidence for unique methanol reactivity resulting in new product formation that is associated with mixed Co–Mo sites. The uniform increase in the yield of CO as a function of Co
Fig. 3. Schematic of methanol reaction on Co-covered MoŽ110. ŽqCo s1–1.3 ML..
example, O–H bond dissociation competes with methanol desorption at 210 K on the 1.0 ML Co overlayer, whereas all O–H bonds are broken at 100 K on MoŽ110.. Methanol evolution, attributed to methoxide recombination with surface hydrogen, has been reported around 175 K for PtŽ111. w54x and RhŽ111. w48x, and around 200 K for NiŽ110. w52x, PdŽ100. w55x, and RuŽ001. w56x. These differences in the ability of the surface to promote O–H bond scission are important in methanol synthesis reactions, since it is a necessary condition for methanol production. The Co overlayer exhibits a significant activity for C–O bond dissociation. Although CO does not dissociate on CoŽ0001., C–O bond scission is observed on stepped Co surfaces w57–59x. Our studies on the 1.3 ML Co overlayer show that methanol
Fig. 4. Electron energy loss data for a saturation exposure of methanol on 1.0 ML of Co after heating to Ža. 250 K, Žb. 350 K and Žc. 760 K.
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coverage demonstrates that methoxide decomposition to CO is characteristic of reaction on Co. The decrease in the H 2 yield as a function of Co coverage reflects decreased methanol reactivity due to the lower activity for O–H bond dissociation on Co compared to MoŽ110.. The presence of mixed Co– Mo sites for the 0.5 ML Co overlayer does not significantly influence CO evolution. The onset of surface CO formation as well as the evolution temperature is approximately the same on the 0.5 ML vs. the 1.3 ML Co overlayer. This result is consistent with our recent observations using scanning tunneling microscopy that show that two-dimensional islands form, which limits the number of mixed Co– Mo sites to the island edges. The lack of special reactivity associated with the Co overlayer is qualitatively similar to our earlier studies of methanethiol w17x and 2-propanol w19x on Co-covered MoŽ110.. In each case, the chemistry of the 1.0–1.3 ML Co overlayer was the same as what is predicted on bulk Co, based on the reactivity of bulk mid-transition metals with similar metal–O and metal–S strengths. The only difference is that more C–O bond dissociation in methanol reaction occurs on the Co overlayer compared to that expected on the bulk Co surface. 4. Conclusions The mechanisms of methanol reaction and methyl addition to oxygen have been investigated on thin films of Co and oxidized Mo in order to probe the sensitivity of specific elementary steps to the nature of available reaction sites. The utility of surface science experiments as a means of probing elementary steps in technologically important surface reactions is clearly demonstrated by our work. On oxidized MoŽ110., methoxy is reversibly formed from either methyl radical addition to oxygen or via methanol decomposition. Oxygen in high-coordination sites reacts readily with methyl radicals and is also deposited in the microscopic reverse, methoxy decomposition to methyl radicals and surface oxygen. These studies show that terminally bound oxygen is not necessary for facile reaction of methyl with oxides of molybdenum. Methanol reaction on 1.0–1.3 ML Co overlayers produces CO and H 2 , which are the products ex-
pected from reaction on bulk Co, based on the Co–O bond strength. C–O bond scission to produce atomic carbon and oxygen also accounted for ; 54% of the total methanol reaction. No new products were formed as a result of Co–Mo interactions at submonolayer Co coverages. Methoxy is identified as the intermediate at 250 K, and O–H bond scission and methanol desorption are competing processes below this temperature.
Acknowledgements We gratefully acknowledge support of this work by the US Department of Energy under Grant No. DE-FG02-84-ER13289.
References w1x w2x w3x w4x w5x w6x w7x w8x w9x w10x w11x w12x w13x w14x w15x w16x w17x w18x w19x w20x w21x w22x w23x w24x w25x
P.J. Spencer, United States Patent, 1986. R. Pitchai, K. Klier, Catal. Rev.-Sci. Eng. 28 Ž1986. 13. E.M. Stuve, R.J. Madix, J. Phys. Chem. 89 Ž1985. 3813. Y. Tong, J.H. Lunsford, J. Am. Chem. Soc. 113 Ž1991. 4741. M.A. Banares, J.L.G. Fierro, Catal. Lett. 17 Ž1993. 205. M.A. Banares et al., New Frontiers in Catalysis, 1993, pp. 1131–1144. N.D. Spencer, J. Catal. 109 Ž1988. 187. M.A. Banares, J.L.G. Fierro, J.B. Moffat, J. Catal. 142 Ž1993. 406. W.J. Meng, T.A. Perry, J. Appl. Phys. 76 Ž1994. 7824. M.R. Smith, U.S. Ozkan, J. Catal. 141 Ž1993. 124. A.M.B.Y.Q. Cai, Q. Guo, D.W. Goodman, Surf. Sci. 399 Ž1998. L357. D.W. Goodman, J. Phys. Chem. 100 Ž1996. 13090. C.X.S.C. Street, D.W. Goodman, Annu. Rev. Phys. Chem. 48 Ž1997. 43. X. Xu, C.M. Friend, J. Am. Chem. Soc. 113 Ž1991. 8572. J.T. Roberts, C.M. Friend, J. Am. Chem. Soc. 108 Ž1986. 7204. P. Uvdal et al., J. Chem. Phys. 97 Ž11. Ž1992. 8727. D.A. Chen, C.M. Friend, H. Xu, Langmuir 12 Ž1996. 1528. D.A. Chen, C.M. Friend, Surf. Sci. 371 Ž1997. 131. D.A. Chen, C.M. Friend, J. Phys. Chem. 100 Ž1996. 17640. K.T. Queeney, D.A. Chen, C.M. Friend, J. Am. Chem. Soc. 119 Ž1997. 6945. M.L. Colaianni et al., Surf. Sci. 279 Ž1992. 211. S.C.S.a.D.W. Goodman, J. Vac. Sci. Technol. A 15 Ž1997. 1717. M.A. Vuurman et al., J. Chem. Soc., Faraday Trans. 92 Ž1996. 3259. M.d.B.A. Andreini, M.A. Vuurman, Gautam Deo, I.E. Wachs, J. Chem. Soc. 92 Ž1996. 3267. D.J. Driscoll et al., J. Am. Chem. Soc. 107 Ž1985. 58.
C.M. Friend et al.r Applied Surface Science 142 (1999) 99–105 w26x D.J. Driscoll, J.H. Lunsford, J. Phys. Chem. 89 Ž1985. 4415. w27x K.T. Queeney, C.M. Friend, J. Phys. Chem. B 102 Ž1998. 5178. w28x K.T. Queeney, C.M. Friend, J. Chem. Phys., submitted, 1998. w29x J.G. Serafin, C.M. Friend, J. Am. Chem. Soc. 111 Ž1989. 8967. w30x S.C. Street, G. Liu, D.W. Goodman, Surf. Sci. 385 Ž1997. L971. w31x M.K. Weldon, C.M. Friend, Rev. Sci. Instrum. 66 Ž1995. 5192. w32x P. Schiller, A.B. Anderson, J. Phys. Chem. 95 Ž1991. 1396. w33x T. Lindner et al., Surf. Sci. 203 Ž1988. 333. w34x O. Schaff et al., Surf. Sci. 331–333 Ž1995. 201. w35x D. Zeroka, R. Hoffmann, Langmuir 2 Ž1986. 553. w36x P. Shiller, A.B. Anderson, J. Phys. Chem. 95 Ž1991. 1396. w37x H. Yang, J.L. Whitten, C.M. Friend, Surf. Sci. 313 Ž1994. 295. w38x K. Fukui, T. Aruga, Y. Iwasawa, Surf. Sci. 295 Ž1993. 160. w39x N.D. Spencer, C.J. Pereira, AIChE Journal 33 Ž1987. 1808. w40x R. Prins, V.H.J. de Beer, G.A. Somorjai, Catal. Rev.-Sci. Eng. 31 Ž1989. 1. w41x P. Grange, Catal. Rev.-Sci. Eng. 21 Ž1980. 135. w42x E. Furimsky, Catal. Rev.-Sci. Eng. 25 Ž1983. 421.
105
w43x M. Tikhov, E. Bauer, Surf. Sci. 232 Ž1990. 73. w44x J.-W. He, D.W. Goodman, Surf. Sci. 245 Ž1991. 29. w45x W.K. Kuhn, J.-W. He, D.W. Goodman, J. Vac. Sci. Technol. A 10 Ž4. Ž1992. 2477. w46x T.S. Rufael, J.D. Batteas, C.M. Friend, Surf. Sci. 384 Ž1997. 156. w47x M.R. Albert et al., Surf. Sci. 221 Ž1989. 197. w48x C. Houtman, M.A. Barteau, Langmuir 6 Ž1990. 1558. w49x F. Solymosi, A. Berko, T.I. Tarnoczi, Surf. Sci. 141 Ž1984. 533. w50x J.L. Davis, M.A. Barteau, Surf. Sci. 187 Ž1987. 387. w51x J.B. Benziger, R.J. Madix, J. Catal. 65 Ž1980. 36. w52x S.R. Bare, J.A. Stroscio, W. Ho, Surf. Sci. 150 Ž1985. 399. w53x J.N. Russell, I. Chorkendorff, J.T. Yates, Surf. Sci. 183 Ž1987. 316. w54x B.A. Sexton, K.D. Rendulic, A.E. Hughes, Surf. Sci. 121 Ž1995. 181. w55x K. Christmann, J.E. Demuth, J. Chem. Phys. 76 Ž1982. 6308. w56x J. Hrbek, R.A. de Paola, F.M. Hoffmann, J. Chem. Phys. 81 Ž1984. 2818. w57x H. Papp, Surf. Sci. 149 Ž1985. 460. w58x K.A. Prior, K. Schwaha, R.M. Lambert, Surf. Sci. 77 Ž1978. 193. w59x K. Jagannathan et al., Surf. Sci. 99 Ž1980. 309.
Structure and reactivity of thin-film oxides and metals C.M. Friend ) , K.T. Queeney, D.A. Chen HarÕard UniÕersity, Department of Chemistry, 12 Oxford St. Cambridge, MA 02138 USA
Abstract The structure and reactivity of thin-film oxides of molybdenum and Co metal supported on oxidized MoŽ110. is discussed. Reactions of interest in heterogeneous oxidation catalysis, in particular hydrocarbon oxidation is the focus of the work. A combination of electron energy loss, infrared, and X-ray photoelectron is used to characterize the structures of the oxides and Co films. Oxidation conditions are used to control the nature of the oxygen coordination sites available as well as the thickness and morphology of the oxide. Accordingly, the reactivity of specific types of oxygen coordination sites was investigated. In the case of the Co overlayers, thermal treatment was used as a means of varying the structure and morphology of the metal supported on the oxidized Mo. Oxygen bound to MoŽ110. in low-symmetry, high-coordination sites was found to play an important role in the hydrocarbon oxidation process. For example, gaseous methyl radicals selectively add to oxygen in these sites, but not to terminal oxygen. In the microscopic reverse of methyl radical oxidation, vacancies at high-coordination sites are necessary for methanol reaction to methoxy to occur. The site-specific oxidation chemistry is modeled in selected cases using first-principles electronic structure calculations. The reactions of alcohols on various Co thin films were also investigated. The selectivity for alcohol reaction is altered by electronic and structural modification of the film. The reactions of ethanol and methanol were used to illustrate these principles. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Oxidation; Catalysts; Methanol reaction
1. Introduction One of the most important problems in heterogeneous catalysis is the partial oxidation of alkanes, especially methane, to oxygenates such as alcohols. The catalytic partial oxidation of alkanes poses a particular challenge because they are very unreactive and do not readily form strong bonds to other species. As a result, high temperatures are generally required for C–H bond activation—the rate-limiting step in alkane reaction. Unfortunately, the high temperatures also favor nonselective secondary reactions, such as )
Corresponding author. Tel.: q1-617-495-4052; Fax: q1-617496-8410; E-mail: [email protected]
combustion. An effective catalyst must therefore balance the desire to achieve high conversion rates with the goal of high selectivity for partial oxidation. In recent years, a commercial catalytic process for methane conversion to formaldehyde based on molybdenum trioxide has been patented w1x. As a result, there has been considerable interest in understanding what controls selectivity and activity in methane oxidation catalyzed by MoO 3-based materials w2–8x. Studies of working catalysts have shown that the activity and selectivity for formaldehyde production is dependent on the nature of the oxygen species present w3,9x. For example, higher selectivity for formaldehyde formation on catalysts with a larger
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fraction of exposed MoO 3 Ž010. planes, which contain Mo5O moieties, was reported w10x. These results suggest that selectivity and activity can be manipulated by controlling the material properties of the oxide catalyst. However, due to the complex nature of the catalyst, it is not possible to determine how these factors affect the elementary steps in methane oxidation. The goal of our work is to gain an understanding of how different types of reaction sites affect selectivity and activity in model systems. Model thin-films of oxides and metals have been targeted for these studies because the type of coordination sites populated on the surface can be systematically varied in ways that may not be possible in bulk systems. In addition, it is possible to use surface science techniques to define the geometric and electronic structure of the thin films and to identify reactive intermediates w11–13x.
tion temperature. The types of sites populated are determined using vibrational Žhigh-resolution electron energy loss. spectroscopy ŽFig. 1. w20,21x. Oxygen is confined to the surface and only occupies multiple coordination Žlong-bridge and lowsymmetry three-fold. sites when the surface is oxidized at low temperatures and low O 2 pressures
2. Experimental Experiments were performed in two separate ultrahigh vacuum chambers, which have been described previously w14–16x, each with base pressures of - 1 = 10y1 0 Torr. Both chambers are equipped with a computer-controlled UTI mass spectrometer for temperature-programmed reaction experiments and low-energy electron ŽLEED. optics for characterizing the Co overlayers. Furthermore, one chamber has a computer-controlled high-resolution energy loss spectrometer ŽLK2000. and the other has an infrared spectrometer ŽNicolet 800.. Sample cleaning and preparation procedures are described elsewhere w15,17x. Detailed descriptions of the experimental procedures are given in our previous papers w16–20x.
3. Results 3.1. Preparation and characterization of oxidized Mo surfaces The reactivity of specific types of oxygen can be addressed by using model thin-film oxides because the population of different oxygen moieties can be controlled by variation of the oxygen flux and oxida-
Fig. 1. Electron energy loss spectra showing the characteristic vibrational frequencies for different oxygen coordination sites. The top spectrum is for a chemisorbed oxygen layer prepared by low-temperature oxidation. Oxygen is bound to high-coordination Žlong-bridge and quasi-three-fold. sites in the chemisorbed layer. Vibrational spectra for thin-film oxides without Žmiddle. and with Žbottom. terminal oxygen are shown.
C.M. Friend et al.r Applied Surface Science 142 (1999) 99–105
Ž; 1 = 10y9 Torr.. The long-bridge site is identified on the basis of the symmetric Õ ŽMo–O. mode at 546 cmy1 , while the lower-symmetry three-fold site is signified by vibrations at 369 cmy1 Ž d ŽM–O.. and 605 cmy1 Ž Õ ŽMo–O.. ŽFig. 1. w21x. Under these conditions, no oxygen dissolves into the Mo lattice. Thin-film oxides, defined as systems where oxygen dissolves into the lattice, can be formed via oxygen transport into the bulk induced by high-temperature oxidation Ž1200 K.. Subsurface oxygen is signified by a vibration in the range of 745–782 cmy1 , in the electron energy loss spectrum ŽFig. 1. w21x, a range generally associated with lattice oxygen in oxides w22–24x. A major advantage of using these thin-film oxides is that they can be prepared both with and without Mo5O w20x, enabling us to systematically investigate the effect of these terminal sites on chemical reactivity. The Mo5O sites are readily identified on the basis of the characteristic Õ ŽMo5O. peak near 1000 cmy1 in the electron energy loss spectrum ŽFig. 1. w21–24x.
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Fig. 2. Electron energy loss spectra demonstrating the formation of methoxy from methyl radical addition to oxygen bound in low-symmetry, high-coordination sites on MoŽ110.. The top spectrum was obtained after exposure of methyl radicals to MoŽ110. containing ; 0.75 ML of oxygen bound in high-coordination, low-symmetry sites corresponding to the top spectrum of Fig. 1. The bottom spectrum provides a reference spectrum for methoxy formed from methanol reaction on the same surface. In both cases, the adsorption temperature was 100 K.
3.2. Reaction mechanisms on oxidized Mo The conversion and selectivity of the methane oxidation process is dependent on both the rates of C–H bond breaking and C–O bond formation. The first and rate-determining step in the oxidation process is C–H bond dissociation of methane, yielding methyl radicals w3,4,25,26x. In this work, we have specifically investigated the C–O bond formation step, which is important in determining the selectiÕity for methane activation. Our overall goal is to relate specific elementary steps to particular active sites on the thin-film oxides. Our studies demonstrate that oxygen in high-coordination sites readily reacts with methyl radicals to form methoxy. Methoxy forms following exposure of MoŽ110. containing only oxygen in high-coordination sites to gaseous methyl radicals at a surface temperature of 100 K w20x. Methoxy is readily identified in the low coverage limit on the basis of its characteristic Õ ŽC–O. at 967 cmy1 in the electron energy loss spectrum as well as the other vibrations of methoxy, d ŽCH 3 . at 1450 cmy1 and Õ ŽCH 3 . at 2980 cmy1 ŽFig. 2.. The assignment of the peak at 967 cmy1 to the Õ ŽC–O. is confirmed by the iso-
topic shift of y30 cmy1 observed for methyl reaction with an 18 O-labelled surface. Our investigations of the reactions of methanol on oxidized MoŽ110. provide further insight into the methyl oxidation step w20,27,28x. Under the conditions of our experiments, methoxy formed from methanol primarily decomposes to liberate gaseous methyl radicals and surface oxygen—the microscopic reverse of methyl radical addition to surface oxygen. Methyl radical production has been welldocumented using both mass spectrometric w29,30x and infrared measurements w31x. Methyl radical evolution from methoxy occurs for a wide range of oxygen coverages, including the thin-film oxides containing terminal oxygen. A single, common methoxy species was detected using infrared spectroscopy for all oxygen coverages investigated w28x. Unfortunately, infrared spectroscopy does not provide information about the symmetry of the methoxy coordination site. However, theoretical studies indicate that methoxy preferentially binds to high-coordination sites on MoŽ110. w32x.
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Our recent work also illustrates that Õacant highcoordination sites are necessary for methanol to react with oxidized Mo w27x. This result is in agreement with experimental w33,34x and theoretical w35–37x studies which conclude that methoxy preferentially binds to high-coordination sites on metal surfaces, including MoŽ110. w32x and with our observation that only highly coordinated oxygen is deposited from the H 3 C–O bond dissociation w27x. Importantly, the number of vacancies at high-coordination sites were varied on thin-film oxides that do not contain terminal oxygen ŽMo5O. in order to isolate their effect w27x. While our studies of methyl radicals and methanol clearly demonstrate that oxygen in high-coordination sites is involved in the C–O bond formation step, it is also important to understand the dehydrogenation of methoxy, since these ensuing dehydrogenation steps control selectivity. Formaldehyde production was observed with vastly different selectivities from the reaction of methanol on oxygen-covered MoŽ110. w27,30x and MoŽ112. w38x, indicating that surface structure is important in determining selectivity. In these cases, the oxidation state of Mo is extremely low, since only chemisorbed oxygen Ž uo - 1 ML. is present under reaction conditions. Extremely high selectivity Ž; 90%. for formaldehyde production from methanol reaction was reported for MoŽ112. containing chemisorbed oxygen w38x. Studies of MoO 3-based catalysts also indicate that the oxide structure affects selectivity w3,5–7,39x. 3.3. Methanol reaction on Co thin films Bimetallic materials play an exceedingly important role in heterogeneous catalysis because of the potential for combining desirable properties of both metals and of the possibility for synergy. The central theme in our work on mixed metal systems has been to understand how to manipulate their electronic and geometric structure and thereby affect reactivity and selectivity. The reactions of methanol provide an excellent test case for the study of bimetallic systems because it has been investigated in detail on a wide range of transition metal surfaces and it is a desirable product of the partial oxidation of methane. Although many of these commercial processes employ Co-containing
multimetallic catalysts, the superior properties of multimetallic over single-metal catalysts are not well-understood w40–42x. We have investigated the reactivity of methanol on Co overlayers deposited on MoŽ110. in order to develop a fundamental understanding of oxygenate chemistry on bimetallic surfaces. We are specifically interested in determining whether C–O bond retention is favored on Co overlayers and whether there is any special reactivity associated with mixed Co–Mo sites. The Co-on-MoŽ110. system is ideal for these investigations, since it has been studied in detail previously w17,43–45x. At coverages below 1.0 monolayer ŽML., the Co atoms are believed to adopt the Mo lattice structure so that a pseudomorphic ˚ by 3.15 layer with a Co–Co lattice spacing of 2.73 A ˚ is formed. As the Co coverage is increased to 1.3 A ML, the Co atoms compress such that the Co–Co lattice spacing is decreased to that of bulk Co Ž2.51 ˚ .. A The chemistry of methanol on the continuous Co overlayers is similar to the reactivity on bulk midtransition metal surfaces ŽFig. 3.. On the 1.3 ML Co overlayer, approximately 46% of the methanol reacts via C–O bond retention, yielding gaseous CO at 430 K. The evolution of CO is generally consistent with the intermediate Co–O bond strength Ž; 90 kcalrmol. and is similar to the reactions on other metals with intermediate metal–O strengths, such as FeŽ110. w46x, FeŽ100. w47x, RhŽ111. w48,49x, PdŽ111. w50x, NiŽ100. w51x, NiŽ110. w52x, NiŽ111. w53x, and PtŽ111. w54x. Decomposition to atomic carbon and oxygen also occurs and is partially attributed to the dissociation of adsorbed CO. Electron energy loss and temperature-programmed reaction studies show that methanol desorption competes with O–H bond breaking at ; 200 K and that adsorbed CO is first formed from dehydrogenation of methoxy around 300 K ŽFig. 4.. Furthermore, there is no special reactivity associated with the mixed Co–Mo sites present at low Co coverages, since no new products are formed. There is also a periodic trend in the ability of different surfaces to promote O–H bond breaking correlated with metal–O bond strengths. Specifically, the temperature required for O–H bond dissociation on mid-transition metal surfaces, including the Co overlayers, is higher than on MoŽ110.. For
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reaction results in ; 54% decomposition to atomic carbon and oxygen. However, based on the amount of decomposition for CO itself on the 1.3 ML Co overlayer, approximately 63% of the atomic carbon and oxygen is attributed to the direct decomposition of methoxy. A possible explanation is that the Co overlayer leaves some Mo sites exposed, since Mo is known to induce C–O bond dissociation. On FeŽ110. w46x, C–O bond scission in methanol reaction accounts for ; 25% of the total reactivity, while the major products are CO and H 2 , but in this case, C–O bond scission can be completely attributed to the dissociation of adsorbed CO. C–O bond scission is also a minor reaction pathway for methanol reaction on FeŽ100. w47,51x and RuŽ001. w56x. Notably, there is no evidence for unique methanol reactivity resulting in new product formation that is associated with mixed Co–Mo sites. The uniform increase in the yield of CO as a function of Co
Fig. 3. Schematic of methanol reaction on Co-covered MoŽ110. ŽqCo s1–1.3 ML..
example, O–H bond dissociation competes with methanol desorption at 210 K on the 1.0 ML Co overlayer, whereas all O–H bonds are broken at 100 K on MoŽ110.. Methanol evolution, attributed to methoxide recombination with surface hydrogen, has been reported around 175 K for PtŽ111. w54x and RhŽ111. w48x, and around 200 K for NiŽ110. w52x, PdŽ100. w55x, and RuŽ001. w56x. These differences in the ability of the surface to promote O–H bond scission are important in methanol synthesis reactions, since it is a necessary condition for methanol production. The Co overlayer exhibits a significant activity for C–O bond dissociation. Although CO does not dissociate on CoŽ0001., C–O bond scission is observed on stepped Co surfaces w57–59x. Our studies on the 1.3 ML Co overlayer show that methanol
Fig. 4. Electron energy loss data for a saturation exposure of methanol on 1.0 ML of Co after heating to Ža. 250 K, Žb. 350 K and Žc. 760 K.
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coverage demonstrates that methoxide decomposition to CO is characteristic of reaction on Co. The decrease in the H 2 yield as a function of Co coverage reflects decreased methanol reactivity due to the lower activity for O–H bond dissociation on Co compared to MoŽ110.. The presence of mixed Co– Mo sites for the 0.5 ML Co overlayer does not significantly influence CO evolution. The onset of surface CO formation as well as the evolution temperature is approximately the same on the 0.5 ML vs. the 1.3 ML Co overlayer. This result is consistent with our recent observations using scanning tunneling microscopy that show that two-dimensional islands form, which limits the number of mixed Co– Mo sites to the island edges. The lack of special reactivity associated with the Co overlayer is qualitatively similar to our earlier studies of methanethiol w17x and 2-propanol w19x on Co-covered MoŽ110.. In each case, the chemistry of the 1.0–1.3 ML Co overlayer was the same as what is predicted on bulk Co, based on the reactivity of bulk mid-transition metals with similar metal–O and metal–S strengths. The only difference is that more C–O bond dissociation in methanol reaction occurs on the Co overlayer compared to that expected on the bulk Co surface. 4. Conclusions The mechanisms of methanol reaction and methyl addition to oxygen have been investigated on thin films of Co and oxidized Mo in order to probe the sensitivity of specific elementary steps to the nature of available reaction sites. The utility of surface science experiments as a means of probing elementary steps in technologically important surface reactions is clearly demonstrated by our work. On oxidized MoŽ110., methoxy is reversibly formed from either methyl radical addition to oxygen or via methanol decomposition. Oxygen in high-coordination sites reacts readily with methyl radicals and is also deposited in the microscopic reverse, methoxy decomposition to methyl radicals and surface oxygen. These studies show that terminally bound oxygen is not necessary for facile reaction of methyl with oxides of molybdenum. Methanol reaction on 1.0–1.3 ML Co overlayers produces CO and H 2 , which are the products ex-
pected from reaction on bulk Co, based on the Co–O bond strength. C–O bond scission to produce atomic carbon and oxygen also accounted for ; 54% of the total methanol reaction. No new products were formed as a result of Co–Mo interactions at submonolayer Co coverages. Methoxy is identified as the intermediate at 250 K, and O–H bond scission and methanol desorption are competing processes below this temperature.
Acknowledgements We gratefully acknowledge support of this work by the US Department of Energy under Grant No. DE-FG02-84-ER13289.
References w1x w2x w3x w4x w5x w6x w7x w8x w9x w10x w11x w12x w13x w14x w15x w16x w17x w18x w19x w20x w21x w22x w23x w24x w25x
P.J. Spencer, United States Patent, 1986. R. Pitchai, K. Klier, Catal. Rev.-Sci. Eng. 28 Ž1986. 13. E.M. Stuve, R.J. Madix, J. Phys. Chem. 89 Ž1985. 3813. Y. Tong, J.H. Lunsford, J. Am. Chem. Soc. 113 Ž1991. 4741. M.A. Banares, J.L.G. Fierro, Catal. Lett. 17 Ž1993. 205. M.A. Banares et al., New Frontiers in Catalysis, 1993, pp. 1131–1144. N.D. Spencer, J. Catal. 109 Ž1988. 187. M.A. Banares, J.L.G. Fierro, J.B. Moffat, J. Catal. 142 Ž1993. 406. W.J. Meng, T.A. Perry, J. Appl. Phys. 76 Ž1994. 7824. M.R. Smith, U.S. Ozkan, J. Catal. 141 Ž1993. 124. A.M.B.Y.Q. Cai, Q. Guo, D.W. Goodman, Surf. Sci. 399 Ž1998. L357. D.W. Goodman, J. Phys. Chem. 100 Ž1996. 13090. C.X.S.C. Street, D.W. Goodman, Annu. Rev. Phys. Chem. 48 Ž1997. 43. X. Xu, C.M. Friend, J. Am. Chem. Soc. 113 Ž1991. 8572. J.T. Roberts, C.M. Friend, J. Am. Chem. Soc. 108 Ž1986. 7204. P. Uvdal et al., J. Chem. Phys. 97 Ž11. Ž1992. 8727. D.A. Chen, C.M. Friend, H. Xu, Langmuir 12 Ž1996. 1528. D.A. Chen, C.M. Friend, Surf. Sci. 371 Ž1997. 131. D.A. Chen, C.M. Friend, J. Phys. Chem. 100 Ž1996. 17640. K.T. Queeney, D.A. Chen, C.M. Friend, J. Am. Chem. Soc. 119 Ž1997. 6945. M.L. Colaianni et al., Surf. Sci. 279 Ž1992. 211. S.C.S.a.D.W. Goodman, J. Vac. Sci. Technol. A 15 Ž1997. 1717. M.A. Vuurman et al., J. Chem. Soc., Faraday Trans. 92 Ž1996. 3259. M.d.B.A. Andreini, M.A. Vuurman, Gautam Deo, I.E. Wachs, J. Chem. Soc. 92 Ž1996. 3267. D.J. Driscoll et al., J. Am. Chem. Soc. 107 Ž1985. 58.
C.M. Friend et al.r Applied Surface Science 142 (1999) 99–105 w26x D.J. Driscoll, J.H. Lunsford, J. Phys. Chem. 89 Ž1985. 4415. w27x K.T. Queeney, C.M. Friend, J. Phys. Chem. B 102 Ž1998. 5178. w28x K.T. Queeney, C.M. Friend, J. Chem. Phys., submitted, 1998. w29x J.G. Serafin, C.M. Friend, J. Am. Chem. Soc. 111 Ž1989. 8967. w30x S.C. Street, G. Liu, D.W. Goodman, Surf. Sci. 385 Ž1997. L971. w31x M.K. Weldon, C.M. Friend, Rev. Sci. Instrum. 66 Ž1995. 5192. w32x P. Schiller, A.B. Anderson, J. Phys. Chem. 95 Ž1991. 1396. w33x T. Lindner et al., Surf. Sci. 203 Ž1988. 333. w34x O. Schaff et al., Surf. Sci. 331–333 Ž1995. 201. w35x D. Zeroka, R. Hoffmann, Langmuir 2 Ž1986. 553. w36x P. Shiller, A.B. Anderson, J. Phys. Chem. 95 Ž1991. 1396. w37x H. Yang, J.L. Whitten, C.M. Friend, Surf. Sci. 313 Ž1994. 295. w38x K. Fukui, T. Aruga, Y. Iwasawa, Surf. Sci. 295 Ž1993. 160. w39x N.D. Spencer, C.J. Pereira, AIChE Journal 33 Ž1987. 1808. w40x R. Prins, V.H.J. de Beer, G.A. Somorjai, Catal. Rev.-Sci. Eng. 31 Ž1989. 1. w41x P. Grange, Catal. Rev.-Sci. Eng. 21 Ž1980. 135. w42x E. Furimsky, Catal. Rev.-Sci. Eng. 25 Ž1983. 421.
105
w43x M. Tikhov, E. Bauer, Surf. Sci. 232 Ž1990. 73. w44x J.-W. He, D.W. Goodman, Surf. Sci. 245 Ž1991. 29. w45x W.K. Kuhn, J.-W. He, D.W. Goodman, J. Vac. Sci. Technol. A 10 Ž4. Ž1992. 2477. w46x T.S. Rufael, J.D. Batteas, C.M. Friend, Surf. Sci. 384 Ž1997. 156. w47x M.R. Albert et al., Surf. Sci. 221 Ž1989. 197. w48x C. Houtman, M.A. Barteau, Langmuir 6 Ž1990. 1558. w49x F. Solymosi, A. Berko, T.I. Tarnoczi, Surf. Sci. 141 Ž1984. 533. w50x J.L. Davis, M.A. Barteau, Surf. Sci. 187 Ž1987. 387. w51x J.B. Benziger, R.J. Madix, J. Catal. 65 Ž1980. 36. w52x S.R. Bare, J.A. Stroscio, W. Ho, Surf. Sci. 150 Ž1985. 399. w53x J.N. Russell, I. Chorkendorff, J.T. Yates, Surf. Sci. 183 Ž1987. 316. w54x B.A. Sexton, K.D. Rendulic, A.E. Hughes, Surf. Sci. 121 Ž1995. 181. w55x K. Christmann, J.E. Demuth, J. Chem. Phys. 76 Ž1982. 6308. w56x J. Hrbek, R.A. de Paola, F.M. Hoffmann, J. Chem. Phys. 81 Ž1984. 2818. w57x H. Papp, Surf. Sci. 149 Ž1985. 460. w58x K.A. Prior, K. Schwaha, R.M. Lambert, Surf. Sci. 77 Ž1978. 193. w59x K. Jagannathan et al., Surf. Sci. 99 Ž1980. 309.