5896
J . Phys. Chem. 1986, 90, 5896-5900
Photooxidation of Methanol Using V205/Ti02and MoO,/TtO, Surface Oxide Monolayer Catalysts T. Cadson and G. L. Griffin* Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455 (Received: April 25, 1986)
We have compared the conversion and selectivity obtained using 7 wt 7% V205/Ti02and 4.0 wt 7% Mo03/Ti02 catalysts for the partial oxidation of CH,OH, as a function of reaction temperature and illumination. The thermally controlled reaction is faster and slightly less selective to CH20 on the vanadia/TiO, catalyst. In contrast, the incremental photoassisted reaction rate is much lower on the vanadia catalyst than on the molybdena catalyst, which in turn is less photoactive than pure TiOz. The observed sequence of the relative photoactivities varies inversely with the concentration of V or Mo cations in the surface oxide monolayers, which suggests that the photoassisted alkoxy dehydrogeneration process occurs primarily at exposed TiO, surface sites.
Photoassisted catalysis represents a novel and incompletely understood approach for modifying the reaction selectivity of heterogeneous catalysts.' Photoassisted reactions generally can be performed at lower temperatures than the corresponding thermally controlled reactions, which offers the potential for decreased process energy costs as well as increased catalyst lifetime. The latter advantage may prove especially valuable for reactions on chemically modified surfaces, which are likely to be required for future, highly selective catalytic processes involved in the manufacture of specialty chemicals. A fundamental understanding of photoassisted reaction mechanisms involving simple molecules on surfaces will also be useful in noncatalytic applications, such as photoassisted direct writing processes for etching or depositing solid materials (e.g., in materials processing steps for microelectronics applications). The approach to designing heterogeneous catalysts for photoassisted reactions which we are investigating in our laboratory is the use of a medium band-gap semiconductor as a photoactive support, with a surface that has been chemically modified in an attempt to control reaction selectivity. This approach offers the advantage that the overall efficiency for photon absorption and subsequent chemical conversion steps are quite high on many semiconductors. For example, a quantum efficiency of 0.4 has been reported for the photoassisted oxidation of 2-propanol on TiO,., A potential disadvantage with this approach is the possibility that the surface modifying agent may itself interfere with the efficiency of the charge-transfer mechanism between the valence or conduction band of the semiconductor and the molecular orbitals of the adsorbed reactant. Recently we reported the photoassisted oxidation of C H 3 0 H using a series of molybdena/TiO, catalysts with different Mo loading^.^ Laser Raman and UV-visible spectra of the catalysts, as well as the product distributions measured as a function of Mo loading, all indicated that at loadings up to 4 wt % MOO,, the molybdena species are present as a surface oxide monolayer. The surface concentration of the saturated monolayer is 4 X lOI4 Mo cations/cm2 of TiO,. For the photoassisted partial oxidation reaction, the molybdena monolayer catalyst showed an increased product selectivity toward dimethoxymethane (DMM), while unmodified T i 0 2 yielded methyl formate (MF) as the major product. However, the improved selectivity seen for the molybdena monolayer catalyst was also accompanied by a loss in photoefficiency, to about 25% of the value that could be obtained for pure TiO,. To examine whether this loss in photoefficiency might be a general effect in surface-modified photocatalysts, we decided to (1) Bickley, R. I. In Catalysis; Specialist Periodic Reports; Royal Society of Chemistry: London, 1982; Vol. 5 , p 308. (2) Egerton, T.; King, C. J. J. Oil. Col. Chem. Assoc. 1979, 62, 386. (3) Liu, Y.C.; Griffin,G.L.; Chan, S . S.; Wachs, I. E. J . Catal. 1985, 94, 108.
0022-3654/86/2090-5896$01.50/0
examine the behavior of the V2OS/TiO2system for the same test reaction. The V,05/Ti02 (anatase) catalyst is of commercial interest for the partial oxidation of o-xylene to phthalic anhydridee4v5 Recent laser Raman6s7 and EXAFS* studies have concluded that vanadia forms a surface oxide monolayer when deposited on Ti02, thus showing structural behavior similar to molybdena supported on TiOz. However, the saturation concentration of the vanadia surface oxide monolayer is about 10 X 1014V cations/cm* of Ti02, which is significantly larger than for the molybdena/Ti02 system. As shown below, this higher concentration is accompanied by a still lower photoefficiency than is observed for molybdena/TiO,. These results show that a decrease in photoefficiency due to site blockage of the semiconductor surface is a general concern for surface oxide modified photocatalyst~.~
Experimental Section Experiments were performed using a 7 wt % V205/Ti02catalyst, kindly supplied by Dr. Israel Wachs of the Exxon Corporate Science Research Laboratory. The material was prepared by impregnation of T i 0 2powder (Degussa P-25% surface area = 50 m2/g; composition 67% anatase:33% rutile) with an aqueous solution of VO(OC2Hs)3,followed by drying and calcining in air at 723 K. This loading corresponds to a coverage of 10 X loi4 V cations/cm2 of T i 0 2 support, after some minor loss of volatile V complexes during the calcination step.'O Previous laser Raman studies have confirmed that this procedure results in a catalyst which contains a monolayer of dispersed vanadia that completely covers the TiO, ~ u r f a c e . ~The , ~ surface oxide monolayer species is characterized by a broad V = O vibrational band around 1010 ~ m - ' , while ~ , ~ the absence of V205crystallites is indicated by the absence of the vibrational band at 998 cm-' that is characteristic of bulk V205." Rate measurements were performed by using the thin-layer flow photoreactor described previ~usly.~A 100-mg sample of the catalyst was deposited from a resuspended aqueous slurry onto the illuminated face of the reactor (ca. 20-cm2 light area). The reactant gas stream was a CH30H:02:He mixture with a mole ratio of 4:22:74. Product analysis was performed by using a gas in., chromatograph (Porapak T, column dimension 4 ft X (4) Bond, G. C.; Bruckman, K. Chem. SOC.,Faraday Discuss. 1982, 72, 1.
( 5 ) Wainwright, M. S.; Foster, N. R. Catal. Reu. 1979, 19, 211. (6) Roozeboom, F.; Mittelmeijer-Hazeleger, M. C.; Moulijin, J. A,; Medema, J.; de Beer, V. H. J. J. Phys. Chem. 1980.84, 2783. (7) Wachs, I. E.; Chan, S.S.; Chersich, C. C.; Saleh, R. Y . In Catalysis on rhe Energy Scene; Kaliaguine, S.,Mahey, A., Eds.; Elsevier: New York, 1984; p 275. (8) Kozlowski, R.;Pettifer, R. F.; Thomas, J. M. J. Phys. Chem. 1983,87, 5176. (9) An exception may be made for photocatalysts modified with highly dispersed noble metals; see e.&, ref 1. (10) Wachs, I. E., personal communication. (11) Beattie, I. R.; Gilson, T. R. J. Chem. SOC.A 1969, 2322.
0 1986 American Chemical Society
The Journal of Physical Chemistry, Vol. 90, No. 22, 1986 5897
Photooxidation of Methanol 100
80
30 -
r---
I
Illwninaled
O,A,O llluminoted
Dark
z
0.4.
Dark
-a0l
E 20-
Figure 2. Product yields as a function of CH30H conversion during dark and photoassisted oxidation using 7 wt % vanadia/Ti02 catalyst. I
373
423
473
TEMPERATURE (K)
Figure 1. Comparison of total conversion during CH30H oxidation using indicated catalysts, as a function of reactor temperature and illumination.
column temperature 433 K) with flame ionization detector. The light source was a medium-pressure Hg lamp. The Suprasil window used in the reactor allowed only light with wavelengths greater than 3000 %r. to reach the catalyst. The photoresponse was determined by measuring the product concentrations leaving the reactor with and without a shutter placed over the reactor window.
Results In Figure 1 we show the total conversion of C H 3 0 H for the 7 wt % V20S/Ti02catalyst with and without illumination, as measured at different reactor temperatures. For comparison, we also show the conversions obtained over the same temperature range using an unmodified T i 0 2 catalyst and also a 4 wt % Mo03/Ti02 monolayer catalyst from our previous study.3 The lower, filled symbols indicate conversions measured without illumination, while the open symbols show the conversions measured with the light on. The difference between the two curves for each catalyst, as indicated by the vertical arrows, represents the incremental photoconversion. Several features are noted. First, the activity of the catalysts for the dark reaction decreases in the order vanadia/Ti02 > molybdena/Ti02 > Ti02. In contrast, the magnitude of the incremental photoconversion decreases in the reverse order T i 0 2 > molybdena/TiO, > vanadia/Ti02. For pure Ti02, incremental photoconversions as large as 50% can be achieved by using our reactor configuration, while for the vanadia/Ti02 catalyst the average incremental photoconversion is less than 5%. The average incremental photoconversion obtained for the molybdena/Ti02 catalyst is about 15%. In almost every case, the observed incremental photoconversion is independent of temperature. In particular, there is no evidence that the quantum efficiency of the photoassisted reaction decreases at higher temperature. A decrease in the incremental photoconversion is observed for the molybdena/Ti02 catalyst a t the lowest temperature studied (333 K). We suspect that this is due to the reaction becoming product desorption limited at sufficiently low temperatures. For example, H 2 0 has been shown to adsorb competitively with CH30H on molybdena-based catalyst;12 a similar competitive adsorption might also be expected for vanadia catalysts. In Figure 2 we show the selectivity plots (Le., yield vs. conversion) for the three partial oxidation products observed with the vanadia/Ti02 catalyst. Yields are defined as the moles of product produced per mole of inlet CH30H. Results for both dark and light reactions are shown on the same graph, in order to emphasize the similarity in reaction selectivity. Because the incremental photoconversion is so small for the vanadia/Ti02 (12) Santaccsaria, E.;Morbidelli, M.; C a d , S.Chem. Eng. Sci. 1981, 36,
909.
CH30H CONVERSION (W
Figure 3. Product yields as a function of CH30Hconversion during dark and photoassisted oxidation using 4 wt 5% molybdena/Ti02 catalyst.
catalyst, the value of the conversion for each point is determined almost entirely by the thermal conversion at that temperature. At low conversions the primary product is DMM. We assume that this product is formed by a condensation reaction between an adsorbed C H 2 0 molecule, which is the primary oxidation product in either the dark or photoassisted surface reaction, and two neighboring CH30H,,, species (see Figure 6 ) . At higher conversions, corresponding to T > 400 K , the yield of DMM decreases, and the major products become methyl formate (MF) and CH20. The C H 2 0 yield passes through a maximum around 50% conversion, which is consistent with CH20 being an intermediate product in a series oxidation mechanism. The M F yield also passes through a maximum, but at a higher conversion than the CH20 maximum. The presence of this maximum confirms that M F is also an intermediate in a series oxidation mechanism. However, the fact that the curves for M F and C H 2 0 are parallel for low conversions (Le., below 40%) suggests that M F is not produced primarily from the secondary oxidation of CH20. Instead, it appears that both CHzO and M F are produced from parallel reaction channels of a common surface intermediate, namely CH20,, (see Figure 6 ) . We observed similar evidence for the parallel formation of M F and C H 2 0 during the photooxidation of CH30H on pure Ti02, as welL3 The graph also shows that the selectivity curves for all three products are essentially unchanged by illumination; Le., the points for the dark and light reactions are described by a single yield vs. conversion curve for each product. Moreover, the incremental changes between dark and light conditions (Le., the line segment connecting neighboring open and filled symbols on the graph) are generally parallel to the overall selectivity curve. We interpret this to indicate that the selectivity is insensitive to the excitation mechanism on vanadia/Ti02, at least for the small magnitude of incremental photoconversions obtained with this catalyst. To show that different selectivity curves for dark and light reactions can be obtained in the case of larger incremental photoconversion, in Figure 3 we show the dark and light selectivity ’ curves for DMM and CH20 obtained using the 4 wt % Mo03/Ti02 catalyst at the same reactor conditions. The M F points have been omitted for clarity; by coincidence they lie almost
5898
The Journal of Physical Chemistry, Vol. 90, No. 22, 1986
on top of the points for C H 2 0 . Two differences with respect to the results for the vanadia/Ti02 catalyst are noted. First, the maximum selectivity for C H 2 0 is slightly higher (16% for the molybdena/Ti02 catalyst vs. 14% for the vanadia/Ti02 catalyst). Thus, the more active vanadia catalyst is also somewhat less selective for desorption of the primary oxidation product, CH20,. Second, at low conversions there is a significant difference in selectivity between the dark and light reactions. We have already attributed the latter effect to the difference in reactor temperature required to obtain similar conversions in dark and light conditions. The light reactions (dashed curve) occur at 30-40 K lower temperature for a given conversion (cf. Figure 1). Lower temperature favors the condensation reaction (eq l), so that the yield of DMM reaches a maximum at higher conversion. In both cases (dark and light), the yield of C H 2 0 shows a distinct positive curvature at the same conversion at which the DMM yield passes through its maximum. In effect, the sum of the product concentrations of DMM and C H 2 0 varies almost linearly with conversion. Interestingly, the slope of this line ((CH20 + DMM) vs. % conversion) is about 1/3. This indicates that only one-third of the initially adsorbed CH,O,,, species that undergo a hydrogen abstraction reaction at either dark or photoassisted oxidation conditions are subsequently desorbed as C H 2 0 or its condensation product DMM. The remaining two-thirds of the CH,O(,) species that undergo reaction are converted instead to M F in the parallel reaction channel noted above. We note that it is not possible to construct a graph similar to Figures 2 and 3 for the case of C H 3 0 H oxidation on unmodified Ti02, because the product levels for the dark reaction are too small at the temperatures that can be obtained with our present photoreactor. Our earlier results showed that a marked difference in selectivity exists between the dark and light reaction^.^ For the dark reaction the only significant partial oxidation product is MF, and it reaches a maximum concentration of only a few percent before being converted to CO and/or C 0 2 . In contrast, for the light reaction we observed M F with 80% selectivity and C H 2 0with 20% selectivity. As mentioned above, both of the latter products appeared in parallel. Discussion Surface Geometry. The most significant feature of the results described above is the fact that the photoefficiency of the vanadia/Ti02 monolayer catalyst is much lower than that of either the molybdena/Ti02catalyst or unmodified Ti02 For the thermal reaction, the product selectivities are qualitatively similar for the two catalysts, suggesting that the reaction mechanisms are not significantly different. This implies that the decreased photoefficiency must arise from a difference in either the photoexcitation step or the charge-transfer mechanism. Since the same support is used in both catalysts, and since the absorbance cross section of one monolayer of a surface oxide modifier is negligible relative to the absorbance of bulk Ti02, it appears that a difference in the efficiency of the charge-transfer mechanism which accompanies the surface reaction is responsible for the lower photoconversion obtained on the vanadia/Ti02 catalyst. We propose that the decreased photoconv6rsion observed on the vanadia/Ti02 catalyst can be directly attributed to the higher V cation surface concentration in the vanadia surface oxide layer, relative to the molybdena monolayer. As noted above, the Raman spectra of the two catalysts indicate that the vanadia surface oxide layer is completed at a concentration of 10 x 1014V cation/cm2, while the molybdena layer is completed at a coverage of 4 X 1014 Mo cations/cm2. The TiOz support used in these catalysts contains 67% anatase, for which the dominant crystal plane is reported to be the (001) surface.13 The calculated surface unit cell density of the ideally terminated (001)-anatase plane is 7.00 X 1014/cm2.14 Thus, we estimate that only one-half of the T i 0 2 surface unit cells (13) Boehm, H.P.A h . Catal. 1966, 16, 179. (14) Wyckoff, R. G. In Crystal Structures; Interscience: New York, 1964; Vols. I and 11.
Carlson and Griffin
I
I
I
I
I
Figure 4. Proposed structure for the molybdena/Ti02 (anatase) surface oxide monolayer.
0 V (+5)
0 Ti (+4)
00 (-2)
Figure 5. Proposed structure for the vanadia/Ti02 (anatase) surface oxide monolayer.
contain a molybdena cation in the saturated molybdena/Ti02 catalyst, while there are about one and a half V cations for each unit cell in the vanadia/Ti02 catalyst. Possible structures which illustrate this difference are shown in Figures 4 and 5. The suggested structure for the molybdena/Ti02 surface oxide monolayer (Figure 4) is based on several assumptions: that the thermodynamic driving force which favors formation of the surface oxide monolayer is also sufficient to force a commensurate ordering between the surface molybdena layer dhd the underlying T i 0 2 lattice, that one Mo cation is present for every two T i 0 2 surface unit cells, that each Mo cation is accompanied by three additional 02-anions in the fully oxidized state of the catalyst, and that the electrostatic valence rule15 is satisfied for both the surface Ti(+4) cations and the monolayer Mo(+6) cations. To satisfy the latter criterion, the Mo cations are shown directly above the surface T i 0 2 lattice 0” anions which do not have a Ti cation neighbor immediately below them. In addition, each Mo cation shares two bridging 02-anions with its nearest-neighbor Mo cations and has two outwardly directed terminal 02-anions. The presence of the latter is confirmed by the broad 960-an-’ band in the Raman spectra of the monolayer catalyst. Thus, each Mo cation has two 1-fold coordinated 02-anions and three 3-fold 02anions as nearest neighbors, which provide an effective anionic charge of -6 in the nearest-neighbor coordination shell. In addition, each Ti(+4) cation in the T i 0 2 surface layer has six 3-fold 02-neighbors, which provide an effective anionic charge of -4 in the nearest-neighbor coordination shell. The key feature of the suggested geometry for the molybdena/Ti02 surface oxide monolayer is the requirement of two terminal M d bonds associated with each Mo cation, in order to satisfy the electrostatic valence requirement of the Mo cation. If there were one Mo cation associated with every T i 0 2 surface unit cell (Le., a more densely packed structure), these terminal 02anions would have to be replaced by bridging anions for steric reasons, and the electrostatic valence rule would no longer be (15) Knozinger, H. Catal. Rev.-Sci. Eng. 1978, 17, 31.
The Journal of Physical Chemistry, Vol. 90, No. 22, 1986 5899
Photooxidation of Methanol
-
CH&)
* CH3O14
cH20(g)
CH20(a)
OCt+OCH3(a)
I
CH30CH+CH3(g)
‘7
r6
l5
e
I
OCHOCWa) ‘10
-CHJOQ)
+Hcoccqg) ‘g
co,co2 co Figure 6. Proposed mechanism for dark and photoassisted CH30H oxidation on supported oxide catalysts. Adsorbed H atoms are omitted for clarity. satisfied. Instead, we propose that the monolayer develops with a 1:2 ratio between Mo cations and TiO, surface unit cells, in order to accommodate two terminal 0,- anions for each Mo cation. A structure for the vanadia surface oxide layer is suggested in Figure 5. This structure follows the observation by Vejux and CourtineI6 that a close crystallographic fit exists between the bulk lattice spacings of the (010) plane of VzOs and the (001) plane of Ti0, (anatase). The structure contains four V cations for every three TiOz surface unit cells, corresponding to a surface concentration of 9.3 X l O I 4 V cations/cm2 of Ti0,. Each V(+5) cation has a coordination shell consisting of one 1-fold, one 2-f0ld, and three 3-fold @- anions, which provide an electrostatic valence count of -5. The terminal V V bonds are again included on the basis of the Raman spectra evidence. Kozlowski et a1.* have also analyzed EXAFS data to indicate the presence of V=O double bonds. The distinguishing feature of the vanadia/Ti02 surface oxide monolayer, relative to the molybdena/TiO, monolayer, is the higher concentration of V cations at the saturation loading. This appears to be the result of the lower 0,- anion coordination requirement of the V(+5) cation, which means that only a single terminal V V bond is needed to complete the electrostatic valence rule for anion coordination. The smaller cation radius of V(+5) relative to Mo(+6) may also contribute to the much denser packing structure. Reaction Mechanism. The probable mechanism for C H 3 0 H oxidation on molybdena/Ti02 and unmodified T i 0 2 and unmodified TiOz catalysts has been discussed in our earlier paper.3 As noted above, the similarity in product yields suggests that the same mechanism occurs on the vanadia/TiOz catalyst. The proposed mechanism is reproduced in Figure 6,modified slightly from its earlier form to suggest a more plausible pathway for the parallel desorption of CHzO and MF. The initial step is the dissociative adsorption of CH30H (rl). Several authors have suggested that this step may occur at the terminal M V bonds of the surface oxide monolayer.” On pure TiO, at sufficiently high temperatures, this species may undergo a condensation reaction with a neighboring CH,O,,) species to yield CH30CH3as a desorption product (r2)., This dehydration reaction is not observed in the present work. The primary oxidation step under either dark or light conditions is the abstraction of an H atom from the CH,O,,) intermediate (r,). This is also the rate-limiting step for the overall reaction. The resulting CH,O,,, intermediate then appears to react along one of three parallel channels: (1) Desorption as CHzO (r4). We propose that this occurs if there is neither a neighboring surface 0,- anion nor an 0-containing adsorbate fragment to stabilize the CH,O,,, species by nucleophilic attack a t the C atom. (2) (16) Vejux, A.; Courtine, P. J. SolidState Chem. 1978, 23, 93. (17) Anpo, M.; Suzuki, T.; Kubokawa, Y.; Tanaka, F.; Yamashita, S . J . Phys. Chem. 1984,88, 5778. (18) Yoshida, S.; Matsumura, Y.; Noda, S.; Funabike, T. J . Chem. Sor., Faraday Trans. I 1981, 77, 2237. (19) Anpo, M.; Tanahashi, I.; Kabokawa, Y. J . Chem. Soc., Faraday Trans. 1 1982, 78, 2121.
Stabilization by a neighboring 0,- anion to produce a CH,O,,,) intermediate, which rapidly dehydrogenates to produce a stable HCOO,,) species (rs). (3) Stabilization by a neighboring CH,O,,) species to produce an adsorbed hemiacetal precursor to methyl formate, OCH,OCH,,,) (r6). Evidence that at least reactions r4 and r6 occur in parallel is provided by the parallel selectivity curves observed for CHzO and M F (cf. Figures 2 and 3). If the surface is saturated with CH,O,,,, then the selectivity ratio r4/r6 is expected to be a function only of the catalyst composition and should be independent of conversion. We propose that the hemiacetal intermediate produced in step r6 will either react with a neighboring CH,O,,) group to produce the DMM product, CH30CH20CH3(r7), or else will dehydrogenate to yield the adsorbed methyl formate intermediate (r6). The latter species may either desorb directly (r9) or possibly undergo a surface-catalyzed decomposition to yield CO and CH,O,,) (rl0). Recent evidence suggests that this may be a faster route to C O formation than the HCOO,,, decomposition step (r, 1) Returning to the relative photoefficiency of the various catalysts, we note that the photoexcited H atom abstraction step probably involves migration of a photogenerated hole in the TiO, valence band to the TiOz surface, where it becomes localized at a surface 0,- anion to produce a surface O-,,) radical anion (eq 1). The O-,,) species then abstracts a hydrogen atom from a neighboring CH30,a) group ( e 2). ~ h+ Ob)’- O-,,) (1)
-
+
CH30,a) + O-(s)
-+
CH,O(a) + HO-(s)
(2)
This reaction sequence is readily photoexcited on unmodified TiO, surfaces. Much lower photoefficiencies are observed for the case of molybdena and vanadia modified surfaces. This suggests that a much smaller number of CH,O,,, species are able to react species in the latter catalysts. The fact with photogenerated O-,,) that the observed order of decreasing photoefficiency for the three catalyst compositions correlates with the order of increasing cation concentration in the surface oxide monolayers suggests that the formation of photogenerated O-,,) species is restricted to exposed 0,- anions in the TiO, lattice. For the molybdena/TiO, catalyst, the fact that there is approximately one Mo(+6) cation for every other TiO, surface unit cell means that a significant fraction of the outwardly relaxed TiOz surface 02-anions remain uncovered and are able to react with the CH,O,,, species when the catalyst is illuminated. For the vanadia/TiOz catalyst, the higher V(+5) cation concentration means that few of the TiO, surface 0,- anions are exposed, and the photoassisted reaction is severely reduced. This explanation carries with it the implication that charge transfer does not occur readily between the valence band of the TiO, support and the molecular orbitals centered on the 0” anions that make up the surface oxide monolayer. This may be related to the fact that the energy required to excite the ligand to metal charge transfer in either the V=O or M e 0 terminal double bond exceeds the band gap of Ti02.18J9Thus, one or both of the molecular orbitals that would correspond to the excited state of the surface oxide layer (Le., either the Mo(+5) or V(+4) cation or the 0(-1) anion) lie outside the band edges of the TiOz substrate and cannot accommodate a charge-transfer reaction with the illuminated semiconductor.
Summary Our results show that for the case of vanadia/TiOz and molybdena/Ti02 photocatalysts, the presence of the foreign surface oxide layer severely reduces the efficiency of photoassisted partial oxidation reactions. The apparent cause is steric blocking of the Ti0, surface 02-anions which perform the hydrogen abstraction step from adsorbed alkoxy intermediates following capture of a photogenerated hole in the Ti02valence band. Future work should be directed toward preparing surface modifying layers that do (20) Yamashita, K.; Naito, S.; Tamaru, K. J . Catal. 1985, 94, 353.
5900
J . Phys. Chem. 1986,90, 5900-5907
not block as large a fraction of the T i 0 2 surface 02-anions or which have active centers within the surface modifying layer itself that can accept charge transfer from the T i 0 2 valence or conduction bands to create reactive sites.
V205/Ti02sample and for helpful discussions about the Raman spectra of these materials. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this work.
Acknowledgment. We thank Dr. Israel Wachs of Exxon Corporate Science Research Laboratories for providing the
Registry No. V205,1314-62-1; Ti02, 13463-67-7; Moo3, 1313-27-5; MeOH, 67-56- 1.
Electron Energy Loss and Thermal Desorption Spectroscopy of Pyridine Adsorbed on Pt(l11) V. H. Grassian* and E. L. Muetterties‘ Department of Chemistry and Materials and Molecular Research Division, Lawrence Berkeley Laboratory, University of California, Berkeley, California 94720 (Received: February 20, 1986; In Final Form: May 6, 1986)
The chemisorption behavior of pyridine (NC5H5)on a Pt( 111) surface has been examined by using thermal desorption and electron energy loss spectroscopy as a function of adsorption temperature. The vibrational spectrum of pyridine adsorbed at room temperature on this surface shows intense loss peaks in the specular direction from vibrational modes which can be characterized as in-plane stretching and bending modes. This vibrational spectrum has been interpreted as the formation of an a-pyridyl species (NC5H4)on the surface. The pyridyl moiety is bonded to the platinum surface through the nitrogen and one of the a-carbon atoms with the pyridyl plane perpendicular to the metal surface. When pyridine is adsorbed at low temperature (120 K), it bonds to the surface through both the nitrogen atom and the ?r and A* orbitals of the pyridine ring. As the crystal is warmed to 260 K, at saturation coverage, approximately 50% of the molecules desorb as molecular pyridine. The remaining pyridine molecules partially decompose on the surface to form an a-pyridyl fragment. The electron energy loss spectra of pyridine adsorbed at both low and room temperature is compared to the infrared spectra of two osmium cluster compounds: Os3(CO),,(NC5H5),a pyridine complex, and HOS~(CO),~(NC,H,), a pyridyl complex.
Introduction The bonding of aromatic molecules on many different singlecrystal metals and different crystallographic planes has been a subject of considerable interest. Benzene has been the most studied aromatic molecule so far and has been studied by several different surface science techniques including photoelectron spectroscopy, low-energy electron diffraction, thermal desorption spectroscopy (TDS), and electron energy loss spectroscopy (EELS). The adsorption geometry of benzene on the low Miller index surface planes of many transition metals (Pt, Ni, Pd, Rh, Ir, and Ru) has been established fairly conclusively.’ The ring plane of benzene is oriented parallel to the surface plane. The ?r and ?r* orbitals of benzene are considered to interact with the metal orbitals in forming the chemisorption bond. In the case of heteroaromatics there is the possibility of a similar interaction between the ?r and A* orbitals of the adsorbate and the metal atoms. However, there is now the possibility of an interaction between the metal and the nonbonding lone pair of electrons localized on the heteroatom which can bond to the surface in a conventional a-donor-type interaction. The importance of both of these interactions for heteroaromatics adsorbed on metal surfaces has been observed with pyridine on Ag( 1 11) and Ni( Both vibrational and electronic electron energy loss spectroscopy of pyridine adsorbed on Ag( 11 1) have shown the conversion of a r-bonded to a nitrogen-bonded pyridine as the pyridine coverage is increased. For coverages below 0.5 monolayer, pyridine adsorbs in a flat configuration, Le., with the ring plane parallel to the surface plane in a geometry similar to that of benzene on the Ag( 111) surface. For coverages above 0.5 monolayer there is a *phase transformation” to a nitrogen-bonded species. Recently a study by DiNardo et aL2again using electron *Author to whom correspondence should be addressed. ‘Deceased January 12, 1984.
0022-3654/86/2090-5900$01.50/0
energy loss spectroscopy of pyridine adsorbed on Ni( 100) showed the existence of two nitrogen-bonded species. The surface chemistries of pyridine, deuterium-labeled pyridines, and methyl-substituted pyridines have been studied on the Ni( 100) and Ni( 11 1) surface^.^ The IR and Raman spectra of pyridine and isotopically substituted pyridines have been well studied,- and normal coordinate analysis has been used to assign normal modes to the observed spectral features-’ The infrared spectra of pyridine complexed with a variety of mononuclear transition-metal complexes have been studied as a function of metal and c o l i g a n d ~ . ~ In J ~ all of these metal complexes, pyridine is bonded to the metal atom through the nitrogen lone pair. There were only minor shifts, 15-25 cm-I, in the vibrational frequencies of pyridine in these complexes relative to that of liquid pyridine. This is in contrast (1) (a) Shanahan, K.L.; Muetterties, E. L. J. Phys. Chem. 1984,88, 1996. (b) Koel, B. E.; Crowel, J. E.; Mate, C. M.; Somorjai, G. A. J . Phys. Chem. 1984, 88, 1988. These two publications have a complete reference list for
benzene adsorbed on many different transition metals. (2) (a) Demuth, J. E.; Sanda, P. N.; Warlaumont, J. M.; Tsang, J. C.; Christmann, K.In Vibrations a?Swfaces; Caudana, R., Giles, J.-M., Lucas, A. A., Eds.; Plenum: New York, 1982; pp 391-410. (b) Demuth, J. E.; Christmann, K.; Sanda, P.N. Chem. Phys. Le??.1980,76,201. (c) DiNardo, J. N.; Avouris, P.; Demuth, J. E. J . Chem. Phys. 1984, 81, 2169. (3) (a) Wexler, R. M.; Tsai, M.-C.; Friend, C. M.; Muetterties, E. L. J. Am. Chem. Soc. 1982,104,2034. (b) Wexler, R. M. Ph.D. Thesis, University of California-Berkeley, 1983. (4) Corrsin, L.; Fax, B. J.; Lord, R. C. J. Chem. Phys. 1953, 21, 1170. (5) Wilmhurst, J. K.;Bernstein, H. J. Can. J. Chem. 1957, 18, 1183. (6) Long, D. A.; Thomas, E. L. Trans. Faraday SOC.1963, 59, 783. (7) DiLella, D. P.; Stidham, H.D. J. Raman Spectrosc. 1980, 9, 90. (8) Wong, K. N.; Colson, S . D. J . Molec. Spectrosc. 1984, 104, 129. (9) Gill, N. S.; Nuttall, R. H.; Scaife, D. E.; Sharp, D. W. J. Inorg. Nucl. Chem. 1961, 18,19. (10) Durig, J.; Mitchell, B. R.; Sink, D. W.; Willis, J. N., Jr. Spectrochim. Acra, Par? A 1976, 23A, 1121.
0 1986 American Chemical Society
