5240
J . Phys. Chem. 1985,89, 5240-5246
Adsorption of CO, CO,, H,, and H,O on Tltanla Surfaces with Dtfferent Oxidation States G . B. Rauppt and J. A. Dumesic* Department of Chemical Engineering, University of Wisconsin, Madison, Wisconsin 53706 (Received: April 23, 1985)
The adsorptive properties of titania surfaces with different oxidation states were probed by temperature-programmeddesorption (TPD) of CO, H2, CO,, and H 2 0 . Auger electron spectroscopy and X-ray photoelectron spectroscopy revealed that vacuum annealing an oxidized titanium foil at temperatures from 300 to 800 K was an effective means of systematically varying the average surface oxidation state from Ti4+to Ti2+. Carbon monoxide weakly adsorbed (desorption energy of 44-49 kJmol-’) in a carbonyl fashion on coordinatively unsaturated cation sites. Titania surfaces were inert with respect to H2 adsorption and dissociation. Carbon dioxide adsorbed in a linear molecular fashion at exposed cation sites or as monodentate carbonates at surface oxygen anions (desorption energy of 45 kJmol-I). A constant fraction (ca. 0.8) of the total adsorbed C 0 2dissociated upon heating, releasing CO in two distinct desorption peaks at 240 and 330 K. The atomic oxygen produced in this process remained on the surface. Activation energies for CO, dissociation were estimated to be 58 and 81 kJmol-l corresponding to the peak temperatures of the two reaction-limited desorption peaks. Water adsorbed both molecularly and dissociatively, with molecular hydrogen evolution becoming more extensive on surfaces which were initially more reduced. These results are discussed in terms of the role of titania oxidation state in CO hydrogenation over titania-supported metal catalysts.
Introduction Titania in an important catalytic material not only because of its effectiveness in photoassisted oxidation, reduction, and decomposition reactions’-5 but also because of the activity and selectivity enhancement titania imparts to group metals when used as a support6-8or as a “promoterng-12in CO hydrogenation reactions. This latter phenomenon, known as “strong metalsupport interaction”, provided the primary motivation for the present study. The enhancement in CO hydrogenation kinetics as well as the observed anomalous CO and H2 chemisorption behavior of titania-supported metal catalysts were originally attributed to electron transfer from partially reduced titania (e.g., Ti3+)to the metal.1316 Later, Burch and Flambard postulated the existence of special sites located at the metal-support interface.17*18 At these sites, which consist of a metal atom or ensemble and a Ti3+cation (with an associated anion vacancy), CO dissociation was thought to be facilitated. More recently, it has been reported that titania can modify the chemisorptive and catalytic properties of metallic crystallites by migrating onto the crystallite surfaces.”2,1+2z This explains how a support can influence a large metal particle and may also help explain the behavior of small particles that are thought to exhibit “structure sensitivity”. The role of titania oxidation state remains unclear. Using Mossbauer spectroscopy to characterize titania-supported FeNi catalysts, Dumesic and c o - w ~ r k e r sconcluded ~ ~ , ~ ~ that reduced titania is not present under methanation reaction conditions for which activity enhancement in CO hydrogenation is still observed. It appears that the key difference between reduced and oxidized titania is that the former shows higher mobilityz5 and spreads over metal surfaces. The support may play a role in catalytic reactions if reactants, intermediates, or products chemisorb on the support surface. The best known example of this so-called “bifunctionality” of the support occurs in Pt/A120, reforming catalysts in which carbonium ions may adsorb and rearrange on the alumina itself. In the research described presently, the adsorption characteristics of the simple gases CO, H,, CO,, and HzO were determined by temperature-programmed desorption (TPD) from titania model surfaces of varying oxidation state. The gases studied represent both reactants (CO, H,) and products (CO,, HzO) of CO hydrogenation reactions. Titania oxidation state was varied between 4+ and 2+ (TiO, and TiO) through annealing at different temperatures in vacuo. With this approach, the relative importance of the adsorption of reactant/product gases on titania could be assessed for the full range of titania oxidation states which might Present address: Department of Chemical and Bio-Engineering, Arizona State University, Tempe, A 2 85287.
0022-3654/85/2089-5240$01 SO10
exist in a conventional high surface area titania-supported catalyst. A similar strategy has recently been employed by Udovic and D u m e s i ~for ~ ~the , ~ study ~ of adsorption on model magnetite water-gas shift catalysts.
Experimental Section The TPD experiments were performed in the ultrahigh vacuum (UHV) chamber described previously using a constant heating rate of 20 K-s-1.21927The surface oxidation state of the model titania sample was varied in the following manner. A polycrystalline titanium foil (Alfa, 99.98%) ca. 0.127 mm thick and 20 mm diameter was initially cleaned with numerous cycles of argon ion bombardment (8 X Pa, 25 PA, 3 kV, 30 min) and high-temperature annealing in vacuo (1000 K, 30 min). This procedure has been shown to remove all surface impurities with the exception of ca. 0.05-0.10 monolayer of residual oxygen.28
(1) Sato, S.; White, J. M. J . Phys. Chem. 1981, 85, 592. (2) Pichat, P.; Herrmann, J.-M.f Disdier, J.; Mozzanega, M. N. J. Phys. Chem. 1979,83, 3 122. (3) Fleischauer. P. D.: Allen, J. K. J . Phvs. Chem. 1978, 82. 432. (4j Y0neyam.H.; Toyoguchi, Y.; Tamuia, H. J. Phys. Chem. 1972.76, 3460. (5) Nozik, A. J. Nature (London) 1975, 257, 383. (6) Vannice, M. A.; Garten, R. L. J . Catal. 1979, 56, 236. (7) Vannice, M. A.; Garten, R. L. J . Catal. 1980, 63, 255. (8) Vannice, M. A.; Twu, C. C.; Moon,S. H. J . Catal. 1983, 79, 70. (9) Kugler, E. L.; Garten, R. L. U.S. Patent No. 4273724, June 16, 1981. (10) Vannice, M. A.; Sudhaker, C. J . Phys. Chem. 1984, 88, 2429. ( 1 1 ) Chung, Y.-W.; Xiong, G.; Kao, C.-C. J . Catal. 1984, 85, 237. (12) Raupp, G. B.; Dumesic, J. A,, submitted to J. Catal. (13) Horsley, J. A. J. A m . Chem. SOC.1979, 101, 2870. (14) Greiner, G.; Menzel, D. J . Catal. 1982, 77, 382. (15) Kao, C. C.; Tsai, S. C.; Bahl, M. K.; Chung, Y.-W. Surf. Sci. 1980, 95, 1.
(16) Fung, S. C. J. Catal. 1982, 76, 225. (17) Burch, R.; Flambard, A. R. Stud. Surf. Sci. Caral. 1982, 11, 193. (18) Burch, R.; Flambard, A. R. J . Catal. 1982, 78, 389. (19) Sadeghi, H. R.; Henrich, V. E. J. Cafal. 1984, 87, 279. (20) Simoens, A. J.; Baker, R. T. K.; Dwyer, D. J.; Lund, C. R. F.; Madon, R. J. J. Catal. 1984, 86, 359. (21) Raupp, G. B.; Dumesic, J. A. J. Phys. Chem. 1984, 88, 660. (22) Santos, J.; Phillips, J.; Dumesic, J. A. J. Catal. 1983, 83, 168. (23) Jiang, X.-2.; Stevenson, S.; Dumesic, J. A. J. Catal. 1985, 91, 11. (24) Jiang, X.-2.; Stevenson, S.; Dumesic, J. A., Kelly, T.; Casper, R. J. Phys. Chem. 1984, 88, 6191. (25) Overbury, S . H.; Bertrand, P. A,; Somorjai, G. A. Chem. Reu. 1975, 75, 547. (26) Udovic, T. J.; Dumesic, J. A. J. Catal. 1984, 89, 303. (27) Udovic, T. J.; Dumesic, J. A. J. Catal. 1984, 89, 314.
0 1985 American Chemical Society
The Journal of Physical Chemistry, Vol. 89, No. 24, 1985 5241
Adsorption on Ti Surfaces
b (K) .-0
'j 1 b -
-
.-
I-
5
ll.-
12-
0 f-
460 BINDING ENERGY
450
Figure 1. X-ray photoelectron spectra of the Ti 2p312-2p112region of oxidized titanium foil vacuum-annealed at indicated temperatures, T,.
The foil was initially oxidized at 650 K a t an oxygen pressure of ca. 7 X lo4 Pa for 1 h. Under similar conditions this resulted in an oxide layer ca. 35 nm thick, as determined by in situ ell i p ~ o m e t r y . ~Following ~ initial oxidation, the foil was increased or decreased to the desired annealing temperature T (ranging from 150 to 800 K),the oxygen was evacuated from the chamber, and the sample was annealed for 180 s. During the annealing process, partial reduction of the titania surface takes place at temperature T above 300 KZ9via titanium cation diffusion to the surface and oxygen counterdiffusion and dissolution into the bulk metal. This reduction process is not possible in a fully oxidized single crystal; instead, reduction must proceed through oxygen anion removal from the surface into the gas phase. This latter reduction process proceeds with greater difficulty, and reduction of fully oxidized sample is usually accomplished by argon ion bombardment when working under UHV conditons. The surface oxidation state of the titanium foil was estimated through Auger electron and X-ray photoelectron spectroscopy (AES and XPS) measurements performed on a similarly treated titanium foil in a separate UHV analysis chamber (Physical Electronics, Model 548 ESCA/Auger electron spectrometer). Samples were loaded through a turbo-pumped introduction chamber and heated in situ on the introduction rod by a pancake-type heater. All binding energies were referenced vs. the residual C 1s peak value of 284.4 eV. X-ray excitation was accomplished with a differentially pumped aluminum source anode. Auger electron spectroscopy was performed at a primary beam energy of 3 kV and beam current of 20 wA.
Results XPS and AES Characterization. X-ray photoelectron spectra region for the oxidized polycrystalline titaof the Ti nium foil are shown as a function of annealing temperature from 300 to 800 K (f20 K)in Figure 1. The XPS spectrum following vacuum annealing a t 300 K shows the first several layers of the sample to be T i 0 2 by virtue of the locations of the 213312 and 2p1i: photoemission peaks. Published peak positions of Ti02,30Ti203, Ti0,32*33 and Ti30are indicated in the figure. No oxidation states other than Ti4+were present in the XPS analysis volume of this sample. Vacuum annealing at 400-500 K results in surface (28) Bignolas, J. B.; Bujor, M.; Bardolle, J. Surf.Sci. 1981, 108, L453. (29) Smith, T. Surf. Sci. 1973, 38, 292. (30) Ramqvist, L.; Hamrin, K.; Johansson, G.; Fahlmann, A,; Nordling, C. J. Phys. Chem. Solids 1969, 30, 1835. (31) Sinha, A. P. B.; Honig, J. M. J. Solid State Chem. 1976, 19, 391. (32) Franzen, H. F.; Umana, M. X.; McCreary, J. R.; Thorn, R. J. J. Solid State Chem. 1976, 18, 363. (33) Sayers, C. N.; Armstrong, N. R. Surf.Sci. 1978, 77, 301.
303
LW
503
603
7w
8W
ANNEALING TEMPERATURE (K)
Figure 2. O/Ti atomic ratios estimated from XPS and AES measurements as a function of vacuum-annealing temperature. XPS ratio calculated from 0 l s and Ti 2 ~ , / ~ - 2 ppeak ~ / ~ areas corrected for photoelectron cross sections and constant scaling factor. AES ratio calculated from 0 (KLL) 510 eV and Ti (LMM) 390 eV peak heights corrected for relative sensitivities.
reduction to give measurable concentrations of Ti3+in the analysis volume. For annealing temperatures above 600 K, further reduction to Ti3+ occurs, as well as to Ti2+. The surface annealed at 800 K consists primarily of Ti2+,with lesser amounts of Ti3+ also present. Importantly, no Tio was evident in any of the spectra. The oxygen 1s spectra showed a single peak at ca. 533.0 eV, indicating that all oxygen present on or near the film surface was associated with metal oxide; that is, significant concentrations of hydroxyl groups were not present on the titania surfaces. With increasing extent of surface reduction, the 0 1s peak location did not measurably shift (the total shift between the 300 and 800 K annealed surfaces was ca. f0.2 eV). Oxygen to titanium atomic ratios were calculated from the areas under the Ti 2 ~ 3 / ~ - 2 pand , / ~ 0 1s photoemission peaks. Peak areas were measured by using a linear background base line correction and were normalized by using published photoelectron cross sections.34 This method yielded an O/Ti ratio of 2.2 for the 300 K annealed surface, vs. a value of 2.0 expected for a stoichiometric Ti02film. This discrepancy has been previously encountered in XPS studies of various metal o ~ i d e sincluding ~ ~ , ~ ~titania33 and is thought to be due to improper background subtraction. Titanium XPS peaks for the 2p3,2-2p,/2 transitions contain appreciable backgrounds due to inelastic loss peaks and shakeup satellite^.^^ Rather than attempt more complex background correction schemes, the oxygen to titanium atomic ratios were adjusted by a constant factor which gave a value of 2.0 for the most oxidized (300 K) surface. Ratios calculated in this manner as a function of sample vacuum annealing temperature are shown in Figure 2. The extent of surface reduction increased in a smooth fashion with increasing annealing temperature. Consistent with the Ti 2p3/2-2p1/2 peak positions which showed Ti2+,the O/Ti value of 1.1 for the surface annealed at 800 K suggests that this surface is similar to TiO. Auger electron spectroscopy analysis of these surfaces corroborated the XPS results. Following four sputter-anneal cycles ~
(34) Wagner, C. D.; Riggs, W . M.; Davis, L. E.; Moulder, J . F. 'Handbook of X-ray Photoelectron Spectroscopy", Muilenberg, G. E., Ed.; Perkin-Elmer Corp.: Eden Prairie, MN, 1978. (35) Lin, A. W. C.; Armstrong, N. R.; Kuwana, T. Anal. Chem. 1977,49, 1228. (36) Kim, K. S.; Winograd, N. Chem. Phys. Lett. 1975, 31, 312.
The Journal of Physical Chemistry, Vol. 89, No. 24, 1985
5242
Raupp and Dumesic
f
U
8
w D 1
260
300
Lb 5b
6b
7b
860
A
S1gnai x 2
TEMPERATURE (K) Figure 3. CO desorption from oxidized titanium following vacuum-annealing at (a) 300, (b) 400, (c) 500, (d) 600, (e) 650, and (f) 800 K. All traces represent desorption from saturation coverages.
I
I
I
I
1
I
I
in the AES/ XPS chamber, carbon contamination was reduced 100 200 300 LOO 500 600 700 to less than 5% of a monolayer and S to less than 2%. ApproxTEMPERATURE (K) imately 8% of a monolayer of oxygen remained on the surface. Figure 4. Carbon monoxide and carbon dioxide desorption spectra folNote that in the TPD chamber 25 cycles were employed to clean lowing 50-langmuir C’*02exposure at 150 K on oxidized titanium vacthe surface of the Ti foil. uum annealed at (a) 250, (b) 300, (c) 400, (d) 500, (e) 600, (f) 650, (g) The relative atomic ratio of oxygen to titanium O/Ti was 750, and (h) 800 K. estimated by dividing the “(E)/dE intensities of the oxygen KLL transition at 510 eV by the intensity of the titanium LMM carbonyl species (carbon end down). No peaks due to C and 0 transition at 390 eV, with correction for differences in the accepted surface recombination were observed, suggesting that C O does Auger sensitivities of 0 and Ti.37 For the most-oxidized surface, not dissociate on titania under our experimental conditions. an O/Ti ratio of 1.6 was obtained. This value is typical of those For adsorption temperatures from 303 to 373 K,Gopel et al.& measured for titania powders and air-oxidized titanium observed a more strongly bound CO species, estimated to have and can most likely be attributed to electron-beam r e d u ~ t i o n . ~ * ~ ~an isosteric heat of adsorption of 80 kJ-mol-I. This species adThe O/Ti ratio decreased with increasing annealing temperature sorbed at coverages up to ca. 0.01 monolayer, and a fraction of in a fashion similar to the results from XPS, as shown in Figure the C O desorbed as COz,effectively reducing the titania. This 2. The XPS-derived values are thought to be more representative state was not observed in the present CO desorption experiments, of the actual surface composition. in which adsorption temperatures of 150 K were used exclusively. TPD Results. CO AdsorptionlDesorption. Exposure of the Saturation coverages estimated for the surfaces annealed from titanium oxide surfaces to CO a t room temperature resulted in 300 to 800 K were low, ranging from 0.9 X lOI7 to 8.9 X 10’’ negligible C O adsorption. After the surface was dosed a t 150 molecules.m-2, respectively. If we assume a site density of 1 x K, CO was weakly adsorbed on the surface, as indicated by the 1019 molecules.m-2, these correspond to fractional coverages of low peak temperatures of the TPD spectra shown in Figure 3. At 0.009 to 0.089 monolayer. The increase in coverage as the surface each annealing temperature a single molecular state desorbed in becomes more reduced suggests that CO adsorption may occur the vicinity of 200 K. As the annealing temperature increased, at exposed, low-coordination cation sites. It should be noted that the saturation coverage increased significantly and the peak these fractional coverages are estimates only since the oxidized position shifted to slightly higher temperatures. titanium foil site density is unknown. The density used is apAssuming a first-order desorption process and a preexponential proximately that of a metallic titanium polycrystalline surface. of 10” s-l, we estimate a desorption activation energy of 44 H2 AdsorptionlDesorption. Exposure to hydrogen of the titania kJ.mol-l for the 300 K, or most-oxidized surface. This value surface annealed at temperatures between 300 and 800 K resulted increased to 49 kJ.mo1-I for the 800 K, or most-reduced surface. in negligible desorption under all conditions studied. Sample These energies are similar to the 40 kJ.mol-1 heat of adsorption temperatures during dosing from 150 to 350 K and exposures up reported for C O adsorbed in a carbonyl fashion on and to 1000 langmuirs (1 langmuir = 1.33 X lo4 Pa-s) were used in to the 46 kJ-mol-I desorption energy reported for CO desorption an attempt to adsorb hydrogen. The titania surfaces are evidently from low surface area magnetite (Fe3O4).” Carbon monoxide incapable of dissociating significant amounts of molecular hyhas also been found to adsorb weakly on A1203with a heat of drogen, consistent with the work of Henrich and Kurtz on single adsorption equal to 59 k l - m ~ l - ~ . Thus, “ ~ CO apparently adsorbs crystal titania.45 Gopel et a1.@ reported that H2 adsorbs and on oxidized and reduced titania surfaces as a weakly bound dissociates at defect sites on TiOz (110) for adsorption temperatures from 303 to 348 K. Coverages were low, however, with maximum fractional coverages of ca. 0.005 monolayer. An isosteric heat (37) Davis, L. E.; MacDonald, N. C.; Palmberg, P. W.; Riach, G. E.; of adsorption equal to 83 kl-mol-“ was estimated for this hydrogen Weber, R. E. “Handbook of Auger Electron Spectroscopy”, Muilenberg, G. E., Ed.; Perkin-Elmer Corp.: Eden Prairie, MN, 1976. adsorption state. (38) Armstrong, N. R.; Quinn, R. K. Surf. Sci. 1977, 67, 451. C02AdsorptionlDesorption. The adsorption of CI8O2on the (39) Shik, H. D.; Jona, F. Appl. Phys. 1977, 12, 311. various oxide surfaces held at 150 K yielded CI80desorption upon (40) Chung, Y.-W.; Lo, W. J.; Somorjai, G. A. Sur/. Sci. 1977, 64, 588. heating in addition to desorption of the parent molecule. Figure (41) Lo, W.J.; Chung, Y.-W.; Somorjai, G. A. Surf. Sci. 1978, 71, 71. (42) Gay, R. R.; Nodine, M. H.; Henrich, V. E.; Zeiger, H. J.; Solomon, E. I. J . A m . Chem. SOC.1980, 102, 6752. (43) Della Gatta, G.; Fubini, B.; Ghiotti, G.; Morterra, C. J . Cutul. 1976, 43, 90.
(44) GGpel, W.; Rocker, G.; Feierabend, R. Phys. Reo. B 1983,28,3427. (45) Henrich, V. E.; Kurtz, R. L. Phys. Reo. E 1981, 23, 6280.
Adsorption on Ti Surfaces
The Journal of Physical Chemistry, Vol. 89, NO. 24, 1985 5243
1
The oxidized titanium surfaces were prepared with I6O2so that the possibility of oxygen exchange between the surface and carbon oxides could be investigated. Under no circumstances was CI60 detected in the gas-phase product desorption spectra. An approximately constant fraction of C180160desorbed simultaneously with the CI8O2peak at 200 K, corresponding to ca. 2% of the total C 0 2 desorption. No C t 6 0 2was desorbed. The results described above allow us to make tentative adsorption site assignments to the three desorption states observed. The a-state is similar in heat of adsorption to monodentate carbonate species adsorbed on other oxide s u r f a ~ e s . ~ ’ , ~ ’ - ~ ~ Monodentate carbonates require the presence of a surface base site, or oxygen anion pictured as follows:
L
I
M
ANNEALING TEMPERATURE ( K ) Figure 5. Fractional CI80, and C’*O coverages on oxidized titanium following 50-langmuir C1802exposure at 150 K vs. sample vacuum-an-
nealing temperature. 4 illustrates this behavior. Following 50-langmuir Ci802exposure at 150 K, CI8O2weakly bound to the surface desorbed at 200 K, followed by desorption of Ci80at ca.240 and 330 K. Comparison of the CO peaks with those desorbed following C O adsorption shows that these peaks evolve from a reaction-limited process. In this case the reaction is C 0 2 dissociation to give C O and surface oxygen. We conclude that a minimum of three C 0 2 adsorption states exist on titania surfaces corresponding to the peaks labeled a ( C i 8 0 2200 K peak), &, and O2 (Ct80240 and 330 K peaks). The molecularly adsorbed a state is held to the surface with a desorption binding energy of 45 kJ-mol-l, estimated assuming first-order desorption kinetics and a preexponential factor of lOI3 s-l. Assuming first-order decomposition kinetics and v = lot3s-l, activation energies for C 0 2 dissociation are estimated to be 58 and 81 kl-mol-’, for the Pi and p2 states, respectively. In general, C 0 2 does not dissociate on metal oxides upon desorption. Exceptions are the mixed zinc oxide systems ZnA1204,ZnCr204,and ZnFe204in which exchange between surface oxygen and CO2/CO mixtures occurs through an oxidation-reduction mechanism.46 With increasing annealing temperatures, the initial coverages of all three COz desorption states increased for constant C 0 2 exposure (50 langmuir). This can be viewed more clearly with the aid of Figure 5 which plots the CI8O2fractional coverage desorbed from the a state and fractional coverage desorbed as Ci80from the & plus b2 states as a function of surface annealing temperature. The plotted coverages represent saturation or near-saturation values since large Ci802exposures were used. Initial sticking coefficients for the various surfaces were estimated to be in the 0.1-0.2 range, suggesting that adsorption occurred through a nonactivated process. The figure shows that the a or molelcular state coverages were low on all surfaces, with values less than 0.01 monolayer on 170-300 K surfaces which gradually increased to ca. 0.04 on the 800 K, or most-reduced surface. For all surfaces, the @-statecoverages exceeded a-state coverage by factors of 4-6. The overall dissociation probability, defined as the population of the @-statesdivided by the sum of all states, a p, was approximately constant a t 0.84 f 0.02 regardless of annealing temperature for T between 250 and 700 K. A lower probability equal to 0.78 was found for the 800 K surface. For a given surface, this probability was nearly independent of C 0 2 exposure. The amount of desorbed CO was proportional to the amount of adsorbed C 0 2 . The near-constant dissociation probability suggests that a-CO2 states do not dissociate at 150 K upon adsorption, but instead convert to B-states upon heating.
+
(46) Yoneda, 4503.
Y.; Makishima, S.;Hirasa, K. J. Am. Chem. SOC.1958, 80,
M
If all C 0 2 molecules desorbing as the a-state were originally adsorbed as monodentate carbonates, one would expect a decrease in the population of this state with an increase in surface reduction. In fact, the opposite trend was observed. Hence, only a fraction of the a-state carbon dioxides could have been adsorbed as monodentate carboantes, with the highest fractions occurring for the most oxidized (Le., 250 or 300 K) surfaces. On the basis of the observed trend in site population, it seems reasonable to assign the remainder of the C02 a-state to adsorption at exposed cation sites. Weakly bound, linear C02adsorbed species have been found to adsorb on such strong acid sites on oxidized titania ( a n a t a ~ e ) ~ * ~ ’ as well as on other oxide materiaks2 Gopel et al.44assigned a COz adsorption state with isosteric heat of adsorption equal to 63 kJ.mol-I to adsorption at an oxygen anion site on a Ti02{ 110) surface. We favor assignment of the carbon dioxide @-stateadsorption to strongly held organic or bidentate carbonate species illustrated as follows:
/ \
M
M
181 \ 16
0
I
I
M
Po2
- %d;e I
f 160
0
t
!
M
1
bidentate
M
These adspecies require the presence of a coordinately unsaturated cation adjacent to an oxygen anion, or acid-base pair site. As the degree of surface reduction increases one would expect a greater filling of these adsorption states; this trend was indeed observed. Due to the high affinity of titanium for oxygen, the oxygens bonded to the Ti cations are strongly held to the surface, facilitating C 0 2dissociation to give gas-phase C t 8 0and an oxidized surface containing both I8O and I6O. It is important to note that Tanaka and Whitesi investigated C 0 2 adsorption using infrared spectroscopy on reduced and oxidized titania (anatase) and observed bands due to bidentate carbonate species and to C 0 2 coordinated at Ti sites. (47) Tu&, J. M. D.; Gondez-Tejuca, L.J . Chem. Soc., Faraday Tram. I 1981, 77, 591. (48) Ueno, A.; Hochmuth, J. K.; Bennett, C. 0. J . Cafal. 1977, 49, 225. (49) Kwan, T.; Fujita, Y. J. Res. Imt. Cafal.Hokkaido Uniu. 1953,2, 110. (50) Morterra, C.; Chiorini, A,; Bocuzzi, F. 2.Phys. Chem. 1981, 124, 211. (51) Tanaka, K.; White, J. M. J . Phys. Chem. 1982,86, 4708. (52) Morterra, C.; Ghiotti, G.;Bocuzzi, F.; Coluccia, S. J . Coral. 1978, 51. 299.
5244
The Journal of Physical Chemistry, Vol. 89, No. 24, 1985
Raupp and Dumesic OLE
,
012
s
3
-I
LL
Z
0 I-
n E
/
0 v,
I
W
n
6, D;”O
Figure 6. Temperature-programmed desorption spectra from oxidized titanium prepared with l*OZand vacuum annealed at 650 K,followed by exposure at 150 K to D2l60; (a) 3, (b) 5, (c) 15, and (d) 30 langmuirs.
Finally, it is useful to compare the results of these TPD studies of C 0 2 on titania with analogous studies of COz on iron oxide surfaces prepared with different oxidation states. In short, Udovic and D ~ m e s i c ~showed ~ , ~ ’ that bidentate carbonate species on iron oxides exchanged one oxygen atom with the surface and these carbonate species desorbed as C 0 2 ,not as CO. In contrast, these carbonate species on titania do not exchange an appreciable amount of oxygen with the surface and they desorb as CO. This different behavior can be attributed to stronger metal-oxygen bonding in titania compared to iron oxides. Water Adsorption/Desorption. Water adsorbed on titania, like CO,,is capable of dissociation and oxidation of the titania. Typical desorption behavior as a function of initial water surface coverage for a titania surface annealed at 650 K is depicted in Figure 6. The titania surface was prepared with I8O2 and dosed with D2I60 so that oxygen exchange, if present, could be detected. At D 2 0 exposures up to 3 langmuirs, D2I60 desorbs from a broad distribution of sites, with desorption temperatures ranging from 250 to 500 K. Sequential adsorption of D 2 0 and H 2 0 resulted in isotopic mixing of H and D to give D20, HDO, and H 2 0 upon thermal desorption. Thus, these adsorption states must be dissociative in nature. These states, denoted PI, are also capable of exchanging oxygen with the titania surface as evidenced by the observed D2180peak desorbing nearly simultaneously with the D2I60 B,-states. At low exposures, negligible D2desorption was detected. As exposure was increased above 3 langmuirs, molecular (or a ) D2I60 states filled (corresponding to the desorption peak a t 210 K) as the &-states neared saturation coverage. In addition, D2 desorbed from the surface in a single broad peak centered near 400-425 K. This behavior is indicative of a second type of dissociatively adsorbed water species, which is accordingly denoted the B2-state. Assuming first-order desorption and a preexponential factor of loi3s-l, the activation energy of desorption for the a-state was determined to be 51 kJ-mol-I. This value is close to those found for molecular adsorption of water on other oxide surface^.^^*^^ Moreover, this value is similar to the heat of liquefaction of water (45 kJ-mol-l), suggesting that these most-weakly bound species are molecular in nature and are essentially physisorbed on the (53) Udovic, T. J. Ph.D. Thesis, University of Wisconsin-Madison, 1983. (54) McCafferty, E.;Zettlemoyer, A. C. Discuss. Faraday SOC.1971.52, 239.
( 5 5 ) Morishige, K.; Kittaka, S.;Iwasaki, S.;Morimoto, T. J. Phys. Chem. 1981, 85, 570. (56) Primet, M.; Pichat, P.; Mathieu, M. V. J . Phys. Chem. 1971, 75,
1216. ( 5 7 ) Jackson, P.; Parfitt, G. D. Trans. Faraday SOC.1971, 67, 2469.
Adsorption on Ti Surfaces The behavior of the titania surfaces presently investigated is similar to the behavior of SrTi03 { 11 1) reported by Ferrer and S o m ~ r j a i .Following ~~ exposure of the clean crystal to D20, the D 2 0 thermal desorption peak shape and position suggested that water adsorbed both molecularly and dissociatively to form hydroxyl groups. Nearly 75% of the total initially adsorbed D 2 0 desorbed as D, at higher temperatures, leaving oxygen on the surface. Surface characterization using XPS and UPS revealed that, prior to adsorption, the surface was partially reduced, containing about one monolayer of Ti3+ ions.59 Following adsorption/desorption, the Ti3+surface ion concentration decreased from the original value. Using H2I8Odosing of the clean and reduced SrTi03 (111) surface, we estimated that 15% of the water molecules desorbed had exchanged oxygens with the surface. The present results for well-characterized model surfaces confirm that the presence of surface Ti3+facilitates the dissociative adsorption of water on titania surfaces, as has been suggested by Tanaka and White for anatase powder.51 Undoubtedly, the high affinity of titania for oxygen plays an integral role in the dissociation process.
Discussion Adsorption on Titania. The combination of surface characterization and thermal desorption measurements on titania surfaces annealed in vacuo at various temperatures allows us to make the following conclusions regarding the adsorption characteristics of CO, H2, CO,,and H 2 0 : (i) C O adsorbs weakly on oxidized or reduced titania, with adsorption strength and saturation coverage increasing with increasing reduction of the titania; (ii) H2cannot be dissociated on titania even on the most reduced surfaces, and hence hydrogen does not adsorb strongly; and (iii) both C 0 2 and water adsorb molecularly and dissociatively on titania, and oxidize the surface upon desorption. Based on these observations, the relative importance of direct participation of titania in catalytic C O hydrogenation reactions over titania-supported group 8-10 metals can be assessed. Several research groups have proposed that methanation proceeds through direct hydrogenation of CO adsorbed on acidic sites on the supThe present results on model titania surfaces show that CO does indeed adsorb on acidic sites (Ti3+cations), but only in a weak molecular fashion. This adsorption state would not be significantly populated under moderate pressure CO hydrogenation conditions due to the presence of more-strongly bound adsorbates. Moreover, since they are weakly bound, the small number of adsorbed C O molecules would probably not dissociate, a prerequisite for hydrogenation. Burch and Flambard'7-'8v62favor a mechanism for C O hydrogenation wherein CO adsorbs and is subsequently dissociated at a metal-support interfacial site, with carbon bonding at a metal atom and oxygen bonding at the anion vacancy associated with a Ti3+ cation. This model cannot be evaluated on the basis of the present experiments. In a forthcoming paper, however, titania surfaces containing nickel particles are investigated and this issue is addressed.I2 Using infrared spectroscopy, Palazov et al.63 observed that methoxy and formate groups are adsorbed on alumina following exposure to CO/H2 gas mixtures at room temperature. At higher temperatures these species decomposed to form methane. Brief studies of low-molecular weight hydrocarbon adsorption on the titania model surfaces were conducted but not described in the Results section. The titania surfaces were inert toward methane and ethylene adsorption at room temperature. At lower adsorption temperatures a small amount of ethylene and a lesser amount of methane weakly adsorbed, with adsorption strengths suggestive of physisorbed states. N o decomposition of the adsorbed molecules (58) Ferrer, S.; Somorjai, G. A. Surf.Sci. 1980, 97, L304. (59) Ferrer, S.; Somorjai, G. A. Surf.Sci. 1980, 94, 41. (60) Fajula, F.; Anthony, R. G.; Lunsford, J. H. J. Cutal. 1982, 73, 237. (61) Ichikawa, M. Chemtech. 1982, 12, 674. (62) Burch, R.; Flambard, A. R. J . Cutul. 1984, 85, 8. (63) Palazov, A.; Kadinov, G.; Bonev, c.; Shopov, D. J . Catal. 1982, 7 4 , 44.
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was observed upon performing flash desorption. Following exposure to formic acid at low temperatures, formate intermediates subsequently decomposed upon heating to CO and H,O. A portion of the H 2 0 further decomposed to H2 and surface oxygen. No methane was detected under any conditions. These experiments indicate methoxy or higher hydrocarbon intermediates are not easily formed on titania surfaces under UHV TPD conditions. In addition, it appears that formate intermediates on titania cannot account for the high methanation activities of titania-supported metal catalysts. Hydrogen atoms adsorbed on titania might participate directly in CO hydrogenation reactions. Under high vacuum dosing conditions titania does not dissociatively adsorb hydrogen. However, if a source of hydrogen atoms exists, these atoms can be adsorbed on titania. Using electron-stimulated desorption (ESD), K n ~ t e khas ~ ~shown . ~ ~ that hydrogen atoms are adsorbed on T i 0 2 (001)at Ti3+ sites or as surface and subsurface hydroxyls. In titania-supported group 8-10 metal catalysts, the metal particles can dissociate hydrogen and thus provide a source of hydrogen atoms. Although diffusion of adsorbed H and spillover onto the support appears to be an activated p r o c e s ~ ,pretreatment ~~,~~ and reaction are carried out at elevated temperatures and should result in significant population of these adsorption states. Because of their high adsorption strengths, these atomic hydrogen species would be expected to predominate over C O or hydrocarbon adsorbates at steady-state methanation conditions. These states could provide a source of hydrogen atoms that might participate directly in hydrogenation by reaction with carbonaceous intermediates or active carbon adsorbed at the titania-metal interface. Alternatively, this hydrogen could be transported back onto the metal particle surface and thereon undergo catalytic reaction. This latter mechanism, termed reverse spillover, has been postulated to be important in several other reaction systems.66-68 In summary, the adsorption/desorption behavior of C O hydrogenation reactants C O and H2, possible formate intermediates (from HCOOH exposure), and hydrocarbon products CH4 and C2H4suggest that methanation does not occur to an appreciable degree directly on titania. This conclusion is consistent with that previously made by Vannice and V a ~ c o - J a r a . ~However, ~ hydrogen adsorbed on titania could potentially participate in the overall reaction. Importance of Titania Oxidation State in Methanation. The adsorption/desorption characteristics of methanation products H 2 0 and C 0 2 shed light on the relative importance of the oxidation state of titania in methanation catalysts. It was originally believed that enhanced activities for C O / H 2 reactions in such catalysts was due to an electronic modification of the metal particles by charge transfer from Ti3+cations. However, using Miissbauer spectroscopy characterization of titania-supported FeNi alloy particles, Jiang et al.23924inferred that titanium is present as Ti4+under methanation reaction conditions. Addition of water to the reactor feed had a negligible effect on methanation kinetics. This provided further evidence that appreciable amounts of Ti3+ are not present under reaction conditions, since water would be expected to oxidize Ti3+ to Ti4+. The present results for C 0 2 and H 2 0adsorption and desorption further confirm the above conclusions. Carbon dioxide and water adsorb at reduced titania sites and subsequently desorb as CO and H2, leaving oxygen on the surface. The sites responsible for water dissociation on single crystal Ti203and TiO, have been identified previously as complexes containing Ti3+ and oxygen va~ancies~"'~ or cation-pair sites.73 On single crystal surfaces (64) Knotek, M. L. Surf.Sci. 1980, 91, L17. (65) Knotek, M. L. Surf. Sci. 1980, 101, 334. (66) Sermon, P. A.; Bond, G. C. Cutal. Rev. 1973, 8, 211. (67) Holstein, W. L.; Boudart, M. J . Catal. 1981, 72, 328. (68) Fujimoto, K.; Toyoshi, S. Proc. In?. Cong. Cutal. 7th 1981, 235. (69) Vannice, M. A,; Vasco-Jara, J. Stud. Surf. Sci. Cutal. 1982, 11, 185. (70) Henrich, V. E. Prog. Surf.Sci. 1979, 9, 143. (71) Henrich, V. E.; Dresselhaus, G.; Zeiger, H. J . Phys. Reu. Lett. 1976, 36, 1335.
(72) Henrich, V. E.; Kurtz, R. L. J . Vac. Sci. Technol. 1981, 18, 416.
J . Phys. Chem. 1985, 89, 5246-5250
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these defect sites were formed with inert gas ion bombardment. Identical sites are apparently created by vacuum annealing an oxidized polycrystalline titanium foil. These sites not only dissociate water but also can dissociate carbon dioxide. Therefore, under moderate pressure methanation conditions, H,O and CO, produced would subsequently readsorb on the titania and oxidize any Ti3+cations originally produced during reductive pretreatments. This does not preclude the possibility, however, that under reaction conditions an oxidation-reduction process occurs so that at steady state a small concentration of Ti3+sites may exist on the catalyst surface. Conclusions
Temperature-programmed desorption from well-characterized titania surfaces of systematically varied oxidation state indicated (73) Kurtz, R. L.; Henrich, V. E. Phys. Rev. B 1982, 26, 6682. (74) The group notation is being changed in accord with recent actions by IUPAC and ACS nomenclature committees. A and B notation is being eliminated because of wide confusion. Group I becomes groups 1 and 11. group I1 becomes groups 2 and 12, group I11 becomes groups 3 and 13, etc.
that (i) CO adsorbs weakly with low saturation coverage on oxidized titania, with slight increases in adsorption strength and coverage with increasing extent of surface reduction, (ii) oxidized or reduced titania cannot dissociate H2, and hence does not adsorb hydrogen strongly in the absence of a source of H atoms, and (iii) C 0 2 and H20oxidize reduced titania, releasing CO and H,, respectively. These results suggest that reactants directly adsorbed on titania do not play an important role in CO hydrogenation for titaniasupported catalysts, except possibly as a source of hydrogen adatoms. Second, reduced Ti3+ cations become oxidized by product gases (eoz,HzO)under normal methanation conditions and hence cannot be important in the enhanced activities observed for titania-supported metal catalysts. Acknowledgment. We gratefully acknowledge the financial support of Eastman Kodak who provided a fellowship for G.B.R. Funding from the National Science Foundation was also received and is appreciated. Registry No. Ti, 7440-32-6; Ti02, 13463-67-7; C02, 124-38-9; CO, 630-08-0; H2, 1333-74-0; HZO, 7732-18-5.
Surface Perturbation of Vibrational Transitions of Pyrenesilanes Bound to Silica Gel M. L. Hunnicutt, J. M. Harris,* Department of Chemistry, University of Utah, Salt Lake City, Utah 84112
and C . H. Lochmiiller Department of Chemistry, Duke University, Durham, North Carolina 27706 (Received: April 29, 1985)
Enhancement of weakly allowed vibrational transitions is reported for pyrenesilane molecules covalently bound to porous microparticulate silica. The appearance of these new bands is attributed to adsorptive interactions which alter the symmetry and electron density of the surface bound molecules. The intensity of the surface-perturbed vibrational modes is shown to vary as a function of the bound silane surface concentration and the water content of the chemically modified silica. Thermal pretreatment of the modified silica produces large intensity differences indicating that the proton-donor properties of surface silanols are significantlyinfluenced by the concentration of surface adsorbed water. Differences in the orientation and associative interactions of bound and surface adsorbed molecules are also inferred.
Introduction
Infrared spectroscopy has been used extensively to investigate adsorptive interactions between molecules and surface hydroxyls (silanols) on porous silica gel. Considerable effort has focused on establishing the physical state of adsorbed molecules as well as the changes in symmetry, orientation, and rotational motion of molecules in the field of the adsorbent. Spectroscopic studies of molecules adsorbed on porous particulates and metal surfaces have shown that electrostatic field gradients at adsorptive surfaces can induce infrared activity of vibrational modes that are normally infrared inactive.*-9 The enhancement of weakly allowed vibrational transitions and the appearance of new bands where symmetry-forbidden transitions exist has also been observed for pyrene in the presence of polar solvents and has been attributed to a reduction in symmetry of the pyrene molecule via solvent complex Similar results could be expected for (1) Sheppard, N.; Yates, D.J. C. Proc. R.SOC.London, A 1956,238,69. (2) Little, L. H. J . Chem. Phys. 1961, 34, 342. (3) Galkin, G. A,; Kiselev, A. V.; Lygin, V. I. Russ. J . Phys. Chem. 1962, 36, 951. (4) Abramov, V. N.; Kiselev, A. V. Russ.J . Phys. Chem. 1963, 37, 613. (5) Sidorov, A. N.; Neimark, I. E. Rum. J . Phys. Chem. 1964,38, 1518. ( 6 ) Pons,S.; Korzeniewski, C. J . Phys. Chem., in press. (7) Pons, S.; Korzeniewski, C., manuscript in preparation. (8) Devlin, J. P.; Consani, K. J . Phys. Chem. 1981, 85, 2597. (9) Saas, J. K.; Neiff, H.; Moskovits, M. J.; Holloway, S . J . Phys. Chem. 1981, 85, 621. (10) Lianos, P.; Georghiou, S. Phofochem. Photobiol. 1979, 29, 843.
0022-3654/85/2089-5246%01.50/0
pyrene adsorbed on porous, microparticulate silica gel. The present work describes the Fourier transform infraredphotoacoustic spectroscopy of lo-( 3-pyreny1)decyldimethylmonochlorosilane (3PDS) and 3-(3-pyrenyl)propyldimethylmonochlorosilane (3PPS) covalently bound to microparticulate silica gel. New absorption bands were observed in the photoacoustic spectra of the bound pyrene molecules; these bands correspond to weakly allowed transitions not observed in the neat infrared spectra of the 3PDS or 3PPS silanes. The enhancement of these vibrational transitions is attributed to a reduction in symmetry of the pyrene moiety as a result of complex formation with surface silanols. The intensity of these bands varies as a function of the bound silane surface concentration and the water content of the chemically modified silica. Intensity differences of surface-enhanced vibrational transitions are observed for 3PDS and 3PPS silicas having equivalent silane surface coverages. These differences indicate that the bound 3PDS molecules are less sterically hindered and more free to complex with the underlying surface silanols than the shorter chained 3PPS silane. Little vibrational enhancement was observed for pyrene absorbed at surface coverages of less than or equal to one monolayer. This result was unexpected and reflects differences in the associative interactions of the molecules on the silica surface. The results presented yield valuable information concerning molecular perturbations caused by adsorptive surface interactions as well as (11) Lianos, P.; Georghiou, S. Phorochem. Phorobioi. 1979, 30, 355
0 1985 American Chemical Societv
