J. Phys. Chem. 1994, 98, 11733-11738
11733
Ti3+Defect Sites on TiOz(110): Production and Chemical Detection of Active Sites Guangquan Lu, Amy Linsebigler, and John T. Yates, Jr.* S u ~ a c eScience Center, Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 Received: February 18, 1994; In Final Form: August 16, 1994@
Surface oxygen vacancies (defects) can be created on the TiOz( 110) surface by thermal annealing to high temperatures (above 500 K). Two Ti3+ sites are present at each oxygen vacancy. Adsorbed D20, 13CH20, or 15N0 react reductively at these Ti3+ sites to produce D2, 13C2H4,and 15N20,respectively. The oxygen atoms in the adsorbate molecules are preferentially extracted by the substrate Ti02 surface. The deoxygenation of adsorbates is accompanied by the oxidation of surface Ti3+ sites. The yield of reduction products (D2, l3CZH4,and 15N20)is therefore proportional to the coverage of surface oxygen vacancies. No deoxygenation reactions are observed on the fully oxidized (defect-free) surface. This unique property of the surface provides a convenient method to quantitatively detect the relative coverage of Ti3+ sites on the TiOz( 110) surface.
1. Introduction The structure of single-crystal Ti02 surfaces has been extensively It is well established that Ti3+ defect sites can be created on the Ti02(110) surface by thermal a n ~ ~ e a l i n g . ~On - ' ~ the fully oxidized, defect-free TiOz(ll0) surface, all the surface Ti cations are in the 4+ oxidation state and are 5-fold coordinated to oxygen anions.' Heating the surface to high temperatures induces desorption of surface oxygen, producing oxygen vacancies. One missing oxygen atom at the bridging-0 site leaves two subsurface Ti3+ sites exposed. It is the relative ease in producing such defect sites that makes the Ti02 surface more reactive than other oxides, such as A1203 and Si02, which are also commonly used as catalyst supports. Even lower oxidation states of Ti cations (i.e.: Ti2+ and Ti+) can be produced on the surface under severe reducing or sputtering conditions.10s18 Therefore, the Ti02 surface exhibits not only the acid-base property of most oxides but also oxidative-reductive reactivities. Ti02 is of interest for many reasons including its photocatalytic ~ a p a b i l i t y . ~It~ has - ~ ~been demonstrated that the Ti3+sites play an essential role in the photo-oxidation of organic species on the Ti02 photocatalyst.22 The defect sites also influence the dissociative chemisorption of molecules such as H2016,17,23-29 NH3,30-32H z S , ~and ~ S02.34-36 In this paper, we investigate the reductive reactivity of the surface defect sites on TiO2( 110). Temperature-programmed desorption (TPD) measurements are made to characterize the reduction of D20, 13CH20, and 15N0 molecules at the Ti3+ sites. D20 adsorbs in three different states: dissociative, first layer, and multilayer. D2 is produced from D2O on the annealed TiOz(l10) surface, which contains surface oxygen vacancies. Formaldehyde, 13CH20,is found to undergo reductive coupling to produce ethylene, H213C=13CH2, and 15N0 is reduced to l 5 N 2 O . J h e yields of these products are dependent on the coverage of the surface defects, and no such reactions are observed on the fully oxidized surface.
2. Experimental Section The experiments were performed in a stainless steel ultrahigh vacuum (UHV) chamber with a base pressure of less than 1 x mbar. The system37is equipped with (1) a Perkin-Elmer digitally controlled cylindrical mirror analyzer (CMA) for Auger electron spectroscopy (AES); (2) a home-built low-energy @
Abstract published in Advance ACS Abstracts, October 1, 1994.
0022-365419412098-11733$04.5010
electron diffraction (LEED) apparatus for structural characterization; (3) a UTI IOOCquaddtiole mass spectrometer (QMS) (housed in a differentially pumped shield with a 1.6" aperture38), multiplexed with a VTI computer interface for temperature-programmed desorption (TPD) measurements; (4) an ion gun for Ar+ sputter cleaning; and ( 5 ) a collimated and calibrated microcapillary array doser for the absolute measurement of gas exposure to the crystal s ~ r f a c e . ~ ~ - ~ ~ The TiOz(110) single crystal (10 mm x 10 mm x 1.0 mm) was obtained from Commercial Crystal Laboratories, Inc., and is mounted onto a Ta support plate (10 mm x 10 mm x 1 mm) for heating and cooling. The crystal is mounted in such a way that no part of the support is exposed in the forward direction and only the front face of the crystal is seen in TPD measurements, in Auger measurements, and for gas dosing (using the beam doser). The crystal is heated by means of resistively heating the Ta supporting plate by passing current through two 0.38" W wires spot-welded onto the back of the Ta plate. The temperature of the crystal is measured by a type-K thermocouple inserted into a precut slot at one comer of the crystal. The thermocouple is fixed with a hightemperature ceramic adhesive (AREMCO 571) which has nearly the same thermal conductivity and thermal expansion coefficient as TiO2. The crystal temperature can be linearly ramped between 100 and 1000 K by using feedback control from the thermocouple. The heating rate in the TPD measurements reported here is 0.5 Ws. The crystal is cleaned by 1.5-kV Ar+ sputtering. The defect-free (oxidized) surface is obtained by annealing the clean surface in an 0 2 flux (-1 x 1013Odcmzs) to 900 K followed by continuous exposure to the same 0 2 flux at 300 K for at least 1 h. The surface thus prepared shows a sharp (1 x 1) LEED pattem. Annealing of the TiOZ(110) surface was reproducibly carried out by using temperatureprogrammed heating to the final temperature desired and immediate cooling. The rate of heating was 0.5 Us. Annealing this surface to higher temperatures (above 500 K) will create defect sites which are presumably oxygen vacancies at bridgingoxygen sites. No change in the LEED pattem and the AES ratio of TU0 is detectable after annealing due to the low coverage of the defect sites. An ISS study has given an upper limit of -0.08 monolayer of oxygen vacancies for a surface annealed to 1000 K.16 Common impurities on Ti02 such as S, K, and C were not detected on the clean surface by using AES. The reproducibility of the temperature measurements and defect production procedures in our experiments was carefully 0 1994 American Chemical Society
11734 J. Phys. Chem., Vol. 98, No. 45, 1994
91
Lu et al.
1
1
T
Y
i
Defect Production Temperature
I _
- 1
600K
1 100
A
400K
200
300
400
4 500
TEMPERATURE (K)
Figure 1. Thermal desorption spectra of DZfrom the TiOz(110) surface with different Ti3+density upon exposure to 8.34 x 1014Dz0/cm2. Also shown in the upper panel is a TPD spectrum of DzO from TiOz(1 10) annealed to 900 K. Tad = 105 K. dT/dt = 0.5 IUS.
examined. The design of the UHV system used in this work allows two TiO2( 110) crystals to be mounted on separate heating assemblies and studied at the same time. The TPD behavior of several molecules including CH3C1, D20, and D2 (produced from reduction of D20 by surface defect sites) was measured on both crystals and identical results were obtained. The TPD measurements are made by placing the crystal 1.0 mm in front of the 1.6-mm aperture of the QMS. The electron energy is 70 eV, and the QMS aperture is biased at -80 V to avoid the emission of stray electrons to the crystal. All gas exposures are done with the crystal at 105 K. The clean and oxidized surface is first annealed to the indicated temperatures to produce surface defects before each gas exposure. The oxygen gas ( 0 2 , 99.99+%) used for oxidizing the Ti02 crystal was purchased from Linde gas and was used without further purification. DzO was obtained from Aldrich Chemicals Co. (99.996 atom % D) and was purified with four freezepump-thaw cycles, followed by one vacuum distillation process. Paraformaldehyde (99 atom % 13C) was purchased from Cambridge Isotope Laboratories and three cycles of freeze-pump-thaw are performed after the white powder is transferred into a glass bulb. At room temperature, the vapor pressure of 13CH20 produced from paraformaldehyde is high enough for our experiments. QMS mass analysis indicates that only 13CHz0monomer was present in the vapor phase. ISNO was purchased from Cambridge Isotope Laboratories (99 atom % lSN) and was used without further purification.
3. Results 3.1. The Reduction of Adsorbed D20 To Produce Dz. Figure 1 shows the thermal desorption spectra of D2 produced by reduction of adsorbed D2O on the TiOz(1IO) surface. A defect-free surface has been annealed to the indicated temperatures before exposure to 8.34 x IOl4 DzO/cmZat 105 K. Little change is observed for DzO desorption from the annealed surface
e 100
l 200
300
400
500
600
700
TEMPERATURE (K)
Figure 2. Comparison of thermal desorption spectra from oxidized and annealed TiOz(110) surfaces for mass 31, 30, 29, and 27 upon Tad = 105 K; dTldt = 0.5 IUS. exposure to 1.4 x 101413CH20/cmZ.
as compared to the oxidized surface. For comparison, a TPD spectrum is presented in the upper panel for D20 desorption from a 900 K-annealed TiOz( 110) surface. Two desorption states are observed with a maximum desorption rate at the peak temperature, Tp, at -180 K and -268 K and are assigned to the first layer and the recombinative desorption from the dissociatively adsorbed D2O molecules, respectively. The QMS signal of D20 for the 268 K state does not retum to baseline until -350 K, indicating that some hydroxyl groups are stable even above 300 K. Dissociative Dz0 adsorption occurs on both the defect-free and the defective surface. Defect sites are not necessary for this process. Higher DzO exposures result in the buildup of a multilayer state, which desorbs at -160 K (not shown). No desorption of DzO, OD, or DZwas observed above 500 K. The desorption spectra of Dz (mass 4), as shown in the lower panel, vary dramatically from the oxidized surface to the annealed surface. On the oxidized surface (defect-free) annealed up to 400 K, no Dz is produced upon DzO adsorption. The small peak at -180 K is an artifact from the large DzO desorption at 180 K. On the 500-K-annealed surface, DZ desorption at -243 K occurs. With the increase of preannealing temperature, this Dz desorption peak increases in intensity, indicating that more DzO is reduced to form Dz. The D2 desorption is attributed to the reductive reaction of DzO at the Ti3+ defect sites. Annealing the TiOz(110) surface to higher temperatures creates more surface defects, producing more reduction product, D2. 3.2. The Reduction of Adsorbed I3CHzO To Yield l3C2H4. Figure 2 compares the thermal desorption products from oxidized (defect-free) and annealed (defective) TiOz( 110) surfaces upon 13CHz0adsorption. Only masses at 31, 30, 29, and 27 are detected during TPD measurements. Mass 28 is not monitored due to a relatively high CO background in the vacuum system. No other desorbing species is observed. On both surfaces, a desorption state at -275 K is observed for masses 3 1, 30, and 29. The observed ratio of these three masses is consistent with the cracking pattem of gas-phase 13CH20in
Ti3+ Defect Sites on TiOz(l10)
J. Phys. Chem., Vol. 98, No. 45, 1994 11735
ON TiO,( 1 lo),
ON TiO,(l l o ) , YIELDING "C,H,
tl-
1
YIELDING "N,O'
T
5x 1 0-a A
Defect Production Temperature
Defect Production Temperature
800K
A
400K 400
450
500
550
600
650
1 700
a\
400K 100
surfaces with different densities of Ti3+sites upon exposure to 1.4 x loi4l3CH20/cm2. Tad = 105 K; dT/dt = 0.5 Ws. our mass spectrometer, indicating molecular desorption of l3CH2O at this temperature. Another less intense l3CH2O desorption peak is also observed at higher desorption temperatures (at -390 K for the oxidized surface and at -350 K for the annealed surface). The high-temperature formaldehyde desorption feature is assigned to the 13CH20molecules adsorbed at some of the defect and step sites. More importantly, desorption features for masses 30, 29, and 27 are observed at -562 K on the annealed surface but not on the oxidized surface. From the ratios of the areas of these desorption peaks, they are attributed to ethylene, l3C2H4,on the basis of our measurements of the mass spectrometer cracking pattern. The formation of ethylene on the annealed TiOz( 110) results from the reductive coupling reaction of adsorbed formaldehyde at the defect sites. Figure 3 shows the 562-K desorption peak for mass 30 (l3C2H4+) on the various annealed surfaces. On the surface annealed to 400 K, no 562-K desorption of 13C2H4is observed. When the surface is annealed above 500 K, l3C2H4is detected, indicating that more surface defect sites are produced at higher annealing temperatures. No carbon deposits can be detected on the surface by AES measurements following 13C2H4desorption. 33. 15N0Adsorption. For 15N0adsorption on the Ti@( 110) surface, 15N20 (mass 46) and the parent 15N0 (mass 31) molecules are the only desorption products detected. No molecular 15N2(mass 30) is produced. Here, only the 15N20 desorption results are presented. Figure 4 shows the TPD spectra of l5N2O following 15N0 adsorption on TiOz(l10) annealed to the indicated temperatures. On the 400-K-annealed surface, only one l5N2O desorption state is observed at -250 K. Upon annealing to higher temperatures, this desorption peak changes slightly in intensity. At the same time, another 15N20 desorption feature, designated 15N20*,develops at -169 K and becomes more intense with increasing annealing temperature. The evolution of this low-temperature 15N20 desorption state follows the same trend as the D2 and 13C2H4production reported above. Figure 5 presents the dependence of the relative yield of reduction products on annealing temperature for all three molecules studied. Three sets of experimental data are plotted
150
200
250
300
350
400
TEMPERANRE (K)
TEMPERATURE (K)
Figure 3. Thermal desorption spectra of l3CZH4from TiOz(110)
500K
Figure 4. Thermal desorption spectra of I5N2Ofrom TiO2(110) surfaces with different densities of Ti3+sites upon exposure to 1.4 x 1014I5NO/ cm2. Tad= 105 K dT/dt = 0.5 Ws.
for D2 production and two sets each for 13C2H4and 15N20.For each molecule, the relative yield is calculated by normalizing the integrated TPD peak area of the product from each surface to that for the 500-K-annealed surface. Although the data are somewhat scattered, it is obvious in this plot that the reduction yields for all three molecules are correlated. This implies that a specific type of reactive site on the surface is responsible for those reduction processes. These sites do not exist on the oxidized surface and are created during the annealing process.
4. Discussion 4.1. The Production of Surface-Active Sites. We have demonstrated that a special type of reactive site can be created on the TiOp( 110) surface by thermal annealing. The coverage of these sites increases with increasing annealing temperatures. Reductive reactions occur at these sites. Adsorbed molecular DzO, 13CH20,and 15N0 lose their oxygen atoms at such sites to produce Dz,13C2H4, and 15N20,respectively. It is believed that these sites are Ti3+ sites as a result of the loss of bridging oxygen atoms on the TiOz(ll0) surface. The annealing process produces surface oxygen vacancies by eliminating surface oxygen atoms by thermal desorpti~n.~-l~ Ultraviolet photoemission spectroscopy (UPS) studies have observed the growth of a surface state at 0.8 eV below the Fermi level after the TiO2( 110) surface is annealed to high temperature~.99'~,'~ This state can be eliminated by adsorption of oxygen or water. Using 1 8 0 2 and H2180 adsorption, the incorporation of atoms into the surface lattice was observed in ISS studies.16 These results suggest that oxygen vacancies exist on the annealed surface. The thermal formation of point defects on TiOz( 110) were also characterized by using electron energy loss spectroscopy,9~10~12 electron paramagnetic r e s ~ n a n c eand ,~ surface conductivity measurement^.^ X P S studies show the presence of Ti3+ sites on the annealed s u r f a ~ e .The ~ ~ ~ ~ ~ ~ ~ chemisorption study presented here demonstrates that the preferential extraction of 0 atoms from adsorbate molecules occurs only on the annealed surface and not on the oxidized
Lu et al.
11736 J. Phys. Chem., Vol. 98, No. 45, 1994
CORRELATION
-
OF
REDUCED PRODUCTS YIELD
ON DEFECT SITES FOR THREE MOLECULES
-
6
0 3
P
P
f,
53
8 a 0 LL
3E Y
3
W
0 1 6 300
4
, 500
,
,
,
,
,
,
,
,
600 700 800 900 DEFECT PRODUCTION TEMPERATURE (K) 400
1000
Figure 5. Correlation plot for the relative yield of reduction products for D20,I3CHzO, and I5NO adsorption on TiOl(l10) as a function of defect site production temperature. The relative yield is calculated by normalizing the integrated TPD area of the reduction product against that for the 500 K-annealed surface in the same set of experiments. Here, a total of seven independent sets of experimental data are plotted.
surface. This confirms that the annealed surface is oxygen deficient, consistent with the oxygen vacancy model. The oxidized (defect-free) TiOz(l10) surface is composed of rows of bridging oxygen atoms on top of the in-plane surface layer,' as shown in Figure 6. Two types of defect sites can be produced by the removal of either a bridging oxygen atom or an in-plane oxygen atom. A bridging oxygen atom is coordinated with two sublayer Ti cations. An in-plane oxygen atom is coordinated with three Ti cations. It is energetically more favorable to remove a bridging oxygen atom. Furthermore, the removal of a bridging oxygen atom reduces the coordination number of two 6-fold Ti cations to five, while two 4-fold coordinated Ti cations are produced at an in-plane oxygen vacancy site. Although both types of oxygen vacancies can be produced, the relatively low temperature for producing the active defect sites as shown in this work (starting temperature -500 K) indicates that a bridging-oxygen vacancy is more probable for mild thermal activation. However, a distinction between the two types of defect sites will require further study using other measurement methods. The bridging oxygen vacancy model for the annealed TiO2(1 10) surface is consistent with other observations of the thermal removal of bridging-oxygen atoms. A (1 x 2 ) LEED pattem is produced upon a long period of annealing the Ti02(110) surface at 700-900 K.6s7 It is proposed that the (1 x 2) surface structure is derived from the TiO2( 110) surface with every other row of bridging oxygen atoms missing. This structure results from the thermal desorption of the surface oxygen to vacuum. At temperatures between 700 and 900 K, the surface oxygen desorption process is more rapid than the diffusion of bulk oxygen to the surface. Surface oxygen vacancies are therefore produced. Longer annealing time causes more surface oxygen to desorb and thus facilitates a merging process of the vacancies to form the (1 x 2) structure. In our experiments, the momentary annealing at -900 K produces only point defects. The coverage of these sites is not high enough to induce a LEED pattem change. 4.2. Oxidationof Ti3+Defect Sites by Adsorbed D2O. The adsorption of D2O on TiO2( 110) results in both molecular
adsorption and dissociative a d s o r p t i ~ n . ~ ~ vIn~ addition, ~,~~-~~ reduction of D2O to form D2 occurs at the defect sites. The formation of D2 is very possibly due to the recombination of adsorbed D atoms produced upon dissociation of DzO at the defect sites. Two Ti3+sites are present at each oxygen vacancy. Dissociative adsorption at a vacancy site produces adsorbed D and OD species, each occupying a Ti3+ site. The adsorption of D at a Ti site creates a Ti-D bond. Upon heating the surface, the adsorbed D atom can recombine with another D atom to desorb as a D2 molecule. Another possibility is for D(a) to react with a neighboring OD group to yield DZleaving a lattice oxygen. It is not possible in this experiment to differentiate these two reaction channels. The presence of surface Ti-H bonds was also suggested in an ISS study for the H2180 adsorption on a highly oxygendeficient TiOa(ll0) surface.16 The saturation coverage of '*O for H2180 adsorption is only half of that for 1 8 0 2 adsorption, implying that H2180 dissociates on the surface to form adsorbed 180Hand H, each of which occupies a surface Ti site.16 It was also found in the same study that the Ti-ISS intensity decreases for the highly 0-deficient surface upon low-energy hydrogen ion exposure. This was believed to be due to the blocking of Ti by the adsorbed H on the Ti3+ sites; these effects were not observed on the oxidized surface. However, it was not possible to directly detect the adsorbed H atoms in the ISS experiments. The above observations are consistent with our detection of DZ production only on the annealed surface. The adsorption and desorption process can be summarized in eqs 1-3. The net result of the adsorption is that two Ti3+ (present at one bridging 0 vacancy site) are oxidized to Ti4+ and the missing lattice 0 is filled. This is proven to be true by our observation (not shown) that the D:!desorption can be eventually eliminated after a D20 adsorption experiment. Adsorption Steps: D,O
-
+ OD(a) (only at defect sites) D,O + Olattice -2 OD(a)
D(a)
Desorption Steps: 2D(a) D(a> + 2 OD(a) Net Result:
D,O
-
D,
-
-
-
D,
D,
D,O
+ Olattice
+ Olattice
+ Olatticc
(3)
4.3. Formation of a Carbon-Carbon Bond by Reductive Coupling. The formation of olefins with the structure R-CH--CH-R has been reported for the adsorption on Ti02 of aldehydes, R-CHO, with two or more carb0ns.4~ The deoxygenation of other organic molecules such as alcohols,44 carketones,& and aldehyde^^^,^^ has also been boxylic observed on reduced Ti02( 100) and powdered Ti02 surfaces. It is believed that the olefin products result from the reductive coupling of aldehyde molecules at the defect sites on the Ti02 surface. Formaldehyde is found to decompose on the defective Ti02(100) surface to adsorbed C, H, and 0,which further react to produce the methoxide species, CO, and C02.l8 On the fully oxidized surface, formaldehyde undergoes a Cannizzaro-type reaction to simultaneously produce the adsorbed methoxide species and formate. The relative yield of each product is strongly dependent on the preparation procedures of the surface.
Ti3+ Defect Sites on TiOz( 110)
J. Phys. Chem., Vol. 98, No. 45, 1994 11737
SINGLE BRIDGING VACANCY
LATTICE VACANCY
DOUBLE BRIDGING VACANCY
I
/
Figure 6. Schematic of TiOz(1 IO), showing nondefective surface
regions and two types of oxygen-vacancy defect sites.
Our studies involve the production of C2H4 from formaldehyde. In our studies, the reaction was clean on the Ti3+ sites, as no other gases or surface products were detected. The fact that others have observed [on Ti02(1OO)] the production of other surface species from formaldehyde indicates that the surface reactions are sensitive to the Ti02 surface structure. The structure of the defective Ti02( 110) surface optimizes the reductive coupling reaction of formaldehyde to C2H4, while suppressing other reaction channels. The reductive coupling of formaldehyde to form ethylene requires the presence of surface defect sites (surface oxygen vacancies). Similar results have been observed for the reaction of larger aldehydes, where Ti2+ and Ti3+ were necessary for the formation of olefins.43 For formaldehyde, the interaction may involve a stable intermediate which contains a methylene group. The product l3CZH4is due to the recombination of two methylene groups at higher surface temperatures. Two oxygen atoms from two 13CH20molecules are left on the surface to fill up two oxygen vacancy sites. The formation of dioxymethylene, -0CH20- has been observed on several oxide surfaces upon formaldehyde ad~orption.~*-~l On these porous oxides, the structure is quite complex, and the adsorbed dioxymethylene undergoes total decomposition or self-disproportionation to form methoxy and formate species. On the nondefective TiOz( 110) surface, the stable lattice structure prevents the removal or involvement of lattice oxygen in the reaction. However, on the defective TiO2( 110) surface, the oxygen vacancies have a strong tendency to extract the oxygen atom from the adsorbed molecule. Therefore, the deoxygenation reaction is optimized among all the possible reaction channels. The following scheme, eqs 4-6, is proposed for the 13C2H4 production on the annealed TiO2( 110) surface:
+ vacancy +
Adsorption Step:
CH2=0
Desorption Step:
2/0CH20\
Net Result:
2CH20
-
+ 2 vacancies
CHp=CHp
-
-
/OCH20\
Olanice
+
C2H4(g)
(4)
401anice
(5)
+ 2OlattiCe
(6)
4.4. Deoxygenation of 15N0. The 15N0 adsorption results presented here represent the first study of 15N0 adsorption on single-crystal oxides. In addition to the molecular 15N0 desorption at 120 K, two 15N20desorption features are detected at -169 K and -250 K. The 169-K desorption, designated 15N20*, is observed only on the defective surface and its intensity increases with increasing preannealing temperature, indicating this feature is related to the adsorption and decomposition of 15N0at the Ti3+ defect sites created in the annealing process. However, the 250-K 15N20 desorption process is observed on both the oxidized (“defect-free”) and the annealed surface (defective); the reactive sites responsible for this process are therefore not related to the thermally generated defect sites.
This feature is assigned to the 15N0decomposition at the steps. It is known that annealing the Ti02 surface at 900 K cannot completely merge the steps created during Ar+ s p ~ t t e r i n gA .~~ long period of annealing at higher temperatures is required to reduce the step density. Using 15N0 adsorption, we observed a large lSN20 desorption peak at 250 K for a surface with only one cycle of annealing and oxidation at 900 K after Ar+ sputtering. This peak is gradually reduced after increasing the number of annealing cycles. However, we were unable to completely eliminate this feature by annealing at 900 K, consistent with the findings in the above The 15N20desorption at 169 K is characteristic of the reaction of 15N0with the surface Ti3+ sites. As illustrated in Figure 5, its formation is quantitatively consistent with the results for DZ and 13C2H4 produced from D20 and 13CH20 adsorption, respectively. A lattice oxygen is produced to fill up the oxygen vacancy, resulting in the 15N20 formation from two adsorbed 15N0molecules. The surface produced after l5N2Odesorption is inert toward further oxidation by 15N0, indicating that the surface Ti3+ sites have been oxidized. These results are in good agreement with previous studies of NO adsorption on powdered Ti02 catalyst^.^^-^^ The following scheme, eqs 7-9, is proposed: Adsorption Step: Reaction Step: Net Result:
NO(gas) 2NO(a)
2NO(a)
-
NO(a)
(7)
+ vacancy - Olattice+ N20(a)
+ vacancy - Olanice+ N,O(gas)
(8)
(9) 4.5. Chemical Detection of Surface Ti3+ Sites. We have demonstrated that D20, 13CH20,and 15N0react with the surface Ti3+ sites on the TiO2( 110) and that each produces a single reduction reaction product with characteristic desorption behavior. The deoxygenation products D2, 13C2H4,and 15N20give high sensitivity for mass spectrometric detection and can be easily quantified in a relative manner. Therefore, these reactions can be conveniently applied to measure the relative surface coverage of Ti3+ sites on the TiO2( 110) surface. 5. Conclusions The reductive reactivity of Ti3+ sites on Ti02(110) was investigated for three molecules, D20, 13CH20,and 15N0. The results can be summarized as follows: 1. Surface sites for reductive reactions do not exist on the fully-oxidized TiO2( 110) surface (defect-free). Such sites can be created by annealing the surface to above 500 K. 2. The surface reactive sites are surface oxygen vacancies, most probably missing bridging oxygen atoms. 3. Adsorbed D20, CH20, and NO molecules can be reduced at the defect sites to produce D2, C2&, and N20, respectively. The reactions are clean, without other products. 4. The above reductive reactions of the defect sites can be conveniently applied as a titration method to measure the relative coverage of surface defects.
Acknowledgment. We acknowledge with thanks the support of the Army Research Office. Helpful comments from Dr. David Teveault of Aberdeen Proving Grounds are acknowledged. References and Notes (1) Henrich, V. E. Progr. Sur$ Sci. 1979, 9, 143;Rep?. Progr. Phys. 1986, 48, 1481 and references therein.
Lu et al.
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