- Email: [email protected]
TiO2(110) system
Surface
213
Science 249 (1991) 213-222
North-Holland
Interactions
in the Fe/TiO,( 110) system *
Junzhuo Deng, Dezheng Wang, Xuming Wei, Runsheng Zhai and Hongli Wang DalianInstitute of Chemical Physics, Chinese Academy of Sciences,Dafian 1161323, People’s Rep&c of China Received
3 Juty 1990: accepted
for publication
8 January
1991
Fe deposited on TiO,(llO) has been studied using AES, LEED, JSS, UPS and HREELS. The deposited Fe overlayer is found to grow 3n a layer-by-layer mode. Oxygen migrates from the subsurface layers of Tit&(llO) to the deposited Fe during the deposition process. The chemical species of the deposited Fe depends on the state of oxidation of TQ(110) and the amount of Fe deposition. The state of the adsorbed CO in this system is found to depend remarkably on the amount of Fe deposited on the TiQ,(llO) substrate. CO is found to adsorb weakly on a low Fe coverage system of 0.2 ML Fe/TQ,(llO), while it is strongly chemisorbed an a high Fe coverage system of 1.7 ML Fe/Ti02(110). Difference in the CO chemisorption behavior is attributed to the different structural characteristics of these two states af the Fe,/TiOa(llO) system.
The group-VIII transitionmetal/TiO, system has been quite extensively studied for the various interactions between the metal and the support, particularly with regard to the effect of these interactions on the SMSI (strong metal-support interaction) behavior of these systems [l]. For systems using single crystals, considerable amount of work has been done on the Rh/TiO,(llO) system and much light has been thrown on the behavior of SMSI of these and similar systems as a result. For instance, the formation and migration of a titanium suboxide species appears to be well established ]2-4]. The Ni/TiO,(lOO) system has also been studied recently and the layer-bylayer growth of the Ni overlayer up to 3 monolayers has been reported [5]. Chemical bonding at the Ni/TiO,(llO) interface has been inferred [6J. However, the Fe/TiO, system has received less attention by the surface science community in the past than that it deserves. This is particularly so in view of the growing importance of the role of TiO,
* This work was perFormed at the Laboratory Dalian institute of Chemical Physics, Da&n ple’s Republic of China.
003%60%/91/rSO3.50
for Catalysis, 115023, Peo-
0 1991 - Elsevier Science Publishers
as a promising promoter support for the iron catalysts used in the Fischer-Tropsch synthesis from CO and H, [7]. Study of a model system involving iron and single-crystal ‘HO, has not been reported in the literature to our knowledge. It is therefore the aim of our work to study both the electronic and the geometric interactions manifested in the Fe/TiO,(llO) system. Structural requirement for the activation of CO molecule is inferred by our Auger, UPS and HREELS studies.
The experimental system used in this work was a Leybold-Heraeus UHV chamber equipped with AES, ISS, HREELS, UPS and a Varian LEED optics with a base pressure of 5 X 10-i’ Torr. The AE/E = constant mode was used for the energy analyzer in the Auger measurements. He1 UPS spectra were recorded in the AE = constant mode. 1000 eV incident He ions were used for ISS with a scattering angle of 135*. Before it was instailed in the UHV system, the TiO,(tlO) sample was mechanically polished to an optical finish using various grades of polishing
B.V. (North-Holland)
214
Junzhuo Deng et al. / Int~~a~t~o~ in the Fe/ TiO,(IfOf
powders ending with 0.1 pm alumina. All experiments were carried out after the UHV system was baked at 200” C for 12 h. Argon ion bombardment with 3 keV energy and high temperature (7” = 800-1000°C) heating in 2 X 10F6 Torr oxygen cycles were used to clean all contaminants from the sample and to achieve the full oxidation of the sample surface [S]. Iron of high purity was evaporated from a source which consists of a resistively heated Ta wire wrapped with a spectrally pure iron filament. The evaporation rate was controlled by the current through the Fe filament to maintain the filament temperature at 1190 o C to within 3 o C. To improve the macroscopic uniformity of Fe evaporation on the TiO,(llO) surface, a metal sheath with a circular aperture of 7 mm diameter was used to cover the filament assembly approximately 3 cm in front of the sample. The evaporation source was separated from the sample by a shutter to control the evaporation time. The filament assembly was outgassed many times until no pressure rise above the chamber pressure could be detected during the Fe evaporation. Only a small amount of carbon contamination (ca. 0.05 ML) was observed for the evaporation time as long as 1 h. The sample temperature during Fe evaporation was 25 o C.
3. Results and discussion
The Auger signal ratio of Fe(651 eV)/Ti(383 eV) versus Fe deposition time is shown in fig. 1. It is seen from the figure that the Fe/Ti Auger signal ratio rises linearly with the Fe deposition time and breaks occur after 125 and 250 s, respectively. The first break point is taken to indicate the completion of a monolayer (ML) coverage. That is, 1 ML corresponds to a deposition time of about 125 s. Furthermore, the ratio of the slope for the two successive segments remains to be the same, i.e., the following relation holds for the system under our experimental conditions, namely, (S, + i/S,) = constant, where S stands for the slope of the segment concerned. Thus, the deposition
system
1
Fe(651eWTd383eV)
~
100 Fe Evaporation
200
300 Time (set)
Fig. 1. Fe(651 eV)/Ti(383 eV) Auger peak-to-peak Fe deposition time on TiOz(llO).
ratio versus
behavior of this Fe/TiO~(llO) system fully complies with the criteria for the layer-by-layer growth of a vapor-deposited film (Frank-van der Merwe mechanism) [9]. In contrast, the relationship (S,,+,/S,) = constant does not hold for the plot of Fe(651 eV)/0(510 eV) versus Fe deposition time (fig. 2) although the Auger signal intensity ratio also increases linearly with the deposition time of Fe and
Fe
Evaporation
Time
(secl
Fig. 2. Fe(651 eV)/O(SlO eV) Auger peak-to-peak Fe deposition time on TiO,(llO).
ratio versus
Junzhuo Deng et al. / interactions
breaks also occur at regular intervals. This is understandable as the result of oxygen migration during the Fe deposition to be discussed below. This layer-by-layer growth of Fe on a TiO,(llO) surface is further supported by the evolution of the LEED pattern as the deposition of Fe proceeds. The TiO,(llO) gives a sharp (1 X 1) LEED pattern following annealing at 700 o C under 2 x 10v6 Torr oxygen for 30 min. No change of this pattern was observed up to a deposition of appro~mately 0.2 ML of Fe. However, beyond this coverage the spots in the (1 X 1) pattern become diffuse. At about 0.4 ML coverage, a weak (1 X 2) LEED pattern apparently induced by the deposited Fe begins to appear which persists up to a coverage of about 0.6 ML. When the coverage of the deposited Fe exceeds 0.8 ML, the diffraction spots became so weak that no pattern could be discerned. The appearance of an ordered structure around the coverage of 0.5 ML Fe could be caused by the reconstruction of the TiO,(llO) surface [lo], but this is not likely as no thermal ~eatment was used during our LEED measurements. Thus the LEED study supports the formation of a monolayer of deposited Fe on TiOz(llO). 3.2. Migration of oxygen in the Fe/TiO,(llO) system As the coverage of deposited Fe is increased, the O(510 eV)/Ti(383 eV) Auger peak height ratio increases essentially linearly as shown in fig. 3, although plateaus near the compietion of each ML are discemable. During the Fe deposition, the O(510 eV)/Ti(383 eV) Auger peak height ratio is seen to increase from an initial value of 2.46 to a value higher than 4 when the Fe deposition corresponds to a coverage of 3 ML. This enrichment of oxygen on the surface during the process of Fe-deposition can only take place through the migration of oxygen from the subsurface layers to the surface of TiOz(llO). This migration of 0 may be understood by the affinity of 0 for the Fe deposited on the TiO,(llO) surface. Further informa~on on the migration of oxygen is provided by ISS studies. ISS spectra of TiO,(llO) after Fe deposition times of 0, 60, and 150 s, respectively, are shown in fig. 4. The peaks
215
in rhe Fe/ TiO,(I IO) system
I
O(SlOeV)ITi(383eV)
$ tt t ’ 0
2 3 Fe Coverage (Ml-1 Fig. 3. Variation of O(510 eV)fli(383 ev) Auger peak-to-peak ratio with Fe coverage on TiO,(llOf. t
of Fe, 0 and Ti all appear near the values of E/E, expected from the binary collision model [ll]: 0.783 for Fe, 0.417 for 0 and 0.751 for Ti, where E0 is the incident energy and E is the scattered energy of the He ions used. The spectrum with a deposition time of 150 s corresponds to a coverage exceeding 1 ML of Fe. It is seen that with the increase of Fe deposition time, the peak intensity of Fe grows while that of Ti diminishes. It is interesting to note that the Ti signal is no longer detectable when the Fe deposition time is more than 125 s. The disappearance of the ISS signal of Ti at the break point of the curve for Auger signal ratio versus Fe deposition time lends strong support to the identification of the formation of ML of Fe on TiO,(llO) at a deposition time of about 125 s. The 0 peak intensity also changes somewhat with the Fe deposition time. But it does not disappear altogether even when the coverage of Fe exceeds 1 ML on the TiO,(llO) surface. This seems to suggest that at least part of the oxygen
216
Junzhuo Deng et al. / rn~eracIio~s in the Fe/ TiO,(llO)
0.6
0.4
0.8
0
ElEo Fig. 4. ISS spectra of Fe~iO~(llO) times.
Fe Fig. 6. Amplitude ratio of LMV,(A)
0.4 Fe
at different Fe deposition
migrated outwards from the subsurface layers bonds directly to the Fe and stays on the surface. Fig. 5 gives the difference spectra for Fe and 0 versus Fe coverage. It is seen that signal intensity of Fe increases linearly with the coverage, indicating the process of monolayer formation. While
0.b
0.2
system
0.6 0.8 Coverage (ML) to LMVo(B)
0.6
0.8
Coverage
? (ML)
Fig. 5. ES intensity in terms of peak area for Fe and 0 at different coverages of Fe.
that of 0, which gives the quantity of 0 in excess of the amount originally held by the TiO,(llO) surface as deduced from the intensity of the Ti signal, increases sharply initially but flattens out on further deposition of Fe. This clearly indicates the migration of 0 from the subsurface layers of
1 versus Fe coverage on Ti02(110). the inset following ref. [ll].
E leV1 A and E are defined by Auger spectra shown in
217
TiO,(llO) to its surface in the deposition process similar to the situation often observed in a TiO, sample but with the additional feature that at least part of the 0 appears to be held by the Fe deposited on the surface as mentioned earlier. 3.3. Formation of different Ti 0, (110) surface
Fe species on the
The Auger spectra of Fe give much info~ation on the oxidation state of Fe of the sample studied. Following Hemich et al. [12], the degree of oxidation or reduction of the surface Ti ions can be correlated with the experimental value of the ratio ,4/B graphically defined in the inset of fig. 6. A is a measure of the intensity of the LMVT, peak and B is a measure of the intensity of the LMVo peak in the 416 eV Ti peak which is an overlap of two TiLMV Auger transitions. The A/B ratio increases as the surface is reduced, i.e., more Ti3+ is formed. For a fully oxidized TiOZ(llO) surface, the A/B ratio is near zero. For a reduced sample used in our experiments which was argonbombarded and fully reduced, the A/B ratio is 1.39. No change in this A/B ratio was detected for the reduced surface of TiO,(llO) when different amounts of Fe were deposited to this surface. This would indicate that no new Ti3+ was formed during the Fe deposition process, suggesting that no reduction of Ti4+ and thus no oxidation of Fe0 into Fe3” took place during this process. On a fully oxidized TiO,(llO) surface, however, the A/B ratio rises ,sharply at the first deposition of less than 0.1 ML of Fe as shown in fig. 6, clearly indicating the reduction of Ti4+ and the formation of Fe3+. Then it increases further but less steeply until 0.2 ML of Fe was deposited. Furthermore, according to Leygraf and Ekelund [13], the low energy Auger peak of a clean Fe(100) surface at 47.8 eV which is associated with Fe0 undergoes distinctive change upon adsorption of oxygen at room temperature. The 47.8 eV peak, which corresponds to the Auger transition of M II.111M,v,vMrv.v~ is replaced by two peaks at 52.3 and 44.4 eV which are suggested to be related to the surface con~ntration of Fe3+ and Fe*+ respectively. Interesting changes are found to take place in the second-derivative plot of low energy
l.r
I
i/’ 42
Kinetic
41.5
Energy
cl
53
(eV1
Fig. 7. Change of low-energy Auger peaks at different Fe coverages on TiO&llO). (a) TiO,(llO) or reduced TiO,(llO). (b) 0.2 ML Fe/Ti02(110). (c) 1.2 ML Fe/TiO,(llO). (d) 0.2 ML Fe/reduced TiOz(llO), A/B = 1.39. (e) 0.5 ML Fe/TiO, (110) when oxidized.
Auger peaks for the fully oxidized TiO,(llO) covered with different amount of Fe as shown in fig. 7. Fig. 7a gives the second-derivative plot of the low-energy Auger peak for the TiOz(110) sample, either fully oxidized or fully reduced. It is seen that only one peak at - 42 eV due to Ti ions is present. For a 0.2 ML coverage of deposited Fe (fig. 7b), in addition to the shifting of Ti signal from - 42 to - 43.5 eV which is believed to be also associated with the formation of Fe2+, a peak appears at 53.4 eV which may be assigned to Fe3+ according to Leygraf and Ekelund’s work. With further deposition of Fe to 1.2 ML coverage, in addition to the Fe3” peak, an intense peak appears at 47.5 eV (fig. 7~) which may be assigned to Fe0 as described previously. For a reduced TiO,(llO) surface deposited with 0.2 ML Fe (fig. 7d), no peak ascribed to Fe3+ was found and only the Fe0 peak was present. This is in agreement
218
Junrhuo Deng et al. / Interactions in the Fe/ TiO,(llO)
10 Binding
Ef=O Energy
leV)
Fig. 8. He1 UPS spectra for different Fe coverages on TiOz(llO). (a) TiOz(llO). (b) 0.2 ML Fe. (c) 0.45 ML Fe. (d) 0.65 ML Fe. (e) 0.80 ML Fe. (f) 0.95 ML Fe. (g) 1.1 ML Fe. (h) 1.9 ML Fe. (i) 2.2 ML Fe. (i) 3.0 ML Fe.
with the findings obtained previously that no change in A/B ratio in the Auger spectra was observed indicating that no Ti3+ and thus no Fe3+ was formed in this case. Upon oxidation of a 0.5 ML Fe on the reduced TiO,(llO) surface, the Fe0 peak disappears and a Fe3+ peak is formed instead, as expected (fig. 7e). The formation of different species of Fe in a Fe/TiO,(llO) system was also studied by UPS as shown in fig. 8. Fig. 8a gives the UPS spectrum for TiO,(llO). The UPS spectra for TiO,(llO) surface with different coverages of Fe, ranging from 0.2 ML (fig. 8b) to 3 ML (fig. 8j), are obtained. In addition to the d-band peak of Fe near the Fermi level which increases in intensity with coverage of Fe, a peak is observed initially at 1.5 eV below the Fermi level which is associated with Fe3+ and subsequently merged with the dband peak. A small peak assigned to Fe’+ at 2.5 eV below the Fermi level according to the work of Eastman and Freeout [14] is found in all coverages throughout the whole range of deposition. This is
system
also in agreement with the experimental data of low-energy Auger peaks in the second-derivative mode given previously. it was shown that a small amount of Fe*+ was produced in the early stage of Fe deposition on an oxidized surface of TiO,(llO) besides Fe3+. It is seen from the AES and UPS data that various oxidation states of iron are formed upon deposition of Fe on the TiO,(llO) surface. Depending on the condition of the TiO,(llO) surface, whether in an oxidized or a reduced state, and the amount of Fe deposited, different oxidation states of the deposited Fe may be formed. On a reduced Tiq(llO) surface, only Fe0 atoms were formed, apparently because of lack of electron transfer from the deposited Fe to the Ti3+ present on the reduced surface. This is understandable since further reduction of Ti3+ to Ti2+ would be much more difficult. It is known that only Ti3+ was detected in many transition metal/TiO, systems even upon high-temperature reduction [15). On the fully oxidized TiO,(llO) surface, Fe3+ was initially formed with the concomitant formation of Ti3+ by the reduction of Ti4+ on the surface. A small amount of Fe2+ appears to be also present at the initial stage. As the deposition of Fe proceeds beyond about 0.2 ML, only Fe0 was formed in the deposition process. This behavior of the change in the oxidation state of the deposited Fe for the system Fe/TiO,(llO) in different conditions should be compared with the results reported by Dumesic and his co-workers [16]. They found the formation of Fe3+ for the iron deposited on a TiO, film (oxidized) and this Fe3+ was converted into Fe0 upon high-temperature reduction. 3.4. Chemisorption
of CO molecules
For the adsorbed state of CO on supported Fe, including the Fe/Ti02 system, IR studies have been extensive [17]. But EELS results, particularly those designed to shed some light on the activation of CO in the supported metal system, have been seldom found in the literature. Before we look into the adsorbed state of CO in such a system of Fe/TiO,(llO) with different coverages of deposited Fe, however, the chemical state of Fe
Junthuo Weng et al. / Interactions in the Fe/ TiO,(llO)
42 Kinetic
47.5 Energy
53 (eV)
for this deposited overlayer of Fe is examined first. Fig. 9 shows the second-derivative Auger spectra for the 0.2 ML Fe/TiO,(llO) system with different exposures of CU. Fig. 9a gives that for the 0.2 ML FefTiO,(llO) before exposure to CO.. Since it is an oxidized system, Fe is seen to be present in Fe’+ species as shown, But upon cxposure to CO from 1 to 150 L, as seen from figs, 9b to 9e, Fe’ emerges at - 47.5 eV at the expense of the Fe3* peak at - 53 eV. This suggests that the chemical state of Fe in the 0.2 ML Fe/TiO,(llO) system is mainly Fee atoms in a highly dispersed state. This is fully consistent with the LEED observation that a sharp LEED pattern of the original TiO,(llO) substrate was preserved until the 0.2 ML coverage of Fe was exceeded. We will examine at this point what brought about the ~~sfo~at~on from Fe’* to Fe* when
system
219
F~~j~~(l~O) was exposed to CO. At first glance, this Fe’ formation during the CO chemisorption may be simply attributed to the electron transfer from the CO adsorbate to the Fe3’ which is known to be present in this system of low coverage of 0.2 ML Fe on an oxidized TiQ(ll0) surface. But this simple explanation cannot be reconciled with the change of work function of the system as derived from the UPS data for the system upon chemisorption of CO. The work function of this system was found to be practically unchanged as the result of CO adsorption, only - 0.08 eV upon exposure to as much as 150 L CU. A much greater change in work function would be expected if appreciable ejectron transfer took place from CO to Fe, So we conclude that the electrons required to transform Fe3’ into Fe0 did not come from CO when 0.2 ML Fe/TiO,(llO) was exposed to CO. Two other sources of electrons remain to be further considered, namely, Ti and 0. The electrons may conceivably be furnished by the Ti3’ ions which are known to be formed together with Fe3’ as the result of Fe deposition on the TiUz(10). But what is the driving force for this reversal of electron flow from Ti to Fe, in contrast to from Fe to Ti4+, as the result of CO adsorption? Furthermore, why does it stop if CO adsorption really triggers the reoxidation of Ti3” back to Ti4+? Apparently some Fe3+ still remains on the surface of the Fe/TiO,(llO) system after the CO adsorption as seen from fig. 9. A more reasonable explanation for the production of Fe* appears to be associated with the last source of electrons, the oxygen bound to the surface Fe’+ in such a system. In the presence of CO, the oxide ions formed as the result of the migration of 0 from the subsurface layers of TQflfO) would be removed with the liberation of CO, and leave the electrons to Fe3+ with the production of Fe’. This process would stop when the 0 supply is depleted since the Fe surface is only partially covered by 0 as shown earlier, This appears to be what brought about the formation of Fe0 on the surface of 0.2 ML Fe/TiO,(llO) an exposure to CO. As to the chemical state of the high Fe coverage system of more than 1 ML Fe,/TiO&lO), it is known that beyond 0.2 ML coverage, only Fe0
220
Junzhuo Deng et al. / Interactions
in the Fe/ TiO,(llO)
sysiem
I xl 6
LL
2125cmv’
x64
0
1000 Energy
Fig. 10. HREELS
2000
Loss spectra
3000
(cm-‘) of TiOz(llO).
0
‘500 Energy
atoms were formed. It is also known that the 0 migration in such a system levels off with the Fe deposition after the fast initial stage. A good part of the deposited Fe was not covered by 0. The covered part will be reduced by CO as in the case of a low Fe coverage system. Therefore, upon exposure to CO, this high Fe coverage surface will be expected to be predominately Fe0 also. With the chemical state of the deposited Fe now known to be Fe0 for both the low and the high coverage systems, the adsorbed state of CO in the Fe/TiO,(llO) system with different amounts of deposited Fe will then be discussed in the light of both HREELS and UPS data. Fig. 10 gives the HREELS spectra for the fully oxidized TiO,(llO) sample. The energy loss peaks due to surface optical phonons are obtained in the range of low loss energy of less then 0.5 eV. Spectral features attributed to the fundamental as well as to multiple and combination phonon losses were obtained in excellent agreement with those given in the literature [8,18,19]. The HREELS spectra for the adsorbed CO are found to be remarkably dependent on the coverage of the deposited Fe. For a low Fe coverage system of 0.2 ML Fe/TiO,(llO), as shown in fig. 11, the stretching frequency of CO was found to be 2125 cm-‘, not much different from that for CO in the gaseous phase, 2143 cm-‘, suggesting that a weak chemisorption may be involved. While on a high Fe coverage system of 1.7 ML Fe/TiO,(llO), it was found to be considerably
1000
1500
Loss
2000
2500
(cm-‘1
Fig. 11. HREELS spectra of CO adsorption on 0.2 ML Fe/TiO,(llO). (a) 0.2 ML Fe/TiO,(llO). (b) Exposure to 150 L CO on 0.2 ML Fe/TiO,(llO).
lower, 1870 cm-‘, as shown in fig. 12. Owing to the serious interference by the phonon background of the TiO,(llO) substrate, the HREELS spectra of the adsorbed CO were obtained only with great difficulty. We were unable to find any EELS peaks due to CO adsorption in the low wave numbers which may possibly be also present [20,21]. The UPS spectra of adsorbed CO on these systems were also recorded. UPS spectra of the
1870cm“
cA
0
500 Energy
1000 Loss
1500
2000
2500
(cm-‘1
Fig. 12. HREELS spectra of CO adsorption on 1.7 ML Fe/TiOz(llO). (a) 1.7 ML Fe/TiO,(llO). (b) Exposure to 50 L CO on 1.7 ML Fe/TiO,(llO).
Junrhuo Deng et al. / Interactions
.
I
12
10 Binding
.
8 6 4 Energy (eV]
Fig. 13. He1 UPS spectra of Fe/TiO#lO) CO. (a) 50 L CO on 1.2 ML Fe/TiO,(llO). 0.2 ML Fe/TiO,(llO).
2
Ef’lo
after exposure to (b) 150 L CO on
adsorbed CO on various metal single crystals have been studied quite extensively for group-VIII transition metals [22-241. A general picture of CO adsorption on such metals emerge as the result of such studies. It is known for instance that the 5a orbital of CO shifts toward 18 orbital for quite a few group-VIII metals upon adsorption of CO on these metals. It is also known that the separation between llr and 4a orbitals reflects the tendency of dissociation for the CO molecule adsorbed. The larger the separation, the longer the C-O bond, and thus the higher tendency for dissociation. In our Fe/TiO,(llO) system, the UPS spectra for the adsorbed CO also depends strongly on the amount of Fe deposited on the TiOz(llO) surface as shown in fig. 13. For a low Fe-coverage of 0.2 ML, the peaks ascribed to 5a, le and 4a molecular orbitals of adsorbed CO are found to have the same sequence as in the gaseous phase with not much change in their relative positions. In agreement with what was found in HREELS, this also indicates a weak chemisorption. While for a Fe/TiO,(llO) system covered by 1.7 ML of deposited Fe, two new features in the UPS spectrum emerge. Firstly, the Sa orbital shifts 3-4 eV toward the In orbital and the two orbitah appear to merge into one peak. Secondly, the 17r-4a separation of the adsorbed CO is appreciably increased, suggesting a larger separation between C
in the Fe/ TiO,(llO)
system
221
and 0 atoms in the CO molecule. These two features indicate that CO is strongIy chemisorbed on a densely Fe covered surface of 1.7 ML Fe/TiO,(llO). This is in sharp contrast to a weak chemisorption in the case of a sparsely populated surface of 0.2 ML Fe/TiO,(llO). Why do these two systems differ so much in their behavior toward the chemisorption of CO as they both have Fe0 atoms on the surface? For the low Fe coverage system, there were essentially no change in work function upon exposure to CO, not much change in the C-O stretching frequency as measured by HREELS, and not much change in the UPS spectra as compared with that for a gaseous phase, all these point to a weak chemisorption of CO. While on the other hand, for the high Fe coverage system, there were greatly reduced C-O stretching frequency, greatly changed positions of the molecular orbitals of CO after their adsorption, all pointing to a strong chemisorption. The following possibility seems to stand out. For a low Fe coverage system of 0.2 ML Fe/TiO,(llO), each Fe atom is estimated to occupy an area in the order of 50 A* when 1 ML is taken to correspond to 1 X 10” Fe atoms/cm2. The deposited Fe atoms are thus separated from each other by a distance of about 4 Fe atoms. This would mean that the Fe atoms in this system are essentially isolated individuals. In addition to the evidence produced by the LEED pattern mentioned earlier, this is supported by the stretching frequency of the adsorbed CO, 2125 cm-‘, which is very close to the stretching frequencies of the carbonyls in the mononuclear metal carbonyls, e.g. Fe(CO), [25]. Furthermore, deposition of isolated metal atoms [3] has also been invoked to explain the observed shift in XPS binding energy in the system of Rh/TiO,(llO) at low concentrations of the deposited metal. It is thus reasonably certain that the iron deposited on the 0.2 ML Fe/TiO,(llO) was in the form of isolated Fe atoms. It is possible that the structural require ment for the strong chemisorption of a CO molecule is not satisfied for this low Fe coverage system. As a result, such single Fe0 atoms, individually, are not sufficiently facile to accept electrons from, or backdonate electrons to the ad-
sorbed CO. While for the case of CO adsorbed on the densely covered 1.7 ML Fe/TiO,(llO), this restriction is removed. Each Fe atom has many nearest as well as next-nearest neighbors, and consequently strong chemisorption was observed. Our work thus strongly suggests that an ensemble of Fe atoms may be required for the strong chemisorption of CO in this system. Such a structural requirement has been proposed for some catalytic reactions [26], in which the activation of the reactant molecule via the che~so~tive interaction with the substrate undoubtedly ptays a key part.
4. Conclusions
(1) Fe
deposition on a TiO,(llO) surface takes place in a layer-by-layer mode. (2) Oxygen from the subsurface layers of the TiO,(llO) substrate migrates to the surface during the deposition process and binds to the deposited Fe to the extent of - 0.2 ML. (3) The state of the deposited Fe depends on the coverage of the Fe as well as the state of the TiO,(llO) substrate. On a fully oxidized TiO,(llO) surface, Fe3+ is initially formed together with Ti3+ and Fe0 atoms are deposited during the later stage. A small amount of Fe2+ is also formed in the initial stage. On a reduced surface of TiO,(llO) only Fe0 is formed. (4) When CO is adsorbed on the low Fe coverage system of 0.2 ML Fe/TiO,(llO), Fe3+ is reduced to Fe0 and only weak che~so~tion of CO occurs. For a 1.7 ML Fe/TiOZ(llO) system, strong chemisorption of CO is found to occur. The chemisorbed state of CO on the Fe/TiO,(llO) system is proposed to be related to the structural characteristics of the Fe atoms deposited on the surface of TiO,(110).
Acknowledgements The authors wish to express their sincere thanks to Guoxing Xiong for the single crystal of TiO,(llO) provided by him and valuable discus-
sions during the course of this work. Expert assistance given by Yuming Cao in various aspects of instrumentation is also gratefully acknowledged.
References VI See, for example, B. Imelik, C. Naccache,
G. Coudurier, H. Praliaud, P. Meriaudeau, P. Gallezot, G.A. Martin and J.C. Vedrine, in: Metal-Support and Metal-Additive Effects in Catalysis’ (Eisevier, Amsterdam, 1982). 121H.R. Sadeghi and V.E. Henrich, J. Catal. 87 (1984) 279. 131 H.R. Sadeghi and V.E. Heruich, Appl. Surf. Sci. 19 (1984) 330. [41 H.R. Sadeghi and V.E. Hemich, J. Catal. 109 (1988) 1. I51 S. Bourgeois, D. Diakite, F. Jomard, M. Perdereau and R. Poirault, Surf. Sci. 217 (1989) 78. 161CC. Kao, SC. Tsai, M.K. Bahl and Y.-W. Chung, Surf. Sci. 95 (1980) 1. 171 See, for instance, R. Snel, Catal. Rev. Sci. Eng. 29 (1987) 361. D. Frankel, H. Jachnig, K. I81 W. Gopel, J.A. Anderson, Phillips, J.A. Schafer and G. Rocker, Surf. Sci, 139 (1984) 333. f9l W. Schlenk and E. Batter, Surf. Sci. 93 (1980) 9. [LOI V.E. Henrich. Rep. Prog. Phys. 48 (1985) 148. ILLI E.P.Th.M. Suurmeijer and A.L. Beers, Surf. Sci. 43 (1973) 309. and H.T. Zeiger, Phys. Rev. H21 V.E. Hen&h, G. Dresselhaus B 14 (1978) 4908. H31 C. Leygraf and S. Ekelund, Surf. Sci. 40 (1973) 609. H41 D.E. Eastman and J.T. Freeout, Phys. Rev. Lett. 34 (1975) 395. PI See, for instance, CM. Greenlief, J.M. White, C.S. Ko and R.J. Gorte, J. Phys. Chem. 89 (1985) 5025. P’51 B.J. Tartarchuk and J.A. Dumesic, J. Catal. 70 (1981) 335. N. Sheppard and T.T. Nguyen, in: H71 See, for example, Advances in Infrared and Raman Spectroscopy, Vol. 5, Eds. R.J. Clarke and R.E. Hester (Heyden, London, 1978). 1181 B. Rocker, J.A. Schafer and W. Giipel, Phys. Rev. B 30 (1984) 3604. [I91 W.J. Lo, Y.-W. Chung and G.A. Somorjai, Surf. Sci. 71 (1978) 199. J. Kiippers and G. [201 U. Seip, M.-C. Tsai, K. Christmann, Ertl, Surf. Sci. 139 (1984) 89. VI D.W. Moon, S.L. Bemasek, D.L. Dwyer and J.L. Gland, J. Am. Chem. Sot. 107 (1985) 4363. PI C.F. Brucker and T.N. Rhodin, Surf. Sci. 86 (1979) 638. 1231 G. Broden. T.N. Rhodin, C. Brucker, R. Benbow and 2. Hurych, Surf. Sci. 59 (1976) 593. I241 G. Broden, G. Gafner and I.P. Bonzel, Appl. Phys. 13 (1977) 333. [25] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, 4th ed. (Wiley, New York. 1986) p. 292. (261 G.A. Martin, Catal. Rev. Sci. Eng. 30 (1988) 519.
213
Science 249 (1991) 213-222
North-Holland
Interactions
in the Fe/TiO,( 110) system *
Junzhuo Deng, Dezheng Wang, Xuming Wei, Runsheng Zhai and Hongli Wang DalianInstitute of Chemical Physics, Chinese Academy of Sciences,Dafian 1161323, People’s Rep&c of China Received
3 Juty 1990: accepted
for publication
8 January
1991
Fe deposited on TiO,(llO) has been studied using AES, LEED, JSS, UPS and HREELS. The deposited Fe overlayer is found to grow 3n a layer-by-layer mode. Oxygen migrates from the subsurface layers of Tit&(llO) to the deposited Fe during the deposition process. The chemical species of the deposited Fe depends on the state of oxidation of TQ(110) and the amount of Fe deposition. The state of the adsorbed CO in this system is found to depend remarkably on the amount of Fe deposited on the TiQ,(llO) substrate. CO is found to adsorb weakly on a low Fe coverage system of 0.2 ML Fe/TQ,(llO), while it is strongly chemisorbed an a high Fe coverage system of 1.7 ML Fe/Ti02(110). Difference in the CO chemisorption behavior is attributed to the different structural characteristics of these two states af the Fe,/TiOa(llO) system.
The group-VIII transitionmetal/TiO, system has been quite extensively studied for the various interactions between the metal and the support, particularly with regard to the effect of these interactions on the SMSI (strong metal-support interaction) behavior of these systems [l]. For systems using single crystals, considerable amount of work has been done on the Rh/TiO,(llO) system and much light has been thrown on the behavior of SMSI of these and similar systems as a result. For instance, the formation and migration of a titanium suboxide species appears to be well established ]2-4]. The Ni/TiO,(lOO) system has also been studied recently and the layer-bylayer growth of the Ni overlayer up to 3 monolayers has been reported [5]. Chemical bonding at the Ni/TiO,(llO) interface has been inferred [6J. However, the Fe/TiO, system has received less attention by the surface science community in the past than that it deserves. This is particularly so in view of the growing importance of the role of TiO,
* This work was perFormed at the Laboratory Dalian institute of Chemical Physics, Da&n ple’s Republic of China.
003%60%/91/rSO3.50
for Catalysis, 115023, Peo-
0 1991 - Elsevier Science Publishers
as a promising promoter support for the iron catalysts used in the Fischer-Tropsch synthesis from CO and H, [7]. Study of a model system involving iron and single-crystal ‘HO, has not been reported in the literature to our knowledge. It is therefore the aim of our work to study both the electronic and the geometric interactions manifested in the Fe/TiO,(llO) system. Structural requirement for the activation of CO molecule is inferred by our Auger, UPS and HREELS studies.
The experimental system used in this work was a Leybold-Heraeus UHV chamber equipped with AES, ISS, HREELS, UPS and a Varian LEED optics with a base pressure of 5 X 10-i’ Torr. The AE/E = constant mode was used for the energy analyzer in the Auger measurements. He1 UPS spectra were recorded in the AE = constant mode. 1000 eV incident He ions were used for ISS with a scattering angle of 135*. Before it was instailed in the UHV system, the TiO,(tlO) sample was mechanically polished to an optical finish using various grades of polishing
B.V. (North-Holland)
214
Junzhuo Deng et al. / Int~~a~t~o~ in the Fe/ TiO,(IfOf
powders ending with 0.1 pm alumina. All experiments were carried out after the UHV system was baked at 200” C for 12 h. Argon ion bombardment with 3 keV energy and high temperature (7” = 800-1000°C) heating in 2 X 10F6 Torr oxygen cycles were used to clean all contaminants from the sample and to achieve the full oxidation of the sample surface [S]. Iron of high purity was evaporated from a source which consists of a resistively heated Ta wire wrapped with a spectrally pure iron filament. The evaporation rate was controlled by the current through the Fe filament to maintain the filament temperature at 1190 o C to within 3 o C. To improve the macroscopic uniformity of Fe evaporation on the TiO,(llO) surface, a metal sheath with a circular aperture of 7 mm diameter was used to cover the filament assembly approximately 3 cm in front of the sample. The evaporation source was separated from the sample by a shutter to control the evaporation time. The filament assembly was outgassed many times until no pressure rise above the chamber pressure could be detected during the Fe evaporation. Only a small amount of carbon contamination (ca. 0.05 ML) was observed for the evaporation time as long as 1 h. The sample temperature during Fe evaporation was 25 o C.
3. Results and discussion
The Auger signal ratio of Fe(651 eV)/Ti(383 eV) versus Fe deposition time is shown in fig. 1. It is seen from the figure that the Fe/Ti Auger signal ratio rises linearly with the Fe deposition time and breaks occur after 125 and 250 s, respectively. The first break point is taken to indicate the completion of a monolayer (ML) coverage. That is, 1 ML corresponds to a deposition time of about 125 s. Furthermore, the ratio of the slope for the two successive segments remains to be the same, i.e., the following relation holds for the system under our experimental conditions, namely, (S, + i/S,) = constant, where S stands for the slope of the segment concerned. Thus, the deposition
system
1
Fe(651eWTd383eV)
~
100 Fe Evaporation
200
300 Time (set)
Fig. 1. Fe(651 eV)/Ti(383 eV) Auger peak-to-peak Fe deposition time on TiOz(llO).
ratio versus
behavior of this Fe/TiO~(llO) system fully complies with the criteria for the layer-by-layer growth of a vapor-deposited film (Frank-van der Merwe mechanism) [9]. In contrast, the relationship (S,,+,/S,) = constant does not hold for the plot of Fe(651 eV)/0(510 eV) versus Fe deposition time (fig. 2) although the Auger signal intensity ratio also increases linearly with the deposition time of Fe and
Fe
Evaporation
Time
(secl
Fig. 2. Fe(651 eV)/O(SlO eV) Auger peak-to-peak Fe deposition time on TiO,(llO).
ratio versus
Junzhuo Deng et al. / interactions
breaks also occur at regular intervals. This is understandable as the result of oxygen migration during the Fe deposition to be discussed below. This layer-by-layer growth of Fe on a TiO,(llO) surface is further supported by the evolution of the LEED pattern as the deposition of Fe proceeds. The TiO,(llO) gives a sharp (1 X 1) LEED pattern following annealing at 700 o C under 2 x 10v6 Torr oxygen for 30 min. No change of this pattern was observed up to a deposition of appro~mately 0.2 ML of Fe. However, beyond this coverage the spots in the (1 X 1) pattern become diffuse. At about 0.4 ML coverage, a weak (1 X 2) LEED pattern apparently induced by the deposited Fe begins to appear which persists up to a coverage of about 0.6 ML. When the coverage of the deposited Fe exceeds 0.8 ML, the diffraction spots became so weak that no pattern could be discerned. The appearance of an ordered structure around the coverage of 0.5 ML Fe could be caused by the reconstruction of the TiO,(llO) surface [lo], but this is not likely as no thermal ~eatment was used during our LEED measurements. Thus the LEED study supports the formation of a monolayer of deposited Fe on TiOz(llO). 3.2. Migration of oxygen in the Fe/TiO,(llO) system As the coverage of deposited Fe is increased, the O(510 eV)/Ti(383 eV) Auger peak height ratio increases essentially linearly as shown in fig. 3, although plateaus near the compietion of each ML are discemable. During the Fe deposition, the O(510 eV)/Ti(383 eV) Auger peak height ratio is seen to increase from an initial value of 2.46 to a value higher than 4 when the Fe deposition corresponds to a coverage of 3 ML. This enrichment of oxygen on the surface during the process of Fe-deposition can only take place through the migration of oxygen from the subsurface layers to the surface of TiOz(llO). This migration of 0 may be understood by the affinity of 0 for the Fe deposited on the TiO,(llO) surface. Further informa~on on the migration of oxygen is provided by ISS studies. ISS spectra of TiO,(llO) after Fe deposition times of 0, 60, and 150 s, respectively, are shown in fig. 4. The peaks
215
in rhe Fe/ TiO,(I IO) system
I
O(SlOeV)ITi(383eV)
$ tt t ’ 0
2 3 Fe Coverage (Ml-1 Fig. 3. Variation of O(510 eV)fli(383 ev) Auger peak-to-peak ratio with Fe coverage on TiO,(llOf. t
of Fe, 0 and Ti all appear near the values of E/E, expected from the binary collision model [ll]: 0.783 for Fe, 0.417 for 0 and 0.751 for Ti, where E0 is the incident energy and E is the scattered energy of the He ions used. The spectrum with a deposition time of 150 s corresponds to a coverage exceeding 1 ML of Fe. It is seen that with the increase of Fe deposition time, the peak intensity of Fe grows while that of Ti diminishes. It is interesting to note that the Ti signal is no longer detectable when the Fe deposition time is more than 125 s. The disappearance of the ISS signal of Ti at the break point of the curve for Auger signal ratio versus Fe deposition time lends strong support to the identification of the formation of ML of Fe on TiO,(llO) at a deposition time of about 125 s. The 0 peak intensity also changes somewhat with the Fe deposition time. But it does not disappear altogether even when the coverage of Fe exceeds 1 ML on the TiO,(llO) surface. This seems to suggest that at least part of the oxygen
216
Junzhuo Deng et al. / rn~eracIio~s in the Fe/ TiO,(llO)
0.6
0.4
0.8
0
ElEo Fig. 4. ISS spectra of Fe~iO~(llO) times.
Fe Fig. 6. Amplitude ratio of LMV,(A)
0.4 Fe
at different Fe deposition
migrated outwards from the subsurface layers bonds directly to the Fe and stays on the surface. Fig. 5 gives the difference spectra for Fe and 0 versus Fe coverage. It is seen that signal intensity of Fe increases linearly with the coverage, indicating the process of monolayer formation. While
0.b
0.2
system
0.6 0.8 Coverage (ML) to LMVo(B)
0.6
0.8
Coverage
? (ML)
Fig. 5. ES intensity in terms of peak area for Fe and 0 at different coverages of Fe.
that of 0, which gives the quantity of 0 in excess of the amount originally held by the TiO,(llO) surface as deduced from the intensity of the Ti signal, increases sharply initially but flattens out on further deposition of Fe. This clearly indicates the migration of 0 from the subsurface layers of
1 versus Fe coverage on Ti02(110). the inset following ref. [ll].
E leV1 A and E are defined by Auger spectra shown in
217
TiO,(llO) to its surface in the deposition process similar to the situation often observed in a TiO, sample but with the additional feature that at least part of the 0 appears to be held by the Fe deposited on the surface as mentioned earlier. 3.3. Formation of different Ti 0, (110) surface
Fe species on the
The Auger spectra of Fe give much info~ation on the oxidation state of Fe of the sample studied. Following Hemich et al. [12], the degree of oxidation or reduction of the surface Ti ions can be correlated with the experimental value of the ratio ,4/B graphically defined in the inset of fig. 6. A is a measure of the intensity of the LMVT, peak and B is a measure of the intensity of the LMVo peak in the 416 eV Ti peak which is an overlap of two TiLMV Auger transitions. The A/B ratio increases as the surface is reduced, i.e., more Ti3+ is formed. For a fully oxidized TiOZ(llO) surface, the A/B ratio is near zero. For a reduced sample used in our experiments which was argonbombarded and fully reduced, the A/B ratio is 1.39. No change in this A/B ratio was detected for the reduced surface of TiO,(llO) when different amounts of Fe were deposited to this surface. This would indicate that no new Ti3+ was formed during the Fe deposition process, suggesting that no reduction of Ti4+ and thus no oxidation of Fe0 into Fe3” took place during this process. On a fully oxidized TiO,(llO) surface, however, the A/B ratio rises ,sharply at the first deposition of less than 0.1 ML of Fe as shown in fig. 6, clearly indicating the reduction of Ti4+ and the formation of Fe3+. Then it increases further but less steeply until 0.2 ML of Fe was deposited. Furthermore, according to Leygraf and Ekelund [13], the low energy Auger peak of a clean Fe(100) surface at 47.8 eV which is associated with Fe0 undergoes distinctive change upon adsorption of oxygen at room temperature. The 47.8 eV peak, which corresponds to the Auger transition of M II.111M,v,vMrv.v~ is replaced by two peaks at 52.3 and 44.4 eV which are suggested to be related to the surface con~ntration of Fe3+ and Fe*+ respectively. Interesting changes are found to take place in the second-derivative plot of low energy
l.r
I
i/’ 42
Kinetic
41.5
Energy
cl
53
(eV1
Fig. 7. Change of low-energy Auger peaks at different Fe coverages on TiO&llO). (a) TiO,(llO) or reduced TiO,(llO). (b) 0.2 ML Fe/Ti02(110). (c) 1.2 ML Fe/TiO,(llO). (d) 0.2 ML Fe/reduced TiOz(llO), A/B = 1.39. (e) 0.5 ML Fe/TiO, (110) when oxidized.
Auger peaks for the fully oxidized TiO,(llO) covered with different amount of Fe as shown in fig. 7. Fig. 7a gives the second-derivative plot of the low-energy Auger peak for the TiOz(110) sample, either fully oxidized or fully reduced. It is seen that only one peak at - 42 eV due to Ti ions is present. For a 0.2 ML coverage of deposited Fe (fig. 7b), in addition to the shifting of Ti signal from - 42 to - 43.5 eV which is believed to be also associated with the formation of Fe2+, a peak appears at 53.4 eV which may be assigned to Fe3+ according to Leygraf and Ekelund’s work. With further deposition of Fe to 1.2 ML coverage, in addition to the Fe3” peak, an intense peak appears at 47.5 eV (fig. 7~) which may be assigned to Fe0 as described previously. For a reduced TiO,(llO) surface deposited with 0.2 ML Fe (fig. 7d), no peak ascribed to Fe3+ was found and only the Fe0 peak was present. This is in agreement
218
Junrhuo Deng et al. / Interactions in the Fe/ TiO,(llO)
10 Binding
Ef=O Energy
leV)
Fig. 8. He1 UPS spectra for different Fe coverages on TiOz(llO). (a) TiOz(llO). (b) 0.2 ML Fe. (c) 0.45 ML Fe. (d) 0.65 ML Fe. (e) 0.80 ML Fe. (f) 0.95 ML Fe. (g) 1.1 ML Fe. (h) 1.9 ML Fe. (i) 2.2 ML Fe. (i) 3.0 ML Fe.
with the findings obtained previously that no change in A/B ratio in the Auger spectra was observed indicating that no Ti3+ and thus no Fe3+ was formed in this case. Upon oxidation of a 0.5 ML Fe on the reduced TiO,(llO) surface, the Fe0 peak disappears and a Fe3+ peak is formed instead, as expected (fig. 7e). The formation of different species of Fe in a Fe/TiO,(llO) system was also studied by UPS as shown in fig. 8. Fig. 8a gives the UPS spectrum for TiO,(llO). The UPS spectra for TiO,(llO) surface with different coverages of Fe, ranging from 0.2 ML (fig. 8b) to 3 ML (fig. 8j), are obtained. In addition to the d-band peak of Fe near the Fermi level which increases in intensity with coverage of Fe, a peak is observed initially at 1.5 eV below the Fermi level which is associated with Fe3+ and subsequently merged with the dband peak. A small peak assigned to Fe’+ at 2.5 eV below the Fermi level according to the work of Eastman and Freeout [14] is found in all coverages throughout the whole range of deposition. This is
system
also in agreement with the experimental data of low-energy Auger peaks in the second-derivative mode given previously. it was shown that a small amount of Fe*+ was produced in the early stage of Fe deposition on an oxidized surface of TiO,(llO) besides Fe3+. It is seen from the AES and UPS data that various oxidation states of iron are formed upon deposition of Fe on the TiO,(llO) surface. Depending on the condition of the TiO,(llO) surface, whether in an oxidized or a reduced state, and the amount of Fe deposited, different oxidation states of the deposited Fe may be formed. On a reduced Tiq(llO) surface, only Fe0 atoms were formed, apparently because of lack of electron transfer from the deposited Fe to the Ti3+ present on the reduced surface. This is understandable since further reduction of Ti3+ to Ti2+ would be much more difficult. It is known that only Ti3+ was detected in many transition metal/TiO, systems even upon high-temperature reduction [15). On the fully oxidized TiO,(llO) surface, Fe3+ was initially formed with the concomitant formation of Ti3+ by the reduction of Ti4+ on the surface. A small amount of Fe2+ appears to be also present at the initial stage. As the deposition of Fe proceeds beyond about 0.2 ML, only Fe0 was formed in the deposition process. This behavior of the change in the oxidation state of the deposited Fe for the system Fe/TiO,(llO) in different conditions should be compared with the results reported by Dumesic and his co-workers [16]. They found the formation of Fe3+ for the iron deposited on a TiO, film (oxidized) and this Fe3+ was converted into Fe0 upon high-temperature reduction. 3.4. Chemisorption
of CO molecules
For the adsorbed state of CO on supported Fe, including the Fe/Ti02 system, IR studies have been extensive [17]. But EELS results, particularly those designed to shed some light on the activation of CO in the supported metal system, have been seldom found in the literature. Before we look into the adsorbed state of CO in such a system of Fe/TiO,(llO) with different coverages of deposited Fe, however, the chemical state of Fe
Junthuo Weng et al. / Interactions in the Fe/ TiO,(llO)
42 Kinetic
47.5 Energy
53 (eV)
for this deposited overlayer of Fe is examined first. Fig. 9 shows the second-derivative Auger spectra for the 0.2 ML Fe/TiO,(llO) system with different exposures of CU. Fig. 9a gives that for the 0.2 ML FefTiO,(llO) before exposure to CO.. Since it is an oxidized system, Fe is seen to be present in Fe’+ species as shown, But upon cxposure to CO from 1 to 150 L, as seen from figs, 9b to 9e, Fe’ emerges at - 47.5 eV at the expense of the Fe3* peak at - 53 eV. This suggests that the chemical state of Fe in the 0.2 ML Fe/TiO,(llO) system is mainly Fee atoms in a highly dispersed state. This is fully consistent with the LEED observation that a sharp LEED pattern of the original TiO,(llO) substrate was preserved until the 0.2 ML coverage of Fe was exceeded. We will examine at this point what brought about the ~~sfo~at~on from Fe’* to Fe* when
system
219
F~~j~~(l~O) was exposed to CO. At first glance, this Fe’ formation during the CO chemisorption may be simply attributed to the electron transfer from the CO adsorbate to the Fe3’ which is known to be present in this system of low coverage of 0.2 ML Fe on an oxidized TiQ(ll0) surface. But this simple explanation cannot be reconciled with the change of work function of the system as derived from the UPS data for the system upon chemisorption of CO. The work function of this system was found to be practically unchanged as the result of CO adsorption, only - 0.08 eV upon exposure to as much as 150 L CU. A much greater change in work function would be expected if appreciable ejectron transfer took place from CO to Fe, So we conclude that the electrons required to transform Fe3’ into Fe0 did not come from CO when 0.2 ML Fe/TiO,(llO) was exposed to CO. Two other sources of electrons remain to be further considered, namely, Ti and 0. The electrons may conceivably be furnished by the Ti3’ ions which are known to be formed together with Fe3’ as the result of Fe deposition on the TiUz(10). But what is the driving force for this reversal of electron flow from Ti to Fe, in contrast to from Fe to Ti4+, as the result of CO adsorption? Furthermore, why does it stop if CO adsorption really triggers the reoxidation of Ti3” back to Ti4+? Apparently some Fe3+ still remains on the surface of the Fe/TiO,(llO) system after the CO adsorption as seen from fig. 9. A more reasonable explanation for the production of Fe* appears to be associated with the last source of electrons, the oxygen bound to the surface Fe’+ in such a system. In the presence of CO, the oxide ions formed as the result of the migration of 0 from the subsurface layers of TQflfO) would be removed with the liberation of CO, and leave the electrons to Fe3+ with the production of Fe’. This process would stop when the 0 supply is depleted since the Fe surface is only partially covered by 0 as shown earlier, This appears to be what brought about the formation of Fe0 on the surface of 0.2 ML Fe/TiO,(llO) an exposure to CO. As to the chemical state of the high Fe coverage system of more than 1 ML Fe,/TiO&lO), it is known that beyond 0.2 ML coverage, only Fe0
220
Junzhuo Deng et al. / Interactions
in the Fe/ TiO,(llO)
sysiem
I xl 6
LL
2125cmv’
x64
0
1000 Energy
Fig. 10. HREELS
2000
Loss spectra
3000
(cm-‘) of TiOz(llO).
0
‘500 Energy
atoms were formed. It is also known that the 0 migration in such a system levels off with the Fe deposition after the fast initial stage. A good part of the deposited Fe was not covered by 0. The covered part will be reduced by CO as in the case of a low Fe coverage system. Therefore, upon exposure to CO, this high Fe coverage surface will be expected to be predominately Fe0 also. With the chemical state of the deposited Fe now known to be Fe0 for both the low and the high coverage systems, the adsorbed state of CO in the Fe/TiO,(llO) system with different amounts of deposited Fe will then be discussed in the light of both HREELS and UPS data. Fig. 10 gives the HREELS spectra for the fully oxidized TiO,(llO) sample. The energy loss peaks due to surface optical phonons are obtained in the range of low loss energy of less then 0.5 eV. Spectral features attributed to the fundamental as well as to multiple and combination phonon losses were obtained in excellent agreement with those given in the literature [8,18,19]. The HREELS spectra for the adsorbed CO are found to be remarkably dependent on the coverage of the deposited Fe. For a low Fe coverage system of 0.2 ML Fe/TiO,(llO), as shown in fig. 11, the stretching frequency of CO was found to be 2125 cm-‘, not much different from that for CO in the gaseous phase, 2143 cm-‘, suggesting that a weak chemisorption may be involved. While on a high Fe coverage system of 1.7 ML Fe/TiO,(llO), it was found to be considerably
1000
1500
Loss
2000
2500
(cm-‘1
Fig. 11. HREELS spectra of CO adsorption on 0.2 ML Fe/TiO,(llO). (a) 0.2 ML Fe/TiO,(llO). (b) Exposure to 150 L CO on 0.2 ML Fe/TiO,(llO).
lower, 1870 cm-‘, as shown in fig. 12. Owing to the serious interference by the phonon background of the TiO,(llO) substrate, the HREELS spectra of the adsorbed CO were obtained only with great difficulty. We were unable to find any EELS peaks due to CO adsorption in the low wave numbers which may possibly be also present [20,21]. The UPS spectra of adsorbed CO on these systems were also recorded. UPS spectra of the
1870cm“
cA
0
500 Energy
1000 Loss
1500
2000
2500
(cm-‘1
Fig. 12. HREELS spectra of CO adsorption on 1.7 ML Fe/TiOz(llO). (a) 1.7 ML Fe/TiO,(llO). (b) Exposure to 50 L CO on 1.7 ML Fe/TiO,(llO).
Junrhuo Deng et al. / Interactions
.
I
12
10 Binding
.
8 6 4 Energy (eV]
Fig. 13. He1 UPS spectra of Fe/TiO#lO) CO. (a) 50 L CO on 1.2 ML Fe/TiO,(llO). 0.2 ML Fe/TiO,(llO).
2
Ef’lo
after exposure to (b) 150 L CO on
adsorbed CO on various metal single crystals have been studied quite extensively for group-VIII transition metals [22-241. A general picture of CO adsorption on such metals emerge as the result of such studies. It is known for instance that the 5a orbital of CO shifts toward 18 orbital for quite a few group-VIII metals upon adsorption of CO on these metals. It is also known that the separation between llr and 4a orbitals reflects the tendency of dissociation for the CO molecule adsorbed. The larger the separation, the longer the C-O bond, and thus the higher tendency for dissociation. In our Fe/TiO,(llO) system, the UPS spectra for the adsorbed CO also depends strongly on the amount of Fe deposited on the TiOz(llO) surface as shown in fig. 13. For a low Fe-coverage of 0.2 ML, the peaks ascribed to 5a, le and 4a molecular orbitals of adsorbed CO are found to have the same sequence as in the gaseous phase with not much change in their relative positions. In agreement with what was found in HREELS, this also indicates a weak chemisorption. While for a Fe/TiO,(llO) system covered by 1.7 ML of deposited Fe, two new features in the UPS spectrum emerge. Firstly, the Sa orbital shifts 3-4 eV toward the In orbital and the two orbitah appear to merge into one peak. Secondly, the 17r-4a separation of the adsorbed CO is appreciably increased, suggesting a larger separation between C
in the Fe/ TiO,(llO)
system
221
and 0 atoms in the CO molecule. These two features indicate that CO is strongIy chemisorbed on a densely Fe covered surface of 1.7 ML Fe/TiO,(llO). This is in sharp contrast to a weak chemisorption in the case of a sparsely populated surface of 0.2 ML Fe/TiO,(llO). Why do these two systems differ so much in their behavior toward the chemisorption of CO as they both have Fe0 atoms on the surface? For the low Fe coverage system, there were essentially no change in work function upon exposure to CO, not much change in the C-O stretching frequency as measured by HREELS, and not much change in the UPS spectra as compared with that for a gaseous phase, all these point to a weak chemisorption of CO. While on the other hand, for the high Fe coverage system, there were greatly reduced C-O stretching frequency, greatly changed positions of the molecular orbitals of CO after their adsorption, all pointing to a strong chemisorption. The following possibility seems to stand out. For a low Fe coverage system of 0.2 ML Fe/TiO,(llO), each Fe atom is estimated to occupy an area in the order of 50 A* when 1 ML is taken to correspond to 1 X 10” Fe atoms/cm2. The deposited Fe atoms are thus separated from each other by a distance of about 4 Fe atoms. This would mean that the Fe atoms in this system are essentially isolated individuals. In addition to the evidence produced by the LEED pattern mentioned earlier, this is supported by the stretching frequency of the adsorbed CO, 2125 cm-‘, which is very close to the stretching frequencies of the carbonyls in the mononuclear metal carbonyls, e.g. Fe(CO), [25]. Furthermore, deposition of isolated metal atoms [3] has also been invoked to explain the observed shift in XPS binding energy in the system of Rh/TiO,(llO) at low concentrations of the deposited metal. It is thus reasonably certain that the iron deposited on the 0.2 ML Fe/TiO,(llO) was in the form of isolated Fe atoms. It is possible that the structural require ment for the strong chemisorption of a CO molecule is not satisfied for this low Fe coverage system. As a result, such single Fe0 atoms, individually, are not sufficiently facile to accept electrons from, or backdonate electrons to the ad-
sorbed CO. While for the case of CO adsorbed on the densely covered 1.7 ML Fe/TiO,(llO), this restriction is removed. Each Fe atom has many nearest as well as next-nearest neighbors, and consequently strong chemisorption was observed. Our work thus strongly suggests that an ensemble of Fe atoms may be required for the strong chemisorption of CO in this system. Such a structural requirement has been proposed for some catalytic reactions [26], in which the activation of the reactant molecule via the che~so~tive interaction with the substrate undoubtedly ptays a key part.
4. Conclusions
(1) Fe
deposition on a TiO,(llO) surface takes place in a layer-by-layer mode. (2) Oxygen from the subsurface layers of the TiO,(llO) substrate migrates to the surface during the deposition process and binds to the deposited Fe to the extent of - 0.2 ML. (3) The state of the deposited Fe depends on the coverage of the Fe as well as the state of the TiO,(llO) substrate. On a fully oxidized TiO,(llO) surface, Fe3+ is initially formed together with Ti3+ and Fe0 atoms are deposited during the later stage. A small amount of Fe2+ is also formed in the initial stage. On a reduced surface of TiO,(llO) only Fe0 is formed. (4) When CO is adsorbed on the low Fe coverage system of 0.2 ML Fe/TiO,(llO), Fe3+ is reduced to Fe0 and only weak che~so~tion of CO occurs. For a 1.7 ML Fe/TiOZ(llO) system, strong chemisorption of CO is found to occur. The chemisorbed state of CO on the Fe/TiO,(llO) system is proposed to be related to the structural characteristics of the Fe atoms deposited on the surface of TiO,(110).
Acknowledgements The authors wish to express their sincere thanks to Guoxing Xiong for the single crystal of TiO,(llO) provided by him and valuable discus-
sions during the course of this work. Expert assistance given by Yuming Cao in various aspects of instrumentation is also gratefully acknowledged.
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