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Depth profile analysis of strong metal-support interactions on RhTiO2 model catalysts
Nuclear Instruments and Methods in Physics Research B I 18 (1996) 533-540 LiWiI
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Beam Interactions with Materials 6 Atoms
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ELSEVIER
Depth profile analysis of strong metal-support Rh/TiO, model catalysts Ch. Linsmeier a Max-Planck-lnstituf,ftir h Institwftir
Plasmaphysik,
Physikalische
‘** , H. Kniizinger EURATOM-Association,
Chemie der Universitiit
Miinchen.
interactions on
b, E. Taglauer
Boltzmannstr. Sophienstr.
II,
a
2. D-85748 D-80333
Garching,
Miinchen.
Germany
Germany
Abstract Strong metal-support interactions (SMSI) are regarded to be responsible for the change in selectivity and activity of group 8 metal catalysts supported on transition metal oxides. To investigate the nature of interaction between the catalyst components, model :ystems for Rh/TiO, catalysts were prepared with a metal loading of the order of one monolayer on top of several hundred A of TiO,. The samples showed a sufficiently high electrical conductivity to be examined by low-energy ion scattering spectroscopy (ISS) and sputtering as well as by additional surface physical techniques. The model catalysts were annealed at several temperatures up to 823 K and depth profiles of the first atomic layers were measured after every step. It is shown that the rhodium layer is encapsulated by an oxide layer at a substrate temperature of about 750 K, regardless of the presence or absence of hydrogen. The encapsulation cannot be reversed by an oxidation-reduction cycle, which is in agreement with an overall reduction of the free interface energy of the system during the transition into the SMSI state.
1. Introduction The activity and selectivity of heterogeneous catalysts, in particular of group 8 metals supported on transition metal oxides, is strongly dependent on the choice of both the active and the supporting components. In certain cases, the catalytic properties can be altered by the pretreatment of the catalysts, which leads to a different degree of interaction between the metal component and the substrate. Particularly in the case of group 8 metals supported on titanium dioxide (TiO,), strong differences in the CO and H, chemisorption capacities were observed before and after a high-temperature reduction (hydrogen at 773 K) of the catalysts [l-3]. Strong metal-support interactions (SMSI) were discussed as a reason for this behaviour. Besides the chemisorption properties of these catalysts, activities and selectivities for several chemical reactions changed [4,5]. During the transition of the catalyst from the normal to the SMSI state, morphological (encapsulation of the metal clusters by support material) as well as electronic effects (reduction of support material) are ob-
* Corresponding author. Tel. +49 89 3299 1263, fax +49 89 3299 2591, e-mail: [email protected]. 0168-583X/96/$35.00 Copyright .SSDIOl68-583X(95)01480-2
served [3,6,7]. Despite the numerous studies of metal-support interactions and the SMSI effect [5.8,9], the interpretation regarding the reasons for the SMSI effect, be it mere electronic effects, geometrical or morphological alterations of the catalysts during the high-temperature reduction or a combination of several processes, vary in the literature. In this work, we address the question of the encapsulation of the metal clusters on the catalyst surface during the transition into the SMSI state. We use ISS (see Ref. [lo]) in combination with slow sputter-erosion as a unique tool to determine the composition of the first atomic layers after various sample treatments. Due to its exclusive sensitivity for the outermost atomic layer, ISS provides a precise elemental analysis of the first layer of a solid. To facilitate the simple application of ISS, model catalysts are prepared which exhibit a sufficiently high electrical conductivity for the application of both ion and electron beam techniques. No charge compensation is necessary during the ISS measurements which is usually applied in experiments with oxidic systems to compensate for the distortions in the backscattering spectra due to charging phenomena [I I]. The model systems consist of a thin oxide film, anodically grown on a high-purity polished metal foil. The catalytically active rhodium component is introduced by evaporation under ultra-high vacuum (UHV) conditions. These model catalysts are well-defined and comparable to real powder-supported catalysts.
0 1996 Elsevier Science B.V. All rights reserved VII. SURFACES/INTERFACES
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We investigated the encapsulation of the rhodium layer by support material during thermal treatment with and without hydrogen environment at different temperatures. The catalyst composition is monitored by ISS depth profiling after the individual treatment steps. Furthermore, an oxidation-reduction sequence is applied to a model catalyst in the SMSI state to examine the reversibility of the SMSI transition.
2. Experimental 2.1. Sample
preparation
The model catalysts consist of a rhodium layer with a thickness of the order of one monolayer (ML), supported by a thin titanium dioxide film. This oxide is grown anodically on highly polished, polycrystalline titanium foils (> 99.6% Ti, Goodfellow). The metal foils were polished using a diamond polish with a final grain size of I pm and pellon disks. The oxide films were produced by anodic oxidation at a constant current density of 10 mA/cm’ in a 3% aqueous solution of ammonium diborate ((NH,j2B,0, .4H,O, Sigma Chemie, ca. 99%). The oxidation was stopped at a barrier voltage of 15 V, which should result in an amorphous TiO, film with a thickness of 300 A [I 2,131. The oxidation of the titanium metal led to an increase in the surface area due to the smaller density of the oxide. This roughening was observed by several microscopic techniques (electron microscopy, scanning atomic force and tunneling microscopy). A subsequent calcination of the films for 4 h at 623 K led to a conversion from amorphous TiO, to anatase [l4]. Both modifications were confirmed by laser Raman spectroscopy (LRS) and grazing-incidence X-ray diffraction at an angle of I”. The rhodium metal was evaporated onto the oxide films in a separate UHV chamber without prior cleaning of the substrate. The pressure during the evaporation was kept in the high 10-s hPa range. The pure metal (99.99%, marz grade) was heated by an electron beam and the deposition was monitored by a quartz crystal microbalance. The deposited amount was calculated for a monolayer coverage of the geometrical sample surface. However, since the surface roughness increased during the anodic oxidation, an actually smaller rhodium coverage per real surface area is expected. The samples were characterized by several methods. The metal coverage was measured by Rutherford backscattering spectrometry (RBS) which confirmed metal loadings in the range of one monolayer. The oxide thickness was measured by nuclear reaction analysis (NRA) using the 160(d,p,)‘70 nuclear reaction with D+ at an incident energy of 850 keV [ 1.5-181. The results showed that the oxide thickness of the uncalcined samples is slightly below the expected 300 A. The calcination step not only changed
the crystallinity of the films, as demonstrated both by LRS and X-ray diffraction, but also almost doubled the number of oxygen atoms in the layer. Not anodically oxidized, polished titanium films showed an oxygen content corresponding to the expected approximately 70 A for airoxidized titanium metal [ 121. 2.2. In situ treatments and analysis The prepared model catalysts were introduced in the UHV chamber DESPERADO, where all further treatments were carried out in situ with the ISS and Auger electron spectroscopy (AES) measurements. Both spectrometers are equipped with cylindrical mirror analyzers (scattering angle 137”) and integral ion and electron guns, respectively. The base pressure of the chamber is 1 X IO-” hPa, the samples can be heated by electron impact from the back. The sample temperature is monitored by a NiCr/Ni thermocouple spotwelded on the sample surface. Hydrogen and oxygen can be dosed to the sample surface up to a local pressure of approximately IO-’ hPa by means of a glass capillary array. The ISS spectra were measured with 500 eV 4Het at a current density of ca. 2 pA/cm2. The same ion beam is used to slowly erode the sample surface for the analysis of the elemental depth distribution. Since no experimental sputtering yields were available for TiO, at this helium ion energy, calculations were carried out using the TRIDYN code [l9-211 giving a sputtering yield of 0.03 atoms/ion. This means that under these conditions a monolayer of TiO, is eroded every 85 min by the ‘He+ beam. To examine the temperature dependence of the catalyst morphology, the samples were treated at several temperatures between room temperature and 870 K, with and without gas dosage. The temperature was held for I5 min at every step. After cooling down, AES spectra and ISS depth profiles were taken. Since the ion beam spots are well defined, a whole temperature series can be carried out at one single sample without interference of the depth profile spots. The ISS elemental signals in the spectra were fitted with Gaussian type functions under consideration of a linear background in the respective peak region [22] and the peak integrals used for analysis. Since every ISS spectrum shows the elemental composition of the outermost surface, an elemental depth profile is recorded with increasing erosion of the sample. Besides sputtering, the ion beam can have additional effects on the sample, like preferential sputtering and ion beam mixing. Moreover, the crater effect may obscure the depth resolution. It could be shown with oxidic powder catalysts with their (compared to polished foils and single crystals) large surface area, that this effect is of negligible importance [23-251. Under the constraint that this work covers thin metal clusters on an oxidic substrate, the results can be transferred. According to results with rhodium on Al,O, as well as on TiO,, ion
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ratio should show the depth profile depicted in Fig. Id. At the heginning of the ion bombardment, no backscattering signal from the adsorbate can be detected. By sputter-etching of the encapsulation layer, more and more adsorbate becomes exposed, whereas substrate intensity drops. This leads to an increase of the intensity ratio. Further sputtering removes also the adsorbed layer and leads to a decreasing profile. similar to case la. The positron of the maximum in the depth profile is related to the thickness of the encapsulating layer.
3. Results and discussion
Fig.
I.
Different
models
for the distnbution
(rhodium metal) on a substrate
(TiO, film)
of an adsorbate
and the respectively
expected ISS depth profiles (a) adsorbate monolayer, (b) adsorbate islands, (c) intermixed
adsorbate and substrate. (d) encapsulated
adsorbate.
mixing and preferential sputtering are also of minor importance in the ISS depth profiling experiments. The depth profiles taken in that way enable the determination of the sample morphology in the first atomic layers. Fig. I shows a schematic representation of four different distributions of an adsorbate (rhodium metal) on a substrate (TiO, film) and the respective ISS intensity ratios as a function of primary ion fluence. In the simplest case of a monolayer adsorbate (Fig. 1a), the adsorbate intensity drops exponentially with increasing fluence. At the same time, more and more of the substrate is exposed to the ion beam, leading to a corresponding increase of the substrate intensity. This results in a fast drop in the adsorbate/substrate intensity ratio. In the case of adsorbate islands with several layers thickness on top of the substrate (Fig. lb). their intensity ratio is expected to stay constant until the island thickness reaches one monolayer. Further sputtering leads to the same decrease in the backscattered ion intensity ratio as in the case of the monolayer adsorbate. For a sample morphology where some of the adsorbate is not only located on the surface of the substrate, but is also intermixed with it (case Ic), the intensity ratio should not show large changes over the examined depth (assuming a constant concentration). The slope during the beginning of the depth profile depends on whether the surface layer is enriched or depleted in adsorbate compared to the deeper layers. For the case of an adsorbate layer which is encapsulated hy a thin layer of substrate material, the intensity beam
The model catalysts were examined by ISS and AES after their introduction in the UHV chamber. In order no1 to alter the prepared morphology, no further cleaning steps were applied. Both methods showed only the expected elemental signals of oxygen, titanium and rhodium. AES detected a carbon contamination, which is not visible in the 500 eV He’ KS spectra. However. the presence of a contamination layer consisting of presumably hydrocarbons, originating from the transport through air, can also be deduced from the development of the signal intensities in the ISS spectra. Fig. 2 shows a series of ISS spectra of a Rh/TiO, model catalyst with a rhodium loading of 2.4 ML, taken after introduction in the chamber, prior to any further treatment. The first spectrum after the start of the ion bombardment shows barely any elemental peak: only very small oxygen and rhodium backscattering intensities are noticeable. However, the sputtering peak at low ion energies, produced by sputtered and ionized particles and indicating a surface contamination layer, extends in this first spectrum up to an energy of 2.50 eV. This intensity of those low-energy ions resulting from sputtering is much smaller in the subsequent spectra (cut off in Fig. 2). Both observations indicate that after the transport through the air, the surface of the sample is covered by some light adsorbate which prevents direct hackscattering of the helium projectiles. In the applied backscattering geometry this is only posstble for atoms lighter than helium, i.e. hydrogen from hydrocarbon adsorbates. The large sputteting peak proves the erosion of these light adsorbates. With increasing erosion of the contamination adlayet, the signals originating from the model catalyst increase. This can he seen directly in Fig. 3. where the backscattering intensities of the three elements Rh, Ti and 0 are plotted as a function of ion fluence. The rhodium intensity increases up to a fluence of ca. 6 X IO’” He+/cm2, from where an exponential decay starts. Beginning at this iluence, a linear decrease of the rhodium backscattering intensity is found in a logarithmic plot. This means that the remaining rhodium atoms on the surface are located in islands with a thickness of only one monolayer [26]. The slope of the straight line in the logarithmic plot yields a desorption cross section for rhodium on TiOz of (its = 1.7 X IO-”
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model catalysts [28]. It can be explained with the lighter substrate compared to bulk rhodium, and with the surface roughness of the model catalysts which leads to a reduced sputtering yield due to shadowing and blocking effects. The oxygen and titanium substrate signals increase during the examined depth. However, after the maximum of the rhodium intensity, the Ti and 0 slopes change slightly and towards the end of the depth profile, the substrate signals approach finite values, which means that the rhodium left on the oxide films does not shadow large parts of it. A direct conversion of the fluence into a depth scale is not straightforward, since the sputtering yields of the involved materials are different and also depend on the sample morphology. However, an estimated fluence of 3-6 x lOi He+/cm* seems reasonable for eroding one monolayer of the model catalysts.
3.1. In situ high-temperature
I
I
100
I
200
I,
I
300
I
I
400
I
I
500
Energy [eV] Fig. 2. 1% spectra of a Rh/TiO, model catalyst with a nominal rhodium metal loading of 2.4 ML, as determined by RBS. The spectra are normalized to the same height. The numbers to the right indicate the He+ ion fluence at the beginning of the respective spectrum.
cm*, which can be converted This value is smaller than calculated for rhodium meta similar behavior has already
in a sputtering yield of 0.003. the sputtering yield of 0.048, and 500 eV He+ ions [27]. A been observed for Rh/Al*O,
40
0
II l,,,,,,l,,,,,,,,,,,,,, 0 2 4 6 8 10 12 14 16 18 20 22 24
,
Fluence [ lot6 He+ I cm*] Fig. 3. Depth profile obtained from the spectra in Fig. 2 for a Rh/TiO, model catalyst with a metal Ioading of 2.4 ML. The peak integrals are normalized by the sample current.
reduction
The model catalysts were exposed to hydrogen at a partial pressure of about 10e4 hPa, holding the sample at each temperature for 15 min. This hydrogen atmosphere was maintained during heating up and cooling down. Since the elemental scattering signals are influenced by the sputtering of contaminants, the elemental ratios are used for interpretation. Fig. 4 shows the rhodium intensity, normalized by the total backscattering yield of the Rh, Ti and 0 intensities after a reduction at the indicated temperatures. The model catalyst had a nominal metal loading of I ML rhodium. The intensity ratios of the untreated and the sample reduced at 473 K and 573 K are similar. The rhodium fraction decreases strongly within the first monolayers and reaches a constant value after about 8 X 10’” He+/cm*. This proves that the rhodium is concentrated at the surface of the model catalyst. A first change in the rhodium depth distribution can be noticed after the 623 K treatment, whereas the rhodium fraction in the first layer is still somewhat increased. The rhodium fraction decreases slower with depth than in the first three experiments. This change already indicates a shift of rhodium towards deeper layers. This trend is strongly enhanced after the 723 K treatment. The final rhodium fraction of about IO%, which was reached after 10 X lOI He+/cm* in the 623 K experiment, is found at more than twice the fluence (2.5 X lOI He+/cm*) after the 723 K treatment. From this development a distribution of rhodium to larger depths can be concluded. This is caused by the formation of titanium oxide islands on top of the rhodium. The next reduction step at 773 K leads to a drastically altered rhodium depth profile. Compared to the treatments at lower temperatures, the rhodium fraction in the first spectra of the series is strongly reduced. Within the first 3 X lO’6 He+/cm’ it rises quickly and after reaching the maximum it falls off again at about the same rate as after the 723 K treatment. This shows that during the 773 K
Ch. Linsmeier et al. / Nucl. tns~r. und Meth. in Phvs. Rex B I18
treatment the rhodium content has not changed in depth larger than two monolayers, but the outermost layer now contains only very little rhodium. This change is also reflected in the titanium and oxygen intensities not shown here. Both go through a minimum at a fluence of 3 X IO’” where the rhodium fraction has its maximum. He+/cm’, The rhodium layer beneath a titanium oxide overlayer is still intact after the next 823 K reduction step. Both profiles also show the same depth development, but they differ in the absolute rhodium fraction, which is reduced by almost a factor of 2 after the 823 K step. After another 873 K reduction, the rhodium intensity is further decreased. However, the surface layer stays depleted of rhodium metal. The sequence of depth profiles after hydrogen treatments at increasing sample temperatures shows that the catalyst morphoplogy has changed. In the beginning, a rhodium overlayer is sitting on top of the oxide film. After an initial broadening of the rhodium zone over about 623 K. an encapsulation of the metal layer by an oxide layer
537
f 1996) 533-540
-c- Intensity Ti/O
IO
A
Intensity Ti/Rh
8
20
i;t b r+
15
.g
IO
E s .E
6 4
5
2
0 300
400
500
600
700
800
W"
Reduction Temp. [K] Fig. 5. 1SS Ti/O
(0)
and Ti/Rh
(0)
intensity ratios for an
untreated (first data points) and reduced Rh/TiO?
model catalyst,
respectively, measured after an ion fluence of 3 X
IO’” He’/cm’
at 500 eV.
begins above 723 K. Up to 773 K, the rhodium layer is still basically intact. After 823 K. there is no more metal layer left. From this temperature on, the rhodium is only found dispersed in the first layers. Both the titanium and the oxygen profiles, which are not shown here, support the interpretation based on the rhodium depth profiles. The temperature dependence of the elemental ratios shows that the changes in the composition of the first atomic layer start at slightly different temperatures for the Ti/O and Ti/Rh ratios, respectively. Fig. 5 shows the Ti/O ISS intensity ratios for the experiment from Fig. 4, measured after an ion fluence of 3 X 10lh He+/cm’. This is a fluence where approximately one monolayer (adsorbates and model catalyst) is eroded and therefore represents the composition in the first atomic layer. The increase of the ratio reflects the strong depletion of this layer in oxygen, starting at a temperature of approximately 750 K. This is about 50 K below the temperature where the encapsulation of the rhodium layer begins. From the Ti/Rh ratio it is clear that dramatic changes in the titanium concentration of the first atomic layer start above 750 K. This is consistent with the ISS depth profiles, which show the encapsulation after the 773 K reduction. It should be noted that the observed encapsulation does not necessarily go parallel with a depletion of the oxide film in oxygen. This is shown in reduction experiments at ambient hydrogen pressure and subsequent characterization of the samples with X-ray photoelectron spectroscopy (XPS). In these experiments, no change of the oxide stoichiometry was observed up to a reduction temperature of 830 K, although a similar rise in the Ti/Rh intensity ratio as in Fig. 5 was measured [29]. 3.2. In situ temperature
treatment
Fig. 4. Rhodium intensity fraction at the ISS signals of a Rh/TiO, model catalyst with a 1 ML ments in UHV
at different
metal loading after reducing treat-
sample temperatures.
The depth
files are measured after the stated reactions with 500 eV He+.
pro-
To examine the influence of hydrogen on the high-temperature reductions carried out during the experiments described in the last section, the same treatments were
VII. SURFACES/INTERFACES
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applied without hydrogen dosing. The pressure for all these experiments was 2-3 x IO-” hPa. Similar to the hydrogen experiments, the rhodium intensity fraction increases after the first temperature treatments. The rhodium intensity mainly originates from the first atomic layer up to a sample temperature of 623 K. From 723 K on, rhodium intensity in deeper layers begins to build up. Compared to the hydrogen experiments, the rhodium intensity maximum extends only approximately half as deep and is less pronounced. However, the rhodium layer beneath the titanium adlayer at this temperature is also confirmed by the corresponding titanium depth profile (not shown). This shows its minimum intensity at the same fluence where the maximum appears in the rhodium depth profile. After a 823 K treatment, the rhodium intensity drops further and the maximum in the depth profile is still visible, but not as clear as after the hydrogen reduction.
3.3. In situ oxidation-reduction
treatment
Already in the first work reporting SMSI effects, the reversibility of the reduced chemisorption capacity in the SMSI state was observed after an oxidation at high temperatures and a subsequent low-temperature reduction step [ 11. In order to test whether the encapsulation observed in the ISS depth profiles could be reversed by an oxidation/reduction sequence, Rh/TiO, model catalysts were accordingly treated and examined in situ. The encapsulation of the rhodium layer was achieved already by a single high-temperature reduction (HTR) step at 773 K. The rhodium intensity fraction showed the identical depth profile as after a series of reduction steps in Fig. 3. The rhodium layer was still intact and the depth profile showed the maximum as expected for an encapsulated layer (Fig. Id). After a subsequent high-temperature oxidation (HTO) at 723 K in approximately 10m4 hPa O,, dosed using the glass capillary array, the rhodium intensity fraction decreased strongly. However, the encapsulated layer of rhodium is still visible in the intensity backscattered from rhodium. The titanium and oxygen depth profiles prove that the first atomic layer now is mainly oxygen. This can be interpreted in an oxidation of the first layers, consisting of a titanium oxide species and rhodium. From XPS experiments it is known that the rhodium adlayer on freshly prepared model catalysts is oxidized, presumably by the transport through the air. After a smooth reduction (in diluted hydrogen), however, this thin rhodium oxide layer is converted to rhodium metal [29]. A similar reduction may happen in the in situ low-temperature reduction (LTR) at 473 K at a hydrogen pressure in front of the sample of about 10M4 hPa. The rhodium depth profile again shows the encapsulated metal layer, slightly broadened to larger depths. The titanium profile again goes through a shallow minimum, which is also true for the oxygen profile. However, in the oxygen profile, the mini-
1l014TV0 Ti/Rh 4
300 500 700
32
900
Temperature [K] Fig. 6. ISS Ti/O (0) and Ti/Rh (0) intensity ratios for a Rh/TiO, model catalyst, taken after an ion fluence of 3 X 10lh He+/cm’ at 500 eV. The different points represent sample treatments. 1: untreated sample, 2: HTR at 773 K, 3: HTO at 723 K, 4: LTR at 423 K. The numbers indicate the treatment sequence.
mum is no more as pronounced as after the first 773 K high-temperature reduction. From the depth profiles after the HTR-HTO-LTR treatment sequence, no removal of the encapsulating layer on top of the rhodium metal by an oxidation treatment can be observed. In the oxidized state, the model catalyst shows an enlarged oxygen content in the first atomic layers, which is reduced again to the former state after the low-temperature reduction. Fig. 6 shows the intensity ratios Ti/O and Ti/Rh after several treatment steps. The ratios were measured with ISS after an ion fluence of 3 X 10’” He+/cm*. At this fluence, approximately one monolayer was sputtered off and the ratios represent the composition within the first monolayer of the sample. It can be seen that the Ti/O ratio (open circles) increased after the high-temperature reduction, corresponding to an oxygen loss in the outermost layers of the model catalyst. After the HTO at 723 K, the oxygen content increased slightly, without reaching the value after the sample preparation. The last LTR step at 473 K again led to an oxygen depletion. The Ti/Rh intensity ratio (filled circles) increased during the HTR step at 773 K and again after the HTO at 723 K. This demonstrates that the encapsulation of the rhodium layer was not reversed during the HTO, although the Ti/O ratio approaches the original value. The final LTR at 423 K had only a small influence on the Ti/Rh ratio and hence on the encapsulation, the ratio stayed almost constant.
4. Summary
and conclusions
Rh/TiO, model catalysts treated in a hydrogen atmosphere of about 10e4 hPa at different temperatures between room temperature and 823 K and model catalysts treated in vacuum at the same temperature range showed an encapsulation of the rhodium metal adlayer, beginning at a sample temperature of about 750 K. Already 50 K below this encapsulation threshold, the samples start to lose oxygen. In a different experimental setup where model
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catalysts are reduced in dilute and pure hydrogen at ambient pressure and subsequently analyzed by XPS, the same encapsulation transition temperature was observed 1291. During these experiments, however, no reduction of the TiO, was observed. This correspondence in the transition temperatures in three different experimental setups indicates that the encapsulation is a kinetically controlled process. An empirical quantity which describes the mobility of species in the first atomic layer of a solid is the Hiittig temperature, defined as the third of the bulk melting temperature of the solid (TH = 0.33 r,. in K). The Hiittig temperature describes the mobility onset for surface species to enable surface processes like sintering or agglomeration [30]. The melting point of rutile is T, = 2173 K [31], which leads to a Htittig temperature of TH = 724 K. This corresponds to the onset of the encapsulation process. In the ISS depth profiles, a thickness of about one monolayer is found. This means that at the Hiittig temperature sufficient oxide species are available to form the encapsulating layer. As a driving force for the encapsulation the free surface energy of the components titanium oxide and rhodium may be discussed. From a thermodynamic point of view, which, however, discusses a system in equilibrium. spreading of one phase over a second one is possible, if the free interface energy (A F = yaag - ysg + y,) decreases during this process (A F < 0). Under the assumption of a constant area during the spreading process, the sum of the free surface energy between the adsorbate and the gas phase and the free interface energy between adsorbate and substrate (yal: and ya,) must be smaller than the free surface energy between the substrate and the gas phase (yag + yas < -ysy,,).Free surface energies between gas phase and several materials are available in several publications [32,33]. For titanium dioxide, values of 28-38 pJ/cm* are found. The free surface energies of metals are about one order of magnitude larger (Rh: 275 pJ/cm’). To predict the behavior, however, the free interface energy y,, has to be known, which is almost never the case 1301. The value of y,, depends on the surface energy towards the gas phase, and also on additional factors like the lattice misfit and the chemical interactions between adsorbate and substrate. However, since there is a large difference in the free surface energies between metals and oxides, it can generally be assumed that oxides tend to spread on metals more easily than reverse 1341. For the case of the encapsulation of rhodium on the model catalysts, rhodium plays the part of the substrate and the TiO, is the adsorbate. Provided that the mobility of the oxide species is sufficiently high, the energetic conditions enable the spreading of the oxide over the metal. This makes the corresponding encapsulation temperatures in the different experiments plausible. The sufficiently high mobility is provided by a substrate at the Hiittig temperature. According to these kinetic and thermodynamic considerations, a qualitative understanding of the encapsulation process is possible.
Acknowledgements This work was carried out under the framework of Sonderforschungsbereich 338 of the Deutsche Forschungsgemeinschaft.
References
[II
S.J. Tauster. S.C. Fung and R.L. C&ten. lo0 (1978)
Dl S.J.
Tauster and SC
[31 S.J.
Fung, J. Catal. 55 (1978)
Tauster,
S.C.
Fung,
I
(1981)
1121.
Science 21
J. Am. Chem. Sot.
170. R.T.K.
Baker
29.
and J.A.
Horsley.
[41 M.A. Vannice and R.L. Garten, J. Catal. 56 (1979) 236. 151 G.L. Haller and D.E. Resasco, Adv. Catal. 36 (1989) 173.
[61A.D.
Logan, E.J. Bnunschweig,
Langmuir 4 (1988)
171 B.-H. [81 R.T.K.
A.K. Datye and D.J. Smith,
827.
Chen and J.M. White. J. Phys. Chem. 87 (1983)
Met&Support
Interactions, ACS Symp. Series 298 (Ameri-
can Chemical Society, Washington.
[91 S.A.
ing. and Redispersion. Series, VNR
Baker and E. Rucken-
Interactions in Catalysis, Smter-
(Van
Nostrand
Company, New York,
Reinhold
Catalysis
1987).
Niehus. W. Heiland and E. Taglauer.
(1993) [I
1986).
Stevenson, J.A. Dumesic, R.T.K.
stein (eds.) Metal-Support
1101 H.
1327.
Baker, S.J. Tauster and J.A. Dumesic (eds.). Strong
Surf. Sci. Rep. 17
213.
II H.H. Brongersma, P.A.C. Groenen Interfaces of Ceramic Materials. ed.
and J.-P.
Jacobs, in:
J. Nowotny
(Elsevier,
1995). 1121 A. Aladjem,
Anodic Oxidation
of Titanium
Israel Atomic Energy Commission, LS-254
and its Alloys, (1972).
1131 Y. Sermys, T. Sakout and D. Gorse. Surf. Sci
282 (1993)
279. 1141 F. Cover and M. Musselin. Thin Solid Films 2 (1968)
21 I.
[I51 J.W. Mayer and E. Rimini, Ion Beam Handbook for Material Analysis (Academic
Press, New York,
1977).
[l61 G. Amsel and D. Samuel. Anal. Chem. 39 (1967) [I71 G. Debras (1977)
I181 H.C.
and G. Deconninck,
Kim, R.F. Seiler, D.F. Herring and K.W.
W.
Miiller
(1984)
[201 W.
and W.
Eckstein.
Nucl.
Instr. and
Jeziorowski, KnSzinger
H.
[271 N.
Taglauer.
H. Kniizinger. E. Taglauer and C. Vogdt. J. 286.
and E. Taglauer,
Div. Petr. Chem. Preprints,
357.
Bull. Sot. Chim. Belg. 90 (1981) W. Heiland
168 (1980)
Matsunami,
mata,
and E. Taglauer,
596.
Rodrigo, H. Jeziorowski, H. KnGzinger, X.Z. Wang and
E. Taglauer, Meth.
Knijzinger
Meth. B 64 (1992)
Amer. Chem. Sot. 26 (1981)
[251 B.C.
Phys.
355.
Josek. Ch. Linsmeier,
Catal. 80 (1983)
[261 E.
fiir Plasma-
(1988).
MGller. W. Eckstein and J.P. Biersack. Comput.
Nucl.
[241 H.
lnstr. and Meth.’ B 2
814.
Commun. 51 (1988)
(231 H.
Jones, Nucl.
Mijller and W. Eckstein, Max-Planck-lnstitut
[221 K.
38
526.
physik, Garching, IPP-Report 9/64
[211 W.
1689. Chem.
193.
Phys. 57 (1964)
[I91
1. Radionanal.
1339.
and J. Onsgaard, Nucl.
Instr. and
571.
Y. Yamamura,
S. Miyagawa.
K.
Y. Itikawa
Morita
N. Itoh, Y. Kazu-
and R. Shlmizu,
Energy
VII. SURFACES/INTERFACES
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Ch. Linsmeier et al. / Nucl. Instr. and Meth. in Phys. Res. B I I8 (1996) 533-540
Dependence of Sputtering Yields of Monatomic Solids, Inst. of Plasma Physics, Nagoya University, IPPJ-AM-14 (1980). (281 Ch. Linsmeier, H. Knozinger and E. Taglauer, Surf. Sci. 275 (1992) 101. [29] Ch. Linsmeier, H. KnSzinger and E. Taglauer, in preparation. [30] H. Knozinger and E. Taglauer, Catalysis, Royal Society of Chemistry, London, vol. Jo(J) (1993) 1.
[31] D.R. Lide (ed.), CRC Handbook of Chemistry and Physics, 74th ed. (CRC Press. Boca Raton, 1993). [32] S.H. Overbury, P.A. Bertrand and G.A. Somorjai, Chem. Rev. 75 (1975) 547. [33] A.R. Miedema, 7.. Metallkde. 69 (1978) 455. [34] E. Ruckenstein, in: Ref. [9] (1987) 230.
__
B
Beam Interactions with Materials 6 Atoms
!!@!I
ELSEVIER
Depth profile analysis of strong metal-support Rh/TiO, model catalysts Ch. Linsmeier a Max-Planck-lnstituf,ftir h Institwftir
Plasmaphysik,
Physikalische
‘** , H. Kniizinger EURATOM-Association,
Chemie der Universitiit
Miinchen.
interactions on
b, E. Taglauer
Boltzmannstr. Sophienstr.
II,
a
2. D-85748 D-80333
Garching,
Miinchen.
Germany
Germany
Abstract Strong metal-support interactions (SMSI) are regarded to be responsible for the change in selectivity and activity of group 8 metal catalysts supported on transition metal oxides. To investigate the nature of interaction between the catalyst components, model :ystems for Rh/TiO, catalysts were prepared with a metal loading of the order of one monolayer on top of several hundred A of TiO,. The samples showed a sufficiently high electrical conductivity to be examined by low-energy ion scattering spectroscopy (ISS) and sputtering as well as by additional surface physical techniques. The model catalysts were annealed at several temperatures up to 823 K and depth profiles of the first atomic layers were measured after every step. It is shown that the rhodium layer is encapsulated by an oxide layer at a substrate temperature of about 750 K, regardless of the presence or absence of hydrogen. The encapsulation cannot be reversed by an oxidation-reduction cycle, which is in agreement with an overall reduction of the free interface energy of the system during the transition into the SMSI state.
1. Introduction The activity and selectivity of heterogeneous catalysts, in particular of group 8 metals supported on transition metal oxides, is strongly dependent on the choice of both the active and the supporting components. In certain cases, the catalytic properties can be altered by the pretreatment of the catalysts, which leads to a different degree of interaction between the metal component and the substrate. Particularly in the case of group 8 metals supported on titanium dioxide (TiO,), strong differences in the CO and H, chemisorption capacities were observed before and after a high-temperature reduction (hydrogen at 773 K) of the catalysts [l-3]. Strong metal-support interactions (SMSI) were discussed as a reason for this behaviour. Besides the chemisorption properties of these catalysts, activities and selectivities for several chemical reactions changed [4,5]. During the transition of the catalyst from the normal to the SMSI state, morphological (encapsulation of the metal clusters by support material) as well as electronic effects (reduction of support material) are ob-
* Corresponding author. Tel. +49 89 3299 1263, fax +49 89 3299 2591, e-mail: [email protected]. 0168-583X/96/$35.00 Copyright .SSDIOl68-583X(95)01480-2
served [3,6,7]. Despite the numerous studies of metal-support interactions and the SMSI effect [5.8,9], the interpretation regarding the reasons for the SMSI effect, be it mere electronic effects, geometrical or morphological alterations of the catalysts during the high-temperature reduction or a combination of several processes, vary in the literature. In this work, we address the question of the encapsulation of the metal clusters on the catalyst surface during the transition into the SMSI state. We use ISS (see Ref. [lo]) in combination with slow sputter-erosion as a unique tool to determine the composition of the first atomic layers after various sample treatments. Due to its exclusive sensitivity for the outermost atomic layer, ISS provides a precise elemental analysis of the first layer of a solid. To facilitate the simple application of ISS, model catalysts are prepared which exhibit a sufficiently high electrical conductivity for the application of both ion and electron beam techniques. No charge compensation is necessary during the ISS measurements which is usually applied in experiments with oxidic systems to compensate for the distortions in the backscattering spectra due to charging phenomena [I I]. The model systems consist of a thin oxide film, anodically grown on a high-purity polished metal foil. The catalytically active rhodium component is introduced by evaporation under ultra-high vacuum (UHV) conditions. These model catalysts are well-defined and comparable to real powder-supported catalysts.
0 1996 Elsevier Science B.V. All rights reserved VII. SURFACES/INTERFACES
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We investigated the encapsulation of the rhodium layer by support material during thermal treatment with and without hydrogen environment at different temperatures. The catalyst composition is monitored by ISS depth profiling after the individual treatment steps. Furthermore, an oxidation-reduction sequence is applied to a model catalyst in the SMSI state to examine the reversibility of the SMSI transition.
2. Experimental 2.1. Sample
preparation
The model catalysts consist of a rhodium layer with a thickness of the order of one monolayer (ML), supported by a thin titanium dioxide film. This oxide is grown anodically on highly polished, polycrystalline titanium foils (> 99.6% Ti, Goodfellow). The metal foils were polished using a diamond polish with a final grain size of I pm and pellon disks. The oxide films were produced by anodic oxidation at a constant current density of 10 mA/cm’ in a 3% aqueous solution of ammonium diborate ((NH,j2B,0, .4H,O, Sigma Chemie, ca. 99%). The oxidation was stopped at a barrier voltage of 15 V, which should result in an amorphous TiO, film with a thickness of 300 A [I 2,131. The oxidation of the titanium metal led to an increase in the surface area due to the smaller density of the oxide. This roughening was observed by several microscopic techniques (electron microscopy, scanning atomic force and tunneling microscopy). A subsequent calcination of the films for 4 h at 623 K led to a conversion from amorphous TiO, to anatase [l4]. Both modifications were confirmed by laser Raman spectroscopy (LRS) and grazing-incidence X-ray diffraction at an angle of I”. The rhodium metal was evaporated onto the oxide films in a separate UHV chamber without prior cleaning of the substrate. The pressure during the evaporation was kept in the high 10-s hPa range. The pure metal (99.99%, marz grade) was heated by an electron beam and the deposition was monitored by a quartz crystal microbalance. The deposited amount was calculated for a monolayer coverage of the geometrical sample surface. However, since the surface roughness increased during the anodic oxidation, an actually smaller rhodium coverage per real surface area is expected. The samples were characterized by several methods. The metal coverage was measured by Rutherford backscattering spectrometry (RBS) which confirmed metal loadings in the range of one monolayer. The oxide thickness was measured by nuclear reaction analysis (NRA) using the 160(d,p,)‘70 nuclear reaction with D+ at an incident energy of 850 keV [ 1.5-181. The results showed that the oxide thickness of the uncalcined samples is slightly below the expected 300 A. The calcination step not only changed
the crystallinity of the films, as demonstrated both by LRS and X-ray diffraction, but also almost doubled the number of oxygen atoms in the layer. Not anodically oxidized, polished titanium films showed an oxygen content corresponding to the expected approximately 70 A for airoxidized titanium metal [ 121. 2.2. In situ treatments and analysis The prepared model catalysts were introduced in the UHV chamber DESPERADO, where all further treatments were carried out in situ with the ISS and Auger electron spectroscopy (AES) measurements. Both spectrometers are equipped with cylindrical mirror analyzers (scattering angle 137”) and integral ion and electron guns, respectively. The base pressure of the chamber is 1 X IO-” hPa, the samples can be heated by electron impact from the back. The sample temperature is monitored by a NiCr/Ni thermocouple spotwelded on the sample surface. Hydrogen and oxygen can be dosed to the sample surface up to a local pressure of approximately IO-’ hPa by means of a glass capillary array. The ISS spectra were measured with 500 eV 4Het at a current density of ca. 2 pA/cm2. The same ion beam is used to slowly erode the sample surface for the analysis of the elemental depth distribution. Since no experimental sputtering yields were available for TiO, at this helium ion energy, calculations were carried out using the TRIDYN code [l9-211 giving a sputtering yield of 0.03 atoms/ion. This means that under these conditions a monolayer of TiO, is eroded every 85 min by the ‘He+ beam. To examine the temperature dependence of the catalyst morphology, the samples were treated at several temperatures between room temperature and 870 K, with and without gas dosage. The temperature was held for I5 min at every step. After cooling down, AES spectra and ISS depth profiles were taken. Since the ion beam spots are well defined, a whole temperature series can be carried out at one single sample without interference of the depth profile spots. The ISS elemental signals in the spectra were fitted with Gaussian type functions under consideration of a linear background in the respective peak region [22] and the peak integrals used for analysis. Since every ISS spectrum shows the elemental composition of the outermost surface, an elemental depth profile is recorded with increasing erosion of the sample. Besides sputtering, the ion beam can have additional effects on the sample, like preferential sputtering and ion beam mixing. Moreover, the crater effect may obscure the depth resolution. It could be shown with oxidic powder catalysts with their (compared to polished foils and single crystals) large surface area, that this effect is of negligible importance [23-251. Under the constraint that this work covers thin metal clusters on an oxidic substrate, the results can be transferred. According to results with rhodium on Al,O, as well as on TiO,, ion
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ratio should show the depth profile depicted in Fig. Id. At the heginning of the ion bombardment, no backscattering signal from the adsorbate can be detected. By sputter-etching of the encapsulation layer, more and more adsorbate becomes exposed, whereas substrate intensity drops. This leads to an increase of the intensity ratio. Further sputtering removes also the adsorbed layer and leads to a decreasing profile. similar to case la. The positron of the maximum in the depth profile is related to the thickness of the encapsulating layer.
3. Results and discussion
Fig.
I.
Different
models
for the distnbution
(rhodium metal) on a substrate
(TiO, film)
of an adsorbate
and the respectively
expected ISS depth profiles (a) adsorbate monolayer, (b) adsorbate islands, (c) intermixed
adsorbate and substrate. (d) encapsulated
adsorbate.
mixing and preferential sputtering are also of minor importance in the ISS depth profiling experiments. The depth profiles taken in that way enable the determination of the sample morphology in the first atomic layers. Fig. I shows a schematic representation of four different distributions of an adsorbate (rhodium metal) on a substrate (TiO, film) and the respective ISS intensity ratios as a function of primary ion fluence. In the simplest case of a monolayer adsorbate (Fig. 1a), the adsorbate intensity drops exponentially with increasing fluence. At the same time, more and more of the substrate is exposed to the ion beam, leading to a corresponding increase of the substrate intensity. This results in a fast drop in the adsorbate/substrate intensity ratio. In the case of adsorbate islands with several layers thickness on top of the substrate (Fig. lb). their intensity ratio is expected to stay constant until the island thickness reaches one monolayer. Further sputtering leads to the same decrease in the backscattered ion intensity ratio as in the case of the monolayer adsorbate. For a sample morphology where some of the adsorbate is not only located on the surface of the substrate, but is also intermixed with it (case Ic), the intensity ratio should not show large changes over the examined depth (assuming a constant concentration). The slope during the beginning of the depth profile depends on whether the surface layer is enriched or depleted in adsorbate compared to the deeper layers. For the case of an adsorbate layer which is encapsulated hy a thin layer of substrate material, the intensity beam
The model catalysts were examined by ISS and AES after their introduction in the UHV chamber. In order no1 to alter the prepared morphology, no further cleaning steps were applied. Both methods showed only the expected elemental signals of oxygen, titanium and rhodium. AES detected a carbon contamination, which is not visible in the 500 eV He’ KS spectra. However. the presence of a contamination layer consisting of presumably hydrocarbons, originating from the transport through air, can also be deduced from the development of the signal intensities in the ISS spectra. Fig. 2 shows a series of ISS spectra of a Rh/TiO, model catalyst with a rhodium loading of 2.4 ML, taken after introduction in the chamber, prior to any further treatment. The first spectrum after the start of the ion bombardment shows barely any elemental peak: only very small oxygen and rhodium backscattering intensities are noticeable. However, the sputtering peak at low ion energies, produced by sputtered and ionized particles and indicating a surface contamination layer, extends in this first spectrum up to an energy of 2.50 eV. This intensity of those low-energy ions resulting from sputtering is much smaller in the subsequent spectra (cut off in Fig. 2). Both observations indicate that after the transport through the air, the surface of the sample is covered by some light adsorbate which prevents direct hackscattering of the helium projectiles. In the applied backscattering geometry this is only posstble for atoms lighter than helium, i.e. hydrogen from hydrocarbon adsorbates. The large sputteting peak proves the erosion of these light adsorbates. With increasing erosion of the contamination adlayet, the signals originating from the model catalyst increase. This can he seen directly in Fig. 3. where the backscattering intensities of the three elements Rh, Ti and 0 are plotted as a function of ion fluence. The rhodium intensity increases up to a fluence of ca. 6 X IO’” He+/cm2, from where an exponential decay starts. Beginning at this iluence, a linear decrease of the rhodium backscattering intensity is found in a logarithmic plot. This means that the remaining rhodium atoms on the surface are located in islands with a thickness of only one monolayer [26]. The slope of the straight line in the logarithmic plot yields a desorption cross section for rhodium on TiOz of (its = 1.7 X IO-”
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model catalysts [28]. It can be explained with the lighter substrate compared to bulk rhodium, and with the surface roughness of the model catalysts which leads to a reduced sputtering yield due to shadowing and blocking effects. The oxygen and titanium substrate signals increase during the examined depth. However, after the maximum of the rhodium intensity, the Ti and 0 slopes change slightly and towards the end of the depth profile, the substrate signals approach finite values, which means that the rhodium left on the oxide films does not shadow large parts of it. A direct conversion of the fluence into a depth scale is not straightforward, since the sputtering yields of the involved materials are different and also depend on the sample morphology. However, an estimated fluence of 3-6 x lOi He+/cm* seems reasonable for eroding one monolayer of the model catalysts.
3.1. In situ high-temperature
I
I
100
I
200
I,
I
300
I
I
400
I
I
500
Energy [eV] Fig. 2. 1% spectra of a Rh/TiO, model catalyst with a nominal rhodium metal loading of 2.4 ML, as determined by RBS. The spectra are normalized to the same height. The numbers to the right indicate the He+ ion fluence at the beginning of the respective spectrum.
cm*, which can be converted This value is smaller than calculated for rhodium meta similar behavior has already
in a sputtering yield of 0.003. the sputtering yield of 0.048, and 500 eV He+ ions [27]. A been observed for Rh/Al*O,
40
0
II l,,,,,,l,,,,,,,,,,,,,, 0 2 4 6 8 10 12 14 16 18 20 22 24
,
Fluence [ lot6 He+ I cm*] Fig. 3. Depth profile obtained from the spectra in Fig. 2 for a Rh/TiO, model catalyst with a metal Ioading of 2.4 ML. The peak integrals are normalized by the sample current.
reduction
The model catalysts were exposed to hydrogen at a partial pressure of about 10e4 hPa, holding the sample at each temperature for 15 min. This hydrogen atmosphere was maintained during heating up and cooling down. Since the elemental scattering signals are influenced by the sputtering of contaminants, the elemental ratios are used for interpretation. Fig. 4 shows the rhodium intensity, normalized by the total backscattering yield of the Rh, Ti and 0 intensities after a reduction at the indicated temperatures. The model catalyst had a nominal metal loading of I ML rhodium. The intensity ratios of the untreated and the sample reduced at 473 K and 573 K are similar. The rhodium fraction decreases strongly within the first monolayers and reaches a constant value after about 8 X 10’” He+/cm*. This proves that the rhodium is concentrated at the surface of the model catalyst. A first change in the rhodium depth distribution can be noticed after the 623 K treatment, whereas the rhodium fraction in the first layer is still somewhat increased. The rhodium fraction decreases slower with depth than in the first three experiments. This change already indicates a shift of rhodium towards deeper layers. This trend is strongly enhanced after the 723 K treatment. The final rhodium fraction of about IO%, which was reached after 10 X lOI He+/cm* in the 623 K experiment, is found at more than twice the fluence (2.5 X lOI He+/cm*) after the 723 K treatment. From this development a distribution of rhodium to larger depths can be concluded. This is caused by the formation of titanium oxide islands on top of the rhodium. The next reduction step at 773 K leads to a drastically altered rhodium depth profile. Compared to the treatments at lower temperatures, the rhodium fraction in the first spectra of the series is strongly reduced. Within the first 3 X lO’6 He+/cm’ it rises quickly and after reaching the maximum it falls off again at about the same rate as after the 723 K treatment. This shows that during the 773 K
Ch. Linsmeier et al. / Nucl. tns~r. und Meth. in Phvs. Rex B I18
treatment the rhodium content has not changed in depth larger than two monolayers, but the outermost layer now contains only very little rhodium. This change is also reflected in the titanium and oxygen intensities not shown here. Both go through a minimum at a fluence of 3 X IO’” where the rhodium fraction has its maximum. He+/cm’, The rhodium layer beneath a titanium oxide overlayer is still intact after the next 823 K reduction step. Both profiles also show the same depth development, but they differ in the absolute rhodium fraction, which is reduced by almost a factor of 2 after the 823 K step. After another 873 K reduction, the rhodium intensity is further decreased. However, the surface layer stays depleted of rhodium metal. The sequence of depth profiles after hydrogen treatments at increasing sample temperatures shows that the catalyst morphoplogy has changed. In the beginning, a rhodium overlayer is sitting on top of the oxide film. After an initial broadening of the rhodium zone over about 623 K. an encapsulation of the metal layer by an oxide layer
537
f 1996) 533-540
-c- Intensity Ti/O
IO
A
Intensity Ti/Rh
8
20
i;t b r+
15
.g
IO
E s .E
6 4
5
2
0 300
400
500
600
700
800
W"
Reduction Temp. [K] Fig. 5. 1SS Ti/O
(0)
and Ti/Rh
(0)
intensity ratios for an
untreated (first data points) and reduced Rh/TiO?
model catalyst,
respectively, measured after an ion fluence of 3 X
IO’” He’/cm’
at 500 eV.
begins above 723 K. Up to 773 K, the rhodium layer is still basically intact. After 823 K. there is no more metal layer left. From this temperature on, the rhodium is only found dispersed in the first layers. Both the titanium and the oxygen profiles, which are not shown here, support the interpretation based on the rhodium depth profiles. The temperature dependence of the elemental ratios shows that the changes in the composition of the first atomic layer start at slightly different temperatures for the Ti/O and Ti/Rh ratios, respectively. Fig. 5 shows the Ti/O ISS intensity ratios for the experiment from Fig. 4, measured after an ion fluence of 3 X 10lh He+/cm’. This is a fluence where approximately one monolayer (adsorbates and model catalyst) is eroded and therefore represents the composition in the first atomic layer. The increase of the ratio reflects the strong depletion of this layer in oxygen, starting at a temperature of approximately 750 K. This is about 50 K below the temperature where the encapsulation of the rhodium layer begins. From the Ti/Rh ratio it is clear that dramatic changes in the titanium concentration of the first atomic layer start above 750 K. This is consistent with the ISS depth profiles, which show the encapsulation after the 773 K reduction. It should be noted that the observed encapsulation does not necessarily go parallel with a depletion of the oxide film in oxygen. This is shown in reduction experiments at ambient hydrogen pressure and subsequent characterization of the samples with X-ray photoelectron spectroscopy (XPS). In these experiments, no change of the oxide stoichiometry was observed up to a reduction temperature of 830 K, although a similar rise in the Ti/Rh intensity ratio as in Fig. 5 was measured [29]. 3.2. In situ temperature
treatment
Fig. 4. Rhodium intensity fraction at the ISS signals of a Rh/TiO, model catalyst with a 1 ML ments in UHV
at different
metal loading after reducing treat-
sample temperatures.
The depth
files are measured after the stated reactions with 500 eV He+.
pro-
To examine the influence of hydrogen on the high-temperature reductions carried out during the experiments described in the last section, the same treatments were
VII. SURFACES/INTERFACES
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applied without hydrogen dosing. The pressure for all these experiments was 2-3 x IO-” hPa. Similar to the hydrogen experiments, the rhodium intensity fraction increases after the first temperature treatments. The rhodium intensity mainly originates from the first atomic layer up to a sample temperature of 623 K. From 723 K on, rhodium intensity in deeper layers begins to build up. Compared to the hydrogen experiments, the rhodium intensity maximum extends only approximately half as deep and is less pronounced. However, the rhodium layer beneath the titanium adlayer at this temperature is also confirmed by the corresponding titanium depth profile (not shown). This shows its minimum intensity at the same fluence where the maximum appears in the rhodium depth profile. After a 823 K treatment, the rhodium intensity drops further and the maximum in the depth profile is still visible, but not as clear as after the hydrogen reduction.
3.3. In situ oxidation-reduction
treatment
Already in the first work reporting SMSI effects, the reversibility of the reduced chemisorption capacity in the SMSI state was observed after an oxidation at high temperatures and a subsequent low-temperature reduction step [ 11. In order to test whether the encapsulation observed in the ISS depth profiles could be reversed by an oxidation/reduction sequence, Rh/TiO, model catalysts were accordingly treated and examined in situ. The encapsulation of the rhodium layer was achieved already by a single high-temperature reduction (HTR) step at 773 K. The rhodium intensity fraction showed the identical depth profile as after a series of reduction steps in Fig. 3. The rhodium layer was still intact and the depth profile showed the maximum as expected for an encapsulated layer (Fig. Id). After a subsequent high-temperature oxidation (HTO) at 723 K in approximately 10m4 hPa O,, dosed using the glass capillary array, the rhodium intensity fraction decreased strongly. However, the encapsulated layer of rhodium is still visible in the intensity backscattered from rhodium. The titanium and oxygen depth profiles prove that the first atomic layer now is mainly oxygen. This can be interpreted in an oxidation of the first layers, consisting of a titanium oxide species and rhodium. From XPS experiments it is known that the rhodium adlayer on freshly prepared model catalysts is oxidized, presumably by the transport through the air. After a smooth reduction (in diluted hydrogen), however, this thin rhodium oxide layer is converted to rhodium metal [29]. A similar reduction may happen in the in situ low-temperature reduction (LTR) at 473 K at a hydrogen pressure in front of the sample of about 10M4 hPa. The rhodium depth profile again shows the encapsulated metal layer, slightly broadened to larger depths. The titanium profile again goes through a shallow minimum, which is also true for the oxygen profile. However, in the oxygen profile, the mini-
1l014TV0 Ti/Rh 4
300 500 700
32
900
Temperature [K] Fig. 6. ISS Ti/O (0) and Ti/Rh (0) intensity ratios for a Rh/TiO, model catalyst, taken after an ion fluence of 3 X 10lh He+/cm’ at 500 eV. The different points represent sample treatments. 1: untreated sample, 2: HTR at 773 K, 3: HTO at 723 K, 4: LTR at 423 K. The numbers indicate the treatment sequence.
mum is no more as pronounced as after the first 773 K high-temperature reduction. From the depth profiles after the HTR-HTO-LTR treatment sequence, no removal of the encapsulating layer on top of the rhodium metal by an oxidation treatment can be observed. In the oxidized state, the model catalyst shows an enlarged oxygen content in the first atomic layers, which is reduced again to the former state after the low-temperature reduction. Fig. 6 shows the intensity ratios Ti/O and Ti/Rh after several treatment steps. The ratios were measured with ISS after an ion fluence of 3 X 10’” He+/cm*. At this fluence, approximately one monolayer was sputtered off and the ratios represent the composition within the first monolayer of the sample. It can be seen that the Ti/O ratio (open circles) increased after the high-temperature reduction, corresponding to an oxygen loss in the outermost layers of the model catalyst. After the HTO at 723 K, the oxygen content increased slightly, without reaching the value after the sample preparation. The last LTR step at 473 K again led to an oxygen depletion. The Ti/Rh intensity ratio (filled circles) increased during the HTR step at 773 K and again after the HTO at 723 K. This demonstrates that the encapsulation of the rhodium layer was not reversed during the HTO, although the Ti/O ratio approaches the original value. The final LTR at 423 K had only a small influence on the Ti/Rh ratio and hence on the encapsulation, the ratio stayed almost constant.
4. Summary
and conclusions
Rh/TiO, model catalysts treated in a hydrogen atmosphere of about 10e4 hPa at different temperatures between room temperature and 823 K and model catalysts treated in vacuum at the same temperature range showed an encapsulation of the rhodium metal adlayer, beginning at a sample temperature of about 750 K. Already 50 K below this encapsulation threshold, the samples start to lose oxygen. In a different experimental setup where model
Ch. Linsmeier
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Instr. and Meth. in Phys. Res. B I18 (1996) 533-540
catalysts are reduced in dilute and pure hydrogen at ambient pressure and subsequently analyzed by XPS, the same encapsulation transition temperature was observed 1291. During these experiments, however, no reduction of the TiO, was observed. This correspondence in the transition temperatures in three different experimental setups indicates that the encapsulation is a kinetically controlled process. An empirical quantity which describes the mobility of species in the first atomic layer of a solid is the Hiittig temperature, defined as the third of the bulk melting temperature of the solid (TH = 0.33 r,. in K). The Hiittig temperature describes the mobility onset for surface species to enable surface processes like sintering or agglomeration [30]. The melting point of rutile is T, = 2173 K [31], which leads to a Htittig temperature of TH = 724 K. This corresponds to the onset of the encapsulation process. In the ISS depth profiles, a thickness of about one monolayer is found. This means that at the Hiittig temperature sufficient oxide species are available to form the encapsulating layer. As a driving force for the encapsulation the free surface energy of the components titanium oxide and rhodium may be discussed. From a thermodynamic point of view, which, however, discusses a system in equilibrium. spreading of one phase over a second one is possible, if the free interface energy (A F = yaag - ysg + y,) decreases during this process (A F < 0). Under the assumption of a constant area during the spreading process, the sum of the free surface energy between the adsorbate and the gas phase and the free interface energy between adsorbate and substrate (yal: and ya,) must be smaller than the free surface energy between the substrate and the gas phase (yag + yas < -ysy,,).Free surface energies between gas phase and several materials are available in several publications [32,33]. For titanium dioxide, values of 28-38 pJ/cm* are found. The free surface energies of metals are about one order of magnitude larger (Rh: 275 pJ/cm’). To predict the behavior, however, the free interface energy y,, has to be known, which is almost never the case 1301. The value of y,, depends on the surface energy towards the gas phase, and also on additional factors like the lattice misfit and the chemical interactions between adsorbate and substrate. However, since there is a large difference in the free surface energies between metals and oxides, it can generally be assumed that oxides tend to spread on metals more easily than reverse 1341. For the case of the encapsulation of rhodium on the model catalysts, rhodium plays the part of the substrate and the TiO, is the adsorbate. Provided that the mobility of the oxide species is sufficiently high, the energetic conditions enable the spreading of the oxide over the metal. This makes the corresponding encapsulation temperatures in the different experiments plausible. The sufficiently high mobility is provided by a substrate at the Hiittig temperature. According to these kinetic and thermodynamic considerations, a qualitative understanding of the encapsulation process is possible.
Acknowledgements This work was carried out under the framework of Sonderforschungsbereich 338 of the Deutsche Forschungsgemeinschaft.
References
[II
S.J. Tauster. S.C. Fung and R.L. C&ten. lo0 (1978)
Dl S.J.
Tauster and SC
[31 S.J.
Fung, J. Catal. 55 (1978)
Tauster,
S.C.
Fung,
I
(1981)
1121.
Science 21
J. Am. Chem. Sot.
170. R.T.K.
Baker
29.
and J.A.
Horsley.
[41 M.A. Vannice and R.L. Garten, J. Catal. 56 (1979) 236. 151 G.L. Haller and D.E. Resasco, Adv. Catal. 36 (1989) 173.
[61A.D.
Logan, E.J. Bnunschweig,
Langmuir 4 (1988)
171 B.-H. [81 R.T.K.
A.K. Datye and D.J. Smith,
827.
Chen and J.M. White. J. Phys. Chem. 87 (1983)
Met&Support
Interactions, ACS Symp. Series 298 (Ameri-
can Chemical Society, Washington.
[91 S.A.
ing. and Redispersion. Series, VNR
Baker and E. Rucken-
Interactions in Catalysis, Smter-
(Van
Nostrand
Company, New York,
Reinhold
Catalysis
1987).
Niehus. W. Heiland and E. Taglauer.
(1993) [I
1986).
Stevenson, J.A. Dumesic, R.T.K.
stein (eds.) Metal-Support
1101 H.
1327.
Baker, S.J. Tauster and J.A. Dumesic (eds.). Strong
Surf. Sci. Rep. 17
213.
II H.H. Brongersma, P.A.C. Groenen Interfaces of Ceramic Materials. ed.
and J.-P.
Jacobs, in:
J. Nowotny
(Elsevier,
1995). 1121 A. Aladjem,
Anodic Oxidation
of Titanium
Israel Atomic Energy Commission, LS-254
and its Alloys, (1972).
1131 Y. Sermys, T. Sakout and D. Gorse. Surf. Sci
282 (1993)
279. 1141 F. Cover and M. Musselin. Thin Solid Films 2 (1968)
21 I.
[I51 J.W. Mayer and E. Rimini, Ion Beam Handbook for Material Analysis (Academic
Press, New York,
1977).
[l61 G. Amsel and D. Samuel. Anal. Chem. 39 (1967) [I71 G. Debras (1977)
I181 H.C.
and G. Deconninck,
Kim, R.F. Seiler, D.F. Herring and K.W.
W.
Miiller
(1984)
[201 W.
and W.
Eckstein.
Nucl.
Instr. and
Jeziorowski, KnSzinger
H.
[271 N.
Taglauer.
H. Kniizinger. E. Taglauer and C. Vogdt. J. 286.
and E. Taglauer,
Div. Petr. Chem. Preprints,
357.
Bull. Sot. Chim. Belg. 90 (1981) W. Heiland
168 (1980)
Matsunami,
mata,
and E. Taglauer,
596.
Rodrigo, H. Jeziorowski, H. KnGzinger, X.Z. Wang and
E. Taglauer, Meth.
Knijzinger
Meth. B 64 (1992)
Amer. Chem. Sot. 26 (1981)
[251 B.C.
Phys.
355.
Josek. Ch. Linsmeier,
Catal. 80 (1983)
[261 E.
fiir Plasma-
(1988).
MGller. W. Eckstein and J.P. Biersack. Comput.
Nucl.
[241 H.
lnstr. and Meth.’ B 2
814.
Commun. 51 (1988)
(231 H.
Jones, Nucl.
Mijller and W. Eckstein, Max-Planck-lnstitut
[221 K.
38
526.
physik, Garching, IPP-Report 9/64
[211 W.
1689. Chem.
193.
Phys. 57 (1964)
[I91
1. Radionanal.
1339.
and J. Onsgaard, Nucl.
Instr. and
571.
Y. Yamamura,
S. Miyagawa.
K.
Y. Itikawa
Morita
N. Itoh, Y. Kazu-
and R. Shlmizu,
Energy
VII. SURFACES/INTERFACES
540
Ch. Linsmeier et al. / Nucl. Instr. and Meth. in Phys. Res. B I I8 (1996) 533-540
Dependence of Sputtering Yields of Monatomic Solids, Inst. of Plasma Physics, Nagoya University, IPPJ-AM-14 (1980). (281 Ch. Linsmeier, H. Knozinger and E. Taglauer, Surf. Sci. 275 (1992) 101. [29] Ch. Linsmeier, H. KnSzinger and E. Taglauer, in preparation. [30] H. Knozinger and E. Taglauer, Catalysis, Royal Society of Chemistry, London, vol. Jo(J) (1993) 1.
[31] D.R. Lide (ed.), CRC Handbook of Chemistry and Physics, 74th ed. (CRC Press. Boca Raton, 1993). [32] S.H. Overbury, P.A. Bertrand and G.A. Somorjai, Chem. Rev. 75 (1975) 547. [33] A.R. Miedema, 7.. Metallkde. 69 (1978) 455. [34] E. Ruckenstein, in: Ref. [9] (1987) 230.