- Email: [email protected]
ceria model planar catalysts
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Surface Science North-Holland
287/288
(1993) 222-227
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Growth of Ce on Rh, surface alloy formation and the preparation and properties of Rh/ceria model planar catalysts J.P. Warren,
X. Zhang
‘, J.E.T.
Andersen
* and
R.M.
Lambert
*
Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 IEW, UK Received
21 August
1992; accepted
for publication
7 October
1992
Ce deposition on Rh has been studied over a range of conditions: at sufficiently high temperatures a stable surface alloy (Ce,Rh,) is formed at all Ce coverages. Oxidation and thermal treatment of Ce overlayers leads to agglomeration and the formation of cerium oxide crystallites on an essentially bare Rh surface. The chemisorption and reactive behaviour of this latter system towards carbon monoxide very closely mimics that of practical CeO,/Rh catalysts: CO, is efficiently produced by oxygen spillover from the rare-earth oxide phase.
1. Introduction Catalytic removal of automobile exhaust emissions is one of the most active fields in catalysis [1,2]. Much work has focussed on improving the three-way catalyst (TWC) systems currently employed [3], most of which utilise a precious metal in conjunction with a rare-earth oxide, usually ceria, supported on a high surface area, thermally stable alumina. Rhodium is particularly effective for NO, reduction, whilst ceria provides oxygen storage capabilities enabling the platinum-group metal to oxidise CO and hydrocarbons; however, the reaction mechanisms are not well understood. Studies of Ce film growth under UHV conditions are very rare, despite the importance of CeO,-based catalytic systems. This probably reflects the difficulty of producing truly clean films of this refractory (T, = 1071 K) and highly reactive metal. Here, we describe the growth of Ce on polycrystalline Rh as a means of producing
’ Present address: Department of Chemical Engineering, Pennsylvania State University, University Park, PA 16802, USA. ’ Present address: CAG/Procesteknisk Institut, DTH bgn. 425, DK-2800 Lyngby, Denmark. * To whom correspondence should be addressed. 0039-6028/93/$06.00
0 1993 - Elsevier
Science
Publishers
Rh/ceria model catalysts with a high density of the interfacial sites which are crucial to the TWC. Annealing the bimetallic system leads to surface alloying and appropriate oxidative treatment yields a model catalyst which shows very similar behaviour to real catalysts. By such modelling, one can in principle characterise the function of each component, with the aim of elucidating the origin of the enhanced performance due to Rhceria interaction.
2. Experimental Experiments were carried out in a UHV chamber described previously [4]. Briefly, it was equipped with AES, LEED, TDS, and ion bombardment facilities; a combination of diffusion, ion, and sublimation pumps routinely gave a base pressure of < 1 X lo- ‘” Torr. The rhodium foil sample (> 99.9.5%, Johnson Matthey) was cleaned by heating to 1300 K in flowing hydrogen gas, at 1 atm, for a period of - 60 h to remove boron and carbon from the bulk. It was then mounted by means of molybdenum support wires, and a chromel-alumel thermocouple was attached to the centre of the sample. The only other contami-
B.V. All rights
reserved
J. P. Warren et al. / Rh / ceria model systems
nant observed (phosphorus) was effectively sputtered away after N 20 h of argon bombardment at N 1100 K. The UHV cleaning procedures followed the recommendations of ref. [5]. The collimated cerium evaporation source employed electron bombardment to heat high-purity Ce (> 99.99%) 161, monolayer (ML) calibration being obtained from observations on the growth of cerium monitored by the Ce(88 eV> intensity and the attenuation of the Rh(305 eV> intensity [7,8]. Following cerium deposition and subsequent alloy formation, the sample was cleaned by Ar+ etching at ambient temperature, followed by continued etching at elevated temperatures (N 700 K>. All gases used were research grade.
3. Results Auger spectroscopy was used to follow the substrate and adsorbate signals in the standard fashion. Ce growth was monitored by following the RhM,N,,,N,,,(305 eV) and the CeN,,,O,,, NJ88 eV) Auger intensities [6], as shown in fig. 1. A fairly clear break in the Ce data is apparent at a deposition time of N 28 min and we take this as a calibration for the monolayer point: Ce/Rh intensities derived from fig. 1 are therefore used to evaluate Ce loadings in subsequent experiments. In passing we note that this calibration of the monolayer point agrees within 6% of the value which is deduced based on attenuation of
0
10
223
20 30 40 50 Ce deposition time (min)
60
-I 70
Fig. 1. Rh(305 eV) and Ce(88 eV) Auger intensity variation as a function of Ce-deposition time at room temperature.
the Rh (305 eV> electrons. The small threshold apparent in the figure has been observed previously in similar systems [91 and may be associated with subsurface penetration of Ce at very low surface coverages. It appears that the first layer of Ce grows following the Frank-van der Merwe mode; growth of the second layer appears slightly more complex, probably involving some contribution from the “simultaneous multilayer” mode. Heating these Ce overlayers causes very substantial changes in the Ce : Rh ratio. Fig. 2a illustrates the temperature dependence of this quantity for a series of different initial cerium loadings. The data were obtained by heating the specimen to the relevant temperature for 30 s in each 2.5 x .=: 2 B 2.0 .5 B F 1.5 d g B
I.0
.o 0.5 z z Oi 0.0 500
700
900
Temperature (K)
1100
1300
0
5
10 15 20 25 30 Ce deposition time (min)
35
40
Fig. 2. (a) Ce : Rh ratio variation with temperature as a function of initial Ce-coverage: (0) 3 ML; (0) 2.2 ML; (0) 2.0 ML; ( n ) 1.6 ML, (A ) 1.3 ML. (b) Ce : Rh ratio variation as a function of Ce deposition time at 300 and 850 K.
J.P. Warren et al. / Rh / ceria model systems
224
180 ,
(a)
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Ce(8X eV)
120 i 0
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Rh(305 eV)
Y
OI 0
30 1
2
3
4
5
6
7
Oxygen exposure (L)
w
200
400
Fig. 3. (a) Rh, Ce, and 0 Auger intensity variation as a function of oxygen exposure. a function of annealing temperature for an oxygen-saturated
case before recording the Auger spectrum. Control experiments showed that this heating interval was long enough in that increased heating times led to no change in the results. It can be seen that for initial coverages 2 1.6 ML Ce, the Ce/Rh ratio starts to decline rapidly at N 800 K,
400 500 600 700 Temperature/K
600
800
1000
I200
Temperature (K)
(b) Rh, Ce, and 0 Auger Ce/Rh alloy surface.
intensity
variation
as
eventually reaching a limiting value 2 1100 K. This temperature-dependent behaviour could be due to alloy formation, or grain boundary diffusion or agglomeration of Ce. Depth profiling/ AES measurements strongly suggest that surface alloy formation is responsible, a conclusion which
400
500
600
700
800
900
Temperature/K
Fig. 4. Desorption spectra following CO adsorption on (a) a practical Rh/CeO,-supported catalyst obtained at atmospheric pressure (desorption into He), (b) a freshly prepared CeO,/Rh model system following exposure to 0.2 L CO in UHV and (c) the same model system as (b), but after three CO adsorption/desorption cycles. Figure continued on next page.
J.I? Warren et al. / Rh /ceria model systems
(c)
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...........
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400
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.
500
02 ._ ,......._ .__.......e-v
600
700
800
900
Temperature/K Fig. 4. Continued.
is supported by related measurements (see below). Additionally, as indicated in section 4, analysis of these data in terms of the crystallographic electron attenuation model [8] shows that they are consistent with the formation layers of a Ce,Rh, surface alloy of varying thickness. This alloy appears to be stable in vacuum over prolonged periods of heating at 2 1100 K. Fig. 2b shows variations in the Ce: Rh ratio as a function of deposition time when the substrate was maintained at 850 K (note that this is the temperature at which the data in fig. 2a indicate that significant metal transport occurs>. Also shown in fig. 2b are the corresponding data obtained at room temperature and the point at which a monolayer of Ce has been deposited is indicated. The uptake of oxygen by Ce overlayers on Rh and the thermal behaviour of the oxidised bimetallic system are illustrated in figs. 3a and 3b. Fig. 3a shows the variations in Rh, Ce and 0 Auger intensities for a - 1.1 ML deposit of Ce as a function of oxygen exposure. It can be seen that virtually all the transformations are complete by - 1.5 L 0, exposure: there are significant decreases in both the Ce and Rh intensities while
225
the O(KLL) intensity rises to a limiting value. Fig. 3b shows the response of the oxidised system to successive increments in sample temperature. In each case the sample was held at the relevant temperature for 30 s before recording the spectra. In this case, there is a very marked decline in both the Ce and 0 signals in the interval 700-1050 K which is accompanied by a corresponding rise in the Rh intensity. No thermal desorption of 0, was detectable during any of these measurements. It is believed that the results in fig. 3b correspond to agglomeration of the oxidised cerium film to yield crystallites of ceria on the rhodium surface. In order to evaluate the properties of this system in terms of its usefulness as a model catalyst, we compared the CO adsorption/ desorption and reactive behaviour of the model system with that of a CeO,/Rh practical catalyst. Fig. 4a shows the CO, desorption spectrum which is obtained following exposure of the Rh/CeO, catalyst to CO at atmospheric pressure; very similar results were obtained when these measurements were carried out in ultra-high vacuum. Note the complete conversion of CO to CO,. Fig. 4b shows the results of corresponding experiments carried out on the model catalyst in ultrahigh vacuum. Initially, there is very extensive conversion of CO to CO, (fig. 4b). After a number of CO adsorption/ desorption cycles (> 3) (fig. 4c) it can be seen that the CO, desorption yield is drastically decreased whereas that of CO shows a corresponding increase; note also the good correspondence in CO, desorption temperature between the model catalyst and the real catalyst.
4. Discussion Ce appears to grow on Rh in a layer-by-layer fashion, at least for the first two monolayers. The exact details of the growth mode are not of critical importance for present purposes; these data serve primarily to provide a calibration for the loading of Ce on Rh in any given experiment. The calibration of a monolayer dose based on the Ce data is usefully confirmed by a simple calcula-
226
J.P. Warren et al. / Rh /ceria
tion based on the attenuation of Rh intensities in terms of the mean free path of the Rh Auger electrons: the two values agree within - 10%. The temperature-dependent behaviour illustrated in fig. 2a shows that for Ce loadings in excess of - 1.6 ML, there is a marked change in surface composition of temperatures 2 850 K, tending in every case to a high-temperature limiting value whose magnitude depends on the initial Ce loading. These high-temperature limiting ratios may be analysed quantitatively in terms of a recently described crystallographic electron attenuation model [lo], the essence of which is as follows. Electron attenuation is calculated on a layer-bylayer basis for parallel crystal planes within the solid; in the present case we assume that each layer has the same elemental composition. The results indicate that the data in fig. 2a correspond to the formation of a surface alloy film with stoichiometry Ce,Rh,: the thickness of the film (and therefore the magnitude of the limiting high-temperature ratio) being dependent on the initial Ce loading. Ce,Rh, is a stable phase in the Ce/Rh bulk phase diagram [ll]. Note that the data illustrated in fig. 2b are fully consistent with the data shown in fig. 2a. It is postulated that oxidation of Ce overlayers on Rh leads to the formation of a system in which the Rh surface is decorated by microcrystallites of ceria, as suggested by the data in fig. 3b. The results illustrated in fig. 3a show that oxygen uptake by the 1.6 ML Ce film is essentially complete after - 2 L exposure. Even if the 0, sticking probability on the Ce film is unity and independent of coverage, the fig. 3a data show that the amount of oxygen taken up is only sufficient to oxidise the Ce. It is therefore reasonable to propose that subsequent heating of the system leads to agglomeration into crystallites of cerium oxide and an essentially bare Rh surface. This conclusion is strongly supported by the CO adsorption/ desorption and reaction data. As shown in fig. 4, the practical catalyst completely converts CO to CO, without the necessity for adding gaseous oxygen. Detailed work on this system suggests that the source of the oxygen required for CO, formation is the CeO, lattice from which oxygen adatoms “spillover” onto the Rh surface
model systems
where they react with chemisorbed CO. The model catalyst data shown in figs. 4b and 4c appear to mimic this very well: CO chemisorbed on rhodium in the absence of any added gaseous oxygen reacts to form CO, yielding a desorption peak at essentially the same temperature to that observed with the real catalyst. It seems likely that the necessary oxygen is supplied by spillover and diffusion from CeO, microcrystallites which are present on the surface, indicating that the model system indeed provides a very good simulation for the practical system, not just in terms of the solid phases nominally present but also in terms of the way in which they interact with each other to produce active oxygen species. As noted in section 1, such oxygen storage and release by CeO, is an extremely important aspect of practical platinum metal/CeO, oxidation catalysts.
5. Summary (1) At room temperature Ce growth on Rh occurs initially in a layer-by-layer mode. Deposition at higher temperatures or annealing of room-temperature-deposited films leads to interdiffusion and ultimately the formation of a surface alloy whose composition is Ce,Rh,. (2) Oxidation of Ce overlayers at room temperature leads to formation of a continuous oxide film. Subsequent thermal treatment causes agglomeration of the oxide film, probably yielding crystallites of CeO, on an essentially bare Rh surface. (3) The chemisorption and reactive properties of CO on this system closely resemble those of practical Rh/CeO, catalysts in which important oxygen spillover phenomena are known to occur. (4) These model planar systems show considerable promise for mechanistic investigations of CO and hydrocarbon oxidation and NO reduction.
Acknowledgements
Financial support by Johnson Matthey plc. (J.P.W. and X.Z.1, the European Community and
J.P. Warren et al. / Rh /ceria model systems
the Danish National Research Council (J.E.T.A.) is gratefully acknowledged.
References [II K.C. Taylor, in: Catalysis, Science and Technology, Vol. 5, Eds J.R. Anderson and M. Boudart (Springer, Berlin, 1984) p. 119. [21A.M. Thayer, Chem. Eng. News 70 (1992) 27. [31J.T. Kummer, J. Phys. Chem. 90 (1986) 4742. [41E.A. Shaw, R.M. Ormerod and R.M. Lambert, Surf. Sci. 275 (1992) 157.
227
[5] R.G. Musket, W. McLean, C.A. Colmenares, D.W. Makowiecki and W.J. Siekhaus, Appl. Surf. Sci. 10 (1982) 143. [6] A.P. Walker and R.M. Lambert, J. Phys. Chem., in press (1992). 171 C. Araile and G.E. Rhead. Surf. Sci. Rep. 10 (1989) 277. i8] J.E.T.-Andersen, J.P. Warren, X. Zhang-and R.M. Lambert, in preparation. [9] K. Harrison, R.M. Lambert and R.H. Prince, Surf. Sci. 176 (1986) 530. [lo] J.E.T. Andersen, Surf. Sci. 243 (1991) 337. [ll] T.B. Massalki, in: Binary Ahoy Phase Diagrams, Vol. 2, Eds. H. Okamoto, P.R. Subramanian and L. Kacprzak, 2nd ed. (Scott, Glenview, IL, 1990).
,.:...:
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:.:.:. :(.:.:>
,..:.:
.:..:,“:::::.~::::..~:~... ..... spy:.
Surface Science North-Holland
287/288
(1993) 222-227
3
. . ..:y
:j ..j.
.:..
,:j:.‘::j;
.. . .,
:.
::.,, ,..
.
surface science :. ..:,::::j ... ‘::::::k:): ::.: .,.:. ...,.,. .. ....-:::” ..:,:.,;; .y.:. ::..;,,:.:.:: .. ....> ..... .....,I. ......::...:. .,:::y::.> ““:.:.:.:::..:::i::::::::::.::,:::.:: ‘::i:i:~::::ii~~,._.
Growth of Ce on Rh, surface alloy formation and the preparation and properties of Rh/ceria model planar catalysts J.P. Warren,
X. Zhang
‘, J.E.T.
Andersen
* and
R.M.
Lambert
*
Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 IEW, UK Received
21 August
1992; accepted
for publication
7 October
1992
Ce deposition on Rh has been studied over a range of conditions: at sufficiently high temperatures a stable surface alloy (Ce,Rh,) is formed at all Ce coverages. Oxidation and thermal treatment of Ce overlayers leads to agglomeration and the formation of cerium oxide crystallites on an essentially bare Rh surface. The chemisorption and reactive behaviour of this latter system towards carbon monoxide very closely mimics that of practical CeO,/Rh catalysts: CO, is efficiently produced by oxygen spillover from the rare-earth oxide phase.
1. Introduction Catalytic removal of automobile exhaust emissions is one of the most active fields in catalysis [1,2]. Much work has focussed on improving the three-way catalyst (TWC) systems currently employed [3], most of which utilise a precious metal in conjunction with a rare-earth oxide, usually ceria, supported on a high surface area, thermally stable alumina. Rhodium is particularly effective for NO, reduction, whilst ceria provides oxygen storage capabilities enabling the platinum-group metal to oxidise CO and hydrocarbons; however, the reaction mechanisms are not well understood. Studies of Ce film growth under UHV conditions are very rare, despite the importance of CeO,-based catalytic systems. This probably reflects the difficulty of producing truly clean films of this refractory (T, = 1071 K) and highly reactive metal. Here, we describe the growth of Ce on polycrystalline Rh as a means of producing
’ Present address: Department of Chemical Engineering, Pennsylvania State University, University Park, PA 16802, USA. ’ Present address: CAG/Procesteknisk Institut, DTH bgn. 425, DK-2800 Lyngby, Denmark. * To whom correspondence should be addressed. 0039-6028/93/$06.00
0 1993 - Elsevier
Science
Publishers
Rh/ceria model catalysts with a high density of the interfacial sites which are crucial to the TWC. Annealing the bimetallic system leads to surface alloying and appropriate oxidative treatment yields a model catalyst which shows very similar behaviour to real catalysts. By such modelling, one can in principle characterise the function of each component, with the aim of elucidating the origin of the enhanced performance due to Rhceria interaction.
2. Experimental Experiments were carried out in a UHV chamber described previously [4]. Briefly, it was equipped with AES, LEED, TDS, and ion bombardment facilities; a combination of diffusion, ion, and sublimation pumps routinely gave a base pressure of < 1 X lo- ‘” Torr. The rhodium foil sample (> 99.9.5%, Johnson Matthey) was cleaned by heating to 1300 K in flowing hydrogen gas, at 1 atm, for a period of - 60 h to remove boron and carbon from the bulk. It was then mounted by means of molybdenum support wires, and a chromel-alumel thermocouple was attached to the centre of the sample. The only other contami-
B.V. All rights
reserved
J. P. Warren et al. / Rh / ceria model systems
nant observed (phosphorus) was effectively sputtered away after N 20 h of argon bombardment at N 1100 K. The UHV cleaning procedures followed the recommendations of ref. [5]. The collimated cerium evaporation source employed electron bombardment to heat high-purity Ce (> 99.99%) 161, monolayer (ML) calibration being obtained from observations on the growth of cerium monitored by the Ce(88 eV> intensity and the attenuation of the Rh(305 eV> intensity [7,8]. Following cerium deposition and subsequent alloy formation, the sample was cleaned by Ar+ etching at ambient temperature, followed by continued etching at elevated temperatures (N 700 K>. All gases used were research grade.
3. Results Auger spectroscopy was used to follow the substrate and adsorbate signals in the standard fashion. Ce growth was monitored by following the RhM,N,,,N,,,(305 eV) and the CeN,,,O,,, NJ88 eV) Auger intensities [6], as shown in fig. 1. A fairly clear break in the Ce data is apparent at a deposition time of N 28 min and we take this as a calibration for the monolayer point: Ce/Rh intensities derived from fig. 1 are therefore used to evaluate Ce loadings in subsequent experiments. In passing we note that this calibration of the monolayer point agrees within 6% of the value which is deduced based on attenuation of
0
10
223
20 30 40 50 Ce deposition time (min)
60
-I 70
Fig. 1. Rh(305 eV) and Ce(88 eV) Auger intensity variation as a function of Ce-deposition time at room temperature.
the Rh (305 eV> electrons. The small threshold apparent in the figure has been observed previously in similar systems [91 and may be associated with subsurface penetration of Ce at very low surface coverages. It appears that the first layer of Ce grows following the Frank-van der Merwe mode; growth of the second layer appears slightly more complex, probably involving some contribution from the “simultaneous multilayer” mode. Heating these Ce overlayers causes very substantial changes in the Ce : Rh ratio. Fig. 2a illustrates the temperature dependence of this quantity for a series of different initial cerium loadings. The data were obtained by heating the specimen to the relevant temperature for 30 s in each 2.5 x .=: 2 B 2.0 .5 B F 1.5 d g B
I.0
.o 0.5 z z Oi 0.0 500
700
900
Temperature (K)
1100
1300
0
5
10 15 20 25 30 Ce deposition time (min)
35
40
Fig. 2. (a) Ce : Rh ratio variation with temperature as a function of initial Ce-coverage: (0) 3 ML; (0) 2.2 ML; (0) 2.0 ML; ( n ) 1.6 ML, (A ) 1.3 ML. (b) Ce : Rh ratio variation as a function of Ce deposition time at 300 and 850 K.
J.P. Warren et al. / Rh / ceria model systems
224
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(a)
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Y
OI 0
30 1
2
3
4
5
6
7
Oxygen exposure (L)
w
200
400
Fig. 3. (a) Rh, Ce, and 0 Auger intensity variation as a function of oxygen exposure. a function of annealing temperature for an oxygen-saturated
case before recording the Auger spectrum. Control experiments showed that this heating interval was long enough in that increased heating times led to no change in the results. It can be seen that for initial coverages 2 1.6 ML Ce, the Ce/Rh ratio starts to decline rapidly at N 800 K,
400 500 600 700 Temperature/K
600
800
1000
I200
Temperature (K)
(b) Rh, Ce, and 0 Auger Ce/Rh alloy surface.
intensity
variation
as
eventually reaching a limiting value 2 1100 K. This temperature-dependent behaviour could be due to alloy formation, or grain boundary diffusion or agglomeration of Ce. Depth profiling/ AES measurements strongly suggest that surface alloy formation is responsible, a conclusion which
400
500
600
700
800
900
Temperature/K
Fig. 4. Desorption spectra following CO adsorption on (a) a practical Rh/CeO,-supported catalyst obtained at atmospheric pressure (desorption into He), (b) a freshly prepared CeO,/Rh model system following exposure to 0.2 L CO in UHV and (c) the same model system as (b), but after three CO adsorption/desorption cycles. Figure continued on next page.
J.I? Warren et al. / Rh /ceria model systems
(c)
‘\. i
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il\
,.......... i
co a.--
co2 %,.....,..... ...% ...... ..eiCII a
...........
,- _.... ..,.
400
,._....I,.
.
500
02 ._ ,......._ .__.......e-v
600
700
800
900
Temperature/K Fig. 4. Continued.
is supported by related measurements (see below). Additionally, as indicated in section 4, analysis of these data in terms of the crystallographic electron attenuation model [8] shows that they are consistent with the formation layers of a Ce,Rh, surface alloy of varying thickness. This alloy appears to be stable in vacuum over prolonged periods of heating at 2 1100 K. Fig. 2b shows variations in the Ce: Rh ratio as a function of deposition time when the substrate was maintained at 850 K (note that this is the temperature at which the data in fig. 2a indicate that significant metal transport occurs>. Also shown in fig. 2b are the corresponding data obtained at room temperature and the point at which a monolayer of Ce has been deposited is indicated. The uptake of oxygen by Ce overlayers on Rh and the thermal behaviour of the oxidised bimetallic system are illustrated in figs. 3a and 3b. Fig. 3a shows the variations in Rh, Ce and 0 Auger intensities for a - 1.1 ML deposit of Ce as a function of oxygen exposure. It can be seen that virtually all the transformations are complete by - 1.5 L 0, exposure: there are significant decreases in both the Ce and Rh intensities while
225
the O(KLL) intensity rises to a limiting value. Fig. 3b shows the response of the oxidised system to successive increments in sample temperature. In each case the sample was held at the relevant temperature for 30 s before recording the spectra. In this case, there is a very marked decline in both the Ce and 0 signals in the interval 700-1050 K which is accompanied by a corresponding rise in the Rh intensity. No thermal desorption of 0, was detectable during any of these measurements. It is believed that the results in fig. 3b correspond to agglomeration of the oxidised cerium film to yield crystallites of ceria on the rhodium surface. In order to evaluate the properties of this system in terms of its usefulness as a model catalyst, we compared the CO adsorption/ desorption and reactive behaviour of the model system with that of a CeO,/Rh practical catalyst. Fig. 4a shows the CO, desorption spectrum which is obtained following exposure of the Rh/CeO, catalyst to CO at atmospheric pressure; very similar results were obtained when these measurements were carried out in ultra-high vacuum. Note the complete conversion of CO to CO,. Fig. 4b shows the results of corresponding experiments carried out on the model catalyst in ultrahigh vacuum. Initially, there is very extensive conversion of CO to CO, (fig. 4b). After a number of CO adsorption/ desorption cycles (> 3) (fig. 4c) it can be seen that the CO, desorption yield is drastically decreased whereas that of CO shows a corresponding increase; note also the good correspondence in CO, desorption temperature between the model catalyst and the real catalyst.
4. Discussion Ce appears to grow on Rh in a layer-by-layer fashion, at least for the first two monolayers. The exact details of the growth mode are not of critical importance for present purposes; these data serve primarily to provide a calibration for the loading of Ce on Rh in any given experiment. The calibration of a monolayer dose based on the Ce data is usefully confirmed by a simple calcula-
226
J.P. Warren et al. / Rh /ceria
tion based on the attenuation of Rh intensities in terms of the mean free path of the Rh Auger electrons: the two values agree within - 10%. The temperature-dependent behaviour illustrated in fig. 2a shows that for Ce loadings in excess of - 1.6 ML, there is a marked change in surface composition of temperatures 2 850 K, tending in every case to a high-temperature limiting value whose magnitude depends on the initial Ce loading. These high-temperature limiting ratios may be analysed quantitatively in terms of a recently described crystallographic electron attenuation model [lo], the essence of which is as follows. Electron attenuation is calculated on a layer-bylayer basis for parallel crystal planes within the solid; in the present case we assume that each layer has the same elemental composition. The results indicate that the data in fig. 2a correspond to the formation of a surface alloy film with stoichiometry Ce,Rh,: the thickness of the film (and therefore the magnitude of the limiting high-temperature ratio) being dependent on the initial Ce loading. Ce,Rh, is a stable phase in the Ce/Rh bulk phase diagram [ll]. Note that the data illustrated in fig. 2b are fully consistent with the data shown in fig. 2a. It is postulated that oxidation of Ce overlayers on Rh leads to the formation of a system in which the Rh surface is decorated by microcrystallites of ceria, as suggested by the data in fig. 3b. The results illustrated in fig. 3a show that oxygen uptake by the 1.6 ML Ce film is essentially complete after - 2 L exposure. Even if the 0, sticking probability on the Ce film is unity and independent of coverage, the fig. 3a data show that the amount of oxygen taken up is only sufficient to oxidise the Ce. It is therefore reasonable to propose that subsequent heating of the system leads to agglomeration into crystallites of cerium oxide and an essentially bare Rh surface. This conclusion is strongly supported by the CO adsorption/ desorption and reaction data. As shown in fig. 4, the practical catalyst completely converts CO to CO, without the necessity for adding gaseous oxygen. Detailed work on this system suggests that the source of the oxygen required for CO, formation is the CeO, lattice from which oxygen adatoms “spillover” onto the Rh surface
model systems
where they react with chemisorbed CO. The model catalyst data shown in figs. 4b and 4c appear to mimic this very well: CO chemisorbed on rhodium in the absence of any added gaseous oxygen reacts to form CO, yielding a desorption peak at essentially the same temperature to that observed with the real catalyst. It seems likely that the necessary oxygen is supplied by spillover and diffusion from CeO, microcrystallites which are present on the surface, indicating that the model system indeed provides a very good simulation for the practical system, not just in terms of the solid phases nominally present but also in terms of the way in which they interact with each other to produce active oxygen species. As noted in section 1, such oxygen storage and release by CeO, is an extremely important aspect of practical platinum metal/CeO, oxidation catalysts.
5. Summary (1) At room temperature Ce growth on Rh occurs initially in a layer-by-layer mode. Deposition at higher temperatures or annealing of room-temperature-deposited films leads to interdiffusion and ultimately the formation of a surface alloy whose composition is Ce,Rh,. (2) Oxidation of Ce overlayers at room temperature leads to formation of a continuous oxide film. Subsequent thermal treatment causes agglomeration of the oxide film, probably yielding crystallites of CeO, on an essentially bare Rh surface. (3) The chemisorption and reactive properties of CO on this system closely resemble those of practical Rh/CeO, catalysts in which important oxygen spillover phenomena are known to occur. (4) These model planar systems show considerable promise for mechanistic investigations of CO and hydrocarbon oxidation and NO reduction.
Acknowledgements
Financial support by Johnson Matthey plc. (J.P.W. and X.Z.1, the European Community and
J.P. Warren et al. / Rh /ceria model systems
the Danish National Research Council (J.E.T.A.) is gratefully acknowledged.
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