Langmuir 1994,10,4530-4533
4530
Activation of Pt-Rh( 100) Alloy and Pt/Rh(100) and Rh/Pt(100) Bimetallic Surfaces by Chemical Reconstruction Hiroyuki Tamurat and Ken-ichi Tanaka* The Institute for Solid State Physics, The University of Tokyo, 7-22-1 Roppongi, Minatoku, Tokyo 106, Japan Received March 8,1994. In Final Form: August 31, 1994@ When pto.25Rh0.75(100)alloy and Pt/Rh(lOO),Rh/Pt(lOO) bimetal surfaces were heated in NO or 0 2 a t
T > 400 K, a hybrid surfaceof Rh-O/Pt-layer/bulk was built up on these surfacesby a chemicalreconstruction. The hybrid surface of the Rh-Opt-layer gives a characteristic p(3x 1)LEED pattern. A Pt-deposited Rh(100)surface, Pt/Rh(lOO),annealed at 1000 K was poorly active for the reaction of NO with Ha, but the reconstructed p(3x l)Rh-O/Pt/Rh( 100) surface was highly active for this reaction. On the basis of these results, it was concluded that the formationof the Rh-Opt hybrid surfaceduringthe catalysis is responsible for the prominent activity of the Pt-Rh three-way catalyst.
Introduction Catalytic activity of metal surfaces is some times markedly influenced by a small amount of foreign metals. The three-way catalyst for automotive exhaust gas is a typical example; that is, a small amount of Rh improves markedly the catalytic activity and selectivity of the Ptbase catalyst for this reaction. The roles of Rh atoms, however, are still controversial. In order to throw light on this interesting phenomenon, the adsorption of NO and the reaction of NO with H2 were performed on a PtRh(100) alloy surface1V2 as well as on the Rh/Pt(lOO) bimetal ~ u r f a c e . ~ The reaction of NO with Ha on the catalyst can be described by the following two steps.
-N(a) + H,O N(a) + H, - NH,, N,
NO + H,
_.
(ii)
When step i proceedsrapidly, but the followingprocesses described by the step ii are slow, accumulation of the N(a) intermediates will take place during the catalysis. In fact, when the reaction of NO Ha was performed on Pd(100) and Rh(100) surfaces, the accumulation of the N(a) intermediates occurred to give a c(2 x2) LEED pattern.'^^ Contrary to them, no accumulation of the N(a) intermediates occurred on the Pt(100)surface although the reaction of NO with H2 proceeded on it. Therefore, it is an interesting question whether the accumulation of N(a) intermediates will occur or not on the Pt-Rh(100) alloy surface,because Rh and Pt atoms are randomly distributed over the surface. To get an answer, the in situ adsorption of N(a) and O(a) on a Pt-Rh(100) alloy surface was
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* To whom correspondence should be addressed. + KAO Corp.Wakayama
Research Laboratory, Wakayama 640, Japan. Abstract published in Advance ACS Abstracts, November 1, 1994. (UYamada, T.; Tanaka, K. J . Am. Chem. Soc. 1991,113,1173. Hirano, H.;Yamada,T.;Tanaka,K.; Siera,J.; Cobden,P.;Nieuwenhuys, B. E. Surf. Sci. 1989,262,97. (2)Hirano, H.; Yamada, T.; Tanaka, IC;Siera, J.; Nieuwenhuys, B. E.Vacuum 1990,41,134.Hirano, H.; Yamada, T.; Tanaka, K.; Siera, J.; Nieuwenhuys,B. E.Surf. Sci. 1989,222,L804. Hirano, H.; Yamada, T.; Tanaka, K.; Siera, J.; Nieuwenhuys, B. E. Su$. Sci. 1990,226,1. (3)Taniguchi,M.;Kuzembaev, E.; Tanaka, K. Surf.Sci. 1993,290, L711. (4)Matsuo, I.; Nakamura, J.; Hirano, H.; Yamada, T.; Tanaka, K.; Tamaru, K. J . Phys. Chem. 1989,93,7747.Tanaka, K.Prog. Theor. Phys. 1991,106,419.Tanaka, IC; Yamada, T. Res. Chem. Zntermed. 1991,15,213. @
0743-7463/94/2410-4530$04.50/0
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Figure 1. Segregation of Rh atoms on R-Rh(100) surface Torr 0 2 . (PtRh = 1in the initial surface) heated in 1 x Each plot was obtained by heating in 0 2 for 5 min.
measured by using AES and LEED.lv2 When a Pt-Rh(100)alloy surface was heated in 0 2 , the Pt/Rh ratio started to decrease a t ca. 400 K with increase of the O/Rh ratio as shown in Figure 1. The LEED pattern changed from p(1 x 1)to a characteristic p(3 x 1)pattern when the O/Rh reached at a constant value. The same ~ ( 3 x 1pattern ) appeared on the Pt-Rh( 100)surface in NO at 520K, which indicated the segregation of the Rh atoms. When Ha was added onto the ~ ( 3 x 1 Pt-Rh(100) ) alloy, the ~ ( 3 x 1 ) pattern disappeared with the decrease of O(a),and a c(2x 2) pattern appeared with the increase of N(a).2 On the other hand, it was shown that a Pt0.45Rho.55alloy tip annealed at 700 "C has a Pt enriched topmost layer (5) Tsong,T. T.; Ren, D. M.; Ahmad, M. Phys. Rev. B 1988,38,7428. Ren, D. M.; Tsong, T. T. Surf. Sci. 1985,184,L439. (6) Tamura, H.; Sasahara, A.; Tanaka, K. Surf. Sci. Lett. 1994,303, L379. (7)van Delft, F.C. M. J. M.; Nieuwenhuys, B. E.; Siera, J.; Wolf, R. M. ZSZJ Znt. 1989,29,550. (8)Hahn, E.; Schief, H.; Marsico, V.; Fricke,A.; Kern, K. Phys. Reu. Lett. 1994,72,3378.
0 1994 American Chemical Society
Catalytic Activity of Pt
Langmuir, Vol. 10, No. 12, 1994 4531 In this paper, it will be shown that the Rh-O/Pt-layer structure is also formed on a pt/Rh( 100)surface by reacting with NO or 0 2 , and the formation of the Rh-OPt-layer on the Pt-Rh(100) alloy, Rh/Pt(lOO), and Pt/Rh(lOO) surfaces is responsible to the promintent catalytic activity of these surfaces for the catalytic reaction of NO with H2.
Experimental Section A bimetallic Pt/Rh(lOO)surface was prepared by an electrochemical deposition of Pt ions on a Rh(100)surface. Before the
< + ~(2x2)
2*40
t
~(3x1+ ) p4g ~ ( 2 x 2 )
electrochemical deposition of Pt ions, the Rh(100) surface was Torr)at subjected to Ar ion sputtering, 0 2 treatment (5 x 800-1000 K for 10 min, and annealing at 1000-1200 K for 20 min. After the cleaning, the Rh(100) crystal was transferred into a small volume cubic cell, and the cubic cell was filled with 1atm of Ar gas. Then, an electrochemical glass cell filled with M Ptc14 was electrolyte solution of 0.05 M H2S0.4 + 5 x positioned to make contact with the one side of the crystal disk. After the electrochemical deposition of Pt ions, the crystal was washed with highly pure water and transferred back to the ultrahigh vacuum (UHV) chamber for measurement of LEED and XPS as described in ref 8. The reaction was carried out in the cubic cell.
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Results and Discussion 1.40
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Exposure (min) Figure 2. Segregation of Rh atoms on Pt/Rh(lOO)bimetallic Torr at 780 K. LEED surface by heating in 0 2 of 1 x pattern at 60 eV shows p(3x 1)+ p4g spots. and a R depleted (Rh atom enriched) second layer.5 Taking these facts into account, it was speculated that the Rh atoms are extracted form the second layer by making Rh-0, and the Rh-0 is ordered on a Pt layer in the p(3 x 1)~ t r u c t u r e . ~ The diffusion of R and/or Rh atoms is slow a t temperatures lower than 900 K,' and it is in good agreement with the temperature (900 K)for the thermal evolution of a Pt(997)surface morphology.8 This fact suggests that the Pt enriched second layer is pinned beneath the Rh-0 at the working temperatures (ca. 500 K), and the RhOpt-layer structure is rather stable during the catalysis.
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(5~2O)Pt(100)
deposition
Pt and Rh atoms may be randomly distributed on the Pt-Rh alloy surfaces but the depth distribution of them depends on the annealing temperature. So far, it has been accepted that the higher the annealingtemperature, the higher the surface Pt fraction, and it was shown that the ptO.45Rho.55alloy tip surfaces annealed a t 700 "C (ca. 1000 K) have a Pt-enriched topmost layer and a Ptdepleted (Rh-enriched) second layer.5 Contraryto such commonly accepted expectation,it was shown that the pto.25Rh0.,5(100) surface seems to take higher Pt fraction a t lower temperature, and the R-Rh(100)surface annealed a t 1300 K assumes almost equal Pt fraction to that of the bulk.' It was also clarified that the R-Rh( 100) surface is hardly attained a t an equilibrium composition a t temperatures lower than 900 K. On the other hand, we showed that the Pt-Rh( 100) surface undergoes chemical reconstruction even a t ca. 400 K when the surface is exposed to NO or 02,2p3 that is, the Rh atoms in the second layer are easily extracted by reacting with oxygen.
07 (lxl)Rh/Pt(100)-L p(3xl)Rh-O/Pt(100) (400K)
Figure 3. Changes of the Rh/Pt(lOO)surface by chemical restructuring. Reversible change between p(3x 1)and p(1x 1)at room temperature by H2 or 0 2 of 10-7-10-8 Torr.
Tamura and Tanaka
4532 Langmuir, Vol. 10, No. 12, 1994 Taking these facts into account, we predicted the formation of such a hybrid surface as “Rh-0 compound on a Pt-metal layer” on the Pt-Rh alloy and Rh/Pt bimetallic surfaces during catalysis. By use of a Rh-deposited Pt(100) surface as a model ~ u r f a c eit, ~was demonstrated that the Rh-0 compound formed on a Pt layer takes a n ordered arrangement. A very sharp p(3 x 1)LEED pattern appeared when the Rh deposited Pt(100) surface, Rh/Pt(100), was heated in low7 Torr of 0 2 or NO a t 340-400 K, which indicates the ordering of the Rh-0 on Pt(100) surface. A very similar p(3x 1)pattern appears on the Pt-Rh( 100) alloy surface when it is exposed to NO or 0 2 a t T > 450 K.2 Therefore, it was assumed that a n ordered Rh-0 overlayer was formed on a Pt layer with Pt(lOO)-likestructure when the Pt-Rh(100) surface was heated in NO or 0 2 . In making the Rh-Opt layer on the Pt-Rh(100) surface, the Rh atoms in the second layer might be extracted by the chemical reaction. The chemical reconstruction taking place on the Pt-Rh alloy surface is evidently shown on a model surface of the Pt-deposited Rh(100) surface.6 As shown in Figure 2, the Rh atoms are extracted by making a Rh-0 compound over the Pt/Rh(100) surface (0, = ca.4 monolayer) when the surface was Torr of 0 2 a t 780 K. Upon increasing heated in 1 x the Rh atom fraction on the surface, the LEED pattern changed from c(2x 2) to p(3 x 1) p4g pattern. We found that the ~ ( 3 x 1 ) p4g surface is highly reactive with respect to H2, and the surface changes quickly to a p(1x 1) surface, but the Rh atoms remain at the surface. In the case of the Rh/Pt(lOO) surface, the Rh atoms are on the surface so that no extraction of the Rh atoms is required for making the p(3 x 1)Rh-Opt-layer surface. Therefore, the p(3 x 1) Rh-OPt(100) surface is more readily formed on the Rh/Pt(100) surface (0, = 0.4-1.0 monolayer) compared to that on the PtLRh(100)or on the Pt-Rh(100) alloy surface when these surfaces are heated in 02. The p(3 x 1)Rh-Opt layer formed on either the Rh/Pt(100) or PtdRh(100)is highly active with respect to H2 so that the ~ ( 3 x 1pattern ) is erased immediately by exposing to H2 a t room t e m p e r a t ~ r e .Furthermore, ~ the ~ ( 3 x 1 surface ) is recovered a t room temperature by exposing to 0 2 as shown schematically in Figure 3. In order t o know the thermal stability of the clean Pt/ Rh(100) and the Rh/Pt(100) surfaces, they were annealed in UHV a t ca. 1000 K. As shown in Figure 4, the Pt XPS peak of the PtLRh(100) surface changes little by the annealing, spectra ii and iii are annealed a t 1000 K for 20 min and a t 1050 K for 10 min, respectively. Contrary to this, the Rh X P S peak for the Rh/Pt(lOO) surface is markedly weakened by annealing and completely disappeared by annealing at 1000 K for 15 min. These results are quite consistent with the depth distribution of the Pt-Rh alloy surface annealed a t 700 0C.5 It should be pointed out that the Rh on Pt(100) is not stable at high temperature but the Rh-0 on Pt(100) is stable up to the decomposition temperature of Rh-0. Therefore, when the Rh-Opt layer is formed on the catalyst surfaces during the reaction, the surfaces are stable but the PtRh(100) alloy surface and the Rh/Pt(lOO) or PtLRh(100) bimetallic surface might be indiscernible. In fact, the p(3 x l)Rh-O/Pt/Rh(lOO), p(3 x 1)Rh-OW Pt-Rh(100) alloy, and p(3 x l)Rh-O/Pt( 100)surfaces have almost equal catalytic activity for the reaction of NO with Hz. Therefore, it should be finally shown that the formation of the p(3 x l)Rh-O/Pt layer corresponds to the activation of the surface for the reaction of NO with H2.
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(9) Tanaka, K.; Taniguchi, M. Top. Cutal. 1994, 1, 95.
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Figure 4. XPS spectra of wRh(100) and Rh/Pt(100)surfaces annealed in UHV: (a,top) FWRh(100)surfacebefore annealing (i, top spectrum) and annealed at 1000 K for 20 min (ii, middle spectrum) and 1050 K for 10 min (iii, bottom spectrum);(b, bottom) Rh/Pt( 100)surface before annealing (i, top spectrum) and annealed at 1000 K for 15 min (ii, bottom spectrum).
Curve i in Figure 5 shows a temperature programmed reaction, in a flow of a mixture of 1 x 10+ Torr of NO and 2x Torr of H2 where a Pt-deposited Rh( 100) surface, Pt/Rh(lOO), was pretreated by annealing in UHV a t 1000 K for 5 min, and then it was heated up. The PtLRh(100) surface is inactive for the formation of N2 a t temperatures lower than ca. 600 K. On the contrary, the p(3 x l)Rh-O/PtJRh(lOO) surface, which was precedingly prepared by heating the PtLRh(100) surface in 0 2 (lo-’ Torr) a t 780 K for 10 min, was so active as to evolve the NZ even a t ca. 400 K as
Langmuir, Vol. 10,No.12, 1994 4533
Catalytic Activity of Pt
0.233 for run i at 780 K, which was very close to the value of 0.277 on the active surface of run ii. From these results, we can conclude that the F'tfRh(100)surface is activated by making a hybrid surface such as the Rh-O/Pt layer on the Rh(100) surface. The formation of a common hybrid surface of Rh-O/Pt layer during catalysis by the chemical reconstruction may be responsible for the prominent catalytic activity ofthe PtRh three-way catalyst. Therefore, we could expect that the Rh-deposited Pt-base catalyst will be structure insensitive for the reaction of NO with Hz. In fact, the NO HZ reaction on Pt(100) and P t ( l l 0 ) is entirely structure sensitive, Pt(100)>> Pt(110),but Rh/Pt(lOO)and Rh/Pt(llO) have almost equal catalytic activity for this reaction.1°
.sm c
E e N
z
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450 550 650 Temperature (K) Figure 5. Catalytic reaction of NO (1 x
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Torr) Hz (2 x Torr) on PtJth(100) surfaces with different constructions: (i)p(1x 1)FWRh(100)pretreated by annealing in UHV at 1000 K for 3 min; (ii) p(3xl)Rh-O/Pt/Rh(lOO) prepared by heating Torr 0 2 at 780 K for 10 min; (iii)repeated run after in 1 x run ii. represented by curve ii. Curve iii is the repeated run after run ii. The selectivity for the ratio of NHJN2 was
Acknowledgment. This work was supported by the Grant-in-Aid for Scientific Research (05403011) of the Ministry of Education, Science and Culture of Japan, and the authors acknowledge the Asahi Glass Foundation for finantial support to our work. One of the authors (Tamura) acknowledges KAO Corporation for giving the opportunity to study this work. (10)Sasahara, A.; Tamura, H.;Tanaka, K Catal. Lett. 1994, 28, 161.
