4598
J . Phys. Chem. 1992, 96, 4598-4602
CTRIP process involving bulk band states of metal is the dominant route for the photon emission. The surface plasmon excitationdecay process can also be considered as the possible process and seems to affect the properties of the emitted photons in this system, but to a lesser extent. To explain the details of the emission properties, we are now developing a more quantitative model35 as well as a new experimental a p p r ~ a c h . ~ ~ , ~ ~ (35) Murakoshi, K.; Uosaki, K. To be submitted for publication
Acknowledgment. Thanks are due to Messrs. T. Kiya and M. Kohiyama of Hokkaido University for constructing the solution transfer system and the electrochemical cell, respectively. This work was supported by a Grant-in-Aid for Scientific Research of the Ministry of Education, Science and Culture, Japan (02453001). (36) Murakoshi, K.; Uosaki, K. J . Electroanal. Chem. 1991, 308, 351. (37) Murakoshi, K.; Uosaki, K. J . VUC.Sci. Techno/. A, in press.
Absorption Spectrum and Chemical Reactions of Colloidal Cadmium in Aqueous Solution A. Henglein,* M. GutiBrrez, E. Janata, and B. G. Ershovt Hahn-Meiter-Institut GmbH Berlin, Bereich S , 100 Berlin 39, FRG (Received: November 20, 1991, I n Final Form: January 29, 1992)
A cadmium sol is prepared by high-energy electron irradiation of a cadmium perchlorate/sodium formate/sodium polyphaphate solution. The absorption spectrum of the sol consists of a narrow band at 260 nm ( E 2.0 X lo4 M-' cm-' ) and a low-intensity tail toward longer wavelengths. The sol contains single cadmium particles (mean diameter 50-70 A). The narrow absorption band is positioned at a slightly shorter wavelength than that calculated by Creighton and Eadon for IO-nm spherical particles. Upon aging, the particles grow and cluster together, and broad absorption bands at longer wavelengths develop. The cadmium particles have reducing properties. They dissolve upon exposure to air, methyl viologen, nitrous oxide, and carbon tetrachloride. In the two latter cases, nitrogen and chloride are formed, respectively. Changes in the optical absorption of the particles occur during their chemical reactions which are attributed to declustering. Pulse radiolysis experiments on the reduction of Cd2+were also carried out. The reduction is initiated by the hydrated electron. The Cd+ ion formed reacts with COY with a specific rate of 2 X lo9 M-I s-'.
Introduction Recent calculations by Creighton and Eadon have shown that cadmium particles in aqueous solution should absorb in a rather narrow wavelength range around 280 nm.' However, the absorption spectra of the cadmium sols which have been reported to date show strong absorptions in the visible, sometimes even extending into the infrared range.* Similarly, layers of cadmium on silver3 and on CdS4 particles were found to absorb strongly at longer wavelengths. It is the purpose of this paper to decribe the preparation of a cadmium sol whose absorption spectrum possesses the predicted narrow UV band. In addition, some chemical properties of colloidal cadmium are reported as well as the changes that occur upon aging. The colloid was prepared radiolytically as in the previous work? making use of the strong reducing hydrated electrons and carboxyl radicals that are generated in the radiolysis of aqueous formate solutions. Dissolved cadmium ions are reduced upon y-irradiation of such solutions. In the absence of a stabilizer, colloidal cadmium precipitates, the yield amounting to 2.5 Cd atoms formed per 100 eV of absorbed radiation energy.5 In the presence of sodium poly(viny1 sulfate) as stabilizer, a yellow solution results upon y-irradiation, and the absorption spectrum contains a weak band at 260 nm and stronger absorption in the visible.2 It was already observed in these earlier studies that the intensity ratio of the 260-nm band to the longer wavelength absorptions increased with increasing intensity of the y-radiation.2 Following this observation, very much higher intensities are applied in the present work by using high-energy electrons as ionizing radiation. Intensities higher by more than 1 million times than in the yirradiation experiments can readily be produced this way. (In y-radiolysis, the rate of free radical generation is about Mss-'; in electron irradiation it is about 2 M-s-I). The beam of highenergy electrons is not applied continuoilsly but in the form of *To whom correspondence should be addressed. 'On leave of absence from the Institute of Physical Chemistry of the Academy of Sciences, Moscow, USSR. 0022-3654/92/2096-4598$03.00/0
a pulse train to avoid heating of the sample.
Experimental Section The energy of the electron beam was 3.8 MeV. The pulse trains consisted of 1.5-ps pulses, each pulse producing a concentration of hydrated electrons plus carboxyl radicals of M. The interval between the pulses was 0.1 s. Ten-milliliter solutions were irradiated in an evacuated glass vessel at a distance of 20 an from the outlet window of a Van de Graaff generator. The irradiation vessel is sketched in Figure 1. It had a side arm carrying a 1.Oor 0.5-cm cuvette for optical measurements. Dosimetry was carried out by measuring the charge of each single pulse of the train, integrating the current of a beam pickup located behind the vessel! The values measured for the various pulses were added in a computer. To calibrate this dosimetry procedure, a chemical actinometer was applied: the vessel was filled with an aqueous solution of M tetranitromethane plus 0.1 M propanol-2. In such a solution nitroform is formed with a yield of 6 molecules per 100 The concentration of nitroform was determined spectrophotometrically (t at 350 nm: 1.4 X lo4 M-' cm-' ). Reagents were added to the irradiated solution under exclusion of air by sucking a measured amount of the deaerated solution into the evacuated vessel. Nitrous oxide was purified by triple freezing-thawing under high vacuum. Nitrogen was determined on a vacuum line (Toepler pump and McLeod manometer) combined with a gas chromatograph. Anions were determined ion chromatographically. A sample was prepared for electron microscopy by putting a drop ( I ) Creighton, J . A.; Eadon, D. G.J . Chem. SOC.,Faraday Truns. 1991, 87, 3881.
(2) Henglein, A.; Lilie, J. J . Phys. Chem. 1981, 85, 1246. (3) Henglein, A. Ber. Bunsen-Ges. Phys. Chem. 1980, 84, 253. (4) GutiCrrez, M.; Henglein, A. Ber. Bunsen-Ges. Phys. Chem. 1983.87, 474. ( 5 ) Kelm, M.; Lilie, J.; Henglein, A. J . Chem. SOC.,Faraday Trans. J 1975, 71, 1132. ( 6 ) Janata, E. Radiat. Phys. Chem., in press. ( 7 ) Henglein, A.; Jaspert, J . 2.Phys. Chem. 1957, 12, 324.
0 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 11, 1992 4599
Colloidal Cadmium in Aqueous Solution
2.0 plus HV'*
n
"200
400 600 800 A lnml Figure 3. Absorption spectrum after application of different numbers of pulses. The solution contained 0.1 M propanol-2 as O H radical scavenger instead of formate. Dashed line: spectrum after the addition of an equal volume of MV2+ solution.
Figure 1. Vessel for high-energy-electron irradiation. EB, electron beam;
S,solution; V, high-vacuum valve; C, optical cuvette; W, grounded wire to lead off electrical charges produced by the stopping of electrons in the vessel; R, reagent solution to be sucked into the evacuated vessel; G, scale for the addition of reagent solution; J, joint to high-vacuum line.
"200
400
600
800
A [nml Figure 4. Absorption spectrum of a cadmium sol at various times of aging. Solution as in Figure 2.
"200
400
600
800
A lnml Figure 2. Full line: absorption spectrum of a solution after the application of 600 pulses. Dashed line: spectrum after the addition of an equal volume of MV2+ solution. Solution: 4 X lo4 M Cd(C104)2,2 X lo-' M N a H C 0 2 , 2.5 X lo4 M (NaPO,),, 2 X lo4 M NaOH.
-
of the irradiated solution on a copper-carbon mesh and drying in an argon-filled glovebox (residual oxygen 1 ppm).
Results Absorption Coefficient of Cad". Figure 2 shows the narrow absorption band which is present after the irradiation of a solution containing 4 X lo4 M Cd(C104)*, 2 X M NaHC02, 2.5 X lo" M (NaP03), and 2 X l@ M NaOH. The pH of the solution was 8.2. The solution was irradiated with 600 pulses. In order to determine the absorption coefficient of cadmium, an equal volume of a 1 X M solution of methyl viologen, MV2+,was added. The blue color of the radical cation, MV+, developed immediately as the cadmium was oxidized: Cdo
+ 2MV2+
-
Cd2+ 4- 2MV+
(1)
The dashed curve in Figure 2 shows the absorption of MV+ formed. As the absorption coefficient of this radical cation is known (e at 600 nm: 1.2 X 104 M-I cm-I), and taking into account the stoichiometry of reaction 1 and the dilution factor of 2 (caused by the addition of the MV2+solution), one calculates a maximum absorption coefficient of 2.0 X lo4 M-l cm-' per Cd atom.
Irradiation of solutions which did not contain polyphosphate also resulted in the formation of colloidal cadmium. However, the maximum absorbance that could be reached was very much smaller than in the presence of the stabilizer and the solutions had a stronger absorption tail a t longer wavelengths. Similarly, solutions containing polyphosphate but propanol-2 as OH radical scavenger had a broader absorption band after irradiation. This can be seen from Figure 3, where the absorption spectrum of a solution is shown after irradiation with various pulse numbers. It can also be seen that the peak of the absorption band shifts during the buildup to longer wavelengths. The absorption spectrum after addition of an equal amount of methyl viologen solution is also shown in Figure 3. A comparison with Figure 2 shows that the ratio of the 260-nm peak (before MV" addition) to the MV2+ peak is smaller; i.e., the absorption coefficient of cadmium is lower than in Figure 2. We attribute the greater width of the absorption band to the presence of larger cadmium particles which have less absorption a t 260 nm but absorb more strongly at longer wavelengths than the smaller particles formed in the experiment of Figure 2. Aging of the Sol and Electron Microscopy. Figure 4 shows the absorption spectrum of a sol immediately after preparation and a t various times of aging. It can be seen that the narrow band at 260 nm decreases and a broad absorption at longer wavelengths appears. When methyl viologen was added after a few days, the amount of M V formed was the same as that formed upon addition of MV2+ to a freshly prepared sol. This shows that practically no dissolution of cadmium had taken place. The changes in the absorption spectrum have rather to be attributed to changes in the size and/or structure of the colloidal particles. Electron micrographs were taken of the freshly prepared solution and of a solution aged for 6 days. The fresh solution contained almost spherical particles 50-70 8, in diameter which were separated from each other. The aged solution contained
4600 The Journal of Physical Chemistry, Vol. 96, No. 11, 1992
resulted in a decrease of the cadmium absorption band, this decrease being again very rapid in the beginning and then much slower. It was also observed that the absorption band of cadmium shifted slightly toward longer wavelengths during this decay. When about two-thirds of the cadmium absorption had disappeared, the vessel was opened and the solution analyzed for chloride. The ratio of [CI-]/-A[CdO] (-AICdo] = decrease in cadmium metal concentration) was 1.7. This ratio is a little smaller than expected from the stoichiometry of the reaction
- 1.5 c 2 gm 1.0
Cdo + 2CCI,
v)
0.5
) 1 1 1 1 1 1 1 1 1 1
O200
300
400
H20
X [nml Figure 5. Absorption spectrum of a freshly prepared solution before and at various times after the addition of nitrous oxide (600 mbar). 1.0
I
-
+
CdZ+ 2C1-
+ C2Cl,
(3) Radiation-ChemicalExperhents. The first elementary reactions that occur in the radiolysis of aqueous Cd2+solutions containing an alcohol as OH radical scavenger have been investigated in previous pulse radiolysis ~tudies.~ The hydrated electrons formed in the radiolysis of the aqueous solvent
f 0 n
Henglein et al.
-
+ ea; + O H
H+
-
(4)
reduce cadmium ions to yield monovalent cadmium: eaq-
+ Cd2+
Cd+
+ + -
(5)
The OH radicals react with the alcohol to yield organic radicals: O H + RHOH H 2 0 + ROH (6)
1
Subsequently, these intermediates react among each other: Cd+
:0.5
CdZ2+(or Cdo
+ Cd2+)
H 2 0 CdZ++ R H O H OH- (8) Colloidal metal is finally formed via dismutation and association reactions of CdZ2+. Because of the partial reoxidation of half reduced cadmium (eq 8), the yield of metal is rather low in the presence of an alcohol. However, in the presence of formate, a much higher yield was observed, which shows that the carboxyl anion, which is formed in the O H attack on formate
H
P m
600
400
800
OH
A [nml
Figure 6. Absorption spectrum of an aged solution before and at various
times after exposure to nitrous oxide. larger particles with diameters around 150 A, the shape of the particles still being spherical; however, these particles appeared to a large extent in the form of clusters. In both samples, lattice planes could be seen; the distances measured were 2.63.08,which agrees with the value of 2.95 A of hexagonal cadmium metal. Energy-dispersive X-ray analysis revealed that the particles consisted of cadmium. Chemical Reactions. Exposure of the irradiated solution to air results in the disappearance of both the absorption band at 260 nm of fresh samples and of the longer wavelength absorption of aged samples. Figure 5 shows the absorption spectrum of a sol before and at various times after the addition of 600 mbar of nitrous oxide. At the beginning, there is a rapid decrease in the cadmium absorption band, which then is followed by a much slower one. During the decay of the cadmium absorption band, the peak shifts toward longer wavelengths. After 2 days the sample was analyzed for nitrogen. The amount found corresponded to the amount of cadmium present (as calculated from the intensity of the 260-nm absorption). Note that no long-wavelength absorption builds up during the disappearance of the cadmium absorption. Nitrous oxide obviously oxidizes colloidal cadmium: Cdo + N 2 0 + H 2 0
-
Cd2+ + N,
+ 20H-
(7)
+
Cd+ + ROH
m
e
OZOO
Cd+
(2)
Unexpected changes in the absorption spectrum occurred when nitrous oxide was added to an aged sample. As can be seen from Figure 6, the broad absorption a t longer wavelengths disappeared relatively rapidly to leave a broadened 260-nm band that subsequently decayed even more slowly than in the freshly prepared solution. Addition of a tiny amount of carbon tetrachloride to the freshly prepared solution (to produce a solution saturated with CC14) also
-
+ HC02-
H20 + C02-
(9)
contributes to the reduction of Cd2+.5 It has not yet been proven whether this contribution consists of a reaction of COT with Cd2+ itself
-
C02- + Cd2+
CO,
+ Cd+
COz- + Cd+
Cdo + CO,
(10) or of a reaction with the half reduced cadmium formed in reaction 5:
(1 1)
To investigate this problem, pulse radiolysis experiments were carried out with deaerated solutions of 5 X lo4 M Cd(C104)z plus M NaHCO,. When the solution was pulsed under nitrous oxide, the weak absorption of Cd+ (UV band at 250 nm5) was seen immediately after the pulse. However, only 8% of the hydrated electrons generated reacted with the Cd+ ions under these conditions, the majority of e,; reacting with nitrous oxide: e,; + N 2 0 H20 N2 + OH OH-. No additional Cd+ was produced after the pulse. It can therefore be said that reaction 10 did not occur. The Cd+ absorption decreased rapidly after the pulse, the first half-life of this decay becoming shorter with increasing dose of the pulse. This is attributed to reaction 11, which occurred under conditions where the concentration of COT generated during the pulse was much higher than that of Cd+. The analysis of the decay curves leads to a rate constant for reaction 11 of 2 X lo9 M-' s-l. A solution pulsed in the absence of N 2 0 showed the strong UV signal of Cd+ immediately after the pulse; this absorption decayed via second-order kinetics, primarily through reactions 11 and 7 . The solution of Figure 2, which at the beginning contained 2 X M formate and 4 X M Cd2+, had the following composition after irradiation with 600 pulses: HCOF, 7.6 X 10-4 M; (C02)2-, 3.8 X M; C 0 2 , 3.8 X lo4 M; Cd2+, 2 X lo4 M. It can be seen from these numbers that the material balance with respect to formate consumption and formation of the oxi-
+
-
+
The Journal of Physical Chemistry, Vol. 96, NO. 11, 1992 4601
Colloidal Cadmium in Aqueous Solution dation products of formate is satisfactory. The formation of oxalate is explained by the reaction
c02- + CO;
-
(C02)22-
6o
(12)
However, the efficiency of the Cd2+ reduction is low under our irradiation conditions. One can see this from the fact that more C 0 2 was formed than corresponds to the cadmium formation, and M hydrated electrons that the total radical production (6 X plus carboxyl radicals) was much greater than the concentration of the products.
Discussion Small cadmium particles have a rather narrow absorption band in the UV as predicted by Creighton and Eadon.' The band maximum for the 50-70-Aparticles was found to lie at 250-260 nm, i.e., at a wavelength slightly shorter than predicted for 10-nm particles. We regard this difference as a first indication for the band position being dependent on the size of the cadmium particles. The strong shift in the absorption spectrum upon aging (Figure 4), where the mean size of the particles increased by a relatively small factor of 2, shows that this size dependence is rather drastic. The Mie theory predicts that the absorption spectrum should be rather independent of the size in the size range of 5-20 nm, where the size is significantly smaller than the wavelength of light. The broad absorptions at longer wavelengths of the aged samples are understood in terms of clustering of the larger particles. The narrow band at 250-260 nm reminds one of a similar band (at 380 nm) in the spectrum of silver particles, which is attributed to the surface oscillation of the electron gas. In both cases, the absorption coefficient per atom is close to 2 X lo4 M-' cm-I. If the band in cadmium particles is due to a collective electron excitation, these particles may be suitable for surface enhanced Raman scattering experiments. Theoretical considerations have predicted a moderate enhancement factor for cadmium particles.* The weak long-wavelength tail in the absorption spectrum (Figure 2) is attributed to interband transitions in the colloidal particles. Chemicd Reactivity. The cadmium particles are strong reducing agents, which react with oxygen, methyl viologen, nitrous oxide, and carbon tetrachloride. The first step in these reactions is thought to consist of the transfer of one or two electrons to adsorbed molecules of these substances. It was always observed that addition of the oxidizing reagent resulted in an initial rapid decrease in the absorption band of the cadmium particles, followed by a slower and slower decrease at longer times. At the same time, the peak of the absorption band was slightly red-shifted. We explain these observations by a higher reactivity of the smaller particles in the size distribution of the aggregates. They absorb more strongly and at slightly shorter wavelengths than the larger particles in the size distribution; thus, as the chemical reaction proceeds, the absorption coefficient per atom of the remaining cadmium becomes lower and lower. This explains why the full stoichiometric ratio [Cl-]/-AICdo] of 2:1 was not obtained in the reaction of the particles with C C 4 (eq 3). The rather rapid decrease in the long-wavelength absorption during the reaction of the aged solutions with N20 (Figure 6) can only be understood if the reaction leads to a dissociation of the particle clusters at an early stage of reaction. The reactions are initiated by the transfer of electrons from the Cd particles to the dissolved substances (N20or CC14). The particles get positively charged this way and repel each other, which explains the dissociation of the particle clusters. The isolated particles which are still relatively large then react more slowly than the smaller particles in the freshly prepared solutions. The concentration of excess CdZ+ions in the freshly prepared solution was about 2 X lo-" M. However, as most of them are bound to the polyanion chains, the concentration of free Cd2+ions is very much lower. Thus, the redox potential of the cadmium particles is more negative than one would calculate from Nernst's equation using the standard potential of the bulk cadmium electrode (-0.4 V) and the nominal Cd2+ concentration in the (8) Zeman, E. J.; Schatz, G. C. J . Phys. Chem. 1987, 91, 634.
- 0
0
200
400
pulse number Figure 7. Percent reduction of cadmium as a function of the number of pulses applied. Solution as in Figure 2.
solution. In addition, small metal particles have a more negative potential because of their high free surface energy? Thus, it is understandable that carbon tetrachloride, a substance with a rather negative standard reduction potential (Eo(CC14/CC13 Cl-) = -0.6 V) reacts with small cadmium particles. A few thoughts may also be given to the stabilizing effect of polyphosphate: The effect is explained by the weak nucleophilic behavior of the anion groups of this polymer. Atoms on the surface of the metal particles are unsaturated coordinatively. Thus, unoccupied electronic orbitals exist on the surface into which a nucleophile can donate a fraction of an electron pair. In the case of silver particles, it has recently been shown that the adsorption/desorption equilibrium of nucleophilic compounds is strongly determined by the position of the Fermi level in the particles.'0 The Fermi level in cadmium particles (Le., particles of a non-noble metal) is positioned at a rather negative potential (as compared to silver particles of equal size); this may explain why only rather large cadmium particles can be stabilized by the weakly nucleophilic anion groups of polyphosphate (in the case of silver, it was even possible to stabilize oligomeric clusters of very high negative potential). Radiation-ChemicalConsiderations. It was already mentioned that the reduction of Cd2+was rather inefficient under the conditions applied. This inefficiency is also manifested by the fact that not all the Cd2+ions could be reduced upon longer irradiation. As is shown by Figure 7, the concentration of cadmium metal formed first increases with the number of applied electron pulses and later reaches a limiting value. Two reasons for the inefficiency of reduction can be given: In the first place, it has to be remembered that the reaction of hydrated electrons with Cd2+ions (eq 5) is the initiating step of reduction. Whereas this reaction is very fast when the Cd2+ ions are free in solution (k = 2.5 X 1O1OM-' s-I), it occurs much more slowly in the presence of the stabilizing polyphosphate chains to which the Cd2+ ions are strongly attached." At the high intensities of the Van de Graaff irradiation, the hydrated electrons can partly undergo reaction with other radicals (such as 2eq- + 2 H 2 0 H 2 20H- and/or ea; C02- H+ HC02-), this effect becoming more and more important as the Cd2+ions are consumed. One could argue that an increase in the Cd2+ concentration of the solution would improve the situation. However, at higher concentrations of Cd(C104)*,the colloid formed already has a strong contribution for larger particles absorbing at longer wavelengths. Secondly, one must expect that the radicals generated in the later pulses of the train react in part with the products formed in the earlier pulses. The rather large amount of carbon dioxide
+
-
+
+
+
-
(9) Plieth, W. J. J . Phys. Chem. 1982, 86, 3166. (10) (a) Henglein, A,; Linnert, T.; Mulvaney, P. Eer. Bunsen-Ges. Phys. Chem. 1990, 94, 1449. (b) Mulvaney, P.; Linnert, T.; Henglein, A. J. Phys. Chem. 1991,95,7843. (c) Henglein, A.; Mulvaney, P.; Linnert, T. Faraday Discuss. 1991, 92, 31. (11) Haase, M.; Weller, H.; Henglein, A. J . Phys. Chem. 1988, 92, 4706 (see Figure 4).
4602
J . Phys. Chem. 1992, 96, 4602-4608
formed could be explained by the attack of the OH radical on oxalate anions formed in the earlier pulses: (13)
The optical properties of the colloidal particles can be compared with the theoretical spectra, and the redox properties of the particles can also be studied. The method should-al& be applicable to other metals.
It may finally be concluded that-although not very efficient from the radiation-chemical point of view-high-energy electron irradiation is a useful tool to prepare colloidal solutions of cadmium metal with properties that cannot be obtained by other methods.
Acknowledgment. We thank Dr. M. Giersig for preparing and interpreting electron microscopic pictures, Dr. P. Mulvaney for valuable discussions, and Mrs. H. Pohl for assistance in the laboratory work.
OH
+ (CO&?-
-
OH-+ co2 + COT
Adsorption and Thermal Decomposition of Benzene on the Pd( 1IO)( 1X2)-Cs Surface: High-Resolution Electron Energy Loss Spectroscopy, Thermal Desorption, and Low-Energy Electron Diffraction Studies M. Fujisawa, T. Takaoka, T. Sekitani, and M. Nisbijima* Department of Chemistry, Faculty of Science, Kyoto University, Kyoto 606, Japan (Received: November 21, 1991; In Final Form: January 17, 1992)
The adsorbed state of benzene on the Pd( 1 lo)( 1X2)-Cs surface at 90 K and its thermal decomposition procass in the temperature region up to 650 K have been investigated by using high-resolution electron energy loss spectroscopy, multiplexed thermal desorption spectroscopy, and low-energy electron diffraction. Vibrational spectra show the existence of two chemisorbed states of benzene on Pd(1 lO)(lX2)-Cs at 90 K. For a small exposure (50.3langmuir, fractional coverage BC6H6 S 0.07), benzene is predominantly adsorbed away from the Cs adatoms. With increasing exposure, the adsorbed benzene located near the c s adatoms is formed additionally. The saturation coverage corresponds to 0.2 C&6 molecule per Pd atom of the unreconstructed Pd(ll0) surface (0% = 0.2). For large exposure (>0.3langmuir), a part of the adsorbed benzene is desorbed intact at about 300 and 330 K. The thermal decomposition of C6H6occurs above 300 K, and the C,H, species ( x = 1 or 23,y = 0, 1) and H atoms are formed on the surface. Only carbon adatoms remain on the Pd( 1lo)( 1X2)-Cs surface by heating to 650 K. These results are compared with thosc for the Pd( 110) clean surface, and the effect of Cs adatoms on the surface reactions are discussed.
I. Introduction The adsorption and thermal decomposition of benzene on well-defined transition-metal (1 1 1) and (100) surfaces have been an object of many studies as a prototype for the interaction of aromatic compounds with catalysts.’ According to these studies, benzene is chemisorbed associatively with its C-ring plane parallel to the metal surface, bonding being through ?F orbitals. On the other hand, the interaction between benzene and the (1 10) surface has been studied only by a few research groups.24 Recently, we have studied the adsorption and thermal decomposition of benzene on the Pd( 110) clean surface by the use of high-resolution electron energy loss spectroscopy (EELS), low-energy electron diffraction (LEED), and thermal desorption spectroscopy (TDS).5 Vibrational spectra show the existence of two adsorbed states of benzene on Pd(ll0) at 300 K. In one state, benzene is adsorbed with its C ring nearly parallel to the surface ( T a t ” benzene), and in the other state, with some angle (“tilted” benzene). The tilted benzene ) and the flat benzene outside of is located in the ~ ( 4 x 2 domains the ~ ( 4 x 2 domains. ) The decomposition temperature is shifted toward higher temperatures by the site-blocking effect of benzene ( I ) Sheppard, N. Annu. Rev. Phys. Chem. 1988,39, 589, and references therein. (2) (a) Friend, C. M.; Muetterties, E. L. J. Am. Chem. Soc. 1981, 103, 773. (b) Gentle, T. M.; Muetterties, E. L. J . Phys. Chem. 1983, 87, 2469. (3) (a) Netzer, F. P.;Rangelov, G.; Rosina, G.; Saalfeld, H. B.; Neumann, M.; Lloyd, D. R. Phys. Rev. B 1988, 37, 10399. (b) Ramsey, M. G.; Steinmiilltr, D.; Netzer, F. P.; Santanielo, A.; Lloyd, D. R. Surf. Sci. 1991, 2511252, 979. (4) Huber, W.; Weinelt, P.; Zeibisch, P.; Steinriick, H.-P. Surf. Sri. 1991, 253. -..,
72. ( 5 ) Fujisawa, M.; Sekitani, T.; Morikawa, Y.; Nishijima, M. J . Phys. Chem. 1991, 95, 7415. ~
admolecules themselves. For C&I,-Saturated Pd( 1 lo), the decomposition occurs above 400 K, and C,H, (x = l or 13, y = 0,1) species are observed on the Pd( 110) surface as the decomposition products for 400-650 K. Only carbon adatoms remain on Pd( 110) by heating to 650 K. Alkali metal has been used in catalysis (the Fischer-Tropsch synthesis of hydrocarbons, etc.) to promote the selectivity and reactivity of metal catalysts. The promotion mechanism is of interest in catalysis and surface chemistry. The effect of alkali metal on the adsorbed state of hydrocarbons has been studied mainly for the (1 11) surface of facecentered-cubic (fcc) metalsw Although it is well-known that the alkali-metal-induced (1 X 2) reconstruction occurs on the Pd( 110) surface,I*Jl there are only a few works on the adsorbed state of hydrocarbons on the (1 X 2) surface. Recently, we studied the adsorption and thermal decomposition of ethylene on the Pd( 11.0)(lX2)-Cs surface.I2 Compared with ethylene on the Pd(ll0) clean ~urface,’~ ethylene ~~~
_______
~~
~
(6) Garfunkel, E. L.; Maj, J. J.; Frost, J. C.; Farias, M. H.; Somorjai, G. A. J. Phys. Chem. 1983,87, 3629. (7) (a) Windham, R. G.; Bartram, M. E.; Koel, B. E. J. Voc. Sci. Technol. 1987, A5, 457. (b) Windham, R. G.; Bartram, M. E.; Koel, B. E. J . Phys. Chem. 1988,92,2862. (c) Windham, R. G.; Koel, B. E. J. Phys. Chem. 1990, 94, 1489. (8) Zhou, X.-L.; Zhu, X.-Y.; White, J. M. Surf. Sci. 1988, 193, 387. (9) (a) Hugenschmidt, M. B.; Dolle, P.; Jupille, J.; Cassuto, A. J . Vac. Sci. Technol. 1989, A7, 3312. (b) Cassuto, A.; Mane, M.; Hugenschmidt, M. B.; Dolle, P.; Jupille, J. Surf. Sci. 1990, 237, 63. (c) Cassuto, A.; Mane, M.; Kronneberg, V.;Jupille, J. Surf. Sci. 1991, 2511252, 1133. (10) Barnes, C. J.; Ding, M. A.; Lindroos, M.; Diel, R. D.; King, D. A. Surf. Sci. 1985, 162, 59. (11) Barnes, C. J.; Lindroos, M.; King, D. A. Sut$ Sci. 1988, 201, 108. (12) Sekitani, T.;Yoshinobu, J.; Onchi, M.; Nishijima, M. J . Phys. Chem. 1990, 94, 6847.
0022-365419212096-4602%03.00/0 0 1992 American Chemical Society
