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J . Phys. Chem. 1992, 96, 350-357
the depth at which the surface was studied. Thus, in our experimental system, we cannot determine D with an error less than 0.1. From eq 2, D values of 2.5 f 0.1 and 2.3 f 0.1 result for the nonaged and aged gold surfaces, respectively, in agreement with the smoothing of the deposit surface as t, increases (Figures l a and 2a). These values indicate a marked reconstruction of the gold surface in order to decrease the surface area energy of the system. On the other hand, the D value for nonaged and aged platinum surfaces is 2.5 f 0.1, showing that platinum deposits are more stable than gold deposits under similar experimental conditions. This conclusion is consistent with the slower mobility of the platinum surface (surface diffusion coefficient, Ds,Rzz 1 X cm2/s)I7in relation to the surface mobility of gold (Ds,Au LZ 1 X cm2/s)I7in this electrolyte and temperature. Similar results were recently reported on the surface diffusion of gold and platinum in air at room temperature.I8 The effect of the aging process on the D values was verified by measuring the time dependence of the diffusional current of the Fe(CN),4-/Fe(CN)63reaction.6 We obtained D = 2.5 f 0.1 and D = 2.3 f 0.1 for recently prepared and aged gold electrodepositsin the 5 X and 0.1 s time range,lg respectively, in good agreement with the STM results. On the other hand, the value of D = 2.5 f 0.1 obtained for our platinum electrodeposits by the STM method is also in good agreement with D = 2.46 reported for a thicker platinum deposit exhibiting self-similar patterns obtained by the same electrochemical procedure.6 Finally, it should be noted that information concerning the growth mechanism of the deposits should be derived from the analysis of the D value of nonaged samples ( D 2 2 . 5 ) . For diffusion-limited deposition with DLA patterns one expects D = 2.S3 whereas for a simple ballistic deposition D'gives 1.67,20both (17) Alonso, C.; Salvarezza, R. C.; Vara, J. M.; Ania, A. J.; Vizquez, L.; BartolomC, A,; Bare, A. M. J. Elerrrochem. SOC.1990, 137, 2161. (18) Sommerfeld, D. A,; Cambron, R.; Beebe, T., Jr. J. Phys. Chem. 1990, 94, 8926.
(19) Ocon, P.; Herrasti, P.;Vizquez, L.; Salvarezza, R. C.; Vara, J. M.; Arvia, A. J. J. Electroanal. Chem., in press.
in three dimensions. Our experimental values of D are closer to that expected for DLA model. Computer simulations and experimental work are in progress in our laboratory to verify this point.
Conclusions (1) We have used STM images to measure the fractal dimension of the surface of thin gold and platinum electrodeposits by using perimeter/area relationships. This island type analysis, which applies to both self-similar and self-affine fractals, is based on the fact that STM is able to give real-space three-dimensional images of the surface. Also the good lateral resolution of STM allows us to reach the nanometer range, and the data show that the fractal nature of the surface reaches this small unit. (2) For freshly prepared gold and platinum electrodeposits, a D value equal to 2.5 f 0.1 was obtained. The aging of the gold films in the electrolyte at 325 K produced a decrease in the D value from 2.5 f 0.1 to 2.3 f 0.1, whereas no change in D was detected for aged platinum films. The correlation of this with surface diffusivities suggests that diffusion of atoms on surfaces can cause changes over time of fractal dimension,*' and different values of this dimension should be obtained by changing the metal, electrolyte composition, and temperature. Acknowledgment. A fellowship from the ComisiBn Interministerial de Ciencia y Teqnologia (Spain) and Consejo Nacional de Investigaciones Cientificas y Tecnicas (Argentina) to R.C.S. is gratefully acknowledged. Financial support was obtained from the CICYT through Contract No. MAT89-0204. We acknowledge the critical reading of the manuscript by F. Guinea. We also thank J. G6mez-Herrero for valuable discussions. Registry No. Au, 7440-57-5; Pt, 7440-06-4; H2S04,7664-93-9; Fe(CN)63-,13408-62-3; Fe(CN),+, 13408-63-4; gold oxide, 39403-39-9; platinum oxide, 11129-89-8. (20) Vicsek, T . Fractal Growth Phenomena; World Scientific: Singapore, New Jersey, London, 1989. (21) Hepel, T. J. Elerrrochem. Soc. 1987, 134, 2687.
Catalytic and Spectroscopic Studies of the Water Gas Shift Reaction over RuNa-X and RuH-X Zeolites Guan-dao Lei and Larry Kevan* Department of Chemistry, University of Houston, Houston, Texas 77204-5641 (Received: May 7, 1991)
The low-temperaturewater gas shift reaction has been studied over RU(NH,)~CI~ exchanged into the H-X and Na-X zeolites. The RuNa-X and RuH-X zeolite catalysts are investigated under water gas shift reaction conditions by electron spin resonance, electron spin echo modulation, and infrared and diffuse reflectance spectroscopies. A Ru( 1I)dicarbonyl complex is found to be the catalytically active species for the reaction, and the evidence suggests that it is located at a four-ring site in the a-cage of the X zeolite. In the RuH-X zeolite, an inactive Ru species is formed inside the 8-cage, which is responsible for its lower activity compared to the RuNa-X zeolite.
Introduction The conversion of carbon monoxide and water to produce carbon dioxide and hydrogen as in eq 1 is known as the water gas CO + H2O -* CO2 + H2 (1) shift (WGS) reaction. Its industrial importance derives from its role in the production of ammonia gas,' which requires high-punty hydrogen, and from its ability to increase the H2:C0 ratio in (1) Newsome, D. S. Coral. Reu.-Sci. Eng. 1980, 21, 275.
gaseous feedstock for methanation and Fisher-Tropsch synthesis.2 Commercially, the WGS reaction is carried out over iron-based and copper-based catalysts, which operate at temperatures of 300-400 and 200-250 OC and are called high-temperature shift catalysts and low-temperature shift catalysts, respectively. Attempts have been made to use homogeneous catalysts for the WGS reaction such as rhodium carbonyl iodide and ruthenium carbonyl supported on faujasite-type in alkaline s ~ l u t i o n . ~Ruthenium *~ (2) Storch, H. H.; Golumbic, N.; Anderson, R. B. The Fisher-Tropsch and Related System; Wiley: New York, 1951.
0022-365419212096-350$03.00/0 0 1992 American Chemical Society
The Water Gas Shift over Zeolites zeolitesSshows activity as a low-temperature shift catalyst, which is better than the best homogeneous catalyst on the basis of turnover frequency.6 While the catalytic activity of ruthenium zeolites for the conversion of synthesis gas has been extensively studied,' only a few investigations have reported about the WGS reaction over this catalyst. In these reports, two intermediates, Ru(1) biscarbonyl and Ru(1) tricarbonyl species located in the a-cage, have been suggested to be involved in a catalytic cycle in the a-cage of a faujasite-type zeolite.* However, the structure and location of the active ruthenium species are still not clear. In this study, the WGS reaction is carried out over the RuNa-X and RuH-X zeolites as catalysts with simultaneous investigations by electron spin resonance (ESR), electron spin echo modulation (ESEM), and infrared (IR) and visible-ultraviolet diffuse reflectance (DR) spectroscopic techniques. The catalytically active species is characterized with respect to its structure, oxidation state, and probable location within the zeolite. The effect of the H-X zeolite versus the Na-X zeolite on the catalytic activity is also assessed.
Experimental Section Linde 13X zeolite was washed with 0.1 M sodium acetate solution to give Na-X zeolite. This zeolite was then exchanged with 1 M NH4N03solution four times at 70 "C and calcined in air at 450 "C for 8 h to obtain H-X zeolite. R u ( N H ~ ) was ~~+ exchanged into the Na-X and H-X zeolites by using [Ru(NH3)6]C13(Strem Chemicals). The Ru-exchanged samples were stirred for about 24 h and filtered, and the zeolite was then washed with distilled water and dried at room temperature. The Ru contents were determined by commercial analysis as 1.59 weight % for the H-X and 1S O weight % for the Na-X zeolites. The gases I3C-enriched CO (MSDIsotopes, 99.4% 13C),CO (Matheson research grade), and hydrogen (Linde, UHP) were used without further purification. The catalytic studies were conducted using a fixed-bed-type reactor with continuous gas flow at atmospheric pressure. A measured flow of deionized water was supplied to the gas inlet by a peristaltic pump. The water is converted to the vapor form in the gas inlet tube heated at 120 OC. The entire system was heated to 120 OC with heating tape to prevent H 2 0condensation. The flow system was designed so that the reactor could be bypassed when necessary. The catalyst (-0.2 g) was placed in an electrically heated Pyrex U-tube reactor, and the reaction temperature was monitored by a thermocouplein a thermowell located at the center of the reactor tube. The WGS reaction was studied on the zeolite samples by flowing the reactant mixture over the catalyst at a specific reaction temperature. Stoichiometric amounts of reactants diluted with helium in a CO:H20:He ratio of 1:1:2 were used, and the gaseous hourly space velocity (GHSV) for CO was 220 h-I. The reactants and products were analyzed on-line with use of a sampling valve to a Varian Model 1400 gas chromatograph using a 1/8 in. 0.d. X 6 ft stainless steel column packed with 100/120 mesh of Haysep Q and operating at 120 OC. In order to monitor the zeolite under WGS conditions by electron spin resonance spectroscopy, the reactor was connected to a Suprasil quartz ESR tube maintained at 120 OC. At suitable points during reaction, the sample was quenched at room temperature and transferred to the ESR tube to record a spectrum. ESR spectra at the X-band were recorded at 77 K with a Bruker ESP-300 or Varian E-4 spectrometer. The microwave frequency was measured with a Hewlett Packard Model 5352B microwave frequency counter, and the magnetic field measurements were made with Bruker ER-035M nuclear magnetic resonance (3) Laine, R.;Linker, R.; Ford, P. J. Am. Chem. SOC.1977, 99, 252. (4) Baker, E. C.; Hendriksen, D. E.; Eisenberg, R. J. Am. Chem. SOC. 1980, 102, 1020. (5) Verdonck, J. J.; Jacobs, P. A.; Uytterhoeven, J. B. J . Chem. SOC., Chem. Commun. 1979, 181. (6) Cheng, C. H.; Eisenberg, E. J . Am. Chem. SOC.1978, 100, 5968. (7) For examples, see: Hastings, W. R.;Cameron, C. J.; Thomas, M. J.; Baird, M.C. Inorg. Chem. 1988, 27, 3024 and references therein. (8) Jacobs, P. A.; Chautillon, R.;De Laet; Verdonck, J. J. Inrruzeolire Chemistry; American Chemical Society: Washington, D.C., 1983; p 439.
The Journal of Physical Chemistry, Vol. 96, No. 1, 1992 351
'O
100
O
200
300 b
TEMPERATURE ("C)
Figure 1. Effect of reaction temperature on the conversion of carbon monoxide: (0)RuNa-X zeolite; (A)RuH-X zeolite. Reaction conditions: catalysts, -0.2 g; reactant mixture, CO:H,O:He = 1:1:2; GHSV = 220 h-I for CO; reaction time, 1 h after reaching each temperature. RuNa-X
FeUII) Fresh
X8 4
X8
200°C 250" C
e
Figure 2. ESR spectra at 77 K of the RuNa-X zeolite: (a) after ion exchange; (b) after the WGS reaction at 120 OC; (c) after the WGS reaction at 150 O C ; (d) after the WGS reaction at 200 O C ; (e) after the WGS reaction at 250 O C . Reaction conditions are the same as those in Figure 1.
gaussmeter. Electron spin echo spectra were recorded at 4.2 K with a home-built spectrometer that was described elsewhere."l Infrared spectra were obtained with a Nicolet FTIR system 740 spectrometer equipped with a DTGS detector and interfaced with a Nicolet 620 spectroscopy workstation. Zeolite sample disks for IR studies were prepared by pressing the zeolite powder under 1 ton/cm2 into a self-supporting disk having a diameter of 14 mm and weight between 10 and 20 mg. The disks were then inserted into a Pyrex cell equipped with CaF2 windows connected to the flow system. The IR cell was designed so that the sample disk can be heated in the pretreatment region while the reactant gases are passing over it. The entire cell was maintained at 120 OC with heating tape to prevent H 2 0 condensation, and the IR spectra were taken at this temperature with a resolution of 4 cm-l. Diffuse reflectance spectra in the ultraviolet-visible region were obtained with a Hitachi integrating sphere in a Perkin-Elmer 330 spectrophotometer. The design of the reactor for reflectance studies is similar to that of the ESR cell except the reactor is connected to a quartz reflectance cell instead of the ESR tube. BaS04 was used as a standard, RWikS When RuNa-X and RuH-X zeolites were heated under water gas shift conditions, the only products are hydrogen and carbon dioxide in the temperature range 100-280 OC. Trace amounts of methane were observed after reaction at 280 "C for both the RuNa-X and RuH-X zeolites. The formation of methane indicates that the high-temperature WGS reaction is taking place in which the methanation reaction occurs.12 Figure 1 shows the percent conversion of CO at increasing temperatures for the (9) Ichikawa, T.; Kevan, L. J . Phys. Chem. 1979, 83, 2384. (10) Narayana, P. A.; Kevan, L. Phozochem. Photobiol. 1983, 37, 105. (1 1) Narayana, P. A.; Kevan, L. Magn. Reson. Rev. 1983, 7, 239. (12) Gustafson, B. L.; Lunsford, J. H. J . Coral. 1982, 74, 393.
352 The Journal of Physical Chemistry, Vol. 96, No. 1, 1992
Lei and Kevan
