J. Phys. Chem. 1987,91,6669-6673 agreement with the value calculated from kexo
Conclusions The application of the mixed monolayer approach to gold electrodes coated with highly organized long-chain hydrocarbon amphiphiles enables us at this point to obtain complete blocking of the electrode, or, by controlling the adsorption conditions, to leave a certain amount of natural pinholes. Both cases are of considerable interest from an electrochemical point of view. In the present work we have exploited the latter possibility, Le., the electroactive pinholes in Au/monolayer electrodes with 0 close to (but less than) unity. Such electrodes provide unique arrays of ultramicroelectrodes with an average diameter of 5-10 nm, probably the smallest microelectrodes which have so far been characterized and used. It was shown that Au/monolayer electrodes can effectively facilitate various electrochemical measurements, including kinetic studies of very rapid electrode reactions. We have used such electrodes for the determination of
6669
several very large heterogeneous rate constants ko, the highest being 5.0 cm/s. However, ko values greater than 10 cm/s can be measured quite conveniently with the present system. A complete blocking of electrodes with a single organized monolayer will be important in electrochemical experiments involving electron transfer via electron tunneling, as well as the future implementation of selective ionic or electronic conductivity in organized monomolecular systems. These directions are presently pursued in our laboratory. Acknowledgment. We are pleased to acknowledge numerous helpful discussions with Drs. J. Sagiv and R. Maoz. I.R. is the Incumbent of the Victor L. Erlich Career Development Chair. Registry No. OTS, 112-04-9; OM, 2885-00-9; AA, 50-81-7; PB, 14038-43-8;Fc’, 12125-80-3;Fco, 102-54-5;Me3N+MeFct,51 150-57-3; Me3N+MeFco,33039-48-4; Au, 7440-57-5; Fe(CN):-, 13408-63-4; Ru(bpy)3’+, 15158-62-0; R u ( N H ~ ) ~ * 19052-44-9; +, Ru(NH~)~’+, 18943-33-4.
CO Methanation and Ethane Hydrogenolysis over Ni Thin Films Supported on W(110) and W(100) C. Michael Greenlief,? Paul J. Berlowitz,* D. Wayne Goodman,* Surface Science Division, Sandia National Laboratories, Albuquerque, New Mexico 871 85
and John M. White Department of Chemistry, University of Texas, Austin, Texas 78712 (Received: June 2, 1987)
The kinetics of methanation and ethane hydrogenolysis have been used to probe the surface chemistry of submonolayeramounts of Ni deposited on W( 110) and W( 100). Ni adsorption up to 1 monolayer is pseudomorphic, and submonolayer films are strained to conform to the W substrate lattice dimensions. Methanation rates per Ni atom on both W substrates are equal to rates found on Ni single crystals and on supported Ni catalysts. From these results we conclude that CO methanation is structure insensitive and that the rate-limiting step likely requires no more than one Ni atom. For ethane hydrogenolysis over Ni/W(110), the activation energy was 21.9 f 1 kcal mol-’ and independent of Ni coverage, but the overall reaction rate (per Ni atom) decreased with increasing Ni coverage. Over Ni/W(100) the activation energy was 22.3 & 1 kcal-’ and was independent of Ni coverage. The results are consistent with the strained (more open) Ni overlayers being more active toward C-C bond breaking and with only a single, unhindered Ni atom being required for the ratedetermining step in ethane hydrogenolysis.
Introduction There has been considerable interest in bimetallic systems over the years because of the commercial success of these materials in altering the catalytic selectivity and/or activity in desirable ways.’” Many fundamental studies have centered on trying to define the roles of “ligand” and “ensemble” effects in the catalysts.+12 Ensemble effects are usually defined in terms of the number and geometry of atoms needed for a catalytic process to occur, for example, C-C bond cleavage. Ligand effects refer to those involving electronic interactions. In an effort to evaluate the relative importance of ensemble and ligand effects, we have undertaken a series of studies involving the addition of an active metal to a relatively inactive metal with the intent of systematically varying the geometry of the local ensemble and its electronic character. The prepared surfaces are then probed by a variety of methods, ranging from ultrahighvacuum (UHV) surface investigations to elevated pressure reaction kinetics. In this paper we present results for two different reactions over strained Ni overlayers (
10-1
0
2
U
10-2 F W
3
1.1
1.3
1.5
1.7
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1000/T (K-l) Figure 2. Comparison of the rate of CO methanation over Ni supported on W(110) and W(lOO), Ni(lOO)I4 supported Ni catalysts,"*'* and a thin-film model cata1y~t.l~The total reactant pressure is 120 Torr
(H2/CO = 4:l). over the temperature range studied. An Arrhenius plot of the specific rate of CHI production over various separately prepared Ni coverages on W( 110) and W( 100) is shown in Figure 1. For a total pressure of 120 forr, Arrhenius behavior is observed over the entire temperature range studied (45C-700 K) as the CH4 production rates vary by almost 3 orders of magnitude. The similarity between Ni/W(llO) and Ni/W(100) at all coverages studied is evident in both the turnover frequencies and the activation energy, 18.4 & 1 kcal mol-'. That there is no significant change in the turnover frequencies at each temperature as the Ni coverage and morphology change is consistent with the conclusions of previous studies on ~ingle-crystal'~ and supported catalysts16 that C O methanation is a structure insensitive reaction. Figure 2 is an Arrhenius plot showing previously published data from two nickel single crystals,I4 three aluminum-supported Ni catalyst^,^^^^^ and a Ni/alumina thin-film model ~ata1yst.l~Also
(14) Goodman, D. W. Acc. Chem. Res. 1984, 17, 194. (15) Houston, J. E.; Madey, T. E. Phys. Reo. E : Condens. Matter 1982,
26, 554.
'
1000/T (K-')
(16) Boudart, M. Adu. Catal. 1968, 20, 153.
The Journal of Physical Chemistry, Vol. 91, No. 27, 1987
Ni Thin Films Supported on W(110) and W(100) CO + H z + C H ~ P-120 Torr
0
z W
a
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6671
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1 0 0 0 / T (K-l) Figure 3. Arrhenius plot for CHI synthesis over several different Ni
1000/T ( K - l ) Figure 4. Arrhenius plots of the rates of ethane hydrogenolysis versus
coverages on W(110) at total reactant pressures of 1, 10, and 120 Torr (H2/CO = 4:l).
Ni coverage on W(l lo), at a total pressure of 100 Torr (H2/C0 = 100).
included is the best-fit line through the data from Figure 1. The turnover frequencies measured for the Ni/W system at each reaction temperature are close to those for the previously published work. The activation energy for methanation obtained for Ni/ W(110) and Ni/W(100) of 18.4 f 1 kcal mol-' is lower than the value of 24.7 kcal mol-' reported for Ni(lOO).I4 However, the comparison between turnover frequencies for the four different systems is remarkable, particularly noting the range of Ni coverages and the altered Ni morphologies. The similarities among the different catalysts suggest that the mechanism for the methanation reaction on each is very likely identical. Lowering the total pressure has a significant effect on the rate of methane production (Figure 3) for Ni supported on W(110). At a pressure of 120 Torr, Arrhenius behavior is observed over the entire temperature range studied (450-700 K). However, lowering the total pressure to 10 Torr results in deviation from Arrhenius behavior at 500 K. Raising the temperature causes the methane production rate to increase, but not as quickly as observed for 120 Torr. At 1 Torr, the departure from Arrhenius behavior occurs at even lower temperatures, -440 K. This type of behavior has also been observed on Ni(100)14 and was attributed to the relationship between temperature, pressure, and surface hydrogen atom concentration on the Ni crystal surface. Goodman and co-workers argued that the departure from Arrhenius behavior at the different total pressures was due to the inability of the surface to maintain a critical hydrogen coverage as the pressure was lowered and the temperature was increased. The slope of a plot of the maximum temperature to which Arrhenius behavior was observed over Ni( 100) versus reciprocal temperature correlated with the activation energy for desorption of H2from Ni( 100).14 A similar analysis applied to this data yields a slope of 15 kcal mol-'. This compares well with the activation energy of desorption of H2 from Ni/W( 110) of 17 kcal mol-'.'3 Ethane Hydrogenolysis. Figure 4 shows the turnover frequencies for methane formation from the hydrogenolysis of ethane for three different Ni coverages on W(110). The values shown are for a 99:l H2/C2H6ratio and a total pressure of 100 Torr. At any temperature the rate of methane production over the initially clean Ni layer was constant with no apparent induction period. The turnover frequency during a fixed reaction period of time was determined by using the W(110) atom density of 1.4
-
(17) Vannice, M. A. J . Caral. 1976, 44, 152. (18) Vannice, M. A. Catal. Reo.-Sci. Eng. 1976, 14, 153.
(19) Bischke, S.D.; Goodman, D. W.; Falconer, J. L. Surf.Sci. 1985,150, 351.
-
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0 0.7 ML
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(100)
A 1.1 ML NI/W ( 1 0 0 )
1.8
1.7
1.8
l.g
2.0
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1000/T (K-l) Figure 5. Comparison of the methane production from ethane over
several Ni coverages on W(lOO), Ni on W(110) (in the limit of zero Ni coverage), and a single-crystal Ni( 100) catalyst.20 The total reactant pressure was 100 Torr (H2/C2H6= 99). X lo1' atoms cm-2 and the Ni overlayer coverage. Background reactions on exposed W surfaces have been accounted for and are lo2 to lo3 times lower on the clean W surface over the temperature range studied (500-600 K). There are several features in Figure 4 that are noteworthy: Arrhenius behavior is observed for each Ni coverage; the turnover frequencies decrease with increasing Ni coverage; and the activation energy is 21.9 f 1 kcal mol-' and independent of Ni coverage. If the data in Figure 4 are extrapolated to the limit of zero Ni coverage, the predicted turnover frequencies are found to lie very close to those previously determined for Ni(100), as shown in Figure 5. The zero coverage extrapolation for the activation energy yields a value of 22.6 kcal mol-', within the experimental accuracy of 21.9 f 1 kcal mol-' determined for nonzero Ni coverages. Ethane hydrogenolysis under the same experimental conditions as above was performed using four different Ni coverages on W ( 100) (Figure 5). In contrast to the data on W ( 1lo), where the methane production rates droped with increasing Ni coverage, the turnover frequencies on Ni/W( 100) are independent of Ni
-
6672 The Journal of Physical Chemistry, Vol. 91, No. 27, 19‘87 coverage. Also shown in Figure 5 are the data from a Ni( 100) single crystalz0 (solid line) and the rates from Ni/W(l 10) in the limit of zero Ni coverage. For each of the three systems, the activation energies and turnover frequencies are essentially identical.
Discussion CO Methanation. The specific rates for CO methanation are invariant with respect to Ni coverage and structure on W. Furthermore, the activation energy and specific rates for Ni supported on both W(110) and W(100) are identical and correlate well with previous results for supported Ni catalyst^,'^*'^ Ni single crystal^,'^ and a Ni/alumina thin-film model ~ a t a 1 y s t . lThese ~ results are further manifestations of the structure insensitive behavior of catalytic CO methanation and suggest that the mechanistic steps which control the rate of CO methanation are the same in each case. Lowering the total pressure altered the rate of methane production for Ni/W(llO). Similar effects have been observed previously for Ni single ~rysta1s.l~ However, the departure from Arrhenius behavior w u r s at lower temperatures for Ni/W( 110). The pressure-dependent data can be directly correlated with the H2 temperature-programmed desorption data of Berlowitz and Goodman13for Ni/W(llO). A plot of the maximum temperature for which Arrhenius behavior is observed versus reciprocal temperatue yields an activation energy for H2 desorption of 15 kcal mol-]. The activation energy for H2 desorption from Ni/W( 110) is 17 kcal m01-l.’~ This is lower than the Hz desorption energy from bulk Ni (23 kcal mol-’ 21). The lower H2 desorption energy observed here indicates that there is some interaction between the W single-crystal support and the Ni overlayer. However, since the methanation turnover frequencies for Ni single crystals and for Ni on both W crystals are essentially identical, H2 adsorption/dissociation likely is not the rate-limiting step for CO methanation. C(KVV) Auger line shapes measured after reaction showed the presence of a carbidic carbon and carbon monoxide. Heating to between 1000 and 1100 K was necessary to remove the CO. This treatment also removed the carbidic carbon (via diffusion into the bulk); therefore, no assessment of the carbide concentration was possible. Since CO desorbs from 1 ML of Ni/W( 110) and 1 M L of Ni/W(100) below 400 K,I3 and CO has a hightemperature desorption state on W(110) and W(100) at -950 K, heating to 1100 K to remove the CO indicates that part of the CO remaining after reaction is on the W portion of the surface. CO methanation could occur on the mixed Ni-W ensembles. CO and H2 chemisorption experiment^,'^ however, indicate that no new desorption features are observed that cannot be attributed to either pure Ni or W. This suggests that the interactions of H2 and CO with Ni-W sites are not sufficient to significantly alter CO methanation kinetics. As discussed above, the temperature-pressure-rate behavior shown in Figure 3 may be directly related to the strength of H2 chemisorption. Since chemisorbed H2 on Ni/W(llO) is less strongly bound than on bulk Ni, the rollover in rate is expected to occur a t a lower temperature for the bimetallic system. The origin of the change in activation energy for the overall reaction is not as clear. However, a change in the CO binding energy, resulting in a 50-75 K downward shift in the CO desorption temperature versus bulk Ni,I3 may also affect the kinetics of CO bond cleavage. In addition, the bonding of carbon atoms may be altered. Since CO bond cleavage and hydrogenation of surface carbon may influence both the overall rate and the observed activation energy, the electronic perturbation of the Ni layer in Ni/W may account for the subtle differences between Ni/W and bulk Ni single crystals. Ethane Hydrogenolysis. Since the activity for ethane hydrogenolysis depends markedly on the size and orientation of the metal
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(20) Goodman, D. W. Surf.Sci. 1982, 123, L679. (21) Christmann, K.; Schober, 0.; Ertl, G.; Neumann, M. J . Chem. Phys. 1974, 60, 4528.
Greenlief et al. catalyst particles, it is described as a structure sensitive react i ~ n . ~ , ~In”a~kinetic study of ethane hydrogenolysis on Ni( 111) and Ni(100) single crystals,20Goodman has shown that Ni(100) has a higher activity than N i ( l l 1 ) . This was attributed to the spacing of high-coordination sites on the two different Ni crystals. Ni( 100) has a spacing of 2.5 A between fourfold hollow sites, while Ni( 11 1) has a spacing of 1.4 A between threefold hollow sites. Goodman argued that the greater spacing on Ni( 100) was sufficiently large to facilitate breaking of the ethane C - C bond. On the other hand, Ni(ll1) has a spacing that is close to a C-C bond length and might stabilize C2 species. A 15 kcal mol-’ difference in activation energies between Ni( 100) and Ni( 1 11) also strengthened this argument. The results of Goodman suggest that if the spacing between Ni atoms is sufficiently large, the rate of ethane hydrogenolysis will be large and independent of Ni structure. This is indeed what was observed for hydrogenolysis of ethane for Ni on W(100) where the rates were independent of Ni coverage. The strained Ni overlayer on W( 100) is more “open” than Ni on W( 110) with the distance between high-coordination Ni sites on Ni/W( 100) being approximately 2.7 A. The results indicate that contributing factors to the structure sensitivity of ethane hydrogenolysis are the distance between Ni atoms (as proposed earlierz0)and the distribution of high-coordination adsorption sites for ethane. It is useful to review results from field ion and field emission microscopy in discussing the structure sensitivity of ethane hydrogenolysis on W ( 110). These techniques have shown that Ni Recently, another FIM grows pseudomorphically on W study by Kellog31has established the mobility of Ni atoms and islands. At 77 K Ni is first adsorbed randomly on the surface. At 200 K the Ni atoms are mobile and form one-dimensional rows along the ( 11 1 ) direction on the W(110) surface. At 230 K two-dimensional clusters are formed, these clusters become mobile, as a unit, above 345 K. Above 375 K the clusters were observed to “melt” and diffuse over the tip surface. Thus, at our reaction temperatures, Ni is highly mobile on the W( 110) surface. The Ni may form islands on the surface, but the temperature is sufficiently high that individual atoms may easily break away from these islands, then migrate as single atoms, or join another Ni cluster. It is then reasonable to assume that at reaction temperatures there is a coverage-dependent equilibrium between islands and individual atoms. Low coverages would favor smaller islands and a larger proportion of isolated, low-coordinated, island edge atoms. Larger coverages would favor large islands with few isolated atoms and more island interior atoms. Regardless of the Ni coverage on W ( l lo), identical activation energies for ethane hydrogenolysis are observed, and these energies are within experimental error of those observed on both the open Ni( 100) and Ni/W(100) surfaces. The observed activation energy is approximately 17 kcal mol-’ lower than the value observed on close packed Ni( 111). The only change with coverage is in the specific rate per Ni atom. This suggests that the reaction mechanism on Ni/W(1 10) is similar to the mechanism on Ni( 100) and Ni/W(100) but that the proportion of active metal atoms is changing as a function of coverage. Since it has been established that sterically unhindered atoms are needed for high rates and low activation energy, a plausible model is that only isolated and/or low coordination (island edge atoms) exhibit high activity on Ni/W( 110). The activity of island interior atoms may be similar to high-coordination Ni( 111) atoms. If this is the case, then the large activity of the low-coordination atoms would dominate the overall rate, since rates on Ni( 11 1) (22) Carter, J. T.; Cusumano, J. A.; Sinfelt, J. H. J . Phys. Chem. 1966, 70, 2257. (23) Yates, D. J. C.; Sinfelt, J. H. J . Catal. 1967, 8, 348. (24) Martin, G. A. J . Coral. 1979, 60, 345. (25) Martin, G. A. J . Catal. 1979, 60, 452. (26) Jones, J. P. Nature (London)1966, 211, 479. (27) Smith, G. D. W.; Anderson, J. S. Surj. Sci. 1971, 24, 459. (28) Jones, J. P.; Martin, A. D. Surf.Sci. 1974, 41, 559. (29) Bassett, D. W. Thin Solid Films 1978, 48, 237. (30) Flahive, P. G.; Graham, W. R. Thin Solid Films 1978, 51, 175. (31) Kellogg, G. L., submitted for publication in Surf. Sci.
J . Phys. Chem. 1987, 91, 6673-6677 are 1-4 orders of magnitude smaller than rates on Ni( 100) from 600 to 450 K.20 These observations are consistent with our zero-coverage extrapolation of the rates. As zero coverage is approached, the proportion of low-coordination Ni atoms would approach 10076, and the rate should be equal to the less sterically hindered Ni/W( 100) surface, which is the observed behavior.
Conclusions 1. C O methanation rates are independent of both Ni coverage and morphology and are similar to rates observed for Ni single crystals and supported Ni catalysts. An activation energy of 18.4 f 1 kcal mol-' is observed. 2. The methanation rate behavior as a function of total pressure is similar to bulk Ni single crystals, although the decrease in rates occurs at a lower temperature on Ni/W(110). This may be a result of the decrease in H2 binding energy on Ni/W( 110) versus bulk Ni.
6673
3. Ethane hydrogenolysis rates and activation energies for Ni/W(100) and Ni/W(110) in the limit of zero coverage are identical with rates on bulk Ni( 100) but substantially different from N i ( l l 1 ) . 4. The rate of ethane hydrogenolysis on Ni/W(1 10) is highly dependent on coverage, but the observed activation energy of 21.9 1 kcal mol-' is independent of coverage and equal to the activation energy on Ni/W(100) and Ni(100). This may be caused by a "surface area" effect, where only island edge and/or isolated Ni atoms exhibit significant activity.
*
Acknowledgment. This work, performed at Sandia National Laboratories, was supported by the U S . Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, under Contract No. DE-AC04-76DP00789. Registry No. CO, 630-08-0; CH3CH3, 74-84-0; Ni, 7440-02-0; W, 7440-33-1.
Characterization of (SiW,204,)4-/Ti02 Suspensions and Their Activity in the Evolution of Hydrogen from Water J. Kiwi* and M. Gratzel Institut de Chimie Physique, Ecole Polytechnique Fgdgrale, CH- 1015 Lausanne, Switzerland (Received: June 8, 1987)
Tungstosilicate ((SiW120a!4-)-loaded titania is active in the photoproduction of hydrogen under band gap illumination. The role of the heteropoly acid is to enhance water reduction. Absorption studies, potentiometric titrations, and electrophoresis have been carried out to determine the effect of (SiWi20a)" deposition on the acid-base characteristics of the Ti02 interphase. The point of zero potential was found to be lowered by up to 150 mV as a result of polytungstate deposition.
