J. Phys. Chem. 1985,89, 5671-5676
5671
The Structure of the Cu/ZnO Catalyst by an in-Situ EXAFS Study Kazuyuki Tohji, Yasuo Udagawa,* Institute for Molecular Science, Okazaki, Aichi 444, Japan
Takanori Mizushima, and Akifumi Ueno Toyohashi University of Technology, Toyohashi, Aichi 440, Japan (Received: March 21, 1985)
In-situ EXAFS measurements have been made on the Cu/ZnO catalyst under reacting conditions at elevated as well as at ambient temperature. It was found that the local structure around the Cu atoms, which were believed to be the active atomic species, changes reversibly with heating/cooling cycles in a Hzstream. Between about 400 and 550 K highly dispersed Cu metal clusters appear, while metal clusters with metal-oxygen bonding exist below 400 K. Since this catalyst is used industrially around 550 K, the former should be regarded as the carrier of catalytic activity for the system. However, once this catalyst is heated above its active region, the Cu clusters coalesce into large crystalline particles and the change with heating/cooling cycles ceases to be reversible. This provides a rationale for a well-known loss of activity when this catalyst is operated at high temperatures. The results presented here emphasize the importance of characterizing catalysts under working conditions.
Introduction One of the most important problems in understanding heterogeneous catalysis is to identify the active species and to determine the structure of the catalyst. Numerous spectroscopic techniques have been applied to this problem. Aided by improvements in vacuum technology, surfacesensitive methods employing electrons such as X-ray photoelectron spectroscopy (XPS), among others, have added to the list' in recent years. An assumption implicit in the application of these high vacuum methods is that the structure of the catalyst in the vacuum is not different from that under the catalytically active environment, which is often at high temperature and/or under high pressure. The copper/zinc oxide catalyst containing alumina or chromia is industrially important for methanol synthesis from H2 and C0.2-9 It has been reported that the mixed catalyst is over three orders of magnitude more active than each of the components. In spite of the considerable interest in this important catalyst, reports concerning the structure of the active species are in disagreement. Klier et al.2-4 proposed that Cu+ dissolved in the ZnO lattice is the active species. This suggestion was based mainly on the diffuse reflectance spectra of the catalyst, combined with a study of its reactivity over a range of composition. On the other hand, Okamoto et al.*s9 employed XPS in order to study the copper species on the surface and concluded that the two-dimensional Cuo-Cu+ species is the carrier of the catalytic activity. These two studies were done at room temperature. The reflectance spectra were measured without exposure to air after reduction. In the XPS study the reduced catalyst was studied under vacuum. In this study in-situ extended X-ray absorption fine structure (EXAFS) measurements have been made to study the local structure of the Cu/ZnO catalyst under conditions close to those present in the reaction vessel: under a H2 stream and at elevated temperatures. It has been found that the local structure around (1) Somorjai, G. A.; Zaera, F. J . Phys. Chem. 1982, 86, 3070. (2) Klier, K. Adv. Coral. 1982, 31, 243. (3) Herman, R. G.; Klier, K.; Simmons, G. W.; Finn, B. P.; Bulko, J. B.; Kobylinski, T. P. J . Catal. 1979, 56, 407. (4) Mehta, S.;Simmons, G . W.; Klier, K.; Herman, R. G. J . Catal. 1979, 57, 339. (5) Bulko, J. B.;Herman, R. G.; Klier, K.; Simmons, G. W. J . Phys. Chem. 1979, 83, 3 1 1 8. ( 6 ) Klier, K.; Chatikavanij, V.;Herman, R. G.; Simmons, G. W. J . Coral. 1982, 74, 343. (7) Shimomura, K.; Ogawa, K.; Oba, M.; Kotera, Y . J . Catal. 1978, 52, 191. (8) Okamoto, Y.; Fukino, K.; Imanaka, T.; Teranishi, S. J . Phys. Chem. 1983,87, 3740. (9) Okamoto, Y . ;Fukino, K.; Imanaka, T.; Teranishi, S. J . Phys. Chem. 1983,87, 3747.
0022-3654/85/2089-5671$01.50/0
the Cu atom, which is believed to carry the catalytic activity, assumes several forms depending upon the temperature.
Experimental Section and Data Analysis In one preparation, the Cu/ZnO catalyst precursor was prepared by a coprecipitation method. A blue precipitate was obtained by adding an aqueous solution of cupric nitrate and zinc nitrate to a 1.0 M sodium carbonate solution. A Cu:Zn ratio of 3:7 was employed. The precipitate was dried overnight at 383 K, followed by calcination a t 623 K for 3 h. The performance from this catalyst preparation will be reported in a separate paper. In a second method of preparation, the catalyst was prepared by conventional wet impregnation of ZnO using an aqueous cupric nitrate solution, then dried overnight at 383 K, and calcined at 623 K for 3 h. The EXAFS experiments were performed by an in-house EXAFS system which has been described in detail .previously.'&l2 Basically it consists of a high-power X-ray generator (Rigaku RU-200), a spectrometer with a curved Ge(311) crystal monochromator, and a germanium solid-state detector. The X-ray source with a Ag target was operated at 20 kV and 200 mA in order to minimize the effect of higher-order reflections. The resolution of the spectrometer near the Cu K absorption edge is about 4 eV hwhm. A drawing of the in-situ cell used in this work is shown in Figure 1. The fundamental design is similar to the one reported by Lytle et al.I3 The windows of the sample cell were fabricated from 0.5-mm-thick boron nitride in order to minimize X-ray absorption loss. A spacer is placed between the windows to hold a sample of the desired thickness. The sample thickness throughout this work was 1.0 mm. The windows and the spacer are clamped tightly together by two stainless steel mounting blocks. Gas can reach the sample by means of holes in one of the mounting blocks and in one of the windows. The assembly is housed inside copper heating blocks, which can heat the sample to 1300 K. X-ray diffraction (XRD) data were obtained with a Rigaku Geigerflex diffractometer equipped with a copper X-ray tube, operated at 30 kV and 20 mA. The EXAFS spectra were collected as the dependence of photon energy, E , of the ratio of the transmitted photon number, I, to the incident beam intensity, I,. The ratios are converted to the (10) Tohji, K.; Udagawa, Y. Jpn. J . Appl. Phys. 1984, 22, 882. (1 1) Tohji, K.; Udagawa, Y . ;Kawasaki, T.; Masuda, K. Rev. Sci. Zmtrum. 1983.. 54.. 1482. (12) Tohji, K.; Udagawa, Y . ;Tanabe, S.; Ida, T.; Ueno, A. J . Am. Chem. SOC.1984, 106, 5172. (13) Lytle, F. W.; Wei, P. S. P.; Greegor, R. B.; Via, G. H.; Sinfelt, J. H. J . Chem. Phys. 1979, 70, 4849.
0 1985 American Chemical Society
5612 The Journal o/Physical Chemistry, Vol. 89, No. 26. 1985
Tohji et al.
I
Calcined
t
Figure 1. Drawing showing the assembly of the in-situ eel1 and the furnaee: a, gas in; b, gas out: c, thermmuple; d, stainless steel mounting blocks; e, BN windows; f. spacer; g, copper blocks; h, heater; and i, X-ray.
1a o
9.0
I
ENER Q V / b V
absorbance, pt, and plotted against the magnitude of the photoelectron momentum, k, where p(k)t = In k = 2r[2m(E
(Id0
- Eo)]l/Z/h
(1)
(2)
and f is the sample thickness. In this study the absorption edge energy Eo was first set as the energy a t which the absorption coefficient is equal to one-half the absorption maximum and then varied in the least-squares fitting. The monotonous contribution of the background abwrption was estimated hy least-squares fitting of Victoreen's formula to the pre-edge absorption and was subtracted by use of interpolation. The EXAFS function, x(k), is defined as x(k) = ( p - po)/po where p refers to absorption by atoms in the material of interest, and h to that of an isolated atom. It is a matter of debate in the analysis of EXAFS how to estimate flW" Much care is required to avoid problems inherent to the procedure, and still some ambiguity always remains. In order to reduce the ambiguity, w, was determined in this work by a second-order least-squares-fitting over an interval around each data point. If the intervals around the data points are equal, the smooth background po(J) can be expressed as a polynomial in J po(J) = a
+ b J + CP+ ...
(3) where J denotes the running integer of data points. In order to make A = xf..L[p(J) - p&)]z minimum, the coefficients can be determined as follows (where only the first three terms in (3) are retained): a=
I
cc
I
I
I
I
DISTANCE / P I
Figure 2. (a) Observed Cu K and Zn K absorption spectra of the calcined 3 9 Cu:Zn catalyst. (b) Fourier transforms of the EXAFS oscillations assaeiated with (a).
A single scattering model for the EXAFS oscillations is written as15J6 x(k) =
4 x -F.(r,k) kr? '
exp(-2kza? - Zrj/X) sin (2krj + uj(k)) (7)
J
where Nj is the number of atoms in the jth shell, Fj(k) is the scattering amplitude, cj is the DebyeWaller factor, and uj is the phase shift. R, is the distance from the central absorbing atom to the atoms in the jth shell. The uncertainty in Rj, to which static disorder as well as the lattice vibration can contribute, is taken account by ai. The mean free path of the photoelectron in the solid, X, was assumed to be k independent in the present analysis. The extracted EXAFS function was Fourier transformed into real space with a k' weighing factor to yield the radial structure function, @, by use of the equation:
@(R)= Jk'x(k)
exp(-ikr) dk
(8)
A curve-fitting technique was subsequently employed in order to determine the bond distance, Rj; the DebyeWaller factor, aj;
The coefficients are calculated at each data point and so is h(0. In this procedure only one parameter, L, the size of the interval used in the least-squares calculation, is required, leaving less ambiguity in the estimation of po. In this work a k range of 4-5 A-1 was used for the central portion of the EXAFS, and an extrapolation was employed at low k edges. By using the po determined by the above procedure, it was unnecessary to remove low-frequency components by an inverse Fourier transform technique. (14)
Cmk, Jr.. J. W.;Sayers, D. E. J. Appl. Phys. 19%1,52, 5024
as well as the mrdination number, Ni. The main peaks in I@(R)I were back-transformed to k space and a least-squares calculation was carried out by using eq 7. All of the calculations were carried out on an M-200H computer a t the Computing Center of IMS.
Results The Calcined Catalyst. An in situ X-ray absorption study has been carried out at room temperature upon the Cu/ZnO catalyst prepared by coprecipitation and containing 30 mol 90Cu atoms. This composition corresponds to the 3:7 Cu:Zn ratio reported to be the most active? a fact also confirmed by us. The EXAFS spectrum of the Cu/ZnO catalyst a t the calcination step is ob(15) (16)
Stern, E. A. Phys. Re". B 1974, 10, 3027. Stem, E. A,; Sayers, D. E.; Lytle, F.W.Phys. Reo. B 1975, I / . 4836.
rhe Journal of Physical Chemistry, Vol. 89, No. 26, 1985 5673
Structure of the Cu/ZnO Catalyst
TABLE I: Structural Parameters Obtained for the Calcined 30 mol % Cu/ZnO Catalyst and for the Reference Materials at Room Temperature" material bond temp N X R, 8, a, 8, R,C 8,
Reference Materials RT (4) 6.30 1.93, RT (4)b 5.74 2.840 Cui0 RT (2) 6.99 1.856 RT (12) 4.85 2.953 Cu foil 2.514 8.0, RT (12) ZnO 1.952 (4) 5.3, RT 3.25, 5.3, RT (12) Cu/ZnO Calcined Catalyst 3.6 (6.30) 1.929 Cu edge Cu-0 RT CU-CU RT 4.5 (5.74) 2.850 Zn edge Zn-0 RT 4., (5.31) 1.968 Zn-Zn RT l l + (5.31) 3.24, CUO
I
1
3
5
7
I
3
5
7
CU-0 CU-CU CU-0 CU-CU Cu-Cu Zn-0 Zn-Zn
0.04, 0.109 0.049 0.142 0.08* 0.064 0.092
1.95 2.90 1.84 3.01 2.55 1.97 3.24
0.04, 0.110 0.06., 0.Og4
"The numbers in parentheses are those fixed in the least-squares calculations. *The distances to other eight copper atoms are much larger than 3 A. cXRD.
I
9
9.2
9.8
9.4
10.0
9.6
10.2
1
3
5
7
1
3
1
7
b
0 ISTANCE / A E N E R G Y / keV Figure 3. EXAFS spectra and associated Fourier transforms of reference compounds: (a) CuO, (b) Cu20, (c) Cu metal, (d) ZnO.
served in the energy range from about 300 eV below the Cu K absorption edge to 1000 eV above the Zn K absorption edge. It is shown in Figure 2a. The associated Fourier transforms of Cu as well as Zn EXAFS are shown in Figure 2b. EXAFS spectra and the associated Fourier transforms for the reference materials are shown in Figure 3 for comparison. It is immediately evident that the EXAFS spectrum and the associated Fourier transform of the calcined catalyst coincide completely with parts a and d of Figure 3, suggesting, without detailed analysis, that all Cu atoms exist as CuO particles and Zn atoms as ZnO at this stage. In order to confirm the structural parameters of the calcined catalyst, curve-fitting analysis was performed for the main peaks shown in Figure 2b. The best-fit values of the calcined sample are listed in Table I as well as those of reference compounds. From these analyses, it was substantiated that a large peak centered at about 1.4 A in Figure 2b represents the four nearest oxygen atoms which make up the first shell of Cu neighbors at 1.93 A. A second peak centered about 2.2 8, represents the second shell made of four Cu atoms at a distance 2.85 A. The resultant parameters from the calcined sample, such as the coordination numbers and the interatomic distances, coincide completely with that of CuO crystal. The parameters obtained from the Zn EXAFS also shows very good agreement with those of ZnO. The Reduced Catalyst. The calcined sample was reduced in the in-situ cell with hydrogen at 473 K for 2 h. EXAFS measurements were subsequently made at several temperatures under constant H2 flow. Since the Zn EXAFS is almost the same as that shown in Figure 2 and it did not show any change under the conditions employed throughout this work, only Cu EXAFS will be shown henceforth. Cu K EXAFS spectra of the reduced catalyst subsequently observed at 473,373,293,373, and 473 K under constant H2 flow are shown on the left-hand side of Figure 4. The associated Fourier transforms of the EXAFS functions, which were taken over the wave number range of 3 < k < 11 are shown on the right-hand side of Figure 4. A significant local structure change with temperature around the copper atoms is evident from
Figure 4. EXAFS spectra and associated Fourier transforms of Cu/ZnO catalyst prepared by coprecipitation and reduced by H2 at 473 K and subsequently observed at 473, 373, 293, 373, and 473 K with a constant H2 flow.
the X-ray absorption near edge structure (XANES) as well as the EXAFS region; a sharp, strong peak near the edge emerges at lower temperature, and the oscillation, characteristic of the Cu metal, diminishes at thesame time. A phase transformation must exist at a temperature between room temperature and 473 K. These spectral changes are found to be reversible with repeated heating/cooling cycles. A detailed analysis using curve-fitting techniques gives structural parameters. The main peaks shown in Figure 4 were backtransformed, and Nj, Rj,and ai at several temperatures were calculated by use of eq 7. Because it is not an easy task to determine all the parameters independently, the best-fit X values of the reference compounds were computed first with N fixed. Then, other parameters of the catalyst were calculated by use of the A's determined above. The results are summarized in Table 11. The observed peaks centered at about 1.4 and 2.2 A in the Fourier transforms of Figure 4 correspond to the Cu-0 distance
5674
TABLE 11: Structural Parameters at Various Temperatures Obtained for tbe Reduced 30 mol % Cu/ZnO Catalyst Prepared by Coprecipitation as well as by Impregnation (Those for Copper Foil Are Also Shown) material bond temp, K N X R , 8, u, 8, R,”8,
Reference Material Cu foil CU-CU RT (12) (8.01) 2.514 Cu-Cu 373 (12) (8.01) 2.502 CU-CU 473 (12) (8.01) 2.49, CU-CU 573 (12) (8.01) 2.484 Cu/ZnO Reduced Catalyst impregnation Cu-Cu RT 11.8 (8.01) 2.510 CU-CU 373 12.0 (8.01) 2.503
cu-cu
coprecipitation
CU-CU Cu-0
cu-0 cu-0
CU-0 CU-CU Cu-cu
cu-cu
CU-CU CU-CU
cu-cu
CU-CU a
Tohji et al.
The Journal of Physical Chemistry, Vol. 89, No. 26, 1985
473 573
RT 373 393 408
RT 373 393 408 423 473 493
M
50
W
c
0.088 2.556 0.10, 0.115 0.130
II
-
0
= l l W
c
*
b
l
0.099 0.110 11.7 (8.01) 2.49, 0.119 l l . , (8.01) 2.47, 0.127 1.0 (6.99) 1.759 0.057
1.1 (6.99) 1.3 (6.99) 1.0 (6.99) 5., (8.01) 5.4 (8.01) 5.9 (8.01) 8.4 (8.01) 10.3 (8.01) 9., (8.01) IO., (8.01)
1.789 0.057 1.756 0.069 1.765 0.07, 2 . ~ 80~. 1 0 ~ 2.47, 0.105 2.477 0.099 2.475 0.102 2.468 0.107 2.474 0.109 2.46, 0,115
XRD. 100
-
1
A N 6 L E / 28 Figure 6. X-ray diffraction patterns of calcined CuO/ZnO catalysts
prepared by (a) coprecipitation and (b) impregnation. Peaks due to CuO (A) and ZnO (A)are indicated.
Catalyst Prepared by Impregnation. The catalyst with the same composition prepared by impregnation was also studied by the same manner as above. Although not shown here, the EXAFS spectrum of the calcined material is almost the same as that in Figure 2 , and that of the reduced sample is very similar to that of the copper metal. Therefore, only the parameters refined by the curve fitting are shown in Table 11. EXAFS as well 8s X-ray diffraction, ESR, and ESCA studies of calcined copper/alumina prepared by impregnation has been reported by Friedman, Freeman, and Lytle.” They also observed that the CuO phase predominates at higher Cu loadings as is employed in this work.
Discussion
TEMPERATURE/ K Figure 5. Observed intensity change of the Cu-Cu ( 0 )and CuO (A) peaks in the Fourier transforms of the EXAFS of the Cu/ZnO catalyst. Measurements were carried out consecutively as indicated by the numbers in the figure at various temperatures.
of copper oxide and the Cu-Cu distance in copper metal, respectively. In order to determine more precisely the temperature of the phase transition, EXAFS measurements were made at various temperatures. The intensity changes of the Cu-Cu and the Cu-0 peaks with temperature are indicated by circles and triangles in Figure 5, respectively. Experiments 1-5 represent cooling, and 6-10 heating, indicating reversibility. In addition to the continuous intensity changes with temperature, a jump can be observed between experiments 3 and 4 on cooling, and between experiments 7 and 8 on heating, both in the vicinity of 400 K. In order to examine the high temperature structure of the Cu/Zn catalyst under hydrogen an EXAFS spectrum was taken at 673 K (expt 11, Figure 5). After this, EXAFS measurements were made at lower temperatures (expt 12 and 13). A normal increase of the Cu-Cu peak was seen as usually expected from the Debye-Waller factor. The unusual, reversible behavior shown in expt 1-10 is now absent. In addition, after heating the catalyst to 673 K the CuO peak is no longer present below 400 K. Evidently heating to high temperatures has irreversibly altered the catalyst. Since the EXAFS spectrum is that of pure Cu metal at every temperature (subsequent to heating) it seems that the Cu is now in the form of crystalline particles.
We have shown how the local structures around Cu atoms has been probed by EXAFS spectroscopy under various conditions. The most striking observation in the reduced Cu/ZnO catalyst prepared by coprecipitation is that the local structure around the Cu atom varies with temperature, and that the change is reversible as long as the temperature is not too high. We shall discuss the local structural changes of the reduced catalyst as well as the calcined catalyst by combining the structural information obtained in the present EXAFS experiments with that derived by XPS, optical spectroscopy, and XRD reported earlier.3~5~8~9 Calcined Catalyst. On the basis of the XPS results, Okamoto et aL8 proposed that three types of copper species exist on the surface of the catalyst: crystalline CuO, amorphous copper oxide, and Cu+ ions dissolved substitutionally in the ZnO lattice. Their fractional presence is said to depend on the elemental composition of the catalyst. For the catalyst containing about 30 mol 56 CuO studied here, the surface species is reported to be predominantly amorphous copper oxide. The evidence for amorphous CuO comes from the intensity of a satellite line, which appears because of the distortion of CuO from square-planar to octahedral symmetry. The EXAFS spectra, however, do not provide support for such a distortion; the local structure around the Cu atoms is almost the same as that in crystalline CuO. A comparison with the results of EXAFS and XRD on the catalyst prepared by impregnation is helpful in identification of the structure. As was stated previously, the EXAFS spectrum, as well as the radial structure function of the impregnated sample, coincides completely with those from the coprecipitated sample shown in Figure 2. The XRD patterns shown in Figure 6, however, differ depending on how the sample was prepared. The XRD pattern of the impregnated sample clearly shows sharp lines due to the CuO crystal as well as to ZnO, indicating that CuO is not highly dispersed. On the other hand, in the coprecipitated sample relatively broad lines are seen only due to ZnO, suggesting that particle sizes of ZnO are smaller in this case. No trace of the lines due to CuO can be found. (17) Friedman, R. M.; Freeman, J. J.; Lytle, F. W. J . Card. 1978, 55, 10.
Structure of the Cu/ZnO Catalyst These results, which show the same EXAFS spectra but different XRD patterns for the catalysts prepared by coprecipitation and impregnation, should relate to features that distinguish these two methods; EXAFS reflects only the local structure around the central atom, while the XRD is concerned with long-range order. Therefore, the experimental results can be best explained in the following manner; very small CuO crystallites are formed by coprecipitation, while fairly large CuO crystals grow in the impregnated sample. Distortion from square-planar symmetry in the CuO particle is small, if it exists at all. Reduced Catalyst: below 400 K . One of the features in the EXAFS of the sample below 400 K is the appearance of oxygen atoms adjacent to Cu. Unlike the calcined sample, the bond length determined from EXAFS analysis of the reduced catalyst is somewhat shorter than that of CuO and is closer to that of Cu,O as shown in Table 11. The coordination number is estimated to be about 1. This is much smaller than that of CuO and Cu20, where copper atoms are surrounded by four and two oxygen atoms, respectively. Another feature is the main peak at 2.3 8, which shows the presence of metallic Cu. The estimated coordination number is about 6, which also is very small and only half of that of the bulk metal. Both XPS studies by Okamoto et aL9 and optical spectra by Bulko et aL5 indicate the presence of two kinds of Cu, Le., Cu+ and Cuo. Two models that are compatible with those observations are suggested. One model assumes the existence of two kinds of copper atoms in different environments: those dissolved in the ZnO lattice and those contained in well-dispersed copper metal clusters. In this model the peak at 1.48, in Figure 4 comes from Cu atoms dissolved in the ZnO lattice and the peak a t 2.2 8, is due to Cu in the cluster. The apparent reduction in the coordination number is the result of the existence of two kinds of Cu atoms. Such a model has already been proposed by Herman et aL3 They observed a pronounced change of the Cu absorption band and the simultaneous disappearance of the Zn absorption band in the 3:7 Cu:Zn catalyst and took this as an evidence for some copper being dissolved in the ZnO host. They concluded that all copper atoms are accounted for by the two distinct forms, i.e., the small microcrystallites of the pure metal and dispersion of copper atoms in the ZnO crystallites. The latter condition would represent the active species in methanol synthesis. Another model suggests the existence of a quasi-two-dimensional epitaxial copper layer over the ZnO. In this model the Cu-0 bonds are formed at the interfacial region, and the Cu atoms inside the layer must contribute to the peak at 2.2 A. In this case the apparent reduction in the coordination number is attributed to the size and thickness of the epitaxial layer. If it is very small and thin, the fraction of copper atoms located a t the surface and thereby having a smaller codidination number, will be significant. On the basis of the XPS results, which indicate the existence of both Cuo and Cu+, Okamoto et aL9 proposed the coexistence of the Cu metal cluster and the two-dimensional Cuo-Cu+ layer. The latter provides active sites for methanol synthesis in this scheme. It is impossible at the present stage to judge which is the correct description of the Cu/ZnO catalyst a t room temperature. The latter model is preferred from our observations as will be discussed shortly. However, the proposed active catalytic component in methanol synthesis in either model is probably incorrect since catalytic activity increases with temperature and, as we have seen, the structure changes upon heating above 400 K. Reduced Catalyst: above 400 K . The EXAFS spectrum and its Fourier transform of the Cu/ZnO catalyst above 400 K are similar to those of Cu metal foil, indicating that Cu atoms exist in the form of metal particles. From the curve-fitting analysis, the coordination number of this sample was found to be considerably smaller than those of Cu metal foil at the same temperature, as is shown in Table 11. This indicates that the metal particles are very small. On the other hand, in the catalyst prepared by impregnation, the coordination number almost coincided with that of copper foil, showing that fairly large Cu particles are formed. In passing, it is noteworthy that an apparent bond distance contraction with increasing tempdrature, which has been reported
The Journal of Physical Chemistry, Vol. 89, No. 26, 1985 5675 (b)
Figure 7. Schematic structure models of copper clusters in Cu/ZnO catalyst proposed here at various temperatures: (a) epitaxially developed quasi-two-dimensionallayers, (b) small copper metal clusters, and (c) copper metal crystals.
several times in EXAFS studies,18J9has clearly been observed in Table 11. The sizes of small metal particles can be estimated from the measured coordination number by using the procedure proposed by Greegor and Lytle.20 Assuming that the shape is either spherical or cubic and using Figure 6 of ref 20 together with the coordination number shown in Table 11, that is about 10, the average particle size is estimated to be 20-30 A. Although the surface atoms in a cluster of this size represent about 20-30% of all atoms, no unique contribution from them to the EXAFS spectrum (for example, the existence of a Cu-0 bond) was observed. The only differences from the Cu metal EXAFS is the smaller apparent coordination number. A two-layer model is not adequate in this case because it requires a much lower coordination number than is observed. Since the Cu atoms are very mobile and evidently exist in forms that change easily during the heating/cooling cycles, the particles can be expected to be very small and most likely exist on the surface of ZnO. One scheme that explains the complete set of observations is the following. At lower temperatures copper atoms form a quasi-two-dimensional layer on the ZnO support, but at higher temperatures they transform into very small metal clusters at the surface of the support. Since the catalyst is especially active at elevated temperatures, the highly dispersed metal clusters at the surface should be the carriers of catalytic activity. This assumes that additional structural changes do not occur at the high pressures employed in the methanol synthesis and this was not examined in the present work. Once this catalyst is heated over 600 K, the phase change is no longer reversible. As was seen previously, the EXAFS spectra at room temperature after the catalyst had been heated to 673 K is almost the same as that of Cu foil. This likely indicates that small Cu clusters have coalesced to make large crystalling particles at the high temperature, and that once the particle size exceeds some threshold, most Cu atoms are retained in the interior of the pure metal phase and can no longer contribute to the reversible changes that are seen prior to such heating. This might well be an explanation for the known loss of catalytic activity through thermal shock.
Concluding Remarks The temperature dependence of the EXAFS experiments on the Cu/ZnO catalyst is best explained by assuming the following three structures: (a) below 400 K a quasi-two-dimensional layer epitaxially developed over the ZnO support; (b) between 400 K and at least 500 K small copper metal clusters dispersed over the ZnO host; and (c) after heating to high temperature large copper metal crystals dispersed on the support due to a sintering of copper atoms. These three structures of the Cu/ZnO catalyst are illustrated in Figure 7a-c, respectively. It can be concluded that, of the three forms of Cu, the dispersed small copper metal clusters in Figure 7b are the active species in methanol synthesis. The question as to the mechanism of catalytic activity of the Cu/ZnO in methanol synthesis still remains. Herman et aL3 proposed the simultaneous chemisorption and activation of C O on Cu+ and of hydrogen on the adjacent ZnO surface. This mechanism depends on the catalyst model in which Cu+ substi(18) Marques, E. C.; Sandstrom, D. R.; Lytle, F. W.; Greegor, R. E. J . Chem. Phys. 1982, 77, 1027. (19) Eisenberger, P.; Brown,G. S.Solid State Commun. 1974, 29, 481. (20) Greegor, R. B.; Lytle, F. W. J . Catal. 1980, 63, 476.
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J . Phys. Chem. 1985, 89, 5676-5681
tutionally replaces Zn in the ZnO lattice. This model is now found to be inadequate for temperatures corresponding to those of the reaction conditions. Another mechanism must be sought. The small Cu particle size might be important to catalytic activity. For both overheated Cu/ZnO and the catalyst prepared by impregnation the Cu particle size is large and in both cases the catalytic activity is diminished. Whether the function of ZnO is just to support the highly dispersed Cu clusters or actually to alter the electronic states of these particles for improved catalytic activity is still open to question. CO chemisorption has been reportedz1to have significant influences upon the structure of Rh supported on A1,03 from EXAFS studies. The C O molecule was found to strongly interact (21) Van? Blik, H. F. J.; Van Zon, J. B. A. D.; Hizinga, T.; Vis, J. C.; Koningsberger, D. C.; Prins, R. J . Phys. Chem. 1983, 87, 2264.
also with the Cu cluster studied in this work. Under a C O atmosphere, the Cu clusters prepared by reduction at 473 K did not show any structural transformation upon lowering the temperature. Evidently chemisorption of C O has a marked effect upon the nature of the Cu cluster. Therefore, it is likely that the initial step in the catalytic synthesis of methanol is the chemisorption and activation of C O on the surface of the Cu clusters. A study of the effects of several gases upon this catalyst is now in progress and will be reported in the near future.
Acknowledgment. We express our gratitude to Mr. Shinji Kat0 and other staff of the Development Workshop of IMS for their collaboration in the construction of the in-situ cell used in this work. We thank Dr. Endo of IMS for the suggestions on least-squares calculations. Registry No. ZnO, 1314-13-2; Cu, 7440-50-8; methanol, 67-56-1.
Determination of Flat-Band Position of CdS Crystals, Films, and Powders by Photocurrent and Impedance Techniques. Photoredox Reaction Mediated by Intragap States Malcolm F. Finlayson, Bob L. Wheeler, Narioyshi Kakuta, Koon-H. Park, Allen J. Bard,* Alan Campion, Marye A. Fox, Stephen E. Webber, and John M. White Department of Chemistry, The University of Texas, Austin, Texas 78712 (Received: April 29, 198s)
The flat-band potential of CdS has been measured for three different forms of CdS and by three different methods. Good agreement is found for determinations by Mott-Schottky and photocurrent techniques for single crystals and thin films ( V , = -1.3 V vs. SCE in 0.1 M NazS). Photocurrent measurements of powder and colloidal systems yield apparent flat-band positions that are more positive than the above by about 0.7 V. The present work reveals the presence of a number of intragap states and identifies those responsible for the 0.7-V discrepancy. The effect of annealing of films and treatment of powders with Zn2+ on the intragap states and the improvement in CdS particle systems for H, evolution by ZnS coating is discussed.
Introduction This paper deals with the energetics (band gap, band energies) of several different forms of CdS and especially with CdS powders and colloids. To decide what photoelectrochemical processes are possible at a semiconductor/liquid interface,’ one must know the magnitude of the band-gap energy (Ebg), the relative energies of the band edges, and the location and density of states within the gap. E b g is usually measured by spectroscopic and/or electrochemical methods. The band edges are usually located energetically by determination of the flat-band potential (Vb),which can be estimated at single-crystal semiconductor electrodes by capacitance measurements (Mott-Schottky plots) or by the potential dependence of the photocurrent,2 or other technique^.^ For particulate semiconductors the band edge positions have been estimated by several methods;3a4 these basically involve either measuring the extent of reaction of a solution phase species as
a function of pH or direct collection of photogenerated charge at an electrode.3e The presence of intragap states is inferred from luminescence measurements, studies of the impedance (particularly the conductance) of the semiconductor/liquid i n t e r f a ~ e ,or ~~?~ conductivity A knowledge of these parameters for different forms of the semiconductor (single crystal, polycrystalline, thin film, powder, colloids) is helpful in understanding the behavior of these materials. For example, band bending, which is important for charge separation (and thus photoelectrochemical current generation) in semiconductor electrodes is not present in colloids because of their small dimension^,^^ and a different mechanism for charge separation in such particles has recently been postulated.5b Also, since the surface/volume ratio is much greater in particles than crystals, surface and near surface states might be expected to play a more important role in the former. One intensely studied system is CdS. Luminescence measurements of CdS in its various forms6 have determined that, for
(1) Wrighton, M. S. Pure Appl. Chem. 1985, 57, I , 57. (2) Noufi, R. N.; Kohl, P. A,; Bard, A. J. J. Electrochem. SOC.1978, 125, 375. (3) (a) Ellis, A. B.; Kaiser, S. W.; Bolts, J. M., Wrighton, M. S. J. Am. Chem. SOC.1977, 99, 9, 2839. (b) Ward, M. D., White, J. R., Bard, A. J. J. A m . Chem. Soc. 1983, 105, 27. (c) Dimitrijevic, N. M.; Savic, D.; Micic, 0.;Nozik, A. J. J . Phys. Chem. 1984.88, 4278. (d) Petit, J. P.; Alonso Vante, N.; Chartier, P. J . Electroanal. Chem. 1983, 145. (e) Dunn, W. W.; Aikawa, Y . ; Bard, A. J. J . Electrochem. SOC.1981, 128, I , 222.
(4) (a) Nagasubrarnanian, G.;Wheeler, B. L.; Hope, G. A,; Bard, A. J. J . Electrochem. Soc. 1983, 130, 385. (b) Nicollian, E. H.; Goetzberger, A. Bell. Syst. Tech. J. 1967, 46, 1055. (c) Wright, H. C.; Downey, R. J.; Canning. J. R. J . Phvs. D.1968. 1 (2) 1593. (d) Handelman. E. T.: Thomas. D. G . JyPhys. Chem. Solids 1965,*26, 1261.‘(e) Kulp, B. A,; Keliey, R . H: J . Appl. Phys. 1961, 32, 1290. ( 5 ) (a) Henglein, A. In “Photochemical Conversion and Storage of Solar Energy”, Rabani, J., Ed.; Weizmann Science Press of Israel: 1982; p 115. (b) Gerischer, H. J. Phys. Chem. 1984, 88, 6096.
0022-3654/85/2089-5676$01 S O / O
0 1985 American Chemical Society
