J. Phys. Chem. 1986, 90,3183-3187 (2) The monolayer dispersion on TiOl is less well developed than on Alz03,since Ti3+ sites remain detectable in the supported catalyst system. The reducibility of the molybdate is strongly enhanced on TiO,, a phenomenon which is probably related to the oxygen deficiency of this support material. Mo4+ and Mo3+ sites were detected by the C O probe on H2-reduced samples. (3) CeO, as a support shows a fairly complex behavior. However, again the oxygen deficiency seems to facilitate H, reduction of supported molybdate species, and Mo oxidation states less than or equal to +4 are clearly detectable by C O adsorption. (4)In contrast to the above three supports, monolayer formation does not occur on Z r 0 2 . As has been discussed earlier,!' the dispersion of the molybdate is strongly dependent on the pH of
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the impregnation solution relative to the isoelectric point of the support oxide. The present study supports these previous results and clearly demonstrates that the low-temperature infrared spectra of adsorbed C O provide qualitative information on the dispersion of the supported oxide component.
Acknowledgment. Finakial support of this work by the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie is gratefully acknowledged. M.I.Z. expresses his appreciation to the Alexander-von-Humboldt-Stiftung for a fellowship. Registry No. Moo3, 1313-27-5; AI20,, 1344-28-1; Ti02, 13463-67-7;
Ce02, 1306-38-3; Zr02, 1314-23-4; CO, 630-08-0.
Desorption of Zn from ZnO Single-Crystal Surfaces during Temperature Programmed Decomposition of Methanol, Formic Acid, and 2-Propanol K. Lui, M. Vest, P. Berlowitz, S. Akhter, and H. H. Kung* Chemical Engineering Department, and the Ipatieff Laboratory, Northwestern University, Evanston. Illinois 60201 (Received: October 23, 1985: In Final Form: February 25, 1986)
Desorption of Zn atoms at temperatures lower than the sublimation temperature of Zn from ZnO was observed during the temperature programmed decomposition (TPD) of methanol, formic acid, formaldehyde, and 2-propanol on a Zn-polar surface of ZnO. It was observed for formic acid, methanol, and formaldehyde on a stepped (5051) surface, 2-propanol and methanol on an 0-polar surface, but not for 2-propanol on a stepped (5051) surface. The areas of the Zn desorption peaks were usually less than 10%of the areas of the other products, but the decomposition of methanol on the Zn-polar surface was exceptionally efficient in causing Zn desorption. Zn desorption was also enhanced by adsorbed triethylamine and prolonged exposure to D2. It was suppressed by the presence of gaseous 02.It was not enhanced by adsorbed 02, CO, C02,H20, or NH,. The enhanced Zn desorption was attributed to reduction of the ZnO surfaces. The desorption temperatures of various products in this work were lower than those previously reported. These more correct temperatures were reported.
Introduction Sublimation is a common phenomenon for oxides of relatively high vapor pressure, such as MOO,, CdO, and ZnO. for these oxides, it is possible to transport them in the vapor phase from a hot region to a cold region, thereby effecting crystal growth by vapor transport. If the oxide is thermodynamically stable in its surrounding, the components sublime in a stoichiometric ratio. Sublimation of one component may be possible if the other components are removed by chemical reactions with other species. Such conditions may be found during the reduction of an oxide to metal ore, or when an oxide is used as a catalyst in a reducing atmosphere. In the latter case, sublimation would result in a long-term deactivation of the catalyst. Indeed, it has been reported, for example, that a "metallic silver mirror" was formed at the cold part of the reactor when ZnO was used in catalytic methanol decomposition. Stoichiometric sublimation of ZnO takes place at an appreciable rate above 600 "C under ultrahigh vacuum condition~.~JExp u r e to CO or H2at these temperatures increases the sublimation rate of Zn atoms on the polar and prismatic faces due to removal of lattice oxygen by these gases,4vs and Zn sublimation is detected at a lower temperature. Desorption of Zn atoms at temperatures substantially lower than 600 OC is also observed for Zn evaporated onto ZnO surfaces.6 At a submonolayer coverage, the adsorbed Zn desorbs in two peaks at about 300 and 450 OC from the Tawarah, K. M.; Hansen, R. S. J. Catal. 1984, 87, 305. Kohl, D.; Henzler, M.; Heiland, G. Surf.Sci. 1974, 41, 403. Gopel, W. J. Vac. Sci. Technol. 1978, 4 , 15. Grunze, M.; Hirschwald, W.; Krebs, S.2.Phys. Chem. (Frankfurt am Main) 1976, 102, 83. ( 5 ) Grunze, M.; Hirschwald, W.; Hofmann, D. J. Crystal Growth 1980,
52, 241. ( 6 ) Mokwa, W.; Kohl, D.; Heiland, G. Surf. Sci. 1980, 99, 202.
0022-3654/86/2090-31S3$01.50/0
Zn-polar face, and at about 350 and 550 OC from the 0-polar face. In our temperature programmed decomposition (TPD) of methanol and formic acid on ZnO we have noticed recently that Zn atoms desorb in well-defined peaks simultaneously with the other reaction products. Since this appears to be Zn desorption as a result of the interesting phenomenon of corrosive chemisorption, more detailed characterization of the phenomenon appears warranted. We report here the results of experiments which address the question of under what conditions does this Zn desorption take place. In these experiments, a heating method different from the one used in our previous work was used. This new method generated different desorption temperatures and led us to conclude that our previous reported temperatures were io0 high due to radiative heating of the thermocouple by the heating filament. We report here also the set of correct temperatures.
Experimental Section The apparatus and the experimental procedure have been described in detail previously?.*~" Methanol, 2-propanol, and formic acid were introduced to the surface via a doser, while the other gases were introduced as background gases. In the former case, the exposure was calculated from the flux of molecules through the doser which was determined by measuring the pressure drop in the feed line with a pressure transducer. From the flux and the position of the doser with respect to the surface, an effective (7) Cheng, W. H.; Akhter, S.; Kung, H. H. J . Catal. 1983, 82, 341. (8) Akhter, S.; Cheng, W. H.; Lui, K.; Kung, H. H. J. Catal. 1984, 85, 431. (9) Lui, K.; Akhter, S.; Kung, H. H. ACS Symp. Ser. 1985, 279, 205. (10) Akhter, S.; Lui, K.; Kung, H. H. J. Phys. Chem. 1985, 89, 1958. (11) Cheng, W. H.; Kung, H. H., Surf.Sci. 1982, 122, 21.
0 1986 American Chemical Society
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The Journal of Physical Chemistry, Vol. 90, No. 14, 1986
pressure at the surface was estimated. For example, a flux that torr of methanol resulted in a background pressure of 4 X torr effective pressure at the surface. corresponded to 3.3 X torr background pressure corresponded For 2-propanol, 5 X torr. Exposures for these to an effective pressure of 1.3 X gases were estimated from the effective pressures. TPD was performed at 10 K s-l. The Zn desorption peaks were monitored at m / e 64. The assignment of this mass to Zn was confirmed by the isotope distribution of Zn at m / e 66 and 68. The same ZnO single crystals as used previously were used. They were purchased from Atomergic Chemicals. Clean and ordered surfaces were prepared as before.'-" There were two modifications in the experimental procedure from previous work. First, the crystals were heated by contacting the back side of the ZnO crystals with a tantalum foil which was heated directly by electric current. Previously, the crystals were heated radiatively by a tungsten filament. Second, the temperature was measured by a thermocouple inserted into a hole mechanically drilled on the side of the crystal. Previously, the thermocouple was pressed onto the surface of the crystal by its own tension. All features of the TPD results such as the nature of the desorption products, the relative peak sizes, and the sequence of the peaks are identical for the two procedures with one exception, which is the desorption peak temperatures. When heating was achieved by radiation, the peak temperature for a given product was higher than when heating was achieved by contacting the tantalum foil. For example, we previously reported that the TPD peaks in methanol decomposition on the Zn-polar surface were at 400 and 470 O C 8 With the new heating method, the peak temperatures were 250 and 320 O C , respectively. In order to determine the cause for the discrepancy, two sets of experiments were performed. The first set was to determine how sensitive was the dependence of the temperature reading on the contact between the thermocouple and the surface. A thermocouple was spot-welded onto the side of a Pt single crystal and another thermocouple was pressed onto the crystal surface by its own tension. The crystal was heated at 10 K s-l by the new heating method and the readings from both thermocouples were recorded. The difference between the two thermocouple readings depended on how the thermocouple was pressed onto the surface. If only the thermocouple head was touching the surface, the difference increased linearly from no difference at room temperature to about 100 OC at 600 "C. But if the thermocouple head and the wire adjacent to the head were touching the surface (which had been our procedure), the difference was smaller, increasing from no difference at room temperature to about 20 "C at 600 OC. Presumably, the temperature gradient at the head was smaller in the second method. These results showed that a good contact between the thermocouple and the surface should essentially give the correct surface temperature. Therefore in the case of ZnO, it was concluded that the different methods of mounting the thermocouple were not the cause of the observed temperature differences. The second set of experiments was to determine the effect of different heating methods. A ZnO sample was heated radiatively with a tungsten filament and the temperature was measured by the thermocouple inserted into the hole on the side of the crystal. Temperature programmed decomposition of formic acid was used as a probe reaction. With this heating method, formic acid decomposition peaks were at 410 'C. Then the crystal was wrapped by a tantalum foil at the back so that the crystal was heated by thermally contacting the tantalum foil, which in turn was heated radiatively by the tungsten filament. In this case, the decomposition peaks were at 310 OC. These experiments showed that radiation emitted by the tungsten filament could heat the thermocouple to a temperature higher than the surface temperature. Therefore, we believe that the temperatures determined with the new heating method are correct. Finally, we compared the TPD peak temperatures in methanol decomposition when the thermocouple was pressed onto the surface by tension with those when the thermocouple was inserted into the hole in the ZnO crystal. Within experimental error (110 "C), the two sets of temperatures were the same. Therefore, the
Lui et al. TABLE I: TPD Peak Temperatures from Various ZnO Surfaces at Saturation Coverage peak temp. OC desorptn (ioio) (OOOi) and (5051) (0001) adsorbate Prod CH30H oxidation" 200 f 10 280 f 10 320 f 10 dehydrogenationb C C 2 5 0 i 10 CH,OH 8 0 f 10 9 0 f 10 1 0 0 f 10 2-propanol acetone, propene 120 f 10 180 f 10 180 f 10 2-propanol 100 f 10 120 f 10 125 f 10 HCOOH CO, C02, H, d 280 f 10 325 f 10 HCHO oxidation" d 260 f 10 325 f 10 HCHO d 9 0 f 10 1 5 0 f 10 "Oxidation products are CO, CO,, and H2. bDehydrogenation products are CO, H2, and HCHO. 'No dehydrogenation products. d N o t determined.
TABLE 11: The Peak Temperatures and Peak Area Ratios for TPD from Various ZnO Surfaces desorptn adsorbate prod 2-propanol acetone
+ C02
HCOOD
CO
CH,OH
oxidn prod
surface (0001) (5051)
(oooi)
(0001) (5051) (0001)
(OOOT) dehydrogn prod
(0001)
desorptn Zn zn/ prod desorptn desorptnO temp, OC temp, O C prod 180 250 0.02 0 180 none 0.01 120 260 0.05b 320 320 0.06b 280 310 0.04' 320 320 0.13' 200 260 250 d 250
"For saturation coverages: the peak area of Zn is the sum of the areas for m / e 64, 66, and 68, and the peak area of the products is the sum of the areas of the parent peak and cracking fragment peaks. The exposure for saturation coverage is 4.9 i 4 L for methanol, 4.6 f 4 L for 2-propanol. bZn/(CO + C02). C Z n / ( C O CO,), where C O and CO, are oxidation products. dSee Table 111 for these ratios.
+
temperature gradient across the crystal was small and the temperature measured at the crystal interior is a good representation of the surface temperature. Using the new procedure, we repeated the temperature programmed desorption and decomposition of methanol, formaldehyde, formic acid, and 2-propanol. The new and more accurate temperatures are summarized in Table I. It should be noted that TPD of methanol has also been studied on polycrystalline ZnO samples. The activation energy for the decomposition pathway via surface formate was reported to be 38 kcal/mol by Bowker et al. on ZnO powder,12and 37 kcal/mol by Chan and Griffin on ZnO film.13 These were calculated by using Redhead's forrnulal4 and a preexponential factor of 1 X For our data, the activation energy was calculated to be 36 kcal/mol, which compared well with the reported values. Results and Discussion I. Zn Desorption from a Zn-Polar Surface during Methanol, 2-Propano1, and Formic Acid Decomposition. Zn desorption was observed in the TPD of adsorbed 2-propanol, formic acid, and methanol. Desorption from a surface exposed to 4 L of 2-propanol showed a desorption peak for 2-propanol at 130 "C, for acetone at 180 OC, for H, at 180 "C, and for propene at 180 "C. Except for the temperatures, these products were the same as those reported earlier.g A small Zn peak was observed at 250 OC. The peak area ratio of Zn atom to acetone is given in Table 11. Zn atoms were evolved during the decomposition of HCOOD. Similar to earlier reports,8J0 reaction-limited decomposition product peaks of CO, COz,and H, were desorbed at 320 O C . A small Zn peak also appeared at this temperature. A desorption (12) Bowker, W.; Houghton, H.; Waugh, K. C. J . Chem. SOC., Faraday 3023. (13) Chan, L.; Griffin, G. L. Surf. Sci. 1985, 155, 400. (14) Redhead, P. A. Vacuum 1962, 12, 203.
Trans, I , 1981, ,,
The Journal of Physical Chemistry, Vol. 90, No. 14, 1986 3185
Desorption of Zn from ZnO Single Crystals
TABLE 111: Peak Area Ratios as a Function of Methanol Coverage on the Zn-Polar Surface re1 MeOH en/ (Znl Zn(250 "C)/ coverage" CH2O + C0)250 OC co + co2)320OC Zn(320 "C) 5.8 f 1.4 1lO 0.36 f 0.07 0.041 f 0.010 1.o 1.o 0.79 0.57 0.31 0.24
0.39 f 0.08 0.42 f 0.09 0.48 f 0.14 0.53 f 0.19 0.49 & 0.20 0.53 f 0.22
0.034 0.038 0.030 0.042 0.036 0.039
f 0.010
f 0.010 f 0.009
f 0.012 f 0.012 f 0.013
f 2.0 f 1.7 f 2.0 f 1.2 3.8 f 1.4 2.9 f 1.2
6.9 6.7 6.9 4.0
Zn(250 "C)/ (CO + C02)320 'C
Zn(250 "C)/ COz(320 "C)
0.24 f 0.05 0.23 f 0.05 0.26 f 0.05 0.21 f 0.04 0.17 f 0.05 0.14 f 0.04 0.11 f 0.03
0.50 f 0.10 f 0.10
0.52 0.57 0.47 0.40 0.30 0.25
f 0.11
f 0.10 f 0.09 f 0.07
f 0.06
Fraction of saturation coverage.
n
C 1601e0/4
E
'1
0
I
U
1
7
l :
a
7
a
a
H
0
v)
5
2
/
I 0
I
100
200
TEMP
I
I
I
300
400
500
0
1 0 0 2 0 0 3 0 0 4 0 0 ~
C
Figure 1. TPD spectrum of CH30H decomposition on a Zn-polar sur-
Znx2
1 TEMP
'c
Figure 2. TPD spectrum of CH3I80Hdecomposition on a Zn-polar
face.
surface.
limited H 2 0 peak was desorbed at 340 "C. In addition to the mass numbers corresponding to Zn and the cracking fragments of the decomposition products, the following masses were also monitored with a multiplier setting an order of magnitude higher A): m / e 80 (corresponded to ZnO), than m / e 64 (which was 92 (ZnCO), 108 (ZnCO,), 109 (ZnHCOO), and 154 (Zn (HCOO),). No detectable peaks were observed. Between 40% to full coverage of formic acid, the Zn/CO, Zn/C02, and Zn/(CO CO,) ratios remained constant within experimental uncertainties. The last ratio is reported in Table 11. Zn desorption in methanol decomposition was studied in considerable detail because the largest amount of Zn was evolved. As reported before:,8,'0*'' the decomposition products of methanol were evolved in two sets (see Figure 1). The lower temperature set consisted of CO, H,, and CH20. The higher temperature set consisted of CO, CO,, and H,, with H 2 0 desorbing at an even higher temperature. The higher temperature set of products evolved at the same temperature and had the same product distribution as those from formic acid decomposition. It was postulated that they were from the oxidation of surface methoxy to surface formate, which then decomposed.8 This reaction pathway was called the oxidation pathway. The lower temperature set of products was observed for methanol only on the Zn-polar surface. The products were dehydrogenation products of methanol. The reaction pathway was called the dehydrogenation pathway, and it was postulated to take place on the reduced portion of the surface.8 The fraction of decomposition that proceeded via the dehydrogenation pathway was small at low coverage (
