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(24) S. Sharafy and K. A. Muszkat, J. Am. Chem. SOC.,93, 41 19 (1971). (25) L. A. Brey, G. B. Schuster, and H. G. Drickamer, J. Am. Chem. Soc., 101, 129 (1979). (26) H. W n e r and D. Schuhe-Frohlinde, J. phys. Chem., 82, 2653 (1978). (27) (a) D. V. Bent and D. Schulte-Frohlinde,J . Phys. Chem., 78, 446 (1974); (b) ibid., 78, 451 (1974); (c) D. Schulte-Frohlinde and H. Gorner, Pure Appl. Chem., 51, 279 (1979). (28) H. Gijrner and D. Schulte-Frohlinde, Ber. Bunsenges. Phys. Chem., 81, 713 (1977). (29) H. Gorner and D. Schulte-Frohlinde, Ber. Bunsenges. Phys. Chem., 82, 1102 (1978). (30) G. Heinrich, S. Schoof, and H. Gusten, J . Photochem., 3, 315 (1974- 1975). (31) M. N. Pisanias and D. Schulte-Frohllnde, Ber. Bunsenges. Phys. Chem., 79, 662 (1975). (32) A. R. Holzwarth, H. Lehner, S. E. Braslavsky, and K. Schaffner, Justus Liebigs Ann. Chem., 2002 (1978). (33) H. J. Kuhn, R. Straatmann, and D. Schuite-Frohiinde, J. Chem. SOC., Chem. Commun., 824 (1976). (34) H. Jungmann, H. Gusten, and D. SchuRe-Frohlinde, Chem. Ber., 101, 2690 (1968). (35) H. Gusten and L. Klasinc, Tetrahedron Left., 3097 (1968). (36) (a) G. Fischer and E. Fischer, Mol. Photochem., 8, 279 (1977); (b) 0. A. v. Salis and H. Labhart, J . Phys. Chem., 72, 752 (1968); (c)
Bulko et
al.
A. C. Ling and J. E. Willard, bid., 72, 3349 (1968). (37) M. Sumitani, N. Nakashima, K. Yoshihara, and S. Nagakura, Chem. Phys. Lett., 51, 183 (1977). (38) (a) 0. Teschke, E. P. Ippen, and G. R. Holtom, Chem. Phys. Left., 52, 233 (1977); (b) F. Heisel, J. A. Miehe, and B. Sipp, ibid., 81, 115 (1979); (c) B. I. Greene, R. M. Hochstrasser, and R. B. Weisman, ibid., 62, 427 (1979); (d) B. I. Greene, R. M. Hochstrasser, and R. B. Weisman, J . Chem. Phys., 71, 544 (1979); (e) K. Yoshihara, A. Namiki, M. Sumitani, and N. Nakashima, ibid., 71, 2892 (1979). (39) D. J. S. Birch and J. B. Birks, Chem. Phys. Lett., 38, 432 (1976). (40) See, e& T. Bercovici, R. Korenstein, K. A. Muszkat, and E. Fischer, Pure Appl. Chem., 24, 531 (1970). (41) J. 8. Birks, Photochem. Photobiol., 24, 287 (1976). (42) E. Lippert, Z. Phys. Chem. (Frankfurt am Main), 42, 125 (1964). (43) K. Kruger and E. Lippert, 2.Phys. Chem. (Frankfurt am Main), 66, 293 (1969). (44) N. J. Turro, V. Ramamurthy, W. Cherry, and F. Farneth, Chem. Rev., 78, 125 (1978). (45) A. Henne, N. P. Y. Siew, and K. Schaffner, J . Am. Chem. SOC., 101, 3671 (1979). (46) D. Schulte-Frohlinde and H. Blume, Z. Phys. Chem. (Frankfurt am Main), 59, 282 (1968). (47) G. D. Gillispie and E. C. Lim, J . Chem. Phys., 65, 2022 (1976).
Optical Properties and Electronic Interactions of Microcrystalline Cu/ZnO Catalysts John 6. Bulko, Richard G. Herman," Kamll Kller," and Gary W. Simmons Center for Surface and Coatings Research, Sinclair Laboratory, Lehigh University, Bethlehem, Pennsylvania 180 15 (Received June 1 I , 1979) Publication costs assisted by the Department of Energy
Diffuse reflectance spectra of the microcrystalline Cu/ZnO solids provide evidence for interactions between an X-ray amorphous or solute form of copper and the ZnO wurtzite crystals. These interactions result in the emergence of a new absorption band at 17 500 cm-' and the simultaneous spectral disappearance of the 25 800-cm-' fundamental absorption edge of zinc oxide. This behavior is most pronounced when the copper concentration in the zinc oxide phase reaches the saturation value of 17 h 1%. The copper solution in the zinc oxide is a highly degenerate semiconductor with a strong light absorption intensity in the visible region of the spectrum. A comparison with earlier observations of methanol synthesis activity of the Cu/ZnO catalysts shows that specimens containing a maximum amount of copper dissolved in the zinc oxide are also the best catalysts for methanol synthesis.
Introduction Chemical and physical interactions in the microcrystalline binary copper-zinc oxide catalysts were recently reported to give rise to surface chemical properties such as selective activation of carbon monoxide for methanol synthesisal Diffraction studies demonstrated that both the copper and the zinc oxide had their ordinary crystal structures but analyses in the scanning transmission electron microscope (STEM) revealed that considerable amounts of copper were present in particles that showed only a single diffraction pattern of the wurtzite Zn0.2 Preliminary optical measurements indicated a very high absorption in the visible region that occurred in the binary catalysts but could not be obtained by a superposition of the absorption spectra of the pure components. It was proposed that the high optical absorption was due to the copper found in the zinc oxide phase and that, as a result of preparation history, the copper was dissolved in the zinc oxide as a formally monovalent ion. This Cu+/ZnO solution was then suggested to be the carrier of the catalytic activity of the system. The purpose of the present investigation was to establish the relation between the optical parameters, such as the 0022-3654/79/2083-3118$01 .OO/O
occurrence of the high intensity visible absorption and the loss of the fundamental absorption edge of the zinc oxide, and the existence of the copper-zinc oxide solid solution. To accomplish this, the crystallinity of the components of the Cu/ZnO catalysts was quantitatively determined by X-ray diffraction in a wide range of compositions, and the amount of noncrystalline copper assayed by X-ray diffraction was compared with that found dissolved in the zinc oxide by the earlier STEM study2 and was correlated to the present optical parameters. The results reported herein further demonstrate that those catalysts in which the amount of copper dissolved in the zinc oxide is at a maximum are also the most active for methanol synthesis. Experimental Section Materials and Specimen Preparation. The binary CuO/ZnO (0/100,2/98,5/95,10/90,15/85,20/80,30/70, 40/60,50/50,67/33,80/20, and 100/0 wt 5%) specimens were prepared by coprecipitation of the hydroxycarbonates and hydroxynitrates, thermal decomposition to CuO/ZnO, and reduction at 250 "C in flowing 2% H2-98% N2.' The structure and morphologies of the resulting Cu/ZnO catalysts were identical with those reported previouslya2The 0 1979 American Chemical Society
The Journal of Physical Chemistry, Vol. 83, No. 24, 1979 3119
Microcrystalline Cu/ZnO Catalysts 30
specimens, small errors and noise in R, give rise to larger errors in a,/a,,. All spectra presented here were, therefore, multiply scanned and averaged prior to transformation by eq 1. The difference spectra were calculated by the equation
T -
4
i
0
40
20 $
?no
60
80
100
i n the Mixture
Figure 1. The intensity (a,/a,) expressed as the absorption function obtained for the mechanical mixtures of the Cu/ZnO = 30170 catalyst with pure ZnO (0)and as the height (W) of the absorption edge of ZnO determined by eq 2 vs. the percentage of pure ZnO in the mixture. (aJu,),,
reductions of the CuO/ZnO samples were carried out in a glass reaction chamber with an attached glass side arm that terminated in a reflectance cell with an Infrasil quartz window. After reduction, the specimens were transferred to the reflectance cell and optical spectra were obtained in situ without exposure of the specimens to the atmosphere. Calibration standards for X-ray diffraction analyses and for optical spectroscopy were prepared by mechanically mixing pure copper oxide and zinc oxide powders and then subjecting the mixtures to the standard reduction treatment. Optical Spectroscopy. Diffuse reflectance spectra were taken in the range of wavelengths from 200 to 2400 nm with a Cary 14R spectrometer equipped with a diffuse reflectance accessory. The measured reflectances, R,, from the semiinfinite specimen relative to a powdered magnesium oxide standard were digitally converted to the Schuster-Kubeka-Munk (SMK) function? This function, F(R,) = (1 - RJ2/(2R,), was corrected by the relationship
where a,/u, is the ratio of the true absorption (a,)and the scattering (u,) coefficients, and q / x is a numerical factor correcting the SKM function in an intensely absorbing isotropically scattering media: The a,/u, ratio was found to be proportional to the amount of absorbing substance, as indicated by the linear relationships between F(R,)(q/x) and the composition of mechanical mixtures of Cu/ZnO = 30/70 and pure zinc oxide (Figure 1). The intensity of the ZnO absorption edge was also found to be proportional to the amount of ZnO present in this mechanical mixture as shown in Figure 1. The intensity of the ZnO absorption was obtained by the following equation:
where the subscripts 350 and 450 are the wavelengths (in nm) of the top and bottom of the absorption edge (28560 and 22 220 cm-l, respectively). The linear relationships in Figure 1 show that the scattering coefficient aVwas independent of the composition. This is expected when a single component such as ZnO dominates the scattering. Thus,eq 1 represents a conversion of the observed diffuse reflectance (R,) into a quantity a,/u, that is proportional to the true energy absorption. In very highly absorbing
where the subscripts A, Cu/ZnO, Cu, and ZnO indicate the difference spectrum, and the spectra of the composite, pure copper, and pure zinc oxide, respectively. In this expression, ncu and nznOare the molar fractions of Cu and ZnO in the catalyst. Over the 12 000-24 OOO-cm-l region of the spectrum the ZnO contribution, nzno(ay/u,)zn0, was zero. For catalyst compositions in the 0/100-15/85 range, the copper contribution was negligible and therefore ncu was set equal to zero. However, ncuwas set equal to the analytical content for samples having higher copper concentrations. X-ray Diffraction Analyses. X-ray powder patterns were obtained with a Siemens diffractometer with Cu K a radiation. To prevent oxidation, the specimens were transferred in a nitrogen-filled glove bag to aluminum sample holders, which were then sealed with Mylar film. Quantitative amounts of crystalline copper and of crystalline zinc oxide in the Cu/ZnO catalysts relative to the Cu/ZnO mechanically mixed standards with the same nominal concentrations were determined from the integrated intensities of the (111) reflection of copper and of the (101) reflection of zinc oxide. Therefore, a direct comparison of the crystalline components can be given by the ratios ( ~ C / ~ M M ) C y ( l land I ) ( ~ C / ~ M M ) ~ 0 ( * 0 1where ), IC and h M are the intensities in the coprecipitated samples and the mechanical mixtures, respectively. The particle sizes of the crystallites were determined from the broadening of the diffraction lines by using the simplified Scherrer relationq5
Results Spectra of pure copper and zinc oxide and selected spectra of reduced Cu/ZnO catalysts are shown in Figure 2. Spectra of the pure components show the characteristic copper metal d hump located at approximately 18 OOO cm-I and the ZnO absorption edge a t 25800 cm-’. The most pronounced interactions in the reduced composites are seen in the spectrum of Cu/ZnO = 30/70, which has far more intense absorption in the visible region than would correspond to a 30% copper and 70% zinc oxide mixture; a t the same time, the ZnO absorption edge is entirely missing, despite the presence of 70% of crystallographically identifiable zinc oxide in the sample. To pursue further the interactions indicated by the latter spectrum, we calculated the optical absorption difference spectra by using eq 3. Selected difference bands so obtained are shown in Figure 3. The maximum intensities of the difference bands for all of the Cu/ZnO catalysts measured are plotted as a function of their nominal copper concentration in Figure 4. A corresponding plot of the intensity of the ZnO absorption band edge as a function of the nominal copper concentration of the catalysts is presented in Figure 5. The ZnO absorption edge increased in intensity upon slow oxidation of the Cu/ZnO composites in air. This effect was particularly noticeable for the Cu/ ZnO = 30/70 sample, for which the value of ( a , / ~ , ) ~ ~ ~ increased from 0 to 0.36 after exposure to air for 7 days. To determine how the intensities of the difference bands and the attenuation of the zinc oxide absorption edge were connected with the crystallinity of the catalysts, quantita-
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The Journal of Physical Chemistry, Val. 83, No. 24, 1979
Bulk0
et 81.
~1 0
2.0
0' 20
.-
J
%--L.
1
...,.
.
20
0
1
L~
u
~ ~ i - ~ ~ - - -
Irn
E!
conttni
copper
5
~
M
40
Flgure 4. The maximum intensities of the difference bands ( a J u J A as a function of copper content in the CuIZnO composites.
, .
'\-
Flgure 2. The absorption spectra of pure copper (A), pure zinc oxide (E), and the CuIZnO catalysts having compositions of 5/95 IC), 30170 (D), and 67/33 wt % (E). The absorption lunction a,/ouwas obtained from the diffuse reflectance R , value according to eq 1 2.0
-..-.
--T--_I~Ii
0
M
40
20
lm
80
C a Q Q l r LontPnl
5
Figure 5. The Intensities of the zinc oxide absorption edge obtained for the reduced wprecipitated catalysts by utilizing eq 2. The dashed line represents the curve expected if the ZnO were simply being diluted by metallic copper having the same particle size.
i o
0 8
-u. -
0 6
-
Flgure The optical absorption difference bands, obtained by u 3 eq 3.01 reduced CulZnO = 5/95 (A), 10/90 (B), 30170 IC), and 67/33 (D) samples.
0 4
"'L 0. 0
0
LO
20
10
%
tive analyses hy X-ray powder diffraction were carried out. The results of these analyses are shown in Figure 6. The amount of crystalline ZnO in the catalysts and the corresponding mechanical mixtures is shown to he the same for all nominal compositions. In contrast, the amount of crystalline copper is observed to he less in each catalyst sample than in the corresponding reduced mechanical mixture. The shaded area represents the fraction of copper that is X-ray amorphous in the reduced coprecipitated catalysts. This quantity was used to calculate the copper concentration in the zinc oxide crystallites, and the results are presented in Table I The average particle sizes of copper, its CuO precursor, and the zinc oxide, as determined from the X-ray diffrac-
'
I
'
'
'
'
40
50
60
70
BO
PO
100
C o p Q e i contan,
Flgure 6. Comparisons of the crystalline Cuo (0)and ZnO ( 0 )components in the reduced coprecipitated catalysts, I,, with those in the reduced mechanically mixed composites. 1,- as determined by X-ray powder diffraction, where I is the integrated intensity.
tion line broadening, are represented in Figure 7. In the range where X-ray line broadening analysis is applicable, these particle sizes agree very well with those found earlier by electron microscopy.
Discussion The Cu/ZnO catalysts display, to a degree determined by the copper-to-zinc oxide ratio, several features that are
The Journal of Physical Chemistry, Val. 83,No. 24, 1979 3121
Mlcrocrystailine Cu/ZnO Catalysts
TABLE I: Extent of Cu/ZnO Solid Solution Formation in the Reduced Coprecipitated Catalyst, as Determined bv X-rav Powder Diffraction and Electron MicroscoDv nominal catalyst compositiona
% Cu' in solid solution -
X-ray diffractionb
2/98 5/95 lOi90
2.0 2.5 4.8
15/85
8.1 10.5
20180 30170 40160 50150 67/33 80120
16.8 k 12.6k 13.0k 15.4 f 14.2
electron microscopy"
1.5
15.1 f 3.7
1.5
5.0 f 3.7 8.7 f 3.6 11.6 f 2.8
2.0 3.0
Expressed as amorphous copper/ a CuO/ZnO in wt %. (ZnO t amorphous Cu). From ref 2.
0
10
20
30 I Copper
40
50
60
70
Content
Flgure 7. The average particle slzes of copper (O),its CuO precursor (0), and zinc oxide (A)determlned from X-ray diffraction line broadening.
solely due to interactions between the two components: (i) the appearance of the new intense absorption band peaked around 17 500 cm-' and extending over the whole visible region into the near infrared; (ii) the loss of intensity of the ZnO absorption edge a t 25800 cm-'; and (iii) the presence of noncrystalline copper in all composites. Since the amount of noncrystalline copper found here by X-ray analysis agrees fairly well with the amount of copper in the zinc oxide phase determined earlier by microanalysis in the scanning transmission electron microscope (STEM): all copper is now accounted for by the two distinct forms, i.e., the small microcrystallites of the metal and the dispersion in the ZnO crystallites. The amount of the ZnOdispersed copper correlates with, and therefore appears to be the cause of, the intensity of the 17 500-cm-' band. Simultaneously the zinc oxide absorption edge is reduced (Figure 5), suggesting that the electronic energy states introduced by the copper dispersion merge with the valence band edge or conduction band edge of the zinc oxide. Such a change of the electronic spectrum of the host ZnO provides evidence for an interaction of the dispersed copper with the ZnO lattice on the atomic scale and is therefore also evidence that the copper is dissolved in the zinc oxide. The quantitative aspects of the relations between the concentrations of the dissolved copper and the electronic spectra reported here must be discussed separately for the two regions of concentrations, Cu/ZnO = 0/100 to 30/70,
in which the particles of copper and zinc oxide are small (5-20 nm), and Cu/ZnO = 40/60 to 100/0, in which the particles are large (>30 nm). In the first region, the 17 500-cm-l spectral band intensified as the copper content increased (Figure 4), although not linearly except a t low copper concentrations. Difference band spectra that were calculated by assuming an average of 12% Cu/ZnO solid solution resulted in a maximum increase in the intensity of the difference spectra of 7% (0% difference for the low copper containing samples) with no shift in band position nor sharpening of the band edge. The X-ray diffraction analyses demonstrated that the zinc oxide accommodated increased amounts of the copper solute to a maximum value of 17% Cu in the zinc oxide phase (15% by earlier STEM analyses) for the Cu/ZnO = 30/70 composite (Table I). Simultaneously, the ZnO absorption edge drastically decreased in intensity, as shown in Figure 5 where the dashed line represents the expecled curve. Whether the 17 % copper in the zinc oxide is a saturated or a supersaturated solid solution is uncertain but it is striking that the ZnO crystals can retain their structural integrity with such a large amount of dissolved copper. The copper is not in its divalent state since the absorption spectra of the Cu2+/Zn0 solid solutions are different from the spectra observed here.g In the second range of compositions, Cu/ZnO = 40/60 to 80/20, the intensity of the 17 500-cm-' band is proportional to the amount of zinc oxide in the composite (Figure 4). The ZnO particles are large and the X-ray diffraction results in Table I show that 13-15% copper is dissolved in the zinc oxide phase. Thus, in this range the system establishes an average concentration of copper in the zinc oxide that depends very little on the total amount of copper. The part of copper that is not dissolved in ZnO segregates as large metal crystallites, many of which are not even in contact with the ZnO. It is apparent that the zinc oxide absorption edge is not reduced as strongly in the range of compositions Cu/ZnO = 40/60 to 80/20 as it is in the 30/70 composite (Figure 5), which is in qualitative agreement with the relative ratios of intensities of the 17 500-cm-' band and with the amount of copper dissolved in ZnO. A comparison of X-ray analyses of the CuO/ZnO composites prior to reduction,' of STEM analyses of the ZnO phase in CuO/ZnO composites prior to and after reduction,2 and of the presently reported X-ray analyses of the reduced Cu/ZnO composites permits the conclusion that the copper was driven into the zinc oxide during the reduction process. In the partially ionic lattice of zinc oxide, copper is unlikely to dissolve as neutral atoms and therefore, quite formally, the dissolved copper is assigned the valence state of +1, in agreement with the proposal put forward earlier.' The copper species are visualized not as isolated Cu+ ions, such as the ones planted by ion exchange in zeolites,' but rather as electron deficient copper atoms with strong electronic overlap with the host zinc oxide lattice, particularly with neighboring oxygens. Since Cu+ is isoelectronic with Z i P and,like Zn2+,prefers tetrahedral coordination, a zinc lattice site is a likely location for the Cu' species. The charge deficiency of such a site may be compensated by interstitial Cu+ species or oxygen vacancies. Further work will have t~ show how the Cu+ species and oxygen vacancies are distributed. The oxidation of the Cu/ZnO = 30/70 catalyst, which slowly developed the ZnO absorption edge, can be interpreted as a removal of the Cu+ solute and oxygen vacancies through ionosorption of oxygen with the generation of Cu2+,setting up a space charge bilayer, followed by the diffusion of the copper ions
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The Journal of Physical Chemistty, Vol. 83, No. 24, 1979
and oxygen vacancies to the surface. As a result, the bulk Cu'/va~&cy defects, which generate the described energy band overlapping with the ZnO band edge, are neutralized and the band structure of pure zinc oxide gradually reappears.
Acknowledgment. Partial support for this research was provided by Department of Energy Grants AER-7503776 and ET-78-S-01-3177. One of the authors (J.B.B.) acknowledges the receipt of an NSF Fellowship.
Pichat et at.
References and Notes (1) Herman, R. 0.; Klier, K.; Simmons, G. W.; Finn, B. P.; Bulko, J. 6.; Kobylinski, T. P. J . Catal. 1979, 56, 407. (2) Mehta, S.; Simmons, G. W.; Klier, K.; Herman, R. G. J. Cafai. 1979, 57. 339. Klier, K. Catal. Rev. 1967, 7 , 207. Klier, K. J . Opt. SOC.Am. 1972, 62, 882. Innes, W. B. In "Experimental Methods in Catalytic Research", R. B. Anderson, Ed.; Academic Press: New York, 1968; p 84. (6) Chapple, F. H.; Stone, F. S.Proc. Brit. Ceram. SOC.1964, 7 , 45. (7) Texter, J.; Strome, D. H.; Herman, R. G.; Klier, K. J . Phys. Chem. 1977, 87, 333.
Photocatalytic Oxidation of Propene over Various Oxides at 320 K. Selectivity Pierre Pichat, Jean-Marie Herrmann, Jean Disdler, and Marie-Noeile Mozzanega Institut de Recherches sur la Cataiyse, C.N.R.S., 69626 Vileur6anne Cgdex, France (Received Ju/y 9, 1979) Publication costs assisted by Centre National de la Recherche Scienfifique
Propene oxidation has been studied at 320 K over the following series of UV irradiated oxides: Ti02,Zr02, V205, ZnO, Sn02, Sb2O4, Ce02, WOS, and a Sn-0-Sb mixed oxide. None of these solids was active at this temperature in the absence of UV irradiation corresponding to an energy equal to or greater than its band gap. A rise in temperature had a negative effect on the photocatalytic activity. The quantum yields varied widely with the metal oxides; however, only the V205sample used was found photoinactive. The selectivity greatly depended on the catalyst. As for thermal catalysis, the selectivity to mild oxidation products increased with decreasing conversion (modified by varying light intensity). For low and equivalent conversion levels, total oxidation predominated for Ce02and TiOz,and, to a lesser extent, for Zr02and ZnO, whereas partial oxidation products only were obtained over SnOz, W03, and the Sn-0-Sb mixed oxide, and almost solely over Sb204. In addition to water, C 0 2 (but no CO), ethanal, acrolein, acetone and, in some cases, propene oxide and traces of propanal, were produced. Ethanal was generally formed preferentially to acrolein. For a given catalyst, the product distribution was influenced by the activation mode (UV light or increase in temperature). The results, which emphasize the important role of the catalyst sample in determining the selectivity even for the same activation mode of oxygen at near room temperature, are briefly discussed.
Introduction Although the photocatalytic properties of n-type semiconductors in oxidation reactions are well known, most studies dealt with TiOl or ZnO, and, to our knowledge, the influence of the photocatalyst on the selectivity has not been thoroughly investigated. It has only been briefly mentioned that Ti02,Zr02,Sn02,and W03 for isobutane oxidation1 and the same oxides plus ZnO and MooBfor ammonia oxidation2 exhibited photocatalytic activities which varied over a large range, whereas the reaction products remained the same. On the other hand, it has been reported that oxygen isotopic exchange around room temperature over UV-irradiated Ti02,3Zr02, ZnO, and Sn024proceeded only via the same mechanism which involved a surface dissociated oxygen species, whereas, generally, several mechanisms occur simultaneously in the case of thermally activated isotopic exchange. Propene oxidation has often been chosen to study the selectivity of mild oxidation catalysts. Furthermore, although alkenes have been considered as probable intermediates in the photocatalytic oxidation of alkanes and of aliphatic secondary and tertiary alcohol^,^ articles on their own photocatalytic oxidation are not very numerous. Ti02617and ZnO' have been reported to photocatalyze complete oxidation of propene; however, additional formation of methanal has also been pointed out for C3H6and C2H4 oxidations over UV-irradiated Ti02.8 Traces of COZ
were detected upon illumination of Sn02,whereas MgO, V2O5, Cu20,Y2O3, Zr02, Moo3, Sb2O3, GdzO3, Hf02,W03, and PbO were found photoinactive for propene and isobutene ~ x i d a t i o n .Total ~ oxidation of the two n-butenes was achieved over T i 0 2 as a photocatalyst.6 Isobutene yielded mainly acetone when the catalyst was Ti02,UV irradiated either at room t e m p e r a t ~ r eor l ~in~ the ~ ~ 333-423 K temperature ra~~ge,~JO while methylacrolein was formed over ZnO, UV irradiated between 373 and 543 Ke7Finally, very recently,l' the photocatalytic oxidation of the 3methylbutenes over T i 0 2 at room temperature has been interpreted in terms of molecular configurations. The present article reports a more precise analysis of the products of propene oxidation over a series of oxides (Ti02,Zr02, V205,ZnO, Sn02,Sb204,CeOz, W03, and a Sn-0-Sb mixed oxide) UV irradiated a t 320 K, in order to emphasize the role of the catalyst in the selectivity. Experimental Section The catalysts, whose specific area and origin are included in Table I, were irradiated by a Philips SP 500-W mercury lamp through a water containing cell and an optical filter (300-400-nm transmission) so that the 365-nm (3.4-eV) Hg line was essentially used. In order to modify the conversion level of propene, the light intensity was weakened by using various calibrated metallic grids, which did not change the energy distribution of the photons nor the geometry of the
0022-3654/79/2083-3122$01.00/00 1979 American Chemical Society
