J . Phys. Chem. 1990, 94, 5022-5028
5022
measured complexation equilibrium constants by a variety of techniques and a certain consistency of results emerges. Certain small molecules (such as ethanol and acetonitrile have significant constants while others like dimethyl sulfoxide have relatively small constant^.^ Cyclohexanol and phenol were noteworthy for their particularly large constants. Coinclusion phenomena have been noted not only with cyclodextrins but also with Noteworthy in several studies of cyclodextrin complexes is the sensitivity of the structure of the complex as a function of the structure of the cyclodextrin and the included molecules.6,1’,25 (20) Veno, A,; Takahashi, K.; Osa, T J . Chem. Soc., Chem Commun. 1980, 1921-1922.
(21) Lindner, K.: Saenger, W. Angew. Chem.. Int. E d . EngI. 1978. 17. 694-695. (22) Russell. J . C.; Wild, U . P.; Whitten. D.G. J . Phys. Chem. 1986, 90, 1 3 I 9- I 323. (23) Thomas, J. K . Chem. Reo. 1980, 8. 283-299. (24) Jones 11, G.; Jackson, W. R.; Choi, C.; Bergmark. W. R J . Phys. Chem. 1985, 89. 294-300.
Experimental Section
The coumarins were commercially available laser-grade materials obtained from Eastman Kodak Co. or Exciton Chemical Co. and used in most cases as received following TLC analysis for impurities. CI F was synthesized and purified as described p r e v i ~ u s l y . The ~ ~ cyclodextrins were obtained from Aldrich Chemical Co. and used as received. Emission spectra were recorded on a Perkin-Elmer MPF-44A fluorescence spectrophotometer equipped with a differential spectrum correction unit or a Perkin-Elmer MPF-2A instrument. Spectra were recorded at room temperature and integrated intensities were obtained by cut-and-weigh. Acknowledgment. Support for this work by The Office of Naval Research, Research Corporation, and the Charles A. Dana Foundation is gratefully acknowledged. (25) Turro, N J.; Okubo, T.; Chung, C. J Am. Chem Sot. 1982. 104, 3954-3957
Effect .of Structure in Selective Oxide Catalysis: Oxidation Reactions of Ethanol and Ethane on Vanadium Oxide S. Ted Oyama* Department of Chemical Engineering, Clarkson University, Potsdam, New York I3676
and Cabor A . Somorjai Department of Chemistry, University of California, Berkeley, Berkeley, California 94720 (Received: August 31, 1989; In Final Form: November 7 , 1989)
The catalytic activity of well-characterized samples of vanadium oxide supported on silica were studied for the partial oxidation of ethanol and ethane. Surface vanadium atoms were counted by a new oxygen chemisorption technique which allowed measurements of sample dispersions and turnover rates. Ethanol oxidation was found to be a structure-insensitive reaction that produced mostly acetaldehyde. Ethane oxidation was different; while its overall turnover rate changed little with vanadium oxide dispersion, its selectivity did vary with catalyst structure. The products of this reaction were mostly ethylene and acetaldehyde, but at low temperatures on the highest dispersion catalysts there was excess carbon dioxide formation.
Introduction
For catalytic reactions on metals the structure of the surface can exert a profound influence on catalytic activity and selectivity. For catalytic reactions on oxides the effects of structure are known to a much lesser extent but have been receiving increasing attention. Recent studies have investigated the structure sensitivity of stoichiometric reactions on available oxide single crystals like or oxide thin films like Mg063’ or V 2 0 5 . s Other Zn0,’-3 Ti02,4-5 studies have relied on preparation of samples with different ratios of exposed crystal facesg-13 for investigations of the effect of structure. The subject area has been reviewed.14-” A common approach for investigating the effect of structure in metal catalysis is to study the response of reactivity to dispersion, the ratio of surface to total atoms in a small metal particle.’* In the critical region where particle diameters grow from atomic sizes to I O nm. the surface structure evolves from isolated atoms to large multiatomic and multilayered ensembles as found in the exposed planes of macroscopic single crystal^.'^,^^ In this region the dispersion decreases from 100% to very small values, and the effect of structure on reactivity can be assessed. Ilnfortunately, the effect of dispersion has rarely been used in the study of oxide catalysts because of the difficulty in measuring the number of surface atoms in oxides. In this investigation w e *To whoin correspondence should be addressed.
____
0022-3654/90/2094-S022$02.50/0
employ the results of a recently developed method for counting surface sites on supported and unsupported vanadium oxide. The effect of dispersion in the critical range is investigated for two reactions: ethanol and ethane oxidation. We find that the oxi( I ) Berlowitz, P.; Kung, H. H. J . A m . Chem. SOC.1986, 108, 3532. (2) Vohs, J. M.; Barteau, M. A. J . Phys. Chem. 1987, 91, 4766. (3) Vohs, J. M.; Barteau, M. A. Surf.Sci. 1988, 197, 109.
(4) Onishi, H.; Aruga, T.; Egawa, C.; Iwasawa, Y . Surf. Sci. 1988, 193, 33. ( 5 ) Kim, K. S.; Barteau, M. A. Surf. Sci., in press. (6) Martinez, R.; Barteau, M. A. Langmuir 1985, 1, 684. (7) Barteau, M. A.; Peng, X. D. Mater. Chem. Phys. 1988, 18, 425. (8) Lewis, K. B.; Oyama, S. T.; Somorjai, G. A. Surf. Sci., in press. (9) Volta, J. C.; Desquesnes, W.; Moraweck, B.; Coudurier, G. React. Kinet. Catal. Lett. 1979, 12, 241. ( I O ) Volta, J. C.;Forissier, M.; Theobald, F.; Pham, T. P. Faraday Discuss. Chem. SOC.1981, 72, 225. ( 1 I ) Tatibouet, J . M.; Germain, J . E. C. R . Seances Acad. Sci. 1980, 290,
321. (12) Tatibouet, J. M.; Germain, J. E. J . Catal. 1981, 72, 375. (13) Oyama, S. T. Bull. Chem. Soc. Jpn. 1988, 61, 2585. (14) Henrich, V. E. f r o g . SurJ. Sci. 1979, 9, 143. (15) Volta, J . C.; Portefaix, J . L. Appl. Catal. 1985, 18, 1. (16) Kung, H . H. Ind. Eng. Chem. Prod. Res. Deo. 1986, 25, 171. (17) Barteau, M. A,; Vohs, J. M. In Successful Design of Catalysts; Inui. T.. Ed.; Elsevier: Amsterdam, 1988; p 89. ( 1 8 ) Boudart, M. Adc. C a t d 1969, 20, 153. (19) Van Hardeveld, R.; Hartog, F. Surf. Sci. 1969, 15, 189. (20) Ptrez. 0.-L.; Romeu, D.: YacamBn, M. J . J. Catal. 1983, 79, 240.
1990 American Chemical Society
Reactions of Ethanol and Ethane on Vanadium Oxide
The Journal of Physical Chemistry, Vol. 94, No. 12, 1990 5023
TABLE I: Oxygen Uptake on Silica-Supported Vanadium Oxide
before reaction“ sample
SA,” m2 g-I
Si02
99
0.3% V205/S!O2 1.4% v20s/s102 3.5% V205/Si02 5.6% V205/Si0, 7.7% V205/Si02 9.8% V20,/Si02 100% v20s
92 95 82 78 82 2.8
” Reference 21.
0, uptake,c wmol g-l 0.6 15.3 67.0 133 142 191
258 7.3
after ethane oxidation 0, uptake: dispd
SA,b m2 g-I
wmol g-l
dispd
0.93 0.87 0.69 0.46 0.45 0.48 0.001
58 61 50 56 59 63 59 4.9
9.2 26.2 57.4 63.4 126 180 13.0
0.56 0.34 0.30 0.21 0.29 0.33 0.002
bSurface area. COxygenmolecule uptake. dDispersion = fraction of V atoms a t the surface, assuming Oadr/Vaurl = I.
dation of ethanol is structure insensitive. Ethane oxidation, however, exhibits a large change in selectivity (ethylene vs carbon dioxide production) with dispersion while the overall reaction rate is independent of surface structure. Experimental Section
The supported V2Os catalysts were prepared by impregnating
Si02( C a b - O M , 90 m2 8-l) to incipient wetness with vanadium oxalate solutions formed from N H 4 V 0 3 (Aldrich, 99.99%) and oxalic acid (Adrich, 99.99%). Vanadium oxide concentrations are reported as weight percent throughout the paper. The solids were dried for 2 h at 573 K and calcined for 12 h at 773 K. They were used as powders of mesh size < 120 (particle size, L < 0.012 cm), except in diffusion limitation tests where they were pressed to 10 MPa and sieved to 6C-120 mesh ( L = 0.025 cm) and 30-60 mesh ( L = 0.040 cm). Pure unsupported V,Os (Aldrich, Gold Label, 99.999%) in the form of a fine powder ( L < 0.012 cm) was used as received. Surface areas (BET) and oxygen uptakes were measured with a flow apparatus equipped with a thermal conductivity (TC) detector. BET areas were obtained by flashing samples equilibrated in a 30% N 2 / H e stream. Oxygen uptakes were measured at 641 K on samples reduced in H 2 at the same temperature by adsorbing calibrated pulses of 02. Catalytic activity was obtained in a 16-mm-o.d./14-mm-i.d. quartz packed-bed flow reactor. A sealed quartz capillary tube held axially in the center allowed a thermocouple to probe temperature gradients along the reactor length. Axial temperature differences of a few kelvin were detected only at the highest conversions. I n all experiments, the total flow rate was 73-74 Kmol s-I and the total pressure was 101 kPa. (Flow rates in Mmol s-l may be converted to cm3 (NTP) min-’ by multiplying by 1.5.) In the ethanol oxidation studies the partial pressures of reactants were PEtOH = 1.6 kPa, Po, = 28 kPa, PHZO = 10 kPa, and PHe -= 61 kPa. I n the ethane oxidation studies these were PCHtCH, 13 kPa, Po, = 28 kPa, PHlO= I O kPa, and PHe= 50 kPa. The ethane contained a 0.5% ethylene impurity whose presence was taken into consideration in the rate calculations. The ethane was purified from possible sulfur impurities by passage through a reduced N i / S i 0 2 bed. All other gases were research grade (Matheson, >99.995%) and were employed as received with flow rates adjusted by rotometers. The liquids, including the undenatured ethanol, were also research grade (Aldrich, 99.99%). They were loaded into a syringe pump and metered into an on-line evaporator. All inlet and outlet lines were made of stainless steel and were heat-traced. The ethanol partial pressure was chosen to be below the lower flammability limit (5 kPa). The ethane partial pressure was chosen to be above the upper flammability limit (10 kPa) because the lower limit ( 3 kPa) made the analysis for products at low conversions difficult. Although these choices caused the ethanol and ethane feeds to be oxygen-rich and oxygen-lean with respect to combustion stoichiometry, the conversions were such that there was always excess oxygen. For the ethanol and ethane studies the maximum O2conversions were 17% and 33% at the maximum reactant conversions of 100% and 20%, respectively. The H 2 0 partial pressure was kept high in order to reduce variations due to conversion level. H 2 0 is a product of reaction
and an inhibitor in oxidation reactions. In all cases amounts of catalyst corresponding to 7.0 mg of V205 were loaded in the reactor. Si02diluent was also added so that the total sample weight was 500 mg, corresponding to a constant bed volume of 3.1 cm3. The only exceptions were for the 0.3% V 2 0 5 sample for which 7.0 mg of V2Os resulted in 2.33 g of total sample and for the pure unsupported V 2 0 5in which case 5.4 g was used. For analysis a gas chromatograph with a FID (Poropak QS column) and a TC detector (Carbosphere column) was used. Rates are reported as turnover rates based on surface vanadium atoms counted by oxygen chemisorption: 0, =
QYX -
vw2s
The validity of the chemisorption technique has been established elsewhere.2’ In the equation above ut is the total ethanol or ethane turnover rate, Q is the total volumetric flow rate, y is the mole fraction of ethanol or ethane, Vis the molar volume at the condition of flow measurement, w is the weight of catalyst, and 2 s is the oxygen uptake value. The factor of 2 accounts for the stoichiometry of one chemisorbed oxygen molecule per two surface vanadium atoms. The conversion, x, is defined as the fraction of ethanol or ethane that reacts. The other turnover rates are defined as ci =
u,s,
where S,,the selectivity of product i, represents the fraction of ethanol or ethane that gets converted to i and takes into consideration the number of carbon atoms in the product. (Thus, CO or C 0 2 production is divided by 2.) The turnover rates are given in units of ks-I equivalent to IO-) s-l. Particular care was used to ensure that the catalysts were stable at the reaction conditions employed. Before every run the catalysts were pretreated in the 02-H20-He reactant flow mixture for 1 h at the maximum temperature of reaction. The temperature was then lowered and the hydrocarbon added to the feed. Reactivity measurements were then taken at increasng temperatures while verifying that steady state was reached. Each point typically required 1-2 h, and runs of 16-24 h were carried out. Importantly, after the highest temperature was attained, a lower temperature measurement was always repeated to verify catalyst stability and lack of deactivation during the rate data acquisition. These points are indicated by arrows in Figures 1,3, and 4. Carbon and oxygen mass balances closed to 100 f 5%. Results
Oxygen Chemisorption. The chemisorbed oxygen uptake values of the fresh catalysts are reported in Table I. It was found that the catalysts employed in the ethanol oxidation reaction had essentially the same uptakes and surface areas after reaction. However, the catalysts employed in the higher temperature ethane oxidation lost surface area. This loss occurred in the initial hydrothermal activation period and not during reaction, and thus the rate measurements remain accurate. However, for this reason, (21) Oyama, S. T.; Went, G.T.; Lewis, K. B.; Bell, A. T.; Somorjai, G.
A . J . Phys. Chem. 1989. 93, 6786.
5024 The Journal of Physical Chemistry, Vol. 94, No. 12, I990
Oyama and Somorjai
TABLE 11: Representative Conversion and Selectivity Values in Ethanol Oxidation
sample
conversion, %
Si02
0.4 2.0 3.5 5.5 3.9 3.8
I .4% V20s/Si02 3.5% V20s/Si02 5.6% V2OS/S1O2 7.7% V205/Si02 9.8% V205/Si02
SA~OH, %
SCH,CHO% S C H ~%C H ~ T = 490 K 100 84 4 77 2 77 1 65 1 74 1
6
6
11
IO
II 17 14
11
ut, ks-'
%
SCO$
11
0.32 0.97 2.2 1.3 1.6
5 12 14 17 15
2 5 14 14 20 15
0.97 27 5.8 4.1 4.2
3 4 9 12 15 14
IO 4 9 12 15 13
17
T = 530 K
SiO,
I .4% v ~ o ~ / 3.5% V205/Si02 5.6% V205/Sj02 1.7% V205/S102 9.8% V205/Si02
s~o~
1.5 5.0 12 15 13 12
98 77 66 66 59 66
6.8 15 30 35 35 30
87 62 56 61 59 62
13 8 6 4 4
T = 570 K
Si02 1.4% 3.5% 5.6% 7.7% 9.8%
V20s/Sj02 V20s/S102 V20s/Si02 V205/S102 V205/S102
30 26 I5 11 11
3.0 7.3 14 13 12
-_
__
p--€--
b ) I'
7
10
. v)
Y
I
0
c
2
t I"
0
9 8%
x
77% 5 6 %
A
1 1
L
m
t
5
0
i
E
Too Temperature / K Figure 1. Conversion and selectivity in ethanol oxidation: X, conversion; 0, acetaldehyde; 0,ethylene; 0, acetic acid; V, carbon monoxide; A, carbon dioxide. (a) SO,;(b) 1.4% V205/Si02;(c) 3.5% V20s/Si02;(d) 5.6% V205/Si0,;(e) 7.7% V20s/Si0,; (f) 9.8% V20s/Si02. for the ethane oxidation reaction the turnover rates were based on oxygen chemisorption at 641 K on the catalysts after reaction (Table I). Table I also reports vanadium oxide dispersions, defined as the fraction of vanadium atoms on the surface as measured by oxygen chemisorption. These range from 0.93 to 0.45 for the catalysts used in ethanol oxidation and from 0.56 to 0.21 for those used in ethane oxidation. Ethanol Oxidation. Ethanol oxidation was studied between 400 and 700 K on the silica-supported samples. The results are summarized in Figure 1 and Table 11. All samples showed similar behavior regardless of dispersion; at low temperatures the main product was acetaldehyde while at high temperaures it was ethylene. Acetic acid was produced at intermediate temperatures, and CO and COz were produced at the most elevated temperatures. At these highest temperatures traces of methane were occassionally detected, probably from the homogeneous decomposition of the acetaldehyde product. No formaldehyde, ethyl
2
01
I
i t 0.01
1
h ,
h
Acetic Acid
Ethylene I
1.6 1.8 2 . 0 2 . 2 2 . 4
)L2
1
-,
1.6 1.8 2.0 2.2 2 . 4
103 T - ~ I K - ' in ethanol oxidation: V, 1.4% V20s/Si02;0, 3.5% V205/Si02;A, 5.6% V20s/Si02;X , 7.7% V20s/Si02;0,9.8% V20s/Si02. (a) total turnover rate; (b) acetaldehyde rate; (c) ethylene rate; (d) acetic acid rate.
Figure 2. Arrhenius plots
ether, or higher coupling products were ever observed, although these were detectable. Silica had a considerably smaller activity and differed from vanadium oxide in producing mostly acetaldehyde and no ethylene at low conversions. All conclusions in this study will be drawn from data at low conversions where the silica remains chemically inert, as shown in Figure 1 and Table 11, and where product decomposition does not occur. Arrhenius plot for the various products summarize the effect of dispersion on the turnover rate as shown in Figure 2 and Table 111. The activation energies do not vary drastically from sample to sample, and there are no trends. The average values for acetaldehyde, ethylene, acetic acid, and COXare 5 1, 120, 64, and 66 kJ mol-'. The Arrhenius plots are linear except for the acetic acid product (Figure 2d) because it is probably an intermediate
Reactions of Ethanol and Ethane on Vanadium Oxide
60 40
-. >
.
I
1
X 0 0 V A
The Journal of Physical Chemistry, Vol. 94, No. 12, 1990 5025
Conrersion CH3CHO CH2CH2 CO COz
,
20-
5
0 fn 700
,,
1
s
/&
800
.
I
%
900
Lo
0 CHJCHO 0 CHzCH2
401 V CO A Cop
700
800
Temperature / K
Temperature / K
Figure 3. Conversion and selectivity in ethane oxidation:
conversion; 0,acetaldehyde; 0.ethylene; V, carbon monoxide; A, carbon dioxide. (a) S O 2 ; (b) 0.3% V20s/Si02;(c) 1.4% v205/siO& (d) 3 3 % V2OS/ X,
Si02. TABLE 111: Summary of Activation Energies (in kJ mol-’) for Ethanol Oxidation sample ECH~CHO ECH~CH EA~OH ~ Eco, Et 1.4% V 2 0 5 / S i 0 2 3.5% V 2 0 5 / S i 0 2 5.6% V20s/Si02 7.7% V20s/Si02 9.8% V205/Si02
53 48 45 63 47
1 I8 121 1IO 123 1 I4
900
58 63 67 71 60
67 59 51 65 53
62 72 68 66
which is consumed at high temperature. The data tend to cluster, although there is some spread for the acetic acid. There is a small but consistent trend for the low concentration samples (1.4% and 3.5%) to have lower rates.
Figure 4. Conversion and selectivity in ethane oxidation:
X, conversion; 0,acetaldehyde; 0,ethylene; V, carbon monoxide; A, carbon dioxide. (a) 5.6% V20s/Si02; (b) 7.7% V20s/Si02; (c) 9.8% V20s/Si02; (d) 100% V20s.
Ethane Oxidation. As expected, the oxidation of ethane required much higher temperatures than the oxidation of ethanol (Figures 3 and 4), conversions becoming appreciable only above 800 K. The products of the reaction were acetaldehyde, ethylene, CO, and COz. Traces of methane at high temperatures but no formaldehyde, acetic acid, ethyl ether, or higher coupling products were obtained. Unlike ethanol oxidation, where small amounts of CO and COz were observed only at the highest temperatures (>630 K), for ethane oxidation large amounts of CO and COz were produced at all temperatures (Table IV). For the supported samples combined CO and C 0 2 selectivity tended to increase with temperature to just over 50% at 870 K (Table IV). For the unsup-
TABLE IV: Conversion and Selectivitv Values in Ethane Oxidation sample conversion, % SCH,CHO? % S C H ~%C H ~ ~ sco, % T = 800 K 0 Si02 1.o 7 30 0.3% V205/Si02 3 19 5 1.4% V205/Si02 1.5 12 63 0.6 IO 3.5% V205/S102 7 73 9 5.6% V205/Si02 0.5 7 7.7% V205/Si02 9 65 0.8 8 59 0.5 5 9.8% V 2 0 s / S i 0 2 100% V2O5
8.0
0.2
Si02
0.1 2.1 2.2 1.8 1.9 2.5 2.8
100 9 4 11 6 9 8 0.2
26
39
36 28 57 66 59 58 24
2 8 10
SCO*,
7%
ut, ks-I
63 73 15 11 19 28 35
2.4 5.1 2.2 3.3 3.8 2.7 6.2
52 60 22 17 21 29 33
9.3 16 15 22 21 21 8.6
15 35 43 35 29 30 34 30
21 30 63 69 57 49 15
T = 825 K 0.3% V205/Si02 I .4% V 2 0 5 / S i 0 2 3.5% V205/Si02 5.6% V205/Si02 7.7% V205/Sj02 9.8% V205/S102 100% V 2 0 s
10
11
5 8 43
T = 870 K
Si02 0.3% V205/Si02 1.4% V205/Si02 3.5% V 2 0 5 / S i 0 2 5.6% V205/S102 7.7% V205/Si0, 9.8% V , 0 5 / S i 0 2 100% v20s
2.9 14 12 13 12 14 16 18
8 5
5 3 5
5 0.2
81 42 39 38 46 46 44 21
4 15
13 22 22 24 17 49
5026
The Journal of Physical Chemistry, Vol. 94, No. 12, 1990
Oyama and Somorjai
'
j a) 10 1
b)
i
t
I
. L
a! >
1
0
E t-
L 1.1
L 1
1.2
1.3
10
-
r tL i \ , & , i ,
1
1.1
1.2
c)
,
1.3
1.1
1.2
IO3 T - ' i K - '
Figure 5. Arrhenius plots in ethane oxidation: 0, acetaldehyde; 0, ethylene; V, carbon monoxide; A,carbon dioxide. (a) 0.3% V205/Si02; (b) 1.4% V205/Si02;(c) 3.5% V205/Si02.
1.1
1.2
1.3
lo3 T-'/ K - '
Figure 6. Arrhenius plots in ethane oxidation: 0, acetaldehyde; 0, ethylene; V,carbon monoxide; A,carbon dioxide. (a) 5.6% V205/Si02; (b) 7.7% V205/Si02;(c) 9.8% V,0s/Si02; (d) 100% V 2 0 5 .
TABLE V Summary of Activation Energies (in kJ mol-') for Ethane Oxidation Eco2 E, sample ECHf2H0 ECH2CH2 ECO LT HT LT HT 78 162 122 337 223 307 0.3%V20s/Si02 207 96 167 87 217 248 238 1.4% V20s/Si02 229 224 354 323 294 3.5% V205/Si02 184 220 346 344 253 5.6% V205/Si02 181 219 224 7.7% V20s/Si02 192 216 304 9.8% V20s/Si02 176 210 311 252 210 100% V2OS 70 55 93 56 72 ported 100% V 2 0 s sample this figure rose to 80%. In fact, the 100% V2Os sample had much higher selectivity to the deep oxidation products throughout the entire temperature range. It also displayed much higher conversions and less temperature dependence (Figure 4d). The most dramatic difference between ethanol and ethane oxidation was the sudden shift in selectivity in ethane oxidation in going from high to low dispersion samples (Figure 3). For the 0.3% and 1.4% samples at low conversion, the main product was C 0 2 ,and this decreased in favor of ethylene as the temperature was raised (Figure 3b,c). For the 3.5% and higher concentration samples this behavior was entirely reversed, with ethylene being the main product at low conversion and carbon dioxide increasing in its place as the temperature was raised (Figures 3d and 4a-c). I n all cases acetaldehyde was a minority product (5-10%). Arrhenius plots of these data reflect the same peculiar difference between low and high vanadium concentration samples (Figures 5 and 6 ) . For the low-concentration samples the C 0 2 production rates are high and the Arrhenius plots have curvature. For the high concentration samples the C 0 2production rates are low and the Arrhenius plots are linear. Activation energies obtained from these plots by linear regression are summarized in Table V. There is a clear trend toward decreasing activation energies with increasing vanadium concentration. There is a very large drop in activation energies for the 100% V 2 0 5 sample. For the lowconcentration samples values of activation energy are given at low temperature (LT) and high temperature (HT) because of the curvature.
L
1.3
t
T
.'
=
870 K
Ethylene
LA Ld L10 0.3 1.4 3 5 5.6 7.7 9 8 100
0.1
1 t
~
I
c)
T
=
870 K
I
u Acetaldehyde
0'01 0.3 1.4 3.5 5.6 7.7 9.8 100
Vanadium Loading / % Figure 7. Effect of vanadium concentration on ethane oxidation: (a) or +, total rate; (b) 0,ethylene rate; (c) 0,acetaldehyde rate.
X
The effects of V 2 0 5 Concentration on the turnover rates are displayed at three temperatures in Figure 7. The overall rate is seen to be relatively constant at all temperatures. However, the rates for ethylene and acetaldehyde formation increase with concentration, although the increases are modest and tend to be leveled at higher temperatures.
Reactions of Ethanol and Ethane on Vanadium Oxide 100 100%
v*o5
t
40i v
C a r b o n monoxide A C a r b o n dioxide
0
1000
2000
3000
4000
5000
Time I s
Figure 8. Conversion and selectivity in ethane oxidation in the absence of oxygen: catalyst, 100%V205,4.58 g: X, conversion: 0 , ethylene; V, carbon monoxide: A, carbon dioxide.
The behavior of the unsupported 100% VzOs was investigated further by cutting off the gas-phase oxygen supply and following the products of ethane oxidation in time (Figure 8). At zero time the helium flow rate was increased so as to maintain the total flow rate constant at the usual 7 3 pmol s-I. The selectivity to the various products stabilized in -2000 s. The conversion decreased slightly during this time and then gradually rose. The temperature remained constant at 7 5 0 K throughout the experiment. Transport Effects. Lack of mass- and heat-transfer effects were checked by varying the particle size of the catalysts in both the ethane and ethanol oxidations (Figure 9). The particle size refers to the macroscopic catalyst powder agglomefates obtained by sieving the samples, not the supported crystallites of V20s. Plots of conversion and selectivity for catalysts employed in this study ( L < 0.012 cm) are shown on the left panels (Figure 9a,c) and reproduced on the right panels for the pressed and sieved samples (Figure 9b,d) of larger particle size ( L = 0.025 and 0.040 cm). The points for the larger sized particles are seen to fall very close to the plots for the fine particle sizes.
Discussion Oxygen Chemisorption. The oxygen chemisorption technique employed here has been described in a recent publication in this journal.2' It was clearly shown by using X-ray diffraction, laser Raman spectroscopy, and isotopic labeling that the method counts surface atoms and not bulk atoms. The technique relies on finding an optimal temperature where surface but no bulk reduction occurs. The finding that ethanol oxidation is a structure-insensitive reaction is an internal check that also confirms the validity of the method. This will be discussed in the next section. From the oxygen uptake values, vanadium dispersions can be calculated (Table I), and these are in excellent agreement with the Raman spectra of the same samples reported earlier.21 Thus, where oxygen chemisorption shows high dispersion, the Raman shows monatomic vanadium species on the surface; where low dispersions are measured, small crystallites of V2Os of diameter
