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The Journal of Physical Chemistry, Vol. 82, No. 16, 1978
K.4. Aika and J. H. Lunsford
D. C. Grahame, J . Am. Chem. Soc., 76, 4819 (1954). R. Payne, Trans. Faraday Soc., 64, 1638 (1968). A. R. Sears and P. A. Lyons, J. Electoanal. Chem., 42, 69 (1973). R. Payne, J. Phys. Chem., 70, 204 (1966); see erratum in ib/d., 70, 4101 (1966). R. Payne, J . Phys. Chem., 69, 4113 (1965). W. R. Fawcett and J. B. Sellan, Can. J . Chem., in press. C. V. D'Alkaine, E. R. Gonzalez, and R. Parsons, J . Electroanal. Chem., 32, 57 (1971). M. J. Weaver and F. C. Anson, J. Electroanal. Chem., 84, 47 (1977). See Appendix I1 of ref 1. A. N. Frumkin, 0. A. Petry, and N. V. Nikolaeva-Fedorovich, Electrochim. Acta, 8, 177 (1963). L. Gierst, E. Nicolas, and L. Tytgat-Vandenberghen, Croat. Chem. Acta, 42, 117 (1970). D. J. Bieman and W. R. Fawcett, J. Electroanal. Chem., 34, 27 (1972). M. J. Weaver and F. C. Anson, J. Eleciroanal. Chem., 65, 711 (1975). From plots of 4 , vs. qmaccording to the GCS model, coupled with q m vs. E data given in ref 28. S. N. Frank and F. C. Anson, J. Electroanal. Chem., 54, 55 (1974). I f the reaction site lies sufficiently far inside the oHp, the reactants' coordinated ligands may penetrate the "primary hydration layer" of water molecules immediately adjacent to the electrode surface;' such a reaction pathway would then be (conventionally) designated "inner-sphere". However, the available evidence' suggests that ammine complexes that appear to involve reaction sites within the oHp reside in a "secondary hydration layer"' bound by the hydration spheres of the supporting electrolyte ions that populate the oHp. Such a reaction site can be categorized as "outer-sphere'' as the primary inner layer (the electrode's "coordination sphere") remains intact. For a revlew, see H. D. Hurwitz in "Electrosorption", E. Gileadi, Ed., Plenum Press, New York, N.Y., 1967, Chapter 7. D. J. Bieman and W. R. Fawcett, J. E/ectroanal. Chem.,34, 27 (1972). V. A. Fedorov, V. E. Mironov, and F. Ya. Kul'ba, Russ. J . Inorg. Chem., 7, 1311 (1962).
(47) These rate responses wlII be independent of the reaction site If this lies within the diffuse layer In a region domlnated by the supporting electrolyte a n i ~ n . ~ ' - ~ ~ (48) Thus using Boltzmann terms such as in eq 4 to calculate the corresponding concentrations of ion pairs, free cations, and anions in the bulk and at the reaction plane inevitably yields the conclusion that (Klp)bulk = ( K I )rp. (49) See, for example, I-!. W. Nurnbergand G. Wolff, J. E/ectroanal. Chem., 21, 99 (1969). (50) S.Levine, K. Robinson, and W. R. Fawcett, J . Nectroanal. Chem., 54, 237 (1974). (51) E. R. Nightingale, J. Phys. Chem., 63, 1381 (1959). (52) S. Levine and K. Robinson, J. Nectroanal. Chem., 51, 159 (1973). (53) See Appendix I of ref 1. (54) D. J. Barclay and J. Caja, Croat. Chem. Acta, 43, 221 (1971). (55) B. B. Damaskin, V. F. Ivanov, N. I.Melekhova, and L. F. Malorava, Sov. Electrochem., 4, 1205 (1968). (56) B. B. Damaskin, A. N. Frumkin, V. I. Ivanov, N. I. Melekhova, and V. F. Khonina, Sov. Electrochem., 4, 1200 (1968). (57) W. R. Fawcett and S.Levine, J. Electroanal. Chem., 65, 505 (1975). (58) R. A. Marcus, J , Phys. Chem., 67, 853 (1963); J . Chem. Phys., 43, 679 (1965). (59) J. F. Endicott and H. Taube, J. Am. Chem. Soc.,86, 1686 (1964). (60) J. P. Candlin, J. Halpern, and D. L. Trimm, J . Am. Chem. Soc., 86, 1019 (1964). (61) A. A. Vlcek, Proc. Int. Conf. Coord. Chem., 6th, 590 (1961). (62) See, for example, 8.Perlmutter-Hayman, Prog. React. Kinet., 6, 239 (1971). (63) J. F. Endicott and H. Taube, Inorg. Chem., 4, 437 (1965). (64) M. J. Weaver and F. C. Anson, J. Elecfroanal. Chem., 58, 95 (1975). (65) M. J. Weaver, submitted for publication. (66) A. A. Vlcek, Discuss. Faraday Soc., 26, 164 (1958). (67) For a review, see A. A. Vlcek, Prog. Inorg. Chem., 5, 211 (1963). (68) W. R. Fawcett and C. L. Gardner, J . Electroanal. Chem., 82, 303 (1977).
Surface Reactions of Oxygen Ions. 2. Oxidation of Alkenes by 0- on MgO Ken-ichi Aika and Jack H. Lunsford" Department of Chemistry, Texas A&M University, College Station, Texas 77843 (Received February 2 1, 1978) Publication costs asslsted by the Natlonal Science Foundation
Surface reactions between C2 to C4 alkenes and 0- ions on MgO were studied at 25 "C, and a stoichiometry of one alkene reacted per one 0- ion was determined. The reaction of ethylene and propylene with 0- gave no gaseous product at 25 "C; however, hydrocarbons (mainly methane) were obtained when the samples were subsequently heated above 450 "C. Following the reaction of 1-butene with 0-,butadiene appeared as a main product, which had a yield of ca. 40% when thermally desorbed above 300 "C. Alkoxide ions are proposed as intermediates for the formation of butadiene. The thermal desorption pattern following the ethylene/Oreaction was found to be almost the same as those of acetaldehyde and ethylene oxide on MgO. Since infrared spectra indicate that acetaldehyde and ethylene oxide react to form the acetate structure on MgO, a similar intermediate is proposed for the reaction of ethylene with 0-. The reaction of propylene with 0- may also result in the formation of a carboxylate intermediate. With the three alkenes the initial reaction appears to be the abstraction of a hydrogen atom by 0-.
Introduction In the gas phase, reactions between alkenes and 0- ions have been studied by several groups. Bohme and Young1 have shown that the 0- ion adds only to ethylene and that it reacts with propylene and butene by either abstracting a hydrogen atom or a proton. Parkes2 claimed that 0abstracted H2+from ethylene, as well as added to ethylene; whereas, Goode and Jennings3 have reported that the main reaction between 0- and ethylene is H2+ abstraction. Similarly, Neta and Schuler4 have shown that in aqueous solution the addition of 0- to double bonds is relatively 0022-3654/78/2082-1794$01.00/0
slow and hydrogen atom abstraction is the dominant reaction. Heterogeneous reactions involving the 0- ion have been studied in an attempt to understand the role of this ion in catalytic oxidation. Naccache and Che5 have explored the reactivity of 0- on MgO by employing electron paramagnetic resonance (EPR) spectroscopy and have found that propylene and 1-butene caused the 0- signal to disappear without the formation of a free radical product, while ethylene produced a new signal, which they initially assigned as the C2H40-radical. Recently this 0 1978 American Chemical
Society
Surface Reactions of Oxygen Ions
assignment was reconsidered by Ben Taarit et aL6 who suggested that the species produced by ethylene was the H2C=C-- - -HO- radical. This radical reacted rapidly at temperatures greater than -20 "C, forming diamagnetic surface intermediates. Neither the stoichiometry nor the product characterization has been determined for the subsequent surface reactions, probably because the concentrations are small and the products are strongly adsorbed on the MgO surface. In the previous paper of this ~ e r i e sit, ~was shown that the surface reactions between simple alkanes and 0 - ions on MgO can be studied by using EPR and thermal desorption techniques. It was demonstrated that the 0- ion was effective in bringing about the dehydrogenation of the alkanes and forming the corresponding alkenes. The purpose of this research was to extend the study to reactions that occur between simple alkenes and 0- on MgO by applying the techniques reported previously, that is, to determine the reaction stoichiometry, the relative reactivity of alkenes and the thermal desorption spectra of the product. From the thermal desorption spectra and the infrared spectra of model compounds, it is possible to infer the elementary steps in the heterogeneous ion-molecule reactions. Experimental Section As described previously the 0- on MgO was prepared by first irradiating a degassed sample in the presence of H2 to form trapped electronsas Reaction of N 2 0 with the surface centers yielded 0- and gas phase N2. In this work the 0- was also formed by irradiation of MgO in the presence of NzO, thus the sample was never exposed to H2, Four MgO samples and reactors were used: two of them (1.60 g in reactor A and 0.82 g in reactor B) were identical with those used p r e v i ~ u s l ywhereas, ;~ the other two (1.60 g in reactor C, 33.5 mL) and (0.30 g in reactor D, 11 mL) were newly prepared. Reactor D consisted of two 11-mL glass chambers; one of these included the EPR tube containg the MgO and the other glass chamber contained the alkenes. This reactor was used for determining the temperature at which the reaction between 0- and alkenes commenced. Reactors A and B were used for the stoichiometry determinations and competitive reaction experiments. Reactor C was used in the determination of thermal desorption products formed via the reactions of 0- with alkenes and also the decomposition of model compounds. The temperature program for thermal desorption consisted of heating the sample in 1-h increments at 25, 150, 300, 450, and 600 "C, followed by 6 h at 600 "C. Although 500 "C was adequate to obtain maximum yields following the reaction between 0- and alkanes, 600 "C or more was necessary to obtain maximum yields for all alkene reactions except butene. Temperatures in excess of 600 "C were avoided since treatment at 500-600 "C gave maximum concentrations of trapped electrons.8 In these experiments the production of C 0 2 was not followed since it was observed that ca. 2% of the sample was present as MgCO, and a thermal treatment at 600 "C failed to decompose all of the carbonate. Since the remaining fixed C 0 2 (ca. 3%) may be desorbed during the thermal desorption program, it was impossible to determine the exact amount of C 0 2 produced from the surface reaction. Results EPR Evidence for the Reaction between 0- and Alkenes. After recording the EPR spectrum of 0- on MgO at -196 "C 1 Torr of an alkene was introduced to the
The Journal of Physical Chemistry, Vol. 82, No. 16, 1978 1795 gL=2.042
g,, 12.0014 b
a=7s G C
x4
'
1g.2.002
dl7
Figure 1. EPR spectra observed during the reaction between I-butene and 0-on MgO. All the spectra were measured at -196 " C . (a) Spectrum of 0-on MgO before reaction, (b) after introducing 1 Torr of 1-C,H8 at 25 ' C for 5 min, (b') spectrum expanded 4 times that of b, (c) after 10 min at 25 "C.
sample, which was then warmed by transferring it to a constant-temperature bath for 5 min. After cooling the sample to -196 "C the EPR spectrum of 0- was again recorded. The EPR signal of 0- disappeared at -60 "C with ethylene, 0 "C with propylene, 25 "C with 1-butene, and 30 "C with 1,3-butadiene. The fact that these temperatures are parallel to, though 30-50 "C higher than, the boiling points for each alkene indicates that the surface reaction is controlled by the diffusion of the alkene through the column of MgO in the EPR tube. The minimum reaction temperature appeared to depend both on the alkene concentration and on the amount of MgO. A t higher concentrations, ethylene has been reported to react with 0- at -196 0C.5 In this work, the same paramagnetic species as previously 0bserved~9~ appeared a t -60 "C simultaneous with the disappearance of the 0- signal, and the new radical was stable up to -20 "C. The maximum concentration of this radical was ca. 50% of the 0- concentration. No new paramagnetic species were detected after the reaction of propylene and 0-,although contrary to previous reports5 1-butene yielded a product in low concentration (ca. 3% of the 0- concentration) having an EPR spectrum. This spectrum was also observed when 1,3-butadiene reacted with 0- on MgO. The radical was so unstable that it disappeared within 10 min at the same temperature at which it was formed. The spectrum, shown in Figure 1, is characterized by g = 2.002 and a hyperfine doublet with a = 75.0 G, or conceivably a triplet with a = 37.5 G. Another signal near g = 2.002, probably due to a carbon deposit on MgO, overlaps the central region of the spectrum, thus making it difficult to establish whether a triplet indeed exists. Reaction Stoichiometry. When 0.1 Torr of an alkene (ethylene, propylene, 1-butene, or cis-2-butene) was introduced on MgO which had not been treated with N20, no reaction occurred, and the alkene was recovered above 105 "C as reported in the previous paper.7 The amount of alkene that reacted with 0- was determined by subtracting the amount recovered at the thermal desorption temperature of 300 "C from the amount introduced on a sample at 25 "C. Since the 0- on MgO under vacuum had a half-life of 1h a t 25 "C, the 0- concentration could be varied by controlling the period that the sample was kept at this temperature. The amount of propylene reacted was
1796
The Journal of Physical Chemistry, Vol. 82, No. 76, 7978
K.4. Aika and J. H. Lunsford
I
I
i
d
h
0
3
0
50 100 Conc. of 0-, n m o l / g - M g 0
Figure 2. Amount of propylene reacted at 25 0-concentration.
O C
150
as a function of the
L&v
'25
150
360
460
660
1 hr each
600°C 6 hr
Flgure 4. Variation of products as a function of the thermal desorption program following the reaction of propylene with 0-at 25 O C . C4 indicates the sum of butenes and butadiene.
1 hr each
6 hr
Figure 3. Variation of products as a function of the thermal desorption program following the reaction of ethylene with 0-at 25 O C . C4 indicates the sum of butenes and butadiene.
examined as a function of 0- concentration as evaluated from the EPR spectrum, and the results are shown in Figure 2. These results are almost identical with those reported previously for the reaction of 0- with a l k a n e ~ . ~ The slope of unity (solid line in Figure 2 ) suggests a one-to-one stoichiometry for the reaction of propylene and 0-. Limited data on the other alkenes indicated a similar stoichiometry. About 20 nmol of propylene/g of MgO reacted with some oxygen species, probably derived from NzO, which did not contribute to the EPR spectrum. We believe that this oxygen is present as 0- coordinated to a transition metal ion impurity, since it is known that supported transition metal ions may promote such reaction~.~ Product Determination. After 0.1 Torr of an alkene was introduced and circulated for 1h over MgO with 0- at 25 O C , the gas phase products were analyzed. A trace of butadiene was obtained from 1-butene, whereas no gas phase products were obtained from reactions with ethylene or propylene. The reaction systems were then heated, and the gas phase products were transferred by circulating
gi
/
>,
'25
150
300
1 hr each
450
600
6OO0C 6 hr
Figure 5. Variation of products as a function of the thermal desorption program following the reaction of 1-butene with 0-at 25 O C . BD indicates butadiene.
helium to a trap at -196 "C. The amounts of products obtained are shown as a function of thermal desorption temperature in Figures 3-5. As a result of the reaction between 0- and ethylene or propylene, the amount of products obtained below 300 "C was quite small; however, methane as a main product, as well as ethylene, propylene, butene, and butadiene as minor products, were obtained at 450 and 600 "C. In the case of 1-butene, 1,3-butadiene was obtained as a main product and its yield almost reached a maximum (42%) at 300 "C. No other organic products such as alcohols or aldehydes were detected. The amount of the products in Figures 3-5 is shown as "yield" which indicates the ratio of the total carbon atom number of a product to the total carbon atom number of the reactant consumed. Thus it can be seen that more than 50% of the carbon atoms were recovered as hydro-
The Journal of Physical Chemistry, Vol. 82, No. IS, 1978
Surface Reactions of Oxygen Ions
I
TABLE I: Relative Reactivity of Alkanes and Alkenes with 0-Ions 0- on MgO 0- in gas phasea temp, 'C 25 22.5 press., Torr 0.1 0.23 hydrocarbon 0.50 f 0.02 0.14 CH, 1.00 1.00b CZH, 1.33 1.27 i 0.03 C,H, 1.7 0.67 * 0.12 n-C4H10 1.10 1.67 f 0.3 C2H4 1.13 f 0.3 1.43 C,H, 1.30 i: 0.2 2.0 1-C4H, cis-2-C4H, 1.46 f 0.2 1.72 Reference 1.
a
M-1s-l 10
-
1797
- --
'25
160
300
650
600
6OO'C
1 hr each
6 hr
1 hr lath
6 hr
'25
I50
300
150
500
600'C
1 h i each
5 hr
1 nr each
5 hr
Absolute rate constant is 4.2 x 10"
a
/
CHq
i
80
p
::
0
$
8
3 m
$ -
Figure 7. Variations of gas phase composition as a function of the thermal desorption program following the adsorption of acrolein (360-470 nmol/g of MgO), acetone (460-610 nmol/g of MgO), propionaldehyde (360-420 nmol/g of MgO), and allyl alcohol (620-710 nmol/g of MgO) on MgO at 25 'C. C4 lndlcates the sum of butenes and butadlene.
4
z 9 ' 2 >
/'
9 Y
0 I 1 hr each
6hr
1 hr m c h
L-
Bhr
Flgure 6. Variations of gas phase composition as a function of the thermal desorption program followlng the adsorption of acetaldehyde (120-1 100 nmol/g of MgO) and ethylene oxlde (120-330 nrnol/g of MgO) on MgO at 25 "C. C4 lndlcates the sum of butenes and butadlene.
carbons by thermal desorption at 600 "C. The remainder may be present as CO, COz, or a carbonaceous residue. Competitive Reactions. Competitive runs between ethane and an alkene (propylene, 1-butene, or cis-2-butene) were carried out and conversion ratios were determined. Since ethane yields ethylene, a competitive run between ethane and ethylene cannot be evaluated. Instead, a competitive run between ethylene and propane was carried out and the relative rate between ethylene and ethane was calculated using the previous data' for alkanes, The results of the relative rate measurements at 25 "C are shown in Table I, which includes the previous data and the gas phase data.l Thermal Desorption of Oxygen-Containing Species on MgO. The concentration of surface intermediates on MgO was below the detection limits of infrared spectroscopy; therefore it was necessary to characterize them by more indirect methods. Thermal desorption patterns when compared with those of known molecules have been useful in determining surface structure.' Typical oxygencontaining C2 and Cs hydrocarbons, in amounts comparable to those obtained from the reaction between 0- and alkenes, were adsorbed on degassed MgO and the reaction products were thermally desorbed. The amounts of the gas phase products as a function of the thermal desorption program are shown in Figures 6 and 7. Acetaldehyde (curve a in Figure 6) and ethylene oxide (curve b in Figure 6) gave similar product patterns to that observed from the ethylene/O- system (Figure 3). Acetaldehyde is known to polymerize by aldol condensation on basic compounds.1° Although the polymerization may cause the yield to be lower, the adsorbed acetaldehyde monomer seems to form an intermediate similar to those produced from ethylene oxide and the ethylene/O- system. An alkoxide is not
considered to be the intermediate because the thermal desorption pattern is quite different from that reported heres7 Since aldehydes and ketones are known to polymerize on basic compounds,1° the amount of gas phase products obtained after the adsorption of acrolein, propionaldehyde, and acetone were small, as shown in Figure 7. The yield from allyl alcohol was also low, probably as a result of isomerization to propionaldehyde. At 300 "C the characteristic thermal desorption product from allyl alcohol was propylene, which is believed to be derived from alkoxide ions on MgO. Analogous reactions involving alcohols and MgO have been discussed p r e v i ~ u s l y . The ~ formation of propylene at 300 "C from propionaldehyde suggests that the latter molecule isomerizes to allyl alcohol. Acetone is known to form a stable dimer known as mesityl oxide, CH3COCH=C(CHJ2. The product isobutene, which was unique to the decomposition of acetone, was confirmed by both gas chromatography and mass spectrometry. The formation of isobutene may be understood by assuming that mesityl oxide decomposes, leaving the carbonyl CO on MgQ. Acrolein, which yielded decomposition products above 450 "C, has a desorption pattern similar to the propylene/O- system in that a considerable amount of ethylene was formed; however, relatively less methane was formed from acrolein. Propylene oxide was not studied because it has been reported to isomerize to allyl alcohol and acetone at 260 "C on MgO.ll Infrared Study. Since acetaldehyde and ethylene oxide appear to form the same surface complex as the ethylene/O- system, it was of interest to identify the common surface intermediate which was produced by reactions of the former two molecules with MgO. A MgO wafer was heated at 600 "C under vacuum overnight, and 250 Torr of acetaldehyde was introduced to the sample at 25 "C. After 1. h, the gas phase was evacuated and the sample was subjected to the same thermal desorption program as discussed previously. After each temperature increment the infrared spectrum was recorded, and the results are shown in Figure 8.
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The Journal of Physical Chemistry, Vol. 82, No.
IS, 1978
K.4. Aika and J. H. Lunsford
TABLE 111: Observed Frequencies (cm-*)of Carbonato Comdexes
unidentate
bidentate
unidentate
1454-1462 1370-1380 1050-1090
a
bidentate
1453 1373 1070
1590-1593 1245-1 250 1050-1090 Reference 16.
1593 1266 1030
1600
!ZOO CM-'
800
Figure 8. Variation of infrared spectrum of adsorbed acetaldehyde on MgO as a function of the thermal desorption program: (a) background of MgO; after evacuation for 1 h at (b) 25 "C,(c) 150 "C,(d) 300 O C , (e) 450 "C,(f) 600 "C,(b') spectrum of acetaldehyde resin."
TABLE 11: Observed Frequencies of Carboxylate Bands on MgO (cm-l) CH,COOref 13 14 15 15 15 this work this work
starting compd C,H,OH C,H,OH Mg(CH,COO), (CH, ),CH OH (CH,),CO CH,CHO CH -CH, \, I
asym 1582 1580 1570 1586 1586 1575 (1590)"
sy m 1445 1420 1422 1422 1408 1420 1426
0
The actual number is difficult t o determine because of overlapping bands. a
The spectrum following adsorption of acetaldehyde at 25 "C (b in Figure 8) is considered to be mainly due to four compounds. One compound is polymerized acetaldehyde, which has the spectrum depicted as curve b' in Figure 8.12 Polymerization of acetaldehyde is essentially a condensation reaction in which water is formed. This is seen in curve b as hydroxyl groups (the second species) which have bands around 3600 cm-l. The third compound is physically adsorbed molecular acetaldehyde which absorbs at 2732 and 1728 cm-l.I3 The fourth one, which is important for this work, has strong bands at 1572 and 1419 cm-l. Based on the several literature values listed in Table 11,14-16 these bands are assigned as the 0-C-0 asymmetric and symmetric stretching modes of the acetate ion. Upon evacuation at 150 "C the physisorbed acetaldehyde was removed and polyacetaldehyde seems to have decomposed to various hydrocarbons. The peaks at 3020,2962,2930, and 2875 cm-l correspond well with bands assigned to the =CH- stretch, the CH, asymmetric stretch, the CH2 asymmetric stretch, and the CH2 and CH, symmetric stretch, respectively, Since the acetate ion should only have peaks a t 2962 and 2880 cm-l, the above four peaks must correspond to hydrocarbons from the decomposed polymer. Although evacuation a t 300 "C did not change
3800
3000
Flgure 9. Variation of infrared spectrum of adsorbed ethylene oxide on MgO as a function of the thermal desorption program: (a) background of MgO; after evacuation for 10 min at (b) 25 "C;after evacuation for 1 h at (c) 150 "C,(d) 300 "C,(e) 450 O C , (f) 600 O C , (b') spectrum of polyoxyethylene."
the acetate peaks, the other peaks decreased in intensity. Above 450 "C the bands a t 1570-1580 and at 1420 cm-I decreased in intensity while new bands at 1593 and 1454 cm-l were observed. Weaker bands at 1380,1245, and 1050 cm-l were also apparent in the spectra. These values are in good agreement with the spectra of the unidentate and bidentate carbonato complexes, [ Co(NH3)&03]Br and [ C O ( N H ~ ) ~ C Oas~ shown ] C ~ ~ in ~ Table 111. Adsorption of COz on MgO, however, gave a somewhat different spectrum.18 It is interesting to note that methane began to appear in the gas phase at the temperature corresponding to the decomposition of the acetate ion. Polymerization of ethylene oxide on MgO was so rapid that the standing time was limited to 0.5 min in order to avoid excess reaction on MgO after introducing 150 Torr of ethylene oxide at 25 "C. After evacuating the gas phase for 10 min at 25 "C, the infrared spectrum was recorded, and as depicted in curve b of Figure 9, the spectrum shows no molecular ethylene oxide, which should have strong bands near 3000, 1280, 1270, and 870 cm-l.13 Polymerization of ethylene oxide, which is a ring-opening reaction, produces no water as confirmed by the absence of new hydroxyl bands in Figure 9. The spectrum of the product, polyoxyethylene, as reproduced from the literature12 is shown as curve b' in Figure 9. This product seems to account for most of the peaks in b, and the few remaining ones may be attributed to a distorted alkoxide or an acetate ion. This unknown complex and perhaps the
Surface Reactions of Oxygen
The Journal of Physical Chemistry, Vol. 82, No. 16, 1978 1799
Ions
decomposed product from the polymer reacted to form the acetate ion at 150 "C, as characterized by the peaks at 1590 and 1425 cm-l. At higher temperatures the carbonato complexes developed as indicated by the bands at 1590, 1462, 1372, 1250, and 1090 cm-l. The asymmetric vibrations of the acetate ion at 1590 cm-l and the v1 vibration of the bidentate carbonato complex cannot be distinguished in this case. In general it should be noted that above 150 "C the essential features of the spectra of surface complexes from ethylene oxide are almost the same as those observed from acetaldehyde.
Discussion Reaction between Ethylene and 0- o n Mg0. The reactions between ethylene and 0- in the gas phase have been reported to yield two possible products which result from oxygen addition (CzH4O) or hydrogen abstraction (CzH2-).1-3 On supported molybdenum the reaction of ethylene with 0- has been reported to yield the CH2CHzO- radical which decomposes to form the vinyl radical and a hydroxyl The EPR spectrum following the reaction of ethylene with 0- on Mg05 is quite different from the spectra of the .CH2-CHz0- radical on both Mo/Si0J8 and Mg0: and the assignment of the spectrum to the complex H2C=C-- - -HO- by Ben Taarit et ala6seems to be correct. The results of this work have shown that the HzC= C--- -HO- radical reacts at temperatures above -20 "C, forming a diamagnetic surface intermediate. By analogy with the thermal desorption patterns of model compounds and from the infrared spectra of their surface intermediates we suggest that the acetate ion is formed on the surface by the reaction of ethylene with 0- according to the following mechanism: H2C=CH2 H2C=CH H,c=C-
-
+ 0+ 02-
- - - HO- t
02-
H,CdH
+ OH-
HzC=C-- - -HO-+
H,C-c
40.[
'0
t 3e-
(1)
(2) (3)
The negative charge is balanced, of course, by Mg2+ions, and the electrons released may be trapped at surface defects. It is significant that the observed radical (eq 2) had a concentration which was about 50% of the 0concentration and that the percentage of ethylene which reacted to form CH4 was a comparable value. Thus, it appears that about half of the ethylene reacts by a mechanism other than that described by eq 1-3. The subsequent reaction involving the acetate ion is depicted in eq 4. Thermal decomposition data and the H,C-C
40-
"0
t OH-
A -+
CH, t C0,'-
(4)
infrared spectra indicate that the surface acetate ion decomposed to methane and the carbonato complex at 300-450 OC. The work of Yakerson and Rubinshtein20 has demonstrated that acetate ions decompose to form methane, presumably by reacting with water. Since hydroxide ions on MgO begin to form water at 300 "C, a similar reaction could be written in place of eq 4. Acetate ions may also decompose to form :CH2 which would recombine to form ethylene. At temperatures over 450 "C the carbonato complexes form COz. Reaction between Propylene and 0-. Failure to observe radicals formed by the reaction of 0- with C3Hs on MgO
may be attributed to the higher temperature of 0 "C required for this reaction as compared with a temperature of -60 "C for the reaction of 0- with C2H4. It should be recalled that the H,C=C-- - -HO- radical was unstable at temperatures above -20 "C. Although no new EPR spectrum was observed following the reaction between 0and C3H6on MgO, by analogy with the gas phase reaction and the surface reactions between 0- and alkenes or ethylene, it seems reasonable to suggest that the initial step would involve a hydrogen atom abstraction. On the basis of bond strength and other data it is generally accepted that hydrogen atom abstraction occurs at the methyl group in most reactions on metal oxides, thus one might expect either the formation of an allyl radical or an allyl alkoxide. In the latter case the thermal decomposition pattern should resemble that shown in Figure 7d since allyl alcohol probably forms the corresponding alkoxide ion on MgO. Clearly the thermal desorption pattern of Figure 4 is quite different, therefore an alternate surface reaction is required. Instead of forming an alkoxide intermediate the allyl radical may react with oxide ions of the lattice according to the mechanism H,C-CH-CH,
t 40,-
-+
40-
H,C=CH-C
