Radiation effects on COH2 gas mixture in the presence of silica gel

Radiat. Phys. Chem. Vol. 16, pp. 175-181 © Pergamon Press Ltd., 1980. Printed in Great Britain

0020-7055/80/0801-0175/$02.0010

RADIATION EFFECTS ON CO-H2 GAS MIXTURE IN THE PRESENCE OF SILICA GEL S. NAOAI,H. ARAI and M. HATADA Osaka Laboratory for Radiation Chemistry, JAERI, Mii-minami 25-1, Neyagawa, Osaka 572, Japan (Received 18 September 1979) Abstract--Electron beam irradiation of gas mixture of CO and H2 (1:6) in the presence of silica gel exclusively produces carbon dioxide and hydrocarbons in high yields. At the reaction temperature of 1400C, CO: is preferentially produced, most of which are formed independently of hydrocarbons. The yields of hydrocarbons consisting predominantly of low molecular weight paraffins, obtained at 295°C, were higher by more than an order of magnitude than those by the homogeneous radiolysis and comparable to those by catalytic reactions over Fischer-Tropsch catalyst. Results obtained by irradiation of H2 over silica gel pre-irradiated under CO--He gas stream indicate that secondary reactions between H2 and solid deposits produced from CO are responsible for the formation of hydrocarbons in the radiolysis of the CO--H2mixture over silica gel.

INTRODUCTION ELECTRON BEAM irradiation of gas mixtures of carbon monoxide and hydrogen produces a variety of products such as hydrocarbons, alcohols, aldehydes and acids. The yields of these products were determined as functions of composition of the reactant gas mixture, reaction temperature and pressure) j~ but we do not know the reaction conditions yet at which limited kinds of products are selectively produced. By use of the spin trapping technique, we confirmed the presence of two intermediates which are produced from the gas mixture by electron irradiation and are considered to play an important role in the radiation chemical reaction of the gas mixturefl ~In an attempt to see if the presence of solid catalyst could give selectivity to the reaction involving the intermediates including ones we detected, we initiated the study of the radiation effects on the gas mixture in the presence of Fischer-Tropsch catalyst over which catalytic reactions of the gas mixture have been studied most extensively. A preliminary report on this subject ~3~describes the presence of radiation effects on this catalytic reactions as reported by other authors, ~4"5~but later experiments t6~ on this system using a modified apparatus and electron beams of higher dose rates indicated that the product distribution can be explained on the basis of the primary products of catalytic and radiation chemical reactions and the secondary products of subsequent hydrogenation reactions of the primary products. Since the active RPCvol. 16,No.2.--F

catalyst such as Fischer-~Tropsch catalyst was found to be inappropriate for our purpose ~6~in that the products of catalytic reaction obscure those of the radiation chemical reaction, we have carried out the studies of the radiation effects on the gas mixture in the presence of silica gel which is considered to be inactive to the gas mixture at the temperatures where the irradiation experiments are going to be undertaken. There are several reports describing radiolysis in the adsorbed state, c7~which indicate that radiation effect is most prominent when molecules to be studied are adsorbed on insulators of large surface areas such as silica and alumina rather than semiconductors or metals. The enhancement in the radiolysis yield was accounted for by energy transfer from irradiated solid to adsorbed molecules, the efficiency of which correlates with the band gap of solid, c7~ Accordingly, it might be expected that the presence of silica gel would increase the product yields of the radiation chemical reaction of the CO-H2 mixture in addition to the possibility of giving selectivity to the reaction. We have found that low molecular weight hydrocarbons are efficiently produced over silica gel at the reaction temperature close to 300°C. In an attempt to interpret the high yield of hydrocarbons, studies have been carried out of radiolysis of CO not only in the presence but also in the absence of silica gel and of radiation effects on H2 over silica gel which had been irradiated under CO-He gas stream. 175

S. NAGAI et al.

176 .

. . . 500 ram . . . . . TI rod (30pro) window

0~0000000"~-0000000000

/

I

C0*H2 j r

Al,n,i-chro,,,,~

|

thermocouple

J

,

the gaschromatogram, but we did not succeed in obtaining the reliable yield. On increasing temperature of irradiation, the G-values of paraffins increase while those of oxygen containing products decrease. The details of the product yields are included in Table 2 for comparison with those by reactions over silica gel.

| CO *Hz " ~'=

* *product Ga,schro=natograph

FIG. I. Schematic o f flow reactor.

EXPERIMENTAL Figure I shows the flow reactor employed in this study. The reactor is made of stainless steel with an inner volume of 20 ml. A 30 ~m thick titanium foil window was attached at the top of the reactor, through which electron beams could penetrate into the reactor. The temperature of the reactor was controlled by an electric heater attached to the bottom of the reactor, and was recorded through three pairs of chromel-almel thermocouples on a multi-channel plotting recorder. Mixture of CO and H2 (1:6) was introduced at 100 ml/min to the reactor in which 2 g of silica gel were placed. Silica gel supplied by Mallinckrodt, 100 mesh was outgassed at 450°C in vacuo for several hours before use. The surface area determined by the BET method was 550 m2/g. Irradiation was carried out with electron beams of the accelerating energy of 600 keV and the beam current of 2mA. The dose rate determined by radiolysis of N20 with G(N2) = 10.0(mwas 5.96 x 106rad/s for the CO--H2 mixture at the flow rate of 100 ml/min. Product analysis was made by two gaschromatographs equipped with Porapak Q and Porapak N columns, respectively. Radiation-induced reactions over alumina (Nakarai Chemicals Co., 200 mesh) at 295°C were also studied for a comparison purpose, using the same apparatus and methods as those over silica gel. Radiolysis of CO not only in the presence but also in the absence of silica gel was carried out using the same equipments as described above. Gas mixture of CO and He (i:6) was continuously introduced into the reactor. The silica gel irradiated in the CO-He gas stream was further irradiated under H2 gas stream. The flow rate of H2 in this experiment was 100 ml/min.

2. Radiolysis of CO-H., (1:6) mixture in the presence of silica gel In the absence of radiation, only small amounts of methane, ethylene and propylene were detected in the gas stream of the C O - H : mixture passed over silica gel kept at 350°C, but were still lower than those produced by reactions of the gas mixture in the absence of silica gel. This finding indicates that silica gel exhibits no catalytic acitivity for the reactions of CO-H2 mixture without radiation. Results obtained by irradiation experiment over silica gel kept at 140°C are first discussed. The yields of hydrocarbons were determined by analysis of gaschromatograms recorded both during and after irradiation. The desorption after irradiation was made by passing Ar gas stream through the reactor at 350°C. The dominant products were low molecular weight hydrocarbons together with CO_, and H20. Oxygen containing products such as formaldehyde, methanol and acetaldehyde which have been found to be produced predominantly by irradiation of the gas mixture in the absence of solid catalyst were produced in quite low yields. Figure 3 shows the concentration of the dominant products in the effluent observed during irradiation of the gas

1.5

._,~~.0 R E S U L T S AND DISCUSSION 1. Radiolysis of C O - H , mixtures in the absence of

solid catalyst Before discussing reactions over silica gel, our results obtained by irradiation of CO-H2 mixtures in the absence of solid catalyst are mentioned briefly. Figure 2 shows the G-values of dominant products as a function of CO content in the gas mixture. The dominant products with G-values higher than 0.1 are formaldehyde, CO2, methane, acetic acid, methanol and acetaldehyde. In addition to these products, H20 was also detected in

O 05

~

10

044_

20

30

CH3COAOH

40

50

co CONTENT(mole %)

FIG. 2. Effect of CO content in CO-H2 mixtures on the G-values of the products by electron beam irradiation at 600 keV, 2 mA and at 92 ± 5=(2.

177

Radiation effects on CO-H2 gas mixture

,.oo

60

50 2(x1110)

E

?o 40

CH4

z

~_ 3o

-,i

~ 2o

0

30

60 TIME ( MIN )

90

120

8~ o ~ ~

• -

FJc. 3. Concentration of dominant products observed during irradiation of CO-H2 (1 : 6) mixture over silica gel at 140_+5°C as a function of time after initiating irradiation.

3'O

90

120

150

71ME(MIN)

mixture over silica gel at 140°C. It is seen that the concentration of COz which is produced preferentially at this relatively low temperature decreases gradually whereas the concentrations of methane, ethane and propane increase with time. The decrease in the CO2 concentration, which has also been observed in the radiolysis of CO over silica gel as described later, may he due to deposition of the radiolysis products on the silica gel surfaces. The difference in behavior between CO2 and hydrocarbons seen in Fig. 3, therefore, suggests that most of the CO2 produced are formed independently of hydrocarbons leaving carbon rich compound on silica gel.

FIG. 4. Concentration of dominant products as a function of time of Ar gas flow following irradiation of CO-H2 (! :6) mixture over silica gel at 140~. Figure 4 shows the concentration of C , - C 3 hydrocarbons desorbed by Ar flow in the effluent as a function of time after initiating Ar flow. By approximating the concentration change by the straight line for each product, we have estimated the amount of each hydrocarbon desorbed. Table 1 summarises the hydrocarbon yields obtained by 150min irradiation with electron beams of the CO-H2 mixture in the presence of silica gel at 140°C. The first and second columns indicate the amounts detected during and after

T A B L E I. HYDROCARBON YIELD~ ") BY IRRADIATION OF CO-H2 GEL AT 140°C

CH 4

Observed after

Total

B

Irrad.

irrad.

(B)

(A+B)

A+B

31.5

650.5

0.05

6.0

7.2

0.03

18.6

101.7

0.18

(A)

1.22

C2H 6

03.1

C3H 6

0.30

C3H 8

MIXTURE IN THE PRESENCE OF SIUCA

Observed during

619

C2H 4

(I : 6)

23.2

8.0

0.4

0.95

11.9

35.1

0.34

i8o-C4Hl0

3.00

1.5

4.5

0.33

I-C4H 8 + Iso-C4H 8

0

5.2

5.2

1.0

n-C4HI0

4.95

6.6

11.6

2-C4H8

0

1.1

1.1

1.0

0.57

iso-CsHl2

1.25

3.2

4.5

0.71

n-CsHI2

0.01

4.7

5.5

0.85

(a) Yields in l O ' 6 n o l e byelectron beam irradiation of 15~ reactant at the flow rate of i00 ml/min at 600 keV and 2 mA.

178

S. NAGAt et al.

irradiation, respectively, and the third column the total amounts. The hydrocarbons produced consist dominantly of low molecular weight paraffins, the yield of each paraffin decreasing with the increase in the carbon number. In the last column is shown the ratio of the amount desorbed after irradiation to the total amount. As may be seen, the ratio increases with the increase in the carbon number of the product and it is higher for olefins than for paraffins. These trends are in good agreement with those ~gJof selective adsorption of hydrocarbons on unirradiated silica gel. This fact indicates that the hydrocarbons desorbed after irradiation were not produced from any surface intermediates in the desorption process but had been already produced and adsorbed on silica gel during irradiation. Much higher yields of hydrocarbons were obtained when irradiation was carried out over silica gel at 295°C. In contrast to the reactions at 140"C described above, the concentrations of all the products detected during irradiation were nearly independent of time after initiation of the irradiation up to 150 rain. Since desorption by Ar flow after irradiation yields hydrocarbons in concentrations lower than 0.1 of those detected during irradiation for all C~ ~ C~ hydrocarbons, the yields at 295"C were determined only by the product analysis during irradiation ignoring the amounts of hydrocarbons remained on the solid surfaces. The product yields in 10-6 tool produced by irradiation at 295°C of 101 reactant gas are shown in Table 2 together with those over silica gel at 140"C and those in the homogeneous radiolysis.

It may be seen from Table 2 that the yield of each hydrocarbon over silica gel is more than an order of magnitude higher than that in the homogeneous radiolysis while the yield of each oxygen containing product is much lower in the reaction over silica gel. Since oxygen containing compounds are, in general, adsorbed on silica gel more strongly than hydrocarbons, the low yield of the oxygen containing products may be accounted for by their adsorption followed by decomposition over the solid surfaces. Evidence supporting this view may be found, for example, in the formation of ethane and ethylene by y-irradiation of ethanol adsorbed on silica. "°~ The high yield of hydrocarbons is accompanied with the high yield of CO2 as may be seen by comparison of the data in the homogeneous radiolysis and those by reactions over silica gel in Table 2. This provides evidence that CO2 is indeed one of the by-products in the radiation-induced reactions of CO--H2 mixture producing hydrocarbons. However, over silica gel the amount of CO2 produced is higher when the temperature of reaction is lower, contrary to the amounts of hydrocarbons produced. That is, a high yield of CO2 does not necessarily mean a high yield of hydrocarbons. It will be shown below that this fact is consistent with the mechanism proposed for the formation of hydrocarbons. Table 3 shows a comparison of products obtained by reactions of CO--H2 mixtures. The hydrocarbon yield over silica gel is much higher than the sum of the yields of hydrocarbons and

T A B L E 2. PRODUCTYIELDS(a) BYIRRADIA~ON OF C O - H 2 ( ! : 6 ) MIXTURE Solid

Temp.

Silica gel (~5°C}

92

CH 4 C2H 4 C2H 6 C3H 6 C3H 8

27.8 0.33 2.5 0.07 0.94

HCHO CH3OH CH3CHO HCOOCH3(+iso-C4HI0) C2HsOH (+n-C4HI0) CH3COOH

117.4 21.2 9.2 1.5 2.3 17.9

171

297

29.4 0.25 3.6 0.06 1.8

50.2 0.16 4.9 0.04 2.1

434 4.8 68 5.6 23

62.2 7.4 7.9 1.8

16.9 3.2 3.4 2.2

trace 3.5 2.2

iso-C4Hl0 1-C4H8 (+iso-C4H 8) n-C4HI0 2-C4H8 CO 2

0.48 61.5

(a) Yields

31.8

19.1

140

3.0 3.5 7.7 0.73 3920

Silica gel 295 1635 15 222 15 120 3.0 4.0

60 11 31 2120

in 10 -6 mol produced by irradiation of IOE reactant at the flow rate of i00 ml/min

at 600 keV and 2 .mA.

Radiation effects on CO-H: gas mixture

179

TABLE 3. COMPARISON OF PRODUCTS BY REACTIONS OF C O - H 2 MIXTURES H210~ r a t i o

6

6

6

6

Solid ~mction T~p.

('C)

¥iaXd (a) T o t a l hydrocarbons Total organic oxygenates Mole ratio

C2HA/C2H6 C3H6/C3H8 ZCAH8/EC4HIo

Irrad. 92

Irrad. 171

Irrad.

Silica gel

Irrad.

297

6

6

Ahmina

Irrad.

297

5

S i l i c a gel

F-T catalyst

Irrad.

Thenml

295

140

214 - 312

32

35

58

500

275

1055

170

80

26

5

6

7

0.17 0.23 0.21

0.07

0,07

0.44

0.24

0.13

1.86

0.40

0.12

0.48

0.65

0.39

1.19

0.82

1.9h 2.55

0.13 0.07

0.07 0.03

0.03 0.02

iso-C4~10/n-C4Hl0

tso-CsH12/n-CsH12

890

(a) In 10-6moI p e r 10¢ r e a c t a n t (per gram of s o l l d ) .

oxygen containing products in the absence of solid catalyst. Therefore, it is not likely that most of the hydrocarbons produced over silica gel would be formed by decomposition of organic oxygen containing products produced in gas phase. As described later, a considerable part of the high yield of hydrocarbons may be explained by radiation-induced reactions between H2 and solid deposit produced by radiolysis of CO. One of the characteristics in the reaction producing hydrocarbons by radiation over silica gel is the low yield of olefins as seen from the values of mole ratio, olefin/parafl~n in Table 3. Similar values were obtained in the presence of alumina. In addition, we have found ~6~ that the olefin/parafi~n ratio decreased markedly when COH2 mixture was irradiated in the presence of Fischer-Tropsch catalyst which in the absence of radiation, is an excellent catalyst producing olefins in high yield as may be seen from Table 3. Accordingly, the observed low values of the mole ratio may be ascribed to hydrogenation reaction of olefins over the solid surfaces. Another point to be noted in the reaction over silica gel is the high yield of isobutane and isopentane. This finding parallels the predominant formation of branched hydrocarbons observed in the radiolysis of pentane adsorbed on silica gel."" It has already been found that radiation generates acidity of silica gel"2~ which is known to play an important role in the isomerisation reaction of hydrocarbons.

3. Radiolysis of CO in the absence and presence of silica gel The concentration of CO and CO2 in the radiolysis of CO-He (1:6) mixture was monitored periodically by the gaschromatogram of the effluent. Before irradiation, no CO2 was detected in the efltuent irrespective of the presence or absence of silica gel in the reactor. In the absence of silica gel, the concentration of CO~ in the effluent during irradiation of the CO-He mixture was found to decrease slightly with time after initiating irradiation, the data obtained at radiolysis temperature of 300°C being shown in the curve A in Fig. 5. The G-value of CO2 estimated

100C

400 ~ C

B

c.)

30

60 TIME(MIN)

90

120

150

FIG. 5. Concentration of CO2 produced by irradiation of CO-He 0:6) gas mixture as a function of time after initiating irradiation. (A) homogeneous radiolysis, (B) radiolysis over silica Eel at 23YC, (C) radiolysis over silica Eel at 300"C.

180

S. NAGAI et al.

from the concentration lies in the range from 1.71 to 1.39, which are in reasonable agreement with the reported values: TM Decreasing the radiolysis temperature to 210°C results in a slight decrease in G(CO:) to 1.39 _+0.09. Irradiation of the same CO-He gas mixture over silica gel produces CO2 in a yield much higher than that of the homogeneous radiolysis stated above. The curves B and C in Fig. 5 show the change in the concentration of CO: in the effluent when the CO-He mixture was irradiated over silica gel at 235 and 300°C, respectively, as a function of time after initiating irradiation. As may be seen, the concentration of CO: at both temperatures rapidly decreases at an early stage of irradiation and approaches gradually to the steady-state value. It is noted that the change in the CO: concentration with time resembles that shown in Fig. 3. This fact supports the suggestion described in Section 2 that most of the CO: produced by irradiation of the CO-H2 mixture over silica gel at 140°C are independent of the formation of hydrocarbons. The silica gel after the radiolysis of CO changed in color to dark brown, due to the formation of some reaction product on the surfaces, which may explain the initial decrease in the CO: concentration seen in the curves B and C in Fig. 5.

4. Irradiation of H~ over silica gel pre-treated in CO-He stream When H, gas stream at a flow rate of 100 milmin was passed over silica gel at 300°C which had been irradiated in the CO--He gas stream at the same temperature for 150rain, a small amount of methane (~ 10-6 molll) was detected in the effluent in addition to a trace amount (< 10-Tmol/l) of ethylene and ethane. The concentration of these hydrocarbons decreased rapidly with time of the H: flow. Irradiation of the H2 stream over the silica gel at 300°C results in the formation of hydrocarbons consisting dominantly of low molecular weight paraffins in high concentrations. A small amount (< 104 molll) of acetaldehyde was also detected in the effluent but neither CCh nor H20 was produced in a detectable yield. The concentration of each hydrocarbon in the effluent decreased rapidly with time after initiating irradiation, as shown in Fig. 6 for C~ ~C3 paraffins. The approximate yields of the paraffins estimated from the curves in Fig. 6 are 330 x 10-6 moi for methane, 160 x 10-6 tool for ethane and 91 x 10-6 mol for propane. By comparing with the data in the last column in Table 2, it may be seen that these values may account for 0.13 for methane, and 0.5 for ethane and propane,

F-vo z o

IO0

\

o CH4

\

= C2H6

E z

30

60

90

TIME(MIN)

FIG. 6. Concentration of dominant products by irradiation of H~ stream at 301~C over silica gel preirradiated under CO-He gas stream.

respectively, of the yields obtained by irradiation of the CO-H2 mixture over silica gel at 295°(] for 150 min. For other hydrocarbons produced, it was not possible to follow the change in concentration with time but the firstsampling at 6min after initiating irradiation gave the following values in 10-6 mol/I: C2H4(1.5), C31-16(1.2),iso-C4H,o(16.2), n-C4H1o(8.3), iso-CsHt2(7.7), and n-CsH,2(l.8). It is noteworthy that the concentrations of iso-Cd-l,oand iso-CsH,2 are higher than those of the corresponding straight-chain hydrocarbons, which is one of the characteristics of the products by irradiation of C O - H 2 over silicagel as already mentioned. A similar experiment was carried out using silica gel pre-irradiated in the C O - H e stream at 235°(] for 150rain. Irradiation of H2 stream at 2350(] produced small amounts of methane and higher hydrocarbons, but on raising the irradiation temperature to 300"C the concentration of each hydrocarbon in the effluent increased to reach the value close to that observed over the silica gel pre-irradiated at 300"C. This result is consistent with the increasing yield of hydrocarbons at higher temperature in the reaction of C O - H 2 over silica gel.

5. Mechanism for the formation of hydrocarbons by irradiation of CO--H, mixture over silica gel From the observations and discussion described in the preceding section, it is evident that radiation-induced reactions between H2 and some surface species remained after radiolysis of CO play an important role in the formation of hydrocarbons, at least hydrocarbons higher than methane, by irradiation of the CO-H2 mixture in the presence of silica gel. In order to examine the possibility that adsorbed CO would be the surface species in question,

Radiation effects on CO-H, gas mixture

181

analysis was carried out of the products by irradi- obtained in the present study, energy transfer may ation of H2 stream over silica gel which had been play a role in the radiation chemical reactions of treated by C O - H e (1:6) gas stream at 300°C, CO over silica gel which produce CO2 and the surface species and/or in the secondary reactions without irradiation, for more than 1 hr. Methane, ethane and propane were observed to be produced between H2 and the surface species. in small amounts (<3 x 10-~ mol/i) in addition to the by-product, H20. CO2 was not detected, probably due to low adsorption amount of CO at this Acknowledgements--The authors thank Profs. I. Sakurada, temperature. These results indicate that radiation- I. Nitta and S. Ohnishi for their encouragement during induced reactions between H2 and adsorbed CO this study. Thanks are also due to Mr. K. Matsuda for help in experiments. indeed produce hydrocarbons, but they are not responsible for the high yield of hydrocarbons observed. REFERENCES It has been well established that radiolysis of I. Ann. Rep. Osaka Lab. Radial. Chem. 1975, JAERICO produces CO2 and a carbonaceous solid. ~'4~ M 6260, 4; 1976, JAERI-M 6702, 4; 1977, JAERI-M According to the studies of proton radiolysis of 7355, 4; JAERI-M 7949, 4. CO, ~1~ the yields of CO2 and the solid of an 2. S. NAGAI, K. MATSUDAand M. HATADA, J. phys. Chem. 1978, g2, 322. empirical composition (CxO,), are nearly identical 3. M. HATADAand K. MATSUDA,Radial. Phys. Chem. at the dose rate of 5 x 1017eV/ml.s, close to that 1977, le, 195. employed in this study, and at temperatures of 200 4. E. J. GIBSON,R. W. CLARKE,T. A. DOE~JNGand D. and 300°C. Therefore, it is reasonable to assume POPE, Int. Conf. on Peaceful Use of Atomic Energy that the same solid was formed by radiolysis of 1958, 29, 312. 5. H. PICNLERand B. F3RNHARER,Brennst.-Chem. 1963, CO over silica gel in the yield comparable to that 44, 33. of CO2. On this basis, the carbonaceous solid 6. S. NAGAI, K. MATSUDA,H. ARAI and M. HATADA, deposited on the silica gel surfaces would most JAERI-M, 1978, 7875, 53. likely be the surface species which reacts with H2 7. J. G. RACE,B. RAREand A. O. ALLEN,J. phys. Chem. under irradiation to produce hydrocarbons. 1966,70, 1098, G. M. ZHABROVA, V. I.VLADIMIROVA, B. KADENATSI, V. B. KAZANSKII and G. B. PARHSKII, J. The fir,ding that methane was also produced by Catal. 1966, 6, 41 I. thermal reaction between H2 and the surface spe8. F. T. JONESand T. J. SWORSKI,J. phys. Chem. 1966, cies reminds us of the methanation reaction of CO 70, 1546. over various solid catalysts. ~'6~ According to the 9. D. M. YOUNGand A. D. CROWELL,in Physical Adsorption of Gasses, Chap. I I. Butterworth, London recent studies of the reaction over Ni, ''6~ carbon 1962. produced by dissociation or disproportionation of 10. L. ADAMSand A. O. ALLEN,J. phys. Chem. 1969, 73, CO reacts with H2 to produce methane. By 2741. analogy, the surface species by radiolysis of CO I 1. J. W. SUTtlERLANDand A. O. ALLEN,J. amer. chem. Sac. 1961, 83, 1040. over silica gel may also contain carbon atoms which are active for reaction with H2 to produce 12. C. BARTImand C. D. WAGNER,J. phys. Chem. 1964, 68, 2381. mainly methane. 13. A. R. ANDERSON,in Fundamental Process in Radiation It is likely that the high yield of hydrocarbons in Chemistry (Edited by P. Ausloos), Chap. 5. Inter. science, New York, 1968. the radiolysis of the CO-H2 mixture over silica gel would be at least partly due to efficient energy 14. S. C. LIND, in Radial. Chem. of Gases, Chap. 6. Reinhold, New York 1961. transfer from the irradiated solid to adsorbed 15. A. R. ANDERSON,J. V. F. VESTand M. J. WILLLrr, molecules, which has often been invoked to Trans. Faraday Sac. 1966, 62, 595. explain enhancement in the radiolysis yield in the 16. R. L. PALMERand D. A. VROOM,J. Catal. 1977, 50, 244 and references therein. adsorbed state. ~7~Although no direct evidence was