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Hydrogenation of segregated carbon and adsorbed acetylene on iron
Volume
72, number
CHEhIICAL
1
HYDROGENATION
OF SEGREGATED
PHYSICS
CARBON
AND ADSORBED
H-P. BONZEL, H J. KREBS and W. SCHWARTING Instrhtrfir GrenzjEchenforschung und Vakuumphyak. KemforschungsanIage O-51 70 Jiihch. Recewed
15 hlay 1980
LETTERS
ACETYLENE
ON IRON
Jiihch GmbH.
West Getmony
4 March 1980
Monolayers with segregated carbon and/or adsorbed acetylene on iron fork were prepared m vacuum and characterized by theu C Is XPS spectra. The subsequent hydrogenation of these layers m I bar Hz at 500-560 K produced substantial amounts of hrgher molecular weight hydrocarbons, up to butane, m support of a cham growth mechanism not mvolvmg molecular CO
The recent intensification of research m the area of heterogeneous methanation and Fischer-Tropsch synthesis has led to rmportant new informatron with regard to the mechamsm of these reactrons [l-3] _ In partrcular, the finding of dissocrative CO chemrsorptron on such metals as Ni [4], Co [5], Fe [6] and Ru [7] has lent support to the old Idea that atomrc or carbrde-hke carbon should be necessary for the imtratron of methane and hrgher molecular weight hydrocarbon formatron [8] . A number of studies have in fact shown that atomic carbon produced by CO dissocration is readily hydrogenated to methane [4,9-141 m a pure Hz atmosphere. It seems reasonable therefore to state that the initiatron step of methanation and Fischer-Tropsch synthesis consists of the dissociation of CO and the partial hydrogenation of atomic carbon to CH, adsorbed mtermediates (x < 3). The controversial issue III the mechanistic discussron of Fischer-Tropsch synthesis is the chain propagation step [1,2 3 . Here it is not clear whether chain growth occurs predominantly by CH, polymerisation (or CH, “insertion”) or by the insertion of molecularly adsorbed CO into the hydrocarbon chain. There may be other possible mechamsms for chain growth but we restrict ourselves to mentioning these two. The current discussron seems to favor the CO insertron mechanism [2] although considerable experimental evidence for CHx insertion or addition has
also been published ClO,lS] . The present experiments were aimed at testing the CH, polymerisation model. For this purpose clean polycrystalline Iron foils were covered wrth atomic (carbrdic) carbon, adsorbed acetylene or a mixture of both and then hydrogenated in 1 bar H-, at 500-560 K. These reactions produced substantial amounts of higher molecular weight alkanes, up to butane. The highest selectrvities for C,-C4 productron were observed from mrxed carbidic and CzH2 layers whrch were hydrogenated at approxrmately 500 K. The experiments were performed in a combination apparatus featuring an ultrahigh vacuum (UHV) chamber with X-ray photoemission (XPS) and Auger electron spectroscopies (AES), and an attached catalytic microreactor which was run at 1 bar Hz pressure [ 161. The sample was a high purity Fe foil. 6 X 8 mm2 and 0.1 mm thick. The foil was heated resistively and its temperature measured by a chromel-alumel thermocouple pressed against the underside of the foil. As described in detail elsewhere [16], the Fe forl could be transferred between the UH.V chamber and catalytic reactor via a valveless, differentially pumped air-lock system (Leybold-Heraeus). An experimental run was carried out as follows. The Fe foil was cleaned by Ar sputtering and heating in the UHV chamber. It was then covered with a carbonaceous deposit by either segregating carbon from the bulk of the fod or by exposure to C,H, (5 X 10e7 165
Volume
72, number
1
CHEMICAL
mbar for 200 s) at room temperature. A thud preparation procedure was the combination of segregation and C2Hz adsorption. The carbon deposit was then analyzed by XPS in terms of the C Is spectrum. Next the foil was transferred to the cataliytrc reactor and heated to the reaction temperature. The reaction products were analyzed by gas chromatography (CC) utilizing a flame romzatron detector. The first CC was taken after 15 s The punty of the Hz gas was checked by CC molecular sieve column and thermal conductrvrty detection. The amount of CO present in the gas was below the detection limit, i e. < 1 ppm. Frg I shows the C 1s XPS data for carbon segregated Fe surfaces before and after a hydrogenation run. Spectrum (a) was obtamed after the for1 was sputtered and heated in UHV at about 650 K for 1 min. There are two peaks in thus spectrum at C Is bindmg energies of 283.3 eV and 284 8 eV corresponding to atomic (or carbldlc) and Sraphlttc car-
290
2%
286 28L Ea (evl
282
280
276
Fig. 1. C Is photoelectron spectra of carbonaceous layers on lzon taken wrth fiw = 1253 6 cV (a) Fe foil was heated m UHV to 650 K for 1 mm. (b) After hydrogenation of (a) m 1 bar HZ at 560 K for 10 mm (c) Fe foil was exposed to 5 x 10-7 mbar C2 Hz for 200 s at 290 K. (d) Fe foil was heated in UHV to 1000 K for 1 mm and then exposed to 5 x 10-7 mbar CzHo for 200 sat 290 K (e) After hydrogenation of(d) m 1 bar Hz at 500 K for 28 mm.
166
PHYSICS
LETTERS
15 hlay 1980
bon, respectrvely [ I3]_ After hydrogenation at 560 K for 10 mm, spectrum (b), all of the atomic carbon is removed and only the graphxtic peak at about 285.1 eV is left. The hydrogenation products observed during the mitral reactron penod were methane, ethane and propane, with selectivities of approximately 95.7,4.0 and 0.2 mol%, respectively. Sirmlar results were obtained with samples where the carbon was segregated at hrgh temperature (=lOOO K). Spectrum (c) m fig. 1 was measured for Fe foils whrch were exposed to C2H, at room temperature in UHV. The C 1s bmding energy of 283.9 eV indrcates that the CzH, is adsorbed in predominantly moIecuIar form [ 13,173 _This peak is nearly completely removed during subsequent hydrogenatron at 560 K. The initral product selectrvitres are now even higher than before for the h&her moIecular werght specres: 67.7,30.4 and 1.9 mol% for methane, ethane and propane, respectively. Some butane could also be detected in the CC but was not Integrated by the data processor. The highest selectivity for the higher molecular weight products (C3 and C4) was measured durmg the hydrogenatron of mixed layers. The C Is spectrum fd) in fig. 1 was recorded for a Fe surface which was first heated to --~I000 K for 1 mm in order to achieve the maxrmum carbon segregation, and then exposed to 10e4 mbar s of C2H2 at room temperature. The binding energy is 283.8 eV. Thrs fad was then hydrogenated at =500 K yielding 83.1,12.6, 3.5 and 0.8 mol% methane, ethane, propane and butane, respectrvely. As usual, this product distribution was observed in the CC taken after 15 s of reaching the reaction temperature. The C Is spectrum after 28 min of hydrogenation IS shown as trace (e) in fig. 1. A small peak typical for graphitrc carbon can be noted [13]_ The present results summa~zed in table 1 ilfustrate that Cz+ hydrocarbons can be formed from atomrc carbon on Fe and Hz in the gas phase, and originate from a mixsimliarly, that C,+ h y drocarbons ture of atomrc carbon and adsorbed CzH2 heated m H,. We conclude therefore that adsorbed molecular CO on Fe is not necessu~ for chain growth of hydrocarbons in Fischer-Tropsch related reactions. Of course, this statement does not rule out the CO insertion mechanrsm in the presence of adsorbed CO. It is stffl possible that both mechanisms - CH, po-
Volume 72, number 1
Table 1 Dlstnbutlon of products (III mol%) observed during the hydrogenation Deposrt
CZ
c3
(a)
95 7 67.7 83 1
40 30.4 17-6
0.2 19 3.5
(c)
(d)
with recent experimental
re-
the CH, polymerization model [10,12,15]. For example, the HZ pulsmg experiments with mixed carbon and CO layers on Co, NI and Ru catalysts by Rabo et al. [IO], carried out at room temperature, produced large relative amounts of alkanes (up to butane). In contrast to these mixed layers, adsorbed molecular CO subjected to HZ pulses at low temperature produced no hydrocarbons [IO] _ These results are similar to our observations, the only difference being that the carbonaceous layers prepared m the present experiments did not contam any CO because
layers
Cl
lymensatlon and CO msertlon - may be operative in CO-H, reactlons but thus far no posltlve evidence provmg the existence of CO insertion has been produced. On the other hand, our present fimdmg that adsorbed atomic carbon and/or adsorbed acetylene on Fe irutlates and propagates hydrocarbon cham
molecular
of carbonaceous
Fig. 1
spectrum
scgrcgated C adsorbed CzH2 segregated C + C2 H2
growth, 1s m agreement sults that are supporting
15 May L9SO
CHEMICAL PHYSICS LETTERS
the surface was not exposed
to CO gas. Therefore the objection that the slmultaneous presence of atomic carbon and adsorbed molecular CO might be necessary for cham growth to occur does not have any weight m our case. The product dlstnbution and hence selectiwties in the present hydrogenation experiments were a function of tune. The reasons for this time dependence are obvious: firstly, only a fimte amount of carbon was avarlable for hydrogenation, and secondly, the amount of carbon that can be hydrogenated decreased with time, thus makmg the formation of Cz+ products by polymerization more and more improbable. An important question in this context is: at which stage of the reaction are the C,, mtermedlates on the surface being formed? Are they already present m the preparative stage of the surface layer or do they form during the hydrogenation? This question cannot be answered unambiguously now, but one experimental observation may be useful for elucidatmg this problem. In this experiment, C2H2
c4
was adsorbed onto the carbon segregated Fe surface at elevated temperature (=&SO K). The corresponding C Is spectrum showed a binding energy at 284.1 eV indrcatmg the partial decomposition of C,Hz. The subsequent hydrogenation of this layer yielded less ethane and propane than the other experiments where C2Hz had been adsorbed at room temperature. Tha result seems to imply that most intermediates for the formation of C,, products are formed in the preparative stage of the carbonaceous layer. However, more detailed results are needed in order to answer this question more rigorously_
References [ 11 R.W. Joyner, J. Catal. 50 (1977)
176.
Ponec, Catal. Rev. Sci. Eng. 18 (1978) 151. [31 R J. hladon. J. Catal. 60 (1979) 485. (41 P R. Wcntrcek. BJ. Wood and H. Wii. J. Catal. 43 (1976) 363. and refelences therem. ISI K.A. Prior, K. Schwaha and R.M. Lambert, Surface Sci. 77 (1978) 193. [61 K. Klshi and hf.W. Roberts, J. Chcm. Sot. Faraday Trans 171 (1975) 1715. K J. Smgh and H.E. Grenga, J. CataL 47 (1977) 328. F. rlscher and H. Tropsch. BrennstofChem. 7 (1926) 97 PI hf Arakl and V. Ponec, J. Catal. 44 (1976) 439. 1101 J A Rabo. A-P. Rlsch and M-L. Poutsma. J. Catal. 53 (1978) 295. [I11 D J Dwyer and G A. Somoqal, J. Catal. 52 (1978) 291 1121 J.G. Ekerdt and A T. Bell, J. Catal. 58 (1979) 170. 499 1131 H P. Bonzel and HJ. Krebs, Surface Sci. 91(1980) 1141 D W. Goodman, R.D. Kelley. T-E. Madcy and J.M. White, 1. Catal., to be published. [I51 P. Biloen. J-PI. HeUe and W-M-H. Sachtler. J. Catal. 58 (1979) 95. [WI H J. Krebs. H-P. Bonzel and G. Garner, Surface Sci. 88 (1979) 269. [ 17 1 G. Brodin, G. Gafner and H P. Bonzel, Appl. Phys. 13 (1977) 333.
PI V.
167
72, number
CHEhIICAL
1
HYDROGENATION
OF SEGREGATED
PHYSICS
CARBON
AND ADSORBED
H-P. BONZEL, H J. KREBS and W. SCHWARTING Instrhtrfir GrenzjEchenforschung und Vakuumphyak. KemforschungsanIage O-51 70 Jiihch. Recewed
15 hlay 1980
LETTERS
ACETYLENE
ON IRON
Jiihch GmbH.
West Getmony
4 March 1980
Monolayers with segregated carbon and/or adsorbed acetylene on iron fork were prepared m vacuum and characterized by theu C Is XPS spectra. The subsequent hydrogenation of these layers m I bar Hz at 500-560 K produced substantial amounts of hrgher molecular weight hydrocarbons, up to butane, m support of a cham growth mechanism not mvolvmg molecular CO
The recent intensification of research m the area of heterogeneous methanation and Fischer-Tropsch synthesis has led to rmportant new informatron with regard to the mechamsm of these reactrons [l-3] _ In partrcular, the finding of dissocrative CO chemrsorptron on such metals as Ni [4], Co [5], Fe [6] and Ru [7] has lent support to the old Idea that atomrc or carbrde-hke carbon should be necessary for the imtratron of methane and hrgher molecular weight hydrocarbon formatron [8] . A number of studies have in fact shown that atomic carbon produced by CO dissocration is readily hydrogenated to methane [4,9-141 m a pure Hz atmosphere. It seems reasonable therefore to state that the initiatron step of methanation and Fischer-Tropsch synthesis consists of the dissociation of CO and the partial hydrogenation of atomic carbon to CH, adsorbed mtermediates (x < 3). The controversial issue III the mechanistic discussron of Fischer-Tropsch synthesis is the chain propagation step [1,2 3 . Here it is not clear whether chain growth occurs predominantly by CH, polymerisation (or CH, “insertion”) or by the insertion of molecularly adsorbed CO into the hydrocarbon chain. There may be other possible mechamsms for chain growth but we restrict ourselves to mentioning these two. The current discussron seems to favor the CO insertron mechanism [2] although considerable experimental evidence for CHx insertion or addition has
also been published ClO,lS] . The present experiments were aimed at testing the CH, polymerisation model. For this purpose clean polycrystalline Iron foils were covered wrth atomic (carbrdic) carbon, adsorbed acetylene or a mixture of both and then hydrogenated in 1 bar H-, at 500-560 K. These reactions produced substantial amounts of higher molecular weight alkanes, up to butane. The highest selectrvities for C,-C4 productron were observed from mrxed carbidic and CzH2 layers whrch were hydrogenated at approxrmately 500 K. The experiments were performed in a combination apparatus featuring an ultrahigh vacuum (UHV) chamber with X-ray photoemission (XPS) and Auger electron spectroscopies (AES), and an attached catalytic microreactor which was run at 1 bar Hz pressure [ 161. The sample was a high purity Fe foil. 6 X 8 mm2 and 0.1 mm thick. The foil was heated resistively and its temperature measured by a chromel-alumel thermocouple pressed against the underside of the foil. As described in detail elsewhere [16], the Fe forl could be transferred between the UH.V chamber and catalytic reactor via a valveless, differentially pumped air-lock system (Leybold-Heraeus). An experimental run was carried out as follows. The Fe foil was cleaned by Ar sputtering and heating in the UHV chamber. It was then covered with a carbonaceous deposit by either segregating carbon from the bulk of the fod or by exposure to C,H, (5 X 10e7 165
Volume
72, number
1
CHEMICAL
mbar for 200 s) at room temperature. A thud preparation procedure was the combination of segregation and C2Hz adsorption. The carbon deposit was then analyzed by XPS in terms of the C Is spectrum. Next the foil was transferred to the cataliytrc reactor and heated to the reaction temperature. The reaction products were analyzed by gas chromatography (CC) utilizing a flame romzatron detector. The first CC was taken after 15 s The punty of the Hz gas was checked by CC molecular sieve column and thermal conductrvrty detection. The amount of CO present in the gas was below the detection limit, i e. < 1 ppm. Frg I shows the C 1s XPS data for carbon segregated Fe surfaces before and after a hydrogenation run. Spectrum (a) was obtamed after the for1 was sputtered and heated in UHV at about 650 K for 1 min. There are two peaks in thus spectrum at C Is bindmg energies of 283.3 eV and 284 8 eV corresponding to atomic (or carbldlc) and Sraphlttc car-
290
2%
286 28L Ea (evl
282
280
276
Fig. 1. C Is photoelectron spectra of carbonaceous layers on lzon taken wrth fiw = 1253 6 cV (a) Fe foil was heated m UHV to 650 K for 1 mm. (b) After hydrogenation of (a) m 1 bar HZ at 560 K for 10 mm (c) Fe foil was exposed to 5 x 10-7 mbar C2 Hz for 200 s at 290 K. (d) Fe foil was heated in UHV to 1000 K for 1 mm and then exposed to 5 x 10-7 mbar CzHo for 200 sat 290 K (e) After hydrogenation of(d) m 1 bar Hz at 500 K for 28 mm.
166
PHYSICS
LETTERS
15 hlay 1980
bon, respectrvely [ I3]_ After hydrogenation at 560 K for 10 mm, spectrum (b), all of the atomic carbon is removed and only the graphxtic peak at about 285.1 eV is left. The hydrogenation products observed during the mitral reactron penod were methane, ethane and propane, with selectivities of approximately 95.7,4.0 and 0.2 mol%, respectively. Sirmlar results were obtained with samples where the carbon was segregated at hrgh temperature (=lOOO K). Spectrum (c) m fig. 1 was measured for Fe foils whrch were exposed to C2H, at room temperature in UHV. The C 1s bmding energy of 283.9 eV indrcates that the CzH, is adsorbed in predominantly moIecuIar form [ 13,173 _This peak is nearly completely removed during subsequent hydrogenatron at 560 K. The initral product selectrvitres are now even higher than before for the h&her moIecular werght specres: 67.7,30.4 and 1.9 mol% for methane, ethane and propane, respectively. Some butane could also be detected in the CC but was not Integrated by the data processor. The highest selectivity for the higher molecular weight products (C3 and C4) was measured durmg the hydrogenatron of mixed layers. The C Is spectrum fd) in fig. 1 was recorded for a Fe surface which was first heated to --~I000 K for 1 mm in order to achieve the maxrmum carbon segregation, and then exposed to 10e4 mbar s of C2H2 at room temperature. The binding energy is 283.8 eV. Thrs fad was then hydrogenated at =500 K yielding 83.1,12.6, 3.5 and 0.8 mol% methane, ethane, propane and butane, respectrvely. As usual, this product distribution was observed in the CC taken after 15 s of reaching the reaction temperature. The C Is spectrum after 28 min of hydrogenation IS shown as trace (e) in fig. 1. A small peak typical for graphitrc carbon can be noted [13]_ The present results summa~zed in table 1 ilfustrate that Cz+ hydrocarbons can be formed from atomrc carbon on Fe and Hz in the gas phase, and originate from a mixsimliarly, that C,+ h y drocarbons ture of atomrc carbon and adsorbed CzH2 heated m H,. We conclude therefore that adsorbed molecular CO on Fe is not necessu~ for chain growth of hydrocarbons in Fischer-Tropsch related reactions. Of course, this statement does not rule out the CO insertion mechanrsm in the presence of adsorbed CO. It is stffl possible that both mechanisms - CH, po-
Volume 72, number 1
Table 1 Dlstnbutlon of products (III mol%) observed during the hydrogenation Deposrt
CZ
c3
(a)
95 7 67.7 83 1
40 30.4 17-6
0.2 19 3.5
(c)
(d)
with recent experimental
re-
the CH, polymerization model [10,12,15]. For example, the HZ pulsmg experiments with mixed carbon and CO layers on Co, NI and Ru catalysts by Rabo et al. [IO], carried out at room temperature, produced large relative amounts of alkanes (up to butane). In contrast to these mixed layers, adsorbed molecular CO subjected to HZ pulses at low temperature produced no hydrocarbons [IO] _ These results are similar to our observations, the only difference being that the carbonaceous layers prepared m the present experiments did not contam any CO because
layers
Cl
lymensatlon and CO msertlon - may be operative in CO-H, reactlons but thus far no posltlve evidence provmg the existence of CO insertion has been produced. On the other hand, our present fimdmg that adsorbed atomic carbon and/or adsorbed acetylene on Fe irutlates and propagates hydrocarbon cham
molecular
of carbonaceous
Fig. 1
spectrum
scgrcgated C adsorbed CzH2 segregated C + C2 H2
growth, 1s m agreement sults that are supporting
15 May L9SO
CHEMICAL PHYSICS LETTERS
the surface was not exposed
to CO gas. Therefore the objection that the slmultaneous presence of atomic carbon and adsorbed molecular CO might be necessary for cham growth to occur does not have any weight m our case. The product dlstnbution and hence selectiwties in the present hydrogenation experiments were a function of tune. The reasons for this time dependence are obvious: firstly, only a fimte amount of carbon was avarlable for hydrogenation, and secondly, the amount of carbon that can be hydrogenated decreased with time, thus makmg the formation of Cz+ products by polymerization more and more improbable. An important question in this context is: at which stage of the reaction are the C,, mtermedlates on the surface being formed? Are they already present m the preparative stage of the surface layer or do they form during the hydrogenation? This question cannot be answered unambiguously now, but one experimental observation may be useful for elucidatmg this problem. In this experiment, C2H2
c4
was adsorbed onto the carbon segregated Fe surface at elevated temperature (=&SO K). The corresponding C Is spectrum showed a binding energy at 284.1 eV indrcatmg the partial decomposition of C,Hz. The subsequent hydrogenation of this layer yielded less ethane and propane than the other experiments where C2Hz had been adsorbed at room temperature. Tha result seems to imply that most intermediates for the formation of C,, products are formed in the preparative stage of the carbonaceous layer. However, more detailed results are needed in order to answer this question more rigorously_
References [ 11 R.W. Joyner, J. Catal. 50 (1977)
176.
Ponec, Catal. Rev. Sci. Eng. 18 (1978) 151. [31 R J. hladon. J. Catal. 60 (1979) 485. (41 P R. Wcntrcek. BJ. Wood and H. Wii. J. Catal. 43 (1976) 363. and refelences therem. ISI K.A. Prior, K. Schwaha and R.M. Lambert, Surface Sci. 77 (1978) 193. [61 K. Klshi and hf.W. Roberts, J. Chcm. Sot. Faraday Trans 171 (1975) 1715. K J. Smgh and H.E. Grenga, J. CataL 47 (1977) 328. F. rlscher and H. Tropsch. BrennstofChem. 7 (1926) 97 PI hf Arakl and V. Ponec, J. Catal. 44 (1976) 439. 1101 J A Rabo. A-P. Rlsch and M-L. Poutsma. J. Catal. 53 (1978) 295. [I11 D J Dwyer and G A. Somoqal, J. Catal. 52 (1978) 291 1121 J.G. Ekerdt and A T. Bell, J. Catal. 58 (1979) 170. 499 1131 H P. Bonzel and HJ. Krebs, Surface Sci. 91(1980) 1141 D W. Goodman, R.D. Kelley. T-E. Madcy and J.M. White, 1. Catal., to be published. [I51 P. Biloen. J-PI. HeUe and W-M-H. Sachtler. J. Catal. 58 (1979) 95. [WI H J. Krebs. H-P. Bonzel and G. Garner, Surface Sci. 88 (1979) 269. [ 17 1 G. Brodin, G. Gafner and H P. Bonzel, Appl. Phys. 13 (1977) 333.
PI V.
167