J. Phys. Chem. 1992, 96, 5048-5053
5048
that the Pt-benzene distance is similar to that in transitionmetal-arene complexes leads to the conclusion that the pseudodipolar coupling may have an anisotropy of the order of 500 Hz.
I/#-
-2000
-1600
L
-1000
-500
0
600
-
lo00
0.166
Acknowledgment. This work was supported, in part, by the Sponson of the Center for Catalytic Science and Technology and by a Grant-in-Aid from Hercules, Inc. C.D. acknowledgessupport by the National Science Foundation under Grants CPE 82- 17890 and CHEM-9013926. G.N. acknowledges a grant from the Fonds der Chemischen Industrie. RWhy NO. 19'Pt, 14191-88-9; Pt, 7440-06-4; A1203, 1344-28-1; benzene, 71-43-2.
nm
1600
2000
Frequency (Hz) Figure 4. Simulated spectra for benzene in the on-top site at 2.0 and 1.56 A above the surface, respectively.
distance, the pseudodipolar coupling would be smaller; if it were at a greater distance, the pseudodipolar coupling would have to be larger to account for the observed spectrum. Conclusions The NMR spectrum of protons on benzene associatively chemisorbed on Pt/A120s, measured with suppression of homonuclear dipolar couplings, indicates that it interacts with platinum- 195 nuclei in the surface of the metal particle. The observed powder pattern can be interpreted in terms of coupling between protons and nuclei. From comparison with simulated spectra for various geometries, one concludes that the benzene is in the on-top site with the plane of the ring parallel to the surface platinum layer. The benzene is not static on the NMR time scale; to simulate the spectrum requires one to assume it rapidly jumps about the hexad axis. The value of the platinum-benzene interplanar spacing can be determined to be greater than 1.56 f 0.02 A. The effects of pseudodipolar couplings cannot be excluded, which would cause one to underestimate the distance by the assumption that only direct dipole-dipole couplings affect the proton spins. In particular, if one examines simulations of benzene at a typical Pt-benzene distance of 2 A, one finds that the major splitting may have a substantial contribution from pseudodipolar coupling. Thus, 1.56 A must be viewed as a lower limit for the benzeneplatinum distance. Making the reasonable asssumption
References and Notes (1) (a) Haaland, D. H. SwJ Sci. 1981,102,405. (b) Haaland, D. H. Ibid. 1981, 111, 555. (2) Primet, M.; Basset, J. M.; Mathieu, M. V.; Prettre, M. J. Caral. 1973, 29, 213. (3) Moyes, R. B.; Wells, P. B. Adv. Catal. 1973, 23, 121. (4) Palazov, A. J. Catal. 1973, 30, 13. (5) Horsley, J. A,; Stoehr, J.; Hitchcock, A. P.; Newbury, D. C.; Johnson, A. L.; Sette, F. J. Chem. Phys. 1985, 83, 6099, (6) Lehwald, S.; Ibach, H.; Dehmuth, J. E. S u r - Sci. 1978, 78, 577. (7) Jobic, H.; Renouprez, A. Surf. Sci. 1981, I l l , 53. (8) Ogletree, D. F.; van Hove, M. A.; Somorjai,G. A. SurJ Sci. 1987,183, 1. (9) Gland, J. L.; Somorjai, G. A. Surf Sci. 1973, 38, 157. (10) Stair, P. C.; Somorjai, G. A. J . Chem. Phys. 1977,67,4361. (11) Tetenyi, P Babernics, L. J. Catal. 1967, 8, 215. (12) Tirendi, C. F.; Mills, G. A.; Dybowski, C. J . Phys. Chem. 1984,88,
5765.
(13) Garten, R. L. J. Catal. 1976, 43, 18. (14) Rhim, W.-K.; Elleman, D. D.; Vaughan, R. W. J . Chem. Phys. 1973, 58, 1772. (15) Haubenreisser, U.; Schnabel, B. J. Magn. Reson. 1979, 35, 175. (16) Gerstein, B. C.; Dybowski, C. Transienr Techniques in NMR of Solids; Academic Press: Orlando, FL, 1985. (17) Ryan, L. M.; Wilson, R. C.; Gerstein, B. C. J . Chem. Phys. 1977,67, 43 10. (18) Andrew, E. R.; Eades, R. G. Proc. R . Soc. London 1953, A218,537. (19) Englesberg, M.; Yannoni, C. S.; Jacintha, M. A.; Dybowski, C. J . Phys. Chem., submitted for publication. (20) (a) Jesson, J. P. In Transition Meral Hydrides; Muetterties, E. L., Ed.; Dekker: New York, 1971; p 75. (b) Appleton, T. 0.;Clark, H. C.; Manzer, L. E. Coord. Chem. Rev. 1973,10, 335. (c) Powell, J.; Gregg, M. R.; Sawyer, J. F. Inorg. Chem. 1989, 28, 4451. (21) Pople, J. A.; Santry, D. P. Mol. Phys. 1964, 8, 1. (22) (a) Anklin, C. G.; Pregosin, P. Magn. Reson. Chem. 1985, 23,671. (b) Albinati, A.; Pregosin, P.; Wombacher, F. Inorg. Chem. 1990,29, 1812. (23) See, for example: Comprehensive Organometallic Chemistry; Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.;Pergamon: Oxford, U.K.,
1982.
Carbon Filament Growth on Platinum Catalysts W. T. Owens, N. M. Rodriguez, and R. T. K. Baker* Chemical Engineering Department, Auburn University, Auburn, Alabama 36849 (Received: October 18, 1991) A combination of experimental techniques including controlled atmosphere electron microscopy, thermogravimetry, and flow reactor studies have been used to study the formation of carbon deposits resulting from the interaction of platinum with ethylene and acetylene. It was found that in order to produce significant amounts of carbon on the metal it was necessary to perform the reaction in the presence of added hydrogen. In this investigation we have focused attention on all of the steps involved in the reaction,including those which occur at the metal-gas interface: diffusion of carbon through the metal particles and eventual precipitation at the metal-carbon interface to form a carbon filament. In both CBSCS as the amount of hydrogen in the reactant gas was increased, there was a corresponding increase in the degree of crystalline perfection of the carbon filaments which were produced on the platinum particles. This effect was more pronounced for acetylene than ethylene, and if the metal was treated in a mixture of acetylene containing excess hydrogen, it was possible to produce filaments which had identical oxidation characteristics to those of graphite.
Platinum-Catalyzed Decomposition of Hydrocarbons A
Figure 1. Schematic representation of a carbon filament produced from the metal-catalyzed decomposition of a hydrocarbon: (A) metal-gas interface, (B) metal catalyst particle, (C) metal-carbon interface, and (D) ordered platelets of carbon produced by precipitation of carbon from the metal particle B.
ordered arrangement of polyaromatic molecule^.^^^ Since the deposit did not appear to collect preferentially in the vicinity of the platinum particles, it was not clear if the metal played a catalytic role in the formation of the carbon. Other workerseI2 have reported that under certain conditions platinum particles are capable of catalyzing the growth of filamentous carbon structures. The sequence of events leading to the formation of this type of deposit are shown schematically in Figure 1. When a hydrocarbon is adsorbed on a metal surface (A) and conditions exist which favor the scission of a carbon-carbon bond in the molecule, then the resulting species may dissolve in the particle (B), diffuse to the rear faces, and ultimately precipitate at the interface (C) to form a carbon filament. The degree of crystalline perfection of the deposited structure (D) is dictated by the chemical nature of the catalyst particle and the experimental conditions. Fryer and Paa19 found that carbon filaments were formed on platinum particles after heating to 360 OC in the presence of either 1-hexene or cyclohexene. Wu and Phillipdo describe some of the carbon deposits produced on the surfaces of platinum foils which had been exposed to ethylene/oxygen mixtures for prolonged periods of time at temperatures between 500 and 685 OC as being fibrous in nature. Detailed examination of the electron micrographs presented by these workers shows that the structures which are claimed to be fibrous do not resemble those which are normally associated with filamentous carbon. In the absence of oxygen, it was reported that at temperatures above 600 OC the carbonam u s deposit consisted of a mixture of graphite and a less ordered carbon component. Murphy and Carroll" observed the formation of carbon filamentsduring the pyrolysis of acetylene over platinum at temperatures ranging from 345 to 900 "C, with the heaviest deposits being formed between 800 and 900 OC. In a previous investigation from this laboratory, Chang and co-workers'* used controlled atmosphere electron microscopy to continuously monitor the growth of carbon filaments on platinum particles as they were heated in 2 Torr acetylene at temperatures up to 800 OC. The interaction of hydrocarbons with single-crystal platinum surfaces has been the subject of numerous surface science studies (for example, refs 14-25). The consensus of opinion to emerge from the early studies dealing with the interaction of ethylene and acetylene with platinum (1 1 1) surfaces can be summarized according to the following observations: (i) ethylene adsorbs irreversibly and dissociates into acetylenic species which occupy four metal sites, and the mobile hydrogen atoms which are released during this process are also absorbed on the platinum surfaces; (ii) acetylene adsorbs on the metal in a similar manner to ethylene; however, in this case the hydrocarbon does not undergo dissociation; (iii) above 500 "C condensation reactions lead to the formation of a graphite overlayer which is ultimately responsible for catalyst deactivation. In the current investigation we have used a combination of various techniques including controlled atmosphere electron
The Journal of Physical Chemistry, Vol. 96, No. 12, 1992 5049 microscopy, thermogravimetric analysis, and conventional flow reactor methods coupled to a gas chromatograph to examine the events occurring at the gas-metal and metal-carbon deposit interfaces and also follow the manner by which the addition of hydrogen to a hydrocarbon feed stream modifies these processes. Experimental Section Techniques. (a) Controlled Atmosphere Electron Microscopy (CAEM). Details of the CAEM technique are already well therefore, only a brief description will be given here. The key design feature is the ability to perform reactions at high gas pressure in the specimen region while maintaining very low pressure in other parts of the microscope. This operation is accomplished by incorporation of a gas reaction cell into the specimen chamber. With this arrangement it is possible to expose the specimen to a gas environment at pressures up to 400 Torr while at the same time heating the specimen at temperatures up to 1300 OC. In order to collect the dynamic information generated in these types of experiments the changes in the appearance of the specimen are recorded continuously on video tape using a closed-circuit TV system. From frame by frame analysis of reaction sequences it is possible to obtain detailed quantitative kinetic data on the behavior of single catalyst particles. Unfortunately the modifications made to the microscope to accommodate the gas reaction stage coupled with the presence of a gas limit the resolution of the instrument when used in this mode to 2.5 nm. Two types of specimen design were used for the current experiments: platinum supported on graphite and platinum wires. For the supported metal system, a film of platinum was introduced onto transmission specimens of single crystal graphite (Ticonderoga, NY) by evaporation of the metal (99.999% purity) from a multistranded tungsten filament at a residual pressure of 10" Torr. The conditions were selected so as to produce a metal film at least 1 atom in average thickness. The unsupported metal specimens were prepared by spot welding a piece of a 0.025mm-diameter platinum wire across the hole in the center of the specimen heater ribbon. Although this latter form of specimen was too thick to allow transmission by the electron beam, it was possible to follow the changes in the appearance of the profile of the edges and directly observe the events occurring at these regions. (b) Flow Reactor Studies. In these experiments a 20-mg sample of platinum black was evenly spread in the bottom of a ceramic boat, which was then loaded into the constant temperature zone of a flow reactor tube. Prior to reaction in a hydrocarbon environment the platinum was treated in a 10% H2/He mixture at 536 OC for 1.5 h. Following this step the metal powder was reacted in various ratios of either C2H4/H2or C2H2/H2mixtures for periods of up to 2 h. In experiments involving acetylene, the partial pressure of the hydrocarbon was limited to 100 Torr in order to minimize the tendency of gas-phase polymerization reactions. The composition of the gas phase was monitored at regular intervals throughout the reaction by taking samples for gas chromatographic analysis in a Varian 3400 instrument which was equipped with a 30-m megabore column (GS-Q). At the termination of the deposition reaction the amount of carbon which had accumulated on the catalyst was determined by weight difference. Structural characteristics of the carbonaceous deposit were determined by examination in both a scanning electron microscope and a highresolution transmission electron microscope. (c) ThermogravimetricAnalysis. Samples of filaments formed from different gas mixtures were subjected to controlled oxidation carried out in a Cahn 2000 microbalance. The temperature program used consisted of a period of 30 min at 400 OC followed by a 5 OC/min ramp. A flow rate of 80 mL per minute of Ar-C02 (3050) was maintained for the duration of the experiment. For comparison controlled oxidation of both 5% Pt/graphite and 5% Pt/active carbon samples was also performed. Materials. Platinum used in the electron microscopy studies was spectrographicgrade 99.999% purity and was obtained from Alfa Products. The platinum black was kindly donated by Engelhard Corp. (fuel cell grade with a stated surface area of 20
Owens et al.
Soso The Journal of Physical Chemistry, Vol. 96, No. 12, 1992
0 Ptwire
0 PtIGraphite 3.0
-
2.5
1
1.0
1.1
1.2
1.3
1.4
1.5
1.6
i m (K) r103 Figure 2. Arrhenius plot for the platinum-catalyzed formation of carbon filaments from acetylene obtained by in situ transmission electron microscopy for supported and unsupported platinum.
m2/g). The gases used in this work, acetylene (98%~)~ ethylene (99.99%~)~ hydrogen (99.999%), helium (99.999%), argon (99.998%),and carbon dioxide (99.99%),were obtained from Alphagaz Co. and used without further purification. Results (a) contraoed Atmosphere Ektron Microscopy. All specimens were initially treated in 1.0 Torr of hydrogen at 325 "C for 30 min to ensure that the platinum was clean and in the fully reduced state. Following this step, the hydrogen was replaced by 2.0 Torr of acetylene, and the changes in appearance of specimens were continuously observed and recorded as the reaction temperature was progressively raised to a maximum level of 1000 "C. When platinum/graphite samples were reacted in acetylene, the onset of carbon filament formation was observed at 590 OC. The filaments were produced by a whisker-like mode where the metal catalyst particles were camed away from the support surface and remained attached to the tip of the growing filament. A survey of many specimen regions showed that this behavior was very sporadic and appeared to be restricted to platinum particles which were located at edge or step regions of the graphite support. It was also apparent that filaments grew at a faster rate from small metal particles (- IO-nm diameter) than from the larger particles which were present (-55-nm diameter). The growth of this form of carbon tended to proceed in a relatively smooth fashion as the temperature was gradually increased to 700 OC. At higher temperature the number of new filaments being formed decreased significantly, and eventually at 800 "C growth of filaments suddenly ceased. In a series of experimentscarried out with unsupported platinum specimens, growth of carbon filaments was found to commence at 380 OC and continued to increase in both number and rate as the temperature was raised to 675 OC. In this case, however, it was apparent that some of the filaments were being produced by an extrusion mode in which the active catalyst particle remained in contact with the bulk metal surface throughout the growth process. Detailed quantitative kinetic analysis of the growth of carbon filaments on both graphite-supported and unsupported platinum particles is shown in the form of an Arrhenius plot, Figure 2. In order to overcome variations in growth rate due to particle size effects, these data have been normalized to a common particle size of 25 nm using a standard calibration curve.I3 From the slope of this plot it is possible to derive a value of 12.2 f 2 kcal/mol for the apparent activation energy for the growth of carbon filaments from the platinum-catalyzeddecomposition of acetylene. (b) Nature of Macroscale Carbonaceous Deposits. Electron microscopy examinations of a number of samples of the carbonaceous deposit taken from a selection of flow reactor experiments performed in both ethylene and acetylene were found to be composed of networks of carbon filaments. High-resolution trans-
I Figure 3. Electron micrograph of carbon filaments produced from the reaction of a C2H2/H2(1:4)mixture with platinum black at 550 OC. Stacking of the carbon is indicated by lines and arrows. Inset shows the electron diffraction pattern. TABLE I: Perceabge Carbon Distribution at 550 O C as a Function of Ethyk/Hydrogen Ratio in the Presence of Platinum ethylene/hydrogen ratio carbon product l:o 4:1 1:l 1:4 solid carbon 0. I 0.2 2.5 7.7 0.4 0.3 0.4 CH4 0.2 93.5 88.4 76.8 C2H4 94.5 2.6 6.I 14.0 C2H6 1 .o 1.3 0.4 1.7 2.5 c 3 1.6 I .3 0.6 c4 1.7 TABLE Ik Percentage Carbon Distribotian at 550 OC as a F d o n of Etbylene/Hydrogen Ratio in the Absence of Platinum ethylene/hydrogen ratio carbon product l:o 4:1 1:l 1:4 solid carbon 0.0 0.0 0.0 0.0 0.2 0.1 0.1 CH4 0.0 96.0 94.6 91.9 CzH4 97.5 2.0 4.3 7.5 C2H6 0.4 0.9 0.3 0.1 c 3 1.2 0.7 0.4 0.9 0.9 c4
mission electron microwopy studies of individual filaments showed the existence of a "fish bone" stacking arrangement; characteristics normally associated with a well-ordered crystalline structure. These features were particularly prevalent on filaments which had been produced from mixtures of acetylene containing a substantial amount of added hydrogen. Figure 3 is an electron micrograph showing the appearance of carbon filaments from a sample in which platinum black had been-reacted in an acetylene/hydrogen (1 :4). Also included as an inset in this figure is the selected area electron diffraction pattern which shows the existence of sharp rings containing arcs indicative of a crystalline material. (c) Flow Reactor Studies. The effect of hydrogen addition to the hydrocarbon feed on the amount of carbon filaments formed on platinum black is presented in Figure 4. Inspection of these data show a number of points. (a) Hydrogen enhances the dissociation of hydrocarbonson the metal surface to form filamentous carbon in both ethylene and acetylene environments. (b) It appears that for platinum to catalyze the growth of filaments from ethylene it is necessary to add hydrogen to the system. (c) The amount of carbon deposit produced from acetylene is significantly higher than that from ethylene under comparable conditions. The carbon distributions resulting from the decomposition of ethylene and various hydrogen/ethylene ratios in both the presence and absence of platinum are shown in Tables I and 11. As expected, increasing the amount of hydrogen present enhances hydrogenation of ethylene to ethane. This effect is more pro-
The Journal of Physical Chemistry, Vol. 96, No. 12, 1992 5051
Platinum-Catalyzed Decomposition of Hydrocarbons
o^
-0
I0n
'
0
0 CzH2 0 C2H4
30
UI 0 I
4
1
0
Q
20
Y
10
o Filaments (83% CZH4) Filaments 8% C2H4
a
s
20
40 60 YOHydrogen
500
80
TABLE Ilk Percentage Carbon Wtributioo at 550 OC as a Function of Acetylew/Hydrogen Ratio in the Presence of Platinum
0
0.1
36.9 0.1 2.2 53.5
-20
CH4 C2H4 CZHZ CZH6
c3 c 4
900
1000
1100
similar platinum loading.
acetylene/hydrogen ratio 1:o 4: 1 1:l 1:4 1.5 84.3 0.0 1.2 11.1
800
Figure 5. Temperature-programmedoxidation in COz of carbon filaments produced from the platinum-catalyzed decomposition of ethylene/hydrogen mixtures as compared to graphite in the presence of a
carbon product solid carbon
4.9 0.1 2.1 19.5 0.3 1.5 11.6
700
Temperature ("C)
Figure 4. Effect of hydrogen on the decomposition of ethylene and acetylene over platinum black in the formation of solid carbon.
1.8
600
8.3 0.3 3.2 14.5 0.3 1.9 11.5
0.2
v)
0
-I
40
1
0 0
1.I
5.4
0
TABLE Iv: Percentage Carbon Distribution at 550 O C as a Function of Acetylew/Hydrogen Ratio in the Absence of Platinum
carbon product solid carbon
acetylene/hydrogen ratio 1:o 4: 1 1:l 1:4
CH4 CZH4 C2Hz
0.1
C2H6
c3
c 4
0.0 1.9 86.5 0.0 3.5 8.0
0.0 0.1 2.2
85.0 0.0
0.8 11.8
0.0
0.0
0.2 2.9 86.8
0.2
0.0 1.9 8.2
3.2 81.9 0.4 1.3 13.1
nounced when the reaction is camed out over a platinum catalyst. It is interesting to find that although the amount of methane present in the exit gas stream is relatively low, it is significantly higher in the presence of platinum. It is also apparent that the amounts of C3 and C4 hydrocarbons generated is higher when the reaction is conducted over a platinum catalyst. The corresponding product distributions obtained when acetylene was used as the hydrocarbon source are presented in Tables I11 and IV. In this case, it is clear that considerably more carbon is deposited on the catalyst than from the olefin. On the other hand, the amount of methane produced from the platinum-catalyzed decomposition of acetylene is significantly lower than that measured during the decomposition of ethylene under the same conditions. This finding suggests that methane may originate from reactions other than hydrogasification of solid carbon. Attempts to produce carbon filaments from the catalyzed decomposition of ethylene/hydrogen mixtures at temperatures higher than 750 OC were thwarted by the uncatalyzed decomposition of the hydrocarbon which led to the formation of polymeric residues. (d) Tbemogrpvimetric Aoalysis. The characteristics of controlled oxidation of carbonaceous solids in C02provides a very sensitive method of determining the structural perfection of such materials. In the current study we have compared the reactivity of the catalytically produced carbon deposits to that of two forms of carbon which possess extreme structural order: graphite (highly crystalline material) and active carbon (amorphous in nature). In the presence of 5% platinum, graphite was observed to react at 800 OC,whereas amorphous carbon containing the same loading of metal started to undergo oxidation at 550 OC. When a similar
0 Fiiaments (3WbCZH2)
0 Fib" -100 400
500
(33% CZH4)
600
700
800
900
1000
1100
Temperature ("C)
Figure 6. Temperature-programmedoxidation in COz of carbon filaments produced by the decomposition of hydrocarbon/hydrogenmixtures over platinum black as compared to graphite impregnated with an equivalent amount of platinum.
procedure was used to examine carbon filaments produced from various ethylene/hydrogen ratios, it can be seen from the thermograms presented in Figure 5 that the filaments consist of two types of carbon: one which starts to gasify at about 600 OC and a second component which undergoes gasification at temperatures in excess of 800 OC. This trend suggests the existence of a duplex structure, a large fraction of amorphous carbon combined with a smaller amount of graphite. It is clear that the relative amounts of these two components is dependent on the ratio of ethylene/ hydrogen in the reactant feed, with the more graphitic filaments being produced from a mixture containing a relatively large fraction of hydrogen. Inspection of the data presented in Figure 6 shows that the degree of crystalline perfection is also sensitive to the nature of the hydrocarbon source. A comparison of the thermograms obtained from exposure of platinum to hydrocarbon/hydrogen mixtures containing similar amounts of acetylene and ethylene, respectively, shows that in the presence of the former hydrocarbon it is possible to produce carbon filaments which consist entirely of graphite.
Discussion 1. TbeMdaH;esInterface. 1.1. InteraCtioaofHyd" with Platinum Maces. The interaction of ethylene and acetylene with selected metal surfaces has been used in the present investigation to follow the events occurring at the surface of platinum catalyst particles. Under carefully selected conditions, these hydrocarbons can be adsorbed and reacted continuously at the surface of the metal in such way that a relatively large amount of products can be generated for subsequent analysis. From a
5052 The Journal of Physical Chemistry, Vol. 96, No. 12, 1992 comparison of the data presented in Tables I and I11 it is evident that platinum-catalyzed decomposition of acetylene to form solid carbon is a much more facile process than the corresponding reaction with ethylene. It is possible that the lower reactivity of ethylene toward carbon filament formation may be related to the fact that this type of deposit is formed via an indirect step, which requires the participation of an acetylenic intermediate. 1.2. Effect of Added Hydrogen. The presence of added hydrogen in the hydrocarbon feed enhances the decomopsition and consequent formation of solid carbon on platinum surfaces. This effect is found to be more pronounced with acetylene than ethylene. In the absence of platinum, however, this modification in reactant composition appears to have only a minor effect on the hydrocarbon decomposition reactions. It is possible that the increase in the observed yield of solid carbon in the presence of added hydrogen is related to a change in the characteristics of the deposit. In a pure hydrocarbon environment there is a strong tendency for the adsorbed species to undergo condensation reactions on the metal surfaces, and through subsequent dehydrogenation steps it rearranges to form a graphitic overlayer which is responsible for the rapid deactivation of the catalyst. Since this process occurs at virtually a monolayer coverage, the observed increase in weight of the catalyst due to carbon accumulation is probably below the detection limits of the thermogravimetric technique used in this work. It is possible that the presence of adsorbed hydrogen on the metal disrupts the sequence of events leading to surface graphite f o r m a t i ~ n . ~Under ~ . ' ~ these circumstances the carbonaceous intermediates decompose via an alternative pathway to produce species which are capable of dissolving in the platinum and are subsequently precipitated at the rear of the particle to produce a carbon filament. Since this type of deposit is produced in a continuous manner the catalyst maintains its activity for prolonged periods of time, and it is a simple task to monitor the amount of carbon filaments produced. At first sight one might be tempted to speculate that the role of hydrogen in this reaction is merely that of an agent to maintain the catalyst particle surface clean. If this were the case then one would expect to observe much higher yields of methane upon increasing the fraction of hydrogen in the hydrocarbon feed. As can be seen from the data presented in Tables I and I11 methane yields remained almost constant in these experiments. A more plausible explanation may lie in the role played by hydrogen in inducing reconstruction of the platinum surfaces. It has been known for many years that clean platinum surfaces can undergo reconstruction upon heating and that the presence of hydrogen can have a significant effect on this The ramifications of this phenomenon on the nature of the carbon deposit formed on platinum crystal surfaces were examined by Lang,17 who used LEED to show that the Pt (100) surface was most susceptible to deactivation by a graphite overlayer, and since adsorption of hydrogen has been found to alter the structure of this face14*30 the propensity for graphite formation might be eliminated. Examination of the data presented in Table I shows that between 2.0 and 4.6% of the ethylene which decomposes forms methane. This product can be generated from the platinumcatalyzed hydrogasification of the carbon deposits; however, at the prevailing reaction temperatures of 550 O C the rate of this reaction is extremely slow and consequently the amount of methane which might be produced via this route is negligible.33-35 It is possible therefore that some of the ethylene is adsorbed in a conformation where only one of the carbon atoms in the molecule is bonded to the metal surface and subsequently transforms to generate the "ethylidyne" structure. Fragmentation of this structure will lead to the formation of methane and possibly solid carbon. In contrast, at this temperature both carbon atoms in the acetylene molecule appear to be strongly held on the metal surface and the tendency to form an equivalent type of structure is suppressed. 2. The Metal Particle and Diffusion of Carbon. Direct observations of the carbon filament growth process obtained through the use of controlled atmosphere electron microscopy has con-
Owens et al. clusively shown that the rate-determining step in the process is carbon diffusion through the metal parti~1e.I~ The finding that platinum catalyzes the formation of carbon filaments eliminates the possibility that the catalytic entity is a metal carbide since platinum does not form a bulk carbide at 550 "C. By comparison with previous carbon filament growth studies it is probable that the measured activation energy for the growth of filaments on platinum of 12.2 f 2 kcal/mol corresponds to that for diffusion of carbon through the metal. The unavailability of pertinent data in the literature does not allow us to make a direct comparison between these two values; however, recent calculations by Yang and c o - ~ o r k e rare s ~ ~consistent with this relatively low number. 3. The Metal-Carbon Interface. One of the most intriguing aspects to emerge from this study is the fiiding that as the amount of hydrogen in the reactant gas is increased, there is a corresponding increase in the ratio of graphitic to amorphous carbon in the filaments which are produced from the interaction of both ethylene and acetylene with platinum. Moreover, conditions have been identified where it is possible to produce carbon filaments which are entirely graphitic in nature. As shown in the schematic diagram, Figure 1, the crystalline order of the precipitated carbon filament is intimately related to the nature of the interfacial reaction occurring at the metal-carbon interface (region C). For the case of platinum a considerable amount of effort has been devoted to the study of the adsorption of hydrocarbons on the various metal surfaces; however, little information is known about the events occurring at the rear of the particle. Yang and Chen attempted to identify the faces of nickel which favor the precipitation of carbon in the form of graphite for the formation of carbon filaments from hydroc a r b o n ~ It . ~has ~ recently been suggested that in order for such a process to occur conditions must be achieved in which the metal undergoes a wetting action with graphite.j8 Direct observation of the interaction of platinum with singlecrystal graphite surfaces showed that when the system was heated in the presence of hydrogen the metal particles initially underwent a wetting action a t about 650 OC and as the temperature was raised to 755 OC proceeded to spread along the graphite edge regions in the form of a thin film. When the temperature was progressively raised to 845 OC, then the interaction of platinum with graphite was weakened and particles were observed to reform.j4 It is possible that this transition is associated with an increase in the amount of dissolved carbon in the metal. There are many reports claiming that when a critical amount of carbon is dissolved in a metal particle then an abrupt change from a wetting to a nonwetting state takes place at the metal/graphite interfa~e.j'~* In the present systems carbon is dissolved in the platinum particles from the gas phase at significantly lower temperatures than those stated above, and it would appear that the presence of excess hydrogen in the reacQnt mixture is required to maintain a strong interaction between the carbon-containing metal and the precipitated graphite. There is a further factor which should be borne in mind when attempting to explain the structural differences of filaments produced on platinum in these two hydrocarbons. The decomposition of acetylene is highly exothermic, 53.5 kcal/mol, whereas that of ethylene is only 12.5 kcal/mo14j and as a consequence it is possible that in the former case the metal particles are significantly hotter than the set temperature and as a result might be expected to exhibit different wetting characteristics with respect to graphite. Based on this hypothesis one might speculate that it should be possible to produce more ordered carbon filament structures from the platinum/ethylene-hydrogen system if the reaction temperature was raised to about 750 OC. Unfortunately, we were unable to test this notion since the formation of polymeric residues arising from the uncatalyzed decomposition of ethylene interfered with the catalytic activity of platinum. Conclusions We have found that in order to produce carbon filaments in signifcant amounts from the interaction of platinum with ethylene
J. Phys. Chem. 1992,96, 5053-5059
or acetylene it is necessary to add hydrogen to the reactant mixture. As the fraction of hydrogen in the gas feed was increased there was a corresponding increase in both the yield of carbon filaments and the degree of their crystalline perfection. By careful selection of the hydrocarbon/hydrogen mixture it was possible to make carbon filaments which were entirely graphitic in nature at temperatures of the order of 550 OC. Product analysis indicated that in all cases ethylene was less reactive than acetylene. It is suggested that prior to carbon filament formation ethylene can undergo a structural rearrangement on the surface to form either an acetylenic intermediate which eventually leads to the growth of carbon filaments, or an ethylidyne intermediate which is responsible for methane production along with solid carbon formation. Acknowledgment. This work was supported by the Department of Energy, Basic Energy Sciences, Grant No. DE-FGO589ER14076. We would like to thank Engelhard Corp. for the gift of the platinum used in this work. Registry No. C, 7440-44-0; Pt, 7440-06-4; graphite, 7782-42-5; acetylene, 74-86-2; ethylene, 74-85-1; hydrogen, 1333-74-0.
References and Notes (1) Bacaud, R.; Charcosset, H.; Guenin, M.; Torrelas-Hidalgo,R.; Tournayan, L. Appl. Catal. 1981, 1, 81. (2) Barbier, J.; Marecot, P.; Martin, N.; Elassal,L.; Maurel, R. Srud. Surf Sci. Caral. 1982, 6, 53. 13) Parara. J. M.: Figoli. N. S.: Traffano. E. M. J. Catal. 1983. 79.481. (4) Barbier, J.; C0rro:G.f Zhang, Y.; Boumonville, J. P.; Frank, J: P. Appl. Caral. 1985, 13, 245. ( 5 ) Mieville, R. L. J . Catal. 1987, 105, 536. (6) Parera, J. M.; Betramini, J. N. J . Caral. 1988, 112, 357. (7) Cabral, R. A.; Oberlin, A. J. Caral. 1984, 89, 256. (8) Gallezot, P.; Lxqlerq, C.; Barbier, J.; Marecot, P. J. Caral. 1989, 116, 164. (9) Fryer, J. R.; Paal, Z. Carbon 1973, 11, 665. (10) Wu,N. L.; Phillips, J. J . Catal. 1988, 113, 383. (11) Murphy, D. B.; Carroll, R. W . Carbon 1990,28, 733. (12) Chang, T. S.;Rodriguez, N. M.; Baker, R. T. K. J . Catal. 1990,123, 486.
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Photochemistry on Surfaces. Photodegradation of 1,3-Diphenyiisobenzofuran over Metal Oxide Particles K. Vinodgopal* Department of Chemistry, Indiana University Northwest, Gary, Indiana 46408
and Prashant V. Kamat* Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556 (Received: December 13, 1991; In Final Form: March 9, 1992)
Steady-state and diffuse reflectance laser flash photolyses have been camed out to elucidate the mechanism of photodegradation of 1,3-diphenylisobenzofuran (DPBF) on solid surfaces of AlzO3, TiOz, and ZnO. In the absence of oxygen, the semiconductor supports TiOz and ZnO catalyze the photodegradation by accepting electrons from excited DPBF. The fluorescence of degassed DPBF on Ti02 and ZnO is quenched relative to that on alumina, thereby offering independent codmation of charge transfer. In oxygenated samples, the primary mechanism of photodegradation involves reaction with singlet oxygen. This is confirmed by the studies on the insulator surface A1203,where significant degradation is observed only in the case of air-equilibrated samples. The dependence of the rate of DPBF photodegradation on the surface coverage indicates that only the molecules that are in direct contact with the surface undergo photodegradation. The results that highlight the role of the support material in guiding the course of a photochemical reaction are described here.
Introduction Polychlorinated and polybrominated dibenzofurans (PCDF, PBDF) are a group of extremely hazardous chemicals whose toxicity has been well established.'-$ The hazardous nature of these chemicals is rendered more acute by their widespread presence. In addition to being formed as byproducts in the in-
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dustrial manufacture of polychlorinated biphenyls and chlorophenols, etc.,6 these furans are also formed following the thermal combustion of these above compounds. ~OnSequentlY,their presence has been detected as byproducts from the incineration Of both "kip1 and industrial waste? More threatening their presence in aquatic sediments, in marine and freshwater fish, and
0 1992 American Chemical Society
