J. Phys. Chem. 1990, 94,4599-4610
seems to be valid in time and space; i.e., the energy is deposited in the polymer matrix on the same time and space scale. Especially the small carbon clusters are created in surroundings having the same plasma temperature in both desorption modes. Thus it may be that polymer matrices offer the possibility to get a better understanding of what happens during primary ion impact and secondary ion emission. Additionally degradation effects of polymers could be studied by the comparison of different energy-transfer mechanisms from the primary ions to the target atoms.
4599
Acknowledgment. We thank U. Jurgens, who supported this work by the development and maintenance of the excellent registration computer program. We also thank Prof. D. M. Hercules, Dr. M. P. Chiarelli (University of Pittsburgh) and B. Hagenhoff (University of Munster) for reading the manuscript and for valuable discussions. Registry No. PTFE, 9002-84-0; PVDC, 9002-85- I ; PVDF, 2493779-9; PET, 25038-59-9; C , 7440-44-0.
Infrared Identification of Adsorbed Surface Species on Ni/SiO, and Ni/AI2O3from Ethylene and Acetylene Adsorption Mark P. Lapinskit and John G. Ekerdt* Department of Chemical Engineering, University of Texas at Austin, Austin, Texas 78712 (Received: August 3, 1989; In Final Form: January 3, 1990)
The adsorption of ethylene and acetylene was studied on Ni/Si02 and Ni/A1203catalysts over the temperature range 177-300 K with Fourier-transform infrared spectroscopy (FTIR). The general approach involved adsorbing ethylene or acetylene and monitoring the subsequent surface transformationswith FTIR as the catalyst was heated, exposed to hydrogen or deuterium, or subjected to vacuum. For the adsorption of ethylene on Ni/Si02 and Ni/AI2O3 at temperatures below 250 K, two types of ?r-bonded species (A and B), a owethylene species (hybridization between sp2 and sp3) and ethylidyne were identified. Over the temperature range 194-240 K, n-bonded species B and the un-ethylene species were observed to decompose while ethylidyne formed. A mechanism is proposed in which adsorbed H inserts into n-bonded species B and the un-ethylene species leading to ethyl and ethylidene intermediates, and finally ethylidyne. The amount of hydrogen adsorbed on the nickel surface and crowding effects are proposed to be responsible for ethylidyne formation on nickel. For temperatures over 250 K, ethylidyne decomposed while several new species were formed. Some ethylidyne remained on the surface at 300 K. Acetylene was very reactive on Ni/Si02 and Ni/A1203 but an acetylene-type species and ethylidyne were identified.
Introduction In heterogeneous catalysis, many studies have been undertaken to identify reaction mechanisms in an effort to understand what factors influence activity and selectivity. The use of vibrational spectroscopies such as infrared (IR), Raman, or electron energy loss (EEL) on transition-metal surfaces greatly simplify mechanistic possibilities by allowing in situ observation of adsorbed species transformations. These methods offer direct monitoring of elementary reactions and reaction intermediates. A wellcharacterized system, which has a limited number of adsorbed species, is required for in situ reaction studies. Adsorption studies of ethylene and acetylene on supported group VI11 metals'-24 and single-crystal surface^^^-^^ have revealed the types of structures which can form during reaction, the sites at which these structures form, and that C2 systems can be good model systems for reaction studies. In this paper we examine the adsorbed species resulting from ethylene and acetylene adsorption on Ni/Si02 and Ni/AI2O3 over the temperature range 177-300 K with FTIR. In a forthcoming paper we will describe the use of IR spectroscopy to follow the rates of surface reactions and determine the kinetics of these reactions. Infrared spectra for C2adsorbed species on Ni/Si02 have been reported mostly at room temperature with good resolution only in the C-H stretching region. The low-temperature studies of ethylene on Ni/Si02 by Morrow and Sheppard'o*" only reported bands in the C-H stretching region. At 195 K, di-u-bonded ethylene (C2H4)was proposed to form upon initial ethylene adsorption. After time at 195 K or at 293 K, di-a-bonded ethylene was thought to form sp3-hybridized acetylene (C2H2)or more likely a C4 species with multiple carbon-metal bonds. The results
* Author to whom correspondence should be addressed. 'Present address: Exxon Research and Development Laboratories, P.O. Box 2226, Baton Rouge, LA 70821. 0022-3654/90/2094-4599$02.50/0
of SomaI6 for C2H4 and C2D4adsorption on Ni/AI2O3 at low temperatures were in doubt because of the possible presence of ~
~
~~~~
~~~~~
( I ) Pliskin, W. A.; Eischens, R. P. J . Chem. Phys. 1956, 24, 482. (2) Eischens, R. P.; Pliskin, W. A. Adu. Curd. 1958, I O , 1. (3) Crawford, V. Q. Reu. 1960, 14, 378. (4) Little, L. H.; Sheppard, N.; Yates, D. J. C. Proc. R. SOC.A 1960, 259,
242. (5) Peri, J. B. Discuss. Furuduy SOC.1966, 4 1 , 121. (6) Erkelens, J.; Liefkens, TH. J. J. Curd 1967, 8, 36. (7) Blyholder, G.;Wyatt, W. V. J . Phys. Chem. 1974, 78, 618. (8) Blyholder, G.; Shihabi, D.; Wyatt, W. V.; Bartlett, R. J. Curd. 1976, 43, 122. (9) Erkelens, J. J. Catul. 1975, 37, 332. (IO) Morrow, B. A.; Sheppard, N. J. Phys. Chem. 1966, 70, 2406. (11) Morrow, B. A.; Sheppard, N. Proc. R. SOC.A 1969, 311, 391. (12) Sheppard, N.; Avery, N. R.; Morrow, B. A,; Young, R. P. Chemisorption Card. Proc. 1970, 135. (13) Sheppard, N.; Ward, J. W. J . Curd 1969, IS, 50. (14) Prentice, J . D.; Lesiunas, A.; Sheppard, N. J . Chem. SOC.,Chem. Commun. 1976, 76. (15) Soma, Y. J. Chem. SOC.Chem. Commun. 1976, 1004. (16) Soma, Y. J. Cutul. 1979, 59, 239. (17) Sheppard, N.; James, D. I.; Lesiunas, A,; Prentice, J. D. Bulg. Acud. Sci., Chem. Commun. 1984, 17, 95. (18) Bandy, B. J.; Chesters, M. A.; James, D. I.; McDougall, G. S.; Pemble, M. E.; Sheppard, N. Philos. Trans. R. Soc. London A 1986, 318, 141. (19) Cruz, C.; Sheppard, N. J . Chem. SOC.,Chem. Commun. 1987, 1854. (20) Beebe, T. P., Jr.; Albert, M. R.; Yates, J. T., Jr. J . Cum\. 1985, 96, I. (21) Beebe, T. P., Jr.; Yates, J . T., Jr. J. Phys. Chem. 1987, 91, 254. (22) Mohsin, S. B.; Trenary, M.; Robota, H. J. J . Phys. Chem. 1988, 92, 5229. (23) Anderson, K. G.; Ekerdt, J. G. J . Curd. 1989, 116, 556. (24) Lapinski, M. P.; Ekerdt, J. G.J. Phys. Chem. 1988, 92, 1708. (25) Lehwald, S.;Ibach, H. Surf. Sci. 1979, 89, 425. (26) Bertolini, J. C.; Rousseau, J. Surf. Sci. 1979, 83, 531. (27) Stroscio, J. A.; Bare, S. R.; Ho, W. Surf. Sci. 1984, 148, 499. (28) Zaera, F.; Hall, R. B. J . Phys. Chem. 1987, 91, 4318. (29) Ibach, H.; Lehwald, S. J . Vac. Sci. Technol. 1981, 18, 625.
0 1990 American Chemical Society
4600
Lapinski and Ekerdt
The Journal of Physical Chemistry, Vol. 94, No. 1I, 1990 Thermo-
Liquid Nitrogen ln'Out
EEEI
Dating, Vacuum Vacuum
EEil
vacuum, Pressure, Thermocouple
L'nes
TABLE I: Chemisorption Results for Ni/Si02 and Ni/AI2O3 Catalysts 8.6% Ni/A1203a 8.6% Ni/Si02" 643 Kb 723 Kb 643 Kb 723 Kb 94.0 75.4 73.5 H2 uptake, pmol/g of catalyst 99.1 O2 uptake pmol/g of catalyst 613.4 688.1 705.6 728.5 % reduction 83.4 93.6 96.2 99.3 % dispersion 16.2 13.7 10.7 10.1 a Nickel weight percent was determined by the ICP method (Galbraith Labs) and corrected for NiO. Reduction temperature.
S.S. Flange
Fiange
Figure 1. Infrared cell 2 with temperature range of 177-800 K.
nickel oxide and no Ni/A1203 spectra were shown. A .Ir-bonded ethylene (C2H4) species was proposed at 195 K. In a recent study of ethylene adsorption at 225 and 248 K on Ni/A1203, Lapinski and Ekerdt24identified ethylidyne by comparison to single-crystal and organometallic compounds. This was the first observation of ethylidyne on Ni. For acetylene adsorption on supported Ni, Sheppard and WardI3 proposed a sp2-hybridized acetylene species and surface alkyl groups (formed by self-hydrogenation and polymerization processes) on Ni/Si02 at 293 K. C2 adsorption studies on Ni single crystals have shown that different types of surface species exist on the different crystal planes and are a function of temperature and surface conditions. For the adsorption of ethylene on Ni( 1 11) at 150 K, di-a-bonded ethylene (C2H4) was proposed25and at 230-300 K, acetylene (C2H2)was p r o p ~ s e d .On ~ ~Ni( ~ ~1 ~ di-a-bonded ethylene (80 K) and acetylide (CCH, 220 K) were observed while on Ni( sp2-hybridized C2H4 (90 K), vinyl (-CH=CH2, 225 K), and acetylene (275 K) were observed. On the stepped surface, Ni[5(11 l)X(ilO)], vinyl was observed at 150 K.25 For the adsorption of acetylene on Ni single-crystalsurfaces, acetylene-type species,(C2H2)were observed at 80-300 K.25-30 In a recent static secondary ion mass spectroscopy (SSIMS) study, Zhu and White3] found evidence for ethylidyne (CCH3) formation on Ni( 111) from both acetylene and ethylene adsorption. Ethylidyne was only detected when the initial acetylene coverage was more than 70% or the ethylene coverage was more than 80% of a monolayer. Ethylidyne was proposed to form by adsorbateadsorbate repulsion (30) Demuth, J. E.; Ibach, H. Surf. Sci. 1979, 85, 365. White, J. M. Catal. Lert. 1988, I, 247. (31) Zhu, X.-Y.; (32) Steininger, H.; Ibach, H.; Lehwald, S. Surf. Sci. 1982, 117, 685. (33) Gates, J. A.; Kesmodel, L. L. Surf. Sci. 1983, 124, 68. (34) Koel, B. E.; Bent, B. E.; Somorjai, G. A. Surf. Sci. 1984, 146, 211. (35) Hills, M. M.; Parmeter, J. E.; Mullins, C. B.; Weinberg, W. H. J. Am. Chem. SOC.1986, 108, 3554. (36) Bent, B. E. Ph.D. Thesis, University of California, Berkeley, 1986. (37) Kesmodel, L. L.; Gates, J. A. Surf. Sci. 1981, I l l , L747. (38) Hatzikos, G. H.; Masel, R. I. Surf. Sci. 1987, 185, 479. (39) Marinova, Ts. T.; Kostov, K. L. Surf.Sci. 1987, 181, 573. (40) Lehwald, S.; Ibach, H.; Steininger, H. Surf. Sci. 1982, 117, 342. (41) Kesmodel, L. L.; Dubois, L. H.; Somorjai, G. A. Chem. Phys. Lett. 1978, 56, 267. (42) Kesmodel, L. L.; Dubois, L. H.; Somorjai, G. A. J. Chem. Phys. 1979, 70, 2180. (43) Koestner, R. J.; Van Hove, M. A.; Somorjai, G. A. Surf. Sci. 1982, 121, 321. (44) Creighton, J. R.; Ogle, K. M.; White, J. M. Surf. Sci. 1984, 138, L137. (45) Zaera, F.; Hall, R. B. Surf. Sci. 1987, 180, 1. (46) Ibach, H.; Lehwald, S. J. Vac. Sci. Technol. 1978, 15, 407. (47) Avery, N. R. Langmuir 1988,4,445. (48) Dubois, L. H.; Castner, D. G.; Somorjai, G. A. J. Chem. Phys. 1980, 72, 5234. (49) Parmeter, J. E.; Hills, M. M.; Weinberg, W. H. J. Am. Chem. SOC. 1986, 108, 3563. (50) Chesters, M. A.; McDougall, G. S.; Pemble, M. E.; Sheppard, N. Appl. Surf. Sci. 1985, 22/23, 369. (5 I ) Nyberg, C.; Tengstal, C.G.; Anderson, S . Chem. Phys. Lett. 1982, 87, 87. (52) Bent, B. E.; Mate, C. M.; Kao, C.-T.; Slavin, A. J.; Somorjai, G. A. J. Phys. Chem. 1988 92,4720.
TABLE 11: Average Particle Sizes (angstroms) from H2 Chemisorption and Transmission Electron Microscopy
8.6% Ni/AI2O3 8.6% N i / S i 0 2
H2 chemisorption 643 Kb 723 Kb 60 71 91 96
a Arithmetic, surface, and volume averages. ture.
TEM a 673 Kb 62, 104, 127 95, 143, 161 Reduction tempera-
at high coverages, i.e., a crowded surface led to species with the CC axis perpendicular to the surface thereby occupying less surface area. In this study, the surface selection rule, frequency shifts upon deuteration, reactivities, and comparisons to single-crystal studies were used to help identify adsorbed species on supported Ni. A more in depth identification and characterization of the previously proposed ethylidyne species24is included with a reinterpretation of some of the previous literature assignments on supported Ni. Methods A . Apparatus. Transmission IR spectra were obtained with a Digilab FTS-15/90 FTIR. All spectra were obtained at a resolution of 2 cm-l and consisted of 120-200 scans. Infrared spectra were referenced to the respective reduced surfaces just prior to the adsorption of acetylene or ethylene. All IR intensities are reported as peak heights above the base line. Two stainless steel IR cells were used in these studies. Cell 1 was designed by Campione and EkerdtS3and was used in high dose (50 Torr cell pressure) C2H4 adsorption experiments on pressed wafers at 225-285 K. Subambient cooling of the cell was obtained by filling an outer cell jacket with liquid nitrogen. Cell 2 (Figure 1) was designed with a temperature range of ca. 177-800 K. Subambient cooling of the wafers was achieved by circulating N2 liquid and vapor around the catalyst holder assembly inside the cell. As shown in Figure 1, the catalyst was heated by a series of three uniformly spaced 80-W cartridge heaters. Sheathed chromel-alumel thermocouples were placed about 0.2 mm in front and in back of the wafer. The wafer temperature could be monitored by pressing 0.0508-mm-0.d. thermocouple wires into the wafer and connecting them to the chromel-alumel feedthroughs. The cell was sealed with two Harshaw CaF2 optic flanges. Gases could be admitted to the cell by either a 0.635cm-0.d. side tube or by a 0.318-cm-0.d. tube placed about 0.65 cm in front of the wafer. The IR flow system has been described p r e v i ~ u s l y . ~Gas ~ samples were withdrawn from the IR cells by syringe periodically and analyzed on a Hewlett-Packard 5880A gas chromatograph. B. Catalyst Preparation and Characterization. These methods have been described in detail p r e v i ~ u s l y . ~ ~Briefly, . ~ ~ Si02 (Cab-o-Sil, 325 m2/g) or A1203 (Degussa C, 100 m2/g) were contacted with Ni(N03)2*6H20by using a wet impregnation technique. Pretreatment was done in a quartz tube and consisted of 4 h in He (99.995%) at 673 K followed by H2 (99.999%) reduction for 6 h at 673 K. Hydrogen and O2chemisorption were done to calculate the percent reduction, percent dispersion, and average particle diameter. Particle size distributions were de(53) Campione, T. J.; Ekerdt, J. G. J . Catal. 1986, 102, 64.
Adsorption of C2H4 and C2H2on Ni/Si02 and Ni/AI2O3
The Journal of Physical Chemistry, Vol. 94, No. 11, 1990 4601
TABLE 111: Comparison of Band Frequencies and Band Intensities with Time for Physically (A) and Chemically (B) Adsorbed Species from C2H, Adsorption on Ni/Si02 and Ni/A1203 at 180 K intensity (au X 100) Ni/SiOz intensity (au X 100) Ni/A1203 physically adsorbed band, cm-' 1.5 min 8.5 min band, cm-' 1.5 min 11.5 min mode species" A 3000 0.15 0.13 3005 b vsy(CH2) C2H4 2976 0.27 0.27 2975 vsy( C H 2) C2H4 2965 (sh)C 0.16 0.14 ~as(CH3) C2H6 1466 0.17 0.20d 1470 0.36 0.59d 6as(CH,) C2H6 1443 0.33 0.30 1445 0.60 0.53 6JCH2) C2H4 I336 0.09 0. I7d 1337 0.16 0.32d 6,,(CH2) C2H4
B 2095 2892 2853 1545' I525 1410 (b)
0.28 0.24 0.28 0.10 0.22 0.06
0.27 0.15 0.24 0.1 1 0.23 0.10
1547c
1525 (b) 1415 (b) 1250 1227 1 I84 1154 (sh)
0.38 0.26 0.17 0.39 0.72 0.83 0.47
0.25 0.37 0.18 0.50 0.67 1.81 0.60
Ethane assignments'"' were confirmed by dosing C2H6 on Ni/AI2O3 at 180 K. bStretching region was very noisy. '(sh) = shoulder, (b) = broad. dThese absorbances are thought to contain components of chemically adsorbed species. 'Noise level at 1600 cm-' was f0.02 (au X 100) for Ni/SiOz and 1 0 . 0 5 (au X 100) for Ni/A1201.
termined by using a Siemens Elmiskop I transmission electron microscope. Catalyst characterization results are reported in Tables I and I1 for the range of reduction temperatures used in IR cells 1 and 2. C. Experimental Procedures. The in situ treatment and dosing procedure for the catalyst wafers in IR cell 1 can be found in refs 24 and 62. For cell 2, the pretreatment was similar except the degas step was done with He flowing at 50 cm3/min (30 min at 410-500 K). For H2 or D, (99.5%) in situ reductions, a flow rate of 10-50 cm3/min was established and the cell was ramped at 8-10 K/min to 643 K for Ni/SiOz and 643-723 K for Ni/AIzO3 (2 h reduction period). Since the temperature of cell 2 depended strongly on the gas-phase pressure, the cell was evacuated of the reducing gas at ca. 223 K for 1-2 min and then filled with 0.6-30 Torr of He. These pressures enabled temperatures as low as 177 K to be reached (average between front and back thermocouple probes). Experiments that were done with chromel-alumel thermocouple wires pressed in the wafers showed that the wafer temperature was very close (1-3 K) to the average of the front and back thermocouple probes. In some experiments, He was flowed through the cell to a mechanical pump while keeping the cell pressure at 0.6-3 Torr. This procedure was used to reduce the amount of physically adsorbed species on the catalyst wafers. All surfaces were scanned after the pretreatment schedules to check for hydrocarbon impurities prior to the admission of the hydrocarbon gases. Typically (1.0-4.5) X 10l8molecules of C2H4 (99.99%, Matheson), CzD4(99% D4, Cambridge), CzH2(99.6%, Matheson"), or C2D2(99% D2, Cambridge) were dosed by syringe for the I R experiments in cell 2. The maximum dosing error was estimated to be f0.25 X IO'* molecules.
For physically adsorbed C2H4on SiO, and A1203,bands were observed at 2975 (s), 1443 (vs), and 1340 cm-l (w) and correspond to the v,,(CH2), 6(CH2)(v,,), and 6(CH2)(~3)modes, respectively. Analysis of the gas phase was done after desorption of the physically adsorbed species. For Si02, only ethylene was found. For Alz03, there was a very small conversion to ethane (0.23 carbon mol %), the balance being ethylene (99.77%). Both CzH4 and C2D4gave the same G C results for each support. For the adsorption of CzH2and C2Dzon Si02and A1203at 177-1 80 K, only physically adsorbed acetylene bands were observed. GC samples taken at
