J. Phys. Chem. 1985,89, 591-600 Both O3 and CH,C(O)OH were formed in relatively small amounts, and their analysis was judged to be accurate to only f20%. Nevertheless, it is apparent from Table I1 that their relative of those yields were comparable for all runs at approximately of the CH3C(0)OOH and that their absolute yields per -A(CH3CHO) increased significantly with added HCHO. Furthermore, when the irradiated samples were aged in the dark for approximately 15 min, none of these three products exhibited any indication of heterogeneous formation or decay. These results may, therefore, be taken as evidence for the Occurrence of reactions
591
la and 1b possibly via formation of a common adduct intermediate CH3C(0)OOOH and subsequent rearrangement and unimolecular dissociation. However, a more rigorous quantitative analysis of the overall reaction mechanism based on the observed product distribution requires more accurate data particularly on the H 2 0 2 yields. Further work is being planned to refine these data using a new photochemical reactor-IR cell with a much improved surface-to-volume ratio. Registry No. C H 3 C H 0 , 75-07-0; CI2, 7782-50-5.
Catalytic Etching of Platinum during Ethylene Oxidation Nae Lih Wu and Jonathan Phillips* Department of Chemical Engineering, The Pennsylvania State University, University Park, Pennsylvania I6802 (Received: July 24, 1984)
The catalytic etching of platinum foils during ethylene oxidation was studied primarily with electron microscopy (SEM and TEM). It was found that catalytic etching produced dramatically changed surfaces, the structures of which were a strong, consistent function of the foil temperature and the ethylene to oxygen ratio of the input feedstream. Control studies of the effect of etching in each individual reactant (including oxygen) and heterogeneous reaction product indicated that the observed catalytic etching was not the sum of the etching effects of the individual reactant and product molecules. Current models of the catalytic etching of platinum, which assume volatile platinum oxide species are responsible for catalytic etching, appear to be inadequate to explain these observations. A new model involving the interaction between a gas-phase radical, probably methylene, and the platinum surface is proposed.
Introduction It is generally understood that there are two types of surface etching. However, the mechanisms of each type of etching and the differences between the two types are not adequately understood. The first type, known as thermal etching, occurs in nonreactive atmospheres or in vacuum and normally results in the formation of boundary grooves and faceted surfaces. At very high temperatures (> 1500 K) pits may also develop. The study of thermal etching is an old one and is reviewed in many places.'-3 Several models have been postulated to explain thermal etching."'* The second type of etching, known as catalytic etching, occurs only in reactive atmospheres, is much more rapid, and takes place at lower temperatures than thermal etching. Although many studies of catalytic etching have been carried out,'$ surprisingly few are thorough and convincing. That is, in most works it is not adequately demonstrated that reaction conditions are both necessary and sufficient for producing the observed surface changes. These studies often leave open the possibility that either a single reactant or (in particular) a single product gas is responsible for the observed e t ~ h i n g . ' ~ - ' Also, ~ in most studies, inadequate (1) Moore, A. J. W. In "Metal Surfaces"; Robertson, W. D., Gjostein, N. A., Eds.; American Society for Metals: Metals Park, OH, 1963, p 155. (2) Shuttleworth, R. Metallurgica 1948, 38, 125. (3) Flytzani-Stephanopoulos,M.; Schmidt, L. D. Prog. Surf. Sci. 1979, 9, 83. (4) Herring, C. Phys. Rev. 1951, 82, 87. (5) Rhead, G. E.; Mykura, H. Acta Metall. 1962, 10, 843. (6) Rhead, G. E.; Mykura, H. Acta Metall. 1962, IO, 578. (7) Mullins, W. W. In "Metal Surfaces"; Robertson, W. D., Gjostein, N. A., Eds.; American Society for Metals: Metals Park, OH,1963, p 17. (8) Mullins, W. W. Philos. Mag. 1961, 6 , 1313. (9) Moore, A. J. W. Acta Metall. 1962, I O , 579. (10) Moore, A. J. W. Acta Metall. 1958, 6 , 293. (11) Andrade, E. N.; Randall, R. F. Proc. Phys. SOC.,London, Sect. B 1950, 63, 198. (12) Moreau, J.; Benard, J. J. Znst. Met. 1954-1955, 83, 87. (13) Pareja, P.: Amarigilo, A,; Piquard, G.; Amariglio, H.J . Catal. 1977, 46, 225.
0022-3654/85/2089-0591$01.50/0
attention is given to the effect of reaction parameters (e.g., temperature, gas composition) on the final morphology. New models of the catalytic etching process are clearly needed. Most models of e t ~ h i n g ' , ~apply , ' ~ to surface faceting (thermal etching) and not to pitting, fuzzing, or other types of reconstruction which are frequently associated with catalytic etching. Moreover, most models of catalytic etching are simply modifications of models of thermal etching. For example, the most widely cited model of platinum catalytic etching is that volatile platinum oxide is formed at catalytically inactive sites and decomposes depositing platinum at new active site^.'^*'^ Platinum oxide has a fairly high vapor pressure at elevated temperatures18 and can, in some circumstances, transport significant amounts of p l a t i n ~ m . ' ~ - ~ ' However, it has not been directly shown that platinum oxide is the intermediate responsible for etching under reaction conditions nor is it clear why, purportedly, platinum oxide formation will produce pitting and fuzzing under reaction conditions, but only faceting and grain boundary grooving in oxygen only atmospheres. Furthermore, it is unclear why platinum oxide formation is used to explain widely different catalytic etching processes. For example, how can PtO, formation be used to explain both the massive platinum metal loss and fuzzing which occurs during ammonia ~ x i d a t i o n and ~ ~ -the ~ ~formation of broad facets, without (14) Norris, L. F.; Parravano, G. In "Reactivity of Solids"; Mitchell, De Vries, Roberts, Cannon, Eds.; Wiley: New York, 1969; p 149. (15) McCabe, R. W.;Pignet, T.; Schmidt, L. D. J . Catal. 1974, 32, 114. (16) Frohberg, G.; Adam, P. Thin Solid Films 1975, 25, 525. (17) Flytzani-Stephanopoulos,M.; Schmidt, L. D. Chem. Eng. Sci. 1979, 34, 365. (18) Alcock, C. B.; Hooper,G. W. Proc. R. Soc. London, Ser. A 1960,254, 551. (19) Schlfer, H."Chemical Transport Reactions"; Academic Press: New York, 1964. (20) Fryburg, G. C.; Petrus, H. M. J. Electrochem. SOC.1961, 108,496. (21) Nowak, E. J. Chem. Eng. Sci. 1966, 21, 19; 1969, 24, 421. (22) Handforth, S. L.; Tilley, N. J. Znd. Eng. Chem. 1934, 26, 1287.
0 1985 American Chemical Society
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The Journal of Physical Chemistry, Vol. 89, No. 4, I98 5 1
A
I
L u
Wu and Phillips attaching the reactor to a glass high vacuum system. Pressures torr were reached. The vacuum system also as low as 1 X served as a gas purification and introduction system. Calculations of the approximate Reynolds number for a foil setting on the sample holder in the exit tube (see Figure 1) indicate that under the operation conditions employed the flow was laminar, thus satisfying requirement (iv). Experiments showed that the use of Pt-Pt-10% Rh thermocouple spot welded to the downstream side of the foil sample did not effect the final morphology (requirement (v)). Experiments also showed that in nonreactive gas mixtures the set oven temperature (Lindberg Series 5400), the measured gas temperature, and the sample temperature agreed within f 5 OC. Since ethylene oxidation is exothermic, under reaction conditions the sample temperature was found to be much higher than the gas-pressure temperature. Gases used in this experiment were research grade and were further purified whenever possible. For example, oxygen was further purified by passing through a cooled 5-A molecular sieve trap. Nitrogen was passed first through a heated copper turning trap and then through a cooled 5-A molecular sieve trap. Ethylene of 99.98% purity was used as received. Wet nitrogen was produced by passing the prepurified nitrogen through distilled water. Total gas flow was maintained at 300 cm3/min (10 cm/s over the sample) during all experiments. Composition of the gas was controlled with calibrated rotameters. Prior to performing all experiments (both thermal and catalytical etching), each platinum foil sample was activated following a standard procedure or a ‘low-temperature” variation. First, the torr) annealed at platinum samples were vacuum (ca. 1 X a temperature of 800 K for 0.5 h. Next, a mixture of 1:l 02:N2 was passed over the samples for 1 h at 1170 K. In a few instances, this step was performed at only 670 K. This is the only difference between standard activation and low-temperature activation. Following that each sample was first cooled to about 570 K and then heated to the experimental temperature in pure N2. The final gas-phase mixture was not introduced into the reactor until the chosen experimental temperature had been reached. The activation procedure followed is similar to that used by other worke r who ~found ~that a ~high-temperature ~ ~ calcining step is required to “activate” platinum samples. Other w o r k e r ~have ~~,~~ reported that Auger studies indicate that treating platinum at high temperatures in oxygen removes most impurities leaving a clean platinum surface. In general, once the final gas mixture was introduced experimental conditions were maintained for between 40 and 70 h. At the end of each experiment, the sample was cooled rapidly (-20 min) to room temperature in flowing nitrogen. After removal from the reactor, the samples were examined in a SEM and/or a STEM. In some instances, surface features such as (platinum) particles and (carbon) deposits were removed from the sample surfaces for further investigation. The particles were removed from the foil surfaces by replica tape and then transferred to an evaporated thin carbon film by a standard replica technique.35 Thick carbon deposits were removed from the samples by dissolving the platinum foil in aqua regia. Energy dispersive X-ray analysis (EDX) and selected area diffraction (SAD) were used for chemical and structural analysis of these isolated surface structures.
W2FTf-J - -- - -
-- - --
E
Figure 1. Quartz reactor: (A) gas inlet; (B) gas outlet; ( C ) preheater coil; (D) furnace; (E) platinum foil; (F) insulated thermocouple; (G) quartz sample rod.
significant metal 10ss,24325which occurs during H C N synthesis? Oxygen is present during both processes and indeed the temperature is higher during H C N synthesis. Also it is unclear why pitting and fuzzing should result during ammonia oxidation while in the same reactor deep grooves form during propane oxidation.26 In the present work, a thorough study of the etching of polycrystalline platinum foils used as ethylene oxidation catalysts was carried 01’’ The study was designed (i) to distinguish thermal etching el. , f s clearly from catalytic etching effects and (ii) to determine the critical reaction parameters which result in the surface being etched in a particular manner. The principal analytical tools used in this work were scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It was found that catalytic etching, as contrasted to thermal etching, takes place over a limited range of temperature and that the type of etching is also a function of the oxygen to ethylene ratio of the reactant gases. Significantly, at high temperature in excess oxygen where the formation of platinum oxide is most favored, no catalytic etching took place, demonstrating that platinum oxide formation cannot explain the observed etching process. Thus, a new model was developed. The model is consistent with all the results obtained in this study and, with appropriate modification may explain, in general, catalytic etching which takes place during hydrocarbon oxidation reactions.
Experimental Section The main thrust of the experimental work done for this study was the treatment of polycrystalline platinum foil samples (99.99% purity, Goodfellow Metals, Ltd.) under a variety of controlled reaction conditions and the subsequent examination of the foil samples in a scanning electron microscopy (SEM, IS1 SUPER IIIA) or scanning transmission electron microscope (STEM, Philips 420). The reactor used for sample preparation in this study (Figure 1) was designed (i) to permit treatment of the samples at controlled gas-phase temperatures as high as 1473 K, (ii) to allow exposure of the samples only to preheated well-mixed gases of controlled composition, (iii) to allow vacuum annealing, (iv) to produce laminar flow conditions over the sample surface, (v) to allow nonintrusive measurement of the foil temperature, and (vi) to allow easy introduction and removal of the foil samples. The reactor was not designed to allow control of the sample temperature as earlier work suggests that that might effect surface morph~logy.~’-*~ Requirements (i) and (ii) were met by building the reactor from quartz (Quartz Scientific, Inc., Fairport Harbor, OH) and having the gas pass through a long preheater coil (Figure 1) before reaching the sample. Requirement (iii) was met by (23) Bartlet, R. W.J . Electrochem. Soc. 1967, 114, 547. (24) Schmidt, L. D.; Luss, D. J . Catal. 1971, 22, 269. (25) Kauffer, C. T.; Leitenberger, W. Chem. Eng. Tech. 1953, 25, 697. (26) Flytzani-Stephanopoulos,M.; Wong, S.; Schmidt, L. D. J . Caral.
1977. 49. 51
(i7)Miowca, B. A. J . Appl. Phys. 1943, 14, 684. (28) Langmuir, D. B.Acta Metall. 1957, 5 , 13. (29) Hondros, E. D.; Moore, A. J. W. Acta Metall. 1960, 8 , 751
Results Experiments were carried out with two goals, to demonstrate conclusively the existence of a chemical etching process during ethylene oxidation and to determine the critical parameters (e.g., ethylene to oxygen ratio, temperature) which resulted in the (30) Vayenas, C. G.; Georgakis, C.; Michaels, J.; Tormo, J. J . Cutal. 1981, 67, 348. (31) Vayenas, C. G.; Lee, B.; Michaels, J. J. Cutal. 1980, 66, 36. (32) Zuniga, J. E.; Luss, D. J . Caral. 1978, 53, 312. (33) Somorjai, G. A. Cutal. Rev. 1972, 7 , 87. (34) Pignet, T. P.; Schmidt, L. D.; Jarvis, N . L. J . Cutal. 1973, 31, 145. (35) Goodhew, P. J. In ‘Practical Methods in Electron Microscopy”; Glauert, A. M., Ed.; North-Holland: Amsterdam, 1972; p 137.
Catalytic Etching of Platinum
The Journal of Physical Chemistry, Vol. 89, No. 4, I985 593
TABLE I: Thermal Etching: Foil Sample History and Surface Reconstruction sample
pretreatment''
historyb
2
standard standard
3
standard
4
standard
5
standard
6
standard
7
standard
8 9 10
standard standard standard
none 50%02/50% N2 at 973 K for 65 h 50%0 2 / 5 0 % N2 at 893 K for 40 h 8% 0 2 / 9 2 % N2 at 893 K for 48 h 2% C2H4/98% N, at 958 K for 38 h 2% C2H4/98% N2 at 873 K for 48 h 1 % C2H4/99% N2 at 873 K for 48 h wetted N, at 893 K for 4 5 h wetted N2 at 973 K for 4 5 h 1% C o 2 / 9 9 % N2 at 873 K for 4 0 h
1
surface
i
For standard pretreatment procedure, see Experiment Section. bThe etching temperature listed in Table I is the gas-phase temperature. The sample-gas temperature differences in these experiments are less than 10 K. CType A: Two types of facets: (1) round-ridge hill and valley facets with width typically < 0.5 pm, (2) small scale cubelike facets (see Figure 2). dType B: Hill-and-valley facets. Some grains are partially and lightly covered with carbonaceous deposit (see Figure 3).
surface being etched in a particular manner. To accomplish the first goal, an extensive set of thermal etching (individual gases) studies was carried out. To accomplish the second goal, an extensive set of catalytic etching (reaction gas mixtures) studies was conducted. Thermal Etching. Studies were made of the reconstruction of polycrystalline foil samples given the standard activation which occurred in all reaction gases (N2, 02,C2H4) and product gases (C02, H 2 0 ) encountered by a platinum surface during the heterogeneous oxidation of ethylene.36 (A host of other gas-phase species are also present at the temperatures encountered in these studies3' However, as shown in the Discussion section, none of these could be responsible for the observed reconstruction and hence these species were not studied individually.) Each gas was mixed with nitrogen carrier gas such that its concentration was approximately the same as, or somewhat larger than, its concentration over the sample under reaction conditions. The temperatures selected were those at which the greatest etching took place under reaction conditions. The difference between sample temperature and gas temperature during these experiments was found to be negligible. A complete list of the thermal etching experiments and the results obtained are given in Table I. Oxygen. It is particularly important to study the structure of the surface following prolonged treatment in oxygen for two reasons. First, the initial activation procedure is performed in a 1:l mixture of N 2 and O2and it is very important to know the common starting point for all etching studies. Second, it is widely assumed that volatile platinum oxide formation is responsible for the etching of platinum. Certainly if this is true, the greatest etching should take place in the highest oxygen concentrations. It was found that standard activation (see Experimental Section) produced a clean surface with small-scale (
