Influence of Potassium on Carbon Monoxide Hydrogenation over Iron

over Iron: A Surface Analytical Study. D. A. Wesner,* ... clean and K-modified surfaces show reaction poisoning due to the large (up to 10 monolayers)...
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Langmuir 1985,1, 478-487

Influence of Potassium on Carbon Monoxide Hydrogenation over Iron: A Surface Analytical Study D. A. Wesner,* F. P. Coenen, and H. P. Bonzel Znstitut fur Grenzflachenforschung und Vakuumphysik, Kernforschungsanlage Julich, D-5170 Julich, West Germany Received December 27, 1984. I n Final Form: April 1, 1985 The CO hydrogenation reaction was studied on clean and potassium-modified polycrystalline iron foil surfaces. The approach combines on-line gas chromatographic analysis of the reaction products coming from a microreactor with ultrahigh vacuum surface analysis of the catalyst after reaction by X-ray photoemission and Auger spectroscopy. Reaction conditionswere as follows: temperature 498-623 K, CO:H2 ratio 1:20, total pressure 100 Wa, and initial potassium coverages between 0 and about 1monolayer. Both clean and K-modified surfaces show reaction poisoning due to the large (up to 10 monolayers) graphitic carbon buildup within a few hours, as observed in both CHI production and in the production of C2-C5 higher hydrocarbons. Before this graphitic stage, the K-modified surfaces show enhanced selectivity for higher molecular weight products (chain growth factor cy approximately 0.39 vs. 0.30, *0.03), a reduced methanation rate, and an enhanced rate of carbon deposition on the surface, relative to clean Fe. Surface analysis after reaction shows that both surfaces are carbided early in the reaction and have graphitic carbon at later stages, the latter species accumulating more quickly on K-dosed surfaces. Potassium induces an additional C species with a 285.8-eV C 1s binding energy at low reaction temperature and/or short reaction times. This species is active in hydrogenation and its presence correlateswith the enhanced chain growth probability. An initial surface coverage of about 0.7 monolayer of K is sufficient to produce it at a reaction temperature of 498 K. The unpromoted surface is reduced during reaction, but the K-modified surface stabilizes oxygen in a 1:l stoichiometry.

Introduction The CO hydrogenation reaction or Fischer-Tropsch synthesis on transition-metal catalysts is a subject that has received intense scrutiny in studies employing a wide variety of experimental techniques. Relatively early studies tended to be kinetic investigations using catalytic chemistry techniques on samples that were close to real technical catalysts. Reactant pressures were often near or above atmospheric pressure, as in the actual catalytic system. In contrast, rapid progress in surface analysis techniques in recent years has made possible investigation of monolayer amounts of chemisorbed species on transition metals. These studies are carried out in ultrahigh vacuum (UHV) with adsorbate coverages on the order of a monolayer, often on single-crystal surfaces. Still more recent are model experiments which attempt to bridge the differences of sample type and approximately 12 orders of magnitude in pressure between these two classes of experiments.l+ They combine UHV surface analysis with the ability to isolate the sample and expose it to a gaseous reactant stream a t the higher pressures characteristic of the technical catalytic system, often with on-line monitoring of the reaction products via mass spectrometry or gas chromatography. The strength of such "combination" experiments lies in their ability to study a catalytic reaction and to correlate the chemical state of the catalyst surface as determined from UHV surface analysis with the observed reaction products and kinetics. Our study involves this third type of investigation and addresses the question of alkali-metal promotion of CO hydrogenation, specifically, that occurring on K-modified Fe foil surfaces. The promoting effect of K on Fe catalysts has long been known7and has been investigated in detailed (1) Krebs, H. J.; Bonzel, H. P.; Gafner, G. Surf. Sci. 1979, 88, 269. (2) Goodman, D. W. J. Vac. Sei. Technol. 1982, 20, 522. (3) Somorjai, G. A. Catal. Rev.-Sci. Eng. 1978, 18, 173. (4) Somorjai, G. A.; Zaera, F. A. J.Phys. Chem. 1982,86, 3070. (5)Dwyer, D. J.; Hardenbergh, J. H. J. Catal. 1984, 87, 66. (6) Dwyer, D. J.; Hardenbergh, J. H. Appl. Surf. Sci. 1984, 19, 14.

0743-7463/85/2401-0478$01.50/0

kinetic ~ t u d i e s .They ~ ~ ~found that K shifts the product distribution to higher molecular weight hydrocarbons and increases the CO heat of adsorption. Surface analytical studies have been made of the electronic effects of K adsorbed on Fe surfaces and its influence on subsequent CO and H2 adsorption.lOJ1 A general consensus reached among these works is that K promotes the dissociation of CO by weakening the C-0 bond. We are concerned in the present study with the further question of the chemical state of the surface during reaction conditions and in relating it to the reaction activity and selectivity. Previous studies on CO hydrogenation over transition metals have combined surface analysis with kinetic meas u r e m e n t ~ , ~ and , ~ ,some ~ ~ ~have ~ - ~considered ~ K-modified surfaces as ~ e l l . ~ The J ~ 'results ~ of kinetic studies have usually been reproduced, and the surface analysis has revealed different species on the K-promoted surfaces after reaction. Particularly important in this regard has been the distinction between carbidic and graphitic C deposits revealed by electron spectroscopy. The former species is associated with catalytically active and the latter with unreactive poisoned ~ u r f a c e s . ~A~relatively J~ recent stud9 reported a third species, apparently polymeric C, to be dominant after reaction on K-promoted Fe powders, but (7) Anderson, R. B. In "Catalysis"; Emmett, P. H., Ed.; Reinhold New York, 1956, Vol. 4, Chapters 1, 2. (8) Dry, M. E.; Shingles, T.; Boshoff, L. J.; Oosthuizen, G. J. J. Catal. 1969, 15, 190. (9) Dry, M. E.; Shinglea, T.; van H. Botha, C. S. J. Catal. 1970,17,341. (10) BrodBn, G.; Gafner, G.; Bonzel, H. P. Surf. Sci. 1979, 84, 295. Benziger, J.; Madix, R. J. Surf. Sci. 1980, 94, 119. (11) Ertl, G.; Lee, S. B.; Weiss, M. Surf. Sci. 1981, 111, L711. (12) Bonzel, H. P.; Krebs, H. J. Surf. Sci. 1980, 91, 499. (13) Erley, W.; McBreen, P. H.; Ibach, H. J. Catal. 1983, 84, 229. (14) Goodman, D. W.; Kelley, R. D.; Madey, T. E.; Yates, J. T., Jr. J. Catal. 1980, 63, 226. (15) Dwyer, D. J.; Somorjai, G. A. J. Catal. 1978, 52, 291. (16) Bonzel, H. P.; Krebs, H. J. Surf. Sci. 1981, 109, L527. (17) Bonzel, H. P.; BrodBn, G.; Krebs, H. J. Appl. Surf. Sci. 1983,16, 373. (18) Campbell, C. T.; Goodman, D. W. Surf. Sci. 1982, 123, 413. (19) Somorjai, G. A. Surf. Sci. 1979, 89, 496. Somorjai, G. A. Cat. Reu.-Sci. Eng. 1981, 23, 189.

0 1985 American Chemical Society

Langmuir, Vol. 1, No. 4, 1985 479

Influence of Potassium on Hydrogenation

the relationship between this species and the reaction kinetics is not yet clear. A goal of the present study is thus to correlate the species observed in surface analysis following reaction with the increased higher hydrocarbon selectivity and other effects of K. Because there are several effects operating at once, and because CO hydrogenation on Fe foils poisons relatively fast due to graphitic C buildup, this has required time-dependent measurements and a more detailed, systematic study than previously done for this system. Another question we want to address concerns the chemical state of the K promoter species during the hydrogenation reaction. Partially reduced layers of adsorbed K salts (K2C03,KN02, KOH) on Fe foils are thought1' to contain mostly KOH, but other species have been proposed as the active compound under reaction conditions.8,20

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Experimental Details With some exceptions, the apparatus and experimental procedures are similar to those used in previous studies in our lab~ r a t o r y . ' * ' ~ ~The ' ~ J ~apparatus combines an ion-pumped, UHV surface analysis chamber with a catalytic microreactor operating at atmospheric pressure. A valveless sample-transfer mechanism couples the two sections and allows relatively rapid (45 s) movement between them without exposing the sample to atmosphere or interrupting the reactant gas flow. The ultimate base pressure in the UHV section is about 4 X Pa and during Pa, most of the additional measurements was about 1 X pressure being due to the Ar used in ion-bombardment sample cleaning. The microreactor volume is 4 cm3 and the total reactant flow rate was 15 cm3/min (at room temperature and atmospheric pressure). All of the reactions were done with a CO:H2 partial pressure ratio of 1:20 and a total pressure of 100 kPa. Reaction temperatures (i.e., sample temperature during reaction) varied between 498 and 623 K. The temperature was regulated to within i l K by a feedback-controlled resistive heating circuit. Cooling (-300 K) of the reactor housing and sample supports minimized any possible reactions with these parts. As a check, we did control experiments with a catalytically inactive Cu sample and found no significant amounts of reaction products. After leaving the microreactor the reactant-product gas stream was directed to the sampling valve of a gas chromatograph (Hewlett-Packard 5840 A). Products were separated over an 8-ft Porapak Q column run at 423 K and detected by a flame ionization detector. Cl-C5 hydrocarbon peak retention times and sensitivities were identified by calibration with standard gas mixtures. The fraction of CO converted to products under all reaction conditions was low: typically less than lo4. We calculated the turnover frequency for CHI production as in a previous work,' assuming an adsorption site density of 1015sites/cm2 for the clean polycrystallineFe foil. Because the degree of site blocking by K is not known, however, we do not plot the turnover frequency in the CH, production rate graphs below. Instead the ordinates are simply moles of CHI produced per centimeter squared per second. For clean surfaces, then, a CH, production rate of 1.66 X lo4 mol/(cm2s) corresponds to a turnover frequency of 1 molecule/(site/s). The UHV surface analysis part of the apparatus allows X-ray photoemission (XPS) and electron-excited Auger measurements (AES)to be made after transfer from the microreactor. Electrons were energy analyzed by a 100-mm radius hemisphericalanalyzer (Leybold LHS-10). For X P S , the pass energy was 50 eV (constant absolute resolution mode), yielding a total resolution for photons plus electrons of less than 1eV. The XPS excitation source was unmonochromatized Mg KLYradiation, hv = 1253.6 eV. XPS electron energy distribution curves are given in terms of binding energy with respect to the Fe Fermi level EF. For a clean Fe surface the Fe 2p3/, peak came at a binding energy of 706.7 eV, in good agreement w t h previously reported values for this core l e ~ e l . ~ All ~ ~XPS ~ ' , data ~ are shown with ordinates in "arbitrary (20)Ozaki, A,; Aika, K.; Morikawa, N. In "Proceedings of the Fifth International Congress on Catalysis"; Hightower, J. W., Ed.; NorthHolland: Amsterdam, 1973;Vol. 2, p 1251.

units", but the unit sensitivities are the same for all spectra within a given figure. The polycrystalline Fe foil sample was 0.2 mm thick with a catalytically active area of 6 X 6 mm2. This active area was the central part of a 6 X 40 mm2strip, the length ensuring temperature uniformity over the center. Other parts of the foil were either shielded from sputter cleaning and thus passivated by surfacesegregated impurities or were cooled by conduction from the sample supFsrts and thus were at too low a temperature to participate in the reaction. Heating (-1000 K), 4-keV Ar ion sputtering, and H2 reduction (100 kPa, 823 K) were used to produce a surface free of all contaminants other than a small amount of 0. Figure 1 shows an AES spectrum from such a surface and from one with 1ML of K (ML = monolayer). Because of what happens to the 0 species on the surface during the subsequent reaction on either clean or K-modified samples (see below), we believe that the small 0 contamination had no effect on the measurements. Previous investigations of CO hydrogenation on K-modified Fe surfaces have used samples that were impregnated with a K salt solution at atmospheric pressure. The samples then underwent further processing steps such as reduction in H2 or heating in UHV before reaction in order to remove the oxygen contamination and decompose the ~alt.67'~We used instead a resistively heated, commercial K dispenser source (SAESGetters type KNF 2.2/12) to produce clean K overlayers in vacuum which required no further preparation before reaction. Deposition was controlled by varying the dispenser temperature, and the amount deposited could be determined by the height of the K (LMM) Auger peak at 251-eV kinetic energy relative to the Fe (LMM) peak at 651-eV kinetic energy, which was found to correlate well with the K 2plI2-2p3/*peak area measured in XPS. It was previously shownB that on Fe(ll0) this ratio is equal to the K coverage in monolayers (ML) for less than 1ML. Here 1ML is understood to mean the room-temperature saturation K coverage, which on Fe(110) corresponds to 0.31 K atom per Fe surface atom. Although we have a polycrystalline Fe sample, we have assumed that the result is approximately valid in our case and report the K coverages in ML derived from this AES ratio. By taking K 2p Auger spectra from different parts of the surface it was also confirmed that K deposition was spatially uniform over the catalytically active area. As will be seen below, the amount of K did not always stay constant during a reaction. The references to coverages, unless otherwise noted, refer to the initial amount, before reaction. We carried out two types of experiments: either gas chromatography measurements would be taken as the sample continuously underwent reaction in the microreactor, or the reaction would be periodically interrupted by cooling the sample and transferring it back into UHV, where electron spectroscopic measurements would be made. As in a previous study,' we found that the results were not affected by this interruption for mea(21)Fuggle, J. C. J . Electron Spectrosc. Relat. Phemm 1980,21,275. (22)Textor, M.;Gay, I. D.; Mason, R. Proc. R. SOC. London, Ser. A 1977,356,37. (23)Brodgn, G.; Bonzel, H. P. Surf. Sci. 1979,84, 106.

Wesner, Coenen, and Bonze1

480 Langmuir, Vol. 1, No. 4 , 1985 - 1600 I

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Results Gas Chromatographic Measurements. Methane was the dominant reaction product formed over all of the surfaces studied. Figures 2-4 show the CH4 production rates as a function of time during the reaction for several reaction temperatures and over surfaces with K coverages between 0 and 1 ML. Each data point represents the CH, peak area from a gas chromatogram taken at the indicated time. The production rates over clean Fe (top curves in Figures 2-4) are in qualitative agreement with those previously observed.’ Generally there is a delay of about 100 s after the sample is brought to the reaction temperature in the microreactor before any CHI is detected at the gas chromatograph. This represents the time for a volume of gas to flow from the reactor to the gas chromatograph sampling valve. On the clean Fe surface the methane production rate then rapidly rises to a maximum value within the first 10 min, followed by a decline. The maximum value reached and the steepness of the subsequent decline both increase with reaction temperature. This decrease in methanation rate has been observed before1J4 and attributed to the buildup of an inactive carbon species on the surface. From the appearance of the Auger line shape it has been called “graphitic” but may contain bonded H.12 The more active form of carbon observed earlier in the reaction (or at lower reaction temperatures) has an Auger line shape suggestive of a carbide-like species

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Figure 4. As in Figure 2 but for a reaction temperature of 548 K. Note the change in ordinate sensitivity relative to Figures 2 and 3.

but has also been assigned to a hydrogen-containing species.12 Consider next the CHI production rates over an Fe surface with 1 ML K, plotted as the bottom curves in Figures 2-4. They show much different behavior. First, the absolute rate is significantly lower. A lowering of the methanation rate on Fe6J9 and Nil8 surfaces upon the addition of monolayer amounts of K has also been previously observed. Second, the time dependence of the production rate is different. Only for a 573 K reaction temperature (not shown) is something similar to the clean Fe surface behavior observed, but even in that case the decline is less sharp than on the clean surface a t the same reaction temperature. At the lower temperatures plotted in Figures 2-4, a more stable time dependence is observed on the K-modified surfaces. Another difference between K-modified and clean surfaces concerns the higher hydrocarbon reaction products. Hydrocarbons up to C4, and sometimes C5, were detected for reactions on both K-modified and clean surfaces. A clear separation of a given C number product into saturated, unsaturated, or oxygenated species was not always possible, but in general the product distribution favored unsaturated hydrocarbons. In the case of C2 production on clean Fe, typically only about 15% of the total product was C2H@ For K-modified Fe (1 ML of K), the fraction was even less: typically 7-10%. Some methanol was also detected, but the peak was usually small: on the order of the C4 peaks. If the higher C number reaction products follow a Schulz-Flory-type d i ~ t r i b u t i o nthen ,~~ M p = (In C Y ) ~ ~ P . P where M p is the mass fraction of product with C number p and a is the chain growth factor, i.e., the probability that a hydrocarbon chain adsorbed on the surface will grow. The chain growth factor a was obtained from the slope of a least-squares line fitted to a plot of log ( M p / p )vs. p ; results are summarized in Table I for several surfaces. Despite some scatter in the values, possibly related to the small number of data points used in the fit, some general features became clear. The selectivity for higher hydrocarbons increases for the K-modified surface relative to clean Fe and is approximately independent of reaction temperature. A maximum a value of about 0.30 and 0.39 for clean and K-modified (1 ML of K) surfaces, respectively, is observed. This maximum value generally obtains (24) Henrici-Oliv& G.; Oliv6, S. Angew. Chem., Int. Ed. Engl. 1976, 15, 136. Satterfield, C. N.; Huff, G. A., Jr. J. Catal. 1982, 73, 187.

Langmuir, Vol. 1, No. 4, 1985 481

Influence of Potassium on Hydrogenation Table I. Chain Growth Factor a as a Function of Reaction Time" K coverage 0 ML

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548 K

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time, min

a

2.0 7.1 27.8 57.0 77.1 104.8 141.7 2.0 11.5 30.1 48.1 103.7 167.9 2.0 11.5 30.4 58.5

0.28 0.28 0.30 0.31 0.31 0.27 0.27 0.27 0.30 0.29 0.27 0.24 0.22 0.31 0.28 0.23 0.20

time, min

a

4.0 13.1 31.1 69.5 91% 114.1 136.1 2.0 11.5 30.6 57.5 103.1 166.8 2.0 11.6 30.5

0.36 0.39 0.34 0.28 0.33 0.28 0.26 0.39 0.31 0.29 0.28 0.27 0.27 0.38 0.29 0.27

Table 11. Chain Growth Factor CY and Percent Unsaturated Product as a Function of Initial K Coverage after 70 min of Reactiono K coverage OK, ML reaction temn o 0.2 0.25 0.70 0.75 1.m Chain Growth Factor a 498 K 523 K 548 K

0.29 0.29 0.24

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" The estimated uncertainty in a is 10.03. only early in the reaction, usually within the first 20 min. Later a decreases; this decrease is most apparent a t the higher reaction temperatures, e.g., for a 573 K reaction on either clean or K-modified surfaces. At the lower temperatures on both types of surfaces a is more nearly constant with reaction time. Further experiments were done to detail the effects of K on the reaction products. Methanation rates were measured on surfaces with varying amounts of K, between 0 and 1 ML, a t several reaction temperatures, as shown in Figures 2-4. The decrease in the CH4 production rate with K coverage is apparent, as is the change in the time dependence of the rate. It is also clear that the effect of K on CHI production is smallest for higher temperatures and/or long reaction times. The curves for the various K coverages a t a 548 K reaction temperature (Figure 4), for example, merge after approximately 30 min of reaction, and those for 523 K (Figure 3) appear to be doing the same, but apparently after a longer time than is shown. However, the curves for the 498 K reaction temperature (Figure 2) are not merging at all over the 90-min time scale plotted in Figure 2. This was also the temperature and reaction time scale over which C 1s XPS showed most clearly a different species of C on the surface (see below). There is a noticeable effect a t all reaction temperatures after addition of as little as 0.20 or 0.25 ML of K. Higher hydrocarbon production was also studied as a function of initial K coverage, and results are summarized in Table 11. Chain growth factors are given as a function of initial K coverage and reaction temperature. All the values are means of several measurements made between 60 and 80 min of reaction time, when all production rates are reasonably constant. Only for the 498 K reaction temperature is the increased higher hydrocarbon selectivity visible. For the higher reaction temperatures the general decline of a with reaction time mentioned above has already set in, so that the effect of K is not so striking. Table I1 also shows the percentage of unsaturated C2 products produced under the various reaction conditions. This number always increases with the initial K coverage. It remains high even on surfaces with a relatively low a value. Electron Spectrpscopic Measurements. XPS measurements were made on the variously prepared surfaces before and after reaction to identify the surface species present. Spectra were recorded as a function of reaction

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Wesner, Coenen, and Bonze1

482 Langmuir, Vol. 1, No. 4 , 1985 carbidic C peak grows and undergoes a slight shift to higher binding energies. At the same time the second peak is also growing near 284.5-eV binding energy. Intensity near this binding energy region has been attributed to the buildup of graphitic C.536J2We also measured a graphite sample (Union Carbide Corp. "Grafoil" exfoliated graphite) and found the C Is core level at 284.4-eV binding energy. However, a binding energy of 284.2 eV has also been assigned to a carbidic C layer containing bonded H,12 so it is not clear from these XPS measurements alone how much graphitic C has built up. Other experiments (AES and hydrogenation, see below) suggest that there is little early in the reaction, and that it builds up as the reaction proceeds. As will be seen below, the buildup of graphitic C is also a function of reaction temperature, occurring faster a t higher temperatures. Figure 5b shows C 1s XPS spectra after reaction at 548 K on a surface with an initial K coverage of 0.94 ML. Reaction times parallel those in part a. The spin-orbitsplit K 2p line shapes appear a t 293.4- and 296.1-eV binding energy. Comparing to the data on clean Fe, we observe significant differences as a function of reaction time. Before starting the reaction there is slightly more intensity in the C 1s binding energy region than for the clean Fe surface. Part of this stems from Mg K a satellite excitation (at about 10% of the main line intensity) of the K 2p levels. Some could also be due to CO adsorption and dissociation, since adsorbed K increases the probability of CO dissociation.1° Looking next a t the longest times, a peak at 284.5-eV binding energy is seen, implying graphitic C. This species is apparently more graphitic than that produced on a clean surface after an equal reaction time. For example, all the spectra in part a show only a double peak, but already in the 60-min spectrum of part b there is a broad (2.6-eV fwhm) peak centered near 284.5-eV binding energy with only a slight shoulder suggestive of carbidic C. A t 180-min reaction time even this shoulder disappears as the peak narrows (2.0-eV fwhm). For the shorter reaction times also there are differences. Three distinct peaks are visible near 283.1, 285.8, and 289.0-eV binding energy after 0.25 and 1.0 min of reaction. The peak a t 289.0 eV is close to the C 1s binding energies observed for adsorbed K2C03layers on Fe6J7and for bulk Na2C03samples.25 The 283.1-eV peak is slightly shifted from the position of the carbidic peak on clean Fe. The peak at 285.8-eV binding energy is in the binding energy range normally observed for oxygenated C species,25but for reasons given below, we do not believe it is such. At intermediate reaction times a merging of the carbidic peak and the 285.8-eV peak occurs, and the carbonate peak disappears. There is a broad peak centered near the graphitic binding energy of 284.5 eV which then narrows with further reaction (15-min spectrum in Figure 5b). As the C builds up during reaction the K 2p peaks become attenuated, probably corresponding to desorption of K (see below). Although this was difficult to quantify, it appeared that the largest decrease in K 2p intensity occurred when appreciable amounts of graphitic C begin to appear, e.g., with the 15 min and later spectra in Figure 5b. Figure 6 presents further measurements which show how reaction temperature influences the composition of the carbonaceous layer produced by CO hydrogenation. Plotted here are K 2p and C Is XPS measurements on clean and K-modified (1 ML of K) Fe surfaces after reaction at various temperatures. The reaction times were all 1 h, except for some of the 573 K spectra, where dif-

ferent times are indicated. Again there are two components in the clean Fe data in part a. The 283.3-eV binding energy peak appears to be the dominant peak a t low reaction temperatures, e.g., 523 and 548 K. At 498 K very little C is deposited. With higher temperatures the two peaks appear to be of equal strength. After a 3-h reaction at 573 K the graphitic peak is dominant, with only a slight peak asymmetry toward the low binding energy side suggesting the presence of carbidic carbon. Such a line shape is apparently the saturation form of C which appears after long reaction times (or high reaction temperatures), once enough C is on the surface. Figure 6b shows data for a surface with an initial 1 ML of K coverage. The 285.8-eV binding energy peak is largest a t a 498 K reaction temperature, where it is the dominant C species. As was observed on clean Fe surfaces, the effect of higher reaction temperature is similar to that of longer reaction time at a fixed temperature. As the reaction temperature is increased in part b, the spectra show a trend toward a single graphitic C 1s peak similar to that which occurs with reaction time in Figure 5b. The 283.3- and 285.8-eV peaks shift toward each other and begin to merge into a single peak a t the graphitic binding energy, and intensity near the carbonate binding energy disappears. The AFS data in Figure 7 show the C (KLL) region after reaction at various temperatures and provide further evidence for the peak identifications given above. Considering first the clean Fe data in part a, the line shape changes in going from 523 to 573 K are consistent with a carbidic to graphitic transition. In particular, the change in the relative heights of the maxima near 248, 256, and 266 eV and the disappearance of the double-minima structure at 270-275 eV both suggest this. Such differences in AES line shape for carbidic and graphitic surface layers on transition-metal surfaces have been previously observed, both in catalytic systems1J2J4and in C layers on Nb.26 Figure 7b shows AES measurements on the K-modified surfaces after reaction. The C (KLL) line shape is partly obscured by the K (LMM) line (minimum near 250-eV kinetic energy). However, the same qualitative differences occur in the C AES line shape and it is clear that the reaction layer proceeds from one having mostly carbidic character to one with graphitic character. To further illustrate the dependence of the C overlayer on the K coverage, we took C 1s XPS data after a 1-h reaction on surfaces with initial amounts of K varying

(25) Gelius, U.; Heden, P. F.; Hedman, J.; Lindberg, B. J.; Manne, R.; Nordberg, R.; Nordling, C.; Siegbahn, K. Phys. Scr. 1970, 2, 70.

(26) Wesner, D.; Krummacher, S.; Strongin, M.; Car, R.; Sham, T. K.; Eberhardt, W.; Weng, S. L. Phys. Reu. B 1984, 30,855.

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-

Langmuir, Vol. 1, No. 4, 1985 483

Influence of Potassium on Hydrogenation

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(except where otherwise noted),at various reaction temperatures. The reaction conditions correspond to some of the curves in Figure 6. Part (a) is an initially clean surface and part (b) one with 1 ML of K. I

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various temperatures and initial K coverages, indicated in monolayers for each curve. Parts (a), (b), and (c) are for reaction temperatures of 498, 523, and 548 K, respectively. between 0 and 1 ML. The results for 498, 523, and 548 K reaction temperatures are in Figure 8. As with the gas chromatographic data in Figures 2-4, there are clear effects visible even for small amounts of K. For example, a t the 498 K reaction temperature a clean Fe surface shows only a small amount of C buildup after 1 h, but one with 0.23 ML shows a much larger carbidic C peak and a smaller, higher binding energy shoulder. The latter shifts toward higher binding energy and grows as the initial K coverage

is increased, eventually becoming the 285.8 eV feature of Figures 5 and 6. Similar effects are seen for the higher reaction temperatures. A 523 K reaction on clean Fe produces a mostly carbidic peak after 1 h reaction, while on a surface with 0.25 ML of K higher binding energy components are visible. After the 1-h reaction time this surface layer is probably a mixture of C species, having reached the stage where graphitic C begins to grow. For the 548 K reaction temperature, a clean Fe surface shows a mixture of carbidic and graphitic species, while the surfaces with K all show larger amounts of C and an already mostly graphitic C 1s binding energy. For this reaction temperature a 1-h reaction, even with only 0.21 ML of K coverage, is nearly sufficient to reach the graphitic stage, and the effects of additional amounts of K are not as pronounced. Another effect of K on the C deposited during reaction that is visible in Figure 8 or in Figures 5 and 6 is the enhancement of the deposition rate. This was observed previously6J6 and is due to the increased CO dissociation probability on the K-modified surface. The amount of deposited C based on C 1s peak area always increases with the initial K coverage. Typically the 1-ML K-modified surface shows about 2-3 times more deposited C, depending on reaction conditions. From the C 1s peak area and the attenuation of the substrate Fe 2p3/2 line, it is estimated that about 10 ML of C is on the surface for the longest reaction times (e.g., for the 573 K, 3-h reaction on K-modified Fe shown in Figure 5b). Once such a thick C layer is deposited the deposition rate slows. 0 1s XPS peak intensities were also measured on the surfaces after reaction. These data are in Figure 9. As mentioned above, the clean Fe surface as prepared showed some 0 contamination. The 0 1s XPS region for such a clean surface is shown in Figure 9b, both before reaction and after 0.25 min of reaction a t 573 K. The sloping background arises from the tail of the Fe Auger line near 703-eV kinetic energy (-546-eV binding energy). The binding energy is 530.0 eV before reaction, consistent with atomic 0 in a -2 valence state.27 There is apparently not enough 0 to influence significantlythe Fe 2p3,, peak which a t this stage shows no sign of intensity near the 710-eV binding energy that would be expected for an Fe3+species.28 If this 0 were all concentrated on the surface, it would correspond to a coverage of about 0.2 0 atom per Fe surface atom. At the earliest stage of reaction on the (27)Brundle, C. R.IEM J. Res. Deu. 1978,22,235.Kishi, K.; Roberts, M. W. J. Chem. SOC.,Faraday Tram. 1 1975,71, 1715. (28)Brundle, C.R.;Chuang, T. J.; Wandelt, K. Surf. Sci. 1977,68,459.

Wesner, Coenen, and Bonzel

484 Langmuir, Vol. 1, No. 4, 1985

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surface without K this 0 1s intensity is drastically reduced. Apparently the hydrogenation of adsorbed 0 to HzO is very efficient at maintaining the surface in a reduced state. This was the case for all the clean Fe surface reactions, but not for surfaces with K. After K is deposited somewhat more 0 is present than is present on the clean Fe surface before reaction. There are peaks a t 531.4- and 529.5-eV binding energy suggesting molecular absorption and dissociation of CO, r e s p e c t i ~ e l y .We ~ ~ also measured a small 0 1s peak at 531.4-eV binding energy when an Fe surface with K was exposed to pure CO a t room temperature in the microreactor. Again, a drastic change is seen upon starting the CO hydrogenation reaction. In this case, however, the double peak is replaced by a considerably larger, rather wide (1.8eV fwhm) single peak at the earliest stage of reaction. As the reaction proceeds this peak decreases in area. Qy measuring the total peak areas of 0 1s and K 2p levels and correcting for the relative photoionization subshell cross sections of 0 and K29and the kinetic energy dependence of the analyzer transmission ( -E1), we calculated the K/O stoichiometry for the reaction layer. The values in Figure 9 all cluster near one. Potassium/oxygen ratios near one have been observed for partially reduced layers of K2C03on Fe, with the conclusion that the K exists on such surfaces as KOH in the form K+ and OH-.17 Our binding energy of 531.9 eV is also close to that measured for a KOH layer on Fe." The decomposition of C reaction layers was also studied. One method was to heat the layer in 100 kPa of H2in the microreactor to hydrogenate the surface C species. XPS measurements were taken after various lengths of time in the microreactor a t 573 K. Data for hydrogenation of C reaction layers produced on clean Fe surfaces are shown in Figure 10. The time needed to remove the C layer from any surface was in general proportional to the initial amount of C, but sharp differences were observed depending on the starting composition of the surface. For example, Figure 10a shows the hydrogenation of a C layer deposited during a 90-min reaction a t 523 K. The dominant peak is near the carbidic binding energy. Almost all the C is removed after 30 s. In contrast, Figure 10b shows the similar experiment for a C layer deposited in a W-min reaction at 573 K. This reaction layer shows a more graphitic C 1s line shape. Much longer hydrogenation is (29) Scofield, J . H.J . Electron Spectrosc. Relat. Phenom. 1976,8, 129.

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necessary to remove the C; in fact, it could not be fully removed a t 573 K, but finally had to be heated to 673 K before all C was removed. After 60 min of hydrogenation a t 573 K only a single C 1s peak a t 284.6 eV is left, suggesting a mostly graphitic species, and an AES measurement taken at this point showed a graphitic C (KLL) line shape. Such a slow hydrogenation w& generally observed on clean Pe surfaces which had developed appreciable graphitic C 1s XPS character, in agreement with previous studie~.'~~~~ Figure 11shows similar experiments on reaction layers produced on K-modified surfaces (1h L of K). Figure l l a shows spectra taken during the hydrogenation of a layer produced after a 60-min reaction a t 498 K. This is the optimum reaction temperature for producing the 285.8-eV binding energy C 1s peak. This peak is very sensitive to hydrogenation, almost disappearing after 60 s. There is more C on this surface than on a clean Fe surface after the same reaction, and to completely remove it would require longer hydrogenation compared to, e.g., Figure loa. However, the 285.8-eV peak is much easier to hydrogenate than the carbidic peak. Figure l l b shows the results of hydrogenation on a C layer produced after a 90-min reaction a t 523 K. After this reaction the surface layer is probably a mixture of carbidic, graphitic, and the 285.8-eV binding energy C, resulting in a broad peak a t 285.2-eV binding energy and a smaller carbidic secondary peak. The total amount of C deposited on the surface by this reaction is actually comparable to that in Figure lob, but the hydrogenation reveals much different behavior. It takes about the same time to remove all the C from either surface, but on the K-modified surface the higher binding energy part of the line shape is the first to hydrogenate, leaving the carbidic behind, while on the clean surface the reverse is true. In Figure 11, both part a and b, this higher binding energy component is significantly reduced in the first 30 s of hydrogenation. Another observation connected with the hydrogenation data in Figure 11 concerns the K 2p line shape. In both parts a and b it is unchanged while the C species are being removed. The same was seen on other surfaces having larger (several monolayers) amounts of deposited C before hydrogenation. Several explanations are conceivable, the simplest perhaps being that the K layer after reaction is (30)Krebs, H.J.; Bonzel, H.P.Surf. Sci. 1980, 99, 570.

Influence of Potassium on Hydrogenation sitting on top of the C layer but that the C species and H2 can still react to form gaseous products. This behavior also implies that the decrease in K 2p intensity seen during the reaction is not due to attenuation of buried K by the C overlayer but rather to an actual removal of K from the volume probed by XPS. Otherwise one would expect to recover some K intensity in the spectrum when the buried K was uncovered as C was removed by hydrogenation. The second method for decomposing the surface reaction layers, thermal decomposition in UHV, was judged to be less satisfactory. It was not clear whether the C was desorbing or simply undergoing transformation as H was desorbed from hydrogenated C species. However, also in this experiment the 285.8-eV binding energy peak was the most unstable and the first of the C species to desorb (or decompose). The H2which evolved during the heating of reaction layers was measured by a mass spectrometer in the UHV section. Relatively less H2 (by a factor of about 2) evolved when a surface layer with the 285.8-eV binding energy peak was heated, compared to the amount evolving from a layer produced over clean Fe but having about the same total amount of deposited C.

Discussion From the data presented it should be clear that both the gaseous and solid-phase hydrocarbon products formed on Fe foils are part of a relatively complex, time-dependent system. The behavior observed in the first few hours of reaction time spans a large catalytic range, from the early carbidic stage to substantial graphitic poisoning. XPS and AES data reveal that more than one kind of C species is present on the surface at almost every stage of reaction for either clean or K-modified Fe. The measurements clearly show different types of surface C species on Kmodified surfaces than on clean Fe surfaces. Other effects of K are to reduce the conversion rate of CO into hydrocarbons, to shift the product distribution to higher C number, and to increase the amount of C deposited on the surface, accelerating the growth of graphitic C. The time dependence of the CHI production rate is also affected. As mentioned above, we base our assignment of the carbidic and graphitic C peaks on the K-modified Fe surface not only on the C 1 s peak location but also on the AES line shape, which is able to distinguish carbidic and graphitic C on either kind of surface. It is perhaps surprising that K does not shift the binding energy of these two species. With such a highly electropositive adsorbate one might expect some shift in binding energy due, for example, to changed screening properties of the Fe + K surface or to polarization effects of the highly ionic K adatoms on the surface C species. In the case of the graphitic layer these effects might be attenuated because by the time this stage is reached the K signal has decreased. For the carbide peak, which always occurs early in the reaction, one would expect them to be more important, but they apparently are not. Compare, e.g., the carbide peak positions in the sequence of spectra in Figure 8b. The changes in the methanation behavior due to K observed in Figures 2-4 can be correlated with the appearance of the various C species seen in Figure 8. Some of the loss of hydrogenation activity with added K is undoubtedly due to a simple blocking of active sites by the K adatoms. For this reason we cannot report a true turnover frequency for the K-modified surfaces. The exact magnitude of this effect is uncertain because it occurs simultaneously with other effects of the K. Such effects have been studied under UHV conditions by a variety fo surface analytical techniques. It is well-known that K on transition metals lowers the work f ~ n c t i o n ,thus ~ ~ en~~~,

Langmuir, Vol. 1, No. 4, 1985 485 hancing the electron-donating capability of the surface. When CO is then coadsorbed with the K, there is a correspondingly large charge exchange between the adsorbed molecule and the K-modified surface.32 Potassium thus enhances the adsorption energy of CO and the amount of CO dissociation.1° Vibrational spectroscopy reveals that the adsorbed CO stretch frequency is reduced on K-modified surfaces,33consistent with a weakened C-0 bond and increased probability of dissociation. These effects can mostly be explained by an enhanced back donation of charge from the metal d states to the antibonding 27r CO orbital.% The increased CO dissociation with K is evident in our data showing the large enhancement of C deposition rates with K coverage (Figures 5 and 8). At an initial K coverage of 0.23 mL and 498 K reaction temperature (Figure 8a), the largest change is an increased carbidic C peak, suggesting more dissociated CO. Such an increase in the bulk carbidization of the foil with K can help explain the decrease in methanation activity with K coverage seen in Figure 3. Models of the methanation reaction have been proposed35in which an adsorbed C atom deposited on the surface by CO dissociation can take one of a t least three possible reaction paths: bulk carbide formation, hydrogenation to CH, surface species, or combination with other already adsorbed C atoms to form inactive (graphitic) C. The methane and higher hydrocarbons produced are tied to the relative amount of the hydrogenated C species. If the bulk carbidization is high it can compete with the hydrogenation of this species, thus decreasing the methanation activity. On our foil surfaces the addition of K does increase the carbide formation, a t least a t low temperature. At the higher temperatures the tendency to form graphitic C increases. This is also expected to reduce the methanation activity, since the graphitic species is relatively inert, as seen from the hydrogenation behavior. The 523 K data in Figures 3 and 8b represent an intermediate situation. At this temperature there is still not a distinct graphitic peak in the C 1s spectra, and some differences can still be seen as the initial K coverage is increased. This correlates with the curves for different initial K coverages in Figure 3 which are still separate but beginning to merge after 60-min reaction time. Finally, the high-temperature (548 K) C 1s XPS data show only small changes as the initial K coverage increases. After an hour of reaction all the K-modified surfaces have reached a similar graphitic stage. Correspondingly,the methanation curves in Figure 4 have merged. It was mentioned that the amount of K decreases under some reaction conditions, in particular, when graphitic C builds up. This, of course, would help to make the methanation rate similar on clean and Kpromoted surfaces. However, the magnitude of this decrease alone seems insufficient to explain the merging of the curves in Figure 4. A t the time of the merger, for example, there still remains about 0.4 ML of K on the surface that originally had 1 ML. The increased selectivity effect of K is rapidly lost once the surface C layer reaches the graphitic stage. Table I shows how LY decreases with reaction time as graphitic (31)Lee, S.B.; Weiss, M.; Ertl, G. Surf. Sci. 1981,108,357. (32)Kiskinova, M.;Pirug, G.; Bonzel, H. P. Surf. Sci. 1983,133,462. (33)Garfunkel, E.L.; Crowell, J. E.; Somorjai, G . A. J.Phys. Chem. 1982,86,310. Crowell, J. E.; Somorjai, G. A. J. Vac. Sci. Technol.,A 1984, 2,881. Crowell, J. E.; Somorjai, G. A. Surf. Sci. 1982,121,303.Hoffman, F. M.;de Paola, R. A. Phys. Reu. Lett. 1984,52, 1697. (34)Bonzel, H. P.J. Vac. Sci. Technol. A , 1984,2, 866. (35)Bianchi, D.;Tau, L. M.; Borcar, S.;Bennett, C. 0.J. Catal. 1983, 84,358. Niemantaverdriet, J. W.; van der Kraan, A. M.J. Catal. 1981, 72,385. Niemantsverdriet, J. W.; van der Kraan, A. M.; van Dijk, W. L.; ~van ~ der Baan, H. S. J.Phys. Chem. 1980,84,3363.

486 Langmuir, Vol. I , No. 4 , 1985

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Figure 11. As in Figure 10, except K 2p-C 1s spectra during hydrogenation of layers produced on surfaces with initial K coverages of 1 ML. For part (a) the reaction lasted 60 min at 498 K and for part (b) 90 min at 523 K. Note that the hydrogenation times in part (a) are in seconds and those in part (b) are in minutes. poisoning of the reaction sets in. In agreement with the genation experiments (Figures 10 and 11). There the C 1s XPS data, this occurs sooner on the K-modified species is seen to be very easy to hydrogenate relative to surface, with its larger rate of C deposition. One would the carbidic peak. expect the generally less active graphitic layer to be less In contrast to this ease of hydrogenation, we see in effective in initiating the chain growth reactions which Figure l l a and 10a that in the presence of K the hydromust occur among adsorbed hydrogenated C entities in genation of the carbidic peak is actually hindered. In the order to generate the higher hydrocarbon products. K-modified surface layer hydrogenation in Figure l l a the Considering the 498 K reaction temperature, for which carbidic peak is still unchanged after 30 s, while after this there is still little graphitic character in the layer deposited time most of the C layer from the clean Fe surface is gone. during the first hour, there is evidence that the 285.8-eV This behavior is seen even for the thicker C layers in part binding energy peak is responsible for the increased chain b of Figures 10 and 11, where both surfaces have some graphitic C to start with. Sixty minutes of hydrogenation growth probability. Table I1 and Figure 8a show that the increase in a parallels the growth of the 285.8-eV binding removes the low binding energy, carbidic C component energy peak as the initial K coverage is increased. If, for from the surface in Figure lob, but some still remains on the surface in Figure l l b after the same treatment. The example, the increased amount of carbidic C on the surface alone were enough to cause the increase in selectivity to hydrogenation resistance of the carbidic C in the presence higher C number products, then the increase should have of K helps to explain an apparent contradiction: while the already occurred around 0.2 ML of K. This amount of K 285.8-eV binding energy species is readily hydrogenated, significantly enhances carbidic C deposition on the surface. the net effect of K is a lower rate of methanation. Such To induce the increase in CY and the appearance of the behavior could arise from an interaction between the carbidic and high binding energy C, the latter somehow 285.8-eV binding energy C 1s peak about 0.7 ML of K are shielding the former from hydrogenation reactions. Alnecessary. Also visible in Table I1 is a parallel growth of the unsaturated C2 product percentage with K coverage. ternatively, it could be related to the decreased hydrogenation capability of the K-modified surfaces evident in This is consistent with the thermal decomposition measurements showing that the 285.8-eV binding energy the gaseous product distribution. Whether this arises directly from an intrinsic electronic effect of K on H species has less hydrogen than the C species produced on clean Fe. The increase in CY is tied with a decreased hychemisorption or simply from the different numbers of free drogenation capability of the K-modified surface, which adsorption sites on the two surfaces is not clear. is reasonable since methanation is a competing reaction A less drastic time dependence of the CHI production to chain growth on the surface. It has been s ~ o wthat ~ ~ , rate ~ ~ is observed on K-modified surfaces. Since the dethere is an inverse relationship between higher hydrocrease in methanation with reaction time has been ascribed carbon selectivity and the Hzreactant partial pressure to the buildup of graphitic C this might suggest that K prevents graphitization, as reported for oxidized Fe f0i1s.l~ during CO hydrogenation. Apparently the effect of K on The XPS and AES data contradict this, however, because the reaction is similar to that of a lower H2partial pressure. It has been s u g g e ~ t e dthat ~ , ~this ~ could arise because of under equal reaction conditions graphitic C appears sooner competition between adsorbed C and H, and that the on the K-modified surfaces (Figures 5 and 8c) as a result decreased hydrogenation capability of alkali-modified of the enhanced C deposition. This behavior is also obsurfaces could be responsible for their increased higher served on Ni.'* What is true is that for equivalent amounts hydrocarbon selectivity.% Our results are consistent with of deposited C (not equal reaction temperatures) on clean and K-modified surfaces, the layer is more graphitic in the this and further suggest that the increased a can be tied more directly to the 285.8-eV binding energy C species. former case. For example, the reaction layer produced on Evidence that the 285.8-eV binding energy species is active clean Fe after an hour's reaction at 573 K has approxiin the CO hydrogenation reaction comes from the hydromately the same (or even somewhat less) C than that produced on a K-modified surface after an hour's reaction a t 498 K (Figure 6). Yet, the AES spectra after reaction (36) Vannice, M. A. J. Catal. 1975, 37, 449. on clean Fe begin to show graphitic features already at 548 (37) Luftman, H. S.; Sun, Y.-M.; White, J. M. Surf. Sci. 1984, 140, K. In contrast, the AES spectrum of a 498 K, K-modified L259. (38) McLaughlin McCiory, M.; Gonzalez, R. D. J. Catal. 1984,89,392. surface reaction layer is still mostly carbidic (Figure 7 ) .

Influence of Potassium on Hydrogenation Apparently the graphitic C on K-modified surfaces forms only after the 285.8-eV binding energy species, after more C is on the surface. In this sense, then, the K may indirectly delay the start of graphitization by inducing the 285.8-eV binding energy C species. Dwyer and Hardenbergh6 have suggested a “polymethylene” character for the 285.7-eV binding energy species they observed on Fe powder surfaces-i.e., a hydrogenated C chain of high molecular weight. Their argument was based on the C 1s binding energy being the same as that of CzsHSsadsorbed on a K-modified Fe foil and on a suggestive cracking pattern of the reaction layer. We find a similar binding energy of 285.9 eV also for C,H, adsorbed on our K-modified surfaces. While this binding energy is about 1 eV higher than that normally observed for bulk, non-oxygenated polymer sam~les,3~ it is consistent with the idea of an adsorbate species that is loosely coordinated to the surface. In such a case one would expect some increase in binding energy because of the change in relaxation screening by the surface. That the CzsHssis saturated, while the observed reaction species is probably relatively low in H, does not invalidate the comparison. The dependence of hydrocarbon C 1s binding energies on the degree of saturation is relatively small, at least at room temperature and above.40 Unfortunately, we could not detect a polymer cracking pattern, observing only H2in significant amounts during thermal decomposition of our samples. Possibly there is not enough of this species present on a low-surface-areasample (less than of the surface area of Dwyer and Hardenbergh’s powder samples). Our data are thus consistent with this picture but of course do not allow an exact identification of the various surface species present. It is interesting that the 285.7-eV C 1s binding energy species is apparently the dominant one on Dwyer and Hardenbergh’s powder samples, being much larger than the carbidic peak, suggesting that it may play an even more important role in the reaction on dispersed catalysts than it does on foils. The 0 1s XPS data during reaction (Figure 9) show that the presence of the 285.8-eV binding energy peak does not correlate with the presence of 0 and thus rule out an oxygenated species. Specifically, if the 285.8-eV binding energy peak represented an 0-containing C species, a growth of the 0 1s peak should be observed parallel to the growth of this peak. Instead, 0 1s peaks during reaction correlate only with the K peak. While the chain-growth process during CO hydrogenation previously had been proposed to occur through insertion of CO groups into adsorbed hydrocarbon species,4l it is now rather believed to occur via polymerization of surface CH, ( x = 1, 2, 3) species,42consistent with the present results on either clean or K-modified Fe. The K:O ratio of -1 and the 531.9-eV 0 1s binding energy are suggestive of a KOH species1’ for the chemical state of K under reaction conditions but do not constitute proof. In particular, the 0 1s peak is broad, and there is also another oxygenated species, the carbonate, present (39) Clark, D.T.In “Polymer Surfaces”; Lee, L. H., Ed.; Academic Press: New York, 1977; Vol. 11, pp 5-52. (40) Freyer, N.; Pirug, G.; Bonzel, H. P. Surf. Sci. 1983, 126, 487. Thomas, T. D.J.Chem. Phys. 1970,52,1373. Siegbahn, K.;Nordling, C.; Johansson, G.; Hedman, J.; HedCn, P. F.; Hamrin, K.; Gelius, U.; Bergmark, T.;Werme, L. 0.;Manne, R.; Baer, Y. ‘ESCA Applied to Free Molecules”; North-Holland: Amsterdam, 1969. (41) Henrici-OlivC, G.;OlivC, S.J.Mol. Catal. 1982,16, 187. Pichler, H.; Schulz, H. Chem.-Zng.-Tech. 1970, 42, 1162. (42) Brady, R.C.; Pettit, R. J . Am. Chem. SOC.1980,102,6182; 1981, 103, 1287. Bell, A. T. Catal. Reu.-Sci. Eng. 1981,23, 203. Biloen, P.; Helle, J. N.; Sachtler, W. M. H. J. Catol. 1979, 58, 95. Wang, C. J.; Ekerdt, J. G . Zbid. 1984, 86, 239.

Langmuir, Vol. 1, No. 4, 1985 487 during the early stages of reaction. Beyond the 1:l K/O stoichiometry the chemical state is less certain. Pad, Ertl, and Lee43studied K and 0 adsorption on polycrystalline Fe and concluded that 0 thermally stabilized the K adlayer and that the catalytically active species in NH, synthesis was probably not any known bulk K compound, but rather an adsorbed K-0 layer. Similarly, potassium oxide formation, a t least for K coverages of less than a monolayer, has been ruled out in the adsorption of K and 0 on Fe(ll0) under UHV condition^.^^ In the hydrogenation of char or graphite, which is also promoted by alkali-metal compounds, reaction cycles have been proposed4 in which both alkali carbonates and hydroxides play a role, so that no one K compound can be said to be the active species. Something similar may occur during CO hydrogenation. The decrease of K coverage during reaction is puzzling, especially in light of earlier results” which observed no decrease. We have no easy explanation for this decrepancy. Note, however, that the K layers in that study were prepared by a different method than used here. This decrease is most evident once the relatively inert graphitic C begins to build up, and, as mentioned above, represents an actual loss of K, probably by desorption in the microreactor. This suggests that there may be a relationship between K and C as well as between K and 0. However, a more detailed specification of such a relationship of the surface K does not seem to be possible from these data.

Summary K dosing of Fe foil surfaces causes the following changes in the CO hydrogenation reaction: a decrease in activity, an increase in selectivity to higher hydrocarbons, an increase in the rate of C deposition, and the appearance of different surface C species as seen by XPS. While some of the effects are understandable in terms of the known electronic effects of K on CO adsorption, a significant role in the reaction is played by the surface C species with a C 1s binding energy of 285.8 eV. It is connected with the increase in a and is active in hydrogenation. It is not an oxygenated species but may be a polymeric carbon compound loosely coordinated to the surface. The amount of K needed to induce the presence of this species and the increase in higher hydrocarbon selectivity is about 0.7 ML. On Fe foil surfaces under the reaction conditions investigated this species is most prevalent at reaction temperatures of 498 K and reaction times of about an hour. At higher reaction temperatures the buildup of graphitic C occurs, rapidly quenching the reaction and blurring the distinctions between clean and K-modified surfaces. This occurs also on clean Fe foils, but is more rapid on the K-modified ones because of the increased amount of C. The K on the surface during reaction is coordinated with surface 0 in a 1:l stoichiometry. The K 2p signal is attenuated during reaction, especially when the graphitization sets in, probably due to desorption from the surface. Acknowledgments. We thank K. Duckers, N. Freyer, M. Kiskinova, G. Linden, M. Peuckert, and G. Pirug for critical reading of the manuscript and for stimulating discussions. D. J. Dwyer and J. H. Hardenbergh kindly furnished copies of their work prior to publication. The Deutache AGF-Nachwuchs programm is acknowledged for its financial support of one of us (D.A.W.). A. Franken’s very able technical assistance in preparing the manuscript is also appreciated. Registry No. CO, 630-08-0; Fe,7439-89-6; K, 7440-09-7. (43) PaHI, Z.;Ertl, G.; Lee, S. B. Appl. Surf. Sci. 1981, 8, 231. (44) Pirug, G.; BrodCn, G.; Bonzel, H. P. Surf. Sci. 1980, 94, 323. (45) Wen, W.-Y. Catol. Reu.-Sci. Eng. 1980, 22, 1.