Surface science approach to heterogeneous catalysis: CO hydrogenation on transition metals

639

Surface Science 117 (1982) 639-658 North-Holland Publishing Company

SURFACE SCIENCE APPROACH TO HETEROGENEOUS CO HYDROGENATION ON TRANSITION METALS H.P. BONZEL

and H.J. KREBS

Institut fiir Grentfliichenforschung Jiilich, Fed. Rep. of German?, Received

CATALYSIS:

14 September

und Vakuumph_vsik, Kernforschungsanlage

1981; accepted

for publication

20 October

Jiilich GmbH, D-51 70

1981

Modern surface sensitive electron spectroscopies and other surface analytical techniques have in recent years been extensively applied to the study of H, and CO adorption on transition metals. This work has now been extended to include the heterogeneous reaction between adsorbed Ha and CO on these metals. The combination of surface analysis (carried out under ultra-high vacuum conditions) and reaction rate measurements in the range of 100 mbar to 1 bar total pressure is being practiced. This approach yields information on changes of the surface composition of the catalyst as well as data on reaction kinetics and the possible time dependence of the reaction rate. Low surface area samples - either single or polycrystalline - are used for these studies. In the present paper the results obtained by this approach will be reviewed and discussed in the light of the adsorption data. Recent advances in the direction of studying either poisoned or promoted catalytic surfaces will also be mentioned.

1. Introduction The primary objectives of the surface scientist investigating heterogeneous catalysis have been listed some time ago and are still valid today. These are: (1) to carry out basic research on metal surfaces by utilizing the available surface analytical techniques; (2) to select catalytically relevant problems, study them with these techniques and complement the work of the catalytic chemist; (3) to investigate catalytic reactions in combination experiments at low pressure (vacuum) and at high pressure; (4) to propose new mechanisms and concepts in heterogeneous catalysis [l]. A nearly perfect illustration of how these objectives have been pursued can be made by examining the intense research efforts focussed on the heterogeneous hydrogenation of CO and CO,. This reaction is also called the methanation or Fischer-Tropsch reaction depending on the product spectrum. The metals of primary interest are Fe, Co, Ni and Ru [2,3]. Basic research has centered on a detailed study of CO and H, adsorption on these metals. Much is known about molecular versus dissociative adsorption and also about the adsorbate-metal bond strengths. With regard to the second point, the CO + H, reaction is certainly a relevant problem to study because there is a direct context with modern efforts of coal gasification and liquefac0039-6028/82/0000-0000/$02.75

0 1982 North-Holland

tion [4]. The kinetics of this reaction under vacuum conditions are extremely slow such that a high pressure (- 1 bar) reactor is needed in order to carry out kinetic reaction measurements; thus the third objective of studying the reaction at high pressure is a necessity in this case. Finally, we will see in this paper whether new mechanisms or concepts for this particular reaction have already emerged from this work. In addition to being of a high degree of relevancy, the hydrogenation of CO on transition metals is also a very challenging and interesting problem to study. Large differences in specific reactivity of different metals have been found by systematic investigations of Vannice [5,6]. Fig. 1 shows, for example, a plot of the turnover number of methane which is equal to the number of methane molecules produced per surface site per second, versus the adsorption energy of CO for a series of 3d, 4d and 5d metals (“vulcano plot”). A maximum in methanation activity results for the metal Co. The usual interpretation of this result embarks on the assumption that molecularly adsorbed CO is an important intermediate for this reaction. Low energies of adsorption, e.g. for Cu, entail low CO coverages and therefore low rates of methanation. On the other hand, high energies of adsorption cause nearly saturation coverage of CO such that very few sites for hydrogen chemisorption are available; hence low rates of methanation. The optimum is obtained for intermediate values of

CO

Heat

of Adsorption

(kcal/mole)

Fig. I. Semilogarithmic plot of the methane turnover number versus the energy of CO adsorption for several transition metals supported on SiOz (from Vannicc [5]).

the CO adsorption energy. We will see later on that this simple interpretation of fig. 1 is not consistent with our present state of knowledge. Another even more complex behavior in the CO hydrogenation catalysis becomes evident through the use of alkaline promoters [2]. For example, McVicker and Vannice [7] prepared various supported Fe catalysts by adding K compounds or by starting from K-Fe complexes. Fig. 2 shows a few product distributions obtained for Fe and K-promoted Fe catalysts. The differences are obviously striking. There is as yet no basic understanding of these results. The study of such complex Fe-K systems and of their behavior in CO/H, environment is therefore an extremely interesting problem for surface scientists who are making their first strides in this direction [8- 1 I]. In this paper we are going to review briefly the present state-of-the-art situation for CO and H, adsorption on transition metals. New experiments dealing with a combined investigation of surface properties and of the CO + H, reaction kinetics at higher pressure will be presented next. The results will be discussed in the light of the gas adsorption results. Finally, we will have a look at the effect of electropositive and electronegative additives on the reaction behavior. In the closing section of this paper we are going to assess critically our present situation and explore future research activities. At this point it is helpful to remember that a thorough investigation of the fundamentals of heterogeneous catalysis has always been considered very difficult, mainly due to the inaccessibility of the active surface of the catalyst. The advent of the surface science approach with its arsenal of analytical tools has not totally alleviated this problem. However, as this review intends to show, many results

CO Hydrogenation

50

on Fe

CO H,=l 3 P,=lbar T = 537K

1

10%Fe/A120, 35% Conversion

55%K,39%FeIAl,O, 0.07% Conversion

1 2 3 L 5 5+ 55%K. 3,g%FelA1203 2.3% Conversion

Fig. 2. Product distributions of hydrocarbons formed by various Fe catalysts at 573 K and 1 bar total pressure from a 1: 3 mixture of CO and H,. The second distribution was obtained from a conventional K2C0, impregnated Fe catalyst while the third is characteristic of a catalyst impregnated with a K-Fe complex (from McVicker and Vannice [7]).

obtained in recent years by basis of our knowledge of hypotheses and speculations direction of completing our have been made.

2. Chemisorption

the surface science approach have solidified the heterogeneous catalysis by turning a number of into scientific facts. Therefore real advances in the basic understanding of heterogeneous reactions

of H,

The interaction of H, with a large number of well defined transition metal surfaces has been studied by a variety of techniques. The adsorption process

Table 1 Adsorption

energies

Metal

Gas

Fe Fe(ll0) Fe( 100) Fe(ll1)

H*

CO(OO1) co

H*

Ni Ni(lll) Ni(lOO) Ni(ll0)

H*

Ru

of H, and CO at small coverage

on some transition Measuring

E,, (kJ/mole)

technique

metals Reference

Calor. FDh FD FD

1121

67 12

FD FD

1141 [I41

15 96 96 90

Calor. Isostere Isostere Isostere

I151

HZ

100

lsostere

[171

Fe Fe( 110)

co

120-155 105

Calor FD

[‘RI

c0( toio) Co(O01)

co

140 128 103 143

Isostere lsostere FD Isostere

PII P-11 WI

co

112 149 125 -140 125

FD Isostere Isostere FD Calor.

~231 ~241 ~251 WI [I51

co

126 121 118 121

FD Isostere FD FD

[271 WI (291 [301

Co( 1120, Ni(ll1) Ni(lOO) Ni(ll0) Ni Ru( IOiO) Ru(001) Ru(lOl)

95 109 106 88

a Higher value most likely for dissociated b FD= flash desorption peak temperature.

a

C + 0.

[I31 [131 (131

[I61 1161 [I61

[19.20]

v51

H. P. Bonzel, H.J. Krebs / Sur/crce scrence upprouch io heterogeneow cuta!wis

643

results in all cases in the formation of atomically adsorbed hydrogen. This is an important result for the understanding of hydrogenation reactions. The second most important information is the adsorption energy of atomic hydrogen. This quantity is listed in table 1 for the metals of interest here. It follows that E,,(H) is between 70- 110 kJ/mole at low coverage. Other recent summaries of gas adsorption on transition metal surfaces were published by Toyoshima and Somorjai [31] and by Varma and Wilson [32]. The later paper dealt with theoretical aspects of hydrogen (and oxygen) bonding to metals.

3. Chemisorption

of CO

of CO on the same transition metals is Compared to H,, the adsorption rather complex. This complexity arises mainly from the fact that CO can adsorb in either molecular or dissociated form. The transition of molecular to dissociative adsorption is a function of temperature, changes from one metal to another, and even varies with crystallographic orientation for the same metal at constant temperature [33]. For this reason it is sometimes difficult to measure the adsorption energy of molecular CO. Hence there is considerable scatter in the experimental energy values when plotted as a function of element position in the peridic table [31]. The adsorption of CO on transition metals was dealt with in several recent papers, e.g. by Broden et al. [33], Joyner [34], Miyazaki [35], Benziger [36], Rosen et al. [37], Andreoni and Varma [38] and Papp [21]. A summary of measured adsorption energies is also given by Toyoshima and Somorjai [31]. It is clear that the question of molecular versus dissociative CO adsorption will be crucial for the discussion of the mechanism of CO hydrogenation reactions. Broden et al. [33] pointed out a systematic variation of the CO dissociation propensity with the position of the substrate metal in the periodic table. Elements with decreasing electronegativity tend to facilitate CO dissociation. Their correlation was based on electron spectroscopic data of the undissociated, adsorbed CO molecule, namely on the energy separation between the 1~ and 4a molecular orbitals [33]. This energy separation is related to the C-O bond length and the C-O stretch frequency [37] but its relationship to the probability of CO dissociation is questionable. The reason is that these physical quantities - the molecular orbital energies, the C-O bond length, and the C-O stretch frequency - are characteristic of the ground state of the adsorbed CO molecule only, i.e. of the harmonic part of the adsorption potential. These quantities contain therefore no information on the potential energy surface near the activation barrier of CO dissociation. By the same token, the adsorption energy of the molecular CO state is not related to the dissociation propensity of adsorbed CO, as it has been suggested by Joyner (341. This was also pointed out recently by Benzinger [36] who compared the

CO adsorption energies of Pd and Ni. Even though this energy is larger for Pd, the CO dissociation propensity is larger for Ni. In the following we are going to expand this argument. Fig. 3 shows a plot of the energy of molecular CO adsorption and of the observed C-O stretch frequencies of adsorbed CO for the substrate metals Fe, Co, Ni, Ru and Pd. It can be seen that the energy of adsorption increases from Fe towards Ni, and from Ru to Pd, and that the C-O stretch frequencies observed for various coverages and surface orientations do not vary in a systematic fashion for the metals shown in fig. 3. Experimental investigations of CO adsorption by various spectroscopies have shown that CO dissociates at 300 K on Fe( 100) [39], Fe(ll1) [40], polycrystalline Fe [41]. Co( 1120) [21]. Co( 1012) [42] and polycrystalline Co [21]. Also CO dissociates on Fe( 110) at - 385 K [43] and some CO dissociation has been observed on a stepped Ni(ll1) crystal at elevated temperature [44]. On the other hand, no CO dissociation has been reported at 300 K or even higher temperatures for low-index surfaces of Ni, Ru or Pd. It can therefore be concluded that the propensity for CO dissociation decreases from Fe to Co to Ni, Ru and Pd. It follows also that there is no ground for a correlation between CO adsorption bond strength or C-O bond weakening and CO dissociation propensity. It becomes clear from this discussion that the vulcan0 plot of Vannice [5].

200

150

u 5 E

/I--I-

;t w

CO Adsorption

100 -

,I/

,I'

/I

B 50

1700

-

’

I

I

I

I

I

Fe

Co

NI

Ru

Pd

Fig. 3. Plot of the adsorption energy of ntoleculur CO on some 3d and 4d transition metals (ace frequencies table I for references and ref. 1461) and of the range of C-O stretch vibrational observed for CO adsorbed on single crystal surfaces at various coverages [45-50.X0]. The frequencies corresponding to the initial coverages are indicated by filled squares.

fig. I, needs to be re-interpreted. First of all, the measured adsorption energies of CO were not examined as to whether they were influenced by CO dissociation or not. Secondly, the maximumm activity for methanation occurs for those metals which are likely to be the most active for CO dissociation: Ru. Co and Fe. Unfortunately, stepped Ru surfaces have not been investigated in this context. Benzinger [36] points out that one needs to consider the adsorption energy of dissociated CO (C and 0) as a thermodynamic driving force in order to obtain a first order estimate of dissociation behavior with different metals. On that basis previous correlations of dissociative adsorption with element position in the periodic table can be well understood. It follows from this analysis that the relative rates of CO dissociation on different metals go in the sequence Fe (highest rate), Co, Ni, Ru and Pd (lowest rate).

4. Systems with combined surface analysis station and catalytic reactor The heterogeneous hydrogenation of CO and CO, is a reaction which runs with such a low rate that it cannot be studied under vacuum conditions [5 1,521. It is therefore necessary to carry out the reaction in a separate system at higher pressure and to transfer subsequently the catalytic sample into a UHV system for surface analysis. The sample transfer should be possible without exposing the sample to air or any other reactive environment. Combination systems which accomplish this task have first been built by Somorjai and coworkers [53,54] in order to bridge the gap between surface science studies and catalysis. Similar systems have been described by Goodman et al. [55], Fleisch et al. [56], Ott et al. [57], Polaschegg et al. [58] and Krebs et al. [59]. A slightly different experimental arrangement based on a differentially pumped mass spectrometer was used by Palmer and Vroom [60]. The authors have used these sytems to study the hydrogenation of CO on various metals. A very versatile and easy-to-use combination system is shown in fig. 4 as an example [58,59]. This sytem features Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) as surface analytical tools operating in a ultra-high vacuum (UHV) of - 10 p9 mbar. In addition there is a quadrupole mass spectrometer and an Ar ion gun for surface cleaning. The sample transfer is accomplished by a valve-less, differentially pumped air-lock system [59]. The catalytic reactor is part of this transfer system, its volume is about 4cm3 and guarantees high sensitivity for differential operation. The reactor is normally run at 1 bar total pressure. The products of the reaction are detected by gas chromatography. The transfer time between UHV and reactor is 45 s including pump-down. Further experimental details are given elsewhere [58,59]. The combination systems all suffer from a common drawback: Surface analysis with electron’ spectroscopic or similar techniques can only be per-

646

H. P. Bowel, _H.J. Krrbs / Surjurce science upprocrch to heterogeneous

cutu!vsIs

ELECTRON SPECTROMETER II

II

ION SPUTTER

ELECTRO SOURCE

1 ION PUMP

Fig. 4. Schematic of combined UHV and micro-reactor system for surface analysis and catalytic rate measurements. Sample is transferred between UHV. reactor and atmospheric position by a movable stainless steel rod; pairs of black circles on the right denote sealing O-rings (from Krebs et al. [59]).

formed before the reaction and after it has been terminated by lowering the temperature of the sample and by removing the sample from the reactive gas mixture. In other words, the surface is analyzed after the reaction (“post mortem”), not during the reaction. One consequence of this experimental procedure is that the actual surface composition of the sample during the reaction may be different from that during the analysis - a fact that should always be remembered. Weakly adsorbed species will not be present while other species not adsorbed during the reaction may adsorb during cooling and stay on the surface for the analysis. An exception is infra-red spectroscopy which can be applied in situ, i.e. during the reaction in a high pressure cell [69].

5. CO hydrogenation

and surface analysis

Several research groups have utilized combination systems in studying the hydrogenation of CO on different metals, such as Rh [51,62], Ni [55,63,64], Co [60], Ru [61,64] and Fe [59,65-711. In all these experiments polycrystalline foils or single crystals were chosen as catalytic samples in order to facilitate the application of surface spectroscopies. The experimental procedure in most investigations of this kind was as follows: The surface of the catalytic sample was cleaned by heating or

H. P. Bowel, H.J. Krebs / Surface science upprouch

to heterogeneous

t.utcr!vsis

647

sputtering, and its condition was checked by LEED, Auger electron spectroscopy, X-ray photoelectron spectroscopy, secondary ion mass spectroscopy (SIMS) or some other surface analytical tool. The sample was then brought into the CO/H, mixture and heated while reaction products were monitored by gas chromatography or mass spectrometry. Later the reaction was terminated and the change in the surface composition measured. From these data which can be taken repetitively it is possible to obtain: (a) turnover numbers (TON) based on the geometric area of the sample; (b) activation energies of the formation of methane and other products; (c) product distributions; (d) the surface composition versus time; (e) the temperature and partial pressure dependencies of these quantities. Furthermore it is possible to study the effect of additives (either to the surface of the catalyst or to the gas mixture) on the reaction as well as the surface composition of the sample after reaction. Most of the measured characteristics (a)-(e), with the exception of the surface analytical data, are also available for supported catalysts of the same metal. An obvious and important step is therefore the direct comparison of these data with the ones obtained for the idealized but well characterized bulk metal catalysts. Such a comparison provides, first of all, a basis for judging the reasonableness of this approach, and secondly, allows to make a statement about the structure (and support) sensitivity of the reaction. We will illustrate this general discussion by some examples. A comprehensive study of the temperature dependence of the rate of methane synthesis was carried out for several Ni samples [55,64] and compared with corresponding data for supported Ni/Al,O, catalysts [72a]. The Arrhenius plot for the methane TON is presented in fig. 5 for a total pressure of 90 mbar and HZ/CO = 4 (data for the supported catalyst were corrected to the H, and CO partial pressures of the bulk Ni work according to a kinetic rate law [72a]). The agreement between the Ni single crystal and the supported catalyst data is surprisingly good and as such suggestive,of the structure insensitivity of this reacion. The activation energy is about 105 kJ/mole. On the other hand, the data for the polycrystalline foil exhibit a lower activation energy of 66 kJ/mole [55] which is not quite understandable in this context. The general success of this comparison, however, derives from the fact that the CO hydrogenation on Ni is a steady state reaction, i.e. there is no time dependence in either reaction rate or surface composition. In particular, the surface coverage of carbon is constant during the reaction but depends on the HZ/CO ratio, the total pressure [64] and temperature. The situation for Fe is quite different from that for Ni. Remembering the more efficient CO dissociation on Fe as compared to Ni. it is not unexpected that considerably mom C deposition will be encountered during CO hydrogenation on Fe. This is indeed reflected by the experimental results [59.65-711. Non-steady state behavior is observed for the rate of synthesis as well as the

l/T lO'(K-'1

Fig. 5. Comparison of the temperature dependence of the methane turmner number for polycry+ talline and single crystalline Ni samples and for AllO, supported Ni catalyst?, (from Goodman et al. [55.64] and Vannice [72]). Measurements are made for CO : H, ratio of I : 4 and a total pressure of 90 mbar.

surface composition. Fig. 6 shows the rate of CH, formation and carbon deposition versus time at 1 bar total pressure, H/CO = 20 and 560 K on polycrystalline Fe [59]. The carbon concentration was determined by XPS via the area of the C Is core level peak. There is a distinct advantage in using XPS for analyzing the carbon layer because the binding energy E, of the C 1s peak can be used to infer some details about the chemical make-up of this layer. For this reason a few well defined adsorbate layers were produced in UHV on a clean Fe( 110) surface and then investigated by XPS [66]. These layers were segregated graphitic carbon, adsorbed C,H2, adsorbed CO and segregated atomic (“carbidic”) carbon. Their C 1s spectra are shown in fig. 7. It can be seen that the binding energies are different. with 283.3 eV for atomic carbon. 283.9 eV for C2H,. 285.0 eV for graphitic carbon, and 285.9 eV for adsorbed molecular CO. The adsorption of C,H, on Fe( 110) at 300 K is molecular as shown previously by UPS [43] and recently by vibrational spectroscopy [72b]. With the knowledge of these binding energies it is possible to interprete the carbonaceous layers formed during the CO + HZ reaction in some detail.

Fe - Foil co H, =

1 20

Cls - XPS

T 5 560K

Gmphk I

290

I

C

II i ~ 286

282

27%

Fig. 6. CO hydrogenation on Fe foil at I bar and CO : H, = I : 20, 7’= 560K; shown are the time dependencies of methane formation and carbon deposition. The latter was determined by XPS of the C Is peak (from Bonzel and Krebs [69]). Fig. 7. XPS of various well-defined carbon containing layers on Fe( t IO). Heating of the Fe( 1IO) crystal in UHV at 720 K for - 3 min caused the segregation of carbidic (=atomic) carbon: heating at 625 K for 25 min resulted in a graphitic carbon layer: C,H, adsorption on the clean surface was obtained by an exposure to gaseous C,H, of 2X IO-’ mbar for 5 min at 300 K: the exposure for the CO layer was 2 X IO -7 mbar for 5 min at 300 K (from Dome1 and Krehs [NJ]).

Further information is also obtained from the fine structure of the carbon Auger peaks [66]. The outcome of this work is essentially that CO dissociation on Fe is so fast that only some C can be hydrogenated to CH, and higher molecular weight hydrocarbons, but that the excess C precipitates into graphite which has a much lower hydrogenation rate and which therefore acts as a partial poison for CO hydrogenation. The total amount of surface graphite increases with time while the rate of hydrocarbon synthesis decreases. The initially deposited carbon, on the other hand, is very active for hydrogenation 1681 and because of

its C 1s binding

energy

and C Auger peak shape most likely of the form CH,

1661. At this point it has been implicitly assumed that methane and other hydrocarbons are produced via the hydrogenation of atomic carbon and of polymerized carbon-hydrogen species. This “carbide mechanism” [73] is fully consistent with the finding of CO dissociation as well as other carbon hydrogenation experiments [63,68,74,75a,75b]. It follows also that the more atomic carbon is available on the surface, the more higher molecular weight products should be formed. This is exactly the case as one compares Fe and Ni: on Fe CO dissociates more readily, more C is available, relatively more longer hydrocarbon chains are formed, and concomitantly more graphitic carbon is deposited. The latter, of course is undesirable and efforts are being made to hinder the precipitation of surface graphite. In addition to surface graphite formation there is evidence that part of the excess carbon diffuses into the bulk of Fe to form Fe,C, Hagg carbide [76]. This process has been studied by Mossbauer spectroscopy [76]. The other metals Co, Ru and Rh have not been as extensively investigated as Ni and Fe but published work indicates so far that Ru and Rh behave similar1 to Ni [64,62]. A serious C deposition problem does not seem to exist. The situation for Co should be intermediate between Fe and the other metals as judged from the structure sensitivity of CO dissociation.

6. CO hydrogenation on oxidized metals In the work by Dwyer and Somorjai [65] on the hydrogenation of CO on Fe foils it was observed that pre-oxidized Fe showed about a tenfold increase in the initial rate of methane formation. There was also a small change in selectivity towards more higher molecular weight products [65]. Recent studies with oxidized Rh yielded similar results, namely increased initial rates of synthesis and more higher molecular weight products together with some oxygenated hydrocarbons [65]. These results for oxidized metals were mainly attributed to a “promoting” effect of near surface oxygen atoms. However, there are two problems with this interpretation: Firstly, the TONS for clean and oxidized surfaces are both calculated on the basis of geometric surface area and then compared without allowing for an increase in surface area due to oxidation/reduction. Secondly, both Fe and Rh are rapidly reduced in the CO/H, mixture such that oxygen as promoting element may not be available near the surface as long as the increase in rate is observed. We have carried out experiments with oxidized Fe foils and also with magnetite samples, Fe,O, [70], which show increases in surface areas and concomitant increases in synthesis rate. An example for an experiment with an oxidized Fe-foil will be briefly discussed. The experiments were carried out in the combination system de-

H.P.

Bonzel, H.J. Krebs / Surfcrce science upprouch

to heteropleour

cutu(tw

651

scribed elsewhere [59]. Fig. 8 shows a summary of the important results as a plot of the rate of methanation (log scale) versus reaction time for clean, oxidized and oxidized/reduced Fe samples. The reaction conditions were H/CO = 3, T= 560K at 1 bar total pressure, flow rate 5 cm3/min. The samples were oxidized in the atmospheric loading position in a stream of 0, at - 800 K for 15 min. Reduction took place at 1 bar of flowing H, inside the micro-reactor at 570 and 670 K for 15 min, respectively. The data in fig. 8 illustrate the previously observed increase in methanation rate for the oxidized Fe sample but they furthermore show that oxidized and reduced samples exhibit even higher activities than the oxidized samples. The reduction process in this case was monitored by XPS of Fe and 0 1s peaks. The spectra showed that the Fe-foil became partially reduced where the degree of reduction was higher for 760 K reduction temperature. It follows that not oxygen alone but rather an increase in surface area of Fe is most likely responsible for this effect. Support for this interpretation comes also from a comparison of the data for the two reduced samples: the sample reduced at 570 K has a maximum about three times higher than that reduced at 670 K. This means that some sintering of reduced Fe can occur at the higher reduction temperature. The attainment

Fe - FOII CO:H, T. 560K

q

A.

103'

0

’ 10

1 3. P,=lbar

1

20

30

LO

t

50

60

70

80

ImmJ

Fig. 8. Semilogarithmic plot of the rate of methanation versus time for initially oxidized/reduced Fe foils under otherwise identical conditions.

clean, oxidized

and

of the maximum specific surface area for completely reduced Fe is therefore limited by simultaneous sintering. It is clear from these experiments that no quantitative concIusions about a possible enhancement in the rate of methanation on oxidized samples can be drawn without independently checking for changes in total surface area. It is therefore not permissable to calculate TONS for the oxidized samples on the basis of geometric surface area [62,65]. Our work on Fe,O, samples 1701 gave very similar results to those of the oxidized Fe foil except that even higher surface area and porosity led also to an increased probability of readsorbed primary products and hence to more noticeable changes in product distribution.

7. Catalyst additives The combination systems presented in section 3 are ideal for studying the deposition of additives on the surface of the catalytic material and their effect on the CO hydrogenation reaction. Surface analysis can be used directly to monitor the amount of additive on the surface, its interaction with the substrate, and its chemical state after reduction or reaction. We distinguish two main classes of additives: electronegative (generaily “poisons”) and electropositive elements (“promoters”). The effect of C, 0, P, S and Cl on the adsorption of CO and H2 on various metals has been studied by utilizing surface techniques [20,39,77-801. All these investigations show that the strength of the adsorption bond of CO and hydrogen is reduced, that the rate of adsorption is decreased and that the probability for CO dissociation is much lower than for the clean surface - for those metals where CO dissociation is observable. An example for the decrease in adsorption bond strength of CO on Ni( 100) due to P is shown in fig. 9. Kiskinova and Goodman 1791 chose CO thermal desorption as the indicator of changes in bond strength. The lowering of the peak temperature from - 500 to 300 K for the main CO peak corresponds to a decrease in E, of about 60 kJ/mole. In this case the total amount of adsorbed CO is not significantly reduced. It is interesting to note that C and 0 as surface additives also inhibit CO adsorption and dissociation as it was demonstrated for Fe( 100) by Benzinger and Madix 1201. It is therefore also not expected that the hydrogenation of CO should be faster on C covered or oxidized surfaces. Of course, carbon is necessary for the formation of hydrocarbons, and in that sense it plays a dual role in this reaction. It follows from the results of these coadsorption experiments that electronegative eIements, in particular S, P and Cl are going to be active poisons of This is in agreement with experimental the CO hydrogenation reaction. observations in our laboratory and elsewhere [81,82] where a drastic decrease

in the rate of synthesis for small amounts of adsorbed sulfur was observed. A completely different behavior results from the addition of electropositive elements, e.g. alkaline metals. In this case coadsorption experiments of, for example, K and CO or K and H, produce three main results: (a) a decrease in the initial sticking coefficient of CO and H,; (b) an increase in the adsorption bond energy; (c) an increase in the overall rate of dissociation 183,841. Brod&n et al. [84] investigated the adsorption of CO on K promoted Fe(ll0) surfaces at 310 K. Fig. 10 shows a summary plot of the results outlined above; the increase in the adsorption energy is here indirectly contained as an increase in the amount of adsorbed CO at constant temperature. The relationship between adsorption energy of the molecular species (e.g. CO) and dissociation probability as it is derived from these investigations can be understood in a phenomenological sense by using the one-dimensional adsorption potential of fig. 11. A decrease in the adsorption energy induced by adsorbed electronegative elements shifts the crossing point between the potentials of molecular and dissociated states upward. Thus, assuming constant barriers of dissociation, the probability for desorption increases relative to that for dissociation. On the other hand, an electropositive element increases the adsorption energy; this lowers the crossing point of the potentials and hence increases the probability for dissociation relative to that for desorption. We have recently complemented the coadsorption studies by investigating the hydrogenation of CO on K covered Fe [lo]. In this experiment we deposited 5 yl of lop4 molar solution of K,CO, on Fe, heated this layer to 690 K for about 1 min in UHV, and subsequently transferred the sample into the reactor for hydrocarbon synthesis. The reaction was then interrupted at regular intervals for surface analysis by XPS. The time dependence of methane formation and carbon deposition for an initially clean and a K-covered Fe sample are shown in fig. 12. Note the decrease in rate of methanation and the rather substantial increase in the rate of C deposition for the Fe + K sample. This result is consistent with the previous discussion of the effect of K on CO adsorption and dissociation in that faster CO dissociation due to K means also more carbon per unit time, faster rate of ,graphite precipitation and a lower rate of methanation. First results for the CO hydrogenation on K-promoted Ni( 100) [ 1I] indicated an increase in the rate of methanation relative to clean Ni. This result is understandable because on clean Ni the rate of C hydrogenation seems to be faster than the rate of CO dissociation. Hence an increase in the CO adsorption energy due to K also increases the rate of CO dissociation as well as the total amount of carbon available for hydrogenation. Thus the rate of CH, formation is increased by the presence of K on Ni. An interesting new result was obtained in context with K-promoted Fe surfaces. After considerable C deposition due to prolonged CO + H, reaction it was found by XPS [lo] and also by SIMS [9] that the K signal was hardly attenuated. This means that the active K compound is sitting on top of the

654

00

020

010 r

I

06

,

030 I

,

05

P 8

.OL

ihI3 8 02

8

Fe (llO)+K+CO

01 00 ”

100

200

300 400 500 TEMPERATURE

600 (S)

I

10

010 POTASSIUM

030

020 COVERAGE

8~

Fig. 9. Flash desorption traces of CO from Ni(100) surfaces pre-covered with various amounts of phosphorus (coverage of P given by f?). Note change in peak temperature as a function of P coverage (from Kiskinova and Goodman 1791). Fig. 10. Effect of K on CO adsorption on Fe(l10) of 310 K and PC0 <4X IO-’ mbar: increase in CO coverge, decrease in initial CO sticking coefficient and increase in the fraction of dissociated CO after heating adsorbed CO layer to 500 K (from BrodCn et al. [83]).

deposited C layer. The mechanism for this effect is not yet known but a possible explanation could involve the following steps: (a) K is present on Fe as a K-O complex, oxide or hydroxide [85,86]; (b) C is deposited due to the reaction (CO dissociation); (c) C + K-oxide (-hydroxide) react to form elementary K metal; (d) K metal dissolves (“intercalates”) in the graphitic carbon tayer; (e) K metal segregates at the surface of the carbon layer, reacts with oxygen

H.?? Bon:el, H.J. Krehs / Surface science rrpprouch to heterogeneous cutu!vm

1

-

Clean

---

Electronegative

Additive

-.-‘-

Electropositive

Additive

655

Dkzsoclated State

Fig. 11. One-dimensional potential energy plot for molecular and dissociative CO adsorption. Schematic illustration of the effect of electronegative and electropositive co-adsorbates (catalyst additives) on the adsorption energy and the activation barrier for dissociation of CO.

a T = 570 K CO H, = 1 2@ Pr= lbar

O \ 1 I

\ ‘0

\,wlthout

K

‘. /,-\.

b with K ._/-• /-

_/-,

_/.-

, 0

200

LOO

600

800

t lsec)

Fig. 12. Effect of K on the CO+H, reaction on Fe: time dependence and of carbon deposition for initially clean Fe and K,COs promoted Krebs [ IO]).

of the rate of methanation Fe foils (from Bonzel and

656

H. P. Bonrel; H.J. Krehs / Surfuce science upprrmch to heterogetleous cutu!vsIs

and hydrogen to form K-oxide or -hydroxide; (f) steps (c)-(e) cycle. There are two observations in support of this XPS studies of K compounds after reaction suggest [87]; secondly, heating this K compound on top results in possibly K metal [87]. The latter process with K-graphite intercalates [88].

hypothetical cycle: firstly, that it may be K-hydroxide of the C layer to 2720 K is also postulated to occur

8. Outlook

Much has been learned about the adsorption and dissociation of H, and CO on transition metal surfaces. It is equally impressive to realize how much knowledge has recently been obtained on the CO + H, reaction on these metal surfaces. It is important and comforting (for the surface scientist) to realize that the newly obtained results agree well with those of the older catalysis literature. Quite often the route for obtaining such a result is much more direct than by the more conventional catalysis research. For example, it takes only a few minutes of reaction on an Fe foil and a subsequent AES or XPS analysis in order to prove the deposition of carbon during the reaction. Some of the results for clean metal surfaces achieved in this manner are quite novel, i.e. there is no direct evidence for them in the older literature. Those are: (a) dissociative adsorption of H, and CO on clean metal surfaces; (b) direct hydrogenation of deposited carbon to methane and higher molecular weight hydrocarbons; (c) precipitation of graphite on Fe as a partial poison of the reaction; (d) higher rate of hydrogenation for atomic carbon than for graphitic carbon; (e) increased CO dissociation and carbon deposition for K-covered surfaces: (f) segregation of K-compound on top of the deposited carbon layer. There is yet more to come - or at least to expect. This statement can be exemplified by enumerating some questions which are prompted by the reaction results obtained for Fe. A lot of attention has so far been paid to the initial, strongly time dependent stage of the reaction. What happens under steady state conditions? How thick and how continuous are the C deposits and where does the reaction take place? How is the metal specificity maintained for thick C deposits? How do single crystals or polycrystalline Fe foils compare to supported Fe catalysts under steady state conditions? Is there a structural effect? These are some of the questions which need to be answered by performing additional experiments. A similar, perhaps more complex situation exists for the K-promoted or even the multiply (K + Cu + Al + . ..) promoted Fe catalysts. What limits the rate of C deposition? What comprises the “active site” in such a multicomponent catalyst? Is it possible to eliminate graphite precipitation? What is the role of Cu? Only systematic and careful work will eventually provide answers to

H. P. Bonzel, H.J. Krehs / Surfuce science approach to heterogeneous

crrta!vsis

651

these questions but the important point is that the systematics of the route to follow have been clearly outlined.

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