Adsorption Measurements during Surface Catalysis

Ad sorption Measurements during Surface Catalysis KENZI TAMARU* Department of Chemietry, Yokohama National University, Yokohama, Japan

I. Introduction .................................................... 11. General Scope of Adsorption Measurements during Surface Catalysis ...... A. Partial Equilibrium in the Reaction Scheme and the RateDetermining Step .............................................. B. Estimation of the Chemical Potential of Reaction Intermediates ........ C. UseofIsotopeTracers .......................................... D. Dynamic Treatment of Reaction Systems .......................... 111. Experimental Methods. ............................................. A. Gravimetric Method ............................................ B. Volumetric Method ............................................ C. Gas Chromatographic Technique. ................................. D. OtherMethods .................................................. IV. Decomposition of Germane on Germanium ............................ V. Decomposition of Formic Acid on Metal Catalysts ...................... VI. Decomposition of Ammonia on Metal Catalysts ........................ VII. Ammonia Synthesis on Iron Catalysts ................................ VIII. Concluding Remarks ................................................ References ........................................................

page 66 68 68 72 72 73 76 76 76

76 77 79 81 83 86 88 89

1. Introduction It is generally accepted that chemisorption plays an important role in surface catalysis and at least one of the reactants [or activated complexes (I)]should be chemisorbed on the catalyst surface (2). When we study the kinetics of the reaction on a solid catalyst, we analyze the data, in many cases, on the basis of the Langmuir-Hinshelwood, or sometimes, of the Eley-Rideal mechanisms, tacitly assuming that the elementary steps other than the rate-determining surface reaction are all in equilibrium (3). The adsorption on the catalyst surface during the reaction is accordingly estimated from the kinetic data of the overall reaction postulating that Langmuir adsorption isotherms are *Present addreee: Department of Chemistry, The University of Tokyo, Hongo, Tokyo, Japan. 66

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applicable and that the rate of the reaction is proportional to the concentration of the reactants in their adsorbed state (or, in the case of Eley-Rides1 mechanism, one of the reacting spccics being gaseous or physically adsorbed, the rate is proportional to pressure of that species). The Langmuir adsorption isotherm is not always applicable, the heat and entropy of adsorption changing with coverage, and other adsorption isotherms such as proposed by Frumkin and Slygin ( 4 ) and Freundlich (5),sometimes, take the place of the Langmuir isotherm. The postulate concerning the deduction of adsorption data from the kinetic measurements undergoes, however, no fundamental change with these more elaborate treatments, though a better description of the adsorption may well be obtained. The adsorption of the several gases, which participate in the catalysis, onto the catalyst surface has been measured separately, and the thermodynamics and kinetics of chemisorption have been studied in various systems by many investigators. Chemisorption from gas mixtures, however, has been studied only in a limited number of cases and no measurements of adsorption on the catalyst in its working state has been carried out until recent years. In 1957, this author initiated a program of adsorption measurements during surface catalysis with simultaneous measurements of reaction rate ( 6 ) . The adsorption postulated from the reaction kinetics could consequently be compared with the observed results to examine the reaction mechanism. The state and the coverage of the catalyst surface during the reaction could be followed by direct measurements, including data on the pressures of the reacting species and on the reaction rate. Chemisorption on the catalyst surface during the progress of reaction cannot be estimated from the adsorption equilibria of reactants and products measured separately with each species. It depends not only upon the interaction among the adsorbed species and the catalyst Burface, but also upon the mechanism of the reaction, or the “kinetic structure” of the overall reaction. I n all cases the catalytic reaction proceeds through a certain number of elementary steps or reaction intermediates. The chemical potentials of those intermediates depend upon which of the steps is rate determining. Consequently by estimating the chemical potentials of the intermediates during the reaction, which is only possible by studying the reacting system in its working state, the rate-determining step may be identified. The properties of the catalyst surface such as, for instance, work function, heat of adsorption, reactivity of the adsorbed species, markedly

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depend upon its coverage (Z), and the most important properties of the catalyst are not those of the bare surface, but those of the surface in its working state. The latter can be found only by an investigation of the catalyst in its working state. When a reducing and an oxidizing gas react on an oxide catalyst, for instance, the fugacity of the oxygen on the catalyst surface depends not only upon the pressures of the reacting gases, but also upon the kinetic structure of the overall reaction. The fugacity of oxygen over an oxide is one of its inherent thermodynamic properties and its change possibly influences the properties of the oxide itself, or its catalytic activity. In the case of acidic catalysts, the acidity of the catalyst surface, which should be correlated with the catalytic activity is that in the working state and not that under conditions far removed from those prevailing in the actual reaction. In the case of adsorption measurements during reaction the catalyst surface in its working state is treated as one of the reactants. The reactivity of the chemisorbed species depends upon the coverage of the catalyst surface and can be studied as a function of its coverage under reaction conditions. The activity of the chemisorbed species is generally not equal to its surface concentration. I n the case of homogeneous reactions, one of the most orthodox treatments of their kinetics is to measure all the possible elementary reactions separately, and on the basis of this information the overall reaction may be constructed and the kinetic structure of the overall reaction is elucidated accordingly. This method should also be employed in the case of contact catalysis, but the nature of the medium where the reaction takes place changes with surface coverage. If all the simpler processes, however, which make up the overall reaction are studied separately as a function of surface coverage and partial pressures, the kinetic structure of the overall reaction would be elucidated. This is also one of the main developments to be aimed a t by the adsorption measurements during surface catalysis. It is also one of the important fields where the theories of solids would play their important role in catalysis. The nature of the adsorbed species during reaction is not always revealed by the kinetic data analyzed following the ideas of Langmuir and Hinshelwood. For instance, when the reaction is zero order, it appears that the active part of the catalyst surface is saturated with such species as reactants, products, or intermediate compounds, the adsorption being independent of the ambient gas pressures. The kinetic behavior does not tell which of them is really adsorbed. One of the merits of the adsorption measurements during the reaction is to provide such an identification.

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In the example a t hand, a question also arises concerning the saturation of the catalyst surface. The zero-order kinetics only suggests saturation of the active part of the catalyst surface. This is not necessarily the whole surface, especially when the latter is heterogeneous. One of the most fundamental problems in catalysis is to estimate the area of the active part of the surface. Since 1925 when Taylor ( 7 )suggested his concept of “active centers,” many discussions of this problem have been presented from various points of view. Actually it is shown by the field emirrsion microscope that the heats of adsorption on different crystal faces are often largely different. Thus for a zero-order catalysis the adsorption on the whole surface may depend upon the partial pressures of the ambient gases, only a part of the adsorption being pressure independent. From this pressure-independent adsorption, the active part of the catalyst surface as well as the saturating species might be found. From adsorption measurements during reaction one can also examine whether the reaction rate is correlated with the amounts of adsorbed reactants or with the pressure of the reactants, i.e., whether the mechanism is of Langmuir-Hinshelwood type or of the Eley-Rideal variety. I n most discussions of surface catalysis it is tacitly assumed that all steps are in equilibrium except the rate-determining step. With this new approach, the validity of this assumption can also be verified, The importance of adsorption measurements during surface catalysis has been outlined so far in generality. I n the following sections, the fundamental principle of the kinetic study based on this approach will be discussed together with appropriate experimental methods and results. An attempt will be made to classify the application of this approach to various cases, and emphasis will be placed on the principles of this approach rather than the detailed discussion of each reaction.

II. General Scope of Adsorption Measurements during Surface Catalysis

A. PARTIAL EQUILIBRIUM IN THE REACTION SCHEMEAND THE RATE-DETERMINING STEP

Let us suppose a heterogeneous catalytic reaction, L + R, where L stands for reactants and R, for reaction products. I n the course of the reaction, intermediate systems, E, F, G , . . . are formed and in some of these various kinds of adsorbed species are involved, such as L f E + F + G + 1

2

9

4

. . . +R n

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The rates of the forward and opposing consecutive steps are designated +

c

+

- + +

c

as V,, V,, V,, V2,, . . V,, 8,with suffixes representing the number of each step. In the stationary state the overall reaction rate ( V ) is equal to the differences of forward and opposing rates of each step in the above reaction scheme:

V+vI-v~IIvVa-v2+...~vn-v~ -

+

C

-

+

-

t

+

t

If step (2) is rate determining for the overall reaction, all the other steps being rapid enough in comparison with step (2), the rate of each step can be represented, for instance, as shown in Fig. 1 ( 8 ) . As the rates of step ( 1 ) -+

are sufficiently faster than those of step (2), V , becomes approximately t

equal to V,, which suggests that the step (1) is in equilibrium. The greater the difference in the rates, the closer it approaches equilibrium, Analogously it is concluded that all the steps except the rate-determing step are in equilibrium provided that they are rapid enough compared with the rate-determining step; in other words, no changes in free energy accompany those steps, and the free energy change of the overall reaction is accordingly concentrated at the rate-determining step.

I

I c

v,

I

I

FIG.1. Partial equilibrium in the reaction scheme.

I n the case of heterogeneous catalysis, let us take as an example a decomposition reaction, A(g) + B(g) 4-C(P)

and the reaction proceeds via the adsorbed state of the reactant, A(a), further, to those of the products, B(a) and C(a), where (g) and (a)represent gaseous and adsorbed states respectively. The reaction scheme is depicted in Fig. 2, each step being numbered as in the figure. (1) Case I . If the surface reaction, step (2), is rate determining, while

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FIG.2. The reaction scheme of a heterogeneous catalytic reaction,A(g) + B(g) -t- C(g).

all other steps are much more rapid than the step (2) so as to be equilibrated, the reactant A as well as the products B and C are all in adsorption equilibrium: A(g) + A(a), B(g) + B(a), C(g) + C(a). (2) Case 11. I n a similar way, if step (3) is rate determining, C(a) and A(a) are in adsorption equilibrium with C(g) and A(g) respectively, and B(a), on the other hand, with A(g) and C(g) in the following manner; B(a) C(g) + A(g). I n this case, the forward rate of the overall reaction (V,) can be expressed as follows according t o the LangmuirHinshelwood mechanism :

+

f'

=

-

kbl

- 1 $- b,Y,/P,

IpC

+ b,PA +b,P,

which can be expressed as follows when the denominator is approximately unity (the coverage of the catalyst surface is small): vf OC p A I p C

This rate expression can also be obtained on the assumption of a different rate-determining step. I n case I, the reaction rate can be expressed as follows:

If the adsorption of C(a) is so strong that biP, is much larger than 1 + b;PA+ bLP,, the following expression is obtained: v j CC

The same kinetic expression can be derived when the adsorption of A(g) onto the surface mainly covered by C(a) is rate determining. These treatments of the reaction kinetics clearly suggest that a kinetic expression is not enough to estimate the adsorption during the reaction or to elucidate the reaction mechanism, I n order to obtain more information, direct measurements of adsorption during reaction must be undertaken. If, moreover, both adsorption during the reaction and reaction

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rate can be measured simultaneously, we may discriminate between alternative reaction mechanisms. In case I, when B(g) is removed from the reaction system, B(a) should drop to, or at least approach, zero, unless the adsorption of B is extremely strong, desorption of B(a) taking place. As B(a) is in adsorption equilibrium with B(g),in this case, as soon as B(a) is formed from A(a),it goes rapidly to B(g) to be removed from the system. I n case 11.on the other hand, B(a) is not in equilibrium with B(g), but with A(g) and C(g), and the removal of B(g) does not result in a rapid decrease of B(a). I n such a way, the two cases I and I1 could be distinguished by following B(a) with time during this operation. If B can be labeled with an isotope to give B*, and B(g) is replaced by B*(g) during the reaction, following the isotopic abundance in B(a) with time, would give the rate of step (3). I n case I, the mixing should be rapid, while in case I1 it should be slow. The labeled B*(a) would finally mix with A(g) with the corresponding rate. The rate of each of the steps, ( l ) ,( 2 ) , (3), and (4) may also be treated in the same manner. Consequently, the kinetic structure of the overall reaction could be elucidated. To explain the situation in a different way let us suppose a series of water tanks connected by means of tubes, ( l ) , ( 2 ) , (3), . . . of various sizes as shown in Fig. 3. The tank a t the extreme left corresponds to the initial reacting system, L, while that a t the right, to the final reaction product system, R. The reaction intermediate systems, E, F, G, . . . are located between them in order. The water level of L is higher than that of R and water flows from left to right, just as reaction proceeds. I n this case the water level of each tank corresponds to the chemical potential of each system. If the water levels of the adjacent tanks are equal, they are in equilibrium. The flow rate of water moving from left to right depends upon the size of each tube which connects the tanks. If tube (3) is much narrower than any of the other tubes, the water

R FIQ.3. A series of water tanks connected by mews of tubes as 6 model of reaction L

sequence.

E

F

G

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levels of the tanks will adjust as shown in Fig. 3. All water levels preceding tube (3) are at the same height. The same goes for all water levels following tube (3). The difference of the water levels in vessels F and G is equal to that of L and R, which is the water level drop (free energy decrease) causing the overall flow (reaction) from L to R. The location of the drop in water level can accordingly be used as a criterion of ratedetermining step.

B. ESTIMATION OF CHEMICALPOTENTIALS OF REACTION INTERMEDIATES

A method to identify the narrowest connecting tube is to determine the water level in each tank. Similarly, a method to find the rate-determining step is to ascertain the chemical potential of each intermediate. An example of this sort is the decomposition of ammonia on a tungsten catalyst (9). During the reaction the amount of nitrogen chemisorbed on the surface is several times as much as that to form an adsorbed monolayer under the reaction conditions employed, This suggests surface nitride formation during the reaction. The adsorption of molecular nitrogen by the tungsten is not so strong as to form a nitride layer under identical experimental conditions. Consequently, the nitrogen chemisorbed during the ammonia decomposition has a higher chemical potential than the ambient nitrogen gas. Therefore the free energy cascade is located at the desorption process, provided that the whole surface of the catalyst takes part in the catalysis. Recently Apel’baum and Temkin (10) reported a new technique to measure directly the fugacity of the chemisorbed hydrogen during hydrogenation by means of a palladium membrane. This technique will be described in the section on experimental methods.

C. USEOF ISOTOPE TRACERS The size of each tube in Fig. 3 can also be examined by means of a dye. If we put a dye in tank R, it will become colored rapidly. So will tank G. But tanks L, E, and F will be colored after some delay, as tube (3) is very narrow. If the dye is added to tank G, a similar phenomenon will take place, but if it is in I?, tank L as well as E will be colored rapidly, while G and R will be colored at a much slower rate. In this way, by measuring at what rate each tank gets colored, the size of each tube may be estimated. Isotope tracers have been used to study the kinetic structure of the decomposition of germane on a germanium surface and of formic acid on a gold catalyst as will be described later,

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The mixing of an isotope between a n adsorbed species and a reactant is also applicable for investigating the reactivity of the adsorbed species. I n this connection we should recall interesting experiments carried out by Bond (11)and Schuit et al. (12).They studied the hydrogen-deuterium exchange on platinum and nickel catalysts, respectively. Deuterium was first admitted to be chemisorbed by the catalyst, then the ambient gas was replaced with hydrogen and the mixing of hydrogen with the adsorbed species was followed. It was observed that part ofthe adsorbed deuterium is exchangeable very rapidly, while another part mixes at a measurable rate and a residual part virtually fails to undergo exchange. The distribution of those parts markedly depends upon reaction temperature. This behavior suggests that the reactivity of the chemisorbed hydrogen is not uniform. Similar experiments were carried out by Gundry (13)with nickel and tungsten catalysts, including evaporated films. These experiments gave clear evidence for heterogeneity in the reactivity of adsorbed species. The differential isotopic method initiated by Keier and Roginsky (14) belongs to a similar category, though it does not necessarily deal with the catalyst in its working state. Krylov and Fokina (15)developed the method to identify the active region of a catalyst surface.

D. DYNAMIC TREATMENT OF REACTION SYSTEMS The water levels of tanks, E, F, and G in Fig. 3 can also be treated dynamically to examine the sizes of some of the connecting tubes. The rate of response of each tank to 'a rapid change of water level in another tank can be followed to study the size of the tubes between them. When all the water levels are equal in height and the water in tank R is removed rapidly, for instance, the level of each tank undergoes corresponding changes according to the size of the connecting tubes. This is the method described above, when B(a) is followed with time. Tamaru (16)observed that when ammonia was rapidly removed from the equilibrated mixture of nitrogen, hydrogen and ammonia in adsorption equilibrium with a doubly promoted iron catalyst, the nitrogen adsorption underwent no appreciable rapid change as a result, which suggests that, if the major part of the adsorbing surface participates in the reaction, the hydrogenation of the chemisorbed nitrogen to form ammonia is not very fast as has been generally accepted. The development of the method consisting in following the time response to an abrupt change of one of the water levels of the tanks leads to a method with which to examine the rate of each stage in the process which the overall reaction comprises. This method corresponds

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to a study of the size of the connecting tube in any given set of two water tanks. If the sizes of all the tubes throughout the reaction sequence are known, the kinetic structure of the overall reaction can be elucidated. Of course, generally speaking, it is not always possible to further separate the two sets of tanks with equal water levels, but the overall reaction may be separated into the simpler processes which it comprises. In the case of ammonia synthesis on a doubly promoted iron catalyst nitrogen adsorption (or desorption) and the hydrogenation of the chemisorbed nitrogen were separately measured as a function of the coverage and the pressure of reacting gases (17). The change in the properties of the catalyst surface with coverage has been taken into account. Thus a kinetic structure for the overall reaction could be set up and the nature of the rate-determining step under various reaction conditions could be determined as will be discussed later. These methods for elucidating the kinetic structure of the overall reaction have been briefly explained. They are not limited to heterogeneous catalysis, but are also applicable to reaction in general. The general method used hitherto t o study the reaction mechanism consists in following the material balance between L and R. On the other hand, very few kinetic observations on the intermediates have been carried out. Recently new tools, such as ESR, NMR, infrared and ultraviolet spectroscopy*, have been adapted to surface studies and detailed information on the catalytic reaction, as regards both adsorbed intermediates and their reactivity, is becoming available. It is to be expected, therefore, that the dynamic techniques that have been outlined will be effectively applied to elucidate the kinetic structure of the overall reaction. The existence of a certain adsorbed species on the catalyst surface does not necessarily imply that it is really a reaction intermediate which is involved in the reaction sequence. The identification of a reaction intermediate in the adsorbed state can be carried out by treating its kinetic behavior in the reaction as has been explained. I n this way, the elementary steps and reaction intermediates which the overall reaction comprises may be identified. The kinetic behavior of the overall reaction, such as order of reaction, activation energy and so forth, is determined by the constituent elementary steps and also by the kinetic mutual relation or kinetic structure of these steps as has been discussed in connection with Fig. 3.

* Recently Tachibana and Okuda ( 1 8 ) studied the electronic spectrum of cumene adsorbed on a silica-alumina catalyst during its cracking reaction arid suggested that a Bronstcd acid contributes to the cracking reaction through the formation of protoiiated cmen0.

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Similarly, the properties of a molecule are determined by the constituent elements and also by its structure.

111. Experimental Methods A. GRAVIMETRIC METHOD Mars et al. (19)and Scholten (20)constructed anelaborated apparatus for studying the adsorption of nitrogen during ammonia 'synthesis, measuring the weight of the catalyst. The apparatus used for this experiment consists of a gravimetric system; a beam balance is installed in a high-vacuum system and can be operated magnetically. The catalyst, placed in a platinum basket, is suspended on a thin glass wire. The temperature of the catalyst is measured directly with a Pt-PtRh thermocouple connected to the Pt-glass seals by means of 4-p wire loops to avoid weighing errors. Appropriate correction for buoyancy and flow are necessary. The reaction rate can be measured from the concentration of the reaction products in the exit gas. Elaborate studies were carried out by means of this apparatus, but the gravimetric method to measure adsorption under reaction conditions has some inherent disadvantages, only the weight change of the catalyst being measured. I n the case of ammonia synthesis, it was assumed that the hydrogen adsorption on the catalyst surface partly covered by nitrogen would be equal to that on the part of the surface not covered with nitrogen. Due to the low atomic weight of hydrogen the ambiguity in nitrogen coverage is obviously small. But, generally speaking, the ambiguity due to the mutual influence of the chemisorbed gases is not always negligible; it is especially important when the behavior of the adsorbed species depends sharply upon their coverages.

METHOD B. VOLUMETRIC A volumetric method to measure simultaneously adsorption during reaction and reaction rate was proposed by Tarnaru (6,21).It consists of a closed circulating system similar to those used in studying mixed adsorption. The amount of gases adsorbed on the catalyst surface can be calculated from the amount of the reactant introduced into the system and the pressure and the composition of the circulating gas. The composition of the circulating gas can be measured by thermal conductivity or mass spectrometry, and sometimes by condensing gases one by one with liquid nitrogen and solid carbon dioxide traps, successively; for instance, this is possible in the case of a mixture of hydrogen, carbon

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dioxide, and formic acid vapor. Due allowance has t o be made for the analytical samples removed from the system. The amount of the catalyst is preferably large for accurate measurements and the adsorption should be measured under such conditions that the reaction takes place very slowly, while the rate of gas circulation is fast enough so that the composition of the circulating gas is virtually the same throughout the system and isothermal conditions prevail in the reactor. I n these measurements the adsorption is calculated from 8 material balance. The result yields the amount but not the kind of adsorbed chemical species. As to the reaction rate, it is to be noted here that the change in adsorption with time accompanies the change in the amount and composition of the reacting gas. This should be taken into account in the calculation of the reaction rate. I n Tamaru’s studies of ammonia synthesis, the produced ammonia was always trapped in a liquid nitrogen trap and its amount gave the reaction rate.

C. GAS CHROMATOGRAPHIC TECHNIQUE

A third method designed to measure adsorption during surface catalysis is a gas chromatographic technique initiated by Tamaru (22), and Nakanishi and Tamaru (23). In the usual gas chromatographic technique, inert gases such as nitrogen or helium are employed as carriers, but in the present modification, the reacting gases are used as a carrier gas and the catalyst is placed in the adsorption column maintained a t reaction temperature so that a stationary state of the catalytic reaction is established. The gases which participate in the reaction are introduced into the system a t the top of the adsorption (catalyst) column as gas samples and the extent of their adsorption on the catalyst surface in, its working state can be measured by the retention time. In this technique, the reaction rate can be measured simultaneously by analyzing the product in the exit gas. I n this way the adsorption on the catalyst surface in its working state and the reaction rate can be studied simultaneously under various reaction conditions. A necessary requirement is that the reaction proceeds slowly enough to keep the composition of the reacting gas as well as the surface condition of the catalyst virtually the same throughout the catalyst column. A variation of the technique, applicable t o such reactions as decomposition, isomerization or polymerization, consists in using the reactant as a sample gas and an inert gas as a carrier gas, keeping the catalyst column a t reaction temperature. The exit pulse of the sample gas gives

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its retention time and its area, the amount of reactant unreacted in passing through the column. The apparatus employed for this experiment is similar to the usual gas chromatograph. The material of the column walls should be noncatalytic at reaction temperature. A requirement for this technique is that the adsorption be reversible, rapid, and moderate. The retention time in this case corresponds to A x / A N , where A x is the increase in adsorption at the increase of concentration of gas sample by A N . When the sample gas is one of the reactants, its adsorption also takes place from the carrier gas. When the surface is really saturated with a sample gas from the carrier gas, the retention time of the sample should be zero, no further adsorption taking place. Accordingly a small value of A x / A N for a reactant can be due ( 1 ) to weak adsorption or (2) to a strong and nearly saturated one from the carrier gas. I n the former case, A x / AN should increase as temperature becomes lower, while in the latter case, A x / A N should decrease, as the adsorption from the carrier gas approaches saturation so that A x / A N decreases provided the adsorption is equilibrated. Bassett and Habgood (24) deliberately applied this chromatographic method, in addition to the “microcatalytic chromatographic method” of Hall and Emmett (25), to the isomerization of cyclopropane on a molecular sieve. They were able to assess the heat of adsorption of the reactant, the activation energy and the order of the reaction. Ozaki, et at. (26) recently studied the “rapid and reversible” part of the adsorption of hydrogen on nickel-kieselguhr (50 wt-%) by means of a chromatographic technique. They measured the retention time of a deuterium sample using hydrogen as a carrier gas at various temperatures from - 195” up to 300”C,placing the catalyst in the adsorption column. The HD content in the exit pulse of the deuterium sample was analyzed, and the total uptake of hydrogen by the catalyst was separately estimated in a static system a t these temperatures. The amount of hydrogen adsorbed, which can exchange quickly with deuterium (or the fraction of rapid adsorption in total uptake), was thus estimated from the retention time. It sharply decreased with temperature up to -140°C, increased to pass a first maximum around 0°C and then a second maximum at about 120°C. This behavior was associated with different states of adsorption.

D. OTHERMETHODS Adsorption on the catalyst surface during the progress of reaction can be estimated in various ways. In the case of germane decomposition

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on a germanium surface (27) it could be estimated in the following manner. The reaction system was cooled down rapidly to freeze in the state of the catalyst surface. Then, germane and ambient hydrogen were removed, and the catalyst surface with chemisorbed hydrogen was exposed t o higher temperatures to desorb hydrogen. This gave the amount of hydrogen chemisorbed during the reaction. The experiment was possible because the adsorption of hydrogen on a germanium surface is activated and reversible. It was thus concluded that the catalyst surface during the reaction is saturated with chemisorbed hydrogen, the

FIU.4. Sketch of a flow circulating system with a palladium membrane. A: circulation pump. B : palladium-membrane.

number of hydrogen atoms chemisorbed being equal to the number of surface germanium atoms.* I n a special case the fugacity of one of the adsorbed reactants could be measured directly during the reaction. For example, in the hydrogenation of ethylene on a palladium surface, Apel’baum and Temkin (10)successfully used a very thin palladium film through which hydrogen passes during the reaction. The apparatus employed is a flow circulating system schematically shown in Fig. 4. When the hydrogen pressure in the supply side is higher than the fugacity of the chemisorbed hydrogen, hydrogen passes into the reaction system through the palladium membrane, while if it is lower, hydrogen comes out of the reacting

* The analyses of an oxide catalyst after the oxidation of carbon monoxide, according to Voltz and Weller ( H ) ,gave an indication of the state of the catalylyst during the reaction.

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system through the membrane. I n this way the fugacity of the chemisorbed hydrogen during the reaction could be estimated. It turns out that the fugacity is substantially lower than that of hydrogen in the reacting gas a t 0-42", while a t 176°C they are almost equal. Recently the behavior of preadsorbed ethy1ene-C-14 on a nickel film could be followed by counting the /3 radiation from the nickel during the hydrogenation of C-12 ethylene (29). Only a fraction of the preadsorbed ethylene was removed from the film, while the remainder was firmly held during continued hydrogenation of C-12 ethylene. It was thus concluded that only a fraction of the chemisorbed species participates in the hydrogenation.

IV. Decomposition of Germane on Germanium The thermal decomposition of gaseous hydrides on their constituent elements to produce hydrogen is one of the simplest catalytic reactions (27,30). During the reaction the surface of the catalyst element is always renewed by the continuous deposition of fresh surface atoms and only two elements are involved including catalyst. In the case of germane decomposition on germanium it has been shown that (1) the decomposition is a zero order reaction and its activation energy is 41.2 kcal/mole; (2) during the decomposition the entire surface of germanium is virtually covered by chemisorbed hydrogen atoms the number of which is approximately equal to that of surface germanium atoms; (3) no hydrogen deuteride is formed during the decomposition of germane in the presence of an excess deuterium; (4) when a mixture of germanium hydride (GeH,) and deuteride (GeD,) is decomposed, abundant quantities of equilibrated hydrogen deuteride are produced, while (5) no exchange takes place between the two kinds of germane during the reaction to form GeH,D,; (6) the desorption rate of the chemisorbed hydrogen on germanium a t full coverage is equal to the decomposition rate of germane; ( 7 ) the ratio of the decomposition rates of germane and deuterogermane is 1.8 to 1; (8) the adsorption of hydrogen on the germanium surface is reversible with an activation energy of 14.6 kcal/mole for the initial adsorption,* and obeys the Langmuir isotherm for dissociative adsorption a t lower coverage, 0 = bP3:. This suggests a dissociative type of adsorption with a n initial heat of adsorption of 23.5 kcal/mole. At higher coverages the Freundlich adsorption isotherm is applicable, indicating a decrease of the heat of adsorption with coverage.

* Recently Bennett and Tompkins ( 3 1 ) observed that the activation energy for the initial adsorption on an evaporated $Zm is 16.0 kcal/mole.

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According to the adsorption isotherms, for the hydrogen pressures under reaction condition, the fraction of the surface covered with hydrogen would be about only one half, if adsorption equilibrium were established. Consequently, observation (2) leads to the conclusion that the fugacity of the hydrogen adsorbed on the germanium surface during the decomposition is much higher than in the ambient hydrogen gas. I n other words, desorption of hydrogen from the germanium surface is the rate-determining step. This conclusion is supported by observations (3) and (6) and is confirmed by the zero-order kinetics, I n fact, hydrogen from the gas phase can scarcely reach the surface during the decomposition, as shown by the lack of hydrogen deuteride production during the germane decomposition in the presence of deuterium, while exchange between Ha and D, proceeds when the decomposition is over. This lack of HD production during the decomposition of GeH, D, rules out a Eley-Rideal mechanism, involving collisions of deuterium molecules in the gas phase (or those in the van der Waals’ adsorption layer) with the chemisorbed species on the surface. Although the heat of adsorption decreases with coverage, the activation energy for desorption increases only slightly when one passes from a bare surface (14.6 23.5 = 38.1 kcal/mole) to saturation (41.2 kcal/mole). The rate of the hydrogen-deuterium (1:1 mixture) exchange on the germanium surface as HD production (re) is accordingly calculated from the hydrogen coverage of germanium surface (6) during the exchange reaction, obtained from the adsorption isotherm, and the rate of desorption a t full coverage ( r J . This calculation, by means of the formula r, = Oar,/2, gives results in good agreement with experiment.* The basic assumption behind this calculation is that the exchange can take place on all covered sites and not just on a limited number of active centers. Indeed, this agreement conforms to the observation that the surface is fully covered with hydrogen during decomposition, the germanium surface exhibiting no a priori heterogeneity, and all the germanium surface atoms seemingly participating in the decomposition. The fall in adsorption heats is consequently due to “induced heterogeneity” of a type akin to that suggested by Boudart (33) being due for instance, to the change in electronic properties of the surface with adsorption. It should be noted that the observation (5) suggests another free energy drop a t the chemisorption of germane on the germanium surface mostly covered by hydrogen. If the rate determining step is only hydrogen desorption from the germanium surface, the chernisorption rate of

+

+

* Kuchaev and Boreskov (32) studied the isotopic exchange of hydrogen on n- and p-type germanium, in which the density of free electrons or holes was deliberately altered in a wide range. The rate of the exchange waa the same for ell the specimens.

ADSORPTION MEASUREMENTS DURING SURFACE CATALYSIS

81

germane is so strongly hindered by the chemisorbed hydrogen that it even becomes slower than the desorption rate of hydrogen. This finally results in two free energy cascades a t the step of the chemisorption of germane as well as that of hydrogen desorption, the hydrogen chemisorbed still covering most of the surface.

V. Decomposition of Formic Acid on Metal Catalysts The decomposition of formic acid on metal catalysts to form carbon dioxide and hydrogen has been studied extensively by many investigators. The reaction is of interest in connection with selective catalysis, as formic acid also decomposes to carbon monoxide and water on some “dehydrating” catalysts such as alumina. I n recent years, considerable progress has been made in the elucidation of the reaction mechanism using such tools as infrared spectroscopy (34),electric conductivity (35,36) isotopic tracer (37-39) and adsorption measurements during surface catalysis (21, 40). In the case of nickel, it was observed by Tamaru (21) that, when formic acid vapor gets into contact with a clean nickel surface a t 100°C, all the vapor is virtually chemisorbed at first until hydrogen comes off when a first saturation point is reached, no carbon dioxide, on the other hand, being evolved. When more and more formic acid is admitted, carbon dioxide is finally evolved a t the second saturation point where the ratio of hydrogen and carbon dioxide chemisorbed is 1 : 2, and the number of hydrogen atoms chemisorbed is approximately twice as large as the number of surface nickel atoms. These observations strongly suggest that a monolayer of nickel formate is formed on the nickel surface a t the contact with the acid vapor. I n other words, the nickel surface is first saturated according to HCOOH(g) + H(a)

and then following, HCOOH(g)

+ H(a)

--t

+ HCOO(a)

HCOO(a)

+ HAg)

until the second saturation with HCOO(a) takes place. This observation was confirmed by Fahrenfort, et at. (34)on a supported nickel catalyst. The formation of nickel formate monolayer can only be understood sterically if the superficial nickel atoms leave their place in the metal lattice. The formation of a formate layer on the catalyst surface, was originally imagined by Rienacker and Hansen (35),later confirmed by Hirota et al. (34)with infrared spectroscopy of the adsorbed species on copper,

82

KEN21 TAMARU

nickel, and zinc. Their results also agree with the observation of Fahrenfort et al. (34). Fahrenfort et al. (34) not only observed the formation of formate during the acid decomposition, but also studied the decomposition of the formate-covered surface by means of infrared spectroscopy and also by measuring the increase in gas pressure. They found that the rate and the activation energy of the decomposition of the surface formate both coincide with those of the overall reaction of formic acid decomposition. This was not necessarily the case in the data obtained by Tamaru (21) at lower temperatures. They accordingly concluded that the decomposition of formic acid on a nickel surface proceeds via nickel formate as a key intermediate. In the case of the decomposition on noble metals such as gold, the adsorption spectra showed only a weak absorption band which was ascribed to formate ion on the surface. Actually, during the decomposition of formic acid on a silver catalyst, according to Tamaru (Zl),hydrogen is adsorbed as much as carbon dioxide. Kinetically speaking, the decomposition on the catalyst is first order at higher temperatures and lower pressures, while it is zero-order at lower temperatures and higher acid pressures. The adsorption of the acid, on the other hand, seemed to conform to the reaction order, and was proportional to the pressure when it is a first order reaction and reached a saturation in the zeroorder region. We should here recall the work of Sachtler and De Boer (37) on the formic acid decomposition on a gold catalyst. They studied the reaction by means of deuterium tracer and found no hydrogen deuteride formation when HCOOH was decomposed in the presence of deuterium, while abundant (equilibrated) amounts of HD were observed when HCOOH and DCOOD were decomposed simultaneously. The hydrogendeuterium exchange does not proceed on the gold catalyst. These observations led to the conclusion that formic acid first decomposes to formate intermediate and hydrogen atom, the former decomposes yielding carbon dioxide and another hydrogen atom. Thus hydrogen atoms move round on the surface to combine with a partner with which to escape from the surface. The chemical potential of the hydrogen atoms on the gold surface is accordingly higher than that of ambient hydrogen molecule, which shows the hydrogen desorption to be rate determining. The formic acid decomposition on metal catalysts accordingly seems to proceed via surface formate, though the difference in basicity of the metal surface would result in different coverage during the reaction, which in turn leads to a different reaction order. It is widely known that Fahrenfort et al. (34)thus correlated the catalytic activities of the

ADSORPTION MEASUREMENTS DURING SURFACE CATALYSIS

83

individual metals with the heat of formation of their superficial formates. I n the case of a low heat of formation, such as gold, the surface formate is very unstable a t the decomposing temperatures. This results in slow overall reaction rate. With increasing stability of the surface formate, the reaction rate becomes faster until it reaches an optimum stability. If the surface formate is too stable, the catalyst surface is covered by the stable surface formate. The reaction rate, on the other hand, becomes slower, as the surface formate decomposes with difficulties, while the overall reaction becomes zero-order, the surface being saturated with formate. I n this way a “volcano-shaped” curve was obtained in a plot of catalyst activity versus heat of formation of metal formates. It is thus easily realized that the state of the catalyst surface during the decomposition depends upon the nature of the catalyst. I n the case of formic acid decomposition on a copper surface (40), adsorption measurements during the reaction revealed a characteristic behavior of the catalyst surface. The reaction is of zero-order, the rate being independent of the ambient gas pressures. The adsorption, on the other hand, is markedly dependent upon the partial pressure of formic acid. The amount of carbon dioxide adsorbed is comparable in size with that of hydrogen, which is in marked divergence from the case of a nickel surface, where carbon dioxide is adsorbed twice as much as hydrogen, forming a surface formate. The zero-order kinetics imply saturated adsorption on the active part of the catalyst surface, while the adsorption on the whole surface apparently depends upon the pressure of formic acid. The saturated adsorption of the copper surface calculated as one-site adsorption is shown as 8 = 1 in the results. It appears therefore that the catalytically active part is a minor part of the surface available for adsorption. I n this manner, adsorption measurements during surface catalysis in the case of a, zero-order reaction could lead to an estimate of the active part of the catalyst surface.

VI. Decomposition of Ammonia on Metal Catalysts The decomposition of ammonia on tungsten is one of the examples most frequently discussed in textbooks of catalysis (2, 3). It is accepted that the reaction is of zero-order in the initial stage, which is interpreted t o indicate the catalyst surface to be fully covered by ammonia during the reaction. According to Frankenburger and Hodler (41),on the other hand, a rapid first step of the decomposition is the formation metal 4 NH-metal H,, which takes place of surface imide, NH,

+

+

84

KEN21 TAMARU

at temperatures as low as 150°C and is followed by surface nitride formation a t about 200"C, 2(NH-metal) --f 2(N-metal) H,. These results clearly indicate that ammonia molecules are not the species that saturate the surface a t decomposing temperatures generally above 600°C. The decomposition of ammonia on iron catalysts has been extensively studied in relation to the ammonia synthesis and it is generally admitted that nitrogen desorption is the rate-determining step in the overall reaction (42). Consequently, it might also be possible to interpret the zero order kinetics on the basis of saturated adsorption of nitrogen on the catalyst surface, the nitrogen desorption being rate determining. However, this view seems contradicted by additional evidence. Jungers and Taylor (43)and also Barrer ( 4 4 )observed a kinetic isotope effect in the decomposition rates of ammonia and its deuterocompounds; on a tungsten surface NH, decomposed more rapidly than ND,. This isotopic effect of hydrogen cannot be explained by the saturated adsorption of nitrogen on the catalyst surface. One of the methods to cast light on this problem is to measure adsorption of the catalyst surface during the reaction (9). It is shown by the measurements that, a t reaction temperatures, the uptake of nitrogen is more than the monolayer and the formation of surface nitride layers was suggested.* The amount of nitrogen sorbed depends upon the reaction time and ammonia pressure and increases at higher ammonia pressures, or with the addition of ammonia. At lower temperatures, nitrogen comes off with difficulty and its chemisorption increases with reaction time, although the ammonia pressure becomes lower, and hydrogen pressure, higher, suggesting that the ambient ammonia is not in equilibrium with the nitrogen in the nitride and hydrogen. As to the adsorption of hydrogen during the reaction, virtually no hydrogen is adsorbed a t temperatures higher than 600"C, where the decomposition has generally been investigated. Consequently, if the discussion is based on the behavior of the whole surface, the interpretation of the zero-order kinetics as due to saturation with NH,(a) or NH(a) is not likely to be adequate, though this view has generally been accepted. During the course of the reaction, if the temperature was rapidly lowered to 150"C, all the ambient gas was removed and the temperature was then raised again, the pressure and the composition of the gas evolved from the catalyst with time give the rate of desorption of nitrogen (or decomposition of nitride layers). It is of interest to note that

+

* Logan and Kemball ( 4 5 ) also studied the decomposition of ammonia on a tungsten evaporated film and observed nitriding to various extent.

ADSORPTION MEASUREMENTS DURING SURFACE CATALYSIS

85

nitrogen desorption separately measured in the absence of the reactant was almost exactly equal to the rate of nitrogen production in the overall reaction at equal nitrogen uptake, which suggests that nitrogen desorption is rate determining. Thus, the overall reaction can be considered to be a consecutive reaction comprising nitride formation followed by its decomposition,while the amount of the chemisorbed nitrogen (or the thickness of the nitride layers) during the reaction depends upon the relative rates of the two processes, It is accordingly suggested that the simpler processes of which the overall reaction is composed can be studied separately, and also that the rate of nitriding decreases, while that of nitrogen desorption increases, as the nitride becomes thicker, until the supply and the consumption of the chemisorbed nitrogen are dynamically balanced. The isotope effect in the rates of decomposition of NH, and ND, can be explained on the basis of the different rates of nitride formation from these two isotopic molecules, while no hydrogen is being chemisorbed during the decomposition at decomposition temperatures. The decomposition of ammonia on an ammonia synthetic catalyst is similar to that on tungsten (46). At lower temperatures, nitrogen is evolved with extreme difficulty and is increasingly chemisorbed with time and temperatures, though the pressure of ammonia decreases and that of hydrogen increases. At higher temperatures, on the other hand, the description of nitrogen takes place, while adsorption of hydrogen decreases with time and temperatures.

VII. Ammonia Synthesis on Iron Catalysts Ammonia synthesis is a model case of a fundamental investigation in surface catalysis and many efforts have been focused at this problem making use of a variety of the tools available (42, 47). Emmett and his group (48) found that the rate of chemisorption of nitrogen and the rate of ammonia synthesis are both of the same order of magnitude and that the exchange reaction of nitrogen, Nio NP = 2NgQ, proceeds at a speed comparable to that of the synthesis reaction. The exchange reactions between hydrogen and deuterium, and also between deuterium and ammonia, on the other hand, both take place at temperatures far below normal synthesis temperatures. These data have been considered t o support the view that nitrogen chemisorption is the rate-determining step. Temkin and Pyzhev (49) successfully derived a kinetic expression for the ammonia synthesis assuming that the chemisorption of nitrogen is

+

86

KENZI TAMARU

rate-determining . Their work was considered as another strong indication in favor of nitrogen chemisorption as rate-determining step. More recently Dutch workers a t the Staatsmijnen in Limburg (19,50), tried to estimate the adsorption during the reaction using a gravimetric method. After a careful study of nitrogen adsorption on a singly promoted iron catalyst, they compared the nitrogen coverage during the synthesis with that which would give a rate of nitrogen chemisorption equal to the observed rate of ammonia synthesis (at the same temperature and partial pressure). I n this comparison, a very good agreement is observed except for measurements a t very low temperatures. The next step in establishing the kinetic structure of the overall reaction is to confirm whether the chemisorbed nitrogen is in equilibrium with hydrogen and ammonia, N(a) gH,(g) = NH,(g). If it is equilibrated, the following equation will be valid,

+

p$€I*/(p;a * p3,J = K(eq) where P& is the pressure of nitrogen which should be in adsorption equilibrium with the nitrogen coverage under reaction conditions. As

= 7BPN,(1 -x)4

stm

as P,,(eq) = PEI(l- x) atm, PN,(eq)= PNI(l- 2) atm, where 7 = PNH,/PNH,(eq) and x is the molar fraction of ammonia in the 1:3 N2-H, mixture when it is in equilibrium. I n this way, Pi, may be calculated from the partial pressures of nitrogen, hydrogen, and ammonia in the catalyst bed, and, consequently, the nitrogen coverage which equilibrates with P i 9is obtained from the adsorption isotherm, postulating that hydrogen adsorption does not influence the nitrogen adsorption. Both values of nitrogen coverage thus obtained showed “perfect” agreement, in support of the current view that the nitrogen chemisorption is only rate-determining and all the other steps are equilibrated during the synthesis. at I n the calculation of P i 2 , however, the Dutch workers used PNHa the exit of the catalyst bed. I n other words, the ammonia pressure over the catalyst was postulated to be equal t o that a t the exit. I n the flow system P N H I a t the entrance of the catalyst bed is zero. Without knowing, consequently, the distribution of ammonia pressure throughout the catalyst bed, the calculation of Pk,is subject to error, especially since Pisis proportional to the square of PN1,$. Enomoto and Horiuti (51) suggested that the hydrogenation of the

ADSORPTION MEASUREMENTS DURING SURFACE CATALYSIS

87

chemisorbed nitrogen to form ammonia is rate determining. This conclusion was based on the determination of the “stoichiometric number,” of the rate-determining step, In general, the stoichiometric number of a step is the number of times that the step under consideration takes place in one overall reaction. In the case at hand, for the overall reaction N, + 3H, = 2NH,, the stoichiometric number of the ratedetermining step is two rather than one. Bokhoven et al. (52) repeated Horiuti’s experiments and reached an opposite conclusion (a stoichimetric number of one) which conforms to the current view. Their data have been reexamined by Horiuti and Takezawa (53) who insist that the conclusion of the Dutch group is not correctly derived. Recently Tanaka and Matsunaga (54)obtained the stoichimetric number of one by means of a singly promoted iron catalyst. They also confirmed that the rate of isotopic exchange, Nlo Ni8 = 2Nig,is slow during the reaction. Ozaki et al. (55)compared the rate of ammonia synthesis on a doubly promoted iron catalyst with that of deuteroammonia, and found that deuterium reacts markedly faster than hydrogen under the same reaction condition. From the kinetic data, as well as the isotope effect, they reached the conclusion that the rate-determining step of the overall reaction is the chemisorption of nitrogen on a surface mainly covered with NH radicals, and that the isotope effect is due to the fact that NH is adsorbed more strongly than ND. Tamaru (46)measured adsorption on a doubly promoted iron catalyst surface during the course of the synthesis. He found that nitrogen chemisorption undergoes no appreciable rapid change when ammonia is removed from the equilibrated mixture of nitrogen, hydrogen and ammonia all in adsorption equilibrium. This suggests that the hydrogenation step of the chemisorbed nitrogen is not as fast a process as has been accepted, if it is admitted that a significant part of the surface participates in the reaction. Tamaru studied adsorption and desorption of nitrogen in the presence

+

-+

c

of hydrogen (vl and vl, respectively) and also the reaction between the +

chemisorbed nitrogen and hydrogen to form ammonia (v2) all as a function of ambient pressures and coverages. The following expressions are thus obtained; -+

=

AP,, exp( -aN(a))

211

=

B exp(PN(a))

v2

= CPH. exp(yN(a)1

v,

t 3

A , B,C , a,8, and y are constants at a constant temperature, and N(a)

88

KENZI TAMARU

is the amount of nitrogen adsorbed. The criterion of the rate-determini---f

ing step can be the ratio, Vl/vz; if this ratio is much larger than unity, the latter step, the hydrogenation of chemisorbed nitrogen to form ammonia, is rate determining, while if it is much smaller than unity, nitrogen ad- or desorption is rate controlling. The values of fl and y empirically obtained are almost equal, while the activation energy contained in B is much larger than that contained in C. This leads to the conclusion that, at higher temperatures and lower hydrogen pressures, t 3

the ratio vl/va is much larger than unity; in other words, the hydrogenation of the chemisorbed nitrogen is rate determining, on the contrary at lower temperatures and higher hydrogen pressures, nitrogen chemisorption is rate determining.

VIII. Concluding Remarks A new approach, measuring adsorption during the course of catalytic reaction, has been described. The state of the surface which catalyzes the reactions is not that of the surface in the absence of reactants, but that which exerts under reaction conditions. In this sense, the properties of a catalyst surface to be studied should be those in the working state, rather than those of a bare surface. Adsorption measurements during the reaction can be one of the direct ways to obtain information on the working state. As this working state is dependent upon the mechanism of the reaction, it gives direct information toward the elucidation of the reaction mechanism. Methods to study the simple reactions (or elementary steps), as a function of the coverage of the surface and the pressures of reacting gases, would lead to the elucidation of the kinetic structure of the overall reaction, at least in a more straightforward way than has been generally possible thus far. I n recent years techniques to study the properties of adsorbed species have markedly developed, which may provide great assistance in measuring each adsorbed species quantitatively during the reaction. The new method discussed in the review has been fruitful in many respects, but it is still far from complete in itself. It is hoped that this discussion will contribute to the development of new tools to gain a fuller insight into the nature of contact catalysis. ACKNOWLEDQNENT The author owe8 a great debt of gratitude to Sir Dean Hugh Taylor, Professor Michel Boudert, Dr. W. M. H. Sachtler, Professor J. Horiuti, and Professor T. Kwan, a11 of whom read the manuscript, and whose many suggestions have markedly improved the article.

ADSORPTION MEASUREMENTS DURING SURFACE CATALYSIS

89

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