Adsorption and desorption dynamics as seen through molecular beam techniques

/.

surface

Surface Science 2991300 (1994) 261-276 North-Holland

science

Adsorption and desorption dynamics as seen through molecular beam techniques K.D. Rendulic* and A. Winkler

Technische Universitdt Graz, Institut ftir Festkiirperphysik, Petersgasse 16. A-8010 Graz, Austria

Received 2 February

1993; accepted for publication

24 May 1993

A description of the history of knowledge about adsorption and desorption dynamics is given. The individual stations include the encounter with non-cosine, non-Maxwellian distributions of adsorbing and desorbing particles; detailed balancing in its development as a tool to relate adsorption and desorption data is described. A further section treats the concept of precursor mediated adsorption and its verification by molecular beam methods. The problem of surface defects is briefly touched. Refinements in the molecular beam techniques finally lead to the possibility to gain state resolved dynamics data for adsorption and desorption processes.

1.

Introduction

In this report we want to deal with the application of molecular beam techniques both to the investigation of the dynamics of adsorption and the dynamics of desorption. In particular the dissociative chemisorption of reactive gases (Hz, 02, N2 ) will be treated. Dissociative chemisorption from a theoretical point of view is a most interesting topic because the dynamics of adsorption is profoundly influenced not only by the translational energy but also by the quantum state (vibration, rotation) of the impinging molecule. For practical purposes the attempt to elucidate the mechanism of heterogeneous catalysis involves the understanding of dissociative chemisorption. The beginning of the insight into the dynamics of dissociative chemisorption is the legendary paper by Lennard-Jones: “Processes of Adsorption and Diffusion on Solid Surfaces” published in 1932 [ 11. Everybody is familiar with the picture of the one-dimensional model of activated dissociative chemisorption, where the two diabatic potential curves for the molecular and the atomic state intersect and form the crest of an * Corresponding

activation barrier in front of the surface. Yet there is a lot more to be found in this paper: already the variation of the barrier height across the unit cell is dealt with and the question as to the role of the vibrational and rotational states of the molecule in activated adsorption is raised for the first time. Of course, we know now that in the context of the one-dimensional Lennard-Jones model it is impossible to describe many features (e.g. the contribution of internal energy of a molecule) of the adsorption/desorption dynamics. The aim in understanding adsorption in today’s language is to build an appropriate multi-dimensional potential energy surface which describes the dynamics of adsorption and desorption. Such a potential energy surface has to be deduced from the experiment or, if a theoretical model is present, it has to be tested against the experiment. How can we experimentally determine the shape of such a potential energy surface? To probe the potential energy surface difirential sticking experiments are needed for which the particle properties can be chosen or at least analyzed at will. The sticking coefficient for example should be determined as function of the angle of incidence, the translational energy, the quantum state and perhaps even the orientation

author.

0039-6028/94/$07.00 0 1994 - Elsevier Science B.V. All rights reserved SSDI 0039-6028(93)E0281-X

262

K.D. Rend&c,

A. Winkler /Adsorption

of the molecule. A measurement of this type is said to determine the dynamics of adsorption, Dynamics measurements involve the application of molecular beams. Ideally a well collimated beam of molecules for which some or all of the above mentioned parameters are known is made to collide with the sample surface and the fraction of adsorbed particles is determined. In turn also the desorption properties may be deduced by molecular beam techniques: a collimator selects a beam of desorbing particles under a certain desorption angle. Again analytical techniques can be applied to determine the velocity distribution and the quantum state of the desorbing particle. This short sketch of the application of molecular beam techniques for the investigation of the adsorption/desorption dynamics looks quite simple. Nevertheless, tremendous experimental and conceptual difficulties had to be overcome until at least some of the aims could be realized. Presently the most complex experiments in surface physics are employed to elucidate the details of adsorption dynamics. The end of this development is certainly not in sight but one can state that the birth of these techniques coincides with the birth of “Surface Science”. What follows is a limited account of the progressive development in’ our knowledge about the dynamics of adsorption and desorption. It also is a subjective account of the development in the sense that every person has different ideas of what constitutes an important step in science; we do not attempt an encyclopedic approach but limit ourselves to several topics encompassing adsorption of reactive gases, in particular hydrogen. Nevertheless, this is not such a serious limitation after all, since basic ideas and basic experimental accomplishments need only a few characteristic examples for elucidation.

2. Activated adsorption and non-cosine distributions Around 1960 a number of papers started to deal with the measurement of angular distributions of particles leaving a surface after having

and desorption dynamics seen through MBT

been supplied to the surface by a molecular beam [2,3]. Everybody at that time believed in the cosine distribution of desorbing molecules and the experiments seemed to verify this concept. The general theoretical arguments given in favor of a cosine distribution of desorbing particles were equilibrium considerations involving the second law of thermodynamics. Since in thermal equilibrium the particles impinging on a surface have a cosine distribution with respect to the surface normal the particles leaving the surface have to exhibit a cosine distribution too. This is generally labelled the cosine equilibrium law which has been discussed by Maxwell [4], Langmuir [ 5 1, Clausing [ 61 and Knudsen [ 71. The same argument can be made with respect to the energy distribution of particles leaving the surface: it ought to be Maxwellian. These arguments are, of course, correct. But as Comsa [ 8 ] has pointed out, the particles leaving the surface are not only desorbing particles, there are in addition also elastically and inelastically scattered particles. Only the sum of these particles needs to have a cosine/Maxwellian distribution. Nothing can be said a priori about the angular and energy variation for each of the subsets of particles such as desorbing particles on their own. Actually, already in the 1960s there was evidence that the reflecting power of a surface was generally not isotropic, which according to Comsa would directly lead to non-cosine distributions for desorbing particles. A graphic description of the relation between scattered and desorbing particles is given in Fig. 1. A rather nice explicit verification of the fact that scattered particles plus desorbing particles have to add up to a cosine distribution was given quite early in the game through a molecular beam experiment by Palmer etal. [9] forthecaseofDJNi(l11). In one of the most famous experiments in surface physics van Willigen [ lo] in 1968 showed that molecular hydrogen desorbing from Fe, Pd and Ni exhibited strongly forward (towards the surface normal) peaked angular flux distributions. What an ingenious experiment! The hydrogen was permeated from a high pressure area through the heated sample; on the vacuum side one obtained a continuous desorption flux

K.D. Rend&c,

263

A. Winkler / Adsorption and desorption dynamics seen through MBT

D(@)=S(e).I(@) =K~co&~co&

60’

80’

-80” Fig. 1. Polar diagram schematically depicting the relation between total particle flux Z(8) = K cos 8 impinging on a surface (solid line), the scattered flux (dotted line) and the adsorbed flux (dashed line). If for example the sticking coefficient changes like S(B) = cos2 8 the scattering probability R (8) has to vary as ( 1 - cos2 8) to always add up to Z(e). In addition, detailed balancing demands the adsorbing flux S(S)Z(e) to be identical to the desorbing flux D(8). Data from Ref. [18].

without the disturbance of scattered particles present in a gaseous environment. This was a true molecular beam experiment, the desorbing particles were registered in a beam detector. The results of this experiment are depicted in Fig. 2. Not only did van Willigen get the experiment right, he also presented a basically correct interpretation of the data in terms of a one-dimensional barrier model: a Maxwellian hitting a one-dimensional barrier will be attenuated in the low energy end of the spectrum. The velocity component normal to the barrier ( TJcos 63) can only be used to transgress the onedimensional barrier of height EA when the “normal translational energy” E cos2 0 > EA. As the angle with the surface normal increases, fewer and fewer particles out of the Maxwellian are able to cross the barrier. With increasing barrier height the desorption flux D(e) becomes more and more concentrated about the surface normal. van Willigen explicitly formulated:

Fig. 2. Non-cosine desorption flux obtained by van Willigen for a number of permeation sources at a temperature of 900 K. Interpretation of the data was given in terms of a one-dimensional barrier desorption mechanism. An activation barrier of 1 eV height would result in a flux distribution depicted under A. Data from Ref. [lo].

x exp[-(EA/kT)

tan20].

(1)

Traditionally [9 ] the desorption flux D(e) been approximated by the expression

has

D(0)

(2)

= D(0”) cos” 8.

The experimentally obtained distributions are very well represented by this function; as later could be shown [ 111 the cos” 8 functions are closely related to normal energy scaling (see below). The van Willigen paper immediately stimulated a large number of investigations involving activated adsorption, in particular the measurement of differential sticking coeffkients S (~,63) and differential desorption probabilities D (w, 0)~. It was, for example, immediately obvious that the energy distribution of the particles desorbing over the barrier had to exhibit a mean particle energy which is larger than the 2kT expected for a Maxwellian.

3. Detailed balancing EA + kTcos20 D(‘) = D(‘“) (EA + kT) cos8

Detailed balancing, a principle of equilibrium physics frequently applied to problems of physi-

264

K.D. Rend&c, A. Winkler /Adsorption and desorption dynamics seen through MBT

cal chemistry, was first formulated by Kirchhoff [ 12 ] and Richardson [ 13 1. Actually, Kirchhofl’s law relating absorption and emission of radiation is quite similar to the laws relating adsorption and desorption dynamics. To gain access to this important principle in its application to surface physics one best follows the historic, statistical treatment by Langmuir [ 141 describing the properties of adsorption isotherms. This statistical treatment very clearly shows the salient features of detailed balancing. The derivation of the adsorption isotherm is generally obtained from the dynamic equilibrium between the rate of adsorption and the rate of desorption:

sorbing particles as well as particles which are going to be scattered (elastically, inelastically) and particles which have been scattered (elastically, inelastically). Detailed balancing is particularly well suited in its application to molecular beam data. It is therefore not surprising that the widespread use of this principle started in the 1970s along with the development of molecular beam techniques [ 16 1. An especially useful formulation of detailed balancing for the adsorbing and desorbing flux in thermaf equilib~um was introduced by Stickney and Cardillo [ 17 ] :

(3) The variables are temperature T, pressure p and surface coverage N,. The left side of Eq. (3 ) contains the specific kinetics o~adsor~tion (e.g. via a precursor path), the right side the specific kinetics ofdesorption. From this formulation one would expect that the adsorption isotherm will depend on the particular kinetics involved. But as has been pointed out [ 14,151, from a statistical point of view an equilibrium is described by a distribution of particles over a set of available states, free or adsorbed. The physics of transition between these states, that is the particular kinetics, cannot enter in this description. This means the adsorption and desorption kinetics of Eq. {3) have to be related at all times in such a way that they cancel and do not appear in the resulting adsorption isotherm. Or as Langmuir [ 141 already put it in 19 16: Since evaporation and condensation are in general thermodynamically reversible phenomena, the mechanism of evaporation must be the exact reverse of that of condensation, even down to the smartest detail. A general definition of detailed balancing can be obtained using the concept of microscopic reversibility: for each “forward” path there exists an identical “reverse” path related to the former by time reversal. Detailed balancing demands that in thermal equilibrium each forward path occurs with the same probability as the corresponding backward path. This principle relates subsets of particles such as adsorbing and de-

=D(v,@,

Qi)fM(v,

Qi).

(4)

Here zi is the velocity of the particle, 8 the angle with the surface normal and Qi the quantum state (which was not yet contained in the original formulation); the Maxwellian flux distribution is labelled fM. Here, and for all other considerations in this paper, the sticking coefficient is always defined for the zero coverage limit; that means it is the initial sticking coefficient. The right hand term can be considered a definition of the desorption probability D in terms of the generated desorption flux. On the left hand side the already existing definition of the sticking coefficient as ratio of adsorbing to impinging particles introduces the case term. One can also write: S(v,9,Qj)cos8

= I>(a,@,Qi).

(5)

The equation formulated for thermal equilibrium is in this form of little practical use. The impo~ance comes from the fact that one can independently investigate adsorption dynamics and desorption dynamics in certain situations of nonequilibrium, situations of quasi-equilibrium, and still retain the validity of Eq. (5 ) _The term quasi-equilibrium in the context of adsorption has acquired a well-defined meaning: During the separate investigation of adsorption and desorption the the~odynamic parameters of the remaining part of the system are kept as

265

K.D. Rend&c, A. Winkler /Adsorption and desorption dynamics seen through MBT

close as possible to those of the corresponding equilibrium state. This has been discussed in detail by Stickney [ 17 ] and Comsa [ 18 1. What is generally labelled as a “test” of detailed balancing can only involve this last point mentioned: Will the adsorbing and desorbing particles observed in a specific non-equilibrium situation (a situation of quasi-equilibrium) still retain the same adsorption and desorption paths as in thermal equilibrium and obey Eq. (5)? Experimental verification of the principle of detailed balancing for a situation of quasiequilibrium has been performed on several levels [ 191. The simplest type of experiment involves the angular variation of g (8 ) and D (43). If the desorption flux changes according Eq. (2) as D(8)

= 0(0°) cos”@

then Eq. (5) will lead to: S(B) = S(0”) co?-i 8.

Tilt z g

axis:

[211]

1.0

-i $ 0.6 Q) 2 0.6 CI a g 0.4 ._ -ii 6 0.2 g

0.0 -60

-40

-20

o

20

40

60

Angle(Degree) Fig. 3. Detailed balancing requires the differential adsorption flux S(e) case to be equal to the differential desorption flux D(e). The impinging hydrogen beam was generated by a capillary array at 300 K, angle resolved flash desorption was applied to determine the desorption flux from the Ni( 111) surface. Data from Ref. [20].

(6)

Palmer et al. [9] were the first ones to apply molecular beam techniques to verify above relations for the case hydrogen/Ni( 111). Similar data were obtained for HJCu by Cardillo et al. in a series of papers which will be discussed in the next chapter. A systematic investigation of S (8 ) and D (e) was later performed in our laboratory involving Hz adsorption and desorption for Ni [20,21], Pd [21], Cu [ll], Al [22,23] and W [24]. Angular distributions S(e) were measured with nozzle beams or Knudsen beams while D (8) in all cases was determined by angle resolved flash desorption. As an example the measurements for the system Hz/Ni( 111) [ 201 are depicted in Fig. 3. On an other level one can check detailed balancing for the velocity distributions of adsorbing and desorbing particles. For the desorption flux a time-of-flight measurement has to be performed on molecules originating from a permeation source. These experiments were pioneered by Stickney et al. [ 251 and later by Comsa and David [ 261. Complementary adsorption data have to be obtained by the ap-

plication of monoenergetic nozzle beams as described in the next chapter. Actually, there are only a few adsorption systems for which both sets of velocity resolved dynamics data have been obtained: HJCu [ 11,271, H/Ni [21,28] and Hz /Pd + sulfur [ 2 1,29 1. The example of hydrogen adsorbing on and desorbing from a sulfur covered Pd( 100) surface is shown in Figs. 4a and 4b. The occurrence of an activation barrier at about 3 kcal/mol separates the hydrogen flux in a Maxwellian and a non-Maxwellian component, both in the adsorption and in the desorption flux. Finally, most recently validity of detailed balancing also in terms of vibrational quantum states was found to hold. Vibrationally excited molecules have been found to be overrepresented both in the desorption flux as well as in the adsorption flux for some systems of activated dissociative adsorption; this matter is treated in the last chapter. For almost all adsorption systems investigated so far, detailed balancing seems to hold for a situation of quasi-equilibrium. Those cases showing an apparent breakdown of detailed bal-

266

K.D. Rendulic, A. Winkler /Adsorption

and desorption dynamics seen through MBT

ancing can all be traced to some violation of the state of quasi-equilibrium in the experimental procedure: for example, it is easy to see why one can adsorb say CHJOH on a cold surface and obtain CO and Hz during subsequent thermal desorption. These are just non equivalent situations to be related by detailed balancing.

4. Velocity distributions of adsorbing and desorbing particles

0

2000

4Ol~O

0000

Velocit,y(m/s)

b

:,

D,-Pd(

100)

T,=360

K

: _/

OS-o.5 ~~54~~

_

a 00

-~ 200

)P

,-

10

5

Time

of Flight

(x10-”

s)

Fig. 4. Both (a) and (b) demonstrate the validity of detailed balancing in terms of the velocity distributions of the adsorbing and desorbing flux. A nozzle beam was used to determine the velocity dependent sticking coefficient of hydrogen on a sulfur covered (- 0.5 ML) Pd( 100) surface. Because of the barrier for dissociation around 3500 m/s (equivalent to N 3 kcal/mol) an impinging Maxwellian produces a mixture of a Maxwellian and a non-Maxwellian (high velocity), component in the adsorption flux. Data from Ref. [ 211. (b) Time-of-flight measurement on deuterium originating from a sulfur covered (- 0.5 ML) palladium ( 100) permeation source. The situation is roughly complementary to the adsorption experiment of (a). The desorption flux also contains a high velocity component as well as a Maxwellian fraction. Data from Ref. [ 291.

This chapter deals with the most important developments for the understanding of adsorption and desorption dynamics. It comprises a set of seminal experiments by Stickney and Cardillo that profoundly influenced the ideas in surface science, The van Willigen experiment [ lo] described earlier, directly implied that adsorbing and desorbing particles had to have nonMaxwellian energy distributions. The development of nozzle beams as sources of monoenergetic molecular beams (discussed e.g. by Stoles in this volume and in Ref. [30]) opened the door for the investigation of velocity resolved sticking coefficients S (v, e). On the other hand time-of-flight measurements on permeation sources made it possible to check on the velocity distribution D (u,C3) T of desorbing particles. The first of the Stickney papers [ 251 dealt with time-of-flight measurements of D2 molecules desorbing from a hot polycrystalline nickel surface. As in the van Willigen experiment a permeation source served as a sample. The result of this crucial experiment was thefirst

time determination of the non-Maxwellian character of the desorption flux generated in activated associative desorption (Fig. 5 ). Hydrogen molecules (Dz) desorbing from the polycrystalline nickel surface exhibited a mean energy of 3kT instead of the 2kT expected for a Maxwellian beam. In addition, the velocity distribution of the desorbing particles was narrower than expected for a Maxwellian. Both of these results are in qualitative agreement with the van Willigen formula [ lo] (Eq. ( 1) ) and are also reasonably compatible with the angular distribution D (@) determined as COS~.~ 8. Shortly af-

K.D. Rend&c.

A. Winkler /Adsorption

Dz-Ni(poly) T = 1073 K

rv-\

Maxwellian

I 100

200

Time

300

of Flight

400

500

(~3)

Fig. 5. Time-of-flight measurement performed by Stickney et al. on the D2 desorption flux originating from a polycrystalline nickel permeation source. This experiment for the first time showed the non-Maxwellian character of desorbing hydrogen. The desorbing particles have a higher mean energy than expected from a Maxwellian of identical temperature. Data from Ref. [25].

terwards Cardillo and Stickney [ 311 made the next logical step and investigated the adsorption dynamics for HJCu with the help of a nozzle beam. The HD exchange reaction served as a measure for the sticking coefficient; a choice that in retrospect proved to have some serious consequences. The much discussed experiment seemed to indicate an activation barrier of about 4 kcal/mol for Hz/Cu. What made this paper so fascinating was the agreement with some general principles of surface physics. Angular and energy distributions of adsorbing molecules obeyed normal energy scaling, the consequence of a one-dimensional barrier mechanism: S(E,0)

=

S(ECos20,00).

(7)

The picture of a one-dimensional barrier was still more reinforced by the measurement of the Sshaped form of S (E ) as predicted by the model. In a further impressive paper Cardillo and Stickney [ 17 ] discussed the above data with respect

and desorption dynamics seen through MBT

267

to detailed balancing. A well rounded picture of dissociative chemisorption seemed to emerge. What a surprise when Comsa and David [27] determined the velocity distribution of hydrogen originating from a copper permeation source. There was no compatibility in terms of detailed balancing with the Cardillo/Stickney data. What had gone wrong? Only 15 years later it was found out [ 111 that the activation energy of 4 kcal/mol obtained in Ref. [ 171 was much too small. These experiments had just been one step too far for the experimental capabilities of that time. Nevertheless, the Stickney/Cardillo papers were milestones in the investigation of adsorption/desorption dynamics and the general procedures (normal energy scaling, detailed balancing) are still applied in the way presented at that time. During the period of 19751980 Comsa and David performed a series of experiments characterizing the desorption of hydrogen from permeation sources [ 261. These were experiments performed under truly well defined surface and vacuum conditions. Again and again these data have proved to be some of the most accurate characterizations of desorption dynamics. The time-of-flight investigations included nonactivated adsorption for H2/Pd [ 291, slightly activated adsorption for H2/Ni ( 111) [ 28 ] and the highly activated adsorption system Hz/copper [ 27 1, which exhibited mean energies for desorbing hydrogen of about 8kT. At this point, one faced the situation that microscopic reversibility and detailed balancing were up to scrutiny again because of the Hl/Cu dilemma. A beautiful set of desorption data by Comsa and David existed for several adsorption systems but no corresponding adsorption data were available yet. In addition, there was the unanswered question if post-permeation hydrogen really originated from the same initial surface state as in the traditional desorption experiments. Was this perhaps the reason for the apparent breakdown of detailed balancing? It therefore was not surprising that at the same time experimental efforts were directed to gain differential sticking coefficients S (w , f3) to compare them to the permeation data. In

268

K.D. Rendulic, A. Winkler /Adsorption and desorption dynamics seen through MBT

1985 a wealth of information on the adsorption/desorption dynamics of Hz/Ni appeared in the literature [20,32-341. All these molecular beam results indicated that adsorption of Hz on Ni ( 111) was an activated process, although exhibiting a rather small activation barrier of about 3 kcal/mol. What a relief that adsorption [ 201 and desorption [ 281 for Ni( 111) agreed in terms of detailed balancing: the mean energy of adsorbing as well as desorbing hydrogen was found to be identical with (E) x 3kT. Especially good agreement was also obtained for the angle resolved adsorption and desorption data. The COS~.~ 8 obtained in angle resolved flash desorption agreed perfectly with the COS~.~ Q determined for the sticking coefficient in beam experiments [20,32]. A more refined analysis later [21] completed the picture in showing that really all existing adsorption and desorption data for Hz/Ni( 111) could be fitted into one consistent picture: direct, activated adsorption obeying normal energy scaling and detailed balancing. At the same time these experiments proved that permeation/desorption experiments and conventional low temperature adsorption/desorption experiments yield identical results. Post-permeation hydrogen apparently originated from the same initial state as in flash desorption. What about the now famous case of Hz/copper? It still resisted any elucidation. Nozzle beams using the HD exchange reaction and postpermeation had established non-compatible sets of data: here a picture of low activation energy [ 3 1 ] hardly larger than for nickel, there an immense excess energy of 8kT in the desorption flux [27]. In the meantime molecular beam techniques had been sufficiently refined to circumvent the HD reaction and use flash desorption to determine coverage values for adsorption of hydrogen on copper. Anger et al. [ 111 showed that sticking coefficients for Hz/Cu were one to two orders of magnitude smaller than previously assumed. Only above a beam energy of 0.2 eV the sticking coefficient would increase rapidly with particle energy. This rapidly increasing sticking coefficient when folded with a Maxwellian yielded a mean energy for the adsorp-

tion flux of about 7kT, in close agreement with the permeation data. Because of normal energy scaling one could expect a rather sharp angular change of the sticking coefficient; and indeed values of about n = 16 were observed. Both the small value of the sticking coefficient and the high value of n implied a rather high activation barrier to dissociative adsorption. Needless to say that everybody was quite happy with these results, in particular the theoreticians who for long time had proclaimed that noble metals like copper should exhibit large activation barriers, perhaps 1 eV, for hydrogen adsorption [ 35 1. The nozzle beam experiments basically confirmed these ideas. There was one feature though in the nozzle beam results for H2/Cu immediately pointed out [ 36,37 ] : normal energy scaling was only obeyed in an approximate fashion. The sticking coefficients S (E, 8) for high energies and high nozzle temperatures when projected into the 0” sticking curve according to S (E cos2 8,O” ) systematically gave too high values. To understand the implication of this result one has to realize that to produce nozzle beams of varying translational energy E one employs high pressure expansion from a nozzle of varying temperature ( Tnozde), whereby translational energy and nozzle temperature are related through:

(8) But not only translational energy, also the internal energy of the molecules is increased with temperature. Ostensively, the sticking coefficient for H2/Cu as seen in Fig. 6 is not only a function of the translational energy but also of the nozzle temperature. The reason, as suggested by Harris [ 361, is the contribution of the vibrational energy to the dynamics of adsorption. But this is a matter to be discussed in the last chapter.

5. Precursors for dissociative chemisorption At this point one should keep in mind that already the simple Lennard-Jones model [ 1]

K.D. Rendulic, A. Winkler /Adsorption and desorption dynamics seen through MBT

1

1 C”(ll1)

.

ItAl

. Am @A ‘s=

I Cu(llO) I

-

0.02 rcuclooil 0.01

0

... Ao

1

uq=+-, 0.1 Normal

..

0’

0.2

Beam

I

0.4

0.3

Energy

(eV)

Fig. 6. Breakdown of normal energy scaling in the adsorption of Hz&u. Angle resolved sticking coefftcients obtained at high total energies and high nozzle temperatures (solid triangles: T = 1700 K, E = 0.4 eV, solid squares: T = 1400 K, E = 0.33 eV and solid circles: T = 1000 K, E = 0.23 eV), when plotted versus normal energy E cosz 8 yield different values than the sticking coefficients obtained at 8 = 0” incidence (open symbols). This indicates an explicit influence of the nozzle temperature on sticking via a vibrational contribution. Data from Ref. [ 111.

for dissociative chemisorption included the possibility of a molecularly adsorbed state. The molecular state might be a separate adsorption state [ 381 but more frequently can act as an intermediate state to dissociative chemisorption. Because of the shallow well depth the lifetime of such an intermediate state would be very short. Physisorption or trapping was initially investigated for adsorption of rare gases [ 391; as a precursor mode to chemisorption it was studied only considerably later. The seminal work in this area can be traced to Becker and Hartman [ 401, Ehrlich [ 4 1 ] and Kisliuk [ 42 1. Frequently precursors were merely considered conceptual crutches of little physical reality. One still re-

269

members headings: “Precursor states, myth or reality” [ 431 or “How real are precursors”? [44]. Although indirect evidence of precursor mediated chemisorption had been available for a long time, only molecular beam studies could get hold on the specific dynamics involved. The physics determining trapping is the dissipation of the excess translational energy of the impinging molecule to prevent the particle from redesorption. From all we know today the most effective mechanism involved in this energy transfer is the excitation of phonons in the adsorbing surface. Actually, on an uncorrugated smooth surface it is only necessary for the impinging particle to dissipate the normal energy E cos2 8 to remain trapped on the surface; parallel energy may be dissipated during a subsequent movement along the surface. This physics leads to just the opposite result as in the case of activated chemisorption. Normal energy scaling is expected, but the larger the normal energy the lower the trapping probability. The characteristic dynamics of precursor adsorption will be a sticking coefficient falling with particle energy. Normal energy scaling will then lead to sticking coefficients increasing with increasing angle and thus to values n of Eq. (6) smaller than unity. The first application of a molecular beam technique to precursor mediated chemisorption involved the system NJW by King and Wells [45 1. The paper by King and Wells is remarkable in several ways: from an experimental point it introduced the now generally applied “Method by King and Wells” to determine sticking coefficients. The appeal of this method is its capability to determine absolute values of sticking coefficients without the need of any calibration. Although only a limited range of energies was available for the effusive beam source used in the experiments, all of the pertinent features of precursor adsorption emerged from the study: the sticking coefficient clearly decreased with increasing beam energy as expected by the need of energy dissipation. Second, an observed variation of the sticking coefficient with surface temperature Ts was also in line with an intermediate precursor state. The molecule equilibrated in the precursor state can either desorb or trans-

270

K.D. Rendulic, A. Winkler /Adsorption and desorption dynamics seen through MBT

fer into the chemisorbed state. These competing processes will depend on surface temperature. In fact, today the condition aSlaT, # 0 is considered a strong indication of a precursor mediated process. By the years 1985 to 1990 a more systematic application of molecular beam techniques to the problem of precursors took place. Sticking coefficients falling with beam energy have been observed for the adsorption systems Hz/Ni ( 110) and HJNi(997) [21,32,46] in our laboratory as well as in studies by Rettner and Auerbath on the adsorption of 02/W(llO) [47] and Nz/W( 100) [48]. Further results included Oz/Pt ( 111) [49] and propane and butane adsorptiononIr(llO)(l x 2) [50]. What about the angular variation of the sticking coefficient in chemisorption via a precursor state? As mentioned above, as a consequence of normal energy scaling the sticking coefficient should actually increase with increasing angle of incidence. Indeed, such a behavior with values of n < 1 (Eq. (6) ) could be observed for several adsorption systems [ 2 15 1,521. Generally though, normal energy scaling is obeyed only in a qualitative fashion and the values n are usually somewhat larger than expected from the shape of S(E). This seems to imply that also the tangential component of the energy has to be accommodated to some extent. This will occur if the mobile precursor state is confined to a limited surface area. Finally, if complete localization of the intermediate state on the surface is encountered total energy scaling is the result. A rather nice example of this behavior can be seen in Fig. 7 [ 2 11. Adsorption of Hz on a Ni ( 110) surface proceeds via a mobile precursor (n < 1) when the beam is aimed parallel to the surface grooves. This corresponds to an orientational change of the crystal by tilting it about the [ 1001 direction relative to the fixed beam direction. Aiming the beam perpendicular to the grooves (tilt axis [ 1 lo] ) suppresses any tangential movement and results in a near angle independent sticking coefficient. One feature encountered in many dissociative adsorption processes utilizing a precursor path is the presence of a second, direct, activated adsorption channel. This has, for example, explicitly

o^

1.3

x x

1.2

2 rk 8

1.1

._ 5

1.0

Beam

parallel to grooves ” = 0.75

;; -z 4

aQ1

0.9

7g I+

2 k

n = 1.1

0.8

2 0.7

’ 0

HJNi(ll0) 10

20

Adsorption

30

40

Angle

50

60

(Degree)

Fig. 7. Influence of a precursor state on the adsorption of hydrogen on a corrugated surface. With the beam aimed parallel to the grooves of the Ni (110) surface only normal energy has to be dissipated. This results in a sticking coefftcient increasing with angle of incidence. A beam hitting the surface perpendicular to the atom rows has to dissipate the total energy, resulting in a near angle independent sticking coefficient. Data from Ref. [ 2 11.

been shown for hydrogen adsorption on all transition metals (Pd, W, Pt, Fe, Ni) as well as on all other adsorption systems already mentioned above. At low particle energy the precursor path is utilized whereas at higher beam energies the activated, direct adsorption path with a sticking coefficient increasing with increasing beam energy dominates. At the same time the angular variation of the sticking coefficient changes from values n < 1 to values of n > 1 as the particle energy is raised. As an example of a molecular beam study exhibiting most of the features expected for precursor adsorption the system NJW [48] is shown in Fig. 8. As a consequence of detailed balancing one should really also see two desorption channels in the flux of desorbing molecules; and indeed the time-of-flight measurements by Comsa and David [ 26 ] for some cases exhibited a superposition of two sets of particles. The mean energy of desorbing particles was observed to decrease with increasing desorption angle for Hz /Ni ( 111) [28,53]. The real shocker at that time was the mean energy of hydrogen desorbing at angles larger than 70”; they had an even smaller mean

K.D. Rend&c, A. Winkler / Adsorption and desorption dynamics seen through MBT

Nitrogen/W(lOO) oi = 450

Ts:

I

0.0

0.5

Kinetic

l 300K . BOOK A 1000 K .lzoO K

I

1.0

1.5

2.0

Energy(eV)

Fig. 8. The adsorption of Nz/W( 100) serves as an example for precursor mediated adsorption. At low beam energy adsorption is dominated by a precursor path: the sticking coefficient decreases with beam energy. A second, activated adsorption path is utilized at high beam energy (> 0.5 eV). In addition, low surface temperature will aid adsorption via the precursor path. Data from Ref. [48].

energy than the 2kT expected for a Maxwellian [ 281. Once nozzle beam adsorption data on precursor mediated adsorption became available, also this puzzle was solved [ 541. A parallel path involving a precursor will inevitably exhibit mean energies for adsorbing and desorbing particles smaller than 2kT because of the monotonically falling sticking coefficient. The higher desorption angles just separate the precursor path from the activated path. But where had the precursor path for Hz/Ni ( 111) come from? For all one knew, hydrogen adsorption on Ni ( 111) was a purely direct, activated process. The source of the precursor path proved to be surface defects, a topic that deserves a more detailed treatment in this context. 6. Dynamics of adsorption at defect sites: stepped surfaces

If we look back at the short history of surface physics, we can recognize that up to the 1960’s

271

wires and polycrystalline foils of sometimes undefined surface composition served as samples in the adsorption experiments. Although the experiments corresponded to the situation on a real, technologically relevant surface, very little about the microscopic mechanism during adsorption could actually be learned at that time. Only after the physics of adsorption was known for a perfect crystal one could start to introduce surface defects and contaminants to study their effect on the process of adsorption [ 55,561. And at that moment, one has to say, a gift from heaven arrived: the concept of stepped surfaces. Stepped surfaces or vicinal planes are located a few degrees off a smooth low index crystal plane. They consist of terraces of the low index plane several atom rows wide, separated by generally monatomic steps [ 571. Field ion microscopy and the physics of crystal growth have already dealt for a long time with stepped surfaces. It was only in the 1970’s that vicinal planes were introduced as models for well characterized defective, catalytically active surfaces. The true starting point of this development were the beam experiments by Somorjai [58,59]. For the first time one could see that the catalytic rate of dissociation for hydrogen became strongly dependent on the angle of incidence relative to the steps. But what particular process would yield the “promoting” action at the steps? The answer was found later during a detailed examination of the adsorption/desorption dynamics of Hz on Ni ( 111) vicinal planes such as the (997) plane [ 32,46,56]: steps introduce sites for non-activated adsorption via a precursor on a surface which normally only allows activated adsorption. The dynamics of hydrogen adsorption for a beam aimed “step up” and “step down” is completely different [46]. “Step up” the beam encounters mostly step sites and a sticking coefficient falling with beam energy is observed, whereas a beam aimed “step down” will mainly hit the terraces and exhibit most properties of the original ( 111) plane, e.g. activated adsorption. Again the beam energy will determine which adsorption/desorption path is taken. Molecules with low energy (< 0.1 eV) will predominantly take the precursor path via

K.D. Rend&

H, - Ni(997)

200

A. Winkler /Adsorption and desorption dynamics seen through MBT

- 50~ ”

400

+ 50

800

600

Gas Temperature

r

(K)

Fig. 9. Surface steps as active sites for H2 dissociation. Adsorption dynamics for a Knudsen beam of varying temperature is different on the step sites of the Ni(997) surface and on the ( 11 I ) terraces. With the beam aimed “step-up” (8 = -50° ) mostly step sites are encountered and adsorption proceeds via a precursor (aS/i3T < 0). A beam directed “step-down” (C3 = + 50” ) hits mostly the ( 111) terraces exhibiting activated adsorption (aS/aT > 0). Data from Ref. [ 461.

the steps whereas high energy molecules will predominantly adsorb on terraces. This drastic change in the adsorption dynamics with temperature and angle of incidence is clearly manifested in the measurements of S (T)e shown in Fig. 9. While all the evidence points to the paramount role of surface defects in adsorption dynamics, theoreticians have paid little attention to these developments. Aside from a few general ideas relating work function changes at defect sites to barrier heights, detailed work is still missing.

7. Internal energy contributions to the dynamics of adsorption and desorption This section deals with the simple but important question already touched by LennardJones [ I 1: what happens to the internal energy of a molecule during activated dissociative chemisorption? Can this energy perhaps be utilized to aid transgression of an activation barrier? Surface scientists owe many of their

concepts to the description of gas phase chemistry. Not only the elbow type two-dimensional potential energy surface has successfully been transposed to describe adsorption/desorption processes [60], but also the idea of vibrational contributions to the dynamics of a reaction was first explored by gas phase chemistry [ 6 11. Several well documented cases of vibrationally assisted gas phase reactions can be found in the literature [ 62 1. First evidence of an exchange of rotational and vibrational energy at surfaces came from molecular beam scattering data. The role of vibrational energy in adsorption/desorption dynamics was investigated much later. Again it was the famous adsorption system H2/Cu that started it all. Kubiak et al. [ 631, puzzled by the discrepancies between the Cardillo/Stickney molecular beam data [ 3 1] and the Comsa/David time-of-flight experiments [27], proceeded in a now famous paper to explore the influence of rotational and vibrational quantum states on the desorption dynamics of Hz/Cu. A copper permeation source at 850 K served as source for a continuous molecular beam of desorbing hydrogen. Laser spectroscopy on this beam was employed to probe the internal state distribution of the desorbing hydrogen molecules. Whereas the rotational energy levels of hydrogen were slightly underpopulated compared to a Maxwell-Boltzmann distribution defined by the surface temperature, the vibrational state Y = 1 was highly overpopulated by a factor of about 50 to 100. By way of detailed balancing this would mean that a hydrogen molecule impinging on a copper surface would have a vastly superior sticking coefjcient when in the first excited vibrational state. The corresponding molecular beam experiment for the case of hydrogen adsorption on copper would still have to wait another six years. How can one set up such an experiment? Obviously the influence of translational energy and of nozzle temperature have to be separated in the exploration of adsorption dynamics: one has to get away from the physics of Eq. (8). A rather nice method can be employed to change the translational energy even at a constant noz-

K.D. Rendulic, A. Winkler /Adsorption and desorption dynamics seen through MBT

zle temperature: to slow down the beam a second heavier gas, usually an inert gas, is mixed into the primary molecular beam. These seeded beams were developed quite early in the short history of nozzle beams [64,65]. The original applications had of course nothing to do with the vibrational state of the molecules; the aim was strictly to produce high energy beams through the acceleration of heavy molecules seeded into a beam of light molecules. At least in a limited fashion nozzle temperature Tnozzle(which determines the vibrational state of the molecule) and the translational energy E can be adjusted independently. Any time the inequality:

is fulfilled for the sticking coefficient, a contribution of internal energy is present. Molecular beam experiments explicitly probing the influence of vibrational energy were first performed on the system N2/Fe [66]. Even more complex molecules like CH4 [67,68] and CO2 [ 691 were shown to convert vibrational energy into translational energy during dissociative, activated adsorption. But again the adsorption of hydrogen on copper provided a model system for vibrationally assisted adsorption [23,37,70-731. Actually, the adsorption of H2/Cu had earlier already been singled out by theoreticians as a possible candidate for vibrational/translational energy conversion [ 741. This conversion can come about in two different ways: ( 1) A softening of the potential to which the vibrating atoms of the adsorbing molecule (e.g. hydrogen) are subjected will lead to a decreased vibrational energy at constant vibrational quantum number v. This energy can be converted into translational energy [ 36,75,76]. (2) If the adsorption path in the PES is curved, a mixing of vibrational energy and translational energy can occur. Here the vibrational quantum number has to change. One can understand this mechanism even in a classical picture. For example, a desorbing particle possessing translational energy will climb up the potential

213

wall in the curved section of the desorption path leading to a vibrational motion. The reverse process in adsorption, although more difftcult to visualize, is of course equally possible [77,78]. The translational energy gained from the conversion of vibrational energy can be used to overcome an existing activation barrier to dissociative chemisorption. The application of seeded beam techniques to the system H2/Cu was first performed by Hayden and Lamont [72] and demonstrated, at least qualitatively, the importance of vibrational energy in the dynamics of adsorption. In short sequence further investigations on this topic followed [70,71,73,79]. The most striking demonstration of the internal energy contribution to the dynamics of hydrogen adsorption and a clear verification of inequality Eq. (9) is obtained when the nozzle temperature is changed while the translational energy is kept constant [23,73,80], using the seeded beam

technique (Fig. 10). The explicit variation of the sticking coefficient with nozzle temperature can only be caused by a change in occupation for the individual vibrational states of the hydrogen molecules, each state exhibiting a different sticking probability. An evaluation of the activation energy from the Arrhenius plots leads to the conclusion [ 23 ] that for adsorption of hydrogen on Cu( 110) mainly the first vibrational level (activation energy 0.52 eV) and for adsorption of D2 the second vibrational level (activation energy 0.72 eV) results in the observed temperature dependence of the sticking coefficient. Transgression of the activation barrier for dissociative chemisorption is aided by the partial conversion of vibrational energy into translational energy, as a consequence less of the directly supplied translational energy is needed for the particle to get over the barrier. One can say, the effective barrier height to dissociation has been lowered [ 75 1. A lowering of the barrier will directly lead (via normal energy scaling) to a widening of the angular variation of the sticking coefficient S (e)~ with increased nozzle temperature and the increased occupancy of the excited vibrational states. This widening of S (e) has indeed been observed for all cases of vibrationally as-

K.D. Rend&c, A. Winkler /Adsorption and desorption dynamics seen through MBT

214

-2 -$

Hz at Ei;,,, = 0.2 eV 1 E,, = 0.60 eV

1

0 0.0

if 3 y

0.2

Kinetic

cn 7 ._ .z 2

A E,, = 0.84 eV 0.1 ’ , 6

Inverse

\ 7

Nozzle

8

Temp.

9

10

+ I1

(x lO@K-‘)

Fig. 10. An explicit manifestation of the vibrational contribution to the adsorption dynamics of hydrogen on copper. A change of nozzle temperature alone introduces a drastic change in the sticking probability, while the translational energy of the hydrogen molecules is kept constant by a seeding procedure. The activation energy observed in this experiment points to the predominant influence of the first vibrational state in the case of Hz adsorption; deuterium in contrast uses mainly the second vibrational level to facilitate barrier transgression. Data from Ref. [23].

sisted hydrogen adsorption: H2/Cu ( 111) [ 701, H2/Cu( 110) [23] and Hz/Fe(lOO) [73]. In this context it is interesting to note that normal energy scaling has been observed for the sticking coefficient S(E,8)v=0,~,~ in the individual vibrational states [ 731. Seeded hydrogen beam experiments will directly lead to the information S(E)T. It is not too difficult to transform this function into a plot of state resolved sticking coefficients S(E)V=s,1,2 as has been done [73] for H2/Cu(llO), HJCu(ll1) and HZ/Fe. As an example state resolved sticking coefficients for D2/Cu( 111) obtained by Rettner [79] are depicted in Fig. 11. What about an isotope effect in the sticking coefficients? Obviously we are dealing with the quantum nature of the Hz and D2 molecules exhibiting different vibrational levels. As expected,

0.4

0.6

Energy

0.8

1.0

(eV)

Fig. 11. State resolved sticking coefficients for the individual vibrational states in the system D2/Ni( Ill). To obtain the result shown above, a particular S-shaped dependence of the sticking coefficient on the kinetic energy has to be introduced in the deconvolution procedure. Data from Ref. 1791.

the sticking data on H2/Cu for S(E) T obtained in a seeded beam experiment clearly show an isotope effect between Hz and D2 [ 23 1. The sticking coefficient for H2 is larger than the one for D2 at identical translational energy E and constant Lozzle. This is a consequence of the smaller population of the predominantly utilized v = 2 level for D2 compared to the v = 1 level for HZ. In addition this also reflects the smaller tunnelling probability for D2. Interestingly, if a conventional beam experiment to obtain sticking coefficients in the form S(E ( T) ) is evaluated, only relatively small isotope effects are obtained. This is due to partial compensation through opposing behavior of adsorption parameters with translational energy and internal energy as shown by Brenig [81]. Isotope effects have also been observed for the adsorption systems Hz/Fe [73], CHd/Ni [67] and CHJW

[681. While the vibrational contributions leave a clear mark on the adsorption as well as on the desorption dynamics, the detection of rotational contributions has been confined mostly to laser spectroscopy of desorbing molecules. Almost in all cases investigated, rotationally excited molecules are underrepresented in the desorption flux, implying a retarding effect of

K.D. Rendulic, A. Winkler / Adsorption and desorption dynamics seen through h4BT

rotational energy on the process of adsorption [ 63,82,83]. Recently, though, indications appeared that in the adsorption system H2/Cu the high rotational states (j > 8) might actually contribute to lower the effective height of the activation barrier [ 841. It certainly seems to be a safe prediction that adsorption/desorption dynamics in the near future will be investigated along the line of state resolved quantities pointed out in this last section. Also the trend in the theoretical description of adsorption dynamics seems to point into the same direction.

8. Concluding remarks What we have seen is that the advancement in a field generally comes from concepts, not necessarily from the first time formulation of ideas, but from the incorporation of concepts in the general line of thought. Experimental developments per se have a smaller impact than usually assumed. An example are the van Willigen and the Cardillo/Stickney papers. These people were thinking in terms of differential adsorption quantities, that means dynamics, while the rest of the world was still pondering kinetics. In a quantitative fashion their results have been replaced by more accurate measurements; but despite many experimental shortcomings these papers have charted the course to follow. What will the future bring? Certainly the major steps will again happen unexpectedly. Nothing is more difficult to predict than the future especially in advance. Perhaps around the next corner we might meet some unexpected but exciting surprises.

Acknowledgements

The work on this project has been supported by grants from the Austrian “Fonds zur Forderung der wissenschaftlichen Forschung” and from the “Jubillumsfonds der Osterreichischen Nationalbank”.

275

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