The acceleration of a catalyzed reaction by hot electrons

Volume 11, numker 5

CHEhIlCAL

PHYSICS Li3l-ERS

IS November

1971

THE ACCELERATION OF A CATALYZED REACTION BY HOT ELEnRONS W. I’AIDIWGER and J. FIC;A.K BatteIie Geneva Rerc?amh Centre, 1227 Coroy~, Received

11 October

&WCW, Swit:er&m’

1971

Acceleration of a mtalyzed reaction due to hot ehctron flow throufi a catalyst wils predicted and cxperimenttiy proved for formic acid decomposition on gold. Measurements were carried out with current densitks of (! to 1.5 .h!cn2 and with temperatures of 20 to 250°C. During hot electron flow decomposition rates up to 100 times kger than the normal rates were obser,-eG when an effective power of 0.1 W/cm2 was applied, Surprisingly, a flow of Fermi electrons caused a slight decreese in decomposition rates.

1. Introduction The influence of static electrical fields and surface charges on catalysis has been investigated by a number of authors [ 1) . It has been reported that reaction rates could be incrcascd or decreased depending on

the sign of the &hsrge.To the best of ol;r knowledge, however, nothing has been reported about the influence of electron flol;v through
electrons produced by an appropriate tunnel device, then by a flow of electrons slightly (kn above the Fermi energy. A very clear effect was observed in both cases. A strong increase of decomposition rate-was observed during hot elec?ron flow, whereas Fermi erectron flow caused a slight rate decrease.

2. Principles of the electron fiow effect Lennard-Jones pointed out in I937 [3] that conduction electrons may transfer part of their energy to adsorbed particles by surface scattering. He argued that activation of chemisorbed compIexesby thermai vibration only is anunfavourable event because the probability of the simultaneous emission or adsorp tion of several quanta by the solid is small. This is so because the vibrational IeveIs ofchemisorbed species are widely spaced compared with those of the solid, so that many thermal quanta must be transferred simultaneously to the adsorbed species to change its rate of vibration. As an aLternative, L.ennard-Jones considered conduction eIectrc;ns to be a possible source of energy supply. Those eIectrons carry energies of several eV (Fermi energy) and they hit one surface atom with a frequency of IOrs per second. In this respect a flow of hot eIectrons is of special

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meqy bses by unlterirj e&s

Fig. 1. Band @ram

illustrating the hot electron effect.

interest becw they carry an excess energy above the F+rmi IeveSand a supplementary energy s~ctrum is available for energj transfer. A tentative model of the hot electron path is given in fig. 1. When a voltage P’1 is applied across a metal-insulator-metal device, the Fermi level is lowered on the positively biased electrode and electrons may tunncl.th~ough the oxide layer into the second electrode because they find empty states in the conducting band. These electrons will lose part of their excess energy by scattering events in the gold layer, but some of :hem will reach the surface with energy eV1 (fig. 1) above the Fermi level. It is evident that for the energy spectrum of hot -electrons arriving at the surface oh the second gold electrode the thicknesses of the insulating and second gold layers are critica! parameters. The remaining excess energy eV7. may be transferred to the chemisorbed complexes and activate them to higher vibrational states. The hot electron drops back into the Fermi level during this event. Phenomenologically we may describe the energy which may be transferred to the chemisorbnd eomplexes as l’ollows:

E=jerrV2 =jtotfi f, V2 , wherejen is the effectrve current density of the tunnei current, the electrons of which reach the surface and scatter inelastically, transferring their energy to one of the vibrational revels of the chemisorbed complexes. 7lre j,, is related to the total tunnel-current density 546

15 November 1971

jtot by two interaction probabilities, namely fi determining the number of electrons which reach the surface with excess energy, and f2 the fraction of fl which interacts with chemisorbed complexes. For purposes of measuring a sizeable effect in a relatively small reactor, and also to avoid poisoning as.much as possible, we switched five tunnel devices parallel .with each 1 cm* area 80 that 5 cm* of catalyzing gold sur. face was available. It is evident that with devices of such large area only part of the measured current density &, will pass as tunnei current, and that the remainder will be conducted as Fermi electrons along pin holes. It would be premature to attempt a profound. theoretical description of the acrivation and deactivation rates using well-estabiished theories on the tunnel effect because of the lack of knowledge of the kind of interaction mechanism. We therefore first of all attempted to fmd experimental proof whether such an interaction mechanism with electron flow would occur.’

3. The experiments The decomposition of formic acid is a particularly suitable reaction for such a proof. Many authors have studied this reaction, and it is now well established [4] that on gold surfaces the decomposition takes place almost exclusively through dehydrogenation, schematically following HCOOH + Hz + COz. The rate change due to eiectrcn flow is easy to follow, especially since the reaction relay be considered to be of the order of pseudo zero in a sufficient pressure interval. In order to measure the reaction rates with and without electron flow we selected the isothermic pressure-rise in a constant-volume static reactor in which tunnel .davices have been mounted. The tunnel device consists of an Ni substrate 1 mm thick covered with a gold la:ier 1~ thick (first electrode) folIowed by a thin alumina layer (thicknesses of 59 to 250 A have been investigated) and a gold layer 80-l OQA thick (second electrode). The second gold electrode is exposed to the gaseous phase and acts as catalyzing surface. To separate the hot e!ectron effect from other possible ones, the reaction rates with and without electrons must be compared in precisely equal experimental conditions. in order to avoid errors due to poisoning

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I?& 2. Apparatus for reaction rare measurements:

1. Reactor. 2. reservoir, 3. tunnel devices, 4. contact wires, 5. rubber membrane, 6, stop-cock separating the reactor and the reservoir, 7. metal block-thermostat, 8. asbestos insulation, 9. ground join!.

and ageing of gold surfaces, rate measurements were carried out in the following sequence: the pressure rise -wasrecorded as a function of time for five minutes without electron flow, five minutes &&and five minutes after switching off. The rates of reaction before and after efectron ffow should be equal iF there are no irreversible changes iq catalyst surface conditions, and in so far as the concentration of reaction Iproducts are far enough from equilibrium. Thi apparatus shown in fig. 2 consists of two tubes .which are of the same volume (about IO0 cm3) and, ~CIfar as possible, identical in geometric form. The’ first contains the nmnel devices and acts as reactor, the second serves as a reserYo:r in which the formic acid is kept under &e init& pressure for pressurereference throughovt tAe experiment. The two voi- ’ umes ge directly connected through a stop-cock (6 in fig. 2) which is bridged by the differential manometer

197’1

(silicon oil, density I .0X g/cm3). In the open position the stop-cock provides for evacuation of the ap paratus and filling with formic acid. When closed it separates tt;c reactor from ttic memoir, and the pressurp differences (dp) can be recorded OH the differential manometer when the-rwct!on in’the reactor is catalyzed. Since the two ~oIurnes are geometrically similar, side effects,s& as pressure varirrtions due to change of temperature and to decomposition of the acid on the waits are to z large extent compensated for . The apparatus was embedded in a metal block to keep the temperature constant within 51%. Two ~e~o~ouples (PI--PEW) were in~%teCU-rrtorrhc block between reactor and resewair, one tozontril:tempenture, the other to regulate the power sup@y to the thermostat. Formic acid (Merck, pro anaiysi, 98-100% HCOO!r) was defIy$rated ovtzr anhydrous C&I4 and purified by fqieatid upturn s~b~rn~t~on. For the rate measurements, the empty apparatus was preheated up to the desired reaction tem~rature. Ten j.4 (2.2 X ICY4 mole) of formic acid were thun introduced through the rubber membrane (5 in fig_ 2) by mezI?s of a microsyringe. After about five minutes for equilibration of temperature and pre’ssure, the initial pressure (about 20 torr) was checked on the mercury manometer. To start the rate measurements, the stopcock was closed and the pressure differences were recorded on the differential manometer. Most experiments were carried out with tunnel devices as cataiyzing elements because the experiments showed a positive catalysis effect. Since tunnel current de~~d~rna~~y on the height, thickness and shape of the insulating layer, we carried out experime’nts on devices with an oxide thickness ofd = SO, 100,200 and 250 A. This was done mainly to ensure the qua& ty of the oxide layer rather than for f~damenta~ information on the effect. The foliowing current densities and rcmpentures

were investigated,

together

with

all their combinations: j=O.8,

1.0, 1.25, 1.5 A/cm2 ;

t = 20,180,200,

250°c .

For the experiments with Fermi eIectrons, gold layers 80-100 &thick were deposited on top of the Ni substrate, and electron Row across the sample was 547

VaIume i 1, number 5

g;.ometrkally

4. SsuIts

CHEMICALPHYSKSLE'MYERS

similar to that in the tunnel device.

and inierpretations

The essmtiai information obtained from ‘hese experiments may be summarized as fdilows: * (a) The pressure &ff~r~nce!time plot of the experiments with tunnel de*tices showed_a considerable increase of decomposition rate durkng cutrent flow (see fig. 3) indicating the typical change d slope found in all experiments. If we express the rate acc&ration by the ratio of the slopes rZ/rl (see fig. 3) and if we take the proper tunnel currentj,, as a basis, rate increases as high as it20 times per @.I W/cm2

have been observed. (b) As fig. 3 indicates, there was a first tail after the electrons were turned on, which lasted tmtil the

'te!q.=25o"C 90

j=O.8A/,,

15Nowmber1971

accelerated rate became linear. A similar tail, but different in z&a?, occurred after cutting off the efectrons until the original slope ri = rI was obtained. A precise evaluationof these tails shows that the first behaves almost exponentially in time, whereas the second follows a t”’ law within the limits of experimental errors. We attribu:e both tails to a rearrangement of a new steady-state concentration of population of all species involved in the surface reaction on the gold surface. We cannot far the time being expkiin why this arrangement follows an exponential law for the first Llil. For the second tail we suppose that the rate of sueface reaction falls immediately after cutting off the electron ff ow to the reIativeIy low initial rate. The reaction ,Froducts adsorbed on the gold surface are now in excess, compared with the steady-state ~n~ntratior‘, and they leave the surface by desorption. lhis situation is analogous to the diffusion from a limited planar source into the semi-in&& cylinder, and the solution of Fick’s equation, as known, is very weli approximated by the square-root law with time. (c) The experiments carried out with differem sets

d:2CO%

Fig. 3.Prezwre’difference Ap versus time plot shoiring the raK:increase du.r&ghot ~Iectron flow, 548

Fs.4.Ftatcs innu& due to electron f&w as.a functkn of the measured cturent den&g: d = thickness of the Al303 layer.

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Fig. 5. Reaction

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5

rate with and without

electron

AL PHYSlCS

lJ3’TERS

flow as a function of temperature. rate during hot electron flow.

of tunnel devices with varying oxide layer thicknesses are illustrated in fig. 4, where the individual rate increase re = r2 - r, , indicating the pure electronic effect, is plotted against the measured current density. Fromthese experiments we can deduce that rate acceleration changes systematically with the thickness d of the alumina layer, A steep increase of rate acceleration is obtained from d = 50 to 200 A at a given current density, wh.ich is followed by sudden decrease towards d= X0& TheoreticalIy, the hot electron current density should decrease as the thickness of the barrier increases. We attribute the above anomalous behaviour to a quality effect of the alumina layer. The quality of the oxide layer Is poor when it is very thin, and the majority of the current will pass with Fermi electrons. The quality may improve with thickness, and more hot electrons arrive at the gold surface. The drop in rate acceleration from d = 200 A to d = 250 A corresponds to that of the expected drop in the tunnel current. The measured total current is thus again cartied mainly by Fermi electrons. The curves in fig. 4 follow an exponential law. This Iaw can be described qualitatively by assuming that,

IS

Curve (a) rate without

electron

Novcm&r 19?1

ffow-. Curve (b)

owing to the energy transfer by hot eiectrons, the activation energy E. of the decomposition reaction decreases Linearly with the electron current, so that the actual activation energy becomes Ea,&) = E,, crjtot,where Q is a factor andim, the hot electron current density. The total rate is then: ‘tot = A exp(-EolR

T) exp(ckj/R T) ,

where A is the Arrhenius constant. The rate due only to electronic interaction re =r2 - rl is proportional to r; =A exp(-Eo/RT)

(exp(cu/lRT)

- I) .

IThus, this would lead to an exponential relationship between the rate acce!eration and the current density. (d) A very interesting feature was observed during experiments with the tunnel devices at different temperatures, the results of which are shawn in fig. 5. Curve (a) shows the dependence on temperature of the decomposition rate without electron flow. and curve (b) the rate increase due to the electron flow effect only. Below 170°C normai catalysis gives unmeasurably low rates, whereas at 20°C rates due to

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rhc electron flow are fcmnd to be as hi& as rates from normal catalysis at. 29OV. (e) In order to investigate the effect of Fermi electron flow only, experiments were carried out on wmjfes whxe a thin g&d layer was deposited on a pilike\ substrate of I cm2 (thus -withbut an insulating hycr). These were mour.ted in the reactor h the same way as the tunnel diodes had been before, and

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(i) the rate increase should be more or less inde. pendent of the thicL,?essd of the insulator layer in the investig+ed range,,or should at least show another sequence with d than that i~ust~~ted in fig. 4; (ii) the transition events (first and second tail, fig. 3) should obey the stie general law, which is not

the case;

the same !;witching sequence was &lied. Contrary to our executions, ori turning an the e!ectrons we

found a 5tight but definite deceleration of thy decom@tion reaction, which was immediately restored after cutting offtSe current, as shown in fig. 6. This proves that a nonMtia1 interaction takes place, but probabiy a d.Xferent mechanism is responsible than with hot electrons, (f) Whereas the rate deceleration during the f, IW of Fermi electrons cannot be explained otherwise thzn by some ~teraction betwe.en the latter and the chemiso:bed complexes, more evidence is required to show that rate acceleration iiue to hot electrons is not ascribable tc c: trivial side-effect_ One of the most obvious side-effects is local heating of the tunnel devices by heat dissipation in the insulator layer during current flow. We have devoted a number of experiments to this trivial affect and mass4 other arguments against it. These are lhat if rate increme were caused by local heating of rhs insulator layer, then: &P

Fig. 6. PreFurc difference Ap versus time plot showing the rate decrease during Fermi electron flow through the gold layer.

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(iii) to obtain the measured rate increase by Xl’, the temperature in the tunnel device should rise to about 290°C (see fig. S), whe,reas a thermocouple soldered near the contact on the gold layer showed a temperature increase of 18°C when a current of j = 1S A/cm2 was passed;

(iv) a different finai pressure should be obtained wi@ and without current flow at saturation of the reaction (equilibrium), since the final pressure is a measure of the equilibrium constant. For this measurement a smaller quantity of only 2 fi HCOOH was used. Fig. 7 shows that both curves coincide almost at saturation, indicating that the equilibrium constants are nearly equal and only a minor temperature change may be admitted. Although it is reported;in the literature [S] that temperature may rise considerably in tunnel devices, the conditions for heat conduction are much more favourable in our case because our tllnnel devices are deposited on massive NI supports which are in close contact with relatively thick metal wires instead of on glass supports and interconnected with condensed thin metal strips as is usually the case. One may therefore conclude thzt,.!;&obserd . effect on rate acceleraEi6fi ii due to a proper mechanism caused by the electron flow through an insulating layer.

5. Conclusiqns

From the experiments carried out on the decomposition of formic acid, we can conclude that decomposition rates increase considerably owing to electron flow through a metal layer on top of a thin insulator layer, and that thejl decrease slightly owing to electron current flow through a metal layer on top of a metal support, the field being perpendicular to the catalyst in both &es. Experimental evidence showed that the rate increase was not due to a trivial local heating effect during electron flow. Since the opposite behaviour was found in the case of Fermi electron flow, we hzve-to attribute this effect to a different mechanism. Whereas Fe@ electron flow may cause a proper scattering mechanism at or near the surface

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when the latter is covered with chemisorbcd species and thus cause the rate decrease, thcmte increases may be caused by enera transfer fram:the hot eIcctrons to the chemimrbed species. AIihcqh we hsve no real proof of the existenti of hot dk~tm~s in our experiments and other potential explanationsbased on the insulator-metal struciure may &suQes:ed, we find for the time being that the hot ekxtron effect is a reasonable description of the observed phenomenon. To achieve greater certainty about the mechanism which allows such high rate acce!crations more experiments must be carried out. The method described was applied only to formic acid decomposition. Application to other more complex reactions may reveal interesting aspects, espe&ily in selectivity, because activation cross actions will depend on the kind of chemisorbed particles. The relatively high catalytic activity at room temperature may aiso be interesting for the catalysis of moIecuIes which decompose at elevated temperatures.

Acknowledgements

c

The authors wish to thank BatteIIe Deve!opmeqt Corporation for sponsoring this work; Messrs. M. Boon and E. Bergmann of the Wteite Advanced Studies Centre for very helpful discussio!ls; and D. Ioyeux and M. Pongratz for valuable help with experiments.

References Ill P.M.Stadnikand V.P.Fentsik, Kinetikn i KataLiz 2 (1961) 252. 121J.Block and H.Kral, 2. Physik. Chcm. 24 (1960) 66; R.Suhrmann. Advan. Catalysis 8 (1955) 303: G.Wedler and P.Wisrmmn, Surface Sci. 26 (1971) 389, and E.T.Goodwin, Proc. Roy. Sot. (3; J.E.Lennard-Jones Al63 (1937) 101. L.L.van Reijen and W.hf.H.Sachtler. Proc. 141 J.Fahrcnfort, Symp. on tee mechanism of heterogeneous catalysis, ed. J.H.de Boer (Elsevier. Amsterbm, 1960); P.Mass, J.J.F.SchoIten and P.Zwietering, Advan. Caiafysis 14 (1963) 35. 151 T.W.Hickmott, J. AppL Phys. 36 (196.5) 1885.