Contains report on one research project.National Aeronautics and Space Administration (Grant NGL 22-009-091)Joint Services Electronics Programs (U. S. Army, U. S. Navy, and U. S. Air Force) under Contract DA 28-043-AMC-02536(E

Dabiri, A. E.

Stickney, Robert E.

DSpace@MIT

IV. PHYSICAL ELECTRONICS AND SURFACE PHYSICSAcademic and Research StaffProf. R. E. StickneyDr. T. J. LeeGraduate StudentsA. E. DabiriL. J. ForneyS. YamamotoA. EXPERIMENTAL INVESTIGATION OF THE VELOCITYDISTRIBUTION OF HYDROGEN MOLECULES DESORBEDFROM METAL SURFACES1. IntroductionAn essential step in many chemical reactions occurring at gas-solid interfaces isthe associative desorption reaction, A(ad) + B(ad) - AB(g), where two atoms adsorbedon the surface combine to form a molecule that desorbs to the gas phase. This reactionhas been studied in several laboratories1 by a variety of techniques because of its impor-tance to heterogeneous catalysis, high-temperature oxidation, chemical vapor deposition,and the degassing of metals. Existing data are not sufficiently detailed, however, toprovide rigorous tests of theoretical models of the kinetics and energetics of associativedesorption. We believe that more rigorous tests will be possible with the unique dataobtained by the experimental technique described herein.It has been generally assumed that the velocity distribution of molecules desorbedfrom a solid surface corresponds to that of molecules effusing through an orifice froman equilibrium gas phase at a temperature equal to that of the solid. This assumptionleads to the expectations: (i) that the spatial distribution of the desorbed molecules willbe directly proportional to cos 0, where 0 is the angle of inspection measured from thesurface normal; (ii)that the speed distribution at all values of 0 will be of the Maxwellianform with temperature equal to that of the solid. Recent results obtained independently23in two different laboratories show, however, that the spatial distribution of hydrogendesorbed from nickel deviates markedly from the cos 0 relation. More specifically,these data indicate that the fraction of the molecules desorbing in the general directionof the surface normal is much greater than that corresponding to the effusion of anequilibrium gas through an orifice. Van Willigen Z has reported similar data for the*This work was supported principally by the National Aeronautics and SpaceAdministration (Grant NGR 22-009-091, and in part by the Joint Services Elec-tronics Programs (U. S. Army, U. S. Navy, and U. S. Air Force) under ContractDA 28-043-AMC-02536(E).QPR No. 95(IV. PHYSICAL ELECTRONICS AND SURFACE PHYSICS)desorption of H2 from palladium and iron, and he interprets these data by means of thesimple model.We propose to measure the speed distribution, as well as the spatial distribution, ofhydrogen desorbed from different metals, since the form of the speed distribution isexpected to be more sensitive to the desorption mechanism than is the spatial distribu-tion. Also, a more complete description of the state of the desorbed gas is obtained bymeasuring both distributions, and this will enable us to perform a more stringent testof theoretical models. Although the proposed experiments are limited to H2 and D 2 , thegeneral technique could be modified so that it applies to any chemical species.2. Experimental ApparatusThe main components of the apparatus are shown schematically in Fig. IV-1. Thevacuum chamber consists in two crosses made from 15-cm ID stainless-steel pipe. Eachcross is evacuated by a mercury diffusion pump (Edwards 6M3A) equipped with twoApparatus for measuring the speeddesorbed from metals.distribution of hydrogenliquid-nitrogen cooled traps in series plus a thermoelectrically cooled baffle. Coppergaskets are employed so that the chamber may be baked during initial evacuation. At-10present, the ultimate pressure of the system is -8 X 10 Torr, and we expect that evenlower pressures will be attained in the future.In the actual experiment a thin metal membrane (Ni, Pd, or Fe) will be welded tothe end of the tube shown in Fig. IV-1, and the temperature of this membrane will becontrolled by electron-bombardment heating. By filling the tube with H 2 gas, H atomsQPR No. 95Fig. IV-1.(IV. PHYSICAL ELECTRONICS AND SURFACE PHYSICS)will adsorb in the metal and diffuse to the vacuum face of the membrane where they asso-ciate and desorb as molecules. This scheme for supplying hydrogen to the test surfacehas been used successfully by van Willigen, 2 and it has the advantage of minimizing thepressure of H 2 in the vacuum chamber. (If one wished to study gases that do not per-meate readily through metals, then the gas could be supplied to the surface by meansof a molecular beam3 rather than by permeation.)The system is designed to ensure that the pressure is sufficiently low that the veloc-ity distribution of the desorbed molecules will not be altered by gas-phase collisions.A fraction of the desorbed molecules will pass through a collimating aperture(0. 1 X . 0 cm) in the partition between the "beam chamber" and the "detector chamber"(see Fig. IV-l) and then be gated (modulated) by a slotted disc driven by a motor. (Globe#18A103 motor with Barden Bartemp bearing for vacuum service; the motor casing hasbeen provided with water cooling to prevent overheating during operation or bakeout.)These molecules travel 37 cm to a detector where a fraction of the molecules are ion-ized by electron impact. The ions are accelerated by electrostatic fields into an elec-tron multiplier (EMI 9603/2) to amplify the signal by ~105. As indicated in the blockdiagram of Fig. IV-2, the output signal from the multiplier is amplified further and thenSIGNAL FROM REFERENCE SIGNALSIGNAL FROMDETECTOR FROM PHCTO-MULTIPLIERFET PULSEPREAMPLIFIER AMPLIFIERAMPLIFIER SCHMITT(PAR CP-4) TRIGGERFig. IV-2. Detector electronics.WAVE FORM EDUCTOR(PAR TDH-9)OSCILLOSCOPE(556 TEKTRONIX)X-Y RECORDER(D.E.C. 800)fed to a multichannel signal-averaging instrument (waveform eductor, PAR TDH-9) whichenhances the signal-to-noise ratio. The waveform eductor is triggered by a referencesignal derived from the slotted disc that modulates the beam. (We shall return to thispoint.) The output signal from the waveform eductor is viewed on an oscilloscope andpermanent recordings are made by means of an X-Y recorder.QPR No. 95(IV. PHYSICAL ELECTRONICS AND SURFACE PHYSICS)3. Performance TestWe are now performing a preliminary experiment in which the membrane is simu-lated by a small slit (0. 05 0X. 5 cm) cut in the end of the tube shown in Fig. IV-i. Thisprovides a simple means for producing a molecular beam of H2 or D 2 having nearly thesame intensity and speed distribution as what we expect to have in the case of desorp-tion from a membrane. Our principal objectives are to determine whether the signalamplitude is sufficiently large relative to the noise and whether the signal waveform isnot seriously distorted by the detector and amplifier characteristics.If the gas pressure in the tube is sufficiently low that the mean-free path is at leastan order of magnitude greater than the slit width (0. 05 cm), then the molecules effusingthrough the slit will have a Maxwellian speed distribution which, when expressed in the.4time-of-flight coordinates appropriate for the present apparatus, isF -r /xpns t exp(-T 2 /t 2 ) - exp -d co c+ [erf -t  erf(/t) (1)where ns(t) is the density (molecules/cm3) of the molecular beam in the detector ionizerat time t measured from the instant at which the chopper (the slotted disc) allows mole-cules to be transmitted, F is the flux of molecules into the ionizer for the case of a DC(unmodulated) beam, A d is the cross-sectional area of the ionizer (4 cm ), tco is thetime that the chopper remains "open," and T is the characteristic flight timeT = L/a, (2)where L is the distance from chopper to ionizer (37 cm), anda = (2kT/m)1/2 (3)with k being Boltzmann's constant, T the gas temperature, and m the molecular mass.Since the number of ions produced in the ionizer is directly proportional to ns(t), thedetector output signal, S(t), isS(t) = An s(t), (4)where A n is the detector sensitivity. It is convenient to represent ns by an equivalentpressure, ps, defined by-17 (5)ps =nskTc = 3. 1X10 n (Torr), (5)QPR No. 95(IV. PHYSICAL ELECTRONICS AND SURFACE PHYSICS)where Tc is the temperature of the chamber walls (~300 0K). In this case, Eq. 4 becomesS(t) = A pP (t), (6)where A = An/kT c . We have experimentally determined A to be 45 A/Torr for H 2 witha 0. 2 mA electron current, a 50 V electron energy, and a voltage of 2500 V applied to theelectron multiplier.Based on the dimensions and pumping speed of the apparatus, we estimate that Psthe value of ps for the case of a DC (unmodulated) beam, is given byPs = 1. 17 X 10 -6Po (7)where po is the H 2 pressure in the molecular beam source (that is, the pressure up-stream of the slit). Similarly, we estimate the background pressure in the detec-tor chamber to be-5Pc = 3. 3 X 105 Po (8)Therefore,-2Ps/Pc= 3. 5 X 10 - 2  (9)and, since Ps(t) Ps, it follows that-2Ps(t)/pc < 3. 5 X 10 - 2  (10)That is, less than 3.5% of the ions produced in the ionizer are due to the modulated beam,so it is necessary to: (a) place a blocking capacitor after the electron multiplier toreject the DC component of the total signal, and (b) use a waveform eductor to averageout AC components that are due to pc, mechanical vibrations, and other sources of noise.In the present test the reference signal is obtained by placing a photomultiplier tubeat window A and a light source at window B (see Fig. IV-1). The beam of light passingthrough the slit and collimating aperture is modulated by the chopper in the same manneras the molecular beam, and the resulting output signal of the photomultiplier tube servesto trigger the waveform eductor periodically at the instant that a slot in the chopperrotates to the position where light is transmitted. (When the slit is replaced by a mem-brane, the reference signal will be obtained either by placing a magnetic transducer nextto the chopper, as has proved to be satisfactory in another apparatus, 5 or by using thethermal radiation from the heated membrane as the light source for stimulating the pres-ent photomultiplier tube.)QPR No. 95(IV. PHYSICAL ELECTRONICS AND SURFACE PHYSICS)The experimental curve shown in Fig. IV-3 was measured under the following condi--3 -5 -7tions: p 0  3. 5 X 10 Torr, T 300 0 K, t =5 X 10 sec, pc =. 2 X 10 Torr, andO CO -6the background pressure in the beam chamber = 9 X 10 Torr. According to Eq. 7,zD>-I" //0 0.2t, TIMEEXPERIMENTALCURVEFig. IV-3. Time-of-flight data for anH 2 molecular beam.0.4 0.6(10-3sec)-9P - 4 X 10 Torr. The dashed curve labelled "Maxwellian" was computed from Eq. 1sfor T = 300 0 K, and its amplitude has been normalized to fit the maximum of the experi--4mental curve. The agreement is good for t )> 2 X 10-4 sec, and we believe that theagreement for t < 2 x 10-4 sec will be improved when we install a preamplifier adjacentto the electron multiplier output to reduce distortion caused by cable capacitance. (TheFET preamplifier shown in Fig. IV-2 was not employed in the present test.)4. Proposed ProgramThe rate of permeation of hydrogen through a nickel membrane of thickness I (mm)and temperature T ('K) given by 63. 75 X 1017 (1/2 1/2) ep(6970/T) moleculesR l cm sec2cm sec(11)where pl and p 2 are the hydrogen pressures (Torr) on the two sides of the membrane.17 -2 -1Therefore, we would expect the permeation rate to be -1. 5 X 10 cm sec for thefollowing conditions: f = 0. 5 mm, pl = 760 Torr, p2 " 1 X 10-6 Torr, T = 1150'C.following conditions: I[ : 0. 5 mam, P1 760 Torr, P2 < 1 X 10 Torr, T : 150°C.QPR No. 95(IV. PHYSICAL ELECTRONICS AND SURFACE PHYSICS)Based on the dimensions of the present apparatus and an assumed membrane surfacearea of -~0. 1 cm , we estimate that for the above-mentioned conditions the density ofdesorbed hydrogen in the detector ionizer will correspond to an equivalent pressure, P-10 s'of ~3 X 10 Torr. Since this value of Ps is -1/13 the value associated with the pre-liminary experiment described in the preceding section, we are confident that the pro-posed measurements can be performed after slight improvements in the detectorelectronics.We propose to measure the speed distributions of H2 and D 2 desorbed from bothpolycrystalline and monocrystalline surfaces of one or more metals (for example, Ni,Fe, Pd). Spatial distribution measurements will be performed either by (a) modifyingthe apparatus so that the membrane surface may be tilted at different angles relative tothe center line of the detector, or (b) constructing a simple apparatus similar to thatused by van Willigan.2 We plan to determine the effects of surface impurities on thespeed and spatial distributions by intentionally contaminating the membrane with oxygenand/or carbon.2H +MH2 + M2H Fig. IV-4.Schematic representation of the gas-solidinteraction potentials for the case of acti-vated dissociative adsorption of H2 on thesurface of solid M. E is the activationenergy separating the molecular (H 2 + M)and atomic (2H+ M) adsorption states, andH2 D is the dissociative energy of H 2.As an illustration of how the experimental results may be used to test models ofassociative desorption, we consider briefly the model of activated adsorption suggestedby Lennard-Jones,7 in 1932. According to this model, the impinging molecules mustsurmount an energy barrier Ea if they are to dissociate and adsorb as atoms on themetal surface M (see Fig. IV-4.) Therefore, an impinging molecule having speed vand angle of incidence 0 must satisfy the condition1 m(v cos 0) E (12)2 aQPR No. 95(IV. PHYSICAL ELECTRONICS AND SURFACE PHYSICS)in order to reach the atomic adsorption state. Arguments based on the principle ofdetailed balancing lead to the conclusion that the condition represented by Eq. 12 alsoapplies to the desorbed molecules if local equilibrium exists in the adsorbate phase.(These arguments have been presented in detail by Batty and Stickney. ) That is, thespeed distribution function of the desorbed molecules would be zero for v < v and itwould have the Maxwellian form for v > v , where the critical speed, v , is equal to(2Ea/m)1/2 sec 0 according to Eq. 12. Although this model is extremely crude and2ignores the apparent evidence against activated adsorption, van Willigen has demon-strated that it provides a possible explanation of his experimental data on the spatialdistributions of H12 desorbed from different metals. It would be of interest to see if theresults of the proposed speed distribution measurements show a deficiency of slow mol-ecules as predicted by the model.A. E. Dabiri, R. E. StickneyReferences1. For a review, see G. Ehrlich, in Proc. 3rd International Congress on Catalysis(North Holland Publishing Co. , Amsterdam, 1965), pp. 113-145. Also see D. O.Hayward and B. M. W. Trapnell, Chemisorption (Butterworths, London, 1964),Chap. 4.2. W. van Willigen, Phys. Letters 28A, 80 (1968).3. R. L. Palmer, H. Saltsburg, and J. N. Smith, Jr., Bull. Am. Phys. Soc. 13, 1648(1968); and private communication.4. P.. B. Scott, Report No. 65-1, Fluid Dynamics Research Laboratory, M. I. T., Feb-ruary 1965.5. S. Yamamoto and R. E. Stickney, Quarterly Progress Report No. 89, Research Lab-oratory of Electronics, M. I. T. , April 15, 1968. pp. 49-56.6. R. Barrer, Diffusion In and Through Solids (Cambridge University Press, London,1951), p. 168.7. J. E. Lennard-Jones, Trans. Faraday Soc. 28, 333 (1932).8. J. C. Batty and R. E. Stickney, Technical Report 473, Research Laboratory of Elec-tronics, M.I. T., Cambridge, Mass., June 2, 1969.QPR No. 95

Physical Electronics and Surface Physics

http://dspace.mit.edu/bitstream/1721.1/56019/1/RLE_QPR_095_IV.pdf

Physical Electronics and Surface Physics

Abstract

Similar works

Full text

Available Versions

DSpace@MIT