Dynamics of the dissociative adsorption of CO2 on Ni(100)

Surface Science 167 (1986) 451-473 North-Holland, Amsterdam

451

D Y N A M I C S O F T H E D I S S O C I A T I V E A D S O R P T I O N O F CO2 O N Ni(100) M P D ' E V E L Y N *, A V H A M Z A , G E. G D O W S K I ** and R.J M A D 1 X Department of Chemtcal Engineering, Stanford Umverstty, Stanford, Cahforma 94305, USA

Received 12 June 1985, accepted for pubhcahon 1 November 1985

The dynanucs of the dlssoclatwe chemlsorptlon of CO2 on clean Nffl00) has been investigated using supersomc molecular beam and ultrahigh-vacuum techniques The &ssocmtwe sticking probablhty, s o, to form adsorbed CO and O. was found to be enhanced by three orders of magmtude by the addmon of translational and vibrational energy to the incident CO2 molecules SO increased from 4× 10 4 to 0 15 as the component of translational energy normal to the surface was increased from 8 to 100 kJ/tool Translational actwat.on was also seen at a nozzle temperature of 1000 K, but so was greater by a factor of two to ten at each translauonal energy, indicating the part~c~pahon of v.brat~onal excitation m actlvatton of the molecule The magmtude of the enhancement indicates that population of excited bending mode levels slgmficantly increase the dlssoclatwe adsorpUon probabdlty

1. Introduction Th e subject of activated, d~ssoclatlve c h e m l s o r p U o n is of c o n s i d e r a b l e i m p o r t a n c e , f r o m b o t h a f u n d a m e n t a l and a practical p o i n t of view Th er e is c u r r en t l y a great deal o f interest m the d y n a m i c a l state of molecules scattered or d e s o r b e d f r o m surfaces T h r o u g h such stu&es o n e h o p es to gain insight into the nature of the g a s - s u r f a c e m t e r a c u o n p o t e n t m l an d into various energy e x c h a n g e processes T h e dynarmcs of activated a d s o r p t i o n ~s related by detailed b a l a n c e to the translational and internal state p o p u l a t i o n s o f molecules which sample a repulsive p o t e n ti a l following f o r m a t i o n by a surface reaction T h e r e f o r m a t i o n derivable ~s thus c o m p l e m e n t a r y to that o b t a i n a b l e f r o m state-resolved surface reaction e x p e r i m e n ts F r o m a m o r e a p p h e d p o i n t of wew, & s s o c m t w e a d s o r p t i o n reactions constitute the m m a l step of a variety of technologically ~mportant h e t e r o g e n e o u s catalytic reactions In the case of a m m o n m synthesis over ~ron catalysts, for example, the rate-limiting step ~s the & s s o c m u v e a d s o r p t i o n of N 2 [1] * Present address Department of Chemistry, BG-10, Umversny of Washington Seattle, Waslungton 98195, USA ** Present address San&a National Laboratory, DIvlslon 8343 Box 969 Llvermore, Cahforma 94550, USA 0 0 3 9 - 6 0 2 8 / 8 6 / $ 0 3 50 © Elsevier Science Pubhshers B V ( N o r t h - H o l l a n d Physics P u b h s h l n g Division)

452

M P D'E~,ehn et al / Dlsso~tatt~,e adsorptton oJ CO, on Nl(lO0~

Despite their ~mportance, only a hm~ted number of activated adsorption systems have been investigated under well-characterized cond~tions In an early molecular beam study, Balooch et al [2] found that the dissociative sticking probabdity. So. for hydrogen on several single-crystal faces of Cu increased by roughly a factor of five as the mean translational energy. E. was increased from 7 to 45 kJ m o l - f S 0 was found to depend only on the component of m o m e n t u m normal to the surface, that is. so( Zv, 0, ) ~ so( E cos20, ), where 0, is the angle of incidence This behawor is consistent with a quasi-one-dimensional barrier to dissociation, as originally proposed by Lennard-Jones [3] and discussed and modified by others [2.4.5] More recently. Lee. et al [6] and Auerbach. et al [7] have investigated the activated adsorpuon of N. on W(ll0), and Rettner et al [8] have reported studies of C H 4 o n W ( l l 0 ) In each case s o rose by several orders of magnitude as the translational energy of the incident molecules was increased from 8 to 100 k J / m o l The dynamical details of the process were rather different, however In particular, the dependence of s o on the angle of incidence for CHn scaled with normal kinetic energy [8]. just as m the H 2 / C u [2] case, while the dissociation probabdlty for N~ was relatively insensitive to incident angle [7] No evidence for the participation of internal modes of the incident molecules in dissociative adsorption was seen w~th N~ [91, whde for C H 4 i n c r e a s e s in the source temperature led to enhancements in so at low translational energies [8] The interaction of CO~ with nickel surfaces xs of considerable technological importance, as dissociation of CO~ occurs m the hydrogenation of CO 2 to methane CO._ + 4 H . --* C H 4 + 2H20,

(1)

over nickel catalysts [10-12] and m the water gas shift reaction [13.14]. C O 4- H 2 0 ~ C O 2 + H 2

(2)

Reaction (2) typically reaches e q m h b n u m under conditions appropriate to commercial steam reforming of hydrocarbons [13] Dissociative adsorptxon of CO 2, to form CO(a)4- O(a), has been reported on N l ( l l 0 ) [151, NI(100) [16], and on a supported N1/SIO, catalyst [10] As further evidence for dissociative adsorption of CO 2 under catalytic conditions, gas-phase CO ~s evolved during steady-state CO 2 hydrogenation over supported or single-crystal NI catalysts [ 1 1 , 1 2 , 1 7 , 1 8 ] CO2 dissociation on mckel also displays a significant structural sensmwty On the (100) surface s0 has been reported as ~ 0 007 [16] while on the more open (110) face s 0 ~ 0 14 [15] This behavior is quahtatlvely similar to that observed m the dxssoclation of N 2 on different crystal faces of W [6,19,20] Finally, the dxssoclative adsorption of CO. on a N~/S10~ catalyst has been reported to be acuvated Falconer and Za~h [10] found the amount of CO~ dlssoclatwely adsorbed to increase by a factor of 60 as the temperature of the gas/catalyst system was increased from 300 to 440 K

M P D'Et,elyn et al / Dtssoclatwe adsorptwn of CO: on Nt(lO0)

453

We have carried out a molecular beam investigation of the dissocmtwe adsorption of CO 2 on a N1(100) surface, in order to study the d~ssoclat~on dynarmcs in more detail and to determine the effects of the addition of energy separately to the gas molecules or to the surface, respectively By making use of the energy &stributlon m a molecular beam generated by a supersomc expansion [21], we have measured s o as a function of the translational and internal energy of the incident CO 2 molecules We have also determined the dependence of s o on the angle of incidence and on the surface temperature

2. Experimental The experiments were carried out in the apparatus illustrated schematically in fig 1 The stainless-steel ultrahigh-vacuum (UHV) scattering chamber is pumped by a 1500 g' s -] turbomolecular pump, a 220 t s -1 ion pump, and a titanium sublimation pump The base pressure is < 3 × 10-11 Torr, although most of the experiments described here were carried out with a background pressure of ( 1 - 3 ) × 1 0 -l° Torr The U H V chamber is eqmpped with L E E D / A u g e r electron optics, a sputter 1on gun for crystal cleaning, and two quadrupole mass spectrometers, one stationary and the other rotatable about the center of the chamber The distance between the crystal sample and the rotatable detector is 11 9 cm Gas exposures are made with either the molecular beam or a glass capdlary array doser connected to a variable leak valve The supersonic molecular beam is formed from a nozzle source located 101 7 cm from the crystal sample The beam passes sequentially through a skimmer and two dIfferentmlly pumped chambers separated from each other and from the mare chamber by colhmatmg orifices The orifices dmmeters are chosen so as to y~eld a beam spot at the crystal of 1 00 cm dmmeter (the crystal

S upersonlc Nozzle Skimmer Assembly / ~ Ill Chamber

Jill Chamber

LEED Optics and Retarding Potenha/ Analyzer r~7 Isolahon Valve ' \

I ~• / ] I

I / ~ "~ Samole

'

.W

IrP--~--~ \'~/'"7 ~ rj / ~ /

Mass Spectrometer

Seatter,, " Chamber

/~ I~-Statlonary ~!~q MassSpectrometer

Fig 1 Schematic dlustratlon of the reacUve scattering apparatus The source chamber, modulation chamber, lsolahon valve, and scattering chamber are separately differentially pumped

454

M P D'Evel) n e t al / Dt~soctattve adsorptton o] CO. on Nt(lO0)

diameter) The source chamber is pumped by an unbaffled 1500 ( s -1 diffusion pump, the modulanon chamber by a water-baffled 1500 g s -~ dlffusxon pump, and the bakeable all-metal isolation valve by a hquid-mtrogen-trapped 285 ( s ~ diffusion pump The base pressure of the source chamber is 1 × 10 -6 Torr, and that of the latter two chambers ~s m the low-to-mad 10 8 Tort range During beam operanon the pressure in the source chamber typically rises to (1-9) x 10 4 Torr The 100%-duty-cycle CO 2 fluxes at the crystal ranged from 1 x 10 ~3 to 2 × 10 ~4 molecules cm 2 s ~ in this study Several different nozzles were used m these experiments Most of the data reported below for a nozzle temperature of 300 K were taken using a stainless-steel somc nozzle w~th a throat dmmeter, dN, of 0 019 mm. The remainder of the data were obtained using a heatable nozzle source The nozzle ovens consist of stainless-steel tubes, sealed at one end, w~th a small pinhole orifice (40-80 /~m in diameter) laser-drilled [22] m the side The nozzle tubes are held at each end by a water-cooled copper block and are heated reslsnvely The nozzle temperature is measured by a W / 5 % R e - W / 2 6 % R e thermocouple spot-welded to the s~de of the tube The nozzle mount ~s attached to a homebullt x - y - z translation stage feedthrough wnh shdmg O-ring seals, to allow for adjustment of the nozzle position while under vacuum Modulation of the beam is accomphshed by a rotating slotted disk, located 75 cm from the crystal Several different chopper disks, w~th duty cycles of 1, 8, and 50%, were used dunng the course of these experiments A lamp-photodiode system ~s used to provide a reference signal for the detecnon electromcs Also attached to the chopper mount is an electronically operated shutter for precise control of dose times The Nl(100) crystal sample was mounted on the rotation axis of a precision x - y - z manipulator The temperature was measured by a chromel-alumel thermocouple spot-welded to the back of the crystal The electncally-~solated crystal could be cooled to 90 K by thermal contact with a llquld-mtrogen-cooled copper block Heating was achieved by radmtlon from a tungsten filament located behind the crystal The principal surface contaminant encountered m this work was carbon Bulk diffusxon and surface segregation of carbon m nickel are well known [23,24] During the initial stages of the experiments carbon was removed by high-temperature treatment in oxygen [25] The resultant oxide was then reduced by treatment in hydrogen [25] or by prolonged Ar ~ sputtering This procedure depleted the carbon content m the surface region sufficiently so that a clean surface could be obtained before each experiment by the following (mdder) approach. The crystal was dosed with -- 0 1 L (1 L = 10 6 Torr s) of oxygen The temperature was ramped to 800 K~ and the partial pressure of CO momtored If CO desorpnon was detected between 600 and 800 K, the procedure was repeated Any oxygen left on the surface was removed by Ar +

M P D'Evelyn et al / Dtssoclatwe adsorption of CO 2 on Nt(lO0)

455

sputtering The crystal was then annealed at 900 K for 30 mln The carbon coverage resulting from this treatment vaned from undetectable to just above the hmlt of detectablhty (0~ < 1.5% ML; 1 ML = 1.61 x 1015 c m - 2 ) . The peak temperature, Tp, of the saturation CO temperature-programmed desorptlon (TPD) spectrum was found to depend strongly on 0 c We found empmcally that Tp >/440 K (heating rate of 7 K s-1) corresponded to a surface with 0~ < 1 5% M L The translational energy of the CO 2 beam was vaned by seeding into He or H 2 and changing the CO 2 mole fraction a n d / o r the temperature of the nozzle. The vibrational and rotational energy could also be adjusted, albe~t in a cruder manner, by adjusting the nozzle temperature, nozzle dmmeter, and stagnation pressure Hydrogen adsorbs readily on NI(100), desorbmg with T P D peak temperatures between 350 and 425 K at low coverage [26,27]. We found that the presence of H(a) decreased the sticking probablhty for CO2 somewhat. The hydrogen-seeded beam data reported below were taken at a surface temperature, T~, of 407 K, where the hydrogen coverage rapidly reaches an adsorption-desorptlon equilibrium In order to estimate OH, we make use of measurements of the hydrogen sticking probability, sH(On), made in our laboratory [28] and of the desorptlon rate constant reported by Christmann et al [26] For a pure H 2 beam with TN = 300 K and OH < 0 3, we find [28] SH2(OH) = S0,~/2(1 -- 2 0 . ) 2,

(3)

with s o H2--0 4, in good agreement with earlier measurements [26,29] Our estimate of 0 n dunng CO 2 adsorption experiments at T~ = 407 K with hydrogen seeded beams ~s then O. = (4a 2 +

a ) 1/2 - 2 a =

0 04 ML,

a -~

2s0 r~QH~

(4)

kd n2 -- 8So,n2Qi % '

where k d = 8 x 10 -2 e x p ( - 9 6 kJ m o l - 1 / R T ~ ) cm 2 s -1 [26], n o = 1 61 × 101S cm -2 and QH._ = 2 x 1014 c m - 2 s - 1 The use of hydrogen seeding at elevated nozzle temperature was precluded by the production of CO + H 2 0 by the water gas shift reaction (eq (2)) within the nozzle, since we quantify CO2 dissociation on the surface by the appearance of CO(a). During each expenment the molecular beam was charactenzed w~th respect to flux and to average molecular velocity, b. In order to measure the velocity of the molecules, the rotatable mass spectrometer was positioned in the beam path at the inlet of the scattenng chamber The beam was modulated at frequencies of 200-600 Hz, and the chopper-to-detector transit ume measured by &ginzmg the waveform on a multichannel analyzer (MCA) or by measurement of the phase shaft with respect to the reference signal by a lock-m amplifier The procedure was repeated with the detector at the back of the chamber (i.e. rotated by 180 °) The average time-of-flight, g, was than taken as the difference between the two measurements, and extraneous detection t~mes,

456

M P D'E~,eh'n et al / Dtssocmttee adsorptton of CO, on Nl(lO0)

including the 1on transit time and electromc delays, cancelled out The precision of the measurement was hmlted by the resolution of the MCA (20/*s per channel) or by the phase angle measurement ( + 0 0 5 °) with the lock-in a m p l i f i e r T h e t y p i c a l u n c e r t a i n t y in i w a s ~ 5 )< 10 - 6 s, so t h a t t h e u n c e r t a i n t y in the average energy was ~ 3% for an 8 k J / m o l beam and 10% for a

100 k J / m o l beam The spread m velocities was not measured, but spreads of 5-10% (FWHM) are typical of supersonic expansions under the stagnation condmons employed here [21] The flux of the molecular beam was determined by making use of the mass balance for N~(t). the number of molecules of species c~ m the scattering chamber N ~ ( t ) = N~, ~ + Q ~ ( t ) A

- (S/V)N~(t).

(5)

where /Vu ~ is the outgassmg rate. Q ~ ( t ) is the time-dependent incident beam flux, A the beam area, V the chamber volume and S the pumping speed The molecular beam, modulated at variable (angular) frequency ~%, was allowed to enter the scattering chamber with the crystal removed from ~ts path The Fourier amphtude of the parual pressure fluctuations at the modulation frequency were measured as a functmn of ¢00 w~th the stationary mass spectrometer interfaced to a lock-in amphfier Fourier transforming eq (5) and rearranging, we rea&ly obtain ]p.(~0,,)]

2(x(Q0.~])-e[¢00 + ( S / V ) 2 ] ,

(6)

where Fourier transformed quantmes are denoted by tildes, 0~(0:0) = Q~ ~g(°:0)= Q0 ~g, and Qo ~ is the beam flux m the absence of the chopper For square-wave modulation, g = 2 / v For the smaller duty cycle choppers, the gating function g(t) was measured experimentally and ~ calculated by numerical Fourier transformation A sample "flux plot" for a beam of CO2 molecules IS shown m fig 2 The slope of the hne is inversely propomonal to the square of the unchopped flux The propomonally constant was determined by performing this measurement w~th a CO beam whose flux was measured independently using the coverage cahbratlon described below The mass spectrometer sensmwties for CO and CO: were separately calibrated against the ~on gauge, incorporating tabulated ion gauge correction factors Ion gauge sensmvmes relative to N~ for CO and CO2 were taken as 1 05 and 14, respectively The mass spectrometer sensmvlty for CO2 was cahbrated against the ion gauge at the end of each experiment m order to account for changes from day to day During a typmal experiment ] p(o: 0)1 was measured at 4 6 different modulation frequencies The standard error m the slope of the flux plot was typically 5-10% A fracUon of the CO 2 molecules incident on the NI(100) surface was observed to dissocmte into adsorbed CO and O The surface coverage of CO. 0co, was determined by TPD with a heating rate of 7 K s ~ Surface oxygen

M P D'Et,elvn et al / Dis wctatlve adsorptton of CO_, on Nt(lO0)

457

!/ %, 06

/ /o ooo ~'°1 i

~6 o _2

~

o

i

i

J

i

i

i

J

i

5 I0 ~o2 (10 5 s "2)

~

i

i

15

F~g 2 Sample of data for determination of the flux of a CO 2 molet.ular beam Representative error bars (+_20) are included The slope of the hne is reversely proportional to the square of the flu,~ (eq (6))

remained on the surface after the flash, as observed by Auger electron spectroscopy The absolute CO coverage was derived by dwldmg the time-integrated T P D signal by that obtained following a CO exposure sufficient to saturate the surface at 300 K and rnultlplymg by the known CO saturation coverage, 0co,,at, of 0 55 ML [30] Surface CO coverages as small as 0 001 ML could be deterrmned by this method The saturation CO desorption measurement was performed at the beginning of each experiment to account for any changes m the system pumping speed from day to day and also to verify that the surface was clean, as dBcussed above Spot checks ln&cated that the pumping speed did not change by more than a few percent during the course of a beam experiment We measured correctzons due to background CO adsorption by perforrmng blank CO 2 beam exposures with the crystal removed from the beam path Except m cases where T~ < 300 K or when the dlssoclatwe stickang probabdlty was less than 1%, these corrections were neghgtble No evidence was seen for reaction of CO 2 wnh the small ( < 1 5% ML) carbon impurity (reverse Boudouard reaction). The proportlonahty constant relating the time-integrated T P D signal, A p,~ TeD(t), to the surface coverage is equal to that relating the partml pressure rise, Ap~bcam, tO the flux of an unmodulated molecular beam entering the scattering chamber, multlphed by the ratio of the area of the beam at the sample to the area of the sample When the two areas are equal, then for CO (or for any molecule with a known saturation coverage), the beam flux is given by Q,~ =

A Pa beam

fApa

TPD

0~ ,at,

~atdt

independent of any other cahbrat~on factors

(7)

458

M P D'Evelvn et al / Dlssoctatwe adsorption of CO: on Nl(lO0)

3. Results

W h e n the NI(100) surface at T~ < 200 K was e x p o s e d to a CO 2 b e a m with an average translational energy of 7 8 kJ m o l - ~ , b o t h m o l e c u l a r a n d dissociative a d s o r p t i o n was observed A T P D s p e c t r u m o b t a i n e d following an exp o s u r e of 0 7 M L of C O 2 at T, = 85 K is shown in fig 3 Two m o l e c u l a r C O 2 d e s o r p t i o n states ( m / e = 44) are seen, in the low-coverage h m l t the p e a k t e m p e r a t u r e s are 110 a n d 255 K, respectively A t T~ = 85 K the 255 K state fills first for low C O 2 exposures The s a t u r a t t o n coverage of this state is only 0 011 M L , i n d l c a t m g that ~t is associated with m i n o r i t y defect sites The 110 K state is associated with C O 2 molecules a d s o r b e d on m a j o r i t y (100) sites Similar p e a k t e m p e r a t u r e s have been seen for m o l e c u l a r CO~ d e s o r b i n g from P t ( l l l ) [13] a n d from oxygen-covered Ag(110) [32] A fraction of the CO 2 dissociated, the CO(a) thus f o r m e d d e s o r b e d near 470 K (fig 3) T h e decrease in the initial dissociative sticking p r o b a b i l i t y , s 0, from 1 7 × 10 -3 to 4 5 × 10 - 4 between surface t e m p e r a t u r e s of 200 a n d 400 K is shown in fig 4 Both the small value of s o a n d its decrease with T~ have been r e p o r t e d earlier [16] However, we o b t a i n a s o m e w h a t smaller a b s o l u t e value for s o than did Benzager a n d M a d i x (0 007) [16] at T, = 150 K The decrease of s o with T, suggests that dxssociauon of C O 2 at low t r a n s l a t i o n a l energy takes p l a c e through a molecularly a d s o r b e d p r e c u r s o r [19,33] N o t e that s o refers to I

I

I

I

= 7 8 kd m o l q O,= O*

r

COz, CO / COz

E

o

~

m/e

r

T

%1

o g 44 t--,

I

I00

L

I

200

I

I 300

I

I 400

Ts (K)

I

I 500

....

o

25o

'

360

'

460

Vs (K)

Fig 3 Temperatureprogrammeddesorptlon(TPD) spectrumof CO2, and CO followingexposure toCO 2wIth E=78kJmol 1 at T~=85 K Fig 4 Surfacetemperaturedependenceof the d~ssocxatlvestickingprobabilityof CO2 molecules with f=78kJmol ~,mcldentat 0,=0 °

M P D'Evelyn et al / Dls~octattt,e ad6orptton of CO, on N#IO0) CO Thermal Desorpfion subsequent to sequential CO2 beam dosing I00 8co 014

459

Dose I 58

0.'.'50 105 ~n

o 50

0 68

062

045

c :D

~

~ ~

£

0

6

2

0 30 068

050

044- 015

o

097 '

4 ~50

i

015

i 500

T(K) Fig 5 Sequence of C O / C O 2 thermal desorpuon spectra following exposures of the crystal to a translanonally excited CO2 beam The (dlfferentml) CO coverage and CO 2 dose (m monolayers) are indicated beside each curve T, = 407 K during dose, if7 = 79 kJ tool 1 0, = 0 °

the ratio of the surface coverage of CO observed desorblng near 470 K to the CO 2 dose at a specified temperature For I", sufficiently low that molecular CO 2 ts stable on the surface, t e below -- 250 K, we are unable to dlstlngmsh between Immediate d~ssoctatlon during the dose at T, and dlssoctatton durmg the flash A classical precursor model turns out to be quantitatively inadequate to describe the results of fig 4 A klneUc model for dlssoclanon of CO 2 at low translanonal energy, involving competmve diffusion and desorptlon on terrace sites and dissociation at step/defect sites, will be presented elsewhere [34]. The d~ssoclatlve adsorption behavior of CO 2 incident with an average translational energy greater than 20-30 kJ tool-~ ts qualitatively different An example of the raw thermal desorption data is presented in fig 5 Shown ts a sequence of TPD curves obtained by sequential exposures of the surface at 407 K to a beam of CO 2 molecules with E = 79 kJ mol-~ Subsequent to the first exposure, the first desorption curve (fig 5, bottom) was obtained as T, was ramped to 507 K, desorbmg CO The crystal was cooled to 407 K and the experiment repeated Since adsorbed oxygen remains on the surface, the differential CO coverage dwIded by the differential CO2 dose is a first approx~matton to s(Oo) Several features are immediately apparent First, %, the zero-coverage hmlt of s(Oo), is about 007, more than two orders of

460

M P D'E~,eh'n et al / Dts ~octatlt, e advorptlon of ~ 02 on NI(IO0)

T

T

T

.

T T

o o3 o o

3 = 43

kd tool "l

e~= 0 °

26o

'

36o

'

46o

Ts (K) F~g 6 Surfa~.e temperature dependence of the ~,tlt.klng probablllt~ tor CO. at elevated tran~latmnal energy ( E = 43 kJ mol ~. 0 , = 0 °)

m a g n i t u d e larger than s~ at lower energy This increase d e m o n s t r a t e s the activated nature of the dissociation process Second. as oxygen builds up on the surface, ~(0o) dlmxnlshes sharply, b e c o m i n g smaller than ~ by m o r e than an o r d e r of m a g m t u d e as 0o rises to only a few percent of a m o n o l a y e r A n a d d t t i o n a l effect is that the p e a k t e m p e r a t u r e of the d e s o r p t t o n curve decreases, indicating a w e a k e n e d interaction between the surface a n d the a d sorbed CO molecules as oxygen builds up on the surface T h e r a p i d decrease in s with oxygen coverage will be discussed in detail in a s e p a r a t e pubhcatxon [35] We note only that the values o f s o r e p o r t e d here were o b t a i n e d as the initial slope of a least-squares fit to the c u m u l a t t o n C O coverage versus cumulative C O 2 exposure d a t a In this p a p e r we will be c o n c e r n e d with the dissociation p r o b a b i l i t y o f C O 2 on the clean surface and its d e p e n d e n c e on the d y n a m i c a l variables in the system the surface temperature, the angle of incidence, a n d the a m o u n t of energy m translational, vibrational, and r o t a t i o n a l degrees of freedom in the m c t d e n t molecules The surface t e m p e r a t u r e d e p e n d e n c e of % for a 43 kJ m o l - ~ CO2 b e a m ts shown in fig 6 A s i d e from a small decrease near 400 K, % is a p p r o x i m a t e l y c o n s t a n t over a range of 200 K, in contrast to the b e h a v i o r with the 7 8 kJ mol ~ b e a m (fig 4) This constancy of ~o indicates that at elevated CO2 translational energies, a second, dtrect channel b e c o m e s available for dissociation. w h i c h does not Involve a m o l e c u l a r p r e c u r s o r W e have d e t e r m i n e d % at 7~ = 407 K as a function of translational energy, at several angles of m c l d e n c e a n d at nozzle temperatures, Ty, of 300 a n d 1000

M P D'Et,elvn et al / Dtssoctatwe adsorpnon of CO, on Nt(lO0)

F l (kJ tool") 40 60 80

20 ,

,

,

,

,

i

r

I00

i

,

,

461

120 ,

i

i

,

oO .._-----

i0-I

O~

•

TN(K)

,/

-/ g

lO-Z J

/ "~

o

, i0-~

300

looo

•

o

0 °

•

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22

•

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45 °

~

zb

z'~

5°

O,

/ ,Oo

~

,b

3'o

EL= 1~ COS20, (kcol mot ~) Fig 7 Summary of the dependence of % at 7", = 407 K on translatmnal energy (E) incident angle (0,), and nozzle temperature (TN) Solid symbols supersonic beams u, lth TN =300 K Open symbols supersomc beams with TN = 1000 K Open symbols with horizontal bar quash-effusive beams with TN = 1000 K = T~,. Smooth curves are least-squares fits to the data by the empmcal function %(E.. ) = s* exp[- E*/( E~ + D)] Values of v*, E*, and D are 0 21, 83 kJ tool-I and 5 7 k J m o l ~,respectlvely, forthe TN = 3 0 0 K d a t a a n d 0 8 2 l l O k J m o l ~ a n d 1 6 4 k J m o l - I for the Tr~ = 1000 K data

K. T h e r e s u l t s a r e s u m m a r i z e d m fig 7 E r r o r b a r s h a v e b e e n o m i t t e d f o r c l a r i t y , a b r i e f d i s c u s s i o n o f t h e u n c e r t a i n t i e s m v o l v e d ts p r e s e n t e d b e l o w A t b o t h n o z z l e t e m p e r a t u r e s , s o m c r e a s e s b y m o r e t h a n a n o r d e r o f m a g n i t u d e as t h e t r a n s l a t i o n a l e n e r g y ~s r a i s e d f r o m 8 t o 100 k J t o o l - z A t T N = 300 K, so r t s e s s h a r p l y f r o m 4 × 10 - 4 to ( 1 - 4 ) × 10 -~- as E m c r e a s e s f r o m 8 to 3 0 - 4 0 k J m o l - 1 , t h e n c h m b s m o r e g r a d u a l l y t o 10 ~ a t h i g h e r E T h e b e h a v i o r f o r T N = 1000 K ts s t m t l a r , b u t % ts l a r g e r b y a f a c t o r o f t w o to t e n t h a n t h e c o r r e s p o n d i n g v a l u e f o r T N = 300 K at t h e s a m e t r a n s l a t i o n a l e n e r g y , t h e l a r g e s t r e l a t i v e i n c r e a s e o c c u r r i n g a t l o w E w h e r e s 0 is s m a l l T h e i n c r e a s e s m so with translational energy and, separately, nozzle temperature, indicate the partlclpat~on of both translational and internal energy m act~vatton of the incident CO. molecules The behavmr of so(E) at the two nozzle temperatures indicates a non-addmve enhancement of d~ssocmtlon by energy m the form of t r a n s l a t m n a l e n e r g y a n d i n t e r n a l e x c l t a t m n T h e d i f f e r e n c e , s 0 ( E , T N .= 1000 K ) - s o ( £ ' , T N = 300 K), o r t h e n e t f r a c u o n o f i n c i d e n t m o l e c u l e s t h a t d l s s o c l -

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Table 1 Comparison of sticking probabflmes at 0, = 0° and two nozzle temperatures w~th estimates (see section 4 2) of the total populations of excited vlbrauonal states of the incident CO~ molecules

Total populations of excited wbratlonal states (estmlated)

so(ff=17 kj tool 1) so(~= 43 kj mol l) so( E = 72 kJ mol- ~) s0(~= 103 kj moI 1) Sym stretch Bend Asym stretch

TN = 300 K

T,~ = 1000 K

0002 004 0 10 015 0 0002 0 04 l × 10 '

003 010 0 27 035 0 12 0 58 0 02-0 03

ate by virtue of internal excitation, appears generally to increase with i" (see also table 1) For if2 between 60 a n d 100 kJ tool 1 this dltference IS ~ 0 20, a considerable effect, whereas the difference is only 0 0 3 at E = 17 kJ tool -~ T h e d e p e n d e n c e of s 0 on the angle of mcldence has been incorporated into fig 7 by choosing as the abscissa the c o m p o n e n t of translational energy normal to the surface Since the data taken at d i f f e r e n t a n g l e s of incidence fall on a c o m m o n curve, the angular d e p e n d e n c e of s o ( E . 0,) is well accounted for by the scaling parameter, E . = E ¢os20, These points will be elaborated u p o n m section 4 Most of the sources of error in our m e a s u r e m e n t s have been m e n t i o n e d above Based on our estimates of these uncertainties a n d on the least-squared fits to the coverage versus exposure data. we e s u m a t e the standard error m the relatwe values of 6o = h m o , , _ o s ( O o) to be a b o u t _+ 15% E x a m i n a t i o n of fig 7 will reveal the presence of several deviations somewhat larger than this We believe these dewatlons to be due to small, u n c o n t r o l l e d coverages of a surface impurity, most hkely carbon The absolute values of % hinge on the CO coverage c a h b r a t l o n (0.55 M L at 300 K [30]) and on the relanve ion gauge s e n s m v m e s for CO 2 a n d CO The former Is based o n ra&otracer [36] a n d L E E D [37] measurements, a n d ~s almost certamly correct to w~thm 5-10%

Note added in proof S u b s e q u e n t to the completion of this work, we performed a c a h b r a t l o n - m d e p e n d e n t m e a s u r e m e n t of the sticking p r o b a b i h t y A b e a m flag was added to the U H V c h a m b e r [76]. facilitating absolute sticking probability m e a s u r e m e n t s by the "reflection detector" technique described by K i n g [19] We measured s. for CO2 at an average translational energy of 113 kJ mol ~ using both the method described in section 2 a n d the reflection detector techmque The value o b t a i n e d using the same calibrations as for the rest of the data was 0 38, m

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good agreement with the results summarized in fig. 7 The absolute measurement yielded a value of 0 56, l.e larger by 47% We feel that this alternative calibration is to be preferred over that described above, and hence that the sticking probabihtIes reported here should be multlphed by the factor 1 47 None of the arguments presented below are affected by this scahng factor

4. Discussion

4 1 Translational actwatlon and mctdent angle dependence of s o The three-hundred-fold increase in s o with translational energy at Ty = 300 K clearly indicates that the dissociation process is activated and that the activation barrier may be surmounted by translational energy m the incident molecules Enhancement of dlssocmtlon by internal energy as well is indicated by the Increase in s~ at ftxed Et ..... with nozzle t e m p e r a t u r e However. there is stdl an increase by a factor of 30 in s 0 at TN = 1000 K as E is increased from 10 to 100 kJ tool- ~ The approximate constancy of s~ with T, at elevated beam energy (fig 6) shows that excitation of molecularly adsorbed COz molecules by energy transfer from the hot surface is relatively unimportant in promoting &ssoclatlon. Much of the discussion of activated adsorption in the literature to date [2,6-8] has focused on the apphcablhty of a one-dimensional barrier model [2-5] This simple model predicts that s0(~?, 0,) will depend only on L? cos20,. as observed for CO2/NI(100 ), H 2 / C u [2], H2/Nt(100 ) [28], and C H 4 / W ( l l 0 ) [8], but not for N z / W ( l l 0 ) [7] The model in its simplest version also predicts a sharp threshold m the increase of so with E ± , which has not been seen in any of the system investigated to date The absence of such a threshold should not be surprising, given the drastic oversimplification of the potenhal-energy hypersurface introduced by such a model First. there are a number of internal molecular degrees of freedom revolved (four vibrations and two rotations m the case of CO 2). Moreover. the metal surface ~s far from structureless While very weak corrrugat~on ~s seen in the interaction of rare-gas atoms w~th low-index metal surfaces [38], the semi-empirical potentials used to describe dlssoclatwe adsorption [39,40] display considerable variation parallel to the surface In light of the corrugation to be anticipated m the CO2/NI(100) potential (the potentials for the CO(a) and O(a) fragments are certamly strongly corrugated), the observed dependence of so(E, 0,) only on EL therefore provides useful reformation. In particular, this behawor suggests the presence of an activation b a m e r along the reactant channel, m the spirit of gas-phase reaction dynamics [41] Strong couphng of the internal molecular degrees of freedom Introduced by colhslon with the two-dimensional surface comphcates this simple picture, however, as we discuss further below We note

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M P D'Et,ehn et al / Dt~souattt,e adsorpmm o] ( O: ,m Nt(lO0)

that the H y C u system is the only one to d a t e where an a t t e m p t has been m a d e to go b e y o n d simple a r g u m e n t s a n d to try to r e p r o d u c e the e x p e r i m e n t a l s 0 ( E ) behavior by p e r f o r m i n g trajectory calculations with a reahstic m o l e c u l e - s u r f a c e potential [39,40] G e l b a n d G a r d l l l o [42-44] were able to o b t a i n a r e a s o n a b l e fit to s o ( E ) only with a highly c o r r u g a t e d potential, with a b a r r i e r to surface diffusion of H a t o m s of 40 kJ mol ~ However, this p o t e n t i a l d i d not r e p r o d u c e the a p p r o x i m a t e scaling of s 0 with E~ 4 2 Dependence o] so on CO, internal energ~ The nozzle t e m p e r a t u r e d e p e n d e n c e of s~ (fig 7. table 1), a p p e a r s to d e m o n s t r a t e a d e p e n d e n c e on the internal energy of the C O 2 molecules Two possible sources o f error might obfuscate this conclusion C O 2 cluster f o r m a tion in the supersonic e x p a n s i o n a n d C O t o r m a t l o n by wall reaction in the hot nozzle The f o r m a t i o n of C O , clusters in nozzle beams, with as m a n y as 105 molecules per cluster, is well k n o w n [45-47] One might i m a g i n e that the p r o b a b l h t y for dissociation to C O ( a ) + O(a), p e r C O , molecule, w o u l d be c o n s i d e r a b l y smaller for CO2 dlmers a n d larger clusters than for the m o n o m e r , a n d that the nozzle t e m p e r a t u r e effect in fig 7 could be due to r e d u c e d cluster f o r m a t i o n m the hot nozzle Based on a c o m p a r i s o n of the typical s t a g n a t i o n c o n d i t i o n s e m p l o y e d here for 10-100% CO~ b e a m s (dN = 0 055 mm, PN = 500 Torr) to p u b l i s h e d m e a s u r e m e n t s of cluster size a n d c o n c e n t r a t i o n as a function of PN a n d d N [45-47], it is clear that the large m a j o r i t y of CO~ molecules in the b e a m were present as the m o n o m e r . T h e ratio of dimers to m o n o m e r s was estimated to be ~ 1 0 ~ Mass spectrometric m e a s u r e m e n t s m o u r system yielded a d i r e c t - t o - m o n o m e r ratio of 2 × 10-3 in a pure CO2 b e a m , the r a n o in the d d u t e l y seeded C O 2 b e a m s of high t r a n s l a t i o n a l energy will be much less It should be n o t e d that the a f o r e m e n t i o n e d m e a s u r e m e n t s of cluster formation, ours a n d those r e p o r t e d in the literature, m a d e use of electron-imp a c t ionization, which can c o n s i d e r a b l y u n d e r e s t i m a t e cluster c o n c e n t r a t i o n s because of f r a g m e n t a t i o n [48] However, even if the d i m e r c o n c e n t r a t i o n s were larger by an o r d e r of m a g n i t u d e than our estimates, they would still be far too small to be responsible for the increase m so b y 0 2, for 60 < E < 100 kJ mol 1 in going from T N = 3 0 0 t o T N = 1 0 0 0 K If C O were f o r m e d in the 1000 K nozzle, p e r h a p s b y o x i d a t i o n of the stainless-steel wall, clearly the C O , dissociative a d s o r p t i o n p r o b a b i l i t y would a p p e a r larger than the actual value However, there is direct evidence that C O f o r m a t i o n was not occurring to an a p p r e c i a b l e extent As n o t e d earlier, declines precipitously with 0o for a CO~ b e a m with TN = 1000 K a n d E = 74 kJ mol -~ ~(0o = 0 05 M L ) IS 0 0025. two orders of m a g n i t u d e less than so O n the other hand, we find that the (non-dissociative) sticking p r o b a b i l i t y for m o l e c u l a r C O is not greatly r e d u c e d on the p a r t i a l l y o x y g e n - c o v e r e d surface, certainly not by more than a factor of two from the clean surface value of 0 9

M P D'Eeelvn et al / Dt~o~mtt~,e adsorptton of CO: on NttlOO)

465

This places an u p p e r hmlt of 6 × 10 ~ on the C O / C O : ratio in the b e a m Again, this value is much too small to be responsible for the o b s e r v e d variation of % with nozzle t e m p e r a t u r e A l t h o u g h we were u n a b l e to measure the internal state p o p u l a t i o n s of the C O : molecules i m p i n g i n g on the surface, it Js possible to m a k e rather g o o d estimates by reference to studies of relaxation in C O : nozzle b e a m s r e p o r t e d in the h t e r a t u r e T e r m i n a l r o t a t i o n a l energaes in free jets have been investigated b y infrared emission s p e c t r o s c o p y [49], by mtracav~ty R a m a n scattering [50], a n d by time-of-flight m e t h o d s m conjunctaon with an energy b a l a n c e [51,52] While some question exists as to whether the terminal &strlbutlon~ are n o n - B o l t z m a n n [49,50], the average r o t a t i o n a l energies o b t a i n e d b y the various m e t h o d s are m rather g o o d agreement [49] Cooling of vibrational degrees of freedom has been studied by c o m p a r i n g the mass s p e c t r o m e t e r f r a g m e n t a t i o n p a t t e r n s observed In nozzle b e a m s with those o b t a i n e d from an effusive source at a k n o w n t e m p e r a t u r e [51,53] S h a r m a et al [53] have p r e s e n t e d a s u d d e n freezing m o d e l of v~bratlonal coohng, w~th spec~hc a p p h c a t l o n to jets c o n t a i n ing C O : Th~s m o d e l assumes a B o l t z m a n n & s t r l b u t l o n of vlbrataonal states, frozen at a t e m p e r a t u r e d e t e r m i n e d by the p o i n t m the e x p a n s i o n where the h y d r o d y n a m i c derivative of the w b r a t l o n a l relaxation tame becomes less than one A g r e e m e n t between p r e d i c t e d and m e a s u r e d terminal v~bratlonal temperatures for b e a m s ot pure C O : [51,53] and of CO~ seeded into H~ or N : [53] is q m t e g o o d Corrections to the excited v~bratlonal state p o p u l a t i o n s due to fluorescence decay during the transit from the nozzle to the N~ surface can be m a d e , u s i n g the k n o w n [54] radiative hfetlrnes, decay of excited s y m m e t r i c stretch and b e n d i n g levels is neghglble Simple c o n s i d e r a t i o n s indicate that vibrational rather than rotational, excltataon ~s responsible for the increase of s~ w'lth nozzle t e m p e r a t u r e The C O , molecules m the s u p e r s o m c b e a m s were rotataonally cold By c o m p a r i n g the s t a g n a t i o n c o n d i t i o n s e m p l o y e d m th~s study w~th h t e r a t u r e results for pure C O 2 [49-52], average rotational energies of 0 4 and 1 2 kJ tool ~ are estzmated at T N = 300 and 1000 K, respectively It seems very unhkely that such a small c h a n g e m r o t a t i o n a l energy could be i m p o r t a n t m p r o m o t i n g dlssoc.lat~on As a further test, however, we p e r f o r m e d e x p e r i m e n t s wath r o t a t l o n a l l y hot C O . b e a m s (T~,,, = 1000 K, E,.,¢ = 8 3 kJ mol ~) Using a nozzle with d~. = 0 5 7 ram, we p r o d u c e d b e a m s with n ~ d N ~ 3 × l 0 I'~ cm : (n.. is the stagnation density), where rotational cooling should be neghglble [49] The values of s. o b t a i n e d at three values of 0. are included in fag 7 as the open s y m b o l s with a horizontal b a r ( T N = 1000 K, Ex < 20 kJ tool ~) These d a t a a p p e a r to he on an extrapolataon of the results o b t a i n e d at higher E± and show the same general trend at low Ex as the T~ = 300 K results, i n d i c a t i n g that gas-phase rotataonal excitation ms not I m p o r t a n t ~n p r o m o t i n g CO+ dl+soc~atton an this energy regtme V~bratlonal relaxataon of C O : during the s u p e r s o m c expansion was meffi-

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clent under the stagnauon conditions of this study In order to estimate the extent of relaxation we have used the model of Sharma et al [53] and. in the case of pure CO 2 beams, have compared the stagnation conditions with published data [51.53] At 300 and 1000 K. excitation of the doubly degenerate b e n d m g (v 2) mode predominates Non-mode-specific measurements of vibrational coohng, whether in jets [51.53] or m an ambient gas [55]. therefore. probe the relaxation behavior in this mode To understand the relaxation behavior of the other modes It Js necessary to consider the colhslonally reduced V ~ V energy exchange rates In general, energy exchange within the manifold of excited symmetric stretch (vl) and bending (v 2) levels and within the manifold of excited asymmetric stretch (v~) levels ~s fast c o m p a r e d to exchange between these mamfolds or between either m a m t o l d and the (00°0) [56] ground state [57] Therefore. one may safely assume that T,, = T.. within the context of a sudden-freezing model In pure CO 2 decay ot (00(~1) 1,~ faster than that of (0110) ]53.55,57[ and apparently occurs wa the (nml0) manifold [57]. and so in this case we also assume T,,, = T~, For a dilute mixture of C O : in He. the reverse is true [57], and c o o h n g of the v~ mode is negligible under the stagnation conditions encountered here. although v~ and vz do relax The sudden-freezing model [531 predicts that T . , b / T N is determined by the dimensionless quantity ( d * a o i ~h- p), 1 where d * IS the eHecuve source orifice diameter. at) the adlabanc speed of sound and ~'h p the vibrational relaxation time (of the v2 mode in th~s case) at constant enthalpy and pressure To calculate ~'h p we have used tabulated data for vlbranonal relaxanon [53.55,57] in pure CO~ and m dilute C O . - H e mixtures, together with eqs (1). (11), and (22) m the paper by Sharma et al [53] The terminal vibrational temperatures were then read from their fig 7 [53] The radmnve hfetlmes of the lox~-lying (nm/0) levels are of order 1 [54]. and decay due to spontaneous fluorescence is neghglble However. the lifetime of (00°1) IS 2 6 ms [54]. which is of the same order of magnitude as the CO~ transit nine from the nozzle to the surface Consequently between 16 and 48% of the molecules m this level decay, for translational energies ranging from 100 to 8 kJ tool i respecnvely Fortunately, the populations of excited t,~ states are small at T N = 1000 K and their variation with k2 is u n i m p o r t a n t m the analysis below Our estimates of the total populations of e,~c~ted vibrational states, calculated by assuming Boltzmann d~strlbutions at the terminal vibrational temperatures and neglecting anharmonic correcuons, are given in table 1. along with s n c k m g probablhtles at three translational energies Correcnon~ due to radiative decay were included m the case of the v~ mode The uncertainties is the values at Ty = 1000 K due to variation in the stagnation conditions at different E and to uncertainties m the calculanons ol the terminal v~bratlonal temperatures are appro,~lmately 0 0 l , 002. and 001 for the v 1, v.. and v~ modes, respecnvely These uncertainties are small because of the inefficiency of

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vibrational coohng m the case of vl and v2, and to the small degree of excitation in the case of v~. The corresponding total exoted state populations assuming no relaxation are 0 15, 0 62, and 0 03, respectively Some insight into the dissociation dynamics is gamed by a comparison of the two sets of values m table 1 Since the d~ssooation probablhty of an internally excited molecule is less than or equal to umty, the difference, s 0 ( E z , TN = 1000 K) - so ( E z , T N = 300 K), is a lower bound to the excited vibrational state population responsible for the increase m s o The only mode with greater than 20% of the molecules m an excited state at T N = 1000 K is the bending (v2) mode Therefore, the data imply that CO 2 molecules with one or more quanta of bending excitation dlssoclahvely adsorb with slgmficantly higher probability than those in the ground state Unfortunately, we are unable to determine whether excitation in e~ther the symmetric or asymmetric stretch modes promotes dissociation The partlclpatlon of the bending mode m dzssoclatlve adsorption suggests that the transition state to dlssoc~atlon is bent, a hypothes~s that is supported by other considerations The final product state, CO(a) + O(a), Is bent, indicating that a transmon m geometry from the hnear molecule must be made along the reachon coordinate An ~/2-(C, 0) CO 2 configuration has been observed m ~everal organometalhc complexes [58-60], where the metal center is bonded both to C and to one of the oxygen atoms The structures of both the d ~ complex [59] N' [ P(C6HI~ )312( ~2-CO2 ) and the d -~complex [60] Nb(¢

-

have been characterized by X-ray diffraction Substantml rehybr~dlzatlon of the CO x hgand occurs m these complexes The C - O bond closest to the metal atom x~ slgmficantly weakened, as ewdenced by both an increase m the bond length and a lowering of the vibrational frequenoes [59]. and the O - C - O bond angle zs about 133 ° [58-60] An ab m m o SCF study [61] has shown that these effects may be understood as resulting from backbondlng by the metal d electrons It seems h~ghly plauszble that the transition state for d~ssocmt~on on N~(100) resembles such a species A significant activation b a m e r must be overcome to form such a speoes on the surface, as evidenced by the weaknes~ of the interaction between molecular CO. and the surface (fig 3 and related d~scuss~on) On oxygen-covered Ag(ll0), where the molecular binding energy ~s s~mdar, the wbrational frequenoes of molecularly adsorbed CO 2 were found to be only very weakly perturbed from t h o r gas-phase values [32]. Stuve et al. [32] concluded from the surface dipole selection rule that CO~ was randomly omented on the surface, since both the bending and asymmetric stretch modes were observed

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M P D'Eeel)n et al / Dts~oclatlt,e adsorpuon o] CO, on Nt(lO0)

If, as seems likely, the transition state to dissociation is a b e n t species with b o t h the c a r b o n a t o m a n d one of the oxygen a t o m s interacting strongly with the surface, the increase in s o with a d d m o n of energy to the b e n d i n g m o d e is easy to rationalize, since c o n s i d e r a b l e energy is r e q m r e d to access the b e n t configuration The repluslve p o r t i o n of the potential m u s t also extend significantly into the entrance channel so that m o m e n t u m n o r m a l to the surface is required in o r d e r to s u r m o u n t the barrier, as evidenced by the large increase m % with E L at T N = 1000 K a n d by the m o d e s t absolute increase in ,% with nozzle t e m p e r a t u r e at low t r a n s l a h o n a l energy (fig 7, table 1) The E x scahng behavior of % at Ty = 300 K, where the excited v i b r a t i o n a l state p o p u l a u o n s are ms~gmficant, suggests that the cross section for transfer of t r a n s l a t i o n a l energy into the reaction c o o r d i n a t e ( p o s t u l a t e d to the b e n d m g - h k e near the surface) d e p e n d s only on the c o m p o n e n t of m o m e n t u m n o r m a l to the surface To date there ts very httle m f o r m a t l o n in the h t e r a t u r e on conversion of t r a n s l a u o n a l to v i b r a t i o n a l (T ---, V) energy b y surface colhslon T h e d a t a that are avmlable suggest that T---, V energy transfer is not facile in the case of dmtom~e molecules Kleyn et al [621 d e t e r m m e d the internal state p o p u l a t i o n s of N O molecules, with an mitml t r a n s l a t i o n a l energy of up to 200 kJ m o l - ~ , scattered lnelastically from A g ( l l l ) S u b s t a n h a l t r a n s l a n o n a l to r o t a t i o n a l (T ~ R) energy transfer was observed, but no a p p r e c i a b l e v i b r a t i o n a l excitation was seen, despite only 22 kJ mol I being r e q m r e d for excitation to the v = 1 level Classical trajectory simulations s u p p o r t the p r e d o m i n a n c e of T -~ R over T ---, V energy transfer m impulsive colhslons of dlatomIcs with surfaces [63] G a d z u k a n d c o l l a b o r a t o r s [64-67] have investigated in some detml the i m p l i c a t i o n s of a model in which an electron is t e m p o r a r i l y translerred from a metal surface to an incident molecule The m o d e l predicts ~ubstantial w b r a t l o n a l e~cItatlon m the scattered molecules [64,65,67] a n d also e n h a n c e m e n t of &sSOClative a d s o r p t i o n [65-67] The extent to whmh th~s process ~s ~mportant remains to be seen Regardless of the possible i m p o r t a n c e of electromcally n o n - a d i a b a t i c processes, one expects conversion of translational to b e n d i n g vibrational energy in CO~ to be c o n s i d e r a b l y m o r e efficmnt t h a n T -~ V transfer m dIatomics since the large impulsive torques d u r i n g the colhslon event, which gl,,e rise to c o n s i d e r a b l e T ~ R transfer, will also couple strongly to the b e n d i n g m o d e W e note that T ---, R energy transter was f o u n d to d e p e n d only on Ez for N O / A g ( l l l ) [62], which is consistent with the hypothesis that the same ~mpulslve torques are responsible for • - , V(~,z) transfer m the &ssoclaUve a d s o r p t i o n of C O , In related work, substantial v~brat~onal excitation was seen by M a n t e l l et al in CO, CO~ [68] and N O [69] scattered from a hot Pt foil However, no s e p a r a n o n was m a d e between exc~tauon by a direct inelastic collision and trapping, e q m h b r m m with the surface, and subsequent d e s o r p t l o n It seems likely that the latter process was d o m i n a n t as was c o n c l u d e d by A s s c h e r et al [70] m the case of N O scattered from P t ( l l l ) V i b r a t i o n a l de-ex~ttatton by

M P D'Evelyn et al / Dls~ocmnt,e adsorpnon of CO, on Nl(lO0)

469

surface collision has also received attention Mlsewlch et al [71,72] found the deactivation probability for CO 2 prepared in the (00°1) or (10°1) state and for CO( t, = 2) to lie in the range 0 2 - 0 7 for colhslons with a variety of polycrystalhne surfaces The magnitude and surface temperature dependence of the processes suggest that trapping and subsequent deactivation and desorpuon was the dominant mechanism [72] Theoretical studies have Illustrated the importance of V ~ R [73,74] and V-~ T [74] energy transfer in models of vlbrationally excited hydrogen scattering from flat metal surfaces Clearly, further experiments are required, together with theoretical analysis, to elucidate the nature of the surface reaction dynamics in these elementary processes

4 3 Comparison wtth CO2 hydrogenanon kmencs We conclude with some brief comments on the implication of our results for Since so is relatively insensmve to the surface temperature, at least for T~ < 400 K, so(T), the dissociation probability for an ambient gas m contact with a clean NI(100) surface, both at temperature T, may be evaluated to first approximation by performing a flux-weighted convolution of s0(E, 0,, T2 = 407 K) with a Maxwell-Boltzmann distribution In order to carry out such a convoluuon the enhancement of dissociation by vibrational energy is neglected, that is, only the T N = 300 K data are considered The measurements are fit rather well by the empirical function

CO._ hydrogenation.

s 0 ( E . O,) = s* exp

E cos._O~ + D

"

wlths*=021. E*=83kJmol 1 and D = 5 7 k J m o 1 - 1 Note that the result of neglecting the enhancement of s o by vibrational exotation is to underestimate both s0(T) and the apparent activation energy for dissociation Calculauons of s o ( T ) are presented in fig 8 in the form of an Arrhemus plot Between T = 450 and 700 K the plot of % versus 1 / T is fairly linear, and yields an apparent activation energy of 12 kJ tool- ~ Comparison of our results with the CO 2 methanatlon study of Peebles et al [12] on NI(100) is relatively strmghtforward The latter authors have shown the hydrogenation kmetlcs on the Nl(100) model catalyst to be m good agreement with the kinetics on supported N1 catalysts [11.18] The total CO~ reaction probability is given by dividing the sum or the turnover frequencies (molecules (surface metal atom) -~ s -1) for production of CH 4 and of CO by the CO 2 colhslon frequency From the mmal turnover frequencies reported [12] w~th Pco = 1 Torr. p u = 96 Torr at 552 and 710 K. we estimate the CO~ reaction probablhties as 7 × 10 _6 and 1 × 10 -4, respectively, at the two temperatures These values are smaller, by more than an order of magnitude, than our estimates of s0(T) for CO 2 on a clean surface The discrepancy indicates that

470

M P D'Et,ehn et al / Dts~octatwe ad~orptmn o f ( . O , on Nt(lO0)

T (K) 700

600

500

40q

10-3

h-

~0~ I L I I L I I ~ I L I L I 15

2

I000 / T Fig 8 ( alculauons ol the shckmg probabdR) for ambient CO 2 at temperature 7 on clean Ng,100) dissociation of CO~ is n o t the r a t e - h m m n g step, m agreement with the conclusions of the most recent kinetic studies [11,12], and that b u i l d u p of other k | n e n c intermediates (CO(a), O(a), H(a), surface c a r b i d e [74]) is sufficiently r a p i d on the time-scale of the kinetic m e a s u r e m e n t s [121 that CO x dissociation takes place on a p a r t i a l l y covered surface with reduced sticking p r o b a b i l i t y The conclusion that CO~ dissociation is not r a t e - h m t t m g is s u p p o r t e d by the obser,~atlon of eftectl~e actlvatum energm~ for p r o d u c t i o n of C H 4 and CO of 89 and 73 kJ mol i, respectively [12], c o n s i d e r a b l y m excess of the value (12 kJ mol i) o b t a i n e d from s,,(1/] r ) on the clean surface

5. Summary The d e p e n d e n c e ol the m m a l sucking p r o b a b d l t y , ~¢~, for CO~ on Nl(100) on translational and Internal energy, angle of incidence, and surface t e m p e r a ture was investigated by supersonic molecular b e a m techniques S~ increased s m o o t h l y by three orders ot m a g m t u d e as translational and vibrational energy v~as a d d e d to the incident ( ' 0 2 molecules A t a p p r o x i m a t e l y a t r a n s l a h o n a l energy of 43 kJ mol ~, ,~,~ ,aas i n d e p e n d e n t ot the surlace t e m p e r a t u r e between 200 and 400 K A c t i v a t i o n of C O 2 by translational energy d e p e n d e d only on the c o m p o n e n t of m o t i o n n o r m a l to the surfaces The m a g n i t u d e of the increase of ~,, as the nozzle t e m p e r a t u r e was raised from 300 to 1000 K indicates that molecules with one or m o r e q u a n t a of b e n d i n g mode excltat|on dissociate with a slgmflcantly higher p r o b a b i l i t y than molecules m the g r o u n d

M P D'Evelyn et a l / Dlssoctatwe adsorptton of CO., on Nt(lO0)

state

471

W e a r g u e t h a t t h e t r a n s i t i o n s t a t e to d i s s o c m t l o n ~s b e n t a n d t h a t t h e

a c h v a t l o n b a r r i e r e x t e n d s s ~ g n f f l c a n t l y i n t o t h e r e a c t a n t c h a n n e l , so t h a t b o t h translational and vibrational energy are required m order to surmount the bamer efficiently Comparison of the calculated dissociation probablhty of ambient CO 2 in contact with a clean Nffl00) surface with observed reacnon probabdmes under catalytic hydrogenatmn conditions leads to the conclusmn t h a t C O e d l s s o c i a t m n is not t h e r a t e - h m i t l n g s t e p

Acknowledgements The authors gratefully acknowledge the support of the Department of Energy, Basic Energy Sciences (Grant DE-AT03-79ER10490) in this work We a l s o t h a n k D r . D J A u e r b a c h f o r h e l p f u l d i s c u s s i o n s a n d f o r a s s i s t a n c e in t h e design of the heatable nozzle source

References [1] M Grunze, in The Chermcal Physics of Sohd Surfaces and Heterogeneous Catalysis Vol 4 Fundamental Studies of Heterogeneous Catalysis, Eds D A Fang and D P Woodruff ('Elsevier, Amsterdam, 19821 p 143 [2] M Balooch, M J Carddlo, D R Mdler and R E Stlckney Surface Scl 46 (19741 358 [3] J E Lennard-Jones, Trans Faraday Soc 28 (1932) 333 [4] W van Willlgen, Phys Letters 28A (19681 80 [5] G Comsa and R David, Chem Phys Letters 49 11977) 512 [6] J Lee, R J Madlx, J E Schlaegel and D J Auerbach, Surface Sci 143 (19841 626 [7] D J Auerbach, H E Pfnur, C T Rettner, J E Schlaegek J Lee and R J Madlx, J Chem Phys 81 (1984) 2515 [8] C T Rettner, H E Pfnurand D J Auerbach Phys Rev Letters 54(19851 27•6 [9] H E Pfnur, D J Auerbach and R J Madlx, to be pubh~hed [10] J L Falconer and A E Za~li, J Catalysis 62 (19801 280 [11] G D Weatherbee and C H Bartholomew, J Catalysis 77 (19821 460 [12] D E Peebles, D W Goodman and J M White, J Phys Chem 87 (19831 4378 [13] J R Rostrup-Nielsen, Steam Reforming Catalysts (Danish Techmcal Press, Copenhagen 19751 [14] D C Grenoble, M M Estadt and D F Olhs J Catalysp, 67 (1981) 90 [15] J McCarty, J Falconer and R J Madlx, J Catalys~s 30 (1973) 235 [16] J B Benzlger and R J Madix, Surface Scl 79 (1979) 394 [17] M Arakl and V Ponec J Catalysis 44 (19751 439 [18] G D Weatherbee and C H Bartholomew, J Catalysis 68 (1981) 67 [19] D A Fang and M G Wells, Proc Roy Soc ¢London) A339 (1974) 245 [20] S P Smgh-Boparak M Bowker and D A King, Surface Scl 53 (19751 55 [211 J B Anderson, in Molecular Beams and Low Density Gas Dynamics Ed P P Wegener (Dekker New York, 19741 p l [22] Lenox Laser, Lenox, Massachusetts. USA [23] J M Blakely, J S Klm and H C Potter, J Appl Phys 41 (19701 2693 [24] E I Ko and R J Madlx, Appl Surface SCl 3 (19791 236

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[25] R G Musket W McLean C ,~ Colmenares D M Makowmckt and W J Smkhau`, Appl Surlace Scl 10 119821 143 [26] K Chnstmann, O Schober, G Ertl and M Neumann, J (-hem Ph',s 60 (19741 4528 [27] S Johnson and R J Ma&x Surface Sol 1t)8 119811 77 [28] 4 V Hamza and R J Madp~, J Phvs Chem 89 119851 5381 ]29] S Andersson Chem Phvs Letter,, 55 11978)185 [30] M P Klskmo'~a, Surface Sol 111 119811 584 [31] F Mat`,ushlma, Surface Scl 127 11983) 4113 [32] lc M Stuve R J Madlx and B ~, Sexton, Chem Ph)s Letters 89 I1982) 48 [33] P Kp, huk, J Phv`, ('hem Sohd,, 3 (1957)95 [34] M P D'E'~elyn A V Hamza and R J Ma&x to be pubh`,hed [35] M P D'E',el~n A V Hamza G F Gdow,,kl and R J Ma&x, to be pubh`,hed [36] K kher A ( Zettlemover and H Lmdhmser J Chem Phy', 52 119701 589 [37} J ( Frac) J ( h e m Phy`, 56(197212736 [38] J 1~2 Hurst [ Wharton, K ( Janda and D J Auerba~.h J ('hem Ph'/s 78 11983)1559 [39] J l | McCreer5 and G Wolken J Chem Phy', 63 119751 2340 [411] J ( Tully J Chem Ph'Y', 73 119801 633t [41] J (" Polan'~x &co Chem Re', 5 119721 161 [42] & Gelb and M J (ard~llo, Surface S~.L 59 11976) 128 [43] A G e l b a n d M J (ardfllo SurlaceSu 64 (19771 197 [441 A Gelb and M J (ardlllo Surface Sol 75 119781 199 [45] O F ttagena and W ()bert 1 ( h e m Ph'~,, 56 119721 1793 [461 R E Miller PhD Dp,',ertaUon, Department o1 Phb',m', klm' 74, lI 1198l) 8112 I52] R J Gallagher and J B Fenn I Chem Phvs 60 (1974) 3487 153] P K Sharma W S ~oung W F Rodgersand E L Knuth J Chem Phy`, 62(19751341 [541 H Stab" C l. Fang and G F Koqer J Appl Ph,,,', 37 11966) 4278 155] B %te,~ens, (olh,qonal &ctl',atLon m Ga,,e,, (Pergamon Oxlord 19671 [561 The standard Inln~n~) ',pectroscoplc rlotatlon ix employed, where n, i`, the "dbratama[ quantum number lot the ~,, mode and [ ~s the ,dbrat~onal angular momentum quantum number lor the doubl'y degenerate bending mode [57] (" B Moore R 1- Wl,~od B -k Hu and J T Yardle~, I ( hem Ph',`, 46 11967) 4222 [58] D I Darensbourgand R A Kudaroskl Ad'~an Organomet Chem 22 11981)12q [591 M Aresta C F Nobfle \ G Albano E Form and M Mands',ero, I Chem Soc Chem ( o m m u n 1197516~6 kl Areqa and r. F Nobfle ! ( h e m Soc Dalton qran`, (1977)7(18 ]60] G S Bn,,tov, P B Httchcock and M P Lappert J ( h e m Soc Chem ( o m m u n (1981) l14S [61] S Sakakl IX Kitaura and Ix Morokuma Inorg Chem 21 (1982) 760 ]62] A W Ixlevn A ( L u n t z a n d l ) J &uerbad~ SurfaceScl 11711982133 1631 R B Gerber and R k.lber Chem Phxs Letters 102 (19811 466 ]64] I W Gadzuk J ( h e m Phy,, 79119831 6341 i65] J W (JadTuk and J k N~r`,ko,, J Chem Ph'~`, 81 11984) 2828 [661 [W Gad7uk a n d s Ho[[owa', ('hem Phvs Letter,, 114~,1c/85) 314 [~,71 S HolMwa\ and J V~ GadLuk Surface Sc~ 152 153 t19851 838 [681 D A Mantell S B R'~ah G L Hailer and lB Fenn J Chem Ph'v~, 78 (19831 4250 [69] I ) A Mantell h ' - F Maa S B R',ah G L Hailer and J B Fenn J ('hem Ph',s 78 (198~1

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6338 [70] M Asscher, W L Guthrle, T -H Lln and G A Somorjal, J Chem Phys 78 (1983) 6992 [71] J Mlsewlch C N Plum G Blvholder, P L Houston and R P Merrdl, J Chem Phys 78 (1983) 4245 [72] J Mlsewlch, P L Houston and R P Merrill, J Chem Phys 82 (1985) 1577 [73] R B Gerber, L H Beard and D J Kourl J Chem Phys 74 (19811 4709 [74] A E DePnsto, Surface Scl 137 (1984) 130 [75] D W Goodman, R D Kelley T E Madey and J M White J Catalysis 64 (1980) 479 [761 M P D'Evelvn H P Stemruck and R J Madlx to be published