KINETICS OF ADSORPTION, THERMAL DESORPTION AND DISSOCIATION OF NO ON RHODIUM, STUDIED BY PULSED FIELD DESORPTION

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KINETICS OF ADSORPTION, THERMAL DESORPTION AND DISSOCIATION OF NO ON

RHODIUM, STUDIED BY PULSED FIELD DESORPTION

N. Kruse, G. Abend, J. Block

To cite this version:

N. Kruse, G. Abend, J. Block. KINETICS OF ADSORPTION, THERMAL DESORPTION AND

DISSOCIATION OF NO ON RHODIUM, STUDIED BY PULSED FIELD DESORPTION. Journal

de Physique Colloques, 1989, 50 (C8), pp.C8-147-C8-151. �10.1051/jphyscol:1989826�. �jpa-00229924�

COLLOQUE DE PHYSIQUE

C o l l o q u e C8, suppl6ment au n o 11, Tome 50, novembre 1989

KINETICS OF ADSORPTION, THERMAL DESORPTION AND DISSOCIATION OF NO ON RHODIUM, STUDIED BY PULSED FIELD DESORPTION

N. KRUSE, G. ABEND* and J.H. BLOCK*

Technisch-Chemisches Laboratorium, ETH Zentrum, CH-8092 ZiiriCh, pitzerland

Fritz-~aber-Institut der Max-Plank-Gesellschaft, Faradayweg 4 - 6 , 0-1000 Berlin 33, F.R.G.

Abstract -The interaction of NO with stepped surfaces of a Rh field emitter has been studied by means of pulsed field desorption mass spectrometry (PFDMS). At temperatures 300<T<550 K and pressures p<1.3.10-~ Pa, the PFD mass spectra of the adsorbed layer were always dominated by NO+, RhXNn+ and Rhon+ ions (n,x = 1.2). We conclude that part of the adsorbed NO undergoes dissociation with subsequent deposition of oxygen and nitrogen atoms on the surface. ~t low coverages, i.e. Q<0.1 monolayer, NO adsorption was found t o be predominantly molecular.

Using a probe-hole, -1 50 atomic sitesclose t o the (100) pole of the Rh tipsurface were analyzed by varying the field-free reaction interval, t ~ , between the field pulses (100ps<t~< IS). The equilibration between NO adsorption and thermal desorption was monitored. The temperature dependence of the mean lifetimes, T! before thermal desorpt~on was evaluated, assuming first order thermal desorption kinetics, yielding an activation energy Ed = 102 kllmol and a preexponential zo = 4.10-l4 s.

1 - INTRODUCTION

The interaction of nitric oxide with Rh single crystal surfaces has been the subject of a considerable number of investigations during the past few years 11-1 11. This interest is at least partly stimulated by the activity of Rh in NO reduction for air pollution control.

Temperature programmed desorption (TPD) has shown that NO on low index Rh sin le crystal surfaces dissociates during heating. Most of the studies reported complete dissociation of a

8

sorbed NO below i t s desorption temperature provided the initial coverage was small enough. The NO bond breaking was found t o be facilitated by the presence of step sites /1,4,91. In a study with Rh field enlitter tips Hendrickx and Nieuwenhuys I91 prov~ded an actlvity pattern of the vartous crystal planes w ~ t h respect t o NO decomposition.

Thermal desorption of associatively adsorbed NO was seen t o occur after dosing Rh single crystal surfaces close t o saturation 11-1 11. NO,d on Rh(100) desorbed according t o first order kinetics with activation energies Ed = 100-1 18 kJ/mol 16.8.1 11. A desorption preexponential v, =

l o q 4

s-' was determined from temperature programmed electron energy loss spectroscopy (TP-EELS) 11 11. In the present study field emitter tips were used t o probe the influence of step sites on both the decomposition and thermal desorption of Noad. We employed pulsed field desorption mass spectrometry (PFDMS), which has been shown t o provide kinetic data of the thermal equilibration process of NOad on Pt field emitter surfaces 1121.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989826

2

-

EXPERIMENTAL a) General

The PFDMS technique and i t s application t o the study of surface reaction kinetics have been described in detail elsewhere 112-131. In brief, short pulses of high electric field strengths with repetition frequencies up t o 10 kHz are produced in front of a field emitter tip. Species adsorbed on the tip surface are desorbed as ions,and chemically identified by means of time-of-flight mass spegrometry..A channel plate image intens~fier with a probe hole is mounted at the entrance of the f l ~ g h t tube In order t o determine the orientation of the tip, using reversed fields. The tip can be tilted so that ions from selected different surface planes can enter the flight tube through the probe hole. The monitored area of a chosen planecontainsup to -200atomic sites.

In the time, t~ ,between the pulses, an arbitrary field can be maintained. For the measurements t o be reported here, a stead field was not applied. Rh field emitter tips were prepared by etching high purity wirer (99.995 % rrom Goodfellow Metal) using standard etchinp procedures.

N 9

(99.8 % purity) was provided by Messer Gr~eshe~m and used w ~ t h o u t further purif~cat~on. Surface temperatures were measured by a thermocouple spotwelded to the shank of t h e t ~ p .

b) Kinetic measurements

The procedure for studies of surface reaction kinetics is schematically illustrated in Fig. 1.

ce to0 ns pulse width

-rFD

F P

0

tc'

short medium long t~

Fig. 1

-

a) Time scheme of the desorption field pulses,

FD:

desorption field strength; FR: arbitrary base field strength during the reaction interval t ~ . b) Development of surface coverage for different pulse repetition frequencies.

While the emitter surface is co~~tinuously dosed by gaseous NO, adsorption takes place only in the time t~ between two pulses. A t the end

oft^

,the adsorbed layer is desorbed by a field pulse. If the layer is completely removed by each pulse, the measured ion intensity directly represents the surface concentration within the monitored area before a pulse. Consequently, each reaction period has the same starting condition of zero coverage. As indicated on the left of fig. 1 the surface concentration that is reached for short reaction times is far below any saturation or equilibrium limit. Under these conditions, the probability o f consecutive surface reactions i s low and, consequently, the field pulses probe the initial stages of the adsorption process. The longer the reaction time, the more the adsorption process proceeds. Provided the temperature is chosen appropriately, a dynamic adsorption-thermal desorption equilibrium can be established for long reaction times. In this case, the

ionic intensity and, consequently, the surface concentration does not increase further with t~ ,as indicated on the right of fig. 1. Assuming adsorption t o proceed with a constant sticking probability and thermal desorption t o obey first order kinetics, dddt = -d~, the surface concentration develops according to c = 2 ( 1 - e - ~ ) . The mean lifetime before thermal desorpton is given by the time t ~ , when the NOad concentration reaches the (1-lle) level of i t s equilibrium value, 2, after long times. The temperature dependence of T,can be parametrized via Frenkel's equation, T =zo-eEdkT. The activation energy, Ed, for thermal desorption and the preexponential, T,, are accessible i n thisway.

3 - RESULTS AND DISCUSSION

The results presented here were obtained by probing -1 50 atomic sites of the stepped surface region close t o the (100) pole of a Rh field emitter. The experiments were performed by continuously dosing the tip surface at a steady gas pressure of 1.3-10-5 Pa. Fig. 2 presents a typical time-of-flight mass spectrum obtained under pulsed field conditions with zero base field and pulse amplitudes of F ~ = = 2 8 Vlnm and a reaction time t~ = 200 y s between the pulses. As is clearly seen in fig. 2 the mass spectrum isdominated by NO+ ions, suggesting that the surface layer contains mainly molecularly adsorbed NO at a reaction temperature of T=547 K. A small amount of RhNO+ ions possibly indicates field desorption NOad from step sites with the simultaneous removal of a Rh step atom.

Field strength variation measurements1141 have shown that field pulses with amplitudes F ~ z 2 8 Vlnm keep the surface dynamically clean. At a pressure p = 1.3-10-~ Pa, the NO,d concentration that can be reached during t~ = 200 p s is below lP4 monolayer.

mle

Fig. 2 -Time- of-flight mass spectrum of NO on stepped Rh with (100) orientation of the terraces; NO pressure: 1.3 Pa; desorption field FD = 28 Vlnm; no base field (FR = 0); temperature T = 547 K.

As a consequence of the high field strength exerted by the pulses field evaporation of the emitter material itself takes place. This is evidenced by the occurrence of Rh+ and Rh+ ⁺ ions in the mass spectrum.

Moderate intensities are found for Rho+ +, RhN+ +, Rh2N+ ⁺ and Rho'. We conclude that NOad undergoes partial dissociation during t ~ , so that oxygen and nitrogen atoms are deposited on the surface.

We sug est that NOad dtssociation occurs most easily at step sites since the products of this process are mainly Reid desorbed along with the removal of Rh (step) atoms. High activity o f step sites i n N-0 bond scisson was also observed ~n work on macroscopic stepped single crystal surfaces /1,4/ and Rh field emitter tips 191. DeLouise and Winograd I41 studied NO adsorption on Rh (331) and Rh (1 11) by means of XPS (X-ray photoelectron spectroscopy) and SIMS (secondary ion mass spectrometry). At saturation coverage, the authors found -10times moredissociated specieson thestepped (331) surface at 300 K than on the atomically flat (1 11) surface. In contrast t o these results, Dubois et al. 121. in a study of NO interaction with Rh (331) by means of high-resolution electron energy loss spectroscopy (ELS), could not detect any NOad dissociation at 300 K.

The present study, ernploy~ng PFDMS under in-situ reaction conditions, gives clear evidence i n favor o f N - 0 bond rupture on atomically stepped Rh surfaces. An adr;iitional pointwe wish t o make i s that even at T=547 K and very low surface coverages (ONo<10 monolayer, see above), molecular NO adsorption dominates over dissociation. This is in contrast t o most other studies w i t h macroscopic single crystal surfaces 11, 3-7,10 , 111, where small concentrations of Noad were found t o dissociate completely during heating from low temperatures t o above 300 K. N2 and 0 2 (due t o recombination reactions), rather than NO thermal desorption, were observed in these investigations. Obviously, NOad dissociation on Rh is a rather slow process which occurs t o a relatively small extent at short reaction times t~ = 200 ps, b u t is complete i n a TPD measurement with a time constant of t h e order o f some t e n seconds. Thus, reaction time variation measurements should reveal the dissociation kinetics o f adsorbed NO. In fact, we observed increasing amounts o f dissociation products w i t h increasing t~

values. The detailed results o f this study will be presented elsewhere/14/.

In this paper w e focus on the kinetics o f thermal desorption of NOad These measurements were performed according t o the procedure described in the experimental part. The reaction time was varied i n t h e ran e from 100 p s t o 1 sat temperatures between 443 K and 547 K. The mean lifetimes, I;,

o f molecularly a3sorbed NO were measured as those values of t~ where the NO* intensity reached t h e ( ? - l i e ) level o f t h e constant intensity for long t ~ . The temperature dependence o f the r, -values was evaluated according t o Frenkel's equation and plotted i n fig. 3.

Fig. 3 -Temperature dependence of the mean lifetimest of NO on the sametip as i n Fig. 2.

An activation ener y Ed = 102 kJ1mol was obtained from the slope in fig. 3, and a preexponential

~ ~ = 4 . 1 0 - ~ ~ s from t%e intersect with the ordinate. These valuer are i n reasonable agreement w i t h those reported i n the literature for NO on Rh (100) 1 6,8,11/. Villarrubia and Ho /11/ have, e. g., determined Ed = 118 kJ1mol and t o = 10-l4 s by TP-EELS experiments with varying heating rates and an initial surface concentration o f 0.5 monolayer. Under these conditions more than 50 % o f the adsorbed layer underwent decomposition during heating, while the remainder desorbed from a molecular adsorption stale. In our measurements, the surface concentrations that built u p during t~

were always much lower. In spite of these low coverages, most o f the NO was found t o be molecularly adsorbed.

In a recent study w ~ t h Pt field emitter tips PFDMS provided kinetic data for the thermal desorption process o f adsorbed N01121. Interestingly, step sites on the (1 11) plane were found t o be inactive i n N - 0 bond breaking b u t exerted a strong bonding t o Noad. Consequently, the thermal desorption kinetics o f NOad were dominated by the step sites, provided the surface concentrations were far below saturation o f the steps. In the present study the Rh (100) stepped surface was found t o be more

C8-151

active in N-0 bond scisson than any stepped Pt plane probed before. This finding is in accordance with the catalytic behavior known for these metals. On the other hand, any decomposition of NOad with subsequent deposition of nitrogen and oxygen atoms at steps leads to site blocking for NOad.

especially when the decomposition products are not completely desorbed by each field pulse. Thus, step sites of the probed Rh surface do not control the thermal desorption kinetics, and it is not surprising that similar rate data are determined for NOad on Rh (100) /11/and stepped Rh (100).

ACKNOWLEDGEMENT

This work was partially supported by the Sonderforschungsbereich Sfb 6 at the Freie Universitat Berlfn:

REFERENCES

/ 1 / CASTNER, D.G., and SOMORJAI, G.A., Surf. Sci. 83 (1979) 60

111 DUBOIS, L.H., HANSMA, P.K., and SOMORJAI, G.A.J. Catal. 65 (1980) 318 131 BAIRD, R.J., KU, R.C., and WYNBLATT, P., Surf. Sci. 97 (1980) 346

16,1 DELOUISE, LA., and WINOGRAD, N., Surf. Sci. 1 5 9 ^ 9 8 5 ) 199 15/ ROOT, T.W., SCHMIDT, L.D., and FISHER, G.B., Surf. Sci. 134 (1983) 30 16/ HO, P., and WHITE, J.M., Surf. Sci. 137 (1984) 103

ni ROOT, T.W., FISHER, G.B., and SCHMIDT, L.D., J. Chem. Phys. 85 (1986) 4679 18/ HENDERSHOT.R.E., and HANSEN, R.S., J. Catal. 98 (1986) 150

/9/ HENDRICKX, H.A.C.M., and NIEUWENHUYS, B.E.,Surf. Sci. 175 (1986) 185 /10/ BUGYI, L, AND SOLYMOSI, F., Surf. Sci. 188 (1987) 475

/ 1 1 / VILLARRUBIA, J.S., AND HO, W., J. CherrTPhys. 87 (1987) 750

/12/ KRUSE, N., ABEND, G., AND BLOCK, J.H., J. Chem. Phys. 88 (1988) 1307

/13/ BLOCK, J.H., AND CZANDERNA, A.W., IN A.W. CZANDERNA (ED.), METHODS AND PHENOMENA.

VOL. I , ELSEVIER SCIENTIFIC PUBL. COMP., (1975) 379 /14/ KRUSE, N., ABEND, G . , AND BLOCK, J.H ,to be published