INTERACTION OF H2 AND N2 ON Fe, INVESTIGATED BY PULSED LASER ATOM PROBE

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INTERACTION OF H2 AND N2 ON Fe,

INVESTIGATED BY PULSED LASER ATOM PROBE

Y. Li, W. Drachsel, J. Block, F. Okuyama

To cite this version:

Y. Li, W. Drachsel, J. Block, F. Okuyama. INTERACTION OF H2 AND N2 ON Fe, INVES- TIGATED BY PULSED LASER ATOM PROBE. Journal de Physique Colloques, 1986, 47 (C7), pp.C7-413-C7-418. �10.1051/jphyscol:1986769�. �jpa-00225964�

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Colloque C7, supplément au n o 11, Tome 47, Novembre 1986

INTERACTION OF H, AND N, ON Fe, INVESTIGATED BY PULSED LASER ATOM PROBE

Y. LI(^), W. DRACHSEL, J.H. BLOCK and F. O K U Y A M A ( ~ )

Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4 - 6 , 0-1000 Berlin 33, F.R.G.

A b s t r a c t - The i n t e r a c t i o n o f hydrogen and n i t r o g e n has been s t u d i e d on i r o n f i e l d e m i t t e r s a t low temperatures (T<300 K) and low pressures ( p < l ~ - 4 ~ o r r ) . I t i s shown from thermodynamics t h a t chemical e q u i l i b r i u m p e r m i t s NH3-formation a t these temperatures. However, intermediates o r products o f such a r e a c t i o n c o u l d n o t be observed on i r o n c o n t r a r y t o t h e l i t e r a t u r e data on P t . The i r o n surface i s extremely s e n s i t i v e t o t r a c e s o f H20 and undergoes f i e l d enhanced o x i d a t i o n processes. F i e l d enhanced n i t r o g e n a d s o r p t i o n leads t o a d e p l e t i o n o f hydrogen i n t h e adsorption l a y e r .

1 - INTRODUCTION

I n t h e i n d u s t r i a l synthesis o f ammonia from hydrogen and n i t r o g e n , i r o n c a t a l y s t s have been used since t h e e a r l y days o f F r i t z Haber and Alwin Mittasch. A d i s s o c i a t i v e mechanism w i t h t h e d i s s o c i a t i o n o f N2 as r a t e l i m i t i n g step was w i d e l y accepted 111.

However, t h e d i r e c t o b s e r v a t i o n o f r e a c t i o n intermediates by means o f f i e l d i o n mass spectrometry was a tempting task. Thus i n 1968, W.A. Schmidt used a FI-source w i t h a magnetic mass spectrometer t o look a t t h e coadsorption o f Hz and N2 on Fe as w e l l as a t t h e decomposition o f NH3 a t 300 K 121. The products N+, NH+ o r NH; were n o t ob- served; however small amounts o f N*H+ were obtained. I n c o n t r a s t t o these f i n d i n g s , Tsong and coworkers r e c e n t l y gave evidence f o r these intermediates. I n t h e i r f i r s t paper /3/, by u s i n g l o w r e s o l u t i o n pulsed l a s e r atom probe mass analysis, i o n i n t e n - s i t i e s o f NH; o r N H ~ ( d i s t i n c t i o n was n o t p o s s i b l e ) were observed f o r a s e r i e s of metals i n c l u d i n g Fe. The f i e l d and temperature dependences were studied, showing maximum y i e l d a t 120 K. I n a l a t e r experiment, w i t h h i g h mass r e s o l u t i o n 141, NH;

was assigned as t h e predominant i o n species from a Pt(001) s u r f a c e and a d i s s o c i a t i v e mechanism f o r t h e ammonia synthesis was deduced.

Due t o t h e l o w ammonia c o n c e n t r a t i o n i n t h e chemical e q u i l i b r i u m 112 N2 + 312 H2SNH3 a t c a t a l y t i c r e a c t i o n temperatures (673 K), h i q h pressures (W 200 atm.) have t o be

(l)~iSiting scientist from Dalian Institute of Chernical Physics, The Chinese Academy of Sciences.

Dalian. P.R. of China

(2)~isiting scientist from Nagoya Inetitute of Technology, Gokiso-cho Showa-ku. Nagoya 466. Japan

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986769

C7-414 JOURNAL DE PHYSIQUE

a p p l i e d i n t h e i n d u s t r i a l ammonia synthesis. E x t r a p o l a t i o n t o lower temperatures ( f i g . 1 ) shows t h a t t h e e q u i l i b r i u m a t 100 K i s s h i f t e d i n f a v o r o f NH3. I f t h e a c t i - v a t i o n o f n i t r o g e n c o u l d be achieved by means o t h e r than t h e c a t a l y s t surface, which needs 673 K, h i g h ammonia concentrations would be expected a t low temperatures f o r thermodynamic reasons. We pose t h e q u e s t i o n whether an e l e c t r o s t a t i c f i e l d o f t h e order o f 1

~ / 8

6 f

F i g . 1 - l o g K vs temperature and r e c i p r o c a l temperature ( i n s e t ) , e x t r a p o l a t e d t o low temperatures f o r t h e r e a c t i o n 3/2 Hz + 1/2 N2+NH3;

3/2 1/2

PH^

K = ; P [ a t l

.

P N H ~

I I

-

I n s p i r e d by t h e c o n t r a d i c t i n g l i t e r a t u r e r e s u l t s and t h e thermodynamical estimate, we t r i e d t o i n v e s t i g a t e t h i s r e a c t i o n u s i n g Our pulsed l a s e r atom probe. An o l d e r v e r s i o n o f t h i s instrument was described a l r e a d y /5,6/.The m o d i f i e d apparatus ( f i g . 2 ) a l l o w s observation o f t h e e m i t t e r s u r f a c e w i t h a video-camera w h i l e f i e l d d e s o r p t i o n experiments a r e performed. The channelplate ( w i t h two h o l e s ) i n f r o n t o f t h e e m i t t e r g i v e s a f i e l d i o n image o f t h e s u r f a c e w h i l e a second channelplate d e t e c t o r , 40 cm a p a r t from t h e e m i t t e r , images o n l y a few s u r f a c e atoms. This d e t e c t o r i s used f o r t i m e - o f - f l i g h t measurements. The i r o n specimen (99.99 %) was etched i n d i l u t e HC1 and cleaned i n s i t u by annealing i n 3 x 1 0 ' ~ T o r r Hz a t 800 K and subseguent f i e l d evapora- t i o n i n hydrogen a t 110 K, u n t i l t h e (110) and (211) areas were developed. An atom- i c a l l y r e s o l v e d p a t t e r n of t h e (111) plane c o u l d n o t be achieved. About 50-100 atomic s i t e s were probed w i t h our system. The gares 1 4 ~ 2 , 1 5 ~ Z , Hz and D2 were introduced from g l a s s b o t t l e s a t dynamic vacuum c o n d i t i o n s . Residual gas pressure a f t e r bakeout was 3x10-l0 Torr, b u t was e s s e n t i a l l y h i g h e r d u r i n g dynamic gas i n l e t .

F i g . 2 - Pulsed l a s e r atom probe.

III

-

I n o u r f i r s t experiments t h e mass spectra d i s p l a y e d a s t r o n g y i e l d o f mass 16 and 17 ( f i g . 3 ) s i m i l a r t o t h e f i n d i n g s o f L i u and Tsong f o r t h e Pt(001) surface /3/. We mea- sured t h e f i e l d as w e l l as t h e temperature dependence f o r t h e r e a c t i o n products NH2 and NH3. Without d i s c u s s i n g these r e s u l t s f u r t h e r , we wish t o s t a t e a t t h i s p o i n t t h a t these f i r s t r e s u l t s were obtained by i n t r o d u c i n g

a

The obvious disappearance o f surface hydrogen by displacement o f N2 leads t o t h e argument t h a t , due t o t h e h i g h s t i c k i n g c o e f f i c i e n t , o n l y very low Na pressure i s needed. Some N2 i s expected i n t h e r e s i d u a l gas, as conf irmed by t h e 1 5 ~ 2 exchange r e a c t i o n w i t h chamber-wall 14N2 l e a d i n g t o 14815N.

Using D2 i n s t e a d o f H2 ( f i g . 4), i t i s obvious t h a t deuterium dominates on t h e sur- face; so i f m=16 i s l a b e l l e d as NH2, then m=18 as

+

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m/e

Fig. 3 - ToF-spectrum: pH = 3 x l 0 - ~ ~ o r r ,

- 2 ~ 1 0 - ~ ~ o r r , T = 110 2 K, F = 2 ~ / 8 .

Pres. gas-

Fig. 4

-

From these f i n d i n g s some doubts about t h e c o r r e c t mass assignment arose, which we t e s t e d by u s i n g CO, CH4, H20 and O2 as candidates f o r t h e r e s i d u a l gases. Methane gave a s t r o n g peak a t m=16 and a l s o a weak and broad peak a t m=15. The o t h e r oxygen- c o n t a i n i n g species gave r i s e t o a more o r l e s s pronounced s i g n a l a t m=16. Since m=32 was absent, O2 c o u l d be excluded; t h e o r i g i n o f m=16 could be a t t r i b u t e d t o t h e presence o f Fe-oxides, mainly v i s i b l e as ( ~ e ~ 0 ) ~ ~ a t m-64, i n t h e spectrum, These f i n d i n g s revealed a s t r o n g s e n s i t i v i t y o f t h e i r o n specimen t o oxide f o r m a t i o n from i m p u r i t i e s .

Therefore, we performed an experiment under b e t t e r c o n d i t i o n s w i t h t h e r e s i d u a l gas pressure below 5 ~ 1 0 - ~ ~ o r r . S t a r t i n g w i t h o n l y D2 ( f i g . 5 a ) , we added 15N2 a t 1 0 - ~ ~ o r r (fig.5b). I n comparison, no ND; was observed ( f i g . 5 b ) whereas mass=32 appeared as N ~ D + a t t h e (211)-Fe surface. No e x t r a r e a c t i o n products occurred when we increased t h e n i t r o g e n pressure f u r t h e r ; o n l y a decrease o f surface deuterium was observed.

Fig. 5(a)

-

(b)

-

( a

CO FeD3

7 Fe C

I I I I I I I

1 2 4 6 16 aS EBb2-

n/z

I n a l 1 o f Our experiments so f a r , t h e r e was no evidence f o r N2 d i s s o c i a t i o n ; n e i t h e r N+ n o r F ~ N ' was obtained below 300 K. Atomic N was detected i n small amounts a t 2

~ / 8

I V - CONCLUSION

The a p p l i c a t i o n o f pulsed l a s e r f i e l d d e s o r p t i o n t o surface r e a c t i o n s such as those between N2 and Hz has several l i m i t a t i o n s :

1. Surface a n a l y s i s can be performed f o r these systems o n l y a t low temperatures ( i . e . T < 300 K). Otherwise,the coverage o f t h e surface w i t h r e a c t i n g gas mole- cules i s t o o low s i n c e temperature enhanced f i e l d d e s o r p t i o n a t t h e r e q u i r e d d e s o r p t i o n (and r e a c t i o n - ) f i e l d s t r e n g t h depletes t h e surface. F i e l d p u l s e ex- periments w i t h a l o w o r n e g l i g i b l e r e a c t i o n f i e l d s t r e n g t h a r e p r e f e r a b l e f o r i n v e s t i g a t i n g r e a c t i o n intermediates i n t h i s case.

2. Since s u r f a c e r e a c t i o n s a r e f r e q u e n t l y a l t e r e d by h i g h e l e c t r o s t a t i c f i e l d s , l a s e r p u l s e d e s o r p t i o n may l e a d t o products which d i f f e r from those o f f i e l d - f r e e chemical surface r e a c t i o n s .

3. Various i o n s o f t h e mass spectrum may be formed, due t o bond breaking, f r a g - mentation, a s s o c i a t i o n o r proton-capture, which do n o t represent o r i g i n a l neu- t r a l compounds

.

4. The photon pulse can

-

-

C7-418 JOURNAL DE PHYSIQUE

5. We have observed extended i s o t o p e exchange r e a c t i o n s a t t h e chamber w a l l s a f t e r i n t r o d u c i n g d i f f e r e n t i s o t o p e gases. This complicated t h e proper mass assign- ment.

Taking these l i m i t a t i o n s i n t o account, t h e f o l l o w i n g conclusions can be drawn f o r t h e r e a c t i o n o f n i t r o g e n and hydrogen on i r o n surfaces:

1.) Our "low r e s i d u a l gas" experiments gave no evidence f o r NH3-synthesis on l o w index planes o f Fe below 300 K; n e i t h e r was N 2 - d i s s o c i a t i o n observed. The o n l y evidence f o r t h e d i s s o c i a t e d form o f n i t r o g e n came from experiments i n which i r o n was p r e t r e a t e d w i t h n i t r o g e n a t 800 K.

2.) I r o n seems t o be very s e n s i t i v e t o contamination from r e s i d u a l gas (e.g. CO, H20) l e a d i n g t o f i e l d enhanced o x i d a t i o n , which y i e l d s 0' ions, f o r instance.

3.) Adsorbed N2 (as w e l l as CO) blocks t h e a d s o r p t i o n o f H2 ( t h e reverse was n o t observed)

.

4.) A t and below b e s t image voltage, t h e f i e l d i o n i z a t i o n c u r r e n t c o n s i s t s m a i n l y o f f i e l d desorbed H2, recombined from p a i r s o f H atoms which have d i f f u s e d from t h e shank. A s t r o n g i s o t o p i c e f f e c t a t T < 150 K i s i n d i c a t e d by t h e r e l a t i v e l y l o w e r D2 r a t e s . t

I r o n surfaces behaved r a t h e r d i f f e r e n t l y than p l a t i n u m /3,4/. Traces o f H20 and o t h e r oxygen c o n t a i n i n g compounds probably l e a d t o s u r f a c e contamination by o x i d a t i o n r e a c t i o n s w i t h i r o n , which must be f i e l d dependent. A d i r e c t c o r r e l a t i o n o f t h e p a r t i c u l a r p r o p e r t i e s o f Fe and t h e r e a c t i o n o f n i t r o g e n w i t h hydrogen a t h i g h e r tem- peratures and pressures i s s t i l l l a c k i n g .

REFERENCES

/1/ E r t l , G., CRC, C r i t i c a l Rev. S o l i d S t a t e Mater. S c i . - 10 (1982) 349.

/2/ Schmidt, W.A., Angew. Chemie 4 (1968) 151.

/3/ Ai, C.F., and Tsong, T.T., 3.-de Physique, C2-3 47 (1986) 347.

/4/ Liu, W., and Tsong, T.T., S u r f . Sci. 165 (1986) l 7 6 .

/5/ Drachsel, W., Jentsch, Th., and Block,.H., I n t . J. Mass Spectrometry I o n Phys.

46 (1983) 293.

/6/ s a c h s e l , W., Block, J.H., and Viswanathan, B., Springer Ser. i n Chem. Phys.

2

(1983) 221.