A STUDY OF SURFACE CATALYZED AND FIELD ENHANCED FORMATION OF H3 AND NH3 ON METAL SURFACES

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A STUDY OF SURFACE CATALYZED AND FIELD ENHANCED FORMATION OF H3 AND NH3 ON

METAL SURFACES

C. Ai, T. Tsong

To cite this version:

C. Ai, T. Tsong. A STUDY OF SURFACE CATALYZED AND FIELD ENHANCED FORMATION OF H3 AND NH3 ON METAL SURFACES. Journal de Physique Colloques, 1986, 47 (C2), pp.C2- 347-C2-351. �10.1051/jphyscol:1986253�. �jpa-00225687�

A STUDY OF SURFACE CATALYZED AND FIELD ENHANCED FORMATION OF H, AND NH, ON METAL SURFACES

C.F. AI' and T.T. TSONG

' I n s t i t u t e of N u c l e a r Energy R e s e a r c h , T a i w a n , D.R.C.

P h y s i c s D e p a r t m e n t , Penn S t a t e U n i v e r s i t y , U n i v e r s i t y P a r k , Pa 16802, U.S.A.

A b s t r a c t - Formation of H3 and s y n t h e s i s of NH3 on about 20 t r a n s i t i o n m e t a l s u r f a c e s h a s been s t u d i e d by p u l s e d - l a s e r a s s i s t e d f i e l d d e s o r p t i o n i n a p u l s e d - l a s e r i n a g i n g atom-probe. The NH3 molecule i s found t o be formed from chemisorbed H and N atoms. The r e a c t i v i t y of t h e s e r e a c t i o n s depends on b o t h t h e s u r f a c e m a t e r i a l and i t s s t r u c t u r e s .

I

-

The i n t r i n s i c r e a c t i v i t y of a m e t a l s u r f a c e may b e enhanced by a n a p p l i e d e l e c t r i c f i e l d . T h i s h a s been known f o r y e a r s mainly from f i e l d i o n i z a t i o n mass s p e c t r o s c o p i c s t u d i e s . 1 U n f o r t u n a t e l y , t h e s u r f a c e c o n d i t i o n s i n many of t h e s e e a r l y s t u d i e s a r e n o t v e r y w e l l d e f i n e d , and p r o c e s s e s i n v e s t i g a t e d a r e o f t e n complicated chemical re- a c t i o n s . With t h e a d v e n t of t h e h i g h v o l t a g e p u l s e o p e r a t e d t i m e - o f - f l i g h t atom- probe, s u r f a c e r e a c t i v i t i e s can now be s t u d i e d w i t h a t o m i c a l l y w e l l d e f i n e d , f i e l d e v a p o r a t e d s u r f a c e s . U n f o r t u n a t e l y t h e e x c e s s i v e l y h i g h e l e c t r i c f i e l d needed f o r f i e l d d e s o r p t i o n w i l l f i e l d d i s s o c i a t e t h e desorbed s p e c i e s , and t h e d e t e c t e d s p e c i e s do n o t r e f l e c t t h e t r u e r e a c t i o n p r o d u c t s of t h e s u r f a c e . These d i f f i c u l t i e s can b e l a r g e l y overcome by u s i n g t h e p u l s e d - l a s e r atom-probe.2

Two t y p e s of p u l s e d - l a s e r atom-probe have been developed i n our l a b o r a t o r y and b o t h have been used t o s t u d y f o r m a t i o n of H 3 and NH3 on m e t a l s u r f a c e s . One of them i s t h e p u l s e d - l a s e r imaging a t 0 r n - ~ r o b e , 3 which c a n s i m u l t a n e o u s l y p r o v i d e an i n s i t u t i m e - o f - f l i g h t mass spectrum and a time-gated f i e l d d e s o r p t i o n image of a s e l e c t e d s p e c i e s . I n t h i s s t u d y we u s e mainly t h e imaging atom-probe. From time t o time w e r e s o r t t o t h e h i g h r e s o l u t i o n system f o r c o r r e c t m a s s i d e n t i f i c a t i o n .

Our aim h e r e i s t o f i n d o u t t h e mechanism of f o r m a t i o n of H3 and NH3, and a l s o t h e c o n d i t i o n s under which t h e observed d e s o r p t i o n s p e c i e s w i l l t r u l y r e p r e s e n t t h e s u r - f a c e c a t a l y z e d p r o d u c t s and what s p e c i e s a r e t h e f i e l d induced a r t i f a c t s . The con- c l u s i o n s a r e mainly d e r i v e d from t h e f i e l d dependence, t e m p e r a t u r e dependence, and p r e s s u r e dependence of t h e r e l a t i v e abundances of t h e d e s o r p t i o n s p e c i e s , and t h e s u r f a c e m a t e r i a l s p e c i f i t y .

11. FORMATION OF H? AND NHq

H ~ + i s known t o e x i s t s i n c e J. J. Thomson's mass s p e c t r o s c o p i c s t u d i e s . I n f i e l d i o n e m i s s i o n many i n v e s t i g a t o r s have observed H ~ + and concluded t h a t i t i s a f i e l d and s u r f a c e induced p r o d u c t , o r i g i n a t e d from p r o t r u d i n g s u r f a c e atoms i n t h e f i e l d r a n g e of 2.0 t o 2.5

v/A.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986253

JOURNAL DE PHYSIQUE

The s t u d y of H3 b e h a v i o r a l s o prompted u s t o l o o k i n t o t h e f o r m a t i o n of NH3 by i n t r o - ducing a d d i t i o n a l l y N2 i n t o t h e system. We f i n d t h a t i n t h e p r e s e n c e of a n H2-N2 mixed g a s , t h e p u l s e d - l a s e r f i e l d d e s o r p t i o n mass spectrum c o n t a i n s

NV

The q u e s t i o n of i n t e r e s t i s , however, what i s t h e f o r m a t i o n mechanism of NH3 a t s o l i d s u r f a c e s . S i m i l a r works have been done by some o t h e r a u t h o r s , b u t t h e i r r e s u l t s a r e n o t i n complete agreement. Here we r e p o r t a f i e l d i o n e m i s s i o n experiment, i n which we w i l l t r y t o s h e d some l i g h t on t h i s problem. Some of o u r main r e s u l t s a r e l i s t e d below.

A. E x i s t e n c e of a f i e l d a d s o r p t i o n s t a t e of H3

I n p u l s e d - l a s e r f i e l d d e s o r p t i o n of hydrogen, Hf, H2+ andoH3+ a r e o b f e r v e d , b u t H3+

can o n l y be observed i n a narrow f i e l d r a n g e , from 2.0 VIA t o 3.0 VIA. A t y p i c a l ex- ample ks shown i n Fig. 1 from hcp Hf s u r f a c e . The maximum r e l a t i v e abundance of H3+

Hf SURFACE 100

FIG. 1. F i e l d dependence of t h e r e l a - t i v e abundances of Kl', H2+ and H3+

from a n hcp Hf s u r f a c e .

FIELD STRENGTH (v/A)

o c c u r s around 2.3 v/L t o 2.6 VIA f o r v a r i o u s m e t a l s . The time-gated d e s o r p t i o n image of H3+ from MO shows t h a t H3+ comes m o s t l y from p r o t r u d e d s i t e s of t h e s u r f a c e . These o b s e r v a t i o n s s u g g e s t two p o s s i b i l i t i e s . F i r s t , H3 molecules e x i s t i n a f i e l d adsorbed s t a t e o n l y i n t h i s f i e l d range. Second, i t may i n d i c a t e t h a t a1:hough H3 can e x i s t i n a wider f i e l d r z n g e , i t cannot be f i e l d i o n i z e d below 2.0 V/A, and can- n o t be d e t e c t e d above 3.0 V/A s i n c e H3 w i l l be f i e l d d i s s o c i a t e d i n t o H2+ and H+.

At t h e low f i e l d l i m i t of d e t e c t i n g H3+, H3 may e x i s t i n an a d s o r p t i o n g t a t e of zome s u r f a c e s w i t h o u t a n a p p l i e d f i e l d . A s h o r t range s u r f a c e f i e l d of 1 V/A t o 3 V/A always e x i s t s v e r y c l o s e t o a m e t a l s u r f a c e , and a n H3 may e x i s t i n an a d s o r p t i o n s t a t e on a rough s u r f a c e . We, however, cannot answer t h i s q u e s t i o n u n e q u i v o c a l l y w i t h t h e p r e s e n t e x p e r i m e n t a l technique.

I n a h i g h e l e c t r i c f i e l d of 2 VIA ^{t o} 3 v / L , t h e f i e l d induced d i p o l e - d i p o l e i n t e r a c - t i o n between H2 and H amounts t o 0.04 eV t o 0.06 eV. H3 a r e t h e r e f o r e s t a b l e i n high e l e c t r i c f i e l d r e g i o n where a chemisorbed H and a f i e l d adsorbed H2 can bound t o - g e t h e r .4

B. H3+: The mechanism of f o r m a t i o n

I n t h e t e m p e r a t u r e dependence s t u d y o f H3+ f o r m a t i o n , t h e t o t a l s i g n a l i n t e n s i t y , I F ~ + H ~ + + H ~ + , a s w e l l a s H3+ s i g n a l i n t e n s i t y d e c r e a s e w i t h i n c r e a s i n g t i p temperature.

The r e l a t i v e abundance of H3+ i s a l m o s t c o n s t a n t below 120K, b u t d e c r e a s e s more r a p i d l y t h a n Kl' o r H2+ above 120K, a s shown i n Fig. 2. T h i s t e m p e r a t u r e dependence behavior of t h e r e l a t i v e abundance of H3+ a g a i n shows t h a t H ~ + i s d e f i n i t e l y formed from f i e l d adsorbed H3, and t h a t i n p u l s e d - l a s e r f i e l d d e s o r p t i o n H3+ i o n s a r e formed by f i e l d i o n i z a t i o n o f t h e r m a l l y desorbed H3 molecules above t h e c r i t i c a l d i s t a n c e of

C. H3: A s u r f a c e c a t a l y z e d and f i e l d enhanced product

The f i e l d dependence of t h e r e l a t i v e abundances of Kt, H ~ + and H3+ from 20 t r a n s i t i o n

p .:10 X 1 0 ~ ~ Torr

z

FIG. 2. T o t a l abundance of H+

+

+

0 a f u n c t i o n of s u b s t r a t e t e m p e r a t u r e . Absolute

V)

0 L0 80 120 160 m 260 abundance and r e l a t i v e abundance of H ~ + as a f u n c t i o n of s u b s t r a t e temperature. The de- c r e a s e i n t h e s i g n a l i n t e n s i t

a t a f i e l d s t r e n g t h of 2.4 V/

= 10 X 10-~ Torr a thermal d e p o p u l a t i o n of t h e f i e l d adsorbed H3 molecules.

z

V)

0 L0 80 120 160 200 X0 TEMPERATURE ( K )

m e t a l s u r f a c e s h a s been s t u d i e d . 3 On some f c c m e t a l s u r f a c e s , f o r example, N i , Cu, Ag, Ir and Rh, no H 3 i s formed. The maximum o b t a i n a b l e r e l a t i v e abundances of H3 can be a s high a s 30% t o 40% f o r some hcp m e t a l s , and a b o u t 12% t o 20% f o r some bcc m e t a l s . Also t h e s e r e l a t i v e abundances of H ~ + from m e t a l s u r f a c e s have l a r g e r v a l u e s f o r m e t a l s on l e f t hand s i d e of t h e p e r i o d i c t a b l e , w h i l e t h e y have s m a l l e r v a l u e s on r i g h t hand s i d e of t h e p e r i o d i c t a b l e . T h i s m a t e r i a l s p e c i f i c i t y c l e a r l y demon- s t r a t e s t h a t H3 i s a s u r f a c e c a t a l y z e d p r o d u c t , b u t enhanced by t h e a p p l i e d f i e l d . D. NH3: The mechanism of f o r m a t i o n

I n t h e p r e s e n c e of a n H2

-

f i e l d adsorbed s t a t e . T h e r e f o r e t h e p r o b a b i l i t y of d e t e c t i n g NH3+ i s s m a l l e r i n p u l s e d f i e l d d e s o r p t i o n . T h i s a l s o shows t h a t NH3 i s n o t formed by t h e a p p l i e d f i e l d , b u t by a t r u e s u r f a c e c a t a l y z e d r e a c t i o n . NH2 molecules once formed, due t o t h e i r l a r g e r b i n d i n g e n e r g y w i t h t h e m e t a l s u r f a c e t h a n t h a t of NH3 molecules, w i l l s t a y on t h e s u r f a c e . Thus i s more r e a d i l y d e t e c t e d i n pulsed l a s e r f i e l d d e s o r p t i o n . The r e l a t i v e abundance of N H , ~ and N ~ + degends s t r o n g l y on t h e f i e l d s t r e n g t h , a s shown i n F i g . 3. At a f i e l d above 1.7 V/A, e f f e c t i v e i o n i z a t i o n can occur f o r both NHn+ and N At low f i e l d , NH, may s t i l l be formed but cannot be d e t e c t e d . The de- t e c t e d NH,3+lomes from f i e l d a d s o r p t i o n s t a t e , o r from p h y s i c a l a d s o r p t i o n s t a t e . A q u e s t i o n i s what i s t h e f o r m a t i o n mechanism o f t h e %+. When t h e p a r t i a l p r e s s u r e of H2 - N2 mixed g a s r e a c h e s 10-8 T o r r , t h e number of chemisorbed atoms a l r e a d y s a t u - r a t e because of a f i e l d enhanced g a s supply. The r a t e of f o r m a t i o n of NH,,+ cannot be f u r t h e r i n c r e a s e d by i n c r e a s i n g t h e g a s p r e s s u r e . Once a n NH,,+ molecule i s formed, i t w i l l b e bound t o a n apex s i t e of a p r o t r u d i n g s u r f a c e atom. Thus i t can be pulsed- l a s e r f i e l d desorbed. On t h e o t h e r hand, t h e number of N2 i n f i e l d a d s o r p t i o n s t a t e w i l l i n c r e a s e n e a r l y l i n e a r l y w i t h t h e p a r t i a l p r e s s u r e of N2 i n t h e mixed g a s . Thus

JOURNAL DE PHYSIQUE

t h e r e l a t i v e abundance of NY,+ obsenred w i l l d e c r e a s e w i t h i n c r e a s i n g g a s p r e s s u r e a s shown i n Fig. 4. T h i s c o n c l u s i o n i n d i c a t e s t h a t t h e NI+, i s formed from chemisorbed n i t r o g e n and hydrogen atoms.

Fe SURFACE

H2 (75%) - N2 MIXTURE (16' TORR) NH+;

NHS

0.8

t N

z

\ > ^0.6

*I

+>

-

z

g

0

a 4 U

0.0

FIG. 4. The f r a c t i o n , ( N H ~ + + N H ~ + ) / N ~ + , a s a f u n c t i o n of t o t a l g a s p r e s s u r e . The r e l a - t i v e i n t e n s i t i e s of N%+ depend s t r o n g l y on the g a s p r e s s u r e even f o r a g i v e n composition of t h e mixed gas.

- ^Fe _H2(70%) ^SURFACE - ^{N2 MIXTURE}

FIELD STRENGTH 1.78 v/A

- ^{80 K}

, , \ , , , ,

NG

-

1.0

I

,

k ,

1.5 2.0

FIELD STRENGTH (V/&

TO* 2 4 6 IO-? 2 4 6 1 6 ~

TOTAL PRESSURE (TORR)

E. NH3: M a t e r i a l s p e c i f i c i t y

The amount of W+ d e t e c t e d , w i t h i n t h e m a t e r i a l s so f a r we have s t u d i e d , i n c r e a s e s a c c o r d i n g t o t h e sequence,

N i X W < T i 3 Ta << CO < Fe.

Although NH,,+ can be found from a l l t h e above m e t a l s u r f a c e s , s u b s t a n t i a l amounts of

%+ a r e o b t a i n e d o n l y from CO and Fe s u r f a c e s . The above sequence a g r e e s f a i r l y w e l l w i t h t h e known c a t a l y t i c a c t i v i t y o f m e t a l s i n ammonia s y n t h e s i s . A s l i g h t d i s c r e p - ancy may b e due t o t h e v e r y low t e m p e r a t u r e of o u r experiment, o r may be due t o t h e e f f e c t of t h e a p p l i e d f i e l d . I n t h i s experiment t h e a p p l i e d f i e l d i s a l r e a d y con- s i d e r a b l y lower t h a n most o t h e r f i e l d i o n e m i s s i o n experiments. Our observed i s a s u r f a c e c a t a l y z e d p r o d u c t , b u t may a l s o be promoted by t h e a p p l i e d f i e l d .

No NH,+ i s observed below 50K. NHn+ s i g n a l i n t e n s i t y a p p e a r s when t h e t e m p e r a t u r e i s r a i s e d g r a d u a l l y . The N H ~ + s i g n a l i n t e n s i t y i n c r e a s e s r a p i d l y and becomes a dominant one when t h e s u r f a c e t e m p e r a t u r e i s r a i s e d above 120K. The t e m p e r a t u r e dependence of NZ+ and NH,+ a r e shown i n Fig. 5. Above t h i s t e m p e r a t u r e , b e s i d e s NH,+ and NZ+ mass l i n e s , a d d i t i o n a l mass l i n e s of mass 23 t o 84 a r e observed. They a r e t e n t a t i v e l y i d e n t i f i e d t o be (NH,.,)2+, Fe (NH,)*, n=2,3, FeN2+ o r N3+, Fe+, Fe$ and FeN2+, r e - s p e c t i v e l y .

P") /pN* = ¹⁰

S

,S-

Q, e

>

C CO FIG. 5. S i g n a l i n t e n s i t y o f N%+ and N2+ a s

0 10 80 120 160 X X ) 240 280

W ,- a f u n c t i o n of t h e s u b s t r a t e t t m p e r a t u r e a t

( b ) f i e l d s t r e n g t h s of ( a ) 1.8 V / A , and (b)

1.7 V/& 1.7 V/I.

J

S

CO

TEMPERATURE ( K )

*This work was s u p p o r t e d ,jy DOE.

REFERENCES

/l/ See H. D. Beckey, F i e l d I o n i z a t i o h Mass Spectrometry (Pergamon, New York, 1971).

/ 2 / Tsong, T. T., S u r f a c e S c i .

70,

/ 3 / A i , C. F. and son^, T. T., S u r f a c e S c i .

138,

/4/ Tsong, T. T., Kinkus, T. J. and A i , C. F., J. Chem. Phys. 78, (1983) 4763.

/5/ A i , C. F. and Tsong, T. T., J. Chem. Phys.

2,

/6/ L i u , W., L i u , Y. and Tsong, T. T., t o be p u b l i s h e d .