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CHEMISORPTION OF CO AND METHANATION ON Rh SURFACES AT LOW TEMPERATURE AND
LOW PRESSURE, AN ATOM-PROBE FIM STUDY
W. Liu, D. Ren, C. Bao, T. Tsong
To cite this version:
W. Liu, D. Ren, C. Bao, T. Tsong. CHEMISORPTION OF CO AND METHANATION ON Rh SURFACES AT LOW TEMPERATURE AND LOW PRESSURE, AN ATOM-PROBE FIM STUDY.
Journal de Physique Colloques, 1987, 48 (C6), pp.C6-487-C6-492. �10.1051/jphyscol:1987680�. �jpa-
00226888�
CHEMISORPTION OF CO
AND
METHANATION ON R hSURFACES
AT LOW TEMPERATURE AND LOW PRESSURE, AN A T O M - P R O B E FIM S T U D YW. ~iu*, D.M. Ren, C.L. Bao and T.T. Tsong
Physics Department,The Pennsylvania State University, University Park, PA 16802, U.S.A.
ABSTRACT
-
Pulsed-laser imaging atom-probe and high resolution voltage pulsed atom-probe were employed to study the chemisorption behavior of CO on rhodium surfaces at low temperature and low pressure. The results are consistent and interesting. Our results support dissociative chemisorption on stepped sur- faces of Rh and the effect of the surface structures. We also carried out methanation on Rh surfaces under adverse conditions and identified the inter mediates of methanation with an isotope exchange technique. Our results led us to conclude that a dissociative mechanism is responsible for methanation on Rh surfaces under the condition of our experiment.1. INTRODUCTION
The adsorption of diatomic molecules CO, NO, N on transition metal surfaces has been studied by a variety of experimental techniques. Dissociative adsorption of such diatomic molecules has been clearly demonstrated on a variety of transition metal surfaces/l/ and Broden et al. correlated the trend of dissociation with the position of the adsorbent in the Periodic Table. There are also some expectations, such as the anomalous dissociation of CO on high Miller index Pt surfaces/2/ and stepped Rh surfaces/3/, which has created some confusion and controversy. Since Rh has been revealed to have unique ability to selectively produce methane and low-carbon chemi- cals/4/ and the importance of methanation related to modem efforts of coal gasifica- tion and liquefication, we are attracted to this problem.
It is an accepted view that only under high pressure (about or above 1 bar) one can expect to observe methanation/5/. Therefore it is very difficult to obtain informa- tion on the atomic steps of methanation by surface science techniques. As one tries
*~epartment of Physics, The Huazhong Normal University, Wuhan, Hubei, The People's Republic of China.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987680
C6-488 JOURNAL DE PHYSIQUE
to transfer a sample from high pressure chamber to the vacuum for analysis, the pres- sure and temperature are completely changed and the reaction ceases to occur. One consequence of such experimental procedure is that the surface composition of the sample during the reaction may be quite different from that during the analysis
-
afact that should always be remembered for such "post analysis". Weakly adsorbed species will not be present while other species not adsorbed during the reaction may adsorb during the cooling and may stay on the surface for analysis. The success to carry out methanation at low pressure and low temperature made us able to study the atomic steps of methanation in situ. We report here a preliminary study of chemi- sorption of CO and methanation on Rh surfaces.
2. EXPERIMENTAL METHOD
Pulsed-laser imaging atom-probe (PLIAP) and high resolution voltage pulsed atom-probe (HRVPAP), which have been presented elsewhere/6/, were employed in this study.
The chamber is always baked overnight to reach vacuum in the range of 10-10 Torr for the PLIAP, and 10-9 HRVPAP respectively. After field evaporation to clean the sample surface and to develop well defined crystal planes on the atomic scale, we can only detect a few adsorbed particles in one hundred laser pulses. This shows little con- tamination of the surface.
The experiments were carried out under gas flowing conditions (or dynamic gas supply mode) by adjusting the opening of the main valve connecting diffusion pump and the leak valve for admitting reactant gases.
We also would like to point out here that the gas supply toward the emitter surface is greatly enhanced by the polarization force. This effect and field adsorption greatly improve the coverage of reactants on the surface, thus do enhance the reac- tion. In addition, IAP has the exceptional sensitivity to detect even only a few atoms or molecules on the entire emitter surface. Thus we are able to study hetero- geneous catalysis at low pressure without interrupting or changing the reaction.
Such unique capability has been presented in our previous work/7/.
3. RESULTS
THE ADSORPTION BEHAVIOR OF CO ON Rh SURFACES AT 150K (1) With PLIAP we have studied three coverage sequences:
a) A clean surface is saturated at 150K with CO: Only CO+ can be detected even if a mixture of CO and Hg is introduced later.
b) A clean surface is saturated at 150K with Hz, then a mixture of CO and Hg is introduced: H+ and are detected first, and CO+ can be detected after some time.
c) CO is introduced at low pressure to a clean surface and only partial cover- age is allowed: Besides the ions of mass 28(~0+), we detected the ions of the following masses: 12, 16 and 44. Since there are only CO and residual Hg in our chamber, we identified the newly detected ions to be c+, O+ or C@ and C O ~ respectively/8/.
( 2 ) The field dependence of relative abundance of 0+ and CO+: the relative abundance
of O+ is 37% at field Z V / ~ but 19% at field 2.3~11.
(3) We studied three different sample orientations: ~h(001), (111) and (1.13). We find no dissociation of CO from (001) and (111) orientation tips, so that=
of our data were collected from (113) orientation samples.
(4) In order to study the structure effect on dissociation of CO, we adopted HRVPAP to collect data from different areas covering only a few atoms. After aiming the probe hole at a certain region, we evaporated the sample with high voltage pulse to collect data (Fig. 1). The spectra from planes (001) and (111) are the same: little dissociation of CO is evident. The spectrum obtained from (113) plane is basically similar to the previous one, but it now shows a dissociative tendency. However, we detected plenty of C* and 0+, besides CO+ and ~ h + + , from the stepped surfaces of the (001), (111) and (113) planes and vicinities Fig. 1-(C). The most interesting result is that the spectrum from the terrace
b u t remarkably d i f f e r e n t from t h e d a t a of t h e same r e g i o n b u t w i t h a much h i g h e r d e n s i t y of p l a n e edges o r s t e p s .
( 5 ) W e found t h e p r e s s u r e of CO t o be most c r i t i c a l . I n o u r c a s e , o n l y when t h e p r e s s u r e of CO i s m a i n t a i n e d a t about 1x10-8 T o r r o r l e s s , does t h e d i s s o c i a t i o n of CO become s i g n i f i c a n t . When we changed t h e p r e s s u r e o n l y a l i t t l e from 1x10-8 T o r r t o 0.8~10-8 T o r r o r 1 . 2 x 1 0 - ~ T o r r , t h e r e l a t i v e abundance of d e t e c t - ed O+ r o s e t o 5.5% o r dropped t o 1.0% from t h e o r i g i n a l 2.8%.
9 0
Sample: Rn 11111
80 Sample: Rh 11131
Gas: CO ll~1o~Iorrl bar: CO ll.lO+Torrl
8 50
p no
f 30
; 20
0 10 20 , 30 40 5 0
( A ) Mass-to-Charge Ratlo law1 o f 100s ( 5 ) Mars-to-Charge Ratio laaul o f Ions
! Sam~le: Rhllll) Steps
(C) ~arr-to-Charge Ratio lanu1 of tons-
Sample: Rnlllll S ~ C D I Gar: CO Il=iO'Torrl
ti
SO
4
0 10 PO 30 40 50
(D) Wars-to-Charge Ratio Ism) o f 10"s
Fig. 1. The s p e c t r a of d i f f e r e n t probing a r e a s w i t h HRVPAP under t h e same c o n d i t i o n .
METHANATION
( 1 ) S i n c e i t i s hard t o g e t r i d of r e s i d u a l H z , we o f t e n d e t e c t e d i o n s of mass 2 .and 18 b e s i d e s 6 , 1 2 , 1 6 , 28 and 44 when we observed d i s s o c i a t i o n of CO w i t h t h e PLIAP. I n t r o d u c i n g t h e mixed g a s of CO and H2 i n s t e a d of p u r e CO a t t h e same p r e s s u r e , we d e t e c t e d much more i o n s of mass 16 and-18 b e s i d e s 28, b u t few 44.
We a l s o d e t e c t e d i o n s of mass 1 2 , 13, 1 4 , 1 5 , 17 and 19 sometimes. We b e l i e v e we have d e t e c t e d methanation on Rh s u r f a c e . According t o t h e f o l l o w i n g reac- t i o n s :
co
'
cad+
oad ( 1 )H2 2Had ( 2 )
Oad + 2Had + H20 ( 3 )
we i d e n t i f i e d t h e i o n s newly d e t e c t e d t o be
~ 4 ,
H20+, CH+, CH?,~ 3 ,
HO+ and H ~ o H + , r e s p e c t i v e l y .C6-490 JOURNAL DE PHYSIQUE
(2) To confirm our identification, we introduced a mixture of CO and D2 instead of the one of CO and Hz. We got a shift of spectral lines as it should to convince us of the above reactions/8/.
( 3 ) Under certain narrowly defined experimental conditions we observed an "oscilla- tion" between signal intensities of mass about 16 and that of mass 28 (Fig. 2).
The pressure was about 5x10-~ Torr and the repetition rate of laser pulses was 1Hz for the recorded oscil-
lation. As the flight path of the PLIAP is too short
B *---The products of methanation
and the intensities of n--- Carbon monoxide
signals change in a wide
I 1 I 2 I
3 I
range, the peak of mass 16 I
sity of laser pulses, the periods of oscillation are
2
not exactly the same. A 40 large desorption of CO in 2 the middle of third period c? 20
made this look like two
periods. Changing the 0
---
0 5 10 15 20
repeating rate of laser 25
pulses to 0.5Hz or 2Hz, Number o f Laser Pulses the oscillation damped out
but the relative abundance Fig. 2. An oscillation of relative of the products of metha- abundances of CO and the products nation reduced to 65% or of methanation on Rh surfaces.
raised to 96% respectively from the original 84%.
(4) Raising the temperature of Rh tip from 150K up to 300K, the relative abundance of
c H ~
among all the detected ions raised from 18% to 32%.4 . DISCUSSION
First, we can conclude that the detected ions C+ and 0+ are not artifacts of field dissociation of CO. Otherwise, the relative abundance of O+ would increase with in- creasing field strength in the PLIAP case, or the spectra collected from different planes would be the same in the HRVPAP case. Our results are exactly the opposite.
Comparing the spectra gotten with the HRVPAP, we came to the conclusion that only steps and kink sites play a major role in promoting dissociation of CO, as suggested by Carstner and Sommorjai/9/.
Though the thermal desorption is a gentle process, the heating of laser pulses can raise the temperature of the emitter surface to about 300K and then drops down in a few ns/lO/. Such heating should facilitate the reaction as well as desorbing adsor- bates. This may explain why we can observe the dissociation of CO and carry out methanation on Rh surfaces at such low temperature and pressure, and can also detect the intermediates.
Now, we would like to discuss the mechanism of the observed oscillation. Our tenta- tive explanation is the following: Methanation depends on the amount of disso- ciated CO existing on the surface. When the coverage of CO is low, the dissociation of CO is effective and the detected signals show a large amount of methanation pro- ducts and little CO+. As the coverage of CO increases, desorbed signal of CO in- creases gradually, but the one of methanation products remains relatively constant at first, then decreases gradually. Meanwhile, repulsive interaction between adsor-
s u r f a c e i s r e l a t i v e l y c l e a n a g a i n and a new p e r i o d begins. The r e s u l t s a f t e r chang- i n g t h e r e p e t i t i o n r a t e of l a s e r p u l s e s confirmed s u c h a n i n t e r p r e t a t i o n . So t h a t t h e o s c i l l a t i o n i m p l i e s t h a t under o u r e x p e r i m e n t a l c o n d i t i o n s , d i s s o c i a t i o n of CO i s t h e dominant f a c t o r i n t h e methanation on Rh s u r f a c e , and t h e d i s s o c i a t i v e mecha- nism i s r e s p o n s i b l e f o r methanation.
The l a s t q u e s t i o n we a r e g o i n g t o c o n s i d e r i s how o u r s t u d y i s r e l a t e d t o t h e r e a l c a t a l y s i s . The e x p e r i m e n t a l c o n d i t i o n s d i f f e r c o n s i d e r a b l y from t h o s e i n " r e a l "
c a t a l y s i s , mainly i n g a s p r e s s u r e and r e a c t i o n t e m p e r a t u r e as w e l l a s i n s u r f a c e s t r u c t u r e and composition. The b a s i c mechanism are probably v e r y d i f f i c u l t t o inves- t i g a t e under r e a l c a t a l y s i s c o n d i t i o n s . So we s h o u l d t r y t o b r i d g e t h e s e gaps. I n o u r experiment, t h e p r e s s u r e gap i s probably t h e most s e r i o u s problem. I n t h e heterogeneous c a t a l y s i s , t h e main f a c t o r r e l a t e d t o g a s p r e s s u r e i s t h e r e a c t a n t con- c e n t r a t i o n of t h e c a t a l y s t s u r f a c e . It i s o f t e n s t a t e d t h a t a c a t a l y t i c r e a c t i o n , which can o c c u r o n l y under h i g h p r e s s u r e t o have a s a t u r a t i o n s u r f a c e coverage, can- n o t occur a t low p r e s s u r e ; m e t h a n a t i o n i s a n example. Apart from t h e f a c t t h a t high s u r f a c e c o n c e n t r a t i o n , of c o u r s e , c a n a l s o e a s i l y be reached i n low p r e s s u r e s t u d i e s simply by l o w e r i n g t h e t e m p e r a t u r e , t h i s s t a t e m e n t i s n o t g e n e r a l l y c o r r e c t . Accord- i n g t o a complicated i n t e r p l a y of v a r i o u s p r o c e s s e s a t t h e s u r f a c e , t h e s t a t i o n a r y s u r f a c e composition i s dependent i n a complex manner on t h e d i f f e r e n t r a t e p r o c e s s e s i n v o l v e d , s o t h a t e v e n t u a l l y t h e a c t u a l coverage may become r a t h e r low even a t h i g h p r e s s u r e . The m e t h a n a t i o n r e p o r t e d i n t h e p a s t were c a r r i e d o u t a t about o r above 500K. It i s h i g h e r o r around t h e d e s o r p t i o n peak t e m p e r a t u r e of Hp and CO, s o t h a t t h e coverage on Rh s u r f a c e of t h e s e g a s e s may n o t r e a c h t h e s a t u r a t i o n coverage e i t h e r . R e a l i z i n g t h a t o n l y a monolayer r i g h t above t h e c a t a l y s t s u r f a c e can p a r t i - c i p a t e i n t h e c a t a l y t i c r e a c t i o n , we do n o t t h i n k t h e "gap" of g a s p r e s s u r e w i l l c a u s e a completely d i f f e r e n t r e a c t i o n p a t h even i f t h e coverages a r e not q u i t e d i f f e r e n t .
Regarding t h e i n f l u e n c e of t e m p e r a t u r e , q u a n t i t a t i v e c o n s i d e r a t i o n s s u g g e s t t h a t t h e s t e p s i n a d i s s o c i a t i v e mechanism w i l l have a h i g h e r p r o b a b i l i t y ( e n t r o p y f a c t o r ) b u t a l s o h i g h energy b a r r i e r s (energy f a c t o r ) t h a n t h e i r a s s o c i a t i v e c o u n t e r p a r t s . Hence a d i s s o c i a t i v e mechanism may b e f a v o r e d a t h i g h t e m p e r a t u r e w h i l e a t low tem- p e r a t u r e a s s o c i a t i v e mechanism may p r e v a i l . Our r e s u l t shows t h a t a t room tempera- t u r e t h e m e t h a n a t i o n i s much more e f f i c i e n t t h a n a t 150K.
Our s t u d y i s , of c o u r s e , i n t e n d e d t o f i n d t h e d e t a i l e d atomic s t e p s of methanation o n Rh s u r f a c e . Even though, under t h e c o n d i t i o n s of o u r experiment, t h e r e a c t i o n a s w e l l a s t h e d i s s o c i a t i o n i s v e r y i n e f f i c i e n t , n e v e r t h e l e s s , t h e r e a c t i o n mechanism d e r i v e d may shed some l i g h t on t h e mechanism i n r e a l c a t a l y s i s . The v e r y h i g h s e n s i - t i v i t y of t h e atom-probe makes it p o s s i b l e t o s t u d y methanation under extremely ad- v e r s e c o n d i t i o n s .
5 . ACKNOWLEDGEMENTS
The a u t h o r s wish t o e x p r e s s t h e i r s i n c e r e a p p r e c i a t i o n t o S. Brooks McLane f o r h i s h e l p i n e l e c t r o n i c s , T h i s work was s u p p o r t e d by DOE u n d e r Grant Number DE-ACOZ- 81ER10857.
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C6-492 JOURNAL DE PHYSIQUE
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