A STUDY OF THE DECOMPOSITION REACTION OF METHANOL OVER Ru FIELD EMITTER SURFACES

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A STUDY OF THE DECOMPOSITION REACTION OF METHANOL OVER Ru FIELD EMITTER

SURFACES

G.-K. Chuah, N. Kruse, G. Abend, J. Block

To cite this version:

G.-K. Chuah, N. Kruse, G. Abend, J. Block. A STUDY OF THE DECOMPOSITION REACTION

OF METHANOL OVER Ru FIELD EMITTER SURFACES. Journal de Physique Colloques, 1986,

47 (C7), pp.C7-435-C7-440. �10.1051/jphyscol:1986773�. �jpa-00225968�

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A STUDY OF THE DECOMPOSITION REACTION OF METHANOL OVER Ru FIELD EMITTER SURFACES

G.-K. CHUAH, N. KRUSE, G. ABEND a n d J.H. BLOCK

F r i t z - X a b e r - I n s t i t u t der Max-Planck-Gesellschaft, F a r a d a y w e g 4-6.

D-1000 B e r l i n 33. F . R . G .

Abstract

-

The decomwsition of methanol on Ru f i e l d e m i t t e r t i o s has been studied by pulsed f i e l d desomtion mass s m t r o m e t r y i n t h e t e m r a t u r e ranqe from 300 K t o 580 K. Various i o n i c species a r e 4etected includinq t h e narent molecular i o n , CH30H+, t h e ~ r o t o n a t e d molecule, CH30H2+, dehydroqenated svecies , C O H ~ + ( x = 0.. . 3 ) , CHI+ and R u 4 ; f ( y = 1.. .3, n = 1.. .2). A t temperatures b e l a , 460 K, hiah i n t e n s i t i e s of C H ~ + a r e observed, which c o r r e s d t o t h e t o t a l oxyqen content of R d Y n + . These ions a r e oresunably formed from M30ad. For T 7 460 K , t h e intensities of M3+, C O H ~ + and droD while those of COH+ and CO+ increase a d reach maxima j u s t a t t h e thermal d e s o m t i o n t e m e r a t u r e of CO from Ru. It is conclude3 t h a t t h e stea'ly decomwsition o f methanol below 460 K is prevented by adsorbed CO.

I. ImODumION

H i s t o r i c a l l y , t h e i n t e r a c t i o n of methanol with f i e l d e m i t t e r surfaces s t a d s f o r one of t h e f i r s t attemDts t o use f i e l d inmulse techniques t o study chemical Drocesses i n adsorb& layers. Inqhram and Gomer / I / , i n an i n v e s t i s a t i o n on oxidized tunasten, claimed detection of CF1 c)+ ions a& considered t h i s a s roof f o r t h e d i s s o c i a t i v e

3

chemisomtion of t h e methanol molecule. Beckey e t a l . /2/ r e w r t e d on corrosion ef- fects of t h e i r P t e m i t t e r s durinq r e a c t i o n with CLI OH i n the presence of e l e c t r i c a l

3

f i e l d s . This r e s u l t e d i n t h e rapid bluntinq of t h e tiws s o t h a t systematic s t u d i e s under well defined exoerimental conditions were not ~ o s s i b l e a t t h a t time. I n t h e meantime, PFDMS (wised f i e l d tiesorottion mass swctrometry) has been f u r t h e r devel- o d and s u c e s s f u l l y a m l i e d t o e l u c i d a t e t h e k i n e t i c s of o t h e r reaction s y s t e m /3,4/. The r e i n v e s t i q a t i o n of t h e methanol reactions on f i e l d emitter surfaces is p a r t l y prompted by t h e f a i l u r e durina e a r l y attempts t o e s t a b l i s h k i n e t i c d e t a i l s about Drocesses i n t h e adsorbed layer. More generally, t h e favourable oathways of t h e methanol decommsition t w a d . . CO and H2 a r e still unknown. I n a number of in- v e s t i a a t i o n s on mcrosconic s i n n l e c r y s t a l surfaces d i s s o c i a t i v e adsomtion was found t o be associated with t h e formation of a methoxy smxies, CH30 /5

-

^8/.^{W e}

nresent here the f i r s t rp-sults f o r t h e Gi30!i r l e c m s i t i o n cv,. -r Qu f i c l 3 e n i t t - r surfaces.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986773

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C7-436 JOURNAL DE PHYSIQUE

11. EXPERIMENTAL

In PFDMS, fast neqative field mlses (amlitudes u~ to 20 kV, rise time2 100 ns, re- petition rates UD to 100 kHz) are awlied to a counter electrode, which is mounted in front of the Ru tin. Thereby the layer a+sorberl on the tio surface is desorbed and the res-tive ions are chemically analysed by time-of-fliqht mass s~ectrosco~y.

Between the ~ulses an arbitrary field strenqth, FR, can be maintained. A channel plate imaqe intensifier is mounted at the entrance of the fliqht tube and allows the orientation of the tip to be determined. Tip tem~eratures are measured by means of a chromel/alumel thermocouple spotwelded to the emitter tip. Ruthenium was cut by spark erosion from a boule and electrolytically etched with dilute HC1. Cleaninq of the surface was performed in situ by field evawration. The methanol used was Merck analytical qrade and carefully deaassed by several freeze-thaw cycles.

111. RESULTS

The reaction of methanol with Ru has been studied by probinqel50 atomic sites of the stepped region in the vicinity of the (001) pole of the field emitter surface.

Fiq. 1 shows a typical mass spectrum obtained for a pulse fraction of 63 % at a pulse re~etition rate of f = 4000 Hz, i.e. at a r9action time

5

⁼^{2 5 0 ~}see. Variouq ionic soecies are detect&. Besides the parent molecular ion, CH O,'H the protonat- ed molecule, CH30H2+ as well as the dehdmenated snecies , COHx

1

( x = 0.. .3 ) , are observed. Hiqh intensities of CH3+ are seen and found to corresmnd to the total oxygen content of Ru

-

oxides, RuO~+ (y = 1...3, n = 1.2). ~~(03): ions (z = 1...3)

Y

may also be detected. They are absent in Fiq. 1 but anpear at lonqer reaction times, tR b 2ms. Carbon containinq sbecies,

d"

and R&+, are only observed at hiqh temp eratures,T>580 K.

Fiq. 1

-

Schematic mass smctrum obtained by pulsd field desorption of CH30H during the decomposition over Ru. The plotted intensities of the lo2Ru species

refer to the isotope distribution of Ru. Experimental parameters:

desorption field strength = 24 V/nm (pulse heiqht = 15 V/nm, base field FR = 9 ~ / m ) f = 4000 Hz, i.e. tR = 250 ps; p = 1.3-lo5 Pa CW30H; T = 305 K

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desorked a t a hiqh an3 constant r a t e . Within t h e measured temoerature ranqe, T = 300.. -460 K, equal ion i n t e n s i t i e s of CH3+ and L y

. RuO"'

can be understood by

Y

assumina t h a t t h e s e ions a r e formed from adsorbed methoxy smies, CH3Od. Only a t hiqher temoeratures, T,460 K, geviations from t h e observed i n t e n s i t y r e l a t i o n occur. While t h e RuQ

'+

species raDidly disacpear from t h e mass s p e c t r a , t h e CH3+

Y

i o n i c r a t e decreases more slowly. This behaviour is not y e t u n d e r s t d i n d e t a i l .

10

1

3

-

0 0 1

001 . .

3W GO ' 500 600

Temperature [ K 1

( a , b) Ion i n t e n s i t i e s of t h e various swecies as a function of t h e s u r f a c e t m m r a t u r e , with t h e same e x ~ r i m e n t a l Darameters a s i n Fiq. 1.

a+,

a~+,

C O V ~ disolayed s m a t a t e l y f o r k t t e r v i s i b i l - i t y . note t h e l i n e a r i n t e n s i t y scale.

Temperaturh I K I

I

( c ) l'hernal desomtion saect- rum of CO a d !I2 a f t e r CH3QR decomosition a s measure3 by J.Srbek et a l . , /R/.

Tempera lure I K 1

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C7-438 JOURNAL DE PHYSIQUE

The absence of RuO n+ a t hiqh temperatures mav be evidence f o r oxygen d i f f u s i o n o u t Y

of t h e monitored a r e a i n t o t h e low f i e l d regions of t h e e m i t t e r surface. Such a orocess must always be consider& f o r e l e c t r o n m a t i v e a i s o r b a t e s i n ( w s i t i v e ) r2action f i e l d s and has k e n ohserved by Okuyama f o r t h e oxi5ation of rnolyWenum f i e l d emitter surfaces (9).

COH~+ ions a r e a l s o observed, thouc~h, t h e i r i n t e n s i t i e s a r e much smaller than those of C H ~ + . The hiqh i o n i c r a t e s of C O H ~ + f o r T <460 Y can only be understood i n terms of desorwtion from t h e chemisorbed layer. D e t a i l s w i l l be 3iscussed below.

The narrow temperature ranqe T = 460...520 R is characterize3 bv t h e c o m l e t e 3 s - a-arance of (Xi2+ an3 QuO ^"+ and t h e simultaneous cteveloment of i n t e n s i t y maxima of COH+ ancl a+. The occurrence of Y COH+ and orovides evidence f o r t h e c a t a l y t i c . 3 e c o m s i t i o n of t h e Cd3Q61 volecule. The increase of t h e i r i n t e n s i t i e s reflects t h e enhancement of t h e reaction r a t e . I n t e r e s t i n q l y , the maxima of W+ anrl CO+ coin- c i d e with t h e oressure neak 3urina temoerature oroararmed 32somtion of a 3 s o r M

CO f r o m R u (001) as measure3 by H r k k e t a l . / B / .

The thermal a c t i v a t i o n of a3 f a c i l i t a t e s t h e i o n i z a t i o n and desorption as

a+.

A t ad

high temperatures, T>520 K, t h e lifetime ^Tof

mad

with respect t o thermal d e s o p t i o n is s h o r t e r than t h e r e a c t i o n t i m e

5

⁼250 p s e c . Thus t h e CO+ i n t e n s i t y is small. A t low temperatures, T<460 K , ^Tis s u f f i c i e n t l y long. However, t h e f i e l d s t r e n g t h F = 24 V/nm is not l a r g e enough t o y i e l d considerable 00' desorption. The maximum of t h e CO+ i n t e n s i t y a t T h 490 K reflects t h e optimal balance between (therm- a l l y a c t i v a t e d ) f i e l d desorption and thermal desorption.

Special i n t e r e s t concerns t h e behaviour o f t h e COH+ and O H 2 + ion i n t e n s i t i e s i n Fiq. 2. The COH+ species e x h i b i t s e s s e n t i a l l y t h e same temperature dewndence as _{t h e} a + , i.e. it rises f i r s t , w a k s a t T = 460

...

470 K and decreases l a t e r on. On t h e other hand, t h e C0H2+ species s t a r t s t o diminish a t T2460 K and is below t h e d e t e c t i o n l i m i t a t T > 5 1 0 K. It might be suspected t h a t f i e l d d i s s o c i a t i o n of CH30ad accounts f o r t h e observation of COH2+. However, t h i s process requires Ru-O a s w e l l a s C-H bond breaking, which is considered as unlikely. F i e l d induced formation of COH+ and/or ~0I-l~' from

mad

and Had can be excluded s i n c e t h e concentration of Had is small a t temperatures T > 4 2 0 K due t o its thermal desorption as H2. A m o t o n t r a n s f e r during r e a c t i o n of

CO,d

with water o r methanol cannot e m l a i n t h e tempera- t u r e d e c e d e n c e of t h e COH+ species. On account of t h e s e considerations atxi from t h e observed d i f f e r e n c e s i n t h e temperature dependence of COH+ and c0H2+ we conclude t h a t t h e s e species a r e desorbed from d i f f e r e n t chemisorbed s t a t e s .

DISCUSSION

The r e s u l t s obtained s o f a r d i s ~ l a y some i n t e r e s t i n q f e a t u r e s of t h e methanol decom- w s i t i o n . Variable amounts of a H x + ions ( x = 1

. .

- 3 ) have been detected. The measured absolute i n t e n s i t i e s and t h e d i f f e r e n t behaviour of t h e swcies durinq temperature v a r i a t i o n can be understood by f i e l d desomtion from t h e che!nisorh'I

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Vrbek et a l . , by means of e l e c t r o n enerqy l o s s scectroscopy, i d e n t i f i e d adsorbed CY 3 0, bonded v i a t h e oxyqen atom t o t h e surface /a/. The formation of t h i s s m i e s is associated with a decrease of t h e work function suqqestinq a n e t e l e c t r o n trans- f e r , referentially from t h e CH3 qroup, t o t h e metal. Thus t h e r e s u l t i n g s u r f a c e d i p o l e has its s t a b l e o r i e n t a t i o n i n t h e steady e l e c t r i c a l f i e l d , and t h e

referential f i e l d desorption of (TI3+ from CCH3 by C-O bond breakinq becomes under- standable. I n t e r e s t i n q l y , (TI3+-ions w e r e found a l s o rlurinq f i e l d i o n i z a t i o n of t h e '7iqher a l i ~ h a t i c alcohols / 2 / . The CH30+-ion i n / I / is based on a wrong mass s c a l e . Actually mass 32 and 33 a r e present. I n our s t u d i e s on Ru, C O H ~ + i o n s a r e formed i n minor amounts.

F i e l d s t r e n q t h v a r i a t i o n measurements have shown saturated ion i n t e n s i t i e s of CH ⁺ 3 a t r e l a t i v e l y low f i e l d s t r e n q t h values ( d e t a i l s w i l l be ~ u b l i s h e d elsewhere). This oroves both t h e ease of (TI3+ formation an3 its q u a n t i t a t i v e desomtion. Field d i s - sociation of CH30ad and formation of c ~ ~ + leaves oxyqen behind a t t h e surface. How- w e r , continuous f i e l d desomtion a s RuO "+ does not allow f o r its accumulation.

Y

Tince equal i n t e n s i t i e s of CH3+ and E y

.

R ~ O ~ " ' a r e observed, we conclude t h a t Oad is q u a n t i t a t i v e l y desorbed by t h e hiqh frequencv oulses. A t hiqher t e m e r a t u r e s , T 7460 X, t h i s relations hi^ does not hol? any lonqer. The qecrease of t h e RuO "+ in-

Y

"nsitiees is a b m t and cannot be exolained i n chemical t e r n . P r a l i n e e t a l . observed oxvqen oenetration i n t o t h e subsurface reqion of Qu (001) a t temweratures 7.9400 Y /lo/. This process is too slow t o occur under our exoerimental conditions, where we a w l v hiqh frequency oulses t o 3esorb t h e adsorbed soecies continuously.

Moreover, subsequent layer-by-layer f i e l d e v a ~ o r a t i o n qave no evidence f o r dissolved oxyqm. Rather we consider 3 i f f u s i o n of mobile oxyqen o r Ru-oxides out of t h e moni- toreq area. Evi3ence f o r t h e mobilitv of Ru-oxides has been obtained 3urinq a recent stu.ht of t h e oxi3ation of RU emitters by means of PQDMS /11/. Such a orocess may be Womoted by t h e steady reaction f i e l d aoolied durinq our studies. I n t e r e s t i n q l v , t h e decrease of RuO coincides with t h e onset of thermal and f i e l d desomtion of CO

Y ad-

This observation is i n t e r m e t e d i n terms of a blockinq model, where oxyqen d i f f u s i o n is prevented by adsorbed CO a t law t e m e r a t u r e s , T4460 K , and enabled a f t e r thermal

O r f i e l d d e s o m t i o n of COad a t hiqh t e m ~ e r a t u r e s .

The localized chemisomtion of CO a t lw temperatures a l s o irn~edes t h e steady decomposition of t h e methanol. A t T4460 K f u r t h e r sites a r e made a v a i l a b l e f o r t h i s reaction. These s i t e s must be adjacent s i n c e t h e methoxy smcies, X H 3 , is con- verted i n t o

mad

(and Had), which is bonded t o t h e s u r f a c e v i a t h e carbon atom. On account of t h e present r e s u l t s , w e s t a t e t h a t f o r T7460 K t h e r a t e of t h e o v e r a l l decomoosition r e a c t i o n of t h e CH 3 OH is determined by t h e r a t e of thernal desorotion of COad. Brbek e t a l . /8/ a r r i v e 3 a t a s i m i l a r coqclusion f o r t h e i r s t u d i e s on ma-

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C7-440 JOURNAL DE PHYSIQUE

croscopic Ru (001 1. No statement on rate determining steps can be made so far for temperatures T>460 K. Possibly, future reaction time variation measurements will provide further insight into the kinetics of the decomposition steps.

REFERENCES

/1/ Inghram, M.G. and Gcmer, R., 2. Naturforsch., A, (1955) 863 /2/ Beckey, H.D., Schulze, P., 2. Naturforsch., A,

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/3/ Kruse, N., Abend, G., Drachsel, W. and Block, J.H., in: Proc. 8th Int. Congress on Catalysis, Berlin/F.R.G., 1984, Vol. 111, Ed. DECHEMA, Verlag Chemie, Wein- heim, 1984, p. 105

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