Characterization of carbonaceous overlayers on platinum by catalytic oxidation

Characterization of carbonaceous overlayers on platinum by

catalytic oxidation

Citation for published version (APA):

van Langeveld, D., Hertrooy, van, J. P. F. M., Loos, J. B. W. P., & Niemantsverdriet, J. W. (1988).

Characterization of carbonaceous overlayers on platinum by catalytic oxidation. Vacuum, 38(4-5), 393-395.

https://doi.org/10.1016/0042-207X(88)90087-5

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10.1016/0042-207X(88)90087-5

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Published: 01/01/1988

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38/numbers 4/5/pages 393 to 395/1988 0042-207X/88S3.00+ .00

Printed in Great Britain Pergamon Press plc

Characterization of carbonaceous overlayers on

platinum by catalytic oxidation

A D van Langeveld, J P F M van H e r t r o o y , J B W P Loos and J W N i e m a n t s v e r d r i e t ,

Laboratory of

Inorganic Chemistry and Catalysis, Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands

Carbonaceous overlayers formed from ethylene on polycrystalline Pt have been characterized with AES and

SIMS. The reactivity of these deposits towards oxygen depends sensitively on their nature. Hydrogen-rich

hydrocarbon deposits on Pt oxidize at 410-420 K, carbidic C and CH species in the range 450-470 K, and

graphitic carbon at 565-570 K. The results confirm that oxidation is a suitable test for the reactivity of

catalytically relevant carbonaceous overlayers on Pt.

1. Introduction

Every metal which catalyzes a reaction of hydrocarbons is during the reaction covered by an overlayer. This adsorbate layer consists of carbon and hydrogen, and determines together with the metal the properties of the catalytic system t. Four distinct types of carbonaceous deposits can be identified, which differ in reactivity, hydrogen content, and extent ofgraphitization ~-4. We will use the following nomenclature. Deposits consisting of adsorbed hydrocarbons are called molecular, single C atoms or CH species carbidic, and unreactive, polycyclic aromatic carbons graphitic. A less polymerized and more hydrogen-rich precursor to graphitic carbon is called amorphous carbon.

Previous work in our laboratory has revealed that AES and negative SIMS form a powerful combination to characterize carbonaceous overlayers on noble metals and alloys 3'4.

The oxidation of carbonaceous deposits is of practical interest, because catalysts deactivated by excessive carbon deposition are regenerated by burning-off the carbon 5. Oxidation may also provide a convenient test for the reactivity of carbon deposits 6. Model studies of the reaction of oxygen with carbon layers on Ni 6-8 and Fe 9 have been reported. However, we are not aware of model studies on the oxidation of carbonaceous layers on platinum.

In this paper we show that the oxidation of three different types of carbonaceous layers on polycrystalline Pt occurs in well separated temperature regions. The results confirm that oxidation is a suitable reaction to test the reactivity of carbon overlayers on Pt.

2. Experimental

Experiments were carried out in an XPS/AES/SIMS spectrometer (Perkin-Elmer, PHI 550) equipped with a reaction chamber ~°. Auger spectra were measured differentially with a defocused 4.5/zA beam of 2 keV electrons and a spot diameter of 0.5 mm,

the modulation amplitude used was 2 eV. SIMS spectra were measured with a 50 nA beam of I keV Ar ÷ ions, which was rastered over the sample.

A high purity Pt foil (Highways), spotwelded on tantalum wires, was cleaned by several cycles of 5 keV Ar ÷ sputtering and annealing at t000 K. A subsequent Auger analysis showed no contaminants within the range of accuracy. Carbon was deposited on Pt from 67 Pa (0.5 torr) C / H 4 (Messer Griesheim). Oxidation was carried out in 67 Pa of 0.5% O2-in-He. After reaction the sample was cooled to 300 K, and transferred into the uhv chamber after evacuation of the reaction chamber.

3. Results

3.1. Carbonaceous deposits on Pt. Carbonaceous deposits of different nature were made by treating Pt with C2H 4 at 325, 525 and 775 K, and investigated with AES and SIMS. Carbon cover- ages were calculated from the intensity ratio of the C and Pt Auger peaks at 272 and 228 eV, respectively, and are given in Table 1. In the calculations the empirical relations of Biberian and Somorjai ~ were used. SIMS patterns of the carbonaceous deposits are shown in Figure 1. As discussed elsewhere, the negative ions, C2H ~ and C4H ~, display the best sensitivity for the state of the carbona- ceous deposits 3'4. The C2H~- intensities in Figure 1 indicate that deposits formed at 325 K are rich in hydrogen trelatively high intensities of C2H- and C2H ~-), and that carbon formed at 775 K is depleted in hydrogen (high C~ intensity). We express the information on the hydrogen content in one number by defining a parameter h as the ratio (C2H- + CzH 2 )/(C 2 + C2H- + C2H 2 ), where the ions represent the intensities of their corresponding peaks in the SIMS spectra. With the spectrometer used, and the beam conditions specified, the values ofh fall in the range between close to zero (0.03 for graphite on Pt annealed in uhv at 800 K) and about 0.7 for ethylene adsorbed on Ir at 300 K, where ethylidyne ( = C C H 3 ) is expected to form ~2. The extent of polymerization of the carbonaceous deposit is reflected in the 393

A D van Langeve/d et al: Carbonaceous overlayers on platinum SIMS of C a r b o n a c e o u s Deposits on Pt formed at

"1

0 ! C3

i

~ 0 ¼100 525K 12 15 2Z. 26 1.8 /.9 1001 325K 12 15 2/. 26 L8 L9 o m u

Figure 1. Intensity distributions of the most important ions in SIMS of carbonaceous overlayers formed on Pt from 67 Pa (0.5 torr) C2H 4 at the

temperatures indicated.

Table 1. Properties of carbonaceous deposits formed from C2H , at temperature T, as derived from AES, Negative SIMS and their oxidation temperature

Carbonaceous deposit Oxidized at

T(K) 0,. h g T(K)

325 0.5+0.1 0.9 0.012 415+5

525 1.2+0.1 0.5 0.030 465_+ l0

775 2.0__+0.3 0.2 0.050 565_+5

0,. calculated according to ref 1 I.

h = (C 2 H - + C 2 H ; )/(C~ + C 2 H - + C2H 2 ). g = C.,H,/C2H~-.

ratio 3 g = ( C 4 H ~ ) / ( C 2 H ~ ) . The idea behind this is that large aromatic structures have a greater chance to yield heavier secondary ions than smaller carbon species. Note, that this entirely empirical p a r a m e t e r depends on the fragmentation by the ion beam and on the transmission characteristics of the mass spectrometer. U n d e r our conditions the graphitization p a r a m e t e r g varies between 0.01 for adsorbed hydrocarbons and carbidic carbon on Ir, and 0.05 for uhv-annealed graphitic carbon on Pt. The values o f h and 9 for the carbonaceous deposits present on Pt in Table 1 indicate that carbon deposited on Pt at 325 K is molecular, whereas carbon deposited at 775 K is mainly graphi- tic. These results agree with T D S , H R E E L S and S I M S observa- tions on the thermal decomposition of C2H 4 at low coverages on Pt L13't4. The carbon formed at 525 K, where in the low coverage limit mainly C and C H species are expected L~4, is still relatively rich in hydrogen. However, it is also partly polymerized, as the intermediate value of g (0.03, vs 0.05 for graphitic carbon) suggests. F o r a more detailed discussion we refer to ref 3.

3.2. Oxidation o f carbonaceous deposits on Pt. Auger spectra of the graphitic deposit, formed from C2H. , at 775 K, after various treatments in 67 Pa of 0.5% O z - i n - H e are shown in Figure 2. The top spectrum, carbon exposed to O z at 550 K, is still identical to

that of the initial carbonaceous deposit; its shape is characteristic for graphitic or a m o r p h o u s carbon 15. The carbon coverage is

a b o u t 2 monolayers (ML). After oxidation at 560 K a consider- able fraction of the carbon has been removed; the a m o u n t of the remaining carbon corresponds roughly to 0.9 ML. N o t e that the carbon peak exhibits a shoulder around 265 eV (see arrow), which was absent in the spectrum of the original deposit. This fine structure is characteristic of either carbidic or molecular carbon 15. After reaction at 567 K almost all carbon has been removed by oxygen, only about 0.1 M L of carbon is left, which decreases to half that value after reaction at 575 K. A lower C coverage could not be obtained by reaction with oxygen in the reactor. Hence, we conclude that graphitic carbon on Pt can be removed by oxygen at temperatures in the range form slightly below 560-567 K.

Figure 3(a) shows the oxidation of a carbonaceous deposit formed from C2H4 at 525 K. The top spectrum corresponds to the clean Pt foil, the second to that of the carbon overlayer formed at 525 K. The coverage of the latter is 1.2 ML. As molecular and carbidic carbon are more sensitive to degradation by electron irradiation than graphitic carbon, the width of the energy channels was increased and the measuring time per channel shortened, in order to decrease the time of exposure to the primary electron beam. Consequently, the spectra of Figure 3 are less detailed and contain more noise than those of Figure 2. Neverthe- less, the spectra of Figure 3(a) clearly show that the oxidation of the carbonaceous deposit formed from C z H 4 at 525 K starts around 450 K and is completed at 470 K.

Finally, the oxidation of the molecular carbon on Pt, formed from C 2 H 4 at 325 K, takes place between 410 and 420 K (Figure 3(b)). Here a new carbonaceous deposit was prepared after each Auger analysis because electron beam induced degradation of the molecular deposit could be observed after recording m o r e than two Auger spectra. F o r example, a molecular deposit which had been exposed to the electron beam during the time needed for four spectra, was oxidized at 470 K, whereas the oxidation temperature of a freshly prepared deposit was between 410 and 420 K. Note, that this electron beam induced degradation was not observed for carbonaceous layers deposited at 525 K.

4. Discussion

The most i m p o r t a n t conclusion from the present results as collected in Table 1, is that different carbonaceous deposits on Pt oxidize at different temperatures. This means that oxidation is indeed a suitable test reaction for the reactivity of carbon in carbonaceous overlayers on metals. The characteristic oxidation temperatures are sufficiently well separated to m a k e temperature p r o g r a m m e d oxidation (TPO) of carbonaceous deposits feasible. The oxidation temperature of graphitic carbon on Pt, 565 K, is in g o o d agreement with results on coked P t / A l 2 0 3 catalysts reported by Barbier 16 The T P O profile of a deactivated Pt/AIzO 3 catalyst shows a peak at 575 K, attributed to the oxidation of coke on the metal, and a peak at 725 K, attributed to coke on the AlzO 3 support. The gasification of coke on catalysts which are inactive for oxidation reaction requires a temperature of about 775 K 5. The fact that all carbonaceous deposits on Pt studied here can be oxidized at considerably lower temperatures confirms that the oxidation is catalyzed by Pt.

With respect to the oxidation of molecular carbon on Pt, it is i m p o r t a n t to note that it makes a difference whether C z H 4 is d e c o m p o s e d first and then exposed to oxygen, or the other way

A D van Langeveld et al: C a r b o n a c e o u s overlayers on p l a t i n u m dN dE AES o x i d a t i o n of g r a p h i t i c C on Pt O 2, 550K 02 , 560K O 2, 567 K

i ,,++

. + , . . . 575K ¢ ~.k / ~ l,,~pz. . ,.I 02, i i i I I 200 250 EIeV) 300

Figure 2. Auger spectra ofa graphitic deposit on Pt after treatment in O 2 at various temperatures.

a r o u n d as in refs 13, 17, because ethylene i n t e r a c t s differently with o x y g e n - p r e c o v e r e d P t ( l l l ) t h a n with clean P t ( l l l ) . W h e n the system O / P t ( 1 1 1 ) is exposed to C z H +, C O e e v o l u t i o n is a l r e a d y o b s e r v e d between 300 a n d 400 K 13'17. I n o u r e x p e r i m e n t s , the m o l e c u l a r c a r b o n a c e o u s deposit, p r e s u m a b l y ethylidyne, oxidizes at 4 1 5 + 5 K, as the spectra in F i g u r e 3(a) show.

T h e fine s t r u c t u r e in the A u g e r spectra o b s e r v e d d u r i n g the o x i d a t i o n of g r a p h i t i c c a r b o n (see a r r o w in F i g u r e 2) is typical for c a r b i d i c c a r b o n a t o m s , a d s o r b e d h y d r o c a r b o n f r a g m e n t s or a d s o r b e d C O ~5. Its presence indicates t h a t a f r a c t i o n of the g r a p h i t i c c a r b o n has been c o n v e r t e d by oxygen w i t h o u t h a v i n g left the surface. C o n s i d e r i n g t h a t C O z a n d C O d e s o r b at t e m p e r a t u r e s well below 560 K, it seems likely t h a t the A u g e r fine s t r u c t u r e at 265 eV is due to c a r b o n from d e s t r o y e d a r o m a t i c rings. S I M S spectra were of little use in s t u d y i n g the o x i d a t i o n of c a r b o n a c e o u s deposits o n Pt. F o r example, o n e of the effects of e x p o s i n g g r a p h i t i c c a r b o n o n Pt to 0 2 at r o o m t e m p e r a t u r e was t h a t the emission of C 2 H - a n d CEH~- increased with respect to t h a t of C~-, implying t h a t the h p a r a m e t e r loses its m e a n i n g for c a r b o n deposits u n d e r O z. T h e low intensities of P t ÷, P t C . H ~ + a n d P t C O + ions in positive S I M S m a k e a p o t e n t i a l l y i n t e r e s t i n g s t u d y of the o x i d a t i o n m e c h a n i s m with positive S I M S impossible,

dN dE

200

AES Oxidation Carbonaceous Deposits on Pt

P t ,~\

/

/

/

C2H 4 52SK \ ~ , r - ~ ^ , . S ~ \

\

h/

q

Pt + / C2H~32SK ,\ lh/ 02 411K -~ M 02 4 1 6 K -~ .4 02 455 K '\ _%// +~"*~ ."~-" _{' \ ,/"}

\

'4 02 (+20 K '~ 0 2 4701( / t /¢"~ . j ,V,~.% . ~ ' -

o)

b)

i i I i i i I i T i i 250 200 V' +/ i i i i L F---- 25[, E r e . '

Figure 3. Auger spectra of the oxidation of carbonaceous deposits on Pt formed from C2H 4 (a) at 525 K, and (b) at 325 K.

at least with o u r spectrometer. F o r the future we i n t e n d to study the o x i d a t i o n of c a r b o n a c e o u s deposits o n R h a n d Ni, where the desired s e c o n d a r y ions d o h a v e sufficient intensity.

Acknowledgement

J W N is s u p p o r t e d by a Huygensfellowship from the N e t h e r l a n d s O r g a n i z a t i o n for the a d v a n c e m e n t of p u r e research ( Z W O ) .

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