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39 Carbon Monoxide Oxidation on Platinum: Coverage Dependence of the Product Internal Energy D. A.Mantell1,K. Kunimori2, S. B. Ryali3, and Gary L. Haller Department of Chemical Engineering, Yale University, New Haven, CT 06520 Time resolved FTIR emission spectroscopy is used to detect vibrationally excited gas phase CO from catalyzed CO oxidation on a Pt f o i l . A continuous O free jet and a pulsed CO jet (= 200 μsec FWHM) supply the reactants to the surface. The infrared emission of the CO product is analyzed with 30 μ s e c time resolution using the time multiplexing capabilities of a commercial Fourier trans form spectrometer. At low CO pressures the total signal parallels the time dependent flux to the surface with only minimal changes in the infrared spectra. At high CO pressures the reaction can be shut off as the oxygen on the surface is depleted. These IR spectra show large changes in internal energy of the product CO . 2
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The o x i d a t i o n of CO on group V I I I metals i s one of the most studied metal catalyzed reactions and many aspects of i t are w e l l under stood 0 ^ 2 ) . Reaction occurs by a Langmuir-Hinshelwood mechan ism between chemisorbed m o l e c u l a r CO and atomic oxygen. However, the k i n e t i c s are complex because high coverage of CO i n h i b i t s oxygen d i s s o c i a t i o n but CO chemisorbs on top of an oxygen covered surface. At low surface concentration of both species, the reaction i s l i m ited by oxygen chemisorption and the rate i s d i r e c t l y proportional to oxygen p a r t i a l pressure. With high coverage of adsorbed oxygen atoms and low CO coverage, the reaction i s d i r e c t l y proportional to CO p a r t i a l pressure. At r e l a t i v e l y high coverage of both species, the case for most p r a c t i c a l applications of c a t a l y t i c oxidation of CO, the r a t e i s about f i r s t order i n oxygen p r e s s u r e and i n v e r s e f i r s t order i n CO. 1Current address: Physics, B-268, National Bureau of Standards, U.S. Department of Commerce, Washington, DC 20234 2Current address: Institute of Materials Science, University of Tsukuba, Sakura-mura, Ibaraki 305, Japan 3Current address: Aerodyne Research, Inc., Billerica,MA01821
0097-6156/85/0288-0464$06.00/0 © 1985 American Chemical Society Deviney and Gland; Catalyst Characterization Science ACS Symposium Series; American Chemical Society: Washington, DC, 1985.
Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on May 21, 2018 | https://pubs.acs.org Publication Date: October 16, 1985 | doi: 10.1021/bk-1985-0288.ch039
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Carbon Monoxide Oxidation on Platinum
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In the case of CO o x i d a t i o n on P t , there have been performed several experiments which e l u c i d a t e the dynamics of the r e a c t i o n . Because the reaction between adsorbed CO and oxygen atoms is a c t i vated and CO2 has only a van der Waals i n t e r a c t i o n with the surface, a part of the energy acquired to form the activated complex may be d i s t r i b u t e d among degrees of freedom of the product m o l e c u l e . In the l i m i t i n g case where no energy i s exchanged w i t h the s u r f a c e d u r i n g the d e s o r p t i o n event and a complete a n a l y s i s of i t s p a r t i t i o n i n g between t r a n s l a t i o n , r o t a t i o n , v i b r a t i o n and e l e c t r o n i c energies in the product is a v a i l a b l e , i t is in p r i n c i p l e possible to construct a p o t e n t i a l energy surface which would describe the molec u l a r s t r u c t u r e of the a c t i v a t e d complex and the dynamics of the d e s o r p t i o n e v e n t . While t h i s i d e a l i s not yet p r a c t i c a l , we have made s i g n i f i c a n t steps toward gaining t h i s information for CO oxida t i o n on Pt. Several investigations have found that COJ2 molecules l e a v e the surface with a n g u l a r d i s t r i b u t i o n s which are s t r o n g l y peaked i n the d i r e c t i o n of the surface normal which implies excess t r a n s i at ional energy.(3-5). Time-of-f 1 ight analysis of CO oxidation on a p o l y c r y s t a l 1 ine Pt f o i l a l s o demonstrates t h a t the p:>T,I?W 1 leaves the surface with k i n e t i c energy in excess of that expected :'f the m o l e c u l e were i n e q u i l i b r i u m w i t h the surface(6,). Infrared emission experiments show that the CO2 product of CO oxidation i s , moreover, v i b r a t i o n a l l y ( 7 , 8 ) and r o t a t i o n a l ly(7.) h o t t e r than the s u r f a c e . In v e r y recent experiments i t has f u r t h e r been observed that the angular d i s t r i b u t i o n s (transi at ional energy) (9) and v i b r a t i o n a l energy of the product CO2 (15) are s t r o n g f u n c t i o n s of s u r face coverage. We w i l l c o n f i r m here t h a t t h i s i s the case f o r r o t a t i o n a l and v i b r a t i o n a l d i s t r i b u t i o n s . Because the a c t i v a t i o n energy f o r the surface r e a c t i o n between adsorbed CO and adsorbed oxygen atoms depends on the coverage(4), t h i s i s not unexpected. Q u a l i t a t i v e l y one might anticipate that the product C0|2 would have less excess energy at high oxygen coverage because there is l e s s of a b a r r i e r to reaction and therefore the activated complex would have less energy to dissipate. However, the matter is somewhat compli cated by the f a c t that as the coverage i n c r e a s e s , the heat of a d s o r p t i o n of both CO and ©2 d e c r e a s e ( 4 , 1 0 ) , but s u f f i c e i t to say that at low coverage the activated complex is about 1 0 0 kJ above COj i n the gas phase and that t h i s v a l u e i n c r e a s e s as coverage increases. In experiments using a steady-state mixed CO-O^ molecular beam reacting on a Pt f o i l , we observed excess energy in a l l vibrations and rotation and large changes i n the amount of excess energy i n the symmetric stretch of desorbed (X>2 as a function of surface tempera ture ( . 1 1 . 1 2 ) . There was a strong s u g g e s t i o n that the l a t t e r was p r i m a r i l y due to changes i n surface coverage which accompany a change i n surface temperature rather than a d i r e c t coupling between the i n t e r n a l energy states and the surface temperature. In order to test this hypothesis, we i n i t a t e d pulsed molecular beam experiments with time r e s o l v e d - i n f r a r e d emission s p e c t r o s c o p y of the desorbed product. A steady-state beam of Οχ was incident on a Pt f o i l and a second pulsed beam was simultaneously directed onto the f o i l . The CC p u l s e s were 2 0 0 με halfwidth at h a l f maximum height with a 2 0 0 0 με pause between p u l s e s ( 1 2 ) . The 2 0 0 0 μβ pause is long enough such
Deviney and Gland; Catalyst Characterization Science ACS Symposium Series; American Chemical Society: Washington, DC, 1985.
Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on May 21, 2018 | https://pubs.acs.org Publication Date: October 16, 1985 | doi: 10.1021/bk-1985-0288.ch039
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CATALYST C H A R A C T E R I Z A T I O N SCIENCE
that the surface coverage of oxygen c o u l d r e c o v e r to i t s steadys t a t e v a l u e at the g i v e n r e a c t i o n temperature and the p a r t i c u l a r f l u x used i n the oxygen beam. The number of CO m o l e c u l e s i n the pulse could be e a s i l y varyed by changing the stagnation pressure i n the pulsed nozzle source. At low pressures the surface coverage of oxygen did not vary much from i t s steady state value while at high pressures we could e f f e c t i v e l y perform a t i t r a t i o n of the surface oxygen as the p u l s e passed o v e r the s u r f a c e . The l e a d i n g edge of the pulse reacted at the r e l a t i v e l y high oxygen coverage but, since r e a c t i o n removed oxygen f a s t e r than i t c o u l d be r e p l e n i s h e d by a d s o r p t i o n ( i n p a r t due to CO i n h i b i t i o n ) , toward the end of the p u l s e r e a c t i o n o c c u r r e d at v e r y low oxygen coverage. The p u l s e d experiment allowed us to vary the coverage isothermal l y over a much larger range than would have been p r a c t i c a l by varying the f l u x of a steady-state source. T i m e - r e s o l v e d i n f r a r e d emission spectroscopy of the desorbed COg pulse e f f e c t i v e l y provides a coverage-resolved picture of the v i b r a t i o n a l and r o t a t i o n a l energy d i s t r i b u t i o n in the product CO^. The time-resolved spectroscopy is accomplied using a Fourier trans form spectrometer ar>.f!
