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Vibrationally excited CO2 from the reaction of O atoms and adsorbed CO on platinum
Volume
1 IO, number
3
~BRATIONALL~ AND ADSORBED
CHCMICAL
PHYSICS
18 SepteIRbcr 1984
LE-iTERS
EXCITED CO, FROM THE REACTXON OF 0 ATOMS CO ON PLATINUM
M. KORI and B.L. IiALPERN
Kcccivcd
16 hfJy 1984; in final form 26 July 1984
IligitIy excited CO2 is pruduced by the reaction 0 f CO.,da-pt CO,.* Its infrared emission spectrwn indicates that nxwe than Wfof the 4.1 CV cwcrgicity appcxs in asynrn~ctric stretch vtbmtion in a distribution pcsked roughly at icvei (009). CO; is dcstroycd by 0 atcms at high prcssurc (p 2: 10 an’l‘orr).The reaction dynamic\ .md applic.hons arc discussed.
1. Introduction Vibratio~ia~y excited products have been observed in a number of surface catalyzed reactions [I-31 _ In a previous letter 131, for example, we showed by IR emission spectrometry that CO in as high as the 7th level was released in the Ltlnglnuir-Hindielwood (LH) reaction between C and 0 on a hot Pt surface. At cquilibrium, even u = 3 would have been insignificantly popuIated at that surface temperature. Tfle population foIiowed a statistical, prior disrrib~lt~o~l and showed no inversion. Gas phase reactions arc known to give products whose vibrational distributions are either statistical or inverted and non-statistical [4]. One may ask if surface reactions show the same range of behnviar. It is therefore of interest to find a surface reaction whose product exhibits a vibrational population inversion. One candidate is the Rideal reaction between 0 and adsorbed C on Pt, for which trajectory calrulations [S) predict a CO distribution peaked at about u = 1 I. Our study [3] of C oxidation on Pt was intended to test that prediction, but experimental conditions allowed only the LH reaction giving a product with a statistical vibrational distribution_ We report now the first example of a surface reaction yieIding a population inversion: the oxidation of CO on oxidized Pt at room temperature by 0 atoms supplied directly from the gas phase. CO oxidation with 0 atoms is a good test case. If, 0 009-2614/84/S 03.00 0 Elscvier Science Publishers (North-Holland Physics Publishing Division)
B.V.
for example, it occurs at low coverage on Pt( I 1I) by a Rideal process with CO as the target, the energy released would be about 4 eV, given by the exothermicity of the gas reaction CO + 0 -+ CO,, Ai?= 5.5 cV, minus the heat of chemisorption of CO, 1A eV 161. That energy is sufflcicnt to populate CO, asymmetric stretch levels up to (0 0 16). Its less energetic counterpart, the 0, supplied, LH reaction on clean Pt, is known to produce vibrationally excited CO? in several low levels although with no population inversion [2]. Our esperiments show that highly excited CO: is indeed produced even at room temeperature from lhe 0 atom oxidation of CO on Pt oxide. The shape and position of the emission spectrum, and their variation with surface temperature and baclcground pressure indicate
a vibrational
ture of rhe reaction
2. Apparatus
inversion,
and give a distinct
pic-
dynamics.
and procedure
The experiment took place in a 500 P vacuum chamber pumped by a 10000 Q/s diffusion pump at a minimum background pressure cf 1 Ov4 Torr. Oxygen atoms and CO were directed, through separate sources, at an electrical& heatable Pt foil. This target was positioned just beyond the Geld of view of a Fourier transform infrared spectrometer (Nicoler 7 199). The 0 atoms were generated in an auxiliary glass fast flow system, linked to the main chamber, 223
Volume I IO, number 3
CHEMICAL PHYSICS LETTERS
28 September 1984
where oxygen was dissociated
with 10% efficiency by a microwave discharge. Carbon dioxide molecuIes desorbed into vibrational energy loss free conditions and some of them radiated during their transit through the observation zone, Radiation was collected and focused onto the InSb detector of the spectrometer_ The detector has maximal sen-
sitivity near 2300 cm-l; we observe fundamental transitions for the asymmetric stretch mode (u1u2u3 - u1u2u3 - I) in that range. Most of the spectra in
this study were recorded at 0.1 cm-l resolution, which was sufficient to resolve rotational lines in CO, and CO. Rotational relaxation occurred even at low pressures. The Pt surface was pretreated by heating briefly to T= 1700 K and then cooling in a stream of either 0 atomsaIone,or of 0 and CO. This formed an oxide layer above and below [7] the surface. The 0 atom CO reaction was then run at room temperature on that oxidized surface. There is evidence that carbon can be deposited when Pt is heated in CO and molecular oxygen [Sj ; carbon is unlikely to persist in the presence of 0 atoms, howeverOxygen atom pretreatment was essential, and ordy after it was perfomlcd was an infrared emitting product seen at room temperature. There was no gas phase reaction; in the absence of the Pt foil no radiation was detected.
3. Results
Even at room temperature, the 0 atom oxidation of CO on Pt oxide gives vibrationally excited CO:. A high resolution (0.1 cmW1) emission spectrum ofthe asymmetric stretch mode is shown in fig_ I for the case of pretreatment in 0 atoms alone. It has several unusual features. (a) The spectrum is diffuse, even at high resolution, and shifted to lower wave number from the familiar, first asymmetric stretch fundamental at 2350 cm-l. (b) if the background pressure of 0, 02 and CO is allowed to rise by reducing pumping speed, and keeping relative pressures constant, a discrete spectrum of COT, radiating mostly from u = 1, appears. At the same time the diffuse CO3 spectrum diminishes in intensity. This is shown in fig. 2. At 100 mTorr, the CO* component is the dominant feature. It should be 224
Fi. 1. “Diffuse” CO, emission spectrum at hi& (0.1 cm-‘) resolution and low backgound pressure The h&h den&p of overlappinglines is not shown,
noted that while the diffuse CO; component diminishes, it does not shift to higher or lower wavenumber. There are thus two components in the same frequency range. (c) If the surface ten~perature is increased at low pressure the familiar resolvable band centered at 2350 cm-1 appears along with the diffuse spectrum. The band at 2350 cmWi is identical to that seen in the 0, supplied, U-I oxidation of CO [2] on unoxidized Pt metal. This third “?ormaI” CO; component increases and the diffuse CO; component decreases as the temperature rises. At T> 600 K, little of the diffuse component remains, as seen in fig. 3. The oxide layer is beginning to be removed, at higher T, and growth of the 2350 cm-’ band is related to that removal. (d) If the surface is pretreated with CO as well as 0 atoms, the “normal” component also appears when 0 atoms react with adsorbed CO. But now it appears even at room temperature in contrast with 02 supplied oxidation.
Volume 110, number 3
CHEhlICAL
PHYSICS
LE’MXRS
28 September
1984
4. Discussion
t 2350
I
8
I
2151)
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1
2050
2150
CY
1950
-I
Fig. 2_ Emission spectrum at 11th background pressure (28 mlbrr) showing diffuse CO* component and emergtis CO discrete spectrum. T
OK
Fig. 3. Effect of surface temperature on relative intensity of diffuse CO2 component and “normal” CO2 component. Lies in the normal component are resolvable, but the scans shown are at low resolution.
We will try to show that the following themes arc consistent with the observations (a) through (d). (I) CO; is genefateti in very high vibrational levels by reactio: of 0 and CO on Pt oxide. Most of the vibrational energy appears in the asymmetric stretch mode with v,, = 16, and vaverqe = 9. (2) The excited CO; can react with 0 atoms at higher background pressure to give 0; and slightly excited CO*_ The spectrum of Gg. 2 E due to both excited CO; and CO?. Before developing these ideas and explaining results (a)-(d), we consider the possibIe mechanisms by which such highly excited COT can arise_ It is natural to compare the reaction of 0 and CO on oxidized Pt to the commonIy studied CO oxidation on clean Pt. That reaction is known to be a LH process with a small 1x5 kcaljmole) exothermicity, and an activation energy of slightly less than 1 eV [6]_ About 1 eV can therefore appear in product vibration, and, in agreement with that fact, IR emission measurements have shown that no significant amount of CO, is excited to beyond v = 2 in asymmetric stretch [2]. The LH process, in which the chemisorption energies of CO (ml.3 eV) and 0 atoms (z3.9 eV) are both large [6], thus produces a CO, molecule with relatively little excitation. For the 0 atom oxidation of CO on Pt oxide we consider several possibilities that might lead to highly excited COI. It is unlikely to be a LH step with a 4 eV activation energy since such a reaction could not take place at room temperature as we observe. Conceivably it is a step in which both CO and 0 are essentially physisorbed and react as a two dimensional gas. The available energy for the product would then be the gas phase exothermicity x5.3 eV. It is difficult to judge the likelihood of this mecha~m_ The reaction probab~i~y depends critically on the physisorption energies of CO and 0 on the Pt oxide; these energies are not known. We note however that in general 0 atoms do not recombine efficientIy on metal oxides [9] _If a 2-L) mechanism were probable, it might be expected to promote atom recombination too. A simple alternative could be a variant of the Rideal process, as described by Bond [ lOI_ For ex-
225
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1 IO, number 3
CHEMICAL
ample, an 0 atom impacts and physisorbs near a chemisorbed CO, and attacks it in the plane of the surface. If the surface were clean, the 0 atom could
onant transfer of even one quantum [ 121. For nonresonant transfer or at lower pressure, V-V transfer
chernisorb: tfle role of tfte Pt oxide is to prevent chemisorption and permit approach from the side. since tfte direct approach from tile gas phase to the “wrong” end of the CO might not favor reaction. Whatever mechanism is operative, it is clear that both CO and 0 cannot be strongly bound on Pt oxide. To produce a CO: product fn as frigfi as u * 15, the sum of the bindr$g energies of botfr CO and 0 cannot be greater tfran 1.3 eV. ffowever, we know little about the cftemisorption of CO and 0 on a Pt oxide surface prepared by heating Pt in 0 atoms. CO does cfremisorb with about 1 eV adsorption energy on a number of oxides [I If , and the Rideaf type step we described is plausible in that it can account for the Iligfl product excitation. But It is not possrble to decide clearly in favor of tfrat mecfranism by this study atone. It wiff be shown that higher levels of CO; asymnietric stretch vibration are preferentrally exciteh. Thus, dnfrarmonicrty shifts the vibrational transition to lower wavenumber from tftc 23.50 cm-* fundamentaf, so the CO: spectrum is found close to that of CO?_ The spectrum is diffuse because in ahigfrly excited triatomic the number of transitions(u~u~~~ - ZQU+~ --I) is large, and rotational fines cannot be dis&i&iisfted even at 0. I cm-1 resolution because of overlaps. The fact that excited COr is formed at higfrer pressure is a key to our interpretation_ At higher pressure, desorbmg CO3 can collide with incoming 0, 02 and CO. it would seem at first sight that CO;
could direct-
ly excite CO by V-V transfer. The level spacings of CO and CO3 asymmetric stretch are similar; the (009 - 00s) tran2tion of COT is almost exactly resonant with the lowest CO tram&ion. But there are three strong arguments against colfi-
sfonaf excitarion. First, CO; makes too few collisions between surface and observation zone to permit such energy transfer. By a simple random walk argument, the total number of collisions Z is given by 2 =fd”l’lf,
= o.t.&
I
wfrere p is tire chamber pressure in mTorr, CO, diffusion coefficient, d is the distance the observation zone, and fis the collision per mofecufe. Only at the highest pressure mTorr is rfie number of collisions sufficient 226
28 September 1984
PHYSICS LE-ITERS
(0 D is tfte through frequency of 100 for a res-
could not produce the efficient CO; depletion that is observed. Second, even if the number of co&ions did suffice
for energy transfer the CO; spectrum shoufd not merely decrease in intensity; rather it should shift toward higher frequency as CO; molecules accumulate in lower levels. Eventually, one would observe radiation from the resolvable band at 2350 cm-l_ However, this does not occur; the CO1 diffuse spectrum dfminishes but does not shift. Third, the changes in the spectrum do not depend strongly on the nature of the gas used to raise the background pressure, e.g. He, CO,, N2 or CO. These coffision partners frave greatly different energy transfer probabilities, so energy transfer cannot be important in CO; depletion. One is forced to conclude that the pressure induced onset of CO emission and the accompanying decrease of CO? diffuse emission cannot be accounted for by a near-resonant V-V, or any fess efficient energy transfer process. The only remaining possibility is that the excited COf is being destroyed by chemical reaction with one of its collision partners, and that CO is generated as a reaction product. In our system the onfy potential reaction partners are 0, 0, and CO in tfreir ground states, and the excited species 02(lA), O,(lE), O(lD) and O(*S). Of these, only 0, 02, CO and O,(lA) occur at significant concentration. Of the few possible reactions that may be written down the only one that is energeticaffy feasible and consistent with the observed efficient generation of CO is O(3P) + co1 + COT + 0;
.
(0
Both reaction (f) and its reverse have been tfre subject of shock-tube studies [S-15] because of their role in combustion. With ground vibrational state CO,, the reaction is endothermic by 0.34 eV. The activation energy has been reported to be x2.6 eV [ 16f _ For reaction (I) to proceed at room temperature and high pressure, some CO: molecules would have to carry that activation energy in vibration. We cannot in our system demonstrate by an independent experiment that reaction (1) does occur. The CO emission is itself perhaps the best proof; since most of tfie reaction energy should appear in the new
Volume 110. number 3
CHEMICAL PHYSICS LElTERS
0, bond, it is resonable that CO be only slightly excited. We argue that reaction (I) is the only explanation of result (b) that is consistent with improbable collisional excitation and efficient CO production_ In fact reaction (I) proceeds with great efficiency_ At 10 mTorr where CO; makes about one collision with an 0 atom in the observation zone, a noticeable fraction of the diffuse CO: intensity has disappeared_ The diminution is pronounced at 28 mTorr, as seen in fig. 2. The inescapable conclusion is that mosr of the CO; molecules indeed have an energy in vibration sufficient to drive reaction (I) with few collisions: the average CO; vibrational energy must then be =Z6 eV. One could determine the distribution of that energy among the three vibrational modes of CO, by synthesizing a spectrum to match that of fig. I. Before discussing our synthesis, we examine three limiting cases in which all the available energy resides in one of the three modes. The fact that reaction (I) is efficient constrains the choice of energy distribution. Suppose that only asymmetric stretch is excited. Because the spectrum begins 50 cm-l from the first CO, fundamental, the lowest occupied level must be =(003X The highest level must be (0 0 16) to account for the width of the spectrum (350 cm-l)- This is because the spectrum consists of vibrational bands (00~ -+ 00~ - 1) each shifted to lower frequency by x24.5 cm- 1 due to anharmonicity. If the population distribution is roughly symmetric, it would peak at =(009) and since the asymmetric stretch spacing is N-25 eV, the average energy woud be a2.3 eV. Similar reasoning holds if only symmetric stretch or bending is excited. One quantum is required in asymmetric stretch for an emission band near 2350 cm- 1 to be seen at all, but each added quantum in bending or symmetric stretch will provide an additional band shifted from and superposed on the first. The shift per quantum of bending is ~12 cm-l, while the shift per quantum in symmetric stretch is ~34 cm-l. To account for the observed position and width of the spectrum, the lowest bending level would have to be (041) and the highest ( 0 30 I). The lowest symmetric stretch level would be (201) and the highest (15 0 1). The average enerm in both cases would be about 1.3 eV; the equality arises because twice as many bending levels must be occupied to explain the spectral width, but each quantum is half the energy of that in symmetric stretch.
28 September 1984
These extreme cases show that the width and position of the spectrum can be explained by very different distributions. The position of the maximum in the diffuse spectrum immediately implies that the distribution in at least one mode must be peaked. However, only if the energy is overwhelmingly in asymmetric stretch will the average energy suffice to drive reaction (I) with high probability_ To reinforce this picture we synthesized a spectrum according to the following admittedly approximate procedure_ Because of the coincidence of shifts caused by occupied levels in asymmetric stretch, bending and symmetric stretch modes (12, 24 and 24 cm-r, respectively) the centers of bands contributing to the spectrum can only occur at roughly 12 cm-l intervals in the range 2300-1900 cm-l. The spectrum was therefore regarded as being composed of 32 identical bands. whose rotational line positions in the P and R branches were given by standard expressions [ 171, and whose centers were placed at 12 cm-l intervals_ Each rotational line was taken to be triangular with a half width equal to the resolution_ A rotational temperature of 350 K was assumed since relaxation did occur. Each band has contributions from several states. For example, that centered at 2176 cm-l includes transitions from (004), (033), (103), (043) (122) (202) (061), (141) (221) and (301). The band intensities were summed to give the best fit to the experimental spectrum; the summation was done by computer at 5000 points and plotted at 1000. The number of superposed bands that will yield a diffuse spectrum can be estimated as the rotational line spacing divided by the resolution; in our case, even 16 bands would have sufficed_ But it is not possible to derive unique ener,T distributions for all three modes from that synthetic spectrum. For example, because of the shifts described above, the transition (041) to (040) would place emission lines in the same position as (201) to (200). Nevertheless, one can assume distributions for bending and symmetric stretch which involve little energy, and then derive a distribution for asymmetric stretch. Fig. 4 shows one distribution so derived. Bending and symmetric stretch were assumed Boltzmann at T = 1700 K and T = 800K, respectively as shown in ref. [?I; the resulting asymmetric stretch distribution P(v) is consistent with a -2.6 eV average energy required to drive reaction (I). Why should the reaction of CO and 0 on a Pt oxide 227
Volume 110, number 3
CHEMICAL PHYSICS LETTERS
28 September
1984
main areas of the surface that are not heavily oxidized on which 0 and CO can chemisorb strongly_ When 0 atoms are supplied during reaction, the LH mechanism can occur even at room temperature as in result (d) to given a CO, product with low vibrational energy emitting near 2350 cm-l_ This is because 0 atoms can compete for sites with CO [I8 ] _If 0, rather than 0 is supplied then the LH step occurs only at high temperature_ 02, unlike 0, cannot compete for sites with CO and at room temperature the surface is saturated with CO. Reaction proceeds at high T when CO desorption permits dissociative O2 adsorption [IS]. CO, does not adsorb on clean Pt and is unlikely to adsorb on oxidized Pt. Therefore extensive relasation of CO: vibration by electron-hole pair creation is improbable since desorption would occur rapidly following COT formation.
5. Summary 0
10
5
Room tempenture on PI yields excited
V
I‘ig_ 4. Rcl~tive population in asymmetric stretch fevcls .ISsumin~ Iox\ energy in bending and asymmetric stretch.
throw ener,3, prcfercntially into the asynmerric stretch ~athcr than distributing it more equally among all modes” The LH oxidation of CO on clean Pt, with bound 0 atoms, also favors the asymmetric over the symmetric mode. In both reactions the CO: is formed “asynlmetrically” from a short CO bond and an inireally long O-C...0 bond. Evidently the Pt oxide layer has so reduced either or both of the CO and 0 adsorp11on energies, that reaction begms to resemble a gas phase encounter in which selective energy disposal is oflen seen. Fkaction is so rapid that energy randomi/.ation cannot occur as it did. for example, in oxidation of C on hot Pt to give CO. Raising the Pt foil tcmpclaturc breaks down the oxide and exposes bare Pt, whereupon the normal LH mechanism becomes operative. As the oxide disappears fewer highly excited COT molecules are formed on it and emission from the Lp product, formed on the clcnn l’t fractron. becomes dominant, as in result (c) dnd the spectrum of fig. 3. If CO is present during pretreatment.
and conclusions
there will re-
oxidation
of CO with
0 atoms
CO,, bearing on average ~3.6 eV in asymmetric stretch. The asymmetric stretch population peaks at about level (009). Excited carbon dioxide is depleted at high pressure by reaction with 0 atoms to yield CO. The reaction has a high barrier but is driven by the CO1 excitation_ The surface reaction is of interest on several grounds. This is the fist example of a surface reaction whose product has a non-statistical, inverted vibrational energy distribution_ It may be compared to the LH oxidation of C on Pt where 0 atoms were supplied by 0, dissociative adsorption [3] _There CO emerged from a long-lived complex of C, 0 and Pt and desorbed with a statistical prior distribution_ The complex was longlived because all participants were adsorbed or bound, several bonds had to be broken, and the product CO could adsorb. The formation of CO: in this present study is rapid; there is insufficient time for energy to be statistically partitioned among product modes and heavy surface atoms. In the LH oxidation of CO, the 0 atom is bound, and the CO, distribution is statistical, characterized by a vibrational temperature [z]The reaction suggests itself as a possible Rideal. mechanism by virtue of the high energy of its CO; product. It is difficult to confirm Rideal steps in gencral, but they have been invoked to explain incomplete
Volume 110, number 3
CHEMICAL
PHYSICS LETTERS
chemical reaction energy accommodation in atom recombination on metals 1191. If both 0 and CO adsorbed as strongly on Pt oxide as on clean Pt, at most 1 eV would be available for product excitation. That would not permit the energy disposal actually seen. The high vibrational energy, high density of states, and inverted distribution of the product CO; suggest the 0 atom reaction with CO on Pt oxide as the basis for a CO, chemical laser. Such a laser would be tunable over a wide frequency range near 2300-1000 cm-l. There is as yet no laser based on a surface catalyzed reaction. It is interesting that the 0 atom pretreated Pt catalyst used here so strongly catalyzes formation of vibrationally excited molecules. Electronically excited molecules have been formed with low probability on oxygen covered metals [20] and on glass [21] _While electronic excitation is energetically unlikely in our study, we sugest that atom reactions on heavily oxidized metals are a useful source of excited molecules. In particular. the oxidation of CO with 0 atoms on oxidized Pt bears further study.
This work has drawn support from NSF grant CPE8114348, Petroleum Research Fund FPRF 13443-ACS, and Air Force grant OSR F499620-80-C-0026. We are grateful for the invaluable assistance of J.B. Fenn, D.A. Mantel1 and S.B. Ryali.
i 984
References
111S. [?I [31 [41 151 VI 171 181 PI 1101 IllI 1131
I131 1141 LW
Acknowledgement
28 September
1161 [I71 1181 [I91 1201 Pll
Bemasek and S.R Leone, Chem. Phys. Letters 84 (1981) 401. D.A. hlantell, S.B. Ryali, B-J_. Halpern. G.L. Ha&r and J.B. Fenn, Chem. Phys. Letters 81 (1981) 185. hl. Kori and B.L. Halpcrn, Chem. Phys. Letters 98 (1983) 32. S.H. Bauer, Chem. Rev. 78 (1978) 147. J.C. Tully, J. Chem. Phys 73 (1980) 6333. CT. Campbell, G. Ertl, H. Kuipers and J. Segner, J. Chem. Phys. 78 (1980) 5862. H. Niehus and G. Comsa, Surface Sci. 93 (1980) L147. G-Ii. Hori and L-D. Schmidt, J. CataL 38 (1975) 335. G.A. hlelin and R.J. Madix, Trans. Faraday Sot. 67 (1971) 198. G.C. Bond. Catalysis by metals (Academic Press, h’ew York, 1982) ch. 7. F.S. Stone, Advan. CataL 13 (1962) 1. J.T. Yardley, Introduction to molecular enegy transfer (Academic Press, New York, 1980) ch. 5. E.R. BartIe and B.F. Myers. Am. Chem. Sot. Abstract of Papers 1969, Division of Ph> sical Chemistry _4bstmct No. 152. A.M. Dean and G.B. Kistiakowsky, J. Chem. Phys. 53 (1970) 830. KG-P. Sulzmann, L. Leibowitz and S.S. Penner. 13rd Cornbust. Symp. (1970) 137. X1.P. Heap, T.J. Tyson, J.E. Cichanowicz, R. Gel;hman and C-J. Kau, 16th Combust. Symp. (1976) 533. D. Bailey, R. Farrenq, G. Guelachvili and C. Rosetti, J. hlol Spectry. 90 (1981) 74. T. Engel and G. Ertl, Advan CataL 28 (1979) 2. B.L. Halpem and D.E. Rosner, Trans. Faraday Sot. 71 (1978) 1863. R-R. Reeves, G. Mannella and P. Harteck. J. Chem. Phys. 32 (1960) 946. A.M. Pravilov and L.G. Smirnova, Kinct. CataL 22 (1981) 416.
229
1 IO, number
3
~BRATIONALL~ AND ADSORBED
CHCMICAL
PHYSICS
18 SepteIRbcr 1984
LE-iTERS
EXCITED CO, FROM THE REACTXON OF 0 ATOMS CO ON PLATINUM
M. KORI and B.L. IiALPERN
Kcccivcd
16 hfJy 1984; in final form 26 July 1984
IligitIy excited CO2 is pruduced by the reaction 0 f CO.,da-pt CO,.* Its infrared emission spectrwn indicates that nxwe than Wfof the 4.1 CV cwcrgicity appcxs in asynrn~ctric stretch vtbmtion in a distribution pcsked roughly at icvei (009). CO; is dcstroycd by 0 atcms at high prcssurc (p 2: 10 an’l‘orr).The reaction dynamic\ .md applic.hons arc discussed.
1. Introduction Vibratio~ia~y excited products have been observed in a number of surface catalyzed reactions [I-31 _ In a previous letter 131, for example, we showed by IR emission spectrometry that CO in as high as the 7th level was released in the Ltlnglnuir-Hindielwood (LH) reaction between C and 0 on a hot Pt surface. At cquilibrium, even u = 3 would have been insignificantly popuIated at that surface temperature. Tfle population foIiowed a statistical, prior disrrib~lt~o~l and showed no inversion. Gas phase reactions arc known to give products whose vibrational distributions are either statistical or inverted and non-statistical [4]. One may ask if surface reactions show the same range of behnviar. It is therefore of interest to find a surface reaction whose product exhibits a vibrational population inversion. One candidate is the Rideal reaction between 0 and adsorbed C on Pt, for which trajectory calrulations [S) predict a CO distribution peaked at about u = 1 I. Our study [3] of C oxidation on Pt was intended to test that prediction, but experimental conditions allowed only the LH reaction giving a product with a statistical vibrational distribution_ We report now the first example of a surface reaction yieIding a population inversion: the oxidation of CO on oxidized Pt at room temperature by 0 atoms supplied directly from the gas phase. CO oxidation with 0 atoms is a good test case. If, 0 009-2614/84/S 03.00 0 Elscvier Science Publishers (North-Holland Physics Publishing Division)
B.V.
for example, it occurs at low coverage on Pt( I 1I) by a Rideal process with CO as the target, the energy released would be about 4 eV, given by the exothermicity of the gas reaction CO + 0 -+ CO,, Ai?= 5.5 cV, minus the heat of chemisorption of CO, 1A eV 161. That energy is sufflcicnt to populate CO, asymmetric stretch levels up to (0 0 16). Its less energetic counterpart, the 0, supplied, LH reaction on clean Pt, is known to produce vibrationally excited CO? in several low levels although with no population inversion [2]. Our esperiments show that highly excited CO: is indeed produced even at room temeperature from lhe 0 atom oxidation of CO on Pt oxide. The shape and position of the emission spectrum, and their variation with surface temperature and baclcground pressure indicate
a vibrational
ture of rhe reaction
2. Apparatus
inversion,
and give a distinct
pic-
dynamics.
and procedure
The experiment took place in a 500 P vacuum chamber pumped by a 10000 Q/s diffusion pump at a minimum background pressure cf 1 Ov4 Torr. Oxygen atoms and CO were directed, through separate sources, at an electrical& heatable Pt foil. This target was positioned just beyond the Geld of view of a Fourier transform infrared spectrometer (Nicoler 7 199). The 0 atoms were generated in an auxiliary glass fast flow system, linked to the main chamber, 223
Volume I IO, number 3
CHEMICAL PHYSICS LETTERS
28 September 1984
where oxygen was dissociated
with 10% efficiency by a microwave discharge. Carbon dioxide molecuIes desorbed into vibrational energy loss free conditions and some of them radiated during their transit through the observation zone, Radiation was collected and focused onto the InSb detector of the spectrometer_ The detector has maximal sen-
sitivity near 2300 cm-l; we observe fundamental transitions for the asymmetric stretch mode (u1u2u3 - u1u2u3 - I) in that range. Most of the spectra in
this study were recorded at 0.1 cm-l resolution, which was sufficient to resolve rotational lines in CO, and CO. Rotational relaxation occurred even at low pressures. The Pt surface was pretreated by heating briefly to T= 1700 K and then cooling in a stream of either 0 atomsaIone,or of 0 and CO. This formed an oxide layer above and below [7] the surface. The 0 atom CO reaction was then run at room temperature on that oxidized surface. There is evidence that carbon can be deposited when Pt is heated in CO and molecular oxygen [Sj ; carbon is unlikely to persist in the presence of 0 atoms, howeverOxygen atom pretreatment was essential, and ordy after it was perfomlcd was an infrared emitting product seen at room temperature. There was no gas phase reaction; in the absence of the Pt foil no radiation was detected.
3. Results
Even at room temperature, the 0 atom oxidation of CO on Pt oxide gives vibrationally excited CO:. A high resolution (0.1 cmW1) emission spectrum ofthe asymmetric stretch mode is shown in fig_ I for the case of pretreatment in 0 atoms alone. It has several unusual features. (a) The spectrum is diffuse, even at high resolution, and shifted to lower wave number from the familiar, first asymmetric stretch fundamental at 2350 cm-l. (b) if the background pressure of 0, 02 and CO is allowed to rise by reducing pumping speed, and keeping relative pressures constant, a discrete spectrum of COT, radiating mostly from u = 1, appears. At the same time the diffuse CO3 spectrum diminishes in intensity. This is shown in fig. 2. At 100 mTorr, the CO* component is the dominant feature. It should be 224
Fi. 1. “Diffuse” CO, emission spectrum at hi& (0.1 cm-‘) resolution and low backgound pressure The h&h den&p of overlappinglines is not shown,
noted that while the diffuse CO; component diminishes, it does not shift to higher or lower wavenumber. There are thus two components in the same frequency range. (c) If the surface ten~perature is increased at low pressure the familiar resolvable band centered at 2350 cm-1 appears along with the diffuse spectrum. The band at 2350 cmWi is identical to that seen in the 0, supplied, U-I oxidation of CO [2] on unoxidized Pt metal. This third “?ormaI” CO; component increases and the diffuse CO; component decreases as the temperature rises. At T> 600 K, little of the diffuse component remains, as seen in fig. 3. The oxide layer is beginning to be removed, at higher T, and growth of the 2350 cm-’ band is related to that removal. (d) If the surface is pretreated with CO as well as 0 atoms, the “normal” component also appears when 0 atoms react with adsorbed CO. But now it appears even at room temperature in contrast with 02 supplied oxidation.
Volume 110, number 3
CHEhlICAL
PHYSICS
LE’MXRS
28 September
1984
4. Discussion
t 2350
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8
I
2151)
WAVEN”MBER
1
2050
2150
CY
1950
-I
Fig. 2_ Emission spectrum at 11th background pressure (28 mlbrr) showing diffuse CO* component and emergtis CO discrete spectrum. T
OK
Fig. 3. Effect of surface temperature on relative intensity of diffuse CO2 component and “normal” CO2 component. Lies in the normal component are resolvable, but the scans shown are at low resolution.
We will try to show that the following themes arc consistent with the observations (a) through (d). (I) CO; is genefateti in very high vibrational levels by reactio: of 0 and CO on Pt oxide. Most of the vibrational energy appears in the asymmetric stretch mode with v,, = 16, and vaverqe = 9. (2) The excited CO; can react with 0 atoms at higher background pressure to give 0; and slightly excited CO*_ The spectrum of Gg. 2 E due to both excited CO; and CO?. Before developing these ideas and explaining results (a)-(d), we consider the possibIe mechanisms by which such highly excited COT can arise_ It is natural to compare the reaction of 0 and CO on oxidized Pt to the commonIy studied CO oxidation on clean Pt. That reaction is known to be a LH process with a small 1x5 kcaljmole) exothermicity, and an activation energy of slightly less than 1 eV [6]_ About 1 eV can therefore appear in product vibration, and, in agreement with that fact, IR emission measurements have shown that no significant amount of CO, is excited to beyond v = 2 in asymmetric stretch [2]. The LH process, in which the chemisorption energies of CO (ml.3 eV) and 0 atoms (z3.9 eV) are both large [6], thus produces a CO, molecule with relatively little excitation. For the 0 atom oxidation of CO on Pt oxide we consider several possibilities that might lead to highly excited COI. It is unlikely to be a LH step with a 4 eV activation energy since such a reaction could not take place at room temperature as we observe. Conceivably it is a step in which both CO and 0 are essentially physisorbed and react as a two dimensional gas. The available energy for the product would then be the gas phase exothermicity x5.3 eV. It is difficult to judge the likelihood of this mecha~m_ The reaction probab~i~y depends critically on the physisorption energies of CO and 0 on the Pt oxide; these energies are not known. We note however that in general 0 atoms do not recombine efficientIy on metal oxides [9] _If a 2-L) mechanism were probable, it might be expected to promote atom recombination too. A simple alternative could be a variant of the Rideal process, as described by Bond [ lOI_ For ex-
225
Volume
1 IO, number 3
CHEMICAL
ample, an 0 atom impacts and physisorbs near a chemisorbed CO, and attacks it in the plane of the surface. If the surface were clean, the 0 atom could
onant transfer of even one quantum [ 121. For nonresonant transfer or at lower pressure, V-V transfer
chernisorb: tfle role of tfte Pt oxide is to prevent chemisorption and permit approach from the side. since tfte direct approach from tile gas phase to the “wrong” end of the CO might not favor reaction. Whatever mechanism is operative, it is clear that both CO and 0 cannot be strongly bound on Pt oxide. To produce a CO: product fn as frigfi as u * 15, the sum of the bindr$g energies of botfr CO and 0 cannot be greater tfran 1.3 eV. ffowever, we know little about the cftemisorption of CO and 0 on a Pt oxide surface prepared by heating Pt in 0 atoms. CO does cfremisorb with about 1 eV adsorption energy on a number of oxides [I If , and the Rideaf type step we described is plausible in that it can account for the Iligfl product excitation. But It is not possrble to decide clearly in favor of tfrat mecfranism by this study atone. It wiff be shown that higher levels of CO; asymnietric stretch vibration are preferentrally exciteh. Thus, dnfrarmonicrty shifts the vibrational transition to lower wavenumber from tftc 23.50 cm-* fundamentaf, so the CO: spectrum is found close to that of CO?_ The spectrum is diffuse because in ahigfrly excited triatomic the number of transitions(u~u~~~ - ZQU+~ --I) is large, and rotational fines cannot be dis&i&iisfted even at 0. I cm-1 resolution because of overlaps. The fact that excited COr is formed at higfrer pressure is a key to our interpretation_ At higher pressure, desorbmg CO3 can collide with incoming 0, 02 and CO. it would seem at first sight that CO;
could direct-
ly excite CO by V-V transfer. The level spacings of CO and CO3 asymmetric stretch are similar; the (009 - 00s) tran2tion of COT is almost exactly resonant with the lowest CO tram&ion. But there are three strong arguments against colfi-
sfonaf excitarion. First, CO; makes too few collisions between surface and observation zone to permit such energy transfer. By a simple random walk argument, the total number of collisions Z is given by 2 =fd”l’lf,
= o.t.&
I
wfrere p is tire chamber pressure in mTorr, CO, diffusion coefficient, d is the distance the observation zone, and fis the collision per mofecufe. Only at the highest pressure mTorr is rfie number of collisions sufficient 226
28 September 1984
PHYSICS LE-ITERS
(0 D is tfte through frequency of 100 for a res-
could not produce the efficient CO; depletion that is observed. Second, even if the number of co&ions did suffice
for energy transfer the CO; spectrum shoufd not merely decrease in intensity; rather it should shift toward higher frequency as CO; molecules accumulate in lower levels. Eventually, one would observe radiation from the resolvable band at 2350 cm-l_ However, this does not occur; the CO1 diffuse spectrum dfminishes but does not shift. Third, the changes in the spectrum do not depend strongly on the nature of the gas used to raise the background pressure, e.g. He, CO,, N2 or CO. These coffision partners frave greatly different energy transfer probabilities, so energy transfer cannot be important in CO; depletion. One is forced to conclude that the pressure induced onset of CO emission and the accompanying decrease of CO? diffuse emission cannot be accounted for by a near-resonant V-V, or any fess efficient energy transfer process. The only remaining possibility is that the excited COf is being destroyed by chemical reaction with one of its collision partners, and that CO is generated as a reaction product. In our system the onfy potential reaction partners are 0, 0, and CO in tfreir ground states, and the excited species 02(lA), O,(lE), O(lD) and O(*S). Of these, only 0, 02, CO and O,(lA) occur at significant concentration. Of the few possible reactions that may be written down the only one that is energeticaffy feasible and consistent with the observed efficient generation of CO is O(3P) + co1 + COT + 0;
.
(0
Both reaction (f) and its reverse have been tfre subject of shock-tube studies [S-15] because of their role in combustion. With ground vibrational state CO,, the reaction is endothermic by 0.34 eV. The activation energy has been reported to be x2.6 eV [ 16f _ For reaction (I) to proceed at room temperature and high pressure, some CO: molecules would have to carry that activation energy in vibration. We cannot in our system demonstrate by an independent experiment that reaction (1) does occur. The CO emission is itself perhaps the best proof; since most of tfie reaction energy should appear in the new
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CHEMICAL PHYSICS LElTERS
0, bond, it is resonable that CO be only slightly excited. We argue that reaction (I) is the only explanation of result (b) that is consistent with improbable collisional excitation and efficient CO production_ In fact reaction (I) proceeds with great efficiency_ At 10 mTorr where CO; makes about one collision with an 0 atom in the observation zone, a noticeable fraction of the diffuse CO: intensity has disappeared_ The diminution is pronounced at 28 mTorr, as seen in fig. 2. The inescapable conclusion is that mosr of the CO; molecules indeed have an energy in vibration sufficient to drive reaction (I) with few collisions: the average CO; vibrational energy must then be =Z6 eV. One could determine the distribution of that energy among the three vibrational modes of CO, by synthesizing a spectrum to match that of fig. I. Before discussing our synthesis, we examine three limiting cases in which all the available energy resides in one of the three modes. The fact that reaction (I) is efficient constrains the choice of energy distribution. Suppose that only asymmetric stretch is excited. Because the spectrum begins 50 cm-l from the first CO, fundamental, the lowest occupied level must be =(003X The highest level must be (0 0 16) to account for the width of the spectrum (350 cm-l)- This is because the spectrum consists of vibrational bands (00~ -+ 00~ - 1) each shifted to lower frequency by x24.5 cm- 1 due to anharmonicity. If the population distribution is roughly symmetric, it would peak at =(009) and since the asymmetric stretch spacing is N-25 eV, the average energy woud be a2.3 eV. Similar reasoning holds if only symmetric stretch or bending is excited. One quantum is required in asymmetric stretch for an emission band near 2350 cm- 1 to be seen at all, but each added quantum in bending or symmetric stretch will provide an additional band shifted from and superposed on the first. The shift per quantum of bending is ~12 cm-l, while the shift per quantum in symmetric stretch is ~34 cm-l. To account for the observed position and width of the spectrum, the lowest bending level would have to be (041) and the highest ( 0 30 I). The lowest symmetric stretch level would be (201) and the highest (15 0 1). The average enerm in both cases would be about 1.3 eV; the equality arises because twice as many bending levels must be occupied to explain the spectral width, but each quantum is half the energy of that in symmetric stretch.
28 September 1984
These extreme cases show that the width and position of the spectrum can be explained by very different distributions. The position of the maximum in the diffuse spectrum immediately implies that the distribution in at least one mode must be peaked. However, only if the energy is overwhelmingly in asymmetric stretch will the average energy suffice to drive reaction (I) with high probability_ To reinforce this picture we synthesized a spectrum according to the following admittedly approximate procedure_ Because of the coincidence of shifts caused by occupied levels in asymmetric stretch, bending and symmetric stretch modes (12, 24 and 24 cm-r, respectively) the centers of bands contributing to the spectrum can only occur at roughly 12 cm-l intervals in the range 2300-1900 cm-l. The spectrum was therefore regarded as being composed of 32 identical bands. whose rotational line positions in the P and R branches were given by standard expressions [ 171, and whose centers were placed at 12 cm-l intervals_ Each rotational line was taken to be triangular with a half width equal to the resolution_ A rotational temperature of 350 K was assumed since relaxation did occur. Each band has contributions from several states. For example, that centered at 2176 cm-l includes transitions from (004), (033), (103), (043) (122) (202) (061), (141) (221) and (301). The band intensities were summed to give the best fit to the experimental spectrum; the summation was done by computer at 5000 points and plotted at 1000. The number of superposed bands that will yield a diffuse spectrum can be estimated as the rotational line spacing divided by the resolution; in our case, even 16 bands would have sufficed_ But it is not possible to derive unique ener,T distributions for all three modes from that synthetic spectrum. For example, because of the shifts described above, the transition (041) to (040) would place emission lines in the same position as (201) to (200). Nevertheless, one can assume distributions for bending and symmetric stretch which involve little energy, and then derive a distribution for asymmetric stretch. Fig. 4 shows one distribution so derived. Bending and symmetric stretch were assumed Boltzmann at T = 1700 K and T = 800K, respectively as shown in ref. [?I; the resulting asymmetric stretch distribution P(v) is consistent with a -2.6 eV average energy required to drive reaction (I). Why should the reaction of CO and 0 on a Pt oxide 227
Volume 110, number 3
CHEMICAL PHYSICS LETTERS
28 September
1984
main areas of the surface that are not heavily oxidized on which 0 and CO can chemisorb strongly_ When 0 atoms are supplied during reaction, the LH mechanism can occur even at room temperature as in result (d) to given a CO, product with low vibrational energy emitting near 2350 cm-l_ This is because 0 atoms can compete for sites with CO [I8 ] _If 0, rather than 0 is supplied then the LH step occurs only at high temperature_ 02, unlike 0, cannot compete for sites with CO and at room temperature the surface is saturated with CO. Reaction proceeds at high T when CO desorption permits dissociative O2 adsorption [IS]. CO, does not adsorb on clean Pt and is unlikely to adsorb on oxidized Pt. Therefore extensive relasation of CO: vibration by electron-hole pair creation is improbable since desorption would occur rapidly following COT formation.
5. Summary 0
10
5
Room tempenture on PI yields excited
V
I‘ig_ 4. Rcl~tive population in asymmetric stretch fevcls .ISsumin~ Iox\ energy in bending and asymmetric stretch.
throw ener,3, prcfercntially into the asynmerric stretch ~athcr than distributing it more equally among all modes” The LH oxidation of CO on clean Pt, with bound 0 atoms, also favors the asymmetric over the symmetric mode. In both reactions the CO: is formed “asynlmetrically” from a short CO bond and an inireally long O-C...0 bond. Evidently the Pt oxide layer has so reduced either or both of the CO and 0 adsorp11on energies, that reaction begms to resemble a gas phase encounter in which selective energy disposal is oflen seen. Fkaction is so rapid that energy randomi/.ation cannot occur as it did. for example, in oxidation of C on hot Pt to give CO. Raising the Pt foil tcmpclaturc breaks down the oxide and exposes bare Pt, whereupon the normal LH mechanism becomes operative. As the oxide disappears fewer highly excited COT molecules are formed on it and emission from the Lp product, formed on the clcnn l’t fractron. becomes dominant, as in result (c) dnd the spectrum of fig. 3. If CO is present during pretreatment.
and conclusions
there will re-
oxidation
of CO with
0 atoms
CO,, bearing on average ~3.6 eV in asymmetric stretch. The asymmetric stretch population peaks at about level (009). Excited carbon dioxide is depleted at high pressure by reaction with 0 atoms to yield CO. The reaction has a high barrier but is driven by the CO1 excitation_ The surface reaction is of interest on several grounds. This is the fist example of a surface reaction whose product has a non-statistical, inverted vibrational energy distribution_ It may be compared to the LH oxidation of C on Pt where 0 atoms were supplied by 0, dissociative adsorption [3] _There CO emerged from a long-lived complex of C, 0 and Pt and desorbed with a statistical prior distribution_ The complex was longlived because all participants were adsorbed or bound, several bonds had to be broken, and the product CO could adsorb. The formation of CO: in this present study is rapid; there is insufficient time for energy to be statistically partitioned among product modes and heavy surface atoms. In the LH oxidation of CO, the 0 atom is bound, and the CO, distribution is statistical, characterized by a vibrational temperature [z]The reaction suggests itself as a possible Rideal. mechanism by virtue of the high energy of its CO; product. It is difficult to confirm Rideal steps in gencral, but they have been invoked to explain incomplete
Volume 110, number 3
CHEMICAL
PHYSICS LETTERS
chemical reaction energy accommodation in atom recombination on metals 1191. If both 0 and CO adsorbed as strongly on Pt oxide as on clean Pt, at most 1 eV would be available for product excitation. That would not permit the energy disposal actually seen. The high vibrational energy, high density of states, and inverted distribution of the product CO; suggest the 0 atom reaction with CO on Pt oxide as the basis for a CO, chemical laser. Such a laser would be tunable over a wide frequency range near 2300-1000 cm-l. There is as yet no laser based on a surface catalyzed reaction. It is interesting that the 0 atom pretreated Pt catalyst used here so strongly catalyzes formation of vibrationally excited molecules. Electronically excited molecules have been formed with low probability on oxygen covered metals [20] and on glass [21] _While electronic excitation is energetically unlikely in our study, we sugest that atom reactions on heavily oxidized metals are a useful source of excited molecules. In particular. the oxidation of CO with 0 atoms on oxidized Pt bears further study.
This work has drawn support from NSF grant CPE8114348, Petroleum Research Fund FPRF 13443-ACS, and Air Force grant OSR F499620-80-C-0026. We are grateful for the invaluable assistance of J.B. Fenn, D.A. Mantel1 and S.B. Ryali.
i 984
References
111S. [?I [31 [41 151 VI 171 181 PI 1101 IllI 1131
I131 1141 LW
Acknowledgement
28 September
1161 [I71 1181 [I91 1201 Pll
Bemasek and S.R Leone, Chem. Phys. Letters 84 (1981) 401. D.A. hlantell, S.B. Ryali, B-J_. Halpern. G.L. Ha&r and J.B. Fenn, Chem. Phys. Letters 81 (1981) 185. hl. Kori and B.L. Halpcrn, Chem. Phys. Letters 98 (1983) 32. S.H. Bauer, Chem. Rev. 78 (1978) 147. J.C. Tully, J. Chem. Phys 73 (1980) 6333. CT. Campbell, G. Ertl, H. Kuipers and J. Segner, J. Chem. Phys. 78 (1980) 5862. H. Niehus and G. Comsa, Surface Sci. 93 (1980) L147. G-Ii. Hori and L-D. Schmidt, J. CataL 38 (1975) 335. G.A. hlelin and R.J. Madix, Trans. Faraday Sot. 67 (1971) 198. G.C. Bond. Catalysis by metals (Academic Press, h’ew York, 1982) ch. 7. F.S. Stone, Advan. CataL 13 (1962) 1. J.T. Yardley, Introduction to molecular enegy transfer (Academic Press, New York, 1980) ch. 5. E.R. BartIe and B.F. Myers. Am. Chem. Sot. Abstract of Papers 1969, Division of Ph> sical Chemistry _4bstmct No. 152. A.M. Dean and G.B. Kistiakowsky, J. Chem. Phys. 53 (1970) 830. KG-P. Sulzmann, L. Leibowitz and S.S. Penner. 13rd Cornbust. Symp. (1970) 137. X1.P. Heap, T.J. Tyson, J.E. Cichanowicz, R. Gel;hman and C-J. Kau, 16th Combust. Symp. (1976) 533. D. Bailey, R. Farrenq, G. Guelachvili and C. Rosetti, J. hlol Spectry. 90 (1981) 74. T. Engel and G. Ertl, Advan CataL 28 (1979) 2. B.L. Halpem and D.E. Rosner, Trans. Faraday Sot. 71 (1978) 1863. R-R. Reeves, G. Mannella and P. Harteck. J. Chem. Phys. 32 (1960) 946. A.M. Pravilov and L.G. Smirnova, Kinct. CataL 22 (1981) 416.
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