6070
J. Phys. Chem. 1991, 95, 6070-6072
corresponding hydrogenic wave functions are characterized by a strong localization of the H atom which can be analyzed in terms of bending and asymmetric vibrational modes of the 12Hsystem. The use of an attractive electronic hypersurface has confirmed this analysis which has been extended up to v2 = 3. Present results, obtained in independent calculations for various symmetries of the D,* point group, are found to be very consistent. In particular the regular energy spacing of the specific hydrogenic wave = 2 can be functions and the near degeneracy observed for ~r~ considered as a proof of the validity and accuracy of the present method.
The second result is that the above specific hydrogenic wave functions and the corresponding antibonding wave functions are expected to play an important role in the H-atom-exchange process in IH + I collisions. Since the number of these states and their u-g energy differences are strongly dependent on the electronic surface characteristics, the low-energy H-atom-exchange cross sections will be very different assuming either an attractive or a repulsive electronic surface.
Acknowledgment. We are grateful to Professor E. E. Ferguson for a careful reading of the manuscript.
Hydroxyl Radical Formatlon during Methane Oxidation and Water Decomposttlon on Pt Measured by Laser-Induced Fluorescence Charles E. Mooney, Louis C. Anderson, and Jack H. Lunsford* Department of Chemistry, Texas A&M University, College Station, Texas 77843 (Received: May 13, 1991)
Laser-induced fluorescence (LIF) was used to monitor the formation and desorption of OH' radicals over a Pt wire during the catalyzed oxidation of methane. As in the Pt-catalyzed oxidation of hydrogen, the apparent activation energy for OH' desorption, E,, depends strongly on the fuel-to-oxygen ratio, varying from -33 kcal/mol at high 02/CH4 values to -56 kcal/mol at low 02/CH4 ratios. The reaction of surface oxygen with D2 during the decomposition of H 2 0 on Pt shows that the surface oxygen concentration influences E, for OH' desorption on platinum.
Introduction The hydroxyl radical is one of the most important radicals involved in catalyzed chemical reactions and combustion processes. In the work described here, we have used laser-induced fluorescence (LIF) spectroscopy to study the formation and desorption of this reactive radical intermediate during methane oxidation and water decomposition reactions on a polycrystalline Pt wire. Since many of the same surface intermediates in these two reactions are also involved in the H2 O2 reaction, we have examined hydrogen oxidation on Pt as well. The goal of this research is to more fully understand the surface reactions, including the desorption of the OH' radicals, during heterogeneously catalyzed oxidation reactions. Hydroxyl radicals adsorbed on platinum were first identified by Fisher and Sext0n.l They coadsorbed O2 and H 2 0 on a Pt( 1 1 1) surface. The surface species was detected by ultraviolet photoelectron spectroscopy (UPS) and electron energy loss spectroscopy (EELS). The OH' radical has also been detected during H2 oxidation on 02-treated Pt(l11) with EELS2" Platinum facilitates the formation and reaction of OH' on the surface, but also it can produce gas-phase radicals by desorption during catalyzed oxidation reactions. Using an H2/02gas mixture in an Ar buffer, Talley et aLM have shown by LIF spectroscopy that OH' can desorb from Pt. The OH' radical is also found to desorb from other catalytically active metals (Pd, Rh, Ir, Ni) during H2 oxidation as shown by LjungstrBm et a1.P using LIF. Moreover, this technique has been used to detect hydroxyl radicals desorbing from 02-treated Pt during the decomposition of watefl and in the ethane-air boundary layer near a Pt surface during catalytically stabilized thermal comb~stion.~ Although OH' radicals have been detected over Pt during catalyzed hydrogen oxidation, Berlowitz et al. found no evidence for CH3' radical desorption over Pt during the oxidation of methane using the matrix isolation electron spin resonance (MIESR) technique.I0 (The MIESR technique can easily detect methyl radicals emanating from metal oxide catalysts during methane partial oxidation.)I' The absence of gas-phase methyl
+
*To whom all correspondence should be addressed.
radical indicates that the Pt-C bond is strong enough to cause a long residence time on the catalyst allowing for further dehydrogenation of the surface intermediate. In this paper, we present LIF reaults showing that OH' radicals desorb into the gas phase during the Pt-catalyzed oxidation of methane. Recently, Marks and Schmidt12have reported on OH' radical desorption during catalytic combustion on Pt foil, but their experiments were carried out at higher temperature and reactant partial pressures than in our work. The conditions of our experiments were deliberately set to minimize the possibility of reactions in the gas phase; i.e., these are reactions occurring on the Pt surface rather than in gas-phase combustion as previously described by Pfefferle et al.9 We emphasize here the importance of the OH' radical desorption step in the overall energetics of Pt-catalyzed oxidation reactions. To this end, we also present new data on 0-assisted decomposition of H20on Pt in the presence of D2 to illustrate how the surface oxygen concentration affects the apparent activation for the appearance of OH' radicals in the gas phase. Experimental Section In these experiments a CHI (Matheson UHP), 4 (Matheson extra dry), and He gas mixture flowed through a leak at 2 mL min-' (STP) into a Pyrex vacuum chamber with fused silica (1) Fisher, G. B.; Sexton, B. A. Phys. Reu. Lea. 1980, 44, 683. (2) Germer, T. A.; Ho, W. Chem. fhys. Lett. 1989, 163,449. (3) Mitchell, G. E.; White, J. M. Chem. Phys. Lett. 1987, 135, 84. (4) Tevault, D. E.; Talley, L. D.; Lin, M. C. J . Chem. fhys. 1980, 72, 3314. (5) Talley, L. D.; Tevault, D. E.; Lin, M. C. Chem. fhys. Lett. 1979,66, 584. (6) Talky, L. D.; Sanders, W. A.; Bogan, D. J.; Lin, M. C. J. Chem. Phys. 1981, 75, 3107. (7) LjungstrBm, S.; Hall, J.; Kasemo. B.; Rostn, A.; Wahnstrh, T. 1. Catal. 1987, 107, 548. (8) Talley, L. D.; Lin, M. C. Chem. fhys. 1981,61, 249. (9) Pfefferle, L. D.; Griffin, T. A.; Winter, M.; Crosley, D.; Dyer, M. J. Combusr. Flames 1989, 76, 325. (IO) Berlowitz, P.; Drircoll, D. J.; Lunsford, J. H.; Butt, J. B.; Kung, H. H. Combust. Sci. Technol. 1984, 40, 317. (11) Driscoll, D. J.; Martir, W.; Wang, J.-X.; Lunsford, J. H. J . Am. Chem. Soc. 1985, 107, 58. (12) Marks, C . M.; Schmidt, L. D. Chem. Phys. 1991, 178, 358.
0022-3654191 12095-6070302.50/0 0 1991 American Chemical Society
The Journal of Physical Chemistry, Vol. 95, No. 16, 1991 6071
Letters
.-E100 5
100
c'
* '
I
'
'4
80 60
i
CI
e
3
I-
40
Q,
U
-c 20 u, 4
1
0
0 2 4 6 8 10 12 14 16
P, mtorr Figure 1. Gas-phase hydroxyl radical LIF signal as a function of CHI and H2 pressure near a polycrystalline Pt wire at T = 900 OC: 0, CH4 with po2 = 4.8 mTorr; @ H2 with p = 5.3 mTorr. The total pressure was 60 mTorr with the balance as a e . windows. In some experiments H2 (Matheson UHP)and/or H 2 0 was substituted for CH,. The gases passed over a 0.5-mm-diameter coiled Pt wire (Johnson Matthey) which was suspended about 5 mm above the path of a laser beam (3-5-mm diameter). A thermocouple junction was spot-welded on the coil for temperature measurements. Inside the vacuum chamber the total pressure was typically 60 mTorr; He constituted 7591, or more of the gas mixture. Whenever possible, the total pressure was kept below 100 mTorr to reduce the effects of gas-phase reactions. Laser excitation of the OH' radical (A2& u' = 1 X2n, u = 0, near 282 nm) was provided by the frequencydoubledoutput of a tunable dye laser (bandwidth -0.3 cm-', pulse width - 5 ns, energy 1 mJ/pulse in the UV region). The dye laser was pumped by the second harmonic of a Q-switched Nd:YAG laser at 10 Hz. Fluorescence emission (A2& u' = 1 X2n, u"= 1, near 315 nm)', was collected 90° from the laser beam into a spectrometer with a UV-sensitive PMT. Output pulses from the PMT were processed by a gated photon counter (gate width 1 M) which was triggered by the Q-switch of the YAG laser. For most of the data presented here the Ql(4) band was used for excitation.
-
-
-
-
Results and Discussion Although Pt is a good complete combustion catalyst, no C 0 2 was detected under our reaction conditions, as determined by mass spectrometry. The main products of CH4 oxidation in these experiments were CO and H20. T h e results are consistent with studies of methane oxidation on supported Pd and Pt catalyst where CO production increases over C 0 2 production at higher
temperature^.'^ Figure 1 shows the dependence of the LIF intensity on the partial pressure of CH, and H2 at 900 OC. The intensity went through a maximum for each gas, but a plateau was evident with CH,. The variation of LIF intensity with H2 pressure agrees with the results of Tevault et al.' The similarity observed with H2 and CH4 is likely a result of reactions occurring on the surface. With both reagents the platinum surface abstracts hydrogen atoms which can react with surface oxygen, forming OH' radicals. A more complete reaction scheme (proposed by Tevault et a1.4) for hydrogen oxidation with adsorbed surface species (subscript "s") and gas-phase species (subscript "g") is shown below. H24
OH,
2H,
+ -
024
H,
--
+ 0,
OH,
20, OH,
(1) (2) (4)
OH,
(5)
(13) Clyne, M. A.; McDermid, I. S.;Curran, A. H. J. Phorochcm. 1976, (14) Cullis. C. F.; Willat, B. M. J . Curul. 1983, 83, 267.
8.4
8.8
9.2
9.6
1OVT[K) F l g m 2. krhenius plot of gas-phase hydroxyl radical LIF signal during methane oxidation on Pt wire with various O2/CH4 ratios: V, O2/CH4 = 5.4, E, = 33 & 3 kcal/mol; 02/CH4 = 0.22, E, = 48 3 kcal/mol; 0 , O2/CH4 = 0.088, E, = 56 f 3 kcal/mol.
*
For H2 oxidation, the hydroxyl radical LIF intensity increases with increasing p H Iuntil a maximum occurs at a 0 2 / H 2ratio of about 1/1 as shown in Figure 1. The LIF intensity then decreases with increasing pH2.This decrease in the LIF signal is, at least in part, a result of the formation of water as shown in reaction 4.,J Since reactions 4 and 5 are competing reactions for the surface hydroxyl radical (OH,), increasing H, forces a higher production of H20, and hence the LIF signal for gas-phase hydroxyl radical (OH ) decreases. For methane oxidation the hydroxyl radical LIF dependence on pCH,is more complicated. The initial step for Pt-catalyzed methane oxidation is shown in reaction 6 where hydrogen ab-
straction forms H,and Pt-C species. Subsequent dehydrogenation of the adsorbed CH, radical continues to increase the H, concentration. The other pertinent steps in the process are reactions 2-5. As shown in Figure 1, at low pcH,hydroxyl radical production appears to be first order in pea. But as pct4 became >6 mTorr, the LIF signal remained nearly constant, and at 12 mTorr it decreased rapidly. Since CH4 has four H atoms per molecule, one might expect the LIF signal to go through a maximum at a lower partial pressure than observed with H2. However, the sticking coefficient for methaneI5on Pt is much smaller than for hydrogen;I6 thus, much more methane is required to give the H, concentration needed to decrease the OH' LIF signal. The concentration of gas-phase OH' radical is dependent not only on the concentrations of OH, but also on the activation energy for OH' desorption (reaction 5). Figure 2 shows Arrhenius plots of hydroxyl radical LIF intensity for three different 02/CH4 ratios. These data show that the apparent activation energy for OH' desorption, E,, is very sensitive to this ratio which changes from 33 f 3 kcal/mol at 02/CH4 = 5.4 to 56 f 3 kcal/mol at O&H4 = 0.088. Here pcH, was in the range 2.4-36.5 mTorr, while p q was varied from 12.9 to 3.2 mTorr. In like manner, the apparent activation energy for OH' radical desorption depends on the 02/H2 Fujimoto et al. found that E, ratio for H2 oxidation on Pt.17-19 increased from 27.4 kcal/mol at 0 2 / H 2= 100 to 56.2 kcal/mol at 0 2 / H 2 = 0.038." For both the CH, and H2 oxidation on Pt the activation energy for the appearance of OH' in the gas phase appears to be inversely related to the extent of oxygen coverage on the surface. A very similar trend was reported for oxygen desorption from Pt( 1 1 1).
(3)
HI H20,
J, 201.
8.0
(15) Schoofs, G. R.; Arumainayagam, C. R.; McMaster, M. C.; Msdix, R. J. Surf. Sci. 1989, 215, 1. (16) Poelsema, B.; Verheij, L. K.; Ca", G. Surf. Sci. 1989, 152, 496. (17) Fujimoto. G. T.;Selwyn. G. W.; Keiser. J. T.; Lin, M. C. J . Phvs. Chem..1983,87, 1906. (18) Hsu. D. S.;Hoffbauer, M. A,; Lin, M. C. Lmngmuir 1986, 2, 302. (19) Wahnatram, T.; Fridell, E.; Ljungstrbm. S.;Hellsing. B.; Kaaemo, B.; Rostn, A. Surf. Scf. k t r . 1989, 223, L905.
Letters
6072 The Journal of Physical Chemistry, Vol. 95, No. 16, 1991
1 8.1
8.5
8.9
9.3
1o ~ / T ( K ) Figure 3. Arrhenius plot of gasphase hydroxyl radical LIF signal during H20decompasition on R wire with and without D2(or H2)present @ H p = 1.5 mTorr and pq = 8.8 mTorr): 0 , = 0, E, = 32 f 2 kcal/mol; A, pH = 1.7 mTorr, E, = 41 & 2 kcal/mol; W, b2= 1.7 mTorr, E, = 39 &
3 kcnllmol.
on which the heat of desorption for atomic oxygen increased from 40 kcal/mol at high fractional oxygen coverage [in a (2X2)O overlayer] to 110 kcal/mol at low fractional oxygen coverage.20 At the temperature of our experiments, this ordered overlayer was not present. We also studied the oxygen-assisted decomposition of H20on Pt, as shown in reaction 7, in order to establish that E, for OH'
H20,+ 0,-w 20H,
(7)
appearance is affected only by the oxygen coverage. In the gas phase water decomposition is thermodynamicallyunfavorable, but on Pt water reacts exothermically by -8 kcal/mol with surface oxygen to form two hydroxyl radicals.21 The activation energy for OH' desorption formed during reaction 7 also depends on the steady-state 0, concentration as shown below. Figure 3 shows the Arrhenius plot for water decomposition on Pt (reactions 5 and 7), with and without D2present. With no D2present to remove 0, the apparent activation energy for OH' radical desorption was 32 f 2 kcal/mol with pHlo= 1.5 mTorr and p = 8.8 mTorr, which agrees well with literature results.1° Wheny.7 mTorr of D2was added to the system, E, increased to 40 f 3 kcal/mol. At pD,= 4.3 mTorr (not shown in Figure 3), E, increased to 48 f 3 kcal/mol. These phenomena can be (20) Gland, J. L.; Sexton, J. L.;Fisher, G. 8.Surf. Sci. 1980, 95, 587. (21) Anton, A. B.; Cadogan, D. C. Sur/. Scl. Left. 1990, 239, L548.
explained by noting reactions 3 and 4. As the concentration of D2 is increased, 0, decreases with the formation of OD' and D20. Since OD' fluoresces at a different spectral region than OH', its presence does not interfere with the OH' LIF signal which results from H 2 0 decomposition. When H2was substituted for D2during the water decomposition reaction, the LIF intensity increased as shown in Figure 3, but the value of E, was the same, within experimental uncertainty. The activation energy was 41 f 2 kcal/mol at pH, = 1.7 mTorr and 50 f 2 kcal/mol at pH,= 4.3 mTorr. (The latter data is not shown in Figure 3.) These results indicate (i) that H2and D2 remove 0,with equal efficiency and (ii) that reaction 3 is a much more efficient source than reaction 7 for OH' radicals. The observation that E, does not change when H2is substituted for D2 helps confirm that step 5 is the rate-limiting step for the appearance of OH' radical in the gas phase. These results also indicate that the breaking of the Pt-OH bond is the rate-limiting step for the appearance of hydroxyl radicals in the gas phase for the CH, + O2reaction. The largest value of E, = 56 f 3 kcal/mol (for a methane-rich mixture) for our polycrystalline Pt wire catalyst is in quite good agreement with that reported for OD' desorption during the D2 O2reaction on Pt(ll1) in the limit of low oxygen coverage.21 This value of E, for OH' desorption approaches the extrapolated value of 60 kcal/mol previously reported by Fujimoto et al. during hydrogen oxidation on Pt foil under oxygen-depleted conditions."
+
Conclusions We have shown how the detection of gas-phase OH' radicals by LIF can provide information on the surface reactions that occur during CH4 oxidation on a Pt wire. These results indicate that CH, supplies surface hydrogen and that the ensuing reactions are the same as those proposed during H2 oxidation. The effect of surface oxygen on the appearance of gas-phase OH' radicals has been clearly demonstrated by studying the oxygen-assisted decomposition of H20on Pt. In all three systems, hydrogen oxidation, methane oxidation, and water decomposition, the apparent activation energy for hydroxyl radical desorption depends on the surface oxygen coverage, with E, values varying from -30 kcal/mol for Pt with high oxygen coverage to -60 kcal/mol on Pt under oxygen-depleted conditions. Acknowledgment. We acknowledge financial support of this work by the Division of Chemical Sciences, Office of Basic Energy Sciences, U S . Department of Energy. R a e NO. R,7440-06-4; CH,, 74-82-8; H2, 1333-74-0; OH', 3352-57-6; H20,7732-18-5.
