Langmuir 1994,10, 699-708
699
A Laser-Induced Fluorescence Study of OH Desorption from Pt in H20/02 and H20/H2 Mixtures Erik Fridell' and Arne Ros6n Department of Physics, Chalmers University of Technology and University of Giiteborg, S-412 96 Giiteborg, Sweden
Bengt Kasemo Department of Applied Physics, Chalmers University of Technology and University of Giiteborg, S-412 96 Giiteborg, Sweden Received December 7,1992. I n Final Form: January 7, 1994" The desorption of OH from high-temperature-annealedpolycrystalline Pt in H20/02, H20/H2 (D2),and HzO/Oz + Hz (D2) mixtures has been investigated using the laser-induced fluorescence (LIF) technique. The measurements were done in the pressure range 1-lo00 mTorr and temperature interval 900-1300 K. The experiments yield new interesting information about water decomposition on Pt, unimolecularlyand in the presence of 02 and/or H2, and provide complementary results to earlier studies of water formation from H2 + 0 2 . There is a small but significant OH desorption even in pure H2O. In H20/02 mixtures the OH desorption rate has a maximum at 22-30% 0 2 content. The absolute desorption rate increases nearly linearlywith totalpressure. The maximumyield occursat successively lower relative 0 2 concentration, the higher the pressure. In H20/H2 mixtures the OH desorption is strongly suppressed due to reaction of OH with adsorbed hydrogen atoms. H20/D2 mixtures yield OD and OH desorption. The experimental results are well described by a previously developed kinetic model for the H2 + 1/202 H20 reaction. Approximateanalyticalexpressionsare derived for the kinetics and compared with full numerical solutions. Interestingly the dominant path for HzO decompositionseems to be the unimolecular decompositionH2Oa Ha + OH. rather than the bimolecular reaction HzOa + 0 ' e 20Ha (a = adsorbed). The promoting effect of oxygen is kinetic,rather than mechanistic;i.e., the presence of 0 atomsmoves the surfaceequilibrium to higher OH concentration, rather than providing a new reaction path via direct H2O' + 0. interaction. An evaluation procedure is briefly outlined on how to extract the true OH desorption rate from the measured LIF' intensities at pressures where collisional gas-phase quenching and pumping speed effects are important.
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1. Introduction In this work we have investigated the OH formation/ desorption from H2O on Pt at high temperatures (>900 At low temperature, water adsorbs m ~ l e c u l a r l y ~on- ~ K)both in pure H2O gas and in the presence of 0 2 , H2, or platinum surfaces. No decomposition to hydroxyl and D2. The work represents a considerable extension of the hydrogen has been observed with pure H2O. In contrast, first study presented recently.I2 In that work the focus the presence of adsorbed oxygen, atomic or molecular, was on the temperature dependence of the OH desorption induces the decomposition of water to form stable hydroxyl rate in different H20/02 mixtures. The maximum in the species,&g as evidenced by electron energy loss spectrosOH desorption flux (0.04 site-l s-l at 1200K and 50 mTorr) copy (EELS)results at low temperatures. occurs at around 30% oxygen. The "apparent" activation At high temperatures, above 900 K, OH has been energy for desorption was found to vary from 1.6 to 1.9 eV, observed to desorb from Pt due to water dissociation on depending on the gas mixture. These results were well a partially oxygen covered surface, by using the laserby the kinetic modeP employed to analyze . ~ inter~ ~ ~ ~ ~ reproduced ~ induced fluorescence (LIF)t e c h n i q ~ e An the data. In the present work we present more extensive esting question is if H2O dissociates a t high temperatures measurements of OH formation/desorption in H2O/O2 on Pt, even in the absence of adsorbed oxygen.1°J2 (The mixtures and new results from H20/H2 (D2) and HzO/Oz strong promoting effect of oxygen for OH formation makes + H2 (D2) mixtures. it a nontrivial experimental problem to prove that H2O There are several motivations to study the H2O dedissociates in the absence of 0 2 . 9 composition on Pt: (i) To study H20 decomposition is a @Abstractpublished in Aduance ACS Abstracts, February 15, natural complement to previous studies of OH formation 1994. in the water formation reaction (H2 + l/202 HzO), to (1) Creighton, J. R.; White,J. M.Chem. Phys.Lett. 1982,92,435. provide further insight into the kinetics and reaction paths. (2)Fieher, G.B.;Gland, J. L. Surf. Sci. 1980,94,446. (3)Fisher, G. B. General Motom Research Publication, GMR-40071 Several different kinetic models have been developed to PCP-171,1982. describe the catalytic water formation reaction (H2 + l/202 (4)Sexton, B.A. Surf. Sei. 1980,94,435. H2O) on Pt.1P22The present data provide additional (5)Langenbach, E.;Spitzer, A.; Lath, H. Surf. Sci. 1984,147,179. (6)Ibach, H.; Lehwald, S. Surf. Sci. 1980,91,187. tests of some of these models (generally, the models should (7)Fieher, G.B.;Sexton, B. A. Phys.Rev. Lett. 1980,44,683. @)Melo, A. V.; O'Grady, W. E.; Chottiner, G. 5.;Hoffman, R. W. Appl. Surf. Sci. lSSS,21,160. (14)Hellsing, B.;Kaeemo, B.; Ljungstrbm, S.; Roe& A.; Wahmtrbm, (9)Wagner, F. T.; Moylan, T. E. Surf. Sci. 1987,191,121. T.Surf. Sei. 1987,1891190,851. (10)Talley, L. D.;Lin, M. C. Chem. Phys. 1981,61,249. (15)Hellsing, B.;Kasemo, B. Chem. Phys.Lett. 1988,148,466. (11)He,J.; Kaeemo, B.; Ljungstrbm, S.; W n , A.; Wahnetrbm, T. (16)Hellsing, B.;Kasemo, B.; Zhdanov, V. P.J. Catal. 1991,132,210.
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J. Vac. Scr. Technol., A 1987,5,523. (12)Fridell, E.Chem. Phys. Lett. 1992,188,487. (13)Fridell, E.;H e W i , B.; Kaeemo, B.; LjungetrBm, 5.;RoeBn, A.; Wahnstrbm, T. J. Vac. Sci. Technol., A 1991,9,2322.
(17)Hsu, D.S.Y.;Hoffbauer, M. A.; Lm,M. C.Surf. Sci. 1987,184,
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(18)Verheij, L.K.; Hugenachmidt, M. B.; Poeleema, B.; Comea, G.
Surf. Sei. 1990,233,209.
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equally well apply to the decomposition and to the formation of water). (ii) The water decomposition reaction and how it may be influenced by coadsorbatesis of general 0.06 interest in the search for reaction schemes to produce H2 from water. (iii) OH production by catalytic decomposition of water is of interest in catalytic combustion, since it may influence and stabilize gas-phase combustion.23-26 The water formation reaction on Pt is one of the most extensivelystudied catalytic reactions.14 The reactants, H2 and 0 2 , as well as the product, H20, and the reaction intermediate, OH, have been studied thoroughly. The kinetics has been explored using several experimental techniques such as time-resolvedEELS,28molecular beam c I I technique^,^^^^^^^^ and reactions in isotropic gas mix8 3t.8 305.9 307.0 tures.2*3032The importance of OH as a surface intermediate in this reaction is well established by EELS detection as well as by LIF detection of OH at low temperature~~133 molecules in the gas phase by Lin's p 0 ~ p , ~ ~ in J~ our1 ~ 9 ~ l a b ~ r a t o r y ~ and ~ J recently ~ * ~ * by ~ ~Williams, Marks, and 5 0 002 4 ) & I 6 ' S~hmidt.~~~~ I I We have used LIF to monitor OH production on the 307 1 3073 3075 nm surface during this reaction and simultaneously studied Figure 1. A scan of the recorded OH fluorescenceintensity va the water production rate.13J4930996.89The results include laser wavelength at a catalyst temperature of 1300 K, a total OH desorption and water production rates at different pressure of 50 mTorr, and a relative oxygen concentration of K = 0.42( K = pod@% + p ~ p ) )Assignments . of the peaks are done gas mixtures, total pressures, and surface temperatures.30 according to ref 56. A maximum in the water production occurs at 1522% H2, while the OH desorption shows a maximum at 3-8% The significant and important difference between the H2. Both the water production rate and the OH desorption H2O decomposition reaction and the water formation rate increase almost linearlywith pressure up to 100mTorr. reaction is that the latter is a rapid, essentiallyirreversible, The water formation rate decreases slowly with increasing exothermic reaction converting Hz and 02 to HzO while temperature between 900 and 1200 K. The temperature the former is predominantly areuersible reaction at quasidependenceof the OH desorptionrate shows a complicated equilibrium, where H2O and 0 2 landing on the surface dependence on the gas mixture due to kinetic effects. The decompose to OH, 0,and H that react again,to form H2O activation energy for OH desorption was found to be 2.0 and 0 2 that desorb from the surface, except for a minute & 0.16 eV from a kinetic analysis of experimental data.38 fraction that desorbs as OH and H2. Thus, the water The LIF technique also makes it possible to examine the formation reaction occurs far from equilibrium, while in rotational distribution of the desorbing OH."@t37 In the decomposition reaction there is a quasi-equilibrium general,the experimentaldata have been subject to kinetic between the gas phase and the adsorbed phase. m0deling.l3-~6~38 2. Experimental Section (19) Verheij, L. K.; Hugemchmidt, M. B.; Cblln, L.; Poelsema, B.; 2.1. Setup. The experimental arrangement and procedure Comaa, G. Chem. Phys. Lett. 1990,166,523. have been described elsewhere." Briefly,the setup consists of (20) Verheij, L. K.; Freitag, M.; Hugenschmidt, M. B.; Kempf, I.; a resistively heated Pt foil, 99.95% pure, and 22 mm x 3 mm X Poelwma, B.; C o w , G. Surf. Sci. 1992,272,276. 0.025mm in size, mounted in a turbo or Roots (200d / h at 100 (21) Anton, A. B.; Cadogan, D. C. Surf. Sci. Lett. 1990,239, L548. (22) Anton, A. B.; Cadogan, D. C. J. Vac. Sci. Technol., A 1991, 9, mTorr) pumped stainless steel vacuum chamber. The temper1890. ature of the foil is determined by its temperature-dependent (23) Driscoll, D. J.; Campbell, K. D.; Luneford, J. H. Adu. Catal. 1987, resistivity and is kept constant by microcomputer control. The 35,139. reagent grade water ie kept in a stainlesa steel can, connected to (24) Pfefferle, L. D.; Griffin, T. A.; Winter, M.; Crosley, D. R.; Dyer, the main chamber via heated regulating valves. It is heated to M. J. Combust. Flame. 1989, 76,325. (25) Pfefferle, L. D.; Pfefferle, W. C. Catal. Rea-Sci. Eng. 1987,29, about 90 O C and kept at constant temperature (f2 O C ) . The 219. relative partial pressures of the hydrogen, oxygen, and water are (26) Norton, P. R. In The Chemical Physics of Solid Surfaces and measured with a quadrupole maas spectrometer, cross-checked Heterogeneow Catalysis; King, D. A., Woodruff, D. P., Eds.; Elsevier: with a capacitance manometer. The mass flow of Ha and On are Amsterdam, 1982; Vol. IV,p 27. measured and controlledbymass flowmeters. Thetotal pressure (27) Ertl,G. Catal. Sci. Technol. 1983,4, 209. can be increased by gradually closing a valve to the Roots pump, (28) Germer, T. A.; Ho, W. Chem. Phys. Lett. 1989,163,449. (a) D'Evelyn, M. P.; Madix, R. J. Surf.Sci. Rep. 1984,3,413. and thus keeping the mass flow constant, or by increasing the (30) LjungetrBm, 5.;Kasemo, B.; Rash, A.; WahnstrBm, T.; Fridell, mass flow with approximately constant pumping speed (the E. Surf. Sci. 1989, 216, 63. pumping speed of the Roots pump is almost independent of (31) Fisher, G. B.: Gland. J. L.: Schmien, -. S. J. J. Vac. Sci. Technol. pressure in the range of interest here). 1982,20, 518.. A small fraction of the OH molecules which are produced by (32) Gland, J. L.; Fieher, G. B.; Kollin, E. B. J. Catal. 1982, 77, 263. (33) Mitchell. G. E.: White. J. M. Chem. Phvs. Lett. 1987.135. 84. 0 + H recombination or by HzO decomposition are thermally (34) Hsu, D. 8. Y.;Lm,M.'C. J. Chem. Phyk 1988,88, 1.' desorbed from the surface and are detected using the LIF (36)Talley, L. D.; Saunders, W. A.; Bogan, D. J.; Lm,M. C. Chem. technique. A frequency-doubled, excimer-pumped, dye laser, Phva. Lett. 1981. 78. 500: J. Chem. Phvs. 1981. 76. 3107. XaII inducing the 0-0 vibrational transition in the AQ+ 736) Ljungs&m, S.;Hall, J.; Kasemo, B.; &en, A.; WahnstrBm, T. electronicband at 306.3-307.5nm, probes the OH concentration J. Catal. 1987, 107, 648. (37) WahnstrBm, T.; Ljungstrdm, S.; Rosh, A.; Kesemo, B. Surf. Sci. a few millimeters from the catalyst sample. A part of this band 1990.234. 439. taken at 50 mTorr total pressure, a relative oxygen pressure, K , (38) W'ahnatrBm, T.; Fridell, E.; LjungstrBm, S.; Hellsing, B.; Kasemo, of 0.42 (K = PO,/@% + p ~ p ) )and , a surface temperature of 1300 B.; W n , A. Surf. Sci. 1989, 223, L905. K is shown in Figure 1. (39) Rdn,A.;LjungstrBm, S.;WahnstrBm,T.; Kesemo, B. J.Electron 2.2. Rotational Quenching. By measuring the heights of Spectroec. Relat. Phenom. 1986, 39, 15. (40) Marks, C. M.; Schmidt, L. D. Chem. Phys. Lett. 1991,178, 368. different peaks in the LIF spectra, such as those shownin Figure
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Langmuir, Vol. 10, No. 3, 1994 701
1, and normalizing them with the corresponding transition probabilities (Einsteincoefficients),one obtains the correspondinginitial-statepopulation. Boltzmann analysisof the scan given in Figure 1 gives a rotational temperature of 428 f 6 K. Since the surface temperature is much higher than the temperature of the reactant gas mixture,the rotational distribution willchange, with, for example,increasing pressure, due to collisions with the reactants. At the present pressuresthe gas-phasemean free path between collisions is quite small, for example,a few centimeters at 1 mTorr and only 1W cm at 100mTorr. Sincethe temperature of the foil is much higher than the temperature of the reactant gas mixture,there willbe temperature gradients in the gas, close to the catalyst, and thus also considerable rotational-state redistribution of the desorbing OH molecules, due to gas-phase collisions. (We have previously shown that it is still possible to derive the true temperature of the desorbing OH molecules by an extrapolation procedure, to zero pressure.s7) Due to the collisional redistribution of rotational states, the height of a particular peak in the LIF spectra will not correspond to the same fraction of the total numberof desorbedOH radicals, when parameterslike the mixing ratio, total pressure,and surface temperature are varied. In order to calculate the total number of OH radicals in the LIF detection volume element, one should therefore sum up the contributions from all populated levels. This can, as described in ref 37, be achieved using measured population distributions for the various parameter values. Anothermethod is to probe a particular level (N= 4in this case), whose fraction of the total number of OH radicals is insensitive to changes in the rotational distribution, over the range of parameter values of interest. Both methods have been used in the experiments reported here and give consistent results. 2.3. ElectronicQuenching. The fluorescencelight detected in the measurements originates from the spontaneousemission when OH radicals are deexcited back to the ground electronic state after initial laser excitationto the f i t electronicallyexcited state. However, if the excited molecule collides with a gas molecule, before radiative deexcitation, the deexcitation may instead proceed via radiationlessenergytrader in the collision." We denote the probability of this electronic quenching as Q. In order to obtainthe total number of OH radicalefromLIF spectra, one must know the fraction of the laser-excited molecules that fluoresce. Further, to be able to compare the relative number of OH radicals for different pressures and gas mixtures, the quenchingconstant for each component of the gas mixture must be known. The total fluorescenceyield is determinedby the ratio between the radiative deexcitation rate, A (Einstein's spontaneous emission coefficient), and the total deexcitation rate, Q + A
(2.1)
where N a is the number of excited molecules. The value of Q depends on the total pressure and on the gas mixture, and can be written as a sum over all collision partners, i, with quenching rate constants k,(i) times the densities ni of molecules i:
To determine Q, one can measure the lifetime, T , of the excited state. If N w ( 0 ) molecules are laser-excited at time t = 0, the excited state will then decay as (2.3)
Thus,a measurement of the time-dependent decay curve of the fluorescence yield will determine Q + A. Figure 2 shows four such curves at T = 1300 K and three different Hn0/0a mixtures (A-C)and one HJOn mixture (D). T h e decaycurvesare obtained by averaging over lo00 laser shots using a digital oscilloscope. The lifetime is defined as the time it takes for the intensity to decreaseto l/e of the initial intensity, I(0)(Le., r = (Q+ A)-l). Figure 3 shows how T changes with total preseure in H*0/0* (41) Crcmley, D. R. Opt. Eng. 1981,20,311.
v
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00
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Figure 2. Timadependent decay curvesof the fluoregcencefrom OH: A%(u'=O) Xan(u"=O),R1(2)for T = 1300 K in H*0/0* mixtures (A) p = 100mTorr, K = 0.6,(B)p = 100 mTorr, K = 0.2, and (C)p = 26 mTorr,K = 0.2 ( K =PO,/@% + p ~ p )and , the HdOa mixture (D) p = 25 mTorr, a = 0.1 (a PHJ@H,+ PO,)). I
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500s14 K
3000
25
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Figure 3. Lifetime, T = (Q+ A P , of laser-excitedOH molecules as a function of pressure for Hn0/0a mixtures with K = 0.4 (fiied circles) ( K = p d @ q + p ~ p )and ) Hd01 mixtures with a = 0.1 (crosses) (a = PHJ@H* + p a ) ) . The solid lines represent the results expected using the quenchingrate constants given in the text. mixtures (40% 02)and HJOa mixtures (10% Ha). Using the known value for A (0.71 pa in this caseu), Q is found. From severaldecay curves,the k,(i) for the individualcollisionpartners have been calculated with the result k,(HaO) (6.82f 0.39) X 10-l0cms/sand k,(Oa) = (1.10f 0.29)X 10-lo cms/s. The lifetimes calculatedfromthesequenchingrate constantsare ale0illustrated by the curvesin Figure 3. The quenchingconstants determined here agree fairlywell with thosereported for high-pressureflames (k,(HaO) = (3.7-6.6)X 10-10 cms/s, k,(01) = (0.814.91)X 10-10 cms/s, k,(H1) = (0.78-1.37)X W0 ~ m ~ / s )The . ~ l experimental results presented below have been adjusted for electronic quenchingto obtain the correct relative OH concentration. The small temperature dependence of the quenching constants was not c0nsidered.a 2.4. Influence of Pumping Speed. To obtain the OH requires desorption rate,&, fromthe measuredLIF ~ignal,Zo~, a certain evaluationprocedure,which becomesincreasinglymore important as the pressure rises. At suffciently low pressures, where the molecular flowconditionprevails,the situationis simple and straightforward. Here IOHis directly proportional to +OH. This proportionality holds as long as the mean free path for molecule-molecule collisions is considerably larger than the distance, z, between the sample and detection volume (a few millimeters), i.e., up to a pressure of at most -10 mTorr. Provided angular and rotational distributions are u n a f f d , no corrections are required in order to compare results at different pressures, gas mixtures, etc., since the measured signals in this regime are unaffected by gas-phase collisions. (42) Dimpfl, W.L.;Kineey, J. L.J. Quant.Spectrosc.Radiat. ZYamfer 1979,21, 233.
(43)Gudmundson,F.; Fridell, E.;RosBn, A.;Kaaemo,B.J. Phys. Chem.
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