J. Phys. Chem. 1993,97, 12828-12834
128%
Evaluation of OH Desorption Rates from Pt Using Spatially Resolved Imaging of Laser-Induced Fluorescence Fredrik Gudmundson,'*+ Erik FrideU,+ Ame R0-t
and Bengt Kasemo*
Department of Physics, Department of Applied Physics, Chalmers University of Technology and University of Gateborg, S-412 96 Gdteborg, Sweden Received: March 18, 1993; In Final Form: October 13, 1993"
Using laser-induced fluorescence (LIF) to study OH radicals produced and desorbed into the gas phase in catalytic reactions, information is obtained about the OH desorption rate, the O H coverage and surface kinetics. A question of central importance in this context is how the O H L I F intensity can be reliably related to the O H desorption rate. It is demonstrated in this work that simple proportionality between the two is not generally valid in the pressure regimes where the present and previous measurements have been performed. The spatial distribution of OH molecules desorbed from a Pt catalyst during the catalytic water formation reaction, H2 1/202 H 2 0 was in this work measured using the L I F technique. By imaging the fluorescence light from the laser-excited O H radicals with a two-dimensional CCD detector, concentration profiles outside the catalyst were obtained. The experiments were performed under steady-state reaction conditions at pressures of 25-200 mTorr and a catalyst temperature of 1300 K. It is demonstrated that the derivation of the OH desorption rate from OH LIF signals measured outside the catalyst requires careful attention to mass-transport effects (pumping speed, mass flow, flow geometry) and gas-phase quenching. By analysis of the data with a simple equation for diffusion and/or choosing appropriate mass flow conditions, the "true" desorption rate can be obtained from the measurements, as illustrated by comparison with a kinetic model calculation for OH production.
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1. Introduction OH formation on the surface and subsequent desorption into the gas phase in the catalytic reaction H2 + ' / 2 0 2 H2O on Pt at high temperatures has been investigated by several groups, using the laser-induced fluorescence (LIF) technique.'-14 The OH desorption is a side branch in the main water formation reaction, where intermediate OH species occasionally desorb before reacting to form H20. OH also desorbs from the surface in H2O 0 2 mixtures6J1,1sJ6and in pure H20 gas1IJ2J4due to water decomposition. The appearance of desorbed OH has been used to investigate several aspects of the kinetics and energetics of hydroxyl and water formation, desorption,and decomposition. The possibility of obtaining a measure of the surface concentration of the OH intermediatespecies in these reactions, via the recorded LIF signal outside the surface due to desorbing OH, has considerably improved our knowledge about the kinetics of the reactions mentioned above. The phenomenon is also of a practical interest: In catalytic combustion at high temperatures the generation of radicals such as OH may play a significant role for coupled catalytic and gas phase combustion phen~mena.'~J* The latter can be investigated using spatially resolved detection of the OH LIF inter~sity.'~J+~~ In the experiment the LIF intensity, caused by the exciting laser pulse, is measured. Provided corrections can be made for quenching effects, local temperaturevariations, etc., the intensity can be used to derive the local OH density in the excitation volume. This density can in turn be used to derive a desorption flux from the catalyst surface provided a transport model can be formulated, connecting the flux of OH molecules leaving the surface to a local density outside the surface. In this paper we have performed such measurements and address the question of how to relate the measured OH LIF intensity to the OH desorption rate. This relation is not straightforward for the reasons briefly mentioned above and articulated below.
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t Department of Physics.
t Department of Applied Physics. Abstract published in Aduance ACS Abstracts. November 15, 1993.
0022-3654/93/2097-12828$04.00/0
(1) The population distribution among rotational levels of the desorbed OH radicals will change due to collisions with the roomtemperature reactants (rotational quenching) when the molecule mean free path is comparable to or less than the distance between the surface and the LIF excitation volume. This may lead to erroneous results if the transition used to probe the total concentration cccurs from a level whose relative population is sensitive to a change in temperature.5~7 (2) The probability of electronic quenching of laser excited OH radicals depend on the gas mixture? the total pressure, and the gas temperature.22 (3) The diffusion rate of OH molecules from the catalyst to the region where they are probed, will change with pressure, gas mixture, and gas temperature. (4) The pumping speed and mass-flow conditions will strongly influence the concentration gradients of OH outside the surface. If mass transport close to the surface is primarily governed by diffusion, one type of gradient is established. If it is instead governed by, say, laminar flow, the gradients will be steeper. The common situation is mass transport by a combination of diffusion and flow. In this work we have investigated these phenomena experimentally and by simple model calculations. The aim is not to develop a full model to account for the phenomena mentioned above. It is rather to demonstrate and discuss their relative importanceand how to proceed to obtain reliabledesorption rates. Of special interest was to explore how the pressure dependence of the measured OH LIF signal is affected by the different phenomena mentioned above and how it can be used to obtain the actual desorption rate from the surface. For the measurements, a CCD camera was used to get two-dimensional images of the OH fluorescence outside the catalyst. The results suggest procedures to obtain correct desorption rates of OH with respect to its pressure dependence and also questions the quantitative correctness of some earlier published data. 2. Experimental Setup
The main featuresof the experimentalsetuphave been described elsewhere.4 Briefly, it consistsof a simple,turbo- (pumping speed 0 1993 American Chemical Society
OH Desorption Rates from Pt
The Journal of Physical Chemistry, Vol. 97, No. 49, 1993 12829 the sheet. The laser intensity was measured using a photomultiplier, which was moved vertically through the sheet. An optical system arranged in this way gave the possibility for point to point measurementsof the fluorescence light originatingfrom desorbed OH radicals. The data were analyzed using the Mathematica software package. 3. Rotational, Electronic, and Reactive Quenching
Figure 1. Schematic view of the experimental arrangement. The tube for gas inlet is 20 mm in diameter, the distance between the sample and the tube is 30 mm. The sample is situated in a cross between two 100mm4.d. tubes one of which is connected to the pumps.
a few L/s at the pressuresused here) or Roots-pumped (pumping speed 75 L/s), stainless steel, high-vacuum chamber. A quadrupole mass spectrometer (Balzers QMG 420), located in a separatelypumped chamber, was connected to the main chamber by a tubing and was used for partial pressure measurements. All partial pressures given below were obtained from calibrated QMS signals as described in ref 4. The sample was a polycrystalline Pt foil, 3.3 mm X 20 mm X 0.025 mm, purity 99.95%. It was mounted in the main chamber on a simple manipulator allowing vertical motion. The catalyst was resistively heated. The temperaturewas controlledby a four-pointarrangement and kept constant (at 1300 K in the experiments reported here) by microcomputer control. The reactant gases entered the chamber via a tube, 20 mm in diameter, that ends 30 mm below the sample, resulting in a directed gas flow toward the surface. The mass flows were measured using mass flow meters. The positions of the gas inlet and the pumps relative to the sample are shown in Figure 1. Some test experiments were also made with different flow conditions for the reactant gases by mounting an aluminum cap about 5 mm above the end of the gas inlet tube. This changed the gas flow from vertical to horizontal (directed to the left in the view of Figure 1). The concentration of OH radicals in the region outside the foil was measured with the LIF technique. As an excitation source we used the light from a frequency-doubled, pulsed, excimerpumped dye laser (Lambda Physik EMG 102 E and FL 2002 E) in the wavelength region 306.3-307.5 nm, covering an absorption band of OH (A2Z(u’ = 0) X211(u” = O)21). The wavelength of the laser light was tuned to the Rl(4) transition. The laser pulses had a pulse length of 20 ns, energy of 0.1 mJ, and a bandwidth of 0.2 cm-l. For the experiments reported here, the UV laser beam was shaped in the form of a sheet (6 mm X 0.5 mm) in the probed region outside the catalyst by use of two cylindrical quartz lenses (Figure 1). Black-body radiation from the heated Pt foil was eliminated with a bandpass filter (Schott UV-M-A1) centered at 305 nm with a bandwidth of 20 nm. In the imaging experiments reported in this work, the fluorescencelight from desorbed and laser excited OH molecules was measured in the region covered by the laser light, at different distances from the surface, as shown schematically in Figure 1. The LIF emission was collected by a quartz lens (Nikon UV quartz 4.5,f= 105 mm) focused onto a gateable CCD (Princeton Instruments ICCD, 386 X 578 pixels). The width of the gate was 1.5 ps (-twice the natural lifetime of the excited state). The signal was averaged over several shots (typically 300) and the background was subtracted. Extraction of the relative fluorescence intensity of each point required that the registered LIF signals werenormalized with respect to the laser intensity over
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In LIF experiments, the fluorescence light is recorded as a function of the exciting wavelength for transitions between rotational levels in-the electronic-vibrational ground state and an excited electronicstate. Normalization of the peak intensities in fluorescence spectra with the appropriate transition probabilities, also known as the H6nl-London factors, give the population of the rotational levels from which a rotational temperature can be d e r i ~ e d . The ~ total OH concentration is obtained by a rather tedious summation of the populations in all rotational levels. In a LIF experiment, it is therefore common to follow the intensity of only one fluorescence peak as some parameter is varied. This means that only the population of a single level in the ground state is probed. However, due to collisions between the OH radicals and the reactants/products the population of the rotational levels is continuously redistributed (if there are temperature gradients), which for high pressures results in a local thermal equilibrium with the reactants, as investigatedin earlier work.5~~ For example,if R1( 1)23is followed as a function of pressure, an underestimation of the OH concentration would be the case at high temperatures, where the N = 1 level in relative terms is scarcely populated compared to low temperatures. There are fortunately often some levels whose populations are a relatively constant fraction of the total OH concentration even when the rotational distributionchanges, which can then be used to monitor the OH concentration (an example is the N = 4 level, probed in the present ex~eriments).~l~ Electronic quenching takes place when laser excited OH radicals collide with, for example,the reactants.6 In this process, the excitationenergyis dissipated without radiation in the collision and the OH molecules do not fluoresce. The number of OH molecules that fluoresce is proportional to the ratio A / ( A + Q), where A is the Einsten coefficient for spontaneous radiation and Q is the probability for quenching. Q increases linearly with pressureand can be written as a sum over the individual quenching rateconstants for each species times their ~oncentration.~.~~ Thus, a change in gas mixture also influences Q. By measuring the life-time of the excited state, Q can be evaluated as described in ref 6. The electronic quenching of OH radicals has also been shown to depend on temperature.22 In our experiment there is a strong temperature gradient present as the temperature drops from 1300 Kat the surface to 300 K which is the temperature of the incoming gases. From the values of the quenching cross section given in ref 22 we have estimated the decrease in fluorescencequantum yield, A / ( A Q), due to the temperature decrease to be less then 6% when comparing LIF intensities a t distances of 1 and 5 mm from the surface. Our experimental values of the OH LIF intensity have not been corrected for this temperature gradient. In principle one also needs to consider reactive quenching, Le., removal of OH by reactive gas-phase collision on the path from the surface to the LIF detection volume. As argued in ref 6, this effect is of negligible importance at the conditions considered here.
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4. Diffusion
The concentration of OH radicals at a certain point outside its source, i.e., the catalyst surface, can be correlated with the OH desorption rate by solving the two-dimensional diffusion equation25.26 for a net flow of velocity u parallel to the surface.
Gudmundson et al.
12830 The Journal of Physical Chemistry, Vol. 97, No. 49, 1993 I
platinum foil
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Diiect flow, 760 sccm 0 Indirect flow, 760 sccm Direct flow, 65 sccm 0 Indirect flow, 65 sccm x
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Distance from P t surface (mm) I
Figure 2. CCD image of the laser-induced fluorescence from OH outside the Pt foil (indicated in the figure) at 1300 K. The rectangle shows the area used in the extraction of the results presented in Figures 3-1 1.
OO
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Calculated 760 sccm LIfi, 65 sccm Calculated , 6 5 sccm
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Following the geometry of Figure 1, the partial pressure of OH at ( x , z) will be 1
2
3
4
5
Distance from Pt surface (mm)
where D is the diffusion ~oefficient,~’ Iul is the speed of the flow parallel to the surface (see Figure l), POH is the OH desorption rate and xo is the width of the foil. Using a single velocity Iul, proportional to the pumping speed and parallel to the surface is an oversimplification as the gases are directed from below and the flow close to the surface is much more complex. However, as we shall see below, the qualitative influences of diffusion and mass flow on the OH gas phase concentration can be understood from eq 1. 5. Results and Discussion
In the presentation below we illustrate how the various factors mentioned in the Introduction affect the recorded LIF signal of OH. It should be remembered that the source of OH is always the Pt surface, where impinging H2 and 0 2 (or H2O in experiments with water) dissociateand form intermediate OH, some of which desorbs into the gas phase. The recorded LIF signal at a given position outsidethe foil is then a consequence of (i) the desorption rate, (ii) transport by diffusion and/or flow of OH into and out of the volume element excited by the laser beam, (iii) rotational population redistribution due to gas-phase collisions, and (iv) electronic quenching of excited OH. Results will be presented and analyzed either as LIF profiles perpendicular to the surface, for different conditions of flow, pressure, carrier gas, etc., or as LIF signalsvs flow, gas mixture, or pressure, respectively, recorded at a fixed positions outside the surface. The former type of results illustrate the importance of understanding the OH concentration gradients established outside the catalytic surface. The latter type of results illustrate that the mass flow conditions have a profound influence on the measured data. 5.1. Influence of Mass Flow. Figure 2 shows an image of the fluorescence light taken with the CCD camera. It was recorded at a surface temperature of 1300K, a pressure of 100mTorr, and a gas mixture of 10%H2 and 90% 0 2 , i.e., about the gas mixture for maximum OH yield.4 In the following analysis we consider only the area inside the small rectangle in Figure 2 corresponding to an area of 2 mm X 4 mm, i.e., from a distance of 1-5 mm outside the catalyst. The range of distances from the surface that are taken into consideration is limited by the homogeneity of the laser sheet. The width of the rectangle is chosen so that catalyst edge effects will not be important. Values of the measured OH LIF intensity, IoH, vs distance from the Pt surface, z, are obtained by summation of the signal from the pixels in each row parallel to the surface in order to reduce the noise.
Figure 3. Recorded LIF intensity, ZOH, vs distance from the Pt surface at a constant total pressure of 100 mTorr with 10% H2 and 90% 0 2 , a catalyst temperature of 1300 K, at different mass flows (65 and 760 sccm) and (in a, top) for two different flow geometries (see text). The lines in b (bottom) denoted “calculated” represent the partial pressure of OH, POH, calculated from eq 1.
Figure 3 shows four such profiles of OH LIF intensity, IOH, vs distance from the Pt surface, taken at different gas flows. Two of the curves were obtained at a mass flow of 65 sccm (1 sccm = 0.0127 Torr L/s) and the other two with 760 sccm. The two curves marked Direct are taken with the flow geometry shown in Figure 1, while the two curves labeled Indirect were recorded with the gas flow directed to the left after the gas inlet tube (see section 2). The curve with high and direct flow strongly deviates from the others in that the measured OH LIF intensity, 101-1,is higher close to the surface and lower far from the surface than for the other curves. The higher signal close to the surface is simply due to the elimination of gradients in H2 and 0 2 caused by the fast reaction (Le., mass-transport limitations of reactants are avoided). The lower signal far from the surface, in the high flow case, is due to the higher transport rate (u in eq 1) of OH away from the detection zone. The results of Figure 3a dramatically demonstrate the great influence of mass flow on the OH LIF intensity, which consequently paves the way for interpretation errors, especially when measurements are performed only in a single volume element at fixed distance from the catalyst (using e.g., a PMT). It should be noted that the quantitative results presented here are uniquely connected with the employed geometry. However, this does not prevent generalconclusionsto be drawn as further discussed below. Using eq 1, theoretical estimates of the partial pressure of OH, POH, outside the catalyst were calculated for the two different mass flows (Figure 3b). The experimental curves are the same as in Figure 3a (Direct flow). To adapt the absolute value of the calculations to the experiments,we used the following procedure. The calculated and experimental curves for high flow were normalized to the same value at 1 mm from the Pt surface. The value for the calculated, low-flow curve at 1 mm from the Pt surface was then obtained by scaling with the same factor (0.56) as the measured water formation rate (see below) scaled between these two flows, i.e., the water formation rate was taken as proportional to the OH desorption ratein the scaling. The value of the velocity, u, used in the calculationswas obtained by dividing the mass flow with the product of the pressure and the cross section area of the gas inlet tube. The diffusion coefficient is calculated at 300 K. (This is a simplificationsince the diffusion coefficient varies with the distance from the foil due to the
The Journal of Physical Chemistry, Vol. 97, No. 49, I993 12831
OH Desorption Rates from Pt I
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O !
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01 200
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Lpw flow 0 Highflow
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z=lmmO z=5mm+ Chemical energy 0 100
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Figure 4. Recorded LIF intensity, IOH,vs mass flow at a total pressure of 100 mTorr with 10% Hz and 90% 02,and a catalyst temperature of 1300 K for different distances from the Pt surface (1 mm and 5 mm). For comparison,the water formationrate, RH^, on the surface,measured as the dissipated chemical engery, is shown. To illustrate the different dependencies on mass flow, the curves are set equal at 200 sccm. The fluorescence intensity at z = 1 mm was 1.6 times higher that at z = 5 mm at this mass flow.
difference in temperature of the foil and the incoming gas. Calculations using diffusion coefficients for temperatures up to 1300 K shifts the crossing of the two theoretical curves a t 2 to about 4 mm but without any qualitative difference in shape.) There is noother fitting parameter than the absolutevalue of the calculated curve for the high flow. Since the assumptions of a net flow parallel to the surface and the constant diffusion coefficient are oversimplifications, a better agreement than in Figure 3b, is not expected. Note that the calculations predict the most important qualitative differences between high and low flow, namely, a higher partial pressure Of OH,poH, close to the surface and a lower one far from the surface, at the high flow. For the indirect flow, the OH LIF intensity, IOH is quite independent of the mass flow rate, for a fixed pressure, both close to and far from the surface. The reason for this is that the flow rate close to the foil is in this case not determined by the flow velocity in the gas inlet tube but rather by the flow velocity in the tube leading to the Roots pump. The cross section area of the latter is 25 times larger than the area of the inlet tube. The pumping speed is thus not high enough, even a t 760 sccm, to remove concentration gradients in H2 and 0 2 in the Indirect flow case. In other words, themass transport of H2,02, OH is primarily governed by diffusion in this case, explaining the insensitivity to the flow rate. The influence of mass flow (at constant pressure) is further illustrated in Figure 4. It shows the OH LIF intensity, IOH, vs mass flow (for constant total pressure (100 mTorr) and constant gas mixture (10% H2)) at different distances from the surface, taken from profiles such as those in Figure 3. Close to the surface, the OH LIF intensity, IOH,increases with increasing mass flow. This is due to the above-mentioned occurrence of concentration gradients in the gas phase (of H2 and 02) at low pumping speed caused by the fast surface reaction. The buildupof thesegradients reduces the impingement rates of the reactants and thus lowers the surface coverages of all species, leading to a lower OH desorption rate. The problem with increasing mass-transport limitations in the water formation reaction with decreasing mass flow has been reported earlier.4 It is illustrated by the open squares in Figure 4 showing the rate of H2O formation, R H ~ oFar . from the surface, the OH LIF intensity, IOH,decreases with increasing mass flow as discussed above and predicted by the calculated results of eq 1 (see Figure 3b). The complication caused by gradients in the reactants (H2, 02), is eliminated if one instead studies the H20 decomposition and accompanying OH desorption, according to the overall rea~tion6-1~
H20g+ 1/20,B e H20a+ 0 '
20Ha
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20Hg
(g for gas and a for adsorbed). Since this is a reversible reaction leading to equilibrium between gas-phase and surface species, no
,
ill'
800
2
1
3
1
4
Distance from Pt surface (mm)
5
Figure 5. Recorded LIF intensity, IOH,vs distance from Pt surface in 70 mTorr of H20 and 30 mTorr of 0 2 at T = 1300 K for two different mass flows (approximately 153 and 526 sccm). The relative magnitudes
as measured are shown. Helium 0 Neon + Argon 0
(a. u.) 0.4
0.2 1
2
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6
Distance from Pt surface (mm)
7
Figure 6. Recorded LIF intensity, IOH,vs distance from Pt surface in 1 mTorr of Hz, 9 mTorr of 0 2 and 90 mTorr of either He, Ne, or Ar. The catalyst temperature was 1300 K. The curves are normalized at z = 1 mm. The relative magnitude as measured were 1.11:1.24: 1.OO for
He, Ne, and Ar, respectively. reactant gradients are formed. (The OH desorption is a negligible perturbation of the equilibrium6J5.) Figure 5 shows two profiles of the OH LIF intensity, IOH,vs the distance from the Pt surface for this reaction, taken at mass flows of 153 and 526 sccm, respectively. In contrast to the H2 O2reaction (Figures 3 and 4) the OH LIF intensity, ZOH, close to the foil does not change as the pumping speed (mass flow) is increased due to the absence of reactant gradients. Far from the foil, the OH LIF intensity, IOH, decreases in a similar way as for the H2 0 2 reaction as expected from eq 1, Le., a lower OH LIF intensity ZOH, is observed for the high flow case. 5.2. Influence of Carrier Gases. The use of different carrier gases, in which OH has different diffusion rates, further illustrates how OH diffusion influences the measured O H LIF profiles. Some measurements were therefore performed at constant total pressure with noble gases as carrier gas. The gas mixture was 1 mTorr of H2, 9 mTorr of 02 and 90 mTorr of either He, Ne, or Ar, and the mass flow was approximately 750 sccm. The measured profiles are shown in Figure 6. The calculated diffusion coefficients for OH at 300 K in the three gases are (in m2 s-l): He 0.43, Ne 0.24, and Ar 0.17.27 Clearly a lower diffusion coefficient for O H correlates, as expected, with a more rapid decrease in the OH LIF intensity, IOH, with the distance from the Pt surface as predicted by eq 1. OH LIF profiles outside the surface were also measured a t different hydrogen partial pressures, a (a = PH,/(PH~ + PO,), at a total pressure of 100 mTorr, and a gas flow of 250 sccm. Figure 7 shows the measured OH LIF intensity, IOH,VS a,at two different distances from the surface. The curve recorded furthest from the surface shows a small but significant shift to higher a values, caused by the higher diffusion coefficient for OH in hydrogen compared to oxygen. 5.3. Analysis of the Pressure Dependence. Measurements of the pressure dependence of the OH LIF intensity at different flow conditions and distances from the surface further illustrate the effects discussed above and provide guidelines for how to obtain a reliable relation between the measured OH LIF intensity and the desorption rate. Figures 8 and 9 were measured with
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uuamunason et ai.
12832 The Journal of Physical Chemistry, Vol. 97, No. 49, 1993 7
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W f
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J o 50
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cy
Figure 7. Recorded LIF intensity, ZOH, vs CY (a = ~ H , / @ H ~+ PO,)) at 1 and 5 mm outside the surface. The total pressure was 100 mTorr and the catalyst temperature was 1300 K. e 1
1
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a” IOH (a. u.) 0.5
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c;
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B
A 0
+8
II
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B + c x
D O
Q
I”
01 50
100
Pressure (mTorr)
150
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Figure 8. Recorded LIF intensity, IOH,vs pressureat constant pumping speed 1 mm from the surface (A). The catalyst temperature was 1300 K and the gas mixture contained 10% H2 and 90% 02.The same curve
adjusted for electronic quenching is also shown (B) together with the expected OH desorption rate, +OH, calculated by using curve B and eq 1 (C). The calculated OH desorption rate, #OH, using the kinetic model described in ref 16, is shown for comparison (D). The curves are normalized to one at their respective maxima.
50
100
150
Pressure (mTorr)
200
Figure 9. As in Figure 8 but 5 mm from the surface. 1
@
50
100
Pressure (mTorr)
150
I
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Figure 10. Recorded LIF intensity, ZOH, vs pressure at a constant mass flow of 226 sccm 1 mm from the surface (A). The catalyst temperature
was 1300 K and the gas mixture contained 10% H2 and 90% 0 2 . The same curve adjusted for electronic quenching is also shown (B). The
calculated OH desorption rate, +OH, from the kinetic model described in ref 16 is shown for comparison (D). The curves are normalized to one at their respective maxima. constant pumping speed, V, i.e., varying mass flow, 9,at distances of 1 mm (Figure 8) and 5 mm (Figure 9) from the Pt surface. Figures 10 and 11 show the same type of measurement at the same distances from the surface, the only difference being that the mass flow rather than the pumping speed was kept constant. In each figure the measured OH LIF intensity, IOH, is shown (curve A) together with the corresponding curve after correction for electronic quenching (B), and the result of a kinetic model calculation28 predicting the OH desorption rate, +OH ( D ) . In
100
Pressure (mTorr)
150
200
Figure 11. As in Figure 10 but 5 mm from the surface. Figures 8 and 9 we also show the desorption rate, #OH, evaluated from the quenching-corrected measured OH LIF intensity by applying the transport equation (eq 1) (curve C). All curves in Figures 8-1 1 are normalized to unity a t their respective maxima. The measured relative fluorescence intensities at 100mTorr were 1.00:0.37:0.88:0.55 for curves A in Figures 8-1 1, respectively. The first and maybe most important result is that very good agreement is obtained between the measured OH LIF intensity, ZOH, and thecalculated desorptionrate,#OH, when measurements are performed with (i) constant mass flow and (ii) with the probed volume close to the surface (Figure 10). In this case it does not seem necessary to apply eq 1 in the data evaluation. If such corrections are attempted it will require a pressure dependent velocity, u, in eq 1 (see below). (It should be noted that in the comparison in Figures 10 and 11 all curves are normalized at 200 mTorr. In reality, considering all error sources and uncertainties, the deviation between measurements and calculations are probably larger for higher pressure.) In contrast, to the situation with constant mass flow and measurement close to the surface, the OH LIF intensity, IOH,vs pressure,p, with constant pumping speed, and thus varying mass flow, measured 5 mm from the surface (A in Figure 9), deviates dramatically from the kinetic model predictions ( D ) . The main reason is the mass transport effects discussed in section 4. In the uncorrected OH LIF intensity, IOH,there is actually a maximum a t 100 mTorr, which has no correspondence in the calculated desorption rate. The reason for the appearance of the maximum is that with increasing pressure the mass flow increases proportionally to pressure (since V is constant and 9 = Up), making the gradients like the one shown in Figure 3a successively steeper. Equation 1 manages, at least semiquantitatively, to correct this influence of mass transport, and in the corrected curve (curve C in Figure 9) the maximum disappears and the data points are in fair agreement with the kinetic model. The results of Figures 8 and 11 are intermediate between the two extreme cases of Figures 9 and 10. With constant mass flow there isa fairly goodagreement even when measurements are performed 5 mm away from the surface. In Figure 8 (constant pumping speed, z = 1 mm), correction by eq 1 gives very good agreement between measured and calculated data. A final comment in this paragraph concerns the velocity parameter, u, in eq 1. For the results in Figures 8 and 9 the velocity, u, was assumed to remain constant as the pressure changed with constant pumping speed. However, as the pressure is increased, this parameter will probably change even when constant pumping speed is used, due to the complex flow pattern from the input tube to and beyond the sample. With constant mass flow it will of course vary as the pressure is increased. Provided we have a system where we trust the kinetic model the functional dependence of the velocity on the pressure can be derived, and it can then be used to correct data where the kinetic model is unknown. 5.4. Procedure To Evaluate the Desorption Rate. It is evident from the results presented here that derivation of the OH desorption rate from LIF data requires a careful analysis of the LIF experiments. Especially the evaluation of the pressure dependence of the OH desorption rate is difficult. The exper-
OH Desorption Rates from Pt imental conditionsconcerningmass flow, distancebetween surface and detection zone, and concentration gradients must be well defined. To obtain the true OH desorption rate from LIF investigations, the following p i n t s should be given proper attention. The LIF signal should be recorded with constant mass flow as close to the surface as possible or measured at a set of points close to the surface so that the LIF signal correspondingto zero distance can be obtained by extrapolation. The use of a multielement detector is ideal for this, since the behavior of the LIF signal at different distances from the surface can be followed simultaneously. The mass flow should be large enough to minimize reactant gradients. The changes in rotational distribution, taking place as the experimental conditions (e&, the pressure) changes, should be analyzed (the rotational distribution alsovaries with the distance from the surface'). Often, a transition whose relatiue population is fairly insensitive to the rotational redistributions taking place can then be found. If not, several transitionsthat together account for rotational redistribution effects must be recorded. To account for electronic quenching, one can measure the lifetime of the excited state or, alternatively,use published quenching rate constants.6,22~24~30J1 Observe that for gated detection, the measured fluorescence yield relative to the total number of excited OH will depend on the width and delay of the gate. Most straightforward is to record all of the fluorescence (by using a gate of, say, 1.5 times the natural lifetime) and multiply by (A + Q)A. However, also with a shorter gate, the influence of electronicquenching is significantat pressures of 10-1000 mTorr and can easily be accounted for. Measurements in gas mixtures at constant total pressure are less vulnerable to analysis errors but can still be influenced by these effects, because of variations in the probabilityfor electronicquenchingexisting if the quenching rate constantsare different for the different gases in the mixture.15 There is also a temperature d e p e n d e n ~ e ~ and ~ Ja~rotational level dependence31 of the quenching rate constants, which can call for corrections in some cases. It is obviously much better to increase the pressure with the mass flow kept constant. However, this may lead to concentration gradients at high pressures, when the surface reaction is fast. For any reaction, the global turnover number of the reactants should not be more than a few percent, to ascertain that gradients are negligible, if one wants to connect measured OH LIF intensities to reaction and desorption kinetics. 5.5. Some Comments on Previous Studies. The phenomena discussed above have significance for the interpretation of several previous studies. Our groups published early results of the OH LIF signal vs pressure for pressures up to 2 Torr. This signal was recorded about 5 mm from the surface and a strong absolute signal decrease with pressure was observed above 0.8 Torr. This curve does not correctly reflect the OH desorption rate because it was not adjusted for either electronic quenching or masstransport effects. Both of these effects were probably significant. Further results on the pressure dependence were later published by Ljungstram et al.3 showing a LIF maximum at about 0.7 Torr. This result was discussed in terms of electronic and rotational quenching, besides kinetic effects, but the influence of mass flow and diffusion were not considered. In yet another publication by Ljungstr6m et u I . ~an OH LIF signal up to 100 mTorr was presented showing a slightlyless than linear increase with pressure. This curve was also to some extent influenced by mass transport phenomena. In these three publications3.4.S it was pointed out that the curves probably did not reflect the true OH desorption rate. In a recent paper, also from our laboratory, by Fridell et ai.? the pressure dependence of the OH desorption rate in HzO decomposition reactions was presented. The LIF signal was recorded with constant mass flow and the effects of electronic
The Journal of Physical Chemistry, Vol. 97, No. 49, 1993 12833 and rotational quenching were considered, and these results correctly describe the OH desorption rate. In some early LIF measurements of OH produced in the H2 0 2 reaction by Tevault et and in H20 0 2 mixtures by Talley and Lin16 with varying total pressure, the factors mentioned above were not considered. The presented curves are probably significantly influenced by flow effects and by electronic quenching. In a recent article by Williams et several measurements of OH LIF signals for varying pressures are presented. From the presented data it seems unavoidable that the Hz + 02 reaction was in this case influenced both by concentration gradients in the reactants and by the mass flow and quenching phenomena, discussedabove, that strongly affect the relation between the OH desorption rate and the OH LIF signal. This is particularly likely since the measurements were performed with varying total pressure at a relatively large distance from the surface ( 5 mm). This casts doubts on the kinetic analysis since the input data are probably influenced by the effects just mentioned. In general, to use OH LIF data to analyze surface kinetics, it is much more preferable to use data at constant pressure, with varying mixing ratios, if mass transport effects of OH at different pressures cannot be reliably accountedfor. With constant pressure the mass transport effects are small as demonstrated in Figure 7. This is a major reason that we have used such data in most of our kinetic analysis in the
+
+
6. Conclusions
The use of laser-induced fluorescence to detect OH radicals produced and desorbed into the gas phase from a surface, is a powerful method to follow surface kinetics but requires careful attention to phenomena that complicatethe functional relationship between the recorded LIF signal and the desorption rate (this statement can of course be generalized to LIF detection of any surface-producedspecies). These complications appear at pressures above the ones where molecular flow conditions are at hand and consists of (i) influence by gas-phase diffusion and mass flow on the local OH concentration in the probed volume and (ii) quenching effects (rotational and electronic). An additionalerror source, when the measured results are compared with kinetic model calculations, may be gradients established in the reactants due to insufficient mass transfer. All these phenomena can be handled provided sufficient precautions are taken and control measurements are performed.
Acknowledgment. We thank V. P. Zhdanov, Institute of Catalysis, Novosibirsk, for deriving eq 1. We gratefully acknowledge financial support from Carl Tryggers Foundation, from Volvo Research Foundation and Volvo Educational Foundation (Contract No. 92:36), from the Swedish Natural Science Research Council (NFR, contract No E-EG 2560-129),from the National Energy Administration (STEV, Contract No. 276 330-l),and from the Swedish Research Council for Engineering Sciences (TFR, Contract No. 92-538). References and Notes (1) Hsu, D. S. Y.; Hofhauer, M. A.; Lin, M. C. Surf. Sei. 1987, 184,
25. (2) Hellsing, B.; Kasemo, B.; Ljungstrdm,S.;Rostn. A.; Wahnstrdm,T. Surf. Sci. 1987, 1891190, 851. (3) Ljungstrdm, S.; Hall, J.; Kasemo, B.; Rostn, A,; Wahnstrdm, T. J. Coral. 1987, 107, 548. (4) Ljungstrdm, S.;Kasemo, B.; Rostn,A.; Wahnstrdm, T.; Fridell, E. Surf. Sci. 1989, 216, 63. (5) Wahnstrdm, T.; Ljungstrdm, S.; Rostn, A.; Kasemo,B. Surf. Sci. 1990, 234,439.
(6) Fridell, E.; Rostn, A,; Kasemo, B., submitted to Lungmuir. (7) Fridell, E.; Westblom, U.; Aldtn, M.; R d n , A. J. Coral. 1991,128,
92.
12834 The Journal of Physical Chemistry, Vol. 97, No.49, 1993 (8) Rosba, A,; Ljungrtrdm,S.;Wahnsttr(lm,T.; Kascmo, B.; JElecrron.
Spccrrosc. Relar. Phenom. 1986, 39, 15.
(9) Fridell, E.; Hellsing, B.; Kasemo, B.; Ljungstr6m. S.;Rostn, A.; Wahnstrbm, T. 1. Vac.Sei. Technol. A 1991, 9,2322. (10) Wahnstdhn, T.;Fridell, E.; L j u n p t r h , S.; Hellsing, B.; Kasemo, B.; R d n , A. Surf. Sci. 1989,223, L905. (1 1) William, W.R.; Marks, C. M.;Schmidt, L. D. J. Phys. Chem. 1992, 96,5922. (12) Fridell, E.; Elg, A.-P.; Rosba, A,; Kasemo, B.,to be published. (13) Tevault, D. E.; Talley, L. D.; Lin, M.C. J. Chem. Phys. 1980, 72, 3314. (14) Mooney, C. E.; Andenon, L. C.; Lunsford, J. H. J. Phys. Chem. 1991, 95,6070. (15) Fridell, E. Chem. Phys. Lrrr. 1992,188,487. (16) Talley, L. D.; Lin, M. C. Chem. Phys. 1981,61,249. (17) Pfefferle,L. D.;Pfefferle, W. C. Carul.Reu.-Sci.Eng. 1987,29,219. (18) Driscoll, D. J.; Campbell, K. D.; Lunsford, J. H. Adu. Cural. 1987, 35, 139. (19) Pfefferle, L.D.;Griffiin,T.A.; Winter,M. Appl. Opt. 1988, 27,3197. (20) Cattolia, R. J.; Schefer, R. W. NinereenfhSymposium (Inremarional) on Combusrion;The Combustion Institute: Pittsburgh, 1982, p 31 1.
Gudmundson et al. (21) Pfefferle, L. D.; Griffm, T. A.; Winter, M.; Crorley, D. R.; Dyer, M. J. Combust. Flame 1989, 76,325. (22) Diekc, 0. H.;Crosswhite, H. M.J. Qwnrum. Specrmc. Radiar. Trans. 1962, 2, 97. (23) Croslcy, D. R. Opr. Eng. 1981,20, 551. (24) Zhdanw, V. P. Elemeniaty Physicochemical Processes on Solid Surfaces; Plenum Rar: New York, 1991. (25) Jost, W. Dvfusion;Academic Rsu; New York, 1966. (26) D = */&)l, when (u) isthc~~tivespeedofthecollirionpartnen and I is the m a n free path (u) = d-, where p b the reduced maas, I = k P T / p r M , w h m d is the collision diameter. For the case of OH wllidtng with another ipecicr I, we d ’d = 1 / 2 ( & ~ + 4). (27) Hellsing, B.; Kascmo, B.; Zhdanw, V. P . J. Cural. 1991,132,210, (28) Hellsing, 8.;Karcmo, B. Chem. Phys. Leu. 1988, 148,465. (29) Copeland, R. A.; Crosley. D. R. J. Cham. Phys. 1986,84, 3099. (30) Copeland, R. A.; Dyer, M.J.; Croslcy, D. R. J. Chem. Phys. 1985, 82, 4022. (3 1) Atkina, P. W. Physical Chemistry; 3rd ed.;Oxford University Res: Oxford,1987.
