J. Phys. Chem. 1992, 96, 5922-5931
5922
Steps in the Reaction H2 4- 0 2 = H 2 0 on Pt: OH Desorption at High Temperaturest W. R. Williams,t C.M. Marks,$ and L. D. Schmidt* Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455 (Received: October 8, 1991)
OH radical desorption over a polycrystalline Pt foil exposed to mixtures of H2,O,,and H 2 0for surface temperatures between lo00 and 1800 K was measured by laser-induced fluorescence (LIF) and used to determine the elementary steps in this reaction. OH production in H 2 0decomposition varied nearly linearly with H 2 0 pressure, while addition of O2 enhanced and H, reduced OH production. H2oxidation yielded the highest OH desorption signals with a sharp maximum OH desorption occurring in lean Hz/02mixtures. A 12-step mechanism for reversible H2oxidation over a Pt surface is proposed which assumes the reaction proceeds through H, 0, OH, and H 2 0surface intermediates. Model calculations indicate that the observed results in the HzO,H 2 0 0 2 ,and HzO + H2systems are primarily due to the shifting of the equilibrium: H(a) OH(a) s H20(a), and the results in the H2 + 0, system are primarily due relative rates of the reactions H(a) + O(a) OH(a) and H(a) OH(a) H20(a). Pre-exponentials and activation energies for all reactions were determined by fitting the experimental results to model calculations based on the mechanism, and complete potential energy diagrams of reaction pathways are proposed.
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I. introduction Catalytic combustion is important technologically because catalysts alter selectivities to desired products, and the ease of ignition and stability of the ignited reaction at surfaces are frequently superior compared to their homogeneous counterparts.I However, little quantitative information is known about the kinetics of individual steps of these processes because most information about individual steps in surface reactions comes from techniques requiring ultrahigh vacuum (UHV)and/or low surface temperatures, while catalytic combustion is usually carried out at high temperatures and near atmospheric pressures. Thus much of our knowledge of kinetics comes from long extrapolations from the conditions of fundamental studies. Laser-induced fluorescence (LIF) of desorbing species has been previously demonstrated to be a versatile technique for studying catalytic combustion reactions since it can be camed out at almost any pressure and surface temperature. Most work over Pt has focused on the desorption of the OH radical, either to infer the surface concentration of OH and thus the kinetics of a surface combustion reaction at low pressures2" or to examine OH desorption and interaction with a reacting boundary layer by measuring OH concentration profiles at or near atmospheric In this paper, we present experimental results of a kinetic study of the H2 oxidation reaction over a high surface temperature Pt surface using both mass spectrometry and laser-induced fluorescence. Since hydrogen oxidation is characterized by extremely fast rates, determination of the rates of the intermediate surface reaction steps by stable species detection can be difficult. In fact, because of fast rates, much of the kinetic behavior in H2 oxidation is determined by mundane factors such as sticking coefficients and flux l i m i t a t i ~ n s . ~However, ~ ~ J ~ by probing trace amounts of desorbing OH radicals using LIF, we have obtained essential information on the coverage of this intermediate species during H2 oxidation. Using this data, we have developed a model which not only explains the features observed in the H2 oxidation kinetics but fixes the surface kinetic parameters and activation energies of the intermediate steps. The obvious starting point for understanding surface combustion is the simplest combustion reaction, H, 02.This is because all conceivable reactions can be written out in a simple diagram (Figure 1) making a mechanistic study quite tractable. Extensive work has been done by the group of Kasemo and Rosin on this
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'To whom correspondence should be addressed. 'This research partially supported by DOE under Grant DE-FGO288ER13878-AO2. . 'Current address: Du Pont Central Research and Development, Experimental Station, Wilmington, DE 19880-0262. $Current address: Naval Research Laboratory, Code 6522, Washington. DC 20375-5000
and excellent progress has been made in terms of obtaining a working model to determine many of the kinetic parameters for intermediate surface reaction steps by fitting experimental results to a model of Hzoxidation.I2J5 Agreement between the model and experiment was quite good for very lean H2/02mixtures, but the model failed to predict a tail which was observed on the fuel-rich side of the OH desorption maximum. Also, their model did not consider OH formed as a result of H 2 0 readsorption and decomposition. In addition to studying the forward reaction of H2 oxidation, we have studied OH desorption in the reverse reaction, H 2 0 decomposition over Pt. In contrast to more aggressive metals such as W where H20decomposition is rapid,16the rate of this reaction is negligible over Pt and is impossible to study without a trace analytical technique such as LIF, even then only at high surface temperatures (>1150 K). Thus, H 2 0decomposition is limited by surface reaction rates, and O H desorption rates are governed by the surface reaction rates and not flux limits as is often the case in H2 oxidation. By examining both the forward and reverse reactions, we have been able to determine additional surface rate constants as well as determine a complete high surface temperature energy diagram for the reaction pathway which complements the pathway proposed by Anton and Cadogan for low and intermediate surface t e m p e r a t ~ r e . ' ~ Our objective is not only to understand the mechanism of H, oxidation on a Pt surface but to use this system as a prototype in understanding surface combustion in general. Therefore, in addition to fitting the experimental results presented here to a working model of the H2 + O2 ~2 H 2 0 equilibrium over Pt, we will also examine the qualitative behavior of the model. This provides insight not only into the features observed in the H2/ 0 2 / H 2 0equilibrium on the Pt surface but into the processes which dominate surface combustion in general. Reaction Mechanism for H2 $. O2 e H20. Our analysis of this reaction will use a model based on a 12-step mechanism. Using a single set of rates for these reactions, we have been able to obtain good agreement between model and essentially all experimental results. The mechanism and notation used are Adsorption and desorption rates and equilibria:
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0022-365419212096-5922$03.00/0
OH(a)
kdDH
0 1992 American Chemical Society
OH'(g)
The Journal of Physical Chemistry, Vol. 96, No. 14, 1992 5923
OH Desorption at High Temperatures Surface reactions (all species are adsorbates): k
H+O&OH k-l
H
k
+ OH & H20 k-2
20H
k
&E k-3
(2)
H20+ 0
(3)
This mechanism allows for all plausible reactions to occur assuming only H, 0, OH, and H 2 0 as surface species and no readsorption of OH. Although other species have been proposed such as O2(a)I8and H30+(a),19these are only observed at low temperature and were not included in this mechanism. Examination of this mechanism reveals why H 2 0 decomposition is as important to study experimentally as the forward reaction, H2 oxidation. It will be shown that because the rate of reaction is slow, one can reasonably assume that the OH measurement is a direct measure of the surface reaction step (-2) above. This is useful since knowledge of the reverse reaction can potentially give surface energies and equilibria, and this would give additional parameters for surface energy diagrams. More importantly, by studying the effects of H2 addition to OH production in water decomposition, we are able to directly examine the forward reaction (2) since adding H2 shifts the equilibrium (2, -2) toward water, depleting the OH which we can measure. Finally, adding O2 to water decomposition allows us to determine the relative importance of reaction 1 and reaction -3.
Figure 1. Reaction mechanism for the reversible catalytic reaction H2(g)
+ OAg) * H2O(g).
m I1 Sd:YAG Laser
1064 nmU2 1 = 532 nm
Tunable Dye Quadrupole Spectrometer I
616
308
Clechanically Pumped Low Pressure Reactor
1
Pt Foil CaIalysr
< CHV Chamber
Y
T
Lek Valve
11. Experimental Section The reaction was a stainless steel six-way cross with a volume of 0.4 L pumped by a mechanical pump. A resistively heated Pt foil, 0.17 X 3.0 cm in area, was suspended from nickel leads. Surface temperature was monitored with a thermocouple. The pumping speed time constants, determined by measuring the pressure decays in time upon shutting off the leak valves, were 1.3, 3.9, and 4.6 s for H2, 02,and H 2 0 , respectively. Total pressure was measured with a capacitance manometer, and partial pressures in reacting systems were measured by leaking into a chamber equipped with a quadrupole mass spectrometer at lo-’ Torr pumped by a turbomechanical pump (Figure 2). In nonreacting systems and for calibrations, partial pressures were measured by using differences in capacitance manometer measurements. The Q1 1 rotational line of the 2AI;+ = 2Xll (307.844 nm20) transition was saturated by a 10-Hz Q-switched pulsed Nd:YAG pumped dye laser beam which passed -0.5 cm below the Pt foil. Typical laser energy was 500 pJ/pulse, but measurements of OH intensity vs laser intensity indicated that the transition was near saturation at any laser energy above 100 pJ/pulse. The fluorescence signal was filtered and detected using an RCA 1P28B photomultiplier and a boxcar integrator which integrated the fluorescence signal between 50 and 100 ns after the laser pulse. Data were usually taken by integrating 300 laser shots. The fluorescence was also monitored with a 175-MHz digital oscilloscope. This allowed us to monitor fluorescence lifetimes to assure that nonradiative quenching was not affecting our results. Lifetimes ranged from -500 ns with a total pressure at 0.1 Torr, decreasing to below 200 ns at 1 Torr. Since the signal was integrated from 50 to 100 ns, nonradiative quenching should not significantly affect results below 1 Torr. Thus, most of our results are taken below a total pressure of 1 Torr except for the H 2 0 + O2 experiments. In the hydrogen oxidation experiments oxygen conversions approached 80% upon adding H2. This meant that to maintain a constant O2pressure of 0.1 Torr during high surface temperature reaction, we would have to increase the O2leak rate to give total pressures >1 Torr for as little as 0.01 Torr H2 added. Because this would cause problems with nonradiative quenching, we usually fixed the inlet feed of O2and measured the O2pressures during
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Turbo Pump
Gases
H20
Figure 2. Experimental apparatus for LIF experiments.
experiments. A superscript i will be used to denote references to O2 pressures which would exist with no reaction (effectively expressing an O2 feed rate), whereas an O2 pressure with no superscript will refer to the actual reactor pressure. 111. Experimental Results
Our experiments focused primarily on isothermal studies of the H 2 / 0 2 / H 2 0system, but we also have examined the temperature dependence of OH desorption. Although the OH desorption rates Torr have arbitrary units (we estimate a unity LIF signal OH), the relative numbers are comparable in all figures since the LIF OH transition was saturated. In the followingfigures, all points are experimental results, while curves represent model results from fits to be described later. Isothermal H20 Decomposition. Isothermal measurements of OH desorption rates for the decomposition of H 2 0 over a 1700 K Pt foil as a function of H 2 0 pressure are shown in Figure 3a. The effects of O2(Figure 4a) and H2 (Figure Sa) addition on the decomposition of H 2 0 were also determined. Rates of H2 and O2formation were negligible (
