Energetics and mechanism for hydroxyl radical production from the Pt-catalyzed decomposition of water

Chemical Physics 61 (1981) 249-255 North-Holland Publishing Company

ENERGETIC3 AND MECMAiVISM FOR HYDROXYJL RADICAL FROM THJ3 PbCATALYZED DECOMBOSiiTION OF WATER L.D. TALLEYS

PROD’LJCT~ON

and M.C. LIN

Chemistry Division. Naunl Resmelt Laborarmy,

Washington, DC20375,

USA

Received 15 April 1981, in final form 8 June 1981

The catalytic decomposition of Hz0 over a polycrystalline pIatinum surface has been studied in a low pressure flow system. Hydroxyl radicals were detected in gaseous decomposition products above about 820 K by !aser-induced fluorescence at 300 nm. The activation energy for the HO desorption process, HO* + HO(g) i *, was determinedto be 30* 1 kcal/mol, which is in excellent agreement with the valuesreported previouslyfor the Pt-catalyzedoxidationof Hz by O2 and hT,O.The effectsof addedHz. D1 and O2 on HO formationwere also investigatedand the observeddata can be satisfactorily accounted for by a mechanism which involves chemicallyadsorbed H, 0 and HO species. A similar but brief experiment was carried out for the catalytic decomposition of NH3 over the same catalyst. However, no NH or NH2 radicals were detected in gaseous decomposition products up to 1100 EC.

1. Introductiola We have recently studied a series of catalytic oxidation reactions of Hz by 01 [l-3], NzO [4] and NO, [5] over a polycrystalline Pt surface using the matrix isolation and/or laser induced fluorescence (LIF) methods to analyze reaction products. In these experiments we have detected for the first time the presence of hyclroxyl radicals in the reactions presumably via the following thermal desorption process [l-5]: Hs+O*+HO*+*

(1)

HO* + HO(g) + *,

(2)

where “*” represents an active site on the Pt surface. With the exception of the Hz-NO2 system [S], the activation energy for the desorption process (2) was determined to be in the range of 30*2 kcai/mol for the pure Pt as well as a PtRh(lO%) catalyst. In the NO* system, however, t NRC/NRL postdoctoral research associate (January

1979-November 1980). Present address: Exxon Research and Engineering, Products Research Division. Baytown.

TX 77520, USA. 0301~0104/81/0000-0000/$02.50

@ North-Holland

the activation energy for HO radical production was found to be 19* 1 kcaljmol, independent of the total pressure (or NO, concentration) of the system. The lower activation energy observed in the NOz reaction was attributed to a possible simultaneous occurrence of side reactions which directly generate HO(g) with a lower activation energy [S]. Further study is being carried out to confirm this hypothesis_ It should be mentioned that the reaction of HP and NO over the same Pt catalyst up to 1270 K did not generate any detectable quantity of HO radicals by means of the LIF technique [S]. This observation seems to be consistent with the fact that the NO molecule is not dissociatively adsorbed on Pt surfaces significantly C6-81. In this work we report the results of our more recent investigation of the kinetics of HO radical production in the decomposition of Hz0 on the same Pt catalyst used in previous experiments in order to confirm the energetics required for reaction (2) at high temperatures as well as the effects of Hz and 02 on HO formation rates. Additionally, we also carried out preliminary experiments on the possibility of a

250

L.D. Talky, MC. Lin / Pmducrion of hydmxyl radical

similar free radical for&tion process in the decomposition of NH3 on Pt_ at these high temperatures. The results of the HzO decomposition reaction 2nd the observation made for the NH3 system are reported herein. 2. Experimental 2.1. Apparatzcs and proclrdzzre

The experimental apparatus used in this work has been descriied in detail previously [2, 31. AccordingIy, we wiII discuss this only briefly here. The reaction cell is a black-anodized a!uminuti cylinder equipped with a stainless Steel mixing nozzle constructed from two concentric rtibes (0.63 2nd 1.27 cm o.d.). The outer tube was sealed off and drilled with 24 evenly spaced 100~rJ.m diameter holes for sample mixing. The excitation laser pulses passed through two sidearms attached to opposite sides of the cell at about 2 cm below the mixing nozzle. Each sidearm contains one pzir of baffles to eliminate scattered iight. Berpendicular to the axis of the sidearms lies a large-aperture window for collecting LIF signals. A chromatix CMX-4 Eashlamp-pumped dye laser was used to exc’ite the A ‘1 +X ‘lI txansition of the HO radical in the gas phase. The wavelength of the second harmonic output was tuned to the Q1 rotational bandhead frequency (307.8 nrn) of the O-O vibronic transition. The LIF sign21 was detected with an RCA 7265 photomultiplier tube operating, typically, at 2 kV. A dielectric coated interference filter with transmission maximum at 309 nm was employed in the collection optics to minimize stray light at other waveIen&s. The signal from the detector was sent to 2 Nicolet model 1072 signal averager for intensity accumuIation and also to an oscilloscope for real-tine display. &nce the LIF sign21 vaties with the intensity of the excitation beam, the beam intensity (at 307.8 nm) was aIs0 monitored simultaneously with 2 diode whose o&put was accumulated on the signal averager for intensity norrnaiization. Both sign& were accumulated, typically, for sever21 hundred shots.

The catralyst (99.999% Pr wire, 50 cm long, 0.5 mm diameter) was present 2s 2 coil suspended by two 0.3 cm brass heating electrodes in the horizontal plane between the mixing nozzle and the excitation laser beam. Before each sun, the catalyst was cleaned by heating at 1000 K for 30 min in 5 To.rr 02, followed by pumping at 1100 K for 10 min. The catalyst was heated resistively and the wire temperature was measured by its resistance change relative to the room temperature value. No reduction in the activity of ihe Pt catalyst due to poisoning was noticed in these runs. Gas pressures were measured with an MKS model 145 Baratron capacitance manometer. 2.2.

Chemicals

Argon jhlatheson, goId-Iabeled), hydrogen (Ma’theson, ultra-high purity), deuterium (Matheson, CP) and oxygen (Matheson, extra dry) gases were used without further purification. H,O was of distiiled, milIipore deionized grade. &‘a0 (Monsanto, 99%) was used as obtained commercially after degassing.

3. Results Hydroxyl radicals were detected to desorb from the Pt catalyst surface above 820 K, similar to that observed in the catalytic oxidation of Hz by 02 [l-3] 2nd N20 [4]. The intensity of the LIF signal was very reproducible 2nd no apparent poisoning of the catalyst was observed throughout the whole experiment. Fig. 1 shows the effect of O2 on the production of HO radicals at 1070 K. In this experimeni, varying amounts of the 1% OJAR mixture were added to the steady stream of 110 mTorr 1% HzO/Ar mixture. The concentration of HO was found to increase monotonically with the partial pressure of 0, (within the scatter of the data). A similar dependence was observkd in the catalytic oxidation of HZ by 02 and NZO when the partial pressures of both oxidants were increased. The implication of this observation will be addressed !ater in the discussion.

L.D. Tafle,; MC. Lin / Production of hydroxyl radkl

Fig. 1. Dependence of HO production on the partial pressure ot O2 at constant HZ0 pressure (1.1 mTorr) and temperature (1068 K). Both 1% O2 and 1% Hz0 diluted in Ar were used in the experiment.

The effect of added O2 was also studied by using isotopically labeled water, Hzl’O. In this study, a constant pressure of 02 at 0.5 mTorr was mixed with increasing amounts of a 2% H2180/Ar mixture through the nozzle; the temperature of the catalyst was kept at 1100 K. Since

the iinewidth

of the ciye laser

(0.5 cm-‘)

was much narrower than the isotope shift (0.88 cm-‘) in the HO radical, we could readily detect H160 and H’*O species. Fig. 2 shows the increase of both H160 and HI80 radical concentrations as the pressure of Hzl’O was

d

Fig. 3. Dependence of 1-1160 and I-X”0 production OR the pressureof H~“O in the presence of 0.5 mTorr Oz. Catalyst temperature = 1098 K. A 2% H21SO/Ar mixture partial

was used in the experiment.

251

increased. The latter was found to increase smoothly and almost linearly with [H2”O], as was observed in an unlabeled experiment, whereas the former increased rapid!y with [Hz’*O] and reached a maximum value at 3 mTorr Hzl*O pressure. This change in H160 species concentration seems to be qualitatively consistent with the occurrence of exchange processes involving dissociatively adsorbed I60 and “0 atoms, and HI60 and HI80 groups on surfaces. The effect of added Hz was also investigated. The results are presented in fig. 3, in which the intensity of the LIF signal or relative HO concentration was plotted against the partial pressure of added Hz_ The catalyst temperature was maintained constant at 1063 K. The observed bell-shaped dependence, although having a much broader peak, is quite similar to the dependency observed in the catalytic oxidation of HZ by 02 and N20 mentioned before. We have also compared the effect of adding a small amount of Hz to the J&O mixture with that of the DZ addition. The results of this brief test are summarized in table 1. Further discussion on these findings will be made later. The effect of temperature on the production of the HO was studied using a more highly diluted (0.2 % H20/Ar) mixture. The total pressure was kept constant at 100 mTorr. The data points shown in fig. 4 were randomly measured throughout the whole range of tem-

Fig. 3. Eiiect of added H2 on the production of HO radicals at a constant Hz0 (1 mTorr) and catalyst temperature (1063 K). Both 1% Hz and 1% Hz0 diluted in Ar were employed in the experiment.

252

L.D. Talky, MC. LitI / Producrion of AydroxyIradico[

Table 1 _ Effects of added Hz and D2 on HO production catalytic decomposition of H&

from the

HO fl. int. =’

Hz (mTorr)

Dz CmTorr)

5.6 12.0 2.4

a 0.90 0

0 0 0.90

L’ In all three tests, 150 mTorr of 0.1% HZ0 in Ar was used and the temperature of the catalyst wa5 maintained constant at 1053 K.

perature (870-1084 K) to avoid systematic errors that might arise from the deactivation of the catalyst. Least-squares analysis of the data in fig. 4 led to an acti\ration energy of 30? 1 kcal/mol, which agrees very well with other values obtained from catalytic oxidation reactions of Hz (see table 2). It should be mentioned that the observed activation energy was found to be independent of Hz0 concentration because the result of a separate experiment carried out with 240 mTorr of 1% H20/Ar mixture gives rise to essentially the same value (30* 1 kcal/mol). I

Fig. 4. An Arrhenius plot of the observed HO LIF intensity versus the reciprocal Pt catalyst temperature for the decomposition of HzO. Reaction mixture: 103 mTorr of 0.1% &I20 in Ar. In a separate run, 240 mTorr of a 1% H,O/Ar mixture was med. Both sets of data gave the same activation energy of 30~1 kcaI/mol.

Table 2 Activation energies for HO desorption from polycrystalline Pt surfaces in different reaction systems Reaction

Catalyst

Temperature range (IQ

E, (kcaI/mol)

Ref.

HZ0 decomp. Hli-0, H2i0, Hz+NzO H=+NO> H,iNO

Pt

869-1084

30*1

882-1049 933-1373 839-1023 573-1043 573-1373

3151 29&l 25*2 1911 a)

this work 2 3 4 5 5

Pt Pt-10% Pt Pt Pt

Rh

” No HO radicals were detected

in the gas phase.

4. Discussion 4.1. CataZyticdecomposition of Hz0 The results of our previous study on the oxidation of HZ by 02, catalyzed by the same polycrystalline Pt catalyst were shown to be consistent with the following reaction mechanism involving dissociatively adsorbed H and 0 atoms and the chemisorbed HO species [2]: H~+2*+2H*,

(3)

Oz+2*+20*,

(4)

H*tO*+HOa+*,

(1)

HO*+ZTO(g)+*,

(2)

H* + HO* + HZ0 t- 2*.

(5)

For the catalytic decomposition of H20, the reaction may take place via the reverse of reaction (5), or (-S), followed by (-l), (-3) and (-4) prqducing HZ and OZ. Due to the relatively large barrier, 30 kcal/mol, the desorption o>HO via reaction (2) is only a minor process even at the highest temperatures studied in both HZ i O2 and HZ0 decomposition reactions. However, hecause of the constant supply of these reactants (H2+Ot or l&O), the steady state level of HO radical concentrations, alO particIes/cm3, can be readily detected and measured by &IF. The evidence for the existence of the Pt-OH group has been reviewed in ref. [Z]. its

L.D. Talky, MC. Lin / Production of hydroxyl radical

existence has been unambiguously demonstrated by the results of two recent studies using electron energy loss spectroscopy [9, lo]. In an experiment carried out in an ultra high vacuum system using a clean Ptflll) surface, for example, Fisher and Sexton [9] found that Hz0 which has been adsorbed at 100 K desorbs at 180 K with no appreciable decomposition. In the presence of O2 (which dissociatively adsorbs on the surface), the Hz0 molecule dissociates at temperatures above 150 K generating Pt-OH species. The strong, nearIy linear dependence of the gas phase HO radical concentration on the partial pressure of added 02 shown in fig. 1 seems to be in line with Fisher and Sexton’s low temperature results. The presence of 02 increased the concentration of adsorbed 0 atoms, CO*], and thus enhanced the rates of reactions (1) and (2), resulting in a higher gas phase HO concentration, [HO]. This O,-enhancement effect was also demonstrated by the result shown in fig. 2 in which the production of HI60 and H’*O was studied by adding diluted H2’*0-Ar mixture to 0.5 mTorr of 02. The initial rapid increase of H160 in this system can be described by the following reactions: 160 2 + 2* 4 2l60*,

(6) \

H2180+2*+H180*+H~

(7)

+ H’*O* + He + ‘*O*, 160*+H’80*

+ Hr60c

H* + 160* + H=O*

(8)

+ ‘*O*,

+ *,

(9) (10)

followed by the desorption process. However, as the concentration of Hrl’O was increased, the occurrence of the t60-removal processes such as (11) and (12): 160* + ‘SO* + ‘6Ol80 H”+H=0*+Hz’“O+2s,

+ 2*,

(II) (12)

became important and thus reduced the concentration of H160. The steady and aImost linear increase of [H’%] with [H7’eO], however, is expected by the above mechanism.

253

The addition of a small amount of Hz is also expected to increase [HO] by promoting reactions (1) and (2). This is shown by the results given in table 1 and fig. 3. The addition of too much Hz, however, will not only increase the rates of reactions (1) and (2), but also (S), resulting in the inhibition of Hz0 decomposition rate, or equivalently, in the reduction of [HO]. Therefore, the continued addition of the HZ to a constant flow of Hz0 in the system tends to exhibit a promoting as well as inhibiting effect, resulting in a bell-shaped Hz dependence as shown in fig. 3. The fact that Hz is dissociatively adsorbed on Pt is also clearly demonstrated by the drastic reduction in HO concentration upon the addition of Dz (see table l), resulting from the removal of O* and HO* by the reactions,

D*cO*~DO*+r~DO(g)~2*,

(13)

HOr;D*+Hei-DO*.

(14)

The activation energy for HO desorption measured in the present work, 30 I 1 kcal/mol, agrees excellently with the values determined in the catalytic oxidation of Hz by 02 and NrO (see table 2). In these systems, the measured activation energy is believed to be associated primarily with the recornbination-desorption processes involving dissociatively adsorbed H and 0 atoms as represented by reactions (1) and (2). The energetic diagram of the Ptcatalyzed 2H7_+ Oz = 2Ha0 system, which includes Pt-OH as a reactive intermediate is shown in fig. 5. In this diagram, A@ (Pt3H) was estimated by taking D(Pt-OH) = 30 kcal/mol determined in this work. Other energetics were taken from thesis of Ceyer Ill]. 4.2.

Catalytic decompositiortof NW;

Since the Cm-4 dye laser used in the present work can be readily tuned to wavelengths that are in resonance with NH(X ?Z’ + A 31T, 302-368 m-n) and NH2 (% ‘Bt --* A “At, 430-900 nm) electronic transitions, a brief test has been made to detect the presence of these two gaseous species in the catalytic decomposition of NH3 over the same catalyst. Both NH

254

L.D. Talky, MC. Lin / Production: of hydroxgf radical

fir@ H? O-P*

l

0-Pr

b-----ti20-P? &+

Fig. 5. A schematic energy diagram for the H&+Pt system which includes chemically adsorbed H, 0 and HO species as reaction intermediates. Since all thermochemicsl data (except Pr-OH) taken from ref. [ll] were primarily for Ptilll) surfaces, the energetic diagram is therefore only qualitative& vaIid.

and NH2 have been previously reported to be present in the products of NH3 decomposition on Pt and Fe above 770 # [12]. Interestingly, however, we did not detect any evidence of their presence in the gas phase up to 1100 K Pt temperature using a 10% NHJAr mixture. In order to establish the detectivity and validity of the present test, we have also carried out experiments by dissociating NH2 in the inlet tube using microwave discharge. In these experiments, the stainless steel nozzle was replaced by a Pyrex tube (1.25 cm o.d.). When NH3 was electrolessly discharged in the presence of the cold Pt catalyst at room temperature strong LIF signals due to NH (detected at 336 nm) and NH2 (detected at 598 nm) were observed. However, as the temperature of the catalyst was raised from 300 to 1063 K, as much as 60% of the NHz signal was irreversibly reduced. The cooling of the Pt wire to room temperature could not restore the original signal strength. On the other hand, the intensity of the NH signal was not strongly affected by the change in Pt temperature from 300 to 1160 K within: our detection limits. Both NH and NH2 signals were irreversibly diminished after a small arrount of 02 (0.5 Torr) was added to the system via the discharge tube.

The selectivity of the Pt catalyst with regard to the extent of NH and NHz destruction is quite vividly demonstrited by the results of this brief test. The fact that NH3 does not decompose on Pt to produce gaseous NH and-N& up to 2: 1100 K (contrary to what Melton and Emmett [12J observed before) and that NH2 and, only to a much lesser extent, NH are destroyed on hot Pt surfaces seem to imply that the Pt-NH2 species (but not Pt-NH) is a shortlived intermediate in the Pt-catalyzed 2NH; = N2+3H2 system. Further work on this system is presently underway. The absence of either NH or NH2 in the NHJPt system up to 1100 K also indicates that the detection of the HO radical in the HzO/Pt system is not merely the outcome of simple high-temperature pyrolysis, but rather the result of a selective catalytic activity of the Pt surface.

5. Concluding

remarks

We have employed the laser induced fluorescence technique to study the formation of hydroxyl radicals in the catalytic decomposition of water on a polycrystalline Pt surface. The activation energy for HO production was found to be 302 1 kcal/mol, which is in excellent agreement with the values previously determined in the catalytic oxidation of Hz by O2 and NzO using the same technique. In these two systems, the HO radical is believed to be formed by the association of dissociatively adsorbed H and 0 atoms, followed by thermal desorption. The lifetime of the newly formed species HO on Pt surfaces, according to the results of our recent detailed rotational and vibrational population measurements carried out under near collisionless conditions, was quite long in comparison with the time required for complete thermalization on the surface [13]. This is consistent with the observed rather large activation energy for the desorption process. We have also investigated the effects of added gases (Hz, 02, 02) on the production of HO radicals in the decomposition of normal and isotopically labeled water. The observed effects can be

L.D. Talley, MC. Lin / Prodrmion ofhydroxyl radical

accounted for by a mechanism which involves chemisorbed H, 0 and HO species as reactive intermediates in the D-catalyzed 2H2 + O2 = 2H20 system. A similar experiment was carried out for NH3 $ the same temperature range. In this system, however, no NH cr NH2 was detected desorbing from the Pt surface up to liO0 K. A brief test was made to examine the stability of NH and NH2, both generated by microwave discharge of NH3, over the same Pt catalyst up to as high as 1100 K. The former was found to be more stable than the latter which could be irreversibly destroyed by the Pt catalyst. Further work on the catalytic decomposition of NH, is still underway.

detect the desorption of the NH+ radical from either surface at all temperatures studied, contrary to the observation made by Melton and Emmett [12].

References [l] [2] [3] [4]

[5]

Acknowledgement 161

The authors are grateful to Dr. Syivia Ceyer for a very useful discussion, as well as, for a copy of her Ph.D. Thesis.

[7] [8] [9]

Note added in proof We have discovered, since the submission of this work, that the NH radical can desorb from both Pt and Fe catalyst surfaces above 1200 and 1000 K, respectively_ However, we did not

255

[lo] I1 11

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