J. Chem. SOC., Faraday Trans. 1, 1982, 78, 1237-1247 Effects of Pressure and Surface Initiation Efficiency on the Flowing H,O, + NO, + CO + N, Chain Reaction System BY GARY J. AUDLEY, DONALD L. BAULCH AND IAN M. CAMPBELL* School of Chemistry, The University, Leeds LS2 9JT Received 11 th June, 1981 The yields of CO, from the H,O, +NO, + CO reaction system in flowing camer gas at 298 K have been measured at total pressures ( P ) of 13.3,40.0 and 93.3 kPa. The variations of the yield as functions of [NO,] and [S] (S = acetaldehyde or diethyl ether) have been shown to indicate that the efficiency of conversion of H,O, to OH radicals in the heterogeneous initial step H,O, + NO, 4 OH + HNO, (1) is independent of total pressure for a particular surface activity. The results are consistent with the pressure dependence of k , for the propagation step CO+DH 4 CO,+H (2) (4) indicated by other work.For the rate constant of the termination reaction OH +NO,( + M) -+ HNO,( + M) low apparent values (k;) are indicated compared with literature values of k,. At P = 40.0 kPa, for three surfaces of apparent efficiencies of ca. 20%, ca. 53% and ca. 81 % in converting H,O, to OH in reaction (l), k;/k, values were ca. 0.45, ca. 0.65 and cu. 0.85, respectively. This trend is interpreted in terms of decreasing activity of these surfaces for adsorption of H,O,, producing decreasingly localized reaction zones : depletion of [NOJ in the reaction zone depresses k i / k , while increased surface concentrations of H,O, decrease the apparent H,O, to OH conversion efficiency by inducing an additional route for destruction of the active centre involved in reaction (1).Hydroxyl (OH) radical reaction kinetics are of outstanding interest at present due to recognition of the major roles of OH in atmospheric and combustion chemistry. Also, as pointed out in a recent review,l an upsurge in laboratory studies of OH kinetics in the past decade has resulted from developments in methods of generating and detecting the radical. Of newer methods of producing OH radicals, the thermal source in H,O, + NO, + CO gas-phase mixtures at ambient temperatures, developed in this laboratory,2$3 has shown considerable potential in the measurement of rate constants for reactions of OH with vapour-phase molecules such as alcohols,* e s t e r ~ , ~ aldehydess and nitro-compo~nds,~~ * many of which were first measurements.In all of our previous work the conditions used were a partial pressure of hydrogen peroxide considerably less than the saturation vapour pressure at ambient temperature, an excess of nitrogen dioxide and a large excess of CO or a CO+N, mixture, at a total pressure of d 13.3 kPa. The initiation step, occurring on the wall of the reaction vessel, was proposed to be (1) It was shown3 that coating of the walls with boric acid allowed achievement of a ca. 100% efficiency for conversion of H,O, into,OH in step (l), although usually the efficiency was lower. In this work we set out to extend the total pressure range up to approaching H,O, +NO, -, OH + HNO,. 12371238 H,O,+NO,+CO+N, CHAIN REACTION SYSTEM atmospheric. In doing so, one of our interests was in whether the stoichiometry of the heterogeneous process represented by reaction (1) was independent of total pressure or if some other reaction pathway might be induced.This is of considerable importance if reaction (1) is to be applied to the study of OH-induced reaction kinetics under conditions representative of the troposphere at large. It is also of importance to our understanding of the reaction system to attempt to gain some understanding of the reasons why the surfaces often induce H20, to OH conversion efficiencies of less than 100%. We have encountered three surfaces of different efficiencies in this work. Subsequent to initiation by reaction (I), the propagation steps of the chain cycle with OH as the chain carrier are CO+OH +CO,+I-I H+NO,+OH+NO while with excess NO, present the overwhelming termination step is postulated to be (4) The second-order rate constants k, and k, are likely to be pressure-dependent across our total pressure range of 13-94 kPa in CO + (N,) carriers on the basis of a substantial number of studies reported in the literature.' Accordingly, another aim in studying the H,O, +NO, + CO + (N,) system over this pressure range was to examine to what extent the measured yields of CO, from reaction (2) were consistent with the literature values of k, and k, for each pressure.OH + NO,( + M) + HNO,( + M). EXPERIMENTAL In this work we used the flowtube system which has been described in detail before,8 so that only the essential aspects are given here.The main flowtube was constructed of Pyrex glass, with an internal diameter of 24 mm, and was 1.15 m long. It was contained in a jacket through which water, thermostatted at ca. 298 K, was circulated continuously. An inner liner of Pyrex glass, of internal diameter 21 mm and of length 1.0 m, fitted flush within the main flowtube, so that its upstream end was above the point of the jet through which H,O, vapour was added, as shown in fig. 1. The inner surface of the liner was coated with a thin layer of boric acid as before,6 to provide the surface upon which the initiation reaction (1) occurred. The components of the reaction mixtures were added as follows. Nitrogen (B.O.C., White Spot) was taken from a cylinder and passed through soda-lime-packed columns to remove traces of CO,. In the experiments at total pressures of 40.0 and 93.3 kPa, N, was used as part of the main carrier gas and in all experiments it was used to carry H,O, and NO, into the main flow.Carbon monoxide was taken from a B.O.C. Technical Grade cylinder and was passed through soda-lime-packed columns to reduce traces of CO, to below the limits of detectability. At 13.3 kPa, CO itself was used as the carrier gas while at 40.0 and 93.3 kPa the carrier gases were 30% CO in N, and 14% CO in N,, respectively: this resulted in a constant [CO] at all pressures. The NO, flow was added to the carrier gas before it entered the main flowtube. A small flowrate of the N, was bubbled through liquified nitrogen dioxide in a series of three bubblers maintained at a temperature of 263 & 1 K using a salt-water-ice freezing mixture.The nitrogen dioxide was taken from a Matheson lecture bottle and was purified as before.8 The rate of addition of NO, to the flow system was monitored by the on-line spectrophotometric system described previously .8 In a similar manner a small flowrate of N, was bubbled through a set of three bubblers containing ca. 100% H,O, liquid, maintained at a temperature of 293 f 1 K. The hydrogen peroxide was obtained as an 85% aqueous solution (Laporte Industries, High Test Peroxide) and was concentrated as described previously.2v The N, + H,O, mixture was added directlyG. J. AUDLEY, D. L. BAULCH A N D I. M. CAMPBELL 1239 to the flowtube through the jet shown in fig. 1. The rate of addition of H,O, was measured by diversion of this flow through a set of bubble chambers containing acidified ceric sulphate solution, with spectrophotometric determination of the resultant rate of removal of CeIV as before.s The rate of flow of N, through the hydrogen peroxide was maintained at 2.3 pmol s-' in all experiments.The two substances which were used in this work as internal kinetic calibrants were acetaldehyde (B.D.H., 99% stated purity) and diethylether (Koch-Light, 99.9% stated purity). / S FIG. 1.-Diagram of the working section of the flowtube. A, circulating water maintained at 298 K; B, inner tube internally coated with boric acid; C, to spiral gauge. These liquids were degassed by pumping, before the vapour from that being used was introduced into an evacuated bulb of volume ca.5 dm3 to a pressure of ca. 5 kPa: N, was then added to bring the total pressure to ca. 110 kPa and mixing was assumed to be achieved on a time scale of at least overnight. Flows from the bulb were added to the main flow when required prior to its entry to the main flowtube. All gas flowrates were measured by capillary flowmeters, calibrated as described before.e At the outlet of the flowtube the reacted gas mixture passed through a sampling bulb of volume ca. 2.2 dm3. When this bulb was considered to contain a sample of gas representative of the conditions in the flowtube, it was isolated from the flow using a by-pass system and the contents were pumped slowly through a spiral trap packed with glass helices maintained at a temperature of 77 K using liquid nitrogen.The resultant condensate was analysed for its CO, content using the gas chromatographic procedures described previously.e Pressures in the flowtube were measured using a calibrated spiral gauge. RESULTS 1 K. All experiments were conducted at 298 The amount of CO, collected in the sample bulb was converted into the concentration of CO,, [CO,], in the reacted mixture on the basis of prior calibrations with known mixtures of CO, in the carrier gas. The activity of the boric-acid-coated surface of the liner tube was determined from measurements of the variation of [CO,] as a function of [S]/[CO] ( S = acetaldehyde or diethyl ether), when rates of addition of NO, and H,O, and total pressure were maintained constant. During the course of this work we encountered three different surface activities, each being stable for a period of months before an unpredictable and virtually discontinuous rise to a higher activity took place.These surface activities are denoted as 1, 2 and 3 in increasing order of efficiency. Plots of the reciprocal of1 240 8.0- 7.0- 6.0- - H,O,+NO,+CO+N, CHAIN REACTION SYSTEM 6 Y' t-- I I 1 I I I I 1 0 2.0 4.0 6.0 8.0 10.0 12.0 103 [SI /[COI FIG. 2.-Plots of the reciprocal yield of CO,, [CO,]-', against the concentration ratio [S]/[CO]. 0, S = diethyl ether, P = 13.3 kPa, surface activity 3; 0, S = acetaldehyde, P = 40.0 kPa, surface activity 1 ; V, S = acetaldehyde, P = 40.0 kPa, surface activity 2; 0, S = diethyl ether, P = 40.0 kPa, surface activity 3; A, S = diethyl ether, P = 93.3 kPa, surface activity 3.A J 0 0.5 1.0 1.5 2.0 2 . 5 1O2"021 ,"COl FIG. 3.-Plots of the reciprocal yield of CO,, [COJ-', against the concentration ratio [NO,]/[CO]. 0, P = 13.3 kPa, surface activity 3; 0, P = 40.0 kPa, surface activity 1; V, P = 40.0 kPa, surface activity 2; 0, P = 40.0 kPa, surface activity 3; A, P = 93.3 kPa, surface activity 3.G. J. AUDLEY, D. L. BAULCH AND I. M. CAMPBELL 1241 [CO,] against fS]/[CO] in the case of each surface activity were found to be good straight lines, as shown in fig. 2 for conditions of total pressure of 40.0 kPa with ca. 30% CO in N, as the carrier gas. For each surface activity we conducted experiments in the absence of added S, with constant input of H,O,, in a carrier gas of ca.30% CO in N, at a constant total pressure of 40.0 kPa, but varying the input of NO, in its excess r e g i ~ n . ~ Plots of [CO,]-' against [NO,]/[CO] proved to be good straight lines as is shown in fig. 3. TABLE G GRADIENTS (G) OF PLOTS OF FIG. 2 AND 3 Gs GNOr P/kPa surface activity S /lo7 dm3 mol-l /lo7 dm3 mol-l 13.3 3 C2H50C2H5 1.16f0.13 0.46 f 0.04 40.0 1 CH3CH0 18.7 f 0.8 4.02 f 0.38 40.0 2 CH,CHO 6.63 f 0.60 2.02 f 0.08 40.0 3 C2H50C2H5 2.52 f 0.13 1.76 & 0.05 93.3 3 C2H2OC2H5 5.01 f0.57 5.38 f 0.21 Also shown in fig. 2 and 3 are more limited data for experiments conducted at total pressures of 13.3 kPa (CO is the main carrier gas) and 93.3 kPa (14% CO in N, is main carrier gas). These data were only obtained for surface activity 3 since surface activities 1 and 2 had disappeared before we undertook the study of the effect of variation of the total pressure.A constant molar flowrate of N, through the hydrogen peroxide bubbler system was used for the addition of an initial concentration of H,O,, denoted as [H,O,],, to the flowtube (which would correspond to the homogeneous concentration of H,O, in the flowtube if no reaction occurred and adsorption on the walls was insignificant). For the total pressures of 13.3,40.0 and 93.3 kPa, [H20210 corresponded to fractions of ca. 1/13, ca. 1/38 and ca. 1/90, respectively, of the saturated vapour pressure of pure hydrogen peroxide at 298 K. The total flowrates were such that typically the time required for the gases to pass down the length of the tube was ca. 80 s, while the half-life for the initial reaction (1) under the conditions used is believed to be at least an order of magnitude ~horter.~ That the reaction time scale was much shorter than the residence time was confirmed by the fact that halving the total flowrate in a number of experiments resulted in no significant variation in [CO,].Least-mean-squares analysis of the plots shown in fig. 2 and 3 yielded gradients G shown in table 1. Those derived from fig. 2 are denoted as G,, when S = aFetaldehyde (CH,CHO) or diethyl ether (C,H,OC,H,) was present, while those derived from fig. 3 are denoted as GN02, when no S was present. The error limits quoted represent one standard deviation. DISCUSSION Analysis of the chain reaction mechanism represented by steps (1)-(4) H,O, +NO, + OH + HNO, OH+CO + CO,+H H+NO,+OH+NO OH +NO, + HNO,1242 H,O,+NO,+CO+N, CHAIN REACTION SYSTEM has been shown2v3 to lead to the equation where [OH], denotes the effective yield of hydroxyl radicals from the initiation step (l), integrated throughout the course of the reaction, expressed as a concentration and [NO,] represents the actual concentration of NO, in the reaction zone: since NO, is added in substantial excess of hydrogen peroxide, [NO,] is always assumed to be equal to the concentration of NO, as added.Provided that the efficiency of the boric- acid-coated surface for conversion of H,O, into OH by reaction (1) remains constant, in a set of experiments where [NO,] is varied at constant total pressure and [H,O,],, eqn (i) predicts that plots of [CO,]-l against [NO,]/[CO] will be linear with zero intercept.Thus the gradients in fig. 3 are expressed by the equation (ii) Upon addition of a substrate S which reacts with OH according to OH + S -+ non-propagating products (9 the analysis2* leads to the equation (iii) In a set of experiments where only [S] is varied, plots of [CO,]-l against [S]/[CO] are predicted to be linear with a positive ordinate intercept and gradient Gs given by When [OH], is constant (i.e. same [H,O,], and same surface efficiency) and when for the substrate used there are available absolute values of k, from other work in the literature, the value of the product k,[OH], may be deduced. Also eqn (ii) and (iv) combine to eliminate k,[OH], so that with corresponding values of GNOt and G, for the same conditions and an absolute value of ks, the apparent value of k4 (ki) can be deduced.For S = CH3CH0, an absolute value of k, = (9.6+ 1.0) x lo9 dm3 mol-1 s-l [ref. (9)] is available and there is also a value of ks = (9.2 f 1 .O) x lo9 dm3 mol-l s-l derived from relative rate measurements.10 Accordingly we have used a mean value of k, = 9.4 x lo9 dm3 mol-l s-l for S = CH3CH0 in this work. For S = C,H,OC,H, there is only one absolute value of k, = (5.4f 1.1) x lo9 dm3 mol-1 s-l available in the literature." However in previous works we have measured the ratio of the G, value for S = C,H,OC,H, to that for S = CH3CH0 as (3.33f0.12)/(6.63 k0.59) = 0.502+0.068. On the basis of the mean value of k, = 9.4 x lo9 dm3 mol-l s-' above this ratio leads to k, = (4.7 k0.6) x lo9 dm3 mol-l s-l for S = C,H,OC,H,, which is consistent with the absolute value.We have therefore used k, = 5.4 x lo9 dm3 mol-1 s-l for S = C,H,OC,H, as the mean value. The values of k2[OH], and k; obtained from the data of table 1 are shown in table 2.G. J. AUDLEY, D. L. BAULCH AND I. M. CAMPBELL 1243 The essential parameter of present interest is [OH],, with regard to establishing the surface efficiency for conversion of [H,O,], in reaction (1). The rate constant k,, for N, carrier gas, is considered generally to increase almost linearly with total pressure (P) so that it is enhanced by a factor of ca. 2 for P x 0.1 MPa compared with the low-pressure limiting value.12v13 As we have postulated b e f ~ r e , ~ the value of k, at our lowest value of P of 13.3 kPa is likely to be indistinguishable, within the TABLE 2.-APPARENT VALUES OF k, AND k2 [OH], surface k; P/kPa [H20,],/10-6 mol dm-3 activity k2[OH],/102 s-l /lo9 dm3 mol-l s-l 13.3 6.40 f 0.45 3 4.7 + 0.5 2.14 & 0.29 40.0 2.36 f 0.22 1 0.50 f 0.02 2.02 f 0.21 40.0 2.36 f 0.22 2 1.4kO.l 2.86 f 0.28 40.0 2.36 f 0.22 3 2.1 f O .1 3.77 0.23 93.3 0.83 +_ 0.06 3 1 . 1 +O.l 5.80 f 0.70 experimental error limits, from the low-pressure limiting value of k, = (9.0+0.9) x lo7 dm3 mol-l s-l. On this basis, for P = 13.3 kPa and surface activity 3, the value of k,[OH], shown in table 2 yields [OH], = (5.2k0.6) x mol dm-3 when [H,O,], = (6.40 f 0.45) x mol dm-3, corresponding to a surface conversion efficiency for reaction (1) of (8 1 lo)%. If k, were assumed to be independent of P in our experiments, then the values of k,[OH], shown in table 2 would indicate values of the surface conversion efficiency rising to well over 100% at the higher pressures.This appears most unlikely. On the other hand, if we assume that the surface conversion efficiency is independent of P and calculate corresponding values of k,, at P = 40.0 and 93.3 kPa we obtain sensible results in comparison with those from other studies in which the pressure-dependence of k, has been investigated in N, diluent. For example, using a surface conversion efficiency of (81 & lo)% in conjunction with the k,[OH], shown in table 2 for P = 93.3 kPa, we obtain k, = (1.6 f 0.3) x lo8 dm3 mol-l s-l, which is enhanced by the expected factor of ca. 2 compared with the low-pressure limiting value of k,.The fact that the carrier gas is composed of CO as well as N, is not expected to be of significance in this: evidence is presented below in connection with k, that CO and N, have approximately the same third-body efficiencies in three-body combination reactions. We therefore conclude that the most satisfactory interpretation of our results is based on a pressure-independent surface efficiency of reaction (1) and a pressure-dependent k,. Biermann et all3 have shown that k, in N, carriers only shows a pressure-dependent component when traces of 0, (ca. 0.03 % of [N,] for P x 0.1 MPa) or other (undefined) impurities are present to react with the M-stabilized HOCO complex involved. However, in this work we have taken no specific precautions for removal of 0, and it might not be unreasonable to suspect that NO, may be effective in the same role.Accordingly our interpretation that k, shows a pressure dependence in H,O, + NO, + CO systems is not surprising. Turning to reaction (4), it is immediately clear from the three sets of data for P = 40.0 kPa that k; is not independent of the surface conversion efficiency of reaction (1): k; increases as k,[OH], (i.e. [OH],) increases. These values are for N, carrier containing 30% CO but when an N, carrier containing only ca. 2.5% CO was used no significant difference ink; was found, confirming that N, and CO have approximately the same third-body efficiency in reaction (4). Anastasi and Smith14 have obtained1244 H,O,+NO,+CO+N, CHAIN REACTION SYSTEM absolute values of k, as a function of P for M = N, at ambient temperature, which have been closely confirmed by Wine et aZ.,15 from which we interpolate k, = (4.4 & 0.8) x lo9 dm3 mol-l s-l, using the 18% error limit quoted by Anastasi and Smith.’, In table 2 the values of ki for P = 40.0 kPa are significantly lower than the above value of k,, but tend towards it as the surface conversion efficiency increases.Assuming the mean [H,O,], to [OH], conversion efficiency of 8 1 % for surface activity 3, then surface 1 corresponds to a conversion efficiency of ca. 20% and surface activity 2 to ca. 53 %. A plot of ki against surface conversion efficiency for the values obtained at P = 40.0 kPa is approximately linear as is shown in fig. 4. 8.0 7.0 6.0 - 5.0.I 4 E “E 4.0. 2 a 01 + 1.0 0 ; I I I 1 I 0 20 40 60 80 100 surface efficiency (%) FIG. 4.-Plot of ki against surface efficiency. 0, P = 13.3 kPa; 0, P = 40.0 kPa; A, P = 93.3 kPa. The extrapolation of the linear trend to 100% conversion efficiency leads to a value of ki which is in agreement, within relatively wide error limits, with the absolute value of k, for P = 40.0 kPa quoted above. In fact this is the situation which we found in our previous work3 in a static reactor in which the surface conversion efficiency was 100%: apparent values of k , extracted were within experimental error of the absolute values of k , derived from Anastasi and Smith’s results14 for P < 13.3 kPa. Also indicated in fig. 4 is that the two values of ki for P = 13.3 kPa and P = 93.3 kPa for surface activity 3 tend similarly to be lower than the corresponding values of k, derived from Anastasi and Smith’s work.l4 The interpretation of low values of ki compared with k, seems most likely to originate from a physical rather than a chemical effect in the flowtube, stemming from the mixed heterogeneous/homogeneous nature of the H,O, + NO, + CO reaction system.The heterogeneous initiation step (1) generates OH radicals at the boric- acid-coated surface and these diffuse out into the gas phase. In our present experiments a typical chain length is 4 and, since k , is approximately three orders of magnitudeG. J. AUDLEY, D . L. BAULCH A N D I. M. CAMPBELL 1245 less than the collision frequency between CO and OH, the average OH will make ca.lo3 collisions with CO molecules before it reacts and is converted to an H atom. Since the rates of the propagation steps (2) and (3) are equal, the average H atom survives for the same time as the OH radical before it reacts with NO, to be reconverted to OH. On average this cycle can be considered to occur 4 times before the chain propagation centre is extinguished by the termination reaction (4). As we have argued b e f ~ r e , ~ the chain propagation cannot extend to a distance far from the liner wall in comparison with the tube radius, despite the promotion of OH and H diffusion by their respective concentration gradients from the wall. Thus it is necessary that the entire reaction takes place in a thin layer of the gas phase adjacent to the liner tube wall, which contains instantaneously only a very small fraction of the NO, present in the flowtube.If initiation becomes sufficiently rapid, NO, consumption in reactions (1) and (4) and its conversion to NO in reaction (3) could be imagined to overwhelm the diffusional ability of NO, to maintain the bulk gas-phase [NO,] across this reaction zone, Accordingly a concentration gradient of [NO,] would develop in this thin layer towards the wall. Under such circumstances the average [NO,] in the reaction zone would be less than that in the bulk gas phase: this would reduce the termination rate and hence the apparent value of k, deduced under the assumption that [NO,] in the bulk gas phase was applicable to the reaction zone. On the other hand, the actual chain length in the reaction zone, dependent on the ratio k,[CO]/k,[NO,], will increase, offsetting the reduction in initiation rate to match the reduced termination rate.NO, formed in reaction (3), will be a component of the reaction zone : however, the rate constant of the additional termination reaction OH +NO( +M) + HNO,( +M) ( 5 ) is approximately a factor of 3 lower than k, [ref. (16)] under our conditions. Effective conversion of NO, to NO in the boundary layer, exacerbated by the increased chain length, therefore leads to ki < k, in our analysis. CO is always present in such a large excess that a significant gradient of [CO] across the reaction zone is unlikely. Added S molecules are only consumed at a fraction of the termination rate, so that significant S depletion in the reaction zone is also unlikely.On the basis of our model, depletion in the boundary layer will be most developed above surface 1, when the value of k; shown in fig. 4 suggests an effective [NO,] depletion of ca. 50%. Depletion of S in the boundary layer under these extreme circumstances is estimated to be limited to ca. 10% for the range of [S] used in this work and is therefore insignificant for the general conclusions drawn in connection with surface 1. For the higher efficiencies represented by surfaces 2 and 3, depletion of [NO,] and hence [S] in the boundary layer will be less. On the basis of the above model, our experimental observation that ki increases with improving surface efficiency points to a more severe depletion of [NO,] in the reaction zone at lower conversion efficiencies of [H,O,], into [OH],.The additional factor in the system which is required to resolve this apparent anomaly is the activity of the surface for H,02 adsorption, which is the pre-requisite for reaction (1) according to the earlier work of Gray et a1.l’ If the surface is very active in this respect, the rate of adsorption and the surface coverage will be large: the initially added H,O, will then diffuse to the liner wall to produce a high surface concentration of H,O, on a ring of the surface with a small lengthwise spread. The adjacent reaction zone will have a small volume under these circumstances with a high production rate of OH from reaction (1) per unit volume. This situation has an evident potential for depletion of [NO,] in the highly localized reaction zone.Moreover, such a high effective concen- tration of H,O, may promote interaction of the nascent OH with adsorbed H202 at1246 H,O,+NO,+CO+N, CHAIN REACTION SYSTEM the expense of interaction with CO in the gas phase, thus accounting for the inefficiency of [H202], to [OH], conversion. An alternative explanation is that reaction (1) proceeds very rapidly on this type of surface, which produces a similarly localized reaction zone but cannot account so readily for the inefficiency of conversion. The surface with higher [H,O,], to [OH], conversion efficiencies are then seen as less active for adsorption of H,O,. The ring of surface upon which the initially added H,O, is adsorbed extends much further parallel to the axis of the tube.Reversal of the arguments in the preceding paragraph suggests that due to the larger volume of the reaction zone and the lower production rate of OH per unit volume from reaction (l), the potentials for depletion of [NO,] and for interaction of OH with adsorbed H,O, are considerably reduced. Consequently kl and the [H,O,], to [OH], conversion efficiency are both increased. Although this model of the reaction zone is qualitative and could only be substantiated by a much more sophisticated technique, it provides a reasonable interpretation for all the results presently available for the H,O, + NO, + CO reaction system. The sequence of events in our work is then that the initial surface activity 1 was induced by a surface with a high activity for adsorption of H,O,.Discontinuously, for unknown reasons, the surface then deactivated for H,O, adsorption to produce surface activity 2. Finally a further transition, with further deactivation, produced surface activity 3. Deactivation for adsorption over a long period of operation, as demanded by these arguments, seems more in keeping with the general experience with catalytic surfaces than the activation which might at first sight be ascribed to the observed increase of [H,O,], to [OH], conversion efficiency with time. Finally we consider the likelihood that reaction (1) is more complex than is indicated by the reaction equation. Hydrogen peroxide in aqueous solutions decomposes in the presence of mixed insulator oxide materials possessing acidic centres. A mechanism which has been proposedlB for the decomposition is A++H,O, + A+'HO;+H+ (4 A+'HO;+H,O, + A+'OH-+H,O+O, (b) A+'OH-+H+ +A++H,O (4 in which AS represents an acidic centre.Whilst our H,O, + NO, interaction takes place at the boundary between solid and gas phases, a modification of step (b) in the above mechanism appears to offer some rationalization. Boric acid will evidently have acidic centres which can be represented in general function as A+. In their original work on the H20,+N0, systems, Gray et aZ." suggested that an adsorbed H,O, molecule interacted with a gas-phase NO, molecule. Adapting step (b) above we suggest a step A+'HO;+NO, +A+'NO;+OH in which the A+'NO; species does not appear unrealistic and like step (b) atom transfer is involved. Finally the process is completed by the step represented as A+'NO; + H+ + A+ + HNO,.Further work is required to justify such a mechanism, which must be regarded as speculative for the present. One of us (G.J.A.) thanks the S.R.C. for the award of a CASE studentship. We thank Laporte Industries Ltd for gifts of hydrogen peroxide solution and for their interest in the work.G. J. AUDLEY, D . L. BAULCH A N D I. M. CAMPBELL 1247 D. L. Baulch and I. M. Campbell, Gas Kinetics and Energy Transfer (Specialist Periodical Reports, Royal Society of Chemistry, London, 1981), vol. 4, pp. 137-188. I. M. Campbell, B. J. Handy and R. M. Kirby, J. Chem. SOC., Faraday Trans. I , 1975,71, 867. I. M. Campbell and P. E. Parkinson, J. Chem. SOC., Faraday Trans. I , 1979, 75, 2048. I. M. Campbell, D. F. McLaughlin and B. J. Handy, Chem. Phys. Lett., 1976,38, 362. I. M. Campbell and P. E. Parkinson, Chem. Phys. Lett., 1978, 53, 385. G. J. Audley, D. L. Baulch and I. M. Campbell, J. Chem. SOC., Faraday Trans. I , 1981, 77,2541. ' I. M. Campbell and K. Goodman, Chem. Phys. Lett., 1975,36, 382. * G. J. Audley, D. L. Baulch, I. M. Campbell, D. J. Waters and G. Watling, J. Chem. SOC., Faraday Trans. 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