J. Phys. Chem. 1901, 85,840-848
840
Reaction Kinetics of Nitrogen Dioxide with Liquid Water at Low Partial Pressure Y.-N. Lee* and S. E. Schwartz Environmental Chemistry Division, Department of Energy and Environment, Brookhaven National Laboratory, Upton, New York 1 1973 (Received: October 17, 1980)
+
The reaction 2NOz(g)+ HzO(l) 2H+ NO3- + NOz- (1)has been studied for 1 X Ip N O z I 8 X atm. Since this reaction involves the transfer of a reactant from the gas into the aqueous phase, the rate depends upon the following: (a) physical mass transfer of the reactant, (b) the equilibrium solubility of NO2, and (c) homogeneous aqueous-phase kinetics. In order for the observed rate to yield information most sensitive to (c), the rate of (a) must be comparable to that of (c). In the case of a second-orderreaction, this can be achieved by working at low partial pressure of NOz as well as a high rate of physical mixing of the two phases. To facilitate the latter, the gas was brought into contact with the liquid as finely dispersed bubbles produced by flowing through a disk-frit; the mass transfer time constant, determined by uptake of C 0 2 ,was T, = 1.7-5.3 s. The rate of reaction 1,monitored by observing the electrical conductivity of the aqueous solution, exhibited dependence on PNQ and T, consistentwith second-orderkinetics and a steady-stateaqueous-phaseNO2 concentration. Values of HNoa,the Henry's law coefficient, and kl, the second-order aqueous-phase rate constant, were determined M atm-' and (1.0 f 0.1) X los M-'s-l, respectively, at 22 "C. Self-consistencyof the to be (7.0 f 0.5) X gas-liquid reaction model has been demonstrated in terms of the identity of the diffusing species and the extent of mass-transport limitation. +
Introduction Because of its thermodynamic stability, nitric acid is an important sink for nitrogen oxides in the atmosphere.'V2 Consequently, knowledge of the rate and mechanism of formation of HN03from its precursors is important in the description of atmospheric chemistry of the nitrogen oxides, including the incorporation of these species into clouds and precipitation in the ambient atmosphere. Although substantial understanding of the homogeneous gas-phase processes leading to HN03 formation has now been attained,2 comparatively little attention has been given to the corresponding heterogeneous processes. Among these, the reaction of NO2with liquid water to form nitric and nitrous acids, reaction 1,would appear, on the
+
2N02(g) H20(1)
-
2H+
+ NO3- + NO2-
(1)
basis of equilibrium ~onsiderations,3~~ to be an important pathway. However, present knowledge of the physical solubility of NO2 (Henry's law coefficient) and of the aqueous-phase kinetics of reaction 1appears inadequate to permit confident evaluation of the rate of this process in the ambient atmosphere. The kinetics of reaction 1,which is the rate-determining step of the industrially important process of nitric acid man~facture,~ has been investigated by a wide variety of techniques. These include indirect studies via the kinetics of nitrous acid formation and decompositioneJ and studies of the aqueous-phase reaction by pulse radiolysis and flash (1) E. Robinson and R. C. Robbins, J. Air Pollut. Control Assoc., 20, 303 (1970). (2)D.L. Baulch, R. A. Cox, R. F. Hampson, Jr., J. A. Kerr, J. Troe, and R. T. Watson, J. Phys. Chem. Ref. Data, 9,295 (1980). (3)A. E. Ore1 and J. H. Seinfeld, Enuiron. Sci. Teehnol., 11, 1000 (1977). (4)S.E.Schwartz and W. H. White, Aduan. Enuiron. Sci. Eng.,in press. (5)T. H. Chilton, "Strong Water", the M.I.T. Press, Cambridge, MA, 1968. (6) (a) E. Abel and H. Schmid, Z . Phys. Chem., 134,279 (1928); (b) E.Abel, H. Schmid, and S. Babad, ibid., 136,135(1928);(c) E.Abel, H. Schmid, and E. Romer, ibid., 148,337 (1930). (7)H. Komiyama and H. Inoue, J . Chem. Eng. Jpn., 11, 25 (1978).
TABLE I: Mixing Conditions for Limiting Regimes of Gas-Liquid Reactions" regime condition phase mixed 71 2 107, Or T r 2 1 o T d convective mass-transport r m 2 1 o T r , T~ 2 3Tm2/Td controlled diffusive mass-transport T d 2 1oTr, T * 5 0 . 3 8 ~ ~ ' / 7 d controlled " rr, T,, and T d are the characteristic time of reaction, convective mixing, and molecular diffusion, as defined by T ~ {(-d,[A]/dt)/[A]}-', = T~ = ( h ~ a ) - 'and , 7d = ( D a z ) - ' , respectively. In these equations a is the interfacial area per unit liquid volume, D is the aqueous-phase diffusion coefficient of dissolved species A, and k , is the liquid-side mass transfer coefficient as illustrated in the Appendix. p h o t o l y s i ~as , ~well ~ ~ as direct measurements of the rate of absorption of NO2 and/or N204into liquid water.1°'12 These studies have recently been reviewed e1se~here.l~ Unfortunately, despite the abundant research on the kinetics of this reaction, information requisite to describe this reaction under conditions of atmospheric interest is still lacking. The reasons are twofold. First, the studies that have been conducted pertain to partial pressures of NO2 that are higher by at least three to five orders of magnitude than those characteristic of the ambient atmosphere. Consequently, it is impossible to exclude mechanisms potentially important in the low-pressure region that would be masked at higher pressures by processes that are higher order in NOP. Secondly, despite numerous studies, there remain substantial discrepancies (as much as an order of magnitude) in such critical quantities as Henry's law coefficients and rate constants in this system.13 (8) M. Gritzel, A. Henglein, J. Lilie, and G. Beck, Ber. Bunsenges. Phys. Chem., 73,646 (1969). (9)A. Treinin and E. Hayon, J. Am. Chem. Soc., 92,5821 (1970). (10)S.P. S.Andrew and D. Hanson, Chem. Eng. Sci., 14,105(1961). (11)H.Komiyama and H. Inoue, Chem. Eng. Sci., 35, 154 (1980). (12)H.Kramers, M. P. P. Blind, and E. Snoeck, Chem. Eng. Sci., 14, 115 (1961). (13)S. E. Schwartz and W. H. White, manuscript in preparation.
0 1981 American Chemical Society
The Journal of Physical Chemistry, Vol. 85, No. 7, 1981 841
Reaction of NO, with H20
TABLE 11: Apparent Reaction Order with Respect t o NO, for Limiting Regimes as a Function of True Aqueous-Phase Reaction Order (Diffusing Species NO,) reaction order 0 1 2 m
phase mixed
convective mass transport
diffusive mass transport
0 1 112 1 1 1 2 1 312 m 1 ml2 Increasing Reaction Rate +
1
+ 112
Theory Apparent Reaction Order. The rate of a gas-liquid reaction depends upon the physical solubility of the reactant gas, the rate of mass transport of the reactant, and aqueous-phase reaction kinetics. Depending on the rate and mechanism of mass transport relative to the rate of reaction, such reactions may be distinguished into three regimes: (1)molecular diffusion controlled; (2) convective mass-transport controlled; and (3) phase mixed. The criteria for establishing the regime that pertains to a given reaction system under examination have been discussed extensively (e.g., ref 14) and are summarized in Table I; however, for a system of unknown reaction kinetics and mechanism, these criteria are not immediately applicable. Rather, it is necessary to infer the regime on the basis of observation (e.g., the power-law dependence of the rate of reaction upon the partial pressure of the reactant gas, Le., the “apparent reaction order”) and subsequently to confirm the self-consistency of the interpretation. In Table I1 we have displayed the apparent reaction order for various actual reaction orders for each of the several regimes. It should be emphasized that the regimes must be viewed as limiting cases and that intermediate situations, and in turn intermediate observed reaction orders, would be expected. Thus, one should approach such an interpretation with caution. In the case of aqueous-phase reaction of NOz, one has an additional complexity arising from association to form the dimer N204. This association occurs not only in the gas phase, but also (and of relevance to the present study) in the aqueous p h a ~ e Since . ~ ~ ~the apparent reaction order relative to pNOl depends on the competition between mass transport and chemical reaction, it can be influenced by whether the principal aqueous-phase diffusing species is NO2 or N2O4. This effect is illustrated in Table 111, in which is shown the apparent reaction order with respect to N02(g),evaluated for the several limiting cases under the assumption that the principal aqueous-phase diffusing species is N2O4, rather than NOz. It is seen that the identity of the diffusing species affects the apparent reaction order in both the diffusive and convective regimes. Despite this complexity, examination of the apparent reaction order with respect to p N O z nevertheless remains a principal means of discerning the aqueous-phase reaction mechanism and the identity of the diffusing species. ~~~~~
reaction order 0
The present study was directed to a determination of the physical solubility of NOz and the aqueous-phase rate coefficient of reaction 1 at concentrations corresponding to ppm and sub-ppm partial pressures of NO2, by direct measurement of the rate of reactive dissolution of NOz into water. These quantities may be determined from experiments conducted in a gas-liquid reactor for which the rate of mixing of the two phases is quantitatively characterized.
~
TABLE 111: Apparent Reaction Order with Respect t o NO, for Limiting Regimes as a Function of True Aqueous-Phase Reaction Order (Diffusing Species N,O,)
2 m
convective mass transport
phase mixed
diffusive mass transport
0 2 1 1 2 312 2 2 2 m 2 m/2+ 1 Increasing Reaction Rate -+
Slow Reaction Model. Although the dependence of the rate of reaction upon gas-phase partial pressure of the reagent is difficult to treat for intermediate regimes, one case that is amenable to such treatment is that of so-called “slow” reactions, i.e., situations intermediate between the phase-mixed and convective mass-transport controlled regimes. In this regime, the reaction is considered to occur in the “bulk”, the aqueous-phase reagent concentration being controlled by competition between convective mass transport and chemical reaction. Denoting the reagent as A, one has the processes
A, % !-
-
A,
,k
Aa
2A,
ki
As
products
where k, = k-, are stochastic rate coefficients for convective mixing (conventionally denoted as kLa in the chemical engineering literature, e.g., ref 14), kl is the rate coefficient for aqueous phase reaction, and the subscripts s and a denote surface and bulk aqueous concentrations, respectively. (Here a reaction stoichiometry of 2 is explicitly assumed, but the reaction order is left as a parameter of the model, i.e., d[A]/dt = -2kl[A]”.) According to this model the surface concentration is assumed to be in phase equilibrium with the interfacial gas-phase partial pressure of the reagent gas, i.e., [A,] = Hp, and the bulk reagent concentration is assumed to be uniform.14 Assuming steady state one obtains the following equation for [A,]: 2kl[A,]” + k,[A,] - k,Hp = 0 (2) which may be solved for [A,], in order to yield the expression for the rate R1= kl[Aaln.For a first-order reaction (n = 1)
and hence (3) Likewise for a second-order reaction (n = 2)
which gives [ l - (1 + 8~,k~Hp)’/~]
R1 = -
~~~
(14) P. V. Danckwerts, “Gas-Liquid Reactions”, McGraw-Hill, New York, 1970.
In these expressions constant (k,-l).
T,
is the convective mixing time
842
Lee and Schwartz
The Journal of Physical Chemistry, Vol. 85, No. 7, 1981
Equation 2 also gives the appropriate limits for when the rate of reaction is much smaller or much greater than that of phase mixing, viz. R1 = klHnpn
2kl[A,]"
