J. Phys. Chem. C 2010, 114, 20621–20628
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Kinetics of NO + H+ + NO3- f NO2 + HNO2 on BaNa-Y: Evidence for a Diffusion-Limited A + B f 0 Reaction on a Surface† Aditya Savara and Eric Weitz* Department of Chemistry and the Institute for Catalysis in Energy Processes, Northwestern UniVersity, 2145 Sheridan Road, EVanston, Illinois 60208, United States ReceiVed: June 3, 2010; ReVised Manuscript ReceiVed: October 6, 2010
The reaction NO + NO3- + H+ f NO2 + HNO2 on BaNa-Y, between 100 and 300 °C, displays evidence for low dimensionality diffusion-limited kinetics that can be modeled as an A + B f 0 reaction (DLAB0 reaction), where H+ and NO3- are “A” and “B”, and the diffusion of H+ is rate-limiting. DLAB0 reactions are described by fractal kinetics when the reactants are confined to dimensionalities e4. Appropriate plots (alpha plots) of the depletion of surface nitrates versus time show evidence for a time regime in the kinetics that is termed the “Zeldovich regime”, during which reaction occurs at the edges of segregated regions of reactants. The Zeldovich regime manifests itself as a linear region in alpha plots, with a slope characteristic of the dimensionality to which the reactants are confined. Here alpha is ∼0.5. The end time of the Zeldovich regime, τf, was obtained by spline-fitting the kinetic data to extract the linear region of the alpha plot. Less reactive or unreactive nitrates remain at the end-time. The experimental data are consistent with the theoretical prediction that both τf and [NO3-] at τf are independent of [NO3-]0, the initial concentration of NO3-. There is an observed inverse dependence of τf on [NO], which can be rationalized by incorporating [NO] as a “coefficient”, p, for the probability of a reaction following each encounter of H+ and NO3-, rather than directly into keff. The activation energy for the surface diffusion of H+ is calculated to be 30 ( 30 kJ/mol from the temperature dependence of τf, which is within the expected range for such a process. Reaction-limited models with up to three site types were also considered, but did not provide physically realistic results. To our knowledge, this is only the second instance of kinetic evidence for a DLAB0 reaction on a surface, and the first opportunity to experimentally test predictions for τf for a chemical reaction. HNO3(g) f NO3-(ads) + H+(ads)
I. Introduction Kinetic studies of catalytic processes are often performed in flow-reactors which simplify the reaction kinetics by operating under steady state conditions. However, these steady state conditions can also mask fundamental phenomena that can be more easily elucidated in time-dependent experiments in static reactors.1 For several decades there have been predictions that unusual cases of kinetic behavior could occur for reactions on surfaces due to a lack of rapid mixing of reactants.2-4 Here we present evidence of one such phenomenon which would not be apparent under steady-state conditions, kinetic evidence for a diffusion-limited A + B f 0 reaction. These types of reactions are annihilation/elimination reactions, where, in the present case, the “0” represents elimination because the products do not further participate in the reaction system. BaNa-Y and related heterogeneous catalysts have shown potential for practical application in on-board, low temperature, reduction of nitrogen oxides (NOx) from diesel vehicle exhaust.5 More details will be given about BaNa-Y (and zeolites in general) in section III. For the purposes of the introduction, BaNa-Y can be considered a solid upon which NOx reduction reactions can occur. HNO3 is present in all realistic low temperature NOx streams6,7 and, as shown in eq 1, it dissociatively adsorbs on the surfaces of BaNa-Y and related catalysts to produce H+ and NO3-5,8-11 †
Part of the “Mark A. Ratner Festschrift”. * To whom correspondence should be addressed. E-mail: weitz@ northwestern.edu.
(1)
We have previously shown that these thermally stable nitrates are reduced to less thermally stable nitrites by NO (eq 2), and that this reaction with NO plays a pivotal role in the overall mechanism for the catalytic reduction of NOx.9,12-14
NO(g) + NO3-(ads) + H+(ads) f NO2(g) + HNO2(g) (2) An important topic is then to elucidate the kinetics of eq 2. To do this, we performed experiments in a static reactor as a function of temperature and initial nitrate coverage in the presence of an excess of NO, and monitored the depletion of surface nitrates with time. Two different batches of BaNa-Y zeolites were used, as well as a sample of Na-Y, another deNOx catalyst from the same homologous series of catalysts.15 Several kinetic models are considered. The kinetics of reaction 2 displays evidence for a lower dimensional diffusion-limited A + B f 0 reaction (DLAB0 reaction) on BaNa-Y. In this model, H+ and NO3- are “A” and “B”, and H+ is mobile. During depletion of reactants, theory and simulations predict that a region of time will arise termed the Zeldovich regime, during which reactant depletion leads to growing segregated regions of reactants, after which the reaction continues at the edges of these regions.3,4,16-19 It is important to note that the DLAB0 kinetics of the Zeldovich regime, and the periods leading up to the Zeldovich regime, occur even when only one of the reactants
10.1021/jp105110b 2010 American Chemical Society Published on Web 10/29/2010
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(“A” or “B”) is mobile.20-22 The Zeldovich regime is characterized by the appearance of a linear region in alpha plots (described Section III.C) with a slope, R, which is related to the number of dimensions that the reactants are confined to (for a 2D surface, R ) 0.5 in the Zeldovich regime). In the present study, the data in the R plots are spline-fitted to obtain an end time for the Zeldovich regime, τf, for various initial conditions. Theoretical predictions for the relationships between τf and the concentration of the nitrates at time τf can thus be tested. To our knowledge, the present study is the first opportunity to experimentally test the predictions for τf for a chemical reaction. Only a very limited number of systems have been found and characterized that exhibit DLAB0 kinetics in chemical reactions,23-28 of which only one has been a surface reaction.27,28 There has also been a recent paper in which the kinetics of a surface process was described via a different fractal kinetic model, and which involved mobility limitations for electron-hole transport.29 Several features are observed in the kinetics of reaction (eq 2) which deviate from the expectations for reaction-limited kinetics, and which are consistent with a DLAB0 model. These features are discussed explicitly as part of the kinetic analysis presented in this work. We note that it may also be possible for another diffusion limited model, in conjunction with very specific distributions of sites, to produce the observed features in the kinetics, but that such a situation would have to produce the observed kinetics over the wide variety of conditions studied. We are not aware of another kinetic model that produces the features observed in the kinetics reported in this study. In the experiments reported in this work, four parameters were varied: the NO concentration, the reaction temperature, the initial nitrate coverage, and the Ba/Na ratio (the cations upon which the nitrates adsorb). As will be discussed in detail, there is evidence that this system meets the mechanistic requirements for a DLAB0 reaction, and the data are consistent with the expected behavior for DLAB0 kinetics. II. Experimental Section Na-Y was obtained from Aldrich (Aldrich #33,444-8) with a nominal Si/Al ratio of 2.5. BaNa-Y was prepared by a 3-fold wet ion-exchange with a 0.1 M Ba(NO3)2 solution at ambient temperature. During each ion-exchange, the slurry was stirred for ∼24 h followed by vacuum filtration and rinsing with deionized water. Experiments were run using two BaNa-Y samples obtained by this method (BaNa-Y-1 and BaNa-Y2), and one Na-Y sample, which was used as obtained from Aldrich. Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) showed the BaNa-Y samples to have the composition: Si/Al ) 2.62 and Ba/Al ) 0.181 for BaNa-Y-1 and Si/Al ) 2.06 and Ba/Al ) 0.176 for BaNa-Y-2. The reaction cell and sample preparation are described elsewhere.5 The reaction setup allows in situ FTIR spectra to be recorded for both surface and gas phase species. Subsequent to sample preparation and calcining under vacuum to remove zeolitic water, the zeolite sample was heated/cooled to the desired temperature and kept at that temperature for >1 h prior to experiments. HNO3 gas (
