Determining Rate Constants and Mechanisms for Sequential

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Determining Rate Constants and Mechanisms for Sequential Reactions of Fe with Ozone at 500 K +

Joshua J. Melko, Shaun G Ard, Tri Le, Gregory S Miller, Oscar Martinez, Nicholas S. Shuman, and Albert A Viggiano J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b08971 • Publication Date (Web): 06 Dec 2016 Downloaded from http://pubs.acs.org on December 13, 2016

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The Journal of Physical Chemistry

Determining Rate Constants and Mechanisms for Sequential Reactions of Fe+ with Ozone at 500 K Joshua J. Melko1*, Shaun G. Ard2, Trí Lê1, Gregory S. Miller1, Oscar Martinez, Jr.2, Nicholas S. Shuman2, and Albert A. Viggiano2 1

Department of Chemistry, University of North Florida, 1 UNF Dr., Jacksonville, FL 32224, USA

2

Air Force Research Laboratory, Space Vehicles Directorate, Kirtland AFB, NM 87117-5776, USA *

Corresponding author: Email: [email protected]

Phone: 904-620-2565

Abstract We present rate constants and product branching ratios for the reactions of FeOx+ (x = 0 – 4) with ozone at 500 K. Fe+ is observed to react with ozone at the collision rate to produce FeO+ + O2. The FeO+ in turn reacts with ozone at the collision rate to yield both Fe+ and FeO2+ product channels. Ions up to FeO4+ display similar reactivity patterns. Three-body clustering reactions with O2 prevent us from measuring accurate rate constants at 300 K although the data do suggest that the efficiency is also high. Therefore, it is probable that little to no temperature dependence exists over this range. Implications of our measurements to the regulation of atmospheric iron and ozone are discussed. Density functional calculations on the reaction of Fe+ with ozone show no substantial kinetic barriers to make the FeO+ + O2 product channel, which is consistent with the reaction’s efficiency. While a pathway to make FeO2+ + O is also found to be barrierless, our experiments indicate no primary FeO2+ formation for this reaction.

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Introduction The chemistry of iron and iron oxides is rich and diverse, finding relevance in heterogeneous catalysis in the Fischer-Tropsch process, oxygen transport in biological systems, radio wave propagation in the atmosphere, and oxygen activation in cytochrome oxidases.1-9 Common to all these areas is a need for clear mechanistic understanding of the bonding rearrangements involved. However, experiments that try to probe the reaction centers or mechanisms involved are often hindered by the chemistry required to carry out such complicated processes (e.g. solvents, counterions, supporting structures, defects, or aggregates). Gas phase experiments in the laboratory can close this gap by studying physical characteristics and reaction mechanisms of isolated iron or iron oxide species to elucidate the underlying fundamentals. Realistically, these gas phase systems may never represent the exact energetics or kinetics involved in some of the bulk phase systems discussed above. However, several groups have shown that valuable contributions can be made via gas phase insight into bonding patterns, electronic structure, reactivity, mechanisms, and intermediates involved.10-16 Reactions of iron oxides often involve oxygen atom transfer, which in the simplest case can be modeled via the reaction of a single iron atom with an oxidant. Here, we investigate this fundamental process through the experimental and theoretical study of the reaction of Fe+ with one of the simplest and strongest oxidants, ozone. This model system should provide conceptual understanding of the mechanisms of oxygen atom transfer and explore the bonding rearrangements common to reactions of iron oxides. Moreover, the reaction of Fe+ with ozone has direct relevance in the Earth’s mesosphere and lower thermosphere. The primary source of iron in the atmosphere is meteor ablation, and there exists a global background layer of neutral Fe atoms at altitudes between 85 and 95 km.17, 18 In addition, sporadic and more concentrated 2 ACS Paragon Plus Environment

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layers of neutral Fe atoms exist at altitudes between 90 and 100 km.19 Especially pertinent to our current study, Fe+ ions are abundant above 90 km.17 The reaction of Fe+ with ozone contributes to the amount of Fe+ and neutral Fe in these regions.18, 20, 21 The neutral Fe is affected from this reaction because radiative electron recombination to Fe+ is slow compared to dissociative recombination of an electron with FeO+, which is a product of the ozone reaction.21 Therefore, the kinetics of reactions regulating Fe+ and FeO+ concentrations should have direct implications for chemistry of the mesosphere and lower thermosphere. While two experimental studies of the Fe+ + O3 reaction exist in the literature,20, 22 they disagree with each other. We believe our new measurement refines the value of the rate constant, in addition to providing new data for the reactions of FeOx+ (x = 1 – 4) with ozone. Experimental and Theoretical Methods Experiments were performed using the Air Force Research Laboratory’s variable temperature selected ion flow tube (VT-SIFT) instrument, which has been described in detail elsewhere.23 Briefly, ions are generated in an electron impact source using a dilute mixture of Fe(CO)5 in He (∼10%). Ions are extracted and injected into a quadrupole mass filter where Fe+ is mass selected. The Fe+ ions are focused using an einzel lens before introduction to a laminar flow tube via a Venturi inlet, where ∼104 to 105 collisions with a He buffer gas act to thermalize the ions and carry them downstream. Operating pressures of 0.4 Torr are maintained in the flow tube throughout all experiments, other than to explore ternary reaction channels. Typical reaction times are on the order of 4 ms, dependent on the helium buffer flow. The temperature of the flow tube is variable over a large range either by resistive heating devices (300 – 700 K), pulsed liquid nitrogen (120 – 220 K), or recirculating methanol chillers (220 – 300 K).



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The reactant neutral gas (a mixture of oxygen and ozone, discussed below) is added 59 cm upstream of the end of the flow tube. Upon exiting the flow tube, the core of the flow is sampled through a truncated nosecone with a 2 mm aperture. The remainder of the flow is pumped away by a Roots pump through a throttled gate valve that acts to maintain the desired pressure within the flow tube. After the nosecone, the primary ions and product ions are guided by a lens stack to a quadrupole mass filter for analysis and are subsequently detected using an electron multiplier. The buffer gas, reactant neutral, and ion concentrations are such that pseudo-first order kinetics apply. Usually, we expect a single exponential decay in parent ion signal (linear on a semi-logarithmic scale) over several orders of magnitude, indicative of a single rate constant. However, in the present study, fast secondary reactions that reform the parent ion (e.g. FeO+ + O3 Fe+ + 2O2), lead to curvature in the parent ion signal, which levels off at increased concentrations of the reactant neutral. To determine rate constants and product branching, we iteratively solve the set of coupled differential equations describing the reaction scheme throughout the known reaction time, comparing the calculated product ion abundances to those measured through a weighted least squares fit.24 By varying all relevant rate constants through a Monte Carlo procedure, over ranges constrained either by the literature or calculated collision values, the full parameter space was efficiently sampled. The best fit value for each rate constant was determined by identifying the minimum in the least squares parameter as a function of that rate constant. Uncertainties in the rate constant are derived from systematic sources (e.g. measurements of the absolute flow rates, flow tube pressure) that together typically account for ±15% being convoluted with uncertainty from the Monte Carlo fits that can be as much as ~


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50%. The general manner in which uncertainty is determined for the Monte Carlo fits has been discussed previously.25 The ozone was produced using 99.999 purity O2 gas in an Orec V5-0 ozonator, using a pressure of 3 psig and a discharge current of 0.9 A. The gas emerging from the ozonator is approximately 3% O3 in O2, and independent of the reactant gas flow rate. The concentration of O3 is measured by optical absorption using a Perkin-Elmer Lambda 10 UV/Vis spectrometer. The Pyrex absorption cell is 10.2 cm long with a diameter of 1.3 cm. A flow controller delivers a gas flow of 10-100 SCCM into the absorption cell, corresponding to absorbance readings at 248 nm of 40% - 85%. Further details of the ozonator and concentration determination are provided elsewhere.26 The approximately 97% O2 has little bearing on the measured kinetics presented here, as we have previously observed no bimolecular reaction of Fe+ or FeO+ with O2, in agreement with other groups.27-29 In addition, the reaction of FeO2+ with O2 has been shown to be inefficient (k = 5.0 x 10-12 cm3/s).30 We do, however, observe three-body clustering reactions of FeOx+ (x = 0 – 4) with O2 at 300 K and 250 K. Accurate determinations of the rate constants at these temperatures are difficult to ascertain due to these clustering reactions. Thus, results presented here are at 500 K, where the clustering channel is not observed. This allows us to report bimolecular rate constants for the reactions of FeOx+ (x = 0 – 4) with ozone. In order to characterize the potential energy surface of the Fe+ + O3 reaction, we have performed density functional theory (DFT) calculations with the B3LYP functional,31,

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including D3 dispersion corrections with Becke-Johnson damping (GD3BJ),33 and employing the TZVP basis set.34 Although this approach is not expected to be as quantitative as multireference calculations, it provides a useful general picture of the intrinsic reaction mechanism at reasonable computational cost.35-37 To test the performance of this methodology, we have calculated the 5 ACS Paragon Plus Environment


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ionization potential of Fe and the bond lengths of FeO and FeO+ using a variety of basis sets of moderate computational cost. TZVP was chosen because it gave values of 7.907 eV for the ionization potential of Fe and 1.614 Å and 1.641 Å for bond lengths of FeO and FeO+, respectively. These values represent good agreement with the experimental values of 7.902 eV for iron’s ionization potential and 1.619 Å and 1.641 Å for FeO and FeO+ bond lengths, respectively.38-40 In addition, the 6D – 4F energy splitting for Fe+ is found to be 0.29 eV, in reasonable agreement with the experimental spin-orbit average of 0.247 eV.41 Stationary points along the reaction pathway for the sextet state were identified and their harmonic frequencies calculated. Transition states were confirmed to contain a single imaginary frequency. Intrinsic reaction coordinate (IRC) calculations were performed on all possible transitions states to confirm connections between corresponding minima. All calculations were performed using the Gaussian 09 package,42 and reported energies include zero-point corrections. Because spin-crossings are known to play a role in many reactions of iron,43-48 various spin-states were considered. While the quartet spin state structures are closest in energy, they are higher than the sextet energies for all intermediates and transition states, suggesting spin-crossings do not play a role along the reaction coordinate. Results and Discussion For the reactions discussed here, determination of the rate constants is not trivial due to fast sequential reactions and reactions of FeOn+ with O3 producing both FeOn-1+ and FeOn+1+. As such, an example plot of raw data and a discussion of the chemistry involved are presented first. Figure 1 shows a typical set of raw data for the reaction of mass-selected Fe+ reacting with ozone. FeO+ is the only primary product, although ions up to FeO4+ are observed from sequential


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reactions with ozone. Initial decay of the Fe+ ion in Figure 1 levels off at higher flows and reaches a steady state, precluding a simple pseudo-first order analysis for this reaction. Analogous experiments on selected FeO+ show a similar decay pattern, although in that case the primary products are Fe+ and FeO2+. Therefore, the shape of the Fe+ signal in Figure 1 is explained by initial decay to form FeO+, which itself reacts with an additional ozone molecule to produce Fe+ and two oxygen molecules. The respective rate constants for these processes mostly determine the overall shape of the Fe+ signal in Figure 1 (additional sequential chemistry discussed below plays a small role). We note that an alternative explanation for the shape of the Fe+ decay may be a mixed population of ground and excited state Fe+ that react with two different rate constants, a phenomenon we have observed in previous experiments.49 However, we employed source conditions that have previously produced mainly ground state Fe+ and observe no dependence of the shape of the curve on harsher source conditions.



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Figure 1. Typical raw data for the reaction of Fe+ with ozone, showing normalized ion counts as a function of ozone concentration. Primary FeO+ formation leads to secondary production of Fe+ and FeO2+.

Conducting experiments where we select and inject FeO+ into the flow tube, we observe that its primary products from reaction with ozone are Fe+ and FeO2+ only. Consistent with this is the observation that FeO2+ in Figure 1 is identified as a secondary product when extrapolating relative product formation to zero ozone flow. The FeO3+ in Figure 1 is from the tertiary reaction of FeO2+ with ozone, leading to the very small signals at these flows with larger scatter, although the fit is still adequate. Not shown in Figure 1, even smaller amounts of FeO4+ and O2+ are also observed for higher reactant gas flows. The former is likely from FeO3+ reacting with ozone, while the latter is from the reaction of FeO2+.


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Quantifying the rate constants and product branching for all the reactions is accomplished through the fitting approach described in the experimental section. However, this relies on an accurate reaction scheme. Since both O3 and O2 are present, one must consider reactions of FeOx+ (x = 0-4) with these molecules that are allowed by known thermochemistry,50-52 some of which is not well-defined (e.g. FeO3+ only has an upper limit to its heat of formation of 1197 ± 42 kJ/mol).51 In agreement with the thermochemistry, reactions of Fe+, FeO+, and FeO2+ with O2 are inefficient or nonreactive, with rate constants of 0) [>-386 ± 47] FeO+ + 3O2c [-12 ± 20] + + c FeO4 + O3 6.7 (±4.1) 0.82 FeO3 + 2O2 [

Determining Rate Constants and Mechanisms for Sequential

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