Rate Constant and Thermochemistry for K + O₂ + N₂ = KO₂ + N₂

Abstract

The addition reaction of potassium atoms with oxygen has been studied using the collinear photofragmentation and atomic absorption spectroscopy (CPFAAS) method. KCl vapor was photolyzed with 266 nm pulses and the absorbance by K atoms at 766.5 nm was measured at various delay times with a narrow line width diode laser. Experiments were carried out with O 2 /N 2 mixtures at a total pressure of 1 bar, over 748− 1323 K. At the lower temperatures single exponential decays of [K] yielded the third-order rate constant for addition, k R1 , whereas at higher temperatures equilibration was observed in the form of double exponential decays of [K], which yielded both k R1 and the equilibrium constant for KO 2 formation. k R1 can be summarized as 1.07 × 10 ■ INTRODUCTIONThe high-temperature chemistry of alkali metal species has important implications for combustion. It is well established that the presence of alkali metal species may result in flame inhibition 1,2 and alkali-based flame inhibitors are of interest. 3,4Potassium additives are also known to have an impact on gun muzzle flash. 5 Furthermore, most solid fuels contain alkali metals in minor quantities. Biomass such as wood and annual crops releases potassium to the vapor phase during pyrolysis and combustion, 6−8 mainly in the form of KCl. Once released, the alkali metal chlorides may be partially converted to alkali metal hydroxide or alkali metal sulfates.9 During cooling, the alkali metal components will condense, contributing to aerosol formation and/or cause operational problems, such as deposit formation and corrosion in boilers. 10,11 The fate of the alkali metal will depend on interactions with the sulfur and chlorine species of the gas.Formation of potassium superoxide, KO 2 , is potentially important for both flame inhibition and the high-temperature K/S/Cl transformation in combustion. The ability of alkali metals to catalyze radical removal is well documented by data from laminar premixed flames 12−25 and flow reactor experiments.26 Alkali-metal-based flame inhibitors are typically added as particulates 3,4 and heterogeneous effects have been proposed; however, the inhibiting effect of alkali metals is attributed mostly to gas phase radical removal reactions. The mechanism of inhibition is still in discussion. Under reducing conditions, results from flames 16,17,21 and from flow reactors 26 are consistent with the sequence (for K): KOH/KCl + H → K + H 2 O/HCl, K + OH + M → KOH + M. In combustion systems, alkali metal chlorides and alkali metal hydroxides are rapidly equilibrated through the fast reaction KCl + H 2 O ⇌ KOH + HCl. 9 Under lean conditions, the inhibition mechanism is more uncertain. The chain-terminating recombination reaction,has been suggested to be important. 18,27−29 However, it has been questioned whether potassium superoxide was sufficiently stable to play an important role under flame conditions. 2,26Formation of alkali metal superoxides (mainly KO 2 ) may also play a role in the gas-phase transformation of alkali meta...

Pathways to Triplet or Singlet Oxygen during the Dissociation of Alkali Metal Superoxides: Insights by Multireference Calculations of Molecular Model Systems

“…Dissociation energies of molecular KO 2 have been reported in a number of studies, whereby gas‐phase measurements of Sorvajäri et al. represent the most recent work . Whereas the data for the experimentally determined dissociation energies of molecular LiO 2 and NaO 2 differ by 0.9 and 0.7 eV, respectively, the experimental values for KO 2 agree well within 0.2 eV (see Table ).…”

“…The 3 E theo valuesi np arenthesesi nclude the thermodynamic correction at 298 K( the temperature of the kinetic experimentsa t1 100 Kw ouldl ead to an additional energy change of 0.07 eV).All energies are given in electron volts. [20] 3.17 UMP4/6-311G [20] 3.07 flames tudy [46] 2.30 QZ2P + R + fUMP4(SDTQ) [22] :2.56 CCSD(T)/CBS-DTQ [47] 2.59 NaO 2 1.26 (1.30) 2.16gas-phase study [20] 2.09 UMP4/6-311G [20] 1.92 flames tudy [46] :2.43 CCSD(T)/CBS-DTQ [47] 1.58 flames tudy [48] :1.69 flames tudy [49] :1.80 KO 2 1.04 1.83CPFAAS [a] [21] 1.88 CCSD(T)/aug-cc-pwCV5Z [21] 1.85 flames tudy [48] 1.80 CCSD(T)/CBS-DTQ [47] 1.79 flames tudy [49] 1.76 molecular-beams tudy [50] 1.95…”

Detection of gaseous species during KCl‐induced high‐temperature corrosion by the means of CPFAAS and CI‐APi‐TOF

“…However, no K 2 Fe 2 O 4 could be identified. The reaction kinetics of potassium in the gas phase was studied using CPFAAS by fragmenting KCl molecules with a pulsed laser emitting at 266 nm wavelength . Interestingly, despite the absence of KOH formation in the case of the KCl+Cr mixture, the formed atomic potassium and/or atomic chlorine did interact in the gas phase with gaseous potassium species, which can be seen in the prolonged potassium signal compared to pure KCl (Figure ).…”

Sequential Collinear Photofragmentation and Atomic Absorption Spectroscopy for Online Laser Monitoring of Triatomic Metal Species

“…As the chemical reaction kinetics for lead species, to the authors' knowledge, are not sufficiently well known, the simulation values can only be considered as approximate values. CPFAAS technique could be further utilized to study the lead reaction kinetics to improve the chemical kinetics databases and simulations [41]. Regardless, the results show the potential and applicability of the CPFAAS method to measure PbCl 2 in the full-scale power plant environment.…”