Thermochemistry of aluminum species for combustion modeling from Ab Initio molecular orbital calculations

Abstract

High accuracy ab initio methods for computational thermochemistry have been applied to aluminum compounds expected to be present during combustion of aluminum particles. The computed enthalpies of formation at 298.15 K agree well with experimental values from the literature for AlCl, AlCl 3 , AlO, AlOAl, linear OAlO, planar Al 2 O 2 , AlOH, AlH, and AlN. The agreement is fair for AlCl 2 . Major revisions to the recommended thermochemistry must be considered for OAlCl, OAlH, OAlOH, and AlC. This is not surprising since the thermodynamic data for OAlCl, OAlH, OAlOH, and AlC are given in the literature as rough estimates. Calculated thermochemical data are also presented for several species never studied experimentally, including AlH 2 , AlH 3 , AlOO, cyclic-AlO 2 , linear AlOAlO, AlHCl, AlHCl 2 , and others. Based on the performance of the CBS-Q and G2 methods observed in other systems, the calculated enthalpies of formation would be expected to be accurate to within Ϯ1 to 2 kcal mol Ϫ1 . However, relatively large differences between the results from the CBS-Q and G2 methods for the aluminum oxides indicate that the uncertainties are slightly larger for these compounds. The thermochemistry proposed here is shown to predict substantially different equilibrium composition from the thermochemistry previously available in the literature.

BAC-G2 Predictions of Thermochemistry for Gas-Phase Aluminum Compounds

“…6,7,11,42 However, we remark that G2 predictions for aluminum halides that are both corrected 10,11 and uncorrected 7 for atomic spin-orbit energies exist in the literature, which can lead to some confusion regarding the accuracy of this method. The BAC-G2 method does not explicitly correct for the atomic spin-orbit interaction (which would be accomplished by adding experimental spin-orbit interaction energies to the raw G2 atomic energies used in the calculation of the heat of atomization, ΣD 0 ).…”

“…The most comprehensive investigations prior to this one in which a high-level quantumchemistry method was applied to a series of aluminum compounds are those of Swihart and Catoire (SC) 11 and Politzer et al, 35 each of whom examined a series of compounds expected to be present during combustion of aluminum to form particles. SC used two complete basis set methods (CBS-Q and CBS-RAD) as well as DFT (discussed below).…”

Rate constants for the homogeneous gas-phase Al/HCl combustion chemistry

“…The standard enthalpies of formation at 298 K, the standard entropies at 298 K, and the heat capacities for the species AlH x Cl y ( x ϭ 0-3, y ϭ 0-3) have been determined from ab initio molecular orbital calculations [9].…”

Structures and thermodynamic properties of aluminum oxyhalides: a computational study

“…An obvious example is their occurrence in discharge lamps, where polycrystalline alumina (PCA) is preferred over quartz as the material for the envelope of the lamp [1]. During the operation there is some corrosion of the PCA wall.…”

“…Due to the high heat of combustion of aluminum with different oxidizers, aluminum particles are often added to solid propellants to increase motor-specific impulse. During the combustion, one of the molecules formed is AlOCl [1]. Another occurrence of aluminum oxyhalides is on the cathode surface in the aluminum electrolysis cell [2].…”