Article pubs.acs.org/JPCA Electron Radiolysis of Ammonium Perchlorate: A Reflectron Time-of- Flight Mass Spectrometric Study † ‡ † ‡ † ‡ † ‡ Sandoŕ Gobi,́ , Alexandre Bergantini, , Andrew M. Turner, , and Ralf I. Kaiser*, , † ‡ Department of Chemistry and W. M. Keck Laboratory in Astrochemistry, University of Hawaii at Manoa,̅ Honolulu, Hawaii 96822, United States *S Supporting Information fi ABSTRACT: Thin lms of ammonium perchlorate (NH4ClO4) were exposed to energetic electrons at 5.5 K to explore the radiolytic decomposition mechanisms. The effects of radiolysis were monitored on line and in situ via Fourier transform infrared spectroscopy (FTIR) in the condensed phase along with electron impact ionization quadrupole mass spectrometry (EI-QMS) and single-photon photoionization reflectron time-of-flight mass spectrometry (PI-ReTOF-MS) during the temperature-programmed desorption (TPD) phase to probe the subliming molecules. Three classes of molecules were observed: (i) nitrogen bearing species [ammonia (NH3), hydroxylamine (NH2OH), molecular nitrogen (N2), nitrogen dioxide (NO2)], (ii) chlorine carrying molecules [chlorine monoxide (ClO), chlorine dioxide (ClO2), dichlorine trioxide (Cl2O3)], and (iii) molecular oxygen (O2). Decay profiles of the reactants along with the growth profiles of the products as derived from the infrared data were fit kinetically to obtain a reaction mechanism with the initial steps involving a proton loss from the ammonium ion (NH +) yielding ammonia − − 4 (NH3) and the decomposition of perchlorate ion (ClO4 ) forming chlorate ion (ClO3 ) plus atomic oxygen. The latter oxidized ammonia to hydroxylamine and ultimately to nitrogen dioxide. Molecular oxygen and nitrogen were found to be formed via recombination of atomic oxygen and multistep radiolysis of ammonia, respectively. 17 1. INTRODUCTION (NH4ClO4) was also performed. Attempts were also made to ff Unraveling the decomposition mechanisms of solid rocket shed light on the e ectofirradiationonthethermal propellants such as of ammonium perchlorate (NH ClO ) has decomposition of ammonium perchlorate with energetic 4 4 γ been the focus of interest from the chemistry and materials particles; these experiments applied primarily X-ray, - 18,19 20 ff science communities for the last half-century.1 The first results rays, and neutrons. Di erential thermal analysis (DTA) and TGA methods were used predominantly; more recently, on the thermal degradation of ammonium perchlorate were 21 presented by Bircumshaw and Newman,2 Jacobs and White- Dedgaonkar and Sarwade utilized the FTIR technique. Ivanov head,3 and Keenan and Siegmund.4 Ammonium perchlorate et al. exploited broadband ultraviolet (UV) light to produce dislocations and radiolysis products such as chlorate ions (NH4ClO4) was found to exhibit distinct decomposition − mechanisms at “low” (below 240 °C) and “high” (above 350 (ClO3 ) that may alter the mechanism(s) of the low- °C) temperatures;1,5,6 the effects of the presence of catalysts temperature decomposition of ammonium perchlorate 5 7 22,23 such as metal oxides and of pressure were also examined. (NH4ClO4). These studies concluded that the first decomposition step upon Contrary to the thermal degradation experiments, much less thermal degradation is a proton transfer from the ammonium attention has been paid to the radiolytic decomposition of + − cation (NH4 ) expectedly to the perchlorate (ClO4 ) counter- ammonium perchlorate (NH4ClO4) that could also explain why 1 ion yielding ammonia (NH3)(R1). it affects its thermal degradation; these works mainly exploited γ + + - and X-ray irradiation as summarized in Table 1. The earliest NH43→+ NH H (R1) studies utilized X-rays to irradiate ammonium perchlorate 8 samples and monitored the formation of radicals via electron These investigations exploited mass spectrometry (MS), 24,25 9 paramagnetic resonance (EPR). Both studies concluded scanning electron microscopy (SEM), thermogravimetric + ff that the ammoniumyl radical (NH3 ) could be detected; this analysis (TGA), and di erential scanning calorimetry fi fi (DSC),10 as well as Raman spectroscopy.11 The continuous nding was later con rmed by Hegde et al. along with the observation of the chlorine trioxide (ClO3) and chlorine advancement of analytical tools allowed for simultaneous 26 γ dioxide (ClO2) radicals. Experiments using EPR after (and detection techniques of the decomposition products within 27,28 the same machine: TGA, SEM, DSC, and quadrupole MS neutron) irradiation have also been performed. Besides (QMS)12,13 together with temperature-jump Fourier transform infrared spectroscopy (T-jump FTIR)14 and even Received: February 25, 2017 TGA−FTIR/MS15 or TGA/DSC−FTIR/MS.16 A shock- Revised: April 26, 2017 wave-induced decomposition study of ammonium perchlorate Published: April 26, 2017 © 2017 American Chemical Society 3879 DOI: 10.1021/acs.jpca.7b01862 J. Phys. Chem. A 2017, 121, 3879−3890 The Journal of Physical Chemistry A Table 1. Previous Experimental Results on the Radiolysis of Ammonium Perchlorate type of radiation, dose absorbed sample p (mbar) T (K) (eV/molecule) detection products ref + single crystal N/A 4.2, 300 X-ray, N/A EPR NH3 radical 24 − × −2 + single crystal N/A 93 373 X-ray, 4.9 10 EPR NH3 radical 25 − γ 60 − − − single crystal 1013 283 288 ( Co source), 1.5 chemical analysis of products dissolved in Cl ,Cl2, ClO3 , OCl 30 water single crystal ≈10−7 300 UV at 253.7 nm, N/A UV−vis, EI-ReTOF-MS absorption bands at 610, 360, and 295 nm, 610 nm band 31 attributed to free electrons no gaseous products in the MS spectrum − γ 60 ≈ × −2 + single crystal N/A 77 300 ( Co source), 1.2 10 EPR NH3 radical 27 γ 60 0 ≈ × −2 + single crystal N/A 77, 300 ( Co source), n radiation, 4.2 10 for EPR ClO3, ClO2, and NH3 radicals 28 both single crystal ≈10−6 77 X-ray, UV at 253.7 nm, e− (1−12 MeV), thermoluminescence (TL, 80−420 K), UV− TL peaks at 95, 113, 134, 246, 320 K, peak at 246 K attributed to 40 × −2 − 1.2 10 vis ClO3 UV−vis: 610 nm band attributed to F-centers, 300 nm to − − overlapping ClO , ClO2 , ClO2 bands γ 60 ≈ × −1 − − − KBr pellets, N/A N/A ( Co source), 6.1 10 FTIR, UV vis NIR ClO3 formation 29 microcrystalline single crystal N/A N/A LIBS (Nd:YAG, 1064 nm), N/A X-ray photoelectron spectroscopy; optical cracking and “chemical decomposition” of sample 32 microscopy, SEM, AFM + single crystal N/A 4.2, 77, 300 X-ray, N/A EPR NH3 , ClO3 radical 26 + − microcrystalline air, Ar, N2 N/A LIBS (Nd:YAG, 1064 nm), N/A MIR and LWIR parent ions NH4 and ClO4 exclusively 33, 3880 atmospheres 34 J. Phys. Chem. A DOI: 10.1021/acs.jpca.7b01862 2017, 121, 3879 Article − 3890 The Journal of Physical Chemistry A Article EPR, FTIR29 and chemical analysis30 of γ-radiolyzed samples to tackle this open issue exploiting state-of-the-art single- fl fl were also carried out searching for chlorine trioxide (ClO3), photon photoionization re ectron time-of- ight mass spec- − − chlorate (ClO3 ), chlorine dioxide (ClO2), chlorite (ClO2 ), trometry (PI-ReTOF-MS) of the products formed in the − hypochlorite (OCl ), dichlorine molecule (Cl2), chloride radiolysis of ammonium perchlorate at temperatures of 5 K − − − fi fi (Cl ), nitrate (NO3 ), and nitrite (NO2 ). Only (in decreasing allowing for the rst identi cation of the nascent radiolysis − concentration) chloride (Cl ), chlorine (Cl2), chlorate products on line and in situ. This method is also supplemented − − (ClO3 ), and hypochlorite (OCl )couldbedetected. by FTIR and EI-QMS monitoring of the sample during the Alternative radiation sources were rarely applied; irradiations irradiation and in the temperature-programmed-desorption by UV photons with a wavelength of 253.7 nm (4.89 eV) were (TPD) phase as well as by kinetic studies that help elucidate carried out by Maycock and Pai Verneker reporting the the main decomposition pathways. formation of “color centers“ attributed to free electrons within the sample.31 Furthermore, laser-induced breakdown spectros- 2. EXPERIMENTAL METHODS copy (LIBS) was also conducted on ammonium perchlorate The experiments were conducted in a contamination-free crystals (NH4ClO4) revealing mechanical deformations and ultrahigh vacuum (UHV) stainless steel chamber that can be (micro)cracking as well as the radiolytic decomposition of the evacuated to a base pressure of a few 10−11 mbar using oil-free − sample with chlorates (ClO3 ) as primary product. For these, magnetically suspended turbomolecular pumps and dry scroll − Nd:YAG lasers (1064 nm) were utilized; the irradiation backing pumps.41 43 A polished silver wafer is used as a products were detected by X-ray photoelectron spectroscopy, substrate and is mounted on a coldfinger made of oxygen-free optical microscopy, SEM, and atomic force microscopy high conductivity copper (OFHC) with indium foil between (AFM).32 Yang et al. used used the same method to trace them to ensure thermal conductivity. The coldfinger is cooled the mid- and long-wave IR (MIR and LWIR) emission spectra by a closed-cycle helium refrigerator (Sumitomo Heavy of the sample.33,34 Industries, RDK-415E) while the temperature can be Besides the experimental work, theoretical studies on the maintained by the help of a heater connected to a thermodynamical and spectral properties of ammonium programmable temperature controller; the substrate can be fi ± perchlorate (NH4ClO4) have also been applied. The rst cooled down to 5.5 0.1 K. The entire setup is freely rotatable paper by Barber et al. used the ab initio SCF-MO method to within the horizontal plane and translatable in the vertical axis calculate the ionization energy of the perchlorate unit via a UHV compatible bellows and a differentially pumped − 35 fi (ClO4 ), followed by more exhaustive simulations as the rotational feedthrough. A thin lm of microcrystalline computational capacity of computers increased with time. ammonium perchlorate (NH4ClO4, Sigma-Aldrich, 99.5%) Politzer and Lane utilized density functional theory (DFT) to with a thickness of 1050 ± 60 nm (Table 2) was prepared calculate the energetics of the gas-phase decomposition steps of ammonium perchlorate (NH4ClO4, at the B3PW91/6-311+G- Table 2.