Experimental and QM Investigation of Decomposition and Early Chemical Kinetics of Ammonia Borane

Transcription

1 Experimental and QM Investigation of Decomposition and Early Chemical Kinetics of Ammonia Borane Stefan T. Thynell Department of Mechanical and Nuclear Engineering Pennsylvania State University University Park, PA Presentation made at AFOSR Program Review, Los Angeles, CA January 21,

2 Acknowledgements The presentor acknowledges the support from the Air Force Office of Scientific Research under grant number FA Ammonia Borane (AB) experimental studies are described in: M.R. Wi Weismiller, ill S.Q. Wang, A. Chowdhury, S.T. Thynell, and R.A. Yetter, Confined rapid thermolysis studies of ammonia borane, Thermochimica Acta, 551(10) 2013, pp

3 Nanoenergetics & Combustion Instability Objective of Thynell s portion in MURI program 1. Conduct fast thermolysis experiments to identify decomposition products 2. Employ QM to investigate e and identify reaction pathways ays with experimental data as guide 3. Arrive at reaction mechanisms that can be incorporated in combustion instability studies 3

4 Outline 1. Experimental studies on fast thermolysis of AB 2. Use of QM to explain experimental results of AB Ammonia borane (AB) Diammoniate diborane (DADB) ( isomer of AB) 4

5 FTIR & ToFMS Experimental Setups for Fast Thermolysis Electrical feedthrough top heater ZnSe window sample holder High-Watt Density Heater within Copper Rod Ring Specifies Gap between Foils and Seals around Periphery Molecular Beam Sampling Sample Holder torr Sample Gap torr torr purge outlet bottom heater Thermocouple for Temperature Control of Heater 1st: 8,000 l/s diff. pump, 0.1mm orifice 2nd: 700 l/s turbo, 1 diam. mm skimmer 3rd: 150 l/s turbo, 140 l/s ion pumps, 0.5mm slit ZnSe window Heating rates of 2,000K/s FTIR: 20 spectra/s, Δν=2 cm -1, to 900K, to 1,000psig ToFMS: 1,000 spectra/s, 200 amu, 0 psig, He/Ar 5

6 Fast Thermolysis of AB at 160 C Ammonia evolves first Ammonia peaks and then no longer evolves Band near 3570 cm -1 caused by ν(n-h) stretch Bands near 2500 cm -1 from ν(b-h) stretch Bands near 1625 cm -1 from δ(n-h H 2 ) scissoring Bands near 1360 cm -1 from ν(b-n) stretch; many of them from BH 2 NH 2 Later in the event, band at 1465 cm -1 appears from borazine c-(nh-bh) 3 6

7 ToFMS data of AB Decomp. at 180 C Ammonia rarely mentioned as a decomposition product from AB Temporal data from ToFMS suggest Major species: H 2, NH 3, H 2 NBH 2, and borazine Rate of H 2 evolution decreases slightly as NH 3 evolution peaks and then decreases At 180 C, decomposition is very fast; NH 3 evolves first, slightly before H 2. Borazine evolves last. Hydrogen evolves throughout the event. No BH 3 in gas phase. 7

8 Summary of AB Decomposition Decomposition appears straightforward: AB BH 3 + NH 3 (1) AB H 2 NBH 2 + H 2 (2) H 2 NBH 2 HNBH + H 2 (3) 3HNBH c-(n 3 B 3 H 6 ) (ring) (4) but Dixon et al., JPC-A, 2007, suggest autocatalytic effects from BH 3 enhances H 2 evolution in (2) and Yoon et al., PCCP, 2013, suggested pathways for AB conversion to DADB. Both papers appear to assume reactions in gas phase. 8

9 QM Investigation of AB Since AB begins to melt at around 110 C, treat decomposition as occurring in a solution phase. Use the PCM model of Gaussian09, with acetonitrile as solvent (does not seem to matter what solvent is used). Use the standard B3LYP functional and G(d,p) basis set (data not much different if one uses B3LYP/aug-cc-pVTZ basis set, about 2 kcal/mole difference in most cases). IRC calculations performed to connect reactants with products (one imaginary frequency). Investigation begins by search for TS that produces NH 3. This approach has been used by us to examine by QM early kinetics of G-5ATz, GzT, TAGzT, AP/AN/HAN, 4ATN, MMH/HNO 3 and by Goddard et al. to study the early kinetics of MMH/NO 2 (using SMD model in Jaguar). 9

10 QM Investigation of AB Decomposition Primary reaction for NH 3 formation ΔG f ΔG b ΔH r ΔG r (kcal/mol) BH 3 NH 3 +BHNH 3 3 =BHBH 4 2 NH 3 +NH BH 3NH 3 = BH 3 + NH 3 (not important; no TS) BH 4 BH 2 NH 3 = B 2 H 6 + NH Primary reactions for H 2 and BH 2 NH 2 Formations BH 4 BH 2 NH 3 = BH 2 NH 2 + BH 3 + H BH 3 NH 3 + BH 3 = BH 4 BH 2 NH BH 3 NH 3 + BH 3 = BH 2 NH 2 + BH 3 + H BH 3 NH 3 = BH 2 NH 2 + H 2 (not important)

11 Formation of Borazine Borazine Ring Formation B 3 N 3 H 6 ΔG f ΔG b ΔH r ΔG r (kcal/mol) BH 2 NH 2 +BHNH 2 2 =BHNH 3 2 BHNH BH 3 NH 2 BHNH 2 = BH 2 NHBHNH 2 + H BH 3 NH 2 BHNH 2 + BH 3 = BH 2 NHBHNH 2 + BH 3 + H BH 2 NHBHNHBHNH 2 = BH 2 NHBHNHBHNH 2 ring BH 2 NHBHNHBHNH 2 ring = B 3 N 3 H 6 + H BH 2 NHBHNHBHNH 2 ring+bh 3 =BH 3 -BH 2 NHBHNHBHNH 2 ring BH 3 BH 2 NHBHNHBHNH 2 ring =B 3 N 3 H 6 + BH 3 + H

12 Estimate of Initial Consumption Rate of AB The initial rate of change of mass fraction of AB can be written as where dy 2 ρ y y AB WAB dt W W AB AB AB 2 AB AB AB AB kt ΔG k = στ s tun exp h RT f k using the thermodynamic formulation of the rate constant k (see Laidler, 1987). If σ s =6, τ tun =1, T=433K, k/h=2.048e13 (with 1000 conversion factor included) and free energy change of 29,600 cal/mol, we find 3 k = 61.5 cm mol s The density of AB is 0.78 gr/cm 3 and a molecular weight of 31 gr/mol, we find the initial rate of change of AB mass fraction as dy AB 1 5 s dt which is quite reasonable or may be a little fast at 160 C=433K C=433K. 12

13 Summary of Findings Results from QM investigation on AB decomposition seem to be quite reasonable. NH 3 evolves first which lends support to proposed initiation step. Consumption of AB completed quickly, indicated by disappearance of NH 3. H 2, BH 2 NH 2 evolves throughout the event. B 3 N 3 H 6 ring compounds appear in small amounts and delayed. Further work is needed to complete the formulation of a condensed-phase reaction mechanism. The reaction mechanism would be validated against the existing i experimental data. (For approach, see N. Kumbhakarna, ST S.T. Thynell, Thermochimica Acta, vol. 582, 2014, pp ) Please note the QM results obtained within the last month only. To date, close to 100 TSs have been identified, so much more work remains. 13