Bimolecular radical scavenging processes for laser isotope separation: NH2D + O2

Volume

127, number

4

CHEMICAL

PHYSICS

LETTERS

20 June 1986

BIMOLECULAR RADICAL SCAVENGING PROCESSES FOR LASER ISOTOPE SEPARATION: NH,D + 0,

C.T. LIN Department Received

of Chemistry

27 December

Northern iiiinois University, DeKalb, IL 601 IS’, USA 1985; in final form 4 April 1986

Infrared multiphoton photooxidation of NHrD in NH, mixtures was observed to produce exclusively HDO, suggesting a single step deuterium separation efficiency of [D,O]/([D,O]+[HrO]) >, 50% which is significantly higher than that of the theoretical value, 33%. The results are explained by the large rate differences in the radical scavenging steps, i.e. k(D + 0,) = 2.2 x lo9 M-’ s-‘, /c(NH, + 0,) Q 5 X lo6 M-’ s-’ and k(NHr +NHr) = 1.6 x 10” M-i s-‘. With Ti solid powder as a catalyst, we observed that the formation yields of HDO are at least three to four times higher than those without a catalyst.

1. Introduction Infrared laser multiple-photon dissociation (IR MPD) of molecules is by now known as a highly selective method for laser isotope separation [ 1,2] . Several molecular systems such as CFsCDC12 [3,4] , CDF3 [S-7] , CDCl, [8] and CF2DCl [9,10] have been demonstrated as good candidates for photochemical separation of deuterium using IR MPD. A very high isotopic selectivity was reported [3,5,8,9] for those systems mentioned but all in conditions of either low sample pressure, low temperature or short laser pulse. This resulted from the fact that radical scavenging reactions under some unfavorable conditions can seriously reduce the isotopic selectivity. The key to industrialization of laser-based separation processes is high throughput. These include not only the highly selective excitation and decomposition, but also the natural availability of the molecular system, the existing process for redeuteration, the high isotopic natural abundance and most importantly the selectivity in bimolecular radical scavenging reactions. In this communication, we will examine the IR MPD for NH2D and use the preadded 0, as radical scavengers. The experiment will be conducted at a sample pressure where =lO-20 collisions will be expected to occur during the width of laser pulse. This is designed to illustrate the feasibility of the photochemical pro0 009-2614/86/$ 03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)

B.V.

cesses (rate of reaction) over the photophysical processes (rate of energy transfer) in the system selected. It is shown that 0, is a good radical scavenger for D but not for NH,. Moreover, NH2 radicals prefer to recombine themselves at a very fast rate and form N2H4. These lead to an observed increasing rather than decreasing in the isotopic selectivity for radical scavenging reactions of D, NH2 and 0,. We will examine also the catalytic effect on the speed of laser photooxidation of ammonia. Other advantages of this molecular system as compared to the existing systems for laser isotope separation of deuterium will be discussed.

2. Experimental The experimental apparatus consisted of a tunable CO2 pulse laser, a vacuum line coupled with the reaction cell and an IR spectrometer or a mass spectrometer. The pulse CO2 laser (Lumonics,model TEA-801 A) was operated at a repetition rate of 3 pps by a trigger generator. The duration of the laser pulse was 100 ns as specified in the manual. A wavelength selector attachment (Lumonics, model 501 tuning attachment and model 501R CO, master grating) was used to isolate the P(20) = 944.21 cm-l, P(14) = 949.49 cm-l and R(12) = 970.56 cm-l lines as excitation sources. The frequency of the laser output was calibrated with 347

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a model 16-A CO2 spectrum analyzer from Optical Engineering Inc. A barium fluoride (BaF2) lens (f= 5 cm) was used for focusing the laser beam into the center of the reaction cell. The peak power at the focal point was maintained to be < 1 GW/cm2. Using a vacuum line, NH,D molecules were prepared by condensing NH, and ND3 at 77 K. The isotopic exchange was found to reach completion in 3 h after the cold trap was released. The NHZD sample mixtures and 0, gas were introduced into the reaction cell. To prevent vacuum grease contamination, needle type teflon stopcocks were employed to construct the vacuum line as well as the reaction cell. The sample cell used in this experiment was a glass tube 10 cm long and 2.5 cm in diameter. The ends of the reaction cell are a flat surface O-ring type connectors. Two zinc selenide (Irtran IV) windows were fitted to the cell by a pair of metal adapters. A trough shape container was built inside the bottom section of the reaction cell for depositing the solid catalysts. The arrangement was achieved in such a way that the laser beam was adjusted to “near” but not “on” the metal surface to minimize the laser heating effect. The selective laser photooxidation products of NH,D were analyzed by a Perkin-Elmer model 180 spectrophotometer and a Finnigan model 1015 S/L quadrupole mass spectrometer, separately. It is important to mention that the dimension of the reaction cell was constructed to fit perfectly into the sample chamber of the IR spectrophotometer. This allows us to record the spectrum without the need of further transfer the gas products from the reaction cell into the IR sample cell.

3. Results 3.1. Conditions for selective IR laser photooxidation of NH2D Three ro-vibrational transitions of NH, D of V2(u,J_K,K)-type band were reported [ 11-l 31 to coincide with P(20) = 944.21 cm-l, P(14) = 949.49 cm-l and R(12) = 970.56 cm-l lines of the OOOlloo0 transition of a CO, laser. Spectral assignments showed that the V2(0-,‘4& c+V2(1-, 5(&, v2(0-, 404) ++v2(1+,514) and j’2(0+,414) ++r’2(1-, 524) transitions occur at 944.14,949.40 and 970.64 cm-l, 348

20June1986

respectively. The relative absorption coefficients for these transitions are in the order of ~~(944.14cm-l) > (r(949.40 cm-l) % a(970.64 cm-l). On the other hand, there are several rovibrational transitions of NI-$, e.g., 944.14,945.15,949.34,949.92 and 971.89 cm-l, existing in the same spectral range that can interfere and reduce the excitation and dissociation selectivity of NH,D. It was indicated [14-16]$ that the relative absorption strengths for the ro-vibrational transitions of NH2D in this spectral range are higher than those near-by transitions of NH,. Moreover, the relative absorption coefficients among those nearby rovibrational transitions of NH3 are (~(971.89 cm-l) = lOcr(945.15 cm-l);a(949.34 cm-l) = 3cr(945.15 cm-l) and (~(945.I 5 cm-l) a ~~(949.92 cm-l) w 4a(944.14 cm-l). The comparison above suggests that a high excitation and dissociation selectivity of NH,D in the ammonia mixtures can be achieved only for the ro-vibrational transition v2(O-, 4,4) ff vz( 1 - , 505), i.e. P(20) line should be used to excite selectively the NH2D molecules. Since an isotopic-composition mixture of ammonia will be used for IR MPD of NH2D, any ro-vibrational transitions of NI-ID, and ND3 species displayed in the spectral range of m944.21 cm-l will certainly interfere with the observed results. The CO2 laser Stark-tuned spectroscopy of gas mixtures of NI-13,NH,D, NHD, and ND3 showed [1 1,17 -191 that no detectable ro-vibrational transitions of NHD2 and ND3 were found to be in resonance with the P(20) line of CO, laser. Recently, we have demonstrated [20,21] that the reaction characteristic for ERlaser photooxidation of NH, is very sensitive to the total sample pressure of NH, and 0, used as well as to the laser power density applied. When the total sample pressure of NH, and 0, (with partial pressure ratio of 1:9) is equal or higher than 100 Torr and laser peak power at the focal point is greater than 1 GW/cm2, the reaction is characterized by an explosive type and accompanied by a yellow flash luminescence. The main reaction products analyzed by IR and mass spectra are H20 in both vapor and liquid phases and NI-I,NO, .The reaction mechanism for the explosive IR laser photooxidation of NH, was proposed [2 1 ] as follows: nhv

NH, +JH2(~2A,)~NO~N02 H2O -

NH3

HN03 -

NI-I,NG, .

(1)

In eq. (l), the radical NH@ 2A1) is generated in the excited state and the explosive photooxidation of ammonia proceeded through a so called “superexcitation” reaction channel. On the other hand, if IR laser photooxidation of NH3 is carried out at a total sample pressure less than 100 Torr and the laser power density is lower than 1 GWjcm2, the reaction is nonexplosive and no apparent luminescence is observed during the reaction. The nonexplosive reaction products are quite distinct from those of the explosive type, i.e. we observed N,O, N2H4, Hz0 in the vapor phase, N, and Hz as the main products. The reaction mechanism for the nonexplosive IR laser photooxidation of ammonia is probably via the normal ground state [NH&? 2Bl)f low energy channel as’ NH, 2

NH@ 2B1) 2

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CHEMICAL PHYSICS LETTERS

Vohtme 127, number 4

N,O.

(2)

The above reaction characteristics and their respective reaction products would be used as the key conditions for the following isotopic selective CO, laser photochemistry of NH,D and 0,. 3.2. Selective CO, laserphotochemistryof NH2D and Oz

Isotopic-composition mixture of ammonia (ICMA), 44% NH,, 30% NI-I,D, 20% NHD2 and 6% ND3, was prepared and the percentage of each compo~tion in the mixture was obtained from the integrated intensity of the strongest inversion doublets v2 band at 968 cm-1 (NH& 894 cm-1 (NH2D), 823 cm-l (NHD2) and 749 cm-l (ND3), respectively. The percentage of deuterium in ICMA was calculated as ZD/(ZI, + Z,) = 29.3%. The gas mixture of 25 Torr of ICMA and 75 Ton of 0, was introduced into the reaction cell. Infrared spectrum of the gas mixtures is shown in fig, la and no dark reaction was observed. A TEA CO2 laser was tuned to P(20) = 944.21 cm-l line and controlled at a peak power of =5 X lo* W/cm2 at the focal point which was employed to excite selectively the NH2D molecules and to initiate the photooxidation of NH2D. Apparently, the reaction above proceeded with a very slow rate and no luminescence was observed during the reaction, i.e. a nonexplosive type. Fig. lb displays the IR spectrum of the nonexplosive type photooxidation products of NH2D after applying 900 laser pulses. The following results are observed: (i) the

(a)

j--nl---_ t P (b)

g

(c) I

4000

3500

3000

2500

2000

1800

1400

1000

I

600

WAVENUMBER(cm-‘)

Fig. 1. IR spectra: (a) a mixture of 25 Torr of ICMA (44% NH3,30% NHa D, 20% NHDa and 6% NDs) and 75 Ton of 02 recorded with a 10 cm ceil; (b) the photoo~&tion products of NH2 D after mixture (a) is subjected to 900 pulses CO2 laser tuned at 944.21 cm-l and a power density at the focal point of 5 X 1Oa W/cm’; (c) the same as (b) except Ti solid powder was introduced in the reaction cell as catalysts.

relative infrared band intensity of NH2D to NH, is smaller in spectrum (b) as compared to that in spectrum (a); (ii) the unreacted ammonia compositions in spectrum (b) give 46% NH,, 3 1% NH2D, 18% NHD2 and 5% ND3, i.e. ZD/(ZD+ Z,) is calculated to be 27.3% which is smaller than that of the redated ICMA sample; and (iii) several new IR peaks are weakly seen in spectrum (b) at 1403,2211,2224,2730 and 3750 cm-l. The appearance of a doublet at 2211 and 2224 cm-l was assigned 1221 to originate from N20 molecules which confirms the nonexplosive type of reaction for the CO2 laser photochemistry of NH2D and 02. Results (i) and (ii) suggest that we have achieved the deut~rium selective laser photoo~dation of NH2D in ammonia mixtures. Two general observations are noted and needed to clarify further. First, in spite of the occurrence of some unfavorable collisional energy-transfer processes under our experimental conditions we still observed a large isotope selectivity. This indicates that some specific and favorable photochemical processes are proceeding with reaction rates comparable to those of the photophysical processes. Catalytic effects on the efticiency of laser isotope separation should give further evidence which will be discussed later. Second, we observed a 349

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decreasing in the ratios of [NH2D] / [NI-I.J , [NHD2] / [NH31 and [ND31/[NH31 in fig. lb as compared to those in fig. la. The observed decrease in [NHD2] / [IQ-Is] and [ND31/ [NH31 cannot be due to the process of RI MPD because no ro-vibrational transitions of MID2 and ND3 were found [l 1,17-191 to coincide with the P(20) line of CO2 laser. After photolysis and prior to the IR spectral measurements, a new isotopic exchange equilibrium could have been established. While the IR MPD process depletes mainly the NH2D species, the new isotopic exchange equilibrium decreases the concentrations of ND3 and NHD2 which serve to redeuterate the ammonia and to increase the concentration of NH,D. This indicates that the relative concentrations of NH3 , NH2D, MID2 and ND3 shown in fig. lb is not the true result of IR MPD of NI-I,D, i.e. [NH2D] /w,] should be lower before the isotopic exchange proceeded. From the laser isotope separation point of view, the redeuteration can only favor the separation efficiency. More importantly, the observed ZD/(ZD+ ZH) does decrease indicating that the ICMA mixture is leaner in D after photolysis. We have reported [23,24] that a combination of laser-catalyst photochemistry can be useful for a large scale production of isotopes. Using Ti solid powder as catalyst, the IR spectra for the reaction products of CO, laser photooxidation of NH2D are displayed in fig. lc. It shows clearly that the band intensity at 1403 (1300-1500 cm-l), 2211,2224,2720 (26002900 cm-l) and 3750 cm-l (3500-4000 cm-l) in spectrum (c) are definitely much stronger than those in spectrum (b). Since the experimental conditions were identical for both spectra (b) and (c) except the presence of Ti as catalyst in spectrum (c), thus the observed increases in the IR band intensity suggest that Ti powder catalyst has increased the rate of selective laser photooxidation of NHiD. The integrated intensity ratio of N20 peaks at 2211 and 2224 cm-l for spectrum (c) to spectrum (b) is 4: 1, indicating a fourfold enhancement in the rate of catalytic reaction. On the other hand, the composition of the unreacted ammonia in the reaction mixture as shown on the righthand side of spectrum (c), has changed drastically to 5l%NI-I3,3l%l’&D, 15%NHD2 and3%ND3, i.e. ZD/(Zl-,+ ZH) = 23.3%. The relative rate of laser photooxldation of NH,D with and without catalyst can also be calculated as A(l,/(l, +ZH)) without catalyst = (29.3%- 23.3%)/(29.3% - 27.3%)= 3 which isin 350

20 June 1986

fair agreement with that calculated from the reaction product of N,O. Three well-resolved ro-vibrational bands at 1403 (1300-1500 cm-l), 2720 (2600-2900 cm-l) and 3750 cm- 1 (3500-4000 cm-‘) in spectra(b) and(c) of fig. 1 can be assigned [25,26] as the v2, u1 , and v3 modes of HDO vapor, respectively. Spectroscopically, the ro-vibrational bands at 3750 cm-l (3500-4000 cm-l) can be assigned equally well to the v3 and v3 modes of H20 vapor. However, the infrared band intensity of H20 vapor showsZ,,(1595 cm-l) > Zvr(3657 cm-l) w Z,,(3756 cm-l). The fact that no ro-vibrational bands at 1595 cm-l are observed in spectra (b) and (c) of fig. 1, suggests that the ro-vibrational structures at 3750 cm-l should be assigned to the v3 mode of HDO vapor. This indicates that no H20 is produced in the selective CO, laser photochemistry of NH2D and 0,. We also carefully examined the possible IR absorption bands for D20 vapor at 2666 cm-l (vl), 1175 cm-l (v2) and 2789 cm-l (v3). However, no detectable IR bands for D20 vapor are observed, indicating if there is any D20 produced in the selective

(A)

mle

3

Fig. 2. Mass spectra for the laser-catalyst photooxidation of NH2 D: (A) the background spectrum from the chamber of mass spectrometer; (B) and (C) are the spectra obtained before and after the laser irradiation, respectively.

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laser photooxidation of NH2D, the quantity should be very small. Fig. 2 shows the mass spectra of the reaction products for the Ti catalytic laser photochemistry of NH2D and 02, before (spectrum (B)) and after (spectrum (C)) the reaction mixtures were subjected to 900 pulses of a CO, laser tuned at P(20) line. Spectrum (A) of fig. 2 is the background spectrum of the residues that existed in the chamber of the mass spectrometer. By comparing spectrum (B) and spectrum (C) in fig. 2, the following spectral characteristics are observed: (1) two new mass peaks appear at m/e = 3 and 34, suggesting the production of HD and NH20D, respectively; and (2) the spectral peak intensities at m/e = 16,19, 28,32 and 44 are drastically enhanced. The enhancement in the mass peaks at m/e = 16 and 32 can be attributed [20] to the production of N2H4 whereas those at m/e = 28 and 44 could represent the formation of N2 and N,O, respectively. The increase in the spectral intensity at m/e = 19 gives evidence of HDO formation. It is noted that the mass spectral assignments are in excellent agreement with those of the IR spectra.

DtDtM+D2+M,

(5)

NH, t N,H,

(6)

--f NH, + N2H3,

DtN2H4+HD+N2H3,

(7)

N,H,

(8)

t N,H,

j N, + 2NH,,

Dt02+M-tD02+M, NH2 t 0,

(9)

NH,O, HNO + OH E NO + H,O,

DtDO;!+OD+OD,

(11)

OD t NH,D + NH, + D,O,

(12)

NH, t DO, --f NH,D + O,,

(13)

NH,D + DO2 E

w2D202

NHOD + HDO,

DO, + DO, --f D,O, 4. Reaction mechanisms

NHOD t NHOD

Since N2H4 but not N2H2D2 molecules were clearly identified as the principle product in the selective IR laser photooxidation of NI-I,D, the primary species generated in the IR MPD of NH,D should be NH, and not NHD radicals. This is due to the fact that the large rate of recombination of NH2 (or NHD) &as determined [27] to be /@I-I, t NH2) = 1.6 X lOlo M-l s-l. Similarly, the possible generation of NHD radicals in the subsequent reactions should also be considered as unimportant. Pulse radiolysis of gaseous ammonia and oxygen mixtures has been reported [27] in which the mechanisms and rates of photooxidation of ammonia were established [27] . Using those mechanisms and matching them with our experimental results described in section 3, a possible reaction mechanism for the selective laser photooxidation of NH2D can be outlined as follows: NI-12Dtnhv-tNH2 NH,+NI-I,+N,H,,

+D,

(3) (4)

(10)

+ O,,

N, + HDO -c NOD + NH,OD,

NOD t NOD--f N,O + D20.

(14) (15)

(16) (17)

The reaction rate constants at 300 K for some of these proposed reactions have been determined by Pagsberg et al. [27], i.e. k4 = 1.6 X lOlo M-l s-l, k, = 3.1 X lo* M-l s-l, kg = 1 .O X 10” M-l s-l and k,, < 5 X lo6 M-l s-l ; by Trainor and co-workers [28] , i.e.k5<6X 107M-1s-1;andbyMiller [29],i.e.kg = 2.2 X IO9 M-l s-l. The rate constants for the other reactions above are also available for protiated species [30] . However, the method for obtaining the kinetic isotope effect, k,/k,, is readily established [31] . Since D,, H,O, D202 and D20 (very small quantity if any) were not observed in the selective laser photochemistry of NH2D and O,, thus the reactions (S), (10) and (15) as well as (11) and (12) are probably not the important reaction paths. In fact, reactions (5) and (10) were determined [27,28] to proceed with relatively slow rates. Reaction (13) leads only to the regeneration of NH2D and 0, which provides no desired prod351

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ucts. On the other hand, because we have observed a large quantity of N20 formation, therefore the reactions (17), (16) (14) and (9) should be considered as the favorable reaction routes for the selective CO, laser photochemistry of NH,D and 0,. Reaction (9) has a fairly large reaction rate of 2.2 X IO9 M-l s-l, in addition the reaction products of NH20D, N,, HDO were all observed in our experiment which support the proposed reaction mechanisms of (14) and (16).

also increase the isotope selectivity. Other advantages are: (1) the deuterium natural abundance in NH2D (0.045%) is higher than that in CDF, (0.015%), CF,DC1(0.015%), CF,CDCl, (0.015%) and CDC$ (0.015%); (2) ammonia molecules exist naturally in large quantities whereas CDF3, CF,DCl, CF,CDCl, and CDC13 do not; and (3) the deuterium-depleted ammonia can easily be redeuterated [32] to the original natural abundance.

5. Remarks

Acknowledgement

One might argue that the laser-initiated photooxidation of NH,D described above is only due to the IR MPD of the various isotopic species of ammonia (with some conventional isotope effects for the decomposition process) followed by kinetic isotope effects among the complex radical reactions. If this is the case, the observed deuterium separation efficiency of [D20] /( [D20] + [H20]) should be equal or less than 29.3% (the percentage of deuterium in the original ICMA mixture). On the contrary, the selective nonexplosive CO, laser photochemistry of NH,D and 0, produces principally HDO and probably a small quantity of D,O but no trace of H20 was detected. This indicates that the single step deuterium separation efficiency is [D20] /([D20] + [H20])> 50% which is much higher than that of the theoretical value, 33% (i.e. 33% of D in the NH,D molecule). The high isotope enrichment efficiency is blessed by the large differences in bimolecular radical scavenging rates, e.g., high for k(NH2 t NH,) = 1.6 X lOlo M-l s-l and k(D t 02) = 2.2 X 109 M-l s-l and low for /@I-I, + 0,) < 5 X lo6 M-l s-l. That is D reacts with 0, to give D20 whereas NH2 radicals recombine to produce N2H4. It is interesting to mention that if reactions (9) and (10) have the same rate constants or if the rate of reaction (4) is slow, then we should observe a maximum isotope separation efficiency of 33%. On the other hand, if we can design a way of removing DO2 produced in reaction (9) before the occurrence of reaction (14), we should be able to obtain a 100% deuterium separation efficiency. The principle success by using NH,D and 0, as reaction system for deuterium isotope separation is the unusual bimolecular radical scavenging rates which reveal a new thought that radical scavenging reaction can

Financial supports from the Northern Illinois University Foundation and the Northern Illinois University College of Liberal Arts and Sciences are acknowledged.

352

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