Photolytic Decomposition of LiAIH4
The Journal of Physical Chemistry, Vol. 83, No. 13, 1979
1701
(1974);(c) S.Chen and M. 2. Hoffman, Radlat. Res., 82, 18 (1975). (24) D. Behar, G. Czapskl, and I. Duchovny, J. Phys. Chem., 74, 2206 (1970). (25) J. L. Weeks and J. Rabani, J . fhys. Chem., 70, 2100 (1966);S. Chen, V. W. Cope, and M. Z. Hoffman, ibid., 77, 1 1 1 1 (1973). (26) Unfortunately when we realized that we should measure thls rate, we no longer had access to the Llnac. (27) Recently Filby” questioned our report of a higher rate for the gas
(8) J. Hoing6 and H. Bader, Science, 190, 782 (1975).
(9) M. S. Matheson and L. M. Dorfman, “Pulse Radiolysls”, MIT Press, Cambridge, Mass., 1969. (10) K. H. Schmidt, S. Gordon, and W. A. Mulac, Rev. Sci. Insfrum., 47, 35 (1976). (11) B. 8. Saunders, P. C. Kaufman, and M. S. Matheson, J. phys. Chem., 82, 142 (1978). (12) C. D. Jonah, private communication. (13) M. Slmlc and E. Hayon, J. Am. Chem. Soc., 92, 5982 (1971). (14) 0. E. Adams and L. M. Dorfman, Nafi. Stand. Ref. Data Ser., Natl. Bur. Stand., No. 48 (1973). (15) S. Gordon, E. J. Hart, M. S. Matheson, J. Rabani, and J. K.Thomas, Discuss. Faraday Soc., 38, 193 (1963). (16) I. G. Draganic and Z. D. Draganic, ”The Radlatii Chemistry of Water”, Academic Press, New York, 1971,p 187. (17) G. Czapskl, Annu. Rev. fhys. Chem., 22, 171 (1971). (18) B. H. J. Blelski and J. M. Gebickl, Adv. Radlat. Chem., 2, 177 (1970). (19) W. G. Filby and H. Gusten, Atm. Environ., in press. (20) A. R. Forrester, J. M. Hay, and R. H. Thomson, “Organlc Chemistry of Stable Free Radicals”, Academic Press, New York, 1968. (21) J. Helcklen and J. Lowe, private communicatlon. (22) K. Olszyna and J. Helcklen, Sci. Total Environ., 5 , 223 (1976). (23) (a) S.Chen, M. Z. Hoffman, and 0. H. Parsons, Jr,, J. fhys. Chem., 79, 1911 (1975); (b) S.Chen and M. Z. Hoffman, ibid., 78, 2099
phase reaction of OH wlth DEHA noting that In general liquid phase reactions are faster than the corresponding gas phase. However, thls “rule of thumb” is not applicable In thls case since the rate constant In aqueous solutions is near the diffusion limit for OH. (28) As polnted out by one of the referees, this estlmate is only valid If the rate of reactlon of 03-wlth DEHA is slow compared to the rate of reaction of 0-wlth DEHA and can be neglected. If the 03concentratlon were llmted by the rate of reaction of 03wlth DEHA, then the maximum 0 ; concentration would be the steady-state value, O;] = k,,[02][O-ilk(O; +DEHA)[DEHA]. Under our conditions, fDEH7 = 10.9 X 10- and [O,-] = 1.8X lo4, and thus If [O;]Is at the llmlt of our detection, the rate constant for 03-and DEHA would have to be 2.5 X lo8 M” s-’. This would be an unusually fast Os-rea~tlon,“~’~ e.g., k(H202+03-)= 1.6 X loe M-’ s-‘,thus, the fact that we dM not observe 0; 18not likely to be due to a reaction of 03-with DEHA.
Photolytic Decomposition of Lithium Aluminum Hydride Powder P.
J. Herley” and D. H. Spencer
Department of Materials Science and Engineering, State University of New York, Stony Brook, New York 11794 (Received February 1, 1979) fubllcation costs assisted by the U.S. Department of Energy
The ultraviolet photolysis of powdered LiAlH4 has been investigated at room temperature by using pristine material and material subjected to “Co y-ray irradiation. In all samples, similar shaped pressure vs. time curves are obtained that comprise an initial deceleratory period followed by a constant rate of hydrogen gas evolution. The data curves can be resolved into one saturating-exponential component and one linear component. The final (linear) stage approximates a second-order dependence on light intensity. This dependence indicates that two excited sites are required to produce a decomposition center. For a 1.0 x lo6 rd y-ray dose the irradiation increases the deviation from almost second-order dependence. This is consistent with a previously developed phenomenological theory. In addition, low y-ray doses (below 7.0 X lo3 rd) produce a decrease in the magnitude of the exponential component, a decrease in time required to reach the final stage and the steady state rate. In the y-ray dose range from 7.0 X lo3 to 1.0 X lo7 rd the effect is reversed. These results are interpreted to be produced by an increase in the concentration of radiolysis product with increasing dose. Introduction The photolytic decomposition of lithium aluminum hydride (LiA1H4) powder is the first investigation in a series of studies on alkali-metal alanate photodecompositions. These studies will focus on the photolysis kinetics and will include the effects of ionizing radiation on the decomposition process(es). In an earlier report’ preliminary data showed that LiA1H4 powder was readily photolyzed by high-intensity ultraviolet (UV) light. Apart from this study the available literature does not contain any further qualitative information on the photolysis of solid LiA1H4. However, a large number of photodecomposition studies have been made on other pseudostable inorganic compounds: The earlier work centered on the alkali-metal azide^.^-^ (See also ref 1-11 contained in ref 5.) Recent studies include the photolysis of sodium b r ~ m a t e and ~-~ sodium chlorate? In these studies preexposure of the solid to ionizing radiation was shown to affect the photodecomposition kinetics. The kinetic behavior of the decomposition process suggested that the electronic properties of these solids are important in understanding the photolytic process, and, associated with these properties are all of the electronic species normally involved in the 0022-3654/79/2083-1701$01.00/0
surface chemistry of semiconductors and nonmetals. The role of solid state defects and electronic processes in the initiation and growth of nuclei in photolytic decompositions has been discussed extensively by Jacobs and Tompkins,2 Y o ~ n g ,and, ~ , ~ more recently, by Levy and H e r l e ~ Active . ~ ~ ~defects in the pristine solid (e.g., point defects, impurity atoms, dislocations, voids, etc.) can influence or control the kinetics of the photolysis process. The light-induced carriers, viz., electrons, holes, and/or excitons, can interact at or with the defect sites present to deposit their energy at localized sites in the lattice. In time these sites become decomposition nuclei. The process is envisaged to correspond to energy transitions in the band gap of the solid. The energy transfer occurs between specific electron states on or near the surface. Since both defects and carriers can be readily introduced by exposing the solid to y-ray irradiation, the entire decomposition process may be influenced by preirradiation. Experimental Section Materials. LiA1H4powder was prepared as described previous1y.l Commercial 1.12 M LiA1H4 in diethyl ether solution (Alfa Inorganics Inc.) was filtered and then evaporated over a period of several weeks a t room tem@ 1979 American Chemical Society
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The Journal of Physlcal Chemlstry, Vol. 83, No.
P. J. Herley and D. H. Spencer
73, 1979
perature in a stream of prepurified nitrogen gas. The resulting white, crystalline aggregates were less than 0.5 mm in diameter. For the present study, this material was ground in a porcelain mortar and sieved through a brass U.S. Standard Testing sieve, ASTM No. 200, with a mesh opening of 75 pm. Only those particles that were retained on a 63-pm mesh screen were selected for study. After processing, the material was encapsulated in a Pyrex glass ampule and stored in the dark in a vacuum desiccator over Drierite. The y irradiations were carried out in sealed, evacuated glass ampules. Apparatus and Procedure. The experimental apparatus and techniques have been described previ~usly.~The evolved (hydrogen) gas pressure was determined in a constant-volume, Pyrex-glass, high-vacuum system with a Baratron differential micromanometer (M.K.S. Instruments, Inc., Burlington, Mass.). The background pressure in the system was found to be less than Pa when monitored on a Veeco RG75P ionization gauge prior to photolysis. A liquid nitrogen trap protected both gauges. Pa/min The background outgassing rate was 1.05 X when the system contained the LiA1H4 powder. The ambient temperature of the samples did not exceed 27 "C during decomposition or during y-ray irradiation. Each photolysis run was made in a fused silica optical cell containing 100 mg of power evenly spread over a 2 X 1cm area in a fused-silica boat. The UV light source used was a G.E. BH6 high-pressure mercury lamp with a high continuum in the UV region. The spectral distribution of the lamp intensity is shown in Figure 9 of ref 1. The lamp intensity was determined by using a windowless Eppley thermopile with a sensitivity of 0.0675 pV pW-l cm-2. For variable intensity measurements, combinations of fused-silica neutral-density filters (Spectrum Systems and Ealing Optical Co.) were introduced into the light path. A water filter was present at all times during these experiments, to filter out the water-absorbable infrared components of the light. The results were obtained on a Hewlett Packard digital recorder (Model 5055A). Digitized pressure data were obtained at 1-min intervals. In the subsequent figures the data presented as symbols are typical points on continuous curves. For clarity not all the data points are included. However, all data points were used for the mathematical analyses and were processed with a Univac 1100 computer. For y-ray irradiation, the samples were encapsulated under vacuum in aluminum foil-wrapped (light-tight), Pyrex glass ampules. Irradiation was carried out with @Co y rays in the spent fuel facility at Brookhaven National Laboratory. The y-ray dose rates varied from 5 X lo2 to 1.0 X lo' rd h-l.
Results Unirradiated Powder. Typical evolved gas pressure vs. time curves at 24 "C are shown in Figure 1which contains data obtained for three separate photodecompositions of 100 mg powder. The photodecomposition curves span a pressure region of -2.5 Pa in 400 min and are reproducible. Each curve consists of two distinct regions. The first region lasts approximately 240 min and is deceleratory throughout. This component merges into a second, linear pressure vs. time region which persists to the longest measured time (-800 min). If the light is switched off during the linear (constant-rate) period, the rate of gas evolution immediately returns to the background outgassing rate of the system. After being exposed to the light for 400 min, the surface of the powder darkened considerably. A series of experiments were carried out in order to
REPRODUCIBILITV OF T H E PHOTODECOVPOSIT ON OF LiAlhq 3 0 W D E R AT 24°C
I
I
I
. -
dbarew
1
TIME ( M ' Y )
Flgure 1. Three separate determinations of the gas evolved from 100-mg samples of LIAIH, powder at 297 K exposed to ultraviolet light from a high-pressure mercury lamp. These data demonstrate the reproducibility obtained. Also shown is the resolution of the data curves Into llnear and exponential components as descrlbed in the text.
probe the stability of this material to UV light and to elucidate some of the processes occurring during photodecomposition: (a) The effect of interrupting the photolysis process for various lengths of time at room temperature was examined during the steady-state rate period. The light was shut off from the sample for periods ranging from 0.5 to 8.0 h when the decomposition was in the steady state region. As soon as the light is turned off, the evolution of gas ceases. The rate returns to the background value and remains at this value, i.e., no dark rate was detected, for all the interrupted time periods studied. The rate immediately returns to its steady-state value when the light is switched back on to the surface. This suggests that the electronic species produced by the photolysis during the steady-state period at room temperature are either comparatively stable, i.e., do not decompose significantly in 8 h, or, they are very short-lived, and decay virtually instantaneously when the lamp is switched off. (b) The effect of removing the gaseous products from the photodecomposition was examined by pumping continuously on the sample during photolysis. The evolved gas was removed by pumping continuously on the sample for 300 min while photolyzing at room temperature. (After 300 min of photolysis time the reaction is in the constant-rate period.) The Dekker isolation valve was closed at 300 min and the evolved gas pressure was monitored as a function of time. These data show that the photolysis rate after pumping is the same as that found when the system had been isolated from the beginning of the experiment. This result indicates that the decomposition process is not significantly altered by the presence of gas products, Le., the latter do not inhibit the process from proceeding, e.g., by inducing a back reaction.
The Journal of Physical Chemistry, Vol. 83, No. 13, 1979
Photolytic Decomposition of LIAIH,
E F F E C T OF T H E R M A L L Y D E C 3 M P O S I N G 1 0 0 m g OF L i A l H 4 P O W D E 3 AT 130°C IN VACLUV F O R 8 H r s (CORRESPO\CING T O a i l ) A Y D T r E N PHOTOLYSING
E F F E C T OF T H E UV. LIGHT I N T E N S I T Y ON T H E P H O T O D E C O M P O S I T I O N OF L i A l H 4 POWDER AT ROOM T E M P .
RELATIVE LIGHT INTENSITIES
3.5-
-
IO0
-a 3 0 0
LL
525
,
Lo Lo
092
'+
1703
1
. -
'
e-
1 I M TEWPERATJ3E 0 T r E R M A L Y 3EC3MPOSEC
cr A
S A hl FL E PPIS-IYE S A M P - E
J
i
I
a
0
IO0 200 300 U V EXPOSURE T I M E ( m i n )
400
C
I
1 P
/ -
Flgure 2. Plots of evolved gas vs. time for 100-mg samples of LiAIH4 powder photolyzed at 297 K for relative intensities ( I l l o varying from 1.00 to 0.43. Io = maximum intensity = 3.3 mV).
(c) Attempts were made to determine the actinic wavelength, Quantitative determinations of the dependence of the decomposition rate on light wavelength were inconclusive; the light transmitted through the monochromator was not sufficiently intense to induce decomposition rates different from the background rate of the system containing pristine powder. (d) The intensity of the light was varied by using calibrated, neutral-density filters. A series of runs lasting 400 min each were carried out on 100-mg samples at relative light intensities ranging from 1.0 to 0.43. The maximum intensity reading on the thermopile was 3.3 mV. The results are shown in Figure 2. The entire pressure vs. time curve is changed by decreasing the intensity. The initial, deceleratory stage of the curve has been shortened, the linear, steady-state rate has been decreased, and the total pressure of evolved gas has been decreased, at lower intensities. All of these effects decrease systematically with decreasing intensity. (e) The effect of thermally decomposing the material prior to photolysis was investigated. The sample was thermally decomposed for 8 h at 130 "C under vacuum1 to complete the reaction LiA1H4 LiAlH, + Hzt. The solid product (LiAIHz)was subsequently photolyzed. The result is shown in Figure 3 where it is compared with the photolysis of the same weight of pristine material. (Both curves have similar characteristics but they are not superimposable.) The thermally decomposed residue appears to evolve a larger amount of hydrogen and the linear rate is also increased. The additional increase in the quantity of gas evolved may be due to one or a combination of effects. For example, it could result from an increase in the surface area of the product and/or be due to a retention of gaseous product trapped in the residue during thermal decomposition. It has been shown1 that thermal decomposition produces a smaller particle-sized product; the original crystallites break down during decomposition with a concomitant increase in the surface area. This additional surface would be available to generate more gas on exposure to the light. However, this cannot be the only effect as, in that case, the two curves would differ only in the quantity of gas evolved. Thus, the photolysis process per se is probably more efficient in the decomposed material. This could explain the more rapid increase in
-
-
u 400 TIVE (MIV)
Figure 3. Plots of evolved gas vs. time for a photolysis run of 100 mg of pristine LiAlH4 powder compared to a 100-mg sample whlch was initially isothermally decomposed at 130 O C under vacuum for 8 h (corresponding to a = 1 for the reaction LiAIH, LiAIH, 4- H,f) and then subsequently photolyzed under the same conditions as a normal run.
-
the steady-state rate. Additional experimentsg are underway to investigate the effects of coirradiation (simultaneous thermal and photodecomposition)and will be reported elsewhere. (f) The effect of exposing the material to several, different gaseous atmospheres has been examined prior to photolysis. Exposure of the sample to dry air for 24 h did not alter the subsequent photolytic pressure vs. time curve. However, the photolysis curves were markedly affected by exposing the powder to a saturated water-vapor atmosphere for periods of 10 min and of 12 h prior to photolysis. After a 10-min preexposure the sample follows a curve slightly faster than the pristine material up to 100 min when the gas evolution ceases completely. After a 12-h preexposure only a very slow evolution of gas is observed, i.e., the initial monotonic decay stage is completely removed. These observations support those reported earlier on the effect of water vapor on the thermal decomposition of this material; viz, that exposure to water vapor formed, first, a product that accelerates the decomposition reaction and, secondly, a product that does not evolve hydrogen. The physical nature of the product suggests that a hydroxide is formed. The slow gas evolution after a 12-h exposure would seem to indicate that a small fraction of the material remained unreacted after the water-vapor treatment. Irradiated Powder. A sequence of experiments was carried out on samples exposed to 6oCoy-ray irradiation prior to photolysis. The samples were exposed to y-ray doses ranging from 5.0 X lo2 to 1.0 X lo' rd. As these effects often produce small changes in the photolysis curves, the reproducibility of the photolysis curves was confirmed for a typically dosed specimen after irradiation. A series of pressure vs. time curves for powders exposed to a 1.0 X lo6 rd y-ray dose prior to photolysis is shown
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The Journal of Physical Chem/stty, Vol. 83,No. 13, 7979
P. J. Herley and D. H. Spencer
REPRODUC'BILITY OF I O x l O B RAD LIAIH, POWDER PHOTOLYSIS
1
'A
I
I
1
DECOMPOSITION OF LiAIHq POWDER AT ROOM TEMPERATURE
UNIRRADIATED
I5OC
0
200
I00
300
400
TIME, min
Flgure 5. Plots of evolved gas pressure vs. time of samples after exposure to the Indicated 'OCo y-ray doses. The samples consisted of 100-mg samples of LiAIH, powder and were photolyzed at 297 K at constant lamp intensity. The total y-ray exposure varied from unirradiated (zero dose) to 7.0 X lo3 rd. An unirradiated run is included for comparison. 100
A
P
,
O
0
1
100
I 200
200 330 TIME (MIN)
I
300
1
400
EFFECT OF PRIOR 6oCo GAMMA-RUY IRRADIATION ON THE PHOTOLYTIC DECOMPOSITION OF LiAIHq POWDER AT
400
I
ROOM TEMP.
I
2 51
500
TIME, (MINI
Flgure 4. Three separate determinations of the evolved gas pressure vs. time for 100-mg samples of LiAIH4 at 297 K. The samples had been irradiated with a 1.O X 10' rd 'OCo y-ray dose prior to photolysis. These data demonstrate the reproducibility obtained for irradiated specimens. Also shown is the resolution of the data curves into linear and exponential components, as described in the text.
in Figure 4. The density of points on the plots reflects the number of data points obtained. The reproducibility was not altered by the irradiation pretreatment. These curves exhibit all the characteristics observed above for the unirradiated specimens (see (a)-(e) above). However, several additional, significant changes occurred in the overall shape of the curve when the dose was varied. These changes were found to be independent of the dose rate used and, thus, are presented only as a function of the total dose received. As the y-ray dose is increased from 5.0 X lo2to 7.0 X lo3rd, both the initial decay period and the subsequent linear rate decrease monotonically. This effect is illustrated in Figure 5. Then, as the dose increases above 7.0 X lo3 rd, an increase in the initial decay period occurs, accompanied by a corresponding increase in the linear rate. This effect occurs monotonically with increasing dose up to 1.0 X lo' rd (the largest dose used). Eventually, at the highest dose the irradiated curve approaches the curve for pristine, unirradiated material. These effects are illustrated in Figure 6. These results are reasonably free from large pressure perturbations and can be readily analyzed by the methods described below. Data Analysis The pressure vs. time data for both unirradiated and y-ray preirradiated powders were analyzed by using a phenomenological rate equation described in an earlier paper.6 The equation was applied to the data in the integrated form, i.e.
p,(t) = H1t - Hz(l - ehzt) - H3(l - e-hst) + H4(1- e - 9 (1)
U V FXPOSURE TIME I r n i n l
Flgure 6. Plots of evolved gas ressure vs. time of samples photolyzed after exposure to the indicated CO y-ray doses. The samples consisted of 100 mg of LIAIH, powder and were photolyzed at 297 K at constant lamp intensity. The total y-ray exposure varied from 7.0 X lo3 to 1.0 X lo7 rd. An unirradiated run is included for comparlson.
el:
where PI(t)is the evolved gas pressure for a given lamp intensity, I. t is the time and Hiand h, are constants. The derivation of eq 1contains a series of assumptions which must be taken into account when evaluating the results. Briefly, these assumptions are that the photodecomposition process may involve, to some degree, essentially all of the conventional solid-state processes involving electronic carriers and solid-state defects. Also, the decomposition mechanism is assumed to involve not more than a single two-stage electronic process and, furthermore, the decomposition site concentration can change during the early stages of the photolysis. It is also important to note that only kinetic data are currently available, and thus, it becomes impossible to distinguish between the several, different physical processes that are actually occurring, particularly if these processes give rise to the same overall effect and are governed by the same kinetic equation. The number of terms present in eq 1 is determined by the number of processes occurring during the reaction. This theoretical treatment also yields an expression for the
Photolytic Decomposition of LiAIH4
The Journal of Physical Chemistry, Vol. 83, No. 13, 1979
TABLE I: Analysis of the Photolytic Decomposition Curves for LiAlH, Powder a t 297 K re1 int (Z/Zo)
(Pa min-l)a
H , (Pa)b
h, x (min-l)C
1.00 0.92 0.83 0.73 0.62 0.43
0.476 0.447 0.358 0.242 0.164 0.128
1.244 0.888 0.754 0.603 0.517 0.153
0.860 0.660 0.625 0.457 0.357 0.130
HI X l o u 2
TABLE 111: Parameters Obtained by Fitting Eq 3 to Data for the Photolytic Decomposition of 1.0 X l o 6rd 'OCo y R a y Irradiated LiAlH, Powder Sample at 297 K re1 int (Z/Zo)
TABLE 11: Effect of 7-ray Dose on Constants Obtained from Ea 3 in Text dose (rd)
lo-'
(Pa min-l)a H , (Pa)b
h, X lo-' (min-l)C
0.880 0.145 0.800 0.145 0.701 0.142 0.098 0.522 0,473 0.112 0.391 0.120 0.117 0.590 0.704 0.072 0.803 0.049 a Maximum error is i0.003 X Pa min-'. Maximum error is k0.018 Pa. Maximum error is k0.003 X lo-' min-'. unirradiated 5.00 X 10' 1.00 x 103 2.25 x 103 5.00 x 103 7.00 x 103 1.00 x 10' 5.00 X 10' 1.00 x 107
H , lo-' HI lo-' (Pa h, lo-' re1 int (Pa min-l)a H, (Pa)b (min-l)C (Z/Z,,) min-')a
1.00 1.00 1.00 0.93 0.93 0.88 0.83 0.83 0.80 0.79 0.79 0.79 0.79 0.79
a Maximum error is iO.009 x Pa min-I. MaxiMaximum error is i0.017 X mum error is k0.025 Pa. min-'.
H, x
1705
0.146 0.156 0.155 0.106 0.095 0.095 0.104 0.121 0.125
0.840 0.749 0.708 0.620 0.565 0.520 0.475 0.420 0.400 0.450 0.332 0.331 0.325 0.318
0.526 0.547
0.477 0.465
0.577
0.507
0.76 0.74 0.70 0.68 0.67 0.66 0.60 0.58 0.57 0.50 0.48 0.40 0.21 0.18 0.17
0.300 0.370 0.275 0.285 0.225 0.185 0.160 0.225 0.135 0.115 0.165 0.075 0.038 0.050 0.010
Maximum error is *0.007 x lo-' Pa min-l. Maximum error is iO.011 Pa. Maximum error is *0.010 x lo-' min-'. E F F E C T OF UV I N T E N S I T Y O N S T E A D Y S T A T E PHOTOLYTIC R A T E FOR LiAIH,
z
S
\
6Ok
I
v)
POW3ER AT ROOM 'EMP
I
I RUh I RUN 2 RUN 3
I
I
1
0
1
A 0
linear or steady-state rate, &(a), present at long decomposition times as a function of the light intensity, I fir(-) =
~ @ / ( 1 *PO
(2)
lo"
where 01 and p are constants. When eq 1 is fitted to the present data only three terms were obtained for both the unirradiated material (see insert in Figure 1) and for a typical (1.0 X lo6 rd) y-ray preirradiated sample (see insert in Figure 4). Specifically, eq 1 was applied to the remaining data in the form
PI(t)= H,t
+ H4(1- e
3
(3)
indicating that H2, H3, and h2,h3 from eq 1 are negligible for this material. The method for fitting the data is described in detail in ref 8. The constants derived for the fit of eq 3 to the data at various relative intensities are given in Table I. The maximum errors quoted in Tables 1-111 were determined by a linear regression program applied to the data with a 95% confidence level. These data represent several complete runs of samples taken from one preparation of powder. The reproducibility between runs is clearly less than the fit of eq 3 to the data (see, for example, H1in Table 111). This inhomogeneity in Hlbetween runs reflects the combined effects of several variables, such as the distribution of this particle size of the powder, the inability to expose exactly the same surface topography for photolysis, small fluctuations in lamp intensity from run to run, etc. In addition to the steady-state rate data ( R r ( m ) = H,) tabulated in Table I, more values of H I were determined by using the split-run technique.* The composite plot of all the steady-state rate data for pristine powder is plotted in Figure 7 as a function of relative lamp intensity. These data were fitted by eq 2 (continuous line in Figure 7) in the form RI(m) = 01(I/1~)~/[1 - P(I/lO)]where Io is a maximum intensity of 3.3 mV to a 95% confidence level using a non-linear least-squares curve-fitting prograrnl0J1 available in the Univac computer library and yielded 01 =
0
04
06
08
IO
R E L A T I V E I N T E 1SI T Y ( I / I ~ )
Figure 7. Plot of the steady-state photolysis rate vs. relative light intensity for powdered LiAIH4 at 297 K. The solid curve is a least-squares fit to the data of R(!, = CY(I/I~)~/[~ - & I / I O ) ] where CY = 0.00316 f 0.00016Pa min- and p = 0.217 f 0.011,I , = maximum intensity = 3.3 mV.
0.00316 f 0.00016 Pa min-I and = 0.277 (f0.014). Clearly the data points are scattered about the leastsquares fit. However, the overall general shape of the data plot is not inconsistent with this equation. When the complete analysis is applied to the pressure vs. time curves obtained by the photolysis of samples preirradiated with y rays the results are as follows: At a constant lamp intensity, all the curves obtained can be resolved in one exponential and one linear term (eq 3). The constants derived from the fit of eq 3 are listed in Table I1 for various y-ray dosed samples prior to photolysis at constant lamp intensity. Note that the linear rate, HI, decreases monotonically with dose until 7.0 X lo3 rd at which point it begins to increase with dose. The same trend is apparent for the magnitude of the exponential component and the exponential constant.
1700
The Journal of Physical Chemistry, Vol. 83, No. 13, 1979 EFFEC' OF UV INTENSITY O h 5TEA)Y STATE PeOTOLYTIC RATE 'OR LIAIH, PO*DER PQEV OUSLY IRRADIATED WITH 1 0 ~ 1 0R~A D 6oCo S&NMA-RAyS
RUY 2 RUN 3
; ?C,L
0 A
\
q u
3c
4
A A
!
I
3
=c
6 6 RELATIVE NTENS TY
0
Flgure 8. Plot of the steady-state photolysis rate vs. relative light intensity for powdered LiAlH, at 297 K after a 1.0 X 10' rd 'OCo y-ray dose. The solid curve is a least-squares fit to the data of R4,) = a(I/Io)2/[1 - @(I/Io)] where a = 0.00357 f 0.00018Pa min- and 6 = 0.532 f 0.027, I, = maximum intensity = 3.3 mV.
Using a 1.0 X lo6 rd preirradiated sample as a typical example, measurements were made to determine how the constants in eq 3 vary with lamp intensity for the irradiated sample. The results are tabulated in Table 111. Steady-state rate data, supplemented by data from additional "split run" determinations, were plotted as a function of the relative lamp intensity and the results are shown in Figure 8. A non-linear least-squares curvefittinglOJ1procedure was used to fit eq 2 to the data in the same form as for unirradiated material yielding a = 0.00357 0.00018 Pa min-' and ,6 = 0.532 f 0.027. A comparison with the data for unirradiated material shows that the effect of the irradiation is to increase the magnitude of the constants a and p.
Discussion Similar-shaped pressure vs. time curves are obtained at room temperature for all of the samples investigated. These include pristine material and samples exposed to a y-ray dose prior to photolysis. The curves consist of an initial, deceleratory component with the maximum rate occurring at the onset of the photolysis, which is superimposed on a steady-state constant-rate component. A back reaction was not observed on prolonged standing in gaseous product. This indicates that the photolysis process is irreversible within experimental error. After an exposure, reaction doses not continue while the sample is allowed to stand in the dark, i.e., the thermal dark rate is virtually zero at this temperature. Presumably the entities induced by the photolysis are either relatively stable or decay extremely rapidly, resulting in a thermally stable product. Application of eq 1to the data clearly indicates that only the first and last terms are significant. It is not immediately obvious why the other terms in eq 1are effectively zero and several speculations on this point could be at-
P. J. Herley and D. H. Spencer
tempted. As each of the missing terms includes a number of constants representing contributions from a large fraction of possible physical processes, speculation would be pointless without further experimental evidence. However, the results do indicate that the data can be satisfactorily represented by the sum of one saturating exponential and a linear rate component, within experimental error. The location of the optical absorption edge in this material has not been reported but LiH12has an absorption edge at
