J. Phys. Chem. 1980, 84, 3199-3207
3199
Measurements of Fluorine-Atom Recomblnation on a Nickel Surface by Electron Paramagnetic Resonance Ira B. Goldberg Rockwell International Sclence Center, Thousand Oaks, California 9 1360 (Received: June 24, 1980; In Final Form: August 20, 1980)
The recombination of atomic fluorine on nickel over the temperaiture range of 295-800 K was studied. The experimental data were compared to those recently reported in the literature. Observed here were substantially greater recombination efficiencies (7)than measured by others, hysteresis in the curve of y vs. temperature, and. anomalous transient behavior in obtaining steady-state F-atom concentrations following flow through a Ni reactor, These data are interpreted in terms of two different nickel fluoride surfaces. One surface occurs at all temperatures between 296 and 800 K in the absence of atomic fluorine and above 460 K in the presence of atomic fluorine. The second surface is stable at temperatures below 450-470 K in the presence of atomic fluorine. The latter surface is less efficient in promoting fluorinle-atom recombination.
Introduction The heterogeneous recombination of most atoms that form diatomic molecules has been extensively studied.l Exceptions include the halogens, probably as-a result of difficulty in the handling and measurement of these materials. Recent developments of the HF and DF chemical lasers, in which atomic fluorine reacts with hydrogen to form vibrationally excited hydrogen fluoride, has stimulated interest in reactions of atomic fluorine. Recently, several studies of fluiorine-atom recombination on metalsz4 and n o n m e t a I ~ ~have J - ~ ~been reported. Only a portion of the ~ o r kwas ~ - carried ~ out at other than ambient temperature. Most of the atoms studied appear to follow the Rideal mechanism for surface recombination. This is a two-step reaction in which the first step is rapid adsorption of the atom on the surface (eq 1)followed by an incident fluorine F* surface Fa (1)
atom combining with the adsorbed atom (eq 2) in the
F.
+ Fa
kb
F2
(2) rate-limiting step. Fluorine was assumed to follow this behavior. Among the metals studied, nickel exhibited several anomalies. Aruthyunov and Chaikin4 observed that, where most materials exhibit a monotonic increase or decrease in the probability of recombination with temperature, fluorine-atom recombination on nickel reaches a maximum value at 375 K. Furthermore, the low-temperature values were found to be both irreproducible and dependent upon the previous treatment of the surface. Jumper, Ultee, and Dorko3observed both irreproducible values of recombination, unless there were prior long periods of exposure of the Ni surface to F near room temperature, and a very strong temperature dependence of the rate coefficient between 298 and 328 K. In addition, the recombination rates at 300 K determined by Nordeen and LaGrange, were greater than those determined by Arutyunov and Chaikin4which were in turn greater than those of Jumper et aL3 Jiimperll also observed transient behavior in the recombination of fluorine atoms. If the microwave di~chargewas shut off for a few minutes and restarted, times of up to 50 min would be required to reach a steady-state flow of fluorine atoms through the reactor. In view of the importance of nickel in chemical laser research and development, and in order to obtain more detailed data for comparison with the model of F-atom N
0022-3654/80/2084-3199$01 .OO/O
recombination proposed by Jumper et al.,3J2measurements of F-atom recombination were carried out between 295 and 800 K. Experimental Section Flow Reactor. A schematic drawing of the flow reactor is shown in Figure 1. The reactor itself consists of a Ni-200 alloy cylinder, 2.54 cm 0.d. and 2.195 cm i.d. by 61 cm long. Ni-200 contains nominally 99.5% Ni, with maximum limits of 0.1% C, 0.2% Fe, 0.18% Mn, 0.18% Si, and 0.13% Cu. The cylinder was wrapped tightly with heating tape, with several thermocouples placed on the Ni surface. Several layers of aluminum foil were then wrapped around the heating tape, followed by a layer of fluted aluminum foil, which was followed in turn by several loosely wrapped layers of foil in order to keep the assembly together. The downstream end of the reactor was connected to a 2.45-cm 0.d. quartz tube by a Swagelok coupling with Teflon ferrules. The upstream end of the reactor was connected to a flexible stainless steel bellows. An Ultra-torr fitting provided a vacuum seal and permitted the use of an inner tube of 1.85 cm o.d. that allowed the point of injection of F atoms to be varied along the entire length of the nickel reactor. Cooling fans were placed at either end of the reactor: one at the upstream end which kept the Ultra-torr connectors at low temperature, and one at the downstream end which cooled the quartz as rapidly as possible since the coefficient of recombination on quartz increases with temperature. A second injector, 6 mm o.d., was added so that additional gases, such as a titrant or a portion of the F2-He gas mixture, could enter the reactor without flowing through the microwave generator. This was also used to hold a thermocouple to measure the temperature profile of the reactor. A titrant entry port was placed downstream of the Ni tube to allow H, or NO to react with the effluent F or F, from the reactor. Pressure-measurement pork were placed at the entrance to the e_lectron paramagnetic resonance (EPR) cavity, 24 cm upstream, and at the point of entry of the gas from the microwave discharge. At a typical flow rate through the Ni (510 cm/s at room temperature) with the injector fully extended so that the second and third pressure-measuring ports were respectively located 24 and 174 cm from the downstream port, the pressures were 240.0,243.0, and 271.0 P a in each of the three ports. Analysis of the data shows that the pressure drop in each segment is proportional to its length and inversely proportional to its cross-sectional 0 1980 American Chemical Society
3200
The Journal of Physical Chemistry, Vol. 84, No. 24, 1980
Goldberg
EVENSON CAVITY FOR 2.45 GHz DISCHARGE WITH FLEXIBLE CABLE CONNECTION
,/
I
-w
I
II
,
TI&+
OR
P
POQTION 1 OF GAS LOUARTZ TUBE 6mm O D
,
1
r = u h l P s
!BELLOW
QUARTZ IhJECTOR 18mm OD x fficm I W c m FROhl OPENliLG TO PRESSURETAPI
--
COOLED QUARTZ TUBE
NO OR
H1 TlTRAhT
Figure 1. Schematic drawing of the flow reactor used for measurements of fluorine-atom recombination.
area. As a result, the pressure drop in the Ni is -3% of the mean pressure in that segment. Covering the Ni by the injector reduces the pressure in the two downstream ports by 1.1%. The Ni surface was cleaned by washing with trichloroethylene, polishing with 220-mesh A1203, and washing with acetone, 30% HF, and alcohol. This was done before installing the tube, after an initial series of experiments at room temperature, and after several measurements at elevated temperature. Before the set of room-temperature experiments (except for those noted), the tube was exposed to an F/F2/He mixture for 8 h. Before carrying out measurements of the temperature dependence, the tube was preheated to 800 K for 4 h in the presence of F and
F2.
An Evenson 2.45-GHz cavity driven by a 130-W Raytheon magnetron was used to generate a plasma for F2 dissociation. The range of incident powers used was 20-80 W, An alumina tube contained the plasma. A microwave plasma initiator, described elsewhere,13 was used to maintain a continuous discharge. In the flow reactor, without added gases, the residual pressure was -1.9 Pa. However, if the reactor was sealed, the pressure would increase by -3 Pa/h, indicating a leak rate in the entire 8.5-L volume (including the 4-in. pipe leading from the nearest gate valve in the vacuum system to the valves on the flowmeters) of 6.5 X lom5cm3/s, as compared to typical flow rates of 6-7 cm3/s (STP). Measurements over the region of the most intense line of the EPR spectrum of O2 (transition K = 1,J = 2, M J = 0 1)gave no detectable signal when the He diluent was flowing. Gases and Controls. Fluorine (>98% purity Matheson) was passed through a KF column to remove HF. Nitric oxide (CP, Linde) and 24.4% H2 in He (Precision Gas Products) were each monitored and controlled by Tylan mass flowmeter controllers. The flow rates were calibrated by filling an evacuated 2-L stainless steel tank and carrying out least-squares analysis on the pressure-time dependence, Helium (99.995%, Matheson) was controlled manually and monitored with a Tylan mass flowmeter, which was calibrated with a wet test meter. Flow rates were up to 10 cm3/s (STP). Pressure measurements were carried out with a Baratron (Model 170) capacitance manometer with a range of 0-100 torr. Dry O2 (Airco 99.6%) was calibrated in the same way as the He. EPR and Data-Acquisition Systems. The EPR14 and data-acq~isition'~ systems have been described elsewhere. Auxiliary homogeneity coils16were used on the pole pieces of the magnet. The flow reactor passed through a TEoln cavity. The frequency was 8.862 GHz with the 2.45-cm 0.d. quartz tube concentric to the cavity access. The instrument was calibrated by using the O2 (3C;) transition K = 1,J = 2, M J = 0 1and using standard methods,17J8 with probabilities recently cal~u1ated.l~ The data-acquisition system was used for double integration20 of the
-
-
1
OO
10
20
30
40
50
60
LENGTH ALONG Ni FLOW REACTOR I c m )
Figure 2. Temperature profiles along the metal surface as a function of length along the reactor: (0)data recorded on the outside surface; ( 0 )data recorded with the thermocouple near the lnslde surface.
F-atom, 0, and O2spectra and for pressure measurements.
Results Chronologically,the experimental work was done in the following way, using the same Ni-200 flow reactor: First, extensive experiments were carried out at room temperature; then, variable-temperature experiments at up to 800 K were carried out. The reactor was cleaned in the manner described in the Experimental Section. It is helpful to describe the high-temperature results before describing those at or near room temperature. Surface and Gas Temperatures in the Reactor. The variability of temperature along the length of the reactor may be a significant contributor to the uncertainty of the rate of re~ombination.~To check this uncertainty, a thermocouple was placed on the 6-mm quartz tube so that it could be moved parallel to the inside Ni surface, while the approximate flow conditions (i.e., pressure of 265 P a of He and room-temperature linear velocity of 700 cm/s) were maintained in the reactor. Temperature profiles of the gas compared to the four thermocouples on the surface are shown in Figure 2. At a temperature setting of 443 "C,the outside surface of the metal was constant within 8 "C. However, the temperature measured along the inside varied considerably along the length and was never as warm as the outside metal surface. The initial gradient (between 0 and 15 cm) is probably due to cooling by the gas entering the reactor. The inside surface of the Ni was probably only a few degrees cooler than the outside surface, based on the thermal conductivities and heat capacities of the metal and gas. On the basis of these data, the surface temperature of the metal was considered to be that measured by the four thermocouples. The surface temperature is important in assessing the efficiency of F-atom recombination. The gas temperature is important in establishing the residence time of the gas in the reactor. The temperature dependence along the reactor length was represented by three straight lines through data similar to those shown in Figure 2. Gas-temperature profiles for surface temperatures that were not recorded were interpolated from data corresponding to the closest surface temperatures. The gas temperature at the metal surface was assumed to be that of the metal on the basis of the following argument: At the pressures and temperatures used in this series of experiments, the maximum mean free path was
Fluorine-Atom Recombination on a Nickel Surface
The Journal of Physical Chemlstw, Vol. 84, No. 24, 1980
0.014 cm, much smaller than the reactor radius. In addition, more than 1000 collisions between gas-phase F and the wall would be required to induce recombination. Thus, the gas near the wall should be nearly at the wall temperature. Treatmen,t of Data. Equation 3 describes the fluid
1
.
0
r
-
l
-
3201
T
nk,CFn = 0 (3) dynamics of the flow reactor21 where z and r are respectively the axial and radial positions in the reactor, R is the reactor radius, k, values are homogeneous reaction rates coefficients of order n, CF is the fluorine atom concentration, Vo is the average gas velocity, and DFis the diffusion coefficient. Substituting p = r/R, q5 = z/R, f = CF/CFo where CFois the initial F-atom concentration, assuming that axial d.iffusion is negligible, allowing secondorder homogeneou,s recombination, gives
The boundary equ,ations of eq 4 are f(p,q5=O) = 1 (af/aP)p=o = 0 (af/ap),=l = (2kWR/D)f,=1
kwZ
Pv= -
(5) (6)
(7)
where the factor of 2 originates from the consumption of 2 atoms per recombination. Assuming the LangmuirRideal mechanism, eq 1 and 2 k, = k& (8) where OF is the fraction of surface coverage of fluorine. Equation 4 was solved by the finite difference method of Porier and for constant k,. Results show that the extent of homogeneous recombination
F + F + He
kz
F2 + He
(9)
based on k2 =: 2.2 X 1014cm6m o P s-l 23 is negligible. For example, at 300 K, the half-life of the reaction is -8 s, while the residence time in the Ni reactor is less than 0.13 s. A t higher temperature, k2 decreases while the velocity increases, making recombination less significant. At high temperatures, the homogeneous and heterogeneous dissociation of F2 (eq 10 and 11) need to be con-
F2 + He 5
F+F
+ He
(10)
F2 + surface 5 2F (11) sidered. The recom~nended~~ value of k-z at temperatures between 300 and 2600 K is k-2 = 1.0 X 1014 exp(-147 KJIRT) cm3 mol-l s-l, Although shock-tube measurementsz4typically give higher rate coefficients, this value was based on the fact that k2/k-2 must give the thermodynamic equilibrium constant for F2 dissociation. At 800 K and 266 Pa, the plyeudo-first-orderdecomposition rate is of the order of lo4 s-l, so that homogeneous dissociation is not significant even though thermodynamic calculations indicate more than 99% dissociation at these low Fzpartial pressures. The rate of heterogeneous dissociation of fluorine on any metal surface has not, to our knowledge, been reported. As a result, experimental measurements were carried out to ensure that this did not contribute to the total F-atom signal. A maximum of 5% of the F-atom signal at 800 K
"0
Figure 3. Logarithm of the total flux through the flow reactor as a function of position for different values of p = 2k,R/DF.
can be ascribed to dissociation. For convenience the terms p and q defined in eq 12 and 13 can be used .to describe the dynamics of the reactor. P = 2kWR/D~ (12) = DFZ/(~V&~)
(13) The product Pv is then a dimensionless distance along the flow reactor, given by k,z/ V&. Solutions of eq 4 with k2 = 0 in terms of the total flux of F atoms as a function of Pv is shown in Figure 3. Note that for P < 0.05, pseudo-first-order behavior is achieved in the reactor, and the plot is independent of k,. Experimentally, only k,/& can be determined. If the diffusion coefficient is known, the recombination process can also be expressed in terms of the efficiency of recombination according to eq 14,1122where is the mean atomic 9
2 k W = Ey/[4(1-
(14)
velocity of F, givlen by (8kBT/7rm)1/2where m is the mass of an F atom and y is two times the ratio of the wall collisions of F which result in the recombination to the total number of collisions. Diffusion coefficients of atomic fluorine have not been measured. On the basis of the mass 0.5 and temperature 1.75 d e p e n d e n ~ i eof s ~the ~ diffusion coefficients of atomic oxygen in He,25we obtain eq 15. Thus, at room temDF = 9.60 X 104(T/298)1.75cm2 s-l Pa-'
(15)
perature, for y < 2 X recombination can be treated as a pseudo-first-order decay along the flow reactor, provided that OF (eq 8) is constant. If the above conditions are valid, then the rate coefficient, kd, for pseudo-first-order decay of F in the reactor is given by eq 16 for a cylindrical reactor.26 Since the gas k d = yC/2R (16) temperature and hence the gas velocity varied along the length of the reactor, the residence time was calculated by integrating dt = d(z/ Vo) over the length of the reactor. For
3202
The Journal of Physical Chemistry, Vol. 84, No. 24, 1980
t
,\&,
O 001 1360
,
,
,/, , , 400
,
, ~,
,
500 600 TEMPERATURE i°KI
,
,, , ,
, 700
,
,,,,
,
li
800
Figure 4. Temperature dependence of the concentrationof fluorine atoms of the effluent gas relative to that of the gas entering the Ni reactor. Flow rates vary by up to lo%, so that data should not necessarily be superimposed. Lines represent the following: (-- -) increasing temperature; (-) decreasing temperature. Symbols repF, 1.79;Fz, 9.93. (0,O) resent the following initial pressures in Pa: (0) F, 1.37; Fz, 11.79. (0)F, 6.60;F?, 9.47. (A)F, 1.25; F,, 20.3. (A) F, 0.69;Fz, 9.46.
values of y larger than 2 X or for values of the surface coverage of F which depend upon the gas composition along the reactor, finite difference solutions of hydrodynamic equations must be used. At this point, however, the adsorption of F and Fzon Ni is not well established, other than by the theoretical calculations of Jumper, Ultee, and D ~ r k o .Studies ~ of the absorption of F2 on NiF27 did not reveal the state of the adsorbed fluorine. Qualitative Discussion of Variable-Temperature Experiments. In order to examine the affects of the F and F2 concentrations and the temperature on the extent of recombination, the experiments were carried out at essentially constant He pressure, varying the amounts of F and Fz entering the reactor. Steady-state measurements were made as the temperature was slowly changed. Rates of temperature change varied from less than 0.3 to 8 K/ min, depending upon the temperature range of interest. The F-atom concentration of the effluent from the reactor relative to the initial concentration is shown for some of the experiments as a function of temperature in Figure 4. Only semiquantitative comparisons between sets of data can be made by inspection, since the linear flow velocities varied by up to 10% among the measurements. One obvious point is that reproducibility between measurements is much greater for temperatures above 460 K than for lower temperatures. Most significant is that the F-atom concentration of the effluent, measured while increasing the temperature to 450 K, is larger than that made while decreasing the temperature from this value (data sets 0 , 0).Similar trends were observed for all sets of data, including the double minimum shown in points A and B of Figure 4. This double minimum, observed while increasing the temperature, is representative of nearly all experiments. Above 450 K, data taken while increasing or decreasing the temperature were reversible. To test the assumption of a steady-state F-atom concentration, the temperature in a number of experiments was stabilized at points between 350 and 430 K (such as at points C and D of Figure 4) and held constant for up to 0.5 h. In these experiments, the fluorine concentration was found to be constant and dependent on whether the Ni temperature had been approached from above or below the set value. It was found in other experiments that changing the heating or cooling rate by a factor of up to 10 had negligible
Goldberg
influence on the recombination rates for temperatures above 350 K. At slow flow rates and temperatures above 700 K, a small fluorine-atom concentration was observed in the effluent even when the microwave discharge was turned off. At flow rates used here, the residual signal at 800 K was less than 5% of that with the microwave discharge under the same flow; this was probably due to heterogeneous dissociation. The fact that the data in Figure 4 do not show an increase in the relative F-atom signal is further substantiation that dissociation, either heterogeneous or homogeneous, is not significant with respect to the measured concentrations. Several observations on the F and Fz concentration dependence on recombination are evident from the data shown in Figure 4. Comparison of the data in which the pressure of Fz is held constant and the pressure of F is varied (sets 0,0, and A, 0 ) shows that increasing the F-atom composition increases the relative rate of recombination in the high-temperature region. Comparison of the data in which the F2 composition is changed (set 0 , A) shows that increasing the Fz also increases the rate of recombination but to a lesser extent. As the temperature is increased above 450 K, the different curves begin to converge to the same relative F-atom concentration, suggesting that the recombination can become independent of the initial F and Fzcompositions at high temperatures. These results are partially in agreement with the results of Arutanov and Chaikin* and Jumper et aL3 As the temperature is increased, the rate of recombination is expected to decrease because of desorption of gases from the wall. However, in the temperature region studied, the reaction may not completely become first order in F, since the surface coverage may be a strong function of the partial pressure of F. Not consistent with previous hypothesesS is that addition of Fz to the gas increases the rate of recombination. This can be true if the F2is adsorbed with some dissociation. The fact that the data for experiments in which the sum of p F + 2pF is constant (sets 0 , 0, A) do not overlap more closely indicates that the dissociation of Fzon the surface is not complete below 750-800 K. This observation is consistent with studiesz7in which the adsorption on NiF2 was fit to the equation 8 = pF,lln, where n was found to be 1.8 at temperatures between 423 and 623 K. One important aspect of this work was the differences between data obtained during cooling between 350 and 295 K after heating to >450 K. In this region, the apparent rate of cooling has a substantial influence on the effluent F-atom concentration. However, in this region the cooling rate is difficult to control. This phenomenon is illustrated by the example that, if the temperature is decreased to 310 K, and then increased back to 350 K, the temperature dependence of the F-atom signal is similar to the lower curves of Figure 3, as opposed to the one designated by the open circles. Temperature Dependence of y. The temperature dependence of y, calculated as previously described, is given in Table I with the initial partial pressures of F and Fz. A portion of the data was obtained on a fresh Ni surface. These data are essentially indistinguishable from data taken on Ni surfaces that had been exposed to F and Fz at 800 K. These data are in agreement with the qualitative results described in the previous section. Comparison of the data, however, shows that there is sufficient scatter so that it is not a simple matter to relate the initial concentrations to the concentration dependence of y. A detailed model of the entire reaction system, including ad-
The Journal of Physical Chemistry, Vol. 84, No. 24, 1980 3203
Fluorine-Atom Recombination on a Nickel Surface
5 -
0.2-
4 c
-
E
6
y0 u
0.1-
w
-
5
Illin
i
E
0.05
-
u 2 . Y
5
0 03-
2' (L
0.02
-
Flgure 5. Comparison of representative values of y from this work ((Of.) P:, 1.79; PF:, 9.93. (W) ~ F , O ,1.47; PF;, 11.8. (A)PFO,0.92; pFp,16.1. ( 0 )pFo,6 4 p F O, 9.47. Units are Pa) with those other measurements: (e)ref 4; (4) ref 2; (shaded area) ref 3.
sorption, may be necessary to analyze fluorine recombination. It is evident that the effect of p$ on y is stronger than pF: where pxo and pF,O are are the initial partial pressures of F and Fz, respectively. Quantitative comparisons between these results and the results reported in the l i t e r a t ~ r e are ~ - ~shown in Figure 5. It is evident that there is a substantial difference in values of y , the results obtained here being 3-10 fold greater than those given in ref 3 and 4. The temperature dependence of fluorine recombiiiation in ref 4 is qualitatively similar to those reported here; both sets of data have a maximum value of y at 370-4100 K. The results obtained here are more closely in agreement with the theoretical calculations given by Jumper et aL3in the regions between 300 and 475
0
10
20 30 40 50 DISTANCE OF EXPOSURE TO F-ATOMSlcml
60
Figure 6. Relative EPR signal vs. length of exposure of the Ni surface to F atoms, for a surface temperature of 468 K. Initial conditions: pressure, 235 Pa; flow velocity, 1200 cm/s; F-atom pressure, 1.78 Pa; F, pressure, 7.22 Pa.
FLUORINE ATOM LOSS
0.7 0.6
K. Several differences between experiments may account for the differences in experimental values. For example, the actual surfaces on which the recombination is measured may be different. Also the reactor used here was heated to 800 K in F/F2 atmosphere prior to any measurements, whereas the one used by Arutyunov and Chaikin4 was heated only to 575 K. Tt is possible that the higher temperature changes the nature of the surface layer. This would be consistent with the observation that the reactor used by Jumper et al.3 was heated to 675 K, in which higher values of y were observed than by Arutyunov and Chaikin.4 Smaller values of y were observed in experiments in which the temperature was increased rather than when it was decreased. From Figure 5, the values of y shown on the increasing temperature cycle appear to be consistent with those of ref 4, except that at 390 K, rather than obtaining lower values for y, the values increase. This again suggests a transition in the surface coating. One data point, reported by Arutyunov and Chaikin4 in which a fresh Ni surface was used, is in agreement with our data. Their values decreased only after exposing the surface to F and Fzfor 40 h. The fluorine-atom concentrations used by Jumper et d3may have been considerably higher than those used here. This may also have some effect on the surface condition of the reactor. Nevertheless the pressure used by Arutyunov was approximately the same as that used here. Another significant difference between experiments is that the data obtained at different initial conditions, as well as data at constant temperature in which the point where fluorine is injected into the reactor is varied (shown
I
0.1-
0
10
20
I 30
I 40
50
60
DISTANCE OF EXPOSURE TO F-ATOMS (cm)
Flgure 7. Relative EFTI signal vs. length of exposure of a freshly polished Ni surface to F atom for a surface temperature of 296 K. Conditions: pressure, 251 Pa; flow velocity, 706 cm/s. Curve a: initial F pressure, 2.0 Pa; F2 pressure, 8.7 Pa. Curve b: Initial F-atom pressure, 15.0 Pa; F2pressure, 2.5 Pa. Curve b is shifted by 5 unlts on the abscissa.
in Figures 6 and 7), suggest that the recombination process is not pseudo first order. If the entire recombination process were first order, as is often observed,l then the degree of recombination should be independent of the initial concentration, and the logarithm of the signal should be linear with distance of exposure. In these measurements, increasing the initial fluorine-atom pressure typically increased the efficiency of recombination. Figures 6 and 7 show that at 468 K, close to the point at which y appears to be a maximum, and at 296 K the heterogeneous recombination is not pseudo first order in the F-atom concentration. From these data, the decrease in the Fatom concentration is greatest at shorter times. Thus, decreasing the flow velocity, or conversely increasing the length of the reactor, should decrease the overall value of y for a given system. Room-Temperature Measurements. Initial studies of F-atom recombination were carried out at room temperature. Several mleasurements on a fresh Ni surface, in
3204
The Journal of Physical Chemistty, Vol. 84, No. 24, 1980
Goidberg
TABLE I : Recombination Efficiencies (10'7) for Ni and Initial F and F, Pressures POP,) POW
10.12 2.67
T,K 800 775 750 725 700 675 650 625 600 575 550 525 50 0 47 5 450 425 400 375 350 325 31 0 300
3.40 13.4
1.93 3.49
9.93 1.79
9.19 1.56
6.3 6.8 7.3 7.8 8.2 8.7 9.3
6.7 7.0 7.3 7.7 8.3 8.8 9.3 10.4
8.1
11.8 1.47
fresh surface
11.2 12.3 12.6 12.8 13.8 14.4 15.5 16.7 18.4 20.8 22.6 24.3 25.4 24.6 22.2 15.1 8.0
9.47 6.60
20.3 1.25
9.46 0.60
7.9 8.2 8.7 9.1 9.7 10.4
6.7 7.1 7.7 8.2 8.8 9.4
11.1
10.0
12.1 13.2 14.5 16.4 19.7 23.7 25.3 28.7 30.2
10.6 12.1 13.1 15.0 17.5 20.2 22.0 24.7 25.6
exposed surface
13.8 14.9 16.4 17.8 19.7 21.7 24.1 25.0 28.8 30.6 31.1 28.4 21.8 12.0 8.0
10.0 11.1
11.1
12.4 14.5 16.8 19.9 22.8 25.1 27.6 33.3 26.1 20.3 11.5 5.9 3.5
12.2 14.0 15.9 19.0 22.3 25.4 27.5 28.8 30.2 27.6 20.5 10.6 3.6
6.9 7.1 7.2 7.4 7.6 8.2 8.7 8.9 9.4
8.3 8.6 8.8 9.1
10.1 10.7 11.4 12.5 13.9 15.7 18.1 21.1 24.1 26.4 27.4 27.8 27.4 24.7 13.0 5.0
7.2 7.7 8.3 8.6 9.0 9.4 10.1
7.9 8.3 8.7 9.3 9.8 20.3 10.8 11.7 12.4 13.4 14.8 16.6 19.6 22.6 25.9 28.8 30.0
11.0 12.3 13.8 15.6 18.5 21.2 24.0 26.9 29.0 29.1 28.2 21.0 12.1 6.4 2.5
11.0 12.2 14.3 16.8 19.4 21.5 25.0 26.3 27.2 22.2 20.0 12.0 5.6
TABLE 11: Values of 7 for Ni with Different Surface Treatments at 296 K
FLUORINE ATOM TRANSIENT
treatment
7
ref
freshly polished exposed t o HF
11.2 X 9.0 x 10-4 7.2 x 10-4 2.0 x 10-4 0.3 X
this work
reacted with F at 1 3 Pa for more than 24 h or heated with F exposed to air for 3 days after reacting with F
16.1 0.92
1.0x 10-4 3.8 x 10-4 0.1 x
3 2 4
this work 3 4
this work
1.4
"1 \, .
a
-
d P
y
P
I
.
lbl
\
0.8-
2
-
0.6-
-
0.4
0.2
which the point of injection of F was varied along the length of the reactor, are shown in Figure 7. These data show that recombination is neither a pseudo-first-order process in F nor a second-order process. Room-temperature values of the average y are very dependent upon the nature of surface treatment, as shown in Table 11. Evidently, polished Ni is a better catalytic surface than fluorides which will form after long exposure to F or after heating with F and FP If curve a of Figure 7 is interpreted in terms of two consecutive regions of first-order decay, then the first portion of the curve yields y = 21 X while the second portion gives y = 4.2 X or a change of a factor of 5 in the recombination. Increasing the amount of F and decreasing the amount of Fz appears to decrease the initial slope and increase the slope of the downstream portion. Exposure to air after heating with F2also appears to decrease the degree of recombination on the surface. Transient effects on the degree of recombination have also been noted.28 In one case, curve b of Figure 8, after a steady-state F-atom stream was obtained, if the source of generating F atoms was turned off and subsequently restarted without disrupting the Fz flow, it took up to several hours to build up to the same steady-state value. Typical times were of the order of 20 min. Curve a of Figure 8 shows an example of the case in which, after attaining a steady-state, F-atom flux with the Ni surface covered, the Ni was then exposed. An initial rise in the F-atom signal was observed before a decay to the steadystate value of the F-atom concentration in the presence of Ni was obtained. In Figure 8, the value of 1.0 on the
(1.0 steady state signal arnplitudei
w 1.0
-
-
o.o,i/l
5
I
10
'
15
I
20
I
25
'
30
I
35
I
40
45
'
50
'
55
1
60
TIME 116'~I I
Figure 8. Two types of transient behavior of F-atom concentrations at room temperature on Ni surfaces. Unity on the ordinate indicates the steady-state F-atom signal at the start of each experiment.
ordinate represents the steady-state value with the Ni covered for curve a and with the Ni exposed for curve b. Experiments were carried out to determine whether F is chemically reacted during the transient shown in curve b of Figure 8. NO was added stoichiometricallyto the F/F2 flow, slightly downstream of the Ni reactor (Figure 1). Reactions 17 and 18 can then take place. In this way, F + NO 4 He FNO + He (17) Fz NO --NO + F (18) changes in the total fluorine can be monitored by changes in the EPR signal of NO. An example is shown in Figure 9. In this experiment, the F-atom signal was first allowed to stabilize (Figure 9a). At t = 0, the microwave discharge was turned off (Figure 9b) for a period of 300 s. When the discharge was restarted, -20 min was required to again reach a steady state. This experiment wa8 then repeated, the NO signal being monitored. In the case where there was no F2 or F in the reactor, the relative NO signal was unity. When NO was added to the stream after Fz and a steady-state F-atom concentration was allowed to build up, the relative NO signal decreased to 0.15. Turning off the discharge caused only a small increase of the signal because of a slightly faster reaction of F2 with NO than -+
+
The Journal of Physical Chemistty, Vol. 84, No. 24, 1980 3205
Fluorine-Atom Recombsination on a Nickel Surface
was adsorbed, it is possible that F2could react to form HF F HO at the surface. For this to occur, however, -200-400 molecular layers would be required to generate the increased amount of F above the point S = 1. I t is possible that in the absence of F, 0, was adsorbed on the surface, thus changing the surface activity, although this would not lead to enhanced EPR signals.
+ +
Y
u
lb)
$
ON OFF
a
1
-.
n n.
I
NO : 1 IN ABSENCE
I
i -
Flgure 9. Schematic view of experiments designed to determine whether chemical reaction takes place during the transients in which F-atom concentrations increase: (a) Fatom signal during the transient 1 = steady state; (b) microwave discharge conditions during experiment; (c) NO signal during experiment parallel to that shown in a, where signal = 1 in the absence of F and F,.
TABLE 111: Time Required to Regain One-Half of F-Atom Steady-State Concentration after Stopping Dissociation of Fluorine time to regain time dissociation is stopped, min
50% of F-atom
40
24.5 0.50 2.20
2! 5 10 20 39
concn, min
expt in sequence 1 5
2 3
7.0 8.0
4
12.4
6
of F with NO. If the discharge was restarted, the NO signal gradually returned to its original level. If, however, the F atoms were consumed in a chemical reaction other than recombination, the NO signal would rise significantly be., to 0.5) when the discharge was turned on at t = 5 min. These experimenits show that the transient is due to enhanced atorn recombination. Apparently, the short time that the discharge was off permitted a significant change to take place in the surface layers on the Ni. Without supporting ex,periments,the nature of this surface cannot be defined. The duration of this transient was a function of the history of the surface and also of length of time that the discharge was turned off. Examples of the duration of the transient (timle to regain 1/2 of F-atom signal intensity) as a function of time that the discharge was off is shown in Table 111. The most common impurities in this system could be O2 and HF. The O2 concentration determined by EPR was less than 50 mPa. Small amounts of O2added into the upstream port (Figure 1) did not appear to affect the duration of the transient. HF was removed from the F, by a KF trap, and organics and H20 were removed from the He by passing it, through a coil immersed in liquid nitrogen. The transient shown in Figure 8a was an unanticipated result. The increase in the F-atom signal upon uncovering the Ni was not always observed, although similar periods of time were required to achieve the steady state with the Ni exposed as in the previous case where the microwave discharge was interrupted. Since, in this case, the Ni is covered by the large injector, it would be possible that 0, and HzO can enter the reactor through the seals. If water
Discussion The temperature dependence of y measured in this work gave substantially higher values than those reported in the l i t e r a t ~ r eas , ~Eihown ~ ~ in Figure 5. Several reasons may account for this: the surface treatments were slightly different; the time during which temperature dependencies were recorded as well as the temperature ranges were different in each of the experimental systems; the reactions were not necessarily pseudo first order in fluorine. Particularly important is that in these experiments the Ni surface was pretreated at 800 K with F/Fz mixtures. This may have caused some surface changes which have not yet been defined. A second major difference is that in the experiments carried out here, once a steady-state F-atom signal was attained after the microwave discharge was initiated, the duration of the experiment varied from 8 to 24 h; this is in contrast to those reported in ref 3,28 in which experiments were completed in a few hours. Examination of the data in Figures 4 and 5 shows that the average rate of recombination determined while increasing the temperature is smaller than when decreasing the temperature. While no values of y were obtained between 320 and 460 K in the study reported in ref 3, our studies show extensive hysteresis. The double minimum in the F-atom signal of Figure 4 suggests that there is a change in the surface layer of the Ni reactor between 370 and 390 K. One explanation for the hysteresis is that the kinetics of the surface transformation are more rapid at higher temperatures. For example, consider a slow transformation between two surface configurations (SI and S,) that occurs at high temperature; from the data, y ( S J > y(Sl). The reverse transformation Sz SI occurs at low temperature at a lower rate. This is borne out by the fact that data taken over the range 315-300 K show greater dependence upon time rather than on temperature. Once the reactor is cooled to room temperature and the signal is allowed to return to the initial value, the temperature dependence will follow the curve indicated for the increasing-temperature scans (open circles of Figure 4), but, if the system is rapidly cooled and then reheated, the data fall closer to the decreasing-temperature data (solid circles of Figure 4). Studies on the 1%-F2chemistry have shown some surface reactions that take place. Nickel is particularly resistant to oxidation by Fzup to 1465 KZ9because of formation of a very stable fii,which increases in thickness with time.% Studies31of film-thickness formation in gaseous fluorine at 1-atm pressure indicate film growths of 0.2-0.5 A/h between 300 and 360 K. X-ray and microscopy studies32 of Ni exposed to the F2show that NiF, is formed in fibrous epitaxial films at 800 K. Additional studies33have shown that, at 970-1270 IK,fluorine migrates through the fluoride layer to attack the Ni. Heating a fluorinated Ni surface above 850 K liberates only NiF, until 1100-1200 K, where F and NiF are liberated.34 Extensive amounts of F,, however, are desorbed from the surface up to 1400 K. These results strongly suggest that, at high temperatures, NiF2 is a stable film. At low temperatures, fluorine atoms may also have a greater tendency to oxidize the Ni than does F2 While complex compounds containing Ni(II1) and Ni(1V) are known, NiF3 has never been isolated although it has been
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The Journal of Physical Chemistry, Vol. 84, No. 24, 1980
found in mixtures with NiFz by reacting NiC1, with F2 above 470 K, but no warmer than 500 K for several hours,% and in electrolysis experiments.% The region in which we observe the surface transition is between 400 and 470 K. This suggests that different Ni oxidation states can be on the surface at different temperatures. Thermodynamic calculation^^^ indicate that, once formed, NiF3 should be marginally stable. These reports therefore suggest that at temperatures between 500 and 800 K the main surface film is essentially NiF2. Between 300 and 450 K, in the presence of atomic fluorine, a different surface, possibly NiF3, may form. These surfaces could have different adsorption isotherms. In studies of the adsorption of F, on NiF2,n the nature of the adsorption isotherm changed from that following the BET38equation at temperatures less than 380 K to that of 8 a plln at greater than 420 K. If a more highly fluorinated surface occurs at the lower temperatures, the adsorption of F would be expected to be smaller because of a greater repulsion than on NiF,. The transient observed in which the fluorine-atom signal increased slowly when the discharge was turned on (Figure 8 curve b and Figure 9) is consistent with the above model. It is possible that a chemical reaction takes place on surface S1,which may be an oxidized form of the Ni, to alter the surface perhaps to S2, which is NiF,. During the oxidation of metallic Ni, the fluorine diffuses to the surface of the In the absence of F, there is no impetus for the fluorine to oxidize the NiFP It is therefore possible that the role of F in poisoning the surface is to establish a steady state in which the nickel in the fluorine film is more highly oxidized at the lower temperature. When the fluorine atom source is removed, the film tends toward equilibrium by ionic conduction or diffusion so that, after a few minutes, the surface can be reoxidized. If this is a slow kinetic process, as indicated by the transients and by the slow change in 6 as the temperature is reduced from elevated temperatures to near 300 K, then only a small fraction of the F is consumed in converting the surface. These room-temperature experiments do not give sufficient information to determine whether the transient is due to differences in recombination efficiencies on S1 or S2, or to enhanced recombination on the intermediate surface formed during the slow transient S2 S1 in the presence of F. If this intermediate is of the nature of a mixed-valence compound, it can have different catalytic activity than either S1 or S2. However, in this case, the transient would be expected to show a maximum value in recombination efficiency, which corresponds to a maximum coverage by the intermediate surface. When the nickel was heated rapidly, we were not able to get sufficient reproducibility to determine y vs. T. However, values of y would be greater than those obtained by Jumper et al.3 but smaller than those presented in Figure 5 or Table I. Values of y reported for increasing temperature up to 380 K agree well with the data determined by Arutyunov and Chaikin.4 Extrapolating their high-temperature values along with those of Jumper et aL3 appears to be consistent with the curve that would be expected if there were not a double maximum in y. At low temperatures the reaction was neither pseudo first order nor second order. We did not measure the fluorine atom concentration as a function of the surface exposure of Ni above 470 K because of possible reactions of F with the quartz injector. Increasing the F or F2 concentrations tended to increase the rate of recombination. If one assumes the Rideal mechanism,l the rate coefficient of surface recombination is given by eq 8. If the surface is not completely covered by the reacting atom,
-
Goldberg
then OF may be a strong function of the pressure. Since both F and F, enhance the recombination, it seems likely that F2 is partially dissociated on the surface. Studies of adsorption on NiF, show that the surface coverage is fit 0 = KplIn, Typically this by the Freundlich i~otherrn,,~ represents adsorption on a variety of sites of different energy. However, this particular form, for low surface coverage, is consistent with the equation 0 = Kp1I2obtained for complete dissociation on the surface. The re0 < 20%,and n was typically 1.8, suggest that s u l t that ~~~ there could be dissociative adsorption. The isotherm in which there is partial dissociation of an adsorbed gas and both the diatomic and monatomic species are in the gas phase has a complicated form. In the case in which OF and OF2 are small, using the methods of Laidler,39it can be shown that
where kl, k-l, kd, and k-d are respectively the rate coefficients of adsorption and desorption of atomic fluorine, and dissociative adsorption and recombinative desorption of F,, and Kd is kd/k4. The assumption that the quadratic term in OF is negligible leaves an adsorbtion isotherm of the form 8F iz: ApF + BPFz (20) This changes the surface boundary condition of eq 7 to an equation of the form
where A' = ARTCFO and the dependence of recombination on the gas-phase components may be slightly different. This creates a combination of first- and second-order terms in the rate equation. Modeling with this boundary condition has not been carried out, partially because the variations with pressure are not sufficiently large to define reasonable values. This model, when applied to the reactor, is similar to jumper'^,^ with the exception that F2 is considered to be dissociatively adsorbed and thus to enhance the recombination, rather than to displace F and thus inhibit recombination. More extensive work needs to be done to fully characterize the nature of the nickel fluoride surface and the surface coverage of adsorbed atomic and molecular fluoride. Studies to evaluate the surface must be carried out in the presence of fluorine. Results here indicate that there are two distinct surface conditions that can occur on nickel fluorides in the presence of atomic fluorine and in the temperature range of 300-800 K. Surface Sz is stable at high temperature and is more effective in promoting recombination of atomic fluorine than is surface S1. It appears also that surface S1 is unstable and slowly reverts back to surface S2 in the absence of atomic fluorine. Acknowledgment. I acknowledge helpful discussions with Eric Jumper (U.S. Air Force Academy), Casper Ultee (United Technologies), Karl Christe, and Robert Coombe and the technical assistance of Harry Crowe. The gift of the Ni-200 pipe from Jack Feiffer of Huntington Alloys is gratefully appreciated. References and Notes (1) H. Wise and 9. J. Wood, Adv. At. Mol. Phys., 3 , 291 (1967). (2) P. C. Nordine and J. D. LeGrange, AIAA J., 14, 644 (1976). (3) E. J. Jumper, C. J. URee, and E. A. Dorko, J. Phys. Chem., 84, 41 (1980). (4) V. S. Arutyunov and A. M. Chaikin, Kinet. Catal., 18, 267 (1977).
J. Phys. Chem. 1900, 84, 3207-3210
(5) V. S.Arutyunov arid A. M. Chaikin, Kinet. Catal., 18, 263 (1977): (6) W. Valence, B. Birnng, and D. I.MacLean, Boston College, Boston, MA, Report FRK-116, NTIS AD 732-932, Oct 1971. (7) T. L. Pollack and W. J. Jones, Can. J. Chem., 51, 2041 (1973). (8) P. S.Qnguli and M. Kaufman, Chem. Phys. Lett., 25, 221 (1974). (9) P. C. Nordlne and D. E. Rosner, J. Chem. Soc., Faraday Trans. 1 , 72, 1526 (1976). (10) C. J. Uitee, Chem, Phys. Lett., 46, 366 (1977). (11) E. J. Jumper, personal communication. (12) E. J. Jumper, Ph.D. Dissertation, Air Force Institute of Technology, Wright Patterson Air Force Base, OH, 1975. (13) H. R. Crowe and I. t3. Goldberg, Rev. Sci. Insbum., 49, 1211 (1978). (14) I. B. Goldberg and H. R. Crowe, J. Phys. Chem., 80, 2407 (1976). (15) I. B. Goldberg, H. R. Crowe, and R. S.Carpenter, J. Magn. Reson., 18, 84 (1975). (16) I. B. Goldberg and H. R. Crowe, J. Magn. Reson., 18, 497 (1975). (17) A. A. Westenberg, Prog. React. Kinet., 7, 23 (1973). (18) I. B. Goldberg and A. J. Bard in “Treatlsb on Analytical Chemlstj”, I.M. Kolthoff, P. J. Elving, and M. M. Bursey, Eds., 2nd ed., Wiley, New York, in pres$;. (19) I. B. Goldberg and H. 0. Laeger, J . Phys. Cbem., in press. (20) I. B. Goldberg, J. Magn. Reson., 32, 233 (1978). (21) A. H. Shapiro, “The Dynamlcs and Thermodynamics of Compressible Fluid Flow”, Ronald Press, New York, 1953. (22) R. V. Polrler and R. W. Carr, J . Phys. Chem., 75, 1593 (1971). (23) N. Cohen and J. F. Wott, El Segundo, CA, April 1976, Aerospace Corp. Report SAMSO-TR-76-82; N. Cohen, Supplement to above, June 1978, Rep& SAMSO-TR-7-41; N. W e n and J. F. Bott in ”Handbook
(24) (25) (26) (27) (28) (29) (30) (31) (32) (33) (34) (35) (36j (37) (38) (39)
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of Chemical Lasers”, R. W. F. Gross and J. F. Bott, Eds., Wliey, New York, 1976, Chapter 2. L. S.Blair, W. D. Breshears, and G. L. Schott, J . Chem. Phys., 50, 1582 (1973). T. R. Marrero and E. A. Mason, J . Phys. Chem. Ref. Data, 1, 3 (1972). F. W. Kaufman, Prog. React. Klnet., 1, 1 (1961). N. Watnabe and M. Takashim, Kogvo KagakuL?sSu, 74,321 (1971). C. J. Ultee, private communication. C. E. Holcombe, G. W. Weber, and L. Kovach, Bull. Am. Ceram. Soc., 58, 1185 (1979). 0. L. Hunt, I M, Ritchie, R. J. Esdalle, and J. V. Sanders, J. Catal., 25, 460 (1972). H. W. Schmidt, Washington, D.C., 1967, NASA Report SP-3037. R. Riwan and E3. Auggln, C . R. Hebd. Seances Acad. Scl., Ser. C. 264. 725 (1967). R.’L. Jarry, J.’Fischer, and W. H. Gunther, J. Electrochem. Soc., 110, 346 (1963). J. D. McKinley, J. Chem. Phys., 45, 1690 (1966). L. Steln. J. M. Neil. and 0. R. Alms. Inom. Chem., 8, 2472 (1969). T. L. Court and M. F. A. Dove, J . Chem.-Soc., Dalton Trans., 1995 (1973). M. Barber, J. VV. Linnett, and N. H. Taylor, J . Chem. SOC.,3323 (1961). P. H. Emmett In “Catalysis”, Vol. 1, P. H. Emmett, Ed., Relnhokl, New York, 1954, Chapter 2. K. J. Laidler in ”Catalysis”, Vol. 1, P. H. Emmett, Ed., Reinhold, New York, 1954, Chapters 3 and 4.
Heterogeneous Photocatalytic Oxidation of Hydrocarbons on Platinized TiOp Powders Ikulchiro Iruml, Wendell W. Dunn, Keith 0. Wllbourn, Fu-Ren F. Fan, and Allen J. Bard” Department of Chemlstv, The Unlverslty of Texas at Austln, Austin, Texas 78712 (Recelved: July IS, 1980)
The photodecompositionof hydrocarbons in oxygen-containing solutions at platinized Ti02yields predominantly C02as the reaction product, with intermediate production of hydroxylated compounds. A mechanism for the reaction based on photogeneration of hydroxyl radicals at the Ti02 surface is proposed.
Introductiom Recent investigations from this laboratory have described the application of platinized titanium dioxide powders (Pt/Ti02) to heterogeneous photocatalytic and photosynthetic prwewes, such as the photo-Kolbe reaction, in which aliphatic mlono-l and dicarboxylic2acids are decomposed to give thie corresponding alkane as the main product, as well as other reaction^.^ These investigations have clearly demonstrated the usefulness of Pt/Ti02 in photoelectrochemica1 experiments and the potential for accomplishing reactions requiring highly oxidizing conditions. In previous works we found that the decarboxylation of benzoic acid at Pt/Ti02 involved the preceding hydroxylation of the benzene ring to form hydroxylated benzoates as reaction intermediates.2 However, the main product of this reactiion was Cog,suggesting that photodecomposition of hydrocarbon intermediates was possible. Thus in this paper wte extend these studies to the photocatalyzed decomposition of hydrocarbons at Pt/TiOa. Phenol is shown to be a reaction intermediate for the photodecomposition of benzene, with the final product COP. This unusual and efficient photocatalyzed breakdown of benzene a t room temperature to yield C02 thus provides a possible pathway in the decarboxylation mechanism proposed for the photooxidation of benzoic acid.2 We also describe the photodecomposition of several aliphatic hydrocarboins, in which alcohols have been detected as intermediates. 0022-3654/80/2084-320?$01 .OO/O
Experimental Section Materials. Benzene (reagent grade, Matheson Coleman and Bell (MCB)), hexane (MCB, 99’ mol %), cyclohexane (spectrophotometricgrade, MCB), heptane (reagent grade, Eastman), nonane and decane (Fisher Certified), barium hydroxide (reagent grade, MCB), and phenol (reagent grade, Fisher) were used without further purification. Decanol, nonanol, heptanol, hexanol, and cyclohexanol (reagent grade, ‘Eastman) were used as received for standards in GC--mass spectroscopic analysis. The kerosene was extracted repeatedly with distilled water before illumination to simplify subsequent analysis of the aqueous phase. All solvents and other chemicals were reagent or spectrophotometric grade and were used without further purification. The platinized Ti02powders were prepared by photodecomposition of hexachloroplatinic acid onto Ti02 powders (reagent grade, MCB, 125-250 pm) and contained 10% platinum by p eight.^ Apparatus. The irradiation source for the photodecomposition of aliphatic hydrocarbons was an Atlas Weatherometer, Model 6000, equipped with a 6OOO-W Xe lamp. The described experiments were performed at an unfocused power output of 5000 W, which corresponds to a total radiant intensity of -35 mW/cm2 a t the location of the reaction cell. The Xe lamp was jacketed with a water cooling cell to remove infrared irradiation. A constant reaction temperature was maintained with a large water bath fitted with a coil for water cooling. A 2500-W @ 1980 American Chemical Society
