DEFECTSTRUCTURE OF CUPROUS IODIDE AND ITSCATALYTIC PROPERTIES
calculations are approximately correct, it would have a small effect on the esr spectra.
Experimental Section Rubrene was obtained from K & K Chemical Co. It was vacuum sublimed five times with the purest component being taken each time for resublimation until dark ruby red crystals were obtained. Its purity was then established by carbon-hydrogen analysis, melting point, mass spectrometry, polarography, and fluorescence spectrometry. The methods of purification of DMEG and DMFI4 have been previously described. Methylene chloride was obtained from Aldrich Chemical Co. and used as received as its purity was established by other electrochemical studies.I5 A Varian Associates V-4502 spectrometer employing 100-kc field modulation was used. The field sweep was calibrated by using Fremy’s salt in one side of a V4532 dual-sample cavity, and the low field splitting of
4517
this spectrum was taken to be 13.0 gauss. The spectra were recorded on a Moseley 7100B dual channel recorder. The HMO calculations were done on a Control Data Corp. 6600 computer and were then plotted on the CDC 160 plotter. The electrochemical cell has been described previously.6 Acknowledgments. The support of this research by the National Science Foundation (Grant No. GP-1921) is gratefully acknowledged. The esr spectrometer was purchased with funds provided by the National Science Foundation (Grant No. GP-2090). We are extremely grateful t o Dr. K. S. V. Santhanam for his advice and assistance in the electrochemical aspects of this study. (14) K. S. V. Santhanam and A. J. Bard, J. Am. Chem. SOC.,88, 2669 (1966).
(15) J. Phelps, K. S. V. Santhanam, and A. J. Bard, ibid., 89, 1752
(1967).
The Defect Structure of Cuprous Iodide and Its Catalytic Properties’
by Henry Wise and Bernard J. Wood Solid-state Catalysis Laboratory, Stanjord Research Institute, Menlo Park, California (Received June 26, 1967)
Changes in the defect structure of cuprous iodide have been generated by chemical means (exposure to iodine) and by ion migration in an applied electrical field (solid-state electrochemical cell). The hole density has been found to control the rate of cat,alytic decomposition of isopropyl iodide to propylene and propane. Low defect concentration seems to favor propylene formation. The kinetics of the reaction are measured. The mechanism leading to the observed product distribution is discussed.
Introduction The relationship between the defect structure of a solid and the kinetics and mechanism of reaction OCcurring on its surface are of fundamental importance in heterogeneous catalysis. In principle, an electroionic semiconductor chemical a allows control of the chemical potential of its con-
stituents by ion migration in an applied electrical field. In this way the activity ratio of cations t0 anions may be altered (within the limits of the homogeneity range of the solid) and the defect concentration (excess elec(1) Support of this research by a group of industrial sponsors is gratetuliyacknowledged.
Volume 71, Number 13 December 1967
4518
HENRYWISE AND BERNARD J. WOOD
trons or holes) may be adjusted without introducing any foreign chemical species as occurs, for example, during doping. This technique has been applied successfully to a study of the kinetics of reduction of metal sulfides as a function of the metal-to-sulfur ratio.2 In our study of a surface-catalyzed reaction, we employ cuprous iodide as a catalyst. The solid-state properties of this material are profoundly affected by small deviations from the ideal stoichiometric ratio. In the absence of excess halogen, ionic conduction through copper ions prevail^.^-^ A deficit of copper brought about by excem halogen causes hole conduction (p-type conductivity) in addition to ionic conduct i ~ n . ~ v At ~ - ~temperatures above 525"K, ionic conduction predominates in samples equilibrated with copper. Thus by selecting the solid electrolyte galvanic cell, Cu(s)ICuI(s)/Pt(s), the chemical potential of the copper at the surface or at some point in the copper iodide catalyst may be altered by application of an emf of suitable magnitude and direction between the two electrodes embedded in the solid. On imposing a positive potential on the Pt electrode and passing a current across the cell, copper ions in the CUI will move toward the copper cathode and electrons will be transferred to the platinum anode. On changing polarity the opposite effect may be produced. By this means the ratio of Cu+ to I- may be altered, and for an ionic conductor the degree of change of stoichiometry may be obtained from the number of cou~ombspassed through the cek3 The process may be written as Cuf
4cuo
+p +v
(1)
where Cuo represents metallic copper, p a hole carrier (defect electron), and V a lattice vacancy. Alternatively, cuprous iodide containing a deficit of copper may be described as a solid solution of divalent copper ions in CUI. Hole injection into the catalyst may also be brought about by exposure to gaseous Ip,because of the reaction Iz(g)
2cu+
+ 21- + 2p + 2v
(2)
Le., extension of the lattice due to the incorporation of I- ions. These considerations lead to the interesting conclusion that the changes in the electronic properties resulting from an applied potent,ial to an electrochemical cell with CUI as t,he solid-state electrolyte can also be brought about by chemical means, i.e., exposing the CUIto 1 2 vapor. Indeed a number of publicat,ionshave demonstrated the effect of Iz on the electrical conductivity of cuprous i ~ d i d e . ~ ~ ~At * "low J * ~Iz~ pressures, the conductivity of the copper iodide has been found to vary as the fourth root of the iodine vapor pressure in The Journol of Physical Chemietry
agreement with the assumption that the holes formed are free.'@ The chemical reaction chosen for our investigation is the catalytic decomposition of isopropyl iodide. This reaction also proceeds in the gas phase by a homogeneous mechanism and produces the following products 2i-C3H,I
C3H6
+ C3Hs + I2
(3)
The kinetics of this reaction have been studied in the gas p h a ~ e . ' ~ If ~ ' ~catalytic decomposition of i-C3H71 over cuprous iodide yields I2 as one of the products, it is to be expected that the steady-state chemical potential of copper in the cuprous iodide electrochemical cell will be modified due to the interaction of I, with the solid. Results of a preliminary study14showed that both exposure to isopropyl iodide and imposition of an electric field in a pellet of cuprous iodide affected the distribution of products obtained from the catalytic decomposition of the iospropyl iodide. I t was of interest, therefore, to examine in detail the mutual interaction of the catalytic reaction and the defect structure of the catalytic surface.
Experimental Details The cuprous iodide used as
8
catalyst in Our expefi-
merits was Prepared by exposing turnings of Copper (99.99% purity) to resublimed iodine vapor at elevated
temperatures in accordance with a procedure described . _. in ref 3. In a die designed to accommodate the metal electrodes, the CUI powder was pressed into a pellet (at 4400 psi). The geometrical details of the catalyst are shown in Figure 1. The central electrode consisted of a copper wire (0.030 in. in diameter). The platinum electrode, in the shape of a wire-mesh cylinder, was embedded in the pellet close to the surface. The geometric surface area of the solid catalyst was 11 cm2. This cell was similar to that employed in the earlier (2) H. Kobayashi and C. Wagner, J . Chem. Phys., 26, 1609 (1957). (3) J. B. Wagner and C. Wagner, ibid., 26, 1597 (1957). (4) C. Tubandt, E. Rindtorff, and W. Jost, 2. Anorg. Allgem. Chem., 165. 195 (1927). (5) J. N . Frers, Ber. Deut. Chem. Ges., 60, 864 (1927); (1928).
61, 377
(6) K. Nagel and C. Wagner, 2.Physik. Chem., BZ5, 71 (1934). (7) R. J. Maurer, J. Chem. Phys., 13, 321 (1945). (8) B. H. Vine and R. J. Maurer, 2. Physik. Chem., 198, 147 (1951). (9) C. Wagner, J . Chem. Phys., 18, 62 (1950). (10) K. Nagel and C. Wagner, Z . Physik. Chem., B25, 71 (1934). (11) K . Weisa, ibid., 12, 68 (1957). (12) 9. W. Benson, J . Chem. Phys., 38, 1945 (1963). (13) H. Teranishi and S. W. Benson, ibid., 40, 2946 (1964). (14) H. U. D. Wiesendanger, J. Catalysis, 7, 283 (1967).
DEFECTSTRUCTURE OF CUPROUS IODIDE AND ITSCATALYTIC PROPERTIES
4519
10'1
C PI
ELECTRODE
0 cu 4 ELECTRODE
0 /.---I10-2
-
t
-1
\ E.43.5
TOP VIEW
Figure 1.
Geometry of catalyst pellet.
He
I crn3 SAMPLE
MULTIPORT
{.
I
/
10.5
1.4
1.5
1.6
(YT)
1.7
1.8 (OK)
1.9
2.0
2.1
x 103
Figure 3. Catalytic decomposition of CrHd &s a function , homogeneous ratel*; -0-, of temperature (heterogeneous rate). Values of e in kcal/mole.
Figure 2. Schemrttic diagram of apparatus.
study.14 The potential between the electrodes was measured with a vacuum tube millivoltmeter, the current with an electrometer. The experimental apparatus was designed so that it would operate as a continuous flow system or a pulse microreactor (Figure 2). The catalyst was located in a Pyrex reactor (25 ml in volume) surrounded by a cylindrical furnace. The vessel was provided with leads to establish electrical contact with the electrodes embedded in the pellet. The reactant, isopropyl iodide (Eastman-Kodak reagent grade), was introduced into the helium carrier gas by a vapor transpiration method. The partial pressure of reactant was adjusted by varying the temperature of a water bath surrounding a saturator through which helium was bubbled. A schematic diagram of the apparatus is given in Figure 2. When the multiport valve, A, was actuated from the position
shown to the operating mode, the reactant aliquot cont,ained in the sample loop was swept over the catalyst, through a trap kept at 190"K, and into the gas-chromatographic column containing silica g e P maintained at 420°K. The mass of products (C3Haand CaHs)was measured quantitatively by means of a conductivity bridge. The total residence time of reactant in the reactor volume was estimated to be about 20 sec under our experimental conditions. With the aid of a second multiport valve (B in Figure 2), the apparatus could be operated in a continuous-flow mode during which the catalyst could be exposed to reactants for various lengths of time. In this manner, the variation in catalytic properties was investigated as a function of exposure time, reactant concentration, temperature, and applied electrical field.
Experimental Results Pulse Microreactor Experiments. By exposing the catalyst to individual aliquots containing different (15) Davidson 08-30/60 mesh.
Volume 71, Number 13 December 1967
4520
HENRYWISEAND BERNARD J. WOOD
published for the vapor-phase decomposition of isopropyl iodide,13we noted that in the temperature region of interest the heterogeneous reaction exceeds the homogeneous one by an appreciable factor eo that the
-20
-
-
u
-
-
2m
-10 0
I
I
-
Preteatment of the catalyst with iodine vapor demonstrated a measurable change in catalytic activity. The exposure to iodine led to a gradual enhancement in electrical conductivity due to hole injection (eq 2). At the same time the degree of catalytic conversion of i-C3H71to CaH6decreased markedly as can be seen from the data presented in Figure 4. A similar trend in catalytic behavior was noted on prolonged exposure of the catalyst to i-C3H71vapor preceding the injection of a sample of i-C3H7I into the pulse microreactor (Table I). However, in this case the formation of C3H8in measurable quantities became detectable.
Table I: Effect of GCIH~IPretreatment on Catalyst Properties" at 625°K
Pretreatment
Electrical conductivity. ohm-'
None i-CaHd (15 min) i-CsH,I (15 min) ~ - C I H ~(15 I min)
0.25 1.2 1.0 1.0
Sample size of i-CIH7I = 3.75 X
Products, mole X 10s CsHa CiHs
0.70 0.45 0.40 0.43
0 0.10 0.08 0.10
mole.
Modification of the cat,alyst by means of an applied emf was carried out, both in terms of hole injection (Cu electrode as the cathode) and hole depletion (Cu electrode as the anode). In the cathodic case, little change in catalytic properties was noted when a catalyst depleted in Cu+ ions was exposed to a sample of isopropyl iodide. The absence of any effect undoubtedly resulted from the overwhelming contribution of the The Journal of Physical Chemistry
chemical reaction occurring on the surface of the catalyst, which by itself caused local hole injection of much larger magnitude than that brought about by Cu+ migration, as can be seen from the variation in electrical conductivity with time associated with the catalytic reaction (Figure 5 ) . However, under anodic condition (positive Cu electrode) a marked reduction in conductivity due to the approach to a stoichiometric Cu+/I- ratio was observed. At the same time the influence of the electrically modified solid-state properties on the catalytic reaction could be det,ected (Table 11). In order to achieve the physical conditions of the catalyst for which the data are presented, a positive emf of 300 mv was imposed on the Cu electrode for 24 hr preceding injection of the isopropyl iodide sample. It should be noted that in all our experimental studies the applied voltage was kept below the decomposition potential3 of CUI (E < 0.3 v). Table 11: Effect of Emf on Catalytic Activity a t 625°K Polarity of copper electrode
Electrical conductivity, ohm-'
Negative Positive Positive
1.3 0.09 0.10
Products, mole X 106 CiHa CaHs
0.46 0.76 0.78
0 0 0
Continuous-Flow Reactor. The observation of the modification of catalytic properties by exposure to i-C3H71 led to a more detailed invest,igation of this phenomenon. For this purpose, the continuous flow reactor was employed which allowed pretreatment of
DEFECTSTRUCTURE
I .5
I
I
OF
CUPROUS IODIDE AND ITSCATALYTIC PROPERTIES
I
I
I
I
1
452 1
40
I
ap
30
>
z
ga
20
w
> z
0
"
10
1."
0
1
2
3
4 TIME
5
- rnin
6
7
8
9
Figure 5. Electrical conductivity changes associated with catalytic reaction.
0 0
IO
20
30
EXPOSURE TIME
40
50
60
(min)
Figure 6. Product distribution from catalyzed reaction during continuous exposure to i-CsH7I at 495"II.
the catalyst for prescribed lengths of time with i-CaH71 and analysis of the reactor effluent. A typical result of such experimental measurements is shown in Figure 6 . Two important observations can be made: (a) the formation of propylene goes through a maximum and (b) the formation of propane occurs after a short induction period. It is apparent that the intermediates or products of reaction have a profound influence on the catalyst and on the course of the reaction. Modification of the electronic properties of the catalyst is apparent from an inspection of the current-voltage characteristics during the continuous exposure of the catalyst to isopropyl iodide. A large increase in conductivity takes place, undoubtedly due to the injection of holes into the solid. The change in catalytic reaction rate and product distribution caused by chemical means can be further augmented by ion migration in an applied electrical field as reported in ref 14 where formation of C3He and C3Hs followed a similar pattern to that shown in Figure 6.
Discussion The experimental results obtained in our studies lead to the conclusion that changes in solid-state properties of the catalyst can be brought about by application of an electrical field to the catalyst and also by chemical reaction with one of the intermediates or products formed in the catalytic decomposition of isopropyl iodide. The evidence points to the electronic hole carriers as the species responsible for the catalyst modification. It is apparent that field-enhanced migrations of Cu+ toward one of the electrodes result in changes of solid-state properties analogous to those encountered in contacting the catalyst with iodine vapor (eq 1 and 2). However, modification of the surface defect structure by electrochemical means is a much slower process than modification by chemical surface reaction with iodine. In the presence of an applied
voltage, with the Cu electrode as the cathode, copper ions move away from the region of the Pt, electrode (Figure 1) while electrons move in the opposite direction. As a result, a copper deficit originates at the Pt electrode. However, the movement of copper ions due to the electrical field is counterbalanced by mass diffusion of copper ions due to the concentration gradient. When steady-state conditions prevail, the current is carried entirely by electronic carriers, ie., excess electrons and holes. Excess electron conduction is negligible under our experimental conditions. The pronounced effect of the hole carrier density on the catalytic properties may be interpreted in terms of a reaction mechanism. It appears that cuprous iodide of low conductivity, i.e., a relatively small excess of iodine above the stoichiometric ratio, favors the production of propylene. However, as the hole density is increased the formation of propylene is suppressed. A tentative mechanism is proposed which takes into account these experimental observations
According to this reaction scheme the initial step in the decomposition of isopropyl iodide leads to hole injection and the formation of the propyl radical (a). Further reaction with I- on the surface of the solid yields propylene by hydrogen abstraction (b). HowVolume 71, Number 13 December 1067
4522
M. HALMANN AND I. PLATZNER
ever, with increasing hole densities, step d may begin to compete for the I-(s) and, thereby, reduce the formation of the olefin as a primary product. One would expect, therefore, that the production of propylene as a function of hole-carrier density would go through a maximum. Similarly, the production of propane (step
c) is governed by the hole-carrier density on the catalyst surface, since its formation depends on intermediates which can be depleted by competitive reactions. It is apparent therefore that the defect structure of the solid plays an important role in the kinetics and mechanism of the catalytic reaction.
Ion-Molecule Reactions of Phosphine in the Mass Spectrometer
by M. Halmann and I. Platzner Isotope Department, The Weizmann Inatitute of Science, RehoEoth, Israel
(Received June 88, 1967)
The production of the ion PH4+ in the electron-impact mass spectrum of phosphine was shown by appearance potential measurements to be due to the ion-molecule reaction, PH3+ PH3+ PH4+ PH2. From the dependence of the intensity of the secondary ion peak PH4+ on the gas pressure and the repeller potential in the ion source, the energyindependent factor of the reaction cross section 139 X 10-l6 cm2 molecule-' (v/cm)"' and the second-order rate constant 9.8 X 10-lo cm molecule-' sec-' were derived.
+
+
Introduction The electron impact mass spectrum of phosphine was reported previously in several An ionmolecule reaction product, the phosphonium cation, PH4+,was discovered by Giardini-Guidoni and Volpi.'O The purpose of the present work is to provide a mechanism for the production of the phosphonium cation. One or several of the following reactions may be proposed to account for the formation of PH4+ PHI+
+ PH3
4PHI+
+ PH2
+ P H + H) PH2+ + PH3 +PH4+ + P H P H + + PH3 +PH4+ + P PH3* + PHx +PH4+ + PHz(or PH4+ + PH2 + e-) (or PH4+
(1) (2)
(3) (4)
I n (1) to (3) the reactive species is an ion, while in (4) it is an excited molecule Of phosphine (excited species such as PH2 and PH can also not be excluded).
The ion-molecule reactions 1 to 3 can be differentiated from the chemiionization reaction (4) by measurements of appearance potentials, pressure, and ion-repeller dependence.
Experimental Section Most experiments were performed using an Atlas (1) H. Neuert and H. Clasen, Z . Naturforsch., 71, 410 (1952); 0. Rosenbaum and H. Neuert, ibid., Pa, 990 (1964). (2) "Mass Spectral Data," American Petroleum Institute, Research Project 44, No. 1219 (1955). (3) D. P. Stevenson, Radiation Res., 10, 610 (1959). (4) M. Halmann, J. Chem. SOC.,3270 (1962); J. Fischler and M. Halmann, ibid., 31 (1964). (5) F. E. Saalfeld and H. Svec, I W T ~Chem., . 2 , 46 (1963). (6) A. A. Sandoval, H. C. Moser, and R. W. Kiser, J . Phys. Chem., 67, 125 (1963). (7) Y.Wada and R. W. K. Kiser, I m r g . Chem., 3 , 174 (1964). (8) N. V. Larin, G. G. Devyatykh, and I. L. Agafonov, Zh. Neorg. Khim., 9, 205 (1964). (9) H. Ebinghaus, K. Kraus, W. MClller-Duysing, and H. Neuert, z . Naturforsch., 19a, 732 (1964). G.volpi, Nwvo Cimedo, 17, 919 (10) A, Gairdini-Guidoni and (1960). (7.
