DEFECT STRUCTURE OF CUPROUS IODIDE AND ITS CATALYTIC PROPERTIES 4517

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 com- ponent being taken each time for resublimation until dark ruby red crystals were obtained. I ts purity was then established by carbon-hydrogen analysis, melting point, mass spectrometry, polarography, and fluores- cence 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 electro- chemical 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 V- 4532 dual-sample cavity, and the low field splitting of

this spectrum was taken to be 13.0 gauss. The spectra were recorded on a Moseley 7100B dual channel re- corder.

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 pre- viously.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 to 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 electro- chemical cell). The hole density has been found to control the rate of cat,alytic decom- position 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 OC-

curring on its surface are of fundamental importance in heterogeneous catalysis. In principle, an electro- chemical ionic semiconductor 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-

a (1) Support of this research by a group of industrial sponsors is grate- tuliyacknowledged.

Volume 71, Number 13 December 1967

4518 HENRY WISE AND BERNARD J. WOOD

trons or holes) may be adjusted without introducing any foreign chemical species as occurs, for example, dur- ing doping. This technique has been applied success- fully 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 em- ploy 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 conduc- t i ~ n . ~ v ~ - ~ At temperatures above 525"K, ionic con- duction predominates in samples equilibrated with copper. Thus by selecting the solid electrolyte gal- vanic cell, Cu(s)ICuI(s)/Pt(s), the chemical potential of the copper at the surface or a t 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 impos- ing a positive potential on the Pt electrode and passing a current across the cell, copper ions in the CUI will

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 po- tential of copper in the cuprous iodide electrochemical cell will be modified due to the interaction of I, with the solid. Results of a preliminary study14 showed 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.

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~ombs passed

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 . _.

through the cek3 The process may be written as

Cuf 4 cuo + p + v (1)

where Cuo represents metallic copper, p a hole carrier (defect electron), and V a lattice vacancy. Alterna- tively, 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 CUI to 1 2 vapor. Indeed a number of publicat,ions have demonstrated the effect of Iz on the electrical conduc- tivity of cuprous i ~ d i d e . ~ ~ ~ * " J * ~ ~ At low Iz pressures, the conductivity of the copper iodide has been found to vary as the fourth root of the iodine vapor pressure in

in ref 3. In a die designed to accommodate the metal elec-

trodes, 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); 61, 377 (1928). (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).

The Journol of Physical Chemietry

DEFECT STRUCTURE OF CUPROUS IODIDE AND ITS CATALYTIC PROPERTIES 4519

cu 4 ELECTRODE 0

/.---I-

C PI ELECTRODE

0

10'1

10-2

- t \ -1

E.43.5

TOP VIEW

Figure 1. Geometry of catalyst pellet.

I crn3 He SAMPLE I /

MULTIPORT {. 10.5

1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.1 (YT) ( O K ) x 103

Figure 3. Catalytic decomposition of CrHd &s a function of temperature (- , homogeneous ratel*; -0-, 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 cur- rent 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 tem- perature of a water bath surrounding a saturator through which helium was bubbled. A schematic dia- gram 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 con- t,ained in the sample loop was swept over the catalyst, through a trap kept a t 190"K, and into the gas-chro- matographic column containing silica g e P maintained at 420°K. The mass of products (C3Ha and 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 HENRY WISE AND BERNARD J. WOOD

published for the vapor-phase decomposition of iso-

of interest the heterogeneous reaction exceeds the -10

-20 2 m u propyl iodide,13 we noted that in the temperature region

homogeneous one by an appreciable factor eo that the 0

- -

- -

I I

- Preteatment of the catalyst with iodine vapor demon-

strated 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-C3H71 to CaH6 decreased 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-C3H71 vapor preceding the injection of a sample of i-C3H7I into the pulse microreactor (Table I). However, in this case the formation of C3H8 in measurable quantities became detectable.

Table I: on Catalyst Properties" at 625°K

Effect of GCIH~I Pretreatment

Electrical conduc- tivity. Products, mole X 10s

Pretreatment ohm-' CsHa CiHs

None 0 .25 0.70 0 i-CaHd (15 min) 1 .2 0.45 0.10 i-CsH,I (15 min) 1 . 0 0.40 0.08 ~ - C I H ~ I (15 min) 1 . 0 0.43 0.10

Sample size of i-CIH7I = 3.75 X 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 elec- trode as the anode). In the cathodic case, little change in catalytic properties was noted when a catalyst de- pleted in Cu+ ions was exposed to a sample of isopropyl iodide. The absence of any effect undoubtedly re- sulted from the overwhelming contribution of the

chemical reaction occurring on the surface of the cata- lyst, 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 electri- cal conductivity with time associated with the catalytic reaction (Figure 5) . However, under anodic condition (positive Cu electrode) a marked reduction in con- ductivity due to the approach to a stoichiometric Cu+/I- ratio was observed. At the same time the influence of the electrically modified solid-state proper- ties 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

Electrical Polarity of conduc-

copper tivity, Products, mole X 106 electrode ohm-' CiHa CaHs

Negative 1 .3 0.46 0 Positive 0.09 0.76 0 Positive 0.10 0.78 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

The Journal of Physical Chemistry

DEFECT STRUCTURE O F CUPROUS IODIDE AND ITS CATALYTIC PROPERTIES 452 1

I .5 I I I I I I 1 I

1."

0 1 2 3 4 5 6 7 8 9 TIME - rnin

Figure 5. Electrical conductivity changes associated with catalytic reaction.

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. Modi- fication 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 con- ductivity takes place, undoubtedly due to the injection of holes into the solid. The change in catalytic reac- tion 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 prop- erties of the catalyst can be brought about by applica- tion 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 iso- propyl iodide. The evidence points to the electronic hole carriers as the species responsible for the catalyst modification. It is apparent that field-enhanced migra- tions 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

40

- 30 ap > - z g 20 a

" 10

w > z 0

0 0 IO 20 30 40 50 60

EXPOSURE TIME (min)

Figure 6. during continuous exposure to i-CsH7I at 495"II.

Product distribution from catalyzed reaction

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 direc- tion. 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 gradi- ent. 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 injec- tion and the formation of the propyl radical (a). Further reaction with I- on the surface of the solid yields propylene by hydrogen abstraction (b). How-

Volume 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 forma- tion 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 energy- independent 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 ion- molecule reaction product, the phosphonium cation, PH4+, was discovered by Giardini-Guidoni and Volpi.'O The purpose of the present work is to provide a mech- anism 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 4 PHI+ + PH2 (or PH4+ + P H + H) (1)

PH2+ + PH3 + PH4+ + P H

PH+ + PH3 + PH4+ + P (2)

(3) PH3* + PHx + PH4+ + PHz-

(or PH4+ + PH2 + e-) (4)

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); 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).

J. Fischler and M.

I n (1) to (3) the reactive species is an ion, while in (4) i t

species such as PH2 and PH can also not be excluded).

(9) H. Ebinghaus, K. Kraus, W. MClller-Duysing, and H. Neuert, z . Naturforsch., 19a, 732 (1964). is an excited molecule Of phosphine (excited (10) A, Gairdini-Guidoni and (7. G. volpi, Nwvo Cimedo, 17, 919 (1960).