JOHND. COTTONAND PETERJ. FENSHAM
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Magnetic Susceptibility Changes during the Adsorption of Oxygen and Carbon Monoxide on Cuprous Oxide
by John D. Cotton and Peter J. Fensham Department of Ch.emistry, University of Melbourne, Parkville, Victoria, Australia
(Received October l.dv1068)
Measurements of the magnetic susceptibility of cuprous oxide films on copper powder have been made during the adsorption of oxygen and carbon monoxide. A paramagnetic form of ionized oxygen appears and reaches an equilibrium coverage which is less than a monolayer. KOchange in susceptibility occurs when carbon monoxide adsorbs on an evacuated surface of the oxide, but on a previously oxygenated surface, carbon monoxide adsorbs and reacts with some of the oxygen and a t the same time the paramagnetism is destroyed. The implications of these results for the nature of the adsorbed species are discussed.
The processes occurring during the reaction of oxygen and carbon monoxide on the surface of cuprous oxide have been studied by numerous experimental techniques.'-j From these results it is knoFn that oxygen chemisorbs on a thin layer (100-500 A,) of cuprous oxide to something more than a monolayer coverage and that the reaction involves transfer of electrons from the solid to the oxygen. The positive holes left behind in the surface region of the solid give rise to enhanced conductivity, and the system is a classic case of what has been called cumulative adsorption.'j It is also known that such a surface can be regenerated for further absorption by treating in vacuo at 470°K., and during this anneal the extra oxygen becomes part of the oxide layer by the normal mechanism for oxidation a t this temperature. Some earlier attempts to explore this adsorption reaction had shown that there was no change in the magnetic state of the system after the cycle of adsorption and oxidation was complete in accord with reproducible chemical properties of the system.7~8 However, owing to insufficient sensitivity,' the question was left unanswered as to whether a paramagnetic species appears on the surface during the cycle. In the present investigation, a refined technique does lead to evidence of a paramagnetic intermediate.
Experimental The copper was prepared by reduction of cupric hydroxide with hydrazine. This preparation has also The Journal of Phwical Chemistry
been used in earlier magnetic studies of the copperoxygen system7v8and measurements a t varying field strengths and by e.p.r. techniques have shown that the samples were suitably free of ferromagnetic impurities. After final reduction of the surface with hydrogen a t 450°K., it was allowed to oxidize under controlled conditions until oxide films of appropriate thickness (100-300 d.) were obtained. The surface area of these powdered preparation$ ranged from 2 to 4 m.2 g.-1 measured by B.E.T. adsorption of krypton at 77OK. A thin glass bucket (0.8 cm. diameter X 3.0 cm.), perforated with tiny holes, was loosely packed with 1.5 g. of the powdeied copper, and suspended from one arm of a Gulbransen type ba1ance.O The balance beam was constructed from quartz rod (0.18 em. diameter), and it was supported by tungsten wire (0.005 em. diameter). On the other arm of the balance were hung glass counterweights or a small iron dust core surrounded by a coarse and a fine solenoid. In the (1) W. E. Garner, T . J. Gray, and F. S . Stone, Proc. Roy. Soc. (London), A197, 294 (1949). (2) W. E. Garner, F. S. Stone, and P. F. Tiley, ibid., A211, 472 (1952). (3) T. J. Jennings and F. S . Stone, Advan. Catalysis, 9,441 (1957). (4) E. R. S. Winter, ibid., 10, 196 (1958). (5) A. W. Czanderna and H . Wieder, J. Phys. Chem., 66, 816 (1962). (6) K. Hauffe, Advan. Catalysis, 7, 213 (1955). (7) P. J. Fensham, Thesis, Bristol University, 1952. (8) M. 0. O'Keefe and F. S.Stone, Proc. Roy. SOC.(London), A267, 501 (1962). (9) E. A. Gulbransen, Rev. Sci. Instr., 15, 201 (1944).
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M A G N E T I C S U S C E P T I B I L I T Y C H A N G E S D U R I N G ADSORPT1:ON
former situation the balance was used as a deflection instrument covering a small range of weight, and the position was obsewed with a traveling microscope. I n the latter situation the balance was used as a nul1 instrument and weight changes (real and magnetic) were observed as current changes applied to the solenoids to restore the beam to its original position. The sensitivity of this balance was 3.0 pg. per 0.01 mm. deflection with a load of 5-6 g., while above a load of 8 g. the balance became unstable. Under deflection conditions the maximum weight change was 4 mg. The whole balance was enclosed in a glass envelope with a flat window for observation and connected to a vacuum and gas handling line. The Gouy method was employed for measuring susceptibilities and a permanent magnet with shaped pole pieces (2.0 cm. diameter and 1.5 cm. gap) provided the magnetic field. At the center of the gap the field strength was 4850 oersteds and 2.5 cm. from its center it was 1230 oersteds. With the sa,mple tube placed reproducibly with its two ends in these respective fields, the position was maintained during weight changes by coupling the permanent magnet to the travelling microscope. The magnet could be rotated away from the sample on a hinge thereby providing an “on-off” switch for the magnetic field. The minimum volume susceptibility ( K ) which could be detected c.g.s. unit. was thus 0.0005 X Oxygen was prepared by heating A.R. grade KMn04 and purified through KOH pellets and a liquid air trap. Carbon monoxide was prepared by reacting 90% formic acid with A.R. grade sulfuric acid in vacuo followed by purification over solid KOH, heated copper, sputtered sodium, and a liquid air trap.
earlier observations of Jennings and Stone.8 A series of adsorptions were made a t 295°K. in which the weight of the sample was measured as a function of time with the magnetic field alternatively “on” and “off.” Typical results are shown in Fig. 1. The difference in the observed weight, AW, between the two curves represents the increase in paramagnetism of the sample during the adsorption time, and this paramagnetic increase rises sharply to a maximum value which is reached before the adsorption is complete, and sometimes before even a coverage of a monolayer (assuming dissociation) is reached (Fig. 2). After this constant value, AW,, had been reached, evacuation for several hours had little effect, but on raising the temperature for a short time, the susceptibility a t 295°K. rapidly fell to a value iridistinguishable from the magnetic state of the sample prior to adsorption. Variation of the oxygen pressure in the system changed the constant value, AW,, and also the point on the pure uptake curve, W,, at which the constancy set in. The ratio, AW,: W,, however remained
*F
e;a 900
Results I n expressing the results of adsorption experiments in terms of surface coverages, it is assumed2 that each copper site in the surface can be regarded as a potential adsorption center, and that there are 5.2 X 10l8 such sites per m.2 which is the mean figure for the site density in the three principal planes (loo), (110), and (111). Adsorption of Oxygen. Preliminary experiments on a sample with an oxide layer showed that the uptake of oxygen a t room temperature could be readily measured gravimetrically and the weight changes corresponded to the manometric changes in the system. The surface was regenerated by heating in vacuo a t 450°K. for 12 hr., but evacuation at 520°K. led to sintering and loss of activity. No desorption was detected on reducing the pressure a t 295”K., or when the temperature was raised to 450°K. The kinetics and the extent of the uptake agreed with the
L
I
1
10
20
a
*
30 40 Time, min.
I
50
1
60
Figure 1. Weight increases during adsorption of oxygen on cuprous oxide a t 295’K.: 0, weight of oxygen adsorbed with magnet field off; A, apparent weight with magnetic field on.
constant over the pressure range studied as shown in Table I. This ratio is a measure of the apparent weight increase due to paramagnetic species relative to the real weight of the oxygen adsorbed by the time the constancy is established. Adsorption of Carbon Monoxide. When an evacuated oxide surface was exposed to CO a t 295”K., adsorption Volume 68, Number 6 Mag, 1.964
JOHN D. COTTON A,,I~D PETER J. FENSHAM
1054
8t
.-a" a'
'2 0 .*
$
140
120 100
80 60
40
1
20
s
o 10
20
30
40
60 0 20 40 60 80 Time, min.
50
100
120 140
Figure 2. Increase of paramagnetism during adsorption of oxygen on cuprous oxide a t 295'K. (left curve) and its decay during evacuation a t 450'K. (right curve). ~~
~
Table I : Oxygen Uptakes and Paramagnetic Increases during Adsorption on Cuprous Oxide
Expt.
1 2 3 4
Pressure, mm. Initial Final
AWc,
We, fig.
A Wo
pg.
1.46 0.64 0.52 0.12
156 165 111 84
510 510 345 255
0.31 zk0.04 0.32f0.04 0.32 f 0 . 0 6 0.33fO.08
1.30 0.47 0.17 0.04
wc
was very rapid and reached completion within 5 min. In this case there was no observable change in susceptibility, that is, AW was zero. The results of a number of experiments a t different pressures are summarized in Table 11, in which the extent of desorption by evacuation can also be seen. The surface could be regenerated by heating to 450'K., oxidation at 295'K., and re-evacuation at 450'K. On exposing a freshly oxygenated surface to CO a t 295'K., adsorption occurred rapidly and the weight ~~
Table 11: Carbon Monoxide Uptakes and Coverages during Adsorption and Desorption on Cuprous Oxide
Expt.
Pressure, mm.
Total uptake of GO, fig.
0.11 0.22 0.03 1.00 0.46 0.19 0.22 0.25
375 342 216 447 369 354 330 354
The Journal of Physical Chemistry
Coverage by CO, % Total Reversible
50 49 29 61 50 48 45 49
38 37 25 47 1 38 35 38
of the sample increased, passed through a maximum a t 3 min., and at 30 min. reached a constant value when the adsorption was complete. This behavior is consistent with the familiar adsorption of CO, and the subsequent desorption of some of it and some of the preadsorbed oxygen. During the time of the adsorption, A W , the apparent weight increase due to the paramagnetic species from the oxygen adsorption, decreased steadily to zero. For example, a sample on which 414 pg. of oxygen had been adsorbed, and for which AWc was 99 pg., adsorbed CO in the above way giving a net increase in weight of 294 pg. by which time AW was zero. Adsorption of Oxygen on Cepper. Oxygen was admitted to the reduced surface of the powdered copper at 295'K. and adsorption ensued. However a t no stage in the process could any sign of a differential, AW, be detected. Since the sensitivity was considerably better than that used by one of us previously when a transient paramagnetism was reported,? it must now be concluded that paramagnetic ions do not have an appreciable lifetime on the surface.
Discussion The adsorption of oxygen on cuprous oxide occurs with electron transfer from the metal ions to the oxygen converting diamagnetic Cu+ into paramagnetic Cu2+.' Several types of adsorbed oxygen may be present, paramagnetic 02-and 0-,4and diamagnetic 02-(ads)10 and 02--the last, essentially the same as the surface oxide ions. Adsorption at room temperature proceeds beyond a monolayer, and is explained by the incorporation of 02-(ads) ions into the lattice. Not all the adsorbed oxygen on the surface will react with CO to give COz. Garner, Stone, and Tiley2 showed that not more than 66% of the adsorbed oxygen could be released as COz after 24 hr., and that some CO, even on surfaces saturated with oxygen, was adsorbed by other processe? than by forming the COa complex. There are clearly several possible equilibria inherent in this chemisorption process. First, there is an equilibrium between the process of adsorption and incorporation, and second, between the various forms of adsorbed oxygen. The increase, in Fig. 2, of AW to a constant value before adsorption is complete, is indicative of an equilibrium associated with the emergent paramagnetism. It is strong evidence too, that this paramagnetism is essentially associated with the adsorbed oxygen and not with the paramagnetic CuJ+ since these ions are steadily produced throughout (10)F . 8. Stone, Advan. Catalysis, 13, 1 (1962).
MAGNETIC SUSCEPTIBILITY CHANGES DURING ADSORPTION
the whole adsorption process. O’Keefe and Stone8have measured the effect of Cu2+ions on the susceptibility of cuprous oxide down to Cuz01.06,and the lack of a contribution from these ions in our case is surprising although their coilcentration in the surface may lead to the quenching. This absence of a measurable contribution from the cations is further supported by the removal of all the paramagnetic increase during reaction with CO which is only sufficient to reduce a fraction of the Cu2+. If then the paramagnetism is due to oxygen, the data suggest an equilibrium on the surface between a singly charged and a doubly charged species of the adsorbed gas. At high pressures, when the adsorption rate is higher, the magnetic equilibrium is not achieved until higher coverages, which indicates that the transfer of the second electron is the rate-determining step. That a concentration of negatively charged ions on the surface would inhibit the transfer of further electrons to complete the ionization of the oxygen is not surprising since these electrons must come from the ionization of Cu+ ions. However, conversion to 02on the metal surface could occur because of the availability of electrons in the free-electron band of the metal. The details of the reactivity with CO suggest that the equilibration inferred from the constancy of AWc/ Wc is related with that between the oxygen which. is chemically reactive with CO and that which is not. The association of a singly charged species with reactivity and the doubly charged form with inactivity avoids such a less clear distinction between 02-(ads) and 0 2 - of the oxide surface. It also enables an estimate of the susceptibility of the paramagnetic oxygen to be calculated. In the example given earlier, 99 pg. of paramagnetism was associated with 414 pg. of 0 2 . Following Garner, Stone, and Tileyt2it is reasonable to use 50% as an approximate figure for the amount of adsorbed oxygen which reacts with CO in the experiment. Thus, 207 pg. of oxygen correspond to the AW, value of 99 pg., that is 165 X lop6 c.g.s. unit of volume susceptibility. Since the sample density was 1.5 g. om.-* the estimated gram susceptibility xg, is (50 f 20) X c.g.s. unit. We have
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assumed that all or most of the paramagnetic oxygen is desorbed, or that the concentration of COS complex on the surface is small as expected in the presence of a large excess of CO. 0 2 - ions in the alkali metal superoxides have a measured xg of 33 X lop6 unit, which agrees well with the figures of 39 X loM6unit calculated for a single spin free electron in the molecule ion. The experimental susceptibility, xg, of 0- is not available, unit can be but a calculated estimate of 165 X obtained for room temperature using the general formula of Van V1eck.l’ The effective susceptibility of the oxide ions on the surface might be expected to be reduced by magnetic interactions; however, JuzaI2 did not find any appreciable reduction in the first monolayer of physically adsorbed oxygen from the value for gaseous oxygen molecules. At this stage, it is impossible to differentiate between the two paramagnetic species beyond suggesting that the observed value would seem to be too low if the dominant stable intermediate on the surface is 0-, but that to postulate 02- only, raises difficulties with the number of sites involved in the adsorption of the oxygen which is reactive with CO. The existing model for the adsorption of CO on the evacuated surface of the oxide is supported by the absence of any observed paramagnetism. This rules out any essentially ionic type of bonding involving a charged paramagnetic form of CO. Likewise, the COa complex formed with active oxygen is diamagnetic and must contain an even number of electrons, all of which are paired.
Acknowledgments. We are grateful to Dr. C. G. Barraclough for the gift of the microbalance and J. D. C. is indebted to the C.S.I.R.O. for a research scholarship. P. J. F. wishes to acknowledge his memory of many helpful discussions with the late Prof. W. E. Garner and Dr. T. J. Gray (now of Alfred University, Alfred, N. Y.) during his earlier investigation of this system. (11) J. H. Van Vleck, “The Theory of Electric and Magnetic Susceptibilities,” Oxford IJniversity Press, London, England, 1932. (12) R. Juza and F. Grrtsenick, 2. Elektrochem., 54, 145 (1950).
Volume 68, Number 6 May, 1964
