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The catalytic activity of silver crystals of various orientations after bombardment with positive ions
J. Phys. Cite.
Solids
Pergamon
Press 1959. Vol. 10. pp. 304-310.
THE CATALYTIC VARIOUS
ACTIVITY
Printed in Great Britain.
OF SILVER CRYSTALS OF
ORIENTATIONS
AFTER
VVITH POSITIVE
BOMBARDMENT
IONS
H. M. C. SOSNOVSRY Division of Tribophysics,
Commonwe~th Scientific and Industrial Research Organization, University of Melbourne, Australia*
(Received 31 Decembm 1958) Ab&rsmt--Single crystals of silver with surfaces oriented parallel to (1 ll), (110) and (100) have been bombarded with positive argon ions at voltages between 14 and 4000 V. The catalytic decomposition of formic acid has been used as a test reaction in the temperature range 1 SO-250%. The parameters log A and E found from Arrhenius plots change appreciably with bombarding ion energy and for each ion energy are different for the three orientations. It is concluded that the reaction occurs at sites where dislocation lines intersect the surface and that a compensating effect occurs which is associated with interaction between dislocations when their density is high.
1. IN3!RODUCTION IT
is well known that the activity of a catalyst can be greatly influenced by treatment of its surface. Ion bombardment has frequently been used to clean surfaces, but, in addition to being cleaned, a surface is damaged by bombardment even with low-energy ions, FARNSWORTHand WOODCOCK(~)prepared clean surfaces by bombarding nickel and platinum with positive argon ions in a gaseous discharge at 250 V and found that the rate of hydrogenation of ethylene had been thereby increased by factors of 100 for nickel and 10 for platinum. They examined their surfaces by low-energy electron diffraction and reported that defects of a complex nature were present after bombardment. Ion bombardment of silver produces disoriented regions(s) and thus increases the number of dislocations at the surface. This effect is similar to the increase in dislocation density due to plastic deformation. A change in the activity of a catalyst by plastic deformation was found by ECKELL@), according to whom the reaction velocity of the hydrogenation of ethylene increased by a factor of from 600 to 1000 when the polycrystalline nickel
catalyst was cold-rolled from 3 to O-1 mm. On subsequent annealing the effect of cold rolling disappeared. CRATTYand G~ATo(~) considered that these results may be due to the introduction of dislocations by rolling. RIEMZKER(~) found an increase in the rate of decomposition of formic acid on cold-rolling nickel and also a simultaneous increase in the activation energy. However, these surfaces were probably contaminated and apparently were not cleaned satisfactorily after being rolled. The purpose of the present study was to in-. vestigate the effect of surface damage produced by ion bombardment on the rate of the catalytic decomposition of formic acid at various temperatures. The crystal plates of silver used had (111), (100) and (110) planes, respectively, parallel to their surfaces, and for each of the crystals the catalytic activity was measured as a function of the energy of the bombarding ions. A preliminary report of this work has already been published.@ 2, BUNTS
PROCRDURR
(a) Pregwzratioonof single cvystah of predetermined orienta-
tion
+ Now at the Institute for the Study of Metals, University
of Chicago, U.S.A.
The silver single crystals (99.97 per cent purity) were grown from the melt in thin sheets (5 x 1.8 x O-05 om) in
304
THE
CATALYTIC
ACTIVITY
OF SILVER
a graphite mould. The mould described previously(r) was modified to accommodate a seed crystal and thereby make possible the growth of a crystal of predetermined orientation.(s) The seed crystals were prepared using two other types of mould (Fig. 1). Several moulds of type (b) were made, each with a different angle. By repeated operations with
CRYSTALS
OF VARIOUS
305
ORIENTATIONS
after which polishing for a few more minutes produced a smooth surface. The surfaces were then bombarded by positive argon ions in a low-pressure discharge.@) The specimen surface was set perpendicular to the axis of the discharge tube, which was then evacuated to 10-s mm Hg. The tube was flushed with argon which had passed through a liquid-air trap to remove traces of HsS from the bladder storing the argon. A steady flow of argon at a low pressure was then established through a leak valve and the discharge started. The discharge was run at an argon pressure of less than 2 x 10-s mm Hg whatever voltage was applied, so that the mean free path of the ions was high enough to prevent re-deposition of the silver on the PUMP
t
K u 4
FIG. 1. Graphite moulds for growing thin seed crystals. Close-fitting upper part is not shown. (a) For kinked crystals with sections of different orientations. (b) For changing the direction of growth.
0
these moulds, using each time a suitable section cut from the previous long crystal, a seed of the required orientation could be grown. Figure 2 shows the vacuum furnace into which the appropriate mould containing the silver was placed vertically. To remove oxygen from the silver, the tube E was kept evacuated at 10m3 mm Hg during the whole period of heating and cooling. The heating of the silver was carried out by induction, the coil being wound more closely over the upper half of the specimen, thereby producing a temperaNre gradient. The specimen was positioned so that half of the seed crystal remained solid while the other half and the silver above it were melted. The temperature was then slowly decreased over a period of about 15 min by reducing the induction current until the whole specimen solidified. Then the heating was stopped and the mould allowed to cool to room temperature in wacuo. (b) Surface treatment of singIe crystals and positive-ion bombardment
by electropolishing
After removing a crystal from the mould, it was chemically polished to a bright finish in hot nitric acid with up to 20 per cent of water added, and then electrolytically polished in a cyanide bath. The electrolyte used was 30 g KCN, 35 g AgCN and 38 g KaCOs in 11. of distilled water. It was found advisable to carry out the polishing process in two stages. After approximately 20 min, the polishing was interrupted to remove a solid film which adhered strongly to the surface and largely inhibited further electrolytic polishing. This film was removed by treatment for a short time with hot nitric acid,
Lrc FIG. 2. Vacuum furnace for growing single crystals. KEY
M
Graphite mould held together by graphite rings R.
TC
Thermocouple
P, F
Metal plates.
0
O-ring.
leads.
E, T Silica tubes. C
Water-cooled
induction coil.
306
H.
M.
C.
specimen by back scattering. The voltages used varied from 14 to 4000 V. The maximum energy of the bombarding ions, which was less than that to be expected from the voltage applied, was derived in each case from
measured ion-energy distributions@) and the values are indicated in Table 1. Very low discharge currents were used. The energy dissipated was always less tban O-2 W, and the specimen temperature was found to remain below 100°C. The duration of the bombardment was varied between 30 min at 4000 V and 12 hr at 22.5 V to ensure that equilibrium conditions had been reached (see Section 3(d)). Every surface was cleaned initially by bombardment at a high voltage (between 300 and 4000 V) before being treated at lower ion energies. In all cases after the ion bombardment, and also after being used as catalysts, the surfaces appeared smooth and clean under. the microscope. Electron-diffraction patterns of some of these surfaces confirmed that they were clean, but the appearance of spotty rings indicated that structural changes had taken place as a result of the ion bombardment. In every case the specimen was transferred to the reaction vessel immediately after ion bombardment. It was then annealed in oacuo at 250°C for at least 18 hr before rate measurements were begun. One crystal of each orientation was used over the whole range of discharge voltages. However, a few experiments were made on different crystals with the same orientations and under the same bombarding conditions, with comparable results. (c) Measurement of the reaction rate The formic acid used in the experiments was prepared from A.R. 98-100 per cent HCOOH by shaking it for two days with anhydrous oxalic acid. It was then distilled at atmospheric pressure through a fractionating column, purified by crystallization, and afterwards distilled in mcuo into a storage vessel attached to the apparatus. The dried and purified acid had a melting point of 8.4”C. The method of investigating the catalytic decomposition of formic acid on silver surfaces was the same as that described previously, a similar flow apparatus being used.(r) The acid decomposed at zero order into COaand Hs exclusively, but on the glass walls of the apparatus the decomposition occurred by two reactions, forming COa, Hz, CO, and H20. A McLeod gauge was used to measure the total pressure of the Hs and CO, and the correction for the decomposition on glass was determined by measurements without the catalyst. Reaction rates on the ionbombarded silver surfaces were determined in this way at temperatures between 150 and 25O”C, and the logarithm of the corrected rate of reaction (p) was plotted against 1 /T. A straight line was obtained in each case within the experimental error. The gradient of this line was calculated by the method of least squares and also the intercept at l/T = 0, the gradient being proportional to the experimental activation energy E and the intercept the logarithm of the frequency factor A in the Arrhenius equation p = A exp( -E/RT).
SOSNOVSKY
(a) (111)
surface
3. RESULTS
The Arrhenius plot of log p against l/T is given in Fig. 3 for applied voltages between 22.5 and 4000 V (maximum ion energies between 15 and 3000 eV). It is seen that within the temperature 4.5
“Q 35 5 $
30
20
I ---__2;i__
200
_+__ 22.0
L?%l 7-
__
24.0
(l/T) MO4
FIG. 3. Arrhenius plots for a (111) surface bombarded with argon ions for various applied voltages. For the corresponding values of maximum ion energy see Table 1.
range used, the lowest values of log p are found after bombardment at 22.5 V and the highest values after bombardment at 46 V, the rate changing by a factor of up to 25. On further increasing the voltage, however, the rate decreases. The values of log10 A and E obtained from these plots are given in Table 1. The values of E lie between 12.2 and 22.5 kcal/mole and the corresponding values of logrs A between 2.3 and 7-7. Thus, an increase of about 10 kcal/mole in E is accompanied by an increase in A by a factor of about 10s. The change in both log A and E is most pronounced in going from a bombarding voltage of 22.5 V to one of 46 V, with a smaller increase after treatment at 86 V. Bombardment of the surface with higherenergy ions (1304000 V) changes log A and E only slightly. (b) (110) surf&e The lowest values of log p were found when the surface had been bombarded at 14 V, but the reaction was so slow that a sufficiently accurate rate determination was not possible. The lowest bombarding voltage for which results were plotted (Fig. 4) was 22.5 V. The rate increased continuously with voltage up to 500 V by a factor of up to
THE CATALYTIC
ACTIVITY
OF SILVER
CRYSTALS
100. A slight decrease in the rate was found after bombardment at 4000 V. The values of log10 A and E are given in Table 1. The values of E lie between 24.1 and 34.5 kcal/ mole and those of log10 A between 7.1 and 13.4. These increases occur continuously over the voltage range 22.5-500 V, but the values decrease after bombardment at 4000 V.
OF VARIOUS ORIENTATIONS
307
which were too low for accurate determinations). At voltages of 46 and 300 V, the rate increased by a factor of 100, but decreased when the voltage was raised to 4000 V. For this orientation the values of log A and E appear to increase continuously as the voltage is increased from 22.5 to 300 V, although the changes in these values are much smaller for this orientation.
FIG. 4. Arrhenius plots for a (110) surface bombarded with argon ions for various applied voltages.
(c) (100) sufface
(d) Reproducibility of results
The Arrhenius plots are shown in Fig. 5 and the calculated values of log10 A and E in Table 1. As in the previous two cases, the lowest values of p were associated with surfaces bombarded at 22.5 V (argon bombardment at 14 V resulted in rates
The time of bombardment for a given voltage was chosen so that an equilibrium state had been reached. For example, after a 30-min bombardment at 4000 V, the Arrhenius plot gave a certain straight line of best fit. Where a different specimen
Table 1. Effect of bombarding voltage 012the catalytic activity of silver crystals of different orientations ___ _ Applied voltage
Max. ion energy (ev)
22.5 46 86 130 300 z
15 38 77 118 280 3z
(111) E
log1oA
(100)
(110) (kcsl/mole)
2.3f0.6* 6.1&0*5t 7.3kO.4 6.9&0.7 6.7hO.9
12*2f1*2* 17*6&0*9t 20.4f0.8 19.9f1.5 19.7+1-8
7.7hi.l
22.5f2.3
E log1aA
(kcal/mole)
7.1 kO.2 9-l&0.3 9.5*1.0 12.4hO.6
24.1 &O-S 26.8kO.7 27.1 f2.2 32.8h1.3
13.450.4 11.1*0.3
34.5kO.8 30.2kO.6
I
log1aA
E (k&/mole)
9.2f0.9 12.4f0.8
29*9&3-O 32.5&1*7
13.2f0.3
34,4&0.6
13.0*0*5
35*2&l-1
The & figures are standard errors and, unless otherwise indicated, were based on’four measurements, * on seven and t on five measurements.
measurements
308
H.
M.
C.
with the same orientation and the same bombarding conditions was used, or the same specimen was used and only the bombarding time changed, in nearly every case the Arrhenius plot resulted in a straight line almost coincident with the previous one. A slight change in the gradient was found, corresponding to a change in E of
SOSNOVSKY I
I
I
i
I
I
I
I
32
34
36
“i
9”
0
FIG.
10
6.
1
,*
14
Log
16
I8
20
26 22 24 kCd,~Oi~
A vers$‘E for (ill), surfaces.
it should be remembered obtained by independent
28
30
(110)
and (100)
that A and E are not measurements.
4. DISCUSSION
U/iW04
FIG. 5. Arrhenius plots for a (100) surface bombarded with argon ions for various applied voltages and for various times at 22.5 V.
about Al kcal/mole. This reproducibility was found for all three orientations and all bombarding voltages when equilibrium conditions could be reached. However, for the (110) and the (100) specimens bombarded at 22.5 V, equilibrium conditions were not reached even after 12 hr. For these two specimens after each increase in bombarding time the new Arrhenius plot gave a straight line nearly parallel to the previous one but displaced above it. This behaviour is shown in Fig. 5 for the (100) surface. However, the (111) specimen bombarded at 22.5 V gave the same rates for bombarding times of 3 and 6 hr. (e) Relationship between log A and E Taking the figures given in Table 1 at face value for all three surfaces after bombardment, an increase in E is always associated with an increase in log A. Fig. 6 shows that a linear relationship exists for each of the three orientations. However,
The results show that the catalytic activity of a silver surface can be altered considerably by bombarding it with positive argon ions. With the same crystal, after altering the bombarding energy, systematic changes in A and E were observed which must be associated with some change in the surface produced by the bombardment. The changes in A of several orders of magnitude may imply changes in various factors, including the number of active sites. The surfaces remained smooth, and therefore the surface area is probably not altered appreciably. Assuming that the reaction mechanism remains the same, it seems likely that a change in the number of active sites is the predominant factor. This could be associated with changes in the numbers of dislocations or of point defects such as vacancies, interstitial atoms or argon atoms embedded in the lattice. Changes in the number of point defects cannot be responsible for the change in A, since each crystal was annealed at 250°C in vacua for at least 18 hr before the catalytic measurements were made; after this treatment the concentration of vacancies and interstitial atoms should be negligible,os) and the concentration of argon atoms should have attained the same value whatever the bombarding conditions had been. However, the possibility that dislocations are responsible for the change in A cannot be eliminated. Striking evidence has been given that ion bombardment produces disoriented regions(s) bounded by stable arrays of dislocations. This was found by observations on electron-diffraction transmission patterns of single crystals of silver. In our work transmission patterns could not be taken, since the
THE CATALYTIC
ACTIVITY
OF SILVER CRYSTALS
presence of the holes required for this technique would have made the specimens unsuitable for the rate measurements. However, by reflection electron diffraction, disorientation was detected on some of the crystals used as catalysts. The single crystals used by 0GILVIE(2) had orientations close to those used in the present work, and his patterns showed that a change in the structure takes place after bombardment, even with ions of very low energies, with one exception, viz. the case in which a (111) surface is bombarded at 22.5 V. The appearance of extra spots, the arcing of spots and, in some cases, the presence of spotty rings were interpreted as indicating the formation of a surface layer in which small crystalline blocks are rotated through different angles. Thus, ion bombardment gives rise to a greatly increased number of dislocations. It is now proposed that the active sites for the reaction are those where dislocation lines come to the surface. Furthermore, the transmission diffraction patterns of a crystalfs), when compared with those of HIRSCHet &.(ls), show that the disorientations intraduced by ion bombardment are at least as great as those resulting from cold roiling. Heavy cold work on a polycrystalline aggregate results in an increase in the dislocation density by a factor of about 104.(13) This increase is comparable to the maximum increase in A on varying the bombarding conditions for a given crystal. The fact that for the (111) surface (which is obtainable without disorientation) A increases owing to ion bombardment in approximately the same ratio as the dislocation density, strongly suggests that the increase in A is predominantly due to the increase in the number of active sites and not in other factors such as entropy and steric factors. As shown in Fig. 6, for each crystal an increase in A was always accompanied by an increase in E. Thus, the increase in A by several orders of magnitude on bombardment does not give rise to a similarly large increase in reaction rate, but is, at least partly, compensated.@n This increase in E with the number of active sites, i.e. with the density of dislocations at the surface, could be due to an interaction between the dislocations. As the new dislocations introduced by bombardment are confined to the boundaries of the small crystalline blocks, their local density is very high and their interaction correspondingly strong. The energy of X
OF VARIOUS ORIENTATIONS
309
each site could therefore be lowered, giving an increased value of E associated with the increase in A. It should be emphasized that the presence of dislocations changes not only the geometry of the surface but also the local electronic properties. To make these ideas more definite, we may note that as different blocks in the surface are rotated through various angles by the ion bombardment, the dislocation density is not the same in all boundaries. Hence, because of the interaction, active sites in different parts of the surface may be characterized by different energies. However, if the interaction between the dislocations, which is responsible for the change in their energy as their number increases, operates only when they are close together, one would predict that the sites in the tilt boundaries would have a range of activation energies distributed fairly closely about a mean. While some of the dislocations intersecting the surface in an undeformed crystal will be incorporated in tilt boundaries after bombardment, a proportion will remain isolated and unaffected. Thus, we may assume for the sake of simplicity that only two types of site are present in the surface, viz. those with a lower activation energy, El, typical of the isolated dislocations, and those with a higher activation energy, Es, typical of the new dislocations crowded into boundaries. If we then have a concentration q of sites with El and a eoncentration cs of sites with Ez, the observed rate of the reaction should be expressed by: p = k{cl exp(-El/RT)+cz
exp(-&/RT)t,
where k may depend on the orientation. The observed values of A and E depend on the relative magnitude of the two terms. If, for a given orientation, we choose for El and Es the extreme experimental values of E, we can discuss the significance of the measured A values in terms of the concentrations cl and cs. These E values are: for (111) El = 12, Es = 22; for (110) El = 24, Es = 34; and for (100) El = 30, Es = 35 kcal/ mole. Let us assume t&t in aI1 cases the concentrationcz is 106sites/cm~. For an undeformed crystal, only this one type of site is present in the surface and ca = 0; then and only then are the observed values of A and E representative of aII the sites. (hh~.&&on~~~~~ show that if the new
310
H.
M.
C.
dislocations with activation energy Ez are introduced in increasing numbers, their effect on the measured values of A and E will be negligible until cs has reached 100,s sites/cm2 for the (111) and (110) surfaces, and 107.5 sites/cm2 for the (100) surface. A further increase in cs by a factor of 10 makes the two terms of comparable magnitude and the measured A and E values are composite and larger. When ca is greater still (1011’s or 109.5 sites/cm2, respectively), the second term predominates to such an extent that the measured values of A and E are representative only of the higher-energy sites. This analysis shows that, although a substantial amount of deformation may be present in the surface without having any effect on its catalytic activity, heavy deformation should produce increases in the values of A and E provided that this deformation gives rise to a large number of sites of activation energies at or closely distributed around Ez. It is now possible to associate the change in catalytic properties of the surface with its structure as revealed by electron diffraction. The (111) crystal. Electron diffraction shows that the crystal bombarded at 22.5 V is not deformed, but that there is disorientation at all other voltages. Thus, at 22.5 V, cz = 0, and the only sites are the 10s dislocations/cm2 with activation energy 12 kcal!mole. The changes in A by a factor of 105 and in E of 10 kcal/mole after bombardment at 4000 V can be explained by the introduction of about 10s x 105 = 10” sites/cm*, having an activation energy of about 22 kcal/mole. The analysis shows that if the original 10s sites of low activation energy remain, their effect is swamped by the 10” new dislocations. It is seen that for this orientation, k = 102.3,/106 = 10-3.7 (in arbitrary units). The (110) crystal. Electron-diffraction evidence indicates that this surface is disoriented by bombardment at all voltages, even the lowest. Catalysis results show increases in A by a factor of lo6 and in E of 10 kcal/mole when the voltage is increased. This can be explained if low-voltage bombardment results in a surface containing lo6 sites/cm2 with EI = 24 k&mole and up to loo’s sites/cm2 with Es = 24 k&/mole. The latter number of dislocations does not affect the catalytic rate, but should be sufficient to provide the disorientation detected by electron diffraction. With increased bombarding voltage, more dislocations are introduced, and cs increases from 1OO.sto 10”‘s sites/cm2 and the higher activation energy is observed. A maximum concentration of 10’s sites/cm2 occurs after bombardment at 500 V. For this orientation k = 107’l/lOs = 10”‘. The (100) crystal. Arguments similar to those used for the (110) crystal apply. Indeed, additional evidence from a thermally etched crystal (4” from (lOO)), which gave E = 30 kcal/mole and log& = 10, supports the conclusions that the lowest-voltage bombardment produces a surface whose catalytic behaviour is indistinguishable from that of an annealed crystal. The maximum wncentration of dislocations is introduced at the highest voltages, when cs = lad x 104 = lOlo sites/ems. The uncertainty in the values is large, but, at face value, k = 109~s/lOs = 108.2.
SOSNOVSKY The variations of k, El and EO with orientation are still unexplained, but may be due to an anisotropy of properties associated with a dislocation line intersecting a free surface of differing orientation. However, the values of A and E change together, so that a compensating effect is already present before the number of sites is increased by the bombardment. 5. CONCLUSIONS
1. The large changes in the catalytic activity obtained after bombarding silver surfaces with positive argon ions are most likely due to an increase in the number of dislocation lines intersecting the surfaces. 2. For a given crystal there is a compensating effect (increase of E is always associated with an increase in A) which implies that as the number of dislocations increases, the energy of the sites decreases. This may be due to an interaction between the dislocations. 3. For the same bombarding conditions the values of A and E depend on the orientation of the surface. Ackno&dgements-I wish to thank Mr. E. GILLAM, Dr. G. J. OCILVIE and Mr. J. F. NICHOLASfor their help in discussing the work, and Mr. H. JAEGBR and Mr. U. D. hhNARIN for their assistance with the experiments.
RJZFERENCI?S 1. FARNSWORTHH. E. and W~OLXOCK R. F., Advances in Catalysis Vol. 9, p. 123. Academic Press, Inc., New York (1957). 2. OGILVIE G. J., J. P/tys. Cha. Soti 10, 222 (1959). 3. ECKELL J., 2. Electrochem.39, 433 (1933). 4. Curry L. E. and GRANATOA. V., J. Chews. Phys. 26, 96 (1957). RIENXCKERG., 2. Electrochem.46, 369 (1940). SOSNOVSKY H. M. C., OCILVIE G. J. and GILLM~ E., Nature, Land. 182, 523 (1958). SOSNOVSKYH. M. C., J. Chetn. Pkys. 23, 1486 (1955). Gow K. V. and CHALMERSB., Brit. J. Appl. Phys. 2, 300 (1951). 9. GILLAM E. and OGILVIB G. J., To be published. 10. Sar~z F., Advances in Phys. 1, 43 (1952). 1.1. LB Curve A. D. and ROWE A. H., Reu. MttaZl. 52, 94 (1955). 12. HIRSCH P. B., KELLY A. and MENTER J. W., Rut. Phys. Sot. E%8,1132 (1955). 13. HIRSCH P. B., Progress in Metal Physics Vol. 6, p. 236. Pergamon Press, London (1955). 14. CR~~ER E., Advances in CataZysis Vol. 7, p. 7:. Academic Press, Inc., New York (1955). 15. NICHOLASJ. F., Private communication.
Solids
Pergamon
Press 1959. Vol. 10. pp. 304-310.
THE CATALYTIC VARIOUS
ACTIVITY
Printed in Great Britain.
OF SILVER CRYSTALS OF
ORIENTATIONS
AFTER
VVITH POSITIVE
BOMBARDMENT
IONS
H. M. C. SOSNOVSRY Division of Tribophysics,
Commonwe~th Scientific and Industrial Research Organization, University of Melbourne, Australia*
(Received 31 Decembm 1958) Ab&rsmt--Single crystals of silver with surfaces oriented parallel to (1 ll), (110) and (100) have been bombarded with positive argon ions at voltages between 14 and 4000 V. The catalytic decomposition of formic acid has been used as a test reaction in the temperature range 1 SO-250%. The parameters log A and E found from Arrhenius plots change appreciably with bombarding ion energy and for each ion energy are different for the three orientations. It is concluded that the reaction occurs at sites where dislocation lines intersect the surface and that a compensating effect occurs which is associated with interaction between dislocations when their density is high.
1. IN3!RODUCTION IT
is well known that the activity of a catalyst can be greatly influenced by treatment of its surface. Ion bombardment has frequently been used to clean surfaces, but, in addition to being cleaned, a surface is damaged by bombardment even with low-energy ions, FARNSWORTHand WOODCOCK(~)prepared clean surfaces by bombarding nickel and platinum with positive argon ions in a gaseous discharge at 250 V and found that the rate of hydrogenation of ethylene had been thereby increased by factors of 100 for nickel and 10 for platinum. They examined their surfaces by low-energy electron diffraction and reported that defects of a complex nature were present after bombardment. Ion bombardment of silver produces disoriented regions(s) and thus increases the number of dislocations at the surface. This effect is similar to the increase in dislocation density due to plastic deformation. A change in the activity of a catalyst by plastic deformation was found by ECKELL@), according to whom the reaction velocity of the hydrogenation of ethylene increased by a factor of from 600 to 1000 when the polycrystalline nickel
catalyst was cold-rolled from 3 to O-1 mm. On subsequent annealing the effect of cold rolling disappeared. CRATTYand G~ATo(~) considered that these results may be due to the introduction of dislocations by rolling. RIEMZKER(~) found an increase in the rate of decomposition of formic acid on cold-rolling nickel and also a simultaneous increase in the activation energy. However, these surfaces were probably contaminated and apparently were not cleaned satisfactorily after being rolled. The purpose of the present study was to in-. vestigate the effect of surface damage produced by ion bombardment on the rate of the catalytic decomposition of formic acid at various temperatures. The crystal plates of silver used had (111), (100) and (110) planes, respectively, parallel to their surfaces, and for each of the crystals the catalytic activity was measured as a function of the energy of the bombarding ions. A preliminary report of this work has already been published.@ 2, BUNTS
PROCRDURR
(a) Pregwzratioonof single cvystah of predetermined orienta-
tion
+ Now at the Institute for the Study of Metals, University
of Chicago, U.S.A.
The silver single crystals (99.97 per cent purity) were grown from the melt in thin sheets (5 x 1.8 x O-05 om) in
304
THE
CATALYTIC
ACTIVITY
OF SILVER
a graphite mould. The mould described previously(r) was modified to accommodate a seed crystal and thereby make possible the growth of a crystal of predetermined orientation.(s) The seed crystals were prepared using two other types of mould (Fig. 1). Several moulds of type (b) were made, each with a different angle. By repeated operations with
CRYSTALS
OF VARIOUS
305
ORIENTATIONS
after which polishing for a few more minutes produced a smooth surface. The surfaces were then bombarded by positive argon ions in a low-pressure discharge.@) The specimen surface was set perpendicular to the axis of the discharge tube, which was then evacuated to 10-s mm Hg. The tube was flushed with argon which had passed through a liquid-air trap to remove traces of HsS from the bladder storing the argon. A steady flow of argon at a low pressure was then established through a leak valve and the discharge started. The discharge was run at an argon pressure of less than 2 x 10-s mm Hg whatever voltage was applied, so that the mean free path of the ions was high enough to prevent re-deposition of the silver on the PUMP
t
K u 4
FIG. 1. Graphite moulds for growing thin seed crystals. Close-fitting upper part is not shown. (a) For kinked crystals with sections of different orientations. (b) For changing the direction of growth.
0
these moulds, using each time a suitable section cut from the previous long crystal, a seed of the required orientation could be grown. Figure 2 shows the vacuum furnace into which the appropriate mould containing the silver was placed vertically. To remove oxygen from the silver, the tube E was kept evacuated at 10m3 mm Hg during the whole period of heating and cooling. The heating of the silver was carried out by induction, the coil being wound more closely over the upper half of the specimen, thereby producing a temperaNre gradient. The specimen was positioned so that half of the seed crystal remained solid while the other half and the silver above it were melted. The temperature was then slowly decreased over a period of about 15 min by reducing the induction current until the whole specimen solidified. Then the heating was stopped and the mould allowed to cool to room temperature in wacuo. (b) Surface treatment of singIe crystals and positive-ion bombardment
by electropolishing
After removing a crystal from the mould, it was chemically polished to a bright finish in hot nitric acid with up to 20 per cent of water added, and then electrolytically polished in a cyanide bath. The electrolyte used was 30 g KCN, 35 g AgCN and 38 g KaCOs in 11. of distilled water. It was found advisable to carry out the polishing process in two stages. After approximately 20 min, the polishing was interrupted to remove a solid film which adhered strongly to the surface and largely inhibited further electrolytic polishing. This film was removed by treatment for a short time with hot nitric acid,
Lrc FIG. 2. Vacuum furnace for growing single crystals. KEY
M
Graphite mould held together by graphite rings R.
TC
Thermocouple
P, F
Metal plates.
0
O-ring.
leads.
E, T Silica tubes. C
Water-cooled
induction coil.
306
H.
M.
C.
specimen by back scattering. The voltages used varied from 14 to 4000 V. The maximum energy of the bombarding ions, which was less than that to be expected from the voltage applied, was derived in each case from
measured ion-energy distributions@) and the values are indicated in Table 1. Very low discharge currents were used. The energy dissipated was always less tban O-2 W, and the specimen temperature was found to remain below 100°C. The duration of the bombardment was varied between 30 min at 4000 V and 12 hr at 22.5 V to ensure that equilibrium conditions had been reached (see Section 3(d)). Every surface was cleaned initially by bombardment at a high voltage (between 300 and 4000 V) before being treated at lower ion energies. In all cases after the ion bombardment, and also after being used as catalysts, the surfaces appeared smooth and clean under. the microscope. Electron-diffraction patterns of some of these surfaces confirmed that they were clean, but the appearance of spotty rings indicated that structural changes had taken place as a result of the ion bombardment. In every case the specimen was transferred to the reaction vessel immediately after ion bombardment. It was then annealed in oacuo at 250°C for at least 18 hr before rate measurements were begun. One crystal of each orientation was used over the whole range of discharge voltages. However, a few experiments were made on different crystals with the same orientations and under the same bombarding conditions, with comparable results. (c) Measurement of the reaction rate The formic acid used in the experiments was prepared from A.R. 98-100 per cent HCOOH by shaking it for two days with anhydrous oxalic acid. It was then distilled at atmospheric pressure through a fractionating column, purified by crystallization, and afterwards distilled in mcuo into a storage vessel attached to the apparatus. The dried and purified acid had a melting point of 8.4”C. The method of investigating the catalytic decomposition of formic acid on silver surfaces was the same as that described previously, a similar flow apparatus being used.(r) The acid decomposed at zero order into COaand Hs exclusively, but on the glass walls of the apparatus the decomposition occurred by two reactions, forming COa, Hz, CO, and H20. A McLeod gauge was used to measure the total pressure of the Hs and CO, and the correction for the decomposition on glass was determined by measurements without the catalyst. Reaction rates on the ionbombarded silver surfaces were determined in this way at temperatures between 150 and 25O”C, and the logarithm of the corrected rate of reaction (p) was plotted against 1 /T. A straight line was obtained in each case within the experimental error. The gradient of this line was calculated by the method of least squares and also the intercept at l/T = 0, the gradient being proportional to the experimental activation energy E and the intercept the logarithm of the frequency factor A in the Arrhenius equation p = A exp( -E/RT).
SOSNOVSKY
(a) (111)
surface
3. RESULTS
The Arrhenius plot of log p against l/T is given in Fig. 3 for applied voltages between 22.5 and 4000 V (maximum ion energies between 15 and 3000 eV). It is seen that within the temperature 4.5
“Q 35 5 $
30
20
I ---__2;i__
200
_+__ 22.0
L?%l 7-
__
24.0
(l/T) MO4
FIG. 3. Arrhenius plots for a (111) surface bombarded with argon ions for various applied voltages. For the corresponding values of maximum ion energy see Table 1.
range used, the lowest values of log p are found after bombardment at 22.5 V and the highest values after bombardment at 46 V, the rate changing by a factor of up to 25. On further increasing the voltage, however, the rate decreases. The values of log10 A and E obtained from these plots are given in Table 1. The values of E lie between 12.2 and 22.5 kcal/mole and the corresponding values of logrs A between 2.3 and 7-7. Thus, an increase of about 10 kcal/mole in E is accompanied by an increase in A by a factor of about 10s. The change in both log A and E is most pronounced in going from a bombarding voltage of 22.5 V to one of 46 V, with a smaller increase after treatment at 86 V. Bombardment of the surface with higherenergy ions (1304000 V) changes log A and E only slightly. (b) (110) surf&e The lowest values of log p were found when the surface had been bombarded at 14 V, but the reaction was so slow that a sufficiently accurate rate determination was not possible. The lowest bombarding voltage for which results were plotted (Fig. 4) was 22.5 V. The rate increased continuously with voltage up to 500 V by a factor of up to
THE CATALYTIC
ACTIVITY
OF SILVER
CRYSTALS
100. A slight decrease in the rate was found after bombardment at 4000 V. The values of log10 A and E are given in Table 1. The values of E lie between 24.1 and 34.5 kcal/ mole and those of log10 A between 7.1 and 13.4. These increases occur continuously over the voltage range 22.5-500 V, but the values decrease after bombardment at 4000 V.
OF VARIOUS ORIENTATIONS
307
which were too low for accurate determinations). At voltages of 46 and 300 V, the rate increased by a factor of 100, but decreased when the voltage was raised to 4000 V. For this orientation the values of log A and E appear to increase continuously as the voltage is increased from 22.5 to 300 V, although the changes in these values are much smaller for this orientation.
FIG. 4. Arrhenius plots for a (110) surface bombarded with argon ions for various applied voltages.
(c) (100) sufface
(d) Reproducibility of results
The Arrhenius plots are shown in Fig. 5 and the calculated values of log10 A and E in Table 1. As in the previous two cases, the lowest values of p were associated with surfaces bombarded at 22.5 V (argon bombardment at 14 V resulted in rates
The time of bombardment for a given voltage was chosen so that an equilibrium state had been reached. For example, after a 30-min bombardment at 4000 V, the Arrhenius plot gave a certain straight line of best fit. Where a different specimen
Table 1. Effect of bombarding voltage 012the catalytic activity of silver crystals of different orientations ___ _ Applied voltage
Max. ion energy (ev)
22.5 46 86 130 300 z
15 38 77 118 280 3z
(111) E
log1oA
(100)
(110) (kcsl/mole)
2.3f0.6* 6.1&0*5t 7.3kO.4 6.9&0.7 6.7hO.9
12*2f1*2* 17*6&0*9t 20.4f0.8 19.9f1.5 19.7+1-8
7.7hi.l
22.5f2.3
E log1aA
(kcal/mole)
7.1 kO.2 9-l&0.3 9.5*1.0 12.4hO.6
24.1 &O-S 26.8kO.7 27.1 f2.2 32.8h1.3
13.450.4 11.1*0.3
34.5kO.8 30.2kO.6
I
log1aA
E (k&/mole)
9.2f0.9 12.4f0.8
29*9&3-O 32.5&1*7
13.2f0.3
34,4&0.6
13.0*0*5
35*2&l-1
The & figures are standard errors and, unless otherwise indicated, were based on’four measurements, * on seven and t on five measurements.
measurements
308
H.
M.
C.
with the same orientation and the same bombarding conditions was used, or the same specimen was used and only the bombarding time changed, in nearly every case the Arrhenius plot resulted in a straight line almost coincident with the previous one. A slight change in the gradient was found, corresponding to a change in E of
SOSNOVSKY I
I
I
i
I
I
I
I
32
34
36
“i
9”
0
FIG.
10
6.
1
,*
14
Log
16
I8
20
26 22 24 kCd,~Oi~
A vers$‘E for (ill), surfaces.
it should be remembered obtained by independent
28
30
(110)
and (100)
that A and E are not measurements.
4. DISCUSSION
U/iW04
FIG. 5. Arrhenius plots for a (100) surface bombarded with argon ions for various applied voltages and for various times at 22.5 V.
about Al kcal/mole. This reproducibility was found for all three orientations and all bombarding voltages when equilibrium conditions could be reached. However, for the (110) and the (100) specimens bombarded at 22.5 V, equilibrium conditions were not reached even after 12 hr. For these two specimens after each increase in bombarding time the new Arrhenius plot gave a straight line nearly parallel to the previous one but displaced above it. This behaviour is shown in Fig. 5 for the (100) surface. However, the (111) specimen bombarded at 22.5 V gave the same rates for bombarding times of 3 and 6 hr. (e) Relationship between log A and E Taking the figures given in Table 1 at face value for all three surfaces after bombardment, an increase in E is always associated with an increase in log A. Fig. 6 shows that a linear relationship exists for each of the three orientations. However,
The results show that the catalytic activity of a silver surface can be altered considerably by bombarding it with positive argon ions. With the same crystal, after altering the bombarding energy, systematic changes in A and E were observed which must be associated with some change in the surface produced by the bombardment. The changes in A of several orders of magnitude may imply changes in various factors, including the number of active sites. The surfaces remained smooth, and therefore the surface area is probably not altered appreciably. Assuming that the reaction mechanism remains the same, it seems likely that a change in the number of active sites is the predominant factor. This could be associated with changes in the numbers of dislocations or of point defects such as vacancies, interstitial atoms or argon atoms embedded in the lattice. Changes in the number of point defects cannot be responsible for the change in A, since each crystal was annealed at 250°C in vacua for at least 18 hr before the catalytic measurements were made; after this treatment the concentration of vacancies and interstitial atoms should be negligible,os) and the concentration of argon atoms should have attained the same value whatever the bombarding conditions had been. However, the possibility that dislocations are responsible for the change in A cannot be eliminated. Striking evidence has been given that ion bombardment produces disoriented regions(s) bounded by stable arrays of dislocations. This was found by observations on electron-diffraction transmission patterns of single crystals of silver. In our work transmission patterns could not be taken, since the
THE CATALYTIC
ACTIVITY
OF SILVER CRYSTALS
presence of the holes required for this technique would have made the specimens unsuitable for the rate measurements. However, by reflection electron diffraction, disorientation was detected on some of the crystals used as catalysts. The single crystals used by 0GILVIE(2) had orientations close to those used in the present work, and his patterns showed that a change in the structure takes place after bombardment, even with ions of very low energies, with one exception, viz. the case in which a (111) surface is bombarded at 22.5 V. The appearance of extra spots, the arcing of spots and, in some cases, the presence of spotty rings were interpreted as indicating the formation of a surface layer in which small crystalline blocks are rotated through different angles. Thus, ion bombardment gives rise to a greatly increased number of dislocations. It is now proposed that the active sites for the reaction are those where dislocation lines come to the surface. Furthermore, the transmission diffraction patterns of a crystalfs), when compared with those of HIRSCHet &.(ls), show that the disorientations intraduced by ion bombardment are at least as great as those resulting from cold roiling. Heavy cold work on a polycrystalline aggregate results in an increase in the dislocation density by a factor of about 104.(13) This increase is comparable to the maximum increase in A on varying the bombarding conditions for a given crystal. The fact that for the (111) surface (which is obtainable without disorientation) A increases owing to ion bombardment in approximately the same ratio as the dislocation density, strongly suggests that the increase in A is predominantly due to the increase in the number of active sites and not in other factors such as entropy and steric factors. As shown in Fig. 6, for each crystal an increase in A was always accompanied by an increase in E. Thus, the increase in A by several orders of magnitude on bombardment does not give rise to a similarly large increase in reaction rate, but is, at least partly, compensated.@n This increase in E with the number of active sites, i.e. with the density of dislocations at the surface, could be due to an interaction between the dislocations. As the new dislocations introduced by bombardment are confined to the boundaries of the small crystalline blocks, their local density is very high and their interaction correspondingly strong. The energy of X
OF VARIOUS ORIENTATIONS
309
each site could therefore be lowered, giving an increased value of E associated with the increase in A. It should be emphasized that the presence of dislocations changes not only the geometry of the surface but also the local electronic properties. To make these ideas more definite, we may note that as different blocks in the surface are rotated through various angles by the ion bombardment, the dislocation density is not the same in all boundaries. Hence, because of the interaction, active sites in different parts of the surface may be characterized by different energies. However, if the interaction between the dislocations, which is responsible for the change in their energy as their number increases, operates only when they are close together, one would predict that the sites in the tilt boundaries would have a range of activation energies distributed fairly closely about a mean. While some of the dislocations intersecting the surface in an undeformed crystal will be incorporated in tilt boundaries after bombardment, a proportion will remain isolated and unaffected. Thus, we may assume for the sake of simplicity that only two types of site are present in the surface, viz. those with a lower activation energy, El, typical of the isolated dislocations, and those with a higher activation energy, Es, typical of the new dislocations crowded into boundaries. If we then have a concentration q of sites with El and a eoncentration cs of sites with Ez, the observed rate of the reaction should be expressed by: p = k{cl exp(-El/RT)+cz
exp(-&/RT)t,
where k may depend on the orientation. The observed values of A and E depend on the relative magnitude of the two terms. If, for a given orientation, we choose for El and Es the extreme experimental values of E, we can discuss the significance of the measured A values in terms of the concentrations cl and cs. These E values are: for (111) El = 12, Es = 22; for (110) El = 24, Es = 34; and for (100) El = 30, Es = 35 kcal/ mole. Let us assume t&t in aI1 cases the concentrationcz is 106sites/cm~. For an undeformed crystal, only this one type of site is present in the surface and ca = 0; then and only then are the observed values of A and E representative of aII the sites. (hh~.&&on~~~~~ show that if the new
310
H.
M.
C.
dislocations with activation energy Ez are introduced in increasing numbers, their effect on the measured values of A and E will be negligible until cs has reached 100,s sites/cm2 for the (111) and (110) surfaces, and 107.5 sites/cm2 for the (100) surface. A further increase in cs by a factor of 10 makes the two terms of comparable magnitude and the measured A and E values are composite and larger. When ca is greater still (1011’s or 109.5 sites/cm2, respectively), the second term predominates to such an extent that the measured values of A and E are representative only of the higher-energy sites. This analysis shows that, although a substantial amount of deformation may be present in the surface without having any effect on its catalytic activity, heavy deformation should produce increases in the values of A and E provided that this deformation gives rise to a large number of sites of activation energies at or closely distributed around Ez. It is now possible to associate the change in catalytic properties of the surface with its structure as revealed by electron diffraction. The (111) crystal. Electron diffraction shows that the crystal bombarded at 22.5 V is not deformed, but that there is disorientation at all other voltages. Thus, at 22.5 V, cz = 0, and the only sites are the 10s dislocations/cm2 with activation energy 12 kcal!mole. The changes in A by a factor of 105 and in E of 10 kcal/mole after bombardment at 4000 V can be explained by the introduction of about 10s x 105 = 10” sites/cm*, having an activation energy of about 22 kcal/mole. The analysis shows that if the original 10s sites of low activation energy remain, their effect is swamped by the 10” new dislocations. It is seen that for this orientation, k = 102.3,/106 = 10-3.7 (in arbitrary units). The (110) crystal. Electron-diffraction evidence indicates that this surface is disoriented by bombardment at all voltages, even the lowest. Catalysis results show increases in A by a factor of lo6 and in E of 10 kcal/mole when the voltage is increased. This can be explained if low-voltage bombardment results in a surface containing lo6 sites/cm2 with EI = 24 k&mole and up to loo’s sites/cm2 with Es = 24 k&/mole. The latter number of dislocations does not affect the catalytic rate, but should be sufficient to provide the disorientation detected by electron diffraction. With increased bombarding voltage, more dislocations are introduced, and cs increases from 1OO.sto 10”‘s sites/cm2 and the higher activation energy is observed. A maximum concentration of 10’s sites/cm2 occurs after bombardment at 500 V. For this orientation k = 107’l/lOs = 10”‘. The (100) crystal. Arguments similar to those used for the (110) crystal apply. Indeed, additional evidence from a thermally etched crystal (4” from (lOO)), which gave E = 30 kcal/mole and log& = 10, supports the conclusions that the lowest-voltage bombardment produces a surface whose catalytic behaviour is indistinguishable from that of an annealed crystal. The maximum wncentration of dislocations is introduced at the highest voltages, when cs = lad x 104 = lOlo sites/ems. The uncertainty in the values is large, but, at face value, k = 109~s/lOs = 108.2.
SOSNOVSKY The variations of k, El and EO with orientation are still unexplained, but may be due to an anisotropy of properties associated with a dislocation line intersecting a free surface of differing orientation. However, the values of A and E change together, so that a compensating effect is already present before the number of sites is increased by the bombardment. 5. CONCLUSIONS
1. The large changes in the catalytic activity obtained after bombarding silver surfaces with positive argon ions are most likely due to an increase in the number of dislocation lines intersecting the surfaces. 2. For a given crystal there is a compensating effect (increase of E is always associated with an increase in A) which implies that as the number of dislocations increases, the energy of the sites decreases. This may be due to an interaction between the dislocations. 3. For the same bombarding conditions the values of A and E depend on the orientation of the surface. Ackno&dgements-I wish to thank Mr. E. GILLAM, Dr. G. J. OCILVIE and Mr. J. F. NICHOLASfor their help in discussing the work, and Mr. H. JAEGBR and Mr. U. D. hhNARIN for their assistance with the experiments.
RJZFERENCI?S 1. FARNSWORTHH. E. and W~OLXOCK R. F., Advances in Catalysis Vol. 9, p. 123. Academic Press, Inc., New York (1957). 2. OGILVIE G. J., J. P/tys. Cha. Soti 10, 222 (1959). 3. ECKELL J., 2. Electrochem.39, 433 (1933). 4. Curry L. E. and GRANATOA. V., J. Chews. Phys. 26, 96 (1957). RIENXCKERG., 2. Electrochem.46, 369 (1940). SOSNOVSKY H. M. C., OCILVIE G. J. and GILLM~ E., Nature, Land. 182, 523 (1958). SOSNOVSKYH. M. C., J. Chetn. Pkys. 23, 1486 (1955). Gow K. V. and CHALMERSB., Brit. J. Appl. Phys. 2, 300 (1951). 9. GILLAM E. and OGILVIB G. J., To be published. 10. Sar~z F., Advances in Phys. 1, 43 (1952). 1.1. LB Curve A. D. and ROWE A. H., Reu. MttaZl. 52, 94 (1955). 12. HIRSCH P. B., KELLY A. and MENTER J. W., Rut. Phys. Sot. E%8,1132 (1955). 13. HIRSCH P. B., Progress in Metal Physics Vol. 6, p. 236. Pergamon Press, London (1955). 14. CR~~ER E., Advances in CataZysis Vol. 7, p. 7:. Academic Press, Inc., New York (1955). 15. NICHOLASJ. F., Private communication.