Exoelectron emission from metals subjected to friction and wear, and

Exoelectron Emission from Metals Subjected to Friction (7) (8) (9) (70)

J. Weiss, Adv. Catal., 4, 343 (1952). E. Saito and B. H. Bielski, J . Am. Chem. SOC.,83, 4467 (1961). W. T. Dixon and P. 0. C. Norman. J . Chem. Soc.. 31 19 (1963). Y. S. Chiang, J. Craddook, D. Mickervich, and J. Turkevich, J : Phys. Chern., 70, 3507 (1966). (11) M. S. Bains, J. C. Arthur, Jr., and 0. Hinojosa, J . Phys. Chem., 72, 2250 (1968). (12) . . M. Setaka, Y. Kirino, T. Ozawa, and T. Kwan, J. Catal., 15, 209 (1969). (13) Y. Shimizu, T. Shiga, and K. Kuwata, J. phys. Chem., 74, 2929 (1970).

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(14) M. Bains, J. Indian Chern. Soc., L 111, 83 (1976). (15) Y. Ono, T. Matsumura, N. Kitajima, and S.Fukuzumi, J. Phys. Chem., 81, 1307 (1977). (16) U. Matsuura, H. Kubokawa, and 0. Toyama, Nippon Kagaku Zasshi, 81, 1209 (1960). (17) J. Rabani and S. 0. Nielsen, J . Phys. Chern., 73, 3736 (1969). (18) M. G. Evans and N. Uri, Trans. faraday SOC., 45, 224 (1949). (19) I. Mochida and K. Takeshita, J . Phys. Chem., 78, 1653 (1974). (20) W. M. Latimer, "Oxidation Potential", Prentice Hall, New York, N.Y., 1952.

Exoelectron Emission from Metals Subjected to Friction and Wear, and Its Relationship to the Adsorption of Oxygen, Water Vapor, and Some Other Gases Yoshihiro Momose" and Takashi Namekawa Department of Industrial Chemistry, Faculty of Engineering, Ibaraki University, Hitachi, Ibaraki, Japan (Received November 2, 1977)

A modification of a Geiger counter permitted continuous measurement of exoelectron emission (EEE) from rubbing surfaces of metals (Fe, Ni, Al, and Cu). All metals during friction in the Geiger-countergas (C2H60H-Ar) gave two quite distinct types of EEE, namely, EEE in the dark (this will be termed "dark emission") observed more strongly at the start of friction, and optically stimulated EEE (OSEE) becoming stabilized in the latter stage of friction. The former occurred only while the metal was rubbed both with and without optical stimulation. The latter occurred both during friction and even after cessation of friction, being strongly influenced by the adsorption of oxygen and water vapor on the freshly exposed metal surface. The order of decreasing activity of the OSEE after exposure to oxygen and water vapor following friction was H 2 0 > p2(Fe); 0 2 > H20 (Ni); O2 > H20 (A1 rubbed only for a short time); H20 > O2 (A1 rubbed repeatedly); H 2 0 = O2 (Cu). The activity of OSEE from iron and nickel after exposure to oxygen at various pressures decreased with increasing pressure, particularly the former being strongly suppressed by the adsorption of a small amount of oxygen. The activity of OSEE from nickel was also enhanced by the interaction of hydrogen. The OSEE from iron rubbed in various gaseous environments was well correlated with the amount of iron wear particles produced during this friction process except for the environment of oxygen and water vapor. The ratio of the OSEE to the amount of the wear particles with the water vapor was much larger than that with the other environments.

Introduction EEE from new mechanically created surfaces contributes to enhanced activity of the surfaces, which can lead to rapid surface oxidation in the presence of oxygen, or other chemical reactions with environmental species.1#2However, to our knowledge, only a few attempts have been made to establish the use of mechanically induced EEE as a technique in trib~logy.~ The use of a Geiger counter, which exposes the surface under test to the counter gas, makes unequivocal interpretation of the adsorption processes difficult. However, the technique is considered to be useful in providing an empirical method of attack on problems associated with friction and wear. Previous investigations4 have demonstrated the dependence of EEE from ground aluminum powder on the adsorption of gases. It is of considerable interest to investigate EEE not only after grinding but also during mechanical treatment such as sliding friction, when there is intensive development of defects and formation of new surfaces. The present paper is associated with practical metal surfaces interacting with the environments, not with well-defined surfaces. A new procedure was developed for studying the interaction of gases with rubbing surfaces, and a rig, which permitted continuous measurement of EEE during and after friction, was constructed by modifying the Geiger counter. The following tests were conducted: (1)the emission behavior during and after friction, ( 2 ) the behavior of OSEE after exposure to gaseous

environments such as oxygen and water vapor, and (3) the relationship between the OSEE from iron rubbed in various environments and the amount of iron particles produced during this friction process.

Experimental Section (a) Materials. Metal specimens were rolled sheets of iron (purity, >99.7%), nickel (>99.7%), aluminum (> 99.5%), and copper (>99.9%). The flat metal specimen was 30 X 30 X 0.1 mm (Fe, Ni, and Cu) and 30 X 30 X 0.3 mm (Al). These specimens were degreased with benzene solvent and then annealed in vacuo (about 0.13 N/m2) for 1 h a t 955 OC (Fe), for 2 h at 350 "C (Fe, Ni, and Cu) (iron specimens of this type were usually used), and for 1h a t 300 "C (Al) before use. Gaseous environments used were argon (purity, >99.99%), oxygen (>99.95% 1, redistilled water vapor, hydrogen (>99.99% ), ethanol vapor, and an atmosphere under vacuum (the pressure in this case was usually about 0.67 N/m2). The composition of the Geiger-counter gas was a mixture of organic vapor (C2H50H,CH,CN, C6H6,or n-C3H7NH2)(2700 N/m2) and argon (11200 N/m2), as shown in Table I, and Q gas (helium plus 1%isobutane) a t atmospheric pressure. The latter was used as a gas-flow counter gas for some tests with nickel. A counter gas of C2H50H-Arwas usually used for all the metals. Here, it should be noted that the gas compositions of these counter gases and other mixtures such as argon-oxygen mentioned later were obtained by use of a mercury manometer.

0022-3654/78/2082-1509$01 .OO/O 0 1978 American Chemical Society

1510

Y. Momose and T. Namekawa


TABLE I: Relationship between Exoelectron Emission from Iron during Friction and Geiger-Counter Gas Background Intensity of Intensity of counting EEE in the stabilized Stability of the Anode Counter voltage, rate, dark,b OSEE,C counter gas: gasa V count/s count/s count/s min (countls) C,H,OH + Ar 1350 1.6 450 2000 >600 (>15000) CH,CN t Ar 1420 1.4 350 2000 >400 (>2500) C,H, t Ar 1640 1.6 400 specimen annealed in vacuo for 1h at 955 "C (820 count/s) > specimen annealed in vacuo for 2 h a t 350 "C (450 count/s) > untreated specimen (100 count/s). In this connection the dark emission was also

Exoelectron Emission from Metals Subjected to Friction

The Journal of Physical Chemistry, Vol. 82, No. 13, 1978 1511

5 n

540000

V

U

z3000C

al

!I c

C

c1

g 2000

w

43 L

1h-h

I

1;-3

4 -

1p

10-1 Amount of Iron Particles(d)

1000

LLlzZezd 30 40

0

I

10

20

Time (rnin) Figure 3. Effect of exposure (E, 0.5 min) to argon-oxygen mixtures on the change of OSEE from iron after cessation of friction: (a) without exposure; (b) exposure under vacuum for 5 min; (c) 0.35 N/m2 O2 (510 N/m2); (d) 0.76 N/m2 O2 (1100 N/m'); (e) 4.7 N/m2 O2 (6900 N/m2). The values in parentheses indicate the total pressure.

observed when an iron bar coated with Pyrex glass was rotated on the metal sheet surface. The OSEE was found to occur only while the specimen was optically stimulated and to decay gradually with time after cessation of friction. On restarting the friction the OSEE rapidly rose to give the original intensity. The OSEE was almost uninfluenced by the pretreatment of the specimen, but apparently depended on friction time. A stronger emission intensity of 15 000 count/s or more was obtained with the specimen subjected to repeated rubbing. It was also found that the OSEE intensity with a new specimen increased with a phased increase of the speed of the rotator and became stabilized a t each speed. The intensity for each speed was 400 (100 rprn), 700 (300 rprn), 900 (400 rprn), and 1400 count/s (550 rprn). Table I shows the comparison of two types of EEE obtained from iron during friction in the presence of various counter gases. It is apparent that both C2H50H-Ar and CH,CN-Ar counter gases were more stable to the friction of iron than the other counter gases, and therefore the C2H50H-Ar counter gas alone was used. The OSEE intensity ( I ) after cessation of friction was represented approximately by the equation I = where t is the time and k a decay constant, and the OSEE with a stronger intensity decreased more slowly. The behavior of OSEE from the specimen, which had been exposed to vacuum for a time in the course of the decay process, was examined. When the counter gas was readmitted after the exposure, an emission ascending to a maximum (Figure 3, curve b) and subsequently decaying with time was observed. The emission curve differed markedly from that without the exposure (Figure 3, curve a). The time required to reach the maximum became longer with the specimen with a stronger emission intensity before the exposure. Also, when the counter gas containing a small amount of oxygen (11N/m2) was used after the exposure, only a rapidly decaying emission of a much weaker intensity was obtained. On the other hand, the counter gas containing water vapor (400-670 N/m2) did not appreciably affect the recovery behavior of OSEE.

Figure 4. Relationship between the total count of OSEE from iron after friction in various environments and the amount of iron particles produced in this process: (0)Ar (10700 N/m'); (A)O2 (1300 N/m'); ( 0 )H$O (2700 N/m2); (A) counter gas (CzH,OH 2700 N/m' -t Ar 11 200 N/m ); (0)vacuum; (I C2H,0H ) (2700 N/m2). The values in parentheses indicate the pressure of thk environment.

Figure 3 also shows the recovery behavior of OSEE after exposure to mixtures of argon-oxygen. This experiment was conducted using the same specimen by repetition of a series of the following procedure, that is, in sequence the friction, the measurement of decaying OSEE (10 rnin), the exposure to the mixture, the measurement of recovering OSEE (25 min), and again the friction to increase the OSEE. The pressure ratio of oxygen to argon in each mixture was constant. The higher the partial pressure of oxygen became, the more weakly the OSEE intensity was restored. In order to represent quantitatively the recovery of OSEE after exposure, the ratio of the total count of OSEE measured for 20 min after readmission of the counter gas following the exposure to the environment to that estimated from the decay curve of OSEE without the exposure was determined. For example, the recovery in percent of OSEE for each partial pressure of oxygen was 76 (0.12 N/m2), 29 (0.54 N/m2), 16 (1.3 N/m2), and 5% (4.7 N/m2). Here a recovery of 100% refers to the OSEE without exposure. In addition, after exposure to oxygen a t a pressure of 13 300 N/m2 for 1 min, there was no recovery of OSEE. On the other hand, the recovery of OSEE after exposure to water vapor a t pressures varying from 1100 to 2700 N / m 2 for 4 min was about 85% (median value). The recovery of OSEE after vacuum exposure for 5 rnin was about 73% (median value). Here, it should be noted that the median value of the recovery of OSEE after exposure was obtained from the data of two to six tests in each case in the present work. It is apparent that the activity of OSEE was suppressed by the adsorption of oxygen, but that the water vapor interaction slightly increased the activity compared with vacuum exposure. Furthermore, the recovery of OSEE after exposure to hydrogen a t 13300 N / m 2 for 1 min was about 20% (median value). Figure 4 shows the relationship between the amount of iron particles produced by friction for 120 min in the presence of various gases and the total count of OSEE obtained for 20 min after admitting the counter gas following removing these gases for 5 min. In this experiment a new specimen was used in each case. It can be seen that the experimental data gave a nearly linear plot on a double logarithmic scale with the exception of the data for oxygen and water vapor. The data for oxygen and water vapor were located on both sides of the straight line. The order

1512



1 ';T----------_4 I

0

5 '10

I

20

30

I

I

40

50

----------60

I

I

70

80

Time (rnin) Time (rnin)

Figure 5. Effect of vacuum exposure on the behavior of OSEE from nickel after cessation of friction: (a) when the anode voltage was reduced to 0 V for 5 min (D) without exposure under vacuum; (b) exposure under vacuum for 5 min (E); (c) exposure under vacuum for 5 rnin (E),and then to ethanol vapor (2700 N/m') (F).

of decreasing amount (median value) of the iron particles for each environment was Ar (10570 pg) >> O2 (2510 pg) > H 2 0 (500 pg) > counter gas (C2H,0H-Ar) (100 pg) > vacuum (50 pg) = CzH50H(50 pg). Also, it turned out that the total count of OSEE expressed in units per microgram of the amount of iron particles in these environments differed widely. The decreasing order of this value (median) was H20 (1130 count/pg) > Ar (360 countlyg) > vacuum (90 countlpg) > counter gas (C2H,0H-Ar) (65 count/pg) = C2H50H(65 count/pg) > O2 (zero count/pg). Here, it should be noted that the OSEE originated in the metal sheet as well as the iron particles. A striking observation is that the adsorbed water tended to increase the activity of OSEE, while the adsorption of oxygen completely destroyed the OSEE active sites in spite of the greater amount of wear particles. Furthermore, the ethanol vapor was found to give only a smaller activity of OSEE comparable to that under vacuum in spite of having a hydroxyl group. (b) Exoelectron Emission from Nickel. When the specimen had been rubbed in the counter gas (C2H50H-Ar), some welding took place between the metal sheet and the metal particles produced by friction. The emission behavior during friction was similar to that of the iron specimen. The dark emission was observed a t the start of friction, giving the intensity of about 400 count/s. The rise of OSEE occurred within 1 min after the commencement of friction. On stopping the friction the OSEE decayed with time through a small maximum, which distinctly appeared 2 min after, as shown in Figure 5. Some tests were conducted concerning the behavior of recovering OSEE, as shown in Figure 5. After reducing the anode voltage to 0 for 5 min without substitution of the counter gas a smoothly decaying OSEE was obtained (curve a), and even after previous exposure to ethanol vapor as a component of the counter gas the recovering OSEE was observed (curve c). In addition, a counter gas of Q gas was found to give a recovering OSEE after vacuum exposure similar to that of the counter gas (C2H,0H-Ar). These suggest that the recovering behavior is primarily associated with the evacuation process before vacuum exposure, not with the counter gas. The recovery of OSEE after repeated friction and exposure (1min) to oxygen a t various pressures, which was obtained in the same manner as with the iron specimen (Figure 3), was 280 (1300 N/m2), 220 (6700 N/m2), 200 (20000 N/m2), and 160% (26700 N/m2) for each pressure. In addition, the recovery after vacuum exposure for 5 rnin

Figure 6. Exoelectron emission from rubbing aluminum: (I, 111) during friction; (11) without friction; (-) with optical stimulation; (- - -) without optical stimulation.

was about 88% (median value). A striking observation is that the nickel specimen having been exposed to oxygen gave rise to an increasing of OSEE in contrast with the iron specimen, but the recovery of OSEE tends to decrease with increasing pressure of oxygen. In connection with the action of oxygen, it should be noted that when the counter gas containing a small amount of oxygen (64 N/m2) was substituted for the normal counter gas in the course of the OSEE decay process, the OSEE intensity decreased to about 1/5 of its original value, but the OSEE was restored to the original intensity by removing this counter gas and readmitting the normal counter gas. On stopping the friction the counter gas containing oxygen gave only a decaying OSEE without yielding a small maximum as shown in Figure 5. The recovery in percent (median value) of OSEE after repeated friction and exposure to various environments was 80 (vacuum, 5 min), 140 (02,14000 N/m2, 1rnin), 74 (HzO, 2000 N/m2, 4 min), and 155% (Hz, 13 300 N/m2, 1 min). The values in parentheses indicate both the gas pressure and the exposure time. The hydrogen as well as the oxygen gave a much higher activity of OSEE, but the activity for water vapor was almost the same as that for vacuum exposure when considering the deviation of the data. The emission behavior was also examined using Q gas. This counter gas gave rise to both dark emission and OSEE with a steady intensity of about 40 count/s. The OSEE from the same specimen was compared using both C2HjOH-Ar and Q counter gases. The emission intensity for the former was greater than that for the latter by a factor of about 20-30. This suggests that the OSEE was strongly influenced by the counter gas.4 On stopping the friction Q gas gave a rapidly decaying OSEE without giving a small maximum as shown in Figure 5. (c) Exoelectron Emission from Aluminum. Some welding occurred between the metal sheet and the metal particles produced by friction. Figure 6 shows the emission behavior with the counter gas (C2H50H-Ar)both with and without optical stimulation. The dark emission continuously occurred, rising to a maximum a t the beginning of friction, subsequently decreasing, and then becoming stabilized (Figure 6, dotted line), but only while the specimen was rubbed. The aluminum specimen yielded a dark emission with a considerably stronger intensity than the other metals; further the dark emission increased with increasing speed of the rotator. On the other hand, a strong OSEE was also observed in spite of the lower speed of the rotator. On stopping the friction the OSEE increased steadily over a period of more than 2 h, reached a maximum (Figure 6, solid line), and then decayed slowly.

Exoelectron Emission from Metals Subjected to Friction

This behavior differed extremely from that of the other metals. In connection with this emission behavior, the effect of the counter gas containing a small amount of oxygen on the OSEE while the friction was stopped was examined. With the counter gas containing oxygen (8 N/m2) the OSEE decreased to 1 / 2 of that for the normal counter gas, increasing with time more slowly. With the counter gas containing oxygen (33 N/m2) the OSEE decreased to about 1/15, and only decayed with time. The recovery in percent of OSEE after exposure to oxygen, water vapor, or under vacuum was examined using two types of the test specimen: the specimen rubbed in the counter gas only for 10 min before each exposure and the specimen repeatedly subjected to friction and exposure in the same manner as with the iron or nickel specimen. The former specimen was treated in the following procedure, that is, friction (10 min), measurement of OSEE (20 min), exposure to the environment (20 rnin), and then measurement of OSEE (20 min). The color of the metal surface remained unchanged. The recovery in percent (median value) of OSEE after each exposure was 58 (vacuum), 94 (02,13300 N/m2), and 71% (HzO, 2000 N/m2). With the latter specimen the metal surface turned dark along with some welding of the wear particles. The darkly colored particles on the surface were soluble in a solution of hydrochloric acid, and so this material was the specimen itself. The recovery in percent (median value) of OSEE in this case was 91 (vacuum, 5 rnin), 77 (02,13300 N/m2, 1min), and 100% (HzO,2000 N/m2, 4 rnin). It was also found that the activity of OSEE from the wear particles alone produced by friction was more strongly increased by the interaction of water vapor than that of oxygen. These results revealed that oxygen and water vapor had an opposite effect with respect to each other on the activity of OSEE for each type of the specimen. The recovery of OSEE after exposure to hydrogen (13300 N/m2, 1 min) obtained in the same manner as with the specimen rubbed only for 10 min was over 100%. (d) Exoelectron Emission from Copper. A firm metal-metal welding occurred between the wear particles and the metal sheet, and therefore the rotator was finally brought to a stop by increased frictional force. Under these circumstances some results were obtained. A dark emission of about 200 count/s was continuously observed for a long time, and the OSEE became stabilized a t the intensity of 800 count/s. On stopping the friction the OSEE decayed more rapidly than that of the iron or nickel specimen. The recovery of OSEE after exposure to oxygen (13300 N/m2, 1rnin), water vapor (1300 N/m2, 4 min), or under vacuum (5 min) was about 35% in all cases.

Discussion (a) Exoelectron Emission in the Dark (Dark Emission). It is well known that very high surface temperatures can be reached in sliding systems where some metal-metal , ~ the metal surface in the present contact O C C U ~ S . ~ On experiments which was rubbed at a load of about 60 g and at sliding speeds varying from 0.5 (Fe) to 0.2 m/s (Al), high local temperatures appear to have developed. The dark emission may be associated with the state of high excitation on the rubbing surface. Specifically with aluminum and copper specimens readily causing the metal-metal welding, the emission was continuous during friction, and therefore may be due to the relative softness of these specimens. Wei and Lytle6 have reported that the intensity of the dark emission from abraded metals can be well correlated with the standard heat of formation of the stable oxide, further the emission being of a long lifetime nature and

The Journal of Physical Chemistry, Vol. 82, No. 13, 1978 1513

decreasing with aging. On the other hand, the emission in our experiments ceased abruptly once the friction was stopped. Therefore, the emission is unlikely to be caused by a gas interaction between newly excited regions of metal and some species such as oxygen, which must have remained as small amounts of impurities in the counting gas atmosphere. Gieroszydski and Sujak7 and Arnott and Ramseys have reported that the dark emission from aluminum occurs discontinuously during tensile deformation, the emission intensity being greater the thicker the oxide film and the higher the strain rate.7 Further, the dark emission has been found to occur from oxide-covered nickel during tensile d e f o r m a t i ~ n . ~This J ~ type of EEE has definitely been shown to be associated with cracking or rupture of the oxide film.1° In our experiments the iron specimen oxidized in air gave the strongest intensity of emission of all types of the iron specimens. Therefore, it appears that with all the metals thin, adherent oxide film on the metal specimen may take part in the dark emission, probably being strongly associated with the much stronger emission observed a t the beginning of friction. (b) Optically Stimulated Exoelectron Emission (OSEE). The following values are recommended as the work function of each polycrystalline rneta1s:ll Fe, 4.31 eV (threshold wavelength, 278 nm); Ni, 4.50 eV (275 nm); Al, 4.25 eV (291 nm); Cu, 4.40 eV (281 nm). The wavelength of light irradiated was above 295 nm. Himmel and KellylO have suggested that unless gases such as water vapor are rigorously excluded, any intrinsic enhancement in the photoyield, such as that associated with surface roughening, may be completely overshadowed by environmental effects. Therefore, the OSEE in our experiments is associated with the metal surface with lower work function resulting from the adsorption of ambient reactive gases. The alterations of the work function due to the adsorption of oxygen or water vapor, causing a shift of the threshold wavelength and thus enhancing or reducing the electron yield, can account satisfactorily for the development and decay process of OSEE from the metal surface subjected to friction and wear. Oxygen and water vapor which can easily interact with the surface must have been contained as residual impurities in the counting gas atmosphere. To begin with, we will explain the emission behavior of the iron specimen shown in Figure 2 as a result of the adsorption of oxygen and water vapor (Figure 4). The thin oxide film with which the metal surface has been initially covered is ruptured and subsequently a fresh metal surface is continuously created during the friction process. During the friction time required before the OSEE starts to rise, the adsorption of oxygen on the freshly exposed surface predominates, thus failing to generate the OSEE. At the end of this stage the adsorption of water vapor starts to become more effective on the freshly exposed surface because of an increased consumption of oxygen due to oxide formation, causing the lowering of the work function and thus the OSEE being able to develop. A saturation of the OSEE in the latter stage of friction as shown in Figure 2 implies that two competitive processes are occurring a t the same time, namely, the forming of OSEE active sites by producing the freshly exposed surface and the vanishing of these sites by the adsorption of oxygen leading to oxide formation. The latter process causes a decaying OSEE once the friction is stopped, as shown in Figures 2 and 3. In the case of nickel, the OSEE was found to decay with time through a small maximum observed 2 min after

1514



cessation of friction, as shown in Figure 5. In connection with this maximum, we will refer to the paper of Quinn and Roberts12who have studied the interaction of oxygen with nickel surface using the photoelectric emission technique. They found that the photocurrent from the metal decreased rapidly on admitting oxygen (region AB), and that this decrease in photocurrent was followed by an appreciable increase and a subsequent decrease (region BCD) while the oxygen pressure gradually increased. These regions refer to Figure 2 of their paper.12 Therefore, they have suggested that by interpretation of changes in the work function the region AB corresponds to a buildup of a chemisorption layer and the region BCD indicates incorporation of oxygen to form nickel oxide. Since the adsorption of water vapor on the rubbed nickel specimen was of less importance in the increase of OSEE activity, the measurable maximum in our experiments may be associated with the place exchange of adsorbed oxygen with the underlying metal, followed by the subsequent formation of oxide. The effect of reactive gases such as water vapor and oxygen on OSEE from mechanically treated aluminum has been studied in detai1,13J4and is now reasonably well understood. According to a review by S ~ h a r m a n n ,at ' ~first a fresh surface produced by mechanical treatment is covered by a monolayer of oxygen. Probably the two atoms of an oxygen molecules are dissociated a t this stage, one of them exchanging its place with a metal atom. This layer adsorbs water vapor or hydroxide ions which due to their dipole moment lowers the work function. A t the end of an incubation time the OSEE intensity increases. Additional interaction of oxygen increases the work function, causing the decay of the emission with time. This decay is believed to be due to the nucleation and growth of a discrete oxide film on the exposed metal surfaces, along with the gradual incorporation and disappearance of the chemisorbed water monolayer.1° Therefore, the emission becoming stabilized with friction time, as shown in Figure 6, may be controlled by the adsorption of oxygen and water vapor on the newly exposed metal surface. Scheibe and Feller16 have reported that OSEE from aluminum during friction starts to increase when the coefficient of friction becomes larger, which means that the friction between fresh metals exposed by the cracking of oxide layer begins to occur. A very gradually increasing emission over a long period after cessation of friction, as shown in Figure 6, may be attributable to the fact that the initial adsorption of oxygen, which develops the OSEE, becomes much slower on the freshly exposed surface, probably because oxygen having been contained in the counting gas atmosphere was increasingly consumed to form aluminum oxide (the amount of oxygen molecules contained was about 7.5 X 1015on the assumption that the partial pressure of oxygen was 0.13 N/m2). In this connection the consumption of oxygen in the counting gas atmosphere may be associated with the fact that with the iron specimen the stronger OSEE intensity decreased more slowly. (c) Behavior of O S E E after Exposure t o Gaseous E n vironments. The recovery of OSEE after exposure to oxygen and water vapor obtained under present experimental conditions is compared with that after vacuum exposure in Table 11. It can be seen from Table I1 that the activity of OSEE for all the metals was increased or maintained almost unchanged by the interaction of water vapor, but that the adsorption of oxygen had a widely different effect on the activity of OSEE. On the basis of the results in Figure 5 , the behavior of recovering OSEE after vacuum exposure, the intensity of

TABLE 11: Comparison of Recovery of OSEE after Exposure to Oxygen and Water Vapor with that after Vacuum Exposure Exposure environment Metal Fe Ni Ala A1 cu

0,

-_

Jv t

tt tt

0

0

0

-

t

+

a Specimens rubbed for only 10 min before each exposure. Specimens repeatedly subjected to friction and exposure. The symbols + t , t, 0, -, and -- represent very much larger, larger, nearly equal, smaller, and very much smaller compared with the recovery of OSEE after vacuum exposure, respectively.

which gradually increased and then became stabilized, may be accounted for as a result of the desorption of some negatively charged species1' during the evacuation process, which have been generated on the metal surface, followed by the reproducing of the species along with the adsorption causing the lowering of the work function. The OSEE in our experiments is basically considered to correspond to the adsorption-controlled photoelectric effect on the newly created metal surface, but from the above consideration electrons localized at oxide defects and adsorbed molecules on the surface also seem to participate in the emission.18 In the case of nickel the interaction of oxygen with the fresh metal played a very important role in the increase of OSEE activity. This increase may be associated with the incorporation of chemisorbed oxygen,12but an observation of the emission activity decreasing with increasing pressure of oxygen may be explained as a result of the growth of oxide film on the metal surface. In this connection Smithlg has reported that photoelectron emission from nickel sheets heated in oxygen for 3 h at 200 "C and for 1 h a t 500 "C increases with increasing oxide thickness, thus originating in the oxide. This behavior on oxygen differs from that in our experiments. This suggests that the OSEE from nickel in the present work may be associated with the adsorbed layer with many imperfections. In connection with the interaction of hydrogen with nickel, Quinn and Roberts12have reported that hydrogen adsorbed on nickel changes the photocurrent by only 1070, but that the photoactivity of oxidized nickel is enhanced by a factor of 10 by adsorbed hydrogen. Therefore, the increase in the OSEE activity after hydrogen interaction may be attributable to previously chemisorbed oxygen on the fresh metal surface. The results for two types of the aluminum specimen can be related to the elementary process of metal oxidation. With the test specimen rubbed for only 10 min, first, the freshly exposed metal surface must be sufficiently covered with a monolayer of oxygenlj in order to lower the work function, and therefore the adsorption process of oxygen becomes a more dominant factor in the increase of OSEE than water vapor, thus resulting in the OSEE activity in the order O2 > H20. With the specimen repeatedly subjected to friction and exposure, the discolored metal surface was covered t o a great extent with reaction products such as oxide and hydroxide. The OSEE in this case is likely to be strongly associated with the oxide defects. The work function of such a surface tends to increase by the growth of oxide film after oxygen interaction, but to decrease by the adsorption of water vapor. Thus the OSEE activity decreased in the order HzO > 0 2 .

XPS of

Nickel Catalysts


Acknowledgment. The authors thank N. Kobayashi for the construction of parts of the apparatus and Professor Y. Tamai of Tohoku University for his interest and encouragement. References and Notes (1) C. N. Rowe and W. R. Murphy, "Proceedings of the Tribology Workshop", Oct 1973 (published Apr 1974). (2) I. L. Goklblatt, I d . Eng. Chem., Prod. Res. Develop., 10, 270 (1971). (3) V. D. Evdokimov. Sov. Phvs.-Dokl. Tech. Phvs.. 13. 475 (1968). i4) Y. Momose, Y. Iguchi, S.Iihii, and K. Komatsuiaki, J. Phys. Chem:, 80, 1329 (1976). (5) F. P. Bowden and P. H. Thomas, Proc. R. Soc. London, Ser. A , 223, 29 (1954). (6) P. S. P. Wei and F. W. Lytle, J . Chem. Phys., 64, 2481 (1976).

(7) (8) (9) (10) (11) (12) (13) (14) (15)

1515

A. Gieroszyhki and B. Sujak, Acta Phys. Polon., 28, 311 (1965). D. R. Arnott and J. A. Ramsey, Surf. Sci., 28, 1 (1971). B. Sujak and A. Gieroszyhki, Acta Phys. Polon., A37, 733 (1970). L. Himmel and P. Kelly, Comments Solid Sfate Phys., 7, 81 (1976). V. S. Fomenko, "Handbook of Emission Properties of Materials", G. V. Samsonov, Ed., Naukova Dumka, Kiev, 1970, a Japanese

translation from Russian. C. M. Quinn and M. W. Roberts, Trans. Faracby Soc., 61, 1775 (1965).

J. A. Ramsey, Surf. Sci., 8, 313 (1967). E. Linke and K. Meyer, Surf. Sci., 20, 304 (1970). A. Scharmann, Int. Symp. Exoelectron Emission Dosimetry(Proc.), 4th, 1973, 12 (1974). (16) A. Scheibe and H. G. Feller, 2. Metallkd., 65, 268 (1974). (17) I. V. Krylova, Phys. Status SolidiA, 7, 359 (1971). (18) A. G. Gel'man and A. I. Fainshtein, Sov. Phys.-Solid State, 14, 1752 (1973). (19) T. Smith, J . Appl. Phys., 46, 1553 (1975).

Investigations of Antigorite and Nickel Supported Catalysts by X-ray Photoelectron Spectroscopy Jacques C. Vedrine, * Institut de Recherches sur la Catalyse, C.N.R.S., 79,boulevard du 1 1 novembre 1918, 69626 Villeurbanne Cedex, France

Guy Hollinger, and Tran Minh Duc Institut de Physique Nucleaire, Universite de Lyon I, 43, boulevard du I 1 novembre 1918, 69621 Villeurbanne, France (Received December 19, 1977: Revised Manuscript Received April 7, 1978) Publication costs assisted by Centre National de la Recherche Scientifique

XPS studies have been carried out on Ni(OH)2,NiO, and Ni supported catalysts after preparation and after heating or reduction by hydrogen. The precursors of the catalysts were a lamellar clay of nickel antigorite and Ni(OH)2impregnated at roughly 15 wt % on different high surface area supports such as Ti02,SOz,Si02-A1203, A1203,and MgO. It was observed that transformation of antigorite into NiO supported over amorphous silica led to an increase of 0.5-1.0 eV in the binding energies measured and also a small increase of Ni2+widths and main peak to satellite splittings. A comparison of the whole range of NiO supported catalysts showed that the binding energies for Ni2+(XPS and Auger peaks) may vary over a 2-eV range while the hydrogen reduction ease decreased with the following sequence of supports: no support > TiOz > SiOz -3i02-A1203 A1203 > MgO. The maximum binding energy value reached that of Ni(OH)2catalyst and it was found, moreover, that the stronger the interaction of NiO with a support, the closer the energy levels to those of Ni(OH)> Such an effect was interpreted not in terms of definite compounds such as silicate or aluminate at the interface, but rather as due to modification of the electronic properties of NiO which looses its individuality as a Mott insulator, Le., with a narrow Ni 3d band and undergoes a much larger unpaired spin delocalization toward the support, resulting in larger NiO-to-support interaction. The spectroscopic features of the 2p peaks of Ni2+ (shake-up lines and multiplet splitting effects) have been emphasized and correlated with the ease of reduction. By contrast no interaction between the silica support and metallic nickel particles could be detected no matter what the particle orientation. Agglomeration of NiO and Ni particles at the surface under the given treatment conditions could be determined following the decrease of the ratios of intensities of the Ni to the Si lines and was found to be in very good agreement with electron microscopy data.

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I. Introduction X-ray photoelectron spectroscopy (XPS) provides an attractive method for studying the electronic structure of solids and is generally regarded as a surface technique because of the characteristic electron mean free path in the 10-20-A range. In heterogeneous catalysis porous solids are generally used, their catalytic properties depending mostly on surface active sites, whose nature is indeed unknown. Catalytic materials are usually oxides or metal dispersed on the surface of an oxidic support in order t o increase the surface area of the active phase accessible to reactants. Unfortunately such supports are mostly insulators resulting in a charge effect because of electron ejection, which broadens and shifts the XPS lines and thus impedes their complete resolution. Nevertheless, being surface sensitive the XPS technique has already been 0022-3654/78/2082-1515$01 .OO/O

successfully applied in a large number of cases for characterizing solid cata1ysts.l Determinations of the oxidation state of surface sites,2 of surface segregation of a given p h a ~ eand , ~ of electronic transfer between an active phase and its support4are the main points of interest for catalytic purposes. Ni supported catalysts are important because of their catalytic role in hydrogenation and hydrogenolysis react i o n ~ .Commercial ~~~ catalysts are mostly prepared by impregnating a Ni(I1) salt onto an oxidic support and further reducing Ni(I1) into the metallic state. Other catalysts with oriented Ni plaquettes6 could also be prepared starting from a clay such as antigorite Ni3(OH)4Si205. Previous experimental studies' have shown that NiO supported catalysts are more or less easily reduced by

0 1978 American Chemical Society

Exoelectron emission from metals subjected to friction and wear, and

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