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Interactions of NO with iron deposited on alumina
Interactions of NO with Iron Deposited on Alumina 1 RANJANI V. SIRIWARDANE 2 Department of Chemical Engineering, West Virginia University, Morgantown, West Virginia 26506 AND
JASON M. COOK Morgantown Energy Technology Center, U. S. Department of Energy, Morgantown, West Virginia 26505
ReceivedMay 10, 1985;acceptedAugust 7, 1985 The interaction of nitric oxide with single-crystal surfaces of alumina at temperatures of 298, 473, and 673 K, which had been covered by various amounts of iron, was studied using X-ray photoelectron spectroscopy. The iron was deposited onto A1203 in the Fe° state. At low coverages, iron was partially oxidized due to its interaction with A1203. Scanning auger mapping analysis showed that the iron was randomly distributed on the A1203 surface. The amount of adsorbed NO increased with increasing iron coverage. However, at very high iron coverages, there was a decrease in adsorption. This indicated that the aluminum ions may have activated the NO adsorption on the iron atoms. For increasing temperature there was also an increase in adsorption for high iron coverages, but the adsorption decreased with increasing temperature for low iron coverages. Sticking probability calculations indicated that the adsorption was mobile and dissociative. Binding energy of the nitrogen peaks indicated that NO was adsorbed onto the Fe/A1203 surface as a nitride. © 1986AcademicPress,Inc.
INTRODUCTION
Nitric oxide is produced during coal combustion processes. The removal of pollutant gases such as NO from the combustion stream is important for environmental reasons. In order to understand the combustion process and to be able to reliably predict the NO concentrations in the combustion stream, it is necessary to study the reactions which occur between this gas and the constituents which would typically be found in entrained ash (1), such as alumina and iron. Various workers (2-6) have studied the adPaper was presented at the 59th Colloid and Surface Science Symposium at Clarkson University, Potsdam, New York, June 24-29, 1985. 2 Present address: Morgantown Energy Technology Center, U. S. Department of Energy, Morgantown, W. V. 26505.
sorption of NO on A1203. Although physisorption was observed in most cases (3-5), Douglas (6) reported chemisorption of NO on alumina. Only a few studies were reported for NO interactions with iron. Honda and Hirokawa (7) used X-ray photoelectron spectroscopy (XPS) and observed the formation of N, NO, NO2, and N O 3 when iron surfaces at 273 K were exposed to NO at pressures of 200 and 760 Torr. Infrared spectroscopic studies (8, 9) indicated the formation of NO + on an iron surface. The interaction of NO at 5-20 Torr with iron evaporated onto stainless steel was studied by Kishi and Ikeda (10). The interaction of NO with iron deposited on silica has been studied by the authors (11). However, none of the above studies dealt with the interaction of NO with iron deposited on A1203. Also, no work was reported at very 504
0021-9797/86 $3.00 Copyright © 1986 by Academic Press, Inc. All rights of reproduction in any form reserved.
Journal of Colloid and Interface Science, Vol. 110, No. 2, April 1986
NO-Fe/AI203 INTERACTIONS
low gas exposures or elevated temperatures in a flow system using XPS. The purpose of this work is to investigate the interactions of NO with both a clean and an iron-covered, single-crystal surface of AlzO3 using XPS. Iron was evaporated onto the A1203 surface, and gas exposures were carried out at each iron coverage. Since the exposure levels of the gases were on the order of several Langmuirs (1 L = 10-6 Torr-s), the results are representative of the early stages of the reaction. The gas exposures were performed on surfaces which were at 298, 473, and 673 K.
505
heating the probe. After the preparation chamber was pumped out the sample was transferred to the detection chamber for data acquisition. These temperatures were maintained even during data acquisition. Charge correction of the XPS peaks was accomplished by using C(1 S) as the standard with a binding energy of 284.6 eV. The experimental uncertainty in the binding energies was +1.0 eV. Scanning Auger mapping analysis of iron deposited on silica was performed using a Perkin-Elmer SAM-590 system. RESULTS AND DISCUSSION
EXPERIMENTAL
Deposition of Iron on Alumina The XPS spectra were recorded with a Vacuum-Generators (VG) ESCA 3 spectrometer. This system consisted of a separately pumped, liquid-nitrogen trapped, spectrometer and sample preparation chambers which were routinely operated in a pressure range of 10 -9 to 10-l° Torr. The preparation chamber was equipped with a resistively heated sample probe, an argon ion etch gun, and an evaporator (all standard VG items). The detector was a retarding grid, hemispherical analyzer. An aluminum anode (1486.6 eV) was used as the X-ray source. Single crystals of A1203, obtained from Atomergic Chemical Corporation, were cut to produce a 1 X 10 X 10-mm sample with a (110) surface. The sample was rinsed with acetone and was cleaned further by argon etching in the vacuum system. After 30 rain of argon etching, the residual carbon on the surface was negligible. After the AlzO3 crystal was cleaned, the iron evaporations onto the surface were performed using a resistively heated tungsten basket containing iron foil. Then high-purity NO (99%), supplied by Matheson, was transferred to the preparation chamber. During each gas exposure the pressure was held constant by using a leak valve. The exposures, which ranged from 1 to 72 L, were carried out on surfaces heated to 298, 473, and 673 K by resistively
An arbitrary value for the Fe:A1 ratio was obtained by dividing the amount of iron [i.e., peak area/photoelectron cross section (12)] by the amount of aluminum. The actual number of iron atoms deposited will be calculated considering factors other than the photoelectron cross section, as shown in Eq. [5]. The amount of iron deposition on alumina was varied by changing the total time of evaporation and is expressed by the Fe:A1 ratio. This ratio was varied from a value of 0.0 (i.e., a clean surface) to a value of ~ (i.e., a surface that is completely covered by iron). The amount of iron deposited during a given exposure time depended upon both the amount of iron in the evaporator and the current used to heat the basket during evaporation. It is assumed that all of the iron atoms remain on the surface. These iron atoms can be removed completely by a light argon etching. There is a drastic increase in the intensity of the aluminum signal during etching. Hence, it is reasonable to assume that the iron atoms remain on the surface. The binding energy value o f the Fe (2P3/2) peak for iron deposited at 298 K was 706.5 ___ 1.0 eV, while the full width at half-maxim u m (FWHM) of the Fe (2P3/2) peak was 4.0 eV for low coverages and 3.0 eV for high coverages of iron as shown in Figs. la and b. The Journal of Colloid and Interface Science, Vol. 110,No. 2, April 1986
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SIRIWARDANE AND COOK
388
low iron coverage was larger than that for high iron coverage. When iron was deposited at 473 and 673 K, the F W H M of Fe (2P3/2) was 5.5 eV for low iron coverages and 3.0 eV for high iron coverages. This indicates that more iron interacted with alumina and that iron was oxidized to Fe 2+ or Fe 3÷ at 473 and 673 K for low iron coverages. In order to determine how the iron was distributed on alumina, scanning Auger electron mapping analysis was done on the alumina surface at both low and high iron coverages. The Auger electron maps for iron are shown in Fig. 2. The light dots indicate the areas where iron is present. At high iron coverage (map b which has Fe:A1 = 5.7), iron was closely distributed everywhere, while at low iron coverage (map a which has Fe:A1 = 0.1), iron was randomly distributed everywhere. Therefore, there are no specific regions where iron was favorably deposited on alumina.
403
(a)
700
705
8.E. (eV)
7 o
715
B,,
FIG, 1, Fe (2P3/2) peak of iron deposited on alumina at 298 K for (a) low and (b) high iron coverages and (c) N (IS) peak on Fe/AI203 after exposure to NO at 298 K.
F W H M of 3.0 eV indicated that there was only one form of iron present on the A1203 surface at high iron coverages. In order to identify the form of iron present on A1203, the binding energy of the Fe (2P3/2) peak was compared with those of the standard compounds given in Table I and was found to be closest to the value of the metallic form on an etched iron foil. The binding energy of iron on A1203 also compared well with the binding energy of metallic iron reported in the literature (13). Similar observations were made when iron was deposited on silica (11). Thus, iron was deposited on A1203 in the metallic form for all evaporation times at 298 K. At lower iron coverages there is a possibility of having larger amounts of iron at higher oxidation states relative to metallic states since the F W H M at Journal of Colloid and Interface Science, Vol. 110, No. 2, April 1986
Interaction of NO with Iron Deposited on Alumina When the single-crystal surface of A1203 at temperatures of 298, 473, or 673 K was exposed to 500 L of NO, no nitrogen peak was detected in the binding energy region of 388.0
TABLE I Binding Energies of Fe (2P3/2) Peak of Standard Compounds Compound
Unetched Fe foila Etched Fe foila FeSO4 a FeSa FeO Fe203 FeOOH Fe304 Fe Fe/A1203 Work performed by authors.
BE (eV)
711.6 708.5 713.4 713.4 709.5-710.5 (13) 710.5-711.5 (13) 710.5-711.5 (13) 711.5 (13) 706.5 (13) 706.5
NO-Fe/A1203 INTERACTIONS
FIG. 2. Scanning Auger mapping analysis of iron deposited Fe:A1 = 5.7.
to 418.0 eV. This is the region in which the N ( I S ) peak occurs, indicating that N O did not chemisorb on pure A1203. When the iron-covered alumina system (Fe/ A1203) at 298 K was exposed to NO, only one form of nitrogen was observed, as shown in Fig. 1c. The Fe (2P3/2) peak had a F W H M of
on A1203.
507
(a) Fe:AI = 0.I and (b)
about 3.0 eV before the N O exposure, but it increased to 5.4 eV after the N O exposure. Iron may have been oxidized to either the Fe 2+ or the Fe 3+ state during the N O exposure because the broadening of the iron peak was towards the higher binding energy side. There was broadening of the oxygen peak on alumina Journal of Colloid and Interface Science, Vol. 110, No. 2, April 1986
508
SIRIWARDANE AND COOK
~-
r
7 I""
.~
8
~. Fe: AI = 0.2 o Fe: AI = 0.5
|
n
Fe: AI
= 1.4
~
•
Fe:AI
=,~o
. n
.
,,
,.,.,.....,,~Q
o
~
0 0
5
10
15
20
70
EXPOSURE (LANGMUIRS)
FIG. 3. Intensity of N (IS) peak as a function of NO exposure on Fe/A1203 at 298 K. after N O adsorption. It was difficult to distinguish between the oxygen in adsorbed N O and the oxygen in A1203. The intensity of the nitrogen peak, corrected for its cross section (12), at 396.0 eV versus cumulative exposure is shown in Fig. 3. The intensity reached a saturation value rapidly with increasing exposures. When the Fe:A1 ratio was increased from 0.2 to 1.4, there was a doubling of the nitrogen intensity. However, when the Fe:A1 ratio was further increased to oo, there was a drastic decrease in the saturation intensity. It is possible that the alumin u m ions were activating the iron sites for N O adsorption even though a l u m i n u m by itself cannot adsorb NO: This behavior was not observed when N O was adsorbed on iron deposited on silica (11) in that there was no decrease in saturation nitrogen intensity at higher iron coverages. In order to identify the form of nitrogen present on Fe/A1203, the binding energies were compared with standard compounds containing nitrogen, as listed in Table II. The nitrogen binding energy compares well with the nitride binding energy. Thus, it is possible that N O Journal of Colloid and Interface Science, Vol. 110, No. 2, April 1986
dissociated on Fe/A1203 system to form iron nitride. The reaction of N O with Fe/AI20 3 at 298 K can be summarized as NO
Fe/A1203 /xY ' ' ±'qads-t- O a d s .
A similar dissociation was observed when N O was reacted with Fe/SiO2 (11). However, in addition to the dissociation of NO, adsorbed
TABLE II Binding Energies of N (1S) Peak for Standard Compounds Compound
Fe2(NO3)3a Nitrates Fe4N + Fe2Na Nitride Pt-NO (molec) Ni-NO (molec) Nitrosyl Nitrite NO/Fe/SiO2 a NO/Fe/AI203" Work performed by authors.
BE (eV)
407.1 407.0-408.0 (14) 397.4 396.5-398.5 (14) 401.0 (16) 399.9 (16) 400-403 (14) 404-405 (14) 396.7 (11) 400.6 396.0
NO-Fe/A1203 INTERACTIONS molecular NO was also observed on Fe/SiO2. Similar dissociations of NO on other group elements, including platinum, nickel, and palladium, have been observed by previous workers (14-18). When the reaction was carried out at 473 K, only one form of nitrogen was observed, with a peak located at 396.7 eV. The plot of the intensity of nitrogen versus exposure at 473 K is shown in Fig. 4. There was an increase in nitrogen intensity when Fe: A1 ratio was increased from 0.4 to 3.8. When the Fe:A1 ratio was further increased to 6.6 (the A1 signal was very low and Fe:A1 was very close to ~ at this point), there was a drastic decrease in saturation nitrogen intensity. A similar observation was made at 298 K. This may also be due to the activation of iron atoms by aluminum atoms at low iron coverages. It is not clear how the presence of aluminum atoms could activate the adsorption of NO. Bonzel and Pirug (16) observed the dissociation of NO on Pt and suggested that nitrogen is bonded via charge transfer from the 4a orbital to the metal and back donation of metal electrons into the 21r* orbital which is principally located on the oxygen atom. It was ob-
509
served that the electron donation from the metal to CO was important in the dissociation of adsorbed CO (19, 20). A13+ atoms are strong Lewis acids. They could withdraw electrons from adjacent iron atoms and make iron orbitals available for bonding with nitrogen. The oxygen in NO can also be attracted to aluminum sites which act as Lewis acids, and its electron density could be withdrawn toward the aluminum ions. Thus, the presence of aluminum could enhance the interaction between iron and NO by promoting a bonding. It is also possible that the oxygen that is formed by the dissociation reaction is adsorbed on aluminum ions rather than being adsorbed on iron atoms. Therefore, more iron atoms are available for further reaction with NO. The saturation nitrogen intensities seem to be higher at 473 than at 298 K for high iron coverages. When the reaction was carried out at 673 K, no nitrogen peaks were observed for NO exposures up to 22 L for an Fe:A1 ratio of 1.6; however, a very small peak was observed at 64 L for this iron coverage. It is possible that the iron interacted partially with the alumina
12 oFe:
AI = 0 . 4
• Fe: AI = 3 . 8 z
10
~, Fe: AI = 6 . 6
<
8 <
/
v <
e z
z
2
°1"--"
z 0
I
I
I
5
10
15.
,$ 20
70
EXPOSURE (LANGMUIRS)
FIG. 4. Intensityof N (IS) peak as a function of NO exposureon Fe/AI203at 473 K. Journal of Colloid and Interface Science, Vol. 110, No. 2, April 1986
510
SIRIWARDANE A N D C O O K
surface and was oxidized at 673 K since the F W H M of Fe (2P3/2) was larger than that at 298 K. Thus, the reactivity of iron was decreased at 673 K for this low iron coverage. The binding energy of N (IS) was 395.0 eV. The intensity of the nitrogen peak versus exposure at 673 K is shown in Fig. 5. At an Fe: A1 ratio of 2.3, the saturation intensity was significantly lower compared to that at 298 and 473 K. However, at Fe:A1 = oo the saturation intensity did not decrease as was observed for the other two temperatures, 298 and 473 K. Temperature dependence of the adsorbed nitrogen intensity is shown in Fig. 6. At high iron coverages, an increase in the nitrogen intensity was observed with increasing temperature. However, a decrease in the nitrogen intensity with increasing temperature was observed at low iron coverages. As was stated earlier for low iron coverages the iron interacted and was oxidized more on alumina with increasing temperature. Thus, this decrease in activity and lower adsorption of NO may be due to the oxidation of iron at low iron coverages with increasing temperature. It is not clear whether this drastic decrease is completely due to oxidation of iron at 673 K
because if oxidation of the majority of iron occurred, there should be a large shift in binding energy in addition to the peak broadening. However, previous workers have also observed that, after SO/(11, 19) or NO (11) exposures on iron, there was only a broadening of the iron peak even though oxidation of a major portion of iron due to the reaction was expected. In contrast to 298 and 473 K, the presence of aluminum ions at 673 K has decreased the reactivity of iron on the surface. As was explained earlier the iron atoms were oxidized at 673 K due to the interaction with aluminum atoms. However, if iron remains in the metallic state at 673 K in the vicinity of aluminum, it may be activated for nitride formation since this metallic state is necessary for the NO dissociation.
Monolayer Coverages and Sticking Probability Calculations The sticking probability of NO on Fe/AIaO3 may be obtained using the NO coverage versus exposure data. Following Hayward and Trapnell (21), it may be shown that
12 o Fe:AI = 1.6 • Fe:AI = 2.3 zx Fe: AI = oo
F z) >re
10
N
8
~
6
O
~ z
4
8 N
2
I
I
,
10
15
,+ 2O
70
EXPOSURE (LANGMUIRS)
FIG. 5. Intensity of N (IS) peak as a function of NO exposure on Fe/A1203 at 673 K.
Journal of Colloid and Interface Science, Vol. 110, No. 2, April 1986
NO-Fe/AI203 INTERACTIONS 10 9
0 HIGH IRON COVERAGE O rl LOW IRON C
1 1 -
~
7
b~
6 5
z_~
5
4
2
3
S0~ NNO
0
L + C
f i F e " N Fe IM NNo = - - '
~N
2 1 0
[3]
where C is a constant of integration. Thus, if there is mobile adsorption after dissociation, a plot of 1/(1 - O) versus L should yield a straight line with a slope given by --Sov/Nno. From this slope, the magnitude of the initial sticking probability m a y be calculated if NNO, the n u m b e r of N O molecules for a monolayer of coverage, is known. NNo can be calculated using
8
o
51 1
0
I 1O0
I
I
200
I
I
300 400 500 TEMPERATURE(K)
l
600
700
FIG.6. Saturation intensity of N (IS) peak as a function of temperature.
dO
Sv
dL
N1
where l°e is the intensity of iron at zero N O coverage, a is the photoelectron cross section, N w is the n u m b e r of iron atoms, and IM is the intensity of the nitrogen peak corresponding to monolayer coverage. Since the plots of nitrogen intensity versus exposure are similar to Langmuir adsorption isotherms, the Langmuir equation (22) was used to calculate IM. The N F~ term in Eq. [4] was calculated using (23, 24)
[1] NFe = aAl" PAhO3" hAl" NO" COS ~b IFe
~Fe" MA1203 where v = (27rmkT) -1/2, L is the exposure, S is the sticking probability, Nl is the n u m b e r of molecules adsorbed at monolayer coverage, m is the mass of the gas phase molecule, k is the Boltzmann constant, and Tis the gas temperature. The sticking probability is, in general, a function of coverage. The XPS data indicated that the N O is dissociated into N and O on Fe/A1203 surface. If it is assumed that the N O dissociates and adsorbs on rand o m sites (i.e., mobile adsorption) and that precursors are not involved in adsorption, then [2]
where So is defined as the initial sticking probability (i.e., the sticking probability at zero coverage). Substituting Eq. [2] into Eq. [1] and integrating yields
[5]
I°
where X is the mean free path, No is Avogadro's number, ~b is the angle between the direction
TABLE III
N re, NNO,and F Values for NO on Fe/AlzO3 Temperature (K)
N w X 10-14 (atoms/cm 2)
NNo × 10 t4 (molecules/cm 2)
F
298
0.79 1.38 3.32 3.61 2.19 3.31 4.57 2.20 2.80 3.03
0.36 0.56 0.72 0.53 0.38 0.80 0.74 0.12 0.30 1.03
0.97 0.92 0.96 0.96 0.99 0.90 0.92 --0.99
(21) S = So(1 - 0 ) 2
[4]
1%
473 673
Journal of Colloid and Interface Science. Vol. 110, No. 2, April 1986
512
SIRIWARDANE AND COOK
to the detector and the normal to the sample surface (for the VG ESCA-3, ~b = 45°), IFe is the intensity of the iron peak, I ° is the intensity of the aluminum peak at zero iron coverage, 0A~203and MA~2o3are the density and the molecular weight of aluminum oxide, respectively. The values of N w, N~, and the coefficient of determination (F) for the least-squares linear fit of 1/(1 - O) versus L calculated using Eqs. [3], [4], and [5] are listed in Table III. Several important results can be noted in this table. The maximum monolayer NO coverage (NNo) was observed when the N Fe was about 3 × 1014 atoms/cm 2 for all three temperatures. The adsorption of NO was low at 673 K for low iron coverages, due to the interaction of iron with the alumina surface. However, at high iron coverages the monolayer NO coverage was higher than that at 298 or 473 K. The values of the linear fit (F) for 1/(1 - O) versus L were greater than 0.9 for all iron coverages at all three temperatures. This indicated that there is dissociation of NO duri-ng adsorption and that the adsorption is mobile. This agrees with the binding energy data which gave evidence for the formation of nitride on Fe/A1203. The initial sticking probabilities fell in the range of 0.2 to 0.5. Sticking probability calculations for the second-order adsorption (i.e., adsorption involving two adsorption sites) have been performed by several workers (25-29). In most cases, they indicated that precursors were involved in the adsorption (25, 26). However, in our work, the calculations seem to indicate that the precursors were not involved in NO adsorption on Fe/A1203. A simple theoretical equation, similar to the equation derived in this work, has been developed for dissociative adsorption of oxygen on cuprous oxide for premonolayer coverage by previous workers
(21, 30). CONCLUSIONS Iron was deposited on surfaces of alumina in the Fe ° state. Scanning Auger mapping inJournal of Colloid and Interface Science, Vol. 110, No. 2, April 1986
dicated that iron was randomly distributed on the A1203 surface at low coverages and that the surface was fully covered with iron at high coverages. At low iron coverages there was an interaction of iron with the A1203 surface. This interaction increased with increasing temperature. Nitric oxide interacted with Fe/A1203 at 298, 473, and 673 K. The adsorbed nitrogen was identified as nitride which indicated that NO was dissociated into nitrogen and oxygen on the Fe/A1203 surface. The NO adsorption increased with increasing iron coverage. However, when the A1203 surface was fully covered with iron, there was a decrease in NO adsorption. This indicated that the aluminum ions activated the iron atoms for NO adsorption even though pure alumina did not adsorb NO. At high iron coverages the adsorptivity increased with increasing temperature. However, NO adsorption on Fe/A1203 decreased with increasing temperature for low iron coverages due to the interaction and oxidation of iron with the alumina surface. Sticking probability calculations indicated that the NO adsorption of Fe/AI203 was mobile and dissociative. REFERENCES 1. Stinespring,C. D., and Stewart, G. W., Atmos. Environ. 15, 307 (1980). 2. Lunsford,J., J. Catal. 14, 379 (1969). 3. Lunsford,J., Zingery,L., and Rosynek,M., J. Catal. 38, 179 (1975). 4. Pozdnyakov,D., and Filimonov, V., Adv. Mol. Relaxation Processes 5, 55 (1973). 5. Brown,C., and Hall, P., Surf. Sci. 36, 569 (1973). 6. Douglas,T. B., "The Physicaland ChemicalAdsorption of Nitric-Oxide on Alumina and Other Oxides," Ph.D. dissertation, Stanford University, 1975. 7. Honda, F., and Hirokawa, K., J. Electron Spectrosc. Relat. Phenom. 8, 199 (1976). 8. Blyholder,G., and Allen, M. C., Y. Phys. Chem. 65, 3998 (1965). 9. Terenin, A., and Roev, L. M., Spectrochim. Acta 13, 946 (1959). 10. Kishi, K., and Ikeda, S., Bull. Chem. Soc. Japan 47, 2532 (1974). 11. Siriwardane,R. V., and Cook, J. M., J. Colloid Interface Sci. 104, 250 (1985).
NO-Fe/A1203 INTERACTIONS 12. Scofield, J. H., J. Electron Spectrosc. Relat. Phenom. 8, 129 (1976). 13. Wagner, C. D., Riggs, W. M., Davis, L. E., Moulder, J. F., and Muilenburg, G. E., "Handbook of XRay Photoelectron Spectroscopy," Perkin-Elmer Corp., 1979. 14. Brundle, C. R., J. Vac. Sci. Technol. 13, 301 (1976). 15. Comrie, C. M., Weinberg, W. J., and Lambert, R. M., Surf Sci. 57, 619 (1976). 16. Bonzel, H. P., and Pirug, G., Surf. Sci. 62, 45 (1977). 17. Price, G. L., Sexton, B. A,, and Baker, B. G., Surf Sci. 60, 506 (1976). 18. Conrad, H., Ertl, G., Kuppers, J., and Latta, E. E., Surf. Sci. 50, 296 (1975). 19. Furuyama, M., Kishi, K., and Ikeda, S., J. Electron Spectrosc. Relat. Phenom. 13, 59 (1978). 20. Kishi, K., and Roberts, M. W., Faraday Trans. 71, 1715 (1975). 21. Hayward, D. O., and Trapnell, B. M. W., "Chemisorption." Butterworths, Washington, D.C., 1964.
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22. Adamson, A. W., "Physical Chemistry of Surfaces," 3rd ed. Wiley, New York, 1976. 23. Cook, J. M., "Coal Mineral Interaction on Coal Conversion and Combustion Processes," Quarterly Report, WVU Task Order No. 32, Contract No. DE-AT21-77MC08087, May 31, 1981. 24. Madey, T. E., Yates, J. T., Jr., and Erickson, N. E., Chem. Phys. Lett. 19, 487 (1973). 25. Gasser, R. P. H., Roberts, K., and Stevens, A. J., Trans. Faraday Soc. 65, 3105 (1969). 26. Eisinger, J., and Law, J. T., J. Chem. Phys. 30, 410 (1959). 27. Redhead, P. A., Trans. FaradaySoc. 57, 641 (1961). 28. Ertl, G., Lee, S. B., and Weiss, M., Surf Sci. 114, 515 (1982). 29. Udovic, T. J., and Dumesic, J. A., J. Catal. 89, 314 (1984). 30. Jennings, T. J., and Stone, F. S., Adv. Catal. 9, 441 (1957).
Journal of ColloM and Interface Science, Vol. 110, No. 2, April 1986
JASON M. COOK Morgantown Energy Technology Center, U. S. Department of Energy, Morgantown, West Virginia 26505
ReceivedMay 10, 1985;acceptedAugust 7, 1985 The interaction of nitric oxide with single-crystal surfaces of alumina at temperatures of 298, 473, and 673 K, which had been covered by various amounts of iron, was studied using X-ray photoelectron spectroscopy. The iron was deposited onto A1203 in the Fe° state. At low coverages, iron was partially oxidized due to its interaction with A1203. Scanning auger mapping analysis showed that the iron was randomly distributed on the A1203 surface. The amount of adsorbed NO increased with increasing iron coverage. However, at very high iron coverages, there was a decrease in adsorption. This indicated that the aluminum ions may have activated the NO adsorption on the iron atoms. For increasing temperature there was also an increase in adsorption for high iron coverages, but the adsorption decreased with increasing temperature for low iron coverages. Sticking probability calculations indicated that the adsorption was mobile and dissociative. Binding energy of the nitrogen peaks indicated that NO was adsorbed onto the Fe/A1203 surface as a nitride. © 1986AcademicPress,Inc.
INTRODUCTION
Nitric oxide is produced during coal combustion processes. The removal of pollutant gases such as NO from the combustion stream is important for environmental reasons. In order to understand the combustion process and to be able to reliably predict the NO concentrations in the combustion stream, it is necessary to study the reactions which occur between this gas and the constituents which would typically be found in entrained ash (1), such as alumina and iron. Various workers (2-6) have studied the adPaper was presented at the 59th Colloid and Surface Science Symposium at Clarkson University, Potsdam, New York, June 24-29, 1985. 2 Present address: Morgantown Energy Technology Center, U. S. Department of Energy, Morgantown, W. V. 26505.
sorption of NO on A1203. Although physisorption was observed in most cases (3-5), Douglas (6) reported chemisorption of NO on alumina. Only a few studies were reported for NO interactions with iron. Honda and Hirokawa (7) used X-ray photoelectron spectroscopy (XPS) and observed the formation of N, NO, NO2, and N O 3 when iron surfaces at 273 K were exposed to NO at pressures of 200 and 760 Torr. Infrared spectroscopic studies (8, 9) indicated the formation of NO + on an iron surface. The interaction of NO at 5-20 Torr with iron evaporated onto stainless steel was studied by Kishi and Ikeda (10). The interaction of NO with iron deposited on silica has been studied by the authors (11). However, none of the above studies dealt with the interaction of NO with iron deposited on A1203. Also, no work was reported at very 504
0021-9797/86 $3.00 Copyright © 1986 by Academic Press, Inc. All rights of reproduction in any form reserved.
Journal of Colloid and Interface Science, Vol. 110, No. 2, April 1986
NO-Fe/AI203 INTERACTIONS
low gas exposures or elevated temperatures in a flow system using XPS. The purpose of this work is to investigate the interactions of NO with both a clean and an iron-covered, single-crystal surface of AlzO3 using XPS. Iron was evaporated onto the A1203 surface, and gas exposures were carried out at each iron coverage. Since the exposure levels of the gases were on the order of several Langmuirs (1 L = 10-6 Torr-s), the results are representative of the early stages of the reaction. The gas exposures were performed on surfaces which were at 298, 473, and 673 K.
505
heating the probe. After the preparation chamber was pumped out the sample was transferred to the detection chamber for data acquisition. These temperatures were maintained even during data acquisition. Charge correction of the XPS peaks was accomplished by using C(1 S) as the standard with a binding energy of 284.6 eV. The experimental uncertainty in the binding energies was +1.0 eV. Scanning Auger mapping analysis of iron deposited on silica was performed using a Perkin-Elmer SAM-590 system. RESULTS AND DISCUSSION
EXPERIMENTAL
Deposition of Iron on Alumina The XPS spectra were recorded with a Vacuum-Generators (VG) ESCA 3 spectrometer. This system consisted of a separately pumped, liquid-nitrogen trapped, spectrometer and sample preparation chambers which were routinely operated in a pressure range of 10 -9 to 10-l° Torr. The preparation chamber was equipped with a resistively heated sample probe, an argon ion etch gun, and an evaporator (all standard VG items). The detector was a retarding grid, hemispherical analyzer. An aluminum anode (1486.6 eV) was used as the X-ray source. Single crystals of A1203, obtained from Atomergic Chemical Corporation, were cut to produce a 1 X 10 X 10-mm sample with a (110) surface. The sample was rinsed with acetone and was cleaned further by argon etching in the vacuum system. After 30 rain of argon etching, the residual carbon on the surface was negligible. After the AlzO3 crystal was cleaned, the iron evaporations onto the surface were performed using a resistively heated tungsten basket containing iron foil. Then high-purity NO (99%), supplied by Matheson, was transferred to the preparation chamber. During each gas exposure the pressure was held constant by using a leak valve. The exposures, which ranged from 1 to 72 L, were carried out on surfaces heated to 298, 473, and 673 K by resistively
An arbitrary value for the Fe:A1 ratio was obtained by dividing the amount of iron [i.e., peak area/photoelectron cross section (12)] by the amount of aluminum. The actual number of iron atoms deposited will be calculated considering factors other than the photoelectron cross section, as shown in Eq. [5]. The amount of iron deposition on alumina was varied by changing the total time of evaporation and is expressed by the Fe:A1 ratio. This ratio was varied from a value of 0.0 (i.e., a clean surface) to a value of ~ (i.e., a surface that is completely covered by iron). The amount of iron deposited during a given exposure time depended upon both the amount of iron in the evaporator and the current used to heat the basket during evaporation. It is assumed that all of the iron atoms remain on the surface. These iron atoms can be removed completely by a light argon etching. There is a drastic increase in the intensity of the aluminum signal during etching. Hence, it is reasonable to assume that the iron atoms remain on the surface. The binding energy value o f the Fe (2P3/2) peak for iron deposited at 298 K was 706.5 ___ 1.0 eV, while the full width at half-maxim u m (FWHM) of the Fe (2P3/2) peak was 4.0 eV for low coverages and 3.0 eV for high coverages of iron as shown in Figs. la and b. The Journal of Colloid and Interface Science, Vol. 110,No. 2, April 1986
506
SIRIWARDANE AND COOK
388
low iron coverage was larger than that for high iron coverage. When iron was deposited at 473 and 673 K, the F W H M of Fe (2P3/2) was 5.5 eV for low iron coverages and 3.0 eV for high iron coverages. This indicates that more iron interacted with alumina and that iron was oxidized to Fe 2+ or Fe 3÷ at 473 and 673 K for low iron coverages. In order to determine how the iron was distributed on alumina, scanning Auger electron mapping analysis was done on the alumina surface at both low and high iron coverages. The Auger electron maps for iron are shown in Fig. 2. The light dots indicate the areas where iron is present. At high iron coverage (map b which has Fe:A1 = 5.7), iron was closely distributed everywhere, while at low iron coverage (map a which has Fe:A1 = 0.1), iron was randomly distributed everywhere. Therefore, there are no specific regions where iron was favorably deposited on alumina.
403
(a)
700
705
8.E. (eV)
7 o
715
B,,
FIG, 1, Fe (2P3/2) peak of iron deposited on alumina at 298 K for (a) low and (b) high iron coverages and (c) N (IS) peak on Fe/AI203 after exposure to NO at 298 K.
F W H M of 3.0 eV indicated that there was only one form of iron present on the A1203 surface at high iron coverages. In order to identify the form of iron present on A1203, the binding energy of the Fe (2P3/2) peak was compared with those of the standard compounds given in Table I and was found to be closest to the value of the metallic form on an etched iron foil. The binding energy of iron on A1203 also compared well with the binding energy of metallic iron reported in the literature (13). Similar observations were made when iron was deposited on silica (11). Thus, iron was deposited on A1203 in the metallic form for all evaporation times at 298 K. At lower iron coverages there is a possibility of having larger amounts of iron at higher oxidation states relative to metallic states since the F W H M at Journal of Colloid and Interface Science, Vol. 110, No. 2, April 1986
Interaction of NO with Iron Deposited on Alumina When the single-crystal surface of A1203 at temperatures of 298, 473, or 673 K was exposed to 500 L of NO, no nitrogen peak was detected in the binding energy region of 388.0
TABLE I Binding Energies of Fe (2P3/2) Peak of Standard Compounds Compound
Unetched Fe foila Etched Fe foila FeSO4 a FeSa FeO Fe203 FeOOH Fe304 Fe Fe/A1203 Work performed by authors.
BE (eV)
711.6 708.5 713.4 713.4 709.5-710.5 (13) 710.5-711.5 (13) 710.5-711.5 (13) 711.5 (13) 706.5 (13) 706.5
NO-Fe/A1203 INTERACTIONS
FIG. 2. Scanning Auger mapping analysis of iron deposited Fe:A1 = 5.7.
to 418.0 eV. This is the region in which the N ( I S ) peak occurs, indicating that N O did not chemisorb on pure A1203. When the iron-covered alumina system (Fe/ A1203) at 298 K was exposed to NO, only one form of nitrogen was observed, as shown in Fig. 1c. The Fe (2P3/2) peak had a F W H M of
on A1203.
507
(a) Fe:AI = 0.I and (b)
about 3.0 eV before the N O exposure, but it increased to 5.4 eV after the N O exposure. Iron may have been oxidized to either the Fe 2+ or the Fe 3+ state during the N O exposure because the broadening of the iron peak was towards the higher binding energy side. There was broadening of the oxygen peak on alumina Journal of Colloid and Interface Science, Vol. 110, No. 2, April 1986
508
SIRIWARDANE AND COOK
~-
r
7 I""
.~
8
~. Fe: AI = 0.2 o Fe: AI = 0.5
|
n
Fe: AI
= 1.4
~
•
Fe:AI
=,~o
. n
.
,,
,.,.,.....,,~Q
o
~
0 0
5
10
15
20
70
EXPOSURE (LANGMUIRS)
FIG. 3. Intensity of N (IS) peak as a function of NO exposure on Fe/A1203 at 298 K. after N O adsorption. It was difficult to distinguish between the oxygen in adsorbed N O and the oxygen in A1203. The intensity of the nitrogen peak, corrected for its cross section (12), at 396.0 eV versus cumulative exposure is shown in Fig. 3. The intensity reached a saturation value rapidly with increasing exposures. When the Fe:A1 ratio was increased from 0.2 to 1.4, there was a doubling of the nitrogen intensity. However, when the Fe:A1 ratio was further increased to oo, there was a drastic decrease in the saturation intensity. It is possible that the alumin u m ions were activating the iron sites for N O adsorption even though a l u m i n u m by itself cannot adsorb NO: This behavior was not observed when N O was adsorbed on iron deposited on silica (11) in that there was no decrease in saturation nitrogen intensity at higher iron coverages. In order to identify the form of nitrogen present on Fe/A1203, the binding energies were compared with standard compounds containing nitrogen, as listed in Table II. The nitrogen binding energy compares well with the nitride binding energy. Thus, it is possible that N O Journal of Colloid and Interface Science, Vol. 110, No. 2, April 1986
dissociated on Fe/A1203 system to form iron nitride. The reaction of N O with Fe/AI20 3 at 298 K can be summarized as NO
Fe/A1203 /xY ' ' ±'qads-t- O a d s .
A similar dissociation was observed when N O was reacted with Fe/SiO2 (11). However, in addition to the dissociation of NO, adsorbed
TABLE II Binding Energies of N (1S) Peak for Standard Compounds Compound
Fe2(NO3)3a Nitrates Fe4N + Fe2Na Nitride Pt-NO (molec) Ni-NO (molec) Nitrosyl Nitrite NO/Fe/SiO2 a NO/Fe/AI203" Work performed by authors.
BE (eV)
407.1 407.0-408.0 (14) 397.4 396.5-398.5 (14) 401.0 (16) 399.9 (16) 400-403 (14) 404-405 (14) 396.7 (11) 400.6 396.0
NO-Fe/A1203 INTERACTIONS molecular NO was also observed on Fe/SiO2. Similar dissociations of NO on other group elements, including platinum, nickel, and palladium, have been observed by previous workers (14-18). When the reaction was carried out at 473 K, only one form of nitrogen was observed, with a peak located at 396.7 eV. The plot of the intensity of nitrogen versus exposure at 473 K is shown in Fig. 4. There was an increase in nitrogen intensity when Fe: A1 ratio was increased from 0.4 to 3.8. When the Fe:A1 ratio was further increased to 6.6 (the A1 signal was very low and Fe:A1 was very close to ~ at this point), there was a drastic decrease in saturation nitrogen intensity. A similar observation was made at 298 K. This may also be due to the activation of iron atoms by aluminum atoms at low iron coverages. It is not clear how the presence of aluminum atoms could activate the adsorption of NO. Bonzel and Pirug (16) observed the dissociation of NO on Pt and suggested that nitrogen is bonded via charge transfer from the 4a orbital to the metal and back donation of metal electrons into the 21r* orbital which is principally located on the oxygen atom. It was ob-
509
served that the electron donation from the metal to CO was important in the dissociation of adsorbed CO (19, 20). A13+ atoms are strong Lewis acids. They could withdraw electrons from adjacent iron atoms and make iron orbitals available for bonding with nitrogen. The oxygen in NO can also be attracted to aluminum sites which act as Lewis acids, and its electron density could be withdrawn toward the aluminum ions. Thus, the presence of aluminum could enhance the interaction between iron and NO by promoting a bonding. It is also possible that the oxygen that is formed by the dissociation reaction is adsorbed on aluminum ions rather than being adsorbed on iron atoms. Therefore, more iron atoms are available for further reaction with NO. The saturation nitrogen intensities seem to be higher at 473 than at 298 K for high iron coverages. When the reaction was carried out at 673 K, no nitrogen peaks were observed for NO exposures up to 22 L for an Fe:A1 ratio of 1.6; however, a very small peak was observed at 64 L for this iron coverage. It is possible that the iron interacted partially with the alumina
12 oFe:
AI = 0 . 4
• Fe: AI = 3 . 8 z
10
~, Fe: AI = 6 . 6
<
8 <
/
v <
e z
z
2
°1"--"
z 0
I
I
I
5
10
15.
,$ 20
70
EXPOSURE (LANGMUIRS)
FIG. 4. Intensityof N (IS) peak as a function of NO exposureon Fe/AI203at 473 K. Journal of Colloid and Interface Science, Vol. 110, No. 2, April 1986
510
SIRIWARDANE A N D C O O K
surface and was oxidized at 673 K since the F W H M of Fe (2P3/2) was larger than that at 298 K. Thus, the reactivity of iron was decreased at 673 K for this low iron coverage. The binding energy of N (IS) was 395.0 eV. The intensity of the nitrogen peak versus exposure at 673 K is shown in Fig. 5. At an Fe: A1 ratio of 2.3, the saturation intensity was significantly lower compared to that at 298 and 473 K. However, at Fe:A1 = oo the saturation intensity did not decrease as was observed for the other two temperatures, 298 and 473 K. Temperature dependence of the adsorbed nitrogen intensity is shown in Fig. 6. At high iron coverages, an increase in the nitrogen intensity was observed with increasing temperature. However, a decrease in the nitrogen intensity with increasing temperature was observed at low iron coverages. As was stated earlier for low iron coverages the iron interacted and was oxidized more on alumina with increasing temperature. Thus, this decrease in activity and lower adsorption of NO may be due to the oxidation of iron at low iron coverages with increasing temperature. It is not clear whether this drastic decrease is completely due to oxidation of iron at 673 K
because if oxidation of the majority of iron occurred, there should be a large shift in binding energy in addition to the peak broadening. However, previous workers have also observed that, after SO/(11, 19) or NO (11) exposures on iron, there was only a broadening of the iron peak even though oxidation of a major portion of iron due to the reaction was expected. In contrast to 298 and 473 K, the presence of aluminum ions at 673 K has decreased the reactivity of iron on the surface. As was explained earlier the iron atoms were oxidized at 673 K due to the interaction with aluminum atoms. However, if iron remains in the metallic state at 673 K in the vicinity of aluminum, it may be activated for nitride formation since this metallic state is necessary for the NO dissociation.
Monolayer Coverages and Sticking Probability Calculations The sticking probability of NO on Fe/AIaO3 may be obtained using the NO coverage versus exposure data. Following Hayward and Trapnell (21), it may be shown that
12 o Fe:AI = 1.6 • Fe:AI = 2.3 zx Fe: AI = oo
F z) >re
10
N
8
~
6
O
~ z
4
8 N
2
I
I
,
10
15
,+ 2O
70
EXPOSURE (LANGMUIRS)
FIG. 5. Intensity of N (IS) peak as a function of NO exposure on Fe/A1203 at 673 K.
Journal of Colloid and Interface Science, Vol. 110, No. 2, April 1986
NO-Fe/AI203 INTERACTIONS 10 9
0 HIGH IRON COVERAGE O rl LOW IRON C
1 1 -
~
7
b~
6 5
z_~
5
4
2
3
S0~ NNO
0
L + C
f i F e " N Fe IM NNo = - - '
~N
2 1 0
[3]
where C is a constant of integration. Thus, if there is mobile adsorption after dissociation, a plot of 1/(1 - O) versus L should yield a straight line with a slope given by --Sov/Nno. From this slope, the magnitude of the initial sticking probability m a y be calculated if NNO, the n u m b e r of N O molecules for a monolayer of coverage, is known. NNo can be calculated using
8
o
51 1
0
I 1O0
I
I
200
I
I
300 400 500 TEMPERATURE(K)
l
600
700
FIG.6. Saturation intensity of N (IS) peak as a function of temperature.
dO
Sv
dL
N1
where l°e is the intensity of iron at zero N O coverage, a is the photoelectron cross section, N w is the n u m b e r of iron atoms, and IM is the intensity of the nitrogen peak corresponding to monolayer coverage. Since the plots of nitrogen intensity versus exposure are similar to Langmuir adsorption isotherms, the Langmuir equation (22) was used to calculate IM. The N F~ term in Eq. [4] was calculated using (23, 24)
[1] NFe = aAl" PAhO3" hAl" NO" COS ~b IFe
~Fe" MA1203 where v = (27rmkT) -1/2, L is the exposure, S is the sticking probability, Nl is the n u m b e r of molecules adsorbed at monolayer coverage, m is the mass of the gas phase molecule, k is the Boltzmann constant, and Tis the gas temperature. The sticking probability is, in general, a function of coverage. The XPS data indicated that the N O is dissociated into N and O on Fe/A1203 surface. If it is assumed that the N O dissociates and adsorbs on rand o m sites (i.e., mobile adsorption) and that precursors are not involved in adsorption, then [2]
where So is defined as the initial sticking probability (i.e., the sticking probability at zero coverage). Substituting Eq. [2] into Eq. [1] and integrating yields
[5]
I°
where X is the mean free path, No is Avogadro's number, ~b is the angle between the direction
TABLE III
N re, NNO,and F Values for NO on Fe/AlzO3 Temperature (K)
N w X 10-14 (atoms/cm 2)
NNo × 10 t4 (molecules/cm 2)
F
298
0.79 1.38 3.32 3.61 2.19 3.31 4.57 2.20 2.80 3.03
0.36 0.56 0.72 0.53 0.38 0.80 0.74 0.12 0.30 1.03
0.97 0.92 0.96 0.96 0.99 0.90 0.92 --0.99
(21) S = So(1 - 0 ) 2
[4]
1%
473 673
Journal of Colloid and Interface Science. Vol. 110, No. 2, April 1986
512
SIRIWARDANE AND COOK
to the detector and the normal to the sample surface (for the VG ESCA-3, ~b = 45°), IFe is the intensity of the iron peak, I ° is the intensity of the aluminum peak at zero iron coverage, 0A~203and MA~2o3are the density and the molecular weight of aluminum oxide, respectively. The values of N w, N~, and the coefficient of determination (F) for the least-squares linear fit of 1/(1 - O) versus L calculated using Eqs. [3], [4], and [5] are listed in Table III. Several important results can be noted in this table. The maximum monolayer NO coverage (NNo) was observed when the N Fe was about 3 × 1014 atoms/cm 2 for all three temperatures. The adsorption of NO was low at 673 K for low iron coverages, due to the interaction of iron with the alumina surface. However, at high iron coverages the monolayer NO coverage was higher than that at 298 or 473 K. The values of the linear fit (F) for 1/(1 - O) versus L were greater than 0.9 for all iron coverages at all three temperatures. This indicated that there is dissociation of NO duri-ng adsorption and that the adsorption is mobile. This agrees with the binding energy data which gave evidence for the formation of nitride on Fe/A1203. The initial sticking probabilities fell in the range of 0.2 to 0.5. Sticking probability calculations for the second-order adsorption (i.e., adsorption involving two adsorption sites) have been performed by several workers (25-29). In most cases, they indicated that precursors were involved in the adsorption (25, 26). However, in our work, the calculations seem to indicate that the precursors were not involved in NO adsorption on Fe/A1203. A simple theoretical equation, similar to the equation derived in this work, has been developed for dissociative adsorption of oxygen on cuprous oxide for premonolayer coverage by previous workers
(21, 30). CONCLUSIONS Iron was deposited on surfaces of alumina in the Fe ° state. Scanning Auger mapping inJournal of Colloid and Interface Science, Vol. 110, No. 2, April 1986
dicated that iron was randomly distributed on the A1203 surface at low coverages and that the surface was fully covered with iron at high coverages. At low iron coverages there was an interaction of iron with the A1203 surface. This interaction increased with increasing temperature. Nitric oxide interacted with Fe/A1203 at 298, 473, and 673 K. The adsorbed nitrogen was identified as nitride which indicated that NO was dissociated into nitrogen and oxygen on the Fe/A1203 surface. The NO adsorption increased with increasing iron coverage. However, when the A1203 surface was fully covered with iron, there was a decrease in NO adsorption. This indicated that the aluminum ions activated the iron atoms for NO adsorption even though pure alumina did not adsorb NO. At high iron coverages the adsorptivity increased with increasing temperature. However, NO adsorption on Fe/A1203 decreased with increasing temperature for low iron coverages due to the interaction and oxidation of iron with the alumina surface. Sticking probability calculations indicated that the NO adsorption of Fe/AI203 was mobile and dissociative. REFERENCES 1. Stinespring,C. D., and Stewart, G. W., Atmos. Environ. 15, 307 (1980). 2. Lunsford,J., J. Catal. 14, 379 (1969). 3. Lunsford,J., Zingery,L., and Rosynek,M., J. Catal. 38, 179 (1975). 4. Pozdnyakov,D., and Filimonov, V., Adv. Mol. Relaxation Processes 5, 55 (1973). 5. Brown,C., and Hall, P., Surf. Sci. 36, 569 (1973). 6. Douglas,T. B., "The Physicaland ChemicalAdsorption of Nitric-Oxide on Alumina and Other Oxides," Ph.D. dissertation, Stanford University, 1975. 7. Honda, F., and Hirokawa, K., J. Electron Spectrosc. Relat. Phenom. 8, 199 (1976). 8. Blyholder,G., and Allen, M. C., Y. Phys. Chem. 65, 3998 (1965). 9. Terenin, A., and Roev, L. M., Spectrochim. Acta 13, 946 (1959). 10. Kishi, K., and Ikeda, S., Bull. Chem. Soc. Japan 47, 2532 (1974). 11. Siriwardane,R. V., and Cook, J. M., J. Colloid Interface Sci. 104, 250 (1985).
NO-Fe/A1203 INTERACTIONS 12. Scofield, J. H., J. Electron Spectrosc. Relat. Phenom. 8, 129 (1976). 13. Wagner, C. D., Riggs, W. M., Davis, L. E., Moulder, J. F., and Muilenburg, G. E., "Handbook of XRay Photoelectron Spectroscopy," Perkin-Elmer Corp., 1979. 14. Brundle, C. R., J. Vac. Sci. Technol. 13, 301 (1976). 15. Comrie, C. M., Weinberg, W. J., and Lambert, R. M., Surf Sci. 57, 619 (1976). 16. Bonzel, H. P., and Pirug, G., Surf. Sci. 62, 45 (1977). 17. Price, G. L., Sexton, B. A,, and Baker, B. G., Surf Sci. 60, 506 (1976). 18. Conrad, H., Ertl, G., Kuppers, J., and Latta, E. E., Surf. Sci. 50, 296 (1975). 19. Furuyama, M., Kishi, K., and Ikeda, S., J. Electron Spectrosc. Relat. Phenom. 13, 59 (1978). 20. Kishi, K., and Roberts, M. W., Faraday Trans. 71, 1715 (1975). 21. Hayward, D. O., and Trapnell, B. M. W., "Chemisorption." Butterworths, Washington, D.C., 1964.
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22. Adamson, A. W., "Physical Chemistry of Surfaces," 3rd ed. Wiley, New York, 1976. 23. Cook, J. M., "Coal Mineral Interaction on Coal Conversion and Combustion Processes," Quarterly Report, WVU Task Order No. 32, Contract No. DE-AT21-77MC08087, May 31, 1981. 24. Madey, T. E., Yates, J. T., Jr., and Erickson, N. E., Chem. Phys. Lett. 19, 487 (1973). 25. Gasser, R. P. H., Roberts, K., and Stevens, A. J., Trans. Faraday Soc. 65, 3105 (1969). 26. Eisinger, J., and Law, J. T., J. Chem. Phys. 30, 410 (1959). 27. Redhead, P. A., Trans. FaradaySoc. 57, 641 (1961). 28. Ertl, G., Lee, S. B., and Weiss, M., Surf Sci. 114, 515 (1982). 29. Udovic, T. J., and Dumesic, J. A., J. Catal. 89, 314 (1984). 30. Jennings, T. J., and Stone, F. S., Adv. Catal. 9, 441 (1957).
Journal of ColloM and Interface Science, Vol. 110, No. 2, April 1986