H. E. FARNSWORTH
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Combined Low-Energy Electron Diffraction and Mass Spectrometer Observations on Some Gas-Solid Reactions and Evidence for Place Exchangelasb by H. E. Farnsworth Brown University, Providence, Rhode Island
02912
(Received March 9 , 1970)
Combined LEED and MS observations were used to obtain information on lattice structure and reaction products when an atomically clean Ni (100) crystal was exposed t o gases such as O2 and CO. Measurements of changes in work function and of diffraction spot intensity, as a function of electron energy, were obtained to determine if place exchange or simple adsorption occurred. It was concluded that place exchange occurred when Ni (100) was exposed to 02 at 22’. Two different place exchange structures occurred on M o (100) when the oxygen-contaminated surface was heated at two different temperatures. Adsorption, without and CO on Re (0001). place exchange, occurred at 22’ for CO, COZ,and NO on Ni (100) and for 02,
Introduction The combination of low-enerrrv electron diffraction (LEED) and mass spectrometer (MS) equipments permits one to obtain information on both surface structures and desorbed reaction products. However, it requires that contamination of the reaction product from sources other than the crystal surface be minimized.
Experimental Section The above requirement is satisfied in our equipment by an arrangement in which gas desorbed from the crystal face by heat treatment is transferred preferentially through a fused-silica tube to the ionization chamber of the quadrupole mass ~pectrometer,~“ as shown in Figure 1. The LEED system is the rapid-scan electrical detection type.3b A narrow beam of electrons strikes the crystal at normal incidence and the elastically diffracted electrons are measured by means of a movable Faraday cage. In addition to obtaining spot patterns by a rapid scan, involving simultaneous rotations of the crystal and the Faraday collector, the intensity vs. voltage (I vs. V ) curve for any selected spot or beam may be obtained over a wide range of voltage in a few minutes. This curve, showing variation of spot intensity with primary voltage or wave length for normal incidence, contains significant information. Because the lowenergy electrons penetrate only a few monolayers, with the effect of the surface monolayer predominating, the shape of the I vs. V curve may be strongly affected by an adsorbed monolayer. In this paper we shall consider some of the factors which influence the shape of this curve. Changes in work function are obtained by measuring the crystal current as a function of retarding crystal The J O U T ~of ~Physical Z~ Chemistry, Vol. 74, No. 16, 1970
potential for different surface conditions and observing the change in the cut-off voltage.
Results and Discussion Spurious results due to small amounts of surface contaminants must be avoided. As an example, the influence of small amounts of surface contamination on CO adsorption on Ni (100) is shown in Figure 2. The upper curve shows the development of the CO-peak intensity with exposure for a clean surface. The lower curve was taken after slight bulk contamination (less than 1 monolayer) had diffused to the surface by heating the crystal. This result indicates the great sensitivity of surface reactions to minute amounts of contaminant and the precautions that must be observed in obtaining a clean surface. We shall consider first the interactions of CO and 0 2 with Ni crystal surfaces. I n Figure 3, the upper frame contains a copy of the reciprocal lattice spot pattern for CO on Ni (100). The large solid circles are from the nickel lattice and the small solid circles are from the CO lattice on the surface. A model which will produce this pattern is shown below. The open circles represent the nickel atoms and the solid circles represent the CO molecules. (1) (a) This paper was presented at the “Diffraction and Structure,” symposium of the Division of Physical Chemistry, American Chemical Society National Meeting, New York, Sept 1969. (b) This work was supported by the National Science Foundation, the Advanced Research Projects Agency, and the Department of the Air Force, Wright-Patterson Air Force Base. (2) H. E. Farnsworth, Adcan. Catal., 15, 31 (1964); “The Solid Gas Interface,” Vol. I, E. Alison Flood, Ed., Marcel Dekker, New York, N. Y., 1967, Chapter 13; “Experimental Methods in Catalytic Research,” R. B. Anderson, Ed., Academic Press, New York, N. Y., 1968, pp 265-285; J. W. May, I n d . Eng. Chem., 57, 18 (1965). (3) (a) M. Onchi and H. E. Farnsworth, Surface Sci., 11, 203 (1988); (b) R. L. Park and H. E. Farnsworth, Rew. Sci. Instrum., 35, 1592 (1984).
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LOW-ENERGY ELECTRON DIFFRACTION AND MASSSPECTROMETER OBSERVATIONS
2913
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Figure 3. A, Copy of recipracel lattice spot pattern from a Ni (100)crystal face after exposure of the clean surface to CO a t 10-8 Tam min (60 eV). Large solid circles are from the nickel lattice. Small solid eiroles me fmm the adsorbed CO lattice. B, Model of Ni atoms positions (large open circles) and CO molecule positions (small solid circles) which account for the pattern in Figure 2A.
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Figure 1. Combined LEED and mass spectrometer system (courtesy of Surface Sci.).
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Figure 2. Development of 34-eV peak in CO exposure (courtesy of Surfaee Sci.).
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Figure 4 shows the I us. T' curve for one of the spots due to CO after exposure at room temperature. A slight heating of this surface in uacuo intensified the pattern, thus suggesting that the CO lattice had been improved. When this surface was exposed to 02,a similar but weaker curve was obtained, thus indicating that structureless oxygen had partially covered the CO lattice. After slight heating of this surface in uacuo, a new intensity curve was obtained which was the same as that for oxygen on a clean Ni (100) surface. Mass spectrometer tests showed that during the slight heating the adsorbed oxygen had combined with adsorbed CO to form COS which desorbed with an excess of oxygen remaining on the crystal surface.
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Figure 4. Intensity US. voltage curves for ('/s '/s) beam from Ni (100) (courtesy of J . Chem. Phys.): A, after exposure of clean Ni (100) to CO a t 22"; B, after heating crystal to 300' and cooling to 22"; C, after the CO covered surface had been exposed to Os at 22"; D, after heating the CO Ox surface to 250' and cooling to 22'.
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Figure 5. NisO lattice showing (loo), (110), and (111) faces. Open circles indicate 0 atoms. Solid circles indicate Ni atoms (courtesy of A p p l . Phys. Lett.). The Jouvml of Phwicol Chemistry. Vel. 74.No. 16,1970
2914
H. E. FARNSWORTH
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Figure 6. A, (0 ' 1 2 ) beam after 1 X lo-' Torr min CO exposure of a clean Re (0001) surface at 22' and subsequent vacuum beating at 425" for 10 min (courtesy of Surfoce Sei.); B, (0)>',I beam after 1 x 10- Torr min 0 2 exposure of a clean Re (0001) surface at 22' and subsequent vacuum beating at 425" for 10 min (courtesy of Surface Sci.).
It is important to note that the spot patterns for CO and 0 2 on Ni (100)are the same in this case, but the I us. V curves are entirely different. Because of the large binding energy of oxygen and the work function decrease associated with the formation of this structure, the mechanism of place exchange between oxygen and nickel atoms has been suggested.' I n fact, this result, combined with those for Ni (110) and Ni ( I l l ) , leads to the interpretation of a place exchange structure of NisO in which the interatomic spacing is essentially the same as that for pure nickel (Figure 5).6 I n contrast with the results on the intensity curves for CO and O2on nickel, we have found that CO and 0% adsorbed on Re (OOO1) not only have the same spot patterns but also similar I us. V curves. Figure 6 shows the I os. V curves from double-spaced structures for these two cases. These results suggest that in the case of Re, CO and 0%are both adsorbed m the surface and that multiple scattering, as discussed by McRae,' results in similar intensity curves. I n the case of Ni, there is place exchange between oxygen and Ni atoms instead of adsorption m the surface, thus altering the multiple scattering conditions. To test further the validity of this hypothesis, we The Joumol of Phu&d
Chemietw. Vol. 74,No. 16.1870
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Figure 7. A, ('12 ' 1 3 ) beam after 1 x 10-8 Torr min NO exposure of a clean Ni (100) surface at 22', B, ( I / $ 1 1 % ) beam after 2.5 X Torr min COS exposure of a clean Ni (100) surface at 22".
have made observations with other gas;-solid eombinations. Figure 7 shows I us. V curves from similar multiple-spaced structures for NO and CO, on Ni (100). It is seen that here also the curves are very similar, except for a displacement of the NO curve toward lower voltages. Figure 8 contains curves for C02 and CO. Here there is considerable similarity also, except for the double peak between 50 ar.d 75 eV in the CO curve. From these results, it appears that although place exchange occurred with oxygen and Ni (loo), this was not the case with CO, COS, and NO, while with Re (Oool), no place exchange occurred with either O2or CO. Because there have been differences of opinion concerning the concept of place exchange' (or reconstruction), details of another case are considered, namely, the oxygen-molybdenum reaction, which also supports (4) H. E. Famsworth and H. H. Madden, Jr.. Bull. Aner. Phya. Soc., 5, 349 (1960); J . A p p l . Phys.. 32, 1933 (1961). (5) H. E. Farmworth, A p p l . PAya. Lett.. 2, 199 (1963).
(6) E. G. McRae. S w f e e Sci., 11. 492 (1968). (7) E. ~ ~ as,, ~ 15. 152 e (1986). ~ .
LOW-ENERGY ELECTRON DIFFRACTION AND A k s s SPECTROMETER OBSERVATIONS
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Figure 8. A, (I/% I/z) beam after 2.5 X 10-5 Torr min COz exposure of a clean Ni (100) surface a t 22'; LI, (I/% '1%) beam after 1 X lo-' Torr min CO exposure of a clean N i (100) surface et 22'.
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Figure 10. A, (10) beam from a clean Mo (001) surface (courtesy of Surface Sei.); B, (10) beam after 6 x 10-6 Torr sec 0 2 exposure of a clean Mo (001) surface at 22". I
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Figure 9. Work function changes as functions of oxygen exposure (courtesy of Swjaee Sd.).
Figure 11. Place exchange models of molybdenum and oxygen atom positions in the surface plane of Mo (001). Open circles represmt molybdenum rttoms. Solid circles represent oxygen atoms: A, onehalf monolayer of oxygen atoms. B, two-thirds monolayer of oxygen atoms.
this concept. Exposure of a Mo (100) surface, at room temperature, to O2in the lo-* to 10" Torr range did not produce new diffraction beams characteristic of multiple spacing, that is, the spot patterns were identical. However, the work function and the I us. V curve of the integral-order beams were changed appreciably. Figure 9 shows the change in work function with increasing oxygen exposure for both Mo (100) and Mo (110) surfaces. Limiting values were obtained after a total exposure of 6 X lo-' Torr sec with no appreciable changes for additional exposures to 6 X 10W8 Torr sec. Figure 10 shows I us. V curves for the (10)
beam from clean and O2 exposed Mo (100) surfaces. The increased work function plus absence of fractional orders indicates that a surface monolayer of oxygen was formed with a unit mesh having the same dimensions as that of the underlying molybdenum. Further exposure caused a decrease of diffracted intensity, suggesting the formation of a second amorphous layer. Heating the oxygen-covered (100) surface at 500" produced intense half-order ('/2 beams, suggesting the presence of a cc (2 X 2) (one-half monolayer) structure, and further heating at between 500 and 1OOO" caused these beams to disappear and the The Jounrol oj Physiml Chembtw, Vol. 74. No. 16, 1870
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P. R. GRIFFITHS, P. J. SCHUHMANN, AND E. R. LIPPINCOTT
appearance of strong ("8 l/3) beams (one-third monolayer structure). Both of the structures which produced these two sets of beams are believed to be exchange structures for the following reasons. (1) The work functions for the two structures were the same or less than that of a clean surface, whereas exposures a t room temperature produced work functions greater than the clean-surface value. (2) Adsorption a t room temperature produced no fractional-order beams, but the 1 vs. V curves of the integral order beams were altered, thus showing that oxygen was adsorbed on the surface in the same lattice as that of the molybdenum substrate. (3) Because of the activation energies involved, it appears probable that the 1/2 and '/3 monolayer structures were formed with atomic oxygen while the monolayer formed at room temperature was in the form of molecular oxygen.
It appears unreasonable to expect only monolayer of oxygen atoms above the Mo surface in its most dense form when a complete monolayer of molecular oxygen forms at room temperature. Models for the 1/2 and monolayers are shown in Figures l l a and l l b , respectively. Place exchange structures for the Mo (110) surface were observed after oxygen exposures a t room temperature. In conclusion, it has been shown that a surface reaction between two consitutents on a solid surface may be monitored by an LEED and &!IS system to obtain the lattice structures before and after the reaction and also the nature of the reaction product. Evidence is presented to support the view that place exchange can occur between adsorbed oxygen and the metal substrate for both Ni and Mo in a manner which results in multiple-spaced structures.
Thermodynamic Equilibria from Plasma Sources. 111. Carbon-Hydrogen-Nitrogen Systems1 by P. R. Griffiths, P. J. Schuhmann, and E. R. Lippincott Department of Chemistry, University of Maryland, College Park, Maryland
(Received August WO, 1969)
Some organic nitrogen compounds have been subjected to a radiofrequency electrodeless discharge, and the products analyzed. Although the exact composition can be explained only by invoking kinetic factors, the approximate product distribution as the species emerge from the plasma may be computed by assuming a high-temperature limited thermodynamic equilibrium. The final distribution of the hydrocarbons corresponds to a temperature of about 1300°K, while that of the major nitrogenous compounds corresponds to a higher temperature, owing to the high activation energy needed to break the triple bond of the cyano radical which is formed in the plasma.
Introduction I n the first two papers in this series,2a,bit was shown that the distribution of the reaction products formed when organic oxygen compounds2&or hydrocarbonszb are subjected to a radiofrequency electrodeless discharge is approximately that corresponding to a limited chemical equilibrium for temperatures between about 1400 and 1000°K. The mechanism postulated involves the complete breakdown of the reactant molecules into atomic, radical, and ionic fragments in the center of the plasma, with the subsequent recombination of these highly reactive fragments as the mixture flows through the reaction tube, the product distribution Corresponding to that at thermodynamic equilib~iiuma d the translational temperature of the gaseous
species. When the kinetic and electronic energy of the molecdes is sufficiently low that the activation energy of the reactions involved cannot be overcome, the equilibrium becomes frozen at this temperature. There is an apparent range of as much as 400°K over which this process can occur. The method used to calculate the distribution of species a t equilibrium was that of the minimization of the total free energy of the system as described by White, Johnson, and DantzigS and used by Dayhoff, (1) This research has been sponsored in part by the National Aeronautics and Space Administration. (2) (a) C. K. Wieffenbach, P. R. Griffiths, P. J. Schuhmann, and E. R. Lippincott, J . Phys. Chern., 73, 2526 (1969); (b) P.R.Griffiths, P. J. Schuhmann, and E. R. Lippincott, {bid., 73, 2532 (1969).
