Nitrogen Desorbs with Complex Kinetics Tightly bound state may involve superposition of two or more unresolved substates 150TH
ACS
NATIONAL
MEETING
Colloid and Surface Chemistry
The kinetics of desorption of the tightly bound ^-nitrogen states on polycrystalline tungsten seem more complex than previously thought by chemists. Of the two overlapping binding states—/?! and (32—the fix state desorbs by first-order kinetics, and the more tightly bound /32 state desorbs with higher-order kinetics. The f32 state may involve the superposition of two or more unresolved substates, according to Dr. John T. Yates, Jr., and Dr. Theodore E. Madey of the surface chemistry section of the National Bureau of Standards Institute for Basic Standards (Washington, D.C.). The NBS scientists used ultrahighvacuum techniques of isotopic mixing and flash-filament desorption. They also found that, above 1500° K., the sticking coefficient of nitrogen on clean tungsten surfaces is less than 0.02. (The sticking coefficient is defined as the ratio of the adsorption flux to the incident molecular flux.) The nitrogen-tungsten system, the NBS pair points out, has these advantages for study: • Much is already known about the properties of nitrogen chemisorbed on tungsten, and about the surface and bulk properties of clean tungsten. • Nitrogen has several binding states on tungsten. • The system may be used as a model for studies of surface phenomena in more complex gas-solid systems. Temperature-programed flash-filament techniques and steady-state methods are not new to studying nitrogen adsorption on tungsten, Dr. Yates says. He and Dr. Madey found in earlier work [J. Chem. Phys., 43, 1055 (1965)], using an equimolar mixture
of nitrogen-14 and nitrogen-15 (isotopic mixing method), that the most weakly held state, y-nitrogen (desorbing below 200° K.), remains isotopically unmixed during adsorption, since no 1 4 N 1 5 N forms. Dr. L. J. Rigby of the National Research Council of Canada (Ottawa) reached the same conclusion [Can. J. Phys., 43, 532 (1965)] with another weakly held state, a-nitrogen (desorbing near 400° K.). The NBS scientists conclude that these weakly held states are molecularly adsorbed on most crystallographic planes of tungsten. By contrast, they find the strongly held /3-nitrogen (above 1100° K.) completely mixed isotopically; 50% of all nitrogen desorbed is 1 4 N 1 5 N. This generally indicates atomic adsorption. Using the flash-filament technique, Dr. Gert Ehrlich of General Electric Research Laboratory identified (in 1961) the ft state of nitrogen on tungsten (stable to about 1100° K.) and measured second-order kinetics for the desorption. He explained this with an atomic recombination process. In 1963, Dr. Takeo Oguri of the department of physics, Tokyo Metropolitan University, recognized the splitting of the f3 phase into /?x and f32 states. He assumed first-order kinetics for the fti state and second-order for the p2 state. He used a mass spectrometer to study the system. However, it was a modification of the linear-temperature-programed flash-desorption technique suggested by Dr. P. A. Redhead of Canada's National Research Council that enabled the NBS scientists to obtain more detailed information on the desorption kinetics of the overlapping fi2 state. Dr. Redhead suggested in 1962 that instantaneous desorption rates and coverages could easily be measured by using a relatively slow linear temperature flash, and simultaneously evaluating both the pressure pulse (AP) and its first time derivative (
1
START-UP. Dr. Theodore E. Madey (left) and Dr. John T. Yates, Jr., fill Dewar of apparatus for studying lowtemperature binding states of nitrogen or tungsten
However, this very simple method has not been used by others so far. In the new technique, the pressure pulse is measured with a Bayard-Alpert gage connected to a high-speed picoammeter, and it is displayed on a rapid-response recorder. The time derivative of the pressure is obtained by an analog method and is displayed on a second recorder. Using two temperature scan rates (12.5 and 25 K. ° per second) in the range of 1225° to 1440° K., the NBS pair took a large number of desorption spectra generated over a wide range of initial coverages. (Coverage, cr, is defined as the total number of species adsorbed on the filament unit area. The desorption rate is then — = dt —kcr11, where k is the rate constant and n is the apparent desorption order.) From the desorption spectra, the NBS scientists measured da the
dP desorption rate, and plotted it against coverage on a log-log scale. On the same diagram, they also plotted values of the isothermal desorption flux from isotopic mixing. They find that the two independent techniques agree. The scan rate does not seem to influence the results in the range tested. Dr. Yates points out that these isotherms disprove simple second-order desorption kinetics for /32-nitrogen. SEPT.
2 0, 1965
C&EN
43
132 -Nitrogen on Tungsten Desorbs in a Pressure Pulse
Apparent Desorption Order (n) Decreases with Increasing Temperature
Time Derivative of Pressure During Desorption
Legend • Scan rate — 12.5 K.°/sec. A Scan ra|e = 25.0 K.°/sec. * Rate by isotopic mixing method
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SEPT.
2 0, 1 9 6 5
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Log coverage, molecules/cm, 2
Temperature, °K.
He thinks that the higher-order kinetics indicate the superposition of two or more unresolved substates. If fi2nitrogen were a single atomic binding state, or a distribution of atomically bound substates in rapid equilibrium with each other, pure second-order desorption should have been found. The apparent desorption order (n) decreases from an initial high value with increasing temperature, as shown by the slopes of the isotherms. He interprets the variation with temperature in the apparent desoiption order of nitrogen as arising from competition (lack of equilibrium) between different binding substates during desorption. This competition is due to simultaneous independent desoiption from several spatially separate crystallography regions (patches) on the heterogeneous tungsten surface. Another interpretation is the gapfilling mechanism. According to this theory, the adsoiption of /^-nitrogen depends on prior formation of an appreciable /? 2 -nitrogen layer, where / ? r nitrogen occupies sites created by /3 2 nitrogen.
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The field emission results of Dr. Oguri suggest that both surface heterogeneity and gap-filling processes may lead to the formation of coexistent /?!- and /? 2 -nitrogen states. Thus, /?! and j32 may be coadsorbed on certain regions of the tungsten surface, but may also be adsorbed singly on spatially separated patches of the surface. This possible dual nature of fii- and /? 2 -nitrogen states may be the underlying reason for the complex kinetics found, Dr. Yates comments. The first-order kinetics found for /31 -nitro gen suggest molecular adsorption. However, the isotopic mixing experiments point to atomic adsorption. But complete isotopic mixing in the fa state can be consistent with molecular adsoiption, Dr. Yates says. In an earlier study, the NBS scientists found [/. Chem. Phys., 42, 1372 (1965)] that, at temperatures above 850° K., the nondissociative chemisorption of CO on tungsten was accompanied by rapid isotopic mixing in the strongly held j3 states. They assumed a quadrangular bimolecular adsorption complex for CO, involving
a bimolecular exchange reaction. In this complex, each CO molecule interacts with at least two tungsten atoms on the surface to form bridges. The isotopic mixing behavior of the isoelectronic /? r nitrogen can be explained similarly with a quadrangular nitrogen complex formed on tungsten which involves isotope exchange reaction between nitrogen-14 and nitrogen-15: 1 4 N
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-15N
All previous arguments advanced by others for the existence of chemisorbed nitrogen atoms in the ft state have depended solely on the experimentally found second-order desorption kinetics, Dr. Yates emphasizes. But the complex kinetics he finds indicates that a simple atomic recombination mechanism for nitrogen is inadequate. Both atomic and molecular nitrogen species may be chemisorbed on different crystallographic faces of tungsten in the strongly held fa phase, he concludes.
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