Low energy electron diffraction research of chemical interactions of gases with single crystal metal surfaces, and gas adsorption on nickel and tungsten surface

May, J. W.

English

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I '  
I 
Annual Progress Report f o r  1966-1967 
to the 
National Aeronautical and Space Administration 
by John W. May ; 
f!Adsorption of Atoms and Molecules upon the Surface 
of a Single Crystal and 
Chemical Reactions that take place upon the Surface!! 
(Low Energy Electron Diffraction) ' ,' 
l 
Research Contract NGR 33-010-029 
Principal Investigator: Lester H. Germer 
Department of Engineering Physics. 
Cornell University 
Ithaca, New York 14850 
.. L 
https://ntrs.nasa.gov/search.jsp?R=19670016952 2020-03-12T11:25:32+00:00Z
Distinction between adsorption and beginning of corrosion 
and how these processes are related are not yet properly under- 
stood. In the past year we have devoted about half of our 
research towards this problem, dealing specifically with the 
interaction of oxygen with nickel and tungsten surfaces. We 
are also studying adsorption of other gases on these surfaces. 
Our other main effort is towards basic understanding of catalysis, 
and such understanding is necessarily based on knowledge obtained 
by studying adsorption itself. Our LEED research program is 
proceeding on a broad front and is concerned with closely 
connected aspects of the general problem of chemical interactions 
of gases with single crystal metal surfaces. 
We have been mainly concerned with the (110) face of nickel 
and the (112) face of tungsten. The projects described here are 
either still being investigated o r  have been finished within the 
last year. During this time M r .  C. C. Chang has completed 
experimental work leading to a Ph.D. degree and M r .  J. C. Tracy, 
Jr. is beginning research towards that degree. In addition to 
these graduate students there are now two research associates 
working with Dr. L. H. Germer who is the principal investigator. 
They are Dr. J. W. May, and Dr. R. J. Szostak (on leave from the 
Max-Planck-Institut, Berlin). 
SUMMARY OF WORK 
Adsorption and Corrosion 
W(112) + co C. C. Chang and L. H. Germer, J.  Electro- 
chem. SOC. (to be published) 
C. C. Chang and L. H. Germer, Surface w(112) + o2 
Science (in press) 
J. W. May and L. H. Germer 
L. H. Germer, J. W. May and R. J. Szostak, 
Bull. Am. Phys. SOC. 12 549 (1967) 
(paper submitted to Surface Science) 
Ni(ll0) + O2 
Ni(ll0) + O2 
-
Catalysis 
I Ni (110 ) H2 + 8 02+H20 J. W. May and L. H. Germer 
I 
W(112) " 3 3 Q  N2 + 3/2  H2 J. W. May and R. J. Szostak 
Work Function Changes after Adsorption 
02 + W(110), W(lOO), w(112), w(111) J. C. Tracy, Jr. 
and L. H. Germer 
DESCRIPTION OF WORK 
(Low Energy Electron Diffraction Studies) 
Adsorption and Corrosion 
(a) ~(112) + C o  (C. C. Chang and Lo H. Germer) 
LEED studies of adsorption of CO o r  of O 2  on a W(ll2) 
surface are a continuation of earlier work in this laboratory 
dealing with a W(110) surface [J. W, May and L. H. Germer, J. 
Chem. Phys. - 44 2895 (1966); L. H. Germer and J, W, May, Surface 
Sei. - 4 452 (1966); J. W. May, I,. H. Germer and C. C. Chang, J, 
Chem. Phys. - 45 2383 (1966)I. 
face has yielded results of considerable interest. Adsorption 
of CO at room temperature is disordered. The CO is adsorbed 
This extension to a different 
with very high sticking probability in two forms, a strongly 
held pphase whose coverage is pressure independent and a 
weakly held 4 phase whose coverage is about half that of 
at 
coverage is 1.0 x molecules/cm2. Disordered GO molecules 
torr and about 0.1 of (3 at 5 x 10 -lo torr. The 
adsorbed to saturation coverage at room temperature can be 
ordered by raising the crystal temperature to about lOOOOK 
(CO valve closed). The LEED diffraction pattern is then 
c(2 x 4) and is believed to represent a total coverage of 11/8 
of a monolayer (O(not present). Heating slightly hotter causes 
some of the (3CO to be desorbed, leaving a complex pattern which 
is not yet understood but which is produced by the remaining 
3/4 of a monolayer of CO. The latter is desorbed at about 
120O0K and a clean surface regenerated. Two other structures 
have been produced by heating CO-covered surfaces not having 
saturation coverage. These are a c(6 x 4) structure and a 
p(2 x 1) structure believed respectively to contain 114 and 
1/2 a monolayer of strongly adsorbed ?phase. It is believed 
that the more firmly adsorbed of the e components is produced 
by thermal activation and is not present in disordered form 
after adsorption at room temperature. 
(b) W(112) + O2 (C. C. Chang and L. H. Germer) 
Adsorption proceeds at room temperature with a sticking 
probability close to unity until the coverage is 1/2 a 
monolayer of adsorbed 0 atoms. Thereafter coverage increases 
more slowly and two more patterns, representing a complete 
monolayer and 3 / 2  of a monolayer, form in succession. Adsorption 
then stops. Two interesting features are worth particular 
mention (1) in the first stages the adsorbed oxygen atoms 
gather into long thin islands of the 1/2 monolayer structure, 
the islands lying parallel to the close-packed [111] surface 
direction. These islands are adjacent to areas with no regular 
structure, and as more and more oxygen is adsorbed the islands 
grow in width until the surface is completely covered with the 
1/2 monolayer structure. 
along the [111] surface direction and slow at right angles to 
this direction, and newly adsorbed O 2  molecules very likely 
migrate preferentially along the fftroughslf in the [111] 
Evidently island growth is fast 
direction until they can be incorporated into a growing island 
(2) in the later stages of adsorption the last 1/2 monolayer is 
apparently adsorbed quite differently from the full preceding 
monolayer. This oxygen is believed to be in a second layer. 
This weaker adsorption is correlated with the lltroughsll present 
in a W(112) surface but not found on a W(110) face, explaining 
why second layer adsorption is not found with oxygen on a W(110) 
surface. 
Adsorption of O2 on a W(112) crystal heated between 1000 
and 1 5 0 O 0 K  yields results entirely different. The original 
(112) face is destroyed and replaced by corrugations parallel 
to the [111] surface direction and whose surfaces are {llO] 
planes. These 110 facets are covered with superficial oxygen 
but exact coverages have not been determined. 
I )  
The facets grow 
in size with increasing oxygen exposure and are very small 
compared with llthermalll facets normally observed with the 
ordinary light microscope. A remarkable feature is that at a 
particular exposure all facets are of approximately equal area 
so that crests of the corrugations are equally spaced. 
Separations between crests have been measured as multiples n 
of 4.5A = 2 'l2a o, with n = 2, 3,  4, 5, 6 (i.e. largest separation 
between crests of 27A for n = 6). Growth of 110 facets is 
consistent with other published work which has shown that heated 
E 3  
tungsten surfaces in the presence of oxygen can facet into 
denser planes which are stabilized by adsorbed oxygen, 
. 
(c) Ni(ll0) + O 2  (J. W. May and L. H. Germer) 
This is an extension of earlier work [ L. H. Germer and 
A .  U. MacRae, Robt. A .  Welch Found. Res. Bull. - 11 (1961) and J, 
Appl. mys. 2 2923 (1962) ; A .  U. MacRae, Science - 139 (1963) 
and Surface Sci. 1319 (1964); R. L. Park and H. E. Farnsworth, 
J. Chem. mys . - 40 2354 (1964) and J. *PPI Phys - 35 2220 
Points of particular interest concerning corrosion have been 
investigated in considerably more detail. These are (1) 
competition for 0 atoms between the surface and bulk phases 
as a function of pressure, temperature and total oxygen 
concentration in the crystal (2) development of the first thin 
layers of a true bulk oxide and the transition from an adsorbed 
oxygen layer to a thin layer which is properly called oxide. 
This second problem has yielded the new result that an epitaxial 
oxide layer develops at room temperature which has considerable 
disorder. Good ordering is achieved by very mild annealing and 
the thin film of oxide, estimated to be less than 10A thick, 
has the NiO rock salt structure but is compressed by about 5% 
compared to bulk NiO. The problem of competition for adsorbed 
oxygen between the surface and bulk phases has been discovered 
to be very complex, and much more work will be necessary for a 
good understanding. We have found that adsorbed 0 is readily 
soluble at very low temperatures in the original ultra-pure 
crystal, but that as saturation is approached, surface 
structures become stable and resist thermal decomposition, even 
at temperatures approaching the melting point of nickel. 
Evidently the first oxygen has a very high heat of solution, 
although oxygen is not particularly soluble in bulk nickel. 
We estimate about 1 0 atom per 1000 Ni atoms is saturation 
solubility at 700°C and torr of 02. A publication is 
now being prepared. 
(d )  Ni(ll0) + O 2  (L.  H. Germer, J. W. May and 
R. J. Szostak) 
This work deals with adsorption of oxygen at 300OC on a 
Ni(ll0) crystal pre-saturated with oxygen (see part ( c )  above) e 
Prior to adsorption the surface can be cleaned at 2 0 O O C  by 
reduction with H2,  because diffusion of 0 from the saturated 
interior is comparatively slow at this temperature. Coverages 
of oxygen from zero to 2/3 of a monolayer of 0 atoms have been 
investigated and two striking new results obtained. (1) One 
third of a monolayer of adsorbed 0 atoms makes a ( 3  x 1) 
structure that covers the surface at room temperature, but 
raising the temperature to 300°C causes a reversible clustering 
phenomenon. Patches of the (2 x 1) half-monolayer structure 
form, necessarily leaving 1/3 of the surface bare. Cooling the 
crystal to room temperature causes reversion to the original 
(3 x 1) arrangement, heating to 30OoC produces the (2 x 1) 
patches, and so on quite reversibly. We believe this is the 
first such surface transformation of this kind so far discovered, 
(2) The second and possibly more important discovery is that 
instead of the disorder found at room temperature between 1/2 
and 2 / 3  coverage, one finds ordered superstructures at 3 0 O O C .  
These are believed to be composed of the unit building blocks 
of the 1/2 and 2/3  monolayer structure in proper ratio for a 
given coverage. These blocks are arranged so that the most 
uniform arrangement of surface oxygen is favored at every 
intermediate coverage between 0.5 and 0 .67 .  We believe that 
this flprinciple of maximum uniformityll may have far reaching 
implications for all adsorption work, and possibly may be 
applicable to structures in three dimensions as well. 
Catalysis 
(a) Ni(ll0) : H2 + 1/2 0 2 3 H 2 0  J. W. May and 
L. H. Germer 
A nickel (110) surface pre-exposed to oxygen can be 
readily reduced with H2 at moderate temperatures [L. H, Germer 
and A. U. MacRae, J. Chem. Phys. - 37 1382 (1962)I. This is one 
step to be considered in the larger problem, which we can not 
yet attack, of the catalysis of H20 formation or decomposition 
with H2, O2 and H20 all present in the gas phase. It is first 
necessary to isolate the catalytic behavior for certain extreme 
cases, such as reduction of oxygen-covered surfaces with pure 
H 2 9  which is the subject of this part of our research program. 
Extension of this project w i l l  hopefully follow in the future. 
Work has mainly been confined to coverages of oxygen of half a 
monolayer or less, and most of the experiments have dealt with 
an initial concentration of just 0.5 monolayer of 0 atoms, 
Prior to these experiments the crystal was saturated with oxygen 
at 70OoC and The surface was 
then cleaned of oxide by a combination of argon ion bombardment 
torr of O2 for many hours. 
and mild reduction in lo-' torr of H2. This did not materially 
affect the saturation amount of dissolved 0 atoms, Heating the 
crystal in this condition (clean crystal LEED pattern) 
invariably caused 0 atoms to diffuse from the interior to make 
a full half-monolayer coverage. 
Reduction experiments could then be carried out, with the 
crystal held during reduction at a temperature lower than the 
preheating temperature. Each reduction experiment was initiated 
by preheating to produce an initial coverage of just 0.5. 
interesting features were observed. (1) With the crystal at 
280°C and hydrogen introduced to 1 x 
layer of 0 was removed in just 30 sec, or one 0 atom was removed 
for every 75 incident H2 molecules 
Diffusion then ceased, 
Several 
torr, the half mono- 
(2) With the crystal at 
130°C the removal rate was slower by a factor of about 10 3 . ( 3 )  
With the crystal at 550°C the rate was apparently immeasurably 
slow, This apparently gross violation of the Arrhenius law is 
an illusion, and at 550°C the rate of diffusion of 0 atoms f r o m  
the interior to the surface just balanced removal by hydrogen, 
(4) The most effective temperature for cleaning only the 
surface was 28OOC. At this temperature it was discovered that 
the removal was 3/2 order in H2 pressure. This means a more 
complex mechanism than simple collision and capture of a surface 
0 atom by a molecule of H2. 
steps involving more than one H2 molecule. 
at hand which have not yet been fully analyzed, and a publication 
is presently being prepared, 
I 
The process probably involves two 
We have more data 
(b) W(112) : NH3+1/2 N2 + 3/2 H2 (J. W. May and 
R. J. Szostak) 
Catalytic synthesis and decomposition of NH3 is a subject 
that has interested chemists for 150 years but is not yet 
understood in spite of its great commercial significance. We 
have adsorbed NH3 at room temperature on a clean W(112) surface, 
evacuated the system and then heated the crystal. We observe 
high sticking probability for the NH3 and believe that each 
adsorbed molecule dissociates into NH2 and H until the surface 
is full. 
The surface structure is simple. Heating the surface to about 
5OO0C causes evolution of H2 only, and this hydrogen originates 
from the H atoms originally produced by dissociation. 
point re-exposure to NH3 will cause more to be adsorbed. 
a succession of 5 0 O o C  heatings and re-exposures to NH3 the 
surface reaches a saturation coverage of NH2 complexes, with 
3 no adsorbed atomic H. 
exposure to saturation in one dose, heating the crystal to 
5 0 O o C  causes evolution of H2 as described. 
produces a succession of three complex ordered structures, 
There is then one H atom for each adsorbed NH2 complex. 
At this 
After 
If the crystal is given a Large NH 
Further heating 
Very little H2 or N2 is evolved during these transformations, 
and the final LEED pattern closely resembles the pattern I 
. produced by nitrogen alone at these temperatures. Finally, 
heating to about 8 5 O o C  causes evolution of a large pressure 
burst containing both N2 and H2 and leaving a clean surface. 
Evidently the single N atoms present in the NH2 complexes 
produced at 5 0 O o C  come together via a complicated route at 
higher temperatures to produce an arrangement characteristic 
of nitrogen adsorption. 
We have more observations too detailed for this report but 
which are to be included in a publication now under prepara- 
tion. 
Only then can evolution of N2 occur, 
Work Function Measurements 
~ 
(J. C. Tracy, Jr. and L. H. Germer) 
There is great need at present for an approach to surface 
structure independent of LEED intensities. Measuring the work 
function changes produced by given surface adsorption structures 
will hopefully remove some of the ambiguity inherent, at the 
present state of the theory, in the interpretation of LEED 
intensities [I,. H. Germer, Surface Sci. - 5 147 (196611. 
has been constructed for making measurements, by the Kelvin 
vibrating reed method, of changes in work function produced by 
adsorption. 
oxygen on tungsten and nickel surfaces. Work function changes 
Apparatus 
Attention is being focused on the adsorption of 
will be followed as adsorption proceeds, and will also be made 
with a given coverage after annealing the crystal. It is hoped 
to correlate work function changes with surface structure. 
Experiments are also planned, and apparatus is being constructed, 
for measuring work function changes at liquid nitrogen temper- 
ature. Such experiments will hopefully allow more general 
conclusions to be drawn regarding surface rearrangements 
stimulated by adsorpttion, thermal nucleation of adsorption 
structures and the effect of ordering after annealing. 
t 
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Adsorption of atoms and molecules upon the surface of a single crystal and chemical reactions that take place upon the surface /low energy electron diffraction/ Annual progress report, 1966-1967

Adsorption of atoms and molecules upon the surface of a single crystal and chemical reactions that take place upon the surface /low energy electron diffraction/ Annual progress report, 1966-1967

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