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The Surface Composition of Nickel-Zirconium Intermetallic Compound Catalysts
811
.J.W. Ward (Editor), Catalysis 1987 © 1988 Elsevier Science Publishers B.Y., Amsterdam - Printed in The Netherlands
THE SURFACE COMPOSITION OF NICKEL-ZIRCONIUM INTERMETALLIC COMPOUND CATALYSTS M. C. Deibert l and R. B. Wright 2 IDepartment of Chemical Bozeman, Montana 59717
Engineering,
Montana
State
University,
2Idaho National Engineering Laboratory/EG&G Idaho, Inc., P.O. Box 1625, Idaho Falls, Idaho 83415
ABSTRACT The compositions of the surface and subsurface strata of uniquely active zirconium-nickel alloy hydrogenation catalysts have been investigated by high resolution Auger spectroscopy and Ar ion sputter ing. Sequential 400 0C pretreatments of the intermetallic compounds ZrNi3 and Zr2Ni7 with H2, 02 and finally H2 are shown to produce an essentially pure reduced Ni surface supported on a sublayer of oxidized zirconium, which in turn is supported on the unaltered alloy. This reduced nickel constitutes the highly active catalyst surface. Evidence is developed related to the thickness of the reduced nickel surface layer and its morphology. INTRODUCTION Recent
studies
structure of (ref.
2)
actinide
(ref.
1)
of
the
surface
composition
and
intermetallic alloys have been prompted by earlier
findings
that certain alloys consisting of
and
earth
rare
metals
are
good
catalysts
transition, for
various
reactions of possible industrial relevance. These surface analysis studies observed structure
have
established
catalytic of
the
a
activity alloy.
definite and
the
Generally,
correlation surface the
between
the
composition
and
catalytically
active
surface is the result of a chemically induced decomposition of the alloy into a reduced metal surface phase consisting of one of the alloy components that in turn is dispersed on a second subsurface phase composed of an oxide of the second alloy component. Thus, in a general sense,
the active catalyst consists of a reduced metal
dispersed on a metal oxide, which acts as a support, both of which in turn reside on the underlying bulk intermetallic alloy. Previous catalytic activity studies (ref. 2)
have demonstrated
that the Zr/Ni alloy system exhibits high hydrogenation activity
812
if the alloy is treated in oxygen gas to temperatures up to 550 0C prior
to
a
reducing
treatment
in
hydrogen
of
earlier
at
temperatures
exceeding 200 oC, both of which precede the catalytic testing. The
present
study
is
a
continuation
(ref.
1)
surface
characterization work which used x-ray photoelectron spectroscopy (XPS) to investigate the influence of various elevated temperature oxygen and
hydrogen
treatments on
the surface composition of
a
number of Zr/Ni alloys. The earlier XPS studies showed that even in their as-prepared state, the surface of the alloys are depleted in nickel and consist mainly of Zr02' Upon exposure to oxygen at temperatures exceeding 200 oC, the surface becomes enriched in nickel, as NiO, which upon reduction in hydrogen produces a nickel rich surface, as Ni,
that is supported on a thin layer of Zr02'
The Auger studies presented in the current paper were conducted to elucidate
in
a
more
quantitative
manner
the
compositional
stratification that occurs in this alloy system as a result of the oxidation/reduction pretreatments. EXPERIMENTAL PROCEDURE The Zr-Ni alloys used in this study were prepared by the direct arc melting of the appropriate stoichiometric amounts of pure Ni (99.9%)
and
Zr
(99.9%)
powders.
The
arc melted
specimens
were
sliced and polished using 0.05 micron alumina abrasive to obtain the
final
surface
purity Ni ZrNi3,
and Zr
ZrNi,
reagent
and
finish. foils,
The
samples analyzed
included
as well as polished samples of
Zr2Ni.
All
samples
were
grade acetone and methanol prior
to
solvent
99.99% zr2Ni7,
cleaned
insertion
into
in the
analysis system. The analyzer was a PHI 595 scanning Auger microprobe with Ar+ sputter profiling capability. The 200 nA, 3.0 keV primary electron
beam was rastered over a 0.02xO.02 mm 2 area. This area was central to a 1. 8x1. 8 mm 2 area impacted by a rastered 1. 0 keV Ar+ beam during ion milling. The Ar+ beam current was approximately 0.6 uA or about 18.5 uA/cm 2 over the area of the rastered beam. The E*N(E)
vs.
E data collected during the AES scans were stored on
magnetic disks for software
was
later display, manipulation and analysis.
utilized
to
process
the
E*N(E)
vs.
E
data
PHI as
required. After
characterization
of
the
Auger
spectra
of
a
sputter
cleaned area on each sample in the untreated condition, the sample was withdrawn
to the preparation chamber of
the PHI
595
system
813
which was then backfilled to atmospheric pressure with ultra-high purity H2' The temperature of the sample was then raised to 400 0C for
30
minutes.
A slow purge of
H2 was maintained
through
the
preparation chamber during the high temperature treatment. After cooling,
the
procedure
repeated
using
spectra
were
gas.
Auger
preparation
chamber
was
ultra-high measured
re-evacuated
purity 02
on
the
as
sample
the in
and
the
treatment
the
treated
condition and then after each of a sequence of surface sputters with 1 keV Ar+ to develop a high temporal resolution depth profile of the sample. After
the
first
sputter
depth
profile
was
completed
on
the
sample treated in H2 and then 02, the sample was withdrawn to the preparation chamber and treated for 30 minutes with H2' A second high
resolution
sputter
depth
profile
was
then
conducted
on
a
previously unsputtered portion of the sample surface. EXPERIMENTAL RESULTS Examples this
of
study
the
are
principal AES spectra which are
presented
in
Figures
1
for
the
utilized
energy
in
region
between 80 and 180 eV within which many principal Zr Auger peaks occur and in Figure 2 for the energy regions between 45 and 75 eV and between 816 and 856 eV within which two major Ni Auger peaks occur.
Spectrum
(a)
in Figure 1 is
for
sputter
cleaned pure Zr
foil and exhibits peaks near 91 eV, 116 eV, 124 eV, 146 eV and 173 eV.
After
is oxidized by exposure to 1000 L (lL=l Langmuir=exposure of 10- 6 torr of gas for 1 sec) of 02 at 20 oC, spectrum
the
(b)
Zr
foil
is observed.
The 146 eV peak
is diminished by the
surface oxidation of the Zr and is partially replaced by a new peak at 140 eV. The peak at 173 eV is strongly diminished. These changes provide convenient indications of the oxidation state of Zr
from its Auger spectra. Spectrum (c)
in Figure 2 was measured
on
a
a
sputter
cleaned
ZrNi3
surface
on
sample
which
had
not
received high temperature H2 or 02 treatments. The Auger peaks are essentially the same as spectrum (a)
except for the introduction
of a weak Ni peak near 101 eV. The intensity of the Zr peaks are diminished due to the dilution effect of the Ni. Spectrum (d)
in
Figure 1 was measured on the same ZrNi3 surface after exposure to 1000 L of 02 at 2l oC. Essentially the same influences of the surface oxidation of evident.
The Ni
several
of
the
Zr
peak at Zr
that were observed 101 eV is
peaks
are
in spectrum
(b)
are
substantially eliminated and
stronger
due
to
the
selective
814
segregation of
the
Zr
to
the surface to form Zr02 by
the
room
temperature surface oxidation process.
-
>-
-...
'iii c::
Z'
'Cii
Q)
t::
...
c::
Q)
5... Q)
Cl :::l
,
''' \
,...
/ ' Zr + \... 'b)
« W 1 \\ ..",J
Z «
l'--
W
II
1000 L 02 /
f, __
W
\
LLJ
d) ZrNi3
1000 L
\.
' ..... ....
--
\b) Ni
+--
\ 1000 L 02
\
c) Sputter cleaned
+.
ZrNi3
02\.,
+ 1000 L02
. ,./
120
80
\.
.\
oO
c) Sputter cleaned '-"" ZrNi3 ( , _ . J d ) ZrNi3
r-'
,
"-'\
Z-
,r'
1\."
A
Q)
C'l ::l
\ ......... /
/\.--/.'\
................. ,
140
180
Electron Energy (eV)
Electron Energy (eV)
Figure 1 80 to 180 eV region of E*N(E) Auger spectrum for a) sputter cleaned Zr; b) Zr after 1000 L 02 exposure; c) sputter cleaned ZrNi3; and d) ZrNi3 after 1000 L 02 exposure. Figure 2 45 to 75 eV and 816 to 860 eV regions of E*N(E) Auger spectrum for a) sputter cleaned Ni; b) Ni after 1000 L 02 exposure; c) sputter cleaned ZrNi3; and d) ZrNi3 after 1000 L 02 exposure. Spectrum (a)
in Figure 2 was measured on a sputter cleaned Ni
foil surface and spectrum (b) is this same surface after a 1000 L exposure at 2l oC. The peak at 841 eV is not significantly
02
changed by the surface oxidation process, but the energy level of the low energy Ni peak is shifted from about 57 eV to about 55 eV. This shift of the low energy Ni Auger peak provides an indication of the oxidation state of Ni. Spectra (e) and (d) in Figure 2 were measured on a
sputer
cleaned and an oxidized surface of
ZrNi3,
respectively, The same influences of surface oxidation observed on the pure Ni
foil are evident
in the Auger
spectra of Ni
on the
surface of the alloy, The
measured
AES
Zr,
Ni,
oxygen
and
carbon
spectra
were
quantified by measuring the integrated counts under the E*N(E) vs,
815
E
spectral
curves
procedure.
The
between a
using
program
straight
spectral curve.
the
PHI
provides
software the
designed
area
for
(integrated
this
counts)
line connecting selected end points and
the
These peak area intensities are measures of the
surface abundance of the respective elements. The intensity of the combined Zr 146 eV and 140 eV peaks (when the latter is present) was utilized for
the Zr surface concentration measurements.
integration utilized
end points at
the minimum of
the
This
spectral
curves between 129 and 133 on one end and 150 to 153 on the other. The authors have previously established that this measure of zr Auger
intensity varies proportionately with the area
(intensity)
under the 90 eV peak and that under the combined 116 eV and 124 eV peaks for pure Zr (ref. 3). The combined 146-140 eV peak intensity is utilize6 in this work since it is not interfered with by the 101 eV Ni Auger peak. The Ni peak intensity was measured for the 841 eV transition by integrating from the minimum of the spectra near 850 eV to the point of intersection of a horizontal (constant count rate) line from that minimum on the low energy side of the peak. the
The oxygen Auger spectra
between
intensities
(not
between
eV and
243
intensities were measured by integrating 500
eV
reported 278
and
here)
eV.
515 were
eV,
while
measured
The major
oxygen
the
by
and
carbon
integration carbon
Auger
transitions occur within these energy windows. The spectral alloys
are
intensities of
normalized
by
zr and Ni
dividing
reported here
each
by
the
intensities
therefore
are
a
measure
the
corresponding
intensity measured on pure, sputter cleaned Zr and Ni reported
for
of
foils.
the
The
surface
abundance of Zr or Ni
relative to that determined on the surface
of
The
the
pure
metals.
oxygen
spectral
intensities
measured
between 500 and 515 eV are normalized by division by a consistent factor profile
which
brings
plots.
There
them is
to no
a
reasonable
specific
scale
on
the
depth
surface concentration
with
which these normalized oxygen intensities can be referenced. The depth profile treatment at
400 0C
results obtained on ZrNi3 after
sequential
with H2 and then 02 are presented in Figure 3a.
The near surface regionis shown to be essentially pure oxidized Ni. After somewhat more than 10 minutes of sputtering (log of the sputter maximum
time slightly above 1.0) simultaneously
with
a
the Ni minimum
concentration reaches a in
the
surface
oxygen
concentration. The Ni is in the reduced state after the maximum is passed.
The
concentration
of
reduced
Ni
then
decreases
as
the
816
>-+Ul
c
Q)
-+-
c
L
Q)
m
:J
Q)
N
0
E I.a
z
1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 START
0-0
Sputter cleaned values
lJ-D 0-0
-1.0
0.0
1.0
log 1O(Sputter time, min)
Zr(reduced); e- - e Zr(oxidized); A-A
0-0
Oxygen
2.0
Ni
Zr
3.0
Ni(reduced); . - - . Ni(oxidized)
Figure 3a. Sputter depth profiles of Zr, Ni, and oxygen for ZrNi3 after two 400 0C treatments; H2 for 30 minutes and then 02 for 30 minutes. Auger intensities of Ni and Zr for sputter cleaned, unreacted ZrNi3 are shown on the right.
>-+Ul
c
Q)
-+-
c
L Q)
m :J
-c -0
Q)
N
0
E
L
a
z
1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 START
Sputter cleaned values 0-0
0-0
-1.0
0.0
1.0
2.0
Ni Zr
3.0
log 1O(Sputter time, min) 0-0
Zr(reduced); e- - e Zr(oxidized);
0-0
Ni(reduced);
A-A
Oxygen
Figure 3b. Sputter depth profiles of Zr, Ni, and oxygen for ZrNi3; sample from Figure 3a after an additional 400 0C treatment with H2 for 30 minutes. Auger intensities of Ni and Zr for sputter cleaned, unreacted ZrNi3 are shown on the right.
817
concentration
of
oxidized
Zr
and
of
oxygen
increases.
Finally
there is a transition to near the bulk concentrations of Ni and Zr in
the
unoxidized
ZrNi3'
with
the
concentration
of
reduced
Ni
increasing and the oxygen concentration falling within a matrix of reduced Zr. The spectral intensities of Ni and Zr as measured on the surface of pure sputter cleaned ZrNi3 are shown on the right of Figures 3a and 3b. The depth profile results determined on a different section of the
surface of
with
H2
are
the
ZrNi3 after a
presented
in
Figure
essentially pure reduced Ni slightly
more
concentration
than of
10
subsequent 3b.
The
surface
400 0C
region
is
(normalized intensity near 1.0) After minutes
reduced
treatment at
Ni
of
sputtering
decreases
as
the
the
surface
concentration
of
oxygen and oxidized Zr increase. The depth profiles of zr2Ni7 after 400 0C treatments with H2 and then 02' Figure
Figure
4b,
observed
for
time were treated
4a,
zrNi3'
a
subsequent
essentially
Again,
treatment
equivalent
approximately
10
wi th
results
minutes
as
of
H2'
those
sputter
required to penetrate both the oxidized Ni on the 02
sample and
treated sample. point
and after
demonstrate
of
the
nearly pure
The concentrations of Ni
penetration
approximate
reduced Ni
that of a
of
the
and
oxidized
sputter cleaned,
on the Zr
at
final
H2
the deepest
sample,
Figure
untreated Zr2Ni7
4a,
sample.
The increasing surface concentration of reduced Ni observed in the initial portion of the profile,
Figure 4b, most probably results
from
to
a
surface
simultaneous carbon
decrease,
concentration.
near
The
zero,
surface
of
the
becomes
observed partially
carbonated either during the reduction process or while evacuating the preparation chamber and transferring the sample into the main analysis chamber. The depth profile of the ZrNi sample after treatment at 400 0C in H2 and oxygen was
then 02
is presented in Figure
incorporated
into
this
sample
5.
Significantly more
than was
observed
for
ZrNi3 or Zr2Ni7' The concentration of oxidized Zr near the surface was also not reduced to near zero, as had been noted for ZrNi3 and zr2Ni7'
This
is
certainly
the
result
of
the
higher
Zr
concentration in the bulk alloy which forms Zr02 upon oxidation. Correspondingly, the concentration of oxidized Ni near the surface and of reduced Ni further within the sample did not reach the high levels sample
noted
on
received
the its
alloys final
which
are
treatment
richer with
in H2
Ni. at
When 400 0C
this it
818
>-
;!::
1.1 1.0
Sputter cleaned values
0.9 0.8
Ul
c
....c Ql
0.7
LQl
0.6 0.5
OJ :::J
4:
0-0 0--0
0.4 0.3 0.2 0.1
"U
Ql
N
o
E Lo
Z
Ni Zr
0.0
START 0-0
-1.0
0.0 1.0 2.0 log 1O(Sputter time, min)
3.0
Zr(reduced); .- - . Zr(oxidized); 0 - 0 Ni(reduced); .- - . Ni(oxidized) .. - .. Oxygen
Figure 4a. Sputter depth profiles of Zr, Ni, and oxygen for zr2Ni7 after two 400 0C treatments: H2 for 30 minutes and then 02 for 30 minutes. Auger intensities of Ni and Zr for sputter cleaned, unreacted Zr2Ni7 are shown on the right.
....>Ul
c:
....c: Ql
LQl
OJ :::J
4:
"U
Ql
N
o
E
L-
°
Z
1.1 1.0
0.9 0.8
Sputter cleaned values
0.7
0.6 0.5
0--0 0--0
0.4 0.3 0.2 0.1
Ni Zr
0.0
START 0-0
-1.0
0.0
1.0
1091 O(Sputter time, min)
Zr(reduced); .- - . Zr(oxidized);
0-0
2.0
Ni(reduced); .. -
3.0 .. Oxygen
Figure 4b. Sputter depth profiles of Zr, Ni, and oxygen for Zr2Ni7: sample from Figure 4a after an additional 4000C treatment with H2 for 30 minutes. Auger intensities of Ni and Zr for sputter cleaned, unreacted Zr2Ni7 are shown on the right.
819
disintegrated
into
a
fine
powder
which
could
not
be
further
analyzed by the methods utilized in this study. The Zr2Ni sample
1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0
>-
+-
1Il
c
Q)
+-
c
LQ)
OJ :J
<{
l:J Q)
N
0
E
L
a
z
ZrNi
0-0
-1.0
START 0-0
Sputter cleaned values
3.0
0.0 1.0 2.0 log 1O(Sputter time, min)
Zr(reduced); e- - e Zr(oxidized); ~-~
0-0
Oxygen
Ni
Ni(reduced); . - - . Ni(oxidized)
Figure 5. Sputter depth profiles of Zr, Ni, and oxygen for ZrNi after two 400 0C treatments; H2 for 30 minutes and then 02 for 30 minutes. Auger intensities of Ni and Zr for sputter cleaned, unreacted ZrNi are shown on the right. disintegrated during its initial treatment at 400 0C in H2. Both of the
Zr
rich
alloys,
ZrNi
and
Zr2Ni,
apparently
react
with
or
absorb the treatment gases with sufficient intensity to break down their structures,
presumably due to the formation of a
hydride phase upon hydrogen treatment. Pebler et al.
zirconium
(ref. 4) have
noted that zr2Ni disproportionates into ZrNiH3 and ZrH2 when it is treated with hydrogen. DISCUSSION Prior XPS studies by Wright et al. initial H2 treatment of these Zr/Ni
(ref. 1) have shown that the intermetallic alloys results
in a thin layer of oxidized Zr supported on a base of the reduced alloy. The oxygen required to form this thin oxidized Zr layer may have been present either as a trace impurity in the H2 treatment gas
either
as
Zr02 or as interstitially dissolved oxygen in the bulk alloy.
or
initially
It
is known that, can
dissolve
in
the bulk of
the
untreated alloy,
under certain high temperature conditions, bulk Zr up
to
28
atomic pe r ce n t
oxygen
as
an
interstitial
820 solute in its hcp lattice (ref. 5). The untreated Zr/Ni alloys may also, therefore, have absorbed some oxygen in the process of their being prepared. The surface of ZrNi3 and Zr2Ni7 following treatment with H2 and then 02 at 400 0C are essentially Zr-free oxidized Ni. There is a region rich in oxidized Zr
below the oxidized Ni
followed
by a
transition to concentrations of reduced Ni and Zr characteristic of the untreated sputter cleaned alloys. Note: During the sputter cleaning step the alloy surface should be enriched in zirconium due
to
the
preferential
sputtering
of
nickel.
After
the
final
surface reduction with H2, the catalyst surface is observed to be essentially pure reduced Ni supported on an underlayer of oxidized Zr
which
is
in
turn
supported
on
the
unaltered
intermetallic
alloy. The sequence of
reductions and oxidations,
simultaneous
interpenetrations
results
uniquely active Ni
in a
pretreatment process
and
together with the
migrations
of
catalyst surface
initially involves
components,
(ref.
2).
The
the
formation of a thin layer of oxidized Zr during the first H2 treatment at 400 oC. Ni
segregates to the surface and is oxidized during the subsequent 02 treatment at 400 oC. At the same time a sublayer of oxidized Zr is formed as some of the Zr near the surface of the alloy is oxidized and
is
joined
following in
H2
at
by
the
oxidized
Zr
which
was
on
the
surface
the first H2 pretreatment step. The final pretreatment 400 0C
reduces
the
surface
Ni.
Zr02
is
sufficiently
thermodynamically stable to not be reduced under these conditions. Scanning electron microscope studies reveal that the sequence of pretreatments produces a
"sponge like" surface morphology on the
alloys with surface features on the order of several microns in size. The information developed in this study provides a preliminary basis
to
estimate
an
upper
bound
to
the
thickness
of
surface layer on the pretreated surface. Tapping et a I ,
the
Ni
(ref.
6)
have calibrated the sputtering rates of eight metal oxides using 4 keV Ar ions. These rates, expressed in nm'min- l/ua'cm- 2, ranged in magnitude
from
0.101
for
Cr203
to
0.195
for
Ta205
(Zr02
was
0.120). The sputtering rate utilizing 1 keV Ar ions in this study can
be expected
to be
lower
than
for
4 keV Ar
assumed that the sputtering rate of oxidized Ni
it
is
in this work
ions.
If
is
somewhat less than the range of values reported by Tapping et al. for metal oxides, the 18.5 ua/cm 2 Ar ion current density utilized
821
during sputtering will have removed less than 2 to 4 nm/min of the surface oxide. The thickness of oxidized Ni removed during the initial' 10 min. of sputtering, Figures 3a and 4a, is therefore less than 20 to 40 nm. If no additional Ni migration occurs during the
final
pretreatment
step,
the
difference
in
atomic
concentration of nickel in NiO and bulk Ni will decrease the thickness of the final reduced Ni surface layer to about half of that
of
the
oxidized
Ni
layer
from
which
it
was
formed.
A
reasonable upper bound on the thickness of the reduced Ni layer is therefore
10
to
20
nm,
This
study
provides
no
basis
for
the
estimation a lower bound on this thickness. ACKNOWLWDGMENTS We are grateful to the staff members of CRISS (supported by the NSF, Grant No. DMR-83-09460) for their excellent technical support. Portions of this work were supported by NSF(EPSCoR) Grant No. ISP-8011449, with matching funds from the State of Montana, and by the Interior Department's Bureau of Mines under contract No.
J0134035
through Department of Energy Contract No.
DE-AC07-
76ID01570.
REFERENCES 1 2 3 4 5 6
R. B. Wright, M. R. Hankins, M. S. Owens and D. L. Cocke, J. Vac. Sci. Technol., To be published August, 1987, and references therein. R. B. Wright, J. G. Jolley, M. S. Owens and D. L. Cocke, J~ Vac. Sci. Technol., To be published August, 1987, and references therein. Deibert, M. C., and Wright, R. B., App1. Surface Sci. ,To be published. A. Pebler and E. A. Gulbransen, Electrochem. Tech., 4 (1966) 211. E. Gebhardt, H. D. Seghezzi and W. J. Durrscchnabel, Nuc. MatI. 4 (1961) 241. R. L. Tapping, R. D. Davidson and T. E. Jackman, Surf. Interface Anal. 7 (1985) 105.
.J.W. Ward (Editor), Catalysis 1987 © 1988 Elsevier Science Publishers B.Y., Amsterdam - Printed in The Netherlands
THE SURFACE COMPOSITION OF NICKEL-ZIRCONIUM INTERMETALLIC COMPOUND CATALYSTS M. C. Deibert l and R. B. Wright 2 IDepartment of Chemical Bozeman, Montana 59717
Engineering,
Montana
State
University,
2Idaho National Engineering Laboratory/EG&G Idaho, Inc., P.O. Box 1625, Idaho Falls, Idaho 83415
ABSTRACT The compositions of the surface and subsurface strata of uniquely active zirconium-nickel alloy hydrogenation catalysts have been investigated by high resolution Auger spectroscopy and Ar ion sputter ing. Sequential 400 0C pretreatments of the intermetallic compounds ZrNi3 and Zr2Ni7 with H2, 02 and finally H2 are shown to produce an essentially pure reduced Ni surface supported on a sublayer of oxidized zirconium, which in turn is supported on the unaltered alloy. This reduced nickel constitutes the highly active catalyst surface. Evidence is developed related to the thickness of the reduced nickel surface layer and its morphology. INTRODUCTION Recent
studies
structure of (ref.
2)
actinide
(ref.
1)
of
the
surface
composition
and
intermetallic alloys have been prompted by earlier
findings
that certain alloys consisting of
and
earth
rare
metals
are
good
catalysts
transition, for
various
reactions of possible industrial relevance. These surface analysis studies observed structure
have
established
catalytic of
the
a
activity alloy.
definite and
the
Generally,
correlation surface the
between
the
composition
and
catalytically
active
surface is the result of a chemically induced decomposition of the alloy into a reduced metal surface phase consisting of one of the alloy components that in turn is dispersed on a second subsurface phase composed of an oxide of the second alloy component. Thus, in a general sense,
the active catalyst consists of a reduced metal
dispersed on a metal oxide, which acts as a support, both of which in turn reside on the underlying bulk intermetallic alloy. Previous catalytic activity studies (ref. 2)
have demonstrated
that the Zr/Ni alloy system exhibits high hydrogenation activity
812
if the alloy is treated in oxygen gas to temperatures up to 550 0C prior
to
a
reducing
treatment
in
hydrogen
of
earlier
at
temperatures
exceeding 200 oC, both of which precede the catalytic testing. The
present
study
is
a
continuation
(ref.
1)
surface
characterization work which used x-ray photoelectron spectroscopy (XPS) to investigate the influence of various elevated temperature oxygen and
hydrogen
treatments on
the surface composition of
a
number of Zr/Ni alloys. The earlier XPS studies showed that even in their as-prepared state, the surface of the alloys are depleted in nickel and consist mainly of Zr02' Upon exposure to oxygen at temperatures exceeding 200 oC, the surface becomes enriched in nickel, as NiO, which upon reduction in hydrogen produces a nickel rich surface, as Ni,
that is supported on a thin layer of Zr02'
The Auger studies presented in the current paper were conducted to elucidate
in
a
more
quantitative
manner
the
compositional
stratification that occurs in this alloy system as a result of the oxidation/reduction pretreatments. EXPERIMENTAL PROCEDURE The Zr-Ni alloys used in this study were prepared by the direct arc melting of the appropriate stoichiometric amounts of pure Ni (99.9%)
and
Zr
(99.9%)
powders.
The
arc melted
specimens
were
sliced and polished using 0.05 micron alumina abrasive to obtain the
final
surface
purity Ni ZrNi3,
and Zr
ZrNi,
reagent
and
finish. foils,
The
samples analyzed
included
as well as polished samples of
Zr2Ni.
All
samples
were
grade acetone and methanol prior
to
solvent
99.99% zr2Ni7,
cleaned
insertion
into
in the
analysis system. The analyzer was a PHI 595 scanning Auger microprobe with Ar+ sputter profiling capability. The 200 nA, 3.0 keV primary electron
beam was rastered over a 0.02xO.02 mm 2 area. This area was central to a 1. 8x1. 8 mm 2 area impacted by a rastered 1. 0 keV Ar+ beam during ion milling. The Ar+ beam current was approximately 0.6 uA or about 18.5 uA/cm 2 over the area of the rastered beam. The E*N(E)
vs.
E data collected during the AES scans were stored on
magnetic disks for software
was
later display, manipulation and analysis.
utilized
to
process
the
E*N(E)
vs.
E
data
PHI as
required. After
characterization
of
the
Auger
spectra
of
a
sputter
cleaned area on each sample in the untreated condition, the sample was withdrawn
to the preparation chamber of
the PHI
595
system
813
which was then backfilled to atmospheric pressure with ultra-high purity H2' The temperature of the sample was then raised to 400 0C for
30
minutes.
A slow purge of
H2 was maintained
through
the
preparation chamber during the high temperature treatment. After cooling,
the
procedure
repeated
using
spectra
were
gas.
Auger
preparation
chamber
was
ultra-high measured
re-evacuated
purity 02
on
the
as
sample
the in
and
the
treatment
the
treated
condition and then after each of a sequence of surface sputters with 1 keV Ar+ to develop a high temporal resolution depth profile of the sample. After
the
first
sputter
depth
profile
was
completed
on
the
sample treated in H2 and then 02, the sample was withdrawn to the preparation chamber and treated for 30 minutes with H2' A second high
resolution
sputter
depth
profile
was
then
conducted
on
a
previously unsputtered portion of the sample surface. EXPERIMENTAL RESULTS Examples this
of
study
the
are
principal AES spectra which are
presented
in
Figures
1
for
the
utilized
energy
in
region
between 80 and 180 eV within which many principal Zr Auger peaks occur and in Figure 2 for the energy regions between 45 and 75 eV and between 816 and 856 eV within which two major Ni Auger peaks occur.
Spectrum
(a)
in Figure 1 is
for
sputter
cleaned pure Zr
foil and exhibits peaks near 91 eV, 116 eV, 124 eV, 146 eV and 173 eV.
After
is oxidized by exposure to 1000 L (lL=l Langmuir=exposure of 10- 6 torr of gas for 1 sec) of 02 at 20 oC, spectrum
the
(b)
Zr
foil
is observed.
The 146 eV peak
is diminished by the
surface oxidation of the Zr and is partially replaced by a new peak at 140 eV. The peak at 173 eV is strongly diminished. These changes provide convenient indications of the oxidation state of Zr
from its Auger spectra. Spectrum (c)
in Figure 2 was measured
on
a
a
sputter
cleaned
ZrNi3
surface
on
sample
which
had
not
received high temperature H2 or 02 treatments. The Auger peaks are essentially the same as spectrum (a)
except for the introduction
of a weak Ni peak near 101 eV. The intensity of the Zr peaks are diminished due to the dilution effect of the Ni. Spectrum (d)
in
Figure 1 was measured on the same ZrNi3 surface after exposure to 1000 L of 02 at 2l oC. Essentially the same influences of the surface oxidation of evident.
The Ni
several
of
the
Zr
peak at Zr
that were observed 101 eV is
peaks
are
in spectrum
(b)
are
substantially eliminated and
stronger
due
to
the
selective
814
segregation of
the
Zr
to
the surface to form Zr02 by
the
room
temperature surface oxidation process.
-
>-
-...
'iii c::
Z'
'Cii
Q)
t::
...
c::
Q)
5... Q)
Cl :::l
,
''' \
,...
/ ' Zr + \... 'b)
« W 1 \\ ..",J
Z «
l'--
W
II
1000 L 02 /
f, __
W
\
LLJ
d) ZrNi3
1000 L
\.
' ..... ....
--
\b) Ni
+--
\ 1000 L 02
\
c) Sputter cleaned
+.
ZrNi3
02\.,
+ 1000 L02
. ,./
120
80
\.
.\
oO
c) Sputter cleaned '-"" ZrNi3 ( , _ . J d ) ZrNi3
r-'
,
"-'\
Z-
,r'
1\."
A
Q)
C'l ::l
\ ......... /
/\.--/.'\
................. ,
140
180
Electron Energy (eV)
Electron Energy (eV)
Figure 1 80 to 180 eV region of E*N(E) Auger spectrum for a) sputter cleaned Zr; b) Zr after 1000 L 02 exposure; c) sputter cleaned ZrNi3; and d) ZrNi3 after 1000 L 02 exposure. Figure 2 45 to 75 eV and 816 to 860 eV regions of E*N(E) Auger spectrum for a) sputter cleaned Ni; b) Ni after 1000 L 02 exposure; c) sputter cleaned ZrNi3; and d) ZrNi3 after 1000 L 02 exposure. Spectrum (a)
in Figure 2 was measured on a sputter cleaned Ni
foil surface and spectrum (b) is this same surface after a 1000 L exposure at 2l oC. The peak at 841 eV is not significantly
02
changed by the surface oxidation process, but the energy level of the low energy Ni peak is shifted from about 57 eV to about 55 eV. This shift of the low energy Ni Auger peak provides an indication of the oxidation state of Ni. Spectra (e) and (d) in Figure 2 were measured on a
sputer
cleaned and an oxidized surface of
ZrNi3,
respectively, The same influences of surface oxidation observed on the pure Ni
foil are evident
in the Auger
spectra of Ni
on the
surface of the alloy, The
measured
AES
Zr,
Ni,
oxygen
and
carbon
spectra
were
quantified by measuring the integrated counts under the E*N(E) vs,
815
E
spectral
curves
procedure.
The
between a
using
program
straight
spectral curve.
the
PHI
provides
software the
designed
area
for
(integrated
this
counts)
line connecting selected end points and
the
These peak area intensities are measures of the
surface abundance of the respective elements. The intensity of the combined Zr 146 eV and 140 eV peaks (when the latter is present) was utilized for
the Zr surface concentration measurements.
integration utilized
end points at
the minimum of
the
This
spectral
curves between 129 and 133 on one end and 150 to 153 on the other. The authors have previously established that this measure of zr Auger
intensity varies proportionately with the area
(intensity)
under the 90 eV peak and that under the combined 116 eV and 124 eV peaks for pure Zr (ref. 3). The combined 146-140 eV peak intensity is utilize6 in this work since it is not interfered with by the 101 eV Ni Auger peak. The Ni peak intensity was measured for the 841 eV transition by integrating from the minimum of the spectra near 850 eV to the point of intersection of a horizontal (constant count rate) line from that minimum on the low energy side of the peak. the
The oxygen Auger spectra
between
intensities
(not
between
eV and
243
intensities were measured by integrating 500
eV
reported 278
and
here)
eV.
515 were
eV,
while
measured
The major
oxygen
the
by
and
carbon
integration carbon
Auger
transitions occur within these energy windows. The spectral alloys
are
intensities of
normalized
by
zr and Ni
dividing
reported here
each
by
the
intensities
therefore
are
a
measure
the
corresponding
intensity measured on pure, sputter cleaned Zr and Ni reported
for
of
foils.
the
The
surface
abundance of Zr or Ni
relative to that determined on the surface
of
The
the
pure
metals.
oxygen
spectral
intensities
measured
between 500 and 515 eV are normalized by division by a consistent factor profile
which
brings
plots.
There
them is
to no
a
reasonable
specific
scale
on
the
depth
surface concentration
with
which these normalized oxygen intensities can be referenced. The depth profile treatment at
400 0C
results obtained on ZrNi3 after
sequential
with H2 and then 02 are presented in Figure 3a.
The near surface regionis shown to be essentially pure oxidized Ni. After somewhat more than 10 minutes of sputtering (log of the sputter maximum
time slightly above 1.0) simultaneously
with
a
the Ni minimum
concentration reaches a in
the
surface
oxygen
concentration. The Ni is in the reduced state after the maximum is passed.
The
concentration
of
reduced
Ni
then
decreases
as
the
816
>-+Ul
c
Q)
-+-
c
L
Q)
m
:J
Q)
N
0
E I.a
z
1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 START
0-0
Sputter cleaned values
lJ-D 0-0
-1.0
0.0
1.0
log 1O(Sputter time, min)
Zr(reduced); e- - e Zr(oxidized); A-A
0-0
Oxygen
2.0
Ni
Zr
3.0
Ni(reduced); . - - . Ni(oxidized)
Figure 3a. Sputter depth profiles of Zr, Ni, and oxygen for ZrNi3 after two 400 0C treatments; H2 for 30 minutes and then 02 for 30 minutes. Auger intensities of Ni and Zr for sputter cleaned, unreacted ZrNi3 are shown on the right.
>-+Ul
c
Q)
-+-
c
L Q)
m :J
-c -0
Q)
N
0
E
L
a
z
1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 START
Sputter cleaned values 0-0
0-0
-1.0
0.0
1.0
2.0
Ni Zr
3.0
log 1O(Sputter time, min) 0-0
Zr(reduced); e- - e Zr(oxidized);
0-0
Ni(reduced);
A-A
Oxygen
Figure 3b. Sputter depth profiles of Zr, Ni, and oxygen for ZrNi3; sample from Figure 3a after an additional 400 0C treatment with H2 for 30 minutes. Auger intensities of Ni and Zr for sputter cleaned, unreacted ZrNi3 are shown on the right.
817
concentration
of
oxidized
Zr
and
of
oxygen
increases.
Finally
there is a transition to near the bulk concentrations of Ni and Zr in
the
unoxidized
ZrNi3'
with
the
concentration
of
reduced
Ni
increasing and the oxygen concentration falling within a matrix of reduced Zr. The spectral intensities of Ni and Zr as measured on the surface of pure sputter cleaned ZrNi3 are shown on the right of Figures 3a and 3b. The depth profile results determined on a different section of the
surface of
with
H2
are
the
ZrNi3 after a
presented
in
Figure
essentially pure reduced Ni slightly
more
concentration
than of
10
subsequent 3b.
The
surface
400 0C
region
is
(normalized intensity near 1.0) After minutes
reduced
treatment at
Ni
of
sputtering
decreases
as
the
the
surface
concentration
of
oxygen and oxidized Zr increase. The depth profiles of zr2Ni7 after 400 0C treatments with H2 and then 02' Figure
Figure
4b,
observed
for
time were treated
4a,
zrNi3'
a
subsequent
essentially
Again,
treatment
equivalent
approximately
10
wi th
results
minutes
as
of
H2'
those
sputter
required to penetrate both the oxidized Ni on the 02
sample and
treated sample. point
and after
demonstrate
of
the
nearly pure
The concentrations of Ni
penetration
approximate
reduced Ni
that of a
of
the
and
oxidized
sputter cleaned,
on the Zr
at
final
H2
the deepest
sample,
Figure
untreated Zr2Ni7
4a,
sample.
The increasing surface concentration of reduced Ni observed in the initial portion of the profile,
Figure 4b, most probably results
from
to
a
surface
simultaneous carbon
decrease,
concentration.
near
The
zero,
surface
of
the
becomes
observed partially
carbonated either during the reduction process or while evacuating the preparation chamber and transferring the sample into the main analysis chamber. The depth profile of the ZrNi sample after treatment at 400 0C in H2 and oxygen was
then 02
is presented in Figure
incorporated
into
this
sample
5.
Significantly more
than was
observed
for
ZrNi3 or Zr2Ni7' The concentration of oxidized Zr near the surface was also not reduced to near zero, as had been noted for ZrNi3 and zr2Ni7'
This
is
certainly
the
result
of
the
higher
Zr
concentration in the bulk alloy which forms Zr02 upon oxidation. Correspondingly, the concentration of oxidized Ni near the surface and of reduced Ni further within the sample did not reach the high levels sample
noted
on
received
the its
alloys final
which
are
treatment
richer with
in H2
Ni. at
When 400 0C
this it
818
>-
;!::
1.1 1.0
Sputter cleaned values
0.9 0.8
Ul
c
....c Ql
0.7
LQl
0.6 0.5
OJ :::J
4:
0-0 0--0
0.4 0.3 0.2 0.1
"U
Ql
N
o
E Lo
Z
Ni Zr
0.0
START 0-0
-1.0
0.0 1.0 2.0 log 1O(Sputter time, min)
3.0
Zr(reduced); .- - . Zr(oxidized); 0 - 0 Ni(reduced); .- - . Ni(oxidized) .. - .. Oxygen
Figure 4a. Sputter depth profiles of Zr, Ni, and oxygen for zr2Ni7 after two 400 0C treatments: H2 for 30 minutes and then 02 for 30 minutes. Auger intensities of Ni and Zr for sputter cleaned, unreacted Zr2Ni7 are shown on the right.
....>Ul
c:
....c: Ql
LQl
OJ :::J
4:
"U
Ql
N
o
E
L-
°
Z
1.1 1.0
0.9 0.8
Sputter cleaned values
0.7
0.6 0.5
0--0 0--0
0.4 0.3 0.2 0.1
Ni Zr
0.0
START 0-0
-1.0
0.0
1.0
1091 O(Sputter time, min)
Zr(reduced); .- - . Zr(oxidized);
0-0
2.0
Ni(reduced); .. -
3.0 .. Oxygen
Figure 4b. Sputter depth profiles of Zr, Ni, and oxygen for Zr2Ni7: sample from Figure 4a after an additional 4000C treatment with H2 for 30 minutes. Auger intensities of Ni and Zr for sputter cleaned, unreacted Zr2Ni7 are shown on the right.
819
disintegrated
into
a
fine
powder
which
could
not
be
further
analyzed by the methods utilized in this study. The Zr2Ni sample
1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0
>-
+-
1Il
c
Q)
+-
c
LQ)
OJ :J
<{
l:J Q)
N
0
E
L
a
z
ZrNi
0-0
-1.0
START 0-0
Sputter cleaned values
3.0
0.0 1.0 2.0 log 1O(Sputter time, min)
Zr(reduced); e- - e Zr(oxidized); ~-~
0-0
Oxygen
Ni
Ni(reduced); . - - . Ni(oxidized)
Figure 5. Sputter depth profiles of Zr, Ni, and oxygen for ZrNi after two 400 0C treatments; H2 for 30 minutes and then 02 for 30 minutes. Auger intensities of Ni and Zr for sputter cleaned, unreacted ZrNi are shown on the right. disintegrated during its initial treatment at 400 0C in H2. Both of the
Zr
rich
alloys,
ZrNi
and
Zr2Ni,
apparently
react
with
or
absorb the treatment gases with sufficient intensity to break down their structures,
presumably due to the formation of a
hydride phase upon hydrogen treatment. Pebler et al.
zirconium
(ref. 4) have
noted that zr2Ni disproportionates into ZrNiH3 and ZrH2 when it is treated with hydrogen. DISCUSSION Prior XPS studies by Wright et al. initial H2 treatment of these Zr/Ni
(ref. 1) have shown that the intermetallic alloys results
in a thin layer of oxidized Zr supported on a base of the reduced alloy. The oxygen required to form this thin oxidized Zr layer may have been present either as a trace impurity in the H2 treatment gas
either
as
Zr02 or as interstitially dissolved oxygen in the bulk alloy.
or
initially
It
is known that, can
dissolve
in
the bulk of
the
untreated alloy,
under certain high temperature conditions, bulk Zr up
to
28
atomic pe r ce n t
oxygen
as
an
interstitial
820 solute in its hcp lattice (ref. 5). The untreated Zr/Ni alloys may also, therefore, have absorbed some oxygen in the process of their being prepared. The surface of ZrNi3 and Zr2Ni7 following treatment with H2 and then 02 at 400 0C are essentially Zr-free oxidized Ni. There is a region rich in oxidized Zr
below the oxidized Ni
followed
by a
transition to concentrations of reduced Ni and Zr characteristic of the untreated sputter cleaned alloys. Note: During the sputter cleaning step the alloy surface should be enriched in zirconium due
to
the
preferential
sputtering
of
nickel.
After
the
final
surface reduction with H2, the catalyst surface is observed to be essentially pure reduced Ni supported on an underlayer of oxidized Zr
which
is
in
turn
supported
on
the
unaltered
intermetallic
alloy. The sequence of
reductions and oxidations,
simultaneous
interpenetrations
results
uniquely active Ni
in a
pretreatment process
and
together with the
migrations
of
catalyst surface
initially involves
components,
(ref.
2).
The
the
formation of a thin layer of oxidized Zr during the first H2 treatment at 400 oC. Ni
segregates to the surface and is oxidized during the subsequent 02 treatment at 400 oC. At the same time a sublayer of oxidized Zr is formed as some of the Zr near the surface of the alloy is oxidized and
is
joined
following in
H2
at
by
the
oxidized
Zr
which
was
on
the
surface
the first H2 pretreatment step. The final pretreatment 400 0C
reduces
the
surface
Ni.
Zr02
is
sufficiently
thermodynamically stable to not be reduced under these conditions. Scanning electron microscope studies reveal that the sequence of pretreatments produces a
"sponge like" surface morphology on the
alloys with surface features on the order of several microns in size. The information developed in this study provides a preliminary basis
to
estimate
an
upper
bound
to
the
thickness
of
surface layer on the pretreated surface. Tapping et a I ,
the
Ni
(ref.
6)
have calibrated the sputtering rates of eight metal oxides using 4 keV Ar ions. These rates, expressed in nm'min- l/ua'cm- 2, ranged in magnitude
from
0.101
for
Cr203
to
0.195
for
Ta205
(Zr02
was
0.120). The sputtering rate utilizing 1 keV Ar ions in this study can
be expected
to be
lower
than
for
4 keV Ar
assumed that the sputtering rate of oxidized Ni
it
is
in this work
ions.
If
is
somewhat less than the range of values reported by Tapping et al. for metal oxides, the 18.5 ua/cm 2 Ar ion current density utilized
821
during sputtering will have removed less than 2 to 4 nm/min of the surface oxide. The thickness of oxidized Ni removed during the initial' 10 min. of sputtering, Figures 3a and 4a, is therefore less than 20 to 40 nm. If no additional Ni migration occurs during the
final
pretreatment
step,
the
difference
in
atomic
concentration of nickel in NiO and bulk Ni will decrease the thickness of the final reduced Ni surface layer to about half of that
of
the
oxidized
Ni
layer
from
which
it
was
formed.
A
reasonable upper bound on the thickness of the reduced Ni layer is therefore
10
to
20
nm,
This
study
provides
no
basis
for
the
estimation a lower bound on this thickness. ACKNOWLWDGMENTS We are grateful to the staff members of CRISS (supported by the NSF, Grant No. DMR-83-09460) for their excellent technical support. Portions of this work were supported by NSF(EPSCoR) Grant No. ISP-8011449, with matching funds from the State of Montana, and by the Interior Department's Bureau of Mines under contract No.
J0134035
through Department of Energy Contract No.
DE-AC07-
76ID01570.
REFERENCES 1 2 3 4 5 6
R. B. Wright, M. R. Hankins, M. S. Owens and D. L. Cocke, J. Vac. Sci. Technol., To be published August, 1987, and references therein. R. B. Wright, J. G. Jolley, M. S. Owens and D. L. Cocke, J~ Vac. Sci. Technol., To be published August, 1987, and references therein. Deibert, M. C., and Wright, R. B., App1. Surface Sci. ,To be published. A. Pebler and E. A. Gulbransen, Electrochem. Tech., 4 (1966) 211. E. Gebhardt, H. D. Seghezzi and W. J. Durrscchnabel, Nuc. MatI. 4 (1961) 241. R. L. Tapping, R. D. Davidson and T. E. Jackman, Surf. Interface Anal. 7 (1985) 105.