Applications
of Surface Science 19 (19X4) I-13
North-Holland.
Amsterdam
CHEMICAL MODIFICATION
PROPERTIES
OF NICKEL *
Received
17 October
1983: accepted
OF CHEMISORPTIVE
for publication
AND CATALYTIC
1X June 1984
Several reactions representing
important categories of catalytic systems have been studied on
chemically
modified single crystal surfaces. These reactions are methanation
of CO and CO:.
hydrogenolysis
of ethane. hydrogenation
of ethylene. and q&propane
ring-opening
and hydrogenolysis. Poisoning of the above reactions by ordered. suhmonolayer
coverages of sulfur shou
large nonlinear effects for sulfur coverages versus reactivity attenuation.
These data together with
related chemisorption
results are reviewed with emphasis on the author’s own work. These studies
suggest that the dominant
effect in poisoning by sulfur is an electronic one and extends over
distances larger than the atomic radius. Related studies have addressed
the role of potasstum
promoters in nickel catalysts for methanation.
Potassium decreases the rate of methane formation
and increases the rate of higher hydrocarbons
relative to the clean nickel surface. Similar results
have been reported for supported nickel catalysts suggesttng that support effects play a small role
in catalytic promotion by potassium.
1. Introduction
It has long been recognized that the addition of impurities to metal catalysts
can provide large effects on both activity, selectivity, and resistance to poisoning of the pure metal [l]. For example, the catalytic properties of metals can be
altered greatly by the addition of a second transition or group IB metal or by
adding promoters such as potassium or sulfur. On the other hand catalytic
processing is often plagued by loss of activity due to the inadvertent
contamination
of catalysts by undesirable
impurities. In either case the catalytic
properties are dramatically
altered by the modification
of the chemistry by the
surface impurity. Although these effects are well recognized in the catalytic
industry, the details of how these chemical alterations come about are not at all
understood.
Difficulties are encountered
in interpreting
much of the existing
literature addressing this question because of the uncertainty
in the location of
the impurity (support or metal particle). the impurity concentration
at the
metal surface, and the chemical nature of the impurity.
In general these
* This work was performed
at Sandia National Laboratories
and supported
ment of Energy under contract number DE-AD04-76DP00789.
037%5963/84/$03.00
0 Elsevier Science Publishers
(North-Holland
Physics Publishing Division)
B.V.
by the US Drpart-
difficulties can be circumvented
if model catalytic systems such as metal single
crystals can be used in conjunction
with surface analytic iechniques.
This
combination
of kinetics at elevated pressures coupled with surface spectroacopies has been used extensively to study methanation
of CO [2] and CO1 [3] as
well as alkane hydrogenolysis
reactions [4,5]. The appropriateness
of these
idealized systems as models for supported catalysts has now been well-documented for several reactions [2-41 and will not be discussed here. We wish in
this discussion
to survey our recent studies in probing
the fundamental
mechanism by which impurities, either electropositive
or electronegative
relative to nickel, change the chemisorptive
or catalytic properties of single crystal
surfaces of this material. This will include a comparison of the results obtained
on these model systems with similar data derived by others on supported metal
systems.
2. Experimental
These studies used the specialized apparatus
[2.5] shown in fig. 1. The
custom-built
catalytic reactor, contiguous
to the surface analysis system, employs a retraction belows that supports the metal crystal and allows translation
in situ from the reactor to the surface analysis chamber. A liquid-nitrogen
cooled manipulator
in conjunction
with a line-of-sight mass spectrometry
was
used for the thermal desorption experiments
described. The base pressure in
MASS
SPECTROMETER
ELECTRON
I
GAS
CATALYTIC
GUN
CHROMATOGRAPH
REACTOR
3
IRGY
I
ION
Fig.
1.
operation
An
ultrahigh
at atmospheric
vacuum
apparatus
pressure
ANALYSIS
AND SURFACE
PREPARATION
CHAMBER
PUMP
for
in a catalytic
studying
reactor.
single
crystal
catalysis
before
and
after
the analysis chamber
and the reactor is lo- “’ Torr. The single crystals of nickel
(- 1.0 cm’ surface area) are mounted on tungsten leads and heated resistively.
The samples were cleaned by oxidation at 1400 K in lop6 Torr 0, followed by
reduction at 800 K in 5 Torr H,. A typical Auger spectrum of a clean nickel
surface
is given in refs. [2] and [5].
3. Discussion
3. I. Electronegative
modifiers
3. I. I. Modification
of chemisorptioe
properties
The effect of preadsorbed
electronegative
adsorption-desorption
using thermal
behavior
programmed
atoms
Cl,
S, and
P on
the
of CO and H, on Ni(lOO) has been studied [6]
desorption
(TPD),
low energy electron
diffraction
(LEED), and Auger spectroscopy (AES). It has been found that the presence of
the electronegative
atoms causes a reduction
of the adsorption
rate, the
adsorption bond strength and the capacity of the Ni(lOO) surface for CO and
H, adsorption. The poisoning effect becomes stronger with increasing electronegativity of the preadsorbed atoms.
The observed effect of preadsorbed submonolayers
of Cl, S, and P on CO
TPD curves is shown in figs. 2a-2c. The additives Cl. S, and P were dosed
the surface was
from Cl,, H,S, and PH,, respectively. When appropriate,
flashed to a sufficient temperature to eliminate any residual surface hydrogen.
Coverages were based on a combination
of Auger, LEED, and TPD measurements. The CO curves represent
sorbed adlayer coverages
CO
coverage.
presence
As can be seen the CO
of the preadsorbed
pronounced
the total CO desorption
after a CO exposure
adlayer.
sufficient
uptake
The effects
decreases
markedly
of P, however,
than for Cl, or S. Fig. 3 shows the observed
total CO uptake on the corresponding
Cl no CO desorption
was detected
for different
pread-
to reach the stationary
in the
are much less
dependence
of the
precoverage of impurity. In the case of
at coverages
> 0.4 monolayers,
ML,
whereas
even for saturated
S and P precoverages,
some low temperature
desorption peaks were detected.
Adlayers of Cl, S and P cause a reduction of hydrogen uptake and a shift of
the TPD peak maxima to a lower temperature. As shown in figs. 4a-4c,
at
higher foreign atom precoverages
a lower temperature
state becomes more
pronounced.
The extent of the effect increases in the sequence P, S, Cl. As
shown in fig. 5, the reduction of H, coverage is most rapid in the presence of
Cl atoms such that at chlorine coverages higher than 0.2 a negligibly small
amount of desorbing hydrogen is detected.
Since the Cl, S and P atomic and covalent radii are similar (0.99, 1.04 and
1.10 A, respectively [20]) it was concluded that the electronegativity
factor
plays a major role in explaining the difference in their poisoning effect.
L? I+: Goodnw~
4
/ C‘henmol
nmh/rutrot~
ofproprrtrr~
o/ .%‘I
A similar
study [7] to that discussed
above has been carried
out
presence
of C and N. These impurities
have the same electronegativities
and Cl, 2.5 and 3.0. respectively.
The comparison
N and those for S and
electronegativity
effects
Cl are entirely
consistent
dominate
poisoning
of
with
occupying
close
atomic
size.
the
same
between
the results
with
the interpretation
chemisorption
adsorption
sites.
in the
as S
for C and
that
for adatoma
In the case of
APco
El,,
=o
=o.os
=0.17
=o.z&?
loo
200
300
‘ma
500
TEMPERATURE
SM)
(K)
S
APCC
APcc
200
TEMPERATURE
Fig.
2. Effect
deaorption
of varying
from
(K)
(a) chlorine,
Ni( 10). CO exposure
300
400
500
600
TEMPERATURE (K)
(h) sulfur,
of 6 L.
and
(c) phosphorus
precovemgr
c,n CO
thermal
D. W. Goodmm
/
Chemrcul
modifcutwn
ofpropertresof
5
Nr
adatoms with the same electronegativity
but with different atomic radii (S and
C, Cl and N), the effect becomes less pronounced
with decreasing
atomic
radius.
eT
co
0
0.1
0.2
0.3
ADlll-fIVE
Fig
3.
Dependence
0.4
0.5
COVERAGE
0.6
0.7
(ML)
of total CO adsorption
on additive
1
100
precoverage
A
200
zo.17
300
400
TEMPERATURE(K)
500
AA
P
*pIi,
ep=o
25
fi
I
lO\
200
Fig. 4. Effect
desorption
400
300
TEMPERATURE
(K)
of varying
200
500
(a) chlorine,
from Ni(lOO). H, exposure
1
I
500
400
300
TEMPERATURE IKI
(b) sulfur. and (c) phosphorus
of 10 L.
1
precoverage
on
H,
thermal
D. W. Coodmun / Chemrwl modi’icutml
6
3.1.2.
Modification
Kinetic
function
reaction
similar
of cata(vtic actiuig
studies
[3,6,8,9]
of sulfur
coverage
over Ni(lOO),
to results
of properr~es
O/ NI
have
been
carried
over
Ni(lOO)
the sulfided
surface
for the clean
surface
out
and
for
Ni(lll).
For
(fig. 6) shows
at considerably
n
several
reactions
as a
the
methanation
behavior
remarkably
reduced
hydrogen
partial
Cl
.S
AP
0
Fig. 5. Dependence
0.1
0.2
0.3
0.4
ADDITIVE COVERAGE (ML)
of H, adsorption
on additive
I
0.5
precoverage
Id
NCH4
10
IO
1 /T -1O3 (K-l)
Fig. 6. An Arrhenius plot of the rate of methanation
over a sulfided Ni(lOO) catalyst at 120 Torr
and a Hz/CO
ratio equal to four. 0,‘s are expressed as fractional monolayers. NC,, is the turnover
frequency or the number of methane molecules produced per nickel atom site per4second.
D. W. Goodman
/
Chenucd
ofpropertresof Ni
modification
7
pressure.
For clean Ni(lOO) [2] a departure
from Arrhenius
linearity
is observed at 700 K. Associated
with the negative deviation
of this plot is a rise in
the surface carbon
level. This rise in carbon
level continues
until the carbon
level reaches 0.5 ML, the saturation
level. This deviation,
or rollover,
of the
Arrhenius
plot
has been
interpreted
as reflecting
or critical
hydrogen
coverage
of 4% the reaction
linearity
increase
This
from
a saturation
adsorbed
behavior
indicates
surface
the chemisorption
the kinetics
that
atomic
of the rate of surface
sulfur
sulfur
conditions
at 600 K, some 100 K lower reaction
in surface carbon level is associated
with
steady-state
surface.
Both
rate at identical
the departure
coverage.
carbon
results
and
the
hydrogen
sulfur
hydrogenation.
discussed
above
the TPD
studies
departs
effective
which
These
results
results
for Hz on a sulfur
show
that
surface
similarly
temperature.
this deviation
is very
coverage
of atomically
For a sulfur
from
Here too, an
from linearity.
in
reducing
the
in an attenuation
are consistent
poisoned
the poisoning
is very nonlinear.
Fig. 7 shows this nonlinear
relationship
coverage
and the methanation
rate at 600 K. A precipitous
with
Ni(lOO)
effect
of
between
the
drop is seen
for the catalytic activity at the lower sulfur coverages. The poisoning effect
quickly maximizes and no further reduction in reaction rate is found at sulfur
levels exceeding 0.2 monolayers. An identical reduction of methanation
activity
for supported Ni/AI,O,
has been observed by Rostrup-Nielsen
and Pedersen
for sulfur poisoning
[lo]. These authors also observed a nonlinear
effect of
sulfur on the reaction rate and, as here, a constant activation
energy with
sulfur coverage. The initial change in the reaction rate poisoning
in fig. 7
suggests that ten or more equivalent nickel sites are deactivated by one sulfur
atom. There are two possible causes for this effect: (a) a long-range electronic
I
I
0.2
0.3
0.4
Additive Coverage (ML)
Fig. 7. Methanation
rate as a function of phosphorus
and sulfur coverage
Pressure = 120 Torr, Hz/CO
= 4. Reaction temperature = 600 K.
on a Ni(100)
catalyst.
effect (ligand effect) or (b) an ensemble effect, the requirement
that a certain
number of surface atoms are necessary for a reaction to occur. Experimentally
these two possibilities can be distinguished.
If long-range electronic effects are
most important.
then the reation rate should be expected to be a function of
the relative electronegativity
of the poison. If indeed a t.en nickel atom
ensemble is required for methanation
then changing the electronic character of
the poison should have little effect on the reaction rate. Substituting
phosphorus for sulfur in a similar set of experiments [9] results in a marked change
in the magnitude of poisoning at low coverages as indicated in fig. 7. Phosphorus. presumably
because of its less electronegative
character, poisons only
the four nearest neighbor metal atom sites. These results support the conclusion that long-range electronic effects are playing a major role in the sulfur
deactivation
Similar
catalytic
ethylene
of a nickel methanation
catalyst.
nonlinear
poisoning
of nickel by sulfur
reaction
including
ethane
hydrogenation
[ll]. and
suggest that the dominant
and extends over distances
3.2. Electropositive
has
been
seen
for
other
[ll] and cyclopropane
hydrogenolysis
CO1 methanation
[3]. These studies
influence
of sulfur poisoning
larger than the atomic radius
is an electronic
of sulfur.
[Xl.
also
one
modifier5
Alkali atoms on a transition
metal
ionic state, donating
a large fraction
surface are known to exist in a partially
of their valence electron
to the metal,
resulting
transition
This
been
in a work function
decrease.
metal
surface
atoms
has
explaining
adsorbed
activity
the role of alkali
molecules
in ammonia
electronegative
adatoms
additional
electron
density
on the
shown
to be a major
factor
in
in altering
the chemisorption
bonding
of
such as N, [ 121 or CO [13]. and in promoting
catalytic
synthesis
[14]. We have discussed
above
the role of
impurities
in poisoning
nickel
toward
methanation
activity.
These results have been ascribed,
to a large extent, to an electronic
effect. In
the context of this interpretation
it is expected
that an electropositive
additive
such as potassium
might have the opposite
effect,
i.e. to increase
nickel’s
methanation
although
activity.
certain
steps
A recent
study
in the reaction
[15] has shown
mechanism
that
this is not
are strongly
the case
accelerated
by
the presence
of potassium.
Kinetic
measurements
of methanation
over a Ni(100) catalyst
containing
well-controlled
submonolayer
quantities
of potassium
adatoms
show a decrease
in the steady-state
rate under a variety of reaction
conditions
(fig. 8). The
presence
of potassium
did not alter the apparent
activation
energy associated
with the kinetics. The potassium did, however,
change the steady-state
carbide
coverage which increased
from 10% of a monolayer
for clean Ni(lOO) to 30% of
a monolayer
Adsorbed
for a potassium
coverage
of 10% of a monolayer.
potassium
caused a marked increase
in the steady-state
rate
and
selectivity
of Ni(lOO) for higher hydrocarbon
studied,
the overall rate of higher hydrocarbon
potassium-dosed
surface; so that potassium
with respect to this reaction,
Fischer-Tropsch
synthesis.
At all temperatures
production
was faster on the
may be considered
synthesis.
In a manner identical
to that used for the clean
of carbide
formation
via CO disproportionation
01
I
0
8.
Relative
Ni(lOO) surface [16], the rate
(2 CO + C.,,, + CO,)
was
171
0.05
0.10
POTASSL&l
Fig.
a true promoter
methanation
0.15
COVERAGE
rate
as
a
0.20
(ML)
function
of
potassium
coverage
at
various
reaction
conditions.
r
=
1
.02
0
.04
.08
.06
.l
POTASSIUM COVERAGE (ML)
Fig.
9.
function
The
relative
of potassium
initial
rate
coverage.
of
reactive
carbon
formation
PC.,, = 24 Tom, T = 500 K.
from
CO
disproportionation
as
a
measured
for the potassium-covered
surfaced
carbide (as determined
by Auger spectroscopy)
the carbon-free
tion.
The
surface.
increase
potassium
coverage
reduction
Potassium
of
the
markedly
initial
is shown
of the activation
in
energy
increases
rate
of
fig.
9. Of
from
by observing
the growth
in
with time in CO. starting
from
carbide
the rate of CO dissociabuildup
at
particular
23 kcal
mol-
500
K with
significance
is the
’ for the clean
Ni(100)
surface to 10 kcal mol-’
The
model,
for a 10% surface coverage of potassium.
effects of potassium
upon the kinetics
of CO hydrogenation
on this
single-crystal
Ni(lOO) catalyst
are to: (I) decrease
the rate of methane
formation,
and (2) increase
the rate of higher hydrocarbon
production.
These
same effects have been reported
for high-surface
area-supported
nickel catalysts. This agreement
between
bulk.
indicates
that the major mechanism
catalyst’s
activity
and selectivity
it is rather a consequence
been drawn in the case
found
iron-free
that
atoms
of the support
Potassium
adatoms
then
disproportionation
reaction
carbon
coverages.
is not related
of direct K-Ni
of iron catalysts
the potassium
areas
single crystal nickel and supported
nickel
by which potassium
additives
alter the
to the support
material,
but that
interactions.
A similar conclusion
has
for ammonia
synthesis
where it was
reside
upon
patches
of iron
and
not
upon
[17].
cause a very large increase
in the rate of the CO
and a decrease
in its activation
energy
for low
At low carbon
coverages
and the conditions
of our measure-
ments the surface should be covered with adsorbed
CO so that CO adsorption
does not limit the disproportionation
rate. Similarly,
oxygen removal
via CO,
formation
is relatively rapid so that CO disproportionation
must be rate-limited
instead by the dissociation
of adsorbed
CO into
atoms.
The observation
of a potassium-induced
disproportionation
is then consistent
with the
adsorption
observed
Ni(lOO) [13]. These
been explained
tive potassium
adsorbed
increase
increase
carbon and oxygen
in the rate of CO
in the heat of CO
in the thermal
desorption
studies
from alkali-covered
effects are also observed
on other metals, and they have
in terms
donates
of an electronic
extra electron
ligand effect. whereby
density
to the nickel
which in turn donate
electron
density
to the adsorbed
CO
increases
the extent of backbonding
in the metalLC0
complex,
increased
metalLC0
bond
strength
and a decrease
in GO
bond
the electroposisurface
atoms.
molecule.
resulting
This
in an
strength.
This
model satisfactorily
explains
the decrease
in the activation
energy for carbide
build-up
(rate-limited
by CO dissociation)
brought about by potassium.
This is
entirely
analogous
to the explanation
for dissociative
N2 adsorption
on potassium-promoted
iron [18].
In spite of this increase
in the rate of CO dissociation
or carbide
buildup,
the overall
rate of methanation
decreases
and the activation
energy
is unchanged
in the presence
of potassium.
This indicates
that other step in the
methanation
sequence,
either hydrogen
adsorption
or hydrogenation
but not
CO dissociation,
is rate-limiting
for methane
production
under these condi-
tions. It should be noted that an increasing
carbide
level has been associated
with a decreasing methanation
rate on clean Ni(lOO) [19]. The most noticeable
influence of potassium addition upon surface coverages is to increase markedly
the coverage of molecular CO since potassium increases its heat of adsorption.
On clean Ni(lOO), it was shown [16] that increasing the carbide level increases
the rate of carbide removal by hydrogenation
with Hz (in the absence of CO
where the hydrogen
addition
step is clearly rate-limited).
The decrease in
methanation
activity brought about by potassium must therefore be related to
a poisoning of either the hydrogen adsorption or hydrogen addition steps by a
combination
of adsorbed potassium and the consequently
higher CO coverages. Potassium was shown to decrease the rate of hydrogen adsorption
on
iron, and CO is known to decelerate hydrogen adsorption on Ni(100). Surface
carbide is also known to decrease the hydrogen adsorption
rate on Ni(100).
The
effects
of potassium
and
addition
step are not known.
The influence
of adsorbed
carbons
is likely
is consistent
that carbon
adsorbed
CO
potassium
upon
upon
the
rate
of the
the synthesis
with results on supported
catalysts.
chain growth is rate-limiting.
Thus
hydrogen
of higher
hydro-
For these reactions,
the observed
effect
it
of
potassium
to increase
the steady-state
carbide
coverage
can be related to the
increase
in supply
of reactants
for chain growth.
That is. the more carbon
present
on the surface,
the greater
the chances
C-C
bond formation.
This satisfactorily
activity for higher hydrocarbon
production
sium.
It is also consistent
with the observation
for reaction
explains
increases
events
leading
to
the observation
that the
upon dosing with potas-
on clean
Ni(lOO)
that conditions
0
/
K=O.lOML
iw;;
=
0
20
TIME
40
60
80
(MINUTES)
Fig. 10. Methane production
from a C02/H,
(b) a sulfided Ni(lOO) catalyst, (c) a potassium
covered Ni(lOO) covered Ni(lOO) catalyst.
reaction mixture over (a) a clean Ni(lOO) catalyst.
covered Ni(100) catalyst, and (d) a potassium + sulfur
leading
toward
to higher equilibrium
higher hydrocarbons.
Intrinsic
electronic
to
interpreting
effects
should
moderate
Recent
experiments
[3]. Referring
carbide
catalytic
is the inference
or compensate
have shown
coverages
poisoning
for the effects
promotion
in terms
of an electropositive
of an electronegative
of sulfur
decreases
01
impurity
impurity.
methanation
the rate of methane
of potassium
in the presence
the effects of sulfur.
of sulfur
and conclusions
and promoters
integrating
kinetic
can be quite useful
modify
have been used to examine
tive and reactive properties
atoms
distributions
this to be true in the case of CO,
to fig. 10, the adsorption
Catalytic
studies
analytical
techniques
poisons
the product
and
that adsorption
formation
significantly.
The adsorption
shows that the potassium
can neutralize
3. Summary
shifts
surface
measurements
with modern
surface
in detailing
the mechanisms
by which
chemistry.
Model
single
crystal
catalysts
the effects of surface impurities
on the chemisorpof nickel. The effect of preadsorbed
electronegative
Cl. N, S. C. and P on the adsorption-desorption
on a Ni(lOO) surface have been addressed.
behavior
of CO and
H1
It is found that the presence of these
impurities causes a reduction of the adsorption
rate. the adsorption
strength
and the capacity of the surface for CO and Hz. The poisoning effect becomes
stronger with increasing electronegativity
of the preadsorbed
atoms.
Reactivity
studies on the methanation
reaction over a sulfided Ni( 100)
catalyst indicate that poisoning by sulfur at low coverages is long-range in that
one sulfur atom deactivates ten or more nickel atom sites. Ancillary studies
with phosphorus demonstrate
a correlation between the electronegativity
of the
poison and its ability to attenuate methanation
activity. Potassium addition to
a Ni(100) catalyst results in a marked increase in the ability of the surface to
dissociate CO together with a significant decrease in the associated activation
energy. The overall methane formation rate falls with a corresponding
increase
in the higher hydrocarbon
production.
These results suggest that the promotion mechanism of potassium does not require metal-support
interaction
or a
support material.
The results outlined here are but a few that have been obtained
in the
surface science area, addressing
the effects of surface impurities on surface
chemistry. These kinds of studies, particularly
when coupled through kinetics
with similar ones on supported catalysts. can be invaluable
in defining the
fundamental
mechanisms
by which surface modifiers change the course of
catalytic reactions. The insights so developed will be helpful in the design of
new generations of more efficient practical catalysts.
Acknowledgement
We would like to acknowledge
the partial support
Department
of Energy, Office of Basic Energy Sciences,
Sciences.
of this work by the
Division of Chemical
References
[I] Metal-Support
and Metal-Additive
Effects m Catalysis. Eds. B. Imelik. C. Naccache. G.
Coudurier, H. Praliaud. P. Meriaudeau,
P. Gallerot. G.A. Martin and J.<‘. Vrdrine (Elsevier.
Amsterdam.
1982) p, 315.
[2] D.W. Goodman.
R.D. Kelley. T.E. Madey and J.T. Yates. Jr.. J. Catalyuc 63 (19X0) 226.
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