Catalytic oxidation of CO by platinum

Transcription

Electrochimica Acta 47 (2002) 3595 /3609
www.elsevier.com/locate/electacta
Catalytic oxidation of CO by platinum group metals: from ultrahigh
vacuum to elevated pressures
A.K. Santra, D.W. Goodman *
Department of Chemistry, Texas A&M University, P.O. Box 30012, College Station, TX 77842-3012, USA
Received 10 October 2001; received in revised form 15 February 2002
Abstract
CO oxidation over platinum group metals has been investigated for some eight decades by many researchers and is considered to
be the best understood catalytic reaction. Nevertheless, there has been a renewed interest in CO oxidation recently because of its
technological importance in pollution control and fuel cells. Removal of COx from automobile exhaust is accomplished by catalytic
converters using supported Pt, Pd and Rh catalysts. Catalysts are used in fuel cells to remove traces of COx from the H2 feed gas to
the few ppm level necessary for their efficient operation. Efforts have been made in our laboratory to understand the adsorption of
CO and the kinetics of CO-oxidation on both single crystals and supported metal catalysts over a wide temperature (100 /1000 K)
and pressure (1/10 7 /10 Torr) range. By comparing the results of single crystals, model supported catalysts, and supported
technical catalysts the relationship between particle size and catalytic activity can be better understood. Also discussed is CO
oxidation on model supported Au catalysts, a promising new candidate for low temperature CO oxidation. # 2002 Published by
Elsevier Science Ltd.
Keywords: CO oxidation; Catalysts; Automobile exhaust
1. Introduction
Catalytic oxidation of CO over platinum group
metals (Pt, Ir, Rh and Pd) has been the subject of
many experimental and theoretical investigations [1 /46]
since the classic work of Langmuir [47] in 1922 and has
been extensively reviewed [10,48/50]. Recently, CO
oxidation has attracted renewed attention due to its
technological importance in the area of pollution control [51] and fuel cells [52,53]. Currently the removal of
CO from automobile exhaust is accomplished by the
oxidation of CO in catalytic converters using supported
Pt, Pd and Rh catalysts. It is well established that for
optimum operation of low temperature fuel cells it is
essential to have a continuous supply of CO-free
hydrogen. Although, proton exchange membrane fuel
cells can tolerate a few ppm level of CO in the hydrogen
stream, alkaline fuel cells require CO-free hydrogen. The
* Corresponding author. Tel.: /1-979-845-6822; fax: /1-979-8450214
E-mail address: [email protected] (D.W. Goodman).
conventional hydrogen production technologies such as
steam reforming, partial oxidation and autothermal
reforming of hydrocarbons produce large amounts of
CO as a by-product [52,53]. Therefore, it is extremely
important to have a CO oxidation catalyst with very
high efficiency and one that can preferentially oxidize
CO for the production of CO-free hydrogen stream.
Numerous adsorption and kinetic studies on single
crystals and supported metal catalysts have been reported in the last two decades from our laboratory as a
function of O2 and CO partial pressure from ultrahigh
vacuum (UHV) to elevated pressures (10 Torr) over a
wide temperature range of 100 /1000 K [1 /6,10 /12,54 /
59]. Although the CO oxidation reaction is the best
understood among the industrially important catalytic
reactions, there are many complex aspects to be resolved
with respect to a unified reaction mechanism.
Historically, gold is chemically inert compared with
the other Pt group metals, however, recently it has been
shown that Au, deposited as finely dispersed, small
particles (B/5 nm diameter) on reducible metal oxides
like TiO2, is an excellent catalyst for CO oxidation at
relatively low temperatures [54,55,60 /63]. Furthermore,
0013-4686/02/$ - see front matter # 2002 Published by Elsevier Science Ltd.
PII: S 0 0 1 3 - 4 6 8 6 ( 0 2 ) 0 0 3 3 0 - 4
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A.K. Santra, D.W. Goodman / Electrochimica Acta 47 (2002) 3595 /3609
this catalytic activity has been shown to be a critical
function of the Au cluster size.
In this article, we will compare CO adsorption and
oxidation on single crystals with model supported
catalysts of Pt, Ir, Rh, Pd and Au. Primary emphasis
will be given to the results obtained in our laboratory
over wide temperature (100 /1000 K) and pressure
ranges (1 /107 /10 Torr). The goal of this work is
an understanding of the effects of temperature, pressure
and particle size on CO oxidation that spans the
material and pressure ‘gaps’ between ‘real world’
catalysis and ‘surface science’.
2. Experimental
The UHV systems used for this work were equipped
[64] with Auger electron spectroscopy (AES), a quadrupole mass spectrometer for temperature programmed
desorption (TPD), low energy electron diffraction
(LEED), scanning tunneling microscopy/spectroscopy
(STM/STS) and an Ar ion sputter gun, contiguous to a
high-pressure reaction chamber. The single crystal
samples were mounted on a retractable manipulator,
allowing the sample to be moved between the two
chambers in situ. Pt(100), Ir(110) and Pd(110) crystals of
0.92 cm in diameter and 0.11 cm in thickness were used
for the experiments. The Ir(111) sample was elliptical in
shape and 0.75 /0.55 /0.03 cm3 in size. The samples
were heated resistively by two high purity (0.051 cm)
tungsten leads spot-welded to the back of the crystal;
sample temperature was measured by a 0.08 cm
chromel-alumel thermocouple spot-welded to the sample edge.
AES was used to check the cleanliness of the samples.
In addition, the Pt(100) sample was cleaned of carbon
by O2 adsorption/desorption and of Si and Ca impurities by high temperature oxidation (1123 K, 1 /10 7
Torr O2) and Ar sputtering. The Pd crystal was cleaned
in the reactor with 8 Torr of CO and 8 Torr of O2 and
heating to 600 K for 1 /2 min. One to three cycles of this
treatment produced a clean surface. Large carbon
impurities were cleaned from the Ir samples by oxidation in 1 /105 Torr O2 at 1000 K for 5 /10 min,
followed by a 3 min anneal to 1600 K. Small traces of
carbon were removed by reaction in 8 Torr O2 and 4
Torr CO for 2 min at 600/625 K, followed by a brief
flash to 1600 K. The Pt sample was cleaned by
sputtering at 1100 K in 5/105 Torr of Ar for 30
min (1 kV beam energy) mainly to remove Si and Ca
impurities. This treatment was followed by heating in
0.1 Torr of O2 at 1100 K for 30 min to remove traces of
carbon and then annealing at 1300 K. Repeated cycles
of this procedure produced a clean Pt surface, which
could not be oxidized under UHV conditions at high
temperature. Rh crystals were cleaned by oxidation (2 /
107 Torr O2 at 1300 K) followed by annealing at 1500
K. The Ru(0001) crystal was cleaned by oxidation (2 /
107 Torr O2 at 1450 K for 3 min) followed by
annealing at 1500 K.
The IR cell with CaF2 window is connected to the
UHV chamber through a double differentially pumped
sliding seal. The configuration of the elevated pressure
cell is similar to that described by Campbell et al. [65].
This arrangement allows IR experiments in the pressure
range of 10 8 /103 Torr and also provides convenient
access to the sample without opening the UHV chamber. The pressure in the IR cell was determined using an
ionization gauge and a capacitance manometer at their
respective working pressure ranges.
TiO2(110) single crystals (Commercial Crystal Laboratories) were used in these studies mainly due to their
suitability for atomically-resolved STM and STS experiments. The n-type semiconductor form, sufficiently
conductive for STM and electron spectroscopic measurements, was prepared by cycles of Ar sputtering
and annealing to 700/1000 K. Deposition of the metal
was typically accomplished by resistive evaporation of
high-purity metal wire wrapped around a W or Ta
filament in vacuum. Such dosers provide an excellent
means of obtaining a clean and stable metal flux after
thorough outgassing. By controlling the filament current, doser to substrate distance and the substrate
temperature, fine control can be exercised over clustersize and density.
Gas chromatography with flame ionization detection
(FID) was used to analyze the reaction products in
which CO and CO2 were catalytically converted to
methane before analysis. Rates of reaction are expressed
as turnover frequencies (TOF), defined as the number of
CO2 molecules produced per active metal site per
second. For Pd, Rh or Ir, the entire crystal (front,
back and edge) were included in determining the total
number of sites; for Pt, only the front face was included,
as the back and edge of the crystal were not subjected to
the sputter cleaning procedure. Research grade CO
(99.99%) and O2 (99.995%) were supplied by Matheson.
The CO was further purified by slowly passing it
through a molecular sieve trap at 77 K. No metal
carbonyls (e.g. Ni(CO)4 were detected in any experiment
in post-reaction AES analysis.
The experimental procedure has been described in
detail elsewhere [64]. Briefly, after cleaning, the sample
was moved to the reactor and charged with reactants.
Most experiments were performed with 16 Torr of CO
and 8 Torr O2. The sample is heated to the desired
temperature for a specified time, the products are then
allowed to mix for 15 min and then an aliquot of the
product mixture was analyzed by GC. The reactor was
then evacuated, and the sample returned to the UHV
chamber for post-reaction analysis.
Fig. 1. IR spectra of CO on Pd(111) at Pco /10.0 Torr as a function of
temperature. The spectra were collected from high temperature to low
temperature [85a,85b].
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0.86 ML. The species has been identified as the a-top
type and three different ordered structures at uCO /
0.33, 0.5 and 0.80 ML have been identified by LEED.
A regular increase in the C /O frequency in the IRAS
data along with an increase in the CO desorption
temperature with respect to the increase in coverage
(uCO) have been observed and understood to be due to
lateral CO /CO repulsion. On the reconstructed (5 /1)
as well as on the (1 /1) Ir(100) surface, formation of a
c(2 /2) overlayer structure has been reported [76]. In
this article, we would like to concentrate on the
adsorptive behavior of CO over a wide pressure range
from UHV to 10 Torr and a temperature range of 100/
1000 K using Pd as an example. We will also discuss the
effect of particle size on the catalytic activity.
CO adsorption on Pd(100) and Pd(111), studied under
UHV conditions using an array of surface science
techniques, exhibits structure sensitivity depending on
the face of the Pd crystal. For example, on Pd(111), at a
CO coverage (uCO)B/1/3 ML, CO adsorbs at 3-fold
hollow sites, with a structure corresponding to (3/
3)R 308 as revealed by LEED and a carbon /oxygen
stretching frequency of 1850 cm 1 [77,78]. However at a
coverage (uCO) of 0.5 ML, a c (2 /2) LEED pattern with
a C /O stretching frequency of 1918 cm 1 has been
observed and is assigned to a CO adsorbed onto bridgebound sites. At very high coverages, a-top and bridgebound CO co-exist yielding a (2 /2) LEED pattern with
C /O frequencies near 1951 and 2097 cm 1, respectively. In contrast, only bridge-bonded CO has been
3. CO adsorption
Carbon monoxide adsorption under UHV conditions
especially on transition metal surfaces has been studied
extensively [10,48/50,66 /68]. Three different types of
adsorption sites have been observed, in general, namely
a-top, bridge-bound and 3-fold hollow. The interaction
of a CO molecule with a transition metal can be
understood by a simple donor /acceptor model first
described by Blyholder [69].
Both linear and bridge bonded CO has been observed
on Rh(100) for the entire coverage and temperature
range of 90 /300 K [70]. A transition from a well ordered
c (2 /2) structure at uCO /0.5 ML to a p (42 /
42)R 458 coincidence structure at uCO /0.7 ML has
been observed by LEED and infrared reflection absorption spectroscopy (IRAS). On Pt single crystal surfaces,
however, all three types of CO have been observed
depending on the CO coverage (uCO), adsorption
temperature, pressure and crystal face [20,35,45,67,71/
74]. Three different desorption features of CO have been
observed on the Ir(110) surface [75], whereas the
corresponding IR spectra showed a continuous increase
in the C /O stretching frequency from 2001 cm 1 at the
lowest coverage to 2086 cm 1 at a coverage (uCO) near
Fig. 2. IR spectra of CO on Pd(111) at Pco /1.0 /10 6 Torr as a
function of temperature. The spectra were collected from high
temperature to low temperature [85a,85b].
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Fig. 3. CO /Pd(111) equilibrium pressure /temperature phase diagram
showing the different CO adsorption phases: a-top/3-fold hollow,
bridging, 3-fold hollow and non-adsorbed [85a,85b].
observed on Pd(100) even up to uCO /0.5 ML with a
c (22 /2)R 458 LEED pattern [79 /84]. In this study,
for uCO /0.5 ML, a uniaxial compression takes place
resulting in the formation of incommensurate overlayers. As a consequence of this compression of the
CO overlayer, a sharp decrease in the heat of adsorption
has been observed at uCO /0.5 ML due mainly to lateral
repulsive interactions.
3.1. Effects of temperature and pressure
A series of IR spectra of adsorbed CO on Pd(111)
obtained at a CO pressure of 10 Torr is shown in Fig. 1
[85a,85b]. At high temperatures (i.e. low uCO) only 3fold hollow sites are occupied. With decreasing temperature (i.e. increasing uCO) the population at the
bridging sites increases. At temperatures near 200 K, a
sudden change in the spectrum is apparent, corresponding to a transition from bridging sites to a mixture of atop and 3-fold hollow sites. The bridging0/a-top/3-fold
hollow transition takes place within a very narrow
temperature range while the transition from the 3-fold
hollow to bridging sites is much less defined. The narrow
spectral line-width, due to the 3-fold hollow sites,
indicates the presence of a very well ordered CO
adsorbate layer, and has been confirmed by a very
sharp (2 /2) LEED pattern.
In addition to the 10.0 Torr data shown in Fig. 1,
isobaric adsorption data were collected at every decade
from 107 to 10 Torr. In each of these isobaric series,
adsorption site progression was identical; the only
apparent difference was the temperature at which the
adsorption site transformation took place. As a repre-
sentative case, 1/106 Torr data are shown in Fig. 2
[85a,85b]. Transition from 3-fold hollow to bridging
sites can clearly be seen as the C /O stretching frequency
changes from 1855 to 1900 cm 1. As the population of
CO increases the frequency shifts toward higher values
and the peak becomes sharper. This blue-shift has been
attributed to repulsive lateral interactions between CO
molecules. The frequencies of the a-top and 3-fold
hollow sites are 2110 and 1895 cm 1. The highest
frequency observed for the bridge-bound CO was 1962
cm 1.
An equilibrium phase diagram for CO on Pd(111) is
shown in Fig. 3, based on the adsorption data collected
at the temperature range 90/1000 K and a CO pressure
range of 107 /10 Torr [85a,85b]. It is important to
mention that this phase diagram is valid only at
equilibrium conditions. This is the case for the
bridging 0/a-top/3-fold hollow phase transition. Nonequilibrium CO adsorption can also occur at low
adsorption temperatures (crosshatched region of Fig.
3). In this region appropriate adsorption conditions
have to be used to ensure a fully equilibrated adsorbate
layer. This equilibrium phase diagram illustrates that
low pressure/low temperature adsorption information
can be extrapolated into the high pressure/high temperature regime provided that certain initial adsorption
conditions are used.
In Fig. 4 two series of IR spectra of CO adsorption at
Pd(100) surface are shown at CO pressures of 1/106
and 1 Torr [85a,85b], respectively. In agreement with
previous studies [83,84] only bridge-bound CO was
detected near the C /O stretching frequency of 1895
cm 1 at the lowest coverage. The vibration frequency
that corresponds to a coverage near 0.5 ML was blueshifted to 1950 cm 1, which is further blue-shifted to
1995 /1998 cm1 as the coverage is increased to 0.8 ML.
3.2. Particle size effects
As we have seen in the previous section CO adsorption behavior changes dramatically with respect to the
structure of the crystal surface indicating particle size
effects. In Fig. 5, IRAS data for CO adsorbed on Pd/
Al2O3/Ta(110) model catalysts for uPd /5.0 and 1.0 ML
as a function of temperature is presented [86]. These
results were independent of the direction of temperature
variation provided the samples were annealed in CO to
600 K. On the uPd /5.0 ML catalyst (Fig. 5a), a peak at
1894 cm1 was observed at 500 K. With a decrease in
temperature, this peak shifted towards higher frequency,
became broader, and at 300 K split into two peaks at
1984 and 1942 cm.1 In addition, a peak at 2076 cm 1
appeared at 400 K. These three features continued to
grow, gradually shifting to higher frequency until, at 150
K, a new peak appeared at 1890 cm1. These peaks
have striking similarities to those observed on single
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Fig. 4. IR spectra of CO on Pd(100) as a function of sample temperature at CO pressures of (a) 1.0 Torr and (b) 1/10 6 Torr [85a,85b].
crystal Pd(100) and Pd(111) surfaces as discussed in the
previous section. The broad peak corresponding to 1894
cm 1 at 500 K can be attributed to a combination of 3fold hollow and bridge-bound CO species, shifting
Fig. 5. Temperature dependent IR spectra of CO adsorbed on uPd /5.0 and 1.0 ML Pd/Al2O3/Ta(110) catalysts at 1/10 5 Torr [86].
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Fig. 6. Arrhenius plot of the CO/O2 specific rates of reaction (TOF) for (a) single crystal (Pd, Ir and Pt) and their supported catalysts and (b) for Rh
single crystals [10,11].
towards exclusively bridge-bound species with decreasing temperature. At 300 K, the features at 1984 and 1942
cm 1 are assigned to contributions from bridging CO
from Pd(100) to Pd(111) facets, respectively. The feature
at 2076 cm 1 and 400 K is associated with an a-top
species on 111 facets. Near saturation coverage, the
peak at 1998 cm 1 corresponds to a bridging species on
100 facets while the features at 2108, 1960 and 1890
cm 1 correspond to a-top, bridging and 3-fold hollow
sites, respectively, on 111 facets.
The results obtained for a uPd /1.0 ML catalyst (Fig.
5b) differ significantly from the uPd /5.0 ML sample.
The broad peak near the region of bridge-bound species
never splits cleanly into individual components as is the
case for larger particles. Moreover, no 3-fold hollow
feature is evident at saturation coverage and the ratio of
a-top/bridge intensity is higher compare to the larger
particles indicating that the smaller particles contain a
higher proportion of edge/defects sites. The broader
features observed at saturation coverage for the smaller
particles are due to higher surface curvature on the small
particles and a correspondingly less compressed CO
overlayer assuming roughly hemispherical shape of the
particles. That the types of transitions (bridging 0/a-top/
3-fold hollow) occur on the Pd(111) single crystal and,
to a more limited extent, on the uPd /5.0 ML catalyst
do not occur on the uPd /1.0 ML particles is consistent
with the reduced CO density on the smaller particles.
The effect of particle size with respect to CO oxidation
will be discussed subsequently.
4. CO oxidation
4.1. Pt, Ir, Rh and Pd single crystals and model supported
catalysts
4.1.1. Steady-state reaction kinetics
The CO2 formation rate as a function of inverse
temperature (1/T ) for Pd, Pt and Ir single crystals is
shown [10] in Fig. 6a and compared with the data
obtained on several supported metal catalysts [38]. Data
obtained on Rh(100) and Rh(111) single crystals are
shown [11] in Fig. 6b. The Pd, Pt and Ir single crystal
data are for a (1:2) O2: CO mixture at a total pressure of
24 Torr, whereas, the Rh single crystal data are for a
(1:1) O2: CO mixture at a total pressure of 16 Torr.
Within this pressure range the reaction rate is zero-order
with respect to total pressure. The TOF for the singlecrystal catalysts traverse four orders of magnitude over
a temperature range of 450/600 K. Kinetic measurements over such a wide temperature range with supported catalysts are not possible due to heat and mass
transfer limitations encountered at high temperatures.
Thus a direct comparison between the two types of
catalysts is limited to a relatively small temperature
range. Nevertheless, it is very clear from Fig. 6a and b
that there is excellent agreement between the single
crystal and model supported systems with respect to the
specific reaction rates and apparent activation energies
[28,38].
Fig. 7a /c [10,11] show reaction rate dependence on
CO partial pressure for Pd(110), Ir(111) and Rh(111) /
Fig. 7. CO partial pressure dependence at constant oxygen pressure
and temperature: (a) on Pd(110), (b) on Ir(111) and (c) on Rh(111) and
Rh(100) [10,11].
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Rh(100), respectively. In these experiments the CO
pressure was allowed to vary keeping the oxygen
pressure fixed. In the case of Pd and Ir, up to a CO:
O2 ratio of approximately 1:12 the reaction was firstorder with respect to CO partial pressure; whereas below
this ratio the reaction became negative-first-order with
respect to CO. For Rh(111) and Rh(100) such a roleover with respect to the partial pressure of CO has been
observed with a maximum activity occurring approximately at a O2: CO ratio of 30:1. However, at a lower
partial pressure of O2 (8 Torr), for Pd(110), Rh(100) and
Rh(111), the reaction order has been observed to be
negative-first-order with respect to CO partial pressure
(Fig. 3a and c, respectively). It is noteworthy (Fig. 7c)
that the rate of CO2 formation on Rh(100) is higher
compared with that on Rh(111) at any given CO partial
pressure indicating that the reaction may be structure
sensitive.
For Pd, Ir and Rh the reaction order with respect to
the partial pressure of O2 (Fig. 8a/c) is generally
positive-first-order [10,11]. Above an O2:CO ratio of
12:1, the same ratio at which the CO order changes from
negative to positive, the reaction rate begins to decrease,
becoming negative-order in O2 partial pressures. For
example, at extremely high O2:CO ratios, the order of
reaction for O2 is /0.79/0.2 on Ir(111). For Pd,
changing the CO partial pressure at a constant temperature only shifts the curve, i.e. the maximum rate and
the ratio at which the rate turns over, remain unchanged. Whereas, for Ir the O2:CO ratio at which the
rate varies from first-order oxygen dependency changes
somewhat (from 12:1 to 16:1) as does the order of the
reaction. The reaction rate on both Rh surfaces (Fig. 8c)
increases linearly at low oxygen partial pressures. This
first-order dependence is altered at high oxygen pressures where the rates roll over and become negativeorder with respect to the partial pressure of oxygen.
Note that this rollover occurs at different oxygen partial
pressures on the two single crystal surfaces indicating
the possibility of structure sensitivity.
On Pt(100), the order of the reaction with respect to
CO partial pressure changes with temperature (Fig. 9)
[10]. In the range where the activation energy is
changing (425 /490 K), the reaction order varies from
0.0 to 0.6. Above 500 K the order becomes more
negative and rapidly approaches negative-first-order.
The reaction never becomes positive-first-order with
respect to CO partial pressure even at an O2: CO ratio of
200:1, as observed for Pd, Ir and Rh.
The Pt(100) surface shows (Fig. 10) [10] only positivefirst-order behavior with respect to the partial pressure
of oxygen. The range of O2:CO ratios studied at a given
temperature was limited to TOF’s where there was
differential conversion (B/5%) of CO. No decrease in
the order of reaction is observed for O2:CO ratios of 1:5
to 150:1, and temperatures from 475 to 650 K, indicat-
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Fig. 8. O2 partial pressure dependence at constant CO pressure and temperature: (a) on Pd(110), (b) on Ir(111) and (c) on Rh(111) and Rh(100)
[10,11].
ing that under these experimental conditions no strongly
bound, deactivating oxygen species is present. In order
to form a similar species on Pt, much higher O2 pressure
and/or temperatures would be necessary, conditions not
accessible in our experiments.
4.1.2. Structure sensitivity and particle size effects
Although it appears from the data in Fig. 1 that the
CO-oxidation reaction is structure insensitive, it should
be noted that the single crystal rates are compared with
the least dispersed supported catalyst. Very recently,
under UHV conditions structure sensitivity has also
been observed by Niemantsverdriet and co-workers [90]
on Rh single crystal surfaces wherein the CO2 desorption temperature differs by /50K on Rh(100) and
Rh(111) surfaces. The Rh(100) surface shows significantly higher reaction rates compared with that on the
Rh(111) surface. The other difference between the two
systems is that on Rh(100) all CO is oxidized to CO2,
whereas on Rh(111) a fraction of the CO desorbs. The
reason for such high activity and selectivity on Rh(100)
towards CO2 formation is assumed to be due to the
surface reaction step, COads/Oads 0/CO2(g), being intrinsically faster on Rh(100) than on Rh(111). In
contrast to the high-pressure data (Fig. 7c and Fig.
8c), the reaction exhibits first order kinetics with respect
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Fig. 9. CO partial pressure dependence as a function of temperature at constant oxygen pressure on Pt(100). (a) The Arrhenius plot for CO oxidation
on Pt(100) is shown to illustrate the temperature regimes in which (b) the CO pressure dependence data were obtained [10].
Fig. 10. O2 partial pressure dependence at constant CO pressure and
temperature on Pt(100). In all cases the reaction order is 1.09/0.1 [10].
to both the partial pressure of CO and O2 under UHV
conditions, however, the structure sensitivity is consistent with the high-pressure data (Fig. 7c and Fig. 8).
Dramatic structure sensitivity has been observed on
Ir/SiO2 catalyst [38] (Fig. 11). It is clear from Fig. 11
that larger particles are more active for CO oxidation
and the data more comparable to the single crystal data.
The data for the larger particles were calculated from
Fig. 11. Effect of particle size on the CO2 formation rate on Ir/SiO2
and Ir single crystal catalysts [38].
the dispersion values obtained by Cant and co-workers
[38]. It is noteworthy that the single crystal data in Fig.
11 corresponds to the supported data extrapolated to a
particle size of 40 nm. This is close to the ‘effective’
particle size of the single crystals used in this study,
3604
Fig. 12. CO oxidation with O2 over model Pd/SiO2/Mo(100) and a
conventional 5% Pd/SiO2 catalyst. Reaction conditions were PTorr /
0.5 Torr and CO /O2 /0.2 [56,86].
Fig. 13. Dependence of the rate of CO oxidation on metals on the
oxygen bond energy EMO (793 K, PO2 /PCO /10 7 Torr) [94].
taking into account step densities and edge effects. The
decreasing activity of the reaction with Ir particle size
can be understood as due to a preferential poisoning of
the active sites by carbon formed by CO dissociation, a
competing process to CO oxidation under such highpressure conditions. This reaction would be expected to
occur more rapidly on defect sites and step edges,
present at much higher concentration on the smaller
particles.
CO oxidation has also been investigated over Pd/
SiO2/Mo(100) model catalyst [56,86]. The reaction
conditions for the catalysts were 10 Torr CO, 5.0 Torr
O2, and reaction temperatures in the range 540 /625 K.
The conversions were maintained at less than 50% and
were measured by monitoring the pressure decrease in a
static reactor of known volume (750 cm3). The average
cluster sizes shown in Fig. 12 were determined by CO
TPD, O2 TPD and ex situ STM/AFM and all were in
good agreement. The specific reaction rates were somewhat higher for the model catalysts than the highsurface-area catalysts, but the activation energies are
remarkably similar. There was no noticeable dependence of the CO2 formation rate on the Pd cluster size,
indicating that CO oxidation over Pd/SiO2 is structure
insensitive.
Structure sensitivity has also been observed for Pt.
Yates and co-worker [91 /93] using IRAS and TPD have
shown that on Pt(335) surface dissociation of CO and
O2 occurs preferentially at step sites. It also has been
observed that CO adsorbed at 111 terraces is more
active compared with CO at 100 step sites whereas
chemisorbed oxygen atoms at step sites is more active
compared with CO adsorbed at terraces. Using isotopically labeled 13C18O molecules, it has been shown that
the oxidation rate at step sites is twice the oxidation rate
at terrace sites. However, recent STM studies of CO
oxidation on Pt(111) have shown that the reaction
occurs exclusively at the boundaries between (2 /
2)Oads and c(4 /2)COads domains when co-adsorbed
under UHV condition [17,87 /89]. Therefore, to increase
the reactivity of the surface it is necessary to increase the
interface between the Oads and COads domains, i.e. to
increase the coverage of Oads.
4.1.3. CO oxidation versus metal /oxygen bond energy
The oxidation rate of CO under steady-state conditions on various Pt group metals are compared in Fig.
13 [94]. The temperature of the reaction was chosen
specifically as 793 K in order to avoid surface site
blocking due to CO adsorption; the pressure of CO and
O2 was 1 /107 Torr. From the data of Fig. 13 it is
clear that the most active metals (Pd, Pt, Ir and Rh) for
CO oxidation have M /O bond energies within the range
320 /390 kJ mol 1. In the case of metals having M /O
bond energies less than 320 kJ mol 1, e.g. Ag or Au, the
rate-determining step for CO oxidation is the adsorption
and dissociation of oxygen. On the other hand, for
metals having M/O bond energies larger than 390 kJ
mol 1, the rate-determining step is the reaction between
COads and Oads. In other words those metals on the right
hand side of Fig. 13 have a higher tendency to form
stable oxides. In fact, on Rh(111) and (100) single
crystals, high O2:CO ratios result in a decrease in the
overall rate and a change from positive-order in oxygen
to negative-order. This change was directly correlated
with the formation of an oxide-like species, as determined by post-reaction AES and TPD [10,11]. Pd(110),
Ir(111) and Ir(110) exhibit partial pressure dependence
and high oxygen pressure behavior similar to Rh. For
Rh, the formation of a near surface oxide (probably
Rh2O3 [10,11] which is much less active) is responsible
for the deactivation. The similar behavior of Pd and Ir
suggests a similar deactivation mechanism. In contrast,
on Ru the oxide was found to be substantially more
active than the clean surface, and the reaction order in
oxygen pressure increases by approximately 3-fold
[9,13,80,95].
4.1.4. Reaction mechanism
The Langmuir /Hinshelwood reaction between COads
and Oads is well established as the dominant reaction
mechanism for conditions where CO is the primary
surface species [48,49]. This mechanism has been con-
Fig. 14. Reactive temperature programmed desorption (RTPD) of
CO2 as a function of emission angle after predosing 1.5 /1015 cm 2
molecules of O2 and 2.7/1015 cm2 molecules of CO successively at
Ts /100 K [99].
firmed by numerous UHV studies of the co-adsorption
of reactants, transient kinetic studies, and steady-state
kinetics [1 /46]. The reaction steps can be written as
CO(g) 0 COads
O2 (g) 0 2Oads
COads Oads 0 CO2 (g)
(1)
(2)
(3)
where the recombinative desorption of adsorbed Oads
atoms (reverse of reaction 2) and the dissociative
chemisorption of CO2 (reverse of reaction 3) are
neglected. It is assumed that CO is the dominant surface
species [96]. Considering the reaction steps (1/3) and
using the above assumptions, an approximate rate
expression, originally proposed by Langmuir [47] for
Pt can be derived as:
d[CO2 ]=dtk exp (Edes;CO =F )PO2 =PCO
(4)
where the reaction rate is independent of total pressure,
first-order in O2 pressure and negative-first-order with
respect to the CO pressure. The rate of the reaction is
then governed by the desorption of CO or the lifetime of
CO (tCO) on the surface [48], depending on the reaction
temperature, whereas the pressure dependence simply
reflects the competition for adsorption sites between
oxygen and CO. It has been found [10] that the kinetics
on Pd, Ir and on Pt at high temperatures, are consistent
with this model in that the pressure dependencies
predicted by equation 4 are observed. In addition, a
correlation of the activation energies between supported
and single crystal data, and among different singlecrystal planes [28,38] reflect the fact that the binding
energy of CO does not vary greatly among these metal
catalyst surfaces.
3605
4.1.5. Angle-resolved temperature programmed
desorption
In search of the possible reaction mechanism of CO
oxidation on both polycrystalline and Pt(111) surfaces,
angle resolved reactive temperature programmed desorption (RTPD) has been performed using time of flight
(TOF) mass spectrometry [32,97 /99]. The results show
dramatic angular and velocity dependence of CO2
desorption from the reaction of CO and oxygen coadsorbed on a Pt(111) surface at 100 K. The velocity
integrated desorption spectra [99] (Fig. 14) show four
different peaks (a, b3, b2 and b1) for CO2 formation at
145, 210, 250 and 330 K, respectively. These feature
have been attributed to four different reaction mechanisms operating at various temperatures depending upon
the relative binding of oxygen and the geometric
arrangement and coverage of the adsorbed species.
Although the precise origin of b3 and b2 processes are
not clear at present, the a-CO2 formation temperature
coincides with that of the molecular O2 desorption. The
b1-CO2 formation is most likely the mechanism proposed in the previous section due to the reaction at the
overlapping regions of COads and Oads island boundaries. Very interesting oscillatory behavior of the CO
oxidation reaction has been observed on Pt single crystal
under UHV conditions [15,16,100 /102], however, this
behavior is beyond the scope of the present article.
4.2. CO oxidation on Au
In the bulk form, Au is known to be chemically inert
compared with the other Pt group metals. However
recently it has been shown that Au clusters, deposited as
finely dispersed, small particles (B/5 nm diameter) on
reducible metal oxides like TiO2, Fe2O3 and Co3O4,
dramatically enhance the rate of a number of industrially important reactions. Reactions catalyzed by Au
particles on TiO2 supports are CO oxidation, hydrogenation and partial oxidation of hydrocarbons and the
selective oxidation of higher alkenes [54,55,60 /63]. It
has also been observed that catalytic activity of these
catalysts is a function of cluster size.
4.2.1. Characterization of the Au clusters by STM and
STS
The constant current STM micrographs in Fig. 15
show [103] changes in the clusters with respect to the
amount of Au deposited. At a relatively low coverage of
Au (uAu /0.1 ML) hemispherical 3D clusters are
observed with diameters of 2 /3 nm and heights of 1/
1.5 nm. Interestingly the clusters mainly grow along the
step edges. Well-dispersed quasi-2D clusters, having a
height of 0.3 /0.6 nm and a diameter of 0.5 /1.5 nm, can
be seen on the terraces. With increasing Au coverage
(uAu), the clusters steadily grow larger while the increase
3606
Fig. 15. A set of 50 /50 nm2 STM images (2.0 V, 1.0 nA) of TiO2(110) /(1/1) with different Au coverage (uAu): (A) 0.10 ML, (B) 0.25 ML, (C) 0.50
ML, (D) 1.0 ML, (E) 2.0 ML and (F) 4.0 MLE. With increasing coverage, Au clusters grow and gradually cover the surface [103].
in cluster density is minimal. However, even at uAu /4.0
ML, some portions of the TiO2 substrate are still visible.
In Fig. 16A, the constant current STM micrograph of
Au (uAu /0.25 ML) deposited onto single crystal
TiO2(110) /(1 /1) [55,103] is shown. The metal deposition was performed at 300 K, followed by annealing to
850 K for 2 min to stabilize the clusters. In Fig. 16A only
the Ti cations are visible; whereas the O2 anion are not
seen. The inter-atomic distance between the 001 rows
is /0.65 nm, which can be observed along the terraces
corresponding to the length of the unit cell along the
[110] direction of the unreconstructed TiO2(110) /(1 /
1). Three-dimensional Au clusters, imaged as bright
protrusions, have average diameters of /2.6 nm and
heights of /0.7 nm (corresponding to 2/3 atoms thick)
and are preferentially nucleated at the step edges. Quasitwo-dimensional clusters are characterized by heights of
1 /2 atomic layers. Previous studies have shown that the
Au clusters upon annealing form large microcrystals
with well-defined hexagonal shapes.
Fig. 16B shows STS taken over various clusters on the
surface, where the tunneling current (I) as a function of
bias voltage (V) across the STM tip is measured. The I/
V curves correlate with the Au cluster size on the TiO2
3607
Fig. 17. CO oxidation TOFs as a function of the Au cluster size
supported on TiO2. (A) The Au/TiO2 catalysts were prepared by a
precipitation method, and the average cluster size was measured by
TEM, 300 K. (B) The Au/TiO2 catalysts were prepared by vapordeposition of Au on planner TiO2 films on Mo(100). The CO /O2
mixture was 1:5 at a total pressure of 40 Torr, 350 K [55].
Fig. 16. (a) A CCT /STM image of a Au (uAu /0.25 ML) deposited
onto TiO2(110) /(1/1) prepared just prior to a CO:O2 reaction. The
sample had been annealed to 850 K for 2 min; (b) STS data acquired
for Au clusters of varying sizes on the TiO2(110) /(1/1). An STS of
TiO2 substrate, having a wider band-gap than the Au cluster, is also
shown as a point of reference [55].
surface. The length of the observed plateau at the zero
tunneling current is a measure of band-gap (along the
bias voltage axis) of electrons tunneling between the
valence and conduction band of the cluster and tip. The
electronic character of these clusters vary between that
of a metal and a non-metal depending on their size.
With an increase in size the clusters gradually exhibit
metallic character with an enhanced density of states at
the Fermi level. Note that clusters of 2.5 /0.7 nm2 size
have a larger band gap than that for a cluster 5.0 /2.5
nm2 in size. Smaller clusters have a non-metallic
character resulting in significant band-gap and a reduced density of states near the Fermi level. A similar
metal to non-metal transition with respect to cluster size
has also been observed for Fe clusters deposited on
Fig. 18. The specific activity for CO conversion as a function of
reaction time at 300 K on a model Au/TiO2/Mo(100) catalyst. The Au
coverage (uAu) was 0.25 ML, corresponding to an average cluster size
of /2.4 nm [103].
GaAs(110) [104]. Very small clusters then are nonmetallic and exhibit electronic and chemical properties
unlike those of the corresponding bulk metal.
4.2.2. Particle size effects
Interestingly, a marked correlation between the cluster size and catalytic activity has been observed for CO
oxidation over Au/TiO2 system [54,55,60 /63]. Studies
have been carried out on Au/TiO2/Mo(100) as well as on
Au/TiO2(110) /(1 /1) for comparison. Fig. 17a and b
show plots of CO oxidation activity (TOF) at 350 K as a
function of Au cluster size supported on TiO2(110) /
(1 /1) and TiO2 /Mo(100) substrates, respectively.
3608
These results show similarities in the structure sensitivity
of CO oxidation with a maximum activity evident at /3
nm Au cluster size on both TiO2 supports. For each
catalyst, the activity and the selectivity of the supported
Au clusters are markedly size-dependent. Although the
TiO2 supported Au catalysts exhibit a high activity for
the low-temperature CO oxidation, rapid deactivation
was observed as a function of reaction time. Fig. 18
shows a plot of TOF versus time for CO oxidation at
300 K on a Au(uAu /0.25 ML)/TiO2/Mo(100) model
catalyst. The model catalyst, which exhibited a high
initial activity, deactivated after a CO/O2 (1:5) reaction
of /120 min at 40 Torr. This deactivation is due to
agglomeration of the Au clusters with reaction time, as
detailed by post-reaction STM measurements. The STM
data clearly demonstrate [54,55,60 /63] that under reaction conditions the Au clusters ripen via an Ostwald
mechanism; i.e. large clusters grow at the expanse of
small ones leading to a bimodal size distribution. This
ripening mechanism depends upon the strength of the
cluster/support interaction as well as gas pressure. The
TOF of CO oxidation, which maximizes with respect to
cluster size, correlates with a metal to non-metal
transition at a particle size of /3 nm, as revealed by
a detailed STM /STS investigation. This behavior has
been discussed in the previous section and there is no
unified theory to understand the catalytic activity of the
small gold particles at the moment. However, the fact
that CO oxidation activity increases with decreasing
cluster size as long as the particles are metallic suggests
that an overall catalytic activity that depends on both
electronic as well as geometric factors. Electronic factor
means the size-induced changes in the electronic levels of
the small clusters namely electronegativity, charge state
and metallicity leading to changes in the cluster /CO
and /O2 interaction strength thereby the overall reaction itself. On the other hand, geometric factor leads to
changes in the shape with respect to the changes in the
particles size leading to change in the number of steps,
size of the terraces and facetes.
5. Conclusions
An approach that combines both surface science and
traditional catalytic methodologies has been shown to
be extremely advantageous in bridging the material and
pressure ‘gaps’ between ‘real world catalysis’ and ‘surface science’. Although, it appears that the CO oxidation reaction follows a simple Langmuir-Hinshelwood
mechanism, many complicating factors influence the
overall activity. The example of Pd shows that the
knowledge acquired from single crystal data can be used
to understand the results for model supported catalysts,
including structure sensitivity, reaction order determination, nature of the bonding of adsorbed molecules, the
optimum reaction temperature, deactivation, etc. Finally it has been shown that ultra-small gold particles,
unlike bulk gold metal, are promising catalysts for low
temperature CO oxidation.
Acknowledgements
The support of this work by the Department of
Energy, Office of Basic Energy Sciences, Division of
Chemical Sciences, and the Robert A. Welch Foundation is gratefully acknowledged.
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Catalytic oxidation of CO by platinum

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