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 CO x 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 CO x from the H 2 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 con- trol [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 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 re- ported in the last two decades from our laboratory as a function of O 2 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 TiO 2 , is an excellent catalyst for CO oxidation at relatively low temperatures [54,55,60 /63]. Furthermore, * Corresponding author. Tel.: /1-979-845-6822; fax: /1-979-845- 0214 E-mail address: [email protected](D.W. Goodman). Electrochimica Acta 47 (2002) 3595 /3609 www.elsevier.com/locate/electacta 0013-4686/02/$ - see front matter # 2002 Published by Elsevier Science Ltd. PII:S0013-4686(02)00330-4
15
Embed
Catalytic oxidation of CO by platinum group metals: from ultrahigh
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
Catalytic oxidation of CO by platinum group metals: from ultrahighvacuum 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 con-
trol [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
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 re-
ported 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
towards exclusively bridge-bound species with decreas-
ing 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 (bridging0/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 crystaldata 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 single-
crystal catalysts traverse four orders of magnitude over
a temperature range of 450�/600 K. Kinetic measure-
ments over such a wide temperature range with sup-ported 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. 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].
A.K. Santra, D.W. Goodman / Electrochimica Acta 47 (2002) 3595�/36093600
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 first-order 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 role-
over with respect to the partial pressure of CO has been
observed with a maximum activity occurring approxi-
mately 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 benegative-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 generallypositive-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 tem-
perature only shifts the curve, i.e. the maximum rate andthe ratio at which the rate turns over, remain un-
changed. 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 pres-
sures where the rates roll over and become negative-order 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 from0.0 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 positive-
first-order behavior with respect to the partial pressure
of oxygen. The range of O2:CO ratios studied at a giventemperature 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-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