Kinetics and mechanism of ethylene hydrogenation poisoned by CO on silica-supported monodisperse Pt nanoparticles

Article abstract of DOI:10.1016/j.jcat.2007.10.015

The influence of particle size on the poisoning of ethylene hydrogenation by CO was studied over a series of catalysts composed of nearly monodisperse Pt nanoparticles (1.7-7.1 nm) encapsulated in mesoporous silica (SBA-15). The turnover frequency at 403 K in the presence of 0.5 Torr CO was ～2 × 10^-2 s^-1 (compared with ～10² s^-1 in the absence of CO). The apparent activation energy in the absence and presence of 0.2 Torr CO was ～10 and 20 kcal mol^-1, respectively. The pressure dependency changes significantly in the presence of CO; reaction orders in hydrogen were 1/2 in the presence of CO at 403 K and noncompetitive with regard to co-adsorption with C₂H₄. In the absence of CO at similar temperatures, H₂ adsorption was primarily irreversible (first-order dependence), and H₂ and C₂H₄ compete for the same sites. Ethylene orders at 403 K were first order in the presence of 0.2 Torr CO and remained unity with increasing CO pressure. At similar reaction conditions in the absence of CO, ethylene had an inhibitory effect (negative reaction order) on the overall hydrogenation reaction. The change in C₂H₄ and H₂ kinetics suggests strong competitive adsorption between C₂H₄ and CO for the same type of site, whereas H₂ apparently adsorbs on distinct surface sites due either to steric hindrance or H₂-induced CO desorption. Incorporation of a quasi-equilibrated CO adsorption step into a noncompetitive Langmuir-Hinshelwood mechanism predicts the experimentally observed pressure dependencies and a doubling of the apparent activation energy. Hydrogenation of ethylene in the presence of 1 Torr CO was examined under reaction conditions at 403 K by infrared spectroscopy; the only surface species identified under reaction conditions was linear-bound CO. The hydrogenation of ethylene on clean Pt catalysts was structure-insensitive and remains insensitive in the presence of CO; rates decreased only by a factor of two with increasing particle size.

Full text of DOI:10.1016/j.jcat.2007.10.015

Journal of Catalysis 254 (2008) 1–11
www.elsevier.com/locate/jcat
Kinetics and mechanism of ethylene hydrogenation poisoned by CO
on silica-supported monodisperse Pt nanoparticles
Robert M. Rioux 1, Russell Komor 2, Hyunjoon Song 3, James D. Hoefelmeyer 4, Michael Grass,
Krisztian Niesz 5, Peidong Yang, Gabor A. Somorjai ∗
Department of Chemistry, University of California, Berkeley and Lawrence Berkeley National Laboratory, Materials and Chemical Sciences Divisions,
Berkeley, CA 94720, USA
Received 28 August 2007; revised 14 October 2007; accepted 20 October 2007
Available online 26 December 2007
Abstract
The influence of particle size on the poisoning of ethylene hydrogenation by CO was studied over a series of catalysts composed of nearly
monodisperse Pt nanoparticles (1.7–7.1 nm) encapsulated in mesoporous silica (SBA-15). The turnover frequency at 403 K in the presence of
−2 −1
2
−1
0.5 Torr CO was ∼2 × 10
s
(compared with ∼10 s in the absence of CO). The apparent activation energy in the absence and presence of
−1
0.2 Torr CO was ∼10 and 20 kcal mol , respectively. The pressure dependency changes significantly in the presence of CO; reaction orders in
hydrogen were 1/2 in the presence of CO at 403 K and noncompetitive with regard to co-adsorption with C H . In the absence of CO at similar
2
4
temperatures, H adsorption was primarily irreversible (first-order dependence), and H and C H compete for the same sites. Ethylene orders
2
2
2 4
at 403 K were first order in the presence of 0.2 Torr CO and remained unity with increasing CO pressure. At similar reaction conditions in the
absence of CO, ethylene had an inhibitory effect (negative reaction order) on the overall hydrogenation reaction. The change in C H and H
2
4
2
kinetics suggests strong competitive adsorption between C H and CO for the same type of site, whereas H apparently adsorbs on distinct surface
2
4
2
sites due either to steric hindrance or H -induced CO desorption. Incorporation of a quasi-equilibrated CO adsorption step into a noncompetitive
2
Langmuir–Hinshelwood mechanism predicts the experimentally observed pressure dependencies and a doubling of the apparent activation energy.
Hydrogenation of ethylene in the presence of 1 Torr CO was examined under reaction conditions at 403 K by infrared spectroscopy; the only
surface species identified under reaction conditions was linear-bound CO. The hydrogenation of ethylene on clean Pt catalysts was structure-
insensitive and remains insensitive in the presence of CO; rates decreased only by a factor of two with increasing particle size.
© 2007 Elsevier Inc. All rights reserved.
Keywords: Pt; Nanoparticles; Monodisperse; Ethylene hydrogenation; CO; Poisoning; Kinetics; Mechanism; Structure insensitivity
1. Introduction
*
Corresponding author.
E-mail address: somorjai@berkeley.edu (G.A. Somorjai).
Current address: Department of Chemistry and Chemical Biology, Harvard
Designing heterogeneous catalysts that are capable of the
highest possible selectivity at relevant conversions under envi-
ronmentally benign conditions is an important research objec-
tive in heterogeneous catalysis. Toward that end, the design
and synthesis of high-surface area Pt catalysts with tunable
parameters (i.e., particle size and/or shape) has been pur-
sued [1–3]. Monodisperse platinum nanoparticles are synthe-
sized by solution-based reduction methods [2,4,5], followed by
encapsulation of the nanoparticles in the pores of a mesoporous
SBA-15 silica matrix [3]. The properties of these catalysts
can be tuned systematically to understand the influence of
1
University, Cambridge, MA 02138, USA.
2
Current address: Department of Chemical Engineering, California Institute
of Technology, Pasadena, CA 91125, USA.
3
Current address: Department of Chemistry and School of Molecular Sci-
ence (BK21), Korea Advanced Institute of Science and Technology, Daejeon,
305-701, Korea.
4
Current address: Department of Chemistry, University of South Dakota,
Vermillion, SD 57069, USA.
5
Current address: ThalesNano Inc., Zahony u. 7, H-1031, Budapest, Hun-
gary.
0021-9517/$ – see front matter © 2007 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2007.10.015

2
R.M. Rioux et al. / Journal of Catalysis 254 (2008) 1–11
catalyst structure on catalytic activity [1–3]. Detailed con-
trol of morphology enables the investigation of structure–
function (activity and selectivity) relationships, and also en-
ables the elucidation of surface structure (i.e., particle size)
on the resistance of a catalyst to poisoning/deactivation resis-
tance.
2. Experimental
2.1. Catalyst synthesis and characterization
The synthesis and characterization of catalysts used in this
study have been described in detail previously [2,3] and here
discussed briefly. Pt particles were synthesized according to
literature methods [4,5]. The 1.7-nm Pt particles were synthe-
sized by adding NaOH solution (12.5 mL, 0.5 M) in ethylene
glycol (EG) to a solution of dihydrogen hexachloroplatinate
(H2PtCl6·6H2O, Alfa Aesar, 250 mg) in 12.5 mL of EG. The
mixture was heated at 433 K for 3 h with bubbling N2. Af-
ter reaction, particles were precipitated by the addition of 2 M
HCl (1 mL) and dispersed in ethanol containing 12.2 mg of
poly(vinylpyrrolidone) (PVP; Mw = 29,000; Sigma-Aldrich).
The 2.9-nm particles were synthesized by refluxing a mixture
of PVP (266 mg) and H2PtCl6·6H2O (124.3 mg) in a wa-
ter (40 mL)–methanol (360 mL) solution for 3 h. The 3.6-nm
Pt particles were formed by mixing the 2.9-nm Pt colloidal
solution (100 mL) in a water/methanol (1:9) mixture with
10 mL of 6.0 mM H2PtCl6·6H2O aqueous solution and 90 mL
of methanol, followed by refluxing for 3 h. The 7.1-nm Pt
particles were synthesized by adding 93.8 µL of 0.375 M
PVP (Mw = 55,000, Sigma-Aldrich) and 46.9 µL of 62.5 mM
H2PtCl6·6H2O solutions EG at reflux every 30 s over 16 min,
followed by an additional 5 min reflux [17]. Pt colloidal solu-
tions were purified by sequential precipitation/redispersion and
dispersed in an appropriate amount of deionized water to make
a 3 × 10−3 M solution (based on Pt salt concentration). Esti-
mated particle sizes were 1.73 0.26 (1.7), 2.80 0.21 (2.9),
3.39 0.26 (3.6), and 7.16 0.37 (7.1) nm by transmission
electron microscopy (TEM) and X-ray diffraction (XRD) (in
parentheses), indicating high uniformity and monodispersity of
each particle smaller than σ ∼ 8% [2].
The mechanism by which an adsorbate poisons a catalytic
reaction is site blocking, perturbation of the surface electronic
structure (by either charge screening or surface reconstruc-
tion), or a combination of both. The mechanistic complex-
ity of poisoning is tractable when the influence of a poi-
son is examined during a well-studied reaction; an example
is the influence of CO poisoning on olefin conversion reac-
tions [6–10]. Suppression of turnover frequency (TOF) is se-
vere in some cases; whereas other catalysts appear resistant
to poisoning by CO [8–10]. CO is one of the most widely
studied adsorbates because it serves as an indicator of local
surface structure [11,12], making it an ideal probe molecule
for investigating the influence of particle size on poison resis-
tance. The CO stretching frequency is sensitive to the iden-
tity of neighboring adsorbates [13] and may provide insight
into the interaction between poison and reactant molecules
on the catalyst surface. CO adsorption perturbs the electronic
structure of the surface Pt atoms by significant donation from
the 2p orbital into the metal d-orbital and back-donation to
the anti-bonding 2π∗ orbital of adsorbed CO [14]. Strong
adsorption of CO on Pt surfaces predisposes it to compete
for sites and inhibit turnover during olefin conversion reac-
tions. The co-adsorption of CO and ethylene on monodisperse
Pt(X)/SBA-15 (X = 1.7, 2.9, 3.6, and 7.1 nm) catalysts was
studied by volumetric adsorption measurements and infrared
spectroscopy [15]. The results of study demonstrated that CO
and ethylene compete directly for surface sites. Co-adsorption
experiments revealed that the composition of the surface layer
is dependent on the order gas of exposure. CO adsorbs on an
ethylene-covered surface, although modified by its presence;
however, in the case of ethylene adsorption on a CO-covered
surface, the adsorption of ethylene is eliminated almost com-
pletely.
In this paper, the hydrogenation of ethylene in the presence
of 0.2–1 Torr CO was examined on Pt/SBA-15 catalysts. Ki-
netic measurements demonstrated that the ethylene hydrogena-
tion rate decreased by ∼4 orders of magnitude on the addition
of 0.5 Torr CO at 403 K. Kinetic parameters were influenced
by the presence of CO; the most notable change was a dou-
bling of the apparent activation energy in the presence of CO.
The sequence of elementary steps proposed for the noncompet-
itive Horiuti–Polanyi mechanism [16] with the inclusion of an
equilibrated CO adsorption step enables formulation of a rate
expression that captures the experimentally observed kinetics.
This work demonstrates that the rate of olefin hydrogenation,
which is insensitive to the size of the metal nanoparticles in
the absence of CO, remains structure-insensitive in the pres-
ence of CO, even though the adsorption of CO is sensitive to
the catalyst structure.
The synthesis of Pt(X)/SBA-15 catalysts by the nanoparticle
encapsulation (NE) method has been reported previously [3].
Pluronic P123 (2.5 g, EO20PO70EO20, BASF) was completely
dissolved in deionized water (50.5 mL). The Pt colloidal aque-
ous solution (27.0 mL, 3 × 10−3 M) was mixed with the poly-
mer solution and stirred for 1 h at 313 K. Then 0.375 mL
of 0.5 M NaF aqueous solution was added, and 3.91 mL of
tetramethyl orthosilicate (TMOS, 98%, Aldrich) was quickly
added to the reaction mixture, followed by stirring for a day
at 313 K. The resulting slurry was aged for an additional day
at 373 K. The brown precipitate was separated by centrifuga-
tion, thoroughly washed with ethanol, and dried in an oven
at 373 K. The Pt(1.7 nm)/SBA-15 was calcined at 623 K for
24 h, the Pt(7.1 nm)/SBA-15 was calcined at 723 K for 36 h,
and the 2.9 and 3.6-nm catalysts were calcined at 723 K for
24 h. Typically, 300–400 mg of Pt(X)/SBA-15 was calcined in
a horizontal tube furnace at the specified temperature and time
in 100 mL (NTP) min−1 of 20% O2/He (both gases ultra-high
purity (UHP), Praxair). Calcined samples were stored in scin-
tillation vials before use.
A 3.2% Pt/SiO2 catalyst prepared by ion exchange of
Pt(NH3)4(OH)2·xH2O [18] and a UHP Pt powder (Alfa Aesar,
1 µm particle size) were used as standards for reaction stud-

R.M. Rioux et al. / Journal of Catalysis 254 (2008) 1–11
3
ies. The UHP Pt powder was cleaned by heating at 473 K for
0.5 h in 10% O2/He to remove any surface contaminants before
normal catalyst pretreatment. A 2% Pt/Al2O3 (Exxon Research
and Engineering) also was included in this study to investi-
gate whether the support influenced the poisoning of ethylene
hydrogenation by CO. All standard catalyst samples were re-
duced at 673 K by the same procedure used for the SBA-15
catalysts [2,3].
XRD spectra were measured on a Bruker D8 GADDS dif-
fractometer using CoKα radiation (1.79 Å). Selective gas ad-
sorption measurements were conducted in a Pyrex volumet-
ric apparatus pumped by a liquid nitrogen–cooled diffusion
pump to sample cell pressures of ꢀ3 × 10−6 Torr. Total and
reversible isotherms were collected at room temperature, with
an interim 1 h evacuation between isotherms. Monolayer up-
takes were determined by extrapolating adsorbate uptakes to
zero pressure. Catalysts were reduced for 75 min at 673 K in
50 mL (NTP) H2 min−1 (Praxair, UHP, 99.999%), followed by
evacuation at 623 K before beginning the adsorption measure-
ments.
amorphous SiO2, ca. 20–30 mg SBA-15 samples) was loaded
into the cell and subjected to an identical pretreatment as
used for the catalytic studies with a gas-handling manifold
equipped with mass flow controllers (Porter Instruments Co.)
connected to the diffuse reflectance cell. He (99.999%, Praxair),
H2 (99.999%, Praxair), CO (99.9%, AirGas), and C2H4 (99.9%,
Praxair) were used without further purification. Single-beam
spectra (128 scans, 2 cm−1 resolution) of the freshly reduced
catalyst were obtained at 300 or 403 K under 30 mL (NTP)
He min−1 and used to subtract silica features from in situ spec-
tra. Samples were heated to reaction temperature (300 or 403 K)
in He and then exposed to a 90 mL (NTP) min−1 mixture of
10 Torr C2H4, 100 Torr H2, and 1 Torr CO (which, because of
experimental constraints, was the lowest obtainable CO pres-
sure). Separately, a spectrum of gas-phase CO (1 Torr) and
ethylene (10 Torr) obtained in the DRIFTS cell using a Au mir-
ror was subtracted from the in situ spectrum. The only results
presented herein are for a 2.69% Pt(2.9 nm)/SBA-15 catalyst,
which is representative of all samples.
3. Results
2.2. Kinetic measurements of C2H4 hydrogenation in the
presence and absence of CO
3.1. Catalyst characterization
The characterization of the Pt(X)/SBA-15 series catalysts
has been reported previously [2,3]. The X nomenclature in
Pt(X)/SBA-15 represents the size of the free-standing (un-
supported) nanoparticle determined by TEM [2]. Catalysts
were prepared with a nominal weight loading of 1%; actual
metal loadings determined by elemental analysis (inductively-
coupled plasma optical emission spectroscopy (ICP-OES); Gal-
braith Laboratories, Knoxville, TN) were between 0.6 and
0.8 wt%. Estimation of particle size based on XRD was not suc-
cessful because of the low signal-to-noise ratio of the Pt Bragg
reflections. Only in the case of the Pt(7.1 nm)/SBA-15 catalyst
could a particle size be determined (Table 1).
Removal of PVP from encapsulated nanoparticle surfaces is
achieved by sample-dependent long calcination times (24–36 h)
at relatively high temperatures (623–673 K). Selective gas ad-
sorption (H2–O2 chemisorption) measurements of the exposed
Pt surface area are summarized in Table 1.
Kinetic measurements were conducted in a quartz plug-flow
reactor (PFR) operating under differential conditions. Conver-
sions were maintained below 5% for most catalysts. Pt/SBA-15
catalysts (10–50 mg) diluted in acid-washed low-surface area
quartz (∼50–250 mg) (Aldrich) were reduced by the same pre-
treatment protocol used for chemisorption samples. He (Prax-
air, UHP, 99.999%), H2 (Praxair, UHP, 99.999%), C2H4 (Air-
Gas, CP grade), and CO (Matheson, UHP, Al cylinder) were
delivered to the reactor with mass-flow controllers (Unit In-
struments Corporation, model UFC 1200) at a total flow rate
of 90 mL (NTP) min−1. Reactant and product concentrations
were measured by a gas chromatograph (Hewlett Packard 5890
series II) equipped with a 10-way sampling valve that allows si-
multaneous monitoring of C2H4 and C2H6 with a flame ioniza-
tion detector (FID) and CO and H2 with a thermal conductivity
detector (TCD). All TOF values (corrected to standard condi-
tions of 10 Torr C2H4, 100 Torr H2, 0.5 Torr CO and 403 K)
were reproducible to 10% (determined by multiple measure-
ments) and calculated using the dispersion from the total H2–O2
uptake [18]. No conversion of CO to methane through a CO/H2
reaction was detected.
3.2. Kinetics of C2H4 hydrogenation on Pt/SBA-15 catalysts
poisoned by CO
The kinetics of C2H4 hydrogenation in the presence of CO
(0.2–0.9 Torr) was studied over a series of SBA-15 supported
Pt catalysts. In the absence of CO, the TOF for ethylene hy-
drogenation is ∼102 s−1 at 403 K, while the catalytic activity
varied from 1.7–3.5 × 10−2 s−1 at 10 Torr C2H4, 100 Torr H2,
0.5 Torr CO, and 403 K. The activity (per gram catalyst) and
TOFs are reported in Table 2. TOFs were reproducibly higher
on the Pt(1.7 nm)/SBA-15 and 2% Pt/Al2O3, two samples with
the same particle size (∼2.7 nm), as determined by H2–O2 titra-
tion. The difference in TOF varied by only a factor of two over
the entire particle size range studied (1–290 nm); in the pres-
ence of CO, ethylene hydrogenation is considered structure-
2.3. In situ infrared spectroscopy measurements of C2H4
hydrogenation in the absence and presence of CO
In situ diffuse–reflectance infrared Fourier transform spec-
troscopy (DRIFTS), using a Nicolet Nexus 670 spectrometer
equipped with a Thermo Spectra-Tech controlled-atmosphere
diffuse reflectance cell, was used to study the reaction between
ethylene and hydrogen in the presence of CO at 403 K. The
IR cell was operated at conditions similar to those used for
microreactor studies. Typically, a Pt/SiO2 catalyst (ca. 50 mg

4
R.M. Rioux et al. / Journal of Catalysis 254 (2008) 1–11
Table 1
Adsorption uptake and particle size determined by chemisorption and XRD
c
−1
a,b
Probe gas uptakes (µmol g
)
Dispersion, D
Particle size, d (nm)
Catalyst
d
e
H
CO
O
H –O
2
H –O
2 2,total
Chemisorption
XRD
2
2
2
total
Total
Irrev.
irrev.
total
H
H –O
2
2
2
f
g
3.2% Pt/SiO -IE
133.1
43.5
4.5
6.0
4.0
166.7
80.6
13.3
11.3
7.9
152.2
70.1
12.0
10.0
7.4
24.2
17.4
4.4
4.4
3.1
262.0
66.1
19.1
21.8
12.5
5.6
1
1.0
1.0
2.6
2.7
3.1
4.2
–
2
g
2% Pt/Al O -Exxon
0.43
1.3
3.8
3.7
4.4
11.9
173.5
–
2
3
g
0.6% Pt(1.7 nm)/SBA-15
0.77% Pt(2.9 nm)/SBA-15
0.6% Pt(3.6 nm)/SBA-15
0.62% Pt(7.1 nm)/SBA-15
UHP Pt powder
0.416
0.368
0.270
0.117
0.004
–
g
–
g
–
1.5
16.7
2.9
15.1
2.3
13.7
2.4
8.6
9.4
286.5
7.8
>100
30.3
a
Elemental analysis determined by ICP-OES.
b
c
d
e
f
Number average particle size determined by TEM. Determined by counting a minimum of 200 free-standing particles.
Conducted at 295 K.
Determined from 1.13/(Pt /Pt ).
s
T
Based on the Scherrer–Debye equation after subtracting SBA-15 baseline.
Dispersion calculated >1, assumed D = 1 for turnover frequency calculations.
Not detectable.
g
Table 2
Ethylene hydrogenation reaction kinetics on supported Pt catalysts in the presence of CO
a
Catalyst
Kinetic parameters in the presence of CO
b
b,c
d
E
Reaction orders
f
Activity
TOF
a
−1 −1
−1
−1
e
g
(µmol g
s
)
(×100 s
)
(kcal mol )
C H
2
H
CO
4
2
3.2% Pt/SiO -IE
2
3.2
1.5
2.0
3.5
3.4
2.7
2.3
1.7
2.0
20.3
22.1
21.6
20.1
20.7
22.0
13.6
0.89
0.88
0.97
0.96
0.99
0.98
1
0.67
0.62
0.60
0.52
0.44
0.46
0.44
−1.1
−1.1
−1.2
−1.1
−1.1
−1.0
−1.1
2% Pt/Al O -Exxon
2
3
0.6% Pt(1.7 nm)/SBA-15
0.77% Pt(2.9 nm)/SBA-15
0.6% Pt(3.6 nm)/SBA-15
0.62% Pt(7.1 nm)/SBA-15
UHP Pt powder
0.43
0.38
0.19
0.06
0.39
a
Actual Pt weight loading determined by ICP-OES.
b
c
d
e
f
Rates corrected to 10 Torr C H , 100 Torr H , 0.5 Torr CO and 403 K.
2
4
2
Based on number of surface atoms determined by total H –O titration.
2
2
Conditions were 10 Torr C H , 200 Torr H , 0.2 Torr CO and 373–433 K.
2
4
2
Reaction conditions were 5–40 Torr C H , 200 Torr H , 0.2 Torr CO and 403 K.
2
4
2
Reaction conditions were 10 Torr C H , 140–540 Torr H , 0.2 Torr CO and 403 K.
2
4
2
g
Reaction conditions were 10 Torr C H , 200 Torr H , 0.2–0.9 Torr CO and 403 K.
2
4
2
insensitive. The CO pressure must be stated when reporting
TOFs for ethylene hydrogenation poisoned by CO, because a
change in CO pressure has a significant influence on the mea-
sured rate (see below).
energies ranging from 8 to 12 kcal mol−1 in the absence of CO
at 273–323 K [2,3].
Apparent reaction orders in ethylene, hydrogen, and CO
have been measured for all catalysts; these values are com-
piled in Table 2. Reaction orders in ethylene, hydrogen, and
CO at 403 K are shown in Fig. 2. Reaction orders were ∼ first
order in ethylene, ∼1/2 order in H2, and −1 in CO for all
catalysts (regardless of particle size). The inverse first-order de-
pendence of CO (Fig. 2c) suggests that adsorbed CO is the most
abundant surface intermediate (masi) [20]. Comparison of re-
action kinetics in the presence and absence of CO suggests that
both ethylene and hydrogen surface chemistry are influenced
by adsorbed CO. The H2 dependence on the overall reaction
were 1/2 at all CO pressures, whereas in the absence of CO at
slightly lower temperatures (273–323 K), the hydrogen order is
reported to be unity [21]. The first-order dependence on H2 in
the absence of CO is related to competitive adsorption between
C2H4 and H2, which becomes insignificant in the presence of
CO because adsorbed CO displaces H2 to surface sites inac-
The influence of temperature on the overall reaction rate was
measured at 10 Torr C2H4, 100 Torr H2, 0.2 Torr CO, and 373–
423 K. At these conditions, all conversions were ꢀ10% and
verified to be free of mass- and heat-transfer artifacts by the
Madon–Boudart test [19]. Apparent activation energy measured
at similar pressure conditions (but slightly lower temperatures)
in the absence of CO on Pt/SiO2 catalysts have been reported
to be 8–12 kcal mol−1. The apparent activation energy for
the CO-poisoned ethylene hydrogenation was ∼20 kcal mol−1
for supported catalysts, whereas the Pt powder activation en-
ergy was ∼14 kcal mol−1. The Al2O3-supported Pt and 0.62%
Pt(7.1 nm)/SBA-15 catalysts had an apparent activation energy
of 22 kcal mol−1. Arrhenius plots for catalysts are shown in
Fig. 1. The values of the apparent activation energies are com-
piled in Table 2. The same catalysts had apparent activation

R.M. Rioux et al. / Journal of Catalysis 254 (2008) 1–11
5
cessible to ethylene. However, in the case of CO and C2H4, the
adsorption is competitive. The reaction order in ethylene is zero
order in the absence of CO near 273 K [22–24] and becomes
more negative at higher temperatures (>323 K) [21]. In the
presence of CO, the reaction order is first order at 403 K. Under
the reaction conditions of this study, the coverage of ethylene
was low [25], and the surface was dominated by atop adsorbed
CO (see Section 3.3). The pressure dependence in ethylene
remains first order for gas-phase pressures of CO ꢂ0.2 Torr
(Fig. 3), suggesting that the surface chemistry of ethylene at
high temperatures in the presence of CO changes from inhi-
bition to a dependence directly proportional to the gas-phase
ethylene pressure.
3.3. In situ diffuse reflectance spectroscopy under reaction
conditions
The hydrogenation of ethylene under reaction conditions
(10 Torr C2H4, 100 Torr H2, 1 Torr CO) was monitored in
the diffuse reflectance cell to probe the surface under reac-
tion conditions. Fig. 4 presents a DRIFTS spectrum of a 2.69%
Pt(2.9 nm)/SBA-15 catalyst under reaction conditions at 403 K.
The rate determined in the DRIFTS cell under these conditions
was ∼1 × 10−2
s
−1. Assuming differential operation using
∼10 mg of catalyst, the estimated conversion in this DRIFTS
cell was 0.001%. The only adsorbed species under reaction
conditions was linear-bonded CO; the band position was red-
shifted relative to CO adsorbed on a clean surface at the same
temperature [15]. No surface species are observed under reac-
tion conditions at 300 and 403 K for the unpoisoned reaction
(spectra not shown), in agreement with previous results from a
Pt/Cabosil catalyst [21]. The lack of an ethylidyne peak is most
likely due to the high H2:ethylene ratio (= 10) used and the low
ethylene coverage expected at these temperatures.
4. Discussion
4.1. Comparison of unpoisoned and CO-poisoned ethylene
hydrogenation kinetics
Fig. 1. Arrhenius plot for the hydrogenation of ethylene in the presence of CO.
Reaction conditions were 10 Torr C H , 200 Torr H , 0.2 Torr CO and 373–
2
4
2
−1
433 K. Apparent activation energies of 20 kcal mol
are measured in the
Steady-state activities and TOFs for the hydrogenation of
ethylene in the presence of 0.5 Torr CO are compiled in
Table 2. Ethylene hydrogenation rates ranged from 1.7 to
presence of CO pressures up to 0.9 Torr. (Q) 3.2% Pt/SiO ; (") Pt powder;
2
(F) Pt(1.7 nm)/SBA-15; (2) Pt(2.9 nm)/SBA-15; (ꢁ) Pt(3.6 nm)/SBA-15;
(a) Pt(7.1 nm)/SBA-15; (#) Pt/Al O .
2
3
Fig. 2. Reaction orders in (A) C H , (B) H and (C) CO during C H hydrogenation in the presence of CO. Reaction conditions were (A) 5–40 Torr C H , 200 Torr
2
4
2
2
4
2 4
H , 0.2 Torr CO and 403 K; (B) 10 Torr C H , 140–540 Torr H , 0.2 Torr CO and 403 K; and (C) 10 Torr C H , 200 Torr H , 0.2–0.9 Torr CO and 403 K. (Q) 3.2%
2
2
4
2
2
4
2
Pt/SiO ; (") Pt powder; (F) Pt(1.7 nm)/SBA-15; (2) Pt(2.9 nm)/SBA-15; (ꢁ) Pt(3.6 nm)/SBA-15; (a) Pt(7.1 nm)/SBA-15; (#) Pt/Al O .
2
2 3

6
R.M. Rioux et al. / Journal of Catalysis 254 (2008) 1–11
tial on a Pt(8 nm)/SiO2 during benzene hydrogenation at 298 K;
a CO pressure ꢂ30 Torr was required for complete poison-
ing of hydrogenation. This result differs considerably from that
for ethylene hydrogenation in the presence of CO, where cat-
alysts were not active until the temperature was ꢂ353 K, and
low CO pressures (ꢀ0.1 Torr) completely poisoned room tem-
perature hydrogenation. Scanning tunneling microscopy studies
have shown that the presence of adsorbed CO on Pt(111) leads
to the formation of static structures and results in no H2–D2
exchange at room temperature [29]. A plausible mechanism
for CO poisoning is hindrance of adsorbate mobility, a critical
component for catalytic turnover [30]. The influence of CO on
ethylene hydrogenation is complex; adsorbate segregation may
play a key role and different types of active sites are apparently
available on the surface.
4.2. Effect of CO on the apparent activation energy
The second consequence of the addition of CO addition was
a doubling of the apparent activation energy, which increased
from ∼10 to ∼20 kcal mol−1 in the presence of 0.2 Torr CO.
This doubling of the apparent activation energy was also ob-
served on a Pt(111) single crystal in the presence of 1 Torr
CO [7,10]. The gas-phase pressure of CO had little influence
on the measured value of the apparent activation energy; in 0.3,
0.5, and 0.9 Torr CO, apparent activation energies of 20.6, 20.1,
and 20.8 kcal mol−1 (respectively) were measured. Similarly,
the apparent activation energy for cyclohexene hydrogenation
on Pt(111) doubled in the presence of 0.015 Torr CO [31]. Butt
and co-workers found that the apparent activation energy for
methylcyclopropane hydrogenolysis increased with CO cover-
age on a Pt/Al2O3 catalyst [32], which may reflect the increas-
ing influence of CO in progressively displacing the reaction to
less favorable sites. In a separate study of the same reaction
on Pd/SiO2 [33,34], the authors found the opposite trend—the
apparent activation energy decreasing with the addition of CO
(θ ꢀ 0.5). In the case of Pd, the authors suggested that low lev-
els of CO immediately remove a population of high activation
energy sites from the reaction, although the TOF for n-butane
formation is an order of magnitude lower than the rate on a
clean surface. In a separate study, the apparent activation energy
tripled in the presence of CO (1000 ppm) during plug-flow mi-
croreactor studies of ethylene hydrogenation on Pd/SiO2 [35].
It appears that a small amount of CO is sufficient to block most
of the active sites for ethylene hydrogenation.
Fig. 3. Reaction orders in C H during C H hydrogenation in (2) 0.2 Torr,
2
4
2 4
(") 0.5 Torr and (Q) 0.9 Torr CO on 0.6% Pt(3.6 nm)/SBA-15. Reaction con-
ditions were 5–40 Torr C H , 200 Torr H , stated CO pressure and 403 K.
2
4
2
Fig. 4. In situ DRIFTS spectra of C H hydrogenation (10 Torr C H , 100 Torr
2
4
2 4
H ) at 403 K in the presence of 1 Torr CO on 2.69% Pt(2.9 nm)/SBA-15. The
2
4.3. CO dominates surface chemistry: C2H4–CO competitive
adsorption and H2 surface site distinction
−1
frequency for linear-bound CO is 2055 cm , peaks associated with ethylidyne
−1
−1
(∼1340 cm ), di-σ bonded (1420 cm ) ethylene and π-bonded ethylene
−1
(∼1500 cm ) are absent from the surface during catalytic turnover.
Reaction orders in C2H4 and H2 were ∼0 and 1/2, respec-
tively, in the absence of CO at low temperatures (<300 K)
[3,21], suggesting saturation of the surface with ethylene and
gas-phase hydrogen in equilibrium with surface H atoms ad-
sorbed to sites not accessible to ethylene. It is proposed that the
distinction between sites is purely steric and not energetic (al-
though a combination of both cannot be ruled out). At higher
3.5 × 10−2 s−1 at standard conditions. Extrapolated rates at
standard pressure conditions in the absence of CO to 403 K
are ∼200 s−1. Previous work demonstrated that only ppm
quantities of CO are necessary to completely deactivate a
Pd/charcoal catalyst for ethylene hydrogenation [26]. Chen
et al. [27,28] have shown that CO is significantly less influen-

R.M. Rioux et al. / Journal of Catalysis 254 (2008) 1–11
7
temperatures (>340 K), in the absence of CO, the reaction or-
der in hydrogen approaches unity, and the ethylene coverage
decreases as both species begin to adsorb to the same type
of site. In the presence of CO, the reaction order in hydro-
gen remains ∼1/2 while the ethylene dependence becomes
first order, suggesting that C2H4 and CO compete for the same
adsorption sites. H2 is able to adsorb and dissociate on a CO-
covered surface because of the availability of adsorption sites
that cannot be accessed by CO or ethylene due to steric rea-
sons or more complicated co-adsorption (adsorption-assisted
desorption) phenomena. Vacuum and high pressure studies of
CO–H2 co-adsorption have suggested that both adsorbates seg-
regate into islands, in which the local coverage within CO is-
lands is near saturation coverage (θ = 0.68 ML) [36]. These
locally high CO coverage regions cause a substantial decrease
in CO desorption energy (∼30 kcal mol−1 at low coverage to
∼10 kcal mol−1 at saturation) [37]. Gland and co-workers [37]
have shown that the hydrogen-induced displacement of CO has
an apparent activation energy of ∼11 kcal mol−1 above room
temperature. If the heat of adsorption of H2 is greater than
the CO desorption energy at saturation, then hydrogen can dis-
place the more weakly bound CO [37]. The differential heat
of adsorption of H2 at saturation on Pt powder measured by
microcalorimetry was 13 kcal mol−1 [38]; this is greater than
vibrational spectroscopy studies of C2H4/CO co-adsorption on
Pt(111) under hydrogenation conditions has demonstrated that
linear-bound CO and ethylidyne are present on the surface.
A significant interaction between neighboring ethylidyne and
CO molecules is likely, as indicated by the red shift of the
linear-bound CO vibration after co-adsorption [7]. The most
plausible explanation for the increased ethylene pressure de-
pendence is a direct competition between CO and ethylene for
surface sites. Only as the pressure is increased can ethylene
displace CO from the compressed, mixed adlayer to adsorb
and react with H2 to form ethane. Therefore, the state of the
adsorbate-covered surface may be very complex with islands
of pure CO and a mixed C2H4–CO adlayer, which are suffi-
ciently open to enable dissociative adsorption of hydrogen and
diffusion of H atoms in the adlayers. Pure H adlayers are not
believed to exist on the surface because the reaction order in
hydrogen is ∼1/2, and if patches of a H-covered surface ex-
isted, then competitive adsorption between ethylene and hydro-
gen would occur on this fraction of the surface. This situation
would be reflected in a H2 reaction order >1/2. Table 2 shows
that the reaction order in H2 is >1/2 on the small Pt parti-
cles (1–2.7 nm), which may suggest that a small percentage
of ethane formation occurs through a competitive mechanism
or a smaller fraction of three-fold hollow sites, the preferred
adsorption site of H2 [41]. In fact, H2 dependencies at room
temperature on the same catalyst are >1/2, whereas the H2 re-
action orders on the larger particles are ∼1/2 in the absence
of CO [3]. Chen and co-workers [27,28] have identified three
types of hydrogen adsorption sites on a Pt/SiO2 catalyst with Pt
particles with a mean diameter of 8 nm. They suggest that the
lower benzene hydrogenation activity of a supported Pt catalyst
in the presence of CO is related to the elimination of the most
reactive low-coordination sites for H2 activation due to CO ad-
sorption at these sites [28].
the previously reported ꢀHads,CO (θ = θmax) (−10 kcal mol−1
)
[39], and in the present case with hydrogen in 1000-fold ex-
cess (relative to CO), hydrogen should displace CO from the
surface. However, kinetic and infrared spectroscopy measure-
ments suggest that rather than displacing CO, H2 adsorbs on
sites available on a CO-covered surface on which additional CO
is unable to adsorb due to steric hindrance of adjacent CO. The
formation of CO islands that presumably would open up small
ensembles of clean Pt atoms for H2 dissociation is discounted,
because the measured ethylene reaction order is unity for the
Pt/SBA-15, and these small clean ensembles would represent a
small fraction of surface capable of high-temperature ethylene
hydrogenation behavior; that is, ethylene and hydrogen adsorp-
tion would be competitive, leading to ethylene and hydrogen
partial pressure dependencies different from those observed ex-
perimentally.
The influence of CO on ethylene adsorption on Pt has been
studied to a lesser extent, but the observed ethylene adsorption
kinetics suggest that CO competes with ethylene for surface
sites, and kinetic studies on clean Pt/SiO2 catalysts in the ab-
sence of CO demonstrate different kinetics (ethylene inhibits
H2 adsorption). A recent scanning tunneling microscopy study
of CO adsorption on a Pt(111) surface precovered with C2H4
and H2 at room temperature demonstrated that densely-packed
(θCO = 0.68 ML [36]), large mixed C2H4–CO unit cells exist
with H atoms apparently capable of diffusing within the unit
cell [40]. The authors suggested that the rate of H2 diffusion
in the densely packed adlayer is inhibited but not eliminated,
suggesting that turnover will occur when hydrogen and eth-
ylene are located on adjacent sites. The integrity of this ad-
layer may be diminished or less well defined under the high-
temperature reaction conditions used in this study, where dif-
fusion is more facile. Sum frequency generation (SFG) surface
4.4. A noncompetitive ethylene hydrogenation mechanism in
the presence of CO can explain the doubling of apparent
activation energy and partial pressure dependencies
A mechanism in the presence of CO is proposed and the
corresponding rate expression derived in Appendix A. A non-
competitive (with respect to ethylene and hydrogen) mecha-
nism with the incorporation of an equilibrated CO adsorption–
desorption step is sufficient to describe the kinetics of ethylene
hydrogenation in the presence of CO. Noncompetitive mech-
anisms are possible when adsorption of a reactant requires
multiple sites and the maximum surface coverage of the re-
actant is reached before all sites are occupied, but does not
preclude adsorption of smaller molecules on the remaining iso-
lated vacant sites. The effect of noncompetitive adsorption on
the hydrogenation of propylene and isobutylene has been doc-
umented [42]. For Pt, atop sites represent the preferential ad-
sorption site for both π-bonded ethylene and CO [15,43,44].
This competition for the same type of active site was confirmed
by transient pulse methods; the introduction of additional hy-
drocarbon led to an increase in the CO desorption rate and vice
versa [45,46].

8
R.M. Rioux et al. / Journal of Catalysis 254 (2008) 1–11
Diffuse reflectance infrared measurements of ethylene hy-
4.5. Influence of particle size on poisoning of ethylene
hydrogenation by CO
drogenation at standard conditions (Fig. 4) confirmed that the
surface is dominated by linear-bound CO species, with no evi-
dence of π-bonded C2H4, ethylidyne, or the half-hydrogenated
ethyl (C2H5) species. The Pt surface under ethylene hydrogena-
tion conditions (without CO) is covered with carbonaceous ma-
terials, primarily ethylidyne. Radioisotope 14C ethylene stud-
ies [47] demonstrate that ethylidyne reacts to form ethane six
orders of magnitude slower than π-bonded C2H4. In the pres-
ence of CO and the high H2:C2H4 ratios used in this study,
the coverage of all potential ethylene-derived intermediates is
low (at least below the detection limit of the DRIFTS experi-
ment). The proposed reactive intermediate, π-bonded ethylene,
is present as only 4% of a monolayer under reaction conditions
(35 Torr C2H4, 100 Torr H2, 295 K) [48], and the coverage
of such species is significantly lower when co-adsorbed with
CO, as reported recently [15]. CO influences the overall re-
action rate by inhibiting ethylene adsorption and/or displacing
adsorbed ethylene from the surface. The rate-determining step
(the addition of hydrogen to the half-hydrogenated interme-
diate) is unchanged by the presence of CO in the gas phase,
although the kinetic dependence on ethylene changes due a
change in the masi. Kinetic (Figs. 2 and 3) and spectroscopic
(Fig. 4) measurements suggest that CO is the masi, and thus
CO must desorb to allow ethylene adsorption and hydrogena-
tion. The particle size insensitivity to poisoning by CO suggests
that the rate-determining step requires only one or two sites to
convert ethylene to ethane, supporting the proposed mechanism
in which conversion can occur on a single site adjacent to a
H atom source, namely S–H (see Appendix A). Both π-bonded
ethylene and ethyl stability is maximized on atop sites [49], and
H atom diffusion on the surface is facile at these temperatures.
Kinetic studies demonstrated that the rate of ethylene hydro-
genation is independent of the arrangement of the underlying
metal atoms and thus is considered a structure-insensitive reac-
tion [49,52]. One proposal for this insensitivity is the formation
of an alkyl–metal “complex” [53] that effectively eliminates
the surface’s identity, with the resultant ethylidyne becoming
the active site for hydrogenation of ethylene by weakly binding
incoming ethylene in a π-bonded form and shuttling H atoms
from the Pt surface to the weakly bound ethylene species. Other
studies have proposed various possibilities for the structure in-
sensitivity, each of which involves either a one-site or a two-site
mechanism [49,52–55].
The influence of particle size on the ethylene hydrogenation
TOF for the poisoned (0.5 Torr CO) and unpoisoned reaction is
shown in Fig. 5; the rate follows a trend in which larger parti-
cles are influenced more by the presence of CO, although the
difference in rate on small and large particles is only a fac-
tor of two compared with the rate of the unpoisoned reaction,
which is independent of particle size (see Fig. 5). A signif-
icant number of reports suggest that ethylidyne formation is
favored on larger Pt particles. The formation of ethylidyne on
more open Pt single crystals with coordination numbers similar
to those on small Pt crystallites (1–2.5 nm) has been observed
less frequently due to the formation and stabilization of more
highly dehydrogenated species [56,57]. The formation of this
ethylidyne layer is structure-sensitive [58], but its presence is
not critical for catalysis on smaller particles [23,59]; however,
The energetic penalty paid for CO removal is ∼10 kcal mol−1
,
in agreement with previous studies on the heat of adsorption
for CO on Pt surfaces [39,50]. The doubling of the apparent
activation energy can be explained in terms of an equilibrium
between gas-phase CO and a CO-saturated Pt surface (Appen-
dix A, Eq. (5)). The true activation energy (Etrue) for ethylene
hydrogenation remains the same because the rate-determining
step does not change, and only the apparent activation energy
increases. The apparent activation energy in the absence of CO,
Eapp depends on Etrue and the enthalpy of adsorption of ethyl-
ene and hydrogen, respectively, whereas in the presence of CO,
Eapp,CO is equal to Eapp plus the contribution of the enthalpy of
adsorption of CO (ꢀHads,CO). The apparent activation energy
for the hydrogenation of ethylene on clean Pt ranges from 8
to 12 kcal mol−1 [3,51]; including the heat of adsorption of
CO at saturation coverage (∼10 kcal mol−1), Eapp,CO increases
to ∼20 kcal mol−1, in agreement with experimental observa-
tions. A similar argument has been proposed for the increase
in apparent activation energy for ethylene hydrogenation in the
presence of CO on Pd/SiO2 catalysts [35]. A mechanism based
on competitive H2 adsorption could not reproduce the observed
pressure dependencies and predict a doubling of the apparent
activation energy.
Fig. 5. Influence of Pt particle size on the TOF for ethylene hydrogenation in
the absence or presence of CO. (1) Reaction conditions were 10 Torr C H ,
2
4
100 Torr H , 0 Torr CO and 298 K. (2) Reaction conditions were 10 Torr C H ,
2
2 4
100 Torr H , 0.5 Torr CO and 403 K.
2

R.M. Rioux et al. / Journal of Catalysis 254 (2008) 1–11
9
whether this is true for larger particles is not known. This may
Table 3
Compilation of C H hydrogenation kinetics in the presence of CO on Pt cata-
lysts
2
4
be of particular importance if ethylidyne is necessary for eth-
ylene turnover, because ethylidyne is believed to require the
3-fold symmetry of (111) facets rather than random trimer as-
semblies of Pt atoms [60]. The presence of adsorbed CO would
have a greater affect on catalysts composed of larger crystal-
lites (ꢂ5 nm), because adsorbed CO would inhibit ethylidyne
formation by breaking up 3-fold hollow sites.
Catalyst
C H hydrogenation in the Reference
2 4
presence of CO at standard
a
−1
)
conditions TOF (s
−2
Pt powder
3.3 × 10
This work
This work
This work
This work
This work
This work
This work
[9,10]
−2
3.2% Pt/SiO
2% Pt/Al O
3.3 × 10
2
−2
6 × 10
2
3
The results are not conclusive (although a significant amount
of single crystal work supports this claim), but the hydrogena-
tion of ethylene on larger crystallites may require the ethyli-
dyne adlayer, the formation of which would be suppressed in
the presence of adsorbed CO due to site blocking and dilu-
tion of favored 3-fold hollow sites for adsorption. This layer
may not be necessary on small Pt nanoparticles because of
the high concentration of low-coordination atop sites, the pre-
ferred adsorption site for π-bonded ethylene [61]. Rates of
the poisoned ethylene hydrogenation reaction on large Pt parti-
cle catalysts—Pt(7.1 nm)/SBA-15 and Pt powder—are in good
agreement with the rate measured on Pt(111) [6,7,10], confirm-
ing that large particles are slightly more sensitive to poison-
ing by CO for the hydrogenation of ethylene. The apparent
structure-insensitivity of ethylene hydrogenation is upheld in
the presence of CO, even though infrared measurements sug-
gest that the nature of CO adsorption changes with Pt particle
sizes. The particle size insensitivity to poisoning by CO sug-
gests that the rate-determining step requires only one or two
sites; the same scenario postulated for the “clean” reaction.
−2
0.6% Pt(1.7 nm)/SBA-15
0.77% Pt(2.9 nm)/SBA-15
0.6% Pt(3.6 nm)/SBA-15
0.62% Pt(7.1 nm)/SBA-15
Pt(111) single crystal
28 nm Pt nanoparticle array on Al O
6.2 × 10
−2
4.7 × 10
−2
4 × 10
−2
2.8 × 10
2.8 × 10
b
−2
9
[9]
2
3
28 nm Pt nanoparticle array on SiO
33
0.5
0.2
0.4
[9]
[9]
[9]
[9]
2
22 nm Pt nanowire array on Al O
2
3
25 nm Pt nanowire array on SiO
64 nm Pt nanowire array on SiO
2
2
a
Standard conditions are 10 Torr C H , 100 Torr H , 0.3 Torr CO and 403 K.
2
4
2
b
The rate of ethylene hydrogenation was extrapolated from 10 Torr C H ,
2
4
100 Torr H , 1 Torr CO (assuming inverse first order dependence in CO) and
2
403 K to standard conditions.
increased by only ∼2 kcal mol−1 on CO addition. The authors
suggested the enhanced activity of the nanoparticle arrays is
due to the rapid hydrogenation of CO adsorbed to these inter-
face sites [63]. The rate of CO hydrogenation is enhanced at the
Pt–TiO2 interface [64], but no such enhancement was observed
for CO hydrogenation on Pt–SiO2 [65,66]. A nominal TOF for
CO hydrogenation of 10−5 s−1 is estimated (using the kinetics
reported in Refs. [65,66]) for conditions similar to those used
in this study (1 Torr CO, 100 Torr H2, and 473 K). Calcula-
tion of the CO hydrogenation TOF based only on metal–oxide
interface sites (assuming no structure-sensitivity or enhanced
reactivity at the interface), a rate of ∼10−3 s−1 is calculated.
This is significantly lower than the olefin hydrogenation rate
(∼7 s−1) in the presence of CO, suggesting that the enhanced
activity of the Pt nanoparticle array catalysts is probably due to
a reason other than rapid CO hydrogenation.
4.6. Comparison of CO-poisoned ethylene hydrogenation
rates with previously reported Pt catalysts
The poisoning of ethylene hydrogenation by CO on a num-
ber of different Pt-based catalysts has been studied by Somorjai
and coworkers [6,8–10,28]. TOFs corrected to standard condi-
tions are compiled in Table 3. Rates vary by three orders of
magnitude at identical poisoning conditions. The reported eth-
ylene hydrogenation rate in the presence of ∼0.5 Torr CO at
403 K on a Pt(111) single crystal is 1.7 × 10−2
s
−1, which
5. Conclusion
is ∼5 orders of magnitude lower than the rate in the case of
the unpoisoned reaction [62]. The presence of 1 Torr CO in
the feed leads to a doubling of the apparent activation energy
on Pt(111) [6,7,10]. Rates on lithographically generated SiO2-
and Al2O3-supported Pt nanowire arrays are an order of mag-
nitude more active than Pt/SBA-15 and the Pt single crystal.
In the case of the nanowire arrays, the apparent activation en-
ergy in the absence and presence of 0.3 Torr CO is ∼13 and
∼21 kcal mol−1, respectively. For the Pt nanoparticle arrays on
SiO2 and Al2O3 supports, the TOFs are three orders of magni-
tude higher than the Pt/SBA-15 catalysts. The authors suggest
that increased activity of these nanoparticle arrays is due to sites
at the metal–oxide interface that are not poisoned by CO [8,9].
Correction of the nominal TOF using simple geometric argu-
ments that account only for the sites located at this metal–oxide
interface demonstrate that the TOF (7.1 s−1) is similar to the
unpoisoned catalyst (7.3 s−1) [8]. The apparent activation en-
ergy on the lithography generated nanoparticle array catalyst
The kinetics of the hydrogenation of ethylene poisoned by
CO was found to differ substantially (∼4–5 orders of magni-
tude) from that of the unpoisoned reaction. The apparent acti-
vation energy, ethylene order, and hydrogen order changed in
the presence of CO (0.2–1 Torr). In the presence of CO, the
reaction rate was first-order in ethylene and 1/2 order in H2.
Kinetic measurements demonstrated that C2H4 pressure depen-
dence is invariant with CO pressures >0.2 Torr, suggesting sat-
uration of the surface with CO, leading to the inverse first-order
dependence on CO pressure. The apparent activation energies
double regardless of particle size and do not change with CO
pressure. This increase is a consequence of the adsorption equi-
librium maintained between adsorbed and gas-phase CO, not a
change in the rate-determining step. The only surface species
observed by in situ diffuse reflectance spectroscopy during the
hydrogenation of ethylene in the presence of CO was linear-

10
R.M. Rioux et al. / Journal of Catalysis 254 (2008) 1–11
bound CO, the frequency of which was red-shifted relative to
CO adsorption on a clean catalyst surface. The inclusion of a
CO adsorption–desorption equilibrium to the noncompetitive
Horiuti–Polanyi mechanism for ethylene hydrogenation repro-
duced the experimentally observed kinetics. The hydrogenation
of ethylene on Pt catalysts was structure-insensitive in the ab-
sence and presence of CO.
Steps 1–3 are quasi-equilibrated (
), and step 4 is rate-
determining. Steps 1–4 constitute a catalytic cycle, a single
turnover of ethylene to ethane, and regeneration of the * site
and S site, respectively. Surface sites denoted by * are adsorp-
tion sites for hydrocarbons only, whereas sites denoted by S are
for the adsorption of hydrogen only. The distinction between *
and S is due purely to steric considerations. In the absence of
CO, the surface coverage of carbonaceous species is high at low
temperatures but decreases at higher temperatures (>340 K),
due to poor adsorption thermodynamics. Adsorption in a
π-bonded or di-σ configuration requires at least a single site (*)
or a pair (*–*) and potentially more due to steric considera-
tions [27]. Hydrogen atoms adsorb in three-fold hollow sites
on Pt(111) [41], and if ethylene adsorbs randomly on the
surface, then hydrogen adsorption sites exist on the ethylene-
covered surface because pairs of threefold sites near ethylene
are sterically blocked from adsorbing another ethylene mole-
cule [27].
Acknowledgments
This work was supported by the Director, Office of Science,
Office of Advanced Scientific Computing Research, Office of
Basic Energy Sciences, Materials Sciences and Engineering
Division of the U.S. Department of Energy under contract DE-
AC02-05CH11231. The authors thank Professor M.A. Vannice,
Pennsylvania State University for donating the 3.2% Pt/SiO2
catalyst and Dr. Samrat Mukherjee for its preparation. They also
thank the Exxon Research and Developmental Laboratory for
donating the Pt/Al2O3 sample. R.M.R. thanks the Ford Motor
Company for financial support through a graduate fellowship
administered by the Berkeley Catalysis Center.
Formulation of a rate expression assuming the foregoing
sequence of elementary steps with adsorbed carbonaceous
species as the most abundant * species and adsorbed hy-
drogen saturating the S sites leads to a mechanism consis-
tent with low-temperature kinetics: zero-order ethylene and
reversible half-order hydrogen kinetics. The coverage of hy-
Appendix A. Incorporating CO into a noncompetitive
ethylene hydrogenation mechanism
drogen (θH–S) is determined by step 1 and scales with P1/2
H
(θH–S = K1/2 PH1/2θS), being independent of the ethylene pres-
sure due toHt–hSe tw2o-site mechanism. A competitive (S and * are
identical) mechanism leads to a rate expression that is unable
to capture the experimentally observed kinetics at low temper-
ature.
Horiuti and Polanyi proposed a mechanism for the hydro-
genation of ethylene over Ni catalysts, in which ethylene is
hydrogenated to ethane through a half-hydrogenated intermedi-
ate [17]. In this competitive mechanism, ethylene and hydrogen
compete for the same type of site. This mechanism does not
capture the macroscopic reaction kinetics at low temperatures.
A noncompetitive variant of the Horiuti–Polanyi mechanism
has been proposed in which hydrogen and ethylene adsorp-
tion on separate types of sites can explain the low-temperature
macroscopic kinetics, but this variant is incapable of reproduc-
ing high-temperature kinetics. Therefore, both mechanisms are
unable to reproduce the experimental kinetics over a range of
conditions (ethylene:H2 ratios and temperature). At high tem-
peratures (403 K) and low ethylene pressures (5–40 Torr) used
in this work, by combining results from steady-state kinetic
and temperature-programmed measurements, deuterium trac-
ing experiments, and microkinetic modeling, Dumesic et al.
showed that a H2-activated noncompetitive–competitive mech-
anism can reproduce the experimental kinetic data [27]. In this
appendix, a rate expression that accounts for the observed ki-
netics in the presence of CO is derived by the addition of a CO
adsorption–desorption equilibrium step to the noncompetitive
mechanism.
The introduction of CO leads to different experimental ki-
netics than those observed in the hydrogenation of ethylene in
the absence of adsorbed CO. As shown in Table 2, the reaction
order in C2H4 is unity, 1/2 in H2 and −1 in CO. Fig. 3 demon-
strates that the reaction order in ethylene is first order regardless
of CO pressure. The hydrogen reaction order remains ∼1/2, re-
gardless of CO or ethylene pressure (not shown). This leads to
the conclusion that adsorption of H2 and C2H4 is noncompeti-
tive at high temperatures in the presence of CO, in contrast with
the competitive nature of C2H4 and H2 adsorption in the ab-
sence of CO at similar temperatures; for example, Cortright et
al. measured a −0.43 order dependence for C2H4 in the absence
of CO on a Pt/SiO2 catalyst at 336 K [23]. The adsorption of CO
and ethylene is competitive, with the rate of hydrogenation di-
rectly proportional to the ethylene pressure and adsorbed CO
dominating the surface under reaction conditions (see Figs. 3
and 4). The adsorption of CO on the surface is governed by the
following equilibrium:
The noncompetitive Horiuti–Polanyi ethylene hydrogena-
tion mechanism consists of four elementary steps:
5. CO adsorption: CO + *
CO*.
1. Non-competitive hydrogen adsorption: H2 + 2S
C2H4*.
2H–S.
The coverage of CO is defined as θCO = KCOPCOθ*, where
KCO is the equilibrium adsorption constant and θ* is the frac-
tion of empty * sites. Retaining the uncompetitive mechanism
with the addition of a CO adsorption–desorption equilibrium
step leads to the half-order H2 dependency and inverse first-
order CO dependency if CO is the most abundant surface
2. Ethylene adsorption: C2H4 + *
3. First surface hydrogenation reaction: C2H4* + H–S
C2H5* + S.
4. Surface reaction of half-hydrogenated intermediate: C2H5*
+ H–S → C2H6 + * + S.

R.M. Rioux et al. / Journal of Catalysis 254 (2008) 1–11
11
species on * type sites (as suggested by infrared measurements;
see Fig. 4). The rate of C2H6 formation is determined by the
rate-determining addition of adsorbed hydrogen to the half-
hydrogenated intermediate, r = kappθC H *θH–S, and the com-
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5
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law, r = kappPC H P1/2PC−O1, assuming that [CO*] and [H–S]
2
4
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