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Birck Nanotechnology Center
9-2012
Counting Au catalytic sites for the water-gas shift
reaction
Mayank Shekhar
Purdue University, [email protected]
Jun Wang
Purdue University
Wen-Sheng Lee
Birck Nanotechnology Center, Purdue University, [email protected]
M. Cem Akatay
Birck Nanotechnology Center, Purdue University, [email protected]
Eric A. Stach
Birck Nanotechnology Center, Purdue University; Brookhaven National Laboratory, [email protected]
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Shekhar, Mayank; Wang, Jun; Lee, Wen-Sheng; Akatay, M. Cem; Stach, Eric A.; Delgass, W. Nicholas; and Ribeiro, Fabio H.,
"Counting Au catalytic sites for the water-gas shift reaction" (2012). Birck and NCN Publications. Paper 1138.
http://dx.doi.org/10.1016/j.jcat.2012.06.008
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Authors
Mayank Shekhar, Jun Wang, Wen-Sheng Lee, M. Cem Akatay, Eric A. Stach, W. Nicholas Delgass, and Fabio
H. Ribeiro
This article is available at Purdue e-Pubs: http://docs.lib.purdue.edu/nanopub/1138
Journal of Catalysis 293 (2012) 94–102
Contents lists available at SciVerse ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Counting Au catalytic sites for the water–gas shift reaction
Mayank Shekhar a, Jun Wang a, Wen-Sheng Lee a, M. Cem Akatay b,c, Eric A. Stach b,c,d,
W. Nicholas Delgass a, Fabio H. Ribeiro a,⇑
a
School of Chemical Engineering, Purdue University, West Lafayette, IN 47907, USA
School of Materials Engineering, Purdue University, West Lafayette, IN 47907, USA
c
Birck Nanotechnology Center, West Lafayette, IN 47907, USA
d
Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, NY 11973, USA
b
a r t i c l e
i n f o
Article history:
Received 2 April 2012
Revised 6 June 2012
Accepted 9 June 2012
Available online 20 July 2012
Keywords:
Water–gas shift
Gold
Bromine poisoning
WGS kinetics
Transient isotopic switch
Operando FTIR
a b s t r a c t
We have developed various techniques to count catalytic sites of Au/TiO2 catalysts for the water–gas shift
(WGS) reaction. Addition of Br in an amount that is only 16% of the moles of the surface Au on a
2.3 wt.%Au/TiO2 catalyst decreases the majority of its WGS reaction rate per total mole of Au but does
not result in an appreciable change in the average Au particle size, Au particle shape, apparent activation
energy, or reaction orders. From transient isotopic switch experiments, the WGS turnover frequency
(TOF) for Au/TiO2 catalysts with and without Br, based on the operating active sites counted in the experiment, is 1.6 ± 0.5 s1 under 6.8% CO, 8.5% CO2, 11.0% H2O, 37.4% H2 at 120 °C. The estimated number of
potential active sites, 2% of the total amount of Au on the 2.3 wt.%Au/TiO2 catalyst, best correlates with
the Au corner atoms (2%) of the cubo-octrahedral particles. From operando FTIR spectroscopy, the normalized IR peak area of CO adsorbed on Au0 near 2100 cm1 is proportional to the WGS reaction rate for
Au/TiO2 catalysts with and without Br. Thus, the dominant active sites on Au/TiO2 catalysts for the WGS
reaction are taken to be the metallic corner Au sites with Au–Au coordination number of 4.
Ó 2012 Elsevier Inc. All rights reserved.
1. Introduction
While bulk Au is often regarded to be chemically inert, supported Au nanoparticles catalyze a wide variety of reactions such
as CO oxidation [1], water–gas shift (WGS) [2], and selective and
total oxidation of hydrocarbons [3,4]. Thus, the study of the source
of catalytic activity of Au nanoparticles presents an opportunity to
understand the unique characteristics possessed by metal nanoparticles. In the literature, the catalytic activity of supported Au
nanoparticles is claimed to be due to cationic Au [5], bilayers of
Au [6,7], perimeter sites [8], and low coordinated corner sites
[2,9]. Fu et al. [5] found that the WGS rate for Au/CeO2 catalysts
was not affected by the removal of metallic Au particles by cyanide
leaching and thus that metallic nanoparticles were mere spectators
in the WGS reaction. They concluded that the nonmetallic Au species embedded in ceria catalyze the WGS reaction [5]. In the work
by Herzing et al. [6], the CO oxidation catalytic rates for Au/FeOx
catalysts correlated with the presence of bilayer clusters that are
0.5 nm in diameter and contain only 10 gold atoms. The dependence of the rate on the average Au particle size has been extensively used to determine the identity of active sites on Au/TiO2
catalysts [2,7–9]. Valden et al. [7] found the CO oxidation rate
per total mole of Au for Au/TiO2 catalysts to be maximum at an
⇑ Corresponding author. Address: School of Chemical Engineering, Purdue
University, 480 Stadium Mall Drive, West Lafayette, IN 47907-2100, USA.
E-mail address: [email protected] (F.H. Ribeiro).
0021-9517/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved.
http://dx.doi.org/10.1016/j.jcat.2012.06.008
average Au particle size of 3 nm. On the contrary, HD exchange
[8], CO oxidation [9], and WGS reaction rates per total mole of
Au for Au/TiO2 catalysts were found to increase with decrease in
average Au particle size. While the HD exchange rate correlated
with the perimeter Au sites [8] as being active, the CO oxidation
[9] and WGS [2] rates correlated with the low coordinated corner
Au sites being active.
Halide poisoning has been used as an effective tool to study the
active sites for the CO oxidation reaction and CO adsorption on Au
catalysts. Addition of chlorine [10], bromine [11,12], and fluorine
[13] has been shown to suppress the catalytic activity of Au catalysts by either promoting agglomeration of Au nanoparticles or
poisoning the active Au sites. In the work by Oxford et al. [11], it
was shown that when 5–10% of the total moles of Au, or 10–20%
of the surface moles of Au, in a Au/TiO2 catalyst were bound to
Br, the CO oxidation catalytic activity was completely blocked,
although, at 60 °C, 35% of the original CO adsorption capacity remained. The perimeter Au atoms at or near the particle-support
interface were claimed to be the potential active sites in the oxidative environment of the CO oxidation reaction [11].
Here, we have used the WGS reaction as a model to study the
origin of catalytic activity of supported Au nanoparticles. In this
work, we have used poisoning by Br to confirm the identity of
the catalytic sites on Au/TiO2 catalysts. Gold nanoparticles supported on TiO2 have been shown to have superior catalytic activity
for the WGS reaction [2]. Model non-porous and crystalline rutile
95
M. Shekhar et al. / Journal of Catalysis 293 (2012) 94–102
TiO2 support with a BET surface area of 28 m2 g1 was used to
study the active sites of Au/TiO2 catalysts for the WGS reaction.
Since the support was non-porous, all the active Au nanoparticles
were accessible on the surface of the support. Further, the support’s crystallinity enhanced the contrast between the Au nanoparticles and the support in transmission electron microscopy (TEM)
images, allowing precise determination of the Au particle size distribution and the Au particle shape. A 2.3%Au/TiO2 catalyst, prepared using this model support, was poisoned by KBr to prepare
2.3%Au–(1Br:25Au)/TiO2 and 2.3%Au–(1Br:12Au)/TiO2 catalysts
such that the total moles of Br present were 4% and 8% of the total
moles of Au, respectively. A physical model of Au nanoparticles as
truncated cubo-octahedra was used to determine the percentage
Au that are surface sites, perimeter sites and corner sites from
the entire Au nanoparticle size distribution determined from
TEM. The Au content of these catalysts was determined to be
2.3 ± 0.2 wt.% by atomic absorption. Due to the strong dependence
of the rate on the Au particle size of Au/TiO2 catalysts, it was essential to compare the rates of the brominated Au/TiO2 catalysts to Au/
TiO2 catalysts at the same particle size. While poisoning by Br and
transient isotopic switch experiments were used as tools to count
Au catalytic sites, operando Fourier transform infrared (FTIR) spectroscopy was used to determine the nature of Au catalytic sites.
2. Experimental methods
2.1. Catalyst preparation
The Au/TiO2 catalysts were prepared by the deposition precipitation (DP) method. HAuCl43H2O was used as the Au precursor
and was added to deionized water to give a 0.0015 M Au solution.
A solution of 0.1 N NaOH was added dropwise to the Au solution so
that the mixture maintained a pH = 6 at 35 °C for approximately
6 h. The support material was then added to the solution, and
the suspension was heated to 85 °C at a rate of 1.7 °C per minute
and maintained at 85 °C for 1 h. The mixture was then cooled, centrifuged, washed, and dried. The detailed procedure is discussed in
our previous work [2]. Non-porous and crystalline, ‘‘TEM friendly’’
rutile TiO2 support was used to ensure that all Au was deposited on
the outside of the support, accessible to TEM analysis. The rutile
TiO2 support used was corporation lot number E3-692-011-005
from Sachtleben Chemie GmbH, Germany, and had a stable BET
surface area of 28 m2 per gram (after steaming at 500 °C using a
30% water in air mixture for 48 h).
One gram of 2.3%Au/TiO2 catalyst (synthesized by the procedure stated above) was impregnated by the incipient wetness
method. A KBr solution containing 4% and 8% Br of the total moles
of Au was added dropwise to the solid catalyst while it was stirred
to enhance the mixing in order to synthesize 2.3%Au–(1Br:25Au)/
TiO2 and 2.3%Au–(1Br:12Au)/TiO2 catalysts, respectively. The
impregnation was followed by drying the catalyst overnight under
vacuum at room temperature. Since the brominated Au catalysts
are prone to sintering with time at room temperature, these materials were immediately (in a period of 1 h) transferred to the tubular reactor unit for kinetic measurements. The Au loadings of the
catalysts were determined by atomic absorption spectroscopy
(AAS), performed on each sample with a Perkin–Elmer AAnalyst
300 instrument. Prior to AAS measurements, the catalysts were digested (2 mL/1 mL/100 mg = aqua regia/HF/catalyst) in a NalgeneÒ
amber high-density polyethylene bottle for at least 3 days, and this
solution was then diluted to the desired concentration for the AAS
measurement. The absorption results were compared to those of
known standards to obtain the Au content. We note that the Br
content was not measured. Thus, the nominal Br loadings represent upper bounds on the Br content.
2.2. TEM
The TEM images illustrate the imaging advantages of having
high Z contrast between a metal and its non-porous support. The
use of the non-porous support material ensures that no metal particles are hidden from view within a pore structure. Au particle size
can change during exposure to WGS reaction conditions due to sintering. Thus, used samples (samples after the kinetic measurements) were imaged by TEM using an 80–300 kV S/TEM FEI Titan
operating at 300 kV. Prior to the TEM experiments, the catalyst
samples were dispersed in ethanol and sonicated for 10 min. The
suspensions were then dropped on 200 mesh lacey carbon coated
copper grids. The grids were dried in air for 15 min at room
temperature.
The size of each gold cluster was determined from the longest
measureable distance for that cluster. The particle size distributions of the used Au/TiO2 and brominated Au/TiO2 catalysts were
calculated from the acquired TEM images. From the particle size
distributions, the number (d), surface (ds), and volume (dv) average
Au particle sizes were determined using the following equations:
P
d¼
i di
n
;
P 3
d
ds ¼ Pi i2 ;
i di
P 4
d
dv ¼ Pi i3
i di
Here, di is the Au particle diameter (defined as the longest distance measure for each particle), n is the total number of Au particles counted from the TEM images of a given sample, and the
summation is performed over the entire particle population identified by the TEM images. These equations are also provided in
the literature [14].
2.3. Operando FTIR spectroscopy and transient isotopic switch
experiments
The FTIR spectrometer was a Bruker Vertex 70 FTIR and the
transmission IR cell was a homemade reactor, the details of which
are provided elsewhere [15,16]. About 70 mg of catalyst sample
was pressed in the form of a thin wafer with diameter about
2 cm for the transmission IR study. The IR backgrounds of 100
scans were collected when the catalyst was exposed to 11% H2O,
balance Ar, and He at desired temperatures. All spectra taken at
WGS steady-state conditions were collected at a resolution of
4 cm1 and averaged over 50 scans. For isotopic transient experiments, the IR spectra were collected in the rapid scan, time-resolved mode with the scan rate about 8 spectra per second. For
the operando studies, the CO, H2, and Ar in He balance were bubbled through a H2O saturator heated to a temperature at which
the vapor pressure gave the desired concentration (6.8% CO,
11.0% H2O, 8.5% CO2, and 37.4% H2). The concentrations of CO,
CO2, and H2 were the same as for the kinetic measurements done
in the tubular reactor unit in this study, and the total flow rate
was 50 sccm. For the transient isotopic switch experiments, the details are provided elsewhere [16]. In short, CO + Ar in the reaction
mixture was switched to 13CO + Ne at the same flow rate and pressure, with Ar and Ne acting as tracers. An Agilent 5973 N mass
spectrometer (MS) was used to track the gas phase changes during
the isotopic transient experiments. To determine the error associated with these measurements, the procedure was repeated three
times for each catalyst. The standard deviation calculated from the
three measurements is the reported error.
Fresh catalyst from the same batch used in the kinetic measurements was loaded into the IR reactor cell. It was pretreated with
the same procedure as used in the kinetic measurements. The total
flow rate of gases through the IR cell was kept constant at 50 sccm
throughout the experiments. The IR spectra and concentration
96
M. Shekhar et al. / Journal of Catalysis 293 (2012) 94–102
measurements by gas chromatography during the pretreatment
and under steady-state reaction conditions were then collected.
CasaXPS version 2.3.12 was used for integration of the IR peak
areas in the CO stretching region. Gaussian Lorentzian symmetric
line-shape curves – GL (30), that is, 70% Lorentzian and 30% Gaussian, were used. The peak position, area, and Full Width at Half Maximum (FWHM) were optimized by minimizing the root mean
square (RMS) error through Levenberg Marquardt iterations in
CasaXPS.
2.4. Kinetic measurements – tubular reactor unit
For each of the kinetic experiments, 100–1300 mg of catalyst
was added to a reactor in our automated, four independent parallel
tubular plug flow reactor setup, described elsewhere [17]. The
2.3%Au/TiO2, 2.3%Au–(1Br:25Au)/TiO2 and 2.3%Au–(1Br:12Au)/
TiO2 catalysts were reduced in a 25% H2, 75% Ar mixture with a
flow rate of 50 sccm at 200 °C for 2 h followed by a pretreatment
at the standard WGS conditions (6.8% CO, 21.9% H2O, 8.5% CO2,
37.4% H2, and balance Ar) with a flow rate of 75.4 sccm at 140 °C
for 20 h. After this WGS pretreatment, the temperature was lowered to 120 °C so that conversion was less than 10% and the WGS
rates were determined. We refer to the rates measured immediately after pretreatment as the initial rates, and, for uniformity,
these are the values presented as the results.
The apparent reaction orders with respect to the reactants and
products were measured by varying one gas concentration at a
time (4–21% CO, 5–25% CO2, 11–34% H2O, and 14–55% H2) at
120 °C, and the apparent activation energy was measured by varying the temperature over a range of 30 °C, with the concentrations
kept at the standard conditions. The WGS reaction rate for the catalysts reported here decays by less than 5% of the initial rate during
the kinetic measurements. The catalysts were then exposed to Ar
gas as the temperature was lowered to room temperature. Once
at room temperature, the catalysts were passivated with a 2% O2
in Ar mixture for 2 h. A more detailed discussion of the procedure
for the WGS kinetic measurements is provided in our earlier work
[2]. Since the brominated Au catalysts are prone to sintering with
time at room temperature [11], these passivated samples were
analyzed by TEM on the same day that the kinetic measurement
experiments ended.
3. Results
3.1. Determination of Au particle sizes
The particle size distributions of the used Au/TiO2 and brominated Au/TiO2 catalysts were calculated from their TEM images,
typical examples of which are shown in Fig. 1. The number average
Au particle sizes of the used 2.3%Au/TiO2 and 2.3%Au–(1Br:25Au)/
TiO2 catalysts after PFR and operando FTIR reactor measurements
are reported in Tables 1 and 2, respectively. The number, surface,
and volume average Au particle sizes of the used 2.3%Au/TiO2
and 2.3%Au–(1Br:25Au)/TiO2 catalysts after PFR and operando FTIR
reactor measurements are reported in Fig. 2 and S1. The number,
surface, and volume average Au particle size of the used
Fig. 1. Typical TEM images of used (A) Au/TiO2, (B) 2.3%Au–(1Br:25Au)/TiO2 and (C) 2.3%Au–(1Br:12Au)/TiO2 catalysts used to determine the Au particle size distributions.
Table 1
Summary of PFR results for 2.3%Au/TiO2 and 2.3%Au–(1Br:25Au)/TiO2 catalysts at 120 °C, 6.8% CO, 21.9% H2O, 8.5% CO2, and 37.4% H2.
Catalyst
Number average Au particle diameter
(nm)
Rate/103 (mol H2)
(mol Au)1 s1
Ea
(kJ mol1)
Apparent reaction orders
H2O
CO2
CO
H2
2.3%Au/TiO2
2.3%Au–(1Br:25Au)/
TiO2
3.4 ± 0.9
4.0 ± 1.5
8.3 ± 0.1
0.9 ± 0.1
60 ± 3
57 ± 3
0.30 ± 0.05
0.35 ± 0.05
0.10 ± 0.05
0.05 ± 0.05
0.75 ± 0.05
0.85 ± 0.05
0.20 ± 0.05
0.15 ± 0.05
Table 2
Summary of operando FTIR reactor results for 2.3%Au/TiO2 and 2.3%Au–(1Br:25Au)/TiO2 catalysts at 120 °C, 6.8% CO, 11.0% H2O, 8.5% CO2, and 37.4% H2.
a
Sample
Number average Au particle diameter
(nm)
Rate/10–3 (mol H2)
(mol Au)1 s1
Operating active sites as% of total Aua
(%)
Ea
(kJ mol1)
TOFa
(s1)
2.3%Au/TiO2
2.3%Au–(1Br:25Au)/
TiO2
3.1 ± 0.9
2.8 ± 0.8
11 ± 1
6±1
0.74 ± 0.10
0.37 ± 0.07
56 ± 3
52 ± 3
1.5 ± 0.4
1.6 ± 0.5
Determined by transient isotopic switch experiments.
Rate / (mol H2) (mol Au)-1 (s)-1 at 120o C
M. Shekhar et al. / Journal of Catalysis 293 (2012) 94–102
1.E-01
2.3% Au/TiO2 IR
2.3% Au/TiO2 PFR
1.E-02
2x lower rate
Au/TiO2
Rate = 0.22d-2.7±0.1
2.3%Au-(1Br:25Au)/TiO2 IR
6x lower rate
1.E-03
2.3%Au-(1Br:25Au)/TiO2 PFR
97
resulted in a 6.7 nm number, 7.0 nm surface, and 7.2 nm volume
average Au particle size after 20 h under WGS reaction mixture.
These average particle sizes (6.7–7.2 nm) are significantly higher
than those caused by the addition of Br equal to 4% and 8% of the
total moles Au, that is, the 2.3%Au–(1Br:25Au)/TiO2 and 2.3%Au–
(1Br:12Au)/TiO2 catalysts (2.6–4.5 nm) after 120 and 20 h under
WGS, respectively. Therefore, the addition of Br at 4% and 8% of
the total moles Au does not result in sintering of Au particles, possibly due to the low content of Br on them.
3.2. Determination of Au particle shape
1.E-04
1
3
5
7
Number Average Au Particle Size / nm
Fig. 2. WGS reaction rate per total mole of Au calculated at 120 °C, 6.8% CO, 21.9%
H2O, 8.5% CO2, and 37.4% H2 versus number average Au particle size for Au/TiO2 and
2.3%Au–(1Br:25Au)/TiO2 catalysts in PFR and operando FTIR reactor measurements.
The data for Au/TiO2 catalysts (blue squares) was first reported in our earlier work
[2], and it has been re-plotted here for comparison with catalysts presented in this
work. (For interpretation of the references to color in this figure legend, the reader
is referred to the web version of this article.)
2.3%Au–(1Br:25Au)/TiO2 is 4.0 nm, 4.2 nm, and 4.5 nm and
2.3%Au–(1Br:12Au)/TiO2 is 2.6 nm, 2.7 nm, and 2.7 nm after plug
flow reactor (PFR) measurements, respectively. The WGS kinetic
measurements lasted 120 and 20 h for 2.3%Au–(1Br:25Au)/TiO2
and 2.3%Au–(1Br:12Au)/TiO2 catalysts, respectively. Therefore,
the average Au particle size after the PFR measurements was found
to increase with time on stream.
The WGS reaction rate per total mole of Au for Au/TiO2 catalysts
varies with the number (d), surface (ds) and volume (dv) average Au
2:80:1
2:90:1
particle size as d2.7±0.1, ds
and dv
, respectively (Fig. 2
and S1). Figure S1 shows that the difference between the values
of d, ds, and dv for the used Au/TiO2, 2.3%Au–(1Br:25Au)/TiO2 and
2.3%Au–(1Br:12Au)/TiO2 catalysts is small. This confirms a narrow
Au particle size distribution on these catalysts. To eliminate the errors associated with using an average Au particle size, the entire Au
particle size distribution identified by TEM was used to compute
the percentage of corner, perimeter, and surface sites on these catalysts. This was done by performing a summation of the fraction of
corner, perimeter, and surface sites, determined from the truncated cubo-octahedra geometry [2], over the entire particle size
distribution.
The addition of halides to supported Au catalysts is commonly
reported [10,11,13] to promote sintering of Au nanoparticles. We
observed that the addition of an amount of Br equal to 16% of
the total moles of Au, that is, 2.3%Au–(1Br:6Au)/TiO2 catalyst,
Fig. 3 shows typical high resolution (HR)-TEM images used to
determine the Au nanoparticle shapes of the Au/TiO2 and brominated Au/TiO2 catalysts. From the HR-TEM images, the Au nanoparticles on these catalysts formed well faceted truncated cubooctahedrons. A truncated cubo-octahedral shape for Au nanoparticles was reported in our previous work on Au/Rutile [2] and Au/
Al2O3 [15] catalysts. Based on this result, the geometry of Au nanoparticles was assumed to be truncated cubo-octahedral, that is,
cubo-octahedral geometry terminated at the midline, to estimate
the percentage of corner, perimeter, and surface sites on Au/TiO2
and brominated Au/TiO2 catalysts. It should be noted that Fig. 3B
shows a Au nanoparticle with a cubo-octahedral geometry truncated at approximately 3/4th its height. However, the density of
such nanoparticles was less than 10% for the catalysts used in this
work (3 out of 31 Au nanoparticles observed in HR-TEM). The
remaining Au nanoparticles possessed cubo-octahedral geometry
truncated approximately at the midline.
3.3. Kinetic studies on Au/TiO2 and brominated Au/TiO2 catalysts
The WGS kinetics in the PFR’s was measured under 6.8% CO,
21.9% H2O, 8.5% CO2, and 37.4% H2. In order to compare the rates
measured in the operando FTIR under 6.8% CO, 11.0% H2O, 8.5%
CO2, and 37.4% H2 to PFRs, the WGS reaction rates measured in
the operando FTIR reactors were adjusted to the WGS reaction conditions in PFRs (Fig. 2 and S1), using the kinetic parameters reported in Table 1. In general, this procedure required only
accounting for an increase in water concentration using the water
order and small adjustments to the temperature using the apparent activation energy. Fig. 2 shows the calculated WGS reaction
rate per total mole of Au at 120 °C under 6.8% CO, 21.9% H2O,
8.5% CO2, and 37.4% H2 in PFR and operando FTIR reactors for Au/
TiO2 and 2.3%Au–(1Br:25Au)/TiO2 catalysts. The WGS reaction rate
per total mole of Au for Au/TiO2 catalysts varies with the number
Fig. 3. Typical HR-TEM images of used (A) Au/TiO2, (B) 2.3%Au–(1Br:25Au)/TiO2 and (C) 2.3%Au–(1Br:12Au)/TiO2 catalysts, respectively, used to determine the particle shape.
M. Shekhar et al. / Journal of Catalysis 293 (2012) 94–102
Ln [Rate / (mol H2) (mol Au)-1 (s)-1]
98
- 3.50
- 4.00
- 4.50
- 5.00
2.3%Au/TiO2
Eapp = 60 ± 3 kJ (mol)-1
- 5.50
- 6.00
- 6.50
- 7.00
- 7.50
2.3%Au-(1Br:25Au)/TiO2
Eapp = 57 ± 3 kJ (mol)-1
- 8.00
2.40
2.45
2.50
2.55
2.60
2.65
1000 T-1 / K-1
ders for 2.3%Au/TiO2 and 2.3%Au–(1Br:25Au)/TiO2 catalysts in the
PFR measurements. The H2O, CO2, CO, and H2 reaction orders of
2.3%Au/TiO2 and 2.3%Au–(1Br:25Au)/TiO2 catalysts at 120 °C in
PFR measurements are 0.30 and 0.35, 0.10 and 0.05, 0.75
and 0.85, and 0.20 and 0.15, respectively (Table 1 and Fig. 5).
It was not possible to measure the apparent activation energy
and reaction orders for the 2.3%Au–(1Br:12Au)/TiO2 catalyst because its WGS reaction rate was undetectable.
The WGS reaction kinetics for the 2.3%Au/TiO2 catalyst measured in the PFR and operando FTIR reactors is in the kinetically
controlled regime [16]. This was established by satisfying the
Koros–Nowak (K–N) criterion, that is, the WGS reaction rate was
found to be proportional to the concentration of active sites for a
series of Au/TiO2 catalysts [16]. Additionally, the TiO2 used is
non-porous [16] and thus should be free of internal pore diffusion
transport limitations.
Fig. 4. Arrhenius plots for 2.3%Au/TiO2 and 2.3%Au–(1Br:25Au)/TiO2 catalysts in
PFR measurements.
1
1
13
CO
0.6
0.4
13
COads Au0
-5.0
H2 order = -0.20
H2O order = -0.35
-7.0
CO2 order = -0.05
-7.5
-3.5
-3.0
-2.5
-2.0
-1.5
H2 order = -0.15
-1.0
-0.5
13
CO-Auδ-
0.4
2100
2050
2000
1950
1900
Wavenumber / cm - 1
0.5
1
1.5
2
2.5
3
0
3.5
4
Time / s
CO order = 0.85
-6.5
CO-Auδ-
0
-5.5
-6.0
0.6
0.2
0
CO2 order = -0.10
CO-Au0
13
0.2
COads Auδ-2150
H2O order = -0.30
0.8
CO-Au0
Ne
13
-4.5
CO2
13
0.8
Normalized IR band intensity / a.u.
CO order = 0.75
-4.0
Fig. 6 shows the mass spectrometer (MS) response and time-resolved FTIR spectra during transient isotopic switch (12CO to 13CO)
experiments on 2.3%Au–(1Br:25Au)/TiO2 catalyst during WGS at
120 °C. The area between the envelope made by the Ne and
13
CO2 can be used to estimate the operating and potential active
sites and will be discussed later. The area between the envelope
made by the Ne and 13CO is small due to rapid switch of isotopic
13
CO with CO and the relatively small uptake of CO on the catalyst.
The time-resolved FTIR spectra during transient isotopic switch in
Fig. 6 shows the fast switch of CO adsorbed on Au0 and Aud (IR
peaks near 2100 cm1 and 2030 cm1, respectively, assigned in
the literature [20] to CO adsorbed on Au0 and Aud, respectively)
to isotopic 13CO adsorbed on Au0 and Aud (IR peaks near
2052 cm1 and 1992 cm1, respectively). Fig. 7 shows the FTIR
spectra (CO region) for 2.3%Au/TiO2, 2.3%Au–(1Br:25Au)/TiO2 and
2.3%Au–(1Br:12Au)/TiO2 catalysts during WGS after 20 h exposure
Absorbance / a.u.
-3.5
3.4. Operando FTIR spectroscopy and transient isotopic switch
experiments
Normalized MS signal / a.u.
Ln [Rate / (mol H2) (mol Au)-1 (s)-1 at 120 C]
(d), surface (ds), and volume (dv) average Au particle size as
2:80:1
2:90:1
and dv
, respectively (Fig. 2 and S1). The data
d2.7±0.1, ds
for Au/TiO2 catalysts was first reported in our earlier work [2], and
it has been re-plotted in Fig. 2 and S1 for comparison with catalysts
presented in this work. The Au/TiO2 catalyst with a number average Au particle size of 1.2 nm has a 100 times higher rate per total
mole of Au than that with a number average Au particle size of
6.7 nm. The variation of rate with Au particle size has been used
to conclude that the potential active species for Au/TiO2 catalysts
are the low coordinated Au sites for both the CO oxidation [9,18]
and the WGS reactions [2]. The rate for the 2.3%Au/TiO2 catalyst
used in this work (prior to addition of Br) at 120 °C is similar to that
for Au/TiO2 catalysts at the same number, surface or volume average Au particle size in PFR, and operando FTIR reactor measurements (Fig. 2). The rate for the 2.3%Au–(1Br:25Au)/TiO2 catalyst
at 120 °C is 6 and 2 times lower than for Au/TiO2 catalysts at the
same number, surface or volume average Au particle size in PFR,
and operando FTIR reactor measurements, respectively (Fig. 2 and
S1). The rate for the 2.3%Au–(1Br:12Au)/TiO2 catalyst was undetectable at 120 °C in PFR and operando FTIR measurements.
Fig. 4 shows the Arrhenius plots for 2.3%Au/TiO2 and 2.3%Au–
(1Br:25Au)/TiO2 catalysts in PFR measurements. The apparent activation energies of 2.3%Au/TiO2 and 2.3%Au–(1Br:25Au)/TiO2 catalysts near 120 °C are 60 and 57 kJ (mol)1 in the PFR
measurements (Table 1 and Fig. 4) and 56 and 52 kJ (mol)1 in
operando FTIR reactor measurements (Table 2), respectively.
Fig. 5 shows the plot used to determine the apparent reaction or-
0.0
Ln [Pressure / atm]
Fig. 5. Plots used to calculate the apparent reaction orders for 2.3%Au/TiO2 and
2.3%Au–(1Br:25Au)/TiO2 catalysts in PFR measurements.
Fig. 6. Comparison of normalized MS response and IR band area during transient
isotopic switch experiments of 2.3%Au–(1Br:25Au)/TiO2 catalyst during WGS at
120 °C: Ne (red solid line), 13CO (blue solid line) and 13CO2 (pink solid line); and
surface species (normalized IR band area): 13CO adsorption on Au0 (13COads Au0,
green square with dotted line, 2052 cm1), 13CO adsorption on Aud (13COads Aud
orange triangle with dashed line, 1992 cm1). Feed: 11% H2O, 37.5% H2, 6.8% 13CO,
13.5%Ne in He. The synchronization between IR and MS signals was done by
shifting the response curve of the normalized IR band intensities of 13CO gas phase
to best match the response curve of the normalized Ne MS signal. The synchronization procedure is detailed in our previous work [16]. Inserted figure shows the
time-resolved FTIR spectra while the isotopic transient experiment is switching
from CO to 13CO at time = 0, 0.28, 0.42, 0.56, 0.7, 0.84, 0.98, 1.12, and 2 s, relative to
the switch. (For interpretation of the references to color in this figure legend, the
reader is referred to the web version of this article.)
M. Shekhar et al. / Journal of Catalysis 293 (2012) 94–102
2.3%Au/TiO2
Absorbance / a.u.
2.3%Au-(1Br:25Au)/TiO2
2.3%Au-(1Br:12Au)/TiO2
2150
2100
2050
2000
1950
-1
Wavenumber / cm
Fig. 7. FTIR spectra of 2.3%Au/TiO2, 2.3%Au–(1Br:25Au)/TiO2, and 2.3%Au–
(1Br:12Au)/TiO2 catalysts during WGS at 200 °C.
to the reaction mixture at 200 °C. While the peak area of the IR
peak near 2100 cm1 decreases, the area of the IR peak near
2030 cm1 increases upon addition of more Br to 2.3%Au/TiO2
catalyst.
4. Discussion
4.1. Counting Au catalytic sites
4.1.1. Bromine poisoning
Fig. 3 shows that the Au particles on 2.3%Au/TiO2, 2.3%Au–
(1Br:25Au)/TiO2 and 2.3%Au–(1Br:12Au)/TiO2 catalysts formed
well faceted shapes that resembled truncated cubo-octahedra.
Thus, a physical model of Au nanoparticles as truncated cubo-octahedron, similar to that used by Williams et al. [2], can be used to
determine the percentage Au that are surface, perimeter, and corner sites as a function of Au nanoparticle size distribution according to the equation:
P
sðdÞ
Percentage of site of interest ¼ P
tðdÞ
here s(d) is the number of atoms for a particle of diameter d that
correspond to the site of interest, t(d) is the total number of atoms
for that cluster, and the summation was carried out over all the Au
particles identified from the TEM images of that catalyst. Thus, we
estimate that the Au clusters in 2.3%Au–(1Br:25Au)/TiO2 and
2.3%Au–(1Br:12Au)/TiO2 catalysts after PFR measurements had 1%
and 3% of the total Au atoms as corner sites in contact with support,
3% and 8% as perimeter sites and 25% and 40% as surface sites,
respectively. The Au clusters in 2.3%Au–(1Br:25Au)/TiO2 catalyst
after operando FTIR reactor measurements had 2% of the total Au
atoms as corner sites in contact with support, 6% as perimeter sites
and 35% as surface sites. The 2.3%Au–(1Br:25Au)/TiO2 and 2.3%Au–
(1Br:12Au)/TiO2 catalysts had a number of moles of Br equal to 4%
and 8% of the total moles of Au. Therefore, if we assume that 1 Br
atom blocks 1 Au atom, the 2.3%Au–(1Br:25Au)/TiO2 and 2.3%Au–
(1Br:12Au)/TiO2 catalysts after PFR measurements had sufficient
Br to cover all the perimeter sites and 16% and 20% surface Au
atoms, respectively, whereas the 2.3%Au–(1Br:25Au)/TiO2 catalyst
after operando FTIR reactor measurements had sufficient Br to cover
only 66% of the perimeter sites and 11% of the surface Au atoms. In
the physical model, the corner sites in contact with the support
have Au–Au coordination number of 4, and the perimeter sites in
contact with support include the corner sites and the remaining
perimeter sites that have Au–Au coordination number of 5 [2].
99
These low coordinated Au sites have been shown to bind more
strongly to CO and O as compared to bulk Au [9].
Fig. 2 shows the WGS reaction rate per total mole of Au at
120 °C under 6.8% CO, 21.9% H2O, 8.5% CO2, and 37.4% H2 in PFR
and operando FTIR reactors and the number average Au particle
size for Au/TiO2 and 2.3%Au–(1Br:25Au)/TiO2 catalysts. The WGS
reaction rate per total mole of Au for Au/TiO2 catalysts varies with
the average Au particle size (d) as d2.7±0.1 at 120 °C (Fig. 2) and
correlates with the fraction of corner Au sites (Au–Au coordination
number of four) to the total Au sites that vary as d2.9 in a truncated cubo-octahedral geometry truncated at the midline [2]. The
rate per total mole of Au for 2.3%Au/TiO2 catalyst (prior to addition
of Br) at 120 °C is similar to Au/TiO2 catalysts at the same number,
surface or volume average Au particle size in PFR, and operando
FTIR reactor measurements (Fig. 2). As already mentioned, it is
essential to compare the rates of the brominated Au/TiO2 catalysts
to Au/TiO2 catalysts at the same particle size. The WGS reaction
rate per total mole of Au for 2.3%Au–(1Br:25Au)/TiO2 catalyst at
120 °C under 6.8% CO, 21.9% H2O, 8.5% CO2, and 37.4% H2 is 6
and 2 times lower than for Au/TiO2 catalysts at the same number,
surface or volume average Au particle size in PFR, and operando
FTIR reactor measurements, respectively. The rate for 2.3%Au–
(1Br:12Au)/TiO2 catalyst is undetectable at 120 °C in PFR and operando FTIR measurements.
From the density functional theory (DFT) results provided in the
literature [12], bromine is unlikely to be mobile during the WGS
reaction over Au/TiO2 catalysts because it binds more strongly to
the coordinatively unsaturated Au sites than to the coordinatively
saturated Au sites. The WGS reaction rate for the 2.3% Au/TiO2 and
2.3%Au–(1Br:25Au)/TiO2 catalysts decays by about 15% in the first
20 h under WGS reaction mixture. After this initial deactivation,
the rate remains stable and decays by less than 5% during the kinetic measurements that last for 4–5 days. Thus, based on the
DFT computations and the stability of our catalysts, Br is not mobile during WGS on Au/TiO2 catalysts.
Potassium has been reported [21] to be a promoter of the WGS
reaction rate for Pt/Al2O3 and Pt/SiO2 catalysts. The effect of addition of K on Au/TiO2 catalyst was determined by impregnating a
2.3%Au/TiO2 catalysts with a solution containing 16% moles KNO3
of total moles Au, that is, a 2.3%Au–(1KNO3:6Au)/TiO2 catalyst.
The WGS reaction rate per total mole of Au for 2.3%Au–
(1KNO3:6Au)/TiO2 catalyst, with a number average Au particle size
of 2.7 nm, is 1.8 103 mol H2 (mol Au)1 s1 at 120 °C. From the
dependence of WGS reaction rate per total mole of Au with number
average Au particle size (Fig. 2), a Au/TiO2 catalyst with a number
average Au particle size of 2.7 nm should have a WGS reaction rate
per total mole of Au of 1.5 103 mol H2 (mol Au)1 s1 at 120 °C.
Therefore, K does not promote the WGS reaction rate of Au/TiO2
catalysts.
The apparent activation energies of 2.3%Au/TiO2 and 2.3%Au–
(1Br:25Au)/TiO2 catalysts near 120 °C are similar in PFR measurements at 60 and 57 kJ (mol)1, respectively (Table 1 and Fig. 4) and
operando FTIR reactor measurements at 56 and 52 kJ (mol)1,
respectively (Table 2). The H2O, CO2, CO, and H2 reaction orders
of 2.3%Au/TiO2 and 2.3%Au–(1Br:25Au)/TiO2 catalysts at 120 °C in
PFR measurements are similar at 0.30 and 0.35, 0.10 and
0.05, 0.75 and 0.85, and 0.20 and 0.15, respectively (Table 1
and Fig. 5). Therefore, these catalysts have a different number of
active sites but with the same chemical nature. Thus, the 6 and 2
times decrease in the WGS reaction rate per total mole of Au for
the 2.3%Au–(1Br:25Au)/TiO2 catalyst in PFR and operando FTIR
reactor measurements, respectively, is primarily due to poisoning
of the active sites by Br. Since there is not enough bromine to poison all Au sites and Br may interact with sites on the titania and Au
surface atoms that are not catalytically active, only some active
sites are poisoned by Br while others remain unpoisoned. The
100
M. Shekhar et al. / Journal of Catalysis 293 (2012) 94–102
residual rate of 2.3%Au–(1Br:25Au)/TiO2 catalyst is due to the unpoisoned active sites.
In summary, although the location of the Br on 2.3%Au–
(1Br:25Au)/TiO2 and 2.3%Au–(1Br:12Au)/TiO2 catalysts is unknown, it is established that addition of an amount of Br corresponding to the number of low coordinated perimeter atoms
decreases most of the catalytic activity exhibited by the 2.3%Au/
TiO2 catalyst without altering the apparent activation energies
and reaction orders. Thus, bromine poisons the active sites of the
2.3%Au/TiO2 catalyst, and it is clear that not all surface atoms of
Au exhibit the same rate.
4.1.2. Transient isotopic switch experiments
Fig. 6 shows the mass spectrometer (MS) response and time-resolved FTIR spectra during transient isotopic (12CO to 13CO) switch
experiments of the 2.3%Au–(1Br:25Au)/TiO2 catalyst during WGS
at 120 °C. If it is assumed that the WGS reaction rate is proportional to the coverage of 13C on the surface and that the reaction
pathway is irreversible, a 13C mass balance on the reactor shows
that the area of the envelope made by the Ne (scaled to represent
13
CO) and the 13CO2 MS response curves multiplied by the WGS
reaction rate per total mole of Au is the fraction of the total moles
of Au that are the operating active sites on the catalyst [16,19]. The
operating active sites are defined as the multiplication of all potential active (L) sites and h13C.
From the transient isotopic switch experiments, 0.74 ± 0.10%
and 0.37 ± 0.07% of the total Au atoms were determined by the procedure above to be the amounts of operating active Au sites on
2.3%Au/TiO2 and 2.3%Au–(1Br:25Au)/TiO2 catalyst, respectively
(Table 2). These used catalysts have a similar number, surface,
and volume average Au particle size, 3.1 nm, 3.2 nm and 3.4 nm
for 2.3%Au/TiO2 and 2.9 nm, 3.0 nm and 3.1 nm for 2.3%Au–
(1Br:25Au)/TiO2, respectively, after operando FTIR reactor measurements. Although, during operando FTIR reactor measurements,
the WGS reaction rate per total mole of Au for 2.3%Au–(1Br:25Au)/
TiO2 catalyst is half that of 2.3%Au/TiO2 catalyst (Fig. 2), the turnover frequency (TOF) calculated based on the number of operating
active sites determined by transient isotopic switch experiments
on the 2.3%Au–(1Br:25Au)/TiO2 catalyst is 1.6 ± 0.5 s1 at 120 °C,
which is within experimental error of the equivalent TOF of
2.3%Au/TiO2 catalyst (1.5 ± 0.4 s1) at 120 °C (Table 2). Thus, the
decrease in the WGS reaction rate upon the addition of Br to
2.3%Au/TiO2 catalyst is primarily due to the poisoning (blocking)
of the active sites by adsorbed Br. It is noted that the particle size
distribution for the Au in 2.3%Au–(1Br:25Au)/TiO2 catalyst after
operando FTIR reactor measurements yields corner sites, perimeter
sites, and surface sites equal to 2%, 6%, and 37% of the total number
of Au atoms. Thus, there is not enough Br (at 4% of the total number
of Au atoms) to cover all of the perimeter or surface sites for
2.3%Au–(1Br:25Au)/TiO2 catalyst after the operando FTIR reactor
measurements.
The percentage of Au operating sites determined by transient
isotopic switch experiments was used to estimate the percentage
of potential active sites on the 2.3%Au/TiO2 catalyst by the following reasoning. The coverage of CO (hCO) for Au/Al2O3 catalysts at
120 °C was reported to be 0.35 ± 0.04 of the adsorption of CO on
that catalyst at room temperature in our previous work [22]. The
adsorption strength of CO for Au catalysts has been shown to be
independent of the support by CO TPD [23] and the fact that the
CO order is in the range of 0.8–0.9 for gold on alumina and on titania in WGS kinetic measurements [15]. The CO reaction order on
the 2.3%Au/TiO2 catalyst at 120 °C of 0.85 (Table 1) then indicates
weak adsorption of CO on the surface and is consistent with loss of
2/3rd of the CO coverage when the temperature was increased
from room temperature to 120 °C, as was the case for Au on alumina. The area between the envelope made by the Ne and 13CO
in transient isotopic switch experiments is small (Fig. 6) due to rapid and reversible adsorption of isotopic 13CO and a small uptake
capacity of the catalyst for CO, again in agreement with the weak
adsorption on the Au. We assume further, on the basis of the fact
that operando IR showed no other carbon-containing intermediates
than adsorbed CO, that the adsorbed CO represents an upper bound
on the number of potentially active sites and that CO covers all the
potentially active sites at room temperature. On the basis of the
discussion above, that, at 120 °C, only about 1/3rd of those sites
are occupied, the percentage of the total Au atoms that can be potential active sites on the 2.3%Au/TiO2 catalyst can then be estimated to be the measured number operating sites at 120 °C
(0.74% of the total Au atoms) divided by 1/3, or 2%. Thus, the percentage of the total Au atoms that are potential active sites on the
2.3%Au/TiO2 catalyst best correlates with the percentage of corner
atoms in contact with the support (also 2%) determined from the
truncated cubo-octahedron model. It is noted that less than 10% of
the Au nanoparticles in these catalysts, observed by HR-TEM, possess cubo-octahedral geometry truncated at approximately 3/4th
its height while the remaining are truncated at approximately
the midline. This conclusion regarding the number of sites, however, does not change with the choice of geometry of Au nanoparticles because the fraction of corner sites in contact with the
support to the total Au sites for a cubo-octahedral geometry truncated at 3/4th its height is approximately 3/5th than that for the
cubo-octahedral geometry truncated at the midline. Therefore,
our estimation of corner sites in contact with the support can vary
within a factor of two at most.
It has been argued that the calculated percentage of operating
sites may include contributions from readsorption or secondary
reactions of CO2 [24,25]. Due to these effects, the calculated percentage of operating active sites presented here could be overestimated and can be considered an upper bound. It has been shown in
our previous work [16] that the measured average residence time
of surface intermediates (s) for the 2.3%Au/TiO2 catalyst when cofeeding 8% CO2 was 0.6 ± 0.1 s at 120 °C, which is within experimental error of the calculated s without co-feeding CO2,
0.65 ± 0.05 s. In addition, the calculated s in transient isotopic
(CO2 to 13CO2) switch experiments for 2.3%Au/TiO2 catalyst was
as low as 0.05 s, indicating an insignificant contribution from readsorption of CO2. Further, the similar TOF, calculated based on the
number of operating active sites, for 2.3%Au/TiO2 and 2.3%Au–
(1Br:25Au)/TiO2 catalysts indicates that readsorption or secondary
reactions of CO2 did not significantly affect our results.
To conclude, the TOF calculated based on the number of operating active sites determined on the 2.3%Au/TiO2 and 2.3%Au–
(1Br:25Au)/TiO2 catalysts is similar at 1.6 ± 0.5 s-1at 120 °C, indicating poisoning of the potential active sites by Br. The estimated
potential active sites (2% of the total number of Au atoms) on
the 2.3%Au/TiO2 catalyst best correlates with the corner atoms of
the cubo-octahedral particles being active.
4.1.3. Operando FTIR spectroscopy
Fig. 7 shows the CO region of the steady-state operando FTIR
spectra of the 2.3%Au/TiO2, 2.3%Au–(1Br:25Au)/TiO2 and 2.3%Au–
(1Br:12Au)/TiO2 catalysts at 200 °C. The IR peaks observed near
2100 cm1 and near 2030 cm1 are assigned in the literature to
CO adsorbed on Au0 and Aud, respectively [20]. The normalized
area of the IR peak observed near 2100 cm1 was found to correlate
with the WGS reaction rate on Au/TiO2 catalysts with different
average Au particle sizes [2], during deactivation of Au/Al2O3 [15]
and Au/CeZrO4 [20] catalysts. Fig. 8 shows that the normalized
peak area of CO adsorbed on Au0 near 2100 cm1 determined by
operando FTIR passes through the origin and varies linearly with
the steady-state WGS rate at 200 °C on 2.3%Au/TiO2, 2.3%Au–
(1Br:25Au)/TiO2 and 2.3%Au–(1Br:12Au)/TiO2 catalysts. The nor-
Rate / 10-2 (mol H2) (mol Au)-1 s -1
M. Shekhar et al. / Journal of Catalysis 293 (2012) 94–102
16
CO on Au0
Series1
14
2.3%Au/TiO2
CO on AuδSeries2
12
Total CO adsorbed
Series3
10
2.3%Au-(1Br:25Au)/TiO2
8
6
4
2
2.3%Au-(1Br:12Au)/TiO2
0
0
20
40
60
80
100
120
140
Normalized CO peak areas from operando IR / %
Fig. 8. WGS reaction rate per total mole of Au versus normalized FTIR peak area of
CO adsorbed on Au0, Aud, and total CO adsorbed during operando FTIR reactor
measurements on 2.3%Au/TiO2, 2.3%Au–(1Br:25Au)/TiO2 and 2.3%Au–(1Br:12Au)/
TiO2 catalysts at 200 °C.
malized peak area of CO adsorbed on Aud near 2030 cm1 and the
total CO adsorbed also vary linearly with the rate but do not pass
through the origin. The normalized peak areas were calculated
by assigning the peak areas of the 2.3%Au/TiO2 catalyst as 100%.
Thus, the rate correlates with the concentration of CO on Au0 indicating that the active Au sites, viz. the low coordinated corner Au
sites, are metallic in nature.
For the 2.3%Au–(1Br:12Au)/TiO2 catalyst, the WGS reaction rate
was undetectable and only 5% of the original CO adsorbed on Au0
remained. Therefore, the assumption that at 200 °C, all of the CO
adsorption on the 2.3%Au/TiO2 catalyst occurs on the active low
coordinated corner Au atoms is good to within 5%. We note further
that the decrease in area of the 2100 cm1 CO peak to 50% for the
(1Br:25Au) sample versus the unpoisoned sample is in good agreement with the 50% decrease in the number of operating active sites
measured in the isotope switch experiment. In addition, the CO
vibrational frequency does not vary in the operando FTIR measurements on the poisoned and unpoisoned catalysts. This constant frequency with changes in CO coverage indicates the absence of
dipole–dipole interactions for CO on these catalysts. This result
could be related to the low coverage of CO on these catalysts and
that CO molecules are adsorbed on sites separated from each other,
as would be expected for the corner atom sites.
Counting active sites can depend on how each reactant is
accommodated on the surface of the catalyst. However, for our
specific case, that is, the WGS reaction over Au/TiO2 catalysts, it
is shown from Figs. 7 and 8 that the WGS reaction rate scales linearly with the CO adsorbed on Au0, and therefore, since the coordinatively unsaturated corner atoms are the rate controlling active
sites, the counting of such sites is not affected by the accommodation of H2O on the surface. The analysis of the reaction orders for
the catalysts reported here shows that H2O is more strongly bound
to the surface than CO because the apparent H2O order is 0.3
compared to the CO order that is 0.8. Negative H2O order implies
a high relative surface coverage of hydroxyl species such as H2O,
OH, and O. Since we have already shown that the rate is proportional the number of Au corner atoms, this information that the
water activation sites are highly covered shows that water activation sites do not become the rate controlling site population over
our range of operating conditions. Thus, while the water mechanism is an interesting and still unanswered question, whether
the water is competitive with CO on the Au sites or resides on
101
TiO2 sites proximate to the Au does not change the fact that the
rate is proportional to the number of coordinatively unsaturated
Au species.
In our work here, we conclude that the WGS reaction rates are
proportional to the number of metallic corner Au sites. However,
based on the data presented here, we cannot draw a precise model
for TiO2 showing sites responsible for water activation. In a recently published paper from our group [15] and DFT calculations
in the literature [26], the support sites are important for H2O
adsorption and dissociation over Au/TiO2 catalysts. The apparent
H2O order varies with the support and correlates with the reaction
rate for Au/TiO2, Au/Al2O3 and Au/Al2O3-World Gold Council
(WGC) catalysts [15]. The activation barrier for dissociation of
water on Au(1 0 0) is 1.5 eV [27], on Au29 nanoparticle is 1.3 eV
[27] and on TiO2–Au interface of a TiO2/Au(1 1 1) system is 0.6 eV
[26]. We note, however, that the dependence of the rate on Au particle size and on the amount of CO adsorbed on coordinatively
unsaturated Au, together with the low water order that indicates
high coverage of water activation sites, indicate that the Au corner
atoms are the sites that control the rate over our entire operating
space and that this conclusion is, therefore, not affected by the
mechanism of water activation.
5. Conclusions
The addition of Br at a level of 4% of the total moles of Au to a
2.3%Au/TiO2 catalyst decreased its WGS rate by six times in PFR
and 2 times in operando FTIR reactor measurements as compared
to Au/TiO2 catalysts at the same number, surface or volume average Au particle size. The addition of Br did not result in an appreciable change in the apparent activation energy or the reaction
orders. Operando FTIR and transient isotopic experiments together
with the Br poisoning results were used to confirm that the dominant active sites on Au/TiO2 catalysts for the WGS reaction are
the low coordinated metallic corner Au sites. The TOF for Au/TiO2
catalysts, based on the operating active sites (corner Au atoms),
determined from transient isotopic switch experiments under
6.8% CO, 8.5% CO2, 11.0% H2O, 37.4% H2, and atmospheric pressure,
was determined to be 1.6 ± 0.5 s1 at 120 °C.
The low coordinated corner Au sites are present due to the truncated cubo-octahedra geometry of the Au nanoparticles. For Au/
TiO2 catalysts with an average Au particle size of 3 nm or above,
less than 1 percent of the total Au atoms in the Au nanoparticles
are responsible for most of the catalytic activity. Although these
conclusions pertain to the WGS catalysis over supported Au nanoparticles, this knowledge can be extended to develop a twofold
strategy to further improve the catalytic rate for Au catalysts for
reactions catalyzed by Au. First, it is important to increase the percentage of low coordinated Au atoms in Au nanoparticles by synthesizing them either with smaller size or with different
nanoparticle shapes. Secondly, from the Sabatier principle, developing supported Au species with Au–Au coordination less than 4
is needed to determine the optimum catalytic rates exhibited by
supported Au nanoparticles.
Acknowledgments
Support for this research was provided by the U.S. Department
of Energy, Office of Basic Energy Sciences, through the Catalysis
Science Grant No. DE-FG02-03ER15466. The authors would like
to thank Mr. Leonardo Maciel for his help in conducting the WGS
kinetic experiments. E.A.S. acknowledges additional support
through the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886.
102
M. Shekhar et al. / Journal of Catalysis 293 (2012) 94–102
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.jcat.2012.06.008.
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