www.sciencemag.org/cgi/content/full/333/6043/736/DC1
Supporting Online Material for
Spectroscopic Observation of Dual Catalytic Sites During Oxidation of
CO on a Au/TiO2 Catalyst
Isabel Xiaoye Green, Wenjie Tang, Matthew Neurock, John T. Yates, Jr.*
*To whom correspondence should be addressed. E-mail: [email protected]
Published 5 August 2011, Science 333, 736 (2011)
DOI: 10.1126/science.1207272
This PDF file includes:
Materials and Methods
SOM Text
Figs. S1 to S12
Table S1
References
Supporting Online Material
for
Spectroscopic Identification of Dual Catalytic Sites during
CO Oxidation on a Au/TiO2 Catalyst
Isabel Xiaoye Green, Wenjie Tang, Matthew Neurock, and John T. Yates, Jr.
I.
Table of Contents:
Supporting Text -- Background
II.
Experimental Materials and Methods.
III.
Model System and Calculation Details.
IV.
Supporting Text -- Additional Calculation Results.
V.
References for the Supporting Online Material
VI.
Supporting Figures.
VII. Supporting Tables.
I. Background
Determining the atomic and electronic structure of the active site in heterogeneous
catalysis is a unifying theme for almost all work in the field (31-35). Understanding the
cooperative behavior between dual catalytic centers comprised of distinguishable active
sites is particularly challenging due to the complexity in resolving the mechanistic role of
each site and their interactions under catalytic reaction conditions. For the Au/TiO2
catalyst, some have proposed that the novel catalytic activity originates predominantly
from nanometer size Au particles, and that the role of the oxide support is to stabilize
these particles. Highly dispersed Au clusters thus provide a large fraction of active
coordinatively unsaturated Au surface sites and can also possibly exhibit quantum size
effects (21, 22, 26, 36). Others have proposed that the electronic interaction between the
Au nanoparticles and the underlying support is the key feature since cationic Au is
thought to be more active for the CO oxidation reaction (37, 38). Two atom thick Au
films, supported on macroscopic-width thin TiOx films, where no perimeter interface
between materials is present, have been reported to show high relative activity supporting
the idea of a favorable electronic interaction between Au and TiO2 (4). Considering the
known weak interaction between Au and O2, it was also proposed that the support may
enhance the catalytic reaction by localizing O2 or O atoms at Au perimeter sites (6, 9,
39). Several experiments carried out with inverse catalysts demonstrated that the oxides
might be more important than just supporting the Au or enhancing the reaction on low
coordination number (CN) Au sites, by being involved directly in the catalytic reaction:
inactive Au single crystals or nano-sized Au powders become active when they come in
contact with particular oxide supports such as TiOx or CeOx (5, 8, 40). The active sites
for the catalytic reaction are therefore postulated in this view to be at the Au/oxide
interface. A number of theoretical studies have also been reported which help to elucidate
the CO oxidation pathways and mechanisms over the supported Au catalyst (24, 41), and
specifically examine the perimeter sites (10-14, 42).
II. Experimental Materials and Methods.
In our experimental setup, a stainless steel high vacuum transmission infrared cell
with a base pressure of 1 × 10-8 Torr after bakeout is employed. A schematic drawing of
the cell and the design of the sample holder placed inside the cell is shown in Figure S1.
The high vacuum in the cell is maintained by a 55 L/s (N2) turbomolecular pump, which
can be isolated from the cell by a gate valve. A MKS I-MAG cold cathode ionization
gauge for low pressure reading (10-10-10-3 Torr) and a MKS Baratron capacitance
manometer for high pressure reading (0.001-1000 Torr) are connected to the cell. The
cell has two differentially pumped KBr windows that allow the transmission of a
focussed IR beam through the center of the cell, as shown in Figure S1A. A quadrupole
mass spectrometer (QMS) is connected to the cell via a gate valve for gas analysis. The
Au/TiO2 catalyst and a reference sample of pure powdered TiO2 are pressed separately
into 7 mm spots, one directly above another, on a tungsten mesh support, as shown in
Figure S1B. Each sample spot is pressed with 33,500 psi pressure and holds
approximately 0.005 g of material. The W-mesh is 0.051 cm thick with 1370 window
apertures (0.022 cm × 0.022 cm) per cm2 that allows ~70% IR transmission. The W-mesh
plus pressed samples is then mounted by a pair of nickel clamps that are connected to the
copper rods from a reentrant Dewar for fast electrical heating and cooling. Since the Wmesh mounted this way generates no vertical temperature gradient upon heating and
cooling (43), a type K thermocouple providing 0.1 K temperature resolution is welded on
the top center of the W-grid, at the same vertical position as the samples’ centers. The
sample temperature can be controlled accurately to ± 0.1 K between 90 and 1000 K, via a
combination of resistive heating and l-N2 cooling. The sample holder is arranged so that
the samples are placed in the center of the chamber to fully intercept the IR beam. Both
samples can be examined by IR in one experiment by moving the z-direction manipulator
up and down.
The protocol for Au deposition-precipitation with urea on TiO2 is adopted from
Zanella et al. (44). Commercial TiO2 P25 powder (49 m2 g-1) is provided by Evonic
Industries (previously Degussa). Urea (99.5%) and hydrogen tetrachloroaurate (III)
trihydrate (HAuCl4⋅3H2O, Au precursor) were purchased from Acros. Before deposition,
the TiO2 powder was dried and activated at 373 K in a 100 mL/min air flow for at least
24 hours. The rest of the preparation process was carried out in the absence of light, due
to the known fact that the gold precursor may decompose under illumination. A gold
precursor aqueous solution was made (4.2 × 10-3 M HAuCl4 and 4.2 M urea). One gram
of activated TiO2 powder was mixed with 100 mL of the gold precursor solution and
reacted at 353 K with vigorous stirring for 8 hours. The pH of the reaction mixture
increased from ~2 at the beginning of reaction to ~7 at the end. The Au/TiO2 powder was
separated from the suspension by centrifuging at 3800 rpm for 20 min. To wash off the
residual Cl- anion, which is suspected to compete with O2 for active sites during CO
oxidation (10), the Au/TiO2 powder was washed by 5 cycles of vigorous stirring in 100
mL double-deionized water at 323 K, followed by centrifugation at 3800 rpm for 20 min.
Auger spectra, measured in a separate UHV system, show no Cl- contamination on the
samples after washing. The powder was then dried at 373 K for over 24 hours under 100
mL/min air flow. The theoretical Au loading on the catalyst is 8 wt%.
After the samples were installed in the vacuum cell and before initial use, they were
heated to 473 K with a temperature ramping rate of 0.2 K/s and were held at 473 K in
vacuum for 2.5 hours. Then 20 Torr of O2 was introduced into the cell at 473 K,
remaining for 1 hour. After pumping the cell down to high vacuum, the samples were
held at 473 K for another 0.5 hour. The goal of the heating in vacuum and in O2 is to
remove most of the hydrocarbon impurities from the sample surfaces. Complete removal
of all impurities including hydroxyl groups requires higher treatment temperature which
is known to cause Au particle agglomeration and thus was not used. Between each
separate experiment, heating at 473 K in vacuum for 20 min was performed to bring the
samples back to the original condition.
Transmission electron microscope (TEM) measurements were carried out on a
Au/TiO2 sample that has been treated as described above. Data were collected on a JEOL
2000FXII microscope with 200 kV acceleration voltage. The average Au particle size
was ~3 nm. A TEM image is shown in Figure S2A, along with a histogram of measured
Au particle sizes.
The IR spectra were recorded with a Bruker TENSOR 27 FTIR spectrometer
equipped with a l-N2 cooled MCT detector. The spectrometer, the detector, and the entire
IR beam pathway are purged with CO2- and H2O- free air constantly. In order to
eliminate the influence of the W-grid and the gas phase composition, a reference
spectrum taken through the empty region of the W-grid and the gas phase (vacuum or
reaction mixture) is subtracted from every spectrum recorded through the samples under
the same condition. Each spectrum is obtained by averaging 512 interferograms at 2 cm-1
resolution which takes about 203 s.
Figure S3 shows the IR spectra of CO interacting with the reference TiO2 sample,
both in vacuum and in oxygen. Figure S4 shows kinetic plots of the rate of formation of
CO2(a)/TiO2 from 110 K to 130 K, fitted to first-order kinetics, to complement Figure 1C
which shows the kinetics of CO(a)/TiO2 consumption. An Arrhenius plot in the inset
shows that the kinetics exhibit an apparent activation energy of 0.16 ± 0.01 eV, in
agreement with the measurements in Figure 1C.
Figure S5 shows CO diffusion studies on both TiO2 and Au sites at temperatures from
140 to 160 K. By holding the CO-covered Au/TiO2 sample at elevated temperatures (140
to 160 K) in vacuum, we observed the surface diffusion/desorption of CO by IR. Here
CO molecules present in the interior of the powdered samples as CO/TiO2 and CO/Au
surface species escaped by a combination of surface diffusion and desorption processes,
as observed by this IR method which has also been used for surface diffusion
investigations of other molecules adsorbed on similar high surface area powdered
samples (25, 45). The catalyst surface was saturated with CO at 120 K and then
evacuated for 1 hour before beginning the kinetics study at 140 K or above. For ease of
comparison, the diffusion/desorption studies were all normalized to the saturation point at
120 K. The CO diffusion/desorption on TiO2 was more rapid at all temperatures than on
Au. At 160 K, after 3 hours, 70% of the CO on the TiO2 surface desorbed while only
20% of the CO has desorbed from the Au surface.
III. Model System and Calculation Details.
To simulate the ~3 nm diameter Au particles supported by TiO2 system, we used a Au
nanorod structure which has a height of 3-atomic layers and is 3-atoms-wide covalently
bonded on top of a rutile TiO2(110) slab (10, 11), as shown in Figures S6A and S6B. This
structure is similar to that used in previously reported DFT calculations (3, 4). The Au
nanorod structure was chosen herein to model the sites on the Au particles supported on
TiO2 due to its computational tractability. It appropriately mimics the terrace and edge
sites as well as the Ti sites located at the Au/TiO2 interface as well as within the local
vicinity. The model, however, is an oversimplification of the nm-sized Au clusters on
TiO2 as there are no corner sites. These sites are likely to be somewhat more reactive for
CO oxidation due to their ability adsorb and activate O2. As such the Au-nanorod
structure will somewhat underestimate the actual reactivity.
Two unit cells comprised of four O-Ti-O tri-layers were used to model the rutile
TiO2(110) surface. All of the simulations reported where carried out using a (2×3) unit
cell to simulate CO adsorption and diffusion and the larger (4×3) unit cell to model CO
oxidation. The atoms in the top two trilayers of the TiO2 slab were allowed to fully relax
while the atoms in the bottom two trilayers of the TiO2 slab were fixed to their lattice
positions for both unit cells. All of the Au atoms were allowed to relax in the Zdirection.
All of the calculations reported herein were carried out using plane wave gradientcorrected density functional theory (DFT) calculations as implemented in the Vienna ab
initio Simulation Package (VASP) (46, 47). The wavefunction was expanded in a planewave basis set with energy cutoff of 400 eV. The PW91 gradient approximation (GGA)
functional was used to describe the exchange-correlation energy (48). The core-electronic
states were described by pseudopotentials constructed with the projector augmentedwave (PAW) method (49, 50). The DFT+U method was implemented in our calculations
to correct on-site Coulomb interactions (51). The value of U was chosen to be 4.0 eV
because it can generate similar electronic structure that was experimentally observed
(52). Spin-polarization was considered for all calculations and was used when necessary.
A vacuum gap of 10 Å was used in the z- direction between slabs. The (2×3) and (4×3)
unit cells were sampled with (2×2×1) and (1×2×1) k-point mesh, respectively (53).
Geometries were optimized until the force on each atom was less than 0.03 eV/Å.
The reaction pathway and activation barriers were found by the nudged elastic band
(NEB) method with image climbing (54, 55), combined with the dimer method to isolate
transition states (56). The NEB method was used to follow the path between the reactant
and product states to the point where the perpendicular forces on all of the images along
the band were lower than 0.1 eV/Å. The dimer method was subsequently used to isolate
the transition state to the point where the force acting on the transition state dimer was
lower than 0.03 eV/ Å.
The charge density difference, ρdiff , on the TiO2 surface before and after Au
deposition was calculated following Equation 1.
ρ diff = ρ (Au /TiO2 ) − ρ (TiO2 ) − ρ (Au)
Eq. 1
The electronic charge difference at the perimeter site is plotted in Figure S6C, showing
that negative charge accumulates on the Ti5c site at the perimeter after Au deposition.
This change in charge is responsible for the strong adsorption and activation of O2 at
these sites (7, 13, 17, 57-59).
IV. Additional Calculation Results.
A. CO Binding on Au/TiO2.
The calculated adsorption energies for CO at different sites at the Au/TiO2 interface
are shown in Figure S7. The adsorption energy of CO at the Ti5c site of TiO2 adjacent to
adsorbed Au was calculated to be -0.41 eV as shown in Figure S7A. For the adsorption of
CO on Au, the CO adsorption energy varies with the CN of the Au atoms to which the
CO is bound. The Au atoms examined are shown from the cross section view of the
nano-rod and labeled sites 1-6 on Figure S7A for easy identification. In general, and as
expected, CO adsorbs more strongly on low CN Au sites compared to high CN Au sites
with the exception of sites 1 and 6. These two sites are attached to the TiO2 support
directly, and exhibit minimal interaction with CO molecules due to the electronic effects
and the local structure. The calculations show weak CO binding with an adsorption
energy of -0.26 eV at site 1 and no binding at site 6. The adsorption energy for CO at the
terrace sites (CN=9) was calculated to be -0.2 eV, which also indicates a very weak
interaction. This is consistent with the experimental observation that CO does not
chemisorb on Au terrace sites even at low temperatures as 25 K (7, 17, 57-59). For Au
atoms with CN = 7, the CO binding energies ranges from -0.67 eV to -0.89 eV, as shown
in Figures S7D-F. These sites are likely to be the CO adsorption sites on Au.
B. O2 Activation at Different Sites.
Density functional theory calculations carried out on the adsorption of O2 on the ideal
rutile TiO2(110) surface revealed that O2 does not adsorb on these surfaces which is
consistent with experimental results (60). This lack of adsorption indicates that it would
be very difficult to dissociate O2 directly over the ideal TiO2(110) surface, and as such,
this was not considered. The adsorption of Au on TiO2 leads to a significant charge
transfer from the Au to the neighboring Ti5c cation as is shown in Figure S6C. The charge
transfer along with the unique di-σ binding of O2 at the perimeter Ti-Au site significantly
enhances its adsorption (Figure S8) and promotes the oxidation of CO (11, 13). A
thorough search for the adsorption of O2 at all of the sites along the Au/TiO2 perimeter
and on the Au nano-rod was carried out to help identify the low barrier O2 activation
sites. The co-adsorption of CO at neighboring sites was found to help facilitate the
activation of O2 either indirectly via through-surface electronic interactions or directly by
the formation of a reactive O*-O-C*=O intermediate (where * denotes the atoms bound
to the surface), as was recently reported for CO oxidation on CO covered Pt particles
(61). The results for the coadsorption of CO and O2 and the activation of O2 are
summarized in Figure S9.
The 5 possible sites investigated for O2 activation are: A) the bridge oxygen O
vacancy site on the TiO2 perimeter; B) the Ti5c sites at the perimeter; C) the Au atoms on
the nanorod at the TiO2 interface; D) the Ti5c-Au perimeter site for the CO-O2
mechanisms; and E) the Ti5c-Au perimeter site for the direct O2 dissociation mechanism.
The initial, transition (TS), and the final states for O2 activation, together with the energy
required to break the O-O bond are reported for all of the adsorption states in Figure S9.
The configurations shown in Figures S9B-D were found to follow the CO-O2 bimolecular
mechanism, while the remaining configurations involve direct O-O bond scission. The
activation barriers reported for the CO-O2 bimolecular dissociation steps on the oxide
alone (0.65 eV -Figure S9B) and the metal alone (0.7 eV - Figure S9C) were found to be
much higher than the other three paths.
While the direct activation of O2 at an O vacancy site adjunct to Ti5c has the lowest
barrier (0.12 eV Figure S9A), it is very unlikely to be catalytically active, as one of the
dissociated O atoms will fill this site eliminating it from the catalytic cycle. The most
active sites therefore appear to occur at the Ti5c-Au perimeter sites. The bimolecular COO2 mechanism for the dissociation of O2 has a barrier of only 0.16 eV (Figure S9D)
which is less than half that required for the direct O2 dissociation (0.39 eV – Figure S9E).
The results in Figure S9 clearly indicate that O2 activation and CO oxidation proceed at
the dual Ti5c-Au perimeter site via a CO-assisted path. This is the only site likely to be
able to contribute to the oxidation at temperatures as low as 110 K in the experiments.
C. Supply of the reactants to the active sites.
The oxidation of CO by O2 subsequently results in the formation CO2(g) and an
adsorbed oxygen atom which resides either at the Au or Ti5c site. For the next step to
proceed, a second CO molecule must diffuse to the adsorbed oxygen atom or the
adsorbed oxygen atom must diffuse to a bound CO. The calculations reported in Figures
S10-S12 provide insights into the mobility of CO and O at the Au/TiO2 interface and may
help to explain the IR observation of the selective oxidation of CO/TiO2.
Figure S10 shows the calculated barriers for the diffusion of atomic oxygen on Au
and TiO2 to be 0.65 eV and 0.75 eV, respectively. These high diffusion barriers indicate
that it is unlikely for the atomic O to diffuse away from either type of adsorption site.
Figure S11 displays the possible diffusion barriers for CO on TiO2. CO can diffuse on the
TiO2 surface along paths that connect surface Ti sites. The activation energies for the CO
diffusion paths on TiO2 identified in Figure S11 (including a pathway through an O
vacancy), were all calculated to be within the range of 0.28-0.38 eV. At O2-rich
conditions there are likely very few O vacancies present and as such the calculated
diffusion barrier for CO/TiO2 should be close to 0.33 eV. This result agrees with the 0.35
eV CO/TiO2 diffusion energy calculated by Zhao et al. (62). Finally, Figure S12 shows
the possible diffusion paths for CO along the Au nano-rod. As was discussed above, CO
only chemisorbs on the low CN Au sites. The calculated barriers for CO diffusion
indicate that CO can diffuse with barriers of 0.1 eV (from site 1) and 0.26 eV (from site
2) to the strongest adsorption site (site 3). CO becomes trapped at this site as the barriers
to diffuse to the site 1 and 2, were calculated to be 0.73 (Figure S12A) and 0.50 eV
(Figure S12B), respectively. At low temperatures CO is therefore likely localized at these
Au sites and can not diffuse to the Au/TiO2 perimeter sites.
D. The Effect of CO Coverage
DFT calculations were carried out for all of the elementary steps shown in Figure 3 at
CO coverages of θCO =1/3 and 2/3 ML CO on TiO2 to establish the influence of coverage
on the activation and overall reaction energies. The results reported in Table S1 indicate
that the changes in the calculated barriers and reaction energies in moving from low
coverage to high coverage are less than 0.05 eV. The small changes in the activation
barriers and reaction energies with coverage are typical of reactions on oxides as there are
few lateral interactions between adsorbates as they are much further apart (63).
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VI. Supporting Figures.
Figure S1. Schematic drawings of the high vacuum system for transmission IR studies. A.
Top view of the IR cell. B. A closeup view of the sample holder.
Figure S2. A. TEM image of the Au/TiO2 catalyst. Image was taken after all experiments
are done. B. Plot of the Au particle size distribution.
Figure S3. FTIR spectra of (a) CO(a)/TiO2 in 0.060 Torr of CO at 120 K; (b) CO(a)/TiO2
in vacuum at 120 K; and (c) CO(a)/TiO2 in 1 Torr of O2 for 120 min at 120 K. The CO2
intensity increase is caused by impurity re-adsorption from the cell rather than CO
oxidation reaction, judging from the stable intensity of CO(a)/TiO2.
Figure S4. Plots of the integrated absorbance of CO2 at different temperatures (110-130
K) against time for the kinetics study of CO2 generation over the Au/TiO2 catalyst.
Smooth curves under the experimental data points are the best first-order fit. The inset
shows an Arrhenius plot of the kinetics which results in an activation energy of 0.16 ±
0.01 eV.
Figure S5. Plots of normalized integrated absorbance of CO/TiO2 (left) and CO/Au
(right) against time at various temperatures showing the diffusion/desorption processes of
CO from the two surfaces, where CO/TiO2 diffuses and desorbs more rapidly than
CO/Au.
Figure S6. A. Top view and B. side view of the 3-atomic-layer Au nano rod on rutile
TiO2 (110) surface model. Au atoms are yellow. Ti atoms are light grey. O atoms in TiO2
lattice are pink. C. Charge density difference on the Ti5c sites noted by the dotted
rectangle in B before and after Au deposition. Red regions exhibit the highest electronic
charge.
Figure S7. CO adsorption energies and configurations on different Au sites or on the Ti5c
sites. The distances between the CO and the adsorption site are noted. C atoms are black.
O atoms in CO are red.
Figure S8. O2 adsorption energy at different Au sites.
Figure S9. The initial, transition and final states for the adsorption and activation of O2
along with the corresponding activation energies for the dissociation of the O-O bond at 5
different sites: A) O-vacancy on the TiO2 support; B) Ti5c sites at the perimeter; C) two
Au atoms on the periphery of the Au nanorod ; D) Ti5c-Au perimeter site via CO-O2
mechanism; E. Ti5c-Au perimeter site via direct O2 dissociation mechanism. Oxygen
atoms in CO and O2 molecule are red. Hydrogen atoms are cyan.
Figure S10. Atomic O diffusion barrier on Au (A) and along Ti5c row (B).
Figure S11. CO diffusion barriers on TiO2. The position of the O-vacancy under
consideration is highlighted by a green circle.
Figure S12. CO diffusion barriers on Au sites.
VII. Supporting Tables.
________________________________________________________________________
Ea(θCO=2/3 ML) eV
Elementary Step
Ea (θCO=1/3 ML) eV
CO(a) diffusion along TiO2 surface
0.33
0.26
0.16 (-0.28)
0.13 (-0.32)
CO(a) + O2(a) -> O-C(a)-O-O(a)
O-C(a)-O-O(a) -> CO2(g) + O(a)
0.10 (-3.68)
0.10 (-3.66)
CO(a) + O(a) -> CO2(g) + 2a
0.11 (-1.23)
0.11 (-1.20)
CO2(a) -> CO2(g) + a
0.20
0.20
________________________________________________________________________
Table S1. The effects of coverage on the activation and overall reaction energies shown
in ( ) for elementary CO and O2 adsorption, CO oxidation, CO diffusion, and CO2
desorption steps. DFT-calculated activation energies and for overall reaction energies for
1/3 ML and 2/3 ML of CO on TiO2.