Low pressure CO2 hydrogenation to methanol over gold nanoparticles activated on a CeOx/TiO2 Interface

Article abstract of DOI:10.1021/jacs.5b06150

Capture and recycling of CO₂ into valuable chemicals such as alcohols could help mitigate its emissions into the atmosphere. Due to its inert nature, the activation of CO₂ is a critical step in improving the overall reaction kinetics during its chemical conversion. Although pure gold is an inert noble metal and cannot catalyze hydrogenation reactions, it can be activated when deposited as nanoparticles on the appropriate oxide support. In this combined experimental and theoretical study, it is shown that an electronic polarization at the metal-oxide interface of Au nanoparticles anchored and stabilized on a CeO_x/TiO₂ substrate generates active centers for CO₂ adsorption and its low pressure hydrogenation, leading to a higher selectivity toward methanol. This study illustrates the importance of localized electronic properties and structure in catalysis for achieving higher alcohol selectivity from CO₂ hydrogenation.

Full text of DOI:10.1021/jacs.5b06150

Page 1 of 15
Journal of the American Chemical Society
1
2
3
4
5
6
Low pressure CO hydrogenation to methanol over gold nanoparti-
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
2
cles activated on a CeO /TiO interface
x
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
1
1
2
3
Xiaofang Yang, Shyam Kattel, Sanjaya D. Senanayake, J. Anibal Boscoboinik, Xiaowa Nie,
4
1
1
1,*
1,5,
Jesús Graciani, José A. Rodriguez, Ping Liu, Darío J. Stacchiola and Jingguang G. Chen *
1
Chemistry Department, Brookhaven National Laboratory, Upton, NY 11973
2
Center for Functional Nanomaterials (CFN), Brookhaven National Laboratory, Upton, NY 11973
3
Dalian University of Technology, Dalian 116024, Liaoning Province, China
4
Department of Physical Chemistry, University of Seville, Eꢀ41012, Seville, Spain
5
Department of Chemical Engineering, Columbia University, NY 10027
the reduced states of oxides can greatly impact their surface
chemistry and catalytic activity for CO2 activation. Previous
ABSTRACT: Capture and recycling of CO into valuable chemꢀ
icals such as alcohols could help mitigate its emissions into the
2
studies of the CeO /TiO system have shown that at small coverꢀ
x
2
atmosphere. Due to its inert nature, the activation of CO2 is a
critical step in improving the overall reaction kinetics during its
chemical conversion. Although pure gold is an inert noble metal
and cannot catalyze hydrogenation reactions, it can be activated
when deposited as nanoparticles on the appropriate oxide support.
In this combined experimental and theoretical study, it is shown
that an electronic polarization at the metalꢀoxide interface of Au
nanoparticles anchored and stabilized on a CeO /TiO substrate
3+
ages of ceria, the CeOx nanoparticles at TiO (110) favor Ce
2
12,15
3+
cations.
Meanwhile, the Ce sites interact extensively with
admetals (Pt, Cu and Au) through electronic metal–support interꢀ
actions, causing high dispersion of the active metals and changing
1
4
their chemical activity.
While bulk gold is catalytically inert, Au nanoparticles can be
very active when deposited on oxides.
1
6ꢀ21
. The formation of
x
2
generates active centers for CO adsorption and its low pressure
multifunctional active sites at the metal/oxide interface can impact
the activity and selectivity of catalytic reactions. An example of
the highly important role of the interfacial sites can be seen from a
2
hydrogenation, leading to a higher selectivity towards methanol.
This study illustrates the importance of localized electronic propꢀ
erties and structure in catalysis for achieving higher alcohol selecꢀ
2
2
recent study of CO
hydrogenation over CeO
/Cu(111). The
2
x
tivity from CO hydrogenation.
interfacial sites between CeO
x
and Cu provide a unique capability
2
δꢀ
to stabilize a carboxylate (CO2 ) intermediate and the subsequent
hydrogenation steps also become facile and are characterized by
relatively small activation energies (<0.6 eV). In the study of
A rising CO concentration in the atmosphere has led to conꢀ
cerns about adverse global climate changes and ocean acidificaꢀ
tion. A potential way to alleviate this problem is to capture and
2
1
CO hydrogenation over oxideꢀsupported Au catalysts, the nature
2
of the supporting oxides greatly impacts the overall CO converꢀ
sion and the selectivity to methanol. For example, in comparing
the performance of Au/CeO2 and Au/TiO , Haruta found that
2
convert a fraction of the emitted CO into inexpensive and readily
2
2
,3
available feedstock to produce chemicals or fuels. For instance,
2
a number of valuable chemicals can be produced from CO , inꢀ
2
while Au/TiO was more active than Au/CeO , the later exhibited
2
2
cluding shortꢀchain olefins (ethylene and propylene), syngas (CO
and H , coꢀfed with methane), formic acid, methanol, dimethyl
ether, and other hydrocarbons. Two of the most attractive routes
involve reaction with H , generated from renewable sources, to
convert CO into CO through the reverse water gas shift (RWGS)
reaction and to directly synthesize methanol through further CO
18
higher selectivity in hydrogenating CO to methanol. In contrast
2
2
to the wellꢀstudied oxidation reactions on goldꢀbased catalysts, the
role of Au in hydrogenation reactions has not been clarified. Here
we report the synergistic effect of mixed CeO and TiO catalysts
2
2
2
2
4
on the stabilization and activation of Au for the selective hydroꢀ
genation of CO to methanol, which occurs at very low pressures
1
,5,6
2
hydrogenation.
instead of the several atmospheres of hydrogen required as reportꢀ
ed in the literature. Surprisingly, Au is promoted to have similar
Due to the high thermodynamic stability of CO , splitting of a
CꢀO bond in the molecule is characterized by a high energy barriꢀ
17
2
activity and better selectivity toward methanol than the traditionꢀ
er. Thus, effective activation of CO is a critical step in improving
22
2
ally used Cu catalyst. More importantly, the combined APꢀXPS
the overall reaction kinetics of the process. It has been proposed
analysis and DFT calculations allow us to resolve details in the
link between the unique surface electronic properties and the CO2
activation mechanism at an active goldꢀoxide interface.
that activation of CO occurs at the oxide support or the interfacial
2
7
sites between the active metal and the oxide support. Here, we
will explore in detail the interaction of CO with Au nanoparticles
2
Deposition of ceria on TiO (110) results in the formation of
2
supported on CeO /TiO using a combination of ambientꢀpressure
x
2
well dispersed ceria dimers, where all the cerium is present as
Xꢀray photoelectron spectroscopy (APꢀXPS) and calculations
based on density functional theory (DFT). Metal oxides form a
major category of active support materials and their capability to
3+ 12,14
Ce .
face before and after deposition of Au. Au nucleates preferentially
on defects of TiO (110), but the high dispersion of ceria nanoclusꢀ
Figure 1 shows STM images of a CeO /TiO (110) surꢀ
x 2
8ꢀ14
2
activate CO largely depends on their basicity and reducibility.
2
ters makes Au/CeO interfaces abundant through the surface.
x
Reduced oxides have a strong tendency to react with CO or H O,
2
2
Previous STM studies indicate that the dispersion of Au is much
even causing direct CꢀO or HꢀO bond scission. Thus, stabilizing
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 2 of 15
1
2,14
larger on CeOx/TiO (110) than on TiO (110).
After sample
been raised by one order of magnitude, comparable to that of
Cu/CeO /TiO . The dominating reaction pathway is the RWGS
reaction and the activity for CO production increases by 2ꢀ3 folds
2
2
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
preparation, it was then moved to the XPS position, where both
temperature programmed reaction mass spectroscopy (TPRꢀMS)
and in-situ ambient pressure XPS (APꢀXPS) were measured. The
combination of APꢀXPS and mass spectrometry provides a unique
capability to study the reaction kinetics and in-situ characterizaꢀ
tion of the surface intermediates. For comparison we also present
similar experiments with Cu nanoparticles as a reference system,
since Cuꢀbased catalysts are commercially used for the synthesis
x
2
after CeO modification. The detection of methanol at this low
x
hydrogen pressures (700 mTorr H ) is unprecedented and demonꢀ
2
strates the potential of using Au/CeO /TiO as active and selective
x
2
catalysts for converting CO to methanol.
2
To address the promoting role of CeO in activating CO and
x
2
the nature of the surface intermediates, APꢀXPS of carbonꢀbased
7
of methanol from syngas.
species were measured in the presence of CO and H under in-
2
2
situ reaction conditions. Figures 3A and 3B show the C1s regions
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
after the surfaces were heated in a 100 mTorr CO /700 mTorr H
2
2
gas mixture up to 573 K and then cooled down to 323 K. The C1s
spectra in Figure 3C and 3D were taken at 573 K. A peak at 284.5
eV is commonly observed during APꢀXPS experiments and it is
associated with adventitious carbon. The peak at ~292.8 eV is
due to the gas phase CO2. The peak at 288~290 eV is typically
δꢀ
associated with carbonate, formate or carboxylate species (CO
2
22
).
According to similar measurements on a CeO /Cu(111) surꢀ
x
face, the XPS features in Figure 3 are assigned to a combination
of carboxylate (~288.5 eV) species and carbonate (~289.5 eV)
species at low temperatures, and coꢀexistence of the formate
2
2
(~289.2 eV) and carboxylate species at higher temperatures. It is
worth noting that the peak intensity of 289.6 eV is much larger on
the CeO /TiO support than on TiO . At 573 K, this peak on the
x
2
2
Figure 1 (Left) STM image of CeO /TiO (110) (pink arrows
x
2
Au/TiO2 surface nearly disappears, but it is still visible on
Au/CeO /TiO . Similar C1s features were observed for Cu cataꢀ
indicate ceria nanoparticles). (Right) STM image of
Au/CeO /TiO (110) (Red arrows: Au near ceria; Green arrows:
x
2
x
2
lysts, indicating that the surface species are most likely independꢀ
ent of the metallic components.
Au near defects of TiO ). (Images size: 20x20 nm; V: 1.3 V and
2
I: 0.05 nA)
8.00
7
6
5
4
.00
.00
.00
.00
CO Methanol x 100
3.00
2
1
0
.00
.00
.00
CCuu//TTiiOO22 CCuu//CCeeOOx x/ /TTiOiO22 AAuu//TTiOiO22
AAu/uC/eCOexO/ Tx i/OT2iO
2
Figure 2. TPR of CO2 hydrogenation over Cu/TiO2 and
Cu/CeO /TiO , Au/TiO and Au/CeO /TiO for producing CO
x
2
2
x
2
and methanol. 100 mTorr CO and 700 mTorr H at 573 K. The
2
2
partial pressures of the products at 573 K are plotted.
Figure 3. XPS of C1s regions measured under the presence of
gases of 100 mTorr CO and 700 mTorr H . A) and C). Cu nanoꢀ
The TPR results of the activity and selectivity in converting
2
2
CO to CO and methanol are compared in Figure 2 for the surfacꢀ
2
particles; B) and D). Au nanoparticles; Spectra in A) and B) were
taken at 323 K; Spectra in C) and D) were taken at 573 K. Xꢀray
photon energy: 538 eV.
es of Au/TiO , Cu/TiO , Au/CeO /TiO and Cu/CeO /TiO . No
2
2
x
2
x
2
activity is found for the bare surface of either TiO or CeO /TiO
2
x
2
(
not shown). Even though bulk gold is inactive, the Au/TiO2
surface exhibits similar activity as Cu/TiO for CO production.
2
However, the activity for methanol production is very low for the
The XPS analysis of Au 4f and Ce 4d spectra provides further
insights into the promoting role of CeOx. The Au 4f spectra
before reaction and under reaction conditions are compared in
Figure 4A. A small shift (0.3 eV) to lower binding energy is
observed for Au 4f after reaction, indicating that the Au nanoparꢀ
Au/TiO sample. This observation is consistent with the results
2
1
9
from Au nanoparticles on TiO powders. A large enhancement
2
for methanol production is observed after adding 0.1 ML CeO to
x
both surfaces. At this small coverage of CeO , the ceriaꢀtitania
x
δ+
interactions are maximized and the best catalytic performance is
ticles were partially oxidized (Au ) before reaction and then were
1
2,14
expected.
The activity of Au/CeO /TiO for methanol has
x 2
reduced under reaction conditions. The Ti Auger line and Ce 4d
ACS Paragon Plus Environment

Page 3 of 15
Journal of the American Chemical Society
peaks are shown in Figure 4B. The broad peak in spectrum 1 is
the Ti Auger line. After depositing 0.1 ML CeO on TiO (110)
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
A
x
2
under O , there are multiple Ce 4d peaks in spectrum 2, which are
2
due to the spinꢀorbit splitting in the core level 4d and different
2
3
final state effects in the 4f orbitals.
The peaks at 127.1 and
1
23.9 eV are originated from the final state without any f electron
0
(f ), which is the typical XPS feature of CeO . Thus, after CeO
2
x
4+
4+
3+
deposition in O , cerium is present as Ce or mixed Ce /Ce .
2
Spectrum 3 shows the Ce 4d under reaction conditions. It can be
seen that the features at 127.1 and 123.9 eV disappear, indicating
4
+
3+
that all Ce is converted into Ce . Therefore, the activation of
B
3
+
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
CO2 requires both metallic Au and reduced Ce and the most
favorable adsorption sites are likely located at the interfacial
region between Au and Ce .
0
3+
Figure 5. Charge transfer and reaction energetics calculated by
DFT. A. The net Bader charges of Au and Ce. +: electron loss, ꢀ:
electron gain. B. DFTꢀoptimized potential energy surface (PES)
Figure 4. XPS of Au 4f and Ce 4d regions. A). Au 4f, before
reaction (dash line) and after reaction (solid line), B). 1. Before
depositing CeO , 2. After depositing CeO , and 3. Under reaction.
x
x
for
CO2
hydrogenation
on
Au /TiO (110)
and
3
2
Au /CeO /TiO (110). “TS” corresponds to transition state. The
3
x
2
DFT calculations (see Supporting Information for details) were
performed to gain a better understanding of the promoting effect
of CeO on the catalytic activity of Au/TiO observed experimenꢀ
corresponding geometries for each reaction intermediates were
shown in Figures S3 and S4.
x
2
tally. To this end, the TiO support was modeled using a rutile
2
To determine the reaction mechanism and activity, DFT calcuꢀ
lations were performed to optimize the potential energy surface
(PES) for CO2 hydrogenation on Au /TiO (110) and
TiO (110) surface, and CeO /TiO mixed oxide was described by
2
x
2
depositing Ce O dimer on TiO (110) according to a previous
2
3
2
12
3
2
study. A gold trimer (Au ) supported on both oxide surfaces was
3
Au /CeO /TiO (110) (Figure 5). The corresponding configuraꢀ
3
x
2
considered. According to DFT calculations, the Au cluster is only
3
tions of reaction intermediates involved are shown in Figures S3
and S4. The DFT results show that the reaction occurs at the
metalꢀoxide interface (Au/TiO interface in Au nanoparticle supꢀ
ported on TiO : Figure S3, and Au/CeO interface in Au nanoparꢀ
ticle supported on CeO /TiO : Figure S4). The CO hydrogenation
starts with the RWGS reaction to produce CO via *HOCO interꢀ
mediates on both Au /TiO (110) and Au /CeO /TiO (110), which
is followed by CO hydrogenation to methanol via *HCO, *H CO
and *H CO intermediates (Figure 5). On Au /TiO , the production
of CO via the RWGS reaction is hindered by the relatively large
activation barrier for of the activation of *CO to **CO (E =
weakly physisorbed on stoichiometric TiO (110) and O vacancies
2
(Ovac) are required for stabilizing Au with a binding energy (BE)
2
of ꢀ2.17 eV; in contrast the binding is stronger (BE= ꢀ2.51 eV) on
a CeO /TiO (110) surface even without O . The increased bindꢀ
2
x
x
2
vac
3+
x 2 2
ing of Au on CeO /TiO is attributed to the presence of Ce
cations. The DFT calculations show that the Au cluster binds
x
2
15
3
3
2
3
x
2
almost on top of CeOx supported on TiO (110) (Figure S4a)
2
,
2
which is consistent with the STM measurements (Figure 1).
3
3
2
An electronic metalꢀsupport interaction has been considered as
the origin for the enhanced oxidation activity of the supported Au
nanoparticles, where the charge transfer occurs from Au to the
2
2
a
1
(
.65 eV), which is significantly reduced by the presence of CeOx
24
support. The current results for Au /TiO indicate that there is a
3
2
Ea = 0.50 eV). All the subsequent intermediates are also stabiꢀ
redistribution of electrons within the Au3 unit, but the charge
transfer between Au and TiO is essentially zero. .In the case of
lized by the presence of the ceria, while the barriers for their
conversion remain unaltered with respect to the system without
ceria. Consequently CO production should be greatly enhanced on
Au /CeO /TiO compared to Au /TiO , which is consistent with
3
2
the Au ꢀCeOx interface, the charge redistribution is significant as
3
shown in Figure 5, while the overall charge of the Au. particle
remains close to zero. This is further corroborated by a density of
states plot presented in Figure S2, which shows an increase in Au
states near the Fermi level in Au /CeO /TiO compared to that in
3
x
2
3
2
the experimental findings in Figure 2. As shown in Figures S3 and
S4, Ce of CeO /TiO (110) is much more active than Ti of
3+
4+
x
2
3
x
2
TiO (110) in stabilizing the tilted O of **CO , leading to a reducꢀ
2
2
Au /TiO . This unusual redistribution of electrons facilitate the
3
2
tion in the corresponding barrier by 1.07 eV (Figure 5). Both CO
and methanol production are promoted by the presence of CeOx.
strong adsorption of CO . The DFT results also reveal that the
2
δꢀ
positively charged carbon of CO tends to bind to Au and the
negatively charged oxygen binds to Ce .
2
The barrier for the last hydrogenation step, the conversion of
methoxy species to methanol (TS7 in Figure 5), is also decreased
by the presence of ceria, which is consistent with the improveꢀ
δ+
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 4 of 15
ment on the selectivity to methanol observed experimentally
(Figure 2).
(4)
Porosoff, M. D.; Yang, X.; Boscoboinik,
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
J. A.; Chen, J. G. Angew. Chem. Int. Ed. 2014, 53, 6705.
In summary, the reduction of CO2 by hydrogen was studied
over Au/TiO and Au/CeO /TiO surfaces. It was found that the
presence of small coverages of CeOx (~0.1 ML) stabilizes the
formation of small Au nanoparticles, significantly promoting their
activity in both CO and methanol production and improving their
(5)
Rodemerck, U.; Holeňa, M.; Wagner, E.;
2
x
2
Smejkal, Q.; Barkschat, A.; Baerns, M. ChemCatChem
2013, 5, 1948.
(
6)
Melaet, G.; Ralston, W. T.; Li, C.ꢀS.;
Alayoglu, S.; An, K.; Musselwhite, N.; Kalkan, B.;
Somorjai, G. A. J. Am. Chem. Soc. 2014, 136, 2260.
selectivity towards methanol The existence of carboxylate species
.
is supported by APꢀXPS under in-situ reaction conditions. In the
(7)
2014, 43, 7813.
(8)
1996, 38, 175.
(9)
Pakhare, D.; Spivey, J. Chem. Soc. Rev.
Zhang, Z.; Verykios, X. Catal. Lett.
Wang, H. Y.; Ruckenstein, E. Appl.
Au/CeO /TiO surface an electronic metalꢀsupport interaction
x
2
leads to a charge redistribution in the metal near the Auꢀceria
interface. Such surface polarization at the metalꢀoxide interface
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
promotes both CO adsorption and activation. The DFT calculaꢀ
2
tions further reveal that Au nanoparticles supported on the Ceꢀ
O /TiO mixed oxide support decreases the reaction barriers for
x
2
Catal., A 2000, 204, 143.
CO and methanol production. The interaction of CeO with Au
x
(10)
Staudt, T.; Lykhach, Y.; Tsud, N.; Skála,
nanoparticles allows this noble metal to hydrogenate CO under
2
T. s.; Prince, K. C.; Matolín, V. r.; Libuda, J. r. J. Phys.
Chem. C 2011, 115, 8716.
unprecedented low pressures of hydrogen.
(11)
(12)
Trovarelli, A. Catal. Rev. 1996, 38, 439.
Park, J. B.; Graciani, J.; Evans, J.;
ASSOCIATED CONTENT
Supporting Information
Stacchiola, D.; Ma, S.; Liu, P.; Nambu, A.; Sanz, J. F.;
Hrbek, J.; Rodriguez, J. A. Proc. Natl. Acad. Sci 2009, 106,
4975.
Experimental details including surface preparation and characteriꢀ
zation in UHV, kinetic study by TPR under ambient pressures and
more detailed analysis of the APꢀXPS data and DFT results. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(13)
Mater. 2007, 19, 1215.
14) Park, J. B.; Graciani, J.; Evans, J.;
Zhou, K.; Yang, Z.; Yang, S. Chem.
(
Stacchiola, D.; Senanayake, S. D.; Barrio, L.; Liu, P.; Sanz,
J. F.; Hrbek, J.; Rodriguez, J. A. J. Am. Chem. Soc. 2010,
32, 356.
AUTHOR INFORMATION
Corresponding Author
1
(
15)
P.; Rodriguez, J. A. J. Chem. Phys. 2010, 131, 104703.
16) Haruta, M.; Yamada, N.; Kobayashi, T.;
Iijima, S. J. Catal. 1989, 115, 301.
Graciani, J.; Plata, J. J.; Sanz, J. F.; Liu,
Dr. Dario Stacchiola Eꢀmail: djs@bnl.gov
Prof. Jingguang G. Chen Eꢀmail: jgchen@columbia.edu
(
Present Addresses
Chemistry Department, Brookhaven National Laboratory (BNL)
Upton, NY 11973 (USA)
(17)
Haruta, M.; Tsubota, S.; Kobayashi, T.;
Kageyama, H.; Genet, M. J.; Delmon, B. J. Catal. 1993,
144, 175.
Department of Chemical Engineering, Columbia University
5
00 W. 120th St., New York, NY 10027 (USA)
(18)
Sakurai, H.; Tsubota, S.; Haruta, M.
Appl. Catal., A 1993, 102, 125.
Notes
(
19)
996, 29, 361.
20)
Sakurai, H.; Haruta, M. Catal. Today
The authors declare no competing financial interests.
1
(
Remediakis, I. N.; Lopez, N.; Nørskov, J.
ACKNOWLEDGMENT
K. Appl. Catal., A 2005, 291, 13.
The work was sponsored under Contract No. DEꢀAC02ꢀ
(21)
Lopez, N.; Nørskov, J. K. J. Am. Chem.
9
8CH10886 with the U.S. Department of Energy, Office of
Soc. 2002, 124, 11262.
Science. This research used resources of the Center for Functionꢀ
al Nanomaterials and National Synchrotron Light Source, which
are U.S. DOE Office of Science User Facilities at Brookhaven
National Laboratory under Contract No. DEꢀSC0012704 and the
National Energy Research Scientific Computing Center (NERSC)
supported by the Office of Science of the US Department of
Energy under Contract No. DEꢀAC02ꢀ05CH11231.
(
22)
Graciani, J.; Mudiyanselage, K.; Xu, F.;
Baber, A. E.; Evans, J.; Senanayake, S. D.; Stacchiola, D.
J.; Liu, P.; Hrbek, J.; Sanz, J. F.; Rodriguez, J. A. Science
2
014, 345, 546.
23)
D. R. Surf. Sci. 1998, 409, 307.
24) Liu, X.; Liu, M.ꢀH.; Luo, Y.ꢀC.; Mou, C.ꢀ
(
Mullins, D. R.; Overbury, S. H.; Huntley,
(
Y.; Lin, S. D.; Cheng, H.; Chen, J.ꢀM.; Lee, J.ꢀF.; Lin, T.ꢀ
S. J. Am. Chem. Soc. 2012, 134, 10251.
REFERENCES
(
1)
A. J. Am. Chem. Soc. 2011, 133, 12881.
2) Dorner, R. W.; Hardy, D. R.; Williams,
F. W.; Willauer, H. D. Energy Environ. Sci. 2010, 3, 884.
3) Kondratenko, E. V.; Mul, G.;
Olah, G. A.; Prakash, G. K. S.; Goeppert,
(
(
Baltrusaitis, J.; Larrazabal, G. O.; PerezꢀRamirez, J. Energy
Environ. Sci. 2013, 6, 3112.
ACS Paragon Plus Environment

Page 5 of 15
Journal of the American Chemical Society
Table of Content (TOC)
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
CO + H2
CH OH
2
3
6
5
4
3
2
1
0
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
A 1u /TiO2
Au/C 2e O /TiO
x 2
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 6 of 15
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
585x329mm (120 x 120 DPI)
ACS Paragon Plus Environment

Page 7 of 15
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 8 of 15
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
101x101mm (300 x 300 DPI)
ACS Paragon Plus Environment

Page 9 of 15
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
101x101mm (300 x 300 DPI)
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 10 of 15
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
101x101mm (300 x 300 DPI)
ACS Paragon Plus Environment

Page 11 of 15
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
101x101mm (300 x 300 DPI)
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 12 of 15
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
101x101mm (300 x 300 DPI)
ACS Paragon Plus Environment

Page 13 of 15
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
101x101mm (300 x 300 DPI)
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 14 of 15
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
54x164mm (120 x 120 DPI)
ACS Paragon Plus Environment

Page 15 of 15
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
203x153mm (300 x 300 DPI)
ACS Paragon Plus Environment