Highly Active Subnanometer Au Particles Supported on TiO2 for Photocatalytic Hydrogen Evolution from a Well-Defined Organogold Precursor, [Au5(mesityl)5]

Article abstract of DOI:10.1021/acs.inorgchem.6b00341

A highly efficient H₂ evolution photocatalyst based on TiO₂ supported subnanometer Au particles was developed on the basis of the reaction of a gold(I) molecular precursor [Au₅Mes₅] (Mes = 2,4,6-trimethylphenyl), with titanium dioxide partially dehydroxylated at 120 °C. IR, UV-vis, elemental analysis, XANES, and STEM-EDX show that the deposition of [Au₅Mes₅] onto TiO₂ leads to the formation of both subnanometer Au particles and chemisorbed [Au₅Mes₅]. The remaining organic ligands are removed via a mild treatment under H₂, yielding subnanometer gold(0) particles. A range of Au loadings (0.3, 0.9, 2.4 wt %) with similar particle sizes (<1 nm) on TiO₂ are obtained and tested in methanol-assisted photocatalytic hydrogen production under UV light. These catalysts display significantly higher activity than a commercial reference Au-TiO₂ catalyst. The presence of chemisorbed [Au₅Mes₅] in the as-synthesized catalyst further improved activity, albeit at the expense of stability. This work demonstrates a simple synthetic route to obtain subnanometer Au particles on TiO₂ that display exceptional activity in photocatalysis.

Full text of DOI:10.1021/acs.inorgchem.6b00341

Article
pubs.acs.org/IC
Highly Active Subnanometer Au Particles Supported on TiO for
2
Photocatalytic Hydrogen Evolution from a Well-Defined Organogold
Precursor, [Au (mesityl) ]
5
5
́
Georges Siddiqi, Victor Mougel, and Christophe Coperet*
̈ ̈
Department of Chemistry and Applied Biosciences, ETH-Zurich, Vladimir Prelog Weg 1-5, CH-8093 Zurich, Switzerland
*
S Supporting Information
ABSTRACT: A highly efficient H evolution photocatalyst based on TiO supported
2
2
subnanometer Au particles was developed on the basis of the reaction of a gold(I)
molecular precursor [Au Mes ] (Mes = 2,4,6-trimethylphenyl), with titanium dioxide
5
5
partially dehydroxylated at 120 °C. IR, UV−vis, elemental analysis, XANES, and STEM-
EDX show that the deposition of [Au Mes ] onto TiO leads to the formation of both
5
5
2
subnanometer Au particles and chemisorbed [Au Mes ]. The remaining organic ligands
5
5
are removed via a mild treatment under H , yielding subnanometer gold(0) particles. A
2
range of Au loadings (0.3, 0.9, 2.4 wt %) with similar particle sizes (<1 nm) on TiO are
2
obtained and tested in methanol-assisted photocatalytic hydrogen production under UV
light. These catalysts display significantly higher activity than a commercial reference Au-
TiO catalyst. The presence of chemisorbed [Au Mes ] in the as-synthesized catalyst
2
5
5
further improved activity, albeit at the expense of stability. This work demonstrates a
simple synthetic route to obtain subnanometer Au particles on TiO that display
2
exceptional activity in photocatalysis.
2
9,30,39−46
INTRODUCTION
cocatalysts.
Furthermore, there is no consensus on the
■
effect of Au particle size on photocatalytic activity. While
studies investigating the energetics of charge transfer across the
metal semiconductor interface have indicated the benefits of
decreasing Au particle,
small Au nanoparticles are detrimental to photocatalytic
performance.
Here, we develop a robust method to generate subnanometer
Au particles supported on TiO via the reaction of a gold(I)
organometallic precursor [Au Mes ] (Mes = 2,4,6-trimethyl-
phenyl), with partially dehydroxylated TiO followed by a
treatment under H . To demonstrate the potential of
subnanometer Au for photocatalysis, we study the photo-
catalytic H2 production from water and a CH OH/H O
Photocatalytic hydrogen production offers a sustainable energy
source via the direct conversion of sunlight into chemical
energy. First reported in 1972 using a TiO single crystal anode
to photoelectrochemically split water, this class of catalytic
2
17,47,48
1
others have claimed that very
2
−5
systems has been extensively studied since. Materials such as
TiO , SrTiO , and Ta N as well as many other combinations
19,30,31,49
2
3
3
5
of mixed oxides and dopants have been developed to capture
either the UV or visible spectrum of sunlight by tuning the
bandgap of the material.
2
6
−14
5
5
However, analogously to solar
50
2
cells, the primary factor limiting the efficiency of these
15,16
2
semiconductors is electron−hole recombination.
This can
be mitigated by dispersing a metal cocatalyst onto the
semiconductor surface to spatially separate the electrons from
3
2
7
13,15,17−22
mixture.
the holes, greatly increasing photocatalytic activity.
Current research is focused both on the synthesis and
performance of novel semiconductors to capture light, as well
as on improving photocatalysis by tuning the metal
EXPERIMENTAL SECTION
General Procedures. All experiments were carried out under an
■
1
0,23−28
inert atmosphere of argon using standard Schlenk techniques or using
cocatalyst.
−
5
high vacuum (10 mbar) techniques. Tetrahydrofuran (THF) was
distilled from purple Na/benzophenone ketyl under Ar and stored
over activated molecular sieves; toluene was dried by passage through
Noble metals such as Pt and Au are the most common metal
cocatalysts, significantly improving hydrogen production by
5
,14,29
facilitating the separation of electrons from the holes.
Although the photocatalytic performance of Pt can be slightly
two columns of activated alumina and degassed prior to use. H was
purified over R3-11 BASF catalyst/MS 4 Å prior to use. [Au Mes ]
2
5
5
3
0
better than Au, Au boasts several notable benefits: it makes
and reference materials (Au nanoparticles supported on SiO ) for
2
50
31,32
the reverse reaction of 2H + O → 2H O less favorable,
XANES analysis were synthesized as previous reported. [Au Mes ]
5 5
2
2
2
was synthesized in 55% yield via the addition of 2 equivalent of
MesMgBr to a THF solution of AuCl , followed by purification and
and Au itself can generate electrons via plasmon reso-
3
3−37
3
nance.
Furthermore, recent studies have also found that
38
Au-TiO can be more active than Pt-TiO . However, Au bears
2
2
several synthetic challenges, in particular the greater difficulty to
Received: February 10, 2016
control particle size and distribution by comparison to Pt
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.6b00341
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Inorganic Chemistry
Article
5
0
−2
recrystallization. The 1 wt % Au supported on P25 reference catalyst,
the last washing. Volatiles were evacuated in vacuo (10 mbar), and
the solid was further dried for 3 h under high vacuum (10− mbar) to
afford 2. After drying the catalyst is stored under an inert Ar
atmosphere in absence of light at −40 °C.
5
AUROlite Au/TiO , from Project AuTek was purchased from Strem
2
Chemicals. The sample is stored in a glovebox at −40 °C under Ar
atmosphere, ground in an agate mortar and pestle, and weighed in Ar
atmosphere before use.
Preparation of 3. The material 3 was synthesized in a manner
identical to that of 2, except the amount of [Au Mes ] was reduced to
target ca. 0.3 wt % Au (full consumption of [Au Mes ] is noted). A
Catalyst Characterization. TEM images were collected with a
Philips CM12 transmission electron microscope. STEM-EDX analysis
was conducted on a Tecnai F30 STEM. Samples were exposed to air
for only a short time (<10 min) prior to insertion into the microscope.
IR measurements were performed using a Bruker Alpha-T FTIR
spectrometer inside an Ar filled glovebox. Samples were pressed into
self-supporting disks using a manual press. Elemental analyses were
performed by the Mikroanalytisches Labor Pascher; Remagen,
Germany. NMR spectroscopy was conducted using 200 and 250
MHz Bruker spectrometers. Nitrogen adsorption experiments were
performed on a BELsorp-mini II. UV−vis was acquired on a Cary
000 UV−vis−NIR spectrometer. X-ray absorption spectroscopy
XAS) measurements at Au L -edge were performed at the SuperXAS
beamline at the Swiss Light Source (Paul Scherrer Institute, Villigen,
Switzerland). The SLS is a third generation synchrotron, which
operates under top up mode, 2.4 GeV electron energy, and a current of
5
5
5
5
solution of [Au Mes ] (1.0 mg, 0.004 mmol [Au Mes ]/g TiO ) in
5
5
5
5
2
toluene (5 mL) was contacted with TiO2‑(120) (171 mg) at room
temperature, affording 3.
Preparation of 1-H , 2-H , and 3-H . The materials 1, 2, and 3
were treated under flowing H (60 mL/min H for 12 h at 300 °C, 0.5
/min ramp) to afford a dark gray powder, 1-H , 2-H , and 3-H .
Quantification of the Formation of Mesitylene. TiO2‑(120) (10−20
mg) samples were placed in contact with a quantity of [Au Mes ]
corresponding the appropriate loadings for 1, 2, and 3 dissolved in ca.
mL of C D . The reaction was allowed to proceed as described
2
2
2
2
2
o
2
2
2
5
5
5
1
6
6
(
III
above for the preparation of 1−3. After 4 h under stirring at room
temperature, the supernatant is separated via pipetting and the solid
washed once with ca. 0.5 mL of C D , and all solutions were
6
6
combined. Quantitative 1H NMR using ferrocene as an internal
4
00 mA. The SuperXAS beamline is positioned on one of three
standard was used to quantify mesitylene: 0.52 μmol mesitylene
produced (0.34 μmol [Au Mes ] consumed), 0.25 μmol (0.15 μmol
Au Mes ] consumed), and 0.10 μmol (0.05 μmol [Au Mes ]
5 5 5 5
superbent ports. The incident beam was collimated by Si-coated
mirror at 2.8 mrad, monochromatized using a double crystal Si(111)
monochromator, and focused with Rh coated toroidal mirror (at 2.8
mrad) down to 100 × 100 μm . The beam intensity was of 4−5 × 10
ph/s. We calibrated the beamline energy using Au reference foil. We
5
5
[
consumed) for 1, 2, and 3, respectively.
2
11
NMR Characterization of Methanol Oxidation. A 2.9 mg
sample of catalyst 2 was mixed with 1.08 mL of D O and 0.12 mL of
CH OH (Sigma-Aldrich). In a comparison to photocatalytic tests,
2
used ion chambers filled with He−N gas mixtures for XAS detection
13
2
3
in transmission. To avoid contact with air, all samples were sealed in a
glovebox. Each pellet of sample (with optimized thickness for
transmission detection) was placed in two aluminized plastic bags
excess catalyst was used due to a small scale of reaction and difficulty
to accurately weight such small masses. A magnetic stir bar was
inserted into the tube for agitation during the photocatalysis
experiment. The sample was exposed to the same UV lamp used for
the photocatalytic studies, with a cooling water bath around it to
prevent overheating. Gas bubbles and H were observed to form via
GC. Photocatalysis was carried out under exclusion of air. The
following standards used for NMR peak assignments were validated
(
polyaniline (15 μm), polyethylene (15 μm), Al (12 μm), poly-
ethylene (75 μm) from Gruber-Folien GmbH & Co. KG) using an
impulse sealer inside a glovebox; one sealing layer was removed just
before the measurements. Powder samples for fluorescence detection
were sealed in quartz capillaries (0.01 mm wall thickness, 0.9 mm
outer diameter, Hilgenberg GmbH) in a glovebox using vacuum grease
Apiezon Products) and wax, and stored in sealed glass tubes under
argon that were opened just before the measurements. Au(0)-SiO and
were sealed in pellets, but due to difficulties of pressing pellets with
the molecular precursor [Au Mes ], this was placed in a quartz
capillary. We collected XANES data in transmission mode at 298 K.
Preparation of TiO2‑(120). TiO is obtained from Evonik (P25,
2
1
13
using HMBC H− C NMR (solvent D O): formic acid (HCOOH),
C δ = 165 ppm, H δ = 8.15 ppm; methyl formate (HCOOCH ),
C δ = 167, 51.4 ppm, H δ = 3.16, 3.78 ppm; methylene glycol
2
(
1
3
1
3
2
13
1
1
13
1
(
hydrated formaldehyde) (CH (OH) ), C δ = 81.7 ppm, H δ =
2 2
5
5
1
3
4
5
.19, 3.34 ppm; dimethoxymethane (CH (OCH ) ), C δ = 96.8,
2 3 2
1
4.8 ppm, H δ = 4.64, 3.38 ppm; methoxymethanol (CH (OH)
(OCH ), formed by mixing an equimolar aqueous solution of
formaldehye and methanol at room temperature for 19 h)), C δ =
2
2
2
mixture of anatase and rutile, 60 m /g). Prior to deposition, P25 is
3
1
3
calcined in a quartz reactor in a flow of synthetic air at 450 °C for 16 h
1
9
0.1, 55.2 ppm, H δ = 4.72, 3.38 ppm.
(
5 °C/min). After calcination the reactor is sealed while hot, cooled to
Investigation of Photocatalytic Activity. The photoreactor
room temperature under static atmosphere, and then evacuated for 20
−5
consisting of a 500 mL jacketed reactor with a 220 mm jacketed quartz
immersion well was purchased from Ace Glass, Inc. A quartz mercury
low pressure UV arc lamp (450 W, Ace Glass, Inc.) was used to
illuminate the photocatalysts. The energy output of the lamp after
passing through a quartz cooling jacket with ca. 5 mm deionized water
flowing was measured to be 190 mW/cm using a Thorlabs PM100
optical power and energy meter. All joints of the reactor were
lubricated with fluorinated grease (Krytox LVP, Dupont), as other
min under high vacuum (<10 mbar). The calcined P25 is then
exposed to vapor pressure of water for 30 min. Following hydration,
the P25 is evacuated at room temperature for 1 h, followed by a
thermal treatment under high vacuum for 16 h at 120 °C (5 °C/min
temperature ramp). This P25 sample is referred to as TiO2‑(120)
Au-TiO Nanoparticle Deposition. Preparation of 1. A yellow
solution of an excess of [Au Mes ] (76 mg, 0.08 mmol [Au Mes ]/g
TiO ) in toluene (20 mL) was contacted with TiO
room temperature. The suspension was stirred for 4 h, during which
time the solid turned gray and the solution became paler yellow. After
h the filtrate was separated from the solid, which was washed with
fresh toluene (3 × 20 mL), leaving a yellow filtrate after the last
washing. Approximately 0.03 mmol of [Au Mes ] is noted by H
NMR to remain in the filtrate after rinsing. Volatiles were evacuated in
vacuo (10 mbar), and the solid was further dried for 3 h under high
vacuum (10 mbar) to afford 1. After drying the catalyst is stored
under an inert Ar atmosphere in absence of light at −40 °C.
Preparation of 2. A yellow solution of [Au Mes ] (8.1 mg, 0.01
.
2
2
5
5
5
5
(613 mg) at
2
2‑(120)
greases are observed to decompose and leach H
illumination. H production was monitored using a micro-gas-
chromatograph (490 Micro GC, Agilent Technologies, AG) with a 1
m CP-COX column used to separate H from O . An oil filled bubbler
under UV
2
2
4
2
2
1
was used to prevent backflow of ambient air into either the micro-GC
or the reactor. Photocatalytic tests were performed under a flow (26
mL/min) of N , with a T junction going to the microGC allowing
online sampling of the gases evolved. Photocatalysis was performed
within a sealed (nonreflective) light-tight enclosure, and the reactor
and UV lamp were cooled with 20 °C cooling water mantle. Agitation
was provided via a Teflon stir bar spinning at 255 rpm. Prior to
starting photocatalysis, the reactor was filled with a 10 vol % mixture of
CH OH and H O, and degassed under dynamic vacuum (ca. 20 mbar)
5
5
−2
2
−5
5
5
mmol [Au Mes ]/g TiO ) in toluene (20 mL) was put in contact with
5
5
2
TiO2‑(120) (480 mg) at room temperature. The suspension was stirred
for 4 h, during which time the solid turned gray and the solution
colorless. After 4 h the filtrate was separated from the solid, which was
washed with fresh toluene (3 × 20 mL), leaving a colorless filtrate after
3
2
for 30 min, followed by deaeration with bubbling N (26 mL/min) for
1 h. The catalyst (14.7 mg for 1, 13.8 mg for 1-H , 15.3 mg for 2, 16.6
2
2
B
DOI: 10.1021/acs.inorgchem.6b00341
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Inorganic Chemistry
Article
mg for 2-H , 17.6 mg for 3, 13.8 mg for 3-H , 23.1 mg for 4, and 18.7
particle size, but with a noticeable tail of nanoparticles larger
than 3 nm (Figure 1). Alternative modes of activation of the
titania support (e.g., higher temperatures of dehydroxylation,
2
2
mg for TiO2‑(120)) was then added under a flow of N , and deaerated
2
with bubbling N for 1 h more. Examination of overall water splitting
2
was conducted in an identical matter except only pure H O was used.
2
cooling under N or vacuum) lead to less reproducible gold
2
Oxygen production could not be quantified because of its absorption
in degassed water.
GC analysis of the gas stream is conducted to verify that no oxygen
is present in the outlet stream. NMR analysis of the reaction solution,
following either the standard deaeration procedure or 24 h exposure to
deposition, yielding in particular larger particles and broader
particle size distribution, indicating that surface state and the
level of dehydroxylation of titania are important parameters.
1
H NMR of the supernatant confirms that [Au Mes ] is
5
5
UV light and 26 mL/min N bubbling without any photocatalyst,
partially consumed, corresponding to a deposition of ca. 3 wt %
2
shows minimal loss of methanol. During photocatalysis, the micro-GC
is programmed to sample the outlet gas stream on regular intervals,
and the UV light is turned on at the same moment of the first GC
sample. Once photocatalysis begins the reactor enclosure is not
opened, and gas production is regularly monitored. Quantification of
of Au on TiO (see Supporting Information for details),
2
consistent with 2.4 wt % loading determined by elemental
analysis (Table 1). NMR of the filtrate after reaction reveals
that approximately 1.4 equiv of mesitylene is released per
[
Au Mes ] consumed upon contact with titania.
the H production is done via TCD, which is calibrated prior to use.
5 5
2
H production is calculated with the following entities: v = volumetric
2
flow rate (mL/min), c = concentration (vol %), n = molar flow rate
Table 1. Au-TiO Synthesis Conditions and Effect on Au
Loading and Nanoparticle Size
2
(
mol/min). Known quantities follow: vN2 (N flow rate, 26 mL/min)
and cH2 (concentration of H2 measured by TCD, vol %), P =
atmospheric pressure, 1.01 bar, T = 300 K.
2
targeted Au/nm2
a
Au wt %
b
C/Au
c
NP size (nm)
d
The route for calculating volumetric H flow follows:
2
1
1
2
4
2.4
2.4
0.9
0.9
0.3
0.3
1.2
8.5
0.9 ± 0.1
e
e
e
-H2
f
0.9 ± 0.06
0.7 ± 0.05
0.8 ± 0.04
0.6 ± 0.05
0.8 ± 0.06
1.7 ± 0.08
vH2(mL/min) = vN2 × cH2
0.5
0.2
9.3
Converting volumetric H flow to molar H flow using the ideal gas
2
2
2-H2
3
f
f
f
law
PvH2
RT
3-H2
4
nH2(mol/min) =
f
a
2
Targeted surface loading of Au atoms per nm of TiO , calculated
from the concentration of Au used. Weight loading of Au on TiO ,
determined by elemental analysis (EA). Atomic ratio of C/Au present
on TiO , determined by EA. Average particle size of Au, determined
by TEM analysis, 95% confidence interval reported. Prepared via a
post-treatment under 10 mL/min H flow at 300 °C (0.5 °C/min) for
0 h. Amount of carbon present was too low (<0.2 wt %) for accurate
This number is then either divided by the moles of Au present in
the catalyst used, or by the mass of catalyst used.
Schematic Representation of the Experimental Setup. We show a
representation of the experiment, although some items are not
pictures: light tight box for photoreactor when UV light is illuminated,
and jacket for cooling water for both lamp holder and photoreactor.
2
b
2
c
d
2
e
2
f
1
EA.
We reasoned that the larger particles could arise via the
growth of small nanoparticles from the excess of [Au Mes ]
5
5
present in solution during the deposition step. To limit this
possibility, we used a default of [Au Mes ] corresponding to 10
5
5
(
(
0.9 wt %, 2) and 5 (0.3 wt %, 3) μmol [Au Mes ] per g TiO
0.5 and 0.2 Au/nm , respectively). From quantitative mass
5 5 2
2
balance analysis of the filtrate by NMR, we observe that in both
cases [Au Mes ] is fully consumed, corresponding to the 0.9
5
5
and 0.3 wt % Au in 2 and 3, respectively (Table 1). Similar to 1,
NMR spectra of the filtrates of 2 and 3 after reaction reveal that
1.7 and 2 equiv of mesitylene, respectively, are released per
equiv of [Au Mes ] consumed. TEM pictures of catalysts 2 and
RESULTS AND DISCUSSION
■
Synthesis and Characterization of Au-TiO . Contacting
2
2
an excess of [Au Mes ] (4 Au/nm TiO ) on Evonik P25 TiO
5
5
2
2
5
5
partially dehydroxylated at 120 °C (TiO2‑(120)) affords a gray
solid, 1. Transmission electron microscopy (TEM) on this
sample shows the presence of particles with a 0.9 nm average
3 reveal an average particle size of 0.7 nm for 2 and 0.6 nm for
3 and an elimination of the tail in the PSD (Figure S1),
51
corresponding to ca. 15 atom clusters.
Figure 1. Representative TEM images (left) and particle size distributions (right) of (a) 1 and (b) 1-H2.
C
DOI: 10.1021/acs.inorgchem.6b00341
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UV−vis DRS under inert atmosphere of 1 and 1-H shows
still be adsorbed at the surface along with chemisorbed
2
only a very weak plasmon resonance peak, probably associated
with the very small particle size, which increases upon exposure
[Au Mes ]. The consumption of −OH and −OH bands of
5
5
2
−
1
TiO
(3600−3700 cm ) following deposition indicates
2
‑(120)
52,53
to air (Figure S13).
IR spectroscopy after deposition shows
that deposition occurs primarily via reaction with surface
hydroxyl groups of TiO (Figure 2). In addition, the
the presence of stretching and bending (C−H) vibrations in
2
−1
the 3080−2800 and 1500−1350 cm regions, respectively
Figure 2).
disappearance of the peak in the IR spectra of TiO2‑(120) at
1624 cm− corresponding to adsorbed water suggests that
1
54
(
nanoparticles are formed via the controlled hydrolysis of
[
Au Mes ] with the adsorbed water, yielding unstable Au O
5
5
x
y
55
clusters, which disproportionate to give Au(0) clusters. This
is supported by the lack of Au nanoparticle formation on TiO2
calcined at 450 °C or dehydroxylated silica that does not
5
0
contain adsorbed H O. From this analysis and the
2
subnanometer Au particles observed by TEM, we can
hypothesize that Au is present on the surface as either mesityl
capped Au nanoparticles and/or chemisorbed [Au Mes ].
5
5
To elucidate this further, we turned to XANES, which shows
the presence of a mixture of gold(0) and gold(I) species in 1,
0
I
using Au -SiO and [Au Mes ], as references (Figures S3−5,
2
5
5
50
Supporting Information). In addition, STEM-EDX has a
detectable Au EDX signal in regions without observable Au
nanoparticles, suggesting the presence of chemisorbed Au
species not present as nanoparticles. XANES and STEM-EDX,
combined with the TEM, EA, and IR, suggest that chemisorbed
Figure 2. Transmission IR measurement of (a) partially dehydroxy-
lated TiO2‑(120) and (b) TiO2‑(120) contacted with [Au Mes ] (1).
5
5
Spectra are not normalized.
[
Au Mes ] surface species coexists on the surface with Au
Due to the lack of post-treatment performed on 1−3,
significant amounts of organic moieties remain adsorbed on the
surface, as evidenced by IR and EA with a C/Au ratio of ca. 9
5 5
particles (bare or mesityl capped).
To remove remaining mesitylene and mesityl ligands, 1, 2,
(
Table 1). In a comparison of the IR spectra of 1 to [Au Mes ]
and 3 were treated under a flow of H
forming 1-H , 2-H , 3-H , respectively. After this step, no C−H
stretches are observed in the 2900 cm region of the IR
(Figure S8), and only trace amounts of carbon (<0.2 wt %) can
be detected by elemental analysis (Table 1). XANES shows the
presence of only gold(0) species (Figure S5). TEM indicates a
minor increase in mean particle size by 0−0.2 nm, but also an
2
at 300 °C for 10 h,
5
5
50
and [Au Mes ] physisorbed onto SiO , the spectra of 1 is
2
2
2
5
5
2
−1
slightly shifted and distorted, consistent with the presence of
adsorbed mesityl containing species in a different chemical
environment than in [Au Mes ] (Figure S2). The observation
5
5
of mesitylene (ca. 1.5−2 mol per mol [Au Mes ]) in the filtrate
5
5
5
0
and its ability to adsorb on oxide supports suggest that it can
Figure 3. H production of 4 and the H -treated catalysts (top) and as-synthesized catalysts (bottom) normalized to the amount of Au present in the
2
2
sample (left), and the mass of catalyst (right). TiO2‑(120) has been omitted for clarity. Reaction consists of a deaerated 10 vol % mixture of CH OH +
3
H O with 15−25 mg of catalyst, exposed to a 450 W Hg vapor UV light under a 26 mL/min N flow.
2
2
D
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increase in nanoparticle surface density compared to the as-
experienced rapid deactivation similar to benchmark catalyst
and previous studies, 2 produces as much as ca. 3−4 times
more hydrogen than 4 (Figure S12), but it was not possible to
56
31
synthesized catalysts (Figure S1) and the disappearance of the
larger particle (>3 nm) in the PSD of 1 (Figure 1). This result
is consistent with a redispersion of Au after reduction as
quantify the O produced with our experimental setup.
2
50,57
recently observed on various supports.
particle density is consistent with the presence of chemisorbed
Au Mes ] species prior to the treatment under H , which are
This increase in
A summary of the total amount of hydrogen produced for
each catalyst after 100 h reaction time is presented in Figure 4.
[
5
5
2
then converted into additional Au nanoparticles.
Overall, the deposition of [Au Mes ] onto TiO followed by
5
5
2
a mild treatment under H allows for a simple and robust
2
synthesis for subnanometer Au particles supported on TiO . In
2
view of the ease of preparation of [Au Mes ] in g-scale, this
5
5
approach provides a scalable route to a narrow particle size
distribution of Au particles which are in the range of the
smallest reported Au nanoparticles supported on TiO to our
2
3
9,58−64
knowledge.
Photocatalytic Performance of Au-TiO . Having synthe-
2
sized these subnanometer Au-TiO catalysts, we investigated
2
their photocatalytic performance for methanol-assisted hydro-
gen evolution (Figure 3). To allow long-term catalyst testing
Figure 4. Total H generated after 100 h reaction time per mol (left)
2
Au
we operated our reactor with a continuous flow of N bubbling
and per gcat (right). Total H
2
generation calculated by numerical
2
integration of the curves in Figure S10.
through the reactor during photocatalysis, and quantified the
production of H by gas chromatography. Since differences
2
such as lamp power, photocell design, and catalyst loading can
We observe that the same trends hold as with the peak activity
yield significantly different activities, commercial AUROlite 1.2
(
Figure 4, Table S1): H -treated catalysts produce less H than
2
2
65
wt % Au-TiO (4) was studied as a reference catalyst. XRD
2
the as-synthesized catalysts, and H generation per mol basis
2 Au
shows 4 is a mixture of anatase and rutile identical to P25, and
the support particle size calculated by the Debye−Scherrer
equation of 4 is 27 nm, confirming that the support is P25
increases with decreasing weight loading, with the reverse
occurring per g . By comparison, the bare support, TiO2‑(120),
cat
has negligible activity.
TiO . The average Au particle size from TEM was measured to
2
In agreement with previous studies showing the benefit of
47,71−75
be 1.7 nm (Table 1, Figure S9), similar to other published
reducing particle size,
these TiO -supported subnan-
2
3
9,40
catalysts.
To facilitate photocatalysis, methanol was selected
ometer Au particles also display some of the best reported
5,66
5,66,76
as an electron donor, as commonly described in literature.
photocatalytic activities.
This exceptionally high activity is
With similar weight loading and identical support of 2-H2
and 4, one can make a direct comparison and show that the
peak activity of 2-H , both per molAu and g , is 3-fold higher
most likely due to the presence of subnanometer Au particles
with a relatively narrow particle size distribution. Although
further work is required to elucidate the mechanism by which
these subnanometer particles enhance activity, the high activity
of subnanometer particles parallels what has been found for a
range of metals (Cu, Ag, Pt, Au) and reactions (propylene
oxidation, hydrogen generation, CO oxidation), in particular for
2
cat
than that for 4. The most active catalysts are 3-H and 1-H ,
2
2
per molAu and per g , respectively. NMR analysis of the
cat
reaction mixtures after 69 h of photocatalysis shows the
formation of CH (OH) , CH (OH)(OCH ), HCOOH,
2
2
2
3
77−82
HCOOCH , and dissolved CO (Table S3), consistent with
Au, which is typically inactive as a bulk metal.
3
2
the photooxidation of methanol (reactions 1−13 in Supporting
Deactivation. Despite their high activity, these subnan-
ometer Au-TiO2 catalysts undergo significant deactivation
(Figure 3, Table 2). In all catalysts, the sizes of the Au particle
increase, by 1.1−1.3 nm for 1, 2, and 3 and the 0.7−0.9 nm for
1-H , 2-H , and 3-H . This size increase parallels the level of
6
7−69
Information).
This data confirms that methanol acts as a
scavenging agent for oxidants during photocatalysis.
Since TiO photocatalysts are very efficient at degrading
2
70
organic molecules under UV light, we suspected that the
presence of mesityl groups on the surface of the as-synthesized
Au nanoparticles would not significantly hinder catalytic
performance, and tested the performance of 1, 2, and 3 (Figure
2
2
2
deactivation of 63−68% versus 35−59% observed for the as-
synthesized and H -treated catalysts, respectively (Tables 1 and
2
2), supporting sintering as a deactivation mechanism. This is
3). In fact, all the as-synthesized catalysts 1, 2, and 3 display
further evidenced by the lower deactivation of 3-H , which has
2
significantly greater activity than their H -treated counterparts,
lower weight loading (0.3 wt %) and particle density. It is
noteworthy that Au sintering requires both water and UV light
(a catalyst exposed to only one of them does not suffer from
sintering). This suggests that Au sintering probably results from
radical species formed in solution or at the interface between
the solid and the solution, and not only electron−hole pairs.
In addition, examining the photocatalytic performance of
TiO2‑(120) indicates that even the bare support experiences
deactivation, losing 40% of its activity after 60 h of
photocatalysis, which suggests the participation of alternative
deactivation pathway. Previous studies have found similar
2
1
-H , 2-H , and 3-H , respectively. Finally, although activities
2 2 2
are higher, the trends in activity of the as-synthesized catalysts
are the same as those previously noted. Since the average
observable particle sizes of the H -treated and as-synthesized
2
catalysts are similar, the increase of photocatalytic activity is
associated with the presence of chemisorbed [Au Mes ] prior
5
5
to photocatalysis and probably originates from the in situ
formation of small Au clusters from the decomposition of
[
Au Mes ] under photocatalysis conditions.
5 5
To confirm that hydrogen evolution occurs even in the
absence of methanol, photocatalytic hydrogen evolution was
conducted on 2 and 4 in pure water. Although the catalysts
deactivation of bare TiO , and have correlated this to corrosion
2
83,84
of the TiO surface by photogenerated holes.
In fact,
2
E
DOI: 10.1021/acs.inorgchem.6b00341
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Table 2. Summary of Deactivation and Sintering During Photocatalysis
a
b
c
d
deactivation (%)
dNP before photocat (nm)
dNP after photocat (nm)
ΔdNP (nm)
1
1
2
2
3
3
4
68
59
69
63
54
35
30
0.9 ± 0.1
2.0 ± 0.1
1.8 ± 0.08
1.9 ± 0.1
1.7 ± 0.07
1.9 ± 0.1
1.5 ± 0.1
1.9 ± 0.1
1.1
0.9
1.2
0.9
1.3
0.7
0.2
-H2
-H2
-H2
0.9 ± 0.06
0.7 ± 0.05
0.8 ± 0.04
0.6 ± 0.05
0.8 ± 0.06
1.7 ± 0.08
a
b
Deactivation calculated by percentage change between peak activity and averaged photocatalytic activity from 98 to 100 h. Average Au particle size
c
before reaction, taken from Table 1. Average Au particle size after 100 h reaction, with representative TEM images and PSD shown in Figure S11.
d
(
Final Au particle size) − (initial Au particle size).
investigation of Au-TiO catalysts after photocatalysis by TEM
EDX analysis, XRD and TEM characterization of 4,
photocatalytic data after 140 h reaction time, and tables
summarizing photocatalytic performance (PDF)
2
shows the formation of an amorphous layer after reaction which
encapsulates the material, which could be related to this
phenomenon (Figure S11 and Figure S18). TEM studies of
bare TiO after photocatalysis in pure water also showed layer
formation (Figure S18). This layer is likely a hydroxide layer on
2
AUTHOR INFORMATION
■
top of TiO , which probably forms by leaching−redeposition of
Corresponding Author
*E-mail: ccoperet@ethz.ch.
2
67,85,86
TiO under water splitting conditions.
2
Author Contributions
CONCLUSIONS
■
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
Subnanometer Au particles with a narrow particle size
distribution were prepared with up to 2 wt % loading through
a two-step process involving the reaction of [Au Mes ] with
5
5
Notes
partially dehydroxylated TiO at 120 °C, followed by a mild
2
The authors declare no competing financial interest.
treatment under H . Characterization of the material via EA, IR,
2
and XANES and STEM-EDX shows that the surface of TiO2
after the deposition step is already decorated with gold(0)
nanoparticles along with residual chemisorbed [Au Mes ] and/
ACKNOWLEDGMENTS
■
We acknowledge the Paul Scherrer Institut, in Villigen,
Switzerland, for provision of synchrotron radiation beamtime
at the SuperXAS beamline of the SLS and would like to thank
Dr. Olga Safonova and Dr. Maarten Nachtegaal for their
assistance. We are grateful to ScopeM for the use of their
microscopy facilities and Mr. Dmitry Lebedev for assistance
with XRD spectroscopy and analysis. G.S. would like to thank
the Swiss National Science Foundation (SNF 200021_137691/
1 and SNF 200020_159801) for funding. V.M. was supported
5
5
or gold(0) nanoparticles stabilized by mesityl ligands. The
formation of Au(0) presumably arises from the controlled
hydrolysis of [Au Mes ], with water chemisorbed on TiO , into
5
5
2
Au O , which is then converted into Au(0) particles. Post-
x
y
treatment under H efficiently removes the organic groups and
2
converts residual [Au Mes ] to selectively afford subnanometer
5
5
narrowly dispersed metallic Au particles, thus providing a
simple synthesis route to subnanometer Au particles on TiO2.
These H -treated Au nanoparticles display high activity in H2
̈
by an ETH fellowship (cofunded ETH Zurich-Marie Curie
action for people, FEL-08-12-2).
2
evolution, showing that lowering the nanoparticle size into the
subnanometer range is a viable strategy to significantly improve
photocatalytic performance. The as-synthesized Au-TiO2
catalysts display even higher activity, demonstrating the
potential of smaller Au clusters generated from chemisorbed
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