Photocatalytic performance of Au/ZnO nanocatalysts for hydrogen production from ethanol

Article abstract of DOI:10.1016/j.apcata.2015.10.013

Gold was loaded on several ZnO samples with different morphologies: needle-like (ZnO-n), flower-like (ZnO-f), rods (ZnO-r), and prepared by a thermal calcination process (ZnO-t). A commercial ZnO sample (ZnO-c) was also used for comparison. Bare ZnO and Au/ZnO materials were used for the photocatalytic production of H₂ from a water-ethanol solution under UV-vis irradiation (λ > 300 nm). During the photocatalytic runs, hydrogen was generated at a constant rate for all samples, as well as CH₄ and CO. The obtained results show that bare ZnO materials were able to promote H₂ photogeneration, being ZnO-f the most efficient in terms of H₂ generation. Loading with Au, in general, increased the photoefficiency for all materials. The most active gold loaded photocatalysts were the ones with the lowest gold nanoparticle sizes: Au/ZnO-f (427 μmol h^-1 g_cat^-1; 5.4 nm) and Au/ZnO-n (280 μmol h^-1 g_cat^-1; 5.8 nm). The samples with largest size (Au/ZnO-r and Au/ZnO-t with 6.2 nm, and Au/ZnO-c with 12.5 nm) were not so efficient (values below 230 μmol h^-1 g_cat^-1). Au/ZnO-f seems to be the most promising photocatalyst, generating the highest amount of H₂ and the lowest quantities of CH₄ and CO. It is also possible that the intricate morphology of ZnO-f may scatter the incoming light, enhancing light absorption efficiency and increasing the photoefficiency of this material.

Full text of DOI:10.1016/j.apcata.2015.10.013

Accepted Manuscript
Title: Photocatalytic performance of Au/ZnO nanocatalysts
for hydrogen production from ethanol
Author: Maria J. Sampaio Jo a˜ o W.L. Oliveira Caroline I.L.
Sombrio Daniel L. Baptista S e´ rgio R. Teixeira S o´ nia A.C.
Carabineiro Cl a´ udia G. Silva Joaquim L. Faria
PII:
S0926-860X(15)30190-3
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2015.10.013
APCATA 15589
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
26-7-2015
14-9-2015
9-10-2015
Please cite this article as: Maria J.Sampaio, Jo a˜ o W.L.Oliveira, Caroline
I.L.Sombrio, Daniel L.Baptista, S e´ rgio R.Teixeira, S o´ nia A.C.Carabineiro,
Cl a´ udia G.Silva, Joaquim L.Faria, Photocatalytic performance of Au/ZnO
nanocatalysts for hydrogen production from ethanol, Applied Catalysis A, General
http://dx.doi.org/10.1016/j.apcata.2015.10.013
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Photocatalytic performance of Au/ZnO nanocatalysts for hydrogen
production from ethanol
a
b
b
b
Maria J. Sampaio , João W.L. Oliveira , Caroline I. L. Sombrio , Daniel L. Baptista , Sérgio
b
a
a
a*
R. Teixeira , Sónia A.C. Carabineiro , Cláudia G. Silva , Joaquim L. Faria jlfaria@fe.up.pt
a
b
LCM - Laboratório de Catálise e Materiais - Laboratório Associado LSRE-LCM, Faculdade
de Engenharia, Universidade do Porto, Rua Dr. Roberto Frias s/n 4200-465 Porto, Portugal
b
Instituto de Física, Universidade Federal do Rio Grande do Sul, Porto Alegre, RS 91501-
9
70, Brazil
*
Corresponding author: Tel: 00351 225 081 645; Fax: 00351 225 081 449
1

Graphical Abstract
2

Highlights
H was photocatalytically produced at a constant rate, from a water-ethanol mixture, using
2
ZnO and Au/ZnO catalysts
Loading ZnO materials with gold increased the photoefficiency towards H production.
2
Au/ZnO materials with variable morphology are obtained by a double impregnation method
for gold loading.
Gold nanoparticles size plays a major role in the photoefficiency of Au/ZnO materials.
Abstract
Gold was loaded on several ZnO samples with different morphologies: needle-like (ZnO-n),
flower-like (ZnO-f), rods (ZnO-r), and prepared by a thermal calcination process (ZnO-t). A
commercial ZnO sample (ZnO-c) was also used for comparison. Bare ZnO and Au/ZnO
materials were used for the photocatalytic production of H from a water-ethanol solution
2
under UV-visible irradiation (λ > 300 nm). During the photocatalytic runs, hydrogen was
generated at a constant rate for all samples, as well as CH and CO. The obtained results
4
show that bare ZnO materials were able to promote H photogeneration, being ZnO-f the
2
most efficient in terms of H generation. Loading with Au, in general, increased the
2
photoefficiency for all materials. The most active gold loaded photocatalysts were the ones
-1
-1
with the lowest gold nanoparticle sizes: Au/ZnO-f (427 mol h g ; 5.4 nm) and Au/ZnO-n
cat
-1
-1
(
280 mol h g ; 5.8 nm). The samples with largest size (Au/ZnO-r and Au/ZnO-t with 6.2
cat
-1
-1
nm, and Au/ZnO-c with 12.5 nm) were not so efficient (values below 230 mol h g ).
cat
Au/ZnO-f seems to be the most promising photocatalyst, generating the highest amount of H2
and the lowest quantities of CH and CO. It is also possible that the intricate morphology of
4
ZnO-f may scatter the incoming light, enhancing light absorption efficiency and increasing
the photoefficiency of this material.
Keywords: Photocatalysis; Hydrogen production; Zinc oxide; Gold catalysts; Photoreforming
3

1
. Introduction
Hydrogen energy and hydrogen economy are at the forefront of any nation’s commitment and
intentions when addressing the clean energy agenda. Hydrogen gas promises a redly available
source, without harmful emissions, non-toxic (a rarity among the most used fuel sources),
more powerful (it takes less to accomplish more), more efficient (more energy per mass of
fuel) and potentially inexhaustible. Like always, every silver lining has a cloud. Hydrogen,
although abundant and present pretty much everywhere, is never alone and needs to be
separated from other elements or substances. Then unlike oil and gas that are easy to store
and transport, hydrogen requires an all new set of economic and technical challenges – even a
new infrastructure to distribution and commercial use. Finally, most of the industrial
processes for isolating hydrogen still require the use of fossil fuels – it is but ironic that for
reducing our dependency on the non-renewable sources we must rely on them.
Reforming (partial oxidation of hydrocarbons) is a well know of such processes [1], but it
produces CO and needs large amounts of energy. A very simple way to obtain H is through
2
2
electrolysis of water [2]. It consists in exposing water to an electrical current, which will
separate it into hydrogen and oxygen. The visible drawback is the need for an intensive
supply of energy. Hydrogen can also be obtained from the gasification of biomass [3-6], but
the application is limited by the economical viability of the process.
Applying photon induced processes to the already available procedures can be a noteworthy
improvement, leading to the production of hydrogen from sustainable and renewable natural
sources. Naturally this requires the use of solar energy to become really independent of non-
renewable sources. Thus, photocatalytic water splitting [7, 8] and photocatalytic reforming of
biomass [9-11] are promising alternatives to generate hydrogen. The later has advantages
since hydrogen is produced with concomitant (photocatalytic) degradation of organic
molecules present in water at mild conditions, also contributing to the treatment of industrial
4

waters. If other natural renewable resources (like solar energy, widely abundant in south
European and south American countries) are used, then process becomes even more
attractive.
Titanium dioxide (TiO ) has been extensively used in photo-related processes [12, 13]. The
2
enormous consumption of this material has prompted the search for alternative catalysts with
improved characteristics [14]. Zinc oxide (ZnO) is a promising alternative, since it has a
bandgap similar to that of TiO , and has low cost and morphological versatility [15]. ZnO
2
materials with several shapes can be obtained, like nanospheres, nanowires, nanotubes,
nanorings, and nanotetrapods [16-18]. These materials are normally designated has wide
bandgap semiconductors, meaning that they mostly absorb on the UV range of the
electromagnetic spectrum, which represents only 3-5% of the solar irradiance at Earth’s
surface. Due to limited activity of these materials under visible light excitation, the scientific
community was prompted to search for modified ZnO and TiO materials with reduced
2
bandgaps, therefore capable of absorbing a wider range of visible light range. Making these
materials efficient photocatalysts implied structural and morphological modifications that
were achieved by many ways. The use of TiO hybrids containing carbon materials, such as
2
activated carbon, carbon xerogel, carbon nanotubes, nanodiamonds, activated carbon fibres
and graphene, has been reported as an effective approach to enhance the semiconductor
activity, both under UV and visible/solar light conditions [19-30]. Modification by dye
photosensitization [31] or mixing with other semi-conductors [32] has also been tested.
Doping with metals, such as Au, has proven to be successful in enhancing photocatalytic
activity of both TiO and ZnO [33-37]. Gold is a noble metal that does not undergo
2
photocorrosion and can be strongly anchored on surfaces, exhibiting a characteristic surface
plasmon band in the visible region, due to the collective excitation of electrons in the gold
nanoparticles [38, 39].
5

Although Au/TiO based catalysts have been recently used for the photocatalytic reforming
2
of organic molecules, including ethanol [38-43], to the best of our knowledge that has not
been yet performed using Au/ZnO materials (even though this type of catalysts has been used
for several photocatalytic applications mostly in the fields of water and air decontamination
[
37, 44-49]). In this work, we loaded gold on several ZnO samples with different
micro/nanoscale morphologies. A commercial ZnO sample was also used for comparison
purposes. The synthesized materials were used for the photocatalytic production of H from a
2
water-ethanol solution under UV-visible irradiation ( > 300 nm). During the photocatalytic
runs, H was generated at a constant rate for all the catalytic systems.
2
2
2
. Experimental
.1 Synthesis of ZnO materials
Different ZnO materials were synthesized by diverse techniques. Needle-like ZnO (ZnO-n)
was prepared by a hydrothermal process: a 4 M NaOH (Aldrich, > 97%) aqueous solution
was heated to 70 ºC under vigorous stirring, and then a 2 M Zn(NO ) ·6H O (Aldrich, >
3
2
2
9
9%) aqueous solution was added dropwise to the first with 1h stirring. The resulting solid
precipitate was filtered, washed and allowed to dry at 60 °C in an oven [50]. Rods of ZnO
ZnO-r) were obtained through a similar process, but the molar ratio between the solutions
(
was 4:1 (instead of 2:1). The temperature was 80 ºC and the stirring time was 6 h. Flower-
like ZnO (ZnO-f) was produced from a mixture with equal molar amounts of Zn(NO ) ·6H O
3
2
2
and hexamethylenetetramine (Aldrich, 99%). The pH was adjusted to 10.0 using an ammonia
solution (Aldrich, 25%). The mixture was heated to 90 ºC for 1h in an autoclave. The
obtained solid was washed and dried in air. Another sample was prepared by a thermal
process (ZnO-t), i.e., calcination of the precursor: zinc acetate dihydrate (Aldrich > 99.5%)
-1
was heated at 5 ºC min up to 600 ºC in static air, and maintained there for 2 h [51]. A
6

commercial sample (ZnO-c) from Strem Chemicals (85–95% ZnO, 3–7% Al O , 0.5–3%
2
3
CaO) was used for comparison purposes.
2
.2 Gold deposition on ZnO materials
Au was loaded by double impregnation method (for a nominal loading of 1 wt. %), using
HAuCl ·3H O (Alfa Aesar) as precursor. After impregnation with a chloroauric acid solution
4
2
-3
(
5×10 M) with sonication, a second impregnation step with addition of a solution of 1 M
Na CO (Aldrich > 99%) followed with stirring [52]. This was needed for chloride removal,
2
3
which is well known to cause sintering of gold nanoparticles, making them inactive [53].
2
.3 Materials characterization
The morphology of bare ZnO and Au/ZnO was analyzed by scanning electron microscopy
SEM) and energy-dispersive X-ray spectroscopy (EDS) using a FEI Quanta 400 FEG
(
ESEM/EDAX Genesis X4M (15 keV) instrument.
The materials of Au/ZnO were visualised through high-resolution transmission electron
microscopy (HRTEM). High Z-contrast images were also acquired using scanning
transmission electron microscopy (STEM) and a high-angle annular dark-field (HAADF)
detector. The analyses were carried out in a Cs-corrected FEI Titan 80/300 microscope at
INMETRO.
The different forms of ZnO used as support were also analysed by X-ray diffraction (XRD)
on a Siemens D-5000 diffractometer, at room temperature, using CuKα radiation (=1.5418
Å). Textural characterisation, namely determination of the specific surface area (SBET), was
performed by analysis of N adsorption isotherms at -196ºC using the BET method, in a
2
Quantachrome NOVA 4200e multi-station apparatus.
7

The optical absorption spectrum on the UV-Vis range was obtained with a Jasco V-560 UV-
Vis spectrophotometer, equipped with an integrating sphere attachment (JASCO ISV-469).
The various spectra were recorded in diffuse reflectance mode and transformed by the
JASCO software to equivalent absorption Kubelka–Munk units. The bandgaps (Eg) were
obtained from the UV-Vis spectra.
Diffuse reflection infrared Fourier transformed (DRIFT) spectroscopic analysis was
performed on a Nicolet 510P FTIR Spectrometer. The interferograms were converted by the
OMINC software to equivalent absorption units in the Kubelka–Munk scale.
The gold loading was determined by Inductively Coupled Plasma Optical Emission
Spectroscopy (ICP-OES) using a Horiba Jobin Yvon Ultima 2 apparatus.
The metal dispersion was calculated as D = (6.n .M)/(.N.d ), where n is the number of
M
s
p
s
1
9
-2
atoms at the surface per unit area (1.15  10 m for Au), M is the molar mass of gold
-1
-3
23
(
196.97 g mol ),  is the density of gold (19.5 g cm ), N is Avogadro’s number (6.023  10
-1
mol ) and d is the average particle size (determined by STEM, admitting that particles are
p
spherical).
Photoluminescence (PL) spectra were acquired at room temperature on a Crylas CW 266 nm
laser as an excitation source. The laser power was kept in 1 mW and the sample emission was
collected by a CCD camera attached to a grating monochromator.
2
.3 Photocatalytic reactions
The photogeneration of hydrogen was carried out in a double-wall quartz photochemical
reactor. A 7 mg sample of the photocatalyst was suspended in 8 mL of a 1:4 (volume ratio)
ethanol:water solution with the help of magnetic stirring. The temperature was maintained at
2
5°C, by water circulation through the photochemical reactor, using a thermostatic bath.
Prior to irradiation, the system was deaerated by Ar bubbling for 30 min to remove any traces
8

of air. A 300 W Hg–Xe lamp (PerkinElmer; Cermax-PE300) was used as excitation source.
The gases produced during the experiments were quantified using gas chromatography at
room temperature in an Agilent 6820 GC chromatograph equipped with a thermal
conductivity detector (TCD) with a Porapak Q (80/100 mesh) column by using argon as the
carrier gas. The amounts of gases produced were measured at intervals of 0.5 h using a gas-
tight syringe with a maximum volume of 100 μL. To check the reproducibility of the
photocatalytic activity, each sample was measured three times.
3
3
. Results and discussions
.1 Catalysts characterization
The ZnO materials were characterised by X- ray powder diffraction. The observed diffraction
peaks of all samples match well with the hexagonal wurtzite structure, except in the case of
ZnO-c that reveals the presence of alumina (-Al O phase), which is in in agreement with
2
3
the Strem Chemicals manufacturer information. Loading with gold did not affect the
crystallinity of ZnO. Also no Au phase was detected (Figure 1), most likely due to the low
loading and low nanoparticle size.
The crystallite sizes of the ZnO samples were derived from the XRD analysis. ZnO-n
presents the lower crystallite dimensions (39 nm), followed by ZnO-f (47 nm), ZnO-t (54
nm), ZnO-r (60 nm) and ZnO-c (72 nm). The materials present very low surface areas
2
-1
ranging from 2 to 30 m g (Table 1), which is very common for zinc oxide [33, 37].
Although the gold loading was predicted as 1 wt.%, ICP analysis revealed the presence of
lower percentages of the metal and varying depending on the ZnO material (Table 1).
Previous works with Au/CeO , Au/MnO and Au/ZnO catalysts also showed that DIM
2
x
method produces catalysts with gold loadings lower than the expected, due to the leaching of
part of the metal during the deposition process [33, 54]. For ZnO-n, ZnO-r and ZnO-t, Au
9

loadings range from ca. 0.30-0.40 wt. %, while 0.74% and 0.90 wt. % were obtained for
ZnO-c and ZnO-r.
The morphology of the different ZnO materials was observed by SEM. Selected images can
be found in Figure 2.
It is interesting to note that the synthesis method and preparation conditions have a
significant influence in the morphology of the samples. ZnO-r, ZnO-n and ZnO-f were
obtained by hydrothermal routes, but using different temperatures, pressures, reaction times
and molar proportions of the reactants. ZnO-n consists in needle-like structures having ca.
6
00 nm in length (Figure 2a), while ZnO-r shows hexagonal prismatic particles with lengths
varying between 800 and 1300 nm (Figure 2b). ZnO-n and ZnO-r were obtained by similar
procedures, but at different temperatures (70 ºC and 80 ºC, respectively), and diverse sodium
hydroxide to zinc nitrate molar ratio (2:1 and 4:1, respectively) and dissimilar synthesis times
(1h and 6 h, respectively). The morphological differences between both materials are
associated with the different velocities of crystal growth, ZnO-n crystals growing faster than
ZnO-r, which results in the disappearance of the hexagonal planes that can be observed in
ZnO-r [37]. In ZnO-f, needle-like structures of 2 m of average length grow as bundles
coming from a same core (Figure 2c). ZnO-t (Figure 1d) is constituted by a mixture of
spheroidal particles (175 nm average diameter) and needle-like structures (700 nm average
length). Finally, ZnO-c is constituted by a mixture of larger ZnO particles (100–500 nm) with
smaller (10–20 nm diameter) ‘‘needle”-like particles of Al O (not shown), as already
2
3
reported in an earlier publication [33].
STEM micrographs (Figures 2e-h) reveal the presence of Au particles of very small
dimensions at the surface of the ZnO materials. The average Au particle sizes varied between
5
.2 nm (for Au/ZnO-f) and 6.2 nm (for Au/ZnO-r and Au/ZnO-t), as shown in Figures 2i-l.
Gold nanoparticles deposited over ZnO-r had mostly spheroidal shape, while metal
1
0

nanoparticles with elongated form were observed for the remaining materials (Figures 2i-l).
For Au/ZnO-c, spheroidal Au nanoparticles with sizes ranging from 6 to 20 nm (average size
of 12.5 nm) were observed (not shown), as already reported in an previous paper [33].
The UV-Vis spectra of ZnO materials can be observed in Figure 3a, depicting a broad
absorption in the UV range (Eg > 3.10 eV), characteristic of ZnO materials [33]. After
loading with gold, a new absorption band is shown between 450 and 650 nm (2.76 eV > Eg >
1
.91 eV), which is typical of gold surface plasmon (Figure 3b) [36, 55]. Moreover, a shift to
lower energies was observed for ZnO-r, ZnO-f and ZnO-c after the introduction of Au
nanoparticles (Table 1).
The normalized photoluminescence (PL) spectra of the synthesized materials are shown in
Figure 4. An intense PL band common to all the spectra is observed in the visible range,
being associated to deep-level-emissions (DLE) originated from atomic point defects such as
oxygen vacancies (V ), oxygen interstitials (O ), zinc antisites (Zn ), oxygen antisites (O )
O
i
O
Zn
or zinc vacancies (V ) [56, 57]. Also, the near-band-edge (NBE) emission at UV region is
Zn
observed. In general, the relative intensity of both bands is a reasonable indicative of the
defect amount in the atomic structure [56, 57]. ZnO-t shows the highest emission in the UV,
meaning that this material has the lowest concentration of point defects, probably due to the
high calcination temperature used in its synthesis (Figure 4a).
The other synthesized ZnO samples show a higher degree of defects justifying the lower
emission in the UV. In the case of ZnO-r, some residual emission can be observed, being
absent for ZnO-f and ZnO-n, meaning that recombination of photogenerated charges undergo
preferably via defect levels rather than via gap (UV).
The PL spectra of the Au-loaded materials (Figure 4b) are similar to the ones of bare ZnO,
indicating that recombination mechanisms are similar. The same behaviour was observed for
the ZnO-c and Au/ZnO-c materials (not shown).
1
1

3
.2 Photocatalytic hydrogen generation
The synthesized materials were used for the photocatalytic production of H from a water-
2
ethanol solution under UV-visible irradiation ( > 300 nm). H was generated at a constant
2
rate for all the catalytic systems.
Results show that bare ZnO materials were capable to promote H photogeneration (Table 2
2
and Figure 4). However, the rate of hydrogen production varied among the materials. The
-1
-1
catalysts with lower performances for H generation were ZnO-c (66 mol h g ),
2
cat
-1
-1
followed by ZnO-r (97 mol h g ). ZnO-n and ZnO-t show similar rates of H production
cat
2
-1
-1
(
c.a. 230 mol h g ) while ZnO-f was the best performing catalyst capable to promote H2
cat
-1
-1
generation at a constant rate of 301 mol h g . Apparently the BET surface area of the
cat
materials does not play a role here, since ZnO-c has the highest value (Table 1) and the worse
performance. It might be that, even present in a low percentage, the Al O present in this
2
3
sample (as reported earlier [33]) has a detrimental role in its photocatalytic activity. In fact,
the higher bandgap of ZnO-c comparing to the other bare ZnO materials (Table 1) can be
attributed to the presence of alumina, a wide bangap material (~8.8 eV), resulting in a
decrease in the photoefficiency of this sample. It was reported that light-harvesting or light
trapping properties were found for certain ZnO surface structures or nanomaterials such as
hollow spheres and pore-array structures [58-60]. It is possible that the intricate morphology
of ZnO-f, constituted by bundles of needle-like structures deriving from a same core, may
scatter the incoming light, enhancing light absorption efficiency, thus increasing the
photoefficiency of this material. In fact, we have measured the reflectance of ZnO
suspensions (not shown) and observed that for ZnO-f the reflectance is much lower than for
ZnO-t and ZnO-n (both presenting similar spectra and also similar photoefficiency), which
may result from multiple interface light scattering.
1
2

Addition of gold, in general, improved the photocatalytic activity towards H production
2
(Table 2 and Figure 4), with the exception of ZnO-t, where addition of gold was detrimental.
The efficiency of H production appears to be related to the size of Au nanoparticles. The
2
-1
-1
most active gold loaded photocatalysts were Au/ZnO-f (427 mol h g ) and Au/ZnO-n
cat
-1
-1
(
280 mol h g ), as shown in Table 2 and Figure 4. Those had the lowest gold
cat
nanoparticle sizes (5.4 nm and 5.8 nm, respectively). The samples with largest size (Au/ZnO-
r and Au/ZnO-t with 6.2 nm, and ZnO-c with 12.5 nm) were not so efficient (values below
-1
-1
2
31 mol h g ). The influence of gold nanoparticle size in photocatalytic activity is well
cat
documented [35], although other factors, such as the nature of the support [61, 62], and the
interaction of the support with gold [63] also play an important role, just like in
heterogeneous catalysis by gold in general [64, 65]. However, Idriss and co-workers [61, 62]
reported that gold nanoparticle size did not affect the photoreaction rate of hydrogen
production from water-ethanol mixtures over the 3-12 nm range on Au/TiO catalysts. The
2
same authors also noticed that the nature of the support has also a great deal of influence, as
they found that anatase was much more efficient that rutile as a support, producing more
active materials, for the same gold nanoparticle size [61, 62]. That does not seen to the case
in our work, as for the same gold nanoparticle size (6.2 nm) the difference between Au/ZnO-r
-1
-1
and Au/ZnO-t performance is not so different (226 and 211 mol h g , respectively).
cat
Other authors also found that the nanoparticle size of silver on ZnO could affect the
photocatalytic activity [66].
A synergy factor (R), defined by the ratio between rAu/ZnO, H2 and rZnO, H2, has been determined
in order to quantify the effect of loading gold on ZnO materials (Table 2). The highest R was
obtained for ZnO-c and ZnO-r, with 2.98- and 2.33-fold increase in the rate of H production
2
after loading with Au nanoparticles, respectively. For Au/ZnO-n and Au/ZnO-f, an increase
1
3

of 20 and 42% was observed, when compared to the bare materials, respectively. However no
increase in the rate of hydrogen production was obtained for Au/ZnO-t material.
For better comparison of the results obtained with the different Au-loaded materials, the rates
of H production were normalized by the mass of gold present in each catalyst (r[Au/ZnO, H2]Au)
2
Table 2) and by the metal dispersion (r[Au/ZnO, H2]Au, DM), since ICP and STEM analysis
revealed that distinct percentages of gold with different nanoparticle sizes had been loaded on
the ZnO samples (Table 1 and Figure 1). Results show that, even normalizing the rates of H2
production by the mass of gold and by the metal dispersion, Au/ZnO-f remains as the most
active photocatalyst (Table 2), which is must be related to the lower dimensions of the Au
nanoparticles and to enhanced light trapping properties of the ZnO-f material.
The above results show that several factors such as ZnO purity, morphology, and the amount,
shape and dimensions of Au nanoparticles, have influence in the efficiency of ZnO and
Au/ZnO materials towards H generation by photoreforming of ethanol.
2
After being irradiated with light at  > 300 nm, it is expected that charge separation occurs,
–
+
with electrons (e ) and holes (h ) migrating to the surface of ZnO. Since the Fermi level of
Au is lower than that of ZnO, photo-excited electrons can migrate from the surface of the
semiconductor to Au nanoparticles, where they can reduce water-protons to produce H2.
In some cases, Au nanoparticles have also the ability of being photoexcited by visible light,
due to surface plasmon band resonance. In fact, in a previous work, we have demonstrated
that Au/ZnO-t shows photocatalytic activity towards phenol oxidation under 546 nm
irradiation, i.e., exclusively exciting gold nanoparticles [37]. In the irradiation conditions
used in this work ( > 300 nm), it is expected that both ZnO-t and gold nanoparticles may be
simultaneously photoexcited. This may explain the detrimental effect observed by the
1
4

introduction of Au in ZnO-t for photocatalytic H generation, as a result of the simultaneous
2
electron transfer between the semiconductor and the metal nanoparticles.
Sakata and Kawai reported the photocatalytic hydrogen generation from water-ethanol
mixtures over Pt-loaded TiO [67]. The obtained products were H , CH and CO in the gas
2
2
4
2
phase, and acetaldehyde (CH CHO) and acetic acid (CH COOH) in the aqueous phase. These
3
3
authors suggested that surface trapped holes would interact ethanol to form acetaldehyde:
ꢀꢁ
ꢂꢃ ꢅ ꢇꢅꢈꢉ ꢅ ꢂꢃꢅ ꢃꢅꢇ
ꢁ
(1)
ꢄ
ꢆ
ꢊ
Then acetaldehyde is further oxidizer to acetic acid, following eq. (2):
ꢀꢁ
ꢂꢃꢅ ꢃꢅꢇꢂꢅ ꢇꢈꢉ ꢅ ꢂꢃꢅ ꢃꢇꢇꢅ
ꢁ
(2)
ꢊ
ꢄ
ꢊ
+
While H produced via eqs. (1) and (2) can be reduced by photogenerated electrons
producing H , acetic acid is fully decomposed into CO and CH through a photo-Kolbe
2
2
4
route.
Nadeem et al. [37] proposed a different reaction pathway for the photocatalytic reforming of
ethanol over Au/TiO . In that case, ethanol was deprotonated upon adsorption over the
2
catalyst and then each ethoxide injects two electrons into the conduction band of TiO to
2
produce an -hydroxyethyl radical, which is then oxidized to form acetaldehyde. In other
work by Wahab et al. on H production from ethanol using metal-loaded SrTiO catalysts,
2
3
authors proposed that ethanol decomposition undergo through CH CO radical, which may
3
react with surface oxygen resulting in acetate species, or may split to CH radical and CO
3
[
68]. Then CH radicals could react with OH radicals yielding methanol, which would easily
3
decompose into H and CH .
2
4
This later mechanism is believed to be similar to the one occurring for the reactions with the
ZnO and Au/ZnO materials used in this work since, apart from H , also CH and CO were
2
4
detected in the gas phase and produced at a constant rate (Figure 5). For bare ZnO materials
the rate of gas formation followed the order r > rCH4 > r (Figure 5a).
H2
CO
1
5

Addition of gold to ZnO also increased the amounts of CO and CH by-products generated
4
for all systems (Figure 5b). This increase is particularly noticeable for Au/ZnO-r and
Au/ZnO-c (some of the samples with the largest Au nanoparticle sizes). In general, lower
gold nanoparticle sizes lead to more production of both CH and CO. It has been reported that
4
Au nanoparticle size has a central influence in the selectivity towards CO formation by
photocatalytic reforming of alcohols since CO oxidation occurs at the interface between the
metal and the semiconductor [69]. Thus, the catalysts showing smaller ZnO nanoparticles
would be more efficient towards CO conversion to CO . However, there is a striking
2
difference between Au/ZnO-r and Au/ZnO-t, that have the same gold nanoparticle size (6.2
nm) and large differences in CH and CO production (being the former much more active
4
than the latter for the production of these by-products), which indicates that gold particle size
cannot be the only factor accounting for the different product yields. It was also reported that
defect sites in the semiconductor act as active sites for CO formation [70]. The PL spectrum
of Au/ZnO-t reveals the presence of a lower amount of defects comparing with Au/ZnO-r,
which may contribute to the lower amount of CO (and CH ) produced using Au/ZnO-t. All
4
Au/ZnO systems showed amounts of CH larger than those of H . Au/ZnO-f was the
4
2
exception, which may be attributed to its lowest gold nanoparticle size. Therefore this
material is a promising photocatalyst, but further research needs to be carried out to make it
more selective to H2.
Conclusions
Bare ZnO samples promoted H photogeneration, being ZnO-f the most efficient in terms of
2
H generation. That can be due to its intricate morphology that may scatter the incoming
2
light, enhancing light absorption efficiency and increasing the photoefficiency.
Loading with Au increased the photoefficiency for all materials, especially that of ZnO-f,
-1
-1
which was the most active photocatalyst with a rate of 427 mol h g . That can also be
cat
1
6

related with its lower gold nanoparticle size (5.4 nm). Catalysts with largest sizes (like
Au/ZnO-r, Au/ZnO-t with 6.2 nm, and Au/ZnO-c with 12.5 nm) were not so efficient and had
highest amounts of CO and CH production, compared to H .
4
2
A mechanism involving ethanol decomposition through a CH CO radical, which may react
3
with surface oxygen resulting in acetate species, or may split to CH radical and CO seems to
3
take place. Then CH radicals react with OH radicals, yielding methanol that can easily
3
decompose into H and CH .
2
4
Acknowledgements
This work was co-financed by FCT/MEC and FEDER under Program PT2020 (Project
UID/EQU/50020/2013) and NORTE-07-0162-FEDER-000050 co-financed by FEDER
through ON2 and QREN. MJS acknowledges FCT for her Ph.D. grant
(
SFRH/BD/79878/2011), while SACC and CGS acknowledge the FCT Investigator Program
IF/1381/2013 and IF/00514/2014, respectively) with financing from the European Social
(
Fund and the Human Potential Operational Program. This work was partially supported by
Brazilian agencies CNPq and CAPES. DLB and JWLO thank DIMAT/NULAM for the use
of Electron Microscopy facilities at INMETRO, Brazil.
1
7

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6

Figure Captions
Figure 1. XRD diffractograms of the gold loaded ZnO materials: Au/ZnO-c (a), Au/ZnO-t
(b), Au/ZnO-f (c), Au/ZnO-r (d) and Au/ZnO-n (e). *Alumina peaks of ZnO-c.
2
7

a
e
i
500 nm
20 nm
b
f
j

m
50 nm
c
g
k
50 nm
5
m
d
h
l
500 nm
50 nm
Figure 2. SEM micrographs of ZnO-n (a), ZnO-r (b), ZnO-f (c) and ZnO-t (d); STEM micrographs
of Au/ZnO-n (e), Au/ZnO-r (f), Au/ZnO-f (g) and Au/ZnO-t (h); histograms for Au particle size
distribution (i-l).
2
8

Figure 3. Diffuse reflectance UV-Vis spectra of ZnO (a) and Au/ZnO (b) materials.
2
9