Making H2 from light and biomass-derived alcohols: The outstanding activity of newly designed hierarchical MWCNT/Pd@TiO2 hybrid catalysts

Article abstract of DOI:10.1039/c6gc01979j

Hydrogen evolution is among the most investigated catalytic processes given the importance of H₂ from an industrial and an energy perspective. Achieving H₂ production through green routes, such as water splitting or more realistically photoreforming of alcohols, is particularly desirable. In this work, we achieve a remarkable H₂ productivity through photoreforming of either ethanol or glycerol as a sacrificial electron donor by employing a hybrid nanocatalyst where the properties of multi-walled carbon nanotubes (MWCNTs), Pd nanoparticles and crystalline TiO₂ are optimally merged through appropriate engineering of the three components and an optimised synthetic protocol. Catalysts were very active both under UV (highest activity 25 mmol g^-1 h^-1) and simulated solar light (1.5 mmol h^-1 g^-1), as well as very stable. Critical to such high performance is the intimate contact of the three phases, each fulfilling a specific task synergistically with the other components.

Full text of DOI:10.1039/c6gc01979j

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Making H from light and biomass-derived alcohols: the
2
outstanding activity of newly designed hierarchical
Received 00th January 20xx,
Accepted 00th January 20xx
MWCNTs/Pd@TiO hybrid catalysts
2
a
a,*
a, b
c
a, b, *
a,d,e, *
DOI: 10.1039/x0xx00000x
A. Beltram, M. Melchionna, T. Montini, L. Nasi, P. Fornasiero,
and M. Prato
www.rsc.org/
Hydrogen evolution is among the most investigated catalytic processes given the importance of H
an energy perspective. Achieving H
photoreforming of alcohols, is particularly desirable. In this work, we achieve a remarkable H
photoreforming of either ethanol or glycerol as sacrificial electron donor by employing a hybrid nanocatalyst where the
properties of multi-wall carbon nanotubes (MWCNTs), Pd nanoparticles and crystalline TiO are optimally merged through
appropriate engineering of the three components and optimised synthetic protocol. Catalysts were very active both under
2
from an industrial and
2
production through green routes, such as water splitting or more realistically
productivity through the
2
2
-
1
-1
-1 -1
UV (highest activity 25 mmol g h ) and simulated solar light (1.5 mmol h g ), as well as very stable. Critical to such high
performance is the intimate contact of the three phases, each fulfilling a specific task synergistically with the other
components.
3
driven synthesis of benzimidazole. In this complex scenario,
Introduction
development of efficient photocatalysts, in particular highly
2
engineered nanocatalysts, for the generation of H has become a
4
prominent field of research. Indeed, although there are several
H
2
is a crucial molecule for many industrial processes including oil
refining, hydrodesulfurization (HDS), hydrodenitrogenation (HDN),
synthesis of NH and related fertilizers and many organic
hydrogenation reactions in fine chemicals production. In addition,
is indicated as one of the promising energy vectors of the future,
5
methods currently available for hydrogen production, particularly
desirable are those strategies based on sustainable and
3
6,7,8,9,10
environmental friendly resources such as water and light.
1
Pure photocatalytic water splitting, however, still suffers from a
number of shortcomings that are not of easy solution. A more
realistic “clean” strategy relies on the utilization of biomass-derived
H
2
2
even if in combination with a pool of different renewable fuels.
From a sustainability perspective, utilization of H -based energy,
2
species as sacrificial electron donors in the photocatalytic H
2
generated in a sustainable manner, implies a carbon-neutral
footprint and therefore represents an extremely attractive option
for a future switch to green methodologies. Surely, there are still
many technical problems which are slowing down the
implementation strategies, including the requirement for large
11,12,13,14
production process.
Ethanol and glycerol are two biomass-derived products, the former
also known as bioethanol when sustainably obtained from next-
generation biomasses, such as agriculture residues or lignocellulose,
and the latter representing around 10 wt % byproduct of biodiesel
infrastructures investments and safety issues related to H
and distribution. Another highly interesting prospect is the in-situ
photocatalytic production to harness the green synthesis of
industrially relevant molecules, a recent example being the photo-
2
storage
15
industry. Hence, the photoreforming of ethanol and glycerol
H
2
catalyzed by heterogeneous catalysts opens a very attractive route
16,17,18
2
to H production.
As far as general heterogeneous photocatalysts are concerned, TiO2
still represents a benchmark material due to its high activity, low
1
9
cost, large availability, low toxicity and stability to corrosion.
However, its utilization still suffers from inherent limitations, above
all the poor absorbance in the visible spectrum and therefore poor
visible light photoactivity, as well as fast recombination of the
photogenerated charge carriers (electrons and holes). The most
popular method to improve hydrogen photo-catalytic production
relies on the deposition onto the TiO
2
surface of noble metal (Au,
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Pt, Pd, Ag) and base metal (Ni, Cu) nanoparticles, that are able to carbonaceous support. Remarkably, while the vast majority of
capture photoexcited electrons and retard the electron/holes photocatalytic systems for H evolution relies on the use of the
There have been numerous reports on generally most active Pt active phase, we were able to surpass H
2
2
0,21,22
recombination.
2
successful photoreforming of oxygenated compounds by TiO
evolution rates by adopting the less rare and costly Pd, establishing
a definite stride forward towards market feasibility. It is noteworthy
that these novel catalysts can also generate H from ethanol and
2
1
1,23,24
loaded with metal nanoparticles.
2
One recent advance in the photocatalysis by TiO
the combination of TiO
2
-based catalysts is
glycerol under simulated sunlight irradiation with excellent activity,
thus further expanding the significance of these catalysts for
2
with carbon nanotubes (CNTs) to form
25
nanocarbon-inorganic hybrids with improved performances. CNTs
have emerged as intriguing active supports in a range of catalytic
applications due to their fascinating mechanical, electronic, optical
2
possible solar driven H production.
2
6,27
and thermal properties.
In particular, as far as photocatalytic Results and discussion
processes are concerned, reports show CNTs as active supports able
to scavenge away photoexcited electrons and retard charge
2
8,29
Synthetic procedures
recombination, therefore promoting reaction rates.
Other
The assembly of the nanocarbon-inorganic hybrid catalysts was
carried out utilizing three building blocks: a) benzoic acid-
functionalised multi-wall carbon nanotubes (t-MWCNTs), b)
palladium nanoparticles functionalised with mercapto-undecanoic
acid, c) titanium tetra-(n-butoxide). The synthetic scheme to access
the final hybrid can be summarised in three phases (A, B and C in
Figure 1). The nanotubes building block was accessed via radical
addition of in situ formed diazonium salt of the benzoic acid (Figure
possible mechanisms are however postulated, based for instance
on a photosensitizing effect by the CNTs, which undergo the light-
induced charge separation and inject the photoexcited electrons
3
0,31
into the TiO
2
conduction band
or formation of intermediate
states arising from Ti-O-C bondings such as those observed in
3
2,33
carbon-doped TiO2.
The effective result of the CNT-TiO2
hybridization, when appropriately achieved, is an increased
photoactivity or a significant shift of the absorbance into the visible
light, both important requisites for the further developments of
3
9
1
, Step A). As opposed to the oxidation treatment of CNTs, we
chose this strategy for the modification of MWCNTs in order to
have a more controlled functionalisation, endowing the nanotubes
with COOH groups while minimising any extra damage of the
polyaromatic framework by inserting additional oxygenated groups
of no utility, such as epoxides or ketones. Freshly prepared
2 2
TiO -based material for H photo-production. Ahmad et al
reported on alcohol photoreforming active catalysts composed of
single-walled carbon nanotubes (SWCNTs) combined with TiO2,
ascribing the improved activity to the electron sink effect generated
3
4
by the SWCNT component. In another report, the main factor for
Pd@TiO
2
precursors were obtained via slow addition of Pd-MUA to
the successful H2 production from triethanolamine solutions by
a THF solution of Ti(n-OBu) , causing the self-assembling of the Ti(n-
multi-walled carbon nanotubes (MWCNTs)/TiO
with Pt nanoparticles was discussed
2
composites loaded
in terms of the
4
4
OBu) around the Pd-MUA in a core-shell configuration (Figure 1,
4
0
35,36
Step B). The as-functionalised MWCNTs were then reacted with
photosensitizing by the MWCNTs.
A more recent advance in H
2
the Pd-MUA@Ti(n-OBu) : ligand exchange between the alkoxide
production by MWCNTs/TiO composites explored the loading with
2
4
3
7
and the benzoic acid groups guarantee initial attachment of the
inorganic nanoparticles to the t-MWCNTs sidewalls, with the t-
MWCNTs then templating the growth of the oxide layer along the
cylindrical scaffold. In contrast, poorer interaction between
other metals such as Ir, Au, Pd as well as Pt. We have previously
shown how a hierarchical system based on CNTs hybridized with a
TiO
2
shell incorporating Pd nanoparticles shows superior
production with respect to the conventional
photocatalytic H
2
MWCNTs and Pd-MUA@Ti(n-OBu)
4
units was observed when
separated components in a model reaction test of photoreforming
of methanol. The synergy between the properties of the MWCNTs
unfunctionalised pristine nanotubes were employed, confirming
that the attached benzoic acid groups serve as essential anchor
points for immobilising the inorganic precursors. The relative
and the Pd@TiO
2
structure is the key determinant for the good
3
8
photocatalytic ability.
4
amount of MWCNTs and Ti(n-OBu) was varied in order to have two
Based on the above concept, in this work we have elaborated a new nominal compositions with 10 and 20 wt % of nanotubes, while the
design for a hybrid nanomaterial incorporating functionalised % of Pd remains fixed at 1.5% wt. Final hydrolysis with H O/THF
2
MWCNTs, Pd nanoparticles and TiO
2
2
, which fully exploits the affords the MWCNTs/Pd@TiO2: 10-CNTs/Pd@TiO -fresh (10%
individual component’s property As a result, we achieved high MWCNTs), 20-CNTs/Pd@TiO -fresh (20% MWCNTs)) (Figure 1, step
.
2
activity in photoreforming processes exploiting the use of C). As part of step C, the hybrid materials were subjected to
industrially relevant biomass-derived alcohols, such as ethanol and calcination at 350°C for 5 hours to give the corresponding calcined
glycerol. By careful tuning of the MWCNTs characteristics and by materials (10-CNTs/Pd@TiO -calc, 20-CNTs/Pd@TiO -calc). For
2
2
2
controlling the thermal formation of crystalline TiO around the reference, the Pd@TiO2 without MWCNTs was also prepared
MWCNTs scaffold, we were able to prepare hybrid materials that (Pd@TiO -calc).
2
can compete as a novel benchmark catalysts in H
2
photo-
production. The catalyst is among the most active of those based on
2
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Fig 2. TGA analysis under air of the p-MWCNTs, (Ph-COOH)-MWCNTs and fresh samples
after drying (top) and of the samples after calcination at 350 °C in air (bottom).
Fig 1. General Synthetic scheme of hierarchical CNTs/Pd@TiO
CNTs/Pd@TiO
-calc. A) Covalent functionalisation of the MWCNTs with benzoic acid Raman spectra of PhCOOH-MWCNTs shows the characteristic bands
molecules through radical addition of in-situ made diazonium salt; B) Self-assembly of of the CNTs: the disorder-induced D band (
the Pd@TiO precursor; C) hydrolysis to obtain the final hybrid, fresh and calcined.
2
-fresh and
2
-
1
∼
1300 cm ) with its
-1
2
first (
∼ 1600 cm , D’ band) and second-order related harmonics
-1
-1
(
∼
2300 cm , 2D-band), and the G-band (
∼
1590 cm ), due to the
in-plane vibrational mode of the sp graphitic framework. After the
hybridization with Pd@TiO , the Raman spectra of the fresh
samples (both 10-CNTs/Pd@TiO -fresh and 20-CNTs/Pd@TiO
fresh) suggest that TiO is predominantly present as an amorphous
material, as no extra peak due to crystalline TiO
Calcination induces the crystallization of the TiO
Characterisation
2
TGA analyses under air confirm the successful organic
functionalisation of the MWCNTs and the incorporation of the three
building blocks in well-defined ratios in the final hybrids. The weight
2
2
2
-
2
loss at
∼ 350 °C in the (Ph-COOH)-MWCNTs (about 3 wt%) is due to
2
are observed.
shell, as the
the removal of benzoic acid moieties on the nanotube sidewalls.
TGA under air (Figure 2, top) exhibits the weight losses due to the
oxidation of residual organic ligands (mostly butanol) around 200 °C
and of the MWCNTs around 500 °C, with the remaining weight
given by the inorganic components. Consistently, the calcined
samples are free from the first weight losses, as the organics have
been removed during calcination prior to the analysis. The content
2
anatase fingerprint becomes evident in the Raman spectra, with an
-1
-1
-1
intense band at 146 cm and minor bands at 198 cm , 395 cm ,
-1
-1 41, 42
5
13 cm and 639 cm .
2
In agreement, the sample Pd@TiO -calc
prepared without MWCNTs showed only the bands related to the
anatase phase (Figure 3, top).
of MWCNT was estimates as
CNTs/Pd@TiO -calc and 20-CNTs/Pd@TiO
Figure 2, bottom).
∼
9 and
∼
14 wt% for 10-
In agreement with Raman analysis, XRD patterns of the samples
calcined at 350°C show the typical reflections related to the anatase
phase (Figure 3, bottom). Notably, no reflections can clearly be
related with Pd nanoparticles, reasonably because of the low metal
loading and the small average size of Pd nanoparticles, as observed
2
2
-calc, respectively
(
2
by HR-TEM. The mean crystallite size of TiO is not significantly
affected by the formation of the hybrid materials with MWCNTs,
indicating that the growth of anatase crystallites is mainly directed
by the self-assembling around Pd nanoparticles.
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analysis, resulting from the higher loading of oxide (Figure S2).
Despite being covered by the layer of TiO , HR-TEM analysis of 20-
2
CNTs/Pd@TiO -calc evidences the crystalline Pd nanoparticles,
2
whose size ranges between 3 and 5 nm (Figure S3). The structure of
the hybids after catalysis remained completely unaltered as
qualitatively observed by TEM.
Fig 4. Representative HAADF-STEM of 20-CNTs/Pd@TiO
2
-fresh (A) and 20-
-calc (B). Representative TEM of 20-CNTs/Pd@TiO -fresh (C) and 20-
-calc (D). The morphology of catalysts 10-CNTs/Pd@TiO -fresh and 10-
-calc are similar.
CNTs/Pd@TiO
CNTs/Pd@TiO
CNTs/Pd@TiO
2
2
2
2
2
Fig 3. Comparison of the Raman spectra of the PhCOOH-MWCNTs, CNT/Pd@TiO
fresh, CNT/Pd@TiO -calc (top) and powder XRD patterns of 10-, 20-CNT/Pd@TiO -calc
and Pd@TiO -calc (bottom). CS = mean crystallite size.
2
-
2
2
2
TEM micrographs confirm the successful assembly of the hybrid
materials, with the calcination treatment not affecting the intimate
contact between the three phases, but only the crystallinity of the
2
TiO layer. Thus, a clearly visible morphological difference between
the fresh samples and the calcined samples is the diverse texture,
with the latter appearing rougher, an indication of the crystallinity
of the oxide layer (Figure 4). A few small regions where the
nanotubes have not been covered by oxide are also observed. A
combined STEM and EDX analysis provides the compositional
structure, confirming the incorporation of the three components
2
Fig. 5 A) Representative HAADF-STEM of a CNTs/Pd@TiO -fresh; B-E) EDX mapping
showing the tight contact of the three phases: respectively palladium, oxygen,
titanium, carbon; F) superimposition of EDX of the C and Ti confirming their colocation.
(
MWCNTs, Pd and TiO ) and their co-location (Figure 5). The non-
2
2
calcined samples bear almost only amorphous TiO while, as
The textural properties of the investigated materials have been
confirmed by Fast Fourier Transform (FFT) of a selected area, the
calcined samples present a polycrystalline anatase phase for the
2
analysed by N physisorption (Table 1). According to the IUPAC
43
classification, all the samples present type IV physisorption
isotherms, typical of mesoporous materials (Figure 6 top). The
contribution of micropores is negligible in all the investigated
samples. Pristine and Ph-COOH functionalised MWCNTs present
high surface areas and extended mesoporous textures, with large
mesopores (10-100 nm) and high pore volumes. The textural
TiO
morphologies of 10-CNTs/Pd@TiO
fresh and calcined samples, appear similar. As expected, an average
thicker layer of TiO in the former (150 nm for 10-CNTs/Pd@TiO vs
20 nm for 20-CNTs/Pd@TiO ) was measured via HAADF-STM
2
shell, whose particle size averages 10 nm (Figure S1). The
2
and 20-CNTs/Pd@TiO , both
2
2
2
1
2
4
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properties of the hybrid materials calcined at 350 °C are different:
pore sizes are much smaller (in the range of 2 – 4 nm) and, as a
consequence, pore volumes are reduced while specific surface
20% MWCNT
228
0.274
3.6
Accessibility of Pd phase has been checked by H
2
chemisorption.
2
areas significantly increased (Figure 6 bottom). Pd@TiO -calc
Samples were reduced at moderate temperature (see the
Experimental section for details) to avoid significant deactivation of
material presents textural properties comparable with that the
hybrid materials, confirming that the surface area and pore volume
2
H adsorption on Pd surface by strong metal-support interaction
of the CNTs/Pd@TiO
component.
2
samples are mainly related with the inorganic
44, 45
(
SMSI) induced by the reduced TiO2,
and the analyses were
conducted at low temperature (around -90°C using a solid/liquid
4
6
acetone bath) to minimize H spillover to the reducible support.
Under these conditions, H interaction with Pd nanoparticles results
in the adsorption of H atoms on the surface and migration of H into
2
4
7
X
the bulk of Pd forming Pd hydride (PdH ). The contribution of H
adsorbed on the surface has been calculated by extrapolation of the
linear part in the 10-20 mmHg range while the overall hydrogen
consumption has been calculated after subtraction of the
physisorbed H
2
by linear extrapolation of the isotherms in the range
2
00–400 mmHg (Figure S4). The amount of hydrogen involved in
PdH
X
formation has been calculated by difference of the two
summarizes the main results of
quantities. Table
2
H
2
chemisorption measurements. The accessibility and the dispersion
of the Pd phase is significantly higher in the nanocarbon-inorganic
2
hybrid materials with respect to the Pd@TiO -calc material.
Consistently, the formation of the hybrid structure allows a more
efficient encapsulation and stabilization of Pd nanoparticles,
avoiding their sintering. Notably, the amount of H absorbed in the
formation of PdH species is higher for the fully inorganic material.
X
This result is in agreement with the dependence of solubility
hydrogen in small Pd nanoparticles reported by Boudart and
4
8
Hwang: the larger the Pd nanoparticles, the higher the solubility
of H forming PdH species.
X
Table 2. Summary of the metal textural properties of the investigated materials
Metallic
Apparent
Particle
Size (nm)
X in
Sample
H
surf/Pd surface area
2
-1
PdH
0.77
0.45
X
(
m g )
Pd@TiO2
0.20
1.4
5.5
2.2
1
0-
0
.50
3.4
3.5
2
Fig 6. (top) N physisorption isotherms and (bottom) BJH pore size distributions for the
CNT/Pd@TiO
2
2
various materials.
2
0-
Table 1. Summary of the textural properties of the investigated materials
0.52
2.1
0.33
CNT/Pd@TiO
Specific
D
(nm)
max
Sample
Surface Area
CPV (mL/g)
2
Catalytic experiments
(
m /g)
Pristine-
MWCNT
UV-Vis irradiation
1
1
42
17
0.634
0.741
3.4/22
37
Upon light irradiation, the PdO likely formed after the calcination
treatment is reduced in situ leading to activation of the catalyst. As
discussed in the introduction, the exact role of the carbon
nanostructure in enhancing photocatalysis is still under debate. A
plausible effect could be that of electron scavenging (Figure 7).
However, given the presence of bare parts of CNTs (see TEM), the
(
Ph-COOH)-
MWCNT
Pd@TiO
2
323
222
0.290
0.236
3.6
3.6
1
0% MWCNT
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photosensitizing could also be partially contributing, and therefore enhanced activity. Because of the positive effect of the thermal
the CNTs could have multiple effects. The reaction generating H treatment on the performance of the present hybrid materials, only
implies the reduction of H by the photogenerated electrons, that the calcined samples have been employed in the following
from the TiO conduction band and are injected on Pd photocatalytic studies for H production using more sustainable
nanoparticles, where they are accumulated and/or consumed for biomass-derived sacrificial agents, i.e. ethanol and glycerol.
2
+
2
2
4
9
reactions. (Figure 7):
Under UV-vis irradiation, the amount of produced H increases over
2
+
-
2
H
(ads) + 2e → H
2
time with all the investigated catalysts using both ethanol and
glycerol aqueous solutions (Figure 8), as already observed for
51, 52, 53
With this consideration in mind, in order for the reduction to be
efficient, another important factor is therefore the rate of diffusion
catalytic systems based on metal modified TiO2.
-2
of the protons, which are produced on the TiO
oxidation of the alcohol, to the Pd sites. All this implies a catalyst for 20-CNTs/Pd@TiO
with very specific morphological characteristics. In this scenario, the while the photocatalyst with a lower CNTs content exhibit lower
2
through the The H
2
production reaches an outstanding value of 2.4 mmol mcat
2
-calc after 24 h under UV-vis illumination,
-
2
MWCNTs would act as electron scavengers, retarding the charge activity (2.0 mmol mcat ). This is a clear indication of the beneficial
recombination rates. Other possible roles for the MWCNTs, as effect of the carbon nanotube scaffold. By comparison, the activity
-
2
explained in the introduction, are however plausible.
2
of the reference catalyst Pd@TiO -calc was only 0.5 mmol mcat . If
reported by gram of catalysts (Figure S5), activities are among the
highest ever reported for catalysts based on titania and
-
1
carbonaceous supports (Table S1), with values above 25 mmol g
-1
-1
-1
h
with 20-CNTs/Pd@TiO
2
-calc and 20 mmol gcat
-calc. The quantum efficiency (QE) at 365 nm of 10-
-calc and 20-CNTs/Pd@TiO -calc, respectively 17%
h for 10-
CNTs/Pd@TiO
CNTs/Pd@TiO
2
2
2
and 21%, confirm the promising performance of these hybrid
materials.
2
Fig 7. Scheme of a possible mechanism for H generation with concomitant scavenging
of the photogenerated holes by the alcohol.
The benefits of the designed synthetic protocol has been first
demonstrated studying the photocatalytic H2 production from
aqueous solutions containing methanol (Figure S5). Despite not
being the most sustainable choice in terms of sacrificial agent, the
use of methanol allows the direct comparison of the activity of the
present materials with literature data (Table S1). Together with H
the production of CO and CO has been observed (data not shown).
Fresh catalysts show a moderate activity, due to the presence of
amorphous/partially crystalline TiO . After calcination, the H
production strongly increases. A similar behaviour was already
2
,
2
2
2
50
reported and can be related to the crystallization of the oxide
shell, forming the nanocrystalline anatase with high surface area
and good accessibility of the Pd active phase. Calcination also
results in a tighter contact between TiO
2
and MWCNT and an
increased number of heterojunctions which are key to the
6
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2
Fig 8. H production over time under UV-vis illumination using aqueous solution of
The detection of large quantities of by-products confirms that, in
the present experimental conditions, the reforming reaction is not
complete. The photodehydrogenation of ethanol to acetaldehyde is
ethanol (top) and glycerol (bottom) as sacrificial donor. Activities are normalised by the
catalyst surface areas reported in Table 1.
2
the major process involved in H evolution. The stepwise oxidation
Investigation of the reaction by-products reveals the complexity of
of ethanol can proceed either through direct interaction with the
the network involved in the photocatalytic H
evolution was observed, implying that the water splitting was not
significantly contributing to the photocatalytic H production under
the present conditions. In fact, only a very small evolution of H (~
10 µmol g h ) was measured using pure water, while the
2 2
production. No O
+
photogenerated h or indirectly by interaction with ∙OH formed
+
+
54, 55
.
from the reaction H
2
O
ads + h → ∙OHads + H ads
Most likely, the
2
two oxidation pathways are simultaneously proceeding. Oxidation
of acetaldehyde to acetic acid is considered to be the rate
2
-1
-1
2
5
8
determining step with TiO
Given the low adsorption capability of the acetaldehyde onto the
TiO , it can easily reacts with ethanol forming the acetal 1,1-
2
catalysts in alcohol photoreforming.
addition of methanol lead to a considerable increase in H
rates (Figure S8).
2
evolution
2
When ethanol is used as sacrificial agent, the analysis of the gas diethoxyethane while the formation of traces of acetic acid is
phase evidences the presence of vapours of acetaldehyde and 1,1- expected to occur through oxidation in solution by some radical
4 2
diethoxyethane (its acetal with ethanol), CH and CO
4 2
in equimolar species formed in situ (mainly ∙OH). The formation of CH and CO
amounts, together with ethane and traces of ethylene (Figure 9 and in equimolar amounts indicates that these gases are formed by
Figure S7). Importantly, we observed only very minor amounts of direct decomposition of acetic acid or by decomposition of
CO, this being an aspect of relevance for the application of acetaldehyde to CH
photoreforming in fuel cells (Figure 9). Moreover, the analysis of through a “photocatalytic Water Gas Shift Reaction”. Together with
liquid solutions recovered after photocatalytic experiments showed methane and CO , the presence of ethane could also arise through
that the major part of acetaldehyde and 1,1-diethoxyethane were a radical mechanism, namely from the radical coupling of ∙CH , or
4 2
and CO, followed by oxidation of CO to CO
2
3
accumulated in the solutions, together with minor amount of acetic by hydrogenation of ethylene formed from ethanol on the acid sites
acid, 2,4,5-trimethyl-1,3-dioxolane, 3-hydroxy-2-butanone and 2,3- of the oxide. The contribution of acetaldehyde photodecomposition
butandiol (Table S2).
was confirmed by performing a photocatalytic experiment using
acetaldehyde as sacrificial agent, added after 3 h of irradiation of
the photocatalysts in pure water (Figure S9). After acetaldehyde
addition, H
2 4 2
is produced in major amount, together with CH , CO
and traces of ethane.
Among the other by-products accumulated in solution during
ethanol photoreforming, 2,3-butandiol is probably formed through
radical C-C coupling of two ethanol molecules. From this
compound, 3-hydroxy-2-butanone is formed by dehydrogenation
from one OH group while 2,4,5-trimethyl-1,3-dioxolane is formed
by reaction of 2,3-butandiol with acetaldehyde.
In the case of glycerol, lower H
all the investigated catalysts (Figure 8 and Figure S6). Both 10- and
0-CNT/Pd@TiO displayed an activity normalised by the surface
2
production rates were observed for
2
2
-2
2
area of 0.6 mmol mcat , while that of Pd@TiO -calc was only 0.3
-2
mmol mcat . Analysis of by-products in the gas phase revealed that
both CO2 and CO are present together with trace amounts of
ethane, with their formation rate increasing over time (Figure 9 and
Figure S10). Semi-quantitative GC/MS analysis of the solutions
recovered after photocatalytic experiments evidenced the
accumulation of non-volatile by-products. Hydroxyl acetaldehyde,
1
-hydroxy-2-propanone and 1,3-dihydroxy-2-propanone were the
most abundant, followed by formic acid and 2,3-dihydroxy-propanal
and trace amounts of acetic acid.
Differently from the case of ethanol, the carbonylic compounds
formed by the first dehydrogenation of glycerol – 2,3-dihydroxy-
propanal and 1,3-dihydroxy-2-propanone – still contain OH groups,
Fig 9. Amounts of by-products in the gas phase accumulated in 24 hours with 10- and
2
that allow their adsorption on the TiO surface and the competition
2
2
0-CNT/Pd@TiO for the photoreforming of ethanol and glycerol.
with glycerol to be oxidised by the holes. The results clearly indicate
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that oxidation/degradation of 2,3-dihydroxy-propanal is much quenched by the (respectively) electron acceptor (protons) and
easier than that of 1,3-dihydroxy-2-propanone. CO and CO electron donors (alcohol). As expected, when ethanol is used as
2
formation are usually related to the photocatalytic degradation of sacrificial donor, analysis of the liquid phase shows two main
carbonylic and carboxylic intermediate compounds, formed by products, acetaldehyde and 1,1-diethoxyethane in amounts
2
progressive dehydrogenation of the glycerol molecule, mainly proportional to the produced H if compared to the UV experiments
through 2,3-dihydroxy-propanal and leading to intermediate (Table S4). With glycerol, we found as main liquid by-product one of
smaller compounds (hydroxyl acetaldehyde, formic acid and acetic the expected first oxidation products, namely 1,3-dihydroxy 2-
5
2
acid). On the other hand, 1-hydroxy-2-propanone is presumably propanone, 1-hydroxy-2-propanone and hydroxyl-acetaldehyde, in
formed on the surface of the titania via dehydration/hydrogenation analogy with the UV experiment (Table S5).
from 1,3-dihydroxy-2-propanone, hardly oxidised/decomposed on
2
TiO surface by holes, or directly by dehydration of glycerol. The
formation of added value organic by-products in both ethanol and
glycerol photoreforming can lead to extra valorization of
18
biomasses. However, this is only viable if the products can be
easily separated or if the oxidation of the alcohol proceeds
selectively. This aspect opens the doors to studies on contact times
between the reactant and the catalyst, sparking exploration of the
correct engineering of flow reactors.
Simulated solar irradiation
When irradiated with
a
simulated solar light, both 10-
-calc display an
CNTs/Pd@TiO -calc and 20-CNTs/Pd@TiO
2
2
appreciable activity for the photoreforming of ethanol, although
lower than the corresponding UV-irradiated samples. The drop in
activity is somehow expected as the TiO can only absorb the small
2
UV percentage of incident radiation. However, the detected activity
provide a platform for these hybrid systems to be used with solar
light (Figure 10, top).
When EtOH is employed as sacrificial electron donor, the catalyst
-
2
2
0-CNTs/Pd@TiO
2 2
-calc shows a H productivity of 120 μmol mcat in
2
4 h, while for the 10-CNTs/Pd@TiO -calc the productivity was
2
-2
slightly higher (140 μmol mcat ). Interestingly, as compared to the
UV experiments, the activity of the two catalysts with different CNT
contents is inversed under simulated solar irradiation. Both
2
catalysts are still more active than the reference catalyst Pd@TiO -
-2
calc (87 μmol mcat ). It is also worth noting that the two CNT-based
catalysts undergo a progressive activation over the initial 4 hours
2
before reaching a constant H evolution rate, while the CNT-free
2
Fig 10. H production over time under simulated solar illumination using ethanol (top)
material shows stable activity from the beginning of the and glycerol (bottom) as sacrificial donor. Activities are reported normalised by the
experiment. Remarkably, and in contrast to the UV experiments, catalyst surface area as calculated in table 1.
the activity of all the catalysts with glycerol used as sacrificial donor,
Experimental
does not decrease significantly (Figure 10, bottom). On the
contrary, for 10-CNTs/Pd@TiO -calc and Pd@TiO -calc the H
2
2
2
-
2
Functionalisation of the multi-wall carbon nanotubes (PhCOOH-
MWCNTs)
productivity is even slightly higher (149 μmol mcat and 107 μmol
-2
m
2
cat ), while for 20-CNTs/Pd@TiO -calc slightly diminishes to 95
-2
μmol mcat . This suggests that the rate-determining step does not
depend on the sacrificial electron donor or on the diffusion of the
1
00 mg of pristine MWCNT purchased from Nanoamor (20–30 nm
diameter and 0.5 2 µm length) were dispersed in 100 mL of H O by
̶
2
2
adsorbed protons from TiO to the Pd sites. Rather, it must be
sonication for 20 minutes. Then the p-amino-benzoic acid (3.3
equivalents respect to carbon moles in the MWCNTs) was added
and mixed by sonication for further 10 minutes. The mixture was
put under vigorous stirring and 2.25 mL of isopentyl nitrite were
added just before start the heating treatment at 80 °C for 6 h under
associated with the rate of formation of the charge carriers
generated upon irradiation, which is considerably lower than that
under UV. It is therefore acceptable to assume that once the
fraction of electrons and holes are formed, they are immediately
8
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ARTICLE
reflux. Caution: diazonium salts should be handled with care since The samples were degassed in vacuum at 120 °C for 12 h before
they can be explosive. The mixture was naturally cooled down at analysis.
room temperature and collected by filtration on a 0.1 µm
2
H chemisorption was performed using a Micrometrics ASAP 2020C
polytetrafluoroethylene Millipore membrane. Finally the material
was washed with DMF, MeOH, H O, EtOH and Et O sonicating the
solid for 5 minutes in each solvent and dried overnight at 80 °C.
apparatus. A chemisorption stoichiometry H:M = 1:1 was assumed
for the calculation of the exposed metal surface area. The materials
2
2
has been pre-reduced in a flow of H
2
(5%)/Ar (40mL/min) at 100°C
chemisorption
Synthesis of MWCNT-Pd@TiO
2
hybrids
for 30 minutes and degassed at 250°C for 5 hours. H
2
isotherms has been recorded in 1 – 400 torr pressure range at
around -90°C (solid/liquid acetone bath).
The composite materials were prepared employing 0, 10 and 20 mg
of f-MWCNT and respectively labelled 10-CNTs/Pd@TiO , 20-
CNTs/Pd@TiO and Pd@TiO . The f-MWCNT were dispersed in Catalytic tests and quantum efficiency measurement
2
2
2
absolute ethanol (EtOH mL/f-MWCNT mg ratio: 2.5) by sonication
It is known that the mechanism and therefore the H
2
evolution
for 30 minutes meanwhile a Pd-MUA THF solution (containing 1.5
5
6, 57
depend on the sacrificial agent concentration.
In the present
mg of Pd) was slowly added to a THF solution of Ti(O-n-Bu)
4
experiments, ethanol and glycerol concentrations have been
chosen in order to better represent aqueous solutions deriving from
plants of the treatment of biomasses. Ethanol/water 50:50
(
containing 98.5, 88.5 and 78.5 mg of TiO
2
respectively). Then the
Pd@TiO
2
precursors solution was slowly added under sonication to
the f-MWCNT dispersion and the mixture was further sonicated for
0 minutes. Finally a 10% solution of H O in EtOH (Ti(n-OBu) /H O
solutions are representative of
a
solution coming from
3
2
4
2
fermentation plants without being subjected to deep purification by
distillation (a highly energy consuming industrial step). Glycerol 1M
solution has a composition close to that deriving from biodiesel
production plants. Moreover, the duration of each experiment has
been optimized to minimize evaporation of the solution, in order to
avoid significant variations of the alcohol/water ratios.
molar ratio: 1/120) was dropped and the mixture sonicated for 30
minutes. The materials were collected by filtration on a 0.45 µm
polytetrafluoroethylene Millipore membrane, washed with ethanol
and dried overnight at 80 °C. In order to eliminate the organic
ligands and improve the crystallization of the TiO
below) the catalyst were subjected to calcination at 350 °C for 5 hrs
+3 °C/min; -4.5 °C/min).
2
(see discussion
(
The materials were tested as photocatalyst for hydrogen
production by photoreforming of ethanol (50% v/v) and glycerol (1
M) under two different light source apparatus: a Teflon-lined
Characterisation
X-ray Diffraction (XRD) patterns were collected on a Philips X’Pert photoreactor illuminate with a Lot-Oriel Solar Simulator equipped
diffractometer using a monochromatized Cu Kα (λ = 0.154 nm) X- with a 150 W Xe lamp and an Atmospheric Edge Filter with a cut-off
ray source in the range 20° < 2θ < 100°. The mean crystallite sizes of at 300 nm and a Pyrex photoreactor illuminated with a 125 W
the TiO were calculated by applying the Scherrer equation to the medium pressure Hg lamp (model UV13F, Helios Italquartz, Italy). In
2
(
101) reflection of anatase phase.
a typically catalytic test 10 mg of the calcined material was prior
suspended in the photoreactor by sonication for 10 minutes in 60
mL of alcohol solution and subsequently purged from air with Ar
Thermogravimetric analysis (TGA) was performed on TGA Q500 (TA
Instruments) under air or nitrogen, equilibrating the temperature at
-1
flow of 15 mL min for 40 minutes and thermostated at 20 °C.
During the purge and the catalytic test the materials were
magnetically stirred.
-1
1
00°C and heating at 10 °C min up to 800 °C.
Raman spectra were recorded on
a
inVia Renishaw
microspectrometer equipped with a Nd:YAG laser using an
excitation wavelength of 532 nm. The dispersed samples were drop
casted onto silicon wafers and analysed in 5 different point
subsequently averaged.
The on-line detection of volatile products was carried out using a
Gas Chromatograph equipped with two analytical lines and a 10
way-two loops injection valve was employed for injection during
on-line analysis of the gaseous products. In the former apparatus an
Agilent 7890A Gas Chromatograph equipped with a Carboxen 1010
PLOT (Supelco, 30 m x 0.53 mm ID, 30 μm film) column followed by
a Thermal Conductivity Detector (TCD) was used for gaseous
products quantification using Ar as carrier and a DB-225ms column
TEM measurements were performed on a TEM Philips EM208, using
an acceleration voltage of 100kV. Samples were prepared by drop
casting the dispersed particles onto a TEM grid (200 mesh, copper,
carbon only). High resolution TEM (HRTEM) were acquired on a
JEOL 2200FS microscope operating at 200 kV, equipped with an
Energy Dispersive Spectrometer (EDS), in-column energy (Omega)
filter, and High-Angle Annular Dark-Field (HAADF) detector.
(
J&W, 60 m x 0.32 mm ID, 20 μm film) using He as carrier followed
by a mass spectrometer (MS) HP 5975C was employed for the
detection of the volatile organic compounds. In the later apparatus
an Agilent 6890N Gas Chromatograph equipped with a MolSieve 5Å
(
Restek, 30 m x 0.53 mm ID) column followed by a Thermal
N
2
physisorption isotherms were recorded at liquid nitrogen
Conductivity Detector (TCD) was used for gaseous products
quantification using Ar as carrier and a PoraPlot Q (Agilent, 30 m x
temperature using a Micrometrics ASAP 2020 automatic analyser.
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0.53 mm ID, 40 μm film) column using Ar as a carrier followed by a The research leading to these results has received funding from the
methanator and a Flame Ionization Detector (FID) was employed University of Trieste (project FRA2015), INSTM, the Seventh
for the detection of the volatile organic compounds.
Framework Programme [FP7/2007- 2013] under grant agreement
n° 310651 (SACS project). Dr. A. Aneggi and prof. A. Trovarelli
After the photocatalytic tests, the liquid phases were separated
from the catalyst by filtration on a 0.45 µm PVDF Millipore
membrane and subsequently analysed by GC/MS to detect the by-
products accumulated. For a semi-quantitative analysis, 1-butanol
(
University of Udine, Italy) are kindly acknowledged for access to
XRD facility.
and 1-esanol were used as internal standard in the liquid phases Notes and references
recovered from ethanol and glycerol tests, respectively.
‡ Footnotes relating to the main text should appear here. These
might include comments relevant to but not central to the
Quantum efficiency (QE) was calculated with equation:
QE = 2∙mol H / absorbed photons
matter under discussion, limited experimental and spectral data,
and crystallographic data.
§
§§
etc.
2
irradiating the sample with a 4 W Hg Penray.
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Development of new nanostructured catalysts for the sustainable
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