Effects of WO3 nanoparticle size on ethylene-butene metathesis activity

Article abstract of DOI:10.1016/j.apcata.2019.04.019

WO₃ nanoparticles (NPs)of 2.64 ± 1.11 nm, 16.6 ± 3.7 nm, and 54.3 ± 18.5 nm were investigated in the cross-metathesis reaction of ethylene and trans-2-butene to form propylene. Measurements of reaction rates normalized by exposed WO₃ surface area showed that decreasing WO₃ particle sizes resulted in increasing propylene formation rates. X-ray photoelectron spectroscopy of post-reaction catalysts showed that smaller WO₃ NPs featured lower O/W surface atomic ratios, and thus higher degrees of surface unsaturation. Based on these results, high-temperature synthetic treatments aimed at increasing the degree of surface unsaturation of WO₃ NPs showed significant improvements in metathesis activity when compared to baseline values. This work demonstrates that control of particle size for industrially-relevant catalysts drastically impacts reactivity.

Full text of DOI:10.1016/j.apcata.2019.04.019

Accepted Manuscript
Title: Effects of WO3 Nanoparticle Size on Ethylene-Butene
Metathesis Activity
Authors: Shiran Zhang, Daniel F. Consoli, Sohel K. Shaikh,
´
Yuriy Roman-Leshkov
PII:
S0926-860X(19)30171-1
DOI:
Reference:
https://doi.org/10.1016/j.apcata.2019.04.019
APCATA 17051
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
30 January 2019
12 April 2019
16 April 2019
´
Please cite this article as: Zhang S, Consoli DF, Shaikh SK, Roman-Leshkov Y, Effects
of WO3 Nanoparticle Size on Ethylene-Butene Metathesis Activity, Applied Catalysis
A, General (2019), https://doi.org/10.1016/j.apcata.2019.04.019
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Effects of WO3 Nanoparticle Size on Ethylene-Butene Metathesis Activity
Shiran Zhang,† Daniel F. Consoli,† , Sohel K. Shaikh,§ and Yuriy Román-Leshkov*,†
† Department of Chemical Engineering, Massachusetts Institute of Technology, 77 Massachusetts
Avenue, Cambridge, Massachusetts 02139, United States
§ Research and Development Center, Saudi Aramco, Dhahran 31311, Saudi Arabia
Graphical abstract
Highlights:
 The surface-specific reaction rate increases with the decrease of WO3
nanoparticle size in ethylene-butene cross-metathesis
 Smaller WO3 nanoparticles have a higher degree of surface
unsaturation during metathesis
 The degree of surface unsaturation of WO3 nanoparticles can be tuned by
pretreating conditions
 A higher degree of surface unsaturation corresponds to a higher
metathesis activity
ABSTRACT
WO3 nanoparticles (NPs) of 2.64±1.11 nm, 16.6±3.7 nm, and 54.3±18.5 nm were
investigated in the cross-metathesis reaction of ethylene and trans-2-butene to form propylene.
Measurements of reaction rates normalized by exposed WO3 surface area showed that decreasing
1

WO3 particle sizes resulted in increasing propylene formation rates. X-ray photoelectron
spectroscopy of post-reaction catalysts showed that smaller WO3 NPs featured lower O/W surface
atomic ratios, and thus higher degrees of surface unsaturation. Based on these results, high-
temperature synthetic treatments aimed at increasing the degree of surface unsaturation of WO3
NPs showed significant improvements in metathesis activity when compared to baseline values.
This work demonstrates that control of particle size for industrially-relevant catalysts drastically
impacts reactivity.
Key Words: Metathesis; Tungsten Oxide; Size Effect; Ethylene; Butene
1. INTRODUCTION
Due to the growing demand of propylene globally, the cross-metathesis of ethylene and 2-butene
to propylene has received renewed interest as it provides a feasible avenue for large-scale
propylene production [1-3]. The most common heterogeneous catalysts for this reaction include
Re2O7, MoO3, and WO3 nanoparticles supported on silica or alumina. While supported Re2O7 and
MoO3 are able to catalyze the cross-metathesis at lower temperatures, the supported WO3 catalyst
continues to be attractive by virtue of its low cost, high robustness, and tolerance to trace amount
of oxygenates [1]. Consequently, extensive research efforts have been dedicated to the
investigation and optimization of WO3/SiO2 catalysts both experimentally and theoretically to
identify the nature of active sites [4, 5], the effects of catalyst support [6-9] and W loading [10,
11], the intrinsic reaction kinetics [12, 13], as well as the various catalyst activation/deactivation
processes [14-17]. However, the effects of WO3 particle size on catalytic performance have not
been investigated in a systematic fashion.
Indeed, industrial catalysts usually possess a wide distribution of particle sizes [1]. The measured
catalytic performance represents an average of the various contributions from all the sites present
in NPs with different sizes and surface structures [18]. In order to improve performance, it is of
practical significance to understand the effects of particle size on metathesis activity and
selectivity. The surface of the WO3 NP mainly consists of terraces, step edges, and corners, where
W atoms at these locations have different coordination environments. W atoms at terraces are
generally more saturated than those at step edges and corners. Compared with highly-coordinated
oxygen atoms at terraces, those at edges and corners are more prone to form vacancies (i.e.,
unsaturated sites). Such differences in coordination environment impact the oxidation state of W
and the geometry of the surface sites, both of which mediate the adsorption energy of olefin
molecules and/or reaction intermediates, ultimately leading to different catalytic performance [19].
More importantly, the surface composition of these surface structures changes as a function of
particle size, with a decrease in particle size favoring step edges and corners at the expense of
terrace sites [20].
2

In this study, the size effect of WO3 NPs on the metathesis activity of the cross-metathesis of
ethylene and trans-2-butene to form propylene was investigated. In order to minimize support
effects, the NPs were used in the absence of a support or were supported on inert graphite, given
that oxide supports, such as SiO2 and Al2O3 have been shown to influence the metathesis activity
of WOx [1]. The metathesis activity showed a strong dependence on particle size when comparing
propylene production rates normalized by surface area. Characterization studies revealed that this
size dependence was related to the degree of surface unsaturation.
2. EXPERIMENTAL SECTION
2.1. Catalyst synthesis
Small-sized WO3 NPs were prepared by a reverse microemulsion (RME) protocol at room
temperature described in literature [21]. In a typical synthesis, 8.8 ml of Igepal CO-520 surfactant
(Sigma Aldrich, used as received) was mixed in 50 ml of n-heptane (Sigma Aldrich, 99%, used as
received) followed by 1.1 ml of 0.2 M HCl aqueous solution to form a clear and colorless solution,
termed Solution I. In another setup, 8.8 ml of Igepal CO-520 were mixed in 50 ml of n-heptane
followed by 1.1 ml of 0.2 M Na2WO4 aqueous solution to form a clear and colorless solution,
termed Solution II. Next, Solution II was transferred to Solution I by way of a syringe in 30 min,
and the mixture was stirred for 8 h. After that, 30 ml of methanol (Sigma Aldrich, used as received)
was added to break the reverse microemulsion by forming a two-phase system. After decanting
the heptane phase, white precipitate was collected by centrifugation of the alcohol phase and re-
dispersed in 30 ml of methanol. Then 0.5 g of graphite powder (Sigma Aldrich, 99.99% trace
metals basis, used as received) powder was added and the mixture was loaded onto a rotovap. After
the vaporization of methanol at 40°C, the remaining solid was collected and calcined in air at
350°C for 3h to obtain 1 wt% small-sized WO3 NPs/graphite, termed 1% s-WO3 NPs/graphite.
The small-sized WO3 NPs on the graphite support were termed s-WO3 NPs. Note that graphite
does not undergo thermal degradation at this temperature, as confirmed by thermogravimetric
analysis of the bare support [22]. The small nanoparticles were supported in graphite in order to
avoid agglomeration and fully expose their surface area.
Medium-sized WO3 NPs were prepared with a modified hydrothermal protocol described in
literature [23]. Briefly, 0.99 g of Na2WO4·2H2O (Sigma Aldrich, >99%, used as received) was
dissolved in 20 ml of deionized water and 2 M HCl was added into the solution dropwise forming
a yellow precipitate. The precipitate was collected by centrifugation and re-dissolved in 40 ml of
1 M oxalic acid solution. After that, 4.37 g of cetyltrimethyl ammonium bromide (CTAB) (Sigma
Aldrich, used as received) was added to the solution, which was then transferred into a 50 ml
Teflon-lined stainless steel autoclave. The mixture was then heated at 180°C for 12 h, and cooled
down to room temperature. The solids were collected by centrifugation, dried at 100°C overnight,
followed by calcination in air at 350°C for 3 h. The final product is termed m-WO3 NPs.
3

Large-sized WO3 NPs (Sigma Aldrich, nanopowder, <100 nm, used as received), were purchased,
and calcined in air at 350°C for 3 h before use. This material is termed lg-WO3 NPs.
2.2. Catalyst characterization
The size and morphology of WO3 NPs were characterized using an FEI Tecnai Multipurpose
transmission electron microscope (TEM) operated at an accelerating voltage of 120 kV. TEM
images were analyzed by Digital Micrograph (Gatan) software. Powder X-ray diffraction (PXRD)
patterns of l-WO3 NPs and m-WO3 NPs were recorded with a Bruker D8 X-ray diffractometer
using Cu Kα radiation (λ=1.54056 Å) operated at 40 kV and 40 mA in a continuous mode in the
2θ range from 20° to 60°. Surface areas of l-WO3 NPs and m-WO3 NPs were measured on a
Quantachrome AutosorbiQ automated gas sorption analyzer using low temperature N2 adsorption.
The loading percentage of 1% s-WO3 NPs/graphite was confirmed by inductively coupled plasma-
optical emission spectrometry (ICP-OES) (Agilent ICP-OES VDV 5100).
X-ray photoelectron spectroscopy (XPS) was performed using a PHI VersaProbe II X-ray
photoelectron spectrometer with monochromated Al Kα X-ray source. Air-free measurements
were performed by loading post-reaction catalysts into a transfer vessel in the glove box, which
could be attached to the XPS without exposure to air. A dual-beam charge neutralizer was applied
to compensate for the charging effect. The acquired spectra were calibrated with C 1s peak at 284.5
eV, and analyzed using CasaXPS software. The spectra fitting follows the peak positions reported
in literature [24]. All fitted components have the same Full-Width-Half-Maximum (FWHM), and
identical fitting parameters were applied to all spectra.
2.3. Reactivity tests
Reactivity tests of the as-prepared catalysts were carried out in a ¼” stainless steel reactor within
an insulated single-zone furnace (Applied Test Systems series 3210, 850W/115V). 200 mg of a
catalyst sample (pelletized into 40-60 mesh size) was packed in between two quartz wool plugs
and two sets of 100 mg of SiC (46 grit, Alfa Aesar, used as received). Reaction temperatures were
monitored and maintained by a K-type thermocouple (Omega Engineering), placed underneath the
catalyst bed, and a PID temperature controller (Cole Parmer Digi-Sense 89000 series). All gases
were delivered via calibrated mass flow controllers (Brooks Instrument SLA 5800 series) and
purified through a trap containing Q5 copper catalyst (Research Catalysts, Inc.) and 3A molecular
sieves (Sigma Aldrich) to remove oxygen and moisture, respectively, before entering the reactor
tube. The catalyst was pretreated in He (100 ml/min) at 550°C for 1 h, and then cooled down to
the reaction temperature of 350°C in He. All pretreatments and reaction conditions were kept at
atmospheric pressure. The reactant gas mixture consisted of equimolar 10% ethylene and 10%
trans-2-butene balanced with He, introduced at a rate of 100 ml/min. Effluent gases were analyzed
4

with an on-line gas chromatograph (Shimadzu GC-2014) equipped with an Agilent HP-PLOT
Al2O3-S (30 m × 0.25 mm) column and a flame ionization detector (FID) for the analysis of both
the reactants and products. All catalytic studies were measured in terms of the formation rates of
propylene for metathesis in the kinetically-controlled regime, i.e., in the absence of mass transfer
limitations, checked with different flow rates and mesh sizes of catalyst particles. The mass balance
closure was kept above 99%.
3. RESULTS AND DISCUSSION
3.1. Characterization of WO3 NPs before reaction.
As shown in Fig. 1, the average sizes of large-, medium- and small-sized WO3 NPs were 54.3±18.5
nm, 16.6±3.7 nm, and 2.64±1.11 nm, respectively, as obtained by a Gaussian fitting of the size
distribution measured from TEM images. The nanoparticles of 1% s-WO3 NPs/graphite have a
wider size distribution relative to the size of lg-WO3 NPs and m-WO3 NPs due to the challenge in
controlling ultrasmall sizes within a very narrow distribution. Fig. 2 presents the diffraction
patterns of lg-WO3 NPs and m-WO3 NPs that is consistent with hexagonal WO3 (PDF No. 43-
1035). Due to the small size and low loading of WO3 NPs in 1% s-WO3 NPs/graphite, the
diffraction pattern did not contain any features associated with crystalline WO3. We therefore
(a)
(b)
(c)
100 nm
50
40
30
20
10
50
40
30
20
10
40
30
20
10
Size: 54.3 18.5 nm
Size: 16.6 3.7 nm
Size: 2.64 1.11 nm
0
0
0
0
0
0
20
40
60
80
100
5
10
15
20
25
30
1
2
3
4
5
Size Distribution (nm)
Size Distribution (nm)
Size Distribution (nm)
Fig. 1. TEM images of (a) lg-WO3 NPs, (b) m-WO3 NPs, and (c) 1% s-WO3 NPs/graphite. The
figures below each TEM images are the corresponding statistical counting of nanoparticle sizes (100
particles were used for counting). The average sizes of small, medium, and large WO3 NPs are
2.64±1.11 nm, 16.6±3.7 nm, and 54.3±18.5 nm, respectively.
5

worked under the assumption that small-sized WO3 NPs would have a similar crystallographic
structure to that of the lg- and m-WO3 NPs, since identical calcination conditions were used.
lg-WO3 NPs
m-WO3 NPs
1% s-WO3 NPs/graphite
PDF 43-1035
PDF# 43-1035
20
25
30
35
40
45
50
55
60
2 Theta (degree)
Fig. 2. X-ray diffraction patterns of lg-WO3 NPs, m-WO3 NPs, and 1% s-WO3 NPs/graphite.
Table 1 lists the surface areas of different-sized WO3 NPs calculated both from average particle
size in TEM and from N2 adsorption using the Brunauer-Emmett-Teller (BET) method (except for
s-WO3). Both techniques showed comparable surface areas for lg-WO3 NPs and m-WO3 NPs.
Therefore, TEM surface areas were used for subsequent normalization of reaction rates.
Table 1. Surface areas of different-sized WO3 NPs calculated with average particle sizes obtained both from
TEM images and N2 adsorption.
TEM Surface Area
(m2/g)
BET Surface Area
(m2/g)
Catalyst
6

lg-WO3
m-WO3
s-WO3
15.5
50.5
316
14.47
56.31
-
3.2. Catalytic performances of different-sized WO3 NPs in ethylene and trans-2-butene
cross-metathesis.
Propylene was produced from the cross-metathesis of ethylene and trans-2-butene at 350°C over
WO3 NPs. As shown in Fig. 3a, lg-WO3 NPs, m-WO3 NPs, and 1% s-WO3 NPs/graphite yield
specific propylene formation rates of 2.5×10-3, 5.4×10-2, and 3.4 mmol/g/h, respectively, which,
while lower than metathesis rates typically obtained in industry using supported catalysts, are
consistent with the metathesis activities reported for bulk WO3 [25]. In order to eliminate the
potential differences caused by differences in surface areas, the propylene formation rates were
instead normalized by the respective calculated surface areas of different-sized WO3 NPs, which
are presented in Fig. 3b. Surface-specific/Areal formation rates of propylene exhibited a clear
increasing trend with the decrease of particle size. Most notably, the areal propylene formation
rate increased by two orders of magnitude from 1.62×10-4 mmol/m2/h for lg-WO3 NPs (54.3 nm)
to 1.07×10-2 mmol/m2/h for s-WO3 NPs (2.64 nm). Given that the sizes of WO3 NPs remained
102
101
100
10-1
10-2
10-3
10-1
10-2
10-3
10-4
10-5
lg-WO3 NPs
m-WO3 NPs
1% s-WO3 NPs/graphite
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Large
Medium
Small
Time on Stream (h)
(a)
(b)
Fig. 3. Catalytic performance of lg-WO3 NPs, m-WO3 NPs, and 1% s-WO3 NPs/graphite in terms of (a)
propylene formation rates normalized by mass of WO3, and (b) propylene formation rates normalized by
exposed surface area of WO3. Reaction conditions: 200 mg of catalyst was pretreated in He at 550°C for
1 h before introducing the reactant mixture consisting of 10% ethylene and 10% trans-2-butene balanced
with He with a flow rate of 100 ml/min.
7

after catalysis (Fig. S2), we hypothesize that the WO3 NPs must possess a drastically different
composition of active sites as a function of particle size.
3.3. XPS studies of fresh and post-reaction catalysts.
The difference in areal reaction rate as a function of WO3 particle size is indicative of distinctly
different surface structures for different-sized WO3 NPs. Due to the high oxophilicity of tungsten,
unsaturated W sites with a reduced electronic state will be readily oxidized upon exposure to
ambient conditions. In order to capture the authentic surface structures, post-reaction catalysts
were characterized by XPS without exposure to air. Fig. 4 shows the W 4f photoemission features
of lg-WO3 NPs, m-WO3 NPs, and s-WO3 NPs before and after reaction. The main photoemission
peak of W 4f7/2 for all the three fresh catalysts was 35.5 eV, consistent with the feature of W6+ in
8

bulk WO3 [24, 26]. A small portion of W5+ at 34.3 eV was observed on lg-WO3 NPs and m-WO3
NPs, suggesting their original surfaces were not completely saturated.
W 4f
W 4f
after catalysis
after catalysis
fresh
fresh
40
38
36
34
32
40
38
36
34
32
Binding Energy (eV)
Binding Energy (eV)
(a)
(b)
3.00
2.95
2.90
2.85
2.80
W 4f
after catalysis
fresh
40
38
36
34
32
Large
Medium
Small
Binding Energy (eV)
(c)
(d)
Fig. 4. Photoemission features of (a) lg-WO3 NPs, (b) m-WO3 NPs, and (c) s-WO3 NPs before and
after catalysis. (d) Calculated atomic ratios of O to W at near-surface region of different-sized WO3
NPs after catalysis. In the peak fitting, the contributions of W6+, W5+ and W4+ are fitted with green,
blue, and orange curves, respectively.
The photoemission features of post-reaction catalysts generally showed a higher portion of reduced
W states, which was consistent with reported partial reduction of WO3 during olefin metathesis
reactions [27]. The higher proportion of reduced W states suggests an increase in the degree of
9

surface unsaturation. For lg-WO3 NPs and m-WO3 NPs, the portion of W5+ slightly increased,
while in the case of s-WO3 NPs, the portion of W5+ increased dramatically with the emergence of
W4+. The degree of surface unsaturation can be described by the O/W surface atomic ratio (Fig.
4d). Based on stoichiometry, the atomic ratio of O to W of a saturated WO3 surface should be 3.
A decrease of the surface atomic ratio of O to W is indicative of an increase in the number of
under-coordinated sites, as surface lattice oxygen is partially removed. Fig. 4d presents a
decreasing trend of surface atomic ratio of O to W of post-reaction WO3 NPs with a decrease of
particle size. Specifically, the measured surface O/W atomic ratios for post-reaction lg-WO3, m-
WO3, and s-WO3 NPs were 2.97, 2.93, and 2.87, respectively. These data show that smaller WO3
NPs have a higher degree of surface unsaturation during catalysis. By correlating the degree of
surface unsaturation of different-sized WO3 NPs with the corresponding catalytic performance,
the change in metathesis activity with the decrease of particle size can be rationalized by the
increased degree of surface unsaturation.
3.4. Tuning the degree of surface unsaturation
Many synthetic protocols can be used to alter the degree of surface unsaturation of metal oxides.
For instance, treating the catalysts in an inert gas or oxygen deficient atmosphere at a higher
temperature can remove surface lattice oxygen atoms via thermal reduction, leading to an increase
in the degree of surface unsaturation [28]. On the other hand, annealing unsaturated surfaces of
WO3 in air is able to saturate the surface with oxygen close to a stoichiometric ratio of O to W.
Therefore, we employed different thermal treatments on freshly calcined samples prior to catalysis
to control O/W surface ratios and thus gain further insight on the correlation between surface
unsaturation and catalytic activity. Note that oxygen is used in industrial metathesis process for
occasional coke burnoff, which also saturates the tungsten sites. A subsequent treatment of inert
gas or diluted hydrogen at a high temperature is typically applied in order to create an unsaturated
W site again for increased metathesis activity.
Fig. 5a shows the air-free XPS results of the W 4f photoemission features of lg-WO3 NPs after
different treatment conditions. The W 4f photoemission feature of lg-WO3 NPs after treatment in
He at 350°C was very similar to that of the fresh lg-WO3 NPs (Figs. 5a1 and 4a), in which the
majority of surface W atoms were in the +6 state at 35.4 eV [24]. Treatment in He at 700°C
dramatically changed the W 4f photoemission spectrum, clearly indicating the appearance of
reduced W states as W5+ at 34.3 eV and W4+ at 33.4 eV (Fig. 5a2) [24]. A subsequent annealing
in air at 350°C of the catalyst treated in He at 700°C restored the catalyst surface to a fully oxygen-
saturated state (Fig. 5a3). Consistent with the photoemission spectra, the calculated surface atomic
ratios of O to W also showed a decrease from 2.99 to 2.88 for the treatment in He at 700°C,
followed by an increase for the subsequent treatment in air at 350°C (Fig. 5b). These results
suggest that the degree of surface unsaturation can be tuned by annealing WO3 NPs at different
temperatures and gas atmosphere.
10

3.00
2.95
2.90
2.85
2.80
W 4f
He 700oC
(a3)
then Air 350oC
(a2)
He 700oC
(a1)
He 350oC
He 700oC
Air 350oC
He 350oC
He 700oC
40
38
36
34
32
Binding Energy (eV)
(a)
(b)
Fig. 5. (a) Photoemission features, peak fitting, and (b) calculated surface atomic ratios of O to W of
l-WO3 NPs under different treatment conditions. All spectra were taken on post-treatment catalysts
without the exposure to ambient conditions. In the peak fitting, the contributions of W6+, W5+ and W4+
are fitted with green, blue, and orange curves, respectively.
The catalytic performance of lg-WO3 NPs was measured after the above treatments, as shown in
Fig. 6. Treatment in He at 350°C yielded a propylene formation rate of ~3.3×10-5 mmol/m2/h,
which increased by three orders of magnitude to 1.3×10-2 mmol/m2/h after the treatment in He at
700°C. We hypothesize that such a significant enhancement in cross-metathesis activity results
from an increased degree of surface unsaturation after high-temperature annealing in an inert gas.
The subsequent treatment in air at 350°C increased surface oxygen saturation of the catalyst and
resulted in the decrease of metathesis activity to 2.5×10-4 mmol/m2/h. Notably, this formation rate
of propylene did not decrease fully to the original level observed after the He treatment at 350°C.
This effect could result from weakly-bond oxygen atoms after air treatment at 350°C that could be
removed more easily during propylene metathesis compared with those strongly-bond oxygen
atoms formed after calcination at 550°C in air.
11

101
100
He 350oC
He 700oC
He 700oC then Air 350oC
10-1
10-2
10-3
10-4
10-5
10-6
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Time on Stream (h)
Fig. 6. Catalytic performances of lg-WO3 NPs after different treatments. The catalysts were purged with
He after treatments at 350°C before a reactant mixture of 10% ethylene and 10% trans-2-butene balanced
with He with a flow rate of 100 ml/min was introduced. Formation rates of propylene normalized by
surface area are plotted as a function of time on stream.
4. SUMMARY
Different-sized WO3 NPs show distinctly different surface-specific activities in the catalytic
reactions of cross-metathesis of ethylene and trans-2-butene to propylene. The metathesis activity
increases significantly with the decrease of particle size. Air-free XPS studies reveal that a smaller
size of WO3 NPs leads to a higher degree of surface unsaturation during catalysis, which is
positively related to the cross-metathesis activity. Further investigation showed that the degree of
surface unsaturation not only is related to the particle size, but can be tuned with different treatment
conditions as well. This study revealed the essence of size effect of WO3 NPs and established a
correlation between catalytic activities of cross-metathesis and the degree of surface unsaturation.
It also provided new insights and approaches in tuning catalytic performance via structural factors.
Acknowledgements
The authors acknowledge the financial support from Saudi Aramco through MIT Energy Initiative
Grant No. 6930839.
12

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