Ligand effects in the stabilization of gold nanoparticles anchored on the surface of graphene: Implications in catalysis

Article abstract of DOI:10.1016/j.jcat.2020.12.027

Gold nanoparticles (Au NPs) functionalized with N-heterocyclic carbene (NHC) ligands immobilized onto graphene are obtained via spontaneous decomposition of well-defined gold-NHC complexes by reduced graphene oxide (rGO) without reducing agents. NHC ligands are responsible for the formation of air-stable, crystalline and small (3.0–4-0 nm) Au NPs homogeneously distributed on the surface of graphene. The catalytic properties of three Au NPs functionalized with different ligands were tested in two benchmark reactions (hydration of alkynes and intramolecular hydroamination of alkynes). The results reveal a pronounced ligand effect on the stability of Au NPs on graphene, by acting as a bridge between them. The Au NPs functionalized with a NHC ligand lacking a polyaromatic group or having a naphthyl tag displayed limited stability and fast deactivation in the first run. On the contrary, the Au NPs functionalized with a NHC ligand containing a pyrenyl handle showed superior catalytic activity and can be recycled at least ten times. The particle size of the Au NPs is preserved after the recycling process indicating a high stability. These results illustrate the use of purposely designed ligands having affinity for both Au NPs and graphene to increase the stability of the hybrid catalyst.

Full text of DOI:10.1016/j.jcat.2020.12.027

Journal of Catalysis 394 (2021) 113–120
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Ligand effects in the stabilization of gold nanoparticles anchored on the
surface of graphene: Implications in catalysis
David Ventura-Espinosa , Santiago Martín b,c, Hermenegildo García d, , Jose A. Mata a,
a
⇑
⇑
a
Institute of Advanced Materials (INAM), Centro de Innovación en Química Avanzada (ORFEO-CINQA), Universitat Jaume I, Avda. Sos Baynat s/n, 12006 Castellón, Spain
Instituto de Nanociencia y Materiales de Aragón (INMA), CSIC-Universidad de Zaragoza, Zaragoza 50009, Spain
Departamento de Química Física, Universidad de Zaragoza, Zaragoza 50009, Spain
b
c
d
Instituto de Tecnología Química, Consejo Superior de Investigaciones Científicas-Universitat Politècnica de València, Avda. Los Naranjos s/n, 46022 Valencia, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
Gold nanoparticles (Au NPs) functionalized with N-heterocyclic carbene (NHC) ligands immobilized onto
graphene are obtained via spontaneous decomposition of well-defined gold-NHC complexes by reduced
graphene oxide (rGO) without reducing agents. NHC ligands are responsible for the formation of air-
stable, crystalline and small (3.0–4-0 nm) Au NPs homogeneously distributed on the surface of graphene.
The catalytic properties of three Au NPs functionalized with different ligands were tested in two bench-
mark reactions (hydration of alkynes and intramolecular hydroamination of alkynes). The results reveal a
pronounced ligand effect on the stability of Au NPs on graphene, by acting as a bridge between them. The
Au NPs functionalized with a NHC ligand lacking a polyaromatic group or having a naphthyl tag displayed
limited stability and fast deactivation in the first run. On the contrary, the Au NPs functionalized with a
NHC ligand containing a pyrenyl handle showed superior catalytic activity and can be recycled at least
ten times. The particle size of the Au NPs is preserved after the recycling process indicating a high stabil-
ity. These results illustrate the use of purposely designed ligands having affinity for both Au NPs and gra-
phene to increase the stability of the hybrid catalyst.
Received 18 November 2020
Revised 22 December 2020
Accepted 22 December 2020
Available online 2 January 2021
Keywords:
Heterogeneous catalysis
Gold nanoparticles as catalysts
Ligand effect
Graphene as support
Hybrid materials
Ó 2020 Elsevier Inc. All rights reserved.
1
. Introduction
Catalytic applications of gold nanoparticles (Au NPs) is a grow-
polymers or ligands [11–16] and the second one is the use of sup-
ports where the MNPs are stabilized by strong interactions with
the material [17]. Supported MNPs are highly active heterogeneous
catalysts which have found numerous applications in chemical
transformations, including oxidations, reductions and couplings
[18–20]. Understanding the metal-support interaction provides
the principles to develop more selective and efficient catalysts
[21–24]. Stability may represent a bottleneck for the development
of efficient catalytic systems. The introduction of ligands on the
surface of MNPs provides a tool to control activity and selectivity
that combined with the selection of appropriate supports can
increase stability [25–29]. Among all the ligands used in the func-
tionalization of MNPs, N-heterocyclic carbene ligands (NHCs) are
prominent and have enabled selective transformations [30–32].
We described the benefits in the use of graphene as support to
immobilize organometallic complexes by non-covalent interac-
tions [33–35]. Design of ligands containing polyaromatic tags
ing research area, since it constitutes a relevant example of the cat-
alytic activity of metal nanoparticles (MNPs) [1–4]. The focus in the
field relies on understanding the structure–activity relationship in
order to further increase catalytic activity [5]. However, develop-
ment of efficient catalysts is also directly related to stability and/
or deactivation pathways, two important issues that are barely
considered in catalysis research [6–8]. In order to improve the life-
time of a catalyst it is high important to understand the factors that
increase stability and the processes that govern deactivation path-
ways [9,10].
One of the general issues in catalysis by MNPs is the limited sta-
bility and the tendency of NPs to coalesce. In order to increase sta-
bility of MNPs two main approaches are commonly employed. The
first one implies the use of capping agents such as surfactants,
allows strong
p interactions with the graphene basal plane that
retains under reaction conditions the metal complexes anchored
onto the surface of graphene. The metal complexes immobilized
onto graphene are efficient catalysts in a variety of transforma-
tions, which include oxidations and couplings [36–38]. Decompo-
Abbreviations: NPs, nanoparticles; NHC, N-heterocyclic carbene; rGO, reduced
graphene oxide.
⇑
Corresponding authors.
E-mail addresses: hgarcia@qim.upv.es (H. García), jmata@uji.es (J.A. Mata).
https://doi.org/10.1016/j.jcat.2020.12.027
021-9517/Ó 2020 Elsevier Inc. All rights reserved.
0

D. Ventura-Espinosa, S. Martín, H. García et al.
Journal of Catalysis 394 (2021) 113–120
Schlenk flask and under the protection of light, complex 1, 2 or 3
(
8
0.107 mmol) and silver triflate (0.117 mmol) were dissolved in
mL of dry CH Cl and the mixture was stirred at room tempera-
2
2
ture for 15 min (Fig. 2). Then the reaction was filtered through a
long pad of celite. The solvent was reduced in the rotatory evapo-
rator until ca. 5 mL. Then, this mixture was added to the suspen-
sion of rGO in CH
temperature for 72 h. The black solid was isolated by filtration
and washed with 200 mL of CH Cl affording the hybrid material
2
Cl
2
and was stirred at the selected
2
2
Au-NPs-rGO. The hybrid material was characterized by HRTEM
and XPS (see S6-S7), and the exact amount of gold was determined
by ICP-MS analysis. The results accounted for
Au/100 mg rGO of gold in the hybrid material
a
0.013 mg
-NPs-rGO,
a
0
.016 mg Au/100 mg rGO of gold in b-NPs-rGO and 0.033 mg
Au/100 mg rGO of gold in -NPs-rGO.
c
Fig. 1. Model representation of Au NPs adsorbed onto graphene due to the
simultaneous interaction of the ligand with both Au NPs and graphene.
2.2. General procedure for the hydration of alkynes.
In a PyrexÒ tube and under air, 4-octyne (1 eq.), water (2 eq.),
sition of organometallic species under reducing conditions pro-
motes the formation of MNPs immobilized onto graphene. Studies
on the catalytic activity of palladium and gold nanoparticles func-
tionalized with ligands supported onto graphene reveal the impor-
tance of the support in the stability of the MNPs. Herein we now
report that ligands can increase the stability of Au NPs on graphene
by simultaneous interaction with both MNPs and graphene. The
synthesis of Au NPs anchored onto graphene is performed by
decomposition of well-defined gold complexes bearing different
NHC ligands. Preparation of three NHC ligands containing different
polyaromatic tags conclusively establishes their role in holding
together the Au NPs onto graphene (Fig. 1). The selection of the
NHC ligand determines the stability of Au NPs onto graphene
and, therefore, their catalytic activity. The Au NPs derived from
the NHC ligand containing a pyrenyl handle shows better catalytic
properties in terms of activity and recyclability.
catalyst and methanol (alkyne concentration 0.15 M) were mixed.
The reaction was stirred at 50 ⁰C in an oil bath. Conversion of
alkyne into the corresponding ketone was monitored by GC-FID
using anisole as an internal standard. When the reaction was com-
pleted the solvent was removed and the yield of isolated product
1
13
analyzed by H and C NMR. Recycling experiments were carried
out under the same reaction conditions for all runs without cata-
lyst regeneration. After completion of each run, the reaction mix-
ture was allowed to reach room temperature and the solid
catalyst was isolated by decantation. The remaining solid was
washed thoroughly with MeOH (4 Â 5 mL), dried and reused in
the following run.
2
.3. General procedure for the cyclisation of 2-alkynil anilines.
In a PyrexÒ tube and under air, 2-(phenylethynyl)aniline (1 eq.)
water (2 eq.), catalyst and toluene (substrate concentration
.0625 M) were mixed. The reaction was stirred at 50 ⁰C in an oil
2
. Experimental section
0
bath. Conversion of substrate was monitored by GC-FID using ani-
sole as an internal standard. When the reaction was completed the
solvent was removed and the yield of isolated product analyzed by
Detailed synthetic procedures for ligands containing different
polyaromatic groups and Au-NHC complexes are included in the
supporting information (SI).
1
13
H and C NMR. Recycling experiments were carried out under the
same reaction conditions as described before. After completion of
each run, the reaction mixture was allowed to reach room temper-
ature and the catalyst was isolated by decantation. The remaining
solid was washed thoroughly with MeOH, dried and reused in the
following run.
2
.1. Synthesis of hybrid materials a, b or c-NPs-rGO.
A suspension of 470 mg of rGO in 350 mL of CH
2 2
Cl in a round
bottom flask was immersed in an ultrasounds bath for 30 min. In a
Fig. 2. Bottom-up approach for the synthesis of Au NPs capped with NHC ligands attached to graphene: a) NHC-Au-X complexes (Dipp = 2,6-diisopropylphenyl), b) ORTEP
structure from single crystal X-ray diffraction of complex 2 (hydrogen atoms omitted for clarity) and c) reaction scheme for synthesis and deposition of Au NPs.
114

D. Ventura-Espinosa, S. Martín, H. García et al.
Journal of Catalysis 394 (2021) 113–120
Fig. 3. TEM characterization of gold nanoparticles supported onto graphene. First column: Dark field STEM micrographs. Second column: HRTEM micrographs. Third column:
Size distribution histograms of Au NPs and fourth column: XPS analysis of the core-level peaks (eV) for the Au4f region.
3
. Results and discussion
Microscopic characterization by HRTEM of the gold hybrid
materials confirmed the presence of spherical Au NPs (Fig. 3). Dark
field TEM micrographs facilitates evaluation of the shape, size and
position of Au NPs on the surface of graphene. We have observed
that spherical Au NPs are homogeneously distributed all over the
graphene surface and not only located at the edges or wrinkles.
The particle size histogram reveals a size distribution with an aver-
3.1. Synthesis and characterization of Au-NPs-rGO.
Three NHC ligands having different handles were tested for
their properties on reinforcing the anchoring of Au NPs onto the
surface of graphene, namely , b or -NPs-rGO. The hybrid mate-
a
c
rials are composed of graphene that acts as a carbonaceous 2D sup-
port and Au NPs capped with the NHC ligands. The synthetic
procedure corresponds to a bottom-up approach starting from
well-defined organometallic Au(I) complexes, which allows a bet-
ter control of the nature and composition of Au NPs (Fig. 2) [39].
First, Au(I) complexes containing NHCs ligands with different pol-
yaromatic tags were prepared from imidazolium salts using gold
age diameter of 3.35 ± 1.7 nm (n = 296) for
a-NPs-rGO, 4.83 ± 1.
6 nm (n = 120) for b-NPs-rGO and 3.05 ± 1.0 nm (n = 467) for
c-
NPs-rGO (Fig. 3). HRTEM micrographs shows the expected inter-
planar distances for the crystalline Au NPs in the three hybrid
materials. The morphological analysis reveals that the remote
functionalization using different tags at the NHC ligands does not
play an important role on the shape, size and crystallinity of the
formed Au NPs. We also assessed the effect of temperature in the
formation of Au NPs by decomposing complex 3 in the presence
of rGO at 25 and 45 °C. The results reveal that in the range of tem-
perature studied, this parameter does not affect the morphology of
the NPs indicating that the growth and dispersion of Au-NPs is pre-
dominantly controlled by the interaction with the NHC ligands and
graphene surface. We have previously observed that graphene
plays an important role in the formation of MNPs. For instance,
the synthesis of PdNPs without graphene using molecular hydro-
gen produces cylindrical particles, while in contrast, in the pres-
ence of graphene the shape of the NPs is spherical [49].
2 3
chloride (or bromide) and K CO (Complexes 1, 2 and 3) [40,41].
Au(I) complexes containing NHCs ligands have been widely used
in homogeneous catalysis and represent a convenient starting
point for the preparation of Au NPs [42–44]. Previous to the forma-
tion of Au NPs, NHC-Au-X complexes were converted into the cor-
responding triflate (OTf) derivatives using a silver salt. The
presence of a labile triflate ligand induces spontaneously the for-
mation of Au NPs in the presence of reduced graphene oxide
(
rGO) by the organometallic approach as we have previously
observed [45]. The NHC-Au-OTf complexes decompose in the pres-
ence of rGO resulting in the formation of Au NPs on the surface of
graphene. These Au NPs are covered by NHC ligands (vide infra)
which can or cannot also interact strongly with rGO, providing sta-
bility and control the interaction with substrates that may result in
selective catalysis [46–48]. The preparation of the Au NPs requires
long times but the process is carried out under mild conditions
which preserve the inherent properties of the support and the
capping ligands (Fig. 2).
X-ray photoelectron spectrometry (XPS) is a characterization
technique that provides valuable information about the elemental
composition and oxidation states of the elements located on the
surface (Fig. 3). The survey XPS spectrum of the three hybrid mate-
rials [a, b or c-NPs-rGO] is dominated, as expected, by the presence
of two intense peaks attributed to the oxygen and carbon from the
rGO used as a support. In addition, the presence of fluor, nitrogen,
115

D. Ventura-Espinosa, S. Martín, H. García et al.
Journal of Catalysis 394 (2021) 113–120
sulphur and gold confirms the presence of NHC ligands and triflate
-
anions (OTf ) on the surface of Au NPs (see SI for the high-
resolution spectra and assignment of these elements). The high-
resolution spectrum of Au(4f) region for
doublet peaks, due to the spin–orbit splitting effect (4f7/2 and
5/2) at binding energies of 88.7 and 85.0 eV attributed to Au(I)
a-NPs-rGO shows two
4
f
[
(
50,51] and at 88.0 and 84.3 eV attributed to Au(0) [52,53]
Fig. 3d). Analysis of the corresponding areas associated with each
signal in the convoluted spectrum shows an area ratio of 2:1 for
the peaks attributed to Au(0) respect to the peaks attributed to
Au(I). In the case of the hybrid material containing a naphthalene
group (b-NPs-rGO), the two doublet peaks appear at the same
binding energies than that for a-NPs-rGO, confirming the presence
of Au(I) and Au(0), albeit in this case the relative area ratio is
inverted with Au(I) being the prevalent (Fig. 3h). Surprisingly, in
the case of c-NPs-rGO only a doublet peak at the binding energies
corresponding to Au(I) is observed without detecting the presence
of Au(0) (Fig. 3i). This last result is in agreement with the findings
of Toste and Somorjai who reported that XPS of Au NPs functional-
ized with chiral-NHC ligands showed only the presence of Au(I). A
further study using EXAFS revealed a major contribution Au(0)
[
48]. In our case, the differences in the XPS analysis are ascribed
to a ligand effect. The three hybrid materials have the same Au
environment but different remote ligand functionalization and in
the case of the bulkier polyaromatic group (pyrenyl) only Au(I) is
observed.
3.2. Ligand effects in the catalytic hydration of alkynes.
The evaluation of ligand effect in the stabilization of Au NPs was
assessed in hydration and intramolecular hydroamination of
alkynes. Gold is an efficient catalysts in the hydration of alkynes
for a variety of different substrates at low catalysts loadings and
under mild reaction conditions [54–56]. We chose 4-octyne as a
model substrate. In all cases the catalytic reactions were carried
out under the same conditions and using a low catalyst loading
(
[Au] 0.05 mol%). The activity and stability of the three hybrid
materials was evaluated by monitoring the reaction progress by
gas chromatography and by reusing the hybrid materials (Fig. 4).
After each run, the catalysts is removed from the solution by
decantation, washed with MeOH and dried with pentane. The
Fig. 4. Performance of Au-NPs-rGO as catalysts in hydration of alkynes. Reaction
results in the first run using
a
-NPs-rGO show full conversion in
conditions: 4-octyne (0.2 mmol), catalyst loading (0.05 mol% based on Au), MeOH
(1.4 mL) as solvent, H O (2 Eq, 7.2 lL) at 50 °C. Conversion determined by GC/FID
2
and using 1,3,5-trimethoxybenzene as an internal standard.
the hydration of 4-octyne in less than 100 min at 50 °C. In the sec-
ond run an important decrease in activity was observed, but still
full conversion was achieved in 200 min. The activity decrease is
more pronounced in run 3, where less than 50% conversion was
achieved in 500 min (Fig. 4a). These results indicate that in the case
conversions were obtained in less than 150 min. In runs 9 and 10,
there is a considerable catalyst deactivation but still full conversion
is achieved in 300 min. The differences in the activity and stability
using the three catalytic systems indicates a pronounced effect of
the ligand in the stabilization of the Au NPs on the surface of
graphene.
of the hybrid material
a-NPs-rGO deactivation in the first run
occurs in a significant extent. Similar results were obtained using
b-NPs-rGO as catalyst. Thus, there was a considerable catalyst
deactivation from run 1 to run 2, but the decrease in activity was
even more important in run 3, where only 50% conversion could
be achieved in 500 min (Fig. 4b). Therefore, the catalytic behaviour
of a-NPs-rGO and b-NPs-rGO is similar and characterized by a high
activity in the first run at low catalyst loadings and a notable deac-
3.3. Characterization of Au NPs after the hydration of alkynes.
tivation in the second and third runs.
Completely different behavior was observed for
terms of activity, -NPs-rGO is even a better catalyst than
NPs-rGO or b-NPs-rGO, a fact that can be attributed to the major
amount of Au(I) as observed by XPS. Using -NPs-rGO full conver-
sion of 4-octyne was achieved in 60 min. The -NPs-rGO material
could be recycled up to ten times (Fig. 4c) indicating its high stabil-
ity. The results show that from run 1 to run 3 the activity is main-
tained according to the similar apparent rate constants and the
coincidence in the reaction-time profiles (Fig. 4c inset). Then, there
is a gradual catalyst deactivation from run 4 to run 8, but still full
c
-NPs-rGO. In
In order to clear up the differences in activity/stability and
understand deactivation pathways of Au NPs, the hybrid materials
after the recycling experiments were characterized by HRTEM
microscopy, XPS and ICP/MS (Fig. 5). First, we focused on the mor-
phological assessment of the support in the three hybrid materials.
The HRTEM images before and after the recycling experiments
show similar properties for the graphene, indicating that the sup-
port is not altered under the conditions of hydration of alkynes,
c
a-
c
c
even in the case of the material
c-NPs-rGO after ten consecutive
runs. Regarding the presence of Au NPs, it is important to note that
116

D. Ventura-Espinosa, S. Martín, H. García et al.
Journal of Catalysis 394 (2021) 113–120
Fig. 5. Characterization of Au NPs supported onto graphene after recycling. First column: Dark field TEM micrographs. Second column: HRTEM micrograph showing the
crystallinity and same morphology for Au NPs after recycling and third column: XPS analysis of the core-level peaks (eV) for the Au4f region.
few Au NPs could be observed in the case of
catalytic runs (Fig. 5a). The scarce number of Au NPs in
explains the sharp drop of activity in this material from run 2 to
a
-NPs-rGO after three
a
-NPs-rGO as can be deduced also, for example, from the low sig-
nal/noise ratio of the core-level peak for the Au4f region (Fig. 5c).
On the contrary, in the case of -NPs-rGO the noise signal is lower
a-NPs-rGO
c
run 3 (Fig. 4a). On the contrary, the number of Au NPs is still abun-
dant in the case of c-NPs-rGO even after ten runs (Fig. 5g). TEM
characterization suggests that the role of NHC ligands consist in
attaching Au NPs to the surface of the support and in consequence
increasing stability under catalytic conditions, particularly impor-
revealing a greater amount of gold. Although XPS may only provide
a semiquantitative analysis, gold loading was confirmed by ICP/MS
(see discussion below). These results point towards a potential deac-
tivation of the hybrid material a-NPs-rGO caused by leaching. The
XPS oxidation state analysis of gold after recycling reveals that the
core-level peaks of Au4f appear at the same binding energy indicat-
ing a similar nature of the Au NPs in the three hybrid materials.
Finally, the remaining amount of gold in the three hybrid mate-
rials after recycling was analysed by ICP/MS. Gold content in used
tant in the case of c-NPs-rGO containing a pyrenyl tag. Addition-
ally, high magnification HRTEM micrographs confirm the same
morphology for Au NPs after recycling and that the shape (spheri-
cal) and size (ca. 3.5 nm) of Au NPs in the three hybrid materials is
preserved after the recycling experiments. In the case of hybrid
material b-NPs-rGO the shape and size is preserved after three
a-NPs-rGO could not be detected by ICP/MS, indicating that all the
Au NPs are lost by leaching in only three catalytic runs. The gold
amount in b-NPs-rGO is 54 wt% of the initial content implying a
fast deactivation by leaching in only three runs. The remaining gold
runs but in
c-NPs-rGO the morphology is preserved even after
ten catalytic runs (Fig. 5h).
The XPS characterization after recycling confirms a similar nat-
ure and composition of the three hybrid materials. The survey XPS
spectrum of the hybrid materials Au-NPs-rGO is again dominated
by two intense peaks corresponding to the carbon and oxygen of
the support. The presence of fluor, nitrogen, sulfur and gold, which
confirm the nature and composition of the Au NPs is also observed.
The relative intensity of the peaks corresponding to Au decreases vs.
the peaks assigned to the support compared to the as prepared
hybrid material. This effect is more pronounced in the case of
in c-NPs-rGO is 65 wt% of the initial content after ten catalytic
runs. The characterization techniques employed give important
information about the hybrid materials after recycling under cat-
alytic conditions. The results clearly support a deactivation path-
way caused by leaching and negligible Ostwald effects. The
hybrid material
c-NPs-rGO containing a pyrenyl group that
favours a strong interaction with the graphene resulted in a better
performing catalysts derived from the higher stability and the
diminished Au leaching.
117

D. Ventura-Espinosa, S. Martín, H. García et al.
Journal of Catalysis 394 (2021) 113–120
3.4. Ligand effects in the catalytic hydroamination of alkynes.
after 5 runs is 28% of the initial amount and in c-NPs-rGO the
remaining gold is more than 70% after 10 runs. These results are
in agreement to Au leaching as the prevalent deactivation path-
way. Notably, as we have previously observed in the case of hydra-
tion of alkynes, the morphology of Au NPs is preserved after ten
runs in the intramolecular hydroamination (Fig. 6i). The XPS anal-
The catalytic properties of Au-NPs-rGO in the intramolecular
hydroamination exhibited similar trends as in the hydration of
alkynes (Fig. 6). The three Au-NPs-rGO are efficient catalysts in
intramolecular hydroamination affording quantitative yields of
indol in short reaction times (less than 100 min) at low tempera-
tures (50 °C) and using a low catalyst loading of 0.05 Au mol%.
ysis of
a-NPs-rGO reveals a low amount of gold meanwhile the XPS
spectrum of
c
-NPs-rGO confirms a similar elemental composition
The materials
a
-NPs-rGO and b-NPs-rGO could be reused four runs
and at the same binding energies for the spent catalyst in compar-
ison to the fresh sample. The catalytic results in the intramolecular
hydroamination indicate a clear difference between the three
hybrid materials in terms of stability depending on the nature of
with only a gradual decrease in activity. This deactivation was
more pronounced in run 5 as reflected in the corresponding tem-
poral reaction profiles (Fig. 6a, 6d). The behaviour of the hybrid
material
c
-NPs-rGO was significantly different. In this case, the
the NHC ligand. The material containing a pyrenyl group (c-NPs-
catalyst was reused up to six times without significant deactiva-
tion. Then, there was a gradual deactivation but still full conversion
was achieved in the tenth run (Fig. 6g). Microscopic characteriza-
tion by STEM of the spent catalysts after recycling experiments
confirmed the spherical shape of Au NPs and provided a visual
indication of the amount of particles remaining in the hybrid mate-
rGO) is more efficient as a consequence of a stronger interaction
with graphene that again avoids deactivation by leaching. Thus,
we have observed in two benchmark reactions, the importance of
ligand design in order to obtain more efficient catalytic systems
based on avoiding deactivation caused by leaching.
rial. For instance, images of
amount of particles after run 5. On the contrary, the number of
Au NPs in -NPs-rGO (Fig. 6h) is higher even after ten catalytic
runs. These results revealed a first hint that leaching should be
a-NPs-rGO (Fig. 6b) showed a small
4. Conclusions
c
We have assessed the ligand influence in the catalytic proper-
ties of Au NPs immobilized onto graphene. For this purpose, three
types of Au NPs containing different NHC ligands have been syn-
thesized by decomposition of well-defined Au(I)–NHC complexes.
the main deactivation pathway in
of
catalytic materials. In the case of
a
-NPs-rGO, but not in the case
c-NPs-rGO. These results were confirmed by ICP/MS of the used
a
-NPs-rGO the remaining gold
Fig. 6. Catalytic properties of Au-NPs-rGO in intramolecular hydroamination of alkynes. Reaction conditions: substrate (0.2 mmol), catalyst loading (0.05 mol% based on Au),
toluene (2 mL) as solvent, at 50 °C. Conversion determined by GC/FID and using 1, 3, 5-trimethoxybenzene as an internal standard. Characterization of Au NPs supported onto
graphene after recycling. Dark field TEM and HRTEM micrographs.
118

D. Ventura-Espinosa, S. Martín, H. García et al.
Journal of Catalysis 394 (2021) 113–120
The ligands contain different functionalities at a remote position
based on polyaromatic groups that permit p-stacking interactions
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[
with the graphene support of different strength. The synthetic pro-
cedure of Au NPs allows the formation of similar NPs that are crys-
talline and small in size (3.0 – 4.0 nm) independently of the ligand
used. The catalytic properties of the three Au NPs types immobi-
lized onto graphene revealed important ligand effects in hydration
of alkynes and intramolecular hydroamination. The Au NPs func-
tionalized with an NHC ligand containing a pyrenyl as polyaro-
matic group showed superior stability versus the Au NPs
functionalized with a naphthyl or lacking of a polyaromatic group.
The presence of a pyrenyl handle allows a stronger interaction with
graphene increasing the stability of Au NPs. We have observed that
Au NPs are deactivated by leaching from the support and not by
Ostwald ripening effects. It is important to note that the Au NPs
functionalized with a pyrenyl group preserved their morphology
even after ten catalytic runs. We believe that these results clearly
evidence the possible contribution of the ligand in the develop-
ment of efficient catalytic materials by attaching simultaneously
to the Au NPs and to the surface of graphene, diminishing leaching
and increasing catalyst stability. This unexplored role of ligands
may inspire other research groups in the development of efficient
catalysts.
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Declaration of Competing Interest
The authors declare that they have no known competing finan-
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The authors thank the financial support from MICIU/AEI/FEDER
RTI2018-098237-B-C21, RTI2018-098237-B-C22 and PID2019-
05881RB-I00) and Universitat Jaume I (UJI-B2018-23). S. M.
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Aragón) for funding the research group Platón (E31_17R). D. V-E.
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