Revealing Hydrogenation Reaction Pathways on Naked Gold Nanoparticles

Article abstract of DOI:10.1021/acscatal.7b00391

Gold nanoparticles (AuNPs) display distinct characteristics as hydrogenation catalysts, with higher selectivity and lower catalytic activity than group 8-10 metals. The ability of AuNPs to chemisorb/activate simple molecules is limited by the low coordination number of the surface sites. Understanding the distinct pathways involved in the hydrogenation reactions promoted by supported AuNPs is crucial for broadening their potential catalytic applications. In this study, we demonstrate that the mechanism of the hydrogenation reactions catalyzed by AuNPs with "clean" surfaces may proceed via homolytic or heterolytic hydrogen activation depending on the nature of the support. The synthesis of naked AuNPs employing γ-Al₂O₃ and ionic liquid (IL)-hybrid γ-Al₂O₃ supports was accomplished by sputtering deposition using ultrapure gold foils. This highly reproducible and straightforward procedure furnishes small (～6.6 nm) and well-distributed metallic gold nanoparticles (Au(0)NPs) that are found to be active catalysts for the partial and selective hydrogenation of substituted conjugated dienes, alkynes, and α,β-unsaturated carbonyl compounds (aldehydes and ketones). Kinetic and deuterium labeling studies indicate that heterolytic hydrogen activation is the primary pathway occurring on the AuNPs imprinted directly on γ-Al₂O₃. In contrast, AuNPs supported on IL-hybrid γ-Al₂O₃ materials cause the reaction to proceed via a homolytic hydrogen activation pathway. The IL layer surrounds the AuNPs and acts as a cage, influencing the frequency of the interaction of the catalytically active species and the metal surface and, consequently, the catalytic performance of the AuNPs. The IL layer is shown to improve the product selectivity by the enhancement of the substrate/product discrimination, and to decrease the catalytic activity by shifting the rate-determining step to the H₂ and substrate competitive adsorption/activation on the same active sites. A series of kinetic experiments suggest that AuNPs imprinted on an IL-hybrid γ-Al₂O₃ support are more efficient (lower activation energy, E_a) than group 8-10 metal based catalysts for hydrogenation reactions at moderate to high temperatures (75-150 °C).

Full text of DOI:10.1021/acscatal.7b00391

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Article
Revealing Hydrogenation Reaction Pathways on Naked Gold Nanoparticles
Leandro Luza, Camila P. Rambor, Aitor Gual, Jesum Alves Fernandes, Dario Eberhardt, and Jairton Dupont
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b00391 • Publication Date (Web): 13 Mar 2017
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ACS Catalysis
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Revealing Hydrogenation Reaction Pathways on Naked Gold Nano-
particles
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Leandro Luza,‡ Camila P. Rambor,‡ Aitor Gual,∫ Jesum A. Fernandes,∫ Dario Eberhardtƒ and
Jairton Dupont*,‡,∫
‡Institute of Chemistry, Universidade Federal do Rio Grande do Sul, Av. Bento Gonçalves, 9500, Porto Alegre,
Brazil. Eꢀmail: jairton.dupont@ufrgs.br, jairton.dupont@nottingham.ac.uk.
∫GSK Carbon Neutral Laboratory for Sustainable Chemistry, University of Nottingham, NG7 2TU, Nottingham, UK.
ƒPhysics Faculty, Pontifícia Universidade Católica do Rio Grande do Sul, Av. Ipiranga, 6681, Porto Alegre, Brazil.
ABSTRACT: Gold nanoparticles (AuNPs) display distinct characteristics as hydrogenation catalysts, with higher selecꢀ
tivity and lower catalytic activity than group 8ꢀ10 metals. The ability of AuNPs to chemisorb/activate simple molecules is
limited by the low coordination number of the surface sites. Understanding the distinct pathways involved in the hydroꢀ
genation reactions promoted by supported AuNPs is crucial for broadening their potential catalytic applications. In this
study, we demonstrate that the mechanism of the hydrogenation reactions catalyzed by AuNPs with ‘clean’ surfaces
may proceed via homolytic or heterolytic hydrogen activation depending on the nature of the support. The synthesis of
naked AuNPs employing γꢀAl2O3 and ionic liquid (IL)ꢀhybrid γꢀAl2O3 supports was accomplished by sputteringꢀ
deposition using ultrapure gold foils. This highly reproducible and straightforward procedure furnishes small (~6.6 nm)
and wellꢀdistributed metallic gold nanoparticles (Au(0)NPs) that are found to be active catalysts for the partial and seꢀ
lective hydrogenation of substituted conjugated dienes, alkynes and α,βꢀunsaturated carbonyl compounds (aldehydes
and ketones). Kinetic and deuteroꢀlabelling studies indicate that heterolytic hydrogen activation is the primary pathway
occurring on the AuNPs imprinted directly on γꢀAl2O3. In contrast, AuNPs supported on ILꢀhybrid γꢀAl2O3 materials
cause the reaction to proceed via a homolytic hydrogen activation pathway. The IL layer surrounds the AuNPs and acts
as a cage, influencing the frequency of the interaction of the catalytically active species and the metal surface, and
consequently, the catalytic performance of the AuNPs. The IL layer is shown to improve the product selectivity by the
enhacement of the substrate/product discrimination, and to decrease the catalytic activity by shifting the rateꢀ
determining step to the H2 and substrate competitive adsorption/activation on the same active sites. A series of kinetic
experiments suggest that AuNPs imprinted on an ILꢀhybrid γꢀAl2O3 hybrid support are more efficient (lower activation
energy, Ea) than group 8ꢀ10 metalꢀbased catalysts for hydrogenation reactions at moderate to high temperatures (75–
150 °C).
KEYWORDS: Gold Nanoparticles • Supported Ionic Liquid • Hydrogenation • Disproportionation • Sputtering
of reports on the use of supported/soluble Au hydroꢀ
1. INTRODUCTION
genation catalysts, what determines the reaction mechaꢀ
Understanding the relationship between the electronic
nism on AuNPs is not well understood. Studies involving
and structural properties of supported metal nanopartiꢀ
a Au model catalyst are still scarce since many variables
cles (MNPs) and their catalytic performance is one of the
may affect the frequency factors of the surface species
biggest challenges for their application in organic transꢀ
(see SI, Sections S3–S6). The presence of additives,
formations at the industrial scale.1ꢀ3 Gold nanoparticles
modifiers and individual functionalized unsaturated subꢀ
(AuNPs) exhibit unique and distinct catalytic activity and
strates, as well as the size/distribution of AuNPs and the
selectivity when compared to group 8ꢀ10 MNPs (see SI,
nature of the metalꢀsupport interaction, can facilitate the
Section S1).4ꢀ6 Despite their relatively low catalytic activiꢀ
dissociation of H2.11ꢀ14 The term “naked” refers to cataꢀ
ty, AuNPs are one of the most selective catalysts for the
lysts (homogeneous, heterogeneous or nano) surroundꢀ
partial hydrogenation of, for example, alkynes and α,βꢀ
ed by weak coordination species and hence containing
unsaturated carbonyl substrates.5 It is assumed that their
ease accessible coordination catalytically active sites
poor catalytic activity for hydrogenation reactions is
(metal surface or metal centre).15ꢀ19
mainly caused by the lowꢀfrequency factor of hydrides
Scheme 1 displays an illustrative description of the
most plausible hydrogenation pathways on an AuNP
surface. The first and most commonly observed pathway
(catalytically active species) on the metal surface, and
not due to the high activation energy of dihydrogen (H2)
(see SI, Section S2).7ꢀ10 Even with the growing number
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(i) involves the ionic hydrogenation mechanism in which on γꢀAl2O3 (one of the most used oxide supports for Au)
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8
the support, additive or substrate collaborate with the Au
surface sites, producing the direct heterolytic activation
of H2 and the transference of one proton to the support,
additive or substrate, with formation of the metal hyꢀ
dride.20ꢀ25 Among others, this mechanism has already
been observed to operate with AuNPs supported onto
inorganic oxides (TiO2, Al2O3 and CeO2),26ꢀ28 AuNPs
and also on ionic liquid (IL)ꢀhybrid γꢀAl2O3. The former
catalyst displayed a strong supportꢀmetal interaction
between the AuNPs and γꢀAl2O3, whereas for the latter,
a thin IL layer is acting as a cage for the AuNPs, avoidꢀ
ing direct contact between the AuNPs and γꢀAl2O3
(Scheme 2). For the preparation of these model cataꢀ
lysts, we used a magnetron sputtering chamber that
allows constant mixing of the solid support, which leads
to the randomly deposition of the MNPs in all spatial
planes of the inorganic oxide. In fact, the obtained
AuNPs are small and uniformly well distributed on all the
distinct supports. Similarly to group 8ꢀ10 MNPs (Pd and
RuNPs),40,41,49 the AuNPs generated by this technique
are imprinted onto the most external layers of the inorꢀ
ganic oxide support. Their surfaces are clean and free of
contaminants since they are directly obtained from high
purity (99.999%) metallic foils. The selective hydrogenaꢀ
tion of conjugated dienes, alkynes and α,βꢀunsaturated
carbonyl compounds (ketones and aldehydes) were
used as probe reactions for testing the 6.6 nm face cenꢀ
tered cubic (fcc) metallic gold nanoparticles (Au(0)NPs)
model catalysts. The kinetic and deuteroꢀlabelling studꢀ
ies herein described provide experimental evidences that
the H2 activation by AuNPs imprinted directly on γꢀAl2O3
proceeds via an ionic hydrogenation mechanism (i),
whereas a dissociative pathway (ii) operates on AuNP
surfaces deposited on ILꢀhybrid γꢀAl2O3. Moreover, an
outerꢀsphere hydrogenation mechanism (iv) is also eviꢀ
denced in the hydrogenation of 1,3ꢀcylohexadiene by
AuNPs catalysts.
29,30
stabilized by secondary phosphine oxides (SPOs)
and AuNPs catalyzed hydrogenations of substrates with
heteroatoms in the reducible function (Nꢀdonor subꢀ
strates, such as imines, nitriles and quinolines).31,32 The
second mechanism (ii) results in the dissociative chemiꢀ
sorption of H2 by the low coordinated Au surface sites,
with the formation of H atoms, which are in bridge posiꢀ
tions sharing the low coordinated Au atoms without inꢀ
ducing any significant deformation of the AuꢀAu distancꢀ
es.33ꢀ35 Although, this mechanism is quite probable (simiꢀ
lar to that observed for other noble metals) from a theoꢀ
retical point of view36, it has not yet been firmly estabꢀ
lished experimentally.
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2. RESULTS AND DISCUSSION
2.1. Synthesis and characterization of supported
naked AuNPs. The hybrid support B1 (Scheme 2) was
prepared by reaction of γꢀAl2O3 with 1ꢀnꢀbutylꢀ3ꢀ
(trimethoxysilylpropyl)ꢀimidazolium chloride. The hybrid
support B2 was afford by simple B1 anion exchange with
LiNTf2 (NTf2 = bis(trifluoromethylsulfonyl)imide).48,49
Scheme 1. Possible hydrogenation mechanisms operatꢀ
ing at the AuNP surfaces (M = Al, Ti, Si, Zr and Ce, B =
ligand or substrates with heteroatoms): (i) heterolytic H2
activation; (ii) homolytic H2 activation; (iii) generation of
subꢀnanometric Au catalytic active species and (iv) outꢀ
erꢀsphere.
The third mechanism (iii) consists of the activation of
H2 with the ejection of AuꢀH subꢀnanometric clusters
(HxAumLn, L: ligand) as catalytically active species. Altꢀ
hough this pathway has not yet been reported, it is wellꢀ
known that isolated Au atoms37ꢀ40 and subꢀnanometric
clusters32,41ꢀ44 display improved performances regarding
activity and selectivity on hydrogenations of butadienes,
internal alkynes and α,βꢀunsaturated carbonyl comꢀ
pounds. Finally, and not yet experimentally proved, is the
involvement of the outerꢀsphere mechanism (iv), similar
to the disproportionation of cyclohexadiene observed for
PdNP hydrogenation catalysts.45ꢀ48 Hence, more detailed
experiments are crucial to obtain deeper information
regarding mechanism(s) occurring during the hydrogenaꢀ
tion reactions catalyzed by AuNPs and the pathways
involved in the hydrogen/substrate activation, which will
allow obtaining more efficient Auꢀbased catalysts.
Scheme 2. AuNPs model catalysts prepared by sputterꢀ
ing deposition: Au/γꢀAl2O3, Au/B1 (X: Cl) and Au/B2 (X:
NTf2).
For this purpose, we have designed surface clean,
small size and wellꢀdistributed AuNPs supported directly
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Table 1. Summary of the physical and chemical characteristics of the Au/γꢀAl2O3, Au/B1 and Au/B2 catalysts.
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Entry
Catalyst
Au/γꢀAl2O3
Au/B1
IL[a]/ mmol g−1
Au[b]/ wt.%
0.34 ± 0.03
0.34 ± 0.04
0.32 ± 0.04
∅
[c]/ nm
SBET[d]/ m2 g−1
∅
[d]/ nm
Pore
Au 4f7/2[e]/ eV
83.8
AuNPs
1
2
3
―
6.7 ± 2.0
6.5 ± 2.2
6.6 ± 1.8
194
142
143
7.0
6.3
6.2
0.38
83.6
Au/B2
0.35
83.7
[a]Determined by CHN elemental analysis; [b]Determined by Xꢀray fluorescence (XRF) spectrometry; [c]Determined by transꢀ
mission electron microscopy (TEM); [d]Determined by the Brunauer–Emmett–Teller (BET) multipoint method and Barrettꢀ
JoynerꢀHalenda (BJH) method; [e]Determined and Xꢀray photoelectron spectroscopy (XPS).
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The γꢀAl2O3 and the ILꢀhybrid γꢀAl2O3 (B1 and B2)
were decorated with AuNPs using a 3D mixing sputtering
device (Scheme 2).48ꢀ50 The asꢀprepared catalysts (Au/γꢀ
Al2O3, Au/B1 and Au/B2) were characterized by solidꢀ
state crossꢀpolarization magic angle spinning nuclear
magnetic resonance (CPꢀMAS NMR), Fourier transform
infrared (FTꢀIR), N2ꢀphysisorption, transmission electron
microscopy (TEM), CHN elemental analysis, scanning
electron microscopy with energyꢀdispersive Xꢀray specꢀ
troscopy (SEMꢀEDS), Xꢀray diffraction (XRD), Xꢀray
fluorescence (XRF), Rutherford backscattering specꢀ
trometry (RBS) and Xꢀray photoelectron spectroscopy
(XPS).
13C and 29Si CPꢀMAS NMR analysis revealed the presꢀ
ence of unreacted SiꢀC bonds and the existence of T1
and T2 species, thus confirming that, even under the
acidic reaction conditions, the IL cation moiety has been
effectively supported by the grafting method (see SI,
Tables S5 and S6 and Figures S3 and S4). The presꢀ
ence of the IL moieties (cation and anion) in the Au/ILꢀ
hybrid γꢀAl2O3 was confirmed by the observation of their
characteristic IR bands (see SI, Table S7 and Figure
S5). The N2ꢀphysisorption isotherms of Au/γꢀAl2O3 and
Au/ILꢀhybrid γꢀAl2O3 (Au/B1 and Au/B2) exhibited similar
typeꢀIV isotherm patterns with surface areas between
142 and 194 m2 g−1 and pore diameters between 6.2 to
7.0 nm (Table 1 and Figures 1(a) and 1(b)).
Au/B2 catalysts with mean diameters of 6.7, 6.5 and 6.6
nm, respectively (Table 1 and Figures 1(d) and 1(e)). In
addition, the RBS results clearly indicate that in all cases
the metal concentration is much higher at the most exꢀ
ternal layers (≈ 10 nm) of Au/γꢀAl2O3 and Au/B1 and
Au/B2 catalyst models, and therefore, the accessibility of
the reactants to the AuNP active sites is expected to be
superior compared to Au catalyst synthesized by chemiꢀ
cal methods (Figure 1(f)). Similar depth profile was alꢀ
ready observed for PdNPs imprinted in ILꢀhybrid SiO2
supports.45,46These results suggest that such characterꢀ
istics are unaffected by the nature of the support (γꢀAl2O3
or hybrids γꢀAl2O3 containing hydrophilic or hydrophobic
moieties). It has already been demonstrated that deꢀ
pending on sputtering parameters, it is possible to obtain
AuNPs of small sizes (1–10 nm) on pure IL,53ꢀ55 pure
inorganic oxides (TiO2)56 and ILs supported on inorganic
oxides (ILsꢀdispersed on TiO2).57 It is also worth noting
that in all cases, the AuNP surface is exclusively comꢀ
posed of metallic gold (Au(0)), as determined by XPS
(Figure 1(g)). Furthermore, there is a positive shift in the
Au binding energies (for instance, see Au 4f components
in Table 1 and Figure 1, and Au valence band (VB) in SI,
Figure S6) of the AuNPs onto γꢀAl2O3, with respect to
those on the ILꢀhybrid γꢀAl2O3 (B1 and B2), suggesting
that in the latter case, the interaction of the AuNPs with
the γꢀAl2O3 support was decreased by the presence of
the IL pair layers.58,59 Therefore, it is much likely that the
IL moiety forms an IL cage, surrounding the AuNPs, akin
to that already observed for other metal catalysts on ILꢀ
containing supports.45,60,61
The surface area, pore volume and pore diameter valꢀ
ues slightly decreased in the catalysts bearing ILs
(Au/B1 and Au/B2), indicating that ILs partially fill the γꢀ
Al2O3 pores. These findings are of crucial importance,
since they suggest the presence of a homogeneous and
wellꢀdispersed IL layer on the B1 and B2 supports,
which can provide efficient stabilization for AuNPs. Simiꢀ
lar characteristics were obtained in the immobilisation of
a functionalized imidazolium IL on SiO2 used for stabiliꢀ
zation of Ruꢀ and FeRuNPs.51,52
−
The complete exchange of Cl− by NTf2 was confirmed
by XPS analysis of the hybrid support B2 and also on
support AuNP containing material Au/B2 (Figure S7).
XPS analyses were also used to estimate the thickness
of the IL layers, and the presence of 10 and 7 Au atoms
per IL pair in Au/B1 and Au/B2, respectively, was conꢀ
firmed (see SI, Table S8 and Figure S7). Although these
IL layers are thin, previous reports on hydrogenation
processes described that similar thin IL layers produce
the best compromise between the hydrogenation activity
and the selectivity enhacement by IL substrate/product
discrimination.55,57,60 These behaviours were ascribed to
the effect on the concentration of reactants available at
the catalytic centre by the distinct reactant solubiliꢀ
ty/miscibility in the IL62ꢀ64 or to the IL interaction with
specific catalytically active particle sites, i.e., a proposed
ligandꢀlike interaction with different surface sites on Pt or
As expected, the metal amount (~0.3 wt.%), shape,
size (~6.6 nm) and distribution of the AuNPs are similar
in all catalysts, since identical sputtering conditions were
employed (Table 1 and Figure 1). The XRD analysis
displayed the typical pattern of the fcc Au(0) lattice with
characteristic diffraction peaks at 2θ = 38.1, 44.2, 64.6,
77.5 and 81.4°, corresponding to the (111), (200), (220),
(311) and (222) planes, respectively (Figure 1(c)). Interꢀ
estingly, TEM and SEMꢀEDS analyses showed wellꢀ
distributed and small AuNPs on the Au/Al2O3, Au/B1 and
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.
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Figure 1. Characterization of the AuNPs model catalysts prepared by sputtering deposition: (a) BET N2ꢀphysisorption isoꢀ
therms; (b) BJH pore diameter distribution; (c) XRD patterns; (d) TEM images and size histograms; (e) EDSꢀSEM images; (f)
RBS depthꢀprofile distribution of Au in the supports and (g) Au 4f XPS spectra.
PdNPs
by
the
use
of
1ꢀnꢀbutylꢀ3ꢀmethylꢀ
eV in the Cl 2p region was observed (198.3 and 195.6
eV for B1 and Au/B1, respectively). In the case of NTf2
bearing supports, the F 1s regions of B2 and Au/B2
displayed a peak related to uncoordinated NTf2 (688.8
eV),66 and a new component appeared at 685.4 eV for
Au/B2. These behaviours were ascribed to the interacꢀ
imidazolium—bis(trifluoromethylsulfonyl)imide (BMI∙NTf2)
IL.60,62,65 The IL pair interaction with the AuNPs was
further confirmed by comparing the XPS spectra of B1,
B2, Au/B1 and Au/B2 (see SI, Figure S8). For the supꢀ
ports bearing chloride as an anion, a negative shift of 2.7
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tion of the contact ion pairs67 of the IL with the AuNPs,
minute) calculated from the slope of plots of TON vs. time at
low substrate conversions.70
similar to those observed for thiolꢀmodified AuNPs.58
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2
2.2. Catalytic hydrogenation properties of support-
ed naked AuNPs. The hydrogenation of 1,3ꢀ
cyclohexadiene (1) was used as a probe to check the
catalytic properties of the asꢀprepared supported AuNPs
(Table 2). It is clear that there is an influence of the supꢀ
port on the catalytic performance of the imprinted
AuNPs. Au/γꢀAl2O3 is more active than Au/B1 and Au/B2
under identical reaction conditions, whereas Au/B2 is
slightly more selective to cyclohexene (2) formation (Taꢀ
ble 2, Entries 3, 6 and 9). It is also noteworthy that
Au/B1 and Au/B2 are almost inactive at lower termperaꢀ
tures (50 °C) and lower pressures (5 bar H2) (Table 2,
Entries 4, 5, 7 and 8), while Au/γꢀAl2O3 exhibits higher
hydrogenation activities (Table 2, Entries 1 and 2). This
fact indicates a strong metalꢀsupport interaction (SMSI)
and probably a distinct H2 activation pathway (see beꢀ
low). More interestingly, when compared to the results
obtained using H2 and D2, a change in the kinetic isotopꢀ
ic effect (KIE) and cyclohexene (2) selectivities was
observed for the AuNPs catalysts surrounded by IL
(Scheme 3, Table S9 and Figure S9). The KIE observed
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Scheme 3. Hydrogenation of 1,3ꢀcyclohexadiene cataꢀ
lyzed by Au/γꢀAl2O3, Au/B1 and Au/B2 under H2/D2.
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Conversely, high KIEs were obtained by the Au/ILꢀ
hybrid γꢀAl2O3 catalysts (kH/kD of 4.6 and 3.8 with Au/B1
and Au/B2, respectively) indicating that, in these cases,
the the H2/D2 activation is occurring during the RDS.
1,2
1,2
Moreover, the formation of cyclohexeneꢀd2
and cyclohexeneꢀd2
(2ꢀd2
)
(2ꢀd21,4) products by (1,2ꢀ/1,4ꢀ
1,4
additions ratios of 4.5, 4.6 and 5.3 for Au/Al2O3, Au/B1
and Au/B2, respectively) suggests the involvement of πꢀ
allyl metastable intermediates in the reaction mechaꢀ
nism, and the formation of clohexeneꢀd0 (2ꢀd0) and benꢀ
zeneꢀd0
(3ꢀd0)
by
disproportionation
of
1,3ꢀ
cyclohexadiene (1) under D2 suggests the occurrence of
the outerꢀsphere mechanism (Scheme 1, pathway (iv)).
The existence of 1,3ꢀcyclohexadiene (1) disproportionaꢀ
tion is a clear indication of the involvement of Auꢀ
heterotopic species (AuꢀNP surfaces) as the catalytically
active sites,47 a fact that discards the presence of subꢀ
nanometric Au clusters as active species (Scheme 1,
pathway (iii)). In summary, the IL thin layer acts as a
catalytically active membraneꢀlike device, influencing the
frequency factor of active species on the Au surface,
thus changing the AuNP catalytic performance and even
the reaction mechanism and RDS.
for the Au/γꢀAl2O3 catalyst (kH/kD = 1.1) is comparable to
45,46
those observed in Pd/Al2O3,48 Pd/SiO2
and Pd/C1
catalysts in similar hydrogenation reactions and they are
consistent with the zeroꢀpoint energy difference between
isotopic isomers. Therefore, this is an indication that the
H2/D2 activation is not the rateꢀdetermining step (RDS) of
the hydrogenations catalyzed by Au/γꢀAl2O3, but rather
an involvement of hydrogen bond/formation pathways
during the reaction mechanism. Similar behaviours were
reported for other Auꢀcatalyzed processes (kH/kD = 1.5–
2.0)32,68,69 and hydrogenation of 1,3ꢀcyclohexadiene by
PdNPs supported on similar ILꢀhybrid supports (kH/kD of
1.3 and 2.0 for Al2O348 and SiO2,45,46 respectively).
The scope of the most selective catalyst (Au/B2) was
evaluated by carrying out the hydrogenation of 1,3ꢀ
dienes with distinct degrees of substitution, alkenes
conjugated with an arene system, internal aliphatic alꢀ
kynes, α,βꢀunsaturated aldehydes and α,βꢀunsaturated
ketones. In general, our catalytic system, Au/B2, disꢀ
played high selectivities in a broad range of hydrogenaꢀ
tions under 25 bar H2 and 100 °C (Table 3, Table S10
and Figure S10). The high alkene selectivities (96–
>99%) and activity orders obtained are related to the
steric congestion of the double C=C bonds, and thus,
the coordination ability to the Au surface: (8) > (1) > (13)
~ (5) > (17) ~ (20) (Table 3, Entries 1–6). In the hydroꢀ
genation of a primary alkene conjugated with an arene
system (24), much lower activity was obtained (Table 3,
Entry 7) compared to 1,3ꢀdienes, thus suggesting that
substrate adsorption/activation is occurring during the
RDS. The hydrogenation of an internal aliphatic alkyne
(26) achieved an selectivity of 99% to the internal alkene
with a Z:E ratio of 2.6. The activity in the hydrogenation
of internal alkyne is similar to those obtained with interꢀ
nal 1,3ꢀdienes (1 and 5), which indicates that the reactivꢀ
ity of 1,3ꢀdienes and internal alkynes should be very
similar (Table 3, Entry 8). The double bond is hydrogenꢀ
ated with high selectivity in α,βꢀunsaturated carbonyl
compounds (Table 3, Entries 9–12) and the activities
obtained are related to the electronic and steric effects of
the substrates. These results confirm that the coordinaꢀ
Table 2. Selective hydrogenation of 1,3ꢀcyclohexadiene
catalyzed by Au/γꢀAl2O3, Au/B1 and Au/B2.
Enꢀ
T/
ºC
PH2/ 2/
3/ 4/ TOF[c]/
Catalyst
try[a,b]
bar
%
%
%
min−1
0.7
1
2
3
4
5
6
7
8
9
Au/γꢀAl2O3
Au/γꢀAl2O3
Au/γꢀAl2O3
Au/B1
50
5
88
9
1
100
5
86 10
4
1.0
100 25
89
―
92
93
―
91
96
1
10 6.9
50
5
5
―
7
―
1
< 0.01
Au/B1
100
0.1
Au/B1
100 25
2
5
3.2
Au/B2
50
5
5
―
8
―
1
< 0.01
0.1
Au/B2
100
Au/B2
100 25
2
2
4.1
[a]Reaction
conditions: Au
(0.5
ꢁmol),
1,3ꢀ
cyclohexadiene/Au = 250, nꢀhexane (5 mL) and 250 rpm;
[b]Selectivity determined by GC analysis at >99% converꢀ
sion; [c]TOF = mol substrate converted/(mol Au surface ×
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Page 6 of 39
tion ability of the substrate to the Au surface plays a the alkene hydrogenation of involves three steps: (i)
1
2
3
4
5
6
7
8
significant role in the reaction rate.
alkene adsorption on the metal surface catalyst saturatꢀ
ed with hydrogen, (ii) hydrogen migration to the βꢀcarbon
of the alkene with formation of a σꢀbond between the
metal and αꢀC, and lastly (iii) reductive elimination of the
alkane.71 AuNPs, in contrast to group 8ꢀ10 metals, exꢀ
hibit an increasing capacity of H2 dissociation and H2
uptake with increasing temperature.72 H2 adsorption is
limited by kinetic factors at moderate temperatures (25–
100 °C) with two possible kinetic scenarios: (a) low conꢀ
centration of adsorbed hydrogen due to the limited H2
dissociation and (b) H2 dissociation favoured and high
concentration of adsorbed atomic H.73
Table 3. Selective hydrogenations catalyzed by Au/B2.
Conv./
%
[time/ h]
TOF[c
Enꢀ
Subꢀ
Products
(Selectivity/ %)
]/min−
try[a,b] strate
1
9
1
>99 [16]
>99 [16]
4.1
3.3
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
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40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Given the evidence obtained by the kinetic isotopic efꢀ
fects and catalytic experiments, the kinetics of the 1,3ꢀ
cyclooctadiene (5) hydrogenation was investigated using
a modified LangmuirꢀHinshelwood kinetic model.14,35,73,74
The application of the linear form of the Langmuirꢀ
Hinshelwood equation suggests that under our reaction
conditions (15–30 bar H2 and 75–150 °C), the RDS conꢀ
sists in the surface reaction between competitively adꢀ
sorbed 1,3ꢀcyclooctadiene (5) and H2 molecules on acꢀ
tive sites of the same nature. In these conditions, the
1,3ꢀcyclooctadiene (5) hydrogenation rate is independꢀ
ent (zero order) of the fraction of catalyst surface covꢀ
ered by the substrate and H2 (see SI, Tables S11 and
S12 and Figures S11ꢀS14). The reaction kinetic is differꢀ
ent at lower pressures (10 bar) and temperatures (50
°C), which is dependent on the fraction of Au surface
covered by 1,3ꢀdiene and H2. This probably occurs by
the lower fraction of H atoms absorbed in these condiꢀ
tions, which highlights the effect of the IL membrane on
the concentration of the species at the Au surface. This
behaviour was previously observed in hydrogenation
reactions catalyzed by PdNPs45 and indicates again the
presence of IL thin layers in the support. Therefore, theꢀ
se small thinness of these IL layers excluded the inꢀ
volvement of mass transfer limitations during the catalytꢀ
ic process, and they increase the solubility of the H2 at
moderate pressures (>10 bar) and temperatures (>50
°C).
2
3[d]
>99 [10]
>99 [16]
6.5
3.5
4
5
>99 [30]
96 [30]
1.3
1.1
6[e]
7
8
9
79 [30]
0.9
2.9
1.8
>99 [16]
>99 [24]
10
11
94 [30]
1.0
2.1
>99 [24]
12
21 [30]
0.6
[a]Reaction conditions: Au (0.5 ꢁmol), substrate/Au = 250, 5
mL of solvent (nꢀhexane for dienes and alkynes, and mꢀ
xylene for α,βꢀunsaturated carbonyl compounds), 25 bar H2,
100 °C and 250 rpm; [b]Selectivity determined by GC analyꢀ
sis; [c]TOF = mol substrate converted/(mol Au surface ×
minute) calculated from the slope of plots of TON vs. time at
low substrate conversions;70 [d] Selectivity for nꢀbutane (12):
Scheme 4. Proposed mechanism for the selective hyꢀ
drogenation of 1,3ꢀcyclooctadiene catalyzed by Au/B2.
[e]
3%; Selectivity for 1ꢀisopropylꢀ4ꢀmethylcyclohexane (23):
2%.
Scheme 4 displays the proposed mechanism for the
1,3ꢀcyclooctadiene (5) hydrogenation catalyzed by
2.3. Hydrogenation kinetics by supported naked
AuNPs. According to the HoriutiꢀPolanyi mechanism,
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ACS Catalysis
Au/B2. This mechanism is based on the HoriuatiꢀPolanyi
hydrogenation model catalysts. These catalytic systems
1
mechanism for the hydrogenation of alkenes but inꢀ
cludes an additional step: (iv) decoordination of the moꢀ
noene, i.e., cyclooctene (6) (Scheme 4 and Section
S10). A plot of the natural log of kr against the inverse of
different reaction temperatures allowed the estimation of
the apparent activation energy (Eaꢀapp) for the 1,3ꢀ
cyclooctadiene (5) hydrogenation (Figure 2). The obꢀ
served Arrhenius plot exhibits a convex curve, although
quite anomalously, it expresses the effect of the temperꢀ
ature on the H2 and substrate competitive adsorpꢀ
tion/activation on the same active sites, and the presꢀ
ence of absorbed mestastable πꢀallyl as reactive interꢀ
mediates on the AuNP surface (kr = k3K2). At lower temꢀ
peratures (50–100 °C), this reactive intermediate preꢀ
dominates and exhibits a preꢀexponential factor (A) of
658 s−1. However, as the temperature get higher (100–
150 °C), the πꢀallyl intermediate dissociates, which afꢀ
fects the reaction rate, leading to a small A value (1.03
s−1). The Eaꢀapp at low temperatures (29 kJ mol−1) is simiꢀ
lar to those observed for PdNPs catalysts supported
onto similar supports (32 kJ mol−1), but the Eaꢀapp at high
temperatures (9 kJ mol−1) is much lower for Au than for
Pd (24 kJ mol−1).45 Considering thermodynamics, higher
temperatures favoured the hydrogen desorption and the
reduction of the H/M ratio for group 8ꢀ10 metals. Interꢀ
estingly, the H/Au ratio is maintained, owing to the kinetꢀ
ic factors, i.e., higher temperatures facilitate the H2 disꢀ
sociation on AuNPs and the H/Au ratio reaches a maxiꢀ
mum at a certain temperature.72 Therefore, these studies
suggest that Au is more efficient (lower Ea) than group 8ꢀ
10 MNPs for hydrogenation reactions at moderate to
high temperatures (75–150 °C). Finally, the selectivity is
not modified by changing any parameter (subꢀ
strate/product ratio, hydrogen pressure or reaction temꢀ
perature), which emphasises again that the selectivity
Au/ILꢀhybrid alumina is inherent to the Au(0)NPs with IL
thin layers and attributed to a confinedꢀlike environment.
are active for the hydrogenation of a wide scope of subꢀ
strates with high selectivity. The distinct hydrogenation
pathways (heterolytic, homolytic and outerꢀsphere) disꢀ
played by the AuNP surfaces are directly related to the
nature of the support. More specifically, with the AuNP
location, those interacting directly with the oxide support
display a synergic behaviour in which the H2 is heterolytꢀ
ically activated by both catalyst components (support
and AuNPs). Whereas the AuNPs located preferentially
away from the oxide support, i.e. in the IL cage, present
the reactivity of naked AuNPs. The ILꢀlike membrane
acts in two ways: (1) by controlling the access of the
reagents to the AuNP surfaces and (2) by avoiding the
interaction between AuNPs and Al2O3, thus exchanging
the reaction kinetics. We present evidence for the: (1)
heterolytic activation of H2 by collaboration of AuNPs
and Al2O3; (2) homolytic activation of H2 occurring in
AuNPs surrounded by IL thin layers; (3) outerꢀsphere
mechanism (disproportionation of 1,3ꢀcyclohexadiene)
operating on AuNPs; and (4) better gold efficiency when
comparing its hydrogenation Ea (moderate to high temꢀ
peratures) to group 8ꢀ10 metals. These studies open
new opportunities for the industrial application of Au
catalysts prepared by sputtering, since technologies for
the production of related Auꢀmaterials by this technique
have been already applied at industrial scale, for inꢀ
stance, a continuous flow production of Auꢀrecovered
glasses.75
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
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53
54
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58
59
60
4. EXPERIMENTAL
4.1. General. All syntheses were performed using
standard Schlenk techniques under argon atmosphere.
Chemicals were purchased from SigmaꢀAldrich and
purified by standard procedures.76 H2 (>99.999%) and D2
(D> 99.8%) were purchased from WhiteꢀMartins.
Petrobras provided the γꢀAl2O3 used in this study. Synꢀ
thesis and characterization of the ILꢀhybrid γꢀAl2O3 supꢀ
ports (B1 and B2) were reported elsewhere.48,49 Fourier
transform infrared (FTꢀIR) spectra were obtained using a
ABB FTLA 2000 instrument with a resolution of 4 cm−1
with 128 cumulative scans. 13C and 29Si solidꢀstate
crossꢀpolarization magic angle spinning nuclear magnetꢀ
ic resonance (CPꢀMAS NMR) spectra were performed
using
a
Bruker 400 MHz spectrometer at the
CNANO/UFRGS. N2 isotherms of the catalysts, previꢀ
ously degassed at 100 °C under vacuum for 3 h, were
obtained using Tristar 3020 Micromeritics equipment.
Specific surface areas were determined by the Brunauꢀ
er–Emmett–Teller (BET) multipoint method and the avꢀ
erage pore size was obtained by the BarrettꢀJoynerꢀ
Halenda (BJH) method. Au content was determined by
Xꢀray fluorescence (XRF) carried out using a Shimadzu
XRFꢀ1800 sequential Xꢀray fluorescence spectrometer.
Samples were prepared in KBr and calibration was perꢀ
formed using bromine as an internal standard. Rutherꢀ
ford backscattering spectroscopy (RBS) measurements
were carried out in a 3 MV Tandetron accelerator using
a He+ ion beam of 1.5 MeV at IF/UFRGS. The Si surꢀ
faceꢀbarrier detector was positioned at a scattering angle
of 165°. Scanning electron microscopy with energyꢀ
dispersive Xꢀray spectroscopy (SEMꢀEDS) was perꢀ
formed with an EVO 50 from Zeiss, operated at 10 kV.
The images were obtained from the scratched area of
Figure 2. Arrhenius plot of the selective hydrogenation of
1,3ꢀcyclooctadiene catalyzed by Au/B2.
3. CONCLUSIONS
The sputtering deposition approach allowed the synꢀ
thesis of clean surface ~6.6 nm fcc Au(0)NPs recovered
by IL likeꢀmembranes (IL thin layers) for application as
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4.4. Hydrogenation reactions. As a general proceꢀ
the samples. Transmission electron microscopy (TEM)
1
2
3
4
5
6
7
8
was performed on a JEOL–JEM 1200ExII electron miꢀ
croscope operating at 200 kV. The samples were preꢀ
pared by the slow evaporation of a drop of each colloidal
solution deposited under an argon atmosphere onto a
holey carbonꢀcovered copper grid. Xꢀray diffraction
(XRD) analyses were carried out using a Philips X’Pert
MPD diffractometer with BraggꢀBrentano geometry using
a graphite curved crystal with Cu Kα Xꢀray radiation
(1.5406 Å).
dure for the hydrogenation reactions, the catalyst (0.5
ꢂmol Au), substrate (substrate/Au = 250) and solvent (5
mL) were placed in a 50 mL reactor (Parr Multiꢀreactor
4590). The reaction vessel was pressurised under 25
bar H2 and warmed at 100 °C. Aliquots were regularly
taken during the reaction. After the reaction time, the
reactor was cooled to room temperature and then deꢀ
pressurized. The conversion and selectivity were deterꢀ
mined by GCꢀanalysis of the reaction samples.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
4.2. X-ray photoelectron spectroscopy (XPS). XPS
measurements were performed using a Kratos AXIS
Ultra DLD instrument. Samples were mounted on the
standard Kratos sample bar using doubleꢀsided tape
(sellotape brand) and inserted into the airlock and
pumped down to ~3 × 10−7 Torr overnight before transfer
into the instrument analysis chamber. The analysis
chamber pressure during the measurements was better
than 5 × 10−9 Torr. Wide energy range survey scans
were collected at a pass energy of 80 eV in hybrid slot
lens mode and a step size of 0.5 eV. Highꢀresolution
data on the Au 4f, Au VB, Al 2p, Cl 2p, F 1s, O 1s, N 1s
and C 1s photoelectron peaks was collected at pass
energy 20 eV over energy ranges suitable for each peak,
and collection times of 5 min, step sizes of 0.1 eV. The
charge neutraliser filament was used to prevent the
sample charging over the irradiated area. The Xꢀray
source was a monochromated Al Kα emission, run at 10
mA and 12 kV (120 W). The data were captured using
Kratos VISIONII software and exported into vms format
for data processing with CASAXPS (Version 2.3.17).
The highꢀresolution spectra were analysed using a Loꢀ
rentzian asymmetric line shape convoluted with a
Gaussian function for each chemical component. The
high resolution data was charge corrected to the referꢀ
ence peak of Al 2p signal of Al2O3 at 74.5 eV.49 The
energy range for each ‘pass energy’ (resolution) was
calibrated using the Kratos Cu 2p3/2, Ag 3d5/2 and Au 4f7/2
threeꢀpoint calibration method. The transmission function
was calibrated using a clean gold sample method for all
lens modes and the Kratos transmission generator softꢀ
ware within Vision II.
4.5. Identification and quantification of the hydro-
genation reactions. GC analyses were run with an Agꢀ
ilent Technologies GC System 6820: injector and detecꢀ
tor (FID) temperature of 260 °C; N2 as the carrier (1 mL
min−1); column head pressure of 10 psi; temperature
program from 40 °C (10 min) to 250 °C at a heating rate
of 10 °C min−1; DBꢀ17 column (30 m × 0.25 mm × 0.25
ꢁm) for the hydrogenation of 1,3ꢀcyclohexadiene (1),
1,3ꢀcyclooctadiene (5), styrene (24), crotonaldehyde
(29), cinnamaldehyde (32), 2ꢀcyclohexenone (34) and 2ꢀ
methylꢀ2ꢀcyclohexenone (39); Alumina column (30 m ×
0.43 mm) for the hydrogenation of 1,3ꢀbutadiene (8), 2ꢀ
methylꢀ1,3ꢀbutadiene (13), 2,3ꢀdimethylꢀ1,3ꢀbutadiene
(17) and 2ꢀpentyne (26) and HPꢀ5 column (30 m × 0.20
mm × 0.30 ꢁm) for the hydrogenation of αꢀterpinene
(20). H and H NMR analyses of the samples obtained
by D2 reduction of 1,3ꢀcyclohexadiene (1) catalyzed by
AuNPs were performed using a Varian 400 MHz specꢀ
trometer at CNANO/UFRGS. The incorporation of D in
the reaction products was quantified by comparing the
1H NMR spectra with these obtained from a standard
sample (relaxation delay = 1–10 s) and by performing 2H
NMR experiments (relaxation delay = 1–10 s).45,46,48
1
2
SUPPORTING INFORMATION
Additional literature data (tables and comments), further
characterisation and kinetic data. The Supporting Inforꢀ
mation is available free of charge on the ACS Publications
website.
AUTHOR INFORMATION
Corresponding author
4.3. Preparation of AuNPs. As a general procedure
for the AuNP preparation by sputtering deposition, 1.0 g
of each support (γꢀAl2O3, B1 and B2) was placed into a
conical aluminium flask and inside a vacuum chamber
containing an electroꢀmagnetic oscillator with variable
controlled frequency, which allows for constant moveꢀ
ment of the conical flask. Then, the chamber was closed
and its pressure lowered to a base pressure of 4 ꢂbar,
with the supports being evacuated at this pressure for 4
h. Then, the vacuum chamber was placed under a sputꢀ
tering working pressure of 4 mbar by adding an argon
flow. The supports were continuously homogenized by
revolving the aluminium flask at a vibration frequency of
24 Hz. The Au was sputtered onto the revolving support
at 35 mA of discharge current during 4.5 min to give the
Au/Al2O3, Au/B1 and Au/B2 catalysts. After the deposiꢀ
tion, the chamber was vented with nitrogen and the red
powders were recovered and stored under argon atmosꢀ
phere for further characterization and application.
*GSK Carbon Neutral Laboratory for Sustainable Chemisꢀ
try, University of Nottingham, NG7 2TU, Nottingham (UK).
Eꢀmail: Jairton.Dupont@nottingham.ac.uk.
Author contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version
of the manuscript.
ACKNOWLEDGMENTS
We would like to thank CNPq, CAPES, INCTꢀCatal.,
Petrobras, EPSRC: LiPPS XPS system and EP/K005138/1
‘University of Nottingham Equipment Account’ for providing
financial support for this work.
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ACS Catalysis
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Graphical Abstract
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Figure 1. Characterization of the AuNPs model catalysts prepared by sputtering deposition: (a) BET N2-
physisorption isotherms; (b) BJH pore diameter distribution; (c) XRD patterns; (d) TEM images and size
histograms; (e) EDS-SEM images; (f) RBS depth-profile distribution of Au in the supports and (g) Au 4f XPS
spectra.
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ACS Catalysis
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Figure 2. Arrhenius plot of the selective hydrogenation of 1,3-cyclooctadiene catalyzed by Au/B2.
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Scheme 1. Possible hydrogenation mechanisms operating at the AuNP surfaces (M = Al, Ti, Si, Zr and Ce, B
= ligand or substrates with heteroatoms): (i) heterolytic H2 activation; (ii) homolytic H2 activation; (iii)
generation of sub-nanometric Au catalytic active species and (iv) outer-sphere.
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ACS Catalysis
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Scheme 2. AuNPs model catalysts prepared by sputtering deposition: Au/γꢀAl2O3, Au/B1 (X: Cl) and Au/B2
(X: NTf2).
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Scheme 3. Hydrogenation of 1,3ꢀcyclohexadiene catalyzed by Au/γꢀAl2O3, Au/B1 and Au/B2 under H2/D2.
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ACS Catalysis
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Scheme 4. Proposed mechanism for the selective hydro-genation of 1,3-cyclooctadiene catalyzed by Au/B2.
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ACS Catalysis
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