Influence of Synthesis Conditions on the Structure of Nickel Nanoparticles and their Reactivity in Selective Asymmetric Hydrogenation

Article abstract of DOI:10.1002/cctc.201901955

Unsupported and SiO₂-supported Ni nanoparticles (NPs) were synthesised via hot-injection colloidal route using oleylamine (OAm) and trioctylphosphine (TOP) as reducing and protective agents, respectively. By adopting a multi-length scale structural characterization, it was found that by changing equivalents of OAm and TOP not only the size of the nanoparticles is affected but also the Ni electronic structure. The synthetized NPs were modified with (R,R)-tartaric acid (TA) and investigated in the asymmetric hydrogenation of methyl acetoacetate to chiral methyl-3-hydroxy butyrate. The comparative analysis of structure and catalytic performance for the synthetized catalysts has enabled us to identify a Ni metallic active surface, whereby the activity increases with the size of the metallic domains. Conversely, at the high conversion obtained for the unsupported NPs there was no impact of particle size on the selectivity. (R)-selectivity was very high only on catalysts containing positively charged Ni species such as over the SiO₂-supported NiO NPs. This work shows that the chiral modification of metallic Ni NPs with TA is insufficient to maintain high selectivity towards the (R)-enantiomer at long reaction times and provides guidance for the engineering of long-term stable enantioselective catalysts.

Full text of DOI:10.1002/cctc.201901955

Accepted Article
Title: Influence of synthesis conditions on the structure of nickel
nanoparticles and their reactivity in selective asymmetric
hydrogenation
Authors: Rosa Arrigo, Simone Gallarati, Manfred E Schuster, Jake
Seymour, Diego Gianolio, Ivan da Silva, June Callison,
Haosheng Feng, John E Proctor, Pilar Ferrer, Federica
Venturini, dave grinter, and Georg Held
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ChemCatChem
10.1002/cctc.201901955
FULL PAPER
Influence of synthesis conditions on the structure of nickel
nanoparticles and their reactivity in selective asymmetric
hydrogenation
Rosa Arrigo,*[a,b] Simone Gallarati, Manfred E. Schuster,
[c]
[b,d]
Jake Seymour, Diego Gianolio, Ivan
[e]
[b]
[f]
[g,h]
[i]
[a]
[b]
[b]
da Silva, June Callison, Haosheng Feng, John E. Proctor, Pilar Ferrer, Federica Venturini,
[b]
[d,b]
David Grinter and Georg Held
Abstract: Unsupported and SiO
were synthesised via hot-injection colloidal route using oleylamine
OAm) and trioctylphosphine (TOP) as reducing and protective
2
-supported Ni nanoparticles (NPs),
catalysts,[2] e.g. methyl acetoacetate (MAA) to methyl-3-
hydroxybutyrate (MHB), shown in scheme 1. MHB is an
important intermediate in the synthesis of carbonic anhydrase
inhibitor MK-0507, used in the treatment of glaucoma;[
moreover, 3-hydroxyacids are useful building blocks in chiral
synthesis.[4]
(
3]
agents, respectively. By adopting a multi-length scale structural
characterization, it was found that by changing equivalents of OAM
and TOP not only the size of the nanoparticles is affected but also
the Ni electronic structure. The synthetized NPs were modified with
(R,R)-tartaric acid (TA) and investigated in the asymmetric
hydrogenation of methyl acetoacetate to chiral methyl-3-hydroxy
butyrate. The comparative analysis of structure and catalytic
performance for the synthetized catalysts has enabled us to identify
a Ni metallic active surface, whereby the activity increases with the
size of the metallic domains. Conversely, at the high conversion
obtained for the unsupported NPs no impact of particle size on the
selectivity was observed. (R)-selectivity was very high only on
catalysts containing positively charged Ni species such as over the
Scheme 1. Hydrogenation of pro-chiral β-ketoester methyl acetoacetate gives
two enantiomers, (R)- and (S)-methyl-3-hydroxybutyrate. When the surface of
the nickel catalyst is modified with (R,R)-tartaric acid (also in combination with
an inorganic salt such as NaBr), the (R)-enantiomer is preferably obtained.
This reaction can be carried out with high enantiomeric excess
SiO
2
-supported Ni oxide NPs. This work shows that the chiral
(ee) if the surface of the nickel catalyst, e.g. Raney Ni or
modification of metallic Ni NPs with TA is insufficient to maintain high
selectivity towards the (R)-enantiomer at long reaction time and
provide guidance for the engineering of long-term stable
enantioselective catalysts.
supported nanoparticles (NPs), is modified with a chiral
molecule, typically (R,R)-tartaric acid or an amino acid, or by
selective poisoning using inorganic salts, such as sodium
bromide.[5] A molecular level understanding of this reaction
would be beneficial to tackle the challenge of achieving total
selectivity.
Particularly,
the
mechanism
by
which
Introduction
enantioselectivity is realized at the Ni catalyst surface aided by
modifiers is still subject to much debate.[6-9] Investigations of
adsorbates structures of tartaric acid and MAA on model single
crystal Ni surfaces have highlighted two possible pathways
through which the chiral molecule of the modifier facilitates the
formation of the (R)-enantiomer: a) through a one-to-one
molecular interaction, the chiral tartrate molecules adsorbed on
the Ni surface controls the adsorption geometry of the pro-chiral
reagent, which is then reduced on the adjacent exposed metallic
Ni surface by chemisorbed H atoms; b) the chiral molecules
forming a supramolecular chiral arrangement on the Ni surface
desorb from the largely ordered Ni terraces leaving a chiral Ni
surface structure responsible for the specific chemisorption of
the reagent with the favorable geometry;[7] c) the interaction of
tartrate with oxidised Ni particles facilitates etching of Ni cations
from the surface leaving naturally chiral kinks sites on the Ni
The enantioselective asymmetric hydrogenation of unsaturated
molecules is broadly applied for the synthesis of
pharmaceuticals and fine chemicals, where the biggest
challenge is attaining total selectivity to one specific
enantiomer.[ One of the most studied examples of such
reactions is the hydrogenation of β-ketoesters over nickel-based
1]
[
[
a]
b]
Dr R. Arrigo, and Dr J. E. Proctor, School of Sciences, Engineering
and Environment, University of Salford, Manchester M5 4WT, UK
Correspondence to E-mail: r.arrigo@salford.ac.uk
Dr R. Arrigo, Dr M. E. Schuster, Dr D. Gianolio, Dr P. Ferrer
Escourieula, Dr F. Venturini, Dr D. Grinter, Prof. G. Held, Diamond
Light Source, Harwell Science and Innovation Campus, Didcot
OX11 0DE, UK
[
[
[
[
[
c]
d]
e]
f]
S. Gallarati, School of Chemistry, University of St Andrews, North
Haugh, St Andrews KY16 9ST, UK
Dr M. E. Schuster, Johnson Matthey Technology Centre, Reading
RG4 9NH, UK
J. Seymour, Department of Chemistry, University of Reading,
Reading RG6 6AD, UK
Dr I. da Silva, ISIS Facility, Rutherford Appleton Laboratory, Chilton,
Didcot OX11 0QX, UK
[
9]
surface responsible for enantioselectivity.
[
9]
Moreover, Keane et al.
found that molecular Ni species
leached out during modification manifest enantio-selective
activity. This suggests that cationic Ni species might also be
involved in the catalytic reaction.
Despite the large volume of literature available on the adsorption
and ordering of chiral molecules on single crystals a direct
structure-activity correlation remains undisclosed. On the one
hand, the complexity of a real catalytic system makes it difficult
to unveil the local arrangement of these on their surface. On the
other hand, catalytic performances are only available for real
catalysts such as supported Ni catalysts. [10] In contrast to single
crystals, structurally ill-defined nanoparticles (NPs) enable
measurable conversions but suffer from poor enantiomeric
g,h] Dr J. Callison, Department of Chemistry, University College London,
0 Gordon Street, London, WC1H 0AJ, UK, and UK Catalysis Hub,
2
Research Complex at Harwell (RCaH), Rutherford Appleton
Laboratory, Harwell Oxon, OX11 0FA, UK
[
i]
H. Feng, Department of Chemistry, University of Cambridge,
Cambridge, CB2 1EW, UK
Supporting information for this article is given via a link at the end of
the document.
1
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ChemCatChem
10.1002/cctc.201901955
FULL PAPER
selectivity if compared to Raney Ni.[11] The surface modification
proved also in the case of the NPs to influence selectivity and
conversion.[ Yet, it is unclear whether macroscopically achiral
symmetrically round-like NPs with high-index surface sites might
create more opportunities for the generation of enantioselective
surface ensembles upon chiral modification. Moreover, round-
like NPs of different sizes enable a different population of kinks
sites as well as a different size of the facets, both leading to a
different mechanism of action of the TA modifier. An
understanding of the impact of NPs size, electronic and
crystallographic structure and morphology on the catalytic
performance of chirally modified NPs is missing, whilst it can
provide important insights for the design of improved
enantioselective catalysts. Our aim is to contribute a systematic
study of NPs structure-activity correlations, which will enable
new mechanistic insights into the catalyst’s selective chiral
function and the structural transformations responsible for
performance degradation. In this work, we synthetize supported
and unsupported nickel nanoparticles of different size and
investigate their catalytic performance and physicochemical
properties, such as morphology, size and structure, using
synchrotron-based X-ray absorption near edge structure
been used to prepare nanoparticles to be modified with tartaric
acid and to catalyze the hydrogenation of methyl acetoacetate.
12]
Table 1. Synthetic conditions and resulting size of Ni
NPs.
OAm[b]
(x) [eq.] (y) [eq.]
TOP[c]
Samples
Size[d/e] [nm]
Ni_2.5x1.5y
Ni_5x1.5y
Ni_10x1.5y
2.5
5
1.5
1.5
1.5
1.5
1.3/8.0±1.4
4.0/7.0±1.3
5.6/10.6±1.9
4.7/9.9±1.8
10
5
Ni/SiO
Ni/SiO
Ni/SiO
2
2
2
_5x1.5y[a]
_5x0.75y[a]
_5x3y[a]
[
f]
7.3 ;
5
0.75
[
g]
2.5 /2.2±0.6
5
3
3.8/6.5±0.7
[a] 10 wt% loading on CAB-O-SIL 5M silica. [b] Total number
of equivalents of oleylamine (OAm) and 1-octadecene (21)
kept constant by varying amount of solvent. [c]
Trioctylphosphine. [d] Determined by XRD. [e] Determined by
HAADF-STEM. Average particle size of Ni metal [f] and Ni
oxide [g], respectively.
Traditionally, this synthetic method is performed on a small scale
(
XANES), extended X-ray absorption fine structure (EXAFS)
(
mmol of nickel precursors, mg of NPs produced) and the NPs
spectroscopies and X-ray photoelectron spectroscopy (XPS), in
combination with bright field scanning transmission electron
microscopy (BF-STEM), high angle annular dark field scanning
transmission electron microscopy (HAADF-STEM) and X-ray
diffraction (XRD).
are isolated unsupported via flocculation procedure, which
removes the high-boiling solvent and excess ligands.[16] We
employ the mid-to-high temperature thermal decomposition of
Ni(acac) with OAm-TOP as it has been reported to be one of
2
the best strategies for controlling the production of highly
For a precise control over materials properties at the nanometer
level (NPs with narrow size distribution, morphology and
composition), we employed the hot-injection colloidal
synthesis,[13] which is based on the reduction of metallorganic or
metal salt precursors in high-boiling nonpolar solvents in the
presence of surfactant agents.[14] Rapid injection results in a
supersaturated solution where the nucleation of the particles
occurs within a short time period. The surfactant molecules
capped around the nanocrystals prevent their agglomeration and
protect them from possible oxidation upon exposure to
atmospheric conditions. Moreover, the surfactant molecules
endow the NPs with good colloidal stability.[15]
monodisperse NPs with narrow size distribution, morphology
and composition.[
17]
However, as it is preferable to produce
larger amounts of catalyst for testing, multiple spectroscopic and
structural characterizations, we attempted to scale up this well-
known synthesis (gram scale) and also support the colloidal NPs
on silica for ease of handling.
Structural characterization of unsupported NPs
The low magnification (LM) scanning transmission electron
microscopy characterization of the unsupported Ni_5x1.5y
catalyst is reported in Figures S1a and b of the supporting
information. Accordingly, the NPs appear spherical in shape, or
slightly elongated, and have a core-shell structure, where the
core is composed of Ni encapsulated in a less than 1 nm thick
and rough shell of lower contrast, presumably originating from
the capping agents as indicated by XPS and NEXAFS analysis
at the C1s and O1s core levels (not shown).
We will show that by modifying the synthetic composition, it is
possible to tune the NPs’ size, as well as the Ni electronic
structure. These have significant implications on the resulting
reactivity in the investigated reaction.
Results and Discussion
A closer inspection using high resolution high angle annular dark
field scanning transmission electron microscopy (HAADF-STEM)
2
A series of unsupported and SiO -supported Ni NPs were
(
Figure 1a and 1b) enables us to identify detailed structural
synthesized using the hot-injection method to control the size of
the NPs by changing the ratio between the reducing (oleylamine,
OAm) and protecting agents (trioctylphosphine, TOP).
Table 1 lists the synthetic conditions and samples’ notation. The
detailed synthetic procedure is reported in the experimental
section. Firstly, we will discuss the impact of the composition of
the synthesis mixture on the structural features of the resulting
features for this catalyst. Particularly, Figures 1a shows that the
oval NPs is in fact a twinned cuboctahedron with several parallel
twin lamellae (as indicated by the arrow) accounting for the
anisotropic elongation of the NPs along the C2 axis of the twin.
Figures 1b shows in detail the shell structure, where Ni
nanocrystallites are embedded within the organic shell, giving it
the roughened aspect seen in the LM STEM images. The
Ni_10x1.5y sample shows the biggest NPs with more
abundantly elongated particles (LM HR STEM in Figures S1c
and S1d). The HR-HAADF STEM images in Figures 1c shows a
round NPs marked by the arrow imaged along common 5-fold
axis of a multiply twinned decahedron, composed of 5 face-
2
unsupported and SiO -supported Ni NPs. These will then be
correlated with the catalytic performance reported subsequently.
To the best of our knowledge, this is the first time that the hot-
injection oleylamine-trioctylphosphine Ni NPs synthesis has
2
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ChemCatChem
10.1002/cctc.201901955
FULL PAPER
sharing tetrahedrons. Figure 1d shows an example of twinned
XRD. The different trends in average particle size determined by
NPs composed of face-sharing tetrahedrons and cuboctahedron. STEM and XRD led us to conclude that significant fractions of
Note that in this case, the faces of the octahedron and the
tetrahedrons do not match perfectly at the grain boundaries, but
an interfacial layer of lower contrast is observed, which is
probably due to the residual capping agent (Figure 2f).
the particles are an amorphous phase and that this is the
predominant phase in the Ni_2.5x1.5y sample.
a
b
b
c
d
d
Figure 2. STEM characterization of Ni_2.5x1.5y. LM BF-STEM a) and
corresponding HAADF-STEM b); HR BF-STEM of
corresponding HAADF-STEM d).
a Ni NPs in c) and
Figure 1. a) and b): HR-HAADF-STEM characterization of Ni_5x1.5y. The
arrow indicates the twinned lamellae of the NPs in a) and a smaller Ni NPs
embedded in the organic shell in b; c) and d): HR-HAADF-STEM
characterization of Ni_10x1.5y. In a) the arrow indicates the 5-fold axis of a
multiply twinned decahedron. In b) the arrow indicates the Ni atoms of a
smaller NPs embedded in the organic shell.
The trend in particle size observed for Ni_5x1.5y and
Ni_10x1.5y is consistent with our expectation upon the
systematic variation of the x:y ratio (with x and y being the
number of equivalents of OAm and TOP, respectively), as
investigated in these experiments. In the colloidal synthesis of
nanoparticles, the growth mechanism is generally divided into
several stages, occurring at different time frames: nucleation,
aggregation growth, and possibly also Ostwald ripening
process.[19] While OAm is expected to control the nucleation rate
and growth, TOP acts as capping agent and provides tunable
surface stabilization through coordination with the Ni surface,
thus hindering the growth. A small OAm:TOP (x:y) ratio during
the synthesis leads to small NPs, as in the Ni_5x1.5y catalyst,
since the relatively more abundant capping agent TOP further
hinders aggregation and growth. Conversely, increasing the x:y
ratio leads to larger particles, as in Ni_10x1.5y, since the
amount of capping agent is relatively decreased, along with its
ability to kinetically control the growth step by steric hindrance.
Furthermore, by increasing the amount of OAm, the nucleation
rate is enhanced, thus larger quantities of nuclei are produced in
solution; such small nuclei are unstable due to the high surface
energy and intermolecular forces, leading to growth or rapid
aggregation of the nickel nanoparticles[20] and even formation of
twinned structures, as observed in this study. The amount of
reducing agent also influences the geometry of the primary
nucleus: whilst at a low concentration, the nuclei grow to form
small tetrahedrons, which aggregate forming multiply-twinned
NPs, with larger amounts of reducing agent the bigger and more
thermodynamically favored cub-octahedrons are formed as well
A roughened shell of abundant nano-clusters embedded in the
organic shell (Figures 1c and 1d) is also seen in this sample.
In comparison, the LM STEM images of the Ni_2.5x1.5y sample
(
Figure 2) show mainly round amorphous core-shell NPs where
the presence of additional nano-clusters embedded in the
organic shell is rarely encountered. Peculiarly in this sample,
some of the NPs show an inner region of lower contrast,
indicating the presence of voids (Figure 2a and b) within the Ni
NPs or an area of localized low-density elements such as
encapsulated organic matter.
XRD (Figure 3a) confirms the successful formation of metallic Ni
nanoparticles without the need of further reduction steps during
the synthesis. Particularly, in samples Ni_5x1.5y and Ni_10x1.5y,
the peaks at 44.4 and 51.7 degrees 2θ are the characteristic
(
111) and (200) reflections of metallic nickel, respectively.[18] In
Ni_2.5x1.5y, only the most intense (111) reflection is
distinguishable, due to peak broadening caused by the smaller
crystallite size. Furthermore, no significant contribution from
ordered Ni oxide was observed in the XRD patterns of the
unsupported NPs.
The average crystallite size determined by STEM for the three
samples follows the order of Ni_5x1.5y ≤ Ni_2.5x1.5y <
Ni_10x1.5y, as summarized in Table 1. This is in sharp contrast
with the size of the coherent crystalline domains determined by
3
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10.1002/cctc.201901955
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as largely elongated twinned particles (Figures S1c and S1d).
Interestingly, when the amount of reducing agent is as low as
2
2.5 equivalents with respect to the Ni(acac) precursor, as in
Ni_2.5x1.5y, the NPs’ formation mechanism is different from the
one described above. To understand the formation of
amorphous NPs in this condition, one should bear in mind that
TOP not only acts as a capping agent to prevent NPs’ growth,
but also as a source of phosphorous in the synthesis of metal
phosphides.[21] In the case of Ni phosphides, following the initial
formation of the Ni metallic core upon Ni precursor
decomposition, a subsequent phosphidation step occurs, during
which, the voids observed are formed through the Kirkendall
effect, where outward diffusion of metal atoms from the core is
faster than inward diffusion of reactive species.[22]
HAADF-STEM energy dispersive X-ray (EDX) mapping in Figure
3b confirms the colocation of P and Ni atoms, whereas little
signal from O or C is observed, confirming the presence of a
predominant Ni phosphide phase, which is amorphous as shown
in Figure 2c and 2d.
Figure 4. Fitted Ni 2p XP spectra (KE 570 eV) for Ni_10x1.5y (a); Ni_2.5x1.5y
0
(
(
b) and Ni_5x1.5y (c). Fitting: N1 (852.6 eV) is Ni ; N2 (852.9 eV) is Ni-P; N3
853.7 3V) is Ni2+ and Ni4 (856.1 eV) is Ni Ni in oxide and oxyhydroxide,
2+/
3+
respectively. Ni L edges NEXAFS spectra for all samples in (d).
The spin-orbit split Ni2p XPS and NEXAFS characterizations of
the as synthetized samples in Figure 4 confirm these findings,
whilst providing additional information about the nature of the Ni
phosphide phase. We first describe the surface sensitive Ni2p
XPS spectrum of the Ni_10x1.5y sample (Figure 4a). The main
metallic Ni 2p3/2 peak (Ni1) is found at the binding energy of
52.6 eV, consistently with the literature.[
24]
An additional
8
satellite peak attributed to 3d → 4s excitation[ was found at 5.6
eV above the main line in a reference Ni polycrystalline foil (not
shown) and included here in the fitting of the NPs spectra.
A relatively less abundant component Ni2 was found at 853.7
eV, whilst a more abundant component Ni3 was found at 856.1
24]
eV, which is accompanied by two intense satellite peaks at
24-25]
8
61.1 eV and 864 eV. Based on literature assignments,[
the
Ni2 component at 853.7 eV is due to Ni( in NiO, whereas the
Ni3 component at 856.1 eV is due to a mixed Ni( Ni oxy-
hydroxide phase. A closer inspection of the spectrum of the
Ni_2.5x1.5y sample, which is compared to that of Ni_5x1.5y in
Figure S3, reveals the presence of an additional Ni component
II)
II)/ (III)
Figure 3. a) XRD characterization; b) HAADF-STEM and EDX elemental
mapping for the sample Ni_2.5x1.5y.
+
(
Ni4) at 852.95 eV and corresponding low intensity satellite
component at 860.5 eV, previously attributed to Ni phosphides
[26].
Note that the same 4 component fitting was applied to all
Raman spectra of the Ni_2.5x1.5y sample (Figure S2) exhibited
a peak corresponding to the most intense mode in amorphous
three samples; while only a small contribution of the Ni4
component was needed to afford a good fit of the Ni_5x1.5y
spectrum (Figure 4c), the fit of the Ni_10x1.5y spectrum (Figure
-1 [23]
NiPS3 at ca. 600 cm , confirming the existence of Ni-P bonds
in this sample. We did not observe the weaker peak at ca. 380
-1
cm seen in amorphous NiPS3, suggesting that the Ni
phosphide in this sample is more amorphous in character than
that in the previous work.[21] We collected 5 spectra from different
locations of the sample and all spectra exhibited similar features.
Raman spectra of the other samples revealed no evidence for
the presence of a Ni phosphide phase.
4a) suggests that this component is apparently absent in this
case, within the volume probed by these XPS measurements.
Since the kinetic energy of the photoelectrons in the present
XPS analysis (h = 1420 eV, Ekin = 570 eV) corresponds to an
information depth of approximately 3 nm,[ only a fraction of the
NPs volume can be characterized. These results are however
consistent with the bulk characterization performed by Raman
spectroscopy, where only the Ni_2.5x1.5y sample (Figure S2)
27]
4
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ChemCatChem
10.1002/cctc.201901955
FULL PAPER
shows the vibration breathing mode typical of Ni-P bonds. The
analysis of the satellites provides additional information: for
Ni_5x1.5y, the satellite relative to the metallic component N1
was found with an area of 35% of the main peak, which is
consistent with results in the literature;[26a] moreover, the satellite
component relative to the Ni phosphide phase at 860.5 eV in
Ni_2.5x1.5y is indeed 15% of the main peak, as it was found for
amount of protective agent TOP (y) influences the nanostructure
of the Ni NPs. The same range of OAm: TOP ratios were
investigated as in the case of the unsupported NPs. In the
subsequent immobilization step, the metal loading was kept low
to facilitate the NPs’ dispersion on the support. The metal
loadings on these samples determined by EDX confirm the
quantitative immobilization of the preformed NPs onto the
support (table S1 of the supporting information). While for
a phosphorous enriched Ni
2
P phase.[26a]
Another difference amongst the samples is the ratio between the
components contributing to the core of the NPs (Ni1 metallic and
Ni4 Ni-P components) and the those relative to the oxy-
hydroxide layer at the interface with the organic shell (Ni2 and
Ni3). Particularly, the higher intensity of the components of the
Ni_2.5x1.5y core would be an indication of a thinner organic
shell, which enables to probe a larger volume of the metallic
core. Also, the smaller clusters embedded within the organic
shell of Ni_5x1.5y and of Ni_10x1.5y, which are apparently
absent in Ni_2.5x1.5y, might be fully detected within the probing
depth of these measurements, as opposed to the main NPs’
core, which is only partially characterized. This explain the
higher abundance of the surface components in Ni_5x1.5y and
in Ni_10x1.5y. [28] This reasoning is also confirmed by the Ni/C
ratio as determined by XPS quantification which was found to be
Ni/SiO
Figure S4 show the expected nanoparticulate morphology, the
Ni/SiO _5x0.75y sample shows a cauliflower-like morphology as
a result of the low amount of capping agent used, whose role is
also to facilitate nuclei agglomeration and sintering.
2 2
_5x1.5y and Ni/SiO _5x3y the HAADF-STEM images in
2
2.2, 1.4 and 0.4 for the Ni_2.5x1.5y, Ni_5x1.5y and Ni_10x1.5y.
respectively.
The spin-orbit split Ni L-edge absorption spectra of these
samples in Figure 4d present the L
3
and L
2
resonances maxima
of Ni(0) at 852.6 eV and 870.2 eV, respectively,
[29]
whereas in
9
nickel oxide they are reported to be at 853.2 eV (2p3/23d ) and
9
[8b,30]
8
70.3 eV (2p1/23d ).
reported in literature for Ni metal, the NPs characterized in this
study show broadening of the L and L resonances towards
higher photon energies. According to Watson and co-workers,
However, compared to the spectra
3
2
[8b]
this broadening is a result of the greater contribution of a surface
oxide layer formed under ambient conditions. The most striking
difference amongst the spectra reported here is the shift of the
main resonance to higher photon energy, by about 0.5 eV, and
the significant broadening to the high energy side of the
Figure 5. Ni K edge XANES (a) and EXAFS (b) in transmission mode; XRD
characterization of the samples(c). Colour code: Ni/SiO
Ni/SiO _5x3y (blue line); Ni/SiO _5x0.75y (red line).
2
_5x1.5y (black line);
Ni_2.5x1.5y sample.
Similar results were found for Ni
2
2
phosphides,[26a] which is indicative of the partially cationic
character of Ni in these compounds, where the higher the P
content, the higher the energy shift.
Due to the SiO
2
component of these materials making the
We thus conclude that the reduction of the Ni2+ precursor to Ni0
by the action of OAm and formation of crystalline Ni NPs occurs
below the temperature of the TOP decomposition. Similar
conclusions have been reported elsewhere.[31] However, if the
amount of OAm is lower than the quantity needed to reduce
quantitatively the Ni2+ precursor, as in the case of Ni_2.5x1.5y,
the molecules of the TOP capping agent will coordinate to the Ni
complex in excess in solution. Only when the TOP decomposes
at the maximum temperature reached in these experiments
spectroscopic structure analysis by means of electron detection
challenging, the electronic structure of the supported samples
was investigated by bulk-sensitive XANES and EXAFS at the Ni
K-edge in X-ray transmission mode. The samples are compared
with a nickel standard (Ni foil) in Figures 5a and 5b, respectively.
The XANES spectra exhibit two relevant features: the peak P1
at ca. 8336 eV corresponding to either the quadrupole
transitions of 1s photoelectrons to Ni3d–O2p hybridized orbitals
for the oxide phase or the edge for the metallic phase or a
(
220 °C), the amorphous Ni phosphide particles are formed. The
combination of both; the peak P2 at 8352 eV corresponding to
transitions of 1s to Ni4p–O2p orbitals[32] for the oxide phase.
presence of hollow NPs in Ni_2.5x1.5y suggests that the Ni
phosphide phase can be formed via phosphidation of the initially
formed Ni nuclei.
2 2
With respect to the Ni foil, Ni/SiO _5x1.5y and Ni/SiO _5x3y
0
[31]
clearly show a less intense pre-edge region P1 and a more
intense P2 peak. The position of the K-edge (defined as the
position of maximum first derivative) was previously found for Ni
phosphides between NiO and Ni standards, indicating that the
formal oxidation state for phosphides ranges between 0 and 2,
where the higher the amount of P, the higher the energy position
of the edge.[33] However, the spectra of these samples (Figure
Structural characterisation of SiO
2
-supported Ni NPs
In the case of supported NPs, the NPs were first synthetized and
subsequently immobilized onto the silica support. We used 5
equivalents (x) of OAm and investigated how changing the
5
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ChemCatChem
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5
a) do not show significant edge-shifts but rather a systematic
Furthermore, we note that previous literature on Ni phosphides
has indicated that the radial distribution function presents
pronounced changes with the content of P and that the FT
increase of the peak P2 intensity and correspondingly a
decrease of the P1 resonance. P2 is significantly pronounced in
the Ni/SiO
2
_5x0.75y spectrum, resembling XANES spectra of
EXAFS of the Ni/SiO
reported for a Ni12 phase with a calculated oxidation state of
0.43.[33-34] Therefore, it could also be possible that the
2
_5x3y sample is similar to the spectrum
NiO reported in the literature.[26,33] Based on this, we propose
that the samples are composed of a mixed Ni oxide and metallic
phases, whereby spectral variations amongst the samples
account for changes in composition of the two phases. Although
a Ni phosphide phase cannot be completely excluded, its
contribution should be negligible. The absence of the Ni-P
signature in the Raman spectra of the samples (not shown)
corroborates this assumption. If we assume that the oxide
component is a surface overlayer of the metallic core, the
measured increase in the P2 feature corresponds to a higher
fraction of NiO and therefore to smaller particles size. [34]
P
5
Ni/SiO
2
_5x3y sample might contain a small amount of P as an
amorphous phase.[ This could be expected considering that
the OAm:TOP ratio used in its synthesis was similar to the one
used in the preparation of the unsupported Ni _2.5x1.5y sample.
However as discussed above, the Ni phosphide phase can be
safely neglected in this data treatment.
35]
X-Ray diffraction was used to investigate the structure and
phase composition of the samples and to calculate the mean
NPs size. Figure 5c compares the XRD patterns of the three
supported samples between 35 and 65 degrees 2θ. Similarly to
2 2
the unsupported NPs, the Ni/SiO _5x1.5y and Ni/SiO _5x3y
Table 2. XANES Linear Combination Fit and EXAFS fit
samples show the characteristic peaks at 44.4 and 51.7 degrees
2
θ, which correspond to the nickel (111) and (200) reflections,
XANES[a]
Ni(0) NiO
R-factor
Red. 2
[18]
respectively.
2
Only the Ni/SiO _5x0.75y sample shows a
mixture of Ni( and fcc NiO phases (NiO (111), (200) and (220)
reflections at 37.0, 43.2 and 62.5 degrees 2θ, respectively).[36]
Moreover, the diffractograms do not show reflections reported
for crystalline Ni phosphides. Instead, broadening of the main
peak indicates a smaller crystallite size.
0)
0
0
0
.00063
NiSiO
NiSiO
NiSiO
2
2
2
_5x1.5y
_5x0.75y
_5x3y
0.61
0
0.39
1.
0.005202
0.007883
0.006709
R-factor
.001398
.000847
0.48
0.52
EXAFS[b]
Ni(0) NiO
Red. 2
While trends in the crystallite size calculated from the XRD data
for the 5x1.5y (4.7 nm) and 5x3y (3.8 nm) samples are in
agreement with what can be imaged by STEM (5x1.5y particles
are larger than 5x3y) as summarized in Table 1, the absolute
value is very different. Moreover, there is an apparent
3
6
1
614.764
511.449
175.56
Ni/SiO
Ni/SiO
Ni/SiO
2
2
2
_5x1.5y
_5x0.75y 0.2
_5x3y 0.50
0.53
0.47
0.8
0.032123
0.05524
0.5
0.030047
[
a] Linear Combination Fit using Ni metal and NiO as
references. [b] EXAFS fit using combination of two models: Ni-
Ni scattering paths for metal and Ni-O scattering paths for
oxide and a variable ratio between the two.
discrepancy
Ni/SiO _5x0.75y sample shows sharp Ni reflections whereby the
mean crystallite size was calculated to be 7.3 nm. Whilst these
peaks are sharper in the Ni/SiO _5x0.75y sample than in the
for
the
Ni/SiO
2
_5x0.75y
sample.
The
2
2
other samples, its main phase is likely to be NiO. The estimated
crystallite size for the NiO phase from XRD is around 2.5 nm.
However, STEM images clearly show that the catalyst consisted
of particles that are much smaller than the other two samples,
with a particles size consistent with the size of NiO phase
determined by XRD. We therefore propose that for the
To prove our assumption, we perform a linear combination fit of
the XANES spectra using Ni metal and NiO as standards, which
also enabled a quantitative analysis of the two phases, as
summarized in Table 2.
The Fourier transforms of the Ni K-edge EXAFS spectra are
presented in Figure 5b. The standard nickel foil is also included
for comparison. The first peak in face-centered cubic (fcc) Ni is
2 2
Ni/SiO _5x1.5y and Ni/SiO _5x3y a large fraction of the NPs is
in XRD-amorphous state or twinned NPs with coherent size of
the twins as determined by XRD, whereas for the
Ni/SiO _5x0.75y, the large Ni crystallites are covered by smaller
2
oxide NPs forming the cauliflower-like morphology seen in the
HAADF-STEM image (Figure S4).
found at 2.48 Å, which is also found in the Ni/SiO
2
_5x1.5y and
5
x3y samples, although with attenuated intensity, while it is
barely visible in 5x0.75y. The peaks at higher distances
correspond to main single scattering paths of metallic Ni-Ni
higher coordination shells (at 3.50, 4.29, 4.96 Å respectively). In
Similarly to the unsupported NPs, the size and the structural
characteristic of the supported nanoparticles can be rationalized
taking into account the synthetic conditions and the systematic
2
the EXAFS spectra of Ni/SiO _5x0.75y, other peaks assigned to
single scattering of Ni-O (2.09 Å) and Ni-Ni (2.96 Å) in nickel
oxide are clearly visible, indicating that this is the predominant
phase in this sample. The EXAFS data are fitted for a
quantitative interpretation and reported in Table 2. The results of
the fit confirm the trend observed for the XANES linear
combination fits. Moreover, in the radial distribution function for
variation of the x:y ratio. When comparing the Ni/SiO
2
_5x3y and
Ni/SiO _5x1.5y samples, as the number of TOP equivalents (y)
2
is increased, the size of the nanoparticles is reduced, since the
growth step is further hindered by relatively more abundant
capping agent.[19,37] We explain the peculiar morphological and
structural characteristics of the Ni/SiO _5x0.75y sample by
2
taking into account the current opinion in the synthesis of
NPs.[38] Accordingly, in the relative excess of the reducing agent
the Ni/SiO
while other samples also show the Ni-Ni higher coordination
shell. This means that the Ni/SiO _5x3y sample is only short-
2
_5x3y sample, only the first shell peak is observed,
2
range ordered. This is also confirmed by the fact that the
obtained Debye-Waller factors from the fit is much higher for all
shells (σ21st shell = 0.011 vs. 0.008, σ2higher shells = 0.0243 vs.
(
oleylamine), a large number of metal nuclei are formed; these,
assisted by TOP, are prone to growth or agglomeration during
the high temperature reaction, forming a small amount of
0.0140).
0
relatively bigger Ni NPs. However, it is reasonable to assume
6
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that with low concentration of TOP (y<1), the nanoparticles
being formed in the high temperature reduction of Ni(acac) with
2
samples at 22h of reaction did not vary significantly, indicating
that there is no size-effect on the activity at this reaction time
and at such variations of the crystalline domains (4.0 nm vs. 5.6
nm from XRD); in contrast, the activity of the Ni_2.5x1.5y was
significantly inferior (36% tot. conv.). Note that the as prepared
sample is composed of large particles of average diameter of 8
nm composed of a predominantly amorphous Ni phosphide
phase whereas the size of metallic crystallites is much smaller
(1.3 nm circa in table 1).
OAm are not all being encapsulated by TOP thus remaining not
sufficiently protected from oxidation upon exposure to the
environment. The peculiar cauliflower-like morphology of the
main oxidic crystallites in the sample also suggests that TOP
favors the packing of the small NPs to form bigger twinned NPs.
Catalytic performances of the synthetized unsupported and
2
SiO -supported NPs.
Table 3. Catalytic performance of TA modified Ni NPs in
[
a]
hydrogenation of methyl acetoacetate.
Tot. MAA Inst. (R)-
Final ee
(
Samples
conv.
MHB selec.
%)[
d]
(
%)[
b]
(%)
[c]
Ni_5x1.5y
Ni_2.5x1.5y
Ni_10x1.5y
69
36
74
7
48
48[e]
46
62[f]
48[g]
–3
–3
–4
24
7
Ni/SiO
Ni/SiO
2
2
_5x0.75y
_5x3y
3
[
a] Reaction conditions: 0.5 g of unsupported NPs or 1 g of
Ni/SiO . MAA (0.15 mol), MeOH solvent (15 mL), H 20 bar,
2
2
T 60 °C. [b] Conversion after 22 h of reaction. [c] Unless
otherwise stated, selectivity at the same value (50%) of total
MAA conversion. [d] Positive values indicate (R)-selectivity,
negative (S)-selectivity. [e] Value at 36% conversion. [f]
Value at 7% conversion. [g] Value at 3% conversion.
The synthetized NPs were tested in the enantioselective
hydrogenation of MAA. Before the catalytic tests, the NPs
catalysts were pretreated with (R,R)-TA and NaBr following a
modification procedure which proved to be effective in
chemisorbing TA on the surface of polycrystalline Ni. The
analysis of the surface chemical compositions upon various
modification procedures was afforded by XPS characterization
and revealed a very complex chemical speciation which was
significantly affected by the experimental conditions. It is thus
expected that the surface chemical composition of the reactive
interface of the catalyst changes dynamically under
hydrogenation conditions. A more detailed discussion is reported
in the supporting information (Figure S5). The combined NaBr
and TA modification was also confirmed to enhance the
enantiomeric selectivity towards the (R)-MHB as well as the
activity in the case of Raney Nickel (Table S2), consistently with
the literature. [1, 13]
Figure 6. Instantaneous conversion of MAA vs. time of reaction (a) and
corresponding selectivity to (R)-MHB (b). Total conversion of MAA (c).
Figures 6a and 6b report the instantaneous conversion of MAA
and selectivity as a function of the reaction time for the
unsupported and SiO -supported NPs, respectively. The total
2
conversion of MAA as a function of reaction time is reported in
Figure 6c. With respect to the total conversion and selectivity,
the instantaneous conversion and selectivity provide a direct
evaluation of changes in the catalytic performances as well as
deactivation. Indeed, we can observe that the activities of the
catalysts are steadily increasing for the unsupported Ni NPs.
This is an indication that the chemisorbed species composing
the surface overlayer detach from the Ni surface with time and
as a consequence new surface is exposed during the reaction,
which is now available for catalysis.
The chirally modified NPs produce both (R)-MHB and (S)-RHB
(
Figure 6). No other product was observed under the
However, the increase of the activity is not reflected in an
increase of the selectivity towards the (R)-MHB. Particularly, for
the unsupported Ni_5x1.5y and Ni_10x1.5y the selectivity
towards the (R)-MHB is maintained constant within the time
frame investigated regardless of the MAA conversion achieved.
Interestingly, we observe for the unsupported Ni_2.5x1.5y that at
the beginning of the reaction the selectivity towards the (R)-MHB
is higher but decreases very rapidly to reach the value of circa -
experimental conditions used.
Table 3 summarizes the catalytic performance of the supported
and unsupported nanoparticles in the hydrogenation of MAA.
The final enantiomeric excess (ee) is calculated according to
equation (1), given the concentration of (R)-MHB and (S)-MHB
at the end of the reaction; where possible, the instantaneous
selectivity towards (R)-MHB is compared at the same value
(
50%) of total MAA conversion, thus the values reported here
3% ee. This value seems to be characteristic for these
are extrapolations from the fits presented in Figure 6.
As it can be seen from table 3, the activity (total MAA
conversion) of the unsupported Ni_5x1.5y and Ni_10x1.5y
unsupported NPs systems, in which we postulate that the chiral
modification is no longer presents.
7
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In general, the measured activities for the SiO
NPs samples were very poor with a very low conversion, in part
due to the low loading of Ni (10% in weight) in these catalysts.
2
-supported Ni
conversion of 7% after 22h of reaction, probably due to the slow
reduction to form the active hydrogenation Ni0 phase.
Interestingly, in this condition, the catalyst shows the highest ee
towards (R)-MHB observed in this study (23%).
2
However, at such a low conversion, the Ni/SiO _5x0.75y sample
shows an ee of 24%, which is the highest ee towards (R)-MHB
observed in this study and similar to the one obtained for Raney
Ni modified using the same procedure (Table S2). Similarly, the
Indeed, in this study we could identify that high selectivity
towards (R)-MHB is obtained when a cationic Ni phase is
present (NiO for the supported or NiP for the unsupported).
It is generally observed that (R)-MHB is favored when (R,R)-
tartaric acid is used as modifier together with NaBr and was
confirmed in this work also for Raney Nickel investigated under
the same reaction conditions (Table S2).
The observation of enantioselective differentiation for some of
the catalysts at the beginning of the reaction further confirms
that during the ex situ modification step with (R,R)-tartaric acid
and NaBr, some level of exchange of the capping agents with
the chiral modifier takes place as also proved by the XPS in
Figure S5 for polycrystalline Ni. However, the fact that the
instantaneous conversion increases steadily with time whereas
the selectivity at higher conversion is maintained at the value of
the racemic composition indicates that the capping agent was
not quantitatively substituted by TA and NaBr.
2
selectivity of Ni/SiO _5x3y towards (R)-MHB is initially high but
decreases with time.
Figure 7 shows BF-STEM and HAADF-STEM images of the
Ni_2.5x1.5y after the reaction. LM images of the same sample
are reported in Figure S5. It is worth noting that, despite the
core-shell structure still being present, the core of the NPs is
now clearly crystalline whereas the shell is much thicker.
Interestingly, the shell is now composed of more abundant
crystalline nano-clusters covering the overall surface and,
imparting a much rougher aspect to the surface shell structure
(
Figure 7). This clearly indicates that Ni NPs, which are initially
in the Ni-P amorphous phase, undergo structural changes
towards the formation of a more crystalline Ni over-layer during
the hydrogenation reaction.
Raman spectra of the Ni_2.5x1.5y after 22 h of reaction suggest
that the sample is highly inhomogeneous in character (Figure
S6). The five spectra taken at different spots were quite
different, with two exhibiting a weak peak at ca. 600 cm ,
suggesting that a small amount of Ni phosphide phase may now
only be present in certain regions of the sample. However, these
structural characteristics seen in Figure 7 are not enabling
enantiomeric differentiation.
In fact, as the achiral capping agent detaches from the round
shaped NPs, the amount of left and right kinks sites or left and
right planes is equal and thus a macroscopically achiral metal
surface is freed. [1] At this point, the selectivity reaches the value
-1
0
which is characteristic of an unmodified Ni surface.
Moreover, we suggest that the lack of enantiomeric
differentiation of the Ni_5x1.5y and Ni_10x1.5y is the result of
the weak chemisorption of the chiral molecule on the Ni metallic
surface which desorb easily from the Ni surface upon
hydrogenation conditions, where the dynamic surface diffusion
of metal atoms and restructuring typical for NPs [28] results in an
inefficient chiral modification. The weak chemisorption of TA on
metallic Ni was also proved for the case of polycrystalline Ni
reported in the supporting information (Figure S5).
a
b
Thus, from the catalytic data presented here, it may be assumed
that the mechanism that involves the formation of a chiral
arrangement of TA molecules on flat Ni surfaces, leaving a chiral
imprint on it upon TA desorption[
39]
is the least probable
mechanism to explain the chiral action of TA and NaBr in these
nanoparticulate systems. The higher enantiomeric selectivity at
low reaction times on oxidized samples can be correlated to the
higher chelating affinity of the carboxylate species of the TA-Na
towards cationic Ni. As discussed in previous literature, [9-10] TA
is highly corrosive and Ni2+ species coordinated to TA can leach
out leaving (R)-kinks behind. However, whilst we cannot role this
mechanism out, we would have expected that the rapid dynamic
restructuring of these (R)-kinks sites would not allow us to be
able to measure such a transient and short-lived enantiomeric
differentiation similarly to the case of the metallic NPs Ni_5x1.5y
and Ni_10x1.5y. We therefore suggest that the most probable
mechanism that describes our results involves strongly
chemisorbed TA on the surface of oxidized Ni. Indeed, TA binds
more strongly on NiO than on Ni metal, and under
hydrogenation conditions prevents its reduction. However, on
Figure 7. BF-STEM images (a) and corresponding HAADF-STEM (b) of
Ni_2.5x1.5y after 22 h of hydrogenation reaction.
Correlation between catalytic performances and structural
characterization
The comparative analysis of the reactivity data of the
unsupported samples shows that large metal crystallites lead to
higher conversion. In contrast, the enantio-selectivity is mainly
unaffected by the size of the NPs. Higher initial value of
selectivity towards the (R)-enantiomer was obtained on the Ni
2.5x1. 5y catalyst, which contains cationic Ni species in the form
of a phosphide. However, the selectivity for (R)-MHB reached a
similar value for all unsupported catalysts after 22h of reaction.
The measured activities for the supported samples were
generally very poor (Figure 7b). This is only in minimal part due
to the low loading of Ni (10% in weight) in these catalysts.
0
adjacent unprotected reduced Ni , which is the active phase for
the hydrogenation, TA can assist the favorable chemisorption of
MAA to form (R)-MHB. This is consistent with previous work on
the adsorption of tartaric acid and MAA on Ni surfaces.[
7]
Particularly,
a hydrogen bonding interaction between the
Indeed, the Ni/SiO
2
_5x0.75y sample, which was initially present
hydroxyl of a TA molecule chemisorbed via the two carboxylates
predominantly as a NiO phase, shows a particularly low
8
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ChemCatChem
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FULL PAPER
on Ni[7] and the ester-oxygen of a MAA molecule chemisorbed
via the ester-carbonyl might occur. This intermolecular
interaction of the chemisorbed species constrains the interaction
of the MAA through the ketone carbonyl with the chemisorbed H
atoms on the Ni from the side that leads to the (R)-enantiomer
formation. However, with time, the NPs is reduced, under
hydrogenation and simultaneously TA will detach more easily
from the surfaces of the NPs leaving achiral NPs.
NPs with respect to number of moles of Ni precursor) were
typically between 70% and 80%.
List of reagents Technical grade nickel(II) acetylacetonate (95%),
oleylamine (70%), trioctylphosphine (90%) and 1-octadecene (90%)
reagents were purchased from Sigma-Aldrich and used as received
without further purification. Oleic acid (≥ 99%, GC grade), L-(+)-tartaric
acid (ACS reagent, ≥ 99.5%), anhydrous sodium bromide (ACS reagent,
≥
99%) and methyl acetoacetate (ReagentPlus®, 99%) were also
purchased from Sigma-Aldrich and used as received. CAB-O-SIL® (M-5
scintillation grade, 200 m /g) was purchased from Acros Organics (Fisher
Conclusions
2
Scientific) and used as received. Polycrystalline nickel discs (10 × 1.5
mm) were sourced from Goodfellow Cambridge. Raney Ni was
purchased from Sigma Aldrich.
2
In this work, we have synthetized Ni NPs and SiO -supported Ni
NPs obtained via hot-injection using different ratios of reducing
and capping agents and characterized the morphology,
nanostructure and chemical composition. Size and phase control
during synthesis was revealed together with the impact of these
characteristics on the resulting catalytic performance in the
selective enantiomeric hydrogenation of MAA. Our findings
imply that the more likely role played by the modifier under the
conditions investigated is not to generate the chiral Ni surface,
but rather to be involved in a chemical interaction with the pro-
chiral substrate, thus assisting a specific configuration of
chemisorption of the latter on the metallic Ni active surface that
leads to the selective (R)-enantiomer formation.
It is therefore evident that to design highly active and (R)-
selective hydrogenation catalysts, a combination of large Ni
metallic surface domains is needed, together with the existence
of Ni cationic islands where the chiral modifier can strongly
chemisorb to assist the side-by-side selective hydrogenation of
MAA. We speculate that, at an optimal concentration, sodium
bromide plays a similar role, whereas an excessive amount
could act as irreversible poison of the catalyst surface.
Unsupported NPs – variation of x (OAm) number of equivalents
Reactions were carried out under an atmosphere of high-purity argon to
protect nickel and trioctylphosphine from oxidation; the temperature was
controlled with a thermocouple placed directly inside the reaction vessel.
In a typical synthesis, Ni(acac)
2
(3.46 g, 13.45 mmol) and oleylamine
(22.13 mL, 67.27 mmol) were mixed together in 68.87 mL of 1-
octadecene and stirred magnetically at 65-70 °C in a flow of Ar under
reflux conditions. Once the mixture was completely dissolved,
trioctylphosphine (9 mL, 20.18 mmol), after having been degassed with
nitrogen for 10 minutes, was rapidly injected into the flask, followed by re-
setting the temperature to 120 °C and holding it for 10 minutes. The
mixture was then rapidly heated to 220 °C over the course of 15 minutes
for nucleation and ripening of the nanoparticles. After 2 hours of reflux,
the synthesis was halted and the flask cooled to room temperature. Oleic
acid (1.35 mL, 4.25 mmol) was then added to the mixture, followed by
ultra-sonication for 10 minutes. The nickel nanoparticles were then
precipitated by adding anhydrous ethanol (150 mL) and centrifuging the
dispersion at 8500 rpm for 30 min. The wet catalyst was then suspended
in a mixture of hexane (10 mL) and anhydrous ethanol (30 mL) and
centrifuged for a further 30 min at 8500 rpm. This process was repeated
twice.
Experimental Section
Materials and Methods
Supported nanoparticles
equivalents
– variation of y (TOP) number of
The synthesis of both unsupported and silica-supported nickel
nanoparticles via thermal decomposition reduction method was
adapted from the literature.[20b] Oleylamine (OAm, x) was used
In the case of supported catalysts, the protocol above was followed but,
instead of precipitating the NPs via treatment with a poor/good solvent
mixture (ethanol/hexane) and centrifugation (flocculation method), they
were supported on silica (CAB-O-SIL 5M). Amounts of Ni(acac)
precursor, OAm, TOP, 1-octadecene and SiO support were adjusted to
obtain ca. 4 g of sample with a theoretical NPs loading of 10 wt%
assuming 100% reduction yield and 100% incorporation of nanoparticles
2
2
(
as reducing agent whereas trioctylphosphine (TOP, y) and oleic
acid were used as capping agents.
on the support). To achieve the impregnation of the nanoparticles on
silica, CAB-OSIL 5 M was first dispersed in absolute ethanol (150 mL);
under vigorous stirring, the reaction mixture was added to the support
dispersion, which was then filtered under suction and washed multiple
times with copious EtOH until the runnings were essentially colourless,
indicating removal of excess surfactants.
Scheme 2. Hot injection synthesis of monodisperse NPs. Adapted from
[
15b]
ref.
The amount of oleylamine or TOP was varied as a strategy to
tailor the particle size, as described in Table 1. The total number
of equivalents of OAm and solvent (1-octadecene) was kept
constant (21 molar equivalents), therefore the volume of solvent
was also systematically varied. All other parameters
Ex situ modification with tartaric acid
All nanoparticulate systems and the Raney Nickel catalysts were
subjected to ex situ modification with (R,R)-tartaric acid (TA). The
2
catalyst (0.5 g of unsupported NPs, 1 g of Ni/SiO sample) was refluxed
at 100 °C for 1 h in 100 mL of the modification solution (1 wt% TA, 20
wt% NaBr co-modifier, pH 5.1 adjusted with 1 M NaOH), followed by
washing with one portion of water (50 mL) and 2 × 25 mL portions of
(
concentration of nickel precursor, temperature, and oleic acid,
reaction time) were not varied. Reaction yields (mass of isolated
9
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methanol. Prior to modification, the supported nanoparticles were treated
under a stream of 10% hydrogen gas in He gas at 300 °C for 1 h. The
structures, Ni0 and NiO, for which the most relevant single and multiple
scattering paths were calculated. The amplitude of each signal was then
described as a function of the ratio of the phases generating the
scattering path and the ratios optimized during the fit of the experimental
data.
2
samples were then cooled to r.t. under N before the modification solution
was injected into the vessel, thus avoiding exposure to air.
Other samples were obtained starting from Raney Ni, which was
modified in an aqueous solution containing either only 20 wt% NaBr or 1
wt% TA, following the same procedure as indicated above.
X-ray photoelectron spectroscopy (XPS) and near edge X-ray absorption
fine structure (NEXAFS) measurements in the soft X-ray regime were
carried out at the VERSOX end station and beamline at the UK
synchrotron facility Diamond Light Source. The freshly prepared samples
from atmospheric environment were deposited on a Si wafer and directly
exposed to vacuum (10-8 mbar) in the XPS chamber. XPS measurements
were performed by applying a suitable excitation energy corresponding to
a kinetic energy (KE) of the photo-emitted electrons of 450 eV for the
core levels Ni2p, C1s and O1s. The energy pass Ep was normally set to
20 eV. The core levels envelopes were fitted using Casa XPS software
after subtraction of a U2 Tougaard background.
Ex situ modifications of polycrystalline Ni for XPS analysis
The Polycrystalline Ni discs were first reduced under a constant steam of
5
% H
under Millipore water for transport to minimise contact with atmospheric
. Following, the discs were subjected to chemical modifications using a
Schlenk line. 2 discs were transferred into each modifying solution; (1)
50 mL of Millipore H O for the unmodified sample; (2) 1.5 g of TA in 150
mL of Millipore H O; (3) 30 g of NaBr in 150 mL of Millipore H O; (4) 1.5
g of TA and 30 g of NaBr in 150 mL of Millipore H O. Millipore water
used in these experiments was either degassed under vacuum prior to
being used (1 and 2) or boiled under N for 30 min before being used (3
and 4). Solution 1 containing the discs was not refluxed, while solutions 2
4 were refluxed under N for 1 h.
After refluxing, a disc was removed from solutions 2 - 4 and rinsed with
Millipore water and stored under N degassed water for at least 60 h.
2
in He in a tube furnace at 450 oC for 16 h before being stored
O
2
1
2
2
2
2
The fittings of the Ni2p spectra were performed by considering four core-
level components. A DS function line-shape was used for both the
metallic Ni1 and phosphide Ni4 components whereas two components
with a Gaussian-Lorentzian (GL) line-shape (Ni2 and Ni3) were included
accounting for Ni2+ and Ni2+/Ni3+ oxyhydroxide components, respectively.
The fitting of the spectra was done by constraining the peak position by ±
2
–
2
2
The samples were then mounted on sample holders under water before
being placed in the load lock to pump away the water. X-ray
photoelectron spectroscopy (XPS) spectra were then taken of the Ni 2p,
O 1s, C 1s and Valance Band regions.
0
.05eV. The peak area ratio between the Ni2p3/2 and Ni2p1/2 spin orbit
split transitions was constrained approximately to the theoretical value of
:1 and the distance between the two-spin orbit split transition was 17.2
2
eV. Binding energies (BEs) were referenced to the maximum of the C1s
core level peak (284.3 eV) measured after every other core level
measurement at the corresponding excitation energy. The satellite
Catalyst characterisation
The supported and unsupported NPs were characterized prior to
0
feature for Ni is 5.7 eV upshift with respect to the core level, whereas its
2
exposure to H and ex situ modification with tartaric acid.
intensity is 35% of the main peak.
X-ray powder diffraction measurements were carried out on a Rigaku
Miniflex 600 diffractometer, using CuK radiation. A silicon NIST SRM
Auger Electron Yield (AEY) NEXAFS spectra were recorded with an
analyser setting of 50 eV pass energy (Ep) and electron kinetic energy
640e standard sample was firstly measured, in order to obtain the
instrumental resolution function of the diffractometer. The obtained
diffraction pattern for each sample was Rietveld refined using TOPAS 5
software[40] and the corresponding crystallite sizes were estimated by
using calculated integral breadth-based volume-weighted column
heights[41] assuming monodisperse spherical systems of particles.
(KE) of 700 eV, 520 eV and 240 eV for Ni L, OK and CK, respectively.
The beam-line setting was 0.5 m x 0.04 m exit slit (ES) and fix focus
constant (cff) 2.0 (cff = cos/cos).
Bright field (BF) and high angle annular dark field scanning transmission
electron microscopy (HAADF STEM) images were acquired on a probe
corrected ARM200F at the ePSIC facility (Diamond Light Source)
equipped with a cold-FEG and operated at an acceleration voltage of 200
keV enabling a resolution of 0.78Å. Measurement conditions were a CL
aperture of 30 μm, convergence semiangle of 24.3 mrad, beam current of
X-ray absorption fine structure spectra at the Ni K-edge were obtained
from experiments at the B18 beamline in Diamond Light Source, UK.[42]
The measurements were carried out using a fixed-exit double-crystal
Si(111) monochromator and the Pt-coated branch of collimating and
focusing mirrors. Harmonic rejection mirrors were placed in the beam
path and used as a low-pass filter to remove higher energy harmonics.
The beam size at the sample position was approximately 1x1mm. All
samples were prepared in forms of pellets (13 mm diameter) mixing 50
mg of sample with 50 mg of cellulose to optimize the edge jump and total
transmission. The measurement was performed at room temperature in
transmission mode using 3 ion chambers filled with following gases: 300
12 pA, and scattering angles of 0-10 and 35-110 mrad for BF and
HAADF STEM respectively. Samples were deposited onto 3mm holey
carbon Cu- grids for TEM analysis.
Raman spectroscopy experiments were performed using 532 nm laser
excitation and a conventional single grating (1200 lines per mm) 0.32 m
Raman spectrometer. The spectra were calibrated using the intense
-
1
phonon mode in Si at 520.0 cm , and the spectral resolution was verified
to be at worst 3.2 cm-1 half width half maximum. The notch filters on the
2
mbar N + 700 mbar He (I0); 150 mbar Ar + 850 mbar He (I1, I2). The
spectra were collected in quick EXAFS mode by continuously scanning
the monochromator with a constant energy step size of 0.3 eV. The scan
covered an energy range from 8133.15 to 9333 eV, corresponding to a k-
-
1
spectrometer allowed collection of data down to 270 cm . This was
verified by the observation of a radial breathing mode peak from a single-
walled carbon nanotube sample centered on this wavenumber.
-1
range of 16Å . For each sample,
3 scans were acquired and
subsequently merged to improve the signal to noise ratio. Data were
normalised using the program Athena[43] with a linear pre-edge and 2nd
order polynomial post-edge. After background subtraction, the resulting
Hydrogenation reaction of MAA under batch conditions
The modified Ni catalysts were then tested for the hydrogenation reaction
of methyl acetoacetate (17.0 g, 0.15 mol) in methanol (15 mL) using a
2
χ(k) functions were k -weigthed and Fourier transformed in a range from
2
batch reactor at a working H pressure of 20 bar, temperature of 60 °C,
-
1
2
to 15.5 Å . The EXAFS fit was performed using the two references
1
0
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ChemCatChem
10.1002/cctc.201901955
FULL PAPER
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Entry for the Table of Contents
FULL PAPER
Rosa Arrigo,* Simone Gallarati, Manfred
E. Schuster, Jake Seymour, Diego
Gianolio, Ivan da Silva, June Callison,
Haosheng Feng, John E. Proctor, Pilar
Ferrer, Federica Venturini, David Grinter
and Georg Held
Page No. – Page No.
Ni nanoparticles with different electronic structure were synthetized via colloidal hot
injection. In the asymmetric hydrogenation of methyl acetoacetate (MAA) to chiral
methyl-3-hydroxy butyrate, (R)-selectivity is higher at low conversion and on oxidized
surfaces of Ni nanoparticles, whereas no impact of particles size was observed. Our
results indicate that during the hydrogenation reaction, the oxide phase enables a
stronger chemisorption of the (R, R)-tartaric acid, which assists the favourable
chemisorption of MAA on metallic Ni for the enantioselective hydrogenation to (R)-
MHB.
Influence of synthesis conditions on
the structure of nickel nanoparticles
and their reactivity in selective
asymmetric hydrogenation
1
3
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