Direct Visualization of Substitutional Li Doping in Supported Pt Nanoparticles and Their Ultra-selective Catalytic Hydrogenation Performance

Article abstract of DOI:10.1002/chem.202101470

It has only recently been established that doping light elements (lithium, boron, and carbon) into supported transition metals can fill interstitial sites, which can be observed by the expanded unit cell. As an example, interstitial lithium (^intLi) can block H filling octahedral interstices of palladium metal lattice, which improves partial hydrogenation of alkynes to alkenes under hydrogen. In contrast, herein, we report ^intLi is not found in the case of Pt/C. Instead, we observe for the first time a direct ‘substitution’ of Pt with substitutional lithium (^subLi) in alternating atomic columns using scanning transmission electron microscopy-annular dark field (STEM-ADF). This ordered substitutional doping results in a contraction of the unit cell as shown by high-quality synchrotron X-ray diffraction (SXRD). The electron donation of d-band of Pt without higher orbital hybridizations by ^subLi offers an alternative way for ultra-selectivity in catalytic hydrogenation of carbonyl compounds by suppressing the facile CO bond breakage that would form alcohols.

Full text of DOI:10.1002/chem.202101470

Communication
doi.org/10.1002/chem.202101470
Chemistry—A European Journal
Direct Visualization of Substitutional Li Doping in
Supported Pt Nanoparticles and Their Ultra-selective
Catalytic Hydrogenation Performance
[a, b]
[a]
[a]
[a]
[b]
Tianyi Chen,
Christopher Foo, Jianwei J. W. Zheng, Huihuang Fang, Peter Nellist,*
[a]
and Shik Chi Edman Tsang*
that apart from hydrogen, some light elements are also able to
Abstract: It has only recently been established that doping
light elements (lithium, boron, and carbon) into supported
transition metals can fill interstitial sites, which can be
observed by the expanded unit cell. As an example,
occupy the same interstitial sites of Pd. For example, carbon or
[3,4]
boron,
can accommodate at the interstitial holes of the Pd
host metal system and form analogous structures to that of its
interstitial hydrides. With the rapid development in analytical
facilities, such as transmission electron microscopy equipped
with aberration corrector, and diffraction techniques served
with high-quality synchrotron and neutron source, have made
significant contributions to establish the structure of these
doped nanoparticle systems. Crucially, correlating doping,
structure, and activity can help to design and synthesise this
int
interstitial lithium ( Li) can block H filling octahedral
interstices of palladium metal lattice, which improves partial
hydrogenation of alkynes to alkenes under hydrogen. In
int
contrast, herein, we report Li is not found in the case of
Pt/C. Instead, we observe for the first time a direct
sub
‘
substitution’ of Pt with substitutional lithium ( Li) in
alternating atomic columns using scanning transmission
electron microscopy-annular dark field (STEM-ADF). This
ordered substitutional doping results in a contraction of the
unit cell as shown by high-quality synchrotron X-ray
[5]
class of light element doped interstitial Pd catalysts.
[3]
Similarly, elements (carbon, boron and nitrogen) with
sufficiently small radii can be placed in the interstitial sites of
[4,6]
[7,8]
other transition metal lattice, such as cobaltand nickel. The
feasibility of filing the interstitial site is empirically assessed
diffraction (SXRD). The electron donation of d-band of Pt
sub
without higher orbital hybridizations by
Li offers an
[9]
using basic geometry in Hägg’s rule. This rule states that the
alternative way for ultra-selectivity in catalytic hydrogena-
tion of carbonyl compounds by suppressing the facile CO
bond breakage that would form alcohols.
ratio of dopant atomic radius (r ) to that of host metal atomic
X
radius (r ) can be used to guide the structural behaviour after
M
alloying foreign elements. When the ratio (r /r ) is less than
X
M
0
.59, the structure of the ‘interstitial’ alloy is expected to form.
However, there is also an electronic effect that critically affects
whether an in interstitial atom can be accommodated. In other
words, the nature of metallic bonding in interstitial solid-
solution transition-metal alloys (d-orbital dominated) signifi-
cantly differ from the pristine metal because of orbitals mixing
between the foreign atoms and the host metal system, for
instance, the s-p orbitals of the foreign element and the s-p-d
Supported Pd and Pt metal nanoparticles are the central focus
for catalytic hydrogenation because both metals have the
[1]
ability to activate hydrogen under mild conditions. Pd can
react with hydrogen to form the interstitial hydride PdH ,
0
.7
which is stable under ambient conditions. Pt is a good
hydrogenation catalyst, but does not seem to form stable
orbitals of the transition metal element can be intermixed.
[2]
[10]
interstitial hydrides.
According to the Engel-Brewer theory of metals,
it was
Even though the catalytic properties of these active metal
hosts can be significantly enhanced by light-element doping,
more details with particular regard to the structure-activity
relationships are required. It has only recently been confirmed
considered that the structure adopted by a metal or interstitial
solid-solution alloy is governed by s-p electron configuration.
[
11]
Furthermore, the presence of foreign atoms in a metal structure
can lead to lattice change of the host structure, which is the key
indicative of interstitial solid-solution alloy formation owing to
[12]
the electrostatic interactions. In addition, under a reducing
environment, the partial catalytic reduction of interstitial
cationic species by the host structure may form neutral or
anionic dopant species that are larger, and no longer able to be
accommodated at the interstitial site. The delicate balance of
these interactions makes it difficult to understand the structure-
activity relationships in supported-metal catalyst systems.
We are particularly interested in the structure of Li doped
transition metals and their catalytic performance in hydro-
genation reactions. General speaking, cationic lithium is theo-
retically small enough to take residence in the interstitial sites
[
a] Dr. T. Chen, C. Foo, Dr. J. J. W. Zheng, H. Fang, Prof. S. C. E. Tsang
Wolfson Catalysis Centre, Department of Chemistry
University of Oxford
OX1 3QR Oxford, (UK)
E-mail: edman.tsang@chem.ox.ac.uk
[
b] Dr. T. Chen, Prof. P. Nellist
Department of Materials
University of Oxford
OX1 PH Oxford, UK
E-mail: peter.nellist@materials.ox.ac.uk
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/chem.202101470
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of most transition metal lattices, but neutral lithium atoms are
too large. The location of Li and the oxidation state (electronic
interaction) with the host transition metal are important. For
different temperatures. X-ray diffraction (XRD) of pristine Pt and
lithiated Pt illustrated the structural changes induced by
annealing at different temperatures, namely a contracted unit
cell and increasing crystallite size. As depicted in Figure 1, it is
shown that the Pt reflections shift to higher angle for samples
example, we reported that interstitial lithium doped palladium
int
(
PdÀ Li NPs) can be synthesised by heating the mixture of
�
Li(OAc)·2H O and Pd/C at elevated temperatures. The Li (nearly
annealed at 650°C: (111), 39.88° to 40.50°; (200), 46.42 to
2
�
metallic) takes residence in structural distorted interstitial sites
under hydrogenation conditions, which can completely sup-
press the formation of the β-hydride phase and avoid over-
47.06°; (220), 67.56 to 68.57°; (311), 81.33° to 82.53°; (222),
86.20° to 87.08°. According to the Bragg equation, the d-
spacing corresponding to each diffraction index contracts from
d(111) =2.258 Å to 2.224 Å; 1.954(200) to 1.928(200) Å; 1.385(220) to
1.367(220) Å; 1.182(311) to 1.167(311) Å and 1.127(222) to 1.117(222) Å,
respectively. Obviously, there is a clear contraction of the lattice
parameter from 3.92 Å to 3.89 Å (see Table S5). This analysis
also suggests that Li species can preferably react with the Pt
metal framework at elevated temperatures, which is consistent
with the TGA profile. (Supporting Information Figure S1 and
Table S2)
[13]
hydrogenation beyond the target ethylene products.
Binary nanoalloys of Pt with other transition metals are well-
researched; however, alloying Pt with light s-block elements
after extensive reduction are hardly explored. To the best of our
knowledge, the only example is reported by Ohsaka et al. that
the electro-deposition of Li on a Pt electrode in propylene
carbonate can result in the formation of a PtÀ Li alloy, which
exhibited enhanced activity in the catalytic oxidation of formic
[14]
acid relative to a pristine Pt electrode.
However, the
Additionally, when the temperature reached to the 750°C,
�
characterization of the obtained catalyst lacked structural and
electronic detail, crystallographic siting, and chemical form of
the lithium. In contrast, the high temperature synthesis of the
PtÀ Li binary system at the bulk scale can be dated back to the
early 20th century. According to the phase diagram of a PtÀ Li
bulk system, Pt and Li can form four different stoichiometric
extra peaks were found at the range of 50–70 . The diffraction
�
�
peaks located at 56.76 and 66.56 are indexed to (220) and
[19]
(311) of the Li O phase. Again, the result agrees with the TGA
2
data (Supporting Information Figure S1 and Table S2) which
showed the decomposition of Li acetate to Li O under the
2
reaction condition. The rest of low-intensity peaks could be
[15]
[16]
[17]
[16]
compounds: LiPt2,
LiPt,
LiPt7,
and Li Pt.
Recently,
indexed to (133), (024), (224) and (115) of the Pt Li, respectively,
2
7
Howies et al. reported a novel intermetallic compound Li Pt ,
on the basis of the literature, but requires for further
evidence.
1
1
2
[18]
[17]
which can be obtained under even harsher conditions. Notice
that these bulk phases are unable to be used as catalysts due to
the low surface area, limited concentration of exposed metal
sites, and high lithium contents. Therefore, it is highly important
to bridge the knowledge gap between these bulk systems and
the analogous nanoparticle system by using the controlled Li
doping of supported Pt nanoparticles for catalytic applications.
In this work, we have attempted to synthesise PtÀ Li NPs
according to a previously reported synthetic strategy with
With the development of high quality X-rays at synchrotrons
through improvements in collimation, flux, detection, and
[20]
energy bandwidth, high-resolution diffraction data can be
collected. In comparison with the pristine Pt SXRD pattern, the
PtÀ Li reflections shift to higher angles, which is consistent with
our observations using lab-source XRD. Additionally, weak low-
[13]
limited Li content. With the aid of SXRD (synchotron X-ray
diffraction) and the associated refinement, we report that the
unit cell of the Pt metal lattice contracts after lithiation whilst
retaining the parent FCC structure. This unique physical change
is attributed to the substitution of Pt atoms with Li atoms to
form an ordered Pt Li phase, which is for the first time
7
confirmed by the STEM-ADF image. In addition, further
characterization techniques such as X-ray photoelectron spec-
troscopy (XPS), solid state nuclear magnetic resonance (ssNMR),
thermogravimetric analysis (TGA) and transmission electron
microscopy (TEM) (see Supporting Information) were used to
support the findings. We highlight that the combination of
characterization techniques clearly provides valuable insights
into the siting of Li in the Pt metal lattice and the impact on the
catalytic hydrogenation of carbonyl compounds, which exhibits
a different mechanism to the analogous PdÀ Li system.
In this study, the preparation of PtÀ Li/C was investigated by
adapting our previously published synthetic strategy, which
Figure 1. XRD patterns of Pt/C with Li(OAc)·2H O after annealing at temper-
2
int
°
°
atures between 550 C and 750 C for 2 h. Dotted lines indicate the original
positions of Pt diffraction patterns. It can be found that the Pt reflections
shift to higher angle when the annealing temperature is above 650°C. The
was developed for PdÀ Li/C, for which Li was confirmed by
[13]
various techniques.
The pristine Pt/C NPs after thermal
treatment were used as the parent framework material.
diffraction peaks of Li O are annotated with symbol *, while the diffraction
2
7
peaks of Pt Li are annotated with symbol !.
Mixtures of the Pt/C NPs and Li(OAc)·2H O were annealed at
2
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angle peaks were detected at the range of 8 to 32°, which were
columns of exclusively Pt atoms and PtÀ Li mixed columns were
observed. The stronger scattering of Pt relative to Li (Figure 2a–
c) result in significant contrast due to the large difference in
electron-scattering ability, and allows their differentiation. The
indexed and assigned following more intensive refinement
(Figures S5 and S6). Following the simulation of various PtÀ Li
based compounds, the broad peaks, observed at 2θ=10.5°,
1
2.2°, 17.2°, 20.2°, 26.7° and 27.3° (Figure S6b), have been well
STEM micrograph was compared with the Pt Li structure
7
matched to the structure of the known Fm-3 m intermetallic
compound Pt Li, which is coincident with the aforementioned
solution as determined by Rietveld refinement of the SXRD
pattern in order to verify the structure of the NP. As the model
7
observation (Table S5).
crystal structure of the Pt Li is superimposed on the micro-
7
The structure of Pt Li results from the ordered substitution
of Pt atoms with Li. In order to confirm the presence of Li
graph, a very good match was indeed found between this
model and the NP structure (Figure 2b). It is shown by the
intensity line profile of the ADF image that alternating atomic
columns are less bright than others, indicating the presence of
lithium at a substitutional site within such columns (Figure 2c).
The contrast of the restored phase image (Figure S9a–c) is
indistinguishable to that of the STEM-ADF imaging. Except for
this, no unusual contrast was found at the interstitial sites of
the Pt metal framework, which suggests that Li dopants are
not located at the interstitial sites. Meanwhile, a spectroscopic
technique, electron energy loss spectroscopy (EELS), was
attempted to confirm the presence of Li at local environment
and resolve the quantification of Pt:Li, but failed to give an
acceptable result (Figure S10).
7
sub
atoms within the Pt Li phase, we then characterized the
7
structure by electron microscope. On the basis of the BF-TEM
image and its corresponding histogram (Figure S8a and b),
particles are well-dispersed on the carbon support, and 50 NPs
were counted. The size distribution is at the range of 2–16 nm.
Aberration-corrected electron microscopy was used to
directly visualize the atomic arrangement of a selected PtÀ Li
NP. Additionally, being a phase imaging technique, electron
ptychography is highly sensitive to light elements, such as Li
[3]
[21]
and is therefore applicable to this system. We thus simulta-
neously carried out ADF imaging and ptychography at the
atomic level through the use of fast pixelated detectors.
Owing to the strong relationship between atomic number
and electron-scattering ability, the intensity of ADF image of an
atomic column is highly dependent on the number and
element of the atoms being imaged. In this case, lithium atoms
are expected to scatter much less than the heavy platinum
atoms given the approximately identical number of atoms in
each column. Consequently, in a typical Z-contrast STEM-ADF
image of PtÀ Li/C orientated in the <110> direction, alternate
To confirm the validity of the STEM-ADF result, images of
Pt Li of varying thickness was also simulated (Figure 2d–f).
These simulated images show that the total integrated intensity
of the Pt columns is also much higher than the adjacent Pt
columns containing Li atoms (see structure and viewing
7
directions of Pt Li in Figure S7a). Thus, this is compelling
7
evidence for lithium occupying framework platinum sites in an
ordered manner. The observation also supports the postulation
Figure 2. Simultaneously acquired Z-contrast STEM-ADF. (a) Z-contrast STEM-ADF micrograph of PtÀ Li/C NP orientated in the <110> zone axis. (b) Enlarged
area of the NP with Pt Li <110> model superimposed (blue: mixed Pt and Li, gray: Pt). (c) ADF intensity line profile taken along the line indicated in (a).
d), (e) and (f) are simulated ADF imaging for Pt Li of different thicknesses.
7
(
7
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[
24]
that the shifted reflections discussed above are the substitu-
tional-doped platinum.
bonds or to an aromatic ring. Here, we used hydroxymeth-
ylfurfural (HMF), which is an important chemical derived from
biomass. It contains a facile carbonyl group that can be used to
On the basis of the above results, a reaction mechanism is
sub
proposed for the formation of the Pt Li. The thermal decom-
investigate partial hydrogenation over our supported PtÀ Li
7
position of the Li(OAc)·2H O can give the intermediate Li CO
3
catalyst. The desired product is the partially hydrogenated 2,5-
bis(hydroxymethyl)furan (BHMF) by selective hydrogenation of
carbonyl to alcohol. Over-hydrogenation commonly takes place
over supported Pt catalysts by hydrogenic activation of the
facile carbonyl group to yield 5-methyl-2-furanmethanol (MFA,
furfuryl alcohol (FA), or even reacting with IPA to 5-[(1-meth-
ylethoxy)methyl]-2-furanmethanol (MEFA). Further hydrogena-
tion or hydrodeoxygenation (HDO) reactions can lead to 2,5-
Dimethyltetrahydrofuran (DMTHF) or 2,5-dimethyl furan (DMF),
or induce ring-opening yielding side products such as hexane,
2-hexanol, 2-hexanone, etc. (Figure 3).
2
2
which might further decompose to a finely divided oxide, Li O
2
[22]
with increasing temperature.
Elevated temperature and
catalytic phase change can promote this process to accelerate
the slow diffusion of ions in the solids, and to overcome the
strong Columbic attractions between the ions. Subsequently,
Li O could be reduced and exchanged with Pt, in the presence
2
[17]
of hydrogen atmosphere at elevated temperature. Since the
material was prepared via a wet-impregnation method, the
source of the hydrogen may come from the ethanol and/or
water of the hydrated Li(OAc)·2H O precursor. The principle of
2
categorizing mixed-alloys into intermetallic compounds or
solid-solution alloys is dependent on the atomic ordering. In
intermetallic compounds, they give both long-range atomic
order and well-defined stoichiometry, however, in mix-alloys,
two metals are randomly and thoroughly mixed. In light of
the SXRD refinement result, the Bragg constituent is Fm-3 m
Thus, the conversion of HMF and the product selectivity of
BHMF were carefully analysed by analysing quenched aliquots
at different times of a batch reaction. The reaction temperature
was initially at 100°C and 120°C, respectively. The Li content
[23]
was optimized for selectivity of BHMF, the result of which is
summarized in the Supporting Information (Table S7). The time-
sub
matching with crystalline Pt Li. Hence, in combination with the
course of hydrogenation HMF over Pt/C and PtÀ Li/C (con-
7
ADF and solid-state nuclear magnetic resonance (ssNMR) results
version and the selectivity to BHMF) is displayed in Table 1.
(
Figure S2), it can be concluded that the mixture of the thermal
At 100°C, Pt/C exhibited only 11% selectivity towards
decomposition of Li(OAc)·2H O in the presence of Pt/C NPs can
yield a Li-substituted Pt intermetallic nanoalloy (PtÀ Li).
BHMF, despite a conversion of 94% after one hour. The major
side-product was MEFA showing that the excessive adsorption
of facile carbonyl group of HMF on unmodified Pt surface can
lead to hydrolytic cleavage, in addition to by-products such as
liquid alkanes, ring-opening products and intermediate frag-
ments (Figure 3). At higher temperature (120°C) or longer
2
sub
Catalytic Hydrogenation for HMF
The selective hydrogenation of unconjugated carbonyl com-
pounds to their corresponding alcohols represents an important
industrial challenge in catalyst development because cleavage
of the facile CÀ O bond must be prevented in the liquid phase.
Doped palladium is known to be one of the most effective
heterogeneous catalysts for selective hydrogenation of carbon-
carbon double bond in unsaturated aldehydes and ketones.
However, such selective reactions are quite problematic when
carbonyl groups of the substrate are conjugated to double
reaction times, it was found that the BHMF and the carbonyl-
cleaved intermediates, such as MEFA and MFA, were substan-
tially decreased at the expense of other further fragmented
hydrocarbon products (Table S7). This result clearly indicates
poor intrinsic selectivity towards BHMF due to the adsorption of
the carbonyl group of the HMF molecule on Pt. In contrast, Pt/C
treated with Li(OAc)·2H O (0.5 eq.) results in lower activity with
2
a 86% conversion of HMF after 1 h, but with a significantly
improved selectivity for the partially-hydrogenated product
Figure 3. Hydrogenation of hydroxyl methyl furan (HMF) to 5-methyl furfuryl alcohol (MFA); furfuryl alcohol (FA); 5-[(1-methylethoxy)methyl]-2-furanmethanol
MEFA); 2,5-bis(hydroxymethyl)furan (BHMF); 5-methyl furfural (MF) and other side products, etc.
(
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sub
Table 1. Time-course regarding the conversion and product selectivity of the selective hydrogenation of HMF over Pt/C and PtÀ Li/C in IPA at 100°C.
Catalyst
Reaction Time
h]
HMF conversion
[%mol]
BHMF Select.
[%mol]
MEFA Select.
[%mol]
MFA Select.
[%mol]
Side products Select.
[%]
[
Pt/C
Pt/C
Pt/C
1
2
4
1
2
4
94
11
9
7
87
93
93
30
32
18
1
4
55
59
75
3
Trace
2
>99
>99
86
98
99
Trace
Trace
9
6
5
sub
PtÀ Li/C
sub
PtÀ Li/C
1
sub
PtÀ Li/C
Trace
BHMF of 87%. Reaction intermediates, such as furfural and
MEFCA, were not observed by GCÀ MS analysis. Prolonging the
reaction time to two hours, a near-total conversion of HMF was
observed (98–99%), for which the BHMF selectivity increased to
dopant can dramatically suppress the formation of side
products by inhibiting the extensive hydrodeoxygenation
(HDO) reaction pathways. It is anticipated that the electron
filling of the d-band centre of the Pt after lithiation without
high-order orbital hybridizations can severely weaken the
adsorption of HMF molecules, leading to retention of the CÀ O
bond and higher selectivity towards the partially-hydrogenated
product, BHMF.
9
3%. Optimization of temperature (120°C) and time (2 h) can
give stoichiometric conversion of HMF to BHMF, accompanied
by only a trace of other products (Table S7).
In contrast to the unmodified Pt commercial catalyst, the
sub
PtÀ Li catalyst exhibits exclusive conversion of HMF to BHMF
without cleavage of the facile CÀ O bond. Instead of blocking
the interstitial sites from hydrogen accommodation and the Experimental Section
resulting excessive hydrogenation, as is the case for the
Experimental Details are available in the Supporting Information.
int
palladium system PdÀ Li, the catalytic activity and selectivity of
Pt metal NPs may also be altered by the position of the d-band
centroid of the metallic structure, e , and the degree of electron
c
filling with respect to the Fermi level. The downshift of the d- Acknowledgements
band centroid will decrease the adsorbate-substrate binding
strength, whereas the degree of electron filling will also reduce
The support of this project from the IUK-EPSRC of UK (DGE
[25]
the binding energy. Substitutional Li doping results in only a
small change in the lattice parameter, hence the effect of d-
102000) is gratefully acknowledged. The authors wish to thank
Diamond Light Source (Diamond, UK) for accessing STEM and
XAS facilities (B18; SP20856-1).
[26]
band down-shifting is weak. In addition, lithium is a highly
electropositive element that does not have available p or d
orbitals for hybridizations with the host framework atoms which
alter the Pt d-band position (in contrast to other light element Conflict of Interest
dopants such as boron and carbon). Hence electron filling is
anticipated to cause substantially weaker adsorption of the
HMF carbonyl, resulting in selective conversion to BHMF, with
total inhibition of over-hydrogenated or related products. It is
believed that this direct correlation between the doped
structure and catalytic hydrogenation activity will allow the
development of new catalytic applications.
The authors declare no conflict of interest.
Keywords: bimetallic
compounds
·
hydrogenation
·
nanoparticles · ordered alloys · substitution
In conclusion, lithium-doping pre-reduced supported Pt NPs
is shown to be substitutional by the formation of a discrete
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Manuscript received: April 25, 2021
Accepted manuscript online: June 22, 2021
Version of record online: ■■■, ■■■■
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These are not the final page numbers!

COMMUNICATION
Dr. T. Chen, C. Foo, Dr. J. J. W. Zheng, H.
Fang, Prof. P. Nellist*, Prof. S. C. E.
Tsang*
1
– 7
Direct Visualization of Substitutional
Li Doping in Supported Pt Nanopar-
ticles and Their Ultra-selective
Catalytic Hydrogenation Perform-
ance
sub
A direct ‘substitution’ of Pt with Li
in atomic columns by electron pty-
chography images with characteristic
contraction in lattice parameters from
advanced materials characterization is
observed for the first time. The
without higher orbital hybridizations
by Li offers an alternative way for
ultra-selectivity in catalytic hydroge-
nation of carbonyl compounds with
detainment of facile CO bond to
alcohols.
sub
electron donation of d-band of Pt