Dual-Site-Mediated Hydrogenation Catalysis on Pd/NiO: Selective Biomass Transformation and Maintenance of Catalytic Activity at Low Pd Loading

Article abstract of DOI:10.1021/acscatal.0c00414

Creating a new chemical ecosystem based on platform chemicals derived from waste biomass has significant challenges: catalysts need to be able to convert these highly functionalized molecules to specific target chemicals and they need to be economical - not relying on large quantities of precious metals - and maintain activity over many cycles. Herein, we demonstrate how Pd/NiO is able to direct the selectivity of furfural hydrogenation and maintain performance at low Pd loading by a unique dual-site mechanism. Sol-immobilization was used to prepare 1 wt % Pd nanoparticles supported on NiO and TiO2, with the Pd/NiO catalyst showing enhanced activity with a significantly different selectivity profile; Pd/NiO favors tetrahydrofurfuryl alcohol (72%), whereas Pd/TiO2 produces furfuryl alcohol as the major product (68%). Density functional theory studies evidenced significant differences on the adsorption of furfural on both NiO and Pd surfaces. On the basis of this observation we hypothesized that the role of Pd was to dissociate hydrogen, with the NiO surface adsorbing furfural. This dual-site hydrogenation mechanism was supported by comparing the performance of 0.1 wt % Pd/NiO and 0.1 wt % Pd/TiO2. In this study, the 0.1 and 1 wt % Pd/NiO catalysts had comparable activities, whereas there was a 10-fold reduction in performance for 0.1 wt % Pd/TiO2. When TiO2 is used as the support, the Pd nanoparticles are responsible for both hydrogen dissociation and furfural adsorption and the activity is strongly correlated with the effective metal surface area. This work has significant implications for the upgrading of bioderived feedstocks, suggesting alternative ways for promoting selective transformations and reducing the reliance on precious metals.

Full text of DOI:10.1021/acscatal.0c00414

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Article
Dual-Site Mediated Hydrogenation Catalysis on Pd/NiO: Selective Biomass
Transformation and Maintaining Catalytic Activity at Low Pd Loading
Sebastiano Campisi, Carine E Chan-Thaw, Lidia E Chinchilla, Arunabhiram Chutia, Gianluigi
A. Botton, Khaled M.H. Mohammed, Nikolaos Dimitratos, Peter Wells, and Alberto Villa
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.0c00414 • Publication Date (Web): 10 Apr 2020
Downloaded from pubs.acs.org on April 13, 2020
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Dual-Site Mediated Hydrogenation Catalysis on Pd/NiO: Selective
Biomass Transformation and Maintaining Catalytic Activity at Low
Pd Loading.
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Sebastiano Campisi†, Carine E. Chan-Thaw†, Lidia E. Chinchilla‡, Arunabhiram Chutia#,
Gianluigi A. Botton‡, Khaled. M. H. Mohammed§,¢, Nikolaos Dimitratos¥, Peter P. Wells§,◊,○* and
Alberto Villa†*
† Dipartimento di Chimica, Università degli Studi di Milano, via Golgi 19, 20133 Milano, Italy.
‡ McMaster University, Department of Materials Science and Engineering, Hamilton, Ontario, L8S 4M, Canada.
# School of Chemistry, University of Lincoln, Lincoln, LN6 7TS.
§ School of Chemistry, University of Southampton, University Road, Southampton, SO17 1BJ, United Kingdom.
¢ Department of Chemistry, Faculty of Science, Sohag University, Sohag, P.O.Box 82524, Egypt.
¥ Dipartimento di Chimica Industriale “Toso Montanari”, Alma Mater Studiorum, University of Bologna, Viale
Risorgimento 4, 40136, Bologna, Italy.
◊ UK Catalysis Hub, Research Complex at Harwell, Rutherford Appleton Laboratory, Harwell Oxon, Didcot, OX11
0FA.
○ Diamond Light Source Ltd., Harwell Science and Innovation Campus, Chilton, Didcot OX11 0DE, United
Kingdom.
KEYWORDS: Furfural hydrogenation, Pd, NiO, heterogeneous catalysis, hydrogen spillover
ABSTRACT: Creating a new chemical ecosystem based on platform chemicals derived from waste biomass has significant
challenges; catalysts need to be able to convert these highly functionalised molecules to specific target chemicals,
economical – not relying on large quantities of precious metals - and maintain activity over many cycles. Herein, we
demonstrate how Pd/NiO is able to direct the selectivity of furfural hydrogenation and maintain performance at low Pd
loading by a unique dual-site mechanism. Sol-immobilization was used to prepare 1 wt% Pd nanoparticles supported on
NiO and TiO2, with the Pd/NiO catalyst showing enhanced activity with a significantly different selectivity profile; Pd/NiO
favours tetrahydrofurfuryl alcohol (72%), whereas Pd/TiO2 produces furfuryl alcohol as the major product (68%). Density
functional theory studies evidenced significant differences on the adsorption of furfural on both NiO and Pd surfaces. Based
on this observation we hypothesised that the role of Pd was to dissociate hydrogen, with the NiO surface adsorbing furfural.
This dual-site hydrogenation mechanism was supported by comparing the performance of 0.1 wt% Pd/NiO and 0.1 wt%
Pd/TiO2. In this study, the 0.1 and 1 wt% Pd/NiO catalysts had a comparable activity, whereas there was a 10-fold reduction
in performance for 0.1 wt% Pd/TiO2. When using TiO2 as the support the Pd nanoparticles are responsible for both hydrogen
dissociation and furfural adsorption, and the activity is strongly correlated with the effective metal surface area. This work
has significant implications for the upgrading of bio-derived feedstocks, suggesting alternative ways for promoting selective
transformations and reducing the reliance on precious metals.
objective necessitates detailed catalytic studies that probes
INTRODUCTION
the performance on an atomistic level and are able to
To expedite the transition from a chemical ecosystem
describe how specific materials promote these complex
based on petroleum to one based on those derived from
transformations.
sustainable bio-derived platform chemicals requires a
In this regard, there have been significant efforts to
concerted effort: catalysts need to be (i) highly active at
understand the transformation of lignocellulosic biomass
mild conditions, (ii) be atom efficient and achieve a high
to fine chemicals and fuels.1–3 Furfural, derived from the
selectivity to a desired product, (iii) maintain performance
hydrolysis of the hemicellulosic fraction of lignocellulose,
over many cycles, and (iv) be economical by not relying on
is considered as a viable platform molecule to many value-
large amounts of scarce and expensive metals. This
added products (Scheme 1).4,5 Furfuryl alcohol (FA),
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obtained via hydrogenation of the furfural carbonyl group,
in tandem with supported metal nanoparticles.26 Indeed,
is widely used for producing resins, lubricants, plasticizers
and fibers.6,7 The hydrogenation of both carbonyl group
and furanic ring produces tetrahydrofurfuryl alcohol
(THFA), a green solvent used in printer inks and
agriculture applications.8 In spite of the relevance of these
compounds, furfural hydrogenation presents a complex
pathway, requiring tailored catalysts to direct the
selectivity towards desired products.
sea urchin-like NiO, was recently demonstrated to possess
excellent catalytic activity in the selective hydrogenation of
furfural to FA.27 The mechanism for the enhancement
enabled by the metal support interaction is very specific –
both for the metal/metal oxide combination and the
reaction under study. For furfural hydrogenation, Somorjai
et al. used sum frequency generation (SFG) vibrational
spectroscopy to reveal the role of O-vacancies at the metal
support perimeter in determining the distribution of
products over Pt/TiO2 catalysts.28 Elsewhere, Seemale et al.
demonstrated that different supports (Al2O3 and TiO2)
induce a different surface composition of bimetallic Cu-Ni,
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and therefore
a different activity in the furfural
hydrogenation.14
It is clear that tailoring the properties of both the metal
nanoparticles and the support has profound consequences
for the resultant performance towards the hydrogenation
of furfural. In this study, our combined experimental and
theoretical approach demonstrates how Pd/NiO not only
directs the selectivity of furfural hydrogenation but
operates through
a unique dual-site hydrogenation
mechanism that allows for high catalytic activity to be
maintained at low (0.1 wt%) Pd loading.
EXPERIMENTAL
Scheme 1. Chemical routes for furfural valorization.
Catalyst synthesis: NiO preparation: Nickel(II) nitrate
hexahydrate(0.134 M, Ni(NO3)2⦁6H2O, Sigma-Aldrich,
purity > 99.9%) and urea (Sigma-Aldrich, purity > 99.5%)
(Ni/urea molar ratio 1:12 mol/mol) were solubilized and
mixed in 0.2 L of water with magnetic stirring for 240 min.
at 80 °C. The suspension was filtered to separate the
precipitated nickel hydroxide, Ni(OH)2, from the solution.
The powder was washed with 2 L of water, dried at 100 °C
for 120 min. and finally calcined at 300 °C for 60 min.
To optimise the hydrogenation reaction of furfural many
noble metal catalysts have been studied.9 Pd and Ir lead to
the formation of FA as the main product10,11 whereas Ni
promotes selectivity to THFA.12 The product distribution
depends on the affinity of the reactant to the metal, as well
as the preferential mode of adsorption on specific sites (e.g.
steps, edges, corners and terraces) of the catalyst.9,13
A
perpendicular adsorption, through the carbonyl group,
results in the formation of furfuryl alcohol and 2-methyl
furan.6,14 A flat adsorption, through the furanic ring and
carbonyl functionality, produces THFA.15 At high
Pd catalyst: Pd(II) salt (0.051 mmol of Na2PdCl4 Sigma-
Aldrich, purity > 99.9%) was dissolved in 0.1 L of water and
sequentially 1 mL of 1 wt% polyvinyl alcohol solution
(Pd/PVA mass ratio 1:0.5 wt/wt) was introduced. As-
obtained yellow solution was maintained under vigorous
magnetic stirring for 3 min, then a precise volume of 0.1 M
NaBH4 solution (Pd/NaBH4 molar ratio 1:8 mol/mol) was
injected. Immediately, the solution assumed a brown
coloration, indicating the formation of Pd(0) colloid. After
few minutes, sulphuric acid was added to adjust the pH to
2 and then the solid support was dispersed in the
vigorously stirring colloid. The amount of the support was
selected in order to achieve a final palladium loading of 1
and 0.1 wt% (on the assumption of a quantitative metal
uptake on the support). After filtration and washing, the
catalysts were dried at 100 °C for 120 min.
temperature,
a
flat orientation results in the
decarbonylation of furfural, producing furan and
tetrahydrofuran.7,16 A strategy for tuning the selectivity is
to selectively block these specific active sites on the metal
surface using thiolates or polymers such as polyvinyl
alcohol (PVA);17–19 the latter of these has a dual purpose as
it is commonly used to limit the growth of colloidal Pd
nanoparticles during sol-immobilzation.18 However, the
effects of the metal sites on directing the transformation of
highly functionalized bio-derived substrates to specific
products cannot be assessed in isolation; the support
material is not benign and has a significant influence on
the catalytic properties.20–22 The ‘so-called’ strong metal-
support interactions (SMSI) at the interface is responsible
for electron transfer, charge redistribution, morphological
changes, metal decoration or encapsulation, metal-
support borderline site creation, and spillover
phenomena.23–25 Furthermore, the supports themselves are
involved in catalytic transformations either in isolation or
Catalytic tests: Catalytic performances in the furfural
(F) (Sigma-Aldrich, purity 99%) hydrogenation reaction
were evaluated at 25 °C, carrying-out batch experiments in
a stainless steel reactor (0.03 L capacity), equipped with
heater, mechanical stirrer, gas supply system and
thermocouple for temperature control. The reactor was
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filled with 0.015 L of furfural solution (0.3 M in 2-propanol)
spectrum was measured for 4 min and on average 6 scans
2
and the proper amount of catalyst (F/metal ratio=500
mol/mol) was suspended in the solution. The reactor was
pressurized with 5 bar of pure hydrogen. The mixture was
kept at room temperature (25°C) under mechanical stirring
(1250 rpm). At the end of the test, gas flow switched from
H2 to N2 for purging the autoclave with flowing nitrogen.
Periodical sampling (200 μL) allowed for monitoring the
progress of the reaction by chromatographic analysis,
using a HP 7820A gas chromatograph equipped with a
capillary column HP-5 30m x 0.32mm, 0.25 µm Film, by
Agilent Technologies. Compound identification was
performed on the basis of retention times obtained from
the analysis of authentic samples. External standard
method (n-octanol) was used for the quantitative analysis.
were measured to improve the data quality of the final
summed spectrum. Data were measured in K space to a
maximum value of 16. Data processing was performed
using IFEFFIT29 with the Horae30 packages (Athena and
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Artemis). The amplitude reduction factor, S0 was
determined by analyzing a Pd foil and determined to be
0.88 and used as a fixed input parameter.
Infrared CO Chemisorption Studies. A Nicolet is10
spectrometer was used for all Fourier transform infrared
(FTIR) spectroscopy measurements. The spectral
resolution was 2 cm−1 and the total number of scans of the
sample and the background was 64. All samples were
measured in transmission as ultra-thin self-supporting
discs; approximately 25 mg of sample was used to form a
disc of 13 mm diameter. A Harrick Dewar cell was used for
the environmental studies. The sample environment was
flushed with He for 30 min to obtain a background
spectrum before CO was introduced using a 10% CO/He
mixture at 70 mL min over a 30 s period. There were 3 doses
of CO for each experiment. Before obtaining a final
spectrum the gas was switched to helium for 30 min at 70
mL min−1, in order to remove gaseous and physisorbed CO.
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Transmission Electron Microscopy (TEM). Holey
copper grids were used for performing TEM studies, where
small quantities of powdered catalysts were deposited onto
the grids prior to analysis. The TEM studies – both for
producing images and elemental analysis through X-ray
energy dispersive spectroscopy (XEDS) - were conducted
in High Annular Dark Field mode using a FEI Titan3 80-
300 microscope. The acceleration voltage was 200 kV. All
data was analyzed using the software packages: Digital
Micrograph, TIA, and INCA. Information on particles size
distribution (PSD) assumed a truncated cubooctahedron
particle shape and was assessed using Gauss software.
Atomic Absorption Spectroscopy (AAS). The
palladium concentration on solid catalysts was determined
by AAS analysis of the post-synthesis filtrate using a Perkin
Elmer 3100 instrument. For each catalyst, 0.01 L of the
filtered solution was withdrawn and analyzed.
X-ray Photoelectron Spectroscopy (XPS). The
acquisition of X-ray photoelectron spectra (XPS) was
performed on a PHI 3056 XPS with an Al anode source
operated at 15kV and an applied power of 0.35 kW. A
sputter cleaned silver foil was used for checking the energy
scale calibration. Powders were hand-pressed between two
clean slices of indium foil; the indium foil, in which the
powders were embedded, was then placed in the sample
holder with a fragment of carbon tape (Nisshin E.M. Co.
LTD). The lowest C1s binding energy species were shifted
about 0.1 eV to 284.8 eV for comparison purposes with
literature values. The XPS analyses did not require any
pretreatments (i.e. cleaning, heating, processing). High
resolution scans were acquired with a 23.5 eV pass energy
and 0.05 eV steps a minimum of 50 times; survey scans
were measured with a 93.9 eV pass energy and 0.5 eV
energy steps a minimum of 25 times. Peak area integration
provided surface concentrations, considering the proper
standard atomic sensitivity factors provided by the
instrument producer. Gaussian-Lorentzian functions and a
Shirley-type background were used for spectra
deconvolution.
Powder X-ray diffraction (XRD). X-ray diffraction
patterns were collected on a PANalytical X’PertPRO X-ray
diffractometer utilizing a CuKα radiation source (40 kV
and 30 mA). XRD patterns were recorded in the 10-80° 2θ
range over a scan time of 1 h.
DFT Studies. DFT Studies. We used the Vienna Ab
Initio Simulation Package (VASP) to perform all the spin
polarized periodic density functional theory based
calculations.31–33 The projector augmented wave (PAW)
method was used and a cut-off energy of 550 eV was
employed for the expansion of the plane-wave basis sets,
which gave bulk energies converged to within 10-5 eV..34 For
the structural optimization we chose a convergence
criterion of 0.01 eV Å-1 and a k-point grid of 3×3×1 was
employed for all slab calculations. The Perdew-Burke-
Ernzerhof (PBE) version of generalized gradient
approximation (GGA) was used to carry out geometry
optimizations and the total energy calculations.35 As the
dispersive effects may be significant for the systems under
consideration we also used Grimme’s dispersion correction
(DFT+D3).36 The ideal surfaces of Pd(111) and NiO(110) and
rutile TiO2(111) were modelled by a 4x4 cell with 5 atomic
layers. Of the five atomic layers, the bottom three layers
were fixed to mimic the bulk of the material and in the
direction perpendicular to the surface we used a vacuum
gap of ~15 Å, which is sufficient to eliminate slab-slab
interactions. The slabs were cut from bulk Pd, NiO and
rutile TiO2 with a calculated energy minimized lattice
constant of 3.904 Å (Exp. 3.891 Å), 4.192 Å (Exp. 4.168 Å)
X-ray
Absorption
Fine
Structure
(XAFS).
Measurements were conducted at the UK national
synchrotron Diamond Light Source, using the core-XAFS
beamline, B18. XAFS data at the Pd K edge were acquired
simultaneously with a Pd foil reference positioned between
It and Iref. Samples were prepared as self-supporting pellets
and were assessed using fluorescence acquisition using a
QEXAFS set-up with a fast-scanning Si(311) double crystal
monochromator and a 36 element Ge detector. Each
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and 4.695 Å (Exp. 4.594 Å), respectively. In all the
product (87 %). This observation is consistent with recent
calculations, the adsorption of furfural was allowed on only
one of the two exposed surfaces. The spurious dipole
moment due to the adsorbed species, was taken into
account by using the methods implemented in VASP
according to the procedures of Markov et al. and
Neugebauer et al.37,38 For the calculations on the
interaction of furfural on the NiO(110) and TiO2(111)
surfaces we used DFT+U. Previous studies have shown that
an U value of 5.77 eV, obtained by using linear response
methods, for the d-orbital of Ni is suitable.39 It has been
also reported that an U value of 4.20 eV is suitable for the
d-orbitals of Ti.40
studies that have already proven that NiO is able to
hydrogenate furfural in the absence of precious metals.27
The time-on-line data for the catalytic testing (Figure S3
and S4) confirmed that Pd/NiO favors THFA over FA as the
major product at all-time points. However, there is a sharp
increase in THFA selectivity as the conversion of furfural
progresses. This arises because of sequential
hydrogenation reactions and the transformation of
isopropyl tetrahydrofurfuryl ether to THFA. To understand
the difference in catalytic properties a series of studies to
establish the physicochemical properties of the Pd
nanoparticles was performed; the intention was to
establish whether the catalytic data could be accounted for
solely by the changes to Pd or if there was a synergistic
effect between the metal and support. The Pd nanoparticle
size data was found using transmission electron
microscopy (Table 1, Figure S5).
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RESULTS
Pd nanoparticles were supported on TiO2 (P25, 50 m2 g-
1) and NiO through an established sol-immobilization
method with a loading of 1 wt%. The metal loading was
confirmed by Atomic Absorption Spectroscopy (AAS). The
NiO support was prepared using an in-house method
involving the precipitation of Ni(OH)2 in aqueous solution,
followed by subsequent calcination. The preparation
resulted in a nanostructured NiO with a particle size of 6
nm and a surface area of 210 m2g-1. The NiO phase was
identified using XRD (Figure S1). X-ray photoelectron
spectroscopy (XPS) analysis provided useful insight on the
nature of Ni surface species (Figures S2). Ni2p3/2 data
disclosed the presence of two main peaks corresponding to
different Ni species on the surface; the first peak centered
around 854.5 eV is due to Ni2+-O lattice species, while the
second peak centered around 856.1 eV confirmed that
Ni(OH)2 was also formed at the surface (Figure S2). The
oxidation state of surface species has been investigated by
XPS. Comparison of Pd 3d XP spectra for Pd/TiO2 and
Pd/NiO (Figure S2) revealed that Pd is present in the
metallic form in both cases with a similar percentage of
Pd0 (85% and 88% for Pd/NiO and for Pd/TiO2,
respectively). The supported metal nanoparticle catalysts
and bare supports were then evaluated for the
hydrogenation of furfural under mild conditions (Table 1);
furfural 0.3 M, furfural/Pd ratio 500 mol/mol, 5 bar H2, 50
°C and 2-propanol as solvent. Prior to the catalytic
evaluation the appropriate Pd/furfural molar ratio was
established to ensure the catalytic performance was in the
kinetic regime and not mass-transport limited (Table S1).
The catalytic activity testing demonstrated that the 1 wt%
Pd/NiO catalyst exhibited superior activity compare to the
1 wt% Pd/TiO2 (470 (metal mol)-1 h-1 vs. 321 (metal mol)-1
h-1) - approximately a 50% increase in activity. Notably, not
only was there a difference in activity, there was also a
pronounced change in the selectivity profile. The Pd/NiO
catalyst had a preferential selectivity towards THFA (72%),
whereas Pd/TiO2 favoured FA (67 %). Catalytic
performance testing of the bare supports was also
performed as a control experiment. Here, under analogous
testing conditions there was no observable conversion of
furfural on TiO2. However, NiO showed a minor level of
activity (39 (metal mol)-1 h-1) with THFA found as the major
The Pd/TiO2 and Pd/NiO catalysts were found to have
comparable Pd particle sizes of 2.7 and 3.1 nm, respectively.
Considering that smaller Pd nanoparticles have a larger
available surface area the difference in catalytic activity
cannot be rationalized by this effect. In this instance, the
catalyst with larger particles, and therefore smaller surface
area, has the greatest catalytic activity. To probe the
properties of the supported nanoparticles in further detail
a high-angle annular dark-field scanning TEM (HAADF-
STEM) study was performed (Figure 1). The Pd/TiO2
catalyst has been characterized previously by HAADF-
STEM with the data reported elsewhere;18 to summarize,
the Pd NPs were found to exhibit a significant number of
sub-nanometer ‘clusters’, as was also found for Au NPs
prepared using an analogous low-temperature sol-
immobilization procedure.40
Figure 1. Representative low magnification STEM-
HAADF images of Pd-NiO catalyst. (ad) Montage of
HAADF image of Pd particles showing the corresponding
EELS elemental maps and RGB reconstructed overlay
maps [GreenPd: PinkNi].
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Figure 1a, shows a representative HAADF-STEM image
lattice planes with a d spacing of 2.26 Å. This is in
agreement with the expected spacing of the (111) planes of
face centered cubic Pd metal. Additionally, high resolution
HAADF-STEM (Figure 2b) provides evidence of the
presence of a sub-nanometer species (marked by yellow
circles). This is in agreement with previous HAADF-STEM
studies on other supported nanoparticle catalysts prepared
using low temperature sol-immobilization.18,41 Figure S6
show the deconvolution of the EELS data into three
components originating from the supporting NiO material
(pink), the background (not shown) and the pure Pd
particle (green). The features present could be assigned to
Pd-M4,5 edge (for Pd with onset at 335 eV), the oxygen K
edge (onset at around 532 eV) and the Ni-L3,2 edge (at 855
eV).
of the Pd/NiO catalyst. In HAADF imaging, the intensity is
approximately proportional to the square of the atomic
number of the scattering element and therefore provides
information of the local composition. In Figure 1a, the Pd
particles (Z = 46) are clearly revealed as bright features on
the nickel oxide matrix (Z ≈ 36). This interpretation was
confirmed by using electron energy loss spectroscopy
(EELS). The EELS maps, Figure 1b-d, illustrate the presence
of well-dispersed Pd nanoparticles (Pd-M4,5 map in color
green) on NiO (Ni-L3,2 map in color pink). The NiO appears
as a collection of agglomerated platelets with diameters
between 20 and 30 nm. Atomic resolution STEM images of
a selected area (Figure 2a) reveals a highly crystalline
particle with multi twinning planes and clearly resolved
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Table 1 Catalytic activity testing for the hydrogenation of furfural at 50°C and TEM particle size data.
Catalysta
Activityb Selectivity (%)c
Pd particle sizee
FA
THFA
2-MF 2-MTHF
Furan
Ethers
Median
(nm)
Deviation
(nm)
1
2
TiO2
-
-
-
-
-
1
-
-
5
-
-
-
1
1
-
-
-
5
-
-
NiO
39
470
321
4d
14
67
87d
72
23
4d
3
-
-
1% Pd/NiO
1% Pd/TiO2
3.1
2.7
0.5
0.9
-
-
FA = furfuryl alcohol, THFA = tetrahydrofurfuryl alcohol, 2-MF = 2-methylfuran, 2-MTHF = 2-methyltetrahydrofuran. a Reaction
conditions: Furfural = 0.3 M; F/metal ratio=500 mol/mol, 50°C, 5 bar H2, solvent 2-propranol. b Mol of furfural converted per hour
per mol of metal, calculated after 15 min reaction mol (metal mol)-1 h-1. Selectivity at 60% conversion. Selectivity at 10%
c
d
conversion. e Calculated using 300 particles. Histograms shown in Figure S5.
intensity of the absorption spectra; the data is normalized
to mass of Pd and the reduced intensity for the Pd/NiO
data is indicative of a smaller available Pd surface area in
agreement with the TEM data. The signals at 2120 cm-1 and
2086 cm−1 are ascribed to CO linearly bound to Pd+ and Pd
particle corners, respectively.42 The signal at 1985 cm−1 is
related to bridge-bonded CO on Pd facets, whereas that at
1945 cm−1 is attributed to bridge-bonded CO on Pd edges.42
The shoulder at 1865 cm-1 can be assigned to 3-fold sites.43
The spectrum of 1 wt% Pd/NiO has additional bands at 2165,
2140, and 1927 cm-1. The band at 2165 cm-1 is attributable to
CO adsorption on NiO,44 with the band at 1927 cm-1
Figure 2. High-resolution STEM-HAADF images of 1 wt%
assigned to bridge-bonded CO on Pd facets.45 The FTIR
spectra with CO probe molecule adsorption do not reveal
profound differences in the surfaces of the Pd
nanoparticles of both catalysts. Previously, the ratio of
linear to bridge-bonded CO has been used to infer the
likelihood of parallel or perpendicular furfural adsorption
and then correlated to the selectivity of furfural
hydrogenation. In the spectra presented in Figure 3, there
are only minor differences in the linear/bridged ratio (0.28
and 0.32 for Pd/NiO and Pd/TiO2, respectively), which
confirms that the observed difference in selectivity for the
hydrogenation of furfural is not linked to the surface
characteristics of the Pd nanoparticles. This observation
led to the hypothesis that there are distinct mechanistic
Pd/NiO catalyst showing a crystalline particle (a) and sub
nanometer species of Pd distributed across the support
(b).
Although particle size is a valuable descriptor for
catalytic activity there are other effects to consider.
Notably, the distribution of discrete sites on metal
nanoparticle surfaces and their inherent ability to promote
catalytic transformations is also of significant importance.
The surface characteristics of the Pd/TiO2 and Pd/NiO
catalysts were assessed by a combination of CO adsorption
and Fourier transform infrared (FTIR) spectroscopy
(Figure 3).
Before assigning the vibrational absorption bands on
both catalysts there is a clear difference between the
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pathways for both catalysts that are directed by differences
in the metal-support interaction.
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Figure 4. The optimised structures of furfural adsorbed
on (a) Pd(111), (b) NiO(110), (c) TiO2(111) and (d)
Pd16/NiO(110) surfaces. For clarity, the furfural molecule
and the first two layers of the surfaces are shown as ball
and stick and lower layers are shown in the CPK format.
Figure 3. CO adsorption FTIR studies for Pd/NiO (black
line) and Pd/TiO2 (red line). Peaks assigned in blue are
present in both catalysts with those is red and black
assigned solely to Pd/NiO and Pd/TiO2 respectively.
The DFT study confirmed distinct differences on the
nature of furfural adsorption on Pd, TiO2, NiO, and Pd16–
NiO systems. These observations, combined with the
selectivity profile of bare NiO, suggested that the
difference in activity is associated with how NiO is able to
activate furfural. In this scenario, it was hypothesized that
the sole role of Pd on the Pd/NiO catalyst is to dissociate
hydrogen. This hypothesis is consistent with recent
hydrogenation studies on Ru/NiO/Ni(OH)2/C catalysts,
where an analogous mechanism was proposed.49
Considering the facile nature of hydrogen dissociation
(Ebarrier = 0.030 eV and 0.854 eV for the reverse reaction),
which agrees well with our previous studies50 (See Figure
S11) over Pd surfaces. Therefore, this process would not be
expected to be rate limiting. By extension, the catalytic
activity would not have a linear correlation with the Pd
metal loading (surface area), i.e. fast hydrogen dissociation
on Pd surfaces, with slower hydrogenation of furfural on
NiO surfaces. To test this hypothesis two further catalysts
were prepared – 0.1 wt% Pd/TiO2 and 0.1 wt% Pd/NiO. The
Pd loading was confirmed using AAS and the particle size
assessed by HRTEM. The 0.1 wt% Pd/TiO2 and 0.1 wt% NiO
catalysts were found to have particle sizes of 2.2 and 3.8 nm,
respectively) (Figure S12).
To probe the effect of the support in greater detail a
density functional theory with dispersion correction
(DFT+D3) study into the adsorption energetics of furfural
on Pd(111), NiO(110), TiO2(111) and Pd16–NiO(110) was
undertaken. We first compare the calculated adsorption
energies (Ead) for the interaction of furfural on Pd(111) and
NiO(110) surfaces. All the optimized structures of furfural
on Pd(111) and NiO(110) surfaces are shown in Figure S7 and
Figure S8. Our calculations show that the furfural molecule
with a slanted configuration is more stable on NiO(110)
surface (Ead = -2.572 eV) as compared to its adsorption on
Pd(111) surface (Ead
= -1.649 eV) with a parallel
configuration. Previous studies have also shown that the
parallel configuration of furfural on Pd(111) surface is more
stable than its other possible orientations and our
calculated adsorption energy value for the adsorption of
furfural on the Pd(111) surface is comparable with the
previously reported values.18,46–48 In the former case, the
furanic ring is adsorbed via the O and C atoms of the -CHO
group and O-atom of the furanic ring (See Table S2) but in
the latter the interaction is via the C-atoms of the furanic
ring and O-atom of the -CHO group. Similar calculations
are also performed on TiO2(111) and on a single atomic layer
of Pd adsorbed on NiO (110) surface i.e., Pd16–NiO(110)
(Figure S9 and S10, Table S3). On the TiO2(111) surface it is
seen that the furfural molecule has a unique mode of
interaction; it is adsorbed through two C-atoms of the
furanic ring with two O-atoms on the surface leaving the –
CHO group non-interactive with the TiO2(111) surface. On
the other hand the adsorption of furfural on Pd16–NiO(110)
is via the furanic C and aldehydic C and O-atoms. The
calculated adsorption energy of the furfural molecule on
TiO2(111) and Pd16–NiO(110) are -4.113 eV and -1.946 eV,
respectively. Figure 4 shows the most stable structures of
furfural on Pd(111), NiO(110), TiO2(111) and Pd16–NiO(110)
surfaces.
Figure 5. a) HAADF Signal, b) HAADF signal f physical
mixture of 1 wt% Pd/TiO2 and NiO after reaction.
The lower loading supported Pd catalysts were assessed
using the same testing procedure reported earlier with the
data presented in Table 1. The 0.1 wt% Pd/TiO2 catalyst
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had an activity of 31 mol (metal mol)-1 h-1, an approximate
to confirm that the enhancement afforded by the NiO
2
10 fold reduction in performance compared to the 1wt%
Pd/TiO2 (321 mol (metal mol)-1 h-1) (Table 2). For the
Pd/TiO2 catalysts, both the hydrogen dissociation and
furfural adsorption happen on the Pd surface. As the Pd
loading decreases there is concomitant, and in this case,
equal reduction in Pd surface area and as a consequence
the rate of turnover. Conversely, the 0.1 wt% Pd/NiO
catalyst shows only a minor loss of activity compared to the
1wt% Pd/NiO catalyst – 421 mol (metal mol)-1 h-1 vs. 471 mol
(metal mol)-1 h-1. The inference is that the hydrogenation
mechanism of furfural is indeed different on Pd/NiO
compared to Pd/TiO2. This supports the proposed
hypothesis that the role of Pd is to dissociate hydrogen and
that the role of NiO is to bind and activate the substrate. It
is worth stating that a minor amount of hydrogenation of
furfural on the Pd surface is also possible, as is suggested
by the subtle change in selectivity profile compared to
NiO. However, seeing as the activity is maintained at such
low Pd loadings it would infer that the vast majority of
furfural hydrogenation occurs on the NiO.
support was linked to the Pd NiO interaction. The catalytic
activity of the physical mixture proved different to that of
the Pd/TiO2 alone. The activity increased from 321 to 436
(metal mol)-1 h-1, a value greater than the addition of the
activity of both separate components (1wt% Pd/TiO2 and
NiO). Furthermore, there was also a change in the
selectivity to THFA, increasing from 23 to 33%. The
enhancement observed for the physical mixture of 1wt%
Pd/TiO2 and NiO can be explained by the detachment of
Ni during the hydrogenation reaction, which decorate Pd
nanoparticles as found with HAADF STEM EDX studies
(Figure 5). To assess further, whether this increase in
performance could be a result of long-range hydrogen
spillover, a control experiment of the 0.1 wt% Pd/TiO2 with
NiO was also performed. Here, the physical mixture
showed broadly consistent catalytic performance to the 0.1
wt% Pd/TiO2, with no discernible increase in rate; the
dissolution and re-adsorption of Pd is negligible at these
low loadings so no promotional effect is observed. These
two control experiments confirmed that the dual-site
hydrogenation mechanism found for Pd/NiO requires the
intimate contact of both Pd/NiO.
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A further series of control experiments were then
performed to assess the hydrogen spillover in greater
detail. Initially, the catalytic activity of a physical mixture
of 1wt% Pd/TiO2 and NiO was assessed; the intention was
Table 2 Catalytic activity testing for the hydrogenation of furfural at 50°C and TEM particle size data for lower
loading Pd/TiO2 catalysts and control experiments.
Catalysta
Activityb Selectivity (%)c
Pd particle sizee
FA
THFA 2-MF 2-MTHF
Furan
Ethers
Median
(nm)
Deviation
(nm)
1
2
0.1 % Pd/TiO2
31
65d 20d
2d
2
-
-
2d
5
3d
-
-
2.2
0.4
0.1 % Pd/TiO2
NiO
+
45
50
38
2
1% Pd/TiO2 + NiO
0.1% Pd/NiO
436
421
56
10
33
81
-
-
2
7
-
-
3
-
1
1
-
-
3.8
1.2
FA = furfuryl alcohol, THFA = tetrahydrofurfuryl alcohol, 2-MF = 2-methylfuran, 2-MTHF = 2-methyltetrahydrofuran. a Reaction
conditions: Furfural = 0.3 M; F/metal ratio=500 mol/mol, 50°C, 5 bar H2, solvent 2-propranol, b Mol of furfural converted per hour
per mol of metal, calculated after 15 min reaction mol (metal mol)-1 h-1. Selectivity at 60% conversion. Selectivity at 10%
c
d
conversion. e Calculated using 100 particles. Histogram shown in Figures S7.
Figure 6. Reusability study for 1 wt% Pd/NiO detailing
changes to conversion and selectivity over 8 cycles,
reaction time 5h. Recycling experiments were performed
by reusing the catalyst in the next run without any
additional pretreatment.
The initial catalytic activity screening has found that
Pd/NiO offers a unique dual site mechanism for the
hydrogenation of furfural. To investigate the catalyst
further a catalytic reusability study, over 8 cycles, was
performed (Figure 6). The reusability study found that
there was an initial reduction in the conversion of furfural,
reducing from ~ 65 to 50% between the first and third runs,
with a more gradual decline in conversion to ~ 40% by the
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eighth cycle. The final three catalytic screening cycles show
Analysis of the extended X-ray absorption fine structure
a plateau region with no further reduction in catalytic
activity. Throughout all the reusability studies, the
selectivity to THFA remains broadly constant indicating
the same mechanistic pathway dominates for every cycle.
It has been established that the hydrogenation of furfural
on Pd/TiO2 deactivates through the formation of Pd
carbide, which promotes a less advantageous adsorption of
furfural.18 To test if the formation of Pd carbide is
responsible for this deactivation ex situ X-ray absorption
fine structure (XAFS) measurements were performed at
the Pd K-edge before and after catalytic testing (Figure 7).
(EXAFS) was also performed with the results shown in
Table S1 and Figure 8. The EXAFS fitting parameters for the
fresh Pd/NiO catalyst confirms that Pd is present as
metallic Pd nanoparticles with an oxidised surface layer.
There are no Pd-Pd scattering paths at longer distances,
indicative of PdO, needed to model the data, and we can
therefore discount the presence of large PdO crystallites.
The EXAFS data for the used Pd/NiO catalyst can be
modelled using a single Pd-Pd scattering path. The
expansion of the Pd lattice from 2.74 Å to 2.79 Å is
consistent with that previously reported for bulk Pd
carbide formation.51 The reduced Pd-Pd coordination
number of the used Pd/NiO catalyst from 12 to 9 is
indicative of small nanoparticles (<5 nm) of Pd.
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Figure 7. Pd K edge XANES of 1% Pd/NiO before and after
reaction and a reference Pd foil.
The X-ray absorption near edge data (XANES) shown in
Figure 7 demonstrate that the height of the main edge of
the fresh Pd/NiO catalyst is greater than both the Pd foil
and used Pd/NiO. This difference in height can be
attributed to a greater degree of oxidized Pd in the fresh
Pd/NiO catalyst. Small Pd nanoparticles have a high
proportion of surface sites that readily adsorb oxygen on
exposure to air. The height of the main edge for the used
Pd/NiO catalyst is the same as the Pd foil, confirming the
presence of Pd0. There are two features in the XANES
spectrum of the used Pd/NiO indicative of Pd carbide;
there is a broadening of the first peak maximum and a shift
to lower energy of the second peak maximum as a result of
lattice expansion. The shift in the position of the second
peak maximum is observed for both hydridic and carbidic
forms of Pd, however, the broadening of the first peak is
only observed for Pd carbide.51,52 Furthermore, the
insertion/removal of hydride into the Pd lattice is such that
without an overpressure of hydrogen present any
incorporated hydride would quickly diffuse out from the
Pd nanoparticle. The lack of oxygen coordination on the
Pd surface, inferred from the XANES, is consistent with
previous studies where heteroatoms are inserted into the
Pd lattice.53
Figure 8. k2 weighted Fourier transform EXAFS data with
simulated fit for (a) the fresh Pd/NiO and (b) the used
Pd/NiO.
The XAFS data confirms that there is a structural change
during furfural hydrogenation on Pd/NiO that is
consistent with the formation of Pd carbide. Previously, it
has been shown that there is a less favourable binding of
furfural on carbidic Pd, which is responsible for the loss of
activity during reusability studies. However, in the case of
Pd/NiO, it has been established that the activity arises
from furfural adsorbed on the NiO surface. In this instance
it is proposed that the loss of activity stems from the
reduced ability of Pd carbide to dissociate hydrogen, which
has also been found previously.54
CONCLUSION
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This study reports a synergistic interaction between Pd
The authors wish to acknowledge Diamond Light Source
for provision of the beamtime (SP10306). The RCaH are
also acknowledged for use of facilities and support of their
staff. AC acknowledges the use of Athena at HPC
Midlands+ in this research, which was funded by the
EPSRC (grant EP/P020232/1) via the EPSRC RAP call of
spring 2018 and 2019. Supercomputer Wales is also thanked
for the computing time. This work also used ARCHER – the
NPs and NiO that is responsible for directing the selectivity
of furfural hydrogenation, alongside being able to maintain
catalytic activity at low Pd loading. This study proposes
that the hydrogenation of furfural proceeds via a dual-site
hydrogenation mechanism with the following steps: i)
hydrogen dissociation on Pd surfaces, (ii) spillover of Hads
onto the NiO support, (iii) parallel adsorption of furfural
through both the furan ring and alcohol group onto the
support, and (iv) hydrogenation of furfural on the NiO
surface to yield THFA as the major product.
UK
National
Supercomputing
Service
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(http://www.archer.ac.uk) via the membership of the UK’s
HEC Materials Chemistry Consortium, which is funded by
EPSRC (EP/L000202). PW acknowledges the UK Catalysis
Hub Consortium and EPSRC (Grants EP/K014706/1,
Initially, the nature of COads on surfaces of Pd NPs
through FTIR spectroscopy was used to assess the available
catalytic sites. However, although there were subtle
differences the changes were not distinct enough to
account for the different selectivity profiles of the 1 wt%
Pd/TiO2 and 1 wt% Pd/NiO samples. This led to the
hypothesis that the NiO surface could be responsible for
furfural adsorption and hydrogenation for the Pd/NiO
catalyst. In this scenario, the role of Pd would be to
dissociate hydrogen, which subsequently spills over onto
the NiO support. Considering that hydrogen dissociation
on Pd is a facile process, and unlikely to be rate limiting,
we determined that the dual-site hydrogenation
mechanism proposed could sustain similar activities at
reduced Pd loading.
EP/K014668/1,
EP/K014854/1,
EP/K014714/1,
and
EP/I019693/1). PW and KM acknowledge the STFC GCRF
START project for funding the position of KM
(ST/R002754/1).
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AUTHOR INFORMATION
Corresponding Author
* Email: p.p.wells@soton.ac.uk.
* Email: alberto.villa@unimi.it.
Author Contributions
2016,
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authors. All authors have given approval to the final version of
the manuscript.
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