Palladium islands on iron oxide nanoparticles for hydrodesulfurization catalysis

Article abstract of DOI:10.1039/c8cy00088c

A four-fold increase in palladium (Pd) mass-based hydrodesulfurization (HDS) activity was achieved by depositing Pd species as nanosized islands on 12 nm colloidal iron oxide (FeO_x) nanoparticles via the galvanic exchange reaction. The highest palladium dispersion was obtained at an optimal Pd/Fe molar ratio of 0.2, which decreased when the ratio increased. The improved dispersion was responsible for the enhanced catalytic activity per the total Pd amount in the HDS of 4,6-dimethyldibenzothiophene at 623 K and 3 MPa as compared to the iron-free Pd/Al₂O₃ catalyst. The lattice strain and modified electronic properties of the Pd islands suppressed deep hydrogenation to dimethylbicyclohexyl and changed the hydrocracking product distribution. Pd nanoparticles deposited on commercial Fe₂O₃ did not provide such an activity enhancement and catalyzed significant cracking. This study demonstrates that FeO_x@Pd structures are a possible alternative to monometallic Pd catalysts with enhanced noble metal atom efficiency for ultra-deep HDS catalysis and points to their great potential to reduce the catalyst cost and move towards more earth-abundant catalytic materials.

Full text of DOI:10.1039/c8cy00088c

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Palladium islands on iron oxide nanoparticles for
hydrodesulfurization catalysis
Received 00th January 20xx,
Accepted 00th January 20xx
Ali Mansouri, Natalia Semagina
DOI: 10.1039/x0xx00000x
A four-fold increase in palladium (Pd) mass-based hydrodesulfurization (HDS) activity was
achieved by depositing Pd species as nanosized islands on 12-nm colloidal iron oxide (FeOx)
nanoparticles via galvanic exchange reaction. The highest palladium dispersion was obtained for
an optimal Pd/Fe molar ratio of 0.2, which decreased with the ratio increase. The improved
dispersion was responsible for the enhanced catalytic activity per the total Pd amount in HDS of
4,6-dimethyldibenzothiophene at 623 K and 3 MPa as compared to the iron-free Pd/Al2O3
catalyst. The lattice strain and modified electronic properties of Pd islands suppressed deep
hydrogenation to dimethylbicyclohexyl and changed hydrocracking products distribution. Pd
nanoparticles deposited on commercial Fe2O3 did not provide such activity enhancement and
catalyzed significant cracking. This study demonstrates that FeOx@Pd structures are a possible
alternative to monometallic Pd catalysts with enhanced noble metal atom efficiency for ultra-
deep HDS catalysis and points to their great potential to reduce the catalyst cost and move
towards more earth-abundant catalytic materials.
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1. Introduction
increased electrocatalytic activity of segregated Pd islands in
bimetallic palladium-tungsten NPs suggesting the lack of
necessity of a complete shell configuration for enhanced
catalysis.
Platinum group-based heterogeneous catalysts are known
for their outstanding catalytic properties among other
transition metals, but they occur at very low levels of
abundance in the earth crust. Since catalytic activity depends
The surface availability of precious metals for catalysis can be
remarkably increased by decorating them on low-cost non-
noble metal cores.3,4,26 The exceptional reactivity of such
structure was ascribed to the modification of electronic
properties and the lattice strain of active metal atoms.26–28
The electronic defects of reducible metal oxides provide
anchoring sites for active metals29,30, where the degrees of
their interactions control dispersion, morphology, and metal
reactivity.29 For example, Pt shell that formed on lattice-
defect-rich amorphous Fe NPs showed better catalytic
performance than crystalline Fe NPs 19, and in another study,
the strong metal-support interactions stabilized more
reactive Pt single atoms over iron oxide support.1 The
electronic impacts of core metal are limited to the first few
layers and decline with layer thickness27, whereas the lattice
strain effects remain for thicker layers26,27 with the possibility
of being tuned by altering the structural properties of core.28
The lattice strain affects the d-band structure and hence the
catalytic activity of heterostructured NPs by modifying the
on the surface fraction of metal nanoparticle atoms1,2
enhancing the utilization of the noble metal atoms has been
scientific and technological matter of paramount
,
a
importance.1,3,4 Accordingly, vast efforts have been invested
to increase the number of surface active atoms by
downsizing nanoparticles (NPs) to subnanometers and single
atoms 1,5–11, introducing high-index crystallographic planes12–
15, and structuring active metal as small particles (dumbbell-
like)16,17 or thin shell (core-shell)3,18–20 over individual
nanoparticles. The core-shell architecture is highly desirable
due to the possibility of tailoring structural and chemical
properties of multiple components at atomic levels within
individual NPs.3,19–25A previous work by Hu et al.4 showed the
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adsorption energy of reactants.26 However, by increasing the 500 ppm, 20 wt% of all sulfur species belong to 4,6-
layer thickness above a critical value, the overgrown layer DMDBT.61–65 Its desulfurization is hindered by the steric
dislocates to recover the strain-free prime-crystal structure, constrains from the alkyl groups, which can be lessened by
which can also change the morphology of NPs.31 This partial hydrogenation of the aromatic rings (HYD path in
explanation elucidates the fact that the activity is essentially Scheme 1). Pt-group metals attract considerable attention as
maximized at a threshold layer thickness, which in some potential catalysts for
a
second-stage ultra-deep
cases is hardly adjustable and/or stable under reaction desulfurization of fuels61,66–68 because they are highly
conditions, especially for metals such as Pd that have a high efficient hydrogenation catalysts. To the best of our
tendency to thermal sintering. For instance, a core-shell knowledge, this is the first report to produce Pd islands on
structure of Fe@Pd has changed into FePd alloy after FeOx nanoparticles and demonstrate their performance in a
catalytic combustion of methane over 530 °C.32
high-temperature catalytic application. The optimized
synthesized material can also be recommended for a variety
of reactions catalyzed by Pt-group metals, where the
efficiency of costly and scarce active metal is paramount.
Iron (Fe), as one of the utmost earth-abundant and
environmentally benign elements, is a promising candidate
as a core structure19,28,32–34 that can also facilitate recovery of
spent catalysts via magnetic separation.19,35–38 The
combinations of Fe with Pt-group metals have shown a
superior performance in a variety of catalytic applications
such as electrooxidation catalysis16,18,39, oxygen reduction
reaction16,28,33,34,40, CO oxidation1,41, hydrodechlorination42,
methane combustion32, and non-syngas-route methanol
production.43 Even a mixture of Pd and iron oxide NPs
exhibited higher activity than Pd NPs in hydrogenation of
alkyne alcohols.36 The Pd-Fe system is especially suitable for
high-temperature applications requiring palladium because
of the lower Pd surface energy and higher atomic diameter
as compared to Fe, which causes large surface segregation of
Pd and similar total surface-based activities of Pd and Pd-Fe
systems in hydrogenations.44 This promising combination has
encouraged researchers to develop different hetero-
Scheme 1. Pathways of 4,6-DMDBT hydrodesulfurization based on
ref. [67, 69, 70] with additional HCK products shown in the
Supporting Information.
configurations such as iron-oxide-supported Pd43,45
dumbbells16,17, urchin-like NPs46, Pt single atoms on FeOx
and cluster-in-cluster Pt-FexOy NPs47 besides their alloys40,48–
50 and in different controlled shapes.51
,
,
1,8
2. Experimental
2.1. Materials
Herein, we report the design of bimetallic FeOx@Pd
nanoparticles using colloidal chemistry technique in which
palladium atoms form islands on the surface of iron oxide
Palladium (II) chloride solution (PdCl2, 5 wt%), iron (III)
chloride hexahydrate (FeCl3·6H2O), polyvinylpyrrolidone
(PVP, average molecular weight of 29,000), sodium
nanoparticles. The synthesis uses
a galvanic exchange
reaction of Pd2+ reduction by Fe0 and/or Fe2+ on preformed
iron oxide seeds.52,53 The developed configuration allowed
for the improved palladium dispersion and catalytic
properties as compared to the monometallic Pd catalyst. The
catalytic performance was evaluated in hydrodesulfurization
borohydrate (NaBH4), gamma-alumina (γ-Al2O3, 150 mesh,
100 μm diameter, average pore diameter of 58 Å, BET=155
m2/g), and Fe2O3 (<5 μm) all from Sigma–Aldrich were used
for the catalyst preparation. Ethanol (95 vol.%, Fischer
Scientific), Milli-Q water, ethylene glycol (≥99.7%, Fischer
Scientific), and acetone (≥99.7%, Fischer Scientific) were
used as received. 720 ppmw sulfur as 4,6-
dimethyldibenzothiophene (4,6-DMDBT, C14H12S, Sigma-
Aldrich) was dissolved in n-decane (Fischer Scientific) as a
(HDS) of
a
refractory sulfur compound, 4,6-
model
dimethyldibenzothiophene (4,6-DMDBT), as
a
reaction in the production of fuels with ultra-low sulfur level
for either transportation or fuel cell applications (Scheme
1).54–60 When sulfur content in fuels has been pre-reduced to
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solvent containing 3.5 wt% n-dodecane (Fischer Scientific) as
the internal standard and was used as a model fuel for HDS
reactions. Ultra-high purity (99.999%) argon and hydrogen
gases were purchased from Praxair Canada.
2.3. Catalyst characterization
The palladium and iron content of the calcined catalysts
were determined using neutron activation analysis NAA
(Becquerel Laboratories–Maxxam Company, Ontario,
Canada). Gamma-ray spectrometer with a high resolution
coaxial germanium detector was used to irradiate the
samples. Transmission electron microscopy (TEM) images
were recorded using a JEOL JEM2100 device operating at 200
kV. Scanning transmission electron microscopy (STEM)
coupled with energy dispersive X-ray spectroscopy (EDS) was
performed using a field emission JEOL 2010F Transmission
Electron Microscope operating at 200 kV. Particle size
distributions (PDS) were measured by counting 200 particles.
2.2. Catalyst synthesis
Monometallic palladium and bimetallic iron-palladium
nanoparticles (FeOx@Pdn, iron oxide as
synthesized as colloidal dispersions in the presence of PVP as
a stabilizer followed by deposition on -Al2O3 by acetone
a core) were
γ
precipitation. An alcohol reduction method was used for the
synthesis of Pd nanoparticles, as described in our previous
work71. Briefly, 500 μl of the Pd precursor solution was
dissolved in 40 ml ethanol and 60 ml Milli-Q water in the
presence of PVP (PVP-to-Pd molar ratio of 30). The mixture
was refluxed for 1 h and then cooled down to room
temperature. For the synthesis of bimetallic FeOx@Pdn
catalysts, 0.14 mmol of iron(III) chloride hexahydrate was
dissolved in 100 ml of ethylene glycol containing 0.47 g of
PVP. The mixture was stirred rigorously for 30 min at room
temperature in a 500-ml three-neck flask while purging with
ultra-high purity nitrogen. After obtaining a transparent
yellowish solution, the temperature was gently increased to
413 K while purging with nitrogen. Excess amount of sodium
borohydrate was added to the solution, changing the color
to black first and then dark-green. The mixture was then
refluxed for 2 h while purging with nitrogen. After cooling
down to room tempeature, hydrogen was flown for 2 hours
followed by drop-wise addition of palladium chloride in
ethylene glycol (40 ml) within 1 h. The system was then
purged with hydrogen for an additional 2 h. No precipitation
was observed at the end of the synthesis procedure, and the
obtained colloids were clear and macroscopycally
homogeneous. All the synthesized nanoparticles were
X-ray diffraction (XRD) patterns of unsupported synthesized
nanoparticles were recorded using Rigaku Ultima IV
diffractometer equipped with a D/Tex detector, an Fe Filter,
and Co Kα radiation (λ = 1.78899 Å). The synthesized
nanoparticles were first washed with Milli-Q water and were
then dried at 333 K in air before XRD measurements. The
diffraction patterns were collected over 5 to 90 degrees on
continuous scan at 2 degrees 2θ per minute with a step size
of 0.02°. Data interpretation was done using JADE 9.6 with
the 2016 ICDD and 2016 ICSD databases.
X-ray photoelectron spectroscopy (XPS) of the calcined and
spent catalysts (after the HDS reaction for 18 h on stream)
was performed using Kratos Axis 165 X-ray photoelectron
spectrometer using Mono Al Kα source operating at 14 kV
and15 mA. Background subtraction and peaks fitting were
performed using CasaXPS software package. All the XPS
core–level spectra were corrected with C 1s at 284.8 eV.
Carbon monoxide (CO) chemisorption analyses were carried
out using a Micromeritics Autochem II 2920 apparatus
equipped with a TCD detector. About 300 mg of calcined
catalysts were pretreated by hydrogen at the temperatures
used for HDS reactions (623 K) for 1 h. This temperature is
known to ensure various Pd species reduction on the
supported catalysts75–77. The catalysts were then purged with
helium (He) at the same temperature for 1 h. After cooling
precipitated on dried γ-Al2O3 using acetone. For comparison,
monometallic Pd nanoparticles were prepared with the same
alcohol reduction method in the presence of PVP and were
deposited on commercial Fe2O3 (<5 μm). All the synthesized
catalysts were dried at room temperature for 2 h and then
373 K overnight with subsequent calcination at 673 K for 4 h
to remove the polymer stabilizer. Prior to the HDS reaction,
the catalysts were reduced in hydrogen at 623 K. Such a
calcination-reduction treatment is known to remove the PVP
stablizer from metal nanoparticles, as shown earlier72–74. The
synthesis repeatability was verified for the most
representative FeOx@Pd0.2/Al2O3 and FeOx@Pd0.6/Al2O3
catalysts, with identical characterization results for two
different batches of each.
down to room temperature,
5 mol% CO in He was
micropulsed to measure the amount of CO uptake.
Temperature-programmed reduction (TPR) was performed
using the same device. About 100 mg of the calcined
catalysts were degassed by argon (Ar) at 473 K for 2 h. After
cooling down to room temperature, TPR analysis was
performed using a 10 ml/min of 10 mol% H2/Ar at the
heating rate of 10 K/min from room temperature.
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to hydrogenation (HYD) path is the summation of
selectivities to dimethylcyclohexylbenzene (DMCHB),
2.4. Catalytic experiments
Hydrodesulfurization of 4,6-dimethyldibenzothiophene (4,6-
DMDBT) was studied under 3 MPa hydrogen pressure 623 K
using a continuous-flow fixed bed reactor, as described in
detail previously60. The calcined catalysts were diluted with
silicon carbide (mesh 120, 22:1 weight ratio) and were then
loaded in a stainless steel reactor (L=22”, i.d.=0.5”). The
dilution ensured the efficient heat conduction in the bed and
negligble effect of axial dispersion. The effect of bed dilution
on the conversion was found below the experimental error78.
The corresponding calculations, along with the verification of
the isothermal kinetic regime and near-ideal plug-flow
dimethylbicyclohexyl
dibenzothiophene
dibenzothiophene
(DMBCH),
dimethylperhydro-
dimethylhexahydro-
(DMPHDBT),
(DMHHDBT),
and
dimethyltetrahydrodibenzothiophene
(DMTHDBT).
Hydrocracking selectivity (HCK) includes benzene (BZ),
toluene (TL), cyclohexane (CH), and methylcyclohexane
(MCH). Yield of all sulfur-free products was calculated as a
product of conversion by a sum of selectivities to each S-free
bicyclic product and 50% of selectivities to each monocyclic
product. The product amounts were identified with the aid
of the internal standard in the feed. No solvent cracking
products (lower alkanes) were found in the product by GC-
MS. The internal standard/solvent peak ratio was 0.0375 for
the feed and the reacted mixture.
behavior can be found in the Supporting Information79–81
.
The catalysts were reduced in situ for 1 h before the HDS
reaction using 100 ml/min H2 at the reaction temperature
(623 K) and operating pressure of 3 MPa. A model liquid fuel
containing 720 ppmw sulfur as 4,6-DMDBT with 3.5 wt% n-
dodecane as the internal standard in n-decane (solvent) was
then introduced into the reactor at 0.05 ml/min downward
continuous-flow using a Series II high-pressure pump. Before
feeding to the reactor, the liquid feed was mixed with 100
ml/min hydrogen gas to reach the hydrogen-to-liquid molar
ratio of 16. All the feed components were in the gas phase at
the reaction conditions, as simulated by Aspen HYSYS
(Version 9). The HDS experiments were performed for 18 h
on stream including overnight catalyst stablization to reach
steady-state conditions indicated by a maximum of 5%
deviation in the conversions at 2 and 18 h on stream.
In Scheme 1 the HCK products are shown to originate from
the products of the hydrogenation path, not from 3,3’-
DMBP. The reaction kinetics and the reaction pathways were
not addressed in the current work but assumed based on
literature that reported the formation of toluene and
methylcyclohexane from the producs of the hydrogenation
path in 4,6-DMDBT HDS.69,70,78 Cracking of the DDS product
can not excluded82 but is less likely because of the very low
observed selectivity to toluene as compared to
methylcyclohexane.
The reported conversions are subject to 10% experimental
error. Two standard deviations in selectivities are 3%. The
carbon mass balance was above 95%.
Indentification of the reaction products was carried out off-
line by
a gas chromatography coupled with mass
3. Results and Discussion
spectrometry (GC-MS) using a Thermo Scientific Trace GC
Ultra, equipped with a Thermo Scientific TR-5 column (30 m,
0.25 mm, 0.25 µm film thickness). The quantitative analyses
of 4,6-DMDBT and all of the reaction products were
3.1. Catalysts characterization
Table 1 illustrates the physicochemical characteristics of
monometallic Pd, Fe, and bimetallic FeOx@Pdn catalysts,
where n is the molar ratio of Pd to Fe measured by neutron
activation analysis (NAA). The bimetallic FeOx@Pdn/Al2O3
structures exhibit higher CO uptakes than those of
monometallic Pd catalysts (Pd/Al2O3 and Pd0.2/Fe2O3), which
also increase with the decreasing Pd concentration in the
catalyst. The CO uptake of Pd0.2/Fe2O3 catalyst, prepared by
deposition of Pd NPs on commercial Fe2O3, was significantly
lower than that of the synthesized FeOx@Pd0.2/Al2O3
catalyst. These results reveal the greater dispersion of Pd
species in bimetallic FeOx@Pdn/Al2O3 catalysts, which could
not be achieved using commercial Fe2O3 with the particle
size <5 μm.
performed using
a
calibrated Agilent 7890A gas
chromatograph equipped with a H-PONA Agilent capillary
column (50 m, 0.25 mm, 0.25 μm film thickness) with high
purity helium as the carrier gas and a flame ionization
detector. The injector and detector temperatures were
maintained at 573 K. The oven temperature was held at 313
K for 2 min, and then ramped up to 573 K at 10 K/min, and it
remained at this temperature for 20 min. Examples of GC
chromatograms can be found in the Supporting Information.
According to the reaction mechanism in Scheme 1, the
selectivity to the direct desulfurization (DDS) path was
calculated based on the amount of dimethylbiphenyl (DMBP)
divided by the amount of converted 4,6-DMDBT. Selectivity
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Figures 1a-c show TEM images of the colloidal FeOx, The Pd lattice compression was also evidenced by XRD (Table
unsupported and Al2O3-supported FeOx@Pd nanoparticles. 1 and Figure S3 in the Supporting Information). This lattice
The image of the colloidal FeOx@Pd in Figure 1b shows the strain changes the filling and the energy level of Pd d-band86–
formation and deposition of ca. 2 nm Pd - nanoislands on the 88, which consequently tunes the surface free energy as well
surface of FeOx particles of ca. 12 nm in size. The spatial as chemisorption and catalytic properties of active
association of Pd and Fe was also confirmed by EDS mapping sites.26,27,86 For instance, a compressed Pt lattice downshifted
(Figure 2). Remarkably, after the calcination at 673 K and an the d-band center and thus reduced the adsorption energy of
18 h catalytic HDS reaction at 623 K, no agglomerates were carbon monoxide.27 The lattice compression of Pd in the
observed for the supported FeOx@Pd catalyst (Figure 1c), FeOx@Pd0.2 sample was greater than FeOx@Pd0.6 suggesting
while the FeOx-free Pd/Al2O3 catalyst exhibited sintered Pd smaller Pd nanoparticles and/or that more Pd atoms were
nanoparticles up to 30 nm in size (Figure S2 in the Supporting involved in the interaction with FeOx probably by formation
Information). Thus, the colloidal FeOx serves as a high- of thinner Pd islands. During synthesis, surface Fe0 and/or
surface area support for the Pd species, which prevents their Fe2+ on the iron oxide core may participate in the galvanic
sintering as opposed to Al2O3 and commercial Fe2O3 supports reaction of Pd2+ reduction to Pd(0) that formed highly-
and explains the enhanced CO uptakes (Table 1). High- dispersed Pd islands.52,53 The interaction between Pd and
resolution TEM images of FeOx@Pd/Al2O3 catalysts, taken FeOx, as evidenced from lattice compression, prevented Pd
after the catalytic reaction, show different areas on the sintering and maintained its dispersion even after high
surface of a single nanoparticle suggesting the partial temperature treatment. This finding is in line with CO
coverage of FeOx surface by Pd species (Figures 1d-f). The chemisorption uptakes (Table 1): 61% of Pd species are
black Pd-containing areas exhibit fringes with spacing of located on the surface in FeOx@Pd0.2 vs. 18% for FeOx@Pd0.6
0.20–0.23 nm, which is different from the fringes on grey and only 5% for the mono Pd. When the layer thickness
areas of 0.24–0.26 nm belonging to magnetite36,83, and the increases above a critical value,31 the overgrown layer
fringe of 0.35 nm to alumina.41 Monometallic Pd recovers the strain-free prime-crystal structure. Thus, the
nanoparticles exhibited lattice spacing of 0.23 nm, which is metal dispersion is maximized at a threshold layer thickness.
characteristic of Pd(111) planes (Figure S2 in the Supporting The highest dispersion in this work was found for Pd/Fe
Information). These lattice compression in bimetallic NPs, as molar ratio of 0.2 as compared to 0.05, 0.6, 1.5, and mono
compared to the monometallic Pd, are characteristic of Pd (Table 1).
surface Pd alloying with Fe32,50,84 and/or smaller Pd
nanoparticle size85.
Table 1. Physicochemical properties of synthesized catalysts.
Catalyst
Pd loadinga Fe loadinga
CO uptake
Pd lattice parameterc
(Å)
(wt%)
0.410
0.410
0.180
0.060
0.012
0.060
0.0
(wt%)
μmolCO/gcat molCO/molPd
Pd/Al2O3
0.0
2.03
5.92
3.02
3.43
0.60
1.34b
0.097
0.053
0.153
0.179
0.608
0.532
0.150
–
3.893
FeOx@Pd1.5/Al2O3
FeOx@Pd0.6/Al2O3
FeOx@Pd0.2/Al2O3
FeOx@Pd0.05/Al2O3
Pd0.2/Fe2O3
0.160
0.160
0.160
0.160
65.7
–
3. 885
3.826
–
–
–
FeOx/Al2O3
0.160
a measured by NAA; b CO uptake of commercial Fe2O3 is 0.44 μmolCO/gcat
on the XRD profiles of as-prepared unsupported nanoparticles
;
c calculated from the Pd(111) peak position
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Figure 1. Electron microscopy images of fresh colloidal FeOx (a) and FeOx@Pd nanoparticles (b) and spent FeOx@Pd0.6/γ-Al2O3 after HDS
reactions at 623 K (c-f).
the decomposition of Pd β-hydride formed at ambient
temperature as a result of PdO reduction.68,91,92 The peak
intensity is known to be higher for the catalysts with lower
Pd dispersion.68,91 No β-hydride decomposition was observed
for other Pd catalysts due to the higher Pd dispersion68,91, in
agreement with CO chemisorption (Table 1).
XPS analyses were performed on the fresh catalysts after
calcination in air and on the spent catalysts (after calcination,
Figure 2. STEM-EDS mapping of spent FeOx@Pd0.6/Al2O3.
reduction, and HDS reaction at 623 K) (Table 2 and Figure S4
in Supporting Information). Iron oxide sulfidation is
thermodynamically and kinetically favourable at the reaction
The TPR analyses (Figure 3) showed that the reduction of the
temperature.85,93 The sulfide detection by XPS is known to be
commercial iron oxide proceeded in two steps: a low-
masked by iron oxides at ca. Fe 2p3/2 711 eV94 but an
temperature peak at ca. 700 K (Fe2O3 to Fe3O4) and a larger
observed positive shift in the Fe 2p3/2 binding energies (BE) in
high-temperature peak at ca. 830 K (Fe3O4 to FeO).89,90 The
the spent versus calcined samples and a sulfide peak at 163
first peak was shifted by 100 K to lower temperatures in the
eV (Figure S5 in the Supporting Information) are in line with
presence of Pd due to hydrogen spillover90. The synthesized
the iron sulfide formation.95 Palladium sulfide was not
FeOx core nanoparticles deposited on Al2O3 are reduced in a
expected to be formed because the HDS reaction was
similar temperature range as the commercial Fe2O3 but with
conducted at a p(H2S)/p(H2) ratio of 10-4 while metallic Pd is
a shift towards a lower temperature, since smaller particles
thermodynamically stable at the ratio below 10-2 at 600 K.93
are reduced at lower temperatures. The XRD (Supporting
Indeed, the XPS of the spent catalysts (Table 2 and Figure S4
Information) also revealed the presence of magnetite and
in Supporting Information) revealed metallic Pd with
hematite in the synthesized FeOx@Pd materials. The TPR
negligible presence of Pd sulfides at 337 eV96. The Pd BE in
profile of the calcined Pd/Al2O3 catalyst (Figure 3) shows a
the FeOx@Pd samples shifted to higher values as compared
hydrogen evolution peak at ca. 350 K that can be ascribed to
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to Pd/Al2O3 sample, which is expected for the Pd Thus, the characterization results confirm the importance of
nanoparticles with higher dispersion.85,97 Similar Pd(0) the developed synthesis method as means to increase and
species were found by XPS in an earlier reported FeOx@Pd Pd dispersion, which was maximized at Pd/Fe molar ratio of
system with highly dispersed Pd.53
0.2. The formation of Pd islands and increased Pd dispersion
resulted in the lattice contraction. The following section
explores how such modified electronic properties affected
the catalytic function in HDS of 4,6-DMDBT at 623 K.
3.2. Catalytic performance in HDS of 4,6-DMDBT
The catalytic activity and selectivity of the synthesized
catalysts were assessed in the HDS of 4,6-DMDBT at 623 K
and 3 MPa for 18 h on stream (Figure 4 and Table 3). The
catalytic performance stabilized after 2 h on stream. Pd-free
FeOx/Al2O3 sample did not show measurable activity in the
reaction after the typical sulfidation procedure, which is
expected for iron sulfides known for hydrogenation activity96
but very low HDS activity98.
HDS of 4,6-DMDBT follows a pseudo-first-order kinetics,
including on Pd catalysts.67,99,100 Initial reaction rates in
Figure 4 were calculated according to r = –[ln(1-X)]FAo/W
equation in which X is 4,6-DMDBT conversion after 18 h on
stream, FAo is entering 4,6-DMDBT molar flow rate, and W is
the weight of Pd in the reactor. The catalytic activities of
FeOx@Pdn/Al2O3 samples depend on the Pd/Fe ratio in the
catalysts and is maximized at the ratio of 0.2, which is in line
with the maximized Pd lattice compression and Pd dispersion
(Table 1). The high Pd dispersion leads to a greater metal
atoms efficiency of the noble metal, and there is only a
narrow window of Pd/Fe ratio (below 0.5) that allows for the
dispersion and activity increase. The reaction rate per
amount of Pd is 4-fold higher for the FeOx@Pd0.2/Al2O3
catalyst, 0.06 molDMDBT/kgPd/s, compared to the
monometallic Pd/Al2O3 catalyst. The reported activity for
commercial NiMo/Al2O3 catalyst (9 wt.% Mo) in HDS of 4,6-
DMDBT at 613 K and 3 MPa was 5.07×10-3 molDMDBT/kgMo/s.82
When Pd was deposited on a commercial Fe2O3 support (< 5
μm), initial rate of 4,6-DMDBT consumption per Pd amount
was similar to the Pd/Al2O3 catalyst, which shows the
necessity of the applied Pd deposition method on the nano-
FeOx for the observed benefits.
Figure 3. TPR profiles of calcined supported catalysts. The inverted
TCD signal reflects hydrogen consumption.
Table 2. Binding energy values of Pd 3d and Fe 2p in the fresh
(calcined) and spent catalysts.
Binding Energy (eV)
Pd 3d5/2
Pd 3d3/2
Fe 2p3/2
Fe 2p1/2
Catalyst
fresh spent fresh spent fresh spent
fresh spent
Pd/Al2O3 336.3 335.4 341.5 340.6
FeOx@Pd0.2
–
–
–
–
336.7 335.7 341.9 340.9 710.9 711.1
336.6 335.8 341.9 341.1 710.8 711.0
724.0 724.1
723.8 723.9
Figure 4 also shows turnover frequencies (TOFs) as initial
rates calculated based on the amount of surface Pd, as
determined by CO chemisorption (Table 1) assuming CO:Pd
stoichiometry of 1. Although the use of FeOx@Pd structures
allowed for higher dispersion and 4-fold higher Pd mass-
/Al2O3
FeOx@Pd0.6
/Al2O3
FeOx/Al2O3
–
–
–
–
710.6 711.7 723.6 724.5
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based catalyst activity, the modified electronic properties of suppressed phenylacetylene hydrogenation as compared to
Pd in FeOx@Pdn/Al2O3 catalysts led to the decreased intrinsic monometallic Pd catalyst.102 The developed FeOx@Pdn/Al2O3
site activity for 4,6-DMDBT conversion at 350 °C (Figure 4). catalysts allowed for the efficient sulfur removal, as seen
Thiophene HDS at 400 °C was also inhibited when Pd was from the S-free product yields (Table 1), at a three-fold lower
diluted with Fe, which was assigned to the formation of less Pd loading in the second-stage hydrotreater as compared to
active Fe-Pd alloy.101
Pd/Al2O3, but they suppressed complete hydrogenation. This
effect could be beneficial for deep desulfurization of ultra-
The presence of nano-FeOx also affected the product clean gasoline to minimize octane losses103, as opposed to
distribution. As seen in the HDS reaction scheme (Scheme 1), the preferred cetane-boosting hydrogenation path in
the detected reaction products include the direct desulfurization of diesel fuels.104
desulfurization product (DDS), hydrogenation products with
and without extracted sulfur (HYD) and sulfur-free
hydrocracking products with 6- membered rings (HCK). All
Al2O3-supported catalysts yielded negligible amounts of
hydrogenated sulfur-containing intermediates of 4,6-
DMHHDBT and 4,6-DMPHDBT (Table 3), which is in line with
the earlier findings by Prins and co-workers65. Pd/Al2O3 and
FeOx@Pdn/Al2O3 catalysts showed 27±3% selectivity to 3,3’-
DMBP (direct desulfurization DDS product), 50±5% selectivity
to hydrogenation (HYD) products (Scheme 1) and 20±5%
selectivity to hydrocracking (HCK) products at similar ca. 33%
4,6-DMDBT conversion at 623 K. As compared to Pd/Al2O3,
the FeOx@Pdn/Al2O3 catalysts suppressed the complete
hydrogenation to DMBCH with a larger remaining fraction of
semi-hydrogenated DMCHB (Scheme 1), and also diminished
the formation of cyclohexane (CH) in favor of the toluene
(TL) and methylcyclohexane (MCH) (Table 3). Similarly, the
Figure 4. Initial rate of 4,6-DMDBT HDS at 623 K and 3 MPa as a
formation of Pd-Fe alloy in SiO2-supported Fe-Pd
nanoparticles, along with large bimetallic particle size,
function of Pd/Fe molar ratio.
Table 3. Catalytic performance in HDS of 4,6-DMDBT at 623 K and 3 MPa at 18 h on stream.
Yield
(%)
Selectivity (mol %)
Catalyst
DMDBT
DDS
HYD
DM- DM- DM- DM-
HCK
conversion
(%)
Total
HCK
g(cat)/mg(Pd)
in the reactor
Total
HYD
All S-
free
DM-
BCH THDBT HHDBT PHDBT
DMBP
CH
BZ MCH TL
CHB
Pd/Al2O3
0.09/ 0.37
31
33
36
6
29
27
24
34
18
12
27
4
12
17
16
24
2
1
0
0
0
7
1
0
53
48
50
39
20
13
6
1
1
2
5
5
3
1
8
6
2
0
18
26
26
27
60
24
24
26
4
FeOx@Pd0.6/Al2O3
27
29
15
0
11
11
11
26
0.18 / 0.32
FeOx@Pd0.2/Al2O3
5
0
7
0.18 / 0.11
FeOx@Pd0.05/Al2O3
0
0
9
0.18 / 0.02
Pd0.2/Fe2O3
0.18 / 0.10
11
1
10
29
5
8 | J. Name., 2012, 00, 1-3
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Remarkably, when Pd was deposited on the commercial data acquisition at the U of A. The STEM-EDS research
Fe2O3 support, the Pd/Fe2O3 catalyst showed twice-higher described in this paper was performed at the Canadian
hydrocracking selectivity of 60% even at 11% conversion vs. Centre for Electron Microscopy at McMaster University,
26% HCK selectivity at ca. 40% conversions for all Al2O3– which is supported by NSERC and other government
supported catalysts (Table 3). Iron catalysts are known for agencies.
high cracking selectivity, as reported for thiophene HDS.101
However, when the Pd islands were formed on the nano-
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TOC entry
Deposition of thin Pd islands on iron oxide nanoparticles results in a 4-fold activity
enhancement in HDS and suppresses cracking