Promotion effects of Pd on tungsten carbide catalysts: Physiochemical properties and cellulose conversion performance

Article abstract of DOI:10.1039/c6ra15819f

A multi-functional catalyst, which is able to perform both retro-aldol reactions followed by hydrogenation, is required to convert cellulose into value-added ethylene glycol (EG) in a one-pot reaction. Herein, we show the results of a catalyst comprising Pd-promoted tungsten carbide supported on activated carbon. Pd acts to hinder carbon deposition onto the catalyst surface during the carburization process, through the inhibition of methane decomposition species on the catalyst surface. XPS and TEM analysis show a cleaner surface which improves the W/C ratio. The enhanced interaction between Pd and W₂C, results in higher cellulose conversions, and higher ethylene glycol (EG) yields when compared to non-promoted or Ni promoted tungsten carbide catalysts reported elsewhere. XAS, HR-TEM and SI-XEDS analysis showed Pd nanoparticles are well dispersed on the surface and in contact with W₂C. Despite higher carburization temperatures, Pd-promoted tungsten carbide catalysts enhance the catalytic properties of the one-pot cellulose conversion reaction towards EG production when compared to Ni-promoted catalysts under the same catalytic reaction conditions.

Full text of DOI:10.1039/c6ra15819f

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Moya, D. M. Meira, D. H. Barrett, E. Teixeira Neto, A. A. D. S. Curvelo, V. Teixeira da Silva and C. B. Rodella,
RSC Adv., 2016, DOI: 10.1039/C6RA15819F.
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Promotion effects of Pd on tungsten carbide catalysts: physiochemical properties and
cellulose conversion performance
a,b
a
a,c
a
Glauco F. Leal , Silvia F. Moya , Debora M. Meira , Dean H. Barrett , Erico Teixeiraꢀ
d
b,e
f
a
Neto , Antonio Aprígio S. Curvelo , Victor Teixeira da Silva and Cristiane B. Rodella
a
Brazilian Center for Research in Energy and Materials (CNPEM) - Brazilian
Synchrotron Light Laboratory (LNLS/, C. P. 6192, 13083-970, Campinas, SP, Brazil
b
Institute of Chemistry of São Carlos (IQSC)/University of São Paulo (USP)/ C.P. 780,
CEP 13560-970, São Carlos, SP, Brazil
c
European Synchrotron Radiation Facility (ESRF), 71 Avenue des Martyrs, 38000
Grenoble, France
d
CNPEM – Brazilian Nanotechnology National Laboratory (LNNano)-CNPEM, CP 6192,
CEP 13083-970, Campinas, SP, Brasil
e
CNPEM - Brazilian Bioethanol Science and Technology Laboratory (CTBE), C. P. 6192,
1
3083-970, Campinas, SP, Brazil
f
Universidade Federal do Rio de Janeiro/COPPE/Chemical Engineering Program/
NUCAT, P.O. Box 68502, Rio de Janeiro, RJ 21945-970, Brazil
Abstract
A multiꢀfunctional catalyst, which is able to perform both retroꢀaldol reactions followed by
hydrogenation, is required to convert cellulose into the valueꢀadded ethylene glycol (EG) in
a oneꢀpot reaction. Herein, we show results of a catalyst comprising Pdꢀpromoted tungsten
carbide supported on activated carbon. Pd acts to hinder carbon deposition onto the catalyst
surface during the carburization process, through the inhibition of methane decomposition
species on the catalyst surface. XPS and TEM analysis show a cleaner surface which
improves the W/C ratio. The enhanced interaction between Pd and W C, result in higher
2
cellulose conversions, and higher ethylene glycol (EG) yields when compared to nonꢀ
promoted or Ni promoted tungsten carbide catalysts reported elsewhere. XAS, HRꢀTEM
and SIꢀXEDS analysis showed Pd nanoparticles are well dispersed on the surface and in
contact with W C. Despite higher carburization temperatures, Pdꢀpromoted tungsten
2
1

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carbide catalysts enhance the catalytic properties of the oneꢀpot cellulose conversion
reaction towards EG production when compared to Niꢀpromoted catalysts under the same
catalytic reaction conditions.
Introduction
Heterogeneous catalysts have been at the forefront of the chemical industry for over
two centuries. These catalysts are now making an impact on the new emerging economy
1
–9
based on biomass conversion to produce bioꢀfuels and chemicals . Cellulose is the most
abundant form of biomass and a key substrate in catalytic reactions and chemical processes
2
,5,6,8–10
that provide bioꢀbased products
. In the past 15 years, significant advances have
been made in developing and understanding the correlation between cellulose processing
and catalyst properties towards an efficient, profitable and environmentally friendly
2
,8–12
production of chemicals
have been shown to be a potential catalyst for lowꢀcost and sustainable ethylene glycol
EG) production in oneꢀpot cellulose conversion reactions. Many papers deal with cellulose
transformation pathways using complex and multiꢀfunctional Niꢀpromoted tungsten carbide
. Tungsten carbide catalysts supported on activated carbon
(
1
3–19
catalysts
. However, only a few give insights into the structural, electronic and surface
properties of these catalysts from catalyst preparation to the reaction steps. Our previous
2
0
work , focused on the physical and chemical effects of Ni on tungsten carbide catalysts
and revealed direct correlations between the metal promoter performance and the
carburization process. The work showed an improvement in the Ni/W C/C interaction
2
which improved selectivity towards ethylene glycol. The highly dispersed Ni improved
properties such as hydrogen spillover, lower carburization temperature and improved
ability to perform retroꢀaldol reactions. The drawback however, was enhanced methane
decomposition which deposits carbon on the catalyst surface and lowers the catalysts ability
to convert cellulose.
Although noble metals are more costly than base metals they provide exceptional
2
,10,12,21–25
hydrogenation and retroꢀaldol properties in catalytic reactions involving biomass
.
Moreover, noble metals are less thermodynamically and kinetically active towards CH (1 ≤
x
2
6–29
x ≤ 4) decomposition when compared to Ni
. This characteristic is important in
controlling carbon deposition during the carburization step during the catalyst synthesis.
2

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The use of a noble metal as a tungsten carbide promoter may offer a profitable balance
amongst catalyst cost, physical and chemical properties and catalytic activity to obtain high
valueꢀadded platform chemicals from cellulose. Thus, we turned our attention to the much
30
less explored promoter Pd . Herein, our research strategy was to perform in situ Xꢀray
diffraction (XRD), Xꢀray absorption spectroscopy (XAS) and mass spectroscopy analysis
during the carburization processes as well as Xꢀray Photoelectron Spectroscopy (XPS), N2
Physisorption, High Resolution Transmission Electron Microscopy (HRꢀTEM) and XꢀRay
Energy Dispersive Spectrum Imaging (SIꢀXEDS) to provide a comprehensive insight into
the structural, electronic and surface properties of the Pdꢀpromoted tungsten carbide
supported on activated carbon catalysts. In addition, the catalytic behavior was substantially
investigated in oneꢀpot cellulose conversion reactions for ethylene glycol production.
Experimental
Catalyst Preparation
Tungsten and palladium impregnation on activated carbon was performed using a
2
0
method previously reported . The Pd precursor (PdCl – SigmaꢀAldrich) was dissolved in
2
o
hot HCl (60 C) under magnetic stirring due to the low solubility of PdCl . The solution was
2
then evaporated and water was added and stirred for 5 min followed by evaporation to
remove the excess HCl. The final aqueous solution was prepared at room temperature and
impregnated into the W/C sample to obtain 1 and 2 wt.% Pd in the final composition.
ꢀ1
The carburization process was carried out under a total flow of 100 mL.min using
o
a mixture of CH (10% v/v) and H (90% v/v) from room temperature to 850 C at a rate of
4
2
o
ꢀ1
8
C min and held isothermally for 40 min. The CH flow was stopped after 20 min of
4
isothermal treatment while maintaining the H flow during the final 20 min followed by
2
cooling down to room temperature. The samples were then passivated under
ꢀ1
1
% (v/v) O /He flow (50 mL min ) overnight before exposure to the atmosphere. The gas
2
phase composition at the exit of the reactor was continuously monitored using a quadrupole
mass spectrometer (Pfeifer GDS 320). The samples were coded as W C/C in the case of
X
nonꢀpromoted tungsten carbide supported on carbon, 1PdꢀW C/C and 2PdꢀW C/C,
2
2
promoted with 1 and 2wt.% of Pd, respectively.
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XAS
Xꢀray absorption nearꢀedge structure (XANES) and Extended XꢀRay Absorption
Fine Structure (EXAFS) at the Pd Kꢀedge (24350 eV) were carried out at BM23 at the
European Synchrotron Radiation Facility (ESRF). Data was collected in fluorescence mode
using a 13 element Ge detector. The samples were placed in quartz capillaries of 1.5 mm
ꢀ1
ꢀ
diameter. The catalysts were carburized in situ under CH (2mL min ) and H (18 mL min
4
2
1
o
o
ꢀ1
)
up to 850 C (10 C min ) and maintained at this temperature for 15 min. CH was then
4
o
removed from the atmosphere and the catalysts were kept at 850 C for an additional
1
0 min. The sample was then cooled down under hydrogen and three consecutive EXAFS
spectra were collected and averaged for data analysis.
The EXAFS oscillations of two samples were analyzed: 1PdꢀW C/C and 2Pdꢀ
2
W C/C. EXAFS oscillations were extracted using Athena code and analyzed using Artemis
2
3
1
2
software . The range used to transform the EXAFS oscillations (k χ(k)) was ꢁk =
−
1
(
3−8.5) Å and the fits were performed in the interval ꢁR = (1.5−3.2) Å.
The local environment of the Pd atoms was determined using the phase shift and
0
amplitude functions for PdꢀPd calculated for Pd . An additional PdꢀW contribution was
included to improve the fit. The calculation was done via randomly substituting 6 Pd atoms
0
in the first coordination shell of the Pd structure by W. A test was also performed
including PdꢀC, PdꢀO or PdꢀCl with the PdꢀPd contribution.
The photoelectron energy origin correction (∆E ) and the passive electron amplitude
0
2
0
reduction factor (S ) were found for Pd standard and used as fixed parameters to fit the
0
samples. For each contribution, a coordination number (N ) and a distance (R ) were fitted
i
i
2
independently while the mean square relative displacement (σi ) were set to be the same for
all paths.
In-situ XRD
The in-situ XRD analysis measured during the carburization process was performed
at the I11ꢀHigh Resolution PXRD beamline at Diamond Light Source (UK) using a
sapphire capillary cell, hot air blower and PSD detectors. The carburization was carried out
ꢀ1
under a total flow of 20 mL min of 10% CH /H gas mixture. The samples were heated to
4
2
°
°
ꢀ1
9
99 C using a heating rate of the 8 C min in order to determine the carburization
4

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temperature of each sample. The experiments were performed at 15 KeV with 2ϴ ranging
o
from 3ꢀ150 .
Ex-situ XRD analyses were obtained at the XPD beamline at the Brazilian
Synchrotron Light Laboratory (LNLS). A wavelength of 1.62551 Å was used and the XRD
patterns were obtained via a Mythen ꢀ 1K detector (Dectris) installed 1 m from the sample.
Identification of crystalline phases was obtained by comparison to the JCPDS files. The full
width at half maximum (FWHM) of the peaks was determined using a pseudoꢀVoigt
function with the crystal size for tungsten carbide phases estimated using the Scherrer
equation.
XPS
Xꢀray Photoelectric Spectroscopy (XPS) measurements were performed at the
LNLS using an electron analyzer SPECS, model Phoibos HSAꢀ3500 150, with Al Kα
radiation (1486.61 eV), pass energy of 20 eV and 14 h acquisition. The powdered samples
were supported on copper doubleꢀsided tape and fixed on the XPS sample holder. The
spectra were analyzed using Casa XPS software, version 2.2.99. The binding energies were
referenced to the C 1s line at 284.5 eV from carbon. The curveꢀfittings used in the
deconvolution for W 4f, Pd 3d, O 1s and C 1s levels were Gaussian line shapes using the
Shirley function in the background simulation. The quantitative evaluation of each peak
was calculated by dividing the integrated areas under the peak by the atomic sensitivity
factors considering 1.0 the sensitivity factor of C 1s (binding energy of the 284.5 eV).
HR-TEM and SI-XEDS
The morphology of the PdꢀW C/C crystallites was investigated by HRꢀTEM using a
2
JEM 2100F microscope (JEOL) operated at 200 kV at the LNNano/CNPEM, with a pointꢀ
to point resolution of 0.19 nm. Samples were prepared by applying the dry PdꢀW C/C
2
powder onto a carbon film surface of standard 400 mesh TEM cooper grids. Catalyst
compositional maps were acquired by XEDSꢀSI with an AZtec 3.0 SP2 (Oxford
Instruments) system. Electron probe size of ca. 0.7 nm was used to guarantee enough
current density at each sample point, in order to obtain statistically significant Xꢀray counts
for the elements of interest. SIs were acquired by scanning an area of interest of the sample
with a pixel size of ca. 0.5 nm and a total dwell time of 0.6 s/pixel. Complete XEDꢀspectra
were acquired at each SI pixel in the 0ꢀ20 keV energy range.
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N Physisorption
2
Surface area and pore volume were determined by adsorptionꢀdesorption isotherms
o
of N at 77 K (ꢀ196 C) using a Quantachrome Autosorb 1C automatic analyzer. The wet
2
o
samples were degassed at 120 C for 12 h before the isotherms were measured. The
apparent BET surface area was determined by BET method using a multipoint method from
the data collected at the relative pressures 0.1 ≤ P/P ≤ 0.25 due to the presence of
0
micropores in the samples. The total pore volume was derived from the amount of gas
adsorbed at a relative pressure of P/P = 0.95. The pore size distribution was determined
0
3
2,33
using BJH method.
Cellulose Conversion Reactions
The cellulose catalytic reactions were conducted using a stainless steel batch reactor
(
300 mL, Parr Instruments) at 4.3 MPa of H , 150 mL of an aqueous suspension contend
2
1
g (dry basis) of cellulose (Avicel microcrystalline Merck) and 0.300 g of catalyst. Details
2
0
of the catalytic reaction are reported elsewhere . The reactions were performed at 190,
o
2
20 and 250 C and the reaction time varied from 0 to 180 min.
HPLC analysis were performed using a Waters 1525 chromatograph at the
CTBE/CNPEM equipped with Aminex HPXꢀ87H – Bioꢀrad capillary column and Acclaim
1
20 C18 Dionex column for analysis using refractive index (RI) and UV detectors.
A quantitative method using thermogravimetric analysis (TGA) has been
developed to determine the quantity of cellulose after a catalytic reaction, which was used
3
4
to calculate cellulose conversion .
Cellulose conversion (X) and product yield (PY) were determined according to the
equations:
ꢅꢆ − ꢅꢆ
ꢂꢇꢃ
ꢂꢈꢃ
ꢀ
ꢁꢂ%ꢃ = ꢄꢁ
ꢉ ꢁꢊꢁ100%
ꢅꢆꢂꢇꢃ
ꢌꢍꢎꢍ
PY (% mol C) = ꢋꢌꢏꢐꢑꢎ ꢁꢓ ꢁꢊꢁ100%
ꢂꢒꢃ
where CM is the dry cellulose mass measured prior to the catalytic reaction, CM is the
(i)
(f)
dry cellulose mass measured after catalytic reaction, n C is the amount of moles of carbon
j
j
6

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of identified product j (acetol, ethylene glycol, 1,2ꢀpropylene glycol, etc.), n C is the
cel (i)
amount of moles of carbon carried in the reactor .
3. Results and Discussion
3.1 Characterization
XAS
XAS measurements were performed to investigate the local structure and oxidation
state of Pd in the catalyst after the carburization under hydrogen flow. The Fourier
transform (FT) of the EXAFS spectra at the Pd kꢀedge of the 1PdꢀW C/C and 2PdꢀW C/C
2
2
samples are shown in the supplementary material, Fig. S5. The EXAFS quantitative
parameters derived from the fitting are presented in Table 1.
Both samples exhibited smaller coordination numbers (N) (Table 1) compared to Pd
bulk (N=12), indicating the presence of nanosized Pd particles. This is also reinforced by
2
the high value obtained for the DebyeꢀWaller factor (σ ), which is characteristic of high
degree of disorder in the Pd lattice.
Interestingly, the EXAFS simulations for both samples improved when PdꢀW
contributions were added. This indicates that palladium nanoparticles are interacting with
W in the catalyst. Moreover, an increase in the PdꢀPd interatomic distance (RPdꢀPd) was
0
observed compared to Pd (2.75 Å) (Table 1). This is in agreement with the presence of
3
5
very small Pd particles, which are exposed to hydrogen . Palladium lattice expansion
3
5–39
during thermal treatment under hydrogen has been reported
and it likely in this case
o
due to the final step of the carburization process at 850 C under H flow.
2
Table 1. EXAFS fit results for the first coordination shell of the Pd in the Pdꢀpromoted
tungsten carbide catalysts after the carburization process under H flow.
2
2
σ
PdꢀPd/PdꢀW
Sample
NPdꢀPd
*
NPdꢀW
*
RPdꢀPd (Å)
RPdꢀW (Å)
2
(
Å )
1
2
PdꢀW C/C
4 ± 2
4 ± 3
3 ± 2
3 ± 3
2.78 ± 0.01 2.91± 0.02
2.77 ± 0.04 2.90 ± 0.08
2
0
.011 ± 0.005
PdꢀW C/C
2
E0 = 4, amp = 0.83.
2
*
High values in the errors of the coordination numbers are due to the high correlation between N and σ .
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The intensity of the XANES spectra of the 1PdꢀW C/C shows enhanced damping
2
0
compared to the Pd foil in Figure 1 (b). This shows structural disorder and/or nanosize
structures, as already determined by EXAFS analysis (Fig. S6 and Table 1). Furthermore,
theoretical calculations of the palladium electronic structure indicate that the increase in
intensity of feature B is associated with the mixing of Pd dꢀstates with hydrogen sꢀ and pꢀ
3
7
unoccupied states . This may be associated with Pd nanoparticles and H interaction, as
2
also pointed out by EXAFS analysis.
Pd-W
simulated
nd
2
derivative
a)
b)
0
24364
Pd
24391
C
Pd-W
simulated
1
Pd-W C/C
2
A
B
0
Pd
1
Pd-W C/C
2
2
4340 24360 24380 24400
24340 24360 24380 24400
Energy (eV)
Energy (eV)
0
Figure 1. (a) Experimental XANES spectra of Pd and 1PdꢀW C/C sample and simulated
2
XANES spectra of PdꢀW and (b) corresponding second derivatives.
Feature A, at lower energy is assigned as a dꢀpeak, associated with 1s → 4d transitions, giving rise to preꢀ
40
edge features in the XANES spectra .Features B and C are assigned to pꢀpeaks, which correspond to s → p
40
transitions . The main feature (B) corresponds to the absorption edge and reflects dipoleꢀallowed Pd 1s ꢀ 5p
transitions. Peak (C) is associated with shakeup processes involving Pd 1s ꢀ 5p transitions or multiple
41
36
scattering and also 5p → 4f transitions .
Comparing the positions of feature B, the 1PdꢀW C/C catalyst coincides with the
2
simulated PdꢀW specie, agreeing with the presence of Pd nanoparticles in contact with W in
the sample. Additionally, an energy reduction in feature C could point to a slightly
3
6
increased electron density in the external orbitals of the Pd , which is related to the
interaction of Pd and hydrogen.
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XRD
In-situ XRD analyses were conducted to investigate the influence of Pd on the
carburization temperature, carbide phase formation and crystallinity of the samples.
In general, the carburization process follows the steps: WO →WO
3
x
2
0,42–45
(
3>x>2)→WO →W→W C→WC
. The in-situ XRD results showed the formation of
2
2
o
o
metallic W at ≈ 561 C and the carbide W C phase transition occurring at ≈ 805 C (Fig. 2).
2
The W C phase can clearly be identified as well as the peaks related to Pd species.
2
*
W C
2
*
O
Pd
* O
*
*
*
*
* 8
43
O
*
805
700
W
561
120
26
16 18 20 22 24 26 28 30 32 34 36 38
2
Θ
Figure 2. In-situ carburization of Pd promoted tungsten carbide catalyst supported on
activated carbon (2PdꢀWxC/C). XRD patterns were collected at a wavelength, λ=
0
.82656Å.
2
0
In a previous work , in-situ XRD analysis of the nonꢀpromoted tungsten carbide
o
catalyst showed that metallic tungsten was formed at higher temperature (≈ 690 C) with
9

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o
carbide formation happening at similar carburization temperature (≈ 850 C). Moreover, a
mixture of three nanosized carbide phases W C, WC , and WC were formed. However,
2
1ꢀx
o
when Ni was used as a promoter, the carburization temperature was reduced by 100 C and
much like the Pdꢀpromoted catalysts studied here, only the W C phase was formed.
2
o
XRD patterns of all samples after the carburization process at 850 C are shown in
Fig. 3. The predominant carbide phase in the promoted catalysts is W C, while in absence
2
of palladium, a mixture of carbide phases are formed. Nanoꢀcrystalline palladium is also
formed during the carburization, corroborating with XAS results.
*
W C
*
2
WC
Pd
o
* o
*
*
*
o
o
*
* *
o
(c)
b)
(
(a)
30
35 40 45 50 55 60 65 70 75 80 85
2θ
Figure 3. XRD analysis of (a) W C/C; (b) 1PdꢀW C/C; (c) 2PdꢀW C/C. XRD patterns
X
2
2
were collected at λ=1.6255Å.
XPS
The W 4f coreꢀshell spectra of the catalysts are shown in Fig. S6. XPS analysis
shows only tungsten oxide species on the surface of the Pdꢀpromoted catalysts after the
carburization and passivation. Only the nonꢀpromoted sample exhibits very small doublet
peaks related to tungsten carbide species, which may indicate a thinner oxide layer on the
2
0
surface of the catalyst .
The XPS spectra in Fig. S7 shows the Pd 3d coreꢀlevel spectra characterized by two
components, Pd 3d5/2 and Pd 3d , which appear due to spinꢀorbital splitting. The noise and
3/2
1
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weak signal can be related to the low Pd loading (1 and 2wt.%) and/or carbon deposition.
The binding energies of the Pd 3d3/2 peaks around 336 and 336.7 eV, respectively, are
4
6
related to palladium oxide species , which is in accordance with the carburization and
passivation process applied in the catalysts synthesis.
Table 2: Relative surface concentration between W and C determined by XPS analysis
(±2% error).
Catalyst
W/C ratio
0.028
W C/C
x
1
2
PdꢀW C/C
0.035
x
PdꢀW C/C
0.038
x
The relative surface concentration was determined considering only the presence of
W and C on the surface. The Pd loadings are lower than the detection limit of the XPS.
According to quantitative XPS analysis (Table 2), a significant increase in the
concentration of W on the surface is observed in both of the Pdꢀpromoted tungsten
carbides, which indicates that more tungsten was exposed on the surface in the promotedꢀ
catalysts in comparison with the nonꢀpromoted.
HRꢀTEM and SIꢀXEDS analysis
Brightꢀfield (BF) TEM images of the 2PdꢀW C/C catalyst revealed a bimodal
2
distribution of particle sizes. BFꢀSTEM images (Fig. 4A, C and D) show extensive regions
with a large number of small particles (< 10 nm), homogeneously distributed over the
support, along with a second population of larger particles varying in size from 20 to
1
00 nm (Fig. 4A, B, and D). These large particles have a coreꢀshell structure, with a dark
core and a gray shell. The contrast of the HAADF image (Fig. 4E) is taken from the same
area as Fig. 4D and has been inverted, revealing a particle structure with a high Z atomic
number core surrounded by a lower Z shell. XEDS chemical mapping (Fig. 4F and S10)
and HRꢀTEM images (Fig. S9) of the larger particles show a crystalline core enriched with
W, which is surrounded by a shell formed by small crystallites giving a mixture of W and
Pd. Interestingly, Pd was dispersed widely over the sample as well as W. Importantly,
filamentous and/or graphitic carbon was not detected on the catalyst surface (Fig. S9).
1
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A)
B)
C)
E)
F)
D)
W
Mapping
Pd
Mapping
Figure 4. Brightꢀfiled STEM (A and D), BFꢀHRꢀTEM (B and C), HAADFꢀSTEM (E)
images and SIꢀXEDS chemical mapping (F) of the Pdꢀpromoted tungsten carbide catalyst.
Insert of figure (D) shows a low magnification BF image of a carbon grain, with the arrow
indicating the central particle shown in higher magnification in (D).
N Physisorption
2
Apparent BET surface area and pore volume values decreased with increasing Pd
loading in the samples (Table 3). The mean pore diameter was the same for all samples.
The adsorptionꢀdesorption isotherms are characteristic of materials comprising both micro
and mesoporosity (Fig. S1) as expected for an activated carbon (70wt%. of the catalyst).
However, the adsorption volume at low relative pressure (P/P ≤0.1), the pore size
0
distribuition (SI Fig. S2) as well as total adsorption capacity (P/P ≈1) decrease when Pd
0
loading increases. This shows that the activated carbon and consequently the catalyst
microporosity and surface are were affected by the carburization process of the tungsten.
1
2

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Table 3: N physisorption analysis.
2
Catalyst
Apparent BET Surface
Total pore
Mean pore
2
ꢀ1
3
ꢀ1
Area (m g )
volume (cm g )
0.577
diameter (Å)
W C/C
649
502
366
38
39
38
X
1
PdꢀW C/C
0.490
2
2
PdꢀW C/C
0.340
2
There are two fundamental steps in the carburization process: i) tungsten oxide
13,16,20,42,43
reduction and ii) tungsten carburization
exhibited by metals, such as Ni, Co, Pt and Pd the reduction of tungsten occurs at lower
. Due to the hydrogen spillover ability
1
3,16,18,20,42,43
temperature when compared with nonꢀpromoted catalysts
. The same trend was
observed here, the in-situ XRD analysis revealed that crystallites of metallic tungsten were
o
formed around 561 C in the 2PdꢀW C/C as well as our previous work with Niꢀpromoted
x
2
0
o
20
catalysts . In the nonꢀpromoted catalyst case tungsten was only formed above 690 C .
Furthermore, besides the reduction step, the activated hydrogen should play a role in the
second step of the carburization where it may interact with the carbon from the support
*
and/or the CH to form CH and C species. These species are also formed through the
4
x
4
7
thermodynamic decomposition of methane. According to Giraudon et al. , the species are
adsorbed onto the W surface and carbon is inserted into the W lattice to produce the carbide
phases.
Another factor to consider is the promoter interaction with CH and its effect on the
4
2
6
carburization. Density functional theory (DFT) studies by Kai Li et al. , showed that CH
x
(1 ≤ x ≤ 4) decomposition is an endothermic process. Therefore, the reaction energy as well
as the energy barrier is lower with Ni than that of Pd. Thus, the methane decomposition is
thermodynamically and kinetically favoured in a Ni promoted catalyst. This explains the
higher carburization temperature in the Pdꢀpromoted tungsten carbide catalysts when
2
0
compared to the Niꢀpromoted catalysts previously studied . The mass spectrometry results
obtained during the carburization process (Fig. S4ꢀS5) also show an increase in CH4
o
production at 850 C for the Pdꢀpromoted and the nonꢀpromoted catalyst. The methane
o
o
level at 850 C is higher than its inlet flow at lower temperatures up to 400 C. This implies
that atomic carbon from the catalyst support (C*) may be interacting with H to form CH
2
4
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3

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in the gas phase, which flows off the catalyst bed and consequently, from the catalyst
surface. This result is confirmed by TEM (Fig. 4) and XPS results (Table 2), showing that
the Pd and tungsten carbide particles are not covered by carbon with W being more
exposed on the surface in the Pdꢀpromoted catalysts. The results indicate that the main
carbon source in the carburization process of the Pdꢀpromoted catalysts is the activated
carbon itself. According to the N physisorption analysis, the microporosity was affected
2
13,20,43
by the carbothermal process
adsorption capacity (Table 3). On the other hand, an intense CH decomposition and carbon
, giving lower surface area, total pore volume and lower
x
2
0,29,48
deposition was identified in our previous work with Niꢀpromoted tungsten carbide
,
resulting in a lower carburization temperature and less sintered W C particles (Fig. S9).
2
However, Ni is also highly active in forming polymeric, filamentous and graphitic carbon
13,16,18,20,42–45
deposits on the catalyst surface, which inhibits the active sites of the catalysts
.
3
. 2 Catalytic Performance
Catalytic activity of 1 and 2 wt.% Pdꢀpromoted tungsten carbide catalysts were
o
evaluated at 190, 220 and 250 C as a function of reaction time. Conversion and product
distribution results are shown in Figures 5, 6 and 7.
The identified reaction products consisted of ethylene glycol (EG), 1,2ꢀpropelyne
glycol (1,2ꢀPG) and acetol for all reaction temperatures. In addition, glucose, sorbitol,
methanol, ethanol, 1ꢀpropanol, 1ꢀbutanol and levulinic acid were also analyzed. Glucose
was not detected in any of the experiments performed, which indicates the catalyst high
reactivity under the catalytic parameters tested.
1
4

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A)
B)
Figure 5: Catalytic activity for the conversion of cellulose and yields of EG, 1,2ꢀPG and
o
acetol for reactions at 190 C for A) 1PdꢀW C/C and B) 2PdꢀW C/C.
2
2
1
5

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A)
B)
Figure 6: Catalytic activity for the conversion of cellulose and yields of EG, 1,2ꢀPG and
o
acetol for reactions at 220 C for A) 1PdꢀW C/C and B) 2PdꢀW C/C.
2
2
1
6

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A)
B)
Figure 7: Catalytic activity for the conversion of cellulose and yields of EG, 1,2ꢀPG and
o
acetol for reactions at 250 C for A) 1PdꢀW C/C and B) 2PdꢀW C/C.
2
2
In general, conversion increased as reaction temperature increased. Taking into
account the longer reaction time, conversion improved from ~30% to ~94% when the
o
o
o
reaction temperature increased from 190 C to 220 C and reached 100% at 250 C.
o
Cellulose reactions carried out at 190 C (Fig. 5) showed similar performances for both
catalysts, giving low conversion values with a slight increase after 120 min. EG was the
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7

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major product, with a maximum yield of ~10% for both catalysts. 1,2ꢀPG, acetol, sorbitol,
ethanol and levulinic acid were also detected with yields lower than 1.5%.
These results are in agreement with previous studies of catalysts based on tungsten
o
13,49
carbide in a oneꢀpot cellulose reaction at low temperature as 190 C
. It is notable that in
the case of noble metals or bimetallic catalysts applied in cellulose hydrogenation reactions,
2
3,24
sugar alcohols, like hexitols, are the preferred product formed
o
Notably, results from the catalytic reaction carried out at 220 C showed the
conversion increasing considerably with reaction time, especially when the 2% PdꢀW C/C
2
catalyst is considered. Cellulose conversion reached 94% for 2% PdꢀW C/C in 150 min and
2
8
4% for the 1% PdꢀW C/C catalyst after 180 min of reaction.
2
Interestingly, both catalysts gave 20% conversion at time zero of the reaction. Time
zero corresponds to the reactor heating period under a low hydrogen pressure of 50 psi.
o
Once the reaction temperature was reached (220 C), the hydrogen pressure was
immediately increased to 800 psi and the reaction time taken as zero at this point. For both
samples, during this initial heating step, acetol was obtained as the major product (~4.5%
yield) and EG production was negligible.
The conversion increased ~2.5 times after 30 min of reaction. The product
distribution changed due to hydrogen availability, as expected, but also because of the Pd
content in the catalyst (Fig. 6 A and B). When the 1PdꢀW C/C sample was used, EG yield
2
increased, especially during the 60 min of reaction (~28%), reaching its highest value ~37%
after 150 min (84% of conversion). Acetol yield followed the opposite trend, forming the
major product with a yield of ~15% in the first 30 min but decreased to ~4% after 150 min.
Over the same reaction time interval 1,2ꢀPG increased from ~2 to 7%. It is well known that
1
5
acetol is an intermediate in 1,2ꢀPG formation from sugars and cellulose . Thus, acetol
1
7,30,50
was hydrogenated to 1,2ꢀPG, in agreement with literature
.
In the case of the 2wt.% Pdꢀpromoted catalyst, EG production was more favored.
EG increased from ~24 (30 min) to ~58% in 150 min of reaction while the conversion
varied from 51 to 93%. Moreover, acetol depletion was followed by the 1,2ꢀPG formation
with more pronounced formation after 90 min of reaction.
Ethanol (~6%) and levulinic acid (~4%) was observed as secondary products with
similar yields for both catalysts. Methanol, 1ꢀpropanol and 1ꢀbutanol were also detected in
1
8

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trace quantities. The presence of shorter chain alcohols, organic acids and other products
such as furans and CO can be attributed to subsequent degradation reactions of
2
monosaccharides as well as reaction intermediates, which can undergo further hydrolysis
13,19,51,52
and hydrogenolysis of C–O and C–C bonds under the reaction conditions
.
o
Reactions performed at 250 C with both catalysts gave 100% cellulose conversion
after 30 min of reaction. Acetol was the main detected product during the reactor heating
step (at time 0). Since H partial pressure is low (50 psi) the catalyst containing 2wt.% of
2
Pd exhibited higher EG yields compared to 1wt.% of the Pdꢀpromoted catalyst. This
tendency was emphasized once the H pressure was increase and at longer reaction times. It
2
indicates a higher hydrogenation efficiency of the catalyst when 2% of Pd was used in the
tungsten carbide catalyst. On the other hand, acetol hydrogenation to 1,2 PG and other
o
degradation products also took place at 250 C for both catalysts but was more pronounced
in the case of the 1PdꢀW C/C catalyst. It was also observed that an increase in the quantity
2
of secondary products when compared at milder temperatures. Yields of levulinic acid, 1ꢀ
butanol, 1ꢀpropanol, ethanol and methanol were approximately 8%, 2%, 4%, 3% and 1%
respectively, with similar results for both catalysts.
The catalytic conversion of cellulose in a oneꢀpot reaction involves various reaction
pathways as showed in Scheme 1. Firstly, cellulose is hydrolyzed to form oligomers and
+
glucose. This step is controlled by the H ions generated from the hot water and also from
1
0,20
the acidic surface properties of the catalyst
. Secondly, glucose can be transformed into
a variety of intermediates and products by various side reactions. These routes are strictly
dependent on the catalytic properties of the catalyst as well as the reaction parameters
1
5,17,19–21,50,53,54
.
Supported metallic catalysts such as Ru/C and Pt/C readily transform glucose into
sugar alcohols by the reduction of the carbonyl groups on the glucose with H on the
2
metallic active site leading to the formation of hydrogenated products such as sorbitol and
10,23,24,55
mannitol
. Here, sorbitol was detected in a concentration lower than 1.5%,
indicating that the direct glucose hydrogenation was not favored under the reaction
conditions.
Metal promoted tungsten carbides are considered multifunctional catalysts
providing high EG yields from cellulose in oneꢀpot reactions. The catalyst is selective in
1
9

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transforming glucose into glycolaldehyde through a retroꢀaldol reaction catalyzed by
tungsten carbide. Secondly, the glycolaldehyde can be hydrogenated to EG over the
1
3,15,20,21,24,53
metallic promoter and W C active sites of the catalyst
. Glucose can be also
2
isomerized to fructose in the reaction conditions. Furthermore, acetol can be obtained by
15
fructose transformation through a retroꢀaldol reaction on the tungsten carbide sites . This
o
was observed at time zero of the reaction at 220 and 250 C (Fig. 6 and 7), exhibiting acetol
as the major product. Once hydrogen pressure is added in the catalytic reaction, acetol and
glycoaldehyde (not identified) are hydrogenated to 1,2ꢀPG and EG, respectively by Pd and
W active sites.
OH
H
H
H
HO
OH
H
H
O
OH OH
O
HO
OH OH
O
O
OH
H O
H
OH
HO
OH
HO
H
H
H
OH OH
Mannitol
OH
OH OH
Sorbitol
Cellulose
n
H
2
H
2
O+
H
2
H
3
Hydrolysis
HO
OH OH
OH
O
Isomerization
O
OH
HO
OH OH
Glucose
OH OH
Fructose
RetroꢀAldol
RetroꢀAldol
OH HO
HO
O
H
H
2
2
OH
OH
OH
OH
OH
O
OH
OH
O
Ethylene Glycol
Glycolaldehyde
OH
O
O
HO
O
2
ꢀ3H O
OH
Dihydroxyacetone
Glyceraldehyde
OH
OH
HMF
HO
O
HO
OH
ꢀ
H
2
O H
2
H
2
OH
OH
Fructofuranose
Erythritol
Erythrose
O
O
OH
O
HO
HO
H2
OH
RetroꢀAldol
OH
Levulinic Acid
Acetol
Glycerol
HO
H2
O
H2
2
OH
2
OH
2
ꢀH O
Ethylene Glycol
2
ꢀH O
Glycolaldehyde
OH
O
O
HO
,2ꢀPropylene Glycol
1
αꢀAngelica Lactone
1
5,19
Scheme 1: Reactions pathways in cellulose conversion, adapted from
.
The stability of the major products, EG and acetol, were tested under the catalytic
conditions given in table 4. EG degraded somewhat over time but was much more stable
than acetol. Acetol underwent hydrogenation to 1,2ꢀPG in high proportions. Our results are
1
5,21,50
in agreement with previous studies with tungsten based catalysts
.
2
0

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Table 4: Catalytic activity of PdꢀW C/C over different substrates. Reactions conditions:
2
o
2
20 C, 150 min, 4 MPa of H .
2
Conversion
Product Yield (%, mol C)
MB*
Substrate
(
%)
84
71
6
EG
1,2ꢀPG
6.6
Acetol
Ethanol
Lev. Ac.
Cellulose
Acetol
EG
36.9
1.7
6.1
1.7
ꢀ
6.0
60.3
58.4
ꢀ
ꢀ
ꢀ
55.4
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
*MB = Mass Balance. Low quantities of methanol, 1ꢀpropanol and butanol were detected which are not
reported in the table.
The contribution of tungsten and palladium species on the conversion of cellulose
over activated carbon, nonꢀpromoted tungsten carbide catalyst (W C/C) and 1% wt. Pd
x
supported on activated carbon (1Pd/C) was also performed. The activated carbon, used as
the catalytic support, was also tested in order to determine its contribution to cellulose
conversion. The results are show in Table 5.
Table 5: Cellulose conversion and product yields over different catalysts. Reaction
o
conditions: 220 C, 150 min, 4 MPa H .
2
Conversion
Product Yield (%, C mol)
Entry Catalyst
(
%)
89
73
86
93
26
EG
1.7
1.7
36.9
58.2
0
1,2ꢀPG
0.3
Acetol
22.5
20.5
1.7
Ethanol
1.1
Lev. Ac.
0.6
MB*
28.6
24.7
60.3
67.5
8.3
1
2
3
4
W
x
C/C
1% Pd/C
0.1
0.0
1.2
1PdꢀW
2PdꢀW
C/C
C/C
6.6
6.1
6.0
2
2
4.9
0.4
0.1
3.6
A
5
C
0
8.0
0
0.3
A
6
0 min of reaction.
*MB = Mass Balance. Low quantities of methanol, 1ꢀpropanol and butanol were detected which are not
reported in the table.
Despite providing relatively high and comparable cellulose conversion, both Pd/C
and W C/C catalysts showed poor selectivity to EG (Table 5, entries 1 and 2). The
x
combination of both metals to obtain a Pdꢀpromoted tungsten carbide catalyst (1PdꢀW C/C
2
and 2PdꢀW C/C) were able to achieve a high EG yield as shown in Table 5 (entry 3 and 4).
2
2
1

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However, both tested catalysts performed retroꢀaldol reactions to produce acetol from
cellulose (Entry 1 and 2). This was not expected from the 1% Pd/C catalyst due to its
1
0,23,24,55
hydrogenation ability to convert glucose into sugar alcohols
. However, the lower
Pd concentration as well as the lower hydrogen pressure used seems to disfavor
hydrogenation reactions. The activated carbon used as the support in all samples, when
applied in a pure form in the cellulose conversion, gave low conversions (26%) and
produced only a small quantity of acetol (8% yield).
Catalyst reusability is a key factor for industrial applications and due to the variety
of recovery procedures and additional structural and catalytic investigations it should be
considered as a separate study. Here, the evaluation of the potential reusability of the 2Pdꢀ
W C/C catalyst was performed. According to the XRD data obtained after the catalytic
2
reaction (SI, Fig. S12), the majority of the W C phase as well as small and broad peaks
2
0
relating to Pd remain, which indicates structural stability for longꢀterm usage of the
catalyst. However, the catalytic results in general exhibited lower conversion and
selectivity to EG after 4 recycling runs (Fig. SI 11). The total polyol yield (EG + acetol +
1
,2ꢀPG) also decreased after 4 cycles. Due to the greasy nature of the catalyst after the
catalytic reaction, even after multiple water washing procedures, evidence points towards
nonꢀreactive oligosaccharides or partially decomposed sugar and/or furans oligomers
5
6
attached to the catalyst . These byꢀproducts were not removed during a simple water
washing and probably blocked the catalysts active sites decreasing its efficiency. This result
indicates that the washing process needs to be improved to remove byꢀproducts and recover
the catalytic activity of the Pdꢀpromoted tungsten carbide catalyst.
From the results presented here, it is clear that the tungsten carbide catalysts
promoted with 2wt.% of Pd give the best catalytic performance under the reaction
o
conditions tested. This system reached the highest EG yield (58%) at 220 C and 150 min of
o
reaction. At higher temperature (250 C), competitive side reactions become more favored,
which results in a less selective catalytic system towards EG production. However, the
multifunctional catalyst still performs the retroꢀaldol reactions followed by hydrogenation.
This catalytic property comes from the interaction between Pd and W C, which forms
2
during the carburization process in the preparation steps of the catalyst. Using less promoter
2
2

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(
1wt.% Pd) in the catalytic system, the interaction between Pd and W C decreased,
2
resulting in a multifunctional catalyst giving lower hydrogenation potential.
In terms of cellulose conversion and EG yield, Pdꢀpromoted tungsten carbide
catalysts are much more efficient than Niꢀpromoted catalysts when comparing the same
2
0
cellulose conversion reaction parameters . 1PdꢀW C/C and 2PdꢀW C/C catalysts
2
2
converted 72% and 75% of cellulose after 90 min of reaction time, producing ~31% and
9% EG yield, respectively, while 2% Niꢀpromoted tungsten carbide catalyst showed
65% conversion and 26% EG yield. Moreover, Niꢀpromoted catalysts gave ~22% acetol
3
∼
yield, while 1% Pd and 2% Pdꢀpromoted only gave 9.7 and 0.6,% respectively. These
results point out a superior hydrogenation ability exhibited by Pd and W C interaction in
2
the cellulose conversion reactions.
Characterization using in-situ XRD, XANES/EXAFS and EDS analysis (Fig. 1, 2
and Table 1), showed Pd and W interactions, as well as the formation of tungsten carbide
W C phases during the carburization process. Pd nanoparticles help reduce the tungsten
2
0
oxide to W due to hydrogen spillover. Moreover, palladium is less thermodynamically and
2
6–29
kinetically active towards CH (1 ≤ x ≤ 4) decomposition
. This hinders carbon
x
deposition on the catalyst surface during the carburization, according to the XPS analysis
and TEM. The carburization temperature did not decrease, as unanimously
1
0,13,16,18,20,30,50,54,57,58
observed in the literature when Ni is applied as a promoter. Nickel
favors the formation CH decomposition species, which contribute to increasing and
x
speeding up the process of atomic carbon (C*) formation and/or CH deposition on W
x
surfaces for carburization. However, the catalytic as well as XPS results obtained here and
20
in our previous work show that a reduction in methane decomposition produces a cleaner
catalyst surface and consequently, a higher conversion of cellulose. This gives Pd an
advantage over Ni when its promotional effect on tungsten carbide is considered.
The catalytic efficiency as well as structural, surface and electronic properties of the
Pdꢀpromoted tungsten carbide catalysts have not been explored in the literature up to now.
1
7,30
Only few catalytic tests are reported
. From the studies here as well as in the wellꢀ
1
0,13–20,30,49,50,54,57–60
explored Niꢀpromoted tungsten based catalyst studies
, the interaction of
the promoter and W C is a key point in the efficiency of cellulose conversion of towards
2
EG production. Palladium hindered carbon deposition on the catalyst surface. Carbon
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3

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2
0,42,43,45
deposition is a drawback when transition metals carbide catalysts are considered
.
This may reveal a better and more profitable balance amongst catalyst cost, physical,
chemical and catalytic properties to obtain higher EG selectivity from cellulose than Niꢀ
promoted tungsten based catalysts.
Conclusions
The promotional effect of Pd on the tungsten carbide catalyst provided
advantageous physical and chemical properties for catalytic application. A carbide catalyst
with a cleaner and less inhibited surface was obtained due to less thermodynamically and
kinetically active palladium reducing CH (1 ≤ x ≤ 4) decomposition during the
x
carburization process. Thus, higher cellulose conversion was observed when compared to
Niꢀpromoted tungsten carbide catalysts.
The interaction of Pd and W also enhances the ability of the catalyst to perform
multiꢀstep reactions resulting in higher conversion and selectivity towards ethylene glycol
production. Thus, Pd seems to offer a better balance amongst catalyst composition,
structural and surface properties as well as catalytic activity to obtain high valueꢀadded
platform chemicals from cellulose.
Acknowledgements
The authors are grateful to LNLS/CNPEM and CAPESꢀCNPEM CNPq for the
financial support, CTBE/CNPEM for chromatographic analysis of reaction products,
LNNano for the TEM microscope and LNLSꢀBrazil and DiamondꢀUK for the synchrotron
radiation facilities.
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