Tungsten carbides as selective deoxygenation catalysts: Experimental and computational studies of converting C3 oxygenates to propene

Article abstract of DOI:10.1039/c3gc41256c

The deoxygenation activity and selectivity of tungsten monocarbide (WC) have been investigated using a combination of DFT calculations, surface science experiments, and reactor evaluations of catalyst particles. Both WC surfaces and particles are very selective in breaking the C-O/CO bond of propanol and propanal, leading to the production of propene as the main product. The consistency of DFT, surface science and reactor studies in predicting the high selectivity in C-O/CO scission suggests that fundamental studies on model surfaces can be extended to more practical applications. Results from the current paper also identify research opportunities in synthesizing nanoparticle WC and W₂C as effective deoxygenation catalysts.

Full text of DOI:10.1039/c3gc41256c

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Tungsten carbides as selective deoxygenation
catalysts: experimental and computational studies
of converting C3 oxygenates to propene
Cite this: Green Chem., 2014, 16, 761
Hui Ren,a Ying Chen,a Yulin Huang,a Weihua Deng,a Dionisios G. Vlachosa and
Jingguang G. Chen*b
The deoxygenation activity and selectivity of tungsten monocarbide (WC) have been investigated using
a combination of DFT calculations, surface science experiments, and reactor evaluations of catalyst
particles. Both WC surfaces and particles are very selective in breaking the C–O/CvO bond of propanol
and propanal, leading to the production of propene as the main product. The consistency of DFT, surface
science and reactor studies in predicting the high selectivity in C–O/CvO scission suggests that funda-
mental studies on model surfaces can be extended to more practical applications. Results from the
current paper also identify research opportunities in synthesizing nanoparticle WC and W2C as effective
deoxygenation catalysts.
Received 28th June 2013,
Accepted 22nd August 2013
DOI: 10.1039/c3gc41256c
www.rsc.org/greenchem
chemicals without reducing the number of carbon atoms. A
good catalyst for deoxygenation should be active enough to
1. Introduction
The production of fuels and chemicals from renewable cleave the C–O bond while remaining relatively inert toward
biomass resources is a promising approach to meet some C–C bond cleavage. In the current study, we use propanol and
of the energy and environmental challenges.1–3 However, the propanal as probe molecules to demonstrate that tungsten
derivatives of biomass, such as sugars and polyols, are over monocarbide (WC) is a promising catalyst for the selective
functionalized with a large number of oxygen atoms per mole- scission of the C–O and CvO bonds. The reactions of pro-
cule,4,5 which need to be removed via selective deoxygenation panol have been reported on several transition metals13,14 and
to produce more valuable fuels and chemicals.6
bimetallic systems.15–20 To the best of our knowledge, none of
For example, biodiesel is one of the most important these materials are selective toward deoxygenation without
biomass-derived biofuels as a replacement of a blending agent breaking some of the C–C bonds.
to petroleum diesel. However, the oxygen content leads to
Transition metal carbides, in particular, tungsten and
higher density and polarity,4 resulting in higher viscosity and molybdenum carbides, have been evaluated for applications
lower cold flow properties compared to conventional diesel.7 in both heterogeneous catalysis21,22 and electrocatalysis.22,23
Furthermore, the presence of oxygen also reduces the energy Although these materials often show “platinum-like” pro-
density of the fuel.4,8–10 If biomass-derived fuels can be selec- perties in hydrocarbon transformation reactions,21,22 recent
tively deoxygenated, biodiesel can potentially be used as a studies reveal that for the reactions of oxygenate molecules,
direct replacement of conventional petroleum resources.11
carbides are often different from either their parent metals or
Deoxygenation can proceed via either decarbonylation or platinum.24,25 Of particular relevance to deoxygenation is the
selective C–O bond cleavage.4,5 The former route often leads to reaction of methanol and ethanol on the WC surface, which
the reduction in carbon chain length, which is often undesir- shows that the WC surface is active towards C–O bond clea-
able for conversion to fuels and chemicals;4 moreover, the CO2 vage.24,25 More recently, it is also reported that these carbides
produced from this route is not environmental friendly.12 are active in deoxygenation reactions.26–30
Therefore, selective deoxygenation via C–O bond cleavage
In the current study, we use a combination of density func-
is crucial in converting biomass derivatives to fuels and tional theory (DFT) calculations, surface science experiments,
and reactor studies to investigate the deoxygenation reactions
of propanol and propanal on WC, which demonstrates as a
case study starting from fundamental theoretical studies and
then extending to more practical catalysis applications. The
mechanistic insights into the deoxygenation pathways and the
aCatalysis Center for Energy Innovation, University of Delaware, Newark, DE 19716,
USA
bDepartment of Chemical Engineering, Columbia University, New York, NY 10027,
USA. E-mail: jgchen@columbia.edu
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excellent selectivity of WC are predicted from DFT calculations;
the deoxygenation reactions on model WC surfaces are verified
by temperature programmed desorption (TPD) and the
additional information of surface reaction intermediates is
provided by surface vibrational spectroscopy; the selective
deoxygenation over WC particles is then demonstrated by
reactor studies. The selection of C3 oxygenates is mainly due
to their relatively high vapor pressure, which allows one to
introduce them into ultrahigh vacuum (UHV) systems for fun-
damental surface science studies and for gas-phase reactor
evaluations. The comparison of propanol and propanal pro-
vides useful information on whether and how the deoxygena-
Table 1 Calculated chemisorption energies of CH3CH2CH2OH,
CH3CH2CH2O, and CH3CH2CHO and enthalpy changes (ΔH) of some
key reactions on WC(0001) using SIESTA and VASP codes
Ead and ΔH (eV)
Species and reactions
VASP code
SIESTA code
CH3CH2CH2OH
CH3CH2CH2O
0.69
4.05
1.04
4.11
CH3CH2CHO
CH3CH2CH2OH → CH3CH2CH2O + H
CH3CH2CHO → CH3CH2CH + O
1.69
−1.77
−2.39
2.04
−1.84
−2.42
tion activity differs between C–O and CvO bonds, which often with previous studies of the characterization of W terminated
coexist in biomass derived molecules.
WC(0001).38
The exposure of propanol and propanal was achieved
through leak valves. The samples were purified by successive
freeze–pump–thaw cycles before use. The TPD measurements
were taken from 105 K to 800 K with a linear heating rate of
2. Methods
2.1. DFT methods
3 K s−1
.
The DFT calculations were performed using the SIESTA
program.31 The pseudo-potentials were generated using the
Troullier–Martins scheme.32 For W, the electrons of 6s2 and
5d4 were included as valence electrons. For all of the atoms, a
double zeta with polarization quality basis set was employed.
The exchange-correlation energy was determined using the
generalized gradient approximation (GGA) functional proposed
by Perdew, Burke, and Ernzerhof (PBE).33 The localization
radii of the basic functions were determined from an energy
shift of 0.01 eV. In this work, the spacing of the grid points
corresponded to an equivalent of 200 Ry energy cutoff in the
associated reciprocal space at which value the geometries and
energies have converged. Spin polarization was included when-
ever necessary. The calculated equilibrium lattice parameters
for WC (hexagonal, a = b = 2.92 Å and c = 2.85 Å) are very
similar to the experimental values (a = b = 2.91 Å and c =
2.84 Å).34 The supercell approach was used to model the W-ter-
minated WC(0001) surface (hereby referred to as WC(0001));
six layers of WC(0001) surface were modeled with 15 Å vacuum
spacing between slabs. The bottom four layers of metal atoms
were fixed at their bulk truncated position and the top two
layers and the adsorbates were allowed to relax. A 3 × 3 unit
cell was used in all the calculations and a Monkhorst–Pack
mesh of 3 × 3 × 1 was employed in the k-point sampling. In
this work, some of the structures and energetic results
obtained from the SIESTA code were further checked using the
pseudopotential plane-wave VASP code35,36 (see Table 1). As
can be seen from Table 1, the calculated results using SIESTA
are very close to those from the VASP code.
High-resolution electron energy loss spectroscopy
(HREELS) measurements were recorded with the angles of
incidence and reflection at 60 degrees with respect to the
surface normal in the specular direction and with a primary
beam energy of 6 eV. After the adsorption of propanol or pro-
panal at 100 K, the surface was flashed to the next temperature
of interest and allowed to cool back to 100 K before proceeding
with the subsequent scan.
2.3. Synthesis and characterization of WC catalyst particles
The synthesis of WC particles was conducted in a sealed
quartz tube reactor using ammonium metatungstate hydrate
((NH4)6H2W12O40, Sigma-Aldrich) as tungsten precursor and
cetyltrimethylammonium bromide (CTAB, Sigma-Aldrich) as
the carbon source. Typically, 0.297 g of (NH4)6H2W12O40 was
mixed with 0.112 g of CTAB and placed in the bottom of the
quartz tube. The quartz tube was then sealed under high
vacuum before it was placed in an electrical resistance furnace
to be carburized. The sample in the quartz tube was heated to
1273 K from room temperature with a heating rate of 1 K per
minute. It was naturally cooled to room temperature after
being held at 1273 K for one hour. The black powder at the
bottom of quartz tube was collected.
Powder X-ray diffraction patterns (XRD) were obtained
using a Phillips Norelco powder diffractometer using Cu Kα
irradiation to examine the purity of the WC catalyst. Particle
morphology was measured using a JOEL JEM-2010F trans-
mission electron microscope (TEM) with a 200 kV acceleration
voltage.
2.2. Surface science studies
2.4. Reactor studies
The preparation of the WC surface was described in detail else-
where.37 Briefly, the WC surface was prepared by carburizing a The vapor phase deoxygenation of propanol/propanal, both
polycrystalline W foil using cycles of ethylene sputtering at with and without hydrogen, was carried out by flow reactor
300 K, followed by annealing the surface to 1200 K. The stoi- studies in a quartz glass reactor under atmospheric pressure at
chiometry of the W : C ratio was around 1 : 1, as confirmed by 653 K. Prior to reaction, the WC catalyst (0.043 g) was reduced
the Auger Electron Spectroscopy (AES), and was consistent under a H2 (25 mL per minute) and He (25 mL per minute)
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mixture at 723 K for 1 hour. During the reaction, a constant reaction pathway for propanol decomposition is, aside from
flow of propanol/propanal was obtained by bubbling He kinetic considerations, also thermodynamically favored. The
through
a
saturator containing propanol/propanal. In decomposition of propanol goes through two elementary
the experiments with co-feed H2, the molar ratio of H2 to dehydrogenation steps to form the key intermediate
propanol/propanal was calculated to be 10 : 1. The products (CH3CH2CHO) rather than having the C–O bond breaking first.
were analyzed using an online GC (Agilent 7890A) equipped Once CH3CH2CHO forms, CvO bond breaking occurs readily
with a flame ionization detector (FID) and a thermal con- to produce CH3CH2CH, as shown in Fig. 1(a). The reaction
ductivity detector (TCD).
network suggests that propanol and propanal share very
similar deoxygenation intermediate and pathway, which are
further confirmed in surface science studies as described
below. Other dehydrogenation and C–C bond scission path-
ways show higher activation barriers, as illustrated in Fig. 1(a).
As the deoxygenation product is CH3CH2CH, which has a very
similar formula and structure with propene, it is a reasonable
prediction that propene would be the main product of the
deoxygenation reaction.
In summary, DFT calculations predict that the WC(0001)
surface should exhibit high activity towards selective C–O/
CvO bond scission of propanol and propanal to produce
propene, and adsorbed CH3CH2CHO is the key intermediate
that undergoes CvO bond cleavage with a low activation
barrier of 0.33 eV. It should be pointed out that the current
study used the closed-packed WC(0001) surface to obtain acti-
vation barriers and reaction networks. Additional calculations
on the more open surface structures should provide additional
insight into correlating model surfaces with polycrystalline or
amorphous WC catalysts.
3. Results and discussion
3.1. DFT calculations
DFT calculations were performed to explore the decomposition
pathways of propanol and propanal on WC(0001). The reaction
network and the energy profile for propanol decomposition on
WC(0001) are presented in Fig. 1. Fig. 1(a) shows that the
chemisorption energy of propanol is 1.05 eV. The initial
decomposition of adsorbed propanol on WC(0001) via O–H
bond scission (a reaction barrier of Ea = 0.66 eV) is kinetically
favorable over C–H bond scission (Ea = 1.50 eV), which is con-
sistent with previous DFT results for methanol on WC(0001).39
Based on the activation barriers, DFT results show that the
propoxy (CH3CH2CH2O) intermediate prefers to be further
dehydrogenated to CH3CH2CHO rather than being dissociated
into CH3CH2CH2 and O. Propanal adsorbs on the surface
through O to the bridge site and C to the top site, with a
binding energy of 2.04 eV. A relatively low activation barrier
(Ea = 0.33 eV) is obtained for the CvO bond scission of
propanal on WC(0001). As shown in Fig. 1(b), the overall reac-
tion is highly exothermic (ΔH = −4.40 eV), indicating that this
3.2. Surface science studies
The TPD results following the adsorption of propanol on the
WC surface at ∼100 K are shown in Fig. 2, with Ni-modified
WC also shown as reference to show the different reaction
pathways of propanol on the two surfaces. As predicted in the
reaction network in Fig. 1, selective C–O bond scission should
lead to the formation of the CH3CH2CH intermediate, which
can undergo isomerization to produce propene. In contrast,
the C–C bond scission of propanol should lead to the
formation of CO, C2 or C1 oxygenates and C2 or
C1 hydrocarbons. As shown in Fig. 2, a broad desorption peak
of propene (C3H6, 42 amu) is observed at 409 K, indicating the
cleavage of the C–O bond. Relatively weak peaks are also
detected in the 28 and 27 amu spectra on the WC surface,
which are due to either the cracking fragments from propene
or trace amount of ethylene. Because the 27 amu and 28 amu
peaks are most likely from the cracking patterns of propene,
the TPD results also suggest that CO (28 amu) is not produced.
The absence of the 15 amu and 16 amu peaks suggests that
there is no methane generated on the surface. The TPD results
reveal that propene is the dominant reaction product from
WC, indicating the selective deoxygenation pathway on the WC
surface, which is also consistent with the DFT prediction. In
comparison, on the Ni-modified WC surface, propene is not
produced. Instead ethylene (C2H4, 27 amu) and CO (28 amu)
are the main products from the decomposition of propanol,
Fig. 1 (a) Schematic of the possible reaction pathway for propanol and pro-
panal decomposition on the WC(0001) surface. The activation barrier (and
the corresponding reaction energy in parenthesis) is in eV. The energetically
favorable pathway is highlighted in red. (b) Energy profile of the most favor-
able reaction pathway for propanol decomposition on the WC(0001) surface. demonstrating that C–C bond cleavage is the main reaction
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Fig. 2 TPD results following the decomposition of propanol on WC. Similar TPD spectra are also included for a Ni-modified WC surface to provide
a reference of mass spectrometry sensitivity for different masses and to highlight the unique gas-phase product distribution from the WC surface.
pathway on the Ni/WC surface. In summary, the reactions of
propanol decomposition on WC are as follows:
In order to investigate whether WC is also active toward
breaking the CvO double bond, as predicted by DFT calcu-
lations, propanal TPD experiments were performed. As shown
in the TPD results in Fig. 3, similar to propanol, propene is
again observed as the only hydrocarbon product from the WC
(a) C2H5CH2OH → C3H6 + H2 + O(ads) deoxygenation
(b) C2H5CH2OH → 3C(ads) + O(ads) + 4H2 total decomposition
Integrating the peak areas of the reaction products allows surface, indicating that WC is similarly active to CvO bond
for quantification of activity for each decomposition reaction cleavage without breaking the C–C bond, consistent with the
pathway. The yield of H2 was determined by converting the H2 DFT prediction. The quantified yield of the amount of propa-
TPD peak area to H2 coverage based on the calibration of mass nal undergoing deoxygenation and total decomposition is
spectrometer saturation coverage using a Pt(111) surface.40,41 0.091 and 0.001 molecule per surface W atom, respectively. In
The yield of C3H6 was determined by using an experimentally comparison, on the Ni/WC surface, propanal primarily under-
determined sensitive factor relative to CO, which was obtained goes C–C bond cleavage to produce ethylene and CO, which is
by back filling the UHV system with equal concentrations of consistent with the previous study on a Ni(111) film.15
C3H6 and CO based on the ionization sensitivity factors of
The unique selectivity of WC toward C–O or CvO bond
these molecules. The quantification of TPD peak areas indi- scission can be explained by the fact that oxygenates are
cates that the amount of propanol undergoing deoxygenation absorbed onto the WC surface via strong bonding through
and total decomposition is 0.061 and 0.002 molecule per the oxygen atom.24,42 The similarities between propanol and
surface W atom, respectively, as summarized in Table 2.
propanal in the TPD experiments suggest that these two
Table 2 Quantification of TPD results of propanol and propanal through different reaction pathways on WC surfaces: (a) deoxygenationa, (b) total
decompositionb, (c) reformingc and (d) methane formationd
Propanol
Propanal
Surface
a
b
c
d
Total
a
b
c
d
Total
WC
1ML Ni/WC
0.061
0.000
0
0.000
0.095
0.002
0.062
0.063
0.175
0.091
0.000
0
0.000
0.093
0.001
0.057
0.092
0.165
0.018
0.015
a C2H5CH2OH → C3H6 + H2 + O(ads); C2H5CHO → C3H6 + O(ads) b C2H5CH2OH → 3C(ads) + O(ads) + 4H2; C2H5CHO → 3C(ads) + O(ads) + 3H2.
.
c C2H5CH2OH → CO + 4H2 + 2C(ads); C2H5CHO → CO + 3H2 + 2C(ads)
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d C2H5CH2OH → 2CH4 + CO; C2H5CHO → CH4 + 2C(ads) + O(ads) + H2.
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Fig. 3 TPD results following the decomposition of propanal on WC. Similar TPD spectra are also included for a Ni-modified WC surface to provide
a reference of mass spectrometry sensitivity for different masses and to highlight the unique gas-phase product distribution from the WC surface.
molecules may have similar decomposition pathways on the
WC surface. HREELS experiments were performed to deter-
mine the surface reaction intermediates from the decompo-
sition of propanol and propanal. Fig. 4 compares the HREELS
results after the adsorption at 100 K and after heating to
300 K, which is below the onset temperature for the desorption
of the propene product.
As summarized in the vibrational assignment in Table 3,
the spectrum following the adsorption of propanal at 100 K is
characteristic of molecularly adsorbed propanal. Upon heating
to 300 K, the intensity of the (CvO) mode at 1670 cm−1 signifi-
cantly decreases, indicating that the bonding of propanal on
WC occurs through the formation of the di-σ C–O bond. In
comparison, the adsorption of propanol on WC leads to the
dissociation of the O–H bond at 100 K, as indicated by the
absence of the (O–H) mode, expected at 3687 cm−1 for gas-
phase propanol and typically observed at above 3300 cm−1 for
hydrogen-bonded alcohols. This observation is consistent with
the previous studies of methanol and ethanol decomposition
on WC, where dissociation of the O–H bond also occurs at
Fig. 4 HREELS spectra from exposure of the WC surface to propanol or
propanal.
100 K.24,25,42 This observation is also consistent with the DFT
calculations of the initial decomposition of propanol on WC
(Fig. 1). As compared in Table 3, the frequencies of all other
The similar nature of the two surface species is consistent with
cating that the propoxy species is produced on WC at 100 K. the similar TPD peak shape of propene from both molecules
vibrational features remain similar to those of propanol, indi-
(Fig. 2 and 3).
Furthermore, the general similarity between the 300 K spectra
of propanol and propanal suggests that a similar surface inter-
Summarizing the results on the WC surface, DFT calcu-
mediate is produced, most likely a propoxy (CH3CH2CH2–OW) lations predict that WC exhibits high activity towards selective
C–O/CvO bond scission to produce propene, which is further
and a di-σ bonded propanal (CH3CH2CHW–OW), respectively.
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Table 3 Vibrational assignment for adsorbed propanal and propanol
Propanal
Propanol
Frequency (cm−1
)
Frequency (cm−1
)
Mode
Liquid
WC
Mode
Gas
WC
ν(CH3)
ν(CH3)
2966
2909
2929 ν(OH)
2929 νa(CH3)
1670 νs(CH3), νs(CH2) 2892
1441 δ(CH2)
1380 δ(CH3)
3687
2978
—
2929
2929
1441
ν(CO)η1-EtCHO 1730
δ(CH2)
δ(CH3)
νa(CCC)
ρ(CH3)
δ(CCO)
1458
1418
1464
1464, 1393 1380
1393, 1300 n.r.
1092, 1120 1090 ρ(CH2)
898
660
866 δ(COH)
656 ρ(CH3)
νa(CCO)
1218
—
1063, 898
1047, 971
898
685
463
1055
1055
880
n.r.
480
480
νs(CCO)
γ(CH2)
δ(CCC)
δ(CCO)
463
confirmed by the TPD experiments. DFT calculations also
suggest that propanol and propanal have a similar reaction
intermediate, and undergo similar decomposition pathway on
WC surface, which is verified by the HREELS measurements.
Both computational and experimental results show that the
WC surface is highly selective to C–O/CvO bond cleavage,
suggesting that WC may be potentially used as a selective
deoxygenation catalyst, as verified below.
Fig. 5 Characterizations of powder WC: (a) XRD; (b) TEM; (c) HRTEM.
3.3. Characterization and reactor studies of WC catalysts
with a high steady-state conversion, clearly indicated that co-
feeding H2 led to the production of H2O by removing the
surface oxygen from the carbide catalysts.29
In order to extend model surfaces to catalyst particles, WC
catalyst was synthesized and tested in a flow reactor. The XRD
pattern in Fig. 5(a) shows that the catalyst particle can be
indexed to a hexagonal WC phase.43 The 2θ of 31.54, 34.84,
48.48, 64.16, 64.58, 73.28, 75.62 and 75.86° corresponds to
(001), (100), (101), (110), (002), (111), (200) and (102) facets of
WC, respectively. The TEM images of the WC catalyst are
shown in Fig. 5(b). The high resolution TEM micrograph in
Fig. 5(c) clearly indicates that the WC particle is highly crystal-
line in nature with lattice fringes with d spacing of 0.2942,
0.2452 and 0.1981 nm, which closely matches the (001), (100)
and (101) planes of hexagonal WC.43,44 The average size of the
catalyst particle is around 100 nm from Fig. 5(b). The large par-
ticle size is most likely related to the high synthesis tempera-
ture (1273 K in the current study) that is required to achieve
the WC phase.
The results of propanal deoxygenation are shown in Fig. 7.
The product distribution is more complicated in the case of
propanal compared to that of propanol. As reported before,
propanal can undergo ketonization and condensation reac-
tions on transition metal oxide catalysts,45,46 a similar situ-
ation is also observed on WC. Apart from higher ketones and
aldehydes, C6 hydrocarbons, produced from the coupling reac-
tions of C3, are also detected in the products. Although there
are many side products observed from the reaction, the
highest selectivity remains to be the selective deoxygenation of
propanal to produce C3 hydrocarbons (mainly propene), with
only trace amounts of products from the C–C bond cleavage.
3.4. Insight from comparison of model surfaces and catalysts
The results of reactor evaluation of propanol deoxygenation
are shown in Fig. 6. Fig. 6 reveals that C3 hydrocarbons Both DFT calculations and UHV experiments on WC surfaces
(mainly propene) and propanal are the main gas products, predict that propanol and propanal should undergo selective
with trace amounts of C2 hydrocarbons, CO, CH4 and C6 C–O/CvO bond scission to produce propene. Such prediction
hydrocarbons. The high selectivity (>95%) of C3 hydrocarbons is confirmed by the excellent selectivity to produce propene
suggests that deoxygenation is the dominant reaction with from the reactor evaluation of WC catalysts. These general
little C–C bond scission, which is consistent with the DFT similarities between model surfaces and catalyst particles illus-
calculations and surface science results. Due to the low steady- trate the usefulness of combining theory and experiments in
state conversion over the WC catalysts, it is difficult to deter- identifying catalytic materials.
mine the exact role of H2 in the current study. However, results
On the other hand, there are two main inconsistencies
from previous studies of C3 oxygenates over Mo2C catalysts, between model surfaces and catalysts, which in turn provide
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Fig. 6 Flow reactor results of propanol deoxygenation reactions at
653 K (reaction conditions: H2–propanol = 10 : 1, catalyst 43.5 mg): (a)
conversion; (b) selectivity with co-feed H2; and (c) selectivity without
co-feed H2.
Fig. 7 Flow reactor results of propanal deoxygenation reactions at
653 K (reaction conditions: H2–propanal = 10 : 1, catalyst 43.5 mg): (a)
conversion; (b) selectivity with co-feed H2; and (c) selectivity without
co-feed H2.
useful insight and future research opportunities in the utiliz-
ation of WC catalysts for deoxygenation. The first is the differ-
ence between the deoxygenation activity of propanol and
propanal. As suggested in DFT calculations (Fig. 1), propanol
undergoes two elementary dehydrogenation steps to produce
propanal before deoxygenation. This is consistent with the
TPD measurements that the yield of propene is higher from
the deoxygenation of propanal than from propanol on WC sur-
faces (Table 2). In contrast, in the reactor evaluation over WC
catalyst the deoxygenation rate of propanal is significantly
lower than propanol. One possible explanation is the presence
of oxygen species over the WC catalyst, which is not present in
the DFT and UHV studies of clean WC surfaces. The presence
of oxygen on the WC catalyst should introduce acidic function
from the oxidized W or WC, which should in turn enhance the
dehydration pathway of propanol to directly produce propene.
Further DFT and UHV studies on oxygen-modified WC
surfaces are necessary to determine the role of surface oxygen
in controlling the deoxygenation pathways.
(1273 K in the current study) that is required to achieve the
WC phase. To highlight the importance of particle size, Fig. 8
compares the conversion and selectivity of propanal deoxy-
genation over WC and Mo2C particles. The Mo2C particle is in
the range of 3 to 5 nm and correspondingly leads to a much
higher steady-state conversion.29 Interestingly, both WC and
Mo2C catalysts produce propene as the dominant product,
suggesting similar deoxygenation pathways over the two
carbide catalysts. The comparison in Fig. 8 suggests future
opportunities in synthesizing and utilizing WC nanoparticles,
preferably in the same particle range as that of Mo2C, as
effective deoxygenation catalysts. Another possibility is to
utilize W2C instead of WC, which should be easier to obtain in
smaller particle size to the lower synthesis temperatures.
4. Conclusions
We explored the utilization of WC for the deoxygenation reac-
The second inconsistency between model surfaces and cata- tions of propanol and propanal using a combination of DFT
lysts is the very low steady-state conversion of propanal (Fig. 7 calculations, UHV studies on model surfaces, and reactor
(a)). This is most likely due to the large size of the WC par- evaluations of catalyst particles. These results demonstrate
ticles, ∼100 nm, resulting from the high synthesis temperature that WC is very selective in breaking the C–O/CvO bond. The
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