Transcript MS PowerPoint - Catalysis Eprints database

The role of tungsten carbide as support for Pt
in electrochemical reactions
B. VISWANATHAN
NATIONAL CENTRE FOR CATALYSIS RESEARCH
INDIAN INSTITUTE OF TECHNOLOGY, MADRAS
CHENNAI, INDIA
18th July 2008
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1
Three important electrochemical reactions receiving
considerable attention in recent years are:
(a) Hydrogen evolution Reaction (HER)
(b) oxygen Reduction Reaction (ORR)
( c) Fuel oxidation reaction in Fuel cells
All the catalysts used for these reactions are based on
noble metals loaded on carbon.
References:
H Mei Wu, Pei Kang Shen, Zidong Wei, Shuqin Song and Ming Nie, J.Power Sources, 166 (2007) 310.
Chunan Ma, Jiangfeng Sheng, Nigel Brandon, Cheng Zhang and Guohua Li, Int.J.Hydrogen Energy, 32 (2007) 2824.
Ming Nie, Pei Kang Shen,and Zidong Wei, J.Power Sources, 167 (2007) 69.
R.Ganesan,
D. J.Ham
9 (2007) 2576.
18th July
2008and J.S.Lee, Electrochem. Commun,,NCCR
2
M.K.Jeon, H.Daimon, K.R.Lee,A.Nakahara and S.I.Woo, Electrochem. Commun., 9 (2007) 2692.
The electrochemical HER is catalyzed most effectively by Pt group metals.
The necessary criterion for high catalytic activity ( based even on enzymes
like nitrogenase and hydrogenase) is the binding energy of atomic hydrogen
which should be close to zero. This provides clue for research of new
catalysts. The criterion enables us to search for new catalysts, and inspired
by the nitrogenase active site, we find that MoS2 nano-particles supported
on graphite are a promising catalyst. They catalyze electrochemical
hydrogen evolution at a moderate over-potential of 0.1-0.2 V.
Figure 1 Calculated free energy diagram for hydrogen evolution at a potential U = 0 relative to the standard hydrogen electrode at
pH = 0. The free energy of H+ + e- is by definition the same as that of 1/2 H2 at standard conditions. The free energy of H atoms
bound to different catalysts is then found by calculating the free energy with respect to molecular hydrogen including zero-point
energies and entropy terms.
Biomimetic
Hydrogen
for Hydrogen Evolution , Berit Hinnemann, Poul Georg Moses,
18th July
2008 Evolution: MoS2 Nanoparticles as Catalyst
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3
Jacob Bonde, Kristina P. Jorgensen, Jane H. Nielsen, Sebastian Horch, Ib Chorkendorff, and Jens K. Norskov* . J.Am. Chem. Soc.,
127 (15), 5308 -5309, (2005).
MoS2 nanorods for hydrogen evolution reaction
T.F.Jaramillo et al., Science 317,100-102, 6 July 2007
They have shown that the exchange current density
correlates linearly with the MoS2 edge length rather
than the area of MoS2. This is anothr evidence that
one dimensional structures will have been dispersion
for HER and other electrochemical reactions.
(samples annealed at 400 and 550oC (red annealed at
5500C and open circules annealed at 4000C)
Biological H2 production – enzymes –
nitrogenase and hydrogenase
Nitrogenase and hydrogenase based electrodes
– electrochemical water splitting for the
production of hydrogen
They have free energy closer to Pt – hence the
activity – search for similar inorganic analogs
MoS2 – lies closer to Pt
The active sites in nitrogenase and hydrogenase
- similar to MoS2
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The Role of Supports
• In the electrochemical reactions the support is
mainly based on the conductivity of the
support and hence carbon is mostly used.
• However generally the supports are used for
effective dispersion but also in electron
transfer reactions, the electronic factor of the
support also plays a role.
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The Role of Supports
The support active phase interaction has been usually
considered in terms of
Decorative effect (effective dispersion)
Electronic effect (charge transfer concept)
New phase formation (alloy formation)
Generation of new interface sites
However though these concepts can be applicable which one is
important has not yet been established and it is generally
considered depending on the system under investigation
For example, in the case of Strong Metal Support interaction,
it is usually interpreted depending on the experimental
conditions and the net effect observed.
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One dimensional architecture
[reproduced from the presentation of I.
Chorkendorff et al on 22nd November 2007]
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• It has been shown that
edge state atoms can be
effectively promoting
the electrochemical
reactions – One
Dimensional nanoarchitectured
compounds of W for
electrochemical
reactions.
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To synthesize metal (M = W, Mo and V) oxide nano rods by the thermal
decomposition of tetrabutylammonium salt of isopolyacids
To synthesize metal sulfide (M = W, Mo) nano materials by thermal
decomposition of tetrabutylammonium salt of isopolyacids followed by
treatment of the oxides in H2 S atmosphere at elevated temperatures
To load Pt on the as-prepared (i) WO3 nano-rods (WO3 (NR)), (ii) WO3 nanorods and Vulcan XC-72R carbon composite (WO3 (NR)- C ) and (iii) WO3 nanorods-carbon nano-tube composite (WO3 NR-CNT) and to study their electro
catalytic activity towards methanol oxidation
To study the electro-catalytic activity of Pt supported on tungsten oxide systems
for oxygen reduction reaction from linear sweep voltammetry
To examine the catalytic behaviour of WO3 nano-rods and MoS2 nano-tubes
towards hydrogen evolution reaction by linear sweep voltammetric studies
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Synthesis and characterization of WO3 nanorods
TEM images of WO3 nanorods
Na2WO4.2H2O
Synthesis of metal oxide nanorods
Method employed
Thermal decomposition of tetrabutylammonium salt of polyoxoacids
((C4H9)4N)4M10O32 (M = W, Mo, V)
Advantages over the existing reports
Relatively low temperature; Short reaction
time; Easy and economical route for the
synthesis of precursor
3M
HCl
TEM images of WO3 (a & b)
130-480 nm and 18-56 nm of
length and width respectively
; (c) HRTEM image of a WO3
nanorods (d) EDX of WO3
H2WO4
((C4H9)4)NBr
(5:2 mol ratio of Na2WO4:
((C4H9)4)NBr)
((C4H9)4)N)4W10O32
(a)
(c)
N2, 450 C, 3h
WO3 nanorods
JCPDS No: 75-2072
18th July 2008
WO3 – Monoclinic phase
(a) tetrabutylammonium decatungstate
(b) WO3 nanorods obtained from the
pyrolysis of tetrabutylammonium
decatungstate; (c) WO3 obtained from the
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pyrolysis of ammonium paratungstate, ;
(NH4)10H2W12O 42.XH2O (d) commercially
(b)
(d)
9
HER reaction on WO3 nanorods
xH+ + xe- + WO3  HxWO3 (1)
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2008 + WO3
HxWO3
 x/2H2
(2)
30
-2
Noble metals such as Pt, Pd and Ru or
Raney Ni, Ni-Mo – used as electrode
materials
High activity shown by noble metals – not
commercialized – expensive – search for
newer materials or reduction of the loading
of noble metals
Hydrogen evolution reaction by electrocatalysis proceeds through the steps:
H+ (aq) + M + e-  M –Had
2M- Had  H2 + 2M
Where M = surface site of the electrocatalyst
Current Density (mAcm )
Hydrogen evolution reaction on WO3
nanorods
An overlay of cyclic voltammograms of
tungsten trioxide nanorods and bulk WO3
WO3 nanorods
bulk WO3
20
10
0
-10
-20
-30
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
Potential (V)
Anodic peak current density: WO3 Nanorods: 24.7 mAcm2; Bulk WO3 : 3.5 mAcm-2; Peak current density of WO3
nanorods is ~ 7 times higher than the bulk WO3
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J.Rajeswari et al, Nanoscale Res Lett.,2,496(2007)
Electrochemical Studies
Linear sweep voltammograms for Hydrogen
Evolution Reaction
(a) WO3 nanorods
(b) bulk WO3
© bare glassy carbon
electrode
•
Platinum when supported on WO3,
shows synergism towards hydrogen
evolution
•
•
Pt + H+ + e-  PtH
2PtH  2Pt + H2
•
•
x Pt-H + WO3  x Pt + HxWO3
HxWO3  x/2 H2 + WO3
•
Tungsten trioxides can thus function
as an active support for Platinum
•
Pt/C, Pt/WO3nanorods-C, Pt-bulk
WO3-C were employed to study HER
1M H2SO4 at a scan rate of 5mVs-1
Current
18thdensity:
July 2008WO3 nanorods - 23mAcm-2; NCCR
Bulk WO3 -15mAcm-2
J.Rajeswari et al, Nanoscale Res Lett.,2,496(2007)
11
ELECTROCHEMICAL
MEASUREMNTS
Cyclic voltammograms of (a) 20% Pt/WO3 NR-CNT, (b)
20% Pt/WO3 NR-C and (c) 20% Pt/C in 1MCH3 OH- 1M
H2 SO4 at a scan rate of 25mVs-1
Fig. Cyclic voltammograms of (a) 20% Pt/WO3
NR, (b) 20% Pt-Ru/C (J. M) and (c) 20%
Pt/WO3 B in 1MCH3OH- 1M H2SO4 at a
scan rate of 25mVs-1
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Table. 2 Comparison of electro-catalytic activity of Pt supported on
various carbon supports for methanol oxidation
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Electro-catalyst
Current density
(mA cm-2)
Mass activity (mA
mg-1)
20% Pt/WO3NR-CNT
322
452
20% Pt/WO3NR-C
272
382
20% Pt/C
130
180 12
HER and Oxygen Reduction
Linear sweep voltammograms of (A): (a)
WO3 NR, (b) WO3 B and (c) bare glassy
carbon electrode, for HER
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• The oxygen reduction
reaction of 20% Pt
supported on WO3 NR-C
was evaluated by linear
sweep voltammetry and its
activity has been compared
with that of Pt supported on
WO3 B-C and only C. WO3
NR-C supported Pt showed
a higher ORR activity (2.77
mA.cm -2) than WO3B-C
(1.88 mA.cm-2 ) and C (1.88
mA.cm-2 ).
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Tungsten Sulphide nanorods
Synthesis of WS2 nanomaterials
Transition metal chalcogenides (MX2)
((C4H9)4)N)4W10O32
One layer of metal M is sandwiched between two
layers of X, where, M = Mo, W, Ta or Nb
and X =S, Se or Te structural analogy to graphite
N2, 450 C, 3h
Hollow fullerene like nanoparticles ( inorganic
fullerenes IFs) and nanotubes (inorganic
nanotubes (INTs) are synthesized from these
WO3
materials
Several applications: solid lubricants, hydrogen
H2S, 800 C, 1h
storage, hydrodesulfurization catalytsts, electroCooled to room
chemical intercalation, Li batteries
temperature
under N2 atm
WS2 nanomaterials
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20 nm
50 nm
Intensity
Transition metal chalcogenides
Energy (keV
14
Linear sweep voltammograms for hydrogen evolution reaction
in the presence of platinum
MoS2 nanorods for hydrogen evolution reaction
T.F.Jaramillo et al., Science 317,100-102, 6 July 2007
Sample
HER activity at 0.8V
(mA/cm2)
20% Pt/WO3nanorods-C
185
20% Pt/Bulk WO3-C
135
20% Pt/C
110
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J.Rajeswari
al, Nanoscale Res Lett.,2,496(2007)
Biological H2 production – enzymes –
nitrogenase and hydrogenase
Nitrogenase and hydrogenase based electrodes –
electrochemical water splitting for the production
of hydrogen
They have free energy closer to Pt – hence the
activity – search for similar inorganic analogs
MoS2 – lies closer to Pt
NCCR The active sites in nitrogenase and hydrogenase
15 similar to MoS2
Electrochemical Hydrogen Evolution Reaction
Summary
Pt/C when modified with WO3
nano rods have shown
enhanced HER activity than
the unmodified Pt/C
MoS2 nanotube showed higher
activity than its bulk
counterpart
MoS2 nanotube showed better
performance for HER in terms
(a) – Glassy carbon electrode
(b) - commercial MoS2 (Sigma Aldrich) -13.7 mAcm-2 of lower onset potential and
(c) - MoS2 nanotube – 37.4 mAcm-2
higher current density than
Onset potential for nanostructured MoS2: -0.2 V vs
Ag/AgCl ; commercial MoS2:
-0.4 V vs.
WO3 nanorods
Ag/AgCl; Three times increase in activity is observed
in case of MoS2 nanotubes
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Which support and why?
• Tungsten based systems appear to be one of
the suitable candidates for exploitation as
supports for noble metals
• One dimensional architecture can be convenient
for use as electrodes.
• Among the possible tungsten based systems
tungsten carbide has a special place.
Conventionally in catalysis in place of Pt
tungsten carbide has been employed.
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The desire for higher dispersion as well as the need to reduce the
amount of noble metal required has led to the development of
alternate support materials. In recent years, tungsten carbide has been
examined as support for Pt in a variety of reactions like hydrogen
evolution reaction (HER), oxygen reduction reaction (ORR) and
methanol oxidation. WC based materials have received attention due
to its resemblance to Pt in various catalytic reactions. In
electrochemical reactions like methanol oxidation, ORR and HER,
WC supported systems exhibit nearly 3-6 times higher activity as
compared to Pt/C electrodes. This has also led to the development
of high surface area (micro sphere) tungsten carbide by a variety of
methods. Another important observation recorded in literature is that
WC supported Pt is tolerant to CO poisoning in DMFC applications
and this has been attributed to the enhanced reactivity towards CO
oxidation. It has also been envisaged that alternate metallic systems
supported on tungsten carbide may also be exploited in future for
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2008
18
these
electrochemical
reactions NCCR
H Mei Wu, Pei Kang Shen, Zidong Wei, Shuqin Song and Ming Nie, J. Power
Sources, 166 (2007) 310.
Chunan Ma, Jiangfeng Sheng, Nigel Brandon, Cheng Zhang and Guohua Li,
Int.J.Hydrogen Energy, 32 (2007) 2824.
Ming Nie, Pei Kang Shen,and Zidong Wei, J.Power Sources, 167 (2007) 69.
R.Ganesan, D. J.Ham and J.S.Lee, Electrochem. Commun,, 9 (2007) 2576.
M.K.Jeon, H.Daimon, K.R.Lee,A.Nakahara and S.I.Woo, Electrochem. Commun.,
9 (2007) 2692.
H.Zheng, J.Huang, W.Wang and C.Ma, Electrochemistry Communication, 7
(2005)1045-1049
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HER Activity of WC
(Ma et al, Int J Hydrogen Energy,32(2007)2824);H.Zheng et al,
Electrochem.,7(2007)1045)
Tungsten carbide also shows a certain catalytic activity for HER. It resists
catalytic poisons like CO, HC and H2S. But activity has to be improved for
replacement of Pt. The synergistic effect of Pt and WC[Fig (a) for WC in
H2SO4 and (b) for Pt/WC; the activity of Pt/WC shows stability The
degradation for WC alone for 100 cycles is 23% while no such degradation is
observed for Pt/WC system. The electrochemical activity of WC for HER is in
between that of Pd and Pt and the exchange current density is of the same order
of magnitude of that of Pt(10(-3)A/cm2)
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Au-Pd nanobimeallic particles supported on
nanocrystalline tungsten carbide as electrocatalysts for
oxygen reduction offer activities that surpass the state of
art Pt based electro-catalysts. The advantage comes
from the novel support WC which itself has the catalytic
activity to enhance the activity of the metal systems
Nie et al, J.Power sources,167 (2007)69.
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WO3 based electrode for HER
• WO3 nanoparticles prepared using Chitosan
biopolymer as a template.
• WO3 nanoparticles are able to intercalate 2.1
times the number of proton than bulk WO3 can
intercalate and also show four fold higher
activity as regards HER in acidic medium than
bulk WO3.
• R Ganesan and A.Gedanken,Nanotechnology,
19 (2008) 025702.
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Pt/WC for methanol oxidation
• In the methanol electro-oxidation the presence of
tungsten carbide had a positive effect on the catalytic
performance of Pt catalyst. Tungsten carbide
dispersed system promotes the removal of CO, which
resulted in higher catalytic performance of the Pt/WC
than Pt/C system
• Ji Bong Joo et al, Materials letters, 62(2007)3497.
• R.Ganesan, D.J.Ham and J S
Lee,Electrochem.Commun.9 (2007)2576
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Tungsten carbide examined as support for Pt in a variety of reactions
• Hydrogen evolution reaction (HER)1-2
• Oxygen reduction reaction (ORR)3
• Methanol oxidation4-5.
Observations
• Resemblance of WC to Pt in catalytic reactions
• 3-6 times higher activity for electro-chemical reactions like
methanol oxidation, ORR and HER, for Pt/WC compared to Pt/C
electrodes.
• Alteration of onset potential
• Pt/WC is tolerant to CO poisoning in DMFC
1.
2.
H Mei Wu, Pei Kang Shen, Zidong Wei, Shuqin Song and Ming Nie, J. Power Sources, 166 (2007) 31.
Chunan Ma, Jiangfeng Sheng, Nigel Brandon, Cheng Zhang and Guohua Li, Int. J. Hydrogen Energy, 32 (2007)
2824.
3. Ming Nie, Pei Kang Shen,and Zidong Wei, J.Power Sources,167, (2007) 69.
4. R.Ganesan, D. J.Ham and J.S.Lee, Electrochem. Commun,, 9 (2007) 2576.
5. 18th
M.K.Jeon,
H.Daimon, K.R.Lee,A.Nakahara and S.I.Woo,
July 2008
NCCRElectrochem. Commun., 9 (2007) 2692.
24
The rationalization can be achieved in terms of
alternate electrochemical routes involving WC
surfaces.
It is necessary that the electronic structure of WC
be examined to identify its resemblance or
otherwise to that of Pt.
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Methodology
•Plane-wave-based DFT calculations using CASTEP
•Ultrasoft pseudopotentials,
•Kinetic energy cutoff maximum of 300eV.
•GGA approximation with PW91 exchange correlation
functional
•All the electronic band structures and the brillioun zone
figures were calculated on the corresponding optimized
crystal geometries.
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Band diagram and Brillioun zone of Pt:
DOS of Pt:
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Partial density of states for Pt
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WC Partial density of states
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The comparison of the total Density of States (DOS) for Pt
and WC is shown in Fig.1. The shape of the total DOS for
Pt and WC is similar near the d-band center of Pt (2.25eV). Partial DOS of WC, shown in Fig.1(b), illustrates
a distinct p-d band overlap, with the C p-PDOS and W dPDOS having the same shape and intensity between 2.5eV and -8eV, and a split of the d-band into bondinganti-bonding states. The increased occupancy of the states
involved in electron transfer steps is responsible for the
observed increased activity.
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The postulates arising from this study are: (i) the
compatibility of the electronic structure of Pt and WC
facilitates the electron transfer reactions as seen by the
similarity in shape of the DOS near the d-band center of Pt.
(ii) the overlapping p-d bands can alter the free energy of
adsorbed hydrogen (DGH ~ 0) thereby accounting for the
increased current density for HER. (iii) the favoured
orientation of the Pt particles and the consequent edge-site
creation may be responsible for CO tolerance or facile
oxidation on the surfaces. (iv) the existence of equipotential surface favours facile transport of the species
between Pt and the support.
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Binding energy of H
• Presence
of WC is
expected to take the
∆GH closer to 0
thereby
enhancing
activity for HER
• Effect of Pt/WC is
expected
to
be
intermediate to that of
Pt and PtW
Kitchin
et al, Catalysis Today 105 (2005) 66
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The postulates arising from this study are:
i. The compatibility of the electronic structure of Pt and
WC facilitates the electron transfer reactions as seen by
the similarity in shape of the DOS near the d-band center
of Pt.
ii. The overlapping p-d bands can alter the free energy of
adsorbed hydrogen (GH ~ 0) thereby accounting for the
increased current density for HER.
iii. The favoured orientation of the Pt particles and the
consequent edge-site creation may be responsible for CO
tolerance or facile oxidation on the surfaces.
iv. The existence of equi-potential surface favours facile
transport of the species between
Pt and the support. 35
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Thank you for your kind attention
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36
DOS (Density of States) has been calculated by utilizing primitive unit cell of Pt and WC crystal structure
Pt metal
Results after the optimization:
The optimized geometry data
Lattice parameters(A)
Cell Angles
a = 2.937706
alpha = 60.000000
b = 2.937706
beta = 60.000000
c = 2.937706
gamma = 60.000000
Current cell volume = 17.927097 A**3
Atomic Populations
-----------------Species Ion s
p
d
f Total Charge (e)
==============================================================
Pt
1 0.70 0.51 8.79 0.00 10.00 0.00
==============================================================
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Band diagram and Brillioun zone of Pt:
DOS of Pt:
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Partial Density of States - Pt
d-band center -1.81eV
All contributions to total
DOS in the region
between 0 and -6 eV is
from d-band.
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WC:
WC
Results after the optimization:
Lattice parameters(A)
Cell Angles
a = 2.940342
alpha = 90.000000
b = 2.940342
beta = 90.000000
c = 2.847037
gamma = 120.000000
Current cell volume = 21.316683 A**3
Atomic Populations
-----------------Species Ion s
p
d
f Total Charge (e)
==============================================================
C
1 1.39 3.23 0.00 0.00 4.62 -0.62
W
1 2.47 6.49 4.42 0.00 13.38 0.62
==============================================================
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WC Partial density of states
• Significant p-d mix near the d-band center region of Pt
• Shape and intensity of C p-PDOS and W d-PDOS similar between -2.5 eV to
-8 eV
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Comparison of density of states (DOS)
• Total DOS of Pt and WC quite
similar near the d-band center
of Pt (-1.81 eV)
• Presence of electronic
interaction near the d-band
center between WC and Pt
• Description of d-band center
for WC is difficult as one must
exclude the effects of C but not
completely
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Binding energy of H
• Presence of WC is
expected to take the ∆GH
closer
to
0
thereby
enhancing activity for HER
• Effect
of
Pt/WC
is
expected
to
be
intermediate to that of Pt
and PtW
Kitchin
et al, Catalysis Today 105 (2005) 66
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46
The postulates arising from this study are:
i. the compatibility of the electronic structure of Pt and
WC facilitates the electron transfer reactions as seen by
the similarity in shape of the DOS near the d-band center
of Pt.
ii. the overlapping p-d bands can alter the free energy of
adsorbed hydrogen (GH ~ 0) thereby accounting for the
increased current density for HER.
iii. the favoured orientation of the Pt particles and the
consequent edge-site creation may be responsible for CO
tolerance or facile oxidation on the surfaces.
iv. the existence of equi-potential surface favours facile
transport of the species between
Pt and the support. 47
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The enhanced activity can be attributed to the
catalytic behaviour of WC itself. The aspects
requiring rationalization are:
•The increased activity for typical electrochemical
reactions to the extent of 3-6 times on Pt/WC
system as compared to Pt/C.
•The alteration of the onset potential on Pt/WC
indicating a change of the overpotential.
•The increased resistance to CO poison in
methanol oxidation.
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The enhanced activity can be attributed to the
catalytic behaviour of WC itself. The aspects
requiring rationalization are:
•The increased activity for typical electrochemical
reactions to the extent of 3-6 times on Pt/WC
system as compared to Pt/C.
•The alteration of the onset potential on Pt/WC
indicating a change of the overpotential.
•The increased resistance to CO poison in
methanol oxidation.
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