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 NCCR

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 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. 18th July 2008 NCCR

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 Evolution: MoS2 Nanoparticles as Catalyst for Hydrogen Evolution , Berit Hinnemann, Poul Georg Moses, 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). 18th July 2008 NCCR

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 18th July 2008 NCCR

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. 18th July 2008 NCCR

The Role of Supports Decorative effect (effective dispersion) 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. 18th July 2008 NCCR

One dimensional architecture It has been shown that edge state atoms can be effectively promoting the electrochemical reactions – One Dimensional nano-architectured compounds of W for electrochemical reactions. [reproduced from the presentation of I. Chorkendorff et al on 22nd November 2007] 18th July 2008 NCCR

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 nano-rods and Vulcan XC-72R carbon composite (WO3 (NR)- C ) and (iii) WO3 nano-rods-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 18th July 2008 NCCR

Synthesis and characterization of WO3 nanorods TEM images of WO3 nanorods Synthesis of metal oxide nanorods Na2WO4.2H2O 3M HCl H2WO4 ((C4H9)4)N)4W10O32 ((C4H9)4)NBr (5:2 mol ratio of Na2WO4: ((C4H9)4)NBr) WO3 nanorods N2, 450 C, 3h 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 Method employed Thermal decomposition of tetrabutyl-ammonium 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   d020 0.375nm (a) (b) (c) (d) (a) tetrabutylammonium decatungstate (b) WO3 nanorods obtained from the pyrolysis of tetrabutylammonium decatungstate; (c) WO3 obtained from the pyrolysis of ammonium paratungstate, ; (NH4)10H2W12O 42.XH2O (d) commercially obtained WO3 JCPDS No: 75-2072 WO3 – Monoclinic phase 18th July 2008 NCCR

HER reaction on WO3 nanorods An overlay of cyclic voltammograms of tungsten trioxide nanorods and bulk WO3 Hydrogen evolution reaction on WO3 nanorods 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 electro-catalysis proceeds through the steps: H+ (aq) + M + e-  M –Had 2M- Had  H2 + 2M Where M = surface site of the electro-catalyst Anodic peak current density: WO3 Nanorods: 24.7 mAcm-2; Bulk WO3 : 3.5 mAcm-2; Peak current density of WO3 nanorods is ~ 7 times higher than the bulk WO3 J.Rajeswari et al, Nanoscale Res Lett.,2,496(2007) xH+ + xe- + WO3  HxWO3 (1) HxWO3  x/2H2 + WO3 (2) 18th July 2008 NCCR

Electrochemical Studies Linear sweep voltammograms for Hydrogen Evolution Reaction 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 (a) WO3 nanorods (b) bulk WO3 © bare glassy carbon electrode 1M H2SO4 at a scan rate of 5mVs-1 Current density: WO3 nanorods - 23mAcm-2; Bulk WO3 -15mAcm-2 J.Rajeswari et al, Nanoscale Res Lett.,2,496(2007) 18th July 2008 NCCR

ELECTROCHEMICAL MEASUREMNTS 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 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 Table. 2 Comparison of electro-catalytic activity of Pt supported on various carbon supports for methanol oxidation 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 18th July 2008 NCCR

HER and Oxygen Reduction 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 ). Linear sweep voltammograms of (A): (a) WO3 NR, (b) WO3 B and (c) bare glassy carbon electrode, for HER 18th July 2008 NCCR

Tungsten Sulphide nanorods Transition metal chalcogenides Synthesis of WS2 nanomaterials Transition metal chalcogenides (MX2) 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 Hollow fullerene like nanoparticles ( inorganic fullerenes IFs) and nanotubes (inorganic nanotubes (INTs) are synthesized from these materials Several applications: solid lubricants, hydrogen storage, hydrodesulfurization catalytsts, electro-chemical intercalation, Li batteries ((C4H9)4)N)4W10O32 WS2 nanomaterials N2, 450 C, 3h H2S, 800 C, 1h Cooled to room temperature under N2 atm WO3 Energy (keV) Intensity 20 nm 50 nm 18th July 2008 NCCR

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 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 J.Rajeswari et al, Nanoscale Res Lett.,2,496(2007) 18th July 2008 NCCR

MoS2 nanotube showed higher activity than its bulk counterpart 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 of lower onset potential and higher current density than WO3 nanorods – Glassy carbon electrode - commercial MoS2 (Sigma Aldrich) -13.7 mAcm-2 - MoS2 nanotube – 37.4 mAcm-2 Onset potential for nanostructured MoS2: -0.2 V vs Ag/AgCl ; commercial MoS2: -0.4 V vs. Ag/AgCl; Three times increase in activity is observed in case of MoS2 nanotubes 18th July 2008 NCCR

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. 18th July 2008 NCCR

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 these electrochemical reactions 18th July 2008 NCCR

R.Ganesan, D. J.Ham and J.S.Lee, Electrochem. Commun,, 9 (2007) 2576. 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 18th July 2008 NCCR

HER Activity of WC (Ma et al, Int J Hydrogen Energy,32(2007)2824);H 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) 18th July 2008 NCCR

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. 18th July 2008 NCCR

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. 18th July 2008 NCCR

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 18th July 2008 NCCR

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 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. 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. 18th July 2008 NCCR

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. 18th July 2008 NCCR

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. 18th July 2008 NCCR

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Band diagram and Brillioun zone of Pt: DOS of Pt: 18th July 2008 NCCR

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Partial density of states for Pt 18th July 2008 NCCR

WC Partial density of states 18th July 2008 NCCR

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 d-PDOS having the same shape and intensity between -2.5eV and -8eV, and a split of the d-band into bonding-anti-bonding states. The increased occupancy of the states involved in electron transfer steps is responsible for the observed increased activity. 18th July 2008 NCCR

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 equi-potential surface favours facile transport of the species between Pt and the support. 18th July 2008 NCCR

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 18th July 2008 NCCR

The postulates arising from this study are: 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. The overlapping p-d bands can alter the free energy of adsorbed hydrogen (GH ~ 0) thereby accounting for the increased current density for HER. 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. The existence of equi-potential surface favours facile transport of the species between Pt and the support. 18th July 2008 NCCR

Thank you for your kind attention 18th July 2008 NCCR

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 18th July 2008 NCCR

Band diagram and Brillioun zone of Pt: DOS of Pt: 18th July 2008 NCCR

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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. 18th July 2008 NCCR

18th July 2008 NCCR 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 18th July 2008 NCCR

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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 18th July 2008 NCCR

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 18th July 2008 NCCR

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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 18th July 2008 NCCR

The postulates arising from this study are: 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. the overlapping p-d bands can alter the free energy of adsorbed hydrogen (GH ~ 0) thereby accounting for the increased current density for HER. 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. the existence of equi-potential surface favours facile transport of the species between Pt and the support. 18th July 2008 NCCR

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The increased resistance to CO poison in methanol oxidation. 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. 18th July 2008 NCCR

The increased resistance to CO poison in methanol oxidation. 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. 18th July 2008 NCCR