WO2014111717A1 - Catalytic and composite materials - Google Patents

Abstract

Mixed metal oxide materials of titanium and tungsten are provided for use in fuel cells. The mixed metal oxides are useful as catalyst support materials and as composite materials comprising a catalyst supported on or within support materials. The mixed metal oxides may comprise up to 40at.% tungsten. The catalyst, supported by the mixed metal oxide may be palladium or a palladium alloy. An alloy of palladium and cobalt is preferred.

Description

Catalytic and Composite Materials The present invention relates to materials for use in a fuel cell and particularly to catalytic materials for use in fuel cells, more particularly to catalyst support materials and composite materials comprising a catalyst supported on or within support materials. A fuel cell comprises an anode where fuel is oxidised and a cathode where an oxidising species such as oxygen is reduced. An electrolyte allows for the transport of ions between the electrodes. Fuel and oxidiser are supplied separately to each electrode. As fuel is oxidised at the anode, electrons are released and pass through an external circuit to the cathode where they are consumed in the reduction of the oxidising species. In a polymer electrolyte membrane fuel cell (PEMFC) the fuel used is usually hydrogen and the oxidising species is usually oxygen; the polymer electrolyte is capable of allowing the flow of protons from the anode to the cathode.

Platinum and platinum alloys have been found to be the most efficient catalysts for facilitating the oxygen reduction reaction (ORR) which takes place at the cathode. Platinum, however, has a high cost and there has been much interest in reducing the amount of precious metal. Usually, platinum is dispersed over a carbon support to increase the surface area relative to the mass of platinum used. Figure 1 shows a plot of the mass activity in ORR obtained at 0.85 V vs. RUE from the steady state measurement experiments versus Pt particle size, on a carbon support. The measurements were performed in 0.5 M HCI0_4(aq) and at 25°C, following the procedure set out in the examples below. Previously, the inventors have studied the effects of particle size for a series of commercially available Pt nano-particles supported on carbon which were supplied by Johnson Matthey pic [Reference 1]. Both studies and others in the literature suggest that the particle size associated with optimum mass activity for carbon supported platinum is ca. 3.5 - 4 nm [References 1- 4]. This is also in good agreement with predictions made by Kinoshita, that suggested a maximum at 3.5 nm in mass activity corresponding to an optimum in size for the exposure of (100) facets for cubo-octahedral particles [5]. This suggests that the high throughput methodologies and thin-film models used by the current inventors are a good approximation for powder catalysts as would be conventionally used in fuel cells.

It is also of interest to compare and contrast the particle size dependence of the ORR activity of carbon supported platinum to carbon supported palladium. Palladium is considered to be a potentially cheaper alternative to platinum in fuel cell cathode catalysts. There are a number of published reports on electrochemical studies of palladium and palladium alloys in acidic electrolytes, in particular, Pd [7-13] PdAu [14], PdCo [15-22], PdCr [18], PdFe [23], PdTi [24], PdCoAu [24, 25], PdCoMo [26], PdV [27], PdMn [27], PdPt [28-33], PdRh [30], PdRhPt [30, 34], and PdNi [18].

Figure 2 shows a plot of the mass activity in ORR obtained using the same procedure at 0.65 V vs. RHE from the steady state measurement experiments versus Pd particle size, on carbon support. Note that the Pt nanoparticle data were shown at 0.85 V vs. RHE, but the Pd data shows considerably lower current at this potential, which means that Pd has intrinsically lower activity than Pt on carbon. There appears to be no clear maximum for Pd supported on carbon, however looking at Figure 3 : The specific activity at 0.70 V vs. RHE of a thin film of Pd (see dashed line) is lower than that obtained for particles with a diameter between 9 and 12 nm (at the same potential), which means that there must be an optimum particle size, but that this has not been reached by 12 nm.

Although pure Pd is less active than Pt, recent studies have suggested that alloying of Pd with a second element such as Co or Fe can lead to activities which are comparable to, or even better than Pt [20, 21]. The main disadvantage of palladium based catalysts is the intrinsic instability of palladium under ORR conditions, although there are also suggestions that alloying Pd can considerably improve its stability [15, 35]. The ORR activity of PdCo alloys with approximately 30 at.% Co has been investigated by Savadogo et al [20, 21]. They found good stability of sputtered PdCo films during extended potential cycling. It was proposed that this could be due to an electronic interaction between Pd and Co in the amorphous alloy. The onset potential measured for the ORR for the PdCo alloy was similar to that for Pt (0.90 V vs. RHE) whilst the current observed at 0.80V vs. RHE for PdCo (0.58 mA cm^"2) was higher than that obtained with Pt (0.25 mA cm^"2). Similar conclusions were obtained by Fernandez et al. [17] who prepared a series of PdCo particles with compositions ranging from 0-100 at.% Pd on glassy carbon, and screened them using a Scanning Electrochemical Microscopy (SECM) technique. They concluded that the addition of 10 - 30 % Co to Pd produced a material with a superior catalytic activity to pure Pd and close to that of pure Pt, although the activity was observed to degrade over time.

The inventors have previously studied PdCo thin films, the steady-state current (b) and surface area (a) were published in a previous patent application [15] and are shown in Figure 4. Combined they suggest an optimum activity (specific current density per unit area -(a)) for PdCo alloys with 40-50 at.% Co (50-60 at.% Pd). This is similar to the maximum found (in both specific and mass specific current density) for PdCo particles deposited on carbon as shown in Figure 5. The data was obtained from steady state potential step measurements at 0.75 V vs. RHE. There does not seem to be any influence of the particle size. The main trend seems to follow the composition with increased activity as the percentage of Co increases, until 60 at.% Co in PdCo at which the particles become unstable. In this work, palladium and palladium cobalt alloy particles with a tungsten doped titanium oxide support have been produced. The effect of cobalt is studied in terms of improvements of the stability (durability) and activity of the catalyst in low temperature fuel cells. Metal oxides have previously been investigated for use as fuel cell catalyst supports [36-48]. Carbon can be oxidised under fuel cell operating conditions leading to degradation of the catalyst and limiting the lifetime of the fuel cell [49]. Oxides may be less prone to oxidative corrosion and therefore more stable than carbon in a fuel cell environment [45]. Several patents have described the use of metal oxides as supports for fuel cell catalysts and methods of synthesising them [36, 37, 39, 42, 47]. Traditionally, dopants have been added to metal oxides in order to increase conductivity by creating defects (such as oxygen vacancies). However, the inventors believe that in addition to increased conductivity, dopants can also help to create defects on the surface of the metal oxides and hence more sites for nucleation of the active catalyst. Moreover it was found that the propensity for large palladium particles formation is significantly enhanced by increasing the substrate temperature during deposition, which means that the number of particles is also reduced. All depositions of palladium and palladium cobalt particles were carried out at 350°C.

In general, published reports of Pd and Pd alloys studies focus on a limited number of compositions due to the time taken to prepare the materials using standard laboratory techniques. An inherent advantage of the inventors High Throughput Physical Vapour Deposition (HT-PVD) synthesis method is that a wide range of compositions can be prepared in a single experiment and the resulting materials can subsequently be screened in a parallel electrochemical experiment [50]. This approach varies from others in that thin film materials of varying composition are prepared via simultaneous deposition of the component elements[51]. This method allows for the preparation of graded compositional thin films of alloy materials through the controlled and simultaneous deposition of the component elements on a substrate at room temperature. A significant advantage of this method is that the alloys formed are not subjected to annealing steps which could alter their relative bulk and surface compositions and cause the formation of thermodynamically stable phases. Rather, it is possible to prepare thin film alloys with compositions determined by the deposition conditions. Previously, these have been demonstrated to be continuous polycrystalline solid solutions with good agreement between the surface and bulk compositions for the as-deposited films. The high-throughput methodologies also provide a means of obtaining good statistical data under identical electrochemical conditions to provide both particle size and support effects in electrocatalysis [44, 52-55]. For example, the effects of particle size and substrate support interactions on the activity of Au nano- particles supported on carbon and on sub-stoichiometric titanium dioxide (also referred to as titania) (TiO_x) has been successfully demonstrated using the high- throughput synthetic and screening methodology [52]. In the current work, high throughput methodologies have been used to optimise the particle size and/or composition of Pd or PdCo particles on W-doped titanium oxide supports.

In its broadest sense, the present invention provides a mixed metal oxide material of titanium and tungsten.

Preferably, the mixed metal oxide material comprises tungsten in an amount of 40 atomic percent (at.%) or less, calculated on a metals only basis. Preferably, the material comprises an oxide of titanium doped with tungsten, with an oxygen content near to stoichiometric.

In one embodiment, the material is formed in an amorphous phase. Preferably, the atomic percentage of tungsten is 26 at.% or less.

In an alternative embodiment, the material is formed in a crystalline phase.

Suitably, the material is formed in a crystalline phase by annealing of an amorphous material as described above.

More preferably, in a crystalline phase, the atomic percentage of tungsten is 23 at.% or less, even more preferably, 18 at.% or less, most preferably about 7 at.%.

The present invention also provides a catalytic material comprising a catalyst support and a catalyst supported by the catalyst support, wherein the catalyst support is a mixed metal oxide material of titanium and tungsten

Preferably, the catalyst is palladium or a palladium alloy. Preferably, the catalyst is formed as particles on the catalyst support.

Preferably, the particle size is 50nm or less, preferably from lnm to lOnm; more preferably from 5nm to 8 nm.

Preferably, the catalyst is a palladium alloy of palladium and cobalt.

More preferably, the palladium alloy consists essentially of palladium and up to about 70 at.% cobalt; preferably up to about 40 at.% cobalt, more preferably up to about 30 at.%) cobalt; most preferably, 15-27 at.%> cobalt.

The present invention also provides a fuel cell comprising a catalytic material. The present invention also provides the use of a mixed metal oxide material as defined above or of a catalytic material as defined above in a fuel cell.

For convenience, mixed metal oxide materials of tungsten and titanium will also be referred to by the shorthand formulation TiWO_x. It will be appreciated that this is not used as a chemical formula to indicate any specific stoichiometry and should not be taken to be so limiting in any way.

The term mixed metal oxide indicates an oxide of a mixture of metals; a mixture of metal oxides or a combination thereof.

The above and other aspect of the invention will now be described in further detail, by way of example only, with reference to the following examples and the accompanying figures, in which: Figure 1 : Mass activity per g of platinum at 0.85 V vs. RHE (obtained from steady state step measurements at discrete potentials) versus Pt particle diameter for particles supported on carbon. Measurements performed in 0.5 M HC10₄(_aq) and at 25°C; Figure 2: Mass activity per g of palladium, at 0.65 V vs. RHE (obtained from steady state step measurements at discrete potentials) versus Pd particle diameter for particles supported on carbon for 2 different arrays. The measurements were performed in similar conditions in 0.5 M HC10₄(aq) and at 25°C;

Figure 3 : Specific activity obtained at 0.70 V vs. RHE (from steady state step measurements at discrete potentials) versus Pd particle diameter for particles supported on carbon for 2 different arrays. The measurements were performed in similar conditions in 0.5 M HC10₄(aq) and at 25°C. The dashed line represents the limit of a thin film of Pd calculated by CO stripping area;

Figure 4(a): Electrochemically active surface area, (b) Steady state current and (c) Specific current density (raA cm^"2) of PdCo thin film for ORR at 0.8 V vs. RHE;

Figure 5(a): Mass activity (A gpd^"1) and (b) Specific current density (A cm^"2) of PdCo particles on Carbon at 0.75 V vs. RHE. [15]

Figure 6: Schematic view of an electrochemical array substrate, highlighting the x and y coordinates 1, 4, 7 and 10;

Figure 7: XRD patterns of TiO_x on glass after annealing at 450 °C for 6 hours in 1 atm of oxygen. The dashed black line corresponds to Ti0₂ anatase (00-021-1272) (Reference 6);

Figure 8: XRD patterns of TiWO_x on glass after annealing at 450 °C for 6 hours in 1 atm of oxygen. The dashed black line corresponds to Ti0₂ anatase (00-021-1272);

Figure 9: TEM images for TEM grids/TiO_x(amorphous)/Pd, obtained at an equivalent x-coordinate position of a) 1; b) 4; c) 7; d) 10; Figure 10: TEM images for TEM grids/TiWO_x (amorphous)/Pd, obtained at an equivalent x-coordinate position of a) 1; b) 4; c) 7; d) 10 with 25 at.% W.

Figure 11 : Average particle diameter versus the grid position (x) for Pd particles on amorphous TiO_x and TiWO_x (25 at.% W);

Figure 12: Selected fields: Slow cyclic voltammetry (5 mV s^"1) in oxygenated 0.5 M HC10₄ obtained between -0.05 and 1.10 V vs. RHE for sample #6621 - Pd particles on amorphous TiWO_x (5 at.% W). The mean particle sizes range from approximately 2-7 nm. The black dashed line shows the chosen current (0.7 μΑ) for the ignition potential analysis. The markers on some of the plots do not represent real data points; they are only there to guide the eye;

Figure 13 : a) 10 x 10 plot of oxygen reduction ignition potential for Pd particles on amorphous TiWO_x with 7 at.% W; b) 10 x 10 plot of oxygen reduction ignition potential for Pd particles on amorphous TiWO_x with 10 at.% W; c) data per column shown in a) ; and d) data per column shown in b). The black dashed lines in c) and d) show the limit of braking down of the Pd particles around 4 nm diameter. The current at which the ignition potential was taken is 0.7 μΑ;

Figure 14: Cyclic voltammetry (100 mV s^"1) in deoxygenated 0.5 M HCIO₄ obtained between -0.05 and 1.00 V vs. RHE for Pd particles on amorphous TiWO_x (7 at.% W) performed before and after slow cycling experiments in saturated oxygen electrolyte. The black dashed line shows the Pd particle diameter at which Pd breakdown is believed to start occurring;

Figure 15: a) Raw oxygen reduction current; b) Specific activity and c) Mass activity per g of palladium, all at 0.65 V (obtained from cathodic sweep of slow sweep voltammetry (5 mV s^"1 in 0₂ saturated electrolyte) versus Pd particle diameter for particles supported on amorphous TiWO_x Figure 16: TEM images for TEM window//TiO_x(crystalline)/Pd, obtained at an equivalent x-coordinate position of a) 1; b) 4; c) 7; d) 10;

Figure 17: TEM images for TEM window/TiWO_x (crystalline)/Pd, obtained at an equivalent x-coordinate position of a) 1; b) 4; c) 7; d) 10 with 25 at.% W;

Figure 18: Average particle diameter versus the grid position (x) for Pd particles on crystalline TiO_x and TiWO_x (25 at.% W); Figure 19: Selected fields: Slow cyclic voltammetry (5 mV s^"1) in oxygenated 0.5 M HCIO₄ obtained between -0.05 and 1.00 V vs. RHE for Pd particles on crystalline TiWO_x (8 at.% W). The mean particle sizes range from approximately 2-8 nm. The black dashed line shows the chosen current (0.7 μΑ) for the ignition potential analysis. The markers on some of the plots do not represent real data points; they are only there to guide the eye;

Figure 20: a) 10 x 10 plot of oxygen reduction ignition potential for Pd particles on crystalline TiWO_x with 8 at.% W; b) 10 x 10 plot of oxygen reduction ignition potential for Pd particles on amorphous TiWO_x with 29 at.% W; c) data per column shown in a); and d) data per column shown in b). The black dashed line in d) show the limit of breaking down of the Pd particles around 5.5 nm diameter;

Figure 21 : a) Raw oxygen reduction current; b) Specific activity and c) Mass activity per g of palladium, all at 0.65 V vs. RHE (obtained from cathodic sweep of slow sweep voltammetry (5 mV s^"1 in 0₂ saturated electrolyte) versus Pd particle diameter for particles supported on crystalline TiWO_x.

Figure 22: Mass activity per g of palladium at 0.65 V vs. RHE (obtained from cathodic slow sweep voltammetry (5 mV s^"1 in 0₂ saturated electrolyte) versus Pd particle diameter for particles supported on both amorphous and crystalline TiO_x supports; Figure 23 : A (10 x 10) macro of slow cyclic voltammetry (5 mV s^"1) in oxygenated 0.5 M HC10₄ electrolyte obtained between -0.05 and 1.00 V vs. RHE for PdCo particles on amorphous TiWO_x (20 at.% W). Average particle sizes range from approximately 2-9 nm;

Figure 24: Selected fields: Slow cyclic voltammetry (5 mV s^"1) in oxygenated 0.5 M HCIO₄ obtained between -0.05 and 1.00 V vs. RHE for a sample with PdCo particles on amorphous TiWO_x (20 at.% W), with the atomic percentage of cobalt varying between 0 and 100 at.% in PdCo (corresponding to row 5 highlighted by the plane rectangle selection in Figure 23) and the particle size varies from 4.5 to 7.3 nm. The markers on some of the plots do not represent real data points; they are only there to guide the eye; Figure 25: Selected fields: Slow cyclic voltammetry (5 mV s^"1) in oxygenated 0.5 M HCIO₄ obtained between -0.05 and 1.00 V vs. RHE for a sample with PdCo particles on amorphous TiWO_x (20 at.% W), with the atomic percentage of cobalt constant at 35 at.%), and the particles size varying from 2.7 to 8.0 nm (corresponding to column 4 highlighted by the dashed rectangle selection in Figure 23). The markers on some of the plots do not represent real data points; they are only there to guide the eye;

Figure 26: a) Plot of ignition potential against field position for PdCo particles on amorphous TiWO_x (20.0 at.% W); b) Plot of atomic percentage of Pd in PdCo; c) Plot of particle diameter (nm);

Figure 27: Activity per unit mass of Pd, at 0.65 V vs. RHE (obtained from the cathodic sweep of slow sweep voltammetry 5 mV s^"1) as a function of atomic percentage of Co, for the 3 equivalent thickness of particles deposited (from 5.3 to 8.3 nm diameter) supported on amorphous TiWO_x supports;

Figure 28: A (10 x 10) macro of slow cyclic voltammetry (5 mV s^"1) in oxygenated 0.5 M HCIO₄ electrolyte obtained between -0.05 and 1.00 V vs. RHE for PdCo particles on crystalline TiWO_x (13 at.% W). Average particle sizes range from approximately 2-10 nm;

Figure 29: Selected fields: Slow cyclic voltammetry (5 mV s^"1) in oxygenated 0.5 M HCIO₄ obtained between -0.05 and 1.00 V vs. RHE for PdCo particles on crystalline TiWO_x (13 at.% W), with the atomic percentage of cobalt varying between 0 and 100 at.% in PdCo (corresponding to row 5 highlighted by the plane rectangle selection in Figure 28) and the particle size varies from 4.4 to 5.6 nm. The markers on some of the plots do not represent real data points; they are only there to guide the eye;

Figure 30: Selected fields: Slow cyclic voltammetry (5 mV s^"1) in oxygenated 0.5 M HCIO₄ obtained between -0.05 and 1.00 V vs. RHE for PdCo particles on crystalline TiWO_x (13 at.%) W), with the atomic percentage of cobalt constant at 35 at.%>, and the particles size varying from less than 3.0 to 8.1 nm diameter (corresponding to column 4 highlighted by the dashed rectangle selection in Figure 28). The markers on some of the plots do not represent real data points; they are only there to guide the eye;

Figure 31 : a) Plot of ignition potential against field position for PdCo particles on crystalline TiWO_x (13.0 at.%> W) and b) zoom of plot a) in the range 0.6 to 0.67 V vs. RHE highlighting the optimum; and

Figure 32: Activity per unit mass of Pd, at 0.65 V vs. RHE (obtained from the cathodic sweep of slow sweep voltammetry 5 mV s^"1) as a function of atomic percentage of Co, for the 3 equivalent thickness of particles deposited (from 5.3 to 8.3 nm diameter) supported on crystalline TiWO_x supports.

Experimental

Synthesis of samples:

Thin film samples were deposited using a high throughput physical vapour deposition system described in Reference 51. The depositions were carried out in a cryo-pumped ultrahigh vacuum system with a base pressure of 1 x 10^"10 Torr, incorporating 6 off- axis PVD sources (3 electron beam sources and 3 Knudsen cells). A range of oxide samples were synthesised onto different substrates including silicon wafers, glass wafers, SSTOP (Si/Si0₂/Ti0₂/Pt where the Ti0₂ layer is 10 nm thick and Pt layer is 100 nm thick) wafers and electrochemical arrays (10 x 10 arrays of gold contact pads as represented schematically in Figure 6). Titanium (Alfa Aesar Puratronic 99.995%) and tungsten (Alfa Aesar 99.97%) were both deposited from an electron beam (E- gun) source, in the presence of either molecular oxygen or an atomic oxygen source (operating at 400 - 600 W power) at a pressure in the range of 5 x 10^"6 to 5 x 10^"5 Torr to form TiWO_x films. Amorphous thin films were prepared by depositing TiWO_x at 25°C. Crystalline thin films were prepared by post-annealing the as-deposited oxide films for 6 hours at 450°C in an atmosphere of oxygen in order for crystallisation to take place. For comparison, carbon (Alfa Aesar 99.995%) films were deposited from an E-gun source.

The Pd and PdCo particles were deposited onto the pre-deposited oxide thin films from a Knudsen cell source (Pd (Allgemeine 99.95%))) and an E-gun source (Co (Alfa Aesar 99.95%)), during deposition the oxide substrates were heated to 350°C in order to dehydroxylate the surface and ensure a good distribution and size of particles. A shutter that was moved during deposition allowed different equivalent thicknesses of catalyst to be deposited onto different fields. These deposition procedures allowed for a varying PdCo alloy composition in one direction and a varying size of particles in the other (orthogonal) direction, while the oxide support composition was left uniform across the substrate. The amounts of Pd and Co deposited were calibrated by depositing thicker films onto silicon substrates and measuring the thickness of the films by optical profilometry and producing a calibration curve against deposition time. For comparison, Pt (Algemein 99.9%) particles were deposited onto pre- deposited carbon films using an E-gun source using a similar procedure.

Oxide characterisation:

The composition of the oxide films was determined using a Laser Ablation Inductively Coupled Plasma Mass Spectrometer (LA-ICP-MS, New Wave 213 nm laser & Perkin Elmer Elan 9000). This method gives the relative composition of the metallic elements, but is not capable of measuring the oxygen concentration. Therefore oxide compositions shown are the relative atomic masses of the two metallic elements only within the oxide (i.e. Ti and W). As the ICP-MS measurements are destructive, composition measurements were made on samples deposited onto silicon wafers. The same deposition conditions were then used to deposit onto equivalent electrochemical arrays. The samples used for stability test of the support were made by depositing the oxide thin films onto SSTOP substrates with a varying oxide composition across the substrate. The composition range on each substrate was measured by ICP-MS on the four corners of the film. X-Ray diffraction (XRD) patterns were obtained using the Bruker D8 Discover diffractometer, a powerful XRD tool with a high-precision, two-circle goniometer with independent stepper motors and optical encoders for the Theta and 2 Theta circles. The D8 diffractometer system is equipped with a GADDS detector operating at 45 kV and 0.65 mA. A high intensity X-ray Ιμ8 Incoatec source (with Cu Ka radiation) is incorporated allowing high intensity and collimated X-rays to be localised on thin film materials providing an efficient high throughput structural analysis. This analysis was carried out on oxide films deposited onto glass substrates. Electrochemical stability tests have been carried out on the oxide films deposited on SSTOP substrates as continuous thin films. The samples were placed in an electrochemical cell and left at 1.2 V vs. RHE the reversible hydrogen electrode (RHE for a period of 100 hours. Cyclic voltammograms were performed between 0.025-1.2 V vs. RHE at 25 hour intervals, at a sweep rate of 50 mV s^"1 to determine the initial, intermediate, and final state of the oxide layers. The temperature was maintained at 25°C and a mercury/mercury sulphate (MMSE) reference electrode was used. A Pt mesh counter electrode was used, in a glass compartment separated from the working electrode compartment by a glass frit. The electrolyte used for all experiments was 0.5 M HCIO₄ prepared from concentrated HCIO₄ (double distilled, GFS) and ultrapure water (ELGA, 18 ΜΩ cm). The apparatus used for these measurements has been previously reported [1], and the high-throughput cell was simply adapted to provide data for a continuous film through a single channel. Photographs and ICP-MS measurements of the samples were obtained before and after this test.

Electrochemical screening:

A range of electrochemical arrays have been prepared, amorphous and crystalline TiO_x and TiWO_x supports combined with either Pd or PdCo particles; and, for comparison, Pt and Pd particles on carbon supports.

High-throughput electrochemical screening equipment enables electrochemical experiments on 100 independently addressable electrodes arranged in a 10 ^χ 10 array to be carried out in a parallel screening mode which has been described in detail elsewhere [1]. The geometric areas of the individual working electrodes on the electrochemical array are 1.0 mm². The samples were screened using the procedure described in Table 1, in 0.5 M HCIO₄ at 25 °C, prepared from concentrated HCIO₄ (double-distilled, GFS) and ultrapure water (ELGA, 18 M Ω cm). The gases used (Ar, 0₂ and CO) were of the highest commercially available purity (Air Products). Unless stated otherwise, experiments were performed under an atmosphere of argon. Unless noted otherwise, the maximum potential applied to the electrodes was 1.0 V vs. RHE.

Table 1: Electrochemical screening procedure.

Results and Discussion

XRD Characterisation:

TiO_x and TiWO_x samples deposited at 25°C showed no XRD features, suggesting the direct formation of an amorphous oxide.

Figure 7 shows XRD patterns of TiO_x deposited on glass substrates at 25°C after annealing at 450°C for 6 hours in one atmosphere of oxygen. Crystallisation is much more effective for sub-stoichiometric, as-deposited, oxide, compared to the oxides deposited with a higher oxygen content (or using the atom source). Only the titanium oxide deposited using a pressure of oxygen of 1 χ 10^"5 Torr with the molecular source show XRD peaks after annealing. The peaks seen are in good agreement with those of TiC"2 anatase space group 141/amd (card 00-021-1272), which displays a single peak at 29=25.28° for the Miller indices (hkl)≡ (101), a triplet around 29=37.80° for (004), and single peaks at 29=48.05° for (200) and at 29=53.89° for (105) (see dashed black line). The experimental data was also compared to the diffraction patterns of Ti0₂ rutile (space group P42/mnm) (card 00-021-1276) reported in the database used. However no assignment of diffraction peaks to the rutile phase could be made. Indeed none of the present features matched any other phase reported in this database.

Figure 8 shows the XRD patterns of TiWO_x on glass substrates deposited at 25°C after annealing at 450°C for 6 hours in one atmosphere of oxygen. As observed for the pure titanium oxide, samples deposited using a lower pressure of oxygen lead to an oxide which gives better crystallisation after annealing. The dashed black line corresponds to Ti0₂ anatase (00-021-1272).

The data shown here have been separated into groups of oxides deposited under the same conditions for the oxygen source, but with a varying compositional range of Ti vs. W. It is clear that with a higher titanium content the XRD features are in good agreement with that of the Ti0₂ anatase space group (see dashed black line). As the content of W increases (i.e. the content of Ti decreases) the formation of the Ti0₂ anatase phase is disrupted. No assignment of any peaks associated with any tungsten and/or tungsten oxide phases was possible. Hsu et al. [56] synthesized these oxides by sol-gel methods and further annealed their samples. They made similar observations and concluded that during the heat treatment of the mixed Ti0₂ and W0₃ precursor gels, nano-crystalline Ti0₂ outgrew the counterpart WO₃ phase.

Electrochemical stability tests:

Tests have been carried out on oxide films deposited on SSTOP substrates. The samples were placed in an electrochemical cell and left at 1.2 V vs. RHE for a period of 100 hours as detailed in the experimental section.

TiO_x was found to be very stable both in amorphous and anatase phases. The amorphous binary TiWO_x showed that adding up to 30 at.% W only leads to a small loss of tungsten, with a final atomic percentage of W of 27 at.% after 100 hours. The crystalline samples showed no significant loss when adding up to 17 at.% W. Pd particles on amorphous TiO_x and TiWO_x:

High-throughput methodologies have been used to investigate Pd particles on amorphous TiO_x and TiWO_x supports. The effect of Pd particle size and the W- content of the support have been observed.

TEM data has been obtained for a range of different palladium particle sizes on amorphous supports as shown in Figure 9 and Figure 10 for TiO_x and TiWO_x (25 at.% W), respectively. The images were obtained for positions equivalent to coordinates x=l, 4, 7 and 10 on the electrochemical array (Figure 6), therefore providing an insight into the variations observed across the range of electrochemical measurements performed. Figure 11 shows that the average particle size of the Pd varies from approximately 1.6 to 6.8 nm. Very little difference is observed between Pd on TiO_x and TiWO_x (with 25 at.% W).

Figure 12 shows cyclic voltammograms obtained from slow voltammetric cycling (5 mV s^"1) in oxygenated 0.5 M HCIO₄ electrolyte obtained between -0.05 and 1.10 V vs. RHE for sample #6621 for selected fields from a (10 x 10) array (Pd particles supported on amorphous TiWO_x (5.0 at.% W)), with varying particle size. The mean particle sizes range from approximately 2-7 nm. The black dashed line shows the chosen current (0.7 μΑ) for the ignition potential analysis. An ignition or onset potential is defined as the potential at which the absolute value of the current starts to increase from the background (double layer) level in a cyclic voltammetry experiment indicating that an oxidation or reduction reaction is taking place at that potential. The markers on some of the plots have been added to help differentiate the different data sets. It will be appreciated that the data actually includes many more data points than indicated by the markers.

In the slow cycling experiment the main feature observed is a reduction current associated with the oxygen reduction reaction. This has an onset at approximately 0.68 - 0.71 V vs. RHE for the largest Pd particles on all samples. The onset (ignition) potential appears to shift gradually to lower voltages with decreasing particle size indicating a decrease in activity for the oxygen reduction reaction. The ignition potential is also altered depending on the support. Figure 13a) and b) show maps of the ignition potential on 2 (10 x 10) arrays of Pd particles deposited on amorphous TiWO_x supports with 7 and 10 at.% W respectively measured at a fixed current of 0.7 μΑ. Figure 13 c) and d) show the ignition potential versus Pd particle diameter on both supports. Below 4 nm particle diameter (as shown by the black dashed lines) a large distribution in the data is clearly apparent which is likely to represent the limit at which a thin film or larger islands of Pd begin to break down into particles of inhomogeneous size. In comparison, the Pd thin film on carbon studied showed an ignition potential of around 0.83 V vs. RHE at the same current of 0.7 μΑ. The increase in the over-potential can be attributed to an increase in the reversibility of the Pd/PdO couple on oxide supports analogous to the previously observed Pt on Ti0₂ [44, 55].

Figure 14 shows cyclic voltammograms (CVs) (100 mV s^"1) in deoxygenated 0.5 M HCIO₄ obtained between -0.05 and 1.00 V vs. RHE for Pd particles on amorphous TiWO_x (7 at.% W) performed before and after slow cycling experiments in saturated oxygen electrolyte. The black dashed line shows the Pd particle diameter at which Pd breakdown is believed to start occurring. A comparison of the CVs before and after oxygen reduction gives an indication of the stability or instability of the Pd particles depending on their size and the substrate on which they were deposited. Figure 14 shows that at the larger particle sizes there is no or little difference in the voltammetry before and after oxygen reduction as shown for electrode (2,3). For small particle size the difference between the cyclic voltammograms is clear, in certain cases (which correspond to the lowest ignition potential observed) the PdO to Pd surface reduction peak is not even present anymore which suggests an increased irreversibility of the palladium oxide formation and reduction reaction, possibly due to break down of the Pd particles on the titanium tungsten oxide surface as shown for electrode (9,2). In comparison for electrode (9,7) which has Pd particles of the same size as electrode (9,2), the difference between the two cyclic voltammograms is small, suggesting that the Pd particles were more stable. Electrode (6,3) highlights the size of particles at which the instability starts to occur. The correlation between change in the cyclic voltammograms and the ignition potential seen during the oxygen reduction reaction (an indicator of activity) is clear.

Figure 15 shows plots of the raw oxygen reduction current (a), specific activity (b) and mass activity (c) in ORR obtained at 0.65 V from the cathodic sweep of the slow cycling (5 mV s^"1 in 0₂ saturated electrolyte) experiments versus Pd particle size, on the amorphous TiWO_x supports. A maximum in specific activity and mass activity is found around 5.5 - 6 nm particle diameter, decreasing rapidly towards 3 nm diameter particles. This is similar to the results obtained for Pt particles on carbon shown in Figure 1, where a maximum in mass activity and specific activity is found at 3.5 - 4 nm particle diameter but is in contrast with results obtained for Pd particles on carbon shown in Figure 2 which do not shown any clear optimum. The increase seen at lower particles size is likely to be due to some uncertainty when calculating the surface area from TEM data. There is no obvious trend with the W doping of the support, however only a limited number of TiWO_x compositions were studied. Hence, a more detailed study of the effect of the support composition might highlight a complex trend in the activity of Pd.

Pd particles on crystalline TiO_x and TiWO_x:

High-throughput methodologies were also used to investigate Pd particles on crystalline TiO_x and TiWO_x supports. The effect of Pd particle size and the W- content of the support have been observed.

Initially, TEM data has been obtained for a range of different palladium particle sizes on the crystalline supports as shown in Figures 16 for TiO_x and TiWO_x (25 at.% W), respectively. The images were obtained for positions equivalent to coordinates x = 1, 4, 7 and 10 on the electrochemical array, therefore providing an insight into the variations observed across the range of electrochemical measurements performed. Figure 18 shows the average particle diameter versus grid position x for Pd particles on crystalline TiO_x and TiWO_x (25 at.%w) and shows that the average particle size of the Pd varies from approximately 2.6 to 9.4 nm. Very little difference is observed between Pd on crystalline TiO_x and TiWO_x (with 25 at.% W).

However a slight difference between the particle sizes is observed between the amorphous and crystalline substrates (see Figure 11 and Figure 18), with the particles smaller on the amorphous substrate which may be due to a higher number of nucleation sites on the amorphous substrate, due to a higher degree of disorder, leading to a higher number of smaller particles for the equivalent metal loading.

Figure 19 shows CVs from selected fields from the oxygen reduction slow cycling experiment (5 mV s^"1) in oxygenated 0.5 M HC10₄ obtained between -0.05 and 1.00 V vs. RHE obtained from one sample (Pd particles supported on crystalline TiWO_x (8.0 at.%) W)), demonstrating the change with varying particle size. The mean particle sizes range from approximately 2-8 nm. The black dashed line shows the chosen current (0.7 μΑ) for the ignition potential analysis. The markers on some of the plots have been added to differentiate different data sets. It will be appreciated that the data actually includes many more data points than are indicated by the markers.

The main feature observed is the current associated with the oxygen reduction reaction which has an onset at approximately 0.67 - 0.70 V vs. RHE for the largest Pd particles on all samples. The onset (ignition) potential appears to shift to lower voltages with decreasing particle size as can be seen from two different samples on Figure 20, suggesting decreasing activity.

At particles with a diameter of below around 5.5 nm (as shown in Figure 20 by the black dashed line) a steep switching in ignition potential is seen (Figure 20 (d)) for a TiWO_x crystalline support with 29 at.%> W (3.5 nm for 18 at.%> W and 6.5 nm for 24 at.%) W (not shown here). A gradual decrease in ignition potential is seen for a support with only 8 at.%> W. In fact, looking at various supports (from 0 to 40 at.%> W) the behaviour of the ignition potential varies significantly with varying W content, which is probably due to a) the Pd particles wetting very differently, b) the distribution of particle size varying and/or c) the stability of the Pd particles on the substrate varying depending on the crystalline support. Hence by optimising the composition of the crystalline support it is possible to minimise the size of particles (hence the amount of material used) and still keep an ignition potential around 0.70 V vs. RHE.

Figure 21 shows plots of the raw oxygen reduction current (a), specific activity (b) and mass activity (c) in ORR obtained at 0.65 V vs. RHE from the cathodic sweep of the slow cycling experiments (5 mV s^"1 in 0₂ saturated electrolyte) versus Pd particle size, on the crystalline TiWO_x supports. All variables are seen to increase as the particle diameter increases in the range studied, although a maximum at higher particle sizes cannot be ruled out. Below 3 nm particle diameter the oxygen reduction current is negligible. Contrary to the amorphous support, the W-doped crystalline materials show a higher activity than the undoped anatase titanium oxide, therefore the same specific or mass activity can be obtained for smaller particle size than on the pure anatase TiO_x, by W-doping.

Comparing data from the amorphous and crystalline supports it is clear that the activity is higher on the amorphous supports than on the crystalline, as can be shown for undoped titanium oxide in Figure 22, which shows mass activity per g of palladium at 0.65 V vs. RHE (obtained from cathodic slow sweep voltammetry (5 mV s^"1 in 0₂ saturated electrolyte) versus Pd particle diameter for particles supported on both amorphous and crystalline TiO_x supports.

The results obtained here contrast with previous data obtained for carbon supported Pt shown in Figure 1 but show similar behaviour to that obtained on carbon supported Pd particles shown in Figure 2 where no clear maximum in specific or mass activity were found for the range of size of particles studied. PdCo particles on amorphous TiO_x and TiWO_x:

Figure 23 shows a typical 10 x 10 data set obtained from slow voltammetric cycling (5 mV s^"1) in oxygenated 0.5 M HC10₄ electrolyte obtained between -0.05 and 1.00 V vs. RHE for PdCo particles supported on amorphous TiWO_x (20 at.% W). Average particle sizes range from approximately 2-9 nm. The data demonstrates a clear trend in activity relating to both particle size and PdCo composition which is repeated across all samples.

Figure 24 shows selected fields from row y=5 in Figure 23 (as highlighted by the solid black rectangle). The percentage of cobalt in the particles varies from 0 to 100 at.%, while the particle size varies from 4.5 to 7.3 nm. In fact, although it has been demonstrated (not shown here) a constant thin film thickness can be obtained during deposition across a row, the particles size may vary with composition, that is palladium and cobalt do not wet the support in the same way, leading to differences in the particle size and density. Figure 25 shows selected fields from Figure 23 (see dashed rectangle selection) where the percentage of cobalt has been kept constant at 35 at.% in PdCo and the size of particles varies from 2.7 to 8.0 nm diameter. Together these figures show that by adding a small amount of cobalt to palladium (15 at.%) Co) the current improves and that the optimum particles size appears around 7 nm.

The ignition potential measured (at 0.7 μΑ) for the array (with 20 at.%> W) is plotted against the field in Figure 26 a). The white area represents the region of highest ignition potential hence giving the conditions in particle alloy composition and size of particles for the most promising materials. Figure 26 b) and c) show the alloy composition and the particle diameter across the array, respectively. Electrodes with high Co content gave a very poor signal due to the unstable nature of particles very rich in cobalt (see column x = 10). Still it is clear that the area of interest for the highest ignition potential is represented by the white region which is for a particle diameter between 4.5 and 8.0 nm and for 0 to 35 at.%> Co in PdCo. An in-depth analysis of all data for PdCo particles with varying size and composition on amorphous TiWO_x supports with a varying amount of W was performed. The specific activity, the mass activity and the ignition potentials where extracted from these slow cyclic voltammetry data.

Comparison of PdCo particles on amorphous TiWO_x support (3^rd equivalent thickness):

Figure 27: shows the Activity per unit mass of Pd, at 0.65 V (obtained from the cathodic sweep of slow sweep voltammetry 5 mV s^"1) as a function of atomic percentage of Co, for the 3 equivalent thickness of particles deposited (from 5.3 to 8.3 nm diameter supported on amorphous TiWO_x supports.

Particles size diameter varies from: 5.3 to 8.3 depending on the ratio of PdCo and the amount of W within the substrate (5.3-5.9 for TiO_x and 6.1 to 8.3 for TiWO_x), although the equivalent thickness of material deposited is identical.

This plot demonstrates the effect of the atomic percentage of Co on the mass activity (for mass of Pd) obtained at 0.65 V vs. RHE on amorphous TiWO_x. The behaviour observed suggests that there is a slight enhancement on the addition of some Co for some of these systems, and certainly, there is no considerable negative effect of adding up to approx. 30 at.% Co. At higher Co contents there is a sharp decrease in the activity, and by 60-70 at.% the activity is very low. This trend is observed for the range of samples investigated. For the amorphous materials, the TiO_x support appears to lead to the most active systems.

PdCo particles on crystalline TiO_x and TiWO_x:

Figure 28 shows a typical 10 x 10 data set obtained from slow voltammetric cycling (5 mV s^"1) in oxygenated 0.5 M HC10₄ electrolyte obtained between -0.05 and 1.00 V vs. RHE for PdCo particles supported on TiWO_x (13 at.% W). Average particle sizes range from approximately 2-10 nm. This demonstrates a clear trend in activity relating to both particle size and PdCo composition which is repeated across all samples.

Figure 29 shows corresponding CVs from selected fields from the oxygen reduction slow voltammetric cycling experiment, obtained from PdCo particles supported on crystalline TiWO_x at 13.0 at.% W (see Figure 28 row 5 highlighted by the solid rectangle selection). The percentage of cobalt in the particles varies from 0 to 100 at.%, while the particles size varies from 4.4 to 5.6 nm. As mentioned before, although it has been demonstrated (not shown here) that we could obtain a constant thin film thickness during deposition across a row, the particles size may vary with composition. Figure 30 shows selected fields from Figure 28 (see dashed rectangle selection) where the percentage of cobalt has been kept constant at 35 at.% in PdCo and the size of particles varies from 3.0 to 8.1 nm diameter. Together these figures show that by adding a small amount of cobalt to palladium (15 at.% Co) the current improves and that the optimum particle size appears around 7 nm, similarly to the amorphous support.

The ignition potential (measured at 0.7 μΑ) for the array (with 13 at.% W) is plotted against the field in Figure 31a). The white area represents the region of highest ignition potential hence giving the conditions in particle alloy composition and size of particles for the most promising materials. Figure 31b) is an enlargement in the ignition potential scale of Figure 31a), in the range 0.6 to 0.67V vs. RHE which highlights the hotspot which corresponds to a peak activity at approximately 6-7 nm for PdCo with 15 to 27 at.% Co.

An in-depth analysis of all data for PdCo particles with varying size and composition on crystalline TiWO_x supports with a varying amount of W was performed. The specific activity, the mass activity and the ignition potentials where extracted from these slow cyclic voltammetry data. Comparison of PdCo particles on Crystalline TiWO_x support (3^rd equivalent thickness):

Figure 32 shows mass activity of Pd, at 0.65 V vs. RHE (obtained from the cathodic sweep of slow sweep voltammetry 5 mV s^"1) as a function of atomic percentage of Co, for the 3 equivalent thickness of particles deposited (from 5.3 to 8.3 nm diameter) supported on crystalline TiWO_x supports.

Particles size diameter varies from: 5.3 to 8.3 depending on the ratio of PdCo and the amount of W within the substrate (5.3 to 5.9 for TiO_x and 6.1 to 8.3 for TiWO_x) although the equivalent thickness of material deposited is identical. These plots demonstrate the effect of the atomic percentage of Co on the raw current (proportional to the geometric current density), specific activity and mass activity (for mass of Pd) obtained at 0.65 V vs. RHE on amorphous TiWO_x. The behaviour observed suggests that there is no loss of activity on the addition of up to approx. 30 at.% Co for some of these systems. At higher Co contents there is a sharp decrease in the activity, and by 70 at.% the activity is very low. This trend is observed for the range of samples investigated. For the crystalline materials, a binary oxide support with 7 at.% W appears to lead to the most active systems.

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Claims

Claims

1 A mixed metal oxide material of titanium and tungsten comprising up to about 40 atomic percent tungsten on a metals basis.

2 A mixed metal oxide material as claimed in Claim 1 comprising up to about 30 atomic percent tungsten.

3 A mixed metal oxide material as claimed Claim 1 or Claim 2 wherein the material is formed in an amorphous phase.

4 A mixed metal oxide material as claimed in any preceding claim wherein the material is formed in a crystalline phase.

5 A mixed metal oxide material as claimed in Claim 4, wherein the material is formed in a crystalline phase by annealing of a material as claimed in Claim 3.

6 A mixed metal oxide material as claimed in any preceding claim wherein the atomic percentage of tungsten is 26 at.% or less, preferably 23 at.% or less, more preferably 18 at.% or less.

7 A mixed metal oxide material as claimed in Claim 6 wherein the atomic percentage of tungsten is about 7 at.%.

8 A catalytic material comprising a catalyst support and a catalyst supported by the catalyst support, wherein the catalyst support is a mixed metal oxide material of titanium and tungsten.

9 A catalytic medium as claimed in Claim 8 wherein the mixed metal oxide material is a material as claimed in any one of Claim 1 to 7.

10 A catalytic material as claimed in Claim 9 wherein the catalyst is formed as particles on the catalyst support.

11 A catalytic material as claimed in Claim 10 wherein the particle size is 50 nm or less; preferably from 1 nm to 10 nm; more preferably from 5 nm to 8 nm. 12 A catalytic material as claimed in any one of claims 9 to 11 wherein the catalyst is palladium or a palladium alloy.

13 A catalytic material as claimed in Claim 12 wherein the catalyst is a palladium alloy of palladium and cobalt. 14 A catalytic material as claimed in Claim 13 wherein the palladium alloy consists essentially of palladium and up to about 70 at.% cobalt; preferably up to about 40 at.%) cobalt, more preferably up to about 30 at.%> cobalt; most preferably 15- 27 at.% cobalt.

15 A fuel cell comprising a catalytic material as claimed in any one of claims 9 to 14.

16 Use of a mixed metal oxide material as claimed in any one of claims 1 to 7 or of a catalytic material as claimed in any one of claims 8 to 14 in a fuel cell.