Patent Application
20090250353
The invention relates to nanoporous compositions, and more particularly to nanoporous materials comprising platinum, processes of making them, and methods of using them.
Nanoporous materials have high surface areas that impart unique physical and chemical properties to the materials. Some of these materials have a number of applications in the area of, for example, catalysis, fuel cells, and chemical sensors. Nanostructured platinum (Pt) materials have particularly useful electrocatalytic properties.
Several species such as tin (Sn), iridium (Ir), osmium (Os), ruthenium (Ru), tungsten (W), and Pt have been extensively studied in an effort to improve catalytic performance for such applications as the direct methanol fuel cell (DMFC). Currently, bimetallic platinum-ruthenium particles and thin films have been shown to exhibit the best activity. One limiting factor in the DMFC is the formation of the carbon monoxide (CO) intermediate from the partial oxidation of methanol. CO, a catalyst poisoning species, is chemisorped and significantly degrades the overall performance of the fuel cell. The addition of ruthenium to platinum has been shown to promote the oxidation of CO to carbon dioxide (CO2) by way of a bifunctional mechanism. Alternatively, the ligand effect mechanism, proposes an enhancement of platinum catalytic activity by way of a PtRu orbital overlap, Which modifies the electronic properties of platinum.
Nanoporous materials show promise as electrochemical sensors. For example, there is interest in developing precise and quick methods to monitor blood sugar levels for purposes of controlling and treating diabetes. Electrochemical biosensors in particular have received much attention recently for glucose detection due to their quick, concise readings using a disposable tip, which eliminates the possibility of instrument contamination.
Several processes have been developed for the preparation of nanoporous materials. For example, the present inventors have disclosed a process for making Pt nanoporous materials using electrochemical depositions (Peng, X. S., Koczkur, K., Nigro S., and Chen, A. C., 2004, Chem. Commun. 2872, incorporated herein by reference). Other processes involve multiple steps to fabricate the nanoporous materials, such as electrochemical deposition(s), template directed synthesis, multiple hydrothermal treatments, and drying the electrodes under heated air streams or under argon at elevated temperatures.
The present invention provides Pt-based nanoporous materials, processes of fabricating thereof, uses thereof, and methods of using thereof.
According to one aspect of the present invention, a method of fabricating Pt-based nanoporous materials is provided, the method comprising the steps of optionally washing a substrate, the substrate comprising titanium, tantalum, zirconium, platinum, gold, or carbon; etching the substrate to substantially remove any oxide layer from the substrate; heating the substrate in an apparatus such as an autoclave, the apparatus containing at least one reducing agent and at least one precursor solution; and cooling and removing the resulting nanoporous material from the autoclave. The nanoporous material may optionally be washed again with a suitable solvent. Optionally, Pt nanoparticles are electrodeposited on the etched substrate at a suitable current after the etching step.
According to another aspect of the invention, nanoporous Pt-based materials fabricated by the method above are provided.
According to another aspect of the invention, uses of the nanoporous Pt-based materials are provided. The uses include detection of a substrate, wherein the substrate is a biochemical substrate, wherein the biochemical substrate can be a saccharide such as glucose, galactose, fructose, lactose, maltose, or sucrose. The substrate may also be an alcohol such as methanol, ethanol, isopropanol, or the like, or may be carbon monoxide or carbon dioxide. The nanoporous Pt-based materials provided herein can also be used as electrodes in fuel cells, such as in a direct methanol fuel cell. They may also be used as catalysts in the electro-oxidation of species, such as organic species.
According to another aspect of the invention, a method of detecting a biochemical substrate using the nanoporous Pt-based materials provided herein are provide. The method comprises the steps of obtaining the nanoporous Pt-based material that reacts with the biochemical substrate to produce a signal, contacting a sample solution expected to contain the biochemical substrate with the nanoporous Pt-based material, and detecting the signal. The signal may be an electric signal and it may be detected by a response current generated by applying a voltage to the nanoporous Pt-based material. It is preferred that the nanoporous Pt-based material is substantially free of any immobilized enzyme. The biochemical substrate can be a saccharide such as glucose, galactose, fructose, lactose, maltose, or sucrose. The biochemical substrate can be measured in a sample of water, blood, urine, serum, or PBS buffer. The current can be measured amperometrically. The range of applying the voltage can be between about −0.4 V and 0.5 V versus a reference electrode, and the reference electrode can be Ag/AgCl. It is preferred that the current generated is proportional to glucose present in the sample from a range of about 0 to about 50 mM, more preferably in the range of about 0 to about 30 mM, and even more preferably in the range of about 20 mM glucose.
According to another aspect of the invention, there is provided a method of manufacturing a biochemical substrate detector, comprising the step of providing the detector with the nanoporous Pt-based material provided by the present invention. The biochemical substrate can be a saccharide such as glucose, galactose, fructose, lactose, maltose, or sucrose.
According to another aspect of the present invention, there is provided a biochemical substrate detector comprising a nanoporous Pt-based material provided by the present invention. The biochemical substrate can be a saccharide such as glucose, galactose, fructose, lactose, maltose, or sucrose.
According to another aspect of the present invention, there are provided fuel cell electrodes comprised of nanoporous Pt-based materials provided herein. The fuel cell electrodes can be used in direct methanol fuel cells.
According to another aspect of the present invention, there is provided a method of using fuel cells comprising a nanoporous Pt-based material provided herein, the method comprising the step of contacting a fuel to the nanoporous Pt-based material. The fuel may be a lower alcohol such as methanol or ethanol.
The invention is illustrated in the figures of the accompanying drawings which are meant to be exemplary and not limiting, and in which:
The present invention relates to nanoporous materials comprising Pt, PtxAy or PtxAyBz, wherein A and B are the same or different, and represent Ru, Ir, Os, Bi (bismuth), Pb (lead), Pd (palladium), Rh (rhodium) or W; x can be selected from the percent range of about 10% to about 100%, or from the percent range of about 20% to about 80%, or from the percent range of about 40% to about 60%; and (y+z) can be selected from the percent range of about 90% to about 0%, or from the percent range of about 80% to about 20%, or from the percent range of about 60% to about 40%. This invention also relates to processes for making and methods of using the nanoporous materials.
The nanoporous materials comprising Pt of the present invention are synthesized according to the process represented by the following steps:
(i) a suitable substrate is washed with a suitable solvent. The substrate can be manufactured from or comprise Ti, Ta (tantalum), Zr (zirconium), Pt, Au (gold), or suitable alloys thereof, or may be carbon, such as carbon nanotube. The substrate can be any shape, such as foil, sheet, rod, wire, and mesh. For example, the substrate can be Ti foil. Solvents suitable for washing the substrate include, but are not limited to, acetone, methanol, ethanol, and iso-propanol. It will be recognized by a person skilled in the art that there are many solvents and mixtures of solvents, including organic solvents, that can used. The substrate can be rinsed with purified water following the solvent wash. The water can have a purity of at least 10 MΩ·cm (megaohm centimeter), and can have a level of purity of at least 15 MΩ·cm. It will be recognized by the skilled person that another pure solvent could be used instead of water, such as ethanol.
The washing step (i) is optional and is performed as required to ensure that the surface of the substrate is suitably clean.
(ii) If the substrate has or is thought to have an oxide layer on its surface, as is often the case when using substrates comprising Ti, Ta, or Zr, the substrate can be etched using an acidic solution in order to substantially remove the oxide layer. Suitable acids include hydrochloride acid, nitric acid, or sulfuric acid, or mixtures thereof with strengths ranging from about 3% to about 50%, or in the range of about 5% to about 40%, or in the range of about 10% to about 35% of suitable strength and at a suitable temperature for a suitable time. The etching process can be conducted at a temperature in the range of room temperature to about 100° C. (degree Centigrade), or in the range of about 40° C. to about 90° C., or in the range of about 50° C. to about 85° C. The etching time can be a few seconds to a few hours depending on the temperature and the concentration of the acid, and it is within the ability of the skilled person to determine suitable length of time.
(iii) Optionally, Pt nanoparticles can be electrodeposited on the etched substrate at a suitable current (milliamps per centimetres squared) for a suitable length of time from a solution comprising a Pt based compound and an acid, such as H2PtCl6 and HCl. The present invention also includes processes which omit an electrodeposition step.
(iv) This step can be referred to as the hydrothermal step. The substrate can be transferred into an apparatus such as an autoclave, the apparatus containing at least one reducing agent and at least one precursor solution. Suitable reducing agents include ethylene glycol, formaldehyde, formic acid, and NaBH4. It is preferred that ethylene glycol is not used for reducing both platinum and ruthenium at the same time, as incomplete results may be obtained. However, the inventors have found that formaldehyde can reduce Pt and Ru, for example, in a single step. The precursor solution can comprise M1, M1+M2 or M1+M2+M3, where M1 can be H2PtCl6.xH2O (x is in the rang between 0 and 6), PtCl4 or PtCl2 X g/L (X can be in the range of about 0.1 and 32, or in the range of about 0.4 to about 10, or in the range of about 0.6 to about 5, or in the range of about 0.8 to about 3.8); M2 and M3 can be IrCl3.yH2O (y in the range 0 to 3), RuCl3.yH2O (y in the range 0 to 3), BiCl3, AuCl3, RhCl3, PbCl2, PdCl2, WCl6 X g/L (X can be in the range of about 0.1 and 32, or in the range of about 0.4 to about 10, or in the range of about 0.6 to about 5, or in the range of about 0.8 to about 3.8). As the skilled person will appreciate, other compounds will be suitable for use with this step of the method. The pH of the precursor solution can be lower than 7, and can be lower than 5. The pH of the precursor solution can be adjusted by adding such components as HCl, H2SO4, H3PO4, HCOOH, CH3COOH or HOOC—COOH, The substrate can be heated in the presence of the at least on reducing agent and at least one precursor solution for a suitable temperature and for a suitable period of time. The temperature can be in the range of about 80° C. and about 240° C., or in the range of about 100° C. to about 220° C., or in the range of about 120° C. and about 200° C. For example, the temperature can be about 180° C. The time period of this step can be in the range of about a few minutes to about several days, or in the range of about 5 minutes to about 24 hours, or in the range of about 10 minutes to 15 hours. The time period depends in part on the temperature used.
(v) After cooling to room temperature, the sample is taken out from the autoclave, and dried. It can optionally be washed again with a suitable solvent, such as acetone or pure water to produce a nanoporous Pt-based material, also referred to as an electrode.
The nanoporous materials described herein comprise pores with diameters in the range of about 0.1 nm (nanometer) to about 1 micrometer, or in the range of about 0.5 nm to about 500 nm, or in the range of about 1 nm to about 100 nm, in at least one dimension. The nanoporous materials are substantially non-mesoporous. The term “mesoporous” in reference to a material will be recognized by the skilled person as meaning a porous material with regularly arranged, uniform pores usually having a diameter in the range of about 2 nm to about 50 nm. As used herein, the term “non-mesoporous” refers to a material that substantially does not have regularly arranged, uniform pores as does a mesoporous material. As used herein, the term “non-mesoporous” is not intended to refer to materials having or not having pore diameters of a particular size.
The nanoporous materials described herein have relatively large surface areas and exhibit
improved electrochemical activity.
The nanoporous materials described herein can be useful for a number of applications. In particular, the nanoporous materials can be highly sensitive and selective biochemical compound sensors for biochemical compounds such as glucose, ethanol, and carbon monoxide. They can be suitable for sustaining sensitivity and selectivity under physiological glucose levels (3-8 mM), for example. The nanoporous materials can also be useful for use in fuel cells, such as a direct methanol fuel cell, for example, and in catalysis.
Embodiments of the invention are described in the following examples. The examples are meant to be exemplary only, and are not intended to limit the scope of the invention.
Products made by the following process are useful for the applications disclosed herein.
A titanium (Ti) foil was washed with acetone followed by Nanopure water (18.2 megaohm cm), then etched in 30 weight % hydrochloric acid (HCl) at 80° C. for 10 minutes to remove the thin oxide layer on the titanium surface. Then Pt nanoparticles were electrodeposited in the etched Ti substrate at −20 milliamps (mA) centimetres-2 (cm-2) for 3 minutes from a solution composed of H2PtCl6 0.8 g/L and HCl 0.3 g/L. The treated Ti substrate was transferred into an autoclave containing ethylene glycol (EG), H2PtCl6 0.8 g/L and HCl 0.3 g/L, and heated at 100 degC for 10 hours. After cooling to room temperature, the sample was washed again in acetone and Nanopure water to produce the nanoporous Pt network electrodes.
Electrodes, with varying compositions of PtRu, were produced by changing the amount of precursor solution prior to hydrothermal treatment. Activities in 0.1 M phosphate buffered saline (PBS, pH 7.4) were compared with and without 0.15 NaCl.
Nanoporous PtRu nanomaterial electrodes were made in the following manner: pieces of titanium foil were washed with Nanopure water (18.2 MΩ cm), then etched in an 18% HCl solution at 85° C. for 10 minutes to remove the oxide layer on the titanium surface. The etched substrate pieces were then washed once more and transferred to a teflon autoclave containing formaldehyde (CH20), H2PtCl6*6H2O, RuCl3*3H2O, and HCl and heated at 180° for 10 hours. After cooling to room temperature, the coated substrates were removed, air dried, then rinsed with Nanopure water. This process resulted in the synthesis of the 3D nanoporous PtRu nanomaterial electrodes.
Electrochemical experiments including cyclic voltammometry (CV) and chronoamperometry (CA) were performed with a three-electrode cell system which includes the working electrode, platinum wire as the counter electrode, and silver chloride as the reference electrode. Data acquisition and analysis were done with a CH Instruments CH1660B potentiostat. The geometric surface area of each electrode was used to calculate the current density. The electrolytes were prepared with KH2PO4 (Aldrich, 99%), NaCl (Anachemia, 99%), glucose (BDH, analytical grade) and pure water (Nanopure, 18.2 Mg cm). Ultrapure argon (BOC GASES, 99.999%) was used to deaerate all solutions prior to measurements, and passed over top of the solution during testing. All measurements were conducted at room temperature.
Scanning electron microscopy (SEM) was used to characterize the surface morphologies of 3D nanoporous PtRu alloy network electrodes after hydrothermal treatment.
Based on the SEM analysis, without being bound the theory, the inventors propose a seed growth method. At appropriate temperature and pressure, the formaldehyde reduces the platinum from Pt4+ to Pt0 and ruthenium from Ru3+ to Ru0 resulting in deposition on the titanium substrate surface. As reduction continues, the surface of the substrate becomes covered until the nanoparticles start forming on top of one another culminating in the formation of the nanoporous PtRu nanomaterial electrodes.
Electrochemical properties of the electrodes fabricated in Example 2 were determined by methods including cyclic voltammometry and chronoamperometry.
The electrocatalytic activity of the electrodes at fixed potentials and for time after injections of glucose were studied in order to further evaluate the electrodes sensing ability.
To further characterize and test the electrodes fabricated in Example 2, electrochemical experiments including CV, CA, & EIS, were performed with a 3 electrode cell system as described above. Data acquisition and analysis were done with a Solartron 1287 potentiostat, a Solartron 1252A frequency response analyzer in addition to CorrWare and Zplot software. The geometric surface area of each electrode was used to calculate the current density. EIS measurements had an amplitude of modulation potential of 10 mV and a frequency range of 40 kHz to 40 mHz. The electrolytes were prepared with H2SO4 (Aldrich, 99.999%), methanol (Calcdon, 99.9%) and pure water (Nanopure, 18.2 MD cm). Carbon monoxide (PRAXAIR, 99.9%) was used for the CO study. Ultrapure argon (BOC GASES, 99.999%) was used to deaerate all solutions prior to measurements, and passed over top of the solution during testing. All measurements were conducted at room temperature.
Electrochemical studies, as described above, were used to evaluate the electrochemical properties of the 3D alloy electrodes. The adsorption and desorption of hydrogen is a technique used to determine the active surface area of the platinum electrode. As the ruthenium concentration of the electrodes increased, the double layer capacitance increased while the structure of the hydrogen region became less defined. These observations were consistent with those of the prior art.
One factor in the DMFC is efficient removal of the CO catalyst poison species.
Another factor towards the DMFC, was the catalyst's activity towards methanol oxidation.
Charge transfer resistance and capacitance measurements were done by way of electrochemical impedance spectroscopy.
In conclusion, the inventors have successfully fabricated novel 3D nanoporous Pt and PtRu alloy network electrodes from a one step, hydrothermal method. The electrochemical studies performed indicate these samples contain a higher surface area, over 100 times for S3 and S4, than polycrystalline Pt as well as improved activity for CO oxidation and methanol oxidation. S3 and S4, with ruthenium concentrations of 38% and 56% showed desirable eletrocatalytic activity for electrochemical sensor design and fuel cell applications.
Nanoporous PtIr nanomaterial electrodes were made in the following manner: pieces of titanium foil were washed with Nanopure water (18.2 MΩ cm), then etched in an 18% HCl solution at 85° C. for 10 minutes to remove the oxide layer on the titanium surface. The etched substrate pieces were then washed once more and transferred to a teflon autoclave containing formaldehyde (HCHO), H2PtCl6*6H2O dissolved in water and IrCl3 dissolved in isopropyl alcohol), and heated at 180° for 10 hours. After cooling to room temperature, the coated substrates were removed, air dried, then rinsed with Nanopure water. This process resulted in the synthesis of the 3D novel nanoporous PtIr materials as shown in the SEM image in
Nanoporous PtPb nanomaterial electrodes were made in the following manner: pieces of titanium foil were washed with Nanopure water (18.2 MΩ cm), then etched in an 18% HCl solution at 85° C. for 10 minutes to remove the oxide layer on the titanium surface. The etched substrate pieces were then washed once more and transferred to a teflon autoclave containing formaldehyde (HCHO), H2PtCl6*6H2O and Pb(NO3)2, and heated at 180° for 14 hours. After cooling to room temperature, the coated substrates were removed, air dried, then rinsed with Nanopure water. This process resulted in the synthesis of the 3D novel nanoporous PtPb materials as shown in the SEM image in
It will be appreciated by those skilled in the relevant arts, from a reading of the disclosure, that various changes in form and detail can be made without departing from the true scope of the invention in the appended claims.
While the invention has been described and illustrated in connection with preferred embodiments, many variations and modifications, as will be evident to those skilled in the relevant arts, may be made without departing from the spirit and scope of the invention; and the invention is thus not to be limited to the precise details of methodology or construction set forth above as such variations and modifications are intended to be included within the scope of the invention. Except to the extent necessary or inherent in the processes themselves, no particular order to steps or stages of methods or processes described in this disclosure, including the Figures, is implied. In many cases the order of process steps may be varied without changing the purpose, effect, or import of the methods described.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/CA07/00935 | 5/25/2007 | WO | 00 | 6/10/2009 |
| Number | Date | Country | |
|---|---|---|---|
| 60803325 | May 2006 | US |
