SI24372A - A method of deposition of Pt or Pd catalysts using gaseous reducing agents - Google Patents

Abstract

The present invention provides a practical method for the simple application of metal catalytic nanoparticles or layers at low temperatures. Using a reducing gas in the gaseous phase, it is possible to apply metal platinum or palladium to various substrates. The present invention solves the problem of applying a catalytic platinum or palladium to flexible thermoplastic substrates that are not stable at higher temperatures. In this way, prepared platinum or palladium is useful as an immobilized catalyst or electro catalyst in Graz cells, fuel cells, and for electro-oxidation / reduction of organic compounds. Furthermore, the present invention provides a practical method for applying electrically conductive metal layers at low temperatures with excellent grip and low resistance.

Description

Method for the application of a platinum or palladium catalyst using a gaseous phase reducing agent

FIELD OF THE INVENTION

The present invention relates to a method of depositing platinum or palladium nano-particles or layers at low temperature. More specifically, the invention relates to a novel method of applying platinum or palladium by chemical reduction of a platinum or palladium precursor (complex) using a gaseous phase reduction reagent.

A technical problem

Among perspective types of solar cells, color-sensitive solar cells (DSSCs) are relatively inexpensive and have a favorable production-cost ratio (B. O'Regan, M. Gratzel, Nature 353 (1991) 737-740; TN Murakami , M. Gratzel, Inorg. Chim. Acta 361 (2008) 572-580). Part of the DSSC is a counter electrode that catalyzes the reduction of triiodide to iodide, thus regenerating the electrolyte and allowing electricity to be produced. A typical electrode is prepared by applying a thin layer of catalyst to an optically transparent electrode (OTE), typically SnO ₂ : F coated glass. Of the various catalysts that can catalyze said reaction, platinum is the most effective.

Lately, a lot of effort has been put into applying thin layers of platinum at low temperature and developing flexible DSSCs. We would like to extend the possibility of their use and remove restrictions due to the rigidity of the glass OTE used in conventional solar cells. The main problem in achieving this is the relatively high temperature required to reduce the deposited platinum precursor to metal platinum (above 400 ° C), which is too high for plastic substrates (P. Li, J. Wu, J. Lin, M. Huang, Z. Lan, Q. Li, Electrochim. Acta (2008) 53, 4161-4166).

There are several different methods for applying platinum particles at low temperature to different substrates, but most require complex and expensive equipment or the procedures are too complicated for practical use. It is important to develop a simple and effective method of depositing platinum nano-particles at low temperatures. One way to achieve this is to use a reducing agent, but this method has several disadvantages:

(1) poor adhesion of platinum particles to the surface (2) the layers are not uniform due to the pooling of the particles in the liquid phase and point application due to the droplet formation during solution drying (3) the platinum particles thus prepared have a poor adhesion and can be easily wiped off.

Description of the invention

The present invention is the result of the needs mentioned above and allows for a simple reduction of the platinum or palladium precursor at low temperatures and maintains or maintains. improves platinum good properties reduced at temperatures above 400 ° C. In the presented manner, it is possible to prepare electrocatalytic platinum or palladium particles on transparent electrodes that can be used in electrochemical solar cells.

In order to achieve this, the present invention proposes a method of chemical reduction of a platinum or palladium precursor at low temperatures using reducing agents in the gaseous phase. Applicants have found that applying a liquid reducing agent directly to the substrate causes agglomeration of platinum or palladium nano-particles. The precursor dissolves in the reducing solution and high surface tension prevents the formation of a uniform layer during drying. For more evenly distributed platinum or palladium metal particles, the samples were exposed to reducing vapor. Since the precursor cannot be dissolved in the reduction process, no particle movement occurs and the quality of the thin layer depends only on the application of the precursor solution and on the degree of reduction at the solid-gas interface. The applied platinum or palladium nano particles have excellent adhesion and cannot be wiped off or removed in an ultrasonic bath. Good adhesion results from the interaction of nano-particles with the substrate surface and is related to the surface energy of the particles. As the surface energy decreases rapidly after particle formation due to the redistribution of atoms on the surface, the adhesion can be improved by slowing the aging of the nanoparticles before their binding. This was achieved by forming nanoparticles directly on the substrate surface at the solid-gas phase boundary.

Applicants have found that the present invention allows the application of an electrically conductive layer of platinum or palladium on flexible substrates. By increasing the amount of platinum or palladium deposited above 30 pg cm ' ^2, platinum or palladium particles prefer to form a continuous layer rather than isolated particles, resulting in the ability to conduct electricity.

It is further possible to extend the present invention to the preparation of palladium metal nanoparticles or palladium conductive layers in the same manner as platinum.

Short description of the pictures

Figure 1 shows the XPS spectrum of reduced platinum samples.

Figure 2 shows the XPS spectrum of reduced palladium samples.

Description of the process according to the invention

The method for depositing the platinum nano-particles according to the invention comprises two steps. The first stage is the application of the platinum precursor complex. First, we dissolve the precursor complex, usually H ₂ PtCl6, in a suitable solvent. Typically, it is an organic solvent with low surface tension, for example ethanol, which allows a uniform layer of precursor to be applied. Typically, a rotation or smear method is used to apply the precursor to the substrate, but it is also possible to apply the platinum complex immersion, spray or adsorption technique. The concentration and volume of the precursor solution depends on the amount of platinum desired. The layers are then dried at room temperature in air.

The second step involves exposing the platinum precursor to the gaseous phase, usually formic acid or ethylene glycol, at temperatures above 100 ° C, between 100 ° C and 170 ° C, which is best accomplished by placing the substrate and a small amount of the reducing agent together in not a tightly closed container. The reducing agent may initially be in a liquid state but evaporate while heating the vessel to the desired temperature. It is desirable for the vapor of the reducing agent to slowly escape from the container. This prevents condensation of the reducing agent on the sample surface. The presence of air does not affect the reaction. After reduction, the prepared layers are cooled in air.

The method described in the present invention can be divided into two separate steps:

(1) application of precursor complex using various techniques (2) reduction of precursor complex using gaseous phase reducing agent.

In order to prepare conductive platinum layers, the amount of platinum applied must be increased, which in turn increases the thickness of the layer and results in greater agglomeration of the particles during the application of the precursor solution. To achieve the desired layer uniformity and good adhesion of the applied layers, a thin transparent layer of a hydrophilic layer such as TiO ₂ or ZnO was first applied. This layer has two roles. First, it is hydrophilic, its hydrophilicity increasing further during drying of the sample under UV light, which prevents the formation of droplets of precursor solution during drying and allows a uniform application (R. Sun, A. Nakajima, A. Fujishima, T. VVatanabe, K. Hashimoto, J Phys. Chem. B, 105 (2001) 1984-1990. Second, it acts as a bonding element between the platinum layer and the substrate, allowing a thicker layer to be applied with good adhesion.

Platinum translation layers are prepared by smearing with a suitable amount of precursor solution on a TiO ₂ coated substrate and dried under UV light. The dry precursor layer is exposed to the gaseous reducing agent at temperatures above 100 ° C.

The palladium nano-particles or conductive layers are prepared in the same way as platinum, replacing only the platinum precursor complex with that containing palladium. Typically, it is H ₂ PdCl4 or salts thereof.

XPS was used to confirm that the reduction of the applied platinum (Fig. 1) and palladium (Fig. 2) had expired to the metallic state.

Implementation examples

The present invention is described in detail in the examples which do not, however, limit the delayed invention.

Example 1: Deposition of electro-catalytic platinum nanoparticles on conductive substrates

A suitable amount of precursor was applied to a transparent glass with a fluorine doped SnO ₂ (SnO ₂ : F) translucent glass. The substrate is then air-dried at room temperature. After drying, the precursor layer is exposed to formic acid vapor at 100 ° C for 15 minutes.

Example 2: Application of electrically conductive platinum layers

The substrate is first coated with a thin compact TiO ₂ layer and then the smear complex is applied to the platinum precursor complex. Approximately 20 μΙ_ cm ′ ² 0.01 M H ₂ PtCl6 solution is brought to the substrate surface and air-dried under UV light. After drying, the precursor complex is reduced by subjecting the substrate to formic acid vapor at 100 ° C for 15 minutes.

Claims (8)

Claims: A method for depositing a platinum or palladium nano-particle or layer, comprising the application of a precursor complex solution to a substrate and the chemical reduction of a precursor complex, using a gaseous reducing agent formulation for the chemical reduction of the precursor complex. 2. The method of claim 1, wherein the precursor complex is a soluble platinum complex, preferably H ₂ PtCl ₆ or a soluble palladium complex, preferably H ₂ PdCl4. 3. The method of claim 1, wherein the reducing agent is formic acid, ethylene glycol, methanal, ethanal, propanal or propanenediol. Method according to claim 1, characterized in that the chemical reduction is carried out at a temperature between 100 ° C and 170 ° C, preferably at 150 ° C for 15 minutes. 5. Grazl cell counter electrode prepared according to claim 14. 6. Platinum or palladium electrocatalyst prepared according to claims 1-4. An immobilized platinum or palladium catalyst prepared according to claims 1-4. An electrically conductive platinum or palladium layer prepared according to claims 1-4, wherein the thin transparent hydrophilic layer is applied to the substrate prior to application of the precursor complex solution.