WO2009057992A2

WO2009057992A2 - Process for making catalyst by impregnation of platinum on activated carbon

Info

Publication number: WO2009057992A2
Application number: PCT/MY2009/000008
Authority: WO
Inventors: Abu Bakar Mohamad; Wan Ramli Wan Daud; Abdul Amir Hassan Kadhum; Ramli Sitanggang; Mimi Hani Abu Bakar
Original assignee: Universiti Kebangsaan Malaysia
Priority date: 2007-11-02
Filing date: 2009-01-02
Publication date: 2009-05-07
Also published as: MY154540A; WO2009057992A3

Abstract

A method of impregnating platinum on activated carbon comprises bringing into contact an activated carbon sample with a dissolved platinum solution, adding a base into the platinum solution to raise the pH to a range of 9.7 to 9.9, boiling the platinum solution with distilled water and evaporating a mixture of the base and the distilled water from the platinum solution by heating it to a calcining temperature.

Description

PROCESS FOR MAKING CATALYST BY IMPREGNATION OF PLATINUM

ON ACTIVATED CARBON

Field of Invention

This present invention relates to a method of producing a catalyst for the hydrogen production. More particular, the present invention relates to a method of producing a catalyst by impregnating a platinum on an activated carbon by the application of heat and chemicals.

Background of The Invention

Platinum-impregnated activated carbon is one of the most widely used catalysts in the Membrane Electrode Assembly (MEA) fabrication of the Proton Electrolyte Membrane Fuel Cell (PEMFC). The platinum is capable of actively oxidizing hydrogen gas and methanol to provide fuel which serves as an energy source for generating the system of PEMFC.

There are several methods to deposit the platinum on an activated carbon. These methods include precipitation, adsorption, ion exchange and impregnation. Out of all these methods, impregnation is considered the most direct and simplest approach to deposit platinum on activated carbon.

The impregnation of platinum on the activated carbon is more efficient as compared to impregnation of platinum on other types of supporting material such as aluminium or any common unactivated carbon material.

Canada Patent No. '2299602 discloses a method of impregnating a supporting material. This invention relates to a powdered aluminium oxide which is stabilized with basic oxides as support material and impregnated with a solution containing at least one precursor compound of alkaline earth and rare earth metals. After drying and calcining, this post-impregnated material is further impregnated with a solution containing precursor compounds of catalytically active noble metals from the platinum group to obtain an impregnated support material.

An invention relating to a process to prepare a catalyst for the use in a hydrocarbon conversion process is claimed in United Kingdom Patent No.1348653. This prior art discloses a process for the preparation of a catalyst comprising depositing one or more noble metals and tin on a porous carrier, in which the tin deposited on the carrier is in the form of stannate.

United States Patent No.4431750 discloses a method for the preparation of supported noble metal catalysts having the noble metal impregnated substantially at the surface of a porous support. This prior invention explains a method for preparing a supported noble metal catalyst which comprises impregnating a porous support with an aqueous solution of a platinum-group metal sulfite and a rhenium compound, drying the thus- prepared composite, and heating the thus-dried composite at a temperature sufficient to decompose said sulfite, thereby producing the supported platinum-group metal catalyst.

An invention describes a method of preparing a heterogeneous catalyst is disclosed in United Kingdom Patent No.1263258. This prior invention discloses a method of preparing a heterogeneous catalyst comprising the steps of impregnating a supporting material by non-aqeuous solution, removing excess solvent from the impregnated supporting material and heating the impregnated support material under an inert atmosphere and under anhydrous condition. The supporting material may be referred as boria, silica, an oxide of a metal, clay, a diatomaceous earth, a molecular seive or carbon. Basically, these prior arts relates to various production method of impregnating a selection of noble metals on a supporting material.

With the use of suitable chemicals, the amount of platinum impregnated on an 5 activated carbon can be increased by maximizing the surface area of the activated carbon. By developing an optimum pH condition during the impregnation process, a higher quality catalyst can be obtained with an increasing amount of platinum loaded on the activated carbon. A lower cost but higher quality catalyst can be obtained by carrying out this efficient method of impregnating platinum on activated carbon. 10

Summary of The Invention

The primary object of the present invention is to develop an efficient method of impregnating platinum on activated carbon so as to produce a catalyst of higher 15 quality by using acid and alkali treatments to increase the loading of platinum powder on the activated carbon.

Another object of the invention is to produce a catalyst of platinum-impregnated activated carbon which is lower in production cost.

20

At least one of the preceding objects is met, in whole or in part, by the present invention, in which the embodiment of the present invention describes a method of impregnating platinum on activated carbon comprises bringing into contact an activated carbon sample with a dissolved platinum solution, adding a base into the

25. platinum solution to raise the pH to a range of 9.7 to 9.9, boiling the platinum solution with distilled water and evaporating a mixture of the base and the distilled water from the platinum solution by heating it to a calcining temperature. Detailed Description of The Invention

One skilled in the art will readily appreciate that the present invention is well adapted to carry out the objects and obtain the ends and advantages mentioned, as well as those inherent therein. The embodiment described herein is not intended as limitations on the scope of the invention.

The present invention discloses a method of impregnating platinum on activated carbon comprises bringing into contact an activated carbon sample with a dissolved platinum solution, adding a base into the platinum solution to raise the pH to a range of 9.7 to 9.9, boiling the platinum solution with distilled water and evaporating a mixture of the base and the distilled water from the platinum solution by heating it to a calcining temperature.

To increase the amount of platinum impregnated on the activated carbon, a chemical treatment is necessary to oxidize the activated carbon so that its surface area exposing to the platinum will be increased. This chemical treatment is achieved by oxidizing the activated carbon by nitric acid.

Based on the preferred embodiment of the present invention, the method of impregnating platinum on activated carbon is initiated by bringing into contact an activated carbon sample with a dissolved platinum solution. The platinum solution is preferably prepared by dissolving pure platinum powder in a mixture of strong acids. According to the preferred embodiment of the present invention, an aqua regia which is a mixture of hydrochloric acid and nitric acid is preferably used as the mixture of strong acids to dissolve the pure platinum powder. The aqua regia contains 65% concentrated nitric acid and 37% concentrated hydrochloric acid. The volume ratio of concentrated nitric acid to concentrated hydrochloric acid of the aqua regia is 1:3. According to the preferred embodiment of the present invention, the pure platinum powder is dissolved in aqua regia by heating the platinum solution to a boiling temperature until no more trace of platinum is to be seen in the solution. As described in the preferred embodiment, the preferred boiling temperature is in a range from 55°C to 75°C and the time for dissolving the platinum powder is at least 45 minutes before a platinum solution can be formed. In the present invention, the most preferred boiling temperature to dissolve the platinum powder is 350°C. In the preferred embodiment, the type of platinum powder used is preferably a platinum hydrogen chloride (HaPtCl₆) or platinum ammonium chloride [Pt(NHs^Cl₂].

According to the preferred embodiment of the present invention, a mixing step of activated carbon with a solution of platinum in aqua regia is initiated in a method to impregnate platinum on activated carbon. During the mixing process, the platinum chloride anions (PtCl₆ ^2") in the solution will be adsorbed by protonated oxygen groups appeared on the surface of the activated carbon. This protonated oxygen groups are referring to protonated carboxyl groups (-COOH₂ ⁺) and protonated hydroxyl groups (OH₂ ⁺). A mechanism showing the dissociation of a protonated hydroxyl group (OH₂ ⁺) on the surface of an activated carbon is shown as below:

Activated Carbon -OH₂ ⁺ > Activated Carbon -OH + H⁺

The activated carbon used is preferably of a powder form and it is oxidized with nitric acid to increase its exposing surface area to the loaded platinum particles. The preferred particle size of the activated carbon powder is in a range from 20 μm to 30μm in length according in the preferred embodiment of the present invention. The most preferred particle size of the activated carbon to be used in the present invention is 30μm in length. In the preferred embodiment, the platinum solution is heated slowly at a temperature range from 55⁰C to 75°C for at least 5 hours at a range of stirring rate from 95rpm to 105rpm until the solution is appeared as partially dried, ensuring a complete reaction has occurred. According to the present invention, the most preferred temperature is 60⁰C and the most preferred stirring rate is lOOrpm when heating the platinum solution.

A base is added into the platinum solution to raise the pH to a range of 9.7 to 9.9 as disclosed in the embodiment of the present invention. The base is added into the platinum solution so that an optimum alkaline pH condition can be reached to allow better loading of platinum on the activated carbon. According to the present invention, the base is preferably a weak alkali which added into the platinum solution by titration. The preferred pH condition for this invention is in between pH9.7 and pH9.9 whereas the most preferred pH value is pH9.8 as described in the preferred embodiment. The base used in the preferred embodiment of the present invention is preferably ammonium hydroxide (NH₄OH).

According to the preferred embodiment, when the base is added into the platinum solution, the ammonium cations (NH₄ ⁺) in the base will replace the proton of the hydroxyl groups on the activated carbon surface. This replacement of ions is shown in the following mechanism:

Activated Carbon -OH + NH₄ ⁺ > Activated Carbon -OTS[H₄ ⁺ + H⁺

At the same time, the platinum chloride ions (PtClβ^2") will be adsorbed on the activated carbon by the attraction of cations OTSTH₄ ⁺ . As described in the preferred embodiment of the present invention, the platinum solution is preferably heated at a temperature range from 55°C to 75 °C for at least 20 hours at a range of stirring rate from 95rpm to 105rpm until the solution is partially dried, ensuring a complete reaction has occurred. As described in the most preferred embodiment of the present invention, the most preferred temperature is 60°C and the most preferred stirring rate is lOOrpm for heating the platinum solution. In one of the embodiments of the present invention, a step of boiling the platinum solution with distilled water is disclosed. The main purpose of this boiling step is to wash away acid residues and ammonium salts. The boiling step of the platinum solution is achieved by adding distilled water into it and heating it at boiling temperature until a lesser amount of solution volume is left. As disclosed in the preferred embodiment of the present invention, the platinum solution will be heated until a three quarter of the solution volume is left. The heated platinum solution is allowed to cool, resulting in one layer of clear liquid and another layer of suspension particles, in which the layer of clear liquid is to be discarded. These steps of heating and cooling are repeated for several times to obtain a platinum solution with a stabilized pH. According to the preferred embodiment, the preferred pH of the platinum solution is in a range of pH3 to pH4 whereas the most preferred pH in the present invention is pH3.7.

In a preferred embodiment of the present invention, a step of evaporating a mixture of the base and the distilled water from the platinum solution by heating it progressively to a calcining temperature is disclosed. The platinum solution is dried progressively in an oven at a temperature range from 100⁰C to 110⁰C for 4 hours to 6 hours to evaporate the excess water that clogs the pores of the activated carbon. In the most preferred embodiment of the present invention, the platinum solution is dried progressively at a temperature of HO⁰C for 2 hours. This evaporating step is necessary to achieve homogeneity of the platinum impregnated on the activated carbon.

According to the present invention, the calcining step is taken place in an inert environment where nitrogen gas is allowed to flow through the whole calcination process. The step of evaporating the platinum solution by heating it progressively to a calcining temperature is performed at two different temperatures. It is preferably carried out at a low temperature and then followed by a higher temperature. The platinum solution is evaporated and calcined by pre-heating it from 120°C to 130°C for a time range from 3 hours to 4 hours and then followed by a heating step from 340°C to 360°C for 2 hours to 3 hours in an inert environment as stated in the preferred embodiment..

According to the most preferred embodiment, the step of evaporating the platinum solution by heating it progressively to a calcining temperature is performed at a lower temperature of 100°C for an hour and then followed by a higher temperature of 500°C for 4 hours in helium atmosphere as stated in the preferred embodiment. The main purpose of the calcination step is to drive off volatile impurities from the bulk of platinum-impregnated activated carbon, since the volatile impurities can interfere with the performance of the activated carbon sample impregnated with platinum.

The platinum-impregnated activated carbon is used as a catalyst in the Polymer Electrolyte Membrane Fuel Cell (PEMFC), Phosphoric-acid Fuel Cell (PACF) and industries that need to attract hydrogen for its system usage. The fuel cell is an efficient energy converter and an alternative electrical power source for the future use.

In this system, the catalyst is used to expedite the hydrogen ionization to proton and electron at the anode while the reduction of oxygen at the cathode. Consequently this will produce electricity.

According to the present invention, an activated carbon sample impregnated with a loading of platinum at least 40% by weight of the activated carbon is produced. The activated carbon is used as a catalyst support which allowed high degree of platinum dispersion, stable under reaction and regeneration conditions and not adversely affected by solvents, reactants or reaction products. According to the embodiment of the present invention, the activated carbon used is preferably in a powder form with particle size of 30μm, surface area of 1686.88m²/g and micro-pore diameter oil.91 K. In the present invention, there are three techniques being performed for treating the activated carbon before carrying out the process of impregnation to produce a catalyst. In the first technique for treating the activated carbon, distilled water is used to treat the activated carbon by boiling and filtration. This process has to be carried out for several times until the pH of the activated carbon is reduced from 10 to a pH range of 6.28 to 7.58. The activated carbon is then transferred to an oven and heated at a range of temperature from 100⁰C to 110°C for 12 hours to dry the activated carbon.

After the drying step, the activated carbon is heated in a horizontal furnace to remove impurities from the activated carbon. Besides, this heating step will also increase the surface area and the number of pores on the activated carbon. This heating step is performed at a temperature of 350⁰C for 2 hours and then followed by a second step of heating at a temperature of 900⁰C for 12 hours. Both of these heating steps are occurred with gas nitrogen flowing across the horizontal furnace. The first technique of treating the activated carbon is involved a step of grinding the activated carbon at a speed of 30rpm for about an hour.

The second technique for treating the activated carbon as described in the present invention is included a step of oxidizing the activated carbon with acid such as nitric acid or hydrochloric acid. The step of oxidizing the activated carbon is performed by heating it in 100ml of 6M nitric acid solution at the boiling point of the acid. The purpose of performing this step is to remove the unwanted metal residues from the surface of the activated carbon. The activated carbon is separated from the acid solution by filtration and boiled with distilled water for several times to reduce the pH of the activated carbon. The activated carbon is then dried in an oven at a temperature of 110⁰C for 4 hours. After the drying step, the activated carbon is heated in a furnace at a temperature of 300⁰C for 2 hours and then followed by a second step of heating at a temperature of 900⁰C for 12 hours. As discussed in the present invention, the activated carbon is mixed with sodium chloride salt to prevent clumping of the activated carbon particles. The mixture of activated carbon and sodium chloride salt is soaked in distilled water to dissolve the salt and form a mixture solution. The top layer of activated carbon particles in the mixture solution is collected and centrifuged at a speed of 15,500rpm for 10 minutes. A top layer of colorless dissolved salt solution and a bottom layer of activated carbon particles suspension are formed after the centrifugation. The layer of dissolved salt solution is to be discarded and replaced with distilled water. The centrifugation step is then repeated for four more times, in which at the fourth time, the centrifugation speed is at 4000rpm for 10 minuets. After the centrifugation, the smaller particle size of the activated carbon is formed on top of the solution whereas the bottom layer of the solution will contain larger particle size of the activated carbon. Both particle sizes of the activated carbon can be used as the support catalyst for impregnation of platinum during analysis.

In a preferred embodiment of the present invention, the platinum powder impregnated on an activated carbon to produce a catalyst is preferably having a mean particle size of 28μm.The loading of platinum on activated carbon is controlled by using the appropriate ratio of platinum and activated carbon in the formulation during impregnation. For a 10% loading of platinum by weight of the platinum-impregnated activated carbon catalyst, the ratio of platinum to activated carbon is 1:10 according to the preferred embodiment of the present invention. If a 20% of platinum is loaded on the activated carbon by weight of the catalyst, the ratio of platinum to activated carbon will be 1 :4.

Claims

1. A method of impregnating platinum on activated carbon comprises: a) bringing into contact an activated carbon sample with a dissolved platinum solution; b) adding a base into said platinum solution to raise the pH to a range of 9.7 to 9.9; c) boiling said platinum solution with distilled water; and d) evaporating a mixture of said base and said distilled water from said platinum solution by heating it to a calcining temperature.

2. A method as claimed in claim 1, wherein said dissolved platinum solution is obtained by dissolving platinum powder in aqua regia.

3. A method as claimed in claim 1 or claim 2, wherein said base is ammonium hydroxide.

4. A method as claimed in any of the claims 1 to 3, wherein said evaporating step is performed by pre-heating said platinum solution at a temperature ranged from 12O⁰C to 13O⁰C followed by a heating step at a temperature ranged from 340⁰C to 360⁰C.

5. An activated carbon sample impregnated with platinum produced by any of the preceding claims characterized in that loading of the platinum is at least 40% by weight of the activated carbon.