WO2009040539A2

WO2009040539A2 - A method of activating a composition

Info

Publication number: WO2009040539A2
Application number: PCT/GB2008/003265
Authority: WO
Inventors: Aleksander Jerzy Groszek; Jerzy Haber; Erwin Lalik
Original assignee: Microscal Limited
Priority date: 2007-09-26
Filing date: 2008-09-25
Publication date: 2009-04-02
Also published as: GB2453140B; WO2009040539A3; GB0718829D0; GB2453140A

Abstract

A method of activating a composition comprising a transition metal selected from at least one of gold, nickel, copper, ruthenium, molybdenum and platinum, the method comprising: exposing the composition to at least one pulse of an atmosphere comprising a noble gas; and exposing the composition to at least one pulse of an atmosphere comprising oxygen.

Description

A method of activating a composition

The present invention relates to methods of activating compositions comprising a transition metal selected from at least one of gold, nickel, copper, ruthenium, molybdenum and platinum.

Methods of activating catalysts are known in the art . EP0034403, for example, discloses activation of a titanium/iron alloy by initially exposing the alloy to hydrogen, followed by an outgassing step and then alternate pressurizing of the catalyst bed with hydrogen and outgassing. The process of successively hydriding and dehydriding the catalyst is described as breaking up the catalyst granules into smaller particles, producing multiple cracks in the surface of each individual granule, thereby greatly increasing the reactive surface area of the catalyst bed. EP0034403 teaches that when helium is used in the dehydriding step, extreme care must be taken to avoid oxygen impurities. This document teaches that such impurities may permanently poison the catalyst by forming a stable oxide upon contact .

A further example of the use of gas to activate catalysts is described in PCT/GB2005/002212. This document discloses a method for preparing a catalyst comprising a metal having absorbed therein hydrogen, wherein the metal is selected from at least one of, gold, ruthenium, or molybdenum, The method comprises the step of exposing a sample to an atmosphere comprising nitrogen or a noble gas, followed by exposing the sample to an atmosphere comprising hydrogen and/or a hydrogen source, whereby hydrogen is absorbed into the sample. The sample is then exposed to an atmosphere comprising noble gas or an atmosphere comprising nitrogen, followed by an atmosphere comprising hydrogen and/or a hydrogen source. There is , however, no disclosure or suggestion in this document of the use of oxygen to activate a catalyst.

Compositions comprising transition metals selected from at least one of gold, nickel, copper, ruthenium, molybdenum and platinum which are capable of reversibly absorbing hydrogen may be of use in many fields of technology. For example, such compositions may be of use as catalysts and are capable of being used in hydrogen fuel generation and processing. Therefore methods of improving the reactivity and/or absorption properties of compositions comprising a transition metal selected from at least one of gold, nickel, copper, ruthenium, molybdenum and platinum are of commercial interest.

Physical and chemical interactions of solid surfaces with gases result in the evolution of heat. The existence of this thermal effect has long been recognised. The heat evolution can be measured using a flow-through microcalorimeter . A flow-through microcalorimeter can also be used to measure concurrently the uptake of the interacting gases, heat evolution, the sorption of gases and their displacement with an inert carrier gas, such as nitrogen, at a range of temperatures and pressures.

Flow-through microcalorimetry can be used to measure heat evolution produced when compositions comprising transition metals are placed in contact with gases at temperatures ranging from 20⁰C to 240⁰C. The gas absorbed and/or adsorbed in this process can be slowly desorbed from the transition metal by passing a carrier gas over the sample, for example, pure nitrogen. This generates a negative heat effect. The desorption could, however, be triggered by a simple absence of the specified gas.

The present inventors have surprisingly found that the properties of compositions comprising a transition metal selected from at least one of gold, nickel, copper, ruthenium, molybdenum and platinum can advantageously be modified if the composition is treated using the method of the present invention. In particular, it has surprisingly been found that the properties of the composition may be improved by exposing it to small amounts of both a noble gas and oxygen.

In one aspect of the present invention there is provided a method of activating a composition comprising a transition metal selected from at least one of gold, nickel, copper, ruthenium, molybdenum and platinum, the method comprising: exposing the composition to an atmosphere comprising at least one pulse of an atmosphere comprising a noble gas,- and exposing the composition to at least one pulse of an atmosphere comprising oxygen.

In a preferred embodiment, the method of the present invention further comprises exposing the composition to an atmosphere comprising hydrogen or a hydrogen source after exposing it to an atmosphere comprising at least one pulse of a noble gas and to at least one pulse of oxygen. Preferably, the amount of hydrogen absorbed by the composition after being exposed to an atmosphere comprising at least one pulse of a noble gas and to at least one pulse of an atmosphere comprising oxygen is greater than the amount of hydrogen absorbed prior to activation of the composition. The atmosphere comprising hydrogen may comprise hydrogen or a hydrogen source. It will be understood that the hydrogen source may be, for example, methane or methanol .

It is the combined effect of exposure of the composition to both an atmosphere comprising a pulse of a noble gas and to an atmosphere comprising a pulse of oxygen which the inventors have found leads to the surprisingly large and advantageous increase in surface reactivity. Increases in hydrogen absorption of compositions comprising a transition metal selected from at least one of gold, nickel, copper, ruthenium, molybdenum and platinum after activation with both a noble gas and oxygen are much larger than any activation observed after treatment with either a noble gas without oxygen treatment, or treatment with oxygen and without a noble gas .

The description of the present invention is, unless otherwise stated, applicable to each aspect of the invention.

It will be understood that the term absorption as used herein does not preclude adsorption of gases from the composition. Preferably, after treatment of the composition comprising a transition metal selected from at least one gold, nickel, copper, ruthenium, molybdenum and platinum by a noble gas and oxygen as described herein, the absorption (sorption) properties of the composition will be improved. For example, in a preferred embodiment the quantity of absorbed hydrogen per volume unit of the composition following treatment of the composition is greater than the quantity of absorbed hydrogen per volume unit of the composition prior to treatment. The sorption of hydrogen can be measured by the heat evolution, which accompanies sorption. This can be measured using a flow microcalorimeter. The experiments have revealed that the amount of heat released from the activated composition upon hydrogen sorption may be comparable to thermal effects measured in chemical reactions. This indicates that by exposing the composition to an atmosphere comprising at least one pulse of a noble gas and to an atmosphere comprising at least one pulse of oxygen the behaviour of the composition comprising a transition metal selected from at least one of gold, nickel, copper, ruthenium, molybdenum and platinum affects strongly the transition metal-hydrogen system.

The gases are preferably pre-dried before use.

As used herein the term "pulse" is used to describe exposing a composition to a specified gas for a short period of time, typically milliseconds, seconds or minutes. The length of exposure will depend on the desired amount of gas that is to be exposed to the composition and, for example, the flow rate of the gas etc. A pulse as used herein is not a meant to describe a continuous or extended period of exposure of a gas to the composition.

Preferably the composition is exposed to an atmosphere comprising from 5 to 500 μmol of a noble gas per gram of transition metal, more preferably from 50 to 250 μtnol of a noble gas per gram of transition metal, more preferably still from 80 to 150 μmol of a noble gas per gram of transition metal .

The inventors have found that typically if the composition is exposed to an atmosphere comprising less than 5 μmol of a noble gas per gram of transition metal then the advantageous activation of the composition when combined with oxygen exposure is not observed. The inventors have discovered that the advantageous improvement in the sorption properties/ activation of the composition may be achieved without saturating the composition with a noble gas.

The composition may be exposed to an atmosphere comprising from 0.01 to 10 μmol of a oxygen per gram of transition metal . Preferably the composition is exposed to an atmosphere comprising at least to 0.2 μmol of a oxygen per gram of transition metal, more preferably from 0.5 to 5 μmol of a oxygen per gram of transition metal, more preferably still from 1.0 to 5 μmol of a oxygen per gram of transition metal .

The inventors have found that typically if the composition is exposed to less than 0.01 μmol of a oxygen per gram of transition metal then the advantageous activation of the composition is not observed. If the composition is exposed to greater than 10 μmol of oxygen per gram of transition metal there is an increased risk that the oxygen will adversely effect the surface activity of the composition.

Preferably, the atmosphere comprising a noble gas comprises at least 50% by volume of noble gas, more preferably at least 70% by volume of noble gas, more preferably at least 99% by volume of noble gas.

All noble gases may be used. The noble gas preferably comprises argon, neon, helium, or a mixture of two or more thereof. More preferably the noble gas comprises one of at least argon and neon. Most preferably the noble gas comprises argon.

Preferably, the atmosphere comprising oxygen comprises at least 5% by volume of oxygen, more preferably at least 20% by volume of oxygen, more preferably at least 509% by volume of oxygen. The atmosphere comprising oxygen may comprise oxygen gas or an oxygen source. The atmosphere comprising at least one pulse of oxygen may comprise an atmosphere comprising air. Preferably the oxygen source is oxygen gas.

In one embodiment of the present invention the composition is exposed to an atmosphere comprising a noble gas before it is exposed to an atmosphere comprising oxygen.

In an alternative embodiment the composition is exposed to an atmosphere comprising a noble gas before it is exposed to an atmosphere comprising oxygen. In an further embodiment the composition is exposed to an atmosphere comprising a pulse of a noble gas and simultaneously it is exposed to an atmosphere comprising a pulse of oxygen. This embodiment has the advantage that it simplifies the treatment procedure.

In one embodiment of the present invention, preferably prior to activation of the composition, the composition is exposed to a vacuum. Treatment of the composition in this way has the advantage that unwanted water and gaseous impurities are removed from the composition prior to activation. In another embodiment, the composition is exposed to an atmosphere comprising nitrogen prior to activation. For example, the composition may be saturated with pure nitrogen for 20 hours at a flow rate of 1 cc/ minute. Preferably the atmosphere comprising nitrogen comprises at least 90% by volume of nitrogen, more preferably at least 99% by volume, more preferably still at least 99.999% by volume of nitrogen.

Without wishing to be bound by any theory, the nitrogen is believed to act as an inert gas. Exposing the composition to a flow of nitrogen prior to activation (i.e. prior to exposure to a pulse of noble gas and a pulse of oxygen) is also thought to help remove gaseous impurities that may be present in the composition.

In a preferred embodiment of the present invention, the composition is exposed to a substantially continuous flow of nitrogen, into which at least one pulse of a noble gas and at least one pulse of oxygen is introduced. After activation of the composition, the flow of nitrogen may be stopped and replaced by a flow of hydrogen.

The composition may be exposed to an atmosphere comprising hydrogen after being exposed to an atmosphere comprising at least one pulse of a noble gas and to an atmosphere comprising at least one pulse of oxygen. The composition may be exposed to an atmosphere comprising hydrogen prior to activation with a noble gas and/or oxygen. However, the present inventors have found that improved hydrogen absorption is observed when the composition is exposed to hydrogen after having been exposed to an atmosphere comprising at least one pulse of a noble gas and at least one pulse of oxygen. Preferably, the amount of hydrogen absorbed by the composition after being exposed to an atmosphere comprising at least one pulse of a noble gas and to an atmosphere comprising at least one pulse of oxygen is greater than the amount of hydrogen absorbed prior to activation of the composition.

The transition metal in the composition is preferably in the form of powders, particles, fibres, flakes or sponges or may be deposited on a catalyst support. The composition may also be a metal alloy. The composition is preferably in the form of a pure metal powder. The metal may be selected from at least one of gold, nickel, copper, ruthenium, molybdenum and platinum. Preferably the composition comprises only one metal selected from gold, nickel, copper, ruthenium, molybdenum and platinum. More preferably the composition comprises gold or platinum. More preferably, the composition comprises gold. The metal may be in the form of deposits on catalyst supports, such as TiO₂, silica, graphite or iron oxides. The metal preferably has a purity of at least 99% and most preferably a purity of at least 99.99%. The purity of the metal may be measured using atomic spectroscopy.

In one embodiment of the present invention there is provided a method of improving the hydrogen absorption properties of a catalyst comprising a transition metal selected from at least one of, gold, nickel, copper ruthenium, molybdenum and platinum, the method comprising activating the catalyst using the method described herein and subsequently exposing the pre-treated catalyst to an atmosphere comprising hydrogen or a hydrogen source, wherein the amount of hydrogen absorbed by the catalyst is more that the amount of hydrogen absorbed prior to pre-treatment of the catalyst.

A suitable temperature range for the present invention is 20⁰C to 300⁰C. The present invention may also be carried out at room temperature. When the composition comprises gold preferably the temperature range for the present invention is from 2O⁰C to 150⁰C. When the composition comprises molybdenum and/or ruthenium preferably the temperature range for the present invention is from 200⁰C to 300⁰C. When the composition comprises platinium preferably the temperature range for the present invention is from 20 to 300 C.

The present invention may be carried out at pressures from atmospheric pressure (approximately 10⁵ Pa/g) to 150 bar/g (1.5 x 10⁷ Pa/g) . Most preferably the pressure is between atmospheric pressure (approximately 10⁵ Pa/g) and 30 bar/g (3 x 10⁶ Pa/g) .

In one embodiment of the present invention, there is provided a method of activating a composition comprising pure gold metal, the method comprising, optionally- saturating the gold metal with an atmosphere comprising nitrogen, preferably pure nitrogen gas, then exposing the gold to at least one pulse of an atmosphere comprising a noble gas and to at least one pulse of an atmosphere comprising oxygen gas (preferably in the form of air) . Preferably, the pulse of an atmosphere comprising noble gas comprises from 80 to 150μmol of noble gas per gram of gold, and the pulse of an atmosphere comprising oxygen comprises from 0.2 to lOμmol of oxygen per gram of gold. When the gold has been activated in this way, subsequent exposure of the gold to an atmosphere comprising hydrogen or a hydrogen gas source results in a higher absorption of hydrogen than an equivalent sample of gold which has not undergone the activation steps described.

Examples :

The present invention will now be described further, by way of example only, with reference to the following Examples .

Equipment : The surface energy measurements were carried out using a Microscal Flow-trough Microcalorimeter as described in Chemistry and Industry 25th March 1965, pages 482 to 489 and Thermochimica Acta, 312, 1998, pages 133 to 143. In the experiments described the tubes were extensively purged with the gases under examination to remove the oxygen adsorbed on the walls of the tubes.

The adsorption experiments described herein were conducted by exchanging the flow of nitrogen for those of pure hydrogen, oxygen, noble gas or the gas under investigation. The resulting exposures of the metals to the gases were maintained for seconds or minutes for the pulse experiments, or hours to achieve complete saturation, i.e. until no further uptake of the interacting gases was recorded by the thermal conductivity detector. The pulses were separated by nitrogen flows long enough to remove any oxygen or noble gas that was not retained (absorbed) by the metal powders.

In order to ensure that the pulses of the gases passing through stainless steel capillaries are free from any impurities, especially any adsorbed oxygen on the internal walls of the steel tubing, purification of the internal walls of the tubing was carried out in each case, before the exchanges, for example, by passing at least 100 cc of each gas through the tubing before their exchanges with nitrogen flows .

Example 1

In this example, all the exposures of the composition were carried out by sequentially exchanging the flow of a nitrogen carrier gas to the gases specified below. The gas flow rates for all the gases used were 60 cc per hour. The experiment was carried out at 125⁰C and atmospheric pressure . In this experiment, 561tng of pure gold was first exposed to a flow of nitrogen. 180 μmol of pure helium gas was then introduced into the chamber containing the composition. The composition was then exposed to a flow of pure (99.999%) hydrogen gas for 10 minutes. The flow of nitrogen was then restored and 3 μmol oxygen gas in the form of 0.33 cc of air was introduced into the chamber comprising the gold powder. A pulse of 180 μmol of helium gas was then introduced into the chamber followed by nitrogen. The composition was then exposed for ten minutes of hydrogen gas . As can be seen from Table 1, the uptake of hydrogen and the heat evolution were not significantly increased after exposure of the gold to a pulse of noble gas. However when the composition was exposed to a pulse of noble gas (helium) , then to a pulse of oxygen followed by exposure to a flow of hydrogen a very high heat evolution occurred accompanied by a substantial increase in hydrogen uptake. After a 20 hour nitrogen purge followed by a pulse of helium, without a repeated pulse of 02, the repeated exposure to hydrogen produced a much reduced hydrogen uptake and heat evolution.

The heat evolutions and hydrogen uptakes on the 561 mg of pure gold powder are shown in Table 1.

Table 1

As can be seen from Table 1, the addition of oxygen (step 3) and helium in step 4 (above) to the gold sample can be seen to dramatically increase the heat evolution on exposure of the sample to hydrogen in step 5. However, the subsequent hydrogen exposure without the addition of oxygen gives a much lower heat evolution (see step 7) .

The composition was then exposed to nitrogen for 72 hours to remove any remaining gases in the composition and then exposed to a pulse of oxygen (3 μmol) (step 9) , without the exposure to a noble gas, and subsequently to ten minute exposure to hydrogen (step 10) . As can be seen the heat of evolution and the uptake of hydrogen after activation with oxygen alone was relatively small (see Table 2) . However, after exposure of the composition to a 180 μmol pulse of helium (step 11) and then to ten minutes of hydrogen (step 12) , a large uptake of hydrogen accompanied by a high heat evolution was observed. After step 12 the gold sample was purged with nitrogen, after which it was exposed to 3 μmol of oxygen (step 13) , followed by exposure of the composition to 10 minutes of hydrogen (step 14) . This resulted in further uptake of hydrogen , but not as large as the uptake seen in step 12. Finally after a 72 hour purge with nitrogen a 10 minute exposure to hydrogen gave a much reduced uptake, demonstrating again the importance of the activating effect of the combined oxygen/noble gas treatments.

Table 2

Similar activation of gold reactivity to hydrogen by the combined action of argon and oxygen is exemplified by sequential experiments in Table 3. The gold sample was the same as that described above . Table 3

Table 3 shows similar improved absorption of hydrogen after activation with argon and oxygen.

Example 2

Example 2 shows that the reactivity of platinum towards hydrogen is also increased by treatments with a combination of small amounts of oxygen and helium.

A 0-620 g sample of pure Pt powder (99-99 % purity reported by the suppliers (Aldrich) ) was used. The sample was purged with nitrogen flowing through it at 60 cc/hr at room temperature and atmospheric pressure. The flow of N₂ was then replaced by the same flow of pure hydrogen and its uptake recorded by a thermal conductivity detector (TCD) . At the same time the heat evolution generated by consecutive exposures of the sample to H₂ were determined in the microcalorimetric cell in which the Pt sample was situated. The flow of hydrogen was exchanged to that of nitrogen after 20 to 30 minutes of the exposure to H₂. at an increased flow rate (7 cc/min) The flow of N₂ was continued until there was no TCD indication of continued displacement of H₂ by N₂. The exposure of the sample to hydrogen was then repeated: (a) after brief exposures to air, 0-2 cc containing 9 micromoles of O₂, followed by the hydrogen flow, and (b) 0-2 cc of air containing 9 micromoles of O₂, followed by a 135 micromole pulse of He before the repeated exposure of Pt to the H₂ flow. The resulting heat evolutions and H₂ uptakes during the 10 minutes exposures are listed below in Table 4. All the experiments were conducted at 24⁰C.

Table 4

It is evident that a combination of small amounts of O₂ and He, added to Pt before exposure to H₂ gives a marked increase in its uptake by platinum and the heat of its interaction with H₂. Example 3

Example 3 shows the interactions of H₂ with a pure gold powder, indicating that for a 0.561 gram sample additions of 0.5 μmole and of 0.1 μmole O₂ pulses, representing the oxygen content of 0.05 cc of air and 0.1 cc of air, produce the activating effects. However, enhanced activation is observed when gold is exposed to both helium and oxygen.

In experiment (a) the gold sample was purged with nitrogen, then the heat evolution and hydrogen uptake were recorded.

In experiment (b) the gold sample was purged with nitrogen.

0.5 μmole of O₂ was then introduced into the sample . The heat evolution and hydrogen uptake were recorded.

In experiment (c) the gold sample was purged with nitrogen.

0.1 μmole of O₂ was then introduced into the sample . The heat evolution and hydrogen uptake were recorded.

In experiment (d) the gold sample was purged with nitrogen.

0.1 μmole of O₂ followed by 225 μmole of He was introduced into the sample. The heat evolution and hydrogen uptake were recorded.

The results are given below:

Claims

CLAIMS :

1. A method of activating a composition comprising a transition metal selected from at least one of gold, nickel, copper, ruthenium, molybdenum and platinum, the method comprising: exposing the composition to at least one pulse of an atmosphere comprising a noble gas; and exposing the composition to at least one pulse of an atmosphere comprising oxygen.

2. The method of claim 1 wherein the composition is exposed to an atmosphere comprising from 5 to 500 μmol of a noble gas per gram of transition metal .

3. The method of claim 2 wherein the composition is exposed to an atmosphere comprising from 80 to 150 μmol of a noble gas per gram of transition metal.

4. The method of any one of the preceding claims wherein the composition is exposed to an atmosphere comprising from 0.01 to 10 μmol of oxygen per gram of transition metal.

5. The method of claim 4 wherein the composition is exposed to an atmosphere comprising from 0.01 to 5 μmol of oxygen per gram of transition metal.

6. The method of any one of the preceding claims wherein the composition is exposed to an atmosphere comprising hydrogen or a hydrogen source.

7. The method of claim 6 wherein the composition is exposed to an atmosphere comprising hydrogen after being exposed to at least one pulse of an atmosphere comprising a noble gas and to at least one pulse of an atmosphere comprising oxygen.

8. The method of any one of the preceding claims wherein the composition is a catalyst.

9. The method of any one of the preceding claims wherein the composition is a hydrogen storage material .

10. The method of any one of the preceding claims wherein the composition is exposed to an atmosphere comprising a noble gas before it is exposed to an atmosphere comprising oxygen.

11. The method of any one of claims 1 to 9 wherein the catalyst is exposed to an atmosphere comprising oxygen before it is exposed to an atmosphere comprising a noble gas .

12. The method of any one of claims 1 to 9 wherein the composition is exposed to an atmosphere comprising both a noble gas and oxygen.

13. The method of any one of the preceding claims wherein the composition is exposed to an atmosphere comprising nitrogen.

14. The method of claim 13 wherein the composition is exposed to nitrogen prior to exposure to an atmosphere comprising a noble gas.

15. The method of claim 13 or 14 wherein the composition is exposed to nitrogen prior to exposure to an atmosphere comprising a oxygen.

16. The method of any one of the preceding claims wherein the composition is exposed to oxygen by exposing the composition to an atmosphere comprising air.

17. The method of any one of the preceding claims wherein the atmosphere comprising noble gas comprises helium, neon argon, or mixtures thereof.

18. The method of any one of the preceding claims wherein the composition comprising a transition metal selected from gold, nickel, copper, ruthenium, molybdenum and platinum is in the form of a powder, particle, fibre, flake, sponge or is deposited on a catalyst support.

19. The method of any one of the preceding claims wherein the composition is gold.

20. The method of any one of the preceding claims wherein the metal comprises an alloy of said metal .

21. A method of improving the hydrogen absorption properties of a catalyst comprising a transition metal selected from at least one of gold, nickel, copper, ruthenium, molybdenum and platinum, the method comprising activating the catalyst using the method defined in any one of claims 1 to 20 and subsequently exposing the pre-treated catalyst to an atmosphere comprising hydrogen or a hydrogen source, wherein the amount of hydrogen absorbed by the catalyst is more that the amount of hydrogen absorbed prior to pre-treatment of the catalyst.