WO2005118137A1

WO2005118137A1 - Method for activating a catalyst

Info

Publication number: WO2005118137A1
Application number: PCT/GB2005/002212
Authority: WO
Inventors: Aleksander Jerzy Groszek; Jerzy Haber; Erwin Lalik
Original assignee: Microscal Limited; Institute Of Catalysis And Surface Chemistry
Priority date: 2004-06-04
Filing date: 2005-06-03
Publication date: 2005-12-15
Also published as: GB0700064D0; GB2430394A; GB2416137A; GB2430394B; GB0412563D0

Abstract

A method for preparing a catalyst comprising a metal having absorbed therein hydrogen; wherein the metal is selected from at least one of gold, iron, ruthenium, or molybdenum; the method comprising the steps: (i) exposing a sample comprising said metal to an atmosphere comprising nitrogen or a noble gas; (ii) exposing the sample from step (i) to an atmosphere comprising hydrogen and/or a hydrogen source, whereby hydrogen is absorbed into the sample; (iii)exposing the sample from step (ii) to one of the following: (a) an atmosphere comprising noble gas; or (b) an atmosphere comprising nitrogen; (iv) exposing the sample from step (iii) to an atmosphere comprising hydrogen and/or a hydrogen source, whereby hydrogen is absorbed into the sample.

Description

METHOD FOR ACTIVATING A CATALYST

The present invention relates to gold, iron, ruthenium and molybdenum based catalysts and methods for their preparation.

It is known that palladium is used in many industrial applications as a catalyst. Palladium absorbs hydrogen very strongly and it is often referred to as a "hydrogen sponge". The sorption of hydrogen by palladium is reversible and is related to palladium having catalytic activity. For example, the interactions of hydrogen with metallic palladium are used in the catalytic hydrogenation of acetylene to ethylene. Palladium wire is used in hydrogen sensors.

It is known from Polish Patent Application No. P353550, that the interaction of palladium with hydrogen can be modified. This patent application describes that the properties of palladium within the palladium-hydrogen system can be advantageously modified, if the hydrogen-activated palladium is treated with noble gases. The reason behind this enhanced activity is not yet known. It is known that gold catalysts show high activity under mild conditions. Recent research suggests that gold is capable of being advantageously employed, for example, in hydrogen fuel generation and processing. Potential uses for gold catalysts include catalysing the water gas shift reaction or hydrogen production from water or methanol . It is also known for iron, ruthenium and molybdenum to be used in catalytic reactions. A few examples of which are given below.

The Haber process is perhaps the most well known chemical reaction involving an iron catalyst. In this process hydrogen combines with nitrogen to form ammonia. Recent research suggests that iron may be advantageously employed for low pressure hydrogen storage.

The use of ruthenium catalysts is well known in fuel cell technology, where, for example, ruthenium and platinum are used to convert methanol to electricity. Recent research also suggests that carbon aerogel supported ruthenium catalysts may be useful for hydrosulfurization of dibenzothiophene in hexadecane . This catalytic reaction is thought to have useful applications in the development of a diesel fuel processor for integration into portable polymer electrolyte membrane (PE ) fuel cells.

Furthermore, alloys of molybdenum and platinum are known to be useful as platinum-based anode catalysts for polymer electrolyte membrane (PEM) fuel cells. Methods of improving the metal surface reactivity and/or its absorption properties would be of potential commercial interest for use in catalysts.

Physical and chemical interactions of solid surfaces with gases result in the evolution of heat . The existence of this thermal effect has been recognised at least from the early nineteenth century. The heat evolution can be measured using a flow microcalorimeter . A flow 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 or helium, at a range of temperatures and pressures.

Flow microcalorimetry can be used to measure heat evolution produced when catalysts comprising gold are placed in contact with hydrogen at temperatures ranging from 20 °C to 240 °C. The hydrogen is absorbed and/or adsorbed in this process can be slowly desorbed from the gold by passing a carrier gas over the sample, for example, pure nitrogen or a noble gas. This generates a negative heat effect. The desorption could, however, be triggered by a simple absence of hydrogen. Thus, the presence of hydrogen in the gas phase in contact with the hydride phases is necessary for their stability. Surprisingly, it has been found that the properties of catalysts comprising a metal selected from one or more of gold, iron, ruthenium, or molybdenum, can be advantageously modified if the catalyst is treated using the method of the present invention.

The aim of the present invention is to address at least some of the problems associated with the prior art .

In one aspect, the present invention provides a method for preparing a catalyst comprising a metal having absorbed therein hydrogen; wherein the metal is selected from at least one of gold, iron, ruthenium, or molybdenum; the method comprising the steps:

(i) exposing a sample comprising said metal to an atmosphere comprising nitrogen or a noble gas;

(ii) exposing the sample from step (i) to an atmosphere comprising hydrogen and/or a hydrogen source, whereby hydrogen is absorbed into the sample;

(iii) exposing the sample from step (ii) to one of the following: (a) an atmosphere comprising noble gas; or (b) an atmosphere comprising nitrogen;

(iv) exposing the sample from step (iii) to an atmosphere comprising hydrogen and/or a hydrogen source, whereby hydrogen is absorbed into the sample. In a preferred embodiment, the present invention provides a method for preparing a catalyst comprising a metal; wherein the metal is selected from at least one of gold, iron, ruthenium, or molybdenum; wherein one or both of steps (i) and (iii) (as defined above) comprise exposing the sample to an atmosphere comprising a noble gas.

Preferably, the amount of hydrogen absorbed into the sample in step (iv) is greater than the amount of hydrogen absorbed into the sample in step (ii) .

In another aspect, the present invention provides a catalyst comprising a metal; wherein the metal is selected from at least one of gold, iron, ruthenium, or molybdenum; whenever prepared by a method as defined above.

In another aspect, the present invention provides a method for preparing a catalyst precursor comprising a metal; wherein the metal is selected from at least one of gold, iron, ruthenium, or molybdenum; the method comprising the steps : (i) exposing a sample comprising said metal to an atmosphere comprising nitrogen or a noble gas;

(iii) exposing the sample from step (ii) to one of the following: (a) an atmosphere comprising noble gas; or (b) an atmosphere comprising nitrogen.

Another aspect of the invention provides a catalyst precursor comprising a metal; wherein the metal is selected from at least one of gold, iron, ruthenium, or molybdenum; whenever prepared by a method as defined above.

The description of the present invention is, unless stated otherwise, applicable to each aspect of the invention. For example, where step (i) is defined, these features are applicable to the method of preparing the catalyst and the catalyst precursor. Preferably, the method steps of the present invention (as defined above) are performed sequentially in the order (i) , (ii) , (iii) and (iv) . More preferably, steps (i) , (ii) , (iii) and (iv) are performed sequentially and immediately after one another.

It will be appreciated that the term absorption used herein does not preclude adsorption of the gases by the catalyst .

The catalyst prepared using the method described in accordance with the present invention shows new and unexpected properties upon performing step (iv) . The absorption (sorption) of hydrogen can be measured by the heat evolution, which accompanies absorption. This can be measured using a flow microcalorimeter. The experiments have revealed that the amount of heat released from the activated catalyst upon hydrogen absorption (sorption) in step (iv) may be comparable to thermal effects measured in chemical reactions. This indicates that by using the methods of step (i) , (ii) and (iii) the behaviour of the catalyst comprising metal; wherein the metal is selected from at least one of gold, iron, ruthenium, or molybdenum; affects strongly the metal-hydrogen system.

Preferably, the method of the present invention improves the hydrogen absorption (sorption) properties of the catalyst . Preferably, the quantity of absorbed hydrogen gas per volume unit of the catalyst following the second exposure of the catalyst comprising a metal to an atmosphere comprising hydrogen gas is greater than the quantity of absorbed hydrogen gas per volume unit of the catalyst following the first exposure of the catalyst to an atmosphere of hydrogen gas. Preferably, in this embodiment the metal is gold, ruthenium or molybdenum. More preferably, the metal is gold.

In step (i) of the present invention the sample is exposed to an atmosphere comprising nitrogen or a noble gas. This exposure aims to substantially remove weakly held water and oxygen from the gold surface. For example, the sample may be saturated with nitrogen for 20 hours at a flow rate of 1 cc/ minute. In step (ii) of the present invention the catalyst comprising a metal; wherein the metal is selected from at least one of gold, iron, ruthenium, or molybdenum; is exposed to hydrogen or a hydrogen source. It will be understood that the hydrogen source may be, for example, methane. This exposure displaces some of the nitrogen, or noble gas which has been absorbed into the sample in step (i) . In the present invention it is preferable for the catalyst comprising metal to be exposed to at least 10 μmol of hydrogen per 2500 μmol of metal in step (ii) . Most preferably the sample is completely saturated with hydrogen. Step (ii) is also thought to reduce the amount of oxygen that may be held by the catalyst comprising metal . Complete penetration of the metal may occur when the metal is dispersed in thin layers (one or more mono-atomic layers) on catalyst supports. Complete saturation of the sample is defined as no further uptake of hydrogen. This may be shown by the use of a thermal conductivity detector. Appropriate temperatures for saturation may be from 20 to 240 °C, preferably from 70 to 150 °C. It will be understood that the preferred temperature for saturation may vary with the metal used. The preferred temperature for the saturation of the sample comprising gold with hydrogen is about 120°C, because it is easier to remove water from the catalysts at a temperature above the boiling point of water. On the other hand temperatures approaching 240 °C tend to reduce the amount of hydrogen interacting with gold.

Saturation may be achieved, for example, by exposing the metal sample to an atmosphere of pure hydrogen at a rate of 1 cc/min. The interaction of hydrogen with the metal surface is associated with the evolution of heat . Whilst hydrogen is continuously flowed over the sample this heat evolution may last several hours, indicating that the interaction of hydrogen with the metal sample continues.

The heat evolution usually continues long after the hydrogen uptake becomes undetectable .

Any noble gas or nitrogen may be used in step (i) and/ or in step (iii) , but the preferred gas is nitrogen. When the metal of the present invention is gold, low temperatures not exceeding 150°C are preferred for this work in view of the reported good performance of gold catalysts at such relatively low temperature levels. Preferably, when the metal is gold the temperature will be in the range of from 20 to 150 °C. More preferably, the temperature will be in the range of 100 to 130 °C. The gases are preferably pre-dried before use, and preferably contain less than 10 ppm of water and oxygen. Additionally, it is preferable for nitrogen gas and hydrogen gas not to contain significant quantities of noble gases, and most preferable that they contain less than 1 ppm of noble gases .

Step (iii) of the present invention involves exposing the sample from step (ii) to one of the following: (a) an atmosphere comprising noble gas; or (b) an atmosphere comprising nitrogen.

Preferably in step (iii) the sample from step (ii) is exposed to one of the following: (a) at least 10 μmol of noble gas per 2500 μmol of metal ; or (b) at least 10 μmol of nitrogen per 2500 μmol of metal .

It will be appreciated that much larger quantities of noble gas or nitrogen may be used in step (iii) for a given quantity of metal. For example, 100 μmol of gas may be used in step (iii) per 5 μmol (1 milligram) of metal on an inert support .

More preferably in step (iii) the sample is exposed to one of the following: (a) from 10 to 200 μmol of noble gas per 2500 μmol of metal ; or (b) from 10 to 200 μmol of nitrogen per 2500 μmol of metal . Most preferably in step (iii) the sample is exposed to one of the following: (a) from 20 - 100 μmol of noble gas per 2500 μmol of metal ; or (b) from 20 - 100 μmol of nitrogen per 2500 μmol of metal .

It has been found, for example, that upon exposure of a sample of 0.5g of gold (2500 μmol) to a continuous flow of nitrogen, followed by 10 μmol of hydrogen, then 10 μmol of neon, and finally hydrogen in step (iv) , that the heats of interaction of the sample with hydrogen in step (iv) are increased significantly compared with those in step (ii) .

Preferably, the atmosphere comprising hydrogen and/or a hydrogen source as described in step (ii) and/or step (iv) of the present invention contains at least 50% by volume hydrogen and/or a hydrogen source. More preferably, the atmosphere comprising hydrogen and/or a hydrogen source comprises at least 70% hydrogen and/or a hydrogen source. Most preferably, the atmosphere comprises at least 99% by volume hydrogen and/or a hydrogen source . Hydrogen gas of at least 99% purity by volume is defined in this patent as pure hydrogen.

Preferably, the atmosphere comprising nitrogen as described in step (i) and/or step (iii) of the present invention contains at least 50% by volume nitrogen. More preferably, the atmosphere comprising nitrogen contains at least 70% by volume nitrogen. Most preferably, it comprises at least 99% by volume nitrogen. Nitrogen gas of at least 99% purity by volume is defined in this patent as pure nitrogen.

Preferably, the atmosphere comprising a noble gas as described in step (i) and/or step (iii) of the present invention contains at least 50% by volume of noble gas. More preferably it contains at least 70% by volume of noble gas and most preferably it contains at least 99% by volume of noble gas. A noble gas of at least 99% purity by volume is defined in this patent as a pure noble gas.

All noble gases can be used, but the beneficial results improve with the atomic weight of a noble gas. The noble gas preferably comprises argon, neon or helium, or a mixture of two or more thereof. More preferably the noble gas comprises one of at least argon or neon and most preferably argon.

The metal in the sample is preferably in the form of powders, particles, fibres, flakes or sponges or may be deposited on a catalyst support. The sample may also be a metal alloy. The sample may comprise metal oxide. The sample is preferably in the form of a pure metal powder. The metal may be selected from one or more of gold, iron, ruthenium or molybdenum. Preferably, the catalyst or catalyst precursor comprises only one metal, selected from gold, iron, ruthenium or molybdenum. More preferably, the metal comprises gold or iron. Most preferably the metal is gold.

Preferably the metal is in the form of deposits on catalyst supports, such as Ti0₂, silica, graphite or iron oxides. The metal preferably has a purity of at least 99% and more preferably a purity of at least 99.99%. The purity of the metal is measured using atomic spectroscopy. A suitable temperature range for the present invention is from 20°C to 300°C. The present invention may also be carried out at room temperature. Wherein the catalyst comprises gold preferably the temperature range for the present invention is from 50 to 150 °C. Wherein the catalyst comprises iron preferably the temperature for the present invention is in range of from 150 to 300°C, and most preferably the temperature is approximately 200 °C. Wherein the catalyst comprises molybdenum or ruthenium preferably the temperature for the present invention is in range of from 200 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 an another aspect, the present invention provides a method of modifying the interaction of hydrogen with a catalyst comprising metal; wherein said metal is selected from one or more of gold, iron, ruthenium or molybdenum; the method comprising the steps :

(i) exposing a sample comprising said metal to an atmosphere comprising nitrogen or a noble gas; (ii) exposing the sample from step (i) to an atmosphere comprising hydrogen and/or a hydrogen source, whereby hydrogen is absorbed into the sample; (iii) exposing the sample from step (ii) to one of the following: (a) an atmosphere comprising noble gas; or (b) an atmosphere comprising nitrogen; (iv) exposing the sample from step (iii) to an atmosphere comprising hydrogen and/or a hydrogen source, whereby hydrogen is absorbed into the sample.

In another aspect, the present invention provides a method of catalysing the conversion of carbon monoxide to carbon dioxide; the method comprises exposing the catalyst as prepared herein to an atmosphere comprising carbon monoxide and oxygen, whereby at least some of the carbon monoxide is converted to carbon dioxide.

It will be understood that the catalyst of the present invention may be used to catalyse other oxidation and/or hydrogenation reactions. 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-Microcalorimeter as described in Chemistry and Industry 25th March 1965, pages 482 to 489 and Thermochimica Acta, 312, 1998, pages 133 to 143.

The experiments were conducted by switching the flow of nitrogen or noble gas, used as a carrier gas, to that of, for example, hydrogen, nitrogen or noble gases for, preferably, 10 to 300 seconds, which introduces from 0.25 cc to 5.0 cc of hydrogen or noble gases into the samples. All the experiments were carried out at atmospheric pressure.

Example 1 A 0.501g sample of pure gold powder, which was supplied by Aldrich Co. with reported purity of 99.99%, was loaded into the flow microcalorimeter. The temperature was maintained at 25 °C throughout the experiment. Pure nitrogen (99.999% purity) was then passed through the sample at the rate of 1 cc/min for 20 hours to remove absorbed water, oxygen and other volatile impurities from the sample. The flow of nitrogen was then switched to that of pure hydrogen (99.995% purity, supplied by Aldrich Co.) flowing at a constant rate of 1 cc/min. Heat evolution took place as soon as hydrogen came into contact with the sample and continued to pass through it. Hydrogen uptake was concurrently measured by continuously monitoring its concentration in the effluent. The flow of hydrogen was continued until there was no indication of its uptake by the sample . The flow of hydrogen was then switched again to nitrogen, which passed through the sample for 1200 minutes and caused a negative heat effective and partial removal of the hydrogen absorbed by the sample. The saturation with hydrogen was then repeated and followed by desorption with nitrogen for 600 minutes.

Hydrogen desorption with nitrogen gave a much smaller negative heat effect than the absorption, which suggests strong hydrogen retention. The results indicate that pure hydrogen is strongly absorbed by the gold powder and can only be partly desorbed by an extended flow of pure nitrogen.

The flow of nitrogen was subsequently switched to that of helium, which generated an extended heat evolution. The results of the above sequence of experiments are summarised in Table 1. It should be noticed that, as in the case of hydrogen interactions, the heat of desorption of helium with nitrogen was a small fraction of the heat of helium absorption indicating strong retention of some of the helium uptake by the gold sample.

Table 1

As mentioned above, it can be seen in Table 1, for cycle 1 and cycle 2 that the desorption of hydrogen gave very much smaller endothermic heats than the exothermic heats of the absorption of gold with hydrogen. Additionally, the interaction in the second cycle produced a somewhat higher heat of evolution than was observed for the first cycle.

Example 2 This experiment was carried out at 25 °C and at atmospheric pressure. The 0.5g sample of pure gold sample was loaded into the calorimeter. The sample was then purged with lcc/min nitrogen for 42 hours. Two 90 μmol pulses of pure hydrogen were then passed through the sample, followed by a 90 μmol of pure helium. The helium was partially desorbed from the sample by the flow of nitrogen. The sample was then saturated with hydrogen, which gave a heat evolution of 317 J/g and an uptake of hydrogen of 2.93 mmol/g. The latter hydrogen uptake exceeded that occurring on untreated gold by a factor of 42. Desorption of the absorbed hydrogen yielded only 0.04 mmol/g of hydrogen accompanied by an endothermic heat of desorption of 9.5 J/g.

Example 3 Experiments have been carried out with a 0.757g fresh gold powder sample consisting of particles having an average particle diameter of 2 micrometers and reported purity of 99.99%. The sample was exposed to nitrogen for 20 hours. Saturation of this material with hydrogen at 112°C gave a relatively low heat evolution (0.13 J/g) and hydrogen uptake of 0.004 mmol. Subsequent interaction with three x 45 μmol of helium, generated a total heat of evolution of 0.76 J/g. This was followed by saturation of the sample with hydrogen causing heat evolution of 33.6 J/g associated with hydrogen uptake of 0.186 mmol. The gold sample was then purged for 20 hours with 1200 cc of nitrogen and the experiment was continued by contacting the sample with four pulses of argon which produced a total heat evolution of 0.24 J/g. The saturation with hydrogen was then repeated leading to evolution of heat amounting to 42.2 J/g.

Table 2

Comparison Example 1 A 0.500g sample of 99.99% pure gold was exposed to a continuous flow of nitrogen at 1 cc/min. 45 μmol Argon pulses were introduced into nitrogen carrier gas. The experiments were conducted at 25 °C and atmospheric pressure. It was established that on a fresh sample of gold that had no contact with hydrogen, there was insignificant interaction with argon as evidenced by very small heat effects produced by 45 μmol pulses of argon introduced into nitrogen carrier passing through the gold sample (less than 0.05 J/g) . Example 4 This experiment was conducted on a gold catalyst containing 1% weight of gold supported on titanium dioxide. A 0.098g sample of the catalyst was exposed to nitrogen for 2 hours and then saturated with hydrogen at 24 °C, which generated a heat evolution of 5.0 J/g and a hydrogen uptake of 0.2 mmol/g. The sample was then purged with a nitrogen carrier gas for 20 minutes and contacted with 55 μmol of helium. Subsequent saturation with hydrogen generated an extended heat evolution of 167 J/g associated with a hydrogen uptake of 1.2 mmol/g by the catalyst.

Example 5 This Experiment was carried out using helium as a carrier gas at 101 °C and a pure gold sample of 0.766g. The flow rate of all the gases in this experiment was 1 cc/min at atmospheric pressure. Before starting the first absorption of hydrogen, helium was passed through the sample for ca. 20 hours until equilibration was reached.

The sample was then saturated with hydrogen. This was achieved by exposing the sample to hydrogen for 4500 seconds, which produced a heat evolution of 988 mJ. Desorption of the hydrogen with helium followed for 1800 seconds. During this desorption process three x 45 μmol of argon were injected into the Helium carrier, giving consecutive heat effects of 84mJ, 42mJ and 14mJ.

In the final step, the sample was saturated with pure hydrogen for 15000 seconds, which yielded a heat evolution of 46,552 mJ. As can be seen from the results, the increase in the heat of absorption observed for the final exposure of hydrogen compared to the initial exposure implies that the uptake in hydrogen was significantly larger in the final step.

Example 6 This experiment was carried out with the same sample as in Experiment 5 after heating at 240 °C for 24h in a flow of helium. The flow rate of all the gases in this experiment was 1 cc/min at atmospheric pressure.

The sample was saturated with helium as in experiment 5. The sample was then saturated with hydrogen for 3500 seconds. Injection of three 45 μmol pulses of nitrogen gave consecutive heat effects of 39 mJ, 40 mJ and 27 mJ. Saturation of the sample with pure hydrogen for 17,200 seconds caused a heat evolution of 60,955 mJ. It can be seen that, as for Experiment 5, the huge increase in the heat of sorption following the injection of the three pulses of nitrogen suggests a substantial increase in the hydrogen uptake . Examples 7 to 11 Examples 7 to 11 show the enhanced catalytic activity of gold on a 75% Ti0₂:25%SiO₂ (Au/ (75%Ti0₂) ) solid catalyst, when the catalyst has been prepared using the method of the present invention, in the reaction of conversion of CO to C0₂ with oxygen. The catalytic activity measurements were carried out using the equipment described in detail in the paper by M. Gasior, B. Grzybowska, K. Samson, M. Ruszel, J. Haber, Oxidation of CO and C₃ hydrocarbons on gold dispersed on oxide supports, Catalysis Today, 91-92C (2004) pp. 131- 135, which also contains a description of the catalyst preparation. The low temperature oxidation of CO to C0₂ with gaseous oxygen on the supported Au catalyst was used as the test reaction. The catalyst was prepared using the solution of hydrogen tetrachloroaurate for impregnation of the support containing the mixture of oxides Si0₂ (25%) and Ti0₂ (75%) with the total of the gold content in the catalyst being 1%. The flow-mode microreactor with gas- chromatograph (GC) was used to determine the degree of conversion of CO (%) which can be used as a measure of activity of the catalyst.

Each experiment (7 to 11) begins with the degree of the CO conversion being measured, with the gaseous reaction mixture flowing through the fresh sample of the catalyst at the reaction temperature, preferably 50 °C (see "fresh sample" entries in Tables 3 and 4) . The catalyst is then subjected to activation in the flow of pure hydrogen for 0.5h at the activation temperature, preferably 150 °C. The sample is then cooled down to the reaction temperature in a flow of pure nitrogen. After having reached the reaction temperature the sample's activity is tested again by switching the gas flow from nitrogen to the gaseous reaction mixture and measuring the CO conversion degree (cf . second rows in Tables 3 - 5) . Afterwards the flow of nitrogen is switched back again, and the sample is contacted with pure argon gas, by switching the flow from nitrogen to the flow of pure argon at the same flow rate, either as pulses of argon for, preferably, 1 min each of three separate pulses, or as a continuous flow of argon for, preferably, 10 min. Following the argon contact, the sample is again subjected to activation in the flow of hydrogen for 30 min at the activation temperature, after which it is cooled down in the nitrogen flow, and afterwards on switching back to the flow of reaction mixture the degree of CO conversion is being measured with GC (see third rows in Tables 3 and 4) .

All experiments (7 to 11) were carried out at the atmospheric pressure, and the flow rates of all gases were set at 30cm³/min.

Example 7 A 0.5287g sample of Au/ (75%Ti0₂+25%Si0₂) catalyst was loaded in the microreactor and the procedure described above was followed. In this example three pulses of argon were introduced to the sample. The results are summarised in Table 3. It should be noted that after the argon contact the measured degree of CO conversion increases substantially (78.9% compared to 26.6%). Example 8 A 0.5238g sample of Au/ (75%Ti0₂+25%Si0₂) catalyst was loaded in the microreactor and the procedure described above was followed. In this example argon was introduced to the catalyst sample by switching the gas flow to pure argon flow for 0.5h. The results are summarised in Table 4. A dramatic increase of the degree of CO conversion after the sample is contacted with argon (98.9% compared to 27.4%) should be noticed.

Examples 7 and 8 show that the hydrogen activation of the Au/ (75%Ti0₂+25%Si0₂) catalyst is greatly enhanced when the catalyst is prepared using the method of the present invention.

Table 3. Example of the enhancing action of argon pulses on the catalytic activity of Au/ (75%Ti0₂+25%Si0₂) solid catalyst in the reaction of low temperature conversion of CO to C0₂ with oxygen in the flow-mode microreactor. Reaction conditions: sample mass 0.5287g; reaction temperature 50°C; reaction mixture flow rate 30cm³/min, reaction mixture molar content: 2.3 CO, 23.3 0₂, 74.4 N₂ (vol. %) ; catalyst particle size 0.2-0.5mm; residence time 1.5s duration of Ar pulses 1 min at the flow rate 30cm³/min, surface area of the catalyst 40.4m²/g.

Table 4. Example of the enhancing action of Ar flow on the catalytic activity of Au/ (75%Ti0₂+25%Si0₂) solid catalyst in the reaction of low temperature conversion of CO to C0₂ with oxygen in the flow-mode microreactor. Reaction conditions: sample mass 0.5238g; reaction temperature 50 °C; reaction mixture flow rate 30cm³/min, reaction mixture molar content: 2.3 CO, 23.3 0₂, 74.4 N₂ (vol. %) ; catalyst particle size 0.2-0.5mm; residence time 1.5s duration of Ar flow 10 min (at the flow rate 30cm³/min) , surface area of the catalyst 40.4m²/g.

Comparative Example 2. A 0.5253g sample of Au/ (75%Ti0₂+25%Si0₂) catalyst was loaded in the microreactor and the catalytic activity of the fresh sample measured. In this example the first hydrogen treatment was omitted and the hydrogen activation procedure was started from contacting the sample with argon for 10 min, after which the catalytic activity of the sample was measured again, followed by the second hydrogen treatment, with the final catalytic activity of the sample tested afterwards. The results are summarised in Table 5. It can be concluded first that the catalytic activity after the argon contact is the same as that before the argon exposure (ca. 11% of CO conversion) . Secondly, the hydrogen treatment that follows the argon contact is, in this case, not more effective than the hydrogen activation performed without any argon exposure (see Examples 7 and 8) , with the CO conversion increased only to 32.8%, a figure similar to 26.6% and 27.4% found in Tables 3 and 4 respectively. These results point to the crucial importance of the first hydrogen treatment, that must precede the argon exposure, in order for the argon to be able to enhance the effectiveness of the whole hydrogen activation.

Table 5. Example of the argon/hydrogen activation of the Au/ (75%Ti0₂+25%Si0₂) catalyst with the initial step of hydrogen treatment omitted. After such a "partial" activation the catalyst was tested in conversion of CO to C0₂ with oxygen in the flow-mode microreactor. Reaction conditions sample mass 0.5253g; reaction temperature 50 °C; reaction mixture flow rate 30cm³/min, reaction mixture molar content: 2.3 CO, 23.3 0₂, 74.4 N₂ (vol. %) ; catalyst particle size 0.2-0.5mm; residence time 1.5s; duration of Ar flow 10 min at the flow rate 30cm³/min, surface area of the catalyst 40.4m²/g.

Comparative Example 3 and Example 9 This experiment was a continuation of Comparative Example 2. After having measured the catalytic activity of the sample treated as described, the catalytic test was continued for two days. The sample was kept constantly in a stream of reagents during this period. This experiment made it possible to compare the timing of argon-enhanced hydrogen activation with that of non-enhanced hydrogen activation. The argon-enhanced hydrogen activation is instantaneous, that is, the catalyst performance is greatly improved immediately after the hydrogen/argon/hydrogen treatment. On the other hand, following the non-enhanced activation the performance of the catalyst was also improved by its remaining in a stream of the reaction mixture for a prolonged period of time. This point is illustrated in

Figure 1. Figure 1 shows an instantaneous increase of the CO conversion degree up to ca. 90% (filled squares) following the argon-enhanced hydrogen activation, compared to the slow lingering increase of the CO conversion after the non-enhanced activation (open circles) which eventually reaches the 90%-level only after ca 2500 minutes of time in stream.

Figure 1 gives a comparison of the timing for the argon- enhanced hydrogen activation of the Au/ (75%Ti0₂+25%Si0₂) catalyst and the hydrogen activation of the same catalyst without argon. The filled squares represent the CO conversion degree (%) after the hydrogen activation preceded by three pulses of argon (each of 1 min duration) . The open circles represent the CO conversion on the same catalyst measured for a prolonged period of time. Reaction conditions: see the caption to Table 5. Example 10 In a separate series of experiments, the argon effect on the hydrogen treatment of the Au/ (75%Ti0₂+25%Si0₂) catalyst was demonstrated by evaluating the differences in heat evolution accompanying the hydrogen sorption on the Au/ (75%Ti0₂+25%Si0₂) sample, performed before and after argon exposure, and monitored in si tu with gas flow-through microcalorimeter. Each measurement begins with the sample being purged with pure nitrogen to reach the thermal equilibrium, upon which the nitrogen carrier is replaced by hydrogen flow and the thermal effect of hydrogen sorption on Au/ (75%Ti0₂+25%Si0₂) is being measured. After approximately two hours the nitrogen flow is being switched again and three pulses of argon each containing 45 micromol of argon are made to pass through the sample. The catalyst is then subjected again to sorption of hydrogen by replacing the nitrogen flow by hydrogen, and thus the thermal effect of the past-argon hydrogen treatment of the Au/ (75%Ti0₂+25%Si0₂) sample is being measured. The results are summarised in Table 6. It is clear that the amount of heat evolving during the hydrogen treatment after the argon contact surpasses significantly the amount of heat evolving upon the hydrogen sorption before the argon admission, preferably at the temperature of 100°C or higher. The catalytic tests reveal that the hydrogen activating Au/ (75%Ti0₂+25%Si0₂) is much more effective if the sample is exposed to argon after the initial hydrogen treatment, as the catalytic performance of such sample in the oxidation of CO increases substantially and instantaneously and at the same time, the thermal effect of the hydrogen treatment of Au/ (75%Ti0₂+25%Si0₂) , measured with microcalorimeter, increases dramatically if the second hydrogen admission is preceded by pulses of argon.

Example 11 shows a comparison of the enhancing action of the argon contact with that of helium contact under the same conditions.

Table 6. Comparison of the initial heat evolution rates during the hydrogen treatment of the Au/ (75%Ti0₂+25%Si0₂) catalyst before and after the Ar exposure, at different temperatures. More than one figure in a row, separated with semicolons, means that experiments were carried out more than once at the same temperature . Experimental conditions : sample mass 0.1140g; the flow rate lcm³/min, duration of Ar pulse 1 min (at the flow rate of lcm³/min) .

Example 11. A 0.5091g sample of Au/ (75%Ti0₂+25%Si0) catalyst was loaded in the microreactor and the procedure described above was followed with the difference that helium gas was used instead of argon. In this example helium was introduced to the catalyst sample by switching the gas flow to pure He flow for 0.5h. The results are summarised in Table 7. Here the increase of the degree of CO conversion after the sample is contacted with helium is 32.4% compared to 27.2%. In this case, the helium contact produced a much smaller effect on the catalytic performance of the catalyst than that observed for argon.

Table 7. Example of the action of He flow on the catalytic activity of Au/ (75%Ti0₂+25%Si0₂) solid catalyst in the reaction of low temperature conversion of CO to C0₂ with oxygen in the flow-mode microreactor. Reaction conditions: sample mass 0.5091g; reaction temperature 50 °C; reaction mixture flow rate 30cm³/min, reaction mixture molar content: 2.3 CO, 23.3 C0₂, 74.4 N₂ (VOL %) ; catalyst particle size 0.2-0.5mm; residence time 1.5s; duration of He flow 30 min (at the flow rate of 30cm³/min) , surface area of the catalyst 40.4m²/g.

Example 12 Finely divided iron composed of thin flakes having a high aspect ratio and thicknesses ranging from 10 to 50 nanometres were prepared as described in A.J. Groszek, Weear, 18, 279-289,1971. The surfaces of the flakes were protected from reaction with atmospheric oxygen by the adsorbed layers of n-heptane, which was removed by nitrogen carrier at temperatures over 100°C. Examination of the flakes by XRD after storage in air revealed a predominant presence of a-iron and a minor proportion of Fe₃0₄.

Subsequent pulse and saturation experiments on the flakes with hydrogen and noble gases revealed their interactions with the iron surfaces accompanied by heat evolution measured in the flow micro-calorimeter. Examples of the interactions with helium and argon at 220°C are shown in Figures 2 and 4 respectively. Hydrogen uptakes by the iron flakes are listed in Tables 8 and 9. The tables show progressive uptakes of molecular hydrogen by the iron flakes at 10 minute intervals, which are accompanied by heat evolution. Similar effects were previously observed for fine gold powders. Repetition of hydrogen saturation, after its desorption by nitrogen over a period of 20 hours, resulted in an increase of the uptakes shown in Table 8 and Figure 3.

The total uptakes of hydrogen by the iron flakes at 221°C and atmospheric pressure are of the order of 40 mmol per gram, which is equivalent to 8% by weight, or, 4.5 atoms of hydrogen per atom of iron.

The results listed in Table 9 indicate that at 221°C the additions of argon to nitrogen, prior to saturation of the iron flakes with hydrogen, do not increase its uptake and, if anything, argon seems to have initially a reducing effect on the hydrogen uptake, as if was able to block the access of hydrogen to the iron sorption sites.

The interactions of argon pulses with the iron flakes in nitrogen carrier at 221°C, following an extended desorption of hydrogen and nitrogen, are shown in Figure 4. The first argon pulse produces a high positive heat effect and a high positive response of the thermoconductivity detector (TC detector) suggesting displacement of hydrogen that was still retained by the flakes after the desorption treatment. The second larger pulse of argon produces an even higher heat effect, but this time some of the injected argon breaks through the bed of the iron flakes resulting in a negative response by the TC detector.

The above results show clearly that molecular hydrogen is absorbed by iron nanoflakes at 220°C and that noble gases, such as helium and argon, also interact with this finely divided iron resulting in displacement of some of the absorbed hydrogen.

Table 8. Absorption of hydrogen by iron nanoflakes at 220°C

Table 9. Absorption of hydrogen by iron flakes at 220°C Effect of argon

Example 13 0.323g of ruthenium was exposed to two times 10 micromol pulses of helium, followed by three times 10 micromol pulses of hydrogen, followed by two times 10 micromol pulses of helium. Gas flow rates of 1 ccm/min were used at room temperature and atmospheric pressure. Experimental measurements were taken after each pulse of gas, using a flow micro-calorimeter and a thermoconductivity detector. The results are shown in Figure 5. Comparison of peaks one and two with peaks six and seven in Figure 5 shows that the interaction of helium with the ruthenium powder is increased after the adsorption of hydrogen. It appears that the first pulse of hydrogen (peak three) displaces a small amount of helium which has interacted with the fresh ruthenium. This is shown by the relatively small heat effect produced by the first pulse (peak three) compared to the much larger heat effects produced by the other pulses of hydrogen (peaks four and five) .

It has also been observed that peak six produces a bigger heat effect than that produced by hydrogen peak five. This indicates that hydrogen is displaced by helium. Otherwise, it would be expected that the helium heat effect would be closer to the value recorded by peak seven. Example 14 Molybdenum powder was exposed to one pulse of hydrogen, followed by three pulses of argon in a nitrogen carrier gas.

Gas flow rates of 1 ccm/min were used. The reaction temperature was 200°C at atmospheric pressure. Experimental measurements were taken after each pulse of gas, using a flow micro-calorimeter and a thermoconductivity detector.

The results are shown in Figure 6.

Figure 6 shows that a pulse of 11 μmol of hydrogen produced a large heat effect of 811 mJ. Most of the hydrogen was retained by the molybdenum on passing the nitrogen carrier. This can be seen by the low response of the T. C. Detector. Three subsequent pulses of Ar (11 μmol each) all produced positive heat effects of 347 mJ, 67 mJ, and 9 mJ respectively, indicating a displacement of the previously absorbed hydrogen from the Molybdenum powder - decreasing with the progressive pulses.

The thermoconductivity detector (T. C. detector) , which normally gives a negative effect when argon is passed through it, produced a distinct positive effect after its first pulse confirming displacement of hydrogen from the molybdenum powder. The second and third pulses of argon produced negative effects as the amount of the displaced hydrogen was reduced. These results indicate a strong affinity of molybdenum surfaces for Argon. Other noble gases were also found to have a strong affinity for Molybdenum surfaces after the powder's contact with hydrogen at 200°C.

Claims

CLAIMS : 1. A method for preparing a catalyst comprising a metal having absorbed therein hydrogen; wherein the metal is selected from at least one of gold, iron, ruthenium, or molybdenum; the method comprising the steps:

(i) exposing a sample comprising said metal to an atmosphere comprising nitrogen or a noble gas; (ii) exposing the sample from step (i) to an atmosphere comprising hydrogen and/or a hydrogen source, whereby hydrogen is absorbed into the sample;

(iv) exposing the sample from step (iii) to an atmosphere comprising hydrogen and/or a hydrogen source, whereby hydrogen is absorbed into the sample.

2. The method of claim 1, wherein one or both of steps (i) and (iii) comprise exposing the sample to an atmosphere comprising a noble gas.

3. The method of claim 1 or claim 2 , wherein the amount of hydrogen absorbed into the sample in step (iv) is greater than the amount of hydrogen absorbed into the sample in step (ii) .

4. The method of any one of the preceding claims, wherein the sample is saturated with hydrogen in step (ii) .

5. The method of any one of the preceding claims, wherein in step (iii) the sample is exposed to one of the following: (a) at least 10 μmol of noble gas per 2500 μmol of said metal; or (b) at least 10 μmol of nitrogen per 2500 μmol of said metal.

6. The method of claim 5, wherein in step (iii) the sample is exposed to one of the following: (a) from 10 to 200 μmol of noble gas per 2500 μmol of said metal; or (b) from 10 to 200 μmol of nitrogen per 2500 μmol of said metal .

7. The method of claim 6, wherein in step (iii) the sample is exposed to one of the following: (a) from 20 - 100 μmol of noble gas per 2500 μmol of said metal ; or (b) from 20 - 100 μmol of nitrogen per 2500 μmol of said metal .

8. The method of any one of the preceding claims, wherein the atmosphere comprising hydrogen in step (ii) and/or step (iv) contains at least 99% by volume hydrogen.

9. The method of claim 8 , wherein the atmosphere comprising hydrogen in step (ii) and/or step (iv) contains at least 99.99% by volume hydrogen.

10. The method of any one of the preceding claims, wherein the atmosphere comprising nitrogen in step (i) and/or step (iii) contains at least 99% by volume nitrogen.

11. The method of claim 10, wherein the atmosphere comprising nitrogen in step (i) and/or step (iii) contains at least 99.99% by volume nitrogen.

12. The method of any one of the preceding claims, wherein the atmosphere comprising a noble gas in step (i) and/or step (iii) contains at least 99% by volume a noble gas.

13. The method of claim 12, wherein the atmosphere comprising a noble gas in step (i) and/or step (iii) contains at least 99.99% by volume of a noble gas.

14. The method of any one of the preceding claims, wherein the noble gas comprises helium, neon or argon or a mixture of two or more thereof.

15. The method of any one of the preceding claims, wherein the catalyst comprising said metal is in the form of powders, particles, fibres, flakes or sponges or may be deposited on a catalyst support .

16. The method of claim 15, wherein the catalyst support comprises Ti0₂, silica, graphite or iron oxides.

17. The method of any one of claims 1 to 14, wherein the catalyst comprising said metal comprises an alloy of said metal .

18. A catalyst comprising said metal whenever prepared by a method as defined in any one of the preceding claims .

19. A method for preparing a catalyst precursor comprising metal; wherein said metal is selected from at least one of gold, iron, ruthenium, or molybdenum; the method comprising the steps:

(ii) exposing the sample from step (i) to an atmosphere comprising hydrogen and/or a hydrogen source, whereby hydrogen is absorbed into the sample; (iii) exposing the sample from step (ii) to one of the following: (a) an atmosphere comprising noble gas; or (b) an atmosphere comprising nitrogen.

20. A catalyst precursor whenever prepared by a method as defined in claim 19.

21. A method of modifying the interaction of hydrogen with a catalyst comprising metal; wherein said metal is selected from at least one of gold, iron, ruthenium, or molybdenum; the method comprising the steps: (i) exposing a sample comprising said metal to an atmosphere comprising nitrogen or a noble gas;

22. A method for preparing a catalyst comprising metal; wherein said metal is selected from at least one of gold, iron, ruthenium, or molybdenum; substantially and herein described with reference to any one of the Examples excluding the comparison examples.

23. A method of catalysing the conversion of carbon monoxide to carbon dioxide; the method comprises exposing the catalyst as claimed in claim 18 to an atmosphere comprising carbon monoxide and oxygen, whereby at least some of the carbon monoxide is converted to carbon dioxide.