GB2416137A

GB2416137A - Preparation of a gold catalyst

Info

Publication number: GB2416137A
Application number: GB0412563A
Authority: GB
Inventors: Aleksander Jerzy Groszek; Jerzy Haber; Erwin Lalik
Original assignee: Microscal Ltd; INST OF CATALYSIS AND SURFACE
Current assignee: Microscal Ltd; INST OF CATALYSIS AND SURFACE
Priority date: 2004-06-04
Filing date: 2004-06-04
Publication date: 2006-01-18
Also published as: GB2430394B; WO2005118137A1; GB0700064D0; GB2430394A; GB0412563D0

Abstract

A method for preparing a catalyst comprising gold having absorbed therein hydrogen, the method comprising the steps: <SL> <LI>(i) exposing a sample comprising gold to an atmosphere comprising nitrogen or a noble gas; <LI>(ii) exposing the sample from step (i) to an atmosphere comprising hydrogen, whereby hydrogen is absorbed into the sample; <LI>(iii) exposing the sample from step (ii) to one of the following: <SL> <LI>(a) an atmosphere comprising noble gas; or <LI>(b) an atmosphere comprising nitrogen; </SL> <LI>(iv) exposing the sample from step (iii) to an atmosphere comprising hydrogen, whereby hydrogen is absorbed into the sample. </SL>

Description

Preparation of a catalyst The present invention relates to gold and gold

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 hydrogenactivated 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.

Methods of improving the gold surface reactivity and/or its absorption properties would be of potential commercial interest for use in gold catalysts. - 2 -

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 powders by passing a carrier gas over the sample, for example, pure nitrogen or a noble gas. This generates a negative heat effect. The Resorption 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 gold 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 prior art.

In the first aspect, the present invention provides a method for preparing a catalyst comprising gold having absorbed therein hydrogen, the method comprising the steps: (i) exposing a sample comprising gold to an atmosphere comprising nitrogen or a noble gas; (ii) exposing the sample from step (i) to an atmosphere comprising hydrogen, whereby hydrogen is absorbed into the samples (iii)exposing the sample from step (ii) to one of the following: (a) an atmosphere comprising noble gash or (b) an atmosphere comprising nitrogen) (iv) exposing the sample from step (iii) to an atmosphere comprising hydrogen, whereby hydrogen is absorbed into the sample.

In an another aspect, the present invention provides a method for preparing a catalyst comprising gold, wherein one or both of steps (i) and (iii) (as defined above) comprise exposing the sample to an atmosphere comprising a noble gas.

In an another aspect, the present invention provides a method for preparing a catalyst comprising gold, 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 -

In an another aspect, the present invention provides a catalyst comprising gold whenever prepared by a method as defined above.

In an another aspect, the present invention provides a method for preparing a catalyst precursor comprising gold, the method comprising the steps: (i) exposing a sample comprising gold to an atmosphere comprising nitrogen or a noble gash (ii) exposing the sample from step (i) to an atmosphere comprising hydrogen, whereby hydrogen is absorbed into the samples (iii)exposing the sample from step (ii) to one of the following: (a) an atmosphere comprising noble gasi or (b) an atmosphere comprising nitrogen.

Another aspect of the invention provides a catalyst precursor comprising gold 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. - 5 -

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 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 catalyst upon hydrogen 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 gold catalyst affects strongly the gold- hydrogen system.

Preferably, the method of the present invention improves the hydrogen sorption properties of a catalyst comprising gold.

Preferably, the quantity of absorbed hydrogen gas per volume unit of the catalyst following the second exposure of the catalyst comprising gold 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.

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 6 - 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 gold is exposed to hydrogen. 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 gold to be exposed to at least 10 umol of hydrogen per 2500 umol of gold 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 gold. Complete penetration of the gold may occur when the gold 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 between 20 to 240 C. The preferred temperature for the saturation of the sample 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 gold sample to an atmosphere of pure hydrogen at a rate of 1 cc/mint The interaction of hydrogen with the gold surface is associated with the evolution of heat. Whilst hydrogen is continuously flowed over the sample this heat - 7 evolution may last several hours, indicating that the interaction of hydrogen with the gold 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. 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.

The gases are preferably pre-dried before use, and preferably contain less than lO 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 l 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 lO Amok of noble gas per 2500,umol of gold; or (b) at least lO,umol of nitrogen per 2500 Amok of gold.

It will be appreciated that much larger quantities of noble gas or nitrogen may be used in step (iii) for a given - 8 - quantity of gold. For example, lOO,umol of gas may be used in step (iii) per 5,umol (l milligram) of gold on an inert support.

More preferably in step (iii) the sample is exposed to one of the following: (a) from lO to 200,umol of noble gas per 2500,umol of gold; or (b) from lO to 200,umol of nitrogen per 2500,umol of gold.

Most preferably in step (iii) the sample is exposed to one of the following: (a) from 20 - lOO,umol of noble gas per 2500,umol of gold; or (b) from 20 - lOO,umol of nitrogen per 2500 Pool of gold.

It has been found, for example, that upon exposure of a sample of 0.5g of gold (2500,umol) to a continuous flow of nitrogen, followed by lO Pool of hydrogen, then lO,umol 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).

It is preferable that the atmosphere comprising hydrogen described in the present invention contains at least 99% by volume hydrogen and most preferably at least 99.99% by volume hydrogen. Hydrogen gas of at least 99.99% purity by volume is defined in this patent as pure hydrogen. - 9 -

It is preferable that the atmosphere comprising nitrogen described in the present invention contains at least 99% by volume nitrogen and most preferably at least 99.99% by volume nitrogen. Nitrogen gas of at least 99.99% purity by volume is defined in this patent as pure nitrogen.

It is preferable that the atmosphere comprising a noble gas described in the present invention contains at least 99% by volume of noble gas and most preferably at least 99.99% by volume of noble gas. A noble gas of at least 99.99% 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 thereof. More preferably the noble gas comprises one of at least argon or neon and most preferably argon.

The gold 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 gold alloy. The sample is preferably in the form of a pure gold powder. Most preferably the gold is in the form of deposits on catalyst supports, such as TiO2, silica, graphite or iron oxides. The gold preferably has a purity of at least 99% and most preferably a purity of at least 99.99%. The purity of the gold is measured using atomic spectroscopy. - 10

A suitable temperature range for the present invention is 20 C to 240 C and preferably 50 C to 150 C. The present invention may also be carried out at room temperature.

The present invention may be carried out at pressures from atmospheric pressure (approximately 1Os Pa/g) to 150 bar/g (1.5 x 107 Pa/g). Most preferably the pressure is between atmospheric pressure(approximately 1Os 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 gold, the method comprising the steps: (i) exposing a sample comprising gold to an atmosphere comprising nitrogen or a noble gas; (ii) exposing the sample from step (i) to an atmosphere comprising hydrogen, 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, whereby hydrogen is absorbed into the sample.

- 11 -

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/mint 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 12 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 Resorption 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 summarized 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

Heat Effects - J/g Uptake - Time of the Cycle Absorption/Desorption mmol/g absorption mins (N2) 1 (H2) 11.5 - 0.07 270 (N2) 2 (H2) 21.6 4.8 0.05 83 (N2) 3 (He) 5.2 1.2 0.10 233 - 13 As mentioned above, it can be seen in Table 1, for cycle 1 and cycle 2 that the Resorption 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 0.5 g sample of pure gold sample was loaded into the calorimeter. The sample was then purged with lcc/min nitrogen for 42 hours. Two 90,umol pulses of pure hydrogen were then passed through the sample, followed by a 90,umol 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.

- 14 - Subsequent interaction with three x 45,umol 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

Cycle Heat effects - J/g Uptake of H2 mmol/g 1. (N2) 2 (H2) 0.13 0.004 3 (He) 0.76 3 x 45,umol pulses 4 (H2) 33.6 0.186 (N2) furs. 1200cc. _.

6 (Ar) 0.24 7 (H2) 42.2 0.174

Comparison Example

A 0.500g sample of 99.99% pure gold was exposed to a continuous flow of nitrogen at 1 cc/mint 45,umol 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,umol pulses of argon introduced into - 15 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 0.098 g 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 Jig 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 umol of helium. Subsequent saturation with hydrogen generated an extended heat evolution of 167 Jig associated with a hydrogen uptake of 1. 2 mmol/g by the catalyst.

Experiment 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 cat 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 Resorption process three x 45,umol of argon were injected into the Helium carrier, giving consecutive heat effects of 84mJ, 42mJ and 14mJ. 16

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.

Experiment 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 umol 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.

Claims

- 17 - CLAIMS: 1. A method for preparing a catalyst comprising gold having

absorbed therein hydrogen, the method comprising the steps: (i) exposing a sample comprising gold to an atmosphere comprising nitrogen or a noble gas; (ii) exposing the sample from step (i) to an atmosphere comprising hydrogen, whereby hydrogen is absorbed into the sample; (iii)exposing the sample from step (ii) to one of the following: (a) an atmosphere comprising noble gash or (b) an atmosphere comprising nitrogen; (iv) exposing the sample from step (iii) to an atmosphere comprising hydrogen, whereby hydrogen is absorbed into the sample.
2. The method of claim l, 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 l 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). - 18
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,umol of noble gas per 2500,umol of gold; or (b) at least 10,umol of nitrogen per 2500,umol of gold.
6. The method of claim 5, wherein in step (iii) the sample is exposed to one of the following: (a) from 10 to 200,umol of noble gas per 2500,umol of gold; or (b) from 10 to 200,umol of nitrogen per 2500,umol of gold.
7. The method of claim 6, wherein in step (iii) the sample is exposed to one of the following: (a) from 20 - 100,umol of noble gas per 2500,umol of gold; or (b) from 20 - 100,umol of nitrogen per 2500,umol of gold.
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. 19
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 gold 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 TiO2, silica, graphite or iron oxides.
17. The method of any one of claims 1 to 14, wherein the catalyst comprising gold comprises an alloy of gold.

- 20 -
18. A catalyst comprising gold whenever prepared by a method as defined in any one of the preceding claims.
19. A method for preparing a catalyst precursor comprising gold, the method comprising the steps: (i) exposing a sample comprising gold to an atmosphere comprising nitrogen or a noble gas; (ii) exposing the sample from step (i) to an atmosphere comprising hydrogen, 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 gold, the method comprising the steps: (i) exposing a sample comprising gold to an atmosphere comprising nitrogen or a noble gas; (ii) exposing the sample from step (i) to an atmosphere comprising hydrogen, whereby hydrogen is absorbed into the sample; - 21 (iii) exposing the sample from step (ii) to one of the following: (a) an atmosphere comprising noble gash or (b) an atmosphere comprising nitrogen) (iv) exposing the sample from step (iii) to an atmosphere comprising hydrogen, whereby hydrogen is absorbed into the sample.
22. A method for preparing a catalyst comprising gold substantially and hereby described with reference to any one of the Examples excluding the comparison examples.