The interaction of O2 with silver

A survey on the molecular processes in the passage of O2 through a silver membrane

Jaap Bergwerff

Supervised by Bibi Dauvillier Dr. A.J. van Dillen

1

Abstract

In a number of industrial processes, the separation of oxygen from a mixture of gasses plays an important role. Examples are the production of pure oxygen from air and the recycling of oxygen-rich gas streams after oxidation reactions. The conventional way to achieve gas separation is by distillation at high pressures and low temperatures. However, the low energy efficiency of this cryogenic air separation process and the safety risks that are caused by the presence of pressurised pure oxygen in the system are major disadvantages. An alternative way to separate oxygen from a gas stream is by selective diffusion of oxygen through a membrane. A mixed gas stream is led over a membrane at high pressures. A low oxygen pressure on the other side of the membrane forces diffusion of oxygen through the membrane. Silver is one of the materials, which is able to permeate oxygen selectively. In this essay, the possibilities to apply silver as an oxygen separation membrane are explored. This is done by discussing publications made by a large number of researchers in this field. First of all, to provide some background, the advantages and drawbacks of several air separation processes, as well as, several techniques to produce thin silver films are discussed. The interaction between silver and oxygen at high temperatures and pressures will be the subject of chapter 3. Chapter 4 consists of an overview of oxygen transport parameters, as measured in permeability and diffusion coefficient measurements over the years. In chapter 5, the possibilities to increase the permeability of silver membranes to oxygen are discussed. Adsorption and dissociation of oxygen at the surface is required to enable diffusion of atomic oxygen through the silver membrane. The interaction of oxygen with the silver surface causes the surface to reorganise in a less densely packed terminating structure. Diffusion probably takes place via the octahedral holes in the silver lattice, though grain boundary diffusion may also occur. Bulk diffusion is regarded to be the rate-determining step in the overall process. An increase in permeability may be achieved by increasing the oxygen concentration gradient over the membrane. This can be done by either decreasing the membrane thickness or by increasing the oxygen pressure. Glow discharge facilitated adsorption, and forced desorption of oxygen by reaction with a suitable reactant on the low-pressure side are more sophisticated ways to increase the concentration gradient. However, doping of the surface with a metal, having a high affinity towards oxygen (e.g. Zr) seems to be the most promising option.

2

Contents

Abstract 1 1 Introduction 3 2 Backgrounds on gas separation using silver membranes 4

2.1 Gas separation techniques 4

2.1.1 Cryogenic air separation 4 2.1.2 Adsorption processes 4 2.1.3 Membrane technology 5

2.2 Preparation of silver membranes 7

2.2.1 Rolling 7 2.2.2 Vapour deposition 7 2.2.3 Plating 8

3 A qualitative description of the diffusion of oxygen through a silver membrane 9

3.1 Morphology of the silver surface 10 3.2 Adsorption & dissociation 11 3.3 Solution of oxygen into the bulk 12 3.4 Sub-surface oxygen species in silver 13 3.5 Bulk diffusion 13

4 Determination of diffusivity and permeability parameters of oxygen in silver 16 4.1 Diffusion: Fick’s laws 16 4.2 Permeability measurements 17 4.3 Diffusion coefficient measurements 20

5 Discussion & recommendations 24

6 References 28

3

1 Introduction

The catalytic partial oxidation (CPO) of hydrocarbons to form higher value hydrocarbon species is an important industrial process. To avoid complete oxidation, which is a highly exothermic side reaction, partial oxidation is always carried out at high temperatures (>900°C). The flow coming out of the reaction chamber contains a low concentration of the reaction product, the hydrocarbon starting product, oxygen, and carbon dioxide. After the removal of the reaction product and starting product, separation of the remaining gasflow is required to enable recycling of oxygen. As the main supply of oxygen to the CPO is produced from air, a second gas separation step is involved in the overall process [1]. Gas separation usually takes place by distillation of the gas mixture. For this cryogenic gas separation process, low temperatures and high pressures are required. The high temperature, at which the CPO process is carried out, requires (i) cooling of the gas flow leaving the reaction cycle to enable distillation, and (ii) heating of the distilled oxygen before entering the reaction cycle. These two steps are very energy inefficient, and, therefore, highly expensive. Furthermore, the high pressure required for gas distillation implies considerable safety risks [2]. For these reasons, efforts are made to develop a gas separation process that can be run at high temperatures. One of the most promising alternatives is gas separation using oxygen permeable membranes. The gas mixture is led over a membrane that is able to permeate oxygen selectively. A low oxygen pressure on the other side of the membrane induces an oxygen flux through the membrane. One of the metals that are able to dissolve oxygen selectively is silver. Oxygen permeability measurements on silver membranes were already performed in 1924 [3]. Permeability of these silver membranes increases with increasing temperature, making them well suited for integration in the CPO process. Up till now, the short lifetime of silver membranes when applied at high temperatures has been the main obstacle for their introduction in industrial gas separation processes [4]. In this essay, an attempt is made to give an indication of the possibilities and limitations of the application of silver membranes in gas separation processes. For this purpose, relevant publications on the progress of research in this field were critically reviewed. The next chapter provides some background information on the subject. In the first part of chapter 2, an overview of the different techniques for gas separation is given as well as an evaluation of their strengths and weaknesses. The second part deals with an investigation of the techniques applied by different authors to produce silver membranes. In chapter 3, the interaction between silver and oxygen is discussed on a molecular scale. The mechanism of the different steps in the passage of oxygen atoms through a silver membrane, such as adsorption, dissolution and diffusion, will be explained. Over the years, the physical permeability characteristics of silver membranes have been studied by a large number of authors. An overview of these permeability and diffusion coefficient measurements is given in chapter 4. Finally, in chapter 5, several possibilities to increase the permeability and stability of silver membranes are discussed.

4

2 Backgrounds on gas separation using silver membranes.

In this chapter, several techniques that are applied in the field of gas separation are presented. Evaluating the strengths and weaknesses of the different options, the possibility to apply silver membranes in gas separation is reviewed. An overview of various preparation techniques for metal, and more in particular silver membranes, is given in paragraph 2.2. The (dis)advantages of the different preparation techniques are discussed.

2.1 Gas separation techniques

There are a number of ways to separate oxygen from a mixture of gasses. In this paragraph, the principles of three fundamentally different set-ups are discussed. Separation may be achieved by making use of differences in properties of the different gasses, such as boiling point, chemical affinity, or atomic radius. 2.1.1 Cryogenic air separation Cryogenic air separation has been successfully employed for many years to produce oxygen in industrial set-ups. For this reason the technique is the most developed of the three described in this paragraph. After a pre-treatment step, distillation of the air takes place at high pressures and low temperatures. In this pre-treatment, contaminants, such as carbon dioxide, water and hydrocarbons are removed from the gas-stream. High purity oxygen may be obtained. It is also possible to produce high quality nitrogen and argon as useful by-products. The products are obtained at high pressures, ready for storage and use at a different site. Alternatively, the heat of the incoming gas stream may be used to accomplish expansion of the compressed gasses. The purified gasses may then be used in an integrated chemical process. This heat-exchange step is illustrated in figure 2.1. The presence of the highly reactive, pure oxygen, which is compressed at high pressures, imposes considerable safety risks [2]. Figure 2.1: cryogenic air separation process. 2.1.2 Adsorption processes The differences in affinity of several adsorbents towards nitrogen and oxygen can be used in a gas separation set-up. For instance, nitrogen is easily adsorbed in the void spaces of zeolites. A non-uniform electric field exists in the pores of this material, causing preferential adsorption of nitrogen due to its higher polarisability compared to oxygen. When air is passed

air pre-treatment

air compression

heatexchange

cryogenicseparation

storage/ product delivery

industrial process

5

through a zeolite bed, nitrogen is retained and an oxygen-rich stream exits the bed. On the other hand, carbon molecular sieves may be used as oxygen adsorbents. Oxygen molecules, being slightly smaller than nitrogen molecules, diffuse more readily in the cavities of this material. A typical flow sheet of an adsorption-based air separation process, using zeolite as a nitrogen adsorbent, is represented in figure 2.2. Pressurised and filtered air is passed through a vessel containing the zeolite. Nitrogen is adsorbed and an oxygen-rich effluent stream is produced. When the bed has been saturated with nitrogen, the feed air is switched to a fresh vessel and regeneration of the saturated bed can begin. This is done by desorption of the nitrogen by either increasing the temperature or decreasing the pressure. By using multiple vessels, the process can be run continuously. Figure 2.2: Adsorption based air separation process. Another way to bind oxygen is by reversible chemical reaction with a suitable reactant. A number of molten salt materials have been developed which have the ability to bind oxygen at certain pressure and temperature conditions. Changing the temperature or pressure leads to desorption of the oxygen. The flow sheet of separation procedures using chemical processes will resemble that of the set-ups in which physical adsorption is used [2]. 2.1.3 Membrane technology Membrane processes are based on the difference in rates of diffusion of the components in a gas mixture through a membrane separating high pressure and low pressure process streams. In general, it can be stated that the permeation flux through the membrane increases with decreasing membrane thickness and increasing pressure difference. This process, illustrated in figure 2.3, can be run continuously, while no special pre-treatment of the incoming gas stream is necessary. The membrane units consist of a large number of cylinder-shaped membranes to create a high surface area. The simplicity of the system makes it possible to integrate this type of gas separation process in all kinds of industrial set-ups.

filter air

waste

oxygen

adsorption vessels

air blower

6

Figure 2.3: Membrane air separation process Microporous materials, such as polymeric materials may be used as a membrane material. Due to the smaller size of the oxygen molecule, these membranes are more permeable to oxygen than, for instance, to nitrogen. However, the selectivity of this type of membranes is far from complete and the polymers are not resistant to high temperatures. In principle, materials that are able to incorporate oxygen into their crystal structure and allow the diffusion of oxygen through the lattice, provide complete selectivity towards oxygen. For this purpose, inorganic oxide ceramic materials have recently been developed. Oxygen transport in these compounds occurs in the form of ions, as dissociation of molecules takes place on the surface [2]. Another material long known for its permeability to oxygen is silver. Already in 1924, reports on oxygen permeability measurements on silver membranes appear in literature. The permeability of these silver membranes increases with temperature [3]. However, application of the silver membranes at high temperatures is reported to result in restructuring of the silver and loss of selectivity. As was already discussed, the CPO process is carried out at high temperatures. For energy efficiency reasons, the required gas separation steps should be run at high temperatures as well. In this case, no inefficient heating and cooling processes would be required. Furthermore, one should be able to operate the gas-separation system in a continuous mode. As both cryogenic air separation and adsorption processes require low operating temperature, the gas-separation set-up using ceramic or silver membranes may be able to meet the conditions, mentioned above [2].

filter air

O2

wastemembrane

units

vacuum pump

air blower

7

2.2 Preparation of silver membranes A large number of techniques to produce thin layers of metal exist. These different procedures may be divided into three categories. Rolling is a technique in which the thickness of a metal foil is reduced by mechanical force. Another way to create a thin layer of metal is by the deposition of a metal vapour on a substrate. Deposition of a metal layer by reduction of metal ions in solution is a third way to achieve membrane growth. The principles of the different techniques will be discussed briefly in this paragraph. 2.2.1 Rolling The simplest way to produce a metal membrane is by applying high pressures on foils to reduce their thickness. This is achieved by alternate cold rolling and heat-treatment steps. In the cold rolling, the foil is passed through a laboratory mill, reducing its thickness. At the same time the structure of the metal is disturbed resulting in an increase of its hardness. Heat treatment in inert atmosphere causes a reorganisation of the metal structure. The hardness of the metal is reduced and another cold rolling step may be carried out. In this way the thickness of a Pd membrane was reduced from 750 to 50 µm in five steps [5]. The charm of this technique lies in its simplicity. After each rolling or heating step, the properties of the membrane can be investigated; i.e., the thickness and hardness of the membrane can be carefully determined. The resulting membrane will obviously be a thin sheet of metal. To obtain differently shaped membranes, further treatment is required. 2.2.2 Vapour deposition In the deposition of a vaporous metal on a substrate, three steps may be distinguished. First of all, a vapour of metal drops must be created. This can be done by evaporation of solid metal by heating it in vacuum. The metal can be heated in a number of ways. An electric current may be passed through a high resistance metal. Electron bombardment is another way to create high temperatures on the surface of the metal source. A completely different way to create a vapour of silver drops is by spraying an aqueous solution of silver nitrate in a H2/O2 flame. The water is evaporated and pyrolysis of the silver nitrate takes place resulting in a metal vapour [6]. In the second step, the metal drops must be transported from the metal source to the substrate. Nucleation and subsequent condensation takes place on the substrate surface. Hereafter, diffusion of the silver is necessary to acquire a homogeneous metal layer. Diffusion is facilitated by the feed of energy into a growing metal film. For this reason, heat treatment increases the density of the metal membrane, and the adhesion of the metal to the substrate while the amount of voids is reduced. The feed of energy can easily be achieved by heating of the substrate. Since a number of frequently used substrate structures is not resistant to high temperatures, alternative ways to supply energy have been developed. Bombardment of the growing metal film with inert gas ions of high kinetic energy is frequently applied. An elegant example of a vapour deposition technique is the so-called sputtering process. The sputtering atoms are ejected from the metal source through energetic particle bombardment. Figure 2.4 shows the standard configuration for diode sputtering. A gas discharge is created between two plates of a capacitor in a 10-2 mbar Ar atmosphere. Ar+ ions with kinetic energies

8

of 500 to 3000 eV are generated. These ions are attracted to the cathode, with the metal source attached to it, and upon collision target particles are released. Deposition of these target particles takes place on all the surfaces inside the process chamber, including the substrate attached to the anode [7]. This undirected deposition is probably one of the most important disadvantages of this technique. Coating only takes place on surfaces that are exposed to the target particle bombardment. For this reason, homogeneous coating of a substrate with a more complex shape is very difficult. Figure 2.4: The mechanism of diode sputtering. 2.2.3 Plating Generally, the term plating refers to the deposition of metal on a substrate surface by the reduction of metal ions in solution. In electroplating the reduction is brought about by the use of electrical energy. Deposition takes place on the cathode of an electrochemical cell. Electroless plating deposition takes place as a reductive agent is added to a solution of metal ions. To achieve controlled reduction, complexing agents, such as EDTA are frequently used to stabilise the metal ions in solution. When deposition has taken place, the metal formed acts as a catalytic surface for further reduction and a metal layer is being formed. Hydrazine is often used as a reducing agent for silver deposition in an alkaline solution [8]. Reaction takes place according to equation 2.1.

3

03 444)(4 NHAgeNHAg +→+ −+ (0.61 eV)

−− ++→+ eOHNOHHN 444 2242 (1.12 eV) (2.1)

OHNNHAgOHHNNHAg 223

0423 4444)(4 +++→++ −+ (1.73 eV)

Since silver has a low activity in the catalysis of reaction, a precursor is required to initiate deposition. Pd nuclei can act as a starting point for silver deposition [9]. The possibility to coat articles of any shape is an important advantage of electroless plating. Electroless plating may therefore be the appropriate technique to produce cylinder shaped silver membranes for gas separation set-ups.

- Ar+T

T

Ar+

-

Kathode

Anode

-

+

Argon ions

Electrons

Target i l

Substrate

Metal source

9

3 A qualitative description of the diffusion of oxygen through a silver membrane Literature on the behaviour of silver, applied as a membrane for gas separation has been focussed on the determination of transport parameters such as permeability and diffusion coefficients. The results of all measurements reported up to date are presented in chapter 4. In the reports on transport parameter measurements, the molecular processes, from which the unique selective permeability of silver to oxygen arises, are left out of consideration. Furthermore, little work has been done on the changes that occur in the silver membrane as a result of the exposure of this material to high temperatures and oxygen pressures. Industrially, silver is used as a catalyst for the partial oxidation of methanol to formaldehyde (Eq. 3.1) and the oxidation of ethylene to ethylene epoxide (Eq. 3.2). The interaction of oxygen with silver plays a major role in these reactions [10]. Dissociation of oxygen takes place at the silver surface and oxidation occurs via atomically adsorbed oxygen. Moreover, it was found that oxygen dissolves into the silver to form a reservoir of oxygen atoms. In this context, much research has been done on the adsorption of both molecular and atomic oxygen on silver surfaces, as well as on the diffusion of oxygen through bulk silver. A whole range of characterisation techniques, as well as, quantum mechanical calculations were applied by several authors to study the interaction of oxygen with silver. The industrial processes mentioned above are carried out at high temperatures and oxygen pressures. For this reason the silver/oxygen system was studied over a temperature range of –150°C to 800°C and at oxygen pressures up to 1 bar.

OHOCHOOHCH 2223 222 +→+ (3.1)

OHCOHC 42242 22 →+ (3.2)

The industrial process of gas separation using a silver membrane, as described in paragraph 2.1.3, also takes place at high temperatures and oxygen pressures. Using the insights that were acquired in the study of silver catalysts, an attempt will be made to describe the processes that are likely to play a role in the diffusion of oxygen through a silver membrane. One can distinguish a number of essential steps in the passage of oxygen through a silver membrane. On the high-pressure side of the membrane, adsorption of oxygen onto the silver surface must occur in one way or another. Dissociation is required as the membrane is considered to be non-porous. In the next step, oxygen dissolves into the bulk, where diffusion of the oxygen to the low-pressure side takes place. On the low-pressure side, the several processes occur in reversed order. A schematic representation of the overall process is presented in figure 3.1.

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Figure 3.1: Schematical representation of the four steps in the transport of oxygen through silver.

3.1. Morphology of the silver surface The morphology of the surface of the silver at both the high-pressure and the low-pressure side of the membrane is expected to play an important role in the overall diffusion process. The adsorption, dissociation, and dissolution steps all take place on or near the silver surface. In general, densely packed surfaces exhibit the highest degree of bonding and are therefore the thermodynamically preferred surface structures. Densely packed surfaces have a lower surface free energy as compared to more open surfaces. As silver crystallises in a face centred cubic (f.c.c.) lattice, the (111) plane is the most densely packed crystalline plane and therefore the preferred surface structure. Other low-indexed surfaces, such as (100) and (110) crystalline planes, already show an increase in surface free energy with decreasing density. Representations of low-indexed crystalline planes are presented in figure 3.2. Figure 3.2: representation of the first atomic layer in low-indexed f.c.c. surface structures. It turns out that oxygen adsorbs preferentially on the more open terminating crystal surfaces. This phenomenon is illustrated by the sticking coefficients1 for oxygen on various crystal

1 the chance of a molecule to adsorb onto a certain surface upon collision

2

1

3 4

1 adsorption of O2 2 dissociation 3 dissolution 4 diffusion

√6

√2 √2

2 2

2

(111) (110) (100)

silver

11

planes: 10-7 for (111), 10-4 for (100) and 10-3 for (110) [11, 12]. Preferential adsorption of oxygen on more open surfaces results in a decrease of their surface free energy in oxygen atmosphere [13]. A system will always strive to achieve a minimisation of its surface free energy. The ability to achieve this thermodynamically most stable formation is limited by the significant mass-transfer that is required in the rearrangement of the surface structure. Mass transfer is, however, facilitated at higher temperatures. Heating of a silver foil to 750°C in inert conditions results in annealing of the surface [13]. In SEM images, the surface appears to consist of broad, flat areas of monocrystalline material. This feature indicates the reorganisation of the surface in a densely packed surface morphology resulting in a decrease in surface free energy Heating of a silver foil in oxygen atmosphere to 750°C, resulted in massive restructuring of the silver surface. Pyramidal like structures in the µm size range were reported to be formed on the surface. Apparently, planes in a monocrystalline area prefer to be orientated in the same direction. From this feature a different preferred morphology of the surface structure is suggested [13]. This so-called oxygen induced restructuring of the silver surface may be expected to occur in silver membranes, when applied in a gas separation set-up. In such system, one side of the silver membrane is exposed to high oxygen pressures, whereas the other side faces a low oxygen pressure. Regarding the high temperatures at which air separation takes place, restructuring of the surface is to be expected. A more open surface will be formed on the high-pressure side, while on the low-pressure side a densely packed structure is the thermodynamically most stable surface. It should therefore be stressed that, although the processes that play a role on both surface sides of the membrane are basically the same, differences in the rate at which the several steps take place may occur due to the possible differences in surface structure. 3.2. Adsorption & dissociation Literature on the interaction of oxygen with a silver surface is mainly addressed to single crystals. In this way, the adsorption of oxygen on a specific Ag-surface can be studied. The researchers’ attention is focussed on the adsorption of oxygen on (110) and (111) surfaces [11, 12]. As illustrated in the previous paragraph, adsorption of oxygen causes massive restructuring of the surface at elevated temperatures. For this reason, experiments described in literature were always carried out applying low oxygen pressures. The relevance of the results of this kind of surface studies for the understanding of adsorption of oxygen on a silver membrane at high oxygen pressures and temperatures is therefore limited. Forms of molecular oxygen, adsorbed on silver surfaces were only observed at temperatures under –100°C. The oxygen molecule is adsorbed with its axis parallel to the silver surface. Chemisorption of this species results in a weakening of the O-O bond strength. This is confirmed by IR measurements [11, 12]. It is caused by the donation of electrons from the metal to the antibonding π* orbital of oxygen and backdonation of electrons from the oxygen bonding π orbital to the metal.

12

As a result of these processes, dissociation of molecular oxygen adsorbed on the surface takes place at temperatures higher than –100°C. To facilitate the resulting atomic oxygen (labelled as Oα), twice as much adsorption sites are required. Dissociation of molecular oxygen on a surface, that is densely covered with O2, will enhance desorption of part of the molecular oxygen. In the temperature range of -100°C to 300°C, the silver surface is completely covered with atomic oxygen. Above 300°C no stable adsorbed molecular or atomic oxygen species were observed [11, 12]. Industrial gas separation by silver membranes will take place at high temperatures. Adsorption measurements show no stable adsorbed species on the silver surface under these conditions. Exposure of silver to high temperatures and oxygen pressures does, however, result in restructuring of the silver surface, as mentioned in the previous section, and the dissolution of atomic oxygen into the bulk. The manifestation of both processes shows that adsorption of oxygen must take place at high temperatures. Adsorption and dissociation is apparently followed by immediate solution in the bulk and the formation of sub-surface oxygen species. 3.3 Solution of oxygen into the bulk Due to the short lifetime of the adsorbed oxygen species, in situ measurements on the dissolution process are extremely difficult. Quantum mechanical calculations provide an alternative way to study possible pathways for dissolution [14, 15]. Milov et al. have calculated the energetic characteristics of the transport of oxygen through the first layers of a (111) surface. First, the energetically most favourable position for atomic adsorption on a (111) surface was determined. Subsequently, the activation energy for the incorporation of the oxygen atom between the first and second atomic layer of the silver lattice was calculated. Silver crystallises in a f.c.c. lattice with lattice parameter 4.08 Å. In a f.c.c. lattice, octahedral holes and smaller tetrahedral holes exist. Two threefold sites exist for the atomic adsorption on a (111) surface, as illustrated in figure 3.3. One (a) is situated above an octahedral hole between the first and second silver layer and the other (b) above a tetrahedral hole. The former one proves to be the energetically favoured position. From this position, one can easily picture the migration of the O-atom to the octahedral hole. Given the Goldschmidt atomic radii of silver (1.44 Å) and oxygen atoms (0.6 Å), it can be shown that oxygen atoms fit into the octahedral holes of the silver lattice, while the tetrahedral holes prove to be too small. Figure 3.3: Schematical representation of the first (grey circles) and second (dotted line) atomic layers of the (111) surface of a f.c.c. lattice, illustrating atomic adsorption above (a) an octahedral hole and (b) a tetrahedral hole.

(b)

(a)

13

The calculated activation energy for desorption from the preferred adsorption site is found to be larger than the activation energy for migration to an octahedral hole between the first and second silver layers. The energy barrier for oxygen penetration through the first layer of the Ag(111) surface was calculated to be 100 kJ/mole. This value appeared to be much smaller than that for desorption, which was determined at 222 kJ/mole. From these values, the authors conclude that at elevated temperatures, atomically adsorbed oxygen does not desorb from the surface, but most likely dissolves into the bulk occupying octahedral holes [16]. 3.4. Sub-surface oxygen species in silver In thermal desorption spectroscopy (TDS) measurements carried out by several authors, silver was exposed to oxygen at high temperatures. After exposure, the dosing chamber was evacuated and the sample cooled to room temperature. The sample was then subsequently heated at a certain heating rate, while the amount of oxygen was monitored by a mass spectrometer. As discussed in paragraph 3.2, at dosing temperatures of 300°C or higher, no surface adsorbed oxygen is present. The oxygen desorption, detected in TDS measurements may therefore be attributed to the liberation of sub-surface adsorbed oxygen. The temperature at which desorption takes place is a measure for the stability of the adsorbed species. Two peaks were observed in the TDS spectra when dosing was carried out at high temperatures and oxygen pressures. A peak at ± 400°C was reported, which is generally attributed to desorption of bulk-dissolved oxygen (labelled Oβ) [10, 13, 17, 18]. This oxygen species is assumed to be situated in the octahedral holes of the silver lattice, or in the areas between monocrystalline domains, the so-called grain boundaries. The possible mechanisms for diffusion of this bulk-dissolved oxygen are discussed in paragraph 3.5. At temperatures of 675°C or higher, a second maximum in the oxygen desorption was observed, indicating the existence of another more tightly bound sub-surface oxygen species. This species (labelled Oγ) was found to be situated near the surface. Oxygen was assumed to be present between the first silver surface layers. This oxygen is present as negatively charged ions in a structure that resembles that of Ag2O. A closely packed surface structure is formed [13, 19, 20]. Diffusion to the surface on the low-pressure side of the gas-separation silver surface may be hindered by this layer.

3.5. Bulk diffusion

Considering the crystal structure, one can instinctively come up with a possible diffusion path for oxygen atoms through monocrystalline silver. Transport of an oxygen atom between two octahedral sites may be expected to occur in the [110] direction between Ag atoms occupying adjacent faces of the unit cell. The proposed diffusion process is illustrated in figure 3.4.

14

Figure 3.4: diffusion of oxygen atoms (open circles) via octahedral holes in the f.c.c. ordered silver lattice (solid circles). Other metals to crystallise in a f.c.c. lattice are gold and copper. Oxygen is observed to diffuse readily through copper and silver, while gold shows no oxygen diffusivity. As gold and silver have nearly identical atomic radii, stearic effects cannot explain this difference in diffusivity. Electronic calculations were performed on the model presented above in an attempt to explain these features. It was shown that the presence of oxygen in the octahedral sites of the silver and copper lattice, weakens the bonds between adjacent metal-atoms. As a result, the electron density between neighbouring atoms diminishes and diffusion resistance is decreased. In gold an opposite effect was observed; the presence of oxygen caused an increase in the metal-metal bond strength resulting in an increased diffusion resistance [21]. So far, only diffusion of oxygen through a perfect silver crystal is discussed. Defects in the silver lattice, such as Ag-atoms vacancies will undoubtedly be present. Such defects should be able to accommodate oxygen atoms with less electronic repulsion, as compared to octahedral holes. Moreover, a silver membrane is constructed of an accumulation of a large number of silver grains. Between these, monocrystalline, f.c.c. ordered, areas, so called grain boundaries exist, where the lattice shows a lower degree of ordering and is, therefore, less closely packed. These grain boundaries could also provide an alternative pathway for diffusion. To find out which of the diffusion pathways mentioned is dominant, in situ measurements on silver material, showing bulk diffusion of oxygen are required. XRD is an excellent technique to monitor changes in the bulk structure of crystalline material. Nagy et al. reported the effects of heating in reductive and oxidative atmosphere on the structure of the silver lattice. Ag-powder was heated from 100°C to 600°C while XRD patterns were recorded at 100°C, 200°C, …up to 600°C. Measurements were made in pure oxygen atmosphere, as well as, a 14% methanol in helium mixture, representing extremes in oxidation and reduction. At all circumstances, only reflections corresponding to a f.c.c. ordered silver lattice were observed in the XRD-patterns. The silver lattice did not undergo serious changes in morphology. Heating in oxygen atmosphere resulted, however, in a slight shift in the position of the reflections. Starting from 300°C, an increase in the lattice parameter was calculated from the shifted positions of the reflections. Figure 3.5 shows the

15

values for the unit cell constant (a) as a function of temperature under reductive and oxidative conditions. The expansion of the lattice upon exposure to oxygen at elevated temperature, indicated by an increase of a, provides direct evidence for the incorporation of oxygen into the silver bulk [13].

Figure 3.5: Variation of the unit cell constant (a) as a function of temperature under oxygen (solid circles) and methanol (open circles) conditions. An increase in the integral half width of peaks in an XRD pattern indicates the formation of defects or the building up of stress in the crystal lattice. Expansion of the lattice at high temperatures in oxygen is accompanied by an increase in the integral half width of the (111), (311) and (220) reflections. However, no increase in the half width of the (200) reflection was observed however. The [110] direction is the common zone axis for the (111), (311) and (220) reflections. Apparently, incorporation of oxygen causes an increase of stress or a decrease in crystal size along the [110] direction in the lattice. Nagy et al. stated that diffusion of oxygen atoms in the [110] direction, causing an increase in the amount of stress along this axis, could explain the observed increase in integral half width [13]. At low temperatures, grain boundary diffusion is expected to be the prevalent diffusion route of oxygen through silver. At elevated temperatures, crystal growth causes a decrease in the concentration of grain boundaries. Grain boundary diffusion is therefore more and more hindered with increasing temperature. Meanwhile, the activation barrier for the alternative diffusion of oxygen in the crystal volume is overcome. An increase in unit cell constant, caused by the incorporation of oxygen into the silver lattice, is monitored at 300°C by in situ XRD measurements on Ag-powder heated in oxygen atmosphere. The expansion of the silver lattice further decreases the diffusion resistance. Intuitively, diffusion of oxygen atoms through monocrystalline silver domains is expected to take place along the [110] direction. Other diffusion pathways, in which defects in the crystal are involved, may, however, not be disregarded.

4,085

4,086

4,087

4,088

4,089

4,09

4,091

4,092

4,093

4,094

4,095

0 100 200 300 400 500 600 700

Temperature (°C)

a-A

xis

valu

e (Å

)

16

4 Determination of diffusivity and permeability parameters of oxygen in silver

Over the years, a large number of techniques to determine permeability and diffusion coefficients for oxygen transport in silver have been developed. Already in 1924, Johnson studied the permeability of oxygen through a silver membrane [3]. More recently, these experiments were repeated using more sophisticated techniques. In this chapter, the theory behind these measurements will be discussed. A summary of the experimental methods that were applied over the years will be given. Furthermore, the results of measurements on the diffusion behaviour of oxygen through silver will be discussed. The principles of the diffusion process will be discussed in paragraph 4.1. Fick’s first and second law to describe diffusion will be introduced. In paragraph 4.2, an overview of several ways to determine the permeability of a silver membrane to oxygen will be explained. The values for the permeability obtained by a number of authors will be presented as well. In paragraph 4.3, different methods for diffusion coefficient measurements are discussed. Again, an overview of values for the diffusion coefficient, as found in research over the years, will be presented. 4.1 Diffusion: Fick’s laws Diffusion can be regarded as a displacement of matter from a region with a high concentration to a region with lower concentration. Diffusion stops when the concentration in the whole system has become constant. The chemical potential of a particle increases with increasing concentration. On a molecular scale, diffusion may therefore be explained by the tendency of a particle to minimise its chemical potential. Equilibrium is reached when all particles in the system have the same chemical potential. In a non-equilibrium system, the concentration (c) will be a function of time and place. As a concentration gradient is present in the system, a flux of matter (J) will result from a region with a high concentration to a region with a low concentration. The direction of this flux will be perpendicular to the concentration gradient. In mathematical terms, this process is expressed in Fick’s first law (Eq. 4.1). The flux of matter is expressed as the number of atoms that cross a certain area in a certain unit time. As the concentration is expressed in the number of atoms per unit volume, the dimension of the proportionality factor D, called the diffusion coefficient, is unit area per unit time.

( ) ( )x

txcDtxJ∂

∂−= ,, (4.1)

J(x,t) = flux of matter (mol*m-2*s-1) D = diffusion coefficient (m2*s-1) C(x,t) = concentration (mol*m-3) The diffusion coefficient is considered to be independent of concentration. When atoms diffuse through a solid, transport from one stable site to another is required. For diffusion of oxygen through silver, transport of an oxygen atom from one octahedral site to another is required. Often an energy barrier exists between two adjacent sites. A certain amount of

17

thermal energy is required to overcome this energy barrier. For this reason, the diffusion coefficient is found to increase with temperature. The relation between the diffusion coefficient and absolute temperature (T) may be expressed with an Arhennius type function (Eq. 4.2), in which the activation energy (Ea) for transport of an atom from one site to the next is usually expressed in kJ*mol-1.

RTEa

eDD−

= 0 (4.2)

The change in concentration of a system at an arbitrary time and place can be described by Fick’s second law (Eq. 4.3). The change in concentration is found to be linearly dependent on the “slope” of the concentration gradient. Since Fick’s second law is a second order partial differential equation in two variables, it can only be solved, when certain boundary conditions are imposed on the system [22].

2

2 ),(),(x

txcDt

txc∂

∂=∂

∂ (4.3)

4.2. Permeability measurements

The solution of Fick’s second law is reasonably straightforward, when the concentration profile of system is considered to be constant with time. For this “steady state approximation” equation 4.3 is simplified to:

0)(2

2

=dx

xcdD (4.4)

In the diffusion of oxygen atoms through a silver membrane, a steady state system may be achieved when the oxygen pressure at both sides of the membrane is kept constant. When this is the case, it can be shown that the oxygen concentration gradient in the membrane is constant with time and decreases linearly when going from the high-pressure side (left) to the low-pressure side (right). A schematic representation of the situation is given in figure 4.1. Figure 4.1: Schematic representation of diffusion of oxygen atoms through a silver membrane of thickness d. The concentration of oxygen atoms in the silver (c) decreases linearly when going from the high-pressure (hp) to the low-pressure (lp) side.

plp

php

chp

clp

x=0 x=d

18

The concentration of oxygen atoms at the surface (c0) is dependent on the solubility of the oxygen atoms in the silver (S) and the oxygen pressure applied (

2OP ) (Eq. 4.5). The adsorption and subsequent dissociation of one oxygen molecule results in the solution of two oxygen atoms in the bulk. The concentration of oxygen atoms at the surface is therefore expected to be linearly dependent on the square root of the molecular oxygen pressure. As for the solution of oxygen atoms into the silver bulk an energy barrier exists, the solubility shows Arhennius behaviour and its dependence on temperature is expressed in equation 4.6, in which S0 is an entropy factor and Hsol is the enthalpy of solution of oxygen in silver.

21

2Os SPc = (4.5)

RTH sol

eSS 0= (4.6) Combining Fick’s first law and equation 4.5, we find an expression for the steady state flux of oxygen atoms through a silver membrane (Jss) (Eq. 4.7). The product of diffusion and solubility coefficients is called the permeability coefficient (K) with dimension mol*m-1*s-1. Regarding the dependence on temperature of both D and S, K also shows Arhennius-type behaviour.

d

ppK

d

ppDS

dcc

Ddx

xdcDJ lphplphplphpss

−=

−=

−=−= )( (4.7)

Values for the permeability coefficient may be determined in experiment by measuring the amount of molecular oxygen diffusing through a membrane with time. A vacuum is created on one side of the membrane, while the pressure on the high-pressure side is kept at a constant value. Since √php >>√plp, the latter may be disregarded. Taking into account that the atomic oxygen flux is twice as large as the molecular oxygen flux, a value for the permeability coefficient may be derived using equation 4.8, in which

2On (mole) is the number of oxygen molecules that have passed the silver membrane with surface area A (m2) in a certain time period t (s) [23].

Apdt

ddnK

hp

O

∗∗

∗∗= 2

2 (4.8)

Over the years, the permeability of a silver membrane to oxygen has been measured by a large number of researchers, applying the method described above. Already in 1924, Johnson and Larose measured the flux of oxygen through a silver membrane. In their experimental set up, a thin silver film was attached to a porous porcelain plate. This disk formed the barrier between two chambers with high and low oxygen pressures. A certain oxygen pressure was applied on one side of the silver disk. The increase in oxygen pressure at the low-pressure side was measured over time. From this increase in pressure, the silver flux was calculated, using the ideal gas equation. A schematic representation of the experimental set-up is given in figure 4.2.a [3].

19

Another way of measuring permeability is by applying a high oxygen pressure inside a silver capillary. This method was applied by Coles in 1963. The experimental set-up is slightly different, as is illustrated in figure 4.2 b. Again the amount of oxygen diffusing to the low-pressure side is measured with time. In this method, the determination of the permeability constant is more reliable since the surface of the silver membrane is dramatically increased [24]. Figure 4.2: schematic representations of experimental set-ups for permeability measurements using (a) silver disk or (b) capillary. Other authors have repeated these experiments using better pumps and more sophisticated methods (e.g. mass spectrometry) for measuring the oxygen flux. In addition, the purity of the silver membrane, increased over the years. An overview of the different publications in this field is presented in table 4.1[3, 24-26]. Table 4.1: Specifications of permeability measurements over the years[3, 24-26].

author Year Exp. set-up Method of oxygen flow measurement

Temp. (°C)

Pressure (mbar)

Membrane thickness

(mm)

Membrane area (cm2)

Johnson 1924 silver disk pressure measurement 400-630 212-1013 0.248 2.68 Coles 1963 capillary pressure measurement 500-850 160-2733 0.127 28.6

Gryaznov 1973 capillary mass spectrometer 250-400 33-947 0.2 21 Outlaw 1988 silver disk mass spectrometer 400-800 6.7-333 0.254 8.29

The pre-exponential factor (K0) and activation energy (Ea) for permeation can be derived, by plotting log K against 1/T. In figure 4.3, the permeability coefficients measured by different authors over the years are presented in such an Arhennius plot. The activation energy for permeation may be regarded as the sum of the activation energies for solution and diffusion. Values for the pre-exponential factor and activation energy as found by the different authors are presented in table 4.2. [3, 24-26] Table 4.2: Values for pre-exponential factor and activation energy for permeation [3, 24-26].

Author Temp. (°C) Pre exp. Factor (K0) (mol*cm-1*s-1)

Activation energy (Ea) (kJ*mol-1)

Johnson 400-630 1.56*10-6 90.7 500-630 1.24*10-5 96.4 Coles 630-850 4.58*10-8 54.4

Gryaznov 250-400 6.55*10-7 85.2 Outlaw 400-800 1.68*10-6 91.5

high

high

low low

O2 supply O2 supply vacuum vacuum

a b

20

The values for the activation energy for permeation, found by Gryaznov, Johnson, and Outlaw, are in reasonable agreement with each other. A considerable decrease in activation energy was found by Coles at higher temperatures. The formation of leaks in the membrane, as a result of grain growth at high temperatures, was reported by this author. The reliability of these measurements is therefore questionable. At 800°C, the permeation coefficient found by Outlaw is 6*10-11 mole*cm-1*s-1.

Figure 4.3: Values for permeability coefficients as measured by several different authors. The activation energy for permeation can be derived from the slope of the lines (-Ea/R). The value for the permeability coefficient at 1/T=0 is called the pre-exponential factor [3, 24-26]. 4.3. Diffusion coefficient measurement. The measurement of diffusion coefficients is much more difficult than that of permeability coefficients. Investigation of a system in which the concentration gradient changes with time is required. For this reason, solution of Fick’s second law is no longer straightforward. Usually, in diffusion coefficient measurements, a silver specimen is exposed to a certain oxygen pressure for a certain period of time. In this way, a constant oxygen concentration is established throughout the silver, since the specimen is saturated with oxygen. Diffusion of dissolved oxygen out of the silver is forced by introducing this saturated silver specimen into vacuum. Figure 4.4 shows the decrease of the oxygen concentration in a silver cylinder with time after introduction into the vacuum at t=0 [27]. An expression for the decay of the oxygen flux out of the silver specimen with time may be obtained by solving Fick’s second law (Eq. 4.3) with the appropriate boundary conditions. These boundary conditions depend on the shape of the silver specimen. For example, oxygen diffusion perpendicular through the cylinder jacket requires the boundary conditions given in equation 4.9. The resulting oxygen flux as a function of time is given in equation 4.10 [27].

1E-15

1E-14

1E-13

1E-12

1E-11

1E-10

0,0000000010,0008 0,001 0,0012 0,0014 0,0016 0,0018 0,002

1/T (K-1)

K (m

ol c

m-1

s-1

)

800°C 600°C 400°C 250°C

Outlaw

Gryaznov

Johnson

Coles

21

Figure 4.4: Oxygen concentration profile in a saturated silver cylinder at different points in time, after introduction in vacuum at t=0. c = c0 for 0 < r < a t = 0 (4.9)

c = 0 for r = a t ≥ 0

τt

eDconstJ−

∗∗= with D

a∗

=784.5

2

τ (4.10)

a = radius of the cylinder c0 = oxygen concentration in the saturated silver

When a silver specimen of different shape is used, another set of boundary conditions is required. As a result, a different solution of Fick’s second law is obtained. In general, fitting a certain flux function to experimental flux data allows one to determine the diffusion coefficient. The experimental flux data can be obtained by oxygen pressure tests or by mass spectrometry. An alternative way to measure the oxygen flux from a silver specimen is electrochemistry. In this method, the silver specimen is covered with a layer consisting of ZrO2 doped with 8-10 at.% CaO2. This material has high conductivity towards oxygen ions. As in the evaporation method, a silver specimen is saturated with oxygen by exposure to high temperatures and oxygen pressures. The system is integrated in an electric circuit, as illustrated in figure 4.5. When a potential difference is applied over the circuit, an electrochemical potential well is created at the surface and the oxygen is forced out of the silver object. The magnitude of the electric current is a measure for the oxygen flux out of the silver [28].

c

r surface (r=a)

0

c0 t=0

t=t1

t=t2

t=t3

bulk (r=0)

r=0 r=a

22

Figure 4.5: Electrochemical cell for oxygen diffusion coefficient measurements.

An overview of diffusion coefficient measurements by different authors, using different techniques, is presented in table 4.3. Determination of diffusion coefficient is carried out in a larger temperature interval as compared to permeation measurements [25-31].

Table 4.3: Specifications of the measurements of diffusion coefficients of oxygen in solid silver over the years [25-31].

Author

Year Method of measurement Silver shape Temp

(°C) Pressure (mbar)

Eichenauer 1962 pressure cylinder/cone 418-862 600 Rickert 1966 electrochemical cylinder 760-900 - Bazan 1968 electrochemical cylinder 810, 820 -

Ramanarayanan 1972 electrochemical cylinder 750-950 - Gryaznov 1973 mass spectrometer membrane 250-400 33-947 Outlaw 1987 mass spectrometer membrane 400-800 6.7-333

Park 1990 electrochemical membrane 740-915 -

In table 4.4 and figure 4.6, the results of several diffusion coefficient measurements are presented. Generally, an activation energy of 46-48.5 kJ*mol-1 is found in the temperature range of 600-950°C. At lower temperatures, Gryaznov and Outlaw find considerably different values. In both cases, diffusion of oxygen out of a silver membrane was monitored. Obviously, the surface to bulk ratio of a membrane is much higher than that of a cylinder. When a membrane is used, the formation of oxygen present at (Oα) or near (Oγ) the surface will be of much more influence on the diffusion coefficient measurement.

A

Ag O 2 e-

O2-

O2-

ZrO2 + 8-10 at% CaO2

2 e-

ZrO2 + 8-10 at% CaO2

23

Table 4.4: Values for pre-exponential factor and activation energy for diffusion of oxygen through silver [25-31].

Author

Temp (°C)

Pre exponential factor (cm2*s-1)

Activation energy (kJ*mol-1)

Eichenauer 418-862 2.72*10-2 46.0 Ramanarayanan 750-950 4.9*10-3 48.5

Gryaznov 250-400 4.67*10-4 33.9 400-630 3.2*10-2 64.1 Outlaw 630-800 2.96*10-3 46.2

Park 740-915 3.2*10-3 48.2 Diffusion coefficient (cm2*s-1)

760 1.5*10-5 Rickert 900 2.9*10-5

810 2.7*10-5 Bazan 820 2.9*10-5

As can be seen in figure 4.6, the values for diffusion coefficients measured by Bazan and Rickert are in good agreement with values found by other authors in this temperature range. At 800°C, the diffusion coefficient for oxygen through silver is 1-3*10-5 cm2s-1

.

Figure 4.6: Values for diffusion coefficients for oxygen in solid silver as measured by several different authors [25-31].

OutlawEichenauer

Park

Ramanarayanan ■ Bazan ● Rickert

0,0000001

0,000001

0,00001

0,00010,0007 0,0009 0,0011 0,0013 0,0015 0,0017 0,0019

1/T (K-1)

D (c

m2

s-1)

Gryaznov

800°C 600°C 400°C 250°C

Outlaw

Ramanarayanan

Park

Eichenauer

■ Bazan ● Rickert

24

5 Discussion & recommendations Before attempts can be made to increase the O2 permeability of silver membranes, elucidation of the kinetics of the permeation process is necessary. As was discussed in chapter 3, several steps in the oxygen transport process are distinguished. The rates at which these steps take place are all functions of concentration and temperature. To obtain information on the nature of the rate-determining step, one must know the rate constant of each individual step. It is impossible to measure the rates at which the separate steps take place. However, some information on the transport kinetics may be obtained by monitoring the rate of the overall process, while changing a parameter that is likely to influence one of the separate reactions. Careful interpretation of the results may lead to a better understanding of the kinetics of the permeation process. In this chapter, an overview of publications by a number of authors is presented in which the permeability and diffusion coefficients are measured as a function of several parameters. In permeability measurements, the overall process of oxygen transport through a silver membrane is monitored. In diffusion coefficient measurements, evaporation of oxygen from an oxygen saturated silver specimen into vacuum takes place. The diffusion coefficient measured is therefore only determined by the events taking place in the bulk and on the low-pressure side of the membrane. Generally, the oxygen flux through a silver membrane is proportional to the square root of the oxygen pressure on the high-pressure side [3, 25]. From this, one can conclude that oxygen atoms are involved in the rate-determining step of the overall process. Johnson and Larose measured the permeability of silver membranes with different thickness. The oxygen flux through the membrane proved to be linearly dependent on the inverse of this parameter. In other words, the permeability constant, as expressed in equation 4.8, is independent of membrane thickness. In figure 5.1, this phenomenon is illustrated at different temperatures are presented in the same figure. Changing the thickness of the membrane does not affect the processes taking place at the surface of the membrane. One may conclude that the rate at which bulk diffusion takes place determines the rate of the overall process [3]. When diffusion is considered to be the rate-determining step, an expression for the oxygen flux J is given in equation 4.1. From this relation, it is clear that the oxygen flux through a silver membrane may be increased by either increasing the diffusion coefficient or by applying a larger oxygen concentration gradient over the membrane. The oxygen concentration at the high-pressure surface is linearly dependent on

2Op . As a result, the same dependency is found for the permeability. The concentration gradient may also be increased by decreasing the thickness of the membrane. For this reason, the oxygen flux through the membrane is proportional to the inverse of its thickness.

25

Figure 5.1: Permeability constants of oxygen through silver as measured by Johnson varying temperature and membrane thickness [3]. Outlaw et al. have measured the permeability and diffusion coefficients of silver membranes with different structures in the temperature interval of 400-800°C. Both polycrystalline (grain size ≈ 1 mm) and nanocrystalline (grain size ≈ 10 nm) silver was used as a membrane, as well as a (111) silver single crystal. They found that both permeability and diffusion coefficients were independent of the degree of crystallinity of the silver. As diffusion is expected to take place through grain boundaries and other defects in the silver lattice, one would expect a decrease in diffusion constant with increasing crystallinity [32]. When Zr doped silver was used as a membrane, an increase in permeability was measured. However, the diffusion coefficient was drastically lower. Auger electron spectroscopy (AES) measurements showed that the Zr in an Ag 2.0 Zr alloy was predominantly situated near the surface of the membrane. The presence of Zr in the surface layer seems to increase the rate at which surface processes, such as adsorption and dissolution of oxygen, take place. Due to the reactivity of Zr towards oxygen, the adsorption rate may be increased. ZrO2 is formed in the surface layer at the high-pressure side. Increased adsorption causes a larger oxygen concentration gradient over the silver membrane, resulting in higher permeability. The decrease in diffusion coefficient was explained by the formation of a ZrO2 layer on the low-pressure side of the membrane. This layer would act as a kind of trap for oxygen atoms, hindering the dissolution and desorption of oxygen from the silver surface [32].

1E-13

1E-12

1E-11

1E-100,001 0,0011 0,0012 0,0013 0,0014 0,0015 0,0016

1/T (K-1)K

(mol

cm

-1 s

-1)

600°C 400°C 500°C

▲ 0.0787 mm

□ 0.135 mm ○ 0.205 mm ♦ 0.248 mm

26

Another way to increase oxygen permeability is by glow-discharge assisted dissociation of oxygen on the high-pressure side of the membrane. In this procedure, gas phase atomic oxygen is created by the collision of oxygen molecules with electrons, created in the electric field between two plates of a capacitor. The sticking coefficient for atomic oxygen is much higher than for molecular oxygen, leading to facilitated adsorption and dissolution of oxygen. In this way, a higher concentration of oxygen near the surface is obtained, leading to an increase in the concentration gradient and enhanced permeability [33]. Gryaznov et al. studied the transport of oxygen through silver, when oxygen was allowed to react with a reactant on the low-pressure side of the membrane. The permeability of a silver membrane was monitored while oxidation of ammonia, ethylene or propylene took place at the low-pressure side of the membrane. When this silver surface was exposed to one of the reactants mentioned above, the oxygen permeability was found to be larger than when a vacuum was applied on the low-pressure side. This process was found to be reversible. When the reactants were removed, and the vacuum was restored, the permeability again decreased. Hence, the increased permeability may not be explained by changes in the surface structure upon exposure to one of the reactants [34, 35]. As was mentioned in paragraph 3.4, sub-surface oxygen species play an important role in oxidation reactions taking place at a silver surface. In this paragraph, the formation of near-surface Oγ was discussed. This species was found to be very stable as was indicated by it’s high desorption temperature in TDS measurements. Oxygen permeation through a silver membrane may lead to the formation of an Oγ surface layer on the low-pressure side of the membrane, even when a vacuum is created. A considerate oxygen concentration results at this side of the membrane, leading to a smaller concentration gradient and limited permeability. Exposure to ammonia, ethylene or propylene leads to a depletion of the oxygen surface layer. A larger concentration gradient and higher permeability result. In general, it may be concluded, that the permeability of a silver membrane may be increased by either increasing the diffusion coefficient, or the oxygen concentration gradient in the membrane. Since the diffusion coefficient is a material constant, increasing the concentration gradient seems to be the only option. This may be easily achieved by decreasing the thickness of the silver membrane or by increasing the oxygen pressure. On the high pressure side, the oxygen concentration may be increased by glow-discharge assisted dissociation, or doping of the surface with a metal having a high reactivity towards oxygen, such as Zr. Formation of a sub-surface Oγ layer on the low pressure side causes a surface limitation at this side of the membrane. Even when a vacuum is created at this surface, a considerate concentration of oxygen is present. Reaction of this Oγ with ammonia, ethylene or propylene causes a decrease in oxygen concentration on the low-pressure side, resulting in a larger concentration gradient. In figure 5.2, the influence of the different procedures to increase permeability on the concentration gradient in a silver membrane is illustrated.

27

Figure 5.2: Different ways to increase oxygen permeability of a silver membrane by increasing the oxygen concentration gradient

a. increasing oxygen pressure at the high-pressure side b. decreasing membrane thickness c. depletion of Oγ surface layer at the low-pressure side d. facilitating adsorption and dissolution

Most of these different methods to increase the permeability are not suited to be applied in a gas separation set-up. However, doping of the surface of the silver membrane with a metal having a high reactivity towards oxygen is possible. Already in 1967, a patent for a gas-separation membrane existing of silver with low concentrations of transition metal contaminants was granted. It was claimed that both permeability and stability of the permeation membrane was increased by the introduction of small amounts of Cu, Zn ,Pt or Pd [4].

a c b d

php

plp

28

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