The effect of air exposure on palladium–copper composite membranes

Applied Surface Science 240 (2005) 85–104

The effect of air exposure on palladium–copper composite membranes Fernando Roa, J. Douglas Way* Chemical Engineering Department, Colorado School of Mines, 1500 Illinois Street, Golden, CO 80401-1887, USA Accepted 8 June 2004 Available online 21 August 2004

Abstract It was found that when electrolessly deposited thin Pd and Pd–Cu membranes were exposed to air at temperatures above 350 8C, their H2 flux increased substantially immediately after the air exposure, then decreased to a new steady-state value. While this was a quasi-reversible change for the H2 flux, the flux of insoluble species, such as N2, irreversibly increased with every air exposure but by a much smaller extent. The extent of these changes was found to be dependent on the exposure time and the temperature of the tests. Thus, we decided to investigate the effect of gas exposures on the properties of these materials. Palladium and palladium–copper films, prepared by electroless deposition on ceramic supports, and commercial foils were exposed to air, hydrogen and helium at 500 and 900 8C for times varying from 1 h to 1 week with the objective of determining the effect of the different exposure conditions on the surface morphology, the flux of different penetrants and the crystalline structure of the materials. Atomic force microscopy (AFM) and X-ray diffraction (XRD) were used to study the changes occurring in the films under those conditions. It was observed that the exposure of both the electroless films and the foils to hydrogen and air markedly modified their surface morphology. The hydrogen exposure tended to smooth the surface features whereas the oxygen exposure created new surface features such holes and large peaks. Additionally it was found that the air exposure produced some oxidation of the film to create PdO. These results suggested that a common hypothesis stating that air oxidation just cleans the surface of the membrane might not be sufficient to explain all of those changes. A contributing effect of air exposure may be the increase in surface area due to the formation of palladium oxide. However, the extent of the surface area increase was insufficient to explain the increase in steadystate H2 flux. # 2004 Elsevier B.V. All rights reserved. Keywords: Palladium thin films; Air exposure; Surface topology; Solid state diffusion; Hydrogen; Self-assembly morphology; Crystalline structure

1. Introduction In previous publications [1–3] we have described in detail the fabrication, characterization and the per* Corresponding author. Fax: þ1-303-273-3730. E-mail address: [email protected] (J.D. Way).

meation properties of Pd and Pd alloys composite membranes. It has been a common practice to execute a short air purge at temperatures above 350 8C before commencing the permeation experiments for those membranes in the belief that this procedure cleans the surface of those membranes of any residual carbonaceous material left from their preparation.

0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.06.023

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Continuous improvement in our deposition technique has made possible for us to keep decreasing the thickness of the metal films. Once air purges have been applied to these thin films (5 mm), some interesting changes in their permeation behavior that might have been unnoticed with thicker films were then observed. For instance, we have seen that the pure component hydrogen fluxes increase sharply right after they are exposed to air. Eventually though, the continuous contact of the membrane with hydrogen causes the H2 fluxes to come back down to new steady-states, higher than those measured before the air exposure. Also, it is observed that the membrane’s ideal selectivity between hydrogen and nitrogen dropped after repeated air exposures. The purpose of this work was to study the influence of air exposure on the permeation behavior of palladium and palladium–copper films, and to try to determine the mechanisms by which these changes occur. The term ‘air purge effect’ is coined here to encompass all the changes that occur in the membrane and in its permeation behavior after it has been exposed to air at high temperature. Several researchers in diverse fields of study have noticed this effect and some theories had been proposed to explain what goes on. A widely held view among the Pd membrane community is that air burns away leftover contaminants, primarily organic, from the membrane preparation. An example of this argument can be found in the paper by Yang and coworkers [4]. They analyzed surface carbon content before and after executing an air purge on a Pd–Ag membrane and concluded that air treatment reduced it by half. Yet, there were some observations from our experiments that this hypothesis fails to explain, which led us to believe that there might be more processes at work besides the removal of contaminants from the surface. In contrast, the paper by Uchikawa and collaborators [5] is unique in that they describe how when they exposed thick palladium foils to air at 600 8C for 6 min, the H2 volumetric absorption saturation of 70% was obtained in a matter of minutes. In contrast, non-activated samples only reached this level of absorption after soaking them in H2 for several days. Also, they reported the presence of a blue color on the surface of the sample after the activation with air. That was later identified by XRD as being palladium oxide.

Research on the interaction between gases and metals has also been carried out by a number of groups in other fields, many of which have focused their investigations on metal properties changes resulting from exposure to air. As particularly relevant to this discussion, Aggarwal and coworkers [6], for example, described how they deposited ultra thin Pd films (40– 200 nm) via laser sputtering on oxide substrates such as MgO and LaAlO3, and annealed the films at 900 8C for an hour under vacuum and oxygen. Then, observing the resulting film using atomic force microscopy (AFM) a pattern of conical structures was revealed for the films exposed to oxygen, shown in Fig. 1, whereas less pronounced features where observed for the vacuum-exposed films. They concluded that the film had undergone a structural rearrangement caused by expansion of the Pd lattice as it oxidized and by anisotropy of the crystalline planes in the resulting oxide. Although the conditions used to treat the Pd films were very harsh compared to ours (the maximum temperature used to test our membranes was 500 8C and air rather than pure oxygen was used), we believed that the observations made and conclusions reached by Aggarwal’s group might still offer some clues which would help us explaining the air purge effect in Pd and Pd–Cu membranes. In another example, Pluym and coworkers [7] investigated the production of palladium oxide by spray pyrolysis at various conditions of temperature and environment. What they noticed is that the oxide particles were produced at low temperatures (400 8C) and they were agglomerations of single crystallites. For example, at 500 8C and under air these particles were composed of nanocrystalline grains 5–15 nm in diameter as calculated from the widths of the X-ray diffraction (XRD) patterns and from transmission electron microscopy (TEM) images. At high temperatures (800 8C and above in N2 and 900 8C and above in air) these particles decompose into dense-single crystal pure metallic palladium spheres. Comparing those papers, we think they complement each other and provided us with some tools to analyze our specific situation. The work of Pluym and coworkers sheds light on what might be the initial steps in the transformation process that leads a flat Pd film to become the sort of shapes observed by Aggarwal. In the presence of air, polycrystalline, nanosize palla-

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Fig. 1. AFM image showing the result of annealing a 80 nm thick Pd film under oxygen at 900 8C for 1 h. Reprinted with permission from Aggarwal et al., Science, 287: 2235–37 (2000). Copyright [2000] AAAS.

dium oxide crystallites might be formed at moderate temperatures (like the ones used for our permeation experiments, 350–450 8C); in time, this reaction should make the Pd film rearrange to accommodate the extra volume associated with the oxide formation. Finally, the surface topography reported by Aggarwal and collaborators at much higher temperatures could be an extreme manifestation of that rearrangement. Given that, in our opinion, the cleaning hypothesis failed to fully account for the pattern of changes observed when exposing Pd–Cu composite membranes to air, we intend to study the changes that the Pd and Pd–Cu membranes undergo when exposed to air at high temperature. Also, we will use the information extracted from the literature as a framework for the analysis of our observations. This knowledge is a valuable tool that can be used not only to improve membrane performance but also to fine-tune membrane operation parameters, which put us a step further in the direction of a real application for these membranes.

2. Experimental We studied the air purge effect from two different perspectives. The first set of data was extracted by observing and quantifying the changes caused on the gas permeances for actual membranes that were air purged while in testing. The other source of information for this study was collected by examining the changes in physical properties of pieces of metal films, which were exposed to different gases at high temperatures. A brief description of the procedure used to make, test and air purge Pd and Pd alloy composite membranes is given below. For a more in depth description of the materials, preparation and characterization techniques used, other publications by our group should be reviewed [1–3]. Tubular porous ceramic substrates were cleaned, end-glazed, plumbed into stainless steel fittings with graphite ferrules, activated and plated using electroless solutions of both palladium and copper. Mem-

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branes were leak tested at room temperature using N2 and plated additionally until no leaks were detected. Membranes to be tested were loaded into a stainless steel shell, which in turn was mounted in a tube furnace. The membrane was then heated slowly under helium up to 350 8C, at which temperature the H2 embrittlement of pure Pd is no longer a problem [8]. Single gas permeability tests were conducted at transmembrane pressure differentials of 50 and 25 psig and temperature ranging from 350 to 500 8C, using either hydrogen or nitrogen, which are nominally 99.999% pure (UHP grade). The permeate pressure (shell side) is the local atmospheric pressure (12 psia). No sweep gas is used on the permeate side during the single-gas permeation experiments. The procedure followed to perform an air purge started by removing any hydrogen left in the permeation apparatus by flowing an inert gas for at least 5 min. Air was then introduced at a pressure of between 10 and 20 psi for some short period of time, usually between 5 and 30 min. Finally, the membrane was flushed with inert gas again to allow the continuation of subsequent permeation tests. All this is done while the temperature is held constant. The metal films used to study changes in properties were made in similar manner as described above, such that the results of these studies could be applied to the operation of our membranes. The films were deposited by electroless plating in tubular ceramic porous supports, which had been prepared as before the only difference being that the seeding or activation solution

was diluted so that the adhesion between the support and the film was weaker. The metal films came off from the support by breaking it or after heating the resulting membranes to 350 8C under helium. Pieces of these metal films were then placed in a glass sample holder, which in time was secured within a stainless steel shell, heated under inert gas to the test temperature and exposed to a flowing gas for periods of time varying from 1 h to 1 week according to the needs of the particular test.

3. Results and discussion 3.1. Air purges on membranes In accordance to the prevailing hypothesis in the literature, air purges were routinely applied to our Pd– Cu composite membranes with the basic objective of cleaning whatever impurities might have ended in them [2]. However, as we discussed in the introduction, air purge treatments on membranes having thin metal films (5 mm) exhibited unusual permeation behavior. One of the membranes that best showed the effect that air purges have on permeability and selectivity was membrane 20. The visible thickness of the metal film for this membrane was about 1 mm from SEM images while its alloy composition was found to be 9 wt.% Cu. The permeation data set for membrane 20, including three air purges, are presented in Fig. 2. The

Fig. 2. Permeation data for membrane #20 with a transmembrane pressure difference of 50 psig. Dotted lines indicate timing of air purges.

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timing of the air purges is indicated as dotted lines. The first air purge lasted half-an-hour, the second 2 h and the third half-an-hour again. It is evident from Fig. 2 that after performing every air exposure, there is a surge in the hydrogen flux for membrane 20 and, less obviously at first, in its nitrogen flux as well. However, all these H2 flux maxima are short lived and the H2 flux always comes slowly back down, yet when a new steady-state H2 flux is reached its value is higher than the H2 flux before the purge. In contrast, the N2 flux increase is irreversible and the magnitude of the change seems to be proportional to the cumulative time of the individual air purges, i.e. is dramatically affected by multiple air exposures. In regard to the validity of the cleaning-only hypothesis, the behavior exhibited by membrane 20 and the other membranes with multiple air purges does not seem to back it up beyond the initial air purge. Clearly, since the high temperature tests were carried out using pure gases, there could be no significant accumulation of carbon or any other kind of contaminant on the membrane’s surface once the first purge was done. In other words, if the air purge just ‘cleans’ the surface, the hydrogen flux of the membrane should not change because its surface was freed of contamination by the air purge and cannot get contaminated

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again. However, that is not the case as we see in Fig. 2, i.e. the H2 flux comes down after any air purge is carried out. Further, that the membrane shows a different steady-state H2 flux after it undergoes an air exposure might indicate that changes in the metal film other than cleaning have taken place. In addition, it has also been observed that when a palladium–copper membrane, previously exposed to H2S and showing a severely depressed H2 flux, is put in contact with air for a short period of time, its hydrogen flux not only recovers but its value is greater than that before the membrane was sulfur-exposed. This is shown in Fig. 3. This fact serves as further evidence that air purges cause additional changes in the membrane that lie beyond the surface cleaning hypothesis. An important corollary was derived from this particular observation: air purges can be used to regenerate poisoned membranes, an application of profound consequences since potential feedstocks usually carry in finite amounts of sulfur compounds and other contaminants [9], which typically reduce the hydrogen flux. 3.2. Preliminary findings on planar films In order to start identifying possible causes for the air purge effect, we wanted to know if our films

Fig. 3. Permeation data for various tests done at 450 8C on a Pd–Cu membrane. The filled triangles represent H2 flux data for pure H2 feed. H2 fluxes after exposing the membrane to a stream containing 1000 ppm of H2S are represented by the crossed squares. Finally, the data represented by the circles are H2 fluxes measured after a 1 hr air purge and 2 hr H2 soaking.

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behaved similarly to those studied by Aggarwal and coworkers [6]. To do so, we needed to replicate their experiments but under conditions adapted to reflect our own materials and applications. This suggested some experiments such as testing unsupported Pd films made by electroless deposition as opposed to sputtered films, as was their case, and using air instead of pure oxygen. There was also the need to temporarily modify our traditional plating technique because the AFM instrument available to characterize those films could only accept flat thin samples. This would not allow us to test samples taken from our tubular membranes because of their curvature, although later on we were able to overcome this challenge. Thus, metal films were grown by electroless deposition on alternative flat supports like alpha-alumina chips, silicon wafers, alumina-based Anodiscs and planar stainless steel pieces. These support materials were chosen to obtain reasonable adhesion between the Pd films and the substrate. The support materials also needed to maintain dimensional stability at the high temperatures required for the tests. However, once these samples were exposed to the stringent testing conditions (900 8C under air), most of the substrates showed poor dimensional stability, implying that morphological anomalies if observed

could not be attributed only to the ‘air purge effect’ acting on the palladium film. Overall, silicon wafers exhibited the best performance. Fig. 4 is an AFM image taken from a 0.5 mm-Pd film supported on a piece of silicon wafer and tested at 900 8C under air for 1 h. It is possible to see the relatively mild distortions on the substrate’s surface caused possibly by thermal relaxation of dimensional stresses. Nonetheless, the presence of features resembling the Pd hillocks reported by Aggarwal and coworkers is evident, though they are only about 0.2 mm in height compared with the almost 1 mm tall features observed by them. The reason for that along with why the conical shapes are not homogeneously distributed throughout the surface of the sample may be due to weak adhesion of the Pd film to the silicon surface at such conditions. If this morphological change occurred even at some partial extent under current membrane testing conditions (between 350 and 500 8C), as the observations of Pluym and coworkers might suggest, we believed it might be connected to the observed surge in H2 flux after air purges are applied to the membrane because it will add more surface area for hydrogen dissociation. Therefore, we postulated the following hypothesis as an explanation for the ‘air purge effect’: the exposure of palladium and palladium–copper alloy thin

Fig. 4. AFM image of a Pd film supported on a piece of silicon wafer and annealed under air at 900 8C for 1 h. Vertical scale is 500 nm.

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films to air would cause them to rearrange in conicalshaped structures and, collaterally, would also clean the surface from carbonaceous materials and other contaminants if present. The H2 flux of the resulting metal film would be higher than before the air purge because of the additional surface area. Additional contributions to the permeation rate could also come from alternative transport paths like pinholes, very likely created during the rearrangement of the film. This idea is better illustrated in Fig. 5, which is our graphical representation of what would happen to a Pd film exposed to air at high temperature. At first, the as-deposited film would have almost no defects or pinholes; the transport of hydrogen atoms would occur mostly by solid-state diffusion, while the N2 molecules, which are not soluble in the Pd film, would permeate only through defects on the film. Once the film is exposed to air though, its morphology would change radically. The air-exposed metal film would rearrange itself forming conical structures that would create more area for the H2 to dissociate on the surface of the membrane. As a consequence of this process, the metal film might get very thin in the troughs or even recede from the support, thus creating leak paths for non-hydrogen gases. The opening of pinholes would be consistent with the N2 flux increase observed after the membrane is exposed to air. From a chemical standpoint, this hypothesis might be justified as well. It is noted that the oxidation of metallic Pd is accompanied by an experimentally observed 38% volume increase [6]. Thus, metal oxide material would be pushed off of the metal film bulk to relieve the dimensional stress created by the molecular volume expansion; that would result in the formation of the conical-shaped structures on the surface of an oxidized film as Aggarwal and coworkers suggested [6].

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3.3. Metal films from membranes Although other planar supports were used, a procedure was devised to allow the use of tubular supports to be analyzed by AFM. We reasoned that even though the tubular supports had the curvature problem, the metal films deposited on them could be easily flattened out, if they could be removed. Consequently, we needed to figure out a way to get the films to come off the support to study them without damaging them or compromising in any way their characteristics. As it turned out, by impregnating a new ceramic support with an spent or diluted activation solution while following the other original preparation steps, the films came off after heating the resulting membranes under helium purge. In essence, using the depleted activation solution created a much weaker attachment between the film and the support that could rather easily be broken. In all other aspects the unattached films closely resembled the metal films on our other membranes. For instance, Fig. 6 shows top and backside SEM images taken from a Pd film produced as described above. The features on the top surface of the film, Fig. 6a, (commonly referred to as ‘cauliflowers’ in reference to their particular shape) are similar to those observed on films for whose preparation the standard procedure was followed and so were still strongly attached to their support as seen, for example, in the SEM images published in the article by Paglieri and coworkers [2]. The picture of the back of the film, Fig. 6b, shows a morphology closely mirroring the top surface of the support. Fig. 7 shows AFM images taken from the corresponding surfaces of an as-prepared sample. As was the case with the SEM image of the back, the AFM images of the back of the metal film closely resemble the morphology of the top surface of the support. Similarly, the top-side of the film shows a fairly

Fig. 5. Graphical representation of the air purge effect on a Pd–ceramic composite membrane according to our initial hypothesis.

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Fig. 6. Top and backside SEM images of a Pd film grown on a tubular ceramic support. Scale bar for both is 10 mm.

smooth underlying texture and a number of much larger features growing out of it that correspond to the cauliflowers seen in the SEM image. In the discussion that follows, all of the analyses were performed only on the top surface of the film unless otherwise noted. This is because that surface is ordinarily the one exposed to gases when the membranes are tested in the permeation apparatus.

3.4. Air exposures on films at 900 8C Some of the films were tested at temperatures of 900 8C for one hour under helium, hydrogen and air to simulate the conditions used by Aggarwal and collaborators. Table 1 shows the effects of these exposures on the morphology of the film. In agreement with Aggarwal’s observations, the surface of the membrane

Table 1 The effect of environment conditions on surface structure of electroless Pd films at 900 8C for 1 h under He, H2 and air View

Helium

Hydrogen

Air

135

149

293

Top

Side

RMS (nm)

Roughness expressed in nanometers.

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Fig. 7. AFM images from an as-prepared electroless Pd film. (a) and (c) are the side and top views of the metal side next to the ceramic top surface while (b) and (d) are the side and top views of the top surface of the film, respectively.

that was exposed to helium underwent a minor sharpening of the original surface features, however, no big changes in the overall morphology were detected under this condition. However, exposing the film to H2 and air produced remarkable changes in the surface structure. In the case of the exposure to H2, it can be seen that the original features on the surface of the film are largely smoothed out forming but a few new large features, whereas some sinking occurs as well. In contrast, the air exposure definitively transforms the landscape on the surface, clearly forming new features that look like curved cones. These features can be seen as the brighter spots in the top view while in the side view, they can be distinguished as bullet-shaped features, sprouting from the troughs of the surface and homo-

geneously distributed throughout it. Nonetheless, they show some resemblance to the shapes observed in Figs. 1 and 4. These results suggested that the selfassembly noted by Aggarwal could indeed occur in our metal films. One question that might be asked at this point is why the overall morphology in Table 1-Air did not completely look like that in Fig. 1. There are two possible factors that may help explain the differences. First, our Pd film is much thicker than the Pd films Aggarwal’s group used for their experiments (2000 versus 120 nm); it can be thought as if the source of Pd in our film is essentially infinite and a lot more time would have been necessary to complete the selfassembly of the film. Additionally, our Pd films lack the ceramic support that creates the contrast in Fig. 1,

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thus making it more difficult to distinguish the surface features. Nonetheless we think that by using the harsh and extreme conditions employed to obtain the results shown in Table 1, it is possible to qualitatively describe what changes on surface morphology might be expected after the exposure of a Pd film to a certain gas. Inert gases like He and N2 would produce a sharpening of the original features on the surface of the film without drastic changes in the surface area. Hydrogen would smooth out the original surface features, which will cause a loss of surface area. That could certainly explain why the H2 flux surge after an air purge is short lived. Finally, air would create new and large surface features, which are essentially different from the original features of the film. These new features might add significant surface area that, according to our hypothesis, could be the cause of the observed H2 flux increase.

3.5. Air exposures of films at membrane testing conditions The final objective was to understand the actual behavior of the metal film under the conditions of an air purge. To this extent, further testing of metal films was carried out at conditions that resembled more closely the actual conditions encountered when air purging and testing our membranes. Tables 2 and 3 show AFM images of samples of a 2 mm Pd film tested at 500 8C for one hour under different gases. It is noteworthy to mention that the air-exposed sample, unlike the other samples and all of the samples tested at 900 8C and whose AFM images were shown in Table 1, exhibited a blue color rather than the typical gray. As mentioned in the introduction, Uchikawa and coworkers noted that palladium oxide is blue in color [5]. This fact provided further evidence that some portion of the Pd film was being oxidized at moderate

Table 2 The effect of annealing gas on surface structure of electroless Pd films at 500 8C for 1 h under He View

As prepared film

Helium

132 5.7

165 6.6

Top

Side

RMS (nm) DArea (%)

Roughness expressed in nanometers and the surface area relative to the area of a flat sample of the same dimensions.

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Table 3 The effect of annealing gas on surface structure of electroless Pd films at 500 8C for 1 h under H2 and air View

Hydrogen

Air

48.8 2.8

266 34.8

Top

Side

RMS (nm) DArea (%)

Roughness expressed in nanometers and the surface area relative to the area of a flat sample of the same dimensions.

temperatures and that it decomposed at higher temperatures, evidently in agreement with Pluym’s group observations [7]. In looking at the AFM images in Tables 2 and 3, and especially at the values of roughness and relative surface area, we can see that exposing the film to both air and hydrogen caused the more drastic changes to its surface. Indeed, as was the case for the exposures at 900 8C, here we see again that if the film is exposed to an inert gas, in this case He, the film experiences little if any change. However, when the film was exposed to H2, it underwent a sort of smoothing out of its surface features, causing a coarsening or enlargement of its visible grains. This translated in much less roughness compared to the unexposed sample (49 versus 132 nm in RMS). Conversely, air exposure caused another sample to double its roughness, from 132 to 266 nm and, in turn,

creating a significant 30% more surface area that it exhibited originally. This area increase must be driven by the molecular volume increase theoretically expected from the oxidation of palladium. What is most interesting, the film is rearranging itself under these conditions, forming new features and opening large pores as seen also in the top view of the airexposed sample. Another piece of information, confirming that the air exposure promotes a pore-opening process on palladium and palladium alloys films, was found looking at the surface of an actual membrane exposed to air at 450 8C for hours and immediately cooled down. Fig. 8 shows a couple of SEM images of the metal film surface after this treatment. Looking at these images it is evident the presence of a significant number of visible pores, which clearly resulted from the air purge performed on this membrane.

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Another noteworthy observation derived from the permeation data for that same membrane is that, while initially it exhibited a high H2 to N2 selectivity, after the air purge was done its value dropped to about 3.7, near the predicted Knudsen selectivity for those two gases [10]. The significance of this result lies in that although the pores might look like they are microns in diameter, as shown for example in Table 3 (air) and Fig. 8, we might just be looking at their wider entrance. In other words, the pores formed on metal films under air exposure could be conically-shaped as well, with their diameter dwindling down to a few nanometers at the interface with the ceramic support. We think that the support’s own pores, whose diameter is also in the order of nanometers, might constrain the ability of the metal film to rearrange, resulting in the geometry and dimensions it adopted after the air exposure. Thus, we have confirmed that there are changes in the surface morphology of the film after an air purge is done. From these observations we deduced that there are at least two visible changes in the film capable of impacting the transport mechanism: there is the rearrangement of the metal film that adds surface area and thus more sites for H2 dissociation, and there is the pore opening process that creates unselective transport paths. Are these two processes enough to explain the observed permeation changes when our membranes are exposed to air? To answer this, we should be able to calculate their individual contributions to the overall hydrogen transport. As it turns out, these contributions can be approximately estimated. As the first Fick’s Law states, the flux is directly proportional to the transfer area. It follows that any increase in area will be reflected in a comparative flux increase. In other words, given that the increase in surface area for a Pd film after an air purge at 500 8C for 1 h averages 30% according to our morphological studies, the expected increase in H2 flux, after discounting the flow contribution coming through the new pores, should be around 30% for membranes exposed to air at similar conditions. In addition, the amount of H2 going through the pores might be estimated using the increase on N2 flux. Indeed, as noted above, after one of our membranes was exposed to air, it developed pores which were presumably responsible for its selectivity approaching the Knudsen diffusion selectivity value. Thus, the

portion of H2 flow going through pores could be approximately estimated by multiplying the difference between the N2 flux before and after the air exposure by 3.74, the expected Knudsen selectivity of H2 over N2. Both of these fluxes were measured at the same pressure differential, which justifies this calculation. Having thus derived a formal framework based on our assumptions to quantitatively evaluate the implications of air exposure on the permeation behavior of membranes exposed to air, we then proceeded to perform the calculations included in it. Table 4 shows permeation data collected from several membranes that were air-purged during their permeation experiments and also the results of the proposed calculations based on this same set of data. The first observation from looking at Table 4 is that the N2 flux change depends strongly on membrane thickness, with the magnitude of these changes being most noticeable on the thinner membranes. That increase in the diffusion of insoluble gases must be a consequence of the formation of pores, explaining the deterioration of the selectivity of membranes undergoing air purges. However, the same is not true for hydrogen since the N2 flux changes in absolute terms are insignificant if compared with the measured hydrogen flux increases. In other words, the transport of molecular hydrogen through pores is not the mechanism that accounts for the bulk of the hydrogen flux increase after an air purge. Once the H2 flux change is reduced by the small fraction of H2 molecules going through the pores, we can see that it varies a lot depending upon the duration of the air exposure, the thickness of the metal film and even the kind of support used for the membrane. In most cases, however, the increase in the H2 flux for airpurged membranes is larger than the expected 30% even though most air purges were only 30 min long and at temperatures between 350 and 400 8C. Thus, the ultimate conclusion from the air purge data is that morphological changes alone are not enough to explain the observed H2 flux behavior of air-exposed membranes. As to why there is such important disagreement between what our hypothesis predicted and the experimental data, we can only suggest some ideas at this time based upon recent developments in our continuous membrane development program. These are that air causes some oxidation of the metal film and

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Table 4 Permeation data for air-exposed membranes showing duration of air purges and changes in pure gases fluxes Membrane

Film thickness (mm)

Air-purge duration (min)

H2 flux (mol/m2 s)

N2 flux (mol/m2 s)

Before

After

Before

After

DN2 flux (mol/m2 s)

Change H2 flux (%)

USF-10

11

30 30 30

0.1218 0.1495 0.1848

0.2109 0.2812 0.3537

0.00053 0.00049 0.0005

0.00051 0.00049 0.00051

0 0 0.00001

73 88 91

GTC-14

12

30 30 30

0.0998 0.1382 0.1974

0.1343 0.1809 0.2785

0.0005 0.00081 0.00111

0.0005 0.00088 0.00111

0 0.00007 0

35 31 41

USF-20

1

30 120 30 60

0.2862 0.3291 0.4989 0.4605

0.3427 0.7072 0.7149 0.9155

0.00014 0.00018 0.00049 0.0018

0.00016 0.00044 0.00164 0.00329

0.00002 0.00026 0.00115 0.00149

20 115 42 98

USF-25b

1.5

10

0.6853

1.0965

0.00735

0.01384

0.00649

56

USF-28

1.5

40

0.0843

0.2193

0.00103

0.00228

0.00125

154

Before and after fluxes mean the fluxes before and after the air exposure was performed. H2 flux is corrected by Knudsen diffusion before calculating the percentage change caused by the air-purge.

consequently a change in its chemical and crystalline structure. Additionally, rethinking the ‘‘cleaning hypothesis’’, we think carbon is indeed removed as the film is exposed to air but this process is not limited to the surface as thought initially but it also affects the bulk of the metal film and, thus, its properties. Only these two possibilities will be briefly reviewed although this clearly does not discount that there might be even more possibilities to explain the transport changes observed in membranes exposed to air (Fig. 8).

Before attempting to discuss these ideas though, it is worth to elaborate a bit more in the pore opening process caused by air exposure and the impact that the metal film thickness has on it. For instance, the portion of H2 molecules crossing the membrane through pores is negligible in thick film membranes such as membranes 10 and 14, as noted in that the nitrogen flux hardly changed after the air exposure. On the other hand, some of the sinking spots on a thin film undergoing an air purge will end up reaching the other side of the film and becoming pores, proof of which is the

Fig. 8. SEM images of an electroless deposited Pd membrane tested and air-exposed at 450 8C which show the distribution and typical size of pores formed. Both images are from the same Pd membrane sample.

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significant increase in molecular flow for all of the thin films in Table 4. It is also clear from the data that the extent of this pore opening process is time-dependent as the N2 flux for membrane 20 only increased a 14% when the membrane was exposed to air for 30 min while it went up almost one and a half times when the air purged lasted 2 h. 3.6. Hydrogen effect on an air-purged film Finally, experiments were performed to investigate the effects of hydrogen exposure on metal films that have been air-exposed. Several pieces of Pd film were exposed to air for 30 min at 500 8C. At the end of the

air exposure period, we switched the flowing gas to hydrogen with a short helium purge for safety purposes. One of the samples was cooled down after just an hour whereas the other was left for one week under H2. AFM images of these two samples are presented in Fig. 9. The surface morphology of the sample cooled after just one hour resembles closely the morphology of films exposed to air as seen in Table 3-Air. Holes and other characteristic features formed while the film was exposed to air can still be seen in the AFM image for the sample exposed to H2 for 1 h. In contrast, the image of the sample that was exposed to hydrogen for a week shows a smooth surface with no visible holes.

Fig. 9. Effect of H2 exposure on a 2 mm-thick electroless film previously exposed to air (30 min, 500 8C). (a) and (c) AFM top and side images of sample exposed for 1 week at 500 8C; (b) and (d) AFM images sample exposed for 1 h at 500 8C.

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This behavior is consistent with what happened to the films exposed to hydrogen at 900 8C; hence we can conclusively say that the hydrogen exposure of a film causes a considerable smoothing of its surface. In other words, holes and features developed during air purges are covered and leveled off respectively, during H2 exposure. We believe that these observations might be the reason why the hydrogen flux goes steadily down when an air-purged membrane is kept under H2. We reasoned that by flattening the surface, this process decreases the surface area, which in turn reduces the number of active sites available to dissociate and transfer hydrogen into the metal bulk. Simultaneously, by leveling and covering up the holes, the hydrogen exposure might decrease the leakage of molecular species going through the holes. Though this might be particularly advantageous in failing membranes, in practice, only seldom we have been able to detect a significant degree of reduction on the N2 leakage rate during permeation tests. This may be due to the fact that we rarely perform permeation tests above 400 8C. As these results convey, it is important to note that some of the changes the metal film undergoes when exposed to air are transient in nature while others modify the membrane permanently regardless of the changes in operation conditions. The morphological changes caused by the air exposure on the surface of the film belong to the former category because they go away as the film is exposed to hydrogen once more.

4. Effect of gas exposure on bulk metal structure 4.1. Electroless Pd films It has been shown so far that morphological changes alone are not enough to explain all the modifications that occur in the transport properties of an air-purged film. As noted in the previous discussion, the increase in surface area failed to account for the entire H2 flux increase in air-exposed membranes. That led us to think that perhaps the bulk structure of the metal film, which had been neglected in the earlier analysis, might be also playing a significant role in the air purge phenomenon. In order to determine what type of changes where occurring at that level, we studied the crystalline

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structure of metal samples exposed to the gases of interest by X-ray diffraction (XRD). The same type of Pd film samples as used for the morphology studies were employed for these studies. The samples were heated to 500 8C under inert gas and subsequently exposed to air, H2 or He for 1 h and then quickly cooled down. Subsequently, they were examined by means of XRD. Table 5 presents the XRD spectra for these samples and the respective lattice parameters and dominant crystal phases. Looking at these spectra it is evident that only the air exposure caused a significant difference in the crystalline structure of the resulting film. Indeed, common to all four spectra shown in Table 5 is a well-defined set of five high-intensity reflections from the crystalline planes 111, 200, 220, 311 and 222 in the 308 to 958 2y range, this diffraction pattern belongs to a face centered cubic lattice with a lattice parameter (a) equal to 0.388 nm, which corresponds to pure palladium in the so-called a phase. In the air-exposed sample, however, in addition to the fcc peaks there is a series of not so intense reflections from the planes 110, 111, 200, 210, 211 and 220 in the same 2y range. This pattern corresponds to a CsCl lattice (commonly referred as a bcc lattice) with a ¼ 0:373 nm and thus consistent with the PdO reflections [5]. The point to highlight from these findings is that, as predicted by Pluym’s group [7], the processing conditions at which membranes are air-purged are sufficient to partially oxidize the metal film. The presence of palladium oxide might alter the permeation properties of an air-exposed film in a way that is beyond the scope of the present investigation to examine. What can be said is that this change is temporary as the oxide is clearly susceptible to be reduced to the metal state once H2 is reintroduced. For the reader’s reference, some theories from the literature that might illuminate the role that oxide plays in the air purge effect are given next; however, no effort is made to verify their validity. According to Pluym and coworkers, the formation of palladium oxide causes a reduction in the size of the original Pd crystallites to nanosize, probably increasing the volume of grain boundaries. More grain boundaries would enable more surface diffusion of H2; in turn, the solid-state diffusion of atomic hydrogen would be obstructed by those additional boundaries.

The downward behavior of the H2 flux after an airpurged membrane is exposed back to hydrogen for testing could fit within this hypothesis because if it is assumed that the air purge develops a lattice of nanocrystallites, they are forced to agglomerate into bigger grains when the film is exposed to H2, as we have seen, for example, in Fig. 8. The smoothing of the film surface must diminish greatly the grain boundaries and the beneficial effect that these initially had on the H2 permeation rate should gradually fade away. Furthermore, other researchers have found that interstitial oxygen increases the hydrogen solubility in certain metals by selectively occupying octahedral sites in the metal lattice [11]. In sum, additional work is needed in this area to establish the validity of these theories. Yet, to complete our understanding of the air purge effect it is important to consider not only the surface changes associated with the exposure of metal films to air but also the effects of it on their internal structure. 4.2. Pd60Cu40 foils

Source: JCPDS–ICDD card files.

a-phase Pd (fcc) PdO (bcc) Pure a-phase Pd Pure a-phase Pd Pure a-phase Pd

0.388 nm (fcc) 0.373 nm (bcc) 0.388 nm 0.388 nm 0.388 nm

fcc (large peaks) bcc (small peaks) fcc fcc

Crystalline structure Lattice parameter Identification

XRD spectrum

fcc

Air As prepared

Helium

Hydrogen

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Table 5 XRD spectra and crystalline structure parameters and identification for electroless Pd film samples exposed to air, H2 and He at 500 8C for 1 h

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Cold rolled 25 mm-thick foils were used instead of electroless films to study the effects that a short air exposure had on their crystalline structures. The reason was that these foils exhibited a very uniform alloy composition unlike our samples that showed some local variation in composition. Table 6 shows the XRD patterns for foil samples exposed to H2, He and air at 500 8C for 1 h. At the temperatures that these samples were tested (500 8C) and at their particular alloy composition (40 wt.% Cu) two phases can coexist, according to the phase diagram for the palladium copper alloy [12]: a NaCl/fcc phase called the a-phase and a CsCl/bcc phase called the b-phase. Eventually, however, in the presence of hydrogen the phase boundaries are displaced and only the b-phase should remain at temperatures below 600 8C [13]. We can see that with the exception of the diffraction pattern for the initial foil, all the diffraction patterns definitively present reflections corresponding to the two expected phases for this alloy composition. This simply tells us that the a-to-b phase transition is a thermodynamically spontaneous process at these conditions and that eventually the entire crystalline domain will be in the b-phase.

fcc (small peaks) bcc (large peaks) two other phases 0.375 nm (fcc) 0.297 nm (bcc)

Pd60Cu40 a-phase (fcc) Pd60Cu40 b-phase (bcc) palladium/copper oxides

fcc (large peaks) bcc

0.375 nm (fcc) 0.297 nm (bcc)

Pd60Cu40 a-phase (fcc) Pd60Cu40 b-phase (bcc)

Yet two different things happened to the foil that saw air. First, unlike the XRD spectra for H2 and He exposed foils where the a-phase reflections show higher intensity, the diffraction pattern for the airexposed sample displays more intense peaks for the bphase. In other words, this latter phase is in bigger proportion within the lattice than the a-phase; this implies that the expected a-to-b phase transition was taking place at a faster rate when the film was being exposed to air. Second, two more sets of less-intense peaks are discernable in the XRD spectrum for the air-exposed foil. They correspond to oxides of palladium and copper but the low intensity of their peaks did not allow us to complete the indexing of their diffraction patterns and calculation of their lattice parameters. Nevertheless, we believe that these oxides should behave in very much the same way as the palladium oxide did in the electroless Pd films. 4.3. Grain sizes from the XRD spectra

Pd60Cu40 a-phase (fcc) Pd60Cu40 b-phase (bcc) Pd60Cu40 a-phase

Bcrystallite ¼

kl Lcos y

Bcrystallite is the broadening of X-ray diffraction peaks due to crystallite size, l the wavelength of the X-rays used, y the Bragg angle, L the ‘average’ crystallite size measured in a direction perpendicular to the surface of the specimen, and k a constant usually taken as close to unity. Table 7 summarizes the results obtained for the different exposures. It is evident from the grain size data presented in Table 7 that this parameter is strongly influenced by Source: JCPDS–ICDD card files.

0.375 nm (fcc) 0.297 nm (bcc)

fcc

Crystalline structure Lattice parameter Identification

XRD spectrum

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From the X-ray diffraction patterns, information about grain sizes for each phase and under the different environments can be calculated using the Scherrer equation [14]:

0.375 nm

fcc (large peaks) bcc

Air Hydrogen Helium As received

Table 6 XRD spectra and crystalline structure parameters and identification for Pd60Cu40 foil samples exposed to air, H2 and He at 500 8C for 1 h

F. Roa, J.D. Way / Applied Surface Science 240 (2005) 85–104

Table 7 The effect of annealing gas on the average grain size in nanometers for a 2 mm-thick Pd film and a 25 mm-thick Pd60Cu40 foil at 5008C for 1 h Crystal phase

Helium

Air

Hydrogen

Pd film (a-Pd) Pd film (PdO–fcc) Pd60Cu40 foil (fcc) Pd60Cu40 foil (bcc)

30

18.8 16.4 28 38

38

46.7 51.4

35.8 90.6

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the annealing environment. One can see that the air exposure creates the smallest crystallites both in the Pd film and in the Pd–Cu foil and this fact correlates very well with the findings of Pluym’s group [7]. This is further evidence that the crystal structure of a Pd or Pd–Cu film is considerably but probably just temporarily modified when its surface is exposed to air at permeation conditions. As discussed before, smaller grain sizes mean more grain boundaries, this entails that hydrogen atoms diffusing through the metal bulk will encounter them more often and will be slowed down by them. On the other hand, surface diffusion of molecular hydrogen will be incremented as a result of the larger boundaries. These competing processes combined with the new phases or species created in the solid phase under air purge conditions must surely affect the balance between the different transport mechanisms for both soluble and insoluble penetrants. Further study is required still to clarify how these changes relate to the measured fluxes.

5. Effect of gas exposure on carbon content and removal rate As opposed to the metal film changes described above, carbon removal by gas exposure at high tem-

perature might permanently modify the properties of the membrane. The origin of this carbon can be traced back to the electroless plating method used to fabricate these membranes. Indeed, for the chemistry of this process to work, the formation of certain complex ions between the metal to be plated and organic ligands is required prior to the metal deposition. As the plating proceeds, some ligands inevitably end up getting attached and then trapped inside the growing metal film. When membranes prepared this way are annealed and tested at high temperatures, the organic compounds still in the film get decomposed to, presumably, some form of elemental carbon. Total carbon analyzers have shown that in some instances the carbon content of these electrolessly deposited films can reach upto 7 wt.% of the metal film. Several thermogravimetric analysis (TGA) experiments have also been carried out using pieces of metal film to determine what the effects of atmosphere and temperature conditions have on the rate of extraction of this carbon. Fig. 10 shows a typical TGA test for our membranes. Initially, a sample of metal membrane was tested under argon to establish a baseline. Then, a second sample was tested under CO2, chosen instead of air to allow a slower removal rate. Both samples were ramped to 400 8C at 1 8C/min, held at 400 8C for

Fig. 10. TGA tests carried out with pieces of electrolessly deposited Pd films. Samples were ramped to 400 8C at 1 8C/min, held at 400 8C for 5 h and then cooled at 1 8C/min to room temperature, as the red curve shows. The blue curve is the change of weight of the sample exposed to CO2.

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5 h and then cooled at 1 8C/min to room temperature. Then the argon baseline was subtracted from the CO2 TGA and the result is presented in Fig. 10. Carbon removal only starts when the sample reaches about 160 8C and proceeds rapidly from there till it reaches 400 8C. After that, carbon is removed at a slower rate for the remainder of the test. The total weight loss of the membrane under CO2 was 6.4% after 18 h. The visible evidence of two different extraction rates might indicate that two different mechanisms are subsequently dominating the carbon removal. The initial behavior probably signals the removal the most accessible carbon, that in and near the surface of the film while the slower rate of removal might be due to the slow diffusion of carbon trapped in the bulk of the metal film to its surface, where it is reacted away. Future work will probably include more detailed kinetic studies of these processes to further refine this distinction.

6. Conclusions Air purges were previously thought of as simple surface cleaning procedures for Pd and Pd alloy composite membranes. However, multiple air purges performed on thin membranes, such as Membrane 20, clearly showed that air exposure on membranes also caused a sudden quasi-reversible increase in the H2 flux and a small irreversible change in the flux of insoluble species. Air purges were also found very effective in recovering membrane activity in membranes that had been poisoned while being exposed to gas streams containing hydrogen sulfide. The air exposure of films made by electroless deposition on tubular ceramic substrates at 900 8C for 1 h created surface features and morphological changes such as the formation of conical hillocks. These changes were similar to the results obtained by Aggarwal and collaborators [6] who exposed laser sputtered Pd films to oxygen at similar conditions. Air purges at 500 8C lasting less than 1 h were shown to cause the opening of pores in the surface of Pd films as well as the self-assembly of new surface features. Those two processes account for the creation of up to 30% more surface area. The new pores created when a thin film membrane was exposed to air might explain the observed irre-

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versible increase in the flux for non-hydrogen penetrants. Conversely, the large increase in the H2 flux observed after those membranes are exposed to air cannot be understood solely in terms of the transformations occurring in the surface of a thin film exposed to air. The lattice changes observed on a metal film exposed to air might also have some bearing in their H2 flux increase. X-ray diffraction patterns showed the production of palladium oxide during air treatment of a Pd film. In turn, the characteristic nano-sized grains of this oxide could modify the H2 transport mechanism through the film by possibly introducing much more surface diffusion of H2 while slowing its solid-state diffusion by creating a more tortuous transport path. Evidence was shown indicating that gas exposures of electrolessly plated Pd–Cu films might remove carbon not only from the surface of the film but also from the metal bulk. The air treatment of cold rolled foils of Pd60Cu40 alloy composition was found to induce the a-to-b phase transition at a greater rate that in atmospheres of hydrogen or inert gas.

Acknowledgements This work was possible thanks to the funding provided by several sources. The US Department of Energy provided most of the funds used in this investigation through Grants DE-FG2699-FT40585 and DE-FG-03NT41792 from the University Coal Research Program, and Grant DE-FG03-93ER14363 from the DOE Office of Science, Division of Basic Energy Sciences. Also, additional funding came from the CO2 Capture Project consortium (DOE contract DE-FC26-OINT41145). The authors are grateful to Dr. David Edlund, from IdaTech LLC for the generous donation of Pd60Cu40 foil and to the Pall Corporation for their donation of ceramic and stainless steel supports. References [1] F. Roa, D. Way, S. Paglieri, Chem. Eng. J. 93 (1) (2003) 11. [2] S. Paglieri, J.D. Way, J. Collins, Ind. Eng. Chem. Res. 38 (1999) 1925. [3] J. Collins, D. Way, Ind. Eng. Chem. Res. 32 (1993) 3006.

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[4] L. Yang, O. Sakai, S. Kosaka, T. Kawae, T. Takahashi, in: Proceedings of the Fifth International Conference on Inorganic Membranes, Nagoya, Japan, 1998. [5] H. Uchikawa, T. Okazaki, K. Sato, Jpn. J. Appl. Phys. 32 (1993) 5095. [6] S. Aggarwal, A.P. Monga, S.R. Perusse, R. Ramesh, V. Ballarotto, E.D. Williams, B.R. Chalamala, Y. Wei, R.H. Reuss, Science 287 (2000) 2235. [7] T.C. Pluym, S.W. Lyons, Q.H. Powell, A.S. Gurav, T.T. Kodas, L.M. Wang, H.D. Glicksman, Mater. Res. Bull. 28 (1993) 369. [8] F.A. Lewis, Platinum Met. Rev. 40 (1996) 180. [9] A. Criscuoli, A. Basile, J. Membr. Sci. 181 (2001) 21.

[10] M. Mulder, Basic Principles of Membrane Technology, Kluwer Academic Publishers, Dordrecht, The Netherlands, 1991. [11] S. Yamanaka, Y. Fujita, M. Uno, M. Katsura, J. Alloys Comp. 293 (1999) 42. [12] T.B. Massalski, J.L. Murray, L.H. Bennett, H. Baker, L. Kacprzak, B.P. Burton, T. Weintraub, J. Bhansali, in: T.B. Massalski, H. Okamoto, P.R. Subramanian, L. Kacprzak (Eds.), Binary Alloy Phase Diagrams, vol.2, ASM International, Materials Park, OH, 1990. [13] E. Wicke, K. Fro¨ lich, Z. Phys. Chem. N. F. 163 (1989) 35. [14] C. Suryanarayana, M. Grant-Norton, X-ray Diffraction: A Practical Approach, Plenum Press, New York, 1998.