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Investigation of the degradation mechanism of catalytic wires during oxidation of ammonia process
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Investigation of the degradation mechanism of catalytic wires during oxidation of ammonia process a a ´ ´ Jarosław Pura a,∗ , Piotr Wiecinski , Piotr Kwa´sniak a , Marta Zwolinska , Halina Garbacz a , a b b Joanna Zdunek , Zbigniew Laskowski , Maciej Gierej a b
Faculty of Material Science and Engineering, Warsaw University of Technology, Wołoska 141, 02-507 Warsaw, Poland Precious Metal Mint, Weteranów 95, 05-250 Radzymin, Poland
a r t i c l e
i n f o
Article history: Received 17 January 2016 Received in revised form 14 April 2016 Accepted 13 May 2016 Available online xxx Keywords: Platinum Catalysis Degradation
a b s t r a c t The most common catalysts for the ammonia oxidation process are 80 m diameter platinum-rhodium wires knitted or woven into the form of a gauze. In an aggressive environment and under extreme conditions (temperature 800–900 ◦ C, intensive gas flow, high pressure) precious elements are drained from the surface of the wires. Part of this separated material quickly decomposes on the surface in the form of characteristic “cauliflower-shape protrusions”. The rest of the platinum is captured by palladium-nickel catalytic-capture gauzes located beneath. In our investigation we focused on the effects of the degradation of gauzes from one industrial catalytic system. The aim of the study was to compare the degree and the mechanism of degradation of gauzes from a different part of the reactor. The study covered PtRh7 catalytic and PdNi5 catalytic-capture gauzes. X-ray computer microtomography investigation revealed that despite strong differences in morphology, each Pt-Rh wire has a similar specific surface area. This indicates that the oxidation process and morphological changes of the wires occur in a self-regulating balance, resulting in the value of the specific surface area of the catalyst. Microtomography analysis of Pd-Ni wires revealed strong redevelopment of the wires’ surface, which is related to the platinum capture phenomenon. Scanning electron microscope observations also revealed the nanostructure in the cauliflower-shape protrusions and large grains in the wires’ preserved cores. The high temperature in the reactor and the long-term nature of the process do not favor the occurrence of the nanostructure in this type of material. Further and detailed analysis of this phenomena will provide a better understanding of the precious metals etching and deposition processes during oxidation. © 2016 Elsevier B.V. All rights reserved.
1. Introduction The process of oxidation of ammonia proceeds in temperatures of 800–900 ◦ C and under extreme conditions. The most common material for this process is platinum-rhodium alloy [1,2]. During six months’ exploitation PtRh7 wires are subjected to strong processes occurring both on their surface and inside the wires. The most spectacular process is the formation of the characteristic “cauliflower-shape” growths. They can differ significantly in terms of shape, size, number and placement, depending on parameters of use: pressure, nitrogen load, temperature and gas flow direction [3]. This phenomenon has been studied [4] and several mechanisms were introduced. In an aggressive environment and under extreme
∗ Corresponding author. E-mail address: [email protected] (J. Pura).
conditions elements are drained from the surface of the wires and transform to gaseous species such as PtO2 and RhO2 . Most of these oxides quickly decompose, forming characteristic growths (Fig. 1). During use, part of the oxides (especially platinum oxide, which is more stable) is torn away by the gas flow and decomposes on other PtRh7 gauzes located beneath. This process occurs on every subsequent gauze in the gauze stack. To reduce the size and material cost of the catalyst, the last gauzes in the stack, are made of palladium based alloys. Their function besides catalysis is to capture platinum drained from Pt-Rh gauzes [5–7]. The platinum capture process is complex and preceded by an intensive redevelopment of wire morphology [8]. All these phenomena cause a strong change in morphology and chemical composition of the wires. The rate of these changes is related to the exposure time and conditions in the reactor. These conditions may vary depending on the part of the reactor. Due to the intensive processes occurring both on the surface and inside the
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140–170 ◦ C. The reactor efficiency—the degree of conversion of ammonia to nitric oxide was about 97–98%.
3. Results
Fig. 1. Schematic representation of the ammonia oxidation process assisted by Pt transport phenomenon (based on X-ray computer microtomography data).
Fig. 2. Ammonia oxidation reactor and gauzes placement.
wires, mechanical and catalytic properties drop during exploitation. Improving the exploitation behavior of the gauzes would achieve a noticeable economic impact on nitrogen acid production process. 2. Methodology Observations of the surface were executed using a Hitachi S-3500N scanning electron microscope (SEM) with an energy dispersive spectroscopy device (EDS) to obtain the chemical composition. A Hitachi NB-5000 focused ion beam microscope (FIB) and Hitachi IM4000 ion milling/polishing devices were used to reveal wire cross-sections for further detailed analysis. Cross section observations were performed using Hitachi SU-70 SEM microscope working in YAG-BSE mode to reveal microstructure. Changes in the spatial morphology of the wires were examined using Xradia XCT400 X-ray computer microtomography (XCT). The XCT research enabled three-dimensional models of studied wires and their quantitative characterization to be obtained. Complex analysis of this type of material is problematic due to the many difficulties encountered in acquiring material for samples. Ammonia oxidation is one of the first and most important steps in the production of nitric acid. This process continues uninterrupted for almost exactly six months. In most situations there is no possibility to obtain any samples from the reactor before the expiry of that period. In our investigation we were able to perform a detailed analysis of gauzes from both the catalytic and catalyticcapture parts of the reactor. For a better understanding of the processes occurring during oxidation—especially the dependence between gas flow direction and change in the spatial morphology of the wires—the first and last gauze of the catalytic stack and first and last gauze of the catalytic-capture stack (Fig. 2) were analyzed. All samples were obtained from the center of gauzes. In this case, the reactor temperature during exploitation was 870–910 ◦ C and pressure was 500 kPa. The atmosphere was a mixture of 10% (volume) of ammonia and 90% air, pre-heated up to
SEM observations and EDS chemical composition analysis showed that wires from the top and bottom of the PtRh7 stack differ in terms of their degradation rate and the morphology of the characteristic cauliflower-shape growths (Figs. 3 and 4). Laspecially the dependenceore developed growths were seen on sample B—the last gauze in the catalytic stack. Those protrusions are located on the gas outflow side and on the sides of the wires. Protrusions on sample A are smaller and located only on the gas outflow side. There are also some parts of the gauze where there is almost no trace of growths and the surface is covered only by pores and small, sharp protrusions. These protrusions do not have the typical cauliflower shape: thinner bottom and wide top – they are rather narrow and hunched. The surface of the wires core is furrowed with shallow, elongated pits characterized by grainy topography (Fig. 3b,c). These pits can also be seen also on sample B, but only on the gas inflow side of the wires (Fig. 4b). EDS analysis confirmed that these pits are significantly enriched with rhodium (Fig. 5). Those Rh-rich areas might be remains of larger growths which were torn off by the gas flow. However, it is worth noticing that Rh enrichment might be related to the rhodium diffusion to grain boundaries and then to the surface [4,9]. In such case, the observed pits placement corresponds to the areas where grain boundaries cross the surface of the wire. It is also worth noticing that these areas are almost pure rhodium (Fig. 6a). Rhodium diffusion is a known fact, but such high, local concentration of rhodium in this specific sample indicates unique diffusion-favorable conditions in the reactor. On sample B the segregation of rhodium is quite typical – it is concentrated in characteristic cauliflower-shape protrusions. Rhodium enrichment of characteristic protrusion on Pt-Rh wires is a well-known phenomenon and is observed even in the very early stages of exploitation [9]. EDS point analysis (Fig. 6b) also revealed the reduced content of Rh near the surface of the core and increased rhodium content with the height of protrusion. This depletion of rhodium near the wires’ surface clearly indicates that the Rh-enriched spots visible on sample A cannot be the remains of larger growths (at least ones that are observed on sample B). PdNi5 catalytic-capture gauzes, during exploitation, change their morphology in a different way. The platinum capture phenomenon results in a strong increase in the volume of the wires. On the first gauze of the PdNi5 stack (sample C) the gas-inflow side of the gauzes is completely rebuilt and the original layout is hard to identify. There is more visible weave on the gas-outflow side of this gauze (Fig. 7). The surface of this gauze is covered by globular pallets, partially made of captured material. Depending on the direction of the gas flow, pallets have sharp or soft edges. This is probably related to the way the gas flows across the overgrown structure. Gauze from the bottom of the reactor (sample D) retained their original layout on both sides. Wires changed their morphology to a lesser degree. Individual wires did not merge and it is possible to separate them (unlike wires from top of the catalytic capture stack). The surface of the wires is covered by regular lumps with sharper edges and rather low porosity (Fig. 8). It is worth noticing that captured platinum accumulates on the surface of the lumps in a terrace-like way. This indicates that the crystal structure of these protrusions is not random but is related to the crystal structure of the initial wire [9]. Processes occurring on the capture gauzes are not only surface processes. Between the globular pallets there are
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Fig. 3. SEM images of sample A—first PtRh7 catalytic gauze. (a) gas inflow side – wires layout, (b) gas inflow side – typical wire, (c) gas outflow side – typical wire.
Fig. 4. SEM images of sample B—last PtRh7 catalytic gauze. (a) gas inflow side – wires layout, (b) gas inflow side – typical wire, (c) gas outflow side – typical wire.
Fig. 5. EDS mapping of the surface of the PtRh7 wires.
Fig. 6. EDS point analysis of the surface of the PtRh7 wires: (a) sample A—core, (b) sample B—protrusion.
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Fig. 7. SEM images of sample C—First PdNi5 catalytic gauze. (a) gas inflow side – wires layout, (b) gas inflow side – typical wire, (c) gas outflow side – typical wire.
Fig. 8. SEM images of sample D—last PdNi5 catalytic gauze. (a) gas inflow side – wires layout, (b) gas inflow side – typical wire, (c) gas outflow side – typical wire.
cavities that reach deeper than the initial diameter of the initial wires. EDS mapping showed that, after long-term exploitation, the distribution of the elements on the surface of both catalytic-capture gauzes is quite homogenous (Fig. 9). Acquired data show that the process of platinum capture is not followed by the rhodium capture phenomenon. There is no trace of nickel on the surface of the gauzes. The platinum capture rate is significantly higher on the gas-inflow sides of both gauzes (Table 1) and in accordance with the earlier SEM observations much higher on the first gauze of the catalytic-capture stack. EDS chemical composition of the surface (Table 1) revealed a similar, considerable increase in rhodium concentration on both catalytic gauzes. On the first gauze, it is related to small, but highly enriched spots (almost 100 wt% of rhodium), while on the last gauze it is related to the rhodium enrichment of the cauliflower-shape protrusions. Micro X-Ray computer tomography is a great tool to analyze changes in the spatial morphology of catalytic wires. Operating on 3D reconstructions helps identify details that are hard to see using a microscope and gives an opportunity to quantitatively describe changes in volume, surface area and cross section shape – parameters which strongly affect the catalytic behavior of the wires. On each wire there is a significant difference in surface development between the gas inflow and gas outflow side of the wires (Fig. 10). For PtRh7 catalytic gauzes it is very easy to indicate the gas flow direction – protrusion are mainly directed one way. Also, on both samples, there is one side which is particularly smooth and this area corresponds to the direction of gas inflow. As was shown during SEM analysis, most protrusions are located on the gas outflow side, but on sample B growths are also located on the sides of the wire. It is easy to notice that the preserved core of sample A (from the top of the gauze stack) is significantly thinner. Also, the cross section of the wire changed its shape from regular round into triangular. This change might be related to the specific phenomenon of mass transport, including diffusion processes and decomposition of Pt(O) and Rh(O) on the gas outflow side mostly. The morphology of palladium-nickel gauzes changed even more. As was observed previously, the initial layout of the wires
of sample C (first from the catalytic-capture stack) is barely visible. Due to the platinum capture process, individual wires merged together, creating a massive, porous structure. As the first gauzes took over the main role of the capture of platinum, sample D morphology changed to a lesser degree. Calculations based on micro tomography data (Table 2) revealed many differences in sample parameters. For better understanding, all samples were compared to a new, unused Pt-Rh wire. Despite many differences in shape, volume and cross section area, the specific surface area of both PtRh7 samples is the same. This observation is discussed further in the article. For samples A and B it was possible to calculate actual mass loss during exploitation. Taken into account were volume change and change of the chemical composition related to the platinum capture. Mass loss is significantly higher for the sample from the top of the gauze stack, which indicates stronger degradation processes. In the case of PdNi5 catalytic-capture gauzes, it was impossible to indicate definitively which part of the porous structure is related to a particular wire, so there is no calculation of mass change. The most important observation is the highest specific surface area calculated for the first capture gauze. It is important, because the entire process of platinum incorporation proceeds via the Pd-Ni gauze surface. The most recent analysis concerns YAG BSE contrast observations of cross sections of the PtRh7 wires, which revealed strong grain growth in the wires core and nanometric/submicron structure in all cauliflower-shape growths (Fig. 11). The presence of this type of microstructure merits further detailed investigation.
4. Discussion It is well known and clearly indicated that degradation/wear processes of catalytic PtRh7 and catalytic-capture PdNi5 wires proceed in a different way. Earlier studies, complemented by recent results, enable one to describe the very complex process degradation of palladium-based catalytic-capture wires [8]. It begins with a very intensive diffusion of Ni to a certain type of grain boundary (mostly high disorientation angle) and to the surface (Fig. 12 – point
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Fig. 9. EDS mapping of the surface of the PdNi5 wires. Table 1 EDS chemical composition analysis on the surface of the wires.
Gauze
Side of the gauze
Pt
Rh
Pd
Ni
A - First PtRh7 gauze
Gas inflow Gas outflow
89.04 81.78
10.96 18.22
– –
– –
Gas inflow Gas outflow
89.05 84.52
11.05 15.48
– –
– –
Gas inflow Gas outflow
34.88 (captured) 24.91 (captured)
– –
65.12 75.09
– –
Gas inflow Gas outflow
17.11 (captured) 13.42 (captured)
– –
82.89 86.58
– –
– B - Last PtRh7 gauze – C - First PdNi5 gauze – D - Last PdNi5 gauze
Fig. 10. 3D reconstruction of the wires (based on XCT analysis).
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Table 2 Parameters of the wires after exploitation (based on XCT analysis) volume [103 m3 ].
new Sample A Sample B Sample C Sample D a
Volume [103 m3 ]
Volume lossa [103 m3 ]
Mass lossa [mg]
Surface area [103 m2 ]
Specific surface area [m−1 ]
Average cross section area [m2 ]
Calculated mass loss of 1 m wire sample [mg]
1724.97 584.10 905.70 27,383.44 11,887.43
– 1140.87 819.27 – –
– 0.0241 0.0173 – –
125.57 78.75 117.81 8793.63 2915.66
0.07 0.13 0.13 0.32 0.24
5749.90 1947.00 3019.00 – –
– 962.44 691.14 – –
If possible, estimated for 320 m length wire.
Fig. 11. YAG-BSE contrast SEM observations of protrusions cross sections: (a, b) sample A—first catalytic gauze, (c, d) sample B—last catalytic gauze.
Fig. 12. Schematic presentation of proposed degradation model of palladium-nickel catalytic capture gauzes (based on the XCT data).
1). In high temperatures and an aggressive environment those Nienriched areas dissolve rapidly. The nickel oxidizes almost entirely and forms small, feathery wisps, which are torn away by the gas flow. On the other hand, the palladium released this way forms massive protrusions, which lead to an increase in the special surface area of the wires (Fig. 12 – point 2). The high surface area of strongly rebuilt wires supports the platinum capture phenomenon. The porous structure of the original wire is quickly covered in its entirety by captured platinum (Fig. 12 – point 3). As the process
proceeds, captured platinum becomes the main matter of the wires. The platinum capture process is so intensive that the initial layout of the wires is almost impossible to identify, especially on the gas inflow-side of the capture gauze stack (Fig. 12 – point 4). Degradation of PtRh7 catalytic gauzes is also partially connected with the diffusion processes, but in this case rhodium diffusion occurs a lot more slowly. In PdNi5 gauzes after 1 month’s exploitation there is almost no trace of nickel inside and on the surface of the wires. Diffusion of Ni and strong etching of Ni-rich areas
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Fig. 13. Schematic presentation of proposed degradation model of platinum-rhodium catalytic gauzes (based on the XCT data).
across wires core is only a starting point for the platinum capture phenomenon. In the case of Pt-Rh wires, diffusion of rhodium across the wire core occurs during the whole service time. Depending on exploitation parameters it can be a diffusion aligned with gas flow direction, but it is mainly a diffusion to grain boundaries, and then to the surface [3]. No matter the service time of gauzes, there is still a noticeable amount of rhodium in the preserved wire cores. Moreover, because of the platinum loss phenomenon, the total percentage of rhodium in the wires increases. This investigation revealed the second part of the processes occurring inside the wires – strong grain growth in the wires’ preserved cores. Systematic movement of grain boundaries is related to the direction and intensity of rhodium diffusion. The combined effect of rhodium diffusion and grain growth is observed on the wires – the whole surface is covered with high Rh-content areas whose placement corresponds to the areas where grain boundaries escape the surface. It is also worth noticing that these rhodium-rich areas do not reach deep into the wire core and are visible on the surface as clusters of small (even submicron) grains/crystallites. Still more intensive processes occur on the surface of the wires. Cauliflower-shaped protrusions start to form at very early stages of gauze exploitation. They can differ significantly in terms of shape, size, number and placement, depending on the parameters of exploitation: pressure, nitrogen load, temperature and gas flow direction [3]. This investigation revealed that the morphology of these growths also depends on where the gauze is placed in the reactor. Protrusions on gauzes located at the top of the catalytic stack are smaller and have a less regular shape. Gauzes from the bottom of the catalytic stack are still covered by numerous, welldeveloped, cauliflower-shape growths. Earlier it was observed that, despite many differences in exploitation parameters, the specific surface area of Pt-Rh catalytic wires is almost the same. This investigation, however, showed that the specific surface area of wires from the top of the catalytic stack is the same as the specific surface area of wires from the bottom of the stack. This indicates that ammonia oxidation and all processes occurring in the catalytic wires might be in a self-regulating balance, which gives the maximum conversion rate of ammonia (of approximately 96–97%). Their microstructure of characteristic cauliflower-shape protrusions makes them even more interesting. Observations performed on wires cross sections revealed a submicron/nanostructure in both kinds of growth: in small, irregular (from the top of the gauze stack) and in large, cauliflower-shaped protrusions (from the bottom of the gauze stack). Extreme conditions, long service time and especially high temperatures do not favor this kind of microstructure, even in such temperature resistant material. There is no indica-
tion as to structure stability in those protrusions, especially as, in preserved cores, grain growth is quite significant. The observed microstructure is related to platinum transport phenomenon: At the beginning of the service time, extreme conditions (especially high temperature) initiate the platinum transport phenomenon (Fig. 13 – point 1). Protrusions start to form (Fig. 13 – point 2), which leads to an increase in the surface area of the catalyst. Growing protrusions are significantly enriched with rhodium and have a submicron microstructure. During exploitation, protrusions grow to their final size and probably start to lose their fine microstructure. YAG-BSE observations revealed that mostdeveloped protrusions (Fig. 11d) are made of both, nano- and micrometric size grains. At this stage (Fig. 13 – point 3), the protrusion becomes a good source of material for platinum and rhodium evaporation – initiated on the wires’ surface. Large protrusions start to dissolve. Part of the material forms new protrusions (Fig. 13 – point 4) and part is captured by Pd-Ni gauzes located underneath. The processes of formation and dissolution of protrusions proceed after each other throughout the whole exploitation time. The gradual enrichment of protrusions with rhodium has considerable importance in the whole process. This, combined with the reduction in wire diameter is a symptom of wire wear as a catalyst (Fig. 13 – point 5). The material soon loses its ability to form large, well-developed protrusions. Rhodium enrichment leads to reduced catalytic and mechanical properties. During the last stage of exploitation, entire protrusions are torn away by the gas flow – they can be observed on the surface of capture gauzes [10]. After that, on the surface of wires, small, almost 90–100% rhodium spots are observed (Fig. 6a). This kind of degradation does not occur in every gauze in the stack at the same time. Initially, the strongest change in spatial morphology takes place on the gauzes at the top of the stack (on the gas-inflow side). This gauze is also first to be exploited, as is clearly visible on SEM images. During long-term operation, each successive gauze is subjected to intense transformations. As strong transformations proceed downwards through the gauze stack, they are followed by a zone of highest platinum content, highest surface area and also strongest catalytic properties in the gauze stack. Summary This work revealed that placement of the Pt-Rh catalytic gauzes in the reactor strongly influence their morphology and chemical composition. Gauzes located on the top of the catalytic stack are covered by less-developed and less numerous “cauliflower shape” protrusions. EDS analysis also revealed in those gauzes stronger rhodium enrichment and segregation.
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XCT analysis showed, that despite many differences in shape, volume and cross section area, specific surface area of all Pt-Rh wires is almost the same. It indicates that ammonia oxidation and all processes occurring in the catalytic wires are in the selfregulating balance. This paper also showed that microcomputer tomography can be a powerful tool for analyzing this type of catalyst. Scanning electron microscope observations also revealed the nanostructure in the cauliflower-shape protrusions and large grains in the wire’s preserved cores. The high temperature in the reactor and the long-term nature of the process do not favor the occurrence of the nanostructure in this type of material. This indicate short lifetime of the observed protrusions. This implies that protrusions form and dissolve repeatedly throughout whole exploitation and accompanying phenomena such as microstructure change and gradual rhodium enrichment leads to the slow degradation of the catalyst. Obtained data allowed to present the description of two degradation processes: degradation of Pt-Rh catalytic gauzes and degradation of Pd-Ni catalytic-capture ones. Both these processes are linked by a platinum transport and merits further detailed analysis. A better understanding of phenomena occurring in the reactor will inform future improvement of catalyst performance. Due to the high cost of catalysts, it is important to reduce platinum loss rate and extend the service time of gauzes.
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Acknowledgements This work was conducted by the Faculty of Material Science and Engineering at Warsaw University of Technology in the project POIG.01.01.02-00-015/09
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Investigation of the degradation mechanism of catalytic wires during oxidation of ammonia process a a ´ ´ Jarosław Pura a,∗ , Piotr Wiecinski , Piotr Kwa´sniak a , Marta Zwolinska , Halina Garbacz a , a b b Joanna Zdunek , Zbigniew Laskowski , Maciej Gierej a b
Faculty of Material Science and Engineering, Warsaw University of Technology, Wołoska 141, 02-507 Warsaw, Poland Precious Metal Mint, Weteranów 95, 05-250 Radzymin, Poland
a r t i c l e
i n f o
Article history: Received 17 January 2016 Received in revised form 14 April 2016 Accepted 13 May 2016 Available online xxx Keywords: Platinum Catalysis Degradation
a b s t r a c t The most common catalysts for the ammonia oxidation process are 80 m diameter platinum-rhodium wires knitted or woven into the form of a gauze. In an aggressive environment and under extreme conditions (temperature 800–900 ◦ C, intensive gas flow, high pressure) precious elements are drained from the surface of the wires. Part of this separated material quickly decomposes on the surface in the form of characteristic “cauliflower-shape protrusions”. The rest of the platinum is captured by palladium-nickel catalytic-capture gauzes located beneath. In our investigation we focused on the effects of the degradation of gauzes from one industrial catalytic system. The aim of the study was to compare the degree and the mechanism of degradation of gauzes from a different part of the reactor. The study covered PtRh7 catalytic and PdNi5 catalytic-capture gauzes. X-ray computer microtomography investigation revealed that despite strong differences in morphology, each Pt-Rh wire has a similar specific surface area. This indicates that the oxidation process and morphological changes of the wires occur in a self-regulating balance, resulting in the value of the specific surface area of the catalyst. Microtomography analysis of Pd-Ni wires revealed strong redevelopment of the wires’ surface, which is related to the platinum capture phenomenon. Scanning electron microscope observations also revealed the nanostructure in the cauliflower-shape protrusions and large grains in the wires’ preserved cores. The high temperature in the reactor and the long-term nature of the process do not favor the occurrence of the nanostructure in this type of material. Further and detailed analysis of this phenomena will provide a better understanding of the precious metals etching and deposition processes during oxidation. © 2016 Elsevier B.V. All rights reserved.
1. Introduction The process of oxidation of ammonia proceeds in temperatures of 800–900 ◦ C and under extreme conditions. The most common material for this process is platinum-rhodium alloy [1,2]. During six months’ exploitation PtRh7 wires are subjected to strong processes occurring both on their surface and inside the wires. The most spectacular process is the formation of the characteristic “cauliflower-shape” growths. They can differ significantly in terms of shape, size, number and placement, depending on parameters of use: pressure, nitrogen load, temperature and gas flow direction [3]. This phenomenon has been studied [4] and several mechanisms were introduced. In an aggressive environment and under extreme
∗ Corresponding author. E-mail address: [email protected] (J. Pura).
conditions elements are drained from the surface of the wires and transform to gaseous species such as PtO2 and RhO2 . Most of these oxides quickly decompose, forming characteristic growths (Fig. 1). During use, part of the oxides (especially platinum oxide, which is more stable) is torn away by the gas flow and decomposes on other PtRh7 gauzes located beneath. This process occurs on every subsequent gauze in the gauze stack. To reduce the size and material cost of the catalyst, the last gauzes in the stack, are made of palladium based alloys. Their function besides catalysis is to capture platinum drained from Pt-Rh gauzes [5–7]. The platinum capture process is complex and preceded by an intensive redevelopment of wire morphology [8]. All these phenomena cause a strong change in morphology and chemical composition of the wires. The rate of these changes is related to the exposure time and conditions in the reactor. These conditions may vary depending on the part of the reactor. Due to the intensive processes occurring both on the surface and inside the
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140–170 ◦ C. The reactor efficiency—the degree of conversion of ammonia to nitric oxide was about 97–98%.
3. Results
Fig. 1. Schematic representation of the ammonia oxidation process assisted by Pt transport phenomenon (based on X-ray computer microtomography data).
Fig. 2. Ammonia oxidation reactor and gauzes placement.
wires, mechanical and catalytic properties drop during exploitation. Improving the exploitation behavior of the gauzes would achieve a noticeable economic impact on nitrogen acid production process. 2. Methodology Observations of the surface were executed using a Hitachi S-3500N scanning electron microscope (SEM) with an energy dispersive spectroscopy device (EDS) to obtain the chemical composition. A Hitachi NB-5000 focused ion beam microscope (FIB) and Hitachi IM4000 ion milling/polishing devices were used to reveal wire cross-sections for further detailed analysis. Cross section observations were performed using Hitachi SU-70 SEM microscope working in YAG-BSE mode to reveal microstructure. Changes in the spatial morphology of the wires were examined using Xradia XCT400 X-ray computer microtomography (XCT). The XCT research enabled three-dimensional models of studied wires and their quantitative characterization to be obtained. Complex analysis of this type of material is problematic due to the many difficulties encountered in acquiring material for samples. Ammonia oxidation is one of the first and most important steps in the production of nitric acid. This process continues uninterrupted for almost exactly six months. In most situations there is no possibility to obtain any samples from the reactor before the expiry of that period. In our investigation we were able to perform a detailed analysis of gauzes from both the catalytic and catalyticcapture parts of the reactor. For a better understanding of the processes occurring during oxidation—especially the dependence between gas flow direction and change in the spatial morphology of the wires—the first and last gauze of the catalytic stack and first and last gauze of the catalytic-capture stack (Fig. 2) were analyzed. All samples were obtained from the center of gauzes. In this case, the reactor temperature during exploitation was 870–910 ◦ C and pressure was 500 kPa. The atmosphere was a mixture of 10% (volume) of ammonia and 90% air, pre-heated up to
SEM observations and EDS chemical composition analysis showed that wires from the top and bottom of the PtRh7 stack differ in terms of their degradation rate and the morphology of the characteristic cauliflower-shape growths (Figs. 3 and 4). Laspecially the dependenceore developed growths were seen on sample B—the last gauze in the catalytic stack. Those protrusions are located on the gas outflow side and on the sides of the wires. Protrusions on sample A are smaller and located only on the gas outflow side. There are also some parts of the gauze where there is almost no trace of growths and the surface is covered only by pores and small, sharp protrusions. These protrusions do not have the typical cauliflower shape: thinner bottom and wide top – they are rather narrow and hunched. The surface of the wires core is furrowed with shallow, elongated pits characterized by grainy topography (Fig. 3b,c). These pits can also be seen also on sample B, but only on the gas inflow side of the wires (Fig. 4b). EDS analysis confirmed that these pits are significantly enriched with rhodium (Fig. 5). Those Rh-rich areas might be remains of larger growths which were torn off by the gas flow. However, it is worth noticing that Rh enrichment might be related to the rhodium diffusion to grain boundaries and then to the surface [4,9]. In such case, the observed pits placement corresponds to the areas where grain boundaries cross the surface of the wire. It is also worth noticing that these areas are almost pure rhodium (Fig. 6a). Rhodium diffusion is a known fact, but such high, local concentration of rhodium in this specific sample indicates unique diffusion-favorable conditions in the reactor. On sample B the segregation of rhodium is quite typical – it is concentrated in characteristic cauliflower-shape protrusions. Rhodium enrichment of characteristic protrusion on Pt-Rh wires is a well-known phenomenon and is observed even in the very early stages of exploitation [9]. EDS point analysis (Fig. 6b) also revealed the reduced content of Rh near the surface of the core and increased rhodium content with the height of protrusion. This depletion of rhodium near the wires’ surface clearly indicates that the Rh-enriched spots visible on sample A cannot be the remains of larger growths (at least ones that are observed on sample B). PdNi5 catalytic-capture gauzes, during exploitation, change their morphology in a different way. The platinum capture phenomenon results in a strong increase in the volume of the wires. On the first gauze of the PdNi5 stack (sample C) the gas-inflow side of the gauzes is completely rebuilt and the original layout is hard to identify. There is more visible weave on the gas-outflow side of this gauze (Fig. 7). The surface of this gauze is covered by globular pallets, partially made of captured material. Depending on the direction of the gas flow, pallets have sharp or soft edges. This is probably related to the way the gas flows across the overgrown structure. Gauze from the bottom of the reactor (sample D) retained their original layout on both sides. Wires changed their morphology to a lesser degree. Individual wires did not merge and it is possible to separate them (unlike wires from top of the catalytic capture stack). The surface of the wires is covered by regular lumps with sharper edges and rather low porosity (Fig. 8). It is worth noticing that captured platinum accumulates on the surface of the lumps in a terrace-like way. This indicates that the crystal structure of these protrusions is not random but is related to the crystal structure of the initial wire [9]. Processes occurring on the capture gauzes are not only surface processes. Between the globular pallets there are
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Fig. 3. SEM images of sample A—first PtRh7 catalytic gauze. (a) gas inflow side – wires layout, (b) gas inflow side – typical wire, (c) gas outflow side – typical wire.
Fig. 4. SEM images of sample B—last PtRh7 catalytic gauze. (a) gas inflow side – wires layout, (b) gas inflow side – typical wire, (c) gas outflow side – typical wire.
Fig. 5. EDS mapping of the surface of the PtRh7 wires.
Fig. 6. EDS point analysis of the surface of the PtRh7 wires: (a) sample A—core, (b) sample B—protrusion.
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Fig. 7. SEM images of sample C—First PdNi5 catalytic gauze. (a) gas inflow side – wires layout, (b) gas inflow side – typical wire, (c) gas outflow side – typical wire.
Fig. 8. SEM images of sample D—last PdNi5 catalytic gauze. (a) gas inflow side – wires layout, (b) gas inflow side – typical wire, (c) gas outflow side – typical wire.
cavities that reach deeper than the initial diameter of the initial wires. EDS mapping showed that, after long-term exploitation, the distribution of the elements on the surface of both catalytic-capture gauzes is quite homogenous (Fig. 9). Acquired data show that the process of platinum capture is not followed by the rhodium capture phenomenon. There is no trace of nickel on the surface of the gauzes. The platinum capture rate is significantly higher on the gas-inflow sides of both gauzes (Table 1) and in accordance with the earlier SEM observations much higher on the first gauze of the catalytic-capture stack. EDS chemical composition of the surface (Table 1) revealed a similar, considerable increase in rhodium concentration on both catalytic gauzes. On the first gauze, it is related to small, but highly enriched spots (almost 100 wt% of rhodium), while on the last gauze it is related to the rhodium enrichment of the cauliflower-shape protrusions. Micro X-Ray computer tomography is a great tool to analyze changes in the spatial morphology of catalytic wires. Operating on 3D reconstructions helps identify details that are hard to see using a microscope and gives an opportunity to quantitatively describe changes in volume, surface area and cross section shape – parameters which strongly affect the catalytic behavior of the wires. On each wire there is a significant difference in surface development between the gas inflow and gas outflow side of the wires (Fig. 10). For PtRh7 catalytic gauzes it is very easy to indicate the gas flow direction – protrusion are mainly directed one way. Also, on both samples, there is one side which is particularly smooth and this area corresponds to the direction of gas inflow. As was shown during SEM analysis, most protrusions are located on the gas outflow side, but on sample B growths are also located on the sides of the wire. It is easy to notice that the preserved core of sample A (from the top of the gauze stack) is significantly thinner. Also, the cross section of the wire changed its shape from regular round into triangular. This change might be related to the specific phenomenon of mass transport, including diffusion processes and decomposition of Pt(O) and Rh(O) on the gas outflow side mostly. The morphology of palladium-nickel gauzes changed even more. As was observed previously, the initial layout of the wires
of sample C (first from the catalytic-capture stack) is barely visible. Due to the platinum capture process, individual wires merged together, creating a massive, porous structure. As the first gauzes took over the main role of the capture of platinum, sample D morphology changed to a lesser degree. Calculations based on micro tomography data (Table 2) revealed many differences in sample parameters. For better understanding, all samples were compared to a new, unused Pt-Rh wire. Despite many differences in shape, volume and cross section area, the specific surface area of both PtRh7 samples is the same. This observation is discussed further in the article. For samples A and B it was possible to calculate actual mass loss during exploitation. Taken into account were volume change and change of the chemical composition related to the platinum capture. Mass loss is significantly higher for the sample from the top of the gauze stack, which indicates stronger degradation processes. In the case of PdNi5 catalytic-capture gauzes, it was impossible to indicate definitively which part of the porous structure is related to a particular wire, so there is no calculation of mass change. The most important observation is the highest specific surface area calculated for the first capture gauze. It is important, because the entire process of platinum incorporation proceeds via the Pd-Ni gauze surface. The most recent analysis concerns YAG BSE contrast observations of cross sections of the PtRh7 wires, which revealed strong grain growth in the wires core and nanometric/submicron structure in all cauliflower-shape growths (Fig. 11). The presence of this type of microstructure merits further detailed investigation.
4. Discussion It is well known and clearly indicated that degradation/wear processes of catalytic PtRh7 and catalytic-capture PdNi5 wires proceed in a different way. Earlier studies, complemented by recent results, enable one to describe the very complex process degradation of palladium-based catalytic-capture wires [8]. It begins with a very intensive diffusion of Ni to a certain type of grain boundary (mostly high disorientation angle) and to the surface (Fig. 12 – point
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Fig. 9. EDS mapping of the surface of the PdNi5 wires. Table 1 EDS chemical composition analysis on the surface of the wires.
Gauze
Side of the gauze
Pt
Rh
Pd
Ni
A - First PtRh7 gauze
Gas inflow Gas outflow
89.04 81.78
10.96 18.22
– –
– –
Gas inflow Gas outflow
89.05 84.52
11.05 15.48
– –
– –
Gas inflow Gas outflow
34.88 (captured) 24.91 (captured)
– –
65.12 75.09
– –
Gas inflow Gas outflow
17.11 (captured) 13.42 (captured)
– –
82.89 86.58
– –
– B - Last PtRh7 gauze – C - First PdNi5 gauze – D - Last PdNi5 gauze
Fig. 10. 3D reconstruction of the wires (based on XCT analysis).
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Table 2 Parameters of the wires after exploitation (based on XCT analysis) volume [103 m3 ].
new Sample A Sample B Sample C Sample D a
Volume [103 m3 ]
Volume lossa [103 m3 ]
Mass lossa [mg]
Surface area [103 m2 ]
Specific surface area [m−1 ]
Average cross section area [m2 ]
Calculated mass loss of 1 m wire sample [mg]
1724.97 584.10 905.70 27,383.44 11,887.43
– 1140.87 819.27 – –
– 0.0241 0.0173 – –
125.57 78.75 117.81 8793.63 2915.66
0.07 0.13 0.13 0.32 0.24
5749.90 1947.00 3019.00 – –
– 962.44 691.14 – –
If possible, estimated for 320 m length wire.
Fig. 11. YAG-BSE contrast SEM observations of protrusions cross sections: (a, b) sample A—first catalytic gauze, (c, d) sample B—last catalytic gauze.
Fig. 12. Schematic presentation of proposed degradation model of palladium-nickel catalytic capture gauzes (based on the XCT data).
1). In high temperatures and an aggressive environment those Nienriched areas dissolve rapidly. The nickel oxidizes almost entirely and forms small, feathery wisps, which are torn away by the gas flow. On the other hand, the palladium released this way forms massive protrusions, which lead to an increase in the special surface area of the wires (Fig. 12 – point 2). The high surface area of strongly rebuilt wires supports the platinum capture phenomenon. The porous structure of the original wire is quickly covered in its entirety by captured platinum (Fig. 12 – point 3). As the process
proceeds, captured platinum becomes the main matter of the wires. The platinum capture process is so intensive that the initial layout of the wires is almost impossible to identify, especially on the gas inflow-side of the capture gauze stack (Fig. 12 – point 4). Degradation of PtRh7 catalytic gauzes is also partially connected with the diffusion processes, but in this case rhodium diffusion occurs a lot more slowly. In PdNi5 gauzes after 1 month’s exploitation there is almost no trace of nickel inside and on the surface of the wires. Diffusion of Ni and strong etching of Ni-rich areas
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Fig. 13. Schematic presentation of proposed degradation model of platinum-rhodium catalytic gauzes (based on the XCT data).
across wires core is only a starting point for the platinum capture phenomenon. In the case of Pt-Rh wires, diffusion of rhodium across the wire core occurs during the whole service time. Depending on exploitation parameters it can be a diffusion aligned with gas flow direction, but it is mainly a diffusion to grain boundaries, and then to the surface [3]. No matter the service time of gauzes, there is still a noticeable amount of rhodium in the preserved wire cores. Moreover, because of the platinum loss phenomenon, the total percentage of rhodium in the wires increases. This investigation revealed the second part of the processes occurring inside the wires – strong grain growth in the wires’ preserved cores. Systematic movement of grain boundaries is related to the direction and intensity of rhodium diffusion. The combined effect of rhodium diffusion and grain growth is observed on the wires – the whole surface is covered with high Rh-content areas whose placement corresponds to the areas where grain boundaries escape the surface. It is also worth noticing that these rhodium-rich areas do not reach deep into the wire core and are visible on the surface as clusters of small (even submicron) grains/crystallites. Still more intensive processes occur on the surface of the wires. Cauliflower-shaped protrusions start to form at very early stages of gauze exploitation. They can differ significantly in terms of shape, size, number and placement, depending on the parameters of exploitation: pressure, nitrogen load, temperature and gas flow direction [3]. This investigation revealed that the morphology of these growths also depends on where the gauze is placed in the reactor. Protrusions on gauzes located at the top of the catalytic stack are smaller and have a less regular shape. Gauzes from the bottom of the catalytic stack are still covered by numerous, welldeveloped, cauliflower-shape growths. Earlier it was observed that, despite many differences in exploitation parameters, the specific surface area of Pt-Rh catalytic wires is almost the same. This investigation, however, showed that the specific surface area of wires from the top of the catalytic stack is the same as the specific surface area of wires from the bottom of the stack. This indicates that ammonia oxidation and all processes occurring in the catalytic wires might be in a self-regulating balance, which gives the maximum conversion rate of ammonia (of approximately 96–97%). Their microstructure of characteristic cauliflower-shape protrusions makes them even more interesting. Observations performed on wires cross sections revealed a submicron/nanostructure in both kinds of growth: in small, irregular (from the top of the gauze stack) and in large, cauliflower-shaped protrusions (from the bottom of the gauze stack). Extreme conditions, long service time and especially high temperatures do not favor this kind of microstructure, even in such temperature resistant material. There is no indica-
tion as to structure stability in those protrusions, especially as, in preserved cores, grain growth is quite significant. The observed microstructure is related to platinum transport phenomenon: At the beginning of the service time, extreme conditions (especially high temperature) initiate the platinum transport phenomenon (Fig. 13 – point 1). Protrusions start to form (Fig. 13 – point 2), which leads to an increase in the surface area of the catalyst. Growing protrusions are significantly enriched with rhodium and have a submicron microstructure. During exploitation, protrusions grow to their final size and probably start to lose their fine microstructure. YAG-BSE observations revealed that mostdeveloped protrusions (Fig. 11d) are made of both, nano- and micrometric size grains. At this stage (Fig. 13 – point 3), the protrusion becomes a good source of material for platinum and rhodium evaporation – initiated on the wires’ surface. Large protrusions start to dissolve. Part of the material forms new protrusions (Fig. 13 – point 4) and part is captured by Pd-Ni gauzes located underneath. The processes of formation and dissolution of protrusions proceed after each other throughout the whole exploitation time. The gradual enrichment of protrusions with rhodium has considerable importance in the whole process. This, combined with the reduction in wire diameter is a symptom of wire wear as a catalyst (Fig. 13 – point 5). The material soon loses its ability to form large, well-developed protrusions. Rhodium enrichment leads to reduced catalytic and mechanical properties. During the last stage of exploitation, entire protrusions are torn away by the gas flow – they can be observed on the surface of capture gauzes [10]. After that, on the surface of wires, small, almost 90–100% rhodium spots are observed (Fig. 6a). This kind of degradation does not occur in every gauze in the stack at the same time. Initially, the strongest change in spatial morphology takes place on the gauzes at the top of the stack (on the gas-inflow side). This gauze is also first to be exploited, as is clearly visible on SEM images. During long-term operation, each successive gauze is subjected to intense transformations. As strong transformations proceed downwards through the gauze stack, they are followed by a zone of highest platinum content, highest surface area and also strongest catalytic properties in the gauze stack. Summary This work revealed that placement of the Pt-Rh catalytic gauzes in the reactor strongly influence their morphology and chemical composition. Gauzes located on the top of the catalytic stack are covered by less-developed and less numerous “cauliflower shape” protrusions. EDS analysis also revealed in those gauzes stronger rhodium enrichment and segregation.
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XCT analysis showed, that despite many differences in shape, volume and cross section area, specific surface area of all Pt-Rh wires is almost the same. It indicates that ammonia oxidation and all processes occurring in the catalytic wires are in the selfregulating balance. This paper also showed that microcomputer tomography can be a powerful tool for analyzing this type of catalyst. Scanning electron microscope observations also revealed the nanostructure in the cauliflower-shape protrusions and large grains in the wire’s preserved cores. The high temperature in the reactor and the long-term nature of the process do not favor the occurrence of the nanostructure in this type of material. This indicate short lifetime of the observed protrusions. This implies that protrusions form and dissolve repeatedly throughout whole exploitation and accompanying phenomena such as microstructure change and gradual rhodium enrichment leads to the slow degradation of the catalyst. Obtained data allowed to present the description of two degradation processes: degradation of Pt-Rh catalytic gauzes and degradation of Pd-Ni catalytic-capture ones. Both these processes are linked by a platinum transport and merits further detailed analysis. A better understanding of phenomena occurring in the reactor will inform future improvement of catalyst performance. Due to the high cost of catalysts, it is important to reduce platinum loss rate and extend the service time of gauzes.
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Acknowledgements This work was conducted by the Faculty of Material Science and Engineering at Warsaw University of Technology in the project POIG.01.01.02-00-015/09
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