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Effect of α-Fe2O3 surface coating on reconstruction of platinum–rhodium catalysts during oxidation of ammonia
Applied Catalysis A: General 284 (2005) 177–184 www.elsevier.com/locate/apcata
Effect of a-Fe2O3 surface coating on reconstruction of platinum–rhodium catalysts during oxidation of ammonia Lenka Hannevold *, Ola Nilsen, Arne Kjekshus, Helmer Fjellva˚g Department of Chemistry, University of Oslo, P.O. Box 1033 Blindern, N-0315 Oslo, Norway Received 8 November 2004; received in revised form 21 January 2005; accepted 24 January 2005 Available online 17 February 2005
Abstract Surface coating of a-Fe2O3 has been shown to affect the reconstruction of platinum–rhodium catalysts during oxidation of ammonia at 900 8C and atmospheric pressure. Surfaces of wire-formed Pt–Rh specimens with 0–30 and 100 wt.% Rh were coated with thin layers of aFe2O3, deposited by thermal decomposition of iron(III) nitrate or by atomic layer chemical vapor deposition. Scanning electron microscopy, electron microprobe analysis, and powder X-ray diffraction were used to examine the catalyst wires before and after use in ammonia oxidation. The reconstruction on Pt–Pt/10 wt.% Rh and Rh catalysts with a-Fe2O3-coated surfaces involves ‘‘cauliflower’’-like excrescences similar to those observed on corresponding materials without coatings. The reconstructions on a-Fe2O3-coated catalysts of Pt/20 wt.% Rh and Pt/30 wt.% Rh carry the same characteristics. However, for these alloys the reconstruction process becomes much faster and the resulting patterns more extensive than for corresponding materials without a-Fe2O3 coating. The boundary zones between the a-Fe2O3 cover and the liberated Pt–Rh surfaces appear to stand out as spots (hotspots in the thermal sense) with enhanced catalytic activity. A certain decrease in activity and selectivity with time is observed for all tested specimens. This is attributed to gradual degradation of a-Fe2O3 to more inactive Fe3O4. The progressing degradation of a-Fe2O3 to Fe3O4 shows that the temperature in the hotspots must exceed some 1400 8C or that reducing conditions prevail locally at the surface. # 2005 Elsevier B.V. All rights reserved. Keywords: Platinum–rhodium catalyst reconstruction; Platinum–rhodium catalyst with a-Fe2O3 coating; Ammonia oxidation; Catalytic etching; ‘‘Cauliflower’’-shaped excrescences
1. Introduction Oxidation of ammonia to nitrogen monoxide is a vital step in the production of nitric acid, and the industrial process is usually promoted by Pt–Rh and/or Pt–Pd–Rh catalysts. The surfaces of such catalysts are strongly reconstructed with numerous pits, grain boundary grooves, and ‘‘cauliflower’’-like excrescences. These appear already after a very short time of operation and depend strongly on the surface morphology and composition of the catalyst. This affects the activity, selectivity, and operation lifetime. In addition, small amounts of impurities may affect the activity and act as poison for the catalyst. Such impurities may stem from various sources. The reactor, reactant supply * Corresponding author. Tel.: +47 228 555 75; fax: +47 228 555 65. E-mail address: [email protected] (L. Hannevold). 0926-860X/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2005.01.035
pipes, and gauze support can introduce constituents of steel (above all Fe) as impurities. The reactant gases can, e.g., introduce salts with basic cations (Na, Mg, Ca, etc.) and nonmetal ingredients (e.g., Cl and S). Impurities of Mg, Al, S, Cl, Ca, Mn, Fe, Ni, Cu, Ba, and Au and are all reported [1,2] to decrease the activity of Pt–Rh catalysts. However, some impurities can be oxidized under ammonia oxidation conditions to solid oxides that are stable at these high temperatures and are themselves catalytically active. One impurity of this kind is iron that can be oxidized to Fe2O3. The catalytic active a-Fe2O3 ([3–6] and references therein) has, in fact, been considered as an alternative to platinumbased catalysts for ammonia oxidation, but has proved to be less efficient. We have reported [7,8] that surface reconstructions on wire-shaped catalysts of noble-metal alloys (in particular Pt–Rh) during ammonia oxidation, depend strongly on the
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initial composition of the catalyst material. For example, the surfaces of Pt–Pt/13 wt.% Rh (numbers refer to wt.%) and pure Rh were found to be extensively reconstructed with numerous ‘‘cauliflower’’-like excrescences, whereas the reconstruction processes on Pt/20 wt.% Rh and Pt/30 wt.% Rh proved to be very much slower. This variation in reconstruction characteristics makes Pt–Rh alloys good candidates for exploring the effect of a-Fe2O3 surface coatings on such catalysts.
2. Experimental
active for the reaction. Only qualitative and semiquantitative analyses were possible since the pressure in the analysis chamber was subjected to oscillation because of condensation of water or precipitation of salt in the capillary sampling tube from time to time. The maximum temperature of the catalyst specimens was kept at 900 8C (measured by an optical pyrometer; estimated uncertainty 20 8C). The temperature and the flow rate of the reactant gas mixture were held at steady values during each run (336 h, unless otherwise stated). The experiments were terminated by replacing the reaction gas mixture by nitrogen and cooling the system to room temperature by turning off the resistance heating.
2.1. Catalyst materials 2.3. Characterization Ammonia oxidation was studied along wire-shaped catalysts of Pt, Rh, and Pt–Rh alloys (see Table 1 for specification). Before each experiment the wires were washed with trichloroethylene, acetone, methanol, and distilled water and, after drying, coated with a thin layer of a-Fe2O3. One set of specimens was prepared by dipping the wires in a saturated solution of iron(III) nitrate followed by thermal decomposition of the nitrate on heating in an oxygen–hydrogen flame at some 1000–1200 8C (here referred to as dip coating; DIPCO). This coating procedure was repeated 7 times, giving a layer thickness of ca. 1 mm. Another set was coated with a-Fe2O3 using atomic layer chemical vapor deposition (ALCVD) [9]. The films were deposited using 10 000 cycles of Fe(thd)3 (Hthd = 2,2,6,6tetramethylheptan-3,5-dione) and ozone at a reactor temperature of 186 8C, which gave a a-Fe2O3 film thickness of 120 nm (12 pm/cycle). 2.2. Reactor and gas handling The construction of the reactor is described in Ref. [7]. The catalyst specimen was arranged vertically in the reactor, and the gases, entering at the top, flowed parallel to the resistance heated catalyst. The reactant gas mixture contained 13.8 vol.% NH3, 17.2 vol.% O2, and 69.0 vol.% N2 (high purity gases, all >99.999%; AGA), and the total gas-flow rate was 290 cm3 min 1 (linear flow velocity 0.25 m s 1), as controlled by calibrated mass-flow meters (Brooks 5850S). The composition of the product gas mixture was checked with an HPR20 gas analyzer equipped with a Faraday detector to verify that these catalyst materials are
All catalyst specimens were studied before and after testing with scanning electron microscopy (SEM) using a Philips XL30 instrument equipped with an EDAX energydispersive X-ray analysis system. The microscope was operated at 15–25 kV, depending on the samples. Powder X-ray diffraction (XRD) was used for analyses of the catalysts before and after use. Data were collected in two modes, in transmission geometry with the catalyst wires mounted inside boron–glass capillaries (0.3–0.5 mm bore), or in reflection mode with surface abrasions from the catalysts mounted on a zero-background Si-plate sample holder. The XRD data were collected with a Siemens D5000 diffractometer (2u range 10–908), using Cu Ka1 radiation from a Ge monochromator and a position-sensitive detector. Determination of composition was made by means of an automatic wavelength-dispersive CAMECA SX100 electron microprobe (EMP; Department of Geology, University of Oslo) equipped with a PGT energy-dispersive system for element analysis. The EMP was operating at an acceleration voltage of 20 kV and a sample current of 20 nA. Pure samples of Pt, Rh, and Fe were used as standards.
3. Results The results are presented in order of increasing Rh content on the a-Fe2O3-coated catalysts specimens, and are compared with the findings for corresponding catalysts without a-Fe2O3 coating. 3.1. The a-Fe2O3 coating
Table 1 Specifications of Pt–Rh catalyst test materials (wires of length 100 mm; d: diameter as specified) Rh content (wt.%)
Formula
Manufacturer
Purity (wt.%)
d (mm)
Pt Pt/10 wt.% Rh Pt/20 wt.% Rh Pt/30 wt.% Rh Rh
Pt Pt83Rh17 Pt68Rh32 Pt55Rh45 Rh
Rasmussen Rasmussen Engelhard Rasmussen Goodfellow
99.99 99.95 99.95 99.99 99.9
0.20 0.20 0.20 0.25 0.25
Fig. 1 shows typical XRD diffractograms for Pt–Rh specimens with and without iron oxide coating prepared by DIPCO. The coating is identified as a-Fe2O3, with no sign of other oxides prior to testing. For all DIPCO-treated samples, an asymmetric broadening of the diffraction peaks is observed on the 2u high-angle side. The inset of Fig. 1 shows the 2u range for the observed (2 0 0) reflection together with two deconvoluted peaks, one with the same characteristics as
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Fig. 1. XRD diffractograms of Pt/30 wt.% Rh specimens: (a) with and (b) without surface coating of a-Fe2O3 prepared by thermal decomposition of iron(III) nitrate. Reflections marked with asterisk belong (according to indexing) to a-Fe2O3. Inset shows enlargement of the (2 0 0) reflection from (a) with two deconvoluted profiles (see text).
the corresponding reflection for the original Pt–Rh alloy and one weaker and considerably broader peak. The latter signals a solid solution between Fe and the original Pt–Rh alloy close to the surface. The broadening appears to reflect strain and/or inhomogeneous distribution of the dissolved iron. The approximate content of Fe in the Pt–Rh matrices is according to relations between unit-cell dimensions and Fe content [10] some 3–6 wt.%. After removal of the a-Fe2O3 coating by diluted HCl, an iron content of some 3–5 wt.% was established by EMP in the surface region of the DIPCO-
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treated Pt–Rh wires (down to a depth of ca. 5 mm). Solid solution formation between Fe and the Pt–Rh matrix was avoided by the use of ALCVD to prepare the coatings. This was proved by XRD in the sense that there was no alteration in the peak widths in XRD diffractograms for Pt–Rh specimens with or without coating, and by EMP in the way that the Fe content was below the detection limit. a-Fe2O3 is virtually stoichiometric at and below 900 8C [11] and decomposes thermally above 1400 8C [12]. Hence, its bulk physical properties are believed to be little affected by heat treatment at 900 8C in nitrogen or oxygen. However, when heated at 900 8C, cracks appear in the a-Fe2O3 coatings along the length direction of the wires. This is a well-known phenomenon in corrosion science [13] and results from thermally induced stress in coatings with poor elastic properties (mismatch in thermal expansion between the coating and the underlayer). On the catalytically tested specimens, the a-Fe2O3 coatings had become porous, thus resembling the products obtained by wet corrosion of iron. XRD analyses showed furthermore that the oxide layers of the used specimens comprised Fe3O4 in addition to the original a-Fe2O3. The uncovered Pt–Rh surfaces along cracks and thoroughgoing pores in the oxide layer are clearly of great importance for the coupled catalyzed ammonia oxidation and catalyst reconstruction processes. 3.2. Pt catalysts with a-Fe2O3 coating Fig. 2 shows typical SEM images of surface reconstructions on ALCVD-a-Fe2O3-coated Pt catalysts after exposure
Fig. 2. SEM images of a Pt-wire surface coated with a 120 nm thick a-Fe2O3 layer (made by ALCVD) after exposure to ammonia oxidation for 336 h; approximate location in mm from gas inlet: (a) 2, (b) 30, (c) 40, and (d) 45.
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Fig. 3. SEM images of a Pt-wire surface coated with a 1 mm thick a-Fe2O3 layer (made by thermal decomposition of iron(III) nitrate) after exposure to ammonia oxidation for 336 h; approximate location in mm from gas inlet: (a) 8, (b) 18, (c) 35, and (d) 45.
to ammonia oxidation at 900 8C for 336 h. Between 1 and 2 mm from the gas inlet, regions with uncoated Pt are easily recognized (Fig. 2a). In these liberated Pt domains the surface adopts similar morphologies as observed in the same location on uncoated Pt catalysts (see Ref. [7]). The reconstruction pattern consists of terraces separated by steps and facets with different orientations. Small holes (0.1–0.2 mm) with four-fold symmetry are located close to the steps. Further down the wires (Fig. 2b and c), regions with ‘‘cauliflower’’-like structures are observed. The morphology of these features is similar to those for uncoated Pt catalysts, but the size and shape of the excrescences are less uniform. There are regions where ‘‘cauliflowers’’ alternate with less reconstructed domains, and well-defined Pt crystals can be identified on the a-Fe2O3 coating (Fig. 2d). The DIPCO specimens gave reconstructions of the same type as described for the ALCVD-treated catalysts. Typical ‘‘cauliflower’’ excrescences are developed on surface areas mainly covered with a-Fe2O3 (Fig. 3a). In nearby locations fairly well-defined crystals of Pt are recognized (see inset of Fig. 3b). The surface patterns on the two types of iron-oxidecoated Pt catalysts show a certain difference ca. 15 mm down the wires. Numerous large holes (with four-fold symmetry and cross-section up to 15 mm) are seen (Fig. 3c) for the DIPCO specimens, whereas hole formation is less perceptible for the ALCVD-coated specimens. Further down the wire, patterns with ‘‘cauliflowers’’ and well-defined platinum crystals (see Fig. 3d) reappear repeatedly over wide ranges of the specimens.
Typical temperature profiles along a Pt catalyst wire are shown in Fig. 4 (note: temperatures refer to averages, not to the situation at hotspots). The position and extension of the 900 8C region change with the time of operation. At the onset of the reaction, distinctly colder regions are clearly visible, probably associated with uneven oxide coating on the wire surface. After 8 h of operation these are less pronounced and after ca. 48 h the temperature profiles adopts a parabolic shape (as observed on uncoated Pt–Rh specimens [7]), which remained throughout the experiments.
Fig. 4. Typical temperature profile along a catalyst wire as measured with an optical pyrometer; illustrated with results for an a-Fe2O3-coated Pt catalyst as representative examples; solid line: after 5 min, dash line: after 8 h, and dotted line: after 48 h of reaction.
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3.3. Pt/10 wt.% Rh, Pt/20 wt.% Rh, and Pt/30 wt.% Rh with a-Fe2O3 coating Fig. 5 shows SEM images of surfaces of Pt/10 wt.% Rh catalysts coated with a-Fe2O3 after ammonia oxidation for 336 h. The surfaces appear virtually identical for coatings prepared by DIPCO and ALCVD. In the first 1–2 mm from the gas inlet, terrace structures with numerous pits are developed (Fig. 5a). Cavities of different size and shape are mostly located along the grain boundaries or close to edges or terrace steps. Small nodule-formed undulated structures are clearly visible on elevated points. Further down the wires, the pattern consists of grain boundary grooves, numerous facets of different orientation, curved features, and excrescences of irregular shapes, together with some unreconstructed grains (Fig. 5b). In the region from 15 mm and downward, ‘‘cauliflower’’-like excrescences are well developed (Fig. 5c), corresponding to those seen on Pt/10 wt.% Rh catalysts without a-Fe2O3 coating [7]. Regions with the morphology shown in Fig. 5b reappear further down the wires, together with regions with fully developed ‘‘cauliflowers’’. This situation is maintained up to 80 mm from the gas inlet. Irregularly-shaped structures of iron oxide protrude clearly out from the wire surfaces (Fig. 5d). The temperature profiles along the Pt/10 wt.% Rh catalyst wires change with time during the progressing operation, with similar characteristics to those described for the Pt catalysts (see Fig. 3), however, the extension and position of the warmest region vary only by some 2–3 mm.
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The reconstruction patterns of a-Fe2O3-coated Pt/ 20 wt.% Rh and Pt/30 wt.% Rh wires are similar to those described above. Fig. 6a shows an a-Fe2O3-coated Pt/20 wt.% Rh surface ca. 4 mm from the gas inlet. The characteristic feature is a large number of pits with mainly four-fold symmetry, located not only around edges, but extending also into the middle of the facets. Further down along the wires, nodules on elevated points are clearly visible. In the region starting around 15 mm, non-uniform ‘‘cauliflower’’-like structures develop (Fig. 6b). Less reconstructed regions with a-Fe2O3 coating largely intact, alternate with more heavily reconstructed regions (Fig. 6c). Distinct features are seen with needle-shaped iron oxide protruding from the surfaces (Fig. 6d). The most heavily reconstructed surfaces of Pt/20 wt.% Rh with and without aFe2O3 coating are compared in Fig. 6e and f. A difference in character is evident for the numerous ‘‘cauliflower’’-like features all over the disengaged surfaces on the a-Fe2O3coated specimen. Similar observations are made for a-Fe2O3-coated Pt/30 wt.% Rh catalysts (Fig. 7a and b). 3.4. Rh with a-Fe2O3 coating Fig. 8 gives a comparison of SEM images of surfaces of Rh catalysts, with and without a-Fe2O3 coating after exposure to ammonia oxidation for 60 h. These experiments were maintained only for a short period because of the rapid mechanical degradation of the Rh wires [7]. The reconstruction process is distinctly slower on the a-Fe2O3-coated surfaces (Fig. 8b), but the patterns retain the same basic character (Fig. 8a). Rh2O3 was found in different regions
Fig. 5. SEM images of reconstructed surfaces on a Pt/10 wt.% Rh catalyst wire coated with a-Fe2O3 after exposure to ammonia oxidation for 336 h; approximate location in mm from gas inlet: (a) 2, (b) 8, (c) 15, and (d) 80.
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Fig. 6. SEM images of reconstructed surfaces on a Pt/20 wt.% Rh catalyst wires with a-Fe2O3 coating after oxidation of ammonia for 336 h; approximate location in mm from gas inlet: (a) 4, (b) 15, (c) 25, and (d) 60. Comparison of the most reconstructed surfaces on Pt/20 wt.% Rh catalyst wires (e) with and (f) without a-Fe2O3 coating.
along the a-Fe2O3-coated Rh wires: approximately 1–7, 22– 33, and 80–100 mm from gas inlet. This contrasts the findings for Rh catalysts without coating, where the Rh2O3 deposits only occur at the end of the wires where the temperature is below the decomposition temperature of Rh2O3 (1000 8C in oxygen [14]).
4. Discussion The introduction of a-Fe2O3 coating on Pt–Rh catalyst surfaces brings new aspects into the ammonia oxidation reaction. The surface-layer characteristics of a-Fe2O3 should be considered both in the as-deposited state and
Fig. 7. SEM images of reconstructed surfaces on a Pt/30 wt.% Rh catalyst wire after ammonia oxidation for 336 h: (a) with and (b) without a-Fe2O3 coating.
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Fig. 8. SEM images of reconstructed surfaces on pure Rh catalyst wires after ammonia oxidation for 60 h: (a) Rh catalyst coated with a-Fe2O3 and (b) Rh catalyst without coating.
under the ruling high-temperature conditions during the oxidation process. Furthermore, attention should be paid to the fact that although a-Fe2O3 is a well-recognized catalyst for ammonia oxidation it has a detrimental effect on the catalytic properties of the Pt–Rh support. The as-deposited a-Fe2O3 coatings have imperfections in the form of pores, cracks, blisters, etc. due to stress in the layers (in particular lateral stresses induced by differences in thermal expansion and/or crystal structure between the oxide and the Pt–Rh matrix). Such macroscopic defects can hardly be controlled. Hence, the characteristics for coatings deposited under equal conditions on assumed identical supports may differ significantly. Owing to the very different conditions during the coating processes, in particular with respect to temperature (see Section 2), the a-Fe2O3 layers prepared by DIPCO contain more and larger macroscopic defects than those obtained by ALCVD. This distinction is prominent at the as-deposited layer stage, but disappears largely after heating at 900 8C where pores grow, cracks open up and increase in size and number, and blisters burst. During operational testing, the situation is modified further, and notably, the pore structure changes owing to the presence of H2O as reaction product. The distinction between the DIPCO- and ALCVD-prepared a-Fe2O3 layers becomes hence less important, and the two sets of results will hereafter be treated jointly. The mechanisms for ammonia oxidation on a-Fe2O3 catalysts are not clearly understood. According to Griffiths et al. [3] gaseous or weakly adsorbed NH3 reacts directly with surface oxygen on a-Fe2O3 to yield surface intermediates that contain the nitrosyl group. This process does apparently not involve stripping of hydrogen from NH3 and formation of NH and NH2 radicals as surface intermediates (viz. rejection of a route that is debated for ammonia oxidation on Pt–Rh catalyst, see Ref. [7] and references therein). In the present study the reactants have been subjected to the combined influence of the Pt–Rh and a-Fe2O3 catalysts, of which Pt–Rh has the distinctly higher activity and selectivity for oxidation of NH3 to NO. It is likely that an interplay between these catalyst materials occurs in the
present system. Oxygen is believed to be easily and more strongly chemisorbed on Pt–Rh surfaces than on the aFe2O3. On the other hand, NH3 floats around with a likely tendency to accumulate at the boundaries between the different catalyst phases. These are precisely the regions on the a-Fe2O3-coated Pt–Rh catalysts that we envisage as particularly active with respect to ammonia oxidation. The pile-up of oxidation events for individual NH3 molecules in limited surface regions leads to hotspots which, in turn, promote the formation of gaseous PtO2 and RhO2 species. These are in due course transported a short distance under the influence of a temperature gradient and deposited on colder spots on the Pt–Rh support (mainly characteristic ‘‘cauliflower’’-like Pt–Rh excrescences) or the a-Fe2O3 cover (usually more distinct Pt–Rh crystals). For a more detailed outline of the involved chemical-vapor-transport process, see Ref. [15]. The designation of the boundary zones between the oxide and the liberated alloy surface as favorable regions for ammonia oxidation provides a simple explanation for why the coated wires do not exhibit the major Rh-contentdependent distinction in reconstruction patterns seen on uncoated catalyst specimens of, say, Pt/10 wt.% Rh and Pt/ 30 wt.% Rh [7]. The oxide/alloy boundary zones are largely determined by the properties of the coating. Hence, such favorable regions are equally probable all over the mixedcatalyst specimens. These interface regions show no a priori correlation with the defects governing the ammonia oxidation as well as reconstruction in the Pt–Rh alloy matrix itself [7]. Three other findings of the present study should be briefly discussed. First, the alloying of iron into the surface region of the Pt–Rh master matrix of DIPCO-treated specimens is a trivial consequence of the use of thermal decomposition of iron(III) nitrate in a (reducing) H2–O2 torch to produce the coatings, and also in accordance with the available information [11] for the phase diagrams concerned. Second, the decrease in activity/selectivity for the mixed oxide/alloy catalyst as function of time on stream appears to correlate with an increasing amount of Fe3O4 in the oxide layer (as observed by XRD). It is well recognized that Fe3O4
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at best is a poor catalyst for ammonia oxidation. The occurrence of Fe3O4 on used iron-oxide-coated catalysts also provides information on the conditions experienced by the oxide coating. The thermal decomposition of a-Fe2O3 to Fe3O4 takes place at some 1400 8C [12], depending on the oxygen partial pressure in the surrounding atmosphere. The inference is accordingly that the hotspot regions of these catalysts obtain a temperature of around 500 8C above the average catalyst temperature, or that the conditions at the surface are locally rather reductive. Third, the observation of Rh2O3 on different and timedependent locations along the wire contrasts the findings for the catalysts without a-Fe2O3 coating, where Rh2O3 was observed only at the terminal end of the catalysts. These findings concur with the time-dependent temperature profiles in Fig. 4, and reflect merely the fact that some parts of catalyst surface take temperatures below the decomposition temperature of Rh2O3 during the ammonia oxidation reaction. Such colder regions, with temperatures below the average 900 8C, are typically found on the oxidecovered parts where the temperature is not controlled by the current through the wire owing to the poor conductivity of the a-Fe2O3–Fe3O4 coating. Hence, hotspot temperatures on oxide/alloy boundary zones are fully compatible with the occurrence of colder regions a little away from the zones.
5. Conclusions Pt–Pt/30 wt.% Rh and Rh specimens coated with thin layers of a-Fe2O3 show catalytic activity to ammonia oxidation, but are less active than the corresponding uncoated Pt–Rh alloys. Extensive surface reconstructions are observed for all tested catalysts. The reconstruction patterns on liberated surfaces of Pt–Pt/10 wt.% Rh and Rh catalysts are similar to those observed on corresponding specimens without a-Fe2O3 coating. The introduction of aFe2O3 coating on Pt/20 wt.% Rh and Pt/30 wt.% Rh specimens makes these materials more susceptible to surface reconstruction under ammonia oxidation conditions than corresponding uncoated specimens. The main part of the entangled ammonia oxidation reaction and Pt–Rh reconstruction process are suggested to occur on boundary zones between the a-Fe2O3 cover and the liberated Pt–Rh
surface. A gradual thermal conversion of a-Fe2O3 to Fe3O4 causes a certain decrease in activity and selectivity of the mixed oxide/alloy catalysts.
Acknowledgements The financial contribution of Norsk Hydro ASA (the earlier Agri division, now separate company Yara ASA) is gratefully acknowledged. The authors extend their cordial thanks to staff members of the company for fruitful and stimulating discussions. The authors also express their gratitude to Cand. Scient. S. Jørgensen for assistance with SEM and helpful discussions, to Dr. M. Erambert (Department of Geology, University of Oslo) for help with EMP analyses, and to Cand. Scient. M. Lie for performing the ALCVD coatings.
References [1] G. Lomic, E. Kis, M. Grabovac, R. Marinkovic-Neducin, Scanning 24 (2002) 140. [2] R.W. McCabe, T. Pignet, L.D. Schmidt, J. Catal. 32 (1974) 114. [3] D.W.L. Griffiths, H.E. Hallam, W.J. Thomas, J. Catal. 17 (1970) 18. [4] V.A. Sadykov, L.A. Isupova, I.A. Zolotarskii, L.N. Bobrova, A.S. Noskov, V.N. Parmon, E.A. Brushtein, T.V. Telyatnikova, V.I. Chernyshev, V.V. Lunin, Appl. Catal. A 204 (2000) 59. [5] N.I. Zakharchenko, Kinet. Catal. 42 (2001) 679. [6] N.I. Zakharchenko, Russ. J. Appl. Chem. 74 (2001) 229. [7] L. Hannevold, O. Nilsen, A. Kjekshus, H. Fjellva˚ g, Appl. Catal. A 284 (2005) 163. [8] O. Nilsen, A. Kjekshus, H. Fjellva˚ g, Appl. Catal. A 207 (2001) 43. [9] O. Nilsen, M. Lie, S. Foss, H. Fjellva˚ g, A. Kjekshus, Appl. Surf. Sci. 227 (2004) 40. [10] W.B. Pearson, A Handbook of Lattice Spacings and Structures of Metals and Alloys, Pergamon Press, London, 1958. [11] E.M. Levin, C.R. Robbins, H.F. McMurdie, Phase Diagrams for Ceramists, The American Ceramics Society, Columbus, 1964. [12] N.N. Greenwood, A. Earnshaw, Chemistry of the Elements, Butterworths/Heinemann, Oxford, 1997. [13] O. Kubaschewski, B.E. Hopkins, Oxidation of Metals and Alloys, Butterworths, London, 1967. [14] Gmelin Handbook of Inorganic Chemistry, Rhodium, System No. 84, vol. B1, Springer, Berlin, 1982. [15] L. Hannevold, O. Nilsen, A. Kjekshus, H. Fjellva˚ g, J. Cryst. Growth, in press.
Effect of a-Fe2O3 surface coating on reconstruction of platinum–rhodium catalysts during oxidation of ammonia Lenka Hannevold *, Ola Nilsen, Arne Kjekshus, Helmer Fjellva˚g Department of Chemistry, University of Oslo, P.O. Box 1033 Blindern, N-0315 Oslo, Norway Received 8 November 2004; received in revised form 21 January 2005; accepted 24 January 2005 Available online 17 February 2005
Abstract Surface coating of a-Fe2O3 has been shown to affect the reconstruction of platinum–rhodium catalysts during oxidation of ammonia at 900 8C and atmospheric pressure. Surfaces of wire-formed Pt–Rh specimens with 0–30 and 100 wt.% Rh were coated with thin layers of aFe2O3, deposited by thermal decomposition of iron(III) nitrate or by atomic layer chemical vapor deposition. Scanning electron microscopy, electron microprobe analysis, and powder X-ray diffraction were used to examine the catalyst wires before and after use in ammonia oxidation. The reconstruction on Pt–Pt/10 wt.% Rh and Rh catalysts with a-Fe2O3-coated surfaces involves ‘‘cauliflower’’-like excrescences similar to those observed on corresponding materials without coatings. The reconstructions on a-Fe2O3-coated catalysts of Pt/20 wt.% Rh and Pt/30 wt.% Rh carry the same characteristics. However, for these alloys the reconstruction process becomes much faster and the resulting patterns more extensive than for corresponding materials without a-Fe2O3 coating. The boundary zones between the a-Fe2O3 cover and the liberated Pt–Rh surfaces appear to stand out as spots (hotspots in the thermal sense) with enhanced catalytic activity. A certain decrease in activity and selectivity with time is observed for all tested specimens. This is attributed to gradual degradation of a-Fe2O3 to more inactive Fe3O4. The progressing degradation of a-Fe2O3 to Fe3O4 shows that the temperature in the hotspots must exceed some 1400 8C or that reducing conditions prevail locally at the surface. # 2005 Elsevier B.V. All rights reserved. Keywords: Platinum–rhodium catalyst reconstruction; Platinum–rhodium catalyst with a-Fe2O3 coating; Ammonia oxidation; Catalytic etching; ‘‘Cauliflower’’-shaped excrescences
1. Introduction Oxidation of ammonia to nitrogen monoxide is a vital step in the production of nitric acid, and the industrial process is usually promoted by Pt–Rh and/or Pt–Pd–Rh catalysts. The surfaces of such catalysts are strongly reconstructed with numerous pits, grain boundary grooves, and ‘‘cauliflower’’-like excrescences. These appear already after a very short time of operation and depend strongly on the surface morphology and composition of the catalyst. This affects the activity, selectivity, and operation lifetime. In addition, small amounts of impurities may affect the activity and act as poison for the catalyst. Such impurities may stem from various sources. The reactor, reactant supply * Corresponding author. Tel.: +47 228 555 75; fax: +47 228 555 65. E-mail address: [email protected] (L. Hannevold). 0926-860X/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2005.01.035
pipes, and gauze support can introduce constituents of steel (above all Fe) as impurities. The reactant gases can, e.g., introduce salts with basic cations (Na, Mg, Ca, etc.) and nonmetal ingredients (e.g., Cl and S). Impurities of Mg, Al, S, Cl, Ca, Mn, Fe, Ni, Cu, Ba, and Au and are all reported [1,2] to decrease the activity of Pt–Rh catalysts. However, some impurities can be oxidized under ammonia oxidation conditions to solid oxides that are stable at these high temperatures and are themselves catalytically active. One impurity of this kind is iron that can be oxidized to Fe2O3. The catalytic active a-Fe2O3 ([3–6] and references therein) has, in fact, been considered as an alternative to platinumbased catalysts for ammonia oxidation, but has proved to be less efficient. We have reported [7,8] that surface reconstructions on wire-shaped catalysts of noble-metal alloys (in particular Pt–Rh) during ammonia oxidation, depend strongly on the
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initial composition of the catalyst material. For example, the surfaces of Pt–Pt/13 wt.% Rh (numbers refer to wt.%) and pure Rh were found to be extensively reconstructed with numerous ‘‘cauliflower’’-like excrescences, whereas the reconstruction processes on Pt/20 wt.% Rh and Pt/30 wt.% Rh proved to be very much slower. This variation in reconstruction characteristics makes Pt–Rh alloys good candidates for exploring the effect of a-Fe2O3 surface coatings on such catalysts.
2. Experimental
active for the reaction. Only qualitative and semiquantitative analyses were possible since the pressure in the analysis chamber was subjected to oscillation because of condensation of water or precipitation of salt in the capillary sampling tube from time to time. The maximum temperature of the catalyst specimens was kept at 900 8C (measured by an optical pyrometer; estimated uncertainty 20 8C). The temperature and the flow rate of the reactant gas mixture were held at steady values during each run (336 h, unless otherwise stated). The experiments were terminated by replacing the reaction gas mixture by nitrogen and cooling the system to room temperature by turning off the resistance heating.
2.1. Catalyst materials 2.3. Characterization Ammonia oxidation was studied along wire-shaped catalysts of Pt, Rh, and Pt–Rh alloys (see Table 1 for specification). Before each experiment the wires were washed with trichloroethylene, acetone, methanol, and distilled water and, after drying, coated with a thin layer of a-Fe2O3. One set of specimens was prepared by dipping the wires in a saturated solution of iron(III) nitrate followed by thermal decomposition of the nitrate on heating in an oxygen–hydrogen flame at some 1000–1200 8C (here referred to as dip coating; DIPCO). This coating procedure was repeated 7 times, giving a layer thickness of ca. 1 mm. Another set was coated with a-Fe2O3 using atomic layer chemical vapor deposition (ALCVD) [9]. The films were deposited using 10 000 cycles of Fe(thd)3 (Hthd = 2,2,6,6tetramethylheptan-3,5-dione) and ozone at a reactor temperature of 186 8C, which gave a a-Fe2O3 film thickness of 120 nm (12 pm/cycle). 2.2. Reactor and gas handling The construction of the reactor is described in Ref. [7]. The catalyst specimen was arranged vertically in the reactor, and the gases, entering at the top, flowed parallel to the resistance heated catalyst. The reactant gas mixture contained 13.8 vol.% NH3, 17.2 vol.% O2, and 69.0 vol.% N2 (high purity gases, all >99.999%; AGA), and the total gas-flow rate was 290 cm3 min 1 (linear flow velocity 0.25 m s 1), as controlled by calibrated mass-flow meters (Brooks 5850S). The composition of the product gas mixture was checked with an HPR20 gas analyzer equipped with a Faraday detector to verify that these catalyst materials are
All catalyst specimens were studied before and after testing with scanning electron microscopy (SEM) using a Philips XL30 instrument equipped with an EDAX energydispersive X-ray analysis system. The microscope was operated at 15–25 kV, depending on the samples. Powder X-ray diffraction (XRD) was used for analyses of the catalysts before and after use. Data were collected in two modes, in transmission geometry with the catalyst wires mounted inside boron–glass capillaries (0.3–0.5 mm bore), or in reflection mode with surface abrasions from the catalysts mounted on a zero-background Si-plate sample holder. The XRD data were collected with a Siemens D5000 diffractometer (2u range 10–908), using Cu Ka1 radiation from a Ge monochromator and a position-sensitive detector. Determination of composition was made by means of an automatic wavelength-dispersive CAMECA SX100 electron microprobe (EMP; Department of Geology, University of Oslo) equipped with a PGT energy-dispersive system for element analysis. The EMP was operating at an acceleration voltage of 20 kV and a sample current of 20 nA. Pure samples of Pt, Rh, and Fe were used as standards.
3. Results The results are presented in order of increasing Rh content on the a-Fe2O3-coated catalysts specimens, and are compared with the findings for corresponding catalysts without a-Fe2O3 coating. 3.1. The a-Fe2O3 coating
Table 1 Specifications of Pt–Rh catalyst test materials (wires of length 100 mm; d: diameter as specified) Rh content (wt.%)
Formula
Manufacturer
Purity (wt.%)
d (mm)
Pt Pt/10 wt.% Rh Pt/20 wt.% Rh Pt/30 wt.% Rh Rh
Pt Pt83Rh17 Pt68Rh32 Pt55Rh45 Rh
Rasmussen Rasmussen Engelhard Rasmussen Goodfellow
99.99 99.95 99.95 99.99 99.9
0.20 0.20 0.20 0.25 0.25
Fig. 1 shows typical XRD diffractograms for Pt–Rh specimens with and without iron oxide coating prepared by DIPCO. The coating is identified as a-Fe2O3, with no sign of other oxides prior to testing. For all DIPCO-treated samples, an asymmetric broadening of the diffraction peaks is observed on the 2u high-angle side. The inset of Fig. 1 shows the 2u range for the observed (2 0 0) reflection together with two deconvoluted peaks, one with the same characteristics as
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Fig. 1. XRD diffractograms of Pt/30 wt.% Rh specimens: (a) with and (b) without surface coating of a-Fe2O3 prepared by thermal decomposition of iron(III) nitrate. Reflections marked with asterisk belong (according to indexing) to a-Fe2O3. Inset shows enlargement of the (2 0 0) reflection from (a) with two deconvoluted profiles (see text).
the corresponding reflection for the original Pt–Rh alloy and one weaker and considerably broader peak. The latter signals a solid solution between Fe and the original Pt–Rh alloy close to the surface. The broadening appears to reflect strain and/or inhomogeneous distribution of the dissolved iron. The approximate content of Fe in the Pt–Rh matrices is according to relations between unit-cell dimensions and Fe content [10] some 3–6 wt.%. After removal of the a-Fe2O3 coating by diluted HCl, an iron content of some 3–5 wt.% was established by EMP in the surface region of the DIPCO-
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treated Pt–Rh wires (down to a depth of ca. 5 mm). Solid solution formation between Fe and the Pt–Rh matrix was avoided by the use of ALCVD to prepare the coatings. This was proved by XRD in the sense that there was no alteration in the peak widths in XRD diffractograms for Pt–Rh specimens with or without coating, and by EMP in the way that the Fe content was below the detection limit. a-Fe2O3 is virtually stoichiometric at and below 900 8C [11] and decomposes thermally above 1400 8C [12]. Hence, its bulk physical properties are believed to be little affected by heat treatment at 900 8C in nitrogen or oxygen. However, when heated at 900 8C, cracks appear in the a-Fe2O3 coatings along the length direction of the wires. This is a well-known phenomenon in corrosion science [13] and results from thermally induced stress in coatings with poor elastic properties (mismatch in thermal expansion between the coating and the underlayer). On the catalytically tested specimens, the a-Fe2O3 coatings had become porous, thus resembling the products obtained by wet corrosion of iron. XRD analyses showed furthermore that the oxide layers of the used specimens comprised Fe3O4 in addition to the original a-Fe2O3. The uncovered Pt–Rh surfaces along cracks and thoroughgoing pores in the oxide layer are clearly of great importance for the coupled catalyzed ammonia oxidation and catalyst reconstruction processes. 3.2. Pt catalysts with a-Fe2O3 coating Fig. 2 shows typical SEM images of surface reconstructions on ALCVD-a-Fe2O3-coated Pt catalysts after exposure
Fig. 2. SEM images of a Pt-wire surface coated with a 120 nm thick a-Fe2O3 layer (made by ALCVD) after exposure to ammonia oxidation for 336 h; approximate location in mm from gas inlet: (a) 2, (b) 30, (c) 40, and (d) 45.
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Fig. 3. SEM images of a Pt-wire surface coated with a 1 mm thick a-Fe2O3 layer (made by thermal decomposition of iron(III) nitrate) after exposure to ammonia oxidation for 336 h; approximate location in mm from gas inlet: (a) 8, (b) 18, (c) 35, and (d) 45.
to ammonia oxidation at 900 8C for 336 h. Between 1 and 2 mm from the gas inlet, regions with uncoated Pt are easily recognized (Fig. 2a). In these liberated Pt domains the surface adopts similar morphologies as observed in the same location on uncoated Pt catalysts (see Ref. [7]). The reconstruction pattern consists of terraces separated by steps and facets with different orientations. Small holes (0.1–0.2 mm) with four-fold symmetry are located close to the steps. Further down the wires (Fig. 2b and c), regions with ‘‘cauliflower’’-like structures are observed. The morphology of these features is similar to those for uncoated Pt catalysts, but the size and shape of the excrescences are less uniform. There are regions where ‘‘cauliflowers’’ alternate with less reconstructed domains, and well-defined Pt crystals can be identified on the a-Fe2O3 coating (Fig. 2d). The DIPCO specimens gave reconstructions of the same type as described for the ALCVD-treated catalysts. Typical ‘‘cauliflower’’ excrescences are developed on surface areas mainly covered with a-Fe2O3 (Fig. 3a). In nearby locations fairly well-defined crystals of Pt are recognized (see inset of Fig. 3b). The surface patterns on the two types of iron-oxidecoated Pt catalysts show a certain difference ca. 15 mm down the wires. Numerous large holes (with four-fold symmetry and cross-section up to 15 mm) are seen (Fig. 3c) for the DIPCO specimens, whereas hole formation is less perceptible for the ALCVD-coated specimens. Further down the wire, patterns with ‘‘cauliflowers’’ and well-defined platinum crystals (see Fig. 3d) reappear repeatedly over wide ranges of the specimens.
Typical temperature profiles along a Pt catalyst wire are shown in Fig. 4 (note: temperatures refer to averages, not to the situation at hotspots). The position and extension of the 900 8C region change with the time of operation. At the onset of the reaction, distinctly colder regions are clearly visible, probably associated with uneven oxide coating on the wire surface. After 8 h of operation these are less pronounced and after ca. 48 h the temperature profiles adopts a parabolic shape (as observed on uncoated Pt–Rh specimens [7]), which remained throughout the experiments.
Fig. 4. Typical temperature profile along a catalyst wire as measured with an optical pyrometer; illustrated with results for an a-Fe2O3-coated Pt catalyst as representative examples; solid line: after 5 min, dash line: after 8 h, and dotted line: after 48 h of reaction.
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3.3. Pt/10 wt.% Rh, Pt/20 wt.% Rh, and Pt/30 wt.% Rh with a-Fe2O3 coating Fig. 5 shows SEM images of surfaces of Pt/10 wt.% Rh catalysts coated with a-Fe2O3 after ammonia oxidation for 336 h. The surfaces appear virtually identical for coatings prepared by DIPCO and ALCVD. In the first 1–2 mm from the gas inlet, terrace structures with numerous pits are developed (Fig. 5a). Cavities of different size and shape are mostly located along the grain boundaries or close to edges or terrace steps. Small nodule-formed undulated structures are clearly visible on elevated points. Further down the wires, the pattern consists of grain boundary grooves, numerous facets of different orientation, curved features, and excrescences of irregular shapes, together with some unreconstructed grains (Fig. 5b). In the region from 15 mm and downward, ‘‘cauliflower’’-like excrescences are well developed (Fig. 5c), corresponding to those seen on Pt/10 wt.% Rh catalysts without a-Fe2O3 coating [7]. Regions with the morphology shown in Fig. 5b reappear further down the wires, together with regions with fully developed ‘‘cauliflowers’’. This situation is maintained up to 80 mm from the gas inlet. Irregularly-shaped structures of iron oxide protrude clearly out from the wire surfaces (Fig. 5d). The temperature profiles along the Pt/10 wt.% Rh catalyst wires change with time during the progressing operation, with similar characteristics to those described for the Pt catalysts (see Fig. 3), however, the extension and position of the warmest region vary only by some 2–3 mm.
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The reconstruction patterns of a-Fe2O3-coated Pt/ 20 wt.% Rh and Pt/30 wt.% Rh wires are similar to those described above. Fig. 6a shows an a-Fe2O3-coated Pt/20 wt.% Rh surface ca. 4 mm from the gas inlet. The characteristic feature is a large number of pits with mainly four-fold symmetry, located not only around edges, but extending also into the middle of the facets. Further down along the wires, nodules on elevated points are clearly visible. In the region starting around 15 mm, non-uniform ‘‘cauliflower’’-like structures develop (Fig. 6b). Less reconstructed regions with a-Fe2O3 coating largely intact, alternate with more heavily reconstructed regions (Fig. 6c). Distinct features are seen with needle-shaped iron oxide protruding from the surfaces (Fig. 6d). The most heavily reconstructed surfaces of Pt/20 wt.% Rh with and without aFe2O3 coating are compared in Fig. 6e and f. A difference in character is evident for the numerous ‘‘cauliflower’’-like features all over the disengaged surfaces on the a-Fe2O3coated specimen. Similar observations are made for a-Fe2O3-coated Pt/30 wt.% Rh catalysts (Fig. 7a and b). 3.4. Rh with a-Fe2O3 coating Fig. 8 gives a comparison of SEM images of surfaces of Rh catalysts, with and without a-Fe2O3 coating after exposure to ammonia oxidation for 60 h. These experiments were maintained only for a short period because of the rapid mechanical degradation of the Rh wires [7]. The reconstruction process is distinctly slower on the a-Fe2O3-coated surfaces (Fig. 8b), but the patterns retain the same basic character (Fig. 8a). Rh2O3 was found in different regions
Fig. 5. SEM images of reconstructed surfaces on a Pt/10 wt.% Rh catalyst wire coated with a-Fe2O3 after exposure to ammonia oxidation for 336 h; approximate location in mm from gas inlet: (a) 2, (b) 8, (c) 15, and (d) 80.
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Fig. 6. SEM images of reconstructed surfaces on a Pt/20 wt.% Rh catalyst wires with a-Fe2O3 coating after oxidation of ammonia for 336 h; approximate location in mm from gas inlet: (a) 4, (b) 15, (c) 25, and (d) 60. Comparison of the most reconstructed surfaces on Pt/20 wt.% Rh catalyst wires (e) with and (f) without a-Fe2O3 coating.
along the a-Fe2O3-coated Rh wires: approximately 1–7, 22– 33, and 80–100 mm from gas inlet. This contrasts the findings for Rh catalysts without coating, where the Rh2O3 deposits only occur at the end of the wires where the temperature is below the decomposition temperature of Rh2O3 (1000 8C in oxygen [14]).
4. Discussion The introduction of a-Fe2O3 coating on Pt–Rh catalyst surfaces brings new aspects into the ammonia oxidation reaction. The surface-layer characteristics of a-Fe2O3 should be considered both in the as-deposited state and
Fig. 7. SEM images of reconstructed surfaces on a Pt/30 wt.% Rh catalyst wire after ammonia oxidation for 336 h: (a) with and (b) without a-Fe2O3 coating.
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Fig. 8. SEM images of reconstructed surfaces on pure Rh catalyst wires after ammonia oxidation for 60 h: (a) Rh catalyst coated with a-Fe2O3 and (b) Rh catalyst without coating.
under the ruling high-temperature conditions during the oxidation process. Furthermore, attention should be paid to the fact that although a-Fe2O3 is a well-recognized catalyst for ammonia oxidation it has a detrimental effect on the catalytic properties of the Pt–Rh support. The as-deposited a-Fe2O3 coatings have imperfections in the form of pores, cracks, blisters, etc. due to stress in the layers (in particular lateral stresses induced by differences in thermal expansion and/or crystal structure between the oxide and the Pt–Rh matrix). Such macroscopic defects can hardly be controlled. Hence, the characteristics for coatings deposited under equal conditions on assumed identical supports may differ significantly. Owing to the very different conditions during the coating processes, in particular with respect to temperature (see Section 2), the a-Fe2O3 layers prepared by DIPCO contain more and larger macroscopic defects than those obtained by ALCVD. This distinction is prominent at the as-deposited layer stage, but disappears largely after heating at 900 8C where pores grow, cracks open up and increase in size and number, and blisters burst. During operational testing, the situation is modified further, and notably, the pore structure changes owing to the presence of H2O as reaction product. The distinction between the DIPCO- and ALCVD-prepared a-Fe2O3 layers becomes hence less important, and the two sets of results will hereafter be treated jointly. The mechanisms for ammonia oxidation on a-Fe2O3 catalysts are not clearly understood. According to Griffiths et al. [3] gaseous or weakly adsorbed NH3 reacts directly with surface oxygen on a-Fe2O3 to yield surface intermediates that contain the nitrosyl group. This process does apparently not involve stripping of hydrogen from NH3 and formation of NH and NH2 radicals as surface intermediates (viz. rejection of a route that is debated for ammonia oxidation on Pt–Rh catalyst, see Ref. [7] and references therein). In the present study the reactants have been subjected to the combined influence of the Pt–Rh and a-Fe2O3 catalysts, of which Pt–Rh has the distinctly higher activity and selectivity for oxidation of NH3 to NO. It is likely that an interplay between these catalyst materials occurs in the
present system. Oxygen is believed to be easily and more strongly chemisorbed on Pt–Rh surfaces than on the aFe2O3. On the other hand, NH3 floats around with a likely tendency to accumulate at the boundaries between the different catalyst phases. These are precisely the regions on the a-Fe2O3-coated Pt–Rh catalysts that we envisage as particularly active with respect to ammonia oxidation. The pile-up of oxidation events for individual NH3 molecules in limited surface regions leads to hotspots which, in turn, promote the formation of gaseous PtO2 and RhO2 species. These are in due course transported a short distance under the influence of a temperature gradient and deposited on colder spots on the Pt–Rh support (mainly characteristic ‘‘cauliflower’’-like Pt–Rh excrescences) or the a-Fe2O3 cover (usually more distinct Pt–Rh crystals). For a more detailed outline of the involved chemical-vapor-transport process, see Ref. [15]. The designation of the boundary zones between the oxide and the liberated alloy surface as favorable regions for ammonia oxidation provides a simple explanation for why the coated wires do not exhibit the major Rh-contentdependent distinction in reconstruction patterns seen on uncoated catalyst specimens of, say, Pt/10 wt.% Rh and Pt/ 30 wt.% Rh [7]. The oxide/alloy boundary zones are largely determined by the properties of the coating. Hence, such favorable regions are equally probable all over the mixedcatalyst specimens. These interface regions show no a priori correlation with the defects governing the ammonia oxidation as well as reconstruction in the Pt–Rh alloy matrix itself [7]. Three other findings of the present study should be briefly discussed. First, the alloying of iron into the surface region of the Pt–Rh master matrix of DIPCO-treated specimens is a trivial consequence of the use of thermal decomposition of iron(III) nitrate in a (reducing) H2–O2 torch to produce the coatings, and also in accordance with the available information [11] for the phase diagrams concerned. Second, the decrease in activity/selectivity for the mixed oxide/alloy catalyst as function of time on stream appears to correlate with an increasing amount of Fe3O4 in the oxide layer (as observed by XRD). It is well recognized that Fe3O4
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at best is a poor catalyst for ammonia oxidation. The occurrence of Fe3O4 on used iron-oxide-coated catalysts also provides information on the conditions experienced by the oxide coating. The thermal decomposition of a-Fe2O3 to Fe3O4 takes place at some 1400 8C [12], depending on the oxygen partial pressure in the surrounding atmosphere. The inference is accordingly that the hotspot regions of these catalysts obtain a temperature of around 500 8C above the average catalyst temperature, or that the conditions at the surface are locally rather reductive. Third, the observation of Rh2O3 on different and timedependent locations along the wire contrasts the findings for the catalysts without a-Fe2O3 coating, where Rh2O3 was observed only at the terminal end of the catalysts. These findings concur with the time-dependent temperature profiles in Fig. 4, and reflect merely the fact that some parts of catalyst surface take temperatures below the decomposition temperature of Rh2O3 during the ammonia oxidation reaction. Such colder regions, with temperatures below the average 900 8C, are typically found on the oxidecovered parts where the temperature is not controlled by the current through the wire owing to the poor conductivity of the a-Fe2O3–Fe3O4 coating. Hence, hotspot temperatures on oxide/alloy boundary zones are fully compatible with the occurrence of colder regions a little away from the zones.
5. Conclusions Pt–Pt/30 wt.% Rh and Rh specimens coated with thin layers of a-Fe2O3 show catalytic activity to ammonia oxidation, but are less active than the corresponding uncoated Pt–Rh alloys. Extensive surface reconstructions are observed for all tested catalysts. The reconstruction patterns on liberated surfaces of Pt–Pt/10 wt.% Rh and Rh catalysts are similar to those observed on corresponding specimens without a-Fe2O3 coating. The introduction of aFe2O3 coating on Pt/20 wt.% Rh and Pt/30 wt.% Rh specimens makes these materials more susceptible to surface reconstruction under ammonia oxidation conditions than corresponding uncoated specimens. The main part of the entangled ammonia oxidation reaction and Pt–Rh reconstruction process are suggested to occur on boundary zones between the a-Fe2O3 cover and the liberated Pt–Rh
surface. A gradual thermal conversion of a-Fe2O3 to Fe3O4 causes a certain decrease in activity and selectivity of the mixed oxide/alloy catalysts.
Acknowledgements The financial contribution of Norsk Hydro ASA (the earlier Agri division, now separate company Yara ASA) is gratefully acknowledged. The authors extend their cordial thanks to staff members of the company for fruitful and stimulating discussions. The authors also express their gratitude to Cand. Scient. S. Jørgensen for assistance with SEM and helpful discussions, to Dr. M. Erambert (Department of Geology, University of Oslo) for help with EMP analyses, and to Cand. Scient. M. Lie for performing the ALCVD coatings.
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