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Effect of Acid Treatment on the High-Temperature Surface Oxidation Behavior of FeCrAlloy Foil Used for Methane Combustion Catalyst Support Dong Zhang,†,‡ Lihong Zhang,† Bin Liang,§ and Yongdan Li*,† Tianjin Key Laboratory of Applied Catalysis Science & Technology and State Key Laboratory for Chemical Engineering (Tianjin UniVersity), School of Chemical Engineering, Tianjin UniVersity, Tianjin 300072, People’s Republic of China, National Academy of Nanotechnology & Engineering, No. 80, Fourth AVenue, TEDA, Tianjin 300457, People’s Republic of China, and College of Chemical Engineering, Sichuan UniVersity, Chengdu 610065, People’s Republic of China
The effect of acid treatment on the high-temperature oxidation behavior of Fe-Cr-Al alloy has been examined with oxidation tests and various characterization techniques. Acid treatment effectively improves the stability of the alloy in a high-temperature oxidation environment. After the treatment, the alloy oxidizes at a higher rate in the induction period and a lower rate in the subsequent time period. A passivative oxide layer was determined to be better formed during high-temperature oxidation on the surface of the acid-treated alloy, which suppressed further oxidation. It was observed that the oxidation rate of the alloy at high temperature can be described precisely by the Quadakkers equation. 1. Introduction The catalytic flameless combustion of low alkans improves the efficiency of energy production and reduces NOx emission.1,2 Monoliths that are made of metal foils have been used as the support for high-temperature combustion catalysts.3-5 Fe-Cr-Al alloy (FeCrAlloy) has superior stability to other iron-based alloys at high temperatures and has been used in the fabrication of gas burners, industrial heaters, and other high-temperature devices.6-9 Metal monoliths have several advantages over ceramics ones,3-7 including, for example, low environmental impact, high radiation efficiency, very low pressure drop, and good resistance to thermal and mechanical shock. However, the combustion atmosphere is challenging for metallic materials, because of the high oxidation potential. Although FeCrAlloy is comparatively thermally resistant and oxidation-resistant, its exposure to a high-temperature and high-oxidation environment leads to a preferential migration of the Al atoms toward the surface and formation of an oxide layer. This intermediate layer is crucial to the stability of the final catalyst and a structure of the layer with good affinity to the high temperature catalyst (i.e., hexaaluminate) is preferred.10,11 The Al2O3 layer formed is preferably as dense as possible, which further enhances the oxidation resistance.12 Nevertheless, if the Al atoms in the alloy are overconsumed, the Cr atoms will migrate outward to the surface of the alloy, which results in the collapse of the alloy. Therefore, the growth rate of the Al2O3 layer becomes a measure of the oxidation resistance of the FeCrAlloy.13,14 A similar case has been observed from the thermal barrier coatings, where an Al2O3 layer formed on the alloy exhibits a high resistance against high-temperature oxidation.15-18 Several methods have been adopted to improve the oxidation resistance of the alloy, such as doping with one or two rareearth elements (Ti, Zr, Hf, La),19-24 and preannealing treatment in a hydrogen atmosphere.12,25 It was proposed that the * To whom correspondence should be addressed. Tel.: +86-2227405613. Fax: +86-22-27405243. E-mail address:
[email protected]. † Tianjin Key Laboratory of Applied Catalysis Science & Technology and State Key Laboratory for Chemical Engineering (Tianjin University), School of Chemical Engineering, Tianjin University. ‡ National Academy of Nanotechnology & Engineering. § College of Chemical Engineering, Sichuan University.
preannealing treatment reduced the growth rate of Al2O3 layer by limiting the oxygen diffusion along the Al2O3 grain boundaries.25 In the preparation of the metallic monolithic catalyst, hightemperature calcination in air is often employed, which results in the formation of an oxide layer on the surface. This layer has the same functions as the aforementioned ones. There have been some published works on the effect of preparation factors on the catalyst performance.26-33 However, the effect of the oxidation rate on the thermal stability of the catalyst has not yet been explored. Zhai et al.34,35 found that FeCrAlloy that was experiencing an acid treatment showed enhanced resistance to thermal shock. Wagner36,37 proposed a parabolic model to describe the oxidation rate of metals, which is based on a hypothesis that the rate-determining step is the diffusion of the reactant under a chemical potential gradient. In reality, it was found that few metals obey the parabolic law.38 Quadakkers39 proposed a modified equation to describe the surface oxidation rate of a metal alloy, which is written as ∆m ) kt1/n
(1)
where ∆m is the mass gain, k the oxidation rate constant, n the exponent, and t the oxidation time. In this work, the model is used to describe the oxidation rate of FeCrAlloy. The effect of the acid treatment on the oxidation resistance of the alloy at high temperatures is examined. 2. Experimental Section 2.1. Acid Treatment. FeCrAlloy foil with a thickness of 0.1 mm and a chemical composition of 6.1 wt % Al, 25.1 wt % Cr, and Fe (balance) was supplied by Shanghai Daoda Electric Alloy Material Co., Ltd., PRC. Before the acid treatment, the alloy surface was polished with 600-grit SiO2 sandpaper, to remove the oxide layer and increase the roughness of the surface. It then was cleaned with ultrasonic bath for 30 min in acetone and washed by deionized water. Finally, the alloy was immersed in a mixed solution of 100 g/L H2SO4 and 100 g/L NaCl at 60 °C for 5 min. The foil then was washed with deionized water.
10.1021/ie8019664 CCC: $40.75 2009 American Chemical Society Published on Web 04/24/2009
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Figure 1. SEM micrographs of FeCrAlloy with and without acid treatment and after high-temperature oxidation: (a) FCA, without acid treatment; (b) FCAA, with acid treatment; (c) oxidized FCA, at 1100 °C for 10 min; and (d) oxidized FCAA, at 1100 °C for 10 min.
2.2. Oxidation Test. The oxidation of the alloy was performed at 1100 °C for a period of time in air. The heating rate was 10 °C/min. The mass gain of the alloy after a certain period of oxidation was measured using an analytical balance with an accuracy of (1 mg at room temperature. 2.3. Characterization. The surface morphology of alloy foil and the fractured cross-section of the oxide layer formed by oxidation at 1100 °C for 24 h and then immersion in liquid nitrogen were observed by scanning electron microscopy (SEM). Two instruments (PHILIPS XL 30 and FESEM1530 VP) were employed, respectively, for analysis of the samples before and after oxidation. X-ray diffraction (XRD) patterns were recorded using a PANalytical X’Pert PRO diffractometer with Co KR radiation at accelerating conditions of 40 kV and 40 mA. X-ray photoelectron spectroscopy (XPS) measurements were performed using a PE PHI-1600 system and Mg KR (hv ) 1253.6 eV) as the X-ray source. The hemispherical analyzer functioned with constant pass energy of 50 eV for high-resolution spectra. The binding energies were referenced to the C 1s peak at 284.6 eV. The experimental curves were fitted with a program that used a combination Gaussian-Lorentzian method, and the intensity of each peak was estimated from integration after being smoothed and subtracted from an S-shaped background.
3. Results 3.1. Morphology of the Alloy. SEM micrographs of the surface of the alloy without acid treatment (FCA) and with acid treatment (FCAA) are shown as Figures 1a and 1b, respectively. The surface of the FCA seems to be fairly rough, with some stripes on the surface. For the FCAA, the stripes become unclear and the surface of it seems rougher than that of the FCA. Micrographs of the surface of the two samples after oxidation at 1100 °C for 10 min are given as Figures 1c and 1d, respectively. The untreated alloy grows a layer of welldeveloped and porous oxide grains, whereas the acid-treated alloy surface also grows Al2O3 grains, but they are somewhat buried in a dense Al2O3-like layer. Figure 2 gives the SEM micrographs of the cross-section of the alloy showing two oxide layers formed during the oxidation at 1100 °C for 24 h in air. The results of the XRD indicate that the two layers are both considered to be the Al2O3 phase (vide post). The outer-layer oxide grains show an equiaxed morphology. The inner layer grains have a columnar grain morphology that is similar to that reported by Fukuda.40 The inner layers of the cross-sections of the two samples are quite different. The inner layer of FCA has some porous channels, whereas the thin inner layer of the FCAA seems to be very dense. 3.2. Mass Gain during Oxidation. The mass gain versus time curves of the two samples during oxidation are plotted in
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Figure 2. SEM micrographs of the cross-section of the Al2O3 layer formed on the alloys after oxidation at 1100 °C for 24 h.
Figure 3. Oxidation kinetic curves of FCA and FCAA at 1100 °C in air. Table 1. Fitting Parameters for the Oxidation Rate of FeCrAlloy with and without Acid Treatment Using eq 2 ln ∆m ) ln k + 1/n ln t alloy FCA FCAA
k
n
2.12 2.12
2.12 2.90
Figure 3, in which each point is the mean value of three measurements. It shows that FCAA has a higher oxidation rate than that of FCA in the first 0.5 h, and after that point, FCA oxidizes faster than FCAA. The difference in oxidation rate becomes larger, along with the increase of the oxidation time. The data of 0.5-5 h were correlated with eq 2, which is obtained by taking the logarithm of the Quadakkers equation, and the result is given in Table 1 and shown in Figure 4. The FCA and FCAA have a same k value, whereas the n values for the two samples are different. ln ∆m ) ln k +
( n1 ) ln t
(2)
3.3. XRD Patterns. Figure 5 depicts the XRD patterns of the FCA and FCAA surfaces after oxidation at 1100 °C for different times. Figure 5a gives four patterns of the FCA for 0.5, 1, 5, and 10 h. It shows clearly that R-Al2O3 (Joint Committee on Powder Diffraction Standards (JCPDS) File Card No. 01-074-1081) is the major oxide phase formed during oxidation. The diffraction pattern of R-Al2O3 is already welldeveloped for the sample that has been oxidized for 0.5 h,
Figure 4. Fitting lines for the oxidation rate of the FeCrAlloy using eq 2.
although diffraction peaks of the FeCr alloy (JCPDS File Card No. 00-034-0396) are also detected in the same sample. The respective patterns of the sample after oxidation for 1-5 h are very similar, in which the peaks of FeCr alloy disappear completely and only the peaks of R-Al2O3 are detected. Figure 5b shows a similar trend, that R-Al2O3 is also the major oxide phase in FCAA after oxidation for different times. However, the development of the R-Al2O3 phase is much slower than that on FCA. The intensity of the FeCr alloy phase is much stronger for the samples oxidized for 0.5 and 1 h than that of the FCA treated for a same time period. Even after 5 h, the major peak of the alloy still persists. The relative intensities of the peaks of R-Al2O3 phase are also much weaker than that of the FCA oxidized for the same time period. The intensities of the peaks of R-Al2O3 are even rather weak after oxidation for 10 h. 3.4. Chemical State of the Elements on Surface. Table 2 summarizes the proportions of the different chemical states of iron, chromium, aluminum, and oxygen, as determined from the M 2p3/2 (M ) Fe, Cr, Al) and O 1s regions analysis for the FCA and FCAA. As an illustration, the Fe 2p3/2 XPS spectra for the FCA and FCAA are displayed in Figure 6. The Fe 2p3/2 spectrum recorded for the FCA has been deconvoluted into three peaks at binding energies (BE) of 707.0, 708.5, and 710.3 eV, which are associated with Fe metal, Fe2+, and Fe3+, respectively. 41-44 It shows that both Fe2+ and Fe3+ are present on the alloy surface, which can be attributed to the coexistence of different
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Figure 6. XPS spectra of the representative Fe 2p3/2 regions in the FCA and FCAA.
Figure 5. XRD patterns of the samples oxidized at 1100 °C for different times: (a) FCA and (b) FCAA. Table 2. XPS Characteristics of Fe 2p3/2, Cr 2p3/2, Al 2p3/2, and O 1s Regions for FeCrAlloy with and without Acid Treatment FCA
sample Fe 2p3/2 Cr 2p3/2 Al 2p3/2 O 1s
Fe Fe2+ Fe3+ Cr CrA3+ CrB3+ Al3+ O2OHH2O
FCAA
binding energya (eV)
peak intensityb (%)
binding energya (eV)
peak intensityb (%)
707.0 (1.4) 708.5 (1.7) 710.3 (2.1) 574.2 (2.0) 576.3 (2.1) 577.8 (1.6) 74.3 (2.5) 530.7 (1.4) 532.0 (1.4) 533.2 (1.6)
56.1 24.0 19.9 51.3 40.1 8.60 100.0 43.0 32.5 24.5
707.1 (1.7) 709.2 (1.8) 710.6 (1.8) 574.5 (2.4) 576.6 (1.8) 577.8 (1.6) 74.9 (2.4) 531.0 (1.6) 532.1 (1.4) 533.0 (1.6)
47.4 30.7 21.9 41.4 43.3 15.3 100.0 42.8 31.7 25.5
a Number in parentheses refers to the fwhm (in eV). b Intensity of the peaks (expressed as a percentage) of the total M 2p3/2 (M ) Fe, Cr, Al) or O 1s area.
oxides and hydroxides. For the FCAA, the Fe 2p3/2 spectrum also includes three fitted peaks at BE ) 707.1, 709.2, and 710.6 eV, respectively, with the same attribution. The spectra of Cr 2p3/2 region of the samples can be fitted into three relevant peaks in the ranges of 568.0-582.0 eV, which represent three different types of surface species. The first peak at ∼574.0 eV is attributed to Cr metal, whereas the peak at BE ≈ 576.0 eV is attributed to Cr3+ (A). The peak close to that of Cr3+ at
∼577.0 eV may be associated with a different coordination environment of the Cr3+ (B). These BE values are consistent with the previous literature.45-48 The percentages of the metallic Fe and Cr in FCAA is lower than those in FCA, which implies that a relatively denser oxide film was formed in the acid-treated sample. The XPS spectra of Al in Al 2p3/2 region for the FCA and FCAA are simple. Only one peak, located at BE ≈ 74.3 eV, for the FCA attributed to Al3+ ions in the Al2O3 phase. The BE value increases to 74.9 eV for the acid-treated sample. Note that the BE values of elemental Fe and Cr for FCAA are higher than that of the FCA, which may indicate a stronger interaction between surface metallic ions and OH- on the surface of FCAA. This strong interaction is helpful to the formation of a dense oxide layer. The O 1s spectra can be deconvoluted into three peaks. One contribution to the spectrum is the oxygen present in the form of oxide (i.e., O2-) at 530.7 eV.49,50 Another, at 532.0 eV, is associated with the hydroxide group OH-.49,51 The third, at 533.2 eV, is assigned to weakly bonded oxygen in water.52,53 For the FCAA, the O 1s signals exhibit similar results, while the peaks of O2- and OH- are shifted to slightly higher values and the three peaks become closer. The analysis results about the M 2p3/2 (M ) Fe, Cr, Al) and O 1s XPS spectra for the FCA and FCAA oxidized at 1100 °C for 5 h are listed in Table 3. The results show that, after oxidation at 1100 °C for 5 h, no metal atoms or water molecules exist on the surface of FeCrAlloy. In addition, the other chemical states of each element still exist on the surface of the alloy after oxidation at 1100 °C for 5 h and the increased BE value is also ascribed to the stronger interaction among each element on the surface of the alloy.
Ind. Eng. Chem. Res., Vol. 48, No. 10, 2009 Table 3. XPS Characteristics of Fe 2p3/2, Cr 2p3/2, Al 2p3/2, and O 1s Regions for the FeCrAlloy Oxidized at 1100 °C for 5 h FCA1100
sample Fe 2p3/2 Cr 2p3/2 Al 2p3/2 O 1s
Fe2+ Fe3+ CrA3+ CrB3+ Al3+ O2OH-
FCAA1100
binding energya (eV)
peak intensityb (%)
binding energya (eV)
peak intensityb (%)
709.2 (1.5) 711.7 (2.7) 576.4 (1.5) 578.1 (2.1) 77.5 (1.8) 530.7 (1.6) 531.8 (1.4)
60.7 39.3 64.3 35.7 100.0 58.7 41.3
709.2 (1.6) 711.1 (2.5) 576.5 (2.0) 578.5 (2.4) 77.5 (1.8) 530.7 (1.8) 531.8 (1.6)
68.6 31.4 73.5 26.5 100 58.4 41.6
a Number in parentheses refers to the fwhm (in eV). b Intensity of the peaks (expressed as a percentage) of the total M 2p3/2 (M ) Fe, Cr, Al) or O 1s area.
4. Discussion 4.1. Surface Composition and Morphology. From the XPS data presented in Table 2 and Figure 6, it is observed that the amount of Fe and Cr oxide/hydroxide species on the surface of the FCAA is greater than that on FCA. It is likely that, by dipping the alloy in the acid solution, the anodic oxidation reactions of undissociated H2SO4 molecules and metal atoms occurred. Although the amount of oxide on the surface of the FCAA can be detected by XPS and is higher than that on FCA, the oxide phase is still too thin to be detected by XRD. Nevertheless, the surface of the FCAA became sufficiently rough, as seen in Figures 1a and 1b. In the high-temperature calcination process in air, the Al atoms in the alloy migrate to the surface and are oxidized there. The results of XPS and XRD indicate that the Al2O3 is the major oxide phase for the alloy after oxidation at 1100 °C for 5 h in air. Meanwhile, the XPS data show that the surface of the alloy has been covered by the Al2O3 layer with a certain thickness, because XPS is used to measure the elemental composition of the surface and the analyzing depth is usually 2-10 nm. In the oxide layer, FeO and Cr(A)2O3 are still the major forms of elemental Fe and Cr and the ratios of FeO to Fe2O3 and Cr(A)2O3 to Cr(B)2O3 of the FCA1100 are smaller than that of the FCAA1100. By comparison of the XRD patterns of the samples, the relative weaker diffraction peaks of R-Al2O3 phase and slower disappearance of the major peaks of the alloy phase for the FCAA suggests that the Al2O3 layer on the surface of the FCAA is thinner but denser than that on the FCA. This is further proven by SEM observation of the cross-section. 4.2. Oxidation Kinetics. In the Quadakkers equation,39 the exponent n reflects the significance of the oxidation time t on the reaction rate. The constant k is determined by the oxidation reaction itself. The n value for FCAA is 2.90, and this value is larger than that of FCA; both values are >2. This agrees with the explanation given by Quadakkers39 that the growth of the oxide layer is controlled by oxygen grain-boundary transport. The rate of diffusion is influenced by the size of the Al2O3 grains across the oxide layer. The k values of the FCA and FCAA are the same, which means the same rate-determining factor for the two samples. In other words, the acid treatment did not change the oxidation mechanism, but rather suppressed the inward diffusion of oxygen through the Al2O3 layer. 4.3. Oxidation Process. Using inspections of the mass gain data, the oxidation process of FCA can be divided into two stages, including the faster oxidation process in the initial 0.5 h and the subsequent slower one. The inverse trends of the mass
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gains between the FCA and FCAA in the period suggest that the influences of the rate-determining factors on the alloy oxidation are opposite at the different oxidation time periods. In the first stage, only a thin oxide layer is formed on the surface of the alloy and the contact between the elemental metals and oxygen is very easy. The rate-controlling step of the oxidation process is the surface reaction between Al and O atoms. The results from XPS analysis and SEM observation reveal that the acid-treatment process is helpful for alloy surface coarsening and Fe-, Cr- and Al- oxide/hydroxide nucleation to accelerate the surface oxidation of the FCAA and, thus, form a dense oxide layer. In the second stage, the mass-transfer process (i.e., the inward diffusion of the oxygen54,55) becomes the major influencing factor for the oxidation rate. On the surface of the FCAA, the dense oxide layer acts as a barrier for further oxygen diffusion. The acid treatment has an activation effect for the formation of a dense oxide layer, which is beneficial to protect the metal phase from further fast oxidation. This explanation is consistent with that proposal by Fukuda et al.40 Note that, in catalyst preparation, the alloy is washcoated with catalytically active components (e.g., alumina, hexaaluminate, etc.); these materials, on the alloy surface, may change the oxidation behavior, irrespective of the acid treatment. 5. Conclusions The effect of acid treatment on the oxidation behavior of FeCrAlloy was investigated. From this preliminary work, two main conclusions can be attained: (1) the acid treatment is an effective method to improve the oxidation resistance of FeCrAlloy at high temperatures, and (2) the oxidation rate can be described by the Quadakkers equation and the oxidation mechanism is the same for FeCrAlloy with and without the acid treatment, even in the initial oxidation stage. Acknowledgment This work has been supported by the NSF of China (under Contract Nos. 20425619 and 20576097). The work has been also supported by the Program of Introducing Talents to the University Disciplines (under File No. B06006), and the Program for Changjiang Scholars and Innovative Research Teams in Universities (under File No. IRT 0641). Literature Cited (1) Pfefferle, L. D.; Pfefferle, W. C. Catalysis in combustion. Catal. ReV. Sci. Eng. 1987, 29, 219. (2) Saint-Just, J.; der Kinderen, J. Catalytic combustion: from reaction mechanism to commercial applications. Catal. Today 1996, 29, 387. (3) Thevenin, P. O.; Menon, P. G.; Ja¨rås, S. G. Catalytic processes to convert methane: partial or total oxidation, part II. Catalytic total oxidation of methane. CATTECH 2003, 7, 10. (4) Cybulski, A.; Moulijn, J. A. Structured Catalysts and Reactors; Marcel Dekker: New York, 1998. (5) Fornasiero, P.; Montini, T.; Graziani, M.; Zilio, S.; Succi, M. Development of functionalized Fe-Al-Cr alloy fibers as innovative catalytic oxidation devices. Catal. Today 2008, 137, 475. (6) Padture, N. P.; Cell, M.; Jordan, E. H. Thermal barrier coatings for gas-turbine engine applications. Science 2002, 296, 280. (7) Gulbransen, E. A.; Andrel, K. F. Oxidation studies on the ironchromium-aluminum heater alloys. J. Electrochem. Soc. 1959, 106, 294. (8) Klower, J.; Brill, U.; Heubner, U. High temperature corrosion behaviour of nickel aluminides effects of chromium and zirconium. Intermetallics 1999, 7, 1183. (9) Satyanarayana, D. V. V.; Pandey, M. C. The role of active elements in Fe-Cr-Al alloys for heating element applications. Bull. Mater. Sci. 1995, 18, 207.
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ReceiVed for reView December 20, 2008 ReVised manuscript receiVed March 1, 2009 Accepted April 16, 2009 IE8019664
