Ind. Eng. Chem. Res. 2000, 39, 547-549
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RESEARCH NOTES Interfacial Reactions in Pt/Ti, Pt/Zr, Pt/TiO2, and Pt/ZrO2 Couples Sinn-wen Chen* and Shyh-jye Luo Department of Chemical Engineering, National Tsing-Hua University, Hsin-Chu, Taiwan, Republic of China
Interfacial reactions in four kinds of diffusion couples, Pt/Ti, Pt/Zr, Pt/TiO2, and Pt/ZrO2, are examined. Ti3Pt, TiPt, and γ phases are observed in the Pt/Ti couples annealed at 800 °C. PtZr and Pt3Zr5 are formed in the Pt/Zr couples reacted at 900 °C. No noticeable reaction are found when TiO2 and ZrO2 are annealed in contact with Pt with relatively high oxygen activities. Introduction Platinum is one of the most commonly used precious metal catalysts in the hydrogenation processes of aromatic hydrocarbons. It has been observed that the chemisorption and catalysis of Pt are dramatically reduced at high temperatures, and the effect varies with the choice of substrates. This so-called strong metalsupport interaction (SMSI) phenomenon has gained widespread research interests.1-10 It is usually believed, yet still not fully understood, that the SMSI phenomenon could be caused by two different mechanisms. The electronic effect mechanism refers to the changes of the electronic structures of Pt resulting from interaction with substrates, and the geometric effect mechanism refers to the migration of atoms of Pt and substrates. When two dissimilar materials are brought into contact, there is a tendency for interfacial reaction, and even new phases will form, because of the diffusion of atoms that result from the chemical potential difference of the atoms in the two materials.11-14 However, in addition to the thermodynamic driving forces, the migration of atoms depends on their kinetic behaviors as well. Therefore, it is not necessary that significant reaction occurs at the interface of Pt and the substrates, even though they are not thermodynamically in equilibrium. Yet if Pt does react significantly with its supporting substrates, TiO2 and ZrO2, that is, Pt, O, Ti, or Zr elements interdiffuse significantly, the geometric effect will definitely be an important factor in the SMSI phenomenon. This work examines the interfacial reactions in the Pt/Ti, Pt/Zr, Pt/TiO2, and Pt/ZrO2 systems by using diffusion couples. The catalyst Pt is usually a very tiny particulate with an irregular shape; thus, examination of interfacial reactions in the real catalyst system is extremely difficult and hence the results are not reliable. This difficulty can be overcome by using the diffusion couples which are made of foils of known thicknesses and planar interfaces. Experimental Method High-purity metal foils, 99.99 wt % Pt (0.1-mm thick), 99.99 wt % Ti (0.25-mm thick), and 99.8 wt % Zr (0.25* To whom correspondence should be addressed. Tel.: 8863-572-1734.Fax: 886-3-571-5408.E-mail:
[email protected].
Figure 1. Schematic diagram of the diffusion couple.
mm thick), and hot-pressed TiO2 (2-mm thick) and ZrO2 (2-mm thick) disks were used. In the preparation of Pt/ Ti diffusion couples, a piece of 3 × 3 mm2 Pt foil and a piece of Ti foil of the same size were cut and clamped together by two stainless steel screws in a stainless steel tube, as shown in Figure 1. BN powders were sprayed on the screws to prevent possible reactions between the screws and foils. Similar procedures were followed in the preparation of other kinds of diffusion couples. The stainless steel tube was then encapsulated in a quartz tube with 10-3 Torr vacuum, placed in a furnace, and annealed at either 800 or 900 °C. After a predetermined period of reaction time, the sample tube was removed from the furnace and quenched in water. The heat-treated diffusion couple was then taken out from the tube and mounted carefully so that the exact crosssectional interface was exposed. The mounted specimen was polished by using progressively finer SiC sand papers and alumina slurries, finishing with a 0.05-µm slurry. Optical microscopy and scanning electron microscopy (SEM) were used for microstructural examination. To enhance phase contrast, an etching solution with 10 mL of HF + 5 mL of HNO3 + 85 mL of H2O composition was used for most of the specimens prepared for optical microscopic examination. Compositions of phases formed at the interfaces were determined by using electron probe microanalysis (EPMA). Results and Discussion Figure 2 shows the microstructure of the Pt/Ti couple reacted at 800 °C for 7 days. Three intermetallic phases can be observed. The composition of each phase was
10.1021/ie990463h CCC: $19.00 © 2000 American Chemical Society Published on Web 01/07/2000
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Figure 2. Optical micrograph of a Pt/Ti couple reacted at 800 °C for 7 days.
Figure 3. SEM micrograph of a Pt/Zr couple reacted at 900 °C for 20 days.
determined by using EPMA probing on a point located at the center of each phase layer. The phase adjacent to the Ti phase is about 9-µm thick. Its composition determined by using EPMA is Pt-72 at. % Ti which indicates the phase is PtTi3. The thin and bright phase in the middle has a composition of Pt-51 at. % Ti and is likely the PtTi phase. The thickness of the third phase adjacent to the Pt phase is approximately 11 µm. Its composition is Pt-80 at. % Ti and is the γ phase. Standard deviations of the EPMA results are about 3%. According to the equilibrium Ti-Pt phase diagram,15 there are five intermetallic compounds at 800 °C, that is, Ti3Pt, TiPt, TiPt3, γ, and TiPt8. It is usually accepted11-14 that stable intermetallic phases can be formed by interfacial reactions. However, it is not uncommon that not all of the stable intermetallics are found at the contact.14 The absence of some phases is probably due to either the nucleation difficulties which inhibit their formation or the slow growth rates of certain phases which make them too thin to be visible. Rezkii et al.16 studied the diffusion bonding of Pt to Ti at 900 °C. They found a layer of Ti3Pt at the interface. Dzhur et al.17 also studied the diffusion bonding of Pt to Ti at 900 °C, but they found three intermetallic compounds, Ti3Pt, TiPt, and TiPt3. Onagawa et al.18 examined alloys prepared from Ti and Pt powders by using spark plasma sintering, and they observed the existence of Ti3Pt, TiPt, and TiPt8. All three previous studies and this present work agree that Ti reacts with Pt when in contact with each other, although the results of compound formation are not in agreement. Because the three studies of Pt/Ti interfacial reactions were not conducted in the same conditions, it is possible that in some cases semi-infinite boundary conditions were not met for all the annealing conditions, and the phase formation could then be different. Another possibility is that because the existence of TiPt3 and TiPt8 phases is not confirmed,15 it is likely that the TiPt3 and TiPt8 phases found by Dzhur et al.17 and Onagawa et al.18 are mislabeled as γ phases so that the phases formed are Ti3Pt, TiPt, and γ. It should be pointed out that the phase identification of this study was done by using EPMA under conditions near instrumental resolution limits, so it is possible that some phases were misidentified in this study as well. Figure 3 is the micrograph of the Pt/Zr couple reacted at 900 °C for 20 days. Two intermetallic layers, PtZr and Pt3Zr5, can be observed. Their compositions have
been determined to be Pt-49 at. % Zr and Pt-63 at. % Zr, respectively. One stable compound, Pt3Zr, at 900 °C is found to be absent.19 The thickness of the PtZr layer is about 50 µm and is much thicker than that of the Pt3Zr5 layer. Although a marker experiment has not been carried out, interfacial morphology and voids found at the Pt3Zr5/Zr interface as shown in Figure 3 indicate that the original Pt/Zr interface is located near the Pt3Zr5/Zr interface. The growth front was at the PtZr/Pt interface and Zr was the primary moving element. It can be noticed in Figure 3 that there are two layers in the region which is labeled as the Pt3Zr5 phase. However, the compositional results determined by using EPMA cannot be distinguished in these two layers. It is thus believed that these two layers are both the Pt3Zr5 phase, and the different levels of brightness of the two layers might be caused by different orientations of the grains. No previous results in the literature were available concerning interfacial reactions of Pt/Zr and Pt/TiO2 systems. No directly visible reactions were observed for all the Pt/TiO2 and Pt/ZrO2 couples annealed at 800 and 900 °C for 7, 10, and 14 days. The diffusion couples fell apart when they were removed from the screws. The surface of Pt was examined metallographically and no reaction products can be observed. Lu et al.20 studied the reactions of Pt with ZrO2 at temperatures between 1100 and 1400 °C. They found that Zr dissolved from ZrO2 in a reducing environment. The reduced Zr metal reacted with Pt, but Pt did not diffuse into the ZrO2 substrate. On the basis of the observation of this work and that by Lu et al.,20 it can be concluded that there is no significant reaction when Pt is annealed in contact with TiO2 or ZrO2 at relatively high oxygen activities. But in a reducing environment, chemical potentials of the metallic components in the oxides become higher, and the high-activity metals can then react with Pt. Thermodynamic calculations and more reaction couple studies at various oxygen activities are thus of high interest to quantitatively determine the critical oxygen activity for significant interfacial reactions. Conclusions Pt reacted with Ti and formed three compounds, Ti3Pt, TiPt, and γ phase, at the interfaces of Pt/Ti couples annealed at 800 °C. Two compounds, PtZr and Pt3Zr5,
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were found in the Pt/Zr couples annealed at 900 °C. Pt did not react significantly with TiO2 and ZrO2. However, in a reducing environment, such as the hydrogenation processes of aromatic hydrocarbons, the high-activity metals may react with Pt. Thus, although it has been found that Pt has no significant reaction with TiO2 and ZrO2 with relatively high oxygen activities, it does not exclude the possibility of SMSI phenomenon caused by the geometric effect. Acknowledgment The authors wish to acknowledge fruitful discussion with Professor I-Kai Wang in Tsing-Hua University and the financial support of the Chinese Petroleum Corp. and National Science Council of Taiwan ROC (NSC-87CPC-E-007-010). Literature Cited (1) Tauster, S. J.; Fung, S. C.; Garten, R. L. Strong MetalSupport Interactions: Group and Noble Metals Supported on TiO2. J. Am. Chem. Soc. 1978, 100, 170. (2) Tauster, S. J.; Fung, S. C. Strong Metal-Support Interactions: Occurrence among the Binary Oxide of Groups IIA-VB. J. Catal. 1978, 55, 29. (3) Alnot, M.; Cassuto, A.; Ducros, R.; Ehrhardt, J. J.; Weber, B. XPS UPS Study of the Reaction of Carbon Monoxide with Oxygen and Nitrogen Monoxide on Platinum-Rhenium Alloy. Surf. Sci. 1982, 114, L48. (4) Bardi, U.; Ross, P. N. Initial Stages of Oxidation of the Pt3Ti(111) and (100) Single Crystal Surfaces. J. Vac. Sci. Technol. 1984, A2, 1461. (5) Beard, B. C.; Ross, P. N. Pt-Ti Alloy Formation from HighTemperature Reduction of a Titania-Impregnated Pt Catalyst: Implication for Strong Metal-Support Interaction. J. Phys. Chem. 1986, 90, 6811. (6) Anderson, J. B. F.; Burch, R. The Reversibility of Strong Metal-Support Interactions. A Comparison of Pt/TiO2 and Rh/ TiO2 Catalysts. Appl. Catal. 1986, 25, 173. (7) Ocal, C.; Ferrer, S. A New CO Adsorption State on Thermally Treated Pt/TiO2 Model Catalysts. Surf. Sci. 1986, 178, 850. (8) Ocal, C.; Ferrer, S. The Strong Metal-Support Interaction.(SMSI) in Pt-TiO2 Model Catalysts. A New CO Adsorption State on Pt-Ti Atoms. J. Chem. Phys. 1986, 84, 6474.
(9) Belzunegui, J. P.; Sanz, J.; Rojo, J. M. Contribution of Physical Blocking and Electronic Effect to Establishment of StrongMetal-Support Interaction in Rh/TiO2 Catalysts. J. Am. Chem. Soc. 1992, 114, 6749. (10) Xu, W.-X.; Schierbaum, K. D.; Goepel, W. Ab Initio Study of the Effect of Oxygen Defect on the Strong Metal-Support Interaction between Pt and TiO2 (Rutile)(110) Surface. J. Solid State Chem. 1995, 119, 237. (11) van Loo, F. J. J. Multiphase Diffusion in Binary and Ternary Solid-State Systems. Prog. Solid State Chem. 1990, 20, 47. (12) Romig, A. D., Jr.; Chang, Y. A.; Stephens, J. J.; Frear, D. R.; Marcotte V.; Lea, C. Solder Mechanics: A State of the Art Assessment; Frear, D. R., Jones, W. B., Kinsman, K. R., Eds.; TMS: Warrendale, PA, 1991. (13) Luo, H.-T.; Chen, S.-W. Phase Equilibria of the Ternary Ag-Cu-Ni System and the Interfacial Reactions in the Ag-Cu/ Ni Couples. J. Mater. Sci. 1996, 31, 5059. (14) Su, L.-H.; Yen, Y.-W.; Lin, C.-C.; Chen, S.-W. Interfacial Reactions in Molten Sn/Cu and Molten In/Cu Couples. Metall. Mater. Trans. B 1997, 28, 927. (15) Murray, J. L. Phase Diagrams of Binary Titanium Alloys; Murray, J. L., Eds.; ASM: Materials Park, OH, 1987. (16) Rezkii, Yu. A.; Petrov, G. L.; Shapovalov, G. D. Diffusion Bonding of Platinum Titanium Anodes. Svar. Proiz. 1981, 8, 15. (17) Dzhur, E. A.; Kvasha, A. N.; Kedrin, I. D.; Ogdanskii, N. F.; Fesenko, A. G. Special Features of the Formation of the Joint between Platinum and Titanium during Vacuum Diffusion Bonding. Svar. Proiz. 1982, 3, 14. (18) Onagawa, J.; Goto, T.; Ise, O.; Ishii, N.; Horikawa, T.; Sawada, K. Corrosion Resistance of Titanium-Platinum Alloy Prepared by Spark Plasma Sintering. Mater. Trans. JIM 1996, 37, 1699. (19) Okamoto, H. ASM Handbook, Vol. 3: Alloy Phase Diagrams; Baker, H., Eds.; ASM: Materials Park, OH, 1992. (20) Lu, F.-H.; Newhouse, M. L.; Dieckmann, R.; Xue, J. Platinum-a Non Inert Material Reacting with Oxides. Solid State Ionics 1995, 75, 187.
Received for review June 24, 1999 Revised manuscript received November 15, 1999 Accepted November 18, 1999 IE990463H
