J. MATER. CHEM., 1994, 4(3), 373-377 Composite Materials based on Ti and Ru Oxides M. I. Ivanovskaya, V. V. Romanovskaya and G. A. Branitsky Scientific and Research Institute for Physical and Chemical Problems of Belarus State University, 7 4 Leningradskaya Street, 220080 Minsk, Belarus The interaction and the electronic state of the components in the Ru0,-TiO, film system, obtained by the sol-gel method from poly(buty1 titanate) and ruthenium chloride, are considered. After the films had been heat-treated in air at 770 K a composite oxide film was formed, which is mostly a phase of solid solution of TiOP in RuO,. Further annealing of the film in H, led to deterioration of the solid solution and partial reduction of both components, Ti4+ and Ru4+ ions. Within the reduced system ruthenium in oxidation states 0-VIII was identified by XPS.Titanium appeared in the form of non-stoichiometric Tino%-, oxides, mostly Ti,O,, but Ti0 and Ti,O are also possible. Ru0,-TiO, films can serve as effective anodes for some electrochemical processes, in particular, in the manufacture of chlorine and in the production of highly stable and selective catalysts for thermocatalytic sensors, which are designed for detecting reducing gases. As catalysts in thermocatalytic sen- sors, Ru02-Ti02 and anode coatings offer some advantages over palladium and platinum. The reduced Ru-Ti02 systems on supports (Al,O,, SiO,) are attractive as catalysts for many reactions: Fischer-Tropsch, oxidation, isomerization and others.The characteristic of such film catalysts is that active components are concentrated in the pre-surface layer of the support granules, thus a decrease in ruthenium concentration is achieved.' This study is concerned with the formation of the RuO,-TiO, film under joint thermal decomposition of poly( butyl titanate) (PBT) and ruthenium chloride and the electronic state of ruthenium and titanium in these systems after oxidation and reduction. Experimental We investigated Ru02-Ti0, film systems, deposited over quartz substrates and over a porous support of 2-3 mm spherical y-Al,O, granules. In order to form the films, a solution of PBT and ruthenium chloride in isopropyl alcohol-ethanol was deposited over the quartz substrates or y-A1203 granules. Commercial RuC1,.3H2O and PBT [empiri- cal formula Ti01.4(OC4H9)1,2] were used as starting materials.The concentration and volume of the solution were selected such that TiO,:RuO, =2.5 and a film thickness of 150 nm was obtained while the Ru and TiO, contents were 0.3% and 0.6% (by weight), respectively, of the total weight of the support. After they had been dried in air at 350-370K the PBT films containing ruthenium were annealed in air for 2 h at 773 K, this temperature being attained after heating for 1h. The annealing temperature was chosen taking into account the results of differential thermal analysis (DTA). DTA was performed with an OD-102 differential thermal analyser in a static air atmosphere with a heating rate of 10K min-' within the 273-973 K temperature range.The precipitates obtained after evaporation in air at 353-373 K of the solvent from the solution, comprised PBT and ruthenium chloride in the same ratio as in the initial films. Reduction of the samples was performed at 723 K for 3 h at heating rate of 30 K min-' and a cooling rate of 2 K min-'. The particle size and structural and phase transformations in the films were determined by means of transmission electron microscopy (TEM) (EM-100CX microscope). For TEM stud- ies the RuO,(Ru)-TiO, films were removed from the quartz substrates and A120, granules with HF solution. Electron diffraction data were obtained (HZG diffractometer, Cu-Ka radiation, Ni-filter). For the purpose of XRD studies a thin layer of RuO,(Ru)-TiO, film was peeled off the Al,O, granules, resulting in a decrease of the Al,03 line intensities that interfere with the analysis of phase compositions of titanium and ruthenium oxide structures.X-Ray photoelectron spectra (XPS) were recorded directly from the catalyst granule surface prior to and after reduction and after bombardment of the surface with Ar' ions to a depth of 0.10-0.15 pm. Secondary-ion mass spectra (SIMS) of the reduced samples were recorded at the same time. An LAS-300 'Riber' spectrometer using A1-Ka radiation was utilized for recording these spectra. EXAFS measurements were performed with a double-crystal X-ray spectrometer. A double monoblock Si( 11 1) crystal was used as the monochromator.Analyses of EXAFS data were carried out as described in ref. 2. Results and Discussion According to DTA data the thermal decomposition of PBT takes place in several stages [Fig. l(a)]. As the temperature is increased, removal of the residues of organic solvents and Q I Fig. 1 DTA and TG for (a) PBT and (b)PBT +RuCl.nH,O powders; heating rate 10 K min-' J. MATER. CHEM., 1994, VOL. 4 water takes place (543 K), followed by removal of hydroxy groups (663 K) and TiO, phase crystallization (788 K). When RuC1, is added to PBT the pyrolysis of PBT is intensified, leading to completion of TiO, crystallization at a lower (by 150K) temperature than during pyrolysis of PBT on its own [Fig. l(b)]. It follows from DTA data that in the pyrolysis of PBT-RuC1, the formation of titanium and ruthenium oxide is observed within the same temperature range (588-733 K).Ruthenium chloride is oxidized in air at 623 K., When a PBT film, deposited over a quartz substrate, is annealed in air (770 K, 4h) a continuous crystalline TiO, film with 5-20 nm grains is formed [Fig. 2(a)]. In order to obtain the particle size distribution the dimensions of at least 1000 particles on three different TiO, film areas were determined. Note that the particle size distribution in the films under investigation was satisfactorily reproducible. By means of XRD, two structural modifications in the TiO, film (rutile and anatase) were detected. Annealing PBT films containing added ruthenium chloride under similar conditions resulted in a crystalline film, com- posed of ununiformly sized particles.Unlike TiOz films, which are characterized by a narrow range of particle dimensions, Ru0,-TiO, films contain particles ranging from 5 to 80nm [Fig. 2(b)]. An electron micrograph of such a film is shown in Fig. 3(a). In the XRD pattern of such a film RuO, reflections predominate, while rutile and anatase reflections (TiO,) are weak. Heat treatment in H, at 473 K (for 3 h) does not alter the Ti0, film structure and phase composition. Increasing the temperature to 723 K does not result in any noticeable changes either. However long-term heating in H, at 723 K leads to the appearance of a single signal of low intensity, which can be assigned to the Ti407 phase.Annealing the RuO,-TiO, films in an H2 atmosphere results in substantial changes in dispersity of the particles: large crystallites split into small grains, forming films with particles an average size of 7 nm [Fig. 2(c)]. An electron micrograph of an RuO,-TiO, film reduced in H, at 723 K is shown in Fig. 3(b). Note that the large grains observed in the pattern 601 n 40120 I iI. ,I h?f. 40-::I 40 201 I L=L--LL20 40 60 80 100 particle sizehm Fig. 2 Particle-size distribution of (a) TiO,, (b) Ti0,-RuO, after treatment in air at 770 K for 4 h and (c) TI02-Ru0, after treatment in air at 770 K for 4 h, then in H, at 720 K for 3 h Fig. 3 Electron micrographs of RuO,( Ru)-TiO, thin films after treatment (a) in air at 770 K for 4 h; (b) in air at 770 K for 4 h then in Hz at 720 K for 3 h are really composed of small particles which are very well discriminated when viewed by an electron microscope. The phase composition of the film changes simultaneously with the dispersity during treatment with H,.Diffraction patterns show signals that can be attributed to TiO, (rutile) and partially reduced oxides of the type Ti,Oz,- l(n =4, 6, 8, 9) and, possibly, Ti0 and Ti,O. In this case TiO, peaks are higher in intensity than those of RuO,. The metallic ruthenium phase was detected by electron diffraction in Ru02-Ti02 after it had been reduced in H, at 723 K for 3 h; however, RuO, also remained. Thus, according to electron diffraction data, complete reduction of RuO, into Ru is not observed even after a long period (3 h) of reduction in H, at high tempera- ture (723 K). However, in the Ru0,-TiO, system, unlike undoped TiO,, a deeper reduction of TiOz takes place in an H, atmosphere: more Ti3+ ions are formed and these participate in Tin02n-1, Ti2+, Tif (TiO, Ti20) non-stoichiometric oxides. The reduced films of Ru0,-TiO, incorporate the phases RuO,, TiO, (rutile), Ru and Ti,,02fl-1.Because of the broad lines in the diffraction patterns and superposition of various phases, as well as the low precision of the TEM method, it was not possible to state for certain that a solid solution of TiO, in RuO, had formed in the J. MATER. CHEM., 1994, VOL. 4 Ru0,-TiO, films.However, the very much greater peak intensity of RuO, than that of TiO, with a high content of the latter in the film, may indicate the formation of a solid solution of Ti0, in RuO,. The changes in dispersity and phase composition that occur in the reduction atmosphere may also prove the existence of a solid solution of TiO, in RuO, and of decomposition thereof under H,, followed by formation of a new structural phase. The formation of a solid solution of TiO, in RuO, in the process of joint pyrolysis of PBT and ruthenium chloride confirm the results of the XRD study of Ru0,-TiO, films deposited over y-A1203 (Table 1). The values of the interplanar distance, d, obtained for Ru0,-TiO, samples annealed in air, are slightly higher than those in the RuO, unit cell (P/4mnm).In this case no intense TiO, peaks are present in the XRD. The parameters of the RuO, unit cell, measured by the ASTM method, are as follows: u =4.490 A, c =3.106 A. The samples studied !ad the following uyit-cell parameters: a =4.496 f0.002 A, c = 3.118 0.003 A. Thus, the presence in XRD patterns of intense of RuO, peaks without any TiO, peaks, along with slightly expanded dimensions of the RuO, unit cell allow us to infer the formation of a solid solution of TiO, in RuO,. An XRD study of RUO,-TIO,~,~showed only rutile-type oxide peaks with 28 values intermediate between those of RuO, and TiO,, showing that these oxides must form a solid solution. The TiO, peak cannot be lost due to high dispersity. According to TEM data, Ti0, particles have dimensions sufficient to diffract X-rays and electrons.When the Ti0, phase is present in an Ru0,-TiO, film it is easily detected by TEM and XRD. Thus, using TEM we have established that only a thin surface layer is in the TiO, (rutile) phase. This is affirmed by XPS data for an un-reduced sample of Ru02-TiO, (Fig. 4). XPS spectra of such an Ru0,-TiO, sample show Ti 2p,,, and Ti 2p,,, peaks with binding energy (E)of 458.5 and 464.2 eV, corresponding to Ti4+ in TiO,. In this case the XPS spectrum has hardly any peaks corresponding to Ru 3p3,, with an E value characteristic for Ru4+ in RuO, [Fig. 4(u)]. The highest-intensity line with E=462.5 eV in the XPS spectrum could correspond to the Ru2+ state.However, in view of the results given above, this line could also be attributed to the Ru4+ state in the solid solution of RuO, in TiO,. The intensity of this line increased slightly after the surface had been sputtered with Ar' ions, indicating that the Table 1 Ru0,-TiO, XRD data annealed at 773 K for 4 h in air 3.1821 0.0042 3.1700 25 RuO, 110 2.5632 0.0035 2.5500 25 RuOz 101 1.6897 0.0014 1.6850 16 RuO, 211 annealed at 773 K for 4 h in air and at 723 K for 3 h in H,3.3811 0.0044 3.3800 8 Ti,O,, -In 120 (Ti,%)3.1537 0.0040 3.1700 8.5 Ru02,(Ru0,) 110 2.7867 0.0039 2.8010 2 T1,O, 022 2.4405 0.0032 2.42.30 2 Ti,Oz,-l 024 (Ti&)2.3508 0.0030 2.3430 27 Ru 100 2.1456 0.0024 2.1420 4 Ru 002 2.0627 0.0021 2.0560 42 Ru 101 1.3546 0.0009 1.3530 2 Ru 110 1.2243 0.0007 1.2189 14.5 Ru 103 1.1448 0.0005 1.1434 14 Ru 112 "n=4-9.Ru6+ R u2+ Rue+ Ru4+ Ru" \, *...'i'... %'--.\..I I. 474 466 458 450 binding energy, EdeV Fig. 4 Ti 2p and Ru 3p,,, XPS spectra of Ti02-Ru0, (-1 treated (a)in air at 770 K for 4 h; (b) in air at 770 K for 4 h then in H,at 720K for 3 h. Dashed lines represent the effects of argon-ion bombardment. Dotted lines represent the separation of the spectrum into Ru- and Ti-components thickness of the TiO, film on the surface of the Ru0,-Ti02 solid solution phase is small. After reduction of the film in H,, Ru and Ti,O, phases were detec!ed (Table 1). A single line of low intensiity with d=3.1537 A is close to the RuO, line of maximum intensity (d=3.1700 A).A decrease in d can be attributed to the formation of partially reduced oxide RuO, (x=0.5, 1, 1.5). No data are available in the literature on the unit-cell param- eters for such ruthenium oxides (Ru2O, RuO, Ru203). Partial reduction of both Ru4+ and Ti4+ under H, is probable, accompanied by decomposition of the solid solution structure. XPS data indicate the formation of RuO, and TiO,. From an analysis of the complex peak in the XPS spectrum of the reduced sample of Ru0,-TiO, [Fig. 4(b)] it follows that titanium and ruthenium are present in different valence states. Low-intensity lines with E =455.3, 457.1 and 458.5 eV can be attributed to Ti2+, Ti3+ and Ti4+ states in oxides, respe~tively.~ In the Ru 3p3/, XPS spectrum we can identify lines corre- sponding to various valence states of ruthenium, from Ruo to RuS+. Owing to superposition of signals from different states it is not possible to estimate quantitative state ratios of titanium and ruthenium.After bombardment of the surface with Ar+ ions, the intensity of the peak corresponding to highly oxidized ruthenium states (Rus+, Ru6+) decreases, while the intensity of the peaks for Ruo and partially reduced ruthenium ions (Ru2+, Ru') increases. The Ru8+ and Ru6+ states are probably surface-bound. XPS peaks with high E values (470 eV and above) may be considered to be the main satellites of the Ru 3p3/, peak.' In SIMS (Fig.5), besides Ru', Ti', RuO' and TiO+ clusters, which are normally found for Ru and Ti oxides, we also found RuOTi+ and RuTi' clusters.The presence of RuOTi' clusters in SIMS confirms the existence of a solid solution in the Ru0,-TiO, system, while the presence of RuTi+ clusters illustrates the possibility of formation of Ru-Ti J. MATER. CHEM., 1994, VOL. 4 mlz Fig. 5 SIMS spectrum (fragment) of RuO2-TiO, intermetallic species in the reduction atmosphere, indicative of strong electronic interaction between ruthenium and titanium, and thus of a strong mutual influence upon each other's state within the Ru0,-TiO, system. The presence of RuOAl' and RuAl+ clusters may be the result of a strong metal-support interaction (SMSI) between ruthenium and Al,03.' This interaction could provide an obstacle to reduction of ruthenium ions to the metal and could promote the formation of highly dispersed ruthenium particles. In the SIMS spectrum there are also low-intensity lines corresponding to TiOAl+ and TiAl+ clusters.The presence of such clusters in SIMS implies the interaction of titanium ions with the support (A1203). The results of the EXAFS study of the Ru02-Ti0,/y-A1,0, sample, treated in air, showed that oxygen and ruthenium form a local structure around Ru. A long oscillation [Fig. 6(a), (b)] is characteristic of heavy backscatterers like Ru.~ Interatomic distances (R)in Fourier transforms of Ru0,-TiO, are referred to Ru-0 and Ru-Ru bonds in RuO, [Fig. 6(c)]. However, values of R and the amplitude (I) for the Ru02-Ti0,1y-A1,0, sample display certain differences from 4 6 8 10 12 14 2468 k IA-' (R-s)IA Fig.6 EXAFS oscillations (a), (b) and the associated Fourier transform (c), (d)of RuO,(Ru)-TiO,/A1,0, after treatment: (a), (c) in air at 770 K for 4 h; (b),(d) in air at 770 K for 4 h then in H, at 720 K for 3 h those for the Ru02/y-Al,0, sample (the reference sample, obtained by impregnating the support with RuC1, solution without adding PBT and processed under identical conditions; Table 2). These structural changes point to reformation of an RuO, array, caused by the introduction of titanium ions in the process of formation of the solid-solution structure. In the Fourier transform of the reduced Ru--TiO, film only one high-intensity peak is present [Fig.6(d)].The interatomic distance (RRu-RRu) obtained from EXAFS is 2.58 Jr0.02 A, which is less than the interatomic Ry-Ru distance in the bulk metal" (RRu--Ru =2.68-2.70 A). A decrease in interatomic distance is a characteristic feature of the formation of metallic clusters. Conclusions By means of joint thermal decomposition of PBT and ruthenium chloride at 773 K, RuO,-TiO, films were obtained which have an Ru0,-based solid-solution structure with Ru02:Ti02=0.3. DTA data indicate that crystallization of TiO, from PBT and RuO, from RuC1, takes place within the same temperature interval and promotes formation of a solid solution. In the presence of ruthenium, the processes of pyrolysis of PBT and crystallization of TiO, are accelerated, while the temperature of the polymorphic transformation of anatase into rutile is decreased.Partial reduction of both Ru4+ and Ti4+ accompanied by decomposition of the Ru02-TiO, solid-solution structure takes place in an H2 atmosphere at 473-723 K. In this case an effect of Ru and Ti upon the electronic states of each other is observed. The reduction of Ti4+ in the Ru0,-TiO, system occurs more readily, while reduction of Ru4+ is hampered. Table 2 Structural parameters of supported Ru02--Ti02 and RuO, after treatment in air Ru0,-TiO, RuO, Rlk Ib R/k 2.02 0.50 2.00 2.96 0.22 2.96 3.60 0.27 3.48 4.59 0.14 4.50 5.42 0.09 5.42 'Error, f0.02 A.bError, *20%. Ib assignment ~ 0.40 Ru-0 ( RuO,) 0.20 Ru-RU (RuO,) 0.35 Ru-Ru (RuO,) 0.08 Ru- RU ( RuO,) 0.12 Ru-RU (RuO,) J.MATER. CHEM., 1994, VOL. 4 Owing to the strong mutual influence of the components in the Ru0,-TiO, system ruthenium in oxidation states from Ru' to Ru8+ and partially reduced Ti states Ti2+ and Ti3+ are stabilized. As a result of the high-temperature reduction of Ru0,-TiO, films under H,, finely dispersed ruthenium particles are formed and these preserve high dispersity after a long duration in a reducing atmosphere at high temperature (720 K). Highly dispersed ruthenium particles obtained in this way have the features of metallic clusters: decreased interatomic distance in comparison to the bulk metal and strong interaction with titanium oxides and oxygen in the air.The authors are grateful to Prof. V. V. Sviridov for valuable discussions and editing of the text. The financial support of this work by the Belarus Fundamental Research Fund is also gratefully acknowledged. References 1 M. I. Ivanovskaya, V. V. Romanovskaya and G. A. Branitsky, Dokl. Acad. Sci. Belarus, 1992,36, 140. 2 D. 1. Kochubey, Yu. A. Babanov and K. I. Zamaraev, X-Ray Spectral Method of Amorphous Bodies Structure Study, Nauka, Novosibirsk, 1988. P. G. J. Koopman, A. P. G. Kieboom and H. van Bekkum, Red. Trat.. Chim. Pays-Bas, 1983,102,429. J. Augustynski, L. Balsenc and J. Hinden, J. Electrochem. Soc., 1978,125,1093. K. Kameyama, S. Shohij, S. Onoue, K. Nishimura, K. Yahikozawa and Y. Takasu, J. Electrochem. Soc., 1993, 140, 1034. 6 T. Sheng, X. Guoxing and W. Hongli, J. Catal., 1988, 111, 136. 7 Practical Surface Analysis by Auger and X-ray Phot !,electron Spectroscopy, ed. D. Briggs and M. P. Seach, Mir, MOSCOW, 1987, p. 147. 8 S. J. Tauter, S. C. Fung and R. L. Garten, J. Am. Chem. Soc., 1978,100,170. 9 T. Mizushima, K. Toji, Y. Udagdwa and A. Ueno, J. Am. Chem. Soc., 1990, 112, 7889. 10 K. Asakura, N. Kosugi, Y. Iwasawa and H. Kuroda, in Proc. Inr. Conf. EXAFS and Near Edge Structure Ill, Stanford, CA, Springer Proc. Phys., 1984, p. 190. Paper 3/04254E; Received 20th July, 1993