J. Chem. SOC., Faraday Trans. I, 1987, 83 (8), 2271-2277 Electron Spin Resonance Investigations of Ruthenium supported on ?-Alumina Maria Grazia Cattania Sabbadini Centro C.N.R. per lo Studio sulle- Relazioni tra Struttura e Reattivita Chimica, Universita di Milano, Via Golgi 19, 20133 Milano, Italy Antonella Gervasini, Franca Morazzoni* and Donatella Strumulo Dipartimento di Chimica Inorganica e Metallorganica e Centro C.N.R., Universita di Milano, Via Venezian 21, 20133 Milano, Italy We report an electron spin resonance investigation performed on alumina- supported ruthenium, Ru/y-Al,O,, obtained by decarbonylation (pyrolysis or H, reduction) of Ru,(CO),,/y-Al,O,. The spectra observed after Ru/ y- A1,0, was in contact with o,, CO, NO ‘probe’ molecules allow one to distinguish superoxide, carbonyl and nitrosyl paramagnetic derivatives of ruthenium, and show that ruthenium centers in formal oxidation states other than the zero metal state become stable by interaction with y-Al,O,.The strength of the interaction between paramagnetic ruthenium centres and n*-acceptor molecules (02, CO and NO) is dependent both on the n*- acceptor molecules and on the decarbonylation method. In recent years several investigations have been made in order to characterize the electronic properties of supported transition-metal centres. A specific effort has been made to relate the electronic state of the metal to the activity and selectivity of the dispersed metal system used as catalyst in a given reaction. Associated with the above- mentioned properties is the interaction between the metal component and the support; this plays a decisive role owing to the close relationship between the metal and the support, which greatly affects the interaction of the metal with the chemisorbed reactants.’ Since it was discovered that the supported metal particles are not completely reduced and that dispersed metal ions are stable at the metal-support interface, the investigation of the paramagnetic species by e.s.r.spectroscopy has come to be considered as a possible way of understanding the electronic properties of the system. We report here the results of an e.s.r. study of Ru/yA1203 samples, obtained by Ru3(CO),,/lj-A1203 decarbonylation, performed with the following objectives : (i) to characterize the paramagnetic oxidation states of ruthenium originating from the metal-oxide interaction ; (ii) to study the chemical interaction between metal centres and appropriate ‘probe’ molecules (02, CO and NO).The use of such molecules may help to increase the sensitivity of the e.s.r. technique when the resonance lines of simple transition-metal ions are too broad to be assigned; it is already known that when compared to the resonance lines of noble-metal ions those characteristic of superoxo or carbonyl complexes are narrower. Moreover, the same molecules are frequent substrates in catalytic reactions involving ruthenium, i.e. the Fisher-Tropsch reaction3 and reduction of NO with CO and H2,4 so that any electronic studies of their interaction with the metal should increase our knowledge of the reaction mechanism.227 I2272 E.S.R. Study of Ruthenium on y-Alumina Ex primen t a1 Preparation of the Samples Ru/y-Al,O, samples composing 1.57 g Ru per 100 g y-Al,O, were obtained by thermal decarbonylation of the precursor Ru,(CO),,/y-Al,O, at 673 K. An Ru,(CO),, solution (containing the appropriate concentration of the cluster) in anhydrous deaerated dichloromethane was added dropwise, under an inert atmosphere, to deaerated y-Al,O, suspended in the same solvent. When the addition was complete, the mixture was allowed to equilibrate for 2 h while being stirred; the solid was then filtered off. Samples were dried in vacuo at room temperature for 2 h and stored under an inert atmosphere. Contact with air was avoided throughout the procedure.Ru,(CO),,/y-A120, gave Ru/ y-Al,O, by two different methods : (a) pyrolysis in vacuo (Ru,/y-Al,O,) and (b) reduction in an H, stream (Ru,/y-Al,O,). Using a standard gas-vacuum line the pyrolysis was carried out in vacuo ( loA3 Pa) at 673 K for 2 h, in a small flask connected to an e.s.r. tube (internal diameter 3 mm). The complete decarbonylation was monitored by i.r. spectroscopy. Reduction treatments were performed in a special tube connected to an e.s.r. cell; the H, stream (166 cm3 min-') flowed at 673 K for 2 h through a porous septum on which the catalyst precursor Ru3(C0),,/ y-Al,O, was placed. Ru,(CO),, was a pure reagent from Strem Chemicals. y-Al,O, was Ketjen grade-A from Akzo-Chemie. It was thermally pretreated in an 0, stream at 773 K for 4 h in order to avoid carbonaceous impurities and then cooled from 773 K to room temperature in an N, stream.Pretreated y-Al,O,, as well as metal- containing samples, was pyrolysed [y-Al,O,(P)] or reduced [y-Al,O,(R)] under the conditions already described. After decarbonylation and before the gas treatments Ru,,,/y-Al,O, were kept in an inert atmosphere. Gas Treatment Contact with gases (O,, CO and NO) at controlled pressures was carried out on a gas- vacuum line. Pure 0, was dried over molecular sieves. Samples were contacted at room temperature with a 26.6 kPa 0, pressure; after 5 min contact the 0, was pumped off to 13.3 Pa to eliminate the major part of the paramagnetic physisorbed gas; the e.s.r. spectrum was then recorded. Pure CO from SIO Blu Gas was used without further purification.All samples were contacted with a 26.6 kPa CO pressure at room temperature and the e.s.r. spectrum was recorded under the same CO pressure. Pure NO from SIO Blu Gas was used without further purification. NO has introduced into the samples at room temperature and at pressures of 9.3 and 26.6 kPa. Samples were then cooled at the temperature of liquid nitrogen for 30min before recording the e.s.r. spectrum; this was to enable the physisorbed NO to be converted into solid diamagnetic N,O,. Specific details are given in the results section. Apparatus A conventional X-band E-109 Varian spectrometer, equipped with an automatic temperature control, was employed. The g values were standardized against 2,2- diphenyl-1-picrylhydrazyl. Computational Methods The spin concentration of the magnetically diluted species were determined by double integration of the area of the resonance lines.The reference area was that of the Varian weak pitch (lo1, spin cm-l). The sensitive region of the e.s.r. cavity was 1 cm in lengthM. G. Cattania Sabbadini, A . Geruasini, I - 0 0 0 0 0 0 9 9 9 9 < v 1" d 0 0 0 0 m I 2 O ,c.l X x m I 53 X x d 0 0 F. Morazzoni and D. Strumolo 22732274 E.S.R. Study of Ruthenium on y-Alumina Fig. 1. X-Band e m . spectra recorded at 123 K of (a) Ru,/y-Al,O, and (6) Ru,/y-Al,O, in an argon atmosphere. Lines with asterisks are due to Fe3+ impurities. and the internal radius of the e.s.r. tube was 1.5 mm. The apparent density of the catalysts was 1 g ern-,. All e.s.r. data are reported in table 1.The spin concentration of the paramagnetic species containing ruthenium was ca. 1015 spin g-l. Results Vacuum-pyrolysed Samples Y -A12 0, The e x . spectrum of y-Al,O,(P) recorded in an argon atmosphere shows the resonance lines of Fe3+ impurities in a tetragonal field symmetry ( g = 4.2, lines marked by an asterisk in fig. 1) and of a radical species (g = 2.00) which was not observed on y-Al,O, before vacuum treatment. Treatments with 0, and CO gases did not affect the spectrum of y-Al,O,(P). After NO absorption (9.3 kPa), y-Al,O,(P) changed colour from white to pale yellow. The spectrum was recorded after the sample, contacted with NO (9.3 Pa) at room temperature, was cooled at liquid N, temperature for 30 min. The shape of the lines and the g tensor components (8, = 1.99 g,, = 1.95) [fig.4(a) later] are in agreement with the data reported by Lunsford for NO adsorbed on an A1,0, surface as A13+-N0.5 No hyperfine structure was observed. The paramagnetic species could be easily removed by degassing the samples for 1 min to 133 Pa at room temperature. A fresh adsorption of NO (9.3 kPa) restored the signal. Ru,ly-A120, The pyrolysis of pale brown Ru,(CO),,/y-Al,O, samples lead to a dark-grey dispersed metal system Ru,/y-Al,O, whose e.s.r. lines, in an argon atmosphere, were the same as those observed on the y-Al,O, support [fig. l(a)]. No paramagnetic species due to transition-metal centres were observed. The e.s.r. spectrum of Ru,/y-Al,O,, contacted with 0, (see experimental section) shows new strong resonance lines [fig.2(a)]. Two paramagnetic 0, species with axial symmetry were observed, distinguishable by theirM. G. Cattania Sabbadini, A . Gervasini, F. Morazzoni and D. Strumolo 2275 H 40 G --t Fig. 2. X-Band e.s.r. spectra recorded at 123 K of (a) RuJy-Al,O, and (b) RuJy-Al,O, in an 0,(13.3 Pa) atmosphere. different g,, values : species A (6 = 2.063) and species B (ga = 2.034). The shapes of the lines and the values of the magnetic tensor components suggest that the resonances are due to 0; attached to different positively charged centres.' Unfortunately the absence of any hyperfine interaction precludes further details. The formation of superoxide is promoted by ruthenium, as the signals described above are not present after contacting y-Al,O,(P) with 0,. Both A and B species cannot be removed by vacuum (lo-, Pa, room temperature, 1 h) or thermal vacuum treatments Two different paramagnetic species are observed in the e.s.r.spectra of Ru,/y-Al,O, contacted with CO[fig. 3(a)]. One, referred as species C, is also formed by CO contact on RuJy-Al,O, (see later), and its axial symmetry (gy = 2.045 gC, = 2.00) is clearly visible. For the other species, referred as D, only the g feature at 2.055 can be quoted with certainty. The signals of the two species are very similar in shape and width to those reported by Knozinger el al. for osmium carbonyl derivatives' on Os/y-Al,O,; we suggest, on the basis of this work, that Ru is present in our systems as a carbonyl derivative with two different metal electronic configurations, probably Ru"' and Ru', both e.s.r.-active.Different stabilities were observed for the two derivatives: D is removed by vacuum treatment (loY3 Pa, room temperature, 1 h), while C is thermally and vacuum stable (lo-, Pa, 100 OC, 1 h). The adsorption of NO gas on Ru,/y-Al,O, (contact pressure 9.3 and 26.6 kPa, see experimental section) leads to e.s.r. spectra like those of y-Al,O,(P) in the same conditions. The resonance lines are very strong (gl = 1.99 g,, = 1.95) and are not distinguishable from those assigned to NO bonded to the surface A13+ centres. However, if we lower the NO pressure to 133 Pa, the spectrum of Ru,/y-Al,O, [fig. 4(b)J resolves into signals characteristic of NO metal adducts,8 with a detectable, although not well resolved, magnetic interaction between the unpaired electron of NO and the 14N ( I = I ) nucleus.Magnetic tensor components are almost isotropic, with g,, x g,, x g,, x g, and Aiso x 35 G. The decrease in NO pressure does not affect the stability of the paramagnetic species, unlike the case on the y-Al,O,(P) support. The paramagnetic species is also stable to vacuum treatment (lo-, Pa, room temperature). It may be suggested that an Ru-NO bond interaction is active and that the NO molecule supplies a covalent contribution to the interaction with the metal centre. The Pa, 100 "C, 1 h).2276 E.S.R. Study of Ruthenium on y-Alumina 40 G 40 G Fig. 3. X-Band e.s.r. spectra recorded at 123 K of (a) Ru,/y-Al,O, and (b) Ru,/y-Al,O, in a CO (26.6 kPa) atmosphere. magnetic parameters can be interpreted as being due to Ru" nitrosyl adducts.If the contact of Ru,/y-Al,O, with NO follows that with CO, all the species stable after CO contact are removed in favour of those stable after NO contact, in agreement with the i.r. data of the literat~re.~ Reduced Samples In an argon atmosphere the e.s.r. spectrum of y-A1203(R) shows no large differences from that of y-Al,O,(P). Contacts with NO at a pressure of 9.3 and 26.6 kPa leads to the disappearance of the lines at g = 1.93. Paramagnetic A13+-NO species were not observed. With regard to this absence it is probable that the H, treatment induces a lower surface acidity of A13+ centres in y-Al,O,(R) with respect to y-Al,O,(P), so that the strength of the A1-NO bond interaction decreases. The signals at g = 1.93 are restored by degassing the samples to 133 Pa for 1 min at room temperature.In an argon atmosphere Ru,/y-Al,O, shows additional slightly anisotropic lines at g = 1.93 [fig. 1 (b)] visible both at room temperature and at - 150 "C. The attribution of this signal to Ru paramagnetic species can be excluded on the bases of its narrowness and of the appearance of the same signal, although less strong, in the y-Al,O,(R) spectrum. Once the attribution to Ru centres was excluded, further investigation of this signal was not considered. The conventional contact of 0, with RuR/y-Al,O, induces the formation of only one 0; derivative, already referred as species B [fig. 2(b)]. On the other hand species B is not stable to vacuum treatment (lo-, Pa, room temperature, 1 h), probably because of the general lower Lewis acidity of the positive surface centres in the H,-reduced catalysts.M. G.Cattania Sabbadini, A . Geruasini, F. Morazzoni and D. Strumolo 2277 J DPPH Fig. 4. X-Band e.s.r. spectra recorded at 123 K of (a) pAl,O,(P) in an NO (9.3 kPa) atmosphere and (b) Ru,/y-A120, in an NO (13.3 lo-, kPa) atmosphere. Contact with CO gave only the Ru carbonyl derivative referred as species C [Fig. A paramagnetic adduct resulting from contact with NO was also observed in RuJy- Al,O,; however, the resonances are not easily distinguishable because of the presence of support lines in the same region and because of their intrinsic lower intensity with respect to those observed on RuJy-Al,O,. 3 (4. Conclusions The paramagnetic species which become stable after Ru/ y-Al,O, was contacted with ‘probe’ molecules allow us to establish that some ruthenium centres are present on the alumina surface in oxidation states different from zero. Positively charged metal centres arise from the interaction between ruthenium and acidic centres of the y-Al,O, support.The use of a carbonyl precursor, instead of the salt, precludes the suggestion that positive Ru centres could originate from an incomplete reduction of the salt. Strong interactions are observed between the metal centre and z*-acceptor molecules ; the type and degree of stability of the surface species depend on the choice of chemisorbed molecule and on the decarbonylation method which is employed. References 1 T. Hulzinga and R. Prins, J. Phys. Chem., 1983, 87, 173 and references therein. 2 B. L. Gustafson, Mei-Jan Lin and J. H. Lunsford, J. Phys. Chem., 1980, 84, 3211. 3 M. A. Vannice, in Catalysis Science and Technology, ed. J. R. Anderson and M. Boudart, (Springer- 4 T. P. Kobylinski and B. W. Taylor, J. Catal., 1974, 33, 376. 5 J. H. Lunsford, J. Phys. Chem., 1968, 72, 4163; J. Catal., 1969, 14, 379. 6 J. H. Lunsford, Catal. Rev., 1973, 8, 135. 7 V. A. Shvets, A. L. Tarasov, V. B. Kazansky and H. Knozinger, J . Catal., 1984, 86, 223. 8 P. H. Kasai and R. M. Gaura, J . Phys. Chem., 1982, 4257; P. H. Kasai and R. J. Bishop Jr, J . Am. 9 A. Davydov and A. T. Bell, J. Catal., 1977, 49, 345. Verlag, Berlin, 1981), vol. 3, chap. 3, pp. 154-159. Chem. SOC., 1972, 94, 5560. Paper 61728; Received 14th April, 1986