J. CHEM. SOC. FARADAY TRANS., 1994, 90(18), 2809-2813 Ru-Cu/SiO, Catalysts :Characterization by FTlR Spectroscopy Carmelo Crisafulli, Rosario Maggiore and Salvatore Scire Dipartimento di Scienze Chimiche, Universita di Catania , Wale A. Doria 6,/-95127Catania , Italy Signorino Galvagno Dipartimento di Chimica lndustriale, Universita di Messina, Cas. Post. 29, 1-98166 Sant'Agata di Messina , Italy Silica supported Ru-Cu bimetallic catalysts have been studied by FTlR spectroscopy. Experiments have been carried out on two series of samples prepared from RuCI, and Ru(NO)(NO,), . The systems studied contained a total amount of metal of ca. 2 wt.%. On the monometallic Ru/SiO, samples, adsorption of CO led to an asym- metric band centred at 2036 cm-' and two small bands at a higher frequency.The band intensity was greater for the samples prepared from Ru(NO)(NO,),. CO adsorption on Cu/SiO,, leads to a band at 2117 cm-' which easily disappears upon outgassing at room temperature. On the bimetallic samples, the IR spectra indicate the formation Ru-Cu particles having a surface covered mainly by Cu; there is more Cu on the surface of the samples prepared from RuCI,. In recent years supported bimetallic catalysts have received considerable attention because of their ability to control the activity, selectivity and stability of many reactions of indus- trial interest. Among the bimetallic catalysts, Ru-Cu samples have been found to be of particular interest as model systems because the two metals are practically immiscible in bulk.The large influence of the inert copper on the activity of ruthenium indicates, however, that the two metals form bimetallic aggregates.' To investigate the nature of the bimetallic particles, a large number of investigations have been carried out on powder samples'-6 and on single crys- tal~.~.~Recent papers on supported Ru-Cu catalysts have also shown that the surface composition, and therefore the chemisorption properties and the catalytic activity, are strongly influenced by the precursor salts used and by the nature of the ~upport.~.~ In this paper we report a detailed FTIR study of CO adsorbed on Ru-Cu/SiO, samples pre- pared from Ru(NO)(NO,), . The results are compared with similar samples prepared from RuC1,. The main purpose of this work was to determine the surface composition and to find a correlation between the catalytic properties and the degree of the Ru-Cu interaction.Experimental Ru-Cu samples were prepared by incipient wetness impreg- nation of the support with aqueous solutions of Ru(NO)(NO,), or RuCl, (Johnson Matthey) and Cu(NO,), (Carlo Erba) having an appropriate concentration of metals. The concentration of salts in the solution was adjusted to yield a total (Cu + Ru) metal content of ca. 2 wt.%. The support used (supplied as powder by GRACE) was a silica gel with a BET surface area of 306 m2 g-'. After impregna- tion catalyst samples were dried at 393 K for about 24 h and reduced in a flow reactor at 673 K for 1 h with a stream of pure H,.Chemisorption of CO was measured in a conventional pulse system operating at room temperature. Pulses of 10 vol.% CO in He were used. Under these conditions negligible amounts of CO were chemisorbed by the support and on Cu/SiO, . Details of the experimental conditions used are reported elsewhere.' Chemical composition and the CO :Ru ratio of the Ru-Cu/SiO, samples are reported in Table 1. Chemisorption of CO, instead of H, , was used to avoid spill- over phenomena. Spillover of hydrogen from Ru to Cu has been reported previously.' The sample code used has the following meaning: the first two letters indicate the support used (SD = silica type D) while the three digits indicate the atomic percentage of ruthe- nium in the metallic phase.The last letters indicate the pre- cursor used [Cl = RuCl, ,N = Ru(NO)(NO,),]. For IR studies the powdered samples were pressed into thin self-supporting discs of about 25 mg cm-' and 0.1 mm thick using a pressure of 15 x lo3 bar. Pellets were evacuated and reduced in pure H, raising the temperature slowly to 673 K over a period of 2 h and held there for another 1 h. The sample was then evacuated for 30 min at 673 K and cooled at Table 1 Chemical composition and CO uptakes of Ru-Cu/SiO, samples code RU (wt.96) cu (wt.%) [Ru : (Ru + Cu)] (at.%) CO uptake /cm3(STP) (g cat.)-' CO :Ru SDlOON 2.0 - 100 4.41 0.994 SDOSON 1.7 0.3 80 3.52 0.934 SD04ON 1 .o 1 .o 40 1.76 0.794 SD020N 0.6 1.4 20 1.33 1 SD1 00Cl 2.0 - 100 0.67 0.152 SD080Cl 1.7 0.3 80 0.61 0.163 SD020CI 0.6 1.4 20 0.04 0.032 SDOOO - 2.0 0 a - Not detectable.2810 room temperature. CO was passed over the reduced sample at a pressure of 20 mbar, unless otherwise specified. Sub- sequent evacuations were performed at room temperature (RT) or at higher temperatures. The IR spectra were collected on an FTIR spectrophotom- eter Perkin-Elmer System 2000 with a resolution of 2 cm- '. No spectrum of adsorbed carbonyl species was revealed on the pure SiO, support. Data are reported as difference spectra obtained by subtracting the spectrum on the sample recorded before the interaction with CO and are normalized to the same amount of catalyst per cm2 (25 mg ern-,). Results MonometalicRu and Cu Catalysts Fig.1A shows the IR spectra of CO adsorbed at room tem- perature on the monometallic Ru/SiO, sample prepared from Ru(NO)(NO,) (SD100N) and reduced within the IR cell at 673 K for 1 h. After admission of CO [Fig. lA(a)] a very intense asymmetric band centred at 2036 cm-' (LF) is observed. A shoulder at 2083 cm-' (MF) and a band of smaller intensity at 2142 cm-' (HF) are also found. The band at higher frequency (ca. 2165 cm-') is due to 2200 2000 1800 wavenumber/cm -' Fig. 1 A, FTIR spectra of co adsorbed on the 'reduced' SDlOON sample; (a)20 mbar of co;(b) after l5 min Outgassing at RT; (') after 15 min outgassing at 473 K; (d)after 15 min outgassing at 573 K.B, FTIR spectra of co on the 'reduced' SDlml sample: (a) 20 mbar of CO; (b) after 1 min outgassing at RT; (c) after 10min outgassing at RT. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 residual CO in the gas phase. Upon evacuation at 298 K [Fig. lA(b)] the bands at 2036 and 2142 cm-' are quite stable, whereas the shoulder at 2083 m-' disappears. Evac- uation at 473 K [Fig. lA(c)] removes a large fraction of the chemisorbed CO and causes a shift to lower frequency of the LF band. After evacuation at 573 K [Fig. lA(d)] almost all chemisorbed CO is removed. The band at 2036 cm- ',which shifts to lower frequency as the coverage decreases, can be assigned to CO linearly adsorbed on metallic Ru. The coverage dependence of the carbonyl frequency on Ru/SiO, is attributed to dipoleaipole interactions between neighbouring adsorbed CO oscil-lators." However, note that before evacuation the band at 2036 cm-' is already a broad band.Therefore, it may be possible that a band at a lower frequency already exists, but it could not be assigned to Ru,CO-bridged species since a band at a significantly lower frequency (1870-1910 cm-') has been reported for these species.' The medium-frequency (MF) and high-frequency (HF) bands have been previously assigned to a small fraction of carbonyl species adsorbed on partially reduced RU.'~,'~,'~ The three bands observed in Fig. 1A(a) have an intensity ratio similar to that previously reported for Ru/SiO, prepared from Ru(NO,), .lo The spectra of CO adsorbed on the Ru/SiO, sample pre- pared from RuCl, (SD100C1) are reported in Fig.lB.-AlsO in this case three bands at 2036, 2080 and 2145 cm-' are observed. However, differences in shape and intensity are evident from a comparison with the spectra of Fig. 1A. On SDlOOCl the peak at 2036 cm-' is sharper and its intensity is lower than that found on SD100N. Note also that in contrast to what is observed on SD100N, the band at 2036 cm-' is less stable to evacuation (Fig. 1B). The differences observed between the two samples are likely to be related to different metal particle sizes. Smaller particles lead to a larger amount of Ru surface atoms and most likely to greater heter-~geneity.'~Chemisorption data (Table 1) have shown that samples prepared from Ru(NO)(NO,), chemisorb a larger amount of CO and therefore they have a larger fraction of Ru surface atoms. Formation of smaller metal particles on samples prepared from Ru(NO)(NO,), ,with respect to those prepared from RuCl, ,has been previously reported on silica- supported samples4 Fig.2A shows the IR spectra of CO adsorbed on the SDlOON sample after interaction of the sample with 0, at room temperature ('oxidized' sample). After evacuation of the sample previously used to record the spectra of CO adsorbed on the 'reduced' catalyst, oxygen (20 mbar) was passed over the sample at RT. After equilibration, CO was admitted, in the presence of O,, into the cell. The same procedure was used for all the 'oxidized' samples.Two strong bands at 2073 and 2130 cm-' are observed. Minor features are the three shoulders at 2043, 2021 and 2009 cm-'. The shift upon evac- uation of the bands at 2043 and 2021 cm-' to lower fre- quency can be explained by assuming that these bands are related to CO adsorbed on reduced ruthenium slightly per- turbed by the presence of oxygen. Removal of oxygen (by evacuation) shifts the bands to lower wavenumbers. Fig. 2B shows three well resolved peaks for the 'oxidized' SDlOOCl sample at 2138, 2083 and 2028 cm-'. For both samples, the two bands at higher frequency are quite stable to evacuation whereas the band at 2028-2030 cm- decreases in intensity and shifts to a lower frequency. These results confirm the assignment of the MF and HF bands to CO adsorbed on oxidized species.10 The band at 2028 cm-1 is likely to be related to metallic Ru still remaining on the sample after oxidation at room temperature.This is con- firmed by the shift to lower frequency observed upon evac- uation. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 2073 A 2083 B 1 2200 2000 1800 wavenumber/cm -' Fig. 2 A, FTIR spectra of CO adsorbed on the 'oxidized' SDlOON sample: (a) 20 mbar of CO; (b) after 10 min outgassing at RT. B, FTIR spectra of CO adsorbed on the 'oxidized' SDlOOCl sample: (a) 20 mbar of CO; (b) after 1 min outgassing at RT; (c) after 16 rnin outgassing at RT. Fig. 3 shows the IR spectra of CO chemisorbed at room temperature on Cu/SiO, (SDOOO).On the 'reduced' sample (Fig. 3B) only a band at 2117 cm-'is observed. The band is weak and broad and disappears upon evacuation at room temperature. According to Millar et a1.,15 this band can be ascribed to linear CO adsorbed on a stepped surface of a high-index plane of copper. As shown in Fig. 3B the fre- quency of the band is not influenced by evacuation. This is in contrast with the results of ref. 15 which show that the v(C0) position is a function of coverage. In the presence of 0, (Fig. 3A) the CO band increases and shifts to 2123 cm-'. An analogous band, attributed to CO adsorbed on CuO,' 5-' has been described previously. However, note that Hoffmann and Paul18 have reported a band at 2123 cm-' on Cu/Ru(OOl) which has been assigned to CO adsorbed on small clusters of Cu on top of Ru(OO1).Also on the 'oxidized' sample the band due to CO adsorbed on copper disappears after evacuation at RT. The frequency of CO adsorbed on 'oxidized' Cu is relatively similar to that of the HF band on Ru. However, the differences in stability towards evacuation would allow the metal sites on which CO is chemisorbed to be identified. This is in agreement with our chemisorption measurements which have shown no uptake of CO on Cu/SiO,. In fact, by using a flow system only CO irreversibly chemisorbed can be detected. 2123 A B2117 I ln cu8 I I 2200 2000 waven u mber/cm -' Fig. 3 A, FTIR spectra of CO adsorbed on the 'oxidized' SDOOO sample: (a)50 mbar of CO; (b) after 15 s outgassing at RT; (c) after 15 min outgassing at RT.B, FTIR spectra of CO adsorbed on the 'reduced' SDOOO sample: (a)85 mbar of CO; (b) after 15 s outgassing at RT; (c) after 1 rnin outgassing at RT. Bimetallic Ru-Cu Catalysts Fig. 4 shows the results obtained from the bimetallic samples prepared from Ru(NOXNO,), . Note that by increasing the Cu : Ru ratio there is a progressive decrease of the broad band at 2036 cm-'. The band at higher frequency (2132-2142 cm-') increases with the Cu :Ru ratio, then decreases with increasing Cu content. A shift of this band to lower wave- numbers is also observed. The band at 2036 cm- ' is charac- teristic of CO chemisorbed on reduced Ru. The band at higher frequency is in the same spectral region as the band due to CO adsorbed on Ru'+ and of CO adsorbed on Cu sites.However, it cannot be assigned to an Ru'+-CO species because it has quite a low stability towards outgass- ing. Fig. 5 shows the behaviour of the CO bands to evac- uation on the monometallic SDlOON and on the bimetallic SD020N samples. On the sample SDlOON the band at 2036 cm -remains practically constant upon evacuation whereas the intensity of the band at 2142 cm-' decreases to ca. 70%. A much larger decrease of the band at 2132 cm-' is observed on the SD020N catalyst. Upon the same evacuation treat- ment the intensity of the band at 2132 cm-' is reduced to <25%. This suggests that this band is related to CO chemi- sorbed on Cu surface atoms, which agrees with previous results of Knozinger and co-workers.' Moreover, a fre-quency of 2138 cm-' has been reported for single Cu atoms adsorbed on Ru(OOl)." Note also that the intensity of the band at 2132 cm-' measured on the SD020N sample is higher than that observed on the SDOOO (monometallic Cu/SiO,) sample despite the lower amount of Cu present in the sample.J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Q) Cm e n8 2200 2000 1800 wavenumber/cm-' Fig. 4 FTIR spectra of CO adsorbed on Ru-Cu/SiO, prepared from Ru(NO)(NO,), .20 mbar of CO. The effect of addition of Cu to the Ru samples prepared from RuCl, is reported in Fig. 6. On the sample SDOgOCl, the band of CO adsorbed on metallic Ru (2036 cm-l) decreases and shifts slightly to higher frequency.A strong band at 2138 cm-' is also observed. This latter band can be easily eliminated by evacuation at room temperature and therefore is assigned to CO chemisorbed on Cu. On the sample with the highest Cu : Ru ratio, only a strong band at 2138 cm-' is visible. In the spectral region where the band of CO adsorbed on Ru is expected, only a very weak and broad peak is observed. It can be therefore concluded that the amount of Ru atoms on the surface of this sample is very small which agrees with the low CO : Ru ratio measured by chemisorption. This suggests that on this sample all Ru atoms are encapsulated by Cu. Q) m gn 2200 2000 1800 2200 2000 1800 wavenumber/cm-' wavenumber/cm-' Fig. 5 A, Influence of outgassing at RT on the SDIOON sample: (a) 1 mbar of CO; (b) after 5 min outgassing at RT; (c) after 35 min outgassing at RT.B, Influence of outgassing at RT on the SD020N sample: (a) 1 mbar of CO; (b)after 5 min outgassing at RT; (c)after 35 min outgassing at RT. Q) cm ze 0 n2 m 2200 2000 wavenumber/cm-' Fig. 6 FTIR spectra of CO adsorbed on Ru-Cu/SiO, prepared from RuCl, . 20 mbar of CO. Discussion Before discussing the IR results for the bimetallic Ru-Cu/SiO, samples let us review briefly the results of cata- lytic activity obtained in the hydrogenolysis of propane over the two series of Ru-Cu/SiO, samples.20 On addition of Cu the turnover frequency (TOF) of hydrogenolysis of propane measured at 200 "C (expressed as moles of propane converted per second and per Ru surface atom) was found to decrease for both series of catalysts.20 However, the addition of Cu to samples prepared from RuCl, was much more effective; samples having a Cu : Ru ratio greater than four showed a decrease of more than three orders of magnitude in TOF.For samples prepared from Ru(NO)(NO,), the decrease in cata- lytic activity for the same variation of the Cu :Ru ratio was only one order of magnitude.,' To explain the lower catalytic activity of the Ru-Cu cata-lysts it was suggested that the active sites for propane hydro- genolysis are made of ensembles of n adjacent Ru atoms present at the surface and that the catalytic activity is related to the probability of finding such ensembles.Therefore, it has been concluded that Ru and Cu, even though they are immis- cible in the bulk state, form bimetallic crystallites. The pres- ence of inert copper on the surface of these aggregates would decrease the fraction of exposed Ru atoms and, more impor- tant, the number of active ensembles, which varies as (1 -a)" (where a is the fraction of inert copper present on the surface).,l This explains the fact that upon addition of Cu, the catalytic activity decreases much more rapidly than the Ru surface atoms. On the basis of this hypothesis it was suggested that the amount of Cu interacting with Ru (which lowers the number of active ensembles) depends, in part on the support and on the precursor used for catalyst preparation.The formation of bimetallic Ru-Cu particles having a surface covered by Cu is favoured by using RuCl, as precursor. A similar conclusion was drawn by Damiani et aL3from similar Ru-Cu samples. The formation of bimetallic Ru-Cu aggregates is confirmed by the present IR investigation. The adsorption of CO on bimetallic Ru-Cu catalysts gives spectra which are not a J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 simple combination of the bands of Ru and Cu. Further con- firmation of the interaction between Ru and Cu comes from the shift in the frequency of CO chemisorbed on Cu which is 2117 cm-' on the monometallic sample and drives up to 2138 cm-' on the bimetallic catalysts. In agreement with a previous investigation on Ru-Cu/SiO, ,l9 this shift indicates, that the Cu surface atoms are modified by an interaction with Ru which leads to a positive polarization of the Cu sites. However, in this case we would expect v(C0) to shift towards lower wavenumbers for CO adsorbed on Ru in interaction with Cu.Since this shift is not observed, it is concluded that the electronic interaction is weak. The change in the position of the band of CO adsorbed on Cu can therefore be attrib- uted to the presence of small clusters of Cu. Isolated small clusters of Cu on Ru have been reported to give a v(C0) of 2138 cm-'.I8 Moreover, note that when Cu is added to Ru the intensity of the band attributed to CO chemisorbed on Cu is greater than that for the monometallic Cu sample. This indicates that in the bimetallic particles, a large fraction of copper atoms is located on the surface.This results either from the formation of smaller particles of copper and/or through the formation of bimetallic Ru-Cu particles having a surface mainly covered by Cu. This latter hypothesis is in agreement with the lower sublimation heat and surface tension of Cu with respect to Ru which would favour a segregation on the surface of the Group 11 metal. The possibility that Cu forms smaller aggregates in the bimetallic catalysts compared with the monometallic SDOOO sample is not easy to verify. A transmission electron micros- copy (TEM) analysis of the bimetallic samples does not allow discrimination between Ru and Cu. Moreover, even in the monometallic sample, the contrast of the Cu particles in the TEM micrographs is too low to detect the Cu particles easily.The use of chemisorption techniques, such as chemisorption of N20, cannot be employed in the bimetallic samples since the Ru atoms would also interact with the probe molecule. However, note that on the SD020C1 sample, it was not pos- sible to use FTIR to detect bands due to CO adsorbed on Ru. This is in agreement with the low CO :Ru ratio mea-sured on this sample (Table l). A previous investigation by HREM of similar Ru-Cu samples prepared from RuCl, has shown that the decrease of the amount of Ru on the surface cannot be ascribed to a sintering of the Ru particles. The bimetallic catalysts do not in fact show any large Ru par- ticles.22 It can therefore be concluded that Ru and Cu form bimetallic particles on which Cu is mainly located on the surface.The FTIR results reported in this paper have confirmed that the surface composition of the bimetallic Ru-Cu par-ticles is strongly dependent on the ruthenium precursor used. The use of RuCl, favours the formation of bimetallic particles with a surface enriched by Cu atoms. The preferential surface enrichment on the samples prepared from RuCl, cannot be attributed to their lower metal dispersion. In fact, previous investigations carried out on Ru-Cu samples prepared by using the same ruthenium precursor have shown that the highest degree of interaction between the two metals and the highest Cu surface coverage are obtained on the samples with higher di~persion.~, The results reported in this communica- tion do not allow us to draw any conclusions on the role of the chemical nature of the ruthenium precursor on the surface composition.However, previous TPR (temperature- programmed reduction) experiments indicate that the differ- ent surface composition could be related to the different reduction temperature of the precursor salts. TPR experi- ments carried out on RuCl,/SiO, , Ru(NO)(NO,),/SiO, and Cu(NO,),/SiO, have shown reduction peaks at 130, 211 and 240°C, re~pectively.~~.~~ It is proposed that, owing to the lower reduction temperature of RuCl, with respect to the Cu precursor, Ru nucleation centres are formed first, and Cu atoms are deposited on top of them when the Cu salt reduction begins.In the case of the samples prepared from Ru(NO)(NO,), , the reduction of the ruthenium precursor starts at a temperature closer to that observed for Cu(NO,),/SiO, . This suggests that for the samples prepared from Ru(NO)(NO,),, the nucleation centres of Ru and Cu are formed in a very narrow range of temperatures and there- fore the bimetallic particles which are formed are likely to be more homogeneous than those prepared from RuCl, . This work was partially supported by a financial contribution from MURST and the Progetto Finalizzato Chimica Fine 11. The authors also thank Prof. G. Ghiotti for helpful dis- cussions. 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