J. CHEM. SOC. FARADAY TRANS., 1994, 90(9), 1335-1344 Manganese-promoted Rhodium/NaY Zeolite Catalysts An IF?Spectroscopic Study Tilman Beutel and Helmut Knozinger" lnstitut fur Physikalische Chemie, Universitat Munchen, Sophienstr. 1I,80333 Miinchen, Germany Horacio Treviiio, 2. Conrad Zhangt and W. M. H. Sachtler V. N. lpatieff Laboratory, Center for Catalysis and Surface Science, Departments of Chemistry and Chemical Engineering, Northwestern University, Evanston, IL, USA 60208 Carlo Dossi, Rinaldo Psaro and Renato Ugo CNR Center and Dipartimento di Chimica lnorganica , Metallorganica e Analitica , Universita degli Studi, Via G. Venezian, 21-20133 Milano, Italy Manganese-promoted rhodium/NaY catalysts have been prepared by ion exchange and subsequent H, reduction and by chemical val ' deposition (CVD) of [Rh(CO),(acac)] and Mn,(CO),.followed by thermal decomposition. Carbonyl FTIR spectra reveal significant differences between monometallic Rh/NaY and Mn/NaY and their bimetallic counterparts and they demonstrate profound effects of the synthetic methodology. The H,-reduced Rh/NaY samples prepared by ion exchange contain an appreciable amount of protons. As a consequence Rh+(CO), complexes are formed in the presence of CO. In contrast, samples prepared by CVD contain only Rh,(CO),, and Rh,(CO), 6 which can easily interconvert. The samples prepared by ion exchange and containing both Rh and Mn, exhibit bands at 1800 and 1830 cm-' characteristic of bridging CO ligands which are always accompanied by low-frequency bands at 1684 and 1700 cm-'.These bands are attributed to an Rh2-CO-Mn2+ complex. It is suggested that these q2-C0 species are formed by interaction between bridging CO ligands and Rh,-carbonyl clusters (n = 4 and 6) with Mn2+ ions via the oxygen end. This complex is totally absent in the bimetallic samples prepared by CVD of neutral organometallic precur- sors. These materials do not contain Mn2+ ions since the Mn,(CO),, precursor is decomposed on previously deposited Rho particles, thus forming bimetallic particles, the surfaces of which are presumably enriched in Mn. The number of Rh, ensembles is therefore low and bridging CO ligands are not formed in the presence of CO. The possible relevance of these results for the catalytic conversion of CO-H, mixtures on manganese-promoted Rh/NaY catalysts is discussed. The selectivity of supported rhodium catalysts towards oxygen-containing products in CO + H, reactions can be enhanced by transition-metal oxide promoter^.'-^ Man-ganese oxide has been shown to be an active promoter, and C-and 0-bonded carbon monoxide species (Rh-C-0 -+ MnX+) have been detected by IR spectros- cop^.^.^ It has been argued that these species might be crucial intermediates in oxygenate f~rmation.~q~.~ The supports used in these studies were exclusively high-surface-area oxides such as SiO, or A1,0,, while the application of zeolites has not been reported as yet.Zeolites are known for a variety of well defined cage/ channel structures and for a wide range of controllable acid- ities.The unique structural and acidic properties have led to some important industrial applications of zeolites. Metals located in the open channels or pores of zeolites are particu- larly attractive, as with these materials the metal and acid functions can be controlled independently. Acid protons interact chemically with metal clusters and impede their migration ;''-I3 moreover, the metal-proton adducts are able to act as bifunctional sites.I4 While zeolite-encaged noble metals are mostly used as bifunctional catalysts, the accessibility of metal particles to reactant molecules during catalysis is strongly determined by zeolite structures and pretreatment conditions. For faujasite zeolites, such as Y, the high tendency for metal ion migration into the small cages, such as sodalite cages and hexagonal prisms, and low accessibility of these cages to reactants under reaction conditions create the undesirable situation of ineffi- t Present address : Catalytic System Division, Johnson Matthey, 456 Devon Park Drive, Wayne, PA 19087, USA.cient use of the metal catalysts. For this reason multivalent ions such as Ca2+ or Cr3+ have been applied to block the small cages. Manganese ions can also be accommodated in the small cages; however, the extraordinary stability of the MnZ+(H,0)6 complex retains most of these ions in the super- cages of zeolite Y even after extended heating in dry helium, as demonstrated by Pearce et ~1.'~Accordingly, it is assumed that also in bimetallic samples that have been calcined at 773 K, most of the Mn2+ ions will reside in large cages.In the present study the effect of MnZ+ ions on Rh clusters in sugercages of zeolite Y is followed by FTIR spectroscopy using carbon monoxide as a probe. CO was adsorbed at liquid-N, temperature at reduced pressures so as to probe the initial state of the metal-containing zeolite free from any CO-induced morphological or structural changes. This was subsequently followed by increasing the temperature in the presence of CO. When the conventional ion-exchange tech- nique is used in the preparation of zeolite-encaged metal par- ticles, protonic acidity will be generated upon hydrogen reduction.'6 In the presence of CO, the intrazeolitic rhodium chemistry was dependent on surface acidity.The problem of obtaining non-acidic metal-in-zeolite cata- lysts has been approached recently in a completely different way, using neutral organometallic complexes as metal precur- sors. The organometallic molecule can be introduced selec- tively into zeolite cages uiu chemical vapour deposition (CVD), without altering the distribution of intrazeolitic cations. Metal particles are then simply formed oia thermal removal of the volatile ligands under reducing conditions.' ' For comparison purposes with the ion-exchanged materials, volatile organometallic complexes, such as [Rh(CO),(acac)] (acac = acetylacetonate) and Mn,(CO),, , were deposited into zeolite via CVD.These systems were characterized by diffuse reflectance FTIR spectroscopy (DRIFTS) of adsorbed CO at room temperature. Experimental Sample Preparation by Ion Exchange The starting zeolite was NaY (LZ Y-52, Linde, lot No. 968087061020-S-8) containing 56 Na+ ions per unit cell. The Mn/NaY zeolite was prepared by dropwise addition of a 0.20 mol dm-, aqueous solution of Mn(NO,), * 4H20 (Aldrich) to a 200 ml g-' slurry of NaY and doubly deion- ized water at room temperature for 24 h. The pH of the slurry at the end of the exchange was 6. This was followed by calcination in a flow of pure oxygen (250 ml min-') with a temperature ramp of 0.5 K min- 'from room temperature to 773 K and keeping the temperature at this value for 6 h.This sample is denoted MnY. From ICP analysis, it contains ca. 15 Mn2+ and 25 Na+ ions per unit cell (see Table 1). Rh-containing NaY samples were prepared by dropwise ion exchange in an aqueous solution of [Rh(NH,),Cl]Cl, for 24 h at 355 K. The equilibrium between [Rh(NH,)5C1]2+ and [Rh(NH,),H20l3+ is shifted toward the latter at this temperature, thus minimizing the content of C1 in the zeolite.'* The [Rh(NH3)H,0I3+ ions are retained in the supercages because of their size. Two samples with Rh con- tents 0.4 and 3 wt.% were prepared. They were denoted RhY04 and RhY3, respectively. These samples were air dried at room temperature for 12 h, but not precalcined. For preparation of the bimetallic sample, NaY was first loaded with manganese and calcined as described above.Subsequently, the MnY sample was exchanged with [Rh(NH,),C1]2+. This sample will be denoted RhMnY, while the same sample after reduction at 523 K will be indicated by the acronym RhMnY523. Sample notations and metal contents are summarized in Table 1. Sample Preparation by Chemical Vapour Deposition [Rh(CO,)(acac)] (acac = acetylacetonate) and Mn,(CO),, have been purified by sublimation in oucuo (lo-, mbar) at 363 and 343 K, respectively. The NaY zeolite has been pretreated in flowing argon at 673 K prior to the deposition of the organometallic precur- sor. Deposition of [Rh(CO),(acac)] and Mn2(CO),, has been conducted under CVD condition^'^ in a U-tube glass reactor in flowing argon at 353 and 378 K, respectively.After deposi- tion, the supported precursors were decomposed in flowing hydrogen from room temperature to 673 K with a heating rate of 10 K min-'. The two CVD-based monometallic samples, both having a final metal loading of 1 wt.%, will be denoted throughout the text as RhY(CVD) and MnY(CVD). A bimetallic Rh-Mn/NaY sample has been similarly pre- pared by a two-step procedure. First, [Rh(CO),(acac)] was deposited from the vapour phase into NaY zeolite and subse- Table 1 Catalyst notations and compositions sample Mn content (wt.%) Rh content (wt.%) RhY04 - 0.4 RhY3 RhY (CVD) -- 3 1 MnY 4.9 - MnY (CVD) RhMnY 1 4.6 -2.8 RhMnY (CVD) 1 1 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 quently decomposed to metallic particles in flowing hydrogen as described above.After purging in argon to remove physi- sorbed hydrogen, Mn,(CO),, was introduced from the vapour phase, and the resulting material heated in flowing hydrogen with the same temperature profile as previously indicated. The final catalyst, with a 1 wt.% metal loading both for rhodium and for manganese, will be denoted as RhMnY(CVD). Transmission IR Spectroscopy For transmission IR measurements the samples were pressed at 200 bar into self-supporting wafers and placed in a cell which can be cooled to 83 K and which can be evacuted to the pressure of mbar. The samples were treated in situ in 0, at a flow rate of 100 ml min-' at 773 K using heating rates between 0.5 and 8 K min-'.After 20 min purging with N, at 773 K the samples were cooled to 330 K and then reduced for 20 min in a flow of H, (100 ml min-') at 523 K at a heating rate of 8 K min-'. Prior to adsorption of COY the wafer was evacuated for 1 h at the temperature of reduction. When CO was dosed at 85 K, the sample adsorbed the first portions almost completely such that the total pres- sure dropped below 0.1 mbar. In order to control the amount of CO added, a volume of 489 ml was filled at a given pres- sure of CO at room temperature and then expanded into the cold cell (total volume 883 ml). The corresponding pressures within the cell as room temperature were measured; these are denoted as room-temperature CO pressures. Transmission spectra were recorded on a Bruker IFS-66 FTIR spectrom- eter.64 scans were accumulated, corrected for background absorptions and gas-phase contributions. Diffuse Reflectance IR Spectroscopy (DRIFTS) In situ diffuse reflectance spectra were recorded on an FTS-40 Digilab spectrophotometer fitted with a Harrick DRA-2CI diffuse reflectance attachment and a Harrick HVC cell which allows the spectra of the sample in granular form to be recorded under controlled temperature and pressure condi- tions. Samples prepared by CVD of the organometallic precursor were transferred into the DRIFT cell and reduced in a flow of H, at 673 K at a heating rate of 10 K min-'. DRIFTS mea- surements of adsorbed CO were carried out at atmospheric pressure in a flow of CO at room temperature. All spectra were recorded against a KBr standard at 4 cm-' resolution with accumulation of 100 scans per spec- trum.The spectra were converted into the Kubelka-Munk function and plotted against wavenumber. Results and Discussion Ionexchanged Zeolites Hydroxy-group Stretching Region Fig. 1 shows transmission FTIR spectra of Rh- and Mn- containing zeolite samples after in situ reduction in flowing H, at 523 K for 20 min followed by evacuation at 523 K for 1 h. The spectra were recorded at 87 K. The NaY zeolite does not contain any characteristic zeolitic hydroxy groups, except silanol groups located at the external surface of the crys- tallites. These groups give rise to a band at 3749 cm-'. Reduction in H, results in the formation of protons which are attached to bridging oxygens and give rise to the typical 0-H stretching frequencies at 3649 (high frequency, HF) and 3550 (low frequency, LF) cm-' that are attributed to OH groups located in supercages and sodalite cages, respec- J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 d-3645 I 21771 /I 10.050.5 10 t10M I I I I 3900 3700 3500 3300 3100 wavenumber/cm- Fig. 1 Hydroxy-group stretching spectra of RhY04 (a), RhY3 (b), MnY (c) and RhMnY523 (d)recorded at 87 K. Sample pretreatment: oxidation at 773 K for 1 h followed by reduction in flowing H, at 523 K for 20 min and evacuation at 523 K for 1 h. ti~e1y.l~The development of these bands can be seen in Fig. l(a) and (b).Sharp bands are formed at 3653 and 3649 cm-' for samples RhY04 and RhY3, respectively.The intensity ratio of these bands is very close to the ratio of the Rh load- ings of the two materials. In addition, a weaker broad LF band at 3560 cm-' is observed for RhY3. The 0-H stretching spectra observed for the Mn2+-containing samples are also shown in Fig. l(c) and (4. Besides the characteristic bands of the SiOH groups, the Rh-free sample, MnY, exhibits a band of Al(0H)Si groups at 3648 cm- ', with relatively low intensity. In addition, very weak bands can be detected at 3694, 3673 and 3609 cm-'. These bands do not appear in any Mn2+-free samples but are consistently observed (although with varying relative intensities) in all samples containing Mn2+ ions.Their occurrence therefore suggests that they are related to the presence of Mn2 + ions, probably in different coordination environments. The position of the weak band at 3609 cm-' is in the frequency range characteristic of bridging OH groups. This band may therefore tentatively be attributed to Mn(0H)Mn or Mn(0H)Al species. The reduced samples containing Rh in addition to Mn [Fig. l(d)], clearly develop the characteristic HF and LF bands at 3642-3645 and 3545 cm-' of Al(0H)Si groups in supercages and soldalite cages, respectively. The behaviour of the 0-H stretching bands on low-temperature adsorption of CO is complex and will be re-ported independently. Carbonyl Stretching Region Fig. 2 shows the carbonyl stretching region of CO adsorbed on MnY at 88 K.The major feature observed at the lowest CO pressures is a narrow band at 2177 cm-'. This band must be attributed to CO coordinated to Na+ cations, as proposed by Bordiga et a1.*' who observed a carbonyl band at 2178 em-' on Na-ZSM-5. The band shifts to lower wave- numbers (2173 cm-') and becomes broader as the CO con-centration in the zeolite pores increases. This may be due to the formation of Na+. -CO complexes with Na+ ions in dif- ferent positions, but solvent-like effects may also contribute. Minor contributions to this band may also be due to H- bonded CO molecules;" the OH population of this sample, however, is very low. , I I 2230 2190 2150 2110 2070 wavenumber/cm- Fig.2 Carbonyl stretching spectra of CO adsorbed on MnY at 87 K and increasing pressures: (a) 0.05, (b) 0.3, (c) 0.55, (6)0.8, (e) 1.05 mbar In addition to the dominant band at 2177 mi-', a weak band at 2209 and a shoulder at 2188 cm-' can be discerned. These are presumably due to A13+- -CO complexes" which result from the presence of some extraframework A13+ sites. A weak pair of bands at 2128 and 2118 cm-', which coalesce at increasing CO pressures, is also seen in the spectra of Fig. 2. The 2128 cm-' component can be interpreted as the 13C0 satellite to the principal band at 2177 cm-' at natural I3C abundance (theoretical isotope shift, -49 cm-'). The origin of the second component at 2118 cm-' is still an open ques- tion. Bands close to this band position have been observed by Angel1 and S~haffer~~ who proposed an interaction of CO via the oxygen end with lattice oxygen atoms of the zeolite.Bordiga et al." suggested an q2-interaction of CO with Na+ ions and a second Lewis acid site. Fig. 3 shows the carbonyl spectra of RhY04 as they develop in 50 mbar CO at increasing temperatures. At 85 K there is a broad feature around 2100 cm-' in the region of terminally bonded CO on rhodium. No bridging CO is al C mf! $P 2250 2150 2050 1950 1850 1750 1650 1550 wavenumber/cm-Fig. 3 Carbonyl spectra of RhY04 after 1 h oxidation at 773 K and 20 min reduction in H, at 523 K followed by 1 h evacuation at 523 K: (a) after admission of 0.05 mbar CO at 85 K. (6)-(f)50 mbar CO with rising temperature: (6)223, (c) 263, (d)283, (e) 303, (f)323 K.detectable which indicates high metal dispersion. When the sample is warmed in 50 mbar CO significant changes occur only as the temperature rises beyond 223 K. At 263 K [Fig. 3(c)] bands at 2122, 2102, 2086, 2050, 2026 and 1831 cm-' are formed. The bands at 2102 and 2026 cm-' can be as- signed to an Rh gem-dicarbonyl structure.24 The formation + of these species in the zeolite pores is consistent with the high Rho dispersion and the presence of protons. As the tem- perature was raised to 323 K the band at 2122 cm-' is reduced in intensity and a new band at 2117 cm-' becomes evident. The bands at 21 17 and 2050 cm-'can be interpreted as a second dicarbonyl species, which is in line with the results of Bergeret et dZ5who claimed the existence of two rhodium dicarbonyl structures with different support inter- actions in the zeolite framework.Bands at 2122 and 2086 cm-',which start to develop at 223 K and decrease at tem- peratures higher than 263 K, may be assigned to Rh"+(CO),.26*27 Near room temperature this species loses one CO ligand thereby being transformed into an Rh+(CO), species. The narrow absorbance at 1831 cm-' is character- istic of bridging CO on metal clusters. Generally, the number and position of bands for a distinct carbonyl cluster depend strongly on the support. A band at 1830 cm-l has been assigned by Gelin et dz8to a hexanuclear rhodium cluster in Nay, whereas Rode et aL2' ascribed a band at 1834 cm-' to a tetranuclear rhodium cluster in Nay.Ichikawa and co- worker~~~assigned the band at 1830 cm-' to edge-bridging CO of the unstable isomer Rh,(CO),,(p(,-CO)flaY. It was thermally transformed into the isomer Rh,(CO)' 2@3-CO)4, in which the triply bridging CO ligands were characterized by a band at 1760 cm-'. Terminal bands centred at 2088 and 2086 cm-' were con- sidered to be characteristic of the hexa- and tetra-nuclear cluster species, respectively. In Fig. 3, we observed a shoulder around 2090 cm-',which grows in concert with the bridging band at 1831 cm-'. It seems plausible to assign these bands to the same cluster molecule. The antisymmetric stretching vibration of the Rh+(CO), species at 2050 cm-' is much more intense than the symmetric vibration at 2117 cm-'.Moreover, the band at 2047 cm-' in Fig. 3(c) is shifted by 3 cm-' downwards when the temperature is raised from 263 to 283 K. This is reasonable if one assumes that there is a second contribution to that band from the cluster species. A band between 2040 and 2050 cm-' is often reported to be characteristic of rhodium carbonyl cluster^,^ 'v3 1-33 although it is not specific for a certain nuclearity. Hanlan and O~in,~~using matrix-isolation techniques, reported bands at 2060 (s), 2040 (s, sh), 1852 (m) and 1830 (m) cm-' as being characteristic of an Rh,(C0)8 cluster in its bridge-bonded form. The formation of this species requires either high CO pressures, as studied by Wh~man,~' or low temperatures.At 220 K this species is transformed into Rh,(CO) ,under high-vacuum condition^.^^ We find bands at 2050, 2026, 1879 and 1830 cm-' which grow in parallel. However, these bands start to develop at 263 K. This tem- perature is rather more in the stability regime of Rh,(CO),, than of Rh,(CO), . Therefore, the cluster transformations of Rh,(CO),, into Rh,(CO), 6 occur readily around room temperature under vacuum condition^.^' In the presence of CO, however, the tetranuclear species is stable.33 The nuclearity of the cluster species in RhY04 cannot be determined unambiguously, but Rh4(C0),, is the most likely candidate. Fig. 4 shows the carbonyl spectra of RhY3 with increasing temperature. In contrast to RhY04 there is a broad feature at 1928 cm-',which is typical for doubly bridging CO on larger Rho particles.At 85 K a weak band is observed at 1830 cm-' which might be indicative of triply bridging CO. Its intensity J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 AIr"" 2250 2150 2050 1950 1850 1750 1650 1550 wavenumber/cm-' -IH $1 I l'1'1'1'I'I'I'l 2250 2150 2050 1950 1850 1750 1650 1550 wavenumber/cm-' Fig. 4 Carbonyl spectra of RhY3 after 160 min oxidation at 773 K followed by 20 min reduction at 523 K and 1 h evacuation at 523 K. A: (a) after admission of 0.05 mbar CO at 85 K, (b)at 0.7 mbar CO pressure, (c) 2.5 mbar at 143 K, (d) 3.7 mbar at 183 K, (e)50 mbar at 213 K, (f)50 mbar at 233 K. B, Warm-up in 50 mbar CO: (a) at 233 K as in A(f), (b)at 243 K, (c) at 253 K, (6)at 263 K, (e)at 273 K, (f) at 283 K and (9)at 296 K.decreases and a new band grows in at 1810 cm-' when the temperature is increased to 233 K. On further warming, the latter band disappears at 263 K and a bridging cluster band at 1830 cm-' develops, which keeps growing with rising tem- perature. The corresponding linear CO band at 2091 cm-' is clearly visible at 323 K when the tricarbonyl band intensities at 2122 and 2088 cm-' have been depleted. At the same time the evolution of two pairs of twin bands located at 2101 and 2024 cm-' and at 2114 and 2047 cm-' is observed, which are attributed to two Rh+(CO), species with different support interactions as mentioned above and reported earlier.24*2 Fig.5 demonstrates the influence of a further temperature increase (323-523 K) on the cluster chemistry of RhY3. When the sample is heated to 423 K in 50 mbar CO for 10 min [Fig. 5(c)], the bands at 1830 cm-' and 2090 cm-' disappear and new cluster bands at 1760 cm-' and 2086 cm-' grow in. The latter bands seem to belong to one species, most prob- ably an Rh6(CO),, cluster. The hexanuclear cluster is still stable at 523 K [Fig. 5(d)] but is decarbonylated under vacuum at that temperature. Hence, the temperature- and J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 2250 2150 2050 1950 1850 1750 1650 1550 wavenumber/crn -Fig. 5 Carbonyl spectra of RhY3 after 10 min exposure to 50 mbar CO at (a) 323, (b) 373, (c) 423, (d) 523 K pressure-dependent changes observed on RhY3 confirm the assignment of the tetranuclear cluster bands in RhY04. Fig.6 shows the carbonyl spectra of RhMnY523 as the sample is warmed from 85 to 333 K in the presence of CO. Besides the bands at 2168 and 2121 cm-' due to CO inter-acting with Na+ cations and/or acidic protons there is a broad feature in the bridging carbonyl region around 1870 cm-', which indicates bridging CO ligands on Rho metal particles. The corresponding linear CO species give rise to broad bands at 2079 cm-' and at 2050 cm-'. At CQ. 183 K [Fig. 6A(c)], new peaks at 2125 and 2089 cm-' appear and grow in parallel when the system is heated to 333 K [Fig. 6B(f)]. This behaviour is in contrast to that of unpromoted Rh catalysts for which the respective bands at 2122 and 2088 cm-' vanished at room temperature.This is evidence for the presence of additional species in the Mn-promoted Rh samples giving rise to the bands at 2125 and 2089 cm-'. In the bridging carbonyl region we observe the evolution of bands at 1798 and 1684 cm-'. While the former band is typical for bridging CO on rhodium carbonyl clusters, the second band might be attributed to a bridging CO group which forms an additional bond to an adjacent Lewis acid site via the oxygen end. Such tilted CO species have been suggested to exist on silica-supported Mn-promoted rhodium catalyst^.^.^^^^ At 253 K [Fig. 6B(b)], the band at 1799 starts to decrease and a new absorption at 1831 cm-' grows in, together with a small peak at 1869 cm-'.Simultaneously, the band at 1689 cm-' is shifted up to 1700 cm-'. The growth of the band at 1830 cm-' is accompanied by the evolution of sharp peaks at 2098,2052 and a shoulder at 2039 cm- '. The sharpness of the bands at 2125 and 2089 cm-' sug-gests the formation of a well defined carbonyl species. Their assignment to a cluster species, however, is doubtful for several reasons. First, a band as intense and high in energy as that at 2125 cm-' has not yet been observed to our know- ledge for a rhodium carbonyl cluster. Secondly, several changes in the bridging carbonyl region in Fig. 6 indicate that the cluster size and/or geometry is altered with increas- ing temperature. This should influence the position of the linear CO ligands as well.The bands at 2125 and 2089 cm-', however, do not shift. Note that the intensities of the pair of bands at 2052 and 2039 cm-' begins to increase when the bridging CO species at 1831 cm-' stops growing. This sug- gests that the linear bands at 2052 and 2039 cm-' may belong to another cluster species which does not contain I 2250 2150 2050 1950 1850 1750 1650 1550 wavenurnber/cm-t B 0.2 2250 2050 1850 1650 wavenumber/cm-' Fig. 6 A, Carbonyl spectra of RhMnY523 after 1 h oxidation at 773 K, 20 min reduction at 523 K and 1 h evacuation at 523 K: (a) after admission of 0.05 mbar CO (room-temperature pressure) at 85 K, (b) at 0.7 mbar pressure at 85 K, (c) at 2.8 mbar and 183 K, (d)at 5 mbar and 203 K, (e) at 50 mbar and 223 K.B, Warm-up spectra in the carbonyl region of RhMnY523 at a CO pressure of 50 mbar: at (a) 243, (b) 255, (c)273, (6)293, (e) 313, (f)333 K. bridging CO ligands. Besides, symmetry lowering may lead to additional bands. On the other hand, a stepwise carbon- ylation of a given cluster framework can be ruled out. The adsorption of linearly bonded CO onto a bridging carbonyl cluster should exert significant energy shifts on the bridging CO ligands. The band position of the latter, however, does not change. Fig. 7 demonstrates the effect of cooling the sample in the presence of 50 mbar of CO. When the temperature is decreased from 333 to 233 K [Fig. 7C(u)],the bands at 2125 and 2088 cm-' keep growing, while the peaks at 2052 and 2039 cm-l decrease in intensity.Only further cooling to 123 K [Fig. 7B(b)] leads to the depletion of the band intensity at 2088 cm-'. On the other hand, the bands at 2098, 1865, 1830 and 1700 cm-' do not change in intensity or position during the entire cooling procedure. From this fact it may be con-cluded that the cluster species which is characterized by these bands has a constant nuclearity, symmetry and coordination. This, in turn, proves that the bands at 2088, 2052 and 2039 cm-' cannot be attributed to one cluster species alone. 2250 2150 2050 1950. 1850 1750 1650 1550 wavenumber/cm-2250 2150 2050 1950 1850 1750 1650 1550 wavenumber/cm-l Fig. 7 A, Cooling spectra of RhMnY523 at 50 mbar CO pressure at (a) 333, (b) 293, (c) 233 K.B, Cooling spectra of RhMnY523 at 50 mbar CO pressure at (a)233, (b) 123,(c) 173 K. Samples prepared by CVD IR carbonyl spectra of the samples prepared by CVD have been measured by the DRIFTS technique, since it is hardly possible to prepare self-supporting wafers for transmission spectroscopy without exposing the samples to air. It has been demonstrated in advance using standard Rh catalysts that both techniques gave identical carbonyl spectra when compa- rable treatment conditions were applied. It might be argued that the precursor complexes used in the CVD technique would be deposited on the external zeolite surface only. However, the methodology described earlier17 has been shown to produce materials with organometallic precursor molecules being selectively depos- ited in the cages of large-pore Y-zeolites.Evidence for this is also provided by the carbonyl spectra of Rh, and Rh6 car- bonyl clusters shown in Fig. 9 (see later). These spectra are practically identical to those reported by Rao et ~1.~'who prepared their samples by ion exchange. Rhodium clusters located on the external surface of zeolite crystals have totally different spectra, which closely resemble those of rhodium carbonyl clusters adsorbed on amorphous silica supports.33 Hence, we are confident that the organometallic precursor complexes and the species derived thereof are located inside the zeolite cages in the present samples. J. CHEM. SOC. FARADAY TRANS., 1994, VOL.90 MnY(CVD) The carbonyl spectra obtained for the MnY(CVD) sample after exposure to CO at 1 bar and 300 K are significantly different from those described above for the ion-exchanged sample MnY. In particular, bands characteristic of man-ganese carbonyl species are developed in contrast to the ion- exchanged sample, suggesting an entirely different manganese location in the zeolite cages. After 10 min contact with CO, two bands at 2044 and 1948 cm-' begin to appear. Within a 60 min period, these two bands are fully developed, together with additional weaker features at 1959, 1985, 2014 and 2060 cm-' (Fig. 8). The appearance of such bands of the recarbonylated MnY(CVD) sample is likely to be attributed to the formation of zeolite- entrapped Mn(O)(CO), and Mn+(CO), species (see Table 2).However, from simple comparison with the IR spectra of pure reference compounds, it is still not possible to identify unequivocally the exact chemical nature of the entrapped car- bonyl being formed. RhY(CVD) Decomposition to metal of the [Rh(CO),(acac)] complex, vapour-deposited inside the zeolite, was easily performed in an H, atmosphere by thermal removal of ligands without the formation of protonic acidity, i.e. 4 -[Rh(CO),(acac)/NaY + 3 H, 1 Rhi/NaY + Hacac + 2CO n (1) On contacting this sample with CO at room temperature (Fig. 9), bands at 2095 and 1832 cm-' start to appear in the DRIFT spectrum, together with weaker features around 2060 and 2020 cm-'. With increasing time in flowing CO, the 2095 cm-' band grows in concert with other weaker bands at 2060, 2043, 2020, 1987 and 1947 cm-'.In parallel, a new bridging band at 1763 cm-' appears after 180 min.This spectrum is then stable in flowing nitrogen or hydrogen at room temperature. Note that the bands typical for rhodium dicarbonyl structures (2102,2026 cm- 'and 21 17,2050 cm-') are totally absent with this material. This confirms the pre- vious suggestion that the 2090 cm-' shoulder in the IR spectra of Fig. 3 and 4 is related to the bridging CO bands at 1830 and 1763 cm-', being attributed to Rh, and Rh6 entrapped carbonyl clusters. As shown previously, the CO-induced disruption of Rh clusters is, in fact, an oxidation of Rho by protons yielding Rh' ions and H,.37 The Rh+ ions react with CO to form Rh+(CO),.Evidently, this process is impossible in the absence of protons; it is therefore quite reasonable that in the CVD samples only neutral carbonyl clusters are formed. Their interaction with cage walls is a likely cause for the upward shift in the IR bands of the terminal CO groups with respect to the spectra of the unsupported clusters, and also for the downward shift of the bands in the bridging CO region. Table 2 Stretching frequencies (cm-') of some Mn carbonyls" Mn,(CO) 10 2044 m, 2012 s, 1982 m Mn,(CO),(PPhMe,) 2094 w, 2016 s, 1993 vs, 1969 sh, 1938 m Mn,(CO),(PPh,Me), 1983 w, 1954 vs Mn(CO),+ 2090 CMn(CO)sINa 1912 s, 1883 s, 1803 s Mn(CO),Cl 2139, 2083, 2055, 1998 Mn(W,(?-C,H,) 2028, 1945 " Ref.38; my medium intensity; s, strong; vs, very strong; sh, shoul-der; w, weak. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 60-50-C .-*.' C= 40-Y C ?2 30-0)9 3 Y 20-10-t I I I I I I I I I I I 2300 2250 2200 2150 2100 2050 2000 11950 1900 1850 1800 1750 wavenumber/cm-Fig. 8 DRIFT spectra of MnY(CVD) in flowing CO at room temperature taken after (a)10, (b)30, (c) 60min After recarbonylation of RhY(CVD) a temperature-programmed desorption experiment in flowing H, was carried out by monitoring the intensities of the three most significant features in the DRIFT spectrum, namely the ter- minal band at 2094 cm-' and the two bridging ones at 1834 and 1763 cm-', as a function of increasing temperature (Fig.10). The rapid decrease of the 1834 cm-' band is paralleled by the increase in intensity of the 1763 cm-' band. This transformation does not alter significantly the intensity of the terminal band at 2094 cm-', thus precluding the possibility that an extensive decarbonylation of the intrazeolitic carbon- yl clusters is taking place at these low temperatures. At about 373 K, the transformation of the 1834 cm-' band into the 1763 cm-' band is complete. A new IR spectrum with bands at 2094, 2069 and 1763 cm-' is obtained, in very good agree- ment with that of pure Rh,(CO),, inside the Con-version of Rh,-carbonyl to Rh,-€arbOnyl is much easier in u, m 0 N 8-0 0C Nfl-.-tj 6-2 Y-Y Cr' CL 4-r2 Y 2-an atmosphere of H, or N, than in CO, since it is accompa- nied by evolution of carbon monoxide: 3Rh4(C0),, -+ 2Rh6(CO),, + 4CO.(2) By increasing the temperature further, both bands decrease in intensity, until they disappear completely at cu. 470 K. RhMnY(CVD) After flowing CO over the sample at room temperature for 5 min, a set of bands starts to appear at 2071, 2036, 2005 and 1946 cm-' (Fig. 11). At this temperature, the carbonylation process proceeds slowly and requires almost 120 min until the spectrum is fully developed, with additional small bands appearing at 2059, 2014 and 1962 cm-' and shoulders at 2042 and 2020 cm -'. rgI\ h \/ Al II 2300 2250 2200 2150 2100 2050 2000 1950 1900 1850 1800 1750 wavenumber/cm-' Fig.9' DRIFT spectra of RhY(CVD) in flowing CO at room temperature taken after (a)20, (b)90, (c 180 min J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 12*00I"5 9.60 2.40 cr \ ,0.00 I I OYUO 250 300 350 400 450 500 ternperature/K Fig. 10 IR band intensities as a function of increasing temperature in flowing H, of the RhY(CVD) sample after contacting with CO at room temperature for 180 min: (+) 2094, (A) 1834, (0)1763 cm-' 0 A comparison of these spectra with those of the ion- exchanged RhMnY sample of Fig. 6B immediately shows that both the high-frequency bands at 2125, 2088 cm-', and the bridging band at 1831 cm-' are dramatically reduced in intensity. In addition, absorption bands due to C0.a .MnX+ interactions were completely absent.This substantial change in the IR spectrum thus indicates that a completely different chemical interaction between Rh and Mn is taking place in the CVD-based RhMnY sample. The Mn,(CO),, precursor is introduced and decomposed onto the entrapped rhodium clusters previously formed inside NaY zeolite. The effect of metal-promoted decomposi- tion of vapour-deposited carbonyls is well documented in the literat~re.,~In order to ascertain the stability of the carbonyl species giving rise to the complex band pattern in the 2080- 1900 cm -region, a temperature-programmed desorption experiment was carried out, under the same experimental conditions as used for sample RhY(CVD) (Fig. 12). Decarb-P 0 0N 2300 2250 2200 2150 2100 2050 2000 1950 1900 1850 1800 1750 wavenurnber/crn -' Fig.11 DRIFT spectra of RhMnY(CVD) in flowing CO at room temperature taken after (a) 5, (b) 10, (c) 60, (d) 120 min m70. me mo ON 60. .-5 50-c C m2 hw-40-j 30-I]3 Y 20-10-t I I I I I I I I I I I 2300 2250 2x10 2150 2100 2050 2000 1950 1900 1850 iaoo 1750 wavenumber/cm-' Fig. 12 DRIFT spectra as a function of increasing temperature in flowing H, of the RhMnY(CVD) sample after contacting with CO at room temperature for 120 min: at (a) 318, (b) 343, (c) 353, (d) 363, (e) 373, and (f) 423 K J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 onylation of the supported species begins around 343 K and is terminated at around 373 K.In addition, from plotting the band intensities against temperature, it can be seen that the intensities of all bands decrease with increasing temperature, without any substantial frequency shift occurring during the decarbonylation process. This behaviour indicates that a single carbonyl species is formed by contacting the RhMnY(CVD) sample with CO at room temperature. This suggests to us that Rh-Mn bimetallic particles are being formed. The reactivity of the CO-covered Rh-Mn particles is therefore very different from that of the pure Rh clusters, indicating once again the intimate interaction between man- ganese and rhodium in the RhMnY(CVD) sample, and perhaps the formation of bimetallic clusters. Unfortunately, no molecular Rh-Mn carbonyl clusters are known in the lit- erature; a direct assignment of the intrazeolitic carbonyls is therefore impossible at present.The very low intensities of the IR bands at 2094, 1834 and 1763 cm-' in Fig. 9 and at 2044 and 1948 cm-' in Fig. 8, i.e. of the characteristic features for CO-covered Rh only and Mn only samples indicate that no discrete monometallic aggregates are present upon decompo- sing Mn,(CO),, onto prereduced Rh particles inside Y zeolite. Conclusions The CO FTIR spectra presented in this paper reveal signifi- cant differences between the monometallic zeolite-supported samples Rh/NaY or Mn/NaY and their bimetallic counter- part Rh-Mn/NaY; they also demonstrate profound effects of the preparation methodology. As for Rh/NaY, the samples prepared by ion exchange, fol- lowed by reduction, contain an appreciable concentration of protons; by consequence Rh ions coexist in equilibrium + with Rhi clusters.Upon admission of CO this equilibrium is shifted in favour of the Rh+ ions as a consequence of the high stability of the gem my1 ion, Rh+(CO),. While this complex ion is form lution of H, takes place, as was shown previou~ly.~~ ected, the gem-dicarbonyl ion is completely absent in the spectra of the proton-free samples prepared by CVD; only the neutral clusters Rh,(CO),, and Rh,(CO),, are visible. These clusters can interconvert rather easily; the Rh,(CO),, cluster with CO : Rh = 3 : 1 prevails at high CO pressure, but in the absence of gaseous CO the Rh,(CO),, cluster with co :Rh = 8 :3 predominates.As for Mn/NaY, only carbonyl clusters of Mn" are detected; these can be prepared only by the CVD technique, because it is impossible to reduce zeolite-entrapped Mn2+ ions with H, under the conditions used here. As mentioned in the Introduction, the main focus of the present work is on the bimetallic samples, containing both Rh and Mn. A remarkable feature of the samples prepared by ion exchange is the interconversion of bridging CO with a band at 1800 cm-',into a species absorbing at 1830 cm-'. These bands are always accompanied by low-frequency bands at 1684 and 1700 cm-'. As the latter bands are totally absent from the CO FTIR spectra of the monometallic samples, they apparently reveal some chemical interaction of Rh, Mn2+ and CO.Moreover, the absence of these bands from the spectra of the samples prepared by CVD shows that these bands discriminate between Mn" (or Mn+?) and Mn2+. The latter ion is absent in the CVD-prepared samples but predominates in the samples prepared via ion exchange and reduction. This gives high potential relevance to the bands at 1684 and 1700 cm-'. From the work of Pearce et a2.l' it follows that the majority of the Mn2+ ions will be located in super- cages and sodalite cages; only a minority is trapped in hex- agonal prisms. As the reduced Rh, clusters are also located in supercages, it appears that carbon monoxide probes for adducts of Rh, clusters with Mn2+ ions; the IR bands that are so exclusive for the bimetallic system prepared by ion exchange are typical for this interaction. The 1700 cm-' band is supposed to be characteristic of an q2-C0 species which bridges two atoms of an Rh, cluster and simulta- neously coordinates to an Mn2+ cation via the oxygen.Since there is a Coulomb interaction between Mn2+ and the uncompensated negative charge of the zeolite wall, this Rh2-C-O-Mn2+ complex will be anchored to the cage wall. An alternative attribution of these low-frequency bands to formyl species, which have characteristic vibrational modes near 1700 an-', must be ruled out since the corresponding C-H stretching mode near 2720-2760 cm-' could never be detected. The nuclearity of the Rh, clusters is an open question at this stage, but EXAFS work is in progress to clarify this aspect.The conversion of Rh, to Rh6 clusters is expected to cause a downward shift of the bands due to bridging CO at 1800 and 1684 cm-'. FTIR spectra of Rh6(CO),, in NaY have been reported to include a strong band at 1756 cm-', suggesting the presence of a three-fold bridging C0.28*29*31-40 In the present work, a band at 1760 cm-' was observed only for Rh/Y at 273 K. The fact that the 1684 and 1800 cm-' bands increase in intensity in concert suggests that the Rh,CO and Rh,-C-O-Mn2+ entities are parts of the same complex. It therefore appears that the Rh,(CO),, complex migrates from one absorption site in the zeolite to another. The observed changes in the number and position of the carbonyl bands probably reflect the different chemical environments at these sites, in addition to changes in sym- metry.The major bands at 2125 and 2088 cm-' in the Mn- promoted sample increase when the system is brought to room temperature under CO partial pressure. The bands at 2122 and 2088 cm-' evident in RhY3 at low temperature vanished at 300 K. The latter bands, indicating a rhodium tricarbonyl species, happen to be located very close to the former bands typical for RhMnY523. The features at 2125 and 2088 cm-' in RhMnY523 may thus be ascribed to CO on top of isolated rhodium ions in different oxidation states. A band near 2130 cm-' has been stressed to be indicative of Rh3+-C0 complexes by Rice et aL4' Since the absorptions at 2125 and 2088 cm-' are con- fined to Mn-exchanged RhY samples it may be inferred that Mn ions might play a role as adsorption sites for these species.Alternatively, metal atoms that are strongly polarized by protons may serve as coordination centres for CO, as was evidenced by Zhang et al.,, for Pd-exchanged zeolite Y. Note that all of these distinguishing features, both in the high- frequency (2130-2080 cm- ') and in the low-frequency (1850- 1600 cm-') region are almost completely absent from the IR spectrum of the RhMn(CVD) sample; they are replaced by a totally new set of bands between 2075 and 1940 cm-'. Rhodium and manganese thus experience a totally different structural and electronic situation when they are introduced via the vapour phase as volatile organometallics. The very low intensity of the 2095 and 1832 cm-' bands is significant in this respect.The decomposition of Mn,(CO),, on the pre- formed Rh, particles inside the supercages of NaY strongly resembles the decomposition of Re,(CO),, on Pt particles in the same zeolite, which was reported previ~usly.~~ In this case no Mn2+ ions are formed, as is actually confirmed by the present IR spectra. The bimetallic particles formed in this way will presumably have surfaces rich in Mn. As Mn prefer- entially ligates with CO in the linear mode and as its pres- 1344 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 ence at the surface of the bimetallic particles strongly reduces the number of Rh, ensembles, no bridging CO is expected for these systems. This is confirmed by the absence of the bands typical of bridging CO.Summarizing, comparison of mono-and bi-metallic samples, and the concomitant comparison of samples pre- 15 16 17 18 J. R. Pearce, W. J. Mortier, J. B. Uytterhoeven and J. H. Luns- ford, J. Chem. SOC., Faraday Trans. 1, 1979,75,898. T. T. T. Wong, Z. Zhang and W. M. H. Sachtler, Catal. Lett., 1990,4, 365. C. Dossi, R. Psaro, A. Bartsch, E. Brivio, A. Galasco and P. Losi, Catal. Today, 1993, 17, 527. R. D. Shannon, J. C. Vedrine, C. Naccache and F. Lefebvre, J. pared by ion exchange and by chemical vapour deposition, provides evidence for the chemical interaction of Mn2+ ions with Rh, clusters. 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