Gem-dicarbonyls

Van t Blik et al. [3] exposed a highly dispersed 0.57 wt% RI1/AI2O3 catalyst (H/M=1.7) to CO at room temperature and measured a CO uptake of 1.9 molecules of CO per Rh atom. Binding energies for the Rh 3ds/2 XPS peak increased from 307.5 eV for the reduced catalyst under H2 to 308.7 eV for the catalyst under CO. The latter value equals that of the [Rh+(CO)2Cl]2 complex, in which rhodium occurs as a Rh+ ion. The infrared spectrum of the Rh/Al203 catalyst under CO showed exclusively the gem-dicarbonyl peaks at 2095 and 2023 cm-1. All results point to the presence of rhodium in Rh+(CO)2 entities. However, how can a rhodium particle accommodate so much CO ... 

Figure 7 is a differential tunneling spectrum of CO chemisorbed on alumina supported rhodium particles. The identification of the peaks is also shown below and consist of three separate species. These are a gem dicarbonyl Rh (C0)2 a linear carbonyl RhCO and a bridging carbonyl RhxC0. The dicarbonyl is characterized by a peak at 4l3 cm 1 and the linear species by a bending mode at 465 cm-1. 

Fig. 9.8 Infrared spectra of CO adsorbed on a well-dispersed Rh/Si02 catalyst at 80 K, 295 K, 370 K, and 470 K. The spectra show the peaks of linear and bridged CO at 2078 and 1908 cm-1, and the bands of the gem-dicarbonyl species at 2098 and 2030 cm-1, respectively. The bands above 2100 cm-1 in the spectrum at 80 K are due to physisorbed and H-bonded CO. (From [26]).

Equilibration with carbon monoxide at room temperature and low pressure (a few torr ) yielded the rhodium(I)-dicarbonyl compound (13) in addition to the Rh(I)(C0) paramagnetic complexe (11). The structure of this complex was elucidated by ESCA and UV measurements (13) which showed that the trivalent rhodium was indeed reduced to the monovalent state and by infrared spectroscopy which provided evidence for a gem dicarbonyl (14). Use of 1 1 C0 ... 

IR studies showed that each band of the VCO doublet characteristic of the Rh(I) gem dicarbonyl was split into two components. Recent experiments (14) showed that the presence of residual water in variable contents-significantly altered the intensity ratio of the two components of each of the two bands. Only the low frequency components appeared in the case of the strictly anhydrous zeolite. As the residual water content increased (as monitored by the vOH absorptions at 3640 and 3550 cm- ), the high frequency components grew simultaneously. 

Fig. 19. Two suggested adsorption geometries for rhodium gem-dicarbonyl (Rh(CO)2) on Ti02(l 10)1x1 [104].

Fig. 11. Azimuthal dependence of FT-RAIRS spectra for TiO2(110)-Rh(CO)2 [72], The azimuthal angle (j) is defined as 0° when the incident radiation is aligned in a plane parallel to the <110> direction. The Vsym(C-O) dynamic dipole is aligned normal to the surface and couples to Pn (transmission band), and Vasym(C-O) is aligned parallel to the surface in the <110> direction, and couples to Pt (absorption band). Two possible adsorption geometries consistent with the FT-RAIRS azimuthal dependence are shown for the gem-dicarbonyl.

The azimuthal dependence of the intensity of Vasym(C-O) in the P-polarised radiation shows a maximum at 9=90° indicating an alignment of the Rh(CO)2 in the <110> direction. Since the S and Pt fields are orthogonal, using S-polarised radiation at 9 = 0° Vasym(C-O) is not observed, but is observed at 9 = 90°. The two most likely adsorption geometries of the adsorbed gem-dicarbonyl are shown in Fig. 11, both with the C-0 bonds in a plane aligned in the <110> direction. 

Fig. 12. A series of RAIRS spectra taken following deposition of increasing amounts of Rh on an alumina film grown on NiAl(llO) and subsequent CO saturation at 90K[74]. The average number of Rh atoms per particle is indicated. Vsym(C-O) of the gem-dicarbonyl is observed (indicated) as an absorption band at low Rh coverages, while Vasyni(C-O) is screened by the underlying metal substrate. The large broader band observed at higher Rh coverages (particle sizes) results from v(C-O) of CO adsorbed on Rh" particles.

Initial adsorption of the precursor produces the spectrum of the rhodium gem-dicarbonyl species (Rh(CO)2) with the characteristic Vsym(C-O) and Vasym(C-O) modcs (Fig. 10). Heating the surface to 500K results in the desorption of the CO and the formation of metallic Rh" aggregates of a range of sizes on the Ti02(l 10) surface. 

The reactions of hydrogen with carbon monoxide and carbon dioxide over Rh/Al203 and Rh/Ti02 films, some of which contained potassium as an additive, have been investigated. For the CO hydrogenation reaction the presence of potassium caused the dissociation or desorption of the gem dicarbonyl, linear CO, and carbonyl hydride species, while it led to an enhancement of the bridged carbonyl species. 

The interaction of CO with supported Rh catalysts has been well characterized by infrared spectroscopy (13). The primary surface species obtained are shown below. The gem dicarbonyl species (l) exhibits two sharp infrared bands at ca. 2030 and 2100 cm" ... 

Figure 3 shows the IR spectrum of CO adsorption at 0.1 MPa and 298 K as well as the styrene hydroformylation over 4.5wt% Rh 2-phenylpropanal-imprinted catalysts at 353 K and 0.83 MPa. Observation of gem-dicarbonyl at 2086 and 2015 cm-i upon CO adsorption indicated that Rh sites are in the highly dispersed state. 2-phenylpropanal and 3-phenylpropanal were formed after 1 min of the reaction in the batch model as shown by the appearance of the phenylpropanals carbonyl band at 1717 cm-i. The IR intensity of phenylpropanals increased with the reaction time. Table 2 summarizes the conversion, selectivity, and turnover frequency (TOF) of hydroformylation over... 

As expected, the total normalized area for 20 mg samples decreased as a function of dispersion and Ge grafted amount. The evolution of brigded CO species frequencies indicated a modification in the CO coordination to Rh as the wide band corresponds at least to two species Rh 3(CO)2 and Rh aCCO) respectively at about 1920 and 1870 cm [9]. Thus, an increase of band frequencies can be correlated to a higher contribution of Rh 3(CO)a species and consequently to a growth of particle size. A decrease of band frequencies corresponding to identical particles can be attributed to the presence of Ge inducing site isolation and favored low-coordinated CO species. Finally, the frequencies of linear and gem-dicarbonyl species exhibited no modification that can be explained by the absence of an electronic effect between Rh and Ge in our catalysts whereas such an effect was proposed for Pt-Ge (Ge as electron acceptor) [15] and Rh-Sn (Sn as electron donor) [16]. 

Table 1 summarizes the most important surface complexes formed when NO and CO are adsorbed on noble metal catalysts. According to the literature NO and CO are adsorbed as nitrites, nitrates and carbonates on alumina [2]. The most important surface complexes for CO and NO adsorption on rhodium are a gem-dicarbonyl (Rh(CO)2) and a linear Rh-NO complex [1]. However, tricarbonyl and bridged Rhx-CO complexes have been proposed to be formed and different kinds of linear Rh nitrosyl complexes are possible [1-4]. The adsorption of CO on R and Pd catalysts depends much on the oxidation stage of R and Pd [5]. CO adsorption on R forms mostly linear and bridged carbonyls [6-11]. NO is adsorbed linearly on R [12]. In the case of Pd the most common surface complexes are linear carbonyls [13], strong multilaterally-bonded carbonyls, bridged carbonyls [5,14,15] and triply-bonded CO [5]. Isocyanate, nitrous oxide or nitrogen dioxide are proposed to be coimected to the reaction mechanism of the NO-CO reactions [2,16-19]. 

According to static experiments CO was adsorbed on reduced Rh catalysts mostly as gem-dicarbonyl species which were observed at around 2088 and 2022 cm" (Fig.l). At 300 to 400 C traces of linearly bonded carbonyls at around 2060 cm are proposed to arise. Increasing of the sample temperature did not have any effect on the band positions of the gem-dicarbonyl complex. The band intensities increased as the temperature was increased, which may be caused by the increasing adsorption ability of CO. At 400 C the asymmetric stretching of the C-0 bond (2022 cm" ) increased while the symmetric stretching (2088 cm" ) decreased. 

When air was introduced into the chamber with CO, Rh gem-dicarbonyl bands appeared at 150 C (Fig.2). The formation of CO2 was observed at 200 C. At 400 °C gem-dicarbonyl bands were no more present indicating the reaction of CO to CO2. The possible reaction path for the CO-O2 reaction is proposed to be the following ... 

When gases were introduced into the chamber in the following order CO-NO the gem-dicarbonyl complexes could not be seen clearly (Fig.4). Gaseous NO was observed clearly at 1877 cm whereas the bands for gaseous CO were very weak. The formation of a Rh-NO" complex became more and more obvious at 300 and 400 °C giving rise to the absorption band at 1896 cm V The formation of the isocyanate surface complex was not as intense as in the case with NO-CO addition into the chamber. 

The GC-IR spectra for the NO-CO-air and CO-NO-air adsorption on a Rh catalyst are shown in Fig.5 and 6. NO was adsorbed on Rh as a linear nitrosyl (Rh-NO at 1911 cm (Fig.5) and at 1894 cm (Fig.6)) whereas CO formed gem-dicarbonyl surface complexes on Rh (Rh(CO)2 at 2092 and 2020 cm , Fig.6). The isocyanate surface complex formation was strong when air was added into the chamber after NO and CO (Fig.5). In the case of CO-NO-air, the absorption bands for the isocyanate surface complex were very weak. This proves the proposed reaction path for the Rh-NCO formation [2], Isocyanate surface complex formation is possible when the catalyst surface is preadsorbed with NO whereas if CO is preadsorbed on the surface the reaction intermediate is nitrous oxide ... 

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