Surface characterization of the Ru3(CO)12/Al2O3 system II. Structure and reactivity of the surface carbonylic complexes

FTIR Studies of CO Adsorption on Al2O3- and SiO2-Supported Ru Catalysts

“…The fitting results further suggest the presence of an additional dicarbonyl species, giving rise to a pair of bands in the HF 2 and LF regions, at frequencies approximately 20−30 cm -1 lower at room temperature than those of the dicarbonyl species described previously (i.e., 2062 and 1990 cm -1 ; bands 6 and 3). , These bands are very stable and remained in the spectra at temperatures up to at least 300 °C (Figure and Table ). Similar to the tricarbonyl species described above, the band positions are comparable to those observed for Ru dicarbonyl complexes having a Ru(CO) 2 X 2 stoichiometry (where X is Cl, Br, or I), which appear at 2066−2053 and 1995−1988 cm -1 . , On the basis of the same reasoning as above, and the results of 12 CO− 13 CO isotopic substitution experiments that yielded a total of six bands, these bands were previously assigned to a dicarbonyl species adsorbed on Ru n + , where n is suggested to be equal to 2 for the species giving rise to the pair of bands at higher frequencies (i.e., 2080 and 2015 cm -1 ) and 0 for the species giving rise to the pair of bands at lower frequencies (i.e., 2062 and 1990 cm -1 ). , The latter pair of bands experiences a gradual red shift of approximately 10 cm -1 , with decreasing surface coverage up to approximately 210 °C, suggesting that the corresponding species are not isolated. At temperatures above 150 °C, the shift becomes minimal because of a limited dipole−dipole coupling interaction at low surface coverages.…”

“…Although no definite conclusion can be drawn from the results of the current study regarding the type of species that gives rise to this pair of bands and, more specifically, the number of carbonyl groups involved, a careful review of the literature has led us to believe that a tricarbonyl species is formed. Previous studies that concluded the presence of such a species were conducted using a series of varying concentrations of 12 CO and 13 CO, and, hence, were able to discern peaks that would otherwise be overlapped by stronger neighboring bands. ,, In contrast, studies that concluded the presence of a dicarbonyl species were performed using only a single concentration of a 12 CO− 13 CO mixture and the poorly resolved spectra obtained may have led to false conclusions. , Furthermore, the positions of the bands are very similar to those observed for ruthenium tricarbonyl complexes having a stoichiometry Ru 2 (CO) 6 X 4 (where X is Cl, Br, or I), which exhibit strong absorbances at 2143−2128 and 2075−2069 cm -1 and a weak absorbance at 2015−2010 cm -1 . ,, The exact positions of these bands are dependent on the electronegativity of the halogen ligands, with a downshift observed with decreasing electronegativity. , In the case of supported Ru catalysts, the formation of the tricarbonyl species is believed to be the result of a CO-induced oxidative disruption process of finely dispersed Ru clusters with the participation of hydroxyl groups from the support, forming electron-deficient Ru n + ( n ranging between 1 and 3) sites with Y−O−Ru linkages (Y = Al, Si, or Ti). ,, …”

“…The symmetric and asymmetric stretching vibrations of such a species give rise to a pair of bands in the HF 2 (2070−2080 cm -1 ) and LF (2010 ± 10 cm -1 ) regions. , Hence, the HF 2 band at the 2070−2080 cm -1 region also contains a contribution from this species. Two bands at 2015 and 2080 cm -1 similar to the ones observed in the current study (bands 4 and 7, respectively) were observed previously as “shoulders” and were assigned to a dicarbonyl species adsorbed on Ru 2+ ).…”

“…Most previous literature reports agree that upon CO adsorption on supported Ru catalysts characteristic absorption bands appear in the spectra in three different regions: HF 1 (high-frequency 1) at 2120−2156 cm -1 , HF 2 at 2060−2110 cm -1 , and LF (low frequency) at 2000−2060 cm -1 . Analysis of the available literature data indicates that there are at least three different types of adsorbed CO species associated with these bands: type I carbonyls (i.e., Ru 0 −CO), which are characterized by an LF band; − type II carbonyls (i.e., Ru n + (CO) x ), which are characterized by an HF 1 −HF 2 set of bands, with the exact values of n and x under debate; − and type III carbonyls (i.e., Ru n + (CO) 2 ), which are characterized by an HF 2 −LF set of bands. ,− As a result, the nature of the bands in the HF 2 region is fairly complex because of contributions from several different species. Furthermore, it has been reported that the intensity of the HF 2 bands is enhanced in the presence of O 2 , leading to the hypothesis that some contributions in this region can be assigned to monocarbonyl species adsorbed on Ru δ+ . , Notwithstanding these previous studies, some of the band assignments remain controversial.…”

CO Adsorption on Hydrated Ru/Al2O3: Influence of Pretreatment

“…Several models have been presented to explain the appearance of HF and MF bands when CO adsorbs on supported Ru catalysts. These bands have been assigned as the symmetric and asymmetric stretches of di- ,− ,,, or tricarbonyl, ,,,,,, as CO adsorbed on low coordination edge and corner metal atoms, , as CO adsorbed on a surface oxide or on a Ru atom perturbed by a nearby electronegative element such as O or Cl, ,,,,,,,, and as CO bonded to Ru 2+ or Ru 3+ ions covalently bonded to oxygen atoms on the support. ,, …”