Determining the Location of Co2+ in Zeolites by UV-Vis Diffuse Reflection Spectroscopy: A Critical View

Abstract

UV–Vis spectroscopy as well as in situ FTIR spectroscopy of pyridine and CO adsorption were applied to determine the nature of Co species in microporous, mesoporous, and mixed oxide materials like Co–ZSM-5, Co/Na–ZSM-5, Co/Al–SBA-15, and Co/Al2O3–SiO2. Because all sample types show comparable UV–Vis spectra with a characteristic band triplet, the former described UV–Vis band deconvolution method for determination and quantification of individual cationic sites in the zeolite appears doubtful. This is also confirmed by results of pyridine and CO adsorption revealing that all Co–zeolite samples contain two types of Co2+ species located at exchange positions as well as in oxide-like clusters independent of the Co content, while in Co/Al–SBA-15 and Co/Al2O3–SiO2 only Co2+ species in oxide-like clusters occur. Consequently, the measured UV–Vis spectra represent not exclusively isolated Co2+ species, and the characteristic triplet band is not only related to γ-, β-, and α-type Co2+ sites in the zeolite but also to those dispersed on the surface of different oxide supports. The study demonstrates that for proper characterization of the formed Co species, the use of complementary methods is required.

Experimental and Theoretical Assessments of Aluminum Proximity in MFI Zeolites and Its Alteration by Organic and Inorganic Structure-Directing Agents

“…Co 2+ ion exchange has been used to indicate the fraction of Al sites in close proximity to one another, yet the conditions reported to achieve saturated Co 2+ exchange of MFI samples vary widely among these literature reports (0.02–0.5 M Co 2+ , 298–353 K, 6–24 h, 1–3 sequential exchanges with fresh Co 2+ ion-exchange solutions). ,,,− Thus, we built on our prior work to perform an extensive set of aqueous-phase Co 2+ ion-exchange experiments under a variety of conditions (i.e., temperature, molarity, time, number of sequential exchanges) to measure Co 2+ ion-exchange isotherms on Na-forms of a commercial MFI sample (MFI­(13,C)) to determine when Co 2+ saturation was achieved (section S.7, SI). The resulting Co/Na-form MFI zeolites were characterized by a cation site balance to provide evidence for a 2:1 Co 2+ exchange stoichiometry at saturation (Figure S.20b, SI), which together with DRUV–vis spectra of the dehydrated Co/Na-MFI indicate that cobalt oxides are absent (or present in minority amounts relative to Co 2+ ions) on all Co-exchanged MFI samples (section S.8, SI); importantly, such spectra were not inappropriately used to identify or quantify distinct Co 2+ species …”

“…Such interpretations are inaccurate, however, because even a single metal cation in one lattice binding site will adopt a distribution of coordination geometries caused by thermally induced structural changes in M–O f bonds and (alumino)­siloxane rings as we showed recently for the case of isolated Cu 2+ ions in 6-MR of CHA zeolites . Moreover, such d–d transitions are not unique to Co 2+ at lattice binding sites in crystalline zeolitic frameworks and are also observed for Co 2+ exchanged on amorphous supports (e.g., Co/Al 2 O 3 , Co-doped ZnO, Co/Al 2 O 3 SiO 2 ). − Other experimental efforts to identify proximal Al–Al arrangements in zeolites have used 2-dimensional 27 Al MAS NMR spectroscopy, , but such data provide nonquantitative information about the number of different Al–Al site ensembles present.…”

Suppressing C–C Bond Dissociation for Efficient Ethane Dehydrogenation over the Isolated Co(II) Sites in SAPO-34

“…For the three Co/SAPO-34 catalysts with Co contents of about 0.4 wt % by different preparation methods, the UV–vis spectra were quite similar, suggesting that they had similar chemical states of Co species in the zeolite. In the range of 400–700 nm, a slightly enhanced absorbance could be observed, which can be ascribed to the existence of Co oxide clusters . However, the amount should be quite tiny.…”

“…In the range of 400−700 nm, a slightly enhanced absorbance could be observed, which can be ascribed to the existence of Co oxide clusters. 55 However, the amount should be quite tiny. For most transition metals such as Cu 56 and Fe, 57 the absorption band in this range can be characteristic for a d−d CT of metal ions in their oxide clusters.…”

Recent Advances in Cobalt and Related Catalysts: From Catalyst Preparation to Catalytic Performance

“…The importance of the different local structure of the Co species on the surface of different supports was pointed out in the study of Bellmann et al [6]. In this study, different supports were used for the preparation of Co-supported catalysts.…”