X-ray Absorption Spectroscopic Study of Brønsted, Lewis, and Redox Centers in Cobalt-Substituted Aluminum Phosphate Catalysts

Abstract

The structure of acid sites in cobalt-substituted aluminophosphates (AlPOs) catalysts has been investigated, with EXAFS spectroscopy. Data obtained, using in situ methods, for the Co K-edge spectra of cobalt-substituted AlPO-5, AlPO-18, AlPO-36, and APSO-44 in as-prepared, calcined, and reduced states yield the local structure of the Co ions. Whereas the as-prepared materials clearly contain Co(II) ions in regular four-coordinated sites, complex behavior is exhibited by their calcined analogues, with essentially complete oxidation of Co(II) to Co(III) in CoAlPO-18, with the local coordination of the high-spin Co(III) being distorted. By contrast incomplete oxidation of the Co(II) is observed for CoAlPO-5 and CoAlPO-36. A combination of EXAFS data analysis with the results of computer modeling studies suggests that this behavior is interpretable in terms of the formation of oxygen vacancies by dehydroxylation which lead to undercoordinated Lewis acid, Co(II) sites. IR data of the reduced samples reveal the presence of Brønsted acid sites which, from the EXAFS analysis, are seen to correspond to Co(II) species in a distorted environment owing to the presence of a neighboring protonated oxygen ion. The implications of the results for the catalytic properties of the systems are considered.

Synthesis of FeCoMnAPO-5 Molecular Sieve and Catalytic Activity in Cyclohexane Oxidation by Oxygen

“…The vibration bands of the FeCoMnA-PO-5 (trace b) framework, 1109 cm À1 and 472 cm À1 , are shifted to lower wavenumbers with respect to those of AlPO-5, which are 1113 cm À1 and 477 cm À1 , respectively (trace a). This is because that the Fe-O, Co-O and Mn-O bonds are longer than the Al-O bond [9]. At the same time, it also confirms that the three metals are substituted into the framework of AlPO-5.…”

Effect of the impregnation order on the nature of metal particles of bi-functional Pt/Pd-supported zeolite Beta materials and on their catalytic activity for the hydroisomerization of alkanes

“…In a typical experiment, about 100 mg of the metal/zeolite samples were pressed into a self-supporting disc of 20 mm diameter. In order to simulate the in situ pre-treatment prior to the catalytic runs, the samples were placed into a quartz cell designed to allow measuring XAS data in various atmospheres and under various conditions [36]. The samples were then calcined and reduced using the above-mentioned conditions which were similar to those used to pre-treat the catalyst before catalytic reaction.…”

Structural and Functional Characterization of Redox Mn and Co Sites in AlPO Materials and Their Role in Alkane Oxidation Catalysis

“…H 2 O was not detected concurrently with H 2 consumption in any of the samples, but small amounts of water appeared at higher temperatures, apparently from condensation of vicinal OH groups at reduced cations to form extraframework cations or distorted cations within the framework. 21,22 The absence of H 2 O during H 2 consumption indicates that reduction occurs exclusively via formation of protons at cation sites (reaction 1) and that O-atom removal occurs at higher temperatures, if at all. Also, the absence of H 2 O confirms the absence of bulk Mn or Co oxides, which reduce to MnO or Co 0 below 773 K with the concurrent removal of lattice oxygens and formation of H 2 O.…”