Gas‐Phase Photooxidation of Alkenes by V‐Doped TiO₂‐MCM‐41: Mechanistic Insights of Ethylene Photooxidation and Understanding the Structure–Activity Correlation

Abstract

Nanoparticles of Ti(0.95)V(0.05)O(2) were found to be impregnated in the hexagonal channels of the MCM-41 host, with a distribution of some particles on the surface, thus leading to an effective variation in the particle size as a function of loading host MCM-41 matrix. These catalysts were subjected to the photocatalytic degradation of alkenes under the ambient conditions in which the photocatalytic activity varied as a function of the loading percentage of Ti(0.95)V(0.05)O(2) in the host MCM-41.This is explained in light of the structure-activity correlation, and the better catalytic activity can be attributed to an electronic interaction between the host and guest molecules, as established from X-ray photoelectron spectroscopy. To understand the mechanistic aspect of the photooxidation of ethylene on the vanadium-doped titania dispersed in the MCM-41 matrix, extensive in situ FTIR experiments were undertaken. The intermediate species produced on bare Ti(0.95)V(0.05)O(2) are different from that produced on the Ti(0.95)V(0.05)O(2)/MCM-41 surface. Moreover, different intermediates were produced during ethylene oxidation under UV and visible irradiation, thus leading to different rates. The ethylene decomposition over bare Ti(0.95)V(0.05)O(2) occurs by means of formation of ethoxy groups, transformed to acetaldehyde or enolates, subsequently to acetates, and then to CO(2) under both UV and visible irradiation. However, in the case of Ti(0.95)V(0.05)O(2)/MCM-41 catalyst with UV irradiation, the adsorbed acetaldehyde thus formed undergoes aldol condensation over the Lewis acid sites to lead to the formation of crotonaldehyde, which is subsequently oxidized to acetate and consequently to CO(2). It was observed that during visible irradiation labile ethyl acetate is produced either by the Tischenko reaction or by the reaction between the labile acetic acid and the unreacted ethoxy groups. The ethyl acetate produces acetic acid monomer, which is oxidized to CO(2). Furthermore, in this work the effects of particle size on the intermediate species were also studied.

The Origin of the Special Surface and Catalytic Chemistry of Ga-Rich Ni3Ga in the Direct Dehydrogenation of Ethane

“…Over as-prepared catalyst, ethylene adsorption leads to further dehydrogenation and ethylidyne production at temperatures between 25 and 200 °C. This was evidenced by the absorption peaks at 2925, 2866, and 1383 cm –1 and similar spectra recorded for ethylene adsorption over Pt, Rh, Pd, and Ir. − The vibrations at 2961, 1626, and 1469 cm –1 suggested that molecular ethylene could adsorb intact on the catalyst surface similar to other adsorption studies. − The vibration at 1593 cm –1 observed after heating to 300 °C suggests coke formation in the form of polyaromatic carbon species. , This study suggested the presence of over-reactive surface sites in the as-prepared catalysts, which may be poisoned and made to be catalytically inert through the propane pretreatment. Unfortunately, propane-treated catalysts could not be reasonably analyzed by DRIFTS due to coke deposition making the catalyst opaque.…”

V-doped TiO2(110): Quantitative structure determination using energy scanned photoelectron diffraction

“…4,5,6,7]. VO x (specifically including V 2 O 5 ) particles on TiO 2 are known to be particularly catalytically active [8], while doping bulk TiO 2 with transition metals [9], and particularly with vanadium [10,11,12,13], offers a route to lowering the band gap of the pure oxide (~3 eV), thus allowing photoactivation to occur with light in the visible spectral range [14 ].…”

Hierarchical Porous Wood Cellulose Scaffold with Atomically Dispersed Pt Catalysts for Low-Temperature Ethylene Decomposition

“…Their study demonstrated that ethylene decomposition occurred via the formation of ethoxy groups, which subsequently transformed to acetaldehyde or enolates, then to acetates/formate, and finally, to CO 2 . The vanadium-doping enhanced the catalytic activity by forming more chemisorbed hydroxyl species which might have increased the average coverage and residence time of intermediates for further oxidation …”

“…5 The vanadiumdoping enhanced the catalytic activity by forming more chemisorbed hydroxyl species which might have increased the average coverage and residence time of intermediates for further oxidation. 6 Besides the fundamental understanding of how to improve the turnover frequency of the active sites, the rational design of a nanostructured substrate with higher specific surface area and porosity, which is beneficial to mass transport, is of equal importance. For this reason, C 3 N 4 was used as a template for the synthesis of oxygen-deficient, porous TiO 2 nanosheets with Pt decoration.…”