Abstract
The catalytic performance with high conversion and high selectivity of Ti-based oxide catalysts have been widely investigated. Besides, stability, which is an essential parameter in the industrial process, lacked fundamental understanding. In this work, we combined computational and experimental techniques to provide insight into the deactivation of P25 and TS-1 Ti-based oxide catalysts during the methyl oleate (MO) epoxidation. The considered deactivation mechanisms are fouling and surface oxygen vacancy (OV). The fouling causes temporary catalyst deactivation through active site blockage but can be removed via calcination in air at high temperature. However, in this work, the OV formation plays an important role in the overall performance of the spent catalyst as the deactivated catalyst after regeneration, cannot be restored to the initial activity. Also, the effects of OV in spent catalysts caused (i) the formation of more Ti3+ species on the surface as evident by XPS and Bader charge analysis, (ii) the activity modification of the active region on the catalyst surface as the reduction in energy gap (Eg) occurred from the formation of the interstates observed in the density of states profiles of spent catalyst modeled by the O-vacant P25 and TS-1 models. This reduction in Eg affects directly the strength of Ti–OOH active site and MO bonding, in which high binding energy contributes to a low conversion because the MO needed an O atom from Ti–OOH site to form the methyl-9,10-epoxy stearate. Hence, the deactivation of the Ti-based oxide catalysts is caused not only by the insoluble by-products blocking the active region but also mainly from the OV. Note that the design of reactive and stable Ti-based oxide catalysts for MO epoxidation needed strategies to prevent OV formation that permanently deactivated the active region. Thus, the interrelation and magnitude between fouling and OV formation on catalyst deactivation will be investigated in future works.
Highlights
The catalytic performance with high conversion and high selectivity of Ti-based oxide catalysts have been widely investigated
We investigate the deactivation scheme on P25 and TS-1 Ti-based oxide catalysts during the liquid-phase methyl oleate (MO) epoxidation reaction at low temperature based on the evidence from experimental data, surface characterizations, and computational data obtained via the density functional theory-based (DFT) analysis 14–19
(ii) The catalyst surface deactivated through the loss of oxygen atom, forming the surface O V which induced the formation of the interstates in conduction and valence states of the catalyst, where these interstates resulted in the decrease in energy gap for both catalysts leading to the modification of the activity of the active region on the surface
Summary
The catalytic performance with high conversion and high selectivity of Ti-based oxide catalysts have been widely investigated. The effects of OV in spent catalysts caused (i) the formation of more Ti3+ species on the surface as evident by XPS and Bader charge analysis, (ii) the activity modification of the active region on the catalyst surface as the reduction in energy gap (Eg) occurred from the formation of the interstates observed in the density of states profiles of spent catalyst modeled by the O-vacant P25 and TS-1 models This reduction in Eg affects directly the strength of Ti–OOH active site and MO bonding, in which high binding energy contributes to a low conversion because the MO needed an O atom from Ti–OOH site to form the methyl-9,10epoxy stearate. We investigate the deactivation scheme on P25 and TS-1 Ti-based oxide catalysts during the liquid-phase MO epoxidation reaction at low temperature based on the evidence from experimental data, surface characterizations, and computational data obtained via the density functional theory-based (DFT) analysis 14–19. The information of such deactivation schemes would be used to construct the guideline towards the design of reactive and stable Ti-based oxide catalysts for epoxidation reactions
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