Investigation of different modifiers for nanocrystal zirconia on W/ZrO2 catalysts via esterification

Performance controlled via surface oxygen-vacancy in Ti-based oxide catalyst during methyl oleate epoxidation

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 (O_V). 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 O_V 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 O_V 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 (E_g) 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 E_g 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 O_V. Note that the design of reactive and stable Ti-based oxide catalysts for MO epoxidation needed strategies to prevent O_V formation that permanently deactivated the active region. Thus, the interrelation and magnitude between fouling and O_V formation on catalyst deactivation will be investigated in future works.

A computational-experimental investigation on high ethylene selectivity in ethanol dehydration reaction found on WOx/ZrO2-activated carbon bi-support systems

The high ethylene selectivity exhibited on the zirconia-activated-carbon bi-support catalyst is investigated by experiment and density functional theory-based (DFT) analysis. This bi-support catalyst systems prepared by the physical mixing method for the tungsten catalyst show a significant increase in ethylene selectivity up to 90% compared to the zirconia single support system (~58%) during the ethanol dehydration reaction. Besides, the optimal percent weight ratio of zirconia to activated carbon, which results in the highest ethanol conversion is 50:50. The DFT-based analysis is used to investigate high ethylene selectivity in the bi-support system. It shows that the WO₅/zirconia is the most stable model for the zirconia single-support tungsten catalyst represented by the zirconia (101) facet of the tetrahedral phase. The carbon atoms were added to the WO₅/zirconia to model the tungsten catalyst on the bi-support system. The Bader charge analysis is carried out to determine the electron transfer in the catalyst. The bonding between ethylene and the WO₅ active site on the catalyst is weakened when the system is bi-support, where the added carbon atoms on the catalyst in the ZrO₂ region decrease the ethylene adsorption energy. Thus, the desorption and the selectivity of ethylene are promoted. The decrease in adsorption energy can be explained via the analysis of the projected density of states (PDOS) profiles of atom involving the adsorption. It was found that the added carbon in the ZrO₂ region induces the electron transfer from the ethylene molecule to the surface, especially to the ZrO₂ region. The depletion of the electron around the ethylene molecule weakens the bonds, thus, promote desorption. Hence, the advantages of using the bi-support system in the tungsten catalyst are that the catalyst exhibit (1) high conversion due to the zirconia support and (2) high ethylene selectivity due to the added carbon promoting the desorption of ethylene via the induction of electron from an ethylene molecule to surface.