Support Effect and Surface Reconstruction in In₂O₃/<i>m-</i>ZrO₂ Catalyzed CO₂ Hydrogenation

We investigate the chemical and structural dynamics at the interface of In2O3/m-ZrO2 and their consequences on the CO2 hydrogenation reaction (CO2HR) under reaction conditions. While acting to enrich CO2, monoclinic zirconia (m-ZrO2) was also found to serve as a chemical and structural modifier of In2O3 that directly governs the outcome of the CO2HR. These modifying effects include the following: (1) Under reaction conditions (above 623 K), partially reduced In2O3, i.e., InOx (0 x < 1.5), was found to migrate in and out of the subsurface of m-ZrO2 in a semireversible manner, where m-ZrO2 accommodates and stabilizes InOx by serving as a reservoir. The decreased concentration of surface InOx under elevated temperatures coincides with significantly decreased selectivity toward methanol and a sharp increase of the reverse water–gas shift reaction. The reconstruction-induced variation of InOx concentration appears to be one of the most important factors contributing to the altered catalytic performance of CO2HR at different reaction conditions. (2) The strong interactions and reactions between m-ZrO2 and In2O3 result in the activation of a pool of In–O bonds at the In2O3/m-ZrO2 interface to form oxygen vacancies. On the other hand, the high dispersity of In2O3 nanostructures onto m-ZrO2 prevents their over-reduction under catalytically relevant conditions (up to 673 K), when bare In2O3 is unavoidably reduced into the metallic phase (In0). The relationship between the extent of reduction of In2O3 and catalytic performance (CO2 conversion, CH3OH selectivity, or yield of CH3OH) suggests the presence of an optimum coverage of surface InOx and oxygen vacancies under reaction conditions. The conventional model that links catalytic performance solely to the coverage of oxygen vacancies appears invalid in the present case. In situ analysis also allows the observation of surface reaction intermediates and their interconversions, including the reduction of CO3* into formate, a precursor for the formation of methanol and CO. The combinative ex situ and in situ study sheds light on the reaction mechanism of the CO2HR on In2O3/m-ZrO2-based catalysts. Our findings on the large-scale surface reconstructions, support effect, and the reaction mechanism of In2O3/m-ZrO2 for CO2HR may apply to other related metal oxide catalyzed CO2 reduction reactions

Enone as a Process Aid for the Highly Efficient Synthesis of Karstedt’s Catalyst: Probing the Mechanism of Dissolution of Platinum(II) Chloride

Homogeneous platinum complexes such as Speier’s catalyst and Karstedt’s catalyst are some of the most commonly used catalysts for hydrosilylation reactions. The synthesis of Karstedt’s catalyst from anhydrous PtCl2 requires the presence of a polar solvent (methyl ethyl ketone/MEK) and divinyltetramethyldisiloxane (dvtms) as the reagent. Despite being practiced over several decades, the reaction suffers from several limitations such as moderate conversion (80–85%), long reaction time (8–10 h), and thermal decomposition of the catalyst over longer periods. Through an approach that relies mostly on mechanistic insights and systematic investigation of all reaction parameters, we identified that presoaking or milling PtCl2 in MEK at room temperature led to the formation of crystalline Pt6Cl12·(MEK)1.5 (characterized by powder X-ray diffraction), which drastically improved the reaction conversion (4 h, 99% conversion of PtCl2). As our understanding of the mechanism of this reaction improved, we discovered that small amounts of PtCl2(enone) complexes (isolated and fully characterized by X-ray crystallography) were formed in situ from the preheated mixture of PtCl2 and MEK in the absence of dvtms. These enone compounds were likely formed via aldol condensation of MEK, followed by a dehydration reaction. We have since found that these β,γ-enones are superb process additives and can be independently added (as low as 1 wt %) to improve the reaction rate (98% conversion of PtCl2). Computational studies further suggest that enones behave as phase-transfer additives. Once MEK disrupts the PtCl2 lattice, enones facilitate the dissolution process by complexing with the individual molecular PtCl2 moieties, thus stabilizing them in the homogeneous phase. In addition, the calculated energy landscape suggests that once the solid PtCl2 is brought into the homogeneous liquid phase, the formation of Karstedt’s catalyst itself is energetically downhill, overcoming only moderate activation barriers for a Pt(II) to Pt(0) reduction process

Enone as a Process Aid for the Highly Efficient Synthesis of Karstedt’s Catalyst: Probing the Mechanism of Dissolution of Platinum(II) Chloride

Homogeneous platinum complexes such as Speier’s catalyst and Karstedt’s catalyst are some of the most commonly used catalysts for hydrosilylation reactions. The synthesis of Karstedt’s catalyst from anhydrous PtCl2 requires the presence of a polar solvent (methyl ethyl ketone/MEK) and divinyltetramethyldisiloxane (dvtms) as the reagent. Despite being practiced over several decades, the reaction suffers from several limitations such as moderate conversion (80–85%), long reaction time (8–10 h), and thermal decomposition of the catalyst over longer periods. Through an approach that relies mostly on mechanistic insights and systematic investigation of all reaction parameters, we identified that presoaking or milling PtCl2 in MEK at room temperature led to the formation of crystalline Pt6Cl12·(MEK)1.5 (characterized by powder X-ray diffraction), which drastically improved the reaction conversion (4 h, 99% conversion of PtCl2). As our understanding of the mechanism of this reaction improved, we discovered that small amounts of PtCl2(enone) complexes (isolated and fully characterized by X-ray crystallography) were formed in situ from the preheated mixture of PtCl2 and MEK in the absence of dvtms. These enone compounds were likely formed via aldol condensation of MEK, followed by a dehydration reaction. We have since found that these β,γ-enones are superb process additives and can be independently added (as low as 1 wt %) to improve the reaction rate (98% conversion of PtCl2). Computational studies further suggest that enones behave as phase-transfer additives. Once MEK disrupts the PtCl2 lattice, enones facilitate the dissolution process by complexing with the individual molecular PtCl2 moieties, thus stabilizing them in the homogeneous phase. In addition, the calculated energy landscape suggests that once the solid PtCl2 is brought into the homogeneous liquid phase, the formation of Karstedt’s catalyst itself is energetically downhill, overcoming only moderate activation barriers for a Pt(II) to Pt(0) reduction process

Enone as a Process Aid for the Highly Efficient Synthesis of Karstedt’s Catalyst: Probing the Mechanism of Dissolution of Platinum(II) Chloride

Homogeneous platinum complexes such as Speier’s catalyst and Karstedt’s catalyst are some of the most commonly used catalysts for hydrosilylation reactions. The synthesis of Karstedt’s catalyst from anhydrous PtCl2 requires the presence of a polar solvent (methyl ethyl ketone/MEK) and divinyltetramethyldisiloxane (dvtms) as the reagent. Despite being practiced over several decades, the reaction suffers from several limitations such as moderate conversion (80–85%), long reaction time (8–10 h), and thermal decomposition of the catalyst over longer periods. Through an approach that relies mostly on mechanistic insights and systematic investigation of all reaction parameters, we identified that presoaking or milling PtCl2 in MEK at room temperature led to the formation of crystalline Pt6Cl12·(MEK)1.5 (characterized by powder X-ray diffraction), which drastically improved the reaction conversion (4 h, 99% conversion of PtCl2). As our understanding of the mechanism of this reaction improved, we discovered that small amounts of PtCl2(enone) complexes (isolated and fully characterized by X-ray crystallography) were formed in situ from the preheated mixture of PtCl2 and MEK in the absence of dvtms. These enone compounds were likely formed via aldol condensation of MEK, followed by a dehydration reaction. We have since found that these β,γ-enones are superb process additives and can be independently added (as low as 1 wt %) to improve the reaction rate (98% conversion of PtCl2). Computational studies further suggest that enones behave as phase-transfer additives. Once MEK disrupts the PtCl2 lattice, enones facilitate the dissolution process by complexing with the individual molecular PtCl2 moieties, thus stabilizing them in the homogeneous phase. In addition, the calculated energy landscape suggests that once the solid PtCl2 is brought into the homogeneous liquid phase, the formation of Karstedt’s catalyst itself is energetically downhill, overcoming only moderate activation barriers for a Pt(II) to Pt(0) reduction process