Amorphous, single-site, silica-supported main group metal catalysts have recently been found to promote olefin oligomerization with high activity at moderate temperatures and pressures (∼250 °C and 1 atm). Herein, we explore the molecular level relationship between active site structures and the associated oligomerization mechanisms by developing amorphous, silica-supported Ga3+ models from periodic, first principles calculations. Representative Ga3+ sites, including three- and four-coordinated geometries, are tested for multiple ethylene oligomerization pathways. We show that the three-coordinated Ga3+ site promotes oligomerization through a facile initiation process that generates a Ga-alkyl intermediate, followed by a Ga-alkyl-centered Cossee–Arlman mechanism. The strained geometry of the three-coordinated site enables a favorable free-energy landscape with a kinetically accessible ethylene insertion transition state (1.7 eV) and a previously unreported β-hydride transfer step (1.0 eV) to terminate further C–C bond formation. This result, in turn, suggests that Ga3+ does not favor polymerization chemistry, while microkinetic modeling confirms that ethylene insertion is the rate-determining step. The study demonstrates the promising flexibility of the main group ions for hydrocarbon transformations and, more generally, highlights the importance of the local geometry of metal ions on amorphous oxides in determining catalytic properties.
We demonstrate that single-atom alloy catalysts can be made by exposing physical mixtures of monometallic supported Cu and Pd catalysts to vinyl acetate (VA) synthesis reaction conditions. This reaction induces the formation of mobile clusters of metal diacetate species that drive extensive metal nanoparticle restructuring, leading to atomic dispersion of the precious metal, smaller nanoparticle sizes than the parent catalysts, and high activity and selectivity for both VA synthesis and ethanol dehydrogenation reactions. This approach is scalable and appears to be generalizable to other alloy catalysts.
Alloying is an important strategy for the design of catalytic materials beyond pure metals. The conventional alloy catalysts however lack precise control over the local atomic structures of active sites. Here we report on an investigation of the active-site ensemble effect in bimetallic Pd-Au electrocatalysts for CO2 reduction. A series of Pd@Au electrocatalysts are synthesized by decorating Au nanoparticles with Pd of controlled doses, giving rise to bimetallic surfaces containing Pd ensembles of various sizes. Their catalytic activity for electroreduction of CO2 to CO exhibits a nonlinear behavior in dependence of the Pd content, which is attributed to the variation of Pd ensemble size and the corresponding tuning of adsorption properties. Density functional theory calculations reveal that the Pd@Au electrocatalysts with atomically dispersed Pd sites possess lower energy barriers for activation of CO2 than pure Au and are also less poisoned by strongly binding *CO intermediates than pure Pd, with an intermediate ensemble size of active sites, such as Pd dimers, giving rise to the balance between these two rate-limiting factors and achieving the highest activity for CO2 reduction.
A precise understanding of the catalytic surface of nanoparticles is critical for relating their structure to activity. For silica-supported Pt–Cr bimetallic catalysts containing nominal Cr/Pt molar ratios of 0, 1.9, and 5.6, a fundamental difference in selectivity was observed as a function of composition for propane dehydrogenation, suggesting different surface structures. The formation of bimetallic catalysts and the phases present were confirmed by synchrotron in situ X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) of the nanoparticle as a function of reduction temperature. With the increasing reduction temperature, there is a systematic increase in the Pt LIII edge X-ray absorption near edge structure (XANES) energy, which is consistent with the incorporation of more metallic Cr into the nanoparticles. Pt LIII edge extended X-ray absorption fine structure (EXAFS) shows that the nanoparticles are Pt rich regardless of the reduction temperature, and XRD shows the presence of both Pt and Pt3Cr phases at temperatures below about 700 °C. For the latter, a full Pt3Cr intermetallic alloy forms after reduction at 800 °C. This work also presents a method for the characterization of the catalytic surface by the analysis of XAS difference spectra and XRD difference patterns of the (reduced and oxidized) catalysts. The surface analysis suggests that Pt3Cr formation begins at the surface, and at low reduction temperatures, a core–shell morphology is formed containing a Pt core with a Pt3Cr surface. By combining the XAS and XRD analyses with transmission electron microscopy (TEM) particle sizes, the thickness of the shell can be approximated. All evidence indicates that the shell thickness increases with the increasing reduction temperature until a full alloy is formed after reduction at about 800 °C but only if there is enough Cr2O3 available near Pt nanoparticles to form Pt3Cr. Catalysts containing a full monolayer coverage of Pt3Cr have higher olefin selectivity (>97%) compared with partially covered Pt surfaces (88%).
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