The diffusion of saturated and unsaturated hydrocarbons is of fundamental importance for many zeolite‐catalyzed processes. Transport of small alkenes in the confined zeolite pores can become ...hindered, resulting in a significant impact on the ultimate product selectivity and separation. Herein, intracrystalline light olefin/paraffin diffusion through the 8‐ring windows of zeolite SAPO‐34 is characterized by a complementary set of first‐principle molecular dynamics simulations, PFG‐NMR experiments, and pulse‐response temporal analysis of products measurements, yielding information at different length and time scales. Our results clearly show a promotional effect of the presence of Brønsted acid sites on the diffusion rate of ethene and propene, whereas transport of alkanes is found to be insensitive to the presence of acid sites. The enhanced diffusivity of unsaturated hydrocarbons is ascribed to the formation of favorable π–H interactions with acid protons, as confirmed by IR spectroscopy measurements. The acid site distribution is proven to be an important design parameter for optimizing product distributions and separations.
The influence of the acid site density on the diffusivities of alkanes and alkenes through the 8‐ring windows of SAPO‐34 was probed using a complementary set of ab initio MD simulations and experimental techniques (PFG‐NMR and TAP pulse‐response measurements). The presence of Brønsted acid sites was found to have a clear promotional effect on alkene diffusion but it does not influence alkane diffusion.
ZnO-ZrO2 mixed oxide (ZnZrOx) catalysts are widely studied as selective catalysts for CO2 hydrogenation into methanol at high-temperature conditions (300-350 °C) that are preferred for the subsequent ...in situ zeolite-catalyzed conversion of methanol into hydrocarbons in a tandem process. Zn, a key ingredient of these mixed oxide catalysts, is known to volatilize from ZnO under high-temperature conditions, but little is known about Zn mobility and volatility in mixed oxides. Here, an array of ex situ and in situ characterization techniques (scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDX), transmission electron microscopy (TEM), powder X-ray diffraction (PXRD), X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), Infrared (IR)) was used to reveal that Zn2+ species are mobile between the solid solution phase with ZrO2 and segregated and/or embedded ZnO clusters. Upon reductive heat treatments, partially reversible ZnO cluster growth was observed above 250 °C and eventual Zn evaporation above 550 °C. Extensive Zn evaporation leads to catalyst deactivation and methanol selectivity decline in CO2 hydrogenation. These findings extend the fundamental knowledge of Zn-containing mixed oxide catalysts and are highly relevant for the CO2-to-hydrocarbon process optimization.
Herein, we report the discovery of a toroidal inorganic cluster of zirconium(IV) oxysulfate of unprecedented size with the formula Zr70(SO4)58(O/OH)146⋅x(H2O) (Zr70), which displays different packing ...of ring units and thus several polymorphic crystal structures. The ring measures over 3 nm across, has an inner cavity of 1 nm and displays a pseudo‐10‐fold rotational symmetry of Zr6 octahedra bridged by an additional Zr in the outer rim of the ring. Depending on the co‐crystallizing species, the rings form various crystalline phases in which the torus units are connected in extended chain and network structures. One phase, in which the ring units are arranged in layers and form one‐dimensional channels, displays high permanent porosity (BET surface area: 241 m2 g−1), and thus demonstrates a functional property for potential use in, for example, adsorption or heterogeneous catalysis.
A toroidal Zr(IV) oxysulfate cluster of unprecedented size with the formula Zr70(SO4)58(O/OH)146⋅x (H2O) (Zr70) was synthesized. Depending on the co‐crystallizing species, it displays different packing of the ring units. One of the phases contains one‐dimensional channels through the rings, showing high permanent porosity (surface area: 241 m2 g−1). The ring entity measures over 3 nm across and displays a pseudo‐10‐fold rotational symmetry.
Herein, we report the discovery of a toroidal inorganic cluster of zirconium(IV) oxysulfate of unprecedented size with the formula Zr
(SO
)
(O/OH)
⋅x(H
O) (Zr
), which displays different packing of ...ring units and thus several polymorphic crystal structures. The ring measures over 3 nm across, has an inner cavity of 1 nm and displays a pseudo-10-fold rotational symmetry of Zr
octahedra bridged by an additional Zr in the outer rim of the ring. Depending on the co-crystallizing species, the rings form various crystalline phases in which the torus units are connected in extended chain and network structures. One phase, in which the ring units are arranged in layers and form one-dimensional channels, displays high permanent porosity (BET surface area: 241 m
g
), and thus demonstrates a functional property for potential use in, for example, adsorption or heterogeneous catalysis.
Propane dehydrogenation on a Pt-based catalyst can be accelerated by cofeeding hydrogen. An extensive reaction network for propane dehydrogenation on Pt(111), including side reactions and deep ...dehydrogenation reactions, is proposed to explain the effect of cofeeding hydrogen. Simulations at 873 K and 1 bar total pressure reproduce the experimental trends at increasing H2/C3H8 inlet ratios and allow exploration of the origin of the positive effect of cofeeding hydrogen. Increasing hydrogen pressure leads to a lower coverage of deeply dehydrogenated coke precursors on the surface: in particular, CCH3 (ethylidyne) and CH (methylidyne). In addition, it increases hydrogen coverage, which decreases the propylene adsorption strength while the energy barriers for the further dehydrogenation of propylene increase. The combined effect of a decreasing coke precursor coverage, facilitated propylene desorption, and increasing deep dehydrogenation barriers explains the higher catalytic activity when hydrogen is cofed.
Bimetallic nanocatalysts are key enablers of current chemical technologies, including car exhaust converters and fuel cells, and play a crucial role in industry to promote a wide range of chemical ...reactions. However, owing to significant characterization challenges, insights in the dynamic phenomena that shape and change the working state of the catalyst await further refinement. Herein, we discuss the atomic‐scale processes leading to mono‐ and bimetallic nanoparticle formation and highlight the dynamics and kinetics of lifetime changes in bimetallic catalysts with showcase examples for Pt‐based systems. We discuss how in situ and operando X‐ray spectroscopy, scattering, and diffraction can be used as a complementary toolbox to interrogate the working principles of today's and tomorrow's bimetallic nanocatalysts.
Probing the invisible: Understanding the mechanisms behind bimetallic catalyst formation and functioning requires in situ and operando interrogation at the nanoscale. The current possibilities and future potential of X‐ray characterization to probe the dynamics of lifetime changes in bimetallic nanocatalysts under catalytically relevant conditions is reviewed.
The alloying of Pt with Ga delivered from a hydrotalcite-like support was investigated as a strategy to produce bimetallic catalysts for propane dehydrogenation. A series of Pt/Mg(Al,Ga)O x ...catalysts (2–3 wt % Pt, Ga/Pt molar ratios between 0 and 10) and a model Pt/Ga2O3 catalyst (4 wt % Pt, Ga/Pt molar ratio of 50) were characterized by means of X-ray diffraction (XRD), transmission electron microscopy, and activity measurements (873 K, W cat/F C3H8,0 = 25 kgcat·s·mol–1 and P C3H8,0 = 5 kPa at a total pressure of 101.3 kPa). XRD patterns taken during temperature-programmed reduction in 5% H2/He and isothermal reduction/oxidation cycling between 5% H2/He and 20% O2/N2 at 873 K revealed dynamic alloy formation and segregation that depended upon the gas environment and Ga content. Alloying on the Pt/Mg(Al,Ga)O x catalyst with a Ga/Pt ratio of 2 could not be observed by XRD. For a Ga/Pt ratio of 10, an alloy with a diffraction peak at 40.2° was formed during the initial reduction. After subsequent reduction/oxidation treatments, this catalyst evolved toward a stable periodic cycling between pure Pt and one or more Pt–Ga alloys with characteristic peaks at 40.2° and 46.5°. The exact composition of the Pt–Ga alloy(s) could not be identified. On the model Pt/Ga2O3 catalyst, an alloy was formed with the same characteristic peak at 40.2° as on the Ga-rich Pt/Mg(Al,Ga)O x . In addition, another Pt–Ga alloy appeared on the Pt/Ga2O3 catalyst, which was identified as a stoichiometric PtGa phase. These alloys were formed on Pt/Ga2O3 at a lower temperature than on Pt/Mg(Al,Ga)O x and they were stable during the reduction/oxidation cycling. Catalytic activity measurements demonstrated that the formation of Pt–Ga alloys on the Pt/Mg(Al,Ga)O x sample with a Ga/Pt ratio of 10 and on the Pt/Ga2O3 catalyst led to pronounced enhancement of the initial selectivity toward propylene, but lower activity per exposed Pt atom.
ZnO-ZrO
mixed oxide (ZnZrO
) catalysts are widely studied as selective catalysts for CO
hydrogenation into methanol at high-temperature conditions (300-350 °C) that are preferred for the subsequent
...zeolite-catalyzed conversion of methanol into hydrocarbons in a tandem process. Zn, a key ingredient of these mixed oxide catalysts, is known to volatilize from ZnO under high-temperature conditions, but little is known about Zn mobility and volatility in mixed oxides. Here, an array of
and
characterization techniques (scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDX), transmission electron microscopy (TEM), powder X-ray diffraction (PXRD), X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), Infrared (IR)) was used to reveal that Zn
species are mobile between the solid solution phase with ZrO
and segregated and/or embedded ZnO clusters. Upon reductive heat treatments, partially reversible ZnO cluster growth was observed above 250 °C and eventual Zn evaporation above 550 °C. Extensive Zn evaporation leads to catalyst deactivation and methanol selectivity decline in CO
hydrogenation. These findings extend the fundamental knowledge of Zn-containing mixed oxide catalysts and are highly relevant for the CO
-to-hydrocarbon process optimization.
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