Structural dynamics of a metal-organic framework induced by CO2 migration in its non-uniform porous structure.

Stimuli-responsive behaviors of flexible metal-organic frameworks (MOFs) make these materials promising in a wide variety of applications such as gas separation, drug delivery, and molecular sensing. Considerable efforts have been made over the last decade to understand the structural changes of flexible MOFs in response to external stimuli. Uniform pore deformation has been used as the general description. However, recent advances in synthesizing MOFs with non-uniform porous structures, i.e. with multiple types of pores which vary in size, shape, and environment, challenge the adequacy of this description. Here, we demonstrate that the CO2-adsorption-stimulated structural change of a flexible MOF, ZIF-7, is induced by CO2 migration in its non-uniform porous structure rather than by the proactive opening of one type of its guest-hosting pores. Structural dynamics induced by guest migration in non-uniform porous structures is rare among the enormous number of MOFs discovered and detailed characterization is very limited in the literature. The concept presented in this work provides new insights into MOF flexibility

Atomic engineering of platinum alloy surfaces.

A major practical challenge in heterogeneous catalysis is to minimize the loading of expensive platinum group metals (PGMs) without degrading the overall catalytic efficiency. Gaining a thorough atomic-scale understanding of the chemical/structural changes occurring during catalyst manufacture/operation could potentially enable the design and production of "nano-engineered" catalysts, optimized for cost, stability and performance. In the present study, the oxidation behavior of a Pt-31 at% Pd alloy between 673-1073 K is investigated using atom probe tomography (APT). Over this range of temperatures, three markedly different chemical structures are observed near the surface of the alloy. At 673 K, the surface oxide formed is enriched with Pd, the concentration of which rises further following oxidation at 773 K. During oxidation at 873 K, a thick, stable oxide layer is formed on the surface with a stoichiometry of PdO, beneath which a Pd-depleted (Pt-rich) layer exists. Above 873 K, the surface composition switches to enrichment in Pt, with the Pt content increasing further with increasing oxidation temperature. This treatment suggests a route for tuning the surfaces of Pt-Pd nanoparticles to be either Pd-rich or Pt-rich, simply by adjusting the oxidation temperatures in order to form two different types of core-shell structures. In addition, comparison of the oxidation behavior of Pt-Pd with Pt-Rh and Pd-Rh alloys demonstrates markedly different trends under the same conditions for these three binary alloys

Importance of the structural integrity of a carbon conjugated mediator for photocatalytic hydrogen generation from water over a CdS-carbon nanotube-MoS2 composite

Incorporation of CdS quantum dots is shown to significantly promote photocatalytic hydrogen production from water over single-layer MoS2 in a remote manner via their dispersions on a carbon nanotube as a nanocomposite: the hydrogen evolution rate is found to be critically dependent on the content and structural integrity of the carbon nanotube such that the double-walled carbon nanotube shows superior H2 production to a single-walled carbon nanotube because the inner carbon tubules survive from the structural damage during functionalization

Entrapped single tungstate site in zeolite for cooperative catalysis of olefin metathesis with Brønsted acid site

Industrial olefin metathesis catalysts generally suffer from low reaction rates and require harsh reaction conditions for moderate activities. This is due to their inability to prevent metathesis active sites (MASs) from aggregation and their intrinsic poor adsorption and activation of olefin molecules. Here, isolated tungstate species as single molecular MASs are immobilized inside zeolite pores by Brønsted acid sites (BASs) on the inner surface. It is demonstrated that unoccupied BASs in atomic proximity to MASs enhance olefin adsorption and facilitate the formation of metallocycle intermediates in a stereospecific manner. Thus, effective cooperative catalysis takes place over the BAS-MAS pair inside the zeolite cavity. In consequence, for the cross-metathesis of ethene and trans-2-butene to propene, under mild reaction conditions, the propene production rate over WO x/USY is ca. 7300 times that over the industrial WO3/SiO2-based catalyst. A propene yield up to 79% (80% selectivity) without observable deactivation was obtained over WO x/USY for a wide range of reaction conditions

Entrapped single tungstate site in zeolite for cooperative catalysis of olefin metathesis with Brønsted acid site

Industrial olefin metathesis catalysts generally suffer from low reaction rates and require harsh reaction conditions for moderate activities. This is due to their inability to prevent metathesis active sites (MASs) from aggregation and their intrinsic poor adsorption and activation of olefin molecules. Here, isolated tungstate species as single molecular MASs are immobilized inside zeolite pores by Brønsted acid sites (BASs) on the inner surface. It is demonstrated that unoccupied BASs in atomic proximity to MASs enhance olefin adsorption and facilitate the formation of metallocycle intermediates in a stereospecific manner. Thus, effective cooperative catalysis takes place over the BAS-MAS pair inside the zeolite cavity. In consequence, for the cross-metathesis of ethene and trans-2-butene to propene, under mild reaction conditions, the propene production rate over WO x/USY is ca. 7300 times that over the industrial WO3/SiO2-based catalyst. A propene yield up to 79% (80% selectivity) without observable deactivation was obtained over WO x/USY for a wide range of reaction conditions

ESICM LIVES 2016: part two : Milan, Italy. 1-5 October 2016.

Meeting abstrac