In Situ Strain Evolution on Pt Nanoparticles during Hydrogen Peroxide Decomposition.

Fundamental understanding of structural changes during catalytic reactions is crucial to understanding the underlying mechanisms and optimizing efficiencies. Surface energy and related catalytic mechanisms are widely studied. However, the catalyst lattice deformation induced by catalytic processes is not well understood. Here, we study the strain in an individual platinum (Pt) nanoparticle (NP) using Bragg coherent diffraction imaging under in situ oxidation and reduction reactions. When Pt NPs are exposed to H2O2, a typical oxidizer and an intermediate during the oxygen reduction reaction process, alternating overall strain distribution near the surface and inside the NP is observed at the (111) Bragg reflection. In contrast, relatively insignificant changes appear in the (200) reflection. Density functional theory calculations are employed to rationalize the anisotropic lattice strain in terms of induced stress by H2O2 adsorption and decomposition on the Pt NP surface. Our study provides deeper insight into the activity-structure relationship in this system.

Synthesis and characterization of ERI-type UZM-12 zeolites and their methanol-to-olefin performance.

A wide variety of different linear, diquaternary alkylammonium ions have been used as supplementary crystallization structure-directing agents (SDAs) in the synthesis of UZM-12, a high-silica version of zeolite erionite, via a charge density mismatch (CDM) approach. When tetraethylammonium is used as a CDM SDA, the crystallization of UZM-12 was found to be critically dependent not only on the type of alkali metal cations added as another crystallization SDA to the synthesis mixture, but also on the size of the groups on the diquaternary ammonium ion employed and the length of its central polymethylene chain that are closely related to the dimensions of cylindrical 23-hedral [4(12)6(5)8(6)] eri cages in this small-pore zeolite. (27)Al MQ MAS NMR measurements reveal a preferential location of Al on the high-multiplicity site over the lower-multiplicity site of the UZM-12 framework. The catalytic results from the methanol-to-olefin reaction over a series of H-UZM-12 zeolites with similar acidic properties but different crystallite sizes (100-2500 nm in length) demonstrate that the nanocrystallinity (probably the ≤100 nm range) may have a detrimental effect on the activity and stability for this reaction, probably due to the fast buildup of large coke molecules on the external surface of zeolite crystallites that inhibits the methanol diffusion to intrazeolitic acid sites, rendering them ultimately inaccessible for catalysis.

Enhanced catalytic performance of copper-exchanged SAPO-34 molecular sieve in methanol-to-olefin reaction.

Methanol-to-olefin (MTO) reaction over copper-exchanged SAPO-34 catalysts was investigated in order to extend their catalyst life. The exchange of copper ions into the cages of an SAPO-34 molecular sieve was confirmed by ESR, XPS, and 129Xe NMR techniques. Copper ions located in its cages considerably reduced its deactivation rate in the MTO reaction, while those dispersed on the external surface of the SAPO-34 molecular sieve accelerated the deactivation due to the limited mass transfer through the pore entrances. The 13C NMR and UV-VIS spectroscopy investigations of the materials occluded on the copper-exchanged SAPO-34 catalysts during the MTO reaction clearly showed that the copper ions exchanged in the cages suppressed the further condensation of alkyl aromatics to large, fused polycyclic aromatic hydrocarbons (PAHs). Theoretical calculations for the SAPO-34 and copper-exchanged SAPO-34 molecular sieves supported this observation because copper ions located in the cages stabilized the alkyl aromatics. Therefore, the exchange of copper ions into the SAPO-34 molecular sieve stabilized the reactive intermediates, alkyl aromatics, of the MTO reaction and suppressed their further condensation to PAHs, thereby slowing the deactivation.

Time-resolved in situ visualization of the structural response of zeolites during catalysis.

Zeolites are three-dimensional aluminosilicates having unique properties from the size and connectivity of their sub-nanometer pores, the Si/Al ratio of the anionic framework, and the charge-balancing cations. The inhomogeneous distribution of the cations affects their catalytic performances because it influences the intra-crystalline diffusion rates of the reactants and products. However, the structural deformation regarding inhomogeneous active regions during the catalysis is not yet observed by conventional analytical tools. Here we employ in situ X-ray free electron laser-based time-resolved coherent X-ray diffraction imaging to investigate the internal deformations originating from the inhomogeneous Cu ion distributions in Cu-exchanged ZSM-5 zeolite crystals during the deoxygenation of nitrogen oxides with propene. We show that the interactions between the reactants and the active sites lead to an unusual strain distribution, confirmed by density functional theory simulations. These observations provide insights into the role of structural inhomogeneity in zeolites during catalysis and will assist the future design of zeolites for their applications.

Understanding the structural characteristics of compstatin by conformational space annealing.

The structural characteristics of the 13-residue compstatin molecule are investigated using the conformational space annealing (CSA) method with CHARMM force field and the GBSA continuum solvent model. In order to sample conformations in the energy range of the minimized NMR structures, we have used the stopping criterion to the CSA search when a conformation whose energy is less than -490 kcal/mol is found. With this stopping criterion, a great variety of conformations are generated around experimentally known structures. Twenty independent CSA runs starting from random states find 1000 representative conformations in the energy landscape of the compstatin, which are classified into thirty-one structural families. The majority of the conformations (94.4%) are in the coil state. Other conformers containing a 3(10)-helix, a pi-helix, a beta-hairpin, and an alpha-helix are also found.