Controlling polymorphism is critical in areas such as pharmaceuticals, biomineralization, and catalysis. Notably, the formation of unwanted polymorphs is a ubiquitous problem in zeolite synthesis. In this study, we propose a new platform for controlling polymorphism in organic-free Na-zeolite synthesis that enables crystal composition and properties to be tailored without sacrificing crystal phase purity. Through systematic adjustment of multiple synthesis parameters, we identified ternary (kinetic) phase diagrams at specific compositions (i.e., Si, Al, and NaOH mole fractions) using colloidal silica and sodium aluminate. Our studies identify multiple stages of zeolite phase transformations involving the framework types FAU, LTA, EMT, GIS, SOD, ANA, CAN, and JBW. We report an initial amorphous-to-crystalline transition of core-shell particles (silica core and alumina shell) to low-density framework types and their subsequent transformation to more dense structures with increasing temperature and/or time. We show that reduced water content facilitates the formation of structures such as EMT that are challenging to synthesize in organic-free media and reduces the synthesis temperature required to achieve higher-density framework types. A hypothesis is proposed for the sequence of phase transformations that is consistent with the Ostwald rule of stages, wherein metastable structures dissolve and recrystallize into more thermodynamically stable structures. The ternary diagrams developed here are a broadly applicable platform for rational design that offers an alternative to time- and cost-intensive methods of ad hoc parameter selection without a priori knowledge of crystal phase behavior.

No abstract

In this communication, we report the synthesis and characterization of TiO 2 -supported PdAu bimetallic nanoparticle catalysts prepared using dendrimer-encapsulated nanoparticles (DENs). 1 The key result is that the compositional fidelity of the original bimetallic DENs, and to some extent their size, is retained in the supported catalyst after removal of the dendrimer template by calcination at 500°C. Additionally, the gas-phase catalytic activity for CO oxidation is higher for the bimetallic catalysts than that for the corresponding Pd-only and Au-only monometallics. 2,3 It has previously been shown that 1-3-nm diameter alloy and core-shell bimetallic PdPt, 4,5 PdAu, 6 PdRh, 7 and AuAg 8 nanoparticles can be synthesized within dendrimer templates. 1 The resulting materials are characterized by a high degree of uniformity in size, structure, and elemental composition, and in some cases they have enhanced solution-phase catalytic activities compared to the analogous monometallic materials. [4][5][6][7] There are also a few examples of dendrimer/nanoparticle composites of various sorts being used to prepare supported Pt, 9 Pd, 10 Au, 10 CuO, 11 and Fe 2 O 3 nanoparticles. 12 In some cases, these were shown to be catalytically active. 9,12 To the best of our knowledge, however, there is only one very recent example of a heterogeneous catalyst prepared from a bimetallic DEN precursor. 13 The DEN approach to bimetallic nanoparticles is potentially simpler than many alternative methods. For example, organometallic cluster precursors containing two different metals have been used to prepare supported bimetallic catalysts, but continuous variation of the elemental composition of the resulting particles requires large precursor libraries. [14][15][16] Most other methods for preparing supported bimetallic nanoparticles in the <5-nm size range lead to phase segregation of the two metals and thus poor control over the composition of individual particles, 17 although limited success has been achieved using surfactant-stabilized nanoparticles. 18 We prepared DENs consisting of PdAu alloys within fourthgeneration, amine-terminated poly(amidoamine) (PAMAM) dendrimers (G4-NH 2 (Pd n Au 55-n )) by co-complexation of the two corresponding metal salts followed by chemical reduction of the dendrimer/metal-ion composite. 6 The synthesis was carried out in a 99% methanol solution using a procedure we previously reported for preparing DENs within amine-terminated dendrimers. 10 For example, G4-NH 2 (Pd 27.5 Au 27.5 ) was synthesized as follows: 0.0924 g of a 14.2 wt % solution of G4-NH 2 in MeOH and 20 µL of 0.3 M HCl were added to 1 L of 99% MeOH, followed by 255 µL of a 0.10 M K 2 PdCl 4 solution. The HCl was added to prevent crosslinking of the G4-NH 2 dendrimer by Pd 2+ (Pd 2+ is used to designate PdCl 4 2-and all its hydrolysis products). 19 After being stirred for 10 min, 255 µL of a 0.10 M HAuCl 4 solution was added. After being stirred for an additional 2 min, 1 mL of a 1.0 M NaBH 4 solution was added (20× molar excess). The s...

No abstract

Inelastic neutron spectroscopy (INS) has been employed to identify surface species formed during the H2-O2 reaction on Au/TiO2 catalysts. Determination of the surface intermediates formed in this reaction is crucial to develop a mechanistic understanding for the direct vapor-phase propylene epoxidation reaction and synthesis of H2O2. Although the presence of intermediate hydroperoxo species (during these reactions) has been suggested in literature, it has never been demonstrated. Our studies provide direct evidence for the formation of surface hydroperoxo species during the H2-O2 reaction.