Dealuminated Beta zeolite reverses Ostwald ripening for durable copper nanoparticle catalysts.

Copper nanoparticle-based catalysts have been extensively applied in industry, but the nanoparticles tend to sinter into larger ones in the chemical atmospheres, which is detrimental to catalyst performance. In this work, we used dealuminated Beta zeolite to support copper nanoparticles (Cu/Beta-deAl) and showed that these particles become smaller in methanol vapor at 200°C, decreasing from ~5.6 to ~2.4 nanometers in diameter, which is opposite to the general sintering phenomenon. A reverse ripening process was discovered, whereby migratable copper sites activated by methanol were trapped by silanol nests and the copper species in the nests acted as new nucleation sites for the formation of small nanoparticles. This feature reversed the general sintering channel, resulting in robust catalysts for dimethyl oxalate hydrogenation performed with supported copper nanoparticles for use in industry.

Hierarchically Porous Ceramics via Direct Writing of Binary Colloidal Gel Foams.

Hierarchically porous ceramics with a high specific surface area and interconnected porosity may find potential application as particulate filters, catalyst supports, and battery electrodes. We report the design and programmable assembly of cellular ceramic architectures with controlled pore size, volume, and interconnectivity across multiple length scales via direct foam writing. Specifically, binary colloidal gel foams are created that contain entrained bubbles stabilized by the irreversible adsorption of attractive alumina and carbon (porogen) particles at their air-water interfaces. Composition effects on foam ink rheology and printing behavior are investigated. Sintered ceramic foams exhibited specific permeabilities that increased from 2 × 10-13 to 1 × 10-12 m2 and compressive strengths that decreased from 40 to 1 MPa, respectively, with increasing specific interfacial area. Using direct foam writing, 3D ceramic lattices composed of open-cell foam struts were fabricated with tailored mechanical properties and interconnected porosity across multiple length scales.

Kinetics of proton-coupled electron-transfer reactions to the manganese-oxo "cubane" complexes containing the Mn4O4(6+) and Mn4O4(7+) core types.

The kinetics of proton-coupled electron-transfer (pcet) reactions are reported for Mn(4)O(4)(O(2)PPh(2))(6), 1, and [Mn(4)O(4)(O(2)PPh(2))(6)](+), 1(+), with phenothiazine (pzH). Both pcet reactions form 1H, by H transfer to 1 and by hydride transfer to 1(+). Surprisingly, the rate constants differ by only 25% despite large differences in the formal charges and driving force. The driving force is proportional to the difference in the bond-dissociation energies (BDE >94 kcalmol for homolytic, 1H --> H + 1, vs. approximately 127 kcalmol for heterolytic, 1H --> H(-) + 1(+), dissociation of the OH bond in 1H). The enthalpy and entropy of activation for the homolytic reaction (deltaH = -1.2 kcalmol and deltaS= -32 calmol.K; 25-6.7 degrees C) reveal a low activation barrier and an appreciable entropic penalty in the transition state. The rate-limiting step exhibits no HD kinetic isotope effect (k(H)k(D) = 0.96) for the first H atom-transfer step and a small kinetic isotope effect (1.4) for the second step (1H + pzH --> 1H(2) + pz(*)). These lines of evidence indicate that formation of a reactive precursor complex before atom transfer is rate-limiting (conformational gating), and that little or no NH bond cleavage occurs in the transition state. H-atom transfer from pzH to alkyl, alkoxyl, and peroxyl radicals reveals that BDEs are not a good predictor of the rates of this reaction. Hydride transfer to 1(+) provides a concrete example of two-electron pcet that is hypothesized for the OH bond cleavage step during catalysis of photosynthetic water oxidation.