Toward digitally controlled catalyst architectures: Hierarchical nanoporous gold via 3D printing.

Monolithic nanoporous metals, derived from dealloying, have a unique bicontinuous solid/void structure that provides both large surface area and high electrical conductivity, making them ideal candidates for various energy applications. However, many of these applications would greatly benefit from the integration of an engineered hierarchical macroporous network structure that increases and directs mass transport. We report on 3D (three-dimensional)-printed hierarchical nanoporous gold (3DP-hnp-Au) with engineered nonrandom macroarchitectures by combining 3D printing and dealloying. The material exhibits three distinct structural length scales ranging from the digitally controlled macroporous network structure (10 to 1000 µm) to the nanoscale pore/ligament morphology (30 to 500 nm) controlled by dealloying. Supercapacitance, pressure drop, and catalysis measurements reveal that the 3D hierarchical nature of our printed nanoporous metals markedly improves mass transport and reaction rates for both liquids and gases. Our approach can be applied to a variety of alloy systems and has the potential to revolutionize the design of (electro-)chemical plants by changing the scaling relations between volume and catalyst surface area.

New Architectures for Designed Catalysts: Selective Oxidation using AgAu Nanoparticles on Colloid-Templated Silica.

A highly modular synthesis of designed catalysts with controlled bimetallic nanoparticle size and composition and a well-defined structural hierarchy is demonstrated. Exemplary catalysts-bimetallic dilute Ag-in-Au nanoparticles partially embedded in a porous SiO2 matrix (SiO2 -Agx Auy )-were synthesized by the decoration of polymeric colloids with the bimetallic nanoparticles followed by assembly into a colloidal crystal backfilled with the matrix precursor and subsequent removal of the polymeric template. This work reports that these new catalyst architectures are significantly better than nanoporous dilute AgAu alloy catalysts (nanoporous Ag3 Au97 ) while retaining a clear predictive relationship between their surface reactivity with that of single-crystal Au surfaces. This paves the way for broadening the range of new catalyst architectures required for translating the designed principles developed under controlled conditions to designed catalysts under operating conditions for highly selective coupling of alcohols to form esters. Excellent catalytic performance of the porous SiO2 -Agx Auy structure for selective oxidation of both methanol and ethanol to produce esters with high conversion efficiency, selectivity, and stability was demonstrated, illustrating the ability to translate design principles developed for support-free materials to the colloid-templated structures. The synthetic methodology reported is customizable for the design of a wide range of robust catalytic systems inspired by design principles derived from model studies. Fine control over the composition, morphology, size, distribution, and availability of the supported nanoparticles was demonstrated.

Assembly, characterization, and electrochemical properties of immobilized metal bipyridyl complexes on silicon(111) surfaces.

Silicon(111) surfaces have been functionalized with mixed monolayers consisting of submonolayer coverages of immobilized 4-vinyl-2,2'-bipyridyl (1, vbpy) moieties, with the remaining atop sites of the silicon surface passivated by methyl groups. As the immobilized bipyridyl ligands bind transition metal ions, metal complexes can be assembled on the silicon surface. X-ray photoelectron spectroscopy (XPS) demonstrates that bipyridyl complexes of [Cp*Rh], [Cp*Ir], and [Ru(acac)2] were formed on the surface (Cp* is pentamethylcyclopentadienyl, acac is acetylacetonate). For the surface prepared with Ir, X-ray absorption spectroscopy at the Ir LIII edge showed an edge energy as well as post-edge features that were essentially identical with those observed on a powder sample of [Cp*Ir(bpy)Cl]Cl (bpy is 2,2'-bipyridyl). Charge-carrier lifetime measurements confirmed that the silicon surfaces retain their highly favorable photoelectronic properties upon assembly of the metal complexes. Electrochemical data for surfaces prepared on highly doped, n-type Si(111) electrodes showed that the assembled molecular complexes were redox active. However the stability of the molecular complexes on the surfaces was limited to several cycles of voltammetry.

Role of the N-terminal helix in the metal ion-induced activation of the diphtheria toxin repressor DtxR.

The metal ion-regulated transcriptional repressor DtxR has been shown to repress the transcription of the diphtheria toxin and other genes associated with ferrous ion homeostasis in Corynebacterium diphtheriae. In vivo studies of single-alanine mutations located in the N-terminal helix of DtxR show that the activity of the mutants is reduced compared to that of the wild type. The three-dimensional structures of the apo and activated forms of DtxR show conformational changes in the N-terminal helix resulting from metal ion activation. We have studied the N-terminal helix mutants DtxR(D6A,C102D), DtxR(E9A,C102D), and DtxR(M10A,C102D) using crystallographic and calorimetric techniques to gain insight into the possible reasons for such behavior at a molecular level. The binding affinities for metal ion extracted from the calorimetric titrations of the mutants DtxR(D6A,C102D) and DtxR(E9A,C102D) are very similar to those found for DtxR(C102D), while the same experiments performed with the mutant DtxR(M10A,C102D), bearing the M10A mutation located in binding site 2, show a decreased binding affinity in a predictable fashion. These results suggest that the decreased activity observed in these mutants cannot be explained exclusively by changes in the binding affinity of the repressor. The crystal structures of Ni-DtxR(M10A,C102D), Ni-DtxR(E9A,C102D), and Ni-DtxR(D6A,C102D) clearly show the presence of two metal ions bound. In the structure of Ni-DtxR(M10A,C102D), a water replaces Met10 in binding site 2. In the structure of Ni-DtxR(D6A,C102D), the nonhelical conformation of the N-terminal region characteristic of the activated form is absent. The side chain of Asp6 is critical in stabilization of the nonhelical conformation. This conformation is identical in all high-resolution structures of activated DtxR with an intact N-terminal helix, suggesting relevance in DtxR's regulatory function.