John R. Regalbuto

Selective Adsorption of Manganese onto Rhodium for Optimized Mn/Rh/SiO2 Alcohol Synthesis Catalysts

Using supported rhodium‐based catalysts to produce alcohols from syngas provides an alternative route to conventional fermentation methods. If left unpromoted, Rh catalysts have a strong selectivity towards methane. However, promotion with early transition metal elements has been shown to be effective to increase alcohol selectivity. Therefore, a key design objective is to increase the promoter–metal interaction to maximize their effectiveness. This can be achieved by the use of the strong electrostatic adsorption (SEA) method, which utilizes pH control to steer the promoter precursor (in this case MnO4−) onto Rh oxide supported on SiO2. Mn‐promoted catalysts were synthesized by both SEA and traditional incipient wetness impregnation (IWI) and subsequently characterized by STEM and extended X‐ray absorption fine structure methods. Using STEM–electron energy loss spectroscopy mapping, catalysts prepared by SEA were shown to have a higher degree of interaction between the promoter and the active metal. The reduction behavior of the catalysts obtained by X‐ray absorption near‐edge spectroscopy and temperature‐programmed reduction demonstrated a minimal change in Rh if promoted by SEA. However, catalytic results for CO hydrogenation revealed that a significant improvement of ethanol selectivity is achieved if the promoter was prepared by SEA in comparison with the promoter prepared by IWI. These results suggest that intimate interaction between the promoter and the metal is a critical factor for improving selectivity to higher alcohols.

Synthesis of Platinum Catalysts over Thick Slurries of Oxide Supports by Strong Electrostatic Adsorption

Platinum uptake by strong electrostatic adsorption on alumina, silica, and titania supports was studied at different slurry thicknesses (or surface loadings). For the anionic platinum chloride precursor, a high counterion concentration at high surface loadings hindered adsorption on the high point of zero charge alumina support, as predicted by the revised physical adsorption model. Owing to this adsorption retardation in thicker slurries, a highly dispersed Pt/Al2O3 catalyst could only be prepared with a lower weight fraction of Pt than that in thin slurries. However, for cationic platinum tetraamine complexes, the effect of counterions in retarding the adsorption at high surface loadings was unexpectedly diminished. As a result, it was possible to synthesize highly dispersed Pt/SiO2 even at a maximum slurry thickness (incipient wetness) without having to sacrifice the Pt loading.

Synthesis of ultrasmall, homogeneously alloyed, bimetallic nanoparticles on silica supports

Dispersing small, bimetallic nanoparticles For applications of nanoparticles in sensing and catalysis, smaller nanoparticles are often more effective because they expose more active surface sites. The properties of metallic nanoparticles can also be improved by creating bimetallic alloys, but typical synthetic methods yield larger nanoparticles where the metals are poorly mixed. Wong et al. show that well-mixed bimetallic ∼1-nm-diameter nanoparticles can be made on silica supports. To do this, they exploited strong electrostatic adsorption, in which the metal precursors are strongly adsorbed onto the surface by controlling pH relative to the surface point of zero charge. Their method was successful for a wide range of metal alloys. Science, this issue p. 1427 A scalable and generalizable synthetic method yields ultrasmall bimetallic nanoparticles that are fully alloyed. Supported nanoparticles containing more than one metal have a variety of applications in sensing, catalysis, and biomedicine. Common synthesis techniques for this type of material often result in large, unalloyed nanoparticles that lack the interactions between the two metals that give the particles their desired characteristics. We demonstrate a relatively simple, effective, generalizable method to produce highly dispersed, well-alloyed bimetallic nanoparticles. Ten permutations of noble and base metals (platinum, palladium, copper, nickel, and cobalt) were synthesized with average particle sizes from 0.9 to 1.4 nanometers, with tight size distributions. High-resolution imaging and x-ray analysis confirmed the homogeneity of alloying in these ultrasmall nanoparticles.

Understanding Uptake of Pt Precursors During Strong Electrostatic Adsorption on Single-Crystal Carbon Surfaces

The formation of Pt clusters deposited by strong electrostatic adsorption (SEA) has been studied on highly oriented pyrolytic graphite (HOPG) as well as on higher surface area graphene nanoplatelets (GNPs), which have well-defined surface structures. Both surfaces can be hydroxylated by treatment with hydrochloric acid, and in the case of HOPG, the surface functional groups are exclusively hydroxyls. When SEA is carried out with the anionic precursor PtCl62− under acidic conditions, the dominant cationic sites for electrostatic adsorption are protonated aromatic rings rather than protonated hydroxyl groups, and therefore the unfunctionalized surfaces exhibit high uptake compared to the HCl-treated surfaces. In contrast, for SEA with the cationic precursor Pt(NH3)42+ under basic conditions, surface hydroxylation leads to higher Pt uptake since deprotonated hydroxyl groups act as negatively charged adsorption sites for the cations. After reduction of the adsorbed ionic precursors by heating in H2, the metallic Pt clusters on HOPG produced from PtCl62− SEA form dendritic aggregates associated with high Pt diffusion rates and irreversible Pt–Pt bonding. For Pt(NH3)42+ SEA on HOPG, subsequent reduction resulted in clusters preferentially located at step edges on the untreated surface, whereas smaller clusters more uniformly distributed across the terraces were observed on the hydroxylated surface. Similar trends in particle sizes and densities were observed on the GNP surfaces, demonstrating that understanding of nucleation and growth on single-crystal HOPG surfaces can be extended to the high surface area GNPs. Oxidative adsorption to Pt4+ was observed for SEA of Pt2+ precursors on both HOPG and GNP surfaces.

Catalytic N-H Bond Activation and Breaking by a Well-defined Co(II)1O4 Site of a Heterogeneous Catalyst

Catalytic N−H bond activation and breaking by well‐defined molecular complexes or their heterogeneous analogues is considered to be a challenge in chemical science. Metal(0) nanoparticles catalytically decompose NH3; they are, however, ill defined and contain a range of contiguous metal sites with varying coordination numbers and catalytic properties. So far, no well‐defined/molecular Mn+‐containing materials have been demonstrated to break strong N−H bonds catalytically, especially in NH3, the molecule with the strongest N−H bonds. Recently, noncatalytic activation of NH3 with the liberation of molecular H2 on an organometallic molybdenum complex was demonstrated. Herein, we show the catalytic activation and breaking of N−H bonds on a singly dispersed, well‐defined, and highly thermally resistant (even under reducing environments) CoII1O4 site of a heterogeneous catalyst for organic (ethylamine) and inorganic (NH3, with the formation of N2 and H2) molecules. The single‐site material serves as a viable precursor to ultrasmall (2.7 nm and less) silica‐supported cobalt nanoparticles; thus, we directly compare the activity of isolated cationic cobalt sites with small cobalt nanoparticles. Density functional theory (DFT) calculations suggest a unique mechanism involving breaking of the N−H bonds in NH3 and N−N coupling steps taking place on a Co1O4 site with the formation of N2H4, which then decomposes to H2 and N2H2; N2H2 subsequently decomposes to H2 and N2. In contrast, Co1N4 sites are not catalytically active, which implies that the ligand environment around a single atom of a heterogeneous catalyst largely controls reactivity. This may open a new chapter for the design of well‐defined heterogeneous materials for N−H bond‐activation reactions.

Hydrocarbon Fuels from Lignocellulose

The main motivation for biofuels at present is to enable transportation fuels which do not contribute to global warming. Biofuels made from non-food biomass, collectively called lignocellulose, dramatically reduce the net carbon dioxide emissions from light and heavy duty vehicles. Lignocellulose consists of agricultural residue such as corn stover and sugarcane bagasse, waste from forest trimming, and energy crops such as switchgrass and short rotation poplar trees grown on marginally arable land with little irrigation or fertilizer.

Continuous-Flow Synthesis of Cu-M (M=Ni, Co) Core-Shell Nanocomposites

AbstractMagnetic nanomaterials have many applications in the fields of catalysis, medicine, and environmental studies. An emerging synthetic method capable of large-scale production of nanomaterials is a continuous-flow microreactor. However, translating known conventional benchtop reactions to a continuous-flow system can be difficult; reaction parameters such as reaction time and viscosity of the solution are significant limitations in flow-based systems. In this study, nanocrystalline Cu-Ni and Cu-Co core-shell materials were successfully synthesized using a capillary microreactor in a one-step process. Ethanol was used as solvent, allowing for faster reaction times and reduced reaction solution viscosity, compared to similar bench top synthetic protocols. Both nanocomposites were tested for activity in Fischer-Tropsch and showed activity above 220 °C. This study shows that a continuous-flow capillary microreactor has the capabilities to make complex metallic nanomaterials at short reaction times with proper selection of reaction solvent systems.

Sophie Hermans and Thierry Visart de Bocarmé (eds.): Atomically-Precise Methods for Synthesis of Solid Catalysts

indeed delivered a relatively comprehensive, well organized survey of state of the art methods to synthesize (primarily) metal catalysts. The sequencing of the material brought to my mind the classic review of the late, great Jim Schwarz, ‘‘Methods for Preparation of Catalytic Materials’’ (Chem. Rev. 95, 1995, 477) in which ‘‘3 dimensional chemistry’’ or synthesis of metal particles in the solution phase, was distinguished from ‘‘2 dimensional chemistry’’ in which metal is deposited and nanoparticles are formed on the support surface. In fact, the material swung back and forth between surface deposition and bulk formation techniques in something of a conical helix as depicted in the figure below. A clever unifying element is that as the chapter numbers increase, the catalysts grow in size and scale.