Metal nanoparticles are commonly supported on metal oxides, but their utility as catalysts is limited by coarsening at high temperatures. Rhodium oxide and rhodium metal nanoparticles on niobate and tantalate supports are anomalously stable. To understand this, the nanoparticle-support interaction was studied by isothermal titration calorimetry (ITC), environmental transmission electron microscopy (ETEM), and synchrotron X-ray absorption and scattering techniques. Nanosheets derived from the layered oxides KCa2Nb3O10, K4Nb6O17, and RbTaO3 were compared as supports to nanosheets of Na-TSM, a synthetic fluoromica (Na0.66Mg2.68(Si3.98Al0.02)O10.02F1.96), and α-Zr(HPO4)2·H2O. High surface area SiO2 and γ-Al2O3 supports were also used for comparison in the ITC experiments. A Born-Haber cycle analysis of ITC data revealed an exothermic interaction between Rh(OH)3 nanoparticles and the layered niobate and tantalate supports, with ΔH values in the range -32 kJ·mol(-1) Rh to -37 kJ·mol(-1) Rh. In contrast, the interaction enthalpy was positive with SiO2 and γ-Al2O3 supports. The strong interfacial bonding in the former case led to "reverse" ripening of micrometer-size Rh(OH)3, which dispersed as 0.5 to 2 nm particles on the niobate and tantalate supports. In contrast, particles grown on Na-TSM and α-Zr(HPO4)2·H2O nanosheets were larger and had a broad size distribution. ETEM, X-ray absorption spectroscopy, and pair distribution function analyses were used to study the growth of supported nanoparticles under oxidizing and reducing conditions, as well as the transformation from Rh(OH)3 to Rh nanoparticles. Interfacial covalent bonding, possibly strengthened by d-electron acid/base interactions, appear to stabilize Rh(OH)3, Rh2O3, and Rh nanoparticles on niobate and tantalate supports.
The thermodynamic adsorption profile at a solvated organic-inorganic interface is probed by following the binding and organization of carboxylic acid-terminated alkanethiols of varying chain lengths (C2, C3, and C6) to the surface of gold nanoparticles (NPs) (5.4 ± 0.7, 9.5 ± 0.6, and 19.4 ± 1.1 nm diameter) using isothermal titration calorimetry (ITC). We discuss the effect of alkyl chain length, temperature, and Au NP size on the energetics at an organic-inorganic interface. ITC allows for the quantification of the adsorption constant, enthalpy of adsorption, entropy of adsorption, and the binding stoichiometry in a single experiment. The thermodynamic parameters support a mechanism of stepwise adsorption of thiols to the surface of Au NPs and secondary ordering of the thiols at the organic-inorganic interface. The adsorption enthalpies are chain-length dependent; enthalpy becomes more exothermic as longer chains are confined, compensating for greater decreases in entropy with increasing chain length. We observe an apparent compensation effect: the negative ΔH is compensated by a negative ΔS as the thiols self-assemble on the Au NP surface. A comparison of the thermodynamic parameters indicates thiol-Au NP association is enthalpy-driven because of the large, exothermic enthalpies accompanied by an unfavorable entropic contribution associated with confinement of alkyl chains, reduced trans-gauche interconversion, and the apparent ordering of solvent molecules around the hydrophobic organic thiols (hydrophobic effect). Understanding the thermodynamics of adsorption at NP surfaces will provide critical insight into the role of ligands in directing size and shape during NP synthesis since thiols are a common ligand choice (i.e., Brust method). The ITC technique is applicable to a larger number of structure-directing ligands and solvent combinations and therefore should become an important tool for understanding reaction mechanisms in nanostructure synthesis.
We obtain H 2 from low temperature NH 3 decomposition using a new hollandite (KRu 4 O 8 ) catalyst supported on c-Al 2 O 3 discovered using high-throughput experimentation and advanced TEM/SEM characterization. Relative to the base Ru°catalyst, this new catalyst shows NH 3 conversion enhancements of 30-50% at T = 350°C and decomposition activity at temperatures decreased by 50-100°C. TEM analysis over the lifetime of the catalyst shows multiple phases and morphologies suggesting that the KRu 4 O 8 behaves either as a new low-temperature decomposition catalyst or as a precursor to the active catalyst.
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