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The structure sensitivity of Pt nanoparticles (PtN) for gas-phase acrolein (AC) hydrogenation was probed for PtN on BaxSr1-xTiO3 nanocuboid supports with (001) facets in a combined theoretical and experimental study. In situ selectivity for allyl alcohol increased with Sr concentration in the support, corresponding to modifications in the stable Winterbottom shape and lattice strain of the Pt nanoparticles as a result of the interfacial energy between Pt and the BaxSr1-xTiO3 supports. "Local model" nanofacets of Pt surface, edge, and corner morphologies were developed as compact representations of adsorption and reaction sites. Density functional theory (DFT) was used as the primary modeling tool of equilibrium adsorption states. We argue that adsorption on edge sites is critically important for the overall allyl alcohol selectivity of PtN catalysts. A simple model was developed to represent PtN strain effects due to its interaction with the substrate. Bader topological atom, spherical volume averaging charge, and modified bond valence sum (BVS) analyses were used to understand bonding structure. Density of states analysis was performed for structures of PtN, adsorbed AC, and intermediate products to examine adsorbate-particle interactions. The simulated hydrogenation of AC on PtN nanofacets was compared to the in situ hydrogenation of AC by PtN on BaxSr1-xTiO3 to examine the effects of facet, edge and corner sites on the overall selectivity.

Here we describe the Wulff shape of strontium titanate nanocuboids prepared by a hydrothermal method and annealed at high temperature. Transmission electron microscopy was used to measure the faceting ratios d(110):d(100) which are compared with surface energy ratios γ(110):γ(100) from first-principles calculations. Internal voids attributed to the Kirkendall effect were also measured and show agreement with the external faceting. Experiment and theory are shown to agree strongly within statistical and density functional theory error.

A catalyst of Pt nanoparticles was prepared by atomic layer deposition on SrTiO3 nanocuboids and tested for methanol decomposition and partial oxidation. The catalyst had uniform nanoparticle size of 1.58 ± 0.37 nm and a Pt (1 1 1) surface. In situ X-ray absorption near-edge spectroscopy (XANES) measured in a temperature-programmed reduction showed that the Pt particles were easily reduced. However, the as-received catalyst, a reduced catalyst, and an oxidized catalyst all had catalytic activity, differing slightly in methanol conversion and product selectivity. In situ XANES also revealed that CO adsorbed on the Pt sites was the only observed surface species during both methanol decomposition and partial oxidation. It seemed that the breakage of CH and OH bonds overwhelmingly occurred once methanol was adsorbed, forming H2 and adsorbed CO. The latter was then released from the catalyst surface or was oxidized to CO2 when O2 was present.

Selective, active, and robust catalysts are necessary for the efficient utilization of new feedstocks. Face-selective catalysts can precisely modify catalytic properties, but are often unstable under reaction conditions, changing shape and losing selectivity. Herein we report a method for synthesizing stable heterogeneous catalysts in which the morphology and selectivity can be tuned precisely and predictably. Using nanocrystal supports, we epitaxially stabilize specific active phase morphologies. This changes the distribution of active sites of different coordination, which have correspondingly different catalytic properties. Specifically, we utilize the different interfacial free-energies between perovskite titanate nanocube supports with different crystal lattice dimensions and a platinum active phase. By substituting different sized cations into the support, we change the lattice mismatch between the support and the active phase, thereby changing the interfacial free-energy, and stabilizing the active phase in different morphologies in a predictable manner. We correlate these changes in active phase atomic coordination with changes in catalytic performance (activity and selectivity), using the hydrogenation of acrolein as a test reaction. The method is general and can be applied to many nanocrystal supports and active phase combinations.

article i nfo Chemical bonding has often been ignored in favor of physics based energetic considerations in attempts to understand the structure, stability, and reactivity of oxide surfaces. Herein, we analyze the chemical bonding in published structures of the SrTiO3, MgO, and NiO surfaces using bond valence sum (BVS) analysis. These simple chemical bonding theories compare favorably with far more complex quantum mechanical calcula- tions in assessing surface structure stability. Further, the coordination and bonding of surface structures explains the observed stability in a readily comprehensible manner. Finally, we demonstrate how simple chemical bonding models accurately predict the adsorption of foreign species onto surfaces, and how such models can be used to predict changes in surface structures.

Pt/SrTiO3 shows promise as a low temperature hydrocarbon combustion catalyst for automotive applications. In this study, SrTiO3 nanocuboid supports were synthesized using sol-precipitation coupled with hydrothermal synthesis, and platinum was deposited on the nanocuboids with 1, 3, and 5 cycles of atomic layer deposition (ALD). The platinum particles have a highly uniform distribution both before and after reaction testing, and range from 1 to 5 nm in size, depending upon the number of ALD cycles. These materials have a >50 °C lower light-off temperature for propane oxidation than a conventional Pt/Al2O3 catalyst, turn over frequencies up to 3 orders of magnitude higher, and show improved resistance to deactivation. The increased activity is attributed to the stabilization of a Pt/PtO core/shell structure during operating conditions by the strong epitaxy between the Pt and the SrTiO3 support.

Raman spectroscopy was used to demonstrate that the lattice dynamics of SrTiO3 (STO) nanoparticles strongly depends on their microstructure, which is in turn determined by the synthetic approach employed. First-order Raman modes are observed at room temperature in STO single-crystalline nanocubes with average edge lengths of 60 and 120 nm, obtained via sol-precipitation coupled with hydrothermal synthesis and a molten salt procedure, respectively. First-order Raman scattering arises from local loss of inversion symmetry caused by surface frozen dipoles, oxygen vacancies, and impurities incorporated into the host lattice. The presence of polar domains is suggested by the pronounced Fano asymmetry of the peak corresponding to the TO2 polar phonon, which does not vanish at room temperature. These noncentrosymmetric domains will likely influence the dielectric response of these nanoparticles.

Concepts in chemical bonding when combined with physics-based energetic considerations can lead to a more complete understanding of the structure, stability, and reactivity of oxide surfaces. While this symbiosis has long been understood for bulk structures, chemical bonding considerations have historically been used less frequently for surfaces. In this chapter, we analyze the chemical bonding of published surface structures of SrTiO3 and MgO using bond valence sum analysis. Bond valence theory compares favorably with complex quantum mechanical calculations in assessing surface structures and explains the experimentally observed surface structures in a readily comprehensible manner. Bond valence theory also helps explain discrepancies between DFT predicted surface stability and experimentally observed surface structures, accurately predicts the adsorption of foreign species onto surfaces, and can be used to predict changes in surface structures.

We report the synthesis and structure determination of single crystals of La4Cu3MoO12 grown from a CuO/KCl flux. This material, whose structure had previously been reported based solely on polycrystalline diffraction data, shows frustrated magnetic behavior and an anti-ferromagnetic ordering of spin-1/2 triangles at low temperatures. The structural and atomic parameters determined from the single crystal data are in very good agreement with those reported previously. However, HREM data showed evidence for disorder in the stacking of the Cu3MoO4 planes, and thus a twinned structural refinement in space group P21/m was replaced by an equivalent disordered structural model in space group Pm. This development of a synthetic route to single crystals of La4Cu3MoO12 will allow a more detailed investigation of its complex electronic and magnetic properties.

Strontium titanate is seeing increasing interest in fields ranging from thin-film growth to water-splitting catalysis and electronic devices. Although the surface structure and chemistry are of vital importance to many of these applications, theories about the driving forces vary widely. We report here a solution to the 3 x 1 SrTiO(3)(110) surface structure obtained through transmission electron diffraction and direct methods, and confirmed through density functional theory calculations and scanning tunnelling microscopy images and simulations, consisting of rings of six or eight corner-sharing TiO(4) tetrahedra. Further, by changing the number of tetrahedra per ring, a homologous series of n x 1 (n > or = 2) surface reconstructions is formed. Calculations show that the lower members of the series (n < or = 6) are thermodynamically stable and the structures agree with scanning tunnelling microscopy images. Although the surface energy of a crystal is usually thought to determine the structure and stoichiometry, we demonstrate that the opposite can occur. The n x 1 reconstructions are sufficiently close in energy for the stoichiometry in the near-surface region to determine which reconstruction is formed. Our results indicate that the rules of inorganic coordination chemistry apply to oxide surfaces, with concepts such as homologous series and intergrowths as valid at the surface as they are in the bulk.