Zur Kristallchemie der Oxoargentate und Silberoxometallate

Ag6Cl4: the first silver chloride with rare Ag6 clusters from an ab initio study

The first diamagnetic semiconducting silver subhalide, Ag₆Cl₄, featuring rare subvalent Ag₆ clusters with 2e^--6c bonds, 1D argentophilic d¹⁰-d¹⁰ intercluster interactions and 3D ionic connectivity ensured by Cl atoms has been predicted employing Density Functional Theory. Ag₆Cl₄ carries all unique features of Ag⁺ so far observed only in selected metal-rich oxides and as such represents an important addition to the discussion of the special bonding properties of silver with a filled d¹⁰-shell. Having appreciable formation enthalpy and dynamical stability, Ag₆Cl₄ should be in principle possible to synthesize as a metastable phase relative to AgCl.

New M+, M3+-arsenates - the framework structures of AgM3+(HAsO4)2 (M3+ = Al, Ga) and M+GaAs2O7 (M+ = Na, Ag)

The crystal structures of hydro-thermally synthesized silver(I) aluminium bis-[hydrogen arsenate(V)], AgAl(HAsO4)2, silver(I) gallium bis-[hydrogen arsenate(V)], AgGa(HAsO4)2, silver gallium diarsenate(V), AgGaAs2O7, and sodium gallium diarsenate(V), NaGaAs2O7, were determined from single-crystal X-ray diffraction data collected at room temperature. The first two compounds are representatives of the MCV-3 structure type known for KSc(HAsO4)2, which is characterized by a three-dimensional anionic framework of corner-sharing alternating M3+O6 octa-hedra (M = Al, Ga) and singly protonated AsO4 tetra-hedra. Inter-secting channels parallel to [101] and [110] host the Ag+ cations, which are positionally disordered in the Ga compound, but not in the Al compound. The hydrogen bonds are relatively strong, with O⋯O donor-acceptor distances of 2.6262 (17) and 2.6240 (19) Å for the Al and Ga compounds, respectively. The two diarsenate compounds are representatives of the NaAlAs2O7 structure type, characterized by an anionic framework topology built of M3+O6 octa-hedra (M = Al, Ga) sharing corners with diarsenate groups, and M+ cations (M = Ag) hosted in the voids of the framework. Both structures are characterized by a staggered conformation of the As2O7 groups.

Oxide-based nanomaterials for fuel cell catalysis: the interplay between supported single Pt atoms and particles

Nanomaterials coated with atomically dispersed platinum on ceria are structurally dynamic and show high potential for applications in fuel cells.

Towards stable single-atom catalysts: strong binding of atomically dispersed transition metals on the surface of nanostructured ceria

Surface oxygen sites on CeO₂ nanostructures are able to bind atoms of various transition metals strong enough to prevent their sintering. This finding opens a knowledge-driven way to prepare stable single-atom catalysts with maximum metal efficiency.

Surface Structure of Silver Nanoparticles as a Model for Understanding the Oxidative Dissolution of Silver Ions

The toxicity of silver nanoparticles (AgNPs) has been related to the release of ionic silver. This process is influenced by a large variety of factors and is poorly understood. The key to understanding Ag(+) release by AgNPs is its subvalency. This is a fundamental property of Ag that can be elucidated by analyzing the crystal structures of a specific class of Ag materials as well as MO/DFT (molecular orbital/density functional theory)-optimized Ag13(OH)4 clusters, being precursors of AgNPs. Semimetallic silver at the (111) faces of AgNPs has a subvalency of +(1)/3 v.u., forming ≡Ag3OH(0) surface groups with a maximum site density of 4.7 sites/nm(2). Oxidative dissolution may remove these groups with the simultaneous formation of oxygen radicals that may further interact with the surface via different pathways. Reactive oxygen species (ROS) can create a circular process with the dissolution of ≡Ag3OH(0), exposure of new metallic sites at the underlying lattice, and subsequent oxidation to ≡Ag3OH(0). This regeneration process is interrupted by the penetration of O(•) radicals into the lattice, forming highly stable Ag6O octahedra with subvalent silver that protects the AgNP from further oxidation. A thermodynamic model has been developed that quantitatively describes the equilibrium condition between ≡Ag3OH(0) and ≡Ag6O(0) and explains a large variety of collectively observed phenomena.

Structure of the mixed-metal carbonate KAgCO3revisited: order–disorder (OD) polytypism and allotwinning

Crystals of KAgCO3belong to an order–disorder (OD) family of structures composed of layers of two kinds. There are two polytypes with a maximum degree of order [MDO1:Pccb; MDO2:Ibca, doubleda-axis compared with MDO1], which are both realised to a different extent in two crystals under investigation [volume fraction MDO1:MDO2in crystal (I): 0.0216:0.9784 (3) and in crystal (II): 0.9657:0.0343 (3)]. Sharp diffraction spots and the absence of diffuse scattering indicate highly ordered macroscopic domains. The structure of KAgCO3was refined concurrently against all reflections using an allotwin model (addition of the intensities of both domains). It is shown that a disorder model refined against reflections of only one domain can lead to a significant overestimation of the volume fraction of this domain.

Chemistry of precious metal oxides relevant to heterogeneous catalysis

The platinum group metals (PGMs) are widely employed as catalysts, especially for the mitigation of automotive exhaust pollutants. The low natural abundance of PGMs and increasing demand from the expanding automotive sector necessitates strategies to improve the efficiency of PGM use. Conventional catalysts typically consist of PGM nanoparticles dispersed on high surface area oxide supports. However, high PGM loadings must be used to counter sintering, ablation, and deactivation of the catalyst such that sufficient activity is maintained over the operating lifetime. An appealing strategy for reducing metal loading is the substitution of PGM ions into oxide hosts: the use of single atoms (ions) as catalytic active sites represents a highly atom-efficient alternative to the use of nanoparticles. This review addresses the crystal chemistry and reactivity of oxide compounds of precious metals that are, or could be relevant to developing an understanding of the role of precious metal ions in heterogeneous catalysis. We review the chemical conditions that facilitate stabilization of the notoriously oxophobic precious metals in oxide environments, and survey complex oxide hosts that have proven to be amenable to reversible redox cycling of PGMs.

Magnetic behaviour of layered Ag(II) fluorides

Fluoride phases that contain the spin-1/2 4d9 Ag(II) ion have recently been predicted to have interesting or unusual magnetochemistry, owing to their structural similarity to the 3d9 Cu(II) cuprates and the covalence associated with this unusual oxidation state of silver. Here we present a comprehensive study of structure and magnetism in the layered Ag(II) fluoride Cs2AgF4, using magnetic susceptometry, inelastic neutron scattering techniques and both X-ray and neutron powder diffraction. We find that this material is well described as a two-dimensional ferromagnet, in sharp contrast to the high-T(C) cuprates and a previous report in the literature. Analyses of the structural data show that Cs2AgF4 is orbitally ordered at all temperatures of measurement. Therefore, we suggest that orbital ordering may be the origin of the ferromagnetism we observe in this material.

Electronic Structure of Ag2Cu2O4. Evidence of Oxidized Silver and Copper and Internal Charge Delocalization

The electronic structure of the recently isolated silver copper oxide Ag(2)Cu(2)O(4) is analyzed along with its precursor Ag(2)Cu(2)O(3) and similar binary oxides, Ag(2)O, AgO, CuO, and NaCuO(2), using X-ray photoemission (XPS) and X-ray absorption (XAS) measurements. The results for Ag(2)Cu(2)O(4) reveal an electronic distribution in which silver and copper share a delocalized valence scheme with both metals in formal oxidation states larger than the usual Ag(I) and Cu(II). Only one type of crystallographic silver or copper is found, but disorder-strain parameters are considerable and the possibilities of thermal disorder, atomic motion, oxygen contribution, mixed valence, and internal charge delocalization are considered. Classical coordination descriptions for oxidized silver are revisited in terms of this new internal charge delocalization framework found for the electronic structure.