Transition metal oxides are capable of a wide range of work functions. This quality allows them to be used in many applications that involve charge transfer with adsorbed molecules, for example as heterogeneous catalysts, as charge-injection layers in organic electronics, and as electrodes in fuel cells. Chemical and structural factors can alter transition-metal oxide work functions, often making their work functions difficult to control. Little is known about the effects of the cation oxidation state and point defects on the oxide work function. It is necessary to understand how such chemical and structural factors affect work functions in order to controllably tune transition metal oxides for desired applications. Here, a correlation between the oxide work function and cation oxidation state is demonstrated. This correlation is attributed to the change in cation electronegativity with oxidation state. A model is presented that relates the work function to the oxygen deficiency for d0 oxides in the limit of dilute oxygen vacancies. It is proposed that the rapid initial decrease in work function, observed for d0 oxides, is caused by an increase in the density of donor-like defect states. It is also shown that oxides tend to have decreased work functions near a metal/metal-oxide interface as a consequence of the relationship between defects and work function. These insights provide guidelines for tuning transition metal oxide work functions.

Transition Metal Oxide Work Functions: The Influence of Cation Oxidation State and Oxygen Vacancies

Systematic trends of the 57Fe Mössbauer isomer shifts in (FeOn) and (FeFn) polyhedra. Evidence of a new correlation between the isomer shift and the inductive effect of the competing bond T-X (→ Fe) (where X is O or F and T any element with a formal positive charge)

The review is a summary and analysis of the data characterizing CO adsorption on surface cationic sites of oxides including supported materials and microporous and mesoporous materials. The contributions of various types of CO bonding to the IR frequency shifts of carbon-bonded molecules are analyzed, namely, the increase of the CO stretching frequency in cases of electrostatic and σ bonding and the decrease of the frequency with π bonding. Polycarbonyls, bridging CO, oxygen-bonded CO, and tilted CO are also considered. The main part of the review is a collection of the experimental results characterizing carbonyls of individual metal ions. The spectral behavior of CO bonded to metal atoms is also assessed in the cases when the metal ions are easily reduced to metal (Cu, Ag, Au, Pd, or Pt) or cationic carbonyls are produced after CO adsorption on supported metals (Ru, Rh, Ir, and Os). The interaction of CO with surface OH groups is also considered. It is demonstrated that IR spectroscopy of adsorbed CO is an efficient methodology to characterize cationic surface sites in terms of their nature, oxidation states, coordination environment and coordinative unsaturation, and location at faces, edges or corners of microcrystallites. When applied to materials with surface hydroxyl groups CO undergoes hydrogen bonding and information can be collected on the proton acid strength.

Characterization of oxide surfaces and zeolites by carbon monoxide as an IR probe molecule

The electronegativities of 82 elements in different valence states and with the most common coordination numbers have been quantitatively calculated on the basis of an effective ionic potential defined by the ionization energy and ionic radius. It is found that for a given cation, the electronegativity increases with increasing oxidation state and decreases with increasing coordination number. For the transition-metal cations, the electronegativity of the low-spin state is higher than that of the high-spin state. The ligand field stabilization, the first filling of p orbitals, the transition-metal contraction, and especially the lanthanide contraction are well-reflected by the relative values of our proposed electronegativity. This new scale is useful for us to estimate some quantities (e.g., the Lewis acid strength for the main group elements and the hydration free energy for the first transition series) and predict the structure and property of materials.

Estimation of Electronegativity Values of Elements in Different Valence States

The structure-performance relationship of α-, β-, γ- and δ-MnO2 catalysts were studied. The four samples exhibited different activities of toluene oxidation in terms of distinct tunnel sizes, surface-active oxygen and redox properties. δ-MnO2 catalyst with K+ in the mezzanines of its layers presented the highest toluene oxidation activity under a GHSV of 60,000 mL·g−1 h−1, as well as good water resistance. HAADF images and EELS results showed that oxygen vacancies preferred to form on δ-MnO2 lattice with layer stack dislocations via Mn4+ reduction rather than β-MnO2 with good crystallization. These inherent-distorted structures with heterocations K+ improved the emerge-annihilate cycling of oxygen vacancies. In-situ DRIFTS results showed that toluene adsorption was facilitated via rapid dehydrogenation of methyl due to abundant surface adsorbed oxygen on δ-MnO2. In addition, benzoate, maleic and manganese carbonate on δ-MnO2 were the key intermediate species during toluene oxidation at relatively low temperatures.

Yonghe. Zhang

Papers

Electronegativities of elements in valence states and their applications. 1. Electronegativities of elements in valence states

Electronegativities of elements in valence states and their applications. 1. Electronegativities of elements in valence states