Doped metal oxides

Semiconductor electrodes seem to be attractive and promising materials for carbon dioxide reduction to highly reduced products such as methanol and methane, in contrast to many metal electrodes at which formic acid or CO is the major reduction product. This potential utility of semiconductor materials is due to their band structure (especially the conduction band level, where multielectron transfer may be achieved)76 and chemical properties (e.g., C02 is well known to adsorb onto metal oxides and/ or noble metal-doped metal oxides to become more active states77-81). Recently, several reports dealing with C02 reduction at n-type semiconductors in the dark have appeared, as described below. 

All containers, processing equipment, and piping to be used in fluorine Service first must be passivated before use and thereafter designated fui fluorine service. These requirements result from the severe oxidizing characteristics of fluorine gas. Passivation removes any easily oxidized materials, such as paint, pipe dopes, metal oxides, grease, and metal filings. 

The introduction of crystal defects such as oxygen vacancies is also an important mechanism for doping metal oxides. In fact, many of the n-type doping processes for... 

Doped metal oxide catalysts are widely used in various catalytic processes. In many cases, the catalytic activity and selectivity of these catalysts may be related to their acidity or basicity. 

SOFCs are based on the use of doped metal oxides. Remarkably, there is production of chemicals in the fuel cell in the course of its operation, so that it can also be considered an electrosynthetic reactor. Formally, the anodic reaction produces the electrons that flow through the external circuit toward the cathode, where they are transferred to the cathodic reactant. The cathode reaction can be written as ... 

Fe S CSCHaPh) as electron carrier. Improvements in the quantum yield for the photodecomposition of water have been recorded using a 0.1% RuOj-n-Ti02(Nb-doped) metal oxide loaded with Pt particles. 

CNT-doped metal oxides. In this case, CNTs are embedded within the MOX matrix. This chapter will focus on this type of CNT and MOX hybrid material. The MOX/CNT thin films can be prepared using various techniques, such as spin-coating, drop-coating, dip-coating, and electron beam evaporation, details of which will be given in the following sections. 

Yeh and Kuwana " were the first to report on the electrochemistry of cytochrome c at doped metal oxide semiconductor electrodes. A nearly reversible electrode reaction was indicated by the cyclic voltammetry and differential pulse voltammetry of cytochrome c at tin-doped indium oxide electrodes. Except for the calculated diffusion coefficient, all of the characteristics of the electrochemistry of cytochrome c at this electrode indicated that the electrode reaction was well-behaved. A value of 0.5 x 10" cmVs was determined for the diffusion coefficient which, like previously determined values at mercury, is lower than the value obtained by nonelectrochemical methods (i.e., 1.1 X 10 cm /s " " ). The electrochemical response of cytochrome c at tin oxide semiconductor electrodes was reported to be quasi-reversible, although no details were given. " ... 

For doped metal oxides, the aliovalent metal ions influence the overall defect concentrations via what is sometimes called the First Law of Doping. This law simply states that adding an aliovalent dopant increases the concentration of defects with opposite charges, and decreases the concentration of defects with charges of the same sign. 

The concentration of dopants in metal oxides is usually limited to 1-2% at most, which corresponds to a concentration of 10 cm . Higher cmicentrations are not likely to be effective, and may even lead to segregation of the dopant phase. If the conductivity of a doped metal oxide photoelectrode is found to be too low, it may be more effective to anneal it at high temperatures and under low p(02) than to increase the dopant concentration. 

Advances in nano-material research have opened the door for transparent conductive materials, each with unique properties. These include CNTs, graphene, metal nanowires, and printable metal grids. Transparent electrodes are necessary components in many modem devices such as touch screens, LCDs, OLEDs, and solar cells, all of which are growing in demand. Traditionally, this role has been well served by doped metal oxides, such as indium tin oxide. A review exploring these innovations in transparent conductors and the emerging trends is presented recently (Hecht et al. 2011). Electrical conductivity in PS nanocomposites with ultralow graphene level was found to enhance significantly (Qi et al. 2011). 

A.2.2 Solid electrodes (gold, carbon, metal oxide electrodes). Cytochrome c is also adsorbed irreversibly at solid electrodes, which mostly results in irreversible kinetices of the electrode reaction. A nearly reversible reaction of strongly adsorbed cytochrome c is, however, observed at electrodes of metal oxides (doped metal oxide semiconductor electrodes, tin-doped indium oxide electrode) [202, 203]. 

Like semiconducting doped metal oxides such as Sn02 or ZnO, doped polymer films also vary their conductivity when exposed to some gases, but polymer based sensors work at ambient temperature. 

It is important that, during simultaneous study of gas-sensing and catalytic properties of doped metal oxides, it was established that, as a rule, the change of sensor response does not coincide with the change of catalytic activity of analyzed material (see Fig. 23.10) (Yamaura et al. 2000). Such behavior of tested characteristics testifies that the observed decrease of sensor response is not connected with reducing catalytic activity of sensing material, as could be assumed at first sight. 

At even higher temperatures in the range of 800-900 °C, ionic conduction can also be observed in solid oxides. Certain doped metal oxides become conductors hence, they can be utilized as electrolytes in the solid oxide fuel cell (SOFC). The most common oxide used is yttria-stabilized cubic zirconia (Zr02), called YSZ, with a doping level of 8% Y2O3. This material is a ceramic the thermal expansion coefficient of its thin layers has to be matched with the other fuel cell components. 

The main classes of catalysts used for heterogeneous WHPCO reaction are clays and anionic clays (hydrotalcites), metal-ion exchanged zeolites and mesoporous silica containing transition metals, and doped metal oxides. Although some other transition metals have been also used (Mn, V), most catalysts contain iron and/or copper as the active elements. Leaching of the active metal is also a significant problem in this case. While different types of catalysts have been reported, only a few of them have been effectively proven to have a stable activity in long-term continuous experiments or at least in several repeated batch tests. Between the stable catalysts, Fe- and Cu-PILC (pillared clays) materials " have the best combination of activity and stability. However, the limited quantity of active elements (around 2% wt. of iron or copper) necessary to achieve stable performances, limits the overall activity. 

Table 1S.6 Base/acid site ratio of Na-doped metal-oxide catalysts and their catalytic performance for the dehydration of glycerol. ...

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