Surface area promoters, effect

The inhibition effect of promoters on the methanation. R is commonly believed that for fused iron catalyst, AI2O3 increases iron surface area (structural effect), while K2O donates electrons to iron atom, and increases electron density and enhances the activity of ammonia synthesis reaction (electronic effect). For the supported ruthenium catalysts, the effect of promoters on performances becomes more complex due to the existence of support. ARhough there are a lot of studies on the role of promoter for ammonia synthesis reaction, the chemical state, the distribution and the mechanism are still unclear. The role of promoters include covering chemisorption s site, donating electron to active metal, direct interacting with the adsorption intermediate and electrostatic field and so For supported... 

Common metals often form mixed oxides with the support compounds. For that reason common metals are usually used as massive metal catalysts. In the case of massive metal catalysts, a few weight percent of a promoter is added. Some promoters make mixed oxides with the active element and influence the reduction process and the surface area (structural promoter). Others are deposited on the metal surface and have an electronic interaction with the surface (chemical effect). Ammonia activity on Fe is known to be enhanced by adding AI2O3 and K2O. It is believed that AI2O3 stabilizes the high surface area of Fe (structural effect) and K2O promotes the ammonia activity per Fe surface area (chemical effect). The structural effect is well studied on Fe single crystal surfaces, where Fe(lll) is the most active plane and Fe(llO) is the next and Fe(lOO) is the least active plane [93]. Such studies have been expanded to other catalysts such as Re, and will be reviewed in Section 3.2.4.2. 

When a liquid or solid substance is emitted to the air as particulate matter, its properties and effects may be changed. As a substance is broken up into smaller and smaller particles, more of its surface area is exposed to the air. Under these circumstances, the substance, whatever its chemical composition, tends to combine physically or chemically with other particles or gases in the atmosphere. The resulting combinations are frequently unpredictable. Very small aerosol particles (from 0.001 to 0.1 Im) can act as condensation nuclei to facilitate the condensation of water vapor, thus promoting the formation of fog and ground mist. Particles less than 2 or 3 [Lm in size (about half by weight of the particles suspended in urban air) can penetrate the mucous membrane and attract and convey harmful chemicals such as sulfur dioxide. In order to address the special concerns related to the effects of very fine, iuhalable particulates, EPA replaced its ambient air standards for total suspended particulates (TSP) with standards for particlute matter less than 10 [Lm in size (PM, ). 

In the case of electrochemically promoted (NEMCA) catalysts we concentrate on the adsorption on the gas-exposed electrode surface and not at the three-phase-boundaries (tpb). The surface area, Ntpb, of the three-phase-boundaries is usually at least a factor of 100 smaller than the gas-exposed catalyst-electrode surface area Nq. Adsorption at the tpb plays an important role in the electrocatalysis at the tpb, which can affect indirectly the NEMCA behaviour of the electrode. But it contributes little directly to the measured catalytic rate and thus can be neglected. Its effect is built in UWr and [Pg.306]

Within the inverse model catalyst approach, the y/7-V309-Rh(l 11) nanostructures have been used to visualize surface processes in the STM with atomic-level precision [104]. The promoting effect of the V-oxide boundary regions on the oxidation of CO on Rh(l 1 1) has been established by STM and XPS by comparing the reaction on two differently prepared y/7-V309-Rh(l 11) inverse catalyst surfaces, which consist of large and small two-dimensional oxide islands and bare Rh areas in between [105]. A reduction of the V-oxide islands at their perimeter by CO has been observed, which has been suggested to be the reason for the promotion of the CO oxidation near the metal-oxide phase boundary. 

The promoter may slow down, or otherwise influence crystal formation and growth, or produce lattice defects. These effects may lead either to a higher activity per unit area or to a higher specific surface area. 

A more robust way to write a rate law for a catalytically promoted reaction is to include the concentrations of one or more surface complexes, in place of the surface area As. In this case, the simulation can account not only for the catalyzing surface area, since the mass of a surface complex varies with the area of the sorbing surface, but the effects of pH, competing ions, and so on. 

The example of uranyl reduction shows the utility of this approach. The concentrations of the two surface complexes vary strongly with pH, and this variation explains the observed effect of pH on reaction rate, using a single value for the rate constant k+. If we had chosen to let the catalytic rate vary with surface area, according to 17.12, we could not reproduce the pH effect, even using H+ and OH-as promoting and inhibiting species (since the concentration of a surface species depends not only on fluid composition, but the number of surface sites available). We would in this case need to set a separate value for the rate constant at each pH considered, which would be inconvenient. 

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