Model catalysts alkali promoters

It is obvious that one can use the basic ideas concerning the effect of alkali promoters on hydrogen and CO chemisorption (section 2.5.1) to explain their effect on the catalytic activity and selectivity of the CO hydrogenation reaction. For typical methanation catalysts, such as Ni, where the selectivity to CH4 can be as high as 95% or higher (at 500 to 550 K), the modification of the catalyst by alkali metals increases the rate of heavier hydrocarbon production and decreases the rate of methane formation.128 Promotion in this way makes the alkali promoted nickel surface to behave like an unpromoted iron surface for this catalytic action. The same behavior has been observed in model studies of the methanation reaction on Ni single crystals.129... 

The book focuses on three main themes catalyst preparation and activation, reaction mechanism, and process-related topics. A panel of expert contributors discusses synthesis of catalysts, carbon nanomaterials, nitric oxide calcinations, the influence of carbon, catalytic performance issues, chelating agents, and Cu and alkali promoters. They also explore Co/silica catalysts, thermodynamic control, the Two Alpha model, co-feeding experiments, internal diffusion limitations. Fe-LTFT selectivity, and the effect of co-fed water. Lastly, the book examines cross-flow filtration, kinetic studies, reduction of CO emissions, syncrude, and low-temperature water-gas shift. 

Structure and Reactivity of Alkali-Promoted NiO Model Catalysts... 

Non-Flory molecular weight distributions have also been attributed to the presence of several types of active sites with different probabilities for chain growth and for chain termination to olefins and paraffins (45). Two-site models have been used to explain the sharp changes in chain growth probability that occur for intermediate-size hydrocarbons on Fe-based catalysts (46,47). Many of these reports of non-Flory distributions may instead reflect ineffective dispersal of alkali promoters on Fe catalysts or inadequate mass balances and product collection protocols. Recently, we have shown that multisite models alone cannot explain the selectivity changes that occur with increasing chain size, bed residence time, and site density on Ru and Co catalysts (4,5,40,44). 

In order to obtain more fundamental catalytic activity data of the catalytic materials of interest a number of model catalysts consisting of alkali metal and precious metal were prepared and tested for their ability to promote the reactions of water and carbon dioxide with solid carbon. These tests provide basic information about the ability of the catalysts to catalyse soot combustion with CO2, H2O and O2. Results are summarized in Table 2. Both alkali metal and precious metal (PM) doped supports were used. Two supports were used which can be categorised as an inert and a reducible oxide support. Clearly the presence of the alkali metal has a significant effect on catalysing the soot combustion as anticipated. The effect of the reducible oxide support is not significant. In addition to the experiments summarised in Table 2 two further samples of alkali metal supported on an alumina foam and cordierite wall flow filter were prepared and coated with soot in a similar manner to that described above. Measurement of the soot combustion characteristics of these samples in O2, CO2 and H2O were very similar to the powder samples. 

Abstract Metal-carbonyl bonds, as found for carbon monoxide either liganded to metalloporphyrins or chemisorbed onto metal surfaces, are discussed. These two classes of systems are compared with emphasis on short range vs. long range perturbations of the vibrational bands, and Vcq. It is concluded that metalloporphyrins serve as models for terminal bondi on metal surfaces and that data obtained for carbonyl-hemes enable the isolation of non-local electronic effects, which are much discussed in connection with alkali promoters coadsorbed with CO on metal catalysts. 

Figure 5.1 shows that the simple model correctly captures that a temperature of the order 700 K is needed for the ammonia synthesis rate to be high enough for a reasonable TOE Industrially, promoters are sometimes added to catalysts in order to increase their TOP significantly. This is also the case for ammonia synthesis where alkali and earth alkali promoters are employed. The role of these promoters is often to decrease the dissociation barrier of the rate-determining step. The issue of promoters will be discussed in Chapter 9. 

Alkalis are the most important electropositive promoters of metal and metal oxide catalysts. They are used in many important industrial catalysts but are also quite suitable for fundamental studies since they can be easily introduced under vacuum conditions on well-characterized model metal surfaces. 

The aim of this work was to apply combined temperature-programmed reduction (TPR)/x-ray absorption fine-structure (XAFS) spectroscopy to provide clear evidence regarding the manner in which common promoters (e.g., Cu and alkali, like K) operate during the activation of iron-based Fischer-Tropsch synthesis catalysts. In addition, it was of interest to compare results obtained by EXAFS with earlier ones obtained by Mossbauer spectroscopy to shed light on the possible types of iron carbides formed. To that end, model spectra were generated based on the existing crystallography literature for four carbide compounds of... 

This section reports a series of examples of application of the cluster model approach to problems in chemisorption and catalysis. The first examples concern rather simple surface science systems such as the interaction of CO on metallic and bimetallic surfaces. The mechanism of H2 dissociation on bimetallic PdCu catalysts is discussed to illustrate the cluster model approach to a simple catalytic system. Next, we show how the cluster model can be used to gain insight into the understanding of promotion in catalysis using the activation of CO2 promoted by alkali metals as a key example. The oxidation of methanol to formaldehyde and the catalytic coupling of prop)me to benzene on copper surfaces constitute examples of more complex catalytic reactions. 

Methanol and higher oxygenate syntheses follow different mechanistic and kinetic patterns over the various catalysts discussed here. Each such pattern is regular, however, and can be modeled with a few kinetic parameters based on fundamental mechanistic steps involved in the C-H, C-C, and C-0 bond forming reactions. Alkali co-catalysts play an important role by promoting... 

---