In catalytic activity

The measures of solid state reactivity to be described include experiments on solid-gas, solid-liquid, and solid-solid chemical reaction, solid-solid structural transitions, and hot pressing-sintering in the solid state. These conditions are achieved in catalytic activity measurements of rutile and zinc oxide, in studies of the dissolution of silicon nitride and rutile, the reaction of lead oxide and zirconia to form lead zirconate, the monoclinic to tetragonal transformation in zirconia, the theta-to-alpha transformation in alumina, and the hot pressing of aluminum nitride and aluminum oxide. 

When studying the kinetics of diffusion of hydrogen through palladium, Farkas (28) noticed the difference in catalytic activity of both sides of the palladium disks or tubes for the parahydrogen conversion the energy of activation was greater on the inlet side than on the outlet side, where due to extensive desorption of the hydrogen its concentration could be lower. 

NO-sensitive GC represents the most important effector enzyme for the signalling molecule NO, which is synthesised by NO synthases in a Ca2+-dependent manner. NO-sensitive GC contains a prosthetic heme group, acting as the acceptor site for NO. Formation of the NO-heme complex leads to a conformational change, resulting in an increase of up to 200-fold in catalytic activity of the enzyme [1]. The organic nitrates (see below) commonly used in the therapy of coronary heart disease exert their effects via the stimulation of this enzyme. 

On the other hand, Sb203 and metal oxides such as Ge02 exhibit a good catalytic activity for the polycondensation step. This explains why the associations of metal acetates with Sb203 are often reported catalytic systems. The following order in catalytic activity was found for the PEN polycondensation step Ti(IV)... 

This linear variation in catalytic activation energy with potential and work function is quite noteworthy and, as we will see in the next sections and in Chapters 5 and 6, is intimately linked to the corresponding linear variation of heats of chemisorption with potential and work function. More specifically we will see that the linear decrease in the activation energies of ethylene and methane oxidation is due to the concomitant linear decrease in the heat of chemisorption of oxygen with increasing catalyst potential and work function. 

This is an important result and shows that the dramatic decrease in catalytic activation energy, EA, upon increasing is due to the decrease in Ed and concomitant weakening of the Pt=0 chemisorptive bond upon increasing UWr and O. 

The second phenomenon, i.e. the change in catalytic activity or selectivity of the active phase with varying catalyst support, is usually termed metal-support interaction. It manifests itself even when the active phase has the same dispersion or average crystallite size on different... 

The peak power density was enhanced when the operating temperature increased, but was limited to 650 C. The reason for the performance decrease in the cell operated over 650 C may result from increase in catalytic activity of LSM for methane partial oxidation. 

Simandi LI (2003) Advances in catalytic activation of dioxygen by metal complexes. Kluwer, Dordrecht... 

It should be noted that the catalytic activity falls markedly with an increase in the time-on-stream. Similar falls in catalytic activity were also observed in the cases of the other W-based mixed oxides, such as W-Sn, W-Ti, and W-Mo. It was also found that the deactivated catalysts can easily be regenerated by a heat-treatment at 500°C in air for 1 h. 

The results of the conversion measurements at atmospheric pressure using 138 mg catalyst are shown in fig. 1. The following sequence in catalytic activity was observed Cs-Ru/MgO > Ru/MgO > CS-RU/AI2O3 > RU/AI2O3. It is noteworthy that the catalytic activity of the Cs-Ru/MgO catalyst exceeds significantly the catalytic activity of a multiply promoted iron... 

Approximate composition of individual hexaaluminate particles was measured by an analytical electron microscope to elucidate the correlation between hexaaluminate phases (Mn-rich and -poor) and Pr contents in the lattice (Figure 6). Although relative concentration of Sr to A1 was a constant at x=0, the composition of the particles in the sample was separated into two groups from their Sr/Al ratios and Mn contents. Pr ions are preferentially doped in the particles with small Mn/ A1 ratios at x=0.2. With an increase in Pr concentration, the number of Pr-poor particles decreased and their Mn/Al ratios approached to those of Pr-rich particles at x=0.4. The substitution of Mn for Al site compensates excess charge of Pr3+ in the hexaaluminate lattice and leads to an increase of Mn in the hexaaluminate particle. It is considered that ease of reduction-oxidation cycle was influenced by the compensation and gave rise to increase in catalytic activity for... 

The difference in catalytic activity between the La- and the Ba-based hexa-aluminates results from the following reasons the first difference is the valence of cation in the mirror pleuie between tri-valent lanthanum ion and di-valent barium ion. The second is the crystal structure between magnetoplumbite and P-alumina, which are different in the coordination of ions and concentration of Frenkel-type defect in mirror plane. The redox cycle of transition metal in hexa-aluminate lattice, which closely related with catalytic activity, is affected sensitively with these two factors. 

When the amount of coke formed as a function of time on stream is compared to the decrease in catalytic activity (see Fig. 3), two regimes of deactivation can be noticed for the strongly deactivating catalysts, i e, a slow initial deactivation which is followed by a rapid loss of activity This first phase is characteristic of a slow transformation of the reactive carbon into less reactive coke. The second phase is attributed to carbon formed on the support which accumulates there and rapidly covers the Pt particles when its amount reaches a critical value causing the sudden decay of catalytic activity. 

In the late 1960s it was discovered (Entina, 1968 Binder et al., 1972) that a strong synergy effect exists in the platinum-ruthenium system. Alloys of these two metals are two to three orders of magnitude more active catalytically for the anodic oxidation of methanol than pure platinum, whereas pure ruthenium is altogether inactive for this reaction. Prolonged exploitation of such anodes is attended by a gradual decrease in catalytic activity of the alloys because of slow anodic dissolution of ruthenium from the surface layer. A similar simation is seen for platinum-tin alloys, which... 

In a number of studies a correlation was seen between the amount of nonstoi-chiometric oxygen in the spinel and the spinel s activity. It appears that excess oxygen consolidates the spinel s defect structure with a large number of active sites. Strong anodic polarization leads to ordering of this structure and thus to a decrease in catalytic activity. 

The simplest case is represented by curve 1. The activity depends linearly on the number of unpoisoned active sites. The interpretation of curves 2 and 3 is less obvious. In the former case the interpretation might be that the least active sites are poisoned first, whereas in the latter case the most active sites are poisoned preferentially. Mass-transfer limitations also play a role in poisoning behaviour. If, for example, the poison is deposited in the outer shell of the catalyst particles, a decrease in catalytic activity can be expected as qualitatively described by curve 3 in Fig. 3.37. 

Recently several pubhcations have examined replacing aqueous solvents with ionic liquids. Since simple and complex sugars are soluble in many imidazolium hahdes, water is not required as a co-solvent and degradation of HMF is minimal. Lansalot-Matras et al. reported on the dehydration of fmctose in imidazolium ionic liquids using acid catalyst (6). Moreau et al. reported that l-H-3-methylimidazolium chloride has sufficient acidity to operate without added acid (7). And we reported that a 0.5 wt% loading (6 mole% compared to substrate) of many metal halides in 1-ethyl-3-methylimidazohum chloride ([EMIM]C1) result in catalytically active materials particularly useful for dehydration reactions (8). 

Wender et al. reported a [5+2] cycloaddition in water by using a water-soluble rhodium catalyst having a bidentate phosphine ligand to give a 7-membered ring product (Eq. 4.69). This water-soluble catalyst was reused eight times without any significant loss in catalytic activity.133... 

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