Activity and surface area

The zinc oxide component of the catalyst serves to maintain the activity and surface area of the copper sites, and additionally helps to reduce light ends by-product formation. Selectivity is better than 99%, with typical impurities being ethers, esters, aldehydes, ketones, higher alcohols, and waxes. The alumina portion of the catalyst primarily serves as a support. 

It is clearly visible from Figure 28.24 that initial moduli of nanocomposites are higher than those of microcomposites at any loading, given that the surface activity and surface area between nanoparticles and microparticles are different. 

In order to investigate the relationship between the surface area of skeletal copper and activity, the same sample of catalyst was tested in four successive runs. Rate constants was compared with that of another sample prepared in the same way but pretreated in 6.2 M NaOH at 473 K before use. Figure 4 shows that the first order rate constants, calculated so as to take into account the mass of catalyst relative to the volume of solution, decreased in the first three cycles but then stabilised. The surface areas, measured on small samples taken after reaction, mirrored this pattern. The rate constant, and the surface area, for the pretreated catalyst was similar to those obtained in cycles 3 and 4. It is apparent that activity and surface area are closely related for the unpromoted skeletal copper catalyst and that the pretreatment in NaOH at 473 K is approximately equivalent to three repeated reactions in terms of stabilising activity and surface area. 

Versus the Nafion catalyst (random copolymer) higher specific catalytic activity and surface area, relative simplicity of the synthetic method. 

A strong caveat to this prediction is the role of active surface area to the actual activity observed in fuel cells. This is often termed the mass activity— that is, the activity per mass of active metal (usually Pt), which has a direct correlation to cost. Mass activity is a combination of specific activity and surface area ... 

Vanadium phosphorus oxides (VPO) are commercially used as catalysts for the s5mthesis of maleic anhydride from the partial oxidation of n-butane. The phase constitution and the morphology of the catalyst are found to be dependent on the preparation routes and the applied solvent [78]. Recently, a method to prepare VPO catalysts in aqueous solution at elevated temperature was reported [79]. In addition to the linear relationship between specific activity and surface area, a small group of catalysts exhibit enhanced activity, which could be due to the combination of a higher proportion of V phases in the bulk of vanadyl pyrophosphate (V0)2P207 catalyst [79, 80]. With high relevance to the catalytic properties, the microstructure characterisation of VPO therefore is of great importance. 

Science. As the researchers noted, There were insignificant changes in the activity and surface area of Au-modified Pt over the course of cycling, in contrast to sizable losses observed with the pure Pt catalyst under the same conditions. ... 

The structure and surface areas (after catalytic tests) of the different solids are given in Table 2. XRD and sulphur to metal ratios indicate that the techniques used for the low-temperature synthesis allow the preparation of the expected most stable binary sulphides. These solids are generally stable under the test conditions, but structural changes occur for some of them in an H2 atmosphere Ni, Co and Pd monosulphides are transformed into Ni3S2, CogS8 and Pd4S. In order to compare the catalytic properties of the different sulphides, the activity are normalized by unit area of the used samples. The proportionality between catalytic activities and surface areas was checked for several samples [7]. 

Fig. 18. Correlation between cracking activity and surface area of Si/Al catalyst after various steam treatments.

Foster et al.90 studied the role of alkali metal additives on Si02 supported catalysts and observed decreasing activity and surface area together with increasing selectivity with increasing atomic number of the alkali metal. No explanation in simple terms could be given for these effects. Westerman et al.91 observed that a sharply defined level of S03 in the reaction mixture is1 needed to obtain optimum activity in naphthalene oxidation and that the selectivity was not influenced by the S03 level. 

Fig. 4A, B C show the activity change of mordenite catalysts as a function of copper content on catalyst for the reduction of NO with the sulfur content deposited on catalyst surface. Note that catalytic activity was defined as the ratio of the reaction rate for a deactivated catalyst to that for a fresh catalyst based on the first-order reaction kinetics a = k/k. The effect of sulfur compounds deposited on the catalysts due to the presence of S02 in the feed gas stream on SCR activity significantly depends on both the reaction temperatures and the copper content of the catalyst. For HM catalyst, the catalytic activity varies with its sulfur content depending on reaction temperatures, i.e., an exponential relationship at 250 °C and a linear relationship at 400 DC as shown in Fig.4A. It has already been investigated that the surface area of deactivated HM catalyst exponentially decreases with sulfur content at lower temperature of 250 °C, while it linearly decreases at higher temperature of 400 aC as shown in Fig. 1 A. Judging from these results between catalytic activity and surface area with their catalyst sulfur content at two different reaction temperatures, the decline of the catalytic activity for deactivated HM catalyst occurs simply due to the decrease of surface area.

The contents of promoters of these catalysts, their conversion activities, and surface areas, as found by Brunauer and Emmett, are given in Table II and based on the values of the ratios of CO chemisorption to a nitrogen... 

By way of example, Figure 3 shews the effect of steaming severity on zeolitic surface area (ZSA) for catalyst A and C. Also identified are typical values for equilibrium catalysts. What is seen is that the conditions needed to deactivate A to typical equilibrium ZSA are different than for C. If C is deactivated using the preferred conditions for A, then activity and surface areas are not in line with commercial experience. If the reverse is true, then A is deactivated too severely. 

FIGURE 26 Relationship between catalyst activity and surface area for standard vanadium phosphate catalysts for the oxidation of n-butane. Reproduced with permission from Ref. (33). Copyright 1997 Elsevier. 

The correlation of a BET surface area loss with sulfur content of the SOj-exposed catalysts suggests that sulfur deposits poison or blocks the surface. A similar trend of decreasing activity and surface area with increasing sulfur content was also observed by Ham et al. [24] for the reduction of NO by NHj over a CuHM catalyst. 

Catalytic activities and surface areas of CoMo nitride with increasing temperature of sulfidation. 

Li, G.J., Zhang, X.H., and Kawi, S., 1999. Relationships between sensitivity, catalytic activity and surface areas of SnO2 gas sensors. Sensors Actuators B., 60, pp. 64-70. 

RCZD catalysts were found [7] to have higher surface areas than the RCZ catalysts as a result of a slower leach rate the sodium zincate [7], The improvements in specific activity and surface area of the RCZD catalyst result in significantly improved methanol yields compared to the RCZ catalyst (Figure 2). The best RCZ catalyst and RCD catalyst and the industrial coprecipitated Cu-ZnO-AljOj catalyst (CAT X) all have similar methanol yields, and the RCZD catalyst has a yield of approximately twice that of the industrial coprecipitated catalyst. Table 2 summarises the best methanol yields achieved using the different techniques for preparing the Raney Cu-ZnO-Al O catalysts,... 

The high catalytic activity and surface area observed for the catalyst CG can be explained by the formation of Pb involved active sites and stabilization of low temperature tetragonal phase for the carrier material ZrOj (XRD results [11]). High oxidation state of Zr(IV) and low Pb Zr ratio will favour the variation of oxidation state of counter cation Pb to higher ones, creating certain active sites M = Pb or M = Pb, M =... 

The major products obtained in the alkylation of toluene on these catalysts are xylenes, tri- and tetra-methylbenzenes. The variation of catalytic activity and surface area of the system CsxHs. XPW12O40 as a function of x is shown in Fig.2. It is seen that the catalytic activity reaches a maximum at x = 2.5 and the surface area increases with increase in the extent of substitution. Similar results were obtained in the case of other salts. It has been reported, in the case of cesium salt, that the activity maximum occured at x = 2.5 for the alkylation of 1,2,3 trimethylbenzene with cyclohexene [ 16] and the high activity has been attributed to the high surface acidity of this catalyst. 

The behavior of amorphous Cu70Zr30 (53) and Au25Zr75 (54) is very similar to that of Pd-Zr. In both cases an increase of activity and surface area were observed and a phase change, resulting from the decomposition... 

Anderson, Hall, Krieg, and Seligman (80) Activities and surface areas of reduced and carburized cobalt catalysts. 

Table IV. DSC Activity and Surface Area of Solid Solutions as a Function of Calcination Temperature...

A series of solid solutions (50 mole % each) was prepared from seven class I and two class II spinels. Each solid solution did stabilize catalytic activity and surface area (cf. Tables IV, I, and II). Furthermore, x-ray diffraction patterns revealed that class I spinels which did not easily form pure spinel phases readily formed a single spinel phase solid solution, e.g. the CuO Fe203 NiO A1203 system. Electron spectroscopy chemical analysis (ESCA) did not indicate any significant... 

Table 6. Phenol hydroxylation activity and surface areas of CuNiAl ternary...

In general, catalyst activity increases with increasing size of the catalyst surface. However, since many reaction rates are strongly dependent on the surface structure, a linear correlation between catalyst activity and surface area can not be expected. In some reactions the selectivity of the catalyst decreases with increasing surface area. 

Most major improvements in performance of Fischer-Tropsch and other catalysts have been achieved through advanced imaging technologies. For example, different types of electron microscopy (Florea et al. 2013 Thomas et al. 2013), X-ray, and neutron powder diffraction (Rozita et al. 2013) facilitate characterization of catalyst particles and the included pores ranging in size from micron to a few nanometers. These advanced tools lend a capability to analyze down to the level of an atom. Further, the same facilitate introduction of efficient promoters and also distribution of smaller catalytic species over greater surface areas. The last feature implies a catalyst with high activity and surface area and should allow higher rates of reaction. 

The catalytic mechanism of the system of Cu-Zn0-Al203 is still not well explained. However, catalysts showed strong relationship between DME activity and surface area, and XRD peaks of ZnO in active catalysts tend to be broadened without Zn and Z11AI2O4 peaks. 

A conq>arative study of the role of the addition of Zn, T1 and Zr on IPIO catalysts has been carried out by Sananes et al. (79). They found a promoter effect for activity, with a maximum in activity as a function of the amount of promoter. These authors did not observe any structural modifications in the (V0)2P20 after die addition of promotes, and no correlation between activity and surface area was found. XPS analysis showed only a surface enrichment of promoters. 

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