Unsupported particle size

The sol—gel technique has been used mosdy to prepare alumina membranes. Figure 18 shows a cross section of a composite alumina membrane made by sHp coating successive sols with different particle sizes onto a porous ceramic support. SiUca or titanium membranes could also be made by the same principles. Unsupported titanium dioxide membranes with pore sizes of 5 nm or less have been made by the sol—gel process (57). 

The performance of a supported metal or metal sulfide catalyst depends on the details of its preparation and pretreatraent. For petroleum refining applications, these catalysts are activated by reduction and/or sulfidation of an oxide precursor. The amount of the catalytic component converted to the active ase cind the dispersion of the active component are important factors in determining the catalytic performance of these materials. This investigation examines the process of reduction and sulfidation on unsupported 00 04 and silica-supported CO3O4 catalysts with different C03O4 dispersions. The C03O4 particle sizes were determined with electron microscopy. X-ray diffraction (XRD), emd... 

Among various methods to synthesize nanometer-sized particles [1-3], the liquid-phase reduction method as the novel synthesis method of metallic nanoparticles is one of the easiest procedures, since nanoparticles can be directly obtained from various precursor compounds soluble in a solvent [4], It has been reported that the synthesis of Ni nanoparticles with a diameter from 5 to lOnm and an amorphous-like structure by using this method and the promotion effect of Zn addition to Ni nanoparticles on the catalytic activity for 1-octene hydrogenation [4]. However, unsupported particles were found rather unstable because of its high surface activity to cause tremendous aggregation [5]. In order to solve this problem, their selective deposition onto support particles, such as metal oxides, has been investigated, and also their catalytic activities have been studied. 

The high sensitivity of the reaction to particle size of Rh is confirmed at 230°C, in a mixture of 0.5% NO+1% CO, the turnover frequency increases from 0.017 s-1 for a highly dispersed catalyst to 0.74 s 1 for a catalyst dispersed at 1.7%, the activity per metal site on unsupported Rh catalysts being still much higher. 

Carbon-supported platinum (Pt) and platinum-rathenium (Pt-Ru) alloy are one of the most popular electrocatalysts in polymer electrolyte fuel cells (PEFC). Pt supported on electrically conducting carbons, preferably carbon black, is being increasingly used as an electrocatalyst in fuel cell applications (Parker et al., 2004). Carbon-supported Pt could be prepared at loadings as high as 70 wt.% without a noticeable increase of particle size. Unsupported and carbon-supported nanoparticle Pt-Ru, ,t m catalysts prepared using the surface reductive deposition... 

In the very active field of unmodified nanoparticles recent discoveries have been made on size-selective Fischer-Tropsch catalysts that convert selectively CO and H2 into hydrocarbons there is a strong dependence of activity, selectivity and Hfetime on Co particle size. This topic of unmodified, supported or unsupported, nanoparticles is outside the scope of this chapter [74, 75]. Nevertheless, we mention discoveries made by Degussa, who have patented a process for H2O2 synthesis from molecular oxygen and molecular hydrogen with nanosized Pd particles (6 A) [76]. 

More recently, Koizumi et al. observed that Mn has an additional beneficial effect in unsupported Fe-based F-T catalysts. These authors studied the sulfur resistance of Mn-Fe catalysts and they observed superior catalysts stabilities, especially when the catalysts were pre-reduced in CO. This group also used IR spectroscopy in combination with CO as a probe molecule to compare Fe and Mn-Fe catalysts. It was found that the addition of Mn led to the appearance of several well-resolved bands upon CO adsorption. The appearance of the bands arising from bridged-bonded CO on Fe indicated that the size of the Fe particles were clearly larger than in the case of the unpromoted catalysts. They attributed the decreased reactivity towards H2S to the observed increase in Fe particle size. 

For unsupported catalysts, where particle sizes are typically an order of magnitude larger than those for supported catalysts, the mobility of various species in the bulk structure may be of interest when considering how the bulk structure and composition are reflected in the surface properties of the particle. In addition, bulk mobility is an important consideration in the understanding of solid state reactions and phenomena such as sintering. 

The development of new and improved electrocatalysts begins with the discovery of materials displaying improved intrinsic electrochemical activity. Intrinsic activity is best observed and compared in a well-controlled catalyst environment where variables that may disguise the intrinsic activity trends are minimized or absent. Particle size, particle size distribution, surface area, catalyst utilization and the distribution of crystallographic phases are parameters that are typically difficult to control. Vapor deposition of unsupported thin film electrocatalysts eliminates many of these variables. This method provides a controlled synthetic route to smooth, single-phase or multi-phase, ordered or disordered metal alloy phases depending on deposition and processing conditions. 

Powders possessing relatively high surface area and active sites can be intrinsically catalytically active themselves. Powders of nickel, platinum, palladium, and copper chromites find broad use in various hydrogenation reactions, whereas zeolites and metal oxide powders are used primarily for cracking and isomerization. All of the properties important for supported powdered catalysts such as particle size, resistance to attrition, pore size, and surface area are likewise important for unsupported catalysts. Since no additional catalytic species are added, it is difficult to control active site location however, intuitively it is advantageous to maximize the area of active sites within the matrix. This parameter can be influenced by preparative procedures. 

In this respect it should be said that even open faces such as the (210) and the stepped (001) surface do not dissociatively adsorb CO at 25-125°C (97). This suggests that unsupported Pd is a rather poor methanation catalyst. Under 1 atm total pressure in a CO + H2 mixture, the Pd black catalyst (210-nm crystallites) produces methane but, here again, the activity level is about two times lower than that of Pd/Si02 catalysts (4.6-nm Pd particle size), and about two orders of magnitude less active than Pd/ A1203 catalysts (4.8-nm Pd particle size) (98). It therefore seems that the effect of dispersion here is not pronounced with respect to the support effect. Silica, as an inert support, does not influence the activity of Pd to the same extent as does more the acidic alumina. 

Vannice (232) measured turnover numbers for methanation on a variety of well-characterized palladium catalysts. The supported catalysts were all more active than unsupported palladium (Pd black) and PdHY was intermediate between Pd/Si02 and Pd/Al203 in specific activity (see Table VII). As is evident from the data there is no obvious correlation betwen particle size and turnover number. It was therefore suggested that the enhanced activity of various supported catalysts was due to a metal-support interaction. Figureas et al. (105) also found good evidence for a support effect during benzene hydrogenation studies. In this case the palladium zeolite... 

Topspe and co-workers (231) studied calorimetrically the adsorption of CO at 303 K on MgO-supported Fe and on two unsupported Fe ammonia synthesis catalysts. These catalysts displayed quite heterogeneous site energy distributions. For example, the differential heat of adsorption on the Fe/MgO catalyst decreased from about 110 kJ moP to a large plateau at 80 kJ moP before decreasing abruptly to near 40 kJ moP. It was found that the amount of weakly held CO increased with decreasing Fe particle size. The authors used IR spectroscopy to demonstrate that the differences in the site energy... 

The effect of Si substitution on the turnover frequency for WGS is shown in Figure 11. The turnover frequencies plotted in this figure were based on the magnetite surface area as determined by the NO chemisorption technique. The turnover frequencies shown for unsupported Fe O indicate that the factor of 10 decline in activity for the silica-supported catalysts is not a particle size effect, but instead is a consequence of the substitution of Si into the lattice. However, when the adsorption of CO/COo at 663 K was used to titrate the surface sites instead of NO, the resulting turnover frequencies were essentially constant as shown in Figure 12. Accordingly, the CO/CO2 mixture apparently titrates the sites active for WGS. Clearly, the number of active sites is decreased markedly as the particle size decreases in the silica-substituted magnetite catalysts. 

Figure 3 shows the Pt NMR spectmm of unsupported Pt-black samples, reduced electrochemically as reported above. The average particle size diameters are ca. 3 (Figure 3A) and 6 nm (Figure 3B), as... 

Decrease in the size of a metal particle below a critical dimension results in dramatic changes in the electronic properties of the bulk metal. Properties like conductivity, magnetism, light absorption, luminescence, electrochemical, and catalytic activity depend on the particle size. Many heterogeneous catalysts are based on finely divided metal particles on various supports. However, this section deals with the catalytic properties of unsupported nanoparticles. 

For single crystal surfaces, a reaction is deemed insensitive if its rate is about the same on all low Miller index planes, but since these differ from small metal particles in not having atoms of very low co-ordination number, the term face sensitivity should be used in this case. Two further approaches to the general problem have been tried (1) systematic variation of particle in supported metal catalysts, and (2) alteration of the composition of the surface of bimetallic catalysts, either supported or unsupported (Section 5.7). These lead respectively to particle size sensitivity and ensemble size sensitivity, but the three types are not necessarily exactly the same. 

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