Surface area metal

Scholten J J and van Montfoort A 1962 The determination of the free-metal surface area of palladium catalysts J. Catal. 1 85-92... 

In general, cavitation damage can be anticipated wherever an unstable state of fluid flow exists or where substantial pressure changes are encountered. Susceptible locations include sharp discontinuities on metal surfaces, areas where flow direction is suddenly altered (Fig. 12.5), and regions where the cross-sectional areas of the flow passages are changed. 

In this article, we will discuss the use of physical adsorption to determine the total surface areas of finely divided powders or solids, e.g., clay, carbon black, silica, inorganic pigments, polymers, alumina, and so forth. The use of chemisorption is confined to the measurements of metal surface areas of finely divided metals, such as powders, evaporated metal films, and those found in supported metal catalysts. 

The metal surface area at the inlet end of the catalyst bed in experiment HGR-12 was smaller than that at the outlet end this indicates that a decrease in nickel metal sites is part of the deactivation process. Sintering of the nickel is one possible mechanism, but carbon and carbide formation are suspected major causes. Loss of active Raney nickel sites could also conceivably result from diffusion of residual free aluminum from unleached catalyst and subsequent alloying with the free nickel to form an inactive material. 

There is little data available to quantify these factors. The loss of catalyst surface area with high temperatures is well-known (136). One hundred hours of dry heat at 900°C are usually sufficient to reduce alumina surface area from 120 to 40 m2/g. Platinum crystallites can grow from 30 A to 600 A in diameter, and metal surface area declines from 20 m2/g to 1 m2/g. Crystal growth and microstructure changes are thermodynamically favored (137). Alumina can react with copper oxide and nickel oxide to form aluminates, with great loss of surface area and catalytic activity. The loss of metals by carbonyl formation and the loss of ruthenium by oxide formation have been mentioned before. 

In principle any standard catalytic metal surface area measuring technique, such as H2 or CO chemisorption can be used to measure the metal/gas interface area Aq or Nq. This is because solid electrolytes such as YSZ chemisorb practically no H2 or CO at any temperature. 

Fig. 3. Metal surface areas of supported Pt catalysts as a function of metal loading.

Mesoporous carbon materials were prepared using ordered silica templates. The Pt catalysts supported on mesoporous carbons were prepared by an impregnation method for use in the methanol electro-oxidation. The Pt/MC catalysts retained highly dispersed Pt particles on the supports. In the methanol electro-oxidation, the Pt/MC catalysts exhibited better catalytic performance than the Pt/Vulcan catalyst. The enhanced catalytic performance of Pt/MC catalysts resulted from large active metal surface areas. The catalytic performance was in the following order Pt/CMK-1 > Pt/CMK-3 > Pt/Vulcan. It was also revealed that CMK-1 with 3-dimensional pore structure was more favorable for metal dispersion than CMK-3 with 2-dimensional pore arrangement. It is eoncluded that the metal dispersion was a critical factor determining the catalytic performance in the methanol electro-oxidation. 

The STEM Is Ideally suited for the characterization of these materials, because one Is normally measuring high atomic number elements In low atomic number metal oxide matrices, thus facilitating favorable contrast effects for observation of dispersed metal crystallites due to diffraction and elastic scattering of electrons as a function of Z number. The ability to observe and measure areas 2 nm In size In real time makes analysis of many metal particles relatively rapid and convenient. As with all techniques, limitations are encountered. Information such as metal surface areas, oxidation states of elements, chemical reactivity, etc., are often desired. Consequently, additional Input from other characterization techniques should be sought to complement the STEM data. 

The present study was initiated to understand the causes of large differences in perfonnance of various catalyst formulations after accelerated thermal aging on an engine dynamometer. In particular, we wished to determine whether performance charaderistics were related to noble metal dispersion (i.e. noble metal surface area), as previous studies have suggested that the thermal durability of alumina-supported Pd catalysts is due to high-temperature spreading or re-dispersion of Pd particles [20-25]. 

Table 2 also lists the noble metal surface areas normalized to the total mass of the catalyst. The surface areas were calculated directly from the dispersion data taking into account the different mass of noble metal in each cataly and assuming a constant site density of 1x10 /m As with dispersion, no clear correlation exists between mass-specific noble metal surface areas and CO/NOx cross-over efficiencies. 

CO/NOx cross-over efficiencies of aged catalysts in dynamometer sweep experiments do not correlate with either noble me dispersion or noble metal surface area. 

Coke formation on these catalysts occurs mainly via methane decomposition. Deactivation as a function of coke content (see Fig. 3 for Pt/ y-AljO,) seems to involve two processes, i e, a slow initial one caused by coke formed from methane on Pt that is non reactive towards CO2 (see Table 3) In parallel, carbon also accumulates on the support and given the ratio between the support surface and metal surface area at a certain level begins to physically block Pt deactivating the catalyst rapidly. The coke deposited on the support very close to the Pt- support interface could be playing an important role in this process. 

Dendrimer-protected colloids are capable of adsorbing carbon monoxide while suspended in solution, but upon removal from solution and support on a high surface area metal oxide, CO adsorption was nil presumably due to the collapse of the dendrimer [25]. It is proposed that a similar phenomena occurs on PVP-protected Pt colloids because removal of solvent molecules from the void space in between polymer chains most likely causes them to collapse on each other. Titration of the exposed surface area of colloid solution PVP-protected platinum nanoparticles demonstrated 50% of the total metal surface area was available for reaction, and this exposed area was present as... 

In calculating the metallic surface area, one has to take proper care of the reaction stoichiometry. In the ideal case, a molecule occupies one site, as shown for terminal adsorbed CO in Fig. 3.46.a. Alternatively, a molecule may chemisorb on more than one metal atom, as shown in Fig. 3.46.b and c for bridged-site adsorbed CO and in Fig. 3.46.d for valley-site adsorbed CO, respectively. In some specific cases of really big molecules, one can imagine that a molecule adsorbs on only one site, while simultaneously blocking adjacent sites for geometric reasons. In case an adsorbate molecule adsorbs dissociatively, it will occupy more than one site as shown in Fig. 3.46.e. 

Metallic surface area and metal dispersions of some samples measured... 

Transmission IR (TIR) spectroscopy if the solid in question is IR transparent over an appreciable range of wavelength. This is often used on supported metal catalysts, where the large metallic surface area permits a high concentration of adsorbed species to be sampled. The sample consist typically of 10-100 mg of catalyst, pressed into a self-supporting disk of approximately 1 cm2 and a few tenths of a mm in thickness. The support particles should be smaller than the wavelength of the IR radiation, otherwise scattering losses become important. 

Catalyst Dispersion (%) Metal crystallite size (run) Metal Surface Area (mV) Pore diameter (mil) PSD (50) (microns)... 

Copper metal surface area was determined by nitrous oxide decomposition. A sample of catalyst (0.2 g) was reduced by heating to 563 K under a flow of 10 % H2/N2 (50 cm min"1) at a heating rate of 3 deg.min 1. The catalyst was then held at this temperature for 1 h before the gas flow was switched to helium. After 0.5 h the catalyst was cooled in to 333 K and a flow of 5 %N20/He (50 cm3mirr ) passed over the sample for 0.25 h to surface oxidise the copper. At the end of this period the flow was switched to 10 % H2/N2 (50 entitlin 1) and the sample heated at a heating rate of 3 deg.min"1. The hydrogen up-take was quantified, from this a... 

It is often found that the ratio R (measured, for instance, by gas adsorption methods) of actual metal surface area accessible to the gas phase, to the geometric film area, exceeds unity. This arises from nonplanarity of the outermost film surface both on an atomic and a more macroscopic scale, and from porosity of the film due to gaps between the crystals. These gags are typically up to about 20 A wide. However, for film thicknesses >500 A, this gap structure is never such as completely to isolate metal crystals one from the other, and almost all of the substrate is, in fact, covered by metal. In practice, catalytic work mostly uses thick films in the thickness range 500-2000 A, and it is easily shown (7) that intercrystal gaps in these films will not influence catalytic reaction kinetics provided the half-life of the reaction exceeds about 10-20 sec, which will usually be the case. 

SS). Data for the metals of group VIII and for rhenium in group VIIA are given in Fig. 1, which is divided into three fields separating the metals of the first, second, and third transition series. The specific activity is defined as the activity per unit surface area of metal. Metal surface areas required for the determination of specific activities are derived from measurements... 

Ee, Eh represent the rates of exchange and the rate of deuteration, respectively (moles per unit time and per unit metal surface area) ... 

S is the metal surface area of catalyst charged into the reactor ... 

Whereas determination of chemisorption isotherms, e.g., of hydrogen on metals, is a means for calculating the size of the metallic surface area, our results clearly demonstrate that IR studies on the adsorption of nitrogen and carbon monoxide can give valuable information about the structure of the metal surface. The adsorption of nitrogen enables us to determine the number of B5 sites per unit of metal surface area, not only on nickel, but also on palladium, platinum, and iridium. Once the number of B5 sites is known, it is possible to look for other phenomena that require the presence of these sites. One has already been found, viz, the dissociative chemisorption of carbon dioxide on nickel. 

The specific metal surface area of our nickel samples was established by means of deuterium chemisorption, the amount of deuterium adsorbed being determined by exchange with a known quantity of hydrogen followed by mass speetrometric analysis. It was assumed in the calculation that 1 cm3 (NTP) of deuterium corresponds to 3.64 m2 of nickel surface area. 

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