Supported catalysts adsorption

Fig. 1. Nj adsoiption-desoiption isothenns of (a) silica template (SB A-15) and carbon supports (Adsorption data for SBA-15, SM-CMK-3, and CMK-3 were offset vertically by 0,200, and 400 cm (STP)/g, respectively) and (b) supported catalysts (Adsorption data for PM012/SM-CMK-3 and PMoj 2/CMK-3 were offset vertically by 0 and 200 cm (STP)/g, respectively).

Adsorption of metal ions and 0x0 ions is an important process for the preparation of supported catalysts. Adsorption of Mo ion is reported to be as follows. ... 

Figure Bl.22.1. Reflection-absorption IR spectra (RAIRS) from palladium flat surfaces in the presence of a 1 X 10 Torr 1 1 NO CO mixture at 200 K. Data are shown here for tluee different surfaces, namely, for Pd (100) (bottom) and Pd(l 11) (middle) single crystals and for palladium particles (about 500 A m diameter) deposited on a 100 A diick Si02 film grown on top of a Mo(l 10) single crystal. These experiments illustrate how RAIRS titration experiments can be used for the identification of specific surface sites in supported catalysts. On Pd(lOO) CO and NO each adsorbs on twofold sites, as indicated by their stretching bands at about 1970 and 1670 cm, respectively. On Pd(l 11), on the other hand, the main IR peaks are seen around 1745 for NO (on-top adsorption) and about 1915 for CO (tlueefold coordination). Using those two spectra as references, the data from the supported Pd system can be analysed to obtain estimates of the relative fractions of (100) and (111) planes exposed in the metal particles [26].

Fig. 4 TOC removal with y-Al203 and y-AljOj supported catalysts during catalytic ozonolysis and ozonolysis followed by adsorption.

Transmission infrared spectroscopy is very popular for studying the adsorption of gases on supported catalysts and for studying the decomposition of infrared active catalyst precursors during catalyst preparation. Infrared spectroscopy is an in situ technique that is applicable in transmission or diffuse reflection mode on real catalysts. 

Propose a strategy for bridging the gap between the world of adsorption and reaction on well-defined single-crystal surfaces and the world of supported catalysts in high-pressure reactors. 

There is a wealth of information available on CO chemisorption over single-crystal and polycrystalline platinum surfaces under ultrahigh-vacuum conditions research efforts in this area have gained a significant momentum with the advent of various surface analysis techniques (e.g., 2-8). In contrast, CO chemisorption on supported platinum catalysts (e.g., 9, 10, 11) is less well understood, due primarily to the inapplicability of most surface-sensitive techniques and to the difficulties involved in characterizing supported metal surfaces. In particular, the effects of transport resistances on the rates of adsorption and desorption over supported catalysts have rarely been studied. 

Parametric sensitivity analysis showed that for nonreactive systems, the adsorption equilibrium assumption can be safely invoked for transient CO adsorption and desorption, and that intrapellet diffusion resistances have a strong influence on the time scale of the transients (they tend to slow down the responses). The latter observation has important implications in the analysis of transient adsorption and desorption over supported catalysts that is, the results of transient chemisorption studies should be viewed with caution, if the effects of intrapellet diffusion resistances are not properly accounted for. 

We illustrate the sensitivity of the C-0 stretching frequency for the bonding configuration with a perhaps somewhat dated but still very instructive study of the adsorption sites of alloy surfaces. Soma-Noto and Sachtler [18] reported an infrared investigation of CO adsorbed on silica-supported Pd-Ag alloys some of their spectra are shown in Fig. 8.5. On pure palladium, CO adsorbs mainly in a twofold position, evidenced by the intense peak around 1980 cm 1, although some CO appears to be present in threefold and linear geometries as well. This is a common feature in adsorption studies on supported catalysts, where particles exhibit a variety of surface... 

The adsorption of CO on Pt is perhaps the most throughly studied system using vibrational spectroscopy. Studies have been made using both supported catalysts (2-5) and single crystals (5-10). Sample environments have included gas phase, vacuum, and aqueous solution (11-13). The similarities between many of these results have led to a remarkably unified understanding of CO adsorption phenomena in all three environments. Features which are relevant to further studies of the metal/electrolyte interface are summarized briefly ... 

Comparative methods may be effectively used for measurements of partial surface areas, Ac, of components in porous composites, for example for active surface area in supported catalysts. The traditional methods of Ac measurements are based on chemisorption of H2, 02, CO, NOr. and some other gases that chemisorb on an active component, and have negligible adsorption on a support [5,54], The calculation of Ac is fulfilled by an equation similar to Equation 9.18 assuming some values of w and atomic stoichiometry of chemisorption [54]. But, unfortunately chemisorption is extremely sensitive to insignificant variations of chemical composition and structure of surface, which alters the results of the measurements. 

Let us consider a supported catalyst of two components a support and an active component (see Problem 5). Adsorption of N2 at 77 K gave a total surface area of the supported catalyst, Asc. Adsorption of C02 at 273 K is different on support and active component, but one knows the reference values of qs(p) and qc(p), which were measured over pure support, and pure powder of active component with known surface areas, correspondingly. Propose a method of measuring of the surface area of the active component in the supported catalyst. 

Recent efforts have concentrated on the immobilization of these materials onto both organic polymers(11) and metal oxides(12) to simplify, by filtration, the separation, recovery and recycle process. These supported catalysts now function in a triphasic environment in either a liquid-solid-liquid or solid-liquid-solid reaction mixture. In this case, the catalyst must transfer the anion from the surface of the crystal lattice to the liquid phase. Here adsorption phenomena often significantly affect the reaction rate(13). 

Paulis, M Gandia, LM Gil, A Sambeth, J Odriozola, JA Montes, M. Influence of the surface adsorption-desorption processes on the ignition curves of volatile organic compounds (VOCs) complete oxidation over supported catalysts, Appl. Catal, B Environmental, 2000, Volume 26, Issue 1, 37-46. 

The adsorption of cyclopropanes at room temperature has been characterized by infrared spectroscopy for a number of silica-supported catalysts, viz., Ni (86), Pt (86), Pd (266), and Rh (91). The spectra are identical with those obtained from the adsorption of propene on the same metals. They give absorptions from CH3 groups showing that the C3 ring has been opened, and the nature of the spectra has already been discussed (140, and Part I, Section Vl.C.l.b). Typical spectra of species formed from cyclopropane on Ni/Si02 and Pt/Si02, obtained by Ward at room temperature, are shown in Figs. 9C and 9D. 

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