Noble metals supported

S.J. Tauster, S.C. Fung, and R.L. Garten, Strong metal-support interactions. Group 8 noble metals supported on T1O2, JACS 100, 170-175 (1978). 

MIDW [Mobil isomerization dewaxing] A petroleum refining process which improves yield and quality by isomerizing and selectively cracking paraffins in waxy oils. The catalyst is a noble metal, supported on a zeolite. Developed by Mobil Corporation from 1991 to 1996. 

Non-noble metals such as Ni, Co, Mo, W, Fe, Ag and Cu have been added to zeolites for use in catalysis. In addition to CO, nitric oxide (NO) has been shown to be a good adsorbate for probing the electronic environment of these metals. When NO chemisorbs on these metals, it can form mononitrosyl (M-NO) and dinitrosyl species (ON-M-NO). The monontrosyl species has a single absorption band and the dinitrosyl species has two bands due to asymmetric and symmetric vibrational modes of the (ON-M-NO) moiety. Again, there have been many studies reported in the literature on the use of NO and/or CO adsorption on non-noble metals supported on zeolites and they are too numerous to list here. Several examples have been selected and summarized to provide the reader with the type of information that can be provided by this method. 

There are a number of examples of tube waU reactors, the most important being the automotive catalytic converter (ACC), which was described in the previous section. These reactors are made by coating an extruded ceramic monolith with noble metals supported on a thin wash coat of y-alumina. This reactor is used to oxidize hydrocarbons and CO to CO2 and H2O and also reduce NO to N2. The rates of these reactions are very fast after warmup, and the effectiveness factor within the porous wash coat is therefore very smaU. The reactions are also eternal mass transfer limited within the monohth after warmup. We wUl consider three limiting cases of this reactor, surface reaction limiting, external mass transfer limiting, and wash coat diffusion limiting. In each case we wiU assume a first-order irreversible reaction. 

Silver, Platinum and Noble Metals Supported On Alumina... 

SMSI is also thought to affect methanation catalysts (normally transition metal or noble metals supported on alumina), which are used in the producton of substitute natural gas (SNG). In general, heating in H2 causes sintering on alumina and silica supports and heating in O2 or steam can cause dispersion and particle coalescence at 200 °C (Rukenstein and Lee 1984,1987, Nakayama et al 1984). The data have been based on ex situ EM studies. Here EM methods, especially under dynamic reaction conditions, can provide a wealth of new insights into metal-support interactions under reaction conditions. 

Johnson and co-workers (62) have come to the conclusion that interaction of lead with Pt crystallites results in the formation of an inactive phase in which the Pt atoms are ionized and soluble in HC1. These data were derived from engine tests, in which the catalysts were exposed to fuels with 0.03-0.1 g Pb/gal. The amount of crystalline Pt in these catalysts was smaller than in catalysts run on lead-free fuels. The authors indicate that noncrystalline forms of Pt are present on A1203 supports under certain conditions, and that lead stabilizes such forms. The question whether the noncrystalline, ionic Pt is a surface or a bulk phase remains unanswered. Bulk mixed Pt-Pb oxides have been described (98, 99), but, again, the dispersed forms of noble metals supported on A1203, which lead (and other elements) may stabilize, are known to be associated with the surface only. Palladium can be expected to form such noncrystalline dispersed phases to a still greater extent since it is more easily oxidized than Pt. 

Hydroisomerization is also a key process. In this process, linear paraffins are converted to isoparaffins. This reaction greatly improves the pour point of the base oil, but results in a loss in VI. The catalyst is often noble metal supported on a controlled acidity support. The catalyst formulations are often proprietary and may utilize an amorphous silica-alumina or a modified molecular sieve. 

We start by giving a short report of the computational methods employed in our calculations, including a geometrical description of the systems investigated. We describe first the cluster calculations on silica based materials, followed by periodic calculations on noble metals supported on oxides. The results for both systems yield detailed information on structure and bonding on these complexes. 

Isomerization of n-paraffin, especially normal pentane to iso-pentane is essential for making high octane gasoline with low aromatics content. Isomerization of lower paraffins has been conducted in the solid catalyzed gas-phase reaction system by using noble metal-supported solid acid under hydrogen atmosphere. The most predominant reaction mechanism for the isomerization of alkane is as follows (1) the dehydrogenation of alkane to alkene on the supported metal (2) proton addition to the alkene to form carbenium ion on the acidic component (3) skeletal isomerization of the carbenium ion on the acidic component (4) deprotonation of the isoraerized carbenium ion to form alkene on the acidic component (5) hydrogenation of the alkene to alkane on the metal [1]. 

Hydrodesulfurization of thiophene over noble metals supported onZSM-5 zeolites... 

The catalytic activity of various noble metals supported on NaZSM-5 and HZSM-5 zeolites was examined in the hydrodesulfurization of thiophene at 400 C (Figure 1). The activity was markedly changed by the the kind of noble metal and the order of the catalytic activities of noble metal/NaZSM-5 catalysts was as follows ... 

Noble metals supported on HZSM-5 zeolite show higher catalytic activity than on NaZSM-5, suggesting that the Brbnsted acid sites of HZSM-5 in noble metal/HZSM-5 catalysts play important role in the hydrodesulfurization of thiophene. 

Tauster S, Fung S, Garten R (1978) Strong metal-support interactions - group-8 noble-metals supported on TiOj. J Am Chem Soc 100 170... 

As summarized in Table 2 base metal oxides can be classified into three groups /7-type semiconductors, -type semiconductors, and insulators, the catalytic activity of which for CO oxidation decreases in this order. As support for Pt group metals, except for Au, semiconductor metal oxides are more effective than insulating metal oxides because the former are partly reducible by CO and H2 at temperatures below 200 °C. However, the degree of reducibility is not directly reflected on the support effect. Although n-type semiconductor metal oxides themselves are less active than /7-type semiconductor ones, their support effect is usually larger. The reason is not clear. However, it seems to be curious that noble metals supported on /7-type semiconductors have not been extensively studied so far, except for a few cases. 

Cu-, Ni-, and noble metal-supported catalysts Fixed-bed reactor, 100-600°C H20/EtOH = 6-10 LHSV = 1.6-2.0 h"1 Catalysts were prepared by impregnation and coprecipitation techniques. Among the catalysts tested in the SRE reaction, the Cu0/Zn0/Al203 exhibited better performance. Sheng et al.266... 

Table 2.30, a wide range of catalysts such as Pd/ZnO, noble metals supported on A1203, or ceramic monolith/foams with or without additives such as Ce02 or La203, as well as bimetallic catalysts such as Ni-Cu, Ni-Co, and Ni-Rh, supported on Zn0-Al203, A1203, and Ce02 have been employed in the OSRE. 

Low levels of SOj in the exhaust gas stream are known to lower the catalytic activity of noble metal-supported catalysts, although this effect is considerably less compared to base metal catalysts [19]. Iwamoto et al. [20] observed a sUght decrease of NO removal activity of the Cu- M-5 for NO reduction by C3H5 upon the addition of SOj to the feed gas stream. The conversion of NO at 300°C, however, completely recovered to the initial state upon termination of the SO2 feed. They suggested that the loss of the removal activity of the catalyst is probably due to the alteration of the copper ionic state on the catalyst surface but provided no evidence in support of this hypothesis. 

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