Supported Metal Catalysts in Reforming

The prime purpose of reforming is to make aromatics from each class of non-aromatic hydrocarbons in naphtha. The core reaction of alkane dehy-drocyclisation in reforming is complex and its optimisation has driven catalyst science and reforming technology since the 1960s. 

The fundamental reforming process is gas phase, at approximately 500°C, using basically a Pt-alumina catalyst. 

Naphtha, consisting typically of a mixture of C6-C10 alkanes, naphthenes and aromatics, is vapourised in a charge heater and passed into the first reactor, where largely endothermic dehydrogenation occurs. This results in a large endotherm (for example AT = 70°C) across the first reactor. The product from this first reactor is reheated in an interheater, and passed into the second reactor. As the process stream passes through the reactor train, more exothermic reactions take place, so that interheater duties decrease down the reactor train.10 

The monofunctional catalytic chemistry of aromatisation over metals is known and has been elucidated by Gault.14 Kugelmann,15 using data from 

16 has calculated the thermodynamic conversions of the aromatisation reactions as a function of temperature, pressure and carbon number. 

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This reaction serves for removal of carbon monoxide from gas mixtures and is usually carried out over supported metal catalysts. In reforming techniques, carbon monoxide, poisonous for the catalyst in fuel cells, is removed in such a way. It is also applied in automobiles for reducing the exhaust gas carbon monoxide to an environmentally acceptable level. 

On the other hand, it is known that catalyst support exerts a great influence on the catalytic properties of the metallic particles deposited on it during the carbon dioxide reforming of methane. So for a given metal, catalytic activities can be changed [5], product selectivities modified [6] and carbon deposition resistivity altered [7]. Also the addition of certain promoters can improve the catalytic behavior of a given supported metal catalyst. In particular we have shown the benefit of the MgO addition to cobalt catalyst [ 8, 9]. 

In many cases there is an interaction between the carrier and the active component of the catalyst so that the character of the active surface will change. For example, the electronic character of the supported catalyst may be influenced by the transfer of electrons across the catalyst-carrier interface. In some cases the carrier itself has a catalytic activity for the primary reaction, an intermediate reaction, or a subsequent reaction, and a dual-function catalyst is thereby obtained. Materials of this type are widely employed in reforming processes. There are other cases where the interaction of the catalyst and support are much more subtle and difficult to label. For example, the crystal size and structure of supported metal catalysts as well as the manner in which the metal is dispersed can be influenced by the nature of the support material. 

In hydrocarbon reforming processes the vapour of an alkane is passed over a supported metal catalyst such as platinum on silica or alumina. Dehydrocyclization, isomerization and cracking reactions all take place to... 

Deactivation of supported metal catalysts by carbon or coke formation, which has its origin in the CH4 dissociation and/or CO disproportionation, is the most serious problem hindering the application of the C02 reforming of methane. Attempts to overcome this limitation have focused on the development of improved catalysts. 

The severe working conditions often encountered in an H2 production process, such as high temperature and high space velocity, combined with the necessity for a long catalyst lifetime, impose the development of an appropriate synthetic procedure to stabilize the catalyst. The reforming activity and product distribution over supported metal catalysts depend on the choice of metal and its content, the presence of promoters, the type of support and method of catalyst preparation. 

The morphology of the carbon on the surface can assume several forms a two-dimensional film or so-called whisker carbon, which is formed when the carbon dissolves in the supported metal catalyst, diffuses through the metal, and forms a growing filament that lifts the metal from the catalyst surface. Whisker carbon is typically associated with Ni-based catalysts because carbon is soluble in Ni at reforming conditions. Whisker carbon tends to form at higher temperatures, low steam to hydrocarbon ratios and higher aromatic content of the feeds. This type of carbon formation may be minimized by the use of precious metals as catalysts, because these metals do not dissolve carbon. On a nickel surface, the whisker mechanism can be controlled by sulfur passivation. 

Steam reforming of hydrocarbons is one of the oldest and most widely used processes in the chemical industry for producing H2. In this process, hydrocarbons react with steam in the presence of a supported metal catalyst at elevated temperatures to generate primarily H2 and CO (reaction (2)). This... 

Without a doubt, a complete picture of the dynamics of dissociative chemisorption and the relevant parameters which govern these mechanisms would be incredibly useful in studying and improving industrially relevant catalysis and surface reaction processes. For example, the dissociation of methane on a supported metal catalyst surface is the rate limiting step in the steam reforming of natural gas, an initial step in the production of many different industrial chemicals [1]. Precursor-mediated dissociation has been shown to play a dominant role in epitaxial silicon growth from disilane, a process employed to produce transistors and various microelectronic devices [2]. An examination of the Boltzmann distribution of kinetic energies for a gas at typical industrial catalytic reactor conditions (T 1000 K)... 

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