Brief Introduction of Metal Catalysts

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The metal catalyst is metals to change the speed of a chemical reaction without itself as the reacting final product component. In 1822, German J.W. Dberei-ner found that platinum powder can make hydrogen crack and burn. This phenomenon is an early example of metal catalysis. In 1835, Swedish chemist J.J. Berzelius introduced the term “catalyze” to the field of chemistry, catalysis and catalyst theory was developed.

Catalysis is divided into homogeneous and heterogeneous catalysis. In the chemical industry, products obtained by heterogeneous catalytic reaction accounts for about 80% to 90% of the total production of catalytic products. Catalysts used in the heterogeneous catalysis are solid catalysts, of which the metal catalyst is an important category. Metal catalysts are mainly used for hydrogenation and dehydrogenation reactions as well as for oxidation reactions. Transition metals in the periodic table are effective hydrogenation and dehydrogenation catalysts, which can divide into precious metals and general metals. Precious metal catalysts base on platinum and palladium, with ruthenium, rhodium, and iridium used frequently. General metal catalysts are nickel, cobalt, iron, copper, and rhenium and so on, among which nickel powder and cobalt powder are used most. Metal catalysts play an important role in the development of the chemical industry and the petrochemical industry. For example, chemical reactions, like ammonia synthesis, hydrocarbons hydrogenation, carbonyl synthesis, alkane dehydrogenation and alcohol dehydrogenation, use different kinds of metals as catalysts.

Key Performance Indicators

a. Activity. It refers to the catalytic capacity of the catalysts. It is represented by products quantity obtained per unit volume (or weight) of catalyst in per unit time under certain conditions in the industry.

b. Selectivity. It refers to catalytic specificity which means that only the catalyst can accelerate a chemical reaction under a certain condition. The selectivity of catalysts allows the synthesis of a variety of products as required.

c. Stability. It refers to service life of a catalyst. In order to maintain catalyst’s activity and selectivity and prolong its service life, catalysts in the course of use are required to maintain stable properties and resist poison (namely without reducing its activity) without the fragmentation.

Requirements of industrial metal catalysts are high activity, good selectivity and strong stability. These properties depend not only on the composition of the metal catalysts, but also on many factors of active metals such as crystal structure, particle size, surface area, pore structure (average pore diameter, porosity), and the dispersion state, etc.

Composition

In addition to the active metal components, catalysts are required to add a small amount of cocatalysts and carriers to improve the catalytic performance. The cocatalyst has no activity or low activity in itself, but it can improve the activity and selectivity of active metal components to extend service life. For example, during the ammonia synthesis process, adding a cocatalyst AlO to the pure iron obtained by the FeO reduction as a catalyst, the cocatalyst can not only significantly slow the growth of the -Fe crystallite, but also increase the service life of the catalyst from hours to years. Carriers are dispersants or supports of catalyst active components. The roles of carriers are to increase the effective surface of catalysts, provide a suitable pore structure, and improve the tensile strength and thermal stability. Using carriers can make precious metals disperse and support on a large volume of loose materials, instead of using a single piece of materials, which can save precious metal materials (such as palladium, platinum, etc.). Frequently-used catalyst carriers are aluminum oxide, silica gel, activated carbon, etc.

Preparations

Preparations of metal catalysts are mainly precipitation, dipping, melting, etc. Based on the nature of raw materials and reaction conditions, metal catalysts can be made into powder, strip, layer and other shapes.

Precipitation is to add precipitant to the metal salt solution under stirring conditions, and the resultant precipitate is made into catalysts through washing, filtration, drying and calcination and other processes. This method is commonly used in the preparation of single-component and multi-component catalysts.

Dipping is to make a solution containing active metal components immerse in carriers, then making into catalysts through drying, calcinations, activation and other processes.

Melting is to melt active metals and metals soluble in alkali into alloys, then crushing the alloy into powder with an alkali to dissolve unwanted metal components, finally obtaining active metals with skeleton structure, also known as a skeleton catalyst.

Applications

The activity of metal catalysts with high activity will gradually decrease in the course of use. This phenomenon causes by the catalyst poisoning, sintering and carbon deposits, etc. Catalyst poisoning usually causes by raw materials mixed in a trace of “toxic” materials which can make catalyst activity lost. Therefore, it is necessary to refine raw materials and remove the poison. Sintering refers to active component particles of catalysts gathering into groups or producing surface melting phenomenon. Poisoning and sintering phenomena can be slowed down by strict control of process parameters to avoid excessive catalyst temperatures. Carbon deposits cause by poor thermal conductivity or very small pores of the catalysts, which occurs on the surface of catalysts. Generally, burning method can remove the carbon deposits phenomenon.