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Терминология в гетерогенном катализепод ред. Р.П.Бурвелла
(окончание )
Русский перевод
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Терминология в гетерогенном катализе
под ред. Р.Л. Бурвелла
(Окончание, начало см. "Каталитический Бюллетень" N33, 2005 и N34, 2005)

1.6 Transport phenomena in heterogeneous catalysis

This section will not attempt to cover the more technical aspects of chemical reactor engineering.

A unique feature of heterogeneous catalytic reactions is the ease with which chemical kinetic laws are disguised by various transport phenomena connected with the existence of concentration and/or temperature gradients in the hydrodynamic boundary layer surrounding the catalyst particles (external gradients) or in the porous texture of the catalyst particles themselves (internal gradients). Additional difficulties arise in batch reactors and in stirred flow reactors if agitation is inadequate to maintain uniform concentrations in the fluid phase. Agitation is particularly critical where one of the reactants is a gas and the catalyst and other reactants and products are in condensed phases, for example, in the hydrogenation of a liquid alkene. Here the agitation must be adequate to maintain the fugacity of the dissolved gaseous reactant equal to that in the gaseous phase.

When external gradients correspond to substantial differences in concentration or temperature between the bulk of the fluid and the external surface of the catalyst particle, the rate of reaction at the surface is significantly different from that which would prevail if the concentration or temperature at the surface were equal to that in the bulk of the fluid. The catalytic reaction is then said to be influenced by external mass or heat transfer, respectively, and, when this influence is the dominant one, the rate corresponds to a regime of external mass or heat transfer.

Similarly, when internal gradients correspond to differences in concentration or temperature between the external surface of the catalyst particle and its centre, the rate in the particle is substantially different from that which would prevail if the concentration or temperature were the same throughout the particle. The catalytic reaction is then said to be influenced by internal mass or heat transfer, and, when this influence is the dominant one, the rate corresponds to a regime of internal mass or heat transfer.

Terms such as diffusion limited or diffusion controlled are undesirable because a rate may be larger in regimes of heat or mass transfer than in the kinetic regime of operation, i.e., when gradients are negligible.

1.7 Loss of catalytic activity

1.7.1 Poisoning and inhibition

Traces of impurities in the fluid to which the catalyst is exposed can adsorb at the active sites and reduce or eliminate catalytic activity. This is called poisoning and the effective impurity is called a poison. If adsorption of poison is strong and not readily reversed, the poisoning is called permanent. If the adsorption of the poison is weaker and reversible, removal of the poison from the fluid phase results in restoration of the original catalytic activity. Such poisoning is called temporary. If adsorption of the poison is still weaker and not greatly preferred to adsorption of reactant, the reduction in rate occasioned by the poison may be called competitive inhibition or inhibition. Here, of course, the poison may be present in much larger than trace amounts, There are, of course, no sharp boundaries in the sequence permanent poisoning, temporary poisoning, competitive inhibition.

In selective poisoning or selective inhibition, a poison retards the rate of one catalysed reaction more than that of another or it may retard only one of the reactions. For example, there are poisons which retard the hydrogenation of olefins much more than the hydrogenation of acetylenes or dienes. Also, traces of sulphur compounds appear selectively to inhibit hydrogenolysis of hydrocarbons during catalytic reforming.

A product of a reaction may cause poisoning or inhibition. The phenomenon is called self-poisoning or autopoisoning.

1.7.2 Deactivation: general

The conversion in a catalytic reaction performed under constant conditions of reaction often decreases with time of run or time on stream. This phenomenon is called catalyst deactivation or catalyst decay. If it is possible to determine the kinetic form of the reaction and, thus, to measure the rate constant for the catalytic reaction k, it is sometimes possible to express the rate of deactivation by an empirical equation such as ,where t is the time on stream, n is some positive constant, and B remains constant during a run but depends upon the temperature and other conditions of the reaction. Alternatively, the decline in k may be assumed to result from elimination of active sites and L may be substituted for k in the preceding equation where L is considered to be the effective concentration of surface centres. It is then common practice to define a time of deactivation (or decay time) as the time on stream during which k falls to a specified fraction of its original value, often 0.5. Times of deactivation may vary from minutes as in catalytic cracking to years as in hydrodesulphurisation.

Catalytic deactivation can sometimes be reversed and the original catalytic activity restored by some special operation called regeneration. For example, coked cracking catalyst is regenerated by burning off the coke (see §§1.7.3, 1.9).

If the catalytic reaction is a network of various processes, deactivation can lead to a change in the distribution of products. In such cases, the deactivation not only reduces the overall rate but it changes the selectivity.

1.7.3 Types of deactivation

Catalyst deactivation can result from deactivation of catalytic sites by poisoning either by impurities or by products of the catalytic reaction (§1.7.1). Many reactions involving hydrocarbons and particularly those run at higher temperatures lead to the deposition on the catalyst of high molecular weight compounds of carbon and hydrogen which deactivate the catalyst. This phenomenon is called coking or fouling. Catalysts so deactivated can often be regenerated.

Catalyst deactivation may also result from changes in the structure or in the texture of the catalyst. Changes of this kind are usually irreversible and the catalyst cannot be regenerated. This type of deactivation is often called catalyst ageing.

Sintering and recrystallisation. Catalysts often suffer during use from a gradual increase in the average size of the crystallites or growth of the primary particles. This is usually called sintering. The occurrence of sintering leads to a decrease in surface area and, therefore, to a decrease in the number of catalytic sites. In some cases, sintering leads to a change in the catalytic properties of the sites, for example, for catalysts consisting of highly dispersed metals on supports, catalytic properties may change on sintering due to a change in the relative exposure of different crystal planes of the metallic component of the catalyst or for other reasons. Thus sintering leads to a decrease in rate and perhaps also to a change in selectivity. Similar phenomena can occur in oxide catalysts as used in catalytic oxidation. The crystal size increases, or the initial structure of the crystals changes. For example, a binary solid compound may decompose into its components or an amorphous mass may crystallise. These processes may be called recrystallisation. In some cases the terms sintering and recrystallisation may refer to the same process. The removal of surface defects may accompany these processes.

In some cases, as for example in catalytic cracking on silica-alumina, processes similar to those involved in sintering and recrystallisation can lead to a change in the texture of the catalyst. Surface areas are diminished and the pore-size distribution is changed.

1.8 Mechanism of catalytic reactions

1.8.1 General

A chemical reaction proceeds by a set of elementary processes (the Manual, §11.3) which are in series and perhaps also in parallel. These processes start and terminate at species of minimum free energy (reactants, intermediates and products) and each elementary process passes through a state of maximum free energy (the transition state). To specify the mechanism, one must specify the elementary processes. This specifies the intermediates. One must also give the nature (energetics, structure, charge distribution) of the transition state. So much is true for chemistry in general. The special features of mechanism in heterogeneous catalysis are those which involve reactions between sorptives and active sites, reactions among adsorbates, and processes which regenerate active sites to give a type of chain reaction.

In general, only partial approaches to the specification of mechanism as given above have been possible.

Mechanism is sometimes used in different senses. For example, consider the two situations.

A + B C vs A + B C
C D C D

It may be said that the two situations have different mechanisms or that they are two variants of the same mechanism.

1.8.2 Elementary processes in heterogeneous catalysis

There are many more types of elementary processes in heterogeneous catalysis than in gas phase reactions. In heterogeneous catalysis the elementary processes are broadly classified as either adsorption-desorption or surface reaction, i.e., elementary processes which involve reaction of adsorbed species. Free surface sites and molecules from the fluid phase may or may not participate in surface reaction steps.

There is no generally accepted classification of elementary processes in heterogeneous catalysis. However, names for a few types of elementary processes are generally accepted and terminology for a partial classification [see M. Boudart, Kinetics of Chemical Processes, Chap. 2 (1968)] has received some currency. The particular reactions used below to exemplify this terminology are ones which have been proposed in the literature but some have not been securely established as occurring in nature at any important rate.

Adsorption-desorption. This includes the process of physical adsorption as well as non-dissociative chemisorption.

* + NH3(g) H3N*
* + H(g) H*

Dissociative adsorption and its reverse, associative desorption.

2* + CH4(g) CH3* + H*

The methane might be supposed to react either from the gas phase or from a physisorbed state.

Dissociative surface reaction and its reverse, associative surface reaction.

2* + C2H5* H* + *CH2CH2*

This involves "dissociative adsorption" in an adsorbate.

Sorptive insertion. This is analogous to the process of ligand insertion in coordination chemistry.

H* + C2H4(g) *C2H5

This reaction might also be imagined to proceed by adsorption of C2H4 followed by ligand migration (an associative surface reaction).

Reactive adsorption and its reverse, reactive desorption. This resembles dissociative adsorption but one fragment adds to an adsorbate rather than to a surface site. In abstraction and extraction processes, an adsorptive or adsorbate species extracts an adsorbed atom or a lattice atom, respectively.

Abstraction process *H + H(g) * + H2(g)

Extraction process + CO(g) 2e + CO2(g)

The following elementary process occurring either on one site or, as shown, on two sites is called a Rideal or a Rideal-Eley mechanism:

D - D(g)   H - D(g)
H  
* * * *

D2 may also be considered to be in some kind of a weakly adsorbed state. It will be noted that one D atom is never bonded to the surface in any minimum Gibbs energy intermediate. It is recommended that the term Rideal or Rideal-Eley mechanism be reserved for this particular elementary process. However, the term has been used for analogous processes in which there is a reactant molecule and a product molecule of nearly the same energy in the fluid phase or in some weakly adsorbed state and in which one or more atoms are never bonded to the surface. An example is the following elementary process, which has been called a switch process. The term might well be used generically for similar processes. The term Rideal or Rideal-Eley mechanism has been further extended to include all elementary processes in which a molecule reacts from the fluid phase or from some weakly adsorbed state. Even the sorptive insertion process and the abstraction process illustrated above fall within this extended definition.

1.8.3 Nomenclature of surface intermediates

Surface intermediates should be named in ways compatible insofar as possible with chemical nomenclature in general.

Adsorbed species may be treated as surface compounds analogous to molecular compounds. For example, *H may be called surface hydride, *=C=O may be called a linear surface carbonyl and may be called a bridged surface carbonyl. H2N* may be called a surface amide and H3C* a surface methyl or a surface s -alkyl.

The species *H may also be called an adsorbed hydrogen atom and *CO, adsorbed carbon monoxide.

Organic adsorbates pose a particular problem because quite particular structures of some complexity are regularly discussed. A nomenclature is recommended in which the surface is treated as a substituent which replaces one or more hydrogen atoms. The degree of substitution is indicated by monoadsorbed, diadsorbed, etc. This terminology does not specify the nature of the chemical bonding to the surface nor does it restrict, a priori, the valency of the surface site *. Thus, both of the following species are named 1,3-di-adsorbed propane. Other examples are:

*CH3 monoadsorbed methane
*CH2CH2CH3 1-monoadsorbed propane
  2-monoadsorbed
2-methylpropane
*OCH2CH3 O-monoadsorbed ethanol
*CH2CH2OH 2-monoadsorbed ethanol
* = CH - CH3 or (*)2CH-CH3 1,1-diadsorbed ethane
* = NH or (*)2NH diadsorbed ammonia
*COCH3 1-monoadsorbed acetaldehyde
Species adsorbed as p-complexes are described as p -adsorbed:
  p-adsorbed allyl

The substitution system of nomenclature should be viewed as showing only how atoms are connected and not as indicating the precise electronic structure. Thus p -adsorbed ethylene is one representation of 1,2-diadsorbed ethane.

Nomenclature based upon the process of formation of a particular adsorbate is to be discouraged. Thus, H* may be "dissociatively adsorbed hydrogen" but the same species is formed in dissociative adsorption of CH4, NH3, H2O.

1.9 Nomenclature of catalytic reactions

In general, a catalytic reaction may be named by adding the adjective "catalytic" to the standard chemical term for the reaction, for example, catalytic hydrogenation (or, if clarity demands, heterogeneous catalytic hydrogenation), catalytic hydrodesulphurisation, catalytic oxidative dehydrogenation, catalytic stereospecific polymerisation.

In general, special terminology for reactions is to be discouraged. However, certain catalytic processes of technological interest have special names in common use. Where such processes involve the simultaneous occurrence of two or more different chemical reactions, special names for the processes are probably inevitable. Some Important examples of such processes of technological interest are:

Catalytic cracking. In this process, a higher boiling cut of petroleum, for example, gas oil, is converted substantially into a lower boiling material of high octane number. Among the processes which appear to

be involved are skeletal isomerisation of alkanes followed by their cleavage into alkane and olefin, and hydrogen transfer reactions which reduce the amount of olefin formed and which lead to coke and aromatic hydrocarbons.

Catalytic hydrocracking. This is similar to catalytic cracking in its industrial purpose but it is effected under hydrogen pressure and on a catalyst containing an ingredient with a hydrogenating function.

Catalytic reforming. Catalytic reforming is a process for increasing the octane number of naphthas. It involves isomerisation of alkanes, dehydrogenation of cyclohexanes to aromatic hydrocarbons, isomerisation and dehydrogenation of alkylcyclopentanes, and dehydrocyclisation of alkanes.

The following reactions may be mentioned because they are rare except as heterogeneous catalytic reactions and have somewhat specialised meanings in catalysis.

Catalytic methanation. This is a process for removing carbon monoxide from gas streams or for producing methane by the reaction

CO + 3H2 CH4 + H2O

Catalytic dehydrocyclisation. This is a reaction in which an alkane is converted into an aromatic hydrocarbon and hydrogen, for example,

heptane toluene + 4H2

Catalytic hydrogenolysis. This is ordinarily used for reactions in which º C-Cº  + H2 gives º CH + HCº , for example,

propane + H2 ethane + methane

toluene + H2 benzene + methane

butane + H2 2 ethane

However, it may also be used for cleavage of bonds other than º C-Cº , for example,

benzyl acetate + H2 toluene + acetic acid

benzylamine + H2 toluene + NH3

Catalytic hydrodesulphurisation. This is a process in which, in the presence of hydrogen, sulphur is removed as hydrogen sulphide.

SECTION 2. LIST OF SYMBOLS AND ABBREVIATIONS

2.1 Catalysis and catalysts   2.1 Катализ и катализаторы
2.2 Adsorption   2.2 Адсорбция
Area of surface A, As, S Площадь поверхности
Specific surface area a, as, s Удельная площадь поверхности
Surface coverage q Поверхностное покрытие
Area per molecule in complete monolayer of substance i am(i) Площадь на молекулу в заполненном монослое вещества i
Surface site * Поверхностный центр
Ion Мn+ (or atom М) of adsorbent or catalyst at the surface (или Ms) Ион Мn+ (или атом М) адсорбента или катализатора на поверхности
Constant in Henry's law K Константа закона Генри
Constant in Langmiur's adsorption isotherms K Константа изотермы адсорбции Лэнгмюра
Constant in Langmiur's adsorption isotherm for substance i Ki Константа изотермы адсорбции Лэнгмюра для вещества i
Constants in Freundlich adsorption isotherms a, n Константы изотерм адсорбции Фрейнлиха
Constants in Temkin isotherms A, B Константы изотерм Темкина
Constant in BET isotherms c Константа изотерм БЭТ
Monolayer capacity nm Емкость монослоя
Constants of Roginskii-Zeldovich equation a, b Константы уравнения Рогинского-Зельдовича
2.3 Composition, structure and texture of catalysts   2.3 Состав, структура и текстура катализаторов
2.4 Catalytic reactors   2.4 Каталитические реакторы
2.5 Kinetics of heterogeneous catalytic reactions   2.5 Кинетика гетерогенных каталитических реакций
Stoichiometric coefficient of
substance B		 nB Стехиометрический коэффициент вещества B
Extent of reaction x Степень реакции
Rate of catalysed reaction xm Скорость катализируемой реакции
Quantity of catalyst Q Количество катализатора
Specific rate of reaction rm Удельная скорость реакции
Specific activity of the catalyst rm Удельная активность катализатора
Rate of reaction per unit volume of catalyst rv Скорость реакции на единицу объема катализатора
Areal rate of reaction ra Поверхностная скорость реакции
Turnover frequency
(turnover number)		 N Частота оборотов
(число оборотов)
Selectivity S, SF, SR Селективность
Rate constant k Константа скорости
Order of the reaction ai Порядок реакции
Frequency factor A Частотный фактор
Activation energy E Энергия активации
Isokinetic temperature (Kelvin scale) Tθ Изокинетическая температура (шкала Кельвина)
Fraction converted x Степень превращения
Feed rate n Скорость подачи
Space velocities nm, nn , na Объемные скорости
Space times tm,tn ,ta Объемное время
Sum of surface concentrations of reaction centres L Сумма поверхностных концентраций центров реакции
Surface concentration of surface intermediate m Lm Поверхностная концентрация поверхностного интермедиата m
Surface concentration of vacant reaction centres Lν Поверхностная концентрация вакантных центров реакции
Energy of activation for activated adsorption Eads Энергия активации активированной адсорбции
Energy of activation
for activated adsorption on
uncovered surface		   Энергия активации активированной адсорбции на свободной поверхности
(Differential) enthalpy of
adsorption		 - q (Дифференциальная)
энтальпия адсорбции
(Differential) enthalpy of adsorption on uncovered surface - q0 Дифференциальная)
энтальпия адсорбции на свободной поверхности
Transfer coefficient a Коэффициент переноса
2.6 Transport phenomena in heterogeneous catalysis   2.6 Транспортные явления в гетерогенном катализе
2.7 Loss of catalytic activity   2.7 Потеря каталитической активности
Constants in equation for rate of deactivation B, n Константы уравнения
скорости дезактивации
Time of run (on stream) t Время опыта (в работе)
2.8 Mechanism   2.8 Механизм
2.9 Nomenclature of catalytic reactions   2.9 Номенклатура каталитических реакций

Перевод Е. Никифорово


Конференция памяти Ю.И.Ермакова

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Международное рабочее совещание "Происхождение и эволюции биосферы"

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2-я Международная Школа-конференция молодых ученых по катализу

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7-й Европейский конгресс по катализу

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За рубежом

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