Modification as a Catalytic Process

N. D. Tyuteva

On the Active Surface

S. Z. Roginskii [1], investigating the nature of the active surface and the role of various types of inhomogeneities of the active surface, explains the distinctive properties of active contacts by the existence on the surface of special active structures. In the theory of catalysis, the activity of a catalyst is usually associated with various kinds of deviations of the structure of a solid from the ideal state without a noticeable change in the chemical composition of the solid.

“If the decisive role in catalysis is attributed to nonequilibrium active states, then it should be borne in mind that states and structures that are nonequilibrium under given conditions will be equilibrium under other conditions” [1].

S. Z. Roginskii [1] distinguishes physical inhomogeneities—physical violations of the ideal structure—and chemical inhomogeneities—structures arising inside a solid or on its surface through the local incorporation of particles of a foreign substance.

Chemical disturbances are structures in which the chemical composition of the solid phase as a whole differs from the thermodynamically most stable one, which in most cases is the composition corresponding to stoichiometry. During casting into a chill mold, the appearance of disturbances typical of rapidly growing crystals is quite probable—for example, incomplete or anomalous faces. Consequently, one may speak of physical inhomogeneities with all transitions from the structure of an ideal lattice to an amorphous cluster of atoms.

In addition, we introduce a modifier into the melt; consequently, one may speak of the influence of chemical inclusions.

With regard to the influence of chemical inhomogeneity on activity, S. Z. Roginskii points to the positive effect of the transition from pure, single-component catalysts to multicomponent catalysts.

“Mixed and promoted catalysts, for the most diverse groups of contacts and for various catalyzed processes, prove to be more perfect than simple contacts. The special features of complex contacts are in most cases associated with the formation of phase boundaries between solid phases of different chemical composition and with the incorporation of impurities into the surface layer of the solid at the point of contact of the phases” [1].

As early as 1935, S. Z. Roginskii [2] was able, for one case, to show by direct experiment the substantial role of the capture of an impurity by a solid in creating high activity.

Small deviations from stoichiometry also have a positive effect on activity. “The positive influence of chemical impurities in itself does not exclude the possibility of positive effects associated with purely physical violations of the ideal structure” [1].

Physical disturbances are usually very unstable. Physical disturbances are more stable in specimens which, at the moment the disturbance is created,

methods of capturing impurities. The presence of impurities greatly hampers the coarsening of crystals.

On the basis of the theoretical propositions listed by S. Z. Roginsky, one can explain the refinement of the grain during modification precisely by this character of the influence of impurities.

“The conclusion concerning the low stability of physical volume disturbances is a fortiori applicable to surface structures, since the mobility of atoms of the surface layer is, as a rule, higher than the mobility of atoms located inside the solid” [1]. Chemical impurities and chemical disturbances are very stable.

Activated adsorbed impurities, as well as those dissolved in the lattice or trapped along intercrystalline boundaries, require more effective methods for their removal.

Activated adsorbed impurities are removed by many hours of annealing at temperatures close to the melting temperature of the crystalline body. Great difficulties are presented, for example, by the removal of metallurgical impurities in steel.

S. Z. Roginsky emphasizes that the less impurity remains in the metal, the more stubbornly it is retained by it. We explain the strong influence of small impurities by their special distribution in the metal, by special conditions of adsorption or dissolution.

The modifying action of a small impurity is explained by another, special character of the electronic bond between the impurity ion and the crystal lattice. The less impurity there is, the more strongly it is held. The strength of the electronic bond is inversely proportional to the concentration of the impurity.

On this basis one may make the assumption: all small impurities are modifiers so long as they are present in the alloy in small concentrations. An increase in concentration leads to overmodification, and the positive influence is no longer observed.

S. Z. Roginsky also points out that “The preservation of impurities in a condensed phase does not, of course, mean the preservation in unchanged form of a definite arrangement of the impurities and of an unchanged structure of these disturbances. Various migration processes, chemical transformations, etc., are possible” [1]. With accelerated cooling (crystallization of the modified alloy in a chill mold), the possibility of impurity migration and the occurrence of chemical reactions is nevertheless not excluded.

S. Z. Roginsky notes the influence of phase transformations on the capture of impurities (an intensification of impurity capture near the points of phase transformations). If by the word “impurities” one understands not only a homogeneous inclusion, but also stoichiometrically excess atoms of the principal substance of the lattice, then near the points of phase transformations processes with the formation of stoichiometrically excess atoms (an increase in the amount of impurity) may occur. In his work S. Z. Roginsky [1] does not recommend strictly distinguishing physical and chemical disturbances, since chemical impurities may be captured at the sites of physical deviations present in the solid.

Thus, it seems to us, we have grounds to regard the process of modification as a catalytic process. The activity of the surface (catalytic activity) depends on disturbances of stoichiometry, on the formation of stoichiometrically excess atoms in the course of the reaction of formation of carbides and of the solid solution.

Stoichiometrically excess atoms and atoms of the modifier, adsorbing on the lattice of growing crystals, promote the reactions taking place. Chemical inhomogeneity (the appearance of stoichiometrically excess atoms) is produced as a result of rapid cooling of the alloy in a metallic mold (chill mold); in this case one may speak of modification by cooling.

When a modifier is introduced, it is captured by the lattice as an impurity, creates additional chemical heterogeneity, and stoichiometrically excess atoms appear which, being adsorbed together with the atoms of the modifier, cause additional modification. With accelerated cooling of the alloy in metallic form, we obtain a nonequilibrium system. The degree of nonequilibrium is increased (substantially) by introducing the modifier.

The formation of a nonequilibrium system during modification has been proved by phase analysis. In modified alloys we have an entirely different distribution of alloying elements between carbides and the solid solution. This indicates a different character of the reaction of formation of carbides and solid solution than in slow cooling of the alloy. It may also be assumed that the surface layer of the lattice is deformed by adsorbed impurity atoms (of the modifier and of stoichiometrically excess atoms of the adsorbate), and this deformation causes a change in the concentration of the solid solution (the degree of alloying), and consequently also in the composition of the carbides during their formation. The phase composition of the alloy thereby changes.

Dependence of catalytic activity on the specific impurity content

Catalytic activity depends on the specific impurity content. Curves of the dependence of catalytic activity on the specific impurity content were first obtained by S. Z. Roginskii and Ablezova [2] in 1935 for nickel and hydrogen.

As a result of the investigation, the conclusion was drawn: the catalytic activity of metallic layers is a direct function of the impurity content. A change in the amount of impurity turns the well-known contacts nickel and platinum into catalytically inactive materials. Conversely, metallic tungsten, with an appropriate dosage of impurities, unexpectedly becomes a highly active catalyst. “It may be thought that, for the hydrogenation of ethylene, pure and at the same time active metallic contacts do not exist. There are only contacts, explicitly and latently promoted. Promotion is a necessary condition for the appearance of high activity” [1].

Analogously to the presence of a maximum on the curve of the dependence of \(K\) (catalytic activity) on \(\gamma\) (atomic fraction of impurity), during modification we also have an optimum on the curves relating one or another property of a cast alloy to the amount of modifier. Such properties as the microhardness of individual structural constituents and cutting properties are optimal at the optimum amount of modifier. Impurities are captured by the solid body (as S. Z. Roginskii indicates) at the moment of crystal formation.

Impurities may enter solid solutions, may be embedded in the lattice in the form of colloidal inclusions, and may concentrate along the intercrystalline boundaries of a polycrystalline aggregate.

“For metals, the study of the electrical properties of the surface also indicates the possibility of several forms of impurity arrangement. With respect to the cross section of the layer, it is possible to find it on the surface, to accumulate it at the depth of several layers of the ‘subcutaneous layer,’ to have it pass under the outer layer, and to have uniform distribution through the thickness of the metal” [1].

On the mechanism of action of promoters

The specific activity of contacts is in a functional dependence on the amount of promoting impurities.

The principal kinetic effect in promotion is a more considerable decrease in the heat of activation of the catalyzed process in the presence of a promoter.

“The simplest assumption explaining the promotion curves is the assumption that the heat of activation changes with change in \(\gamma\), \(E = E(\gamma)\).

At \(\gamma_{opt}\), this \(E\) falls so much that the corresponding stage ceases to be the slowest, and some new stage becomes limiting, for which \(E\) increases with increasing \(\gamma\). Such are the kinetic prerequisites for the appearance of a sharp maximum of activity with respect to \(\gamma\)” [1].

When activity changes with increasing \(\gamma\), the quality of the surface changes. It may be assumed that, as the impurity content in the surface layer increases, the distances between lattice atoms change, in a manner resembling the stretching of the palladium lattice when hydrogen dissolves in it.

In this case \(H\) (the heat of adsorption) and \(E\) (the activation energy) would depend directly on the lattice parameters, which change with change in \(\gamma\). Here the impurity is considered to be uniformly distributed in the lattice. A second model could be a patchy distribution of impurities, at crystallite defects or along intercrystalline boundaries, provided that the degree of surface distortion increases with the growth of the average number of atoms in the inclusion.

To choose between the two models of promoter distribution in the layer described above, direct experiments are needed to study the structure of promoted layers.

In the literature there are indications of a rearrangement of the layer near \(\gamma = \gamma_{opt}\). This is indicated by a hint of a minimum in the work function near \(\gamma_{opt}\). For metallic layers, one and the same picture of promotion is observed, and one and the same type of curves with a sharp maximum of activity.

In studying the dependences of the properties of modified alloys (cutting properties, change in the microhardness of individual components, change in the phase state of the alloy, change in the chemical composition of phases) on the amount of additive, we likewise have a maximum on curves plotted in the coordinates property—amount of additive (modifier-promoter).

Classification of the action of additives introduced into catalysts

In one of his recent works, S. Z. Roginskii [3] proposes an original, rational classification of additives.

All additives may be divided into four large classes.

Class I comprises additives that change the nature and rate of the chemically obligatory stages of the contact process. Such stages are: specific adsorption; chemical reactions in the adsorption layer and between adsorbed molecules of the liquid or gas phase; desorption from the surface; bulk chemical stages for contact processes of the heterogeneous-homogeneous type. We propose to call all additives that affect the chemical stages of catalysis modifying, broadening the content of the concept introduced earlier for additives that simultaneously change the activation energy \(E\) and the pre-exponential factor \(k_0\) of contact processes.

Class II comprises additives that, by means of a corresponding change in the secondary structure and thermal conductivity of the contact grain, regulate the rate and nature of physical microscopic processes ensuring material transport and energy supply in the contact process. We call these additives structure-forming, referring to secondary structures, or distributing.

To Class III we assign all additives that increase the resistance of the contact to mechanical and chemical destruction, calling these additives strengthening.

To the IV class we assign blocking contact poisons, whose effect on a contact is reduced to the exclusion of part of the surface by blocking individual areas or by impeding access to the surface as a whole, or to particular areas of it, through the formation of films or “plugs” that hinder material transport. In this classification, the use of the terms “promoter” and “contact poison” in a broad and vague sense is considered irrational, since the most strongly acting additives may, for one and the same reaction, depending on external conditions and dosage, cause either an increase or a decrease in the rate. Therefore the widespread division into promoters and poisons is obsolete.

Each of the 4 classes includes narrower groups. For modifying additives these are: 1) an acceptor-donor group, mainly regulating the oxidation-reduction properties of the contact; 2) an acid-base group, regulating its acid-base properties; 3) a group of additives affecting the primary chemical stages of the process, chiefly by changing the boundary forms of crystals and lattice parameters, which, as shown by the works of N. D. Zelinsky’s school, very strongly affects the properties of contacts in many organic reactions; 4) additives acting through the formation of new chemical forms, in cases where the character caused by this change cannot be assigned to the 1st or 2nd group.

In the II class, the following groups are distinguished: 1) supports, whose main functions are the advantageous distribution of the active material in the form of isolated and accessible fine particles; 2) pore-forming additives, regulating the porosity of the catalyst grain, and 3) additives that change the thermal properties of the contact. The introduction of additives from each of these groups may lead to a change in the regime of the process, with a transition from diffusion to mixed or kinetic, or from isothermal to non-isothermal.

In the III class of additives we distinguish, as separate groups: 1) additives increasing the mechanical strength of the contact; 2) additives increasing the chemical resistance of the contact to the corrosive action of the reacting substances and of the catalytic reaction; 3) additives slowing the sintering and recrystallization of particles at elevated temperature. Not infrequently, additives may also affect the aggregation of contact particles at low temperatures, causing dispersion or sintering.

In the IV class the following groups are distinguished: 1) microblocking additives and 2) blocking layers. In nature there are no absolute boundaries, and any classification is always to some extent conditional. In our case the limiting extreme cases differ well from one another, but difficulties of a threefold order are encountered: a) additives introduced into a contact may exert simultaneously a twofold or even still more complex action; thus, they may simultaneously be a support, a modifier, and retard sintering.

Modifying microadditives, which mainly change the acceptor-donor properties of the surface, as a rule also change, to some weaker degree, the acid-base properties, etc.; i.e., in all cases one should speak of one predominant or two predominant actions of an additive, taking into account the possibility of weaker actions of other types. The predominant action of one and the same additive may be different with respect to different reactions, and sometimes also with respect to one and the same reaction under sharply different conditions of its conduct.

The character of the action of an additive depends on its method of introduction and on its quantitative content.

Conclusions

1. Surface chemical activity depends on the violation of stoichiometry. The greater the violation of stoichiometry, the greater the activity.

2. Stoichiometry is violated during rapid cooling of the ingot. During casting with accelerated cooling (casting into metal molds), conditions are created for the formation of an active surface on which atoms of various impurities, stoichiometrically excess atoms (ions), and atoms (ions) of the modifier are adsorbed. Stoichiometrically excess atoms are adsorbed by the surface of the crystal. In this case we assume the possibility of a catalytic influence by the stoichiometrically excess atoms.

3. The modifier causes chemical disturbances in the lattice, chemical inhomogeneity, and activity of the surface. The modifier promotes the catalytic reaction of carbide formation and the reaction of solid-solution formation.

4. With accelerated cooling (in a chill mold), crystallization proceeds according to a nonequilibrium diagram. The modifier additionally increases the degree of nonequilibrium.

5. Analogously to the presence of a maximum on the dependence curve \(k_0(\dot{\gamma})\), during modification we also have an optimum on curves plotted in the coordinates alloy property—amount of modifier.

6. The surface layer of the lattice is deformed by the adsorbed modifier atom, and this deformation causes a change in the concentration of the solid solution, the degree of its alloying in the course of formation.

Literature

1. Roginskii S. Z. Active contacts, their formation and nature. Journal of Physical Chemistry, issue 1, p. 6, 1941.

2. Ablezova and Roginskii S. Z. Reports of the Academy of Sciences, 467, 1935.

3. Roginskii S. Z. Classification of the action of additives introduced into catalysts. Reports of the Academy of Sciences of the USSR, vol. 87, No. 6, 1952.