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Chapter IV: General Trends in the Mechanisms of Heterogeneous Catalytic Reactions Involving Molecular Oxygen
104
Chapter IV GENERAL TRENDS IN THE MECHANISMS OF HETEROGENEOUS CATALYTIC REACTIONS INVOLVING MOLECULAR OXYGEN The Significance of Chain Mechanisms The numerous investigations of N.N. Semenov and other scientists have shown that gas-phase reactions involving O2 in the absence of solid catalysts proceed by way of radical chain mechanisms /1-3/. These are typical branched chain reactions or reactions with degenerated branching. The same reactions in the presence of active solid catalysts usually occur at considerably lower temperatures. For example, the noncatalytic oxidation of hydrogen starts around 400 0C while, in the presence of platinum, the reaction can be observed even at the temperature of liquid nitrogen /4/. Another significant difference concerns the product distribution (selectivity) which is different for noncatalytic and catalytic processes if the reaction can follow several paths. A possible explanation of the above facts is that, in the presence of solid catalysts, the reaction mechanism remains the same (i.e. a radical-chain mechanism) but the catalyst facilitates the most difficult step of chain initiation. The latter is caused by the quasi-radical nature of surfaces of metals or semiconductors /1/. This conception corresponds to the idea of heterogeneous-homogeneous catalysis /5/ which was used in discussing the mechanisms of oxidation reactions /6/. For the experimental detection of the homogeneous stages of a reaction, various methods are employed: for example, inves.igation of the dependences of the reaction rate and product distribution on the unoccupied volume of the reaction zone and studies of tne Qependence of the rate on the surface area of the catalyst. If the heterogeneous-homogeneous mecnanj.sm is valid, the ~eac tion kinetics should obey the rate laws 0:.' chain processes but not those of heterogeneous catalytic processes. Th~ latter criterion was used in references /7/ and /8/. Often the method of differentiate calorimetry /9! is use~ to detect homogeneous steps. Radical species could. a.lso ne ae tee ted. directly by the ESR method. These methods have snown that in some
105
cases the oxiJation reactions really proceed by way of heterogeneous-homogeneous mechanism /6/. It should be noted that: I) Homogen~ous steps in these processes are observed at lower temperatures than in case of noncatalytic reactions, so the above steps are evidently associated with the presence of the catalyst. 2) The detection of homogeneous steps does indicates the occurrence of a radical-chain mechanism since other reaction routes in the gas phase are hardly possible. J) The modern theory of noncatalytic chain reactions /1/ suggests that the initiation, as well as the termination, of chains to take place on solid surfaces. It is assumed that the active centers of chains can appear as a result of either homogeneous reactions or the interaction of the initial reagents with the reactor walls. Hence, it is difficult to discriminate between the usual chain reactions and heterogeneous-homogeneous catalytic processes. In principle, one can imagine a situation where the mechanism of both processes is the same. Observed differences in rates and selectivities are caused by quantitative differences in the rate constants for the elementary steps for surface reactions. Another possibility is that, in the presence of an active catalyst, the surface initiation and termination steps of chains are different from those in the absence of the catalyst. The data have been obtained so far do not allow this possibility to be discussed in more detail. One can thus conclude that the heterogeneous-homogeneous mechanism has been shown for some oxidation reactions. At the same time, this mechanism is not a typical route for the reactions under usual conditions of heterogeneous gas-phase catalysis. The validity of the Boreskov rule on the approximate constancy of specific catalytic actiVity of substances with the same chemical composition but with different macrostructure (see Chapter I) is evidence in favour of heterogeneous surface mechanisms. In most cases, under the above mentioned conditions, the critical phenomena which are peculiar to chain processes are not observed. The high selectivity of typical solid catalysts for mild oxidation (which is different from that in noncatalytic chain oxidation) also suggests the heterogeneous character of the processes. The absence of homogeneous steps has often been demonstrated experimentally. However, one should point out that the role of homogeneous steps in heterogeneous catalysis has not been sufficiently studied. In some cases, the methods used are not reliable. Even when a hete-
106
rogeneous-homogeneous mechanism has been provcd, its contribution to the total reaction rate is not clear. Investigations in this field have not been systematic and the selection of topics studied is often accidental. The kinetics of hetrogeneous-homogeneous reactions (both in experimental and theoretical aspects) have been poorly studied. Investigations in this direction have recently commenced /8/. The relationship between heterogeneous and heterogeneous-homogeneous catalysis is of special interest. The development of homogeneous chain steps is favoured by elevated temperatures. It is important to know how the chemical structure of the catalysts and reagents affects the probability of a heterogeneous-homogeneous mechanism. ~ccording to reference /10~ the above probability in CO oxidation is lower for active heterogeneous catalysts which "retain" the process on the surface. However, oxidation reactions on active Pt are often inclined to a heterogeneous-homogeneous mechanism. It has been shown /8/ that under favourable conditions the oxidation of H2 over V2 0 includes homogeneous steps. This 5 is in contrast to the oxidation of CO for which a special requirement, the presence of H-donors, should be satisfied. Thus, comprehensive studies on heterogeneous-homogeneous catalysis are evidently needed. Finally, one should note the foll,'·,'ing. I t is believed that in the presence of a catalyst, the reaction rate is much higher than that without a catalyst. This results in the essentially lower temperatures required for catalytic reactions. Only at these temperatures is the catalytic effect clearly pronounced. At elevated temperatures, in the explosion region where a noncatalytic c~ain reaction proceeds very rapidly, its rate can exceed that of the catalytic reaction. For instance, the rate of the oxidation of a-xylene in an empty reactor at JOOoC is much higher than that of the heterogeneous catalytic oxidation reaction over V20 ' even when S the whole reactor is filled with V~O~ granules /11/; the positive " :J effect of the catalyst is displayed here rather in its selectivity. Nevertheless, although heterogeneous-homogeneous radical-chain mechanisms are not typical for reactions involving 02 under usual conditions, the possibility of chain reaction routes involving surface radicals must remain. Such a concept was put forward by N.N. Semenov, V.V. Voevodskii /13/ and other authors. According' to them, the initial chain-generating sites are free valencies on
107
the surfaces (I"ree electrons, no ... e~, t~c.). These mechanisms shoul~ be conaluered Q~ r.;riJ'" ~l1i:. ~ .tea l -:or which there is no direct tlxfJeriDl~aLal cvi(J.~nct.: .... ~ L; au t'~wu~·:.l"; that the participat.ion of Qdso':"oe~~ raJ.ical SPt:.';;'~f;lf in ~i.i~ r-eac ~.i\);. does not necessarily mean 'that. a >:ha10 mechanlslll l.ni;li:lJ ...l:lC"'.:!~"~' 11.1. Temkin and L.O. Apelbaum /i4/ 1~tel'l11::"uec. t;h~ "~nuil'1 Ifr....:~h" in the h,ydrogenation of e th,ylene \lvol'.i Ii lud~:r I .i. r.:. a chain mechanism does not occur. i~r!verth.t'less, .!i•.:.~:.rbcd ii-ali.JA:.S and C 8 radicals participate in tIlt\! roaction. 2 5 Although surface cDain reacLiona have ~L ~e~n 1r~~,nstra~~~ eAperiJDentally, their existence has not been retl.lteli. ;;le ..:unc~y:' has not been wide-spread becal.lse of tk~ fact Ll~t ex?er~~ll~al ~a ta can usuall,y be explained by noncnain mechaniSll1s. ~hc ~~-cal The same concerns a special type of chain mech~l1G~, led "recuperation catalysis". Acc:Jrding ~o til':'S cc ac c... -:', i.M ~tl:.r gr evolved in an exothermal catalytic reaction is .»t ~i8dipatai but is accumulated in tne surface layer 0: II. ..;\)li~, :'()l' e•..ad.i·le, in the form of labile noneql.l11ibriwa species. 1h.~ Bccu:nl.lla~e;~ .;,'1.';:':"gr is used for the activation of new i>0rtions 'J: cue roagel.liS and it is then accumulated again and so on. SC!wllles of I.h.is type B:'-C likely to be valid for fast highly '9xotnerz.ic reac~1"lI.S like tbe catalytic recombinatioh of oxygen ato~s (0 ~ 0 = ~~;4do=-li; kcal mol~1). ~
tv"'''''
Step-wise and Deformation Kechanisms At equilibrium energy distribution in ~ne rCBe~ing a1'3~ea two general forms o£ interme~atc ci~~ical interac~ion ar~ p~aalble, which arel step-wise and deformation mechanism (see Chapter I). Let us assume that in the presence ~r a catBlyot, ~at, ;hG ~xi dation of R into products, P, ta~e8 place:
If the initial Z"cae;ents fo~.a a ClJ.:JUOO1l activ:o.:c::: c:olil;J.e.x:, o Cat)· willctl is ,iireclily 'l;:811s.1.\:.rill.e.i. int:>:, Ii ";'.:l;.';}r::..;.l"m 2 mechanism occurs I Cat , -. (R o 0
".L ..... ,;
108
The latter is also called a "one-step mechanism" /15/, or "direct catalysis" /16/. Reaction (IV.1) is a redox process. In the formation of the activated complex, electrons are transferred from R to 02 through the electroconductive crystal lattice of the catalyst. 1.V.Pisarzhevskii /17/ and V.A. Roiter /15/ pointed out the possibility of a positive mutual influence of the reagents on the mechanism (IV.2): the donor process of electron transfer from R is stimulated by the simultaneous acceptor process of electron addition to 02 which brings about the advantages of the above onestep mechanism. The alternative step-wise mechanism includes two (or more) steps and one or more surface intermediates, each step having its own activated complex. The simplest example is that given by the mechanism of an alternating surface oxidation-reduction process: 1) 02 + 2Cat -2Cat·0
2) Cat·O
+
R--P
+
Cat
(IV.))
Theoretically, both schemes, deformation and step-wise, are possible. The first may have lower activation energy (due to a positive mutual influence of the reagents) but a highly negative entropy of activation, while the second is characterized by a more favourable entropy of activation but higher activation energies. Hence, one should expect that the first mechanism will change into the second at elevated temperatures /18/. However, we do not today know of any experimentally proven example of "direct catalysis" /16/. At the same time many heterogeneous catalytic oxidation reactions have been distinctly shown to proceed by way of step-wise mechanisms of the type shown in Scheme (IV.)) (see later Chapters). The same is valid for decomposition processes like the decomposition of N20:
In the first step, N20 is an oxidant while, in the second, it is a reductant. The following remarks are noteworthy in connection with the
109
problem discussed. I. The most convincing evidence in favour of mechanism of the type shown in (IV.J) is the coincidence of the kinetic characteristics (rates, activation energies, reaction orders, etc.) for the all-over catalytic process and for the separate steps studied independently. In some cases, this is not observed and this is sometimes used to deduce the validity of a deformation mechanism. However, such facts only allow us to conclude a mechanism such as Scheme (IV.J) does not apply. Several possibilities should then be examined. 1) The reaction does obey a deformation mechanism (IV.2). This is probable in the case of reactions with simple stoichiometry, such as: Cat
..
but is impossible for such processes as
since the simultaneous collision of four molecules and the catalyst is highly improbable. 2) Catalysis can proceed by way of Scheme (IV.J) but the form of adsorbed oxygen participating in the catalytic oxidation step may be different from that in the reduction step. For example, in the first case anion-radicals such as 0- or 02 can react, while, in the absence of 02 in the gas phase during the reduction step, the less reactive 0 2- species takes part. 3) In principle, the catalytic process can proceed by way of a chain mechanism which is not valid for the separate steps of reduction and reoxidation /18/. 4) A step-wise mechanism which is different from Scheme (IV.J) may take place. II. In some cases, the step-wise schemes have been shown experimentally occur which differ from the simple redox mechanism (IV.J). For example, the oxidation of hydrogen on pure platinum films occurs by a Langmuir-Hinshelwood mechanism /19/ :
no 2)
°2 + 3) (H2) +
4) H2
+
)-(°2) (02)----(H 202)
(H202 ) 2H2 + °2
2H2O + 2H2O
)
(
+ (
)
(IV.4)
According to reference /20/, the oxidation of CO and the complete oxidation of aliphatic hydrocarbons over metal oxides at low temperatures involves the formation of carboxylate-carbonate structures, R', from Rand (0) and the sUbsequent decomposition of R' by
°
2:
1)R
+
2) (R')
(O)-(R')
+
02 --- P
+
(IV.5)
Both schemes are step-wise but they differ from the mechanism of Scheme (IV. 3 ) • Schemes of the type of (IV.4)and(IV.5) are sometimes called "associative mechanisms" since they involve steps in which both the initial reagents take part. These "associative mechanisms" are not identical to deformation catalysis since they include intermediate compounds. Besides, the "associative mechanism" cannot be considered as a deformation one because such terms should be general, covering various types of catalytic reactions. In particular, one can imagine a deformation mechanism for tae isomerization of A into B: Cat A
"B
(IV.6)
for which it would be unnatural to use the term "associative". It is believed that "associative mechanisms" are alternatives to those with separate interaction of each of the reagents with the catalyst. The last scheme is presented by mechanism (IV.3). Sometimes, such a classification is possible and advisable. However, it is not clear, from this point of view, what is the mechanism of Scheme (IV.4): on one hand, each reagent is here activated separately but, on the other hand, the associative interaction of
111
ue reagents takes place in the rate-determining step. III. The above considerations are valid for any experimental method of studying the mechanisms. In some cases, the tracer method is used in oxidative catalysis over metal oxides to elucidate whether or not surface lattice oxygen participates in the reaction. For instance, the oxide is labelled by 0 18 and one determines whether or not the tracer occurs in the reaction products. A positive result suggests that surface (0) species participate in the catalysis, but it is not necessarily a redox mechanism (IV.3) since, in Scheme (IV.5), oxygen of the oxide can also take part in the reaction and 0 18 will be transferred into the reaction products. A negative result (i.e. 0 18 is not detected in the reaction products) means that surface oxygen of the oxide is not a reaction intermediate, and this allows one to reject mechanisms involving this form of oxygen. However, a true reaction mechanism cannot be deduced from the last result since many versions are consistent with the absence of 0 18 in the products. It may be a deformation mechanism or a step-wise Scheme (IV.3) involving anion-radicals (0-) or (02-)' which appear rapidly dllring catalysis and which do not have time to exchange with the surface oxygen of the oxide catalyst. Thus, among step-wise mechanisms, the scheme of alternating reduction-reoxidation of the surface is the simplest one and has been proved to occur for many reactions. However, other step-wise mechanisms are also probable. A deformation mechanism is also theoretically possible, especially at low temperatures, but this mechanism has not been experimentally proven. t
Parallel, Consecutive and Parallel-Consecutive Schemes of Complex Reactions Many catalytic oxidation reactions can follow several routes. In these cases, two main situations may be distinguished. In the first extreme, each product is formed only from the initial reagent but not from products (parallel scheme),for example: 1m]
+
~N2 + H20 02---1i20 + H20
~NO
+ H2
°
112
and: CH - CHO + H20 = CH - COOH + H 20
+
+ H20. In the second extreme (consecutive scheme), the initial reagents give initially a reaction product which is less stable thermodynamically than others; it is then converted to more stable products, etc., for example: °2 CH-COOH - C 0 2 + H20. For organic substrates, an increase in the degree of oxidation of the products in such sequences coincides with an increase of the thermodynamic stability of the products. In the oxidation of ammonia, the reverse picture is observed /21/ since the products of deep oxidation (nitrogen oxides) are less stable than the mild oxidation product (N2) and, in this case, different types of consecutive schemes are valid, should such a scheme occur: NH) NH) NH) + ° 2 -NO - N20 - N2, NH) NH) + °2- NO - N2 O - N2 + °2'
NH) + ° 2 - NO - - N2 + 02 In intermediate situation mixed parallel-consecutive schemes are observed, for instance:
----------"
CH2 = CH 2 = CO 2,
C~
- CHO
ck - ~:~Hl
io...==J 2
Schemes of such type are widely discussed in the selective oxidation of organic substances /22, 2)/. The realization of the above schemes over a given catalyst depends upon the reaction conditions. Higher concentration of mild oxidation product (in the oxidation of organic substances) favours
113
its further transformation. Hence, a parallel scheme is more probable at low conversions (low contact times, ~; low temperatures, T). At higher conversions (higher values of t, T), consecutive routes are enhanced. The contribution, f, of a parallel route to a parallel-consecutive scheme can be defined as the ratio of selectivity, Si' at fixed r = t: I to that at r --- 0, (8; )r= 1'1
i.e.
(~ ),&=/7 It is assumed that the concentrations of reagents are constant or their changes are taken into account. Determining the above schemes is useful in elucidating the conditions under which the desirable product is obtained in maximum amounts. However, it should be noted that: 1) such schemes are not detailed mechanisms since they do not involve elementary steps with participation of the catalyst; 2) a parallel scheme of the overall process does not exclude consecutive steps on the catalyst surface /24/. Thus, the reagent R can initially be converted into the surface intermediate (1 1 ) which is then either transformed into a mildly oxidized product P1 or is further converted into a more fully oxidized surface intermediate (1 2 ) which can undergo further similar reactions: P1
t
(0) R -(1 )
1
The above scheme is a parallel one since each product (P P P 1, 2, 3) is formed from R but not from another product. At the same time, the scheme contains series of consecutive surface reactions.
Elementary Steis of Heterogeneous Catalytic Reactions Invo vi~ Molecular O~gen If scheme of the type (IV.3) has been proved, it does not mean that a detailed reaction mechanism has been found. Complex steps of reduction and reoxidation involve elementary steps which should
114
be elucidated. The same concerns other schemes like (IV.)). We can now distinguish several kinds of typical elementary steps which are peculiar to heterogeneous catalytic reaction involving
°2· Reversible Adsorption of Reagents (Without Dissociation) Examples of the adsorption process A + (
)~(A)
are given by the adsorption of H2, 02' CO and NH). In this process, the molecule A is preserved as a whole, while electron density can be shifted from A to the catalyst or in the reverse direction. The adsorption of oxidants (02' N20, etc.) is accompanied by a shift of electron density from the catalyst to A which results in negative charging of (A). In extreme cases, molecular anions are formed: )
+
e _(0-) 2
) + e -(N 0- ) 2
The latter is associated with a decrease in electrical conductivity (i.e. an increase in work function) of the catalyst. To investigate the adsorbed anion-radical species, 02' the electron spin resonance (ESR) method is used /25/. Analysis of the ESR spectra on metal oxides shows that molecular oxygen and metal cation often form a Jor-bond (involving d-orbitals of the cation) and the 0-0 bond is directed in parallel to the surface. The electron transfer from the catal)st to 02 can be considered as the oxidation of Me m+ into Me(m+1 +, so that
The adsorption of the reductants can be accompanied by a shift of the electron from A to the catalyst to form a positively char-
115
ged adsorbed species. pear: ) ~(H2
+) + e
)~(CO+)
+ e
)~(C2H4+)
In the extreme cases,adsorbed A+ ions ap-
+ e.
These processes are associated with an increase in electrical conductivity (a decrease in work function) of the catalyst. Sometimes, one succeeds in elucidating the structure of adsorbed species of the last type. Thus, according to reference /26/, the adsorption of H2 on metal oxides leads to orientation of the H-H bond parallel to the surface. The H2 molecule occupies the top of the truncated octahedron formed by the 0 2- anions surrounding the central metal cation. A similar structure can be assumed for other (A) species. When the A molecule has an unshared pair of electrons, one should expect to find the coordination bond between A and the metal cation (surface ammonia complexes and similar species are formed). Such complexing facilitates further electron transfer from A to the catalyst. The observed increase in the distance during the formation of suggests the loosening of the oxygen-oxygen bonds in 02. A similar effect can be expected in other molecularly adsorbed species (A). Thus, processes such as (IV.7) make the molecule A ready to take part in a subsequent deeper interaction on the catalyst surface. In catalytic oxidation, t he reaction mixture contains bo th oxidant (02) and reductant (R) simultaneously. Their adsorption may cause a positive mutual influence since (02) and (It) are oppositely charged. For example, according to reference /27/, ethylene itself is not adsorbed on pure silver but in the presence of adsorbed oxygen, the adsorption of C2H takes place. This can be due 4 to the fact that electron transfer from Ag to 02 generates vacancies in the d-band of the silver what is favourable for the electron-donating adsorption of ethylene. Adsorption according to Eq. (IV.7) is also possible for the reaction products (H20, CO 2, etc.). Since the latter have oxidative properties (though slight), negative polarization or even ionization is probable during adsorption:
02
°- °
116
+
e -(H 0- )
(IV.10)
+
e -(C0
(IV.11)
2
2-)
The desorption of such species, as in the reactions (IV.8)or (IV.9) results in the liberation of electrons. Some molecules can be both reagents and products. For instance, in the decomposition of N20, nitrous oxide is an initial reagent while, in the oxidation of ammonia,it is the reaction product. In the both cases, the adsorption of N20 is accompanied by electron transfer to N2 0 while removal of the N2 0 from the surface leads to the liberation of electrons (transferred to the catalyst). The H20 molecule (like NH ) can enter coordinative interaction 3 with metal cations, resulting the formation of surface hydrates. Dissociative Adsorption Interaction of the catalyst with adsorbed molecules may cause scission of the chemical bonds in the molecule A. In such cases, the adsorption of oxidants leads to the formation of O-atoms on the surface:
°2
+
2(
°3
+
(
)-2(0) )-(0)
(IV.12) +
°2
+
N2
(IV.13)
or 03 + 3(
) - - 3(0)
N20 + (
)--(0)
Oxygen atoms are negatively charged; in extreme cases, this results in (O-)or (0 2-) species. The (0-) anions are at the same time radicals which can be studied by the ESR-technique /25/. Thus, the activation of 02 can be represented by the sequence
I
II
III
IV
V
117
Here, only the species I is neutral. The species III corresponds to a peroxide anion. Experimental evidence for the formation of (O~-) in 02 adsorption is now absent, but complex compounds of transition metals including O~- ligands are known /28/. The above species can be transformed into one another. Elevated temperatures are favourable for dissociative adsorption and for accepting more electrons, so that heat allows the (02) species to be converted into (0-) and then into (0 2 - ) /25/. Ionization and dissociation are connected: addition of an electron to 02 loosens the 0-0 bond /28/. Higher concentrations of molecular forms are favoured by high surface coverages with oxygen which corresponds to low values of qs' On going from form V to form I, the energy of the oxygen-catalyst bond decreases (see Chapter III).
Fig. 49 - The elementary cell of the V2 0 crystal /31/.
5
o
2.8~
o~
!
The transition from 02 to 2(0 2 - ) requires four-electron transfer. Usually one metal cation cannot satisfy this requirement (i.e. C02+~Co3+ + e; Mn2+~Mn4+ + 2e); hence,the participation of two or more cations is necessary. This is true in the adsorption on rather large crystalline particles. If the cations are separated (as, for example, in dilute solutions of MeXO y in a solid matrix or in dilute supported layers), the four-electron transfer is difficult and one- or two-electron processes leading to (02) or (0-) are more probable. The latter results in weakly bound oxygen species /29/. A decrease in the bond energy of the surface oxygell at high B may also be caused by changes in the degree of oxidation of the metal, ,cation. Such a situation is probable in the adsorption of 02
118
on silver when at higher values of 8 , surface oxides of Ag+ are converted into the less stable surface oxides of Ag3+ /30/. The 0 2- anions formed during the adsorption of O2 on metal oxides are hardly distinguishable from surface anions of oxygen in the oxide. In the oxides of polyvalent metals (which have crystal structures of low symmetry), oxygen atoms are not equivalent. The example of V20 is given in Fig. 49. The longest V - 0 bond 5 o 0 (2,83 A) is the weakest one, while the shortest V - 0 bond (1.54 A) is close to a vanadyl V - 0 bond. The latter is a double bond and is covalent to great extent. Some authors believe that the V - 0 bond is highly reactive in oxidation processes /31/. Similar properties are displayed by the Mo bond in molybdenum oxide catalysts /32/. Reactions (IV.13)and(IV.14) are practically irreversible while reaction (IV.12) becomes reversible at the rather high temperatures which usually correspond to those of isotopic heteroexchange of oxygen. Dissociative adsorption is also possible for the molecules which are oxidized, for example:
°
H2 + 2(
) -2(H),
NH + 2(
) -(NH
3
+ 2( C 2H6
) -(C
2) 2H5
+ (H) ,
) + (H).
Such reactions are peculiar to transition metals /33/. They can be preceded by the adsorption in molecular form (as in the case of 2 ) :
°
2(H) The adsorption of hydrogen has been studied most extensively on Pt, Pd and other transition metals. Several types of hydrogenmetal bonds have been found. A covalent bond, polarized to a definite extent, is likely to be a typical case. Surface hydrides, with a definite contribution for ionic bonds, are also possible, hydrogen being charged negatively or positively. Small hydrogen atoms can be localized over the surface metal atoms or amqng them. Hydrogen adsorbed on such metals as Pt or Pd possesses high surface mobility /34/.
lHJ
Interaction of Various Molecules with Adsorbed Oxygen Even when we restrict ourselves by the reactions with one form of adsorbed oxygen, (0), numerous types of catalytic transformations are possible. It would be advisable to classify the steps considered according to the types of chemical bonds participating in the reactions. The interaction of H2 with (0) involves rupture of the H - H and oxygen-catalyst bonds, leading to the formation of either H or OH-groups: 20
°
(IV.16)
H2 + (0) H2 + H + (0) - ( H 20) 2
H0 +
(IV.17) (IV.18)
2(0)~2(OH)
'-
In the above processes, hydrogen is the reductant (electron-donor). The electrons are added to the metal cation or to the cation and H (or OH): 20
H + (Me m+ ) + (02-)-H + (Me(m-2)+) + ( 20 2
),
H + (Me m+ ) + (0 2-) - ( H + (Me (m-1 )+), 20-) 2 H + (Me m+ ) + 2(02-)~2(OH-) 2
+ (Me(m-2)+).
One can assume two possibilities. The first one involves the reduction of Me m+ by H2 to form H; or H+ with further fast ionic reaction between the positive hydrogen ions and 0 2-. Another possibility /35/ involves an electron transfer from 0 2- to Me m+ to form anion-radicals, 0-, which then rapidly react with H2• In the last case, the reductant is 0 2- but not hydrogen. Such a mechanism may be accepted for systems inclUding easily reduced cations Me m+. However, in most cases, the first mechanism is likely to be valid. The above considerations are also true for other reagents (CO,
120
S02' etc.). Reactions (IV.16)-(IV.18) may be preceded by preliminary activation of hydrogen (reaction (IV.15)) which facilitates sUbsequent interaction of H2 with (0). Other molecules (NH CO, S02' etc.) 3, may behave similarly:
NH
(0) -(NH) + H2O, 3+ NH + (0) + ( ) -(NH) + (H2O), 3 NH +2(0)+( ) (NH) + 2(OH). 3 CO + (0) - C 0 2 + ( ) , CO + (0) --(CO 2), + CO + 2(0)-(C0 3) S02 + ( 0 ) - S03 + ( S02 + (0) -
)
),
(S03)'
S02 + 2 ( 0 ) - (304) + (
)
.
The adsorbed species formed «OH), (C02), (C0 (S03)' (S04)) are 3), charged negatively, for example: CO + (Me m+) + 2(02-)~(CO~-) S02 + (Me m+) + 2(0 2-) _
+ (Me(m-2)+) + (
(30~-)
+ (Me(m-2)+) + (
), ).
Such substances as N20, 03 and H202 may display dual reactivity: in the interaction with the reduced surface, they behave as oxidants (like 02) while in the interaction with oxidized surface, they react as reductants (like H2), for instance: N20 + (0)-N2 + 02 + H202 + (0)-H20 + 02 +
), ), etc.
The reason is that nitrogen in the N:N = group in N2 0 and oxygen in the group in H202 can be both reductant and oxidant. The interaction of paraffins with (0) species may involve the
°- °-
121
scission of C - C or C - H bonds. For olefins, a rupture of the C - H- allylic bond is typical and this leads to allylic complexes /36/, for example: (0)
CH 2 = CH - CH + ( 3
CH - CH
)~(CH2=
.":". CH2 ) + (OH) 3)--(CH2::,.CH
(IV.22) The latter is a J.T-complex attached to a metal cation and oriented in parallel to the surface. The formation of 6-allylic complex (which is perpendicular to the surface) is also possible. If the oxygen attack is directed at the C - C bond, a rupture of the $ - C - C bond is probable with the formation of C - 0 bonds. Activation of cycloolefins and acetylenic hydrocarbons can be imagined in a similar way. Alkylaromatic hydrocarbons, in which the C - H bonds in side groups are activated by conjugation with an aromatic ring, react like olefins in which the C - H-allylic bonds are activated by conjugation with C - C-bond: from toluene, the benzyl complex, (C6H ) , is formed (similar to an allylic one), etc. In the inSCH2 teraction of benzene, toluene, naphthalene and so forth with surface oxygen, scission of tne bonds between t~drogen and the aroInatic ring is also possible, and tais r~sults in surface complexes of the se.ni-quinone type. Heterocycles are likely to be activated like aromatic hydrocarbonds. Reactions between alcohols and (0) species may involve rupture of the 0 - H bond and this leads to surface alcoholates: ( 0)
Thus, for the above mentioned processes involving organic substrates, the formation of ~C - 0 - Me bonds is peculiar. Reactions of aldehydes with adsorbed oxygen lead to more oxidized surface species which are acidic in nature. Thus, in the case of benzaldehyde, the formation of benzoate-ions has been proved
/37/:
o C H C/ 6 S ~ H
+
) + (OH).
122
In the reactions considered, organic molecules behave as reductants (electron-donors) in the same way as H2, CO and NH J ; i.e. the C group in (C6H is attached to the reduced form 6HSCOO-SCOO) of the metal cation. Interaction of Surface Compounds wi th Adsorbed Oxygen Surface compounds appearinG as a result of tne above reactions can be further oxidized on the catalyst. For example, the (NH) species formed from 1ill and (0) can be converted into nitroxyl par3 ticles /J8/: (NH) + (0) --(HNO).
The variety of such types of steps is especially great for organic substances. In most cases, they result in carbonyl or carboxyl surface compounds.
Adsorption - Desorption of Products Adsorption and desorption of H20 and CO 2 (products of the deep oxidation of organic substrates as well as those of the oxidation of H2, CO, lill)' e t c , ) can be described by Eqs (IV.10), (IV.11). In addition, H20 and CO 2 may react with (0 2-): ) + (02-)~ + (0 2-) ~
2(OH-)
(C0 2-).
J The last steps are reversible. Similar reactions can be assumed for SO] and other products:
The desorption of the products of mild oxidation of lecules can take place either directly, e.g.:
0~6~rdc
mo-
123
(CH 2 = CH - CHO) - - - CH 2 = CH - CHO + ( or by yay of more complex transformations: (C-H_COO) + (OH)-Cb"H"COOH + (0) + ( \:) :J
).
J
':rile examples examined above do not cover t ne whole variety of the elementary steps of complex catalytic ::-eactions involving 02. In particular, they may be expanded by the reactions between surface compounds an~ gaseous molecules, for example:
H + 2(OH)---2H + 2( 20 2 02 + (HCOOH)-C0
2
),
+ H20 +
or between different surface
), e to.
inter~ediates,
for
ex~~ple:
Nav er-t he Leaa , the reactions consiciered can present some typical
steps of catalytic processes. Detailed analysis of the corresponding experimental data concerning surface intermediates has been given in reviews /39/. REFERENCES N.N. Semenov, Some Problems of Chemical Kinetics and Reactivity (in Russ.), lzdat. AN SSSR, Moscow, 1958. 2 B. Lewis and G. von Elbe, Combustion, Flames and Explosions of Gases, Acad. Press, New York and London, 1961. 3 V. Ya. Stern, Mechanism of Oxidation of Hydrocarbons in the Gas Phase (in Russ.), lzdat. AN SSSR, ~oscow, 1960. 4 M. Ladacki, T.J. Houser and R.W. Roberts, J. Catal., 4(1965)239. 5 M.V. Polyakov, Uspekhi Khimii, 17(1948)351. 6 ~.V. Polyakov, Kataliz i Katalizatory, 1(1965)35; Ya. B. Gorokhovatskii, T.P. Kornienko and V.V. Shalya, Heterogeneous-Homogeneous Reactions (in Russ.), Tekhnika, Kiev, 1972. 7 n.l. ll'chenko, G.I. Golodets and Yu.l. Pyatnitskii, Doklady AN SSSR, 203(1972)112.
124
8
N.I. Il'chenko, G.I. Golodets and Yu.I. Pyatnitskii, in Proc. All-Union Conference on Kinetics of Catal. Reactions. Processes of Deep Oxidation (in Russ.), Izdat. SO AN SSSR, ~ovosi birsk, 1973, p. 53. 9 M.A. Bogoyavlenskaya and. A.A. Kovalskii, Zhur. Pbys. Khim., 20( 1346)1326. 10 N.A. Eo 1 dyreva, N.A. Stukanovskaya and V.A. Roiter, in IX Ukrainian Conference on Phys. Chem. (in Russ.), llaukova Dumka, Kiev, 1370, p. 23. 11 Yu.I. Pyatnitskii, V.M. Vorotyntaev and G.I. Golodets, Kataliz i Katalizator,y, 11(1374)27. Semenov, in l~ble~s 12 V.V. Voevodskii, P.P. Volkenstein and ~1.N. of Chemical Kinetics, 0atalysis and Reactivity (in Russ.), Izdate ~l SSSR, Moscow, 1955, p. 42.3. 1.3 S.Z. Roginskii, in Chendcal Kinetics and Chain rleactiol1S (in Russ.), Nauka, ~oscow, 1966, p. 46.3. 14 M.I. Temkin and L.O. A»elbaum, Problel£is of Phys. Cham. (in Russ.), Vol. 1, Gozkhimizdat, Moscow, 1958, p. 94. 15 V.A. Roi;er, Introduction into the Theory of Kinetics and Catalysis (in Russ.), Izdat All Ukr. S8ft, Kiev, 1962. 1(1966)65. 16 M.I. Temkin, Izvestiya Otdel Khim. Uauk Holgar. ~l, 17 L.V. Pisarzhevskii, Selected Works on Catalysis (in Russ.), Izdate AN Ukr. SSR, Kiev, 1955. 16 G.K. Boreskov, Kinetika i Kataliz, 11(1970).374. 19 1.1. Tretyakov and Yu. A. Balovnev, Problemy Kinet1ki i Kataliza, 12(1968)164. 20 V.D. Sokolovsk1i, Thesis, Inst. Catal. SO All SSSR, Novosibirsk, 1975. 21 N.I. Il'chenko and G.I. Golodets, Kataliz i Katalizatory, 11(1974).3; N.I. Il'chenko and G.I. Golodets, J. Catal., .39(1975) 73. 22 J.K. Dixon and J.E. Longfield, in P.H. Emmett (Ed.), Catalysis, Vol. VII, Reinhold Publ. Corp., New York, 1960. 23 L. Ya. Margolis, Heterogeneous Catalytic Oxidation of Hydrocarbons (in Russ.), Kh1m1ya, Leningrad, 1967. 24 Yu. I. Pyatnitskii, V.K. Vorotyntsev and G.I. Golodets, Reac. Kin. Catal. Lett., 1(1974)231. 25 V.B. Kazanskii, Kinetika i Kataliz, 11(1970)455. 26 D.A. Dowden and D. Wells, in Actes du 2me Congres Intern. de Catalyse, Vol. 1, Technip, Paris, 1961, p. 1489.
125
27 S.V. Gerei, K.M. Kholyavenko and M. Ya. Rubanik, Problemy Kinetiki i Kataliza, 12(1968)118. 28 E. Bayer and p. Schretzmann; in Structure and Bonding, Vol.II, Springer-Verlag, Berlin - Heidelberg - New York, 1967. 29 A. Cimino, V. Indovina, F. Pepe and M. Schiavello, in Proc. IV Intern. Congress on Catalysis (in Russ.), Vol. I, Nauka, Moscow, 1370, p. 168. 30 V.E. Ostrovskii and M.I. Temkin, Kinetika i Kataliz, 7(1966) 529; L.I. 8hakhovskaya, L.A. Rudnitskii, N.V. Kulkova and M.I. Temkin, Kinetika i Kataliz, 11(1970)467. 31 K. Tarroua, S. Teranishi, S. Yoshida and N. famura, in Proc. 3rd Intern. Congress on Catalysis, North-Holl. Pu aL, Co, , Amsterdam, 1365, p. 282. 32 1:'. Trifiro, P. Centola, I. Pasquon and p. Jiru, in Proc. IV Intern. Congress on Catalysis (in Russ.), Vol.I, Nauka , ;,loscow, 1J10, p. 218. 33 G.C. Bond, Catalysis by Metals, London and New York, 1962. 34 N.I. Il'chenko, Uspekhi Khimii, 41(1972)84. 35 V.B. Kazanskii, Kinetika i Kataliz, 14(1973)95. 36 H.H. Voge and C.R. Adams, Adv, Cat a.l , , 17(1967)151. 37 W.M.H. Sachtler, G.J.H. Dorgelo, J. Fahrenfort and R.J.H. Voorho eve , in Proc. IV Intern. Congress on Catalysis (in Russ.), Vol. I, Nauka, Moscow, 1970, p. 355. 38 N.I. Il'chenko and G.I. Golodets, J. Catal., 39(1975)57. 39 Surface Compounds in Heterogeneous Catalysis (in Russ.), Problemy Kinetiki i Kataliza, Vol. XVI, Nauka, Moscow, 1975.
Chapter IV GENERAL TRENDS IN THE MECHANISMS OF HETEROGENEOUS CATALYTIC REACTIONS INVOLVING MOLECULAR OXYGEN The Significance of Chain Mechanisms The numerous investigations of N.N. Semenov and other scientists have shown that gas-phase reactions involving O2 in the absence of solid catalysts proceed by way of radical chain mechanisms /1-3/. These are typical branched chain reactions or reactions with degenerated branching. The same reactions in the presence of active solid catalysts usually occur at considerably lower temperatures. For example, the noncatalytic oxidation of hydrogen starts around 400 0C while, in the presence of platinum, the reaction can be observed even at the temperature of liquid nitrogen /4/. Another significant difference concerns the product distribution (selectivity) which is different for noncatalytic and catalytic processes if the reaction can follow several paths. A possible explanation of the above facts is that, in the presence of solid catalysts, the reaction mechanism remains the same (i.e. a radical-chain mechanism) but the catalyst facilitates the most difficult step of chain initiation. The latter is caused by the quasi-radical nature of surfaces of metals or semiconductors /1/. This conception corresponds to the idea of heterogeneous-homogeneous catalysis /5/ which was used in discussing the mechanisms of oxidation reactions /6/. For the experimental detection of the homogeneous stages of a reaction, various methods are employed: for example, inves.igation of the dependences of the reaction rate and product distribution on the unoccupied volume of the reaction zone and studies of tne Qependence of the rate on the surface area of the catalyst. If the heterogeneous-homogeneous mecnanj.sm is valid, the ~eac tion kinetics should obey the rate laws 0:.' chain processes but not those of heterogeneous catalytic processes. Th~ latter criterion was used in references /7/ and /8/. Often the method of differentiate calorimetry /9! is use~ to detect homogeneous steps. Radical species could. a.lso ne ae tee ted. directly by the ESR method. These methods have snown that in some
105
cases the oxiJation reactions really proceed by way of heterogeneous-homogeneous mechanism /6/. It should be noted that: I) Homogen~ous steps in these processes are observed at lower temperatures than in case of noncatalytic reactions, so the above steps are evidently associated with the presence of the catalyst. 2) The detection of homogeneous steps does indicates the occurrence of a radical-chain mechanism since other reaction routes in the gas phase are hardly possible. J) The modern theory of noncatalytic chain reactions /1/ suggests that the initiation, as well as the termination, of chains to take place on solid surfaces. It is assumed that the active centers of chains can appear as a result of either homogeneous reactions or the interaction of the initial reagents with the reactor walls. Hence, it is difficult to discriminate between the usual chain reactions and heterogeneous-homogeneous catalytic processes. In principle, one can imagine a situation where the mechanism of both processes is the same. Observed differences in rates and selectivities are caused by quantitative differences in the rate constants for the elementary steps for surface reactions. Another possibility is that, in the presence of an active catalyst, the surface initiation and termination steps of chains are different from those in the absence of the catalyst. The data have been obtained so far do not allow this possibility to be discussed in more detail. One can thus conclude that the heterogeneous-homogeneous mechanism has been shown for some oxidation reactions. At the same time, this mechanism is not a typical route for the reactions under usual conditions of heterogeneous gas-phase catalysis. The validity of the Boreskov rule on the approximate constancy of specific catalytic actiVity of substances with the same chemical composition but with different macrostructure (see Chapter I) is evidence in favour of heterogeneous surface mechanisms. In most cases, under the above mentioned conditions, the critical phenomena which are peculiar to chain processes are not observed. The high selectivity of typical solid catalysts for mild oxidation (which is different from that in noncatalytic chain oxidation) also suggests the heterogeneous character of the processes. The absence of homogeneous steps has often been demonstrated experimentally. However, one should point out that the role of homogeneous steps in heterogeneous catalysis has not been sufficiently studied. In some cases, the methods used are not reliable. Even when a hete-
106
rogeneous-homogeneous mechanism has been provcd, its contribution to the total reaction rate is not clear. Investigations in this field have not been systematic and the selection of topics studied is often accidental. The kinetics of hetrogeneous-homogeneous reactions (both in experimental and theoretical aspects) have been poorly studied. Investigations in this direction have recently commenced /8/. The relationship between heterogeneous and heterogeneous-homogeneous catalysis is of special interest. The development of homogeneous chain steps is favoured by elevated temperatures. It is important to know how the chemical structure of the catalysts and reagents affects the probability of a heterogeneous-homogeneous mechanism. ~ccording to reference /10~ the above probability in CO oxidation is lower for active heterogeneous catalysts which "retain" the process on the surface. However, oxidation reactions on active Pt are often inclined to a heterogeneous-homogeneous mechanism. It has been shown /8/ that under favourable conditions the oxidation of H2 over V2 0 includes homogeneous steps. This 5 is in contrast to the oxidation of CO for which a special requirement, the presence of H-donors, should be satisfied. Thus, comprehensive studies on heterogeneous-homogeneous catalysis are evidently needed. Finally, one should note the foll,'·,'ing. I t is believed that in the presence of a catalyst, the reaction rate is much higher than that without a catalyst. This results in the essentially lower temperatures required for catalytic reactions. Only at these temperatures is the catalytic effect clearly pronounced. At elevated temperatures, in the explosion region where a noncatalytic c~ain reaction proceeds very rapidly, its rate can exceed that of the catalytic reaction. For instance, the rate of the oxidation of a-xylene in an empty reactor at JOOoC is much higher than that of the heterogeneous catalytic oxidation reaction over V20 ' even when S the whole reactor is filled with V~O~ granules /11/; the positive " :J effect of the catalyst is displayed here rather in its selectivity. Nevertheless, although heterogeneous-homogeneous radical-chain mechanisms are not typical for reactions involving 02 under usual conditions, the possibility of chain reaction routes involving surface radicals must remain. Such a concept was put forward by N.N. Semenov, V.V. Voevodskii /13/ and other authors. According' to them, the initial chain-generating sites are free valencies on
107
the surfaces (I"ree electrons, no ... e~, t~c.). These mechanisms shoul~ be conaluered Q~ r.;riJ'" ~l1i:. ~ .tea l -:or which there is no direct tlxfJeriDl~aLal cvi(J.~nct.: .... ~ L; au t'~wu~·:.l"; that the participat.ion of Qdso':"oe~~ raJ.ical SPt:.';;'~f;lf in ~i.i~ r-eac ~.i\);. does not necessarily mean 'that. a >:ha10 mechanlslll l.ni;li:lJ ...l:lC"'.:!~"~' 11.1. Temkin and L.O. Apelbaum /i4/ 1~tel'l11::"uec. t;h~ "~nuil'1 Ifr....:~h" in the h,ydrogenation of e th,ylene \lvol'.i Ii lud~:r I .i. r.:. a chain mechanism does not occur. i~r!verth.t'less, .!i•.:.~:.rbcd ii-ali.JA:.S and C 8 radicals participate in tIlt\! roaction. 2 5 Although surface cDain reacLiona have ~L ~e~n 1r~~,nstra~~~ eAperiJDentally, their existence has not been retl.lteli. ;;le ..:unc~y:' has not been wide-spread becal.lse of tk~ fact Ll~t ex?er~~ll~al ~a ta can usuall,y be explained by noncnain mechaniSll1s. ~hc ~~-cal The same concerns a special type of chain mech~l1G~, led "recuperation catalysis". Acc:Jrding ~o til':'S cc ac c... -:', i.M ~tl:.r gr evolved in an exothermal catalytic reaction is .»t ~i8dipatai but is accumulated in tne surface layer 0: II. ..;\)li~, :'()l' e•..ad.i·le, in the form of labile noneql.l11ibriwa species. 1h.~ Bccu:nl.lla~e;~ .;,'1.';:':"gr is used for the activation of new i>0rtions 'J: cue roagel.liS and it is then accumulated again and so on. SC!wllles of I.h.is type B:'-C likely to be valid for fast highly '9xotnerz.ic reac~1"lI.S like tbe catalytic recombinatioh of oxygen ato~s (0 ~ 0 = ~~;4do=-li; kcal mol~1). ~
tv"'''''
Step-wise and Deformation Kechanisms At equilibrium energy distribution in ~ne rCBe~ing a1'3~ea two general forms o£ interme~atc ci~~ical interac~ion ar~ p~aalble, which arel step-wise and deformation mechanism (see Chapter I). Let us assume that in the presence ~r a catBlyot, ~at, ;hG ~xi dation of R into products, P, ta~e8 place:
If the initial Z"cae;ents fo~.a a ClJ.:JUOO1l activ:o.:c::: c:olil;J.e.x:, o Cat)· willctl is ,iireclily 'l;:811s.1.\:.rill.e.i. int:>:, Ii ";'.:l;.';}r::..;.l"m 2 mechanism occurs I Cat , -. (R o 0
".L ..... ,;
108
The latter is also called a "one-step mechanism" /15/, or "direct catalysis" /16/. Reaction (IV.1) is a redox process. In the formation of the activated complex, electrons are transferred from R to 02 through the electroconductive crystal lattice of the catalyst. 1.V.Pisarzhevskii /17/ and V.A. Roiter /15/ pointed out the possibility of a positive mutual influence of the reagents on the mechanism (IV.2): the donor process of electron transfer from R is stimulated by the simultaneous acceptor process of electron addition to 02 which brings about the advantages of the above onestep mechanism. The alternative step-wise mechanism includes two (or more) steps and one or more surface intermediates, each step having its own activated complex. The simplest example is that given by the mechanism of an alternating surface oxidation-reduction process: 1) 02 + 2Cat -2Cat·0
2) Cat·O
+
R--P
+
Cat
(IV.))
Theoretically, both schemes, deformation and step-wise, are possible. The first may have lower activation energy (due to a positive mutual influence of the reagents) but a highly negative entropy of activation, while the second is characterized by a more favourable entropy of activation but higher activation energies. Hence, one should expect that the first mechanism will change into the second at elevated temperatures /18/. However, we do not today know of any experimentally proven example of "direct catalysis" /16/. At the same time many heterogeneous catalytic oxidation reactions have been distinctly shown to proceed by way of step-wise mechanisms of the type shown in Scheme (IV.)) (see later Chapters). The same is valid for decomposition processes like the decomposition of N20:
In the first step, N20 is an oxidant while, in the second, it is a reductant. The following remarks are noteworthy in connection with the
109
problem discussed. I. The most convincing evidence in favour of mechanism of the type shown in (IV.J) is the coincidence of the kinetic characteristics (rates, activation energies, reaction orders, etc.) for the all-over catalytic process and for the separate steps studied independently. In some cases, this is not observed and this is sometimes used to deduce the validity of a deformation mechanism. However, such facts only allow us to conclude a mechanism such as Scheme (IV.J) does not apply. Several possibilities should then be examined. 1) The reaction does obey a deformation mechanism (IV.2). This is probable in the case of reactions with simple stoichiometry, such as: Cat
..
but is impossible for such processes as
since the simultaneous collision of four molecules and the catalyst is highly improbable. 2) Catalysis can proceed by way of Scheme (IV.J) but the form of adsorbed oxygen participating in the catalytic oxidation step may be different from that in the reduction step. For example, in the first case anion-radicals such as 0- or 02 can react, while, in the absence of 02 in the gas phase during the reduction step, the less reactive 0 2- species takes part. 3) In principle, the catalytic process can proceed by way of a chain mechanism which is not valid for the separate steps of reduction and reoxidation /18/. 4) A step-wise mechanism which is different from Scheme (IV.J) may take place. II. In some cases, the step-wise schemes have been shown experimentally occur which differ from the simple redox mechanism (IV.J). For example, the oxidation of hydrogen on pure platinum films occurs by a Langmuir-Hinshelwood mechanism /19/ :
no 2)
°2 + 3) (H2) +
4) H2
+
)-(°2) (02)----(H 202)
(H202 ) 2H2 + °2
2H2O + 2H2O
)
(
+ (
)
(IV.4)
According to reference /20/, the oxidation of CO and the complete oxidation of aliphatic hydrocarbons over metal oxides at low temperatures involves the formation of carboxylate-carbonate structures, R', from Rand (0) and the sUbsequent decomposition of R' by
°
2:
1)R
+
2) (R')
(O)-(R')
+
02 --- P
+
(IV.5)
Both schemes are step-wise but they differ from the mechanism of Scheme (IV. 3 ) • Schemes of the type of (IV.4)and(IV.5) are sometimes called "associative mechanisms" since they involve steps in which both the initial reagents take part. These "associative mechanisms" are not identical to deformation catalysis since they include intermediate compounds. Besides, the "associative mechanism" cannot be considered as a deformation one because such terms should be general, covering various types of catalytic reactions. In particular, one can imagine a deformation mechanism for tae isomerization of A into B: Cat A
"B
(IV.6)
for which it would be unnatural to use the term "associative". It is believed that "associative mechanisms" are alternatives to those with separate interaction of each of the reagents with the catalyst. The last scheme is presented by mechanism (IV.3). Sometimes, such a classification is possible and advisable. However, it is not clear, from this point of view, what is the mechanism of Scheme (IV.4): on one hand, each reagent is here activated separately but, on the other hand, the associative interaction of
111
ue reagents takes place in the rate-determining step. III. The above considerations are valid for any experimental method of studying the mechanisms. In some cases, the tracer method is used in oxidative catalysis over metal oxides to elucidate whether or not surface lattice oxygen participates in the reaction. For instance, the oxide is labelled by 0 18 and one determines whether or not the tracer occurs in the reaction products. A positive result suggests that surface (0) species participate in the catalysis, but it is not necessarily a redox mechanism (IV.3) since, in Scheme (IV.5), oxygen of the oxide can also take part in the reaction and 0 18 will be transferred into the reaction products. A negative result (i.e. 0 18 is not detected in the reaction products) means that surface oxygen of the oxide is not a reaction intermediate, and this allows one to reject mechanisms involving this form of oxygen. However, a true reaction mechanism cannot be deduced from the last result since many versions are consistent with the absence of 0 18 in the products. It may be a deformation mechanism or a step-wise Scheme (IV.3) involving anion-radicals (0-) or (02-)' which appear rapidly dllring catalysis and which do not have time to exchange with the surface oxygen of the oxide catalyst. Thus, among step-wise mechanisms, the scheme of alternating reduction-reoxidation of the surface is the simplest one and has been proved to occur for many reactions. However, other step-wise mechanisms are also probable. A deformation mechanism is also theoretically possible, especially at low temperatures, but this mechanism has not been experimentally proven. t
Parallel, Consecutive and Parallel-Consecutive Schemes of Complex Reactions Many catalytic oxidation reactions can follow several routes. In these cases, two main situations may be distinguished. In the first extreme, each product is formed only from the initial reagent but not from products (parallel scheme),for example: 1m]
+
~N2 + H20 02---1i20 + H20
~NO
+ H2
°
112
and: CH - CHO + H20 = CH - COOH + H 20
+
+ H20. In the second extreme (consecutive scheme), the initial reagents give initially a reaction product which is less stable thermodynamically than others; it is then converted to more stable products, etc., for example: °2 CH-COOH - C 0 2 + H20. For organic substrates, an increase in the degree of oxidation of the products in such sequences coincides with an increase of the thermodynamic stability of the products. In the oxidation of ammonia, the reverse picture is observed /21/ since the products of deep oxidation (nitrogen oxides) are less stable than the mild oxidation product (N2) and, in this case, different types of consecutive schemes are valid, should such a scheme occur: NH) NH) NH) + ° 2 -NO - N20 - N2, NH) NH) + °2- NO - N2 O - N2 + °2'
NH) + ° 2 - NO - - N2 + 02 In intermediate situation mixed parallel-consecutive schemes are observed, for instance:
----------"
CH2 = CH 2 = CO 2,
C~
- CHO
ck - ~:~Hl
io...==J 2
Schemes of such type are widely discussed in the selective oxidation of organic substances /22, 2)/. The realization of the above schemes over a given catalyst depends upon the reaction conditions. Higher concentration of mild oxidation product (in the oxidation of organic substances) favours
113
its further transformation. Hence, a parallel scheme is more probable at low conversions (low contact times, ~; low temperatures, T). At higher conversions (higher values of t, T), consecutive routes are enhanced. The contribution, f, of a parallel route to a parallel-consecutive scheme can be defined as the ratio of selectivity, Si' at fixed r = t: I to that at r --- 0, (8; )r= 1'1
i.e.
(~ ),&=/7 It is assumed that the concentrations of reagents are constant or their changes are taken into account. Determining the above schemes is useful in elucidating the conditions under which the desirable product is obtained in maximum amounts. However, it should be noted that: 1) such schemes are not detailed mechanisms since they do not involve elementary steps with participation of the catalyst; 2) a parallel scheme of the overall process does not exclude consecutive steps on the catalyst surface /24/. Thus, the reagent R can initially be converted into the surface intermediate (1 1 ) which is then either transformed into a mildly oxidized product P1 or is further converted into a more fully oxidized surface intermediate (1 2 ) which can undergo further similar reactions: P1
t
(0) R -(1 )
1
The above scheme is a parallel one since each product (P P P 1, 2, 3) is formed from R but not from another product. At the same time, the scheme contains series of consecutive surface reactions.
Elementary Steis of Heterogeneous Catalytic Reactions Invo vi~ Molecular O~gen If scheme of the type (IV.3) has been proved, it does not mean that a detailed reaction mechanism has been found. Complex steps of reduction and reoxidation involve elementary steps which should
114
be elucidated. The same concerns other schemes like (IV.)). We can now distinguish several kinds of typical elementary steps which are peculiar to heterogeneous catalytic reaction involving
°2· Reversible Adsorption of Reagents (Without Dissociation) Examples of the adsorption process A + (
)~(A)
are given by the adsorption of H2, 02' CO and NH). In this process, the molecule A is preserved as a whole, while electron density can be shifted from A to the catalyst or in the reverse direction. The adsorption of oxidants (02' N20, etc.) is accompanied by a shift of electron density from the catalyst to A which results in negative charging of (A). In extreme cases, molecular anions are formed: )
+
e _(0-) 2
) + e -(N 0- ) 2
The latter is associated with a decrease in electrical conductivity (i.e. an increase in work function) of the catalyst. To investigate the adsorbed anion-radical species, 02' the electron spin resonance (ESR) method is used /25/. Analysis of the ESR spectra on metal oxides shows that molecular oxygen and metal cation often form a Jor-bond (involving d-orbitals of the cation) and the 0-0 bond is directed in parallel to the surface. The electron transfer from the catal)st to 02 can be considered as the oxidation of Me m+ into Me(m+1 +, so that
The adsorption of the reductants can be accompanied by a shift of the electron from A to the catalyst to form a positively char-
115
ged adsorbed species. pear: ) ~(H2
+) + e
)~(CO+)
+ e
)~(C2H4+)
In the extreme cases,adsorbed A+ ions ap-
+ e.
These processes are associated with an increase in electrical conductivity (a decrease in work function) of the catalyst. Sometimes, one succeeds in elucidating the structure of adsorbed species of the last type. Thus, according to reference /26/, the adsorption of H2 on metal oxides leads to orientation of the H-H bond parallel to the surface. The H2 molecule occupies the top of the truncated octahedron formed by the 0 2- anions surrounding the central metal cation. A similar structure can be assumed for other (A) species. When the A molecule has an unshared pair of electrons, one should expect to find the coordination bond between A and the metal cation (surface ammonia complexes and similar species are formed). Such complexing facilitates further electron transfer from A to the catalyst. The observed increase in the distance during the formation of suggests the loosening of the oxygen-oxygen bonds in 02. A similar effect can be expected in other molecularly adsorbed species (A). Thus, processes such as (IV.7) make the molecule A ready to take part in a subsequent deeper interaction on the catalyst surface. In catalytic oxidation, t he reaction mixture contains bo th oxidant (02) and reductant (R) simultaneously. Their adsorption may cause a positive mutual influence since (02) and (It) are oppositely charged. For example, according to reference /27/, ethylene itself is not adsorbed on pure silver but in the presence of adsorbed oxygen, the adsorption of C2H takes place. This can be due 4 to the fact that electron transfer from Ag to 02 generates vacancies in the d-band of the silver what is favourable for the electron-donating adsorption of ethylene. Adsorption according to Eq. (IV.7) is also possible for the reaction products (H20, CO 2, etc.). Since the latter have oxidative properties (though slight), negative polarization or even ionization is probable during adsorption:
02
°- °
116
+
e -(H 0- )
(IV.10)
+
e -(C0
(IV.11)
2
2-)
The desorption of such species, as in the reactions (IV.8)or (IV.9) results in the liberation of electrons. Some molecules can be both reagents and products. For instance, in the decomposition of N20, nitrous oxide is an initial reagent while, in the oxidation of ammonia,it is the reaction product. In the both cases, the adsorption of N20 is accompanied by electron transfer to N2 0 while removal of the N2 0 from the surface leads to the liberation of electrons (transferred to the catalyst). The H20 molecule (like NH ) can enter coordinative interaction 3 with metal cations, resulting the formation of surface hydrates. Dissociative Adsorption Interaction of the catalyst with adsorbed molecules may cause scission of the chemical bonds in the molecule A. In such cases, the adsorption of oxidants leads to the formation of O-atoms on the surface:
°2
+
2(
°3
+
(
)-2(0) )-(0)
(IV.12) +
°2
+
N2
(IV.13)
or 03 + 3(
) - - 3(0)
N20 + (
)--(0)
Oxygen atoms are negatively charged; in extreme cases, this results in (O-)or (0 2-) species. The (0-) anions are at the same time radicals which can be studied by the ESR-technique /25/. Thus, the activation of 02 can be represented by the sequence
I
II
III
IV
V
117
Here, only the species I is neutral. The species III corresponds to a peroxide anion. Experimental evidence for the formation of (O~-) in 02 adsorption is now absent, but complex compounds of transition metals including O~- ligands are known /28/. The above species can be transformed into one another. Elevated temperatures are favourable for dissociative adsorption and for accepting more electrons, so that heat allows the (02) species to be converted into (0-) and then into (0 2 - ) /25/. Ionization and dissociation are connected: addition of an electron to 02 loosens the 0-0 bond /28/. Higher concentrations of molecular forms are favoured by high surface coverages with oxygen which corresponds to low values of qs' On going from form V to form I, the energy of the oxygen-catalyst bond decreases (see Chapter III).
Fig. 49 - The elementary cell of the V2 0 crystal /31/.
5
o
2.8~
o~
!
The transition from 02 to 2(0 2 - ) requires four-electron transfer. Usually one metal cation cannot satisfy this requirement (i.e. C02+~Co3+ + e; Mn2+~Mn4+ + 2e); hence,the participation of two or more cations is necessary. This is true in the adsorption on rather large crystalline particles. If the cations are separated (as, for example, in dilute solutions of MeXO y in a solid matrix or in dilute supported layers), the four-electron transfer is difficult and one- or two-electron processes leading to (02) or (0-) are more probable. The latter results in weakly bound oxygen species /29/. A decrease in the bond energy of the surface oxygell at high B may also be caused by changes in the degree of oxidation of the metal, ,cation. Such a situation is probable in the adsorption of 02
118
on silver when at higher values of 8 , surface oxides of Ag+ are converted into the less stable surface oxides of Ag3+ /30/. The 0 2- anions formed during the adsorption of O2 on metal oxides are hardly distinguishable from surface anions of oxygen in the oxide. In the oxides of polyvalent metals (which have crystal structures of low symmetry), oxygen atoms are not equivalent. The example of V20 is given in Fig. 49. The longest V - 0 bond 5 o 0 (2,83 A) is the weakest one, while the shortest V - 0 bond (1.54 A) is close to a vanadyl V - 0 bond. The latter is a double bond and is covalent to great extent. Some authors believe that the V - 0 bond is highly reactive in oxidation processes /31/. Similar properties are displayed by the Mo bond in molybdenum oxide catalysts /32/. Reactions (IV.13)and(IV.14) are practically irreversible while reaction (IV.12) becomes reversible at the rather high temperatures which usually correspond to those of isotopic heteroexchange of oxygen. Dissociative adsorption is also possible for the molecules which are oxidized, for example:
°
H2 + 2(
) -2(H),
NH + 2(
) -(NH
3
+ 2( C 2H6
) -(C
2) 2H5
+ (H) ,
) + (H).
Such reactions are peculiar to transition metals /33/. They can be preceded by the adsorption in molecular form (as in the case of 2 ) :
°
2(H) The adsorption of hydrogen has been studied most extensively on Pt, Pd and other transition metals. Several types of hydrogenmetal bonds have been found. A covalent bond, polarized to a definite extent, is likely to be a typical case. Surface hydrides, with a definite contribution for ionic bonds, are also possible, hydrogen being charged negatively or positively. Small hydrogen atoms can be localized over the surface metal atoms or amqng them. Hydrogen adsorbed on such metals as Pt or Pd possesses high surface mobility /34/.
lHJ
Interaction of Various Molecules with Adsorbed Oxygen Even when we restrict ourselves by the reactions with one form of adsorbed oxygen, (0), numerous types of catalytic transformations are possible. It would be advisable to classify the steps considered according to the types of chemical bonds participating in the reactions. The interaction of H2 with (0) involves rupture of the H - H and oxygen-catalyst bonds, leading to the formation of either H or OH-groups: 20
°
(IV.16)
H2 + (0) H2 + H + (0) - ( H 20) 2
H0 +
(IV.17) (IV.18)
2(0)~2(OH)
'-
In the above processes, hydrogen is the reductant (electron-donor). The electrons are added to the metal cation or to the cation and H (or OH): 20
H + (Me m+ ) + (02-)-H + (Me(m-2)+) + ( 20 2
),
H + (Me m+ ) + (0 2-) - ( H + (Me (m-1 )+), 20-) 2 H + (Me m+ ) + 2(02-)~2(OH-) 2
+ (Me(m-2)+).
One can assume two possibilities. The first one involves the reduction of Me m+ by H2 to form H; or H+ with further fast ionic reaction between the positive hydrogen ions and 0 2-. Another possibility /35/ involves an electron transfer from 0 2- to Me m+ to form anion-radicals, 0-, which then rapidly react with H2• In the last case, the reductant is 0 2- but not hydrogen. Such a mechanism may be accepted for systems inclUding easily reduced cations Me m+. However, in most cases, the first mechanism is likely to be valid. The above considerations are also true for other reagents (CO,
120
S02' etc.). Reactions (IV.16)-(IV.18) may be preceded by preliminary activation of hydrogen (reaction (IV.15)) which facilitates sUbsequent interaction of H2 with (0). Other molecules (NH CO, S02' etc.) 3, may behave similarly:
NH
(0) -(NH) + H2O, 3+ NH + (0) + ( ) -(NH) + (H2O), 3 NH +2(0)+( ) (NH) + 2(OH). 3 CO + (0) - C 0 2 + ( ) , CO + (0) --(CO 2), + CO + 2(0)-(C0 3) S02 + ( 0 ) - S03 + ( S02 + (0) -
)
),
(S03)'
S02 + 2 ( 0 ) - (304) + (
)
.
The adsorbed species formed «OH), (C02), (C0 (S03)' (S04)) are 3), charged negatively, for example: CO + (Me m+) + 2(02-)~(CO~-) S02 + (Me m+) + 2(0 2-) _
+ (Me(m-2)+) + (
(30~-)
+ (Me(m-2)+) + (
), ).
Such substances as N20, 03 and H202 may display dual reactivity: in the interaction with the reduced surface, they behave as oxidants (like 02) while in the interaction with oxidized surface, they react as reductants (like H2), for instance: N20 + (0)-N2 + 02 + H202 + (0)-H20 + 02 +
), ), etc.
The reason is that nitrogen in the N:N = group in N2 0 and oxygen in the group in H202 can be both reductant and oxidant. The interaction of paraffins with (0) species may involve the
°- °-
121
scission of C - C or C - H bonds. For olefins, a rupture of the C - H- allylic bond is typical and this leads to allylic complexes /36/, for example: (0)
CH 2 = CH - CH + ( 3
CH - CH
)~(CH2=
.":". CH2 ) + (OH) 3)--(CH2::,.CH
(IV.22) The latter is a J.T-complex attached to a metal cation and oriented in parallel to the surface. The formation of 6-allylic complex (which is perpendicular to the surface) is also possible. If the oxygen attack is directed at the C - C bond, a rupture of the $ - C - C bond is probable with the formation of C - 0 bonds. Activation of cycloolefins and acetylenic hydrocarbons can be imagined in a similar way. Alkylaromatic hydrocarbons, in which the C - H bonds in side groups are activated by conjugation with an aromatic ring, react like olefins in which the C - H-allylic bonds are activated by conjugation with C - C-bond: from toluene, the benzyl complex, (C6H ) , is formed (similar to an allylic one), etc. In the inSCH2 teraction of benzene, toluene, naphthalene and so forth with surface oxygen, scission of tne bonds between t~drogen and the aroInatic ring is also possible, and tais r~sults in surface complexes of the se.ni-quinone type. Heterocycles are likely to be activated like aromatic hydrocarbonds. Reactions between alcohols and (0) species may involve rupture of the 0 - H bond and this leads to surface alcoholates: ( 0)
Thus, for the above mentioned processes involving organic substrates, the formation of ~C - 0 - Me bonds is peculiar. Reactions of aldehydes with adsorbed oxygen lead to more oxidized surface species which are acidic in nature. Thus, in the case of benzaldehyde, the formation of benzoate-ions has been proved
/37/:
o C H C/ 6 S ~ H
+
) + (OH).
122
In the reactions considered, organic molecules behave as reductants (electron-donors) in the same way as H2, CO and NH J ; i.e. the C group in (C6H is attached to the reduced form 6HSCOO-SCOO) of the metal cation. Interaction of Surface Compounds wi th Adsorbed Oxygen Surface compounds appearinG as a result of tne above reactions can be further oxidized on the catalyst. For example, the (NH) species formed from 1ill and (0) can be converted into nitroxyl par3 ticles /J8/: (NH) + (0) --(HNO).
The variety of such types of steps is especially great for organic substances. In most cases, they result in carbonyl or carboxyl surface compounds.
Adsorption - Desorption of Products Adsorption and desorption of H20 and CO 2 (products of the deep oxidation of organic substrates as well as those of the oxidation of H2, CO, lill)' e t c , ) can be described by Eqs (IV.10), (IV.11). In addition, H20 and CO 2 may react with (0 2-): ) + (02-)~ + (0 2-) ~
2(OH-)
(C0 2-).
J The last steps are reversible. Similar reactions can be assumed for SO] and other products:
The desorption of the products of mild oxidation of lecules can take place either directly, e.g.:
0~6~rdc
mo-
123
(CH 2 = CH - CHO) - - - CH 2 = CH - CHO + ( or by yay of more complex transformations: (C-H_COO) + (OH)-Cb"H"COOH + (0) + ( \:) :J
).
J
':rile examples examined above do not cover t ne whole variety of the elementary steps of complex catalytic ::-eactions involving 02. In particular, they may be expanded by the reactions between surface compounds an~ gaseous molecules, for example:
H + 2(OH)---2H + 2( 20 2 02 + (HCOOH)-C0
2
),
+ H20 +
or between different surface
), e to.
inter~ediates,
for
ex~~ple:
Nav er-t he Leaa , the reactions consiciered can present some typical
steps of catalytic processes. Detailed analysis of the corresponding experimental data concerning surface intermediates has been given in reviews /39/. REFERENCES N.N. Semenov, Some Problems of Chemical Kinetics and Reactivity (in Russ.), lzdat. AN SSSR, Moscow, 1958. 2 B. Lewis and G. von Elbe, Combustion, Flames and Explosions of Gases, Acad. Press, New York and London, 1961. 3 V. Ya. Stern, Mechanism of Oxidation of Hydrocarbons in the Gas Phase (in Russ.), lzdat. AN SSSR, ~oscow, 1960. 4 M. Ladacki, T.J. Houser and R.W. Roberts, J. Catal., 4(1965)239. 5 M.V. Polyakov, Uspekhi Khimii, 17(1948)351. 6 ~.V. Polyakov, Kataliz i Katalizatory, 1(1965)35; Ya. B. Gorokhovatskii, T.P. Kornienko and V.V. Shalya, Heterogeneous-Homogeneous Reactions (in Russ.), Tekhnika, Kiev, 1972. 7 n.l. ll'chenko, G.I. Golodets and Yu.l. Pyatnitskii, Doklady AN SSSR, 203(1972)112.
124
8
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125
27 S.V. Gerei, K.M. Kholyavenko and M. Ya. Rubanik, Problemy Kinetiki i Kataliza, 12(1968)118. 28 E. Bayer and p. Schretzmann; in Structure and Bonding, Vol.II, Springer-Verlag, Berlin - Heidelberg - New York, 1967. 29 A. Cimino, V. Indovina, F. Pepe and M. Schiavello, in Proc. IV Intern. Congress on Catalysis (in Russ.), Vol. I, Nauka, Moscow, 1370, p. 168. 30 V.E. Ostrovskii and M.I. Temkin, Kinetika i Kataliz, 7(1966) 529; L.I. 8hakhovskaya, L.A. Rudnitskii, N.V. Kulkova and M.I. Temkin, Kinetika i Kataliz, 11(1970)467. 31 K. Tarroua, S. Teranishi, S. Yoshida and N. famura, in Proc. 3rd Intern. Congress on Catalysis, North-Holl. Pu aL, Co, , Amsterdam, 1365, p. 282. 32 1:'. Trifiro, P. Centola, I. Pasquon and p. Jiru, in Proc. IV Intern. Congress on Catalysis (in Russ.), Vol.I, Nauka , ;,loscow, 1J10, p. 218. 33 G.C. Bond, Catalysis by Metals, London and New York, 1962. 34 N.I. Il'chenko, Uspekhi Khimii, 41(1972)84. 35 V.B. Kazanskii, Kinetika i Kataliz, 14(1973)95. 36 H.H. Voge and C.R. Adams, Adv, Cat a.l , , 17(1967)151. 37 W.M.H. Sachtler, G.J.H. Dorgelo, J. Fahrenfort and R.J.H. Voorho eve , in Proc. IV Intern. Congress on Catalysis (in Russ.), Vol. I, Nauka, Moscow, 1970, p. 355. 38 N.I. Il'chenko and G.I. Golodets, J. Catal., 39(1975)57. 39 Surface Compounds in Heterogeneous Catalysis (in Russ.), Problemy Kinetiki i Kataliza, Vol. XVI, Nauka, Moscow, 1975.