Heterolytic Dissociative Adsorption

Adsorption of some molecules may involve adjacent pairs of Cr +(cus) and 02-(cus). The equivalent of such adsorption was suggested long ago with respect to the adsorption of hydrogen on oxides and it has been speeifieally suggested for chromia 21, 41). Methane could adsorb similarly. 

In these adsorptions, formally, the H—H and the CH3—H bonds undergo heterolytic fission with the proton going to 02 (cus) and or CHa going to Cr8+(cus). This is a crystal field formalism. Electron sharing would substantially reduce the actual charge on H and CH3-. Previous discussion has recognized the importance of coordinative unsaturation of Cr8+ but the importance of coordinative unsaturation of 0 - with consequent increase in its basicity seems not to have been fully appreciated. 

Strength of bonding here will involve the characteristics both of the acidic and of the basic sites. It may be noted that although the proton-O is a hard-hard interaction, that of H or CH3- with 0 + is a soft-hard interaction. Presumably, the strength of adsorptions of H2 and CH4 in this fashion would be greater on Cr2+. However, adsorption of water by heterolytic fission 

We have presented a mechanism for the hydrogenation of olefins and exchange of alkanes based upon heterolytic dissociative adsorption and its reverse, associative desorption (21). Ignoring the question of whether the acidic sites are Cr or Cr2+, the mechanism is represented 

There is no evidence as to whether hydrogen or olefin is adsorbed first. Further, there might be an intermediate adsorption of olefin as a tt-complex. 

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Homolytic and heterolytic relate in the usual sense to the formal nature of the cleavage of a single bond. If the electron pair in the bond of the adsorptive A B is divided in the course of its dissociative adsorption, the adsorption is homolytic dissociative adsorption. If A or B retains the electron pair, the adsorption is heterolytic dissociative adsorption. Examples follow. 

The terms of 1.2.5 have been discussed with reference to metallic surfaces but they can be applied to other adsorbents and catalysts and, in particular, to the pair-sites involved in heterolytic dissociative adsorption. 

Hydrogen goes as two protons to convert two oxide ions to hydroxide ions and the two electrons reduce two Cr + to Cr -. The behavior of carbon monoxide is equivalent. As written, there is no requirement of surface coordinative unsaturation. However, coordinative unsaturation in the oxide ions which are converted to hydroxide ions would favor reductive adsorption. Further, for reasons outlined in Section IV, Cr will be easier to reduce to Cr2+ when it is also coordinatively unsaturated. Further, where this is so, heterolytic dissociative adsorption in the sense of Eq. (7) might subsequently occur at Cr +fcus). 

It should be noted that the species formed by reductive adsorption of hydrogen could, in principle, isomerize to the form at the right of Eq. (12). However, this form is equivalent to that which results from heterolytic dissociative adsorption as in Eq. (7). A priori, it might be difficult to estimate the relative free energies of the two isomeric sorbed species and their rates of interconversion. 

In Sections III-V, we speculated about the nature of active sites on chromia and the relations of such speculations to chemisorption and heterogeneous catalytic reactions. In particular, we suggested that many types of active sites would involve coordinatively unsaturated surface (cus) ions of Cr3+ and 0 - and that the following types of chemisorption might occur at such sites simple coordinative adsorption at Cr3+(cus), adsorption of generalized acids at 02-(cus), heterolytic dissociative adsorption at pair sites of Cr +(cus) and 02-(cus), and reductive adsorption. In addition, we considered the possibility of ligand displacement adsorption which does not depend upon (cus) ions. 

If isomerization does not result from the addition of one hydrogen atom and the removal of another, it could result from the removal of a hydrogen atom and subsequent addition of hydrogen at another position. We suggest, then, that double-bond migration, process (7), results from heterolytic dissociative adsorption of olefin to a proton and adsorbed allyl. Exchange without isomerization, process (5), will not originate in dissociative adsorption to form -011=011—R since such a process should lead to ethylene-d, which is not observed. Both processes (5) and (7) could result from an allylic intermediate of one of two types. 

We suggest that the exchange proceeds by heterolytic dissociative adsorption like other isotopic exchange reactions. Some such intermediate as the following is then involved. 

The mechanistic conclusions may be summarized as follows, reference being made to the classification of processes at the beginning of Section X,A,1. The basic reactions are five heterolytic dissociative adsorptions and one reaction analogous to a ligand insertion reaction. The first five reactions below represent reaction of Cr3+(cus) + 02 (cus) with deuterium or a hydrocarbon. 

Quite similar remarks pertain to the passage (Figure 1) from chemisorbed to absorbed (dissolved) states in the bulk of some solid phases accessible to certain atoms of small radius (H, C, N, O), as exemplified by the solution of hydrogen in palladium or tungsten trioxide. Spillover therefore depends upon the crossing point of the physi- and chemi-sorption potential curves for the two solids, upon the number and kind of the chemisorbed states, and upon the possible interstate transitions. The approach of AB to homolytic or heterolytic, dissociative adsorption along the reaction co-ordinate may be represented as follows—... 

Oxides commonly studied as catalytic materials belong to the structural classes of corundum, rocksalt, wurtzite, spinel, perovskite, rutile, and layer structure. These structures are commonly reported for oxides prepared by normal methods under mild conditions [1,5]. Many transition metal ions possess multiple stable oxidation states. The easy oxidation and reduction (redox property), and the existence of cations of different oxidation states in the intermediate oxides have been thought to be important factors for these oxides to possess desirable properties in selective oxidation and related reactions. In general terms, metal oxides are made up of metallic cations and oxygen anions. The ionicity of the lattice, which is often less than that predicted by formal oxidation states, results in the presence of charged adsorbate species and the common heterolytic dissociative adsorption of molecules (i.e., a molecule AB is adsorbed as A+ and B ). Surface exposed cations and anions form acidic and basic sites as well as acid-base pair sites [1]. The fact that the cations often have a number of commonly obtainable oxidation states has resulted in the ability of the oxides to undergo oxidation and reduction, and the possibility of the presence of rather high densities of cationic and anionic vacancies. Some of these aspects are discussed in this chapter. In particular, the participation of redox sites in oxidation and ammoxidation reactions and the role of redox sites in various oxides that are currently pursued in the literature are presented with relevant references. 

This classification gives a reasonable account of the data except for BaX zeolite found in group B (monovalent adsorption sites). Barium zeolite does not lead to the heterolytic dissociation of water and is not likely to produce monovalent adsorption sites, in contrast to the other alkaline-earth zeolites. 

The adsorption of H2 at RT on high-surface-area MgO sintered at 1073 K leads to heterolytic dissociation of the molecule on a few acid-base sites,... 

The molecular (161) and dissociative (162, 163) adsorption of NH3 on MgO was investigated by IR and UV-VIS spectroscopies (257). The results show that a small fraction of ammonia undergoes heterolytic dissociation on adjacent low-coordinated Mg2+ and O2 ions to form NH2 and OH- groups. The reaction of CO with the NH2 and OH has been characterized by IR emission spectroscopy (164). Formaldehyde and formates are formed first they react to give isocyanate derivatives, and decomposition at high temperatures yields simple (NCO) ions (164). Garrone et al. (165) reported the interaction of N2O with irreversibly preadsorbed ammonia to yield surface azid (Nj) species. The interaction of O2 with preadsorbed NH3 on MgO was described by Martra et al. (166), who used IR spectroscopy the oxidized species Nj, N3, NO, NO2, and NO3 were detected. 

The adsorption of hydrogen on the MgO surface has been studied by Coluccia and Tench (166a). At low temperatures, the adsorption is largely molecular, and the photoluminescence spectra show that both 0 q and O c ions are involved, Infrared evidence (166b) shows that the room-temperature adsorption involves heterolytic dissociation [Eq. (32)] and is associated with 0 c ions,... 

Hydrogen adsorption on MgO can, in principle, be either molecular or dissociative. Dissociative adsorption of hydrogen on high-surface-area MgO has already been reported, and both homolytic and heterolytic pathways have been proposed 12). Homolytic splitting is supposed to operate under UV-irradiation only (117-119) and is not discussed further here. Heterolytic splitting takes place in the dark and at 300 K on coordinatively unsaturated (cus) Mg O surface pairs following the schematic mechanism illustrated in Scheme 2. 

The hydration of acetylene takes place between acetylene and water both adsorbed on similar Cd Z"2 sites [14]. The increase of activity with Si/Al ratio suggests that heterolytically dissociated water is involved in hydration because this dissociative adsorption of water on bivalent metal zeohtes increases with Si/Al ratios as it has been observed for Ca- and Mg-Y,FAU [15]. 

In conclusion, the combined experimental and theoretical study of methanol adsorbed on MgO films with different defect densities allows for a better identification of the surface sites responsible for the MgO reactivity. On the inert terrace sites only physisorption is observed. Molecular chemisorption, activation, and heterolytic dissociation occur on irregular sites. The low-coordinated Mg-O pairs of ions located at edges and steps can lead to strongly activated and even dissociated methanol molecules. Adsorption of CHsO" and H+ fragments seems to be preferred over dissociation into and OH ... 

The data on hydrogen/deuterium exchange in paraffins on oxides indicated that a negatively charged intermediate is formed (26,27). Also the studies of adsorption of ethane and propane on oxide catalysts by IR spectroscopy showed that heterolytic dissociation of the C-H bond takes place with the formation of negatively... 

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