Adsorption heterolytic

Thus, in this reaction the active site functions as an acid-base pair and the adsorption of water is an acid-base reaction. The driving force for this reaction is the resulting reduction of the charge separation. In a similar fashion we can view hydrogen adsorption as heterolytic fission at the highly polar active site, viz ... 

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. 

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. 

The formation of these dinuclear complexes can be impeded by entrapment of the Mn(BPY)22+ complexes in the structure of zeolite Y. Preferably, Mn(BPY)2+ is assembled via ship-in-a-bottle synthesis in zeohte Y, through BPY adsorption on a NaY zeolite partially exchanged with Mn2+. Because a single zeolite Y supercage can contain only one Mn(BPY)2+ complex, the formation of dinuclear complexes is impossible for steric reasons. The reaction of H2C>2 with the zeolite-entrapped Mn(BPY)2+ complex does not lead to the same vigorous peroxide decomposition as occurs in solution. Instead, H2O2 is heterolytically activated on the Mn complex with civ-bipyridine ligands to form a Mn(IV)=0 or Mn(V)=0 species. The latter is a... 

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 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... 

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. 

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