Adsorption sites transition metal cations

Very recently, adsorption of dihydrogen as an IR probe was extended by Kazansky and coworkers to cation-containing zeolites as adsorbents [609-612]. The main aim of their studies was to locaHze the adsorption sites in these materials, especially the cations. This is especially difficult to achieve by other methods in the case of the catalytically important zeolites loaded with low amounts of transition metal cations. In the context of the pertinent investigations by Kazansky and coworkers, the dissociative adsorption of dihydrogen (and light paraffins) in cation-containing zeoHtes was dealt with, too. 

A combination of DRIFT spectroscopy and TPD of CO adsorbed on faujasite-type zeolites, which have been exchanged with transition metal cations (Cu +, Fe +, Co +, Ni +),was employed by Rakic et al. [775]. Except on Cu, Na-Y, disproportionation of CO and carbon deposition occurred. The Lewis acid, charge-compensating sites were assumed to be the sites of adsorption. 

Hydroxyapatite (CajQ(P04)g(0H)2) has also attracted considerable interest as a catalyst support. In these materials, wherein Ca sites are surrounded by P04 tetrahedra, the introduction of transition metal cations such as Pd into the apatite framework can generate stable monomeric phosphate complexes that are efficient for aerobic selox catalysis [99]. Carbon-derived supports have also been utihzed for this chemistry, and are particularly interesting because of the ease of precious metal recovery from spent catalysts simply by combustion of the support. Carbon nanotubes (CNTs) have received considerable attention in this latter regard because of their superior gas adsorption capacity. Palladium nanoparticles anchored on multiwalled carbon nanotubes (MWCNTs) and single-walled carbon nanotubes (SWCNTs) show better selectivity and activity for aerobic selox of benzyl and cinnamyl alcohols [100, 101] compared to activated carbon. Interestingly, Pd supported on MWCNTs showed higher selectivity toward benzaldehyde, whereas activated carbon was found to be a better support in cinnamyl alcohol oxidation. Functionalized polyethylene glycol (PEG) has also been employed successfully as a water-soluble, low-cost, recoverable, non-toxic, and non-volatile support with which to anchor nanoparticulate Pd for selox catalysis of benzyl/cinnamyl alcohols and 2-octanol [102-104]. 

Chlorobenzenes are well known as important precursors of PCDD/Fs and are suitable model compounds for the complete oxidation of chlorinated POPs. Noble metal-based catalysts such as Pt/Al203 show high efficiencies but promote the formation of polychlorinated benzenes. Zeolites have unique properties for the deep oxidation of chlorinated compounds thanks to their well-defined framework and the presence of acid sites or transition metal cations. As stated by Corma, zeohtes present interesting properties of reactant/product partitioning and of molecules preactivated by the molecular confinement effect. Moreover, their adsorptive properties can be modulated by modifying the nature of the extra framework cations and the Si/Al ratio. 

The introduction of monovalent and bivalent transition metal cations into zeolites is also possible and introduces in zeolites sites with redox activity. Several of these systems have wide application in catalysis. In particular, Co-zeolites, such as Co-MFI and Co-FER, have been deeply investigated for their activity in the CH4-SCR reaction [246]. In this case the adsorption of bases such as nitriles and ammonia, followed by IR and by TPD technique, show that they act as medium-strong Lewis acid sites. The current opinion is that these sites are catalytically active for the DeNO c reaction just when they are isolated in the zeolite cavities. A recent investigation provided evidence for the deposition of part of Co ions also at the external surface of the zeolite upon cation exchanging [85] and to their likely nonnegligible catalytic activity [247]. The deposition of Co species at the external cavities can be a reason for only apparent over-exchanging (i.e., production of zeolites with Co +/AP+ atomic ratios >0.5). 

Soma et al. (12) have generalized the trends for aromatic compound polymerization as follows (1) aromatic compounds with ionization potentials lower than approximately 9.7 eV formg radical cations upon adsorption in the interlayer of transition-metal ion-exchanged montmorillonites, (2) parasubstituted benzenes and biphenyls are sorbed as the radical cations and prevented from coupling reactions due to blockage of the para position, (3) monosubstituted benzenes react to 4,4 -substituted biphenyls which are stably sorbed, (4) benzene, biphenyl, and p-terphenyl polymerized, and (5) biphenyl methane, naphthalene, and anthracene are nonreactive due to hindered access to reaction sites. However, they observed a number of exceptions that did not fit this scheme and these were not explained. 

There are several examples where irradiation is not necessary to produce Oj ions. In such cases, a thermal activation is sufficient because of the presence of transition metal ions which can easily transfer one electron to oxygen. Iron is the most common impurity found in zeolites and the formation of OJ depends very much on the iron content (239, 240). Transition metal ions can also be exchanged in zeolites and this will be discussed later. There is also some indication that the types of OJ can be influenced by the level of exchange. When cations of different valence states are involved in the exchange, an incomplete exchange will leave two types of cations present, creating the possibility of at least two types of adsorption sites. This has been observed for both Mg and CaY zeolites (263). 

Chemisorption is irreversible adsorption, which suggests valence bonding at specific sites on a surface. Transition metal ions, protein below its isoelectric point (positively charged), and di- and polyvalent cations are prone to chemisorption. 

Adsorption of cations and cationic complexes of transition metals on carbons is usually accounted for by the presence of surface carboxyl or phenol groups [37]. The number of adsorbed Ag" [112,113] or [Pt(NH3)4] [12,114—116] ions indeed increases with the concentration of these acidic sites. The ion exchange on acidic sites (l -OH) occurs via substitution of protons by cation complexes ... 

Terminal-CO vibrating at 2123 cm, which is stabilized only at <160 K, detects the presence of Nf sites [3]. CO adsorption on metal ions is weak, due to the low ti/ct bonding contribution. Thus, application of low temperature regimes is essential for irreversible adsorption of CO on metal cations [32]. The presence of Ni sites indicates that the catalyst examined is incompletely reduced under the conditions applied. Earlier studies [3] have consistently found, that the complete reduction of Ni ions on alumina is difficult to accomplish. Incomplete reduction of a number of transition metal ions, including Ni, seems to be the trend on alumina and alike electronically "hard" support materials [33]. According to Pearson [33], the electronic hardness of a metal oxide means the occurrence of weakly polarizable metal-oxygen bonds. 

Similarly, IR investigation of CO adsorption on molecular sieves was used to characterize Lewis acidity of cations (C-sites) and true Lewis acidity (L-sites) [ 740]. The interaction of CO with cations (acid C-sites) was dealt with already in Sect. 5.5.2.2. In particular, Angell and Schaffer [595] have carried out a detailed study of CO adsorption on a series of X- and Y-type zeoHtes containing monovalent and divalent cations of alkali, alkaline earth and transition metals. A linear relationship was found between the position of the IR stretching band of adsorbed CO and the Coulomb field, q/r, of the respective cationic adsorption center. This is similar to the observation made by Ward in the case of pyridine attached to cations (vide supra). It should be noted, however, that CO, like pyridine, is not capable of entering the sodalite cages and the hexagonal prisms of the faujasite structure, so that the cations located there are not detected by these probes. 

Since 1970, the perovskite type oxides, typically rare earth oxides with a (ABO3) formula, have been suggested as substitutes for noble metals in automotive exhaust catalysis (1). The most studied perovskites are LaM03 ( M = first row transition metal ) (2,3,4), where M is considered as the active site of the catalyst. The cobaltites show good activity as oxidation catalysts, the reactivity seems to depend on the facility of cobalt to undergo the transition Co Co m, which may be correlated to an oxygen non stoichiometry, and to the spin state of the cation (5). Furthermore, series of LaM03 oxides revealed similar profiles for CO adsorption studies as for NO adsorption, with NO adsorption maxima for M = Mn and Co (6). The reactivity of these catalysts has been shown not only to depend on the surface area, but also on the preparation process (7). 

NO can be employed as a probe to identify Lewis add sites and characterize their density and strength [77]. However, NO may disproportionate into N2O and oxygen and is also very likely to form multi nitrosyl complexes with transition metal ions when present. From a different point of view, calorimetric measurements of CO and N2 adsorption [78] at low temperature also provide a powerful tool for characterizing zeoUtes (li-, Na-, K-MFI and H-MFI), by probing the cations and providing information on their nature and accessibility [79]. 

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. 

An extension of the relative simple formulation used in SCMs for surfaces with permanent eharges (see Section II.B.l) has been published recently [84]. A fictitious surface species (X ) was defined and hypothetical complexation reactions on site X were written, and thus cation-exchange reactions of permanent negative layer charges were easily incorporated into such model. The model showed not only to fit satisfactorily all of the experimental data of transition metal adsorption on montmorillonite but also to explain specific features of adsorption on clays compared to oxides. 

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