Silver clusters adsorption

Coadsorption phenomena in heterogeneous catalysis and surface chemistry quite commonly consider competitive effects between two reactants on a metal surface [240,344]. Also cooperative mutual interaction in the adsorption behavior of two molecules has been reported [240]. Recently, this latter phenomenon was found to be very pronounced on small gas-phase metal cluster ions too [351-354]. This is mainly due to the fact that the metal cluster reactivity is often strongly charge state dependent and that an adsorbed molecule can effectively influence the metal cluster electronic structure by, e.g., charge transfer effects. This changed electronic complex structure in turn might foster (or also inhibit) adsorption and reaction of further reactant molecules that would otherwise not be possible. An example of cooperative adsorption effects on small free silver cluster ions identified in an ion trap experiment will be presented in the following. 

Adsorption of two O2 molecules as observed in this experiment for Ag had been predicted before for the gold cluster anions [356] however, it was never observed experimentally which lead to further discussion in the literature [357,358]. In combination with a systematic theoretical study performed by the group of Bonacic-Koutecky [351] an understanding of the measured rate constant evolution with cluster size depicted in Fig. 1.58a emerges and reasons for the distinct behavior of silver cluster anions can be given [351]. 

Hence, experimental rate constant measurements in combination with theoretical simulations show a pronounced size and structure selective activity of anionic silver clusters toward molecular oxygen due to cooperative effects. In particular, for Ag clusters with odd n, a weakly bound first O2 promotes the adsorption of a second O2 molecule which is then (for n = 3, 5) differently bound with the O2 bond elongated to 1.32 A and thus potentially activated for further oxidation reactions such as CO combustion, which have indeed been observed for larger cluster sizes [361]. 

The redox potential difference between the silver clusters n = 85 and n = 500 is not very large (0.11 V). This su ests that we approach an asymptotic value of ° for the bulk metal °(Ag -Agao) close to +0.40 Vnhe- Note that the redox potential of the bulk metal °( Ag -Agoo) differs from the well known electrochemical potential °(Ag -Aggo) = +0.796 Vnhe by the adsorption energy of the Ag ion on the bulk metal ... 

Original methods of synthesis of highly dispersed silver catalysts (based upon the application of strong reducing properties of electrons solvated in soldium-ammonia solutions, the adsorption-contact method of drying and a weak solubility in nitric acid of the Si02-supported small silver clusters) allowed us to synthesize Si02-supported silver particles of sizes less than 6 nm. It makes possible some unusual catalytic and other physico-chemical properties of these particles to be discovered. 

The above rate equations confirm the suggested explanation of dynamics of silver particles on the surface of zinc oxide. They account for their relatively fast migration and recombination, as well as formation of larger particles (clusters) not interacting with electronic subsystem of the semiconductor. Note, however, that at longer time intervals, the appearance of a new phase (formation of silver crystals on the surface) results in phase interactions, which are accompanied by the appearance of potential jumps influencing the electronic subsystem of a zinc oxide film. Such an interaction also modifies the adsorption capability of the areas of zinc oxide surface in the vicinity of electrodes [43]. 

The dipole moment of the adsorbed water molecules is estimated to be = 0.22 D (unit of D = 3.36 x 10 ° C m) from the slope of the observed curves shown in Fig. 5-25. Since this dipole moment is nearly one tenth of the dipole moment of gaseous water molecules (m = 1.84 D), the dipole of the adsorbed water molecules on the silver surface is suggested to be aligned almost parallel to the metal surface by forming hydrogen-bonded two-dimensional clusters of water molecules. On the other hand, bromine molecules are in the state of dissociative adsorption on the silver surface, producing adsorbed bromine atoms which receive electrons... 

Adsorption of ions or molecules on metal clusters markedly affects their optical properties. It was shown that the intensity and the shape of the surface plasmon absorption band of silver nanometric particles, which is close to 380 nm, change upon adsorption of various substances [125]. The important damping of the band generally observed is assigned to the change of the electron density of the thin surface layer of the... 

Gold is one of the least reactive metals in bulk form. However, in recent years a considerable amount of theoretical and experimental works have studied the reactivity of small neutral and charged Au clusters towards different molecules, like H2, O2, CO, and organic radicals " . The reactivity depends on the size and charge state of the cluster. In the previous section we have studied the reactivity towards oxygen adsorption of anionic silver and gold clusters. In this section we study the reactivity of neutral gold clusters towards molecular O2 (subsection 6.1) and CO (subsection 6.2). 

In all these studies, there is general agreement that silver is the source of the electrons, but the identity of the adsorption site of the oxygen is not clear. Since electron transfer mechanisms of the type described earlier (Fig. 16) can occur, silver ions are not necessarily the adsorption site and the gzz values would also be consistent with adsorption on the support. However, the degree of dispersion of the silver will be very dependent on the method of preparation and clusters of Ag atoms or ions of different sizes are possible. This leads to an alternative explanation of the low gzz values, since these... 

THEORETICAL STUDY OF ADSORPTION OF SILVER DIMER ON TITANIUM DIOXIDE SURFACE COMPARISON OF CLUSTER AND PERIODIC MODELS... 

An adsorption of silver dimer on a rutile (110) surface has been studied using a DFT model within both cluster and periodic approaches. The calculations show that the interaction of silver dimers can occur both with bridging chain of oxygen atoms or with atoms located in the hollows between chains. The bonding of Ag2 in the hollow is characterized by the positive adsorption energy according to the periodic model. On the other hand, the geometry optimization of similar structures within the cluster model leads to desorption or dissociation of silver dimer. The periodic model is shown more appropriate for this system. 

An increasing interest in studying silver particles and clusters adsorbed on titanium dioxide surface is provided by the efficiency of such systems as photocatalysts. Their catalytic properties are more evident than those of individual Ti02, both anatase and rutile-anatase mixture. There are few publications devoted to quantum chemical calculations of metal clusters adsorbed on TiOz surface at the modem level of theory. Besides, existing theoretical data describe adsorption of silver atom on mtile surface only. 

The DFT study of adsorption of silver dimer on rutile (110) surface within the cluster and periodic models shows that the interaction occurs both with chain oxygen atoms of the surface and with atoms located between the chains of 0(2c) atoms. Positive binding energy of Ag2 with rutile surface during adsorption between the oxygen chains was obtained only for the periodic model. The latter is concluded to be the preferable for theoretical study of Ag /Ti02 systems. 

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