The IR technique in combination with adsorption microcalorimetry was used to picture the bonding nature of
silver ion exchanged or supported on solid materials, such as ZSM-5-type zeolite, aluminosilicate, and SiO2,
by utilizing CO as a probe molecule. It has become apparent that there exists an adsorption site on which the
CO molecule is adsorbed to give an IR absorption band: IR band at around 2193 cm-1 for silver-ion exchanged
ZSM-5 (AgZSM-5) and aluminosilicate (Ag/SiO2·Al2O3), and at 2177 cm-1 for silver-ion supported SiO2
(Ag/SiO2). The CO adsorption took place accompanying a large heat evolution of 100−80 kJ mol-1 on the
former two samples and relatively small heat of 70−40 kJ mol-1 on the latter sample; silver ions exchanged
with protons acting as Brønsted acid sites are responsible for the strong adsorption sites for CO adsorption.
Taking account of the relationship between the differential heat of adsorption (q
diff) and the stretching vibrational
frequency of adsorbed CO (ν
CO), it was concluded that the electrostatic interaction is dominantly operative
in these systems. The large adsorption heats in the initial stage of CO adsorption on the AgZSM-5 and Ag/SiO2·Al2O3 samples may be successfully explainable by considering a little contribution of σ-bonding in
addition to the electrostatic interaction. The quantum chemical calculation was performed to justify the two
types of ion-exchange model for silver ion coordinated to two or three lattice oxygen atoms in AgZSM-5, as
well as to clarify the bonding nature between the exchanged silver ion and CO molecule. As the results, the
two-coordinated silver ions in AgZSM-5 can adsorb CO molecules and give the values of about 100 kJ
mol-1 and 2193 cm-1, and the three-coordinated silver ions weakly adsorb CO to give the values of about 80
kJ mol-1 and 2184 cm-1. These adsorption energies are much smaller and the stretching frequencies due to
the adsorbed CO are higher, compared with the case of CO adsorption on copper-ion exchanged ZSM-5
(CuZSM-5). From these results it can be interpreted that the dominant force operating in the AgZSM-5−CO
system is electrostatic attraction, as is different from the case of CuZSM-5−CO system in which the σ-donation
is dominant in the bonding. This difference is explained by taking account of the differences in energy gap
between 4d−5s for silver ion and 3d−4s for copper ion; in the former case the hybridization of orbital is
limited to result in a large σ-repulsion. The present experiment clearly proved that the evaluation of bonding
nature from the relationship between q
diff and ν
CO is useful in the characterization of exchanged ions. The
specific bonding nature between copper ion (Cu+) exchanged in ZSM-5 and CO molecule was also clarified.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.