Low-Temperature CO Adsorption on V-Containing Aluminophosphates: An FTIR Study

Adsorption of CO on VAPO-5 and VMgAPO-5 has been studied by IR spectroscopy. No carbonyl species were formed with V5+sites on oxidized VAPO-5. Mild reduction (673 K) of the sample with hydrogen generated V4+sites which did not form stable complexes with CO at ambient temperature. At 85 K, however, two V4+–CO species were detected, characterized by bands at 2200 (A species) and 2194 cm−1(B species). In these complexes CO is bonded via a σ-bond only. Deeper reduction (773 K and above) led to the formation of V3+sites, predominantly at the expense of the A species. These cations formed two kinds of carbonyl complexes (bands at 2197 and 2186 cm−1) in which a weak π-back bonding is realized. As a result, the carbonyls were more stable than the V4+–CO species and could be detected even at room temperature. The V3+sites were fully oxidized by oxygen even at 85 K thus forming V4+and V5+species. At higher reoxidation temperatures (up to 373 K) the major part of the V4+sites was also oxidized to V5+. CO adsorption on VMgAPO-5 results in the formation of Mg2+–CO species producing intense bands at 2187 and 2179 cm−1which were superimposed on the Vn+–CO bands. For this reason the oxidation–reduction behavior of vanadium on this sample could not be studied in detail. It was found that some V4+sites (producing a band at 2204 cm−1at low temperature CO adsorption) were generated during the evacuation at 673 K. Deeper reduction with hydrogen created new sites (most probably V3+) which were characterized by a carbonyl band at 2197 cm−1. The nature of the Vn+–CO bonds as well as the effect of different factors on its strength are discussed. Some conclusions on the reduction–oxidation behavior of vanadium in the VAPO-5 and VMgAPO-5 are also presented.