Oxygen chemisorption and laser Raman spectroscopy of unsupported and silica-supported vanadium oxide catalysts

Abstract

An oxygen chemisorption method has been developed for measuring the active surface area of supported and unsupported VzOs following reduction in hydrogen. It is shown that to achieve complete reduction of the vanadia surface without reducing the bulk, reduction must be carried out at 640 K. Oxygen uptakes of unsupported samples reduced at close to this temperature yield an oxygen atom site density of 3.2 X 10l8 a value near that expected for a monolayer. The same oxygen chemisorption technique is applied to silica-supported V205. Laser Raman spectroscopy confirms that, near 640 K, oxygen chemisorbs primarily at the surface of the dispersed vanadia but does not exchange with the bulk of the oxide. For very low weight loadings, a limiting stoichiometry of one adsorbed oxygen atom per vanadium atom is obtained. This stoichiometry is used to calculate dispersions ranging from 93% to 50% for supported VzOs samples of 0.3-9.8% weight loading.

In Situ Raman Spectroscopy of Supported Chromium Oxide Catalysts: 18O2−16O2 Isotopic Labeling Studies

“…[18][19][20][21] It is also important to mention here that the presence of mono-oxo surface species is also revealed by a combination of IR spectroscopy and oxygen-18 isotopic exchange experiments in Cr 2 O 3 , MoO 3 /SiO 2 , SO 4 / Al 2 O 3 , and SO 4 /TiO 2 , and V 2 O 5 /TiO 2 catalysts. [22][23][24][25] …”

Use of Oxide Ligands in Designing Catalytic Active Sites

“…The preferential anchoring of the surface M 1 O x species at the surface M 2 O x species over the exposed SiO 2 support sites is a general phenomenon for anchoring of group 5-7 transition metal oxides on the surface-modified SiO 2 support. The surface vanadium oxide species in the dehydrated V 2 O 5 /SiO 2 catalyst exhibit a sharp and intense Raman vibration at 1038 cm À1 , as shown in Figure 1.3 (left), curve labeled A, and has been assigned to the terminal n s (V¼O) stretch of isolated surface VO 4 species [39][40][41][42][43][44][45]. This Raman band position is also independent of exposure to D 2 O at elevated temperatures indicating that there are no VÀOH functionalities (not shown for brevity).…”

“…Therefore, monolayer coverage or maximum dispersion limit is a critical parameter that allows distinguishing between two-dimensional molecularly dispersed surface metal oxide species and the three-dimensional NPs present in supported metal oxide materials. For the purposes of simplifying and focusing the discussion, only SiO 2 -supported metal oxide catalysts at submonolayer coverages will be discussed in this chapter since the catalysis literature has demonstrated that only isolated surface metal oxide species are present at low coverage for supported MO x /SiO 2 catalysts [14,[38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53][54][55][56].…”

A kinetic insight into the activation of n-octane with alkaline-earth metal hydroxyapatites

“…The temperature was ramped from 30°C up to 500°C at a ramping rate of 10°C min -1 and the amount of oxygen chemisorbed was measured at this temperature. 16 The surface morphology of the catalysts was obtained using scanning electron microscopy (SEM) utilizing a Leo 1450 Scanning Electron Microscope. Transmission electron microscopy (TEM) images were viewed on a Jeol JEM-1010 electron microscope and these images were captured and analyzed using iTEM software.…”