Relating methanol oxidation to the structure of ceria-supported vanadia monolayer catalysts

Abstract

Abstract.Vanadia "monolayer"-type catalysts supported on reducible oxides such as ceria previously have shown high activity for the selective oxidation of alcohols. Here, a model system consisting of vanadia particles deposited on well-ordered CeO 2 (111) thin films has been employed. Scanning tunneling microscopy (STM), photoelectron spectroscopy (PES), and infrared reflection absorption spectroscopy (IRAS) were used to characterize the VO x /CeO 2 surface as a function of vanadia loading. The formation of isolated monomeric species as well as two-dimensional vanadia islands that wet the ceria support was directly observed by STM. The vanadia species exhibit V in a +5 oxidation state and expose vanadyl (V=O) groups with stretching vibrations that blue -shift from ~1005 cm -1 , to ~1040 cm -1 with increasing coverage.Temperature programmed desorption (TPD) of methanol revealed three peaks for formaldehyde production. One is correlated with reactivity on the ceria support (565-590 K). Another is correlated with reactivity on large vanadia particles (475-505 K) similar to that previously observed on vanadia/silica and vanadia/alumina model systems. A low temperature reaction pathway (~370 K) is observed at low coverage, which is assigned to the reactivity of isolated vanadia species surrounded by a reduced ceria surface. It is concluded that strong support effects reported in the literature for the real catalysts are likely related to the stabilization of small vanadia clusters by reducible oxide supports.

Methanol Reactivity on Silica-Supported Ceria Nanoparticles

“…For the low-coverage sample, two desorption peaks are observed (i.e., at 305 and 595 K). For the high-coverage sample, the high-temperature signal dominates the spectrum peaked at 575 K, which is close to that observed for CeO 2 (111) surfaces [19,25,26,31], and indicates that the ceria is becoming more "film-like" at high coverages. The supported nanoparticles also…”

“…TPD spectra on the ordered CeO 2 (111) film showed FA desorption at 565 K (not shown here), which is in excellent agreement with that observed in our previous study [31], but considerably lower than that observed above on the ceria nanoparticles (575 -595 K, see Fig. 3).…”

“…Finally, it should be mentioned that low-temperature FA production was previously observed on small vanadyl (V=O) clusters supported on a CeO 2 (111) film, which was proposed to be related to both the high dispersion of the vanadia particles as well as the degree of reduction of the ceria support in the vicinity of the vanadyl species [31]. It seems plausible that the ceria nanoparticles deposited in this study have stabilized similar species without the presence of vanadia.…”

Oxygen activation on oxide surfaces: A perspective at the atomic level

“…We have reported TPD spectra of methanol on ceria supported vanadia have been reported as a function of vanadia coverage [2]. The pristine CeO 2 (1 1 1) surface of a film grown on Ru(0 0 0 1) shows a formaldehyde desorption signal at ∼565 K. In the presence of vanadia, the desorption peak shifts to ∼590 K. Note that in those experiments methanol was exposed to the sample at 300 K. As the coverage of vanadia increases, the integral intensity of the peak decreases and finally becomes negligible for the highest VO x coverage.…”

LEEM and PEEM as Probing Tools to Address Questions in Catalysis

“…For instance, cerium oxide and vanadium oxide are important in catalysis and different model systems have been created in both cases to tackle fundamental questions [33][34][35][36][37][38][39][40]. On one side, CeO x has the ability of easily interconvert the oxidation state of the Ce n+ centres, making the material an interesting candidate as a catalyst for hydrocarbon oxidation reactions.…”