Heterogeneous selective oxidation of formaldehyde to formic acid on V/Ti oxide catalysts: The role of vanadia species

The present work deals with the study of the role of vanadium species in the V/Ti-supported oxide catalysts for the oxidation of formaldehyde to formic acid. Two different series of catalysts were prepared, the first one consisting of catalysts of composition 20 wt.% V2O5/80 wt.% TiO2 calcined at 400 and 450 °C (series 1), and the second one was prepared by washing of series 1 samples with nitric acid (series 2). All the samples were characterized by XRD, IR spectroscopy, temperature-programmed reduction with H2, and differential dissolution methods. In the catalysts of series 1, vanadium is represented by both monolayer species (monomeric and polymeric VOx) and V2O5 phase in crystalline or X-ray amorphous state, depending on the calcination temperature. Washing the samples of series 1 with nitric acid removes the V2O5 phase, and the samples of series 2 contain only monolayer vanadium species. The monolayer vanadia–titania catalysts are highly active in the formaldehyde oxidation to formic acid, while crystalline and amorphous V2O5 exhibit a low activity in that reaction. Moreover, the presence of the V2O5 phase blocks the catalyst active sites and thus decreases the catalyst activity. The monolayer vanadium species are easier to be reduced by H2 as compared to bulk V2O5.

The activity of supported V/Ti oxide catalysts is determined by the nature of vanadium oxide species. The monolayer vanadium–titania catalysts are highly active in the formaldehyde oxidation to formic acid. According to the TPR data, these vanadium species contain a more weakly bonded oxygen as compared to oxygen in bulk V2O5. The bulk V2O5 is highly selective, but low active in respect to the oxidation of formaldehyde to formic acid as compared with the supported V/Ti oxide catalysts. The presence of bulk amorphous or crystalline V2O5 in the V/Ti oxide catalysts leads to a partial blocking of the active sites and hence decreases the catalyst activity in the oxidation of formaldehyde to formic acid.Figure optionsDownload as PowerPoint slide