Selective Oxidation of Propylene to Acrolein over Silver Molybdate-Coated MoO3 Nanobelts

Some of the most active catalysts for the selective oxidation of olefins are based on molybdenum oxides or silver (Ag) systems. Hence, we furnished α-MoO3 nanobelts (NBs) with size-controlled silver nanoparticles (Ag NPs) in order to explore the synergies between these materials. Intriguingly, the components reacted with each other at 350 °C to generate a Ag2Mo4O13 skin on the NBs, which turned them into a very selective catalyst for propylene conversion to acrolein. The selectivity to this aldehyde improved from less than 20% over bare NBs to ca. 75% at 350 °C, in line with the apparent intrinsic limit of other unpromoted molybdates. Propylene oxide formation appears to be completely inhibited because of the encapsulation of residual Ag NPs by silver molybdate. The reaction kinetic characteristics over Ag2Mo4O13 are consistent with a Mars–van Krevelen mechanism. However, the apparent activation energies for reoxidation of silver molybdate (86.9 kJ mol–1) and propylene activation (37.9 kJ mol–1) are considerably lower than those reported for other molybdates. The acrolein formation rate over the coated NBs exceeds that over bulk Ag2Mo4O13 by more than a factor of 10 because of the similarly enlarged surface area. On the basis of the Ag content, the rate gain is even boosted by 4 orders of magnitude because only marginal noble-metal amounts (<0.3 wt %), i.e., Ag2Mo4O13, are needed for optimum catalytic activity of the composite. Altogether, these features render Ag2Mo4O13-coated MoO3 NBs as a prototypal design for highly efficient, nanostructured selective oxidation catalysts.