Influence of feed composition on the activity of Mn and PdMn/Al2O3 catalysts for combustion of formaldehyde/methanol

The activity of supported manganese and palladium-manganese oxides over γ-alumina for the removal of a mixture of formaldehyde/methanol was evaluated using two different feed-streams. One consisted of air feed and the other that surrogates a gas exhaust type feed, with low proportion in oxygen, high concentration in water and a small amount of carbon monoxide. The catalysts were characterised by X-ray diffraction (XRD), temperature-programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS). These techniques pointed to a mixture of MnIV/MnIII oxide phases in both monometallic and bimetallic catalysts together with a PdO phase in the bimetallic systems. The TPR and XPS data of used catalysts depict a change in the active phase for the samples used in a feed-stream poor in oxygen. A decrease in the activity is observed when a feed that surrogates a gas exhaust stream is used. These facts are explained in terms of the mechanism by which VOCs are oxidized by lattice oxygen from Pd and/or Mn oxide phases and the slower re-oxidation of these phases by the oxygen-poor feed-stream. Besides, the high concentration of water in the feed favours deactivation of catalyst due to the competition of VOCs and water molecules for the active site.