Exploiting the synergy of titania and alumina in lean NOx reduction: in situ ammonia generation during the Pd/TiO2/Al2O3–catalysed H2/CO/NO/O2 reaction

In situ DRIFTS, XPS, HREM, XRD, and reactor studies have been used to examine and elucidate the catalytic performance of Pd particles supported on TiO2/Al2O3. These materials deliver 100% NOx conversion under demanding and technically relevant conditions (low temperature, high oxygen excess) in the presence of mixed H2+CO reductant feeds. In particular, they are far superior to the corresponding Pd/TiO2 and Pd/Al2O3 catalysts. It is shown that the synergy that operates between the titania and the alumina components involves the intrinsic surface chemistry of these oxides rather than formation of mixed oxide phases. Specifically, titania is critical for the formation of NCO on Pd while alumina promotes subsequent hydrolysis of NCO to ammonia, which then reduces NOx. At high temperature a second NOx reduction channel operates with Pd/TiO2/Al2O3 which greatly extends the useful temperature range. This also involves in situ formation of ammonia—in this case directly from reaction between H2and NO.