Effect of H2 on deNOx performance of HC-SCR over Ag/Al2O3: Morphological, chemical, and kinetic changes

The enhanced deNOx performance of the Ag/Al2O3 catalyst by addition of H2 was investigated for HC-SCR using a mixture of simulated diesel and ethanol as the reductant. As H2 concentration increased in the feed-gas stream, the low-temperature NOx reduction activity was remarkably enhanced. Addition of H2 promoted the oxidation of both NO and hydrocarbons, resulting in an increased formation of nitrates, enolic, and isocyanate species regarded as reaction intermediates. Results of the UV–vis indicated that H2 promoted the transformation of Ag+ to Agnδ+ and Ag0 on the surface of Ag/Al2O3 catalyst, while those of H2-TPR, O2-TPD, in situ FTIR, and TGA revealed the formation of highly mobile and reactive oxygen species under the (O2 + H2) flow. A reaction mechanism is proposed to elucidate the role of H2 in the enhancement of deNOx performance of HC-SCR in view of morphological, chemical, and kinetic changes of the Ag/Al2O3 catalytic system.

Enhanced deNOx activity of HC-SCR by H2 over Ag/Al2O3 involves morphological, chemical, and kinetic changes. Morphological change is slow but kinetic change on the surface is fast, while chemical change starts fast on the surface but may slowly propagate into the bulk.Figure optionsDownload high-quality image (63 K)Download as PowerPoint slideHighlights
► Addition of H2 dramatically increases the low-temperature deNOx activity of Ag/Al2O3.
► Reduction of Ag+ by H2 involves a morphological change of Ag+ to Agnδ+ and Ag0.
► (H2 + O2) transforms metallic Ag and Ag2O to Ag(OH)2 and AgO (chemical change).
► AgO promotes oxidation of NO/HCs and formation of –NO3/–NCO (kinetic change).
► Surface kinetic change is fast, but morphological/bulk chemical changes are slow.