Effects of Soot Deposition on NOx Purification Reaction and Mass Transfer in a SCR/DPF Catalyst

Experimental studies were carried out to investigate the effect of soot deposition on NOx purification phenomena in an ammonia selective catalytic reduction coated diesel particulate filter (SCR/DPF) catalyst. To study soot deposition effects on the chemical reactions and mass transfer, two types of testing device were used. A synthetic gas bench enabling tests to be conducted with temperature and flow rate ranges relevant to real driving conditions was used to investigate the soot influence on reduction of NOx to N₂ (DeNOx). A micro-reactor that removed the effect of soot deposition on mass transfer in the catalyst layer was used to analyze chemical reactions on a soot surface and their interaction with the SCR catalyst. A filter test brick of a Cu-zeolite SCR/DPF catalyst and a powder catalyst were used for the synthetic gas bench and micro-reactor tests, respectively. Engine soot was sampled in all the tests. The synthetic gas bench results showed that soot deposition had a negative impact on NOx conversion performance. The micro-reactor results showed that NOx purification reactions took place simultaneously with side reactions, e.g., NH₃ oxidation and NO₂ related reactions, even when no SCR catalyst was present. Reactions on the soot surface decreased the NOx purification performance of the SCR catalyst. The influence of soot deposition on the reactivity of side reactions in the filter test brick showed the opposite effect to the powder catalyst: the former effect was intensified as the flow rate was increased. It was concluded that both the effect of reaction on the soot surface and the influence on mass transfer of soot deposition must be considered to describe the internal phenomena of NOx purification in a SCR/DPF catalyst.