The use of vanadia selective catalytic reduction (V-SCR)
catalysts for NOX reduction from diesel engine exhaust is well
known. These catalysts are also active for hydrocarbon (HC) and
particulate matter (PM) oxidation. This dual functionality
(oxidation and reduction) of V-SCR catalysts can help certain
applications achieve the legislative limits with an improved
margin.
In this work, NOX reduction, HC and CO oxidation over V-SCR
were studied independently and simultaneously in microreactor
tests. The effect of various parameters (HC speciation,
concentration, ANR, and NO₂/NOX ratio) was investigated and the
data was used to develop a kinetic model. Oxidation of CO, C₃H₆,
and n-C₁₀H₂₂ is first order in CO/HC, while C₇H₈ oxidation is less
than first order in C₇H₈. All these reactions were zero order in
O₂. Oxidation activity decreased in order: C₇H₈ ≻ n-C₁₀H₂₂ ≻ C₃H₆ ≻
CO. HC oxidation was inhibited by NH₃. The presence of HCs was
found to inhibit NOX conversion, but only after the HC had
started to react. This can be explained by NOX reduction being
inhibited not by the HC itself, but by the CO produced from partial
oxidation of the HC.
The V-SCR catalyst was also tested on a diesel engine in the
absence of an upstream DOC and DPF to investigate its performance.
The results confirm that V-SCR can remove HC and PM effectively:
for temperatures above 230°C, V-SCR showed 45-75% HC conversion and
20-60% PM removal, under the specific engine conditions tested.
This data was used to validate the kinetic model.