In lean-DeNOX catalysis reactions, hydrogen is a good reducing agent in PGM catalysts as well as an effective promoter in selective catalytic reduction reactions over base metal oxide catalysts. However, such a lean-DeNOX system, which uses hydrogen, requires an on-board fuel reforming system applicable to internal combustion engines. In this study, catalytic partial oxidation (CPOx) performance was tested in a laboratory for various reactants and hydrocarbon conditions. Volume concentrations of 5-10% oxygen and 0-5% water vapor were used to simulate diesel exhaust, and n-$C_{12}H_{26}$ was used as the feedstock for the reforming reaction. In the CPOx of n-$C_{12}H_{26}$, the highest hydrogen selectivity was 64% and was achieved at 100,000 h-1 GHSV. Additionally, the C/O ratio was less than unity in the absence of water vapor. However, as the water concentration was increased to 2.5 and 5.0 vol. % in the n-$C_{12}H_{26}$ CPOx reactions, the maximum hydrogen selectivity was increased from 64% in the absence of water to 70% and 75%, respectively. This effect is a consequence of the water-gas shift reaction over the catalyst bed. Regarding oxygen concentration effects, hydrogen selectivity slightly increased with increasing oxygen concentration from 10% to 15%. It was also found that the CPOx reaction of n-$C_{12}H_{26}$ can be ignited at temperatures below 300 C. Accordingly, it can be concluded that CPOx is a useful and feasible device for promoting diesel DeNOx catalysis in terms of hydrogen productivity and reaction initiation.