NOx reduction by H2 on WOx/ZrO2-supported Pd catalysts under lean conditions

This work addresses the H2-deNOx reaction representing a prospective tool for the low-temperature NOx abatement under lean conditions. A novel Pd/WOx/ZrO2 catalyst with a W content of 8.7% was developed. The screening of the Pd load performed by temperature-programmed reaction on a laboratory bench provided highest efficiency at a Pd content of 0.41% (0.41Pd/W/ZrO2). This catalyst showed considerable H2-deNOx activity between 125 and 280 °C with an outstanding overall N2 selectivity of 96%. Drastically higher performance was observed when 0.41Pd/W/ZrO2 was evaluated under steady-state conditions resulting in 90% deNOx at 160 °C and a space velocity of 80,000 h−1. At this temperature, the apparent turnover frequency of Pd amounted to 8.1 × 10−5 s−1, which was very close to that of Pt existing in the highly active Pt/W/ZrO2 system. The variation of the H2 and O2 feed concentration indicated that growing H2 led to increasing H2-deNOx performance accompanied by decreasing N2 selectivity. O2 did not affect the selectivity, but clearly limited the operation window of the catalyst at O2 contents above 6 vol.%. The presence of H2O as well as hydrothermal exposure at 750 °C declined both activity as well as N2 selectivity.The characterisation of the 0.41Pd/W/ZrO2 catalyst by XRD, HRTEM/EDX and XPS showed tetragonal ZrO2, whilst tungsten likely exists in sub-monolayer WOx entities as well as amorphous WO3 nanoparticles. Pd was predominately present in the form of PdO with particle sizes below 2 nm.Mechanistic studies conducted by DRIFT spectroscopy indicated that the lean H2-deNOx reaction 0.41Pd/W/ZrO2 occurs on the Pd sites. The reaction is considered to follow the mechanism according to Burch implying pre-reduction of the active sites by H2 followed by NO dissociation and formation of N2 with minor production of N2O. However, some participation of NOx species located on the W/ZrO2 substrate cannot be ruled out completely.

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► Pd/WOx/ZrO2 catalyst developed for lean H2-deNOx.
► Substantial H2-deNOx performance with high N2 selectivity.
► Operation window ranges from 100 to 250 °C.
► H2-deNOx mainly occurs on the Pd component.