Elucidating the Role of the State of Pd in the H$_2$-SCR of NO$_x$ by Operando XANES and DRIFTS

The selective catalytic reduction (SCR) of NOx with hydrogen is an attractive strategy for NOx removal when H2 is used as a
sustainable fuel in combustion engines. However, the pathway suffers from a strong overconsumption of H2 via direct
oxidation to water. In order to improve the understanding of the SCR mechanism with H2 as the reductant, the state of the
active metal, the reactive surface intermediates, and the conditions which are suited for efficient SCR need to be uncovered.
A 1%Pd/5%V2O5/20%TiO2-Al2O3 catalyst was investigated using operando X-ray absorption spectroscopy (XAS) and diffuse
reflectance infrared Fourier-transform spectroscopy (DRIFTS) to track the temperature dependent stucture and state of Pd,
as well as its gradients and surface intermediates. XAS shows that NO reduces oxidized Pd, forming metal-support interfacial
nitrates according to DRIFTS. Partially reduced Pd becomes oxidized by reducing interfacial NOx species to Pd-nitrosyls.
Limitations for the H2-SCR of NOx arise from the balance of adsorbed Pd-NO and activated H2, which is dependent on Pd
state. The bulk metallic Pd which forms above 250 °C causes a runaway activation of H2, further reduction of Pd, and loss of
... mehr
nitrosyls on Pd. The highest activity occurs when Pd is oxidized enough to promote metal-support interfacial nitrates and
also reduced enough to convert these nitrates to Pd-nitrosyls. The storage of NOx and formation of NHx on vanadia-titania
permits conversion of NO at high temperatures, but does not counteract the deactivation of Pd. In conclusion, activation of
H2 is favored on metallic sites, but must be moderated to allow for PdO being present as well.