Elucidating the role of the state of Pd in the H2-SCR of NOx by operando XANES and DRIFTS

Abstract

The selective catalytic reduction (SCR) of NO_x with hydrogen is an attractive strategy for NO_x 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 structure 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 NO_x species to Pd-nitrosyls. Limitations for the H₂-SCR of NO_x 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 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 NO_x and formation of NH_x 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.