Investigation of NO2 and SO2 adsorption/desorption properties of advanced ternary and quaternary mixed oxides for DENOx catalysis

Abstract

The main premise of the current study is the design, synthesis and functional characterization of novel catalytic materials with superior resistance against sulfur poisoning without compromising NOx storage capacity (NSC) in their NOx Storage Reduction (NSR) catalytic applications. BaO/TiO2-based materials are well known systems in deNOx catalysis, exhibiting promising performance towards sulfur poisoning. However, they suffer from limitations due to poor NSC and high affinity towards unwanted solid state interactionsbetweenTiO2 and BaO storage domains leading to the formation of BaTiOx.The main emphasis of the current work is the design of a novel catalytic system where ZrO2 and Al2O3 act as diffusion barriers between BaO and TiO2 domains while allowing good dispersion and preservation of the individual characteristicsof these active sites within a wide operational temperature window. Along these lines, binary and ternary mixed oxide materials, ZrO2/TiO2 (ZT) and Al2O3/ZrO2/TiO2 (AZT), and their Pt, BaO and/or K2O functionalized counterparts in the form of Pt/ZT, Pt/AZT, Pt/BaO/AZT, Pt/K2O/AZT and Pt/K2O-BaO/AZT with different mass loadings (i.e. 8 and 20 wt. % 20 BaO and 2.7, 5.4 and 10 wt. % K2O) were synthesized via sol-gel synthesis. Surface structure and catalytic properties of the synthesized materials were comprehensively investigated at the molecular level as a function of calcination temperature, catalyst composition, nature of the gas phase adsorbates (e.g. NO2, SO2, O2, H2, N2, N2O C5H5N etc.) interacting with the catalyst surface at various operational temperatures by means of XRD, Raman spectroscopy, BET analysis, in-situ FTIR and TPD. Current results indicate no evidence for the formation of undesired BaTiOx and/or KTiOx. NSC of fresh monolithic catalysts was also quantitatively measured under realistic operational conditions in a tubular flow reactor system. These flow reactor measurements indicated that Pt/8BaO/AZT and Pt/20BaO/AZT materials revealed promising NOx storage and sulfur regeneration performance at low (i.e. 473 K) and moderate (i.e. 573 K) temperatures in comparison to the conventional Pt/20Ba/Al2O3 benchmark catalyst. However, they were found to be surpassed by the conventional Pt/20BaO/Al2O3 benchmark catalyst at higher operational temperatures (i.e. 673 K). Therefore, activity loss at high temperatures was alleviated by incorporating a high-temperature storage functionality (i.e. K2O) to the catalyst structure. Upon this structural enhancement, Pt/5.4K2O/AZT catalyst was found to reveal much higher NSC at high temperatures (i.e. 673 K) as compared to BaO-based materials. An overall assessment of the results presented in the current study suggests that there exists a delicate trade-off between NOx Storage Capacity (NSC) and sulfur uptake/poisoning in NSR systems which is strongly governed by the BaO and K2O loading/dispersion as well as the surface structure of the support material.