Understanding Structure–Function Relationships in Zeolite-Supported Pd Catalysts for Oxidation of Ventilation Air Methane

Abstract

Catalytic combustion of ventilation air methane (VAM) is a potential solution for abatement of this greenhouse gas. In this study, we evaluate the combustion of VAM (with methane concentrations below 1%) spanning over 100 h time on stream (TOS) during reaction over a Pd/HZSM-5 catalyst. The aim is to understand the structural changes that lead to catalyst deactivation. We observe the formation of carbonaceous deposits even under oxygen-rich conditions, which are an important contributor to deactivation. X-ray absorption spectroscopic (XAS) investigation shows that, in addition to carbon deposits, the growth of Pd oxide clusters leads to a reduced number of accessible sites and in turn intrinsic activity. STEM-EDS analysis disclosed the presence of the carbonaceous deposit on the surface of the used catalyst, and TGA confirmed the presence of different carbon species on the used catalyst under very lean conditions. Structural changes show that Pd–O/acid–base interactions have a significant influence on the structure of the active site. This assertion is consistent with findings from acid–base characterization experiments. Although the catalyst displayed a high level of stability over the first 10 h of VAM combustion, long-term reaction, in the presence of water vapor, is associated with partial rearrangement of the zeolite, accompanied by a gradual deactivation of the catalyst. This rearrangement is associated with a decrease in surface area and pore volume, which is consistent with the significant changes observed in the Al-X-ray absorption near-edge spectroscopic (XANES) analysis. A comparison of the NH3-TPD of fresh and used Pd/HZSM-5 catalysts shows that the strengths of the acid sites are significantly reduced. This is a consequence of the changing nature of transition metal interaction with the zeolite, which is accompanied by the dealumination of the zeolite support, thereby enhancing Pd agglomeration and the emergence of two low index surface orientation facet planes identified as PdO(101) and PdO(100). A higher turnover frequency (TOF) (0.031 s–1) for reactivated Pd/HZSM-5 after removing all carbonaceous material compared to the TOF (0.024 s–1) for used Pd/HZSM-5 was observed. The catalyst regained 75% of its initial catalytic activity after removing carbonaceous compound from the used catalyst. We propose the formation of a palladium carbonaceous complex manifesting itself in carbonate and a carbonyl group observed in used Pd/HZSM-5. These species act as an important contributor to catalyst deactivation and cause partial reversible deactivation during long-term VAM combustion.

Will It Be Possible to Put into Practice the Mitigation of Ventilation Air Methane Emissions? Review on the State-of-the-Art and Emerging Materials and Technologies

“…Over time, the measured conversion decreases [24]. Other experiments testing the effect of water on Pd/HZSM-5 catalysts during long-term working showed a gradual deactivation of the catalyst caused by a dealumination and agglomeration of palladium on the surface of the HZSM-5 [81]. A similar conclusion can be drawn on the basis of the study in [84], where tests of Pd/HZSM-5 revealed that the catalyst was unstable in contact with water.…”

“…Catalytic performance is strongly dependent on the nature of the support [75,95,101] and depends on the interaction between the support and active phases [81]. The comparison of the influence of additives in catalysts by various authors on a chemical reaction is not easy.…”

Cobalt-Induced PdO Formation in Low-Loading Pd/BEA Catalysts for CH4 Oxidation

“…Thus, PGMs are still necessary to satisfy the requirements of CH 4 emission control, that is, low CH 4 concentrations (500–1500 ppm) and high GHSV. Among other PGMs, palladium (Pd)-based catalysts have shown the most promising CH 4 oxidation performance. , Different Pd-based catalysts have been evaluated for their CH 4 oxidation performance, such as Pd@CeO 2 , Pd@ZrO 2 , Pd/Al 2 O 3 , and Pd/zeolites (Pd/ZSM-5, , Pd/MOR, and Pd/SSZ-13). Zeolites have attracted significant attention as they are endowed with high porosity, well-defined channels, and enhanced hydrothermal stability.…”

Metal–Organic Framework-Membranized Bicomponent Core–Shell Catalyst HZSM-5@UIO-66-NH2/Pd for CO2 Selective Conversion

“…By creating a composite of MOFs and functional species, it is possible to combine their advantages and circumvent their disadvantages to obtain new properties from the synergistic effects between the individual components. − As reported previously, the MOF@MOF core–shell composite, in which UIO-66-NH 2 with relatively large pore is encapsulated into the molecular-sieving ZIF-8, possesses a size-selective catalysis feature, showing that the obtained characteristics of the MOF@MOF structures are attributed to the existence of the MOF–MOF interface. Zeolites, such as ZSM-5, are crystalline aluminosilicates, which exhibit thermal stability, relatively high surface area, intrinsic acidity, and defined microporosity, and are widely applied in catalysis. − Inspired by the above-mentioned, we propose an MOF-membranized ZSM-5@MOF core–shell composite, which combines the characteristics of MOF and ZSM-5, to produce enhanced or new functionalities. There are a few reports on zeolite@MOF composites; specifically, the previously reported composites of a MOF film on zeolites are very large (i.e., micrometer level) or the MOF particles on zeolite do not continuously grow but rather adhere to it .…”