Modular Pd/Zeolite Composites Demonstrating the Key Role of Support Hydrophobic/Hydrophilic Character in Methane Catalytic Combustion

Abstract

Complete catalytic oxidation of methane in the presence of steam at low temperatures (T < 400 °C) is a crucial reaction for emission control, yet it presents profound challenges. The activation of the strong C–H bond of methane at low temperature is difficult, and the water present in any realistic application poisons the active surface and promotes sintering of Pd particles during the reaction. Finding materials that can deliver high reaction rates while being more resistant to the presence of water is imperative for advancing several technological applications of natural gas-based systems. However, methods to fairly compare the activity of Pd catalysts (the most active metal for methane combustion) are needed in order to perform useful structure–property relationship studies. Here, we report a method to study how zeolite hydrophobicity affects the activity of Pd nanoparticles in the reaction, which led to a significant improvement in the water resistance. Mesoporous zeolites were synthesized starting from commercially available microporous zeolites. In this way, a variety of hierarchically porous zeolites, with different hydrophobic/hydrophilic character, were prepared. Preformed colloidal Pd nanoparticles could be deposited within mesostructured zeolites. This approach enabled the systematic study of key parameters such as zeolite framework, Al content, and the Pd loading while maintaining the same Pd particle size and structure for all the samples. Detailed catalytic studies revealed an optimum hydrophobic/hydrophilic character, and a promising steam-resistant catalyst, namely, 3.2 nm Pd particles supported on mesoporous zeolite beta or USY with a Si/Al ratio of 40, emerged from this multiparametric study with a T 50 of 355 °C and T 90 of 375 °C (where T 50 and T 90 are temperature values at which the samples reach 50% and 90% methane conversion, respectively) in steam-containing reaction conditions. Finally, we verified that the designed catalysts were stable by in-depth postcatalysis characterization and operando diffuse-reflectance infrared Fourier-transform spectroscopy (DRIFTS) analyses confirming that water adsorbs less strongly on the active PdO surface due to interaction with the zeolite acid sites. This method can be of general use to study how zeolite supports affect the reactivity of supported metals in several catalytic applications.

Understanding the Role of SnO2 Support in Water‐Tolerant Methane Combustion: In situ Observation of Pd(OH)2 and Comparison with Pd/Al2O3

“…The Pd/SnO 2 catalyst data in Figure a shows that Pd metal slowly oxidizes with the formation of both PdO as a dominant oxidation product and Pd(OH) 2 as a minor product. At higher temperatures, a complete dehydration of Pd(OH) 2 is seen, which is consistent with the dehydration onset suggested in the literature based on kinetic studies . The maximum observed Pd(OH) 2 fraction on Pd/SnO 2 of 15 % at 350 °C closely resembles the Pd surface area of the catalyst (16 % metal dispersion as determined by CO chemisorption).…”

“…Deactivation of PdO active sites is accepted to occur via the formation of inactive Pd(OH) 2 at temperatures below 450 °C (for Pd/Al 2 O 3 ) . Pfefferle and colleagues advocated that water impedes oxygen mobility on the support, thus, its detrimental effect may be balanced by the use of a support with high oxygen mobility, which was corroborated by other groups for the supports with high oxygen‐storage capacity or hydrophobicity . Our earlier in situ X‐ray absorption spectroscopy study provided evidence of the lack of oxygen at Pd active sites in the wet feed at low temperatures in Pd/Al 2 O 3 system, which was supported by others …”

“…Conventional Pd/Al 2 O 3 catalysts are known for their instability at such conditions due to the water and hydroxyl accumulation on the active sites and/or support . Deactivation of PdO active sites is accepted to occur via the formation of inactive Pd(OH) 2 at temperatures below 450 °C (for Pd/Al 2 O 3 ) . Pfefferle and colleagues advocated that water impedes oxygen mobility on the support, thus, its detrimental effect may be balanced by the use of a support with high oxygen mobility, which was corroborated by other groups for the supports with high oxygen‐storage capacity or hydrophobicity .…”

“…In this work we follow the Pd speciation by X‐ray absorption spectroscopy (XAS) of Pd/Al 2 O 3 and Pd/SnO 2 catalysts during methane combustion in the wet feed to observe the presence of Pd(OH) 2 hydroxyl species on Pd/PdO particles and to understand better the influence of the support on the water tolerance. Typically, hydroxyl formation in methane combustion has been determined by operando or in situ infrared spectroscopy; however that does not typically allow one to distinguish hydroxyl groups presents on the Pd and the support surface . Herein, we were able to track the metallic Pd, PdO and Pd(OH) 2 via in situ Pd L edge and K edge XAS studies at temperatures applicable to lean‐burn methane combustion in the presence of water.…”

Studying Proton Mobility in Zeolites by Varying Temperature Infrared Spectroscopy

“…One [Fe]-ZSM-5 with a nominal Si/Fe ratio of 45, has been synthesized by replacing the Al-source in a reported fluoride mediated zeolite synthesis [17] by Fe(NO 3 ) 3 x 9 H 2 O. The mesostructuring treatment of USY(15) has been performed as previously reported [27].…”