Platinum Inhibits Low‐Temperature Dry Lean Methane Combustion through Palladium Reduction in Pd−Pt/Al₂O₃: An In Situ X‐ray Absorption Study

Abstract

Palladium-platinum bimetallic catalysts supported on alumina with palladium/platinum molar ratios ranging from 0.25 to 4 are studied in dry lean methane combustion in the temperature range of 200 to 500 °C. Platinum addition decreases the catalyst activity, which cannot be explained by the decrease in dispersion or the structure sensitivity of the reaction. In situ X-ray absorption near-edge structure and extended X-ray absorption fine structure spectroscopy measurements have been conducted for monometallic Pd, Pt, and 2:1 Pd-Pt catalysts. Monometallic palladium is fully oxidized in the full temperature range, whereas platinum addition promotes palladium reduction, even in a reactive oxidizing environment. The Pd/PdO weight ratio in bimetallic Pd-Pt 2:1 catalysts decreases from 98/2 to 10/90 in the 200-500 °C temperature range under the reaction conditions. Thus, platinum promotes the formation of the reduced palladium phase with a significantly lower activity than that of oxidized palladium. The study sheds light on the effect of platinum on the state of the active palladium surface under low-temperature dry lean methane combustion conditions, which is important for methane-emission control devices.

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

“…The system was then cooled to 100 °C at which point the gaseous environment was changed to methane/air mixtures for the in situ experiment. 0.1 % CH 4 /N 2 and dry air were used at flow rates of 154 and 103 mL/min respectively, the dry air flowed through two bubblers to saturate it before mixing with the methane mix . Temperature intervals generally started at 100 °C and were increased by 50 °C until 450 °C.…”

“…[9] In the presence of water in the feed, the Pd(0) fraction remained dominant up to about 350°C, [9] which confirmed the earlier hypothesis of Pfefferle and colleagues that water inhibits oxygen migration and exchange. [7] PdO is accepted as a more active phase than metallic Pd in methane combustion, [8,34] or it is at least required in pair with Pd(0) for CÀ H activation. [35] Since the presence of water and nature of the support were suggested to influence the onset and the rate of Pd(0) reoxidation process, [5] we reduced the studied calcined PdO/Al 2 O 3 and PdO/SnO 2 catalysts prior to the in situ XAS analysis.…”

“…Indeed, PdO is accepted as a more active phase than metallic Pd in methane combustion. [8,34,41,42] One should keep in mind, however, that the Pd (re)-oxidation is kinetically limited, and both the kinetics and thermodynamics are particle-size dependent. [41,43,44] A recent operando XAS study of Pd/Al 2 O 3 catalysts with monodispersed Pd particles (3-16 nm range) in wet methane combustion [41] confirmed that at least partially oxidized surface is required for methane combustion.…”

“…0.1 % CH 4 /N 2 and dry air were used at flow rates of 154 and 103 mL/min respectively, the dry air flowed through two bubblers to saturate it before mixing with the methane mix. [34] Temperature intervals generally started at 100°C and were increased by 50°C until 450°C. The ramping rate was 10°C/min, and once at the respective temperature the system was allowed to equilibrate for 10 minutes before measurements.…”

Designing Nanoparticles and Nanoalloys for Gas-Phase Catalysis with Controlled Surface Reactivity Using Colloidal Synthesis and Atomic Layer Deposition

“…PtPd bimetallic catalysts are among the most important methane combustion catalysts. The combined use of Pt and Pd can lead to PtPd alloy formation, for which both positive [112,113] and negative [114,115] effects on combustion activity have been reported. Qu et al [116] used the level of control granted by colloidal methods to devise a strategy to synthesize a PtPd/Mg-Al 2 O 3 catalyst without PtPd alloy formation.…”