Stable complete methane oxidation over palladium based zeolite catalysts

Abstract

Increasing the use of natural gas engines is an important step to reduce the carbon footprint of mobility and power generation sectors. To avoid emissions of unburnt methane and the associated severe greenhouse effect of lean-burn engines, the stability of methane oxidation catalysts against steam-induced sintering at low temperatures (<500 °C) needs to be improved. Here we demonstrate how the combination of catalyst development and improved process control yields a highly efficient solution for complete methane oxidation. We design a material based on palladium and hierarchical zeolite with fully sodium-exchanged acid sites, which improves the support stability and prevents steam-induced palladium sintering under reaction conditions by confining the metal within the zeolite. Repeated short reducing pulses enable the use of a highly active transient state of the catalyst, which in combination with its high stability provides excellent performance without deactivation for over 90 h in the presence of steam.

Fig. 1

Catalytic activity of Pd/Al₂O₃ and various Pd/mordenite catalysts. a Light-off curves, b 65 h stability test without regeneration, and c stability test with continuous short pulse regeneration (averaged values including methane slip during regeneration). Conditions: 1 vol% CH₄, 4 vol% O₂, 0 or 5 vol% H₂O, bal. N₂; gas hourly space velocity (GHSV) = 70,000 h⁻¹; T = 415 °C for stability tests; lean operation: 10 min; regeneration (short rich operation): 3 s; d magnification of four cycles with regeneration over Pd/H-MOR and Pd/Na-MOR. Prior to the tests, the catalysts were degreened at 550 °C for 30 min in 1 vol% CH₄, 4 vol% O₂, 95 vol% N₂

Fig. 2

Representative electron microscopy images of calcined and spent catalysts. a, b Pd/H-MOR, c, d Pd/Na-MOR, e, f Pd/H-MOR-BE. Spent catalysts were aged in 1 vol% CH₄, 4 vol% O₂, 5 vol% H₂O, N₂ bal. at 415 °C for 16 h. STEM and secondary electron (SE) images of g, h Pd/Na-MOR and i, j Pd/H-MOR after 90 h and 65 h on stream, respectively, continuous with short pulse regeneration. Scale bar: 20 nm (a–h) and 50 nm (i, j)

Fig. 3

In situ X-ray absorption spectroscopy of catalyst aging. a X-ray absorption near-edge structure (XANES) spectra and b the corresponding Fourier transforms of in situ Pd K-edge extended X-ray absorption fine structure (EXAFS) spectra (non-phase shift corrected) of the catalysts after pretreatment (10 min in 1 vol% CH₄, 4 vol% O₂, bal. N₂, GHSV = 350,000 h⁻¹ at 410 °C) and after 15, 30, and 90 min aging in a feed of 1 vol% CH₄, 4 vol% O₂, 5 vol% H₂O, bal. N₂ at 410 °C: Pd/H-MOR (black to light gray); Pd/Na-MOR (red to pink); bulk PdO (blue); calcined Pd/H-MOR (black); and palladium foil (green). The spectra and Fourier transforms are offset for clarity

Fig. 4

Transient operando EXAFS experiment with oxygen cut-off. a, b Fraction of oxidized palladium in Pd/Na-MOR and Pd/H-MOR and corresponding catalytic activity in 1 vol% CH₄, 4 vol% O₂, 5 vol% H₂O, bal. N₂, GHSV = 350,000 h⁻¹ at 350 °C. Oxygen was removed from the feed to perform reduction of palladium by methane and then added for subsequent reoxidation in reaction conditions. c, d Fourier transforms of in situ Pd K-edge EXAFS spectra (non-phase shift corrected) of Pd/Na-MOR and Pd/H-MOR upon averaging 15 spectra at 1, 90, 210, 280, and 550 s of the experiment. Corresponding coordination numbers are given in Table 2. Initial rates were obtained from fitting the linear function over the first 10 points during reoxidation. e, f, g STEM images of the quenched Pd/H-MOR at 1, 210, and 280 s. Scale bar: 20 nm