Ageing induced improvement of methane oxidation activity of Pd/YFeO₃

Abstract

The ageing characteristics of flame-made 2 wt% Pd supported on YFeO 3 were analysed in comparison with a Pd/Al 2 O 3-CeO 2-ZrO 2 three-way catalyst (TWC) with respect to structural changes and catalytic performance for methane oxidation under stoichiometric reaction conditions. Thermal treatment under lean conditions (air, 900°C) resulted in slight decrease in the methane oxidation activity of the TWC. In marked contrast, YFeO 3-supported Pd catalysts exhibit an increase in activity after such treatment. Activity enhancement is even higher when the treatment was performed under stoichiometric conditions (air-fuel equivalence ratio, λ = 1, 900°C). To explain this observation, in-depth characterization (BET, STEM, OSCC, XAS, and CO chemisorption) of fresh and aged catalysts was performed. Both thermal and stoichiometric ageing cause a severe sintering of the support particles and the phase transformation from hexagonal to orthorhombic YFeO 3. Despite the absence of a mixed Pd-YFeO 3 phase, the growth of Pd particles appears to be limited under the λ = 1 atmosphere. In contrast to thermally aged catalysts where large PdO particles are formed, well-defined metallic Pd nanoparticles of 10-20 nm are present after stoichiometric ageing along with higher methane oxidation activity. Although it is tempting to conclude that metallic Pd is active for methane oxidation under the given conditions, reversible and periodic partial oxidation of the large metallic particles is observed in modulation excitation high energy X-ray diffraction (HXRD) experiments designed to simulate the oscillating redox conditions experienced during operation. These results indicate that large Pd particles exhibit improved methane oxidation activity but equally confirm that activity under stoichiometric conditions is the result of a delicate equilibrium dictated by the bulk-Pd/surface-PdO pair.

Beneficial effect of adding γ‐AlOOH to the γ‐Al2O3 washcoat of a PdO catalyst for methane oxidation

“…Pd catalysts used for CH 4 oxidation are known to deactivate in the presence of H 2 O and the effect of H 2 O decreases with increased temperature, which is in agreement with the TOS and TPO results of this study. The structural collapse of the catalyst support, sintering and PdO conversion to Pd have all been stated as the reason for the deactivation of Pd catalysts . Kucharczyk et al studied the effect of adding La 2 O 3 and SiO 2 or ZrSiO 4 to the washcoat of a PdO/γ‐Al 2 O 3 catalyst and reported that these promoter oxides improved the activity and stability and enhanced the thermal resistance of the PdO/γ‐Al 2 O 3 catalysts after aging at 1060°C for 24 hours.…”

Frequency analysis for modulation-enhanced powder diffraction

“…One is a Fourier analysis (FA) that gives unambiguous results if the perturbation is periodic and the response is linear (Chernyshov et al, 2011). In practice, an approach called phase-sensitive detection (PSD) has been applied for analysis of diffraction data (Caliandro et al, , 2013Ferri et al, 2014;Lu et al, 2014). In terms of Fourier harmonics A 2Â ½n considered above, it assumes the following integral transformation:…”

“…MED can also serve as a tool that helps to untangle the interference contribution in the detected intensity and, therefore, provides partial information on phases of complex structural amplitudes. So far, a quantitative analysis of frequency spectra of the diffraction response is limited to the Bragg intensities and is only focused on the second harmonics, whereas some promising applications of MED for powder diffraction show a strong and interesting response on the first harmonic Lu et al, 2014). There are no mathematical formulae that could be used to quantify the response for realistic conditions when both structure factors and unit-cell dimensions are varied; this is why many interesting observations are doomed to be discussed only qualitatively.…”

Development of Combustion Technology for Methane Emitted from Coal‐Mine Ventilation Air Systems

“…Ar ecent articler eported that support phase transformation and particle sintering were found after thermal (at 900 8Ci na ir) and stoichiometric( at 900 8C, air/fuel equivalent ratio = 1) aging over aP d/YFeO 3 three-way catalyst. [104] Them echanism of catalyst deactivation has been recently investigated under simulated VA Mg as conditions,i np articular. [88] Catalyst deactivation is initially caused by palladium particle migration, followed by particle growth, and changes in support structure.T his is more prominent in the presence of water vapor.…”