CH₄ Oxidation Activity in Pd and Pt–Pd Bimetallic Catalysts: Correlation with Surface PdO_x Quantified from the DRIFTS Study

Abstract

For Pd-based catalysts, facile and fast interconversion between Pd and PdO occurs continuously during the CH 4 oxidation reaction, which makes it challenging to determine active sites. Herein, we report that the amount of partially oxidized palladium (PdO x ) on the catalyst surface shows a linear correlation with the CH 4 oxidation activity in a series of Pd/Al 2 O 3 and Pt− Pd/Al 2 O 3 catalysts hydrothermally aged under commercially relevant conditions. We characterized the amount of surface PdO x through diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) using CO as a probe molecule. With careful consideration that the redox cycle of Pd particles occurs continuously during the CH 4 oxidation reaction, the Pd/Al 2 O 3 catalysts were reoxidized before obtaining the CO adsorption DRIFTS data to minimize the discrepancy between the catalytically relevant phases and the characterized surface composition. The IR spectra of CO adsorption on reoxidized Pd/Al 2 O 3 catalysts contain peaks at 2135−2145 cm −1 , which correspond to PdO x . The steady-state CH 4 oxidation activities at 300 °C increased proportionally with the normalized surface PdO x . Furthermore, Pt−Pd bimetallic catalysts also showed the same linear correlation between the surface PdO x and CH 4 oxidation activity irrespective of composition, preparation method, and support. Our results indicate that the surface PdO x plays a critical role in determining CH 4 oxidation activity rather than the electronic properties of Pd. Overall, we report a general correlation between the amount of surface PdO x and the steady-state CH 4 oxidation activities in various Pd-based catalysts. This work will greatly help in achieving the fundamental understanding of the CH 4 oxidation reaction on the PdO x surface and the further development of Pd-based catalysts for CH 4 oxidation with better activity.

Effects of Treatment Conditions on Pd Speciation in CHA and Beta Zeolites for Passive NOx Adsorption

“…Surprisingly, even trace water present within the Ar and air cylinders employed has an observable effect on Pd-speciation, the spectrum in which untreated argon is used strongly resembling that presented by Zheng et al The high frequency bands near 2200 cm –1 are the most strongly affected, disappearing in the wetted gas feed and becoming dominant in the rigorously dried feed. In fact, when all water is removed, no metallic Pd 0 –CO bands are observed on 1.4% Pd-Beta, showing that water can either reduce ion-exchanged palladium associated with the bands near 2200 cm –1 and/or leads to the formation of PdO particles whose surfaces are reduced by CO at room temperature. , The absence of similar metallic Pd–CO bands on the Pd–Si-Beta would seem to suggest that the former explanation is more likely; however, lower Pd dispersion on this material, and thus larger PdO particles, could also explain these observations given the ready appearance of PdO–CO bands when PdO is supported on alumina …”

Research Progress and Application of Single-Atom Catalysts: A Review

“…In traditional supported catalysts, only a few metal active components play a catalytic role in the reaction process, and the cost of large-load precious metals is too high, which is not conducive to large-scale development and use. Single-atom catalysts have huge application potential in the rational use of metal resources and the realization of an atomic economy [8]. Because an SAC has a special geometric configuration and electronic structure that is different from traditional catalysts, in particular the valence shell electronic structure of metal atoms, its physical and chemical properties are unique and novel.…”

Reactivity of Pd–MO2 encapsulated catalytic systems for CO oxidation

“…After being exposed to reaction conditions, the three peaks shifted and formed two major peaks centered at 1970 and 2035 cm −1 , corresponding to a-top bound and bridge bound CO on Pd 0 , respectively. 62 The obtained CO DRIFTS results for Pd PEG @ZrO 2 indicate that the rapid oxygen exchange between the Pd surface and ZrO 2 during reaction resulted in similar availability of metal sites and similar CO–Pd interactions, but slightly different CO–ZrO 2 interactions postreaction. Conversely, spectra (v) and (vi) in Fig.…”

“…Both peaks resided in a region corresponding to linearly bound CO on Pd, where the 2076 cm −1 peak is ascribed to atop CO on Pd 0 , and the peak at 2130 cm −1 corresponds to atop CO on Pd + . 62 Interestingly, the peak centered at 2130 cm −1 was observed consistently throughout the experiment, both before and after in situ reaction, but its intensity relative to the 2076 cm −1 peak varied. The changes in relative intensity of the two major peaks under CO oxidation conditions could be explained by the fact that before reaction, the catalyst was mostly reduced and Pd was in a metallic state, in accordance with the prominent peak at 2076 cm −1 .…”

“…3), and ascribed to linearly bound CO on Pd 0 , bridge-bound CO on Pd 0 , and bridge-bound CO on Pd(100). 62,75 These results suggest that Pd species in Pd PEG @ZrO 2 before reaction were completely reduced, with CO binding preferentially to metallic Pd sites. In the case of Pd PEG @CeO 2 (spectrum (iii), Fig.…”