Understanding of Active Sites and Interconversion of Pd and PdO during CH4 Oxidation

Tuning Pd species via electronic metal-support interaction for methane combustion

Utilizing catalytic combustion to convert methane (CH4) into CO2 and H2O stands as one of the most effective approaches for mitigating unburnt CH4 emissions from natural gas engines. Supported Pd catalysts have been extensively researched for their role in low-temperature CH4 combustion, with their catalytic activity greatly influenced by metal-support interactions. Surface interaction Pd phases, as a special type of Pd species originating from metal-support interactions on supported Pd catalysts, show controversial catalytic performance in CH4 combustion. Moreover, the impact of electronic metal-support interactions (EMSI, which refers to metal-support interactions associated with electron transfer) remains unclear. Hence, we opted for Ce-Zr solid solutions with different Ce:Zr molar ratios as supports and synthesized a range of supported Pd catalysts with varying EMSI intensities. Characterization revealed that as the oxygen vacancy concentration on the support increased, electron transfer weakened, leading to a higher Pd-O-Ce content, resulting in a lower CH4 activation barrier and better catalytic performance. This study offers a promising approach for regulating EMSI and active Pd species on supported catalysts in practical applications.

Ultrasmall Pd Clusters in FER Zeolite Alleviate CO Poisoning for Effective Low-Temperature Carbon Monoxide Oxidation

Ultrasmall Pd4 clusters form in the micropores of FER zeolite during low-temperature treatment (100 °C) in the presence of humid CO gas. They effectively catalyze CO oxidation below 100 °C, whereas Pd nanoparticles are not active as they are poisoned by CO. Using catalytic measurements, infrared (IR) spectroscopy, X-ray absorption spectroscopy (EXAFS), microscopy, and density functional theory calculations, we provide the molecular-level insight into this previously unreported phenomenon. Pd nanoparticles get covered with CO at low temperatures, which effectively blocks O2 activation until CO desorption occurs. Small Pd clusters in zeolites, in contrast, demonstrate fluxional behavior in the presence of CO, which significantly increases the affinity for binding O2. Our study provides a pathway to achieve low-temperature CO oxidation activity on the basis of a well-defined Pd/zeolite system.

Identifying the Structure of Supported Metal Catalysts Using Vibrational Fingerprints from Ab Initio Nanoscale Models

Identifying active sites of supported noble metal nanocatalysts remains challenging, since their size and shape undergo changes depending on the support, temperature, and gas mixture composition. Herein, the anharmonic infrared spectrum of adsorbed CO is simulated using density functional theory (DFT) to gain insight into the nature of Pd nanoparticles (NPs) supported on ceria. The authors systematically determine how the simulated infrared spectra are affected by CO coverage, NP size (0.5-1.5 nm), NP morphology (octahedral, icosahedral), and metal-support contact angle, by exploring a diversity of realistic models inspired by ab initio molecular dynamics. The simulated spectra are then used as a spectroscopic fingerprint to characterize nanoparticles in a real catalyst, by comparison with in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) experiments. Truncated octahedral NPs with an acute Pd-ceria angle reproduce most of the measurements. In particular, the authors isolate features characteristic of CO adsorbed at the metal-support interface appearing at low frequencies, both seen in simulation and experiment. This work illustrates the strong need for realistic models to provide a robust description of the active sites, especially at the interface of supported metal nanocatalysts, which can be highly dynamic and evolve considerably during reaction.