Operando Spectroscopy Unveils the Catalytic Role of Different Palladium Oxidation States in CO Oxidation on Pd/CeO 2 Catalysts

Highly Active Oxidation Catalysts through Confining Pd Clusters on CeO2 Nano-Islands

CeO2-supported noble metal clusters are attractive catalytic materials for several applications. However, their atomic dispersion under oxidizing reaction conditions often leads to catalyst deactivation. In this study, the noble metal cluster formation threshold is rationally adjusted by using a mixed CeO2-Al2O3 support. The preferential location of Pd on CeO2 islands leads to a high local surface noble metal concentration and promotes the in situ formation of small Pd clusters at a rather low noble metal loading (0.5 wt %), which are shown to be the active species for CO conversion at low temperatures. As elucidated by complementary in situ/operando techniques, the spatial separation of CeO2 islands on Al2O3 confines the mobility of Pd, preventing the full redispersion or the formation of larger noble metal particles and maintaining a high CO oxidation activity at low temperatures. In a broader perspective, this approach to more efficiently use the noble metal can be transferred to further systems and reactions in heterogeneous catalysis.

Fundamental Structural and Electronic Understanding of Palladium Catalysts on Nitride and Oxide Supports

The nature of the support can fundamentally affect the function of a heterogeneous catalyst. For the novel type of isolated metal atom catalysts, sometimes referred to as single-atom catalysts, systematic correlations are still rare. Here, we report a general finding that Pd on nitride supports (non-metal and metal nitride) features a higher oxidation state compared to that on oxide supports (non-metal and metal oxide). Through thorough oxidation state investigations by X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), CO-DRIFTS, and density functional theory (DFT) coupled with Bader charge analysis, it is found that Pd atoms prefer to interact with surface hydroxyl group to form a Pd(OH)x species on oxide supports, while on nitride supports, Pd atoms incorporate into the surface structure in the form of Pd-N bonds. Moreover, a correlation was built between the formal oxidation state and computational Bader charge, based on the periodic trend in electronegativity.

Influence of Oxygen Vacancy-Induced Coordination Change on Pd/CeO2 for NO Reduction

The byproduct formation in environmental catalysis is strongly influenced by the chemical state and coordination of catalysts. Herein, two Pd/CeO2 catalysts (PdCe-350 and PdCe-800) with varying oxygen vacancies (Ov) and coordination numbers (CN) of Pd were prepared to investigate the mechanism of N2O and NH3 formation during NO reduction by CO. PdCe-350 exhibits a higher density of Ov and Pd sites with higher CN, leading to an enhanced metal-support interaction by electron transformation from the support to Pd. Consequently, PdCe-350 displayed increased levels of byproduct formation. In situ spectroscopies under dry and wet conditions revealed that at low temperatures, the N2O formation strongly correlated with the Ov density through the decomposition of chelating nitro species on PdCe-350. Conversely, at high temperatures, it was linked to the reactivity of Pd species, primarily facilitated by monodentate nitrates on PdCe-800. In terms of NH3 formation, its occurrence was closely associated with the activation of H2O and C3H6, since a water-gas shift or hydrocarbon reforming could provide hydrogen. Both bridging and monodentate nitrates showed activity in NH3 formation, while hyponitrites were identified as key intermediates for both catalysts. The insights provide a fundamental understanding of the intricate relationship among the local coordination of Pd, surface Ov, and byproduct distribution.

Macropore-Size Engineering toward Enhancing the Catalytic Performance of CO Oxidation over Three-Way Catalyst Particles

In recent years, transportation-related air pollution has escalated into a global concern, necessitating the development of a three-way catalyst (TWC) technology to address harmful emissions. However, the efficiency of TWC's performance in mitigating these emissions has been hindered because of limited mass transfer efficiency within their structures. Thus, this study attempted to overcome the existing issue by synthesizing a series of macroporous TWC particles exhibiting various macropore sizes via a template-assisted spray process, aiming to achieve optimal mass transfer efficiency and catalytic performance. The synthesis incorporated various template particles (size of 67-381 nm) to obtain various macroporous structures. Thereafter, these macroporous particles were assessed for their carbon monoxide (CO) oxidation performance, revealing a substantial influence of the macropore size on the catalytic performance of TWC structures. Interestingly, among the investigated samples, those containing the smallest and largest macropores demonstrated the highest CO oxidation performances. Based on these results, a plausible reactant diffusion mechanism was proposed to explain the effect of the macropore size on the diffusion efficiency within the macroporous structures. This work may have significant implications in optimizing the macroporous structure to enhance catalytic performance in the gas purification process.

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.

Highly Selective Activation of C-H Bond and Inhibition of C-C Bond Cleavage by Tuning Strong Oxidative Pd Sites

Improving the product selectivity meanwhile restraining deep oxidation still remains a great challenge over the supported Pd-based catalysts. Herein, we demonstrate a universal strategy where the surface strong oxidative Pd sites are partially covered by the transition metal (e. g., Cu, Co, Ni, and Mn) oxide through thermal treatment of alloys. It could effectively inhibit the deep oxidation of isopropanol and achieve the ultrahigh selectivity (>98%) to the target product acetone in a wide temperature range of 50-200 °C, even at 150-200 °C with almost 100% isopropanol conversion over PdCu_1.2/Al₂O₃, while an obvious decline in acetone selectivity is observed from 150 °C over Pd/Al₂O₃. Furthermore, it greatly improves the low-temperature catalytic activity (acetone formation rate at 110 °C over PdCu_1.2/Al₂O₃, 34.1 times higher than that over Pd/Al₂O₃). The decrease of surface Pd site exposure weakens the cleavage for the C-C bond, while the introduction of proper CuO shifts the d-band center (ε_d) of Pd upward and strengthens the adsorption and activation of reactants, providing more reactive oxygen species, especially the key super oxygen species (O₂^-) for selective oxidation, and significantly reducing the barrier of O-H and β-C-H bond scission. The molecular-level understanding of the C-H and C-C bond scission mechanism will guide the regulation of strong oxidative noble metal sites with relatively inert metal oxide for the other selective catalytic oxidation reactions.

Memory-dictated dynamics of single-atom Pt on CeO2 for CO oxidation

Single atoms of platinum group metals on CeO₂ represent a potential approach to lower precious metal requirements for automobile exhaust treatment catalysts. Here we show the dynamic evolution of two types of single-atom Pt (Pt₁) on CeO₂, i.e., adsorbed Pt₁ in Pt/CeO₂ and square planar Pt₁ in Pt_ATCeO₂, fabricated at 500 °C and by atom-trapping method at 800 °C, respectively. Adsorbed Pt₁ in Pt/CeO₂ is mobile with the in situ formation of few-atom Pt clusters during CO oxidation, contributing to high reactivity with near-zero reaction order in CO. In contrast, square planar Pt₁ in Pt_ATCeO₂ is strongly anchored to the support during CO oxidation leading to relatively low reactivity with a positive reaction order in CO. Reduction of both Pt/CeO₂ and Pt_ATCeO₂ in CO transforms Pt₁ to Pt nanoparticles. However, both catalysts retain the memory of their initial Pt₁ state after reoxidative treatments, which illustrates the importance of the initial single-atom structure in practical applications.