Pentacoordinated Al 3+ ‐Stabilized Active Pd Structures on Al 2 O 3 ‐Coated Palladium Catalysts for Methane Combustion

Formation and Location of Pt Single Sites Induced by Pentacoordinated Al Species on Amorphous Silica-Alumina

Alumina and its mixed oxides are popular industrial supports for emerging supported metal catalysts. Pentacoordinated Al (Al^V) species are identified as key surface sites for anchoring and stabilizing metal single-site catalysts; however, Al^V is rare in conventional amorphous silica-alumina (ASA). Recently, we have developed Al^V-enriched ASA, which was applied as a support for the synthesis of Pt single-site catalysts in this work. Each preparation stage and the interaction between Pt and surface Al species were explored by ¹H and ²⁷Al solid-state nuclear magnetic resonance spectroscopy, and the formation of the dominant Pt single sites on the surface of Al^V-enriched ASA was confirmed by high-angle annular dark-field imaging scanning transmission electron microscopy and energy dispersive spectroscopy line scanning. On the surface of supports without a significant Al^V population (Pt/Al₂O₃ and Pt/SiO₂), mainly Pt nanoparticles were formed. This indicates that Al^V contributes to the strong metal-support interaction to stabilize the Pt single sites on Pt/ASA, which was characterized by diffuse reflectance infrared Fourier transform spectroscopy combined with CO adsorption, X-ray photoelectron spectroscopy, and electron energy loss spectroscopy. Pt single sites supported on Al^V-enriched ASA exhibit excellent chemoselectivity in the hydrogenation of C═O groups, affording 2-3-fold higher yields compared to those of Pt nanoparticles supported on Al₂O₃ and SiO₂.

Pentacoordinated Aluminum Species: New Frontier for Tailoring Acidity-Enhanced Silica-Alumina Catalysts

Silica-alumina catalysts, including zeolites and amorphous silica-aluminas (ASAs), are among the most widely used solid acid catalysts and supports to produce petrochemicals, fine chemicals, and renewable energy. The coordination, distribution, and interactions of aluminum in ASAs have an enormous impact on their acidic properties and catalytic performance. Unsaturated tetracoordinated aluminum (Al^IV) species are commonly accepted as the key sites in generating catalytically active Brønsted acid sites (BASs) in silica-alumina catalysts. Extensive efforts focus on increasing the concentration of Al^IV as the main route to enhance their Brønsted acidity for efficient catalysis. However, increasing the Al^IV concentration either weakens the acid strength in zeolites or lowers Brønsted acidity in ASAs at high Al/Si ratios, impeding acidity enhancement of these popular catalysts."Pentacoordinated aluminum (Al^V) species" are potential unsaturated Al species like Al^IV but rarely observed in silica-aluminas, and thus, are widely considered unavailable for BAS formation or surface reactions. In this Account, we will describe novel strategies for the controlled synthesis of Al^V-enriched ASAs using flame-spray pyrolysis (FSP) techniques and highlight the contribution of Al^V species in acidity enhancement, together with their structure-activity relationship in the conversion of biomass-derived compounds into valuable chemicals. Using various in situ and advanced 2D solid-state NMR (SSNMR) experiments, the studies of the acidic properties and local structure of Al^V-enriched ASAs reveal that Al^V species can highly populate on ASA surfaces, promote BASs formation, and facilitate adaptable tuning of BASs from moderate to zeolitic strength by synergy with neighboring Al sites. Moreover, the BASs with enhanced acidity can work jointly with surface Lewis acid sites or metal active species for bifunctional catalysis on Al^V-enriched ASAs. Compared to zeolites, these Al^V-enriched ASAs are highly active in acid-catalyzed biomass conversion, including alcohol dehydration and sugar conversion reactions, as well as in promoting the performance of supported metal catalysts in chemoselective hydrogenation of aromatic ketones. These new insights provide a state-of-the-art strategy for strongly enhancing the acidity of these popular silica-alumina catalysts, which offers an interesting potential for a wide range of acid and multifunctional catalysis.

Electronic Oxide-Metal Strong Interaction (EOMSI)

Strong metal-support interaction of supported metal catalysts is an important concept to describe the effect of metal-support interactions on the structures and catalytic performances of supported metal particles. By using an example of CeO_x adlayers supported on Ag nanocrystals, herein a concept of electronic oxide-metal strong interaction (EOMSI) is put forward; this interaction significantly affects the electronic structures of oxide adlayers through metal-to-oxide charge transfer. The EOMSI can stabilize oxide adlayers in a low oxidation state under ambient conditions, which individually are not stable; moreover, the oxide adlayers experiencing the EOMSI are resistant to high-temperature oxidation in air to a certain extent. Such an EOMSI concept helps to generalize the strong influence of oxide-metal interactions on the structures and catalytic performance of oxide/metal inverse catalysts, which have been attracting increasing attention.

A Hydrothermally Stable Irreducible Oxide-Modified Pd/MgAl2 O4 Catalyst for Methane Combustion

Catalytic combustion is promising in removing trace amounts of CH₄ to address serious environmental concerns. Supported Pd-based catalysts are most effective but often suffer from low stability in applications owing to the water-vapor-induced sintering. Herein, we develop a universal strategy to prepare irreducible-oxide-modified Pd/MgAl₂ O₄ catalysts which show high activity and excellent stability against both hydrothemal aging at elevated temperatures and deactivation in long-term reaction under wet conditions. The addition of irreducible oxides inhibited the deep oxidation of Pd in the oxygen-rich conditions, which preserved not only the epitaxial structure but also a suitable active phase of Pd-PdO_x on MgAl₂ O₄ , thus promoting both activity and stability. This work provides new insights into the effect of metal-oxide interaction on CH₄ combustion and offers an avenue to design hydrothermally stable and active combustion catalysts for industrial applications.