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Current transition alumina catalysts require the presence of significant amounts of toxic, environmentally deleterious dopants for their stabilization. Herein, we report a simple and novel strategy to engineer transition aluminas to withstand aging temperatures up to 1200 °C without inducing the transformation to low-surface-area α-Al₂O₃ and without requiring dopants. By judiciously optimizing the abundance of dominant facets and the interparticle distance, we can control the temperature of the phase transformation from θ-Al₂O₃ to α-Al₂O₃ and the specific surface sites on the latter. These specific surface sites provide favorable interactions with supported metal catalysts, leading to improved metal dispersion and greatlymore » enhanced catalytic activity for hydrocarbon oxidation. Further, the results presented herein not only provide molecular-level insights into the critical factors causing deactivation and phase transformation of aluminas but also pave the way for the development of catalysts with improved activity for catalytic hydrocarbon oxidation.« less

The analogy between single-atom catalysts (SACs) and molecular catalysts predicts that the specific catalytic activity of these systems is constant. Here we provide evidence that this prediction is not necessarily true. As a case in point, we show that the specific activity over ceria-supported single Pd atoms linearly increases with metal atom density, originating from the cumulative enhancement of CeO₂ reducibility. The long-range electrostatic footprints (≈1.5 nm) around each Pd site overlap with each other as surface Pd density increases, resulting in an observed deviation from constant specific activity. These cooperative effects exhaust previously active O atoms above a certainmore » Pd density, leading to their permanent removal and a consequent drop in reaction rate. The findings of our combined experimental and computational study show that the specific catalytic activity of reducible oxide-supported single-atom catalysts can be tuned by varying the surface density of single metal atoms.« less

Herein, a detailed analysis was carried out using high-field (19.9 T) ²⁷Al magic-angle spinning (MAS) nuclear magnetic resonance (NMR) on three specially prepared aluminum oxide samples where the γ-, δ-, and θ-Al₂O₃ phases are dominantly expressed through careful control of the synthesis conditions. Specifically, two-dimensional (2D) multiquantum (MQ) MAS ²⁷Al was used to obtain high spectral resolution, which provided a guide for analyzing quantitative 1D ²⁷Al NMR spectra. Six aluminum sites were resolved in the 2D MQ MAS NMR spectra, and seven aluminum sites were required to fit the 1D spectra. A set of octahedral and tetrahedral peaks with well-definedmore » quadrupolar line shapes was observed in the θ-phase dominant sample and was unambiguously assigned to the θ-Al₂O₃ phase. The distinct line shapes related to the θ-Al₂O₃ phase provided an opportunity for effectively deconvoluting the more complex spectrum obtained from the δ-Al₂O₃ dominant sample, allowing the peaks/quadrupolar parameters related to the δ-Al₂O₃ phase to be extracted. The results show that the δ-Al₂O₃ phase contains three distinct AlO sites and three distinct AlT sites. This detailed Al site structural information offers a powerful way of analyzing the most complex γ-Al₂O₃ spectrum. It is found that the γ-Al₂O₃ phase consists of Al sites with local structures similar to those found in the δ-Al₂O₃ and θ-Al₂O₃ phases albeit with less ordering. Spin–lattice relaxation time measurement further confirms the disordering of the lattice. Collectively, this study uniquely assigns ²⁷Al features in transition aluminas, offering a simplified method to quantify complex mixtures of aluminum sites in transition alumina samples.« less

Catalytic activity and selectivity of n -hexane reforming are changed significantly by the surface acidic properties of the alumina support following halogen treatment.