UC Santa Barbara

In order to design more effective catalysts, we deployed X-ray absorption spectroscopy (XAS) to develop structure-activity relationships for supported heterogeneous catalysts. Starting from carefully-designed synthesis, the metal ions are uniformly dispersed on the support. The pre-treatment and other synthetic modifications are strategically chosen to alter the structure and reactivity of the metal sites. Specially, we explored catalytic systems in which the active sites are mononuclear metal complexes grafted on the oxide supports. XAS is essential for probing the local structures of the metal centers, such as electronic properties and coordination geometry. The applications of the catalysts include olefin metathesis, propane dehydrogenation and C-O hydrogenolysis. Monomolybdate sites with relatively uniform first coordination spheres were synthesized by a direct solid-solid reaction between MoO2Cl2 and silica. The resulting Mo(=O)2(OSi)2 sites were characterized and compared to the spectra of several well-defined model molybdate compounds. In extended X-ray absorption fine structure (EXAFS) analysis, the attenuation of the long-range EXAFS features at ca. 3 Å in Mo(=O)2(OSi)2 sites suggests the attachment to the silica surface as molybdasiloxane rings of variable ring size. The partially well-defined Mo(=O)2(OSi)2 sites were treated with a few common activation methods, but failed to catalyze propylene metathesis. Only exposure to olefins at high temperatures (>500 C) leads to promoting propylene metathesis at 23 C and resulting in an active site fraction as high as 36%. However, the co-existence of multiple types of metal sites makes it difficult to identify the active sites. GaiBu3/γ-Al2O3 catalyst promotes propane dehydrogenation at 550 °C, and the Ga species present under reaction conditions were unknown. We used operando XAS to monitor the evolution of Ga K-edge from as-prepared to catalytic conditions. The X-ray absorption near edge structure (XANES) reveals that isobutyl ligands are replaced by oxygen-donor ligands derived from the alumina surface in a single, gradual process, leading to a 3-coordinate mononuclear Ga site at 550 °C. Additional, minor Ga(I) species may form simultaneously. DFT-based simulations elucidate the ligand influence on the Ga K-edge XANES features, such as the white line intensity and edge position. This structural insight sheds light on the correlation between the mononuclear Ga site and the subsequent catalytic activity. The direct synthesis of single-atom Fe catalysts supported on nitrogen-doped carbon gives five-coordinate Fe(III), with four in-plane pyridinic nitrogen donors and one axial hydroxyl ligand. These Fe catalysts exhibit high selectivity, activity, and reusability for C–O bond hydrogenolysis using H2 at temperatures as low as 170 °C. The characterization of the catalyst shows the reduction of Fe(III) to Fe(II) sites with loss of the hydroxyl ligand under reaction conditions. The four-coordinate Fe(II) sites are the active sites for H2-splitting. The interaction between metal sites and the support is critical to promote activity.