Spectroscopy and photochemistry of copper nitrate clusters

Rational Design of Mesoporous CuO-CeO2 Catalysts for NH3-SCR Applications Guided by Multiple In Situ Spectroscopies

Efficient nontoxic catalysts for low-temperature NH₃ selective catalytic reduction (NH₃-SCR) applications are of great interest. Owing to their promising redox and low-temperature activity, we prepared CuO-CeO₂ catalysts on a mesoporous SBA-15 support using targeted solid-state impregnation (SSI), guided by multiple in situ spectroscopy. The use of template P123 allowed dedicated modification of the surface properties of the SBA-15 matrix, resulting in a changed reactivity behavior of the metal precursors during the calcination process. To unravel the details of the transformation of the precursors to the final catalyst material, we applied in situ diffuse reflectance infrared Fourier transform (DRIFT), UV-visible (UV-vis), and Raman spectroscopies as well as online Fourier transform infrared (FTIR) monitoring of the gas-phase composition, in addition to ex situ surface, porosity, and structural analysis. The in situ analysis reveals two types of nitrate decomposition mechanisms: a nitrate-bridging route leading to the formation of a CuO-CeO₂ solid solution with increased low-temperature NH₃-SCR activity, and a hydrolysis route, which facilitates the formation of binary oxides CuO + CeO₂ showing activity over a broader temperature window peaking at higher temperatures. Our findings demonstrate that a detailed understanding of catalytic performance requires a profound knowledge of the calcination step and that the use of in situ analysis facilitates the rational design of catalytic properties.

Spin-Orbit Natural Transition Orbitals and Spin-Forbidden Transitions

Natural transition orbitals (NTOs) are in widespread use for visualizing and analyzing electronic transitions. The present work introduces the analysis of formally spin-forbidden transitions with the help of complex-valued spin-orbit (SO) NTOs. The analysis specifically focuses on the components in such transitions that cause their intensity to be nonzero because of SO coupling. Transition properties such as transition dipole moments are partitioned into SO-NTO hole-particle pairs, such that contributions to the intensity from specific occupied and unoccupied orbitals are obtained. The method has been implemented within the restricted active space (RAS) self-consistent field wave function theory framework, with SO coupling treated by RAS state interaction. SO-NTOs have a broad range of potential applications, which is illustrated by the T₂-S₁ state mixing in pyrazine, spin-forbidden versus spin-allowed 4f-5d transitions in the Tb³⁺ ion, and the phosphorescence of tris(2-phenylpyridine) iridium [Ir(ppy)₃].

Photochemistry and UV/vis spectroscopy of hydrated vanadium cations, V+(H2O)n, n = 1-41, a model system for photochemical hydrogen evolution

Photochemical hydrogen evolution provides fascinating perspectives for light harvesting. Hydrated metal ions in the gas phase are ideal model systems to study elementary steps of this reaction on a molecular level. Here we investigate mass-selected hydrated monovalent vanadium ions, with a hydration shell ranging from 1 to 41 water molecules, by photodissociation spectroscopy. The most intense absorption bands correspond to 3d-4p transitions, which shift to the red from n = 1 to n = 4, corresponding to the evolution of a square-planar complex. Additional water molecules no longer interact directly with the metal center, and no strong systematic shift is observed in larger clusters. Evolution of atomic and molecular hydrogen competes with loss of water molecules for all V+(H2O)n, n ≤ 12. For n ≥ 15, no absorptions are observed, which indicates that the cluster ensemble is fully converted to HVOH+(H2O)n-1. For the smallest clusters, the electronic transitions are modeled using multireference methods with spin-orbit coupling. A large number of quintet and triplet states is accessible, which explains the broad features observed in the experiment. Water loss most likely occurs after a series of intersystem crossings and internal conversions to the electronic ground state or a low-lying quintet state, while hydrogen evolution is favored in low lying triplet states.