ACS Catalysis 最新文献

Light-dependent reduction of carbon dioxide (CO₂) can be developed using nonexpensive and abundant molecular catalysts and inorganic photosensitizers based on nonnoble metals. The photoreduction of CO₂ catalyzed by a series of 11 metal-salophen complexes, based on variously functionalized salophen ligands, has been investigated using a Cu-based photosensitizer, [Cu^I(bathocupoine)(xantphos)], for light harvesting. This provides one of the currently few fully earth-abundant systems for efficient CO₂ reduction driven by visible light. Using 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) as the sacrificial reductant in acetonitrile/triethanolamine solution, a maximum turnover number for CO production of 900–1600, a maximum initial turnover frequency of 1300–1700 h^–1 with 93–96% CO/H₂ selectivity, and a high quantum yield of 12–15% (at 420 nm) were achieved with Fe-based complexes. Thorough photophysical studies coupled to DFT calculations allowed tracking of reaction intermediates and provided insights into the reaction mechanism.

Single-atom catalysis is attractive in the context of sustainable chemistry, but single-atom catalysts (SACs) are not always more active than corresponding clusters or nanoparticles. This is the case, inter alia, of CO oxidation on Pt/γ-Al₂O₃, an archetypal catalytic system where SACs are poorly active. In the present work, combining diffuse reflectance infrared spectroscopy experiments and density functional theory calculations, we identify the stable species formed on a Pt/γ-Al₂O₃ SAC compared to its nanocatalyst counterpart. Formates predominantly occupy the alumina support sites, while oxidized Pt₁ species can stabilize carbonyl, carbonate, and bicarbonate species, depending on the temperature regime. Coadsorption of carbonyl and carbonate moieties on the same platinum atom is found likely, based on both experimental and thermodynamic arguments. Unlike the mild adsorption of CO on Pt clusters, allowing for efficient CO oxidation, carbonyl and carbonate species exhibit high stability on the single Pt atoms, which can explain the low activity of the SAC.

Carbon nitride polymers (CNPs) have drawn broad interdisciplinary attention in the arena of solar energy conversion. However, serious charge carrier recombination caused by intrinsic electron–hole Coulomb interaction remains a fundamental and long-standing challenging scientific problem in the CNP photosystem. In this work, we synthesized small-sized CNP (denoted SS-CNP) and constructed an aggregated SS-CNP photosystem by noncovalent self-assembly. The structures, photophysical properties, and photocatalytic activity of SS-CNP aggregates have been carefully analyzed by various characterization methods. Results confirm that the weak noncovalent interactions endow the SS-CNP aggregates with the ability to undergo a continuous change in their structure and thus result in spontaneous symmetry breaking. The spontaneous symmetry breaking with uneven charge distribution of SS-CNP aggregates enables the establishment of a built-in electric field at the interfaces of aggregates, which accelerates charge separation and prolongs charge lifetime. Impressively, the SS-CNP aggregates realize a record-high apparent quantum yield of 76.4% at 420 nm, which is much higher than those of the existing CNP photosystems. The discovery and insights provided in this work are expected to provide some clues for manipulating charge separation and advancing the in-depth understanding of the role of asymmetric aggregation of photocatalysts during photoredox reactions.

Ammonia borane-based transfer hydrogenation mechanisms on copper nanoparticles (CuNPs) are identified and assessed by isotope labeling and Kohn–Sham density functional methods, using the hydrogenation of styrene to ethylbenzene under ambient conditions as the model reaction. The key role of protonic solvents in permitting ammonia borane decomposition is confirmed. Different dehydrogenation pathways are evidenced for the N–H and B–H bonds: while the metal surface always acts as an intermediary in the hydrogen transfer from the B–H bond to the organic substrate, the N–H bond can directly hydrogenate the most negatively charged carbon atom of the unsaturated bond. The styrene to ethylbenzene reaction is here proved to have a >99% conversion with 100% selectivity at ambient conditions, using methanol and pure water as the solvents. The CuNPs are obtained in situ by reduction of the copper source, SION-X (Cu₂[(BO)(OH)₂](OH)₃), by ammonia borane. The catalytic properties of these CuNPs are stable for at least 5 cycles without the need for reduction steps and upon their exposure to air in between subsequent cycles. This is due to ammonia borane’s ability to act simultaneously as the hydrogen source for the reaction and as the reducing agent of copper. Ammonia borane shows then a significant advantage over other hydrogen sources for transfer hydrogenation in combination with CuNPs, eliminating both the catalyst preparation and activation steps and reducing the complexity and operational cost of the process.

The Grignard reagents generated from 2-(1,5-dialkoxypent-3-yl)aryl bromides were treated with a chiral rhodium catalyst with either a segphos or binap ligand to give high yields of 3-aryl-5-alkoxy-1-pentenes with high enantioselectivity (up to 99% ee). Based on deuterium labeling studies, it is proposed that the catalytic cycle consists of (1) transmetalation of the Grignard reagent to a RO-Rh catalyst generating an aryl-Rh intermediate, (2) a 1,4-shift of Rh from aromatic carbon to one of the two homobenzylic carbons, which is not an enantioselectivity-determining step, (3) migration of Rh to a β-alkoxyalkyl position ready for selective β-alkoxy elimination through a sequence of β-hydrogen elimination/hydrorhodations, and (4) β-alkoxy elimination resulting in the formation of an enantioenriched elimination product.

Ammonia (NH₃) has the potential to be a hydrogen carrier because it can be transported and stored with ease, but only if it also can be decomposed easily when needed. Understanding how to control the frequently rate-limiting N–H bond breaking and N–N bond forming on catalytic surfaces may help design efficient means for NH₃ decomposition. Yuan et al. recently demonstrated photocatalytically selective N–H bond breaking in NH₃ on plasmon-driven aluminum–palladium (Al–Pd) antenna–reactor heterostructures [Yuan et al. ACS Nano 2022, 16 (10), 17365]. Using embedded correlated wavefunction (ECW) theory, we predict that the rate-determining step (RDS) for NH₃ decomposition on Pd(111) via thermocatalysis (dissociating the first N–H bond, *NH₃ → *NH₂ + *H, in the ground state, where * means adsorbed) differs from that via photocatalysis (dissociating the second N–H bond, *NH₂ → *NH + *H, in the excited state). This result is consistent with the measured catalytic efficiency and selectivity of NH₃−deuterium (D₂) exchange reactions (an indirect way to measure N–H bond breaking) on Al–Pd heterodimers. We also determine the origin of the observed selectivity of thermocatalysis and photocatalysis on Pd(111) toward doubly deuterated (NHD₂) and monodeuterated (NH₂D) products, respectively, and explore viability of the full NH₃ decomposition path, also via ECW theory. Additionally, we predict that the associative desorption of *N as N₂ from Pd(111) is extremely difficult in thermocatalysis at least at low surface coverages; metal-to-adsorbate hole transfer in photocatalysis stabilizes the transition state for the first N–H bond dissociation, shifting the RDS to the second N–H bond breaking. Furthermore, the redistribution of electrons around *N upon excitation reduces the electron density in the Pd–N bonds, which may lower the barrier for N₂ associative desorption in photocatalysis. Thus, light-induced, plasmon-mediated, excited-state hole transfer may provide an efficient mechanism to accelerate NH₃ decomposition.