A Perspective on Heterogeneous Catalysts for the Selective Oxidation of Alcohols.

Selective oxidation of higher alcohols using heterogeneous catalysts is an important reaction in the synthesis of fine chemicals with added value. Though the process for primary alcohol oxidation is industrially established, there is still a lack of fundamental understanding considering the complexity of the catalysts and their dynamics under reaction conditions, especially when higher alcohols and liquid-phase reaction media are involved. Additionally, new materials should be developed offering higher activity, selectivity, and stability. This can be achieved by unraveling the structure-performance correlations of these catalysts under reaction conditions. In this regard, researchers are encouraged to develop more advanced characterization techniques to address the complex interplay between the solid surface, the dissolved reactants, and the solvent. In this mini-review, we report some of the most important approaches taken in the field and give a perspective on how to tackle the complex challenges for different approaches in alcohol oxidation while providing insight into the remaining challenges.

Photocatalytic Oxidation of α-C-H Bonds in Unsaturated Hydrocarbons through a Radical Pathway Induced by a Molecular Cocatalyst.

To improve the photocatalytic oxidation of α-C-H bonds in unsaturated hydrocarbons, N-hydroxyphthalimide (NHPI) was used as a molecular cocatalyst with CdS as the photoabsorber. Compared with previously reported photocatalysts involving solid cocatalysts, metal-free NHPI offers better sustainability in addition to the significantly enhanced performance as cocatalyst. The photogenerated holes were transferred into the more active phthalimide-N-oxyl radical (PINO) by reacting with NHPI. In this way, α-C-H bond oxidation was significantly improved through the activation by PINO; even for the sluggish toluene oxidation, the apparent quantum efficiency was as high as 36.5 %. The effects of substrates/NHPI concentration ratio, reaction temperature, and time as well as the reaction intermediates were comprehensively studied. It was possible to identify ketones/aldehydes as the primary products, and overoxidation was controlled by adjusting the substrates/NHPI concentration ratio and reaction time. Thus, the radical path induced by the NHPI-PINO redox pair is an efficient alternative to boost the sluggish photocatalytic oxidation of α-C-H bonds.

Anaerobic Alcohol Conversion to Carbonyl Compounds over Nanoscaled Rh-Doped SrTiO3 under Visible Light.

Photocatalytic oxidation of organic compounds on semiconductors provides a mild approach for organic synthesis and solar energy utilization. Herein, we identify the key points for the photocatalytic oxidation over Pt-loaded Rh-doped strontium titanate allowing the conversion of alcohols efficiently and selectively to aldehydes and ketones under anaerobic conditions and visible light with an apparent quantum efficiency of pure benzyl alcohol oxidation at 420 nm of ≤49.5%. Mechanistic investigations suggest that thermodynamically the controlled valence band edge position via Rh doping provides a suitable oxidation ability of photogenerated holes, avoiding the powerful hydroxyl radical intermediates prone to overoxidation resulting in high selectivity. Kinetically, oxygen vacancies induced by Rh3+ substitution in the SrTiO3 lattice not only favor the dissociative adsorption of alcohols yielding alkoxy species but also induce the weakening of the α-C-H bond facilitating its cleavage by the photogenerated holes. Pt nanoparticles deposited as a cocatalyst contribute to the final hydrogen evolution.

Decoupling the Effects of High Crystallinity and Surface Area on the Photocatalytic Overall Water Splitting over ß-Ga2 O3 Nanoparticles by Chemical Vapor Synthesis.

Chemical vapor synthesis (CVS) is a unique method to prepare well-defined photocatalyst materials with both large specific surface area and a high degree of crystallinity. The obtained ß-Ga2 O3 nanoparticles were optimized for photocatalysis by reductive photodeposition of the Rh/CrOx co-catalyst system. The influence of the degree of crystallinity and the specific surface area on photocatalytic aqueous methanol reforming and overall water splitting (OWS) was investigated by synthesizing ß-Ga2 O3 samples in the temperature range from 1000 °C to 1500 °C. With increasing temperature, the specific surface area and the microstrain were found to decrease, whereas the degree of crystallinity and the crystallite size increased. Whereas the photocatalyst with the highest specific surface area showed the highest aqueous methanol reforming activity, the highest OWS activity was that for the sample with an optimum ratio between high degree of crystallinity and specific surface area. Thus, it was possible to show that the facile aqueous methanol reforming and the demanding OWS have different requirements for high photocatalytic activity.

Photodeposition of copper and chromia on gallium oxide: the role of co-catalysts in photocatalytic water splitting.

Split second: The photocatalytic activity of gallium oxide (ß-Ga2 O3) depends strongly on the co-catalysts CuOx and chromia, which can be efficiently deposited in a stepwise manner by photoreduction of Cu(2+) and CrO4 (2-). The water-splitting activity can be tuned by varying the Cu loading in the range 0.025-1.5 wt %, whereas the Cr loading is not affecting the rate as long as small amounts (such as 0.05 wt %) are present. Chromia is identified as highly efficient co-catalyst in the presence of CuOx : it is essential for the oxidation of water.

Enhanced photocatalytic hydrogen generation from barium tantalate composites.

(111)-layered Ba5Ta4O15 photocatalysts were synthesised by a solid state reaction route and a citrate synthesis route, and their structural and electronic properties were investigated. After citrate route preparation, the presence of a second phase, namely Ba3Ta5O15, was determined by X-ray powder diffraction and absorption spectroscopy. The existence of this phase had a profound effect on the photocatalytic activity of this Ba5Ta4O15/Ba3Ta5O15 composite in comparison to the pure Ba5Ta4O15 materials. The photocatalytic performance of the barium tantalates was evaluated by investigating the capability in ˙OH radical formation and hydrogen generation. Strongly increased hydrogen evolution rates for the Ba5Ta4O15/Ba3Ta5O15 composite, up to 160% higher than for the pure Ba5Ta4O15, were determined, and only very small amounts of Rh co-catalyst, deposited on the photocatalysts by stepwise reductive photo-deposition, were needed to achieve these results.

Optimizing the deposition of hydrogen evolution sites on suspended semiconductor particles using on-line photocatalytic reforming of aqueous methanol solutions.

The deposition of hydrogen evolution sites on photocatalysts is a crucial step in the multistep process of synthesizing a catalyst that is active for overall photocatalytic water splitting. An alternative approach to conventional photodeposition was developed, applying the photocatalytic reforming of aqueous methanol solutions to deposit metal particles on semiconductor materials such as Ga2O3 and (Ga0.6 Zn0.4)(N0.6O0.4). The method allows optimizing the loading of the co-catalysts based on the stepwise addition of their precursors and the continuous online monitoring of the evolved hydrogen. Moreover, a synergetic effect between different co-catalysts can be directly established.