Search

Ammonia synthesis remains one of the most important catalytic processes since it enables efficient hydrogen storage and provides the basis for the production of fertilizers. Herein, complementary bulk and local analytical techniques were combined to investigate the effect of selected promoters (Al, K, Ca) on the reduction of wuestite into α-iron and their catalytic performance for ammonia synthesis. The use of promoters appears to have a positive effect on the wuestite-derived catalyst in ammonia synthesis. The promoters seemingly act as a binder for wuestite grains and impede the reduction and disproportionation events of wuestite precursors resulting in an increased catalytic performance. This effect is associated with an increase of surface area and mesoporosity. The study delivers new insights into the interplay of structure and promoters in wuestite-based catalysts.

The influence of oxygen poisoning on a state-of-the-art multipromoted iron-based industrial catalyst for ammonia synthesis as well as the effectivity of different gas purification methods to prevent oxygen poisoning for experiments on laboratory scale were studied in detail. Additionally, the observed results were compared to a common oxygen poisoning test from literature, which on the one hand confirmed its usability in a wide range of conditions, but on the other hand also demonstrated the limitations of this test.

In this work, composites with different amounts of graphene oxide (GO) and the copper-based metal–organic frameworks (HKUST-1) were synthesized. They have been applied in chemiluminescence (CL) as catalysts using the luminol–H2O2 system as a CL model for the first time. The results showed that the well-dispersed GO@HKUST-1 (GMs) greatly improved the CL intensity of the luminol–H2O2 system, up to 359-fold. Based on ascorbic acid (AA) inhibiting the CL of the luminol–H2O2–GMs system, a flow injection CL method for monitoring AA was developed. Under the optimized conditions, a good linear relationship was obtained from 0.001 μM to 10 μM, and the detection limit was 0.35 nM. Moreover, the method was successfully used to detect AA in commercial food and fruit juices with satisfactory results. Due to their fascinating features, the GMs would offer a suitable catalytic platform for CL systems and provide potential promise for the construction of CL sensors.

In this paper, a simple one-pot hydrothermal procedure to create three-dimensional (3D) graphene/multiwalled carbon nanotubes/Pd (G/MWCNTs/Pd) composite hydrogels with a unique porous nanostructure was reported. During the formation of the G/MWCNTs/Pd composites, 2D graphene sheets and 1D MWCNTs were self-assembled to form interconnected porous microstructures and ultrafine Pd NPs were in situ grown on the 3D carbon-based skeleton simultaneously. In the as-obtained nanocomposites, MWCNTs not only prevented the close restacking of graphene sheets to increase the specific surface area but also provided an additional transport path to the catalyst surfaces which facilitated reactant transport. The G/MWCNTs/Pd composites proved to be an efficient, recyclable, and robust catalyst for the Suzuki cross-coupling reactions under mild aerobic conditions, which was attributed to the 3D macroporous framework with high specific surface area, numerous activation sites, and efficient transport pathways for improving the catalytic performance. Moreover, catalyst separation could be easily achieved by simple filtration, and the catalyst could be reused for at least six runs without significant loss in catalytic activity. Additionally, nearly no Pd species was released from the G/MWCNTs/Pd composites during the catalytic reactions, showing the heterogeneity in the present catalysis system.

A series of ZnO/Cr2O3 catalysts with different Zn:Cr ratios was prepared by coprecipitation at a constant pH of 7 and applied in methanol synthesis at 260–300 °C and 60 bar. The X-ray diffraction (XRD) results showed that the calcined catalysts with ratios from 65:35 to 55:45 consist of ZnCr2O4 spinel with a low degree of crystallinity. For catalysts with Zn:Cr ratios smaller than 1, the formation of chromates was observed in agreement with temperature-programmed reduction results. Raman and XRD results did not provide evidence for the presence of segregated ZnO, indicating the existence of Zn-rich nonstoichiometric Zn–Cr spinel in the calcined catalyst. The catalyst with Zn:Cr = 65:35 exhibits the best performance in methanol synthesis. The Zn:Cr ratio of this catalyst corresponds to that of the Zn4Cr2(OH)12CO3 precursor with hydrotalcite-like structure obtained by coprecipitation, which is converted during calcination into a nonstoichiometric Zn–Cr spinel with an optimum amount of oxygen vacancies resulting in high activity in methanol synthesis. Density functional theory calculations are used to examine the formation of oxygen vacancies and to measure the reducibility of the methanol synthesis catalysts. Doping Cr into bulk and the (10–10) surface of ZnO does not enhance the reducibility of ZnO, confirming that Cr:ZnO cannot be the active phase. The (100) surface of the ZnCr2O4 spinel has a favorable oxygen vacancy formation energy of 1.58 eV. Doping this surface with excess Zn charge-balanced by oxygen vacancies to give a 60% Zn content yields a catalyst composed of an amorphous ZnO layer supported on the spinel with high reducibility, confirming this as the active phase for the methanol synthesis catalyst.