Assessing the Nature of Active Sites on Nanodiamonds as Metal-Free Catalysts for the EB-to-ST Direct Dehydrogenation Using a Catalytic Approach

Folic Acid-Derived Low-dimensional carbons for efficient oxidative dehydrogenation of ethylbenzene

The oxidative dehydrogenation (ODH) of alkane is one of the most attractive routes in alkane production because of its favourable thermodynamic characteristic. Nitrogen-doped nanocarbons have demonstrated great potential in this reaction due to its cost-effective, high catalytic activity and stability. However, the influence of nitrogen on the catalytic properties of carbon materials is poorly understood due to the complexities of surface oxygen and nitrogen functional groups. Here we derive the performance descriptor that account for the nitrogen-dependent carbocatalysis in ODH reaction. To achieve this, we designed a set of nitrogen-doped nanocarbon materials with tunable nitrogen species by hydrothermal carbonization (HTC) treatment of the biomass folic acid (FA), which are applied in ODH of ethylbenzene. Among them, FA-180-1000 catalyst can achieve high ethylbenzene conversion (up to ∼62 %) and styrene selectivity (∼87 %), outperforming other HTC carbon-based catalysts. Structural characterizations and kinetic analyses revealed that nitrogen doping strongly interferes the charge polarization of CO site via electron transfer between CO, and nitrogen (mainly pyridine nitrogen and graphitic nitrogen) thus enhancing the reactivity of CO. Furthermore, the induction period during reaction process can be shortened by applying of sulfuric acid-assisted HTC method for constructing nitrogen-doped carbon catalyst with low crystallinity. The present work provides new insights into the contribution of nitrogen doping to the ODH reaction of carbon nanocatalysts, as well as guidance for the rational design of carbon catalysts for the conversion of hydrocarbons to high-value chemicals.

Identification of Active Sites over Metal-Free Carbon Catalysts for Flue Gas Desulfurization

Carbon-based catalysts have been extensively used for flue gas desulfurization (FGD) and have exerted great importance in controlling SO₂ emissions over the past decades. However, many fundamental details about the nature of the active sites and desulfurization mechanism still remain unclear. Here, we reported the experimental and theoretical identifications of active sites in FGD on carbon catalysts. Temperature-programmed decomposition allowed us to modulate the number of oxygen functional groups on carbon catalysts and to establish its correlation with desulfurization activity. Selective passivation further demonstrated that the ketonic carbonyl (C═O) groups are the intrinsic active sites for FGD reaction. Combined with transient response experiments, quasi-in situ X-ray photoelectron spectroscopy, and density functional theory simulations, it was revealed that desulfurization reaction on carbon catalysts mainly proceeded via the Langmuir-Hinshelwood mechanism, during which the nucleophilic ketonic C═O groups served as active sites for chemically absorbing SO₂ and their adjacent sp²-hybridized carbon atoms dissociatively activated O₂. It also turned out that the formation of H₂SO₄ is the reaction barrier step. The output of this study should not only advance the understanding of desulfurization at the atomic scale but also provide a general guideline for the rational design of efficient carbon catalysts for FGD.

Potential Risk of NH3 Slip Arisen from Catalytic Inactive Site in Selective Catalytic Reduction of NOx with Metal-Free Carbon Catalysts

Ammonia emissions from industrial processes have rapidly increased in the past years. Recent advances have used carbon-based selective catalytic reduction (SCR) technology combined with a reaction-regeneration process to reduce NO_x from sintering flue gas, while NH₃ slip is seldom accounted for in this process. This study demonstrates that although the electrophilic carboxyl groups (-COOH) on metal-free carbon catalysts exhibit strong adsorption toward NH₃, they do not participate in the SCR reaction. As a result of the competitive adsorption of NH₃ in the reaction step, these catalytic inactive carboxyl groups not only prolong the time to the SCR steady state, but also result in the potential risk of NH₃ slip. A linear relationship with the equimolar ratio between carboxyl groups and slipped NH₃ was established in the regeneration steps. The slip of NH₃ could be alleviated by the decomposition of carboxyl groups, and special attention should be paid to the presence of inactive sites with strong NH₃ adsorption on industrial-employed carbon catalysts. In addition to advancing the understanding of the NH₃-SCR mechanism, this work also provides valuable opportunities for the control of ammonia emissions from industrial processes.