Liquid versus gas phase dehydrogenation of formic acid over Co@N-doped carbon materials. The role of single atomic sites

Hydrogen Production from Formic Acid Using KIT-6 Supported Non-Noble Metal-Based Catalysts

The aim of this study is to investigate the activity of KIT-6 supported nickel (Ni) and cobalt (Co) catalysts, and the effect of Co incorporation to the Ni@KIT-6 catalyst in the formic acid (FA) dehydrogenation. Ni and Co are inexpensive and readily available non-noble transition metals that are considered ideal for dehydrogenation reactions due to their high activity against C-C and C-H bond breaking. In this study, KIT-6 supported catalysts were tested for hydrogen production from FA in a conventionally heated packed-bed continuous-flow system. N2 adsorption-desorption isotherms of the samples were found to be consistent with Type-IV according to the International Union of Pure and Applied Chemistry (IUPAC) classification. The introduction of metal loading resulted in the preservation of the mesoporous structure of the support material. X-ray diffraction (XRD) patterns of the catalysts exhibited the characteristic amorphous silica structure. Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFT) analysis, Lewis acidity of Co-based catalysts was found to be higher than the Ni-based catalysts. The complete formic acid conversion was observed at 200-350 °C. The highest H2 selectivity was obtained with the 3Ni@KIT-6 catalyst. The Co-based catalysts exhibited relatively lower catalytic activity, which was linked to increased coke formation within these catalysts.

Efficient Atomically Dispersed Co/N-C Catalysts for Formic Acid Dehydrogenation and Transfer Hydrodeoxygenation of Vanillin

Atomically dispersed catalysts have gained considerable attention due to their unique properties and high efficiency in various catalytic reactions. Herein, a series of Co/N-doped carbon (N-C) catalysts was prepared using a metal-lignin coordination strategy and employed in formic acid dehydrogenation (FAD) and hydrodeoxygenation (HDO) of vanillin. The atomically dispersed Co/N-C catalysts showed outstanding activity, acid resistance, and long-term stability in FAD. The improved activity and stability may be attributed to the high dispersion of Co species, increased surface area, and strong Co-N interactions. XPS and XAS characterization revealed the formation of Co-N3 centers, which are assumed to be the active sites. In addition, DFT calculations demonstrated that the adsorption of formic acid on single-atom Co was stronger than that on Co13 clusters, which may explain the high catalytic activity. The Co/N-C catalyst also showed promising performance in the transfer HDO of vanillin with formic acid, without any external additional molecular H2.

Combination of nanoparticles with single-metal sites synergistically boosts co-catalyzed formic acid dehydrogenation

The development of hydrogen technologies is at the heart of a green economy. As prerequisite for implementation of hydrogen storage, active and stable catalysts for (de)hydrogenation reactions are needed. So far, the use of precious metals associated with expensive costs dominates in this area. Herein, we present a new class of lower-cost Co-based catalysts (Co-SAs/NPs@NC) in which highly distributed single-metal sites are synergistically combined with small defined nanoparticles allowing efficient formic acid dehydrogenation. The optimal material with atomically dispersed CoN2C2 units and encapsulated 7-8 nm nanoparticles achieves an excellent gas yield of 1403.8 mL·g-1·h-1 using propylene carbonate as solvent, with no activity loss after 5 cycles, which is 15 times higher than that of the commercial Pd/C. In situ analytic experiments show that Co-SAs/NPs@NC enhances the adsorption and activation of the key intermediate monodentate HCOO*, thereby facilitating the following C-H bond breaking, compared to related single metal atom and nanoparticle catalysts. Theoretical calculations show that the integration of cobalt nanoparticles elevates the d-band center of the Co single atoms as the active center, which consequently enhances the coupling of the carbonyl O of the HCOO* intermediate to the Co centers, thereby lowering the energy barrier.

Ethanol Dehydrogenation to Acetaldehyde over Co@N-Doped Carbon

Cobalt and nitrogen co-doped carbon materials (Co@CN) have recently attracted significant attention as highly efficient noble-metal-free catalysts exhibiting a large application range. In a similar research interest, and taking into account the ever-increasing importance of bioethanol as a renewable raw material, here, we report the results on ethanol dehydrogenation to acetaldehyde over Co@NC catalysts. The catalyst samples were synthesized by a variety of affordable techniques, ensuring generation of various types of Co species incorporated in carbon, such as subnanosized cobalt sites and nano-sized particles of metallic cobalt and cobalt oxides. The catalytic activity was tested under both oxidative and non-oxidative gas-phase conditions at 200–450 °C using a fixed-bed flow reactor. The non-oxidative conditions proved to be much more preferable for the target reaction, competing, however, with ethanol dehydration to ethylene. Under specified reaction conditions, ethanol conversion achieved a level of 66% with 84% selectivity to acetaldehyde at 400 °C. The presence of molecular oxygen in the feed led mainly to deep oxidation of ethanol to COx, giving acetaldehyde in a comparatively low yield. The potential contribution of carbon itself and supported cobalt forms to the observed reaction pathways is discussed.