Characterization of Coke on a Pt-Re/γ-Al2O3 Re-Forming Catalyst: Experimental and Theoretical Study

The Coking of a Solid Catalyst Rationalized with Combined Raman and Fluorescence Lifetime Microscopy

The formation of carbon deposits is a major deactivation pathway for solid catalysts. Studying coking on industrially relevant catalysts is, however, often challenging due to the sample heterogeneity. That is especially true for zeolite-containing catalysts where fluorescence often hampers their characterization with Raman spectroscopy. We turned this disadvantage into an advantage and combined Raman and fluorescence (lifetime) microscopy to study the coking behavior of an equilibrium catalyst material used for fluid catalytic cracking of hydrocarbons. The results presented illustrate that this approach can yield new insights in the physicochemical processes occurring within zeolite-containing catalyst particles during their coking process. Ex situ analyses of single catalyst particles revealed considerable intra-sample heterogeneities. The sample-averaged Raman spectra showed a higher degree of graphitization when the sample was exposed to more hexane, while the sample-averaged fluorescence lifetime showed no significant trend. Simultaneous in situ Raman and fluorescence (lifetime) microscopy, used to follow the coking and the regeneration of single particles, gave more insights in the changing fluorescence dynamics. During the coking, the rise and decline of the average fluorescence lifetime suggested the prolonged presence of smaller coke species that are quenched more and more by adjacent larger polyaromatics acting as Förster-resonance-energy-transfer acceptors.

Sulfur/carbon cathode material chemistry and morphology optimisation for lithium-sulfur batteries

Lithium-sulfur batteries (LSBs) are a promising alternative to lithium-ion batteries because sulfur is highly abundant and exhibits a high theoretical capacity (1675 mA h g-1). However, polysulfide shuttle and other challenges have made it difficult for LSBs to be commercialised. Here, a sulfur/carbon (S/C) composite was synthesised and cathodes were fabricated via scalable melt diffusion and slurry casting methods. Carbon nanoparticles (C65) were used as both sulfur host and electrical additive. Various carbon ratios between the melt-diffusion step and cathode slurry formulation step were investigated. An increased amount of C65 in melt-diffusion led to increased structural heterogeneity in the cathodes, more prominent cracks, and a lower mechanical strength. The best performance was exhibited by a cathode where 10.5 wt% C65 (TC10.5) was melt-diffused and 24.5 wt% C65 was externally added to the slurry. An initial discharge capacity of ∼1500 mA h g-1 at 0.05C and 800 mA h g-1 at 0.1C was obtained with a capacity retention of ∼50% after 100 cycles. The improved electrochemical performance is rationalised as an increased number of C-S bonds in the composite material, optimum surface area, pore size and pore volume, and more homogeneous cathode microstructure in the TC10.5 cathode.

Axially Modified Square-Pyramidal CoN4-F1 Sites Enabling High-Performance Zn-Air Batteries

Cobalt-nitrogen-carbon (Co-N-C) catalysts with a CoN4 structure exhibit great potential for oxygen reduction reaction (ORR), but the imperfect adsorption energy toward oxygen species greatly limits their reduction efficiency and practical application potential. Here, F-coordinated Co-N-C catalysts with square-pyramidal CoN4-F1 configuration are successfully synthesized using F atoms to regulate the axial coordination of Co centers via hydrothermal and chemical vapor deposition methods. During the synthesis process, the geometry structure of the Co atom converts from six-coordinated Co-F6 to square-pyramidal CoN4-F1 in the coordinatively unsaturated state, which provides an open binding site for the O2. The introduction of axial F atoms into the CoN4 plane alters the local atomic environment around Co, significantly improving the ORR activity and Zn-air batteries performance. In situ spectroscopy proves that CoN4-F1 sites strongly combine with the OOH* intermediate and facilitate the splitting of O-O bond, making OOH* readily decompose into O* and OH* via a dissociative pathway. Theoretical calculations confirm that the axial F atom effectively reduces the electronic density of the Co centers and facilitates the desorption of the OH* intermediate, efficiently accelerating the overall ORR kinetics. This work advances a feasible synthesis mechanism of axial ligands and provides a route to construct efficient high-coordination catalysts.

Nanoscale Chemical Diversity of Coke Deposits on Nanoprinted Metal Catalysts Visualized by Tip-Enhanced Raman Spectroscopy

Coke formation is the prime cause of catalyst deactivation, where undesired carbon wastes block the catalyst surface and hinder further reaction in a broad gamut of industrial chemical processes. Yet, the origins of coke formation and their distribution across the catalyst remain elusive, obstructing the design of coke-resistant catalysts. Here, the first-time application of tip-enhanced Raman spectroscopy (TERS) is demonstrated as a nanoscale chemical probe to localize and identify coke deposits on a post-mortem metal nanocatalyst. Monitoring coke at the nanoscale circumvents bulk averaging and reveals the local nature of coke with unmatched detail. The nature of coke is chemically diverse and ranges from nanocrystalline graphite to disordered and polymeric coke, even on a single nanoscale location of a top-down nanoprinted SiO2 -supported Pt catalyst. Surprisingly, not all Pt is an equal producer of coke, where clear isolated coke "hotspots" are present non-homogeneously on Pt which generate large amounts of disordered coke. After their formation, coke shifts to the support and undergoes long-range transport on the surrounding SiO2 surface, where it becomes more graphitic. The presented results provide novel guidelines to selectively free-up the coked metal surface at more mild rejuvenation conditions, thus securing the long-term catalyst stability.

Raman Spectroscopy Characterization of Amorphous Coke Generated in Industrial Processes

Examples on the real-world field application of Raman spectroscopy with systematic analysis of the intensity variation of D and G bands corresponding to the change of excitation laser energy to characterize and compare coke species from various industrial processes are presented. The findings indicate the different degree of sp² and sp³ hybridized bonding structures of amorphous carbon collected from different industrial processes as well as heavy carbonaceous deposits generated by industrial catalysts. This spectroscopic methodology is practical and highly beneficial in identifying coke formation mechanisms in industrial processes, as well as supporting design strategies to abate the undesired coke formation on industrial catalysts.

Understanding the Impact of Hydrogen Activation by SrCe0.8Zr0.2O3−δ Perovskite Membrane Material on Direct Non-Oxidative Methane Conversion

Direct non-oxidative methane conversion (DNMC) converts methane (CH₄) in one step to olefin and aromatic hydrocarbons and hydrogen (H₂) co-product. Membrane reactors comprising methane activation catalysts and H₂-permeable membranes can enhance methane conversion by in situ H₂ removal via Le Chatelier's principle. Rigorous description of H₂ kinetic effects on both membrane and catalyst materials in the membrane reactor, however, has been rarely studied. In this work, we report the impact of hydrogen activation by hydrogen-permeable SrCe_0.8Zr_0.2O_3−δ (SCZO) perovskite oxide material on DNMC over an iron/silica catalyst. The SCZO oxide has mixed ionic and electronic conductivity and is capable of H₂ activation into protons and electrons for H₂ permeation. In the fixed-bed reactor packed with a mixture of SCZO oxide and iron/silica catalyst, stable and high methane conversion and low coke selectivity in DNMC was achieved by co-feeding of H₂ in methane stream. The characterizations show that SCZO activates H₂ to favor “soft coke” formation on the catalyst. The SCZO could absorb H₂in situ to lower its local concentration to mitigate the reverse reaction of DNMC in the tested conditions. The co-existence of H₂ co-feed, SCZO oxide, and DNMC catalyst in the present study mimics the conditions of DNMC in the H₂-permeable SCZO membrane reactor. The findings in this work offer the mechanistic understanding of and guidance for the design of H₂-permeable membrane reactors for DNMC and other alkane dehydrogenation reactions.

Regeneration of Pt-Sn/Al2O3 Catalyst for Hydrogen Production through Propane Dehydrogenation Using Hydrochloric Acid

Compared with dehydrogenation in conventional petroleum refinery processes, relatively pure hydrogen can be produced by propane dehydrogenation (PDH) without innate contaminants like sulfur and metals. Among the existing catalysts for PDH, Pt catalysts are popular and are often used in conjunction with Sn as a co-catalyst. Coke formation is a major concern in PDH, where catalyst regeneration is typically achieved by periodic coke burning to achieve sustainable operation. In this study, Pt-Sn/Al2O3 catalysts were regenerated after coke burning in three stages: mixing the catalyst with liquid hydrochloric acid, drying, and calcining under air atmosphere. In this process, the optimum concentration of hydrochloric acid was found to be 35% w/w. HCl treatment was effective for enhancing redispersion of the metal catalysts and aiding the formation of the Pt3Sn alloy, which is considered to be effective for PDH reaction. HCl treatment may provide oxychlorination-like conditions under the calcination atmosphere. The characteristics of the catalysts were examined by X-ray diffraction (XRD), transmission electron microscopy (TEM), temperature-programmed reduction (TPR), X-ray photoelectron spectroscopy (XPS), and CO chemisorption.

Mechanistic study of the selective hydrogenation of carboxylic acid derivatives over supported rhenium catalysts

The structure and performance of TiO₂-supported Re (Re/TiO₂) catalysts for selective hydrogenation of carboxylic acid derivatives have been investigated.

Acidic effect of porous alumina as supports for Pt nanoparticle catalysts in n-hexane reforming

Catalytic activity and selectivity of n-hexane reforming are changed significantly by the surface acidic properties of the alumina support following halogen treatment.