Dehydrogenation mechanisms of methyl-cyclohexane on γ-Al2O3 supported Pt13: Impact of cluster ductility

Efficient solvent- and hydrogen-free upcycling of high-density polyethylene into separable cyclic hydrocarbons

Plastic pollution is a planetary threat that has been exacerbated by the COVID-19 pandemic due to the surge in medical waste, personal protective equipment and takeaway packaging. A socially sustainable and economically viable method for plastic recycling should not use consumable materials such as co-reactants or solvents. Here we report that Ru nanoparticles on zeolitic HZSM-5 catalyse the solvent- and hydrogen-free upcycling of high-density polyethylene into a separable distribution of linear (C1 to C6) and cyclic (C7 to C15) hydrocarbons. The valuable monocyclic hydrocarbons accounted for 60.3 mol% of the total yield. Based on mechanistic studies, the dehydrogenation of polymer chains to form C=C bonds occurs on both Ru sites and acid sites in HZSM-5, whereas carbenium ions are generated on the acid sites via the protonation of the C=C bonds. Accordingly, optimizing the Ru and acid sites promoted the cyclization process, which requires the simultaneous existence of a C=C bond and a carbenium ion on a molecular chain at an appropriate distance, providing high activity and cyclic hydrocarbon selectivity.

Heterogeneous Catalysts in N-Heterocycles and Aromatics as Liquid Organic Hydrogen Carriers (LOHCs): History, Present Status and Future

The continuous decline of traditional fossil energy has cast the shadow of an energy crisis on human society. Hydrogen generated from renewable energy sources is considered as a promising energy carrier, which can effectively promote the energy transformation of traditional high-carbon fossil energy to low-carbon clean energy. Hydrogen storage technology plays a key role in realizing the application of hydrogen energy and liquid organic hydrogen carrier technology, with many advantages such as storing hydrogen efficiently and reversibly. High-performance and low-cost catalysts are the key to the large-scale application of liquid organic hydrogen carrier technology. In the past few decades, the catalyst field of organic liquid hydrogen carriers has continued to develop and has achieved some breakthroughs. In this review, we summarized recent significant progress in this field and discussed the optimization strategies of catalyst performance, including the properties of support and active metals, metal-support interaction and the combination and proportion of multi-metals. Moreover, the catalytic mechanism and future development direction were also discussed.

Exploring the Dehydrogenation Reaction Pathway of Perhydro-Polycyclic Aromatic Hydrocarbons over the Pt/Al2O3 Catalyst as Liquid Organic Hydrogen Carriers by In Situ DRIFT

Polycyclic aromatic hydrocarbons (PAHs) have been paid more attention as liquid organic hydrogen carriers (LOHCs) because of their high hydrogen storage, easy transportation, low price, and other advantages. Dehydrogenation is the key point of the PAH hydrogen storage. However, the dehydrogenation reaction rate of perhydro-PAHs is slow, and their pathway is still not clear. To clarify the PAH dehydrogenation pathway, three kinds of perhydro-PAHs containing six-membered rings (methylcyclohexane, perhydro-diphenylmethane, and perhydro-dibenzyltoluene) are selected, and their dehydrogenation processes over the Pt/Al₂O₃ catalyst are carried out by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFT). It was found that the dehydrogenation in the six-membered ring started in the para position of the -CH₃ group, and then, the six-membered ring was transformed into a benzene ring gradually. Between the six-membered rings, dehydrogenation started from the side ring, which has fewer groups.

Dynamical Study of Adsorbate-Induced Restructuring Kinetics in Bimetallic Catalysts Using the PdAu(111) Model System

Dynamic restructuring of bimetallic catalysts plays a crucial role in their catalytic activity and selectivity. In particular, catalyst pretreatment with species such as carbon monoxide and oxygen has been shown to be an effective strategy for tuning the surface composition and morphology. Mechanistic and kinetic understanding of such restructuring is fundamental to the chemistry and engineering of surface active sites but has remained challenging due to the large structural, chemical, and temporal degrees of freedom. Here, we combine time-resolved temperature-programmed infrared reflection absorption spectroscopy, ab initio thermodynamics, and machine-learning molecular dynamics to uncover previously unidentified timescale and kinetic parameters of in situ restructuring in Pd/Au(111), a highly relevant model system for dilute Pd-in-Au nanoparticle catalysts. The key innovation lies in utilizing CO not only as a chemically sensitive probe of surface Pd but also as an agent that induces restructuring of the surface. Upon annealing in vacuum, as-deposited Pd islands became encapsulated by Au and partially dissolved into the subsurface, leaving behind isolated Pd monomers on the surface. Subsequent exposure to 0.1 mbar CO enabled Pd monomers to repopulate the surface up to 373 K, above which complete Pd dissolution occurred by 473 K, with apparent activation energies of 0.14 and 0.48 eV, respectively. These restructuring processes occurred over the span of ∼1000 s at a given temperature. Such a minute-timescale dynamics not only elucidates the fluxional nature of alloy catalysts but also presents an opportunity to fine-tune the surface under moderate temperature and pressure conditions.

Effect of Pt cluster size on CO2 adsorption and activation on (110) and (100) γ-alumina surfaces: insights from DFT using a periodic boundary approach

We performed a structural investigation on the adsorption and activation of CO₂ molecules on different sized Pt clusters to study the effect of Pt cluster size on CO₂ adsorption. Several Pt_x clusters (x = 1-5 and 13) were analyzed to represent metal dispersion on surfaces and nanoscale metal clusters. For (110) γ-alumina surfaces, the Pt₂ cluster site was the most stable. In addition, the Pt₃ cluster site was the most stable for CO₂ adsorption on Pt/Al₂O₃. Charge transfer from Pt clusters to CO₂ was critical for CO₂ activation.

Few-Atom Pt Ensembles Enable Efficient Catalytic Cyclohexane Dehydrogenation for Hydrogen Production

Identification of catalytic active sites is pivotal in the design of highly effective heterogeneous metal catalysts, especially for structure-sensitive reactions. Downsizing the dimension of the metal species on the catalyst increases the dispersion, which is maximized when the metal exists as single atoms, namely, single-atom catalysts (SACs). SACs have been reported to be efficient for various catalytic reactions. We show here that the Pt SACs, although with the highest metal atom utilization efficiency, are totally inactive in the cyclohexane (C₆H₁₂) dehydrogenation reaction, an important reaction that could enable efficient hydrogen transportation. Instead, catalysts enriched with fully exposed few-atom Pt ensembles, with a Pt-Pt coordination number of around 2, achieve the optimal catalytic performance. The superior performance of a fully exposed few-atom ensemble catalyst is attributed to its high d-band center, multiple neighboring metal sites, and weak binding of the product.

Fe/Co/N-C/graphene derived from Fe/ZIF-67/graphene oxide three dimensional frameworks as a remarkably efficient and stable catalyst for the oxygen reduction reaction

The development of non-noble metal catalysts with high-performance, long stability and low-cost is of great importance for fuel cells, to promote the oxygen reduction reaction (ORR). Herein, Fe/Co/N-C/graphene composites were easily prepared by using Fe/ZIF-67 loaded on graphene oxide (GO). The Fe/Co/porous carbon nanoparticles were uniformly dispersed on graphene with high specific surface area and large porosity, which endow high nitrogen doping and many more active sites with better ORR performance than the commercial 20 wt% Pt/C. Therefore, Fe/Co/N-C/graphene composites exhibited excellent ORR activity in alkaline media, with higher initial potential (0.91 V) and four electron process. They also showed remarkable long-term catalytic stability with 96.5% current retention after 12 000 s, and outstanding methanol resistance, compared with that of 20 wt% Pt/C catalysts. This work provides an effective strategy for the preparation of non-noble metal-based catalysts, which could have significant potential applications, such as in lithium-air batteries and water-splitting devices.

Gas phase vs. liquid phase: monitoring H2 and CO adsorption phenomena on Pt/Al2O3 by IR spectroscopy

The adsorption of H2 and CO over Pt/Al2O3 was studied in gas and in liquid phase by FT-IR and ATR-IR spectroscopies under otherwise similar conditions. The solvent competes with hydrogen and CO for terrace and kink metal sites.

Atomically dispersed metal catalysts on nanodiamond and its derivatives: synthesis and catalytic application

Atomically dispersed metal catalysts (ADMCs) have attracted increasing interest in the field of heterogeneous catalysis. As sub-nanometric catalysts, ADMCs have exhibited remarkable catalytic performance in many reactions. ADMCs are classified into two categories: single atom catalysts (SACs) and atomically dispersed clusters with a few atoms. To stabilize the highly active ADMCs, nanodiamond (ND) and its derivatives (NDDs) are promising supports. In this Feature Article, we have introduced the advantages of NDDs with a highly curved surface and tunable surface properties. The controllable defective sites and oxygen functional groups are known as the anchoring sites for ADMCs. Tunable surface acid-base properties enable ADMCs supported on NDDs to exhibit unique selectivity towards target products and an extended lifetime in many reactions. In addition, we have firstly overviewed the recent advances in the synthesis strategies for effectively fabricating ADMCs on NDDs, and further discussed how to achieve the atomic dispersion of metal precursors and stabilize the as-formed metal atoms against migration and agglomeration based on NDDs. And then, we have also systematically summarized the advantages of ADMCs supported on NDDs in reactions, including hydrogenation, dehydrogenation, aerobic oxidation and electrochemical reaction. These reactions can also effectively guide the design of ADMCs. The recent progress in understanding the effect of structure of active centers and metal-support interactions (MSIs) on the catalytic performance of ADMCs is particularly highlighted. At last, the possible research directions in ADMCs are forecasted.

Recent Trends on the Dehydrogenation Catalysis of Liquid Organic Hydrogen Carrier (LOHC): A Review

Considering the expansion of the use of renewable energy in the future, the technology to store and transport hydrogen will be important. Hydrogen is gaseous at an ambient condition, diffuses easily, and its energy density is low. So liquid organic hydrogen carriers (LOHCs) have been proposed as a way to store hydrogen in high density. LOHC can store, transport, and use hydrogen at high density by hydrogenation and dehydrogenation cycles. In this review, we will focus on typical LOHCs, methylcyclohexane (MCH), 18H-dibenzyltoluene (DBT), and 12H-N-ethylcarbazole (NECZ), and summarize recent developments in dehydrogenation catalytic processes, which are key in this cycle.

First-principles microkinetic analysis of Lewis acid sites in Zn-ZSM-5 for alkane dehydrogenation and its implication to methanol-to-aromatics conversion

Stabilities and dehydrogenation activities of butane and cyclohexane on four different Zn sites in ZSM-5 zeolite were theoretically revealed. ZnOH⁺ was identified as the most active site at low temperature and the activity increases with the sequence of dehydrogenation.

Recent developments of nanocatalyzed liquid-phase hydrogen generation

Hydrogen is the most effective and sustainable carrier of clean energy, and liquid-phase hydrogen storage materials with high hydrogen content, reversibility and good dehydrogenation kinetics are promising in view of "hydrogen economy". Efficient, low-cost, safe and selective hydrogen generation from chemical storage materials remains challenging, however. In this Review article, an overview of the recent achievements is provided, addressing the topic of nanocatalysis of hydrogen production from liquid-phase hydrogen storage materials including metal-boron hydrides, borane-nitrogen compounds, and liquid organic hydrides. The state-of-the-art catalysts range from high-performance nanocatalysts based on noble and non-noble metal nanoparticles (NPs) to emerging single-atom catalysts. Key aspects that are discussed include insights into the dehydrogenation mechanisms, regenerations from the spent liquid chemical hydrides, and tandem reactions using the in situ generated hydrogen. Finally, challenges, perspectives, and research directions for this area are envisaged.

Including dispersion in density functional theory for adsorption on flat oxide surfaces, in metal-organic frameworks and in acidic zeolites

We examine the performance of nine commonly used methods for including dispersion interactions in density functional theory (DFT): three different parametrizations of damped 1/Rn terms (n = 6, 8, …) added to the DFT energy (Grimme's D2 and D3 parameterizations as well as that of Tkatchenko and Scheffler), three different implementations of the many-body dispersion approach (MBD, MBD/HI and MBD/FI), the density-dependent energy correction, called dDsC, and two "first generation" van der Waals density functionals, revPBE-vdW and optB86b-vdW. As test set we use eight molecule-surface systems for which agreement has been reached between experiment and hybrid QM:QM calculations within chemical accuracy limits (±4.2 kJ mol-1). It includes adsorption of carbon monoxide and dioxide in the Mg2(2,5-dioxido-1,4-benzenedicarboxylate) metal-organic framework (Mg-MOF-74, CPO-27-Mg), adsorption of carbon monoxide as well as of monolayers of methane and ethane on the MgO(001) surface, as well as adsorption of methane, ethane and propane in H-chabazite (H-CHA). D2 with Ne parameters for Mg2+, D2(Ne), MBD/HI and MBD/FI perform best. With the PBE functional, the mean unsigned errors are 6.1, 5.6 and 5.4 kJ mol-1, respectively.

A naphtha reforming process development methodology based on the identification of catalytic reactivity descriptors

This critical review proposes an original and pragmatic naphtha reforming process development approach aimed at merging catalyst development with kinetic modelling through the identification of “effective” and “measurable” catalytic descriptors.