Liquid-film-type catalytic decalin dehydrogeno-aromatization for long-term storage and long-distance transportation of hydrogen

A new approach for mobile storage of hydrogen has been proposed with the use of a catalytic reaction pair of decalin dehydrogenation/naphthalene hydrogenation. With the complement of the industrialized naphthalene-hydrogenation catalysis, the other endothermic catalysis for decalin dehydrogenation was now performed at around 200°C with carbon-supported platinum-based catalysts. Under liquid-film conditions, hydrogen was evolved from decalin much more efficiently than the suspended ones due to the superheated states of dehydrogenation catalysts. It was confirmed that the catalytic conversions of decalin dehydrogeno-aromatization in the liquid-film states could surpass easily the equilibrium limit, because the conditions of suppressed reactant evaporation and reactive distillation were operative here. Exergy loss in the hydrogen storage system would be reduced tremendously by adopting this catalyst-assisted decalin/naphthalene pair as the medium of hydrogen carrier.     

Comprehensive review of Cu-based CO2 hydrogenation to CH3OH: Insights from experimental work and theoretical analysis

Developing the technology of CO₂ hydrogenation into methanol can not only alleviate environmental problems such as greenhouse effect, but also effectively promote the utilization of CO₂ resources. In general, Cu-based catalysts have been extensively studied due to its low cost and the effective synthesis of methanol. Thus, this review is to be reported based on Cu-based catalysts for methanol synthesis from CO₂ hydrogenation. The specific goal of this review is to provide some insights into the structural and surface properties of Cu-based catalysts and their functions on the reaction mechanisms, and further affecting on the catalytic selectivity, stability, and activity for the CO₂ hydrogenation to methanol. A vital issue discussed is the fundamental understanding of active sites, reaction mechanisms, and interactions (active metal-support, active metal-promoter, bimetal) in determining the catalytic performance. Through a comprehensive overview on Cu-based systems for CO₂ hydrogenation to methanol from both experimental and theoretical perspectives, it could provide some useful information to go into CO₂ hydrogenation to methanol for the outsider, and promote the design and synthesis of novel and efficient catalysts.     

Hydrogen storage in liquid organic hydride: Selectivity of MCH dehydrogenation over monometallic and bimetallic Pt catalysts

Hydrogen storage for mobile and stationary applications is an expanding research topic. One of the more promising storage techniques relies on the reversibility, high selectivity, and high hydrogen density of liquid organic hydrides, in particular methylcyclohexane (MCH). Catalyst evaluation for MCH dehydrogenation to toluene is based on three catalytic parameters: activity, selectivity, and stability. Current catalysts, optimized for catalytic reforming, do not meet the targeted aromatic selectivity (+99%) for MCH dehydrogenation. Therefore, a range of Pt catalysts was prepared and compared with commercially available catalysts in a fixed-bed reactor under operating conditions suitable for mobile and stationary applications. The best overall performance was realized by a particular monometallic Pt catalyst. This catalyst showed superior activity, selectivity, and stability compared with other prepared and commercial catalysts. As an effort to further enhance the aromatic selectivity, this study identified the main side-reactions associated with MCH dehydrogenation, the effect of operating parameters on by-product yields, and the effect of catalyst deactivation on long-term selectivity.     

Methanol dehydrogenation and oxidation on Pt1–XNiX/CNTs at low temperature: Effect of Ni addition

This study reports the effect on catalytic activity resulting from Ni incorporation in Pt nanoparticles supported on carbon nanotubes (CNTs) for electrochemical methanol oxidation at low temperature in acidic conditions. Chemical composition, morphology and structure of the Pt_1–XNi_X/CNTs (X = 0, 0.1, 0.2, 0.3, 0.4, 0.5) catalysts were studied by EDS, SEM, XRD, TEM and TGA. The catalytic activity of the prepared materials in methanol electro-oxidation reaction was investigated by cyclic voltammetry (CV) and chronoamperometry (CA). The results of catalytic activity of the nanostructured materials showed a volcano-type relationship between the Ni relative concentration current density. The enhancement of catalytic activity was attributed to changes in surface electronic structure of Pt nanoparticles that impacted in an increment of active sites for methanol dehydrogenation and oxidation processes. On the other hand, high concentration of Ni (concentration ≥ 30 at.%) in Pt nanoparticles caused a substantial decrease of the catalytic activity due to a depletion of active sites for the methanol dehydrogenation process. The highest catalytic activity was observed when the Ni relative concentration reaches 30 at.%.     

过渡金属碳化物的催化研究

过渡金属碳化物作为一种催化新材料得到了人们广泛的关注，在催化加氢、脱氢、脱硫（HDS）、脱氮（HDN）和重整等方面，表现出优良的催化活性和选择性。本文综述了碳化钒、碳化钼、碳化钨、碳化铁、碳化钛等碳化物的催化性能在国内外的研究进展和碳化物在各个反应中的催化机理。     

Production of hydrogen through the coupling of dehydrogenation and hydrogenation for the synthesis of cyclohexanone and furfuryl alcohol over different promoters supported on Cu-MgO catalysts

Cu-MgO is found to be an efficient catalyst for the coupling reaction of furfural (FAL) hydrogenation and cyclohexanol (CyOH) dehydrogenation. This process is not only efficient in compensating the thermodynamic equilibrium constraints in the cyclohexanol dehydrogenation and improving the yields towards cyclohexanone but also is a economical route for the synthesis of furfuryl alcohol (FFA) and cyclohexanone (Cyone) as the process do not need any external pumping of hydrogen. The effect of incorporation of various promoters viz., Co, Zn, Fe, Cr, Pd and Ni in Cu-MgO over its activity towards this coupling reaction has been studied. Incorporation of Cr in Cu-MgO catalyst is found be an advantageous in enhancing the yields of both FFA and Cyone. All other promoters though found to show higher activity for the individual reactions of FAL hydrogenation and CyOH dehydrogenation, failed to do the same in their coupling reaction. The stabilization of active species (Cu⁺/Cu⁰) by Cr which also seem to increase the synergetic interaction between Cu and MgO as observed from higher dispersion of copper (from XRD results) and easier reducibility of copper oxide (from TPR results) seem to be the factors behind its higher activity over other promoted catalysts.     

Mechanistic insights into hydrogen production from formic acid catalyzed by Pd@N-doped graphene: The role of the nitrogen dopant

The catalytic decomposition of formic acid (HCOOH) is a crucial process for hydrogen production technologies. Herein, periodic density functional theory (DFT) calculations were employed to explore the effect of N-doping on the decomposition of formic acid. We designed a series of single Pd-atoms deposited in the single vacancy of N-doped graphene sheets, namely Pd-DGr, Pd–N1Gr, Pd–N2Gr, and Pd–N3Gr, as the proposed catalysts. Our findings show that H₂ production from HCOOH dehydrogenation on these surfaces proceeds via the formate (HCOO) pathway (Path-I) rather than the carboxylate (COOH) pathway (Path-II). Furthermore, the Pd–N3Gr catalyst shows the greatest catalytic reactivity toward HCOOH dehydrogenation via Path-I, requiring an activation energy (E_a) of 0.38 eV.On the other hand, the undesirable dehydration of HCOOH to carbon monoxide (CO) through COOH (Path-IIIA) or formyl (HCO) (Path-IIIB) intermediates is unlikely to occur on Pd–N3Gr due to a large activation energy. We found that the active species on the catalyst surface increased with N-doping concentration. Additionally, microkinetic simulations of the HCOOH decomposition on these surfaces confirmed the high activity and selectivity of the Pd–N3Gr catalyst toward HCOOH dehydrogenation (Path-I). These calculated results highlight that the Pd–N3Gr catalyst is a promising candidate for the formic acid decomposition reaction to yield hydrogen.     

Engineering PdCu and PdNi bimetallic catalysts with adjustable alloying degree for the dehydrogenation reaction of dodecahydro-N-ethylcarbazole

The NECZ/12H-NECZ (N-ethylcarbazole/dodecahydro-N-ethylcarbazole) system is regarded as the most potential liquid organic hydrogen carrier. However, the low activity, selectivity of NECZ and high cost of catalysts for the dehydrogenation reaction restrict its efficiency and commercial applications. In this work, a series of bimetallic Pd-M(M = Cu, Ni)/SiO₂ catalysts were prepared and employed to enhance catalytic activity and selectivity of NECZ for the 12H-NECZ dehydrogenation reaction. Pd₃Ni₁/SiO₂ exhibited high catalytic performance with 100% conversion, 91.1% selectivity of NECZ and 5.63 wt% hydrogen release amount at 453 K, 101.325 kPa for 8 h. The TOF (turnover frequency) of Pd₃Ni₁/SiO₂ is enhanced by 42.4% compared with Pd/SiO₂. Combined with the characterization analysis, it was found that adjusting the alloying degree or the alloy phase in the PdCu and PdNi bimetallic catalysts could significantly enhance the dehydrogenation activity and selectivity, which were dependent on the component of bimetallic catalysts. This work may provide theoretical guidance for designing the efficient and low-cost bimetallic catalysts for the dehydrogenation of 12H-NECZ, which could boost the commercial applications of liquid organic hydrogen carriers.     

Ni–Mo nanoparticles stabilized by ether functionalized ionic polymer: A novel and efficient catalyst for hydrodeoxygenation of 4-methylanisole as a representative of lignin-derived pyrolysis bio-oils

In the present study, nickel-molybdenum nanoparticles stabilized with ether functionalized ionic polymer were synthesized and utilized as a novel and efficient catalyst for hydrodeoxygenation of 4-methylanisole as a representative of lignin-derived bio-oil. The catalytic upgrading process was performed in the presence of hydrogen with a batch reactor at temperature of 80–200 °C, hydrogen pressure of 10–50 bar, reaction time of 0.5–15 h and catalyst loading of 1–5 mol%. The major reaction classes during 4-methylanisole upgrading were hydrodeoxygenation and hydrogenolysis which resulted in production of 4-methylphenol, toluene, phenol and benzene as the main products. The experimental results indicated that the catalytic activity of Ni–Mo (20%–80%) nanoparticles stabilized with ionic polymer is superior to that with low Mo content. Also, it is observed that the selectivity of deoxygenated products including toluene and benzene improves with increasing the Mo content of the catalyst. Finally, regarding to the excellent catalytic activity of synthesized nanocatalyst during upgrading process of bio-oil at mild operating condition, ether functionalized ionic polymer was introduced as an applicable and effective stabilizers for nickel-molybdenum nanoparticles.     

Dehydrogenation and hydrogenation characteristics of MgH2 with transition metal fluorides

The effect of transition metal fluorides on the dehydrogenation and hydrogenation of MgH₂ has been investigated. Many of the fluorides show a considerable catalytic effect on both the dehydrogenation temperature and hydrogenation kinetics of MgH₂. Among them, NbF₅ and TiF₃ most significantly enhance the hydrogenation kinetics of MgH₂. It is suggested that hydride phases formed by the reaction between MgH₂ and these transition metal fluorides during milling and/or hydrogenation play a key role in improving the hydrogenation kinetics of MgH₂.     

Enhancing selectivity and reducing cost for dehydrogenation of dodecahydro-N-ethylcarbazole by supporting platinum on titanium dioxide

N-ethylcarbazole/dodecahydro-N-ethylcarbazole (NECZ/12H-NECZ) was a promising system for hydrogen storage applications. 1.0 wt% Pt/TiO₂ was regarded as the optimal loading in Pt/TiO₂ catalyst applied in the 12H-NECZ dehydrogenation reaction. The hydrogen release amount, selectivity to NECZ and TOF of 12H-NECZ dehydrogenation are 5.75 wt %, 98% and 229.73 min⁻¹ at 453 K. Compared with the commercial 5.0 wt% Pd and Pt-based catalysts, it exhibited very high activity, selectivity and stability for 12H-NECZ dehydrogenation with low Pt loading. Combined with the XRD, XPS, HRTEM, TPR analysis, it was indicated that the enhanced catalytic performance was due to the SMSI (strong metal-supporting interaction) between Pt and TiO₂ support, which accelerated the rate-limiting step and enhanced the whole dehydrogenation reaction. This work may be beneficial for the commercial application of Pt/TiO₂ catalysts in the Liquid Organic Hydrogen Carrier (LOHC) system.     

Selection of competitive adsorption additives to relieve product inhibition of maleic acid hydrogenation in proton exchange membrane flow cell reactor: A molecular dynamics simulation

Adsorption of products on carbon carriers and the sluggish mass transport of products in the catalytic layer lead to product inhibition in proton exchange membrane flow cell reactor (PEMFCR), which seriously decreases hydrogenation performance. Here, via molecular dynamics simulations, an alleviative mechanism of competitive adsorption additives on product inhibition is proposed by choosing biomass derivatives maleic acid (MA) as hydrogenation model compound and ethanol (EtOH) as an additive model. The enhanced solvation and competitive adsorption synergistically promote the outflow of product succinic acid (SA) from the catalytic layer. Strong hydrophobic-hydrophobic interactions between EtOH and SA promote solvation and solubility of SA in the solvent water, and meanwhile the competitive adsorption steric hindrance of EtOH on adsorption interfaces weakens van der Waals interactions between SA and carbon carriers. Accordingly, the main structural characteristics that influence competitive adsorption of additives, such as, amphiphilic structure, polarity, molecular weight, and size of hydrophobic moiety, are exacted for selection of favorable additives. Among the studied water miscible alcohol and ketone additives, isopropanol (iPrOH) is proved to be favorable, which is further verified by the highest increase (64.3%) of hydrogenation conversion of MA in experiments. This work could provide theoretical guidance for selecting high-performance additives, intensifying heterogeneous hydrogenation in PEMFCR.     

Low-temperature alcohol-assisted methanol synthesis from CO2 and H2: The effect of alcohol type

Carbon dioxide (CO₂) conversion to higher-value products is a promising pathway to mitigate CO₂ emissions. Methanol is a high-value-chain chemical in industries that can be produced through CO₂ hydrogenation, which is an exothermic reaction. Due to thermodynamic limitations, a typical synthesis temperature between 250 °C and 300 °C results in a low conversion of CO₂ at equilibrium. To enhance the CO₂ conversion, high pressures of 50–100 bar are required, which inevitably causes the process to be energy-intensive. In this study, an alternative method called alcohol-assisted methanol synthesis is investigated. In this method, alcohol is used as a catalytic solvent and helps decrease the reaction temperature and pressure (150 °C and 50 bar) and significantly increases methanol yield. Ethanol is used as the alcohol due to its reactivity, providing a high methanol yield (47.80%) with 63.93% CO₂ conversion and 67.54% methanol selectivity. However, due to unwanted side reactions, ethanol generates ethyl acetate as a byproduct that forms an azeotrope with methanol, leading to difficulty in product purification. The effects of alcohol type (molecular weight and structure), including ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, iso-butanol, tert-butanol and 1-pentanol, on CO₂ conversion, methanol yield and byproducts are investigated. It is found that smaller-molecule alcohols provide a higher methanol yield. Moreover, n-alcohols provide a higher methanol yield than branched alcohols, and the byproducts of the reaction with n-alcohols do not form an azeotrope with methanol. Therefore, 1-propanol is compared with ethanol providing 26.55% methanol yield, 69.02% CO₂ conversion and 70.82% methanol selectivity.     

A density functional theory study of CO2 hydrogenation to methanol over Pd/TiO2 catalyst: The role of interfacial site

The interface between metal and support has a very significant influence on the activity and selectivity of the CO₂ hydrogenation to methanol, but there is still lack of investigation in understanding its role in the reaction process. In the current work, the synthesis of methanol through CO₂ hydrogenation on a model Pd/TiO₂ catalyst was studied based on the periodic density functional theory calculation, and the reaction mechanism and active sites were revealed after examining the possible routes. The charge density difference and Millikan charge analysis demonstrate that CO₂ adsorbed at the interfacial site is activated due to obtaining charge from the catalyst, and it is transformed into chemisorbed CO₂^δ−. It is found that interface is the active site for the subsequent hydrogenation process of CO₂ while metal Pd provides an active site to the dissociation of H₂. Moreover, there is a metal-support interaction, where the formed H at the Pd particles reacts with the CO₂ and intermediates adsorbed at interface by the spillover, and the methanol is produced on the support surface. In addition, the RWGS + CO-Hydro route is determined to be the dominant pathway for methanol synthesis, and CO hydrogenation to HCO is the rate-determining step.     

Catalytic transfer hydrogenation of furfural to furfuryl alcohol over Fe3O4 modified Ru/Carbon nanotubes catalysts

In this study, magnetic Fe₃O₄ modified Ru/Carbon nanotubes (CNTs) catalysts were used to achieve the catalytic transfer hydrogenation of furfural (FF) to furfuryl alcohol (FFA), with alcohols as the solvent and hydrogen donors. According to the result of the catalyst characterization, Fe₃O₄ promoted the formation of Ru⁰ species. The effects of Fe₃O₄ loading and different hydrogen donors on the catalytic transfer hydrogenation of FF were tested, and the reaction parameters and catalyst stability were also analyzed. It is found that Fe₃O₄ effectively enhanced the activity of Ru/CNTs in catalytic transfer hydrogenation of FF, the catalytic activity was optimized at the Fe₃O₄ loading of 5 wt%, and the optimal hydrogen donor was i-propanol. Moreover, the Ru–Fe₃O₄/CNTs could be easily collected for further use and possessed excellent stability. The mechanism of the catalytic transfer hydrogenation of FF using Ru–Fe₃O₄/CNTs was discussed, and the corresponding catalyst activity groups included metal Ru sites and RuO_x-Fe₃O₄ Lewis acid sites, which account for the excellent catalytic activity of transfer hydrogenation.     

Recent developments on heterogeneous catalysts for biodiesel production by oil esterification and transesterification reactions: A review

Heterogeneous catalysis is widely applied in industry due to important advantages it offers to chemical processes such as improved selectivity and easy catalyst separation from reaction mixture, reducing process stages and wastes. This is the reason why nowadays heterogeneous catalysts are being developed to produce biodiesel. Several catalytic materials have been showed in bibliography: acid solids capable to carry out free fatty acids esterification reaction, base solids which are able to carry out triglycerides transesterification reaction and bifunctional solids (acid–base character) which show ability to simultaneously catalyze esterification and transesterification reaction. This review discusses the latest advances in research and development related with heterogeneous catalysts used to produce biodiesel.     

Zinc oxide nanorods based catalysts for hydrogen production by steam reforming of methanol

Zinc oxide (ZnO) nanorods were epitaxially grown on porous cordierite support by a hydrothermal process and utilized for catalyzing methanol steam reforming (MSR) reaction. Catalytic activity of ZnO nanorods for MSR process was correlated to the terminated surfaces of ZnO crystallites. Copper (Cu), palladium (Pd) and gold (Au) nanoparticles infused ZnO nanorods were prepared by in-situ precipitation of the metals on the nanorods. 28% hydrogen selectivity was observed with Cu/ZnO nanorods (Cu/10Zn), while Pd/ZnO nanorods and (Pd/10Zn) showed slightly lower activity. Higher catalytic activity of copper and palladium impregnated ZnO nanorods can be attributed to the synergistic combination of bimetallic oxides. In contrast, Au/ZnO nanorods (Au/10Zn) showed very high activity for methanol dehydrogenation and higher than 97% methanol conversion was achieved for operating temperatures as low as 200 °C.     

Boosting the dehydrogenation efficiency of dodecahydro-N-ethylcarbazole by assembling Pt nanoparticles on the single-layer Ti3C2Tx MXene

As the candidates for large-scale hydrogen storage, liquid organic hydrogen carriers (LOHCs) exhibit evident advantages in hydrogen storage density and convenience of storage and transportation. Among them, NECZ (N-ethylcarbazole)/12H-NECZ (dodecahydro-N-ethylcarbazole) is considered as a typical system with the lower hydrogenation/dehydrogenation temperature. However, the low dehydrogenation efficiency restrict its commercial applications. In this work, the single-layer Ti₃C₂T_x MXene was employed as the support to load the Pt nanoparticles for the 12H-NECZ dehydrogenation reaction. The effect of transition metals, loading amounts and morphologies of catalysts were analyzed. It was found that the 3 wt% Pt/S–Ti₃C₂T_x catalyst exhibited the best catalytic performance with 100% conversion, 91.55% selectivity of NECZ and 5.62 wt% hydrogen release amount at 453 K, 101.325 kPa for 7 h. The product distributions and kinetics analysis suggested that the elementary reaction from 4H-NECZ to NECZ was the rate-limiting step. The selectivity of NECZ is sensitive to the dehydrogenation temperature. Combined with the XRD, SEM, HRTEM, XPS, BET and FT-IR results, it could be indicated that the special two-dimension structure of S–Ti₃C₂T_x and electronic effect between Pt and S–Ti₃C₂T_x enhanced the dehydrogenation efficiency of 12H-NECZ. The measurements of cyclic dehydrogenation indicated that the Pt/S–Ti₃C₂T_x catalyst exhibited good stability after 42 h. This work brought a new strategy for the design of efficient catalysts using two-dimensional materials in the applications of the liquid organic storage hydrogen technology.     

A comparative study of catalytic dehydrogenation of perhydro-N-ethylcarbazole over noble metal catalysts

N-ethylcarbazole is one of the most promising liquid organic hydrogen carriers (LOHCs) as it can be catalytically hydrogenated and dehydrogenated at relatively moderate temperatures. In the present work, we report a systematic study on dehydrogenation of perhydro-N-ethylcarbazole over several important supported noble metal catalysts to identify the optimal catalyst for temperature-controlled dehydrogenation. The reaction takes three consecutive stages with two intermediates of octahydro-N-ethylcarbazole and tetrahydro-N-ethylcarbazole. The initial catalytic activity of the selected noble metal catalysts for the dehydrogenation process was found to follow the order of Pd > Pt > Ru > Rh. 100% selectivity toward the final product of N-ethylcarbazole and fully dehydrogenation was achieved over the supported Pt and Pd catalysts. The kinetics of the three stage dehydrogenation processes over the catalysts was studied and the rate constants were derived. The results indicate that the dehydrogenation reaction rate decreases significantly with the reaction stage for all the selected noble catalysts and the conversion from tetrahydro-N-ethylcarbazole to N-ethylcarbazole was found to be the rate-limiting step of the entire reaction process.     

On-board methanol catalytic reforming for hydrogen Production-A review

Hydrogen has become a versatile and clean alternative to meet increasingly urgent energy demands since its high heating value and renewability. However, considering the hazards of hydrogen storage and transport, in-situ production processes are drawing more attention. Among all the hydrogen carriers, methanol has become one of the research focuses due to its high H/C ratio, flexibility and sustainability. Regarded as the core of hydrogen supply system, catalysts with higher activity, selectivity and stability are continuously developed for improved efficiency. In this review, two groups of catalysts were investigated namely copper-based and group VIII metal-based catalysts. Not only macro indicators such as feedstock conversion and product selectivity, but also micro interaction and reaction mechanism were elaborated, with respect to the effects of promoters, supports, synthesis methods and binary metal components. Notably, several reaction pathways and catalysts deactivation mechanisms were suggested based on this series of inspection of the structure-reactivity relationship, along with a general perception that large surface area, well dispersed metals, small particle size and synergy effects significantly improve the catalytic performance. Accordingly, a novel concept of single-atom catalysts (SACs) was introduced aimed at efficient hydrogen production under more moderate conditions, by combining the advantages of heterogeneous and homogeneous catalysis. Additionally, an efficient reforming process is required by properly regulating the feed flow and heat flow through a coupled system. Conclusively, a thorough supply and demand network of hydrogen based on methanol was presented, giving an overview for on-board applications of hydrogen energy.