Study of the use of a Pd–Pt-based catalyst for the catalytic combustion of storage tank VOCs

Catalytic combustion is a very cost-effective way to eliminate volatile organic compounds (VOCs). The components of the VOCs in storage tanks are complex, with alkanes, alkenes and aromatic hydrocarbons being the main components. The aromatic hydrocarbons are the most difficult organic compounds among the storage tank VOCs in terms of catalysing combustion, and they are easy to cause catalyst carbon deposition. In this work, the Pt/γ-Al₂O₃, Pd/γ-Al₂O₃, Pd–Pt/γ-Al₂O₃, Pd–Pt/CeO₂/γ-Al₂O₃ and Pd–Pt/CeO₂/γ-Al₂O₃–N catalysts were prepared using an incipient wetness impregnation method. The performances of the catalysts were investigated using toluene and a simulation of VOCs in gas in a storage tank as model reactants. The study found that Pt has a higher catalytic combustion activity than Pd for the alkanes in the VOC gas in the simulation storage tank, and Pd has a higher catalytic combustion activity than Pt for the alkenes and toluene in the VOC gas in the simulation storage tank. The Pt addition enhances the activity of Pd-based catalysts for VOC catalytic combustion, and the Pd–Pt active component has good active stability. The catalyst prepared by using Pd–Pt alone has a defect in that it exhibits an insufficient oxygen supply performance in the catalytic combustion process. The addition of CeO₂ improves the oxygen supply performance of the Pd–Pt-based catalyst. In addition, the activity of the Pd–Pt/CeO₂/γ-Al₂O₃–N catalyst prepared by reducing the validated amount of Pd–Pt is higher than that of a commercial catalyst.     

A experimental study on the dehydrogenation performance of dodecahydro-N-ethylcarbazole on M/TiO2 catalysts

Liquid organic hydrogen carrier (LOHC) is considered as a promising candidate for large-scale hydrogen storage. In this work, we found that Pt/TiO₂ catalysts exhibited better catalytic activity and selectivity compared to Pd/TiO₂ and commercial Pd/Al₂O₃ catalysts in the dehydrogenation of dodecahydro-N-ethylcarbazole (12H-NECZ) at 453 K. The catalytic activity of the noble metal catalysts followed the trend of Pt/TiO₂ > Pd/TiO₂ > Rh/TiO₂ > Au/TiO₂ > Ru/TiO₂. Compared with the commercial Pd/Al₂O₃, Pt/TiO₂ greatly improved the selectivity and conversion rate, the reaction time was also shortened. In addition, kinetics calculation was carried out to obtain fundamental reaction parameters. It was found that the third step of 4H-NECZ dehydrogenation to NECZ was the rate-limiting step of the entire dehydrogenation reaction for all catalysts.     

Synergetic catalyst effect of Ni/Pd dual metal coating accelerating hydrogen storage properties of ZrCo alloy

Aimed at enhancing the hydrogen absorption/desorption performances of ZrCo system, Ni/Pd dual metal coating is employed on ZrCo alloy combined with the electroless plating and displacement plating. The effects of Ni/Pd dual metal coating on the microstructure, hydrogen storage performance of ZrCo alloys were investigated systematically. The results show that Ni/Pd dual metal coating deposits on the surface of ZrCo sample successfully with the thickness of 500 nm. The hydrogen absorption kinetic property is substantially enhanced for ZrCo alloy after Ni/Pd dual metal coating, which is owing to the catalytic effect of Ni/Pd coating. Further, the activation energies (E_a) for hydrogen absorption and desorption are calculated using the Arrhenius Equation and Kissinger method, respectively. Compared with the bare ZrCo, the activation energies of the Ni/Pd coated samples for hydriding/dehydriding process decrease which facilitate the hydrogenation/dehydrogenation reaction. This work introduces a rational approach by building new catalytic coating on the hydrogen storage materials to improve the hydriding/dehydriding kinetic performance.     

Effect of crystallite size of Al on the reversible hydrogen storage of NaAlH4 and few aspects of catalysts and catalysis

NaAlH₄ has been catalyzed by MWCNT, Ce rich mischmetal (Mm), MmNi₅ and TiO₂ catalysts. The general aspect which relates every catalyst with the hydrogen storage capacity of NaAlH₄ system has been verified through XRD analysis. Interesting features like chemical reduction, grain size variation, hydrogenation/dehydrogenation and phase transformation of the catalytic species are noticed. In the case of reversible hydrogen uptake, an interesting relationship exists between the restored hydrogen capacity, crystallite size of Al (desorbed in the dehydrogenation reaction) and the applied hydrogen pressure. Thus, as far as the reversible hydrogen storage in NaAlH₄ is concerned, the mysterious role of catalyst seems to be a process of restricting the size of Al in a narrow range. The factors considered for analyzing this claim are discussed in detail.     

Enhanced reversibility of fluorine substituted bis-BN cyclohexane for hydrogen storage: A first-principles approach

Liquid organic hydrogen carrier (LOHC) technology, in which hydrogen is captured in a liquid-phase compound, is a key enabler in realizing hydrogen economy. Among the LOHC materials, bis-BN cyclohexane is thermodynamically stable at 150°C and does not release detectable volatile contaminants during catalytic dehydrogenation at room temperature. In this study, we identified other potential bis-BN cyclohexane-based hydrogen storage materials with improved dehydrogenation efficiency and reversibility using density functional theory (DFT) calculations. The dehydrogenation mechanisms of bis-BN cyclohexane with and without functional group (F, Cl, NH₂, CH₃, and CN) substitutions were investigated by calculating the reaction enthalpies and free energies. With the Pt catalyst surface, F-bis-BN cyclohexane has slightly lower dehydrogenation reaction efficiency than the existing bis-BN cyclohexane; however, it was found that the efficiency of the reverse reaction of hydrogenation is significantly improved owing to the F substitution in bis-BN cyclohexane. Therefore, F-bis-BN cyclohexane can exhibit enhanced reversible dehydrogenation–hydrogenation cycles in the presence of a Pt catalyst and is expected to be suitable as an alternative hydrogen carrier. Details of dehydrogenation reaction pathways with activation energies and electronic properties of the dehydrogenation mechanisms have also been discussed herein.     

Pt/CeO2 catalyst synthesized by combustion method for dehydrogenation of perhydro-dibenzyltoluene as liquid organic hydrogen carrier: Effect of pore size and metal dispersion

Liquid organic hydrogen carrier (LOHC) is a chemical hydrogen storage method that stores hydrogen in the form of liquid organics. Dibenzyltoluene (DBT) is a promising LOHC material due to its high storage density, low ignitability, and low cost. In this study, Pt/Al₂O₃ and Pt/CeO₂ catalysts are synthesized using a combustion nanocatalyst synthesis method called the glycine nitrate process (GNP) to obtain high catalytic activity for the dehydrogenation of perhydro-dibenzyltoluene (H18-DBT). Pt/CeO₂ exhibits much faster dehydrogenation than Pt/Al₂O₃, 80.5%/2.5 h versus 3.5%/2.5 h. To investigate the causes of the difference in the dehydrogenation rates, microstructural characterization by N₂ physisorption, CO chemisorption and transmission electron microscopy analysis are conducted, and the catalytic activities are evaluated at various liquid hourly space velocities (LHSVs). The differences in dehydrogenation can be attributed to the mass transport of liquid H18-DBT into the catalyst pores being slow due to the small pores in Pt/Al₂O₃, which is a rarely addressed issue for other LOHC materials. This is because many LOHC materials are dehydrogenated at the gas phase, which has higher diffusivity than that of the liquid phase. Pt/CeO₂ synthesized by the GNP is also compared with a commercial Pt/Al₂O₃ catalyst. The commercial Pt/Al₂O₃ catalyst shows a dehydrogenation of 17.8%/2.5 h, which is much slower than that of Pt/CeO₂ synthesized by the GNP, at 80.5%/2.5 h.     

High active pd@mil-101 catalyst for dehydrogenation of liquid organic hydrogen carrier

Highly dispersed Pd nanoparticles immobilized in MIL-101 (Pd@MIL-101) were prepared and used for the catalytic dehydrogenation of Liquid organic hydrogen carriers (LOHC). The as-synthesized catalysts were characterized and it was found that 3 wt% of Pd@MIL-101 embodied smaller and highly dispersed Pd NPs. The catalytic activities of as-synthesized catalysts were investigated by the dehydrogenation of a representative LOHC compound, perhydro-N-propylcarbazole (12H-NPCZ). The results indicated that 3 wt% Pd@MIL-101 catalyst exhibited good catalytic activity and good reusability for the dehydrogenation of 12H-NPCZ, which is superior to that of commercial 5 wt% Pd/Al₂O₃ catalyst. This study demonstrates that Pd@MIL-101 is a promising dehydrogenation catalyst for the application of LOHC technology.     

Effect of different sized CeO2 nano particles on decomposition and hydrogen absorption kinetics of magnesium hydride

In present paper, different sizes of CeO₂ nanoparticles were synthesized by ball milling and their effect on the absorption kinetics and decomposition temperature of MgH₂ was studied. It was found that a small amount of admixing of the above said catalysts with MgH₂ exhibits improved hydrogen storage properties. Among these different sizes of CeO₂ nanoparticles, 2 weight % admixed CeO₂ with a particle size of ∼10–15 nm led to decrease in desorption temperature by ∼50 K. Moreover, it also shows 1.5 times better absorption kinetics with respect to pure MgH₂. The samples were characterized using SEM, TEM and XRD techniques. The hydrogenation/dehydrogenation properties were measured by gas reaction controller.     

Tuning the alloy degree for Pd-M/Al2O3 (M=Co/ Ni /Cu) bimetallic catalysts to enhance the activity and selectivity of dodecahydro-N-ethylcarbazole dehydrogenation

Hydrogen is a promising candidate to substitute the fossil fuels. However, the efficient hydrogen storage technologies restrict the commercial applications. Developing new catalysts with high activity and selectivity is important for the dehydrogenation reaction in N-ethylcarbazole/dodecahydro-N-ethylcarbazole (NECZ/12H-NECZ) hydrogen storage system. In this work, a series of Pd-M/Al₂O₃ (M = Co, Ni and Cu) bimetallic catalysts are synthesized successfully and show good performance in the dehydrogenation reaction of 12H-NECZ than the commercial Pd/Al₂O₃ catalyst. The Pd₁Co₁/Al₂O₃ catalyst (Practical Pd content = 2.4136 wt%) showed the highest catalytic performance with 95.34% H₂ release amount, TOF of 230.5 min⁻¹ and 85.4% selectivity of NECZ. Combined with the characterization analysis, it can be proposed that the dehydrogenation performance of 12H-NECZ is dependent on the alloy phases, reasonable electronic structures and nanoparticle size of catalysts. The fine-tuned alloy degree and appropriate nanoparticle size of Pd₁Co₁/Al₂O₃ bring the 17.7% increase of H₂ release amount and 99.5% increase of NECZ selectivity than those of Pd/Al₂O₃. For the bimetallic catalysts, the enhancement of selectivity of NECZ is mainly from the increase of the kinetic constant of rate-limiting step.     

Tuning the mesopore-acid-metal balance in Pd/HY for efficient deep hydrogenation saturation of naphthalene

Hydrogenation of polycyclic aromatic hydrocarbons to saturate all rings is of great importance for hydrogen storage and fuel production, which is extremely hard due to the hardly broken π bond. In this work, various bifunctional catalysts of HY zeolite supported Pd (Pd/HY) are prepared by a facile impregnation method for hydrogenation saturation of naphthalene. The system of Pd/HY is finely tuned by changing the SiO₂/Al₂O₃ ratio and mesopore volume of HY. The influences of active metal, acid sites and mesopores on the catalytic performance are revealed. A factor termed mesopore-acid-metal factor (X) is defined to comprehensively and quantitatively describe the states of mesopores, acid sites, and Pd NPs in the Pd/HY catalyst. In our catalytic system, X correlates the yield of decalin well with a R² value of 96.03%. The quantitative structure-activity relationship of the Pd/HY system has been determined. By balancing the mesopores, acid sites and Pd state, high decalin yield and good stability have been achieved. In addition, the kinetic behaviors of naphthalene hydrogenation on the optimal catalyst are preliminarily investigated and the apparent activation energy for the formation of tetralin is calculated to be 61.64 kJ/mol.     

A YH3 promoted palladium catalyst for reversible hydrogen storage of N-ethylcarbazole

N-ethylcarbazole (NEC) is a promising liquid organic hydrogen carrier, while sluggish kinetics of hydrogen absorption and desorption restrict its application. To overcome that, a YH₃ promoted palladium catalyst Pd/Al₂O₃-YH₃ is developed in this work by taking advantage of the fast reversible hydrogenation and dehydrogenation kinetics of YH₃. With the Pd/Al₂O₃-YH₃, NEC can reversibly store 5.5 wt% hydrogen in 4 h below 473 K. The performance is the best compared to that of all the reported catalysts for both hydrogen absorption and desorption. Moreover, there are no gaseous impurities produced and no performance decay during three hydrogen storage cycles. The excellent performance derives from the intrinsic high catalytic activity of Pd/Al₂O₃ and the promoting effect of YH₃ by providing a new hydrogen transfer path, making NEC more attractive for practical application.     

A highly adaptable Ni catalyst for Liquid Organic Hydrogen Carriers hydrogenation

Reducing the cost of hydrogenation/dehydrogenation catalysts and improving the catalytic activity are essential steps to promote the commercial application of Liquid Organic Hydrogen Carriers (LOHCs) technology. We reported a series of highly adaptable 70 wt% Ni supported catalysts prepared by a facile co-precipitation method. The as-prepared catalysts were used in the hydrogenation of several promising LOHCs candidates, including benzene, N-propylcarbazole, N-ethylcarbazole and dibenzyltoluene. By adjusting the ratio of Al and Si, the Ni₇₀/AlSiO-1/1 catalyst with Al and Si in a molar ratio of 1:1 presents highest catalytic activity for hydrogenation of the above LOHCs, indicating the catalyst is highly adaptable for different LOHCs. The characterization results proved that the presence of SiO₂ could significantly weaken the interaction between metal and carrier and decrease the formation of NiAl₂O₄ species, which is beneficial to the reducibility of Ni. The introduced Al₂O₃ can inhibit the agglomeration of Ni and increase the dispersion of the metal. Besides, the Ni₇₀/AlSiO-1/1 catalyst was used to hydrogenate N-propylcarbazole by 5 cycles. In the fifth cycle, the hydrogen uptake reached the theoretical hydrogenation storage within 1.5 h, which suggested the excellent stability of the catalyst. Because of its low cost, high efficiency, high adaptation and highly stable, the self-made Ni catalyst has potential prospect in large-scale LOHCs application.     

Fundamental understanding of the role of potassium on the activity of Pt/CeO2 for the hydrogen production from ethanol

The effect of potassium as a promoter on the activity of Pt/CeO₂ catalysts for hydrogen production from ethanol was studied in this work. The Pt/CeO₂ catalysts with or without potassium were prepared with incipient impregnation method and analyzed using synchrotron-based X-ray diffraction (XRD), X-ray absorption near-edge spectroscopy (XANES), oxygen adsorption, ethanol temperature programmed desorption (TPD) NH₃-TPD, and TPR. We find that Pt particles on all CeO₂-based catalysts were less than 2 nm. Potassium addition did not have benefit on hydrogen production activity. However, potassium modified active sites, acidity, and interaction between Pt and CeO₂. Oxygen storage capacity (OSC) was decreased, while the acidity was neutralized and the interaction between Pt and CeO₂ was weakened when potassium was added to Pt/CeO₂. Potassium promoted ethanol decomposition reaction while inhibited decarbonylation of acetaldehyde and CH₄ formation. This work confirms our previous finding that Pt-CeO₂ synergistic interaction played an important role for hydrogen production from bio-alcohols.     

Pd‐Ni/Cd loaded PPy/Ti composite electrode: Synthesis,characterization, and application for hydrogen storage

A wide compositional range of Pd‐Ni/Cd on polypyrrole (PPy)‐modified Ti plates (Pd‐Ni/Cd/PPy/Ti) was fabricated via electrochemical deposition. The hydrogen absorption properties of the prepared Pd‐Ni/Cd/PPy/Ti electrodes were evaluated using cyclic voltammetry and chronoamperometry in acidic media. The optimal Pd₃₆‐Ni₇/Cd₅₇/PPy/Ti electrode achieved a hydrogen storage capacity of 331.3 mC cm^?2 mg^?1 and an H/Pd ratio of 0.77. The enhancement of the hydrogen storage was attributed to a synergistic effect between the Pd‐Ni/Cd catalysts. The surface morphology, crystallinity, and chemical composition of the Pd‐Ni/Cd/PPy/Ti electrode were characterized using scanning electron microscope (SEM), X‐ray diffraction (XRD), and X‐ray photoelectron spectroscopy (XPS), respectively. Hydrogen spillover occurred on the trimetallic catalysts, and secondary hydrogen spillover occurred on the PPy/Ti support. The enhanced hydrogen sorption capacity was due to both the synergistic effect of the trimetallic catalysts and the assistance of PPy, making Pd‐Ni/Cd/PPy/Ti a promising hydrogen storage material.     

Hydrogenation properties of pure magnesium and magnesium–aluminium thin films

We have studied the hydrogenation/dehydrogenation behaviour of multilayered stacks of Pd/Mg/Pd and Pd–Fe(Ti)–Mg–Al–Mg–Fe(Ti)–Pd grown by electron beam physical vapour deposition. The palladium coating was deposited at both sides of the structure to ensure a fast dissociation rate and good transport properties for hydrogen as well as to avoid oxidation of magnesium either from atmosphere as from the substrate surface. Fe and Ti layers were included in the stack composition in order to assess their possible catalyst effect as well as to prevent the formation of Mg_xPd_y intermetallics during the thermal treatments. We have studied the structure evolution after thermal treatments as well as after the hydrogenation and dehydrogenation processes using XRD. We have also followed the reactions kinetics by resistometry and differential scanning calorimetry. The nanostructured Mg films have been hydrogenated at temperature as low as 50 °C in few minutes. Adding aluminium to magnesium has improved its hydrogenation capacity. We have also observed that the formation of an Mg_xAl_y intermetallic before hydrogenation improves the storage capacity. We have confirmed that titanium is a better catalyst for the hydrogenation/dehydrogenation of the Mg films.     

Lanthanide modified Pt/CeO2-based catalysts for methane partial oxidation: Relationship between catalytic activity and structure

In most cases, reasonable design and construction of Pt/CeO₂-based catalysts and detailed exploration of relationship between its structural characteristics and the catalytic activity are crucial to improve the catalytic performance and reduce the cost. In this work, a series of CeO₂ doped with lanthanide metal ions (La, Nd, Er and Yb) has been successfully synthesized, and then Pt is introduced through impregnation. The morphology, structure and component analysis are characterized by SEM, TEM (HRTEM), EDS, XRD, ICP-AES, XPS, UV Raman, O₂-TPD, H₂-TPR and CO or O₂-pulse chemisorption, and the corresponding catalytic performances are developed by partial oxidation of methane. On the basis of the analysis of the structural properties of various catalysts, it is found that the Pt/CeLa catalyst shows the best catalytic performance due to its low valence state of Pt, excellent oxygen migration capacity and oxygen storage capacity, T₅₀ is 510 °C and the selectivity is superiority. What's more, the modification of CeO₂ by lanthanide metal ions especially La³⁺ can effectively change the oxygen activity of supports, so that this catalyst can be used in various redox catalytic reactions.     

Pd/NH2-KIE-6 catalysts with exceptional catalytic activity for additive-free formic acid dehydrogenation at room temperature: Controlling Pd nanoparticle size by stirring time and types of Pd precursors

Pd nanoparticle size is one of important factors to determine the catalytic activity of formic acid dehydrogenation catalysts. Thus various approaches to minimization of Pd nanoparticles have been attempted. In this study, we first tried to decrease Pd nanoparticles size and increase Pd dispersion of Pd/NH₂-mesoporous silica (Pd/NH₂-KIE-6) catalysts by controlling only stirring time and types of Pd precursors. It was demonstrated that the stirring time and types of Pd precursors significantly affect the performance of the catalysts. As a result, the Pd/NH₂-KIE-6 exhibited the highest catalytic activity (TOF: 8185 mol H₂ mol catalyst^?1 h^?1) ever reported for additive-free formic acid dehydrogenation at room temperature. In addition, the Pd/NH₂-KIE-6 provided higher TOF even than the case with additives such as sodium formate. Considering that the catalytic activity of Pd-based catalysts for formic acid dehydrogenation was previously controlled by promoter, support type and surface chemistry of supports, controlling the stirring time and types of Pd precursors is novel and very intriguing solutions to go beyond the current kinetic limitation for formic acid dehydrogenation.     

WO3/C supported Pd catalysts for formic acid electro-oxidation activity

Pd-WO₃/C ternary hybrid was designed as a high-efficient catalyst towards formic acid electrooxidation. WO₃/C hybrids were first prepared with two different synthesis order, and then used as the supports to synthesize two kinds of Pd-WO₃/C catalysts by a quick and facile microwave-assisted ethylene glycol method. Compared with Pd/C, the catalytic performances of two Pd-WO₃/C catalysts towards formic acid electrooxidation are significantly enhanced. We elected the better synthesis order and optimized the best proportion of WO₃ and C in the hybrid catalyst. When the mass content of WO₃ is 20% of the mass of the support, Pd nanoparticles with narrower particle size distribution are more uniformly dispersed on the surface of WO₃/C support than the other counterparts, resulting in the highest performance in terms of activity and stability towards formic acid oxidation among all the samples. The reasons for the performance improvement may be: first, Pd nanoparticles in Pd-WO₃/C catalysts are of small size and evenly distributed; second, there may be the catalyst-support interaction between Pd and WO₃, substantially improving the catalytic capability of Pd-WO₃/C catalysts; finally, the hydrogen spillover effect produced by WO₃ significantly expedites the dehydrogenation of formic acid on the surface of Pd-WO₃/C.     

Reversible hydrogen storage in vapour deposited Mg-5 at.% Pd powder composites

Mg-5 at.% Pd powder composites derived from multilayered films of Mg and Pd deposited in Pd/Mg/Pd/Mg/Pd layer configuration by thermal evaporation reversibly store about 3.5 wt.% hydrogen up to 15 cycles under mild conditions of pressure and temperature. Hydrogenation takes place at 0.15 MPa hydrogen pressure while dehydrogenation occurs in a dynamic rotary vacuum. Each process is completed in about three hours. The temperature of a dehydrogenation or hydrogenation step is about 5–10 K higher than the preceding hydrogenation or dehydrogenation step. The hydrogenation temperature of the first cycle is 343 K whereas the dehydrogenation temperature of the 15th cycle is 423 K. The hydrogen storage capacity of composite is the manifestation of fine-grained microstructure of Mg and the catalytic properties of Pd. It declines beyond 423 K due to the exhaustion of metallic Pd as a result of the formation of Mg–Pd intermetallic compounds. This approach presents a simple and rapid method of preparing Mg–Pd composites for hydrogen storage applications.     

Temperature controlled three-stage catalytic dehydrogenation and cycle performance of perhydro-9-ethylcarbazole

Organic liquid heteroaromatic compounds, e.g. 9-ethylcarbazole, are potentially promising hydrogen storage materials because they can be catalytically hydrogenated and dehydrogenated at relatively moderate temperatures. In the present work, the cyclic hydrogenation of 9-ethylcarbazole and the temperature controlled stage-wise dehydrogenation of perhydro-9-ethylcarbazole were investigated. Full hydrogenation of 9-ethylcarbazole was realized over a 5 wt% Ru/Al₂O₃ catalyst at 180 °C and 80 bar, yielding a gravimetric density of 5.79 wt%. The catalytic dehydrogenation of perhydro-9-ethylcarbazole over a 5 wt% Pd/Al₂O₃ catalyst was found to undergo a three-stage process, i.e. perhydro-9-ethylcarbazole → octahydro-9-ethylcarbazole, octahydro-9-ethylcarbazole → tetrahydro-9-ethylcarbazole, and tetrahydro-9-ethylcarbazole → 9-ethylcarbazole with the initial reaction temperatures of 128 °C, 145 °C and 178 °C, respectively. Our results indicate that 9-ethylcarbazole displays an excellent cycle performance with very little capacity degradation after 10 cycles of catalytic hydrogenation and dehydrogenation. The hydrogen gas produced from the dehydrogenation possesses a high purity of over 99.99% with no carbon monoxide or other poisonous gases for fuel cells.