Hydrogen Reaction Kinetics Research Articles

Boron Nitride Nanotubes for Ammonia Synthesis: Activation by Filling Transition Metals.

Boron nitride (BN), with outstanding stability and robustness in diverse polymorphs, possesses many advantageous properties for industrial applications. Activation of BN materials for nonmetal catalysts is among the most revolutionary and challenging tasks. Taking advantage of quantum size effect and synergistic effect, here we exploit boron nitride nanotubes (BNNTs) encapsulating early transition metal nanowires, which is experimentally feasible, for nitrogen fixation and ammonia synthesis. Using first-principles calculations and microkinetic modeling, we show that the coexisting occupied and unoccupied p states of B atoms in filled BNNTs can effectively mimic the d states of transition metal. They act as electron reservoirs with tunable orbital energies and occupancy, which are beneficial for associative N2 adsorption and hydrogenation. Due to the competition between thermodynamics of gas adsorption and kinetics of hydrogenation reaction, the activity can be optimized by controlling the type of metal filler and size of BN nanotube, achieving a turnover frequency competitive to that of benchmark Fe catalyst. These results manifest a universal strategy for activating BN nanomaterials as a promising family of robust and efficient catalysts and provide vital insights into the activity-band structure relationship for p-block nonmetal catalysts.

In situ long-term membrane performance evaluation of hydrogen-bromine flow batteries

Because of the impracticality of long life time testing of hydrogen bromine flow batteries (HBFBs) under real, cyclic operating conditions, we utilized a high-frequency cycling load accelerated lifetime test (ALT) to evaluate the membrane electrode assembly performance in HBFBs. Four different membrane chemistries were tested to assess the relative long-term performance and durability of the corresponding HBFBs and to understand the main long-term failure mechanism in HBFB technology. The high-frequency cycling load ALT is a highly valuable method to assess long-term HBFB (components) stability and to understand the degradation/failure mechanism. The results showed that the utilization of long side chain perfluorosulfonic acid (LSC PFSA, i.e. Nafion®) membranes result in stable HBFB operation until a sudden cell failure at the end of the cycle life. The use of a more selective (lower bromine species crossover) reinforced LSC PFSA membrane results in approx. five times longer life times. In contrast, the use of a grafted polyvinylidene fluoride (SPVDF) membrane results in a slow incremental performance decrease (or area resistance increase) during the ALT and, again, a sudden cell failure at the end of the cycle life. After the ALT, the LSC PFSA membrane still shows high chemical stability. On the other hand, the grafted SPVDF and SPE membranes show clear membrane degradation visible as a strong decrease in IEC and an increase in ohmic AR. Gradual Pt catalyst degradation-dissolution, that results in insufficient Pt catalyst loading and subsequently low hydrogen reaction kinetics, is the main failure mechanism of the HBFBs. The membrane bromine species crossover rate is directly related to the rate of Pt catalyst degradation-dissolution and scales almost linearly with the cell total ampere-hours. As long as the catalyst loading is sufficient and does not reach a minimum value, this observed Pt degradation-dissolution rate does not significantly impact the cell performance.

Determining Kinetic Barriers in Hydrogen Electrocatalysis in Alkaline Media through Single-Crystal Voltammetry and Kinetic Isotope Effects

The sluggish hydrogen reaction kinetics observed in alkaline media compared to acidic media remain a fundamental challenge in modern electrocatalysis. Despite numerous efforts, the key parameters needed to accurately describe the slow hydrogen reaction kinetics at high pH are still unknown. Thorough understanding of the reaction mechanism and other relevant kinetic parameters is crucial for good electrocatalyst design needed for efficient energy conversion applications. In this work, we undertake an experimental and computational study to determine the effect of solvent dynamics in the kinetics of the hydrogen evolution and hydrogen oxidation reactions (HER and HOR). A possible explanation for the decreased reaction kinetics arises when considering the role of hydroxide on HER/HOR in alkaline media. The Markovic [1,2] group showed that HER/HOR activity can be significantly improved by decorating the catalyst surface with oxophilic sites (such as Ru or Ni(OH)2clusters on Pt), suggesting a “bifunctional” reaction pathway where hydroxide adsorbed on these oxophilic sites is an active participant in HER and HOR. Our past studies [3,4], however, indicate that a bifunctional mechanism where adsorbed hydrogen and hydroxide react is thermodynamically inconsistent, and that kinetic parameters must also be considered for explaining the sluggish HER and HOR kinetics in alkaline media. Other recent efforts have focused on the impact of such oxophilic clusters on the interfacial electric field and water network. The Koper and Feliu [5,6] groups showed that the introduction of Ni(OH)2clusters onto the Pt surface also decreases the ordering of the water network at the interface to accommodate charge transfer through the double layer, the energetics of which are controlled by how strongly water interacts with the interfacial electric field. The introduction of Ni(OH)2clusters therefore decreases the interfacial electric field strength and allows for a more dynamic water network near the interface. In another study, Lia et. al. [7] showed the importance of alkali cations for anchoring reactive species in the double layer region, in accordance with other reports by the Markovic and Koper groups [8,9]. While these studies highlight the profound impact that the interfacial water network rigidity and alkali cations have on the system, the fundamental role of solvent dynamics on adsorption processes and overall reaction kinetics is yet to be deciphered. In this work, we examine through the means of single-crystal voltammetry and microkinetic modeling the effect of solvent dynamics on the overall alkaline hydrogen reaction kinetics via a kinetic isotope effect. By comparing experimentally measured differences between H2O and D2O, we are able to probe the influence of interfacial water dynamics and electrolyte cation choice on adsorption processes (e.g. the Volmer step) and overall reaction kinetics. In addition, we focus on the reaction’s surface sensitivity by comparing Pt(111) and Pt(110) single crystals. Our preliminary results indicate that the HER and HOR kinetics are slower in D2O by approximately 20%, further showing that hydrogen and hydroxide binding energies are not the sole descriptors for HER and HOR in alkaline media, and that solvent reorganization kinetics are important limiting factors. Peak splitting measurements and comparisons with our microkinetic model suggest that the rate-determining step must be hydroxide adsorption, in agreement with the Koper and Feliu groups. The results presented herein reinforce the importance of local barriers and interfacial solvent dynamics for adsorption processes and overall reaction kinetics. [1]Subbaraman et. al. Nat. Mater. 2012, 550-557. [2] Strmcnik et. al. Nat. Chem. 2013, 5, 300–306. [3] Intikhab et. al. ACS Cat. 2017, 7, 8314−8319. [4] Rebollar et. al. JES, 2018, 165 (15), 3209-3221. [5] Ledezma et. al. Nat. Energy 2017, 2(4). [6] Sarabia et. al. ACS App. Mater. Interfaces 2019 11 (1), 612-623. [7] Lia et. al. J. Am. Chem. Soc., 2019, 141 (7), 3232–3239. [8] Subbaraman et. al. Science. 334 (2011) 334, 6060, 1256-1260. [9] Sarabia et. al. ACS App. Mater. Interfaces 2019 11 (1), 612-623.

Synthesis and Characterization of Electrocatalysts for the Hydrogen Evolution and Hydrogen Oxidation Reactions in Alkaline Environment

The development of catalysts for the electrochemical processes of hydrogen systems (e.g. fuel cells and electrolyzer systems) continues to be a significant area of research for renewable energy technologies. One significant challenge has been the development of hydrogen catalysts that can be utilized in alkaline environments as the kinetics of hydrogen reactions decrease by two orders of magnitude in alkaline environments compared to acidic[1,2]. Chemical vapor deposition (CVD) is a conventional tool used in the synthesis of such catalysts[3,4]. This presentation discusses the extending work being done using a variant CVD process known as “Poor Man’s” CVD (PMCVD), to further its impact in electrochemical catalyst application[5]. PMCVD is utilizes an inexpensive vacuum oven to sublime commercially available metal-organic precursors; lowering synthesis cost while providing increased control of particle size and distribution. This work describes the work done to assess the PMCVD mechanisms as well as its use to develop catalysts for the hydrogen oxidation reaction (HOR) and hydrogen evolution reaction (HER) in alkaline environments. Preliminary studies have shown that PMCVD can develop catalysts with highly dispersed nanoparticles on various supports. As depicted in Figure 1, PMCVD synthesized Pt/C and PtRu/C catalysts have shown to have improved kinetics over commercial developed catalysts exhibiting approximately a 40% and 45% increase in average exchange current densities respectively when compared to Tanaka Kikinzoku catalysts of the same loading. An extended function of the PMVCD process is the ability to synthesize alloyed catalysts in a single deposition process. By understanding the parametric effects of temperature and pressure on the deposition process with respect to particle size and distribution, we can optimize the PMCVD process to develop highly active catalysts that can be used in alkaline hydrogen systems. [1] Durst, J. et al. New insights into the electrochemical hydrogen oxidation and evolution reaction mechanism. Energ. Environ. Sci. 7, 2255–2260 (2014). [2]Sheng, W. et al. Hydrogen Oxidation and Evolution Reaction Kinetics on Platinum: Acid vs Alkaline Electrolytes. . Electrochem. Soc. 2010 volume 157, issue 11, B1529-B1536 [3] Garcia, Vargas, et al. “Chemical Vapor Deposition of Iridium, platinum, Rhodium, and Palladium.” Materials Transactions, Vol. 44, No. 9 (2003) pp. 1717-1728 [4] Serp, Philppe, et. al. “Chemical Vapor Deposition Methods for the Controlled Preparation of Supported Catalytic Materials.” Chem. Rev. 2002, 102, 3085−3128 [5] Papandrew, A. et al. Advanced Electrodes for Solid Acid Fuel Cells by Platinum Deposition on CsH2PO4. Chem. Mater. 2011, 23, 7, 1659-1667 Figure 1

Investigating the Effect of Ru(OH)x Surface Decoration on HER/HOR Kinetics

With an increase in energy demand and a rapidly changing climate, there has been an increasing effort within the scientific community to move towards a more sustainable and clean energy system. A clean, safe and sustainable hydrogen energy based system can be provided by electrochemical energy conversion devices such as fuel cells and electrolyzers. Electrolyzers produce hydrogen as a fuel through hydrogen evolution reaction (HER), while fuel cells consume hydrogen to generate energy by hydrogen oxidation reaction (HOR) [1]. It is observed that the more stable alkaline environment is not necessarily the most active for HOR and HER. On Platinum and other catalysts, HOR and HER are extremely fast in acidic electrolytes but several orders of magnitude slower in alkaline electrolytes. The explanation of this unusual pH dependence has been a matter of debate within the scientific community. Understanding the hydrogen reaction kinetics across the pH spectrum will allow the rational design of catalysts for the alkaline electrolyte. Traditionally, hydrogen binding energy (HBE) has been used as the sole activity descriptor, independent of electrolyte pH [2]. This alone, however, is not enough to describe the pH dependence of the HER/HOR activity on metals other than Pt. Markovic et al. have proposed that hydroxide adsorption is necessary to facilitate water recombination/dissociation in base [3]. In our previous work, we have shown that adsorbed hydroxide is not an active participant in HER and HOR reactions but decreases the availability of sites for H adsorption [4][5]. Therefore, any effects of OHad must serve to reduce kinetic barriers associated with the hydrogen reactions. Liu et al. very recently proposed that OHad electrostatically attracts water close to the surface, thereby reducing kinetic barriers [6]. Koper et al. also discussed the role of kinetic barriers in HER kinetics. Through laser-induced temperature-jump experiments, it was shown that Ni(OH)x clusters on the surface of Pt (111), which improves HER kinetics in base, shift the potential of zero free charge (pzfc) at alkaline pH closer to 0 V vs. RHE by ~25 mV [7]. It was hypothesized that the shift of the pzfc closer to the equilibrium potential for HER/HOR increases the water mobility at the surface that leads to a more efficient H/OH-X charge transfer (equivalent to the Volmer step) and consequently improves the HER kinetics. In this work, we discuss the role of Ru(OH)x on Pt in alkaline electrolytes. Surface decoration of Ru(OH)x on Pt leads to a dramatic increase in the HER/HOR activity. The improved activity is correlated to a negative shift in the potential of zero total charge (pztc) towards 0 V vs RHE, where the pztc is measured through CO displacement experiments. Our results indicated that pztc is a valid descriptor for the hydrogen reactions along with pzfc. Two possible hypotheses can explain the effect of pztc. The negative shift in pztc indicates a higher OHad coverage near HER/HOR relevant potentials. While OHad is not an active participant as we have shown in our previous work [4], it decreases the activation energy of the hydrogen reactions by bringing water closer to the interface through the OHad-H2Ox-AM+ adducts, as also indicated by Liu et al. [6]. Alternatively, a shift in pztc may reflect a shift in pzfc, since both follow the same qualitative trends with surface, pH and electrolyte. A negative shift in the pzfc may weaken the electric field at the interface. The weaker electric field leads to a more fluent water structure at the surface and facilitates lower transition-state barriers. More work is required to determine which of these factors plays a greater role in HER/HOR kinetics. Decoration of Ru(OH)x on Pt(111) showed greater improvement of HER/HOR activity compared to when Pt(110) was decorated with the same Ru(OH)x coverage. Based on the structural sensitivity observed on the underlying Pt substrate, we synthesized octahedral Pt nanoparticles with a high density of (111)-like domains. Decorating Octa-Pt/C nanocatalyst with Ru(OH)x clusters yields a higher HER/HOR activity than spherical Pt/C + Ru(OH)x, where spherical Pt/C has a higher density of steps, analogous to the Pt(110) surface. Hence, a highly active nanocatalyst was designed based on the insight from single crystal experiments.

Kinetics Analysis of Multichannel Hydrogen Reactions on Plasmonic-Based Au–GdC Thin-Film Nanocomposites

The reaction kinetics of hydrogen on gold nanoparticles (AuNPs) embedded in gadolinium-doped ceria has been studied in an oxygen-free environment through the time-dependent analysis of the localize...

Quantitative Understanding of the Sluggish Kinetics of Hydrogen Reactions in Alkaline Media Based on a Microscopic Hamiltonian Model for the Volmer Step

The sluggish kinetics of hydrogen evolution/oxidation reactions in alkaline media remains a technical barrier for alkaline membrane fuel cells and a scientific puzzle under heated discussion in fun...

Promoted synergy in core-branch CoP@NiFe–OH nanohybrids for efficient electrochemical-/ photovoltage-driven overall water splitting

Developing low-cost, high-performance electrocatalysts to efficiently convert intermittent solar energy into hydrogen via water splitting is the key to realize sustainable energy cycle but remain challenges. Herein, the three dimensional core-branch CoP@NiFe–OH nanohybrids supported on surface-phosphatized Ni foam (CoP@NiFe–OH/SPNF) are facilely fabricated as the high-performance bifunctional electrocatalysts for alkaline water electrolysis. Theoretical calculation and electrochemical tests manifest that hybridizing CoP with NiFe–OH engenders strong synergistic catalytic effects, leading to fast hydrogen and oxygen evolution reaction kinetics. Meanwhile, hierarchical core-branch arrays and self-supported architecture also result in CoP@NiFe–OH/SPNF with multiple exposing active sites, faster ions, electron transport behavior and superior lifespan. Owing to these typical merits, the bifunctional CoP@NiFe–OH/SPNF electrode expedites overall water splitting with affording a current density of 10, 50 and 100 mA cm−2 only at 1.53, 1.63 and 1.68 V respectively, surpassing the performance of noble (+)IrO2/C//Pt/C(−) and most bifunctional electrocatalysts. As a practical application, a portable solar energy-driven water electrolyzer equipped with the bifunctional CoP@NiFe–OH/SPNF electrode can produce hydrogen at room temperature with a high gas evolution rate of 20.83 μL s−1.

Изучение каталитических свойств мезопористого кремнезема, допированного диспрозием и модифицированного медью на примере реакции гидрирования ксилолов

Catalytic hydrogenation of aromatic hydrocarbons is one of the most important reactions in industry. The use of copper as a modifier of silica is beneficial both from an economic point of view and from the point of view of ease of modification. The catalytic properties of a mesoporous structured material modified by copper (Cu/MCM) and a mesoporous structured material (MCM) doped with dysprosium and modified copper (Dy-Cu/MCM) synthesized by the template method were studied in xylene hydrogenation reactions. The textural characteristics of the catalysts obtained were studied by low-temperature nitrogen adsorption-desorption, ICP, XRF, XRD, SEM, TEM. To determine the specific surface area, the BET model was used with a relative partial pressure P/P0 0.05-0.3. The total pore volume and size distribution of meso- and macroporous was calculated from the desorption curve using the BJH model. With the help of the ICP-method, the metals were also quantitatively determined using the absolute calibration method. For the analysis of microporous, the DFT density functional method was used. The study of the kinetics of hydrogenation of o, m, p-xylenes was carried out in the temperature range 130-170 °C and a hydrogen pressure of 3 atm. The kinetics of gas-phase hydrogenation reactions was studied in a static mode using the method of gas chromatography, on the original setup, which allows on-line analysis of the reaction mixture. The xylene conversions on Cu/MCM and Dy-Cu/MCM catalysts are calculated at various reaction temperatures.

Kinetics and Optimization of Catalytic Transfer Hydrogenation of WCO Using 2-Propanol as a Hydrogen Donor over NiOx–MoOx–CoOx/Zeolite

Process optimization and reaction kinetics of catalytic transfer hydrogenation (CTH) of waste cooking oil (WCO) into jet fuels using zeolite-supported Ni–Co–Mo oxides catalyst in a packed-bed reactor were studied. Experiments were conducted at three different temperatures (360, 390, and 420 °C) to determine the rate constants, the order of reaction, and the activation energy. The kinetics study showed a first-order reaction with the activation energy estimated to be 84 ± 18.7 kJ/mol WCO with 95% confidence. Design of experiment (DOE) was employed to estimate the optimum reaction parameters using the polynomial model (383.7 °C; 14.8 bar; WCO-to-2-propanol ratio = 1.57 mL/mL; weight hourly space velocity (WHSV) = 6.7 h–1). Validation of the model at the optimum operating conditions generated 80% yield of liquid products with 77% alkanes, 3.8% alkenes, and 12.3% aromatics composition, and 6.7% gases, and 100% conversion of WCO. The catalyst was prepared via the wet impregnation method and characterized by X-...

Hydrogen Evolution and Oxidation: Mechanistic Studies and Material Advances.

Electrochemical energy storage and conversion through hydrogen is essential for a clean and sustainable energy system. Highly efficient hydrogen electrocatalysts play a key role in the electrochemical transformation reactions. A comprehensive understanding of the hydrogen reaction kinetics and mechanisms is critical for the catalyst design and development. Especially pH-dependent hydrogen evolution and oxidation reaction (HER/HOR) kinetics receives increasing interest, and understanding its origin adds new knowledge to fundamental hydrogen electrocatalysis. Here, a detailed description of kinetic analysis and reaction mechanisms for HER/HOR, and a brief summary about recent development of highly efficient and cost-effective hydrogen electrocatalysts are presented. Lastly, recent advances in the fundamental understanding of pH-dependent hydrogen electrocatalysis are discussed.

Heterogeneous hydrogenation of phenylalkynes with parahydrogen: hyperpolarization, reaction selectivity, and kinetics.

Parahydrogen-induced polarization (PHIP) is a powerful technique for studying hydrogenation reactions in gas and liquid phases. Pairwise addition of parahydrogen to the hydrogenation substrate imparts nuclear spin order to reaction products, manifested as enhanced 1H NMR signals from the nascent proton sites. Nanoscale metal catalysts immobilized on supports comprise a promising class of catalysts for producing PHIP effects; however, on such catalysts the percentage of substrates undergoing the pairwise addition route-a necessary condition for observing PHIP-is usually low. In this paper, we present a systematic study of several metal catalysts (Rh, Pt, Pd, and Ir) supported on TiO2 in liquid-phase hydrogenation of different prototypical phenylalkynes (phenylacetylene, 1-phenyl-1-propyne, and 3-phenyl-1-propyne) with parahydrogen. Catalyst activity and selectivity were found to be affected by both the nature of the active metal and the percentage of metal loading. It was demonstrated that the optimal catalyst for production of hyperpolarized products is Rh/TiO2 with 4 wt% metal loading, whereas Pd/TiO2 provided the greatest selectivity for semihydrogenation of phenylalkynes. In a study of liquid-phase hydrogenation reaction kinetics, it was shown that reaction order with respect to hydrogen is nearly the same for pairwise and non-pairwise H2 addition-consistent with a similar nature of the catalytically active sites for these reaction pathways.

Multi-objective Operational Optimization of a Hydrotreating Process Based on Hydrogenation Reaction Kinetics

Utility consumptions in hydrotreating (HDT) units are rapidly increasing because more heavy and sour crude oil are being processed to satisfy the increasing market demand of clean fuels, which then cause the direct and indirect emissions of CO2 and SO2. The environmental impacts of utilities should be considered when optimizing an HDT unit. A multi-objective optimization is proposed to reduce the operating cost and environmental impacts of utilities and products based on hydrogenation reaction kinetics. A cracked diesel HDT unit is adopted to illustrate the proposed model which is solved by e-constrains method to obtain the Pareto fronts and three scenarios: minimum operating cost, minimum environmental impacts, and the best compromise. For all scenarios, increasing temperature and decreasing pressure can reduce both the operating cost and environmental impacts. The decreasing of sulfur content in product is at the expense of consuming more utilities. It is not always favorable when the sulfur content of ...

Carbon-Rich Nanomaterials: Fascinating Hydrogen and Oxygen Electrocatalysts.

Hydrogen energy is commonly considered as a clean and sustainable alternative to the traditional fossil fuels. Toward universal utilization of hydrogen energy, developing high-efficiency, low-cost, and sustainable energy conversion technologies, especially water-splitting electrolyzers and fuel cells, is of paramount significance. In order to enhance the energy conversion efficiency of the water-splitting electrolyzers and fuel cells, earth-abundant and stable electrocatalysts are essential for accelerating the sluggish kinetics of hydrogen and oxygen reactions. In the past decade, carbon-rich nanomaterials have emerged as a promising class of hydrogen and oxygen electrocatalysts. Here, the development and electrocatalytic activity of various carbon-rich materials, including metal-free carbon, conjugated porous polymers, graphdiyne, covalent organic frameworks (COFs), atomic-metal-doped carbon, as well as metal-organic frameworks (MOFs), are demonstrated. In particular, the correlations between their porous nanostructures/electronic structures of active centers and electrocatalytic performances are emphatically discussed. Therefore, this review article guides the rational design and synthesis of high-performance, metal-free, and noble-metal-free carbon-rich electrocatalysts and eventually advances the rapid development of water-splitting electrolyzers and fuel cells toward practical applications.

Porous Nickel-Phosphorous Electrode Synthesized By Fast-Simple Electrodeposition Process for Efficient Overall Water Splitting

Water electrolysis is an electrolysis reaction of water, and it is a technology that can produce a large amount of hydrogen environmentally friendly. The purposes of water electrolysis are responding to current hydrogen demand and storing electric energy produced from renewable energy such as solar or wind power to solve their intermittency. Water electrolysis reaction is divided into two half reactions; hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). The theoretical equilibrium potential of the water electrolysis is 1.23 V, but the electrolysis reaction of the actual water requires much higher voltage due to the slow kinetics of oxygen and hydrogen reaction. Therefore, in order to increase hydrogen production efficiency, it is necessary to develop a catalyst having high activity and low cost for OER and HER. At present, transition metal based materials such as iron (Fe), cobalt (Co), and nickel (Ni) are mainly studied as high efficiency and low cost catalysts for oxygen and hydrogen evolution reactions for alkaline water electrolysis. Those transition metals are being studied in the form of various compounds such as oxide, carbide, nitride and phosphide. Among various type of compounds, transition metal phosphides (TMP) have drawn much attention for use in the alkaline water electrolysis system. These TMP catalysts have relatively good catalytic activity and stability for both OER and HER, and have high electrical conductivity compared to oxide based catalyst. In addition, TMP catalysts can be synthesized using non-toxic precursors as compared to nitride and carbide based catalysts. However, since most of TMP catalysts requires several hours to days for synthesis process, including high temperature heat treatment processes, the efficiency problem has not been solved. In this study, we have developed a method for producing nickel-phosphorus electrodes for high efficiency oxygen and hydrogen evolution reaction by fast-simple electrodeposition method. The synthesis procedure of the Ni-P electrodes ends in a few seconds at room temperature. High cathodic current density was applied to promote the generation of hydrogen bubbles and the deposition of Ni-P, thereby obtaining a highly porous structure. Hydrogen bubbles played an important role in forming the porous Ni–P layer by acting as self-dynamic templates. The surface area of highly porous Ni-P (HP Ni-P) was 50 times larger than that of the flat Ni electrode. The HP Ni-P exhibits remarkable electro-catalytic activity and stability towards the HER and OER in alkaline solutions. For the HER, overpotential at 10 mA/cm2 was 180 mV, which is slightly higher than that of Pt/C, but HP Ni-P required a lower overpotential than that of Pt/C at the high current region. For the OER, HP Ni-P shows higher catalytic activity than IrO2 at the entire current region; overpotential at 10mA/cm2 was 287 mV. The durability of HP Ni-P was also superior to the precious metal-based catalysts (Pt/C, IrO2) and Ni electrode in the long-term chronopotentiometry test. This result is ascribed to increase in surface area by hydrogen bubble generation and electronic structure change of Ni. Electronic structure change may lead to change in adsorption energy of intermediate adsorbates such as H*, OH*, O* and OOH*, which are participated in the steps of HER and OER, and can promote the kinetics of reactions in alkaline media. Figure 1

The oxophilic and electronic effects on anchored platinum nanoparticles on sp2 carbon sites: The hydrogen evolution and oxidation reactions in alkaline medium

The impact of sp2 sites of amorphous carbon and carbon nanotubes toward the hydrogen evolution/oxidation reactions on Pt nanoparticles, in alkaline medium, is described. The kinetic enhancement is given through the induced UV-photons of selectively photo-deposited platinum nanoparticles (Pt NPs) onto sp2 domains. The, henceforth, induced sp2 strong metal support interaction favors the electronic, and the oxophilic effects boosting the kinetics of hydrogen reactions.

Optimization of Two-Stage Pygas Hydrogenation Reactor Based on Hydrogen Network Integration

A graphical method is proposed to optimize the two-stage pyrolysis gasoline (pygas) hydrogenation reactor based on the hydrogen network integration. The relation between the hydrogen utility adjustment (HUA) and the parameters of pyrolysis gasoline (pygas) hydrogenation reactor in different situations is deduced based on the hydrogenation reaction kinetics and integration of hydrogen network, and the variation of HUA with temperature is analyzed with the proposed quantitative diagrams. Based on this method, the temperature of first-stage and second-stage reactors can be optimized separately or simultaneously, and the product quality can be taken into consideration to make the results more accurate. The case study shows that this method can identify the optimal reaction temperature in each stage reactor accurately under different conditions and can give significant guidance for pygas hydrogenation unit design and improvement.

Effects of synthetic gas constituents on combustion and emission behavior of premixed H2/CO/CO2/CNG mixture flames

In this study, effects of synthetic gas constituents on combustion and emission behavior of premixed H2/CO/CO2/CNG blending synthetic gas flames were experimentally investigated in a swirl stabilized laboratory scale combustor. Effects of these constituents on flashback and blowout equivalence ratios of respective mixtures were also determined. Firstly, mixtures of CNG/H2/CO with varying H2/CO ratios were tested and then each mixture was diluted with the same amount of CO2 (20% by volume) to better represent synthetic gas. H2/CO ratios of tested gas mixtures were so adjusted that heating value of each gas mixture was low, moderate or high. Combustion behavior of such mixtures was evaluated with respect to measured axial and radial temperature values. Moreover, emission behavior was analyzed by means of emitted CO, CO2 and NOx levels. Flame temperature measurements were conducted with B and K type thermocouples. Emission measurements were performed with a flue gas analyzer, which was equipped with a ceramic coated probe, as well. Results of this study revealed the great impact of gas composition on combustion and emission behavior of studied flames. Two main findings are: H2/CO ratio slightly alters temperature distribution throughout combustor, while hydrogen reaction kinetics play the most significant role in synthetic gas combustion (1), CO2 addition tremendously increases emissions of CO (2).

Optimization of TiH2 content for fast and efficient hydrogen cycling of MgH2-TiH2 nanocomposites

Magnesium hydride is extensively examined as a hydrogen store due to its high hydrogen content and low cost. However, high thermodynamic stability and sluggish kinetics hinder its practical application. To overcome this last drawback, different Ti amounts (y = 0, 0.025, 0.05, 0.1, 0.2 and 0.3) were added to magnesium to form (1-y)MgH2+yTiH2 nanocomposites (NC) by reactive ball milling under hydrogen gas. Thermodynamic stability of the MgH2 phase in NCs was determined using a manometric Sieverts rig. Reversible hydrogen capacity and reaction kinetics were determined at 573 K over 20 sorption cycles under a limited reaction time of 15 min. On increasing Ti amount, reaction kinetics are enhanced both in absorption and desorption leading to a higher reversibility for hydrogen storage with the MgH2 phase. However, titanium increases the molar weight of NCs and forms irreversible titanium hydride. The highest reversible capacity (4.9 wt% H) was obtained for the lowest here studied TiH2 content (y = 0.025).

Hydrogen network optimization by integrating impurity distributions of a fluid catalytic cracker and hydrogenation reaction kinetics

Abstract Minimizing hydrogen consumption in hydrotreating (HDT) units is increasingly important as more heavy and sour crude oil are processed in refineries. A fluid catalytic cracking (FCC) unit commonly links the vacuum gas oil HDT unit with the cracked diesel and the cracked gasoline HDT units. In the course of processing, undesired impurities, such as sulfur, nitrogen and aromatics, pass from the upstream HDT unit, to the FCC unit where they are cracked, and then to the downstream HDT units. As impurity removal can be accomplished in the upstream and downstream HDT units, it is important to understand how the interconnecting FCC unit affects impurity distributions of the adjacent HDT units when minimizing the hydrogen consumption of the whole hydrogen network. A stepwise optimization strategy, using three mathematical models (designated M1, M2 and M3), is proposed to minimize the hydrogen consumption considering impurity distributions within the FCC unit and hydrogenation reaction kinetics. By integrating the FCC unit, M1 is used to investigate the effects of the FCC unit on the upstream and downstream HDT units’ purification degrees and their hydrogen consumption. Based on the hydrogenation reaction kinetics, M2 is used to optimize the operating conditions and to minimize hydrogen consumption within the HDT units according to optimal purification degrees as established by M1. M3 is a hydrogen network optimization model, which is used to obtain the optimal structure of the hydrogen network. Eco-indicator 99 was employed to evaluate the environmental impacts of the hydrogen network. Results show that the hydrogen consumption, the total annual cost (TAC) and the environmental impacts of the hydrogen network are reduced by 44.5%, 34.4% and 38.6%, respectively, compared to the original operation. Optimization without considering the FCC unit and the hydrogenation reaction kinetics only reduced by 32.6%, 19.1% and 28.2% in the hydrogen consumption, TAC and the environmental impacts. Consequently, the effects of the FCC unit and the hydrogenation reaction kinetics should be considered when optimizing the hydrogen network in a refinery.