Hydrogen production by thermocatalytic decomposition of methane over Ni-Al and Ni-Cu-Al catalysts: Effect of calcination temperature

Thermo catalytic decomposition of methane using Ni-Al and Ni-Cu-Al catalyst prepared by fusion of the corresponding nitrates is studied. The effects of catalyst calcination temperature on the hydrogen yields and the characteristics of the carbon obtained are studied. The role of copper has been also analyzed. Whatever the calcination temperature, all the catalysts show a high and almost constant hydrogen yield without catalyst deactivation after 8 h on stream, which confirms the good performance of this kind of catalysts. The presence of copper enhances the hydrogen production and the best results were obtained using catalysts calcined at 600 °C. Cu has a strong influence on the dispersion of Ni in the catalysts and inhibits NiO from the formation of nickel aluminate even at high calcinations temperatures, which facilitates the formation of the metallic Ni active phase during the subsequent catalyst reduction step. All catalysts tested promote the formation of very long filaments of carbon a few tens of nanometers in diameter and some micrometers long. The structural properties of these carbon filaments highly depend on the presence of Cu:Ni-Cu-Al catalysts promote the formation of a well-ordered graphitic carbon while Ni-Al catalysts enhance the formation of a rather turbostratic carbon.     

Optimization hydrogen production over visible light-driven titania-supported bimetallic photocatalyst from water photosplitting in tandem photoelectrochemical cell

Solar hydrogen production was investigated over a Cu-Ni doped TiO₂ photocatalyst from water photosplitting in a tandem photoelectrochemical cell, which was made up by connecting a modified photoelectrochemical cell to dye solar cell in a series. A mathematical representation for preparation parameters for hydrogen production was successfully generated. Optimization of hydrogen production was conducted with varying preparation parameters of Cu-Ni doped TiO₂ photocatalyst including molar ratios of water, acetic acid and Cu to titanium tetraisopropoxide. The optimum preparation parameters of photocatalyst was obtained at molar ratios of water, acetic acid and Cu to titanium tetraisopropoxide of 32, 4.9, and 5.9, respectively. Physical and photoelectrochemical characterization revealed that low content of water and Cu decreased the charge transfer resistance and charge carrier recombination rate on Cu-Ni/TiO₂ surface. This is attributed to the better crystallinity and less degree of agglomeration which led to obtain optimum particle size at this condition. Maximum hydrogen production rate of 2.12 mL/cm². h was achieved under the optimum condition using the tandem photoelectrochemical cell in the aqueous KOH and glycerol solution under visible light irradiation (λ > 400 nm).     

Renewable hydrogen production: A techno-economic comparison of photoelectrochemical cells and photovoltaic-electrolysis

The present paper reports a techno-economic analysis of two solar assisted hydrogen production technologies: a photoelectrochemical (PEC) system and its major competitor, a photovoltaic system connected to a conventional water electrolyzer (PV-E system). A comparison between these two types was performed to identify the more promising technology based on the levelized cost of hydrogen (LCOH). The technical evaluation was carried out by considering proven designs and materials for the PV-E system, and a conceptually design for the PEC system extrapolated to future, commercial scale.The LCOH for the off-grid PV-E system was found to be 6.22 $/kg_H2, with a solar to hydrogen efficiency of 10.9%. For the PEC system, with a similar efficiency of 10%, the LCOH was calculated to be much higher, namely 8.43 $/kg_H2. A sensitivity analysis reveals a great uncertainty in the LCOH of the prospective PEC system. This implies that much effort would be needed for this technology to become competitive on the market.Therefore we conclude that the potential techno-economic benefits that PEC systems offer over PV-E are uncertain, and even in the best case, limited. While research into photoelectrochemical cells remains of interest, it presents a poor case for dedicated investment in the technology's development and scale-up.     

Low Pt loading,wide area electrospray deposition technique for highly efficient hydrogen evolving electrode in photoelectrochemical cell

Environmental compatibility, high flammability, richest in energy per mass unit, and easy conversion into thermal, mechanical and electrical energy are the key advantages of hydrogen fuel, which makes it an idealized vision for future energy as a promising alternative to the diminishing fossil fuels. Unlike the methods very well known in the literature, we used environmental benign photoelectrochemical (PEC) hydrogen production method. Pt is one of the promising electrode materials for PEC method, but high cost makes it impractical for commercialization. A methodology for low Pt loading (7.22 × 10⁻⁵ g cm⁻²) based on electrospray technique is explained for the preparation of hydrogen evolution electrode. The resulted films are annealed at different temperatures and investigated by different characterization techniques that showed surface morphological and compositional changes with annealing temperature. The pores-type structure is transformed to vertically aligned plate-like structure with annealing temperature. After annealing at 400 °C, Pt film is more oxidized and enriching about ∼30% of film surface area with oxidized Pt. The solar to hydrogen conversion efficiency in water splitting was raised from an initial value of 8.4–10.6% and Pt loading was reduced by approximately 1000-fold (from 0.07 to 7.22 × 10⁻⁵ g cm⁻²). Thus, present high efficient hydrogen electrode preparation method utilized less Pt material than the conventional Pt electrode and the efficiency was increased by 26%. This can be scaled up for becoming a volume production low-cost method.     

The application of MOF-derived CeO2 to synthesize the Cu/CeO2 catalyst for the hydrogen production via water gas shift reaction

The development of catalysts for the water-gas shift (WGS) reaction is attracting attention because of the increased interest in on-site small-scale hydrogen production, which requires highly active and stable catalytic performance under severe conditions. In this study, metal–organic frameworks (MOF), which have been adopted in various fields because of their high surface area, diversity of assemblies, and uniform porosity, were applied to prepare Cu/CeO₂ catalysts for the WGS reaction. MOF-derived CeO₂ (MDC) was obtained from a Ce-BTC-based MOF calcined at different temperatures. Various techniques were used to investigate the physicochemical properties of the Cu/MDC catalysts. Important properties that determine the catalytic performance, such as crystallinity, surface area, Cu dispersion, reducibility, and oxygen storage capacity (OSC), were affected by the treatment temperature of MDC. Among the Cu/MDC catalysts, Cu/MDC prepared with MDC that was treated at 400 °C (Cu/MDC(400)) exhibited the highest CO conversion at reaction temperatures of 200–400 °C. In addition, Cu/MDC(400) maintained 80% of its initial CO conversion after 48 h on stream, even at a very high gas hourly specific velocity of 50,233 mL·g_cat⁻¹·h⁻¹. This result was attributed to the high surface area, Cu dispersion, OSC, and easier reducibility of the Cu/MDC(400) catalyst compared to Cu supported on MDC calcined at other temperatures.     

Maximizing the production of hydrogen and carbon nanotubes: Effect of Ni and reaction temperature

With the objective of maximizing hydrogen and CNTs production, the catalytic cracking of naphtha has been carried out at progressive reaction temperatures i.e. from 600 to 750 °C. The ZSM-5 and nickel impregnated ZSM-5 were used as catalysts for cracking purpose in fluidization mode. The catalyst analysis imparted that impregnation of metallic nickel induces a strong adhesion on MFI structure of ZSM-5 associated with an enhancement in textural properties and acid density. In addition, the results disclose that the incorporation of nickel on ZSM-5 leads to increment in stability of catalyst which in turn pushes the yields of H₂, CNTs and conversion to greater values of 3.29%, 4.84% and 90%, respectively. The as-grown carbon structures over the catalyst surface were found to be multiwall carbon nanotubes confirmed by Raman spectra and TGA analysis where they exhibited high quality (I_D/I_G = 0.65) and purity, respectively, at 750 °C.     

Band gap engineering of Gd and Co doped BiFeO3 and their application in hydrogen production through photoelectrochemical route

The present report deals with the synthesis of Gd and Co doped BiFeO₃ (BFO) i.e. Bi_1-xGd_xFe_1-yCo_yO₃ (BGFCO, x = 0.0, 0.1; y = 0, 0.05, 0.10, 0.20, 0.25) nanoparticles by sol–gel method. The co-doping leads to band gap engineering of BiFeO₃ with the band gap varying from 2.23 eV to 1.77 eV. The band gap engineering coupled with UV–Vis spectroscopy has been used to find the optimum material. The significant lowering in the band gap of the doped BFO is attributed to the deformation produced in Fe–O octahedron geometry as well as rearrangement in its molecular orbitals. The band gap engineering leads to materials with improved solar spectral response which in turn results in better harvesting of solar energy. X-ray diffraction (XRD) patterns indicate the formation of pure phase of BiFeO₃ and its doped variants. The surface morphologies and particle sizes of different compositions have been investigated through scanning electron microscope (SEM). The as synthesized BFO as well as its doped variants have been used as photoanodes for hydrogen production through photoelectrochemical (PEC) splitting of water. The optimum material Bi_0.9Gd_0.1Fe_0.75Co_0.25O₃ (BGFCO-25) with band gap of 1.77 eV has been used as photoanode having PEC configuration of 1 mol/L NaOH as the electrolyte solution and the Pt as cathode using 1.5 AM UV–Vis illumination. This has produced the photocurrent density of 2.03 mA/cm² and hydrogen production rate of 74.57 μmol cm^?2 h^?1. The maximum photo-conversion efficiency has been found to be 2.29% for BGFCO-25 which is higher than that of BFO in which it is 0.76%. This noteworthy enhancement in the photoelectrochemical properties is ascribed to narrowing of the band gap which improves the solar spectral response and allows the absorption of higher density of photons. The stability test of the photoanode has been done through chronoamperometry technique.     

Evaluation of hydrogen evolution reaction on chemical bath deposited Cu2O thin films: Effect of copper source and triethanolamine content

The chemical bath deposition method was used to deposit thin films of cuprous oxide. The effect of copper source and triethanolamine content on the optical, morphological, structural, electrochemical and photoelectrochemical properties of the thin films for the development of photocathodes for hydrogen production was investigated. Triethanolamine promotes the complexing of Cu⁺ ions independent of the copper source used, its increase promotes thicker films due to better growth control and reduction of rapid Cu₂O precipitation in the bulk solution. The increase in thickness promoted a change in preferential orientation from (111) plane to (200) plane, which also influenced and reduced the conductivity because there is a decrease in disorder (Urbach energy E_U). The thickness also varied due to copper source used, reaching the thickest films with copper nitrate while the thinnest films with copper acetate, this tendency is in agreement with their solubility in water. The lower solubility reduces the complexing of Cu ⁺ ions which promotes the Cu₂O precipitation in the bulk solution, limiting the growth of the film. Also, electrical properties varied (measured as disorder E_U) with copper source. The most conductive being the thin films deposited with copper acetate and nitrate while the most resistive being the films deposited with copper sulphate. Very little variation in optical properties was observed, estimating the band gap in the range of 2.62–2.66 eV, while high absorption coefficient (>10⁵ cm^?1) was calculated below the absorption edge (460–470 nm). All thin films showed p-type semiconducting behavior with a flat band potential in the range of ?0.10 to 0.18 V (Ag/AgCl sat electrode), which confirms their ability to work as photocathodes for hydrogen production. The best photoelectrochemical performance was observed with the thinnest films, which also are the most conductive and present the highest values of absorption coefficient.     

Enhanced isosteric heat of adsorption and gravimetric storage density of hydrogen in GNP incorporated Cu based core-shell metal-organic framework

The present work explores the hydrogen adsorption potential of graphene nanoplatelet incorporated core-shell metal-organic frameworks (MOFs). Core-shell MOFs [(HKUST-1@Cu-MOF-2 (HM) and Cu-MOF-2@HKUST-1 (MH)] and their graphene nanoplatelets (GNP) incorporated composites [GNP@HKUST-1@Cu-MOF-2 (GHM) and GNP@Cu-MOF-2@HKUST-1 (GMH)] were synthesized by solvothermal method. The core-shell formation and the structural effect of graphene nanoplatelts incorporation was established and studied using X-ray diffraction analysis, thermogravimetric analysis, scanning electron microscopy and X-ray photoelectron spectroscopy. The hydrogen adsorption studies were carried out at maximum pressure of 2 bar and temperature of 100 K using Sievert's apparatus. GHM exhibited the highest storage capacity of 2.3 wt% at 100 K and 2 bar. Significantly, contrary to MH, the average pore size in HM increased from 1.66 nm to 2.58 nm after the addition of graphene nanoplatelets, resulting in Type IV N₂ sorption isotherm as found through BET studies. Additionally, the theoretical isosteric heat of adsorption was estimated using the Clausius Clayperon equation where GHM showed an exceptionally high isosteric heat of adsorption of 14.7 kJ/mol. Therefore these studies on newly developed core-shell MOF/GNP bring out the effect of GNP incorporation on the structure of the MOF, formation of the local active centres and carbon clusters, which are critical to hydrogen adsorption.     

HYDRIDE4MOBILITY: An EU HORIZON 2020 project on hydrogen powered fuel cell utility vehicles using metal hydrides in hydrogen storage and refuelling systems

The goal of the EU Horizon 2020 RISE project 778307 “Hydrogen fuelled utility vehicles and their support systems utilising metal hydrides” (HYDRIDE4MOBILITY), is in addressing critical issues towards a commercial implementation of hydrogen powered forklifts using metal hydride (MH) based hydrogen storage and PEM fuel cells, together with the systems for their refuelling at industrial customers facilities. For these applications, high specific weight of the metallic hydrides has an added value, as it allows counterbalancing of a vehicle with no extra cost. Improving the rates of H₂ charge/discharge in MH on the materials and system level, simplification of the design and reducing the system cost, together with improvement of the efficiency of system “MH store-FC”, is in the focus of this work as a joint effort of consortium uniting academic teams and industrial partners from two EU and associated countries Member States (Norway, Germany, Croatia), and two partner countries (South Africa and Indonesia).The work within the project is focused on the validation of various efficient and cost-competitive solutions including (i) advanced MH materials for hydrogen storage and compression, (ii) advanced MH containers characterised by improved charge-discharge dynamic performance and ability to be mass produced, (iii) integrated hydrogen storage and compression/refuelling systems which are developed and tested together with PEM fuel cells during the collaborative efforts of the consortium.This article gives an overview of HYDRIDE4MOBILITY project focused on the results generated during its first phase (2017–2019).     

Enhanced polyhydroxybutyrate production through incorporation of a hydrogen fuel cell and electro-fermentation system

The integration of a hydrogen fuel cell with an electro-fermentation system represents a novel approach for improving polyhydroxybutyrate (PHB) accumulation in Ralstonia eutropha H16, using a sustainable energy source. In this study, electro-fermentation noticeably affected cell growth, biomass production, substrate consumption, and PHB accumulation. Final residual biomass concentrations and maximum specific growth rates were enhanced by supplying a 10-mA electric current. Furthermore, a remarkable enhancement in PHB content (30% higher than control) was achieved by redox-mediated electro-fermentation with a 10 mA electric current, upon the addition of a redox mediator. Two-stage cultivation limited the growth suppression caused by redox-mediated electro-fermentation, and also increased the maximum PHB productivity of the system. The additional electrons supplied upon supplementation of the redox mediator accelerated the glycolytic pathway and redox cycling of NADH/NAD⁺, led to a spontaneous boost for adenosine triphosphate (ATP) generation, and further facilitated the biosynthesis of PHB.     

Catalytic performance of dioxide reforming of methane over Co/AC-N catalysts: Effect of nitrogen doping content and calcination temperature

The nitrogen doped activated carbon (AC-N) has been successfully prepared with commercial activated carbon as carbon material followed by a simple N-doping method using melamine as nitrogen sources. Using AC-N as the supports, cobalt supported on N-doped activated carbon (Co/AC-N) were developed and used as catalyst for dry reforming reaction (DRM). It was discovered that the Co/AC-N catalysts revealed much higher catalytic performance for DRM reaction in comparison to activated carbon supported cobalt catalyst (Co/AC). Moreover, the catalytic activity was influenced by preparation conditions of AC-N such as calcination temperature and the doping amount of nitrogen. The catalysts were characterized by BET, XRD, XPS, H₂-TPR, Raman spectroscopy and TEM. It was found that catalytic activities of the catalysts with different calcination temperature and nitrogen doping were influenced by catalyst surface defects and disorders, Co²⁺/Co³⁺ molar ratio, the content of nitrogen function groups (graphitic N, pyrrolic-N and pyridinic-N) and interaction between active metal and support. The Raman spectroscopy illustrated that the N-doped catalyst surface defects and disorders increased, which improved the performances of the Redox catalysts. The XPS valence band also revealed that higher Co²⁺/Co³⁺ molar ratio and nitrogen function groups was achieved by decreasing calcination temperature and increasing nitrogen doping. In short, the doping of nitrogen increased the structural defects and the interaction between active metals and supports, modified the surface electronic structure, which were facilitated the oxidation and reduction of methane and carbon dioxide.     

Effect of Au,Ag and Cu thin films' thickness on the electrical parameters of metal-porous silicon direct hydrogen fuel cell

The effect of the thickness of the gold, silver and cupper films on the electrical properties such as open circuit voltage (V_oc) and short circuit current (I_sc), in the direct hydrogen fuel cell, which uses water as a source of hydrogen, is studied by fabricating Metal/Porous Silicon/n-Silicon/Indium structures. The Porous Silicon (PS) layer on n-type (111) oriented silicon wafers were prepared by anodization. The thin films of Au or Ag or Cu with different thicknesses between 120 and 600 nm were deposited onto the PS surface by the electron-beam technique. The obtained results indicated that V_oc and I_sc, strongly depend on the Au, Ag and Cu layer thicknesses. The Au/PS/n-Si structure generated highest V_oc and I_sc values for all thicknesses of Au film. The best values of V_oc and I_sc were obtained at 325 nm as 0.89 V and 0.021 mA for Au, at 350 nm as 0.75 V and 0.017 mA for Ag, at 350 nm as 0.50 V and 0.010 mA, respectively.     

Transformation of methane into synthesis gas using the redox property of Ce-Fe mixed oxides: Effect of calcination temperature

Ce-Fe mixed oxides prepared by co-precipitation were used as oxygen carriers for converting methane into synthesis gas through gas-solid reactions. The structural evolution and reducibility of Ce-Fe oxygen carriers with calcination temperatures from 600 to 900 °C were investigated by XRD, BET, Raman, XPS and TPR techniques and correlated to their activity for methane selective oxidation. The Ce-Fe mixed oxides calcined at low temperatures (e.g., 600 °C) show abundant oxygen vacancies and high specific surface areas, which enhances the concentration of surface adsorbed oxygen and favors the complete oxidation of methane by means of gas-solid reactions. On the other hand, a calcination temperature of 900 °C results in serious sintering and militates against the formation of Ce-Fe solid solution, which brings about catalytic methane decomposition because of the low lattice oxygen mobility. A compromise calcination temperature at 800 °C favors the interaction between iron and cerium oxides, which could improve the lattice oxygen mobility of Ce-Fe oxygen carrier, leading to a high reactivity for methane selective oxidation. More importantly, the lattice oxygen mobility of the oxygen carrier is enhanced by the generation of oxygen vacancies after a repetitive redox treatment (methane reduction/air re-oxidation), which allows the Ce-Fe oxygen carrier to maintain a high activity and stability during the successive production of synthesis gas through a redox process.     

Effect of calcination temperature of mesoporous alumina xerogel (AX) supports on hydrogen production by steam reforming of liquefied natural gas (LNG) over Ni/AX catalysts

Mesoporous alumina xerogel (AX) supports prepared by a sol–gel method were calcined at various temperatures. Ni/mesoporous alumina xerogel (Ni/AX) catalysts were then prepared by an impregnation method, and were applied to the hydrogen production by steam reforming of liquefied natural gas (LNG). The effect of calcination temperature of AX supports on the catalytic performance of Ni/AX catalysts in the steam reforming of LNG was investigated. Physical and chemical properties of AX supports and Ni/AX catalysts were strongly influenced by the calcination temperature of AX supports. Crystalline structure of AX supports was transformed in the sequence of γ-alumina → (γ + θ)-alumina → θ-alumina → (θ + α)-alumina with increasing calcination temperature from 700 to 1000 °C. Nickel species were strongly bonded to the divalent vacancy of γ-alumina, (γ + θ)-alumina, and θ-alumina through the formation of nickel aluminate phase. In the steam reforming of LNG, both LNG conversion and hydrogen composition in dry gas showed volcano-shaped curves with respect to calcination temperature of AX supports. Among the catalysts tested, Ni/AX-900 (nickel catalyst supported on AX that had been calcined at 900 °C) showed the best catalytic performance. The smallest nickel crystalline size and the strongest nickel–alumina interaction were responsible for high catalytic performance of Ni/AX-900 catalyst in the steam reforming of LNG.     

Effect of organic loading rate on continuous hydrogen production from food waste in submerged anaerobic membrane bioreactor

The characteristics of hydrogen fermentation in a membrane bioreactor (HF-MBR) fed with food waste were investigated at thermophilic condition. The HF-MBR was operated at three different organic loading rates (OLRs) of 70.2, 89.4 and 125.4 kg-COD/m³/day. Biogas production rate increased from 22.4 to 32.8 and 62.5 l/day with OLR. The maximum Hydrogen yield and production rate were 111.1 mL-H₂/g-VS _added and 10.7 l-H₂/L/day at an OLR of 125.4 kg-COD/m³/day. The total carbohydrate degradation was better than 96% throughout the experimental runs. Continuous H₂ production from food waste with CH₄-free biogas was successfully sustained in the HF-MBR for 90 days. The microbial community was dominated by Clostridium sp. strain Z6. The H₂ production was significantly improved by shortening the retention time and increasing the OLRs. The HF-MBR showed an H₂ production capacity at the high OLRs due to its higher cell retention.     

Effect of anodization voltage on performance of TiO2 nanotube arrays for hydrogen generation in a two-compartment photoelectrochemical cell

Highly ordered TiO₂ nanotube arrays were prepared by anodic oxidation of Ti foil under different anodization voltages in ethylene glycol electrolyte. The morphology and photoelectrochemical performance of the TiO₂ nanotubes (NTs) samples were characterized by FESEM and electrochemical working station. Hydrogen production was measured by splitting water in the two-compartment photoelectrochemical (PEC) cell without any external applied voltage or sacrificial agent. The results indicated that anodization voltage significantly affects morphology structures, photoelectrochemical properties and hydrogen production of TiO₂ NTs. The pore diameter and layer thickness of TiO₂ samples increased linearly with the anodization voltage, which led to the enhancement of active surface area. Accordingly, the photocurrent response, photoconversion efficiency and hydrogen production of TiO₂ nanotubes were also linearly correlated with the anodization voltage.     

Photoanodic and cathodic role of anodized tubular titania in light-sensitized enzymatic hydrogen production

An anodized tubular titania (TiO₂) electrode (ATTE) is prepared and utilized as both a photoanode and a cathode in a photoelectrochemical system designed to split water into hydrogen (for use in fuel cells) with the assistance of a hydrogenase enzyme and an external bias of 1.5 V. In particular, the cathodic ATTE acts as a substrate for the immobilization of the enzyme due to its large surface area that results from the tubular oxides. The optimum molar concentration of KOH in anode and cathode compartments is 1.0 M and the optimum amount of enzyme for the cathode is ca. 3.66 units per geometrical unit area (1 cm × 1 cm) of the cathodic ATTE. After exposure to air for three weeks, the enzyme shows a hydrogen evolution rate that is 85.8% of that of an argon-purged enzyme. The rate of hydrogen evolution is increased from ca. 65 (in a slurry system) to more than 140 μmol cm⁻² h⁻¹, even after eliminating the electron relay (methyl viologen) and costly platinum counter electrode.     

Poly(acrylamide-co-vinyl sulfonic acid) p(AAm-co-VSA) hydrogel templates for Co and Ni metal nanoparticle preparation and their use in hydrogen production

P(AAM-co-VSA) hydrogel was prepared at different mole ratios form the corresponding monomers and used in absorption of metal ions such as Co and Ni from aqueous environments. Then, these bound metal ions within the hydrogel matrices were reduced to their metal nanoparticles by aqueous NaBH₄ treatment. Finally, p(AAm-co-VSA)–M (M: Co or Ni) composites were used as reactor in the hydrolysis of NaBH₄ for hydrogen generation. The amounts of metal ions before and after metal nanoparticle formation were determined by Atomic Absorption Spectroscopy (AAS). P(AAm-VSA) hydrogel showed greater absorption tendency for Ni(II) ions than Co(II) ions, and the metal ion binding capacity of these hydrogels was increased with an increase in the amount of VSA in the copolymeric hydrogel. It was also found that although the amount of Ni ions loaded into the hydrogel matrices were more than Co ions, Co metal nanoparticle-containing hydrogel produced hydrogen faster than Ni metal nanoparticle-containing hydrogel composites. The activation energy for the Co nanoparticle-embedded p(AAm-co-VSA) was found as 34.505 kJ mol⁻¹k⁻¹, and other thermodynamic parameters were also calculated. The p(AAm-co-VSA)–Co hydrogel can be used up to 5 times repetitively without any loss of yield but with 55% of catalytic activity.