Insight into NiCo-based nanosheets modified MnS/Mn0·2Cd0·8S hybrids for enhanced visible-light photocatalytic H2 evolution

Bimetallic compounds nanocrystals exhibited great potential in catalysis due to the synergistic effects and encouraging performance. Herein, a series of NiCo-based nanosheets, including NiCo LDH/NiCo(OH)₂, NiCo, and NiCo₂O₄, have been developed to modify MnS/Mn_0·2Cd_0·8S (MMCS) nanoparticles for photocatalytic H₂ production under visible light (λ > 420 nm). The two-dimensional (2D) NiCo₂O₄ and NiCo were derived from the oxidation and reduction process of the as-prepared NiCo LDH/NiCo(OH)₂ nanosheets, respectively. MMCS nanoparticles were prepared using a one-pot solvothermal method and then integrated into three different NiCo-based nanosheets through a simple hybridization approach. Compared to pure MMCS, the resultant NiCo-based nanosheets/MMCS hybrids show dramatically improved visible-light photocatalytic activities. Moreover, among the three types of composites, NiCo₂O₄-MMCS (7%NiCo₂O₄-MMCS) displays the highest H₂ production rate of 3.31 mmol g^?1 h^?1 with the apparent quantum efficiency of 6.42% at 420 nm, approximately 22 and 5 times that of pure MMCS (0.15 mmol g^?1 h^?1) and Pt/MMCS (0.67 mmol g^?1 h^?1), respectively. The remarkably enhanced photocatalytic activities of the NiCo LDH/NiCo(OH)₂-MMCS, NiCo-MMCS, and NiCo₂O₄-MMCS are mainly ascribed to the formed type-II, Schottky, and p-n heterojunctions, respectively, which efficiently boost photogenerated charge carrier separation and migration. In this paper, we intensively investigate the roles of three different NiCo-based nanosheets in the Mn_xCd_1-xS-based system. This work provides an effective strategy to design and construct the innovative 2D bimetallic compounds-based catalysts for high-efficiency photocatalytic H₂ production.     

The influence of Ni loading on coke formation in steam reforming of acetic acid

Steam reforming of acetic acid on Ni/γ-Al₂O₃ with different nickel loading for hydrogen production was investigated in a tubular reactor at 600 °C, 1 atm, H2O/HAc = 4, and WHSV = 5.01 g-acetic acid/g-cata.h^?1. The catalysts were characterized by temperature programmed oxidation (TPO) and differential thermal analysis (DTA), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). The results showed that the amount of deposited carbidic-like carbon decreased and graphitic-like carbon increased with Ni loading increasing from 9 to 15 wt%. The Ni/γ-Al₂O₃ catalyst with 12 wt% Ni loading had higher catalytic activity and lower coke deposited rate.     

Ni2P(O)–Fe2P(O)/CeOx as high effective bifunctional catalyst for overall water splitting

The development of cost-effective bifunctional catalysts with excellent performance and good stability is of great significance for overall water splitting. In this work, NiFe layered double hydroxides (LDHs) nanosheets are prepared on nickel foam by hydrothermal method, and then Ni₂P(O)–Fe₂P(O)/CeO_x nanosheets are in situ synthesized by electrodeposition and phosphating on NiFe LDHs. The obtained self-supporting Ni₂P(O)–Fe₂P(O)/CeO_x exhibit excellent catalytic performances in alkaline solution due to more active sites and fast electron transport. When the current density is 10 mA cm^?2, the overpotential of hydrogen evolution reaction and oxygen evolution reaction are 75 mV and 268 mV, respectively. In addition, driven by two Ni₂P(O)–Fe₂P(O)/CeO_x electrodes, the alkaline battery can reach 1.45 V at 10 mA cm^?2.     

An efficiency aqueous hybrid supercapacitor with high working voltage based on porous PbO2/WO3·H2O positive electrode

The PbO₂/WO₃·H₂O composite is prepared by embedding the suspending WO₃·H₂O nanosheets into the PbO₂ matrix during the galvanostatic deposition. The PbO₂ matrix provides a metal-like conductive platform for WO₃·H₂O nanosheets, and the doping of WO₃·H₂O nanosheets improves the utilization rate and specific capacitance of the PbO₂ electrode material. At the same time, the sulfate effect of PbO₂ is reduced and its cycle life is prolonged. Therefore, the PbO₂/WO₃·H₂O composite has a high specific capacitance (～515 F/g), a large electrochemical active area, a large electrochemical window (～2 V), and a long life. The PbO₂/WO₃·H₂O//AC AHS based on the PbO₂/WO₃·H₂O positive electrode presents high working voltage of 2.25 V, high specific capacitance of 114.5 F/g, high energy density of ～49.8 Wh/kg, high power density of ～358.6 W/kg, and long lifespan over traditional aqueous hybrid supercapacitor (at 10 mA/cm²).     

Improvements in hydrogen production from methane dry reforming on filament-shaped mesoporous alumina-supported cobalt nanocatalyst

The mesoporous gamma-alumina (γ-Al₂O₃) synthesized via evaporation-induced self-assembly method (EISA) using inorganic salt, Al(NO₃)₃·9H₂O precursor and water-ethanol solvent mixture was implemented as a support for Co catalyst in methane dry reforming at 973–1073 K under 1 atm. The γ-Al₂O₃ support possessed filament-shaped morphology with surface area of 173.4 m² g^?1 and cobalt nanoparticles were successfully dispersed on support with small crystallite size of 7.8 nm. The stability of 10%Co/Al₂O₃ was evident for CH₄ and CO₂ conversions at 1023 and 1073 K. CH₄ conversion could reach to 76.2% while 81.6% was observed for CO₂ conversion at 1073 K. Although graphitic and amorphous carbons were unavoidably formed on used catalyst, 10%Co/Al₂O₃ exhibited an outstanding performance comparable to noble metals with the desired ratio of H₂/CO for downstream Fischer-Tropsch process.     

Improved proton conduction of sulfonated poly (ether ether ketone) membrane by sulfonated covalent organic framework nanosheets

A type of sulfonated covalent organic framework nanosheets (TpPa-SO₃H) was synthesized via interfacial polymerization and incorporated into sulfonated poly (ether ether ketone) (SPEEK) matrix to prepare proton exchange membranes (PEMs). The densely and orderly arranged sulfonic acid groups in the rigid skeleton of the TpPa-SO₃H nanosheets, together with their high-aspect-ratio and well-defined porous structure provide proton-conducting highways in the membrane. The doping of TpPa-SO₃H nanosheets led to an increased ion exchange capacity up to 2.34 mmol g^?1 but a 2-folds reduced swelling ratio, remarkably mitigating the trade-off between high IEC and excessive swelling ratio. Based on the high IEC and orderly arranged proton-conducting sites, the SPEEK/TpPa–SO₃H–5 membrane exhibited the maximum proton conductivity of 0.346 S cm^?1 at 80 °C, 1.91-folds higher than the pristine SPEEK membrane. The mechanical strength of the composite membrane was also improved by 2.05-folds–74.5 MPa. The single H₂/O₂ fuel cell using the SPEEK/TpPa–SO₃H–5 membrane presented favorable performance with an open voltage of 1.01 V and a power density of 86.54 mW cm^?2.     

Efficient interfacial charge transfer of 2D/2D CoP/CdS heterojunction for extremely boosted photocatalytic H2 evolution performance

Constructing 2D/2D heterojunction photocatalysts has attracted great attentions due to their inherent advantages such as larger interfacial contact areas, short transfer distance of charges and abundant reaction active sites. Herein, 2D/2D CoP/CdS heterojunctions were successfully fabricated and employed in photocatalytic H₂ evolution using lactic acid as sacrificial reagents. The multiple characteristic techniques were adopted to investigate the crystalline phases, morphologies, optical properties and textual structures of heterojunctions. It was found that integrating 2D CoP nanosheets as cocatalysts with 2D CdS nanosheets by Co–S chemical bonds would significantly boost the photocatalytic H₂ evolution performances, and the 7 wt% 2D/2D CoP/CdS heterojunction possessed the maximal H₂ evolution rate of 92.54 mmol g^?1 h^?1, approximately 31 times higher than that of bare 2D CdS nanosheets. Photoelectrochemical, steady photoluminescence (PL) and time-resolved photoluminescence (TRPL) measurements indicated that there existed an effective charge separation and migration over 2D/2D CoP/CdS heterojunction, which then markedly lengthened the photoinduced electrons average lifetimes, retarded the recombination of charge carriers, and caused the dramatically boosted photocatalytic H₂ evolution activity. Moreover, the density functional theory (DFT) calculation further corroborated that the efficient charge transfer occurred at the interfaces of CoP/CdS heterojunction. This present research puts forward a promising strategy to engineer the 2D/2D heterojunction photocatalysts endowed with an appealing photocatalytic H₂ evolution performance.     

Effects of alumina powder characteristics on H2 and H2O2 production yields in γ-radiolysis of water and 0.4 M H2SO4 aqueous solution

The effects of powder characteristics on H₂ and H₂O₂ productions in ⁶⁰Co γ-radiolysis were studied in pure water and in 0.4 M H₂SO₄ aqueous solutions containing alumina powders. In 0.4 M H₂SO₄ solution, the H₂ yields strongly depended on alumina structures and decreased in the order of α > θ > γ-alumina, although the specific surface areas increased as α < θ < γ. The yields increased with increasing specific surface area when compared among α-alumina. In pure water, similar dependence was observed but not as strong as that for 0.4 M H₂SO₄ solution. The H₂O₂ yields were strongly decreased by adding the alumina powders in both water and 0.4 M H₂SO₄ aqueous solution, although the amounts of decrease were almost neither correlated with specific surface areas nor structures. The enhancing H₂ production was discussed in terms of the electron supply from alumina to aqueous solution as well as the adsorption of OH radicals on alumina surfaces.     

Hydrogen production from COx-Free thermocatalytic decomposition of methane over the mesoporous iron aluminate spinel (FeAl2O4) nanopowder supported nickel catalysts

In the present study, the thermocatalytic decomposition of methane (TDM) was performed over the NiO(x)/FeAl₂O₄ catalysts with various contents of nickel oxide. The FeAl₂O₄ catalyst support with mesoporous structure and high S_BET (80.26 m² g^?1) was synthesized according to the carbonate-based mechanochemical method. The calcined catalysts were characterized by the XRD, BET, H₂-TPR, CO₂-TPD, TPO, and FESEM analyses. The obtained results demonstrated that the S_BET of the synthesized catalysts reduced from 62 to 26 m² g^?1 by increasing the nickel loading from 20 to 60 wt.%, which is ascribed to the blockage of FeAl₂O₄ support pores. Furthermore, the activity results showed that the catalytic activity improved by increasing the nickel content from 20 to 50 wt.% due to the rise of active site concentration. However, the methane conversion was reached to 40% at 600 °C over the NiO(50)/FeAl₂O₄ catalyst. The more increment of nickel content decreased the catalytic efficiency due to the decline in active phase dispersion. Moreover, the increment amount of deposited carbon was seen by increasing the weight percentage of NiO. Therefore, the catalyst with 50 wt.% of NiO possessed excellent catalytic potential in the TDM process under the hard operating conditions (GHSV = 50,000 ml.h^?1.g^?1_cat). The influence of GHSV, feed ratio (CH₄:N₂), calcination temperature, and reduction temperature on the textural properties and catalytic activity of the NiO(50)/FeAl₂O₄ catalyst were also evaluated in detail.     

Hollow hexagonal NiSe–Ni3Se2 anchored onto reduced graphene oxide as efficient electrocatalysts for hydrogen evolution in wide-pH range

Integrating transition metal complexes with carbon-based materials, especially graphene, is a useful strategy for synthesizing effective hydrogen evolution catalysts. Herein, we report a design of hollow hexagonal NiSe–Ni₃Se₂ nanosheets grown on reduced graphene oxide (NiSe–Ni₃Se₂/rGO) by a simple hydrothermal method as an effective catalyst for hydrogen evolution reaction (HER) in the full pH range. In 0.5 M H₂SO₄, the NiSe–Ni₃Se₂/rGO possesses 112 mV to achieve 10 mA cm^?2 and a small Tafel slope (61 mV dec^?1). In 1.0 M PBS and 1.0 M KOH, the overpotentials are 261 and 188 mV at 10 mA cm^?2, and Tafel slopes are 103 and 92 mV dec^?1, respectively. Meanwhile, it owns good cycle stability and durability over 20 h in the whole pH range (0-14). In all solutions, the HER performance of NiSe–Ni₃Se₂/rGO is better than that of NiSe–Ni₃Se₂. This is because the rGO substrate accelerates the electron transfer and improves the electrical conductivity, increasing HER activity of catalyst.     

Application of a composite electrolyte in a solid-acid fuel cell system: A micro-arc oxidation alumina support filled with CsH2PO4

A micro-arc oxidation alumina (MOA) support filled with a CsH₂PO₄ proton conductor was investigated as an inorganic composite electrolyte for a H₂/O₂ solid-acid fuel cell (SAFC). The MOA support was polycrystalline and contained α- and γ-Al₂O₃ phases; while, the proton conductor CsH₂PO₄ formed an interlaced network within the MOA support. The single-module SAFC using the fabricated MOA/CsH₂PO₄ membrane delivered a peak power of ∼38.5 m W cm⁻² and a proton conductivity of ∼2.1 m S cm⁻¹ at a low temperature (25 °C). Compared to a SAFC using an anodic alumina membrane composite electrolyte (AAM/CsH₂PO₄ SAFC), which displayed rapid degradation, the SAFC using the MOA/CsH₂PO₄ composite electrolyte showed improved stability with cycling. This was attributed to the crystalline α-Al₂O₃ phase that was part of the MOA support that had increased the chemical resistance.     

Catalytic reforming of glycerol in supercritical water with nickel-based catalysts

The catalytic performance of nickel catalysts supported on La₂O₃, α-Al₂O₃, γ-Al₂O₃, ZrO₂, and YSZ for supercritical water reforming of glycerol was investigated. Experiments were conducted in a tubular reactor made of Inconel-625 with the temperature range of 723–848 K under a pressure of 25 MPa. Carbon formation causing operation failure was observed for α-Al₂O₃, γ-Al₂O₃ and ZrO₂ at temperatures higher than 748, 798 and 823 K, respectively. Ni/La₂O₃ exhibited the highest H₂ yield where almost complete conversion was obtained at 798 K. Moderate space velocities (WHSV = 6.45 h⁻¹) and glycerol feed concentration (5wt.%) favor high hydrogen selectivity and yield. Methanation is favored at a low WHSV or high glycerol feed concentration, resulting in a lower H₂ yield. Increasing Ni loading on the Ni/La₂O₃ catalyst strongly promoted the reforming, water–gas shift, and methanation reactions, which contributed significantly to the product species distribution.     

An efficient ternary photocatalyst via anchoring nickel complex and nickel oxides onto carbon nitride for visible light driven H2 evolution

Developing earth abundant, active and stable photocatalysts for water splitting is a critical but challenging procedure for efficient conversion and storage of sustainable energy. Here, a ternary photocatalyst was rationally prepared for efficient H₂ production by covalently anchoring a nickel molecule cocatalyst (NiL) onto graphitic carbon nitride nanosheets (CN) and introducing nickel oxides (NiO_x) as hole-transport materials. The lower H₂ overpotential by NiL and the faster separation of photoinduced carriers by NiO_x nanoparticles account for the efficient H₂ generation of CN without the help of noble metals. Eventually, the prepared NiL/NiO_x/CN catalyst exhibited excellent performance for H₂ evolution (289 μmol g^?1 h^?1) in TEOA solution under visible light irradiation, which is superior to 3NiL/CN (161 μmol g^?1 h^?1) and CN (Null). Furthermore, a possible mechanism of photocatalytic H₂ production for NiL/NiO_x/CN is proposed based on a series of electrochemical measurements. The noble-metal-free photocatalyst developed in this work will pave a new way to synthesize low-cost multicomponent photocatalysts for solar conversion.     

Study of Ce-Pt/γ-Al2O3 for the selective oxidation of CO in H2 for application to PEFCs: Effect of gases

In order to supply pure hydrogen to proton exchange membrane (PEM) fuel cells and avoid CO poisoning, selective CO oxidation in H₂ was studied over Ce-Pt/γ-Al₂O₃. Adding the Ce promoted the CO conversion and selectivity of Pt/γ-Al₂O₃ with changing loading weights of Pt and Ce, oxygen concentration, residence time, and the composition of gases (H₂O, CO₂, and N₂). At 250 °C, adding H₂O to the feed gas enhanced the CO conversion due to the water–gas shift reaction. While, adding CO₂ to the feed gas suppressed the CO conversion due to the reversible water–gas shift reaction. In situ BET and XRD tests showed that well-dispersed metallic Pt particles (−2 nm) existed on the Ce oxide over the alumina support, which helps to supply oxygen to the Pt for a high activity of CO oxidation and selectivity.     

Catalyst deactivation in on-board H2 production by fuel dehydrogenation

The production of H₂ for on-board application is a very interesting challenge for industrial and academic researchers. The aim is the application of on-board hydrogen production on the airplanes using kerosene as H₂ source. In this work an in depth study into the partial dehydrogenation (PDH) of two hydrocarbons blends and desulfurized JetA1 fuel has been performed by using 1 wt.%Pt–1 wt.%Sn/γ-Al₂O₃ and 1 wt.%Pt–1 wt.%Sn–0.5%K/γ-Al₂O₃ to find a way to produce H₂ “on-board” for the feeding of the fuel-cell apparatus. The mechanism of deactivation by coke was studied in depth combining Raman spectroscopy and Temperature-programmed oxidation (TPO) analyses. Microstructure analysis of metallic particles in fresh and deactivated catalysts was investigated by HRTEM. Relatively high H₂ partial pressure increases catalyst life by controlling full dehydrogenation coke-forming reaction. By feeding model organic molecules, it was possible to identify the contribution of each class of compounds to the H₂ production as well as the amount and type of coke formed. A relatively complex reaction pathway, which is able to evidence the role of different sites and reactions involved in PDH processes, was proposed.     

MoS2 grown in situ on CdS nanosheets for boosted photocatalytic hydrogen evolution under visible light

A novel nano-heterojunction photocatalysts of CdS/MoS₂ with appropriate interfacial contact was successfully obtained by the facile two-step hydrothermal synthesis. The MoS₂ ultrathin layer was well combined with CdS nanosheets and formed the interaction, which facilitated the transfer and separation of charges. The CdS/MoS₂ 15 wt% possessed much higher H₂ evolution photocatalytic performance (35.24 mmol h^?1 g^?1), exhibiting an 85.95 times enhancement as compared to that of pure CdS (0.41 mmol h^?1 g^?1). Moreover, the photochemical stability of CdS/MoS₂ heterojunctions was excellent, which showed no significant decrease in activity after four cycles of experiments. The finding provides a novel method to integrate the structure of MoS₂ with CdS, which exhibits great potential in solar energy conversion.     

High temperature stability and transport characteristics of hydrogen in alumina via multiscale computation

The impact of hydrogen charge states on the stability and transport characteristics of hydrogen interstitials in alumina polymorphs is evaluated by multiscale computational methods including density functional theory (DFT), ab initio molecular dynamics (AIMD) and machine learned force fields. Thermodynamic calculations show that the protonic H_i⁺¹ interstitial is the most stable defect species for most values of the electronic bandgap in both $\alpha$ and amorphous alumina (Al₂O₃). Further, active learned Gaussian approximation potentials (GAP) were developed using AIMD data to study temperature dependent long time proton diffusion in alumina. Diffusivity calculations from GAP-MD simulations are found to be comparable with of the AIMD data, while being ～340 times faster and scalable to larger systems. Comparisons with diffusivity values for other interstitial charge states (H_i⁰ and H_i^?1) and published experimental literature indicate that H_i⁺¹ diffusion is the likely mechanism of hydrogen transport. A good agreement is obtained between H_i⁺¹ diffusivity calculated in $\alpha$-Al₂O₃ from DFT: 5.05 $\times$ 10^?3 exp(-0.81 eV/k_B/T) cm²/s and reported experiment: 9.7 $\times 10$^?4 exp(-0.83 eV/k_B/T) cm²/s. Computationally and experimentally calculated energy barriers (0.81 and 0.83 eV respectively) only differ by 2.5%. Similarly, the pre-exponential diffusion coefficients only differ by 0.5 orders of magnitude. Moreover, the diffusivity of H_i⁺¹ in amorphous Al₂O₃ in the 1000–2000 K range is calculated to be 2.53 $\times$ 10^?2 exp(-0.89 eV/k _B/T), just one order of magnitude higher than the corresponding value in $\alpha$-Al₂O₃. This suggests that local structural disorder does not significantly affect the energy landscape and diffusion behavior of H_i⁺¹ in Al₂O₃. Overall, these results show promise for the application of alumina polymorphs as hydrogen permeation barriers.     

Autothermal reforming of iso-octane and gasoline over Rh-based catalysts: Influence of CeO2/γ-Al2O3-based mixed oxides on hydrogen production

Autothermal reforming (ATR) of iso-octane in the presence of Rh-based catalysts (0.5 wt% of Rh) supported onto γ-Al₂O₃, CeO₂, and ZrO₂ were initially carried out at 700 °C with a S/C ratio of 2.0, an O/C ratio of 0.84, and a gas hourly space velocity (GHSV) of 20,000 h⁻¹. The activity of Rh/γ-Al₂O₃ was found to be higher than Rh/CeO₂ and Rh/ZrO₂, with H₂ and (H₂ + CO) yields of 1.98 and 2.48 mol/mol C, respectively, after 10 h. This Rh/γ-Al₂O₃ material, however, was potentially susceptible to carbon coking and produced 3.5 wt% of carbon deposits following the reforming reaction, as evidenced by C, H, N, and S elemental analysis. In contrast, Rh/CeO₂ catalyst exhibited lower activity but higher stability than Rh/γ-Al₂O₃, with nearly no carbon being formed within 10 h. To combine the superior activity originated from Rh/γ-Al₂O₃ with high stability from Rh/CeO₂, Rh/CeO₂/γ-Al₂O₃ catalysts with different CeO₂ contents were synthesized and examined for the ATR reactions of iso-octane. Compared to Rh/γ-Al₂O₃, the newly prepared Rh/CeO₂/γ-Al₂O₃ catalysts (0.5 wt% of Rh and 20 wt% of CeO₂) showed even enhanced activity during 10 h, and H₂ and (H₂ + CO) yields were calculated to be 2.08 and 2.62 mol/mol C, respectively. In addition, as observed with Rh/CeO₂, the catalyst was further found to be stable with less than 0.3 wt% of carbon deposition after 10 h. The Rh/γ-Al₂O₃ and Rh/CeO₂/γ-Al₂O₃ catalysts were eventually tested for ATR reactions using commercial gasoline that contained sulfur, aromatics, and other impurities. The Rh/γ-Al₂O₃ catalyst was significantly deactivated, showing decreased activity after 4 h, while the Rh/CeO₂/γ-Al₂O₃ catalyst proved to be excellent in terms of stability against coke formation as well as activity towards the desired reforming reaction, maintaining its ability for H₂ production for 100 h.     

Effect of alumina on the enhancement of hydrogen production and the reduction of hydrogen peroxide in the γ-radiolysis of pure water and 0.4 M H2SO4 aqueous solution

The H₂ and H₂O₂ produced by ⁶⁰Co γ-radiation at room temperature were measured in pure water and 0.4 M H₂SO₄ aqueous solution with alumina powder. By increasing the addition of alumina powder, a strong reduction of H₂O₂ concentrations in the solutions was obtained, and the final product H₂ yields were correspondingly enhanced. These enhancement and reduction effects were diminished in the subsequent γ-radiation when irradiated alumina powder was used. The effects were reversibly restored by washing the irradiated powder with purified water. In 0.4 M H₂SO₄ solution with alumina powder, the H₂ yields increased by increasing the absorbed dose rate in the region of 1-5 kGy/h. The radiation-enhanced H₂ production correlated with the reduction of H₂O₂ concentration could be brought about by the reduction of H₂O₂ molecules and OH radicals in the solutions due to alumina powder.     

Mechanistic implications of the solvent kinetic isotope effect in the hydrolysis of NaBH4

The sodium borohydride, NaBH₄, hydrolysis mechanism is studied via the H₂O/D₂O kinetic isotope effect (KIE). This reaction is of importance as NaBH₄ is considered as a hydrogen storage material. Nowadays, hydrogen is thought to be one of the most promising and efficient clean energy carriers. In order to control the rate of the hydrogen evolution reaction (HER), one has to understand the mechanism of its production. The H₂O/D₂O KIE of the reactions of NaBH₄ and NaBD₄ with water was studied in solutions containing a ratio of H₂O/D₂O = 1.00. The separation factor, α, of both reactions is α = 5.0 ± 1.0. The rate of the hydrolysis of BD₄^? in H₂O is faster than that of BH₄^?. The results point out that the rate-determining step in all hydrolysis stages is the H–OH bond scission.