Highly efficient hydrolysis of ammonia borane by anion (-OH, F-, Cl-)-tuned interactions between reactant molecules and CoP nanoparticles

Article abstract of DOI:10.1039/c6cc08120g

The CoP nanoparticle catalyst had excellent catalytic activity and a short catalytic induction period in the presence of anions, and high sustainability in ammonia borane hydrolysis, with an initial turnover frequency of 72.2 mol_(H2) mol_(CoP)^-1 min^-1 at ambient temperature. This value is unprecedented for noble-metal-free catalytic systems.

Full text of DOI:10.1039/c6cc08120g

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Highly efficient hydrolysis of ammonia borane by
anion (ÀOH, FÀ, ClÀ)-tuned interactions between
reactant molecules and CoP nanoparticles†
Cite this:DOI: 10.1039/c6cc08120g
Received 8th October 2016,
Accepted 28th November 2016
Zi-Cheng Fu,a Yong Xu,a Sharon Lai-Fung Chan,b Wei-Wei Wang,c Fang Li,a
Fei Liang,a Yong Chen,a Zhe-Shuai Lin,a Wen-Fu Fu*ac and Chi-Ming Che*d
DOI: 10.1039/c6cc08120g
www.rsc.org/chemcomm
The CoP nanoparticle catalyst had excellent catalytic activity and a under ambient conditions is highly required. In 2006, Xu and
short catalytic induction period in the presence of anions, and high coworkers reported that NPs of earth-abundant metals such as
sustainability in ammonia borane hydrolysis, with an initial turnover Co, Ni, and Cu supported on Al2O3, SiO2, or carbon materials
À1
frequency of 72.2 mol(H ) mol(CoP) minÀ1 at ambient temperature. had good catalytic performances in the hydrolytic dehydrogena-
2
This value is unprecedented for noble-metal-free catalytic systems. tion of AB at room temperature, and stoichiometric H2 evolution
was achieved.14 They also reported that amorphous Fe NPs
The storage and transportation of H2 are major obstacles in produced in situ by the reduction of FeSO4 with NaBH4 had
establishing an H2 economy because H2 has a low volumetric excellent catalytic activity in the dehydrogenation of AB in H2O.15
density under ambient conditions. The development of systems
Metal phosphide NPs, which show low charge transfer
for safe and efficient H2 storage and controlled release of H2 at high from the metal to P, have been extensively used as efficient
rates under mild conditions is therefore important.1 Ammonia electrocatalysts in water splitting16 or as cocatalysts in photo-
borane (AB), which has an H2 capacity of 196 g kgÀ1 and a catalytic reactions.17–19 Recently, our group reported a robust
volumetric density of 146 g LÀ1, is a good candidate for a chemical catalytic system withÀa1 high initial turnover frequency (TOF),
H2-storage material; its aqueous solution is stable in an air 40.4 mol(H ) mol(Ni P) minÀ1, at ambient temperature using
2
2
atmosphere.2 In the past decade, intensive efforts have been made Ni2P NPs to catalyze the hydrolytic dehydrogenation of AB.11
to achieve hydrolytic dehydrogenation of AB, generating 3 mol of H2 Here, we report that the introduction of anions (ÀOH, FÀ, and
per mole of AB, using efficient, economical, and durable catalysts. In ClÀ) during AB hydrolysis greatly reduces the catalytic reaction
this regard, single-metal3 and bimetallic nanoparticles (NPs),4 metal induction period, tunes the competing interactions between
NPs deposited on supports,5–8 transition-metal complexes,9,10 metal CoP and the reactant molecules, and promotes hydrolytic H2
phosphide NPs,11 and organic molecules12 have been used in evolution from AB in the presence of a base.
catalyzed AB hydrolysis. Recently, the development of metal NPs
CoP NPs were prepared using modified versions of previously
for AB hydrolysis has focused on varying the sizes and shapes of the reported methods20 [see ESI†]. The six peaks in the X-ray diffrac-
NPs deposited on the supports to improve the H2-release rates and tion (XRD) pattern (Fig. 1a and Fig. S1, ESI†) correspond to the
reduce NP aggregation.13
(011), (111), (112), (211), (103), and (301) crystal planes of ortho-
The dehydrogenation of molecular hydrides has a wide range rhombic CoP (JCPDS No. 29-0497); the NP size is 10–15 nm
of applications. Therefore the development of sustainable, (Fig. 1b). Energy-dispersive X-ray spectroscopy (EDX) (Fig. 1c)
highly efficient, and noble-metal-free catalysts that can be used and elemental analysis data obtained using inductively coupled
plasma-atomic emission spectrometry confirm the presence of Co
a Key Laboratory of Photochemical Conversion and Optoelectronic Materials and
and P, and the latter showed that the Co/P molar ratio is 1 : 0.98.
The corresponding EDX elemental mappings (Fig. 1d and e) show
that Co and P are uniformly distributed throughout the CoP NPs.
HKU-CAS Joint Laboratory on New Materials, Technical Institute of Physics and
Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China.
E-mail: fuwf@mail.ipc.ac.cn
b Department of Applied Biology and Chemical Technology, The Hong Kong
Polytechnic University, Hung Hom, Hong Kong, P. R. China
c College of Chemistry and Engineering, Yunnan Normal University,
Kunming 650092, P. R. China
High-resolution transmission electron microscopy (HRTEM; Fig. 1f)
shows well-resolved lattice fringes with an interplanar spacing
of 1.9 Å for the (211) plane of CoP; this is smaller than the
lattice fringe distance (2.4 Å) of the (111) plane.
The H2-generating reaction was performed in a closed flask
and aerobic atmosphere, with stirring. The produced gas was
identified using gas chromatography, with Ar as the carrier gas.
d Department of Chemistry, The University of Hong Kong, Pokfulam Road,
Hong Kong, P. R. China
† Electronic supplementary information (ESI) available: Experimental details and
supplementary Fig. S1–S12 and Table S1. See DOI: 10.1039/c6cc08120g
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of an Ni2P catalyst in H2O because strong interactions between
H2O and the catalyst impeded intimate contact between the AB
molecules and the catalyst during the bimolecular reaction.3
He and coworkers showed by using kinetic isotope effect
measurements that the breaking of the O–H bond in H2O is
the rate determining step for the AB hydrolysis reaction.21 But
in the present system, the synergy effect of AB molecules fast
approaching the catalyst surface on the reaction rate should be
noted. The [AB] dependence of the hydrolysis rate was therefore
studied at a constant catalyst concentration at 298 K. Fig. 2b
shows that the hydrolysis kinetics is not zero order in terms of
the AB concentration, and the logarithmic plot of the calculated
initial reaction rate versus AB concentration has a slope of
0.60 (Fig. S3, ESI†), which is in accordance with the increments
of induction time observed iÀn1the dilute AB solution (Fig. 2b).
Fig. 1 (a) XRD pattern and (b) TEM image of the CoP NPs; (c) EDX results
corresponding to a selected HAADF-STEM image (inset); corresponding
EDX elemental mapping images of (d) Co-K and (e) P-K; (f) HRTEM image
of the CoP NPs.
A TOF of 24.9 mol(H ) mol(CoP) minÀ1 was achieved at a CoP/AB
2
molar ratio of 0.043 in air. The results for the temperature depen-
dence of the hydrolysis reaction are shown in Fig. 2c and d. An
Arrhenius plot shows that the activation energy is 46.7 kJ molÀ1
,
comparable to those of Ru-based bimetallic catalysts.22
The volumes of H2 that evolved were determined based on the
displacement of the H2O level in a graduated burette (see ESI†).
The H2 generation rates for AB hydrolysis catalyzed by the CoP
NPs in pure H2O were determined as a function of catalyst
concentration. Fig. 2a shows that H2 evolution depends signifi-
cantly on the amount of catalyst; hydrolysis generates 3 mol of
H2 gas per mole of AB, and follows first-order kinetics with
respect to catalyst concentration (Fig. S2, ESI†). The induction
period for H2 production at a fixed AB concentration in the
initial stage increases with a decreasing amount of CoP. At a
CoP/AB molar ratio of 0.021, the induction period is 3.5 min,
indicating that the chemical environment around the NP
catalyst significantly affects the catalytic performance. This is
consistent with previous results reported by our group;11 a long
catalytic induction period was observed after sonic oscillation
As shown in Fig. 3a, c and d, it was found that the effect of
adsorbed H2O at the catalyst surface on the H2 generation rate
is crucial; high coverage of the catalyst surface by H2O mole-
cules leads to a long induction time. Fig. 3a clearly shows an
extended catalytic induction period when the CoP catalyst in
H2O was stirred for 10 min and used as the starting material
(olive symbol, TOF = 19.5) compared with that of the system
without pretreatment with stirring (black symbol, TOF = 24.9).
The rate of the catalytic reaction was significantly improved by
the presence of ÀOH, and there was almost no induction period
for H2 evolution under the experimental conditions, implying
that the presence of ÀOH inevitably enhances the activation of
the O–H bond in H2O and activates the AB molecule via a
hydrogen bond between ÀOH and the reactants.21,23 Moreover,
the dependence of the AB hydrolytic rate on the anion concen-
tration was investigated in detail (Fig. 3b–d). The effects of
adding FÀ and ClÀ on the TOF were smaller than that of ÀOH,
but the induction period for H2 evolution decreased with
increasing FÀ or ClÀ concentration. The effects of the CoP/AB
molar ratio on the initial steps of hydrolytic H2 evolution from
AB at the optimum ÀOH concentÀra1tion are shown in Fig. 3e.
A high TOF, 72.2 mol(H ) mol(CoP) minÀ1, was achieved at a
2
CoP/AB molar ratio of 0.041 in air. This is much higher than the
literature value (TOF = 70.0) for noble-metal-free heterogeneous
catalytic hydrolysis of AB.24 Fig. 3f also shows that the H2
generation TOFs increased significantly with increasing base
concentration at 298 K. However, the catalytic activity at higher
NaOH concentrations decreased, as shown by the deceleration
of the reaction and the limiting values reached.
A comparison of 1H and 11B NMR spectra of AB in basic
solution with pure D2O shows that AB is stable in aqueous
NaOH solution. A negligible signal was observed at 1.59 ppm in
the 11B NMR spectrum of a sample kept at room temperature
for one week; this corresponds to the conversion of AB to
borate, and shows that the increased H2-release rate observed
for anion-promoted catalytic hydrolysis of AB may result from
Fig. 2 (a) Stoichiometric H2 evolution in aqueous solution (5 mL) containing
a fixed amount of AB (1.62 mmol) at various CoP/AB molar ratios at 298 K;
(b) relationship between the H2 generation rate and AB concentration at a
fixed amount (0.089 mmol) of CoP in H2O at 298 K; (c) H2 generation rate as
function of temperature for the catalytic hydrolysis of AB (1.62 mmol) with
CoP (0.089 mmol) in H2O (5 mL); (d) Arrhenius plot of ln(TOF) versus 1/T.
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Fig. 4 High-resolution XP survey spectra of Co 2p core level for the CoP
NPs (a) before and (b) after adding NaOH; calculated PDOS of Co 4p
orbitals in single CoP surface (c) without and (d) with adsorbed OH; (e) O
2p orbitals in the CoP surface with adsorbed OH.
Fig. 3 (a) Effect of experimental conditions on AB hydrolysis rates at fixed
amounts of CoP, AB in 5 mL of H2O or 0.3 M NaOH aqueous solution at 298 K;
red symbol, CoP in 0.3 M NaOH aqueous solution (3 mL) stirred for 10 min
before adding AB in 0.3 M NaOH aqueous solution (2 mL); dark cyan symbol,
AB in 0.3 M NaOH aqueous solution stirred for 10 min and injected into CoP;
olive symbol, same as for red symbol, but in pure H2O; relationships between
H2 generation rate and (b) ÀOH, (c) FÀ, and (d) ClÀ concentration in AB
(2.14 mmol) hydrolysis with CoP catalyst (0.089 mmol) in H2O (5 mL) at 298 K;
(e) stoichiometric H2 evolution in 0.3 M NaOH aqueous solution (5 mL)
containing a fixed amount of AB (1.62 mmol) at various CoP/AB molar ratios
at 298 K; (f) TOFs at various NaOH concentrations for AB (2.14 mmol)
hydrolysis with CoP (0.089 mmol) in H2O (5 mL) at 298 K.
for CoP is lower than that of 130.2 eV for elemental P, implying a
small charge transfer from Co to P, which results in partially
negatively charged PdÀ and cationic Cod+.27 The broad peaks
located at 133.6 and 132.6 eV can be ascribed to the oxidized
P species.28 It is desirable for the core-level binding energies of
CoP with NaOH to be affected by coordination of the metal
center to ÀOH; this leads to an increased electron density at the
Co sites and moves the Co 2p peak closer to that of metallic Co.
Fig. 4b shows that the Co 2p peak profiles in the fitted valence-
changes in the chemical environment on the catalyst’s surface band spectra differ significantly from those of bare CoP, and
(Fig. S4–S6, ESI†). In addition, an isotopic labeling experiment the Co 2p3/2 peaks are slightly negatively shifted from 781.5 and
using D2O as the solvent for hydrolytic dehydrogenation of AB, 778.7 eV to 781.3 and 778.6 eV, respectively. The significant
confirmed, within the experimental error limits of isotopic increase in the peak intensity at 781.3 eV indicates that there are
exchange, that half of the H2 released comes from H2O rather more valence electrons around the metal Co coordinated to
than ÀOH (Fig. S7, ESI†).
ÀOH. The two enhanced satellite peaks at 786.8 and 784 eV
Anion-accelerated H2 release catalyzed by CoP NPs was confirm that there are intense interactions among different
further studied using X-ray photoelectron spectroscopy (XPS). valence electronic states that are caused by changes in the
XP survey spectra for the bare CoP catalyst show small, sharp, chemical environment around CoP. In contrast, the P 2p XP
low-energy peaks with binding energies of 781.5 and 778.7 eV. spectra with and without NaOH are similar (Fig. S9, ESI†). The
The two peaks are slightly positively shifted from those of results show that the introduction of surface ÀOH improves the
metallic-state Co (778.1–778.2 eV); they are assigned to Cod+ electronic properties of the catalyst.
and are consistent with those from Co 2p3/2 electrons in CoP
First-principles calculations show that the adsorption energies
(Fig. 4a).25 In the high binding energy region, there is an intense for H2O and ÀOH on the CoP (010) surface are 0.259 and 2.046 eV,
peak at 803.4 eV and two peaks centered at 797.7 and 793.7 eV. respectively. This shows that the OH groups are much more stably
The first peak is tentatively attributed to the excitation of high- adsorbed onto the CoP surface than the H2O molecules. Fig. 4c
spin Co2+ ions, originating from the superficial air oxidation and d show plots of the partial density of states (PDOS) of the
of CoP when the samples were removed for measurements, and Co 4p orbitals on the CoP surface without and with OH group
the other two peaks are associated with the Co 2p1/2 state.26 adsorption, respectively. This combined with the PDOS of the
The high-resolution P 2p XP spectra of CoP show two peaks, at O 2p orbitals of the adsorbed OH groups, shown in Fig. 4e
130.3 and 129.5 eV, corresponding to the P 2p1/2 and P 2p3/2 indicates that the OH groups can significantly change the
states, respectively (Fig. S8, ESI†). The binding energy of 129.5 eV energy distribution of the Co 4p orbitals. A new peak appears
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at À7.5 eV and the integrated area of the Co 4p PDOS between Academy of Sciences (XDB17030200). We thank the National
À5.0 and À2.0 eV increases from 0.05 for the bare CoP surface Natural Science Foundation of China (21273257, 21267025,
to 0.12 for the surface with adsorbed OH groups, as a result 21471155, 21367026) for financial support.
of strong hybridization between the Co 4p and O 2p orbitals.
The integrated area for the PDOS of the Co 4p orbitals from À14
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of AB catalyzed by CoP NPs was established for the first time.
The initial TOF of 72.2 mol(H ) mol(CoP) minÀ1 at ambient
À1
2
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