F. Yang, et al.
MolecularCatalysis486(2020)110873
oxidation of aromatic derivatives by high atom-efficiency and quantum
size superiority of CoO [20], CuO [21] and NiO [22] in mesoporous.
When active metal dispersed on the various supports as the supported
catalysts would hinder the agglomerate of the metal active site during
the catalytic reaction, which is beneficial to the catalytic activity and
service life of the catalyst [9,23–28]. But modulating catalytic activity
through interacting catalyst matrix with special property remains one
challenge for this catalyst owing to the purity of silica and intrinsic
activity of single transition metal. Inspiringly, introducing rational
phosphorus species with appreciated metal cation could expect to
generate one more inductive effect [4,29,30].
2.2. Rational fabrication of started redox phosphate catalyst
Two metal salts that NiCl2·6H2O and Cu(NO3)2·3H2O, after separate
or pairwise mixing, were added into a beaker containing 40 ml of
deionized water and 20 ml of ethanol and stirred for 10 min at room
temperature (Solution A). Then, The solution B, containing 15 mmol
(NH4)2HPO4·3H2O were added into 5 ml of deionized water and stirred
at 35 °C to the clear solution, and then was dropwise added into the
solution A. The mixed solution was stirred for 1 h, the product was
collected by centrifugation and washed with deionized water for sev-
eral times before drying at 70 °C for 10 h. These samples were named
Ni-P-RT, Cu-P-RT, and Ni/Cu-P-RT according to the existence and
match of Ni and Cu metal, respectively.
The exploration of these metal phosphates directing at hetero-
geneous thermal catalysis especially for the hydrogenation reaction is
rarest in the current research systems, but, which might provide the
new insight for the hydrogenation of non-precious metal catalysts. The
2.3. Catalytic activity assessment
Ⅰ
Ⅱ
Ⅰ
Ⅱ
well-known series of compounds M M PO4·H2O (M = NH4+, K; M =
Mn, Fe, Co, Ni) [31], has been of interest because of the strongly de-
fined layered crystal structure of this phase which is suitable for special
catalytic reaction due to their open framework and diversity of struc-
ture types. The divalent metal ions are bridged by the oxygen atoms of
the phosphate groups, leading to the formation of magnetic planes se-
2.3.1. Catalytic reduction of 4-nitrophenol
The catalytic hydrogenated performance of the resultant transition
metal phosphate catalysts was evaluated in detail based on the reduc-
tion of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP). The catalytic
reaction was gingerly conducted in a quartz cuvette containing 1 ml of
4-NP(0.0005 M) aqueous solution, 2.5 ml of deionized water, 2 mg
catalysts and 0.2 ml of an aqueous solution of fresh NaBH4 (0.2 M) in
several parallel sequences. As the reaction progressed, the solution
gradually became colorless from yellow. Subsequently, the time-de-
pendent UV–vis absorption spectra were recorded and collected using
the resulted reaction solution, and the catalytic activity was monitored
and calculated based on Lambert beer's law. Besides, the first-recycled
and second-recycled catalyst is labeled as F-Ni/Cu-P-RT and S-Ni/Cu-P-
RT, respectively. Ulteriorly, the completion and the selectivity of the
reaction in the first reaction example using Ni/Cu-P-RT were further
identified by external standard method for SP-6890 Gas Chromatograph
(Lunan Ruihong Chemical Instrument Co. Ltd, China)
+
parated by NH4 ions, and also affected by surrounding phosphates
oxides in the electronic exciting process [32]. In that case, the syner-
getic effects of multiple transition metals are capable of contributing to
the exceptionally high catalytic activity [33]. However, the reduction of
NP catalyzed by such oxygen-bearing metal phosphates especially for
the real reaction process and mechanism remains to be further explored
in deep cater to high catalytic efficiency and greener chemistry.
Hereby, we developed several mon/bimetal-based phosphate cata-
lysts that Ni-P-RT, Cu-P-RT, and Ni/Cu-P-RT and characterized these
resultant samples by virtue of key test techniques to identify their
physicochemical properties. Then, these catalysts were first evaluated
for their catalytic hydrogenated property by the reduction of 4-NP.
Unprecedentedly, the constructed Ni/Cu-P-RT exhibit the ultrafast
catalytic reduction rate within one minute at 30 °C, outperforming the
reported known noble metals catalysts with at least 10 times. Besides,
we observed the existed remarkable reaction induction period namely
evolution of reaction active species in the case of the single metal-
bearing Cu-P-RT/Ni-P-RT, but can be eliminated by the right of both
options of Ni and Cu in the phosphate of Ni/Cu-P-RT. This might as-
sociate the fact that the adjustable electronic band energy effect of
metal phosphate induces the easier reduction of metal species toward
the generation of reaction-preferred active sites. Herein, the in-situ
nascent metal nanoparticles including metallic Cu and Ni were de-
monstrated by the right of transmission electron microscopy (TEM) and
X-ray photoelectron spectra (XPS) techniques. To our satisfaction, by
present efficient characterizations, the resulted first-recycled F-Ni/Cu-
P-RT acted as the stable catalyst was capable of being reused for at least
six cycles without further variation of state and dropping catalytic ef-
ficiency. Our findings provide new insight for the development of
highly-activity transition metal catalysts through the rational design of
heterogeneous structure in the catalytic hydrogenated reactions.
2.3.2. Catalytic stability evaluation
The catalytic stability assessment of the constructed transition
metal-based phosphate catalyst was performed through the specific
recycled catalytic experiment. Generally, after the reaction, the spent
catalyst was retrieved from the reaction solution by centrifugation, the
centrifugal catalyst was dried in an oven of 70 °C for 6 h and directly
reused for the next reaction.
3. Results and discussion
3.1. Synthesis and characterizations of started transition metal phosphate
The synthesis and the microstructure of started transition metal
phosphates (Scheme 1) can be discerned based on our later character-
izations. In reported works, dittmarite-type compounds have a layered
+
structure with metal (II) phosphate sheets separated by NH4 ions in
the interlayer [34]. In the case of the metallic layers, the phosphate
tetrahedral is cross-linked via the M2+O6 octahedral [35]. Besides, in
the bimetallic phosphates, the adjacent MO6 unit was bridged with
another M1O6 by spaced PO4 as shown in the following scheme.
The state information involving structure and the crystal phase of
as-synthesized initial metal phosphate materials were identified by
wide-angle X-ray diffraction (XRD) patterns, and shown in Fig. 1.
Clearly, a strong diffraction peak located at 10° of Fig. 1 (b and c) is
observed and indexed to the existed layered structure of these resultant
metal phosphate samples [36]. The XRD patterns of monometal-bearing
samples all have sharp and narrow diffraction peaks, which indicates
their highly-crystalline state (Fig. 1a and b) corresponding to valid Ni
(NH4)PO4·6H2O (JCPDS no. 21-0034) [37] and Cu(NH4)PO4·H2O
(JCPDS no.89–1303) [38], respectively. Fig. 1c exhibits the dominant
diffraction of Ni(NH4)PO4·H2O (JCPDS no.50-0425) [39] with several
newly-emerging and shifted peaks at 2θ = 30-35° differing from that of
2. Experimental section
2.1. Materials
Nickel chloride hexahydrate (NiCl2·6H2O, AR), copper nitrate hy-
drate (Cu(NO3)2·3H2O, AR) and Ammonium phosphate ((NH4)2HPO4,
AR) were purchased from Nanjing Chemical Reagent Co., Ltd. 4-ni-
trophenol (4-NP, AR), sodium borohydride(NaBH4, AR), 2-Nitrophenol,
2-Nitro-4-tert-butylphenol, 2-Chloro-4-nitrophenol, 2-Amino-5-ni-
trophenol, 2-Amino-4-nitrophenol, and 4-Methyl-2-nitrophenol were
purchased from Aladdin Chemical Reagent Co., Ltd. All these chemical
reagents were directly used without any purification.
2