Facile synthesis of novel, known, and low-valent transition metal phosphatesviareductive phosphatization

Article abstract of DOI:10.1039/d1ta03782j

Novel and known low-valent transition metal phosphates (TMPs) are accessibleviaa novel and facile pathway. The method allows syntheses of TMPs also with reduced oxidation states. The key feature of the new route is the reductive character of a hypophosphite salt melt, which acts as reaction medium and enables directing the oxidation state of the transition metal.

Full text of DOI:10.1039/d1ta03782j

Journal of
Materials Chemistry A
View Article Online
View Journal | View Issue
COMMUNICATION
Facile synthesis of novel, known, and low-valent
transition metal phosphates via reductive
phosphatization†
Cite this: J. Mater. Chem. A, 2021, 9,
8247
1
Received 5th May 2021
Accepted 12th August 2021
Niklas Stegmann,
and Wolfgang Schmidt
Hilke Petersen,
Claudia Weidenthaler
*
DOI: 10.1039/d1ta03782j
rsc.li/materials-a
Novel and known low-valent transition metal phosphates (TMPs) are renewable energy storage via water splitting, using cobalt or
accessible via a novel and facile pathway. The method allows nickel phosphates or mixed cobalt–nickel phosphate (NiPO) for
syntheses of TMPs also with reduced oxidation states. The key feature the oxygen evolution reaction presents an interesting alterna-
11,12
of the new route is the reductive character of a hypophosphite salt tive to classical catalysts for electrochemical water splitting.
melt, which acts as reaction medium and enables directing the The wide range of applications is complemented by the eld of
oxidation state of the transition metal.
nonlinear optics (NLO), where TMP materials are prepared via
crystallization from high-temperature melts of phosphate salts
and used for the production of coherent deep-UV light with
The number of compounds belonging to the class of transition
metal phosphates (TMPs) has constantly been growing for
decades due to persistent interest in their use as functional
materials. Extensively investigated and applied in research and
industry, TMP materials contribute to numerous present and
future key technologies. Based on unique structural features,
TMPs play an important role for clean and efficient power
generation and energy storage in the 21 century. Rechargeable
lithium-ion batteries apply TMPs as cathode materials. Espe-
cially lithium iron phosphate (LiFePO
utilized, combining high power capability and reversibility with
13
wavelengths below 200 nm.
Synthetic phosphate salts are known for most transition
metals. Considering the number of metal cations and the
structural variety of condensed phosphate units, the sheer
quantity of different phases is staggering. The variety of
compounds formed by the class of TMP materials is primarily
limited by the intrinsic chemical nature of the elementary ionic
building units. However, it can be also restricted by availability
of precursors and preparation methods. In literature, versatile
preparation methods, such as hydrothermal procedures, molten
salt processes, or simple precipitation, are established for the
synthesis of highly crystalline bulk TMPs. Application of TMPs as
functional materials oen requires quite sophisticated prepa-
ration methods though. Structure directing techniques are used
to optimize the performance of the materials, porosity is
generated via nanocasting strategies, thin lms are formed via
st
4
) is commercially
1–5
environmental sustainability and low costs. In next genera-
tion fuel cell technology, metal pyrophosphates (MP
Zr, .) serve as proton conducting membrane materials, due to
2
O
7
, M ¼ Ti,
numerous structure-based proton bonding sites and transport
6
pathways. In line with their high density of Brønsted acid sites,
proton conducting TMPs are also serving as effective ion-
gas phase deposition, and nanoparticles are synthesized by
7
exchangers. In the context of heterogeneous catalysis, TMPs
12,14–18
electrochemical approaches.
No matter what kind of tech-
are widely used materials for thermo-, electro- and photo-
catalytic applications due to their unique combination of redox
and/or acidic properties. Among the class of TMP catalysts, the
nique is used or what kind of application is envisaged, all
methods have in common the use of phosphate-based precursor
compounds, very different to the method presented here. In
contrast to conventional syntheses, in our route a hypophosphite
salt acts as precursor in combination with metal oxides. While
the molten hypophosphite salt serves concurrently as reaction
medium and phosphate precursor, the metal oxide provides the
transition metal cation under reductive conditions. Due to the
reducing character of the hypophosphite melt, the transition
vanadyl pyrophosphate (VO) (P O ) is a prominent example,
2
2 7
representing one of the most extensively studied catalysts in
chemistry and the only commercially applied material for the
8–10
conversion of butane to maleic anhydride.
With respect to
Department of Heterogeneous Catalysis, Max-Planck-Institut f u¨ r Kohlenforschung,
Kaiser-Wilhelm-Platz 1, D-45470 M u¨ lheim an der Ruhr, Germany. E-mail: schmidt@ metal gets reduced during the TMP formation.
mpi-muelheim.mpg.de
The molten salt method is thus a simple and versatile
approach, which can be used for the synthesis of known and
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/d1ta03782j
1
This journal is © The Royal Society of Chemistry 2021
J. Mater. Chem. A, 2021, 9, 18247–18250 | 18247

View Article Online
Journal of Materials Chemistry A
Communication
novel rst row transition metal phosphates with low oxidation
ꢀ
states at moderate temperatures (300–500 C). This approach
was tested for a series of rst row transition metals (Ti, V, Cr,
ꢀ
ꢀ
Mn, Fe) under low (300 C) and high (500 C) temperature
conditions. The results, listed in Table 1, indicate that low
temperature conditions tend to form ammonium TMPs, while
higher temperatures lead to the formation of ammonium-free
TMPs with condensed metaphosphate structures. In almost
all cases, the reaction was accompanied by a reduction of the
transition metal.
In a typical reaction, a metal oxide powder, such as TiO2,
V O , Cr O , MnO , or Fe O , is mixed with a surplus of hypo-
2
5
2
3
2
2 3
4 2 2
phosphite (NH H PO ) and heated briey in a tube furnace
under inert gas ow. Then the melt is cooled down and washed
with water to yield the pure TMPs (Fig. S1 in ESI†). Detailed
synthesis parameters for
a series of transition metal
compounds are reported in the ESI.† The process window of the
method is determined by the temperature and time between the
melting point and the thermal decomposition of the hypo-
phosphite precursor (Fig. S2†). The hypophosphite melt medi-
ates the reaction with the dispersed or dissolved metal oxides.
Due to the thermodynamic instability of the hypophosphite
anion, the disproportionation into phosphane gas and phos-
phates limits the reaction. The phosphane acts as spectator
under the presented reaction conditions. Excess phosphate
compounds, as formed on the crystal surfaces, can easily be
Fig. 1 XRD patterns of two novel titanium phosphate phases (a) Ti(III)p,
3 3
(b) Ti(IV)p and (c) Ti(PO ) synthesized via reductive phosphatization of
(
d) TiO (P25) in a melt of ammonium hypophosphite. Red lines indi-
2
removed by washing with water. Formation of metal phos- cate the main reflections of rutile and anatase phases of the TiO (P25)
2
phides via reaction of phosphane and metal oxides, as oen precursor. Images on the right illustrate the colors of the novel and
known product phases.
described in literature, was not observed under the reaction
conditions used here.
A representative example is the reaction of titanium(IV) oxide
3 3
crystallinity of the product. Syntheses of Ti(PO ) with
(
P25) and ammonium hypophosphite, resulting in novel and
ꢀ
increasing weight ratios of hypophosphite precursor result in
different phase compositions as documented by the XRD
patterns in Fig. 2. Lower ratios of the hypophosphite precursor
known crystalline titanium(III) phosphate compounds. At 300 C
an unknown crystalline ammonium titanium(III) phosphate
compound (NH Ti(III)P O ), denoted as Ti(III)p, is formed from
4
2 7
(
1/1–1/2) are apparently not sufficient to convert the TiO2
the melt as illustrated by the respective XRD pattern in Fig. 1a
quantitatively. However, also these conditions cause partial
reduction and the formation of stable titanium(III) oxide species
on the surfaces of the titania particles as indicated by the
characteristic black coloration and conrmed by XPS spectra
and S3.† The product is a pyrophosphate as evidenced by
ꢁ1
a characteristic band at 764 cm
Increasing the reaction temperature to 500 C yields a known
in its Raman spectrum.
ꢀ
titanium(III) trimetaphosphate, Ti(PO
ture the isolated TiO octahedra are linked through innite
chains of PO tetrahedra.
3 3
) (Fig. 1c). In this struc-
(Fig. 3d). Unknown intermediate phases are formed by
6
19
increasing hypophosphite ratios (1/3–1/5 in Fig. 2) before phase
4
pure Ti(PO ) is obtained from the melt (ratio $1/6 in Fig. 2).
The formation of a melt by using an excess of hypophosphite
3 3
In a proposed redox reaction mechanism, the precursors can
be considered as oxygen donor and acceptor pairs. While the
hypophosphite acts as oxygen acceptor to form stable phos-
phate compounds, the metal oxide donates oxygen atoms under
is indispensable for complete conversion of the TiO and high
2
Table 1 TMP product phases and oxidation states (o. s.) for the
reductive phosphatization of first-row transition metal oxides with reductive conditions. Provided that sufficient hypophosphite is
ꢀ
ꢀ
ammonium hypophosphite at 300 C and 500 C
available, all oxygen of the metal oxide will be consumed and
the reduced cations are free to form crystalline TMP phases.
Reports on low-valent titanium(III) phosphates are rather rare in
ꢀ
ꢀ
Product (500 C)
Precursor
TM oxide
Product (300 C)
20
literature, likely due to their strong tendency to oxidize in air.
XPS spectra show the presence of two different titanium species
on the crystal surfaces of Ti(III)p and Ti(PO as shown in Fig. 3a
o. s
TMP
o. s
TMP
o. s
TiO
2
4+
5+
3+
4+
3+
Ti(III)p
3+
3+
3+
—
2+
Ti(PO
3
)
3
3+
3+
3+
2+
2+
3 3
)
V
2
O
5
V(PO
3
)
3
V(PO
Cr (P
Mn (P
Fe (P
3 3
)
and b. They can be attributed to Ti(III) and Ti(IV) indicating that
the Ti(IV) species exist on the crystal surface in both compounds,
likely as the result of oxidation of Ti(III) on the surface by
Cr
MnO
Fe
2
O
3
Cr(NH
—
Fe(II)p
4
)HP
3
O
10
2
6
O
18
12
12
)
2
2
4
O
)
2
O
3
2
4
O
)
18248 | J. Mater. Chem. A, 2021, 9, 18247–18250
This journal is © The Royal Society of Chemistry 2021

View Article Online
Communication
Journal of Materials Chemistry A
the material undergoes a phase transformation to a known
titanium(IV) pyrophosphate (TiP O ). Under non-oxidative
2
7
conditions, thermal treatment of Ti(III)p yields another novel
crystalline Ti(IV) phosphate (Ti(IV)P O ), denoted as Ti(IV)p,
2
7
showing exclusively Ti(IV) species (Fig. 3c). TG/DSC-MS (Fig. S4
and S5†) reveals that the phase transformation is accompanied
by release of ammonia, hydrogen, and water. The XRD pattern
of the novel Ti(IV)p is presented in Fig. 1b and S6.† Reproduc-
ibility and purity of the novel crystalline diamagnetic Ti(IV)p
3
1
compound was conrmed by P MAS NMR spectra (Fig. S7†)
showing no additional crystalline phase or any amorphous
phase in the product (see ESI,† for details). SEM images show
a homogenous distribution of micrometer sized crystals for
pristine Ti(III)p and Ti(IV)p (Fig. S8†).
Overall, the conversion of TiO
2
with ammonium hypo-
phosphite offers reaction pathways to several known titanium
phosphate compounds as well as two novel phases as sketched
in Fig. 4. As representative example, the reductive phosphati-
zation of Ti(IV) to Ti(III) shows the reductive feature and vari-
ability of the hypophosphite route.
Fig. 2 XRD patterns obtained by reductive phosphatization of TiO
2
(
(
P25) with increasing (NH
green arrow) at 500 C in argon. Red lines indicate the main reflec-
4
)H
2
PO
2
weight ratios from 1/1 up to 1/10
Formation of low-valent TMPs, as reported here for the
titanium compounds, is observed also for other transition
ꢀ
tions of rutile and anatase phases of the TiO
on the right illustrate the different colors of reduced products.
2
(P25) precursor. Images
metal compounds. Via identical synthesis procedures V(PO
Cr(NH )HP 10, Cr (P 18), Mn (P 12), Fe(II)p (new phase),
and Fe (P 12) were readily obtained (see ESI,† for details). In
3 3
) ,
4
3
O
2
6
O
2
4
O
2
4
O
literature, low oxidation states of TMPs are generally reported to
be accessible by using metal powders or low-valent metal
compounds as precursors. In comparison, the new route offers
the possibility to direct the oxidation state of the transition
metal via the hypophosphite, which results in quite moderate
reaction conditions.
In summary, rst-row transition metal oxides are converted
in a hypophosphite melt to crystalline metal phosphates. The
Fig. 3 XPS spectra of (a) Ti(III)p, (b) Ti(PO
tively phosphated Ti(III/IV) oxide synthesized from a mixture of TiO
NH PO with a weight ratio of 1/2.
3
)
3
, (c) Ti(IV)p and (d) reduc-
2
and
4
H
2
2
ambient air. XPS analysis probes surface-near regions of the
particles, thus, the Ti(IV) signal is caused by surface species.
19
3 3
Crystalline Ti(PO ) is known to be a pure Ti(III) compound.
The bulk of the novel Ti(III)p compound offers a good
thermal and chemical stability as indicated by XRD data. It is
longtime-stable in acidic aqueous solution even at elevated
Fig. 4 Reaction pathways for the preparation of novel and known
titanium phosphate phases accessible by reductive phosphatization of
TiO in a melt of ammonium hypophosphite.
2
ꢀ
temperature (H
3
PO
4
, pH ¼ 1, 80 C, 72 h) and also thermally
ꢀ
stable in ambient air up to 250 C. At higher temperature in air,
This journal is © The Royal Society of Chemistry 2021
J. Mater. Chem. A, 2021, 9, 18247–18250 | 18249

View Article Online
Journal of Materials Chemistry A
Communication
ꢀ
facile method operates at moderate temperatures (300–500 C)
and offers access to a number of novel, known and, very
specically, low-valent TMPs. The reducing feature of the
hypophosphite precursor is exemplied by the reaction with
2 W.-J. Zhang, J. Power Sources, 2011, 196, 2962–2970.
3 H. Li, S. Xing, Y. Liu, F. Li, H. Guo and G. Kuang, ACS
Sustainable Chem. Eng., 2017, 5, 8017–8024.
4 L. Hong, L. Li, Y. K. Chen-Wiegart, J. Wang, K. Xiang, L. Gan,
W. Li, F. Meng, F. Wang, J. Wang, Y. M. Chiang, S. Jin and
M. Tang, Nat. Commun., 2017, 8, 1194.
5 S. Yang, P. Y. Zavalij and M. S. Whittingham, Electrochem.
Commun., 2001, 3, 505–508.
6 Y. Jin, Y. Shen and T. Hibino, J. Mater. Chem., 2010, 20, 6214–
6217.
2
TiO resulting in a number of known and novel Ti(III) and Ti(IV)
phosphates. Reaction of the hypophosphite with other transi-
tion metal oxides (Cr, V, Mn, Fe) revealed comparable results,
i.e., formation of transition metal phosphates with reduced and
stabilized oxidation states of the metal cation. Overall, the
synthesis of transition metal phosphates via a hypophosphite
melt provides a highly versatile route towards products with
rich structural and compositional variety for a class of materials
that is key for various future technologies, including energy
storage, power generation, catalysis and optical applications.
7 A. Cleareld, Chem. Rev., 1988, 88, 125–148.
8 P. L. Gai and K. Kourtakis, Science, 1995, 267, 661–663.
9 G. Centi, Catal. Today, 1993, 16, 5–26.
10 G. Busca, F. Cavani, G. Centi and F. Triro, J. Catal., 1986, 99,
400–414.
1
1 P. Bhanja, Y. Kim, B. Paul, J. Lin, S. M. Alshehri, T. Ahamad,
Y. V. Kaneti, A. Bhaumik and Y. Yamauchi, ChemCatChem,
2020, 12, 2091–2096.
Author contributions
N. S. conceived and performed the synthetic procedures and
pathways reported here. H. P. worked on the structural charac- 12 N. L. W. Septiani, Y. V. Kaneti, K. B. Fathoni, K. Kani,
terization of the new materials. C. W. evaluated the spectroscopic
data for determination of oxidation states of the novel
materials. W. S supervised collection and interpretation of
A. E. Allah, B. Yuliarto, Nugraha, H. K. Dipojono,
Z. A. Alothman, D. Golberg and Y. Yamauchi, Chem.
Mater., 2020, 32, 7005–7018.
experimental results and conceptually designed the present work. 13 S. Zhao, P. Gong, S. Luo, L. Bai, Z. Lin, C. Ji, T. Chen,
M. Hong and J. Luo, J. Am. Chem. Soc., 2014, 136, 8560–8563.
1
1
4 R. Lin and Y. Ding, Materials, 2013, 6, 217–243.
5 D.-H. Kuo and W.-C. Tseng, Mater. Chem. Phys., 2005, 93,
Conflicts of interest
There are no conicts to declare.
361–367.
1
1
1
6 A. F. Holdsworth, H. Eccles, A. M. Halman, R. Mao and
G. Bond, Sci. Rep., 2018, 8, 13547.
Acknowledgements
7 T. Dobbelaere, F. Mattelaer, J. Dendooven, P. Vereecken and
C. Detavernier, Chem. Mater., 2016, 28, 3435–3445.
8 N. Syed, A. Zavabeti, J. Z. Ou, M. Mohiuddin, N. Pillai,
B. J. Carey, B. Y. Zhang, R. S. Datta, A. Jannat, F. Haque,
K. A. Messalea, C. Xu, S. P. Russo, C. F. McConville,
T. Daeneke and K. Kalantar-Zadeh, Nat. Commun., 2018, 9,
The authors thank S. Leiting for measuring XPS spectra, B.
Zibrowius for supporting us with NMR spectra, B. Mienert and
E. Bill (MPI for Chemical Energy Conversion) for M ¨o ßbauer
spectra, and S. Palm for providing SEM images of selected
samples. Financial support by the Max Planck Society (MPG) is
greatly acknowledged. Open Access funding provided by the
Max Planck Society.
3618.
1
9 J. M. Rojo, J. L. Mesa, R. Calvo, L. Lezama, R. Olazcuaga and
T. Rojo, J. Mater. Chem., 1998, 8, 1423–1426.
References
20 H. Assi, G. Mouchaham, N. Steunou, T. Devic and C. Serre,
Chem. Soc. Rev., 2017, 46, 3431–3452.
1
D. Jugovi ´c and D. Uskokovi ´c , J. Power Sources, 2009, 190, 538–
44.
5
18250 | J. Mater. Chem. A, 2021, 9, 18247–18250
This journal is © The Royal Society of Chemistry 2021