Tetraalkylammonium Salts of Platinum Nitrato Complexes: Isolation, Structure, and Relevance to the Preparation of PtOx/CeO2 Catalysts for Lowerature CO Oxidation

Article abstract of DOI:10.1021/acs.inorgchem.9b00370

A series of tetraalkylammonium salts with anionic platinum nitrato complexes (Me₄N)₂[Pt₂(μ-OH)₂(NO₃)₈] (1), (Et₄N)₂[Pt₂(μ-OH)₂(NO₃)₈] (2), (n-Pr₄N)₂[Pt₂(μ-OH)₂(NO₃)₈] (3b), (n-Pr₄N)₂[Pt(NO₃)₆] (3a), and (n-Bu₄N)₂[Pt(NO₃)₆] (4) were isolated from nitric acid solutions of [Pt(H₂O)₂(OH)₄] in high yield. The structures of salts 2, 3a, 3b, and 4, prepared for the first time, were characterized by X-ray diffraction. The sorption of [Pt(NO₃)₆]^2- and [Pt₂(μ-OH)₂(NO₃)₈]^2- complexes onto the ceria surface from acetone solutions of salts 4 and 1 was examined. The dimeric anion was shown to quickly and irreversibly chemisorb onto the CeO₂ carrier, selectively transforming into Pt(II) centers after thermal treatment, becoming active in the lowerature CO oxidation reaction (T_50% = 110 °C at a space velocity of 240 000 h^-1). By contrast, the homoleptic complex [Pt(NO₃)₆]^2- did not interact with the ceria, which may be attributed to the substitutional inertness of the [Pt(NO₃)₆]^2- anion. We believe that the strategy based on the sorption of polynuclear platinum nitrato complexes is an effective route to prepare ionic platinum species uniformly distributed on an oxide carrier for various catalytic applications.

Full text of DOI:10.1021/acs.inorgchem.9b00370

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Tetraalkylammonium Salts of Platinum Nitrato Complexes: Isolation,
Structure, and Relevance to the Preparation of PtOx/CeO2 Catalysts
for Low-Temperature CO Oxidation
Danila Vasilchenko,*,†,‡ Polina Topchiyan,†,‡ Semen Berdyugin,† Evgeny Filatov,†,§ Sergey Tkachev,†
Iraida Baidina,† Vladislav Komarov,†,§ Elena Slavinskaya,‡ Andrey Stadnichenko,‡,§
and Evgeny Gerasimov‡,§
†Nikolaev Institute of Inorganic Chemistry, Siberian Branch of the Russian Academy of Science, 630090 Novosibirsk, Russian
Federation
‡Novosibirsk State University, 630090 Novosibirsk, Russian Federation
§Boreskov Institute of Catalysis, 630090 Novosibirsk, Russian Federation
S
* Supporting Information
ABSTRACT: A series of tetraalkylammonium salts with anionic
platinum nitrato complexes (Me4N)2[Pt2(μ-OH)2(NO3)8] (1),
(Et4N)2[Pt2(μ-OH)2(NO3)8] (2), (n-Pr4N)2[Pt2(μ-OH)2(NO3)8]
(3b), (n-Pr4N)2[Pt(NO3)6] (3a), and (n-Bu4N)2[Pt(NO3)6] (4)
were isolated from nitric acid solutions of [Pt(H2O)2(OH)4] in
high yield. The structures of salts 2, 3a, 3b, and 4, prepared for the
first time, were characterized by X-ray diffraction. The sorption of
[Pt(NO3)6]2− and [Pt2(μ-OH)2(NO3)8]2− complexes onto the
ceria surface from acetone solutions of salts 4 and 1 was examined.
The dimeric anion was shown to quickly and irreversibly chemisorb
onto the CeO2 carrier, selectively transforming into Pt(II) centers
after thermal treatment, becoming active in the low-temperature
CO oxidation reaction (T50% = 110 °C at a space velocity of
240 000 h−1). By contrast, the homoleptic complex [Pt(NO3)6]2− did not interact with the ceria, which may be attributed to the
substitutional inertness of the [Pt(NO3)6]2− anion. We believe that the strategy based on the sorption of polynuclear platinum
nitrato complexes is an effective route to prepare ionic platinum species uniformly distributed on an oxide carrier for various
catalytic applications.
postpreparation activation effects4,7,12 on performance of
PtOx/CeO2 catalysts was evaluated to find the optimal
synthetic approach. The role of platinum precursors in these
studies was diminished, and in the majority of the cases,
chloroplatinic acid was utilized as a starting reagent to deposit
platinum on the surface of ceria. However, from a preparative
point of view, the more effective route for PtOx site deposition
is the anchoring of the molecularly predesigned precursors on
1. INTRODUCTION
Carbon monoxide is a highly toxic pollutant accumulated in
the urban air. Within enclosed and poorly vented workplaces,
extremely low concentrations of CO (30−50 ppm) can lead to
serious dysfunctions of organisms, blocking the oxygen supply
of tissues.1 Therefore, the development of efficient catalytic
systems capable of neutralizing CO at trace levels in indoor air
and exhaust gases (from vehicles, heaters, generators, etc.) in
the surface of the carrier.
broad temperature and humidity ranges is an important task in
material science.2
Among known platinum compounds, nitrato complexes of
Pt(IV)13,14 seem to be perfect predecessors of PtOx active sites
In the past decade, atomically dispersed platinum on a ceria
in view of the following aspects: (1) These complexes already
surface (CeO2) was found to be an excellent catalyst for CO
include platinum atoms in a desired oxygen coordination
environment. (2) Lability of NO3− ligands toward substitution
provides the way for direct grafting of the complexes by the
reactive groups (O, OH, and H2O) of the surface of an oxide
carrier. (3) Thermolability of nitrato complexes15 implies their
oxidation, providing outstanding activity at low temperatures
(50−100 °C) and high humidity, but also operating at high-
temperature stresses up to 800 °C.3−8 The ionic platinum
species (Pt(II), Pt(IV)) incorporated into the CeO2 surface
were indicated as active sites responsible for low-temperature
CO oxidation by these systems,4,7,9 designated further as
“PtOx/CeO2”. The influence of the carrier preparation method
and morphology,10,11 local coordination structure,3,8 and
Received: February 8, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.9b00370
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
stream of air. Yields for all compounds were in the range of 75−80%.
In the case of (n-Pr4N)NO3, the yield corresponds to the mixture of
compounds 3a and 3b.
(Me4N)2[Pt2(μ-OH)2(NO3)8] (1). Anal. Calcd (Found) for C8H26-
N10O26Pt2: C, 8.99 (9.0); H, 2.45 (2.4); N, 13.11 (13.3). IR (cm−1,
KBr): 2927 (νas(CH3)), 1487 (δas(CH3)), 1385(δs(CH3)), 1584
(ν5(NO3)), 1270 (ν1(O−NO2)), 902 (ν2(NO3)), 532.4 (ν(Pt−
OH)). 195Pt NMR (ppm, acetone): 3980.
(Et4N)2[Pt2(μ-OH)2(NO3)8] (2). Anal. Calcd (Found) for C16H42-
N10O26Pt2: C, 16.28 (16.2); H, 3.59 (3.7); N, 11.86 (12.1).
IR (cm−1, KBr): 2989 (νas(CH3)), 2824 (ν(C−H)), 1385
(δs(CH3)), 1573 (ν5(NO3)), 1263 (ν1(O−NO2)), 932 (ν2(NO3)),
547 (ν(Pt−OH)). 195Pt NMR (ppm, acetone): 3979.
(Pr4N)2[Pt(NO3)6] + (Pr4N)2[Pt2(μ-OH)2(NO3)8] (3a + 3b). Anal.
Calcd (Found) for mixture C24H56N8O18Pt × 0.6 + C24H58N10O26Pt2
× 0.4: C, 26.66 (26.7); H, 5.30 (5.4); N, 11.40 (11.4). IR (cm−1,
KBr): 2979 (νas(CH3)), 2885 (νs(CH3)), 1382 (δs(CH3)), 2947
(νas(CH2)), 2822 (ν(C−H)),1473 (δas(CH3)), 1563 (ν5(NO3)),
1257 (ν1(O−NO2)), 927 (ν2(NO3)), 528.5 (ν(Pt−OH)). 195Pt
NMR (ppm, acetone): 3975 ([Pt2(μ-OH)2(NO3)8]2−), 3991 ([Pt-
(NO3)6]2−).
easy rearrangement into PtnOm species by heating under mild
conditions (∼200 °C, air). (4) No surface poisoning species
(such as Cl− or SOx) are produced during nitrato complex
decomposition; therefore, there is no need for high-temper-
ature annealing.
Moreover, as we reported earlier,13,14 platinum nitrato
complexes with various nuclearity can be prepared ([Pt-
(NO3)6]2−, [Pt2(μ-OH)2(NO3)8]2−, [Pt4(μ3-OH)2(μ2-OH)4-
(NO3)10], [Pt6(μ3-OH)4(μ2-OH)6(NO3)12]2+) that afford the
possibility of constructing PtnOm active sites with a specified n
number.
To isolate individual nitrato complexes from nitric acid
solutions of platinum(IV) hydroxide ([Pt(H2O)2(OH)4]), we
previously used macrocyclic cavitands (crown ethers, cucurbit-
[n]urils) and organic N-containing cations (pyridinium,
tetramethylammonium).13,14,16 In contrast to the majority of
prepared compounds, the tetramethylammonium salt
(Me4N)2[Pt2(μ-OH)2(NO3)8] (1) was found to be sufficiently
soluble in acetone with preservation of the anion structure,13
making it a highly promising precursor for PtOx/CeO2 catalyst
preparation.
Therefore, the main purposes of the current work were (a)
the utilization of a series of R4N+ cations (R = Me, Et, n-Pr, n-
Bu) for the isolation of anionic nitrato complexes from a nitric
acid solution of [Pt(H2O)2(OH)4] to determine how the R-
chain length controls the selectivity in this process, (b) testing
of the isolated salts for platinum deposition onto disperse ceria
via sorption from a solution, and (c) examination of the
resulting materials in the CO oxidation reaction.
(Bu4N)2[Pt(NO3)6] (4). Anal. Calcd (Found) for C32H72N8O18Pt: C,
36.53 (36.5); H, 6.90 (6.8); N, 10.65 (10.6).
IR (cm−1, KBr): 2965 (νas(CH3)), 2877 (νs(CH3)), 2937
(νas(CH2)), 2825 (ν(C−H)), 1383 (δs(CH3)), 1457 (δas(CH3)),
1548 (ν5(NO3)), 1258 (ν1(O−NO2)), 932.1 (ν2(NO3)).
MS (m/z (ESI), ACN): 809.6 (100%, {(Bu4N)[Pt(NO3)6]}−).
195Pt NMR (ppm, acetone): 3987.
Single crystals of compounds 1, 2, 3b, and 4 were picked directly
from the nitric acid solutions. The powder and single crystals of 3a
were prepared by a metathesis reaction as follows. A sample (50 mg)
of 4 was dissolved in acetone (2 mL), and an acetone solution of (n-
Pr4N)NO3 (200 mg in 0.5 mL) was added. To the resulting solution,
2 mL of water was added and the precipitate was isolated by
centrifugation. The precipitate was redissolved in acetone and treated
again with an acetone solution of (n-Pr4N)NO3 and water (the same
quantities) to obtain a pure powder of 3a, which was centrifuged,
washed several times with ethyl acetate, and dried under vacuum.
Single crystals of 3a were grown by slow evaporation of the acetone
solution.
2. EXPERIMENTAL SECTION
2.1. Starting Reagents. All reactants were purchased from
commercial suppliers and used as received. Ultraclean nitric acid (70
wt %, 15.9 M) produced by EuroChem (Novomoskovskiy Azot) was
used in all cases. Platinum(IV) hydroxide ([Pt(H2O)2(OH)4]) was
prepared from hexachloroplatinic acid (“The Gulidov Krasnoyarsk
Non-Ferrous Metals Plant” Open Joint Stock Company, 39.1%
platinum) as described previously.17 The (R4N)NO3 (R = Me, Et, n-
Pr, n-Bu) nitrate salts were prepared by action of concentrated nitric
acid on the corresponding bromide salts ((R4N)Br, Acros Organics,
98%), recrystallized from H2O/HNO3 solutions, and dried in a stream
of air. Acetone was dried prior to use by means of NaI solvate.18
The ceria (CeO2) carrier was prepared by thermal decomposition
of Ce(NO3)3·6H2O (99.9%, “Novosibirsk Rare Metals Plant”) at 220
°C in air for 24 h. The resulting solid material was washed several
times with an acetone/H2O (9:1 v/v) mixture until the acidity of the
solution in which the material is suspended (supernatant) reaches pH
about 3.5 (controlled by a methyl orange probe). The ceria
precipitate was then centrifuged and dried at 100 °C in a vacuum
for 24 h. The resulting material was characterized by X-ray powder
diffraction and high-resolution transition microscopy.
2.2. Complex Salt Preparation. Compounds 1, 2, 3a, 3b, and 4
were synthesized using a facile method described below. A finely
ground powder of the corresponding (R4N)NO3 salt (8.0 mmol) was
added to a freshly prepared solution of [Pt(H2O)2(OH)4] (600 mg,
2.0 mmol) in 5 mL of concentrated nitric acid (15.9 M) under
constant stirring. The resulting solution was evaporated in a stream of
air at room temperature until crystallization began. It should be noted
that, with the addition of (n-Bu4N)NO3 salt to the nitric acid solution
of [Pt(H2O)2(OH)4], layering of the solution was observed, but after
intensive stirring (10 min), the solution became homogeneous and
fine crystals of 3 began to form. The vial containing the solution
(about 4 mL) and the seed crystals was transferred to the desiccator
with solid KOH and evacuated for 24 h. The obtained crystalline
solids were isolated from the mother liquor by filtration, washed with
concentrated nitric acid and ethyl acetate (EtOAc), and dried in a
(n-Pr4N)2[Pt(NO3)6] (3a). Anal. Calcd (Found) for C24H56N8O18Pt:
C, 30.67 (30.7); H, 6.00 (5.9); N, 11.92 (11.8). MS (m/z (ESI),
ACN): 753.5 (100%, {(n-Pr4N)[Pt(NO3)6]}−).
2.3. Apparatus. 195Pt NMR spectra were recorded at 107.5 MHz
using an Avance III 500 Bruker spectrometer with a 5 mm broad-band
probe. A 90° excitation pulse of 15 μs was applied. All spectra were
recorded at 24 °C. δ195Pt (ppm) values are reported relative to the
external reference, which was a 2 M solution of H2[PtCl6] in 1 M
hydrochloric acid. The usual spectral window of 67 kHz (620 ppm)
was used with an acquisition time of 0.1 s and pulse delay of 0.7 s to
allow the 195Pt nuclei to relax completely. A line-broadening factor of
1 Hz was applied in the processing of all experimental FID data.
Electron absorbance spectra of the solutions were analyzed with a
PG Instruments T60 UV−vis single-beam spectrophotometer. Quartz
cells with a 1 cm optical path length were used. Infrared spectra were
recorded in the range of 400−4000 cm−1 on a Scimitar FTS 2000
apparatus in tablets of KBr.
Elemental CHN analysis was carried out on a CHNS analyzer
Vario MICRO cube. The platinum content in the solid Pt−CeO2
composite materials was determined using a high resolution
inductively coupled optical emission spectrometer iCAP-6500, a
Thermo Scientific (λ = 265.9 nm) with a cyclone type spray chamber,
and a “SeaSpray” nebulizer; the samples were dissolved in
concentrated sulfuric acid prior to analysis.
X-ray powder diffraction analysis of the polycrystalline samples was
carried out on a DRON-RM4 diffractometer (Cu Kα radiation,
graphite monochromator in the reflected beam, scintillation detector
with amplitude discrimination). The samples were prepared by
deposition of a suspension in hexane on the polished side of a cell
made of fused quartz. A sample of polycrystalline silicon (a = 5.4309
B
DOI: 10.1021/acs.inorgchem.9b00370
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Å), prepared similarly, was used as an external standard. Refinement
of the lattice parameters was performed by the full profile technique
applied to full-range diffraction data using the TOPAS Academic
program.19 The crystallite sizes of the oxide phases were determined
with the Scherrer equation using WINFIT 1.2.1 software.20
X-ray photoelectron spectra (XPS) were recorded on a KRATOS
ES-300 electron spectrometer using nonmonochromatic Mg Kα
radiation (hν = 1253.6 eV). The photoelectron spectra were recorded
using a low-power X-ray source (78 W) to prevent X-ray damage to
ceria.21,22 The spectrometer was calibrated using the Au 4f7/2 (84.0
eV) and Cu 2p3/2 (932.7 eV) lines of pure gold and copper metallic
surfaces. Spectral calibration for the samples was performed using
Eb(C 3d U′′′) = 916.7 eV.12 Chemical composition was calculated
from XPS data taking into account atomic sensitivity factors.23 The
original XPS-Calc software, which was successfully tested in earlier
works,9,12,24,25 was employed for deconvolution of experimental
spectra into individual components. The spectral background of the
Pt 4f line was subtracted by the Shirley method, and approximation of
the doublets was made using Lorentzian and Gaussian functions.
Before curve fitting, all experimental spectra were smoothed using a
Fourier filter. No significant difference between the smoothed and
experimental curves was observed; the mean-square deviation was less
than 1%.
OH)2(NO3)8]2− anions were then used for single point calculations
performed with the ADF2017 software package40−42 using the B3LYP
functional and COSMO43 solvation method (solvent, acetone, or
acetonitrile). The excitations in the molecule were calculated using
simplified time-dependent DFT (sTD-DFT).44−47 The ZORA*/
TZ2P (Pt) and ET-QZ3P (N, O, H) basis sets48 were chosen due to
their computer efficiency and precision for calculations on the
transition metal complexes.
2.6. Sorption Experiments. The sorption of [Pt2(μ-OH)2-
(NO3)8]2− and Pt(NO3)6]2− complexes on a CeO2 surface was
studied at room temperature (20 1 °C) in a cylindrical vessel with a
nozzle equipped with a porous glass filter (porosity is 16 μm). Five-
hundred milligrams of CeO2 powder was added to the freshly
prepared solution of platinum salt 1 or 4 in dry acetone (5 mL, 3−20
mM), and the resulting suspension was magnetically stirred (300
rpm) in the vessel. At adequate intervals, an aliquot (about 300 μL) of
the solution was filtered off through the nozzle and UV−vis spectra of
the solutions were collected. Then, the aliquot was returned to the
vessel and stirring was resumed. The fraction of Pt adsorbed was
estimated as ω(Ptads) = (D0 − Dt)/D0, where D0 indicates the optical
density of the starting solution of platinum salt at a fixed wavelength
(345 nm) and Dt is the current optical density at reaction time t.
At the end of the experiment, the solid material was isolated by
filtration, washed with acetone, and dried in a stream of air. The Pt
concentration in the solid samples was determined using ICP-ES. In
general, good agreement was observed for the platinum content values
estimated from the UV−vis spectra and determined by ICP-ES, with a
deviation of about 5 relative percent.
2.7. Pt/CeO2 Catalyst Preparation. Catalyst A (Sorption).
About 2.00 g of CeO2 carrier was dispersed in an acetone solution of
complex 1 (161 mg in 20 mL). The suspension was stirred for 5 min,
then centrifuged (4500 rpm) and washed twice with acetone. The
resulting material was dried in an air stream and sequentially calcined
in an oven at 100 °C (10 h) and 220 °C (5 h) in an air atmosphere.
The Pt loading was 2.00 wt % (by ICP-ES).
Catalysts B and C (Incipient Wetness Impregnation). A solution
of the platinum precursor (44 mg of 1 for B and 25 mg of H2PtCl6 for
C) in 1 mL of acetone was added to 2.00 g of CeO2 carrier under
constant stirring of the mixture. The resulting materials were dried in
an air stream while continuing to stir and sequentially calcined in an
oven at 100 °C (10 h) and 220 °C (5 h) in an air atmosphere.
According to ICP-ES, the Pt loading was 2.10 wt % for catalyst B and
2.20 wt % for catalyst C.
High-resolution transmission electron microscopy (HR-TEM)
images were obtained with a JEM-2010 (JEOL, Japan) microscope
with a resolution of 1.4 Å operated at an accelerating voltage of 200
kV. The size distribution of the nanoparticles was calculated based on
a representative set of HR-TEM images taken at different areas on the
sample. EDX analysis was carried out using an energy dispersive
spectrometer with a Si (Li) detector and an energy resolution of 130
eV.
2.4. X-ray Crystallographic Data Collection and Refinement.
Crystal data and experimental details for compounds 2, 3a, 3b, and 4
are given in Table S1 (see the Supporting Information). Experimental
data for the determination of the crystal structure of 2 were collected
on a Bruker APEX-II CCD diffractometer at 240 K using graphite-
monochromated Cu Kα radiation (λ = 1.54178 Å). Experimental data
for the determination of the crystal structures of 3a, 3b, and 4 were
collected on a Bruker-Nonius X8 APEX CCD diffractometer at 296 K
using graphite-monochromated Mo Kα radiation (λ = 0.7107 Å). All
calculations were carried out with the SHELX-97 crystallographic
software package.26 The structure was solved by the standard heavy-
atom method and refined by the anisotropic approximation. The H
atoms were refined in their geometrically calculated positions; a riding
model was used for this purpose. Absorption corrections were applied
empirically using the SADABS software.27 Hirshfeld molecular surface
analysis was performed using the CrystalExplorer 3.1 program.28 The
disordering of propylchains in the structure of 3b causes several level
B alerts in the CheckCIF report due to nonbonding contacts. Also, for
both structures of 3b and 4, residual density alerts (level B) arise
corresponding to the density maxima around Pt centers, which is
often found for structures containing heavy atoms and can be
interpreted as absorption artifacts due to insufficient absorption
correction.
2.8. Catalytic Activity Measurement. Catalytic CO oxidation
was studied using an automated installation with a continuous flow
reactor in a temperature-programmed mode.49 The gas composition
was monitored with mass spectrometry. A sample of a Pt/CeO2
catalyst powder was compacted and crumbled into grains with sizes
0.25−0.5 mm (0.25 cm3 volume) and was then placed in a stainless
steel reactor. The reaction mixture containing 0.2 vol % CO, 1.0 vol %
O2, and 0.5 vol % Ne (balance helium) with a rate of 1000 cm3/min
(GHSV = 240 000 h−1) was fed to the reactor, which was
preliminarily cooled to −10 °C. The catalyst was heated from −10
°C to 450 °C at a rate of 10 °C/min, followed by cooling and heating
in the reaction mixture. The concentrations of CO, O2, and CO2 were
monitored over the course of the reaction at a frequency of 0.34 Hz.
CCDC 1891138 (2), 1891135 (3a), 1891136 (3b), and 1891139
(4) contain the supplementary crystallographic data for this paper.
This data can be obtained free of charge from the Cambridge
Crystallographic Data Centre.
3. RESULTS AND DISCUSSION
2.5. Density Functional Theory (DFT) Calculations. Geometry
optimization and vibrational wavenumber calculation were carried out
using the DFT method implemented in the Gaussian 09C software
package.29 The molecular structures of [Pt(NO3)6]2− and [Pt2(μ-
OH)2(NO3)8]2− (symmetrical and angled conformations) anions
from crystal structures of compounds 4, 2, and 1 were used as initial
system states. The calculations were performed using the B3LYP30−33
method and LANL2DZ34−38 basis set for entire molecules placed in a
cavity within the water reaction field (SCRF solvent method39). The
assignment of the internal vibrations was made by visual inspection of
modes, animated using the GaussView molecular visualization
program. The optimized structures of [Pt(NO3)6]2− and [Pt2(μ-
3.1. Isolation of Platinum Nitrato Complexes with
R4N+ Cations. As reported earlier,13,14 after dissolution of
[Pt(H2O)2(OH)4] in concentrated nitric acid, a rather
complex mixture of platinum species is formed, including a
series of [Pt(NO3)n(H2O)6−n]
4−n mononuclear complexes and
polynuclear complexes with bridging hydroxide ligands and
terminal aqua and nitrato ligands. Addition of excess
(R4N)NO3 salts to the freshly prepared nitric acid solution
of H2Pt(OH)6 ([Pt] = 0.5M) results in rapid (immediate for n-
Bu4N+ salt) deposition of yellow crystalline solids (Figure 1).
C
DOI: 10.1021/acs.inorgchem.9b00370
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
and [Pt2(μ-OH)2(NO3)8]2− can be selectively isolated from
the platinum nitric acid solution using the R4N+ cations.
All prepared salts 1−4 are soluble in acetone, butanone,
acetonitrile, and acetic anhydride, but generally insoluble in
alcohols (MeOH, EtOH, i-PrOH), ethyl acetate, Et2O, and
glacial acetic acid. Complexes 3a and 4 are also slightly soluble
in tetrahydrofuran. It is interesting to note that salts 3a and 4
are completely insoluble in water and do not react with it,
making it possible to prepare 3a via metathesis of 4 and (n-
Pr4N)NO3, as described in the Experimental Section.
3.2. Crystal Structures. The prepared salts are crystallized
in the monoclinic (3b, 4) and triclinic (2, 3a) space groups
(Table S1). The structure of 1 was previously reported with
monoclinic space groups.13 The unit cells of the salts include
only tetraalkylammonium cations and platinum nitrato
complexes anions without additional solvent molecules (Figure
S6). In the structures containing [Pt2(μ-OH)2(NO3)8]2− (1, 2,
3b) dimeric anions with an R alkyl chain, the number of R4N+
cations surrounding a single anion gradually decreases: 10 in 1,
8 in 2, and 6 in 3b (Figure 2). An analogous tendency is
observed in structures 3a and 4 where [Pt(NO3)6]2− anions
are surrounded by eight n-Pr4N+ and six n-Bu4N+ cations,
respectively (Figure 3).
Figure 1. Photographs depicting the typical morphology of as-
prepared products 1−4.
Hirshfeld surface analysis was performed to describe the
main trends of molecular packing and intermolecular
interactions in the studied structures (Table 1). The fraction
of O···O contacts on the Hirshfeld surfaces of the anionic
platinum nitrato complexes decreased with the length of the
R4N+ alkyl chain, while the reverse relationship was observed
for the fraction of O···H contacts. This corresponds to the
partition of complex anions into separate voids (formed by
organic cations) when the size of the R4N+ molecules grows. In
the structures of 3a and 4, [Pt(NO3)6]2− anions are fully
isolated from each other by R4N+ organic cations and their
Hirshfeld surfaces are formed exclusively by contacts with
hydrogen atoms of R alkyl chains.
X-ray powder diffraction analysis (Figures S2−S4) of these
materials using single-crystal X-ray diffraction data (vide infra)
shows that, with Me4N+ and Et4N+ cations, the salts with the
dimeric anion [Pt2(μ-OH)2(NO3)8] (salts 1 and 2, respe-
citvely) are formed, while the n-Bu4N+ cation crystallizes
exclusively with the homoleptic [Pt(NO3)6]2− anion (4). The
n-Pr4N+ cation holds a interjacent position in this row, and
both [Pt(NO3)6]2− and [Pt2(μ-OH)2(NO3)8]2− (compounds
3a and 3b correspondingly) are isolated from the nitric acid
solution of [Pt(H2O)2(OH)4] upon addition of (n-Pr4N4N)-
NO3 (Scheme 1).
Slow crystallization under vacuum from more diluted
platinum nitric acid solution ([Pt] = 0.1 M) also gives an
analogous result: selective crystallization of [Pt2(μ-OH)2-
(NO3)8]2− with Me4N+ and Et4N+ cations and the [Pt-
(NO3)6]2− anion with the n-Bu4N+ cation is observed, while
the two phase mixture is formed in the case of the n-Pr4N+
cation. According to the 195Pt NMR data, the product that
formed upon addition of (n-Pr4N)NO3 to the nitric acid
solution of [Pt(H2O)2(OH)4] contains about 60 mol % of 3a
and 40 mol % of 3b.
In the case of the Hirshfeld surfaces of R4N+ molecules, the
fraction of H···O contacts gradually decreased with the length
of the R4N+ alkyl chain, while the contribution of H···H
contacts corresponding to contacts with adjacent R4N+
molecules dominates. It is interesting to note that, in the
structure of 3a, the fractions of H···O and H···H contacts on
the Hirshfeld surfaces of R4N+ molecules are almost equal and
the contribution of H···H contacts in 3a is the same as in 3b.
Therefore, it can clearly be seen from the results that, for the
salts of R4N+ cations with C3 and C4 carbon chains, the
arrangement becomes a prevailing factor that determines the
overall packing in the structures.
Low solubility of salts 1−4 in concentrated nitric acid
provides excellent yields (up to 80%), especially taking into
account the diversity of possible platinum species in the initial
solution.13 Therefore, the anionic complexes [Pt(NO3)6]2−
Scheme 1. Preparation of Salts 1−4
D
DOI: 10.1021/acs.inorgchem.9b00370
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 2. Packing in the (R4N)2[Pt2(μ-OH)2(NO3)8] (1, 2, 3b) salt structures showing how the R4N+ cations molecules surround the
[Pt2(OH)2(NO3)8]2− anions. (a) Structure 1: two Me4N+ molecules above and two Me4N+ molecules below the plane of the figure are not shown
for clarity. (b) Structure 2: one Et4N+ molecule above and one Et4N+ molecule below the plane of the figure are not shown. (b) Structure 3b: one
n-Pr4N+ molecule above and one n-Pr4N+ molecule below the plane of the figure are not shown.
the structures of salts 3a and 4 displays a trigonal antiprism
(D3d) geometry that is almost identical to that observed
previously for structures with different cations.13,15,16 Two sets
of three nitrato ligands from the “top” and “bottom” faces of
the antiprism are directed to the opposite face (Figure 4a).
The planes of nitrato ligands are perpendicular to the basal
planes of the {PtO6} antiprism and form a propeller-like
structure (Figure 4b).
Figure 3. Packing in the structures of the (R4N)2[Pt(NO3)6] (3a, 4)
salts showing how the R4N+ cations molecules surround the
[Pt(NO3)6]2− anions. (a) Sstructure 3a: n-Pr4N+ molecules are
located in the vertices of a parallelepiped. (b) Structure 4: one n-
Bu4N+ molecule above and one n-Bu4N+ molecule below the plane of
the figure are not shown.
Table 1. Percent Contributions to the Hirshfeld Surface
Area for the Various Close Intermolecular Contacts for
Anions and Cations in the Structures of Salts 1−4
Figure 4. Structure of the [Pt(NO3)6]2− anion in the crystal structure
of 4. (a) Mixed view showing the “top” and “bottom” faces of the
antiprism in the {PtO6} coordination polyhedron. Color code is the
following: gray, platinum; red, oxygen; and blue, nitrogen. (b) The
space-filling model view along the axis perpendicular to the “top” and
“bottom” faces of the antiprism. Oxygen atoms of the nitrato ligands
from the “bottom” faces of the antiprism are marked with a pale red
color.
compound
contacts
1
2
3b
3a
4
anions
O···O
O···N
O···H
H···H
N···O
N···H
N···N
28.2
2.2
65.6
0
2.3
1.7
0
14.8
1.2
77.2
2.5
1.3
2.9
11.7
0.5
84.0
0
0.5
3.2
0.0
0
0
97.2
0
0
2.8
0
0
0
98.0
0
0
2.0
0
The following two different conformations of the dimeric
complex anion [Pt2(μ-OH)2(NO3)8]2− were found in the
structures of salts 1, 2, and 3b: an “angled” conformation with
pairs of nitrato ligands above and below the Pt2(μ-O)2 plain
oriented at an angle of about 100° was observed in the
structures 1 and 3b (Figure 5a), and a “symmetric”
conformation with an almost coplanar orientation of nitrato
ligands above and below the Pt2(μ-O)2 plane was observed in
the structure of 2 (Figure 5b). The nitrato ligands located trans
to the bridging OH ligands in the “symmetric” form of [Pt2(μ-
OH)2(NO3)8]2− also revealed an almost coplanar orientation
(Figure 5d) in contrast to the crossed orientation of the same
ligands in the “angled” conformation (Figure 5c).
0.1
cations
H···O
H···H
H···N
81.8
16.5
1.8
67.1
30.6
2.3
49.4
48.8
1.8
56.9
48.8
1.1
33.5
65.8
0.7
Due to steric hindrances that rise from the arrangement of
six nitrato ligands around the Pt(IV) center, the {PtO6}
polyhedron of the homoleptic complex anion [Pt(NO3)6]2− in
E
DOI: 10.1021/acs.inorgchem.9b00370
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
cm−1) vibrations of nitrato ligands (Tables S9−S11). The
bands attributed to the stretching vibration of Pt−O bonds are
in the low frequency region (250−500 cm−1). An interesting
peculiarity of the dimeric anion [Pt2(μ-OH)2(NO3)8]2− is the
pulsating vibration of the {Pt(μ-OH)2Pt} ring structure, with
calculated IR vibrational frequencies around 520 cm−1. The
DFT calculated frequencies of vibrations deviate notably
(about 100 cm−1) from the experimental frequencies in the
high frequency region, while a reasonable agreement is
observed for bands below 1000 cm−1.
The sTD-DFT calculated vertical transitions for [Pt2(μ-
OH)2(NO3)8]2− in the UV−vis region (Figure S11) include
transitions from π-orbitals of the NO3 ligand to σ*orbitals of
the {O2Pt(μ-OH)2PtO2} fragment in a range of 400−250 nm
(ligand-to-metal charge transfer, LMCT) and a series of
transitions between π orbitals of NO3 ligands, producing an
intense band in a deeper UV range (<250 nm). A similar
situation is observed for the [Pt(NO3)6]2− anion: there are two
weak bands in the ranges of 310−360 nm and 300−250 nm,
related to transitions from π orbitals of the NO3 ligand to σ*
orbitals of the PtO4 fragment (LMCT bands), and an intense
band at 220 nm corresponds to the transitions between ligand-
centered orbitals.
The position and relative intensity of the transitions
calculated with the sTD-DFT method for [Pt(NO3)6]2− are
in good agreement with the experimental UV−vis spectrum of
salt 4 in an acetonitrile solution (Figure S12). Due to the
highly reactive nature of [Pt2(OH)2(NO3)8]2− (see below),
the experimental spectra of this species were acquired only for
acetone and methylethylketone solutions, where the majority
of the mentioned bands are obscured by strong absorbance of
solvents (cutoff wavelength is 320 nm for acetone). At the
same time, for both [Pt2(OH)2(NO3)8]2− and [Pt(NO3)6]2−
in acetone solutions, the edges of LMCT bands are clearly
observable in the range of 320−400 nm (Figure 6), which was
used to track the complex concentration during the sorption
experiments (section 3.5).
Figure 5. Structures of the “angled” (a, c) and “symmetric” (b, d)
conformations of the [Pt2(μ2-OH)2(NO3)8]2− anion from the
structures of 3b and 2, respectively. Panels (a) and (b) show the
view along the axis perpendicular to the Pt2(μ2-O)2 plane. Panels (c)
and (d) show the view along the “Pt−Pt” axis.
The arrangement of nitrato ligands around the Pt center in
the “angled” conformation is closely related to that in the
homoleptic [Pt(NO3)6]2−, while the “symmetric” conforma-
tion shows a principally different mode of the mutual
−
orientation of the NO3 ligands. The “angled” conformation
of the dimeric anion was also observed in the previously
reported structure of the [H3O⊂18-crown-6]2[Pt(μ2-OH)2-
(NO3)8][Pt4(μ3-OH)2(μ2-OH)4(NO3)10] salt.14
Within both conformations of the [Pt2(μ-OH)2(NO3)8]2−
anion and the homoleptic complex [Pt(NO3)6]2−, the Pt−
ONO2 bond lengths are almost identical (2.00 0.01 Å), even
in the case of the nitrato ligands located trans to the bridging
hydroxo ligands (Tables S2−S5, Figures S7−S10). The Pt−
OH bonds in the [Pt2(μ-OH)2(NO3)8]2− dimeric anions
(structures 2, 3b) are slightly longer (2.02 0.01 Å), which
agrees well with previously reported data for 1 and the [H3O⊂
18-crown-6]2[Pt(μ2-OH)2(NO3)8][Pt4(μ3-OH)2(μ2-OH)4-
(NO3)10] salt.13,14
3.3. DFT Calculations. To gain insight into the spectral
properties of the studied platinum complexes, calculations
based on DFT were performed. Experimental molecular
structures of the homoleptic complex [Pt(NO3)6]2− and the
dimeric complex anion [Pt2(μ-OH)2(NO3)8]2− (“angled” and
“symmetric” conformations) were used as initial models for the
energy minimization procedure (Tables S6−S8). Then, vertical
electronic transitions and IR vibrational frequencies (harmon-
ic) were calculated for the optimized structures. Interestingly,
the structure optimization of the dimeric anion [Pt2(μ-
OH)2(NO3)8]2− starting from “angled” and “symmetric”
conformations did not lead to a single conformation, indicating
that both conformers correspond to the local minima on the
energy surface with a sufficiently high barrier for interconver-
sion.
Figure 6. UV−vis absorption spectra of 1 (blue line) and 4 (orange
line) in acetone. Extinction coefficients are calculated per platinum
center. The dashed green line marks the 345 nm wavelength position.
The gray filled area is the spectrum of pure acetone taken against
water.
In the infrared region (4000−100 cm−1), the most intense
DFT calculated transitions for both [Pt2(μ-OH)2(NO3)8]2−
and [Pt(NO3)6]2− complexes were assigned to the stretching
(about 1450 cm−1) and deformation (1150−1200, 800−900
F
DOI: 10.1021/acs.inorgchem.9b00370
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
3.4. Reactivity of Platinum Nitrato Complexes in a
Solution. The solubility of the tetraalkylammonium salts (1−
4) in organic solvents makes them suitable precursors for
platinum deposition on various carriers for preparation of
heterogeneous Pt-containing catalysts via sorption of the
complexes. For example, one would expect that nitrato ligands
in [Pt(NO3)6]2− or [Pt2(OH)2(NO3)8]2− could be easily
replaced by reactive OH− and H2O groups on the surface of an
oxide carrier, resulting in platinum complex grafting that could
be schematically represented by the following equations:
{Pt−NO3} + {H2O−M} → {Pt−(μ‐HO)−M} + HNO3
(1)
{Pt−NO3} + {HO−M} → {Pt−(μ‐HO)−M} + NO3−
(2)
Taking into account the previously noted reactivity of platinum
nitrato complexes,13−15 we first tested the stability of platinum
nitrato complexes in acetone, which was chosen as an
appropriate solvent for sorption experiments. Complexes 4
and 1 were used as a source of [Pt(NO3)6]2− and
[Pt2(OH)2(NO3)8]2− species, respectively. To monitor the
possible transformations in the solutions of salts, 195Pt NMR
spectra were recorded every 1 h for at least 10 h after solution
preparation.
Figure 7. Change in the 195NMR spectrum of the 1 acetone solution
during the first 6 h after preparation (every spectrum is shifted at 10
ppm with respect to the previous one for clarity).
The homoleptic complex [Pt(NO3)6]2− demonstrated
perfect stability in the acetone solution. The only signal of
the [Pt(NO3)6]2− species was registered in the 195Pt NMR
spectra during the experiment, without significant changes in
intensity. Moreover, the addition of water (10%) to the
acetone solution of 4 did not cause the hydrolysis of
[Pt(NO3)6]2−. Taking into account the unexpected stability
of [Pt(NO3)6]2−, we also have managed to record the MS
spectrum of this anion using the solution of 4 in the strongly
coordinating solvent acetonitrile (Figure S13).
On the contrary, in the acetone solution of 1, the
degradation of the [Pt2(μ-OH)2(NO3)8]2− species was clearly
observable after several hours. The 195Pt NMR spectra of
freshly prepared solutions of 1 consisted of a signal of the
[Pt2(μ-OH)2(NO3)8]2− complex at 3980 ppm (Figure 7).
After 2 h, two new signals at 4050 and 3820 ppm appeared,
whereas the fraction of parent [Pt2(μ-OH)2(NO3)8]2−
decreased gradually. New signals located in the range
characteristic of nitrato complexes of platinum(IV) suggest
the solvolysis of [Pt2(μ-OH)2(NO3)8]2− by acetone or traces
of water. After 10 h, the signals in the range of 4200−3700
ppm disappeared completely, while the solution became dark
red. Extended (4500−4500 ppm) examination of the 195Pt
NMR spectrum of the solution after 10 h of aging showed only
the group of three signals in the range of 0−100 ppm, which is
characteristic of Pt(II) aqua-nitrato complexes (Figure
S14A).50
represents a quite complex pattern where only signals of the
{Pt(NO3)3}− anion were reliably ascribed while positive mode
of the spectrum contains a series of signals corresponding to
the {(Me4N)n+1(NO3)n}+ cations.
It can be concluded that the long-term instability of the
dimeric anion in acetone solutions corresponds to the
substitution of nitrato ligands with solvent or residual water
molecules and the reduction of Pt(IV) to Pt(II) species,
presumably by acetone. Such reactivity of [Pt2(μ-OH)2-
(NO3)8]2− in comparison to the mononuclear [Pt(NO3)6]2−
anion is most likely caused by the presence of bridging OH
groups that are known to exert the labilization of ligands
located in the trans position. The tetranuclear complex
[Pt4(μ3-OH)2(μ2-OH)4(NO3)10] was found to react in
acetone solution, even quicker than [Pt2(μ-OH)2(NO3)8]2−,
and after about 30 min from the dissolution of the [H3O⊂18-
crown-6]2[Pt(μ2-OH)2(NO3)8][Pt4(μ3-OH)2(μ2-OH)4-
(NO3)10] salt, the signals of the tetranuclear complex vanished
completely.14
Nevertheless, according to the 195Pt NMR spectroscopy
results, the [Pt2(μ-OH)2(NO3)8]2− complex was apparently
stable within the first 1−2 h after solution preparation, and for
a given period, it can be used for platinum deposition,
assuming that this species is the dominant platinum form in
the solution.
3.5. Sorption of Platinum Nitrato Complexes on
CeO2. A highly dispersed CeO2 powder (80 m2/g) with an
average particle size of 5−10 nm was used for probing
platinum(IV) nitrato complex sorption. Deposition of
platinum complexes from acetone solutions of complexes 1
and 4 was monitored using UV−vis spectroscopy while
decreasing the absorption intensity at 345 nm, where the
shoulders of charge transfer bands of nitrato complexes are
located and the acetone absorption can be neglected (Figure
6).
The new signals were also detected in the 13C spectra of the
aged acetone solution of 1. According to the chemical shift
values of these signals and the results of the additional
examination using the SEFT technique (J-modulated spin−
echo for X-nuclei coupled to H atoms), they were attributed to
CH3, CH2, and CO groups in a product of the reaction
between acetone and the [Pt2(μ-OH)2(NO3)8]2− complex
(Figure S14B). The signal of the CH2 group did not show any
signs of 13C−195Pt coupling, revealing that the given product is
not coordinated to platinum via a Pt−C bond. The ESI-MS
spectrum of the aged acetone solution of 1 in negative mode
The experiments revealed that the homoleptic nitrato
complex [Pt(NO3)6]2− was not sorbed onto CeO2 under the
G
DOI: 10.1021/acs.inorgchem.9b00370
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 8. Left panel is a plot of the amount of the [Pt2(μ-OH)2(NO3)8]2− complex sorbed from the acetone solution of 1 with various initial
concentrations of CeO2 versus time (the dots indicate experimental data, the lines are plotted using eq 3, and the data are from Table 2). The right
panel shows the pseudo-second-order plot for the sorption kinetics of [Pt2(μ-OH)2(NO3)8]2− onto CeO2.
given conditions within a reasonable time (6 h). Examination
of the 4 acetone solution after such interaction by 195Pt NMR
revealed only the signal of the parent [Pt(NO3)6]2− complex in
the spectrum. This result is understandable in view of the
[Pt(NO3)6]2− complex inertness mentioned above. As
substitution of nitrato ligands in [Pt(NO3)6]2− is slow, the
sorption of this species can proceed only by simple
electrostatic binding with the positively charged CeO2 surface.
However, the electrostatic sorption of platinum anionic
complexes from acetone solutions of their salts should be
complicated due to the aprotic nature of this solvent (absence
of sufficient surface charging) and strong ion pairing in such
media. To check this assumption, we studied the interaction of
an acetone solution of the (Bu4N)2[PtCl6] salt with the same
carrier and found no signs of platinum sorption for at least 6 h.
On the other side, effective platinum sorption onto the
CeO2 carrier was observed for the acetone solution of salt 1
containing the dimeric anion [Pt2(μ-OH)2(NO3)8]2−. To
exclude any side processes taking place in the solution (as
mentioned above), all experiments were carried out with a
freshly prepared solution of 1 and the data were collected for
only 60 min after solution preparation.
Figure 8 depicts the time dependences of the amount of
platinum complex adsorbed on CeO2 from the solutions of 1
with different starting concentrations of the complex (3−17
mM). In all cases, fast adsorption of platinum was observed
within the first 30 min, followed by a plateau region. Platinum
desorption was not detected when acetone was used as an
eluent.
Several models were tested to fit the sorption kinetics data,
including pseudo-first-order, pseudo-second-order, intrapar-
ticles diffusion, and Elovich models. Excellent agreement with
the experimental data (R2 > 0.995) was obtained using the
pseudo-second-order model,51 with the integrated rate law
described by the following equation
rate of sorption is determined by both the concentration of the
adsorbate ([Pt2(μ-OH)2(NO3)8]2− denoted as Pt2) and
sorption centers ({Ce}) on the adsorbent surface interacting
in a next net reaction:
Pt2 + 2{Ce} → {Ce}Pt2{Ce}
(4)
According to eq 4, two active centers on the ceria surface are
required to bind the dimeric anion [Pt2(μ-OH)2(NO3)8]2−,
and as the pseudo-second-order rate law holds true, the
concentration of the {Ce} centers determines the sorption rate.
The fitted values of k2 and qe constants together with the R2
values are listed in Table 2.
Table 2. Parameters of the Pseudo-Second-Order Model (eq
3) for the Sorption of [Pt2(OH)2(NO3)8]2− onto CeO2 from
Acetone Solutions of 1
starting concentration of
k2 × 10−3
1 (mM)
(g μmol−1 min−1
)
qe (g μmol−1
)
R2
3.75
7.50
11.25
15.00
16.85
11.0 2.0
53.1 2.4
108.5 10.8
104.1 18.7
105.0 16.8
32
48
65
79
77
1
1
1
2
1
0.998
1.000
1.000
0.997
0.995
Since the sorption kinetics follow the pseudo-second-order
law and platinum desorption was not detected, then the
adsorption of the dimeric [Pt2(μ-OH)2(NO3)8]2− anion onto
CeO2 can be interpreted as a chemical sorption process51−54
involving grafting of the platinum complexes via formation of
Ce−O−Pt covalent bonds with OH− or H2O reactive groups
on the surface of the carrier (eqs 1 and 2; Scheme 2). In
contrast to the inert homoleptic [Pt(NO3)6]2− complex,
bridging hydroxo ligands in the [Pt2(μ-OH)2(NO3)8]2−
−
anion labilize the terminal NO3 groups toward substitution
2
and provide sorption of this complex onto ceria surface via the
mechanism depicted in Scheme 2.
t/qt = 1/(k2·qe ) + t/qe
(3)
where k2 is the pseudo-second-order rate constant of sorption
(g/μmol−1 min−1), qe is the amount of the complex sorbed at
equilibrium (μmol g−1), and qt is the amount of the complex
sorbed at any time t (μmol g−1). This model implies that the
The maximum amount of the complex sorbed (qe) gradually
grows when the starting concentration of 1 is increased from 3
to 15 mM (Table 2, Figure 8), but further increases in the
platinum concentration does not lead to an increase in the
H
DOI: 10.1021/acs.inorgchem.9b00370
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
a
Scheme 2. Proposed Mechanism of [Pt2(μ-OH)2(NO3)8]2− Sorption onto the CeO2 Surface
a
Nitrato ligands are designated as L, and charges are omitted.
value of qe. Moreover, the rate of the sorption process (k2
values in Table 2) also reaches a maximum when the starting
concentration of 1 becomes greater than 11−15 mM. It can be
concluded that this situation corresponds to the saturation of
active sites on the CeO2 surface; thus the sorption rate and the
maximum capacity are both limited by the concentration of
active groups, which can bind the platinum complexes.
platinum(IV) nitrato complexes are substantially reduced upon
sorption onto CeO2, which could be a result of the redox
interaction with Ce(III) ions present on the surface of the
support in a considerable amount (Figure S17, Table S12) due
to the nonstoichiometric properties of CeO2.58 Similar Pt(IV)
ion reduction with Ce(III) was observed on heating (100−150
°C) of PtO2 clusters deposited with radio frequency plasma
sputtering on the ceria surface.57 Nevertheless, the reduction of
the 1 by the solvent molecules during deposition on ceria from
an acetone solution also cannot be excluded.
3.6. CO Oxidation with the PtOx/CeO2 Catalyst. The
sample obtained by deposition of 2.0 wt % platinum on the
CeO2 carrier by sorption from an acetone solution of 1 (1-
CeO2) was calcined at 220 °C, and the resulting catalyst
(catalyst A) was tested for the CO oxidation reaction. For
comparison, two additional catalysts were prepared by the
standard incipient wetness impregnation (IWI) technique
using acetone solutions of 1 and H2PtCl6 as platinum
precursors, followed by calcination at 220 °C (catalysts B
and C). All prepared catalysts were tested in the CO oxidation
reaction at a GHSV of 240 000 h−1.
Within the reliable period (1 h), about 70 μmol (at most) of
the platinum dimeric complex can be loaded per gram of CeO2
carrier, which corresponds to 2.7 wt % of Pt. The IR spectrum
of the composite material containing 2.0 wt % of Pt (1-CeO2)
reveals the weak signals (Figure S15) corresponding to the Pt-
coordinated nitrato ligands at 1550, 1300, and 950 cm−1 and
δ(CH3) vibrations of Me4N+ cations at 1487 and 1385 cm−1.
−
The signals are strongly masked by the peaks of NO3 and
H2O molecules (1600−1200 cm−1) adsorbed on the ceria
surface.55,56 The XRD pattern of the material (1-CeO2) does
not show any reflections different from those of the CeO2
fluorite phase, with crystallite sizes around 4−6 nm (Figure
S16).
The XPS data of the as-prepared 1-CeO2 material contain
two doublet bands in the Pt(4f) core level region (Figure 9),
with Eb(Pt 4f7/2) = 72.8 and 74.3 eV attributed to the Pt(II)
and Pt(IV) states, respectively.9,57 On the other side, in
comparison with starting CeO2, the fraction of Ce3+ on the
surface of 1-CeO2 material is dramatically decreased to about
3% (Figures S17 and S18, Table S12). This implies that the
As shown in Figure 10, when using catalyst A, 50%
conversion of CO is reached at about 110 °C (T50% = 110 °C),
Figure 10. Temperature dependences of CO conversion for catalysts
A (a), B (b), and C (c).
which greatly exceeds the activity of catalyst C prepared using
a conventional precursor (H2PtCl6, T50% = 275 °C). Catalyst B
prepared from 1 by the IWI technique demonstrates interim
T50% values (160 °C). The activation energy (Ea) values
calculated from the Arrhenius plots of ln(rate) vs 1/T for the
Figure 9. Curve-fitted XPS of the core level region of Pt(4f) of the as-
prepared 1-CeO2 material (2 wt % Pt).
I
DOI: 10.1021/acs.inorgchem.9b00370
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 11. TEM micrographs of as-prepared CeO2 carrier (left) and catalyst A after the reaction (right).
CO + O2 reaction over catalysts A, B, and C are 6.8, 9.1, and
14.3 kcal/mol, respectively.
According to the HR-TEM data taken for catalyst A after the
reaction (Figure 11), the morphology of the CeO2 carrier did
not change significantly after deposition of platinum and
treatment in reaction medium (CO + O2 + Ar, 450 °C). It
consists of irregularly oriented CeO2 crystallites with sizes
varying in the range of 5−10 nm. Both Ce(III) and Ce(IV)
states are registered in the XPS spectra of catalyst A (Figure
S19), with the Ce(III)/Ce(IV) ratio sufficiently higher in
comparison to the as-prepared material 1-CeO2 (Table S12).
Only trace amounts of nitrogen with Eb(N 1s) = 398.5 eV were
detected on the surface of catalyst A (Table S12), indicating
−
that the NO3 ions bonded with cerium and platinum
decompose almost completely during catalyst functioning.
The clearly defined particles of any platinum-containing
phases were not detected on the HR-TEM micrographs of
catalyst A, whereas the local EDX analysis shows that platinum
is reasonably uniformly distributed over the carrier surface with
a mean concentration of about 1.8 wt % (Table S13, Figure
S20).
Figure 12. Curve-fitted XPS of the core level region of Pt(4f) of the
catalyst A after the reaction.
The XPS spectrum of catalyst A in the Pt(4f) core level
region reveals the only doublet band, with Eb(Pt 4f7/2) = 72.8
eV attributed to the Pt(II) oxidation state (Figure 12). A small
shift of Eb to the lower binding energy region in comparison to
the reference value can be caused by the retention of the
binuclear structure of the platinum species and further
aggregation into polynuclear assemblies.9 The XPS spectrum
of catalyst C taken after the reaction also demonstrates doublet
band corresponding to the Pt(II) state (Figure S21), but in
this case, an overall analysis reveals high content of chlorides
(Table S14).
Consequently, active sites of catalyst A are represented
exclusively by the Pt(II) ionic species (probably with a
multinuclear structure) incorporated into the surface layer of
the CeO2 carrier. In previously reported studies where low-
temperature activity was found for Pt−CeO2 systems, such
activity also was associated with the formation of Pt(II)
species3,7,59 or polyatomic PtnOm subnanometric clusters.60
The lowest T50% values reported for such systems were about
50 °C3,7,8 at GHSV in a range 10 000−5000 h−1; however, in
more practically oriented conditions (GHSV > 150 000 h−1,
vehicle exhaust conditions), the 50% conversion of CO was
reached at about 100−150 °C,3,4 which is comparable with the
behavior of catalyst A. Sufficiently higher T50% values registered
for catalyst C can be attributed to the poisoning action of
chloride ions.
Thereby, using the dimeric anion [Pt2(μ-OH)2(NO3)8]2−
provides the way for preparation of PtOx/CeO2 catalysts in
mild conditions. Moreover, taking into account that nitrato
complexes of various nuclearity can be isolated from the
platinum nitric acid solution,14 the approach described here
can be used for deposition of polyatomic PtnOm clusters with
the controllable size (n) on ceria or another oxide carrier. It is
also important to note that catalyst A displays nonzero activity
at room temperature (2−5% CO conversion). Analogous room
temperature CO conversion was reported for various Pt−CeO2
catalysts,4,7,8 which makes such systems promising for air
depollution under ambient conditions.
4. CONCLUSIONS
A series of new salts containing tetraalkylammonium cations
R4N+ (R = Et, n-Pr, n-Bu) and anionic platinum(IV) nitrato
complexes [Pt2(μ-OH)2(NO3)8]2− and [Pt(NO3)6]2− were
prepared from a nitric acid solution of [Pt(H2O)2(OH)4]
using a straightforward protocol. It was shown that, depending
on the R-chain length, the mononuclear [Pt(NO3)6]2− (R = n-
Bu) or binuclear [Pt2(μ-OH)2(NO3)8]2− (R = Me, Et)
platinum complex could be selectively isolated from the
J
DOI: 10.1021/acs.inorgchem.9b00370
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Activation of Surface Lattice Oxygen in Single-Atom Pt/CeO2 for
Low-Temperature CO Oxidation. Science 2017, 358 (6369), 1419−
1423.
solution in high yield. The salts (n-Bu4N)2[Pt(NO3)6] and
(Me4N)2[Pt2(μ-OH)2(NO3)8] were tested for platinum
deposition on ceria via sorption from acetone solutions. Fast
and irreversible chemisorption was registered for the binuclear
[Pt2(μ-OH)2(NO3)8]2− anion, while the homoleptic complex
[Pt(NO3)6]2− demonstrated no sorption due to its inertness
toward ligand substitution. The dimeric platinum complex
grafted on ceria after thermal treatment transformed into the
surface-localized Pt(II) species with the oxygen coordination
environment, providing excellent catalytic activity of the
resulting PtOx/CeO2 material for the CO oxidation reaction
at low temperatures. The approach proposed in this work can
be further expanded for preparation of materials, where ionic
platinum states are grafted on the surface of an oxide carrier,
that can be interesting for various applications, including CO
sensing,61 water−gas shift reaction catalysis,62 and photo-
catalytic processes.63,64
́
(5) Neitzel, A.; Figueroba, A.; Lykhach, Y.; Skala, T.; Vorokhta, M.;
̌
́
Tsud, N.; Mehl, S.; Sevcíkova, K.; Prince, K. C.; Neyman, K. M.; et al.
Atomically Dispersed Pd, Ni, and Pt Species in Ceria-Based Catalysts:
Principal Differences in Stability and Reactivity. J. Phys. Chem. C
2016, 120 (18), 9852−9862.
̈
̈
(6) Ganzler, A. M.; Casapu, M.; Maurer, F.; Stormer, H.; Gerthsen,
D.; Ferre, G.; Vernoux, P.; Bornmann, B.; Frahm, R.; Murzin, V.; et al.
Tuning the Pt/CeO2 Interface by In Situ Variation of the Pt Particle
Size. ACS Catal. 2018, 8 (6), 4800−4811.
́
(7) Gatla, S.; Aubert, D.; Flaud, V.; Grosjean, R.; Lunkenbein, T.;
Mathon, O.; Pascarelli, S.; Kaper, H. Facile Synthesis of High-Surface
Area Platinum-Doped Ceria for Low Temperature CO Oxidation.
Catal. Today 2018, in press. DOI: 10.1016/j.cattod.2018.06.032.
(8) Gatla, S.; Aubert, D.; Agostini, G.; Mathon, O.; Pascarelli, S.;
Lunkenbein, T.; Willinger, M. G.; Kaper, H. Room-Temperature CO
Oxidation Catalyst: Low-Temperature Metal-Support Interaction
between Platinum Nanoparticles and Nanosized Ceria. ACS Catal.
2016, 6 (9), 6151−6155.
(9) Stadnichenko, A. I.; Murav’ev, V. V.; Svetlichnyi, V. A.; Boronin,
A. I. Platinum State in Highly Active Pt/CeO2 Catalysts from the X-
Ray Photoelectron Spectroscopy Data. J. Struct. Chem. 2017, 58 (6),
1152−1159.
(10) Singhania, N.; Anumol, E. A.; Ravishankar, N.; Madras, G.
Influence of CeO2 Morphology on the Catalytic Activity of CeO2-Pt
Hybrids for CO Oxidation. Dalt. Trans. 2013, 42 (43), 15343−15354.
(11) Gao, Y.; Wang, W.; Chang, S.; Huang, W. Morphology Effect of
CeO2 Support in the Preparation, Metal-Support Interaction, and
Catalytic Performance of Pt/CeO2 Catalysts. ChemCatChem 2013, 5
(12), 3610−3620.
(12) Slavinskaya, E. M.; Stadnichenko, A. I.; Muravyov, V. V.;
Kardash, T. Y.; Derevyannikova, E. A.; Zaikovskii, V. I.; Stonkus, O.
A.; Lapin, I. N.; Svetlichnyi, V. A.; Boronin, A. I. Transformation of a
Pt-CeO2 Mechanical Mixture of Pulsed-Laser-Ablated Nanoparticles
to a Highly Active Catalyst for Carbon Monoxide Oxidation.
ChemCatChem 2018, 10 (10), 2232−2247.
(13) Vasilchenko, D.; Tkachev, S.; Baidina, I.; Korenev, S. Speciation
of Platinum(IV) in Nitric Acid Solutions. Inorg. Chem. 2013, 52 (18),
10532−10541.
(14) Vasilchenko, D.; Berdugin, S.; Tkachev, S.; Baidina, I.;
Romanenko, G.; Gerasko, O.; Korenev, S. Polynuclear Hydroxido-
Bridged Complexes of Platinum(IV) with Terminal Nitrato Ligands.
Inorg. Chem. 2015, 54 (10), 4644−4651.
(15) Wickleder, M. S.; Gerlach, F.; Gagelmann, S.; Bruns, J.; Fenske,
M.; Al-Shamery, K. Thermolabile Noble Metal Precursors: (NO)-
[Au(NO3)4], (NO)2[Pd(NO3)4], and (NO)2[Pt(NO3)6)]. Angew.
Chem., Int. Ed. 2012, 51 (9), 2199−2203.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.9b00370.
Additional crystallographic data, XRD data, NMR, IR,
XPS, and MS spectra, additional micrographs, and DFT
computed UV−vis spectra (PDF)
Accession Codes
CCDC 1891135, 1891136, 1891138, and 1891139 contain the
supplementary crystallographic data for this paper. These data
can be obtained free of charge via www.ccdc.cam.ac.uk/
data_request/cif, or by emailing data_request@ccdc.cam.ac.
uk, or by contacting The Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44
1223 336033.
AUTHOR INFORMATION
Corresponding Author
*E-mail: vasilchenko@niic.nsc.ru.
■
ORCID
Danila Vasilchenko: 0000-0002-4233-2105
Semen Berdyugin: 0000-0003-3560-6873
Notes
The authors declare no competing financial interest.
(16) Vasilchenko, D.; Tkachev, S.; Baidina, I.; Romanenko, G.;
Korenev, S. Isolation of Homoleptic Platinum Oxyanionic Complexes
with Doubly Protonated Diazacrown Cation. J. Mol. Struct. 2017,
1130, 855−859.
(17) Venediktov, A. B.; Korenev, S. V.; Vasil’chenko, D. B.;
Zadesenets, A. V.; Filatov, E. Y.; Mamonov, S. N.; Ivanova, L. V.;
Prudnikova, N. G.; Semitut, E. Y. On Preparation of Platinum(IV)
Nitrate Solutions from Hexahydroxoplatinates(IV). Russ. J. Appl.
Chem. 2012, 85 (7), 995−1002.
(18) Shipsey, K.; Werner, E. A. CXXXII.The Purification of
Acetone by Means of Sodium Iodide. J. Chem. Soc., Trans. 1913, 103
(c), 1255−1257.
(19) Coelho, A. A. TOPAS Academic, Version 6; Coelho Software:
Brisbane, Australia, 2007.
(20) Krumm, S. An Interactive Windows Program for Profile Fitting
and Size/Strain Analysis. Mater. Sci. Forum 1996, 228−231, 183−190.
(21) Zhang, F.; Wang, P.; Koberstein, J.; Khalid, S.; Chan, S. W.
Cerium Oxidation State in Ceria Nanoparticles Studied with X-Ray
Photoelectron Spectroscopy and Absorption near Edge Spectroscopy.
Surf. Sci. 2004, 563 (1−3), 74−82.
ACKNOWLEDGMENTS
■
This work was supported by the Russian Science Foundation
(RSF grant 18-73-00054). The authors deeply acknowledge
Dr. Alfiya Tsygankova for the help with ICP analysis and Dr.
Dmitriy Sheven’ for the assistance with ESI-MS.
REFERENCES
■
(1) Goldstein, M. Carbon Monoxide Poisoning. J. Emerg. Nurs.
2008, 34 (6), 538−542.
(2) Royer, S.; Duprez, D. Catalytic Oxidation of Carbon Monoxide
over Transition Metal Oxides. ChemCatChem 2011, 3 (1), 24−65.
(3) Chen, J.; Wanyan, Y.; Zeng, J.; Fang, H.; Li, Z.; Dong, Y.; Qin,
R.; Wu, C.; Liu, D.; Wang, M.; et al. Surface Engineering Protocol To
Obtain an Atomically Dispersed Pt/CeO2 Catalyst with High Activity
and Stability for CO Oxidation. ACS Sustainable Chem. Eng. 2018, 6,
14054.
(4) Nie, L.; Mei, D.; Xiong, H.; Peng, B.; Ren, Z.; Hernandez, X. I.
P.; De LaRiva, A.; Wang, M.; Engelhard, M. H.; Kovarik, L.; et al.
K
DOI: 10.1021/acs.inorgchem.9b00370
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(22) Gulyaev, R. V.; Stadnichenko, A. I.; Slavinskaya, E. M.; Ivanova,
A. S.; Koscheev, S. V.; Boronin, A. I. In Situ Preparation and
Investigation of Pd/CeO2 Catalysts for the Low-Temperature
Oxidation of CO. Appl. Catal., A 2012, 439−440, 41−50.
(23) Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, J. F.;
Muilenberg, G. E. Handbook of XPS; PerkinElmer: Waltham, MA,
1979.
(24) Svintsitskiy, D. A.; Kibis, L. S.; Stadnichenko, A. I.; Koscheev, S.
V.; Zaikovskii, V. I.; Boronin, A. I. Highly Oxidized Platinum
Nanoparticles Prepared through Radio-Frequency Sputtering: Ther-
mal Stability and Reaction Probability Towards CO. ChemPhysChem
2015, 16 (15), 3318−3324.
(25) Grayfer, E. D.; Kibis, L. S.; Stadnichenko, A. I.; Vilkov, O. Y.;
Boronin, A. I.; Slavinskaya, E. M.; Stonkus, O. A.; Fedorov, V. E.
Ultradisperse Pt Nanoparticles Anchored on Defect Sites in Oxygen-
Free Few-Layer Graphene and Their Catalytic Properties in CO
Oxidation. Carbon 2015, 89, 290−299.
(26) Sheldrick, G. M. A Short History of SHELX. Acta Crystallogr.,
Sect. A: Found. Crystallogr. 2008, 64 (1), 112−122.
(27) SADABS; Bruker AXS Inc.: Madison, WI, 2001.
Deng, L.; Dickson, R. M.; Dieterich, J. M.; Ellis, D. E.; van Faassen,
M.; Yakovlev, A. L.; et al. ADF2017; SCM, Theoretical Chemistry;
Vrije Universiteit: Amsterdam, The Netherlands, 2017.
(43) Pye, C. C.; Ziegler, T. An Implementation of the Conductor-
like Screening Model of Solvation within the Amsterdam Density
Functional Package. Theor. Chem. Acc. 1999, 101 (6), 396−408.
(44) Jamorski, C.; Casida, M. E.; Salahub, D. R. Dynamic
Polarizabilities and Excitation Spectra from a Molecular Implementa-
tion of Time-dependent Density-functional Response Theory: N2 as a
Case Study. J. Chem. Phys. 1996, 104 (13), 5134−5147.
(45) Bauernschmitt, R.; Ahlrichs, R. Treatment of Electronic
Excitations within the Adiabatic Approximation of Time Dependent
Density Functional Theory. Chem. Phys. Lett. 1996, 256 (4−5), 454−
464.
(46) van Gisbergen, S. J. A.; Kootstra, F.; Schipper, P. R. T.;
Gritsenko, O. V.; Snijders, J. G.; Baerends, E. J. Density-Functional-
Theory Response-Property Calculations with Accurate Exchange-
Correlation Potentials. Phys. Rev. A: At., Mol., Opt. Phys. 1998, 57 (4),
2556−2571.
(47) van Gisbergen, S. J. A.; Snijders, J. G.; Baerends, E. J.
Implementation of Time-Dependent Density Functional Response
Equations. Comput. Phys. Commun. 1999, 118 (2−3), 119−138.
(48) Van Lenthe, E.; Baerends, E. J. Optimized Slater-Type Basis
Sets for the Elements 1−118. J. Comput. Chem. 2003, 24 (9), 1142−
1156.
(28) Spackman, M. A.; Jayatilaka, D. Hirshfeld Surface Analysis.
CrystEngComm 2009, 11 (1), 19−32.
(29) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Petersson,
G. A.; Nakatsuji, H.; Li, X.; Caricato, M.; Marenich, A.; Bloino, J.;
Janesko, B. G.; Gomperts, R.; Mennucci, B.; Hratchian, H. P.; Ort, J.
V.; Fox, D. J.; et al. Gaussian 09, Revision C; Gaussian, Inc.:
Wallingford, CT, 2016.
(49) Slavinskaya, E. M.; Stonkus, O. A.; Gulyaev, R. V.; Ivanova, A.
S.; Zaikovskii, V. I.; Kuznetsov, P. A.; Boronin, A. I. Structural and
Chemical States of Palladium in Pd/Al2O3 Catalysts Under Self-
Sustained Oscillations in Reaction of CO Oxidation. Appl. Catal., A
2011, 401 (1−2), 83−97.
(50) Vasilchenko, D. B.; Tkachev, S. V.; Tsipis, A. C. Aquanitrato
Complexes of Palladium, Rhodium, and Platinum: A Comparative
15N NMR and DFT Study. Eur. J. Inorg. Chem. 2018, 2018 (5), 627−
639.
(30) Vosko, S. H.; Wilk, L.; Nusair, M. Accurate Spin-Dependent
Electron Liquid Correlation Energies for Local Spin Density
Calculations: A Critical Analysis. Can. J. Phys. 1980, 58 (8), 1200−
1211.
(31) Lee, C.; Yang, W.; Parr, R. G. Development of the Colle-
Salvetti Correlation-Energy Formula into a Functional of the Electron
Density. Phys. Rev. B: Condens. Matter Mater. Phys. 1988, 37 (2),
785−789.
(32) Becke, A. D. Density-Functional Thermochemistry. III. The
Role of Exact Exchange. J. Chem. Phys. 1993, 98 (7), 5648−5652.
(33) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J.
Ab Initio Calculation of Vibrational Absorption and Circular
Dichroism Spectra Using Density Functional Force Fields. J. Phys.
Chem. 1994, 98 (45), 11623−11627.
(34) Hehre, W. J.; Stewart, R. F.; Pople, J. A. Self-Consistent
Molecular-Orbital Methods. I. Use of Gaussian Expansions of Slater-
Type Atomic Orbitals. J. Chem. Phys. 1969, 51 (6), 2657−2664.
(35) Collins, J. B.; von R. Schleyer, P.; Binkley, J. S.; Pople, J. A. Self-
consistent Molecular Orbital Methods. XVII. Geometries and Binding
Energies of Second-row Molecules. A Comparison of Three Basis
Sets. J. Chem. Phys. 1976, 64 (12), 5142−5151.
(36) Hay, P. J.; Wadt, W. R. Ab Initio Effective Core Potentials for
Molecular Calculations. Potentials for the Transition Metal Atoms Sc
to Hg. J. Chem. Phys. 1985, 82 (1), 270−283.
(37) Wadt, W. R.; Hay, P. J. Ab Initio Effective Core Potentials for
Molecular Calculations. Potentials for Main Group Elements Na to Bi.
J. Chem. Phys. 1985, 82 (1), 284−298.
(51) Ho, Y. S.; Mc Kay, G. Pseudo-Second Order Model for
Sorption Processes. Process Biochem. 1999, 34 (5), 451−465.
(52) Zhang, B.; Ma, Z.; Yang, F.; Liu, Y.; Guo, M. Adsorption
2‑
Properties of Ion Recognition Rice Straw Lignin on PdCl4
:
Equilibrium, Kinetics and Mechanism. Colloids Surf., A 2017, 514
(947), 260−268.
(53) Mohan, D.; Singh, K. P.; Singh, V. K. Trivalent Chromium
Removal from Wastewater Using Low Cost Activated Carbon
Derived from Agricultural Waste Material and Activated Carbon
Fabric Cloth. J. Hazard. Mater. 2006, 135 (1−3), 280−295.
(54) He, Z. W.; He, L. H.; Yang, J.; Lu, Q. F. Removal and Recovery
of Au(III) from Aqueous Solution Using a Low-Cost Lignin-Based
Biosorbent. Ind. Eng. Chem. Res. 2013, 52 (11), 4103−4108.
(55) Seo, J.; Lee, J. W.; Moon, J.; Sigmund, W.; Paik, U. Role of the
Surface Chemistry of Ceria Surfaces on Silicate Adsorption. ACS Appl.
Mater. Interfaces 2014, 6 (10), 7388−7394.
(56) Vratny, F.; Kern, S.; Gugliotta, F. The Thermal Decomposition
of Cerium (III) Nitrate Hydrate. J. Inorg. Nucl. Chem. 1961, 17 (3−
4), 281−285.
(57) Stadnichenko, A. I.; Muravev, V. V.; Koscheev, S. V.;
Zaikovskii, V. I.; Aleksandrov, H. A.; Neyman, K. M.; Boronin, A. I.
Study of Active Surface Centers of Pt/CeO2 Catalysts Prepared Using
Radio-Frequency Plasma Sputtering Technique. Surf. Sci. 2019, 679,
273−283.
(38) Hay, P. J.; Wadt, W. R. Ab Initio Effective Core Potentials for
Molecular Calculations. Potentials for K to Au Including the
Outermost Core Orbitals. J. Chem. Phys. 1985, 82 (1), 299−310.
(39) Tomasi, J.; Mennucci, B.; Cammi, R. Quantum Mechanical
Continuum Solvation Models. Chem. Rev. 2005, 105 (8), 2999−3094.
(40) te Velde, G.; Bickelhaupt, F. M.; Baerends, E. J.; Guerra, C. F.;
van Gisbergen, S. J. A.; Snijders, J. G.; Ziegler, T. Chemistry with
ADF. J. Comput. Chem. 2001, 22 (9), 931−967.
(58) Montini, T.; Melchionna, M.; Monai, M.; Fornasiero, P.
Fundamentals and Catalytic Applications of CeO2-Based Materials.
Chem. Rev. 2016, 116 (10), 5987−6041.
(59) Cao, F.; Zhang, S.; Gao, W.; Cao, T.; Qu, Y. Facile Synthesis of
Highly-Dispersed Pt/CeO2 by a Spontaneous Surface Redox
Chemical Reaction for CO Oxidation. Catal. Sci. Technol. 2018, 8
(13), 3233−3237.
(41) Fonseca Guerra, C.; Snijders, J. G.; te Velde, G.; Baerends, E. J.
Towards an Order-N DFT Method. Theor. Chem. Acc. 1998, 99 (6),
391−403.
(60) Ke, J.; Zhu, W.; Jiang, Y.; Si, R.; Wang, Y. J.; Li, S. C.; Jin, C.;
Liu, H.; Song, W. G.; Yan, C. H.; et al. Strong Local Coordination
Structure Effects on Subnanometer PtOx Clusters over CeO2
(42) Baerends, E. J.; Ziegler, T.; Atkins, A. J.; Autschbach, J.;
́
Bashford, D.; Baseggio, O.; Berces, A.; Bickelhaupt, F. M.; Bo, C.;
Boerritger, P. M.; Cavallo, L.; Daul, C.; Chong, D. P.; Chulhai, D. V;
L
DOI: 10.1021/acs.inorgchem.9b00370
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Nanowires Probed by Low-Temperature CO Oxidation. ACS Catal.
2015, 5 (9), 5164−5173.
(61) Kutukov, P.; Rumyantseva, M.; Krivetskiy, V.; Filatova, D.;
Batuk, M.; Hadermann, J.; Khmelevsky, N.; Aksenenko, A.; Gaskov,
A. Influence of Mono- and Bimetallic PtOx, PdOx, PtPdOx Clusters on
CO Sensing by SnO2 Based Gas Sensors. Nanomaterials 2018, 8 (11),
917.
(62) Flytzani-Stephanopoulos, M.; Gates, B. C. Atomically
Dispersed Supported Metal Catalysts. Annu. Rev. Chem. Biomol. Eng.
2012, 3, 545−574.
(63) Wang, Q.; Wang, K.; Zhang, L.; Wang, H.; Wang, W.
Photocatalytic Reduction of CO2 to Methane over PtOx-Loaded
Ultrathin Bi2WO6 Nanosheets. Appl. Surf. Sci. 2019, 470, 832−839.
́
́
́
́
(64) Talas, E.; Paszti, Z.; Korecz, L.; Domjan, A.; Nemeth, P.;
́
́
́
Szíjjarto, G. P.; Mihaly, J.; Tompos, A. PtOx -SnOx -TiO2 Catalyst
System for Methanol Photocatalytic Reforming: Influence of
Cocatalysts on the Hydrogen Production. Catal. Today 2018, 306,
71−80.
M
DOI: 10.1021/acs.inorgchem.9b00370
Inorg. Chem. XXXX, XXX, XXX−XXX