Excited-State Chemistry: Photocatalytic Methanol Oxidation by Uranyl?Zeolite through Oxygen-Centered Radicals

Article abstract of DOI:10.1021/acs.inorgchem.0c00388

We have elucidated the complex reaction network of partial methanol oxidation, H₃COH + O₂ → H₂CO + H₂O₂, at a visible-light-activated actinide photocatalyst. The reaction inertness of C - H bonds and O-O diradicals at ambient conditions is overcome through catalysis by photoexcited uranyl units (*UO₂²⁺) anchored on a mesoporous silicate. The electronic ground- and excited-state energy hypersurfaces are investigated with quasirelativistic density-functional and ab initio correlated wave function approaches. Our study suggests that the molecular cluster can react on the excited energy surface due to the longevity of excited uranyl, typical for f-element compounds. The theoretically predicted energy profiles, chemical intermediates, related radicals, and product species are consistent with various experimental findings. The uranyl excitation opens various reaction pathways for the oxidation of volatile organic compounds (VOCs) by hole-driven hydrogen transfer (HDHT) through several exothermic steps over low activation barriers toward environmentally clean or chemically interesting products. Quantum-chemical modeling reveals the high efficiency of the uranyl photocatalysis and directs the way to further understanding and improvement of VOC degradation, chemical synthesis, and biologic photochemical interactions between uranyl and the environment.

Full text of DOI:10.1021/acs.inorgchem.0c00388

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Article
Excited-State Chemistry: Photocatalytic Methanol Oxidation by
Uranyl@Zeolite through Oxygen-Centered Radicals
Yong Li,* Guoqing Zhang, W. H. Eugen Schwarz,* and Jun Li*
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sı Supporting Information
ABSTRACT: We have elucidated the complex reaction network of
partial methanol oxidation, H COH + O → H CO + H O , at a
3
2
2
2
2
visible-light-activated actinide photocatalyst. The reaction inertness of
CH bonds and OO diradicals at ambient conditions is overcome
2
+
through catalysis by photoexcited uranyl units (*UO2 ) anchored on a
mesoporous silicate. The electronic ground- and excited-state energy
hypersurfaces are investigated with quasirelativistic density-functional
and ab initio correlated wave function approaches. Our study suggests
that the molecular cluster can react on the excited energy surface due to the longevity of excited uranyl, typical for f-element
compounds. The theoretically predicted energy profiles, chemical intermediates, related radicals, and product species are consistent
with various experimental findings. The uranyl excitation opens various reaction pathways for the oxidation of volatile organic
compounds (VOCs) by “hole-driven hydrogen transfer” (HDHT) through several exothermic steps over low activation barriers
toward environmentally clean or chemically interesting products. Quantum-chemical modeling reveals the high efficiency of the
uranyl photocatalysis and directs the way to further understanding and improvement of VOC degradation, chemical synthesis, and
biologic photochemical interactions between uranyl and the environment.
INTRODUCTION
environment, thereby influencing the postprocessing of spent
nuclear fuel.
Recently the activation and reactivity of the axial uranyl
■
8
Uranium is a Janus-faced element. It has been in daily use since
antiquity for ceramic glazes and fluorescing glassware. In 1789,
it was recognized as an element and became famous for its
unique rich chemistry and later for its nuclear physics. To the
present public, uranium is known as the material for nuclear
power and for radioactive pollution by the uranium technology
being environmentally problematic, though CO −NO -lean in
energy production. Uranium has an intermediate occurrence
on earth, and so-called depleted 2 U is comparatively cheap as
a hardly used byproduct when recycling spent nuclear fuel.
Due to its slow decay with a half-life of several 10 years, it is
only weakly radioactive (e.g., emitting less α radiation than
natural uranium), but known in biology as one among the
many chemically toxic heavy transition elements.
oxygen atoms (O ) have attracted increasing atten-
ax
1
,3,10,20−25
tion.
Uranyl compounds, in particular in the long-
living electronically excited state, can activate C−H and O−H
bonds, which is of significance to C−C and C−O cleavages,
26
too. Excited uranyl species are also known to photocatalyze
2
x
14,18
the bond breaking in the DNA double-helix.
38
Photoexcitation is the most important method of activating
2
the axial oxygen atoms of uranyl. (In this paper, electronic
9
photoexcitation is designated by an asterisk *; a radicalic
electron or hole by a dot • or circle ° , respectively; and, if
necessary, physical adsorption on a carrier surface by a
chemical formula.) Resulting from the ligand-to-metal charge
1
,2
2
+
One of the most common forms of uranium, both in the
natural environment and in man-made materials, is the uranyl
transfer (LMCT) in the dative [|O⟩U⟨O|] bonds upon
•
2+
photoexcitation, the *[|O⟩ U⟨O°|] species features a
+
6
−2 2+
cation [U O ] . (Throughout this work we write ±n for
•
+5
2
formally pentavalent U center with an additional electron in
the 5f shell, and a monovalent O
Compared with excited s-, p-, and d-
block metal oxides, the excited state of *[O UO°] with
formal chemical oxidation states, and δ± for more realistic
physical or effective charges.) As the functional unit in a
multitude of compounds, uranyl is chemically notoriously
unreactive, with a rigid linear covalent structure of the axial
○
−1
radicalic ligand with an
ax
1,12,27−31
electron−hole.
•
2+
2
+
−2
6
[
|OUO|] group, formed by closed O (2p ) dative
+
6
0
Received: February 7, 2020
shells and well overlapping with the empty U (5f/6d/7s)
3
−7
hybrid shells.
The electron-pair acceptor activity of uranyl
in the equatorial plane is well-known as being decisive for its
coordination chemistry. Determining the interaction modes
between uranyl ions and organic molecules or biochemical
9
−19
systems,
it determines the uranium mobility in the natural
©
XXXX American Chemical Society
https://dx.doi.org/10.1021/acs.inorgchem.0c00388
A
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Article
separated electron and hole is relatively long-lived (at the scale
uranyl derivatives. Therefore, we have chosen the partial
oxidation of methanol to active intermediate products
formaldehyde and hydrogen peroxide by uranyl-photocatalysis,
summarized as
of μs to ms) and slowly luminescing, owing to the significantly
1
weak interaction between the compact U(5f ) electronic
2
+
orbital and the hole in the oxygen shell. Excited *[UO2] is
strongly oxidizing with a high redox potential (E° ∼ +2.6
+
hv
1
,13
V).
H COH + O ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯ →⎯ H CO + H O
3
2
2
2
2
(at uranyl‐silicate)
The electronic excitation can be quenched by various
•
+O
2
organic molecules through hydrogen-atom ( H) transfer to the
(
⎯⎯ ⎯→ CO + 2H O)
(1−4)
2
2
2
+
*
[UO2 ]• unit, saturating the °O hole, thereby leaving
ax
2+
[
|O⟩ U⟨OH] and yielding some organic radicals.
The individual reaction steps of eqs 1−4 are discussed in
Scheme 1 below. Methanol is both a VOC, by which the
biosphere pollutes the environment, as well as a basic industrial
chemical. Formaldehyde and hydrogen peroxide may be either
chemically useful or environmentally harmful but can be easily
changed to the “neutral” products carbon-dioxide and water, if
desirable.
Mesostructured large-surface silicates are ubiquitous in the
environment and are common supports in heterogeneous
catalysis. We have chosen a zeolitic model to anchor the uranyl
unit. Zeolitic materials act as molecular sieves and can steer the
reaction selectivity of organic mixtures. We note that aromatic
compounds and nucleic acids, too, can be modified through
uranyl-catalysis by C−C benzene ring-opening and by C−O
ribose (DNA and RNA) photocleavage. Our proposed
oxidation mechanism will give new insight and understanding
of the photo-physico-chemical interaction phenomena between
uranyl and its environment. In the present work, we have
investigated and determined various mechanistic pathways of
the methanol oxidation applying state-of-the-art quantum-
chemical approaches to uranyl-catalyzed model reactions at the
levels of the “density-functional approximation” (DFA) and of
the ab initio correlated “wave-function theory” (WFT).
This process may be baptized “hole-driven hydrogen transfer”
(
HDHT), the new relative of common “proton-coupled
1
1−13,27,28,32−39
electron transfer” (PCET).
The HDHT is also
called “light-driven oxyl formation causing H-abstraction and
2
4
•
+5
substrate oxidation”. In aqueous solution, two U species
can spontaneously disproportionate to a pair of ••U and
The reduced [OU O] unit can also be directly
oxidized to [OU O] in the presence of oxygen, competing
+4
+
6
35,37,39
+5
1+
U .
+
6
2+
2
7,28,36,38
with the disproportionation.
Concerning O -involving
2
reactions, dioxygen peroxide (O2− ) and superoxide ( O
2
•
−1
)
2
anions and their conjugate acids have been suggested as
27,28,36,38
intermediates in experimental works.
Uranyl cations strongly bind to defect-sites of oxidic support
materials, which restrains disproportionation and mobility,
3
5
thereby also suppressing the toxicity. Uranyl-anchored
silicate sieves have been proven as true visible-light-activated
1
,31,40−42
photocatalysts,
which could efficiently catalyze photo-
oxidation reactions of various organic compounds under
ambient conditions (Figure 1). In experimental photo-
THEORETICAL STRATEGY AND OVERVIEW OF THE
REACTION MECHANISM
■
The density-functional approach has been widely used to
explore the actinide chemistry, owing to low computational
cost and acceptable accuracy in geometric and electronic
structure determinations (concerning uranyl as a typical
48−52
example, see Table S1).
Therefore, we here investigate
the photocatalytic mechanisms of methanol oxidation by O at
2
a uranyl-anchored silicate model (Figure 2). We apply DFA
and TDDFT (time-dependent density-functional theory) at
the scalar-relativistic and spin−orbit coupled levels, in order to
elucidate possible photocatalytic excited-state pathways (Scheme
1
). The geometry of the silicate structure was optimized by a
Figure 1. Uranyl on microporous silicate (colors: gray, Si; red, O;
blue, U) catalyzing the photo-oxidation of volatile organic compounds
quantum+classical mechanical hybrid method (QM/MM).
The energies of the main structures were then accurately
redetermined by a correlated wave function approach (CCSD-
(
VOCs).
(
T)). Details of the applied theoretical strategy and computa-
oxidation experiments of “volatile organic compounds”
VOCs), uranyl doped silicates proved significantly more
efficient than the more common TiO catalysts, in particular in
tional methods including references are described in the
Supporting Information (SI) in Section S1, “Computational
Methodology”.
The whole catalytic cycle, as emerging from our inves-
tigations below, consists of four steps (Scheme 1 and Figure
3): eq 1 represents the photoexcitation and formal photo-
reduction of uranyl(VI) by decoupling a dative U ⇐O
electron pair, yielding formal
radical” °Oax . The respective MO diagram is shown in
Figure 4 (see also Figure S1). Equation 2 describes the
hydrogen activation and transfer from the organic molecule
(
2
40,43−47
the visible region.
Natural biological air pollutants
dominantly comprise hydrocarbons (methane, isoprene,
terpenes), while halogen derivatives and oxygen compounds
+
6
−2
(
such as alcohols, aldehydes, ketones, esters) are among the
ax
•
+5
dominating anthropogenic pollutants, all of which can be
catalytically eliminated with the help of *[UO2]2 from the
environment, or transformed into chemically interesting
intermediate products.
U
and “oxygen-centered
+
−1
−
1
So far, there is little knowledge of excited-state reactions of
actinide compounds, such as the photocatalytic oxidation by
HROH to the uranylic °Oax centered radical, and the
formation of an organic oxyradical HRO , or alternatively an
•
B
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a
Scheme 1. Cyclic Uranyl-Catalytic Photo-Oxidation of H−R−OH with O
2
a
Step 1: reductive photo-excitation of uranyl(VI). Step 2: by a hydrogen transfer, HROH yields an organic radical and hydro-uranyl(V). Bifurcation
to steps 3′ and 4′: at first O oxidizes the hydro-uranyl(V) with back-formation of uranyl(VI); then, H O and RO are formed. Or to steps 3″ and
2
2
2
4
″: at first, O reacts with the organic radical forming RO; then, the hydro-uranyl(V) induces the formation of H O and back-formation of
2
2
2
uranyl(VI).
•
60
hydroperoxyl radical HO is formed, with spin-recoupling
2
•
through the HRO radical, in the vicinity of the heavy U atom.
In the continuing eq 4 the two radicals form the final reaction
products RO and H O .
2
2
On the alternative path of eqs 3″ and 4″, at first an H atom
•
is transferred from the HRO radical to the O , forming
2
product RO and intermediate radical HO2• with spin-coupling
through the heavy U atom. Finally, the uranyl is regenerated,
and hydrogen peroxide H O is formed. In summary, the
2
2
photoreaction induced by light absorption of uranyl effectively
produces radicals which react with each other or with other
species. Eventually, the organic and the hydroperoxyl radicals
are eliminated by forming a carbonyl RO and a hydrogen
peroxide H O molecule.
2
2
Our nomenclature i through vi for the reacting and
intermediate species is displayed in Figure 4 (see also Figure
3
). “i” denotes the silicate-adsorbed uranyl; “ii” the equatorial
uranyl coordination of a methanol molecule; “iii” a hydrogen
atom (either the alcoholic H , or the methylic H )
O
Me
•
transferred from H COH to the radicalic O of uranyl (see
3
ax
reaction step 2 in Scheme 1, and Figure 6); “iv” a dioxygen
molecule being added; “v” a hydrogen atom transferred to the•
dioxygen; and “vi” a second H atom transferred to HO2
yielding HOOH. The respective transition states are denoted
by “ii−iii”, “iv−v”, and so on. At each step, there are several
different options, at ground and electronically excited levels,
with singlet (S), triplet (T), or quintet/“pentet” (P) spin-
coupling, with different order of steps and different H atom
transfers. For further specification we use subscripts G and G1
for the electronic ground states of two isomers, and E and E1
for their electronic excited states.
The elucidated reaction mechanism, whereby uranyl groups,
being nonexcited or excited by one or two photons, convert
one molecule of an alcohol and O into a carbonyl and H O ,
is described below in Figure 12 and the last two sections, the
details being presented in the following extended section.
2
2
2
Figure 2. Geometric structure model, as optimized by a QM/MM
2
+
hybrid procedure: Equatorially pentacoordinated [UO2] on a
hydroxylated SiO layer, bonding also one H COH and two H O
2
3
2
molecules. (a) Top view (note: overlaying OH groups look like
HOH). (b) Side viewthe ovals indicate the quantum mechanically
DETAILED RESULTS AND DISCUSSION
■
(
QM) treated section (enlarged in Figure 5).
Uranyl Photoexcitation (Step 1 in Figure 3). We begin
the discussion of the catalytic cycle with the electronic
excitation of the coordinated uranyl from state ii to ii (see
•
alkyl radical ROH. Either the hydro-uranyl or one of the
organic radicals then reacts next. Equation 3′ represents a
complicated reaction mechanism, where the hydrogen is
transferred from the hydro-uranyl to the diradicalic dioxygen
G
E
Figure 7), assuming that the uranyl is permanently ligated in its
2
equatorial plane to the silica surface (in η fashion) and to two
water ligands. At this stage, the VOC species methanol is
coordinated in addition. The uranyl unit [OUO] remains
2
+
molecule O , whereby uranyl(VI) is regenerated, and a
2
C
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2
+
Figure 3. Schematic uranyl-catalytic cycle of alcohol (HROH) photo-oxidation by O . (a, left) Step 1: uranyl photoexcitation, |OUO|
→
2
•
•
2+
•
•
2+
|
O U O| . Step 2: formation of organic and hydro-uranyl radicals, RO + |O UO−H| . Steps 3 and the following ones: in a complicated
•
net of reactions (see part b, right), products RO and H O are formed through intermediate hydroperoxyl radical (HO ), and uranyl restitution
2
2
2
(
see Scheme 1)The different species are numbered through as i, ii, etc. (nomenclature explained in Figure 4). Subscripts G and E mean ground
and electronically excited states; S , S , T , and T mean lowest singlet (S ), higher singlet (S ), lowest triplet (T ), and higher triplet (T ) states.
(
0
1
0
1
0
1
0
1
•
•
•
b, right) Net of reactions from step 2 through step 3, similar for both cases of ROH with C or O centered radicalic electron, CH OH or CH O .
2
3
2
−2
+6
−2 2+
nearly inversion-symmetric and is only weakly symmetry-
broken by the fields of the five equatorial oxygen ligands
O− ions are partially photo-oxidized, [ |OU O| ]
+
−
2
•
+5
•
−1 2+
−1
•
•
+5
−2 2+
hv → [ |O U  O| ] ↔ [ |O  U O| ] . The
(
Figures 2 and 6). The highest occupied MOs (Figure 5,
UO bonds are weakened and expand by another ca. 0.06 Å
2
+
Figures S1 and S2) are of O (2pσ ) and O (2pπ ) type,
to 1.86 Å (Figure 7, ii ). The excited *[OUO] species has
ax
g,u
ax
g,u
E
•
•
coordinatively stabilized by admixtures from the uranium
uranium oxygen diradicalic character, where the O-centered
valence shell U(5fσ,π , 6dσ,π ). The antibonding virtual orbital
•
−1
u
g
radical O ca1n efficiently activate bonded hydrogens in the
complements at higher energies are of dominant U(5fσπu,
dσπ ) character with some oxygen admixture. In-between are
neighborhood.
6
g
Excited Uranyl Breaks an O−H or C−H Bond (Step 2
in Figure 3). There is vast empirical evidence, and it is
theoretically expected, that photoexcited diradicalic uranyl ions
attract hydrogen atoms.
the reaction cycle is the H-transfer process from methanol to
the lowest virtual nonbonding MOs of U(5fφ,δ ) type, where
u
the 5fφ ones are a bit higher (Figure S1) due to interaction
u
with the equatorial silicatic oxo-ligands (Figure S2). While free
11−13,24,27,28,32−39
Therefore, stage 2 of
2
+
54
[
OUO] has U−O distances of 1.72 Å, equatorial ligation
expands the U−O bonds by ca. 0.08 Å, namely, to 1.80(1) Å
•
•
2+
*
[O − U− O ] . Owing to the well-documented equatorial
ax
ax
(
Figure 7, ii ).
G
oxophilicity of uranyl, the VOC-molecule methanol is bonded
with its oxygen in the equatorial plane of uranyl, and either its
hydroxylic (H ) or its methylic hydrogen (H ) can be
The electronic absorption spectrum of the uranyl@zeolite
system (Figure S3) with maxima in the wavelength region
O
Me
below 500 nm indicates activity under UV−vis sunlight
40
transferred, as shown in Figures 6 and 7.
The energies of the reaction paths for H transfer are shown
irradiation. A comparison of our results with experimental
O
and theoretical literature data (Figure S3 and Table S1) verifies
the reliability of the applied DF approximation and also
documents the relevance of spin−orbit coupling. The lowest-
energy photoexcitations correspond to electronic transitions
in Figure 8. Since the inversion symmetry of the H-uranyl unit
becomes strongly broken, we omit the g, u symmetry indices
from now on. With increasing photoexcitation energy, the
reaction barriers happen to become lower. Further, the
vibrational energy in the lower electronically excited states,
populated by intersystem crossing from the higher ones,
from the highest bonding orbital of dominant O (2p)
ax
character to the lowest empty MOs of U(5f) character,
1
+
3,1
O (2pσπ ) → U(5fδφ ). They are of Σ
→
Δ → Π
ax
u
u
g
g
provides additional energy to overcome the barriers of the iiE
“
symmetry-forbidden” type, becoming “allowed” by U(5f)
to iii H-transfer reactions. The ground-state reaction from iiG
to iii with a higher barrier is known to occur only at elevated
temperatures (i.e., above 400 K).
Different hydrogen transfers from methanol to uranyl are
displayed in Scheme 2. Equation 6 represents the H transfer
spin−orbit coupling and by coordinative and vibrational
geometry-deformations. The lower excited levels become also
populated from the higher UV-excited levels by intersystem
coupling. They have some finite lifetime and cause the
common uranyl luminescence.
E
G
40
O
The formal charge and oxidation state of U is photoreduced
from U (5f ) to U (5f ), while the originally closed-shell
to ground-state uranyl. Equations 2′ and 5 represent H and
O
Me
+
6
0
•
+5
1
H transfers to photoexcited uranyl. Equation 6 yields ground-
D
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Figure 5. Qualitative orbital energy level scheme of uranyl (middle),
on the left the U(5f6d) valence shells, and on the right the two
O (2p) valence shells. Despite (weak) perturbation by the equatorial
ax
ligands, we use unperturbed D nomenclature (5f σ , π , δ , φ and
∞h
u
u
u
u
53
6
d σ , π , δ ) for simplicity.
g g g
Figure 4. Nomenclature of stable and metastable species and
transition states. (a) Reaction species are numbered through from
initial i to final vi. Subscripts: G = ground state, G1 = higher energy
state of ground state structure; E and E1 = lowest and higher energy
electronically excited states. (b) Spin multiplicity: 1, 3, and 5 for
singlets (S), triplets (T), and quintets (=Pentets, P). (c) The
Figure 6. Central cluster model, corresponding to the oval sections in
2
+
Figure 2, treated quantum mechanically. The uranyl unit [O UO ]
ax
ax
is equatorially pentacoordinated, permanently to two H O molecules
2
2
2−
and in η -fashion to an [(OH) Si O ] disilicate group (derived from
4
2
3
the QM/MM optimization of the silicate slab), and to the reacting
organic species (HOCH3 = H O CH HMe). The two possible
O
Me
2
“
permanent” equatorial ligands of uranyl, namely, the two silica
hydrogen transfer pathways are indicated by bent arrows.
−
surface groups (>SiO ) and the two H O ligands, are shown neither
2
here nor in several figures below, in order to avoid overloading.
transfer on the various excited potential energy surfaces all end
up at very similar geometric structures of iiiE.
state products, while eqs 2′ and 5 at first reach some higher-
Concerning the potential energy surface crossings sketched
in Figure 9, the S −S states form a connected surface with the
energy minimum, from where internal energy conversion leads
0
1
to product ground-state (iii ), as shown in Figure 9.
conical intersection in the many-dimensional configuration
G
After the transfer of the methoxy-H to the excited uranyl
space. The T surface becomes also connected to it through
O
1
complex, the originally multiple U−O bond is expanded from
spin−orbit coupling. Accordingly, the excited system can
ax
1
.86 to 2.14 Å (Figure 7, ii to iii ), corresponding to a strong
change over to the ground state without radiation at U−O
E
E
Me
5
5−57
U−O single bond. Pyykko
̈
’s
covalent bond radii incre-
ments of U and O predict U−O triple/double/single bond
distances around 2.9 and 2.5 Å, respectively, whereby the
U(5fδ) and O(2pσ,π) electrons become paired again. The
energy surface crossings in the regions of the red circles induce
radiationless S ↔S and T ↔S transitions that can provide
distances around 1.7/1.9/2.3 Å. The spin-density plot (Figure
7
, iii ) shows unpaired electrons in U(5fδ) and O(2pπ) type
E
1
0
1
0
orbitals (the oxygen spin is distributed over the whole methoxy
unit). If a methyl-HMe is transferred, the unpaired electron
density moves to a C(2sp ) hybrid orbital with only a little
thermal energy for the next reaction steps.
2
+
Experimental EPR investigations of *UO
in liquid
12,32,34
2
3
•
•
methanol detected both H CO and CH OH radicals,
3 2
oxygen admixture (Figure 7, iii ). The reaction paths for HO
while no radicals were found upon irradiation of silicate-
E1
E
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2
+
Figure 7. Mechanistic model for the hydrogen transfer from H COH to O of [UO ] : H transfer in the bottom row for the ground state, and
3
ax
2
(O)
3
in the upper top row for the electronically photoexcited state of (O -2pσ → U-5fδ) type, H
transfer to excited uranyl in the lower top row.
ax
(Me)
Some spin-density contour envelops are shown in blue in the top-left corners of some of the rectangles. ii, ii−iii, and iii indicate the initial,
transition, and final states of the hydrogen transfer reactions (also see Figure 12 below), with G and E subscripts for the ground and excited states,
respectively. Colors of the atomic cores: blue, U; red, O; dark-gray, Si; light-gray, C; white, H; the tube-and-knob models represent the two
spectator-ligand water molecules. The numbers are the bond lengths in Å.
4
0,41,58
adsorbed uranyl alone.
However, when the electron of
H CO as products (in its ground or excited triplet state
2
•
−1
•
3
Oax is coupled with a hydrogen atom H forming an O −H
bond, a surviving electron on U was detected. Another
experimental study could not detect radicals, however, which
we here interpret as due to subsequent radiationless S ↔T
transitions.
Reactions of Radicals [UO H] and (RHO) with
Molecular Oxygen (Step 3 in Figure 3). Although the O
dioxygen molecule is a diradical, it is not particularly reactive.
This has been explained as due to strong static MO correlation,
or VB resonance, of the π π * triplet state. However, O
H
2
CO*).
Reaction 7 is the ground-state transfer of HMe from the
methyl group to O , i.e., from iv via iv −v to v (Figure 10).
ax
•
5+
35
41
2
G
G
G
G
The transition state iv −v shows a single imaginary frequency
G
G
1
1
−
1
of 1423i cm corresponding to the high activation barrier.
•
2+
•
The U−O distance increases by 0.13 Å, and the CO
Me
Me
2
•
•
distance decreases by 0.08 Å. The Mulliken atomic spin-
2
populations (Table S2) of iv −v are 0.72, 0.58, and 0.50 on
G
G
O , O , and the methyl C atom, respectively, corresponding to
a
b
4
u
2
59
•
•
0.7 e charge transfer from methanol to O , accompanying the
2
g
2
H atom transfer. The reduced CO distance of 1.24 Å in
•
2+
•
Me
can be effectively attacked by the [OU OH] , ROH, or
HRO radicals (Schemes 1 and 3, Figures 3b and 10). With
the slightly cationic methoxy group of vG indicates CO
•
double bond character, while the large equatorial U···O
2
+
Me
adsorbed dioxygen, iii (S )CH O−[OUOH] transforms to
G
0
3
distance of 2.53 Å implies a soft coordination bond.
•
•
2+
iv (T ), and iii (T ) CH O ···*[OU OH] transforms to
G
0
E
1
3
The respective reaction from excited state iv also leads to
E
iv (S , T , Q ), from where two reaction paths start. The other
cases with CH OH instead of CH O are considered below.
Reactions 7 and 8 in Scheme 3 yield ground or excited state
3
E
1
1
1
ground state v . Structure iv with O yields energetically
G
E
2
•
•
2
3
near-degenerate singlet (S ), triplet (T ), and quintet (Q)
1 1
states (Figure S6), with spin-populations of 1.02 and 0.98 on
O and O of O , and of 1.10 and 0.90 on U and O ,
•
2
•
+5
2+
uranyl, the OOH and [O U OH] radicals as intermedi-
ates, and H O and H CO in their ground states as products.
a
b
2
Me
2
2
2
respectively (see Table S2 and Figure 10, top right). While
Reactions 9 and 10 yield ground-state uranyl, the two radicals
OOH and OCH as intermediates, and again H O and
there may occur radiationless transitions iv (S , T , Q) → iv
E
1
1
G
•
•
(T ), the reaction can start from excited state iv , if the lifetime
3
2
2
0
E
F
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Figure 8. H -transfer from the hydroxyl group of H COH to one
O
3
O
2+
Oax of [O UO ] : Energy profiles (in eV; form KS-DFA and
ax
ax
TDDFT approximations) for different electronic states vs the H −
O
O distance (in Å, the initial state is on the left side). The geometries
ax
Figure 9. Cuts through the potential energy surfaces (in eV) of the
are optimized along the reaction paths, projected crossings
corresponding to somewhat different substructures. The near-
degeneracy of various excited states for small O −H distances on
2
+
[
(
(
O UO H] ← OCH complex along the R(U−OMe) coordinate
ax
ax
3
2 0
in Å) for ground state S (σ f ) and lowest excited states T and S
0
1
1
ax
O
1 1
σ f , scalar-relativistic KS-DFA, and TDDFT approximation). The
the right is indicated by an ellipse.
energy surface crossings in the regions of the red circles induce
radiationless S ↔S and T ↔S transitions that can provide activation
1
0
1
0
energy for the next reaction steps.
of iv is long, and the reaction is fast enough. A theoretically
E
approximated reaction path is shown in Figure S6. Under light
2
+
irradiation, there is a dynamic equilibrium of iv (T ) ↔ iv
uranyl. The reaction yields UO
catalytic cycle is finished.
2
and H O , whereby the
2 2
G
0
E
(
S , T , Q). Since the energy of iv is above the ground state
1
1
E
reaction barrier iv −v , the reaction should occur sponta-
On the other alternative reaction path, O
hydro-uranyl. When the H is transferred from the uranyl
oxygen to O in reaction 9 from iv via iv −vG1 to vG1 (Figure
2
first oxidizes the
G
G
neously.
The atomic spin-populations of the ground-state species v
O
G
2
G
G
•
2+
•
(
S , T ) containing the [OU OH] and O O H radicals are
10), the “upper” U−Oax length shrinks by 0.15 Å, while the
equatorial U−OMe length expands by 0.3 Å. The transition
0
0
b
a
mainly centered on U (1.13) and on O (0.78); the triplet and
b
singlet couplings of the two radicals have nearly the same
energies and populations. The imaginary frequencies of the
lower and higher barrier transition states v −vi (S ) and v −
state iv −vG1 on the triplet ground state potential surface is
G
again characterized by a large imaginary frequency of 1405i
−
1
3
cm . The original O
1.0 on both O and O
G1 the spin-populations are 0.51, 0.51, and 0.80, respectively,
unit in iv
(T ) has spin-populations of
G
G
0
G
2
G 0
−
1
−1
vi (T) are 848i cm and 1753i cm , respectively. The paths
a
b
atoms, while in the transition state iv −
G
E
of the singlet and triplet reactions 8a,b are shown in Figure S7.
v
They have nearly the same energies for O −H distances from
on O
a
, O
b
, and OMe, and in product vG1 0.83 and 0.31 on
a
O
•
•
2
.8 Å down to 1.6 Å. The singlet (and triplet) path with lower
O
b
O
a
H, and 0.74 on OCH
3
, respectively. Another possible
(
and higher) barrier ends at the ground (and excited) state of
reaction leads from excited state iv via transition state iv −v
E
E
G1
Scheme 2. Transfer of an H (Eqs 2′ and 6) or an H (Eq 5) from Methanol to Uranyl in the Ground State (Eq 6) or Excited
O
Me
State (Eqs 2′ and 5)
G
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Scheme 3. Reaction Mechanism for the Oxidation of the Methoxy Anion on the Lower-Energy Potential Surface
to vG1 (see Figure S6; reaction 9′ in Scheme 3). The reaction
The barriers of the consecutive hydrogen-atom transfers
from the organic hydroxy and methyl groups to the axial uranyl
from excited state iv to ground state v is preferred to that
E
G1
from ground sate iv , if it has reasonably long lifetimes of the
atoms and the O molecule are rather large on the ground state
G
2
excited state.
energy surface, namely, 1.2 eV (1.5 eV) and 1.4 eV (1.3 eV).
Methanol oxidation without irradiation has indeed been
observed only upon thermal activation above 200 °C. On
the lowest excited energy surface, the barriers are much
Furthermore, reactions may continue with reactions 10a or
•
2+
•
40
1
0b from vG1 (S , T ), the complex H CO [UO ] HO O ,
0
0
3
2
a
b
via v −vi (S, T) to vi (S), or via v −vi (S, T) to vi (T),
G1
G
G
G1
E1
E1
as shown in Figure 10 and Figure S8. The spin-populations of
smaller, 0.88 eV (1.1 eV) and 0.1 eV (0.2 eV), respectively. In
3
vG1 are mainly centered on OMe (0.91), and on O (0.70). On
the reaction path from vG1 to viE1 (T), the CH O is
endothermally formed in the excited triplet state, with spin-
the first step, the lowest vertical photoexcitation to Δ
g
−
b
•
2+
[O UfδO°2pσ
]
in the visible supplies some 0.3 eV (0.26
2
1
eV) relaxation energy, while the higher near-UV excited Δ
and the
g
3
,1
density and a C−O single-bond distance of 1.36 Å. The
Φ
g
(O2pσ → Ufφ) states provide more energy to
−
1
imaginary frequency at transition state v −vi is 1793i cm ,
overcome the barrier directly. The photoexcited states were
determined by applying the TDDFT approach, using B3LYP
or SAOP functionals and the spin−orbit coupled relativistic
ZORA approach as implemented in the ADF quantum-
G1
E1
−
1
while for the lower path at v −vi it is only 960i cm . The
G1
G
two reaction paths are again near-degenerate for O −H
b
Me
distances from 2.7 down to 1.5 Å, where they begin to separate
Figure S8).
65
chemical package. In Figure 8, we had also considered the
(
3
next higher Π (O → U ) state with an even lower barrier.
g
2pπ
fδ
The calculations show significant spin-density at the U atom
ENERGETIC ASPECTS OF THE REACTION CYCLE
■
upon visible and near-UV absorption (Table S2), correspond-
We discuss the quantum-chemically determined reaction and
activation free energies of the various reaction steps as
displayed in the upper part of Figure 12. The respective
chemical structures are shown in the lower part of Figure 12.
Our Kohn−Sham density-functional and ab initio coupled-
cluster energies (with standard deviations of 0.3 eV) are listed
in Figure 11. The lowest vertical and adiabatic excitation
energies of uranyl complex ii are 2.37 (2.80) and 2.07 (2.54)
eV, respectively [vertical and adiabatic B3LYP-DFA; in
parentheses vertical EOM-CCSD(T) and adiabatic CCSD-
•
+5
−1
ing to formal U and °O atoms. However, the charge
density distribution and the effective atomic partial charges
change only a little upon photoexcitation. Apparently, one of
•
the pair-bonds is broken, UO| → U °O|, basically
changing the spin-coupling and the two-electron distribution.
The unpaired electron density on the U atom is spatially rather
contracted due to the nature of 5f orbitals and does not play a
big role in the subsequent reactions of the organic ligand that is
weakly bound in the equatorial plane of the uranyl due to U-6d
overlap. However, the open-shell character of the formal
−
1
(
T)]. The experimental adiabatic value is 2.61 eV, showing a
monovalent °O atom makes excited uranyl very electron-
60
minor error of 0.1 eV for the ab initio approach, and 1/2 eV
for the DFA. Concerning the reactants, transition states,
intermediates, and products, the DFA estimates scatter by a
few 0.1 eV around the closed-shell CCSD(T) and open-shell
UCCSD(T) energies. Inasmuch as some species are of
multiconfigurational type, the CC (as well as the DFA)
calculations of single-reference type are less reliable, in some of
these cases. Therefore, we have rounded all energies (in eV) in
Figure 11 to one digit after the decimal point.
attractive, as is well-known experimentally, and eases the
transfer of a H atom that was bound before in methanol.
The states of the intermediate complex iv, H CO−
3
[OUOH]−O on silicate, form the starting point of a complex
2
reaction network (Figure 12), also including radiationless
transitions and absorptive re-excitations, and conversions
between °CH OH and CH O° (a previous study showed an
2
3
6
1
activation barrier around 0.65 eV).
However, some
experimental studies revealed no EPR signals in such
H
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Figure 10. Regeneration of [UO2]2+ and oxidation of the CH O group. Spin-density contours are shown in the upper left corners of some of the
3
boxes.
heterogeneous systems, while both °CH OH and CH O°
radicals to [OUOH], H O, and CO , along the reaction path
2 2
ii → ii → iii → iv → v → vi (Figure 12). Based on our
G E E G/E G G
2
3
40,41
signals were detected in homogeneous solution.
Obviously,
the pinning at the silicate surface conserves the complex and
theoretical model, we predict that easy coupling of unpaired
electrons and the resulting short lifetime of the radicals make
them difficult to be observed in the heterogeneous system.
Such a prediction is consistent with experimental observations.
In ref 40, it was mentioned that the catalytic oxidation reaction
•
eases the energetic stabilization of the [OU OH] CH O°/
3
°
CH OH radical pairs in singlet-coupled form. One also can
2
expect that the abundant O molecules in heterogeneous
2
•
systems easily oxidize the [OU OH], CH O°, and °CH OH
3
2
I
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Figure 11. Relative free energies (ΔF, in eV; 1 eV ≈ 96.5 kJ/mol) and Activation Barriers with respect to the previous minimum (E ), from scalar
a
relativistic B3LYP-D3 density-functional approximations (DFAs) and ab initio EOM-CCSD(T) theory (CC), both with thermal corrections from
DFA. 1) For nomenclature of the species, see Figure 4. For activation by the lowest visible-light excited state.
2)
Figure 12. Relative free energies (in eV; from scalar-relativistic density-functional approximations) of reactant, intermediate, and product species of
2
+
(
photo)catalytic methanol oxidation at UO2 @silicate. (The nondisplayed physical SCF adsorption energy of O is −0.2 eV. The hydrogen
2
2
+
bonding energy between the H O and the H CO−[UO ] is −0.62 eV and has been subtracted from v .)
2
2
2
2
iG
of CH OH occurs above 400 K without photoabsorption. This
The ubiquitous dioxygen is bonded through a hydrogen
bond of 2.0 Å length with an adsorption energy of ca. −0.2 eV.
As discussed in previous sections, there are two broad options
3
is consistent with a reaction path ii → iii → iv → v → vi
G
G
G
G
G
on the ground-state potential energy surface.
J
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for the order of hydrogen transfers and uranyl(VI)
regeneration, from the two species ivG,E to the end point of
uranyl oxygen O(II) to radicalic oxygen °O(I) with
•
simultaneous formal reduction of U(VI) to U(V). This
species vi (Figure 12). The pathways in blue start (i) from the
process has sometimes been labeled as photoinduced ligand-
to-metal charge transfer (LMCT). However, the main point is
electron-spin decoupling of a Lewis electron pair, while there is
little charge-density reorganization on the uranyl. The rather
long-lived excited *uranyl possesses two electrons in two
G
triplet iv (T ) → iv −v → vG1 (T , S ) with a large barrier
G
0
G
G1
0
0
of ∼1.48 eV (∼1.37 eV) and a significant endothermicity of
∼
+1.21 eV (+0.95 eV), which might be overcome only if
previous energy gain is available in direct connection; or (ii)
•
+5
•
from the iv state during its lifetime or after an additional red-
orbitals, one being of compact U (5f) type, and the other of
E
−
1
light absorption of the iv state, with negligible barrier. The
°Oax type with strong oxidative power of the hole in the O-
2p shell. We note the trend from very short-lived excited p-
block (main group) compounds, to medium short-lived excited
d-block (transition metal) complexes, to long-lived excited f-
block compounds (lanthanides and actinides). The f-element
compounds may play an important role in designing
photocatalysts possessing significant lifetime of electron−hole
separation.
G
2+
first result is the regeneration of [UO ] , followed by final
2
•
oxidation of H CO to H CO. The barriers of this last step at
3
2
the v −vi singlet and v −vi triplet states are, respectively,
G1
G
G1
E1
0
.3 eV (0.35 eV) and 1.25 eV, indicating the reaction
possibility for the singlet state.
The other pathways (in red in Figure 12) begin with the
•
oxidation of CH O via iv (T ) → iv −v → v (T , S ) with
3
G
0
G
G
G
0
0
a barrier of 1.4 eV (1.3 eV) and slight exothermicity of −0.1 eV
(iii) Hole-driven hydrogen transfer (HDHT) (right side of
Figure 13): The key point is that the monovalent °Oax− atom
of *uranyl attracts a single electron, that is, an atom supplying
it. To get the reaction rolling, a H gets abstracted from the
methanol molecule, either from the hydroxyl group or from the
methyl group (see Figure 11). H CO° or °CH OH then
1
(
−0.35 eV). The same argument for reaction possibility via the
photoexcited state applies as above. The final steps lead to the
2+
regeneration of [UO ] , via the v −vi singlet transition state
2
G
G
with a barrier of 0.6 eV (0.75 eV), while the v −vi triplet
G
E
transition state has a too high barrier of 1.65 eV. For both red
and blue pathways, an early transition to singlet states is
preferred.
3
2
remains as intermediate species.
(iv) The final steps in the reaction network of organic
molecule didehydrogenation and dioxygen dihydrogenation
2
+
CONCLUSIONS AND PROSPECTS
lead to [UO ] recovery, in various orders: The hydrogen of
2
■
•
2+
•
•
*
[O UOH] can be transferred to OO , which is usually
rather inactive at ambient conditions, forming reactive OOH.
Eventually, [UO ] , H O , and H CO are obtained.
In summary, under the action of light at ambient conditions,
CH OH + O at uranyl-silicate surfaces form H CO and H O
2
The reaction mechanism of oxidation of methanol by O at
2
•
ambient conditions, triggered photocatalytically by excited
uranyl, has here been elucidated in detail. The catalytic cycle
can be split up into three comparatively simple steps i−iii and a
final complex network (iv).
2
+
2
2
2
2
3
2
2
2
•
with intermediate radicalic species HOO and H CO° or
3
(
i) Composing the reacting complex: A methanol molecule
○
°
CH OH. With final products CO and H O, uranyl@zeolite
2 2 2
coordinates at a silicate-attached diaquo-uranyl unit in its
equatorial plane forming a common pentacoordinated
structure. Also, a dioxygen molecule may be adsorbed in the
neighborhood. Now, the complex requires uranyl photo-
excitation to react.
has the potential as a powerful green photocatalyst for cleaning
the air. The porous sizes of the support materials acting as a
molecular sieve can be exploited to control the selectivity for
different applications. The present elucidation of the various
pathways provides insight when searching for possible
applications that can make profitable use of nuclear waste.
Concerning the biological aspect, our studies also provide
insight into the photochemical interaction of uranyl with
biological molecules, such as uranyl triggered photocleavage of
DNA. The wide availability of the depleted uranium and the
special photocatalytic properties of uranyl compounds make it
possible to carry out visible-light photocatalysis. The present
work might provide insights for developing f-block-element
photocatalysts with far longer-lived excited states than the d-
(
ii) Uranyl photoexcitation (left side of Figure 13): The
functional uranyl complex harvests a visible or near-UV light
photon (<500 nm, Figure S3) and jumps to one of its low-
1
,3
lying excited states of Φ ,Δ (O-2pσ → U-5fφ ,δ ) or
g
g
u
u u
1
,3
Π (O-2pπ → U-5fδ ) type. Stronger absorption occurs at
the higher energies, whereby vibrational energy can be gained
for enhanced reactivity. The essence is photo-oxidation of the
g
u
u
block-element compounds (such as TiO ) ever achieved.
2
Because of the well-defined active center and surrounding
atomic structures, this kind of actinide photocatalysts can serve
as highly efficient visible-light single-atom catalysts with long-
lived excited states of actinide atoms at the core of the active
62−64
sites.
ASSOCIATED CONTENT
sı Supporting Information
■
*
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c00388.
Computational methodology and software references,
molecular orbital energy scheme and orbital envelop
plots of the uranyl complex, predicted and observed
UV−vis absorption spectra of the uranyl complex,
Figure 13. Sketch of ground and excited state reaction paths for the
uranyl-catalyzed methanol oxidation.
K
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Inorganic Chemistry
energy profiles of various reaction steps, uranyl cation:
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(
U−O bond lengths and lowest vertical electronic
excitation energies, and atomic charges and spin-
populations at various stationary states (PDF)
(
(
AUTHOR INFORMATION
■
Batista, E. R.; Hay, P. J. Synthesis of Imido Analogs of the Uranyl Ion.
Corresponding Authors
Science 2005, 310 (5756), 1941−1943.
Yong Li − Department of Chemistry and Key Laboratory of
Organic Optoelectronics & Molecular Engineering of the
Ministry of Education, Tsinghua University, Beijing 100084,
China; School of Materials & Energy, Guangdong University of
Technology, Guangzhou 510006, China; Institute of Applied
and Physical Chemistry, University of Bremen, Bremen 28359,
Germany; orcid.org/0000-0002-2774-841X;
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Email: liyong@uni-bremen.de
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W. H. Eugen Schwarz − Department of Chemistry and Key
Laboratory of Organic Optoelectronics & Molecular
Engineering of the Ministry of Education, Tsinghua University,
Beijing 100084, China; Department of Chemistry, University of
Siegen, Siegen 57068, Germany; orcid.org/0000-0001-
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8
730-1508; Email: eugen.schwarz@uni-siegen.de
Jun Li − Department of Chemistry and Key Laboratory of
Organic Optoelectronics & Molecular Engineering of the
Ministry of Education, Tsinghua University, Beijing 100084,
China; Department of Chemistry, Southern University of Science
and Technology, Shenzhen 518055, China; orcid.org/0000-
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002-8456-3980; Email: junli@tsinghua.edu.cn
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Guoqing Zhang − School of Materials & Energy, Guangdong
University of Technology, Guangzhou 510006, China
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Complete contact information is available at:
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The manuscript was written through contributions of all
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Funding
Reveals Strong E.coli RNA Polymerase-DNA Backbone Contacts in
the + 10 Region of the DeoP1 Promoter Open Complex. Nucleic Acids
Res. 1989, 17 (13), 4947−4956.
This work was supported by the National Natural Science
Foundation of China (NSFC grants 21590792, 91645203, and
1433005) and the Guangdong Provincial Key Laboratory of
Catalysis (No. 2020B121201002). We thank the Super-
computer centers at Tsinghua National Laboratory for
Information Science and Technology and North-German
Supercomputing Alliance (HLRN) for providing computa-
tional resources, and we thank the International Science &
Technology Cooperation Program of China (2011DFB91560).
2
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Notes
The authors declare no competing financial interest.
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