Thermal methane conversion to formaldehyde promoted by single platinum atoms in PtAl2O4- cluster anions

Article abstract of DOI:10.1002/anie.201403953

Identification and mechanistic study of thermal methane conversion mediated by gas-phase species is important for finding potentially useful routes for direct methane transformation under mild conditions. Negatively charged oxide species are usually inert with methane. This work reports an unexpected result that the bi-metallic oxide cluster anions PtAl₂O₄ ^- can transform methane into a stable organic compound, formaldehyde, with high selectivity. The clusters are prepared by laser ablation and reacted with CH₄ in an ion trap reactor. The reaction is characterized by mass spectrometry and density functional theory calculations. It is found that platinum rather than oxygen activates CH₄ at the beginning of the reaction. The Al₂O₄^- moiety serves as the support of Pt atom and plays important roles in the late stage of the reaction. A new mechanism for selective methane conversion is provided and new insights into the surface chemistry of single Pt atoms may be obtained from this study. One atom is enough: The negatively charged oxide cluster PtAl₂O ₄^- is an active species in thermal methane conversion. The single platinum atom activates methane and delivers two hydrogen atoms to the "oxide support" Al₂O₄^-. Methane is then transformed to formaldehyde with a high selectivity.

Full text of DOI:10.1002/anie.201403953

.
Angewandte
Communications
DOI: 10.1002/anie.201403953
Methane Conversion
Thermal Methane Conversion to Formaldehyde Promoted by Single
ꢀ
Platinum Atoms in PtAl2O4 Cluster Anions**
Yan-Xia Zhao, Zi-Yu Li, Zhen Yuan, Xiao-Na Li, and Sheng-Gui He*
Abstract: Identification and mechanistic study of thermal
methane conversion mediated by gas-phase species is impor-
tant for finding potentially useful routes for direct methane
transformation under mild conditions. Negatively charged
oxide species are usually inert with methane. This work reports
an unexpected result that the bi-metallic oxide cluster anions
radicals (OCꢀ) to generate methyl radicals, which was also
proposed for oxidative coupling of methane in the condensed-
phase studies:[2c,14]
Cꢀ
C
O
þ CH4 ! OHꢀ þ CH3
ð1Þ
Only a few oxide species such as FeO+,[6,15] CoO+,[16] and
NiO+[17] can react with methane to produce stable C1 oxy-
genates methanol and formaldehyde.[2,3] It is noteworthy that
in addition to oxides, oxygen-free species including atomic
ꢀ
PtAl2O4 can transform methane into a stable organic
compound, formaldehyde, with high selectivity. The clusters
are prepared by laser ablation and reacted with CH4 in an ion
trap reactor. The reaction is characterized by mass spectrom-
etry and density functional theory calculations. It is found that
platinum rather than oxygen activates CH4 at the beginning of
+ [19]
transition metal ions,[18] Au2 , MoC+,[20] and so on[3,21] can
also react with methane to generate stable molecules such as
C2H4 and H2.
ꢀ
the reaction. The Al2O4 moiety serves as the support of Pt
atom and plays important roles in the late stage of the reaction.
A new mechanism for selective methane conversion is provided
and new insights into the surface chemistry of single Pt atoms
may be obtained from this study.
To the best of our knowledge, all of the reported gas phase
ions that can react with CH4 under thermal collision
conditions to generate stable organic compounds are pos-
itively rather than negatively charged species.[4,22,23] It was
reported that the anions are mostly inert or react very slowly
with methane.[2–4,22] Herein, we report an unexpected result
that oxide cluster anions PtAl2O4ꢀ with single platinum atoms
can react with methane quite efficiently to generate the stable
organic compound formaldehyde.
D
irect and selective transformation of methane into value-
added chemicals under mild conditions is of great impor-
tance.[1–4] However, CH4, a highly symmetric molecule, has
ꢀ
a high C H bond energy, a large ionization energy, and a huge
HOMO–LUMO gap, so selective methane conversion is
challenging and has attracted interests of researchers from
most of the disciplines within chemistry.[1–5] Discovery of gas-
phase species that can react with CH4 under thermal collision
conditions serves as a first step to find useful mechanisms to
transform methane under mild conditions.[2–4]
Platinum has been extensively studied for methane
activation. The neutral, cationic, and anionic Pt atomic and
cluster species Ptn (n = 1–24),[24] Ptn+ (n = 1–21),[23,25] and Ptn
ꢀ
(n = 2–5, 7, 11, 12)[23,25] all can activate methane in the gas
phase, for example:[26,27]
q
Ptq þ CH4 ! ½HPtCH3ꢁ
ð2aÞ
ð2bÞ
Oxides are an important type of catalytic materials and
many metal and metal-free oxide species have been identified
to be able to react under thermal collision conditions with
CH4 in the gas phase. These gas phase oxides include not only
diatomic and tri-atomic ions (FeO+,[6] CuO+,[7] SO2+,[8] and so
q
q
½HPtCH3ꢁ ! ½PtCH2ꢁ þ H2, q ¼ 0 and þ 1
In addition, Pt-containing organic compounds[1c,28] and sup-
ported Pt nanoparticles[29] are also effective catalysts for
methane conversion. Recently, single-atom catalysts which
contain isolated Pt atoms[30] dispersed on supports such as
metal oxides, have been developed in order to maximize the
effective use of the noble metal atoms and offer great
potential for achieving high activity and selectivity. As
a result, it is important to study the reactivity of heteronuclear
oxide clusters with single Pt atoms in CH4 activation in order
to find potentially useful mechanisms of methane conversion.
In this study, the Pt-Al-O heteronuclear cluster anions
PtAlxOyꢀ are prepared and reacted with CH4. Once a reactive
cluster is experimentally identified, it is very interesting to
investigate whether the Pt atom or the usually identified OCꢀ
+
on)[2,3] but also polyatomic cluster species such as (V2O5)n
+
+
(n = 1–5),[9] V2O5(SiO2)n (n = 1–4),[10] VxP4ꢀxO10 (x = 0, 2,
3),[11] VxY4ꢀxO6+x+ (x = 1–3),[12] and many others.[2–4,13] Most of
these oxides activate methane through oxygen-centered
[*] Dr. Y.-X. Zhao, Z.-Y. Li, Z. Yuan, Dr. X.-N. Li, Prof. Dr. S.-G. He
Beijing National Laboratory for Molecular Science, State Key
Laboratory for Structural Chemistry of Unstable and Stable Species,
Institute of Chemistry, Chinese Academy of Sciences
Beijing 100190 (P.R. China)
E-mail: shengguihe@iccas.ac.cn
[**] This work was supported by the National Natural Science
Foundation of China (21203208 and 21325314), the Major Research
Plan of China (213CB834603, 2011CB932302), and the Strategic
Priority Research Program of the Chinese Academy of Sciences
(XDA09030101).
radical[2,4,31] is the active center for C H bond activation of
ꢀ
CH4. It is noteworthy that aluminium oxide is an important
q
catalyst-support material and study of PtAlxOy clusters also
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201403953.
serves as a bottom-up strategy to understand single platinum
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2
Figure 2. DFT-calculated structure (left, Cs, A’’) and electrostatic
potential map (right) of PtAl2O4ꢀ. Bond lengths (in pm) and natural
spin densities (in mB and listed in the parentheses) are shown.
efficiency (F)[34] of (7.0 ꢂ 3.5)%. The kinetic isotope effect
ꢀ
ꢀ
[KIE = k1(PtAl2O4 +CH4)/k1(PtAl2O4 +CD4)] is 2.4 ꢂ 0.7.
Density functional theory (DFT) calculations are per-
formed to understand the mechanisms of Reaction (3). The
ꢀ
lowest-energy structure of PtAl2O4 (Figures 2 and S4) is
a doublet and the unpaired spin densities are distributed
around the Pt and two O atoms. The Pt atom as well as the Pt-
O unit with 193 pm bond length is quite strongly bonded with
the remaining part of the cluster:
Figure 1ꢀ. TOF mass spectra for the reactions of mass selected
PtAl2O4 with CH4 (b, c, e) and CD4 (d, f). The maximal reactant gas
pressures[32] are shown. The reaction times are 1.6 ms for (b–e) and
5.6 ms for (f).
ꢀ
PtAl2O4 ! Al2O4ꢀ þ Pt, DH0 ¼ 2:99 eV
ð4aÞ
ð4bÞ
PtAl2O4 ! Al2O3 þ PtOꢀ, DH0 ¼ 5:35 eV
ꢀ
The natural charge on Pt atom is ꢀ0.06 j e j , indicating that
ꢀ
the Pt atom in PtAl2O4 cluster is close to a metallic state
atom catalysis over Al2O3 supports.
(Pt0). The electrostatic potential (ESP) map shown in Figure 2
(right) indicates that the Pt-side of PtAl2O4ꢀ is less negatively
charged than the other side of the cluster, suggesting that the
Pt atom can be the preferred trapping site when the CH4
molecule approaches PtAl2O4ꢀ cluster.
ꢀ
The PtAlxOy cluster anions are generated by laser
ablation, mass-selected, thermalized, and then interacted
with CH4 or CD4 in an ion trap reactor.[32] Pt has six stable
isotopes: 190Pt (0.014%), 192Pt (0.782%), 194Pt (32.967%), 195Pt
(33.832%), 196Pt (25.242%), and 198Pt (7.163%).[33] Figure 1a
presents the time-of-flight (TOF) mass spectrum for the three
major isotopomers of PtAl2O4
The lowest energy reaction pathway found for Reac-
tion (3) is shown in Figure 3 (see Figure S9 for other reaction
paths considered). As predicted by the nature of the ESP map
shown in Figure 2, the CH4 interacts with Pt rather than O or
Al atom in the encounter complex (I1) with a binding energy
of 0.432 eV. The reaction proceeds through insertion of Pt
(
194Pt, 195Pt, and 196Pt). Upon
ꢀ
the interactions with 5 mPa CH4 for about 1.6 ms, product
ꢀ
peaks that can be assigned as PtAl2O3H2 are observed
ꢀ
(Figure 1b). The magnitudes of the PtAl2O3H2 product
peaks increase as the CH4 pressure increases (Figure 1c),
which suggests the following reaction generating formalde-
hyde:
ꢀ
into one C H bond (I1!TS1!I2) with a negligible barrier
ꢀ
(0.003 eV), resulting in the formation of one Pt H and one
ꢀ
ꢀ
ꢀ
Pt C bond and the cleavage of one C H and the Pt Al bond.
PtAl2O4ꢀ þ CH4 ! PtAl2O3H2ꢀ þ HCHO
ð3Þ
The above process follows a mechanism of oxidative addition
driven by the strong bond strengths of Pt C (5.95 eV)[35] and
ꢀ
[36]
ꢀ
The above reaction is confirmed by using the isotopic
labelling experiment with CD4 (Figure 1d) as well as the
experiments by mass-selecting the cluster ions with a single
isotope of platinum (194Pt, Figure 1e,f).
Pt H (3.44 eV). After the oxidative addition, the H atom
can transfer from Pt atom to the “oxide support” to make
a chemical bond with a bridging O (I2!TS2!I3) and this
step releases additional energy (0.52 eV). More energy
(0.83 eV) can be released if the H atom is further transferred
to the terminally bonded O atom (I3!TS3!I4). After
formation of I4, the system has enough energy (2.10 eV) to
transfer a second H atom from the methyl group to Pt atom
(I4!TS4!I5) and then from Pt to O (I5!TS5!I6).
In addition to PtAl2O3H2ꢀ, minor products PtAl2O4H2Oꢀ,
PtOꢀ, and Al2O4ꢀ of which the summed intensity amounts to
3% of the total ion intensity and 11% of the product ion
intensity also appear in PtAl2O4ꢀ + CH4 (Figure S2 in the
Supporting Information (SI)). It turns out that these products
ꢀ
ꢀ
are due to reactions of PtAl2O4 with residual water
After activating two C H bonds of CH4 by the Pt atom
[PtAl2O4ꢀ + H2O!PtAl2O4H2Oꢀ!PtOꢀ + Al2O2(OH)2] and
oxygen (PtAl2O4ꢀ + O2!Al2O4ꢀ + PtO2) in the ion trap (see
SI). It can be concluded that the production of formaldehyde
is the sole channel for PtAl2O4ꢀ + CH4. The pseudo first-order
rate constant (k1) of Reaction (3) is estimated to be (7.3 ꢂ
3.6) ꢀ 10ꢀ11 cm3 moleculeꢀ1 sꢀ1, corresponding to a reaction
and accepting two H atoms by the “oxide support”, the system
ꢀ
is ready to form a C O chemical bond (I6!TS6!I7), which
is the last important step to generate the HCHO moiety. In
order to evaporate HCHO molecule, the reaction complex is
relaxed (I7!TS7!I8!TS8!I9) so that the Pt atom is
bonded with two Al atoms. The cleavage of the Pt-C (I9!
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Angewandte
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Methane activation by free Pt atom and Pt+ ion [Reac-
tion (2)] was extensively studied[26,27] and it has been recently
realized that the neutral (Pt) and cationic (Pt+) reaction
systems follow a similar mechanism, in which the oxidative
ꢀ
addition of one C H bond to Pt atom [Reaction (2a)] is facile
(barrierless for Pt and a tiny barrier for Pt+).[27d] The natural
ꢀ
charge on the Pt atom in PtAl2O4 is only ꢀ0.06 j e j , so the
ꢀ
PtAl2O4 cluster can be considered to be composed of
ꢀ
a neutral Pt atom and a “support” cluster anion (Al2O4 ). It
turns out that the mechanism of methane activation by the
supported Pt atom, the initial stage of Reaction (3), is very
similar to that of the free Pt or Pt+ reaction system: the
ꢀ
formation of the oxidative-addition complex (PtAl2O4
+
CH4!I1!I2, Figure 3) is almost barrierless, indicating that
the supported Pt atom inherits the high reactivity of free Pt in
methane activation. It is noteworthy that the encounter
complex I1 in Figure 3 has a large binding energy (0.43 eV) in
sharp contrast to small adsorption energies (0.05–0.15 eV)
between oxide cluster anions and alkane molecules reported
in literature.[22,37,38] The two C H bonds pointing toward Pt in
ꢀ
ꢀ
I1 are significantly activated as the C H distances are
lengthened from 109 pm of free CH4 to about 113 pm.
The Pt atom plays a crucial role in the activation and
ꢀ
cleavage of methane C H bonds for Reaction (3) on the one
hand. On the other hand, formation of HCHO rather than H2
molecule [as in Reaction (2b)] from PtAl2O4ꢀ + CH4 tells that
the “support” cluster should also play important roles. It is
Figure 3. DFT-calculated potential energy profile for Reaction (3). The
energies of the reaction intermediates (I1–I10) and transition states
(TS1–TS9) with respect to the separated reactants are given in eV. The
structures of I1–I10 are plotted and those of TS1–TS9 can be found in
the Supporting Information.
noteworthy that free Al2O4 is oxygen-rich[4] and has a high
ꢀ
H-atom affinity to abstract an H atom through the OCꢀ center
[as in Reaction (1)] from n-butane under thermal collision
conditions.[38] The high H-atom affinity of Al2O4 is the
ꢀ
driving force to transfer the H atom from Pt to the “oxide
support” (I2!I3!I4, Figure 3), so that enough energy
ꢀ
(2.10 eV) can be released to activate the second C H bond
ꢀ
ꢀ
TS9!I10) and then Al O bond (I10!PtAl2O3H2
+
(I4!I5) and to manage the subsequent transformation to
HCHO) evaporates HCHO. The reaction intermediates,
transition states, and products are all lower in energy than
the separated reactants, so the DFT calculations are consis-
tent with the experimental observation of gas-phase Reac-
tion (3).
produce HCHO.
To further understand the importance of the “oxide
support” in methane conversion, the one-oxygen-atom less
cluster PtAl2O3 is mass-selected and interacted with CH4 in
ꢀ
the ion trap reactor while no evidence of reaction can be
ꢀ
Oxide cluster anions are usually inert with methane and it
has been only recently identified that La6O10 and La8O13
identified (Figure S3). The DFT study indicates that Al2O3
,
which is oxygen-poor,[4] has a much smaller electron detach-
ꢀ
ꢀ
ꢀ
anions can activate CH4 through the OCꢀ centers to generate
CH3 [Reaction (1)] under thermal collision conditions
whereas the reaction efficiencies are quite low (F = 0.02–
0.03%),[22] in sharp contrast to the good efficiency of (7.0 ꢂ
3.5)% for PtAl2O4ꢀ with CH4. This also implies that methane
activation by PtAl2O4ꢀ follows a different mechanism. In the
DFT calculations for Reaction (3), in addition to the facile
ment energy than the oxygen rich cluster Al2O4 (3.3 versus
4.4 eV), so the Pt atom in PtAl2O3 (Figure S7) has a large
ꢀ
negative natural charge (ꢀ0.55 j e j) in contrast to the small
charge (ꢀ0.06 j e j) on the Pt atom in PtAl2O4ꢀ. The Pt-
insertion step in PtAl2O3ꢀ + CH4 (as I1!TS1!I2 in Figure 3)
is subject to an overall barrier of 0.54 eV (Figure S11), which
is in line with the result that the free Ptꢀ anion is inert or
reacts very slowly with CH4.[23] It means that PtAl2O3ꢀ cannot
transform methane under thermal collision conditions and
implies that, in the single-Pt-atom catalysis,[30] the nature of
supports and the charge transfers between Pt atoms and
supports should play very important roles.
In conclusion, selective conversion of methane to stable
organic compounds (formaldehyde) mediated by oxide clus-
ter anions (PtAl2O4 ) under thermal collision conditions has
been identified for the first time. The Pt atom rather than
oxygen radical is the active species for methane activation at
ꢀ
ꢀ
oxidative addition of C H bond to Pt atom (PtAl2O4
+
ꢀ
CH4!I1!I2 in Figure 3), direct C H activation by the
terminal O atom with the unpaired spin density of 0.48 mB
(Figure 2) has also been tested. It turns out that such a process
has an overall barrier of 0.25 eV (Figure S10), which means
that it cannot happen under thermal collision conditions.
Therefore, it is the Pt rather than O atom (or OCꢀ radical)[2,4,31]
in PtAl2O4 acting as the active center for C H activation of
ꢀ
ꢀ
ꢀ
CH4 in the initial stage of Reaction (3).
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O2 seeded in 7 atm He carrier gas (Figure S1). The clusters of interest
are mass-selected by a quadrupole mass filter (QMF) and enter into
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peaks) in Figure 1a–d. Methods of rate constant determination and
DFT calculations are described in the Supporting Information.
Received: April 3, 2014
Revised: May 21, 2014
Published online: July 9, 2014
Keywords: cluster anions · density functional computations ·
.
mass spectrometry · methane · platinum
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