Pseudo-single-atom Platinum Induced by the Promoter Confined in Brucite-like Lattice for Catalytic Reforming

Article abstract of DOI:10.1002/cctc.201501239

Pseudo-single atom Pt catalysts have achieved an excellent selectivity to branched paraffins and cycloalkanes in n-hexane reforming here. Highly dispersed platinum in single-atom and small clusters made up of several separated atoms was produced under the inducement of Sn or Zr as promoting components. The Sn or Zr oxides were topologically transformed from layered double hydroxides as precursors, and thus homogeneously dispersed owing to the confinement of the brucite-like lattices. The lattice confinement also caused the promoter sites charge-transferred, affording electron-rich (Sn) or electron-defected (Zr) sites on the surface, thereby facilitating the inducement to Pt sites. This strategy provides an effective and feasible alternative for the facile preparation of highly dispersed metal catalysts. In n-hexane reforming, a selectivity of up to 95 % to branched paraffins and cycloalkanes was obtained with the pseudo-single-atom Pt catalysts. A site for sore eyes! The excellent dispersion of the promoter components results from the confinement effects of brucite-like lattices, and the selective inducement originates from the strong interactions between Sn^IV or Zr^IV sites and Pt sites. In the catalytic reforming of n-hexane, pseudo-single-atom Pt afforded a selectivity of up to 95 % to iso-alkanes and cycloalkanes, whereas the contrast samples with larger Pt sites performed a lower selectivity to iso-alkanes and cycloalkanes, but generated undesired C1-C5 and benzene.

Full text of DOI:10.1002/cctc.201501239

DOI: 10.1002/cctc.201501239
Communications
Pseudo-single-atom Platinum Induced by the Promoter
Confined in Brucite-like Lattice for Catalytic Reforming
Xiaodan Ma, Zhe An, Yanru Zhu, Wenlong Wang, and Jing He*[a]
Pseudo-single atom Pt catalysts have achieved an excellent se-
lectivity to branched paraffins and cycloalkanes in n-hexane re-
forming here. Highly dispersed platinum in single-atom and
small clusters made up of several separated atoms was pro-
duced under the inducement of Sn or Zr as promoting compo-
nents. The Sn or Zr oxides were topologically transformed
from layered double hydroxides as precursors, and thus homo-
geneously dispersed owing to the confinement of the brucite-
like lattices. The lattice confinement also caused the promoter
sites charge-transferred, affording electron-rich (Sn) or elec-
tron-defected (Zr) sites on the surface, thereby facilitating the
inducement to Pt sites. This strategy provides an effective and
feasible alternative for the facile preparation of highly dis-
persed metal catalysts. In n-hexane reforming, a selectivity of
up to 95% to branched paraffins and cycloalkanes was ob-
tained with the pseudo-single-atom Pt catalysts.
persion of Pt to cluster size. Addition of Re, Sn, or Ge as pro-
moter is especially efficacious because, in addition to altering
the electron density of Pt,[10] the promoter component sepa-
rates Pt atoms from large aggregates by forming PtxM alloys[11]
or Pt-promoter oxides islands.[12]
Recently, (pseudo)-single-atom noble metal catalysts have
attracted significant attention owing to their remarkable cata-
lytic properties.[13–18] A catalyst with Pt atoms completely ex-
posed to MgO surface has been obtained through a mass-se-
lected soft-landing technique, in which each Pt atom counts
for CO oxidation.[13] Isolated Au atoms anchored on Pd clusters
in alloy form exhibit high activity for aerobic glucose oxida-
tion.[14] Single-atom catalysts such as Pt1/FeOx,[15] Pt1/Al2O3,[16]
Au/titania,[17] and Ir1/FeOx,[18] produced by anchoring the metal
atoms on the defect sites of oxide support surfaces, exhibit ex-
traordinary catalytic properties for CO oxidation, WGS, and the
selective hydrogenation of functionalized nitroarenes, respec-
tively. Owing to the tendency of aggregation of single metal
atoms,[19] how to achieve a (pseudo)-single-atom distribution
with a lower cost and more controllable approach remains
a major challenge. Here we report a simple and feasible strat-
egy to fabricate supported pseudo-single-atom Pt using the in-
ducement of Sn or Zr as promoter components, in which the
Sn or Zr were confined in the lattices of the brucite-like layers
of layered double hydroxides (LDHs) as precursors. Sn or Zr
component is supposed to be homogeneously dispersed
owing to the confinement. LDHs are a class of anionic clays,
accommodating a wide range of homogeneously-dispersed di-
valent (M2+) and high-valence (M3+ or M4+) metal cations[20] in
the brucite-like layers. Well dispersed mixed metal oxides
(MMO)[21] can be produced through simple topological transi-
tion of LDHs, and highly dispersed metal catalysts could be ob-
tained by thermal treatment of LDHs in a reduction atmos-
phere.[22,23] The pseudo-single-atom Pt catalysts produced in
this work by the inducement of promoter components exhibit
extraordinary catalytic properties in both of hexane reforming
and cyclohexane dehydrogenation reactions. The strategy is
a promising alternative to obtain a variety of single-atom cata-
lysts taking advantage of the controllable chemical composi-
tions of LDHs.
The catalytic reforming of straight-chain paraffin to branched
paraffin and/or cycloalkanes is a major petroleum refining pro-
cess for the production of high-octane gasoline. The hydroge-
nolysis byproducts, however, need to be avoided for the atom-
economy.[1] The aromatic compounds are also undesired, as en-
vironmental regulations limit the carcinogenic components in
gasoline.[2] The reforming process is industrially accomplished
by use of supported platinum catalysts.[3] It is critical to en-
hance the dispersion of Pt atoms for sake of product selectivi-
ty.[1b,4] Large Pt ensembles have been found to be active for
the hydrogenolysis to form C1–C5[5] by the factor of a 3D struc-
ture, the so-called B5 sites.[5b] The 6-fold coordinated step-like
sites favor the dehydrocyclization to produce aromatic com-
pounds.[6] To improve Pt dispersion, many efforts have been
made during the past decades. The support properties[7] prove
important for the dispersion of Pt sites, such as the surface
properties,[7a] porous structures,[7b] or surface defect types.[7c] Pt
loading is also a critical factor,[8] including Pt precursor,[8a,b]
loaded amount,[8b] and loading method.[8b,c] Strengthening the
Pt-support interactions helps improving Pt dispersion as well.[9]
The formation of a new interaction (e.g. Pt-(ONa)x-SiO2) by ad-
dition of sodium ion[9a] or an enhanced Pt-support interaction
by addition of rare earth ions (e.g. Tm)[9b] has improved the dis-
The Sn or Zr containing LDHs were prepared by a co-precipi-
tation method.[24] The Sn or Zr content was controlled as ꢀ1.5,
3.3, and 5.3 atom% in the total metal cations (Table S1). From
the powder X-ray diffraction patterns (Figure S1), the lattice pa-
rameter a for Sn-containing LDHs (ZnAlSn-LDHs) and Zr-con-
taining LDHs (ZnAlZr-LDHs) was estimated to be enlarged with
the introduction of Sn or Zr and increase with increasing Sn or
Zr content, suggesting that either SnIV or ZrIV cations are locat-
ed in the hydrotalcite layers.
[a] X. Ma, Dr. Z. An, Y. Zhu, W. Wang, Prof. J. He
State Key Laboratory of Chemical Resource Engineering
Beijing University of Chemical Technology
15 Beisanhuandonglu, Chaoyang District, Beijing city (P.R. China)
E-mail: jinghe@263.net.cn
hejing@mail.buct.edu.cn
Supporting information for this article can be found under http://
dx.doi.org/10.1002/cctc.201501239.
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Figure 1. TEM and HRTEM images of ZnAlSn-MMO (a, c) and SnO2/ZnAl-MMO (b, d). HRTEM images of e) ZnAlZr-MMO and f) ZrO2/ZnAl-MMO. g) Z-contrast
STEM image of Sn atoms in ZnAlSn-MMO and single-pixel line profiles across the atom rows marked by rectangles in panels A) and B). The bright dots and
high-intensity single-pixel line profiles correspond to Sn-containing atomic columns, marked with red circles and arrows. h) XPS spectra of Sn3d5/2 of ZnAlSn-
MMO and SnO2/ZnAl-MMO. i) XPS spectra of Zr 3d5/2 of ZnAlZr-MMO and ZrO2/ZnAl-MMO.
Sn or Zr containing mixed metal oxides (the Sn or Zr content
was ꢀ3.3 atom% unless otherwise specified) (Table S1) were
produced (Figure S2) by calcination. No SnO2 or ZrO2 phase
can be observed in the XRD patterns (Figure S2A). In the TEM
image (Figure 1a) for ZnAlSn-MMO, which is produced from
ZnAlSn-LDHs precursor, no SnO2 particle can be resolved. In
contrast, obvious SnO2 nanoparticles are observed (Figure 1b)
for SnO2/ZnAl-MMO, which is produced from the ZnAl-LDHs
with Sn compound impregnated. It is further confirmed by
HRTEM images. No interplanar spacing representing SnO2
phase are discovered in ZnAlSn-MMO (Figure 1c), whereas the
(110) facet of SnO2, with an interplanar spacing of 0.334 nm, is
clearly shown for SnO2/ZnAl-MMO (Figure 1d). The same is the
observation for ZnAlZr-MMO (Figure 1e) and ZrO2/ZnAl-MMO
(Figure 1 f). The interplanar distance for the (100) facet of
ZnOAl in ZnAl-MMO is determined to be 0.273 nm (Figure S2B).
The interplanar spacing for the (100) facet of ZnOAl(Sn) in
ZnAlSn-MMO is enlarged to 0.275 nm (Figure 1c), and that of
the ZnOAl(Zr) in ZnAlZr-MMO to 0.280 nm (Figure 1e), whereas
the (100) facet in either SnO2/ZnAl-MMO or ZrO2/ZnAl-MMO
(Figure 1d and 1 f) retains the interplanar spacing at 0.273 nm.
The enlargement of interplanar spacing indicates the effective
incorporation of SnIV or ZrIV into the ZnO lattice. The Z-contrast
scanning transmission electron microscopy (STEM) image for
ZnAlSn-MMO (Figure 1g) demonstrates an atomic dispersion
of SnIV with a chemical environment of Sn-O-Zn. In the TPR
profiles (Figure S3), it is harder for the SnIV in ZnAlSn-MMO to
get reduced than that in SnO2/ZnAl-MMO, consistent with the
confinement of SnIV in the lattices of ZnAlSn-MMO. Though it
is difficult to obtain a well resolved STEM image for ZnAlZr-
MMO, owing to the similar atomic weight of Zr to Zn, the con-
finement of Zr in the lattices of ZnAlZr-MMO can be deduced
on the basis of the extra peak (8068C) in the TPR profile of
ZnAlZr-MMO (Figure S3).
The Sn3d5/2 XPS (Figure 1h) for ZnAlSn-MMO shifts to low
binding energy (486.5 eV) in comparison to that for SnO2/ZnAl-
MMO (486.8 eV), which reveals that the lattice-confined SnIV
has a higher electronic density. The Zr3d5/2 XPS (Figure 1i) for
ZnAlZr-MMO (182.4 eV) shifts toward high binding energy in
comparison with that for ZrO2/ZnAl-MMO (182.0 eV), showing
a lower electronic density for the lattice-confined ZrIV. The dif-
ference between confined SnIV and ZrIV in the electronic density
is related to the inducement of the lattice of ZnO, as a result
of difference in the electronegativity between SnIV or ZrIV and
ZnII. That means the SnIV sites in ZnAlSn-MMO are electron-
richer than that in SnO2/ZnAl-MMO, whereas the ZrIV sites in
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Figure 2. HAADF-STEM images and the corresponding atom number distribution of Pt clusters of a) Pt (+)/ZnAlSn-MMO, b) Pt(À)/ZnAlZr-MMO, c) Pt(À)/
ZnAlSn-MMO, d) Pt(+)/ZnAlZr-MMO, e) Pt/SnOx/ZnAl-MMO, and f) Pt/ZrOx/ZnAl-MMO.
ZnAlZr-MMO are more electron-deficient than that in ZrO2/
ZnAl-MMO.
pseudo-single-atom Pt dispersion has been achieved with PTA
impregnated ZnAlSn-MMO. For the ZnAlSn-MMO impregnated
with PHC and ZnAlZr-MMO impregnated with PTA, the fraction
of Pt clusters consisting of 2–10 and 11–19 Pt atoms is ob-
served to increase. That means the ZnAlSn-MMO or ZnAlZr-
MMO surface demonstrates selective inducement to Pt precur-
sor. The selective inducement is not surprising by taking into
considerations the difference in the electronic density between
the confined SnIV and ZrIV sites according to the XPS results
(Figure 1h and 1i). For the Pt loaded on SnOx/ZnAl-MMO or
ZrOx/ZnAl-MMO, the clusters consisting of 11–19 Pt atoms and
even larger ones containing above 20 Pt atoms become domi-
nant, in a total fraction of 70 or 68%. So it is clear that it is the
highly dispersed SnIV or ZrIV sites confined in the MMO lattices
that result in better Pt dispersion. The higher Pt dispersion in
Pt/SnOx/ZnAl-MMO (0.74) and Pt/ZrOx/ZnAl-MMO (0.78) than in
Pt/ZnAl-MMO (0.64) (Table S2) proves the role of promoter
components. The STEM-EDS (Figure S4) analysis shows that
more Pt atoms are detected at the sites with higher Sn con-
tent. And in consequence, we could speculate that Sn sites
serve exactly as the adsorption sites for Pt.
So in the following Pt loading (Figure 2 and Table S2), posi-
tively charged platinum (PTA, [(NH3)4Pt]2+) gets better dis-
persed on ZnAlSn-MMO (Figure 2a) whereas negatively
charged platinum hexachloride (PHC, [PtCl6]2À) better on
ZnAlZr-MMO (Figure 2b). As can be seen clearly from the
HAADF-STEM results (Figure 2 and Table S2), though the Pt
sites (in ꢀ0.3 wt%) on ZnAlSn-MMO or ZnAlZr-MMO are well
dispersed, the single-atom Pt percentages are diverse depend-
ing on the charge nature of Pt precursors. More individual Pt
atoms are resolved on ZnAlSn-MMO with PTA as loaded pre-
cursor (Figure 2a) than with PHC as loaded precursor (Fig-
ure 2c), and more individual Pt atoms are observed on ZnAlZr-
MMO with PHC as loaded precursor (Figure 2b) than with PTA
as loaded precursor (Figure 2d). On either ZnAlSn-MMO or
ZnAlZr-MMO, only individual Pt atoms or sub-nanometer-sized
clusters made up of isolated Pt atoms can be observed (Fig-
ure 2a–d). In comparison, larger Pt aggregates are additionally
observed on SnOx/ZnAl-MMO or ZrOx/ZnAl-MMO (Figure 2e–f).
Quantitatively (Figure 2), individual Pt atoms and the clusters
consisting of below 10 Pt atoms were counted up to 75 and
21% for PTA impregnated ZnAlSn-MMO, whereas 66% individ-
ual Pt atoms and 30% small clusters containing only 2–10 Pt
atoms were found for PHC impregnated ZnAlZr-MMO. A
The selective inducement also indicates the stronger interac-
tions between the highly dispersed SnIV or ZrIV sites and the se-
lected Pt precursors, which is confirmed by H2-TPR (Figure S5)
and XPS results (Figure S6). The H2 consumption at 450–5008C
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for Pt reduction in Pt (+)/ZnAlSn-MMO (5008C) or Pt(À)/
ZnAlZr-MMO (5008C) is higher than in Pt(À)/ZnAlSn-MMO
(4908C) or Pt(+)/ZnAlZr-MMO (4928C), demonstrating the
stronger interaction between Sn or Zr component and the se-
lected Pt precursors (Figure S5). The much lower reduction
temperature for Pt/SnOx/ZnAl-MMO (4728C) and the only low-
temperature peak for Pt/ZrOx/ZnAl-MMO (3158C) suggest that
the highly dispersed Sn or Zr component confined in the MMO
lattices provides stronger PtÀSn or PtÀZr interactions than the
Sn or Zr outside the lattices. In the Sn 3d5/2 XPS, more SnII was
estimated to be formed in Pt(+)/ZnAlSn-MMO, owing to the
stronger PtÀSn interaction than in Pt(+)/SnOx/ZnAl-MMO. The
shift of Sn3d5/2 from 486.6 eV (ZnAlSn-MMO) to 487.0 eV
(Pt(+)/ZnAlSn-MMO) reveals that SnIV center in Pt/ZnAlSn-MMO
has a lower electron density, owing to the PtÀSn interaction
(Figure S6). It can be inferred that the SnIV sites serve as elec-
tron donor for Pt. The shift of Zr3d5/2 from 182.0 eV (ZnAlZr-
MMO) to 182.5 eV (Pt(À)/ZnAlZr-MMO) reveals that the ZrIV
center in Pt/ZnAlZr-MMO has a higher electron density, owing
to the PtÀZr interaction (Figure S6). So it can be inferred that
ZrIV sites serve as electron acceptor for Pt. To confirm the elec-
tron density changes of Pt, the in situ FT-IR of CO adsorption
was also performed. According to previous report, the bands
at 2000–2100 cmÀ1 can be ascribed to linearly adsorbed CO on
single-atom Pt sites. When the electron density of such Pt
atoms decreased, the band shifted to higher wavenumber.[15]
In this work, the band of the CO adsorbed on the single-atom
Pt sites was located at 2020 cmÀ1 for Pt/ZnAl-MMO. For Pt/
ZnAlSn-MMO, the band changed lower to 2010 cmÀ1, indicat-
ing Pt species here are electron-rich than that in Pt/ZnAl-MMO.
And for Pt/ZnAlZr-MMO, the band changed higher to
2045 cmÀ1, indicating Pt species here are electron-defect than
that in Pt/ZnAl-MMO.
decreases the TOF, while the electron-defect Pt increases the
TOF. The stronger is the PtÀSn or PtÀZr interaction, the lower
or higher is the TOF. The same trend is observed for the forma-
tion rate of benzene (Figure 3b).
The hexane reforming reaction was carried out at 3608C in
140 Torr n-hexane and 620 Torr H2 (1 Torr=1.33 mbar).[25] To
allow for the selectivity comparison, the total hexane conver-
sion was held between 2 and 6%. As shown in Figure 4, the
Figure 4. The variations of the selectivity to a) C1–C5, b) benzene, and c) i-
C6 +cyclo-C6 alkanes with the fraction of single Pt atom sites in the n-
hexane reforming reaction. To allow for selectivity comparisons, the hexane
conversion was held between 2 and 6%.
selectivities on Pt/SnOx/ZnAl-MMO and Pt/ZrOx/ZnAl-MMO are
quite similar, which suggests that, with a similar Pt dispersion,
the supports exhibit no visible effects. With the increase in the
fraction of single Pt atoms, the selectivities to C1–C5 hydrocar-
bons (Figure 4a) and benzene (Figure 4b) decrease, while the
selectivities to i-C6 alkanes and cyclo-C6 alkanes increase (Fig-
ure 4c). Particularly on the catalyst with a pseudo-single-atom
Pt dispersion (Pt(+)/ZnAlSn-MMO), the selectivity to i-C6 alka-
nes and cyclo-C6 alkanes reaches 95%, and the selectivity to
either C1–C5 hydrocarbons or benzene is below 2%.
The strong interaction between Pt and the confined SnIV and
ZrIV sites was further confirmed with the cyclohexane dehydro-
genation reaction (Figure 3). In the cyclohexane dehydrogena-
tion reaction, in comparison with Pt/ZnAl-MMO, the TOF de-
creases with Sn presence, while increases with Zr presence
(Figure 3a). The PtÀSn or PtÀZr interactions are obviously re-
sponsible for the dehydrogenation activity. The electron-rich Pt
In conclusion, this work has demonstrated that pseudo-
single-atom Pt dispersion can be achieved by the selective in-
ducement of highly dispersed promoter components (SnIV or
ZrIV here) to Pt precursors. The excellent dispersion of the pro-
moter components results from the confinement effects of
brucite-like lattices, and the selective inducement originates
from the strong interactions between SnIV or ZrIV sites and Pt
sites. In the catalytic reforming of n-hexane, pseudo-single-
atom Pt afforded a selectivity of up to 95% to iso-alkanes and
cycloalkanes.
Experimental Section
ZnAlM-LDHs (M=Sn, Zr) were prepared by a co-precipitation
method.[23] Typically, Zn(NO3)2·6H2O, Al(NO3)3·9H2O, SnCl4·5H2O (or
ZrO(NO3)2·xH2O) with the Zn/Al/Sn (or Zn/Al/Zr) ratio of 2:0.9:0,
2:0.95:0.05, 2:0.9:0.1, or 2:0.8:02 were first dissolved in 100 mL of
deionized water ([Zn2+]+[Al3+]+[Sn4+]/[Zr4+]=1.2m). The result-
ing solution was added drop by drop to a round-bottomed flask si-
multaneously with an aqueous solution (100 mL) of NaOH (1.92m)
and Na2CO3 (0.8m) in deionized water. The pH was maintained at
a constant value of ꢀ10.0Æ0.2 in the precipitation. After the pre-
cipitation, the resulting suspension was aged at 608C for 12 h with
stirring. The solid was then isolated by filtration, washed thorough-
Figure 3. a) Turnover frequency (TOF, sÀ1) in the cyclohexane dehydrogena-
tion reaction. TOFs were calculated on the basis of the number of Pt atoms
exposed at the surface. b) Correlation of benzene formation rate with Pt dis-
persion. The Pt catalysts promoted with Sn is shown in black line, while pro-
moted with Zr is shown in blue line. Pt dispersion was determined with the
HOT method.
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ly with deionized water until the pH reached 7, and dried over-
night at 608C in an oven.
Pt was deposited on Zn2Al0.9Sn0.1-LDHs or Zn2Al0.9Zr0.1-LDHs, by in-
cipient wetness impregnation method.[8a] The LDHs with platinum
precursor loaded was then dried overnight at 608C in an oven, cal-
cined in air at 5508C for 2 h, and then reduced in hydrogen at
5508C for 2 h. The sample with platinum hexachloride (PHC,
[PtCl6]2À) as Pt precursor was denoted Pt(À)/ZnAlM-MMO, and the
sample with platinum tetraammine (PTA, [Pt(NH3)4]2+) as Pt precur-
sor was denoted Pt(+)/ZnAlM-MMO.
For contrast, Sn4+ or Zr4+ loaded Zn2Al0.9-LDHs was prepared by in-
cipient wetness impregnation of Zn2Al0.9-LDHs with the aqueous
solution of SnCl4·5H2O or ZrO(NO3)2·xH2O, and then dried over-
night at 608C in an oven (Table S1). The corresponding SnO2/ZnAl-
MMO and ZrO2/ZnAl-MMO were produced by calcining Sn4+
/Zn2Al0.9-LDHs or Zr4+/Zn2Al0.9-LDHs in air at 5508C for 2 h. Pt/SnOx/
ZnAl-MMO was prepared by impregnating Sn4+/Zn2Al0.9-LDHs with
PTA solution, and Pt/ZrOx/ZnAl-MMO by impregnating Zr4+
/Zn2Al0.9-LDHs with PHC solution, followed by calcination in air at
5508C for 2 h and reduction in hydrogen at 5508C for 2 h after
dried overnight at 608C in an oven. ZnAlSn-MMO or ZnAlZr-MMO
were prepared by calcining Zn2Al0.9Sn0.1-LDHs (3.29 atom% Sn) or
Zn2Al0.9Zr0.1-LDHs (3.43 atom% Zr) (Table S1) at 5508C for 2 h in at-
mosphere.
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Detailed procedures for XRD, HRTEM, HAADF-STEM, H2-TPR, XPS,
and procedures for the catalytic reactions are given in the Support-
ing Information.
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Acknowledgements
Financial supports by NSFC, PCSIRT (IRT1205), 973 Project
(2014CB932101) and the Fundamental Research Funds for the
Central Universities (YS1406) are gratefully acknowledged. J. H.
particularly appreciates the financial aid of China National Funds
for Distinguished Young Scientists from the NSFC. This work is
also supported by Beijing Engineering Centre for Hierarchical Cat-
alysts.
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Published online on April 29, 2016
ChemCatChem 2016, 8, 1773 – 1777
www.chemcatchem.org
1777
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