10.1002/anie.201810251
Angewandte Chemie International Edition
COMMUNICATION
The water–gas shift reaction WGSR (CO + H2O → CO2 +
H2) is an industrial reaction that provides an easy source of H2
from the surplus CO generated by other processes.[11] Highly
promising fuel cell technologies rely on the WGSR[12] as long as
new catalysts, operating at temperatures <150 ºC and
compatible with fuel cell applications, are developed. In this
context, several works have reported that Pt1 and Au1 with a
certain positive charge, anchored on solids through strong
metal–support interactions (SMSI) or modified with alkali
precursors, efficiently catalyze the WGSR at temperatures
compatible with fuel cells.[13–14] However, in the case of Pt, the
exact catalytic active species is still under debate, since Pt
nanoparticles (NPs) and not Pt1 have been explicitly claimed as
the only active species of the low–temperature WGSR,
regardless the support.[15]
showed small (TOF <5 h-1) or no activity up to 150 ºC. The
abrupt increase in catalytic activity of MOF 2 can tentatively be
assigned to autoreduction of Pt2+. Previous low–temperature
WGSR Pt catalysts have not shown any catalytic activity below
100 ºC[13,15] and, accordingly, the calculated activation energy for
3 is 5 Kcal lower (12 Kcal mol-1) than previous Pt catalysts, while
the TOF obtained at 200 ºC is similar (0.03 s-1).[13] These results
1+
indicate that Pt1 is a very active SAC for the low–temperature
WGSR, while the Pt2+ species in 2 or 3 are not active at
temperatures below 150 ºC.
PXRD patterns of 3 recovered after 4 hours on the WGSR
stream at 150 ºC (Figure S9) show the retention of crystallinity
and porosity of the used material, and does not show any
characteristic XRD peaks of Pt NPs or metal oxide crystals. AC–
HAADF–STEM images of the same sample (Figures 2c–d
above), in combination with XPS measurements (Figure S11),
Water adsorption isotherms of the activated compounds 2
and 3 at 50 ºC show a similar large water uptake and a complete
rehydration, consisting of the recovery of 26 and 29 mmol of
water per gram of MOF (Figure S16), in agreement with TGA
analyses (Figure S17). However, the CO adsorption isotherms
are different for 2 and 3. The CO uptake is higher for 2 but
irreversible (Figure S18), which contrasts with the reversible
adsorption isotherm for 3. CO FT–IR shows that CO adsorbs at -
178 ºC on the available Pt+ sites of 2 (Pt2+) and 3 (Pt1+) with
different intensity (Figure S19), and that Pt1+–CO signals in 3
disappear when heating to give CO2 as a product, while any
adventitious Pt(0) species do not react.[16] These results indicate
that CO adsorbs very weakly on aqueous Pt11+ and that, at near
room temperature, gives CO2, which makes 3 a potentially
powerful catalyst for the WGSR, since the catalytic activity of Pt
for this reaction is related to its ability to co–adsorb water and
CO without CO poisoning.[15] While it is true that Figure S19 also
indicates that some Pt0 can be present or formed during reaction
conditions, all the data shown above soundly point to the major
Pt species in the MOF, i.e. Pt1+, as the only responsible for the
catalytic activity.
1+
confirm the stability of the Pt1 SAC sites after reaction, which
remain homogeneously distributed along the MOF. A 0.5 nm
resolved image (Figure 2d) shows the persistence and good
distribution of the Pt sites on 3 after reaction. Treatment of 3 in
the XPS chamber with either CO, H2O vapor or H2 does not
produce significant modifications of the original Pt spectrum,
with equimolecular amounts of low–valence Pt1 and Pt2+···Pt2+
1+
dimers still present. The H2O molecules in the first and second
1+
coordination spheres persist bound to Pt1 during reaction, as
assessed by comparison of the FT–IR spectrum of 3 with
samples treated with D2O or H218O during 3 h at 150 ºC and then
evacuated under vacuum (Figure S20). These results illustrate
1+
the robustness of Pt1 stabilized by water clusters within the
MOF during reaction.
The WGSR with labelled H218O as reagent, in batch, forms
mainly C18O2 when catalyzed by 3 (Table S3–4 and Figure S21).
This striking result can only be explained by the incorporation of
two water molecules in the final CO2 product. One is tempted to
think that a scrambling of the CO and water oxygen atom occurs.
However, GC–MS shows <1% of isotopically labelled C18O in
the reaction atmosphere. Indeed, kinetics with 10 times excess
of CO respect to H218O show that C18O2 and regular H2O are still
formed from the very beginning of the reaction, and as the
reaction progresses, regular CO2 and mixed CO18O are formed
statistically, depending on the ratio H2O/H2O18 in the media. The
final mixture contains >50% of isotopically labelled C18O2, a
much higher proportion than expected for a thermodynamic
mixture at these reaction conditions (isotopic molar ratio CO2:
CO18O: C18O2 10:6:1) and more similar to a reaction going
through a long–lived formate or orthoformate intermediate (1:3:5,
Figure S22). 13C magic angle spinning solid nuclear magnetic
resonance (MAS NMR) of 13CO co–adsorbed with water on 3
(CO:H2O 1:2 equivalents respect to Pt) into a sealed rotor and
heated at increasing temperatures, shows the progressive
disappearance of the 13CO signal at expenses of a new signal at
156 ppm, which progressively transforms into 13CO2 when
heating (Figure S23). The signal at 156 ppm also appears in
cross–polarization NMR, which indicates that the 13C atom has H
atoms at 1–2 bond distance, and the NMR value fits that
expected for an intermediate orthoformate. These results
indicate that H2O attacks twice to CO to form the intermediate
orthoformate, which would collapse into CO2 after scrambling the
oxygen atoms of CO and H2O (Figure S24).
30
20
10
0
2
3
Pt(LTA)
Pt(MOR)
Pt2@MOF
Pt/C
50
100
150
Temperature (ºC)
Figure 4. Temperature–programmed WGSR catalyzed by 3 and other Pt–
supported catalysts in–flow.
Figure 4 shows the catalytic results for 2, 3, and other Pt–
supported catalysts for the low–temperature WGSR in
flow(weight hourly spatial velocity, WHSV= 60 000 ml gcat h-1).
MOF 3 catalyzes the reaction from 50 to 200 ºC with a steady
increase in TOF (calculated as mol of CO2 per mol of Pt1 and
hour) from 5 to 108 h-1. In contrast, the rest of Pt–supported
catalysts tested, which includes Pt on carbon, Pt reduced in the
small pores (ca. 0.8 nm) of zeolite KLTA[17] and mordenite
(MOR), a related MOF structure with Pt0 dimers,[18] and MOF 2,
-1
1+
The experimental kinetic rate equation found for 3 is
v0=kapp[CO][3]
for
>0.1
equivalents
of
water
and
v0=kapp[CO][H2O][3] for lower water amounts, and no kinetic
isotopic effect (KIE, ≈1.0) occurs when using D2O as reactant.
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