Journal of the American Chemical Society
Article
would affect certain reaction steps, for example, by lowering a certain
barrier. Trends among catalysts can be studied through the differences
in reaction energy (ΔΔE) for each intermediate and transition state
for a catalyst with respect to a reference catalyst. The procedure is
Catalyst Preparation. CeO was synthesized by calcination of
2
Ce(NO ) ·6H O (Sigma-Aldrich, 99.99%) in air at 623 K for 2 h.
3
3
2
70
MgO was synthesized via a xerogel route. In a typical synthesis, 100
mL of a Mg(II) methoxide solution (7−8% in methanol, Sigma-
Aldrich) was diluted in 1 L of toluene (technical grade). Next, Milli-Q
treated water was added dropwise at RT, and the suspension was
stirred overnight to enable a complete hydrolysis of the alkoxide
precursor. The resulting gel was collected by centrifugation, dried at
4
3
5
53 K, and calcined under air flow in a fixed-bed reactor at 773 K for
h (heating rate, 2 K/min). For the synthesis of oxide-supported Rh
catalysts, Rh(III)-acetylacetonate (Sigma-Aldrich, 97%) was dissolved
in acetone and impregnated on the oxide support in a rotary
evaporator (430 mbar, 313 K). The solid was further dried at 353 K in
an oven overnight and ground into a powder. The resulting powder
was calcined in an uncapped porcelain dish (50 mm diameter) at
1
073 K under a stagnant air atmosphere for 10 h using a temperature
ramp from RT of 2 K/min. Finally, the catalyst was ground and sieved
to retain particles in the 100−200 μm size range. The metal content
was adjusted to achieve a preset surface-specific Rh content (after the
2
high-temperature annealing treatment) of 1.0 ± 0.2 M /nm .
at
Catalyst Characterization. Nitrogen physisorption isotherms
were recorded at 77 K using a Micromeritics 3Flex V4.04 instrument.
Prior to the measurement, samples were dried at 423 K under vacuum
for 5 h. Specific surface areas were derived using the BET method in
the relative pressure (P/P ) regime of 0.05−0.30. Powder X-ray
0
diffraction patterns were collected on a Stoe STADI P transmission
diffractometer equipped with a primary Ge(111) monochromator
(
Mo Kα ) and a position-sensitive detector. Samples were filled into
1
glass capillaries (Ø = 0.5 mm). Data were collected in the 2θ range
between 5° and 50° with a step width of 0.015° and a measuring time
per step of 20 s. For each sample, 8 scans were collected and summed
up. High-angle annular dark-field (HAADF) micrographs were
acquired using a Hitachi HD-2700 dedicated scanning transmission
electron microscope (STEM) with spherical aberration correction,
equipped with a cold field-emission gun and two EDAX Octane T
Ultra W EDX detectors and operated at 200 kV. Powder samples were
dry-cast on Cu grids coated with a lacy carbon film prior to
observation. X-ray absorption spectra were recorded at the RhK edge
(23.220 keV), at the CLÆSS beamline station (BL22) of the ALBA
synchrotron light source, Barcelona (Spain). The beam was
monochromatized using a (311) double crystal monochromator,
and harmonic rejection was performed using Pt-coated silicon
mirrors. Samples were mounted in a multipurpose gas−solid cell
equipped with Kapton windows, and measurements were performed
at room temperature in a fluorescence mode using a fluorescence
solid-state Silicon Drift detector. Reference metal oxide materials were
ground and diluted in powder boron nitride and shaped into pellets
Figure 2. Correlation of the PBE-D3 and the DLPNO−CCSD(T)
approach: (a) Definition of ΔΔE as differences in reaction energy
between different catalysts from intermediate 2 to all other
intermediates; the reaction to itself is not included. (b) Parity plot
showing the correlation of ΔΔE for the two methods employed.
(
Ø = 31 mm) with optimized thickness and measured in transmission
mode employing ion chambers filled with appropriate gases in order
to adsorb 15% and 80% in the I and I , respectively. At least 3 scans
shows how ΔΔE is derived). In Figure 2b, ΔΔE obtained for PBE-D3
is compared to differences calculated with the DLPNO−CCSD(T)
approach in a parity plot. As can be seen from Figure 2, there is
generally a good correlation between ΔΔE for PBE-D3 and
DLPNO−CCSD(T) with a mean absolute error (MAE) of only 5.4
kJ/mol. Calculations employing PBE-D3 are hence able to predict
differences in reactivity between different catalysts. Importantly, the
trends between different catalysts are thus well described by DFT
while absolute values require higher level methods. The fact that
differences in the energetics between different materials are predicted
well by PBE-D3 can be exploited to obtain accurate absolute values
for barriers and reaction energies. If the energies are available at the
DLPNO−CCSD(T) level of theory for a given catalyst, predictions
for other catalysts can be made based on the energy differences
computed with PBE-D3. This approach was applied for Rh1/
MgO(301) and Rh /CeO (111) using Rh /MgO(100) as a reference
0
1
were acquired to ensure spectral reproducibility and good signal-to-
noise ratio. The same cell was applied to study the Rh /CeO catalyst
during and after exposure to a syngas reaction atmosphere. The
1
2
catalyst sample was exposed to a continuous syngas (H /CO = 1)
2
flow of 50 mL/min at atmospheric pressure, and the temperature was
increased from RT to 373 K. XANES spectra were recorded in a
temperature-resolved manner in fluorescence mode through the entire
temperature range, while the EXAFS spectra were recorded, at room
temperature, both prior and after the treatment to study changes in
the coordination environment of the Rh sites. Data reduction and
extraction of the χ(k) function has been performed using Athena
code. EXAFS data analysis has been performed using Artemis
71
(Demeter software package).
Catalyst Testing. Catalytic tests were performed in a stainless
steel autoclave reactor hosting a 20 mL PTFE liner. The powder solid
catalyst (0.002 mmol, Rh basis), 5 mL of solvent, typically 1-octane
1
2
1
catalyst. In these cases, the energies were obtained as
SAC
predicted
ref
SAC
PBE‐D3
ref
PBE‐D3
(>99%, Sigma-Aldrich), and 1 mmol of olefin substrate were added to
E
= E
+ E
− E
DLPNO−CCSD(T)
(2)
the reactor. For experiments requiring time-resolved sampling from
the reactor, the initial olefin concentration and solvent volume were
increased to 5 mmol and 15 mL, respectively, maintaining an olefin/
C
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX