(727f) In Situ XAS/SAXS Study of Al2O3 Coated Pt-Ga/MgAl2O4 Catalysts for Alkane Dehydrogenation

In
situ XAS/SAXS study of Al₂O₃ coated PtGa/MgAl₂O₄
catalysts for alkane dehydrogenation

Nadadur Veeraraghavan
Srinath^1*,
Hilde Poelman¹, Alessandro Longo^1,3, Ji-Yu Feng²,
Jolien Dendooven², Marie-Françoise Reyniers¹, Vladimir V.
Galvita¹

¹Laboratory
for Chemical Technology (LCT), Ghent University, Technologiepark 125, B-9052,
Ghent, Belgium;

²Dept. Of
Solid State Sciences, CoCooN group, Ghent University, Krijgslaan 281/S1, 9000, Ghent,
 Belgium;

³Istituto per
lo Studio dei Materiali Nanostrutturati (ISMN), CNR, UOS Palermo, Via Ugo La
Malfa, 153, 90146 Palermo, Italy font-family:" calibri>.

*Email address: N.V.Srinath@Ugent.be

Introduction

normal">Platinum is
the most effective transition metal to catalyse alkane dehydrogenation to
alkene in chemical industry¹. Addition of promoters
such as Sn and Ga generally improves activity, selectivity and resistance to
coke formation^2,3. PtGa outperforms its
monometallic counterparts due to alloying of Pt and Ga³. Despite significant
improvements of the catalytic performance after alloying, this bimetallic
catalyst still suffers from coke formation and sintering. Regeneration of the
catalyst is usually employed to remove the coke formed on the catalyst by
feeding O₂. This is an exothermic reaction, which can in itself be
cause for sintering. Alternatively, CO₂ can be used as a mild
oxidising agent.

normal">As an advanced
strategy to stabilize the bimetallic nanoparticles (NPs) against sintering, an
alumina (Al₂O₃) coating layer is applied by Atomic Layer
Deposition (ALD), a powerful tool for controlled modifications of solid
surfaces. Providing a coating such as carbon⁴ " calibri>, Al₂O₃⁵ " calibri>, over an active metal surface has shown propensity to
increase the catalyst stability due to significant reduction in sintering of
the metal.

normal">A few
questions remain for such a coated system, especially for bimetallic catalysts.
In order to make the alloyed catalyst accessible to the gas feed, the coating
layer needs to be porous, hence, it requires a calcination treatment, e.g.
under N₂, to crack the layer. A first question is, if there is
access for the reactive gases to the active sites? Is there interaction between
the alloyed elements and the coating during the REDOX process? And in
reaction/regeneration, does the layer prevent sintering?

normal">To examine behaviour
of coated versus uncoated PtGa catalysts, a combined study of in situ XAS and
SAXS was undertaken. SAXS can track the particle size during treatments. Unlike
XRD or XPS, the XAS technique can provide detailed structural and electronic
information about the local environment of both elements in the PtGa NPs under
the Al₂O₃ coating. To examine the PtGa alloy
rearrangement in changing conditions, reduction, regeneration, cycling, XAS is
a necessity as the targeted PtGa particles are small.

Experimental

margin-left:0cm;text-align:justify;line-height:normal">Catalyst preparation:
Preparation of the MgAl₂O₄ support was done by
co-precipitation, similar to the recipe reported by Theofanidis et al⁶. Pt was
loaded onto MgAl₂O₄ by incipient-wetness impregnation
using an aqueous solution of H₂PtCl₆. Ga was sequentially
impregnated on Pt/MgAl₂O₄ in order to form the PtGa/MgAl₂O₄
catalyst. The materials were dried at 393K for 4h and subsequently calcined in
air at 883K for 5h (as-prepared state – uncoated).

margin-left:0cm;text-align:justify;line-height:normal">Prior to coating by
Al₂O₃, the catalyst was reduced under H₂ flow at
878K in order to form the PtGa alloy. The Al₂O₃ coating
was then applied by means of Atomic Layer Deposition (ALD) with trimethylaluminum
(TMA) and water as precursors. The coating thickness depends on the number of TMA
cycles. In this work, a 1 nm coating was applied, obtained after 10 ALD cycles.
Each cycle is alternated with a water vapour cycle which reacts with the TMA
and hydrolyses the residual methyl groups⁷. The
coating was cracked using an inert calcination in N₂ at 973K (as-prepared
state - coated).

In situ XAS/SAXS: In situ XAS
(X-ray absorption spectroscopy) experiments were performed at the Pt L_III
edge (11564eV) at the DUBBLE beam line of ESRF. SAXS (small angle X-ray
scattering) was measured in parallel to XAS, in transmission, so as to monitor
the particle size variation during each treatment.

Catalyst testing: Propane
dehydrogenation reaction to propylene (PDH) was performed at 873K in a
plug-flow fixed-bed quartz tube reactor, with an internal diameter of
7 mm mounted in an electric furnace. All catalysts (~150 µm) were first
reduced in situ in 10% H₂ flow at 873K. Subsequently, the
catalysts were tested for activity measurements. A feed flow of propane,
hydrogen and argon was used with ratio of 1:1:3 and total flow of 200 Nml/min.

Results

Activity tests: Both
catalysts were tested for activity. The uncoated catalyst showed a better
performance in terms of both conversion and selectivity. This could indicate
that the coating may be blocking access of the gas to the active sites.

margin-left:0cm;text-align:justify;line-height:normal">XAS/SAXS: The in situ XAS experiments
allowed comparing the coated and the uncoated sample (PtGa/MgAl₂O₄).
The as-prepared uncoated sample has a high Pt white line, close to PtO₂,
reflecting its calcined state. As for the coated sample, the Pt white line is
lower but higher than after reduction. This indicates that Al₂O₃
deposition and subsequent inert calcination change the PtGa alloy towards a
partially oxidized state, figure 1.

margin-left:0cm;text-align:justify;line-height:normal">H₂-TPR
was applied up to 873K to re-form the PtGa alloy, the active phase for propane
dehydrogenation (PDH). Pt underwent complete reduction for both samples (based
on linear combination fitting with PtO₂ and Pt foil standards).

margin-left:0cm;text-align:justify;line-height:normal">CO₂-TPO was
applied to determine the effect of CO₂ on the catalysts. For
uncoated PtGa/MgAl₂O₄, Pt unexpectedly tended to
partially oxidise based on the increase in white line intensity, in contrast to
the monometallic Pt/MgAl₂O₄. However, the increase in
white line intensity upon CO₂-TPO is lower for the coated sample,
figure 1.

margin-left:0cm;text-align:justify;line-height:normal">Cycling experiments
(4 cycles CO₂/H₂, 2 cycles O₂/H₂)
were performed, mimicking reaction and regeneration, terminating with the H₂-reduction
step. After these REDOX cycles, the Pt white line intensity for the coated
sample is the same as after the H₂-TPR step, whereas for the uncoated
sample it has slightly changed. This indicates that the coating retains the
original alloyed state.

margin-left:0cm;text-align:justify;line-height:normal">The SAXS data were treated
using a monodisperse model with the assumption of spherical particles. This SAXS
modelling gave the average radii of the particles upon completion of each treatment
(Figure 2 a & b). The variation of particle size for the coated sample is
lower than for the uncoated sample which indicates that it is more stable.

Figure 1. XANES spectra after
each treatment for a:
PtGa/MgAl₂O₄ (uncoated) catalyst & b: Al₂O₃/PtGa/MgAl₂O₄
catalyst.

Figure 2. SAXS data after
each treatment for a:
PtGa/MgAl₂O₄ (uncoated) and b: Al₂O₃/PtGa/MgAl₂O₄
(coated) samples. Inset: The average particle size is the radius obtained with
95% confidence interval.

Ga₂O₃
and Al₂O₃ interaction is a possibility based on work by
Afonasenko et al⁸. Hence it was important to
determine if there is any interaction between the coating and the Ga on the
catalyst surface. An in-situ XRD test was performed on Ga₂O₃/Al₂O₃
(17nm & 10nm thickness respectively) bilayer while subjecting this sample
to a reducing, oxidising and an inert environment at 1073K. The tests showed no
clear evidence of any separate phases being formed.

Conclusions

All the data discussed lead
to the following conclusions:

-18.0pt;line-height:normal">·       Access to the active phase
is possible in presence of the overcoat after the inert calcination.

-18.0pt;line-height:normal">·       Based on testing under
different gaseous environments there is no evidence found for any new phase
formation between the Ga₂O₃ and Al₂O₃
coat.

-18.0pt;line-height:normal">·       Reduction and oxidation leads
to formation and decomposition of the alloy. The information on the extent of
alloying is contained in the EXAFS signal.

-18.0pt;line-height:normal">·       The SAXS results point to a
prevention of sintering of the coated catalyst when compared with uncoated
catalyst.

The combined application of
XAS and SAXS is useful to follow the structural changes occurring to the
catalyst during the reaction.

Acknowledgements

We would like to thank
members of the DUBBLE beamline at ESRF for their help. FWO is acknowledged for
funding of travel and subsistence for the DUBBLE beam time (26-01-1169).

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