Reversible restructuring of supported Au nanoparticles during butadiene hydrogenation revealed by operando GISAXS/GIWAXS

Article abstract of DOI:10.1039/c7cc01887h

Periodically arranged, monodisperse gold nanoparticles supported on flat silicon substrates were studied for the hydrogenation of 1,3-butadiene under operando conditions using Grazing Incidence Small- and Wide-Angle X-ray Scattering (GISAXS/GIWAXS). It was found that the composition and shape of the nanoparticles depends very much on the chemical environment; the particles are shown to be dynamic, undergoing reversible size and shape change particularly during catalytic reaction, highlighting a dynamism often not observed in traditional studies. Specifically, the size of the Au nanoparticles increases during butadiene hydrogenation and this is attributed to the partial removal of a Au₂O₃ at the metal-oxide interface and consequential shape change of the nanoparticle from a more hemispherical particle to a particle with a larger height to width ratio.

Full text of DOI:10.1039/c7cc01887h

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Odarchenko, D. Decarolis, J. Herbert, T. Arnold, J. Rawle, C. Nicklin, H. Boyen and A. M. Beale, Chem.
Commun., 2017, DOI: 10.1039/C7CC01887H.
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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc
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3
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=
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Reversible restructuring of supported Au nanoparticles during
butadiene hydrogenation revealed by operando GISAXS/GIWAXS
Received 00th January 20xx,
Accepted 00th January 20xx
1
,2,3
1,2
1,2
David James Martin , Donato Decarolis , Yaroslav I. Odarchenko , Jennifer J.
1,2,4
5
5
5
6
Herbert , Thomas Arnold , Jonathan Rawle , Chris Nicklin , Hans-Gerd Boyen and
DOI: 10.1039/x0xx00000x
1
,2*
Andrew M. Beale
www.rsc.org/
Periodically arranged, monodisperse gold nanoparticles supported hydrogenation reactions amongst others over gold
4
on flat silicon substrates were studied for the hydrogenation of nanoparticle catalysts . However, there are a number of other
1
,3-butadiene under operando conditions using Grazing Incidence pertinent examples of size/support-activity relationships in the
Small- and Wide-Angle X-ray Scattering (GISAXS/GIWAXS). It was literature concerning a range of catalysts, for example cobalt
found that the composition and shape of the nanoparticles based Fischer-Tropsch catalysts and even palladium catalysts
5
–7
depends very much on the chemical environment; the particles for 1,3-butadiene hydrogenation . As a common undesired
are shown to be dynamic undergoing reversible size and shape side product in C streams from naphtha crackers, butadiene
change particularly during catalytic reaction, highlighting can be ‘upgraded’ via hydrogenation over catalysts such as
4
a
dynamism often not observed in traditional studies. Specifically, Au/Pd on oxidic supports. Previous research has hinted at both
the size of the Au nanoparticles increases during butadiene a potentially complex support and particle size dependency
hydrogenation and this is attributed to the partial removal of a due to contrasting reports (of particle size dependency and
Au
2
O
3
at the metal-oxide interface and consequential shape independency), and theoretical studies suggest a Horiuti–
molecules adsorbing
change of the nanoparticle from a more hemispherical particle to Polanyi type reaction process with C
4 6
H
6
,8,9
a particle with a larger height to width ratio.
more strongly at the (100) surface than at the (111)
.
11
1
0
However in the many reports detailing size and/or shape
Heterogeneous catalysts comprising oxide-supported noble
metal nanoparticles (NPs) are the backbone of the chemical
and fuel industries. Obtaining a better understanding of the
properties required of an active catalyst is routinely
investigated in the belief that the traits of an active catalyst
can be identified and translated to yield improved catalytic
materials/catalytic processes. Size and shape dependent
effects such as surface atom fraction and quantum
confinement have been proposed to affect nanoparticle
reactivity, yet there is still a considerable debate within the
community as to the importance/influence of nanoparticle size
effects of nanoparticles, there often lies a problem where the
preparation method yields broad particle size distributions
where the standard deviation ‘σ’ is large (greater than 1 nm).
Therefore, the identification of an ideal particle size for
catalytic reactions has proven challenging and any proposal for
or against particle size/shape sensitivity could at best be
considered partial. Furthermore, in many cases, commonly
used colloidal methods such as those used by Rogers et al to
prepare catalysts appear to offer good control over particle
size (σ ~ 0.6 nm), however, they can also yield secondary (and
1
2
often highly active) sub-nanometer atomic species . Thus, in
order to properly assess the importance of a particular particle
size, more advanced preparation methods are required, such
as those based on reverse micelle encapsulation pioneered by
1
and shape for specific catalytic reactions . Haruta and
Hutchings initially demonstrated the catalytic activity of
supported gold nanoparticles for oxidation of CO and
2
,3
1
3
hydrochlorination of acetylene. A size-activity relationship
has since been shown to exist for oxidation, epoxidations and
Spatz and co-workers. This method yields nanoparticle
populations with extremely small particle size distributions (
< 1 nm) and at the same time by ensuring full metal ion
σ
<
encapsulation, preventing the occurrence of sub-species.
Recently we demonstrated that it is possible to combine this
aforementioned synthesis technique with nano-beam X-ray
Absorption Spectroscopy (XAS) to study the behaviour of just
2
0 nanoparticles with an extremely small particle size
distribution during CO oxidation, almost eliminating statistical
1
averaging effects associated with standard XAS .
4
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four interference peaks in an idealized hexagonal lattice with
the same lattice parameter and particle size distribution.
Fig. 2 (c, d) show the reduced and fitted (using the
distorted-wave Born approximation (DWBA)) 1D GISAXS
profiles under various conditions. All horizontal profiles (at
-1
z
constant q =0.63 nm ) have a main interference (10) peak at
-
1
0
.095 nm (Fig. 2c) that corresponds to the in-plane Au NP
Fig.1 Schematic representation of the model used to fit the GISAXS data for
hexagonal arrangement as shown in Fig. 1. The fitted data
shows this peak corresponds to a real space distance of 76.3
nm (Fig. 2Sb), which is in excellent agreement with the SEM
data (78.8 ± 9.9 nm). Furthermore, the peak is ever-present
throughout the reaction, and therefore there is no evidence of
sintering or Ostwald ripening (inter-particle effects). When the
Au catalyst starts to convert butadiene to but-1-enes and but-
the Au/SiO
support, (b) Au NP hexagonal superlattice with the interparticle distance (Lhex
6.1 nm.
2 2
-Si catalysts; (a) a spherical Au NP submerged in the SiO -Si(111)
)
7
Herein, we demonstrate the benefit of marrying a highly
controlled nano-catalyst preparation procedure with scattering
techniques able to probe nanoparticle behaviour, yielding a
deeper understanding of catalyst behavior under operando
conditions. Specifically, GISAXS and GIWAXS were used to
2
-enes (the two are difficult to distinguish by MS, see Fig. S3),
there is further rise in coherent scattering demonstrated by an
increase in intensity of higher order peaks (Fig. 2c).
Qualitatively, these observed changes could be attributed to
the nanoparticles becoming more identical in some way; akin
uncover
a
previously
unseen dynamic structural
transformation (including size, shape, and crystallinity) of
supported Au NPs under various gas atmospheres and
ultimately during the hydrogenation of 1,3-butadiene under
realistic working conditions.
16
to an inverse melting effect. Furthermore, the Yoneda peak
in Fig. 2d (vertical profile) appears to change shape and
becomes much sharper, shifting to a higher critical angle only
during butadiene hydrogenation (Fig. 2b). Details regarding
GISAXS analysis can be found in the supporting information.
Table S1 summarizes the resulting structural parameters
Operando GISAXS/GIWAXS studies were conducted on I07
at Diamond Light Source, using a dedicated gas flow reactor
1
Fig. S1). The 1x1 cm silicon supported Au nanoparticle
5
2
(
arrays were exposed to various gases at different
temperatures, simulating the industrial catalytic conditions of
butadiene hydrogenation. The Au NP arrays were deposited
for the Au NP arrays on SiO -Si(111) from GISAXS fitting using
2
17
the software package IsGISAXS, based on schematic model
2
onto a Si(111) wafer (with a native SiO layer) using the
shown in Fig. 1. The calculated NPs radius of 4.2 ± 0.1 nm and
height of 4.8 ± 0.4 nm under helium flow are in good
agreement with SEM (R = 4.5 nm) and AFM (H = 6.2 nm) data,
and the radius/height doesn’t significantly change when
exposed to butadiene either. However, during the
hydrogenation of butadiene, the average height of the
nanoparticles increases tremendously, from 4.8 to 9.3 nm (H/R
13
reverse micelle method . The average nanoparticle diameter
as determined by FE-SEM was 9.0 ± 0.9 nm, height as
determined by AFM was 6.2 ± 0.5 nm, and average
interparticle distance in a hexagonal array was 78.8 ± 9.9 nm
(Fig. 1 and Fig. S2) Details regarding the exact methodology are
included within the Electronic Supporting Information (ESI).
Fig. 2 (a) and (b) show experimental and theoretical 2D
operando GISAXS patterns of the Au NP arrays during
butadiene hydrogenation at 548 K. The experimental pattern
displays a number of peaks with the position ratio of
=
2.0). The radius of the NP array only increases by ca. 0.45
nm, meaning that the average particle within the array is
simultaneously growing, and changing shape to have a taller
aspect ratio most probably due to the removal of the gold
Fig. 2 Experimental (a) vs theoretical (b) 2D GISAXS images during butadiene hydrogentation with the diffuse Kiessig fringes typical of NP monolayers with the
-1
-
1
narrow height distribution. (c-d) 1D GISAXS horizontal (q
z
= 0.63 nm ) and vertical (qxy = 0.16 nm ) experimental profiles (points) and corresponding fits (red lines)
using IsGISAXS software of the supported Au NPs under various conditions.
1
: √3 ∶ √4ꢀ: √7ꢀcorresponding to the (10), (11), (20) and (21) oxide layer at the metal-oxide interface. Interestingly however,
reflections of the 2D hexagonal superlattice. The simulated the (10) peak position in Fig. 2c which represents the
pattern using the model from Fig. 1 shows there should also be interparticle distance, does not change, and thus we can rule
out Ostwald ripening as the cause of mass nanoparticle
2
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growth. After the reaction, the atmosphere is switched to become mobile at the Hüttig temperature (T
H m
=0.3T =400 K)
helium, and accordingly, the nanoparticles return to a similar and bulk atoms can start to move by reaching Tammann
1
2
size and height as before the reaction (H/R = 1.15).
temperature (T
T m
=0.5T =600 K). Taking into account that the
GISAXS alone cannot give a complete picture, nor reveal Au/SiO
2
-Si catalyst was heated up to 548 K the solid-state
entirely why the Au NP arrays change shape so drastically, and diffusion of gold atoms can play an important role during
therefore it is useful to collect wide-angle scattering data also. catalysis. Since the butadiene hydrogenation is an exothermic
Fig. 3a shows the operando GIWAXS data under different gas reaction it can result in the temperature rise at the surface of
atmospheres and temperatures, and illustrates the variation in AuNPs that further accelerates the atoms mobility and can
-1
the (200) diffraction peak at q = 3.12 Å , which corresponds to help further explain the observed reversible structural changes
metallic Au (Fm-3m (225), ICSD PDF: 01-071-3755/4-784). in Au/SiO -Si catalyst.
From Fig. 3b, one can see that area of the (200) reflection is It is known that butadiene hydrogenation is facet selective
2
highest during butadiene hydrogenation, and in fact lowest and therefore the expression of a particular facet during
during hydrogen treatment (Table S2). In addition to the signal catalysis could be expected, similar to other reports, e.g. DFT
22
from metallic gold, a relatively weak diffraction peak at q = studies on CO absorption on Au. However, butadiene and
-1
1
.81 Å was observed (Fig. 3c) and indexed as the (111) hydrogen adsorb relatively weakly on Au surfaces (0.01-0.1
23,24
1
8
reflection of the orthorhombic unit cell of Au O . We believe eV), as summarised by Bu or Sault
2
, yet in our findings a
occur according to
3
this signal originates from an oxide layer formed at the signifcant structural changes
interface between gold and the native SiO layer of the Si GISAXS/GIWAXS data. DFT calculations have shown that H2
2
substrate, whose existence was also confirmed on the Au/SiO2- cannot dissociate on gold surfaces alone, and in fact this
Si(111) sample after the reaction using ex situ XPS (Fig. S4, dissociation occurs at the interface between low coordinated
2
5–27
table S3 and discussion on XPS in ESI). Fig. 3c shows that at the Au nanoparticles and support
. In simple single atom DFT
fixed temperature of 473 K the area of the (111) reflection studies, which represent undercoordinated species well,
decreases by 30 % during butadiene conversion and increases metallic Au is unlikely to be present on oxide surfaces (in the
III
again after the reaction (post He and post He (RT)). The study by Liu, ZrO is used), and in fact Au is expected to be
2
2
8
observed changes can be explained by the partial removal of the dominant species . This is in accordance with our
the oxide layer at the Au-SiO interface and correlate well with detection of an Au O layer. At elevated temperatures, DFT
2
2 3
III
I
the increase in intensity of the metallic Au (200) reflection calculations expect Au to be reduced to Au . Experimentally
during hydrogenation (Fig. 3b). Our hypothesis is also however, this possible subtle shift in interfacial species does
supported by the GISAXS fit results showing that the contact not produce any noticeable change in our scattering data at
surface area between the NPs and the support decreases as elevated temperatures alone or in the presence of
shown by the H/R ratio in Table S1. After the reaction, the butadiene/helium (Fig.
2 and 3). However, upon the
Au O layer recovers as indicated by the GIWAXS data in Fig. 3c introduction of hydrogen in addition to that of butadiene,
2
3
whereby an observed increase in the Au O (111) peak occurs. there occurs what we believe is a reversible mass transport
2
3
The subsurface gold oxide buried in the oxide support can also phenomenon, fueled by the exothermic catalytic reaction itself
1
9
contribute to the (111) reflection. As this phenomenon providing the energy for the transformation (Fig. 4).
appears to be reversible, using other, non-operando Theoretical calculations show that the hydrogenation of
techniques would not be able to identify these never-before- butadiene also causes gold-oxygen lattice (Au-Olatt) bonds to
2
8
seen changes.
be broken, which could fuel interfacial reconstruction.
As shown experimentally by Nolte et al, reversible facet
their size. For 9 nm AuNPs the melting temperature was growth/shrinking caused by intraparticle mass transport is one
reported to be below 1200 K that is ca 140 K lower than T of the main reasons behind shape and size change changes in
The melting temperatures of solids strongly depends on
2
0
m
2
0
29
measured for the bulk material. However surface atoms Rh nanoparticles. Therefore in our case it is proposed that a
2
Fig. 3 (a-c) Changes in metallic gold content in the Au/SiO -Si catalyst during butadiene hydrogenation monitored under operando conditions by GIWAXS. (a) 1D
GIWAXS profiles showing the (200) reflection of Au fcc lattice. Histograms showing calculated peak areas (b) under various gas atmospheres, using a Voigt
function for peak fitting. (d) 1D GIWAXS profiles of the (111) reflection of the Au
This journal is © The Royal Society of Chemistry 20xx
2 3
O crystal lattice.
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The authors would like to thank the EPSRC for funding (grant
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Giuseppe Portale (University of Groningen) is also thanked for
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