Quick low temperature coalescence of Pt nanocrystals on silica exposed to NO-the case of reconstruction driven growth?

Article abstract of DOI:10.1039/c3ra48078j

We report an operando XRD/MS experiment on nanocrystalline Pt supported on silica, monitoring quick growth of Pt in an NO atmosphere. XRD data following NO flow show structural changes typical of surface reconstruction. Although a small pressure of NO lifts surface reconstruction of a clean surface, a higher pressure of NO causes the surface to reconstruct again. TEM photographs show formation of clusters of Pt nanoparticles, pointing to growth controlled by coalescence. As surface reconstruction is the phenomenon that we always see accompanying quick growth, we postulate a mechanism of self-lifting cyclic reconstruction that drives the transport of whole clusters and their fusion. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c3ra48078j

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Quick low temperature coalescence of Pt
nanocrystals on silica exposed to NO – the case of
Cite this: RSC Adv., 2014, 4, 14758
reconstruction driven growth?
Zbigniew Kaszkur,* Bogusław Mierzwa, Wojciech Juszczyk, Piotr Rzeszotarski
and Dariusz Łomot
We report an operando XRD/MS experiment on nanocrystalline Pt supported on silica, monitoring quick
growth of Pt in an NO atmosphere. XRD data following NO flow show structural changes typical of
surface reconstruction. Although a small pressure of NO lifts surface reconstruction of a clean surface, a
higher pressure of NO causes the surface to reconstruct again. TEM photographs show formation of
clusters of Pt nanoparticles, pointing to growth controlled by coalescence. As surface reconstruction is
the phenomenon that we always see accompanying quick growth, we postulate a mechanism of self-
lifting cyclic reconstruction that drives the transport of whole clusters and their fusion.
Received 31st December 2013
Accepted 7th February 2014
DOI: 10.1039/c3ra48078j
www.rsc.org/advances
stepwise growth of Pt nanocrystals at temperatures of 353 K and
above, controlled at a given temperature by the time of exposi-
1
Introduction
Due to use of Pt in automotive catalysts for the abatement of tion to NO. During this process, transmission electron micros-
NO from engine vehicle exhausts, the interaction of NO with Pt copy shows clearly separated Pt crystallites forming clusters on
x
surfaces is of great technological importance. Furthermore, the way to growing to a larger size.
high losses of Pt during commercial plant nitric acid produc-
tion still have a high cost problem and the reaction runs at a supported on g-Al
substantial NO concentration. In all these processes Pt surface effect, if support related, the difference between the oxide
A rapid sintering in NO has been already observed for Pt
9
O at 473 K, which may suggest that for the
2 3
9
reconstruction phenomena play a signicant but not fully supports is irrelevant. For Pt/Al
2
O
3
the authors suggest a Pt
understood role. In explaining the atomistic mechanisms of atom transport mechanism through emission (vaporization)
these processes a crucial contribution was achieved by surface from small Pt clusters – a mechanism resembling Ostwald
science vacuum techniques. There is, however, growing aware- ripening. The suggested migrating species are volatile NO-
ness that to fully understand practical reactions, normal to high containing Pt complexes. This may add to the understanding of
pressure experimental techniques are needed and that high the experimental observation of weight loss of a platinum gauze
pressure conditions make a difference for the atomistic picture during ammonia oxidation approaching a constant value at
1
10
of the adsorbate layer. A range of helpful techniques have been temperatures falling from 1170 K to 1000 K. During ammonia
2,3
developed in recent years (e.g. frequency sum generation, high oxidation, NO is an intermediate product interacting with O
pressure STM ) but the versatile and direct technique of and the catalyst. The loss occurs in an oxidizing environment
2
4,5
11
powder diffraction seems to be undervalued and neglected in and may resemble the weight loss of Pt foil heated in air,
surface studies. However, XRD, when applied in situ to nano- attributed to volatile PtO . Oxygen, however, at 473 K affects Pt
crystalline materials, can provide sensitive data that to a large crystal growth with a much lower rate of the process than NO. It
extent relates to the crystalline surface and can be well under- is expected that the Pt emission at a given temperature should
stood and interpreted with little help by atomistic simula- be more likely for Pt nanoclusters than for larger crystals. The
2
9
6,7
tions. Using this technique, for example, we have shown the literature offers not much data on possible complexes of Pt
time evolution of the surface reconstruction of Pt during containing NO although some unstable nitrosyl complexes are
desorption of H and the magnitude of the measured response likely to exist. Our experiments show rapid sintering at
2
8
ꢀ
could be scaled to the particle size.
temperatures as low as 353 K (80 C) and the rate rather weakly
We present here an operando XRD/MS study of the sintering depends on temperature up to 473 K. Above this temperature no
of Pt nanocrystals supported on silica in an NO atmosphere and further sintering was observed. We observe a rapid sintering of
at moderate temperatures. Surprisingly we observe quick, metal nanocrystals supported on silica in NO atmosphere for Pt
but never for Pd metal. It also has not been observed in a CO
ꢀ
atmosphere. During Pt/SiO exposure to NO at 80 C dissocia-
2
Institute of Physical Chemistry PAS, Kasprzaka 44/52, 01-224, Warszawa, Poland.
2 2
tion of NO leads to a noticeable evolution of N O and O .
E-mail: zbig@ichf.edu.pl; Fax: +48 223433450; Tel: +48 223433284
14758 | RSC Adv., 2014, 4, 14758–14765
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
Our results suggest the sintering mechanism of Pt on silica number of top layer Pt atoms. The NO atoms from the depleted
slowly changing with temperature, at the lowest temperatures chemisorbed layer can thus move to more energetically
(
353 K), is a whole cluster transport followed by cluster coales- favourable bridge bonded sites. With further temperature rise
20
cence, which is the rate determining process. This observation the layer is ordered and nally NO desorbs.
contradicts the general opinion that coalescence can be a rate
determining process only at high temperature, with lower One can expect that at high NO pressure maintained during the
The above experiments concern, however, in vacuo evolution.
12
temperature growth controlled by diffusion. Since a moderate heating the scenario can differ as the adsorbate evolution
20
increase of temperature triggers further particle growth, the depends on the starting pressure during initial sorption. With
pore structure of the support here is not an important limiting high pressure and high frequency collisions of NO with the
factor and crystallites are allowed to migrate to grow by coa- surface, frequent adsorption–desorption phenomena may
lescence upon collision. This unexpected mobility and sintering signicantly modify the above picture. Using arguments from
may be ascribed to a specic character of NO chemisorption.
XRD analysis of a Pt surface we postulate here that the surface
NO has a similar electronic structure to CO but with an extra reconstruction occurs for Pt nanocrystals at temperatures of
unpaired electron in its 2p* orbital. The interaction with a 353–423 K under close to atmospheric pressure of NO. On
transition metal surface is via hybridization of molecular 5s missing-raw reconstruction the number of Pt atoms exposed to
levels with unoccupied d states (sigma donation) and hybrid- chemisorption per surface unit increases but the number of top
ization of molecular 2p* levels with occupied d states (pi layer Pt atoms decreases. This may weaken the adsorption,
backbonding). The latter effect dominates for Pd and Pt.
leading to partial desorption of NO. An increased strength of the
NO may dissociate at the Pt crystallite surface even at room remaining NO bonds may be, however, sufficient to li the
13
temperature but this ability strongly depends on the crystal reconstruction. Such a feedback phenomenon may lead to a
face exposed. The dissociated species may associate into semi-oscillatory behaviour driving high surface mobility and
desorbing N
to gaseous N
ciative sorption of NO on silica, small amounts of O
together with N O from the metal may cause formation of an proof seems to exist.
NO layer strongly sorbed on the support, weakening adhesion
2
O with low activation barrier and much less likely enable quick coalescence of the collided nanocrystals even at
14
2
.
Although there is no evidence of non-disso- relatively low temperatures. This possibility, to our knowledge,
released has never been exploited in the literature and no experimental
2
2
15
The interaction of supported Pt clusters cannot be explained
2
forces between Pt clusters and the support and reducing the by magnetic forces although NO carries some magnetic
activation energy for surface migration. On the other hand the moment. Bridge site adsorption of NO on (100) facets may
chemisorption of NO on Pd at RT is mostly associative and only effectively quench the magnetic moment of an NO molecule
a small fraction of molecules may dissociate on defects of the whereas a top-site adsorption may leave residual NO magnetic
1
6
metal structure.
moment on Pd(100) and may induce ferromagnetism on Pt and
24
Pd, as well as other 4d metals, differs from 5d metals in that Pd, which are paramagnetic in the bulk form. The Curie
the latter have the ability of the crystal surface being easily temperature for this induced magnetism is, however, predicted
1
7,18
25
reconstructed.
It is, for example, known that the clean (110) to be rather low (#40 K). This and the occurrence of sintering
19
face of Pt undergoes so called missing-raw reconstruction,
at higher temperatures suggests its magnetic origin to be rather
whereas chemisorption of gases lis this reconstruction with unlikely.
the surface atoms returning to their nearly perfect fcc node
positions. Similarly, (001) facets are known to reconstruct quasi-
2
Experimental
hexagonally, forming hexagonal patches on top of the square
18
structure of the bulk. Under chemisorption, for example, of In the present study we have used samples of 10% (wt) Pt on
2
ꢁ1
CO the surface returns to its square form. At low coverage, NO amorphous silica (Davison 62, 100–120 mesh, 300 m g ). The
adsorbs on Pt atoms with the NO bond perpendicular to the material was obtained by impregnation of silica with a required
surface in bridge positions, whereas on-top positions are the amount of chloroplatinic acid using an incipient wetness
most stable at high coverage. With increasing coverage, surface method. It was then mixed and dried. Samples of about 60 mg
sites are populated nonsequentially, moving to more weakly used for measurements were spread over a thin porous sintered
20,21
adsorbing sites.
Due to mutual NO interactions the adsor- glass disc and mounted vertically on the stainless steel heating
bate reorganizes at higher coverage with NO molecules tilting block of our vacuum-proof XRD camera. The environmental
8
from the vertical position, minimizing their intermolecular camera of 150 ml volume was similar to that described before,
22
repulsion. Interacting NO molecules adsorbed at higher with a removable cap with an X-ray window secured by a
pressures and low temperatures form multilayer Pt coverage 12.5 mm self adhesive Kapton foil. It was connected to the gas
with a dense chemisorbed adlayer fed by a more weakly bonded line via Swagelok vacuum ttings and stainless steel exible
(
NO)2 species that slowly desorbs but multi-layer adsorption tubing, enabling XRD measurements in Bragg–Brentano
20
may remain up to 385 K. The mechanism for the incorporation geometry. The outlet gas was collected from the back side of the
of NO to the chemisorbed layer is unknown although the porous glass through the heating block, assuring close contact
phenomenon was observed also for CO on Cu (100). Heating with the sample load. Between camera and the exible tubing a
the adsorbate layer above 200 K causes liing of the initial union tee tting (Swagelok) provided splitting the stream
surface reconstruction, which causes a two-fold increase in the between the output and the probing capillary of a mass
23
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 14758–14765 | 14759

View Article Online
RSC Advances
Paper
ꢀ
2
spectrometer (Hiden Analytical). The gas feeding system con- was dried in He at 50 C for one day and then reduced in H at
sisted of pressured gas bottles and a gas line containing two the same temperature. The reduction progress was controlled
stainless steel manifolds (A and B in Fig. 1) separated from the via XRD showing a slow transition of PtCl phase (JCPDS card
2
bottles’ entries and from a vacuum rotary pump by normally 85-0840) into Pt metal fcc phase. The appearance of the PtCl2
closed on/off solenoid valves (1–12 in Fig. 1). The manifold phase is in agreement with the suggested path of chloroplatinic
outlets were tted with two mass-ow controllers (MKS1259C, acid hydrolysis to PtCl
1
4
(H
2
O)
2
proceeding slowly at RT during
3 and 14 in Fig. 1) and their outlets linked into one steel line the material’s shelf life. Upon drying, the dehydrated PtCl
feeding the camera. The system enabled a clean switch of a having measurable vapor pressure, dissociates when heated,
2
6
4
,
27
2
supplied gas by pumping off the manifolds as well as a quick evolving chlorine and depositing PtCl . In our procedure the
exchange of gases in the camera by evacuating and ushing reduction to metallic Pt was nearly complete aer a few hours
twice with the selected gas. The gases used were helium of but the sample was le in H for one day. Aer the procedure
2
99.999% purity, hydrogen (99.9995%) and NO (99.5% purity few very weak maxima that could not be attributed to known
with N , N O and CO as a principal contaminants). The gas crystallographic phase remained in the diffraction pattern. A
2
2
2
composition was recorded every 1–2 min during repeating conventional catalysis procedure to stabilize the sample by
ꢀ
powder pattern collection in the angular range 20–140 lasting calcination at higher temperatures was applied to another
4
5 min per pattern. The mass spectrometer (MS) sample con- sample and the results compared. At this stage, as well as at the
ꢀ
sisted of m/z scan from 1 to 50 using a Faraday cup detector and end of further 80 C treatment, part of the sample was been
a second scan with an SEM detector over m/z ranges having investigated by TEM (FEI Titan CUBED 80-300) in SEM mode as
detected partial pressure lower than 10 atm. The time of one well as in high resolution.
MS sample depended thus on the gas composition.
ꢁ
7
The temperature was controlled via two RE-15 controllers
(Lumel) and K-type thermocouples tted to the heating camera
block and to the front side of the porous glass disc.
The powder diffraction patterns were collected on a D5000
powder diffractometer (Bruker AXS) equipped with LynxEye
strip detector, providing good resolution and quick data
collection. The Cu Ka radiation was employed with the X-ray
tube operating at 40 kV and 40 mA.
The whole setup was controlled by an integrated Linux
client-server network architecture and the measurements could
be repeatedly performed under the control of measurement
scripts.
3
Results and discussion
28
Williamson–Hall (W–H) analysis of the XRD pattern of the
reduced sample (Fig. 2) results in an average Pt crystallite size of
5.45 nm and a relatively small strain parameter of 0.002. For the
size calculation we have used a Scherrer constant equal to 1.03
resulting from direct comparison of several peak widths of a
diffraction pattern calculated for model fcc metal cubo-octa-
6
hedra with atomistic model size. This size can be compared to
the one from TEM analysis (Fig. 3E). The images reveal quite
uniform and narrow distribution of well separated Pt nano-
crystals with average diameter of 3.56 nm. As XRD peak
broadening analysis refers to the volume weighted average of
the size distribution, the corresponding average is equal to:
The control of gas purity aer clean exchange of gases to He
routinely shows a decaying low level of H escaping from the
2
stainless steel walls of the camera and low background of water.
The oxygen content remained at the level of several ppm. This
level of contamination in the case of nanocrystals has much less
effect than for single-crystals, where 1 ppm of a contaminant
may fully cover its surface. In the nanocrystalline case, the same
level of impurities may affect only small percent of metal
nanoparticles. The as prepared sample loaded into the camera
N
P
4
n
i
i
d
i
i
i¼1
D
V
¼
N
P
3
n
d
i¼1
Fig. 2 Evolution of the 111, 200 and 220 diffraction peaks during Pt/
Fig. 1 The XRD camera gas feeding system used in the experiments.
SiO
2
sample treatment. Legend refers to the pattern number of Fig. 4.
14760 | RSC Adv., 2014, 4, 14758–14765
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
corresponds to less than 2s, which is common in counting
phenomena. However the D values calculated for the presented
V
log-normal distributions are 4.31 nm (2.78 nm the number
average size – red curve) and 5.45 nm (2.88 nm the number
average size – blue curve) respectively, the second one being in
perfect t with the XRD data.
The above discussion points to different information that
can be obtained from XRD and TEM, where the former shis its
focus toward larger crystallite sizes if they exist in the studied
sample. Different average particle sizes from both methods are
thus quite common.
The reduced sample was subjected to the following treat-
ꢀ
ꢁ1
ment at 80 C: (a) exposed to He stream (40 ml min , 3 h); (b)
1
ꢁ1
6 cycles of: exposure to mixture of NO (20 ml min ) and He
ꢁ1
(
20 ml min ) lasting 1 hour followed by ushing with He (40 ml
ꢁ
1
ꢁ1
min ) for 3 hours; (c) ushing with He (40 ml min , 10 h); (d)
ꢁ
1
exposure to NO (40 ml min , 33 h). The temperature was then
increased to 100 C and (e) kept for the next 26 h in NO (20 ml
min ) and He (20 ml min ) till no noticeable sintering was
ꢀ
ꢁ
1
ꢁ1
seen. Subsequently the sample was ushed with He (40 ml
ꢁ
1
ꢀ
min ) and temperature was further increased to 120 C then
the sample was (f) kept in He for a further 13.5 h and (g) exposed
ꢁ1
ꢁ1
to NO (20 ml min ) and He (20 ml min ) for 25.5 h. It was
followed by a He ush (40 ml min ) and further temperature
rise to 150 C with the sample (h) kept for 22.5 h till no sintering
ꢁ
1
ꢀ
ꢁ
1
was noticed and (i) exposed to NO (20 ml min ) and He (20 ml
ꢁ
1
min ) for 12.5 h till no further sintering observed.
Fig. 3 TEM (A and B) and STEM (C and D) images with histograms of
The whole process lasting nearly 9 days was monitored by
powder diffraction and MS with every pattern analyzed similarly
as described before. For each Pt fcc diffraction peak its posi-
size (E and F) for the reduced sample (left) and the exposed to NO at
ꢀ
8
0 C for two days (right). The particle size histograms include fit to a
8
log-normal distribution (E, red curve; F – two fits: black and blue
curve), to a log-normal distribution slightly modified to provide volume tion, intensity (peak amplitude and integral) and width (full
weighted average in agreement with XRD (E – blue curve) and to an
exponential-power distribution (E – green curve, F – circles).
width at half maximum – FWHM) was determined. All the
analyzed parameters resulted from a procedure tting the
8
diffraction peaks to an analytical prole. As the data evolution
for all reections agree qualitatively, to minimize statistical
where n
population of crystallites. For the presented distribution the 220 reections were recalculated to a corresponding value of
lattice constant and averaged. Fig. 4 collects the time evolution
i i
is a fraction of crystallites of size d
in the whole scatter of the parameters, the peak positions for 111, 200 and
D ¼ 4.24 nm.
V
It may seem a meaningful mist, but to put this comparison of these parameters, together with marked composition of the
on a realistic ground one has to notice that the volume average gas and temperatures. Fragments of the 111 peak width evolu-
D
V
quite strongly depends on very small changes of the distri- tion (Fig. 4C) at constant temperature have been tted to the
bution for higher sizes (e.g. d
> 10 nm). On the other hand the rate–power law curves that are further discussed.
histogram data for this region is either missing or prone to
Several phases of the above described gas and temperature
i
substantial error as within the observed eld there are only a treatment have been repeated on fresh samples of the catalyst to
few (if any) larger crystallites and their statistics are poor. It can check repeatability. For the MS gas analysis the same treatment
be a question of the choice of the area looked at as well as a has been applied to the clean porous glass disc mounted in the
simple economy limiting the number of TEM photographs to camera for a blind test.
ꢀ
tens and not to hundreds. To illustrate this point Fig. 3E
together with the measured size histogram of 608 particles only a small amount of NO dissociates with evolution of N
shows two log-normal distributions. Both t well to the central (m/z ¼ 44, Fig. 5). Aer the 3rd cycle O (m/z ¼ 32) appears in
part of the histogram and differ for larger sizes (D > 5 nm) growing quantities, quickly arriving at the level of approxi-
involving only 3% of counted particles. For example, between mately half of the N O emission in agreement with a global
and 6 nm falls only 12 particles and the observed difference stoichiometry of N : O ¼ 1 : 1. The lacking amount of oxygen
between the distributions equal to 40% corresponds to only agrees roughly with the sorption capacity of silica. The intensity
During NO cycles at the beginning of the experiment at 80 C
2
O
2
2
5
5
counted particles. If standard deviation (s) of the counting of the m/z ¼ 32 MS signal has been corrected for a 0.002 fraction
statistics is proportional to the square root of the count number of the m/z ¼ 30 signal due to the MS fragmentation pattern
then the observed difference between the distributions of NO, also involving the m/z ¼ 32 signal. The minimum level of
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 14758–14765 | 14761

View Article Online
RSC Advances
Paper
evolution from the beginning up to the pattern number 130 in
Fig. 4. Note the nonlinearity of the MS sampling rate with time,
the latter being dependent on the gas composition.
Analyzing Fig. 4A one can notice a slight shi of the lattice
˚
parameter (LP) by 0.003 A (on average) upward on cyclic
ꢀ
exposition to NO at 80 C (phase b). Switching the gas
composition from NO to He should not cause quick desorp-
tion of strongly bonded NO at this temperature. For example,
TPD experiments suggest NO desorption occurring above 150
ꢀ
29
C. This is why we do not interpret the upward shi as a
manifestation of the adsorption phenomenon. The visible sign
of the adsorption is an upward shi of the measured LP, in
agreement with a simple model where adsorption saturates
the surface bonds and causes expansion of the contracted
surface layer, increasing the average LP – the model has been
positively tested in our laboratory on many examples. The
downward shi is, however, visible at the beginning of phase
ꢀ
h at 150 C, which is interpreted as slow (3 h) desorption of
NO ending with by reconstruction of the surface with the LP
shied up. It is accompanied by rise of the peak height, in
agreement with the analogous observation of the process of Pt
8
surface reconstruction on desorption of hydrogen. Here the
NO desorption is slow, running slightly below the thermody-
namic temperature of desorption. Then, it is natural to
interpret the cyclic LP shi upwards as a reconstruction of the
Pt crystallite surfaces.
The observed phenomenon evidently concerns only the
surface of the monitored Pt fcc phase. The upward shi of LP
signies the expansion of the Pt nanocrystal surface. It does not
affect the scattering intensity (Fig. 4B) changing only due to gas
Fig. 4 Evolution of Pt fcc phase control parameters of powder
diffraction pattern. ALP is an apparent lattice parameter as obtained
from the peak position (A), peak height is Pt 111 peak amplitude (B) and
average crystal size is the measured FWHM of Pt 111 recalculated to
size via the Scherrer formula. Red lines are the results of the rate– X-ray absorption, so the observed structural transformation is
power law fit.
not due to a deepening disorder of the surface. The measured
effect is of just the same magnitude as observed by us before for
surface reconstruction during H desorption.
2
It is known, as stated in the introduction, that a multilayer
adsorption of NO at high pressure causes shi of NO molecules
from strongly to more weakly bonding sites while increasing the
20
number of bonded molecules, which may weaken the inter-
action with the surface to the point when it starts to behave like
a clean surface and reconstruct or may interact specically,
promoting reconstruction. Exposition to He would cause weakly
bonded NO to desorb with the remaining molecules moving
back to the strongly bonding sites – the process liing the
reconstruction and moving the surface Pt atoms back to their
lattice nodes.
In our study of Pt nanocrystal reconstruction during
8
desorption of H
2
its effect on diffraction peak position has
been measured. For ꢂ4.5 nm Pt crystallites and a temperature
ꢀ
of 100 C, we recorded the shi of the 111 reection on
Fig. 5 MS sampling of the gases outgoing from the camera. Only
residual gases shown with partial pressure of NO being about 6.1 ꢃ
ꢀ
˚
reconstruction to be about 0.1 corresponding to a 0.01 A shi
in LP. The analogous shi of LP for Pt 3.1 nm crystallites was
.02 A (Fig. 7 of ref. 8). As the square of size ratio is 1 : 2, its
ꢁ7
1
0
atm.
˚
0
decrease with the nanocrystal size is expected to be inversely
m/z ¼ 44 (dashed line) agrees well with the blind test m/z ¼ 44 proportional to the average nanocrystal surface. From Fig. 4A
content in the pressured bottle. A growth of a small quantity of we can estimate LP shi for the beginning of phase (b) (esti-
NO
2
(m/z ¼ 46) to the level of 0.05% of that of NO is also mated size 5.65 nm), transition from the phase (c) to (d) (size
ꢀ
ꢀ
noticeable from Fig. 5. Fig. 5 covers the gas composition 6.95 nm), at 120 C (size 9.45 nm), and at 150 C (size
14762 | RSC Adv., 2014, 4, 14758–14765
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
˚
˚
˚
1
1.05 nm). These values, 0.003(4) A, 0.0018(8) A, 0.0024(3) A these approximations as their evolution remains very similar,
˚
and 0.0018(4) A, correspond to those expected from the and the same set of values could be kept.
˚
˚
˚
˚
growing surface: 0.0064 A, 0.0042 A, 0.0023 A and 0.0017 A,
The best t to the rate-power law results in the following
respectively. The gures in parentheses give the standard powers: phase (b–c–d), n ¼ 14; phase (e), n ¼ 13; phase (g),
deviation of the last gure estimated from averaging LP values n ¼ 20 and phase (i), n ¼ 40. These values for the initial process
ꢀ
30
over 10 points before and aer the shi edge. The observed LP are similar to observed for coalescence at 700 C but for the
shi is about half of that expected for small nanocrystals and following growth are signicantly higher than those reported in
30
agree with the expected for larger sizes. As the expected values the literature (typically 4–5), suggesting that the particle
correspond to a full surface reconstruction it suggests that we growth is very quick and its mechanism differs from the nor-
do not observe full reconstruction of the whole Pt surface, with mally observed diffusion driven growth. They also suggest that
various fragments dynamically changing their state. The the mechanism of growth changes aer phase (e), when
observation that the LP shi approaches its expected value for possibly a diffusion mechanism starts to dominate. The value of
larger crystallites may be due to its growing surface with a rate power does not necessarily characterize a simple process.
decreasing fraction being in contact with other coalescing In practice, a quite complex sequence of processes affecting
26
particles. Thus an increasing with size fraction of the surface sintering may change its value.
undergoes reconstruction. At the end of phase (d), part of the sample was analyzed by
The LP (Fig. 4A) in phase (e) shows a marked increase during TEM and Fig. 3 shows representative pictures (B, D, F)
6
quick crystallite growth. This effect has been discussed before.
compared with the initial sample aer reduction (phase (a) – A,
The further evolution of the LP agrees well with the temperature C, E). The tendency for Pt nanocrystals to group in clusters is
ꢁ
6
ꢁ1
lattice expansion of Pt assumed to be 8.8 10
K
.
striking. Linking this with the mean crystallite growth observed
Fig. 4B shows evolution of the 111 peak amplitude. The rate by XRD brings a natural conclusion that the coalescence is the
of growth is highly correlated with the estimated crystallite size growth rate determining phenomenon up to phase (d).
i.e. inversely proportional to FWHM. The curve was not cor- Considering the evolving powers of the rate-power law, it
rected for the X-ray absorption by gas in the camera but the probably dominates also during phase (e).
visible oscillations as well as shis on gas exchange are well
Quick growth of Pt nanocrystals supported on silica has been
described by this varying absorption of gas and are not inter- observed in our laboratory many times in various experimental
preted structurally.
setups. Fig. 6 shows an example of a quick sintering in NO
Fig. 4C presents evolution of the average crystallite size
estimated directly from the FWHM using the Scherrer formula.
The routine did not account for the material strain. To check its
effect for several patterns the full range W–H analysis has been
done, showing approximately constant strain parameters. This
analysis also provides a slight correction to the values presented
in Fig. 4C. No instrumental broadening was considered as for
the studied particle sizes it provides negligible correction.
As the W–H correction is nearly constant, the values from
Fig. 4C were used to t the phases of quick growth (b–c–d, e, g, i)
to the rate-power law of the form:
dS
n
¼
S
ꢁK S
dt
where S is the exposed metal surface and K
S
is a constant. This
can be rewritten in the form:
dD
2ꢁn
¼
K D
D
dt
nꢁ1
where D is the mean particle diameter D ¼ 6V/S, K ¼ K (6V)
.
D
S
This can be easily solved to the form:
1
DðtÞ ¼ ððn ꢁ 1ÞK
D
t þ CÞnꢁ1
The mean D in the formula above is a surface weighted average
that differs from both the number weighted value (directly from
TEM) and volume weighted average, from the Scherrer formula
and XRD data. Assuming a log-normal distribution of crystallite
size that widens during the process, we have tested several
approximations to follow evolution of the surface weighted size
average. However the best t powers do not depend strongly on
Fig. 6 An example of quick sintering for calcined 10% Pt/SiO
NO at 200 C.
2
kept in
ꢀ
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 14758–14765 | 14763

View Article Online
RSC Advances
atmosphere at 200 C measured using position sensitive
Paper
ꢀ
Crucial for the interpretation is the assumption that NO
ꢀ
detector INEL CPS120. The fresh sample was calcined in O at does not quickly desorb at 80 C. TPD studies, e.g. ref. 29 with
2
ꢀ
ꢀ
ꢁ1
4
00 C and reduced in H at 500 C for 6 h with Pt crystallites the heating rate 7 K s , report desorption of NO at above
2
ꢀ
size of ꢂ8 nm. Then the sample was briey ushed with helium 150 C and possibly higher for very small nanoparticles. The
ꢀ
ꢁ1
at 200 C and exposed to NO (20 ml min ). The presented desorption peak has, however, signicant width and the
diagrams are rather noisy but the effect of surface reconstruc- desorption with low rate can be observed even below the
tion is clearly visible, accompanied by quick crystal growth with desorption temperature. In our case, the process was already
ꢀ

tted rate power n ¼ 6. It suggests that the actual mechanism of sufficiently slow at 150 C to be observed over several hours
growth can be modied but the process is always observed with characteristic signs of surface reconstruction at the nal
together with reconstruction phenomena. Interestingly Fig. 6 stage.
suggests also a slight shape anisotropy of the Pt nanocrystals
with maximum size in the 111 direction. This was also the never observed below 80 C and it is postulated here that the
The observed low temperature growth is specic to Pt. It was
ꢀ
direction of the quickest growth.
reconstruction provides a crucial mechanism driving quick
The experimental data presented above do not fully explain coalescence at low temperature.
the precise atomistic mechanism of the observed coalescence.
An answer to the question of why it occurs for Pt but was never
4
Conclusions
observed for Pd may lay in a mechanism of nanocrystal trans-
port specic to Pt as well as in a mechanism of growth of the
collided nanocrystals specic to Pt. Upon collision, the collided
surfaces should easily link as the chemisorption energies of NO
The presented results from TEM as well as from an operando
XRD/MS study provide strong evidence that the low tempera-
ture, quick growth of Pt nanoparticles supported on silica,
exposed to NO, results from coalescence. As coalescence in
catalysis is known as a high temperature phenomenon, this
opens up a question about a possible mechanism of low
temperature coalescence. Our former experience in observation
31,32
reported in the literature
sion energy. The cohesive energy of Pd is lower (3.94 eV versus
.86 eV for Pt) but still larger than the chemisorption energy of
are by far lower than the Pt cohe-
5
NO (in the range 1–2 eV). The further build-up of larger nano-
crystals requires, however, substantial mass transport and this
may be provided by the reconstructing surfaces. The number of
atoms exposed on reconstructed and unreconstructed surfaces
is different and the phenomenon should involve mass transfer.
It can be speculated that the reconstruction may cause
desorption of a part of the sorbed NO, which can move to
stronger bonding sites. It could then trigger liing of the
reconstruction, initiating cyclic surface rearrangement. This
should provide an efficient mechanism for the motion of the
whole clusters over the support. A traditionally considered
mechanism is the nucleation of new monoatomic layers of the
of reconstruction of Pt nanoparticles surface on desorption of
8
H
2
by XRD allows us to understand the peak position evolution
in NO. It implies reconstruction of Pt surfaces occurring at high
pressure coverage of NO. Such reconstruction at high pressure
of NO at temperatures above 80 C is as yet an unknown
ꢀ
phenomenon. Known phenomena of surface reconstruction of
Pt involve signicant movement of atoms. If the reconstructed
surface, changing number and energetics of chemisorption
sites, desorbs part of weakly bonded NO, that may li the
reconstruction providing necessary feedback for the surface
mobility. This may pose sufficient drive to move whole Pt
clusters over the support as well as means to build up a larger
crystallite out of two smaller collided crystallites. The proposed
mechanism needs further conrmation both experimentally
and theoretically. This low temperature coalescence could be
used for a controlled Pt growth with a predictable size distri-
bution or in selective growth of Pt nanocrystals in mixtures. The
phenomenon and its mechanism is new and has never been
reported previously in the literature.
9
crystal facets, with its rate estimated from nucleation theory.
But the surface reconstruction provides the necessary move-
ment of surface atoms. The traditionally considered coales-
cence could provide sufficient mobility to 6 nm Pt nanocrystals
ꢀ
26,30
on supports at temperatures above 600 C.
This is why it was
rejected in literature as a possible factor affecting Pt sintering at
9
low temperatures. However, the reconstruction phenomena
can provide both a mechanism of cluster transport and a
mechanism of growth.
Some light on the atomistic mechanism of sintering can be
shed by TEM observations of Pd nanocrystals aer heating in Acknowledgements
NO. If the mechanism is limited only to 5d transition metals,
The work has been nancially supported by The Polish National
Science Center (NCN) under Research Grant no. N N204 097839.
the experiments on Ir supported on silica can also add to an
understanding of the phenomenon. Such experiments are in
preparation. The shis of the measured peak position observed
by XRD agree very well with the reconstruction of the Pt nano-
crystal surface studied by us and we know of no other
phenomenon that could explain such behavior. A shi in the
diffraction peak position is always connected to changing the
distribution of atoms in the observed crystallographic phase.
We observed cyclic shis on exposition to NO conserving the fcc
ordering. It is natural to ascribe them to surface phenomena.
References
1 A. G. Somorjai and J. Y. Park, Phys. Today, 2007, 48–53.
2 H. J. Freund, H. Kuhlenbeck, J. Libuda, G. Rupprechter,
M. Baumer and H. Hamann, Top. Catal., 2001, 15, 201–209.
3 X. C. Su, P. S. Cremer, Y. R. Shen and G. A. Somorjai, J. Am.
Chem. Soc., 1997, 119, 3994–4000.
14764 | RSC Adv., 2014, 4, 14758–14765
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
4
5
P. Thostrup, E. K. Vestergaard, T. An, E. Lgsgaard and 19 C.-M. Chan, M. A. van Hove, W. H. Weinberg and
F. Besenbacher, J. Chem. Phys., 2003, 118, 3724–3730. E. D. Williams, Solid State Commun., 1979, 30, 47–49.
R. T. Vang, E. Lgsgaard and F. Besenbacher, Phys. Chem. 20 W. A. Brown, R. K. Sharma, Q. Ge and D. A. King, Phys. Chem.
Chem. Phys., 2007, 9, 3460–3469.
Chem. Phys., 1999, 1, 1995–2000.
6
7
8
Z. Kaszkur, J. Appl. Crystallogr., 2000, 33, 87–94.
Z. Kaszkur, J. Appl. Crystallogr., 2000, 33, 1262–1270.
21 E. H. Backus, A. Eichler, M. L. Grecea, A. W. Kleyn and
M. Bonn, J. Chem. Phys., 2004, 121, 7946–7954.
P. Rzeszotarski and Z. Kaszkur, Phys. Chem. Chem. Phys., 22 B. Hammer and J. K. Norskov, Phys. Rev. Lett., 1997, 79,
009, 11, 5416–5421. 4441–4444.
P. L ¨o ¨o f, B. Stenbom, H. Norden and B. Kasemo, J. Catal., 23 J. C. Cook and E. M. McCash, Surf. Sci., 1996, 356, L445.
2
9
1
993, 144, 60–76.
0 S. L. Handforth and J. N. Tilley, Ind. Eng. Chem., 1934, 26,
287–1292.
1 H. S. Taylor, J. Phys. Chem., 1926, 30, 145–171.
24 K. C. Hass, M.-H. Tsai and R. V. Kasowski, Phys. Rev. B:
Condens. Matter Mater. Phys., 1996, 53, 44–47.
25 G. A. Mulhollan, R. L. Fink and J. L. Erskine, Phys. Rev. B:
Condens. Matter Mater. Phys., 1991, 44, 2393–2395.
1
1
1
1
2 E. Ruckenstein and B. Pulvermacher, J. Catal., 1973, 29, 224– 26 J. R. Anderson, Structure of Metallic Catalysts, Academic
45. Press, 1975, p. 182.
3 W. F. Banholzer, Y. O. Park, K. M. Mak and R. I. Masel, Surf. 27 T. A. Dorling, B. W. J. Lynch and R. L. Moss, J. Catal., 1971,
Sci., 1983, 128, 176–190. 20, 190–201.
4 W. Hong, R. G. Tobin, C. L. DiMaggio, G. B. Fisher and 28 G. K. Williamson and W. H. Hall, Acta Metall., 1953, 1, 22–31;
2
1
1
1
1
1
1
D. K. Lambert, J. Chem. Phys., 1997, 107, 9569–9576.
V. D. Mote, Y. Purushotham and B. N. Dole, J. Theor. Appl.
Phys., 2012, (6), 6.
5 N. D. Parkyns, Catal., Proc. Int. Congr., 5th, 1972, 12, 255–
2
64.
6 D. Loffreda, D. Simon and P. Sautet, J. Chem. Phys., 1998,
08, 6447–6457.
7 K.-M. Ho and K. P. Bohnen, Phys. Rev. Lett., 1987, 59, 1833– 31 W. A. Brown and D. A. King, J. Phys. Chem. B, 2000, 104,
836. 2578–2596.
8 S. Titmuss, A. Wander and D. A. King, Chem. Rev., 1996, 96, 32 H. Tang and B. L. Trout, J. Phys. Chem. B, 2005, 109, 17630–
291–1305. 17634.
29 E. I. Altman and R. J. Gorte, J. Phys. Chem., 1989, 93, 1993–
1997.
1
30 P. J. F. Harris, Int. Mater. Rev., 1995, 40, 97–115.
1
1
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 14758–14765 | 14765