Influence of Metal Deposition and Activation Method on the Structure and Performance of Carbon Nanotube Supported Palladium Catalysts

Article abstract of DOI:10.1002/cctc.201701991

The effects of the metal deposition and activation methods on metal particle size and distribution were investigated for carbon nanotube supported Pd catalysts. The Pd precursor was loaded by incipient wetness impregnation, ion adsorption, and deposition precipitation and was activated by thermal treatment under a nitrogen atmosphere or in the liquid phase by reduction by formaldehyde or sodium borohydride. Regardless of the metal precursor loading method, activation under a N₂ atmosphere at 500 °C led to homogeneously distributed 4 nm Pd particles. Liquid-phase reduction by sodium borohydride provided a bimodal distribution with particle sizes of approximately 1 and >10 nm. A somewhat weaker reducing agent, formaldehyde, yielded particles approximately 1 nm in size. The activities of the catalysts for the hydrogenation of cinnamaldehyde correlated with the particle sizes.

Full text of DOI:10.1002/cctc.201701991

Accepted Article
Title: Influence of metal deposition and activation method on the
structure and performance of Pd/CNT catalysts
Authors: Wouter S. Lamme, Jovana Zečević, and Krijn P. de Jong
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Influence of metal deposition and activation method on the
structure and performance of Pd/CNT catalysts
Wouter S. Lamme,[a] Jovana Zečević,[a] and Krijn P. de Jong*[a]
Abstract: The effects of the metal deposition and activation method
on the metal particle size and distribution were investigated for carbon
nanotubes supported Pd catalysts. The Pd precursor was loaded by
incipient wetness impregnation, ion adsorption and deposition
precipitation and activated by a thermal treatment in nitrogen or in
liquid phase by reduction by formaldehyde or sodium borohydride.
Regardless of the metal precursor loading method, activation in N2 at
500 °C lead to homogeneously distributed 4 nm Pd particles. Liquid-
phase reduction by sodium borohydride provided bimodal particle size
distributions with particles of ~1 nm and >10 nm. A somewhat weaker
reducing agent, formaldehyde, yielded ~1 nm particles. The activities
of the catalysts for the hydrogenation of cinnamaldehyde correlated
with the particle sizes.
However, since Pd/C catalysts are often used in liquid phase,[26]
performing activation in liquid phase is highly desirable.
In this work, we systematically investigated the influence of
the different metal deposition methods and subsequent activation
methods with varying conditions thereof on the structure of
Pd/CNT catalysts. The (NH3)4Pd(NO3)2 precursor was deposited
from aqueous solutions onto LPO-treated CNT either via incipient
wetness impregnation (IWI), deposition precipitation (DP) or ion
adsorption (IA), with a nominal Pd loading of 5.0–6.0 wt%. The
deposited Pd precursor was converted into metallic Pd particles
in the gas phase by a thermal treatment in N2 at 500 °C.[19]
Catalyst precursors prepared by ion adsorption were also
reduced in liquid phase by sodium borohydride or formaldehyde,
both known to act as reducing agents in the preparation of Pd/C
catalysts,[9,10,24,27] with varying conditions such as rate of addition
of the reducing agent, temperature and atmosphere in which the
addition is performed. Details on deposition and reduction
methods can be found in the SI.
As expected, ion adsorption followed by gas-phase activation
in N2 yielded uniformly distributed Pd particles with a unimodal
particle size distribution and an average size of about 5 nm
(Figure 1a). Comparable structures of Pd/C catalysts were
observed when the deposition method was IWI or DP (Figure S1).
Ion adsorption was chosen to investigate further and to compare
liquid phase to gas phase activation. In contrast to N2 activation,
reduction by sodium borohydride led to a bimodal particle size
distribution with 1 nm Pd particles uniformly distributed across the
CNT support and 50 nm Pd particles probably located at the
external surface of the CNT skeins (Figure 1b).[28] Similarly,
reduction by formaldehyde led to 1 nm Pd particles uniformly
distributed over the support, but with only a negligible amount of
the Pd ending up in 10–50 nm particles (Figure 1c).
Carbon supported palladium (Pd/C) catalysts are widely used for
hydrogenation in the fine chemicals industry. Numerous studies
have been performed investigating influences of the support
material,[1–5] the support surface functional groups,[6,7] metal
precursor[2,8] and activation method[2,9–11] on the structure and
activity of the final Pd/C catalyst. Ideally, metal particles have a
narrow size distribution and are uniformly distributed across the
support to arrive at increased activity and stability. However,
particles are often distributed non-uniformly, or the particle size
distributions are broad or multimodal.[12–14]
Carbon supports that are often used include activated carbon,
carbon black, graphite and carbon nanomaterials such as carbon
nanofibers and carbon nanotubes (CNT). In research, CNT are of
particular interest as the material has a well-defined structure with
a rather uniform surface, high purity and no micropores. The
structure and porosity is largely maintained, even if exposed to
aggressive media.[15,16] The well-defined structure makes CNT
suitable for analysis by transmission electron microscopy, with
high contrast between supported metal nanoparticles and the
support. Although CNT are relatively inert, functional groups can
be introduced onto the surface by functionalization treatments
such as liquid phase oxidation (LPO) and gas phase oxidation
(GPO). The surface chemistry can have a strong influence on the
metal dispersion[6,17] and interaction between metal and
support,[7,18] and thus on the performance of Pd/C catalysts.[7,17,19]
Various metal deposition methods such as impregnation,[2]
deposition precipitation,[3,6] deposition reduction,[20] ion
adsorption[21,22] or colloidal routes[7,23] have been studied. The
deposited metal precursor is commonly reduced in the gas phase,
which typically yields well-distributed Pd nanoparticles.[24,25]
XRD and H2 chemisorption were used to determine and
compare the average Pd particle size from the bulk of a sample
with the one obtained by TEM analysis (Table 1). Details on the
characterization methods can be found in the SI. The ion
adsorption sample reduced by the N2 treatment shows relatively
similar particle sizes determined from XRD, H2 chemisorption and
TEM. For the formaldehyde-reduced sample, the absence of Pd
signals in x-ray diffractograms and very low H2 chemisorption
derived Pd particle size further corroborates that the occurrence
of 10-50 nm size Pd particles observed in TEM is rather negligible.
Table 1. Weight loading, average crystallite sizes from XRD and particle
sizes from H₂ chemisorption and TEM (nm).
Activation method
Pd loading
(wt.%)
Pd particle/crystallite size (nm)
[a]
W. S. Lamme, Dr. J. Zečević, Prof. Dr. K. P. de Jong
Inorganic Chemistry and Catalysis
Debye Institute for Nanomaterials Science
Utrecht University
Universiteitsweg 99, 3584 CG Utrecht, The Netherlands
E-mail: K.P.deJong@uu.nl
XRD
2.6
H2
3.6
TEM
N2 treatment
NaBH4
4.0
4.4
3.7
3.2 ± 1.4
1 / 50*
2.9
-
2.0
0.8
Supporting information for this article is given via a link at the end of
the document.
Formaldehyde
1.1 ± 0.3
* Bimodal distribution.
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For the NaBH4-reduced sample, the heterogeneity was quantified
by calculating the contribution of the 1 nm and 50 nm particles to
the average size determined by H2 chemisorption. It was
calculated that ~50% of the Pd ended up in 1 nm particles and
~50% in 50 nm particles.
An important difference between the NaBH4-reduced and the
formaldehyde-reduced catalysts lies in the nature of the reducing
agents. The standard reduction potentials (at pH 0) of Pd2+/Pd,
formic acid/formaldehyde, boric acid/borohydride are 0.9 V,
−0.25 V and −0.5 V, respectively.[29–31] The more negative boric
acid/borohydride potential may explain the observed differences
where Pd particles grow to even 50 nm. This could be a result of
immediate reduction of the metal precursor from the solution onto
the support, while the milder formaldehyde may reduce precursor
species adsorbed on the support only.
All three catalysts were active for hydrogenation of
cinnamaldehyde (Table 2 and Figure S4). Normalized to the
amount of Pd, the formaldehyde-reduced catalyst was much more
active than the N2-activated and NaBH4-reduced catalysts. This
difference could be explained by the particle sizes of the spent
catalysts. The average particle sizes, determined by TEM, were
3.8 ± 1.4 nm, 2.3 ± 1.3 nm and 2.0 ± 1.0 nm for the spent N2-
activated, NaBH4-reduced (sub-10 nm particles only) and
formaldehyde-reduced catalyst, respectively (Figure S6).
Turnover frequencies calculated using these sizes were similar.
The selectivity to hydrocinnamaldehyde was 75% for the
formaldehyde-reduced catalyst and 62% for the NaBH4-reduced
catalyst, the rest was hydrocinnamyl alcohol. A low selectivity of
24% was found for the N2-activated catalyst. No cinnamyl alcohol
was formed as expected,[32] and no other products were observed
in the GC spectra. The difference in selectivity could be caused
by the larger Pd particles in the NaBH4-reduced sample.[33]
Alternatively, the selectivity difference could be explained by
support effects,[34] which could be introduced by a change in
functional groups nature caused by the activation method.
Preparation of Pd/CNT using (NH3)4Pd(NO3)2 as metal
precursor and activation in N2 yields catalysts with ~4–5 nm
particles distributed uniformly across the support regardless of the
metal deposition method. Pd/CNT preparation by ion adsorption
followed by reduction in liquid phase by NaBH4 yields catalysts
with a non-uniform metal particle distribution and bimodal size
distribution, while reduction by formaldehyde yields catalysts with
uniformly distributed ~1 nm metal particles. Various liquid-phase
reduction conditions appear to have no influence on the final
catalyst. It is the nature and probably the strength of the reducing
agent that will determine the final structure of the catalyst, with
weaker reducing agents promoting narrow particle size
distribution and uniform particle distribution across the support.
Figure 1 Representative high-angle annular dark-field scanning transmission
electron microscopy (HAADF-STEM) images and particle size distribution
histograms of Pd/CNT catalysts activated by (a) 500 °C N₂ thermal treatment,
(b) slow addition of NaBH4 at room temperature in air, (c) instant addition of
formaldehyde at room temperature in N2. *) Only sub-10 nm particles were
included in the particle size distribution of the NaBH4-reduced sample.
One of the possible causes for the larger particles in the NaBH4-
reduced sample might be the higher loading of Pd in the sample.
However, ICP analysis confirmed that in all three ion adsorption
samples, as well as in all other samples prepared in this study,
the achieved Pd loading was rather similar ranging from 3.7 to
4.4 wt.% (see Tables 1 and S1). Comparing the nominal loading
of 5.0–6.0 wt.% with the achieved loading 3.7–4.5 wt% in ion
adsorption samples, the extent of Pd deposition was around 70%,
slightly lower than the extents for IWI (85%) and DP (95%).
To further investigate how liquid-phase reduction might have
affected the distribution of the metal nanoparticles over the
support, experiments were performed under different
atmospheres (air or N2), different reducing agent addition
temperatures (RT, 2 °C or 80 °C) and addition rates (fast or slow).
Compared to the samples shown in Figure 1b and 1c, none of the
listed conditions seemed to impact the final catalyst structure
significantly (Figure S2 and S3). The amount of NaBH4 used also
had no significant influence on the catalyst structure observed by
TEM (Figure S2f, S2g). Analysis of metal loading and Pd particle
size determined by XRD, H2 chemisorption and TEM for all
samples can be found in Table S1. These results exclude effects
of concentration gradients as well as Ostwald ripening due to
oxidation–reduction cycles.
Table 2. Catalytic activity and selectivity towards hydrocinnamyl aldehyde.
Activation
method
Activity (mmol gPd-1 s-1)[a]
TOF (s-1)[b]
Selectivity
N2 treatment
NaBH4
2.0
2.4
3.5
0.7
1.0
0.7
24%
63%
75%
Formaldehyde
[a] Activity at 50% conversion, determined from a linear fit on ln(xcinnamaldehyde
)
vs. t (see figure S5).
[b] Calculated as number of cinnamaldehyde conversions per second,
normalized to number of Pd surface atoms calculated from average Pd
particle size of the spent catalysts, determined by TEM,
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Entry for the Table of Contents
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W. S. Lamme, J. Zečević, and K. P. de
Jong*
Page No. – Page No.
Influence of metal deposition and
activation method on the structure
and performance of Pd/CNT catalysts
The particle size distribution of Pd/CNT catalysts is directly influenced by the
activation method, not by the metal deposition method.
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