Preparation of bimetallic catalysts using continuous processing

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Title: Preparation of bimetallic catalysts using continuous processing methods
Participants: John R. Monnier, Akkarat Wongkaew, and D.A. Blom
Motivation: Electroless deposition is a method of preparing bimetallic catalysts in which only bimetallic
particles of known composition are produced [1-4]. Batch methodology has been primarily used thus
far for preparing such catalysts. This method has several drawbacks: (1), only relatively small amounts
of catalyst can be prepared at one time and/or (2), only relatively low levels of the second metal (M) can
be deposited on the primary metal (A). This is because all of the reducing agent (RA) and reducible
metal salt must be added into the vessel before ED begins. ED is governed by a series of kinetic
reactions:
(1), R thermal rxn = ko exp(-Ea/RT) [CRA]x [CM salt]y.
(2), R cat dep = ko’ exp(-Ea’/RT) [CRA]a [CM salt]b [CA sites]c.
(3), R autocat dep = ko” exp(-Ea”/RT) [CRA]d [CM salt]e [CM sites]f.
The thermal reaction (1), or the non-catalytic reduction of the reducible metal salt, is thus a function of
the concentrations of reducing agent and reducing metal salt. In the batch method, both of these
components must be front-loaded and thermal stability in the high initial concentrations of RA and
reducible metal salt often becomes the critical step in the ED process. Further, the rates of catalytic (2)
and autocatalytic (3) deposition are strong functions of the concentrations of reducing agent and
reducible salt concentrations, meaning the rates are much higher at the beginning of the deposition
process than at the end. This can result in heterogeneities in the position of the deposited metal.
Finally, whether or not catalytic deposition (on A sites) or autocatalytic deposition (on just-deposited M
sites) occurs can be a function of the concentration of reducible metal salt M in solution [5-7].
Continuous addition of the reducing agent and/or reducible metal salt minimizes the thermal stability
issue and also provides a method for commercial, larger-scale production of bimetallic catalysts.
Hypothesis: Continuous ED methods can be used to
synthesize high loadings of ED metals and large quantities
of bimetallic catalysts. Micro-processor syringe pumps will be
used to add controlled amounts of reducing agents and/or
reducible metal salts to a vessel containing an aqueous mixture
of a supported, monometallic catalyst to prepare bimetallic
catalysts containing high amounts of the second metal.
Continuous methods are also more amenable to on-line
monitoring of the deposition process, an important factor in largescale production. Preliminary work by co-PI Wongkaew for the
preparation of high weight loadings (5 – 25 wt%) of Pt on 30 wt%
Pd/C have indicated the syringe pump apparatus in Figure 1
works
well
for
deposition of high
weight loadings of
a second metal.
The
UV-visible
spectra for the
PtCl62- complex (at
295 nm) [8] shown
in Figure 2 shows
that the addition of
ethylene diamine
(EN)
as
a
complexing agent
for
PtCl62is
required for thermal
stability in the presence of hydrazine (N2H4) as a reducing agent. At a mole ratio of [N 2H4]/[PtCl62-] =
5:1, bath lifetime is increased from 12 to 26 min when a 2 : 1 molar excess of [EN]/[PtCl62-] is added to
the bath. It also underscores the need to continuously and slowly add N2H4 to the bath so that
unwanted thermal reduction of PtCl62- does not occur. Finally, it indicates that UV-visible spectroscopy
can be used as a real-time probe to monitor the rate of continuous, electroless deposition, an important
feature for large-volume catalyst production. Up to this point, samples have periodically been taken
from the ED bath, filtered, and then analyzed by
atomic absorption spectroscopy. We have recently
designed a continuous-flow UV cell that employs a
pump and fritted filter (to keep the particulate
catalyst in the ED bath) to successfully circulate the
solution between the UV cell and the ED bath.
The results in Figure 3 show kinetic curves for
deposition of high weight loadings of Pt on 30 wt%
Pd/C (Dispersion of Pd = 25%; dPd = 4.5 nm). In
this series of experiments, the N2H4 solution was
pumped at 1.67 ml/min into a bath containing all
other components. Since bath stability is typically
controlled by the concentration of reducing agent,
particularly for active reducing agents, it was
sufficient to pump only N2H4 to a bath already
containing the PtCl62-. The curve for the Pd-free
(blank) sample indicates that thermal stability was
only ~20 minutes before thermal reduction of PtCl62 Pdo occurred. Regardless, deposition of 11.7%,
17.2%, and 22.7 wt% Pt readily occurred within this time frame. Deposition of these amounts of Pt
would not have been possible using batch methods, where all N2H4 would have been added at the
beginning of the deposition experiment.
In order to determine the morphology of Pt deposition, STEM analysis (by co-PI Blom) was
conducted using an aberration-corrected JEOL 2100F scanning transmission electron microscopy
(STEM) and Z-contrast imaging. The images for 17.2 wt% Pt deposited on 30 wt% Pd/C in Figure 4
show clearly that a shell of Pt as a (111) surface layer has been uniformly-deposited on the Pd core.
Because of the relatively shallow depth of field at this high magnification, only one of the core-shell
particles is well focused. The well-ordered Pt shell structures and the fact that all particles exist as Pd
core-Pt shell structures (many other areas of the surface were also imaged) show the selectivity of this
ED process. Energy dispersive x-ray spectroscopy (EDS) analysis was conducted on the particle
shown in Figure 5 and the atomic Pt and Pd positions are presented in Figure 6. The Pt and Pd x-ray
intensities show conclusively that bimetallic particles with Pt shell thickness of approximately 1.5 nm on
a Pd core of about
4 – 6 nm diameter
have been formed,
which is in good
agreement with 2.7
ML thickness of Pt
on 4.5 nm Pd
particle diameters.
This
control
of
bimetallic structure
at high loadings of
deposited
metal
also validates this
Figure 5. EDS image area for particle of 17.2% Pt-30
type of continuous
wt% Pd/C sample.
particle synthesis.
Objective: Evaluate continuous ED
protocols for different combinations
of reducing agents and reducible
metal salts to optimize continuous
ED methods for high deposition
levels and large quantities of
bimetallic catalysts. Develop a facile
method for continuous monitoring of
the deposition process using UVvisible spectroscopy, rather than
analysis by atomic absorption of
aliquots of the ED bath at discrete
time intervals. The ultimate goal is to
develop a continuous methodology
that can be used commercially for
production of bimetallic catalysts.
Research Plan: We will use the
continuous ED set-up shown in
Figure 1. If it is desirable to examine
simultaneous deposition of two metals to the base monometallic catalyst, a third syringe pump will be
installed. Parameters to be examined will include pump rates, deposition temperature, pH values, and
addition of organic stabilizers. Before deposition experiments are conducted, it will be necessary to
determine thermal stabilities of the bath as a function of pumping rates of reducing agent and/or
reducible metal salt. For reducible metal salts with marginal thermal stability, stabilizing agents such as
alkylamines or organic acid salts will be used; the stabilizers function primarily as ligands to lower
reduction rates of the coordinated metal ion. The kinetics in Figure 2 indicate that ethylenediamine (EN)
was required for stability, even when the very active N2H4 was being slowly pumped into the ED vessel
over a 30 min interval. Bimetallic catalysts will not be limited to core-shell structures, but will include
controlled compositions with both metals at the surface.
First year milestone: Several protocols for addition of high loadings of reducible metals such as Pt,
Ru, and Co to supported monometallic catalysts will be developed. Deposition will be extended to
extruded supports. Multiple gram amounts will be prepared.
Cost: $70,000 for year one and successive years at $60,000/yr. First year includes $10,000 for
chemicals, pumps, and computers.
References
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