Tuning Catalytic Selectivity at the Mesoscale via Interparticle Interactions

Article abstract of DOI:10.1021/acscatal.5b02202

The selectivity of heterogeneously catalyzed chemical reactions is well-known to be dependent on nanoscale determinants, such as surface atomic geometry and composition. However, principles to control the selectivity of nanoparticle (NP) catalysts by means of mesoscopic descriptors, such as the interparticle distance, have remained largely unexplored. We used well-defined copper catalysts to deconvolute the effect of NP size and distance on product selectivity during CO₂ electroreduction. Corroborated by reaction-diffusion modeling, our results reveal that mesoscale phenomena such as interparticle reactant diffusion and readsorption of intermediates play a defining role in product selectivity. More importantly, this study uncovers general principles of tailoring NP activity and selectivity by carefully engineering size and distance. These principles provide guidance for the rational design of mesoscopic catalyst architectures in order to enhance the production of desired reaction products.

Full text of DOI:10.1021/acscatal.5b02202

Research Article
pubs.acs.org/acscatalysis
Tuning Catalytic Selectivity at the Mesoscale via Interparticle
Interactions
Hemma Mistry,† Farzad Behafarid,† Rulle Reske,‡ Ana Sofia Varela,‡ Peter Strasser,‡
and Beatriz Roldan Cuenya*,§
†Department of Physics, University of Central Florida, Orlando, Florida 32816, United States
‡Department of Chemistry, Chemical Engineering Division, Technical University Berlin, 10623 Berlin, Germany
§Department of Physics, Ruhr University Bochum, 44780 Bochum, Germany
S
* Supporting Information
ABSTRACT: The selectivity of heterogeneously catalyzed
chemical reactions is well-known to be dependent on
nanoscale determinants, such as surface atomic geometry and
composition. However, principles to control the selectivity of
nanoparticle (NP) catalysts by means of mesoscopic
descriptors, such as the interparticle distance, have remained
largely unexplored. We used well-defined copper catalysts to
deconvolute the effect of NP size and distance on product
selectivity during CO2 electroreduction. Corroborated by
reaction-diffusion modeling, our results reveal that mesoscale
phenomena such as interparticle reactant diffusion and readsorption of intermediates play a defining role in product selectivity.
More importantly, this study uncovers general principles of tailoring NP activity and selectivity by carefully engineering size and
distance. These principles provide guidance for the rational design of mesoscopic catalyst architectures in order to enhance the
production of desired reaction products.
KEYWORDS: copper, nanoparticles, electrocatalysis, CO2 electroreduction, methane, CO
electroreduction reaction, particularly with respect to the
change in binding energy of reactants and reaction
intermediates on different surface sites.16 However, the effect
of nanocatalyst structure on the reaction at the mesoscale, up to
this point, has been overlooked. An important structural
parameter of nanoscale catalysts is the interparticle (IP)
distance, which can significantly affect mass transport during
a reaction. Several studies have investigated mass transport
effects in the oxygen reduction,21−28 CO oxidation,29−31 and
alcohol oxidation32,33 reactions. However, further information
is needed on transport phenomena for small, nanometer-sized
electrocatalysts, which can account for both size and IP distance
effects during multistep, multiproduct reactions.
INTRODUCTION
■
The direct electrocatalytic reduction of CO2 to hydrocarbons is
a highly promising pathway to high-value chemicals and fuels. If
realized on an industrial scale, it would not only provide a
chemical storage technology for renewable electricity (“Power-
to-X” technologies), but the capture and reutilization of CO2
would mitigate the detrimental environmental consequences
caused by rising atmospheric CO2 concentrations. Much
attention has been focused on copper as a CO2 electro-
reduction catalyst, since it is the only metal that can convert
CO2 to appreciable amounts of hydrocarbons. However,
problems remain with high overpotentials, low current
densities, and poor selectivity to desired products.1,2
CO2 electroreduction is a kinetically complex reaction that
can produce CO, HCOOH, CH4, or C2H4; therefore, reactant
mass transport and readsorption phenomena are expected to
play a significant role in catalyst activity and selectivity. Here,
we present a comprehensive analysis on how the IP distance
and size of NP ensembles affect their catalytic product
selectivity for the electrocatalytic reduction of CO2. Our
analysis of reaction-diffusion processes at the mesoscale not
only deconvolutes the individual roles of NP size and distance,
In an effort to improve catalyst design, many recent
theoretical3−7 and experimental8−12 studies have investigated
the mechanism of CO2 reduction over different copper single-
crystal electrodes, and also nanostructured copper cata-
lysts,13−20 revealing highly interesting insights into the
structure-sensitivity of this reaction. Since insights gained
from studies on bulk single-crystal surfaces cannot fully explain
reactivity and selectivity results from nanostructured catalysts,
further work on model, highly controlled nanoscale systems is
needed to uncover the unique reaction mechanism on these
catalysts.
Received: September 30, 2015
Revised: November 29, 2015
Investigations of nanostructured copper catalysts have
focused on the effect of surface structure on the CO2
© XXXX American Chemical Society
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but, more importantly, uncovers previously unaddressed
principles governing the catalysis of NP ensembles in general,
and that of the electroreduction of CO2 in particular. We
propose the “IP distance” as a largely overlooked, yet important
experimental mesoscopic descriptor for selectivity that provides
a deeper mechanistic understanding, as well as predictive
control of catalyst activity and selectivity in the conversion of
CO2 to hydrocarbons.
AFM images and corresponding histograms are shown in
Figure S1 in the Supporting Information.
To characterize the catalytic activity of these NPs as a
function of the IP distance, the NPs were deposited on glassy
carbon plates and used as working electrodes during CO2
electroreduction. Linear sweep voltammetry (LSV) voltammo-
grams were measured up to a potential of E = −1.12 V vs RHE
in 0.1 M KHCO3. The electrolyte was saturated with CO2 and
kept well mixed by bubbling CO2 at a constant flow of 30 mL/
min through the bottom of the cell. LSV measurements on the
4.7 nm NPs with various IP distances are shown in Figure 1c.
Additional LSV data for all samples are provided in Figure S2 in
the Supporting Information. The stability of the NPs during the
reaction was confirmed by measuring AFM on HOPG and
SiO2/Si(111) supported NPs after the electrochemical
measurements (Figures S3 and S4 in the Supporting
Information). The AFM measurements confirmed that no
sintering, loss of material, or change in IP distance occurred
during the reaction.
RESULTS AND DISCUSSION
■
The inverse micelle encapsulation technique allows for the
synthesis of highly ordered NP arrays with a controlled,
uniform size16,34,35 and tunable IP spacing. In this study, three
different Cu NP sizes with various IP distances were
synthesized using this method. Further details are given in
the Supporting Information. Figure 1 shows atomic force
To investigate changes in selectivity with IP distance,
reaction products were sampled using gas chromatography
(GC) after 10 min of reaction at E = −1.12 V vs RHE, and
Faradaic selectivity was calculated as described in the
Supporting Information. The changes in Faradaic selectivity
with IP distance for catalysts with three different sizes are
shown in Figure 2. A change in Faradaic selectivity, as a
function of IP distance, is observed for the 1.5 and 4.7 nm NPs
(Figures 2a and 2b), while a much smaller variation is detected
for the largest (7.4 nm) NPs (Figure 2c). Interestingly, the 1.5
nm particles showed a different trend in selectivity from the 4.7
nm particles. To confirm the trend for the smallest NPs, the
Faradaic selectivity for a similarly sized NP (1.9 nm) with an IP
distance of 27 nm is also plotted in Figure 2a.
The complex trends observed in the Faradaic selectivities
indicate that the activity and selectivity of NP catalysts for the
CO2 electroreduction reaction cannot be understood solely in
terms of NP size and its effect on the binding strength of
reactants. In addition to these nanoscale phenomena, mesoscale
processes such as mass transport through diffusion, the
readsorption of reaction intermediates on a given NP or, if
spaced closely enough, on neighboring NPs should be
considered.
The first mesoscale transport process that we will consider is
the diffusion of reactants to the NP surface. Under our
experimental conditions, the CO2 bubbles fed into the reaction
cell lead to a forced convection in the electrolyte and a uniform
reactant concentration in the bulk of the electrolyte. However,
very close to the electrode surface, the electrolyte may be
considered stationary, and the diffusion process is the only
pathway for mass transfer through this region. Mass-transfer
transport phenomena within the diffusion layer determine how
the reactants diffuse toward the NPs, as well as how the
reaction products leave the NPs and enter the bulk of the
electrolyte. Limitations in mass transfer are expected to hinder
the reaction rate (e.g., current density).
In order to understand how mass-transfer effects may
contribute to the electrocatalytic activity of our NPs,
calculations were carried out to elucidate the changes in the
CO2 reaction rate, because of the diffusion processes. Finite-
element calculations were used to solve Poisson’s equation and
to obtain the spatial distribution of CO2 at the NP surface. The
diffusive flux of reactants, which is linearly proportional to the
concentration gradient, was calculated following Fick’s first law.
Figure 1. Characterization of Cu NPs with varied IP distances. Atomic
force microscopy (AFM) images of 4.7 nm Cu NPs dip-coated
multiple times onto SiO2/Si(111) acquired after ligand removal: (a) 1
coat and (b) 5 coats, corresponding to IP distances of 53 and 24 nm,
respectively. (c) Linear sweep voltammogram at 5 mV/s in CO2-
saturated 0.1 M KHCO3 on 4.7 nm Cu NP samples with different
average IP distances (24, 31, and 53 nm) supported on glassy carbon.
microscopy (AFM) images of 4.7 1.2 nm NPs on a silicon
support with a native oxide surface with the IP distance being
varied from 53 nm to 24 nm. Cu NPs with average sizes of 1.5
0.6 nm and 7.4 1.8 nm were also prepared, with IP spacing
ranging from 22 nm to 10 nm and from 92 nm to 41 nm,
respectively. These IP distances were chosen in order to keep
the ratio of IP distance to NP size (IP/d) in a similar range for
each NP size, since this parameter can be used to compare
diffusion phenomena on samples with different geometry.32
This parameter, along with NP size, IP distances, and
normalized Cu coverages, are provided for all samples in
Tables S1 and S2 in the Supporting Information. Additional
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Figure 2. Faradaic selectivity as a function of IP distance. Faradaic selectivity during the electroreduction of CO2 at E = −1.1 V vs RHE over (a) 1.5
nm, (b) 4.7 nm, and (c) 7.4 nm Cu NPs, as a function of the IP spacing. In panel (a), a data point for 1.9 nm NPs with an IP distance of 27 nm was
added for reference. The top axis displays the copper coverage, defined as the ratio of the copper surface area to the surface area of the glassy carbon
support. Lines are present to serve as guides for the eye. Error bars in panels (b) and (c) are calculated from two or more repeated measurements.
Figure 3. Calculations of CO2 diffusion to the surface of NPs with different size and spacing, and corresponding experimental CO2 electroreduction
activity. (a) Simulation results of the CO2 concentration distribution based on diffusion equations. The red arrows show the reactant flux toward the
NPs. The color scale shows the concentration of CO2 at a given distance from the NPs, as a percentage of its value in the bulk of the electrolyte. A
diffusion layer thickness of 100 nm was assumed. (b) CO2 flux obtained for NPs with different size and IP distances based on diffusion equations.
The data are normalized by the corresponding flux obtained from a flat Cu foil. (c) Experimental current density obtained at −1.1 V (vs RHE)
during the electrochemical reduction of CO2 over 7.4 nm Cu NPs with distinct average IP spacing.
It was assumed that the CO2 concentration is constant outside
the Nernst layer and that the CO2 that reaches the NP surface
reacts very quickly, resulting in zero CO2 concentration at the
NP surface. These two criteria were applied as the boundary
conditions for the diffusion equations. Figure 3a shows
calculation results for 7.4 nm NPs with an IP distance of 36
nm. The surface of the upper hemisphere of the NPs provides a
higher contribution to the reaction, since the electrolyte in
contact with the bottom hemisphere of the NP has a lower
concentration of the reactants (depleted red color region).
Figure 3b shows the normalized CO2 flux to the surface of NPs
with different sizes (d = 1.5, 4.7, and 7.4 nm) and IP distance.
For a given NP size, Figure 3b shows that the normalized CO2
flux is higher for NP arrangements with large IP distance and
reaches a plateau for IP/d values larger than ∼10. The latter
represents a threshold value above which the NPs can be
considered as single NPs on an infinite support. However, for
smaller IP distances, NPs can affect neighboring particles by
depleting their reactant concentrations. This phenomenon is
observed as an overlap in the diffusion profile that limits the
available reactant concentration on the NP surface and reduces
the normalized CO2 flux, which leads to a decrease in the
current density of the CO2 reduction reaction (Figure 3b).
In Figure 3b, it is clear that the CO2 flux saturates at a higher
value for smaller NPs. However, the calculated reactant flux
cannot be directly correlated to the current density, since the
real number of electrons involved in the reaction per CO2
molecule consumed is strongly dependent on the reaction
products. Therefore, a large increase in the current density is
expected when the selectivity shifts from CO and H2 toward
ethylene and methane. However, in cases where the selectivity
remains almost constant, such as for our largest (7.4 nm) NPs,
the electroreduction current will be proportional to the reactant
flux. Figure 3c shows the experimental current density at −1.1
V vs RHE for the 7.4 nm NPs, as a function of the IP/d ratio,
which shows good agreement with the trend in reactant flux
obtained from theory in Figure 3b. The current density toward
H2 evolution will be similarly governed by the diffusion of H+
to the NP surface. As shown in Figure S5 in the Supporting
Information, partial current densities toward CO2 reduction
and H2 evolution both increase with increasing IP/d ratio for
the 7.4 nm Cu NPs.
Figure 3b shows that above an IP/d ratio of ∼10, the reactant
flux to the NP surface reaches a plateau and is not strongly
affected by further increasing the IP distance for each NP size.
Therefore, it is reasonable to assume that, in the large IP
distance regime, the NPs are sufficiently far away from each
other that only the NP size influences the selectivity, not the IP
distance. In order to see this size effect, the Faradaic selectivity
was plotted as a function of NP size only for samples with the
largest IP distances (see Figure 4). In addition to these three
sizes, measurements of 1.9 and 2.3 nm NPs with large IP/d
ratios (14 and 10, respectively) were added to confirm the
trends for the smallest NPs. As expected, the readsorption and
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expected with increasing NP size, in excellent agreement with
our experimental data.
For larger copper coverages, the IP distance may be small
enough to allow the diffusive transfer of reaction intermediates
between neighboring NPs. Therefore, in addition to the NP
size, the IP distance should also be taken into account in order
to understand the selectivity trends. The Faradaic selectivity for
the smallest IP distances of each NP size is shown in Figure 5,
Figure 4. Faradaic selectivity in the large IP distance regime. Faradaic
selectivity as a function of the NP size in the large IP distance regime
(i.e., low Cu coverage), (a) for H2 and CO and (b) for CH4 and C2H4.
Lines are present to serve as guides for the eye.
further reduction of reaction intermediates, such as CO, is
limited by the decreased availability of neighboring particles,
resulting in higher selectivity toward reaction intermediates for
these samples. The H2 and CO production do not vary for large
NPs (4.7 and 7.4 nm), while there is a significant increase in
CO and a suppression in H2 production for the smallest NPs
(Figure 4a). The suppression of H2 may be assigned to CO
poisoning (blockage of active surface sites by CO) that has
been reported in the past for very small NPs with enhanced CO
binding energy. Smaller NPs may have enhanced binding to
reactants and intermediates due to low coordinated sites at the
surface or altered electronic properties, such as the increase in
the work function expected with decreasing NP size. In
addition, the adsorbed CO will weaken the binding strength of
the H*, H2 evolution intermediate on Cu, which further
suppresses H2 production.36
Because of the large IP distances considered here, the
readsorption of reaction intermediate species originating from
one NP onto a neighboring NP is unlikely. Therefore, self-
readsorption processes are likely the only pathways leading to
hydrocarbon production. This explains why the C2H4
production pathway, where the largest number of reaction
steps is involved, is low for all NP sizes (Figure 4b). Also, since
the probability of intermediate self-readsorption is proportional
to the NP size (∝ d2), an increase in the CH4 production rate is
Figure 5. Faradaic selectivity in the small IP distance regime. Faradaic
selectivity as a function of the copper coverage (bottom axis) and IP/d
ratio (top axis) (a) for H2 and CO and (b) for CH4 and C2H4.
Following our diffusion calculations (Figure 3a and Supporting
Information), the Cu coverage was found to be the most
representative parameter to describe the diffusion behavior of the
reactant in a closely spaced NP system. Lines are present to serve as
guides for the eye.
as a function of copper coverage and IP/d ratio, which are
parameters that account for both NP size and IP distance. As
discussed above, the readsorption of reaction intermediates on
neighboring NPs will facilitate the multistep pathway required
for hydrocarbon production, and this process should be
significantly increased in the large copper coverage regime
(e.g., small IP/d ratios). Figure 5 shows that there is a low CO
selectivity and a high selectivity toward CH4 and C2H4 with
increasing coverage, in agreement with such readsorption
phenomena. Very low CO production is expected due to the
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significantly higher hydrocarbon production, which consumes
the CO intermediate.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acscatal.5b02202.
■
S
Despite the fact that the electrolyte used in this study (CO2/
KHCO3) is known to have a buffer effect, the increased
hydrocarbon production is expected to result in a local increase
in the electrolyte pH.37 Considering that the number of H+
species consumed to make hydrocarbon products is large, and
that the normalized current densities on NPs are much higher
than those on Cu foils, the buffer characteristic of the
electrolyte may not be sufficient to counteract the rise in
local pH at the surface of the NPs. Furthermore, an increase in
local electrolyte pH would directly suppress the hydrogen
evolution reaction by shifting its reversible potential more
cathodically. Our experimental results in Figure 5 show a
decrease in H2 production at high Cu coverage where the
hydrocarbon selectivity is highest, possibility due to local
increases in pH near the NP surface.
It is clear from this work, using NP samples with controlled
interparticle distances, that diffusion and readsorption
phenomena are critical processes during CO2 electroreduction.
However, to gain further insight into these phenomena, further
studies of the effect of CO2 mass transport to the electrode
surface would need to be performed. While the experiments
conducted here were done under a constant flow of CO2 in a
well-mixed solution, it is difficult to quantify and control the
transport of CO2 to the catalyst. A possible method to allow
this is to use a flow cell configuration in which the CO2
saturated electrolyte is flowed in a thin layer over the catalysts
at a controllable rate. Other opportunities for further
investigation include getting a better understanding of the
catalyst morphology during the reaction and finding alternative
methods to quantify the electrochemically active surface area.
For the latter, Pb underpotential deposition might be used.
Experimental methods including details on sample
preparation, electrochemical measurements, NP charac-
terization, and diffusion calculations (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: Beatriz.Roldan@rub.de.
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was funded by the Office of Basic Energy Sciences
from the U.S. Department of Energy (under Contract No. DE-
FG02-08ER15995) and by the U.S. National Science
Foundation (No. NSF CHE-1213182). Additional financial
support was provided by the Cluster of Excellence RESOLV
(EXC 1069) at RUB funded by the Deutsche Forschungsge-
meinschaft. Funding by the German Research Foundation
(DFG), through Grant No. STR 596/3-1 under the Priority
Program 1613 “Regeneratively Formed Fuels by Water
Splitting” is gratefully acknowledged. This work received partial
funding by the German Federal Ministry of Education and
Research (Bundesministerium fur Bildung und Forschung,
BMBF) under Grant #03SF0523 - “CO2EKAT”.
̈
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