Supported Pd/Ir bimetallic catalysts were synthesized by the “water-in-oil” microemulsion method at different precursor concentrations and characterized by XRD, XPS, SEM, TEM, and cyclic voltammetry. Depending on the preparation conditions, formation of bimetallic catalysts with different metal segregation and surface composition can be easily obtained, thus tuning the bimetallic structure of catalysts as well as their relative catalytic properties. Bimetallic Pd/Ir systems were efficiently tested in the hydrogenation of cinnamaldehyde showing a better performance than analogous monometallic catalysts.
The synthetic procedure is surely one of the main key factors determining morphology and surface properties of heterogeneous catalysts [
In the present work, supported (carbon and SiO2) Pd/Ir bimetallic catalysts are synthesized by the ME method with a Pd74Ir26 atomic ratio. Their morphological and structural characteristics were determined by XRD, XPS, and electron microscopy (SEM and TEM). Surface properties of PdIr particles, including Pd and Ir surface fractions, and the hydrogen sorption/desorption ability were characterized by means of cyclic voltammetry (CV) measurements. Catalytic tests were performed to gain more insights into Pd/Ir surface properties, taking into account, in particular, the role of Ir in promoting the modification of Pd reactivity. The hydrogenation of cinnamaldehyde was chosen as a model reaction, highlighting the properties of the catalyst surface, such as the distribution/type of the bimetallic structure as well as its role in the activity/selectivity pattern [
Unless otherwise specified, all reagents were purchased from Sigma-Aldrich. Carbon (Vulcan XC72, CABOT) and silica (Davisil 634) were used as supports. The monometallic Pd and Ir catalysts (2 wt.% metal loading) and the bimetallic Pd/Ir systems (2 wt.% of Pd metal and 1.3 wt.% of Ir metal loading) were prepared by the reverse “water-in-oil” microemulsion method as previously described [
All PdIr supported catalysts were synthesized using a total metal loading of 3.3 wt.% (Pd + Ir) and a Pd : Ir atomic ratio of 74 : 26 (Pd74Ir26). During the preparation, a mixture of PdCl42− and IrCl3 solutions having different molar concentrations and cyclohexane as well as surfactant were mixed.
XRD patterns were measured with a Philips X’PERT diffractometer using Cu K
XPS measurements were carried out with a hemispherical analyzer (SES R4000, Gammadata Scienta).
SEM studies were performed with a field emission scanning electron microscope (JEOL JSM–7500 F) equipped with the X-ray energy dispersive system (EDS).
TEM studies were performed on an FEI Tecnai G2 transmission electron microscope operating at 200 kV equipped with EDX analysis.
The cyclic voltammetry (CV) measurements were made by using a CH Instrument (Austin, TX, USA) Model CHI760D workstation with a scan rate of 50 mV using a 0.5 M solution of H2SO4.
Catalytic reactions were performed in a batch glass reactor at a temperature of 50°C and a constant atmospheric pressure of hydrogen (20 cm3 of a cinnamaldehyde solution in toluene having 0.05 mol/dm3 of
The catalyst and the solution were put in the reactor without addition of reagents in order to allow the in situ catalyst activation by hydrogen at a temperature of 50°C before the hydrogenation test. Then, the catalyst was mixed with the reagent solution and the experiment started. The progress of the reaction was measured from the consumption of hydrogen. Samples were withdrawn from the reactor via a tube at intervals of time and analyzed by GC.
XRD patterns of Ir/SiO2, Pd/SiO2, and PdIr-0.2/SiO2 catalysts are shown in Figure
XRD diffraction patterns of Ir/SiO2, Pd/SiO2, and PdIr-0.2/SiO2.
Accordingly, peaks of PdIr-0.02, PdIr-0.04, PdIr-0.08, PdIr-0.14, and PdIr-0.2 are presented in Figure
XRD diffraction patterns of silica-supported PdIr-0.02, PdIr-0.04, PdIr-0.08, PdIr-0.14, and PdIr-0.2 catalysts.
Physicochemical properties of investigated catalysts: metal particle characterization (average size, the overall composition measured by EDS, and the local composition measured by STEM-EDX methods) and the initial rate of cinnamaldehyde hydrogenation.
Catalyst | Metal particle size |
Lattice parameter (Å) | Pd : Ir atomic ratio | Initial rate |
TOF (s−1) | ||
---|---|---|---|---|---|---|---|
XRD | SEM/TEM | EDS | STEM-EDX | ||||
Pd | 6.0 ± 0.5 | 6.0 ± 0.5 |
3.906 | — | — | 24.4 | 0.121 |
PdIr-0.02 | 4.3 ± 0.5 | 4.4 ± 0.5 | 3.883 | 77 : 23 | 75 : 25 | 46.0 | 0.138 |
PdIr-0.04 | 4.4 ± 0.5 | — | 3.883 | — | — | 31.5 | 0.091 |
PdIr-0.08 | 4.5 ± 0.5 | 4.5 ± 0.5 | 3.884 | 74 : 26 | 72 : 28 | 14.0 | 0.042 |
PdIr-0.14 | 4.6 ± 0.5 | — | 3.884 | — | — | 10.1 | 0.031 |
PdIr-0.2 | 4.9 ± 0.5 | 4.8 ± 0.5 | 3.885 | 78 : 22 | 73 : 27 | 7.4 | 0.024 |
Ir | — | 4.2 ± 0.5 |
3.840 |
— | — | — | — |
SEM images indicate that, for PdIr bimetallic catalysts, particles are almost dispersed over the support (Figure
SEM images (magnification 100 000) of bimetallic (a) PdIr-0.02/C, (b) PdIr-0.08/C, and (c) PdIr-0.2/C catalysts.
Particles size distribution, as determined by the TEM analysis, indicates a narrow range of 2–7 nm and the average particles size was calculated by counting over 100 particles for each catalyst (Figure
TEM micrographs of bimetallic (a) PdIr-0.02/C, (b) PdIr-0.08/C, and (c) PdIr-0.2/C catalysts with the corresponding particle-size distribution diagrams.
The catalysts were also examined by X-ray photoelectron spectroscopy (XPS) experiments. The surface composition of Pd and Ir (in at.%) and the binding energies of palladium Pd 3
XPS data and surface fraction of Pd and Ir components determined by CV measurements.
Catalyst | XPS data | Surface fraction (CV) | |||||
---|---|---|---|---|---|---|---|
Pd (at.%) | Ir (at.%) | Pd 3 |
BE shift (eV) | Ir 4 |
Pd | Ir | |
Pd | — | — | 335.3 | — | — | 1 | |
PdIr-0.02 | 1.95 | 1.03 | 335.8 | 0.5 | 61.7 | 0.85 | 0.15 |
PdIr-0.04 | — | — | — | — | — | 0.83 | 0.17 |
PdIr-0.08 | 1.33 | 0.86 | 335.4 | 0.1 | 60.9 | 0.81 | 0.19 |
PdIr-0.14 | 1.04 | 0.75 | 335.5 | 0.2 | 61.0 | 0.80 | 0.20 |
PdIr-0.2 | 0.71 | 0.74 | 335.7 | 0.4 | 61.2 | 0.77 | 0.23 |
Ir | — | — | — | — | 60.7 | 1 |
The binding energy of Pd 3
The catalysts were also characterized by cyclic voltammetry (CV) in order to examine more accurately the structure of bimetallic particles. Voltammograms, recorded for all catalysts, are shown in Figure
Cyclic voltammograms for mono Pd and bimetallic PdIr catalysts in N2-saturated 0.5 M H2SO4 solution with a scan rate of 50 mV/s. The insets are magnified views of the hydrogen desorption region (0.0–0.4 V) and the metal surface reduction region (0.5–0.9 V).
Peaks corresponding to adsorption and desorption of hydrogen are clearly detected in bimetallic systems together with a shift of the hydrogen desorption component (magnification, area A of Figure
For the Pd catalysts, a reduction peak of PdO is observed at the potential of
The surface composition of bimetallic particles (Pd and Ir surface fraction) was calculated from formula (
This method was, so far, used by many authors for the analysis of bimetallic films, as well as for the determination of the surface composition of particles in PdPt/C, PdAu/C, PdIr/C, and PdPtAu/C catalysts [
For all catalysts, the surface fraction of Ir is smaller (0.15–0.23) than that of the bulk composition (0.26 for the Pd74Ir26 sample), indicating the palladium enrichment of the surface and the segregation of Ir in the core of particles. Nevertheless, the effect of the surface increase of iridium on increasing the precursor concentration is evident. The lowest surface fraction is observed in the case of the PdIr-0.02 catalyst, while for the catalyst synthesized from the precursor solution with the highest concentration (PdIr-0.2), the surface composition of Ir is almost the same as that of the bulk composition (0.23). Moreover, by analyzing the hydrogen desorption peak shapes (Figure
Monte Carlo simulations [
In order to verify the effect of both different iridium surface fraction and distribution, the catalytic hydrogenation of cinnamaldehyde was carried out. Simplified possible cinnamaldehyde hydrogenation pathways are presented in Scheme
Possible ways for the hydrogenation of cinnamaldehyde (CAL).
The catalytic test shows that hydrogenation of cinnamaldehyde leads to hydrocinnamaldehyde (HCAL), cinnamyl alcohol (COL), and the corresponding saturated alcohol (HCOL).
Selectivity to saturated alcohol (HCOL) obtained at 80% conversion of CAL with the TOF values for Pd and PdIr catalysts.
Furthermore, the activity decreases on increasing the precursor concentration used in the synthesis, as inferred from the highest TOF value, detected for the sample PdIr-0.02 (0.138 s−1) compared to the lowest one, calculated for the PdIr-0.2 catalyst (0.024 s−1). All bimetallic catalysts, except PdIr-0.02, exhibit also a minor activity than the monometallic Pd catalyst. Yang et al. [
In our catalytic systems, it can be suggested that differences in both activity and selectivity may result from a different promotional effect of Ir, which, in turns, depends on the surface architecture of the PdIr species. There is, in fact, a clear tendency of the surface growth of the Ir fraction from 0.15 to 0.23 with the increasing concentration of the precursor solution (CV studies). Furthermore, in all PdIr catalysts, Ir is present on the surface; however, in the catalyst obtained from the lowest precursor concentration, the fraction of Ir is the lowest. Analogous results were also obtained with Monte Carlo simulations given by Tojo et al. [
Supported PdIr bimetallic catalysts were synthesized by the “water-in-oil” microemulsion method with different precursor concentrations. The concentration of the precursor solutions (0.02 M, 0.04 M, 0.08 M, 0.14 M, and 0.2 M) has a significant influence on the type of PdIr bimetallic particles. XRD, TEM, and cyclic voltammetry (CV) studies confirm the formation of bimetallic structures containing Pd and Ir with different Ir fractions on the surface. For the PdIr-0.02 catalyst obtained from the most diluted precursor solution (0.02 M), the highest activity in the hydrogenation of cinnamaldehyde and the highest selectivity in the hydrogenation of the C=O bond were demonstrated. It can therefore be concluded that not only the difference in reduction rates of both metals but also synthesis conditions may play an important role in the process of nucleation, growth, and formation of bimetallic particles.
The data used to support the findings of this study are included within the article. Other data are available from the corresponding author upon request.
The authors declare that there are no conflicts of interest.
T. S. acknowledges financial support received under the action