Detecting the Genesis of a High-Performance Carbon-Supported Pd Sulfide Nanophase and Its Evolution in the Hydrogenation of Butadiene

Article abstract of DOI:10.1021/acscatal.5b00896

A new procedure for preparation of palladium sulfide nanoparticles, which are deposited and anchored over highly graphitized carbon nanofibers, is presented. The preparation method is based on the use of PdSO₄ as metal precursor or alternatively in the previous functionalization of the carbon surfaces with sulfonic groups by treatment with fuming sulfuric acid. Using an in situ high-energy X-ray diffraction technique, in both cases it is demonstrated that during the reduction treatment, the initially present palladium hydride is transformed into a palladium sulfide (Pd₄S). The catalytic properties of these materials have been tested in the gas-phase butadiene partial reduction to butenes. Although metallic palladium nanoparticles supported in the same carbon fibers produce butane as the principal product, the supported Pd₄S nanocrystals mainly yield different isomers of butenes independently of the conversion level. Furthermore, applying the same X-ray diffraction method reveals that this catalytic phase is stable during reaction.

Full text of DOI:10.1021/acscatal.5b00896

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Detecting the genesis of a high performance carbon
supported Pd sulfide nanoꢀphase and its evolution in
the hydrogenation of butadiene
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a
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b
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Belén Bachiller-Baeza, Ana Iglesias-Juez, Eva Castillejos-López, Antonio Guerrero-Ruiz,*
c
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Marco Di Michiel, Marcos Fernández-García and I. Rodríguez-Ramos *
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Instituto de Catálisis y Petroleoquímica, CSIC, C/Marie Curie 2, 28049 Madrid (Spain).
b
Departamento de Química Inorgánica y Química Técnica, UNED, C/ Senda del Rey 9, 28040
Madrid (Spain).
c
ESRF, 6 Rue Jules Horowitz, 38043 Grenoble (France).
ABSTRACT. New procedure for preparation of palladium sulfide nanoparticles, which are
depositedꢀanchored over highly graphitized carbon nanofibers, is presented. The preparation
method is based in the use of PdSO as metal precursor or alternatively in the previous
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funtionalization of the carbon surfaces with sulfonic groups by treatment with fuming sulfuric
acid. Using “in situ” highꢀenergy Xꢀray diffraction technique, in both cases is demonstrated that
during the reduction treatment the initially present palladium hydride is transformed into a
palladium sulfide (Pd S). The catalytic properties of these materials have been tested in the gas
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phase butadiene partial reduction to butenes. While metallic palladium nanoparticles supported
in the same carbon fibers produce butane as principal product, the supported Pd S nanocrystals
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mainly yield different isomers of butenes independently of the conversion level. Furthermore
applying the same Xꢀray diffraction method is revealed that this catalytic phase is stable during
reaction.
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KEYWORDS: catalyst synthesis, palladium sulfide, Xꢀray diffraction, hydrogenation reaction,
structureꢀactivity relationship.
1. INTRODUCTION
Partial hydrogenation of alkynes and alkadienes in the presence of alkenes is one of the most
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important processes in the chemical industry and selectivity is a relevant issue. Supported
palladium catalysts are currently used for the hydrogenation of 1,3ꢀbutadiene, but because they
lack selectivity to the desired nꢀbutenes when the ratio of monoalkenes to dienes is high,
promoters (Ag, Au, Ga or Cu) or modifiers (CO or sulfur) are needed to enhance the yield of
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, 3, 4, 5, 6
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partially hydrogenated products.
The reaction is known to be structureꢀsensitive and it
has been reported that Pd catalysts under reaction can accommodate carbon atoms in subsurface
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layers that affect the hydrogen diffusion improving the alkene selectivity.
The use of carbon materials as supports of metal nanoparticles is widely employed for
preparing heterogeneous catalysts, among others because as consequence of the moderate metalꢀ
graphite interaction new chemical compositions can be easily synthesized, i.e. intermetallic
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compounds or transition metal sulphides. In addition, modification of carbon surface with
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functionalities has been used for anchoring metal nanoparticles to the carbon surface.
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Supported palladium sulfides catalysts are generally synthesized through preꢀsulfidation of
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supported Pd metal catalysts using H S Na S or organic sulfurꢀcontaining molecules,
but
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such methods have the problems of high environmental pollution and scaling cost. Recently,
thiolate selfꢀassembled monolayers were used to block specific active sites on Pd/Al O during
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the hydrogenation of furfural. The adsorbed thiolate modifiers serve the role of isolating certain
highly active sites for hydrodeoxygenation reaction, restricting selectivity toward undesired
products. Moreover, preformed palladium sulfide nanoparticles grafted on graphene oxide (GO)
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have been reported for catalysis of the Suzuki coupling reaction. In this latter case, the Pd
sulfide nanoparticles were prepared by thermolysis of an organometallic precursor containing
PdꢀS bonds. This method reduces the toxicity risks and contamination. However, the palladium
sulfide/GO composite suffers significant leaching of the active Pd during the catalytic reaction
what hinders its recyclability. Therefore, it seems necessary to develop new simple and safe
methods to synthesize supported palladium catalysts and obtain more detailed insight into their
evolution and stability under reaction conditions.
Herein, we present a study showing the formation and the evolution under reaction conditions
of Pd sulfide nanoparticles supported on carbon nanofibers by coupling time resolved highꢀ
energy Xꢀray diffraction (HEXRD) and mass spectrometry (MS). This approach allows us to
simultaneously investigate in a time resolved manner the structure–activity relationship of the
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supported Pd nanoparticles, and the compounds that are formed or converted over them. We
characterize the nature of the Pdꢀbased nanoparticles generated by reduction under hydrogen
flow at 250 ºC of two metal precursors, Pd chloride and Pd sulfate, deposited (1wt% of Pd) on
carbon nanofibers functionalized with sulfonic or carboxylic groups, respectively. Also, we study
the evolution of these Pd based catalysts under butadiene hydrogenation reaction conditions.
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. EXPERIMENTAL SECTION
Preparation of carbon supports. A commercial carbon nanofibers was used in this study:
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Pyrograph III PR24ꢀHHT (SBET = 32 m /g) provided by Applied Sciences Inc. The PR24ꢀHHT
fibers (denoted as HHT) was originally treated at high temperature (~3030 ºC), possess a
stackedꢀcup morphology with a hollow core through the length of the fiber and also present a
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jagged outer surface with ‘‘round heads’’ or ‘‘loop’’ structures which connect several layers.
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HHT was functionalized according to reported procedures yielding HHTox. Briefly, parent
carbon nanofiber was refluxed in 50% nitric acid (carbon/HNO = 1 g/10 mL) at 100 °C for 48 h
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in order to increase the concentration of functional groups on the surface. HHT was also
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subjected to a sulfonation treatment. Aliquots of 3 g of fibers were immersed in 150 mL of
fuming sulfuric acid (15 wt % SO ) at 80 ºC under a nitrogen gas atmosphere for 7 h. After the
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treatment, the suspension was washed with deionized water to remove any excess sulfuric acid in
the carbon nanomaterial, filtered and dried at 120 ºC. The resulting modified sample was labelled
HHTꢀSO H. Elemental chemical analysis of carbon, hydrogen and sulfur was carried out in a
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LECO CHNSꢀ932 instrument. The results reveal a 1wt% content of sulfur.
Catalyst preparation. Catalysts were prepared by incipient wetness impregnation of the
supports with water solutions of PdCl or PdSO . The catalysts were prepared with a 1 wt% Pd
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loading. In the case of PdCl , the precursor was dissolved in concentrated HCl to generate
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tetrachloride palladium acid (H PdCl ), heated to eliminate chlorine, dissolved in distilled water,
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dried and again diluted in water until the final volume was reached. This solution was dropped to
the carbon nanofiber supports (HHTox and HHTꢀSO H).
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Characterization details. The chemical nature of the functional groups of the two supports
was evaluated by temperature programed desorption coupled with mass spectrometer (TPDꢀMS)
experiments under vacuum in a conventional volumetric apparatus connected to a RGAꢀ200 SRS
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mass spectrometer. The sample was evacuated for 30 min at room temperature and then
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ramped to 1023 K at a 10 K min rate.
All the samples were characterized by Xꢀray photoelectron spectroscopy (XPS),
thermogravimetry and transmission electron microscopy (TEM) analyses. Xꢀray photoelectron
spectra of the supports and catalysts were recorded with an ESCAꢀPROBE P (Omicron)
spectrometer by using nonꢀmonochromatized MgꢀKα radiation (1253.6 eV). The binding energy
of Pd 3d and S 2p were internal referenced to the C 1s line at 284.6 eV. The error in
determination of electron binding energies did not exceed 0.2 eV. Each sample was pressed into
a small pellet of 15 mm diameter and placed in the sample holder and degassed in the chamber
for 6ꢀ8 h to achieve a dynamic vacuum below 10ꢀ8 Pa before analysis. The spectral data for each
sample was analyzed using CASA XPS software. The relative concentrations and atomic ratios
were determined from the integrated intensities of photoelectron lines corrected for the
corresponding atomic sensitivity factor.
Transmission electron microscopy (TEM) images of the catalysts were measured using a JEOL
JEMꢀ2100 fieldꢀemission gun electron microscope operated at 200 kV. The samples were
ground, ultrasonically suspended in ethanol and then dripped onto a carbonꢀcoated copper grid
before TEM images were generated. The mean diameter (d) of the Pd particle size were
calculated based on a minimum of 100 particles.
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Highꢀenergy Xꢀray diffraction (HEXRD) data were collected at the ID15A beamline at the
ESRFꢀThe European Synchrotron with a wavelength of 0.17750 Ȧ (69.85 KeV), using a Perkin
Elmer area detector and a sampleꢀdetector distance of 650 mm. The diffraction patterns were
collected with an exposure time of 30 seconds. Diffraction data were fitted using the Rietveld
method with the XPert Highscore Plus program (Panalytical). The reactor and heat gun were
mounted on a Huber stage capable of translations in the x, y and z directions. The carbon fibres
supported catalysts were pressed into pellets and sieved to a size of 0.075 to 0.150 mm. Aliquots
of these pellet samples were loaded in a fixed bed quartz tube of 2 mm internal diameter, located
between two glass wools. The flow of reaction gases feed to the reactor was constant at 20
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ml·min , with compositions of 15% hydrogen in helium during temperature programmed
reduction experiments and 15% hydrogen+3% butadiene in helium during hydrogenation
reaction conditions. H /H +Bd/H alternate exposures at constant increasing temperatures were
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applied with an exposure time of 30 min for each atmosphere. The reactor was heated with one
Leister LE mini heat gun fitted with a heat spreader. The temperature inside the catalytic bed was
measured using a thermocouple inserted into the carbon supported sample. The range of
operation temperatures was from room temperature up to 550 K. The exhaust products from the
reactor were analyzed using a European Spectrometry ecoSystꢀP ManꢀPortable mass
spectrometer with capillary inlet and heated inlet tubes.
Catalytic measurements. Steady state catalytic experiments for butadiene (Bd) hydrogenation
were carried out in a continuous flow fixedꢀbed reactor (4 mm inner diameter). Before reaction,
the catalyst was pretreated in flowing H at 250 ºC for 1 h. The reactants H (10% vol) and Bd (2
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%
vol) with the balance of N passed through the catalyst bed at a total flow rate of 60 mL/min.
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The hydrogen amount was in large excess. Any significant massꢀtransfer limitation was
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precluded by using this high linear velocity and powder (sieve fraction 150ꢀ250 ꢀm) catalysts.
The occurrence of external diffusion limitations was ruled out by running tests where both the
flow rate and the catalyst amount were significantly changed while keeping the mass/flow rate
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ratio constant (8.3·10 g·h/ml). The reaction temperature was varied between room temperature
and 473 K at atmospheric pressure. The reactor effluent was onꢀline analyzed using a gas Varian
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400 gas chromatograph with flame ion detector (FID) and thermal conductivity detector (TDC)
with a 20% BMEA Chromosorb P80/100 column.
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. RESULTS AND DISCUSSION
Table 1 summarizes the performance of the PdCl ꢀHHTꢀSO H and PdSO ꢀHHTox catalysts in
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the selective hydrogenation of butadiene (Bd) obtained in a fixed bed reactor with the reaction
mixture of H :Bd:N of 10:2:88. The PdCl ꢀHHTox sample is included as a reference for
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comparison. This latter catalyst displays high activity (100% conversion of butadiene is reached
at room temperature) but is totally unselective to partial hydrogenation, as butane was the only
product. However, when sulfur is initially present in the catalyst precursor, either as surface
sulfonic groups on the support or as moieties coming from the Pd precursor, the reduced catalyst
(
250 ºCꢀ2h in hydrogen) shows high selectivity for the hydrogenation of butadiene to butenes
(99% of butenes in the case of PdSO ꢀHHTox at 100% conversion) although the activity is
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somewhat reduced as the selectivity increases from PdCl ꢀHHTꢀSO H to PdSO ꢀHHTox.
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Clearly, the presence of the sulfur modifier has to be related with the selective performance of
the PdCl ꢀHHTꢀSO H and PdSO ꢀHHTox catalysts, in special in the last case. Such marked
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changes in selectivity require further investigation.
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Table 1. Results characterizing the performance of Pd catalysts in the selective hydrogenation of
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butadiene.
Sample
T (ºC)
T100 (ºC)
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S30 (%)
S100 (%)
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PdCl ꢀHHTox
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PdSO ꢀHHTox
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PdCl ꢀHHTꢀSO H
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a
T and T , temperature at which the 30% and 100% conversion of butadiene is achieved.
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S and S , selectivity toward partial hydrogenation at 30% and 100% conversion. Reaction
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performed in a fixed bed reactor with the reaction mixture of H :Bd:N of 10:2:88, 30 mg
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catalyst.
Figure 1 presents the HRTEM pictures and histograms with the size distribution of palladium
particle diameters for the different catalysts reduced at 250 ºC. A broad range of particle sizes
and a relative regular distribution of the Pd particles over the support are observed for the distinct
catalysts. Both features can arise from the high graphitization degree and low defect density
produced by surface functionalization of the support. However, the particle size distribution is
shifted towards smaller sizes for the catalysts containing sulfur in the catalyst precursor. Thus,
between 65ꢀ75% of the metallic particles in PdSO ꢀHHTox and PdCl ꢀHHTꢀSO H catalysts have
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diameters below 4 nm. The micrographs (Figure 1 and Figures S2ꢀS4 in supplementary
information) show the spherical nature of the Pd particles for the different catalysts, particularly
for the PdCl ꢀHHTox, which, as said, could be consequence of a weak interaction between the
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particles and the nanofibers.
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Figure 1. HRTEM micrographs and histograms with the size distribution of particle diameters
for a) PdCl ꢀHHTox, b) PdSO ꢀHHTox and c) PdCl ꢀHHTꢀSO H catalysts reduced at 250 ºC.
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The mean particle diameters (d) of Pd calculated from TEM measurements of the different
samples are displayed in Table 2. The mean volume particle diameters (d ) were also estimated
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and included in Table 2. The mean volume particle diameter, sensitive to the volume of particles,
gives more weight to the larger particles and is more appropriate than mean particle size for
comparison with data obtained from physical techniques such as Xꢀray line broadening or
magnetic methods.
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Table 2. Mean particle size (d) and volumeꢀsurface mean particle size (d ) estimated from TEM
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measurements.
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Catalyst
Σn d /Σn (nm)
Σn d /Σn d (nm)
i i
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PdCl ꢀHHTꢀSO H
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Characterization under realistic reaction conditions of supported nanoparticles with spread
sizeꢀdistribution of Pd nanoparticles and mean size around 4.5 nm (from TEM measurements) is
not an easy task since no so many methods can be applied to this type of materials. Conventional
Xꢀray diffraction techniques (XRD) performed at laboratory diffractometer provide non useful
information on low loading metal supported catalysts as due to the low signal intensity. In
contrast, using synchrotronꢀbased radiation the high energy Xꢀray diffraction (HEXRD) can
provide structural information on finely dispersed palladium on catalysts under reaction
conditions. Physical methods, such as high resolution transmission electron microscopy
(HRTEM), fail when these nanoparticles have to be studied in situ in a catalyzed reaction, in fact
HRTEM measurements should be performed under high vacuum conditions, moreover statistical
measurements of microdiffraction over a large number of particles are difficult to achieve.
To investigate the nature of the Pdꢀbased nanoparticles in the different catalysts during the
temperature programmed reduction in hydrogen we use HEXRD coupled with MS. Color map
representation in Figure 2 shows the variation of the HEXRD patterns for the Pd catalysts from
room temperature to 250 ºC.
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Figure 2. Intensity contour maps of the HEXRD patterns obtained during temperature
programmed reduction in hydrogen/helium from room temperature up to 250 ºC. Initial and final
HEXRD patterns are showed on the left and right of panels, respectively. MS (m/z = 34 and 64)
analysis of gases evolved during the heating treatment is showed in the blue graph insets.
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When the PdCl ꢀHHTox sample (upper panel) is exposed to hydrogen/helium at room
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temperature the HERXD pattern reveals the formation of the β phase of Pd hydride (peak (111)
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at 2.70 Å ). Heating to higher temperature it is seen that above 90ºC the most intense (111)
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reflection shifts to higher Q value (2.80 Å ) due to the lattice contraction as hydrogen is released
from the hydride phase to form metallic Pd (lattice constant at 250 ºC, a= 3.904 Å).
The structural evolution of the sample prepared with palladium sulfate (PdSO ꢀHHTox, middle
4
panel) is different after the transformation into metallic Pd at 90 ºC because as the temperature
approaches the 150 ºC a second transformation occurs, resulting in the formation of the Pd4S
ꢀ1
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phase ((210) and (112) peaks at 2.74 and 2.83 Å , respectively). The formation of the Pd
sulfide phase is further described with the help of MS. In Figure 2 as an inset onto color maps
selected MS signals labelled m/z = 34 and m/z = 64 are plotted. The first one is associated to H2S
while the m/z = 64 corresponds to SO . Both products stem as main gas residues coming from
2
the decomposition/reduction of the sulfate anions of the metal precursor deposited on the support
and appear with a constant level from 100 to 160 ºC. At this latter temperature Pd is completely
transformed to the single well crystalized Pd S structure (lattice constants at 250 ºC, a= 5.137 Å,
4
c = 5.623 Å). Although sulfidation of supported metallic palladium with H S or Na S usually
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leads to mixed crystal phases, formation of a single phase seems to be favored when Pd is
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0
supported on carbon. The formation of palladium sulfides supported on carbon is highly
dependent on the composition of the sufidation atmosphere and temperature. Single phase of
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Pd S was reported at 150ºC under H S/H ratios between 0.008 and 0.25.
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In the case of the PdCl ꢀHHTꢀSO H (lower panel), the complete transformation of β phase of
2
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Pd hydride to Pd is observed at 90ºC. From 150 ºC and above up to 250 ºC only part of the Pd
metal is transformed to Pd S (lattice constants, a = 5.136 Å, c = 5.619 Å). The presence of a
4
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minor metallic Pd phase is successfully tracked out in the high resolution XRD patterns
presented in Figure 2 (bottom). HEXRD fitting using the Rietveld method reveals that a 21 wt%
of metallic Pd remains (see Supporting Information, Table S1). From the MS analysis (inset,
lower panel), we learn that the evolution of H S and SO is appreciably lower than for the
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PdSO ꢀHHTox sample and clearly no enough to transform the whole metallic Pd into Pd S. Both
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fresh samples, PdSO ꢀHHTox and PdCl ꢀHHTꢀSO H, have similar initial sulfur content, 1wt%
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SO and 1wt% SO H, respectively.
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Surface chemical analysis by XPS of fresh samples clearly evidenced the presence of sulfur on
the surface, since the characteristic peak stemming from the overlapped S 2p3/2 and S 2p1/2
doublet (spinꢀorbit splitting of 1.2. eV, see Figure S7 in supporting information) was revealed at
around 169 eV which is characteristic of sulfate (Figure 3) and ꢀSO H groups (result not shown).
3
After reduction at 250 ºC, the XPS spectra show that the PdCl ꢀHHTꢀSO H sample still has
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sulfonic groups on the surface while the surface of PdSO ꢀHHTox sample is practically free of
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sulfur. Moreover, no S 2p peaks emerging from sulfide or S species, expected at 161.5 and
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163.5ꢀ164 eV respectively, were found.
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catalysts reduced at 250 ºC and the fresh PdSO ꢀHHTox.
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The XPS spectra for the doublet Pd 3d5/2 and 3d3/2 of the fresh PdSO ꢀHHTox and the
4
PdSO ꢀHHTox and PdCl ꢀHHTꢀ SO H catalysts reduced at 250 ºC are shown in Figure 4. It is
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+
observed that the Pd 3d5/2 peak shifts from 337.3 eV (Pd in the PdSO ꢀHHTox sample) to
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335.5 eV for the two reduced samples. This value of binding energy (Pd 3d5/2 at 335.5 eV)
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suggests that the oxidation state of Pd is zero in both reduced catalysts.
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Figure 4. XPS spectra of the Pd 3d core level for the PdSO ꢀHHTox and PdCl ꢀHHTꢀ SO H
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catalysts reduced at 250 ºC in hydrogen and the fresh PdSO ꢀHHTox sample.
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It is worth noting that our results are very close to those previously obtained in a study on nonꢀ
supported Pd S nanoparticles where a zero oxidation state for both Pd and S was found by XPS
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15
measurements. In this latter work the possibility of charge transfer from Pd to S in Pd S is also
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discussed based on a marginal shift of the Pd 3d binding energy with respect to that of pure Pd.
In our case the difference between the value of 335.5 eV for the Pd 3d5/2 in the reduced PdSO4ꢀ
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HHTox and PdCl ꢀHHTꢀ SO H catalysts and those found in the literature for Pd zero is of 0.2
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eV as maximum, which is within the error of determination. Therefore, the XRD identified Pd4S
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phase may be described as an alloy type compound with structure basically similar to Pd where
the S atoms occupy substitutional sites resulting in some distortions. In addition, it has been
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reported that Pd S can intrinsically behave as a metal, especially in relation to conducting
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electrons. The values of mean particle size estimated using Scherrer equation for the Pd4S
phase are of 17.4 and 10.8 nm for PdSO ꢀHHTox and PdCl ꢀHHTꢀSO H catalysts reduced at 250
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ºC, respectively.
Figure 5 shows the evolution of Pd phases of reduced catalysts under butadiene
hydrogenation reaction atmosphere at different temperatures and during H /H +Bd/H
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alternate exposure. Once the catalysts were treated in hydrogen at 250 ºC the temperature
was cooled down to the corresponding reaction temperatures which were selected for
each catalyst within the range of activity shown in Table 1.
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Figure 5. HEXRD patterns obtained under consecutive H (), H +Bd (ꢀ ꢀ ꢀ), H (…)
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atmospheres at several constant temperatures and MS results for selected m/z values under
H +Bd. Upper panels: PdC ꢀHHTox; a: RT, b: 150 ºC. Middle panels: PdSO ꢀHHTox; a: 100
2
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ºC, b: 160 ºC. Lower panels: PdCl ꢀHHTꢀSO H; a: 100 ºC, b: 140 ºC, c: RT.
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The structural study of the reference PdCl ꢀHHTox sample reveals at room temperature no
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changes in the lattice parameters of the formed βꢀPd hydride phase during the H /H +Bd/H
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alternate exposure. The same applies for the Pd phase at 150 ºC which remains unchanged under
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the different atmospheres. MS analysis under H +Bd atmosphere confirms that both phases of
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palladium (βꢀPd hydride and metallic Pd) are active at the corresponding reaction temperatures.
In Figure 5 signals labelled m/z = 39, 41 and 43 are plotted. The first one is associated to the
reactant, butadiene, while the other two correspond to the reaction products butenes and butane,
respectively. It is observed the nonꢀselective total hydrogenation of butadiene and major
production of butane. This PdCl ꢀHHTox sample has not preferential growth of particles along a
2
given crystallographic direction and is made of spherical nanoparticles (see HRTEM
micrographs at Figure 1 and at Supporting Information, Figure S2), thus, it could be assumed
that exposes a major fraction of the most thermodynamically stable (111) surfaces. Earliest
comparative studies with different nanostructured Pd catalysts established that the (100) facets
are more active for the butadiene hydrogenation, but also more selective for ceasing the reaction
26,27
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at the butenes than the (111) surfaces.
Similarly, Silvestre et al. also observed that (110)
surfaces are particularly selective for the hydrogenation of dienes into alkenes. Moreover,
selective partial hydrogenation of dialkenes and alkynes to alkenes on Pd catalysts has been
related with the formation of subꢀsurface layers of Pd carbide (PdC ) which limits the diffusion
x
2
,9
of hydrogen and its availability for hydrogenation reaction.
The isotropic palladium
nanoparticles and the absence of Pd carbide phase (detectable in our experimental conditions for
2
8
a chemical composition PdC , with x larger than 0.002) in the PtCl ꢀHHTox sample upon
x
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exposition to the reaction atmosphere at both temperatures (RT and 150 ºC) likely explain its
unselective catalytic behavior in the butadiene hydrogenation.
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The same HEXRD study carried out over PdSO ꢀHHTox sample reveals also no structural
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changes in the phase formed during reduction in hydrogen at 250 ºC (Figure 5). Thus, the lattice
parameters of the Pd S phase remains constant, with variations far below the estimated error of ±
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0
.004 Å, during the time of exposition to reaction mixture and switching to hydrogen at both 100
and 150 ºC temperatures. In addition, the MS analysis shows, in agreement with the previous
steady state catalytic studies (Table 1), the high selectivity of the catalyst for the partial
hydrogenation of butadiene (m/z = 39) into butenes (m/z = 41). Theoretical investigations of H2
dissociation on the faces of Pd S indicate that dissociation barriers are higher than on Pd (111)
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face, although low enough to allow the partial hydrogenation of dialkenes and alkynes.
Therefore, the Pd S phase constitutes an active and selective catalytic phase for partial
4
hydrogenation of polyunsaturated hydrocarbons. Interestingly, the palladium sulfide phase is
stable during the H /H +Bd/H alternate exposures whereby regeneration of the catalyst with
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,3
further feeding a sulfur compound, common practice in hydrorefining plants, is likely
unneeded. The stability of the Pd S phase in the PdSO ꢀHHTox catalyst under reaction
4
4
conditions is also proved by a long term catalytic run performed in fixed bed reactor at 100 ºC,
with 60 ml of the reaction mixture of H :Bd:N (10:2:88) and 30 mg of catalyst (Figure 6). It can
2
2
be seen that activity and selectivity were maintained during the run.
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Figure 6. Stability test carried out at 100 ºC on the PdSO ꢀHHTox catalyst.
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Similarly, in the case of PdCl ꢀHHTꢀSO H sample (Figure 5 lower panels), the two phases
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alternate exposures at 100 and 140 ºC. Strikingly, when the temperature is cooled down to room
temperature in hydrogen atmosphere the foreseeable change in the Pd metal d spacing due to the
βꢀPd hydride phase formation does not occur. Likely, we have nanoparticles with a Pd@Pd4S
coreꢀshell nanostructure where the Pd S shell hinders the diffusion of hydrogen to the Pd core. It
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metallic Pd.
Considering the results of our temperature programmed reduction in hydrogen experiments
where we used HEXRD coupled with MS experiments (Figure 2) and in agreement with the
3
1
literature, we believe that the sulfidation process is a stepwise process which comprise (1)
decomposition of H S and SO precursors (sulfate anions and sulfonic groups for PdSO ꢀHHTox
2
2
4
and PdCl ꢀHHTꢀSO H, respectively), (2) adsorption of these gaseous molecules on the surface of
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3
the metallic Pd nanoparticles initiating the sulfidation at the surface at around 150 ºC and (3)
finally progressing of sulphur into the bulk of Pd particles. Hence, the formation of bulk Pd4S
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nanoparticles in the reduced PdCl ꢀHHTꢀSO H catalyst is likely precluded for the limited
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decomposition, at temperatures below 250 ºC, of the sulfonic groups covalently anchored to the
surface of the support (see Supporting information, Figure S1).
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The MS analysis under reaction (Figure 5 lower panels) shows a high selectivity to partial
hydrogenation at 100 ºC while at 140 ºC some proportion of butane is produced. Moreover, this
PdCl ꢀHHTꢀSO H sample, in agreement with catalytic studies (Table 1), is somewhat more
2
3
active than the PdSO ꢀHHTox. The higher catalytic activity of the former can be clearly
4
attributed to the larger dispersion of the active Pd S phase. Moreover, the smaller particle size in
4
this sample turns into a higher proportion of unsaturated sites at edges and corners on where the
H dissociation barrier becomes likely reduced and therefore the partial hydrogenation of
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,29
dialkenes deprived at higher reaction temperatures.
In summary, we have developed a simple and green method to produce an active, very stable
and highly selective catalyst for the butadiene hydrogenation (here named PdSO ꢀHHTox) based
4
on the pure Pd S phase supported on carbon nanofibers. The palladium sulfide precursor is a
4
common palladium sulfate salt impregnated on the carbon support. The catalyst is produced in a
single step via reduction process in hydrogen atmosphere without further sulfidation.
4. CONCLUSIONS
Based on our observations by combining HEXRD and MS measurements we propose that the
reduction under hydrogen of the carbon supported palladium sulphate precursor, PdSO ꢀHHTox
4
sample, provides the Pd S based catalyst with superior selectivity to partial hydrogenation of
4
dialkenes. This approach supposes an easy and confident method to synthesize a wellꢀdefined
single palladium sulfide structure. Contrarily to Pd metal, where thermodynamically nonꢀstable
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surfaces and morphologies are required to obtain the desired selectivity, here we present a Pd4S
phase with significant activity, appropriate selectivity and high stability under reaction
conditions. Moreover, an effect of particle size of the Pd S phase on the selectivity to partial
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hydrogenation has been also observed, crystallites above 11 nm are likely needed for a full
selectivity to partial hydrogenation of butadiene. Finally we note that the experimental procedure
to obtain the sulfide phase is rather simple, nonꢀrequiring inꢀsitu sulfidation steps.
ASSOCIATED CONTENT
Supporting Information. Figure S1 contains TPD–MS profiles of the HHTox and HHTꢀ
SO H supports, Figures S2ꢀS4 contain TEM micrographs of the different catalysts, Figures S5
3
and S6 contain additional HEXRD patterns for the catalysts and pure species, Figure S7 contains
XPS spectra of the S 2p region for the fresh PdSO4ꢀHHTox sample, Table S1 contains data
derived from application of Rietveld method to HEXRD patterns. This material is available free
of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Authors
*
*
irodriguez@icp.csic.es (I.R.ꢀR.).
aguerrero@ccia.uned.es (A.G.ꢀR.).
Funding Sources
Ministerio de Economía y Competitividad (Spain). ESRF.
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ACKNOWLEDGMENTS
We acknowledge financial support from the Spanish Ministerio de Economía y Competitividad
projects CTQ2011ꢀ29272ꢀC04ꢀ01 and ꢀ03). The ESRF is thanked for granting beamtime at
(
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ID15.
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Bond, G. C. In Metal-Catalysed Reactions of Hydrocarbons, Springer, NewYork, 2005.
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McCue, A.J.; Anderson, J.A. Catal. Sci. Technol. 2014, 4, 272ꢀ294.
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Cooper, A.; BachillerꢀBaeza, B.; Anderson, J.A.; RodríguezꢀRamos, I.; GuerreroꢀRuiz,
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Table of Contents Graphic and Synopsis
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Hard Xꢀray diffraction coupled with mass spectrometry enabled to find out a simple method to
synthesize a catalyst based on palladium sulphide (Pd S) nanoparticles supported on carbon
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which has significant activity, appropriate selectivity and high stability under reaction conditions
of 1,3ꢀbutadiene hydrogenation.
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