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Article

Selective Hydrogenation of Acetylene over Pd-Co/C Catalysts: The Modifying Effect of Cobalt

by
Daria V. Yurpalova
1,*,
Tatyana N. Afonasenko
1,
Igor P. Prosvirin
2,*,
Andrey V. Bukhtiyarov
2,
Maxim A. Panafidin
2,
Zakhar S. Vinokurov
2,
Mikhail V. Trenikhin
1,
Evgeny Yu. Gerasimov
2,
Tatyana I. Gulyaeva
1,
Larisa M. Kovtunova
2 and
Dmitry A. Shlyapin
1
1
Center of New Chemical Technologies BIC, Boreskov Institute of Catalysis, Neftezavodskaya St., 54, Omsk 644040, Russia
2
Federal Research Center Boreskov Institute of Catalysis, Lavrentiev Ave., 5, Novosibirsk 630090, Russia
*
Authors to whom correspondence should be addressed.
Catalysts 2023, 13(4), 739; https://doi.org/10.3390/catal13040739
Submission received: 17 March 2023 / Revised: 8 April 2023 / Accepted: 11 April 2023 / Published: 13 April 2023
(This article belongs to the Special Issue Novel Nanocomposites with Enhanced Catalytic Performance: Synthesis and Structure)
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Abstract

:
Novel bimetallic Pd-Co catalysts supported on the carbon material Sibunit were synthesized by an incipient wetness impregnation method and used for ethylene production by selective acetylene hydrogenation. It has been established that an increase in the Pd:Co molar ratio from 1:0 to 1:2 in 0.5%Pd-Co/C catalysts, treated in hydrogen at 500 °C, leads to an increase in the ethylene selectivity from 60 to 67% (T = 45 °C). The selectivity does not change with a further increase in the modifier concentration. The catalysts were investigated by TPR-H2, XRD, TEM HR, EDS, and XPS methods. It was shown that palladium and cobalt in the 0.5%Pd-Co/C samples form Pd(1−x)Cox phases of solid solutions with different compositions depending on the Pd:Co ratio. The cobalt concentration in the Pd-Co particles increases with an increase in the Pd:Co ratio up to 1:2 and then remains at a constant level. In addition, monometallic Co particles were present in the samples with the Pd:Co ratio higher then 1:2. The optimal combination of catalytic properties (the ethylene yield is 62–63%) is typical for catalysts with a Pd:Co molar ratio of 1:2–1:4. which is mainly due to the presence of bimetallic particles containing ~41–43% by at. of cobalt.

1. Introduction

Ethylene is currently used to produce a wide range of chemical industrial products, including polyethylene used for production of packaging films and plastic tableware [1]. In addition, ethylene is a raw material for the production of ethanol, ethylene oxide, acetaldehyde, vinyl chloride, synthetic rubber, and other substances. Taking into account the importance of ethylene for the chemical industry, scientists are conducting research aimed at finding an effective and economically advantageous way to obtain it. A promising method for ethylene synthesis is the selective hydrogenation of acetylene obtained by the pyrolysis of natural gas or by the carbide method [2,3,4]. Due to the continuous growth of ethylene production in the world, highly efficient catalysts for acetylene hydrogenation to ethylene are in high demand. At the present time, industrial monometallic palladium catalysts for selective hydrogenation have been almost completely replaced by highly efficient bimetallic systems (Pd-Ag, Pd-Zn, Pd-In, and other compositions) supported on oxide materials [5,6,7,8,9]. Since the application of a non-optimal catalyst can lead to significant losses of the target product, continuous renewal and improvements of catalysts, including the developing of new methods of synthesis using, for example, non-traditional supports or new modifiers, are important tasks [10,11,12,13].
A review of the modern literature data showed that cobalt can be a promising palladium modifier in catalysts for selective hydrogenation of organic substances [14,15,16,17,18], including acetylene [10,19]. However, a systematic study of the effect of cobalt on palladium and the catalytic characteristics of such systems in the acetylene hydrogenation process has not yet been done. The modifying effect of the second metal on palladium is usually associated with the formation of bimetallic particles [10,20] or interfaces at the sites of contact between the two metals [18], which leads to a change in the structure and electronic state of palladium. The adsorption properties of the bimetallic surface and the surfaces of individual metals differ significantly. The differences in the behavior can be explained in terms of the electronic (ligand effects) and structural (ensemble composition effects) concepts [21]. Given the identity of the crystal structure of metallic palladium and cobalt, these metals are able to form a continuous series of solid solutions with a face-centered cubic (fcc) crystal lattice [22]. Therefore, the Pd/Co ratio should affect both the composition of bimetallic particles and the state of palladium, as well as the activity and selectivity of the acetylene hydrogenation catalysts. Ma R. and co-authors [10] studying two Pd-Co/Al2O3 catalysts with a Pd:Co mass ratio of 75:25 and 15:85 and showed that the surface of the first sample with a smaller amount of cobalt is characterized by the presence of palladium ensembles, while the surface of the second catalyst containing an excess of cobalt predominantly includes isolated palladium atoms surrounded by cobalt. The authors suggested that such a surface configuration favors the adsorption of acetylene in a weak π-bonded form and promotes the rapid desorption of the target product making it possible to maintain the high ethylene selectivity even at complete acetylene conversion. Menezes W.G. et al. [19] also showed that the use of catalysts based on bimetallic Pd–Co nanoparticles leads to an increase in the selectivity of the triple bond conversion in an acetylene molecule into a double bond compared to monometallic Pd catalysts. At the same time, the composition of the catalyst and Pd-Co nanoparticles, which is optimal for the reaction of selective acetylene hydrogenation, remains unclear.
In connection with the foregoing, the aim of this work was to study the modifying effect of cobalt on palladium in the bimetallic Pd-Co catalysts supported on the carbon material Sibunit and to establish the correlation between the Pd/Co molar ratio, the active component state, and the catalytic properties of the samples in the process of selective acetylene hydrogenation to ethylene. It is known that carbon materials are promising supports for palladium catalysts of acetylene hydrogenation [23]. The Sibunit carbon support is a mesoporous carbon black/pyrocarbon composite characterized by high purity, mechanical strength, stability (chemical and thermal) and tuning textural characteristics [24]. Catalysts based on Sibunit have been successfully used earlier in the processes of selective hydrogenation [24,25,26]. We have also previously shown that bimetallic Pd-Ga [4,27], Pd-Ag [28,29], and Pd-Zn [30,31] catalysts supported on Sibunit (C) exhibit high selectivity in the reaction of acetylene hydrogenation to ethylene, both in the liquid and gas phases.

2. Results

2.1. Catalytic Properties of Pd-Co/C Catalysts

The catalytic properties of Pd-Co/C bimetallic samples prepared by varying the Pd/Co molar ratio at a constant palladium content of 0.5% by wt. were studied in the reaction of gas-phase acetylene hydrogenation. The results obtained in the temperature range of 25–95 °C are shown in Figure 1. It was found during the preliminary experiments that the supported cobalt (0.55% Co/C) does not exhibit catalytic activity under the experimental conditions.
The addition of cobalt to the Pd catalyst in an amount less than the equimolar one does not lead to a significant change in the catalytic properties. The curves describing the dependences of the acetylene conversion and the ethylene yield on the reaction temperature for Pd/C and Pd-Co(1:0.5)/C samples are almost identical (Figure 1a,c). In this case, cobalt promotes a slight decrease in activity, while the selectivity of the target product increases: the ethylene selectivity at T = 45 °C (when the acetylene conversion does not exceed 70%) is 60 and 63% for Pd/C and Pd-Co(1:0.5)/C catalysts, respectively (Figure 1b). It can be assumed that the weak modifying effect of cobalt in the case of the Pd-Co(1:0.5)/C catalyst is associated with an insufficient amount of the modifier, which does not promote a close contact of the Pd and Co particles with the formation of bimetallic sites [28,30,32].
An increase in the amount of cobalt in the catalyst to the molar ratio Pd:Co = 1:1 leads to a noticeable change in the catalytic properties. The acetylene conversion curve shifts to a higher temperature region: 50% conversion is achieved at 38 °C on Pd/C, but in the case of the modified Pd-Co(1:1)/C sample, the same conversion is observed at 45 °C, which indicates the decrease in catalytic activity. At the same time, an increase in the selectivity of ethylene formation is noted (Figure 1b), which provides a significant increase in the maximum yield of the target product from 52% on Pd/C to 59% (Figure 1c). A further increase in the cobalt concentration to the molar ratio Pd:Co = 1:2 and 1:4 is accompanied by a further decrease in activity: 50% conversion is achieved at 50 °C and 62 °C for Pd-Co(1:2) C and Pd-Co(1:4)/C samples, respectively (Figure 1a). The ethylene selectivity continues to gradually increase with increasing Pd:Co molar ratio up to 1:2, reaching 67% at T = 25 °C (Figure 1b). Further addition of cobalt (Pd-Co(1:4)/C catalyst) does not contribute to the change in selectivity. As a result, both Pd-Co(1:2)/C and Pd-Co(1:4)/C samples are characterized by close values of the maximum yield of the target product 62–63% (Figure 1c). It can be assumed that several factors can determine the observed changes in the catalytic characteristics of the samples. First of all, the composition of the active component [10,28] or the dispersion of supported particles [33,34] can be an influence.
It can also be noted that the dependence of the ethylene yield on the reaction temperature differs for Pd/C and bimetallic samples containing different amounts of cobalt (Figure 1c). In the case of a monometallic catalyst, the dependence of the ethylene yield on temperature has an extreme form. The yield passes through a maximum when 100% conversion is achieved, and a further increase in the process temperature is accompanied by a sharp decrease in the C2H4 yield due to the activation of side reactions, such as the complete hydrogenation of acetylene to ethane [10,35,36]. The modification of palladium with cobalt and an increase in the cobalt content in the catalyst (relative to the palladium content) leads to a more gradual decrease in the ethylene yield with an increase in the reaction temperature. It should be noted that the reaction temperature does not affect the ethylene yield in the case of using the Pd-Co(1:4)/C sample. The yield reaches a maximum value of 63% at 75 °C and remains at a constant level up to T = 95 °C. The obtained results indicate a lower tendency of the modified samples, and especially those enriched with cobalt, to activate side processes. Taking into account the similar specific surface area of all studied samples (336 ± 10 m2/g), it can be concluded that the differences in the properties are due to the nature of the active component and its state.

2.2. Temperature-Programmed Reduction

Bimetallic catalysts with different molar ratios of palladium to cobalt, as well as monometallic reference samples Pd/C and Co/C, were studied by temperature-programmed reduction in order to reveal differences in the reduction process of the active component of the Pd-Co/C samples. The TPR-H2 profiles are shown in Figure 2.
The TPR profile of the Co/C catalyst is characterized by weak hydrogen uptake in the temperature range of 200–250 °C with a maximum at 228 °C, as well as hydrogen uptake at the higher temperature range of ~250–500 °C. The second peak includes several components, probably due to the reduction of particles differing in size. It has a higher intensity than the first peak and is characterized by a maximum at 358 °C. Low-temperature absorption of hydrogen is associated with the reduction of Co3O4 to CoO, and the peak that appears at a higher temperature is associated with the further reduction of CoO particles to metallic cobalt [37,38,39,40].
The TPR-H2 curve of the monometallic Pd/C catalyst in the temperature range up to 200 °C, which is typical for the reduction of PdO [41,42], contains only an inverse peak at T = 64 °C, associated with the decomposition of the bulk phase of palladium hydride PdHx [10,27] (which is thought to lead to over-hydrogenation and low selectivity [43]). The absence of hydrogen uptake in this region is probably due to the possibility of the reduction of PdO (formed during preliminary calcination in He) at room temperature when the system is purged with a hydrogen–inert gas mixture. However, the TPR profile of the Pd/C catalyst in the high-temperature region at T > 400 °C includes an intense peak of H2 absorption associated with the methanation of the carbon support [37,40]. It should be noted that this feature is also characteristic of the TPR profiles of all studied catalysts containing palladium. This process is activated by platinum metals; therefore, the corresponding signal is absent on the TPR curve of the Co/C catalyst in the temperature range up to 500 °C.
The intensity of the inverse peak associated with the decomposition of palladium hydride significantly decreases for the modified sample with the Pd:Co = 1:0.5 compared to the monometallic palladium catalyst, while for other Pd-Co/C samples containing a larger amount of Co, there are no signals of hydrogen emission in this region. It can be assumed that the disappearance of this peak at 64 °C is due to the intensification of the interaction between Pd and Co which occurs with an increase in the cobalt concentration in the catalyst [10]. The TPR curve of the Pd-Co(1:0.5)/C sample does not contain cobalt reduction peaks, which is probably due to the insufficient sensitivity of the method for low metal content. An increase in the cobalt concentration leads to the appearance of a diffuse low-intensity peak in the TPR-H2 profile of the Pd-Co(1:1)/C catalyst at ~150–300 °C. The intensity of this peak increases with increasing cobalt content in the samples. This peak appears as a result of the reduction of cobalt oxide by hydrogen activated on palladium, and the formation of bimetallic particles PdxCo(1−x) [10]. It is quite possible that this signal may also include hydrogen uptake to reduce individual Co3O4 particles to CoO for samples with a large amount of cobalt (Pd:Co ≤ 1:2). This is indicated by the appearance of a hydrogen uptake region at ~300–450 °C on the Pd–Co(1:2)/C TPR curve, which is characteristic of a monometallic Co/C catalyst and is associated with the reduction of CoO to Co [39,40]. The TPR profile of the Pd-Co(1:4)/C sample containing an excess amount of cobalt includes a hydrogen absorption peak with a maximum at 283 °C. Its appearance can be explained by the reduction of an excess amount of cobalt oxide (which did not interact with palladium) to a metallic state with the participation of hydrogen activated on palladium. It should also be noted that there is a clear trend towards a decrease in the intensity of H2 uptake at T > 400 °C with an increase in the cobalt content in the catalyst, which also indicates a decrease in the ability of the active component to activate hydrogen and correlates well with both a decrease in the activity of the catalyst and a suppression of a side process of full acetylene hydrogenation to ethane.

2.3. Structural Properties

To study the phase composition of the active component of the Pd-Co/C catalysts, the method of X-ray diffraction analysis was used. The experiments were carried out for model samples with a larger loading of metals in order to increase the sensitivity of the analysis. All catalysts were pretreated in a gas mixture of 10% H2 in He at 500 °C for 1 h to reduce the components oxidized in air. After pretreatment, the reactor was cooled to room temperature, and X-ray diffraction patterns were recorded in a 10% H2/He medium. The obtained diffraction patterns, as well as the model curves, are shown in Figure 3, and the results of X-ray phase analysis are presented in Table 1.
The monometallic palladium catalyst contains a cubic phase with space group Fm(-)3m, which is characterized by a lattice parameter of 4.0271 Å (Table 1) and corresponds to palladium hydride PdHx formed as a result of the dissolution of hydrogen in palladium. The lattice parameter a of the hydride phase can vary from 4.00 to 4.10 Å depending on the amount of absorbed hydrogen [43,44]. To estimate the lattice parameter of the palladium phase in the reduced Pd/C sample, the diffraction patterns were also recorded in air without additional pretreatment in H2. The lattice parameter in the absence of hydrogen was 3.8804 Å, which corresponds to metallic palladium (PDF #46-1043). All bimetallic samples reduced in H2 also contain a cubic phase with space group Fm(-)3m, which can be attributed to the solid solution fcc (α-Co,Pd) [45]. The main reflections of the diffraction patterns of the Pd-Co/C systems gradually shift with increasing cobalt content towards larger diffraction angles due to a gradual change in the lattice parameter from pure palladium a = 3.89019 Å (PDF #46-1043) to pure cobalt (fcc α-Co) a = 3.5447 Å (PDF #46-1043).
The dependence of the lattice parameter a on the cobalt content in the catalyst is shown in Figure 4. The figure also shows the results of estimating the composition of bimetallic phases according to Vegard’s law [46], assuming a linear dependence of the lattice parameter on the atomic composition. An increase in the concentration of cobalt in Pd catalysts leads to a decrease in the lattice parameter a. The stabilization of the lattice constant (a = 3.74 Å) is observed for Pd-Co(1:2)/C and Pd-Co(1:4)/C samples containing an excess of the modifier, which corresponds to ~41–43% by at. of cobalt in the composition of the bimetallic phase [22]. These samples with a high cobalt content (molar ratio Pd:Co ≤ 1:2) also include the phases of metallic cobalt fcc α-Co and hexagonal β-Co (Table 1). An increase in the content of the modifier to the molar ratio Pd:Co = 1:4 is accompanied by a significant increase in the crystallite size of the monometallic Co phases.
The Pd-Co(1:2)/C catalyst, which has a high ethylene yield and higher activity than the activity of the Pd-Co(1:4)/C sample, was studied by high-resolution transmission electron microscopy coupled with EDS mapping. For comparison, we also studied a catalyst with a lower content of palladium Pd-Co(1:0.5)/C, for which the properties are very different from those of Pd-Co(1:2)/C. Figure 5 shows micrographs of the studied catalysts. It can be noted that, in both cases, the particles of the supported component are spherical in shape and are uniformly distributed over the Sibunit surface. EDS mapping shows that in the case of the Pd-Co(1:0.5)/C catalyst containing a small amount of cobalt, the supported particles are predominantly metallic palladium and/or palladium particles with partial substitution of palladium atoms by cobalt, since the Pd/Co atomic ratio for different fields are not saved (Figure S1, Table S1). This thesis is confirmed by the measured interplanar distances d = 0.223–0.224 nm, which are close to the distances characteristic of metallic palladium, but still slightly underestimated (d for Pd(111) is 0.225 nm [47,48,49]), which may indicate a slight compression of the palladium lattice due to the replacement of palladium atoms by cobalt atoms of smaller diameter (Co has a smaller metallic radius (125 pm) than Pd (137 pm) [50]). In this case, cobalt also forms separate small agglomerations of oxidized particles. The interplanar distances of such particles are 0.233 nm [51]. In the case of the bimetallic Pd-Co(1:2)/C catalyst with a higher cobalt concentration, the metals are more uniformly distributed over the carbon surface: there are almost no individual large agglomerates that include only one metal component. The interplanar distances of the particles differ from the distances found for metallic palladium and are 0.216–0.217 nm, which correspond to the PdCo(111) planes of PdxCo(1−x) solid solution [48,52]. The semiquantative estimation showed that the Pd/Co atomic ratio remains ~65/35 for different regions of the sample, indicating the enrichment of the bimetallic phase with palladium (Figure S1, Table S1), which is consistent with the XRD data obtained for the model system. The sample also contains rarer individual particles of oxidized cobalt with d = 0.233 nm [51], which also corresponds to the results of the X-ray phase analysis.
The size of the supported particles in both samples varies from approximately 2 to 8 nm, and the average diameter is 4.2 nm (Figure 6). Thus, the dispersion of the active component of the Pd-Co(1:0.5)/C and Pd-Co(1:2)/C catalysts is approximately the same and does not depend on the cobalt content in the selected range of Pd:Co molar ratios from 1:0.5 to 1:2.

2.4. Electronic Properties of the Active Component

To study the electronic state of the palladium, as well as to evaluate the effects of modifications in the surface layer of the Pd-Co/C catalysts, the XPS method was used. Taking into consideration that catalysts containing different amounts of the modifier are characterized by the presence of bimetallic particles of different compositions, it was assumed that the electronic state of palladium in these samples should also differ [21]. Figure 7 shows the Pd3d XPS spectra.
The binding energy of the Pd3d5/2 peak in the Pd/C sample is 335.4 eV, which is typical for finely dispersed metallic palladium Pd0 [30,53]. The position of the Pd3d5/2 peak in the spectra of the modified Pd-Co/C catalysts shifts to higher binding energies, indicating the appearance of an electronic interaction between the Pd and Co [21,53,54,55,56]. An increase in the cobalt content is accompanied by an intensification of this effect. For example, the Pd3d XPS spectrum of the Pd-Co(1:0.5)/C catalyst is very similar to the spectrum of the monometallic Pd/C sample, while for catalysts with a high cobalt content, the position of the Pd3d5/2 peak is shifted to 335.8 eV, i.e., by +0.4 eV relative to the Pd/C. The core-level shifts are related to the changes in valence state through the hybridization that occurs between metal atoms upon alloying [21]. In Pd-Co/C catalyst, the binding energy originates from the d-band hybridization between Pd and Co upon alloying because the d-orbital forms stronger bonds between metal atoms, reducing the ability to form strong bonds with adsorbed reactants [53], which affects the catalytic properties of the samples. It should be noted that the electronic state of elements in nanosized particles can also depend on their dispersity due to the possibility of interaction with the support [57]. However, based on the TEM data, we can conclude that, in the case of Pd-Co/C samples, the size of the active component should not have a significant effect on the palladium state. Therefore, the observed shifts in the binding energy are primarily due to the alloying of the two metals with the formation of bimetallic particles.
Table 2 shows the ratios of surface concentrations of elements in Pd/C and Pd-Co/C catalysts obtained by varying the Pd/Co molar ratio. An increase in the content of the modifier leads to a regular increase in the Co/Pd surface ratio, which enhances the effect of geometric modification of palladium or the “dilution” of palladium atoms with cobalt [10].

3. Discussion

The study of bimetallic Pd-Co catalysts supported on carbon material Sibunit in the reaction of selective acetylene hydrogenation to ethylene showed that the addition of cobalt has a significant effect on the catalytic properties of Pd/C. The effect of the modifier is very weak at a molar ratio of Pd:Co = 1:0.5, but an increase in the amount of cobalt to a molar ratio of Pd:Co = 1:2 leads to a noticeable increase in ethylene selectivity. This provides a significant increase in the maximum yield of the target product from 52% on Pd/C to 62% despite the decrease in catalytic activity. A further increase in the modifier concentration to the molar ratio Pd:Co = 1:4 does not change the selectivity of ethylene formation, however, it has a negative effect on the catalytic activity, which shifts the temperature at which the maximum ethylene yield (63%) is obtained from 65 to 75 °C. XRD data clearly demonstrate that changes in the catalytic properties of Pd/C due to the addition of cobalt are associated with the formation of a joint Pd-Co phase. According to XRD, the cubic phase of the PdxCo(1−x) solid solution, for which the composition depends on the Pd/Co molar ratio, is formed in all bimetallic samples after a reduction treatment in H2 at 500 °C. An increase in the concentration of cobalt in the sample (relative to palladium) promotes a decrease in the lattice parameter of the bimetallic Pd-Co phase due to its enrichment with cobalt. The lattice parameter of this phase at a molar ratio of Pd:Co ≤ 1:2 does not change significantly, which indicates the stabilization of the composition of the bimetallic phase containing ~41–43% by at. of cobalt. An excess of cobalt (relative to palladium) in the samples with a molar ratio of Pd:Co ≤ 1:2 forms a separate phase of cobalt (or cobalt oxide in air), which did not interact with palladium. The identified phase changes of the active species, which occur with an increase in the cobalt content in the model 3.5% Pd-Co/C catalysts, are in agreement with the TPR-H2 data obtained for the 0.5% Pd-Co/C samples. There is a decrease in the intensity of the inverse peak corresponding to the decomposition of the bulk phase of palladium hydride PdHx, which is characteristic of the Pd/C sample, on the TPR-H2 curves of bimetallic Pd-Co/C catalysts at a cobalt concentration less than equimolar with respect to palladium (Pd:Co = 1:0.5). This indicates a decrease in the content of monometallic palladium in the Pd-Co(1:0.5)/C sample due to its partial interaction with cobalt. The absence of an inverse peak associated with the decomposition of PdHx on the TPR-H2 profile of the Pd-Co(1:1)/C catalyst and the appearance of a peak in the range of ~150–300 °C, the intensity of which increases with an increase in the cobalt content in the samples, indicate the interaction of palladium and cobalt with the formation of bimetallic particles during reduction in H2.
As can be seen from Figure 8, the dependences of the cobalt concentration in the composition of the PdxCo(1−x) solid solution phase and the values of the maximum ethylene yield on the Pd/Co molar ratio have an identical form, which indicates the presence of a direct correlation between them. This fact allows us to state that the increase in the selectivity of ethylene and its yield for the Pd-Co/C samples are due to an increase in the content of cobalt in the bimetallic Pd-Co phase. Apparently, the geometric effect takes place: enrichment of bimetallic particles with cobalt should lead to a dilution of palladium ensembles, i.e., isolation of palladium atoms from each other, which changes the form of acetylene adsorption on active sites and causes an increase in selectivity [28]. Along with a change in the structure of the active phases, the electronic modification of palladium with cobalt also has a significant effect. The gradual shift of the Pd3d5/2 peak position in the Pd3d XPS spectra of the modified catalysts to higher binding energies relative to monometallic palladium (Figure 7) is evidence of the modification of the electronic state of palladium by cobalt. Apparently, there is a weakening of the bond between acetylene, ethylene, and hydrocarbon intermediates formed during hydrogenation with the surface of Pd-Co nanoalloy particles as compared to the surface of unmodified palladium, as it was observed by other authors using the model carbon monoxide molecule [21,54]. The weakening of the bond between acetylene and the active sites of the catalyst explains the decrease in catalytic activity, while the weaker energy of ethylene adsorption favors the rapid desorption of the target product and an increase in the selectivity of the hydrogenation process. It should be noted that the concentration of surface hydrogen will also affect the catalytic activity. It is known that the enrichment of the bimetallic particles’ surface with a modifier is accompanied by a decrease in the ability of the samples to activate hydrogen [10]. In the case of the studied Pd–Co/C catalysts, this is indirectly confirmed by the TPR data (the intensity of the peak at T > 400 °C, associated with the methanation process of carbon support, decreases with an increase in the Co content in the catalysts), as well as by the observed decrease in activity in the reaction of acetylene hydrogenation.
It should be noted that the acetylene hydrogenation reaction is structure-sensitive [33,34]; therefore, the catalytic properties will be determined not only by the nature and structure of the active sites, but also by the particle size of the active component. At the same time, it was shown by TEM that the particle size distribution does not change significantly for Pd-Co/C catalysts with the molar ratios of Pd:Co from 1:0.5 to 1:2 (Figure 6). Thus, an increase in the content of cobalt in the modified samples changes the composition of the bimetallic PdxCo(1−x) particles, but does not significantly affect the average size of the supported particles.
It can be concluded that the factor determining the catalytic properties of the bimetallic Pd-Co samples supported on the mesoporous carbon material Sibunit in the reaction of selective hydrogenation of acetylene is the composition of the bimetallic Pd-Co phase. The highest values of ethylene selectivity and ethylene yield are typical for Pd-Co/C catalysts with a molar ratio of Pd:Co = 1:2–1:4, which is mainly due to the presence of bimetallic particles containing ~41–43% by at. of cobalt.

4. Materials and Methods

4.1. Catalyst Synthesis

The synthetic carbon composite material «Sibunit» (produced industrially in Omsk (Russia) by hydrocarbon pyrolysis, followed by gas activation [24], SBET~336 m2/g) was used as a catalyst support. Initial grains of the carbon support (2–4 mm) were washed with distilled water, dried, milled to a size of 0.07–0.09 mm, and treated by heating with a 5% solution of HNO3 (4 mL of nitric acid per gram of the support) to anchor oxygen-containing groups on the Sibunit surface [58]. The modified samples of the catalysts were obtained by incipient wetness impregnation method from a joint aqueous solution of Pd(NO3)2 and Co(NO3)2. The concentration of palladium solution was chosen to provide the 0.5 wt.% final content of palladium. The amount of Co(NO3)2 was chosen based on the molar ratios of Pd:Co = 1:0.5, 1:1, 1:,2 and 1:4 in the catalysts. Unmodified one was synthesized by a similar method using a Pd(NO3)2 solution. The synthesized catalysts were dried at 120 °C for 2 h and heated in a hydrogen flow (60 mL/min) for 5 h at 500 °C. Heating and cooling rates were 10 °C/min. The obtained catalysts were stored in glass ampoules in a helium atmosphere to prevent their contact with air. Model samples containing a higher content of supported metals were synthesized for studying the phase composition of the catalysts by X-ray diffraction analysis.

4.2. Catalyst Characterization

The specific surface area of the carbon support Sibunit (SBET) was determined by the Brunauer–Emmett–Teller method using nitrogen adsorption isotherms measured at 77 K. The studies were carried out using an ASAP 2400 automated system (“Micromeritics Instrument Corp.”, Norcross, GA, USA).
X-ray diffraction analysis (XRD) of model catalysts 3.5%Pd-Co/C was performed on a Bruker D8 Advance diffractometer equipped with a Lynxeye detector and a XRK900 chamber reactor. The diffraction patterns were obtained in the θ/2θ geometry using Cu Kα radiation (λ = 1.5418 Å, Ni filter) in the 2θ angle range of 20–82° with a step of 0.05°. The exposition time was 4 s per point. The samples were preliminarily reduced in a mixture of 10% H2 in He at 500 °C for 1 h, then the reactor was cooled to room temperature, and the diffraction patterns were recorded in the same gas mixture. X-ray phase analysis was performed using the PDF-4+ database [59]. The structure was refined by the Rietveld method using the MAUD software package [60].
Temperature-programmed reduction (TPR-H2) was carried out in a quartz reactor using a flow unit equipped with a thermal conductivity detector. A gas mixture (10 vol.% H2 in Ar) was fed to the reactor at a rate of 40 mL/min. The heating rate was 10 °C/min from room temperature to 600 °C.
High resolution transmission electron microscopy (TEM HR) was used to study the structure and microstructure of the samples. The electron microscope ThemisZ (Thermo Fisher Scientific, Waltham, MA, USA) with an accelerating voltage of 200 kV and a limiting resolution of 0.07 nm was used for the study. Images were recorded using Ceta 16 CCD sensor (Thermo Fisher Scientific, Waltham, MA, USA). The microscope is equipped with a SuperX (Thermo Fisher Scientific, Waltham, MA, USA) energy dispersive spectrometer (EDX) and a semiconductor Si detector with an energy resolution of 128 eV. The samples were deposited on perforated carbon supports fixed on copper or molybdenum grids using an ultrasonic disperser, which made it possible to achieve a uniform distribution of particles over the support surface. Some samples were examined by using a JEM 2100 JEOL microscope with accelerating voltage 200 kV and resolution 0.14 nm.
X-ray photoelectron spectroscopy (XPS) was used to study the electronic state of elements on the surface. XPS measurements were carried out on a SPECS photoelectron spectrometer (SPECS, Berlin, Germany) using AlKα radiation (hν = 1486.6 eV. 150 W). The binding energy scale (BE) was preliminarily calibrated by the position of the core levels peaks of gold and copper: Au4f7/2—84.0 eV and Cu2p3/2—932.67 eV. The residual gas pressure did not exceed 8 × 10−9 mbar. Samples in the form of a finely dispersed powder were supported on double-sided copper scotch tape. All catalysts were pretreated in H2 at 500 °C for 1 h. To determine the chemical (charge) state of the elements on the surface, the C1s, Pd3d, Co2p, and O1s regions were measured. To calibrate the obtained spectra, the C1s line (BE = 284.5 eV) from carbon in the support (Sibunit) was used as an internal standard. The relative content of elements on the surface and the ratio of their atomic concentrations were estimated from integrated intensities of photoelectron lines corrected for the corresponding atomic sensitivity factors [61].

4.3. Catalyst Tests

Catalytic tests of the samples in the gas-phase acetylene hydrogenation reaction were carried out in flow mode at 0.1 MPa. A 15.0 mg catalyst sample was mixed with an inert diluent SiO2 (the grain size is 0.07–0.09 mm) to obtain a bed of 1 cm3 and transferred to a glass flow-type reactor with the diameter of 1 cm. The reaction temperature was varied from 25 to 95 °C with a step of 10 °C. The reaction gas mixture contained 4 vol.% of C2H2 and 96 vol.% of H2 and was fed to the reactor at a volumetric flow rate of 100 mL/min (GHSV = 400,000 mL/(g × h). The composition of the reaction products was studied chromatographically using a Chromos GC-1000 chromatograph with a flame ionization detector and a capillary column with a SiO2 stationary phase (25 m × 0.32 mm, T = 60 °C). The acetylene conversion (X, %), and the selectivity to ethylene (SC2H4, %) and ethane (SC2H6, %) were calculated from areas of the corresponding peaks by the normalization method.
X = [NinC2H2 − NoutC2H2] × 100%/NinC2H2
SC2H4 = NC2H4 × 100%/[NC2H4 + NC2H6 + NC4+]
SC2H4 = NC2H6 × 100%/[NC2H4 + NC2H6 + NC4+]
where N is the content (mol.%) of the corresponding compounds, and NinC2H2 and NoutC2H2 are the content of acetylene (mol.%) in the inlet and outlet flows.
The selectivity to C4+-oligomers (SC4+, %) was calculated based on a carbon balance:
SC4+ = 100 − SC2H4 − SC2H6
The activity (A) of the catalyst (mL(C2H2)/(gcat × min)) was estimated as follows:
A = [V × C0 × X]/[mcat × 100]
where V is the feed space velocity (mL/min); C0 is the initial C2H2 concentration in the reaction mixture (mLC2H2/mLgas mixture); and mcat is the weight of the catalyst (g).
The ethylene yield (Y, %) was calculated by the formula:
Y = [X × SC2H4]/100.

5. Conclusions

In summary, a detailed study of the modifying effect of cobalt on palladium in Pd-Co catalysts supported on the mesoporous carbon material Sibunit was carried out. The features of the formation, composition and structure of the active phases formed in Pd-Co/C samples are shown, depending on the Pd/Co molar ratio. Using XRD, it was found that palladium and cobalt supported on Sibunit interact in H2 medium to form bimetallic PdxCo(1−x) nanoparticles. The composition of PdxCo(1−x) depends on the palladium to cobalt molar ratio taken at the synthesis stage. The content of cobalt in the Pd-Co nanoparticles increases with increasing Pd:Co molar ratios up to 1:2 (to ~41% by at. of Co in the bimetallic phase) and then remains at a constant level. A correlation has been established between the content of cobalt in bimetallic PdxCo(1−x) nanoparticles and the maximum yield of ethylene in the reaction of selective acetylene hydrogenation on the Pd-Co/C catalysts. The optimal molar ratio of metals Pd:Co = 1:2–1:4 was found, which ensures the formation of bimetallic Pd-Co particles containing ~41–43% by at. of cobalt and leads to the highest ethylene yield (62–63%). It has been proven that the catalytic properties of the studied Pd-Co/C samples in the process of acetylene hydrogenation are due to the influence of the composition of bimetallic Pd-Co sites (geometric effect) and the electronic state of palladium (electronic effect).

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal13040739/s1, Figure S1: EDS mapping pattern for 0.5%Pd-Co(1:0.5)/C (a) and 0.5%Pd-Co(1:2)/C (b) catalysts; Table S1: Results of local elemental EDX analysis of the areas highlighted in Figure S1.

Author Contributions

Conceptualization. D.V.Y., T.N.A., I.P.P. and D.A.S.; formal analysis. D.V.Y., T.N.A., I.P.P., A.V.B., M.A.P., Z.S.V., M.V.T., E.Y.G. and T.I.G.; investigation. D.V.Y., T.N.A., I.P.P., A.V.B., M.A.P., Z.S.V., M.V.T., E.Y.G., T.I.G. and L.M.K.; writing—original draft preparation. D.V.Y. and T.N.A.; writing—review and editing. D.V.Y.; visualization. D.V.Y. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Ministry of Science and Higher Education of the Russian Federation within the governmental order for Boreskov Institute of Catalysis (project AAAA-A21-121011390011-4).

Data Availability Statement

Not applicable.

Acknowledgments

The research was performed using equipment of the Omsk Regional Center of Collective Usage SB RAS and the Shared-Use Center “National Center for the Study of Catalysts” at the Boreskov Institute of Catalysis.

Conflicts of Interest

The authors declare no conflict of interest.

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Catalysts 13 00739 g001 550
Figure 1. Catalytic properties of 0.5%Pd/C and 0.5%Pd-Co/C samples, prepared by varying the Pd/Co molar ratio in the acetylene hydrogenation reaction: (a) acetylene conversion; (b) activity and ethylene selectivity at 25 °C; (c) ethylene yield.
Figure 1. Catalytic properties of 0.5%Pd/C and 0.5%Pd-Co/C samples, prepared by varying the Pd/Co molar ratio in the acetylene hydrogenation reaction: (a) acetylene conversion; (b) activity and ethylene selectivity at 25 °C; (c) ethylene yield.
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Figure 2. TPR-H2 profiles of 0.5%Pd/C, 0.55%Co/C and 0.5%Pd-Co/C catalysts, prepared by varying the Pd/Co molar ratio.
Figure 2. TPR-H2 profiles of 0.5%Pd/C, 0.55%Co/C and 0.5%Pd-Co/C catalysts, prepared by varying the Pd/Co molar ratio.
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Figure 3. Diffraction patterns of model 3.5%Pd/C and 3.5%Pd-Co/C catalysts, obtained by varying the Pd/Co molar ratio. Support model was subtracted from experimental data. λ = 1.5418 Å.
Figure 3. Diffraction patterns of model 3.5%Pd/C and 3.5%Pd-Co/C catalysts, obtained by varying the Pd/Co molar ratio. Support model was subtracted from experimental data. λ = 1.5418 Å.
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Figure 4. FCC lattice parameter of the Pd, Co and PdxCo(1−x) phases vs. the amount of cobalt in the model 3.5%Pd–Co/C catalysts.
Figure 4. FCC lattice parameter of the Pd, Co and PdxCo(1−x) phases vs. the amount of cobalt in the model 3.5%Pd–Co/C catalysts.
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Catalysts 13 00739 g005 550
Figure 5. TEM images and EDS mapping pattern for 0.5%Pd-Co(1:0.5)/C (a) and 0.5%Pd-Co(1:2)/C (b) catalysts.
Figure 5. TEM images and EDS mapping pattern for 0.5%Pd-Co(1:0.5)/C (a) and 0.5%Pd-Co(1:2)/C (b) catalysts.
Catalysts 13 00739 g005
Catalysts 13 00739 g006 550
Figure 6. Particle size distribution in bimetallic 0.5%Pd-Co(1:0.5)/C (a) and 0.5%Pd-Co(1:2)/C (b) catalysts.
Figure 6. Particle size distribution in bimetallic 0.5%Pd-Co(1:0.5)/C (a) and 0.5%Pd-Co(1:2)/C (b) catalysts.
Catalysts 13 00739 g006
Catalysts 13 00739 g007 550
Figure 7. Pd3d XPS spectra of 0.5%Pd/C and 0.5%Pd–Co/C catalysts prepared by varying the content of modifier.
Figure 7. Pd3d XPS spectra of 0.5%Pd/C and 0.5%Pd–Co/C catalysts prepared by varying the content of modifier.
Catalysts 13 00739 g007
Catalysts 13 00739 g008 550
Figure 8. Correlation between the content of cobalt in bimetallic PdCo nanoparticles and the maximum yield of ethylene in the reaction of selective acetylene hydrogenation on Pd-Co/C catalysts.
Figure 8. Correlation between the content of cobalt in bimetallic PdCo nanoparticles and the maximum yield of ethylene in the reaction of selective acetylene hydrogenation on Pd-Co/C catalysts.
Catalysts 13 00739 g008
Table
Table 1. Results of XRD analysis of molel 3.5%Pd/C and 3.5%Pd-Co/C catalysts obtained by varying the cobalt content.
Table 1. Results of XRD analysis of molel 3.5%Pd/C and 3.5%Pd-Co/C catalysts obtained by varying the cobalt content.
CatalystPhaseswt.%Lattice Parameter a, ÅCrystallite Size **, nm
Pd/CPdH, Fm(-)3m1004.0271(3)9.3
Pd-Co(1:0.5)/CPdCo *, Fm(-)3m1003.842(1)2.5
Pd-Co(1:1)/CPdCo, Fm(-)3m1003.798(1)2.5
Pd-Co(1:2)/CPdCo, Fm(-)3m
α-Co, Fm(-)3m
β-Co, P63/mmc		71
20
9		3.744(2)
3.549
a 2.514, c 4.105		2.5
3.7
6.3
Pd-Co(1:4)/CPdCo, Fm(-)3m
α-Co, Fm(-)3m
β-Co, P63/mmc		79
12
9		3.739(2)
3.549(1)
a 2.514, c 4.105		2.5
13.6
10.1
*—the crystallite inclusions, **—obtained from XRD data.
Table
Table 2. Surface atomic concentrations of the elements for the 0.55%Co/C, 0.5%Pd/C and 0.5%Pd-Co/C catalysts, prepared by varying the Pd/Co molar ratio.
Table 2. Surface atomic concentrations of the elements for the 0.55%Co/C, 0.5%Pd/C and 0.5%Pd-Co/C catalysts, prepared by varying the Pd/Co molar ratio.
CatalystPd/CCo/C(Pd + Co)/CCo/Pd
Co/C00.0020.002–
Pd/C0.00100.0010
Pd-Co(1:0.5)/C0.0010.0010.0020.47
Pd-Co(1:1)/C0.0010.0010.0030.93
Pd-Co(1:2)/C0.0010.0020.0031.89
Pd-Co(1:4)/C0.0010.0040.0053.87
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Yurpalova, D.V.; Afonasenko, T.N.; Prosvirin, I.P.; Bukhtiyarov, A.V.; Panafidin, M.A.; Vinokurov, Z.S.; Trenikhin, M.V.; Gerasimov, E.Y.; Gulyaeva, T.I.; Kovtunova, L.M.; et al. Selective Hydrogenation of Acetylene over Pd-Co/C Catalysts: The Modifying Effect of Cobalt. Catalysts 2023, 13, 739. https://doi.org/10.3390/catal13040739

AMA Style

Yurpalova DV, Afonasenko TN, Prosvirin IP, Bukhtiyarov AV, Panafidin MA, Vinokurov ZS, Trenikhin MV, Gerasimov EY, Gulyaeva TI, Kovtunova LM, et al. Selective Hydrogenation of Acetylene over Pd-Co/C Catalysts: The Modifying Effect of Cobalt. Catalysts. 2023; 13(4):739. https://doi.org/10.3390/catal13040739

Chicago/Turabian Style

Yurpalova, Daria V., Tatyana N. Afonasenko, Igor P. Prosvirin, Andrey V. Bukhtiyarov, Maxim A. Panafidin, Zakhar S. Vinokurov, Mikhail V. Trenikhin, Evgeny Yu. Gerasimov, Tatyana I. Gulyaeva, Larisa M. Kovtunova, and et al. 2023. "Selective Hydrogenation of Acetylene over Pd-Co/C Catalysts: The Modifying Effect of Cobalt" Catalysts 13, no. 4: 739. https://doi.org/10.3390/catal13040739

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