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Article

Single-Atom Alloy Pd1Ag10/CeO2–ZrO2 as a Promising Catalyst for Selective Alkyne Hydrogenation

by
Pavel V. Markov
1,
Galina O. Bragina
1,
Nadezhda S. Smirnova
1,
Galina N. Baeva
1,
Igor S. Mashkovsky
1,*,
Evgeny Y. Gerasimov
2,
Andrey V. Bukhtiyarov
2,
Yan. V. Zubavichus
2 and
Alexander Y. Stakheev
1,*
1
N. D. Zelinsky Institute of Organic Chemistry RAS, Leninsky Prospect 47, 119991 Moscow, Russia
2
G. K. Boreskov Institute of Catalysis, Siberian Branch of the Russian Academy of Sciences, Academician Lavrentiev Prospect 5, 630090 Novosibirsk, Russia
*
Authors to whom correspondence should be addressed.
Inorganics 2023, 11(4), 150; https://doi.org/10.3390/inorganics11040150
Submission received: 8 March 2023 / Revised: 24 March 2023 / Accepted: 29 March 2023 / Published: 1 April 2023
(This article belongs to the Special Issue Single Atom Alloys: Modern Trends in Preparation and Application)
Browse Figures
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Abstract

:
The effect of support on the performance of Pd1Ag10/Al2O3 and Pd1Ag10/CeO2–ZrO2 catalysts in the selective hydrogenation of diphenylacetylene (DPA) was studied. Characterization of the catalyst by DRIFTS-CO and HRTEM revealed the formation of a PdAg single-atom alloy (SAA) structure on the surface of PdAg nanoparticles, with Pd1 sites isolated by Ag atoms. It was found that the use of CeO2–ZrO2 as a carrier makes it possible to increase the activity of the Pd1Ag10 catalyst by a factor of three without loss of selectivity compared to the reference Pd1Ag10/Al2O3. According to the HRTEM data, this catalytic behavior can be explained by an increase in the dispersion of Pd1Ag10/CeO2–ZrO2 compared to its Pd1Ag10/Al2O3 counterpart. As evidenced by DRIFTS-CO data, the high selectivity of the Pd1Ag10/CeO2–ZrO2 sample presumably stems from the stability of the structure of isolated Pd1 sites on the surface of SAA Pd1Ag10/CeO2–ZrO2.

1. Introduction

Supported metal catalysts based on noble metals (Pd, Pt, Rh, Ir) are among the most common heterogeneous catalysts [1,2,3,4]. Since they are highly active, thermally stable and easy to separate from the reaction mixture, they are widely used both in industry and in laboratory practice. Thus, supported palladium catalysts are widely used in the selective hydrogenation of alkynes [5,6,7]. A typical example is the large-scale removal of acetylene impurities from pyrolysis ethylene before its polymerization. An important reaction is also the purification of styrene from trace amounts of phenylacetylene in the production of polystyrene, since even insignificant concentrations of alkynes poison metallocene polymerization catalysts. It should also be noted that the selective hydrogenation of substituted alkynes is one of the key routes in the synthesis of many complex alkene compounds, since it can be performed with high stereoselectivity [8,9].
A significant drawback of heterogeneous metal-supported catalysts is their lower selectivity compared to homogeneous metal–complex catalysts. The high inhomogeneity of active sites is one of the key factors that largely determine the catalytic performances in terms of activity and selectivity [10]. In the last decade, the methodology of single-atom alloys (SAAs) has been successfully used to improve the selectivity of hydrogenation catalysts [11,12,13,14]. Typically, they are catalytic systems with single atoms of a catalytically active metal (Pd, Pt) alloyed into the surface of a host metal (usually Au, Ag, In, Zn), whose hydrogenation activity is negligible. Such a structure leads to the formation of active sites with identical adsorption and catalytic characteristics, which makes it possible to obtain target products with extremely high selectivity. Thus, several authors reported on the preparation of efficient SAA Pd-based catalysts for the selective hydrogenation of styrene, butadiene and acetylene, as well as in the dehydrogenation of acetic acid [15,16,17,18,19,20,21]. Despite the high selectivity of SAA catalysts toward the desired products, their activity is usually limited owing to the small number of isolated Pd sites on the surface of PdM nanoparticles. Hence, the development of a method for improving their catalytic activity is an urgent task.
The activity of the catalyst can be significantly affected by the support [22,23,24]. Its choice is usually determined by the degree of interaction of metal nanoparticles with the surface. The strong interaction of the support with the deposited metal particles can provoke a change in their electronic state and enhance their dispersion [7,25]. The use of cerium oxide as a support makes it possible to significantly (3–5-fold) increase the catalytic activity in a number of reactions [24], although the reasons for the change in catalytic activity still remain unclear. As reported earlier for the hydrogenation of phenol to cyclohexanone, an increase in the catalytic activity of the Pd/CeO2 catalyst can be facilitated by an increase in the dispersion of Pd [26,27].
It should be noted that the structural and textural characteristics of CeO2 can be significantly improved by using ZrO2 as a modifier. Even small amounts of ZrO2 can increase the thermal stability of CeO2 through the formation of a CeO2–ZrO2 substitutional solid solution [28,29]. Moreover, the increase in the activity of monometallic M/CeO2–ZrO2 (M = Pt, Pd) catalysts was observed in the hydrogenation of nitroaromatics and carbonyl compounds [30,31,32,33,34,35,36]. The authors suggested that the high catalytic activity of the Pt/CeO2–ZrO2 catalyst may be due to H2 spillover that provokes the formation of adsorption and activation centers of the initial substrate as a result of the partial reduction of CeO2–ZrO2 [33,34,35,36].
However, reported examples illustrate the increase in activity for monometallic catalysts only. To the best of our knowledge, the literature completely lacks data on the improvement of the performance of SAA catalysts on Ce-containing supports. With this in mind, the aim of the current work was to study the effect of a CeO2–ZrO2 carrier on the performance of a PdAg single-atom alloy catalyst in diphenylacetylene (DPA) hydrogenation with special attention paid to the selectivity of the catalyst. PdAg10/Al2O3 was used as the reference catalyst. The catalysts were designated as PdAg10/CZ and PdAg10/A.
Note that one of the efficient ways to control the composition of active sites and, as a consequence, the catalytic performance of single-atom catalysts is to determine the optimal ratio of active and inactive components in its structure. Our previous studies demonstrated significant enhancement of the selectivity in partial alkyne hydrogenation for PdAg/Al2O3 when using a Pd:Ag ratio of 1:1–1:5 [37,38,39]. Here, we focused on catalysts with a Pd:Ag molar ratio of 1:10. Their structural characteristics were studied using DRIFT spectroscopy of adsorbed CO and transmission high-resolution electron microscopy.

2. Results and Discussion

2.1. Mapping Transmission Electron Microscopy

The morphology, phase composition and particle size of the PdAg10 catalysts were obtained using HRTEM. Figure 1 represents the HAADF STEM micrograph (a) and EDX mapping (b) of a PdAg10/CZ catalyst. A narrow particle size distribution was observed with an average particle size of 4.4 nm (Figure 1a). The sample contained rounded particles uniformly distributed on the support surface with no highly enriched areas. The phase composition of the particles was determined from the EDX mapping. The data obtained indicate that the bimetallic particles are PdAg10, which is close to the initial stoichiometry (see Figure S1a and Table S1).
For the PdAg10/A catalyst, a wider particle size distribution (from 2 to 12 nm) was detected with an average size of 6.2 nm (Figure 1c). This catalyst also contained rounded particles with a uniform distribution on the catalyst surface. The EDX study revealed the formation of PdAg nanoparticles for SAA PdAg10/A catalyst with Pd/Ag close to the stoichiometric 1/10 molar ratio (see Figure S1b and Table S2).

2.2. DRIFT Spectroscopy of Adsorbed CO (DRIFTS-CO)

The surface structure of in situ reduced catalysts was studied via the DRIFT spectroscopy of adsorbed CO (Figure 2), which is known as a powerful tool for the analysis of SAA [40,41].
The DRIFTS-CO spectra of the freshly reduced PdAg10/A and PdAg10/CZ samples are very close to each other, indicating the similarity of their surface structure. Both spectra are characterized by the presence of an intensive symmetric absorption band centered at 2044–2047 cm−1, characteristic of linearly adsorbed CO on the Pd atoms. The negligible intensity of the signal in the wavenumber range of 2000–1800 cm−1 confirms practically complete absence of multipoint CO adsorption on the Pd-Ag surface. This indicates the disappearance of multiatomic Pdn surface sites (n ≥ 2) with the formation of a Pd-Ag SAA structure where Pd1 atoms are separated from each other by Ag atoms [42,43,44,45].
Formation of Pd-Ag nanoparticles provokes a redistribution of the electron density between Pd and Ag atoms, which leads to the donation of electron density to the antibonding π-orbital of the adsorbed CO with the C=O bond subsequently weakening [46]. This effect results in a shift of the linear CO band toward lower wavenumbers (~2040–2050 cm−1) compared to the spectrum of monometallic Pd (~2080–2090 cm−1) [47]. Note that both the small width and the symmetry of the band of linearly adsorbed CO points to the formation of highly homogeneous Pd1 active sites.

2.3. Catalytic Tests

Figure 3a,b compare the product distribution of diphenylacetylene (DPA) hydrogenation as a function of the reaction time for the reference PdAg10/A sample and the PdAg10/CZ catalyst. It is evident that, over both catalysts, the reaction proceeds via a two-step sequential scheme (Figure 4). According to this scheme, DPA hydrogenation to cis- (route a) and trans-diphenylethylene (DPE) (route b) first occurred (first step) followed by the alkenes’ hydrogenation to diphenylethane (DPEt) (second step) via routes f and e. It should be noted that trans-DPE formation is also possible through hydro-isomerization (route d) [48,49].
Figure 3 clearly shows the considerable differences in the DPA hydrogenation rates over SAA PdAg10/A and PdAg10/CZ catalysts. Thus, the DPA conversion over the reference PdAg10/A was completed in 80 min, while over the PdAg10/CZ, sample it took 30 min. Calculation of the GC data shows that the rate of DPA to DPE hydrogenation (r1) over PdAg10/CZ was 3.1 times higher than that over the reference PdAg10/A catalysts: 4.21 and 1.36 mmol DPA min−1 gcat−1, respectively (see Table 1). This observation is in agreement with the recent literature data on the positive effect of Ce-Zr support on the hydrogenation rate of different substrates over a Pt catalyst [30,31,33,34,50,51].
On the first reaction step, until complete DPA consumption, the yield of DPEt did not exceed 1% over the reference catalyst. This is the evidence that the DPEt formation through route c (Figure 4) is negligible, indicating the high selectivity of the catalyst toward DPE formation. The “Stilbene selectivity vs. DPA conversion” plot is displayed in Figure 3c. The selectivity of the reference catalyst was extremely high (>99%) and remained essentially constant throughout the whole DPA conversion range. This result agrees well with the previously reported one on the brilliant selectivity of SAA catalysts in partial alkyne hydrogenation [37,52,53,54].
It is remarkable that the selectivity of the PdAg10/CZ catalyst is essentially identical to the selectivity of the reference PdAg10/A (Figure 3c). The shapes of the selectivity profiles are the same for both samples, indicating high selectivity up to 100 % DPA conversion.s
The yields of cis- and trans-stilbenes reached a maximum after complete DPA conversion, giving 95.9% and 3.0% for the reference PdAg10/A catalyst and 93.2% and 4.2% for the PdAg10/CZ sample, respectively. Presumably, the formation of cis- and trans-stilbenes occurred in parallel fashion on the first reaction step. The preferential formation of the cis-isomer results from the syn-addition of hydrogen to the triple bond [55]. Probably, if trans-stilbene formation occurs in parallel to cis-stilbene even before the total consumption of alkyne, it is reasonable to assume that the trans-stilbene may be a product of direct DPA hydrogenation (route b on Figure 4) rather than cis-stilbene hydroisomerization (route d). However, this conclusion is tentative and requires additional detailed study.
On the second reaction step, stilbene hydrogenation leads to the formation of DPEt. It is evident from the reaction profiles (Figure 3a,b) that the rate of the DPEt formation (r2) is much lower than that of the DPA consumption. Thus, over the reference PdAg10/A catalyst, the r2 value was 95 times lower compared to r1 (Table 1, r1/r2). For the PdAg10/CZ sample, this difference was lower (ca. 37).
It is informative to analyze the kinetic profiles of hydrogen uptake in the course of DPA hydrogenation over PdAg10/A and PdAg10/CZ (Figure 5). The profiles exhibit a pronounced downward bending after uptake of 1 equivalent. of H2. It clearly indicates a decrease in the hydrogenation rate after the completion of DPA to DPE hydrogenation and a sharp slowdown in the complete hydrogenation toward alkane.
For both catalysts, the H2 consumption profile on the first reaction step (DPA to DPE) was essentially linear, which suggests a close-to-zero reaction order with respect to the substrate over PdAg catalysts. This observation is consistent with previous studies on the hydrogenation of alkynes on Pd catalysts and can be explained by a much stronger adsorption of initial alkyne than reaction products [56,57,58].
The rates of DPA consumption and DPEt formation calculated on the basis of GC data were in good agreement with the rates of H2 consumption (Table 1). Over the reference PdAg10/A catalyst, the rate of H2 consumption during DPA to DPE conversion was 1.37 mmol H2 min−1 gcat−1 and decreased extremely to 0.014 mmol H2 min−1 gcat−1 resulting in an r1/r2 ratio of ~98 (Table 1). For the PdAg10/CZ catalyst, the H2 consumption rate was significantly higher and reached 4.24 and 0.11 mmol H2/min/gcat on the first and second step, consequently resulting in an r1/r2 ratio of 39.
It should be noted that the stability of the catalysts under the reaction conditions is of great importance. For evaluating the possible impact of the reaction conditions on the catalyst’s performance, both PdAg10/A and PdAg10/CZ were investigated in five repeated catalytic tests. Details of the catalytic test can be found in Section 3.5. After each hydrogenation cycle, the catalysts were recovered from the reaction mixture by centrifugation (10,000 rpm, 10 min) and washed with n-hexane to ensure removal of the reaction products from the catalyst surface, and after that, they were dried overnight. Thereafter, a new run of DPA hydrogenation was performed using the fresh solvent and reactants under the same conditions as the initial experiments. The data obtained demonstrated that the catalytic performances of the fresh catalyst and the catalyst after five catalytic cycles were essentially identical (Figures S1 and S2). This fact suggests the absence of noticeable changes in catalyst’s performance in the course of DPA hydrogenation.

2.4. Discussion

Based on the experimental data, it is obvious that the activity of SAA PdAg10 catalysts can be enhanced by using CeO2–ZrO2 as a carrier. The increase in the catalytic activity of PdAg10/CZ could be attributable to both (1) change in the structure and electronic state of catalytic active sites on the surface of PdAg10 nanoparticles caused by the strong metal–support interaction (SMSI) [59] and (2) change in the number of active sites.
The characterization of single-atom alloy catalysts is a challenging task. However, an analysis of the most recent reviews on this issue [40,41] allows us to identify the most efficient combination of several techniques for studying the structure of the SAA catalyst. HRTEM, EDX, DRIFTS-CO, XRD and XPS are among them. For our catalysts, the use of XPS analysis was restricted by the low Pd content. However, two powerful methods were employed for PdAg10/A and PdAg10/CZ analysis.
DRIFT spectroscopy of adsorbed CO is known as a simple, convenient and informative method to detect the formation of single-atom/isolated Pd1 cites in bimetallic Pd-based structures (such as solid solutions, intermetallic phases, etc.) [40,41]. In the case of a monometallic Pd catalyst, the spectrum usually contains two main adsorption bands. The first one centered at 2100–2050 cm−1 corresponds to linearly bonded CO on top of the Pd atom, and the second one at 2000–1800 cm−1 (usually with higher intensity) is ascribed to bridged and hollow-bonded CO species on two or three neighboring Pd atoms. It should be mentioned that, on the Pd surface, multibonded CO adsorption is preferable due to higher adsorption energy (1.69–1.47 eV for hollow- and bridge-bonded CO vs. 1.15 eV for linearly adsorbed CO) [60]. On the other hand, the DRIFT spectral pattern of the PdAg SAA catalyst displayed only one band of linearly adsorbed CO, since multibonded CO adsorption on Pd1 sites surrounded by Ag atoms is not possible [40,44,52]. The group of Prof. T. Zhang was the first to use the DRIFTS-CO technique for confirming the formation of Pd-Ag nanoparticles with an SAA structure [43], and our data agree well with those obtained by them. The DRIFTS-CO spectra of the reference PdAg10/A and PdAg10/CZ showed almost complete absence of multipoint CO adsorption (Figure 2). This suggests the same SAA structure of active sites for both catalysts where single Pd1 atoms are separated from each other by Ag atoms. The similarity of electronic state of the adsorption sites for PdAg10/A and PdAg10/CZ also stems from the fact that the bands of linearly adsorbed CO are centered at the almost identical frequencies of 2044 and 2047cm−1, having the same symmetry and small width. Thus, the increase in catalytic activity of PdAg10/CZ cannot be explained by the modification of active site properties.
The second technique for studying the structure of PdAg SAA was HRTEM. Here, we used a Themis Z advanced instrument, capable of providing EDX analysis of individual metal nanoparticles (see Section 3.3. for details). The data obtained clearly indicated that the composition of each probed nanoparticle corresponded to the Pd1Ag10, giving solid evidence of PdAg alloyed nanoparticles formation (Tables S1 and S2 for details). The HRTEM data also provided an explanation for the different catalytic activity of PdAg10/CZ and PdAg10/A. The most likely explanation for the higher catalytic activity of PdAg10/CZ is the smaller size of the PdAg nanoparticles compared to the reference PdAg10/A catalyst. Indeed, as the size of the metal particles decreases, the fraction of active metal atoms on the particle surface available for reactants increases, enhancing catalyst activity [61]. Therefore, the higher catalytic activity of the PdAg10/CZ presumably stems from the formation of smaller particles with average size of 4.2 nm compared to the reference PdAg10/A with relatively larger average particle size of 6.2 nm.
In turn, the decrease in the size of the PdAg nanoparticles for PdAg10/CZ can be explained by the increase in the metal–support interaction energy in comparison to the PdAg10/A reference catalysts. It was reported previously that the modification of CeO2 with Zr enhances interaction between the support and metal nanoparticles, thus stabilizing small metal particles [31,62,63].
One more efficient tool to confirm PdAg alloy formation is a powder XRD. In our previous paper, we used this technique to study the structure of PdAg/α-Al2O3 [44]. Owing to the high crystallinity of α-Al2O3 and the relatively large size of the PdAg nanoparticles (10–12 nm), the overlap of signals of the metal nanoparticles and the support was minimized. Unfortunately, XRD is not applicable for PdAg nanoparticles supported on gamma-Al2O3 or CeO2–ZrO2 due to the smaller size of the metal particles and wide reflections of support overlapping those of Pd and Ag. Nevertheless, in the current paper we used the same preparation procedure as in [44], so we dare to hope that catalysts with the same structure were obtained. Moreover, the data from catalytic tests of PdAg10/A and PdAg10/CZ agree well with the previously reported results for the SAA PdAg catalyst with a Pd:Ag molar ratio of 1:3 [38,44,52].

3. Materials and Methods

3.1. Materials

Chemicals: diphenylacetylene (>98%, Sigma-Aldrich, Steinheim, Germany); n-hexane (98%, Merck KGaA, Darmstadt, Germany).
Metal precursors: Pd(NO3)2 (99.9 %, 14.86% Pd, Alfa Aesar, Ward Hill, MA, USA); AgNO3 (≥99.0 %, Sigma-Aldrich, Steinheim, Germany).
Carriers for catalyst preparation: Al2O3 (SBET = 56 m2/g, Sasol, Hamburg, Germany); 80% CeO2–20% ZrO2 (C20Z, SBET = 102 m2/g, Ekoal’yans LLC, Novoural’sk, Russia).
Gases and gaseous mixtures: 5 vol.% H2/Ar (MGPP LLC, Vidnoe, Moscow region, Russia); N2 (99.999% grade, Vidnoe, Moscow region, MGPP LLC, Russia); He (99.9999% grade, Vidnoe, Moscow region, MGPP LLC, Russia); Ar (99.9999%, Balashikha, Moscow region, Linde Gas Rus, Balashikha, Moscow region, Russia); 0.5 vol.%CO/He (Balashikha, Moscow region, Linde Gas Rus, Russia).

3.2. Catalyst Preparation

Two PdAg catalysts with an identical Pd:Ag = 1:10 ratio were synthesized; the catalysts were supported on Al2O3 and 80% CeO2–20% ZrO2. The parent powder supports were precalcined in dry air flow at 500 °C for 4h. Pretreated supports were impregnated by incipient wetness co-impregnation with an aqueous solution of Pd(NO3)2 and AgNO3. The impregnated materials were then treated as follows: (1) drying overnight at ambient temperature, (2) calcination at 550 °C in dry air flow (300 mL/min) and (3) reduction at 550 °C in 5 wt.%H2/Ar flow for 3 h. After reduction, the samples were cooled down to 200 °C in 5 wt.% H2/Ar flow. After that, the gas flow was switched from 5 wt.% H2/Ar to N2 of high purity, and the samples were cooled down to ambient temperature. The reduction temperature was chosen based on our previous data [38]. It has been shown that 550 °C is more than sufficient to reduce metal components and provide the required degree of mobility of Pd and Ag atoms for the formation of uniformly distributed Pd–Ag nanoparticles. According to inductively coupled plasma atomic emission spectrometry (ICP-AES) the metal content was 0.51 wt.% Pd and 5.05 wt.% Ag for Pd1Ag10/Al2O3 (PdAg10/A) and 0.50 wt.% Pd and 5.1 wt.% Ag for Pd1Ag10/CeO2–ZrO2 (PdAg10/CZ), which corresponds to a Pd:Ag molar ratio of 1:10.

3.3. High-Resolution Transmission Electron Microscopy

For the TEM study, all samples were ultrasonically suspended in EtOH and deposited on a holey carbon film mounted on an aluminum grid. The Themis Z instrument was used (Thermo Fisher Scientific, Eindhoven, The Netherlands). The microscope is equipped with a Ceta 16 CCD sensor, a corrector of spherical aberrations and an EDX Super-X spectrometer (Thermo Fisher Scientific, Eindhoven, The Netherlands) with a semiconductor Si detector (128 eV). The instrument operated at an accelerating voltage of 200 kV. The maximum lattice resolution was 0.07 nm.

3.4. DRIFT Spectroscopy of Adsorbed CO

The diffuse reflectance IR spectra of adsorbed CO (DRIFTS-CO) were recorded with a Tensor 27 IR instrument (Bruker, Germany) equipped with an MCT detector and a Diffuse Reflection Catalysis Package for in situ measurements (Harrick Scientific Products, UK). Approximately 0.01–0.015 g of catalyst was placed in a CaF2 cell and heated to 500 °C in flowing Ar with subsequent reduction in a flow of 5% H2/Ar (30 cm3/min) at 500 °C for 1 h. Thereafter, the sample was cooled as follows: to 150 °C in a flow of 5 wt.% H2/Ar and further to 50 °C in a flow of Ar. The background spectrum was acquired at 50 °C under Ar flow. DRIFTS-CO spectra were recorded under a flow of 0.5 vol.% CO/He (30 cm3/min) at 50 °C for 10 min (250 scans, resolution of 4 cm–1).

3.5. Catalytic Tests

A batch-type reactor was used for the diphenylacetylene hydrogenation. The reaction was performed at 25 °C, under 5 bar of H2, with 1000 rpm stirring. n-Hexane was chosen as a solvent. The catalytic unit was equipped with a digital pressure sensor connected to the reactor. This sensor allowed measurement of the hydrogen consumption during the reaction by recording the pressure drop. The reaction mixture was sampled and GC analyzed with a Crystal 5000 instrument (Chromatek, Russia) equipped with a flame-ionization detector. A HP5-MS GC column (30 m × 0.25 mm ID, 0.25 µm film thickness) was used to separate the products.
The H2 consumption profile exhibited a pronounced downward bending after the consumption of 1 eq. of H2 because alkyne hydrogenation proceeds via a two-step sequential scheme (see Figure 5). The reaction rates in mmol H2 min−1 gcat−1 on the 1st and 2nd reaction steps were obtained from the slope of the linear parts of the hydrogen consumption curve. Furthermore, the rates of DPA consumption (1st reaction step rate) and DPEt formation as the 2nd in mmol min−1 gcat−1 were calculated from the time course of reaction mixture composition (Figure 3a,b) on the basis of GC analysis data. Both methods provided consistent results (Table 1).
The efficiency of the kinetic control was evaluated by comparing the rates of diphenylacetylene hydrogenation for the 1st and 2nd steps (r1/r2) in accordance with [64].
The specific activity normalized by Pd content on the 1st (A1, s−1) and 2nd (A2, s−1) reaction steps were calculated as follows:
A n = r n m cat ∗ X Pd ∗ M Pd ∗ 60
Here, rn represents the H2 consumption rate on the 1st or 2nd reaction step in mmol H2/min, mcat and XPd represent the mass of utilized catalyst for the catalytic run and Pd content in wt.% and MPd is the molecular weight of palladium.
The selectivity toward alkene formation (SDPE) was calculated based on the GC data, using the following equation:
S DPE = n cis - DPE + n trans - DPE n cis - DPE + n trans - DPE + n DPEt
Here, ncis-DPE and ntrans-DPE are the molar fractions of the cis- and trans-diphenylethylene, and nDPEt is the molar fraction of diphenylethane.
Our previous experiments with different catalyst loading made it possible to eliminate gas–liquid mass transfer as the limiting factor [56]. Moreover, relying on our previous results, we ensured that the DPA hydrogenation proceeded in the kinetic regime. Thus, special precautions were taken to avoid internal and external mass-transfer limitations. It was shown [56] that, at a stirring speed exceeding 1000 rpm, identical activity and selectivity were obtained. This fact indicates the absence of external mass-transfer limitations. Internal mass-transfer limitations were minimized by carefully grinding the catalysts to obtain a fine powder with a particle size of <10 µm [65]. The Weisz–Prater criterion was used for elucidation of the mass-transfer impact [66]. When calculating the Weiss–Prater criterion for the highest reaction rate, the concentration of hydrogen in the solvent, the effective diffusion coefficient of hydrogen and a ratio of porosity to tortuosity of 0.1 were taken into account. The calculation gives a Weiss–Prater value of <0.02, indicating that internal mass-transfer limitations can be eliminated.

4. Conclusions

In this work, we investigated the effects of the support on the properties of PdAg/Al2O3 and PdAg/CeO2–ZrO2 catalysts in hydrogenation of DPA. Both catalysts with a Ag/Pd molar ratio of 10 were obtained via the incipient wet impregnation technique. The structure of the catalysts was studied using HRTEM and DRIFTS-CO. The formation of SAA nanoparticles was supported by STEM-EDX spectrum imaging. The DRIFTS-CO data confirm that the formation of single-atom Pd1 sites isolated from each other by Ag atoms occurs on the surface of supported bimetallic Pd–Ag nanoparticles for both catalysts. According to catalytic data, the use of a CeO2–ZrO2 support significantly enhanced the activity of SAA PdAg catalysts in DPA hydrogenation while maintaining extremely high target selectivity within the entire DPA conversion range. The high selectivity of the PdAg/CeO2–ZrO2 catalyst was retained due to its high dispersion compared to PdAg/Al2O3, as evidenced by the HRTEM data.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/inorganics11040150/s1, Figure S1: EDX spectrum of PdAg10/CZ (a) and PdAg10/A (b) catalysts.; Figure S2: Products distribution of DPA hydrogenation over PdAg10/A (a) and PdAg10/CZ (b) after 1st and 5th reaction runs; Figure S3: Comparison of selectivity toward stilbene formation for PdAg10/A and PdAg10/CZ catalysts after 1st and 5th reaction runs; Table S1: Quantitative analysis of EDX-spectrum of PdAg10/CZ catalyst; Table S2. Quantitative analysis of EDX-spectrum of PdAg10/A catalyst.

Author Contributions

Conceptualization: A.Y.S., I.S.M., A.V.B. and Y.V.Z.; methodology: A.Y.S. and A.V.B.; validation: P.V.M., N.S.S., G.N.B., G.O.B., A.V.B. and E.Y.G.; formal analysis: P.V.M., N.S.S., G.N.B., G.O.B., A.V.B. and E.Y.G.; investigation: P.V.M., N.S.S., G.N.B., G.O.B., A.V.B. and E.Y.G.; resources: G.N.B. and G.O.B.; data curation: A.V.B. and I.S.M.; writing—original draft preparation: P.V.M., N.S.S., G.O.B. and I.S.M.; writing—review and editing: A.Y.S., I.S.M., A.V.B. and Y.V.Z.; visualization: I.S.M. and P.V.M.; supervision: A.Y.S. and Y.V.Z.; project administration: A.Y.S. and Y.V.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Russian Science Foundation (grant No. 19-13-00285-P).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

All the reported data were taken from the published materials referenced below.

Conflicts of Interest

The authors declare no conflict of interest.

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Inorganics 11 00150 g001 550
Figure 1. HAADF STEM microphotography and HAADF STEM-mode EDX mapping of PdAg10/CZ (a,b) and PdAg10/A (c,d) catalysts. Inserts in Figure 1a,c represent the corresponding particle-size distribution histograms.
Figure 1. HAADF STEM microphotography and HAADF STEM-mode EDX mapping of PdAg10/CZ (a,b) and PdAg10/A (c,d) catalysts. Inserts in Figure 1a,c represent the corresponding particle-size distribution histograms.
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Figure 2. DRIFT spectra of adsorbed CO for the PdAg10/A (1) and PdAg10/CZ (2) catalysts.
Figure 2. DRIFT spectra of adsorbed CO for the PdAg10/A (1) and PdAg10/CZ (2) catalysts.
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Figure 3. DPA hydrogenation products distribution over PdAg10/A (a) and PdAg10/CZ (b) catalysts based on chromatographic analysis data; (c) dependence of selectivity toward stilbene formation. P(H2) = 5 bar, mcat =10 mg, [DPA] = 1 mmol.
Figure 3. DPA hydrogenation products distribution over PdAg10/A (a) and PdAg10/CZ (b) catalysts based on chromatographic analysis data; (c) dependence of selectivity toward stilbene formation. P(H2) = 5 bar, mcat =10 mg, [DPA] = 1 mmol.
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Figure 4. A two-step mechanism of DPA hydrogenation. First step—DPA to DPE hydrogenation (routes a and b). Second step—DPE to DPEt hydrogenation (routes f and e). Route c represents the direct hydrogenation of DPA to DPEt. Route d demonstrates cis- to trans-DPE hydro-isomerization.
Figure 4. A two-step mechanism of DPA hydrogenation. First step—DPA to DPE hydrogenation (routes a and b). Second step—DPE to DPEt hydrogenation (routes f and e). Route c represents the direct hydrogenation of DPA to DPEt. Route d demonstrates cis- to trans-DPE hydro-isomerization.
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Figure 5. Figure 5. Kinetics of hydrogen uptake for DPA hydrogenation over SAA PdAg10/A and PdAg10/CZ catalysts. P(H2) = 5 bar, mcat = 10 mg, [DPA] = 1 mmol.
Figure 5. Figure 5. Kinetics of hydrogen uptake for DPA hydrogenation over SAA PdAg10/A and PdAg10/CZ catalysts. P(H2) = 5 bar, mcat = 10 mg, [DPA] = 1 mmol.
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Table
Table 1. Kinetic parameters of DPA to DPE and DPE to DPEt hydrogenation (first and second reaction steps, respectively) over PdAg10/CZ and reference PdAg10/A catalysts.
Table 1. Kinetic parameters of DPA to DPE and DPE to DPEt hydrogenation (first and second reaction steps, respectively) over PdAg10/CZ and reference PdAg10/A catalysts.
CatalystCalculation on the GC DataCalculation on the Kinetic Data
r1r2r1/r2r1r2r1/r2A1A2
mmol DPA (DPEt) min−1 gcat−1mmol H2 min−1 gcat−1s−1
PdAg10/CZ4.210.1037.04.240.1139.01.5050.039
PdAg10/A1.360.01495.11.370.01497.70.4850.005
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Markov, P.V.; Bragina, G.O.; Smirnova, N.S.; Baeva, G.N.; Mashkovsky, I.S.; Gerasimov, E.Y.; Bukhtiyarov, A.V.; Zubavichus, Y.V.; Stakheev, A.Y. Single-Atom Alloy Pd1Ag10/CeO2–ZrO2 as a Promising Catalyst for Selective Alkyne Hydrogenation. Inorganics 2023, 11, 150. https://doi.org/10.3390/inorganics11040150

AMA Style

Markov PV, Bragina GO, Smirnova NS, Baeva GN, Mashkovsky IS, Gerasimov EY, Bukhtiyarov AV, Zubavichus YV, Stakheev AY. Single-Atom Alloy Pd1Ag10/CeO2–ZrO2 as a Promising Catalyst for Selective Alkyne Hydrogenation. Inorganics. 2023; 11(4):150. https://doi.org/10.3390/inorganics11040150

Chicago/Turabian Style

Markov, Pavel V., Galina O. Bragina, Nadezhda S. Smirnova, Galina N. Baeva, Igor S. Mashkovsky, Evgeny Y. Gerasimov, Andrey V. Bukhtiyarov, Yan. V. Zubavichus, and Alexander Y. Stakheev. 2023. "Single-Atom Alloy Pd1Ag10/CeO2–ZrO2 as a Promising Catalyst for Selective Alkyne Hydrogenation" Inorganics 11, no. 4: 150. https://doi.org/10.3390/inorganics11040150

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