Alkaline-earth-promoted Pd–Ag/Al2O3 for selective acetylene green-oil mitigation, ethylene selectivity, and implications for hydrogen spillover
Authors: Farnaz Rahbar Shamskar, Sajad Mobini, Mehran Rezaei
Abstract
Removing trace acetylene from ethylene streams is critical to protect Ziegler–Natta polymerization. Eggshell Pd–Ag/θ-Al2O3 spheres (0.03 wt% Pd, 0.13 wt% Ag) were promoted with 1 wt% alkaline-earth additives to suppress green oil and improve ethylene selectivity. XRD, N2 physisorption, H2-TPR, NH3/CO2-TPD, TPO, SEM–EDS, and ICP-OES established structure-property links. Fixed-bed tests (10 bar, 40–60 °C) after H2 prereduction showed promoter-stabilized θ-Al2O3, higher surface area and mesoporosity and stronger metal-support interactions. All promoted catalysts achieved ≳ 96% acetylene conversion with higher ethylene selectivity and reduced carbon deposition. A possible contribution from hydrogen spillover is discussed qualitatively based on indirect evidence.
Supplementary Information
The online version contains supplementary material available at 10.1038/s41598-026-46044-5.
Introduction
In the petrochemical industry, ethylene is one of the most widely used raw materials for polyethylene production, typically obtained via steam cracking of hydrocarbons^1–3^. However, ethylene from steam cracking contains trace amounts of acetylene, which acts as a poison for Ziegler–Natta catalysts during ethylene polymerization^4^. The presence of acetylene in ethylene feedstock also negatively affects the quality of the downstream polyethylene product. Therefore, the acetylene content must be reduced to 1–5 ppm^5,6^. The most effective approach for removing acetylene is its selective hydrogenation to ethylene, which is widely recognized as the most practical method.
Hydrogenation catalysts readily convert acetylene to ethylene, but the key challenge lies in designing catalysts with high selectivity. Transition metals such as Pd, Pt, Rh, and Ir exhibit excellent activity in alkyne hydrogenation due to their ability to dissociate H2 efficiently^7^. Among these, Pd-based catalysts are particularly effective for acetylene hydrogenation to ethylene^8–11^. However, the strong hydrogenation activity of Pd also promotes undesired side reactions, including over-hydrogenation of acetylene to ethane and oligomerization reactions over base metals and oxides, which lower ethylene selectivity^12,13^. Consequently, the most critical strategy is the rational design of catalysts that achieve high activity while maintaining superior selectivity toward ethylene.
Borodzinski et al.^14^ reported that small palladium particles enhance catalyst selectivity by favoring acetylene adsorption while suppressing ethylene adsorption. Tew et al.^15^ demonstrated that exposure of reduced metallic catalysts to hydrogen leads to the formation of metal hydrides. When these catalysts are subsequently subjected to alkyne feeds, the hydrides are transformed into unstable carbide-like phases. They showed that the formation of such carbide-like phases is exclusively induced by alkynes and does not occur with alkenes. The role of these carbide phases, along with carbon dissolved in the palladium lattice, was identified as eliminating bulk-dissolved hydrogen, thereby suppressing over-hydrogenation and enhancing ethylene selectivity^16^. In this regard, deposited carbon may act as a selective modifier by isolating metallic sites and preventing subsurface hydrogen from reaching the surface^17,18^.
Improved selectivity can also be achieved by controlling ethylene and acetylene adsorption on Pd surfaces through electronic and geometric modifications^19^. While acetylene adsorbs more strongly than ethylene, the binding energy of ethylene is sufficient to allow retention on the surface after formation^20,21^. Electronic effects involve suppressing side reactions and inhibiting β-PdH phase formation via co-catalyst bonding to Pd, whereas geometric effects hinder the formation of alkylidene intermediates that drive over-hydrogenation to ethane^21^. Doping Pd nanoparticles with organic or inorganic components has been shown to improve selectivity by altering Pd ensemble size and structure^7,22^.
Cao et al.^10^ synthesized Pd–In/Al2O3 catalysts via impregnation, achieving higher activity and selectivity than conventional Pd/Al2O3. They attributed this to the formation of Pd–In (110) intermetallic structures, which block Pd sites and transfer electrons from In to Pd, rendering the Pd surface negatively charged. This electronic modification, combined with weaker adsorption of ethylene and acetylene and suppression of hydride formation, enhanced catalyst selectivity. Overall, it has been proposed that hydride phases shift selectivity toward alkane formation, while Pd–C phases serve as active sites for selective acetylene hydrogenation to ethylene^21^.
Since the 1980s, eggshell Pd–Ag/Al2O3 catalysts have been widely applied industrially for this reaction^2,6^. More recently, Li et al.^23^ investigated α-Al2O3-supported bimetallic catalysts, including Pd–Ag, Ni–Pd, Ni–Zn, Ni–Ag, and Ni–Ga, for acetylene hydrogenation, and demonstrated that Pd–Ag exhibited the highest selectivity under the tested conditions.
Many studies have reported the use of promoters, especially alkali, alkaline earth and rare-earth metals, to prevent the deposition of carbon and carbon chains on the catalyst surface. In steam reforming, alkali (K) added to Ni/Al2O3 demonstrably suppresses coking by modifying surface chemistry^24^. In dry reforming of methane (DRM), alkaline-earth and rare-earth modifiers (MgO, CaO, BaO; La2O3, CeO2, ZrO2) on Ni/Al2O3 cut coke and enhance stability^25–27^; Sr promotion on Ni/ZrO2–Al2O3 likewise increases basicity/dispersion and reduces sintering and coking. Beyond reforming, in propane dehydrogenation (PDH), Sn (and Ca co-promotion) on Pt/Al2O3 lowers coke accumulation while improving propylene productivity—illustrating a general promoter-enabled anti-coking strategy in dehydrogenation systems^28,29^.
In the context of acetylene hydrogenation, the linkage between deactivation and “green-oil” formation establishes carbon-mitigation as a central design target, for which promoter/support strategies have been articulated under tail-end conditions.
In addition to noble-metal modification strategies, several studies have explored alkaline-earth modifications to steer Pd-based catalysts for acetylene hydrogenation. Mg has been introduced either as a basic support or support modifier, altering Pd electronic/ensemble characteristics and improving C2H4 selectivity on Pd–Mg constructs and Pd/MgO systems under acetylene feeds^30,31^. Calcium addition has also been examined on industrially relevant Pd–Ag/Al2O3, where CaO affected Pd dispersion and activity/temperature windows^32^. Strontium promotion of supported Pd has been reported to enhance metal dispersion and hydrogenation performance across multiple oxide supports, suggesting a basic-promoter effect that is likely applicable to acetylene hydrogenation as well^33^.
State-of-the-art reviews summarize strategies for optimizing Pd-based catalysts in acetylene semi-hydrogenation^34,35^. However, composition-controlled, head-to-head comparisons of alkaline-earth promoters on eggshell Pd–Ag/Al2O3 catalysts under genuinely tail-end conditions remain limited. Herein, we compare Mg, Ca, and Sr promoters on an eggshell Pd–Ag/Al2O3 catalyst for tail-end acetylene hydrogenation while keeping Pd and Ag essentially constant. Prior reports (Refs.^30–33^) address alkaline-earth effects, but they do not provide a composition-controlled Mg/Ca/Sr comparison on an eggshell Pd–Ag/Al2O3 under competitive tail-end feed; therefore, the present work provides a useful benchmark for evaluating promoter identity within a fixed Pd–Ag baseline under representative tail-end conditions. Our contribution is not a first report of alkaline-earth promotion, but a controlled promoter-identity benchmark within a fixed eggshell Pd–Ag/Al2O3 architecture under tail-end conditions.
Experimental
Materials
Chemical precursors including Ag(NO3), PdCl2, Mg(NO3)2·6H2O, Ca(NO3)2·4H2O, and Sr(NO3)2 were purchased from Merck. To prepare the PdCl4^2−^ solution, PdCl2 was dissolved in hydrochloric acid (37.5% purity). Deionized water with an electrical conductivity below 10^−6^ S cm^−1^ was used in all synthesis steps. The Al2O3 support in granular form (surface 40 m^2^ g^−1^; 3–4 mm) was supplied by Sasol and employed as the catalyst carrier. A Pd–Ag/Al2O3 catalyst (0.03 wt% Pd and 0.13 wt% Ag) was synthesized via a two-step impregnation method. The resulting eggshell catalyst consisted of palladium and silver deposited preferentially in an eggshell distribution near the outer rim of the Al2O3 granules. The preparation procedure of Pd–Ag/1%M/Al2O3 catalysts is schematically presented in Fig. 1.Fig. 1Schematic representation of the synthesis route for Pd–Ag/Al2O3 catalysts promoted with 1% alkaline-earth metals (Mg, Ca, Sr).
Preparation of promoted alumina support
The alkaline earth promoters (Mg, Ca, and Sr) were introduced onto the spherical Al2O3 support using the dry impregnation method. Aqueous solutions of the respective nitrates (Mg(NO3)2·6H2O, Ca(NO3)2·4H2O, or Sr(NO3)2) were prepared at concentrations corresponding to a final promoter loading of 1 wt%. These solutions were impregnated onto the spherical alumina, followed by drying overnight at 80 °C. The dried samples were then calcined in air at 600 °C for 4 h to obtain the promoted supports.
Preparation of Pd–Ag/M-Al2O3 catalyst
The M–Al2O3 supports (M=Mg, Ca, and Sr) were first impregnated with the desired amount of PdCl2 solution. The impregnated samples were dried in an oven at 120 °C for 12 h. Subsequently, the dried catalysts were impregnated with the required amount of silver nitrate solution. Finally, the catalysts were dried overnight at 80 °C and then calcined in air at 450 °C for 4 h.
Characterization
X-ray diffraction (XRD) analysis was performed using a Rigaku ultima iv diffractometer. Temperature-programmed reduction (TPR), oxidation (TPO), and desorption (TPD) analyses were conducted on a Micromeritics Chemisorb 2750 instrument. For TPR, 500 mg of catalyst was heated at 10 °C·min^−1^ under a H2/Ar mixture (10:90, 30 mL min^−1^), and H2 uptake was monitored using a thermal conductivity detector (TCD). CO2-TPD and NH3-TPD were performed on 500 mg of catalyst. The samples were first saturated with a flow of CO2 or NH3 at 40 °C for 1 h, cooled to 25 °C, and then heated to 900 °C at 10 °C min^−1^ under He flow, with desorbed species detected by TCD. TPO of spent catalysts was carried out on 500 mg of sample under an O2/He mixture (5:95, 30 mL min^−1^), heating to 800 °C at 10 °C min^−1^.
N2 adsorption–desorption isotherms were measured at − 196 °C using a BELSORP-mini II analyzer after degassing the samples at 200 °C for 2 h (Micromeritics VacPrep 061). The elemental composition of the prepared samples was determined by inductively coupled plasma optical emission spectrometry (ICP-OES) using a Varian Vista-MPX instrument. Scanning electron microscopy (SEM) images were acquired on a VEGA TESCAN microscope operated at 30 kV.
Cross-sectional SEM was performed on mechanically cut and polished pellets, with the sectioning plane prepared perpendicular (≈90°) to the pellet surface. Elemental analysis in SEM was limited to EDS mapping.
Catalytic tests
The catalytic tests were carried out in a U-shaped tubular microreactor at 10 bar and various temperatures (40, 50, and 60 °C). Typically, 1.00 g of catalyst granules was loaded into the reactor and pre-reduced with a high-purity H2 stream (20 mL min^−1^, 99.999%) at 150 °C for 1 h prior to the reaction. After pretreatment, the reactor was cooled to 40 °C, and a mixed feed of C2H2 (1.0 vol%), H2 (1.4 vol%), C2H6 (37.5 vol%), and C2H4 (60.1 vol%) was introduced at a total flow rate of 83.3 mL min^−1^, corresponding to a gas hourly space velocity (GHSV) of 5000 mL g^−1^ h^−1^.
The composition of the outlet gas was analyzed online using a gas chromatograph (Young Lin 6500) equipped with a HP-PLOT/Q column and a HID detector. The acetylene conversion (C2H2) and ethylene selectivity were calculated using Eqs. 1 and 2, 1\documentclass[12pt]{minimal}
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X_{{C_{2} H_{2} }} = \frac{{C_{2} H_{2} \left( {in} \right) - C_{2} H_{2} \left( {out} \right)}}{{C_{2} H_{2} \left( {in} \right)}} \times {1}00
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S_{{C_{2} H_{4} }} = \frac{{C_{2} H_{4} \left( {out} \right) - C_{2} H_{4} \left( {in} \right)}}{{C_{2} H_{2} \left( {in} \right) - C_{2} H_{2} \left( {out} \right)}} \times {1}00
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Long-term stability was evaluated at 60 °C for 15 h under constant feed composition.
## Results and discussion
### Structural properties of the fresh catalysts
The XRD patterns of the promoted and unpromoted Pd–Ag/Al~2~O~3~ catalysts are provided in the Supporting Information (Fig. S1). All samples show diffraction features corresponding to θ-Al~2~O~3~, indicating that the support structure is preserved after metal loading and promoter addition^36,37^.
The N~2~ adsorption–desorption isotherms of fresh Pd–Ag/Al~2~O~3~ and alkaline-earth-promoted Pd–Ag/Al~2~O~3~ catalysts are shown in Fig. 2a. All samples exhibit Type IV(a) isotherms with H2-type hysteresis loops according to the IUPAC classification, indicative of a mesoporous structure. Capillary condensation occurs in nearly cylindrical or ink bottle pores of non-uniform size or shape^38^. The pronounced and open hysteresis loops observed, particularly for the Sr- and Ca-promoted catalysts, are attributed to the presence of textural mesopores and macropores formed between aggregates of alumina particles. According to Gun’ko et al.^39^, large voids between aggregated nanoparticles facilitate adsorbate clustering, which reduces enthalpy changes during phase transitions and impedes complete desorption, resulting in open hysteresis loops extending to high relative pressures.Fig. 2(**a**) N~2~ adsorption–desorption isotherms (Minor non-closure near \documentclass[12pt]{minimal}
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P/{P}_{0}\approx 1
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These structural features generally enhance catalytic performance by increasing the accessible surface area and improving mass transport. However, excessive aggregation may lead to the formation of large macropores, which contribute less effectively to the active surface area and may compromise mechanical stability. The N~2~ adsorption–desorption data show that promoter incorporation substantially increases the total N~2~ adsorbed volume, particularly at higher relative pressures, following the trend Sr > Ca > Mg > unpromoted. The incorporation of promoters appears to preserve or induce textural porosity during catalyst preparation, facilitating access to Pd–Ag active sites. Notably, Sr-promoted catalysts display the highest nitrogen adsorption, suggesting the presence of abundant meso-/macropores that enhance mass transport throughout the porous structure during acetylene hydrogenation.
The pore size distribution (PSD) curves derived from the desorption branch using the BJH method (Fig. 2b) indicate a dominant pore size of ~ 10 to 15 nm for all fresh catalysts, confirming mesoporosity as a key structural feature. The measured mesopore volumes follow the trend Sr ≥ Ca > Mg > unpromoted, consistent with the observed high adsorption at elevated relative pressures (P/P~0~). The enhanced textural properties observed for Sr-promoted catalysts can be attributed to the larger ionic radius of Sr^2+^ (≈ 1.18 Å) compared with Ca^2+^ (≈ 1.00 Å) and Mg^2+^ (≈ 0.72 Å). During catalyst synthesis, incorporation of cations with larger ionic radii introduces lattice strain and modifies the distribution of surface hydroxyl groups on the alumina support. Sr incorporation helps mitigate localized sintering and maintains void spaces between adjacent particles. Additionally, the induced lattice strain can disrupt the packing of alumina crystallites, occasionally generating extra pore volume. This dual effect preserves the original mesoporous framework while promoting the formation of broader, interconnected pores, which increase the total pore capacity and enhance mass transport to the Pd–Ag active sites.
Modification of Pd–Ag/Al~2~O~3~ with alkaline earth metals, particularly Sr, increases the total pore volume and mesopore population while maintaining a highly open and interconnected pore network with minimal diffusion limitations. These structural enhancements are expected to improve the accessibility of active metal sites and may enhance catalytic performance in reactions where mass transport within the porous matrix is critical.
The textural properties of the fresh catalysts are summarized in Table 1. Incorporation of alkaline earth metals (Mg, Ca, Sr) into Pd–Ag/Al~2~O~3~ resulted in a noticeable increase in BET surface area, rising from 38 m^2^ g^−1^ for the unpromoted catalyst to 40–45 m^2^ g^−1^ upon promotion. This improvement can be attributed to the role of alkaline earth oxides as structural stabilizers during calcination. Specifically, Mg, Ca, or Sr species deposited on the alumina surface act as spacers between Al~2~O~3~ crystallites, thereby inhibiting particle sintering, preserving the mesoporous network, and maintaining a higher surface area. Furthermore, the addition of promoters modifies particle dimensions and pore-size distribution, leading to an increased external surface area. This enhancement is most likely associated with the dispersion-promoting effect of alkaline earth oxides, which minimize pore blockage and aggregation during catalyst preparation. Among the examined promoters, Sr exhibited the most pronounced effect, primarily due to its larger ionic radius, which increases the spacing between alumina particles and enhances pore-size integrity during calcination.Table 1Structural properties of the fresh catalysts.SamplesS~BET~ (m^−2^ g^-1^)Total pore volume [cm^3^ g^−1^]Mean pore diameter (nm)ICP Results (%wt)MgCaSrPdAgPd–Ag/Al~2~O~3~38.50.0879.060.0250.113Pd–Ag/Mg1%/Al~2~O~3~40.70.0959.331.020.0260.107Pd–Ag/Ca1%/Al~2~O~3~42.80.1079.990.980.0230.107Pd–Ag/Sr1%/Al~2~O~3~45.40.12511.01.080.0250.108
Bulk compositions of the fresh catalysts were quantified by ICP–OES (Table 1). The alkaline-earth promoters closely match the nominal 1 wt% Mg = 1.02 wt%, Ca = 0.98 wt%, and Sr = 1.08 wt%. The noble-metal loadings remain essentially constant across the series at Pd ≈ 0.025 wt% (0.023–0.026) and Ag ≈ 0.11 wt% (0.107–0.113), indicating reproducible deposition and the absence of unintended metal loss or exchange during promoter introduction.
The temperature programmed reduction (TPR) profiles of the catalysts in Fig. 3 exhibited that introduction of alkaline-earth promoters profoundly reshapes the reduction profiles. The unpromoted catalyst exhibits only a weak, broad low-temperature feature, indicating that most PdO is readily reducible and/or that a substantial fraction of Pd is already in the metallic state after calcination. This suggests a relatively weak metal–support interaction (MSI) on Al~2~O~3~ and a correspondingly low overall H~2~ uptake^40^. All promoted catalysts show higher total H~2~ consumption, accompanied by a shift of the main reduction band to higher temperature and a pronounced broadening, consistent with stronger MSI and stabilization of oxidic Pd/Ag species. Incorporation of alkaline-earth oxides (MgO, CaO, SrO) into Pd–Ag/Al~2~O~3~ modifies both the surface properties and electronic structure of the catalyst. These oxides interact with the alumina support to form M–O–Al interfacial sites and, in some cases, surface aluminates. Such modifications can anchor and electronically tune Pd/Ag ensembles, leading to a broader distribution of reducible Pd/Ag environments and more heterogeneous interfacial sites. Because TPR does not directly quantify particle size or dispersion, these interpretations are discussed as qualitative trends. The formation of M–O–Al interfacial sites may enhance metal anchoring and may mitigate sintering and generates more stable and active catalytic sites. In addition, the increased basicity imparted by these oxides suppresses undesired side reactions—such as over-hydrogenation and coke deposition—thereby improving both the selectivity and durability of the catalysts.Fig. 3H~2~-TPR profiles of unpromoted Pd–Ag/Al~2~O~3~ and alkaline-earth metal (Mg, Ca, Sr)-promoted Pd–Ag/Al~2~O~3~ catalysts.
These findings are consistent with previous reports on Ni/γ-Al~2~O~3~ catalysts modified with CaO and MgO, which demonstrated enhanced catalytic performance and stability due to similar modifications of the alumina surface^41^. For the Mg1% sample, the spectrum displays a single, relatively mild maximum at intermediate temperature with limited high-temperature tailing. This behavior reflects a moderate increase in MSI compared to the parent catalyst, yielding PdO species that remain readily reducible. Such moderation is advantageous for start-up because metallic Pd can be generated under relatively mild pre-reduction conditions while avoiding the formation of excessively large Pd ensembles. In the Ca1% catalyst (excluding the terminal sharp event), multiple shoulders are an initial low-temperature component associated with easily reducible PdO, followed by more intense mid-temperature features assigned to PdO/AgOx species interacting with Ca-modified alumina. The larger H~2~ uptake compared to Mg1% indicates that CaO stabilizes a greater fraction of oxidized species and strengthens Pd–support interactions without collapsing into a single, narrow reduction peak.
The Sr1% catalyst exhibits the most intense and most temperature-shifted broad maximum among the series. This profile implies the strongest MSI and the greatest stabilization of oxidic species, likely due to the formation of Sr–Al–O interfacial structures. The breadth of the reduction band further is consistent with highly dispersed oxidic Pd/Ag species and/or a wider distribution of interfacial environments; however, direct particle sizing would require TEM/chemisorption^42^. Overall, the observed trend Sr > Ca > Mg > unpromoted in both temperature shift and band broadening provides clear evidence of progressively stronger MSI and increased support basicity^43,44^. In the context of acetylene hydrogenation (operated at 40–60 °C), these TPR results imply that the promoted catalysts require a deliberate pre-reduction step to generate metallic Pd; however, their stronger MSI and smaller Pd ensembles are expected to suppress over-hydrogenation and enhance ethylene selectivity relative to the parent material^40^.
The NH~3~-TPD profiles in Fig. 4 indicate that alkaline-earth promotion modifies both (i) the population of NH~3~ adsorption sites and (ii) the NH~3~ binding/retention strength, rather than measuring “Brønsted acidity” alone. In alumina-based catalysts, the low-temperature region (≈100–300 °C), typically associated with weakly bound NH~3~ on weak Lewis sites of γ-Al~2~O~3~, can change with surface hydroxyl density and coordination environment. Accordingly, we discuss the low-T region primarily in terms of NH~3~ uptake on weak Lewis sites, and we report the integrated NH~3~ uptake (μmol g^−1^ when calibrated; otherwise, relative TCD area) to enable transparent comparison^45–47^.Fig. 4NH~3~-TPD profiles of unpromoted Pd–Ag/Al~2~O~3~ and alkaline-earth metal (Mg, Ca, Sr)-promoted Pd–Ag/Al~2~O~3~ catalysts.
In the high-temperature region (> ~ 500 °C), the desorption peak should not be directly interpreted as an increase in the density of “strong Brønsted acid sites”. For γ-Al~2~O~3~ and related mixed oxides, high-T NH~3~ release can originate from strongly binding Lewis/interfacial sites (e.g., under-coordinated Al^3+^ centers and promoter–alumina interfacial environments, often described as M–O–Al–type linkages) that retain NH~3~ more strongly. Therefore, an intensified/shifted high-T feature can reflect stronger NH~3~ retention on a minor population of interfacial sites even if alkaline-earth oxides neutralize many alumina acid sites overall^48–50^.
It should be noted that the increase in the low-temperature NH~3~ desorption signal does not necessarily indicate an increase in the overall surface acidity. Rather, it may reflect the formation or redistribution of weak Lewis adsorption sites associated with modified alumina environments or M–O–Al interfacial structures created upon promoter addition.
Figure 5 shows the CO~2~-TPD profiles of the unpromoted and promoted Pd-Ag/Al~2~O~3~ catalysts. Among the promoters examined, Pd–Ag/Sr1%/Al~2~O~3~ exhibits the highest integrated CO~2~ desorption (TCD area) over a broad temperature range, indicating the greatest enhancement in the density/strength of basic sites^51^. Pd–Ag/Ca1%/Al~2~O~3~ and Pd–Ag/Mg1%/Al~2~O~3~ also show increased basicity relative to the unpromoted catalyst, although to a lesser extent. This trend agrees with CO~2~-TPD reports for alkaline-earth oxides dispersed on γ-Al~2~O~3~, where the overall basicity generally increases with the alkaline-earth atomic number^52,53^. Therefore, CO~2~-TPD evidences a net increase in surface basicity (overall neutralization of alumina acidity) upon promoter addition.Fig. 5CO~2~-TPD profiles of unpromoted Pd–Ag/Al~2~O~3~ and alkaline-earth metal (Mg, Ca, Sr)-promoted Pd–Ag/Al~2~O~3~ catalysts.
Finally, consistent with this net-neutralization picture, the high-temperature NH~3~ desorption feature (> ~ 500 °C) should be interpreted mainly as strong NH~3~ retention on a limited population of interfacial Lewis sites (e.g., M–O–Al linkages and undercoordinated Al centers), rather than as a net increase in strong Brønsted acidity (see NH~3~-TPD discussion)^47–49,54,55^.
SEM micrographs of the fresh catalysts are shown on Fig. 6. The SEM micrographs in Fig. 6 primarily reflect the catalyst pellet surface morphology at the micrometer scale. Given the extremely low Pd loading and the SEM magnification/scale used (µm-scale), individual Pd–Ag nanoparticles cannot be resolved; therefore, SEM is used here qualitatively to compare surface texturing/densification and the presence of surface deposits before reaction. The fresh Pd–Ag/Al~2~O~3~ sample (Fig. 6a) shows a relatively homogeneous porous texture.Fig. 6SEM micrographs of fresh (**a**) Fresh Pd–Ag/Al~2~O~3~ and (**b**) Fresh Pd–Ag/Sr1%/Al~2~O~3~.
Similarly, the fresh Sr-promoted catalyst (Fig. 6b) exhibits a comparable porous texture with no obvious dense/dark surface coverage at the micrometer scale.
### Catalytic activity
The catalytic activity and selectivity of Pd–Ag-based catalysts, with and without alkaline-earth metal promoters (Mg, Ca, Sr), were evaluated over time on stream (TOS). The acetylene conversion and ethylene selectivity are presented in Fig. 7a and b, respectively.Fig. 7(**a**) Acetylene conversion and (**b**) ethylene selectivity over unpromoted and alkaline-earth-promoted (1 wt% Mg, Ca, Sr) Pd–Ag/Al~2~O~3~ catalysts as a function of time on stream at reaction conditions.
As shown in Fig. 7a, all promoted catalysts maintained stable acetylene conversion throughout the reaction period, exceeding 96%. This stability is attributed to the modification of surface acidity induced by the alkaline-earth promoters, as revealed by NH~3~-TPD analysis. Low-temperature desorption peaks (≈100–300 °C), corresponding to weak acid sites, increased markedly upon promoter addition. Both Ca- and Mg-promoted catalysts exhibited enhanced weak Lewis acidity, which facilitates selective acetylene hydrogenation by polarizing the C≡C bond without over-stabilizing undesired intermediates. This mild acidity regime promotes reversible acetylene adsorption near Pd–Ag ensembles, may facilitate hydrogen addition and suppresses side reactions such as oligomerization and green-oil formation, thereby enhancing ethylene selectivity. High-temperature NH~3~ desorption features (> 500 °C) also become more pronounced upon promoter incorporation. As discussed in the NH~3~-TPD section, this region should not be interpreted as a net increase in strong Brønsted acidity; rather, it can reflect stronger NH~3~ retention on a minor population of strong Lewis/interfacial sites (e.g., promoter–alumina interfacial environments). Such sites may correlate with a higher propensity for acetylene oligomerization/green-oil formation and carbon deposition at the metal–support perimeter. These experiments were designed for integral industrial benchmarking at high conversion, and therefore intrinsic kinetic parameters are not extracted.
As illustrated in Fig. 7b, ethylene selectivity varied among the catalysts. The unpromoted Pd–Ag/Al~2~O~3~ catalyst displayed the lowest selectivity, consistent with over-hydrogenation of acetylene to ethane in the absence of alkaline-earth promoters. Among the promoted samples, the Mg-promoted catalyst initially exhibited the highest selectivity; however, its performance fluctuated over time. The Ca-promoted catalyst showed intermediate selectivity with less pronounced fluctuations, whereas the Sr-promoted catalyst maintained stable selectivity throughout the reaction period, despite minor initial variations.
Direct numerical comparisons of ethylene selectivity across literature should be made with caution, because performance depends strongly on catalyst architecture and operating protocol. Many prior promoted Pd/Pd–Ag studies used powders and/or simplified feeds and different pressures, whereas this work benchmarks a commercial eggshell Pd–Ag/Al~2~O~3~ catalyst under a representative tail-end feed dominated by ethylene/ethane. Differences in feed composition (H~2~/C~2~H~2~), pressure, space velocity, and pretreatment can shift semi- vs over-hydrogenation and carbon formation. Accordingly, we emphasize the composition-controlled, head-to-head Mg/Ca/Sr trends within a fixed eggshell Pd–Ag baseline.
The observed catalytic behavior correlates with the NH~3~-TPD and CO~2~-TPD results, which together indicate that alkaline-earth promotion partially neutralizes alumina acidity, increases surface basicity, and shifts the remaining acidity toward weaker (mainly Lewis-type) adsorption environments. This acid–base modulation suppresses acid-catalyzed side pathways such as acetylene oligomerization/green-oil formation and coke deposition, while maintaining high acetylene conversion. In addition, the promoted catalysts—particularly Pd–Ag/Ca1%/Al~2~O~3~ and Pd–Ag/Sr1%/Al~2~O~3~—exhibit higher ethylene selectivity than the unpromoted catalyst, consistent with reduced secondary reactions under a more neutral/basic support environment.
Hydrogen spillover may also contribute to selectivity in this system. Hydrogen spillover involves the migration of dissociated hydrogen atoms from Pd–Ag ensembles, where H~2~ dissociation occurs, to nearby acid–base sites on the alumina support. Such spillover could facilitate acetylene activation on weak acid sites, polarizing the C≡C bond without over-stabilizing intermediates. The combination of weak acid sites and hydrogen spillover can help steer hydrogenation toward ethylene rather than ethane. This controlled hydrogenation mechanism is particularly effective in the promoted catalysts, where the balanced acidity and basicity optimize the spillover effect and improve ethylene selectivity. In conclusion, the enhanced catalytic performance of the promoted Pd–Ag-based catalysts result from a synergistic combination of (i) redistribution of surface acidity toward weaker acid sites, (ii) synergistic acid–base interactions, and (iii) more facile hydrogen transfer from Pd–Ag ensembles to the support. Collectively, these factors can rationalize the improved acetylene conversion and ethylene selectivity observed for alkaline-earth-promoted catalysts.
### Unified discussion
In Pd–Ag/Al~2~O~3~ catalyst systems, acetylene (C~2~H~2~) adsorbs more strongly than ethylene (C~2~H~4~) due to its higher binding energy on the active metal sites. This difference in adsorption strength plays a key role in the hydrogenation process. While molecular H~2~ adsorbs only weakly on the surface, it readily dissociates on Pd to form atomic hydrogen (H*), which can co-adsorb with acetylene during hydrogenation. The efficiency of hydrogen utilization, however, can be influenced by hydrogen spillover, i.e., the migration of H* from the metal surface to the support^56–59^. The schematic compares unpromoted and promoted catalysts in acetylene hydrogenation, illustrating a plausible role of alkaline-earth promoters in facilitating hydrogen migration (spillover) improving hydrogen distribution across the catalyst surface, and suppressing undesired side reactions.
In this study, hydrogen spillover is not directly probed (e.g., by H~2~–D~2~ exchange or operando spectroscopy); therefore, it is discussed as a plausible contributor consistent with the observed TPR/TPD/TPO trends.
In the unpromoted Pd–Ag/Al~2~O~3~ catalyst (Fig. 8a), the active metals Pd and Ag are dispersed on the Al~2~O~3~ support. Hydrogen dissociation occurs primarily on the metal sites, but the spillover of hydrogen atoms (H*) onto the support may be limited due to the low basicity of the alumina and the weak metal–support interaction (MSI). As a result, hydrogen may remain more localized near the metal sites, which reduces the uniformity of hydrogen distribution across the catalyst surface and may lead to non-uniform hydrogen availability/coverage and increase the propensity for over-hydrogenation and carbon-forming pathways. If the spillover of hydrogen is limited, this could lead to several possible effects. For instance, restricted migration of H* may reduce the effective utilization of hydrogen on the support surface, which could potentially influence the overall hydrogenation behavior^60,61^. Secondly, localized hydrogen accumulation near active sites may promote over-hydrogenation and undesired side reactions, such as C–C coupling, which could lead to the formation of polymeric deposits commonly referred to as green oil. These deposits may gradually block active sites, potentially contributing to a decline in catalytic activity and partial deactivation. Finally, the combined effects of non-uniform hydrogen distribution and possible over-hydrogenation may also influence ethylene selectivity, as side products such as ethane or oligomeric species could become more favorable along the reaction pathway^62,63^.Fig. 8Conceptual schematic of a spillover-consistent hypothesis (not directly verified).
#### Promoted spillover-consistent features and improved performance
In the promoted Pd–Ag/Al~2~O~3~ system (Fig. 8b), the introduction of Sr, Ca, or Mg into the alumina support may modify the acid–base equilibrium, may strengthens metal–support interactions (MSI), and may create new basic sites that could facilitate hydrogen migration from the metal to the support. This modification may improve hydrogen availability across the catalyst surface, potentially influencing the overall hydrogenation behavior. The addition of alkaline-earth promoters may increase the basicity of the support by generating surface –OH groups and oxygen vacancies, which could act as hydrogen acceptor sites and promote H* migration from the metal to the support^60–64^. This effect may improve hydrogen distribution, potentially leading to more balanced surface reactivity during hydrogenation. Enhanced basicity, combined with stronger MSI, may be associated with more facile hydrogen transfer (spillover), contribute to the activation of additional interfacial surface sites, improving apparent reaction rates, and favoring acetylene hydrogenation toward ethylene, while helping to suppress over-hydrogenation^62,64^. By promoting homogeneous hydrogen distribution, the formation of polymeric green oil through C–C coupling may be reduced, protecting active sites from deactivation and facilitating ethylene desorption, further improving selectivity^63,65^. The –OH groups generated by Sr and Ca may act as effective hydrogen acceptors, potentially enabling interfacial hydrogen transfer via surface –OH/defect sites, thereby contributing to hydrogen activation along the hydrogenation pathway^60,61^. Furthermore, the ionic radius of the promoter may influence the spillover mechanism. Larger ions such as Sr and Ca may promote the formation of oxygen vacancies and modify hydrogen interactions at the surface, which may provide additional migration pathways for hydrogen and potentially improve hydrogen utilization, and optimize catalytic performance^64^. Overall, a plausible spillover-assisted scenario involves H~2~ dissociation on metal sites, migration of H* to the support via basic and oxygen-deficient sites created by the promoters, and participation of H* in selective acetylene hydrogenation to ethylene. The more uniform hydrogen distribution that may arise from this process could mitigate over-hydrogenation and reduce the formation of carbonaceous green oil deposits, thereby enhancing both activity and selectivity^63,64^.
The hydrogenation of acetylene over Pd–Ag/Al~2~O~3~ catalysts can be influenced by the presence of alkaline-earth metal promoters (Mg, Ca, Sr). In the unpromoted catalyst, hydrogen spillover is likely more limited and restricted to short distances, which may limit hydrogen migration across the surface. This can be associated in higher strong acidity at the metal–support interface and may promote the formation of polymeric green-oil by-products. In contrast, promoted catalysts show trends consistent with more facile H* migration via surface –OH defects, combined with moderate acidity and slight basicity. These modifications may improve hydrogen distribution, are consistent with the observed increases in C~2~H~2~ conversion and improvements in C~2~H~4~ selectivity by suppressing over-hydrogenation and carbon deposition, suggesting a potentially important role of alkaline-earth promoters in tunning both catalytic activity and selectivity.
### Structural properties of the used catalysts
The nitrogen adsorption–desorption isotherms of the spent catalysts, obtained after acetylene hydrogenation (Fig. 9a), show a pronounced downward shift in adsorbed N~2~ volume compared to the fresh catalysts, regardless of the promoter type. This decrease is most significant for the unpromoted Pd–Ag/Al~2~O~3~ catalyst, indicating substantial pore blockage and partial collapse. The primary causes of this deterioration are likely (i) deposition of coke within the mesopores and (ii) agglomeration/sintering of metal-containing domains (possible). For example, the Sr-promoted catalyst, which initially exhibited the highest adsorbed volume and the widest hysteresis loop among the fresh samples, retains superior residual porosity relative to other promoted samples, yet still experiences a notable reduction in N~2~ uptake. This observation highlights the unavoidable effects of carbon deposition and thermally induced particle sintering on the textural properties of spent catalysts.Fig. 9(**a**) N~2~ adsorption–desorption isotherms; (**b**) pore size distribution curves for used unpromoted Pd–Ag/Al~2~O~3~ and promoted with 1 wt% Mg, Ca, and Sr.
Analysis of the pore size distribution (PSD) patterns for the used Pd–Ag-based catalysts (Fig. 9b) reveal significant changes relative to the fresh samples, reflecting the structural impact of reaction conditions. The pronounced decrease in peak intensity indicates pore mouth narrowing or partial blockage, primarily due to carbonaceous deposits formed during hydrogenation and aggregation/growth of metal-containing domains. Among the tested compositions, Pd–Ag/Sr–Al~2~O~3~ exhibited the smallest reduction in pore volume and maintained a relatively high mesopore contribution, suggesting enhanced resistance to coke deposition, likely attributable to the surface basicity imparted by Sr. Similarly, Pd–Ag/Ca–Al~2~O~3~ showed good mesopore retention, whereas Pd–Ag/Mg–Al~2~O~3~ experienced the most severe deterioration, with broader PSD peaks shifted toward smaller pore sizes, indicating substantial pore constriction. The unpromoted Pd–Ag/Al~2~O~3~ catalyst displayed an intermediate level of structural decline. This carbonaceous pore blockage can suppress mass transfer and limit access to active sites, accelerating catalyst deactivation under acetylene hydrogenation conditions. The superior mesopore volume retention observed for Sr- and Ca-modified catalysts correlates with the mechanistic role of basicity in coke suppression. By neutralizing Brønsted acid sites on θ-Al~2~O~3~, the basic promoters inhibit acid-catalyzed polymerization of unsaturated hydrocarbons. In addition, improved hydrogen adsorption and hydrogenation of carbon precursors facilitate their removal as desorbable products. Enhanced metal–support electronic interactions, promoted by basic oxides, strengthen Pd–H bonding and may support more effective interfacial hydrogen transfer (spillover). These trends are consistent with the minimal PSD peak suppression and higher N~2~ uptake observed for Sr- and Ca-promoted catalysts, suggesting a potentially important role of surface basicity and structural engineering in enhancing catalyst durability.
The used Pd–Ag/Al~2~O~3~ catalysts exhibit significant alterations in their structural properties compared to the fresh catalysts (Table 2), which directly influence their performance in acetylene hydrogenation. After prolonged reaction, all catalysts show reductions in surface area, total pore volume, and average pore diameter, indicating pore blockage and carbon deposition that restrict active site accessibility and impede mass transfer. For the modified catalysts, the incorporation of alkaline-earth metals (Mg, Ca, Sr) mitigates these effects, preserving relatively higher surface areas and pore volumes compared to the unmodified Pd–Ag/Al~2~O~3~ catalyst. These observations suggest that the alkaline-earth promoters stabilize the catalyst structure, limiting the extent of deactivation and maintaining improved textural properties during hydrogenation.Table 2Structural properties of the used catalysts.SamplesS~BET~(m^2 ^gr^−1^)Total pore volume(cm^3^ g^−1^)Mean pore diameter (nm)Pd–Ag/Al~2~O~3~ -Used36.60.110.91Pd–Ag/Mg1%/Al~2~O~3~-Used42.40.0868.15Pd–Ag/Ca1%/Al~2~O~3~-Used41.20.0807.81Pd–Ag/Sr1%/Al~2~O~3~-Used42.30.0928.71
The Temperature-Programmed Oxidation (TPO) profiles of the promoted and non-promoted catalysts (Fig. 10) reveal marked differences in the nature and oxidation behavior of deposited carbon. Based on the oxidation-temperature windows observed in the TPO traces, the unpromoted Pd–Ag/Al~2~O~3~ catalyst primarily accumulates refractory, graphitic/semi-graphitic carbon, with COₓ evolution restricted to ~ 500–600 °C. In contrast, all alkaline-earth-promoted derivatives (1 wt% Mg, Ca, Sr) display a distinct shift of the oxidation centroid toward lower temperatures (~ 270–420 °C), accompanied by a diminished high-temperature fraction. This indicates a greater proportion of labile, less-ordered deposits and a reduced overall carbon inventory^66,67^. This shift in carbon speciation is consistent with NH~3~-TPD analysis (Fig. 4): promotion increases the population of weak Lewis acid sites (100–300 °C), which enables reversible, orientation-controlled C≡C adsorption and facilitates ethylene desorption. Consequently, residence times are shortened and oligomerization to green oil is suppressed. By contrast, Pd–Ag/Sr(1%)/Al~2~O~3~ exhibits a pronounced > 500 °C band, reflecting stabilized strong acid centers on a support whose acidity is dominantly Lewis-type. Stronger NH~3~ retention at high temperature is a widely accepted proxy for enhanced acidity^68,69^. Functionally, weak metal–support perimeter sites benefit selective acetylene hydrogenation by enabling reversible C≡C adsorption and facile ethylene desorption, thereby suppressing green-oil pathways. In contrast, strong sites over-stabilize π/cationic intermediates and promote oligomerization, cyclization, and aromatization routes that yield refractory coke. This accounts for the residual high-temperature TPO feature unique to Pd–Ag/Sr 1%/Al~2~O~3~^14,22,70^. The ionic-radius sequence Mg^2+^ (0.72 Å) < Ca^2+^ (1.00 Å) < Sr^2+^ (1.18 Å) further rationalizes this dual heavier alkaline-earth oxides are more basic and enhance oxygen mobility and lattice-oxygen participation, converting a larger fraction of deposits into easily oxidized species (low-temperature TPO). However, the stronger lattice perturbation at the Sr–Al–O interfaces stabilize a minority population of strong acid centers, explaining the persistent high-temperature tail observed only in Pd–Ag/Sr1%/Al~2~O~3~^71–73^.Fig. 10TPO profiles of unpromoted Pd–Ag/Al~2~O~3~ and alkaline-earth metal (Mg, Ca, Sr)-promoted Pd–Ag/Al~2~O~3~ catalysts.
Figure 11 illustrates the reduced Pd–Ag/Al~2~O~3~ catalysts, without (Sr-free) and with the Sr promoter.Fig. 11(**a**) Reduced Pd–Ag/Al~2~O~3~ catalyst after H~2~ activation. (**b**) Reduced Pd–Ag/Sr1%/Al~2~O~3~ catalyst after H~2~ activation.
Quantitative eggshell thickness is reported for the Sr case based on mechanically polished cross-sections prepared perpendicular (≈90°) to the pellet surface, which provided the clearest rim contrast. Extending the same penetration-depth quantification to Mg- and Ca-promoted samples is not included in the present study and is left for future work. In both cases the metal phase forms an eggshell architecture, i.e., a metal-rich shell at the outer rim of the alumina grains.
The contrast suggests effective near-surface enrichment of the Pd–Ag phase, consistent with an eggshell-like architecture that may be beneficial for acetylene activation. Notably, the Sr-promoted catalyst (Fig. 11b) exhibits a thinner shell than the Sr-free one. Quantitatively, mean eggshell thicknesses measured in ImageJ (Fiji) were 136.2 ± 4.7 µm (n = 17) for the Sr-free catalyst and 28.83 ± 1.16 µm (n = 17) for the Sr-promoted catalyst (see Methods for calibration details)**.** This trend is consistent with impregnation lower solution acidity and more basic support surfaces reduce penetration depth and bias deposition toward an eggshell distribution, whereas stronger acidity and stronger metal–support interactions favor deeper ingress (thicker shells). Because Sr introduces basic SrO-like surface species and attenuates alumina acidity, the effective acidity decreases, so Pd/Ag deposition remains nearer the outer surface, giving a thinner shell after calcination/reduction. These behaviors are well documented for eggshell catalysts and electrostatic uptake controlled by pH/PZC and acid–base modification of the support^74–76^.
The used Pd–Ag/Al~2~O~3~ sample (Fig. 12a) exhibits smoother/densified regions and darker patches, consistent with partial pore blockage by carbonaceous deposits and/or thermally induced morphological densification. The used Sr-promoted catalyst (Fig. 12b) shows similar macroscopic morphology but with less extensive dense/dark surface coverage after reaction, consistent with its improved resistance to deactivation observed in catalytic testing and TPO trends. SEM–EDS is used to verify elemental presence and does not provide quantitative nanoparticle size/dispersion information. Because the industrial Pd loading is extremely low and the active phase is confined to an eggshell layer on mm-scale pellets, quantitative Pd dispersion/particle-size determination by CO-chemisorption, XPS, or TEM was not performed. Therefore, any discussion of dispersion/ensemble changes is presented cautiously and based on indirect constraints (e.g., the absence of detectable Pd/Ag reflections in XRD at the present loadings and qualitative TPR trends).Fig. 12SEM micrographs of used (**a**) Used Pd–Ag/Al~2~O~3~ and (**b**) Used Pd–Ag/Sr1%/Al~2~O~3~.
SEM–EDS survey spectra (Fig. 13) corroborate the catalyst compositions. Both Pd–Ag/Al~2~O~3~ (Fig. 13a) and Sr–Pd–Ag/Al~2~O~3~ (Fig. 13b) show O Kα (≈0.525 keV) and Al Kα (≈1.486 keV) from Al~2~O~3~ together with Pd Lα (≈2.84 keV) and Ag Lα (≈2.98 keV). Only the promoted sample (Fig. 14b) exhibits Sr Lα at ≈1.81 keV, confirming Sr incorporation. The partial overlap of the Pd/Ag L lines near 3.0 keV was addressed by background subtraction and peak deconvolution. No additional elements were detected above background. EDS is used here qualitatively to verify elemental presence; precise bulk loadings (identical Pd and Ag in both samples and ~ 1 wt% Sr in the promoted sample) were established independently by ICP-OES (Table 1).Fig. 13EDS spectra of (**a**) Unpromoted Pd–Ag/Al~2~O~3~ and (**b**) Sr-promoted Pd–Ag/Al~2~O~3~.Fig. 14(**a**) Pd–Ag/Al~2~O~3~; (**b**) Sr-promoted Pd–Ag/Sr1%/Al~2~O~3~. SEM–EDS elemental mappings acquired under identical conditions.
EDS elemental maps (Fig. 14) show a uniform, near-random spatial distribution of the metal signals in both catalysts. Pd Lα and Ag Lα counts are evenly dispersed across the field of view with no evidence of micron-scale clustering, segregated domains, or banding. In the promoted sample, Sr Lα features are likewise homogeneously distributed and co-located with Pd/Ag, indicating no Sr-rich agglomerates. The C Kα background is largely uniform; although the unpromoted catalyst exhibits more bright C spots, neither Pd/Ag nor Sr signals correlate with carbon hotspots. Secondary-electron images acquired over the same 50 μm field confirm comparable particle beds, and contrast variations do not track any elemental accumulation. Within the spatial resolution and interaction volume of SEM–EDS used here, both catalysts therefore present uniform micron-scale distribution of Pd/Ag (and Sr) signals within the SEM–EDS interaction volume, with no evidence of micron-scale agglomeration.
## Conclusion
The incorporation of alkaline-earth promoters (Mg, Ca, Sr) into Pd–Ag/Al~2~O~3~ catalysts enhances their performance in selective acetylene hydrogenation, particularly by improving ethylene selectivity and suppressing “green oil” formation. All promoted catalysts achieved high acetylene conversion (≈96–98%) and exhibited superior ethylene selectivity compared with the unmodified Pd–Ag/Al~2~O~3~ catalyst. Structural analyses confirmed that the promoters preserve the θ-Al~2~O~3~ phase while increasing surface area and mesopore volume, thereby improving accessibility to Pd–Ag active sites. Complementary TPR, NH~3~-TPD, and CO~2~-TPD measurements demonstrated that promotion strengthens metal–support interactions, enhances basicity, and redistributes acidity toward weaker sites. These modifications suppress oligomerization and coke deposition. Additionally, the promoted catalysts show trends consistent with more facile hydrogen spillover (interfacial hydrogen transfer) across the Pd–Ag/support interface, which may help minimize over-hydrogenation and improve overall catalytic efficiency.
Taken together, these results underscore the importance of balancing promoter loading, surface acidity, and basicity to achieve optimal performance in acetylene hydrogenation. Future studies should focus on fine-tuning promoter concentration and spatial distribution to further improve catalyst stability and durability under reaction conditions.
## Supplementary Information
Below is the link to the electronic supplementary material.
Supplementary Material 1