Metal phthalocyanines as efficient electrocatalysts for acetylene semihydrogenation
Graphical abstract
Introduction
Ethylene (C2H4), predominantly produced through steam cracking of petroleum hydrocarbons, is one of the largest industrial feedstocks for producing ethylene-based polymers such as polyethylene [1], [2]. However, a small amount of acetylene impurity (C2H2, ∼0.5–3%) is unavoidably present in the raw ethylene streams, which irreversibly poisons the catalysts (e.g., Ziegler-Natta catalysts) employed for downstream olefin polymerization [3], [4]. A common approach for removing acetylene is to selectively thermocatalytic hydrogenate acetylene into ethylene using noble-metal Pd-based catalysts at relatively high temperatures (100–250 °C) [5], [6], [7], [8]. However, this thermocatalytic strategy is energy intensive and expensive [9], [10]. Meanwhile, as a hydrogen source, excessive H2 may lead to undesired overhydrogenation of ethylene into ethane and safety issues [11], [12]. Therefore, developing energy- and cost-efficient acetylene semihydrogenation processes is urgently desirable.
Electrocatalytic acetylene semihydrogenation under ambient conditions is an appealing strategy that directly utilizes the rapidly growing amount of electricity generated from renewable energies [13], [14], [15]. Previous studies have revealed that acetylene can be electrochemically reduced to ethylene on some metal electrocatalysts, such as Pt [16], [17], Pd [18], Cu [13], [19], Ag [13] and polymer-modified electrodes [20], [21]. Nevertheless, the selective electrochemical reduction of acetylene suffers from a low current density (<4 mA cm−2) and poor selectivity (current efficiency: <70%) because of the inferior solubility of acetylene in aqueous/or organic solutions (1.06 g/kg H2O) and strongly competitive side reactions, e.g., hydrogen evolution (HER), overhydrogenation and carbon–carbon coupling reactions [13], [17]. The reaction process of electrocatalytic acetylene semihydrogenation in alkaline or neutral solutions is as follows: C2H2 + 2H2O + 2e– → C2H4 + 2OH– (cathode). Specifically, hydrogen (H*ads) generated via in situ water dissociation on cathodic electrocatalyst surfaces in an aqueous solution can be directly employed for hydrogenating acetylene, avoiding the involvement of high-risk hydrogen gas in traditional thermocatalysis [22], [23], [24]. For electrocatalytic acetylene semihydrogenation, four steps are principally involved: 1) initial C2H2 adsorption on catalyst surfaces (*CHCH); 2) the formation of *CHCH2 intermediates (*CHCH + e– + H2O → *CHCH2 + OH–); 3) *CH2CH2 formation (*CHCH2 + e– + H2O → *CH2CH2 + OH–); and 4) ethylene desorption from catalyst surfaces (*CH2CH2 → CH2CH2 (g)) [3], [25], [26]. Accordingly, the adsorption and activation of water molecules, the bonding properties of acetylene, intermediates and ethylene on electrocatalyst surfaces will intrinsically determine the activity and selectivity of acetylene semihydrogenation [26]. Thus, electrocatalysts with individually tailored active centers are promising for suppressing side reactions by atomic engineering.
Herein, we develop well-defined metal phthalocyanines (Co, Cu and Zn) featuring individual metal sites as model electrocatalysts for acetylene semihydrogenation. Cobalt phthalocyanine (CoPc) exhibits particularly high electrocatalytic activity with a very large ethylene partial current density (jethylene) of 150.8 mA cm−2 at –0.7 V vs. reversible hydrogen electrode (RHE) and a peak ethylene faradaic efficiency (FE) of ∼ 96% at –0.4 V under a pure acetylene stream. In-situ electrochemical Raman and theoretical investigations reveal that the Co sites in CoPc facilitate the adsorption/hydrogenation of acetylene and the desorption of ethylene. Eventually, for a long-term stability test in a crude ethylene flow containing 1% (1 × 104 ppm) acetylene impurities, the CoPc catalyst shows a large space velocity (SV) of 2.4 × 105 ml·gcat–1·h−1, a high turnover frequency (TOF) of 4.86 × 10–3 s−1, and a continuous output of ethylene with only ∼ 52 ppm acetylene; these results considerably surpass those for previously reported thermocatalysts.
Section snippets
Chemicals and materials
Phthalocyanine (Pc), Cobalt phthalocyanine (CoPc), Copper phthalocyanine (CuPc), Zinc phthalocyanine (ZnPc) were purchased from Alfa Aesar and used without further treatment. Isopropanol, Nickel nitrate hexahydrate (Ni(NO3)2·6H2O, 98%), urea and iron nitrate (Fe(NO3)3·9H2O, 99%) were obtained from Aladdin (China). Ni foam was purchased from KunShan Kuangxun Ltd. (China). The gas diffusion layer (GDL) (Freudenberg H14C9) and the anion exchange membrane (Fumasep FAB-PK-130) was purchased from
Electrocatalytic performance of acetylene semihydrogenation in the pure acetylene stream
As a type of molecular complex with metal centers coordinated by conjugated planar phthalocyanine ligands, M(Pc)s are ideal model systems for profoundly unveiling the corresponding reaction mechanism and exploring high-activity electrocatalysts (Figure S1) [33], [34]. Accordingly, universally available M(Pc) with different metal centers (Co, Cu, and Zn) were explored as model electrocatalysts for acetylene semihydrogenation. The M(Pc) chemical structures were confirmed using X-ray diffraction
Conclusions
In summary, we first explored metal phthalocyanines (M(Pc)s) as model electrocatalysts for selective alkyne semihydrogenation. Among various M(Pc)s, CoPc presents excellent performance with high ethylene faradaic efficiencies and partial current densities. Electrochemical, in-situ electrochemical Raman and theoretical investigations reveal that the individual Co sites in CoPc are beneficial for acetylene adsorption/protonation and ethylene desorption, unprecedentedly suppressing the side
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
This work was financially supported by the Fundamental Research Funds for the Central Universities (Grant No. 310201911cx028, No. 3102017jc01001), the Natural Science Foundation of Shaanxi Province (No. 2020JQ-141), the National Natural Science Foundation of China (No. 22005245). We would like to thank the Analytical & Testing Center of Northwestern Polytechnical University for the SEM and TEM characterizations. We also acknowledge Prof.Hanchen Liu for the Raman tests at School of Science,
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The authors contributed equally to this paper.