Abstract: Catalysts containing bimetallic Pd-Co particles are widely used. For example, Pd-Co systems have been successfully applied in the processes of selective hydrogenation of various substrates [1, 2]. It is worth noting that the number of studies related to the use of Pd-Co catalysts in the practically important reaction of acetylene hydrogenation to ethylene [3, 4] is very limited, despite the fact that this process is necessary to obtain pure ethylene, which is widely used in petrochemistry for the synthesis of valuable products. The purpose of this work was to study the modifying effect of Co on Pd in Pd-Co/α-Al2O3 catalysts, as well as to establish relationships between the catalyst composition, the state of the active component and the properties of the samples in the reaction of acetylene hydrogenation to ethylene. Al2O3 (Sasol) calcined in air at 1200°C in the form of α-modification (SBET = 5 m2/g) was used as a catalyst support. The catalysts were synthesized by incipient wetness impregnation method with aqueous solution of palladium and cobalt nitrates. The samples were dried at 120°C and reduced in H2 at 400 – 700°C. The Pd content in all catalysts was 0.5 wt.%. The Pd/Co molar ratio was varied from 1/0.5 to 1/4. The state of the active component was studied by XRD, XPS, TPR-H2 and TEM methods. Hydrogenation of C2H2 was carried out in flow mode at 25-95°C. The reaction mixture included 4 vol.% of C2H2 in H2. An increase in the reduction temperature of Pd-Co/α-Al2O3 catalysts (Pd/Co molar ratio is 1/2) in H2 from 400 to 700°C leads to a decrease in activity, but a significant increase in ethylene selectivity (from 47% to 63% at 95°C), which is associated with the formation of PdxCо(1-x) solid solution particles in the samples. An increase in the reduction temperature leads to the enrichment of bimetallic particles with cobalt: according to X-ray diffraction data, a sample reduced at 400°C contains the Pd0.76Co0.24 phase, while reduction at 700°C is accompanied by the formation of the Pd0.58Co0.42 phase with a higher modifier concentration. The binding energy (BE) of Pd3d5/2 in the Pd-Co sample reduced at 700°C is 335.3 eV, which is 0.4 eV higher than BE for Pd in Pd/α-Al2O3. An increase in the amount of Co to Pd/Co = 1/3 leads to a shift of BE of Pd3d5/2 to 335.5 eV, while a further introduction of cobalt to Pd/Co = 1/4 does not affect the electronic state of Pd. The catalytic properties of the samples correlate well with the electronic state of Pd: changing of Pd/Co from 1/0.5 to 1/3 leads to a decrease in activity with an increase in selectivity to 69%. In this case, an increase in the Co content to Pd/Co = 1/4 is accompanied by stabilization of the catalytic characteristics. According to XRD, up to the ratio Pd/Co = 1/3 the concentration of Co in Pd-Co particles increases to ~59 at.% and adding more Co to Pd/Co = 1/4 leads to the formation of two phases – one enriched in palladium Pd0.57Co0.43 and a cobalt phase including an impurity of palladium Pd0.13Co0.87. It is assumed that higher selectivity in the acetylene hydrogenation reaction is ensured by the action of PdxCо(1-x) particles containing an excess amount of Co relative to Pd. The similar characteristics of Pd-Co/α-Al2O3 catalysts with Pd/Co = 1/3 and 1/4 can be explained by the additivity of the action of the bimetallic phases present in the catalyst with the molar ratio Pd/Co = 1/4.

Cite: Yurpalova D.V. , Afonasenko T.N. , Vinokurov Z.S. , Prosvirin I.P. , Bukhtiyarov A.V. , Trenikhin M.V.
Modifying effect of cobalt in Pd-Co/α-Al2O3 catalysts for selective hydrogenation of acetylene to ethylene
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