Abstract: Supported bimetallic catalysts based on palladium are widely used to accelerate practically important reduction processes, for example, the selective hydrogenation of unsaturated compounds. The hydrogenation reaction of acetylene to ethylene is used both to purify ethylene from the C2H2 impurity, which is formed during the pyrolysis of petroleum feedstock, and to directly ethylene obtaining from C2H2, synthesized by pyrolysis of natural gas [1]. It is known that the high selectivity of bimetallic catalysts is achieved due to the formation of bimetallic sites with a specific electronic and geometric configuration. Therefore, it is necessary to use the approaches of catalysts synthesis that improve the efficiency of interaction between palladium and the modifier. At the same time, the modifying effect is also determined by the nature of the second metal and the structure of the bimetallic particles. In this work, Co, Fe, and Mn were used as modifiers because these metals are capable of forming solid solutions and/or intermetallic compounds with palladium. However, there are a small number of works [2-5] on the use of samples based on Pd-Co, Pd-Mn, and Pd-Fe for the hydrogenation of acetylene to ethylene. Sibunit carbon mesoporous material was used as a catalyst support. Samples were prepared by incipient wetness impregnation with joint aqueous solution of palladium and modifier nitrate. The catalysts were dried at 120°С and reduced in hydrogen at 500 – 700°С. The content of palladium in all samples was 0.5 wt.%. The Pd/M molar ratio was varied from 1/0.5 to 1/4. The state of the active component was studied by XRD, XPS, EXAFS, TEM and EDX. Acetylene hydrogenation was carried out in a model gas mixture (4 vol.% C2H2 in a hydrogen) in a flow mode at atmospheric pressure at 25 – 95°C. It was found that Pd and Co supported on Sibunit (С) interact in H2 medium to form bimetallic PdxCo(1-x) nanoparticles. The composition of PdxCo(1-x) depends on the Pd/Co molar ratio. The content of Co in the Pd-Co nanoparticles increases with an increase in the Pd/Co molar ratio up to 1:2 (to ~41% at. of Co in the bimetallic phase) and then remains at a constant level. A correlation has been established between the content of Co in bimetallic nanoparticles and the maximum ethylene yield in the acetylene hydrogenation reaction. The optimal Pd/Co molar ratio of 1:2÷1:4 was found, which ensures the formation of bimetallic Pd-Co particles containing ~41-43% at. of Co and leads to the ethylene yield of 62-63%. It has been proven that the catalytic properties of the studied Pd-Co/C samples in the process of acetylene hydrogenation are not related to the influence of the size effect, but, first of all, are due to the influence of the composition of bimetallic sites and the electronic state of palladium. An increase in the reduction temperature of the Pd-Co(1:2)/C catalyst in hydrogen from 500 to 600 and 700°C is accompanied by a further decrease in activity and an increase in the selectivity and yield of the target product (to 66 and 68%, respectively) due to the formation of PdxCo(1-x) particles containing ~ 50 and 56% at. of Co, respectively. The features of the interaction of components in Pd-Mn samples and the catalytic properties of Pd-Mn/C differ from those observed for Pd-Co/C catalysts. According to EXAFS data, Pd and Mn are predominantly present as individual particles after reduction of Pd-Mn/C at 500°C. An increase in the reduction temperature to 600 and 700°C is accompanied by an increase in the number of bimetallic Pd–Mn bonds, indicating the formation of bimetallic sites. An increase in the reduction temperature leads to a decrease in C2H2 conversion for all series of Pd-Mn samples, which may be due to sintering of the active component and/or the formation of less active Pd-Mn sites. The latter is supported by a significant increase in the ethylene selectivity with an increase in the catalyst treatment temperature from 500 to 700°C. It was found that the highest ethylene selectivity is exhibited by samples with a molar ratio of Pd/Mn = 1/1 regardless of the reduction temperature. The ethylene yield on Pd-Mn(1:1)/C reduced in H2 at 700°C is 75%. In the series of Pd-Fe/C catalysts, the highest yield of the target product (70%) is typical for samples with a molar ratio of Pd/Fe = 1/2 reduced in H2 at 600-700°C. According to XRD, XPS and EXAFS data, the catalytic properties of this sample is associated with the formation of PdxFe(1-x) particles. It can be assumed that the high selectivity is due to the facilitation of ethylene desorption from the surface of PdxFe(1-x) solid solution. Thus, it can be concluded that the catalytic properties of Pd/C can be improved by formation of the mixed Pd-Co, Pd-Mn, and Pd-Fe phases and the maximum yield of the target product increases in the following order: Pd/C (52%) < Pd-Co/C (68%) < Pd-Fe/C (70%) < Pd-Mn/C (75%).

Cite: Yurpalova D.V. , Afonasenko T.A. , Prosvirin I.P. , Bukhtiyarov A.V. , Vinokurov Z.S. , Khramov E.V. , Shlyapin D.A.
Design of Efficient Supported Bimetallic Palladium Catalysts for Selective Hydrogenation of Acetylene
The 8th Asian Symposium on Advanced Materials 03-07 Jul 2023