Abstract: Bimetallic Pd-Zn catalysts are widely used in hydrocarbon conversion processes (steam reforming of methanol and methanol synthesis, selective hydrogenation, oxidation processes, etc.) and are characterized by high yield of target products and stability under the reaction conditions due to the formation of joint Pd-Zn phases. The modifier atoms in the bimetal structure “dilute” neighboring palladium atoms and effectively isolate them from each other, so such Pd–Zn–Pd-ensembles of active sites are formed on the catalyst surface [1]. In the case of selective acetylene hydrogenation for Pd-Zn-Pd the possibility of C2H2 adsorption on two neighboring Pd atoms in the di-σ-bound form, which is mostly converted into alkane, is almost excluded. In these conditions more possible form of the alkyne adsorption is a weaker π-complex, which facilitates the desorption of ethylene and, accordingly, leads to an increase in its yield. It should be noted that the amount of the introduced modifier has a significant effect on the catalyst properties [2]. In addition to that, the optimum of the Pd:Zn ratio is determined not only by the joint phase composition, but also by the textural and structural properties of the support, which affect the degree of the Pd and Zn interaction. In this regard, the aim of this work was to study the effect of the zinc amount on the composition, structure, electronic state of the active component and the catalytic properties of Pd-Zn/Sibunit systems in the selective hydrogenation of acetylene to ethylene. The carbon material Sibunit (Sib) was used as a support [3]. Samples were obtained by an incipient wetness impregnation with joint aqueous solution of Pd(NO3)2 and Zn(NO3)2 with following stages of air drying (120 °C, 2 h) and reduction in a hydrogen stream (500 °C, 5 hours). The modifier content was varied from Pd:Zn = 1:0.1 to 1:4. Catalytic tests were carried out in flow reactor, in a stream of gas mixture containing 4 vol.% C2H2 + 96 vol.% H2 at 35-95 °C. It was found by XRD and EXAFS that in the Pd-Zn(1:0.25)/Sib sample palladium and zinc interact with the formation of a solid substitution solution based on palladium FCC. The crystal lattice parameter of palladium decreases (from 3.886 to 3.844 Å), due to the incorporation of a smaller zinc atom in the lattice [4], and the distance between the neighboring Pd-Pd atoms increases from 2.75 to 2.85 Å. For the catalyst with the equimolar Pd-Zn(1:1)/Sib composition, the bimetallic compound fraction increases, and the crystal lattice transforms into the tetragonal characteristic of the intermetallic PdZn phase. In the Pd:Zn = 1:4 catalyst, the excess of Zn exists as an oxide phase, and bimetallic scattering paths (according to EXAFS) are strongly disordered (Debye–Waller factor — 0.0184 Å) and are determined only by the K-edge of palladium. It also should be noted that increasing of zinc part in the samples also leads to a shift of the Pd3d signal in the XPS spectra towards higher binding energies values compared to the monometallic sample (from 335.5 eV to 335.8 ± 0.05 eV). According to XPS data for the Pd:Zn = 1:0.25 ratio, only a small part of palladium is situated as a PdZn alloy nanoparticles, but with an increase in the amount of introduced zinc, almost all surface of palladium goes into nanoalloy and just a small part exists as individual Pd0 nanoparticles. The introduction of zinc into the Pd/Sibunit composition has a noticeable effect on its catalytic properties. Thus, an increase in the zinc content to Pd:Zn > 1:0.5 is accompanied by a shift of the acetylene conversion curve (X (C2H2) = 50%) to higher temperatures (from 45 to 77°C), which indicates a depression in catalytic activity. It is assumed that the observed effect is caused by blocking a part of the active sites by the formed zinc oxide, found by the EXAFS and X-ray powder diffraction methods. The dependence of ethylene selectivity on the zinc amount in the sample has an extreme character with a maximum at 74% achieved on catalysts with Pd:Zn = 1:0.5 - 1:1. It is ~ 8 % rel. higher than the selectivity obtained on monometallic Pd/Sibunit reference sample (68%). It is assumed that the optimum for the selectivity of ethylene formation is achieved on catalysts containing the highest number of bimetallic particles, and depends on the degree of their structure order.

Cite: Yurpalova D.V. , Afonasenko T.N. , Domanina T.P. , Leontʹeva N.N. , Prosvirin I.P. , Bukhtiyarov A.V. , Shlyapin D.A.
Study of the zinc addition influence on the Pd/Sibunit catalyst of selective acetylene hydrogenation
XI International Conference “Mechanisms of Catalytic Reactions” 07-11 Oct 2019