Yunya Zhang, Weijian Diao, Christopher T. Williams and John R. Monnier
Department of Chemical Engineering, University of South Carolina, Columbia SC 29208 USA
It has long been recognized that Pd provides superior performance on both activity and selectivity for the hydrogenation of acetylene to ethylene [1,2]. However, at high acetylene conversion the formation of ethane, C4 and C6 hydrocarbons is accelerated, which leads to the formation of carbonaceous residues that decrease the catalyst lifetime. To increase the selectivity to ethylene, Pd-based catalysts are usually modified with promoters, such as Ag, Ni, Cu, and K etc [3,4]. However, the bimetallic effects of the above additives are still not well understood, probably due to the conventional catalyst preparation methods that result in both monometallic and bimetallic particles with varying compositions. In our previous work , we used electroless deposition (ED) to prepare a series of Ag- and Au-Pd/SiO2 bimetallic catalysts with selective and controlled coverages of Ag and Au on Pd. The similar performance trends of enhanced selectivity of acetylene to ethylene at high coverages for Ag- and Au-Pd/SiO2 suggested that the bimetallic effect for these catalysts was geometric and not electronic in nature. That is, at high coverages with smaller ensembles of Pd sites, acetylene is weakly adsorbed as a ¦Ð-bonded species, which favors the hydrogenation to ethylene. On the other hand, acetylene is bonded in a multi-¦" mode on larger ensembles of Pd and desorbs only as fully hydrogenated C2H6. Therefore, inspired by these previous results, the primary goal of this work was to prepare a series of reverse bimetallic catalysts where Pd is deposited onto the Ag surface in order to further explore the nature of bimetallic effects for the selective hydrogenation of acetylene.
For the synthesis of Pd-Ag bimetallic catalysts, we use galvanic displacement (GD) of Ago by Pd2+ salts which provides an alternate route for certain bimetallic catalysts. Galvanic displacement occurs when a base material is displaced by a metallic ion in solution that has a higher reduction potential than the displaced metal ion . In this case, it is thermodynamically favorable for Pd2+ reduction to occur by oxidation of Ago to Ag+ and to deposit on the Ag surface (Pd2+ + 2e- = Pd, E˚ = 0.915V; Ag+ + e- = Ag, E˚ = 0.799V). A master batch of 2 wt% Ag/SiO2 was used as the base catalyst. The Ag surface site concentration was determined by H2-O2 titration at 170°C using a Micromeritics 2920 Surface Analyzer. Galvanic displacement was conducted using Pd(NO3)2 in a pH 2 bath (HNO3 to adjust pH) at 25°C. The concentrations of deposited Pd2+ and displaced Ag+ were monitored during the experiment by AA analysis. These bimetallic compositions have been characterized by Fourier-transform infrared (FTIR) spectroscopy of CO adsorption and X-ray photoelectron spectroscopy (XPS). Catalysts have also been evaluated for the selective hydrogenation of acetylene in excess ethylene at the conditions used in our previous publication.
The kinetic GD curves in Fig. 1 indicate that the displacement reaction is initially first order in Pd2+, but as the Ag surface becomes more depleted the rate of displacement decreases. For rigorous GD, the ratio of deposited Pd2+/displaced Ag+ is 1:2 for the reaction Pd2+ + 2Ag° °ú Pd° + 2Ag+. The Pd coverage should then be limited to 0.5 monolayers on Ag, since two Ag surface atoms are required for each Pd2+ deposited. However, the 0.03, 0.09, 0.28, and 0.32 wt% Pd-Ag samples shown in Fig. 1 correspond to 0.3, 0.9, 2.9, and 3.3 theoretical monolayers coverage on Ag, which suggests there is diffusion of Pd into the bulk of Ag, or more likely, migration of Ag to the surface since the surface free energy of bulk Ag is lower than that of Pd. Such migration led to the formation of a Pd-Ag alloy near the catalyst surface. FTIR of CO adsorption in Fig. 2 shows that a single CO stretching band was observed in the 2000-2100 cm-1 region with the peak centered at approximately 2046 cm-1, which was attributed to linearly adsorbed CO on fully reduced Pd sites. This result indicates that some surface Pd is present and that the Pd atoms are distributed in very small ensembles, possibly even atomically, on the Ag surface. Such geometric effects were further confirmed by evaluation studies revealing that the selectivities for C2H4 formation remained high and constant for different Pd loadings. Surface analysis by XPS in Fig. 3 confirms there was Pd at least in the near surface of the catalysts prepared by galvanic displacement. The Pd and Ag peak intensities also indicate, as expected, that near surface Pd concentrations increase and Ag concentrations decrease when more Pd metal is galvanically-exchanged. For all bimetallic compositions, shifts to higher binding energies were observed for the Pd 3d3/2 and Pd 3d5/2 peaks in comparison to 2 wt% Pd/SiO2 catalyst (Figure 3A). Conversely, the Ag 3d3/2 and Ag 3d5/2 peaks were shifted to lower binding energies compared to a 2 wt% Ag/SiO2 catalyst (Figure 3B). These shifts indicate a net electron transfer from Pd to Ag. Unlike for the case of Ag on Pd surfaces, the Pd sites here are significantly influenced by the electron transfer from Pd to Ag. This results in a competing electronic effect in these catalysts that limits the ability to obtain very high selectivity towards C2H4.
Fig. 1 Time-dependent galvanic displacement profiles of deposited Pd2+ from bath solution and at room temperature with different initial Pd2+ concentrations.
Fig. 2 FTIR spectra for CO adsorption on various Pd-Ag/SiO2 bimetallic catalysts.
Fig. 3 XPS spectra of (A) Pd 3d on 2 wt% Pd/SiO2 and Pd-Ag/SiO2 and (B) Ag 3d on 2 wt% Ag/SiO2 and Pd-Ag/SiO2. All samples were reduced in situ at 200 °C in 100% H2 for 2 h.
Fig. 4 Conversion of acetylene and selectivity of acetylene to ethylene as a function of Pd weight loadings on Ag/SiO2. Reaction conditions: 65°C and feed composition of 1% C2H2, 5% H2, 20% C2H4, and balance He at GHSV = 6.0 °Á 105 h-1. Error bars represent maximum and minimum values for each data point; data point is average value.
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