(621bp) Pd-Ag/SiO2 Bimetallic Catalysts Prepared By Galvanic Displacement for Selective Hydrogenation of Acetylene in Excess Ethylene

Pd-Ag/SiO₂
bimetallic catalysts prepared by galvanic displacement for selective
hydrogenation of acetylene in excess ethylene

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 [5], 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 Ag^o 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 [6].  In this case, it is thermodynamically favorable for Pd²⁺
reduction to occur by oxidation of Ag^o 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 3d_3/2 and Pd 3d_5/2 peaks in comparison to 2 wt%
Pd/SiO₂ catalyst (Figure 3A). Conversely, the Ag 3d_3/2
and Ag 3d_5/2 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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