Surface characterisation of Pd–Ag/Al2O3 catalysts for acetylene hydrogenation using an improved XPS procedure

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Abstract

Effects of pretreatment on the surface of alumina-supported Pd–Ag catalysts with oxygen or oxygen-containing compounds (NO, N2O, CO and CO2) have been studied using an improved X-ray photoelectron spectroscopy (XPS). Surface analysis was performed either before or after the selective hydrogenation of acetylene. Analysis of the surface after reduction shows evidence of a Pd–Ag alloy. The binding energy of the Pd 3d is not affected by pretreatment, whereas a significant shift of the Ag 3d is revealed after NO and N2O pretreatment. The surface after reaction shows no state change of either Pd or Ag compared to those measured prior to reaction, which is in agreement with the reactivity test; therefore surface modification occurs after pretreatment and is retained even after 8 h on stream. No carbonaceous deposits are formed after 8 h on stream. Ethylene gain enhancement by NO and N2O pretreatment is a result of strong adsorption on the surface which may block the sites responsible for ethylene hydrogenation without facilitating carbonaceous deposits for hydrogen spillover. On the other hand, pretreatment with O2, CO or CO2 increases the Pd active sites, which increases C2H2 hydrogenation activity.

Introduction

The selective hydrogenation of acetylene over palladium catalysts is used commercially to remove trace amounts of acetylene contaminant in ethylene feedstreams for polyethylene production [1], [2], [3], [4], [5], [6], [7], [8]. Due to poor selectivity at high acetylene conversion and oligomer formation during acetylene hydrogenation, considerable attention has been focused on bimetallic systems in which a second metal such as silver, copper or lead is incorporated into palladium. Substantially increased catalytic performance as well as reduction in green oil formation have been reported with supported Pd–Ag catalysts [9], [10], [11], [12], [13], [14].

In our recent communication [15], we reported the promotion effect of pretreatment with oxygen or oxygen-containing compounds (NO, N2O, CO and CO2) on the catalytic performance of Pd–Ag/Al2O3 for the selective hydrogenation of acetylene. The higher activity was thought to be a result of increasing the Pd working sites. Pretreatment with NO and N2O gave higher ethylene gain, whereas less ethylene gain was observed with the other pretreatment compounds. Gain is used in industry as a measure of selectivity in acetylene hydrogenation. A definition has been given earlier [15].

In this work, we continue our investigation of catalyst pretreatment by oxygen-containing compounds [15]. In our previous study, ex situ XPS was used for surface characterisation. We now employ an improved procedure to ensure our results were not falsified by exposure to air either during storage or sample preparation. This procedural change has caused a correction to our discussion of the promotion effect. Using XPS we now observe a shift in the XPS peak position. The source of this has been studied as explained below. Relevance to catalytic hydrogenation based on our characterisation is also discussed here.

Section snippets

Catalyst preparation and testing

The bimetallic Pd–Ag/Al2O3 (2.8 wt.% Pd and 2.3 wt.% Ag) catalysts used in this study were prepared by sequential impregnation using Pd(NO3)2 and AgNO3 as the Pd and Ag sources, respectively. Details have been given previously [15].

After preparation, the catalyst was pretreated with oxygen or oxygen-containing gases, i.e., N2O, NO, CO2 and CO, prior to use as previously detailed [15].

The selective hydrogenation of acetylene was conducted at 50 °C for 8 h after reduction and pretreatment of the

Pd–Ag surface before reaction

The values of binding energy as well as FWHM of Ag 3d5/2 and Pd 3d5/2 lines obtained from untreated and pretreated Pd–Ag/Al2O3 samples are given in Table 2. Fig. 1, Fig. 2 represent the Ag 3d and Pd 3d spectra obtained from Pd–Ag catalysts before reaction, respectively.

For untreated catalyst, the binding energy of the Pd 3d5/2 is 334.7 eV, which is 0.7 eV lower than the bulk Pd (335.4 eV). The binding energy of Ag 3d5/2 is at 367.4 eV, a −0.6 eV shift from bulk silver (368.0 eV). The core level

Acknowledgements

The project was financially supported by The Thailand Research Fund (TRF) and TJTTP-JBIC. B. Ngamsom would like to thank The Surface Science Technology and Centre of Particles and Catalyst Technologies, School of Chemical Engineering and Industrial Chemistry, The University of New South Wales, Australia, for providing experimental facilities for this work. The technical assistance of and fruitful discussions with Mr. Wilhelm Holzschuh are gratefully acknowledged.

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