Quantitative determination of sites able to chemisorb CO on Au/ZrO2 catalysts

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Abstract

Quantitative volumetric measurements of low temperature CO chemisorption on Au/ZrO2 catalysts and Fourier transform infrared measurements of adsorbed CO in well defined and controlled conditions of temperature and pressure are here presented as widely accessible and reproducible methods for determining the concentration of gold sites able to chemisorb CO on Au/ZrO2 catalysts. HRTEM analyses were also performed in order to support the validity of the measurements.

Introduction

Gold supported on oxides or carbon, once considered catalytically inert, is now firmly established as an effective catalyst and the repertoire of reactions that it can catalyze is really wide. Gold catalysts already have done it out of the lab and into commercial use, and a very large number of papers was published regarding catalysis by gold. It has been shown that the nature and the structure of the support strongly influence the catalytic performances of gold-based samples, but exploration of catalytic activity and selectivity of Au nanoparticles on the basis of sizes was the main focus of most research in this field. Experimentally, the size of gold metallic particles is mainly detected by TEM. However TEM is an expensive method and, even if performed accurately, it is hardly applicable for highly dispersed samples, since the very small metallic particles (<1 nm) are detectable in an over-complicated way. A technique more widely diffuse both in academic and industrial laboratories, reproducible and fast, such as chemisorption test, should be very useful for these systems. Unfortunately, as well known, gold does not chemisorb many molecules easily, and in fact there are only few scientific works related to characterization of supported gold by chemisorption methods [1], [2], [3], [4], [5]. We have recently shown [6] that CO chemisorption by a pulse flow technique and Fourier transform infrared measurements of adsorbed CO in well defined and controlled conditions of temperature and pressure can be taken as effective methods for the quantitative determination of the gold active sites on Au/TiO2 and Au/Fe2O3, two reference catalysts provided by the World Gold Council, and on a Au/CeO2 sample. In particular we have demonstrated that using a pulse flow system after a proper pretreatment, CO chemisorption at 140–180K can be taken as a widely accessible and reproducible technique for determining the concentration of gold active sites and for comparing different samples. The goal of the present work is to extend that study to zirconia supported gold catalysts in order to test further the validity of the method already presented. We choose zirconia because it is of particular interest in a large field of applications, since it possesses desirable properties such as tuneable surface acidity/basicity (controlled by the addition of different dopants), redox properties and tuneable porosity and surface area. Recently, it has been found that zirconia is a very efficient support for gold-based catalyst for the low-temperature water gas-shift (WGS) reaction [7], [8], [9]. Moreover, Au/ZrO2 materials are of significant interest as catalysts for CO oxidation [10], butadiene hydrogenation [11], epoxidation of styrene [12]. We present here a detailed investigation of a series of Au/ZrO2 samples with different metal content and various particle sizes, deeply characterized by a number of techniques, in order to improve, through a systematic study, the possibility of the quantitative determination of gold sites able to activate CO.

Section snippets

Catalyst preparation

The support was prepared by precipitation from ZrOCl2·8H2O (Fluka) at constant pH (pH 8.6), aged under reflux conditions for 20 h [13], [14], washed free from chloride (AgNO3 test) and dried at 383 K overnight. Then the prepared Zr(OH)4 was sulfated with (NH4)2SO4 (Merck) by incipient wetness impregnation (2.5 wt%SO42−/ZrO2) and finally heated (90 K/h) up to 923 K in flowing air (30 mL/min STP) and kept at this temperature for 6 h, followed by slow cooling to room temperature [8]. Gold was deposited

Results and discussion

Sulfated zirconia [17] has been the object of extensive research, due to its characteristics: sulfates cause modifications of the acid properties and affect surface features: they retard crystallization, stabilize the tetragonal phase, improve the surface area and the pore size. These properties make sulfated zirconia interesting both as catalyst and as catalyst support. In the present work we have used sulfated zirconia as support and we have characterized it by N2 physisorption and sulfur

Conclusions

In summary, this work demonstrates that CO chemisorption performed by a pulse flow system at 157 K on prehydrated samples can be taken as a method for the quantitative determination of the gold sites also on Au/ZrO2 catalysts, where gold particles cannot be detected by TEM.

Acknowledgment

Financial support to this work by MIUR (Rome-Cofin 2006) is gratefully acknowledged.

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