Investigation on gold dispersion of Au/ZrO2 catalysts and activity in the low-temperature WGS reaction

doi:10.1016/j.apcatb.2009.02.004

Applied Catalysis B: Environmental

Volume 89, Issues 1–2, 3 July 2009, Pages 303-308

https://doi.org/10.1016/j.apcatb.2009.02.004 Get rights and content

Abstract

Gold over zirconia and sulfated zirconia catalysts were tested in the water–gas shift (WGS) reaction at temperatures between 423 and 453 K. Samples were characterized by N₂ adsorption analysis, XRD, TPO, TG-DTA and pulse-flow CO chemisorption. A very good linear relationship between catalytic activity and dispersion determinated by chemisorption has been evidenced. Sulfated catalysts showed higher activities than samples over plain zirconia. SO₄⁼ do not behave as promoters of the gold active phase, but they yield a larger specific surface area of the zirconia, thus favouring a better dispersion of gold nanoparticles on the support. CO amount at the reactor outlet has been checked and will be discussed.

Introduction

The conversion of hydrocarbons through hydrodesulfurization, steam reforming, water–gas shift (WGS) and preferential oxidation processes is the most common way to produce clean hydrogen for different applications. Aim of the water–gas shift reaction:CO + H₂O = CO₂ + H₂, ΔH = −41 kJ/mol, ΔG = −28.6 kJ/molis to reduce the CO concentration coming from the steam reforming reactor, producing at the same time hydrogen, thus improving the global efficiency of the process. Commercial reaction usually proceeds in two different steps with two types of catalyst: an high temperature (HT) stage over an iron oxide promoted with chromium oxide catalyst, and a low temperature (LT) stage, on a catalyst composed of copper, zinc oxide and alumina [1]. Recently there has been a renewed interest in WGSR as a key step in the conversion of fuel to hydrogen, to be used in proton exchange membrane fuel cells to generate electricity. The development of a new generation of catalysts, showing high activity towards the conversion of CO at low temperatures, easy to activate and with good stability to air and liquid water, is highly desirable, since commercially available LT-WGS catalysts do not meet these requirements [2].

By now it is ascertained that gold becomes a catalyst when supported on a metal oxide, in the form of nanoparticles strongly bound to the oxide surface [3]. Gold catalysts show high activity in some important industrial reactions, such as CO oxidation [4], low-temperature WGS [5], hydrogen peroxide production from H₂ and O₂ [6], selective epoxidation [7]. In particular the catalysis of the WGS reaction by gold nanoparticles supported over metal oxides has recently been the subject of numerous investigations [8], [9]. It has been shown that the nature and the structure of the support and the preparation conditions strongly influence the catalytic activity and selectivity of gold-based samples. Gold catalysts supported on Fe₂O₃ [10], TiO₂ [11], ZnO [12], CeO₂ [13] and ThO₂ [12] showed good catalytic activity in the water–gas shift reaction. Recently gold samples supported on mesoporous zirconia have also been studied [14], [15], [16]. The opportunity of use ZrO₂ as support is due to its intrinsic characteristics: chemical and surface features of zirconia, as surface acidity, redox properties, porosity and surface area, can be adjusted by choosing different precursors and synthesis conditions and by dopants addition. In particular sulfates increase surface acidity, retard crystallization and enhance the surface area and the pore size distribution [17].

The goal of the present work is to investigate the possible correlation between gold dispersion and catalytic activity in the LT-WGSR. Besides the effects of sulfates addition on the support and its effect on the activity and the properties of gold on zirconia catalysts for the low-temperature water–gas shift reaction have been studied. Other aspects, such as the effect of gold content, the use of different bases for the deposition-precipitation procedure (dp) (Na₂CO₃ and NaOH), and the applicability of the newly prepared samples in the fuel processing systems for fuel cells have been examined too.

Section snippets

Catalyst preparation

Zirconia was prepared by a two-step synthesis technique. Zr(OH)₄ was prepared from ZrOCl₂·8H₂O by precipitation in water at constant pH (pH 8.6) by continuous addition of aqueous ammonia 5 M, aged under reflux conditions for 20 h at 363 K, washed free from chloride (AgNO₃ test) and dried at 383 K overnight. Part of the hydroxide was then sulfated with (NH₄)₂SO₄ (Merck) by incipient wetness impregnation, in order to obtain a 2 wt% amount of sulfates on the final support. Sulfated (ZS) and

Catalyst characterization

The choice of the materials is very important in this investigation, since a low surface area of the support would not allow a good dispersion of the gold active phase. So N₂ physisorption analyses were carried out in order to determine surface areas and pore size distributions of the samples. As example, physisorption isotherms and pore diameter distributions of catalysts containing 1 wt% of gold are shown in Fig. 1, and the corresponding data are summarized in Table 1. All samples show type IV

Conclusions

Gold over zirconia and sulfated zirconia catalysts were tested in the low-temperature water–gas shift reaction. Sulfated catalysts showed higher activities than samples on plain zirconia, due to the promotion of zirconia to a higher specific surface area that leads to a better dispersion of gold nanoparticles on the surface. In fact a very good relationship between catalytic activity and chemisorption data has been evidenced. Such close correlation indicates that the chemisorption test is

Acknowledgments

We thank Prof. Giuseppe Cruciani for XRD data. Financial support to this work by MIUR (Rome-Cofin 2006) is gratefully acknowledged.

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Investigation on gold dispersion of Au/ZrO2 catalysts and activity in the low-temperature WGS reaction

Abstract

Introduction

Section snippets

Catalyst preparation

Catalyst characterization

Conclusions

Acknowledgments

J. Catal.

Appl. Catal. A: Gen.

J. Catal.

Catal. Commun.

Appl. Catal. B

Appl. Catal. A

J. Catal.

Appl. Catal. A

Appl. Catal. B

Appl. Catal. A: Gen

J. Catal.

J. Mol. Catal. A: Chem.

Appl. Catal. B

J. Catal.

Investigation on gold dispersion of Au/ZrO₂ catalysts and activity in the low-temperature WGS reaction