Delivered by Ingenta to: Nanyang Technological University
IP: 5.101.219.22 On: Tue, 07 Jun 2016 03:58:58
Copyright: American Scientific Publishers
RESEARCH ARTICLE
Copyright © 2011 American Scientific Publishers
All rights reserved
Printed in the United States of America
Journal of
Nanoscience and Nanotechnology
Vol. 11, 5469–5475, 2011
Influence of Cation Nature on Stabilization of
Gold Nanospecies in Mordenites
I. Tuzovskaya
1 2
, E. Lima
3 4
, P. Bosch
4
, N. Bogdanchikova
2 ∗
,
A. Pestryakov
5
, and J. Fraissard
6
1
Catalytic Processes and Materials, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands
2
Centro de Nanociencias y Nanotecnología, Universidad Nacional Autónoma de México,
Apdo. Postal 2681, C. P. 22800, Ensenada, México
3
Universidad Autónoma Metropolitana, Iztapapala, 09340 México D.F., México
4
Instituto de Investigaciones en Materiales, Universidad Nacional Autónoma de México,
Ciudad Universitaria, México D. F., México
5
Tomsk Polytechnic University, Tomsk 634050, Russia
6
P. and M. Curie University, ESPCI, laboratoire lpem, 75005, Paris, France
Gold species deposited on NH
+
4
- and H
+
-mordenites were studied by X-ray diffraction, xenon
adsorption, NMR of
27
Al and
129
Xe, UV-Visible spectroscopy and TPR. Mordenite ion-exchange
cations exert a significant effect on size distribution and electronic state of gold species supported
on the zeolites. AuNH
4
M contains larger nanoparticles of gold on its external surface than AuHM
because protonic form of zeolite has stronger sites for stabilization of gold nanospecies. In the case
of NH
+
4
-mordenite, Au clusters inside zeolite channels have bigger size and fill the side-pockets com-
pletely after gold deposition due to more complete reduction of gold entities, while in H
+
-mordenite
only half the side-pockets are filled by gold species. H
+
-mordenite favors the stabilization of both
small gold clusters and cations. Even after the reduction of gold by hydrogen flow at 100
C, some
gold species are not completely reduced and have certain effective charge (Au
+
n
clusters).
Keywords: Gold, Zeolites, Clusters, Nanoparticles, Catalysts.
1. INTRODUCTION
The synthesis of small metal particles is very important
since nanoparticles and clusters have different catalytic
properties as compared with bulk metal. For example,
gold nanoparticles smaller than about 3 nm exhibit very
high catalytic activity in low-temperature CO oxidation.
1 2
Different methods of stabilizing gold particles are used,
and the properties of dispersed gold are being studied
intensively.
3
Zeolites are among the most interesting sup-
ports for gold due to their ability to stabilize metal clusters
inside the micropores.
4 5
However, investigations showed that the influence of
zeolite structure and method of sample preparation on the
electronic and structural properties of gold is a complex
function, and that a whole series of different parameters
has to be considered in order to understand the real cause
of the observed effects. Our previous studies revealed that
∗
Author to whom correspondence should be addressed.
the self-assembly of clusters in mordenites depends on
several factors: (1) the type of zeolite crystal structure and
chemical composition (SiO
2
/Al
2
O
3
molar ratio); (2) the
acidity of the zeolite; (3) the pretreatment conditions (first
of all, reduction temperature); (4) additives of metal oxides
(Fe, Ni, Cu, etc.).
5–8
The influence of the gold concentra-
tion, the time and pH of ion-exchange, and the geometry
of zeolites on the state of the gold was also investigated
in some studies.
3 9 10
To control the electronic and structural properties of
nanosized gold particles is very important for the syn-
thesis of highly active catalysts because these parameters
determine the catalytic activity of gold. One of unstudied
parameters influencing gold states in zeolites is the nature
and size of zeolite cation.
The aim of the present study is to investigate the influ-
ence of the nature of the exchangeable cation of zeolite on
the formation of gold species in Au-mordenite. For such a
test we chose two types of exchangeable cations, H
+
and
J. Nanosci. Nanotechnol. 2011, Vol. 11, No. 6 1533-4880/2011/11/5469/007 doi:10.1166/jnn.2011.3432 5469