Journal of Applied Solution Chemistry and Modeling, 2015, 4, 63-71 63
E-ISSN: 1929-5030/15 © 2015 Lifescience Global
Effects of Metal Ions on H
2
Generation during Photolysis of
Suspended TiO
2
in Aqueous Systems
G.R. Dey
*
Radiation & Photochemistry Division, Bhabha Atomic Research Centre, Mumbai 400 085, India
Abstract: Nowadays, photocatalysis is being applied as a promising technique for decontamination, purification,
deodorization, etc. of polluted air and wastewaters. This technique attempts also to generate renewable and non-
polluting fuels utilizing various carbon and hydrogen sources. In this context, studies on photo-catalytic generation of H2
using 0.1% w/v TiO2 photo catalyst as a suspension in water was carried out using 350 nm light. This was explored
further in presence of various metal ions such as Ag
+
, Cu
2+
, Fe
2+
, Au
3+
and V
5+
at different ambient such as air and CO2,
in presence of a hole scavenger (2-propanol and formic acid). H2 yields in CO2-purged systems as analyzed in GC-TCD
were reasonably higher, and increased further when metal ions were added into the systems. Based on H2 yields,
HCOOH was found to be a more supportive hole scavenger as compared to 2-propanol in metal ions containing
systems. Besides the formation of low to high concentrations of various gaseous products, nanoparticles of gold, silver
and copper were also observed as photolysis products. This indicated that the presence of both reduced metal ions and
their in-situ generated nanoparticles at various stages either in the free/unbound and/or associated with TiO2
photocatalyst in aqueous systems, together play a significant role in enhancing the H2 yields.
Keywords: H
2
, TiO
2
, hole scavenger, photolysis, metal ions, nanoparticles.
1. INTRODUCTION
The generation of H
2
, a photolysis product through
the photo catalytic effect of semiconductor in presence
of water is well known [1-4]. Fujishima and Honda have
highlighted their interests on heterogeneous
photocatalysis when they successfully split water into
hydrogen and oxygen in the presence of TiO
2
[4]. After
their success, the research on heterogeneous
photocatalysis has been enhanced tremendously all
over the world. The primary step in TiO
2
photocatalysed oxidation has been understood with the
formation of both hydroxyl radicals (
OH) and super
oxide radical anions (O
2
-
) [5-8] through several
reactions as presented below:
TiO
2
h
h
+
+ e
(1)
h
+
+ H
2
O
OH + H
+
(2)
In presence of air,
e
+ O
2
O
2
(3)
Hence,
OH (E
o
= 2.72 V) [9] a strong oxidizing and
O
2
-
(E
o
= -0.33 V) [10] a weak reducing species are
generated from the above-mentioned photocatalytic
redox processes. In absence of oxygen,
OH radical, a
strong oxidant and e
-
, a strong reductant are the
reactive species generated during photolysis of TiO
2
*Address correspondence to this author at the Radiation & Photochemistry
Division, Bhabha Atomic Research Centre, Mumbai 400085, India; Tel: 91-22-
25595397; Fax: 91-22-25505151/25519613; E-mail: grdey@barc.gov.in
[11, 12] in water containing systems. In the presence of
an
OH or h
+
scavenger, it is possible to use TiO
2
system for photo-reduction processes, similarly in
presence of an e
-
scavenger, the system can be made
favorable for oxidation reaction. During these
conversion processes the respective product yields
increases.
Earlier we have carried out the photo-reduction of
CO
2
in the presence of TiO
2
as suspension in water at
room temperature and under normal atmospheric
pressure with and without 2-propanol (a hole
scavenger) under diverse ambient conditions [13-16].
In continuation to the previous work, the photocatalytic
studies on chemical reduction of CO
2
were carried out
in two different ambient such as Air and CO
2
, using
TiO
2
suspended aqueous solutions both in presence
and absence of 2-propanol/formic acid. In our previous
experiments TiO
2
catalysts of 0.1% (w/v) amount as
suspension in water has been found adequate for
maximum yields of the photolytic products [13],
therefore these experiments were carried out
employing 0.1% (w/v) TiO
2
. The two different ambient
such as air (aerated or air saturated) and CO
2
-purged
systems were chosen because in earlier study in
presence of 2-propanol, the highest yields of methane
was observed as a reduction product of carbon dioxide.
The significant yield of methane has been explained
therein with the active role of in-situ CO
2
(generated
due to mineralization of a part of 2-propanol) in free
radical induced reduction reactions. The yields of CH
4
in both CO
2
-purged as well as aerated systems were
comparable [13]. This work was explored further in the
presence of various metal ions (such as Ag
+
, Au
3+
,