There usually consists of an active phase and is

There are a number of
things that have to be taken into consideration when talking about a catalyst
in any particular chemical reaction. The catalyst usually consists of an active
phase and is numerous times complemented by a porous carries. The reaction
usually takes place on the active phase surface. There are a number of
components such as size, shape, composition, surface properties etc on the
basis of which the behaviour of the nanosized active component is dependent
upon. However they are supported by carrier particles as well and thus they don’t
act in isolation. All of the properties i.e. the physical as well as the
chemical properties rarely work in isolation and most of the time work together
and thus have a direct impact on the catalytic stability, activity and
selectivity. There the study of these properties is of utmost importance if one
seeks to gain an insight as to how a catalytic performance can be optimized.

Selective Catalytic
Reduction

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The selective catalytic
reduction of nitrous oxide is well known for its role in environmental
remediation. Its oxides encompass numerous forms however we will focus on N2o
and nitrogen monoxide. Nitrogen monoxide comprises of two gases i.e. NO and NO2
which are major component of environment mainly air pollution.1 Fossil
fuel combustion can be very well described as its main source and during the
process of combustion atmospheric nitrogen is oxidised as shown by the equation:2

eq1

Now the main role of
SCR if to reduce the NOx by conveting them into molecular nitrogen. Even though
the catalytic decomposition of nitrogen oxide is favored thermodynamically, it
requires a low catalytic barrier as the activation for the decomposition is
very high i.e. 364kj/mol.3
Howsoever a reducing agent such as NH3 has to accompany the catalyst in order
for a speedy reaction to take place

eq2

Photocatalysts

When illuminated to
light, these semiconducting compounds catalyze redox reactions on their
surface. They generally don’t require thermal activation and thus are uniquely
suited for environmental remediation reactions. Photocatalysis has numerous
applications as of today such as cleaning air and water and even for making
self-cleaning surfaces. Even though photocatalyst can catalyze reactions that
are thermodynamically up-hill in nature, we are mostly interested in down-hill
reactions when we talk within the context of evncironmental remediation. The
oxidation of organic species that are present in agricultural, resident and
industrial sewage before being released into lake or sea is one of the most
typical reaction that is carried out by the process of photocatalysis.
Photocatalysis plays another important role of removing volatile organic
compounds from indoor as well as outdoor air i.e. smog. Another major
application of photocatalysis when talked about in the context of environmental
remediation is providing active surfaces which are in turn self-sterlizing and
self-cleaning.

eq74

Principles and
Requirements for Photocatalysts

The creation of an
electron-hole pair by the optical excitation of a semiconductor is the most
basic principle of photocatalysis

eq74

An electron is promoted
to the conduction band in the valence band of the semiconductor while a photon
is absorbed. The promotion of the electron thus in turn leaves behind a ‘hole’
in the valence band. This in turn makes the band gap in the semiconductor equal
to the energy separation which in turn means that the band equal gap of the
semiconductor is equal to the energy stored in the photocatalyst due to the
absorption of one photon. Thus this implies that the band gap of the semi
conductor is directly proportional to the sum of the oxidizing power of the
hole as well as the reductive power of electron. Thus the knowledge of the band
gap of a semiconductor will in turn provide information about the combined
redox power of the electron-hole pair. 
The absolute position of the hole and the electron is however not
deduced by the band-gap. The relation between the conduction band position(CB),
valence band position(VB) and band gap(Eg) is as follows:

eq 75

Another aspect is that
material with lower band gaps can also be utilized for water splitting. CdS
having a band gap of 2.4eV is one obvious candidate. However the reason that
CdS is not used for water splitting is because CdS corrodes.

Fig 76

Even though
photogenerated holes in it should by principle provide the desired oxidation of
water however it is kinetically as well as energetically favourable to oxidize
itself instead.

Eq 77

Thus the corrosion
theory also provides us with a very basic insight that one of the major
requirements for a successful photocatalyst is that it should not corrode under
normal working conditions as well as under illumination. However the provision
of non-corrosion is not that easy as a photocatlyst provides strongly oxidising
holes as well as strongly reducing electron simultaneously.

 Another major requirement in order to be an
effective photocatalyst is that the electron present in the Cb as well as in
the corresponding holes in the VB shall all have a very high probability of
reaching the surface of the photocatalyst.

The above stated
requirements are always relevant and cannot be ignored however the band
requirements that have been explained earlier can be tuned to the relevant
redox chemistry in case the required reaction is anything else but water
splitting.

1 10

2 12

3 12