There are a number ofthings that have to be taken into consideration when talking about a catalystin any particular chemical reaction. The catalyst usually consists of an activephase and is numerous times complemented by a porous carries. The reactionusually takes place on the active phase surface. There are a number ofcomponents such as size, shape, composition, surface properties etc on thebasis of which the behaviour of the nanosized active component is dependentupon. However they are supported by carrier particles as well and thus they don’tact in isolation.
All of the properties i.e. the physical as well as thechemical properties rarely work in isolation and most of the time work togetherand thus have a direct impact on the catalytic stability, activity andselectivity. There the study of these properties is of utmost importance if oneseeks to gain an insight as to how a catalytic performance can be optimized.Selective CatalyticReductionThe selective catalyticreduction of nitrous oxide is well known for its role in environmentalremediation. Its oxides encompass numerous forms however we will focus on N2oand nitrogen monoxide. Nitrogen monoxide comprises of two gases i.e.
NO and NO2which are major component of environment mainly air pollution.1 Fossilfuel combustion can be very well described as its main source and during theprocess of combustion atmospheric nitrogen is oxidised as shown by the equation:2eq1Now the main role ofSCR if to reduce the NOx by conveting them into molecular nitrogen. Even thoughthe catalytic decomposition of nitrogen oxide is favored thermodynamically, itrequires a low catalytic barrier as the activation for the decomposition isvery high i.e. 364kj/mol.3Howsoever a reducing agent such as NH3 has to accompany the catalyst in orderfor a speedy reaction to take placeeq2PhotocatalystsWhen illuminated tolight, these semiconducting compounds catalyze redox reactions on theirsurface. They generally don’t require thermal activation and thus are uniquelysuited for environmental remediation reactions.
Photocatalysis has numerousapplications as of today such as cleaning air and water and even for makingself-cleaning surfaces. Even though photocatalyst can catalyze reactions thatare thermodynamically up-hill in nature, we are mostly interested in down-hillreactions when we talk within the context of evncironmental remediation. Theoxidation of organic species that are present in agricultural, resident andindustrial sewage before being released into lake or sea is one of the mosttypical reaction that is carried out by the process of photocatalysis.
Photocatalysis plays another important role of removing volatile organiccompounds from indoor as well as outdoor air i.e. smog.
Another majorapplication of photocatalysis when talked about in the context of environmentalremediation is providing active surfaces which are in turn self-sterlizing andself-cleaning.eq74Principles andRequirements for PhotocatalystsThe creation of anelectron-hole pair by the optical excitation of a semiconductor is the mostbasic principle of photocatalysiseq74An electron is promotedto the conduction band in the valence band of the semiconductor while a photonis 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 equalto the energy separation which in turn means that the band equal gap of thesemiconductor is equal to the energy stored in the photocatalyst due to theabsorption of one photon.
Thus this implies that the band gap of the semiconductor is directly proportional to the sum of the oxidizing power of thehole as well as the reductive power of electron. Thus the knowledge of the bandgap of a semiconductor will in turn provide information about the combinedredox power of the electron-hole pair. The absolute position of the hole and the electron is however notdeduced by the band-gap. The relation between the conduction band position(CB),valence band position(VB) and band gap(Eg) is as follows:eq 75Another aspect is thatmaterial with lower band gaps can also be utilized for water splitting. CdShaving a band gap of 2.4eV is one obvious candidate.
However the reason thatCdS is not used for water splitting is because CdS corrodes. Fig 76Even thoughphotogenerated holes in it should by principle provide the desired oxidation ofwater however it is kinetically as well as energetically favourable to oxidizeitself instead.Eq 77Thus the corrosiontheory also provides us with a very basic insight that one of the majorrequirements for a successful photocatalyst is that it should not corrode undernormal working conditions as well as under illumination. However the provisionof non-corrosion is not that easy as a photocatlyst provides strongly oxidisingholes as well as strongly reducing electron simultaneously. Another major requirement in order to be aneffective photocatalyst is that the electron present in the Cb as well as inthe corresponding holes in the VB shall all have a very high probability ofreaching the surface of the photocatalyst.
The above statedrequirements are always relevant and cannot be ignored however the bandrequirements that have been explained earlier can be tuned to the relevantredox chemistry in case the required reaction is anything else but watersplitting.1 102 123 12