Abstract
This article reviews the efforts of the last two decades to deNOxify the atmospheric environment with TiO2-based photocatalytic materials supported on various cementitious-like substrates. Prior to undertaking this important aspect of applied photocatalysis with metal-oxide emiconductor photocatalysts, however, it is pertinent to describe and understand the fundamentals of Heterogeneous Photocatalysis. The many attempts done in a laboratory setting to degrade (deNOxify) the major components that make up the NOx, namely nitric oxide (NO) and nitrogen dioxide (NO2), but most importantly the efforts expended in deNOxifying the real environment upon depositing titania-based coatings on various model and authentic infrastructures, such as urban roads, highway noise barriers, tunnels, and building external walls among others, are examined. Both laboratory and outdoor experimentations have been performed toward NOx being oxidized to form nitrates (NO3−) that remain adsorbed on the TiO2-based photocatalytic surfaces (except in tunnels—indoor walls) but get subsequently dislodged by rain or by periodic washings of the infrastructures. However, no serious considerations have been given to the possible conversion of NOx via photocatalytic reduction back to N2 and O2 gases that would restore the atmospheric environment, as the adsorbed nitrates block the surface-active sites of the photocatalyst and when washed-off ultimately cause unduly damages to the environment.
Highlights
The atmosphere is a very complex matrix that, in addition to nitrogen (N2) and oxygen (O2) gases, so important to human survival, consists of various other pollutant gases albeit at much lower concentrations: (i) carbonaceous oxides (COx); (ii) nitrogen oxides (NOx); (iii) sulfur oxides (SOx); (iv) various hydrocarbons (HCs); and (v) particulate matter
The many attempts done in a laboratory setting to degrade the major components that make up the NOx, namely nitric oxide (NO) and nitrogen dioxide (NO2), but most importantly the efforts expended in deNOxifying the real environment upon depositing titania-based coatings on various model and authentic infrastructures, such as urban roads, highway noise barriers, tunnels, and building external walls among others, are examined
Plot made from data reported in Results indicated that irradiation for 60 min and relative humidity of 60% led to ca. 25% of an initial cAonncoetnhterrawtioisne osfim8.i6la1rμrmeaocltoofr NasOsetmo bbleydwegarsaudseeddabt ya flMoawrtirnaetezoafn1d.5cLowmoirnk−e1rs[1[2152]5.]Oton mexoarmtairne anthdegdlaesgsrsaudbastitoranteosf, tNhOe inpflreuseenncteinoftihnecraeiarsbinygmheuamnsidoitfyaopnhtohteodcaetgarlaydtiactiooxnidraattieosndpeproecnedsesdbaosnetdheon naTtiuOr2enoafntohpeasrutibcsletrsattheaatnhdadonbeiennitiinalcoNrOpocroatnecdenintraatpioonlysm; neor-smigantriifixc-abnasteidnflcuoaetnicneg.oTf hheuemxipdeirtiymwenastal obsseeturvpedcoantsiinstiteidaloNf Oa fcloonwcetnytpreatrioeancstoorf 4(F00igaunrde 3140)00adpappbt,ewdhfirloema stihgenIifiSOcan2t21d9e7c-r1eassteanindathrde;ktihneetfiicnsal products detected were NO2 in the gas phase and nitrate ions adsorbed on the photocatalytic surface
Summary
The atmosphere is a very complex matrix that, in addition to nitrogen (N2) and oxygen (O2) gases, so important to human survival, consists of various other pollutant gases albeit at much lower concentrations: (i) carbonaceous oxides (COx); (ii) nitrogen oxides (NOx); (iii) sulfur oxides (SOx); (iv) various hydrocarbons (HCs); and (v) particulate matter. Home to nearly 19 million people and located in a geological basin confined by the Pacific Ocean and mountains, LA is the basin of considerable pollution caused by its car-centric culture, its bustling industries and ports, its sprawling development, and its sunny climate with often stagnant winds It was only in the 1950s that hydrocarbons and NOx were recognized as the source of photochemical smog (Figure 3); with the implementation of mandatory catalytic converters in automobiles in the last two decades, smog has been attenuated somewhat. HHiissttoorriiccaallllyy,, HHeetteerrooggeenneeoouuss PPhhoottooccaattaallyyssiiss iiss aann iinntteerrddiisscciipplliinnaarryy ffiieelldd aatt tthhee iinntteerrsseeccttiioonn ooff CChheemmiissttrryy aanndd PPhhyyssiiccss. Photocatalysis is best described as a change in the rate of a chemical reaction or its initiaCtiuonrruenntdlyer, tphheoatcotciaotnaolyfsuisltrisavbieoslet td, vesiscirbiblee,doraisnafracrheadnrgaediinatitohne irnatteheopf raescehnecme iocfaal rseuabcsttiaonnceorthiatst ianbistoiartbios nliguhntdaenrdthiseiancvtoiolvneodfiunltthraevcihoelemt,icvailsitbralen,soforrimnfartairoendorfatdhieatrieoanctiinonthpearptrneesresn; ctheeopf haostuobcasttaalnycset tishatht easbusobrsbtasnlcieghthtaatncdauissesi,nbvyolavbesdoripntitohneocfhuelmtraicvailolterta,nvsifsoibrmlea, toironinforfartehde rraedaicattiioonn,ptharetncheresm; itchael ptrhaontsofcoartmalaytsiot nisothf ethseurbesatactnicoentphaarttcnaeursse, sr,ebpyeaatbesdolrypctoiomnionfguilnttroaviniotleertm, veidsiibaltee,cohreimnfircaarleidntreardaicattiioonns, wthiethchtheemmicaanl dtrarengsfeonremraattiinogniotsf cthheemreicaaclticoonmppaorstniteiorsn, areftpeeraetaecdhlycyccolme ionfgsuincthoinintteerramcteiodniast[e2c9h].emical interactions with them and regenerating its chemical composition after each cycle of such interactions [29]
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