N\dindex{2}O decomposition over Me/Fe-zeolites – Ing. Kamil Jíša

Abstract:

Summary Nitrous oxide (N2O) is a potent greenhouse gas contributing to ozone layer depletion. Nitric acid production is one of the largest sources of N2O in chemical industry, and thus during the last decade, the urgency of N2O abatement further increased because of the anticipated regulations for its emission. Nowadays there is a quest to find and develop efficient and cost-effective system for N2O abatement. One way that could be feasible is direct catalytic decomposition of N2O to N2 and O2 over metal-exchanged zeolites. The study contributes to assessment of the potential of various iron-containing zeolites for N2O decomposition and of the relationship between their catalytic performance and their specific structures. Attention has also been paid to the influence of NO, O2 and H2O on the decomposition of nitrous oxide over Fe-zeolites. The effect of doping of Fe-zeolites by noble metals (Pt, Rh, Ru) and the effect of zeolite pretreatment at high temperatures (HTP) has been analyzed in detail. Several experimental methods were used to identify the individual iron forms in Fe-zeolites, and their changes due to treatment under various conditions; mainly Fourier transformed infrared spectroscopy (FTIR), UV-Vis-NIR diffuse reflectance spectroscopy and temperature-programmed reduction by hydrogen (H2-TPR) were used for this purpose. High-resolution transmission electron microscopy (HRTEM) is also employed. The decomposition of nitrous oxide was compared over Fe-FER, Fe-MFI and Fe-BEA containing iron in well-established cationic positions and low amounts of oxide species. It was evidenced that, despite a comparable content of Fe(II) in the cationic positions, the catalytic activity of Fe-FER greatly exceeds that of Fe-BEA and Fe-MFI. This was correlated with the concentration of Fe(II) in positions with a specific spatial iron arrangement and optimal Fe?cFe distances, providing suitable conditions for cooperation of the two iron cations on the N2O molecule splitting. Such arrangement is absent in both the Fe-BEA and Fe-MFI structures. Doping of Fe-FER by noble metals (Pt, Rh, Ru) increased catalytic activity in the N2O decomposition in the sequence: Pt < Rh . Ru. The enhancement of the activity has been attributed to a synergic effect between the iron and the noble metal. Addition of NO substantially decreased the decomposition of N2O over monometallic Rh- and Ru-FER samples, but not over Fe-FER and all bimetallic Me/Fe-FER samples. The pretreatment of the catalyst at 700 ??C (HTP) has a strong enhancing effect on the N2O conversion over iron zeolites and particularly over Fe-FER, nearly comparable to the effect of NO addition. Contrary to monometallic Me-FER samples, the bimetallic HTP Me/Fe-FER exhibited increase in the activity. Synergic effect in the decomposition of nitrous oxide, previously identified for Rh/Fe-FER pretreated at low temperature, was observed also after pretreatment at high temperature. This does not hold for Pt/Fe-FER and Ru/Fe-FER, where increase of the activity due to HTP was apparently only due to the positive effect on the Fe component. Combination of the two main approaches for enhancing the catalytic activity of Fe-zeolites in N2O, i.e. doping of iron catalysts by noble metal and high-temperature pretreatment, have been shown to produce the HTP Rh/Fe-FER, the most efficient catalyst for decomposition of nitrous oxide in the series of mono- and bimetallic zeolitic catalysts studied. …víceméně

Abstract:

Souhrn Oxid dusny (N2O) pat.i mezi vyznamne sklenikove plyny, ktere p.ispivaji k ubytku ozonove vrstvy. Jednim z nejv.t.ich zdroj. N2O z chemickeho pr.myslu je vyroba kyseliny dusi.ne. B.hem poslednich desetileti se zvy.ila pot.eba sni.it emise N2O take z d.vod. jeho o.ekavanych regulaci. V sou.asne dob. je snaha nalezt a vyvinout u.inny a cenov. dostupny system pro sni.eni emisi N2O. Jednim z mo.nych zp.sob. je p.imy katalyticky rozklad N2O na zeolitech obsahujicich ionty kov. za vzniku dusiku a kysliku. Tato studie p.ispiva k posouzeni potencialu r.znych Fe-zeolit. v rozkladu N2O a ke vztahu mezi jejich katalytickymi vlastnostmi a specifickymi strukturami. Pozornost byla rovn.. v.novana vlivu NO, O2 a H2O na rozklad N2O na Fe-zeolitech. Byl podrobn. analyzovan vliv dopovani Fe-zeolit. vzacnymi kovy (Pt, Rh, Ru) v.etn. vlivu vysokoteplotniho opracovani zeolitu (HTP). R.zne experimentalni metody byly pou.ity k ur.eni jednotlivych forem .eleza v Fe-zeolitech a jejich strukturnich zm.n v d.sledku opracovani za odli.nych podminek. Zejmena byla k t.mto u.el.m pou.ita infra.ervena spektroskopie s Fourierovou transformaci (FTIR), difuzn. reflexni UV-Vis-NIR spektroskopie a teplotn. programovana redukce vodikem (H2-TPR). Rovn.. byla take pou.ita vysokorozli.ovaci transmisni elektronova mikroskopie (HRTEM). Rozklad N2O byl porovnavan na Fe-FER, Fe-MFI a Fe-BEA zeolitech, ktere obsahovaly .elezo v dob.e uspo.adanych kationtovych pozicich a nizky obsah oxidickych .astic. Bylo dokazano, .e i p.es srovnatelny obsah Fe(II) v kationtovych polohach, katalyticka aktivita Fe-FER zna.n. p.evy.ovala aktivitu Fe-MFI a Fe-BEA. Toto zji.t.ni souvisi s koncentraci Fe(II) iont., ktere se nachazely v pozicich se specifickym prostorovym uspo.adanim a optimalni vzdalenosti Fe?cFe, co. poskytovalo vhodne podminky pro vzajemnou spolupraci dvou kationt. .eleza ke .t.peni N2O molekuly. Takoveto uspo.adani nebylo zji.t.no jak u struktury Fe-BEA, tak i Fe-MFI. Dopovani Fe-FER vzacnymi kovy (Pt, Rh, Ru) zvy.ilo katalytickou aktivitu rozkladu N2O v tomto po.adi: Pt < Rh . Ru. Zvy.ena aktivita byla zp.sobena diky synergickemu efektu mezi .elezem a vzacnym kovem. P.idavek NO vyrazn. sni.il rozklad N2O na monometalickych vzorcich (Rh- a Ru-FER), ale nikoliv na vzorku Fe-FER a na bimetalickych Me/Fe-FER vzorcich. Opracovani katalyzatoru p.i teplot. 700 ??C (HTP) m.lo vyrazny vliv na zvy.eni konverze N2O pro Fe-zeolity, a zejmena pro Fe-FER, kde vliv byl tem.. srovnatelny s vlivem p.idavku NO. Na rozdil od monometalickych Me-FER vzork., u bimetalickych Me/Fe-FER vzork. do.lo k nar.stu aktivity po vysokoteplotnim opracovani. Synergicky efekt v rozkladu N2O nalezeny d.ive pro Rh/Fe-FER, ktery byl opracovan za nizke teploty byl take sledovan u vysokoteplotn. opracovaneho vzorku. Toto v.ak neplati pro Pt/Fe-FER a Ru/Fe-FER katalyzatory, u kterych zvy.eni aktivity po vysokoteplotnim opracovani bylo patrn. zp.sobeno pouze diky pozitivnimu vlivu na Fe slo.ku. Kombinaci t.chto dvou hlavnich p.istup. vedouci ke zvy.eni katalyticke aktivity Fe-zeolit. v rozkladu N2O, tj. dopovanim Fe-katalyzator. vzacnymi kovy a vysokoteplotnim opracovanim, byl p.ipraven HTP Rh/Fe-FER vykazujici ze studovane serie mono- a bimetalickych katalyzator. nejvy..i u.innost pro rozklad N2O. …víceméně