applications in catalysis

8/11/2019 Applications in catalysis

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4.1 Catalytic NO Decomposition

The direct decomposition of NO into N2and O2has large negative Gibenergy and is regarded as one of the most ideal way of NO removal.

Perovskite oxides with an oxygen defect act as promising catalysts for

Oxygen vacancy provide space for NO adsorption.

The redox ability of B-site cation .

are two major factors influencing NO adsorption and activation.

NO-TPD experiment shows that

La2NiO4 with excess oxygen showed no adsorption capacity at lowtemperatures but showed at a temperature above 100 oC.

LaSrNiO4possesses an oxygen vacancy and showed good adsorpteven at room temperature and max. value observed at 200-250oC


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From the nature of the reaction, it is known that NO decomself-redox reaction as no oxidizing or reducing agent is add

The catalyst should account for oxidation and reduction prhence the redox properties of the catalyst should be cruciaparameter.

By measuring the redox potential of two series of Perovskite

La2-xSrxCuO4(x=0,0.5,1) and LaSrMO4 (M= Co,Ni,Cu)

it was observed that the redox potentials is the most cruciaparameter and number of oxygen vacancy is the next infparameter.


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4.2 Catalytic NO Reduction

Although NO direct decomposition is an ideal way of NO removal, Its o

practice however encounters problems due to kinetics limit as well as inhibition by oxygen

Poisoning by other coexisting gases such as CO2, SO2, and H2O .

For practical application, catalytic NO reduction is preferred.

Perovskite oxides are also good catalysts for catalytic NO reduction, usNH3, hydrocarbons or H2as reducing agent.

The presence of reducing agent compensates for the inadequacy of recapacity and mitigates dependence of activity on redox potentials of catalysts.

The type of B-site cation also is a crucial parameter .The redox propertyhas a significant effect on the NO adsorption and the oxygen mobilizatgenerating electrons.


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Using NH3 Catalytic reduction of NO or NO2 by NH3is currently the most effective

technology for NOx removal .

One of the main advantages of using NH3 as a reducing agent is thatreduce NOx with high selectivity even in oxygen-rich atmosphere.

The commercial catalyst V2O5-WO3/TiO2 although shows satisfactory pfor the reaction but problem is the toxicity of vanadia.

Using H2 The reduction of NO by H2is also a promising route for NO removal.

As Hydrogen being a clean reducing agent will not produce secondar

The reduction occurs at lower temperatures than using CO,CxHyand N


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4.3 Catalytic NO oxidation

For removing NO ,NO is oxidized to NO2before reacting with reducing CxHyor NH3.

The ability to oxidize NO into NO2could be a prerequisite in optimizing

Traditional precious metals like platinum are active for NO oxidation buexpensive and prone to aggregate.

Investigation of catalytic performance of LaMeO3(Me = Mn,Fe,Co) Peprepared by sol-gel method found that LaCoO3with stronger reducibil

efficient.

Further studies showed that the nonstoichiometry of Perovskite oxide Lainfluenced the NO oxidation activity.

For e.g. La0.9MnO3with richer Mn atoms and higher Mn4+/Mn3+ratio sho

catalytic performances at lower temperature due to easily regenerateassociated with Mn4+ .


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Besides these, the synthesis conditions are also important in preparing Poxides for NO oxidation.

Samples prepared with faster heating rate during the calcination procevarious impurities, lowered the surface oxygen and caused particle agleading to lower activity.

Consequently, the synthesis with low heating rate conditions is suggeste

It was observed that the activity of LaCoO3can be significantly improv

was doped for La (La0.9Sr0.1CoO3) but there was no improvement in La0


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It is well-known that the oxygenoxygen recombination (2O* = O2+ 2*limiting step in N2O decomposition, it is therefore essential to remove th

adsorbed oxygen immediately after its formation to allow the reaction Surface adsorbed oxygen can be separated using Perovskite membra

ensures removal of surface adsorbed oxygen by reducing agent such

The active site can be rapidly regenerated because of this removal reimproved activity.


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4.5 Catalytic Complete Oxidationof CO and CH4 The oxidation of CO and CH

4represents two surface interactive proces

conducted on Perovskite oxides.

The former is suprafacial, in which the catalyst surface provides a set oforbitals of proper symmetry and energy for bonding of reactant and in

The latter is intrafacial process, in which the catalyst participates as a repurely consumed and regenerated.

From the molecular formula, it is known that the oxidation of CO into Ccapture one external atomic oxygen without rupturing the C-O bond.

Oxygen vacancy of the Perovskite oxides has the capability of splittingmolecular oxygen into atomic oxygen and CO is good reducing agenreact with atomic oxygen.

The material with more oxygen vacancy should benefit the reaction.


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The substitution of Sr2+for La3+in La2xSrxCuO4could lead to a significanthe number of oxygen vacancy and hence increases the CO oxidatio

high temperatures. Besides the number of oxygen vacancy, the redox properties of B-site c

have significant influence on the activity.

the Fe containing samples were substantially more active than LaAlO3,to the essential role of the redox properties of transition metal ion in dehighly active catalysts.

It was found that there existed a dilution effect and concluded that isoions are more active than iron species surrounded by oxygen bound toions.

Optimization of B-site transition metals, based on the series LaCo0.5M0.5OCr, Fe, Ni, Cu), suggested that the composition LaCo0.5Mn0.5O3is the mfavorable.


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In addition to the natural properties of catalyst such as oxygen vacancy properties of B-site cation, the surface crystallinity was also reported to bethe reaction.

It was found that LaCoO3prepared using ethylene glycol and citric acidpossessing low surface area, showed better activity to that prepared usiglycol only, as a result of its better surface crystallinity.

Relative to CO oxidation, the dependence of CH4oxidationon the oxygen vacancy is weak, as it belongs to differenttypes of reactions (suprafacial vs intrafacial).

For CH4 oxidation the CH bond was ruptured before reactingwith the adsorbed oxygen, which required the catalyst toparticipate in as a reagent that was partly consumed and

regenerated in a series of continuous redox cycles.

applications in catalysis

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