Temporal analysis of product (TAP) is used to investigate the effectiveness of CO, C3H6, and C3H8 in the reduction of a La–Zr doped ceria catalyst and NO reduction into N2 over this pre-reduced catalyst. Hydrocarbons are found to be substantially more effective in the reduction of this catalyst at high temperature (above 500 °C) as compared to CO. NO decomposes over oxygen anion defects created upon catalyst reduction. Deposited carbon, in case the catalyst is reduced by C3H6 or C3H8, acts as a delayed or stored reductant and is not directly involved in NO reduction. Instead the oxidation of deposited carbon by an oxygen species derived from lattice oxygen (re)creates the oxygen anion defects active in NO reduction. In situ Raman, in which NO is flown over C3H6 pre-reduced La–Zr doped ceria at 560 °C, additionally shows that re-oxidation of the La–Zr doped ceria catalyst starts prior to the oxidation of deposited carbon, which confirms our TAP findings that firstly NO re-oxidized the La–Zr doped ceria catalyst and that secondly the oxidation of deposited carbon only commences at a higher ceria oxidation state. These findings create a new perspective on the operating principle of Toyota’s Di-Air system.@en

Currently commercial NOx removal (DeNOx) abatement systems for lean-burn engines exceed regulation limits on the road for NOx emissions. Commercial DeNOx catalysts exhibit poor performance in the selective conversion of NO to N2, especially at high temperature and high gas hourly space velocities (GHSV). In this study, oxygen vacancies of reduced ceria and Pt/ or Rh/ceria are found to be the efficient and selective catalytic sites for NO reduction to N2. Even at low concentrations, NO can compete with an excess of O2 at 600 °C and a high GHSV of 170 000 L L−1 h−1, conditions in which SCR and NSR DeNOx system are not able to function well. N2O is not detected over the whole range of conditions, whereas NO2 is only formed upon oxidation of the catalyst, after both NO and O2 start to appear. For consideration of the fuel economy, the working temperature should be between 250 and 600 °C. Above 600 °C, most of the injected fuel was combusted with O2. Below 250 °C, ceria support will not be reduced by fuel and the oxidation rate of the deposited carbon through oxygen from ceria lattice will be too low.@en

Oxygen defects in reduced ceria are the catalytic sites for the NO reduction into N2 in the Toyota Di-Air DeNOx abatement technology. Traces of NO (several hundred ppm) have to compete with the excess amount of other oxidants, e.g., 5% CO2 and 5% O2, in an exhaust gas of a lean burn (diesel) engine. The reactivities of CO2 and NO over a reduced ceria and noble metal loaded reduced ceria have been investigated under ultra-high vacuum system in TAP and under atmosphere pressure in in-situ Raman and flow reactor set-up. The results showed that CO2 was a mild oxidant which was able to oxidise the oxygen defects, but hardly oxidised deposited carbon over both ceria and noble metal loaded ceria. NO was a stronger oxidant and more efficient in refilling the oxygen defects and able to convert the deposited carbon, which acted as buffer reductant to extend the NO reduction time interval. NO was selectively and completely converted into N2. The presence of excess CO2 hardly affected the NO reduction process into N2.@en

In this study, the role of the noble metals Pt and Rh (0.5 wt.%) for the selective reduction of NO into N2 is evaluated by the transient TAP technique and in-situ spectroscopy using a commercial stable ceria support (denoted as CZ) and applying isotopically labelled 15NO and 18O2. The transient operation was mimicked by multi-pulse oxidation (using O2 or NO) and reduction cycles (using CO, H2, C3H6 and C3H8), while following quantitatively the catalyst and reactants response. Pt and Rh significantly lowered the temperature of CZ reduction. CO and H2 only reduce the surface of CZ, while a 2.5 times deeper reduction was achieved by the hydrocarbons C3H6 and C3H8, removing also lattice oxygen. Pt and Rh also promoted carbon deposition after surface reduction. Rh was a more active promoter than Pt, while propene was more reactive than propane over both metals. During the NO reduction the pre-reduced CZ support became gradually re-oxidised and after filling 70–80% of the oxygen vacancies the NO started to appear in the product mixture. In the presence of carbon deposits the lattice oxygen of the CZ reacted with the carbon keeping the CZ in a reduced state, extending the NO decomposition process as long as the carbon was present. The reduction of NO over pre-reduced noble metal/CZ showed a selective formation N2, while N2O and NO2 were never observed. During the NO reduction process some unidentified N-species remained on the catalyst, the amount depending on the type of catalyst, but finally all nitrogen was released as N2. The presence of the noble metal led less unidentified N-species on the CZ surface and to a faster N2 formation rate than that over the bare CZ.@en

Nitrogenoxides (NOx, including NO and NOx) are a group of hazardous, toxic and harmfulgasses, which have an adverse effect on both environment and human health,e.g., acid rain, photochemical smog, and affecting the human respiratorysystem. The NOx concentration in most of the EUcities exceeds the EU annual limit value (40 μg/m3). Around40% of the emitted NOx is attributedto transport related emissions. In currently applicable Euro 6, the real NOx emissionfrom a diesel car is on average 400% than the Euro 6 regulation limit allows ifmeasured under more realistic driving conditions. Although NSR and SCR DeNOx systemshave been broadly investigated and commercially applied with the aim to reduceNOx emissions from lean burn engines,some common problems still exist, e.g., a narrow temperature window and a lowgas hourly space velocity (up to 50.000 L/L/h)in order to convert the NOx selectivelyinto Nx. Due to the in practice high NOxemissionfrom September 2017 additional legislation will be in force to arrive at a morerealistic determination of the highly dynamic NOxemissionby among others the introduction of the real driving emission (RDE) test in thecertification procedure. The Di-Air (Diesel NOxaftertreatment by Adsorbed Intermediate Reductants) system was developed by Toyota(2011-2012) and is still under development. This Di-Air system showed promiseby yielding a high NOx conversion,especially at high temperature (up to 600 ∘C) and high gas hourly space velocity (up to 125.000 L/L/h).This system opts to meet the future stringent NOxreductionrequirements under RDE test conditions (Chapter 1)… @en

Toyota's Di-Air DeNOx system is a promising DeNOx system to meet NOx emission requirement during the real driving, yet, a fundamental understanding largely lacks, e.g. the benefit of fast frequency fuel injection. Ceria is the main ingredient in Di-Air catalyst composition. Hence, we investigated the reduction of ceria by reductants, e.g. CO, H2, and hydrocarbons (C3H6 and C3H8), with Temporal Analysis of Product (TAP) technique. The results show that the reduction by CO yielded a faster catalyst reduction rate than that of H2. However, they reached the same final degree of ceria reduction. Hydrocarbons generated almost three times deeper degree of ceria reduction than that with CO and H2. In addition, hydrocarbons resulted in carbonaceous deposits on the ceria surface. The total amount of converted NO over the C3H6 reduced sample is around ten times more than that of CO. The deeper degree of reduction and the deposition of carbon by hydrocarbon explain why hydrocarbons are the most powerful reductants in Toyota's Di-Air NOx abatement system.@en

The influence of potassium in Rh on a lanthium promoted zirconia stablised ceria (CZ) catalysts was studied toward NOxreduction reactivity and selectivity. The results are compared with a Rh/CZ catalyst. The samples were characterised by N2 adsorption, XRD, SEM, ICP, and H2-TPR. The study highlighted the importance of stored NOx regeneration over potassium in determining the overall performance of the Rh/K/CZ catalyst. The NOx stored over Rh/K/CZ in the previous NO gas stream cannot be regenerated sufficiently during the C3H6 gas stream, and stored NOxgradually decreased from one cycle to the next, resulting in deteriorating performance of Rh/K/CZ. Besides, problem of NOx slip, the formation of both NH3 and N2O (selectivities up to 30% for each side product) were observed by the addition of potassium into the Rh/CZ catalyst system, depending on the reaction conditions applied and the severity of the catalyst deactivation.@en

Simultaneous reduction of N2O in the presence of co-existing oxidants, especially NO, from industrial plants, is a challenging task. This study explores the applications of a hydrocarbon reduced Rh/Zr stabilized La doped ceria (Rh/CLZ) catalyst in N2O abatement from oxidant rich industrial exhaust streams e.g. NO, CO2, and O2. The reaction mechanism was studied by the temporal analysis of products. The obtained results revealed that hydrocarbon pretreatment led to the creation of ceria oxygen vacancies and the formation of carbon deposits on the Rh/CLZ catalyst surface. These ceria oxygen vacancies are the active sites for the selective reduction of N2O into N2, while the dissociated O atoms from N2O fill the ceria oxygen vacancies. The oxidation of the deposited carbon via the lattice ceria oxygen generates new ceria oxygen vacancies, thereby extending the catalytic cycle. The reduction of N2O over C3H6 reduced Rh/CLZ is a process combining oxygen vacancy healing and deposited carbon oxidation. The results obtained from fixed-bed reactor experiments demonstrated that the hydrocarbon reduced Rh/CLZ catalyst provided a unique and extraordinary N2O abatement performance in the presence of co-existing competing oxidants (reactivity order: N2O ∼ NO > O2 > CO2 ∼ H2O).@en

We studied the mechanism of NO reduction as well as its selectivity and reactivity in the presence of excess O2. Results show that fuel injection and/or pretreatment are important for ceria catalyst reduction and carbon deposition on the catalyst surface. Oxygen defects of reduced ceria are the key sites for the reduction of NO into N2. The deposited carbon acts as a buffer reductant, i.e., the oxidation of carbon by lattice oxygen recreates oxygen defects to extend the NO reduction time interval. A small amount of NO showed a full conversion into only N2 both on the reduced Zr-La doped ceria and reduced Pt-Zr-La doped ceria. Only when the catalyst is oxidised NO is converted into NO2.@en