Oxygen Vacancies in Reduced Rh/ and Pt/Ceria for Highly Selective and Reactive Reduction of NO into N₂ in excess of O₂

Abstract

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.

A new dynamic N2O reduction system based on Rh/ceria–zirconia: from mechanistic insight towards a practical application

“…were co-fed over C 3 H 6 reduced Rh/CLZ at 450°C. 21 The current study presented in Fig. 4A suggested that N 2 O was comparably reactive towards ceria oxygen vacancies as NO did, and therefore, N 2 O was a more reactive and competitive reactant towards the oxygen vacancies as compared to O 2 .…”

“…Characterization details of the CLZ support and Rh/CLZ catalyst were reported in detail elsewhere. [16][17][18][19][20][21] In brief, a 0.5 wt% Rh loading was confirmed by ICP-OES (0.0486 mmol g cat…”

“…Details on the characterization and instruments can be found in our previous publications. [16][17][18][19][20][21] 2.2. Catalytic testing 2.2.1.…”

“…15 The comprehensive work by Wang and Makkee has addressed the working principle and application of this Di-Air system in NO reduction. [16][17][18][19][20][21] Oxygen vacancies within the ceria lattice of a reduced ceria, Pt/ceria or Rh/ceria were found to be the selective catalytic sites for the NO reduction into only N 2 (100% selectivity). 16 Even at low NO concentrations (ppm levels), NO could compete for oxygen vacancies with (100×) excess of O 2 and CO 2 .…”

“…16 Even at low NO concentrations (ppm levels), NO could compete for oxygen vacancies with (100×) excess of O 2 and CO 2 . 21 These oxygen vacancies acted as a kind of "oxygen black hole" by catching all oxygen containing species until the holes (vacancies) were completely refilled (re-oxidized), while the captured N species would associate (recombine) into N 2 . In the Di-Air system, the creation of reduced (noble metal) ceria was accomplished by pulsing diesel fuel at a high frequency upstream of the catalyst bed.…”