Ce‐Pr Mixed Oxide Catalysts with a Fibrous Morphology for Low‐temperature PM Oxidation

Abstract

Ceria (CeO2) is an effective precious‐metal‐free catalyst that combusts PM (particulate matter), due to its ability to switch between Ce4+ and Ce3+. In this work, to improve the activity of a ceria‐based catalyst, cerium‐praseodymium mixed oxide catalysts with various Ce−Pr ratios were synthesized with nanofiber morphologies, characterized, and compared with ceria catalysts. Two factors were considered: morphology and Ce−Pr ratio, which result in synergistic effects. In the ceria catalyst, we confirmed that a fibrous morphology is more advantageous for PM oxidation compared to nanocubes and nanorods with cubes, even though these samples have larger surface areas. In addition, the incorporation of Pr into the ceria nanofiber catalyst enhanced its intrinsic properties by promoting the reducibility and formation of oxygen vacancies. The analysis showed improved oxygen storage and supply capacity, especially for oxygen that is chemically adsorbed on the catalytic surface. PM oxidation tests demonstrated the Ce0.5Pr0.5O2‐NF catalyst showed the best activity in air, which was consistent with the characterization results. Thus, the Ce0.5Pr0.5O2‐NF sample was shown to be an effective and promising catalyst for PM oxidation.

Study of Ce/Pr ratio in ceria-praseodymia catalysts for soot combustion under different atmospheres

“…Rico-Pérez et al investigated the effect of copper incorporation to a ceria-praseodymia catalyst, maintaining a Ce/Pr ratio (Ce0.5Pr0.5O2-δ), towards NO oxidation to NO2 and soot oxidation [52], similarly to the investigation of Andana et al, who incorporated Pt nanoparticles [53]. Some other articles published studied the soot combustion process catalyzed by some ceria-praseodymia formulations [54,55]. However, some considerations about these articles reported by Andana et al [54] and Jeong et al [55] should be mentioned: i) the corresponding authors did not study the NO oxidation to NO2 behaviors of these samples, which is of key importance to understand their catalytic activity towards soot combustion under NOx atmosphere; ii) they did not include a praseodymiumrich sample and a pure praseodymium oxide, in order to investigate the whole range of compositions, since their highest Pr atomic content is 0.5; iii) they used a soot/catalyst mass ratio of 1/9 or 1/10, respectively, which is not adjusted to realistic conditions, where soot/catalyst ratios are usually higher, since soot particles deposited onto a real catalytic device are in contact with a limited number of catalytically active sites; iv) they did not consider the effect of calcination temperature in order to study the effect of some high temperature peaks inside the catalytic device, since DPF can achieve 1000-1100ºC due to the highly exothermal soot combustion reaction [23].…”

“…Some other articles published studied the soot combustion process catalyzed by some ceria-praseodymia formulations [54,55]. However, some considerations about these articles reported by Andana et al [54] and Jeong et al [55] should be mentioned: i) the corresponding authors did not study the NO oxidation to NO2 behaviors of these samples, which is of key importance to understand their catalytic activity towards soot combustion under NOx atmosphere; ii) they did not include a praseodymiumrich sample and a pure praseodymium oxide, in order to investigate the whole range of compositions, since their highest Pr atomic content is 0.5; iii) they used a soot/catalyst mass ratio of 1/9 or 1/10, respectively, which is not adjusted to realistic conditions, where soot/catalyst ratios are usually higher, since soot particles deposited onto a real catalytic device are in contact with a limited number of catalytically active sites; iv) they did not consider the effect of calcination temperature in order to study the effect of some high temperature peaks inside the catalytic device, since DPF can achieve 1000-1100ºC due to the highly exothermal soot combustion reaction [23]. Considering these premises, the present investigation deals with the preparation and in-depth characterization of two series of CexPr1-xO2-δ catalysts (with x = 0, 0.2, 0.5, 0.8 and 1), thus ranging the whole Ce/Pr ratios, calcined either at 500ºC or 1000ºC, in order to check their catalytic activities after being exposed to high temperatures such as 1000ºC.…”