1. Diesel Engine Catalyst Deactivation/Ageing
LAT
Durability Task Force
Diesel Engine Catalyst Deactivation/Ageing
TelCo, 15 November 2017

2. Introduction Diesel engine aftertreatment devices
DOC
DPF/cDPF
LNT/SCR/PNA
ASC  used in combination with SCR
Modes of catalyst deactivation
Thermal deactivation
Chemical deactivation
For SCR: It is unclear if hydrothermal ageing effectively replicates the typical “on-road” ageing conditions 2

3. SCR Thermal Ageing
SCR suffers from thermal aging  decrease of active sites
High temperature from DPF regeneration affects SCR  thermal durability is required
Zeolite based base metal (e.g. Cu, Fe) catalysts are deactivated during high- temperature DPF regeneration
Hydrothermal ageing at high temperatures (700⁰C) leads to complete collapse of the zeolite structure
The front of the SCR is exposed to higher temperatures than the rear, it is expected that the front of the SCR catalyst will be more degraded than the rear
When ageing at 750℃, washcoat cracking and the beginning of delamination from the cordierite is visible  The damage to the washcoat is more prominent in the front section of the catalyst than the rear 3

4. SCR Poisoning
Base metal (Fe, Cu)/zeolite SCRs are sensitive to sulphur poisoning
Even with the use of ultra low sulfur fuel, sulfur poisoning is still a durability issue for base metal/zeolite SCR catalysts.
Sulfur poisoning impact is affected by the thermal ageing status
The most thermally durable SCR catalyst is not necessarily the most durable to sulfur poisoning
Two major sulfation modes are most likely involved
Formation of (NH4)2SO4 (ammonium sulfate) or (NH4)HSO4 (Ammonium bisulfate, ammonium hydrogen sulfate)
Formation of CuSO4 (copper sulfate)
NOx activity of Cu/zeolites is significantly reduced for samples poisoned by SO3 compared to those poisoned by SO2
Sulfur poisoning can be an important ageing mechanism for SCR systems
Pt (contained in DOC) contamination in the front section of the SCR catalysts is responsible for the severe decrease in NOx reduction performance  the precious metal is volatilized and deposits on the cooler SCR – Pt deposits lead to NH3 oxidation 4

5. Indicative ageing effects on SCR/SCRF performance.
Exposure of metal-exchanged-zeolites based SCR catalysts to temperatures higher than 500°C and water vapor causes irreversible modifications of catalysts properties, leading to permanent deactivation.
Destruction of the zeolite structure due to dealamination
Aggregation of isolated metallic species to inactive metal oxide particles
Poisoning by sulfur
Sulfation of SCR catalysts negatively affects SCR of
NOx, particularly at low temperatures (T<300°C).
By performing lean heat-up of sulfated catalysts S is
removed and deNOx is restored.
It should be noted that in the case of SCRFs
ash accumulation will further limit the catalytic
performance since the active surface area of the
filter is reduced and space velocity is increased.
Fresh
Aged at 650°C
The chemical removal of alumina or aluminates from a material.
Aged at 750°C
Aged at 850°C

6. LNT
Sulfur effect is comparable to hydrothermal ageing – for high S concentrations
Significant reduction in NH3 production from the LNT – important in combined LNT-SCR systems
Although most effects are reversible (following a desulfation process), in cases of poisoning with high S concentrations the recovery of the system may not be full
The chemical removal of alumina or aluminates from a material.

7. LNT Hydrothermal aged at 800⁰C for 5h under 10% O2, 10% H2O, and N2
Vehicle aged for 80,000km with 10ppm S fuel.
Total desulfation of 4h at maximum temperature 780⁰C
The chemical removal of alumina or aluminates from a material.

8. Combined LNT-SCR Systems
Combined LNT-SCR systems to increase NOx conversion
NH3 production in LNT used in SCR
Significant effect of hydrothermal ageing – the Pt particles grow (sintering) and the reaction activity is reduced
Sulfur poisoning significant at high S concentrations – desulfation did not restore fully the system
The chemical removal of alumina or aluminates from a material.

9. Combined LNT-SCR Systems
Combined LNT-SCR systems to increase NOx conversion
NH3 production in LNT used in SCR
Significant effect of hydrothermal ageing – the Pt particles grow (sintering) and the reaction activity is reduced
Sulfur poisoning significant at high S concentrations – desulfation did not restore fully the system
The chemical removal of alumina or aluminates from a material.

10. Ash deposition and mechanical ageing
Ash deposition represents the most important degradation mechanism for particle filters (Diesel Particulate Filter, Gasoline Particulate Filter), together with soot plugging (from incomplete regeneration) for DPF.