A regenerative aging process for a catalyst, and a program based on such a process

Abstract

The invention relates to a regenerative aging process for a catalyst under a controlled atmosphere, comprising: - a heating step (1) of the catalyst to a target temperature in the presence of at least one aging gas on the catalyst; - an aging step (2) in which an aging profile is applied at a stable temperature or by varying the temperature; - a regenerative reduction step (3) in which at least one reducing gas is applied to the catalyst during the catalyst's cooling; - a temperature reduction step (4) of the catalyst. The invention also relates to a program based on such a vehicle. Figure 6

Description

Title of the invention: METHOD FOR REGENERATIVE AGING OF A CATALYST, AND A PROGRAM BASED ON SUCH A METHOD

[0001] The invention relates to the field of automotive catalysis and more specifically to the aging of catalysts within the context of the design, integration, and validation of emissions control systems. This is an important activity for correctly selecting the catalytic converter that will ensure compliance with regulatory emission standards.

[0002] To perform aging tests on catalysts and verify / validate that the emissions control system is capable of meeting the regulatory durability standard (80,000, 100,000, 160,000, 200,000, or even 240,000 km), it is possible to conduct actual driving tests (vehicle endurance) or to use a faster and more practical approach called "accelerated aging." These "accelerated" aging tests can be carried out using an engine test bench, a burner, or an oven fueled by synthetic gases (this is then referred to as "controlled atmosphere aging"). The latter has the advantage of being faster and more reproducible than other aging methods. Despite some opinions regarding its lack of representativeness, all manufacturers use it for their so-called "OBD" aging tests. It is accelerated oven aging that is the subject of the present invention.

[0003] The main drawback of these oven-based aging processes under controlled atmosphere is their long-term stability (aging processes carried out on an engine test bench, in particular, are more stable over time but much longer). Indeed, depending on the gas composition and the air-fuel ratio used when the aged part is tested on the vehicle or on an engine test bench, its catalytic activity can change. The origin of this change, which generally improves the part's performance, is the modification of its active surface, known as the "washcoat." This modification is all the more problematic because it continues over time, with test after test making the calibration and / or validation of the exhaust system complicated, time-consuming, and sometimes unreliable.

[0004] US6378359B1 describes a method for normalizing an emission threshold catalyst by placing it in an exhaust gas circulation furnace after furnace aging where it is saturated with exhaust gas at a predetermined flow rate and temperature for a specified time.

[0005] This solution uses a "burner" to generate "exhaust gas", but does not offer regeneration of the catalyst.

[0006] An objective of the present invention is to remedy the defects of the prior art, and in particular to propose a solution enabling the regeneration of the catalyst to avoid its instability after thermal aging in the oven in a controlled atmosphere (which entails additional costs and delays because the calibration carried out on these parts must be repeated); limit the impact of thermal aging on the quality of calibrations; and avoid the systematic and sometimes insufficient recourse to the long and tedious vehicle "de-greening" procedure.

[0007] To achieve this objective, the invention proposes a regenerative aging process for a catalyst under a controlled atmosphere, comprising: - a step of raising the temperature of the catalyst to a target temperature in the presence of at least one aging gas on the catalyst; - an aging stage in which an aging profile is applied at a stable temperature or by varying the temperature; - a regenerative reduction step in which at least one reducing gas is applied to the catalyst during the cooling of the catalyst; - a step involving lowering the temperature of the catalyst.

[0008] Advantageously, the invention offers a combined aging and regeneration solution for the catalyst, unlike the prior art.

[0009] This is a process for stabilizing heat-aged catalysts in a controlled atmosphere under controlled conditions, allowing their use without risk of their catalytic efficiency changing over time or depending on the nature of subsequent tests. These aged components can be used a large number of times (> 40 WLTC cycles, for example) without any degradation in their catalytic activity. This eliminates the need for a lengthy exhaust system regeneration procedure on vehicles, known as "burnout," prior to emissions testing.

[0010] Preferably, said reducing gas comprises dihydrogen.

[0011] This makes it possible to effectively restore the properties of the catalyst.

[0012] Preferably, the reduction step is carried out in several phases at different temperatures for different durations.

[0013] This allows the properties of the catalyst to be restored even more effectively.

[0014] Preferably, the reduction step is carried out in three phases, among which: - a first phase between 800 and 1200°C, preferably at 1000°C under an atmosphere of carbon monoxide between 1 and 4%, preferably 3%, and of dihydrogen between 0% and 1.5%, preferably 1%, for 30 to 50 min, preferably 40 min; - a second phase between 700 and 900°C, preferably at 800°C under said atmosphere of carbon monoxide and dihydrogen, for 10 to 30 min, preferably 15 min; and - a third phase between 500 and 700°C, preferably at 600°C under said atmosphere of carbon monoxide and dihydrogen, for 10 to 30 min, preferably 15 min.

[0015] This allows for catalyst regenerations, the tests of which have been conclusive.

[0016] Preferably, the temperature reduction step is carried out in the presence of nitrogen or oxygen.

[0017] This helps to stabilize the catalyst.

[0018] Another object of the invention relates to a computer program comprising program code instructions for the execution of the steps of the aging process according to the invention, when said program is running on a computer.

[0019] The invention will be further detailed by describing non-limiting embodiments, and based on the accompanying figures illustrating variants of the invention, including: - [Fig.l] schematically illustrates hot efficiency synthetic gas bench tests carried out at 450°C after simple thermal aging (evolution of efficiency test after test); - [Fig.2] schematically illustrates vehicle emissions (CO, HC, NOx) after simple thermal aging (evolution of emissions as a function of mileage traveled); - [Fig.3] schematically illustrates hot synthetic gas efficiency bench tests carried out at 450°C after thermal aging followed by the regeneration phase in a furnace (stability of efficiency test after test); - [Fig.4] schematically illustrates vehicle emissions (CO, HC, NMHC and NOx) produced after thermal aging followed by the regeneration phase in a furnace (stability of emissions between the results after 1 cycle and 4 cycles); - [Fig.5] schematically illustrates oxygen storage levels in mg / L at different temperatures before and after catalyst regeneration; - [Fig.6] schematically illustrates an example of a temperature and gas richness profile of thermal aging followed by the specific regeneration phase in the furnace.

[0020] In order to age the catalysts and verify / validate that the pollution control system complies with the "emissions" standard in terms of regulatory durability (80,000, 100,000 or even 160,000 km), it is possible to conduct actual driving tests or use a faster approach called "accelerated aging". These aging processes "Accelerated" can be achieved using an engine test bench, a burner or an oven supplied with synthetic gases (this is called a controlled atmosphere).

[0021] This latter method, carried out within the framework of the invention, has the advantage of being faster and more reproducible than other aging methods. Unfortunately, while it allows us to age / degrade the catalytic performance of the part as we see fit (in particular via an "emissions" target that we set to meet the intended "emissions" standard), it is not capable, on its own, of stabilizing it.

[0022] Indeed, during the aging of the catalyst, its active surface, called the "washcoat," is modified. The sintering phenomenon (or coalescence of precious metal crystallites that ensure the conversion of pollutants into inert species) is the main reason for the degradation of catalytic activity, as it reduces the surface area provided by the precious metals of the catalyst (Platinum, Palladium, or Rhodium).

[0023] Beyond its thermal degradation, the active phase, also composed of oxides such as Cerium oxides or Zirconium oxides, is oxidized by oxidizing species (in particular dioxygen) generally contained in the gases used for thermal aging.

[0024] Therefore, in order to avoid the phenomenon of reduced active phase occurring during vehicle or engine bench tests, which leads to a decrease in catalytic performance, it is necessary to add a reduction phase following oven aging. Otherwise, this stabilization will have to be ensured by a long "burning" phase on the vehicle.

[0025] This oven stabilization phase includes a triple regeneration under reducing conditions preferably at 3% CO + 1% H2 at different temperatures preferably 1000, 800 and 600°C, and for different durations preferably 40, 15 and 15min carried out during the temperature descent.

[0026] The results observed on the synthetic gas bench (SGB) or on a vehicle show that without this regeneration phase, the activity changes from one test to the next (see Figures 1, 2), whereas by adding this specific phase at the end of thermal aging, the activity no longer changes (see Figures 3, 4). The reference "CO-NO p" designates the content of disturbed carbon monoxide and nitrogen monoxide, "HC p" designates the content of disturbed hydrocarbons, and "CV" designates the conversion. In the case of [Fig. 3], the tests are essentially indistinguishable, unlike those of [Fig. 1] where the conversion decreases.

[0027] This approach is robust: - whatever the nature of the thermal aging; - regardless of the catalyst technology used, since the vehicle tests were carried out with a different technology than that used for the synthetic gas bench tests; - regardless of the quantity of precious metals used.

[0028] The graph in [Fig. 5] also confirms the good stabilization of thermal aging by furnace regeneration (curve +RG) via the measurement of the oxygen storage capacity (OSC) level in the catalyst. This is an important criterion used to monitor the condition of the catalyst, in particular. Whether or not regeneration or "greening" has been carried out, the OSC level as a function of the gas temperature (T°) is indeed stable thanks to the regeneration performed in the aging furnace.

[0029] This approach is therefore robust on a synthetic gas test bench and a roller test bench (vehicle) in terms of CO, HC, NOx emissions and OSC levels, regardless of the nature of the thermal aging, the catalyst technology used, and the targeted precious metal content. Furthermore, this approach avoids the time-consuming vehicle "greening" phase prior to vehicle emissions measurements on the roller test bench.

[0030] The steps of the aging process according to the invention are as follows (illustrated in [Fig.6]): - 1°) Temperature increase T° up to the desired temperature for aging the catalyst (in the presence of nitrogen and possibly a reaction gas mixture); - 2°) Creation of the aging profile (stable or on the contrary with variations in temperature and / or gaseous composition); - 3°) Start of the regeneration phase, preferably containing 3 parts: - a first phase RG1 at 1000°C under CO (3%) and H2 (1%) for 40min; then - a second phase RG2 at 800°C under CO (3%) and H2 (1%) for 15min; then - a third phase RG3 at 600°C under CO (3%) and H2 (1%) for 15min; - 4°) Descent to ambient temperature under nitrogen (N2).

[0031] According to other variants of the invention, a similar process can be envisaged with a modification of the duration of the regeneration phases RG1, RG2, RG3. A priori, by increasing them, the stability of the active surface would be maintained.

[0032] Similarly, it is possible to consider modifying the composition of the different regeneration phases RG1, RG2, RG3, by increasing the content of CO and / or H2, the stability of the active surface would be maintained.

[0033] Furthermore, it is also possible to consider modifying the temperature levels of the last two stages (lowering or raising them), or replacing the nitrogen (N2) of the cooling phase with oxygen or a reaction gas mixture.

[0034] Of course, a process similar to that described above can be envisaged, in which water is added during the regeneration phase(s).

[0035] The invention further relates to a control program implementing a method as described above. The program can control, in particular, a furnace and one or more devices for introducing one or more gases.

[0036] Regarding the advantages of the invention, they include in particular: - a time saving for aging tests carried out in the oven compared to those carried out on the engine bench or burner and especially in real endurance, in addition to the elimination of the vehicle degreasing phase (duration = IhlOmin); - a lower production of parts to carry out the activity without needing to redo the calibrations carried out because the part would have degraded in the meantime; - an improvement in the quality / robustness of the calibrations and validations carried out on the aged oven catalysts.

Claims

Demands

1. A regenerative aging process for a catalyst under a controlled atmosphere, comprising: - a heating step (1) of the catalyst to a target temperature in the presence of at least one aging gas on the catalyst; - an aging step (2) in which an aging profile is applied at a stable temperature or by varying the temperature; - a regenerative reduction step (3) in which at least one reducing gas is applied to the catalyst during the cooling of the catalyst; - a temperature reduction step (4) of the catalyst.

2. Aging process according to claim 1, characterized in that said reducing gas comprises dihydrogen.

3. Aging process according to any one of claims 1 to 2, characterized in that the reduction step (3) is carried out in several phases at different temperatures for different durations.

4. Aging process according to claim 3, characterized in that the reduction step (3) is carried out in three phases (RG1, RG2, RG3), among which: - a first phase between 800 and 1200°C, preferably at 1000°C under an atmosphere of carbon monoxide between 1 and 4%, preferably 3%, and of dihydrogen between 0% and 1.5%, preferably 1%, for 30 to 50 min, preferably 40 min; - a second phase between 700 and 900°C, preferably at 800°C under said atmosphere of carbon monoxide and dihydrogen, for 10 to 30 min, preferably 15 min; and - a third phase between 500 and 700°C, preferably at 600°C under said atmosphere of carbon monoxide and dihydrogen, for 10 to 30 min, preferably 15 min.

5. Aging process according to any one of claims 1 to 4, characterized in that the temperature reduction step (4) is carried out in the presence of nitrogen or oxygen.

6. Computer program comprising program code instructions for performing the steps of the aging process according to any one of claims 1 to 5, when said program is running on a computer.

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