This section describes the method for determining the thermal reactivity coefficient(s) used for thermal heat load calculation in the accelerated aging protocol.

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(a) The calculations for thermal degradation are based on the use of an Arrhenius rate law function to model cumulative thermal degradation due to heat exposure. Under this model, the thermal aging rate constant, k, is an exponential function of temperature which takes the form shown in the following equation:

Where:A = frequency factor or pre-exponential factor.Ea = thermal reactivity coefficient in kJ/mol.R = molar gas constant.T = catalyst temperature in K.

(b) The process of determining Ea begins with determining what catalyst characteristic will be tracked as the basis for measuring thermal deactivation. This metric varies for each type of catalyst and may be determined from the experimental data using good engineering judgment. We recommend the following metrics; however, you may also use a different metric based on good engineering judgment:

(1) Copper-based zeolite SCR. Total ammonia storage capacity is a key aging metric for copper-zeolite SCR catalysts, and they typically contain multiple types of storage sites. It is typical to model these catalysts using two different storage sites, one of which is more active for NOX reduction, as this has been shown to be an effective metric for tracking thermal aging. In this case, the recommended aging metric is the ratio between the storage capacity of the two sites, with more active site being in the denominator.

(2) Iron-based zeolite SCR. Total ammonia storage capacity is a key aging metric for iron-zeolite SCR catalysts using a single storage site at 250 °C for tracking thermal aging.

(3) Vanadium SCR. Vanadium-based SCR catalysts do not feature a high level of ammonia storage like zeolites, therefore NOX reduction efficiency at lower temperatures in the range of 250 °C is the recommended metric for tracking thermal aging.

(4) Diesel oxidation catalysts. Conversion rate of NO to NO2 at 200 °C is the key aging metric for tracking thermal aging for DOCs which are used to optimize exhaust characteristics for a downstream SCR system. HC reduction efficiency (as measured using ethylene) at 200 °C is the key aging metric for DOCs which are part of a system that does not contain an SCR catalyst for NOX reduction. This same guidance applies to an oxidation catalyst coated onto the surface of a DPF, if there is no other DOC in the system.

(c)(1) Use good engineering judgment to select at least three different temperatures to run the degradation experiments at. We recommend selecting these temperatures to accelerated thermal deactivation such that measurable changes in the aging metric can be observed at multiple time points over the course of no more than 50 hours. Avoid temperatures that are too high to prevent rapid catalyst failure by a mechanism that does not represent normal aging. An example of temperatures to run the degradation experiment at for a small-pore copper zeolite SCR catalyst is 600 °C, 650 °C, and 725 °C.

(2) For each temperature selected, perform testing to assess the aging metric at different times. These time intervals do not need to be evenly spaced and it is typical to run these experiments using increasing time intervals (e.g., after 2, 4, 8, 16, and 32 hours). Use good engineering judgment to stop each temperature experiment after sufficient data has been generated to characterize the shape of the deactivation behavior at a given temperature.

(d) Generate a fit of the deactivation data generated in paragraph (b) of this section at each temperature using the generalized deactivation equation:

Where:? = aging metric.k = thermal aging rate constant for a given temperature.?EQ = aging metric at equilibrium (set to 0 unless there is a known activity minimum).m = model order (the model order should be set at the lowest value that best fits the data at all temperatures, minimum = 1).

(e) Using the data pairs of temperature and thermal aging rate constant, k, from paragraph (c)(2) of this section, determine the thermal reactivity coefficient, Ea, by performing a regression analysis of the natural log of k versus the inverse of temperature, T, in Kelvin. Determine Ea from the slope of the resulting line using the following equation:

Where:m = the slope of the regression line of ln(k) versus 1/T.R = molar gas constant.