Transient tests to assess the Cu speciation in Cu-CHA catalysts: investigating hydrothermal aging effects

Umberto  Iacobone, Politecnico di Milano

Transient tests to assess the Cu speciation in Cu-CHA catalysts: investigating hydrothermal aging effects

U. Iacobone1, R. Villamaina1, I. Nova1*, E. Tronconi1*, Y. Tang2, A. Kumar2, A. Yezerets2

1Laboratory of Catalysis and Catalytic Processes, Politecnico di Milano (Italy)

2Cummins Inc., Columbus, IN 47201 (USA)

*, *


Cu-exchanged chabazites (Cu-CHA) are recognized as the most efficient catalysts available for the Selective Catalytic Reduction (SCR) of NOx with NH3 from Diesel engines [1-3]. Their small pore structure has a fundamental role for SCR applications, granting superior hydrothermal stability, selectivity and DeNOx activity. Isolated Cu ions are considered the active sites for the SCR abatement reactions and two distinct Cu2+ species with different properties, reactivity and locations, have been proved to exist in the zeolite pores, namely Z2Cu2+ and ZCu2+OH. Their relative amounts change depending on the catalyst formulation and hydrothermal aging (HTA), however their activities in the SCR reactions remain a debated topic [1-3]. In this work, two Cu-CHA catalysts subjected to different hydrothermal aging treatments have been studied to probe the aging effect on Cu speciation. To this purpose, we have applied a protocol recently published by some of us, which relies on simple transient tests, namely NO + NH3 reduction and NO2 adsorption + TPD [3].

Materials and Methods

Two commercial Cu-CHA catalysts samples with similar formulation, provided by Cummins, have been tested: a degreened sample (HTA at 550°C for 4 hours), and an aged sample (HTA at 650°C for 50 hours). 32 mg of catalyst powder, scratched from the original coated monoliths and diluted up to 130 mg with cordierite (75–106 µm mesh), was loaded in a quartz reactor (6 mm ID), with beds of quartz grains placed upstream and downstream of the catalyst bed. The reactor was inserted in an electric furnace and its temperature was controlled by a K-type thermocouple. 500 ppm of reactants (NO/NH3/NO2), with a constant GHSV = 266250 Ncm3/(gcat*h), were firstly sent to bypass lines, then fed to the reactor by pulse valves to simulate a step change in the feed mixture composition. Helium was used as balance gas, and the temporal evolution on the species was followed by a mass spectrometer and a UV analyzer, allowing the measurement of NO, NO2, NH3 and N2. Prior to each test, the samples were pretreated with 8% of O2 for 1 hour at 550°C to remove all impurities.

Results and Discussion

The NO + NH3 reduction tests have been performed at 200°C. As soon as NH3 is fed to the reactor, NO is consumed and N2 is produced according to the known reactions of Cu reduction with Cu:NO:N2 = 1:1:1 stoichiometry [3]. Hence, from integration of the NO and N2 concentration traces it was possible to estimate the content of reducible Cu for both catalysts, and the weight fractions are reported in Table 1. The NO2 adsorption + TPD tests include an isothermal adsorption phase at 150°C lasting till catalyst saturation, an isothermal desorption phase and a Temperature Programmed Desorption (TPD) where temperature was increased up to 550°C with a heating rate fixed to 15°C/min. It has been demonstrated that NO2 is adsorbed on the ZCu2+OH sites only [3], allowing for a direct quantification of the ZCu2+OH cation population by integrating the NO2 released during the TPD. The knowledge of the Cu content and of the ZCu2+OH sites for each catalyst allows to directly compute the Z2Cu2+ ions present in each sample: the corresponding fractions are reported in Table 1.

The different Cu speciation and behavior of the catalysts during the tests can be explained by their different HTA conditions. It is apparent from Table 1 that an increased severity of HTA leads to a transformation of ZCu2+OH into Z2Cu2+, in agreement with the literature [1]. The first evidence is found in the lower NO2 TPD release for the aged sample, corresponding to a lower ZCu2+OH content. Furthermore, considering the temporal evolution of the specie during NO + NH3 reduction tests, upon aging the NO/N2 profile shows a slower trend, coherently with the increased amount of Z2Cu2+, a less reducible species than ZCu2+OH. The results here shown have been further verified and confirmed by means of ICP analysis and of additional transient tests: H2 TPR and NH3 adsorption/TPD [1-3].






Reducible Cu
[% w/w]






Degreened 11.4 11.1 2.26 9.0 79.0


Aged 11.5 11.5 2.28 4.7 41.0


Table 1. Estimates of Cu content and Cu cation speciation for the tested Cu-CHA samples


The simple experimental protocol herein applied provides direct evidence for the variation of the nature of Cu(II) sites upon hydrothermal aging. Furthermore, we demonstrate how a combination of simple well-established transient response methods, requiring no deconvolutions, enables an effective characterization of the Cu active species in Cu-CHJA catalysts, in contrast to more sophisticated spectroscopic techniques.



  1. Luo, F. Gao, K. Kamasamudram, N. Currier et al, J. Catal. 348 (2017) 291, 299
  2. Jangjou, Q. Do, Y. Gu, L.G. Lim, et al, ACS Catalysis. 8 (2018) 1325,1337
  3. Villamaina, S. Liu, I. Nova, E. Tronconi et al, ACS Catalysis 9, 10 (2019) 8916, 8927