Interpretation of H2-TPR from Cu-CHA Using First-Principles Calculations

Temperature-programmed reduction and oxidation are used to obtain information on the presence and abundance of different species in complex catalytic materials. The interpretation of the temperature-programmed reaction profiles is, however, often challenging. One example is H2 temperature-programmed reduction (H2-TPR) of Cu-chabazite (Cu-CHA), which is a material used for ammonia assisted selective catalytic reduction of NOx (NH3-SCR). The TPR profiles of Cu-CHA consist generally of three main peaks. A peak at 220 °C is commonly assigned to ZCuOH, whereas peaks at 360 and 500 °C generally are assigned to Z2Cu, where Z represents an Al site. Here, we analyze H2-TPR over Cu-CHA by density functional theory calculations, microkinetic modeling, and TPR measurements of samples pretreated to have a dominant Cu species. We find that H2 can react with Cu ions in oxidation state +2, whereas adsorption on Cu ions in +1 is endothermic. Kinetic modeling of the TPR profiles suggests that the 220 °C peak can be assigned to Z2CuOCu and ZCuOH, whereas the peaks at higher temperatures can be assigned to paired Z2Cu and Z2CuHOOHCu species (360 °C) or paired Z2Cu and Z2CuOOCu (500 °C). The results are in good agreement with the experiments and facilitate the interpretation of future TPR experiments.


Constrained Molecular Dynamic Simulations
For the reaction of H 2 with [Cu 2 (NH 3 ) 4 O 2 ] 2+ , it was not possible to locate the transition state using CI-NEB.Instead, ab initio molecular dynamics using the slow growth method as implemented in VASP, is used to probe the reaction barrier.The temperature is set to 300 K and is controlled using a Nosé-Hoover thermostat in the NVT ensemble.The mass of hydrogen is set to 3, and the time step is 1 fs.The collective variable is the sum of distance r1 and r2, illustrated in Figure S1.The step size was set to -0.0004 Å giving a total simulation time of ∼ 10 ps.

Adsorption of H
can further react with a second H 2 molecule and this reaction is shown in Figure S2.The barrier is 1.0 eV and the reaction is exothermic by 2.9 eV.During the start of the simulation, a single NH 3 molecule is desorbed from the complex and diffuses into an adjacent cage and is therefore not shown in the structures.The H 2 molecule is dissociated over the Cu ion forming an additional H 2 O and reducing both Cu ions to oxidation state +1.

H 2 Adsorption on ZCu
H 2 can adsorb on ZCu with a low adsorption energy of -0.28 eV.The adsorbed state is shown in Figure S3.A possible reaction could be the formation of a Brønsted acid site and a Cu-H complex.That is similar to the reaction pathway for H 2 over Z 2 Cu (See Figure 4, Structure 5).However, for the case of ZCu, the Cu ion will not have any associated Al.

ZCu + H
Several configurations of ZH + CuH were optimized, however, no minimum was located.
Figure S3: Optimized structure for the adsorption of H 2 onto ZCu.Atomic color codes as in Figure S1.

Reaction of ZCuH with ZCuOH
The product of the reaction between ZCuOH and H 2 is ZCuH and H   Atomic color codes as in Figure S1.

Elementary Steps and Kinetic Parameters
The considered reaction steps and associated kinetic parameters are reported in Table S1.

Figure S2 :
Figure S2: Constrained AIMD simulations of the energy profile for the reaction of H 2 with [Cu 2 (NH 3 ) 4 O(H 2 O)] 2+ .The gray line is the energy and the red line is the rolling average of the energy.Atomic color codes as in Figure S1.
2 O. ZCuH could potentially reduce a second ZCuOH species and the energy landscape for this reaction is shown in Figure S4.The OH-group in ZCuOH can bind to ZCuH forming Z 2 CuOHCuH, which has a barrier of 0.09 eV and is exothermic by -0.2 eV.The next step is a slight movement in the Cu ion and is endothermic by 0.07 eV.The OH-group can subsequently be transferred to ZCuH forming H 2 O and reducing both Cu ions to oxidation state +1 (ZCu + ZCuH 2 O).This process is exothermic by -1.83 eV.

Figure S4 :
Figure S4: Energy landscape for the reaction of ZCuH with ZCuOH.Atomic color codes as in Figure S1.

Figure S5 :
Figure S5: Reaction landscape for the dissociation of H 2 over (a) Z 2 Cu and (b) Z 2 CuOOCu, with alternative Al-configurations.(c) H 2 -TPR simulated profiles of the alternative Al configuration (dashed lines) and the simulated H 2 -TPR profiles in the manuscript (solid lines).Atomic color codes as in Figure S1.

Table S1 :
Elementary steps included in the microkinetic model.Prefactors and barriers are given for the forward (A f , ∆E f ) and reverse (A f , ∆E f ) reaction.The parameters are evaluated at 200 • C for 2000 ppm H 2 in Ar.