Temperature-programmed desorption (TPD) with ammonia is widely used for zeolite characterization revealing information on acidity and adsorption sites. The interpretation of TPD measurements is, however, often challenging. One example is the NH₃-TPD profile from Cu-chabazite (Cu-CHA) which generally is deconvoluted in three peaks with contributions from NH₃ on Lewis acid sites, copper sites and Brønsted acid sites. Here, we use density functional theory calculations combined with kinetic simulations to analyze this case. We find a large number of possible species, giving rise to overlapping features in the NH₃-TPD. The experimental low-temperature peak (below \(200\,^\circ \text {C}\)) is assigned to NH₃ desorption from Lewis acid sites together with NH₃ desorption from a \([\text {Cu(II)(OH)}(\text {NH}_3)_3]^+\) complex. The intermediate-temperature peak (\(250{-}350\,^\circ \text {C}\)) is attributed to decomposition of a linear \([\text {Cu(I)}(\text {NH}_3)_2]^+\) complex and a residual from \([\text {Cu(II)(OH)}(\text {NH}_3)_3]^+\). The high-temperature peak is predicted to have contributions from Brønsted acid sites (\(\text {NH}_4^+\)), \([\text {Cu(I)}\text {NH}_3]^+\) and \([\text {Cu(II)}(\text {NH}_3)_4]^{2+}\). The present work shows that NH₃-TPD from Cu-CHA can be reconciled with copper complexes as NH₃ storage sites.

Hydrogen peroxide (H₂O₂) is an effective green oxidant, which is used in many industrial processes. Here, the reaction mechanism for direct formation of H₂O₂ from H₂ and O₂ over Pd catalysts is studied using density functional theory calculations and mean-field kinetic modeling. The state of the catalyst as a function of reaction conditions is determined from ab initio thermodynamics. It is found that Pd is in a hydride phase during typical reaction conditions. Reaction landscapes are constructed for the reaction over PdH(111) and PdH(211). Formation of H₂O₂ instead of H₂O requires that O₂ adsorbs and that the surface intermediates O₂, OOH, and H₂O₂ do not dissociate. We find that these requirements are fulfilled on the stepped PdH(211) surface. Surface steps are needed for O₂ chemisorption as the adsorption on PdH(111) is endothermic. The high H coverage on the surface of the hydride is important to slow down the unwanted scission of the O–O bond and promote the desorption of the products. Comparative calculations for the Pd(111) surface show that this surface is inactive for both H₂O₂ and H₂O formation below room temperature for typical reaction mixtures. Our findings demonstrate the importance of surface steps and high hydrogen coverage for direct synthesis of H₂O₂ from H₂ and O₂ over Pd catalysts. The results imply that the selectivity of the reaction toward H₂O₂ is enhanced by a high partial pressure of H₂, which is in agreement with experimental observations.

The Al-distribution affects the formation of Cu(NH₃)₂⁺-pairs during NH₃-SCR over Cu–CHA and oxygen dissociates with low barriers over such pairs.