Interpretation of NH3-TPD Profiles from Cu-CHA Using First-Principles Calculations

Abstract

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 NH 3 -TPD profile from Cu-chabazite (Cu-CHA) which generally is deconvoluted in three peaks with contributions from NH 3 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 NH 3 -TPD. The experimental low-temperature peak (below 200 • C ) is assigned to NH 3 desorption from Lewis acid sites together with NH 3 desorption from a [Cu(II)(OH)(NH 3 ) 3 ] + complex. The intermediate-temperature peak ( 250−350 • C ) is attributed to decomposition of a linear [Cu(I)(NH 3 ) 2 ] + complex and a residual from [Cu(II)(OH)(NH 3 ) 3 ] + . The hightemperature peak is predicted to have contributions from Brønsted acid sites ( NH + 4 ), [Cu(I)NH 3 ] + and [Cu(II)(NH 3 ) 4 ] 2+. The present work shows that NH 3 -TPD from Cu-CHA can be reconciled with copper complexes as NH 3 storage sites.

Effect of copper on textural and acidic properties of hierarchical nanocrystalline ZSM‐5

“…The intensity of these peaks corresponding to strong acid sites for high copper loadings is found to be reduced at high temperature 14 . But the moderate acid sites of high copper loading have increased by many folds in the moderate temperature region 58,59 . In short, it can be observed that the copper incorporation into the zeolite structure resulted in a considerable boosting in the acidity of the surface of all synthesized samples.…”

Effect of Modification of Amorphous Silica with Ammonium Agents on the Physicochemical Properties and Hydrogenation Activity of Ir/SiO2 Catalysts

“…Based on the shape of the NH 3 -TPD profiles it can be concluded that the investigated samples contain acid centers of different strengths. The traditional classification [ 28 ] describes the existence of two kinds of acid centers: weak acid centers assigned to low-temperature peaks (below 400 °C) and strong acid centers ascribed to high-temperature peaks (above 400 °C). The low-temperature peaks refer to desorption of ammonia from weakly acidic silanol groups and weak Lewis acid centers, while high-temperature peaks correspond to desorption of ammonia from strong Brønsted and Lewis acid sites [ 28 ].…”

Desorption products during linear heating of copper zeolites with pre-adsorbed methanol

“…22 2.3.1 Calculated methanol desorption profiles. The theoretical methanol desorption profiles were calculated assuming an isolated, independent site where adsorption and desorption of methanol occur, 45,46 i.e. the process:…”