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This article focuses on the surface acidity properties of oxide catalysts and supports (γ-Al2O3, CeO2, ZrO2, SiO2, TiO2, HZSM5 zeolite) and the comparative detection of their surfaces by measuring the temperature-programmed ammonia desorption (ATPD). ATPD is a reliable and simple method in which the surface, after being saturated with ammonia at low temperature, undergoes a temperature change, which leads to desorption of probe molecules as well as temperature distribution.
By quantitative and/or qualitative analysis of the desorption pattern, information can be obtained on the energy of desorption/adsorption and the amount of ammonia adsorbed on the surface (ammonia uptake). As a basic molecule, ammonia can be used as a probe to determine the acidity of a surface. These data can help to understand the catalytic behavior of the samples and even help fine-tune the synthesis of new systems. Instead of using a traditional TCD detector, a quadrupole mass spectrometer (Hiden HPR-20 QIC) was used in the task, connected to the test device through a heated capillary.
The use of QMS allows us to easily distinguish between different species desorbed from the surface without the use of any chemical or physical filters and traps that could adversely affect the analysis. Proper setting of the instrument’s ionization potential helps prevent fragmentation of the water molecules and the resulting interference with the ammonia m/z signal. The accuracy and reliability of temperature-programmed ammonia desorption data were analyzed using theoretical criteria and experimental tests, highlighting the effects of data collection mode, carrier gas, particle size, and reactor geometry, demonstrating the flexibility of the method used.
All materials studied have complex ATPD modes spanning the 423-873K range, with the exception of cerium, which exhibits resolved narrow desorption peaks indicating uniform low acidity. Quantitative data indicate differences in ammonia uptake between other materials and silica by more than an order of magnitude. Since the ATPD distribution of cerium follows a Gaussian curve regardless of surface coverage and heating rate, the behavior of the material under study is described as a linearity of four Gaussian functions associated with a combination of moderate, weak, strong, and very strong site groups. Once all data had been collected, ATPD modeling analysis was applied to help obtain information on the adsorption energy of the probe molecule as a function of each desorption temperature. The cumulative energy distribution by location indicates the following acidity values based on average energy values (in kJ/mol) (eg surface coverage θ = 0.5).
As a probe reaction, propene was subjected to dehydration of isopropanol to obtain additional information about the functionality of the materials under study. The results obtained were consistent with previous ATPD measurements in terms of the strength and abundance of surface acid sites, and also made it possible to distinguish between Brønsted and Lewis acid sites.
Figure 1. (Left) Deconvolution of the ATPD profile using a Gaussian function (yellow dotted line represents the generated profile, black dots are experimental data) (right) Ammonia desorption energy distribution function at various locations.
Roberto Di Cio Faculty of Engineering, University of Messina, Contrada Dee Dee, Sant’Agata, I-98166 Messina, Italy
Francesco Arena, Roberto Di Cio, Giuseppe Trunfio (2015) “Experimental Evaluation of Ammonia Temperature-Programmed Desorption Method for Investigating the Acid Properties of Heterogeneous Catalyst Surfaces” Applied Catalysis A: Review 503, 227-236
Hide analytics. (February 9, 2022). Experimental evaluation of the method of temperature-programmed desorption of ammonia to study the acid properties of heterogeneous surfaces of catalysts. AZ. Retrieved September 7, 2023 from https://www.azom.com/article.aspx?ArticleID=14016.
Hide analytics. “Experimental Evaluation of a Temperature-Programmed Ammonia Desorption Method for Studying the Acid Properties of Heterogeneous Catalyst Surfaces”. AZ. September 7, 2023
Hide analytics. “Experimental Evaluation of Temperature-Programmed Ammonia Desorption Method for Studying the Acid Properties of Heterogeneous Catalyst Surfaces”. AZ. https://www.azom.com/article.aspx?ArticleID=14016. (Accessed: September 7, 2023).
Hide analytics. 2022. Experimental evaluation of a temperature-programmed ammonia desorption method for studying the acidic properties of heterogeneous catalyst surfaces. AZoM, accessed 7 September 2023, https://www.azom.com/article.aspx?ArticleID=14016.
Post time: Sep-07-2023
