Positron Annihilation Spectroscopy

Schmidt, Wolfgang A.

doi:10.1002/9783527618286.ch15

Cited by 5 publications

(5 citation statements)

References 67 publications

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“…[10][11][12][13][14] nometer) to mesopores (several nanometers). 15 These studies have revealed vacancy-type defects in the alumina powders including mono-and di-vacancies, vacancy clusters, and microvoids. However, the size and prevalence of these defects has been found to vary strongly according to the method by which the aluminas were prepared.…”

Section: Introductionmentioning

confidence: 99%

“…Independently of the NMR work, there have been some reports of the use of positron annihilation lifetime spectroscopy (PALS) to probe the surface and defect structure of alumina powders, films, and interfaces. − PALS is a powerful technique capable of characterizing free volume in solids on a scale ranging from atomic vacancies ( subnanometer) to mesopores (several nanometers) . These studies have revealed vacancy-type defects in the alumina powders including mono- and di-vacancies, vacancy clusters, and microvoids.…”

Section: Introductionmentioning

confidence: 99%

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Integrated Study of the Calcination Cycle from Gibbsite to Corundum

et al. 2007

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A comprehensive picture of the gibbsite (Al(OH)3) to corundum (α-Al2O3) calcination process has been developed by multi-technique characterization of an integrated sample set. In 100 °C calcination stages, 27Al nuclear magnetic resonance (NMR), differential thermal analysis (DTA), X-ray diffraction (XRD), and nitrogen sorption surface area measurements were employed to elucidate the structure and chemistry of these calcinates. In addition, positron annihilation lifetime spectroscopy (PALS) has been used to study the defective nature of these aluminas. By exploiting the complementarity of PALS, 27Al NMR, and N2 sorption, new links were established between particle morphology, local atomic coordination, and surface defect chemistry.

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Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Integrated Study of the Calcination Cycle from Gibbsite to Corundum

et al. 2007

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“…The intensity of this lifetime in the PALS spectra reflects the relative number of pores. 31,32 It is worth noting that the PALS technique is able to probe the porosity considered to be closed to adsorptives such as nitrogen because of two features: (i) oPs has a Bohr radius of 0.53 Å or a van der Waals radius of 1.3 Å; therefore, pores too small for N 2 are accessible to oPs and (ii) positrons are injected into the mesocellular material where oPs is subsequently formed; therefore, oPs can probe internal pores that are closed to the external surface (and hence closed to N 2 adsorption).…”

Section: Introductionmentioning

confidence: 99%

“…In a vacuum, the orthopositronium lifetime is 142 ns, but in a porous material with a certain electron density, the orthopositronium (oPs) will annihilate by pick off with an electron from the solid with the opposite spin, with the reduction in the lifetime being a function of the electron density and thus of the pore size. The intensity of this lifetime in the PALS spectra reflects the relative number of pores. , It is worth noting that the PALS technique is able to probe the porosity considered to be closed to adsorptives such as nitrogen because of two features: (i) oPs has a Bohr radius of 0.53 Å or a van der Waals radius of 1.3 Å; therefore, pores too small for N 2 are accessible to oPs and (ii) positrons are injected into the mesocellular material where oPs is subsequently formed; therefore, oPs can probe internal pores that are closed to the external surface (and hence closed to N 2 adsorption).…”

Section: Introductionmentioning

confidence: 99%

“…The conversion of the lifetime to a pore size is often carried out via the quantum mechanical model of Tao and Eldrup: λ = 2 true( 1 + R R + Δ R + 1 2 π sin 2 π R R + Δ R true) where R is the radius of the potential well or the pore radius and Δ R has been found empirically to be 0.17 nm . The model of Tao and Eldrup is limited to pores of a few nanometres or less, and the conversion of longer lifetimes associated with larger pores is the subject of much recent and ongoing work. − For the lifetimes measured in this work, we have used eq to extract assumed spherical-pore sizes for micropores for comparative purposes; however, the exact geometry of the micropores is not known.…”

Section: Introductionmentioning

confidence: 99%

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Micropore Characterization of Mesocellular Foam and Hybrid Organic Functional Mesocellular Foam Materials

et al. 2009

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The micro- and mesoporous structures of microemulsion templated mesocellular foam (MCF) silica materials aged at different temperatures were investigated by using nitrogen sorption αs-comparison plots and positron annihilation lifetime spectroscopy (PALS). The impact of surface amino functionalization with (3-aminopropyl)triethoxysilane (APTES) on these structures was also studied. The nitrogen sorption αs-comparison plots show that the micropore volume of the MCF silica can be controlled by varying the synthesis aging temperature such that the higher the aging temperature, the smaller the micropore volume. The corresponding PALS data showed that this effect of temperature on micropore volume is a direct consequence of a temperature-dependent micropore population, with more micropores in the MCF material aged at lower temperature. After functionalization of the silica MCF materials with APTES, the BET surface area was seen to have reduced substantially, while maintaining the characteristic mesopore sizes. To quantitatively determine the effect on the micropore volumes of the hybrid APTES functional materials, a macorporous amorphous silica material was similarly functionalized with APTES and a high resolution nitrogen sorption isotherm was recorded at 77 K. For the hybrid APTES functionalized MCF silica materials, the nitrogen sorption αs-comparison plots revealed that the materials were no longer microporous. This result was confirmed via PALS completed in both nitrogen and air environments, which indicated that the micropores present in the silica materials prior to functionalization were blocked and replaced by a closed microporosity associated with the APTES layer formed within the mesopores. This study has shown the highly complementary nature of the results obtained from standard nitrogen sorption measurements and PALS. Additionally, the high-resolution nitrogen sorption comparison plot for the APTES functionalized macroporous silica is reported, which is applicable to the study of other meso- and microporous APTES functionalized materials.

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Shedding New Light on Nanostructured Catalysts with Positron Annihilation Spectroscopy

et al. 2018

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Interest in new tools for the analysis of catalytic materials is growing due to the potential to enhance their functionality through the optimal nanostructuring, for example, of pore networks and surface properties. This prompts the need for improved descriptors to discriminate increasingly complex architectures. As a nondestructive, dynamic, and potentially, temporally, and spatially resolved tool, positron annihilation spectroscopy (PAS) can provide valuable complementary insights to already established (e.g., adsorption, spectroscopy, diffraction, and microscopy) methods. This is possible due to the specific sensitivity of positrons to the electronic environment, which determines their annihilation characteristics. However, despite growing enthusiasm, PAS is not widely known in the catalysis community. This review aims to highlight the many unique features, principles, and potential pitfalls of the technique, expanding on the outdated reviews on the topic, which are now over a decade old. After briefly introducing the principles, progress in the application of PAS to investigate various features of relevant catalytic materials is summarized. This includes the crystalline structure, presence of defects, pore connectivity and evolution, chemical properties, and adsorption phenomena. An improved understanding of the response will contribute not only to guiding the design of nanostructured materials but also to positioning PAS as a mainstream method for catalyst characterization.

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Positron Annihilation Spectroscopy

Cited by 5 publications

References 67 publications

Integrated Study of the Calcination Cycle from Gibbsite to Corundum

Integrated Study of the Calcination Cycle from Gibbsite to Corundum

Micropore Characterization of Mesocellular Foam and Hybrid Organic Functional Mesocellular Foam Materials

Shedding New Light on Nanostructured Catalysts with Positron Annihilation Spectroscopy

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