Extended Surface of Materials as a Result of Chemical Equilibrium

Arabczyk, W.; Pelka, Rafał; Jasińska, Izabella

doi:10.1155/2014/473919

Cited by 7 publications

(20 citation statements)

References 14 publications

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“…nanocrystalline materials whose surface is coated with a twodimensional structure of the adsorbent may be in a state of chemical equilibrium. 44 The results presented in this work show plausible the hypothesis outlined earlier.…”

Section: Discussionsupporting

confidence: 89%

“…This dependence was used to determine the size distribution of crystallite (GSD) according to the method proposed by Pelka in an article. 44 For this purpose, on the basis of microscopic observations the SEM iron catalyst was assumed to have the size of the nanocrystallites as 10-70 nm and a shape factor S/V as for the sphere. The shares of individual fractions were chosen in such a way that the area under the curve GSD = f (d) was equal to unity for the specified range of diameters of crystallites.…”

Section: Resultsmentioning

confidence: 99%

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Investigation of nitriding and reduction processes in a nanocrystalline iron–ammonia–hydrogen system at 350 °C

Wilk

Arabczyk

2015

Phys. Chem. Chem. Phys.

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In this paper, the series of phase transitions occurring during the gaseous nitriding of nanocrystalline iron was studied. The nitriding process of nanocrystalline iron and the reduction process of the obtained nanocrystalline iron nitrides were carried out at 350 °C in a tubular differential reactor equipped with systems for thermogravimetric measurements and analysis of gas phase composition. The samples were reduced with hydrogen at 500 °C in the above mentioned reactor. Then the sample was nitrided at 350 °C in a stream of ammonia-hydrogen mixtures of various nitriding potentials, P = pNH3/pH2(3/2). At each nitriding potential stationary states were obtained - the nitriding reaction rate is zero and the catalytic ammonia decomposition reaction rate is constant. The reduction process of the obtained nanocrystalline iron nitrides was studied at 350 °C in the stationary states as well. The phase composition of products obtained in both reaction directions (nitriding and reduction) was different despite the identical concentration of nitrogen in the nitriding mixture. The hysteresis phenomenon, occurring at the iron nitriding degree - nitriding potential system, was explained. In the single-phase areas of α-Fe(N), γ'-Fe4N or ε-Fe3-2N, a state of chemical equilibrium between the ammonia-hydrogen mixture, nanocrystalline iron surface and volume was observed. In the multi-phase areas, between the gas phase and the iron surface a state of chemical equilibrium holds, but between the gas phase and solid phase volume a state of quasi-equilibrium exists. The model of the nitriding process of nanocrystalline iron to iron nitride (γ'-Fe4N) was presented. It was found that nanocrystallites reacted in the order of their sizes from the largest to the smallest.

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Section: Discussionsupporting

confidence: 89%

Section: Resultsmentioning

confidence: 99%

Investigation of nitriding and reduction processes in a nanocrystalline iron–ammonia–hydrogen system at 350 °C

Wilk

Arabczyk

2015

Phys. Chem. Chem. Phys.

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“…This means that the average size of the product nanocrystallites at the beginning of the transformation will be the largest. To sum up, nanocrystallites undergo phase changes in fractions according to their size, and the average size of nanocrystallites of individual phases (substrate/product) after the phase change of individual fractions may change with the progress of the reaction, but the total size distribution (for a sample consisting of 100% of one phase) is practically unchanged (nitriding-reduction reaction cycles: the average size of nanocrystallites is constant) [20,38].…”

Section: Resultsmentioning

confidence: 99%

“…Researchers sought to the lower the internal energy of nanomaterial by decreasing its surface to volume ratio in the sintering processes [16][17][18][19]. However, it has already been shown that nanomaterials can be in the equilibrium state [20], and that industrial catalysts are the best examples of this; the ammonia synthesis iron catalyst maintains its nanocrystalline structure for even more than 15 years despite it working in aggressive atmospheres, under Catalysts 2021, 11, 183 2 of 12 a pressure of 20-30 MPa and a relatively high temperature of 500 • C [21]. In nanocrystalline iron nitriding processes, the minimal nitriding potential is expressed as P 0 = p NH3,pt p 3/2 H2.pt (1) where p NH3,pt is the partial pressure of ammonia, at which phase transformation occurs, and p H2,pt is the partial pressure of hydrogen, at which phase transformation occurs, which is needed to complete transformation of individual nanocrystallites of iron depending on the size of crystallites [22][23][24][25].…”

Section: Introductionmentioning

confidence: 99%

Study of Phase Transitions Occurring in a Catalytic System of ncFe-NH3/H2 with Chemical Potential Programmed Reaction (CPPR) Method Coupled with In Situ XRD

Ekiert¹,

Wilk²,

Lendzion-Bieluń³

et al. 2021

Catalysts

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Nitriding of nanocrystalline iron and reduction of nanocrystalline iron nitride with gaseous mixtures of hydrogen with ammonia were studied at 375 °C and atmospheric pressure using the chemical potential programmed reaction (CPPR) method coupled with in situ XRD. In this paper, a series of phase transitions occurring during the processes is shown, and a detailed analysis of the phase composition and the structure of the material is given. The influence of a variable nitriding potential on the lattice parameters of α-Fe, γ′-Fe4N, and ε-Fe3-2N phases is shown. The α phase interplanar space changes irrelevantly in the one phase area but decreases linearly with average increases in crystallite size when α→γ′ transformation occurs. The nanocrystallite size distributions (nCSDs) were determined, with nCSD of the α phase for nitriding and nCSD of the ε phase for reduction. The reduction of the ε phase can occur directly to α or indirectly with an intermediate step of γ′ formation as a result of ε→γ′→α transformations. The determining factor in the reducing process method is the volume of ε phase nanocrystallites. Those with V < 90,000 nm3 undergo direct transformation ε→αFe(N), and V > 90,000 nm3 transforms to αFe(N) indirectly. It was determined at what value of nitriding potential which fraction of the ε phase nanocrystallites starts to reduce

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“…As a result of overheating, the smallest nanocrystallites transform into larger, stable ones, whose active specific surface A is determined by the equilibrium established under given conditions; therefore, the limit values of nanocrystallites with the largest active specific surfaces A max change in the range 0.15–0.10 nm −1 . The size of larger than minimal nanocrystallites remaining in the sample is not the result of the equilibrium state; only the equilibrium between the promoters and the nanocrystallite surface will be established there, but the S/V relationship of the nanocrystallites will not meet the equilibrium conditions [ 53 ]. The largest nanocrystallite in the catalyst does not change as a result of the overheating process, and its active specific surface area A min in each sample is 0.04 nm −1 .…”

Section: Discussionmentioning

confidence: 99%

Reaction Model Taking into Account the Catalyst Morphology and Its Active Specific Surface in the Process of Catalytic Ammonia Decomposition

Arabczyk¹,

Pelka²,

Jasińska³

et al. 2021

Materials

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Iron catalysts for ammonia synthesis/nanocrystalline iron promoted with oxides of potassium, aluminum and calcium were characterized by studying the nitriding process with ammonia in kinetic area of the reaction at temperature of 475 °C. Using the equations proposed by Crank, it was found that the process rate is limited by diffusion through the interface, and the estimated value of the nitrogen diffusion coefficient through the boundary layer is 0.1 nm2/s. The reaction rate can be described by Fick’s first equation. It was confirmed that nanocrystallites undergo a phase transformation in their entire volume after reaching the critical concentration, depending on the active specific surface of the nanocrystallite. Nanocrystallites transform from the α-Fe(N) phase to γ’-Fe4N when the total chemical potential of nitrogen compensates for the transformation potential of the iron crystal lattice from α to γ; thus, the nanocrystallites are transformed from the smallest to the largest in reverse order to their active specific surface area. Based on the results of measurements of the nitriding rate obtained for the samples after overheating in hydrogen in the temperature range of 500–700 °C, the probabilities of the density of distributions of the specific active surfaces of iron nanocrystallites of the tested samples were determined. The determined distributions are bimodal and can be described by the sum of two Gaussian distribution functions, where the largest nanocrystallite does not change in the overheating process, and the size of the smallest nanocrystallites increases with increasing recrystallization temperature. Parallel to the nitriding reaction, catalytic decomposition of ammonia takes place in direct proportion to the active surface of the iron nanocrystallite. Based on the ratio of the active iron surface to the specific surface, the degree of coverage of the catalyst surface with the promoters was determined.

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Extended Surface of Materials as a Result of Chemical Equilibrium

Cited by 7 publications

References 14 publications

Investigation of nitriding and reduction processes in a nanocrystalline iron–ammonia–hydrogen system at 350 °C

Investigation of nitriding and reduction processes in a nanocrystalline iron–ammonia–hydrogen system at 350 °C

Study of Phase Transitions Occurring in a Catalytic System of ncFe-NH3/H2 with Chemical Potential Programmed Reaction (CPPR) Method Coupled with In Situ XRD

Reaction Model Taking into Account the Catalyst Morphology and Its Active Specific Surface in the Process of Catalytic Ammonia Decomposition

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