ISSN 0036-0244, Russian Journal of Physical Chemistry A, 2016, Vol. 90, No. 6, pp. 1137–1142. © Pleiades Publishing, Ltd., 2016. Original Russian Text © T.F. Sheshko, Yu.M. Serov, T.A. Kryuchkova, I.A. Khairullina, I.V. Chislova, I.A. Zvereva, 2016, published in Zhurnal Fizicheskoi Khimii, 2016, Vol. 90, No. 6, pp. 860–865.

CHEMICAL KINETICS AND CATALYSIS

Interaction between Carbon Oxides, Hydrogen and Fe2O3 and An + 1FenO3n + 1 (A = Gd, Sr, n = 1, 2, …, ∞) T. F. Sheshkoa, Yu. M. Serova, T. A. Kryuchkovaa, I. A. Khairullinaa, I. V. Chislovab, and I. A. Zverevab a People’s b

Friendship University of Russia, Moscow, 117198 Russia St. Petersburg State University, St. Petersburg, 119034 Russia e–mail: [email protected] Received July 13, 2015

Abstract—The interaction between carbon oxides and hydrogen and surfaces of iron(III) oxide and An + 1FenO3n + 1 (where A = Gd, Sr, and n = 1, 2, …, ∞ is the number of perovskite layers) complex oxides is studied for the first time by means of thermal programmed desorption. It is shown that carbon oxides are adsorbed in molecular form with the formation of carbonate–carboxylate complexes, and in dissociative form. The ratios of the adsorption forms of both oxides are determined by the structure of ferrites, the number of perovskite layers, and the valence state and coordinative saturation of iron. The presence of weakly and strongly bonded hydrogen forms is established, and it is suggested that hydrogen dissolves in the bulk of a perovskite. Keywords: adsorption, desorption, carbon monoxide, carbon dioxide, ferrites. DOI: 10.1134/S0036024416060236

INTRODUCTION One of the stages of any catalytic process is the adsorption of interacting compounds on the catalyst’s surface. Studies on the interaction mechanisms of different gases are needed for an analysis of surface structure, chemical composition, and the mechanisms of the processes that occur in adsorbed layers under different conditions. Complex oxides with the structures of perovskites are among the most attractive and interesting mixed oxides, due to their varied functional properties and mixed oxygen ion and electronic conductivities. Owing to their high activity and stability, they find application in the latest technologies and are promising catalysts for high–temperature processes. The aim of this work was thus to investigate the interaction between carbon oxides, hydrogen and surfaces of iron(III) oxide and An+1FenO3n+1 (where A = Gd, Sr, and n = 1, 2, …, ∞) complex oxides used as catalysts for the dry conversion of methane and hydrogenation of carbon oxides, and to determine the effect the chemical composition and structure of ferrites have on their adsorption properties. EXPERIMENTAL Our objects of study, i.e., samples of powdered iron(III) oxide (chemically pure grade; iron oxide content, 99.9%) and complex perovskite-type GdFeO3, GdSrFeO4, and Gd2SrFe2O7 ferrites (An+1FenO3n+1, where A = Gd, Sr, and n = 1, 2, …, ∞),

were prepared by sol–gel technology [1]. The structure and morphology of the obtained samples were determined via X-ray diffraction analysis on a Thermo ARL X’TRA diffractometer with CuKα radiation. Photon correlation spectroscopy measurements were performed on a Malvern Zetasizer Nano analyzer using a helium–neon laser with a power of 4 mW, operating at a wavelength of 633 nm. Scanning electron microscopy micrographs were obtained on a Zeiss EVO®40 instrument with an accelerating voltage of 10 kV, operating in the low–vacuum mode. The Mössbauer spectra were recorded at room temperature on a Wissel spectrometer using a 57Co radiation source in a rhodium matrix with an activity of 10 mCu, and the isomeric shifts were calculated with reference to α-Fe. The surface properties of the samples were measured via the low–temperature adsorption of nitrogen at Т = 77 K on a Nova 4200Е instrument (Quantachrome). The obtained adsorption–desorption isotherms were used to study the specific surfaces of the samples via BET method and the sizes of pores. The chemisorption of carbon oxides was studied by thermal desorption method. Carbon oxides were introduced into a reaction vessel until atmospheric pressure was reached at 293 K. Thermal desorption was conducted in an argon stream in the mode of a linearly programmed rise in temperature from 293 to 823 K. The temperature of the oven was raised at a constant rate using an Oven TRМ1 digital controller

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Table 1. Specific surfaces, sizes, and volumes of pores, and the parameters of Mössbauer spectra of GdFeO3, GdSrFeO4, and Gd2SrFe2O7 complex oxides Ssp, m2/g

V, cm3/g

Mean size of pores, nm

GdFeO3

2.6

0.003

2.4

GdSrFeO4

4.8

0.001

2.0

Gd2SrFe2O7

4.4

0.001

2.2

Compound

with an accuracy of ±2 K. The temperature in the oven was controlled by a chromel–alumel thermocouple. When needed, the thermal desorption products were transferred from the reactor to a liquid nitrogen trap for the removal of water and carbon dioxide. The chemical composition and amount of desorbed gases were controlled using a Kristall 2000 M gas chromatograph (a column 2 m long and 3 mm in diameter, filled with Poropak Q; Тcolumn = 303 K). All of the thermal programmed desorption (TPD) spectra were processed in the coordinates of the Polanyi–Wigner equation [2, 3]:

dθ E n (1) − s = ν nθ S exp ⎛⎜ − des ⎞⎟ , dt ⎝ RT ⎠ where θs is the surface filling, expressed in monolayer fractions or the number of molecules adsorbed by 1 cm2 of a sample; Еdec is the desorption activation energy; Т is the surface temperature; n is the kinetic order of desorption; and νn is the frequency factor, expressed in s–1. The following integral equations for the first and second order thermal desorption were derived by Erlich to determine the energy of desorption activation from changes in the surface concentration of an adsorbate during thermal desorption [2, 3]: ln ln T

θ0 θs 2

= ln

ν1R E des − β E des RT

at n  = 1 ,

(2)

1/ θ s − 1/ θ 0 ν R E = ln 2 − des at n = 2, (3) 2 β E des RT T where β is the rate of the linear rise in temperature, and θо is the maximum degree of surface filling proportional to the area of the entire desorption peak. If a linear dependence was observed when presenting the experimental data in the coordinates of Eq. (2), ln

Fen+

Chemical shift, mm/s

Content, %

Fe3+ Fe3+ Fe4+ Fe3+ Fe3+ Fe3+ Fe4+ Fe3+ Fe3+ Fe3+

0.3 0.37 0.09 0.27 0.17 0.37 0.07 0.36 0.36 0.36

10 90 17 33 30 20 13 27 23 37

desorption was considered to be of the first kinetic order; if one was observed when they were presented in the coordinates of Eq. (3), it was considered to be of the second kinetic order, and the energy of desorption activation was determined accordingly. The samples were subjected to thermal programmed heating up to 823 K in an argon stream before each run to purify them of contaminants. Surface purity was judged from the results of chromatographic analysis. RESULTS AND DISCUSSION The GdFeO3, GdSrFeO4, and Gd2SrFe2O7 complex oxides investigated in this work were Ruddlesden–Popper phases composed of structural blocks of interpenetrating structural fragments of ABO3 perovskite (P) and AO mineral salts (RS) that formed a sequence of alternating …(P)(P)(RS)(P)(P)(RS)… layers. Compounds with similar structures are described by the general formula An+1BnO3n+1, where n = 1, 2, …, ∞ is the number of perovskite layers. The perovskite structure is achieved at the limit n = ∞. X-ray diffraction analysis, photon–correlation spectroscopy, and scanning electron microscopy (Fig. 1) showed that oxides were present in the nanocrystalline state with a 200 nm mean length of crystallites and a porous structure. The specific surface values of the perovskite-type ferrites, determined from data on the low–temperature adsorption of nitrogen and programmed estimates of pore sizes, are given in Table 1. It was established by means of Mössbauer spectroscopy that iron in the samples of GdFeO3 was in the form of Fe3+ ions localized in two types of fields with different symmetry, and that Fe4+ ions with oxygen vacancies coexisted in GdSrFeO4 and Gd2SrFe2O7 complex oxides along with Fe3+ ions localized in three types of fields with different symmetries (Table 1). Another state of iron atoms can stabilize on the sur-

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Table 2. Desorption kinetic orders (n) and activation energies (Еа, kJ/mol) for catalysts after the contact with СО2, СО, and Н2 at 25°C СО2

Н2

СО

Тdes, K

n

Еа

Тdes, K

n

Еа

Тdes, K

n

Еа

423–523 573–700 >700

~1 ~2 ~2

165 169 171

293–423 >623

Fe2O3 ~1 –

27 –

293–323 323–573 >723

1 1 1

87 41 136

423–523 523–573 573–700 >700

~1 ~2 ~2 –

126 162 181 –

293–423 >623

GdFeO3 ~1 ~1

5 69

293–323 >723

1 1

57 129

8 143

293–323 >723

1 –

52 –

49 –

293–323 >723

1 –

45 –

423–523 523–573 573–700 >700

~1 ~2 ~2 ~2

167 171 198 231

293–423 >623

GdSrFeO4 ~1 ~2

293–423 423–523 523–573 573–700 >700

~1 ~1 ~2 ~2 ~2

55 136 201 209 262

293–423 >623

Gd2SrFe2O7 ~1 –

faces of crystallites, where the oxygen environment is distorted most and the Fe3+ ions are in the area with the lowest symmetry. The properties of the perovskite–like oxides were compared to the analogous properties of iron(III) oxide. Two areas are observed in the TPD spectra of all the complex oxides after the adsorption of carbon monoxide at 298 K: low-temperature areas with Тdes = 293 to 423 K and high-temperature areas with Тdes > 623 K (Fig. 2). Analysis of the desorption kinetics using Eqs. (1)–(3) allows us to determine the kinetic

(а)

order and calculate the effective energy of desorption activation (Table 2). According to [2–4], the zero order of desorption corresponds to polymolecular adsorption; the first order, to monomolecular adsorption; and the second order, to dissociative adsorption. Linearly adsorbed CO is normally removed from a surface at temperatures of less than 523 K; CO adsorbed in the bridged form, at temperatures higher than 523 K. The desorption of carbon monoxide from surfaces of Fe2O3, GdFeO3, GdSrFeO4, and Gd2SrFe2O7

100 nm (b)

2 μm

Fig. 1. Microphotographs of GdFeO3 prepared by sol–gel method in (a) nanometer and (b) micrometer scales. RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A

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mol/g 0.16 3

0.12

2 0.08

4 1

0.04 0 273

373

473

573

673

T, K

Fig. 2. Spectra of the thermal desorption of CO from surfaces of (1) Fe2O3, (2) GdFeO3, (3) GdSrFeO4, and (4) Gd2SrFe2O7.

oxides in the first temperature interval proceeds according to the first kinetic order. The energy of desorption activation in this case does not exceed 27 kJ/mol, and no shift of the thermal desorption peaks is observed. This indicates that carbon monoxide is adsorbed on the investigated oxides in linear form. This form of adsorption is labeled as α-CO. The emergence of the second desorption area (β-CO) on surfaces of GdFeO3, GdSrFeO4, and Gd2SrFe2O7 in contrast to iron(III) oxide, and its suppression upon an increase in the number of perovskite layers (Fig. 2) could be due to the heterovalence state of iron (Fe3+ and Fe4+) in different symmetry fields. According to [5, 6], an increase in the coordinative unsaturation of metal atoms thus leads to a change in the energy of carbon–metal bonds, and to the emergence of additional forms of carbon monoxide adsorption (different carbonate and carboxylate complexes). The kinetic order for the second area of desorption is close to second. However, neither О2 nor СО2 are observed among the desorbed products, indicating a two-point adsorption of CO; i.e., β-CO is a bridged form of CO.

I, rel. units

1

2

3

4 270

370

470

570

670

770

870 T, K

Fig. 3. Spectra of the thermal desorption of СО2 from surfaces of (1) Fe2O3, (2) GdFeO3, (3) GdSrFeO4, and (4) Gd2SrFe2O7.

Three areas of desorption with Тdes in the ranges of 423 to 523, 523 to 573, and 573 to 723 K are observed in the TPD spectrum upon the adsorption of СО2 on Fe2O3 at 298 K (Fig. 3). After treating the experimental data in the linear coordinates of the Polanyi– Wigner and Ehrlich equations, the first area of desorption was found to be of the first order, while the remaining two desorption peaks were linearized using both the first and second kinetic orders. All of the obtained kinetic orders and activation energies of desorption of СО2 are given in Table 2. Based on our results, we may assume that carbon dioxide is adsorbed on Fe2O3 in molecular form (Тdes = 423 to 523 K) with the formation of carbonate–carboxylate complexes (Тdes = 423 to 523 K), and perhaps in the

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mol/g 0.008

0.006

0.004

0.002

2

3

1 4

0 273

373

473

573

673

773 T, K

Fig. 4. Spectra of the thermal desorption of Н2 from surfaces of (1) Fe2O3, (2) GdFeO3, (3) GdSrFeO4, and (4) Gd2SrFe2O7.

dissociative form with recombination into СО2 upon desorption (Тdes = 573 to 723 K). Analysis of the TPD profile obtained upon the adsorption of СО2 on GdFeO3 shows that the high– temperature form (573 to 723 K) is missing from the spectrum, but an additional form appears in the range of 523 to 573 K. This form is also linearized using both the first and second kinetic orders. Therefore, it was suggested that it corresponds to another carbonate complex on the surface that could emerge because of the iron in GdFeO3 being present in two types of fields with different symmetries. Four areas of desorption could be distinguished in the TPD spectrum upon the adsorption of carbon dioxide on layered GdSrFeO4. Analysis of the СО2 desorption kinetics (Table 2) showed that the first kinetic order of desorption is observed only in the low–temperature area, and the kinetic order for the remaining areas is close to the second. Based on the results from comparing the experimental and the literature data [2], we may conclude that carbon dioxide is adsorbed on perovskite-type GdSrFeO4 in molecular form with the formation of two types of carbonate– carboxylate complexes, and in the dissociative form. The presence of carbon monoxide in the analyzed samples over the range of temperatures also indicate dissociative adsorption. It should be noted that the desorption peak of the molecular form is shifted toward the area of low temperatures, and its intensity is considerably higher for desorption from Fe2O3 and GdFeO3; i.e., introducing layers of mineral salt into the perovskite structure and the increase in the coordinative unsaturation of iron atoms change the energy of the Mеδ+–СО2 bonds between carbon and metal, and lead to additional forms of carbon dioxide adsorption [5, 6]. The emergence of the dissociative adsorpRUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A

tion form of СО2 could be due to both the heterovalence state of iron (Fe3+ and Fe4+) with oxygen vacancies and the reduced symmetry of iron (three different fields). The emergence of the dissociative adsorption form of СО2 could be due to both the heterovalence state of iron (Fe3+ and Fe4+) in GdSrFeO4 with oxygen vacancies and the reduced symmetry of iron Fe3+ (Table 1). Raising the number of perovskite layers to two in the structure of Gd2SrFe2O7 complex oxide complicates the TPD profile and leads to the emergence of one more sort of weakly bonded СО2, adsorbed in molecular form in the range of 293 to 423 K (Fig. 3). Based on our experimental data, we may conclude that carbon dioxide is adsorbed on layered perovskite– type ferrites in molecular form with the formation of carbonate and carboxylate complexes, and in the dissociative form with recombination into СО2 upon desorption. The ratio of the adsorption forms is determined by the oxide structure, the number of perovskite layers, and the valence state and coordinative saturation of iron. Three areas of desorption with Тdes in the ranges of 293 to 323, 323 to 573, and 573 to 723 K are observed in the TPD spectrum upon the hydrogen adsorption on Fe2O3 at 298 K (Fig. 4). The amounts of hydrogen desorbed in the first two temperature ranges were small, and the greatest fraction corresponds to the third area of desorption. The second area is missing from the TPD spectra obtained for the samples of GdFeO3, GdSrFeO4, and Gd2SrFe2O7. The amounts of hydrogen that evolved in the temperature range of 293 to 323 K are comparable for all of the investigated oxides. The intensity of the desorption peak in the range of 573 to 723 K for perovskite–like GdFeO3 fer-

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rite is much lower than for Fe2O3, and the relevant form is almost missing from the TPD spectra of GdSrFeO4 and Gd2SrFe2O7. The kinetic characteristics of hydrogen desorption for all of the investigated oxides are given in Table 2. Based on our results, we may conclude that there were two forms of adsorbed hydrogen: weakly bonded (molecular) hydrogen, desorbed from the oxide structure when it was heated in an Ar stream up to 423 K, and strongly bonded hydrogen, which remained in the samples at higher temperatures (up to 573 K). Raising the number of perovskite layers in complex oxides lowers the energy of desorption activation for weakly bonded hydrogen; i.e., the Mеδ+–Н2 bond becomes weaker, and we may assume that its ability to migrate over the surface increases. Since the mean size of the pores of complex oxides, determined from the low– temperature adsorption of nitrogen, was 2 to 3 nm, and the diameter of a hydrogen molecule is 0.22 nm [7], we may conclude that the dissolution of hydrogen in the bulk of the perovskites eventually leads to a considerable reduction in the amount of desorbed hydrogen in the high–temperature range. CONCLUSIONS The presence of two molecular adsorption forms of carbon monoxide was detected for the first time while investigating the interaction between carbon and hydrogen oxides and surfaces of iron(III) oxide and An+1FenO3n+1 (where A = Gd, Sr, and n = 1, 2,…, ∞ is the number of perovskite layers) complex oxides by means of thermal programmed desorption. It was shown that carbon dioxide is adsorbed in molecular form with the formation of carbonate–carboxylate complexes, and in the dissociative form with recombination into СО2 upon desorption. The ratio of the adsorption forms of both oxides is determined by the

structure of the ferrite, the number of perovskite layers, and the valence state and coordinative saturation of iron. The presence of weakly and strongly bonded forms of hydrogen on the surface of Fe2O3 was detected. It is suggested that an increase in the coordinative unsaturation of iron in GdFeO3, GdSrFeO4–δ, and Gd2SrFe2O7 reduces the fraction of strongly bonded hydrogen and its dissolution in the bulk of a perovskite. ACKNOWLEDGMENTS This work was supported by the Russian Foundation for Basic Research, project no. 14-03-00940 А. REFERENCES 1. I. V. Chislova, Cand. Sci. (Chem.) Dissertation (St. Petersburg, 2015). 2. N. M. Popova, L. V. Babenkova, G. A. Savel’eva, Yu. E. Kul’evskaya, N. G. Smirnova, and V. K. Solnyshkova, Modern Thermal Desorption, and Its Application to Adsorption and Catalysis (Nauka, Alma-Ata, 1985) [in Russian]. 3. G. Erlich, Catalysis. Physical Chemistry of Heterogeneous Catalysis (Mir, Moscow, 1967) [in Russian]. 4. http://th.fhi–berlin.mpg.de/th/lectures/summer_term_2006/6_desorption.pdf 5. I. A. Kirovskaya, L. N. Pimenova, and I. A. Votyanova, Zh. Fiz. Khim. 52, 2356 (1978). 6. N. L. Levshin, Doctoral (Phys. Math.) Dissertation (Moscow, 2000). 7. Short Handbook of Physical Chemical Values, Ed. by A. A. Ravdel and A. M. Ponomareva (Ivan Fedorov, St. Petersburg, 2002) [in Russian].

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Translated by O.N. Kadkin

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