Pair Distribution Function and Density Functional Theory Analyses of

Communication pubs.acs.org/IC

Pair Distribution Function and Density Functional Theory Analyses of Hydrogen Trapping by γ‑MnO2 Kévin Galliez,†,‡ Philippe Deniard,*,† Christophe Payen,† David Lambertin,‡ Florence Bart,‡ Hyun-Joo Koo,§ Myung-Hwan Whangbo,∥ and Stéphane Jobic† †

Institut des Matériaux Jean Rouxel (IMN), CNRS, Université de Nantes, 2 rue de la Houssinière, BP 32229, 44322 Nantes Cedex 03, France ‡ CEA, DEN, DTCD/SPDE/LP2CMarcoule, 30207 Bagnols-sur-Cèze, France § Department of Chemistry and Research Institute for Basic Sciences, Kyung Hee University, Seoul 130-701, Republic of Korea ∥ Department of Chemistry, North Carolina State University (NCSU), Raleigh, North Carolina 27695, United States S Supporting Information *

ABSTRACT: In the presence of “Ag2O” as a promoter, γMnO2 traps dihydrogen in its (2 × 1) and (1 × 1) tunnels. The course of this reaction was examined by analyzing the X-ray diffraction patterns of the HxMnO2/“Ag2O” system (0 ≤ x < 1) on the basis of pair distribution function and density functional theory (DFT) analyses. Hydrogen trapping occurs preferentially in the (2 × 1) tunnels of γ-MnO2, which is then followed by that in the (1 × 1) tunnels. Our DFT analysis shows that this process is thermodynamically favorable.

M

anganese dioxide (MnO2) is used in a variety of applications, such as lithium batteries, supercapacitors, catalysts, gas absorbers, and oxidizing agents.1 Recently, a new niche application has emerged for γ-MnO2 (i.e., nsutite); in the presence of “Ag2O” (see below), which is used to promote the dissociation H2 → 2H• (see below), γ-MnO2 instantaneously traps dihydrogen (H2), resulting from radiolysis of organic molecules in radioactive waste packages during their transportation.2 γ-MnO2, made up of edge/corner-sharing MnO6 octahedra, has (1 × 1) and (2 × 1) tunnels into which hydrogen insertion takes place (Figure 1a). To develop a more effective material for hydrogen trapping, it is desirable to understand the mechanism of the hydrogen-trapping process in the tunnels of γMnO2. Chabre and Pannetier examined this question by electrochemical experiments, in the context of battery application, to conclude that hydrogen insertion takes place primarily in the (2 × 1) tunnels.3 In this Communication, we explore the hydrogen-trapping process in γ-MnO2 by preparing samples of γ-MnO2/“Ag2O”, by analyzing their X-ray diffraction (XRD) patterns in terms of pair distribution function (PDF) analyses, and by performing density functional theory (DFT) calculations for model structures of hydrogen-trapped γ-MnO2. γ-MnO2 is obtained from a random intergrowth of the pyrolusite (β-MnO2) block containing only (1 × 1) tunnels and the ramsdellite (R-MnO2) block having only (2 × 1) tunnels (Figure 1c,d), explaining the poor quality of its XRD patterns. These tunnels serve as host sites for hydrogen insertion to form O−H bonds, as exemplified by manganite (γ-MnOOH) and groutite (α-MnOOH), which are the hydrogenated derivatives of © XXXX American Chemical Society

Figure 1. Crystal structures of γ-MnO2 (a), an IrSe2 structure type with a Pr value of 0.63 used for the simulation of γ-MnO2 (b), β-MnO2 (c), RMnO2 (d), γ-MnOOH (e), and α-MnOOH (f).

β-MnO2 and R-MnO2, respectively (Figure 1e,f). The relative amounts of the (1 × 1) and (2 × 1) tunnels in γ-MnO2 depend strongly on the synthetic conditions used. Experimentally, one used to quantify the fraction of the (1 × 1) tunnels, Pr = [1 × 1]/ {[1 × 1] + [2 × 1]},3 where [1 × 1] and [2 × 1] represent the numbers of the (1 × 1) and (2 × 1) tunnels, respectively, by using the look-up tables established after meticulous and timeconsuming studies.4 Recently, it was found that PDF analyses of conventional XRD patterns can be used to determine the Pr values.5 This results from the fact that the pair distributions of γ-MnO2 containing both (1 × 1) and (2 × 1) tunnels are well approximated as a mixture of those for β-MnO2 containing only (1 × 1) tunnels and those for R-MnO2 containing only (2 × 1) tunnels. In a similar manner, we simulate the pair distributions of the hydrogen-trapped γ-MnO2/“Ag2O” (namely, HxMnO2/ “Ag2O”, where 0 ≤ x < 1) samples as a mixture of the pair distributions of pyrolusite and ramsdellite [to model the (1 × 1) and (2 × 1) tunnels of unprotonated γ-MnO2, respectively], manganite and groutite [to model the (1 × 1) and (2 × 1) Received: October 31, 2014

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DOI: 10.1021/ic5026334 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry tunnels of protonated γ-MnO2, respectively], and Ag2CO3 and Ag (to model the products that “Ag2O” forms as a result of its reaction with H2CO3 present in a H2O solution, see below). Our study reveals the sequential trapping of hydrogen, first in the (2 × 1) and then in the (1 × 1) tunnels of γ-MnO2, in agreement with the previous investigations of Chabre and Pannetier.3 The γ-MnO2/“Ag2O” hydrogen getter was prepared as follows: First, γ-MnO2 (Merck; precipitated active for the synthesis; specific surface area of 80 m2/g 2) was ground with Ag2O (Merck, 99+%) in an agate mortar for 5 min (with a 87:13 mass ratio). Second, the as-obtained blend was sonicated for 20 min in three different solvents: distilled water, ethanol, and acetone. Then the powder was dried under a dynamic vacuum for 48 h at room temperature. Samples prepared in water exhibit a much greater trapping performance than those prepared in ethanol or acetone.5 Thus, only the getters prepared using water during sonication are considered hereafter. As discussed by Galliez et al.,5 a chemical reaction between Ag2O and CO2 present in distilled water (pH 5.5) forms Ag2CO3 as fine particles (crystallite size of ∼30 Å) that intimately decorate the surface of γ-MnO2, as shown by the scanning electron micrograph in Figure S1 in the Supporting Information (SI). The close contact of the Ag2CO3 particles with the γ-MnO2 surface probably plays a major role in the hydrogen-trapping performance of the getter when distilled water is used as a dispersant because dissociation of H2 would take place in the immediate vicinity of the γ-MnO2 surface. For simplicity, these getters will be referred to as MnO2/Ag2O getters, although the Ag2O precursor is transformed to Ag2CO3 under grinding in water and even if the presence of Ag2CO3 is required to trigger the capture of H2. The as-prepared getters were exposed to a mixture of 4% H2 + 96% N2 gases in a closed vessel, and the hydrogen capture was followed by gas chromatography (for details, see Figure S2 in the SI). The chemical reactions involved in hydrogen insertion can be envisaged as follows: Ag 2O(s) + H 2CO3(l) → Ag 2CO3(s) + H 2O(l) Ag 2CO3(s) → 2Ag(s) + CO2 (g) +

1 O2 (g) 2

ordering. Above 200 K, the susceptibility follows the Curie− Weiss law, χ(T) = C/(T − θ), as evidenced by the linear 1/χ versus T plot (Figure S5 in the SI). Table S1 summarizes the measured Curie constants (Cexp) and θ values. The Cexp values increase with x, in agreement with the theoretical values Ctheo = [n(n + 2)]/8, where n = 3(1 − x) + 4x, expected for HxMnO2 = (H+)x(Mn4+)1−x(Mn3+)x(O2−)2 on the basis of spin-only contributions with Mn3+ cations in the high-spin state. The |θ| values increase with increasing x (see Figure S6 in the SI). We found no magnetic trace of α-Mn2O3 (TN = 80 K), which excludes the possibility of the reaction 2MnO2(s) + H2(g) → Mn2O3(s) + H2O(g), hence showing that hydrogen insertion into γ-MnO2 leads to protonation of O2− to form OH− and reduction of Mn4+ to form Mn3+. To find where the inserted H atoms are located in the γ-MnO2 structure, we perform PDF analyses, a technique well suited for characterization of the strongly disordered γ-MnO2 phase, which was successfully simulated as a mixture of β-MnO2 and RMnO2.5 We carried out PDF analyses (see details in the experimental section and Figures S7 and S8 in the SI). Our PDF refinements of the MnO2/Ag2O getter before hydrogen insertion indicate that it consists of MnO2 and Ag2CO3 (as expected from eq 1) with a Ag/Mn molar ratio of 0.15 (to be compared with the nominal value of 0.11 based on the weighted MnO2/Ag2O precursor ratio) and that the γ-MnO2 material consists of 63% (1 × 1) and 37% (2 × 1) tunnels (to be compared with 66% and 34%, respectively, obtained from the method of Chabre and Pannetier).3 The results analyzed for the hydrogenated getters with the H/Mn ratios x = 0.08, 0.27, 0.4, 0.65, and 0.8 are presented in Figure 2a, where the molar concentrations of

(1) (2)

Ag(s) + H 2(g) → Ag(s) + 2H•

(3)

MnO2 (s) + x H• → HxMnO2 (s)

(4)

where H2CO3 results from the reaction of H2O and CO2 and Ag refers to slightly oxidized AgnO clusters (n ≫ 1). The latter decorate MnO2 particles and dissociate H2 into H• radicals.6 The maximum amount of hydrogen insertion into γ-MnO2 determined by chromatography is x ≈ 0.8. The hydrogen atom trapped in γ-MnO2 acts as a proton and an electron (H• → H+ + e−). Each proton H+ is captured by O2− to form OH−, so hydrogen insertion can be detected by monitoring, as a function of x, either the OH absorption bands in the IR spectra of HxMnO2 or the 2θ angles of the powder XRD peaks of HxMnO2 because their cell parameters are affected by forming OH bonds in the (1 × 1) and (2 × 1) tunnels7 (see Figure S3 in the SI). Each electron from an inserted hydrogen reduces a Mn4+ (d3, S = 3/2) cation to a Mn3+ (d4, S = 2) one, so hydrogen insertion can also be monitored by measuring the temperature-dependent magnetic susceptibilities χ(T). The χ(T) curves for various x values are presented in Figure S4 in the SI, which shows that the susceptibility increases with decreasing temperature with a maximum at ∼20 K for x ≥ 0.27. The occurrence of this maximum signals the presence of an antiferromagnetic spin

Figure 2. Evolution of the molar content of Ag2CO3 and Ag in the γMnO2/“Ag2O” hydrogen getter (a) and evolution of the percentage of empty and filled 1 × 1 and 2 × 1 tunnels (b) versus x in HxMnO2.

Ag2CO3 and Ag are plotted as a function of x. Clearly, with increasing x, the amount of Ag2CO3 decreases with increasing Ag. Figure 2b depicts the fractions of the empty and filled (1 × 1) and (2 × 1) tunnels in the γ-MnO2 structure as a function of x. Within the estimated standard deviation, the sum of the filled and unfilled fractions of the (1 × 1) and (2 × 1) tunnels remains constant and is equal to 1 along the whole series of x. The total fraction of the (1 × 1) tunnels (namely, Pr) should be constant because no major structural reconstruction is expected under hydrogen insertion. However, the Pr value varies from 0.63 to 0.48 as x changes from 0 to 0.8. This discrepancy reflects most probably the limit of our PDF analysis in which the structure of HxMnO2 is simulated by a mixture of four distinctly different structures. Nevertheless, it is clear from Figure 2b that the B

DOI: 10.1021/ic5026334 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

(2 × 1) tunnels is only slightly less stable than the state with three H atoms in the (2 × 1) tunnels. Thus, the former will take place, although less frequently, than does the latter and, hence, inserting the fourth H atom in the (1 × 1) tunnel is more favorable than that in the (2 × 1) tunnel. This explains why hydrogen insertion into the (1 × 1) tunnels starts before the (2 × 1) tunnels are fully hydrogenated for thermodynamical reasons. In summary, our study shows that hydrogen trapping by γMnO2 occurs preferentially in the (2 × 1) tunnels, which is then followed by that in the (1 × 1) tunnels.

inserted H atoms are preferentially located in the (2 × 1) tunnels of the γ-MnO2 structure at the early stage of the insertion process. Namely, with increasing x up to ∼0.4, the (2 × 1) tunnels are preferentially hydrogenated over the (1 × 1) tunnels. When x increases beyond ∼0.4, the protonation of the (2 × 1) tunnels does not increase but that of the (1 × 1) tunnels increases sharply. Thus, trapping H atoms in the (2 × 1) tunnels is more favorable than that in the (1 × 1) tunnels. Nevertheless, it is clear from Figure 2b that hydrogen insertion into the (1 × 1) tunnels starts before all of the (2 × 1) tunnels are fully protonated (at x around 0.4). This may be correlated to a steric effect, i.e., a progressive distortion of the 1 × 1 tunnels induced under the action of the filling of adjacent 2 × 1 tunnels. To account for the above observation, it is necessary to estimate the relative energies of various γ-MnO2 structures with hydrogen inserted in the (1 × 1) and (2 × 1) tunnels. To simplify our discussion, we construct a hypothetical γ-MnO2 structure with an ordered arrangement of the (1 × 1) and (2 × 1) tunnels (Figure 1f). Such an arrangement is found for IrSe2,8 which has four (1 × 1) and two (2 × 1) tunnels per unit cell. Thus, we construct the IrSe2-type γ-MnO2 (hereafter referred to as the ideal γ-MnO2) and determine its optimized structure by DFT calculations (see the SI for calculation details). We generate ideal HxMnO2 (x = 0.125, 0.25, 0.375, and 0.5) structures by inserting H atoms into the (1 × 1) and (2 × 1) tunnels of the ideal γ-MnO2 in several different ways, as depicted in Figure S9 in the SI, and then optimize their structures. As listed in Table S1 in the SI, the calculated magnetic moments of the Mn atoms show the average moment of Mn to increase with increasing x, in agreement with the picture that each inserted hydrogen is partitioned into a proton and an electron, namely, H• → H+ + e−. The energies calculated for several model structures of HxMnO2 (x = 0.125, 0.25, 0.375, and 0.5) are listed in Table S3 in the SI and Table 1.

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Experimental details, Figures S1−S9, and Tables S1−S3. This material is available free of charge via the Internet at http://pubs. acs.org.

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x

hydrogen distribution

eV/HxMnO2

one H in (1 × 1) one H in (2 × 1) one H in (1 × 1) and one H in (2 × 1) two H in (2 × 1)

−24.306 (Figure S9-1-a) −24.325 (Figure S9-1-b) −24.822 (Figure S9-2-c) −24.830 (Figure S9-2-d) −24.869 (Figure S9-2-e) −25.314 (Figure S9-3-d) −25.354 (Figure S9-3-e) −25.368 (Figure S9-3-f) −25.850 (Figure S9-4-c) −25.839 (Figure S9-4-d) −25.873 (Figure S9-4-e)

0.25

0.375

0.5

one H in (1 × 1) and two H in (2 × 1) three H in (2 × 1) two H in (1 × 1) and two H in (2 × 1) one H in (1 × 1) and three H in (2 × 1) four H in (2 × 1)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work at NCSU was supported by the computing resources of the NERSC center and the HPC center of NCSU. K.G. is indebted to AREVA for a grant.

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REFERENCES

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Table 1. Energies Calculated for the Protonated Structures HxMnO2 (See Figure S6 in the SI) 0.125

ASSOCIATED CONTENT

S Supporting Information *

Note from Table 1 that insertion of one H atom prefers the (2 × 1) tunnel, and so does that of two H atoms, with the distribution with two protons in the same tunnel being less stable than the one with two protons in distinct tunnels. Once the two 2 × 1 tunnels of the IrSe2 hypothetical unit cell are occupied by one H atom each, it is still energetically more favorable to insert the third H atom in one of the (2 × 1) tunnel than to start filling the (1 × 1) tunnel. Likewise, if there are three H atoms in the (2 × 1) tunnels, it is energetically more favorable to insert the fourth H atom in the (2 × 1) than in the (1 × 1) tunnel. This explains the preference for H insertion to take place primarily in the (2 × 1) tunnels in the initial stage of hydrogen insertion. Nevertheless, the state with one H atom in the (1 × 1) and two H atoms in the C

DOI: 10.1021/ic5026334 Inorg. Chem. XXXX, XXX, XXX−XXX