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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Remarkable Synergistic Catalysis of Ni-Doped Ultrafine TiO2 on Hydrogen Sorption Kinetics of MgH2 Jiguang Zhang,†,‡ Rui Shi,†,‡ Yunfeng Zhu,*,†,‡ Yana Liu,†,‡ Yao Zhang,§ Shanshan Li,†,‡ and Liquan Li†,‡ †
College of Materials Science and Engineering, Nanjing Tech University, 5 Xinmofan Road, Nanjing 210009, P.R. China Jiangsu Collaborative Innovation Centre for Advanced Inorganic Function Composites, Nanjing Tech University, Nanjing 210009, P.R. China § School of Materials Science and Engineering, Southeast University, Nanjing 211189, P.R. China Downloaded via UNIV OF ARIZONA on July 24, 2018 at 04:23:39 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
‡
S Supporting Information *
ABSTRACT: Catalysts play an extraordinarily important role in accelerating the hydrogen sorption rates in metal−hydrogen systems. Herein, we report a surprisingly synergetic enhancement of metal−metal oxide cocatalyst on the hydrogen sorption properties of MgH2: only 5 wt % doping of Ni into ultrafine TiO2 enables a significant increase in hydrogen desorption kinetics; it absorbs 4.50 wt % hydrogen even at a low temperature of 50 °C. The striking improvement is partially ascribed to the formation of a particular Ni@TiO2 core−shell structure, thereby forming versatile interfaces. This study provides insights into the way of designing high-efficiency catalysts in hydrogen storage and other energy-related fields.
KEYWORDS: Mg hydride, catalyst, synergetic effect, hydrogen sorption kinetics, core−shell structure
T
MHs, hydrogen pump,22 spillover,23 electron transfer or interaction,24 stress-induced desorption,25 and hydrogeninduced interfacial coupling effect26 have been proposed to elaborate the beneficial effect of the additives. Furthermore, the catalysts may act as activation agents or nucleation sites, thereby forming either the trapping centers or intermediate metastable states and accelerating the entire reaction rate of the system.27 One of the most intriguing discoveries in the catalyst field is known as the synergetic effect between the coadding catalytic phases. For instance, bimetallic nanoparticles have been intensively studied for oxidation/reduction reactions due to their higher activity and selectivity than monometallic nanoparticles.28−30 Such a synergistic effect is also thought to be applicable for the catalyzed MH. When two or more catalysts are mutually introduced by combining individuals from different species, unexpectedly astonishing enhancement of the sorption performance occurs. The catalytic efficiency achieved by collaborative doping of nanocatalysts is far superior to that of a single catalyst, and such remarkable promotion does not seem to stem from the superposition of the individual functions. Verón et al.31 reported a study regarding the catalytic enhancement of the coupled addition of
he development of hydrogen energy is mainly driven by the increasing demand to be rid of dependency on traditional fossil fuels and switch to a more clean and sustainable society.1,2 Materials are, however, required to produce, store, and utilize hydrogen in an effective and safe way.3−6 Storing hydrogen in solid-state media via chemical interaction has attracted extensive attention.7−9 For a few decades, as a representative simple metal hydride (MH), MgH2 has been considered to be promising toward automobile or portable applications via an onboard hydrogen storing strategy due to its large hydrogen storage density (7.6 wt %), good reversibility, environmental friendliness, and desirable economic aspects.10 The operating temperature for pure MgH2, however, is always above 300 °C with poor hydrogen sorption kinetics regarding the covered surface oxide layer11−13 and the deficiency of the H diffusion coefficient in bulk MgH2.14,15 Recently, the attempt to simultaneously tune both the thermodynamics and kinetics of Mg-based hydrides via a novel synthesis technique, dielectric barrier discharge plasmaassisted milling, has largely been reported.16,17 Special emphasis has been put on the design of new types of catalysts with unique microstructure, large surface area, and, most importantly, excellent catalytic activity facilitating hydrogen ab/desorption rates.18−21 The catalysts covering carbonaceous materials, transition metal and alloys, metallic oxides, fluorides, sulfides, hydrides, and so forth have been widely used to accelerate hydrogen sorption kinetics. In the field of catalyzed © XXXX American Chemical Society
Received: May 5, 2018 Accepted: July 20, 2018 Published: July 20, 2018 A
DOI: 10.1021/acsami.8b06865 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Letter
ACS Applied Materials & Interfaces
Figure 1. (a) XRD patterns of the commercial Ni and as-synthesized Ni/TiO2 composite; TEM images of the Ni/TiO2 catalyst: (b) bright and (c) dark field images with the objective aperture placed over a portion of the TiO2 (101) diffraction ring.
Figure 2. Isothermal hydrogenation curves of the as-milled (S1) MgH2, (S2) MgH2 + 5 wt % Ni, (S3) MgH2 + 5 wt % TiO2, and (S4) MgH2 + 5 wt % Ni/TiO2 samples at (a) 50, (b) 75, (c) 100, and (d) 150 °C under the initial hydrogen pressure of 3.0 MPa.
tend to decompose into Ni, MgS, and Mg2Ni multiphase, and catalyze the hydrogen reactions. Actually, in our previous works, we have demonstrated the synergetic effect among the individual ingredients. Pronounced improvement on the sorption properties of MgH2 has not only been witnessed by combining MgH2 with other hydrides such as LiAlH434 or Mg2NiH425 but also occurs when introducing multi-catalytic phases, e.g., Ni + Pd,35 MWCNTs + TiO2,36 and MWCNT + Zr0.7Ti0.3Mn2.37 Although some promising results have been achieved, that is, the collaborative enhancement of both the accelerating kinetics and the lower onset temperature of MgH2 desorption, there still exist some limitations in terms of the nanostructuring design, selection
Co and MWCNTs, and they have shown the synergetic effect of Co, Mg−Co−H complex phase and MWCNTs in improving the hydrogen ab/desorption properties of Mg/ MgH2. Ouyang et al.32 combined the catalytic phases of Ni and CeH2.37 NPs through in situ hydrogenation of Mg80Ce18Ni2 alloy. On the basis of the synergistic effect of the in situ-formed CeH2.73 and Ni, the nanocomposite can desorb hydrogen at 232 °C with fast kinetics. In a study by Hou et al.,33 a coupling effect of MWCNTs and TiF3 on the hydrogenation of meltspun Mg−Ni alloy was proposed. Xie et al.20 found that hydrogen sorption kinetics of Mg/MgH2 are significantly enhanced by adding unique flowerlike NiS particles, which B
DOI: 10.1021/acsami.8b06865 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Letter
ACS Applied Materials & Interfaces
Figure 3. Dehydriding curves of the as-milled (S1) MgH2, (S2) MgH2 + 5 wt % Ni, (S3) MgH2 + 5 wt % TiO2, and (S4) MgH2 + 5 wt % Ni/TiO2 measured at (a) 250 and (b) 300 °C under 0.005 MPa hydrogen pressure.
The Ni-doped TiO2 catalyst Ni/TiO2 was then introduced to the MgH2 system by MM to form MgH2 + 5 wt % Ni/TiO2 composite (S4). Three other milled samples, MgH2 (S1), MgH2 + 5 wt % Ni (S2), and MgH2 + 5 wt % TiO2 (S3), were all used for comparative research. As shown in the XRD results (Figure S4), all the milled composites consist of mainly the tetragonal β-MgH2 phase. Signals responsible for fcc Ni were detected in the samples of S2 and S4. The other catalytic-phase TiO2, however, has not been found, probably due to the fact that the ultrafine particle size was further refined during MM and the limited contents. For the synergetic catalytic effect of the Ni/TiO2 to be studied, isothermal hydrogen absorption kinetics at various temperatures were measured (Figure 2), and detailed data are summarized in Table S1. The results show that the as-milled MgH2 can hardly absorb hydrogen even at a high temperature of 150 °C. After catalyst addition, samples of MgH2 + 5 wt % Ni/TiO2, MgH2 + 5 wt % Ni, and MgH2 + 5 wt % TiO2 gradually started the hydrogen absorption at all temperatures. Note that fast hydrogenation kinetics were obtained for the MgH2 + 5 wt % Ni/TiO2 composite independent of the temperature used. This means that catalytic enhancement of Ni/TiO2 is far superior to that of each individual component, indicating the collaborative promoting effect between TiO2 and Ni on the hydrogen uptake rate of Mg. In particular, it even can absorb 4.50 wt % hydrogen at the relatively low temperature of 50 °C and quickly reach the maximum value in less than 60 s at 100 °C. Another interesting phenomena is the temperature-dependent effect on the catalytic activity of TiO2 and Ni individuals: at lower temperatures (50 and 75 °C), Ni demonstrated a superior catalytic effect to that of TiO2; at elevated temperatures (100 and 150 °C), however, TiO2 is better than Ni. The mechanism behind this finding needs to be clarified in future studies. For the amount of hydrogen desorbed as a function of temperature to be evaluated, thermal desorption experiments were performed (Figure S5). The referenced sample initiated hydrogen release at ∼305 °C. This value was substantially decreased to 210 and 228 °C by adding Ni and TiO2, respectively. In particular, the MgH2 catalyzed by Ni/TiO2 started to desorb hydrogen at only ∼190 °C, and the dehydrogenation reaction was completed before 350 °C with a total release of 6.60 wt % of hydrogen, suggesting their prominently positive effects on the dehydrogenation of MgH2
of the catalytic species, as well as in-depth understanding of the synergetic effect between phases. We attempt to verify herein whether the metal−metal oxide nanocomposite has such a collaborative improvement on Mg/ MgH2 by strict contrast experiments. Through a facile chemical method, we have successfully doped Ni into ultrafine TiO2 grains (Ni/TiO2), forming a unique Ni@TiO2 core− shell structure in the absence of extra template or substrate. A striking synergetic effect between Ni and TiO2 has been confirmed on the ab/desorption properties of MgH2 at lower temperatures. The underlying mechanism was discussed. This study is essential for the understanding and design of high efficiency catalysts for hydrogen storage application of light weight MHs, providing insights into collaborative enhancement in catalysis- and energy-related areas. Ni/TiO2 cocatalyst was synthesized via a modified hydrothermal synthesis method. The structural analysis of Ni/TiO2 was carried out using XRD studies (Figure 1a). As for the assynthesized Ni/TiO2, in addition to the face-centered cubic (fcc) phase Ni (JCPDS 04-0850), the other diffraction peaks can be well-indexed as anatase TiO2 (JCPDS 21-1227). Meanwhile, the broadened diffraction peaks of TiO2 indicate its nanocrystalline characteristic with ultrasmall grain size. The Ni doping content is 4.95 wt % in Ni/TiO2 cocatalyst according to Rietveld analysis (Figure S1). The surface configuration of the as-synthesized catalysts was determined by XPS (Figure S2). Two weak peaks at 853.1 and 870.2 eV attributed to Ni 2p3/2 and Ni 2p1/2, respectively, have been recognized, indicating the successful doping of Ni into TiO2. Both the bright and dark field TEM images of the Ni/TiO2 (Figure 1b and c, Figure S3a) have revealed the ultrafine particle size (
