A Multisite Strategy for Enhancing the Hydrogen Evolution Reaction on a Nano‐Pd Surface in Alkaline Media
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Abstract The hydrogen evolution reaction (HER) on a noble metal surface in alkaline media is more sluggish than that in acidic media due to the limited proton supply. To promote the reaction, it is necessary to transform the alkaline HER mechanism via a multisite catalyst, which has additional water dissociation sites to improve the proton supply to an optimal level. Here, this study reports a top‐down strategy to create a multisite HER catalyst on a nano‐Pd surface and how to further fine‐tune the areal ratio of the water dissociation component to the noble metal surface in core/shell‐structured nanoparticles (NPs). Starting with Pd/Fe 3 O 4 core/shell NPs, electrochemical cycling is used to tune the coverage of iron (oxy)hydroxide on a Pd surface. The alkaline HER activity of the core/sell Pd/FeO x (OH) 2−2 x NPs exhibits a volcano‐shaped correlation with the surface Fe species coverage. This indicates an optimum coverage level where the rates of both the water dissociation step and the hydrogen formation step are balanced to achieve the highest efficiency. This multisite strategy assigns multiple reaction steps to different catalytic sites, and should also be extendable to other core/shell NPs to optimize their HER activity in alkaline media.Keywords:
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The abatement of CO by heterogeneous catalysts such as noble and non-noble metal is one of the challenging and interesting routes in environmental catalysis. In this review, the CO oxidation mechanism of noble metal and non-metal based catalyst systems is examined. Oxidation of CO to CO2 by using noble metals (Pt, Pd, Rh, and Au) and non-noble metals (Cu, Co,Mn) has been reviewed carefully regarding their role in support. The CO oxidation by noble metals has shown very high activity. However, a large number of studies have also been devoted to the CO oxidation by non-noble metals, metal oxides, modified binary metal oxides, and the like. Non-noble metals showed CO oxidation activity at lower temperature and were comparable with noble metals. The mechanistic aspects and the role of surface and gaseous oxygen in CO oxidation using transition metals/mixture of metal oxides and Pt, Pd, Rh, and Au have been studied thoroughly.
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Sandwich any one? The bis(butenyltetramethylcyclopentadienyl) complexes of calcium (2), strontium and barium exhibit an interaction of the alkaline-earth metals with olefin double bonds. Whereas the magnesium derivative (1) crystallizes as a sandwich complex with coplanar cyclopentadienyl ligands and free butenyl side chains, these coordinate to the metal in the analogous but open sandwich complexes of the larger metals calcium, strontium, and barium. The coordination of an α-olefin to the metal center of a 14-electron metallocene alkyl complex,1 for example to zirconium complexes of the type [Cp2ZrR]+ is assumed to be the primary reaction step in the polymerization of olefins catalyzed by such metallocenes.2 In the case of zirconium, however, thus far only the isolation and X-ray characterization of complexes with chelating η1,η2 or η5η2 olefin ligands have been successful.3–5 The formation of vanadium,6 niobium,7 and the yttrium complexes investigated by Casey et al.8–10 with non-chelating olefins have until now only been detectable by NMR spectroscopy. Both ourselves and Evans et al. have been able to show recently that chelating η5,η2-cyclopentadienyl ligands such as C5Me4CH2CH2CHCH211 or C5Me4SiMe2CH2CHCH212 in the form of their alkali metal salts form metallocenes with the diiodides of Eu, Sm, and Yb in which the terminal olefin groups of the side chains coordinate to the metal center. No olefin complexes of BaII, which is isoelectronic with these lanthanoids at the oxidation level LnII (apart from the occupation of the 4f shell), and the other alkaline-earth metals are thus far known. An interaction between the alkyne ligand and the magnesium could be detected only with [TiCp][MgCp][μ-η2:η2-C2(SiMe3)2]213 and [(C5HMe4)2Ti(η1-CCSiMe3)2][Mg(thf)Cl]14 from X-ray structural data, and for [(C5Me5)2Ca(Me3SiCC-CCSiMe3)]15 a coordination of the diyne to calcium. Alkaline-earth metallocenes such as [(C5HMe4)2Mg(Me-carb)] or [(C5HMe4)2Sr(Pr-carb)] (Me-carb and Pr-carb=1,3,4,5-tetramethylimidazol-2-ylidene and 1,3-di(isopropyl)-4,5-dimethylimidazol-2-ylidene, respectively) are highly active catalysts for the polymerization of acrylates;16 therefore it appeared obvious to assume that the solvent-free alkaline-earth metallocenes as 12-electron complexes could also seek to compensate their electron deficiency by coordination to olefinic π systems. Herein we confirm this assumption with the synthesis of the first alkaline-earth metal complexes with alkenyl-substituted chelating η5,η2-cyclopentadienyl ligands. C5Me4HCH2CH2CHCH2 may be readily deprotonated with KH in THF at room temperature with the formation of K[C5Me4CH2CH2CHCH2], a pyrophoric, beige-colored powder.11a This reacts with alkaline-earth diiodides (MI2) in THF suspension with loss of KI and formation of the corresponding metallocenes [M(C5Me4CH2CH2CHCH2)2], M=Mg (1), Ca (2), Sr (3), Ba (4) (Scheme 1, A). The latter are isolated as colorless, solvent-free crystals after precipitation from hexane and subsequent crystallization from hexane (1–3) or toluene (4) at −78 °C. Compound 1 may also be obtained by the reaction of dibutylmagnesium with 5-but-3-en-1-yl-1,2,3,4,-tetramethylcyclopenta-1,3-diene in heptane after heating for 12 h under reflux (Scheme 1, B). Compound 1 melts at 37 °C with the formation of a colorless liquid that rapidly turns yellow. Compounds 2, 3, and 4 decompose above 138 (2), 208 (3), and 157 °C (4) with yellow coloration without previously melting. A) Synthesis of the metallocenes 1–4 from alkaline-earth diiodides and K[C5Me4CH2CH2CHCH2]; M=Mg (1), Ca (2), Sr (3), Ba (4). B) Synthesis of 1 from dibutylmagnesium and 5-but-3-en-1-yl-1,2,3,4-tetramethylcyclopenta-1,3-diene. The isolation single crystals was very difficult and required special care since all four metallocenes are highly sensitive and decompose immediately with yellow coloration on contact with very small amounts of moisture and oxygen. The structural investigations show that only the magnesium complex 1 satisfies the strict arrangement of a sandwich structure with a parallel arrangement of the cyclopentadienyl ring planes above and below the central metal atom, an arrangement that was also found for [Mg(C5H5)2]17 and [Mg(C5Me5)2].18 Two independent molecules are located in the unit cell that differ only slightly in the angles Cp1cen-Mg1-Cp2cen (175.84°) and Cp1cen-Mg2-Cp2cen (180°) and in the torsion angles: C1r-Cp1cen-Cp2cen-C6r=−32.1(1)° and C1r-Cp1cen-Cp1′cen-C1r′=−180° (Figure 1, cen=ring centroid). The respective distance between the centers of the two cyclopentadienyl ligands (3.930 and 3.926 Å, respectively, Table 1) corresponds to the value determined for [Mg(C5Me5)2] (3.94 Å).18 The two butenyl side chains are directed away from the magnesium center above and below, respectively. Molecular structure of 1 (ORTEP,19 ellipsoids at 30 % probability). 1 (1)[a] 1 (2)[b] 2 3 (1)[a] 3 (2)[b] 4 MCpcen 1.962(2) 1.963(2) 2.382(2) 2.55(1) 2.55(1) 2.721(3) 1.968(2) 1.963(2) 2.412(2) 2.715(3) MC4s 6.882(2) 6.620(2) 3.045(2) 3.04(2) 3.179(3) MC9s 6.589(2) 6.620(2) 2.941(2) 2.99(2) 3.212(3) MC3s 5.928(2) 5.389(2) 3.230(2) 3.25(2) 3.435(3) MC8s 5.381(2) 5.389(2) 3.078(2) 3.20(2) 3.375(3) C3sC4s 1.306(2) 1.319(2) 1.267(2) 1.32(2) 1.316(3) C8sC9s 1.314(2) 1.319(2) 1.361(2) 1.33(2) 1.316(3) Cp-M-Cp 175.84(5) 180 140.92(6) 139.3(4) 140.4(4) 139.1(1) C4s-M-C9s 81.30(6) 88.9(4) 85.3(4) 98.55(7) C3s-M-C8s 105.52(6) 93.8(4) 92.2(4) 102.17(7) The compounds with the heavy alkaline-earth metals, 2–4, have in principal the same molecular structure with C2 symmetry (3) or approximate C2 symmetry (2, 4; see Figure 2). Unlike in 1, but in agreement with the structures of [Ca(C5Me5)2],20 [Sr(C5Me5)2],21 and [Ba(C5Me5)2],20, 21 the cyclopentadienyl rings in 2–4 are inclined towards each other; furthermore, the butenyl side chains of the cyclopentadienyl ligands are directed towards the metal center. This bent structure, which all metallocenes of the heavy alkaline-earth metals and also metallocenes of EuII,22 SmII,22 and YbII23 exhibit, cannot be attributed here to a lone pair of electrons at the metal center as in the likewise bent metallocenes of Ge, Sn, and Pb. On the contrary, it is accounted for by a slight polarizability of the original spherically symmetrical distribution of the positive charge of the large metal cations caused by the negatively charged Cp ligands, which reduces their mutual repulsion.22 Further explanations for the bent arrangement of the Cp ligands are their Van der Waals attractions24, 25 or the (n−1)d orbital involvement of the metal centers in the sense of an (n−1)dns hybridization.26 Molecular structure of 2 (left) and 4 (right; ORTEP,19 ellipsoids at 30 % probability). The centers of the Cp ligands and the double bonds in 2–4 form a strongly distorted tetrahedral coordination sphere around the metals. The centers of the cyclopentadienyl ligands form an angle of almost exactly 140° with the metal center in all three compounds (Table 1): 140.9° (2), 139.3/140.4° (3), and 139.1° (4). In contrast very different Cpcen-M-Cpcen angles, dependent upon the size of the central metal atom are found for both the solvent-free alkaline-earth compounds [M(C5Me5)2] and the alkaline-earth carbene complexes [(C5Me5)2ML] (L=1,3,4,5-tetramethylimidazol-2-ylidene): 147.7° for [Ca(C5Me5)2] and 131° for [Ba(C5Me5)2] at 116 and 118 K, respectively, in the crystal,20 149° for [Sr(C5Me5)2] and 148° for [Ba(C5Me5)2] in the gas phase,21 and 143.9° for [(C5Me5)2CaL] and 137.0° for [(C5Me5)2BaL] at −70 °C in the crystal.27 This difference and, with consideration of the metal radius, the almost identical MC distances of the olefin-carbon atoms coordinated unsymmetrically to the metal center (MC3s and MC8s=3.23/3.08 (2), 3.25/3.20 (3), 3.44/3.38 (4) and MC4s and MC9s=3.05/2.94 (2), 3.04/2.99 (3), 3.18/3.21 Å (4)) are a clear confirmation that this orientation of the butenyl side chains directed towards the metal center is caused by a coordination of the olefin to the alkaline-earth metal center and not perhaps by crystal packing effects. The distances to the terminal carbon atoms C4s and C9s are generally shorter than to C3s and C8s, but significantly longer than to the cyclopentadienyl carbon atoms. The range of the distances, which is moreover relatively large, and the constant CC lengths in the open magnesium derivative 1 and in 2–4 suggest, however, a very labile coordination of the olefin ligands. Crystallization of the complexes from THF is not possible because they are too soluble. No crystals suitable for X-ray structure analysis could be isolated even by the addition of small amounts of THF to hexane or toluene solutions of 2–4. Therefore it was not possible in this way to determine whether the olefin side chains are displaced from the coordination sphere of the metals by the Lewis basic THF and whether THF complexes with non-coordinated olefin side chains are formed. The 1H NMR spectra of 1–4 recorded in [D6]benzene (selected data in Table 2) show almost identical chemical shifts for the C5Me4, α-CH2, β-CH2, and γ-CH protons of the ligands. The respective chemical shifts of the signals of the cis- and trans-δ-CH2CH protons of 3 and 4 are also practically identical, although they are shifted by up to 0.3 ppm to higher field compared to the corresponding signals of 1 (with the butenyl groups orientated away from the magnesium center). With the addition of [D8]THF, which as σ-donor ligand should displace the olefin from the coordination sphere of the metal, the chemical shifts of all signal groups of 1 change only marginally, in the case of 4, however, the signal of the cis-CH2CH protons shifts by 0.11, and that of the trans-CH2CH proton by 0.17 ppm to lower field and thus in the direction of the signals of 1. With [Yb(C5Me4SiMe2CH2CHCH2)2] too a low field shift of the same order of magnitude for the CH2CH protons was observed with the change from benzene to THF as solvent, although with a simultaneous high field shift of the CH2CH protons.12 These small signal shifts do not allow any prediction about whether the olefin coordinates to the alkaline-earth metal or ytterbium center, since unlike the transition-metal–-olefin complexes no electron density modifying backbonding is possible. In low-temperature 1H NMR spectra of 4 in [D8]THF the position of the CH2CH and CH2CH signals remain constant to −83 °C. Over the same temperature range in [D8]toluene, however, they are shifted successively to higher fields by up to 0.16 ppm. Compound 3 also shows high-field shifts of up to 0.2 ppm for these signals in [D8]toluene at −83 °C. 1H NMR 13C NMR Solvent δCH δ CHCHH (trans) δ CHCHH (cis) δCH δCH2 1 [D6]benzene 5.83 (ddt) 5.07 (ddt) 4.97 (ddt) 138.93 114.64 1 [D8]THF 5.78 (ddt) 4.98 (ddt) 4.89 (ddt) 139.56 114.57 2 [D6]benzene 5.82 (ddt) 4.81 (m) 147.63 115.03 3 [D6]benzene 5.83 (ddt) 4.73 (ddt) 4.70 (ddt) 149.82 115.20 4 [D6]benzene 5.89 (ddt) 4.79 (ddt) 4.73 (ddt) 148.80 115.92 4 [D8]THF 5.83 (ddt) 4.96 (ddt) 4.84 (ddt) 142.17 113.78 The 13C NMR spectra of 1–4 recorded in both [D6]benzene and [D8]toluene as well as in [D8]THF show the expected nine signals, which depending upon the metal and the solvent show no substantial shifts apart from the inner olefin-carbon atom. The γ-C signal in the spectra measured in [D6]benzene shifts from δ=138.93 (1) through 147.63 (2) and 149.82 (3) to 148.80 ppm (4). In contrast in the 13C NMR spectrum of a solution of 4 in [D8]THF the signal for the γ-C atom shows a chemical shift of δ=142.17 ppm, a value that is close to the value for 1. In the coordinating solvent THF the olefin is clearly displaced from the coordination sphere of the barium center. This effect also corresponds to that for the high-field shift found for the inner olefin-carbon atom in [Yb(C5Me4SiMe2CH2CHCH2)2] on comparison of the spectrum in [D6]benzene (δ=147.6 ppm) with that in [D8]THF (δ=140.5 ppm).12 The X-ray structure analyses of these olefin complexes of the alkaline-earth metals (2–4) synthesized for the first time show an orientation of the terminal olefin double bonds of both ligands in the direction of the Ca, Sr, and Ba ions. Owing to the lack of (n−1)d electrons of the alkaline-earth metal ions, this weak coordination cannot be interpreted as a σ-donor–π-acceptor interaction as in classical olefin complexes of transition metals. The NMR spectra measured in [D6]benzene and [D8]toluene show, however, a weak high-field shift of the signals of the δ-CH2 protons; this may be interpreted by the influence exerted upon the double bond attracted towards the alkaline-earth metal center by the electron shell of the metal negatively polarized in the direction of the olefin, as is suggested by the electrostatic model for the explanation of the bent structure of the metallocenes of the heavy alkaline-earth metals.22 A reduction in the Cp-M-Cp angle to 140.92(6)° in 2 and 140.4(4)° in 3 compared with 147.7 and 149°, respectively, for the non-olefin-coordinated analogues [Ca(C5Me5)2] and [Sr(C5Me5)2] is in agreement with this. The widening of this angle of 131° in the case of [Ba(C5Me5)2] to 139.1(1)° in 4 is attributable to the enlarged ionic radius of BaII. σ-Donor ligands such as THF displace the weakly coordinated olefin from the alkaline-earth metal ion, which is demonstrated for the barium derivative 4 by its 1H and 13C NMR spectra in [D8]THF. Analogous structural and bonding relationships are found in the corresponding YbII derivative [Yb(C5Me4CH2CH2CHCH2)2]11a and in [Ln(C5Me4SiMe2CH2CHCH2)2],12 which suggests a remarkable similarity between the organyls of the heavy alkaline-earth metals and the lanthanoids in the oxidation state LnII. This was discussed in detail recently for the chemistry and catalytic behavior of CaII and YbII compounds.28 The synthesis and the comprehensive spectroscopic characterization of the alkaline-earth complexes 1–4 and their crystal data are available in the Supporting Information. CCDC-240854—240857 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or [email protected]). Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2002/2004/z460927_s.pdf or from the author. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.
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Based on the features of alkaline-earth metals,this paper aims to analyze the function of such elements as Be,Ca,Sr,Ba in magnesium alloys.It explores the interactions among different alkaline-earth metals in magnesium alloys,also the interactions between alkaline-earth metals and non-alkaline-earth metals.Further it probes into the roads to development of alkaline-earth magnesium alloys.
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In Pt2Ng2F4and in [Au2Ng2F4]2+, Ng = Kr, Xe, Rn, the noble gas atoms act as links bridging two noble metal atoms.
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Abstract In order to obtain further insight into the influence of Ba replacement on the properties and structures of YBa 2 Cu 3 O 7‐x superconductors, mixed oxides of the composition YM 2 Cu 3 O 7‐x with M: Ba, Sr, Ca, or mixtures of two alkaline‐earth metals are synthesized and characterized.
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ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTSynthesis and spectroscopic properties of bis(trimethylsilyl)amides of the alkaline-earth metals magnesium, calcium, strontium, and bariumMatthias WesterhausenCite this: Inorg. Chem. 1991, 30, 1, 96–101Publication Date (Print):January 1, 1991Publication History Published online1 May 2002Published inissue 1 January 1991https://pubs.acs.org/doi/10.1021/ic00001a018https://doi.org/10.1021/ic00001a018research-articleACS PublicationsRequest reuse permissionsArticle Views4260Altmetric-Citations299LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access optionsGet e-Alertsclose Get e-Alerts
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