Hydrogen evolution by the catalytic decomposition of formic acid in solution is a key reaction for hydrogen storage for which palladium is one of the most efficient catalysts. Based on density functional theory (DFT) computations, we explain why the presence of an anionic promoter renders palladium more active and more selective for formic acid dehydrogenation. The promotion is well-captured by modeling the anion by a negatively charged surface. This promotional effect can be traced back to the modulation of the electric field at the catalyst surface, with a strongly contrasted action on the energy of the various species along the competing pathways through the electrostatic interaction between the electric field and the surface dipole moment. As a result, both the reaction kinetics and selectivity are markedly improved. This opens the door to a rational design of catalytic systems using promoters.Keywords: anionic promoter; DFT; electric field-dipole interaction; formate anion; formic acid; H2 production;

Surface coordination chemistry of nanomaterials deals with the chemistry on how ligands are coordinated on their surface metal atoms and influence their properties at the molecular level. This Perspective demonstrates that there is a strong link between surface coordination chemistry and the shape-controlled synthesis, and many intriguing surface properties of metal nanomaterials. While small adsorbates introduced in the synthesis can control the shapes of metal nanocrystals by minimizing their surface energy via preferential coordination on specific facets, surface ligands properly coordinated on metal nanoparticles readily promote their catalysis via steric interactions and electronic modifications. The difficulty in the research of surface coordination chemistry of nanomaterials mainly lies in the lack of effective tools to characterize their molecular surface coordination structures. Also highlighted are several model material systems that facilitate the characterizations of surface coordination structures, including ultrathin nanostructures, atomically precise metal nanoclusters, and atomically dispersed metal catalysts. With the understanding of surface coordination chemistry, the molecular mechanisms behind various important effects (e.g., promotional effect of surface ligands on catalysis, support effect in supported metal nanocatalysts) of metal nanomaterials are disclosed.

Here we present a comprehensive survey on identification of the active sites for n-butane activation over binary vanadium phosphorus oxides. Density functional theory calculations show that the activity can be spread over all P═O sites through −OP– chain(s). With an increase in the −OP– chain(s), the activities are gradually decayed. We demonstrate that such a tendency can be quantitatively described by the center of P═O lone-pair band (εlp).Keywords: active site; C−H bond activation; DFT; lone-pair band center; PCET; VPO;

•NiS2 nanosheets are transformed into ultrathin metallic Ni nanosheets under alkaline HER conditions.•In situ X-ray absorption spectroscopy is applied to probe the change of electrocatalysts.•Trace surface sulfide on Ni(0) promotes HER performance.•An electrolyzer with a potential of 1.52 V to reach 10 mA cm−2 is built.Transition-metal chalcogenides have attracted great attention for their superior catalytic activity towards hydrogen evolution reaction (HER) as an alternative to platinum. Here we report a facile method for synthesizing two-dimensional nickel disulfide (NiS2) by using Ni(OH)2 on nickel foam as substrate. The as-synthesized NiS2 displayed an activation period during HER with a remarkable structural and compositional change under alkaline conditions. Electrochemical in situ X-ray absorption spectroscopy revealed that metallic Ni acted as catalytic active species with superior activity of 67 mV to reach 10 mA cm−2. The in situ generated metallic Ni were easily oxidized to large-area ultrathin Ni(OH)2 when exposed to air. The overall water splitting device was fabricated by using NiS2-derived metallic Ni and Fe doped NiS2-derived hydroxide as HER and OER electrode with a potential of 1.52 V to reach 10 mA cm−2.Download high-res image (240KB)Download full-size image

•Ni-thiolate coordination polymer is synthesized as a catalyst for alkaline HER•Ni-thiolate transforms into metallic Ni with superior catalytic activity during HER•In situ XAS reveals that metallic Ni is the active species for alkaline HERAlkaline water electrolysis has been emerging as a clean and renewable way to produce H2. The fabrication of active and stable earth-abundant electrocatalysts for the hydrogen evolution reaction (HER) has attracted great attention. Here, we synthesized a Ni-thiolate coordination polymer and found that it can transform into metallic Ni nanosheets under alkaline HER conditions with superior activity. This research highlights the importance of using in situ techniques to investigate catalytic active sites and deepens our understanding of the surface coordination chemistry of electrocatalysts in promoting HER activity.Two-dimensional (2D) nanosheets (NSs) of a Ni-S coordination polymer have been successfully synthesized with the use of 2D Ni(OH)2 NSs grown on conductive carbon cloth as the template and 1,4-benzenedithiol as the ligand. In situ X-ray absorption spectroscopy revealed that the as-prepared catalyst was entirely transformed into ultrathin Ni NSs under alkaline reductive conditions. The in-situ-generated catalysts exhibited superior activity toward the hydrogen evolution reaction (HER) with an overpotential of 80 mV to reach 10 mA cm−2. Studies revealed that the large-area ultrathin Ni NSs served as active sites for H2 generation, and the trace sulfur adsorbed on the Ni surface promoted water dissociation. This work has developed a templating approach for preparing highly active HER electrocatalysts and identified the real active sites under electrocatalytic conditions.Download high-res image (297KB)Download full-size image

Shape-controlled Pd nanocrystals are used as model catalysts to demonstrate facet-dependent catalysis in the hydrogenation of olefins. The close packed Pd{111} shows high catalytic activity for styrene but not for trans-stilbene. However, the open Pd{100} facets hydrogenate both olefins. DFT calculations reveal that the hydrogenation reactivity critically depends on the stability of half-hydrogenated intermediates.

Spin-polarized density functional theory (DFT) calculations are carried out to determine the site preference of H adsorption on Pd(100) surface and subsurface. We carefully scrutinize the energy difference between different patterns at θ=0.50 ML and confirm the LEED observation that surface adsorption can form c(2×2) ordering structure. On the contrary, we disclose that p(2×1) structure become more favorable than c(2×2) for subsurface adsorption. These site preferences are rationalized via an analysis of the layer and orbital resolved density of states. Furthermore, we propose that the interstitial charge as a key factor determining the preferred H adsorbed site.

Carbon monoxide can adsorb specifically on Pd(111) to induce the formation of unique Pd nanostructures. In the copresence of CO and H2, single-crystalline Pd tetrapod nanocrystals have now been successfully prepared. The Pd tetrapods are enclosed by (111) surfaces and are yielded through hydride formation. Density functional theory calculations revealed that the formation of PdHx in the presence of H2 reduces the binding energy of CO on Pd and thus helps to decrease the CO coverage during the synthesis, which is essential to the formation of the PdHx tetrapod nanocrystals. In addition to tetrapod nanocrystals, tetrahedral nanocrystals were also produced in the copresence of CO and H2 when the reaction temperature was ramped to further lower the CO coverage. Upon aging in air, the as-prepared PdHx nanocrystals exhibited a shape-dependent hydrogen releasing behavior. The conversion rate of PdHx tetrapod nanocrystals into metallic Pd was faster than that of tetrahedral nanocrystals.

A new strategy for synthesis of Pt nanocubes on various supports by reduction of a Pt precursor under a CO atmosphere was described. The as-prepared Pt nanocubes supported on multi-walled carbon nanotubes exhibited high activity toward methanol electrooxidation.

Reactions of LGeMe (L = HC[C(Me)N-2,6-iPr2C6H3]2) with 0.25 or 0.5 equiv of (CuC6F5)4 gave the products [LGe(Me)CuC6F5]2 (1) and [LGe(Me)(CuC6F5)2]2 (2), respectively. In situ formed 1 reacted with 0.5 equiv of (CuC6F5)4 to give 2 on the basis of NMR (1H and 19F) spectral measurements. Conversely, 2 was converted into 1 by treatment with 2 equiv of LGeMe. Reactions of LGeC(SiMe3)N2 with 1 or 2 equiv of AgC6F5·MeCN produced the corresponding compounds LGe[C(SiMe3)N2]AgC6F5 (3) and {LGe[C(SiMe3)N2](AgC6F5)2}2 (4). Similarly, 3 was converted into 4 by treatment with 1 equiv of AgC6F5·MeCN and 4 converted into 3 by reaction with 2 equiv of LGeC(SiMe3)N2. X-ray crystallographic studies showed that 1 contains a rhombically bridged (CuC6F5)2, while 2 has a chain-structurally aggregated (CuC6F5)4, both supported by LGeMe. Correspondingly, 3 showed a terminally bound AgC6F5 and 4 a chain-structurally aggregated (AgC6F5)4, both supported by LGeC(SiMe3)N2. Photophysical studies proved that the Ge–Cu metal–metalloid donor–acceptor bonding persists in solutions of 1 and 2 and Ge–Ag donor–acceptor bonding in solutions of 3 and 4 as a result of the clear migration of their emission bands compared to those of the corresponding starting materials. Low-temperature (−50 °C) 19F NMR spectral measurements detected dissociation of 1, 2, and 4 by the aggregation part of the CuC6F5 or AgC6F5 entities in solution. These results provide good support for pentafluorophenylcopper(I) or -silver(I) species having β-diketiminate germylene as a donor because of its remarkably electronic and steric character.

In many previous studies, nonaqueous synthesis of Pt nanocubes with tunable size has been achieved by the use of metal carbonyls (e.g., Fe(CO)5, Co2(CO)8, W(CO)6). The presence of zero-valent metals in the carbonyls was demonstrated as the key factor to the nanocube formation but the role of CO was entirely ignored. By using CO alone, we have now demonstrated that the favorable growth of Pt nanocubes in the presence of CO is mainly owing to the effect that the Pt (100) surface is stabilized by the co-adsorption of CO and amine.

In many previous studies, nonaqueous synthesis of Pt nanocubes with tunable size has been achieved by the use of metal carbonyls (e.g., Fe(CO)5, Co2(CO)8, W(CO)6). The presence of zero-valent metals in the carbonyls was demonstrated as the key factor to the nanocube formation but the role of CO was entirely ignored. By using CO alone, we have now demonstrated that the favorable growth of Pt nanocubes in the presence of CO is mainly owing to the effect that the Pt (100) surface is stabilized by the co-adsorption of CO and amine.

Shape-controlled Pd nanocrystals are used as model catalysts to demonstrate facet-dependent catalysis in the hydrogenation of olefins. The close packed Pd{111} shows high catalytic activity for styrene but not for trans-stilbene. However, the open Pd{100} facets hydrogenate both olefins. DFT calculations reveal that the hydrogenation reactivity critically depends on the stability of half-hydrogenated intermediates.

A new strategy for synthesis of Pt nanocubes on various supports by reduction of a Pt precursor under a CO atmosphere was described. The as-prepared Pt nanocubes supported on multi-walled carbon nanotubes exhibited high activity toward methanol electrooxidation.