Better catalysts are needed to address numerous challenges faced by humanity. In this Perspective, we review concepts and tools in theoretical and computational chemistry that can help to accelerate ...the rational design of homogeneous and heterogeneous catalysts. In particular, we focus on the following three topics: (1) identification of key intermediates and transition states in a reaction using the energetic span model, (2) disentanglement of factors influencing the relative stability of the key species using energy decomposition analysis and the activation strain model, and (3) discovery of new catalysts using volcano relationships. To facilitate wider use of these techniques across different areas, we illustrate their potentials and pitfalls when applied to the study of homogeneous and heterogeneous catalysts.
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The combination of well-defined molecular cavities and chemical functionality makes crystalline porous solids attractive for a great number of technological applications, from catalysis to gas ...separation. However, in contrast to other widely applied synthetic solids such as polymers, the lack of processability of crystalline extended solids hampers their application. In this work, we demonstrate that metal-organic frameworks, a type of highly crystalline porous solid, can be made solution processable via outer surface functionalization using N-heterocyclic carbene ligands. Selective outer surface functionalization of relatively large nanoparticles (250 nm) of the well-known zeolitic imidazolate framework ZIF-67 allows for the stabilization of processable dispersions exhibiting permanent porosity. The resulting type III porous liquids can either be directly deployed as liquid adsorbents or be co-processed with state-of-the-art polymers to yield highly loaded mixed matrix membranes with excellent mechanical properties and an outstanding performance in the challenging separation of propylene from propane. We anticipate that this approach can be extended to other metal-organic frameworks and other applications.
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Systematic evaluation of the accuracy of exchange-correlation functionals is essential to guide scientists in their choice of an optimal method for a given problem when using density functional ...theory. In this work, accuracy of one Generalized Gradient Approximation (GGA) functional, three meta-GGA functionals, one Nonseparable Gradient Approximation (NGA) functional, one meta-NGA, and three hybrid GGA functionals was evaluated for calculations of the closest interatomic distances, cohesive energies, and bulk moduli of all 3d, 4d, and 5d bulk transition metals that have face centered cubic (fcc), hexagonal closed packed (hcp), or body centered cubic (bcc) structures (a total of 27 cases). Our results show that including the extra elements of kinetic energy density and Hartree–Fock exchange energy density into gradient approximation density functionals does not usually improve them. Nevertheless, the accuracies of the Tao–Perdew–Staroverov–Scuseria (TPSS) and M06-L meta-GGAs and the MN12-L meta-NGA approach the accuracy of the Perdew−Burke−Ernzerhof (PBE) GGA, so usage of these functionals may be advisable for systems containing both solid-state transition metals and molecular species. The N12 NGA functional is also shown to be almost as accurate as PBE for bulk transition metals, and thus it could be a good choice for studies of catalysis given its proven good performance for molecular species.
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Membrane-based alkaline water electrolyser is promising for cost-effective green hydrogen production. One of its key technological obstacles is the development of active catalyst-materials for ...alkaline hydrogen-evolution-reaction (HER). Here, we show that the activity of platinum towards alkaline HER can be significantly enhanced by anchoring platinum-clusters onto two-dimensional fullerene nanosheets. The unusually large lattice distance (~0.8 nm) of the fullerene nanosheets and the ultra-small size of the platinum-clusters (~2 nm) leads to strong confinement of platinum clusters accompanied by pronounced charge redistributions at the intimate platinum/fullerene interface. As a result, the platinum-fullerene composite exhibits 12 times higher intrinsic activity for alkaline HER than the state-of-the-art platinum/carbon black catalyst. Detailed kinetic and computational investigations revealed the origin of the enhanced activity to be the diverse binding properties of the platinum-sites at the interface of platinum/fullerene, which generates highly active sites for all elementary steps in alkaline HER, particularly the sluggish Volmer step. Furthermore, encouraging energy efficiency of 74% and stability were achieved for alkaline water electrolyser assembled using platinum-fullerene composite under industrially relevant testing conditions.
Abstract
In-plane sulfur vacancies (Sv) in molybdenum disulfide (MoS
2
) were newly unveiled for CO
2
hydrogenation to methanol, whereas edge Sv were found to facilitate methane formation. Thus, ...selective exposure and activation of basal plane is crucial for methanol synthesis. Here, we report a mesoporous silica-encapsulated MoS
2
catalysts with fullerene-like structure and atomic copper (Cu
/
MoS
2
@SiO
2
). The main approach is based on a physically constrained topologic conversion of molybdenum dioxide (MoO
2
) to MoS
2
within silica. The spherical curvature enables the generation of strain and Sv in inert basal plane. More importantly, fullerene-like structure of few-layer MoS
2
can selectively expose in-plane Sv and reduce the exposure of edge Sv. After promotion by atomic copper, the resultant Cu
/
MoS
2
@SiO
2
exhibits stable specific methanol yield of 6.11 mol
MeOH
mol
Mo
–1
h
–1
with methanol selectivity of 72.5% at 260 °C, much superior to its counterparts lacking the fullerene-like structure and copper decoration. The reaction mechanism and promoting role of copper are investigated by in-situ DRIFTS and in-situ XAS. Theoretical calculations demonstrate that the compressive strain facilitates Sv formation and CO
2
hydrogenation, while tensile strain accelerates the regeneration of active sites, rationalizing the critical role of strain.
The basic bonding mechanisms of graphene on transition-metal surfaces leading to chemisorption and physisorption are identified and studied in the example of graphene adsorption on Ni(111) by means ...of density functional theory at the general gradient approximation level with semiempirical corrections for dispersive interactions. In the more stable chemisorbed graphene, relatively strong Pauli repulsion between graphene and the substrate is compensated by donation/back-donation bonding of the same magnitude. In this case, the electronic interactions with the substrate significantly perturb the electronic structure of graphene, but the adsorption energy is still dominated by van der Waals (vdW) interactions. In physisorbed graphene, weak Pauli repulsion equilibrates the vdW attraction without affecting the electronic structure of graphene. The relative stability of physisorbed and chemisorbed graphene is shown to be changed by carbidic C impurities in the subsurface region.
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•The oxidation states of the CoNi bimetals were monitored during DRM using in-operando X-ray absorption spectroscopy.•While progressive oxidation of monometallic Co catalyst occurred ...during DRM, CoNi and Ni catalysts were largely maintained reduced.•DFT calculation revealed that the Co and Ni atoms in alloy nanoparticles are randomly distributed at high temperature.•DFT calculation showed that heat of oxygen adsorption uniquely increases monotonically with increasing Co content in CoNi alloy.•The CoNi alloy for DRM can balance CH4 and CO2 activation kinetically, preventing both carbon deposition and metal oxidation.
Dry reforming of methane (DRM) proceeds via CH4 decomposition to leave surface carbon species, followed by their removal with CO2-derived species. Reactivity tuning for stoichiometric CH4/CO2 reactants was attempted by alloying the non-noble metals Co and Ni, which have high affinity with CO2 and high activity for CH4 decomposition, respectively. This study was focused on providing evidence of the capturing surface coverage of the reactive intermediates and the associated structural changes of the metals during DRM at high temperature using in-operando X-ray absorption spectroscopy (XAS). On the Co catalysts, the first-order effects with respect to CH4 pressure and negative-order effects with respect to CO2 pressure on the DRM rate are consistent with the competitive adsorption of the surface oxygen species on the same sites as the CH4 decomposition reaction. The Ni surface provides comparatively higher rates of CH4 decomposition and the resultant DRM than the Co catalyst but leaves some deposited carbon on the catalyst surface. In contrast, the bimetallic CoNi catalyst exhibits reactivity towards the DRM but with kinetic orders resembling Co catalyst, producing negligible carbon deposition by balancing CH4 and CO2 activation. The in-operando X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) measurements confirmed that the Co catalyst was progressively oxidized from the surface to the bulk with reaction time, whereas CoNi and Ni remained relatively reduced during DRM. Density functional theory (DFT) calculation considering the high reaction temperature for DRM confirmed the unselective site arrangement between Co and Ni atoms in both the surface and bulk of the alloy nanoparticle (NP). The calculated heat of oxygen chemisorption became more exothermic in the order of Ni, CoNi, Co, consistent with the catalytic behavior. The comprehensive experimental and theoretical evidence provided herein clearly suggests improvement to the catalyst design protocol by selecting the appropriate composition of Co-Ni alloy.
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Electronic interactions between metal nanoparticles and oxide supports control the functionality of nanomaterials, for example, the stability, the activity and the selectivity of catalysts. Such ...interactions involve electron transfer across the metal/support interface. In this work we quantify this charge transfer on a well-defined platinum/ceria catalyst at particle sizes relevant for heterogeneous catalysis. Combining synchrotron-radiation photoelectron spectroscopy, scanning tunnelling microscopy and density functional calculations we show that the charge transfer per Pt atom is largest for Pt particles of around 50 atoms. Here, approximately one electron is transferred per ten Pt atoms from the nanoparticle to the support. For larger particles, the charge transfer reaches its intrinsic limit set by the support. For smaller particles, charge transfer is partially suppressed by nucleation at defects. These mechanistic and quantitative insights into charge transfer will help to make better use of particle size effects and electronic metal-support interactions in metal/oxide nanomaterials.
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The lack of structurally distinct nanoclusters (NCs) of identical size and composition prevented the mechanistic understanding of their structural effects on ion pairing and concomitant optical ...properties. To produce such highly sought NCs, we designed a new monothiolate-for-dithiolate exchange strategy that enabled the selective transformation of the structure of a NC without affecting its metal atomicity or composition. Through this method, a bimetallic PtAg28(BDT)12(PPh3)44– NC (1) was successfully synthesized from PtAg28(S-Adm)18(PPh3)42+ NC (2) (S-Adm, 1-adamantanethiolate; BDT, 1,3-benzenedithiolate; PPh3, triphenylphosphine). The determined X-ray crystal structure of 1 showed a PtAg12 icosahedron core and a partially exposed surface, which are distinct from a face-centered cubic PtAg12 core and a fully covered surface of 2. We reveal through mass spectrometry (MS) that 1 forms ion pairs with counterions attracted by the core charge of the cluster, which is in line with density functional simulations. The MS data for 1, 2, and other NCs suggested that such attraction is facilitated by the exposed surface of 1. The formation of ion pairs increases the photoluminescence (PL) quantum yield of 1 up to 17.6% depending on the bulkiness of the counterion. Unlike small counterions, larger ones are calculated to occupy ≤90% of the volume near the exposed cluster surface and to make the ligand shell of 1 more rigid, which is observed to increase the PL. Thus, the developed synthesis strategy for structurally different NCs of the same size and composition allows us to probe the structure–property relationship for ion pairing and concomitant PL enhancement.
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