No abstract

Intellectually, the advantages that flow from the availability of single-site heterogeneous catalysts (SSHC) are many. They facilitate the determination of the kinetics and mechanism of catalytic turnover-both experimentally and computationally-and make accessible the energetics of various intermediates (including short-lived transition states). These facts in turn offer a rational strategic principle for the design of new catalysts and the improvement of existing ones. It is generally possible to prepare soluble molecular fragments that circumscribe the single-site, thus enabling a direct comparison to be made, experimentally, between the catalytic performance of the same active site when functioning as a heterogeneous (continuous solid) as well as a homogeneous (dispersed molecular) catalyst. This approach also makes it possible to modify the immediate atomic environment as well as the central atomic structure of the active site. From the practical standpoint, SSHC exhibit very high selectivities leading to the production of sharply defined molecular products, just as do their homogeneous analogues. Given that mesoporous silicas with very large internal surface areas are ideal supports for SSHC, and that more than a quarter of the elements of the Periodic Table may be grafted as active sites onto such silicas, there is abundant scope for creating new catalytic opportunities.

Metal oxides, in the form of dispersed powders, have been tested as potential catalysts for the four-electron oxidation of water to 0, under photochemical conditions. The most efficient catalysts were found to be IrO,, Co,O,, RuO,, NiCo,O,, Rh,O, and Mn,O, and, in particular, high activity was observed with IrO,. Comparison of the oxide structure with its observed rate of 0, generation under standard conditions has allowed formulation of a few general requisites for an effective catalyst. Samples of iridium oxide deposited onto the surface of a second (inert) oxide were tested for their 0,-evolving capability. The efficiency of the system depended markedly upon the nature of the support. Materials that favour formation of small deposits of iridium oxide (e.g. ZnO, MgO, TiO,) are the best supports, whilst 0, production is almost completely inhibited with acidic supports. Many metal oxides can be prepared in the form of hydrates of variable composition. These materials also function as 0,-evolving catalysts, the efficiency of the process depending upon any thermal pretreatment. This finding is explained in terms of changes in structure and composition of the oxide that occur upon heating.As part of a protracted research programme, concerned with building a photosystem capable of storing solar energy via the photodissociation of water, a range of 0,-evolving catalysts has been developed. 1-6 Mostly, these catalysts have contained ruthenium dioxide in some form, and all have been prone to corrosion under high anodic bias. homogeneous,'^ colloidal5* and heterogeneou~l.~ Ru0,-based catalysts have been tested for their 0,-evolving capacities under photochemical conditions. In certain cases, quite high efficiencies have been reported. Despite such detailed studies, the poor performance of the catalyst remains as the most serious obstacle in developing a suitable anodic branch of a solar-energy storage device and, if a satisfactory catalyst is to be identified, attention should be given to materials other than RuO,. Indeed, it has been shown that Ir0,,lo Pt0210 and MnO," powders will function as heterogeneous catalysts for water oxidation in related model systems.Using a well established photochemical test system,6 we have evaluated the 0,-evolving capabilities of a range of metal oxides. These materials have been used in the form of powders suspended in aqueous solution. Most of the materials were found to be inferior to RuO, powders, but a few oxides, notably IrO,, were effective catalysts for water oxidation and demonstrated much higher levels of corrosion resistance than the catalysts used previously. Several of these oxides exist in the form of hydrates for which the 0,-evolving capability is increased upon dehydration. In order to extend the scope of the work, a series of catalysts was prepared in which IrO, was deposited onto the surface of a second (inert) oxide. By this means, the effect of the supporting agent on the 2795

Bimetallic nanoparticles (Ru(6)Pd(6), Ru(6)Sn, Ru(10)Pt(2), Ru(5)Pt, Ru(12)Cu(4), and Ru(12)Ag(4)) anchored within silica nanopores exhibit high activities and frequently high selectivities, depending upon the composition of the nanocatalyst, in a number of single-step (and often solvent-free) hydrogenations at low temperatures (333-373 K). The selective hydrogenations of polyenes (such as 1,5,9-cyclododecatriene and 2,5-norbornadiene) are especially efficient. Good performance is found with these nanoparticle catalysts in the hydrogenation of dimethyl terephthalate to 1,4 cyclohexanedimethanol and of benzoic acid to cyclohexanecarboxylic acid or to cyclohexene-1-carboxylic acid, and also in the conversion of benzene to cyclohexene (or cyclohexane), the latter being an increasingly important reaction in the context of the production of Nylon. Isolated atoms of noble metals (Pd, Rh, and Pt) in low oxidation states, appropriately complexed and tethered to the inner walls of nanoporous (ca. 3 nm diameter) silica, are very promising enantioselective hydrogenation catalysts. Nanoporous carbons, as well as other nanoporous oxides, may also be used to anchor and tether the kind of catalysts described here.

Terminally oxidized hydrocarbons are of considerable interest as potential feedstocks for the chemical and pharmaceutical industry, but the selective oxidation of only the terminal methyl groups in alkanes remains a challenging task. It is accomplished with high ef®ciency and selectivity by some enzymes; but inorganic catalysts, although inferior in overall performance under benign conditions, offer signi®cant advantages from a processing standpoint 1 . Controlled partial oxidation is easier to achieve with`sacri®cial' oxidants, such as hydrogen peroxide 2 , alkyl hydroperoxides or iodosylbenzene 3 , than with molecular oxygen or air. These sacri®cial oxidants, themselves the product of oxidation reactions, have been used in catalytic systems involving tailored transitionmetal complexes in either a homogeneous state 4±6 , encapsulated in molecular sieves 7±9 or anchored to the inner surfaces of porous siliceous supports 10 . Here we report the design and performance of two aluminophosphate molecular sieves containing isolated, four-coordinated Co(III) or Mn(III) ions that are substituted into the framework and act, in concert with the surrounding framework structure, as regioselective catalysts for the oxidation of linear alkanes by molecular oxygen. The catalysts operate at temperatures between 373 K and 403 K through a classical freeradical chain-autoxidation mechanism. They are thus able to use molecular oxygen as oxidant, which, in combination with their good overall performance, raises the prospect of using this type of selective inorganic catalyst for industrial oxidation processes.Cobalt(II) is one of the transition-metal ions that, when occupying a small percentage of the framework (tetrahedral) sites in a molecular sieve, may be raised 11±13 to a higher oxidation state (Co(III)) while remaining within the framework as active sites for the catalytic oxidation in air of cyclohexane 14 and other alkanes 15 . By taking advantage of this fact and choosing a molecular sieve that allows only end-on entry of the linear alkanes into the cavities containing the active sites (compare ref. 16) we have been able to design effective catalysts that favour functionalization by oxygen at the terminal CH 3 and penultimate CH 2 groups and operate with air as the oxidant. (We note that in conventional autoxidation of n-alkanes under homogeneous conditions and in the absence of steric constraints the regioselectivity is governed by the relative bonddissociation energies. That is, whereas the bond dissociation energies decrease from 104 kcal mol -1 to 94.6 kcal mol -1 to 91 kcal mol -1 in going from primary to secondary to tertiary carbon atoms 17 , the corresponding selectivities increase.)We selected the molecular sieve known as aluminophosphate (AlPO) number 18 (ref. 18; idealized formula Al 24 P 24 O 96 ), which has pores similar to the zeotypic analogue of the aluminosilicate mineral chabazite. A few atom per cent of various divalent metal ions may be readily accommodated into this material's framework tetrahedral sites,...

Graphitic carbon nitride (g-C3N4) exhibits unique properties as a support for single-atom heterogeneous catalysts (SAHCs). Understanding how the synthesis method, carrier properties, and metal identity impact the isolation of metal centers is essential to guide their design. This study compares the effectiveness of direct and postsynthetic routes to prepare SAHCs by incorporating palladium, silver, iridium, platinum, or gold in g-C3N4 of distinct morphology (bulk, mesoporous and exfoliated). The speciation (single atoms, dimers, clusters, or nanoparticles), distribution, and oxidation state of the supported metals are characterized by multiple techniques including extensive use of aberration-corrected electron microscopy. SAHCs are most readily attained via direct approaches applying copolymerizable metal precursors and employing high surface area carriers. In contrast, although post-synthetic routes enable improved control over the metal loading, nanoparticle formation is more prevalent. Comparison of the carrier morphologies also points toward the involvement of defects in stabilizing single atoms. The distinct metal dispersions are rationalized by density functional theory and kinetic Monte Carlo simulations, highlighting the interplay between the adsorption energetics and diffusion kinetics. Evaluation in the continuous three-phase semihydrogenation of 1-hexyne identifies controlling the metal–carrier interaction and exposing the metal sites at the surface layer as key challenges in designing efficient SAHCs

The ubiquitous challenge of plastic waste has led to the modern descriptor 'plastisphere' to represent the human-made plastic environment and ecosystem.Here we report a straightforward, rapid method for the deconstruction of various plastic feedstocks into hydrogen and high-value carbons. We use microwaves together with abundant and inexpensive iron-based catalysts as microwavesusceptors to initiate the catalytic deconstruction process. The one-step process typically takes some 30-90 seconds to transform a sample of mechanically-pulverised commercial plastic into hydrogen and (predominantly) multi-walled carbon nanotubes. A high hydrogen yield of 55.6 mmol• − is achieved, with over 97 % of the theoretical mass of hydrogen being extracted from the deconstructed plastic. The approach is demonstrated on widely used, real-world plastic waste. This proof-of-concept advance highlights the potential of plastics waste itself as valuable energy feedstocks for the production of hydrogen and high-value carbon materials.

The precise atomic architecture of the catalytically active center is the central topic of the approach presented herein to the design of solid-state catalysts. A wide range of new catalysts may be designed that effect regioselective, shape-selective, and enantioselective conversions, as well as producing high-performance catalysts for the isomerization and hydrogenation of alkenes and the terminal oxidation of alkanes. All the new catalysts described are either microporous or mesoporous solids that have the dual advantages of possessing large specific surfaces and being amenable to delicate structural and compositional variation.