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We present an atomistic understanding of the evolution of the size distribution with temperature and number of cycles in atomic layer deposition (ALD) of Pt nanoparticles (NPs). Atomistic modeling of our experiments teaches us that the NPs grow mostly via NP diffusion and coalescence rather than through single-atom processes such as precursor chemisorption, atom attachment, and Ostwald ripening. In particular, our analysis shows that the NP aggregation takes place during the oxygen half-reaction and that the NP mobility exhibits a size- and temperature-dependent scaling. Finally, we show that contrary to what has been widely reported, in general, one cannot simply control the NP size by the number of cycles alone. Instead, while the amount of Pt deposited can be precisely controlled over a wide range of temperatures, ALD-like precision over the NP size requires low deposition temperatures (e.g., T < 100 °C) when growth is dominated by atom attachment.

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Atomic layer deposition (ALD) is a gas-phase deposition technique that, by relying on self-terminating surface chemistry, enables the control of the amount of deposited material down to the atomic level. While mostly used in semiconductor technology for the deposition of ceramic oxides and nitrides on wafers, ALD lends itself to the deposition of a wealth of materials on virtually every substrate. In particular, ALD and its organic counterpart molecular layer deposition (MLD), have opened up attractive avenues for the synthesis of novel nanostructured materials. However, as most ALD processes were developed and optimized for semiconductor technology, these might not be optimal for applications in fields such as catalysis, energy storage, and health. For this reason, novel applications for ALD often require new surface chemistries, process conditions, and reactor types. As a result, recent developments in ALD technology have marked a considerable departure from the standard set by well-established ALD processes. The aim of this review is twofold: firstly, to capture the recent departure of ALD from its original development; and secondly, to pinpoint the unexplored paths through which ALD can advance further in terms of synthesis of novel materials. To that end, we provide a review of the recent developments of ALD and MLD of materials that are gaining increasing attention on various substrates, with particular emphasis on high-surface-area substrates. Furthermore, we present a critical review of the effects of the process conditions, namely, temperature, pressure, and time on ALD growth. Finally, we also give a brief overview of the recent advances in ALD reactors and energy-enhanced ALD processes.

The reaction-diffusion performance for the Fischer-Tropsch reaction in a single cobalt catalyst particle is analysed, comprising the Langmuir-Hinshelwood rate expression proposed by Yates and Satterfield and a variable chain growth parameter a, dependent on temperature and syngas composition (H 2 /CO ratio). The goal is to explore regions of favourable operating conditions for maximized C 5+ productivity from the perspective of intra-particle diffusion limitations, which strongly affect the selectivity and activity. The results demonstrate the deteriorating effect of an increasing H 2 /CO ratio profile towards the centre of the catalyst particle on the local chain growth probability, arising from intrinsically unbalanced diffusivities and consumption ratios of H 2 and CO. The C 5+ space time yield, a combination of catalyst activity and selectivity, can be increased with a factor 3 (small catalyst particle, d cat = 50 mm) to 10 (large catalyst particle, d cat = 2.0 mm) by lowering the bulk H 2 /CO ratio from 2 to 1, and increasing temperature from 500 K to 530 K. For further maximization of the C 5+ space time yield under these conditions (H 2 /CO = 1, T = 530 K) it seems more effective to focus catalyst development on improving the activity rather than selectivity. Furthermore, directions for optimal reactor operation conditions are indicated.

Nanoparticles (NPs) are applied in a wide range of processes, and their use continues to increase. Fluidization is one of the best techniques available to disperse and process NPs. NPs cannot be fluidized individually; they fluidize as very porous agglomerates. The objective of this article is to review the developments in nanopowder fluidization. Often, it is needed to apply an assistance method, such as vibration or microjets, to obtain proper fluidization. These methods can greatly improve the fluidization characteristics, strongly increase the bed expansion, and lead to a better mixing of the bed material. Several approaches have been applied to model the behavior of fluidized nanopowders. The average size of fluidized NP agglomerates can be estimated using a force balance or by a modified Richardson and Zaki equation. Some first attempts have been made to apply computational fluid dynamics. Fluidization can also be used to provide individual NPs with a thin coating of another material and to mix two different species of nanopowder. The application of nanopowder fluidization in practice is still limited, but a wide range of potential applications is foreseen.Electronic supplementary materialThe online version of this article (doi:10.1007/s11051-012-0737-4) contains supplementary material, which is available to authorized users.

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Reactor design for multiphase catalytic fixed bed reactors is always based on conflicting objectives. In the past, catalyst discovery and development preceded and motivated the selection of an appropriate multiphase reactor type. This type of sequential approach is increasingly been replaced by a parallel approach to catalyst and reactor selection. In nearly all respects, structured catalysts and reactors have the ability to outperform randomly packed reactors. Structured packings, apart from their advantages of high voidage and low-pressure drop, have the benefit of ease of scale-up and accurate description of the fluid mechanics. In this review we have evaluated the potential of using structured internals for multiphase catalytic reactions, which are currently carried out in randomly packed fixed bed reactors. Characteristics of various structured internals such as monoliths, corrugated sheet or gauze packings, knitted wire packings and foams are discussed in detail. Since designing a structured device for gas-liquid-solid contacting requires a sound knowledge of hydrodynamics and transport phenomena, a concise review of the above-mentioned structured packings and their characteristics based on hydrodynamics and transport phenomena is presented. Existing models (empirical, phenomenological and mechanistic) are outlined with respect to flow regime transition, pressure drop, liquid hold-up, gasliquid interfacial area, gas to liquid mass transfer, liquid to solid mass transfer, residence time distribution (RTD), and heat transfer. The models are critically evaluated, and their limitations are discussed. An overview is given about what information is available, what needs to be evaluated and what kind of existing methodology can be applied in order to arrive at quantitative models for the physical parameters. Last, the structured internals are compared with each other and with randomly packed bed reactors, allowing a rational selection of the preferred packing for a given application.