I.3 Title: Statistical design for guided nanoparticle and zeolite synthesis
Participants: Hattrick-Simpers and Lauterbach
Motivation: The development of new and effective catalysts requires an understanding and control of nanoparticles and nano-structured materials, such as zeolites.[1-3] The problem is two-fold; the outcome of a chemical reaction is governed by the nanoscale characteristics, which in turn are determined by the synthesis conditions.[4, 5] Thus, a fundamental and empirical understanding of synthesis is integral to improved catalyst design.
Nanomaterials are inherently difficult systems to study due to their small size and complex hierarchical structure.[6, 7] Additionally, the final product of practical synthesis procedures depends on many different and interacting factors, ranging from the type of synthesis method used over types of precursors to the calcination/oxidation/reduction conditions.[8] Surprisingly, the interactions between variables involved in the synthesis are often poorly understood.
Current research has been successful in identifying response trends with respect to single factors.[9, 10] Many examples documented in the literature obtain the desired nanoparticle characteristics through trial and error synthesis. These efforts have been successful in identifying single factor response trends but do not reflect the complexities of the full design space, where multi-factor interactions are significant.
Understanding the complexities of full design space is of the paramount importance in all stages of catalyst design. One of the most renowned interactions is the interaction between support and metal nanoparticles, strong metal support interaction or SMSI. [11, 12] The support can change the reaction mechanism by participating in the reaction and in some cases can even alter the size and morphology of the supported nanoparticle.
Herein, we employ statistical design of experiments to elucidate the relationship between different design factors in both nanoparticle synthesis and catalyst design, allowing us to achieve the desirable catalytic activity, selectivity, and stability of supported nanoparticle catalysts for ammonia selective reduction of NOx as a first model reaction. The relationship will be investigated and revealed by characterizing the change of the nanoparticles, the support, and the interaction between the two along with the tuning of design factors. This methodology has the potential to provide us with meaningful data, but requires iterative and long term work to achieve a detailed mechanistic understanding of how the synthesis conditions will make an impact on the particle size.
Hypothesis: Statistical design of experiments in combination with in-situ spectroscopy, including IR and EXAFS, can be used to reveal the effects of different synthesis conditions on nanoparticles and supports, generate an optimum interaction between the two, and ultimately guide rational catalyst design. Similar methodologies can also be applied to the microwave-based design of zeolites.
Objective: Insight into the role and interactions of synthesis conditions on catalyst nanoscale size, shape, and composition.
Research Plan:
In order to reveal how the synthesis conditions influence nanoparticle and support formation and control the final particle size, we will employ multi-tiered statistical design of experiments for the desired nanoparticle and support system, which can be chosen in collaboration with the industrial partner. The nanoparticle and support synthesis will be monitored by conducting IR spectroscopy and EXAFS. Specifically, IR will be used to identify the consumption and formation of organic species during the synthesis, such as the reduction of OH peak intensity. IR is also expected to provide information on intermediates, such as adsorbed capping agents. In-situ EXAFS measurements of the coordination number will reveal details on the progression of nanoparticle formation. Once such changes are identified, a kinetic analysis based on the results will be conducted to gain mechanistic understanding of the influence of synthesis conditions and their correlation to the statistical design parameters.
More specifically, the colloid polyol method will be used to synthesize transition metal nanoparticles, such as Fe, Co, and Cu. These transition metals have been reported to be active for ammonia selective reduction, with nanoparticle morphology and particle size governing their activity.[13-16] Colloid nanoparticle synthesis is achieved through the progression of nucleation, growth, agglomeration, and coarsening of the reactant mixture.[8] Effective control of these steps via synthesis conditions results in shape and size control. One important factor is the use of certain molecules as capping agents.[17] Capping agents direct monomer attachment to particular crystal facets and limit agglomeration. While capping agents generally have a thermodynamic affinity for low energy crystal facets and the metal-surfactant strength is negatively correlated with size, the precise effect of the capping agent on the overall synthesis progression is highly sensitive to other factors.[8, 18] For instance, factorial analysis has shown that the limiting effect of surfactant concentration on particle size varies in magnitude between different capping agents studied and can be diminished by using a slower ramping rate. Through continued and careful sampling of the design space, which will include the use of various reactants, reactant molar ratios, and treatments, these types of multi-factor interactions will be discovered and quantified for use in predictive models.
In addition, high surface area supports, such as zeolites, will be systematically synthesized and studied. ZSM-5, for example, is a highly porous alumina-silica zeolite having an intersecting two-dimensional pore structure formed by 10-membered oxygen rings. It has been widely used as a catalyst in ammonia selective catalytic reduction, isomerization, cracking and reforming reactions.[19-22] The synthesis of ZSM-5 proceeds via batch preparation, crystallization, cation-exchange and modification.[23] Different heating methods such as conventional and microwave-assisted heating can be applied for the crystallization step.[24] Conventional heating method requires a heating time up to 24 hours to complete crystallization of ZSM-5, whereas microwave-assisted heating usually takes less than 4 hours. The time difference required for the crystallization between two methods is significant. Applying microwave-assisted heating synthesized method will therefore speed up ZSM-5 synthesis, while reducing the energy necessary for heating.
Crystal size, acidity and pore structure of a zeolite are generally regarded as important characteristic properties.[25-27] For instance, the rate-limiting step of a catalytic reaction often depends on the diffusion of the reactant and product molecules inside the micropores, since the crystal sizes of zeolites are usually much larger than the sizes of their micropores.[28] H-ZSM-5 catalysts, previously synthesized in our group via microwave-assisted heating, indicated that morphology of H-ZSM-5 could vary in comparison to the morphology obtained via conventional heating. As shown in Figure 1, crystal size of the HZSM-5 from conventional heating was found to be around 200 nm forming cubical shapes whereas that from microwave-assisted heating was around 500 nm with cylindrical shapes. Although our results have proven that microwave-assisted heating can be applied successfully to ZSM-5 synthesis, more research is necessary to optimize the parameters used during the microwave-assisted heating and control the particle size, shape and acidity of ZSM-5. During the microwave-assisted syntheses, it was found that a minimum of eight parameters can possibly affect morphology and acidity of a zeolite, i.e., heating power, time, temperature, concentration of gel mixture, ratio of silica and alumina precursor, chemical of alumina precursor, and the nature of the surfactant.
Figure 1. SEM images of the synthesized HZSM-5 materials via conventional hydrothermal heating (A) and microwave-assisted heating (B) methods
After the systematic study of the relationship between synthesis conditions parameters and the properties of the nanoparticles and the support, the nanoparticles will be introduced onto support. Our extensive experience with synthesis will allow us to study, for example, impregnation, reverse micelles, or dendrimer approaches.[29-31] Nanoparticles can be directly introduced onto porous supports by impregnation. Usually, the support will be immersed into a nanoparticle solution under constant sonication.[32] Then the solvent will be gradually evaporated and the nanoparticles will be deposited inside the pore of porous support driven mainly by capillary force. However, since the nanoparticle are physically entrapped in the pore, the interaction between nanoparticles and support are limited. To create a SMSI between the support and nanoparticles, grafting with dendrimers or the use of capping agents were reported to be effective.[33, 34] Firstly, nanoparticles will be coated with the dendrimer or capping agent, such as polyvinyl alcohol.[33] The dendrimer and capping agent have double bonds that can strongly adsorb to the metallic nanoparticle because of electron back donation, while the other functional group, such as the OH group in polyvinyl alcohol, can react with the OH species on the surface of support to graft. After grafting, the dendrimer and capping agent can be removed by microwave assisted solvent extraction or ozone oxidation.[33] It is unclear in literature reports whether a strong or weak interaction between nanoparticle and support will benefit the catalytic activity toward ammonia selective reduction. In this study, we will alternate the interaction between nanoparticles and support from weak to strong by gradually increasing the amount of grafting agent used.
Deliverable: Detailed description of the mechanistic and kinetic perspective of controlling nanoparticles and microwave synthesized zeolites by tuning synthesis reaction conditions.
First year Milestone: Fundamental understanding of synthesis parameter interactions, guidance on how to synthesize nanoparticles with desired properties.
Cost: $60,000 for one year. Possible successive years for other supports, $60,000.
References:
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