New formulations and processes for ceramic layers deposition on complex geometry substrates

In this thesis work, new formulations and processes for structured catalyst production have been developed. A structured catalyst is a multifunctional object which allows reaching the target of process intensification. Its structure can be divided into three main components: the catalytic active phase, which is directly responsible of reduction in process activation energy, the morphologic support, which increases active phase content per unit volume, and the geometric support, which provides three-dimensional structure to the object. In literature, many methods are available in order to deposit active phases on structured supports. The active phase can be directly deposited either onto support surface (i.e. incorporation) or onto a high surface area carrier (i.e. coating). The latter, is a very popular solution that allows enhancing catalytic properties. A variety of procedures are available in order to produce a catalytically active powder. Among others, incipient wetness method allows to easily manage active phase quantity and to produce highly dispersed metal phases. In order to obtain good catalytic performances, a thin catalyst layer (tens of microns) has to be deposited onto structured supports surface. In order to that, dip-coating process from a slurry is widely considered as the best compromise among cost, time and effectiveness. Usually, support surface is pretreated, either thermally or chemically, in order to enhance coating to surface interactions. After slurry deposition, a sequence of subsequent thermal treatment steps is necessary in order to obtain the final product. Many parameters need to be carefully tuned to properly manage washcoating process. Slurry rheology plays a key role in order to produce thin and well adherent washcoat layers on support surface. As a matter of fact, low viscosity slurries usually lead to low loads and high adhesion, while high viscosity formulations promote high washcoat load but poor adhesion. Rheological properties can be modified by acting on formulation components, such as binder, dispersant, solvent and a properly sized powder. Usually, acidic solutions are used in order to obtain stable dispersions. Powder surface is covered by H+ ions and, thus, stability is achieved by surface charging. Unfortunately, this method is not effective both in case of low surface area and chemically inert powders. Moreover, some active phases or supports may be altered in case of acidic solution. For these reasons, many organic compounds have been tested in order to properly stabilize catalyst powder dispersions. The purpose of this work is to develop an acid-free liquid medium in order to avoid acidic solutions issues and to obtain stable powder dispersions. The latter will be used for washcoat deposition onto complex geometry substrates, such as open cell foams and monoliths. After a comprehensive introduction regarding structured catalysts and depositions techniques, in the first part of the research activity, a formulation study has been carried out, in order to stabilize carrier powder dispersions (i.e. cerium oxide and zirconium oxide). Resulting slurries have been used in order to perform preliminary washcoat depositions on ceramic open cell foams and monoliths, by means of a self-assembled dip-coating device. The influence of slurry viscosity, support morphology and procedural parameters (i.e. withdrawal speed and thermal treatments) has been investigated and good results have been obtained in terms of washcoat homogeneity. Then, model catalytic powders have been produced using nickel oxide and cobalt oxide, as active phase precursors, and zirconium oxide, as carrier. An attempt to correlate powder properties, rheological behavior and washcoat properties was made. The same formulation and procedure was validated from the catalytic point of view, by depositing nickel and nickel/rhodium-based catalysts on ceramic open cell foams and monoliths. In all cases, good washcoat properties have been demonstrated, as well as satisfactory results in terms of catalytic activity. Although dip-coating has been proved to be an easy-to-use technique for catalyst production, some limitations have been found. In particular, pore clogging and low adhesion have been recorded, in some cases. In order to overcome these problems, spin-coating has been evaluated as alternative deposition technique. The latter was studied using model water/glycerol solutions at various viscosities. Moreover, the influence of operative parameters (i.e. rotation speed, time, support morphology and size) has been investigated. Finally, cerium oxide was deposited onto 20, 30 and 40 PPI metallic open cell foams; excellent results have been obtained, both from the load management and washcoat adhesion points of view. For these reasons, spin-coating has been identified as a candidate for dip-coating substitution in washcoat deposition procedures.

In questo lavoro di tesi sono stati messi a punto processi innovative e nuove formulazioni per la deposizione di polveri cataliticamente attive su supporti strutturati. In campo industriale, un supporto strutturato è un oggetto multifunzionale che consente di perseguire l’obiettivo dell’intensificazione di processo. La sua struttura è composta da tre parti: la fase cataliticamente attiva, il supporto morfologico e il supporto geometrico. In letteratura sono descritti molti metodi per la deposizione di strati cataliticamente attivi su supporti strutturati; la maggior parte di essi, tuttavia, si basa sulla formulazione di slurry mediante soluzioni acquose acide. Questa metodologia sfrutta il caricamento superficiale della polvere per stabilizzare la dispersione. Nonostante la comprovata efficacia, questo metodo presenta molti limiti, in quanto non può essere utilizzato in caso di polveri chimicamente inerti o di fasi attive suscettibili alla presenza di un ambiente fortemente acido. L’obiettivo di questo lavoro è quindi lo sviluppo di una formulazione priva di acidi per ottenere dispersioni stabili di polveri ceramiche al fine di effettuare deposizioni su supporti a geometria complessa. La tesi, dopo un’ampia introduzione sullo strato dell’arte nell’ambito della produzione di catalizzatori strutturati, si occupa inizialmente della stabilizzazione di dispersioni di polveri ceramiche con soluzioni a base di acqua e composti organici. Successivamente, queste formulazioni sono state utilizzare per ricoprire sia schiume a pori aperti, di diversa natura (metalliche e ceramiche) e porosità, che monoliti ceramici utilizzando la metodologia di dip-coating. A seguito di uno studio estensivo dei parametri operativi e valutati i risultati ottenuti, sono stati evidenziati i limiti della metodologia di dip-coating ed è stata quindi valutata la possibilità di utilizzare lo spin-coating per la produzione di catalizzatori strutturati. Quest’ultima metodologia ha mostrato notevoli vantaggi rispetto al dip-coating, sia in termini di qualità dello strato depositato che di adesione dello stesso al supporto.