| Abstract |
Despite the great improvements in design, efficiency and safety, the majority of vehicles currently on the streets still utilizes fossil fuels as their source of energy. Due to incomplete combustion, a large number of byproducts – like unburned H/Cs, CO and NOx - are released in the atmosphere, significantly degrading air quality at a scale anything but small: the automotive transport sectors’ contribution to CO and NO emissions reaches 30% of the total transport sector (train, marine, aviation etc.) accounting, in turn, for 34% and 56% of the total anthropogenic CO and NO emissions. In an attempt to reduce the negative effects not only on health of living organisms but also on the environment in general, global and European authorities and policy makers, are imposing increasingly stricter emission limits. The relevant legislation in Europe, also known as Euro, requires CO and HCs emissions to be significantly reduced (37% and 89% respectively from EuroIII to EuroV) and almost zeroed in the case of NOx (0.06g/km, EuroVI), trending towards even lower target values for the next years. This trend tries to address the ever-increasing activity of the road transport sector that inevitably increases pollutants concentration in the atmosphere, and places the burden of developing new anti-pollution technologies on the automotive industry.
For the last 45 years, the three way catalyst (TWC) has been the only technology employed for the conversion of automotive gaseous exhaust emissions to more environmentally friendly products. TWCs rely on Platinum Group Metals-PGMs mainly in the form of nanoparticles, to provide the active sites for the necessary catalytic reactions to occur. A typical TWC usually contains a combination of two or three PGMs, namely platinum, palladium and rhodium. However, over the years and due to the constantly increasing demand, the automotive sector turned out to be the largest PGM consumer, requiring today 66% of the global platinum production with percentages rising to 80% for palladium and rhodium.These numbers are expected to rise in order for the existing TWC technology to cope with the decreasing emission limits. At this rate, considering the given depletion of natural resources, the use of PGMs in automotive transport is challenging its sustainability and requires immediate alternatives. Thus, current trends in the field of TWC technology evolution aim to:
- increase efficiency of current TWC systems while keeping metal content at the current levels (0.2%wt. to 0.8%wt. for Pd and Pt)
- reduce PGMs content at a percentage not exceeding 0.1%wt. (<0.08%wt.-0.1%wt.), or use only one PGM– preferably that of the lower cost -, and
- partially or completely replace PGMs by low cost transition metals (TM), while maintaining or possibly exceeding the performance and stability offered by the existing PGM-based catalysts.
In this context, the objective of the work described in this dissertation was the design and development of novel materials that would offer significant advantages over commercial TWCs in terms of efficiency and PGM use, primarily aiming to the reduction and partial or total replacement of PGMs by TM, taking into account the requirements of green chemistry.
The first attempt to prepare active catalytic materials involved commercially available SBA-15 powder as substrate and copper, an inexpensive and abundant transition metal, as the active phase. With its simple porous network, its increased thermal stability and its thick silica walls, SBA-15 constitutes an ideal substrate for metal nanoparticles deposition. For the introduction of the active phase into the porous network, a novel technique was developed, that of assisted impregnation. Unlike common impregnation methods –currently used for the preparation of catalytic systems in the majority of industrial scale applications– this technique ensures high dispersions and controlled deposition and distribution of metals within the substrates’ porous network. This is achieved by employing hyperbranched organic molecules (i.e. polyethyleneimine (PEI), Trilon-P) able to bind metals in specific sites via stable complex formation.
Following their preparation, a systematic characterization of the materials structure, morphology and chemical composition (via FT-IR, N2 adsorption-desorption, XRD, SEM, TEM, etc.) helped assess the factors that potentially affect catalytic performance. In turn, evaluation of the catalytic performance was performed by either investigating the materials behavior during selective reduction of NO by CO (de-NOx reaction), or by catalytic testing under flow, simulating the actual exhaust mixtures and conditions.
The first results showed that the use of an inorganic porous silicate substrate, PEI and copper, results in materials active in oxidation reactions. Thereafter, instead of
employing a commercial silicate substrate, PEI, with its unique chemical and structural properties, was utilized both as a template for the preparation of novel mesoporous silicate materials and as a metal binding agent ensuring high dispersion of particles on the materials surface. The resulting materials containing copper exhibited high activity in oxidation reactions, but fell short of performance in the catalytic NO reduction. For that reason, new batches of catalytic materials were prepared via the assisted impregnation technique, this time using SBA-15 as the porous substrate and palladium as the active phase. All resulting catalysts turned out to be very active in all oxidation reactions, including methane, even at very low metal content (0.13% wt. Pd), whereas surprisingly, at concentrations up to 3%wt. Pd complete NO conversion was observed. However, despite the enhanced performance of the 3% wt. Pd material, its metal content clearly does not meet the automotive industry’s current trends concerning lower PGMs use and probably, an attempt for further reduction of PGM content without compromising performance would require the substrate to somehow contribute to the catalytic activity of the system. To this end, the incorporation of various heteroatoms (such as Al-, Ce-, La-, Zr- and Zr-La or Zr-Ce) into the framework of SBA-15 would turn the inert siliceous matrix into an active carrier, aiming to enhance the materials’ catalytic performance. Hence, a new group of samples was prepared using heteroatom-doped SBA-15 as substrate. Among those samples, the copper containing catalysts apart from oxidation reactions are also very active in NO reduction. Far more importantly, the resulting Pd containing Ce-Zr doped SBA-15 samples are on par with the performance of a commercially available converter (0.17% wt. Pd, 0.03% wt. Rh) but their palladium content is significantly reduced (down to 0.05% wt.). It is worth mentioning that although comparable in performance, there is a marked difference in cost between the commercial bimetallic and the developed monometallic material, not only due to the absence of Rh, but also because of the very low palladium loading and the resulting simplified recycling processes, also in line with the trends set for the next Euro directives.
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