Browsing by Author "Luchters, Niels"

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    Development of bimetallic Pd-Zn catalysts for methanol steam reforming: hydrogen production for fuel cells
    (2015) Xalabile, Philasande; Fletcher, Jack; Luchters, Niels; Malatji, Peter
    Proton exchange membrane fuel cell (PEMFC) has been reported as clean and efficient energy technology from conversion of H₂. However, one of the main challenges remains the storage and transport of hydrogen. The promising alternative is to produce H₂ on site by a reformer using a H₂-dense liquid as a fuel, a technology known as fuel processing. Methanol is an attractive source of H₂ compared to other fuels as it presents several advantages, i.e. it is obtained sulphur-free, has a high H to C ratio and therefore produces a H₂-rich reformate, can be reformed at low temperatures (200 - 300°C) and is a liquid at ambient conditions so that it can be easily handled. Typically, Cu-based catalysts are used for steam reforming of methanol due to their high activity (i.e. H₂ production) and high selectivity towards CO₂. As CO poisons anodic catalyst of PEMFC, high selectivity towards CO₂ is crucial so as to eliminate or at least minimize CO removal load downstream a fuel processor. However, Cubased catalysts are thermally unstable and suffer deactivation due to sintering at high temperatures (> 250°C). Moreover, Cu-based catalysts are pyrophoric and therefore difficult to handle. Recent studies show that PdZn catalysts are very promising as they exhibit comparable activity and selectivity to Cu-based ones. Furthermore, PdZn catalysts are thermally stable in the typically methanol steam reforming temperature range (200 - 300°C). Most literature attributes high CO₂ selectivity of PdZn catalysts to formation of PdZn alloy. It is generally agreed that PdZn alloy is formed when PdZn catalysts are reduced in H₂ at high temperatures (> 250°C). In this work, a Pd/ZnO catalyst aimed at 2.5 wt% Pd was successfully prepared via incipient wetness impregnation and the duplicate preparation of the catalyst was successful. Both impregnation catalysts were confirmed by ICP-OES to contain similar weight Pd loadings i.e. 2.8 and 2.7 wt%, respectively. The actual Pd loading (ICP-OES) was slightly higher than the target loading (2.5 wt%) due to Pd content of Pd salt underestimated during catalyst preparation. Furthermore, crystallite size distribution, i.e. PdO crystallites on ZnO support, was similar (i.e. 6.7 ± 2.4 nm and 6.3 ± 1.9 nm) for both impregnation catalysts.
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    High throughput experimentation: a validation study for use in catalyst development
    (2016) Luchters, Niels; Fletcher, Jack
    High throughput and combinatorial experimentation is becoming more and more used in catalysis research. The benefits of parallel experiments are not only limited to shorten the time - to - market, but also give opportunities to study the process in more depth by performing more experiments. The influence of a parameter, for example the amount of the active metal and/or promoter, to the process is better understood with a broader parameter space investigated. To study the parameter space, multiple experiments need to be performed. It is of paramount importance to understand the variability of the data between these experiments. This is not always defined, specifically when literature gives contradictory results, most often due to the time for duplicate experiments necessary. In this project the reproducibility and variance in high throughput catalyst preparation and testing was determined and the use of parallel experimentation was demonstrated within a catalyst development study. The high throughput equipment was used for catalyst development studies for fuel processing, the production of fuel cell - grade hydrogen from hydrocarbon fuels. Fuel processing consists of three catalytic reactions, namely reforming, water - gas shift and a CO clean - up through either selective methanation or preferential oxidation. Focus has been placed on the first two reactions, steam methane reforming (SMR) and medium temperature water - gas shift (WGS), using platinum group metals (PGM). All catalysts in this study (except for the commercial WGS catalyst) were prepared using automated synthesis robot (Chemspeed ISYNTH) and the activity testing was performed on the Avantium Flowrence. For both reactions two types of studies were performed, one - to - many and many - to - many; referring to one catalyst tested in many reactors or many prepared catalysts (same composition, different batches) tested in many reactors. For the WAGS one - to - many a commercial low temperature shift catalyst was selected and for SMR a single batch of Rh/Al 2 O 3 . The many - to - many experiments comprised of eight batches of prepared catalysts for both reactions. The WGS reaction was performed with 1 wt% Pt/Al 2 O 3 catalysts and for the reforming reaction batches of 0.5 wt% Rh/Al 2 O 3 was used. It was proven that in all these studies the experimental standard deviations in the data is 6%, from preparation to activity measurements. A study on the rhodium metal loading on alumina in the steam methane reforming catalyst was studied between 0.05 and 0.6 wt%. A 0.4 wt% Rh/Al 2 O 3 was found to have the highest activity per amount of rhodium. Lower Rh content would require decreased space velocity, whereas higher metal content does not increase the conversion due to larger crystals sizes. This study has been performed up to a metal loading of 0.6 wt% and it is recommended to follow - up with studying the range of 0.6 to ~2.5 wt% to investigate the optimal metal loading. It was shown that the use of automated experimentation (parallel preparation and evaluation under same condition) for catalyst development results in highly reproducible results with a relative standard deviation of ~6% activity. The high throughput equipment was demonstrate d to be a very powerful tool in catalyst research
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    Investigation into the behaviour of a wash-coated PGM-based catalyst layer onto micro-channel reactors for the steam reforming of methane
    (2017) Van Niekerk, Wesley; Fletcher, Jack; Luchters, Niels
    A wash-coating method which had originally been used for wash-coating a Rh/Al₂O₃ catalyst onto stainless steel micro-channels (MC) for the reforming of propane [24] was tested in the steam reforming of methane. The robustness of this method was unknown and was therefore tested for its possible application in methane steam reforming, which has far harsher reaction conditions. A 1 wt% Rh/Al₂O₃ catalyst was wash-coated onto heat treated MC reactor plates and tested at 700 °C with steam to carbon ratio of 3 at a number of catalyst mass specific space velocities (scc᛫(gcat᛫h)⁻¹). The MC tests yielded conflicting results with some tests having stable catalysts and the majority have unstable catalysts due to poor wash-coat adhesion. The unsuccessful cases were due to a loss of catalyst. The change in catalyst stability was postulated to be the result of the wash-coating suspension size being reduced too much. In the cases where catalyst instability due to poor adhesion and ultimately loss of the catalyst the suspension batch size was reduced such that the surface tension of the viscous suspension now exceeded the intermolecular forces in the liquid. This resulted bubble formation and due to the high viscosity of the suspension due to the presence of the polyvinyl alcohol (PVA) binder the bubbles remained during the wash-coating process which is thought to have adversely affected the wash-coats adhesion. Another possible cause which is thought to have amplified the poor adhesion of the unstable catalyst runs is the thermal expansion of the stainless-steel reactor plates. The results of this study could not give outright and straightforward conclusions as to why there were 2 stable runs and unstable runs due to a loss of catalyst. As a result, further work is required to confirm the postulations and trends seen in this study. Future work should concentrate on using a larger batch of suspension to mitigate bubble formation, adding an alumina primer layer before wash-coating the catalyst to aid adhesion through additional oxide bond formation and the use of a more thermally stable stainless steel reactor plate to mitigate thermal expansion.
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    Solution combustion catalysts for the water-gas shift reaction
    (2018) Van Der Merwe, Marianne Werle; Fletcher, Jack C.Q.; Fletcher, Jack V.; Luchters, Niels
    In the context of a growing world population, more sustainable solutions for energy generation are required. Fuel cells supplied with hydrogen generated from fuel processing trains have emerged as a potential medium-term solution due to their improved efficiency and lower greenhouse gas-emissions. This study focuses on the development of a compact, efficient design for fuel processing trains. More specifically, reduction in the size of the largest component, the Water Gas-Shift (WGS) reactor, which could be achieved by an increase in the catalytic activity via alternative catalyst synthesis techniques. Solution combustion synthesis (SCS) is one such technique that could be used since it allows for the preparation of highly-dispersed Pt oxide particles on low surface area CeO2 with a reproducible metal loading and a defined Pt particle size. The aim of this study was to investigate the “conventional” impregnation approach of supports prepared via SCS versus the one-shot SCS approach (support and active metal prepared in one step) for the synthesis of 1 wt.% Pt/CeO2 catalysts for the WGS reaction using a reformate feed stream. It was hypothesized that the preferential formation of metallic Pt species supported on CeO2 can be achieved using a glycine-nitrate combustion system with excess glycine fuel (stoichiometric ratios of glycine to nitrate oxidants > 1) i.e. the Pt is reduced during the one-shot SCS approach. The catalysts were characterized by XRD, N2-physisorption, ICP-AES, TEM and XPS, and their activity towards the WGS reaction was evaluated with a synthetic reformate stream (50 % H2, 6.67 % CO, 6.67 % CO2, 33.3 % H2O, 3.36 % He). Initial characterization results of the catalysts prepared by the one-shot SCS approach confirmed the reproducible synthesis of Pt particles supported on nano-sized CeO2 with low surface areas. Furthermore, TEM and XPS results of the one-shot SCS prepared catalysts indicated that the Pt species were mainly present as Pt oxide particles on the surface of the CeO2 supports. However, for combustion systems with excess fuel, formation of some metallic Pt was observed together with the more prevalent Pt oxide particles. The catalysts prepared by the “conventional” impregnation approach had higher activities towards the WGS reaction than the one shot SCS catalysts. This was attributed to the smaller Pt particles achieved using this “conventional” synthesis approach (approximately 1 nm compared to 3 nm). One-shot SCS is a viable synthesis approach for the preparation of 1 wt.% Pt/CeO2 catalysts as this method allows for the preparation of highly-dispersed Pt oxide particles on low surface area CeO2 with a reproducible metal loading and a defined Pt particle size. However, the characterization results indicated that using a combustion system with excess fuel resulted in the preferential formation of Pt oxide phases as opposed to the desired metallic Pt phase, therefore refuting the hypothesis of this study. Nevertheless, it is recommended to repeat the synthesis of the 1 wt.% Pt/CeO2 catalysts in an inert atmosphere as this has shown to favour the formation of metallic species (Cross et al., 2014). This study was unsuccessful in preparing catalysts using a glycine-nitrate one-shot SCS system that were more active than the “conventionally” prepared catalysts. However, it is recommended that other fuel types, such as urea, also be investigated. These alternative fuel types could combine the good Pt dispersion achieved using the one-shot SCS approach with potentially smaller Pt particle sizes, thereby increasing the catalyst’s activity towards the WGS reaction (Vita et al., 2015a).