PhD Thesis Defence Presentations - Marios Kourtelesis

The production of renewable hydrogen through ethanol steam reforming reaction at low temperatures is investigated in the present study. It is a very attractive process that has gained interest over the last decades. In the first part, the catalytic behavior of supported Pt and Ni catalysts for the low temperature steam reforming of ethanol is investigated.

First, the effect of Pt addition on the Ni/CeO₂ catalyst is studied. The reaction mechanism was explored using diffuse reflectance infrared spectroscopy under reaction conditions and temperature-programmed surface reaction experiments. The addition of Pt was found to promote the decomposition of dehydrogenated and acetate species to hydrogen, methane, CO and carbonate species. In addition, characterization results in combination with kinetic data suggested that over the PtNi/CeO₂ catalyst, Pt forms a partial monolayer on Ni particles. The presence of Pt on the surface of Ni minimized nickel carbide formation, which is associated with amorphous carbon deposition, while promoting the hydrogenation of reactive carbon species adsorbed on the surface, thus improving the stability of the bimetallic catalyst. Next, the effect of CeO₂ morphology on the performance of Pt/CeO₂ catalysts for the low temperature ethanol steam reforming was investigated, with emphasis on catalyst deactivation. Three different ceria nanostructures (nanocube-NC, nanorods-NR and flower-like-FL) were prepared by the hydrothermal method and were compared with CeO₂ obtained by the precipitation method (CeO₂-PPT). Regardless of the support, the reaction scheme was found to be the same for all catalysts. However, support morphology seems to influence certain reaction routes. Pt/CeO₂-PPT catalyst exhibited the highest activity for ethanol dehydrogenation to acetaldehyde, while Pt catalysts supported on nanostructured supports were found more active for CO methanation and WGS reactions. The latter was attributed to the higher rate of acetate species decomposition over catalysts with structured supports compared to 1% Pt/CeO₂-PPT, which might be attributed to higher Pt dispersion. The stability of the catalysts for the steam reforming of ethanol at 300 ⁰C significantly depended on the type of support. Pt supported on CeO₂-PPT deactivated rapidly, while Pt/CeO₂-NR catalyst remained rather stable. Surface analysis before and after reaction at 300 ⁰C by XPS revealed significant degree of Pt reduction on CeO₂-NR, which can be directly related to the reduction of Ce⁴⁺ to Ce³⁺. DRIFTS experiments under steam reforming conditions at 300 ⁰C revealed the accumulation of acetate species on the catalyst containing ceria prepared by precipitation, whereas, this was significantly less pronounced on Pt catalysts supported on nanostructured ceria. The accumulation of acetate species is likely due to the imbalance between the rate of acetate species decomposition to CH_x, which is not facilitated over Pt^δ+ of the catalyst with the lower Pt dispersion, and their desorption as CH₄.   

The next part deals with the catalytic performance of a reference catalyst 0.5% Pt/γ-Al₂O₃ for ethanol steam reforming in the temperature range of 300-450 ⁰C. Emphasis was given on the stability of the catalyst as a function of reaction temperature. Reaction temperature was found to have a strong impact on the stability of the catalyst, as rapid deactivation was observed at temperatures above 400 ⁰C, while the catalyst remained stable up to that temperature. Metal-support interface was destructed at high temperatures, with the reactions that take place over the surface of the carrier and over the metal, being carried out to a lesser extent. Coke formation near the surface of the metal, probably at the metal/support interface, was favored with increasing reaction temperature, suggesting it is the main cause of the observed deactivation, with its amount increasing by an order of magnitude over the deactivated samples (experiments at 430 and 450 ⁰C). Concerning the source of carbon formed, temperature-programmed oxidation experiments of the used 0.5% Pt/Al₂O₃ catalyst in combination with TPSR experiments under ethanol and acetaldehyde steam reforming conditions, indicated that carbon formation mainly originated from acetaldehyde cracking, with methane decomposition and Boudouard reaction contributing to a lesser extent.

In order to identify the role of acetaldehyde in catalyst deactivation during ethanol steam reforming reaction, stability tests were conducted next, over supported Pt catalysts, under a feed consisting of acetaldehyde and steam. The experiments took place at 340 ⁰C and the results showed rapid deactivation of the catalyst, even at a temperature as low as 340 ⁰C and low partial pressure of acetaldehyde (3.7%). The γ-Al₂O₃ carrier possesses Lewis acid sites, which promote reactions that can lead to the formation of coke precursors. Thus, in order to neutralize the acid sites of the carrier, the effect of modifying γ-Al₂O₃ with the addition of Ca is studied. More specifically, the effect of calcination temperature (500 or 800 ⁰C) and Ca loading (1, 3, 6 and 10 wt.%) was investigated. The results showed that 0.5%Pt-3%Ca/γ-Al₂O₃ catalyst, calcined at 800 ⁰C, presented the optimum performance, as it was found stable under acetaldehyde steam reforming conditions at 340 ⁰C, for more than 18 h TOS. Characterization tests of the modified catalysts showed that the addition of Ca reduced the acid sites of the catalyst, while increasing the number of basic sites. Moreover, the amount of carbon formed over the 0.5%Pt-3%Ca/γ-Al₂O₃ catalyst was the lowest amongst the various 0.5%Pt-X%Ca/γ-Al₂O₃ catalysts that were studied, being 7 fold lower than that of the unmodified Pt/Al₂O₃ sample. The role of Ca is related to the promotion of carbon gasification caused by the improvement of the catalyst’s ability to adsorb and activate H₂O, thus facilitating carbon removal and keeping the Pt sites active for the steam reforming of acetaldehyde. Then, the effect of the nature of the support (CeO₂, ZrO₂, La₂O₃ and SiO₂) on catalytic stability of supported Pt catalysts for the steam reforming of acetaldehyde was studied. Acetaldehyde conversion was found to decrease over all catalysts suggesting the deactivation of the catalysts which is related to the accumulation of carbonaceous deposits, probably at the metal-support interface. The deactivation rate was found to follow the order Pt/ZrO₂>Pt/La₂O₃≈Pt/CeO₂>Pt/SiO₂.

Aiming at the maximization of hydrogen production from ethanol steam reforming reaction, the overall process can be divided in two stages; the first involves ethanol dehydrogenation to acetaldehyde, while the second, acetaldehyde steam reforming. In the last part of the present study, several supported Co catalysts were tested for acetaldehyde steam reforming reaction. At first, the effect of metal loading and mean crystallite size of Co/Al₂O₃ catalysts was studied. The catalytic samples were characterized and tested under steady state experiments in terms of activity and stability at various temperatures. Results showed that the increase of metal loading led to increased activity as more active surface sites were available for the reactions to take place. The formation of the desired products H₂ and CO₂ was found to be favored over 10 and 15% Co/γ-Al₂O₃ catalysts, while, over the γ-Al₂O₃ support, the formation of acetone and crotonaldehyde predominated. 10% Co/γ-Al₂O₃ catalyst showed the best performance during 8 h stability tests carried out at 430 and 480 ⁰C. However, significant carbon deposition was observed over all catalysts, leading to high pressure drop in the system. The characterization of used catalysts indicated that carbon formation is favored over the surface of Co. Concerning the nature of carbon formed, results revealed the formation of amorphous carbon (mainly) as well as filamentous carbon, usually in the form of carbon nanotubes. The amount of deposited carbon was found to increase with increasing metal loading, as it was increased by a factor of 5, with increasing metal loading from 10 to 30 wt.%. Experiments carried out under differential reaction conditions revealed that the structural parameters of the Co/γ-Al₂O₃ catalysts influenced specific catalytic activity, as it was found to increase (almost by a factor of 3) with decreasing the Co crystallite size from 12.4 to 6.3 nm.

The study of 10%Co/γ-Al₂O₃ catalyst modification with the addition of La or B did not lead to improved performance compared to the pristine sample. Moreover, a series of Co catalysts supported on SiO₂ were prepared in order to study the effect of the nature of the support on catalytic performance under acetaldehyde steam reforming conditions. Results showed that the 20% Co/SiO₂ catalyst, even though being less active than those supported on γ-Al₂O₃ and thus, requiring increased contact times to reach the same values of acetaldehyde conversion, remained rather stable at 480 ⁰C. Concerning the product distribution of 20%Co/SiO₂ catalyst, only H₂, CO_x and CH₄ were detected, while the catalyst presented higher values of specific surface area and smaller Co crystallite sizes, compared to Co/γ-Al₂O₃ catalysts. The comparison of the variation of acetaldehyde conversion as a function of temperature over all the catalysts during 8 h stability experiments at 480 ⁰C, showed that 10%Co/γ-Al₂O₃ and 20%Co/SiO₂ catalysts were more stable, while these catalysts also presented the smallest metal crystallite sizes (7.4 and 3.0 nm, respectively). Furthermore, comparison of the quantities of carbon deposition over the catalysts showed that the lowest amount of carbon was formed over Co/SiO₂ catalyst (1.4 mgC/m_cat²), followed by the 10% Co/Al₂O_­3 catalyst (1.6 mgC/m_cat²). Thus, the observed deactivation is probably related to the formation of carbon, which, in turn, depended on the number of terrace sites of Co particles where it is formed.

Finally, the reaction network of acetaldehyde steam reforming over supported Co catalysts was investigated. The catalysts were studied employing transient experiments in order to gain information about the effect of the support, metal and metal loading on reaction mechanism and on the adsorbed intermediate species formed over the catalytic surface as well as the gas phase products. The combination of TPSR experiments with DRIFTS spectra under acetaldehyde steam reforming conditions led to certain conclusions about the steps of the reaction network. Co was found to be active for the steam reforming reaction, as evidenced by the performance of 10% Co/α-Al₂O₃ catalyst, even though the product distribution was found to depend strongly on the support, which can promote certain reactions. Thus, while bare SiO₂ was found inactive under reaction conditions, the addition of Co led to a product distribution containing solely the desired products (CO_x, H₂ and low amounts of CH₄). On the other hand, γ-Al₂O₃ carrier was found to favor acetaldehyde condensation reactions, contributing to the product distribution of Co/γ-Al₂O₃ catalysts which contained several by-products (acetone, crotonaldehyde, ethanol).