The object of this thesis is the study of the catalytic behaviour of copper based catalysts in methanol steam reforming for on board production of hydrogen. In order to study the catalytic properties, an experimental setup which consists mainly of pump device for methanol and water, reactor, separator units for liquid and gas and analytical instruments was established.
The investigations were performed by means of a three channel fixed-bed reactor. Due to the poor long term stability and high CO formation of the commercial CuO/ZnO/Al2O3 catalysts in the methanol steam reforming, a series of novel CuO catalysts supported on ZrO2 has been prepared and their catalytic properties are investigated in this work. These catalysts were synthesized with different preparation methods, such as CuO on nanopowder ZrO2, on mesoporous ZrO2 and on macroporous ZrO2. In order to compare the catalytic properties of these catalysts, the commercial CuO/ZnO/Al2O3 catalyst was used as a reference. The results of this thesis can be divided into three sections (chapter 4, 5 and 6). In chapter 4 the study of the commercial CuO/ZnO/Al2O3 catalyst was focused on the formation of CO. In the next chapter the study of the catalytic properties of the CuO catalyst supported on macroporous ZrO2 was performed and compared to those of the commercial CuO/ZnO/Al2O3 catalyst. In chapter 6 the catalytic behaviour of the six CuO/ZrO2 catalysts was studied and the catalysts are compared to each others.
The main results of these three chapters are summarized in the following:
In chapter 4 the kinetic study of methanol steam reforming over commercial CuO/ZnO/Al2O3
catalyst has been performed at atmospheric pressure over a wide temperature range (230-300
°C). The reaction scheme used is the direct formation of hydrogen and carbon dioxide by steam reforming reaction and the formation of CO as a consecutive product by the reverse water-gas shift reaction. A simulation employing these schemes to describe methanol steam reforming process over a CuO/ZnO/Al2O3 catalyst fit the experimental data measured at 230 to 300°C well.
The monotonic increase of CO partial pressure as a function of contact time measured at the temperature range from 230°C to 300°C as well as the limit of no selectivity for CO as the contact time approaches 0, proves that CO is formed as a consecutive product.
A new finding reported in this work concerns the parameters influencing the formation of CO.
It was found that the CO concentration can be influenced by the particle size of the catalyst through its effect on intraparticle diffusion limitation. This parameter can be added to those reported in the literature as influencing the production of CO, i.e. reaction temperature, contact time, molar ratio of methanol and water, and addition of oxygen to the methanol-steam feed. The greater the mass transport limitation in the catalyst particle the higher the concentration of CO in the product stream.
In Chapter 5 the catalytic properties of the CuO/ZrO2 catalyst which was prepared using a polymer template sol-gel method (CuO on macroporous ZrO2) have been examined. After the reduction in a methanol/water mixture at 250°C for 1h the catalyst showed a very poor activity. After several hours of time on stream, the catalyst can be activated by introducing oxygen. The CO concentration observed as a function of contact time reveals that CO is formed as a consecutive product. The enhancement of the catalytic properties of the CuO/ZrO2 catalyst in comparison to the commercial CuO/Zn/Al2O3 catalyst is described as follows:
(i) higher activity in term of methanol conversion as a function of WCu/Fm,
(ii) more stability in time on stream (i.e. less deactivation), probably due to the higher effectivity of macroporous zirkonia support than ZnO/Al2O3 in preventing sintering of copper particles,
(iii) lower CO formation, especially at high methanol conversion.
In Chapter 6 the catalytic properties of the six CuO/ZrO2 catalysts prepared by different synthesis methods have been studied at the same reaction conditions as employed in chapter 4 and chapter 5. The activity of the catalysts can be improved by introducing oxygen to the feed at reaction condition. The study of the activity of the CuO/ZrO2 catalysts as a function of the copper surface area reveals a relation between the activity and the synthesis. The CuO catalyst prepared on macroporous ZrO2 is the most active followed by the CuO catalysts prepared on mesoporous ZrO2. The CuO catalysts on nanopowder ZrO2 have the lowest activity. There is no significant difference with respect to the formation of CO as a function of methanol conversion determined over all the CuO/ZrO2 catalysts.
The comparison of the catalytic properties between the CuO/ZrO2 catalysts and the commercial CuO/ZnO/Al2O3 catalyst gives the following results:
(i) the measurement of the activity described in term of xs (ratio of methanol conversion and copper surface area) shows that in comparison to the commercial CuO/ZnO/Al2O3 catalyst, all of the six CuO/ZrO2 catalysts exhibit higher activity.
(ii) The CuO/ZrO2 catalysts provide a higher long term stability than the CuO/ZnO/Al2O3
catalyst. This can be seen in the increase of the copper surface area after the catalysts were measured for a long time on stream and with the treatment of oxygen. In contrast, as reported in the literature the activity of the commercial Cu/ZnO/Al2O3 catalyst decreases with oxygen introduction into the feed. This is because of the sintering of the copper particle.
(iii) There is less CO formed in the product stream by using the CuO/ZrO2 catalysts. The difference is becoming more significant at high methanol conversion.
The following paragraphs describe the perspectives with respect to the results presented in this thesis.
The formation of CO as a consecutive product (in case of the commercial catalyst) correlated with the finding that CO concentration increases with the increase of the intraparticle diffusion limitation in the catalyst particle permits potential solutions that can be applied with respect to the chemical engineering process to minimize CO formation are the application of the following methods:
(i) The use of a membrane reactor (ii) The use of a microtube reactor (iii) The use of a monolith reactor (iv) The use of an egg-shell catalyst (v) Dilution of the catalyst.
Referring to the main results in chapter 5 and 6 which describe that the novel CuO/ZrO2
catalysts prepared with various kinds of methods exhibit significantly enhanced catalytic properties (more stability with time on stream, higher activity and less CO formation) in comparison to the CuO/ZnO/Al2O3 catalyst, a knowledge-based preparation of heterogeneous catalysts is feasible permitting the rational design of materials exhibiting an improved catalytic performance.
Furthermore, in order to accomplish a rational design of catalyst, the study of the structure of the catalyst is an important prerequisite bridging synthesis and catalysis. The relation between the synthesis, the structure and the catalytic properties is illustrated in the following Figure.
The treatment condition (e.g. introduction of oxygen), however, is an additional tool to improve the catalyst activity which in turn can influence the catalyst structure.
Synthesis Structure
Treatment
Synthesis Structure Catalysis
Treatment
Figure 7.1: Relationship between synthesis, structure and catalysis.