CuZnAl Derived From Hydrotalcite Precursor And Microfibrous ZnCaAl Catalysts For MSR

Polymeric electrolyte membrane fuel cell (PEMFC) received great interest for its high energy efficiency (60%), no pollution, low noise and transient startup. These clean energy sources have been showed expandable applications, such as passenger propulsion, distributed and auxiliary power systems. Since a widespread hydrogen refueling infrastructure is a long-time project, an on-board hydrogen production from liquid hydrocarbon fuels (e.g. gasoline, diesel oil and methanol, etc.) is an attractive option for fuel cell. Compared with gasoline and diesel oil, methanol is the most prominent liquid fuels due to low reaction temperature (200-250℃), simple molecule with high molar ratio of hydrogen to carbon, lack of sulfur/nitrogen compounds. Furthermore, methanol has been considered by many researchers as reversible source for PEMFC commercial application because of its low cost, classic production technique and attractive possibility to be produced from biomass.Among the catalysts for methanol steam reforming, copper-containing catalysts have been regarded as preferred one due to their good selectivity and high low-temperature activity. However, the sinterring of highly dispered nano-size copper particles, main active phase, after long time reaction limited their practical application greatly. Furthermore, CO concentration in the product reformed by CuZnAl catalysts still beyond the endurance level of proton-exchange membrane. To circumvent the above problems, double layer hydrotalcite-like materials has been used for the development of one-step fuel cell degrade hydrogen catalysts for methanol steam reforming. Good catalytic performance including high stability and selectivity were anticipated based on high CuO dispersion when CuZnAl hydrotalcite-like precursor was pretreated in proper condition with optimize composition and the metal sources. On the other hand, various characterizations have been carried out on catalysts prepared by different methods to illustrate the relationship between catalyst structure and CO selectivity. Furthermore, the way of CO and CO₂ formation were investigated in detail therefore reasonable reaction pathway will be proposed. At last, low efficiency of trade-off reactor would be replaced by new microfibrous catalyst to realize the reaction system miniaturization.In the first part, CuZnAl catalysts derived from hydrotalcite-like precursor were prepared and the influence of calcination temperature, chemical composition and metal salts in the catalyst structure and dispersion of CuO were investigated. The pyrogenation process of CuZnAl hydrotalcite-like precursor was analyzed. It was found CuZnAl-hydrotalcite was almost completely decomposed at 600℃ to isolate nano-sized CuO while being accompanied by the formation of CuAl₂O₄ spinel phase that played a key role in separating and stabilizing the nano-sized Cu and ZnO during the reaction. Excellent activity and stability for the methanol steam reforming would be showed at 250℃ and WHSV of 2.5h^-1 with a H₂O/ CH₃OH molar ratio of 1.3:1. After 25 h, the methanol conversion still over 98% with products composition of H₂ 75mol%, CO₂ 24.8mol%, CO 0.2mol%. Increasing calcination temperature to over 700℃ led to severe sintering of CuO while facilitating the formation of CuAl₂O₄ spinel phase, leading to significant reduction of the number of active sites. With lower calcination temperatures (300-500℃), incomplete decomposition of hydrotalcite precursor occurred to form (Cu,Zn)Al_xO_y(CO₃)_z composite via CO₃^2- rearrangement and meanwhile associated CuAl₂O₄ spinel phase was not formed, which resulted in the ease of sintering of metallic Cu as well ZnO during the reaction thereby providing poor activity. Next, the effect of the Cu/Zn/Al mass ratio on the precursor decomposition and catalyst performance for methanol steam reforming was investigated on the catalysts calcined at optimum temperature. The catalyst with Cu/Zn/Al ratio of 36.7/13.4/ (17-28) was preferred for achieving good activity. Varying Cu ratio barely affected the XRD phase composition of catalysts and reduction temperature of CuO phase. In comparison with the catalyst with optimal metal weight ratio, catalysts with lower Cu ratios provided lower activity mainly due to less active site whereas catalysts with much higher Cu ratios led to big loss of activity probably due to unsuitable Cu/Zn ratio. Both higher and lower Zn ratios led to the transformation of XRD phase composition of catalysts and therefore the catalysts were less active. Higher A1 ratios facilitated the dispersion of CuO and increased the specific surface area of catalysts thereby promoting the activity. It was also found that varying Cu/Zn/Al weight ratio hardly affected the gas product composition. Finally, a series of CuZnAl catalysts were prepared via hydrotalcite-like precursors using various Cu (II) and Zn (II) sources. Among these catalysts, the use of acetates and nitrates facilitated the crystallization of CuZnAl hydrotalcites while the derived catalysts provided large specific surface area, high Cu dispersion and good reducibility of CuO. The use of sulfates and chlorides made against hydrotalcite crystallization while leading to the reduction of the specific surface area and the reducibility of CuO in the corresponding catalysts. It should be noting that the catalyst from acetates provided much lower CO concentration in the dry product gas compared to the one prepared from nitrates: 400 ppm vs. ～2000 ppm with >95% methanol conversion at 250℃ with a WHSV of 3.28 h^-1. Furthermore, over that catalyst prepared from acetates CO concentration could be reduced to <50 ppm with near complete methanol conversion by reducing reaction temperature to 210℃ as well WHSV to 0.5 h^-1.Those prepared from sulfates and chlorides were inactive due to the poisoning of SO₄^2- and Cl^- residues.In the second part, detailed study on the cause of CuZnAl catalysts prepared by several methods with different selecitivity to CO has been made. MSR results over these catalysts show that the preparation method significantly affects the catalyst performance with respect to methanol conversion, H₂ yeild and CO concentration. With modifying by hydrotalcite, high activity and good stability can be obtained. Both the catalysts dirived from hydrotalcite precursor using acetate as Cu/Zn salts and prepared by impregnation have shown high activity and good stability with very low CO concentration (0.04⁰.05%) at 98% methanol conversion while ball milling method increase CO concentration to 0.7% at 50.9% methanol conversion. H₂-TPR/CO₂-oxidation/H₂-TPR cycles and HRTEM measurements over these catalysts suggested that the catalysts with good selectivity have high proportion of CO₂-oxided copper and clear interface between Cu and ZnO, which indicated strong Cu-ZnO interaction exists in the catalyst that is the key factor to catalyst selectivity.In the third part, experiments on in situ DRIFTS for adsorption of CH₃OH, HCOOCH₃, CH₃OH-H₂O, and HCOOCH₃-H₂O on catalysts with significant difference in CO selectivity, i.e., CZA-A drived from HTLcs using acetate metal sources and a CZA-M prepare by high-energy ball milling, have been carried out for clarifying the controversy about the reaction pathway and CO formation. A reasonable reaction route was proposed: CO, the second-product, was produced from decomposition of surface formaldehyde. The catalyst with good selectivity made the transformation of formaldehyde to methyl formate easy due to its good synergistic effect between Cu and ZnO. On the other hand, the rate of methyl formate oxidation to formic acid can be greatly accelerated contributed to the numerous surface active oxygen providing by contacting of H₂O to surface oxygen-ion vacancies in this kind of catalyst. In contrast, poor Cu-ZnO interaction will lead to more residual formaldehyde on catalyst surface, which give rise to CO formation by decomposition of formaldehyde.In the last part, a novel three-dimensional structured support was developed by sintering metal microfibers sheets entrapped with Al₂O₃ support particulates using a high-speed and low-cost papermaking technology. Zn and Ca, active species, were high dispersed onto the particulates by impregnation method. The catalyst performance of such system for methanol steam reforming has been investigated at 470℃ with H₂O/MeOH molar ratio of 1.3. As a result, the special web structure provides high efficient use of the active materials due to its low pressure-drop, high heat transfer and large void volume. Compared with conventional powder filled system, the monolithic composite would raise WHSV from 14.4 to 33.2 h^-1 while product composition of CO and CH₄ remained at relatively low level, 5% and 0.3%, with methanol conversion of 98%.