Microwave-hydrothermal synthesis and characterization of nanostructured copper substituted ZnM2O4 (M = Al, Ga) spinels as precursors for thermally stable Cu catalysts

Nanostructured Cu(x)Zn(1-x)Al(2)O(4) with a Cu:Zn ratio of ?:? has been prepared by a microwave-assisted hydrothermal synthesis at 150 °C and used as a precursor for Cu/ZnO/Al(2)O(3)-based catalysts. The spinel nanoparticles exhibit an average size of approximately 5 nm and a high specific surface area (above 250 m(2) g(-1)). Cu nanoparticles of an average size of 3.3 nm can be formed by reduction of the spinel precursor in hydrogen and the accessible metallic Cu(0) surface area of the reduced catalyst was 8 m(2) g(-1). The catalytic performance of the material in CO(2) hydrogenation and methanol steam reforming was compared with conventionally prepared Cu/ZnO/Al(2)O(3) reference catalysts. The observed lower performance of the spinel-based samples is attributed to a lack of synergetic interaction of the Cu nanoparticles with ZnO due to the incorporation of Zn(2+) in the stable spinel lattice. Despite its lower performance, however, the nanostructured nature of the spinel catalyst was stable after thermal treatment up to 500 °C in contrast to other Cu-based catalysts. Furthermore, a large fraction of the re-oxidized copper migrates back into the spinel upon calcination of the reduced catalyst, thereby enabling a regeneration of sintered catalysts after prolonged usage at high temperatures. Similarly prepared samples with Ga instead of Al exhibit a more crystalline catalyst with a spinel particle size around 20 nm. The slightly decreased Cu(0) surface area of 3.2 m(2) g(-1) due to less copper incorporation is not a significant drawback for the methanol steam reforming.     

Correlation of activity and stability of CuO/ZnO/Al2O3 methanol steam reforming catalysts with Cu/Zn composition obtained by SEM–EDAX analysis

A series of CuO/ZnO/Al₂O₃ catalysts were prepared and characterized by TPR, surface area, metal area, XRD and SEM–EDAX analysis. These systems were evaluated in the development of a methanol steam reforming catalyst (MSR). A correlation of activity and stability of MSR catalysts with the Cu/Zn ratio derived by SEM–EDAX analysis is observed. The stable activity of these catalysts is also supported by the method of preparation, low temperature reducibility and the presence of reversibly oxidizable Cu species observed by TPR of fresh and used catalysts.     

Influence of preparation method on performance of Cu(Zn)(Zr)-alumina catalysts for the hydrogen production via steam reforming of methanol

The selective production of hydrogen via steam reforming of methanol (SRM) was performed using prepared catalysts at atmospheric pressure over a temperature range 200–260^∘C. Reverse water gas shift reaction and methanol decomposition reactions also take place simultaneously with the steam reforming reaction producing carbon monoxide which is highly poisonous to the platinum anode of PEM fuel cell, therefore the detailed study of effect of catalyst preparation method and of different promoters on SRM has been carried out for the minimization of carbon monoxide formation and maximization of hydrogen production. Wet impregnation and co-precipitation methods have been comparatively examined for the preparation of precursors to Cu(Zn)(Al₂O₃) and Cu(Zn)(Zr)(Al₂O₃). The catalyst preparation method affected the methanol conversion, hydrogen yield and carbon monoxide formation significantly. Incorporation of zirconia in Cu(Zn)(Al₂O₃) catalyst enhanced the catalytic activity, hydrogen selectivity and also lower the CO formation. Catalyst Cu(Zn)(Zr)(Al₂O₃) with composition Cu/Zn/Zr/Al:12/4/4/80 prepared by co-precipitation method was the most active catalyst giving methanol conversion up to 97% and CO concentration up to 400 ppm. Catalysts were characterized by atomic absorption spectroscopy (AAS), Brunauer-Emett-Teller (BET) surface area, pore volume, pore size and X-ray powder diffraction (XRPD). The XRPD patterns revealed that the addition of zirconia improves the dispersion of copper which resulted in the better catalytic performance of Cu(Zn)(Zr)(Al₂O₃). The time-on-stream (TOS) catalysts stability test was also conducted for which the Cu(Zn)(Zr)(Al₂O₃) catalyst gave the consistent performance for a long time compared to other catalysts.     

Role of ZnO in Cu/ZnO/Al2O3 catalyst for hydrogenolysis of butyl butyrate

For hydrogenolysis of butyl butyrate (BB), a series of Cu/ZnO/Al₂O₃ catalysts with different metal compositions were prepared, and characterized by N₂O chemisorption for measuring Cu surface area and by chromatographic experiment for determining the heat of BB adsorption. As a result, the presence of ZnO in Cu-based catalysts was found to enhance the catalytic activity of Cu due to dual function of ZnO. The Cu surface area was linearly correlated with the butanol productivity, demonstrating that ZnO exerts the structural function in Cu/ZnO/Al₂O₃ catalysts. Additionally, the role of ZnO as a chemical contributor was revealed such that its presence leads to lower activation energy of the surface reaction, thus resulting in higher Cu catalytic activity obtained at a low temperature such as 200 °C. Consequently, optimizing the Cu/Zn ratio in Cu/ZnO/Al₂O₃ catalyst is required to tune its structural and chemical characteristics of Cu metals, and thus to obtain a higher activity on the hydrogenolysis reaction.     

A zinc nanoparticles-dispersed multi-scale web of carbon micro-nanofibers for hydrogen production step of ZnO/Zn water splitting thermochemical cycle

Water splitting via the two-step ZnO/Zn thermochemical cycle is a promising and environmentally benign method for producing hydrogen from steam. In this study, we focus on the second step of the cycle, which is the exothermic hydrolysis reaction of Zn nanoparticles with steam. The unique Zn nanoparticle-dispersed carbon micro-nanofibers (Zn-CNFs) were prepared by impregnating carbon microfibers (ACFs) with a sodium dodecyl sulfate (SDS)-mediated mono-dispersed aqueous solution of Zn(II), followed by calcinations and reduction. The surfactant increased the metal (average crystal size ∼ 25 nm) loading by approximately two-fold on the ACFs. The CNFs were grown on Zn-ACF at 700 °C by catalytic chemical vapor deposition (CCVD) using acetylene as the carbon source. Zn has dual roles in this system, one as a catalyst for CNF growth and the other as a reactant in the hydrolysis reaction. The water-splitting reaction was performed at different steam and N₂ flow rates and reaction temperatures. The production rate and yield of H₂ at a reaction temperature of 600 °C were calculated as 1.66 × 10⁻⁶ mol/g s and 80%, respectively, which are comparable to or higher than results reported in the literature. The Zn-CNFs prepared in this study are a potential candidate for the H₂ production step of the ZnO/Zn thermochemical cycle.     

Selective Hydrogenolysis of Glycerol to 1, 2 Propanediol Over Cu–ZnO Catalysts

A series of Cu–ZnO catalysts with varying Cu to Zn weight ratio are prepared by co-precipitation method. The catalysts were characterized by surface area, XRD, TPR and N₂O chemisorption to measure Cu metal area. These catalysts were evaluated for hydrogenolysis of glycerol. The catalyst with Cu to Zn ratio of 50:50 is highly active under relatively low H₂ pressure. The catalysts are highly selective towards 1,2 propanediol (>93%). The glycerol conversion depends upon the bifunctional nature of catalyst where it requires both acidic sites and metal surface. The presence of sufficient amount with small particle size of ZnO and Cu are required for high conversion of glycerol and selectivity to 1,2 propanediol. Different reaction parameters are studied in order to optimize the reaction conditions.     

High‐performance HTLcs‐derived CuZnAl catalysts for hydrogen production via methanol steam reforming

A series of CuZnAl oxide‐composite catalysts were prepared via decomposition of CuZnAl hydrotalcite‐like compounds (HTLcs). The catalysts derived from CuZnAl HTLcs (Cu: 37%, Zn: 15%, Al: 48% mol; using metal nitrate or acetate precursors) at 600°C provided excellent activity and stability for the methanol steam reforming. CuZnAl HTLcs were almost decomposed completely at 600°C to form highly dispersed CuO with large specific surface area while forming CuAl₂O₄ spinel that played a key role in separating and stabilizing the nano‐sized Cu and ZnO during the reaction. The CuZnAl catalyst prepared from metal acetates could highly convert H₂O/MeOH (1.3/1, mol/mol) mixture into hydrogen with only ～0.05% CO at 250°C or ～0.005% at 210°C. It is evidenced that the former afforded stronger Cu‐ZnO interaction, which might be the intrinsic reason for the significant promotion of catalyst selectivity. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

A highly efficient Cu/ZnO/Al2O3 catalyst via gel-coprecipitation of oxalate precursors for low-temperature steam reforming of methanol

The impact of preparation methods on the structure and catalytic behavior of Cu/ZnO/Al₂O₃ catalysts for H₂ production from steam reforming of methanol (SRM) has been reported. The results show that the nanostructured Cu/ZnO/Al₂O₃ catalyst obtained by a novel gel-coprecipitation of oxalate precursors has a high specific surface area and high component dispersion, exhibiting much higher activity in the SRM reaction as compared to the catalysts prepared by conventional coprecipitation techniques. It is suggested that the superior catalytic performance of the oxalate gel-coprecipitation-derived Cu/ZnO/Al₂O₃ catalyst could be attributed to the generation of “catalytically active” copper material with a much higher metallic copper specific surface as well as a stronger Cu–Zn interaction due to an easier incorporation of zinc species into CuC₂O₄ · x H₂O precursors as a consequence of isomorphous substitution between copper and zinc in the oxalate gel-precursors.     

Hydrogen production by oxidative steam reforming of methanol using ceria promoted copper–alumina catalysts

The performance of different Cu/CeO₂/Al₂O₃ catalysts of varying compositions is investigated for the oxidative steam reforming of methanol (OSRM) in order to produce the hydrogen selectively for polymer electrolyte membrane (PEM) fuel cell applications. All the catalysts were prepared by co-precipitation method and characterized for their surface area, pore volume and oxidation–reduction behavior. The effect of various operating parameters studied are as follows: reaction temperature (200–300 °C), contact-time (W/F = 3–15 kg_cat s mol^− 1) and oxygen to methanol (O/M) molar ratio (0–0.5). The steam to methanol (S/M) molar ratio = 1.5 and pressure = 1 atm were kept constant. Among all the catalysts studied, catalyst Cu–Ce–Al:30–20–50 exhibited 100% methanol conversion and 179 mmol s^− 1 kg_cat^− 1 hydrogen production rate at 280 °C with carbon monoxide formation as low as 0.19%. The high catalytic activity and hydrogen selectivity shown by ceria promoted Cu/Al₂O₃ catalysts is attributed to the improved specific surface area, dispersion and reducibility of copper which were confirmed by characterizing the catalysts through temperature programmed reduction (TPR), CO chemisorption, X-ray diffraction (XRD) and N₂ adsorption–desorption studies. Reaction parameters were optimized in order to produce hydrogen with carbon monoxide formation as low as possible. The time-on-stream stability test showed that the Cu/CeO₂/Al₂O₃ catalysts were quite stable.     

Influence of preparation method on performance of Cu(Zn)(Zr)-alumina catalysts for the hydrogen production via steam reforming of methanol

The selective production of hydrogen via steam reforming of methanol (SRM) was performed using prepared catalysts at atmospheric pressure over a temperature range 200–260^°C. Reverse water gas shift reaction and methanol decomposition reactions also take place simultaneously with the steam reforming reaction producing carbon monoxide which is highly poisonous to the platinum anode of PEM fuel cell, therefore the detailed study of effect of catalyst preparation method and of different promoters on SRM has been carried out for the minimization of carbon monoxide formation and maximization of hydrogen production. Wet impregnation and co-precipitation methods have been comparatively examined for the preparation of precursors to Cu(Zn)(Al₂O₃) and Cu(Zn)(Zr)(Al₂O₃). The catalyst preparation method affected the methanol conversion, hydrogen yield and carbon monoxide formation significantly. Incorporation of zirconia in Cu(Zn)(Al₂O₃) catalyst enhanced the catalytic activity, hydrogen selectivity and also lower the CO formation. Catalyst Cu(Zn)(Zr)(Al₂O₃) with composition Cu/Zn/Zr/Al:12/4/4/80 prepared by co-precipitation method was the most active catalyst giving methanol conversion up to 97% and CO concentration up to 400 ppm. Catalysts were characterized by atomic absorption spectroscopy (AAS), Brunauer-Emett-Teller (BET) surface area, pore volume, pore size and X-ray powder diffraction (XRPD). The XRPD patterns revealed that the addition of zirconia improves the dispersion of copper which resulted in the better catalytic performance of Cu(Zn)(Zr)(Al₂O₃). The time-on-stream (TOS) catalysts stability test was also conducted for which the Cu(Zn)(Zr)(Al₂O₃) catalyst gave the consistent performance for a long time compared to other catalysts.     

The effect of ZnO in methanol synthesis catalysts on Cu dispersion and the specific activity

The effect of ZnO in Cu/ZnO catalysts prepared by the coprecipitation method has been studied using measurements of the surface area of Cu, the specific activity for the methanol synthesis by hydrogenation of CO₂, and XRD. Although the Cu surface area increases with increasing ZnO content (0–50 wt%) as is generally known, the specific activity of the Cu/ZnO catalysts with various weight ratios of Cu:ZnO is greater than that of a ZnO-free Cu catalyst. These facts clearly indicate that the role of ZnO in Cu/ZnO catalysts can be ascribed to both increases in the Cu dispersion and the specific activity. The XRD results indicate the formation of a Cu–Zn alloy in the Cu particles of the Cu/ZnO catalysts, leading to the increase in specific activity. It is thus considered that the Cu–Zn surface alloy or a Cu–Zn site is the active site for methanol synthesis in addition to metallic copper atoms that catalyze several hydrogenation steps during the methanol synthesis. Furthermore, the advantage of the coprecipitation method through a precursor of aurichalcite is ascribed to both improvements in the Cu surface area and the specific activity. This revised version was published online in July 2006 with corrections to the Cover Date.     

Electrophoretic deposition of ethanol steam-reforming catalysts on metal plates for the development of catalytic-wall reactors

A procedure based on electrophoretic deposition (EPD) was developed to coat metal plates with powder catalysts. The method was tested on stainless-steel plates with three Ni-based catalysts for the steam reforming of ethanol. The catalysts (Ni/La₂O₃/γ-Al₂O₃) contained 15% Ni and 8% La, and were prepared using three types of γ-alumina with different textural properties. The powder catalysts were suspended in isopropanol, and EPD deposition was performed with a voltage of 100 V and a distance between electrodes of 2 cm. Deposition time was varied between 3 and 7 min, which gave a thickness of the catalyst layer from around 30 to 100 μm. The morphology of the catalyst layer was dependent on the textural characteristics of the γ-Al₂O₃ used to prepare the catalyst. The activity of the catalyst plates was tested at 773 K using a steam to carbon molar ratio of 4. Significant differences in the selectivity towards ethanol dehydrogenation, reforming, and dehydration to ethylene could be observed between the three catalysts. Carbon deposition on the surface of the plates could be easily determined by SEM/ESEM.     

Effective parameters for DME steam reforming catalysts for the formation of H2 and CO

Recently, DME has received attention as a clean fuel and is now considered an alternative fuel for diesel engines. DME diesels need de-NOx catalysts such as LNT (Lean NOx Trap) and SCR (Selective Catalytic Reduction) systems. DME is an attractive source of hydrogen because it can be stored easily and is a good transportation fuel. Hydrogen and CO enriched gas as a reductant was used with the LNT catalyst in order to reduce NOx emissions. The steam reforming catalyst of DME was used to formation of hydrogen. It has been reported that Cu-based catalysts have high selectivity and activity in the steam reforming of DME. This research used 600 cPsi cordierite as a catalyst, which was coated with copper. The catalysts were made via a sol–gel and impregnation methods. The formation of H₂ and CO under the prepared catalysts was tested by a model gas. Experimental parameters were considered; the space velocity (SV) and concentrations of H₂O, O₂, and CO₂ were evaluated. The Cu 30%/γ-Al₂O₃ catalyst from the sol–gel method exhibited high and stable activity in the production of hydrogen from the steam reforming of DME. Both DME conversion and the selectivity of hydrogen were affected by SV and the concentrations of H₂O, O₂, and CO₂.     

Effect of H2O on Cu-based catalyst in one-step slurry phase dimethyl ether synthesis

One-step dimethyl ether (DME) synthesis in slurry phase was catalyzed by a hybrid catalyst composed of a Cu-based methanol synthesis catalyst and a γ-Al₂O₃ methanol dehydration catalyst under reaction conditions of 260 °C and 5.0 MPa. It was found that instability of the Cu-based catalyst led to rapid deactivation of the hybrid catalyst. The stability of the Cu-based catalyst under DME synthesis conditions was compared with that under methanol synthesis conditions. The results indicated that harmfulness of water, which formed in DME synthesis, caused the Cu-based catalyst to deactivate at a high rate. Surface physical analysis, elemental analysis, XRD and XPS were used to characterize the surface physical properties, components, crystal structures and surface morphologies of the Cu-based catalysts. It was found that Cu⁰ was the active component for methanol synthesis and Cu₂O might have less activity for the reaction. Compared with methanol synthesis process, crystallite size of Cu became bigger in DME synthesis process, but carbon deposition was less severe. It was also found that there was distinct metal loss of Zn and Al caused by hydrothermal leaching, impairing the stability of the catalyst. In slurry phase DME synthesis, a part of Cu transformed into Cu₂(OH)₂CO₃, causing a decrease in the number of active sites of the Cu-based catalyst. And some ZnO converted to Zn₅(OH)₆(CO₃)₂, which caused the synergistic effect between Cu and ZnO to become weaker. Crystallite size growth of Cu, carbon deposition, metal loss of Zn and Al, formation of Cu₂(OH)₂CO₃ and Zn₅(OH)₆(CO₃)₂ were important reasons for rapid deactivation of the Cu-based catalyst.     

Selective steam reforming of methanol over silica-supported copper catalyst prepared by sol–gel method

Silica-supported copper prepared by a sol–gel method can selectively catalyze methanol steam reforming to hydrogen and carbon dioxide at 250 °C. The catalytic activity increases with the copper content up to 40 wt.%. The selectivity to carbon monoxide with the catalysts containing 20–40 wt.% of copper is significantly lower than that with a commercial Cu/ZnO/Al₂O₃ catalyst. Copper particles are highly dispersed in the catalyst whose Cu content is 20 wt.% or less. After the reaction at 250 °C the particles are present as Cu₂O with the mean crystallite size less than 4 nm. In the catalyst with the Cu content of 30–50 wt.%, the fine Cu₂O particles coexist with large metallic Cu particles whose mean crystallite size is 30–40 nm after the reaction. The large metallic particles are supposed to contribute to the reaction as well as the fine Cu₂O particles although the surface area is estimated to be significantly smaller than that of the latter.     

Hydrogen Production by Steam Reforming of Liquefied Natural Gas Over Mesoporous Ni-Al2O3 Composite Catalyst Prepared by a Single-step Non-ionic Surfactant-templating Method

A mesoporous Ni-Al₂O₃ composite catalyst (Ni-A-NS) was prepared by a single-step non-ionic surfactant-templating method for use in hydrogen production by steam reforming of liquefied natural gas (LNG). For comparison, a nickel catalyst supported on mesoporous alumina (Ni/A-NS) was also prepared by an impregnation method. The effect of physicochemical properties on the performance of Ni-A-NS catalyst in the steam reforming of LNG was investigated. Ni-A-NS catalyst retained superior textural properties compared to Ni/A-NS catalyst. Nickel oxide species were highly dispersed on the surface of both Ni/A-NS and Ni-A-NS catalysts through the formation of surface nickel aluminate phase. Although both Ni/A-NS and Ni-A-NS catalysts exhibited a stable catalytic performance, Ni-A-NS catalyst showed a better catalytic performance than Ni/A-NS catalyst in the steam reforming of LNG. High nickel surface area and high nickel dispersion of Ni-A-NS catalyst played an important role in enhancing the dehydrogenation reaction of hydrocarbon species and the gasification reaction of adsorbed carbon species in the steam reforming of LNG. High reducibility of Ni-A-NS catalyst was also responsible for its high catalytic performance.     

Synergetic effect of combined use of Cu–ZnO–Al2O3 and Pt–Al2O3 for the steam reforming of methanol

Commercial Cu–ZnO–Al₂O₃ catalysts are used widely for steam reforming of methanol. However, the reforming reactions should be modified to avoid fuel cell catalyst poisoning originated from carbon monoxide. The modification was implemented by mixing the Cu–ZnO–Al₂O₃ catalyst with Pt–Al₂O₃ catalyst. The Pt–Al₂O₃ and Cu–ZnO–Al₂O₃ catalyst mixture created a synergetic effect because the methanol decomposition and the water–gas shift reactions occurred simultaneously over nearby Pt–Al₂O₃ and Cu–ZnO–Al₂O₃ catalysts in the mixture. A methanol conversion of 96.4% was obtained and carbon monoxide was not detected from the reforming reaction when the Pt–Al₂O₃ and Cu–ZnO–Al₂O₃ catalyst mixture was used.     

Poly-dimethylsiloxane (PDMS) based micro-reactors for steam reforming of methanol

A miniaturized methanol steam reformer with a serpentine type of micro-channels was developed based on poly-dimethylsiloxane (PDMS) material. This way of fabricating micro-hydrogen generator is very simple and inexpensive. The volume of a PDMS micro-reformer is less than 10 cm³. The catalyst used was a commercial Cu/ZnO/Al₂O₃ reforming catalyst from Johnson Matthey. The Cu/ZnO/Al₂O₃ reforming catalyst particles of mean diameter 50-70 μm was packed into the micro-channels by injecting water based suspension of catalyst particles at the inlet point. The miniaturized PDMS micro-reformer was operated successfully in the operating temperatures of 180-240 °C and 15%-75% molar methanol conversion was achieved in this temperature range for WHSV of 2.1-4.2 h⁻¹. It was not possible to operate the micro-reformer made by pure PDMS at temperature beyond 240 °C. Hybrid type of micro-reformer was fabricated by mixing PDMS and silica powder which allowed the operating temperature around 300 °C. The complete conversion (99.5%) of methanol was achieved at 280 °C in this case. The maximum reformate gas flow rate was 30 ml/min which can produce 1 W power at 0.6 V assuming hydrogen utilization of 60%.     

Effect of surfactant on structure and performance of catalysts for DME synthesis in slurry bed

The Cu-Zn-Al slurry catalysts prepared via modified sol-emulsion-gel method by adding different water-soluble surfactants were studied for CO hydrogenation to produce dimethyl ether (DME) in slurry bed reactor. The results indicated that the catalyst adding Tween80 or PEG600 had higher activity than the catalyst with the PVP additive. All the catalysts had good stability. X-ray diffraction (XRD) results showed that Cu and Cu₂O existed in the pre-reduced Cu-Zn-Al catalysts, while ZnO did not appear. Nitrogen adsorption studies showed that the catalyst prepared with the additives of PEG600 or Tween80 was mesoporous structure with higher specific surface area compared with the one prepared with the additive of PVP. X-ray photoelectron spectroscopy (XPS) results indicated that the Cu/Zn ratio on the surface of all catalysts increased after subject to reduction and reaction. The morphology and size of the catalyst particles could be adjusted by adding water-soluble surfactants.     

Atomically homogeneous dispersed ZnO/N-doped nanoporous carbon composites with enhanced CO2 uptake capacities and high efficient organic pollutants removal from water

Advanced functional composite of ZnO nanoparticles embedded in N-doped nanoporous carbons has been synthesized by a simple one-step carbonization of zeolitic imidazolate framework-8 under a water stream atmosphere. A variety of characterization techniques show that the introduction of water steam during the carbonization process holds the key to obtain the fine and homogeneously dispersed ZnO nanoparticles within the functionalised nanoporous carbon matrix. Possessing a higher specific surface area, a larger pore volume and abundant oxygen-containing hydrophilic functional groups, the resulting composite exhibits a stronger interaction with CO₂ and is more efficient to promote the photocatalytic degradation-adsorption of methylene blue under visible light than the composite obtained without steam treatment. As a result, the steam derived composite exhibits increased CO₂ uptake capacity and excellent methylene blue molecules removal from water. Using different metal-organic frameworks as precursors, this new, simple and green method can be further expanded to generate various new homogeneous dispersed functional metal oxide/porous carbon composites with high efficiency in relevant applications.