CO2 Hydrogenation to Methanol Over Cu/ZnO/Al2O3 Catalysts Prepared by a Novel Gel-Network-Coprecipitation Method

A novel gel-network-coprecipitation process has been developed to prepare ultrafine Cu/ZnO/Al₂O₃ catalysts for methanol synthesis from CO₂ hydrogenation. It is demonstrated that the gel-network-coprecipitation method can allow the preparation of the ultrafine Cu/ZnO/Al₂O₃ catalysts by homogeneous coprecipitation of the metal nitrate salts in the gel network formed by gelatin solution, which makes the metallic copper in the reduced catalyst exist in much smaller crystallite size and exhibit a much higher metallic copper-specific surface area. The effect of the gel concentration of gelatin on the structure, morphology and catalytic properties of the Cu/ZnO/Al₂O₃ catalysts for methanol synthesis from hydrogenation of carbon dioxide was investigated. The Cu/ZnO/Al₂O₃ catalysts prepared by the gel-network-coprecipitation method exhibit a high catalytic activity and selectivity in CO₂ hydrogenation to methanol.     

CO removal from reformed fuel over Cu/ZnO/Al2O3 catalysts prepared by impregnation and coprecipitation methods

A composition of Cu/ZnO/Al₂O₃ catalysts prepared by the impregnation method was optimized for water gas shift reaction (WGSR) coupled with CO oxidation in the reformed gas. The optimum composition of the impregnated catalyst for high WGSR activity was 5 wt.% Cu/5 wt.% ZnO/Al₂O₃. The optimum loading amounts of Cu and ZnO in the impregnated catalyst were smaller than those in the coprecipitated catalyst. Its catalytic activity above 200 °C was comparable to that of the conventional coprecipitated Cu/ZnO/Al₂O₃ catalyst. However, the activity of the impregnated Cu/ZnO/Al₂O₃ catalysts was significantly lowered at 150 °C, whereas no deactivation was observed for the coprecipitated catalyst at the same temperature. It was found that deactivation occurred over impregnated catalysts with H₂O and/or O₂ in the reaction gas; it prevented CO adsorption on the surface.     

Effect of component interaction on the activity of Co/CuZnO for CO hydrogenation

Co/CuZnO is known as a base metal catalyst active for C₂₊ oxygenate synthesis. This study probed the interactions of the different components of Co/CuZnO catalysts on CO hydrogenation using Fischer–Tropsch synthesis (250 °C, H₂/CO = 2) and SSITKA. Only combination of all three metal components produced a catalyst with relatively high C₂₊ oxygenate selectivity, but with much lower activity compared to that for Co/Al₂O₃. In situ reaction characterizations, albeit at somewhat different conditions than alcohol synthesis, helped explain interaction of the components. SSITKA, under methanation conditions, indicated that the most striking feature for the combination of Co with ZnO and/or Cu was a much decreased amount of reaction intermediates. Ethane hydrogenolysis results suggested that the different components for these catalysts were in close contact and few or no large ensembles (n ? 12) of Co atoms existed, confirming that ZnO and/or Cu covered/blocked a substantial number of active sites on Co for CO hydrogenation.     

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.     

Effects of catalyst composition on methanol synthesis from CO2/H2

Effects of catalyst composition have been studied for Cu/support and Cu/ZnO/supports in methanol synthesis from CO₂/H₂. A strong effect of support has been observed. Different supports brought about different behavior in temperature-programmed reduction of copper, different copper surface areas, and different catalytic activity and selectivity. It seemed possible to find catalyst supports that might perform better than commercial Cu/ZnO/Al₂O₃ catalysts. A correlation was observed between catalytic activity and the copper surface area which was varied by using different supports. However, the sup]>orts appeared to influence other catalytic properties as well, for example, the surface oxygen coverage.     

CO/FTIR Spectroscopic Characterization of Pd/ZnO/Al2O3 Catalysts for Methanol Steam Reforming

An as-synthesized 8.8wt% Pd/ZnO/Al₂O₃ catalyst was either pretreated under O₂ at 773 K followed by H₂ at 293 K or under H₂ at 773 K to obtain, respectively, a supported metallic Pd° catalyst (Pd°/ZnO/Al₂O₃) or a supported PdZn alloy catalyst (PdZn/ZnO/Al₂O₃). Both catalysts were studied by CO adsorption using FTIR spectroscopy. For the supported PdZn alloy catalyst (PdZn/ZnO/Al₂O₃), exposure to a mixture of methanol and steam, simulating methanol steam reforming reaction conditions, does not change the catalyst surface composition. This implies that the active sites are PdZn alloy like structures. The exposure of the catalyst to an oxidizing environment (O₂ at 623 K) results in the break up of PdZn alloy, forming a readily reducible PdO with its metallic form being known as much less active and selective for methanol steam reforming. However, for the metallic Pd°/ZnO/Al₂O₃ catalyst, FTIR results indicate that metallic Pd° can transform to PdZn alloy under methanol steam reforming conditions. These results suggest that PdZn alloy, even after an accidental exposure to oxygen, can self repair to form the active PdZn alloy phase under methanol steam reforming conditions. Catalytic behavior of the PdZn/ZnO/Al₂O₃ catalyst also correlates well with the surface composition characterizations by FTIR/CO spectroscopy.     

A novel low-temperature methanol synthesis method from CO/H2/CO2 based on the synergistic effect between solid catalyst and homogeneous catalyst

The activity of a binary catalyst in alcoholic solvents for methanol synthesis from CO/H₂/CO₂ at low temperature was investigated in a concurrent synthesis course. Experiment results showed that the combination of homogeneous potassium formate catalyst and solid copper–magnesia catalyst enhanced the conversion of CO₂-containing syngas to methanol at temperature of 423–443 K and pressure of 3–5 MPa. Under a contact time of 100 g h/mol, the maximum conversion of total carbon approached the reaction equilibrium and the selectivity of methanol was 99%. A reaction pathway involving esterification and hydrogenolysis of esters was postulated based on the integrative and separate activity tests, along with the structural characterization of the catalysts. Both potassium formate for the esterification as well as Cu/MgO for the hydrogenolysis were found to be crucial to this homogeneous and heterogeneous synergistically catalytic system. CO and H₂ were involved in the recycling of potassium formate.     

Core–shell structured Cu@m-SiO2 and Cu/ZnO@m-SiO2 catalysts for methanol synthesis from CO2 hydrogenation

The core–shell catalysts with Cu and Cu/ZnO nanoparticles coated by mesoporous silica shells are prepared for CO₂ hydrogenation to methanol. With the confined effect of silica shell, the size of Cu nanoparticles is only about 5.0 nm, which results in high activity for CO₂ conversion. The CH₃OH selectivity is enhanced significantly with the introduction of ZnO. The core–shell structured catalysts endow the Cu nanoparticles trapped inside with excellent anti-aggregation and no deactivation is observed with time-on-stream. Therefore, the core–shell Cu/ZnO@m-SiO₂ catalyst exhibits the maximum CH₃OH yield with high stability.     

Effect of the hybrid catalysts preparation method upon direct synthesis of dimethyl ether from synthesis gas

Direct synthesis of DME from synthesis gas attains more attention recently due to higher conversion and lower cost in comparison to dehydration of the methanol. In this work Synthesis gas To Dimethylether (STD) conversion was examined on various hybrid catalysts prepared by seven different methods. These catalysts had the same general form as CuO/ZnO/Al₂O₃ with theoretical weight ratio 31/16/53, respectively. A novel preparation method for hybrid catalyst namely sol–gel impregnation has also been developed which showed better performance in comparison with the other methods. Also, in order to find out the effect of various alumina contents at a fixed CuO/ZnO ratio on the performance of the hybrid catalyst, a series of catalysts with different contents of alumina have been prepared by sol–gel impregnation method. The optimum weight ratio for CuO/ZnO/Al₂O₃ catalyst has been found to be about 2:1:5, respectively. These catalysts characterized by TPR, XRD, XRF, BET, TGA, N₂O absorption. The catalysts performance were tested at 240 °C, 40 bar and space velocity 1000 ml/g_cat.h, with the inlet gas composition H₂/CO/N₂ = 64/32/4 in a micro slurry reactor.     

Hydrogenolysis of Dimethyl Maleate on Cu/ZnO/Al2O3 Catalysts

The gas‐phase hydrogenolysis of dimethyl maleate at 10 bar and 513 K was investigated over a series of co‐precipitated Cu/ZnO/Al₂O₃ catalysts. High copper surface areas were obtained with a molar Al content of 5 % in the catalysts. Upon variation of composition at fixed alumina content, copper surface areas increased until the molar ratio exceeded Cu/Zn = 2:1. At the given reaction conditions, dimethyl maleate was completely converted to dimethyl succinate, which further reacted to methanol, γ‐butyrolactone, tetrahydrofuran, and water over all catalysts. Initial deactivation of catalysts was mainly caused by a loss of copper surface area. The catalyst with a molar Cu/Zn ratio of 1:2 was found to be most active and stable under reaction conditions.     

Photoluminescence mechanism of annealed ZnO/Zn/Al2O3 sandwich structures deposited on glass substrates

ZnO/Zn/Al₂O₃ sandwich structures are grown on glass substrates by magnetron sputtering. The effect of Al₂O₃ layers on optical properties of ZnO/Zn/Al₂O₃ sandwich structures is investigated. Results indicated that as the deposition time of Al₂O₃ increases, violet peak centered at 402 nm gradually shifted to 412 nm and the intensity firstly decreases and then increases. We discuss the intensity change and shift of violet peak relating to V_Zn defects and the band alignment of ZnO/Zn/Al₂O₃ sandwich structures, respectively. We proposed that ZnO/Zn/Al₂O₃ sandwich structures can be approximately regarded as a quasiquantum-well-like structure. So the electron tunneling from Zn to Al₂O₃ layer is suppressed and the photogenerated carriers can be confined in the Zn Fermi level. In order to further understand the effect of posttreatment on optical properties of samples, samples are annealed in vacuum at 350 °C for 1 h. PL emissions are weakened with the increase of Al₂O₃ deposition time. Interestingly, at a same deposition condition, PL emissions are still improved after posttreatment. Combined Al₂O₃ layer modulation with annealing treatment, steady PL properties can be effectively improved.     

The synthesis of hydrocarbons and oxygenates during CO hydrogenation on CoCuZnO catalysts: Analysis at the site level using multiproduct SSITKA

This paper addresses the effect of component interaction in CoCuZnO catalysts on oxygenate synthesis during CO hydrogenation. Formation of the various products was investigated for the first time using in-situ multiproduct SSITKA. CO hydrogenation was carried out in a fixed-bed differential reactor at 250 °C and 1.8 atm. Reaction results indicate that the activities for all products decrease with the combination of Co with Cu, possibly, based on SSITKA results, due to partial blockage of the Co surface by Cu. ZnO alone, on the other hand, seems to serve primarily as a support for Co but may increase slightly the intrinsic activities for higher oxygenates. The effects of Cu and ZnO on Co, however, were not additive. The Co–Cu–ZnO combination resulted in a synergy that greatly increased selectivities for higher oxygenates by significantly decreasing the ability for hydrocarbon formation. Interestingly, the rate of synthesis for C₂ oxygenates on Co/CuZnO was identical to that on Co/Al₂O₃ (considered by most to be only a hydrocarbon synthesis catalyst)—but without the high production rate of hydrocarbons.     

Preparation of a highly dispersed Ni2P/Al2O3 catalyst using Ni–Al–CO32 − layered double hydroxide as a nickel precursor

We now report a novel method for the synthesis of a Ni₂P/Al₂O₃-LW catalyst using Ni–Al–CO₃^2 − layered double hydroxide (Ni–Al–CO₃^2 −-LDH) as a nickel precursor and ammonium dihydrogen phosphate as a phosphorous precursor under microwave–hydrothermal (MWH) treatment for 20 min at 363 K. The catalysts were characterized by XRD, TPR, BET, CO uptake and XPS. MWH treatment can promote the formation of smaller and highly dispersed Ni₂P particles and a higher surface area of the catalyst. The Ni₂P/Al₂O₃-LW shows hydrodesulfurization activity of 99.3%, which was much higher than that found for the Ni₂P/Al₂O₃ catalyst obtained via an impregnation method.     

Preparation of Cu/ZnO/Al2O3 catalyst under microwave irradiation for slurry methanol synthesis

Cu/ZnO/Al₂O₃ catalysts with Cu/Zn/Al ratios of 6/3/1 were precipitated and aged by conventional and microwave heating methods and tested in the slurry phase reactor for methanol synthesis. The effect of technological condition of precipitation and aging process under microwave irradiation on the catalytic performance was investigated to optimize the preparing condition of Cu/ZnO/Al₂O₃ catalyst. The results showed that the microwave irradiation during precipitation process could improve the activity of the catalyst, but had little effect on the stability. While the microwave irradiation during aging process has a great benefit to both the activity and stability of the catalyst, the catalyst aged at 80°C for 1 h under microwave irradiation possessed higher methanol space time yield (STY) and more stable catalytic activity. The activity and stability of the catalyst was further enhanced when microwave irradiation was used in both precipitation and aging processes; the optimized condition for the catalyst precursor preparation was precipitation at 60°C and aging at 80°C under microwave irradiation.     

Photocatalytic reduction of CO2 on copper-doped Titania catalysts prepared by improved-impregnation method

Photocatalytic reduction of CO₂ by copper-doped titania catalysts has been investigated. The photocatalysts with various copper species (Cu⁰, Cu^I, Cu^II) were prepared by an improved-impregnation method, where copper nitrate is doped into TiO₂ Degussa-P25. It is likely that copper present on the catalyst surface and the grain size of copper–titania catalysts is uniform, with crystallite size approximately 23 nm. The dispersion capacity of CuO in the vacant sites of TiO₂ is about 4.16 Cu²⁺ nm⁻² (≈2.2 wt% of Cu), as indicated by XRD analysis. The activation energy (E_a) for Degussa-P25 and 3%CuO/TiO₂ is ca. +26 and +12 kJ/mol, respectively. These E_a values suggest that the desorption event is a rate limiting step, and the lower E_a of 3%CuO/TiO₂ may suggest a catalytic role of copper species that enhance the methanol production.     

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.     

Role of CO/CO2 co-feeding in the dehydrogenation of cyclohexanol to cyclohexanone over Cu–ZnO based catalysts

The present work highlights the role of CO/CO₂ co-feeding in the dehydrogenation of cyclohexanol to cyclohexanone over Cu–ZnO–Cr₂O₃ and Cu–ZnO–Cr₂O₃–La₂O₃ catalysts in the temperature range of 448–523 K at atmospheric pressure under vapor phase conditions. Both the catalysts are prepared by coprecipitation technique and are characterized by BET surface area, XRD, TPR and N₂O pulse chemisorption under dynamic conditions. The co-feeding of CO/CO₂ along with cyclohexanol results in enhanced conversion of cyclohexanol and additional formation of methanol. The hydrogen generated in the dehydrogenation of cyclohexanol to cyclohexanone promotes the methanol formation from CO/CO₂. This is the first report where in methanol formation is observed at atmospheric pressure from CO/CO₂ co-feeding in cyclohexanol dehydrogenation.     

Thermogravimetric study on pyrolysis of biomass with Cu/Al2O3 catalysts

The Cu/Al₂O₃ catalysts of three different compositions (10, 20 and 30 wt.% Cu loading), have been investigated with regard to their catalytic effects on pyrolysis of paper biomass species (up to 800 °C) by thermogravimetric analysis (TGA) experiments. The results show that catalysts made devolatilization at lower (below 200 °C) and middle temperature (200–400 °C) regions in the pyrolysis of the biomass species, and the temperature reduction effects follow the order: 30 > 20 > 10 wt.% copper loading. Although the catalysts with 10 and 20 wt.% copper have shown almost similar activity, whereas dehydration reaction was enhanced almost 40% in the presence of 30 wt.% copper-loaded catalyst. At the same time, the amount of residue at the end of the reaction also decreased with increase in the copper loading from 10 to 30 wt.%. At higher temperatures (above 400 °C), the catalyst with greater copper loaded worked more nicely possibly due to the enhancement of the depolymerization reaction over dehydration of cellulose in presence of more basic catalysts. The catalysts were characterized by using X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) surface area analysis and scanning electron microscopy (SEM). XRD results show the formation of CuAl₂O₄ spinel and Cu₂O phase in the catalysts.     

Active sites in Cu/ZnO/ZrO2 catalysts for methanol synthesis from CO/H2

Among various Cu/ZnO/ZrO₂ catalysts with the Cu/Zn ratio of 3/7, the one with 15 wt.% of ZrO₂ obtains the best activity for methanol synthesis by hydrogenation of CO. The TPR, TPO and XPS analyses reveal that a new copper oxide phase is formed in the calcined Cu/ZnO/ZrO₂ catalysts by the dissolution of zirconium ions in copper oxide. In addition, the Cu/ZnO/ZrO₂ catalyst with 15 wt.% of ZrO₂ turns out to contain the largest amount of the new copper oxide phase. When the Cu/ZnO/ZrO₂ catalysts is reduced, the Cu²⁺ species present in the ZrO₂ lattice is transformed to Cu⁺ species. This leads to the speculation that the addition of ZrO₂ to Cu/ZnO catalysts gives rise to the formation of Cu⁺ species, which is related to the methanol synthesis activity of Cu/ZnO/ZrO₂ catalyst in addition to Cu metal particles. Consequently, the ratio of Cu⁺/Cu⁰ is an important factor for the specific activity of Cu/ZnO/ZrO₂ catalyst for methanol synthesis.     

Methanol synthesis over Cu/SiO2 catalysts

The rates of CO and CO/CO₂ hydrogenation at 4.2 MPa and 523 K are reported for a series of Cu/SiO₂ catalysts containing 2 to 88 wt.% Cu. These catalysts were prepared on a variety of silica sources using several different Cu deposition techniques. In CO/CO₂ hydrogenation, the rate of methanol formation is proportional to the exposed Cu surface area of the reduced catalyst precursor, as determined by N₂O frontal chromatography. The observed rate, 4.2×10^–3 mole CH₃OH/Cu site-sec, is within a factor of three of the rates reported by others over Cu/ZnO and Cu/ZnO/Al₂O₃ catalysts under comparable conditions. These results suggest that the ZnO component is only a moderate promoter in methanol synthesis. Hydrogenation of CO over these catalysts also gives methanol with high selectivity, but the synthesis rate is not proportional to the Cu surface area. This implies that another type of site, either alone or in cooperation with Cu, is involved in the synthesis of methanol from CO.