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.     

Evidence for the migration of ZnOx in a Cu/ZnO methanol synthesis catalyst

The behavior and role of ZnO in Cu/ZnO catalysts for the hydrogenations of CO and CO₂ were studied using XRD, TEM coupled with EDX, TPD and FT-IR. As the reduction temperature increased, the specific activity for the hydrogenation of CO₂ increased, whereas the activity for the hydrogenation of CO decreased. The EDX and XRD results definitely showed that ZnO _x (x = 0–1) moieties migrate onto the Cu surface and dissolve into the Cu particle forming a Cu-Zn alloy when the Cu/ZnO catalysts were reduced at high temperatures above 600 K. The content of Zn dissolved in the Cu particles increased with reduction temperature and reached 18% at a reduction temperature of 723 K. The CO-TPD and FT-IR results suggested the presence of Cu⁺ sites formed in the vicinity of ZnO _x on the Cu surface, where the Cu⁺ species were regarded as an active catalytic component for methanol synthesis.     

Creation of the active site for methanol synthesis on a Cu/SiO2 catalyst

The effect of ZnO/SiO₂ in a physical mixture of Cu/SiO₂ and ZnO/SiO₂ on methanol synthesis from CO₂ and H₂ was studied to clarify the role of ZnO in Cu/ZnO-based catalysts. An active Cu/SiO₂ was prepared by the following procedure: the Cu/SiO₂ and ZnO/SiO₂ catalysts with a different SiO₂ particle size were mixed and reduced with H₂ at 523-723 K, and the Cu/SiO₂ was then separated from the mixture using a sieve. The methanol synthesis activity of the Cu/SiO₂ catalyst increased with the reduction temperature and was in fairly good agreement with that previously obtained for the physical mixture of Cu/SiO₂ and ZnO/SiO₂. These results indicated that the active site for methanol synthesis was created on the Cu/SiO₂ upon reduction of the physical mixture with H₂. It was also found that ZnO itself had no promotional effect on the methanol synthesis activity except for the role of ZnO to create the active site. The active site created on the Cu/SiO₂ catalyst was found not to promote the formation of formate from CO₂ and H₂ on the Cu surface based on in situ FT-IR measurements. A special formate species unstable at 523 K with an OCO asymmetric peak at ~1585 cm^-1 was considered to be adsorbed on the active site. This revised version was published online in July 2006 with corrections to the Cover Date.     

Hydrogenation of CO2 Over Skeletal Copper Surfaces Containing Precipitated ZnO

Catalysts have been prepared by precipitating different amounts of ZnO on the surface of skeletal copper particles. The precursor copper catalysts were prepared by fully leaching CuAl₂ particles with concentrated NaOH solutions in the presence and absence of sodium chromate. The resulting catalysts contained surface zinc oxide concentrations in the range 0-1.36 mg per m² of catalyst surface.The influence of different concentrations of ZnO on the surface of skeletal copper was examined by the hydrogenation of CO₂ using a mixture of 24% CO₂/76% H₂ at 4 MPa, 493-533K and space velocities up to 410 700 h^-1. The catalysts were found to be highly active and selective for methanol synthesis. Methanol synthesis activity increased linearly with ZnO loadings with a maximum being reached at a loading of around 1.1 mg m^-2 for higher surface area skeletal copper prepared by leaching in the presence of chromate. ZnO loadings were also shown to improve the selectivity of CO₂ hydrogenation over copper by reducing the formation of CO by the reverse water-gas shift reaction.     

The synergy between Cu and ZnO in methanol synthesis catalysts

The hydrogenation of CO₂ over physically-mixed Cu/SiO₂ and ZnO/SiO₂ was carried out to clarify the synergetic effect between Cu and ZnO in Cu/ZnO methanol synthesis catalysts. The activity of the physical mixtures significantly increased with increasing reduction temperature in the range of 573–723 K. TEM-EDX results definitely showed that ZnO_x moieties migrated from ZnO/SiO₂ particles onto the surface of Cu particles when the physical mixtures were reduced at high temperatures above 573 K. Upon the migration of the ZnO_x species, the oxygen coverage on the surface of Cu, measured after the hydrogenation of CO₂, increased with the reduction temperature. The results clearly showed that the synergetic effect of ZnO in the physical mixtures can be ascribed to the creation of active sites such as Cu⁺ which the ZnO_x moieties stabilize on the Cu surface. Further, XRD results showed that the migrated ZnO_x species partly dissolved into the Cu particles to form a Cu—Zn alloy.     

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.     

The difference in the active sites for CO2 and CO hydrogenations on Cu/ZnO-based methanol synthesis catalysts

The effect of Zn in copper catalysts on the activities for both CO₂ and CO hydrogenations has been examined using a physical mixture of Cu/SiO₂+ZnO/SiO₂ and a Zn-containing Cu/SiO₂ catalyst or (Zn)Cu/SiO₂. Reduction of the physical mixture with H₂ at 573–723 K results in an increase in the yield of methanol produced by the CO₂ hydrogenation, while no such a promotion was observed for the CO hydrogenation, indicating that the active site is different for the CO₂ and CO hydrogenations. However, the methanol yield by CO hydrogenation is significantly increased by the oxidation treatment of the (Zn)Cu/SiO₂ catalyst. Thus it is concluded that the Cu–Zn site is active for the CO₂ hydrogenation as previously reported, while the Cu–O–Zn site is active for the CO hydrogenation.     

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.     

A new method of low-temperature methanol synthesis on Cu/ZnO/Al2O3 catalysts from CO/CO2/H2

A new synthesis method of low-temperature methanol proceeded on Cu/ZnO/Al₂O₃ catalysts from CO/CO₂/H₂ using 2-butanol as promoters. The Cu/ZnO/Al₂O₃ catalysts were prepared by co-impregnation of r-Al₂O₃ with an aqueous solution of copper nitrate and zinc nitrate. The total carbon turnover frequency (TOF), the yield and selectivity of methanol were the highest by using the Cu/ZnO/Al₂O₃ catalyst with copper loading of 5% and the Zn/Cu molar ratio of 1/1, which precursor were not calcined, and reduced at 493 K. The activity of the catalysts increased due to the presence of the CuO/ZnO phase in the oxidized form of impregnation Cu/ZnO/Al₂O₃ catalysts. The active sites of the Cu/ZnO/Al₂O₃ catalyst for methanol synthesis are not only metallic Cu but also special sites such as the Cu–Zn site, i.e. metallic Cu and the Cu–Zn site work cooperatively to catalyze the methanol synthesis reaction.     

The origin of the mediocre methanol selectivity of Cu/ZnO-based catalysts for methanol synthesis from CO2 hydrogenation

Cu/ZnO-based catalysts have been extensively and intensively studied for CO₂ hydrogenation to methanol due to their relatively superior catalytic performance. However, the mediocre methanol selectivity over Cu/ZnO-based catalysts has not been disclosed mainly because the predominant by-product CO formation activity fails to arouse any attention, significantly deterring the further catalyst optimization. The ZnOx-Cu nanoparticles (NP)-ZnO interface, derived from strong metal-support interactions (SMSI), has been recognized to be more active for methanol formation compared with the classical direct contact Cu-ZnO interface. In order to disclose the origin of the mediocre methanol selectivity, these two types of Cu-ZnO interfaces have been designed and constructed through carefully manipulating the synthesis and heat pre-treatment conditions of the powder model catalysts. Then, methanol and CO formation behaviors over these two interfaces have been explored thoroughly in actual reaction conditions. Finally, the origin of the mediocre methanol selectivity over Cu/ZnO-based catalysts has been proposed. This work provides unique insights for designing efficient Cu/ZnO-based catalysts with high methanol selectivity and yield and puts forward an effective strategy to investigate the catalytic behaviors over different interfaces in actual reaction conditions.     

Investigating the effect of metal oxide additives on the properties of Cu/ZnO/Al2O3 catalysts in methanol synthesis from syngas using factorial experimental design

The Cu/ZnO/Al₂O₃ catalysts, prepared by co-precipitation method, have been modified by adding small amount of Mn, Mg, Zr, Cr, Ba, W and Ce oxides using design of experiments (1/16 full factorial design). The structure and morphology of catalysts were studied by X-ray diffraction (XRD) and BET. Performance of the prepared catalysts for CO/CO₂ hydrogenation to methanol was evaluated by using a stainless steel fixed-bed reactor at 5 MPa and 513 K. The oxide additives were found to influence the catalytic activity, dispersion of Cu, Cu crystallite size, surface composition of catalyst and stability of catalysts during their operations. The results showed that the Mn and Zr promoted catalysts have high performance for methanol synthesis from syngas.     

A quantum-chemical study of adsorbed nonclassical carbonium ions as active intermediates in catalytic transformations of paraffins. I. Protolytic cracking of ethane on high silica zeolites

Coprecipitated Cu-ZrO₂ catalysts were found to show higher selectivity to methanol in CO₂ hydrogenation than conventional Cu-ZnO catalysts. Addition of ZnO to Cu-ZrO₂ catalysts of Cu/ZrO₂ = 1 (weight ratio) greatly enhanced the activity at lower temperatures, while keeping the high methanol selectivity of Cu-ZrO₂ catalysts. A remarkable increase in the Cu dispersion with increased amount of added ZnO explains the increased activity at lower temperatures, while the reforming of methanol to CO is accelerated by ZnO at higher temperatures, leading to a lowered yield of methanol. It is suggested that ZrO₂ rather than ZnO in the ternary systems plays a more effective role for the selective formation of methanol.     

Evidence of a metal-support interaction in sol-gel derived Cu-ZrO2 catalysts for CO hydrogenation

Sol-gel derived Cu/ZrO₂ catalysts have recently been shown to have high activity and selectivity toward methanol synthesis. TPR, TEM, in situ XRD and N₂O decomposition have now been used to characterize the active sites in such catalysts over a wide range of Cu concentration. Copper is shown to be in two forms: surface aggregates (or particulate) and dispersed copper in the ZrO₂ substitutional sites. The proportion of the former increases with an increasing Cu content, while the overall strength of the Cu-ZrO₂ interaction simultaneously decreases. The activity in CO/CO₂ hydrogenation showed no evident correlation with the total Cu surface area, but rather with the concentration of highly-dispersed form of copper. This is taken to indicate that the copper in the substitutional sites of ZrO₂ is predominantly responsible for and associated with the active sites on Cu/ZrO₂ for CO/CO₂ hydrogenation.     

Methanol synthesis and reverse water-gas shift kinetics over clean polycrystalline copper

The kinetics of simultaneous methanol synthesis and reverse water-gas shift from CO₂/H₂ mixtures have been measured at low conversions over a clean polycrystalline Cu foil at pressures of 5 bar. An absolute rate of 1.2 × 10^–3 methanol molecules produced per second per Cu surface atom was observed at 510 K, with an activation energy of 77 ± 10 kJ/mol. The rate of CO production was 0.12 molecules per second per Cu surface atom at this temperature, with an activation energy of 135 ± 5 kJ/mol. The rates, normalized to the metallic Cu surface area, are equal to those measured over real, high-area Cu/ZnO catalysts. The surface after reaction was examined by XPS and TPD. It was covered by almost a full monolayer of adsorbed formate, but no other species like carbon or oxygen in measurable amounts. These results prove that a highly active site for methanol synthesis on real Cu/ZnO catalysts is metallic Cu, and suggest that the rate-determining step in methanol synthesis is one of the several steps in the further hydrogenation of adsorbed formate to methanol.     

Adsorption and Deactivation Characteristics of Cu/ZnO-Based Catalysts for Methanol Synthesis from Carbon Dioxide

The adsorption and deactivation characteristics of coprecipitated Cu/ZnO-based catalysts were examined and correlated to their performance in methanol synthesis from CO₂ hydrogenation. The addition of Ga₂O₃ and Y₂O₃ promoters is shown to increase the Cu surface area and CO₂/H₂ adsorption capacities of the catalysts and enhance methanol synthesis activity. Infrared studies showed that CO₂ adsorbs spontaneously on these catalysts at room temperature as both mono- and bi-dentate carbonate species. These weakly bound species desorb completely from the catalyst surface by 200 °C while other carbonate species persist up to 500 °C. Characterization using N₂O decomposition, X-ray diffraction, X-ray photoelectron spectroscopy, and transmission electron microscopy with energy-dispersive X-ray spectroscopy analysis clearly indicated that Cu sintering is the main cause of catalyst deactivation. Ga and Y promotion improves the catalyst stability by suppressing the agglomeration of Cu and ZnO particles under pretreatment and reaction conditions.     

Effects of zirconia promotion on the activity of Cu/SiO2 for methanol synthesis from CO/H2 and CO2/H2

The effect of zirconia promotion on Cu/SiO₂ for the hydrogenation of CO and CO₂ at 0.65 MPa has been investigated at temperatures between 473 and 573 K. With increasing zirconia loading, the rate of methanol synthesis is greatly enhanced for both CO and CO₂ hydrogenation, but more significantly for CO hydrogenation. For example, at 533 K the methanol synthesis activity of 30.5 wt% zirconia-promoted Cu/SiO₂ is 84 and 25 times that of unpromoted Cu/SiO₂ for CO and CO₂ hydrogenation, respectively. For all catalysts, the rate of methanol synthesis from CO₂/H₂ is higher than that from CO/H₂. The apparent activation energy for methanol synthesis from CO decreases from 22.5 to 17.5 kcal/mol with zirconia addition, suggesting that zirconia alters the reaction pathway. For CO₂ hydrogenation, the apparent activation energies (~12 kcal/mol) for methanol synthesis and the reverse water-gas shift (RWGS) reaction are not significantly affected by zirconia addition. While zirconia addition greatly increases the methanol synthesis rate for CO₂ hydrogenation, the effect on the RWGS reaction activity is comparatively small. The observed effects of zirconia are interpreted in terms of a mechanism which zirconia serves to adsorb either CO or CO₂, whereas Cu serves to adsorb H₂. It is proposed that methanol is formed by the hydrogenation of the species adsorbed on zirconia.     

CO2 hydrogenation to methanol over Cu/ZnO/ZrO2 catalysts prepared via a route of solid-state reaction

Cu/ZnO/ZrO₂ catalysts were prepared by a route of solid-state reaction and tested for the synthesis of methanol from CO₂ hydrogenation. The effects of calcination temperature on the physicochemical properties of as-prepared catalysts were investigated by N₂ adsorption, XRD, TEM, N₂O titration and H₂-TPR techniques. The results show that the dispersion of copper species decreases with the increase in calcination temperature. Meanwhile, the phase transformation of zirconia from tetragonal to monoclinic was observed. The highest activity was achieved over the catalyst calcined at 400 °C. This method is a promising alternative for the preparation of highly efficient Cu/ZnO/ZrO₂ catalysts.     

Effect of Pd on Cu-Zn Catalysts for the Hydrogenation of CO2 to Methanol: Stabilization of Cu Metal Against CO2 Oxidation

A palladium–copper–zinc catalyst (PdO:CuO:ZnO=2:28:70), prepared by sequential precipitation of the respective cations, was tested in the hydrogenation of CO₂ at high pressure (conditions: 60 bar, CO₂:H₂=1:3 (molar), W/F=0.0675 kg h/m³, 453-513 K). The methanol yield was improved on using this Pd-containing catalyst at all temperatures with respect to the reference copper–zinc catalyst (CuO:ZnO=30:70). This improvement was not due to an additional effect in which palladium was acting as an independent catalytic site but was caused by a synergetic effect of Pd on the active Cu sites. This effect was explained in terms of hydrogen spillover and an increased stability against CO₂ oxidation of the surface copper. Therefore, the present contribution not only supports previous literature findings concerning the hydrogen spillover mechanism but also resulted in a complementary view regarding the role of palladium in Pd-modified CuO-ZnO-based catalysts.     

Methanol synthesis over a Zn-deposited copper model catalyst

Methanol synthesis by the hydrogenation of CO₂ over Zn-deposited polycrystalline Cu was studied using surface science techniques. The Zn sub-monolayer was oxidized by the reaction mixture during the reaction at 523 K, leading to the formation of ZnO species. The kinetic results definitely showed that the ZnO species on the Cu surface promoted the catalytic activity of methanol formation, where the activity of Cu increased by a factor of 6 at the Zn coverage of 0.17. A volcano-shaped curve was obtained for the correlation between the Zn coverage and the catalytic activity, which was very similar to the correlation curve between the oxygen coverage and the specific activity for methanol formation previously obtained for the Cu powder catalysts. The role of ZnO in Cu/ZnO based catalysts was ascribed to the stabilization of Cu⁺ species by the ZnO moieties on the Cu surface.     

Spectroscopic and Kinetic Analysis of a New Low-Temperature Methanol Synthesis Reaction

The spectroscopy and kinetics of a new low-temperature methanol synthesis method were studied by using in situ DRIFTS on Cu/ZnO catalysts from syngas (CO/CO₂/H₂) using alcohol promoters. The adsorbed formate species easily reacted with ethanol or 2-propanol at 443 K and atmospheric pressure, and the reaction rate with 2-propanol was faster than that with ethanol. Alkyl formate was easily reduced to form methanol at 443 K and 1.0 MPa, and the hydrogenation rate of 2-propyl formate was found to be faster than that of ethyl formate. 2-Propanol used as promoter exhibited a higher activity than ethanol in the reaction of the low-temperature methanol synthesis.