On the Issue of the Active Site and the Role of ZnO in Cu/ZnO Methanol Synthesis Catalysts

The problem concerning the active site and the role of ZnO in Cu/ZnO-based methanol synthesis catalysts can be consistently explained based on the literature results by distinguishing CO₂ and CO hydrogenations. Although only metallic copper has some activities for methanol synthesis by the hydrogenation of CO₂, Cu-Zn alloying in Cu particles is responsible for the major promotional role of ZnO in industrial Cu/ZnO-based catalysts. The morphology effect reported in the literature will probably appear for the system of highly dispersed Cu particles supported on ZnO. As for the hydrogenation of CO, Cu⁺ species or Cu-O-Zn sites are the active sites for methanol synthesis. The spillover effect of the Cu-ZnO system is not significant compared to the effect of ZnO on the creation of the Cu-O-Zn site.     

Partial Oxidation of Ethanol to Hydrogen over Ni–Fe Catalysts

A study has been conducted to identify the influence of zirconia phase and copper to zirconia surface area on the activity of Cu/ZrO₂ catalysts for the synthesis of methanol from either CO/H₂ or CO₂/H₂. To determine the effects of zirconia phase, a pair of Cu/ZrO₂ catalysts was prepared on tetragonal (t-) and monoclinic (m-) zirconia. The zirconia surface area and the Cu dispersion were essentially identical for these two catalysts. At 548 K, 0.65 MPa, and H₂/CO_x= 3 (x = 1, 2), the catalyst prepared on m-ZrO₂ was 4.5 times more active for methanol synthesis from CO₂/H₂ than that prepared on t-ZrO₂, and 7.5 times more active when CO/H₂ was used as the feed. Increasing the surface area of m-ZrO₂ and the ratio of Cu to ZrO₂ surface areas further increased the methanol synthesis activity. In situ infrared spectroscopy and transient-response experiments indicate that the higher rate of methanol synthesis from CO₂/H₂ over Cu/m-ZrO₂ is due solely to the higher concentration of active intermediates. By contrast, the higher rate of methanol synthesis from CO/H₂ is due to both a higher concentration of surface intermediates and the more rapid dynamics of their transformation over Cu/ZrO₂.     

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.     

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.     

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.     

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.     

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.     

Influences of preparation methods of ZrO2 support and treatment conditions of Cu/ZrO2 catalysts on synthesis of methanol via CO hydrogenation

ZrO₂ supports were prepared by different methods (conventional precipitation method, shortened as “CP”, and alcogel/thermal treated with nitrogen method, shortened as “AN”), and Cu/ZrO₂ catalysts were prepared by impregnation method. The supports and catalysts were characterized by BET, XRD, TEM and TPR. The effects of the preparation methods of ZrO₂ supports and the treatment conditions (calcination and reduction temperatures) of the catalyst precursors on the texture structures of the supports and catalysts as well as on the catalytic performances of Cu/ZrO₂ in CO hydrogenation were investigated. The results showed that the support ZrO₂-AN had larger BET specific surface area, cumulative pore volume and average pore size than the support ZrO₂-CP. Cu/ZrO₂-AN catalysts showed higher CO hydrogenation activity and selectivity of oxygenates (C₁–C₄ alcohols and dimethyl ether) than Cu/ZrO₂-CP catalysts. Calcination and reduction temperatures of supports and catalyst precursors affected the catalytic performance of Cu/ZrO₂. The conversion of CO and the STY of oxygenates were 12.7% and 229 g/kg h, respectively, over Cu/ZrO₂-AN-550 at the conditions of 300 °C, 6 MPa.     

Activation mechanism of methane-derived coke (CHx) by CO2 during dry reforming of methane – comparison for Pt/Al2O3 and Pt/ZrO2

The reaction of methane-derived coke (CH_x: intermediate of the reforming reaction and also a source of coke deposition) with CO₂ was studied on supported Pt catalysts in relation with CO₂ reforming of methane. Temperature-programmed hydrogenation (TPH) was performed to investigate the reactivity of coke deposition after the catalyst was exposed to CH₄/He at 1070 K. Coke on Pt/Al₂O₃ could be hydrogenated around 873 K, while for Pt/ZrO₂ this was above 1073 K. The results indicate that the reactivity of coke with hydrogen was higher on Pt/Al₂O₃ than on Pt/ZrO₂, which was different from the reactivity of coke towards CO₂. Thus, the reactivity of CO₂ was studied and compared on these catalysts by several technics. The amount of CO evolution was measured during CO₂ flow at 1070 and 875 K. Rate and amount of converted CO₂ were higher on Pt/ZrO₂ than on Pt/Al₂O₃. Pt/ZrO₂ was proven to react with CO₂ to produce CO and active oxygen (CO₂CO+O) (probably on its oxygen defect site) more easily than Pt/Al₂O₃.     

Production of Higher Hydrocarbons from CO2 over Nanosized Iron Catalysts

The effects of the particle size of a Fe/Cu/K catalyst on CO and CO₂ hydrogenation reactions as well as the variation of crucial factors such as surface area and basicity, reduction, carburization, and catalytic behavior of precipitated Fe/Cu/K catalysts were evaluated. Hematite nanoparticle catalysts with various surface tensions were produced by homogeneous precipitation in alcohol/water solvents. The basicity of the K‐promoted iron catalyst was higher in iron catalysts with lower particle size. The increase in K‐basic sites at the surface of catalysts with smaller particle size was attributed to their higher surface areas. Elevation of catalyst basicity led to considerably stronger dissociative CO adsorption. Shifting the oxygen removal pattern to lower temperature was the consequence of faster nucleation of FeCx crystallites on promoted surface oxides. CO₂ hydrogenation can occur in two distinct direct and indirect routes via the Fischer‐Tropsch mechanism.     

Carbon dioxide hydrogenation over nickel-, ruthenium-, and copper-based catalysts: Review of kinetics and mechanism

This study critically reviews the mechanism of CO₂ hydrogenation over Ni, Ru, and Cu, and the effect of catalyst properties and operating conditions on reaction kinetics. Most studies have reported the presence of CO and formate species on Ni-, Ru-, and Cu-based catalysts, where subsequent conversion of these species depends on the type of catalyst and the physicochemical properties of the catalyst support. Methane is the major product that forms during CO₂ hydrogenation over Ni and Ru catalysts, while methanol and CO are mainly produced on Cu catalysts. A different approach for catalyst formulations and/or process development is required where long chain hydrocarbons are desired.     

Support and additive effects in the synthesis of methanol over copper catalysts

Catalysts containing copper and ZnO in various combinations have been prepared, the copper surface areas have been measured by nitrous oxide frontal chromatography, and the activities in the reaction of CO/CO₂/H₂ and CO/H₂ mixtures to methanol have been determined at 250°C and 10 bar pressure. The results show that there is a strong synergy between copper and ZnO with the area specific rate of Cu/ZnO catalysts being about one order of magnitude larger than that of a Cu/SiO₂ catalyst. The synergy between copper and ZnO is observed both in the presence and absence of carbon dioxide. It is also observed that physical mixtures of Cu/SiO₂ and ZnO/SiO₂ catalysts are significantly more active than either of the components alone. The results are discussed in terms of possible interactions between copper and ZnO in the most active catalysts.     

Influence of titania on zirconia promoted Cu/SiO2 catalysts for methanol synthesis from CO/H2 and CO2/H2

The effects of adding mixtures of titania and zirconia on the methanol synthesis activity and selectivity of Cu/SiO₂ were investigated. The synthesis of methanol from both CO/H₂ and CO₂/H₂ mixtures was examined at 0.65 MPa and temperatures between 448 and 573 K. For CO hydrogenation, the addition of ZrO₂ alone increased the methanol synthesis activity of Cu/SiO₂ by up to three-fold. Substitution of a portion of the ZrO₂ by TiO₂ decreased the methanol synthesis activity of the catalyst relative to that observed when only ZrO₂ is added. ZrO₂ addition also enhanced the methane synthesis activity by as much as seven fold. In the case of CO₂ hydrogenation, the maximum methanol synthesis activity is achieved when a 50/50 wt% mixture of ZrO₂ and TiO₂ is added to Cu/SiO₂. Neither the presence of the oxide additive nor its composition had any effect on the activity of the reverse water–gas-shift reaction, which suggests that this reaction proceeds only on Cu. The observed effects of ZrO₂ and TiO₂ on the catalytic activity of methanol synthesis from CO and CO₂, and methane synthesis from CO, are interpreted in terms of the strength and concentration of acidic and basic groups on the surface of the dispersed oxide.     

Reduction of NO by CO on Cu/ZrO2/Al2O3 catalysts: Characterization and catalytic activities

ZrO₂, γ-Al₂O₃ and ZrO₂/γ-Al₂O₃-supported copper catalysts have been prepared, each with three different copper loads (1, 2 and 5 wt%), by the impregnation method. The catalysts were characterized by nitrogen adsorption (BET), X-ray diffraction (XRD), temperature programmed reduction (TPR) with H₂, Raman spectroscopy and electronic paramagnetic resonance (EPR). The reduction of NO by CO was studied in a fixed-bed reactor packed with these catalysts and fed with a mixture of 1% CO and 1% NO in helium. The catalyst with 5 wt% copper supported on the ZrO₂/γ-Al₂O₃ matrix achieved 80% reduction of NO. Approximately the same rate of conversion was obtained on the catalyst with 2 wt% copper on ZrO₂. Characterization of these catalysts indicated that the active copper species for the reduction of NO are those in direct contact with the oxygen vacancies found in ZrO₂.     

Influences of particle size and crystallinity of highly loaded CuO/ZrO2 on CO2 hydrogenation to methanol

In this study, highly loaded CuO (65.2 wt%) on ZrO₂ support was prepared by flame spray pyrolysis, and the effects of their particle sizes and ZrO₂ crystallinity on CO₂ hydrogenation to methanol were investigated. By varying the precursor feed rate (1–10 mL min⁻¹), the crystallite size (3–7 nm) and the crystallinity of tetragonal ZrO₂ were controlled. After H₂ reduction at 300°C, the Cu species in the catalysts, prepared at the feed rate = 2–10 mL min⁻¹, were converted to Cu particles (approximately 10–20 nm); however, the size and crystallinity of ZrO₂ remained the same. The activity and selectivity of the catalysts prepared at the feed rate = 2–3 mL min⁻¹ were higher than those of the catalysts prepared at the feed rates = 5–10 mL min⁻¹, because the smaller ZrO₂ particles in the former provided more surface to stabilize small Cu particles and from interfacial Cu-ZrO₂ sites (active sites).     

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.     

Selective CO oxidation in the presence of hydrogen over supported Pt catalysts promoted with transition metals

Selective CO oxidation in the presence of excess hydrogen was studied over supported Pt catalysts promoted with various transition metal compounds such as Cr, Mn, Fe, Co, Ni, Cu, Zn, and Zr. CO chemisorption, XRD, TPR, and TPO were conducted to characterize active catalysts. Among them, Pt-Ni/γ-Al₂O₃ showed high CO conversions over wide reaction temperatures. For supported Pt-Ni catalysts, Alumina was superior to TiO₂ and ZrO₂ as a support. The catalytic activity at low temperatures increased with increasing the molar ratio of Ni/Pt. This accompanied the TPR peak shift to lower temperatures. The optimum molar ratio between Ni and Pt was determined to be 5. This Pt-Ni/γ A1₂O₃ showed no decrease in CO conversion and CO₂ selectivity for the selective CO oxidation in the presence of 2 vol% H₂O and 20 vol% CO₂. The bimetallic phase of Pt-Ni seems to give rise to stable activity with high CO₂ selectivity in selective oxidation of CO in H₂-rich stream.     

Effects of Co dopants and flame conditions on the formation of Co/ZrO2 nanoparticles by flame spray pyrolysis and their catalytic properties in CO hydrogenation

The Co/ZrO₂ catalysts with various Co loadings (5–10 wt.%) were prepared by one-step flame spray pyrolysis (FSP) under different flame conditions. As revealed by XRD and TEM, all the resulting Co/ZrO₂ nanoparticles were composed of single-crystalline particles exhibiting the characteristic tetragonal structure of ZrO₂. Varying the amount of Co dopants during FSP synthesis did not alter the primary particle size of ZrO₂ which was determined to be ca. 14 nm. On the other hand, increasing precursor feed rate from 3 to 8 ml/min resulted in an increase of ZrO₂ crystallite size from 10 to 19 nm. The higher precursor feed rate produced higher enthalpy of flame and longer residence times, which increased coalescence and sintering of the particles. Compared to the Co/ZrO₂ prepared by conventional impregnation method, the catalytic activities of the FSP-made catalysts were much higher. Moreover, the hydrogenation rates of the FSP-made Co/ZrO₂ catalysts were increased with increasing Co loading and precursor feed rate. According to H₂ chemisorption and H₂ temperature program reduction results, the improvement of catalytic activity and C₂–C₆ selectivities of the FSP-made catalysts in the CO hydrogenation was attributed to the higher number of Co metal active sites and lower interaction between Co/CoO and ZrO₂ support obtained via the FSP synthesis.     

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.     

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.