The active site for methanol synthesis on a Cu/ZnO/SiO2 catalyst

This note rectifies serious omissions from the references included in a recent paper by Fujitani et al. concerned with methanol synthesis over Cu/SiO₂ containing ZnO. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.     

K2O影响Cu/ZnO催化剂CO2加氢反应性能的原因分析

运用活性评价、XRD、TPR、CO2-TPD、CO-TPD等手段,分析了K₂O影响Cu/ZnO催化剂CO₂加氢反应性能的原因。试验结果表明,甲醇收率大幅度降低的原因是CuO-ZnO催化剂经K₂O改性后,催化剂还原难度增加;提高了金属Cu的电子密度,促进了CO的歧化反应;催化剂还原后,对CO₂的吸附量大为减少。     

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.     

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.     

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.     

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.     

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.     

Effects of space velocity on methanol synthesis from Co2/Co/H2 over Cu/ZnO/Al2O3 catalyst

The space velocity had profound and complicated effects on methanol synthesis from CO₂/CO/H₂ over Cu/ZnO/Al₂O₃ at 523 K and 3.0MPa. At high space velocities, methanol yields as well as the rate of methanol production increased continuously with increasing CO₂ concentration in the feed. Below a certain space velocity, methanol yields and reaction rates showed a maximum at CO₂ concentration of 5–10%. Different coverages of surface reaction intermediates on copper appeared to be responsible for this phenomenon. The space velocity that gave the maximal rate of methanol production also depended on the feed composition. Higher space velocity yielded higher rates for CO₂/ H₂ and the opposite effect was observed for the CO/H₂ feed. For CO₂/CO/H₂ feed, an optimal space velocity existed for obtaining the maximal rate.     

Remarks on the assignments of temperature programmed desorption peaks for the surface species formed on Cu/ZnO and ZnO in the methanol synthesis from CO

Temperature programmed desorption (TPD), IR spectroscopy and chemical trapping of the surface species with H₂O revealed that the TPD peak of CO frequently assigned to zinc formate species, which were formed in the course of the methanol synthesis from CO-H₂, arose from zinc methoxide species.     

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.     

Influence of the calcination on the activity and stability of the Cu/ZnO/Al2O3 catalyst in liquid phase methanol synthesis

Catalyst Cu/ZnO/Al₂O₃ was prepared through co-precipitation method and investigated in a stirred slurry autoclave reactor system for methanol synthesis. The structure of the catalysts was studied by derivative thermogravimetry (DTG), X-ray diffraction (XRD) and H₂ temperature programmed reduction (H₂-TPR). The results show that the average size of CuO crystallite of the catalyst was 3.23 nm when the catalyst was calcined at 623 K for 2.0 h, and the catalyst with this size of CuO crystallite is highly active and stable. Its space time yield and deactivation rate reached 172.2 g/kg_cat·h and 0.43%/d, respectively under the reaction conditions of 513 K, 4.0 MPa, H₂/CO/CO₂ = 68/24/5 and space velocity = 810 ml/g_cat·h.     

Carbon nanotube-supported Pd–ZnO catalyst for hydrogenation of CO2 to methanol

A type of Pd–ZnO catalysts supported on multi-walled carbon nanotubes (MWCNTs) were developed, with excellent performance for CO₂ hydrogenation to methanol. Under reaction conditions of 3.0 MPa and 523 K, the observed turnover-frequency of CO₂ hydrogenation reached 1.15 × 10⁻² s⁻¹ over the 16%Pd_0.1Zn₁/CNTs(h-type). This value was 1.17 and 1.18 times that (0.98 × 10⁻² and 0.97 × 10⁻² s⁻¹) of the 35%Pd_0.1Zn₁/AC and 20%Pd_0.1Zn₁/γ-Al₂O₃ catalysts with the respective optimal Pd_0.1Zn₁-loading. Using the MWCNTs in place of AC or γ-Al₂O₃ as the catalyst support displayed little change in the apparent activation energy for the CO₂ hydrogenation, but led to an increase of surface concentration of the Pd⁰-species in the form of PdZn alloys, a kind of catalytically active Pd⁰-species closely associated with the methanol generation. On the other hand, the MWCNT-supported Pd–ZnO catalyst could reversibly adsorb a greater amount of hydrogen at temperatures ranging from room temperature to 623 K. This unique feature would help to generate a micro-environment with higher concentration of active H-adspecies at the surface of the functioning catalyst, thus increasing the rate of surface hydrogenation reactions. In comparison with the “Parallel-type (p-type)” MWCNTs, the “Herringbone-type (h-type)” MWCNTs possess more active surface (with more dangling bonds), and thus, higher capacity for adsorbing H₂, which make their promoting action more remarkable.     

Methanol steam reforming over paper-like composites of Cu/ZnO catalyst and ceramic fiber

Copper–zinc oxide catalyst powders were successfully prepared into paper-like composites, called catalyst paper in this study, using ceramic fibers as the carrier matrix of the catalyst. A papermaking technique with a dual polyelectrolyte retention system was used. Catalyst particles were supported on the ceramic fiber networks tailored in the catalyst paper having various types of pores. Pieces of catalyst paper were subjected to methanol steam reforming (MSR) to produce hydrogen gas for fuel cell applications. They demonstrated a higher performance for methanol conversion and hydrogen production in the MSR reaction than commercial Cu/ZnO catalyst pellets, exhibiting an efficacy equivalent to that of the original catalyst powders. The concentration of carbon monoxide, which acts as a catalytic poison for fuel electrode catalysts, decreased remarkably, without any carbon monoxide reduction system. Moreover, the porous structure of catalyst paper influenced the MSR efficiency. It was assumed that the macropores ca. 20 μm in diameter greatly contributed to the MSR performance, rather than the mesopores on the catalyst surfaces. Thus, the porous, flexible and easy-to-handle catalyst paper is a promising material for practical MSR applications.     

A thermogravimetric study of the partial oxidation of methanol for hydrogen production over a Cu/ZnO/Al2O3 catalyst

The partial oxidation of methanol for the production of hydrogen was investigated in both a fixed-bed microreactor and in a thermogravimetric analyzer (TG-FTIR) from 180 °C to 250 °C using a commercial Cu/ZnO/Al₂O₃ catalyst. In the microreactor, a hot spot in the undiluted catalyst bed of 4 K and 32 K was observed at 180 °C and 220 °C, respectively. Methanol conversion was strongly accelerated between 180 °C and 220 °C. In the TG-FTIR experiments, the reduced copper was completely oxidized to cuprite, Cu₂O, with increasing time-on-stream in the presence of oxygen and methanol (O₂/MeOH = 0.5) at 180 °C. The selectivity to formaldehyde increased in the same manner as the catalyst was oxidized to cuprite. In contrast, at 250 °C the catalyst remained completely reduced for the same O₂/MeOH ratio. Two main reaction pathways are proposed explaining the influence of the copper oxidation state on the product distribution.     

An EXAFS study of Cu/ZnO and Cu/ZnO-Al2O3 methanol synthesis catalysts

Cu K-absorption edge and EXAFS measurements on binary Cu/ZnO and ternary Cu/ ZnO-Al₂O₃ catalysts of varying compositions on reduction with hydrogen at 523 K, show the presence of Cu microclusters and a species of Cu¹⁺ dissolved in ZnO apart from metallic Cu and Cu₂O. The proportions of different phases critically depend on the heating rate especially for catalysts of higher Cu content. Accordingly, hydrogen reduction with a heating rate of 10 K/min predominantly yields the metal species (>50%), while a slower heating rate of 0.8 K/min enhances the proportion of the Cu¹⁺ species ( 60%). Reduced Cu/ZnO-Al₂O₃ catalysts show the presence of metallic Cu (upto 20%) mostly in the form of microclusters and Cu¹⁺ in ZnO as the major phase ( 60%). The addition of alumina to the Cu/ZnO catalyst seems to favour the formation of Cu¹⁺/ZnO species.     

Methanol synthesis from CO2-rich syngas over a ZrO2 doped CuZnO catalyst

ZrO₂-doped CuZnO catalyst prepared by successive-precipitation method was investigated by ICP-AES, BET, TEM, XRD, EXAFS, H₂-TPR and CO/CO₂ hydrogenation. The active phase of copper in CuZnO catalyst prepared by co-precipitation method was well-crystallized. The presence of ZrO₂ led to a high copper dispersion, which was distinctive from CuZnO. Though the activity for carbon monoxide hydrogenation was little lower than that of CuZnO catalyst, ZrO₂-doped CuZnO catalyst showed much higher activity and selectivity towards methanol synthesis from carbon dioxide hydrogenation. Moreover, ZrO₂-doped CuZnO catalyst showed high performance for methanol synthesis from CO₂-rich syngas.     

Activity and stability of Cu/ZnO/Al2O3 catalyst promoted with B2O3 for methanol synthesis

The addition of B₂O₃ to a Cu/ZnO/Al₂O₃ catalyst increased the activity of the catalyst for methanol synthesis after an induction period during the reaction. The stability of the B₂O₃-containing Cu/ZnO/Al₂O₃ catalyst was greatly improved by the addition of a small amount of colloidal silica to the catalyst. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effect of Constant‐Rate Reduction on the Performance of a Ternary Cu/ZnO/Al2O3 Catalyst in Methanol Synthesis

A multi‐functional flow set‐up was developed for the rate‐ and temperature‐controlled reduction of copper catalysts, their application in high‐pressure methanol synthesis and the determination of the copper surface area by N₂O frontal chromatography. The influence of constant‐rate reduction on the catalytic properties of a ternary Cu/ZnO/Al₂O₃ catalyst was investigated. The temperature during the constant‐rate reduction was found to decrease, indicating autocatalytic kinetics, but no significant catalytic effect of the milder reduction conditions was observed compared with a slow linear heating ramp.     

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.