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.     

An in situ high pressure FT-IR study of CO2/H2 interactions with model ZnO/SiO2, Cu/SiO2 and Cu/ZnO/SiO2 methanol synthesis catalysts

In situ FT-IR spectroscopy allows the methanol synthesis reaction to be investigated under actual industrial conditions of 503 K and 10 MPa. On Cu/SiO₂ catalyst formate species were initially formed which were subsequently hydrogenated to methanol. During the reaction a steady state concentration of formate species persisted on the copper. Additionally, a small quantity of gaseous methane was produced. In contrast, the reaction of CO₂ and H₂ on ZnO/SiO₂ catalyst only resulted in the formation of zinc formate species: no methanol was detected. The interaction of CO₂ and H₂ with Cu/ZnO/SiO₂ catalyst gave formate species on both copper and zinc oxide. Methanol was again formed by the hydrogenation of copper formate species. Steady-state concentrations of copper formate existed under actual industrial reaction conditions, and copper formate is the pivotal intermediate for methanol synthesis. Collation of these results with previous data on copper-based methanol synthesis catalysts allowed the formulation of a reaction mechanism.     

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 from CO2, CO,and H2over Cu(100) and Ni/Cu(100)

The catalytic activity of Cu(100) and Ni/Cu(100) with respect to the methanol synthesis from various mixtures containing CO₂, CO, and H₂have been studied in a combined UHV/high pressure cell apparatus at reaction conditions,P_tot=1.5 bar andT=543 K. For the clean Cu(100) surface it is found that admission of CO to a reaction mixture containing CO₂and H₂does not lead to an increase in the rate of methanol formation, which indirectly suggests that the role of CO in the industrial methanol process relates to the change in reduction potential of the synthesis gas. For the Ni/Cu(100) surface it is found that Ni does not promote the rate of methanol formation from mixtures containing CO₂and H₂. In opposition, admission of CO to the reaction mixture leads to a significant increase in the rate of methanol formation with a turnover frequency/Ni site∼60×the turnover frequency/Cu site at Ni coverages below 0.1 ML making it a rather substantial promoting effect. It is found that the admission of CO to the synthesis gas creates segregation of Ni to the surface, whereas this is not the case for a reaction involving CO₂and H₂. It is suggested that CO acts strictly as a promotor in the system and we ascribe the increase in activity to a promotion through gas phase induced surface segregation of Ni.     

Methanol Synthesis on Potassium-Modified Cu(100) from CO + H2 and CO + CO2 + H2

Methanol cannot be produced from CO + H₂ on a clean copper surface, but a promotional effect of potassium on methanol synthesis from mixtures of CO + H₂ and CO + CO₂ + H₂ at a total pressure of 1.5 bar on a Cu(100) surface is shown in this work. The experiments are performed in a UHV chamber connected with a high-pressure cell (HPC). The methanol produced is measured with a gas chromatograph and the surface is characterized with surface science techniques. The results show that potassium is a promoter for the methanol synthesis from CO + H₂, and that the influence of CO₂ is negligible. Investigation of the post-reaction surface with TPD indicates that potassium carbonate is present and plays an important role. The activation energy is determined as 42 ± 3 kJ/mol for methanol synthesis on K/Cu(100) from CO + H₂.     

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.     

In situ IR studies on the mechanism of methanol synthesis from CO/H2 and CO2/H2 over Cu-ZnO-Al2O3 catalyst

Methanol synthesis from CO/H₂ and CO₂/H₂ was carried out at atmospheric pressure over Cu/ZnO/Al₂O₃ catalyst. The formation and variation of surface species were recorded by in situ FT-IR spectroscopy. The result revealed that both CO and CO₂ can serve as the primary carbon source for methanol synthesis. For CO/H₂ feed gas, only HCOO-Zn was detected; however, for CO₂/H₂, both HCOO-Zn and HCOO-Cu were observed, and without CH₃O-Cu. HCOO-Zn was the key intermediate. A scheme of methanol synthesis and reverse water-gas shift (RGWS) reaction was proposed.     

Promotion through gas phase induced surface segregation: methanol synthesis from CO, CO2 and H2 over Ni/Cu(100)

We have studied the rate of methanol formation over Cu(100) and Ni/Cu(100) from various mixtures of CO, CO₂ and H₂. It is found that the presence of submonolayer quantities of Ni leads to a strong increase in the rate of methanol formation from mixtures containing all three components whereas Ni does not influence the rate from mixtures of CO₂/H₂ and CO/H₂, respectively. The influence of the partial pressures of CO and CO₂ on the rate indicates that the role of CO is strictly promoting. From temperature-programmed desorption spectra it follows that the surface concentration of Ni depends strongly on the partial pressure of CO. In this way the increase in reactivity is interpreted as a CO-induced structural promotion introduced by the stronger bonding of CO to Ni as compared to Cu. It is suggested that this type of promotional behavior will be of general importance in existent catalysts and perhaps even more relevant in the development of new or improved bimetallic catalysts. This revised version was published online in July 2006 with corrections to the Cover Date.     

The effect of ageing time on co-precipitated Cu/ZnO/ZrO2 catalysts used in methanol synthesis from CO2 and H2

The effect of suspension ageing time during the catalyst precipitation process on the performance of co-precipitated Cu/ZnO/ZrO₂ catalysts in methanol synthesis from CO₂ and H₂ has been studied. The ageing time influenced greatly the physical and chemical characteristics of the catalysts as well as their activity in the methanol synthesis. Prolonged ageing was advantageous, mainly due to both lower sodium contents and enhanced crystallinity of the catalysts.     

CO bonding and hydrogenation activity of promoted noble metal catalysts during restructuring in high-pressure CO hydrogenation: FeIr/SiO2

Bimetallic FeIr/SiO₂ catalysts that are active for methanol production from synthesis gas (CO+H₂) bind CO less strongly, and exhibit higher activity for the hydrogenation of ethylene in the presence of CO, than catalysts that produce mainly methane. It is argued that promotion of the noble metal serves to weaken the adsorption of CO, thereby lowering its tendency to dissociate, and, most importantly, enhancing the surface coverage of hydrogen. Both factors are favorable for methanol formation. This revised version was published online in July 2006 with corrections to the Cover Date.     

Co‐Metal Catalysts on SiO2‐TiO2 for Methanol Production from CO2 – Effect of Preparation Methods

The effect of quantity, composition, and different impregnation sequences on the catalytic properties of Cu‐Zn‐Al/SiO₂‐TiO₂ in the CO₂ hydrogenation for methanol production was investigated. The Cu‐Zn‐Al catalysts supported on SiO₂ and TiO₂ were prepared by incipient wetness impregnation. Then, their performances in CO₂ hydrogenation were tested under defined conditions. The composition variation of Cu and Zn catalysts resulted in a high methanol production for Cu catalysts with a higher content of Cu, which was the active site for CO₂ activation. Regarding the metal quantity of catalysts, a relatively low loading of co‐metal (Cu‐Zn‐Al) led to the maximum methanol yield when compared with higher loadings as a result of the largest surface area.     

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.     

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.     

Porous MoxCy/SiO2 Material for CO2 Hydrogenation

The synthesis and characterization of an inexpensive porous Mo_xC_y/SiO₂ material is presented, which was obtained by mixing ammonium hexamolybdate, sucrose, and a mesoporous silica (SBA-15), with a subsequent heat treatment under inert atmosphere. This porous material presented a specific surface area of 170 m²/g. The catalytic behavior in CO₂ hydrogenation was compared with that of Mo₂C and α-MoC_1?x obtained from ammonium hexamolybdate and sucrose, using different Mo/C ratios. CO₂ hydrogenation tests were performed at moderate (100 kPa) and high pressures (2.0 MPa), and it was found that only CO, H₂O and CH₄ are formed at moderate pressures by the three materials, while at higher pressures, methanol and hydrocarbons (C₂H₆, C₃H₈) are also obtained. Differences in selectivity were observed at the high pressure tests. Mo₂C presented higher selectivity to CO and methanol compared with MoC_1?x, which showed preferential selectivity to hydrocarbons (CH₄, C₂H₆). The porous Mo_xC_y/SiO₂ material showed the highest CO₂ hydrogenation activity at high temperatures (270 and 300 °C), being a promising material for the conversion of CO₂ to CO and CH₄.

     

CO methanation over supported Mo catalysts in the presence of H2S

The effect of various Mo catalyst supports, i.e., γ-Al₂O₃, SiO₂, SiO₂–Al₂O₃, ZrO₂, yttria-stabilized zirconia (YSZ), CeO₂, and TiO₂, on CO hydrogenation in the presence of H₂S was examined. At 5 wt.% Mo loading, Mo/ZrO₂ was determined to be the most active catalyst for this reaction; its activity was dependent on the number of active sites, as determined via NO chemisorption. Raman spectroscopy revealed that MoO₃ transforms into MoS₂ during the reaction.     

Role of promoters on Rh/SiO2 in CO hydrogenation: A comparison using DRIFTS

La, V, Zn, Cu, Fe, Li and Ag promoted Rh/SiO₂ catalysts were investigated for the synthesis of ethanol during CO hydrogenation at 230 °C and 1.8 atm. As is well known, the activity and selectivity depend heavily on the choice of promoter. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was used to probe the effects of La, V, Zn and Cu on CO adsorption and hydrogenation. From the IR study, it was found that the behavior of CO adsorbed on the differently promoted catalysts was very different. While La enhanced total CO adsorption, the addition of V, Zn and Cu suppressed CO adsorption to different extents. The doubly promoted Rh-La/V/SiO₂ showed only moderate CO adsorption. Results from DRIFTS suggest that the higher catalytic activity (compared to the non-promoted catalyst) observed for the La singly promoted Rh/SiO₂ catalyst may primarily be caused by an increase in the concentration of the adsorbed CO species in the presence of H₂, possibly due to the formation of new active sites at the LaO_x-Rh interface. The higher catalytic activity of the V singly promoted Rh/SiO₂ catalyst could be ascribed to an increased desorption rate/reactivity of the adsorbed CO species. The La and V doubly promoted catalyst showed both new adsorbed CO species and increased desorption rate/reactivity of the adsorbed species during CO hydrogenation due to a synergistic promoting effect of La and V. The addition of Zn or Cu promoters significantly reduced the desorption rate/reactivity of the adsorbed CO species on Rh/SiO₂, leading apparently to the much reduced activities for CO hydrogenation observed.     

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.     

Mesoporous TiO2/SBA-15, and Cu/TiO2/SBA-15 Composite Photocatalysts for Photoreduction of CO2 to Methanol

A series of mesoporous TiO₂/SBA-15, Cu/TiO₂ and Cu/TiO₂/SBA-15 composite photocatalysts were prepared by sol–gel synthesis for photoreduction of CO₂ with H₂O to methanol. It was found that optimum amount of titanium loading of TiO₂/SBA-15 was 45 wt% which exhibited higher photoreduction activity than pure TiO₂. An addition of copper on TiO₂ or TiO₂/SBA-15 catalyst as cocatalyst was found to enhance the catalytic activity because copper serves as an electron trapper and prohibits the recombination of hole and electron.     

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.