Methanol synthesis surface area

The contribution of different crystal planes to the overall surface area of the particle can thus be calculated and is shown in Fig. 8.12(b). The results have been included in a dynamical micro-kinetic model of the methanol synthesis, yielding a better description of kinetic measurements on working catalysts [C.V. Ovesen, B.S. Clausen, J. Schiotz, P. Stoltze, H. Topsoe and J.K. Norskov, J. Catal. 168 (1997) 133]. 

We have already referred to the Mo/Ru/S Chevrel phases and related catalysts which have long been under investigation for their oxygen reduction properties. Reeve et al. [19] evaluated the methanol tolerance, along with oxygen reduction activity, of a range of transition metal sulfide electrocatalysts, in a liquid-feed solid-polymer-electrolyte DMFC. The catalysts were prepared in high surface area by direct synthesis onto various surface-functionalized carbon blacks. The intrinsic... 

A system has been constructed which allows combined studies of reaction kinetics and catalyst surface properties. Key elements of the system are a computer-controlled pilot plant with a plug flow reactor coupled In series to a minireactor which Is connected, via a high vacuum sample transfer system, to a surface analysis Instrument equipped with XFS, AES, SAM, and SIMS. When Interesting kinetic data are observed, the reaction Is stopped and the test sample Is transferred from the mlnlreactor to the surface analysis chamber. Unique features and problem areas of this new approach will be discussed. The power of the system will be Illustrated with a study of surface chemical changes of a Cu0/Zn0/Al203 catalyst during activation and methanol synthesis. Metallic Cu was Identified by XFS as the only Cu surface site during methanol synthesis. 

Besides supported (transition) metal catalysts, structure sensitivity can also be observed with bare (oxidic) support materials, too. In 2003, Hinrichsen et al. [39] investigated methanol synthesis at 30 bar and 300 °C over differently prepared zinc oxides, namely by precipitation, coprecipitation with alumina, and thermolysis of zinc siloxide precursor. Particle sizes, as determined by N2 physisorpt-ion and XRD, varied from 261 nm for a commercial material to 7.0 nm for the thermolytically obtained material. Plotting the areal rates against BET surface areas (Figure 3) reveals enhanced activity for the low surface area zinc... 

Zn/AljOj catalysts, 31 249 -Zn/Cr Oj catalysts, 31 250 -ZnO/AljO, 31 276, 292-295 -ZnO binary catalyst, 31 257-287 activity patterns, 31 271-274 BET argon surface areas, 31 259 calcination, 31 261-262 catalytic testing, 31 272 chemisorption, 31 268-271 CO2 effects, selectivity, 31 284-285 color spectra, 31 259-261 component comparison, 31 258-259 methanol synthesis, 31 246-247 modifiers, weakening of adsorption energy, 31 283... 

EXAFS has been very useful in the study of catalysts, especially in investigating the nature of metal clusters on surfaces of the supported metal catalysts (Kulkarni et al, 1989 Sinfelt et al, 1984). A variety of systems has been examined already and there is still considerable scope for investigation in this area. Since EXAFS gives bond distances and coordination numbers and is absorber-selective, it is possible to study one metal at a time (Fig. 2.12). Thus, an EXAFS investigation of sulphided Co—Mo— Al20j and related catalysts has shown the nature of the reactive surface species (Kulkarni Rao, 1991). Cu/ZnO catalysts have revealed certain unusual features suggesting the complex nature of the species involved in methanol synthesis (Arunarkavalli et al, 1993). Time-resolved EXAFS is of considerable value for the study of catalysts (Sankar et al, 1992). 

Spinel oxides with a general formula AB2O4 (i.e. the so-called normal spinels) are important materials in industrial catalysis. They are thermally stable and maintain enhanced and sustained activities for a variety of industrially important reactions including decomposition of nitrous oxide [1], oxidation and dehydrogenation of hydrocarbons [2], low temperature methanol synthesis [3], oxidation of carbon monoxide and hydrocarbon [4], and oxidative dehydrogenation of butanes [5]. A major problem in the applications of this class of compound as catalyst, however, lies in their usually low specific surface area [6]. 

Catalysts from Slid Chemie were applied for methanol steam reforming by Stimming et al. [121], G66-MR, a general-purpose catalyst containing 11% Al, 37% Cu and 52% Zn with 121 m2 g 1 surface area, and C18-HA, a catalyst optimized for methanol synthesis containing 2-3% Al, 50-60% Cu and 25-35% Zn with... 

Low-pressure methanol synthesis relies almost exclusively on catalysts based on copper, zinc oxide, and alumina. The catalysts are produced by ICI (now Johnson Matthay), Siidchemie (now Clariant), Haldor Topsoe, in the past also by BASF, and other chemical enterprises and contain 50-70 atomic % CuO, 20%-50% ZnO, and 5%-20% Al203. Instead of alumina, chromium oxide and rare earth oxides have also been used. The mixed oxide catalysts are usually shipped as 4-6 mm cylindrical pellets with specific surface area of 60-100 m2/g. The catalysts are activated in situ with dilute hydrogen, often derived from off-gases from synthesis gas... 

This unique micro structure can be described as an intermediate stage between a supported catalyst and a bulk metallic sponge or skeletal Raney-type catalyst. It enables a reasonably high dispersion of Cu and exposure of many Cu-ZnO interfaces at a high total Cu content. The specific Cu surface area (SACu) of methanol catalysts can be determined by reactive N20 titration [59, 60], which causes surface oxidation of the Cu particles and allows calculation of SAcu from the amount of evolved N2. The SACu of state-of-the-art methanol synthesis catalysts measured by this method... 

Future widespread use of anthropogenic C02 in combination with renewable hydrogen as well as the implementation of coal, biomass, and other nonconventional sources of synthesis gas will lead to suboptimal synthesis gas compositions. Efficient incorporation of these synthesis gas mixtures into the current methanol synthesis infrastructure will necessitate the redevelopment of catalysts to perform stably under high concentrations of C02, water, and impurities. To that end, advanced characterization methods must be implemented to discriminate between surface area loss by... 

To summarize the qualitative findings, the methanol synthesis activity in the binary Cu/ZnO catalysts appears to be linked to sites that also irreversibly chemisorb CO and not to sites that adsorb CO reversibly. Since irreversible adsorption of CO follows linearly the concentration of amorphous copper in zinc oxide, these sites are likely to be that part of the copper solute that is present on the zinc oxide surface. No correlation of the catalyst activity and the copper metal surface area, titrated by reversible form of CO or by oxygen, could be found in the binary Cu/ZnO catalysts (43). In contrast with this result, it has been claimed that the synthesis activity is proportional to copper metal area in copper-chromia (47), copper-zinc aluminate (27), and copper-zinc oxide-alumina (46) catalysts. In these latter communications (27,46,47), the amount of amorphous copper has not been determined, and obviously there is much room for further research to confirm one or another set of results and interpretations. However, in view of the lack of activity of pure copper metal quoted earlier, it is unlikely that the synthesis activity is simply proportional to the copper metal surface area in any of the low-temperature methanol-synthesis catalysts. 

The surface chemistry of zinc oxide is of particular interest in relation to its catalytic and photocatalytic properties. For example the (0001) hexagonal crystal plane appears to have a special role in the catalytic methanol-synthesis reaction (Bowker et al., 1983). The chemisorption of CO and dissociative chemisorption of H2 occur on the exposed Zn2+ cations Bolis et al. (1986) have found that the relative magnitude of this active area of ZnO was highly dependent on the nature of the precursor (oxalate, carbonate of Zn). Similar conclusions can be drawn from the infrared spectroscopic measurements of Chauvin et al. (1986). 

Both catalytic systems even calcined at 550 or 710°C are active in methanol synthesis in presence of CO2 + H2. Comparing the catalysts annealed at 550°C the Cu-LaZr [ex carbonate] sample is the most active due to the higher copper surface area (e.g. 12 m2 compared to 9 m2) as shown in figure 3. 

Catalytic hydrogenation of CO2 into methanol has been investigated over Raney Cu-based catalysts. The Raney catalysts leached in NaOH/ZnO solutions showed high activities and selectivities for methanol synthesis. The deposition of Zn on the surface of Cu particles increased the surface area and the specific activity of Raney Cu-M. Raney Cu-Zr developed was significantly more active than a commercial catalyst. 

As it was reported in others works [6], the increase in copper surface area could be related to the formation of smaller copper particles on the surface of Raney copper due to the slower rate of leaching when the zincate is present. Therefore, both the nature of precursor alloy and the nature of leaching solution were found to be key factors in the preparation of high performance Raney Cu for methanol synthesis from CO2 and H2. 

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