Surface area copper

The predominate role of the 2inc and aluminum oxides in the ICI catalyst is to reduce the rate of sintering and loss of metallic copper surface area, which, in addition to poisoning, is one of the modes of activity loss with time for this catalyst. 

In contrast, certain results indicate that the active centers are located exclusively on metallic copper particles, and that the activity is directly proportional to the copper surface of the metal particles.378-383 This linear relationship is practically independent of the type of support used. However, there are conflicting reports suggesting no general correlation between the activity and the copper surface area.384-386... 

An increasing number of observations also points to the existence of support effects indicating that the support may influence the activity of the catalysts in ways other than by simply increasing the copper surface area. Zinc... 

Co. surface area = 300 m2/g ) with aqueous solutions of Cu, Cr, Mg, Ca, Sr, and Ba in Nitrate. All the catalysts have Cu to Si02 weight ratio of 14/86. For promoted catalyst, the Cr to Cu molar ratio was varied from 1/4 0 to 1/4, and the alkaline earth metal to Cu molar ratio was kept at 1/10. The impregnated catalysts were dried at 100 °C overnight, calcined at 450 for 3 h and then reduced in a stream of 10% H2 in Ar at 300 °C for 2 h. The copper surface areas of catalysts were determined by the N20 decomposition method described elsewhere [4-5J. The basic properties of the catalysts were determined by temperature-programmed desorption ( TPD ) of adsorbed carbon dioxide. Ethanol was used as reactant for dehydrogenation reaction which was performed in a microreactor at 300°C and 1 atm. 

The copper surface areas of fresh (S ) and used (S ) catalysts are demonstrated in Table l. The ratio of S1/S0 exhibits the extent of copper surface area reduced after reaction. The copper surface areas reduce after dehydrogenation reaction. This indicates that sintering occurs in reaction process for all of the catalysts. Chromium promoted catalysts have higher fresh copper surface areas than the unpromoted one as shown in Table 1. The previous results [5] indicated that the catalyst with Cr/Cu molar ratio of 1/10 had the highest stability for unsupported catalyst nevertheless, the catalyst with Cr to Cu molar ratio of 1/40 is the most stable one in Si02-supported case. The stability of chromium promoted catalyst decreases when the Cr/Cu molar ratio increases. 

Copper surface areas of unpromoted and promoted catalysts... 

Chinchen GC, et al. The measurement of copper surface-areas by reactive frontal chromatography. J Catal. 1987 103(1) 79—86. 

The influence of the preparation method of methanol catalysts composed of copper associated with rare earth oxides (eg Cu-La2Zr207 and ZnO promoted Cu-La2Zr20y systems) on the catalytic behaviour is discussed. Good activities and improved aging properties are always associated with a high copper surface area and a reasonnable crystallinity of the La2Zr207 pyrochlore. 

For Cu-La2Zr207, as well as for Cu-ZnO catalysts, an almost linear correlation can be observed between the methanol yield, the copper surface area and the amount of formates located on the catalyst s surface. A similar correlation cannot be evidenced on ZnO promoted Cu-La2Zr207 catalysts. The results are discussed and a mechanism for the hydrogenation of CO2 to methanol is proposed... 

BET and porosity were measured by N2 chemisorption (volumetric technique) on a Coulter SA 3100 equipment. The accessible copper surface area (SCu) is determined by the conventional N2O adsorption technique on reduced catalysts (H2, 270°C, 15 h). XRD measurements were performed on a Siemens D 5000 equipment using the CuKa radiation. 

The characteristics of the catalytic systems depend not only on the preparation technique but also on the annealing temperature. Both catalysts have a poor thermal stability and a calcination above 350°C led to very low BET and copper surface areas e.g. 3m2/g (Cu-Zn [ex carbonate]) and lm2/g (Cu-Zn [ex oxalate]) at 550°C. Since the copper surface area determines for the catalytic activity only the samples calcined at T = 350°C have been used for the catalytic tests. 

The catalytic activity of the catalysts in presence of a CO2 + H2 mixture between 250°C and 320°C (figure 1) can be more or less related to the copper surface areas as observed in table 1. Thus the catalysts prepared using carbonate precipitation are the most active in methanol formation. This can be attributed to the higher selectivity easily related with the high copper coverage of the catalyst. 

The well crystallized product obtained at 900°C has an extremely low BET and copper surface areas for an use as catalyst. The increase of the copper surface area observed on Cu-LaZr [ex oxalate] catalysts after calcination at 710°C can be explained by a phase rejection of CuO described previously [9,16] according to the following reaction pathways ... 

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. 

Considering the catalysts annealed at 710°C it can be observed that the system originating from the carbonates has a lower activity that the sample calcined at 550°C, whereas on the Cu-LaZr [ex oxalate] system the methanol yield is increased if the annealing temperature goes from 550 to 710°C. All these results can more or less be explained by the change of the copper surface area with the preparation and the annealing temperature as described in table 2. Finally each preparation technique needs the use of the best selected annealing temperature labelled in table 2. 

It can also be observed that despite the same copper surface areas of 12 m2/g the Cu-LaZr ([ex carbonate] 550) system has a noticiable different catalytic behavior that Cu-LaZr ([ex oxalate] 710). This proves that, appart the copper surface areas, variation of morphology, presence of impurities as well as other factors related with the preparation can be responsible for difference of catalytic properties related to the preparation technique. 

The physical characteristics of the different samples, summarized in table 3 show that optimal annealing conditions have to be chosen for each catalytic system to have both a good copper surface area and a reasonnable crystallization of the La2Zr207 pyrochlore. 

From table 3 it appears that the highest copper surface area associated with the presence of La2Zr207 pyrochlore stmcture is observed after annealing the CuZn+LaZr catalyst at 350°C, the CuZn-LaZr [ex carbonate] system at 550°C and finally the CuZn-LaZr [ex oxalate] sample at 710°C. The comparison of the catalytic activities can only be achieved by the use of these optimized systems labelled ( ) in table 3. 

In the presence of CO2+H2 the observed conversion and methanol yields can be roughly related to the copper surface areas measured for the different catalyst samples. Thus the CuZn-i-LaZr [ex carbonate] catalyst with 31 m2/g copper surface area shows the best catalytic activity. For the same catalyst, the methanol selectivities illustrated in figure 4 are generally lower than those of the other samples. This phenomenon can be related to the presence of La2Zr07 which is not in interaction with copper and which induces mainly the reverse water gas shift reaction. 

The use of temperature programmed desorption makes possible a qualitative and a quantitative determination of formate species as well as their localization on the copper or on the support [21], Figure 6 shows that after chemisorption of CO2 (or MeOH given the reversibility of the reaction) a CO2 desorption attributed to copper formates can be observed in the temperature range 160-200°C whereas the desorption above 300°C corresponds to formates or carbonates located on the support [21]. The relation between the copper surface areas, the amount of formates and the catalytic activity is given in table 5. 

For copper-zinc catalysts if the same preparation technique [carbonates] or [oxalates] is used a nearly linear correlation can be established between the copper surface area, the amount of formates and the methanol yield. But if the preparation technique is changed the formates and methanol formation, related to a unit of copper surface area, is not maintained and the observed correlation is restricted to catalysts prepared by the same technique as shown in figure 7. 

For copper-pyrochlore catalysts an almost linear correlation can be also established between the copper surface area, formate and methanol formation can be observed. The change of the of copper loading leads in figure 8 to a good correlation between the same values (copper surface area, amount of formates, methanol yield). 

In most of the catalysts composed of copper zinc and copper-pyrochlore a correlation between the copper surface area, the amount of formates located on the copper sites after CO2 or methanol chemisorption, and the catalytic activity can be found, but in most cases the relation is not strictly linear. The promoting effect of ZnO on Cu-LaZr catalysts cannot be ascribed to an enhancement of copper coverage or formate formation. Zinc plays therefore rather a positive role in the formate hydrogenation than its formation. 

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. 

Another example is copper surface area measurement in Raney Cu and Ni-Cu alloys. When reacted with N O. most conveniently in a pulse apparatus, Cu reacts as follows ... 

Fig. 5. Copper surface area for Cat 1 as a function of the amount of potassium added ( ) after reduction ( ) after reaction.

By increasing the amount of potassium, we observed a deactivation of the catalyst which was practically complete for percentages higher than 1%. The decrease in the activity is more significant than that of BET surface area, and may be attributed to a specific interaction with the active phase (16,33). Furthermore, its trend is similar to that observed for the copper surface area after both reduction and reaction ( Fig. 5), even if this parameter did not alone justify the differences observed in the catalytic activity. Worthy of note are the lower values from all the samples after reaction this fact may be attributed to the surface adsorption of higher molecular weight compounds (34). However, the XRD powder patterns evidence a strong interaction of the potassium with the spinellike phase present after calcination (28,35), as was observed for the Zn/Cr catalysts (34,36). 

H.M.A. together with methanol was obtained with low temperature methanol catalysts, without and with the addition of potassium. In this latter case the productivity in H.M.A. increased up to about 0.4% of the added potassium, after which a deactivation was observed with a trend similar to that observed for the copper surface area. It is noteworthy that all the catalysts showed lowest values after reaction, attributable to the presence of high molecular weight compounds adsorbed on the surface. In all cases the deactivation must be attributed to an interaction of the potassium with the active phase. 

J.W. Evans, M.S. Wainwright, A. Bridgewater, D.J. Young, On the determination of copper surface area by reaction with nitrous oxide. Appl. Catal. 7 75-83, 1983. 

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