Results and Discussion

In previous studies, some characterizations of the studied catalysts have already been reported (Bali et al., 1995, 1996).

Thermal analysis in air and in H2, metallic areas determination, in situ in H2 XRD have allowed obtaining some information on the dried, calcined, and partially reduced states of the studied catalysts.

Table 19.1 Surface and metallic areas.

Catalyst

Cu/Zr

Salts

Surface area (m2/g)

Metallic area (m2/g)

(Cu/Zr0.5)K

0.5

CuCl2+ZrOCl2

200

30.5

(Cu/Zr0.5)

0.5

Cu(NO3)2 +ZrO(NO3)2

89

13

(Cu/Zr1)

1

Cu(NO3)2 +ZrO(NO3)2

92

19

(Cu/Zr2)

2

Cu(NO3)2 +ZrO(NO3)2

94

6

(Cu/Zr0)

0

ZrO(NO3)2

93

-

In the oxidized state, a zirconia-like phase is evidenced in all the solids analyzed, while crystallized CuO appears for a specific copper content depending on the calcination temperature and/or the coprecipitation procedure. A solid solution of zirconium and copper is evidenced.

In H2, the appearance of metallic copper depends on the same parameters as previously reported and also on the treatment temperature, meanwhile the crystallization of the zirconia phase increases with the temperature (for temperatures higher than 500°C) and some segregation phenomena occur. Metallic Cu0 is detected by XRD at 150°C for Cu/Zr1 and Cu/Zr2, at 200°C for Cu/Zr0.5, and at 400-500°C for (Cu/Zr0.5)K.

Moreover, for a given temperature, metallic copper appears at lower temperature when increasing the time of treatment in H2. As a matter of fact, small quantities of metallic copper appear in (Cu/Zr0.5)K treated in H2 at 150°C for several hours. Figures 19.1 and 19.2 show the XRD patterns of the (Cu/Zr0.5)K and (Cu/Zr0.5) compounds treated in H2 for 12 h.

Fig. 19.1 XRD patterns of the (Cu/Zr0.5)K catalyst treated in H2 versus time and treatment temperature.

Fig. 19.2 XRD patterns of the (Cu/Zr0.5) catalyst treated in H2 versus time and treatment.

Fig. 19.2 XRD patterns of the (Cu/Zr0.5) catalyst treated in H2 versus time and treatment.

In situ XPS and Auger analysis have been performed on the two Cu/Zr0.5 compounds in order to try to explain the differences observed by thermogravim-etry and XRD, and on the Cu/Zr0 compound used as a reference. The reduction temperatures range is between 100°C and 250°C with 12 h of treatment at each temperature. The Zr3d5/2 signal remains the same either zirconium is alone or with copper, in agreement with an oxidation state +4 of zirconium at the surface of the solid. The «O/«Zr ratio is equal to 2.24 and the binding energy remains equal to 182.1 eV, identical to the one reported by Younes (Younes et al., 2000) and Ka-tona (Katona and Molnar, 1995) for a Cu-Zr intermetallic compound. Besides this value is very different from those obtained by Yang (Yang et al., 1993) for a suboxide ZrOx and for Zr0 which are, respectively, equal to 183.9 and 179.9 eV. Table 19.2 shows the main characteristics of copper found in this study and compared to those reported in the literature for CuO, Cu2O, and Cu0 compounds (Lide, 2002).

After treatment in H2 at 100°C, the Cu2p3/2 line still shows the existence of two peaks typical of the Cu2+ species. The Sat/pp ratio is quite constant in the (Cu/Zr0.5)K sample (0.67-0.66%), while it is seriously decreasing on the (Cu/Zr0.5) solid from 0.60% to 0.55%, showing that the reaction has already started on this solid, in agreement with a huge variation of the kinetic energy reaching 917.1 eV, the typical value of the Cu+ species. This premature reduction may result in the reduction of big crystallites of copper oxide, which are visible by XRD for this compound.

At 150°C the reduction process is started for both compounds. However, the persistence of the satellite peak of the Cu2p3/2 line in the (Cu/Zr0.5)K spectra shows that Cu2+ species is still present and remains particularly well inserted. The Auger line of copper for this solid has been separated into three components with maxima at 915.6, 916.6, and 918.4 eV, corresponding to the three oxidation degrees of copper Cu+, Cu2+, and Cu0. In the spectra of the (Cu/Zr0.5) compound, the disappearance of the component corresponding to Cu2+ species is confirmed. It is important to notice that these values are sharply different from those obtained for Cu+ and Cu2+ species in pure Cu2O and CuO. For instance, the component at 915.6 eV is much lower than that obtained for Cu+ in Cu2O (916.2 eV), in favor of Cu+ species well inserted into the ZrO2 matrix.

After treatment in H2 at 250°C, the typical satellite peak of the Cu2+ species has totally disappeared from the spectra. The width of the Auger line can be attributed to the presence of Cu+ and Cu0 species, but the Cu+ binding energy is still low, assigned to Cu+ species located in the zirconia network.

Thus, XPS results are in good agreement with XRD and thermal reduction analysis. The peak observed at 130°C by thermal reduction (previous study) has been assigned to the reduction of very small particles of CuO that partly cover up the solid solution. Beyond 150°C, the reduction of very big crystallites visible by XRD is obtained on the (Cu/Zr0.5) solid. The reduction of the dissolved copper is slower and starts later, probably following the scheme Cu2+^ Cu+^-Cu0, the two steps being hard to distinguish with the used methods. The low Cu/Zr atomic ratio shows that the main part of copper cations is inserted into the bulk of the (Cu/Zr0.5)K solid. This ratio tends to the nominal value for the (Cu/Zr0.5) compound. So, for the analyzed depth (~20A), the different treatments do not really modify the distribution of copper in both solids. The quantity of surface oxygen species drastically changes during the reducing treatment. On the other hand, the dispersion of the hydroxyl OH" species around metallic cations must decrease because of the reduction of copper, which leads to the redistribution of the surface OH" groups and their scattering at the surface of the network.

Table 19.2 XPS and Auger analysis.

Cu/Zr* obtained by flame ionization analysis; ** according to (Wang et al., 1993).

Finally, after activation in H2, the (Cu/Zr0.5)K catalyst corresponds to a solid solution of copper and zirconium, Cu2+ cations replacing some Zr4+ cations in the quadratic zirconia phase, with a small quantity of metallic copper like clusters, located on the surface. For the other solids, the solid solution also exists, but zirconia is in a cubic phase, and when increasing copper content, surface metallic copper increases.

These observations lead us mainly to assume the existence of two kinds of sites: i) Cu+-O-Zr4+ sites due to the interaction between Cu+ and Zr4+ cations either at the interface of small CuO clusters with the solid solution or in the copper and zirconium solid solution and ii) Cu+-O-Cu+ sites related to the interaction between Cu+ cations in the solid solution or at the metal-solid solution interface. In the (Cu/Zr0.5)K catalyst, the Cu+-O-Zr4+ site is found in highest concentration, whereas the (Cu/Zr0.5) and (Cu/Zr1) catalysts present a great amount of Cu+-O-Cu+ sites with some Cu+-O-Zr4+ sites, as shown in Fig. 19.3.

19.3.1 Reactivity in isoprene hydrogenation in helium

The (Cu/Zr1), (Cu/Zr0.5)K, and (Cu/Zr0.5) catalysts present very good activities, respectively, equal to 38.0, 35.0, and 18.3 x 10"3 mol g1. At 150°C, the isopentane

Calcined

(Cu/Zr0.5)K

934.7

916.7

(Cu/Zr0.5)

934.5

916.7

Tr=100°C

(Cu/Zr0.5)K (Cu/Zr0.5)

915.6

Tr=150°C

(Cu/Zr0.5)K

932.7

916.6 918.4

(Cu/Zr0.5)

932.7

915.6 918.3

Tr=250°C

(Cu/Zr0.5)K

932.3

918.8

(Cu/Zr0.5)

932.4

918.8

**

Cu0

932.6

917.8

**

Cu2O

932.4

916.5

**

CuO

933.6

917.6

selectivity is almost similar to that obtained on industrial catalysts (Sene, 1992). Moreover, the isoprene hydrogenation product selectivity is analogous to that observed on some Cu-M-O catalysts. The existence of particular active sites, involving Cu+ cations of the solid, hydroxyl groups, anionic vacancies and hydrogen species of hydride nature have been proposed, such as xM—M' elementary ensembles (where x and y are the number of unsaturations, i.e., anionic vacancies, on each cation) (Jalowiecki et al., 1987). In particular, it has been shown that three coordinatively unsaturated sites (3 CUS or also noted 3M) are the prerequisite condition to obtain alkadiene hydrogenation activity, while the unsaturated second cation brings its contribution to selectivity. Thus, the production of isopentane could be associated with the 3M-3M' elementary ensemble while the 3M-2M' site leads to the formation of 2-methyl-but-2-ene. It has been shown that in helium, the migration of hydroxyl groups leads to an evolution of the surface active sites. The 3M-2M' site turns out to become an 3M-1M' leading to the formation of 2-methyl-but-1-ene and 3-methyl-but-1-ene. This active site modeling can be applied to the solids studied here, the Zr4+ cation playing the role of the second cation, explained by its lower degree of coordinative unsaturations and its Lewis acidic sites, as it has been reported for Al or Cr3+ (Hubaut et al., 1986). The 2M site has been proposed to be the prerequisite condition to observe isomerization reaction; therefore the 3Cu+-2Zr4+ site noted type I can be related to the formation of 2MB2. The big proportion of isopentane obtained is to be linked to the presence

of a majority of Cu - M' sites, in other words mainly Cu - Zr (Type III) in the (Cu/Zr0.5)K catalyst and 3Cu+-3Cu+(Type II) in the (Cu/Zr 0.5) and (Cu/Zr1) catalysts. Table 19.3 presents the selectivity obtained at different reaction temperatures for the studied catalysts.

Table 19.3 Active sites and isoprene hydrogenation products distribution.

Products

Sites

(Cu/Zr0.5)K

(Cu/Zr0.5)

(Cu/Zr1)

(Cu/Zr2)

Cu0

80°C

150°C

150°C

80°C

150°C

150°C

150°C

2M1B

3Cu+-1Zr4+

22

7.3

0.5

15

0.6

25.1

27

3M1B

8

0.2

0

7

0

0,3

7

2M2B

3Cu+-2Zr4+

43.5

10

4.5

27

4

23.5

55.5

Isopentane

3Cu+-3M +

27

82.5

95

51

95.4

50.8

10

M' = Zr4+ and/or Cu+ depending on the catalyst.

According to the previous modeling, it can be proposed that at 80°C, the 3M2M' sites are more likely to be present in (Cu/Zr0.5)K catalyst, while (Cu/Zr1) compound presents a higher proportion of 3M3M' sites. For these two catalysts there is a sharp evolution between 80°C and 150°C, this means an increase of the surface dehydroxylation and then a creation of a higher proportion of 3M3M' sites. At 150°C the (Cu/Zr0.5) and (Cu/Zr1) catalysts have the same products distribution. The (Cu/Zr2) catalyst acts like metallic copper and its products distribution at 150°C is similar to that of (Cu/Zr1) at 80°C, so the reaction temperature plays an important role in the constitution of the active sites.

(Cu/Zr0.5) and (Cu/Zr1) x : clusters SS : solid solution

• : small agrégats Fig. 19.3 Catalysts modeling.

(Cu/Zr0.5) and (Cu/Zr1) x : clusters SS : solid solution

19.3.2 Reactivity in CO2 hydrogénation

Table 19.4 gives the results of the global CO2 conversion and the selectivities in different products. As we can see, the activities are decreasing according to the following sequence: (Cu/Zr1) > (Cu/Zr0.5)K > (Cu/Zr0.5) > (Cu/Zr2). In our operating conditions (T= 250°C, P=5 atm), we comparatively get few methanol; CH4 and CO are predominant for thermodynamic limits.

Table 19.4 Conversion and selectivities obtained in CO 2hydrogenation.

(Cu/Zr0.5)K

(Cu/Zr0.5)

(Cu/Zr1)

(Cu/Zr2)

(Cu/Zr0)

Conversion (%)

7.2

4.5

11.25

Traces

0

S. methanol (%)

9.54

3.55

4.00

//

0

S. CO (%)

89.13

64.38

72.00

//

0

S. CH4 (%)

1.32

32.01

24.00

//

0

19.3.3 Hydrogen species titration

This titration is performed on a catalyst previously in situ treated in H2 at different temperatures. During the activation step in H2, anionic vacancies created in the bulk and at the surface of the solid, by the loss of H2O, are able to receive hydrogen in a hydride form according to a heterolytic dissociation (O2- M"+ + H2^OH-M"+ H"). The oxides become catalytic hydrogen reservoirs able to hydrogenate isoprene in the absence of gaseous hydrogen. A chemical titration of these hydrogen species can be performed according to a method already published (Jalowiecki et al., 1985). As we can see in Table 19.5, there is no linear correlation between bulk and surface hydrogen storage. However, isoprene hydrogenation activity is directly proportional to surface hydrogen content. CO2 hydrogenation depends on the Hs/SCu ratio in favor of the existence of a particular active site located at the metallic copper and solid solution interface (Bali et al., 1996).

Table 19.5 Bulk hydrogen (QH) and surface hydrogen [Hs] contents (mmol g 1 of oxide) versus treatment temperature in H2.

T red (°C)

(Cu/Zr0.5)K

(Cu/Zr0.5)

(Cu/Zr1)

(Cu/Zr2)

150

9.3 [0.3]

12.9 [1.0]

13.2 [1.2]

11.1 [0.3]

250

12.6 [1.3]

9.2 [0.6]

13.6 [1.3]

13.3 [0.6]

400

7.5 [0.35]

7.3 [0.2]

7.7 [0.6]

6.6 [0.1]

Several studies performed on catalytic systems based on copper have shown that different types of sites, necessary for dienes hydrogenation reactions, can be involved in other reactions, such as the hydrogenation of alpha beta car-bonyl compounds and allylic alcohols. In a study on the hydrogenation activity obtained on these compounds, Hubaut et al. (1986) have shown that the activity is not linked to the presence of Cu0, but is directly proportional to the amount of Cu0 and H- species, (Hubaut et al., 1991). Bechara has also presented a good correlation between activity and existence of Cu+-H- sites in the methanol synthesis reaction from CO + H2 (Bechara et al., 1992). Besides, it is generally admitted that methanol is not formed on copper alone; the addition of a second cation is needed. Several studies performed by in situ IR have evidenced an active ensemble constituted by two cations and have shown some adsorbed species on copper and/or on the second cation, each of these adsorbed species could lead to methanol (Gao et al., 2000, Kung et al., 2002, Ryczkowski, 2001). On the other hand, it is well known that methane can be obtained on catalysts containing a high proportion of copper (Sun and Sermon, 1994, Bartley and Burch, 1988, Gao et al., 2005).

In this study, for the two reactions, the activities are decreasing according to (Cu/Zr1) > (Cu/Zr0.5)K > (Cu/Zr0.5) > (Cu/Zr2). The (Cu/Zr0.5)K catalyst is the most selective toward methanol and the (Cu/Zr0.5) and (Cu/Zr1) catalysts give the highest proportion of methane. The methane selectivity could be explained by a high proportion of Cu0 in close contact with a solid solution, easily oxidized by CO2. The very poor activity of (Cu/Zr2) can be due to the presence of large Cu0 crystallites covering the solid solution. As for the diene, the adsorption of CO2 could be performed on the site presenting three CUS, the second cation being Zr4+ or Cu2+. The 3Cu+-2Zr4+ site is proposed for the formation of methanol and the 3Cu+-3Cu+ site, more unsaturated, for the formation of methane.

As it has been shown in the catalysts characterization section, the active site can belong to the solid solution (SS) or can be located at the interface of clusters or small copper aggregates with the solid solution. Then there are three possibilities as shown in Fig. 19.4.

SS SS SS interface metal-SS

Type I Type III Type II

Fig. 19.4 Types of active sites.

In the case of the (Cu/Zr1) catalyst, the existence of a third type of site such as 3Cu+-3Zr4+ in the solid solution could be considered. However, as it is also present in the (Cu/Zr0.5)K compound, it is unlikely that this kind of site can be greatly involved in the formation of methane. Conversely, the presence of type II sites is highly probable at the interface. This hypothesis is corroborated by an RPE study on Cu-Ce-O catalysts (Wrobel et al., 1996) which evidenced the interactions between copper species belonging to CuO entities of cluster types or small aggregates and the ceria network during reduction.

Differences exist between the two reactions: the hydrogenation of carbon dioxide takes place at 250°C; at this temperature it can be assumed that the surface is less hydroxylated, this could have an influence on the superficial anionic vacancies concentration. For this reaction, CO2 can oxidize copper and particularly Cu0 species. This could be an argument to explain the good selectivity toward methane obtained on the (Cu/Zr1) catalyst which contains a higher proportion of metallic copper compared to the (Cu/Zr0.5)K catalyst. In a previous study, it has been shown that isoprene hydrogenation activity AH varies linearly with the amount of surface hydrogen HS, while CO2+H2 conversion rather depends on the Hs/SCu ratio (Bali et al., 1996). This factor is therefore important and this result is in favor of an interaction between metallic copper and solid solution. As for the isoprene hydrogenation reaction, "elementary ensembles" corresponding to different structures of sites, constituted of a Cu+ cation inserted into the oxide matrix, a hydride species, and anionic vacancies (CUS) localized on the Cu2+ and Zr4+ cations, noted 3Cu+-yM' (where y is the unsaturation degree of M' related to selectivity), are proposed for CO2 hydrogenation reaction. The selectivity toward methanol and methane related to these sites has been discussed and the influence of the metallic area represented by the HS/SCu ratio has been taken into account in order to localize the active sites for the formation of methane at the interface of the solid solution and the copper metal coming from the clusters. Finally, two mechanisms to methanol and methane formation via bidental formats can be proposed as presented in Figs. 19.5 and 19.6.

Methanol Active Sites
Fig. 19.5 CO2 active site modeling for methanol synthesis mechanism on site I.

Fig. 19.6 CO2 active site modeling for methanol synthesis mechanism on site II.

Was this article helpful?

0 0
Getting Started With Solar

Getting Started With Solar

Do we really want the one thing that gives us its resources unconditionally to suffer even more than it is suffering now? Nature, is a part of our being from the earliest human days. We respect Nature and it gives us its bounty, but in the recent past greedy money hungry corporations have made us all so destructive, so wasteful.

Get My Free Ebook


Post a comment