CHAPTER 3 PHYSIO-CHEMICAL CHARACTERISATION

Transcription

1 37 CHAPTER 3 PHYSIO-CHEMICAL CHARACTERISATION 3.1 CHARACTERISATION OF CATALYSTS The purity and nature of the synthesized and supported catalysts were checked using various physio-chemical methods before subjecting them to the catalytic studies X-ray Diffraction (XRD) XRD of copper and chromium hydroxycarbonates The powder X-ray diffraction patterns of hydroxy carbonates of copper, chromium and copper chromite samples are given in Figure 3.1. The drying of the hydroxy carbonates at 110 C led to the formation of microcrystals with the hydrotalcite-like crystalline structure. No trace of the final structures is noted in the XRD pattern of the hydroxy carbonate of chromium and copper chromite respectively. However, the XRD pattern of copper hydroxy carbonate shows the formation of crystalline structures XRD of copper chromite catalysts The XRD patterns of the calcined samples of copper chromite in the increasing order of copper to chromium atom ratio are presented in Figure 3.2. Calcination (650 C) in air transformed all the hydrotalcite like material into a more crystalline solid, showing the patterns of CuO and of CuCr 2 O 4. However, the patterns of only CuO and CuCr 2 O 4 are designated in this diffractograms indicating that no other crystalline material (at least in XRD detectable form) is formed.

2 38 Figure 3.1 XRD patterns of (a) chromium hydroxycarbonate, (b) copper hydroxycarbonate and (c) copper chromium hydroxycarbonate

3 39 [( 2O 4, ( 2O 4 and ( ) CuO] Figure 3.2 XRD patterns of (a) Cu/Cr = 0.5, (b) Cu/Cr = 1, (c) Cu/Cr = 2, (d) Cu/Cr = 3 and (e) Cu/Cr = 4

4 40 ks of copper oxide and copper chromite respectively is considered as a mixture of CuO and CuCr 2 O 4 (Wainwright et al 1984). The XRD patterns show clearly, that for a low value of Cu/Cr (0.5), the only phase detected is the spinel like CuCr 2 O 4. The presence of characteristic peak of The XRD patterns for the ratios Cu/Cr > 0.5 are found to be mixtures of copper oxide and copper chromite. With increase in Cu/Cr ratio, the pattern of CuO phase became evident and the peaks appeared sharper. It is well known that a sharper spectrum is a result of presence of larger and/or perfect crystals XRD of reduced copper chromite The XRD patterns recorded for the initial copper chromite and for the sample after their exposure to the flow of hydrogen for 1 hour and 4 hours at 380 C are given in Figure 3.3. It is clear that there is slight variation in the diffractogram of initial and the hydrogen pretreated samples. In the fresh/untreated sample (Cu/Cr = 2) the peak with 100% intensity is at and CuCr 2 O 4. The height of the peak also reduced almost by two times from to 62.2 cm with decrease in the FWHM. Furthermore the reduction for 4 hours completely transformed the supported CuO to metallic copper and CuCr 2 O 4 to Cu 2 Cr 2 O 4 metallic copper and th chromite phase (Vannice et al 1997, Plyasova et al 1996).

5 41 [( 2O 4, ( 2O 4 and ( ) CuO ( ) Cu/Cu 2 Cr 2 O 4 ( ) Cu] Figure 3.3 XRD patterns of CuO/CuCr 2 O 4 (a) fresh, (b) reduced for 1 h and (c) reduced for 4 h

6 XRD of copper oxide and copper chromite impregnated silica gel The XRD patterns of copper oxide impregnated silica gel are shown in Figure 3.4. The peaks characteristic of copper oxide and copper chromite indicates the formation of spinel on the copper chromite impregnated support. The peaks corresponding to copper oxide are only seen on the support impregnated with copper oxide XRD of copper oxide impregnated supports The XRD patterns of copper oxide impregnated zeolite HY, TiO 2 and K10 clay are shown in Figures 3.5, 3.6 and 3.7. The XRD patterns indicate that the supports are crystalline and the patterns matched with those reported in literature. There was slight decrease in the intensity of the peaks in the XRD pattern of zeolite HY. On the contrary the peak intensity of TiO 2 and the K10 clay catalyst increased after the impregnation of copper oxide, which indicates formation of highly crystalline structure after the impregnation. Additional peaks due to copper oxide 39.5 also appeared in all the diffractograms and the intensity of these peaks increased with increase in loading of copper oxide.

7 43 [( 2O 4, and ( ) CuO] Figure 3.4 XRD patterns of (a) CuO/SG and (b) CuCr 2 O 4 /SG

8 44 Figure 3.5 [( ) CuO] XRD patterns of (a) HY, (b) 5% CuO/HY and (c) 10% CuO/HY

9 45 [( ) CuO] Figure 3.6 XRD patterns of (a) 10% CuO/TiO 2, (b) 5% CuO/ TiO 2 and (c) TiO 2

10 46 [( ) CuO] Figure 3.7 XRD patterns of (a) K10 clay, (b) 5% CuO/K10 clay and (c) 10% CuO/K10 clay

11 Fourier Transform-Infrared Spectroscopy (FT-IR) The FT-IR spectra of the catalysts employed in this study are shown in Figures 3.8 to The data extracted from these spectra are given in Table 3.1. Table 3.1 FT-IR frequencies of various systems studied System studied FT-IR frequencies in cm -1 Copper oxide 3427, 2924, 1384, 1050, 534 CuO/CuCr 2 O , 2925, 1629, 1397, 811, 505 CuO/CuCr 2 O 4 (redcued) 3422, 2924, 1632, 1052, 735, 540 K10 clay 3438, 1639, 1052, 798, 527, 469 5%CuO/K10 clay 3451, 1635, 1063, 799, 478 HY zeolite 3378, 1657, 998, 741, 469 5% Cuo/HY 3445, 1639, 1040, 783, % CuO/HY 3449, 1638, 1039, 775, 458 The band in the region cm -1 in all the spectra is mainly due to surface hydroxyl groups and the intensity differences indicate how these hydroxyl groups interact with other anchoring species. Figure 3.8 shows the IR spectra of K10 clay and also 5%CuO supported on K10 clay. One can see that the essential bands discernable for K10 are also present in the supported system indicating that the support does not change on the loading of the active component. The band at 3451cm -1 present in the original clay changed its shape on loading with copper oxide thus showing that the CuO species are hooked up on the clay surface using surface hydroxyl functions. One would have expected a shift of the band but this is not observed due to the following reasons:

12 48 a) The number density of the surface hydroxyl groups. b) The absorption cross section of the various species involved. Hence, one observes only a change in the shape of this band. Figure 3.8 FT-IR spectra of (a) K10 clay and (b) 5% CuO/K10 clay

13 49 Figure 3.9 FT-IR spectra of (a) CuO, (b) CuO/CuCr 2 O 4 and (c) CuO/ CuCr 2 O 4 (reduced) In Figure 3.9 the spectra for the pure CuO, CuO/CuCr 2 O 4 (Cu/Cr = 2) and CuO/CuCr 2 O 4 (reduced) are shown. The points that emerge are that the essential characteristics of copper oxide phase are easily discernable in both supported and reduced systems though the spectrum for the reduced system is different considerably. The peak intensities are altered as expected for the supported systems.

14 50 Figure 3.10 FT-IR spectra of (a) HY, (b) 5% CuO/HY and (c) 10% CuO/HY Figure 3.10 shows the FT-IR spectra of the copper oxide (5 and 10%) loaded on zeolite HY and in this case also the supported phase identity is essentially retained. The spectrum of HY zeolite reflects the scattering nature of zeolite sample employed. Hence the band positions in this sample cannot

15 51 be precisely known, though absorption regions may be identified and assigned. However, CuO containing HY zeolite samples give rise to different scattering profiles and hence this spectra obtained with these samples are amenable for some identification for the vibrational modes as well as the species involved Thermo Gravimetric Analysis (TGA) The TGA of the copper and chromium hydroxycarbonates along with the complex copper chromium hydroxycarbonate were recorded and the representative traces are given in Figure It is seen from Figure 3.11(a) that the main weight loss is around 200 C and the weight loss corresponds to around 11%. This weight loss manifests itself with a high temperature shoulder showing two stepwise decomposition processes; both the steps may be overlapping to some extent in the shown temperature range. It could be interpreted that the hydroxyl species are first eliminated followed by the expulsion of carbonate. However it is not to explicitly discern these two steps and it can be presumed both the decomposition may be occurring simultaneously in a wide temperature range around 200 C. However, in the case of chromium hydroxycarbonate the decomposition is much more complex as can be seen from the trace given in Figure 3.11(b). There are two weight losses around 60 and 160 C probably corresponding to the decomposition of the hydroxyl species and also removal of some residual adsorbed water. The decomposition of the carbonate is clearly discernable around 300 C with a weight loss corresponding to~22% followed by another weight loss around 500 C which could be due to some residual carbonate and also loss of oxygen from the chromium oxide formed. The crystallization of the oxide formed occurred around 640 C. The situation in the case of complex copper chromium hydroxycarbonate shown in Figure 3.11(c) indicates three weight loss regions centered around 190, 450 and 735 C. It is

16 52 presumed that the initial weight loss centered on 190 C is due to the decomposition of the hydroxyl species and the weight loss around 450 C is due to the decomposition of the carbonate species. The crystallization of the complex oxide is seen clearly around 735 C. This shows that the reaction between copper oxide and chromium oxide is a facile process when the complex hydroxycarbonate is employed as a precursor. Figure 3.11 TGA of hydroxycarbonates of (a) copper, (b) chromium and (c) Copper-Chromium

17 Atomic Absorption Spectroscopy (AAS) The percentage of copper present as copper oxide is determined with the help of AAS on acid-digested samples and the values are presented in Table 3.2. Table 3.2 Percentage of copper as copper oxide in the copper chromite catalysts Catalyst Total Copper Cu as CuO (%) (Cu/Cr) (%) Nitrogen Sorption Studies The BET surface area of all the catalysts obtained by the nitrogen sorption studies is presented in Table 3.3. Though the surface area of the copper oxide and chromium oxide is less, a sharp increase in the surface area of the copper chromite catalysts indicates that chromia increases the surface area by forming the copper chromite spinel (Laine et al 1988, Forni 1988). However, decrease in surface area was observed with increase in copper oxide content indicating that copper oxide higher than the stoichiometric ratio of spinel, deposits on copper chromite and blocks the surface there by decreasing the surface area. A similar decrease in the surface area of the copper oxide impregnated supports was observed with increase in the copper loading. This phenomenon indicates that higher loading of copper oxide leads to agglomeration.

18 54 Table 3.3 BET surface area of synthesized and copper oxide impregnated supports Catalyst Surface area (m 2 /g) Catalyst Surface area (m 2 /g) CuO %Cu/SG Cr 2 O HY 367 Cu/Cr = %Cu/HY 359 Cu/Cr = %Cu/HY 343 Cu/Cr = K10 clay 208 Cu/Cr = %Cu/K Cu/Cr = %Cu/K SG 396 TiO %Cu/SG %Cu/TiO n-butylamine Titration The catalyst acidity measured by n-butylamine titration enables one to evaluate the total number of acid sites and their acidic strength. In order to interpret the results, it is suggested that the initial electrode potential (Ei) indicates the maximum acid strength of the surface sites. The acidic strength of surface sites can be assigned according to the following ranges: very strong site, Ei > 100mV; strong site, 0 < Ei <100 mv; weak site, -100 < Ei < 0 mv and very weak site, Ei < -100 mv. The total acidity values are presented in Table 3.4.

19 55 Table 3.4 Acid strength of the synthesized and CuO impregnated samples Catalyst Acid strength (mv) Catalyst Acid strength (mv) Cu/Cr = % CuO/K Cu/Cr = HY zeolite 192 Cu/Cr = % CuO/HY 230 Cu/Cr = % CuO/HY 206 K % CuO/SG 210 5% CuO/K % CuO/TiO From Table 3.4 it can be deduced that the acid strength is less for the copper chromite samples with the Cu/Cr ratio = 0.5 and the acid strength slowly increases upto Cu/Cr = 2 and later decreases indicating agglomeration. The concept of agglomeration has been also proved from the reduction in the surface area of the catalysts with increasing Cu/Cr ratio. For the CuO impregnated HY and K10 catalysts the acid strength increased from plain support to 5% CuO impregnated supports and later decreased. The protonic sites created by CuO on the catalyst surface might have increased the acidity and when the loading has reached saturation (10%) there might be blocking of the active acid sites which decreased the acid strength Temperature Programmed Reduction (TPR) TPR of the systems have been carried out to find out the phase stability and possible active sites in these systems. The TPR traces obtained are shown in Figures 3.12 to The essential data is given in Table 3.5.

20 56 Table 3.5 TPR characteristics of supported copper oxide catalysts System Reduction temperatures 0 C 5%CuO on silica gel 300, 330, 450 5%CuO on K10 160, 370, 625 5%CuO on TiO 2 230, 340 5% CuO on CuCr 2 O %CuO on zeolite HY 440, 600 The points that emerge from the values given in Table 3.5 are 1. The reduction process is still stepwise in supported systems 2. The reduction temperature is altered depending on the nature of the support. 3. On interacting supports the reduction temperature is shifted to higher values. 4. All the supports employed in the present study are interacting with the active phase. 5. On K10 and silica gel the reduction temperatures are shifted to very high values indicating that the copper species are substituted in the surface functional groups on these supports. This is also true for the zeolite support as well. 6. In the case of copper chromite support the reduction steps are discernable since the support also may undergo reduction thus all the peaks are overlapping in single step as seen from the

21 57 broad nature of the peak. The FWHM for this system is unusually higher than for other systems. 7. In the case of TiO 2 the reduction temperatures are similar to pure copper oxide indicating that the interaction in this support appears to be weak. This is contradictory to other metal support interactions normally observed. Temperature ( o C) Figure 3.12 TPR profile of pure copper oxide Pure copper oxide system shows a two step reduction process centered around 297 and 318 C showing that the reduction of cupric state occurs in stepwise fashion first to cuprous state and then to the metallic state. However the TPR traces of copper oxide impregnated on various support gives a different picture.

22 58 Temperature ( o C) Figure 3.13 TPR profile of copper oxide impregnated silica gel Temperature ( o C) Figure 3.14 TPR profile of copper oxide impregnated K10 Clay

23 59 Temperature ( o C) Figure 3.15 TPR profile of copper oxide impregnated TiO 2 Temperature ( o C) Figure 3.16 TPR profile of CuO/CuCr 2 O 4

24 60 Temperature ( o C) Figure 3.17 TPR profile of copper oxide impregnated HY Electron Spectroscopy for Chemical Analysis (ESCA) The catalyst system employed in this study is a complex one. This system can be conceived as though the active species can be either generated from copper chromite or from the copper oxide used. It has been conceived that the system consists of copper oxide supported on copper chromite, which was also evidenced by the XRD patterns. The ESCA spectra of the fresh and used catalysts have been obtained to support the contention. Since the copper 2p region of the ESCA spectrum has been taken up for the analysis, it must be remembered that we can have Cu 2+ ions in tetrahedral and possibly octahedral sites in the copper chromite as well, as one has Cu 2+ ions in the copper oxide used as the active phase. When there are three different species of the same ion in different environments, it is natural that some broad featured emission is obtained from all the species postulated. The 2p spectrum of the copper region of the fresh catalyst is shown in Figure 3.18.

25 61 Figure 3.18 ESCA spectrum of copper in fresh catalyst Figure 3.19 ESCA spectrum of oxygen in fresh catalyst

26 62 It is seen that the 2p 3/2 peak appears at a binding energy of ev with a corresponding satellite at ev. Another feature observed in the 2p 3/2 emission is an asymmetry in the lower binding energy side. The second observation to be recorded is that the satellite emission is fairly strong. This aspect indicates that the copper is present in the +2 valence state since other valence states of copper do not exhibit satellite features (Fierro et al 1991). The corresponding oxygen 1s peak is shown in Figure This clearly indicates that there are two types of oxygen with binding energies and ev. This is an indication that we have oxide species from the ionic spinel lattice at lower binding and oxide species originating from the copper oxide species supported on the spinel phase. The relative intensities of the 1s peak of oxygen indicates that emission from the spinel is also discernable even though copper oxide is supported on this phase. It is therefore tempting to assign the peak centered at ev in the copper 2p 3/2 region to copper ions in the tetrahedral sites of the spinel lattice of copper chromite and the high binding energy shoulder of this region is attributed to the Cu 2+ of the active phase supported on the spinel. There are reports in the literature where the peak at has been assigned to the tetrahderally coordinated Cu 2+ ions (Murthy and Ghose 1994). Since in the fresh catalyst all the copper is present in the +2 state one observes intense satellite features. These deductions can also be substantiated from the emissions at the 2p 1/2 region, which is also shown in Figure However, the ESCA spectrum of the copper 2p region in the used catalyst is shown in Figure 3.20 and the corresponding oxygen 1s region is shown in Figure 3.21.

27 63 Figure 3.20 ESCA spectrum of copper in used catalyst Figure 3.21 ESCA spectrum of oxygen in used catalyst

28 64 The points that can be noted from Figures 3.20 and 3.21 are: 1. Oxygen 1s region is still resolvable in two or more components but the corresponding binding energies are now and ev. 2. The relative intensities of these two emissions are now comparable or at least it is different from that of the 1s region of the fresh catalyst. 3. The copper 2p 3/2 peak is centered around ev and the corresponding satellite feature is at ev. 4. The p 3/2 emission and the satellite) is around 8 ev and this is not much different from that observed with the fresh catalyst. 5. The intensity of the satellite feature is considerably reduced in the case of the used catalyst. These observations indicate that in the used catalyst one still has copper in the +2 valence state since the satellite feature is observed. However the 2p 3/2 emission is now centered at a lower binding energy as compared to the fresh catalyst. This means that the active copper oxide supported on copper chromite has undergone oxidation state change as a result of the reaction while the copper ions in the copper chromite may be acting as an active support either to disperse the active copper oxide phase or provide active centers for the activation of the substrate molecule without undergoing any change as a result of catalytic reaction. Normally Cu 2+ ions in the cupric oxide will give an emission around 933 ev and for other forms one may not be able to observe considerable shift in the binding energy as has been observed in the case of other ions existing in various valence states.

29 65 However, considerable decrease in the intensity of the satellite feature indicates that copper in copper oxide has undergone reduction to either cuprous state or zero valence metallic state. This indicates that Cu 2+ ions in the copper oxide supported on the copper chromite are the active species for the catalytic reaction and these ions are susceptible for the oxidation state change with the reducing environments of the reaction. Hence, the activity of the catalyst was lost when these Cu 2+ ions are reduced to Cu + state (used catalyst). Alternatively the copper ions in the copper chromite may be activating the substrate (diamine) and the copper in the reduced state may be facilitating the dehydrogenation step.