Thermal decomposition of a hydrotalcite-containing Cu Zn Al precursor: thermal methods combined with an in situ DRIFT study

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1 PCCP Thermal decomposition of a hydrotalcite-containing Cu Zn Al precursor: thermal methods combined with an in situ DRIFT study I. Melián-Cabrera, M. López Granados and J. L. G. Fierro* Instituto de Catálisis y Petroleoquímica (CSIC), Campus UAM, Cantoblanco 28049, Madrid, Spain. jlgfierro@icp.csic.es Received 26th February 2002, Accepted 16th April 2002 First published as an Advance Article on the web 28th May 2002 A Cu Zn Al precursor (CZA) was synthesized efficiently by coprecipitation of the corresponding cations with sodium carbonate at constant ph and temperature. The starting precursor contained a mixture of two hydroxycarbonate phases: rosasite and a Cu Zn hydrotalcite-like phase. The thermal decomposition was studied by conventional thermal methods (TGA, DTA and EGA-MS) as well as by in situ FTIR spectroscopy (DRIFT). Analysis of the CZA precursor showed similar results by both procedures. Dehydration, dehydroxylation and decarbonation of the precursor were analysed in situ by monitoring the hydroxyl and carbonate infrared bands. A Cu Zn hydrotalcite phase, one of the components of the CZA precursor, was also prepared independently. A detailed FTIR study revealed an interesting effect upon heating this hydrotalcite. At K, a carbonate rearrangement in the interlayer space takes place during the loss of interlayer water. Carbonate groups change from their symmetrical coordination with interlayer water molecules to an arrangement involving the OH groups of the octahedral M(OH) m layers. This phenomenon certainly takes place in the CZA material as well but, in this case, it cannot be observed, probably due to the complexity of the material formed by two hydroxycarbonate phases. 1. Introduction The conventional Cu Zn Al mixed oxide catalytic material, which is used for the synthesis of methanol from CO/CO 2 / H 2 mixtures (synthesis gas), is obtained from decomposition of a hydroxycarbonate precursor. This classical hydroxycarbonate route leads to the formation of several mixed metal hydroxycarbonates such as aurichalcite, (Cu,Zn) 5 (CO 3 ) 2 - (OH) 6, zincian-malachite (or rosasite), (Cu,Zn) 2 (OH) 2 CO 3, and a Cu Zn hydrotalcite-like phase (Cu,Zn) 6 Al 2 (OH) 16- CO 3 4H 2 O, all of which decompose to give well-dispersed oxidic phases. 1 3 The Cu Zn hydrotalcite-like phase is a common component of the mixed phase-containing precursor for methanol synthesis catalysts. This material belongs to the socalled hydrotalcite-like compounds (HTlcs), which are a family of materials consisting of layered double hydroxides of general formula [M II 1 xm III (OH) 2 ] X+ (A n ) x/n mh 2 O where M II and M III are di- and trivalent metal cations that occupy octahedral positions in brucite-like layers, A n is the interlayer anion, m the number of interlayer water molecules and x the atomic ratio M III /(M II +M III ). 4 The oxidic phases of such a conventional methanol synthesis catalyst (CuO ZnO Al 2 O 3 ) are obtained by thermal decomposition of the hydroxycarbonate precursor. The calcination temperature mentioned in the literature is K, which is usually chosen on the basis of sintering restrictions. 1 3 Higher calcination temperatures are close to the thermal limit to avoid thermal sintering of the CuO phase. This would have an adverse effect on the catalytic activity for methanol synthesis. However, recent studies on catalyst activation of ex-hydrotalcite mixed oxides have shown that this process is more complex than previously thought. Specifically, the catalytic properties appeared to depend not only on the calcination temperature but also on the activation procedure. 5,6 Therefore, an understanding of the hydroxycarbonate decomposition process is of vital importance to explain the catalytic activity of the resulting oxidic material. The aim of this work was to study in detail the thermal decomposition of a Cu Zn Al hydroxycarbonate precursor by means of classical thermal methods (TGA, DTA and EGA-ms) as well as by the in situ FTIR spectroscopy technique (DRIFT). 2. Experimental 2.1. Synthesis of the precursor The CZA precursor (Cu : Zn : Al ¼ 55 : 30 : 15 atomic ratio) was prepared by coprecipitation at constant ph (ca. 7) and constant temperature (343 K). 7 A solution containing metal nitrates ([Cu 2+ ] + [Zn 2+ ] + [Al 3+ ] ¼ 1.0 M) and a sodium carbonate solution (1.1 M) were simultaneously added to a reaction vessel containing a small amount of deionized water (18 MO cm 1, as supplied by a Millipore deionizer). The suspension was continuously stirred and kept at the desired ph by adjusting the relative flow rates of the two solutions. The final suspension was aged under stirring (343 K for 4 h) and cooled to room temperature overnight. The precipitate was filtered off and repeatedly washed with sufficient deionized water to ensure that the sodium content in the solid was lower than 0.05 wt.%. The solid was dried overnight at 393 K (CZA precursor) and calcined in air at 673 K for 4 h (calcined CZA precursor) Characterisation techniques The chemical composition of the dried CZA precursor was determined by means of inductively coupled plasma atomic emission spectroscopy (ICP-AES), using a Perkin-Elmer Optima 3300DV apparatus. The solid was first treated with aqua regia and aqueous HF at 363 K. The specific area of CZA precursor and calcined samples were calculated by the BET method from the N 2 adsorption 3122 Phys. Chem. Chem. Phys., 2002, 4, DOI: /b201996e This journal is # The Owner Societies 2002

2 isotherms at 77 K, which were acquired using an ASAP 2000 apparatus, taking a value of nm 2 for the cross section of a physically adsorbed N 2 molecule. The CZA precursor was previously outgassed under vacuum at low temperature (323 K, 72 h) to maintain the initial as-synthesized structure. However, CZA calcined material was outgassed at 413 K for 12 h. Thermogravimetric analysis (TGA) was carried out with a Perkin-Elmer TGA-7 analyzer. The sample holder was loaded with 10 mg of specimen and the decomposition of the precursor was monitored under a flow of air (50 ml min 1 ) and at a heating rate of 10 K min 1. Differential thermal analysis (DTA) was performed using a Perkin-Elmer DTA-7 apparatus. The sample, diluted with SiC powder (40 mg, sample/sic ¼ 3/1 by weight), was placed in the sample holder and heated at a rate of 10 K min 1 under a flow of air (50 ml min 1 ). Evolved gases analysis of the precursor decomposition was performed by mass spectrometry (EGA-MS). Samples (20 mg) were placed in a U-shaped quartz reactor incorporated within a flow system. The flow circuit was connected to a Baltzer Prisma TM quadrupole mass spectrometer (QMS 200). A mixture of 21 vol% O 2 in Ar was introduced at a rate of 50 ml min 1 and the temperature was increased by 10 K min 1 while the composition of evolved gases was continuously monitored. In situ diffuse-reflectance infrared spectra (DRIFTS) were collected on a Nicolet-510 FT-IR spectrometer (256 scans at 4 cm 1 ) using a Harrick HVC-DRP environmentally-controlled cell. About 50 mg of the powdered sample was packed into a sample holder and synthetic air (Air liquide) was passed over the sample. A DRIFT spectrum of dry KBr was also recorded as a background. The IR transitions during thermal activation were studied by increasing the temperature from room temperature to 673 K with a heating rate of 5 K min 1. The spectra were recorded at different temperatures with variable increments of 5 50 K. The temperature was kept constant at every selected value for 5 min, the time necessary to accumulate the 256 scans. Powder X-ray diffraction patterns were recorded on a computerized Seifert 3000 X-ray diffractometer using Cu-Ka radiation, a PW 2200 goniometer (Bragg Brentano y/2y geometry), a bent graphite monochromator and an automatic slit. The samples were scanned between 10 < 2y < 75 in the step mode (0.02, 2 s). 3. Results and discussion 3.1. Characterisation of the starting CZA precursor The atomic ratio for the metals (Cu : Zn : Al ¼ 54.9 : 29.7 : 15.4), as determined experimentally by ICP- AES, is in good agreement with the theoretical value (Cu : Zn : Al ¼ 55 : 30 : 15). This indicates that the precipitation procedure was efficient. The XRD pattern of the CZA precursor is presented in Fig. 1A. For the sake of comparison, the JCPDS files for several reference compounds are also included in the lower part of this figure. The typical pattern of the Cu Zn hydrotalcite-like structure [Cu 2 Zn 4 Al 2 (OH) 16 CO 3 4H 2 O] (JPCDS ) 8 can clearly be seen for the base CZA precursor. The remaining diffraction peaks were assigned to a rosasite phase [(Cu,Zn) 2 - CO 3 (OH) 2 ] (JCPDS ). 9 Although the intensities of the two peaks at 2y 32 do not correspond to an intensity observed in any of the rosasite JCPDS files, our assignment is based on other work in the literature. 7,10 In addition, it must be considered that the rosasite phase is a zincian-malachite that can be described as a solid solution in which the Cu/Zn Fig. 1 X-ray powder diffraction profiles for: (A) CZA precursor and (B) calcined CZA precursor (673 K). ratio can influence the intensities of the diffraction peaks in the pattern. Further evidence for the presence of both hydroxycarbonate phases was provided by infrared analysis of the framework vibrations. The spectrum at room temperature of the CZA precursor is displayed in Fig. 2 (labelled CZA). Although this spectrum consists of broad bands, which complicate the interpretation, the analysis of this profile can be facilitated by means of a previous analysis of the spectrum for the Cu Zn hydrotalcite-like phase alone. For this purpose, a Cu 2 Zn 4 Al 2 - (OH) 16 CO 3 4H 2 O hydrotalcite-like compound was prepared according to a method described elsewhere. 4 The spectrum of such a Cu Zn hydrotalcite-like phase is shown in Fig. 2 (labelled HTCu Zn). The features of this latter profile, also recorded at room temperature, include hydroxyl and water vibrations as well as carbonate absorptions (see Scheme 1A for assignments). The broad band at 3500 cm 1 is ascribed Fig. 2 FTIR spectrum of CZA precursor as well as that for the Cu Zn reference hydrotalcite [Cu 2 Zn 4 Al 2 (OH) 16 CO 3 4H 2 O], both recorded at room temperature. Phys. Chem. Chem. Phys., 2002, 4,

3 cannot be ruled out: e.g. the broad zone in the high-wavenumber region can certainly be ascribed to OH stretching of structural hydroxyl groups from both phases Thermal decomposition studies on the CZA precursor Thermal methods. The thermal decomposition of the CZA precursor, as studied by three methods, is represented in Fig. 3 and 4: TGA and DTA analyses (Fig. 3) and EGA-MS analysis, which involved monitoring the H 2 O and CO 2 evolution by detection of fragments at m/z ¼ 18 and 44 with a quadrupole mass spectrometer (solid lines in Fig. 4). For the sake of clarity, the derivative of the TGA curve is also included in Fig. 3A (dashed line). There is good agreement between the processes observed by TG analysis and by MS detection. The precursor undergoes four weight loss processes upon heating in a flow of air. The first weight loss is associated with the removal of water of crystallisation (interlayer water) present in the Cu Zn hydrotalcitelike structure and this is clearly visible in the derivative TGA peak at around 440 K. A second process, shown by the broad peak at 570 K, is due to H 2 O and CO 2 co-evolution during decomposition of the hydrotalcite and rosasite phases. Elimination of structural hydroxyl groups from both phases Scheme 1 Assignments of the Cu Zn hydrotalcite IR bands. Changes upon heating in air. to the OH stretching mode [n(oh) ]: structural hydroxyl groups, water molecules in the interlayer zone and physisorbed and free water. 4 The shoulder at 3080 cm 1 is characteristic of layered hydrotalcite materials and is caused by hydrogen bonding between the water molecules and CO 3 2 groups, both present in the interlayer region. 11,12 The corresponding H O H bending mode of water can also be seen in the spectrum: physisorbed 4 water at 1651 cm 1 and interlayer 13 water at 1751 cm 1. It must be emphasized that this small band at 1751 cm 1 is usually not detected or not mentioned during the assignment of IR spectra. This type of band has recently been ascribed to water bending in the interlayer space, with the assumption that the relatively high frequency is due to symmetry restrictions induced in the interlayer zone. 13 Finally, the carbonate vibrations are observed in the range cm 1. More specifically, the n 3 mode (asymmetric stretching) of the CO 3 2 anions appears around 1404 cm 1. The other carbonate modes, n 2 and n 4, cannot be seen due to the poor resolution obtained in the low-wavenumber region of the infrared spectrum (<1000 cm 1 ). A comparison between the CZA precursor and Cu Zn hydrotalcite spectra clearly shows the presence of some specific absorption bands due to the rosasite phase. The smaller bands at 1106 and 1064 cm 1 are associated with O H deformations 14 while the absorptions at 1550 and 1425 cm 1 are characteristic fingerprints of carbonate species in a malachite-like environment. 15 It should be remembered that rosasite is a zincian-malachite phase, where Zn atoms partially replace the Cu atoms in the malachite structure. The possibility that other rosasite bands are overlapping with the hydrotalcite vibrations Fig. 3 (A) TGA curve of the CZA precursor. The dashed line corresponds to the derivative curve. (B) DTA profile of the same precursor. Fig. 4 Mass spectrometry traces of H 2 O(m/z¼18) and CO 2 (m/ z ¼ 44) evolved for the CZA precursor (solid lines) and Cu 2 Zn 4 Al 2 - (OH) 16 CO 3 4H 2 O hydrotalcite as a reference compound (dashed lines) Phys. Chem. Chem. Phys., 2002, 4,

4 results in the formation of H 2 O. The evolution of CO 2 is only due to the decomposition of the rosasite. Finally, the hydrotalcite carbonate groups decompose in the next two weight loss processes, which occur in the range K. This indicates that at temperatures of K (the commonly used calcination temperature range) the hydrotalcite CO 3 2 units will remain occluded in the solid. The assignment of these peaks to the decomposition of CO 3 2 groups from the hydrotalcite is not only based on data reported in the literature, 4,16,17 but also on a TGA/EGA-MS study of the reference Cu Zn hydrotalcite-like compound [Cu 2 Zn 4 Al 2 (OH) 16 CO 3 4H 2 O]. This analysis showed (see dashed lines in Fig. 4) H 2 O evolution at 440 K (dehydration) and 550 K (dehydroxylation), followed by CO 2 formation between K resulting from carbonate decomposition. A DTA experiment was performed on the CZA precursor (Fig. 3B). Each weight loss was accompanied by an endothermal transformation. The DTA plot shows two large endothermic peaks that are linked to the first and second weight losses observed by TGA. However, the last two weight losses, ascribed to the decarbonation of Cu Zn hydrotalcite, are accompanied by a single weak and endothermic DTA peak centered at around 850 K. The low intensity of this peak is in accordance with results reported by other authors, 18,19 who observed a low change in enthalpy during decarbonation. Thermal decomposition monitored by in situ FTIR spectroscopy. The thermal evolution of the CZA precursor was also monitored by FTIR spectroscopy. Infrared spectra of the CZA precursor acquired at room temperature (discussed previously), as well as during heating, are shown in Fig. 5. The temperature at which each spectrum was recorded is indicated on each line. The first change in the IR spectra upon heating takes place between room temperature (RT) and 448 K. A slight decrease in the intensity of the broad band at 3500 cm 1 and the complete disappearance of the band at 1651 cm 1 can be seen at the higher temperature. These observations are due to the loss of physisorbed water in both precursor phases. In the temperature range K a marked decrease in the intensity of the band at 3500 cm 1 and a slight reduction in intensity in the carbonate region ( cm 1 ) occurred. The first change is related to a loss of interlayer water in the Cu Zn hydrotalcite phase and partial removal of structural hydroxyl groups in both hydroxycarbonate precursor phases. The slight reduction in the carbonate zone is due to partial decarbonation of the precursor. An increase in temperature from 528 to 573 K caused a substantial change in the shape of the spectrum. The band at 3500 cm 1, along with those at 1106 and 1064 cm 1, completely disappeared owing to total dehydroxylation of both rosasite and hydrotalcite phases. In addition, a significant decrease in the carbonate absorption region is found, which is related to complete decarbonation of the rosasite phase. It should be noted that the temperatures at which decomposition was observed by FTIR are in agreement with the previously described TGA and EGA-MS results. Those analyses showed massive decomposition transformations between 373 and 623 K (Fig. 3). The presence of residual carbonate bands at 673 K agrees with previous TGA/EGA-MS studies in the sense that elimination of remaining hydrotalcite carbonate groups required temperatures as high as 900 K. At present, the FTIR analysis points to the same results as obtained using classical thermal methods. In order to complement and support this interpretation of the CZA precursor, IR spectroscopy was also used to monitor the pure Cu Zn hydrotalcite phase upon heating. Infrared spectra of the Cu Zn hydrotalcite phase recorded at room temperature (as explained in section 3.1), as well as during heating, are presented in Fig. 6. The analysis of the spectra is similar to that described for the CZA material. When the sample was heated to 383 K, changes started to appear in the profile. A slight decrease in the intensity of the broad band at 3500 cm 1 and a marked abatement of the band at 1651 cm 1 (complete disappearance at 398 K) were evident. These effects are due to the loss of physisorbed water at the surface. At 383 K there is also a decrease in the intensities of the shoulders at 3080 and 1751 cm 1. This is an indication of the partial removal of interlayer water, which is completely lost at 408 K. A remarkable and surprising effect occurs between 373 and 423 K. The carbonate n 3 band splits into two separate Fig. 5 FTIR spectra for the thermal evolution in air of the CZA precursor. The temperature at which each spectrum was recorded is indicated on each trace. Fig. 6 FTIR spectra for the thermal evolution in air of the Cu Zn reference hydrotalcite. The temperature at which each spectrum was recorded is indicated on each trace. Phys. Chem. Chem. Phys., 2002, 4,

5 bands at 1528 and 1370 cm 1. This rearrangement of the carbonate groups is caused by the loss of interlayer water. Initially, the carbonate groups are symmetrically coordinated to interlayer water but, after the loss of the interlayer water, the carbonate groups will coordinate to hydroxyl groups of the octahedral brucite-like layers (see Scheme 1B). Therefore, the new configuration in the interlayer space gives rise to either mono- or bidentate coordination between carbonates and structural hydroxyl groups, since only two bands are observed. The simultaneous appearance of both configurations requires the observation of four bands since the splitting of monoand bidentate structures should be different. However, from the value of the doublet splitting it cannot be ascertained a priori what type of interaction is present. These two possibilities are taken into account and depicted in Schemes 1B, and 1C by extension. The band at 1528 cm 1 has been assigned to CO vibration in interaction with the OH group of the brucite-like layers, whereas the other band at 1370 cm 1 is assigned to the carbonate C=O vibration. 20 It should be noted that this carbonate rearrangement in the Cu Zn hydrotalcite was not detected for the CZA sample (see Fig. 5), most likely due to the complexity of the carbonate region in the CZA precursor. The presence of the rosasite bands at 1425 and 1550 cm 1 masks the carbonate rearrangement in the hydrotalcite phase. Therefore, analysis of the independent Cu Zn hydrotalcite provides good evidence for the carbonate rearrangement phenomenon. This effect most likely also takes place during decomposition of the CZA precursor. Finally, it can be seen that a marked decrease in the intensity of the band at 3500 cm 1 occurs upon increasing the temperature from 423 to 533 K. This phenomenon is attributed to the partial removal of structural hydroxyl groups. A further increase from 533 to K resulted in complete dehydroxylation. In addition, the intensity of the carbonate bands remains essentially the same (between K), indicating that the carbonate units are still occluded in the structure. The large degree of splitting of the final carbonate doublet (130 cm 1 ) at 623 K indicates that the remaining carbonates are directly and firmly coordinated with the metal cations (Scheme 1C), in the form of mono- or bidentate coordination Characterisation of the calcined CZA precursor The CZA precursor was calcined in air (673 K) according to the literature procedures. However, our TGA and EGA-MS studies (Fig. 3) indicate incomplete decomposition of the precursor at this calcination temperature. Higher calcination temperatures, as required to decompose the remaining CO 2 3, are not recommended as they are close to the higher limit allowed to minimise thermal sintering of the CuO phase. By following this procedure one avoids a detrimental effect on the catalytic performance for the methanol synthesis reaction. Within this frame, the value of BET area is useful. The BET area for the initial as-synthesized CZA precursor was found to be 35.7 m 2 g 1, whereas it reached 41.4 m 2 g 1 for the calcined material, upon heating at 673 K. Therefore, the selected temperature preserves a rather good dispersion for the calcined 2 material. The nature and precise arrangement of the CO 3 units remaining in the mixed oxide, a consequence of incomplete Cu Zn hydrotalcite decomposition, have not been discussed in detail in the literature. In fact, no Cu Zn carbonate is detected by XRD (see Fig. 1B), which indicates that the phase must be amorphous. However, it is believed that the thermally induced changes in the hydrotalcite-like compounds take place through a topotactic decomposition of the brucite-like layers, giving an oxycarbonate in which the metal oxygen framework remains essentially intact and the carbonate anions are still retained in the interlayer region. Such carbonate anion retention in CuO ZnO Al 2 O 3 catalytic systems does not appear to show any effect during reaction: probably because these units are removed during the catalyst activation (H 2 -reduction). However, recent studies have shown that carbonate retention in the CuO ZnO Al 2 O 3 substrate has important implications during the incorporation of a palladium promoter by wet impregnation, which in turn affects the catalytic performance in the hydrogenation of CO 2 to methanol. 21 The XRD pattern of the calcined CZA material is shown in Fig. 1B. Although the most intense diffraction lines at 30 < 2y < 40 are overlapping, the peaks could be assigned to CuO (JCPDS ) 22 and ZnO (JCPDS ) 23 phases. The absence of any crystalline aluminium phase indicates that Al 3+ is incorporated as amorphous alumina. However, the formation of surface Cu Al and/or Zn Al spinels 24 cannot be ruled out. The broad XRD peaks indicate that the Cu and Zn oxides are ill-defined crystalline phases. Similar results have been reported by different authors for other CuO ZnO-based composite materials prepared under similar conditions. 25,26 The coprecipitation results in the formation of Cu Zn mixed phases like rosasite and a Cu Zn hydrotalcite-like phase in which the Cu and Zn are atomically dispersed in a hydroxycarbonate matrix. When these phases are calcined, CuO and ZnO are formed and have a smaller crystal size than those material oxides derived from single phases like malachite (pure Cu-hydroxycarbonate) and hydrozincite (pure Znhydroxycarbonate). 1 3 Moreover, a small amount of Cu(II) cations (ca. 2 atom%) can be dissolved in the ZnO lattice, 27 a situation that can clearly introduce some disorder into the framework, as evidenced by the broad XRD peaks for the ZnO phase. 4. Conclusions The thermal decomposition of a Cu Zn Al hydroxycarbonate precursor (CZA) was studied by conventional thermal methods (TGA, DTA and EGA-MS) as well as by in situ FTIR spectroscopy (DRIFT). These two different methods gave similar results. Dehydration, dehydroxylation and decarbonation of the precursor were found by the thermal methods and these processes were analysed in situ by monitoring the hydroxyl and carbonate infrared bands at different temperatures. The detailed study by FTIR of an independently prepared Cu Zn hydrotalcite phase, one of the components of the CZA precursor, revealed a carbonate rearrangement in the interlayer space between 373 and 423 K. This rearrangement was caused by loss of interlayer water. Initially, carbonate groups are symmetrically coordinated to water molecules but, after loss of the water, the carbonates coordinate to the OH groups of the octahedral M(OH) m layers. This carbonate rearrangement effect was not observed during the analysis of the CZA precursor, most likely due to the complexity of the material formed by two hydroxycarbonate phases. Analysis of the monophasic hydrotalcite system allowed us to establish the carbonate rearrangement phenomenon. This rearrangement effect certainly takes place during decomposition of the CZA precursor itself. Acknowledgement This work was supported by CICYT (Spain) under Grant QUI IMC thanks the Ministry of Education and Science for a fellowship. References 1 K. 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