DRIFT spectra of SnO 2 , Mo/SnO 2 and Sn-Mo/Al 2 O 3

The DRIFT spectra recorded of SnO2, Mo/SnO2 and Sn-Mo/Al2O3 samples are depicted in Fig. 30. The spectra display bands at 1634, 740, 690, 630, 590 and 480 cm^-1.

The bands at 480 νs(Sn-O) and 590 cm^-1 νas(Sn-O) are assigned to symmetric and asymmetric stretching modes of (Sn-O) terminal bonds. The bands centred at 630 and 690 cm^-1 correspond to νs(O-Sn-O) and νas(O-Sn-O) vibrations. Moreover, the peak at 740 cm^-1 is assigned to bridged species of νas(Sn-O-Sn), while the discrete peak at 1031 cm^-1 is related to the lattice of different oxygen-bridged species of tin with the shoulder at 1123 cm^-1 to terminal γ(Sn-OH), besides the band at 1634 cm^-1 observed in all samples due to the deformation vibration mode of the H-O-H bond of water.

The spectrum of Mo/SnO2 exhibits additional band at 995 cm^-1 attributed to νas(Mo=O) terminal stretching in bulk MoO3 indicating strong features for free MoO3 clusters.

On the other hand, in the spectrum of Sn-Mo/Al2O3 the band at 761 cm^-1 is related to νs(Mo-O-Mo) associated with polymolybdates but it is noticeable to observe that the vibrational peak at 995 cm^-1 is absent in this spectrum in agreement with XRD results since no MoO3 reflection peaks were detected by XRD.

Furthermore, no bands arising from Mo–O–Sn vibrations were observed (in line with XRD results) in both Mo/SnO2 and Sn-Mo/Al2O3 samples that undoubtedly appear at 860-870 cm^-1. However, it is important to stress that the Mo–O–Mo unit is associated with polymolybdates, while the Mo–O–Sn is related to isolated tetrahedral Mo species. Therefore, FTIR results allow these two kinds of coordinations at hydrated conditions to be distinguished [91-98].

Fig. 30. In situ DRIFT spectra of SnO2, Mo/SnO2 and Sn-Mo/Al2O3 under vacuum

Table 7. Assignments of IR characteristics of metal-oxygen bonds Assignment Wavenumber (cm-1) Reference Terminal νsCe-O, νasCe-O

Bridging νsCe-O-Ce, νasCe-O-Ce

415, 480

690, 687, 740 65-80, 180-187 Terminal νsAl-O, νasAl-O

Bridging νsAl-O-Al, νasAl-O-Al

416, 485

690, 744 12-31

In plane and out of plane deformation modes of OH groups

1123, 1153,1161

1016, 1017 82, 175-177 Deformation vibration of water 1633, 1634, 1637 25-31, 82 νsMo-O-Al, νasMo-O-Al stretching 415, 423, 490, 512 12-17, 32-37

Terminal νasMo=O 957, 995 12-15, 42-47

Coupled O=Mo=O 995 and 1035 12-15, 34, 65, 80

νsMo-O-Mo 749, 757, 761 12-17

νsMo-O-Ce, νasMo-O-Ce 630, 875 65, 79-81

Terminal νsSn-O, νasSn-O, νsO-Sn-O, νasO-Sn-O

Bridging νsSn-O-Sn νasSn-O-Sn

480, 590 630, 690

740, 1031 82, 90-99 OH groups to tetrahedrally and

octahedrally coordinated Al3+ sites 3200-3800 18-31

OH groups of SnO2 and CeO2 3200-3800 90-99, 180-187

3.5.2. CO chemisorption

Hence, a new sample (50 mg) was used for each experiment and the pre-treatment for all samples as follows:

1. The sample was mounted in the cell fitted with CaF2 disks (which are IR transparent up to 1000 cm⁻¹) followed by evacuation at room temperature and at 1.33x10^-8 bar for 30 min.

2. The sample was reduced at different temperature in hydrogen flow (60 cm³/min, 1 bar, 10°C/min of heating rate) up to 800°C.

3. Then the system was cooled back to room temperature, flushed with N2 and then evacuated at room temperature and 1.33x10^-8 bar for 30 min.

4. Hence, the CO adsorption was carried out at 4x10^-2 bar and in temperature range 20-100°C for one hour. All the DRIFT spectra (MIR) were preserved following evacuation of the cell at room temperature and at about 1.33x10^-8 bar for 30 min.

On the other hand, since no appreciable CO adsorption bands appeared below 100°C only the spectra of CO adsorption at 100°C are subtracted and deconvoluted. The IR assignments of metal-CO bonds are shown in Table 8.

3.5.2.1. CO chemisorption on Mo/Al2O3

The spectra were presented after subtracting the spectra of the gas phase and the sample prior to the adsorption. The spectra of CO adsorbed at 100°C on 20 wt% Mo/Al2O3 reduced at 600°C, 700°C and 800°C are depicted in Fig. 31.

Four different υ(CO) bands can be observed in the spectrum of Mo/Al2O3 reduced at 600°C.

The two bands at νs1388 and νas1497 cm^-1 are originated from monodentate carbonate from CO adsorbed on molybdena, while the other two bands are due to CO species adsorbed on metal cations. Thus, the CO adsorption band at 2048 cm^-1 can be assigned as CO bonded to Mo⁴⁺ and/or Mo³⁺ ions. Furthermore, the band at 2194 cm^-1 is assigned to σ-bonded CO adsorbed on octahedrally coordinated Al³⁺ sites which are present on the surface more than tetrahedral one, having stronger Lewis acidity, since the lower wavenumber has been associated with increasing acid strength [20-28]. However, this band became more prominent and shifted to 2197 cm^-1 after reduction of Mo/Al2O3 at 800°C due to the increase in the CO stretching frequency above of the free CO gas molecule frequency (2143 cm^-1) with strong Lewis acid character.

The spectrum of CO adsorbed on Mo/Al2O3 reduced at 700°C exhibits new bands: (i) two bands at νs1351 and νas1578 cm^-1 due to symmetric and asymmetric vibrations of formate

species that is further confirmed by the broad band at about 2685 cm^-1 attributed to ν(CH) stretching, (ii) two bands at νs1485 and νas1550 cm^-1 are assigned to vibrations of carboxylates, (iii) two bands at νs1385 and νas1447 cm^-1 can be assigned to vibrations of monodentate species. In addition, the bands appearing at νas1415, νs1715 and νs1850 cm^-1 indicate the characteristics of free carbonate species on the surface. However, the band at 2048 cm^-1 appears with a shoulder at 1994 cm^-1 that can be assigned to bridged Mo⁰-CO indicating the presence of small amounts of metallic Mo⁰. These two bands shifted to 2050 and 2002 cm^-1 on the catalyst reduced at 800°C.

It can be seen that further reduction up to 800°C enhanced the adsorption of CO to form bridged carbonates (νas1230 and νs1765 cm^-1) and bicarbonates (νs1447 and νas1600 cm^-1).

The spectrum also reveals bands located at νas1281, νs1653 cm^-1 belonging to bidentates, besides the bands at νs1385, νas1560 cm^-1 assigned to formate. On the other hand, the bands protruding at 2025 and 2050 cm^-1 are created upon the interaction between Mo ions and adsorbed CO molecule. More specifically, the band observed at 2025 cm^-1 with a shoulder at 2002 cm^-1 are very likely associated with the terminally configured CO σ-bonded to metallic Mo⁽⁰⁾ species. Whereas the band at 2048 cm^-1 that was first seen after reduction at 600°C shifted to 2050 cm^-1 due to the presence of lower molybdenum valence states as the result of the higher reduction at 800°C when the average molybdenum oxidation number was estimated to be 1,6 [3-9]. This band can be assigned to CO π-bonding to Mo²⁺ and/or Mo¹⁺ in harmony with some reported results in the literature [12-15].

Another interesting point of CO adsorption behaviour on Mo/Al2O3 surface reduced at 800°C is that the band at 2197 cm^-1 is more prominent and broader than the band at 2194 cm^-1. This band is probably formed by overlapping the band corresponding to octahedral alumina sites with a band generated by interaction of CO with molybdena hydroxyls (Mo-OH bonded).

Consequently, the lower charge and higher reduction of Mo lead to a weaker σ-bond and enhance a π-type electron backdonation to form Mo-CO complexes (carbonates). This contributes to a CO stretching frequency below the CO gas frequency (2143 cm^-1).

Fig. 31. In situ DRIFT spectra of CO adsorbed on Mo/Al2O3 reduced at different temperature