The application of FTIR in situ spectroscopy combined with methanol adsorption to the study of mesoporous sieve SBA-15 with cerium-zirconium oxides modified with gold and copper species

2.1.1 Modification of SBA-15 materials with cerium and/or zirconium oxide and copper and gold

Four different supports for gold and copper were prepared. They were CeO₂, ZrO₂ and mixed CeZrO_x (of different CeO₂:ZrO₂ atomic ratios: 1:2 and 2:1) loaded on SBA-15 molecular sieve. SBA-15 was modified with 10 wt% of a single Ce or Zr metal oxide or with a mixture of Ce and Zr metal oxides. The supports were labelled as follows: CeS, ZrS, CeZrS(1:2) and CeZrS(2:1) for mono-metallic and bimetallic oxide mixtures of different ratios of their content (indicated in brackets), respectively. These supports were subjected at first to functionalization with 3-aminopropyl-trimethoxysilane (APMS), then to gold loading and finally to impregnation with copper species. The details of the syntheses and modifications were described in the earlier papers (Kaminski et al., 2015; Kaminski and Ziolek, 2014). The final monometallic catalysts – Au/CeS, Au/CeZrS(2:1), Au/CeZrS(1:2), Au/ZrS and their bimetallic analogues – CuAu/CeS, CuAu/CeZrS(2:1), CuAu/CeZrS(1:2) and CuAu/CeZrS were obtained and characterised.

2.2.1 The texture and structure properties

The texture and structure properties of these materials using selected methods (ICP-OES, XRD, adsorption and desorption of nitrogen, XPS, UV–vis, FTIR in situ combined with adsorption and desorption of NO, CO or pyridine, TEM) were studied in the earlier study (Kaminski et al., 2015; Kaminski and Ziolek, 2014). In this paper, the most valuable information about the FTIR in situ study will be presented. In this work the results of the FTIR in situ measurements combined with the adsorption of methanol vapours on the surface of catalysts and its oxidation in the selected temperature range are shown. TEM images, XRD patterns, XP spectra recorded for catalysts before and after their using in methanol adsorption and its oxidation are presented in order to show the changes in the structure of materials during methanol oxidation.

2.2.2 The FTIR in situ study combined with the adsorption and oxidation of methanol

For methanol and oxygen adsorption, the Fourier Transform Infrared (FTIR) spectra were recorded at room temperature (RT) using a Nicolet Nexus Fourier Transform Infrared 6700 spectrometer (Thermo Scientific) equipped with DTGS KBr detector and an extended KBr beam splitter at a spectral resolution of 4 cm⁻¹ accumulating 64 scans. The sample powders were pressed into thin self-supporting pellets of ca. 7.5 mg cm⁻² (2 cm² area) and placed in a vacuum quartz cell equipped with KBr windows, where they underwent all activation and adsorption treatments. All spectra during methanol oxidation in the selected temperature range (293–573 K) were recorded at room temperature (RT). The spectrum without any sample (“background spectrum”) was scanned and was subtracted from all recorded spectra. Before the measurements, the samples were activated at 623 K (for 2 h, using a ramp of heating – 3 K min⁻¹) under high vacuum. The first spectrum of the sample was recorded for a fresh sample, before the activation.

Pure methanol (99.9%) in the probe tube was purified by cooling in the liquid nitrogen (for 15 min) before the admission into a cell. Then it was adsorbed at RT. About 1 mbar of methanol was added in three doses (each around 0.33 mbar) and then samples was evacuated in the high vacuum at RT, 323, 373 and 423 K for 5 and 10 min (a heating ramp – 2 K min⁻¹) and at 573 K for 1 h (the heating ramp – 2 K min⁻¹). The details of the procedure of adsorption and desorption were collected in the Supporting Information.

The FTIR spectrum after the out gassing in high vacuum at 573 K for 1 h was used to the subtraction in the case of the FITR spectra recorded after the adsorption of methanol and oxygen, the heating of gas mixture in the temperature range 323–573 K (the heating ramp – 2 K min⁻¹) and the out gassing at 573 K for 5 and 10 min (the heating ramp – 2 K min⁻¹). All presented spectra were normalised to the density of 7.5 mg cm⁻².

2.2.3 Transmission electron microscopy (TEM)

For transmission electron microscopy (TEM) measurements the catalysts before and after their using in methanol oxidation were deposited on a grid covered with a holey carbon film and transferred to JEOL 2000 electron microscope operating at 80 kV.

2.2.4 X- ray diffraction (XRD)

XRD measurements were carried out on a Bruker AXS D8 Advance diffractometer with Cu Kα radiation ( $\lambda$ = 0.154 nm) with a step size of 0.05° in the wide-angle range (20–60°).

2.2.5 X-ray photoelectron spectroscopy (XPS)

X-ray photoelectron spectra were recorded on an Ultra High Vacuum (UHV) System. The study was conducted using X-ray Al K_α = 1486.6 eV with the following lamp parameters: 14.5 kV, 20 mA. Measurements were carried out in the chamber of the analyser under vacuum of approximately 2.0 · 10⁻⁸ mbar. The spectra were recorded for the energy range from 1100 eV to 0 eV in increments of 0.5 eV, the energy transition CAE = 100 eV. Different regions were recorded in increments of 0.1 eV, the energy transition CAE = 30 eV and time counts 50 ms. The number of scans in the measured range was fitted to the observed signal to noise ratio. The powder sample was put on the conductive tape of carbon (graphite), which was adhered to the carrier surface and placed perpendicular to the axis of the analyser. The X-ray source was set at the angle of 60° to the plane of the surface. The area of the samples analysed corresponded to the size of the aperture of the analyser used (large area), ca. 50 mm². Deconvolution of XP spectra was carried out using the OMNIC 8.0™ software. Bands intensities were estimated by calculating the integral of each band after smoothing and baseline correction. Then the deconvolution parameters for the experimental curve, that is the Voigt function, small signal sensitivity and constant base line, were chosen in the OMNIC 8.0™ software. Atomic ratios were computed from the intensity ratios normalized by the atomic sensitivity factors. An estimated error of ±0.1 eV can be assumed for all measurements.

3.1.1 The region of the stretching vibrations of bonds C—H in methanol molecules (3050–2750 cm⁻¹)

The C—H stretching region (3050–2750 cm⁻¹) reveals two pathways for methanol adsorption: undissociated (molecular) and dissociated (methoxy) species. Pairs of bands (Figs. 2, 4–6), which can be seen after methanol adsorption at around 2955 and 2848 cm⁻¹, they are due to the strong asymmetric and symmetric vibrations of C—H bonds in methoxide species —OCH₃ (Engeldinger et al., 2010; Wang et al., 2012). In the literature data (Busca, 1996), these bands were associated to the symmetric methyl stretching mode split due to its Fermi resonance with the first overtone of the corresponding symmetric deformation. In the case of samples with gold and copper, a band at 2848 cm⁻¹ can be the result of adsorbed methoxy species at different copper sites. The band located at around 3000 cm⁻¹ could be assigned to the stretching vibrations in C—H bond of weakly adsorbed methanol molecules (Natal-Santiago and Dumesic, 1998). The band at around 2920–2915 cm⁻¹ can be due to one of the methyl group (—CH₃) bending mode (Bazin et al., 2012; Natal-Santiago and Dumesic, 1998). The bands of stretching vibrations in methanol molecules were disappeared after out gassing at 623 K for 2 h.

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Fig. 5

FTIR spectra were recorded for SBA-15 with ceria or zirconia modified with gold and copper for the regions of 3050–2750 cm⁻¹ (left side) and 1750–1320 cm⁻¹ (right side) after: (a) the adsorption of methanol (∼1 mbar) at RT; (b) 5 min later; (c) after the adsorption of oxygen (∼10 mbar) at RT; (d) 5 min later; (e) 10 min later; heating for 10 min at (f) 323; (g) 373; (h) 423; (i) 473; (j) 523; (k) 573; (l) out gassing at 623 K for 10 min.

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Fig. 6

FTIR spectra were recorded for SBA-15 with ceria or zirconia modified with gold for the regions of 3050–2750 cm⁻¹ (left side) and 1750–1320 cm⁻¹ (right side) after: (a) the adsorption of methanol (∼1 mbar) at RT; (b) 5 min later; (c) after the adsorption of oxygen (∼10 mbar) at RT; (d) 5 min later; (e) 10 min later; heating for 10 min at (f) 323; (g) 373; (h) 423; (i) 473; (j) 523; (k) 573; (l) out gassing at 623 K for 10 min.

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3.1.2 The region of the vibrations of bonds in methanol molecules (1520–1320 cm⁻¹)

After methanol adsorption at RT, the characteristic bands of formate species were appeared at about 1376–1373 cm⁻¹ in FTIR spectra recorded for samples with gold and copper (Fig. 2a–c spectra of samples) and then their shift was observed to about 1382–1381 cm⁻¹ in during out gassing at 623 K (Bazin et al., 2012). These bands can be assigned to the symmetric stretching vibrations ν_s(OCO) of methyl formate and products of it decomposition on surface of catalysts (Engeldinger et al., 2010). Bands around 1451–1450 cm⁻¹ are due to of C—H deformation vibration (Engeldinger et al., 2010), bending vibration (Sun et al., 2007) and they can be assigned to methoxy species (Natal-Santiago and Dumesic, 1998). A band at about 1465–1463 cm⁻¹ can due to the bending vibration in methanol molecules weakly adsorbed on silica via hydrogen bonding interactions (Wang et al., 2012). An according to the literature data (Busca, 1996), bands at 1481–1458 and around 1351–1348 cm⁻¹ are a proof on the occurring of the methyl groups of metoxy species (—OCH₃) on the surface of materials, because are due to asymmetric (ν_asCH₃) and symmetric (ν_sCH₃) stretching vibrations of bonds C—H.

3.1.3 The region of the vibrations of bonds in carbon monoxide (2400–2050 cm⁻¹)

In the case of catalysts modified with copper and gold in the FTIR spectra (Fig. 3.) recorded after the out gassing at RT, the bands of vibrations in CO molecules adsorbed on weakly copper species were observed at 2142–2136 cm⁻¹. It can suggest that copper during the out gassing was reduced from Cu²⁺ to Cu⁺ and this form of copper cations could play the role of active sites. The band at 2142–2136 cm⁻¹ can be attributed also to gold species (as like Au⁺). Methanol molecules could be decomposed in the reducing atmosphere to methane, hydrogen and carbon monoxide on these active sites. This mechanism on copper species could run the according to chemical formula:2 CH₃OH → CH_4(g) + H_2(g) + CO_(g).

3.2.1 The region of the stretching vibrations of bonds in methanol molecules and intermediates of products (1750–1320 cm⁻¹)

The composition of the intermediates created during the methanol oxidation on the surface of catalysts was analyzed using the FTIR in situ spectroscopy by the recoding the spectra in RT (Figs. 5–8 and 3S-4S). The intermediates were created after the adsorption of methanol (∼1 mbar) and oxygen (∼10 mbar) on SBA-15 materials and their heating at 323–573 K.

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Fig. 7

FTIR spectra were recorded for SBA-15 with ceria and/or zirconia modified with gold and copper for the region of 2370–2320 cm⁻¹ and 2400–2100 cm⁻¹ after: (a) adsorption of methanol (∼1 mbar) at RT; (b) 5 min later; (c) after adsorption of oxygen (∼10 mbar) at RT; (d) 5 min later; (e) 10 min later; heating for 10 min at (f) 323; (g) 373; (h) 423; (i) 473; (j) 523; (k) 573; (l) evacuation at 623 K for 10 min.

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Fig. 8

FTIR spectra were recorded for SBA-15 with ceria and/or zirconia modified with gold for the region of 2370–2320 cm⁻¹ after: (a) the adsorption of methanol (∼1 mbar) at RT; (b) 5 min later; (c) after the adsorption of oxygen (∼10 mbar) at RT; (d) 5 min later; (e) 10 min later; heating for 10 min at (f) 323; (g) 373; (h) 423; (i) 473; (j) 523; (k) 573; (l) out gassing at 623 K for 10 min.

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The observed bands were identified. The FTIR bands of the asymmetric stretching vibrations (ν_asOCO) appeared at around 1584–1571 cm⁻¹ (Engeldinger et al., 2010). The bands at about 1578 and 1640 cm⁻¹ belonged to adsorbed formate and monomethyl carbonate species, respectively (Engeldinger et al., 2010). In the region of 1800–1700 cm⁻¹ can be found the vibrations of bands in formaldehyde, which can be one of products in the selective oxidation of methanol. The bands in the region 1750–1700 cm⁻¹ point to the formation of species containing carbonyl groups. A band at about 1714–1711 cm⁻¹, which was observed after the heating of the studied samples at 323–423 K (Figs. 5 and 3S, f-h spectra of samples with cerium, copper and gold), in range of 373–473 K (Fig. 5, g-i spectra of CuAu/ZrS) or at 523–573 K (Figs. 6 and 4S, f-j spectra of samples modified with only gold) in the oxidised atmosphere and in the presence of methanol. It can be assigned to the stretching vibrations (νC⚌O) of methyl formate species adsorbed on catalyst surface (Wang et al., 2012). An according to the mechanism of methanol oxidation, the creation of reagents from methanol molecules during its oxidation is the result of redox properties of active sites and compounds on the surface of catalysts (Tatibouet, 1997). The presence of formaldehyde can lead to the reduction of cationic gold to its metallic form and this is the reason why is used as the reducing agent in the preparation of metallic gold nanoparticles (Choma et al., 2011). The formate species represented by formaldehyde (the possible product of methanol oxidation) can lead also to the reduction of copper species. It means that the presence of formate intermediates on the surface of catalyst can reduced the elements of active phase and these intermediates can play the rule of reactive species in the selected range of temperature.

The band at 1620–1615 cm⁻¹ was due to the stretching vibrations of O—H bond mode of Lewis bound water, which was displaced by the adsorption of methanol (Chesalov et al., 2013; Ladera et al., 2012). These bands, which were attributed to the stretching vibrations of O—H in water molecules, were observed in the FTIR spectra recorded for the bimetallic catalysts after their heating for 10 min at 473, 523 and 573 K. It means that methanol is combusted to carbon dioxide and water. It is according to the observation of the FTIR spectra recorded for the regions of 2200–2090 cm⁻¹. In the case of samples with gold, new bands were showed at around 1620–1615 cm⁻¹ in the FTIR spectra, which were recorded after the thermal treatment for 10 min at 523 K (Au/CeS, Fig. 6, j spectrum) and 573 K (Au/CeZrS(2:1) and Au/CeZrS(1:2), Fig. 4S, k spectra). It can suggest that methanol molecules adsorbed on the surface of catalysts are combusted to carbon dioxide and water at lower temperature over bimetallic catalysts than catalysts with gold.

In the region (1500–1400 cm⁻¹) of vibrations in C—H bonds, the bands are attributed to (C—H) modes of methoxy groups and undissociated methanol molecules mostly overlap. In the FTIR spectra performed for samples after their heating at 373–423 K were weak bands at 1437–1436 cm⁻¹, which were assigned to C—H deformation in methoxy groups. After heating the samples with gold (Figs. 6 and 4S, a spectra recorded for samples modified with only gold), a new band at around 1500 cm⁻¹ was created, which was attributed to the stretching vibrations in methane molecules adsorbed on the surface. The weak band at about 1465 cm⁻¹, which disappears upon out gassing, may be attributed to undissociated methanol species. A band about 1420 cm⁻¹ can due to the deforming vibrations in metoxy groups chemisorbed on copper (Wang et al., 2012). A band at about 1451–1450 cm⁻¹ can be assigned to metoxy species and about 1590 cm⁻¹ was due to formate species (Sun et al., 2007). The bands of vibrations in dimethyl ether couldn’t be identified, because they can give the band at about 1200 cm⁻¹. It isn’t available this region in the study of the materials based on silica.

3.2.2 The region of the stretching vibrations of bonds in CO and/or CO₂ molecules (2400–2050 cm⁻¹)

In the case of catalysts with copper and gold, after the addition of oxygen and methanol to the cell and the heating of samples at 473 K, a new band was appeared at around 2340 cm⁻¹, which is typical for the stretching vibrations in carbon dioxide molecules – the product of methanol combustion (Fig. 7, i spectra of samples). It is worth to note, that in the case of CuAu/ZrS, two bands were created at 2138 cm⁻¹ after the heating at 473 K and at 2342 cm⁻¹ after heating at 523 K (Fig. 7, j spectrum of sample), which were due to the stretching vibrations of bonds in molecules of carbon monoxide and dioxide, respectively. A similar band was appeared at 2341 cm⁻¹ in the FTIR spectrum after the heating at 523 K of Au/CeS (Fig. 8, j spectrum of sample). In the case of Au/CeZrS(2:1) and Au/CeZrS(1:2) (Fig. 4S, k spectra of samples), this band were observed after the heating at 573 K. The bands around 2143 cm⁻¹ can be due to gaseous CO, and the bands at 2342 cm⁻¹ to CO₂ formed mainly during the oxidation of CO (Pérez Vélez et al., 2012). CO molecules can be produced in the reducing conditions from methanol and according to the reaction formula: $$2{\text{CH}}_3\text{OH}\to {\text{CH}}_4+{\text{H}}_2+\text{CO.}$$