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Quantification of hydrocarbon species on surfaces by combined microbalance-FTIR
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 181 (2017) 65–72
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Quantification of hydrocarbon species on surfaces by combined microbalance-FTIR Andrew I. McNab, Tom Heinze, Alan J. McCue, Davide Dionisi, James A. Anderson ⁎ Materials and Chemical Engineering Group, School of Engineering, University of Aberdeen, Aberdeen AB24 3UE, UK
a r t i c l e
i n f o
Article history: Received 15 August 2016 Received in revised form 13 January 2017 Accepted 13 March 2017 Available online 16 March 2017 Keywords: Quantification FTIR Adsorbed hydrocarbons Fischer-Tropsch catalyst Absorption coefficient
a b s t r a c t Absorption coefficients for the asymmetric stretching modes of CH3 and CH2 groups formed by adsorbing alkyl chained species from the vapour phase onto two different adsorbents; a γ-alumina support material and a supported metal catalyst have been determined using a custom made thermogravimetric-infrared cell. Results show that despite variations in the individually calculated absorption coefficients (ca. ±20%), the ratio of the absorption coefficients (CH2:CH3) remained consistent despite employing adsorbates of varying chain length and functionality, and despite the choice of adsorbents which exhibited different surface areas and light scattering characteristics. The use of this absorption coefficient ratio has been shown to be applicable in the quantification of the average chain length of multiple adsorbed species of differing chain length. The potential for applying this to scenarios where reactions on surfaces are monitored is discussed. © 2017 Published by Elsevier B.V.
1. Introduction Fourier Transform Infrared (FTIR) spectroscopy is a well-established technique for the characterisation of surfaces and the monitoring of interactions between an adsorbate and an adsorbent [1]. Information regarding adsorption sites can be elucidated through the use of probe molecules. For example, ammonia and pyridine have been used to determine the nature of acid sites on oxide surfaces [2,3], while CO and NO are routinely employed to investigate supported-metals [4]. In-situ FTIR spectroscopy permits changes to a catalyst surface (e.g. coverage) to be followed as a function of reaction variables such as time, pressure, temperature or reagent composition. In this respect, FTIR spectroscopy is an excellent tool since it is a non-destructive technique which works well under more demanding reaction conditions (i.e., elevated pressures and temperatures). However, there are some limitations including slow scan rates of the instrument relative to the very short life-times of certain surface species (although progress is being made in this area – see reference [5] and references therein), including reaction intermediates and the difficulty in distinguishing between the reactive and non-reactive (spectator) species which can reside at the surface. Additionally, although the technique is well used in obtaining qualitative data, it is somewhat limited in terms of gaining quantitative information [6]. From the Beer-Lambert law, it is well known that spectral absorbance and concentration of a particular species are connected by a constant absorptivity, or absorption coefficient, of that species at a particular wavenumber [7]. In other words, a plot of absorbance against ⁎ Corresponding author. E-mail address: [email protected] (J.A. Anderson).
http://dx.doi.org/10.1016/j.saa.2017.03.030 1386-1425/© 2017 Published by Elsevier B.V.
concentration should produce a linear relationship governed by the Beer-Lambert equation: A ¼ εcl
ð1Þ
where A is the integrated absorbance for a particular vibration, ε is the absorption coefficient, c is the concentration and l is the optical path length. Deviations are known to occur due to unexpected changes to the absorptivity caused by altering the environment (e.g. changes to concentration and hydrogen bonding) in which the analysis is being conducted. In the 1950′s and 60′s, Francis [8] and Wexler [9,10] independently attempted to derive absorption coefficients for absorption bands associated with CH3, CH2 and CH moieties using dilute solutions of hydrocarbons in liquid cells. Although these coefficients have been subsequently employed in studies of adsorbed species on solid surfaces, it is recognised that this may be inappropriate. For example, intermolecular interactions which take place in the liquid phase and are expected to modify the dipole derivative, may be completely eliminated for adsorbates which are spatially isolated on an adsorbent leading to a requirement of different absorption coefficients for the two systems. Many examples exist in literature where molar absorption coefficients have been determined for adsorbed basic probe molecule species on solid acids [6]. Emeis used pyridine in order to determine absorption coefficients and subsequently densities of Lewis and Brønsted sites on silica/alumina samples [11]. By dosing known amounts of pyridine into a cell containing the adsorbent sample in the form of a selfsupporting disc and collecting IR spectra after each dose, absorption coefficients could be determined from the data by a least-squares regression method. Pieta et al. determined absorption coefficients for
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adsorbed pyridine, 2,4-lutidine and 2,6-lutidine using a combined FTIR/ gravimetric cell, in order to calculate the number of Lewis and Brønsted sites available on a silica-alumina sample [12]. Meunier and co-workers investigated the role which formates and carbonates, detectable by DRIFTS, play in the water-gas shift reaction using Au/Ce(La)O2 catalysts. To quantify the amount of formate species present under reaction conditions, the authors employed a calibration curve by loading known amounts of solid formate onto the catalyst support. They found a linear relationship between peak area for the ν(C\\H) stretching mode of a bidentate formate (ca. 2830 cm−1) and formate loading which allowed them to calculate the formate surface concentration [13]. Absorption coefficients have also been applied to quantify the number of available sites on supported-metal surfaces [14,15] and employed under in-situ conditions to monitor changes to surface species during reaction [16]. Few reports [17,18] have investigated the band intensities due to hydrocarbon stretching modes for adsorbed species and even fewer have attempted to calculate absorption coefficients [19]. Subbotina et al. [19] studied the adsorption of ethane on various cationic forms of zeolite Y, using simultaneous volumetric and IR measurements to calculate absorption coefficients for the bands due to CH stretching modes in the 3100–2700 cm− 1 region. In this case, the lower frequency (2880– 2810 cm−1) band was used to calculate one coefficient and the high frequency bands (3000–2900 cm−1) were grouped together to calculate another absorption coefficient. While this method was more than adequate for the aims of their study (to determine the effect of the cationic forms of zeolite Y as well as disc thickness on the absorption coefficients), the transfer of these absorption coefficients to other systems may have limitations. Whilst investigating the use of absorption coefficients for solid surfaces Morterra et al. [20] investigated the methanol/ silica system and derived absorption coefficients for the CH3 stretching modes. The authors studied the impact of methanol being either physisorbed or chemisorbed to non-porous silicas which varied in surface area and discussed various other factors which can affect the resulting coefficients. Other issues which cause difficulties in calculating absorption coefficients in heterogeneous systems include variabilities in dehydration levels and the scattering properties of differing adsorbents as well as reliable deconvolution of overlapping bands [20,21,22]. Selfsupporting discs or wafers are often used for FTIR spectroscopy in transmission mode, but variations in the thickness of these have been shown to have an effect on calculated values due to scattering [19]. Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) has been used to analyse powders and avoids the need to press discs suitable for transmission IR [21]. However, scattering and specular reflectance can induce difficulties in quantification of surface species. Sirita et al. discussed the appropriate transformation function (i.e., either Kubelka-Munk (KM) or absorbance) of the DRIFTS signal for use depending on surface coverage [22]. The authors found that KM was most appropriate when highly absorbing adsorbates were involved and where the relative reflectance (R′) was lower than ca. 60%, whereas an absorbance function was more appropriate for quantification purposes where adsorbates gave a R′ above 60%. Previous reports have employed specially designed IR cells where either volumetric or gravimetric measurements can be simultaneously conducted with the FTIR spectra collection [23,24]. In this study, a custom made thermogravimetric-infrared (TG-IR) cell was used to measure amounts of alkyl chain species adsorbed from the vapour phase while monitoring changes to the FTIR spectrum on pressed powder samples in order to determine the absorption coefficients of the asymmetric (asy) CH3 and CH2 stretching modes (ca. 2960 cm− 1 and 2925 cm−1, respectively). Reagents of varying chain length and functionality were used as adsorbates, whilst two different adsorbents were used. Results reported indicate that whilst the absolute value of an absorption coefficient varied significantly, the ratio of the CH2 to CH3 absorption coefficients were constant with a high degree of reproducibility. It was subsequently shown that this ratio can be used to determine the length of hydrocarbon species adsorbed on a surface and
it is thought that these values could be employed to follow a number of processes where hydrocarbon chain length of an adsorbed species can either increase (i.e., polymerisation or oligomerisation) or decrease (i.e., cracking) during reaction over a solid surface. 2. Experimental 2.1. Samples Two different adsorbents were used in this work; an untreated γalumina (Degussa, 100 m2 g−1) and a typical supported-metal catalyst, 10 wt% Co/Al2O3 (86 m2 g−1) using the same alumina as a support. To prepare the Co/Al2O3 sample, γ-alumina (4.52 g) was added to water (20 ml) and stirred for 20 min at room temperature. Co(NO3)2·6H2O (2.48 g) (BDH Chemicals Ltd.) was dissolved in water (2.4 ml). The resulting solution was added drop-wise to the alumina slurry over 20 min. The solution was left to stir for 40 min, with a further 10 ml of water being added to help with stirring due to the thick consistency. The mixture was left to dry gradually over 24 h. The dried sample was then ground up using a mortar and pestle and allowed to dry for another 24 h. The sample was once again ground into a fine powder before being calcined in a flow (50 ml min−1) of air at 400 °C for 4 h. 2.2. Thermogravimetric-infrared cell All analysis was carried out in a custom made TG-IR cell (Fig. 1) constructed with parts supplied by Allectra. The cell is cross-shaped with opposite facing ports terminated by CaF2 windows for transmission IR spectroscopy, and the path of the IR beam follows the red line (Fig. 1). Occupying the other ports of the cross piece is a thermocouple, gas outlet and a heating cartridge. Above and connected to the cell by glass fittings was a CI Precision MK2-M5 LM 2-01 microbalance where the adsorbent of interest was suspended and followed the path of the blue line. This cell design permitted rapid dismantling for ease of sample exchange and maintenance. 2.3. Determination of absorption coefficients In order to calculate absorption coefficients for CH3 and CH2 modes, a number of molecules which varied in both length and functionality were exposed to an adsorbent sample using the TG-IR cell, combined with a Bruker Vertex 70 FTIR spectrometer (with DLaTGS detector). The samples (either γ-alumina or Co/Al2O3) were pressed into selfsupporting discs using a 13 mm die and ca. 25 mg of the powder and placed in a sample holder which was suspended from the microbalance with the discs resting perpendicular to the FTIR beam. For each experiment, the sample was pre-treated in a flow of H2 (10 ml min−1) while the temperature was raised to 265 °C (maximum achievable temperature of the TG-IR cell) in order to dehydrate the sample as well as to facilitate the surface reduction of the cobalt. Complete reduction of the cobalt would not be expected at this temperature [25], however surface particles are assumed to have undergone reduction. After 2 h at this temperature, the cell was cooled to the required temperature for study (30, 100 or 150 °C). An initial spectrum was collected (16 scans, 4 cm−1 resolution), before exposure was initiated by injecting a hydrocarbon in liquid form (1 μl) into the stream of H2 (heated by tape to avoid condensation). The continued use of a H2 stream rather than inert gas was used to avoid fluctuations in the mass profile. A stability test (Fig. S1, supporting information) of the microbalance was conducted under equivalent conditions as employed for experiments, i.e., H2 flow (10 ml min− 1, 100 °C). A fluctuation of ca. ± 0.005 mg was observed over a 2 h period. The mass change associated with adsorption was recorded using Labweigh software at a sampling rate of 10 min−1 and spectra collected at various intervals. Spectra are displayed as difference spectra (spectra after exposure to adsorbate – sample prior to exposure). The absorption coefficient for a particular vibrational mode
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Fig. 1. Pictures of custom made TG-IR cell. Additional plan and front elevation schematics provide location of individual components.
was determined using Eq. (2):
ε¼
A:C d nT :m
3. Results 3.1. Adsorption on γ-alumina ð2Þ
where ε is the molar absorption coefficient (cm μmol−1), A is the integrated absorbance of the particular mode (cm− 1) – determined by deconvolution of the spectra following adsorption, Cd is the area of the sample disc (cm2), nT is the number of adsorbed species (μmol g−1) and m is the initial mass (g) of the sample (i.e. the mass at the time when the initial spectrum was recorded). Cd was determined using imaging software, ImageJ, which calculates an area based on a user-defined section of a photograph, with known dimensions (in pixels). Eq. (2) is often utilised in quantitative FTIR, for example in the determination of the number of Lewis and Brønsted acid sites on a solid acid catalyst [3, 26]. The asymmetric CH3 and CH2 stretching modes were choosing for investigation due to the better definition/resolution and intensity of bands due to these modes relative to, for example, the bands due to the symmetrical stretching mode of CH3 (2880 cm−1). An example of a spectrum subjected to deconvolution is shown in Fig. S2 (supporting information). Peaks were fitted for asy CH3 and CH2 stretching modes (2960 cm−1 and 2925 cm−1), sym CH3 and CH2 stretching modes (2880 cm−1 and 2850 cm− 1) and a further peak at ca. 2905 cm− 1. The additional peak at 2905 cm− 1 is in the region expected for a methine (CH) stretching mode (not present in any adsorbate used). However, in various tests (discussed later) where coverage varied over the course of a reaction, this peak did not increase or decrease in intensity relative to the bands due to the asy CH2 stretching mode and therefore it is not attributed to a product methine fragment. The inclusion of this peak for all spectra maintained consistency. The accuracy of peak fitting is integral to the tests carried out throughout this study, and therefore it is acknowledged that an element of subjectivity is involved. However, the good resolution and intensity of the spectra collected provided reasonable clarity as to when a suitable fit was obtained.
The tests described in this section involved adsorption onto untreated γ-alumina at 100 °C. For each different adsorbate, multiple tests [2– 3] using freshly pressed discs of γ-alumina are carried out to evaluate reproducibility. - 1-Octanol Fig. 2 shows the recorded mass and the resulting spectra of the CHx stretching mode region for the adsorption of 1-octanol onto γ-alumina at 100 °C. Prior to exposure (0–10 min) a relatively constant mass was
Fig. 2. Mass profile for the adsorption of 1-octanol onto γ-alumina at 100 °C. Inset shows spectra for the CHx stretching mode region (3100–2700 cm−1) collected before (spectrum a) and after (spectrum b) adsorption.
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observed and an initial spectrum (Fig. 2(a)) recorded during this time shows no features due to CHx vibrations. After injection, at t = 10 min, the mass increased rapidly for the first 100 min due to adsorption of 1-octanol. Over the next 900 min the mass slowly decreased, as the system equilibrated. A spectrum recorded at t = 1030 min (Fig. 2(b)), confirms that adsorption had occurred with bands observed at 2962, 2928, 2877 and 2855 cm− 1 which are attributed to CH3 (asy), CH2 (asy), CH3 (sym) and CH2 (sym) modes, respectively. By deconvolution of the spectra to determine the integrated band areas associated with CH3 (asy) and CH2 (asy) modes, absorption coefficients were calculated using eq. 2 (Table 1, entries 1–3). Overall, the results of 3 individual experiments are in good agreement with each other, with an average absorption coefficient of 2.61 cm μmol− 1 ± 10.4% and 0.95 cm μmol−1 ± 13.6% for the CH3 and CH2 modes, respectively. Morterra et al. [20] calculated absorption coefficients for CH3 stretching modes using a combination of the bands found in the 3100–2700 cm−1 region following the adsorption of methanol on silica. For chemisorbed methanol on silicas which were activated at 1073 K with different SAs of 50 and 300 m2 g−1, absorption coefficients of 3.6 and 8.5 cm μmol−1 were found, respectively. Subbotina et al. [19] derived absorption coefficients for ethane adsorbed on various cationic forms of zeolite Y. When grouping all C\\H stretching vibrations together, absorption coefficient values varied from 9.2–26.1 cm μmol−1 depending on the cationic form and the thickness of the wafer used. When only considering the lower frequency, fully symmetric C\\H vibration at 2878–2813 cm−1, the absorption coefficient values varied between 2.6 and 16.6 cm μmol−1. Considering the results of Morterra et al. [20] and Subbotina et al. [19], it is reasonable to say that the variation in the calculated absorption coefficients are as good, if not better, than those reported in previous studies. - 1-Hexanol and decanal
Fig. 3. FTIR spectra collected after a period of exposure to different molecules to γ-alumina and Co/Al2O3 at 100 °C.
modes as a function of increasing chain length although this does not take into account any effect that the change in functionality from a primary alcohol to an aldehyde may have. Despite variation in the individual absorption coefficient values across repeated experiments and different adsorbates, the ratio of molar absorption coefficient (i.e., ε CH2 (asy):ε CH3 (asy)) appears to be remarkably constant with a value of 0.37 ± 4.4%, obtained by combining all the data for the different adsorbates on γ-alumina.
3.2. Stability determination
Two further adsorbates, 1-hexanol and decanal were tested using γalumina as adsorbent in order to investigate the effects of changing chain length and functionality on the calculated absorption coefficients. Fig. 3 shows the resulting difference spectra following exposure to each. Differences in overall absorbance resulted from differences in the extent of adsorption (i.e., spectra are not recorded at fixed, defined coverage). However, differences are observed in the relative intensity of the CH3 (asy) and CH2 (asy) modes consistent with expectations based on the different adsorbate structure. For example, the CH3 (asy) mode is more intense in spectra of 1-hexanol than 1-octanol or decanal (Fig. 3). A summary of the calculated absorption coefficients for different adsorbates on γ-alumina are presented in Fig. 4. Error bars based on repeat experiments for each adsorbate have been included. The bar graph reveals a general shift towards lower absorption coefficients for both
In order to establish whether the ratio of the two absorption coefficients was constant with time/coverage, the ratio was calculated at different times (Fig. 5) during exposure to 1-octanol on γ-alumina. After 33 min, data obtained from the first spectrum gave a ratio of 0.39 for the molar absorption coefficients, CH2:CH3. This ratio decreased to 0.37 after 68 min but thereafter remained constant up to 1032 min. Equivalent tests were conducted with 1-hexanol, 1-pentanol, 1-butanol and butanal (data not shown). These tests produced data showing constant CH2:CH3 ratio of the molar absorption coefficients with time in the cases of 1-hexanol and butanal as adsorbates. However in the cases of 1pentanol and 1-butanol as adsorbates, the value for the ratio of the molar absorption coefficients decreased with contact time/coverage, which may be indicative of gradual transformation of these adsorbates.
Table 1 Calculated molar absorption coefficients from the adsorption of 1-octanol at 30, 100 and 150 °C and the εCH2:εCH3 ratio using either γ-alumina or Co/Al2O3 as adsorbents. Test #
Adsorbent Temperature/°C εCH3 (cm μmol−1)
εCH2 (cm μmol−1)
εCH2:εCH3
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
γ-alumina γ-alumina γ-alumina Co/Al2O3 Co/Al2O3 Co/Al2O3 Co/Al2O3 Co/Al2O3 Co/Al2O3 Co/Al2O3 Co/Al2O3 Co/Al2O3 Co/Al2O3 Co/Al2O3 Co/Al2O3
1.07 1.01 0.77 0.77 1.35 0.79 0.94 0.04 0.04 0.80 0.32 0.91 0.95 1.29 1.23
0.37 0.37 0.34 0.34 0.34 0.35 0.37 0.36 0.40 0.40 0.37 0.38 0.38 0.39 0.38
100 °C 100 °C 100 °C 100 °C 100 °C 100 °C 100 °C 30 °C 30 °C 30 °C 30 °C 30 °C 150 °C 150 °C 150 °C
2.88 2.72 2.24 2.24 4.01 2.28 2.53 0.11 0.10 2.01 0.86 2.34 2.52 3.32 3.21
Fig. 4. Calculated molar absorption coefficients for CH3 (asy) and CH2 (asy) using γalumina as adsorbent. The ratios (CH2:CH3) of the calculated molar absorption coefficients are also displayed and correspond to the right hand y-axis.
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consistent. In this case, for adsorption on Co/Al2O3, the value was determined to be 0.36 ± 6.9% which is comparable to the value (0.37 ± 4.4%) determined using γ-alumina.
3.4. Temperature effect
Fig. 5. Mass profile following exposure of 1-octanol to γ-alumina at 100 °C. Inset shows difference spectra (CHx vibrational mode region) collected before and after adsorption. In brackets are the calculated absorption coefficient ratio (CH2:CH3) calculated at the time indicated.
The influence of temperature on absorption coefficients was studied using 1-octanol on Co/Al2O3 as an example. These studies were carried out at temperatures above (150 °C) and below (30 °C) the previously described experiments conducted at 100 °C. The experiment conducted at 30 °C was repeated 5 times in total (Table 1, entries 8–12) while 3 tests were carried out at 150 °C (Table 1, entries 13–15). Considerable variation was observed between some of the repeat experiments at 30 °C. Tests 8 and 9 are very comparable with each other but are significantly different to tests 10 and 12. At an adsorption temperature of 150 °C, the calculated coefficients were much closer to those calculated at 100 °C and the variation between the repeat tests was far less than observed at 30 °C. However, at all adsorption temperatures, a much more consistent ε CH2 (asy):ε CH3 (asy) ratio was apparent.
3.5. Origin of errors 3.3. Adsorption on Co/Al2O3 A two component system, Co/Al2O3, containing both support and supported phase was also assessed. Fig. 3 displays the difference spectrum collected for the adsorption of 1-octanol onto Co/Al2O3 at 100 °C. The spectrum is very similar to the one collected for 1-octanol on γ-alumina with minimal change in peak positions and CH2:CH3 band area suggesting the inability to discriminate between adsorption on the two components and/or predominance of features due to adsorption on the support. The calculated absorption coefficients for a series of 4 experiments are shown in Table 1 (entries 4–7). Test numbers 4, 6 and 7 are in good agreement whilst test 5 appears to be an outlier. Nevertheless when including all test data, the absorption coefficient ratio remains remarkably consistent. Three additional adsorbates (1-decanol, decanal and octanol) were selected to calculate absorption coefficients using the Co/Al2O3 adsorbent and these are displayed in Fig. 6. Variation in the average value for each adsorbate is observed, but no trend was apparent which would link coefficient values with either chain length or functionality. The average calculated molar absorption coefficients, combining all data are 3.01 cm μmol−1 ± 19.0% and 1.08 cm μmol−1 ± 20.1% for CH3 (asy) and CH2 (asy) modes, respectively. Once again, however, the ratio of the coefficients was observed to be far more
Fig. 6. Calculated molar absorption coefficients for CH3 (asy) and CH2 (asy) using Co/Al2O3 as adsorbent. The ratios (CH2:CH3) of the calculated molar absorption coefficients are also displayed and correspond to the right hand y-axis.
The molar absorption coefficients are derived from Eq. (2) which can be split into two component parts. Cd/m is a representation for the thickness of the disc used. In each case, uniform thickness across the disc is assumed. Discs were pressed with a starting mass of ca. 25 mg each time using 13 mm die and consistency of thickness would infer that data should fall close to the line in Fig. 7 (numbers on the graph relate to the test number in Table 1). This was not always the case as shown with samples 5 and 12 and hence, this could contribute to some error in the calculated coefficient. The second part of Eq. (2), (A/ nT) concerns the concentration of a particular species and the peak area associated with that species. For these examples, the integrated peak area of the CH3 (asy) stretching mode and the amount of adsorbed methyl (μmol g−1) has been plotted (Fig. 8). A linear relationship is expected although this is not apparent for the set of results obtained for adsorbent at 30 °C (Fig. 8a) and hence the calculated absorption coefficients are not constant. However, for the series of tests carried out at 100 and 150 °C, a trend more consistent with expectation was observed in the data (Fig. 8b) and subsequently less variation was found for the molar absorption coefficients determined under these conditions.
Fig. 7. Disc area vs disc mass for Co/Al2O3 discs used for adsorption of 1-octanol at 30, 100 and 150 °C. Numbers refer to test numbers in Table 1.
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γ-alumina in individual experiments at 100 °C. Table 2 compares the results determined from FTIR using the εCH2:εCH3 ratio and the actual molar ratio of the injected liquid. In all cases the calculated value is relatively close to the average CH2:CH3 ratio value of injected liquid, although notably in all three cases, the calculated value is lower than the value predicted based on the liquid composition. 4. Discussion
Fig. 8. Integrated peak area for CH3 (asy) mode vs adsorbed amount of CH3 species on Co/ Al2O3 discs at a) 30 °C and b) results for tests at 100 and 150 °C. The numbers on the graphs relate to the test numbers in Table 1.
3.6. Mixed solution test on γ-alumina Since the ratio of molar absorption coefficients, CH2:CH3, appears consistent across multiple tests using various chain lengths and functional groups (excluding shorter chain alcohols) and both adsorbents tested, it should be possible to determine the average chain length for a mixture of adsorbed species on e.g., γ-alumina using the following equation: Average CH 2 : CH3 ratio of adsorbed species ¼
ACH2 =ACH
3
εCH 2 : εCH3
ð3Þ
where ACHx is the integrated absorbance for a particular mode. A molar absorption coefficient ratio of 0.37 was used. In order to test this hypothesis, three different solutions containing 1-hexanol and 1-decanol with different molar ratios were exposed to
Table 2 Actual and calculated CH2:CH3 ratios for solutions of 1-hexanol and 1-decanol adsorbed on γ-alumina at 100 °C. Molar ratio (Hex:Dec)
Actual CH2:CH3
Calculated CH2:CH3
2:1 1:1 1:2
6.33 7 7.67
6.08 6.65 7.55
Molar absorption coefficients for the CH3 (asy) and CH2 (asy) modes have been determined using a custom made TGIR cell. The individual absorption coefficients determined for molecules of differing chain lengths and functionalites using γ-alumina as adsorbent were shown to vary considerably (Fig. 4), while much more invariant values were observed using varying hydrocarbon chain lengths and functionalities adsorbed on a Co/Al2O3 adsorbent (Fig. 6). The reason for a trend using one adsorbent but not the other is difficult to explain considering that the majority of the adsorption using the Co/Al2O3 adsorbent is expected to take place on the more abundant alumina support material. However, it should be noted that the two materials have different SAs (100 and 86 m2 g− 1 for γ-alumina and Co/Al2O3, respectively) and this may give rise to different light scattering properties although this does not necessarily explain a trend occurring across one sample and not the other. Despite these discrepancies, a constant ε CH2 (asy):ε CH3 (asy) ratio was observed; 0.37 ± 4.4% using the γ-alumina adsorbent and 0.36 ± 6.9% using the Co/Al2O3 adsorbent. The effect of adsorption temperature on the derived absorption coefficients was studied using 1-octanol as an adsorbate and Co/Al2O3 as the adsorbent (Table 1, entries 4–15). Significant variation was observed for the coefficients determined at 30 °C (Table 1, entries 8–12) and the origins of this discrepancy can be elucidated using data plotted in Figs. 7 and 8. Disc thickness in transmission FTIR has been shown to effect the consistency with which absorption coefficients can be calculated [19]. Subbotina et al. [19], observed an increasing absorption coefficient with increasing disc thickness and although the authors comment that this should not be the case, they attributed this trend to the increasing light-scattering characteristics of the thicker discs. They stated that their most reliable results were obtained using wafers of density 7– 9 mg/cm2. In comparison, the discs used in this study were much thicker (≈17–18 mg/cm2). In this work, to limit the influence that disc thickness could have on the calculated molar absorption coefficients, an attempt was made to keep the thickness of each individual disc as constant as possible (Fig. 7), but it can be said that any variation in disc thickness that was apparent, had little or no effect on the ε CH2 (asy):ε CH3 (asy) ratio. If both disc thickness (Fig. 7) and the integrated peak area (CH3 asy) versus amount of methyl adsorbed plots (Fig. 8a) at 30 °C, along with the calculated coefficients in Table 1 (entries 8–12), are considered, it is reasonable to suggest that disc thickness is not the only reason for the variability in the resulting individual molar absorption coefficients. For example, tests 10 and 12, are based on discs of greatest difference in thickness (for adsorption tests at 30 °C), but are comparable in terms of the absorption coefficient determined. Instead, the reason for the variability may be a consequence of the low temperature at which the adsorption was conducted. The plot of peak area for the band due to the CH3 (asy) mode versus the amount of CH3 uptake at 30 °C (Fig. 8a), shows no linearity. When the adsorption temperature was increased to 100 and 150 °C, again an effort was made to keep disc thickness as constant as possible, (Fig. 7) a linear relationship was observed between CH3 (asy) peak area and amount adsorbed (Fig. 8b). Accordingly, the calculated absorption coefficients appear more consistent in comparison to the series of measurements conducted at 30 °C. This could be attributed to a proportion of the species being in a physisorbed state at 30 °C whilst at higher temperatures of 100 and 150 °C the species could be in a more dispersed, chemisorbed state which reduced the extent of intermolecular interactions and therefore gives a truer spectral representation of an adsorbed species. Over the
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three different adsorption temperatures studied, a constant ε CH2 (asy):ε CH3 (asy) ratio was displayed once again (Table 1, entries 4– 15). One other factor which should be considered but was not assessed in any detail here was the possible impact of coke or hydrocarbonaceous deposits on a surface, as these may influence the light absorption and scattering characteristics of the adsorbent. These residues are also likely to perturb coefficients due to the low H:C ratios of these species. In the open literature, there are very few instances where molar absorption coefficients for the modes of CH3 and CH2 have been determined. Wexler [10] and Francis [8] independently carried out studies aimed at determining the integrated intensities of absorption bands for diluted, liquid phase aliphatic hydrocarbons. Wexler calculated coefficients of 4.46 and 3.74 cm μmol−1 for CH3 and CH2 modes, respectively, and compared these results to those of Francis, who obtained values for CH3 and CH2 modes of 3.85 and 3.31 cm μmol−1 although it is unclear in either case whether these are for the asymmetric or symmetric stretching modes. In this study, absorption coefficient values were determined as 3.01 cm μmol−1 ± 19.0% and 1.08 cm μmol− 1 ± 20.1% for CH3 (asy) and CH2 (asy) modes, respectively using a Co/Al2O3 adsorbent (Fig. 6). Absorption coefficients for species in a liquid phase are expected to be larger than their respective adsorbed form, since a more rigid like structure would be apparent, resulting in less of a vibrational response. In comparing the work of Wexler [10] and Francis [8] to this study, the absorption coefficients for chemisorbed hydrocarbon species (Table 1, entries 1–7 and 13–15) are indeed lower than those in the liquid phase, although this does not take into account the differences in methodology for integrating the spectrum. However, notably, some of the derived absorption coefficients in this study which are assigned to more liquid-like, physisorbed species (Table 1, entries 8–12), resulted in a smaller value than the chemisorbed equivalent (Table 1, entries 1–7 and 13–15). In an investigation by the group of Morterra [20], involving the methanol/silica system, the absorption coefficient of a CH3 mode for liquid phase methanol and adsorbed methanol (both chemisorbed or physisorbed) on silica are discussed and although the values differed, they were of the same order of magnitude. On a silica sample with a surface area of 50 m2 g−1 and activated at 1073 K, they found a lower absorption coefficient value for chemisorbed methanol (3.6 cm μmol− 1) than the physisorbed equivalent (11 cm μmol− 1). However, on a higher surface area sample (300 m2 g− 1) the physisorbed species exhibited a lower absorption coefficient (5.4 cm μmol−1) than the chemisorbed example (8.5 cm μmol−1). The authors discussed some of the limitations in transferring absorption coefficients from one system to another which can include sample thickness, surface dehydration levels of the adsorbent and scattering properties of the adsorbent. In this work, however, we have shown, using two different sample adsorbents (with different surface areas, scattering characteristics and disc thicknesses) and using multiple adsorbates, an apparently constant absorption coefficient ratio, ε CH2 (asy):ε CH3 (asy), which may have more widespread applicability. Using the values quoted by Wexler [10] and Francis [8], a ratio (CH2:CH3) of their coefficients give 0.84 and 0.86, respectively. In this work, ratios of 0.36 ± 6.9% (Co/Al2O3) and 0.37 ± 4.4% (γ-alumina) were determined or collectively a value of 0.36 ± 6.6% is found. The reason for this difference could again be attributed to the use of diluted solutions of hydrocarbons by Wexler and Francis to calculate their coefficients compared to a dispersed, adsorbed alkyl species employed here. A range of different adsorbates on γ-alumina were tested to assess for changes to this ratio during adsorption and after steady state had been reached for the amount adsorbed (as indicated by the mass profile). Molar absorption coefficient ratios for 1-octanol (Fig. 5), 1-hexanol and butanal were close to the average value (0.37) and remained within the typical error range (± 4.4%) at all times. The use of 1-pentanol and 1-butanol as adsorbates resulted in an initial (at a point of increasing mass) molar absorption coefficient ratio which was consistent with this average value. However, a decrease in mass after a maximum was attained, lead to a decreasing absorption coefficient ratio which could
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be interpreted in terms of a subsequent transformation of the adsorbed alkoxide. In the carboxylate region (1800–1200 cm−1) of the spectrum (not shown) bands at 1565 and 1440 cm−1 were observed which increased in intensity as a function of exposure time for all adsorbates used (irrespective of chain length), and the changes do not follow changes in the mass profile. This is in contrast to the intensity of the bands observed in the hydrocarbon region (3100–2700 cm−1) which increase and decrease in intensity in parallel with changes to mass (Fig. 5) and result in greater consistency in the absorption coefficients as a function of time/coverage. Knozinger and Stübner [27] reported the appearance of bands at 1565 and 1435 cm−1 following exposure of isobutyl alcohol to η-alumina due to formation of surface carboxylate species. They state that the intensity of these bands did not depend directly on the total surface coverage, consistent with our observations here, where the mass and band intensity did not vary in parallel. Therefore it is difficult to employ differences in the reactivity of short and long chained species in this study to explain variation in molar absorption coefficients for the bands due to CH stretching modes. One manner of testing the value of the coefficient ratio is if it could be employed to determine the average chain length of a group of known hydrocarbon species which contain both CH3 and CH2 groups. To explore this potential, the vapour from a liquid mixture containing two different hydrocarbon species 1-hexanol and 1-decanol, (different chain lengths, same functionality), was exposed to γ-alumina, and the resulting spectra assessed and evaluated (Eq. (3)), using an absorption coefficient ratio value of 0.37. The derived data in Table 2 compares well with the actual ratio of the alcohol species injected although in each case, the calculated value was slightly less than the corresponding predicted average ratio. The predicted value is based on the assumption that both alcohols are adsorbed to the same extent and are present at identical surface coverages. This difference therefore may be attributed to the relative coverage of the two species resulting from differences in reactivity based on the decreasing order of acidity (pKa value increases) as the number of alkyl groups in the chain increase [28]. Therefore, 1hexanol is expected to be more reactive than 1-decanol with respect to forming an adsorbed alkoxide species leading to a relatively higher abundance (surface coverage) of this species and leading to the calculated ratio being lower than the value expected based on the composition of the liquid phase. The use an average CH2:CH3 ratio (Eq. (3)) appears to be robust and overcomes many of the challenges raised by Morterra et al. [20] associated with applying and transferring molar absorption coefficients from one system to another since consistency of the ratio is observed over a range of adsorbates, with two different adsorbents which exhibit different surface areas, as well as different scattering properties. The fact that this ratio remains consistent suggests that there is potential to apply this to other systems, including a potential application in surface reactions which can be monitored by FTIR and where hydrocarbon chains vary in length during the course of a reaction (i.e. a changing CH2:CH3 ratio). Examples could include catalytic processes such as cracking, coupling, as well as polymerisation and oligomerisation reactions. 5. Conclusions A custom made TG-IR cell was implemented in order to determine absorption coefficients for the CH3 (asy) and CH2 (asy) stretching modes of adsorbed species. Some variation in the calculated, individual molar absorption coefficients was observed when using various adsorbates of differing length and using γ-alumina as an adsorbent, but more consistency in the values was observed when using a Co/Al2O3 adsorbent. At adsorption temperatures of 100 and 150 °C a more uniform set of molar absorption coefficients were observed compared to adsorption at 30 °C (using Co/Al2O3) and this is ascribed to the presence of additional physisorbed species for the lower temperature. A constant CH2:CH3 absorption coefficient ratio was found across multiple tests using different adsorbates, two different adsorbents and multiple
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adsorption temperatures. This value has been shown to be applicable to calculating the average CH2:CH3 ratio when surface coverage includes more than one adsorbate. There is potential to apply this methodology to determine average chain lengths of species generated at surfaces using in-situ FTIR, where hydrocarbon chain lengths vary as a function of reaction parameters and/or choice of catalyst. Acknowledgements We thank the Royal Society London for funding the initial prototype of the FTIR/Balance cell through the Paul Instrument Fund, the EU through the ERASMUS scheme (To T. H), and to research students (E. Mitchell) and PDRAs (D. Rosenberg) who were involved in contributions to the design and implementation of the cell. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.saa.2017.03.030. References [1] J. Ryczkowski, IR spectroscopy in catalysis, Catal. Today 68 (2001) 263–381. [2] J.B. Peri, Infrared study of adsorption of ammonia on dry γ-alumina, J. Phys. Chem. 69 (1965) 231–239. [3] D.J. Rosenberg, J.A. Anderson, On determination of acid site densities on sulphated oxides, Catal. Lett. 83 (2002) 59–63. [4] P. Ramamoorthy, R.D. Gonzalez, Surface characterization of supported Pt-Ru bimetallic clusters using infrared spectroscopy, J. Catal. 58 (1979) 188–197. [5] A. Davó-Quiñonero, A. Bueno-López, Lozano-Castelló, A.J. McCue, J.A. Anderson, Rapid-scan operando infrared spectroscopy, ChemCatChem 8 (2016) 1905–1908. [6] A.J. McCue, G.A. Mutch, A.I. McNab, S. Campbell, J.A. Anderson, Quantitative determination of surface species and adsorption sites using infrared spectroscopy, Catal. Today 259 (2015) 19–26. [7] N.B. Colthup, L.H. Daly, S.E. Wiberley, Introduction to Infrared and Raman Spectroscopy, third ed. Academic Press, Inc., London, 1990. [8] S.A. Francis, Absolute intensities of characteristic infra-red absorption bands of aliphatic hydrocarbons, J. Chem. Phys. 18 (1950) 861–865. [9] A.S. Wexler, Integrated intensities of absorption bands in infrared spectroscopy, Appl. Spectrosc. Rev. 1 (1967) 29–98. [10] A.S. Wexler, Infrared determination of structural units in organic compounds by integrated intensity measurements: alkanes alkenes and monosubstituted alkyl benzenes, Spectrochim. Acta 21 (1965) 1725–1742. [11] C.A. Emeis, Determination of integrated molar extinction coefficients for absorption bands of pyridine adsorbed on solid acid catalysts, J. Catal. 141 (1993) 347–354.
[12] I.S. Pieta, M. Ishaq, R.P.K. Wells, J.A. Anderson, Quantitative determination of acid sites on silica-alumina, Appl. Catal. A 390 (2010) 127–134. [13] F.C. Meunier, D. Reid, A. Goguet, S. Shekhtman, C. Hardacre, R. Burch, W. Deng, M. Flytzani-Stephanopoulos, Quantitative analysis of the reactivity of formate species seen by DRIFTS over a Au/Ce(La)O2 water-gas shift catalyst: first unambiguous evidence of the minority role of formats as reaction intermediates, J. Catal. 247 (2007) 277–287. [14] M.A. Vannice, S.-Y. Wang, Determination of IR extinction coefficients for linear- and bridge-bonded CO on supported palladium, J. Phys. Chem. 85 (1981) 2543–2546. [15] A.J. McCue, J.A. Anderson, CO induced surface segregation as a means of improving surface composition and enhancing performance of CuPd bimetallic catalysts, J. Catal. 329 (2015) 538–546. [16] A.J. McCue, J. Aponaviciute, R.P.K. Wells, J.A. Anderson, Gold modified cobalt-based Fischer-Tropsch catalysts for conversion of synthesis gas to liquid fuels, Front. Chem. Sci. Eng. 7 (2013) 262–269. [17] V.B. Kazansky, I.R. Subbotina, R. Schlögl, F.C. Jentoft, Intensities of C\ \H IR stretching bands of ethane and propane adsorbed by zeolites as a new spectral criterion of their chemical activation via polarization resulting from stretching of chemical bonds, J. Phys. Chem. B 110 (2006) 17468–17477. [18] V.B. Kazansky, I.R. Subbotina, F. Jentoft, Intensities of combination IR bands as an indication of the concerted mechanism of proton transfer from acidic hydroxyl groups in zeolites to the ethylene hydrogen-bonded by protons, J. Catal. 240 (2006) 66–72. [19] I.R. Subbotina, V.B. Kazansky, J. Kröhnert, F.C. Jentoft, Intergral absorption coefficients of C\ \H stretching bands in IR spectra of ethane adsorbed by cationic forms of Y zeolite, J. Phys. Chem. A 113 (2009) 839–844. [20] C. Morterra, G. Magnacca, V. Bolis, On the critical use of molar absorption coefficients for adsorbed species: the methanol/silica system, Catal. Today 70 (2001) 43–58. [21] F.C. Meunier, A. Goguet, C. Hardacre, R. Burch, D. Thompsett, Quantitative DRIFTS investigation of possible reaction mechanisms for the water-gas shift reaction on high-activity Pt- and Au-based catalysts, J. Catal. 252 (2007) 18–22. [22] J. Sirita, S. Phanichphant, F.C. Meunier, Quantitative analysis of adsorbate concentrations by diffuse reflectance FT-IR, Anal. Chem. 79 (2007) 3912–3918. [23] E. Roze, P. Gravejat, E. Quniet, J.L. Rousset, D. Bianchi, Impact of the reconstruction of gold particles on the heats of adsorption of linear CO species adsorbed on the Au sites of a 1% Au/Al2O3 catalyst, J. Phys. Chem. C 113 (2009) 1037–1045. [24] C.M. Grill, M.L. McLaughlin, J.M. Stevenson, R.D. Gonzalez, Surface characterization of supported Pt-Pd bimetallic clusters using infrared spectroscopy, J. Catal. 69 (1981) 454–464. [25] G. Jacobs, Y. Ji, B.H. Davis, D. Cronauer, A.J. Kropf, C.L. Marschall, Fischer–Tropsch synthesis: temperature programmed EXAFS/XANES investigation of the influence of support type, cobalt loading, and noble metal promoter addition to the reduction behavior of cobalt oxide particles, Appl. Catal. A Gen. 333 (2007) 177–191. [26] J.A. Anderson, C. Fergusson, I. Rodríquez-Ramos, A. Guerrero-Ruiz, Influence of Si/Zr ratio on the formation of surface acidity in silica-zirconia aerogels, J. Catal. 192 (2000) 344–354. [27] H. Knozinger, B. Stübner, Adsorption of alcohols on alumina. 1. Gravimetric and infrared spectroscopic investigation, J. Phys. Chem. 82 (1978) 1526–1532. [28] W.H. Brown, B.L. Iverson, E.V. Anslyn, C.S. Foote, Organic Chemistry, seventh ed. Cengage Learning, 2013.
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Quantification of hydrocarbon species on surfaces by combined microbalance-FTIR Andrew I. McNab, Tom Heinze, Alan J. McCue, Davide Dionisi, James A. Anderson ⁎ Materials and Chemical Engineering Group, School of Engineering, University of Aberdeen, Aberdeen AB24 3UE, UK
a r t i c l e
i n f o
Article history: Received 15 August 2016 Received in revised form 13 January 2017 Accepted 13 March 2017 Available online 16 March 2017 Keywords: Quantification FTIR Adsorbed hydrocarbons Fischer-Tropsch catalyst Absorption coefficient
a b s t r a c t Absorption coefficients for the asymmetric stretching modes of CH3 and CH2 groups formed by adsorbing alkyl chained species from the vapour phase onto two different adsorbents; a γ-alumina support material and a supported metal catalyst have been determined using a custom made thermogravimetric-infrared cell. Results show that despite variations in the individually calculated absorption coefficients (ca. ±20%), the ratio of the absorption coefficients (CH2:CH3) remained consistent despite employing adsorbates of varying chain length and functionality, and despite the choice of adsorbents which exhibited different surface areas and light scattering characteristics. The use of this absorption coefficient ratio has been shown to be applicable in the quantification of the average chain length of multiple adsorbed species of differing chain length. The potential for applying this to scenarios where reactions on surfaces are monitored is discussed. © 2017 Published by Elsevier B.V.
1. Introduction Fourier Transform Infrared (FTIR) spectroscopy is a well-established technique for the characterisation of surfaces and the monitoring of interactions between an adsorbate and an adsorbent [1]. Information regarding adsorption sites can be elucidated through the use of probe molecules. For example, ammonia and pyridine have been used to determine the nature of acid sites on oxide surfaces [2,3], while CO and NO are routinely employed to investigate supported-metals [4]. In-situ FTIR spectroscopy permits changes to a catalyst surface (e.g. coverage) to be followed as a function of reaction variables such as time, pressure, temperature or reagent composition. In this respect, FTIR spectroscopy is an excellent tool since it is a non-destructive technique which works well under more demanding reaction conditions (i.e., elevated pressures and temperatures). However, there are some limitations including slow scan rates of the instrument relative to the very short life-times of certain surface species (although progress is being made in this area – see reference [5] and references therein), including reaction intermediates and the difficulty in distinguishing between the reactive and non-reactive (spectator) species which can reside at the surface. Additionally, although the technique is well used in obtaining qualitative data, it is somewhat limited in terms of gaining quantitative information [6]. From the Beer-Lambert law, it is well known that spectral absorbance and concentration of a particular species are connected by a constant absorptivity, or absorption coefficient, of that species at a particular wavenumber [7]. In other words, a plot of absorbance against ⁎ Corresponding author. E-mail address: [email protected] (J.A. Anderson).
http://dx.doi.org/10.1016/j.saa.2017.03.030 1386-1425/© 2017 Published by Elsevier B.V.
concentration should produce a linear relationship governed by the Beer-Lambert equation: A ¼ εcl
ð1Þ
where A is the integrated absorbance for a particular vibration, ε is the absorption coefficient, c is the concentration and l is the optical path length. Deviations are known to occur due to unexpected changes to the absorptivity caused by altering the environment (e.g. changes to concentration and hydrogen bonding) in which the analysis is being conducted. In the 1950′s and 60′s, Francis [8] and Wexler [9,10] independently attempted to derive absorption coefficients for absorption bands associated with CH3, CH2 and CH moieties using dilute solutions of hydrocarbons in liquid cells. Although these coefficients have been subsequently employed in studies of adsorbed species on solid surfaces, it is recognised that this may be inappropriate. For example, intermolecular interactions which take place in the liquid phase and are expected to modify the dipole derivative, may be completely eliminated for adsorbates which are spatially isolated on an adsorbent leading to a requirement of different absorption coefficients for the two systems. Many examples exist in literature where molar absorption coefficients have been determined for adsorbed basic probe molecule species on solid acids [6]. Emeis used pyridine in order to determine absorption coefficients and subsequently densities of Lewis and Brønsted sites on silica/alumina samples [11]. By dosing known amounts of pyridine into a cell containing the adsorbent sample in the form of a selfsupporting disc and collecting IR spectra after each dose, absorption coefficients could be determined from the data by a least-squares regression method. Pieta et al. determined absorption coefficients for
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adsorbed pyridine, 2,4-lutidine and 2,6-lutidine using a combined FTIR/ gravimetric cell, in order to calculate the number of Lewis and Brønsted sites available on a silica-alumina sample [12]. Meunier and co-workers investigated the role which formates and carbonates, detectable by DRIFTS, play in the water-gas shift reaction using Au/Ce(La)O2 catalysts. To quantify the amount of formate species present under reaction conditions, the authors employed a calibration curve by loading known amounts of solid formate onto the catalyst support. They found a linear relationship between peak area for the ν(C\\H) stretching mode of a bidentate formate (ca. 2830 cm−1) and formate loading which allowed them to calculate the formate surface concentration [13]. Absorption coefficients have also been applied to quantify the number of available sites on supported-metal surfaces [14,15] and employed under in-situ conditions to monitor changes to surface species during reaction [16]. Few reports [17,18] have investigated the band intensities due to hydrocarbon stretching modes for adsorbed species and even fewer have attempted to calculate absorption coefficients [19]. Subbotina et al. [19] studied the adsorption of ethane on various cationic forms of zeolite Y, using simultaneous volumetric and IR measurements to calculate absorption coefficients for the bands due to CH stretching modes in the 3100–2700 cm− 1 region. In this case, the lower frequency (2880– 2810 cm−1) band was used to calculate one coefficient and the high frequency bands (3000–2900 cm−1) were grouped together to calculate another absorption coefficient. While this method was more than adequate for the aims of their study (to determine the effect of the cationic forms of zeolite Y as well as disc thickness on the absorption coefficients), the transfer of these absorption coefficients to other systems may have limitations. Whilst investigating the use of absorption coefficients for solid surfaces Morterra et al. [20] investigated the methanol/ silica system and derived absorption coefficients for the CH3 stretching modes. The authors studied the impact of methanol being either physisorbed or chemisorbed to non-porous silicas which varied in surface area and discussed various other factors which can affect the resulting coefficients. Other issues which cause difficulties in calculating absorption coefficients in heterogeneous systems include variabilities in dehydration levels and the scattering properties of differing adsorbents as well as reliable deconvolution of overlapping bands [20,21,22]. Selfsupporting discs or wafers are often used for FTIR spectroscopy in transmission mode, but variations in the thickness of these have been shown to have an effect on calculated values due to scattering [19]. Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) has been used to analyse powders and avoids the need to press discs suitable for transmission IR [21]. However, scattering and specular reflectance can induce difficulties in quantification of surface species. Sirita et al. discussed the appropriate transformation function (i.e., either Kubelka-Munk (KM) or absorbance) of the DRIFTS signal for use depending on surface coverage [22]. The authors found that KM was most appropriate when highly absorbing adsorbates were involved and where the relative reflectance (R′) was lower than ca. 60%, whereas an absorbance function was more appropriate for quantification purposes where adsorbates gave a R′ above 60%. Previous reports have employed specially designed IR cells where either volumetric or gravimetric measurements can be simultaneously conducted with the FTIR spectra collection [23,24]. In this study, a custom made thermogravimetric-infrared (TG-IR) cell was used to measure amounts of alkyl chain species adsorbed from the vapour phase while monitoring changes to the FTIR spectrum on pressed powder samples in order to determine the absorption coefficients of the asymmetric (asy) CH3 and CH2 stretching modes (ca. 2960 cm− 1 and 2925 cm−1, respectively). Reagents of varying chain length and functionality were used as adsorbates, whilst two different adsorbents were used. Results reported indicate that whilst the absolute value of an absorption coefficient varied significantly, the ratio of the CH2 to CH3 absorption coefficients were constant with a high degree of reproducibility. It was subsequently shown that this ratio can be used to determine the length of hydrocarbon species adsorbed on a surface and
it is thought that these values could be employed to follow a number of processes where hydrocarbon chain length of an adsorbed species can either increase (i.e., polymerisation or oligomerisation) or decrease (i.e., cracking) during reaction over a solid surface. 2. Experimental 2.1. Samples Two different adsorbents were used in this work; an untreated γalumina (Degussa, 100 m2 g−1) and a typical supported-metal catalyst, 10 wt% Co/Al2O3 (86 m2 g−1) using the same alumina as a support. To prepare the Co/Al2O3 sample, γ-alumina (4.52 g) was added to water (20 ml) and stirred for 20 min at room temperature. Co(NO3)2·6H2O (2.48 g) (BDH Chemicals Ltd.) was dissolved in water (2.4 ml). The resulting solution was added drop-wise to the alumina slurry over 20 min. The solution was left to stir for 40 min, with a further 10 ml of water being added to help with stirring due to the thick consistency. The mixture was left to dry gradually over 24 h. The dried sample was then ground up using a mortar and pestle and allowed to dry for another 24 h. The sample was once again ground into a fine powder before being calcined in a flow (50 ml min−1) of air at 400 °C for 4 h. 2.2. Thermogravimetric-infrared cell All analysis was carried out in a custom made TG-IR cell (Fig. 1) constructed with parts supplied by Allectra. The cell is cross-shaped with opposite facing ports terminated by CaF2 windows for transmission IR spectroscopy, and the path of the IR beam follows the red line (Fig. 1). Occupying the other ports of the cross piece is a thermocouple, gas outlet and a heating cartridge. Above and connected to the cell by glass fittings was a CI Precision MK2-M5 LM 2-01 microbalance where the adsorbent of interest was suspended and followed the path of the blue line. This cell design permitted rapid dismantling for ease of sample exchange and maintenance. 2.3. Determination of absorption coefficients In order to calculate absorption coefficients for CH3 and CH2 modes, a number of molecules which varied in both length and functionality were exposed to an adsorbent sample using the TG-IR cell, combined with a Bruker Vertex 70 FTIR spectrometer (with DLaTGS detector). The samples (either γ-alumina or Co/Al2O3) were pressed into selfsupporting discs using a 13 mm die and ca. 25 mg of the powder and placed in a sample holder which was suspended from the microbalance with the discs resting perpendicular to the FTIR beam. For each experiment, the sample was pre-treated in a flow of H2 (10 ml min−1) while the temperature was raised to 265 °C (maximum achievable temperature of the TG-IR cell) in order to dehydrate the sample as well as to facilitate the surface reduction of the cobalt. Complete reduction of the cobalt would not be expected at this temperature [25], however surface particles are assumed to have undergone reduction. After 2 h at this temperature, the cell was cooled to the required temperature for study (30, 100 or 150 °C). An initial spectrum was collected (16 scans, 4 cm−1 resolution), before exposure was initiated by injecting a hydrocarbon in liquid form (1 μl) into the stream of H2 (heated by tape to avoid condensation). The continued use of a H2 stream rather than inert gas was used to avoid fluctuations in the mass profile. A stability test (Fig. S1, supporting information) of the microbalance was conducted under equivalent conditions as employed for experiments, i.e., H2 flow (10 ml min− 1, 100 °C). A fluctuation of ca. ± 0.005 mg was observed over a 2 h period. The mass change associated with adsorption was recorded using Labweigh software at a sampling rate of 10 min−1 and spectra collected at various intervals. Spectra are displayed as difference spectra (spectra after exposure to adsorbate – sample prior to exposure). The absorption coefficient for a particular vibrational mode
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Fig. 1. Pictures of custom made TG-IR cell. Additional plan and front elevation schematics provide location of individual components.
was determined using Eq. (2):
ε¼
A:C d nT :m
3. Results 3.1. Adsorption on γ-alumina ð2Þ
where ε is the molar absorption coefficient (cm μmol−1), A is the integrated absorbance of the particular mode (cm− 1) – determined by deconvolution of the spectra following adsorption, Cd is the area of the sample disc (cm2), nT is the number of adsorbed species (μmol g−1) and m is the initial mass (g) of the sample (i.e. the mass at the time when the initial spectrum was recorded). Cd was determined using imaging software, ImageJ, which calculates an area based on a user-defined section of a photograph, with known dimensions (in pixels). Eq. (2) is often utilised in quantitative FTIR, for example in the determination of the number of Lewis and Brønsted acid sites on a solid acid catalyst [3, 26]. The asymmetric CH3 and CH2 stretching modes were choosing for investigation due to the better definition/resolution and intensity of bands due to these modes relative to, for example, the bands due to the symmetrical stretching mode of CH3 (2880 cm−1). An example of a spectrum subjected to deconvolution is shown in Fig. S2 (supporting information). Peaks were fitted for asy CH3 and CH2 stretching modes (2960 cm−1 and 2925 cm−1), sym CH3 and CH2 stretching modes (2880 cm−1 and 2850 cm− 1) and a further peak at ca. 2905 cm− 1. The additional peak at 2905 cm− 1 is in the region expected for a methine (CH) stretching mode (not present in any adsorbate used). However, in various tests (discussed later) where coverage varied over the course of a reaction, this peak did not increase or decrease in intensity relative to the bands due to the asy CH2 stretching mode and therefore it is not attributed to a product methine fragment. The inclusion of this peak for all spectra maintained consistency. The accuracy of peak fitting is integral to the tests carried out throughout this study, and therefore it is acknowledged that an element of subjectivity is involved. However, the good resolution and intensity of the spectra collected provided reasonable clarity as to when a suitable fit was obtained.
The tests described in this section involved adsorption onto untreated γ-alumina at 100 °C. For each different adsorbate, multiple tests [2– 3] using freshly pressed discs of γ-alumina are carried out to evaluate reproducibility. - 1-Octanol Fig. 2 shows the recorded mass and the resulting spectra of the CHx stretching mode region for the adsorption of 1-octanol onto γ-alumina at 100 °C. Prior to exposure (0–10 min) a relatively constant mass was
Fig. 2. Mass profile for the adsorption of 1-octanol onto γ-alumina at 100 °C. Inset shows spectra for the CHx stretching mode region (3100–2700 cm−1) collected before (spectrum a) and after (spectrum b) adsorption.
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observed and an initial spectrum (Fig. 2(a)) recorded during this time shows no features due to CHx vibrations. After injection, at t = 10 min, the mass increased rapidly for the first 100 min due to adsorption of 1-octanol. Over the next 900 min the mass slowly decreased, as the system equilibrated. A spectrum recorded at t = 1030 min (Fig. 2(b)), confirms that adsorption had occurred with bands observed at 2962, 2928, 2877 and 2855 cm− 1 which are attributed to CH3 (asy), CH2 (asy), CH3 (sym) and CH2 (sym) modes, respectively. By deconvolution of the spectra to determine the integrated band areas associated with CH3 (asy) and CH2 (asy) modes, absorption coefficients were calculated using eq. 2 (Table 1, entries 1–3). Overall, the results of 3 individual experiments are in good agreement with each other, with an average absorption coefficient of 2.61 cm μmol− 1 ± 10.4% and 0.95 cm μmol−1 ± 13.6% for the CH3 and CH2 modes, respectively. Morterra et al. [20] calculated absorption coefficients for CH3 stretching modes using a combination of the bands found in the 3100–2700 cm−1 region following the adsorption of methanol on silica. For chemisorbed methanol on silicas which were activated at 1073 K with different SAs of 50 and 300 m2 g−1, absorption coefficients of 3.6 and 8.5 cm μmol−1 were found, respectively. Subbotina et al. [19] derived absorption coefficients for ethane adsorbed on various cationic forms of zeolite Y. When grouping all C\\H stretching vibrations together, absorption coefficient values varied from 9.2–26.1 cm μmol−1 depending on the cationic form and the thickness of the wafer used. When only considering the lower frequency, fully symmetric C\\H vibration at 2878–2813 cm−1, the absorption coefficient values varied between 2.6 and 16.6 cm μmol−1. Considering the results of Morterra et al. [20] and Subbotina et al. [19], it is reasonable to say that the variation in the calculated absorption coefficients are as good, if not better, than those reported in previous studies. - 1-Hexanol and decanal
Fig. 3. FTIR spectra collected after a period of exposure to different molecules to γ-alumina and Co/Al2O3 at 100 °C.
modes as a function of increasing chain length although this does not take into account any effect that the change in functionality from a primary alcohol to an aldehyde may have. Despite variation in the individual absorption coefficient values across repeated experiments and different adsorbates, the ratio of molar absorption coefficient (i.e., ε CH2 (asy):ε CH3 (asy)) appears to be remarkably constant with a value of 0.37 ± 4.4%, obtained by combining all the data for the different adsorbates on γ-alumina.
3.2. Stability determination
Two further adsorbates, 1-hexanol and decanal were tested using γalumina as adsorbent in order to investigate the effects of changing chain length and functionality on the calculated absorption coefficients. Fig. 3 shows the resulting difference spectra following exposure to each. Differences in overall absorbance resulted from differences in the extent of adsorption (i.e., spectra are not recorded at fixed, defined coverage). However, differences are observed in the relative intensity of the CH3 (asy) and CH2 (asy) modes consistent with expectations based on the different adsorbate structure. For example, the CH3 (asy) mode is more intense in spectra of 1-hexanol than 1-octanol or decanal (Fig. 3). A summary of the calculated absorption coefficients for different adsorbates on γ-alumina are presented in Fig. 4. Error bars based on repeat experiments for each adsorbate have been included. The bar graph reveals a general shift towards lower absorption coefficients for both
In order to establish whether the ratio of the two absorption coefficients was constant with time/coverage, the ratio was calculated at different times (Fig. 5) during exposure to 1-octanol on γ-alumina. After 33 min, data obtained from the first spectrum gave a ratio of 0.39 for the molar absorption coefficients, CH2:CH3. This ratio decreased to 0.37 after 68 min but thereafter remained constant up to 1032 min. Equivalent tests were conducted with 1-hexanol, 1-pentanol, 1-butanol and butanal (data not shown). These tests produced data showing constant CH2:CH3 ratio of the molar absorption coefficients with time in the cases of 1-hexanol and butanal as adsorbates. However in the cases of 1pentanol and 1-butanol as adsorbates, the value for the ratio of the molar absorption coefficients decreased with contact time/coverage, which may be indicative of gradual transformation of these adsorbates.
Table 1 Calculated molar absorption coefficients from the adsorption of 1-octanol at 30, 100 and 150 °C and the εCH2:εCH3 ratio using either γ-alumina or Co/Al2O3 as adsorbents. Test #
Adsorbent Temperature/°C εCH3 (cm μmol−1)
εCH2 (cm μmol−1)
εCH2:εCH3
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
γ-alumina γ-alumina γ-alumina Co/Al2O3 Co/Al2O3 Co/Al2O3 Co/Al2O3 Co/Al2O3 Co/Al2O3 Co/Al2O3 Co/Al2O3 Co/Al2O3 Co/Al2O3 Co/Al2O3 Co/Al2O3
1.07 1.01 0.77 0.77 1.35 0.79 0.94 0.04 0.04 0.80 0.32 0.91 0.95 1.29 1.23
0.37 0.37 0.34 0.34 0.34 0.35 0.37 0.36 0.40 0.40 0.37 0.38 0.38 0.39 0.38
100 °C 100 °C 100 °C 100 °C 100 °C 100 °C 100 °C 30 °C 30 °C 30 °C 30 °C 30 °C 150 °C 150 °C 150 °C
2.88 2.72 2.24 2.24 4.01 2.28 2.53 0.11 0.10 2.01 0.86 2.34 2.52 3.32 3.21
Fig. 4. Calculated molar absorption coefficients for CH3 (asy) and CH2 (asy) using γalumina as adsorbent. The ratios (CH2:CH3) of the calculated molar absorption coefficients are also displayed and correspond to the right hand y-axis.
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consistent. In this case, for adsorption on Co/Al2O3, the value was determined to be 0.36 ± 6.9% which is comparable to the value (0.37 ± 4.4%) determined using γ-alumina.
3.4. Temperature effect
Fig. 5. Mass profile following exposure of 1-octanol to γ-alumina at 100 °C. Inset shows difference spectra (CHx vibrational mode region) collected before and after adsorption. In brackets are the calculated absorption coefficient ratio (CH2:CH3) calculated at the time indicated.
The influence of temperature on absorption coefficients was studied using 1-octanol on Co/Al2O3 as an example. These studies were carried out at temperatures above (150 °C) and below (30 °C) the previously described experiments conducted at 100 °C. The experiment conducted at 30 °C was repeated 5 times in total (Table 1, entries 8–12) while 3 tests were carried out at 150 °C (Table 1, entries 13–15). Considerable variation was observed between some of the repeat experiments at 30 °C. Tests 8 and 9 are very comparable with each other but are significantly different to tests 10 and 12. At an adsorption temperature of 150 °C, the calculated coefficients were much closer to those calculated at 100 °C and the variation between the repeat tests was far less than observed at 30 °C. However, at all adsorption temperatures, a much more consistent ε CH2 (asy):ε CH3 (asy) ratio was apparent.
3.5. Origin of errors 3.3. Adsorption on Co/Al2O3 A two component system, Co/Al2O3, containing both support and supported phase was also assessed. Fig. 3 displays the difference spectrum collected for the adsorption of 1-octanol onto Co/Al2O3 at 100 °C. The spectrum is very similar to the one collected for 1-octanol on γ-alumina with minimal change in peak positions and CH2:CH3 band area suggesting the inability to discriminate between adsorption on the two components and/or predominance of features due to adsorption on the support. The calculated absorption coefficients for a series of 4 experiments are shown in Table 1 (entries 4–7). Test numbers 4, 6 and 7 are in good agreement whilst test 5 appears to be an outlier. Nevertheless when including all test data, the absorption coefficient ratio remains remarkably consistent. Three additional adsorbates (1-decanol, decanal and octanol) were selected to calculate absorption coefficients using the Co/Al2O3 adsorbent and these are displayed in Fig. 6. Variation in the average value for each adsorbate is observed, but no trend was apparent which would link coefficient values with either chain length or functionality. The average calculated molar absorption coefficients, combining all data are 3.01 cm μmol−1 ± 19.0% and 1.08 cm μmol−1 ± 20.1% for CH3 (asy) and CH2 (asy) modes, respectively. Once again, however, the ratio of the coefficients was observed to be far more
Fig. 6. Calculated molar absorption coefficients for CH3 (asy) and CH2 (asy) using Co/Al2O3 as adsorbent. The ratios (CH2:CH3) of the calculated molar absorption coefficients are also displayed and correspond to the right hand y-axis.
The molar absorption coefficients are derived from Eq. (2) which can be split into two component parts. Cd/m is a representation for the thickness of the disc used. In each case, uniform thickness across the disc is assumed. Discs were pressed with a starting mass of ca. 25 mg each time using 13 mm die and consistency of thickness would infer that data should fall close to the line in Fig. 7 (numbers on the graph relate to the test number in Table 1). This was not always the case as shown with samples 5 and 12 and hence, this could contribute to some error in the calculated coefficient. The second part of Eq. (2), (A/ nT) concerns the concentration of a particular species and the peak area associated with that species. For these examples, the integrated peak area of the CH3 (asy) stretching mode and the amount of adsorbed methyl (μmol g−1) has been plotted (Fig. 8). A linear relationship is expected although this is not apparent for the set of results obtained for adsorbent at 30 °C (Fig. 8a) and hence the calculated absorption coefficients are not constant. However, for the series of tests carried out at 100 and 150 °C, a trend more consistent with expectation was observed in the data (Fig. 8b) and subsequently less variation was found for the molar absorption coefficients determined under these conditions.
Fig. 7. Disc area vs disc mass for Co/Al2O3 discs used for adsorption of 1-octanol at 30, 100 and 150 °C. Numbers refer to test numbers in Table 1.
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γ-alumina in individual experiments at 100 °C. Table 2 compares the results determined from FTIR using the εCH2:εCH3 ratio and the actual molar ratio of the injected liquid. In all cases the calculated value is relatively close to the average CH2:CH3 ratio value of injected liquid, although notably in all three cases, the calculated value is lower than the value predicted based on the liquid composition. 4. Discussion
Fig. 8. Integrated peak area for CH3 (asy) mode vs adsorbed amount of CH3 species on Co/ Al2O3 discs at a) 30 °C and b) results for tests at 100 and 150 °C. The numbers on the graphs relate to the test numbers in Table 1.
3.6. Mixed solution test on γ-alumina Since the ratio of molar absorption coefficients, CH2:CH3, appears consistent across multiple tests using various chain lengths and functional groups (excluding shorter chain alcohols) and both adsorbents tested, it should be possible to determine the average chain length for a mixture of adsorbed species on e.g., γ-alumina using the following equation: Average CH 2 : CH3 ratio of adsorbed species ¼
ACH2 =ACH
3
εCH 2 : εCH3
ð3Þ
where ACHx is the integrated absorbance for a particular mode. A molar absorption coefficient ratio of 0.37 was used. In order to test this hypothesis, three different solutions containing 1-hexanol and 1-decanol with different molar ratios were exposed to
Table 2 Actual and calculated CH2:CH3 ratios for solutions of 1-hexanol and 1-decanol adsorbed on γ-alumina at 100 °C. Molar ratio (Hex:Dec)
Actual CH2:CH3
Calculated CH2:CH3
2:1 1:1 1:2
6.33 7 7.67
6.08 6.65 7.55
Molar absorption coefficients for the CH3 (asy) and CH2 (asy) modes have been determined using a custom made TGIR cell. The individual absorption coefficients determined for molecules of differing chain lengths and functionalites using γ-alumina as adsorbent were shown to vary considerably (Fig. 4), while much more invariant values were observed using varying hydrocarbon chain lengths and functionalities adsorbed on a Co/Al2O3 adsorbent (Fig. 6). The reason for a trend using one adsorbent but not the other is difficult to explain considering that the majority of the adsorption using the Co/Al2O3 adsorbent is expected to take place on the more abundant alumina support material. However, it should be noted that the two materials have different SAs (100 and 86 m2 g− 1 for γ-alumina and Co/Al2O3, respectively) and this may give rise to different light scattering properties although this does not necessarily explain a trend occurring across one sample and not the other. Despite these discrepancies, a constant ε CH2 (asy):ε CH3 (asy) ratio was observed; 0.37 ± 4.4% using the γ-alumina adsorbent and 0.36 ± 6.9% using the Co/Al2O3 adsorbent. The effect of adsorption temperature on the derived absorption coefficients was studied using 1-octanol as an adsorbate and Co/Al2O3 as the adsorbent (Table 1, entries 4–15). Significant variation was observed for the coefficients determined at 30 °C (Table 1, entries 8–12) and the origins of this discrepancy can be elucidated using data plotted in Figs. 7 and 8. Disc thickness in transmission FTIR has been shown to effect the consistency with which absorption coefficients can be calculated [19]. Subbotina et al. [19], observed an increasing absorption coefficient with increasing disc thickness and although the authors comment that this should not be the case, they attributed this trend to the increasing light-scattering characteristics of the thicker discs. They stated that their most reliable results were obtained using wafers of density 7– 9 mg/cm2. In comparison, the discs used in this study were much thicker (≈17–18 mg/cm2). In this work, to limit the influence that disc thickness could have on the calculated molar absorption coefficients, an attempt was made to keep the thickness of each individual disc as constant as possible (Fig. 7), but it can be said that any variation in disc thickness that was apparent, had little or no effect on the ε CH2 (asy):ε CH3 (asy) ratio. If both disc thickness (Fig. 7) and the integrated peak area (CH3 asy) versus amount of methyl adsorbed plots (Fig. 8a) at 30 °C, along with the calculated coefficients in Table 1 (entries 8–12), are considered, it is reasonable to suggest that disc thickness is not the only reason for the variability in the resulting individual molar absorption coefficients. For example, tests 10 and 12, are based on discs of greatest difference in thickness (for adsorption tests at 30 °C), but are comparable in terms of the absorption coefficient determined. Instead, the reason for the variability may be a consequence of the low temperature at which the adsorption was conducted. The plot of peak area for the band due to the CH3 (asy) mode versus the amount of CH3 uptake at 30 °C (Fig. 8a), shows no linearity. When the adsorption temperature was increased to 100 and 150 °C, again an effort was made to keep disc thickness as constant as possible, (Fig. 7) a linear relationship was observed between CH3 (asy) peak area and amount adsorbed (Fig. 8b). Accordingly, the calculated absorption coefficients appear more consistent in comparison to the series of measurements conducted at 30 °C. This could be attributed to a proportion of the species being in a physisorbed state at 30 °C whilst at higher temperatures of 100 and 150 °C the species could be in a more dispersed, chemisorbed state which reduced the extent of intermolecular interactions and therefore gives a truer spectral representation of an adsorbed species. Over the
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three different adsorption temperatures studied, a constant ε CH2 (asy):ε CH3 (asy) ratio was displayed once again (Table 1, entries 4– 15). One other factor which should be considered but was not assessed in any detail here was the possible impact of coke or hydrocarbonaceous deposits on a surface, as these may influence the light absorption and scattering characteristics of the adsorbent. These residues are also likely to perturb coefficients due to the low H:C ratios of these species. In the open literature, there are very few instances where molar absorption coefficients for the modes of CH3 and CH2 have been determined. Wexler [10] and Francis [8] independently carried out studies aimed at determining the integrated intensities of absorption bands for diluted, liquid phase aliphatic hydrocarbons. Wexler calculated coefficients of 4.46 and 3.74 cm μmol−1 for CH3 and CH2 modes, respectively, and compared these results to those of Francis, who obtained values for CH3 and CH2 modes of 3.85 and 3.31 cm μmol−1 although it is unclear in either case whether these are for the asymmetric or symmetric stretching modes. In this study, absorption coefficient values were determined as 3.01 cm μmol−1 ± 19.0% and 1.08 cm μmol− 1 ± 20.1% for CH3 (asy) and CH2 (asy) modes, respectively using a Co/Al2O3 adsorbent (Fig. 6). Absorption coefficients for species in a liquid phase are expected to be larger than their respective adsorbed form, since a more rigid like structure would be apparent, resulting in less of a vibrational response. In comparing the work of Wexler [10] and Francis [8] to this study, the absorption coefficients for chemisorbed hydrocarbon species (Table 1, entries 1–7 and 13–15) are indeed lower than those in the liquid phase, although this does not take into account the differences in methodology for integrating the spectrum. However, notably, some of the derived absorption coefficients in this study which are assigned to more liquid-like, physisorbed species (Table 1, entries 8–12), resulted in a smaller value than the chemisorbed equivalent (Table 1, entries 1–7 and 13–15). In an investigation by the group of Morterra [20], involving the methanol/silica system, the absorption coefficient of a CH3 mode for liquid phase methanol and adsorbed methanol (both chemisorbed or physisorbed) on silica are discussed and although the values differed, they were of the same order of magnitude. On a silica sample with a surface area of 50 m2 g−1 and activated at 1073 K, they found a lower absorption coefficient value for chemisorbed methanol (3.6 cm μmol− 1) than the physisorbed equivalent (11 cm μmol− 1). However, on a higher surface area sample (300 m2 g− 1) the physisorbed species exhibited a lower absorption coefficient (5.4 cm μmol−1) than the chemisorbed example (8.5 cm μmol−1). The authors discussed some of the limitations in transferring absorption coefficients from one system to another which can include sample thickness, surface dehydration levels of the adsorbent and scattering properties of the adsorbent. In this work, however, we have shown, using two different sample adsorbents (with different surface areas, scattering characteristics and disc thicknesses) and using multiple adsorbates, an apparently constant absorption coefficient ratio, ε CH2 (asy):ε CH3 (asy), which may have more widespread applicability. Using the values quoted by Wexler [10] and Francis [8], a ratio (CH2:CH3) of their coefficients give 0.84 and 0.86, respectively. In this work, ratios of 0.36 ± 6.9% (Co/Al2O3) and 0.37 ± 4.4% (γ-alumina) were determined or collectively a value of 0.36 ± 6.6% is found. The reason for this difference could again be attributed to the use of diluted solutions of hydrocarbons by Wexler and Francis to calculate their coefficients compared to a dispersed, adsorbed alkyl species employed here. A range of different adsorbates on γ-alumina were tested to assess for changes to this ratio during adsorption and after steady state had been reached for the amount adsorbed (as indicated by the mass profile). Molar absorption coefficient ratios for 1-octanol (Fig. 5), 1-hexanol and butanal were close to the average value (0.37) and remained within the typical error range (± 4.4%) at all times. The use of 1-pentanol and 1-butanol as adsorbates resulted in an initial (at a point of increasing mass) molar absorption coefficient ratio which was consistent with this average value. However, a decrease in mass after a maximum was attained, lead to a decreasing absorption coefficient ratio which could
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be interpreted in terms of a subsequent transformation of the adsorbed alkoxide. In the carboxylate region (1800–1200 cm−1) of the spectrum (not shown) bands at 1565 and 1440 cm−1 were observed which increased in intensity as a function of exposure time for all adsorbates used (irrespective of chain length), and the changes do not follow changes in the mass profile. This is in contrast to the intensity of the bands observed in the hydrocarbon region (3100–2700 cm−1) which increase and decrease in intensity in parallel with changes to mass (Fig. 5) and result in greater consistency in the absorption coefficients as a function of time/coverage. Knozinger and Stübner [27] reported the appearance of bands at 1565 and 1435 cm−1 following exposure of isobutyl alcohol to η-alumina due to formation of surface carboxylate species. They state that the intensity of these bands did not depend directly on the total surface coverage, consistent with our observations here, where the mass and band intensity did not vary in parallel. Therefore it is difficult to employ differences in the reactivity of short and long chained species in this study to explain variation in molar absorption coefficients for the bands due to CH stretching modes. One manner of testing the value of the coefficient ratio is if it could be employed to determine the average chain length of a group of known hydrocarbon species which contain both CH3 and CH2 groups. To explore this potential, the vapour from a liquid mixture containing two different hydrocarbon species 1-hexanol and 1-decanol, (different chain lengths, same functionality), was exposed to γ-alumina, and the resulting spectra assessed and evaluated (Eq. (3)), using an absorption coefficient ratio value of 0.37. The derived data in Table 2 compares well with the actual ratio of the alcohol species injected although in each case, the calculated value was slightly less than the corresponding predicted average ratio. The predicted value is based on the assumption that both alcohols are adsorbed to the same extent and are present at identical surface coverages. This difference therefore may be attributed to the relative coverage of the two species resulting from differences in reactivity based on the decreasing order of acidity (pKa value increases) as the number of alkyl groups in the chain increase [28]. Therefore, 1hexanol is expected to be more reactive than 1-decanol with respect to forming an adsorbed alkoxide species leading to a relatively higher abundance (surface coverage) of this species and leading to the calculated ratio being lower than the value expected based on the composition of the liquid phase. The use an average CH2:CH3 ratio (Eq. (3)) appears to be robust and overcomes many of the challenges raised by Morterra et al. [20] associated with applying and transferring molar absorption coefficients from one system to another since consistency of the ratio is observed over a range of adsorbates, with two different adsorbents which exhibit different surface areas, as well as different scattering properties. The fact that this ratio remains consistent suggests that there is potential to apply this to other systems, including a potential application in surface reactions which can be monitored by FTIR and where hydrocarbon chains vary in length during the course of a reaction (i.e. a changing CH2:CH3 ratio). Examples could include catalytic processes such as cracking, coupling, as well as polymerisation and oligomerisation reactions. 5. Conclusions A custom made TG-IR cell was implemented in order to determine absorption coefficients for the CH3 (asy) and CH2 (asy) stretching modes of adsorbed species. Some variation in the calculated, individual molar absorption coefficients was observed when using various adsorbates of differing length and using γ-alumina as an adsorbent, but more consistency in the values was observed when using a Co/Al2O3 adsorbent. At adsorption temperatures of 100 and 150 °C a more uniform set of molar absorption coefficients were observed compared to adsorption at 30 °C (using Co/Al2O3) and this is ascribed to the presence of additional physisorbed species for the lower temperature. A constant CH2:CH3 absorption coefficient ratio was found across multiple tests using different adsorbates, two different adsorbents and multiple
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adsorption temperatures. This value has been shown to be applicable to calculating the average CH2:CH3 ratio when surface coverage includes more than one adsorbate. There is potential to apply this methodology to determine average chain lengths of species generated at surfaces using in-situ FTIR, where hydrocarbon chain lengths vary as a function of reaction parameters and/or choice of catalyst. Acknowledgements We thank the Royal Society London for funding the initial prototype of the FTIR/Balance cell through the Paul Instrument Fund, the EU through the ERASMUS scheme (To T. H), and to research students (E. Mitchell) and PDRAs (D. Rosenberg) who were involved in contributions to the design and implementation of the cell. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.saa.2017.03.030. References [1] J. Ryczkowski, IR spectroscopy in catalysis, Catal. Today 68 (2001) 263–381. [2] J.B. Peri, Infrared study of adsorption of ammonia on dry γ-alumina, J. Phys. Chem. 69 (1965) 231–239. [3] D.J. Rosenberg, J.A. Anderson, On determination of acid site densities on sulphated oxides, Catal. Lett. 83 (2002) 59–63. [4] P. Ramamoorthy, R.D. Gonzalez, Surface characterization of supported Pt-Ru bimetallic clusters using infrared spectroscopy, J. Catal. 58 (1979) 188–197. [5] A. Davó-Quiñonero, A. Bueno-López, Lozano-Castelló, A.J. McCue, J.A. Anderson, Rapid-scan operando infrared spectroscopy, ChemCatChem 8 (2016) 1905–1908. [6] A.J. McCue, G.A. Mutch, A.I. McNab, S. Campbell, J.A. Anderson, Quantitative determination of surface species and adsorption sites using infrared spectroscopy, Catal. Today 259 (2015) 19–26. [7] N.B. Colthup, L.H. Daly, S.E. Wiberley, Introduction to Infrared and Raman Spectroscopy, third ed. Academic Press, Inc., London, 1990. [8] S.A. Francis, Absolute intensities of characteristic infra-red absorption bands of aliphatic hydrocarbons, J. Chem. Phys. 18 (1950) 861–865. [9] A.S. Wexler, Integrated intensities of absorption bands in infrared spectroscopy, Appl. Spectrosc. Rev. 1 (1967) 29–98. [10] A.S. Wexler, Infrared determination of structural units in organic compounds by integrated intensity measurements: alkanes alkenes and monosubstituted alkyl benzenes, Spectrochim. Acta 21 (1965) 1725–1742. [11] C.A. Emeis, Determination of integrated molar extinction coefficients for absorption bands of pyridine adsorbed on solid acid catalysts, J. Catal. 141 (1993) 347–354.
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