On the Chemistry of Iron Oxide Supported on Î³-Alumina and Silica

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Article Cite This: ACS Omega 2018, 3, 5362−5374

On the Chemistry of Iron Oxide Supported on γ‑Alumina and Silica Catalysts Sara Mosallanejad,† Bogdan Z. Dlugogorski,‡ Eric M. Kennedy,† and Michael Stockenhuber*,† †

School of Engineering, The University of Newcastle, Callaghan, New South Wales 2308, Australia School of Engineering and Information Technology, Murdoch University, Murdoch, Western Australia 6150, Australia

‡

S Supporting Information *

ABSTRACT: Catalysts of iron oxide on γ-alumina and silica which were prepared by an incipient wetness impregnation technique have been investigated in an effort to understand how the surface chemical properties are influenced by the nature of the supports. Surprisingly, this is the first study to compare in depth the influence of the supports on physicochemical parameters such as acidity, site nuclearity, and reducibility. In this study, surface characterisation techniques including N2 physisorption at −196 °C, ammonia temperature-programmed desorption, inductively coupled plasma optical emission spectrometry, temperature-programmed reduction with hydrogen, CO-chemisorption, scanning electron microscopy, transmission electron microscopy, and NO adsorption by in situ Fourier transform infrared spectroscopy have been performed to understand the different surface reactions occurring over the two different supports. The aim of this study is to ascertain the primary differences between these two catalysts using several catalyst characterization techniques and correlate their chemical and structural differences to their catalytic activity in the conversion of 2-chlorophenol. The results disclose a higher density of acid sites, a smaller particle size of iron oxide, stabilization of Fe(II) aluminate after reduction on the alumina surface, and finally, the formation of isolated iron cations on the surface of alumina which are notably absent on the silica-supported catalyst.

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Thyssen et al.20 reported stronger interactions of metal support with γ-alumina compared to those supported on silica and noted formation of CuO and NiO on the surface of both supports and formation of an additional species, NiAl2O4, on the surface of γ-alumina support; Zahaf et al.21 showed different abundances of Pt species on the surface of alumina and silica; Borgmann et al.12 detected Co3O4 species on the surface of silica and CoO and CoAl2O4 species on the surface of γalumina, and finally, Tanaka et al.22 reported the tetrahedral structure of V on the γ-alumina support and square pyramidal on the silica support. It has been established that a relationship exists between the particle size of metal oxides and the pore diameter of the support. Storsæter et al.23 studied the particle size and dispersion of cobalt (Co0) and Co3O4 on different supports (γ-alumina, silica, and titania) via H2-chemisorption and X-ray diffraction (XRD), respectively, and reported an increase in the particle size of the same order of magnitude as the average pore diameter of the supports. Similar findings were reported by Jean-Marie et al.,24 who found that clusters of Co3O4 particles reduced in size formed on supports with a reduced pore diameter.

INTRODUCTION Iron exists in several oxidation states in the environment. Different phases of iron, such as hematite, magnetite, and goethite (either in their pure forms or supported on carbon, alumina, silica, and zeolite) have been used as catalysts, for a myriad of commercially significant chemical reactions. Iron oxide on various supports has gained particular attention for catalysis applications because of its use in reactions such as Fischer−Tropsch synthesis, synthesis of NH3, and the water gas shift reaction.1−4 However, these catalysts have also been reported to catalyze the formation of a dangerous group of pollutants: polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/Fs, dioxins) under certain conditions where carbon, hydrogen, oxygen, and chlorine (especially a precursor molecule such as chlorophenol) are present in the reacting environment.5−8 In general, the type and structure of the support, method of preparation, and active site loading (e.g., iron percentage) are considered important factors that influence the catalyst’s redox properties and particle size, which in turn has a strong influence on the activity and selectivity of the supported catalysts.9−19 In addition to these variables, the support used is likely to play a vital role in the properties of the metal-supported catalyst. Thyssen et al.,20 Zahaf et al.,21 Borgmann et al.,12 and Tanaka et al.22 all studied copper/nickel (Cu/Ni), platinum (Pt), cobalt (Co), and vanadium (V) catalysts over alumina and silica. © 2018 American Chemical Society

Received: February 2, 2018 Accepted: April 18, 2018 Published: May 18, 2018 5362

DOI: 10.1021/acsomega.8b00201 ACS Omega 2018, 3, 5362−5374

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It was also found that significantly more carbon accumulated on the surface of the spent alumina-supported iron oxide compared to the spent silica-supported iron oxide. An additional chlorine species was also found on the surface of the spent alumina-supported iron oxide which can be specified as either metal chloride or a HCl bond. Finally, reduction of iron(III) is evident in the all spent catalysts. In the current study, we aimed to correlate the structural and chemical properties of iron oxide on silica and γ-alumina to their catalytic activity in the conversion of 2-CPh. To understand more about the physical and chemical properties of the iron oxide on silica and γ-alumina supports, other characterization techniques such as N2 adsorption, ammonia temperature-programmed desorption (NH3-TPD), inductively coupled plasma optical emission spectrometry (ICP−OES), temperature-programmed reduction with hydrogen (H2-TPR), CO-chemisorption, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and NO-FTIR analyses were applied.

With reference to the current available literature, there does not appear to be a comprehensive study of iron catalysts on γalumina and silica supports, prepared by the method of incipient wetness. Only a limited body of literature exists on the relationship between the iron oxide particle size and its dispersion on alumina and silica supports. Some studies have been undertaken to investigate the effect of the support used on the properties of oxidic iron particles. For example, Wan et al.25 reported for amorphous silica alumina that increasing the alumina/silica ratio enhances the size of Fe2O3 crystals and the nature of the Fe−SiO2 interaction, with the consequence that the Fe2O3 to Fe3O4 reduction reaction shifts to higher temperatures. Braga et al.26 observed a reduction in the particle size of Fe2O3 on the surface of alumina than on the silica support. Park et al.11 reported a limited reduction of larger particles of Fe2O3 on the surface of alumina because of the smaller contact area of the metal with the support. A number of techniques have been used to characterize the size and nature of the surface sites of transition metal-containing compounds. They include electron microscopy, XRD, X-ray photoelectron spectroscopy (XPS), and X-ray absorption spectroscopy27,28 to gain information on the size and structure (and also limited information about the chemical nature) of transition-metal sites. The chemical nature, dispersion, and also some structural information can be obtained by adsorption and desorption techniques. Nitric oxide 29−35 alone (or sometimes in combination with other adsorbates, particularly carbon monoxide36−39) is widely applied as a probe molecule to study the oxidation and coordination state of surface cations. In our previous studies,6,7,40,41 we examined the catalytic conversion of 2-chlorophenol (2-CPh, a dioxin precursor) on iron oxide deposited on the surface of γ-alumina and silica. We observed that the conversion of 2-CPh to volatile organic compounds (VOCs), dioxins, and total oxidation products differs significantly on the surface of γ-alumina-supported iron oxide compared to silica-supported iron oxide. On the surface of the former catalyst, we consumed approximately 10 times more 2-CPh and were unable to detect any feed at the outlet of the reactor for the first 3 h, during which we observed CO2, CO, HCl, and chlorobenzenes. After the first 3 h, the catalyst changed color from orange to black, and we observed the feed and chlorinated phenols in the outlet of the reactor. The yield of dioxins increased dramatically once the color change was observed. However, 2-CPh conversion on the surface of silicasupported iron oxide resulted in VOCs, dioxins, and oxidation products from the first hour with a very low concentration of the feed (50 ppm). In another paper,42 we used two techniques of Fourier transform infrared (FTIR) spectroscopy (in situ-FTIR) and XPS to further study the catalysts’ behavior in these particular reactions. In summary, we found that dioxin precursors (chlorophenolate/chlorophenoxy compounds) were formed on the surface of silica-supported iron oxide from the beginning of the experiment at very low pressures (0.01 μbar) of 2-CPh, but over alumina-supported iron oxide, we observed the formation of coke and formate species on the surface. As soon as active sites on the catalyst were masked by oxidation products, dioxin precursors were detected. XPS analysis of fresh catalysts revealed a higher surface coverage of iron on the surface of alumina compared to the silica, despite containing the same concentration of iron in the bulk of the two supports. This indicates that oxidic iron species are present in smaller clusters on the alumina surface compared to the silica support.

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RESULTS AND DISCUSSION N2 Adsorption. Table 1 lists the Brunauer−Emmett−Teller (BET) surface area (m2 g−1), average pore volume (cm3 g−1),

Table 1. BET Surface Area, Average Pore Volume, and Average Pore Diameter oxide sample γ-alumina silica iron oxide on γ-alumina iron oxide on silica

surface area (m2 g−1)

BJH adsorption pore volume (cm3 g−1)

BJH average pore diameter (nm)

191 229 168

0.47 1.0 0.42

9.4 15.4 9.4

218

0.95

15.1

and average pore diameter (nm) obtained by N2 adsorption at −196 °C for γ-alumina, silica, and iron oxide on γ-alumina and on silica catalysts. Typical nitrogen adsorption/desorption isotherms are presented in Figures S2−S5 of the Supporting Information. The isotherms for the samples are type 4 which is indicative of either nonporous adsorbents or adsorbents having relatively large pores. According to Table 1, it was determined that the silica and silica-supported iron oxide samples had a larger average pore diameter compared to the alumina and alumina-supported iron oxide samples. In addition, the pore volume of the silica and silica-supported iron oxide catalysts was significantly higher than that of the alumina and alumina-supported iron oxide catalysts. Similar observations have been reported for cobalt oxide supported on γ-alumina and silica.12 Acidic Properties of the Catalysts. Figure 1 presents the NH3-TPD profiles of all catalysts. The profiles of aluminasupported and silica-supported iron oxide were already published in our previous paper42 and are included here to compare with the data of the alumina and silica supports. Hall et al.43−45 have expressed the total acidity of catalysis in terms of extensive and intensive factors. Extensive factors represent the total number of acid sites and intensive factors indicate the strength of the individual sites. Peak areas in the different temperature regions indicate the relative concentration of acid sites on the surface of the catalysts (extensive factor), and the maximum peak temperature signifies the relative strength of the acid site (intensive factor). The classification of the acid site 5363


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of Fe2O3 → Fe3O4 and Fe3O4 → FeO, respectively. The third peak at 608 °C can be attributed to the transformation of FeO → Fe. However, for the alumina-supported iron oxide catalyst, we observed a two-stage reduction profile, a peak with a shoulder. The shoulder at 275 °C can be assigned to Fe2O3 → Fe3O4, and the peak at 370 °C can be attributed to Fe3O4 → FeO or Fe(II) aluminate (FeAl2O4).50 It is apparent that this structure was stabilized,51 and reduction of Fe(II) to Fe metal was not taking place. Reduction of Fe(III) on the silica support occurs at higher temperatures compared to the one on the alumina support. This can be due to different particle sizes of oxidic iron species and different interactions between iron oxide particles with the two supports.52−54 ICP−OES and CO-Chemisorption. Table 3 compares the Fe loading, (the sum of the bulk and surface iron atoms) of the

Figure 1. NH3-TPD curves for alumina, silica, and alumina- and silicasupported iron oxide.

strength is defined as follows: weak sites (150−300 °C), medium strength sites (300−500 °C), and strong acid sites (greater than 500 °C).46 The γ-alumina support displays a considerably higher concentration of acidic sites compared to silica, at least in terms of weak and medium acidity strength regions. However, the strength of acid sites is higher in the silica support. Ammonia desorbed at temperatures of 300 and 541 °C from the surface of silica and at temperatures of 215 and 470 °C from the surface of alumina. Further, the higher intensity of the modified alumina and silica indicates that iron oxide enhances the concentration of weak and medium acid sites. Table 2 presents the quantities of acid sites of the γalumina, silica, and iron oxide on γ-alumina and silica catalysts.

Table 3. Fe Loading, CO Chemisorption, Fe Dispersion, and Fe Average Diameter oxide sample iron oxide on γ-alumina iron oxide on silica

γ-alumina silica iron oxide on γ-alumina iron oxide on silica

acid site density (μmol m−2)

acid site concentration (mmol g−1)

1.25 0.04 2.48

0.24 0.0090 0.42

0.11

0.025

CO chemisorbed (μmol g cat−1)

Fe dispersion (%)

3.3

16.3

6.2

3.2

4.27

1.95

catalysts based on ICP−OES analysis and provides an estimation of the concentration of accessible iron sites (from CO-chemisorption experiments on the surface of the modified alumina and silica). The dispersion of iron on the surface of silica (1.95%) is less than that determined for the surface of alumina (6.2%). Particle agglomeration on the surface of silica compared to the alumina and the reducibility of iron on the two supports provide possible explanations for this difference. CO chemisorbed much better on the reduced form of iron.55−57 The catalysts were reduced under a flow of H2 at 450 °C prior to COchemisorption experiments. The color of reduced aluminasupported iron oxide changed to greenish, which most likely represents the transformation of the Fe3+ to Fe2+ species. However, a black color observed for silica-supported iron oxide appears to represent a combination of Fe3+ and Fe2+ species on the catalyst. SEM and TEM. SEM and energy-dispersive X-ray (EDX) served to study the morphology and elemental distribution of iron on each of the supports. Figures S6 and S7 in the Supporting Information offer a typical SEM micrograph and an example of EDX elemental mapping. From the SEM image, it is evident that particles of iron oxide supported on the silica are on average larger in diameter when compared to those on the alumina support. The alumina support consists of various agglomerated, relatively small diameter particles of a nearly spherical shape. Moreover, alumina contains smaller pores compared to the silica catalyst. EDX analysis was employed to determine the chemical compositions of silica-supported and alumina-supported iron oxide (Figures S5 and S6, part c). TEM images and the particle size distributions are shown in Figure 3. The lighter regions in the images are composed primarily of the support, while the darker regions are iron particles. The majority of iron oxide particles on alumina and silica supports are in the range of 0.4−2 and 6−8 nm, respectively, as estimated statistically from the TEM data. TEM also confirmed agglomerated and larger particle sizes of iron oxide on the surface of the silica support. The size of iron oxide particles on the surface can be explained by considering the point of zero

Table 2. Ammonia Desorption over Unmodified and Modified Catalysts catalyst

Fe loading (wt %)

Temperature-Programmed Reduction with Hydrogen. The reduction behavior of the two silica-supported and alumina-supported iron oxide catalysts was studied by H2-TPR. Figure 2 presents the reduction profiles of these two catalysts. In the case of the silica-supported iron oxide catalyst, we observed three distinct stages of reduction.47−49 The first reduction region contains two partially overlapping peaks. The peaks at 385 and 438 °C can be ascribed to the transformations

Figure 2. TPR profiles of alumina-supported iron oxide and silicasupported iron oxide. 5364


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Figure 3. TEM images of (a) alumina-supported iron oxide and (b) silica-supported iron oxide and (c) histogram plot of particle size distribution for silica-supported and alumina-supported iron oxide.

Figure 4. (a) IR spectra of adsorbed NO on the surface of vacuum-treated alumina at 30 °C, (b) IR spectra in the range of 1200−1700 cm−1, the numbers correspond to NO adsorption at low coverage on the surface, and (c) IR spectra in the range of 3200−3800 cm−1.

± 0.2 for preparing the silica-supported and alumina-supported iron oxide by the incipient wetness method. Because the pH of the solution was lower than PZC of the two supports, the surface of the two adsorbents was positively charged. However, the surface of alumina is more positive and enhanced by the electrostatic repulsive force, suppresses the agglomeration, and subsequently reduces hydrodynamic size of clusters.61,62 This explanation is in good agreement with the smaller clusters measured, and even isolated cations on the surface of alumina

charge (PZC) of the two supports and the pH of the impregnating solution. The PZC of silica and γ-alumina are well-established in the literature, and these values range between 1.7 to 4.5 and 7 to 9,58−60 respectively. This is consistent with the published PZC and NH3-TPD profiles of the supports (see Figure 1); PZC is related to the extensive factor of catalyst acidity, and higher density of acid sites was measured on the surface of alumina compared to the silica support. We used an iron(III) nitrate solution with a pH of 1.0 5365


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on the catalyst suggests that the reactions 3 and 4 occur on the surface of the catalyst.

(vide infra) were disclosed. In addition, a higher level of iron dispersion was observed on the alumina surface compared to the silica surface with 6.2 and 1.95%, respectively, as measured by CO-chemisorption (see Table 3). NO-FTIR. NO adsorption on catalyst surfaces, in combination with FTIR spectroscopy, is widely used for the characterization of iron species. According to the literature, NO adsorbs more strongly on Fe2+ sites in comparison with Fe3+ sites.63 NO Adsorption on Unmodified γ-Alumina Support. The infrared spectra, corresponding to NO adsorption on the surface of alumina, as recorded at 30 °C, are presented in Figure 4. Bands in the range of 1250−1620 cm−1 have previously been denoted as strongly adsorbed nitrito complexes, nitrite (NO2−), and nitrate (NO3−) species, on an alumina surface.64−70 At very low pressures of NO (less than 1 μbar), several bands at 1231, 1320, 1350, and 1467 cm−1 were observed (see Figure 4b). According to previous studies on the adsorption of NO gas on the surface of alumina, the bands at 1231 and 1320 cm−1 can be assigned to a bridged nitrite species,66,71 while the bands at 1350 and 1467 cm−1 have been attributed to bidentate and monodentate nitrite species bonded to aluminium atoms, respectively.66,70,72,73 Increasing NO coverage results in the appearance and an increase in the intensity of several new bands at 1260, 1294, 1310, 1558, 1591, and 1620 cm−1. Similar results have been reported from studies on the adsorption of NO2 gas on γ-alumina.67,73−77 On the basis of assignments reported, in the adsorption of NO2 on the surface of γ-alumina, bands can be attributed to three types of nitrate species as follows: (i) monodentate (1558 and 1310 cm−1), (ii) bidentate (1591 and 1294 cm−1), and (iii) bridged nitrate (1620 and 1260 cm−1) species.74,78 Scheme 1

3NO(g) → N2O(g) + NO2 (g)

(1)

4NO(g) → N2O3(g) + N2O(g)

(2)

2NO2 (g) + O2 −(surf) → NO3−(ads) + NO2−(ads)

(3)

2NO2 (g) ⇄ N2O4 (gas) ⇄ NO+(ads) + NO3−(ads)

(4)

According to the published literature,79 the band at 2234 cm−1 can be assigned to an NN stretching mode of N2O. Adsorbed N2O exhibits a blue shift in comparison with the infrared spectra of gaseous N2O.80 As presented in Figure 4, the intensity of the absorption band at 1961 cm−1 increased with the increasing NOx coverage. This band can be assigned to an adsorbed N2O3 species. The IR frequencies of N2O3 are widely documented with characteristic absorption bands at approximately 1950, 1300, and 1570 cm−1. The two bands at 1300 and 1570 cm−1 are a combination of N2O3 stretching vibration bands overlapping the bands of adsorbed nitrate species. Absorption bands of a nitrosonium (NO+) on the surface of alumina are characterized by the appearance of the absorption bands at 2264 cm−1. The appearance of negative bands within the 3650−3800 cm−1 region indicates the interaction of isolated hydroxyl groups on the surface of alumina with NOx. Five different bands at 3690, 3733, 3757, 3773, and 3791 cm−1 can be assigned to five different hydroxyl groups with different coordination geometries. The most prominent band at 3733 cm−1 is attributed to hydroxyls groups which are bound to two octahedrally coordinated Al3+ sites, present in a bridging configuration. The shoulder around 3690 cm−1 and the band at 3791 cm−1 are associated with hydroxyl groups bound to three and a single Al 3+ sites with octahedral coordination, respectively. The two remaining features at 3773 and 3757 cm−1 are assigned to the terminal hydroxyl groups adsorbed on a single Al3+ site with a tetrahedral coordination that can exist on two different crystallographic orientations [i.e., (111) and (110)] of the surface of δ-Al2O3.69,77,81 NO Adsorption on Unmodified Silica Support. The IR spectra of NO adsorption on the surface of silica are presented in Figure 5. It is readily observed that the concentration of NO on the surface is much lower on silica compared to the alumina support. Note that the thickness of the solid wafers is very similar. A series of studies reported low adsorption energy of NO and N2 on the surface of silica.36,80,82,83 In addition to the bands at 1480, 1525, and 1731 cm−1, the remaining features are possibly a result of relatively weak adsorption of NOx on silica. According to the literature, the absorption band at 1480 cm−1 is attributed to monodentate nitrates and 1525 cm−1 is in the range of bidentate nitrate.72,80 Kugler et al.84 assigned the bands at 1745 and 1875 cm−1 to adsorbed cis NO dimer (NO)2 during the adsorption of nitric oxide on silica-supported chromium, and in another study by Ghiotti et al.,85 the single band at 1740 cm−1 was attributed to the adsorbed trans NO dimer (NO)2. Moreover, Djonev et al.86 assigned the band at 1744 cm−1 to weakly adsorbed N2O4 when studying the adsorption of NO2 on silica-supported cobalt oxide. This species could form through NO2 dimerization, as discussed previously.

Scheme 1. Structure of Various NOx Species on Alumina

presents the possible conformation of NOx species during adsorption of NO on alumina. Nitrite species are formed at low NO gas coverages, and nitrate species increase in concentration as the pressure of NO increases. The reaction of labile surface oxygen originating from alumina has been postulated as a reason for the formation of nitrate species. The nitrate species were suggested to be formed from NO2.65 It is also possible that, as depicted in reactions 1 and 2, NO2 and N2O arise in the gas phase at elevated pressures of NO gas because of disproportionation. The formation of the absorption bands 5366


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Figure 5. IR spectra of adsorbed NO on the surface of vacuum-treated silica at 30 °C.

Figure 6. IR spectra of adsorbed NO on the surface of vacuum-treated alumina-supported iron oxide at 30 °C.

Figure 7. IR spectra of adsorbed NO on the surface of vacuum-treated silica-supported iron oxide at 30 °C.

The appearances of negative bands in 3743 cm−1 indicate the interaction of isolated hydroxyl groups on the surface of silica with NOx. NO Adsorption on Alumina-Supported Iron Oxide Catalysts. The IR spectra of NO adsorbed on supports (alumina and silica) are recorded, in an effort to differentiate bands that are formed because of the chemisorption of NO on the oxidic iron species (see Figures 6 and 7). Intense bands at 1816 and 1823 cm−1 are evident, and the spectral region between 1800 and 1860 cm−1 is attributed to the formation of adsorbed nitrosyl species on reduced Fe sites.

Figure 6 presents the adsorption of NO gas on the surface of alumina-supported iron oxide. Importantly, it is welldocumented that NO does not adsorb strongly on Fe3+ sites; thus, the peak around 1816 cm−1 reflects bond formation between Fe2+ and NO. The formation of an Fe2+ cation originates from auto-reduction of the iron species; a similar phenomenon has been reported elsewhere.87 At very low concentrations of NO, a new spectral feature at approximately 1816 cm−1 is observed. Increasing the pressure of NO to a level greater than 1 mbar caused an apparent shift of this band to higher wavenumbers. Concomitant with this blue shift, we 5367


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ACS Omega observe the appearance of nitrate (1250−1620 cm−1) and N2O3 bands (1959 cm−1). NO Adsorption on Silica-Supported Iron Oxide Catalysts. The IR spectra of NO adsorption on the surface of silica-supported iron oxide are illustrated in Figure 7. The adsorption of NO on Fe2+ species appears at higher wavenumbers and at a notably decreased intensity compared to alumina-supported catalysts. At low coverage of NO, a band at 1823 cm−1 appeared and, similar to NO adsorption on oxidic iron on alumina, increasing NO coverage resulted in a blue shift of the band to a higher wavenumber (1852 cm−1). At low coverages, NO adsorbs on highly reactive, coordinatively unsaturated Fe2+ species. However, increasing NO coverage results in the formation and co-existence of NO and NO2 surface species. Kayhan et al.88 reported the decomposition of NO2 gas on Fe2+/Fe(3−x)+ sites, producing a surface oxygen atom and oxidizing the reduced iron in the vicinity of the oxygen species. Thus, by increasing NO coverage, the band indicative of reduced Fe sites decreases in intensity and shifts to higher wavenumbers. To understand the nature of these differences, we selected the spectrum obtained at 10 μbar pressure to perform peak deconvolution of distinct surface species assuming a Gaussian peak shape, in the range between 1750 and 1900 cm−1. Four species for the aluminasupported iron oxide (with maxima at 1783, 1807, 1825, and 1841 cm−1) and three for the silica-supported iron oxide (with maxima at 1790, 1805, and 1825 cm−1) were fitted. The results are presented in Table 4.

Indeed, this conclusion is consistent with the results of the investigation by Chen et al.89 who studied the dispersion of Fe2O3 on the surface of different metal oxides by Mössbauer spectroscopy and XRD analysis. They reported that the form and extent of dispersion of iron oxide on the surface of the support depends on the number and availability of vacant sites on the catalysts. They concluded that iron oxide forms crystalline α-Fe2O3 on the surface of silica because of the lack of vacant sites on the surface. This is in contrast to other metal oxides studied, such as γ-alumina, ceria, titania, and zirconia, which possess vacant surface sites, and where the Fe3+ first incorporates in these sites and only once these sites are saturated with Fe3+ in its crystalline α-Fe2O3 form. In another studies, Popova et al.90 and Rangus et al.91 showed that the iron concentration in KIL-2 silica, prepared through direct synthesis, influences the nature of metal species in the matrix. They reported the presence of mostly isolated Fe3+ species in samples with the low concentration of iron, Fe/Si ≤ 0.01, while the formation of oligonuclear iron complex (FexOy) in samples with higher iron content, F/Si > 0.01. In addition, Nechita et al.87 surmised the formation of different types of iron species on an alumina surface compared to a silica surface. Yuen et al.,63 who investigated the NO adsorption on silicasupported iron oxide, attributed the bands at 1810 and 1750 cm−1 to the formation of dinitrosyl and mononitrosyl species on low-coordination tetrahedral Fe2+/Fe(3−x)+ sites, respectively, and the band at 1830 cm−1 to the formation of mononitrosyl at high-coordination octahedral Fe2+/Fe(3−x)+ sites. They reported the low-coordination and high-coordination Fe cations as strongly interacting with the support and small particles of iron oxide on the surface, respectively. Mössbauer spectroscopy disclosed the presence of both outer and inner doublets in various catalysts with different proportions.89 The outer doublet is associated with the iron atoms on the surface (high-coordination), while the inner doublet arises because of iron atoms in the interior of crystallite (low-coordination). According to Figure 7, the ratio of iron as small particles of oxide on the surface to species, which strongly interact with the support in the interior portion of the catalyst, is greater for the silica-supported catalyst compared to alumina. The absorption band at a wavenumber of 1841 cm−1 is assigned to NO adsorbed on isolated Fe2+ cations.33 No isolated Fe2+ was observed on the surface of silica. The overall intensity of the IR absorption bands in the wavenumber region of 1800−1860 cm−1 is higher for aluminasupported iron oxide compared to silica-supported iron oxide. The reasons for this could be either (i) stabilization of Fe2+ as

Table 4. Wavenumber of Each Peak for Both Catalysts

iron oxide on γ-alumina iron oxide on silica a

peak 1, Fe2+(NO) (cm−1)a

peak 2, Fe2+(NO)2 (cm−1)

peak 3, Fe2+(NO) (cm−1)b

peak 4, Fe2+(NO) (cm−1)c

1783

1807

1825

1841

1790

1805

1825

Low-coordination Fe. bHigh-coordination Fe. cIsolated Fe.

Figure 8 illustrates the IR spectra for both silica- and alumina-supported iron oxide between 1750 and 1900 cm−1. On the basis of the curve-fitting analysis, alumina induces an increased number of Fe2+ sites available to NO adsorption compared to silica, most likely because of the existence of isolated Fe2+ species on the alumina-supported catalyst and the larger dispersion of the adsorption sites (iron) on the aluminasupported surface compared to the silica-supported catalyst.

Figure 8. Fitted peaks in the NO adsorbed region at 10 μbar (a) alumina-supported iron oxide and (b) silica-supported iron oxide. 5368


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Figure 9. IR spectra of adsorbed NO on the surface of oxygen-treated alumina-supported iron oxide at 30 °C.

Figure 10. IR difference spectra of the NO adsorption region for oxygen-treated iron oxide/alumina after adsorbing (a) NO at 1 μbar pressure and (b) N2O at 1 μbar pressure.

Figure 11. IR spectra of adsorbed NO on the surface of oxygen-treated silica-supported iron oxide at 30 °C.

NO Adsorption on Oxidized Alumina- and SilicaSupported Iron Oxide Catalyst. Figure 9 illustrates the IR spectra of NO adsorption on the oxidized alumina-supported iron oxide catalyst. Although the intensity of the peak associated with NO adsorbed on Fe2+ decreases, it highlights the presence of Fe2+ in the catalyst (oxygen-treated). The remaining Fe2+ cations on the surface in the oxidized state have been reported as stabilized compounds, such as FeAl2O4.92 The presence of this structure has been confirmed by the H2-TPR experiment in the alumina-supported iron oxide catalyst.

FeAl2O4 during the formation of the catalyst in the interaction of Fe2O3 with Al2O392 or (ii) improvement in the redox property of Fe3+ due to a strong interaction between iron oxide and alumina.93 The interaction inhibits iron particle agglomeration, and as a result, the particle size distribution is relatively narrow on the alumina support, but a significantly broader particle size distribution is reported for iron nanoparticles on the silica support. Moreover, the interaction of iron oxide with the silica support is reported to be weaker than that observed with alumina as a support.93,94 5369


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ACS Omega The presence of Fe2+ on the surface of the oxygen-treated alumina-supported iron oxide catalyst was confirmed by studying the adsorption of N2O on the catalyst surfaces. Grubert et al.95 reported conversion of N2O to NO on the surface of Fe-ZSM-5 and Fe-MCM-41-PS in which iron cations are in their reduced form and iron is present exclusively in the form of Fe2+ species. Figure 10 presents the IR spectra of the adsorbed NO and N2O on Fe2+ cations on the surface of oxygen-treated alumina-supported iron oxide. The peak at 2237 cm−1 is assigned to an NN stretching mode and represents adsorbed N2O on the surface of alumina. NO adsorption on the oxygen-treated silica-supported iron oxide is presented in Figure 11. The absence of a spectral feature in the region between 1750 and 1900 cm−1 indicates a lack of adsorption of NO on the oxygen-treated silicasupported iron oxide, presumably as a result of the oxidation of Fe2+ species to Fe3+. The differences between the iron oxide supported on alumina and silica for 2-CPh conversion and different VOCs and dioxin yields are suggested to originate from different surface acidities as well as the isolated cationic iron species observed on the alumina supported catalyst. A lack of these species and lower acidity were observed on the iron oxide supported on silica. The pKa value of 2-CPh is 8.5; this value includes solvent effects that are not considered in the adsorption of basic molecules on the solid acids. Therefore, proton affinity rather than the pKa value has been adopted as an indicator of relative basicity.96 Proton affinity of 2-CPh and ammonia corresponds to 141097 and 857 kJ mol−1,98 respectively, at 25 °C. According to the Brønsted−Lowry theory, a base is a proton acceptor, and the higher the proton affinity, the stronger the base is. 2-CPh can be considered as a base by comparing its proton affinity with that of ammonia. Therefore, 2-CPh interacts with an acidic catalyst such as alumina in our study more strongly than the silica catalyst which possesses a less acidic characteristic compared to the alumina. In addition, reducibility of transition metals plays a significant role in the formation of dioxins from precursors.99 According to the TPR results, formation of FeAl2O4 species prevents the reduction of iron oxide on the alumina support. It is also established that isolated iron cations, tetrahedrally coordinated FeO4, exhibited higher catalytic activity compared to iron oxide clusters and particles,100,101 and we propose that the presence of isolated iron cations on the alumina support could be responsible for significant decomposition of 2-CPh and accumulation of a large quantity of carbon on the surface of the alumina-supported iron oxide catalyst.

iron on the surface of alumina is greater than that determined for the surface of silica. Finally, the reducibility of the two catalysts of silica- and alumina-supported iron oxide is very different. The former catalyst can reduce to Fe metal under reductant atmosphere; however, the FeAl2O4 structure stabilized in the latter catalyst and prevents further reduction of iron particles. All these differentiation in chemistry between silica-supported and alumina-supported iron oxide can explain the different observed behaviors of 2-CPh conversion on the surface of these catalysts.

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METHODOLOGY Catalyst Preparation. γ-Alumina-supported and silicasupported iron oxide samples were prepared by adopting the method of incipient wetness. A solution of iron(III) nitrate nonahydrate (Chem-Supply) was mixed with alumina (Catal International Ltd) and, in a separate preparation, the silica gel (Davisil grade 645) support, in proportions designed to produce an approximately 3 wt % loading of iron (with respect to the mass of the support) on the surface of each of the supports. The solution was added to the supports drop-wise, while mixing continuously until the mixture assumed paste-like consistency. The sample was then dried at 110 °C and finally, calcined in air at 450 °C for 5 h. The 3% loading of iron on the supports is representative of the concentration of iron in most combustion systems.102 In addition, we use this specific iron concentration to compare our catalyst relative reactivities with the same concentration of iron on the silica support studied by Nganai et al.5 Catalyst Characterization. N2 Adsorption. The BET surface area (m2 g−1), average pore volume (cm3 g−1), and pore diameter (nm) of the samples were determined by nitrogen adsorption at −196 °C using a Micromeritics TriStar 3000 surface area analyser. Prior to any measurement, the samples were degassed overnight at 150 °C using Micrometrics VacPrep 061. The BET theory yielded the surface area of the samples, and the Barrett−Joyner−Halenda (BJH) method served to estimate the pore-volume and pore-size distribution. Acidic Properties of the Catalysts. NH3-TPD experiments were performed in a stainless steel apparatus, as described previously.103 Desorption of gaseous species during controlled heating of the sample was recorded by a Pfeiffer Prisma quadrupole mass spectrometer, using the m/z signal at 16 for quantification of ammonia desorbing from the catalyst during heating. In the TPD measurements, 0.1 g samples were activated in situ, in the desorption tube at 450 °C for half an hour. Following activation, ammonia gas adsorbed onto the catalyst surface at 150 °C. Finally, the ammonia desorption experiments proceeded with the sample being heated between 30 and 750 °C at a heating rate of 5 °C min−1. Figure S1 represents the three primary m/z (17, 16 and 15) ions of ammonia desorbed from the surface of alumina-supported iron oxide. As presented, the m/z = 17 ion current was affected by water vapor at temperatures greater than 450 °C. Thus, the signal of m/z = 16 best reflects the desorption of ammonia (NH2+), as this mass to charge ratio is least contaminated with ions originating from water. For quantitative analysis, ZSM-5 with Si/Al = 15 was used. Inductively Coupled Plasma Optical Emission Spectrometry. The elemental composition of the catalysts was measured using ICP−OES (Varian 715 ES spectrometer ICP− OES). Prior to analysis, samples were dissolved in a solution containing 4.5 mL of HNO3 (65%), 4.5 mL of HCl (37%), and

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CONCLUSIONS The type of supports and catalysts’ preparation techniques have a notable influence on the nature, acidity, dispersion, and reducibility of the formed metal oxide species as well as on their catalytic activity. Characterization and in situ FTIR analysis of catalysts confirms the existence of significant differences between the two catalysts. Having a larger pore diameter on the surface of silica enhanced agglomeration of the iron oxide particle on that surface and the enhanced interaction and vacant sites for iron on the δ-alumina results in formation of isolated iron (Fe2+) species on this support. Moreover, on the basis of the TPD analysis, the acidity of alumina-supported iron oxide is significantly higher compared to that of the silica-supported catalyst. Also, based on CO-chemisorption, the dispersion of 5370


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ACS Omega 3 mL of HBF4 (50%). Thulium (600 μL, 1000 ppmv) was added as an internal standard. To ensure complete sample dissolution, the samples were digested in a Milestone Start D microwave unit for a minimum of 2 h. Temperature-Programmed Reduction with Hydrogen. The temperature-programmed reduction of the samples was performed using 10% H2 mixed with argon (Ar) with a flow rate of 32 cm3 min−1 as a reductant. A sample (600 mg) was charged in a quartz tube, treated in a flow of Ar at 400 °C for 1 h, and then cooled to room temperature. Finally, the sample was heated to 800 °C with a ramp rate of 10 °C min−1 under the reductant mixture. A thermal conductivity detector (Micromeritics AutoChem II 2920) was used to measure the hydrogen consumption. CO-Chemisorption. A static volumetric method allowed measurement of the extent of metal dispersion and an estimation of the active particle size of the metal on the surface of the support using CO as the probe gas. Approximately 0.5 g of the sample was charged into a sample holder placed in a temperature-controlled furnace. The temperature of the sample was increased slowly to 450 °C under a flow of He with a ramp rate of 5 °C min−1, the sample was then treated in the flow of H2 (50 mL min−1) at 450 °C for 16 h, following purging with He for 20 min. CO chemisorption was performed at 50 °C104 in the pressure range between 30 and 90 mbar. Pfeiffer 362-TPG 261 Baratron and an acquisition program developed in LabVIEW version 7.1.1 software were used. The program facilitated vacuum measurements and data logging. Estimation of the dispersion involved an assumption of one atom of CO molecule adsorbing onto two iron surface atoms (Fes), thus the stoichiometric factor used was CO/Fes−1 = 0.5.11,105−107 The Supporting Information presents the formula for estimating dispersion of the active particle size (eq S1). Scanning and Transmission Electron Microscopy. Zeiss SIGMA VP FESEM was used for the collection of SEM images of the catalysts and supports. Images of the samples were taken by a secondary electron and back-scattered electron detector at 15 kV. An EDX spectroscopy detector enabled elemental analysis using Link Isis software. A JEOL 2100 transmission electron microscope was used for TEM imaging of supported catalysts. We measured the particle size by using Gatan DigitalMicrograph software. NO Adsorption Followed by in Situ FTIR Spectroscopy. All transmission spectra were recorded on a TENSOR 27 spectrometer at a spectral resolution of 4 cm−1. The sample powders were transformed into thin, self-supporting wafers, and transferred into an in situ cell as described in a previous manuscript.108 Before the adsorption measurements at 30 °C, samples were activated for 30 min at 450 °C under vacuum (vacuum-treated), followed by 30 min in oxygen (oxygentreated). Subsequent to these treatment processes, NO absorption spectra were obtained over the pressure range of 0.1 μbar to 10 mbar. Finally, difference spectra are reported where the clean activated catalyst is subtracted from the spectrum of the catalyst in contact with the probe molecule. All spectra were recorded at the same temperature (30 °C).

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Three major m/z ratios of ammonia after desorbing from alumina-supported iron oxide; nitrogen adsorption/ desorption isotherm for four samples of γ-alumina, silica, γ-alumina-supported iron oxide, and silica-supported iron oxide; active site dispersion formula; and SEM and EDX (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Bogdan Z. Dlugogorski: 0000-0001-8909-029X Michael Stockenhuber: 0000-0001-5026-2218 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.M. gratefully acknowledges the University of Newcastle for the award of a postgraduate research scholarship. We also acknowledge funding by the Australian Research Council. We thank Jane Hamson for her help with ICP analysis. We are grateful to the University of Newcastle for SEM and TEM analyses at the EM/X-ray unit.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b00201. 5371


Article

ACS Omega

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On the Chemistry of Iron Oxide Supported on Î³-Alumina and Silica

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