Kinetics of CO Oxidation over Unloaded and Pd-Loaded α -Fe 2 O 3 Spherical Submicron Powder Catalysts: Photoacoustic Investigations at Low Pressure

: In this study, α -Fe 2 O 3 spherical particles with an average diameter of approximately 200 nm were synthesized by a solvothermal method for use as both a catalyst and medium for a Pd catalyst. The kinetics of CO oxidation over powders of α -Fe 2 O 3 spherical particles and 14 wt % Pd/ α -Fe 2 O 3 spherical particles were measured in a static reactor by using a CO 2 laser-based photoacoustic technique. The total pressure was ﬁxed at 40 Torr for the CO/O 2 /N 2 mixture for temperatures in the range of 225–350 ◦ C. The variation in the CO 2 photoacoustic signal with the CO 2 concentration during CO oxidation was recorded as a function of time, and the CO 2 photoacoustic data at the early reaction stage was used to estimate the rates of CO 2 formation. Based on plots of ln(rate) vs. 1/ T , apparent activation energies were calculated as 13.4 kcal/mol for the α -Fe 2 O 3 submicron powder and 13.2 kcal/mol for the 14 wt % Pd/ α -Fe 2 O 3 submicron powder. Reaction orders with respect to CO and O 2 were determined from the rates measured at various partial pressures of CO and O 2 at 350 ◦ C. The zero-order of the reaction with respect to P o 2 was observed for CO oxidation over α -Fe 2 O 3 submicron powder, while 0.48 order to P o 2 was observed for CO oxidation over Pd/ α -Fe 2 O 3 submicron powder. The partial orders with respect to P CO were determined as 0.58 and 0.54 for the α -Fe 2 O 3 , and the Pd/ α -Fe 2 O 3 submicron powders, respectively. The kinetic results obtained from both catalysts were compared with those for the α -Fe 2 O 3 ﬁne powder catalysts and were used to understand the reaction mechanism.


Introduction
It is widely known that CO oxidation is one of the most important reactions in environmental research. Extant studies [1] indicate that transition metal oxides, including MnO 2 , Fe 2 O 3 , Co 2 O 3 , NiO, and CuO, as well as noble metals, are highly active for CO oxidation. The catalytic performance of metal oxides is enhanced by increasing their surface area. Thus, metal oxide catalysts were synthesized in the forms of nanoparticles to yield high surface areas. Generally, nanostructured metal oxides are significantly active catalysts when compared with non-nanostructured metal oxides. However, it is not easy to utilize these oxides as a medium for noble metal catalysts due to their extremely small size. Among the transition metal oxides, iron oxide is synthesized in the form of spherical particles with a controlled size, which attracts research attention with respect to its utilization as support. Noble metals on Fe 2 O 3 particles were indicated as appreciably active [2]. Recently, Fe 2 O 3 was frequently used as a model system in studies that focused on strong metal-support interactions [3]. Specifically, Fe 2 O 3 spherical particles with a submicron size have been rarely examined as a catalyst and support to date.
In any kinetics study, monitoring the time-dependent variation of the concentrations of reactants and/or products is considered to be crucial. It has been known that photoacoustic spectroscopy (PAS) is suitable to in situ monitor a reaction processes in its initial stage [4]. Since signals obtained from PAS rely on the concentration of a target species and PAS detects extremely low level molecular gases with high selectivity and large signal intensity, temporal variations in concentration can be readily measured even in a short period of reaction times where rates are high. Thus, low molecular level photoacoustic results are believed to provide more accurate kinetic information for catalytic reactions, especially when the reaction occurs on a relatively clean surface of catalysis.
In this study, we prepared α-Fe 2 O 3 spherical particles (sp) with an average diameter of approximately 200 nm by using a solvothermal method. Subsequently, α-Fe 2 O 3 submicron powder was tested as a catalyst for CO oxidation and as a medium for a Pd metal catalyst. CO oxidation from α-Fe 2 O 3 and Pd/α-Fe 2 O 3 submicron powder catalysts was performed in a static reactor at a total pressure of 40 Torr. A CO 2, laser-based, photoacoustic technique with a differential photoacoustic cell was employed to investigate the kinetics of CO oxidation. Rates of CO 2 formation were calculated from the CO 2 photoacoustic data at an early reaction stage. The rates were measured with a stoichiometric reaction mixture and aided in determining the apparent activation energies in the temperature range of 225-350 • C. The partial orders with respect to CO and O 2 were calculated from the rates that were measured at various partial pressures of CO and O 2 at 350 • C. The catalytic behaviors of both the α-Fe 2 O 3 and Pd/α-Fe 2 O 3 submicron powders for CO oxidation were compared with those of an α-Fe 2 O 3 fine powder. Figure 1A shows the X-ray powder diffraction (XRD) pattern of magnetite spherical submicron particles that were directly prepared by using a solvothermal method. The diffraction pattern indicates that the magnetite phase has a face-centered cubic structure (JCPDS Card No. 00-039-1346). The magnetite powder was calcined at 600 • C in a flow of O 2 (10%)/He mixture, and a red powder was obtained. Figure 1B shows the XRD pattern of the red powder and indicates that the red powder corresponds to an α-Fe 2 O 3 phase with a rhombohedral structure (JCPDS Card No. 01-089-0598). In this study, Pd-loaded α-Fe 2 O 3 spherical submicron particles were prepared by using the impregnation method. Figure 1C shows the XRD pattern of the Pd/α-Fe 2 O 3 particles, which exhibits reflection peaks arising from Pd metal crystallites with a face-centered cubic structure (JCPDS Card No. 87-0641) at 2θ = 40.1 • , 46.6 • , and 68.1 • as denoted by the Miller indices (111), (200), and (220), respectively. By performing energy dispersive X-ray spectroscopy (EDS) on the Pd/α-Fe 2 O 3 sample, the Pd/Fe atomic ratio was determined as 0.123, which corresponds to 14 wt % Pd.    Figure 3A shows the particle size distribution of the α-Fe 2 O 3 spherical submicron particles. From the histogram, the mean diameter of the α-Fe 2 O 3 spherical submicron particles was estimated to be 196 nm. The α-Fe 2 O 3 spherical submicron powder sample is denoted as α-Fe 2 O 3 (sp200). N 2 adsorption measurements were performed for the α-Fe 2 O 3 (sp200) sample at liquid nitrogen temperature. The N 2 adsorption-desorption isotherm is shown in Figure 3B. An International Union of Pure and Applied Chemistry (IUPAC) classification reveals that the hysteresis loop of Figure 3B corresponds to the H3 type [5]. Figure 3C reveals the pore size distribution as determined by the Barret-Joyer-Halenda (BJH) method from the adsorption branch of the isotherm and shows that the pore size is distributed in the range of 3-20 nm. The Brunauer-Emmett-Teller (BET) surface area was calculated to be 19.0 m 2 /g for the α-Fe 2 O 3 (sp200) powder, 7.4 m 2 /g for the Pd/α-Fe 2 O 3 (sp200) powder, and 6.8 m 2 /g for the α-Fe 2 O 3 fine powder (α-Fe 2 O 3 (fp)). The surface area of the Pd/α-Fe 2 O 3 (sp200) is smaller than that of α-Fe 2 O 3 (sp200), suggesting that the openings of some pores could be blocked by the Pd particles. A kinetic study of CO oxidation was performed with the static method, and photoacoustic spectroscopy was employed to measure the rate of CO 2 formation in the early reaction stage of CO oxidation. Generally, the CO 2 photoacoustic signal measured with the CO 2 laser-based photoacoustic method is dependent on the excitation wavelength, the power of the light source, the chopping frequency, and the sensitivity of the microphone. Figure 4A shows that the CO 2 photoacoustic signal as a function of laser power exhibits an optimal linearity in the range from 1-4 W. Figure 4B displays the CO 2 photoacoustic signal measured as a function of CO 2 partial pressure and indicates that the signal is linear for CO 2 partial pressures below 12 Torr. Accordingly, subsequent measurements were performed with a CO 2 partial pressure below 12 Torr and a CO 2 laser power of 3 W. A blank test was performed by using a CO/O 2 /N 2 (20/10/10 Torr) mixture in the temperature range of 25-400 • C, which revealed that variations in the CO 2 photoacoustic signal were absent. The effect of temperature on the rate of CO 2 formation for the catalytic reaction in the static reactor was investigated by using a CO/O 2 /N 2 (20/10/10 Torr) mixture in the temperature range of 175-350 • C.  Figure 5 shows variations in the CO 2 photoacoustic signal relative to time for CO oxidation over catalysts at various temperatures. The rates of CO 2 formation were calculated from the CO 2 photoacoustic signals obtained in the reaction period range of 60-180 s after the injection of the reaction mixture into the static reactor. The rates were plotted as a function of reciprocal temperature according to the Arrhenius-type equation given by rate ∞ exp(−E a /RT) as shown in Figure 6. From the slopes of the linear curves in the temperature range of 225-350 • C, apparent activation energies were calculated as 16.8 (±0.3) kcal/mol for the α-Fe 2 O 3 (fp), 13.4 (±0.1) kcal/mol for the α-Fe 2 O 3 (sp200), and 13.2 (±0.5) kcal/mol for the Pd/α-Fe 2 O 3 (sp200). Reaction orders with respect to P CO and Po 2 were determined from the rates measured at various partial pressures of CO and O 2 at 350 • C. The rates were plotted as a function of partial pressures of CO and O 2 which based on the power rate law should follow rate = k(Pco) m (Po 2 ) n .

Discussion
With respect to CO oxidation over Fe 2 O 3 , it was reported that Fe 2 O 3 acts as a catalyst in the presence of O 2 (g) and as a direct oxidant for CO (g) in the absence of O 2 (g) [6,7]. In the presence of O 2 , the Fe 2 O 3 surface is primarily reduced by CO (g), and the reduced Fe 2 O 3 surface is successively re-oxidized by O 2 (g). In the catalytic oxidation of CO, the CO disproportion reaction (the Boudourd reaction corresponding to 2CO (g) (s) + CO 2 (g)) is considered as a side reaction because it is thermodynamically feasible below 630 • C based on the assumption of the standard free energy change ∆G 0 = 0. According to Li et al. [6], with respect to CO oxidation on Fe 2 O 3 nanopowder, the CO disproportion reaction occurs above 300 • C in the absence of O 2 (g). The photoacoustic measurements in our work were performed by using a CO/O 2 mixture at a total pressure of 40 Torr, and accordingly, the occurrence of CO disproportion is considered as infeasible under the reaction conditions. As shown in Figure 6, the apparent activation energies for CO oxidation were determined as 16.8 (±0.3) kcal/mol for the Fe 2 O 3 (fp) catalyst and 13.4 (±0.1) kcal/mol for the Fe 2 O 3 (sp200) catalyst in the temperature range of 225-350 • C. It is expected from the magnitude of the apparent activation energies that the reactants are chemisorbed on the catalyst surface. Specifically, Fe 2 O 3 is oxygen deficient and oxygen vacancies are considered as the predominant defect for the oxide. Oxygen vacancies are readily generated in the oxide when Fe 2 O 3 is heated at a low pressure [8,9]. Typically, oxygen vacancies in metal oxides act as adsorption sites for O 2 (g) because they serve as electron donors. When O 2 (g) is adsorbed (ads) on the oxygen vacancy, it is dissociated into two oxygen atoms as follows: where e − represents an electron trapped at the oxygen vacancy site. Given that an O − (ads) ion is active, CO (g) reacts with O − (ads) to form CO 2 − (ads): CO (g) + O − (ads) CO 2 − (ads). The resultant CO 2 − (ads) ion is desorbed from the surface to produce CO 2 (g) and the oxygen vacancy is simultaneously regenerated in the oxide. If the O − (ads) ions act as adsorption sites for CO (g) to form CO 2 (g), then the partial order with respect to O 2 is derived to be 0.5 [4]. In this study, as shown in Figure 8, the rates of CO 2 formation for both the Fe 2 O 3 (fp) and Fe 2 O 3 (sp200) powders were obtained as independent of Po 2 , and this indicates that the surface is continuously saturated by O 2 (g). The zero-order kinetics for Po 2 suggests that CO (g) preferably reacts with O 2− (latt) as opposed to the O − (ads) ion. The interaction between CO (g) and the lattice (latt) oxygen of Fe 2 O 3 is described by the following equilibrium: When the CO 2 − (ads) ion reacts further with O 2− (latt), a carbonate ion is formed as follows: The carbonate ion is decomposed into CO 2 (g) according to the following reaction: CO 3 2− (ads) → CO 2 (g) + O 2− (latt). If Equations (3) and (4) are included in the reaction mechanism, then the rate law is derived as first-order kinetics for Pco based on the assumption of constant [O 2− (latt)] [4]. In this study, the partial order to Pco is determined as 0.85 (±0.03)-order for the α-Fe 2 O 3 (fp) catalyst and 0.58 (±0.1)-order for the α-Fe 2 O 3 (sp200) catalyst, thereby suggesting that an inhibition process by CO 2 is included in the reaction mechanism under the reaction conditions. Previous studies of CO 2 adsorption on Fe 2 O 3 [10,11] indicate that CO 2 is chemisorbed on the surface of iron oxide to form bidentate carbonate. Several studies suggested that oxygen vacancies in metal oxide act as adsorption sites for CO 2 as well as O 2, wherein CO 2 (g) is chemisorbed on oxygen vacancies and the resultant CO 2 (ads) further reacts with O 2− (latt) to form carbonate ions [12][13][14][15][16][17]. Oxygen vacancies are considered as predominant defects in Fe 2 O 3 , and thus it is feasible to form a carbonate species on the surface of Fe 2 O 3 . If CO 2 is adsorbed on the lattice oxygen of Fe 2 O 3 , then CO 2 (g) is in equilibrium with carbonate ions according to the following equilibrium: CO 2 (g) + O 2− (latt) CO 3 2− (ads). When both CO and CO 2 competitively interact with the lattice oxygen, the partial order with respect to Pco is observed as a value less than 1. As shown in Figure 7, the partial order to Pco for CO oxidation over Fe 2 O 3 (sp200) was observed as 0.54 (±0.06), and this value is lower than 0.85 (±0.03) for the reaction relative to Fe 2 O 3 (fp). These results suggest that the inhibition process by CO 2 is more feasible on Fe 2 O 3 (sp200) as opposed to the Fe 2 O 3 (fp). Conversely, the apparent activation energy for CO oxidation on the Pd/Fe 2 O 3 (sp200) catalyst is 13.2 kcal/mol, as shown in Figure 6. This value is in good agreement with 13.3 kcal/mol for CO oxidation on 1 wt % Pd/FeO x in the temperature range of 120-160 • C [2] and 12.5 kcal/mol on 5 wt % Pd/Al 2 O 3 in the temperature range of 120-160 • C [18], although it slightly exceeds 8.2 kcal/mol on 1.9 wt % Pd/FeO x in the temperature range of 5-80 • C [19]. As shown in Figure 8, the partial order with respect to Po 2 on the Pd/Fe 2 O 3 (sp200) catalyst is 0.48 (±0.03)-order, and this is entirely different from the zero-order kinetics observed for both the Fe 2 O 3 (sp200) and Fe 2 O 3 (fp) catalysts. The 0.48 (±0.03)-order is close to 0.5-order, and thus it is possible to consider that O 2 (g) is dissociatively adsorbed on the Pd surface to form two oxygen atoms (O − (ads)) based on equilibrium (1). Therefore, CO (g) is easily adsorbed on the active O − (ads) ion to form CO 2 − (ads) as follows: When CO (g) reacts with the O − (ads) ion to produce CO 2 (g) based on Reactions (5) and (6), the partial order to Po 2 is derived to be 0.5-order [8]. The 0.48 (±0.03)-order obtained in this study supports that O 2 (g) is dissociatively adsorbed on the surface of Pd. The partial order to Pco for the Pd/Fe 2 O 3 (sp200) catalyst, determined to be 0.58 (±0.1), implies that the inhibition process by CO 2 is involved in the reaction mechanism. The O − (ads) ions formed on the Pd surface act as an adsorption site for CO 2 (g). If CO 2 (g) is adsorbed on O − (ads), then carbonate ions are formed according to the following equilibrium: This implies that CO 2 (g) inhibits CO 2 formation via the reaction between CO (g) and O − (ads). Although it is not possible to exclude oxygen spillover from Fe 2 O 3 to the Pd surface in the reaction mechanism, the kinetic results for CO oxidation over Pd/Fe 2 O 3 (sp200) suggest that CO (g) favorably reacts with O − (ads) to form CO 2 − (ads). The CO 2 − (ads) further reacts with O − (ads) to produce CO 3 2− (ads), and the resultant CO 3 2− (ads) decomposes into CO 2 (g). When both CO (g) and CO 2 (g) are competitively adsorbed on the O − (ads) on the Pd surface, the partial order to Pco is observed as a value below 1 as obtained for the Pd/Fe 2 O 3 (sp200) catalyst (0.58 (±0.1)-order).

Experimental
The α-  [20,21]. Specifically, NaAc (1.5 g) was dissolved in a mixture of EG (20 mL) and DEG (20 mL) at 60 • C with constant stirring for 30 min. Subsequently, FeCl 3 ·6H 2 O (1.08 g) was dissolved in the NaAc solution with vigorous stirring for 30 min. The formation of viscous slurry was transferred into a teflon-line stainless-steel autoclave with 50 mL capacity. The autoclave was maintained at 200 • C for 10 h and cooled to room temperature to obtain precipitates. The resulting precipitates were rinsed with distilled water and absolute alcohol several times, dried at 60 • C for 6 h, and a black powder was obtained. The black powder (Fe 3 O 4 ) was calcined at 500 • C in air, cooled to room temperature, and a red powder (α-Fe 2 O 3 ) was obtained as a final product. Furthermore, α-Fe 2 O 3 -supported Pd was prepared according to the following procedures: the Fe 2 O 3 submicron particles were dispersed in ethanol, followed by sonication for 3 h at room temperature by using an ultrasonic cleaner, and PdCl 2 was added into the mixture. After sonication for 3 h, a hydrazine solution was dropped into the suspension while stirring, in which Pd 2+ was reduced to Pd 0 by hydrazine [22,23]. The precipitates were separated by centrifugation, washed with ethanol five times, and the Pd/Fe 2 O 3 sample was obtained. In this study, α-Fe 2 O 3 fine powder, used as a reference material, was prepared by the wet method by using FeCl 3 and NH 4 OH solution [24].
The phase identification of the sample was performed by using XRD and using a D/max-RB diffractometer (Rigaku, The Woodlands, TX, USA). The size, morphology, and chemical composition of the spherical particles were determined by using FE-SEM (JEOL-6701F, JEOL, Tokyo, Japan) equipped with an energy dispersive X-ray spectroscopy (EDS). In order to measure the surface area and porosity of samples, N 2 adsorption measurements were conducted with a Micromeritics Autosorb-iQ 2ST/MP instrument (Quantachrome Instruments, Boynton Beach, FL, USA).
Kinetic measurements of CO oxidation were performed in a static reactor by using a CO 2 laser-based photoacoustic technique. As indicated in a previous study [4], the photoacoustic technique with a differential photoacoustic cell was proven as a suitable technique for in situ monitoring of the change in the CO 2 concentration during the catalytic oxidation of CO. The differential photoacoustic cell was composed of a reference cell and a sample cell that were separated from each other by a ZnSe window. The reference cell was filled with a gaseous mixture of CO 2 (0.2 Torr) and N 2 (39.8 Torr), and the sample cell was directly connected to the reactor that was composed of quartz tubing with a volume of 21 cm 3 . The photoacoustic cell was a Helmholtz resonator with a diameter of 19 mm and a length of 33 mm, with an adjoining tube with a diameter of 1 mm and a length of 28 mm. Additionally, CO 2 photoacoustic signals from the microphones attached to the sample (signal A) and reference (signal B) cells were detected by a lock-in amplifier (EG & G Princeton Applied Research Model 5210). The signal ratio (A/B) was recorded as a function of time by using a personal computer. The total pressure (CO/O 2 /N 2 ) in the reactor was maintained at 40 Torr and filled with N 2 as a buffer gas. The purity of gases exceeded 99.999%, and the gases were dehydrated with suitable filters. The output beam of a continuous wave (cw) CO 2 laser (Synrad Series 48-1-28) operating in multilines of 10.6 µm was modulated at the nonresonance condition of 20 Hz. The rates of CO 2 formation for the catalytic reaction were estimated from the CO 2 photoacoustic data in the early reaction stage.

Conclusions
CO oxidations over Fe 2 O 3 (fp), Fe 2 O 3 (sp200), and 14 wt % Pd/Fe 2 O 3 (sp200) catalysts were kinetically measured by using a photoacoustic technique in a static reactor at a total pressure of 40 Torr. The partial order with respect to O 2 was observed as zero-order for both the Fe 2 O 3 (fp) and Fe 2 O 3 (sp200) catalysts and observed as 0.48-order for the 14 wt % Pd/Fe 2 O 3 (sp200) catalyst. The surface of Fe 2 O 3 (fp) and Fe 2 O 3 (sp200) catalysts is considered as continuously saturated by O 2 (g) adsorption to form lattice oxygens. However, in the case of the Pd/Fe 2 O 3 (sp200) catalyst, O 2 (g) is dissociatively adsorbed on the Pd surface to form two oxygen atoms (O − (ads)). The kinetic results suggest that CO (g) is chemisorbed on the lattice oxygen with respect to the Fe 2 O 3 (fp) and Fe 2 O 3 (sp200) catalysts. Conversely, CO is preferably adsorbed on the O − (ads) ion with respect to the Pd/Fe 2 O 3 (sp200) catalyst. With respect to CO oxidation, the Fe 2 O 3 (fp), Fe 2 O 3 (sp200), and Pd/Fe 2 O 3 (sp200) catalysts indicated that the partial orders with respect to Pco are less than 1, which suggests that an inhibition process by CO 2 is included in the reaction mechanisms.
It is considered that the kinetic data collected at the early stages of a catalytic reaction comprising relatively high reaction rates are different from those obtained at later stages that can be controlled by constant activity of the catalyst. It is also noted that the kinetic measurement of a catalytic reaction at a high-pressure condition can be more advantageous to integrated kinetic analysis because the rate of a catalytic reaction is proportional to the surface coverage, which depends on the pressure of gaseous reactants. Nevertheless, it is noticeable that the photoacoustic signals showed a linear relationship with the concentration of species of interest even under low pressure conditions and can be recorded as a function of time-on-stream during the reaction process. Hence, PAS is suitable to study reaction kinetics at the molecular level during the early stages of catalytic oxidation of CO, providing relevant information on the reaction mechanism.