CO Oxidation by Cobalt Oxide: An Experimental Study on the Relationship between Nanoparticle Size and Reaction Kinetics

Review Article

Austin J Chem Eng. 2014;1(2): 1008.

CO Oxidation by Cobalt Oxide: An Experimental Study on the Relationship between Nanoparticle Size and Reaction Kinetics

Bijith D Mankidy, Nianthrini Balakrishnan, Babu Joseph* and Vinay K Gupta

Department of Chemical & Biomedical Engineering, University of South Florida, USA

*Corresponding author: Babu Joseph, Department of Chemical & Biomedical Engineering, University of South Florida, ENB 118, 4202 E Fowler Ave, Tampa, FL 33620, USA

Received: July 11, 2014; Accepted: Aug 04, 2014; Published: Aug 07, 2014


CoO nanoparticles of different sizes (1-14nm) were synthesized using colloidal techniques. Nanoparticle size was controlled by adjusting the ratio of cobalt precursor to surfactant. Activation energies for CO oxidation as a function of nanoparticle size were experimentally measured in an in-situ FTIR reactor using transient state kinetic studies. Experimental results are in agreement with a two-step mechanism for CO oxidation: CO reacts with O2 to form an intermediate OCO followed by the dissociation of the intermediate to CO2 molecule. The activation energies for CO oxidation reaction increased as the size of CoO nanoparticle increased. Experimental measurements of activation energy vary from 9.4kJ/mol to 21.3kJ/mol for step-1 and from 63.6kJ/mol to 95.4kJ/mol for step-2 as the size of CoO nanoparticle increases. The findings reported here shed light on the size effect and mechanistic aspects of CO oxidation on CoO.

Keywords: CO Oxidation; CoO Nanoparticle; Size-effect; In-situ FTIR


The use of nanometer sized catalysts has been of widespread interest as the nanoscale provides several advantages. Nanoparticle catalysts possess an increased number of active sites per unit mass of catalyst material, which helps to reduce the amount of catalyst especially when the catalyst material used is expensive. In addition, as the size of nanoparticle decreases, the percentage of atoms on the surface with low coordination number increases [1], which enhances the effect associated to corners or edges of a nanoparticle that possess unique electronic properties compared to an atom on a flat surface. For example, studies by Dahl and coworkers [2] on single crystal ruthenium, Ru(001) have shown high activity of N2 molecule on edge sites with lower coordination numbers compared to atoms on a flat surface that possess higher coordination numbers. In the study, Dahl and coworkers used experiments and DFT calculations to demonstrate that dissociation of N2 on step sites was at least 9 orders of magnitude higher than on terraces [2].

In a similar manner, the chemistry of different reactions may be tuned by using nanoparticles of different sizes. For example, CO oxidation is an important reaction from a scientific and an industrial point of view due to the interest in purification of indoor air and automotive or industrial exhausts, and in the production of H2 rich feed streams devoid of CO for the fuel cell industry [3-7]. In the case of CO oxidation reaction, gold and other noble metals such as Pt, Ru, Rh, Ir have been extensively studied and the role of catalyst size on the reaction kinetics has been well established. Recently, there have been a few reports on inexpensive non-noble metal catalysts for CO oxidation.[4,8-14] For instance, Xie and coworkers [15] have synthesized unique nanorod-shaped catalysts using cobalt oxide (Co3O4) that oxidize CO at temperatures as low as -77°C. The author describes that this behavior may be due to the higher chemical activity of preferentially exposed atoms on the catalyst surfaces of the nanorods.

These past reports underscore the importance of morphology of base transition metal oxides and of understanding the role of these unique structural features to develop highly efficient oxidation catalysts. To the best of our knowledge, systematic understandings of the impact of CoO nanoparticle size using experiments are limited. To understand the effect of size on CO oxidation, we synthesized CoO nanoparticles of different sizes and systematically examined CO oxidation as a function of nanoparticle size using in-situ FTIR studies. Activation energies of reaction steps were measured experimentally using in-situ FTIR studies under transient conditions to gain insight into the reaction mechanisms [16, 17]. Experimental results suggest that CO oxidizes via a two-step mechanism: CO reacts with O2 to form an intermediate OCO followed by the dissociation of the intermediate to CO2 molecule. The activation energies for CO oxidation reaction increased as the size of CoO nanoparticle increased.

Prior studies suggest 3 possible mechanisms for CO oxidation on metal and metal oxide surfaces: [15,18,19]

  1. CO + OL→ CO2 + OV
  2. CO + O2→OOCO →CO2 + O
  3. CO + O2→ O + OCO → CO2 + O

In Mechanism I, CO adsorbed on the surface reacts with the lattice oxygen OL on the CoO(100) surface to form CO2 and leaves a vacant oxygen site (OV) on the surface [15]. In Mechanism II, CO adsorbed on the surface reacts with adsorbed oxygen to form an intermediate OOCO which later forms CO2 leaving an oxygen atom behind on the surface [19]. In Mechanism III, CO adsorbed on the surface reacts with molecular oxygen to form an intermediate OCO and O on the surface [18]. OCO intermediate then desorbs to form CO2 in the gaseous phase. DFT calculations on the activation barriers for these reactions suggest that Mechanisms I and II have much higher activation barriers compared to Mechanism III suggesting this as the most likely route for the oxidation [20]. In this study, Mechanism III is explored in more detail. Mechanism III is a two-step process with two activation barriers one for the formation of the OCO intermediate and a second barrier for the formation of CO2.


Synthesis of a model CoO/SiO2 colloidal catalyst

Surface modified SiO2

Colloidal non-porous SiO2 substrates were used to support CoO nanoparticle catalysts. In the fundamental study reported here, non-porous support are advantageous because porous catalyst support structures become a barrier for the transfer of reactant/product species from the catalyst surface into the gas phase [21]. Since repeated adsorption and desorption of radicals may take place within the pores, the use of a porous supported catalyst becomes more complex and not ideal for fundamental studies of catalytic reactions such as one studied here. Sub-micron non-porous spherical SiO2 colloids were synthesized using a modified Stöber process [22]. The synthesis procedure is well documented in literature. Typically, 3.14 ml of 28-30 wt% NH4OH was added to an ethanol-water mixture and equilibrated for 30 min. An aliquot of 6ml TEOS was added and stirred for 6 hours at room temperature to yield monodisperse sub-micron SiO2 particles. The NH4OH in solution helps to control the charge of the SiO2 colloids. The SiO2 colloid solution was purified by centrifuging the solution at 7,000 rpm for 30 minutes and the resulting residue was washed 3 times with water. The particles were dried overnight under vacuum at room temperature.

Cobalt oxide nanoparticles were immobilized by tailoring the surface of Stöber SiO2 with favorable chemical functional groups. Surface -OH groups on Stöber SiO2 were covalently modified by methacryloxypropyltrimethoxysilane (MPS), a small molecule ligand that contains a carboxyl (-C=O) functional group.MPS was added to a colloidal (6 wt %) Stöber SiO2 solution dispersed in 70% ethanol in water and stirred for approximately 12 hours. The solution was held at 80°C for 1 hour to promote covalent bonding of the organosilane molecules to the surface of the SiO2 colloids [23,24]. The amount of MPS ligand added was ~50% in excess of that required for full coverage, which was estimated based on 25 Å2 per molecule of MPS on the SiO2 surface. The colloidal solution was purified in a similar manner to Stöber SiO2 colloids. Colloidal suspensions of surface modified SiO2 in toluene were prepared by dispersing ~1mg of solids with 10ml of toluene for further use.

Size control during synthesis of CoO catalysts

The size of cobalt nanoparticles was tuned by varying the ratio between the amount of dicobalt octacarbonyl and AOT surfactant in the recipe for CoO synthesis [25-27]. First, a solution of 30 ml toluene and 150 mg of AOT was refluxed at 110°C under a nitrogen atmosphere. Separately, a 10 ml solution containing 1.2 gm dissolved dicobalt octacarbonyl in toluene was prepared. A 200 μL aliquot of cobalt precursor solution was injected rapidly into the hot surfactant solution. After 2 hours, ~1ml of this solution was withdrawn for mixing with the surface modified SiO2 colloidal solution. An additional 800 μL of cobalt carbonyl precursor was added into the hot refluxing toluene solution and the reaction was continued for an additional 1 hour and a new sample was withdrawn at this point. Thereafter, at every 1 hour interval, additional 1 ml of cobalt precursor solution was injected into the hot solution. Two more samples were collected after a total of 6ml and 10ml of cobalt precursor solution were added to the hot solution. Each cobalt solution that was withdrawn was mixed with ~1gm of surface modified SiO2 colloidal solution dispersed in toluene. The nanoparticles of CoO were immobilized on the surface of the SiO2 support via self-assembly and after a few hours, CoO/ SiO2 nanocomposites settled to the bottom, whereas the excess CoO nanoparticles remained in the supernatant. The CoO/SiO2 residue was separated by decantation. Purification cycles of settling and decantation were repeated 2-3 times by dispersing the residue with pure toluene after every decantation step. The final product was dried at 60-70°C in the vacuum oven for ~1day. As indicated by the above procedure, the samples of CoO of different sizes supported on surface modified SiO2 were obtained by adjusting the molar ratio (R) of cobalt precursor to surfactant. The approximate R values in used in this study were 0.2, 1, 6 and 10.

Temperature programmed reaction

CO oxidation reaction was studied using AABSPEC #2000A module on a Bio-RAD Excalibur FTS3000 IR instrument. AABSPEC #2000A is a stainless steel reactor equipped with zinc selenide (ZnSe) windows for in-situ FTIR spectroscopy measurements. CoO/SiO2 nanocomposites were pressed into pellets and placed along the IR beam on a programmable hot finger inserted into the FTIR reactor. Actual temperatures were monitored by an external thermocouple. After placing the catalyst, the chamber was preheated to 150°C for 30min under a 30 sccm N2 flow to remove any water vapor and then cooled back to room temperature. A gas mixture of CO (10 sccm) and compressed dry air (20 sccm) were introduced using mass flow controllers (approximate ratio of 2.4 for CO to oxygen). Both inlet and outlet valves were closed after 10-15 min of steady flow of gases. A new background signal was collected against which further spectra were compared. Temperature programmed reactions were carried out by increasing the temperature from room temperature to 475°C at a constant heating ramps set at 10, 7.5, 4.5 and 2°C/min. FTIR spectra were collected in transmission mode at regular intervals of time during the temperature programmed reaction. For 10°C/min runs, pure catalyst samples were pressed into ~0.1 mg pellets. In order to get better signal-to-noise ratio, for 7.5, 4.5 and 2°C/min runs, samples were diluted with KBr thereby increasing the transmittance of IR signal.

Material characterization

Electron microscopy measurements were performed on a FEI Morgagni 268D and TECNAI F20 for low resolution and high resolution imaging of CoO/SiO2 colloids. Samples were prepared by drying a drop of the colloidal solution on a carbon-support film TEM grid (Electron Microscopy Sciences, PA). Hydrodynamic diameter and polydispersity of particles were estimated from dynamic light scattering (DLS) technique using a Nano-S Zetasizer (Malvern). A Philips X'pert materials research diffractometer was used to analyze the crystal structure of the cobalt catalyst nanoparticles.

Results and Discussion

Hydrodynamic size of CoO nanoparticles were approximately 1, 2, 6 and 14nm for different samples by DLS measurements as shown in Figure 1. Figures 2 a,b & c depict TEM images of samples of CoO/SiO2 samples with CoO average sizes 14, 6 and 2nm supported on SiO2 colloids. Aggregation of larger CoO nanoparticles was observed that was caused due to the low surfactant concentration. The crystallinity of unsupported CoO nanoparticles prepared using decomposition technique was studied using XRD as shown in Figure 2d. The peak positions were identical in all the samples with different CoO sizes. XRD patterns obtained from previous reports on the synthesis of CoO [27] and a comparison of X-ray diffraction patterns with previous reports suggests that the oxide synthesized from our experiments was CoO in contrast to the other commonly synthesized oxide, Co3O4 [28-30].