Size-Controlled Model Co Nanoparticle Catalysts for CO₂ Hydrogenation: Synthesis, Characterization, and Catalytic Reactions

Abstract

Model cobalt catalysts for CO 2 hydrogenation were prepared using colloidal chemistry. The turnover frequency at 6 bar and at 200−300°C increased with cobalt nanoparticle size from 3 to 10 nm. It was demonstrated that near monodisperse nanoparticles in the size range of 3−10 nm could be generated without using trioctylphosphine oxide, a capping ligand that we demonstrate results in phosphorus being present on the metal surface and poisoning catalyst activity in our application.KEYWORDS: Cobalt nanoparticles, CO 2 hydrogenation, heterogeneous catalysis, catalytic poisoning C obalt-catalyzed processes and specifically the conversion of synthesis gas to hydrocarbons using cobalt (Co), Fischer−Tropsch synthesis, although long established, 1 have recently become a topic of renewed interest. This results from increased demand and declining fossil fuel reserves making both gas-to-liquid and biomass-to-liquid attractive routes to transportation fuels. 2 Especially when derived from biomass, the synthesis gas typically contains a significant fraction of CO 2 , however studies on CO 2 hydrogenation and its catalytic mechanism on Co are much less well developed than the analogous reaction with CO. 3,4 Because of its environmental impact through the greenhouse effect, fixation of CO 2 by reaction (rather than simply capture and storage) also makes studying the possibility of Co-catalyzed CO 2 hydrogenation a topic of considerable interest. 4 Additionally, there is an important technical precedent in terms of producing desirable oxygenated products by incorporation of CO 2 in such reactions (important in producing synthetic fuels). In the case of classical Cu/ZnO catalyzed methanol production, Chinchen et al. demonstrated using isotopic labeling studies that it is CO 2 , rather than CO, that is incorporated in the methanol produced. 5 Few kinetic studies of the chemistry of Co-catalyzed CO 2 hydrogenation exist. In our laboratory the conversion of CO 2 to CH 4 was explored at atmospheric pressure in a batch reactor over high-purity Co foil, 6 subsequently Welder and co-workers also studied the hydrogenation of CO 2 over Co foils in a flow reactor. 7,8 Wetherbee and Bartholomew however have reported on the use of a Co/SiO 2 catalyst prepared by incipient wetness impregnation to obtain various kinetic parameters, 9 although this does not contain information at the atomic scale about the nature of the Co catalyst particles, and it is this problem that we now address.In our laboratory size-and morphology-controlled nanoparticles, which are synthesized using colloidal techniques, have allowed the production of model catalysts via deposition within mesoporous silica supports. This is important because selectivity in multipath reactions has been found to vary with catalyst particle size and shape. In combination with powerful characterization techniques that provide atomic-and molecularlevel information, these model nanoparticle catalysts have been employed in studying fundamental mechanistic questions in many key chemica...

Nanocatalysis I: Synthesis of Metal and Bimetallic Nanoparticles and Porous Oxides and Their Catalytic Reaction Studies

“…Along with the development of nanotechnology, catalytic reactions regarded as structure-insensitive using nanoparticles with broad size distribution, turned out to become structure-sensitive reaction with well-defined nanoparticles. Recently, monodisperse Co nanoparticles with sizes ranging from 3 to 10 nm showed size-dependent activity enhancement of CO 2 hydrogenation which was an analogous reaction of Co catalyzed Fischer-Tropsch reaction [12]. The turnover frequency of CO 2 hydrogenation was significantly higher on the larger Co nanoparticles, which was in good agreement with the values reported for an incipient wetness impregnation prepared catalyst.…”

“…Transition metal nanoparticles including Co and Fe can be obtained through thermal decomposition by injection of organometallic precursors such as dicobalt octacarbonyl and iron pentacarbonyl into the surfactant solution at an elevated temperature [11]. The size of Co nanoparticles can be varied from 3 to 10 nm by the temperature of the oleic acid solution, when the Co precursor is injected rapidly [12]. In order to synthesize small nanoclusters less than 1 nm, a dendrimer which is a macromolecule with a central core surrounded by hyper-branched repeating units can be used as a template as well as a stabilizing agent [13].…”

Combining in Situ NEXAFS Spectroscopy and CO2 Methanation Kinetics To Study Pt and Co Nanoparticle Catalysts Reveals Key Insights into the Role of Platinum in Promoted Cobalt Catalysis

“…16 The use of nanomaterials to address questions specifically in the study of cobalt based catalysts for Fischer Tropsch has recently been reviewed, 17 and we have already used size controlled cobalt nanoparticles to explore particle size effects in CO 2 hydrogenation. 18 To understand better the exact role of precious metals in these types of catalysts, we have recently reported on the preparation of Pt-Co bimetallic nanoparticles, where each nanoparticle contains an approximately 1:1 atomic of the two metals. However, we found that these particles were poor CO 2 methanation catalysts.…”

“…Here, however, we preactivated the cobalt catalyst by reduction at 450 °C in H 2 , a treatment anticipated to afford fully reduced cobalt nanoparticles -we have previously reported a water loss / hydrogen adsorption temperature programmed reduction peak at this temperature for pure cobalt nanoparticle based catalysts. 18 Oxidation by water (a reaction product) is often considered to be a possible deactivation mechanism during cobalt catalyzed hydrogenations, 19,20 although others have been unable to observe this during in situ NEXAFS spectroscopy of cobalt crystallites in a 1 : 1 mix of H 2 O and H 2 . 21 However, if methane (or other hydrocarbon) formation occurs after breaking carbon-oxygen bonds in CO 2 , oxygen may accumulate on (and beneath) the surface eventually leading to near surface oxidation before water is formed (Figure 3(a)).…”

Recent developments in the application of nanomaterials to understanding molecular level processes in cobalt catalysed Fischer–Tropsch synthesis

“…54 Overall the ability to prepare nanoparticles of well-defined sizes (e.g. samples with a particle size distribution of only  0.5 nm 46 ) by a selection of routes has confirmed a particle size effect exists. This is likely to be important in the design of practical catalysts for FT synthesis.…”