Planar oxide supported rhodium nanoparticles as model catalysts

Abstract

C 2 H 4 ∕CO∕H 2 reaction is investigated on Rh∕SiO 2 model catalyst surfaces. Kinetic reactivity and infrared spectroscopic measurements are investigated as a function of Rh particle size under near atmospheric reaction conditions. Results show that propionaldehyde turnover frequency (TOF) (CO insertion pathway) exhibits a maximum activity near hd p i ¼ 2.5 nm. Polarization modulation infrared reflection absorption spectroscopy under CO and reaction (C 2 H 4 ∕CO∕H 2 ) conditions indicate the presence of Rh carbonyl species (RhðCOÞ 2 , Rh(CO)H) on small Rh particles, whereas larger particles appear resistant to dispersion and carbonyl formation. Combined these observations suggest the observed particle size dependence for propionaldehyde production via CO insertion is driven by two factors: (i) an increase in propionaldehyde formation on undercoordinated Rh sites and (ii) creation of carbonyl hydride species (Rh(CO)H)) on smaller Rh particles, whose presence correlates with the lower activity for propionaldehyde formation for hd p i < 2.5 nm.Ethylene hydroformylation | CO insertion | polarization modulation infrared reflection absorption spectroscopy | Rh/SiO2 A complicating effect of understanding the structure-activity relationships of heterogeneous catalytic reactions at ambient pressures (near 1 atm or above) is the known ability of reactant gas environments to alter the morphology, particle dispersion, and even the types of adsorbates present on certain supported nanoparticle (NP) systems (1-4). For example, elevated pressure CO ambients have been shown to oxidatively disrupt and disperse Rh nanoparticles, creating highly dispersed gem-dicarbonyl species RhðCOÞ 2 on oxide supports (1-3, 5-8). Under elevated pressure (CO∕H 2 ) and (CO 2 ∕H 2 ) hydrogenation reaction conditions, Rh(CO)H carbonyl hydride species can also be formed (3, 9-12). The creation and stability of such carbonyl species (RhðCOÞ 2 , Rh(CO)H) can depend on particle size, gas pressure, and surface temperature. The cumulative effect of such factors could potentially have an important effect on the overall catalytic properties of the supported Rh NP surface, especially for catalytic reactions involving surface bound CO. Developing an understanding of how such factors can affect the catalytic properties of supported Rh NPs is important from a fundamental surface chemistry perspective. Unraveling these details will require the ability to conduct spectroscopic and kinetic investigations of an informative probe reaction (i) under near atmospheric reaction conditions and (ii) on supported Rh nanoparticles surfaces with well defined initial particle size distributions.CO insertion into adsorbed alkyl groups (R-C x H y ) to form oxygenates (e.g., alcohols, aldehydes) is an important reaction step in many heterogeneous catalytic reactions. For example, C 2 H 4 hydroformylation (C 2 H 4 þ CO þ H 2 ) is a well known reaction for the synthesis of aldehydes via the CO insertion reaction (13). Insightful studies by Chuang and coworkers (14-17) and others (...

Simulating the Complexities of Heterogeneous Catalysis with Model Systems: Case studies of SiO2 Supported Pt-Group Metals

“…Details of the experimental apparatuses utilized for these studies have been described in detail in previous publications [36][37][38][39][40][41]. Briefly, the primary experimental apparatus consists of a UHV surface analysis system coupled to a contiguous atmospheric pressure reactor/infrared cell, allowing for in situ preparation, characterization, and transfer of model catalyst surfaces from UHV conditions into the batch reactor cell.…”

“…In the following article, we present a review of our recent investigations into the physical and catalytic properties of planar model catalyst surfaces consisting of Pt-group metal (Rh, Pt) nanoparticles supported on a SiO 2 thin film support [36][37][38][39][40]. In the first sections of the review, we discuss our recent results characterizing the physical (e.g.…”

Tracking Rh Atoms in Zeolite HY: First Steps of Metal Cluster Formation and Influence of Metal Nuclearity on Catalysis of Ethylene Hydrogenation and Ethylene Dimerization

“…Rhodium was chosen as the catalytic species because of its rich catalytic chemistry in forms ranging from isolated cationic complexes [17][18][19][20][21][22][23][24][25] to small clusters [26][27][28][29] to larger metallic particles. [30][31][32][33] The catalytic properties of the supported rhodium species were probed with ethylene hydrogenation and ethylene dimerization as test reactions, because these reactions take place under mild conditions that minimize changes in structure of the rhodium species 12, 34-35 and because ethylene and H 2 form ligands on the rhodium that are identifiable by IR spectroscopy.…”

Surface chemistry: Key to control and advance myriad technologies

“…Other crucial aspects include the role of the second metal in alloy or bimetallic catalysts, the role of particle size in supported metal catalysts, and the role of different support materials (different oxides, carbons, etc.). The paper from McClure et al (Goodman group) (22) shows the quantitative effect of rhodium nanoparticle size on the rates of C 2 H 4 hydroformylation (C 2 H 4 + CO + H 2 ) for the synthesis of aldehydes through CO insertion over silica-supported Rh. The Rh particles were prepared on a planar model silica support, and their size was characterized by STM, chemisorptions, and other reaction rates for CO oxidation and C 4 H 10 hydrogenolysis.…”