Research Article Cite This: ACS Catal. 2017, 7, 8247-8254
pubs.acs.org/acscatalysis
Unifying Concepts in Room-Temperature CO Oxidation with Gold Catalysts Frieder Kettemann,† Steffen Witte,† Alexander Birnbaum,† Benjamin Paul,‡ Guylhaine Clavel,† Nicola Pinna,† Klaus Rademann,† Ralph Kraehnert,‡ and Jörg Polte*,† †
Department of Chemistry, Humboldt-Universität zu Berlin, Brook-Taylor-Straße 2, 12489 Berlin, Germany Technische Universität Berlin, Straße des 17. Juni 124, 10623 Berlin, Germany
‡
S Supporting Information *
ABSTRACT: The oxidation of CO is a fundamental model reaction in heterogeneous catalysis. This contribution presents an uncommon approach to investigate a catalytic gas-phase reaction by using colloidal gold and provides a unified picture of the CO oxidation of supported gold nanoparticles at room temperature. Our experiments on ligand-free colloidal gold nanoparticles prove that gold activates molecular oxygen independently from the presence of any support. Isotope experiments along with studies on colloidal stability reveal that the active oxygen species is a stable surface oxide that can be protonated. The role of the support is to provide water for protonation steps. Therefore, the hydrophilicity is the main property of the support which determines the catalytic activity and not, as is often assumed, its acidity or reducibility. The deduced model provides explanations for experimental results described in the literature for various gold catalysts and reaction conditions. KEYWORDS: catalysis, reaction mechanism, CO oxidation, gold nanoparticle, surface oxidation
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AuNPs can activate molecular oxygen and catalyze the oxidation of CO in the absence of any support and that the AuNP surface is almost fully covered with surface oxide species in the presence of molecular oxygen. This reveals that in the CO oxidation all key reaction steps occur exclusively on the Au surface, whereby water acts as an essential cocatalyst. The role of the support in the gas-phase reaction is to provide physisorbed water for protonation/deprotonation of surface species.
or decades gold, as the most inert metal, was considered to be a catalytically inactive material.1 A paradigm shift was induced by the groundbreaking discoveries of Haruta and coworkers concerning the low-temperature CO oxidation of metal oxide supported gold nanoparticles (AuNPs).2 Currently, gold catalysis is a very vivid research topic, experiencing increasing interest and scientific output.3,4 However, the mechanisms of most gold-catalyzed reactions are still a topic of debate.5,6 In particular, the mechanism of the CO oxidation is, after almost 30 years of research, still controversial. In this context, the reactive oxygen species as well as the corresponding activation of O2, the roles of the support and of adsorbed H2O remain unclear. In the last 30 years different and contradictory mechanisms for the CO oxidation have been proposed on the basis of experimental results and theoretical calculations. Commonly suggested reaction mechanisms include the adsorption and activation of both O2 and CO on the AuNP surface (in the following denoted the Au-only mechanism),7−9 the activation of O2 by the oxidic support,10−12 and the activation of O2 on the AuNP−support interface.13,14 Furthermore, H2O as a cocatalyst which can protonate/deprotonate intermediate species7 and the activation of H2O by the support15 have been suggested. In this contribution, colloidal AuNPs with a nonfunctionalized surface (i.e., with water as ligand) are used to investigate the catalytic properties. These experiments provide the unique opportunity to investigate the reaction mechanism decoupled from support influences. For the first time it is shown that © XXXX American Chemical Society
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RESULTS AND DISCUSSION The ultimate test if AuNPs alone are able to activate O2 and to oxidize CO without any support is a sustained CO oxidation with colloidal AuNPs. However, the synthesis of such AuNPs often requires stabilizing agents that covalently bind to the particle surface and can change their catalytic properties. We have recently demonstrated the synthesis of stable aqueous AuNPs with water as ligand.16,17 Such electrostatically stabilized AuNPs with a mean diameter of 3.8 nm (10% polydispersity, size determined by SAXS) were used to investigate if unsupported AuNPs are catalytically active toward room-temperature CO oxidation. The AuNPs were prepared by the reduction of HAuCl4 with NaBH4 (both with highest purity) without the use of any stabilizing agent. Received: August 7, 2017 Revised: September 14, 2017
8247
DOI: 10.1021/acscatal.7b02646 ACS Catal. 2017, 7, 8247−8254
Research Article
ACS Catalysis
power spectrum (Figure 1d) represents 5 pairs of 111 reflections from the 5 subunits. The angles between pairs of reflections are, as expected, 36°. Additional bright field TEM images of the AuNPs can be found in section S2 in the Supporting Information. Colloidal AuNPs were stirred in a closed round-bottom flask equipped with a gas inlet and a gas outlet. The inlet of CO, O2, and Ar was controlled by mass flow controllers which enabled the selective switching of the gas flows. Argon was used as the carrier gas. The gas outlet was linked to a quadrupole mass spectrometer for the time-resolved detection of the different gas species. The experimental setup is schematically displayed in Figure 2a, and details can be found in section S1 in the Supporting Information. In Figure 2b−e the CO, O2, and CO2 flows in the gas outlet are displayed as a function of reaction time. These results were obtained in one experimental run (see section S3.1 in the Supporting Information for complete MS results). In the first part of the experiment the CO flow was kept constant (see Figure 2b). After 15 min a constant O2 flow was switched on (see dotted line in Figure 2b), which resulted immediately in a constant CO2 production. A similar CO2 production was observed if the CO flow was switched off and, after 30 min, on again while the O2 flow was kept constant (see Figure 2c). The two dotted lines in Figure 2c represent the time at which the CO flow was turned off and on. CO2 was only detected if both CO and O2 were fed to the reactor containing AuNPs (for details see section S3.2 in the Supporting Information). These experiments reveal that unsupported and nonfunctionalized colloidal AuNPs are catalytically active for CO oxidation. The corresponding turnover frequency (TOF) amounts to about 1.9 × 10−2 s−1 for particles with a mean diameter of 3.8 nm and remains rather constant for at least 8 h. Supported catalysts at comparable particle sizes, temperatures, and oxygen and CO partial pressures reach TOFs of e.g.
These AuNPs show different structures, including regular cuboctahedra and multiply twinned particles. As a matter of fact, the HRTEM image of the particle in Figure 1a is
Figure 1. HRTEM images of two selected AuNPs (a, c) and their corresponding power spectra (b, d).
characteristic of a cuboctahedron oriented along the [110] direction, as proven by the power spectrum of the lower part of the particle (Figure 1b). A twin plane is indicated by the black arrow. The HRTEM image of the particle in Figure 1c is characteristic of a decahedron in a 5-fold orientation. The
Figure 2. (a) Schematic illustration of the setup for the catalytic testing of aqueous AuNPs (0.25 mM, mean diameter 3.8 nm, polydispersity 10%) toward the oxidation of CO to CO2 at room temperature. (b) Volume flow of CO, O2, and CO2 when O2 is connected to a constant CO flow. (c) Volume flow of CO, O2, and CO2 when CO is switched off and on to a constant O2 flow. (d) Volume flow of CO, O2, and CO2 when the stirring speed of the Au colloid is varied. (e) Volume flow of CO, O2, and CO2 when an MSA solution is added to the Au colloid. All volume flows were calculated from MS data. 8248
DOI: 10.1021/acscatal.7b02646 ACS Catal. 2017, 7, 8247−8254
Research Article
ACS Catalysis
Figure 3. Volume flow (calculated from MS data) of CO, O2, and CO2 during the CO oxidation with colloidal AuNPs (0.25 mM, mean diameter 3.8 nm, polydispersity 10%) at room temperature. The constant flow of Ar (137 mL/min) is not shown. The experiment can be divided into 7 sections: (1) steady-state CO oxidation; (2) O2 flushing to remove dissolved CO and saturate the solution with O2; (3) Ar flushing to remove dissolved O2; (4) CO flushing in order to react with adsorbed oxygen; (5) CO flushing in order to saturate the solution with CO; (6) Ar flushing to remove dissolved CO; (7) O2 flushing in order to react with adsorbed CO. The top bar indicates which gas is switched on (green) or off (red) in the gas inlet.
about 2 to 12 × 10−2 s−1 (AuNPs on TiO2), 2 to 5 × 10−2 s−1 (AuNPs on SiO2), and 1 × 10−2 s−1 (AuNPs on Al2O3).18 Hence, the gold colloids catalyze the CO oxidation with about the same activity and stability in comparison to supported gold particles, despite the fact that in the case of the colloids the reactant molecules have to diffuse through water in order to reach the active gold surface. As displayed in Figure 2d, the production of CO2 is strongly dependent on the stirring speed of the colloidal solution. A systematic variation of the stirring rate confirms that CO oxidation with colloidal AuNPs is limited by the diffusion of the gases from the gaseous to the liquid phase. Catalysis requires the availability of accessible surface sites for the adsorption of reactants. Deliberate addition of strongly adsorbing ligands, i.e. sulfur-containing mercaptosuccinic acid (MSA), should result in poisoning of the surface and thus quenched activity if the reaction is surface-catalyzed. When MSA was added to an AuNP solution during the oxidation of CO, the production of CO 2 readily decreased. The concentration of CO2 in the outlet gas successively decreases if small amounts of MSA (0.25 mL of a 0.025 mM solution) are successively added to the gold colloid (see Figure 2e). In Figure S5 in the Supporting Information the CO2 production is plotted against the addition of MSA equivalents. The CO2 production rate decreases proportionally to the amount of added MSA down to the baseline for a gold atom to MSA ratio of approximately 10:1. Thus, the AuNPs are catalytically inactive if they are functionalized with MSA with an amount which is on the order of surface atoms. Adsorption and Activation of O 2 . It could be unambiguously shown that AuNPs can activate molecular oxygen for the catalytic oxidation of CO to CO2 in the absence of any support, proving the existence of an Au-only mechanism. This means that the adsorption and activation of oxygen occur on the AuNP surface and probably not on the support. On the basis of DFT calculations, this was also assumed by few groups.9,19−21 Key questions in this regard are as follows. Is the oxygen physisorbed or chemisorbed? If the oxygen is chemisorbed, does it dissociatively or molecularly adsorb? Furthermore, it is unclear if only exposed facets, corners, or edges adsorb oxygen.
In order to elucidate if oxygen is physisorbed or chemisorbed, a transient experiment with oxygen exposure followed by Ar purging and subsequent CO exposure was performed (see sections 1−4 in Figure 3). After a steady-state CO oxidation (section 1), CO was switched off. The reactor was flushed with O2 for a further 30 min to remove dissolved CO and CO2 and saturate the solution and NP surface with oxygen (section 2). The O2 flow was then turned off, and the reactor was purged with Ar for 1 h to remove dissolved oxygen from the solution to less than 0.25 ppm22 and weakly adsorbed oxygen (physisorbed) from the AuNP surface (section 3). After 1 h of Ar purging CO was switched on without cofeeding oxygen (section 4). This led to a production of CO2 for approximately 30 min. The amount of stored oxygen required for the observed CO2 production can be estimated from the integration of the CO2 peak area. It corresponds to approximately 1 μmol, which is in the range of the number of Au surface atoms present in the colloid. The residual dissolved oxygen can be estimated to be approximately 0.1−0.3 μmol (
