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Nanofabricated catalyst particles for the investigation of catalytic carbon oxidation by oxygen spillover Carl Justin Kamp, Hector Hugo Perez Garza, Hans O. A. Fredriksson, Bengt Herbert Kasemo, Bengt Andersson, and Magnus Skoglundh Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b04139 • Publication Date (Web): 20 Apr 2017 Downloaded from http://pubs.acs.org on April 26, 2017
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Nanofabricated catalyst particles for the investigation of catalytic carbon oxidation by oxygen spillover Carl Justin Kamp1,2*, Hector Hugo Perez Garza1,3, Hans Fredriksson1,4, Bengt Kasemo1, Bengt Andersson1, Magnus Skoglundh1 1
Competence Centre for Catalysis, Chalmers University of Technology, Gothenburg, Sweden, Massachusetts Institute of Technology, Cambridge, MA, USA, 3Delft University of Technology, Delft, Netherlands, 4Eindhoven University of Technology, Eindhoven, Netherlands 2
Abstract The catalytic oxidation of carbon by molecular oxygen was studied using C/Pt, Pt/C, Pt/Al2O3/C, Pt/CeO2/C, Al2O3/C and CeO2/C model samples prepared by hole-mask colloidal lithography. By this technique the degree of contact between platinum and carbon was controlled with high precision. The oxidation of carbon was monitored using atomic force microscopy and scanning electron microscopy. The results show that Pt in direct contact with carbon catalyzes the oxidation of carbon by spillover of dissociated oxygen from Pt to carbon. By physically separating Pt and carbon with a 10 nm thin spacer layer of Al2O3, the oxygen spillover was entirely blocked. However, through a corresponding spacer layer of CeO2, carbon oxidation was still observed, either by oxygen spillover from Pt to carbon or directly dissociated on the ceria, although at a slower rate compared to the case with no spacer layer between Pt and carbon. Introduction The performance of a catalytic process is ultimately determined by the interaction between the catalyst and the reactants, intermediates and products as described by Sabatier’s principle. Reactants need to adsorb, dissociate and react and the products need to desorb at an appreciable rate. Commercial catalysts are complex structures consisting of an active catalyst, a support and often a promoter. In most cases, it is not clear on what sites the different steps in a catalytic cycle occur, largely due to the complexity of the catalyst. Nevertheless, kinetic models of catalytic processes often assume that different reactants adsorb and dissociate on different active sites followed by surface diffusion and formation of the product species [1]. One reactant can adsorb on the active catalyst material, while the other adsorbs on the support or on the promoter, as suggested for the WGS reaction on Cu/CeOx and Cu/ZnO catalysts [2,3]. Reactant adsorption on different facets of a metal nanoparticle has also been suggested, e.g. for CO-oxidation on Pt [4] or partial oxidation reactions on Ag-catalysts [5]. To test the relevance of such reaction mechanisms experimentally require samples with a higher degree of control than is normally achieved on commercial catalysts. This can be 1 ACS Paragon Plus Environment
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achieved through the aid of lithographic nanofabrication. Designing catalysts where the assumed active sites are in direct contact with one another or where the adsorption site for one reactant is physically separated from the other by a small distance can help in elucidating where reactants adsorb and how they move on the surface. An example is catalytic soot oxidation, where O2 adsorbs and dissociates on a catalyst and where the atomic oxygen diffuses to the carbon particles where it can react to form CO or CO2. Tunable catalystsupport contact [6] has shown to improve catalyst stability in electrochemical reactions. Recently, a wide range of analytical tools including thermogravimetric analysis (TGA) [7-9], in-situ X-ray absorption spectroscopy (XAS) [10], transmission electron microscopy (HRTEM) [6,9], X-ray diffraction (XRD) [9] and X-ray photoelectron spectroscopy (XPS) [9] (among others) have been applied to understand electrochemical catalyst stability and interaction with supports, uniformity of both catalysts and supports and different reaction mechanisms including spillover. Catalytic soot oxidation finds commercial interest primarily in the automotive industry. In many major cities, smog is a growing problem still waiting to be solved and since the introduction of ’aftertreatment forcing’ emissions standards, such as US Tier 4 and Euro V, the use of diesel particulate filters has been almost ubiquitous with diesel engines. In addition, reduced fuel sulfur, such as in the case of Ultra-Low Sulfur Diesel (ULSD) passed in the US in 2006, which limits fuel sulfur to 15 ppm, allows for more efficient use of catalysts due to the reduction in sulfur-related catalyst poisoning. For the aforementioned reasons, the catalyzed diesel particulate filter (DPF) is a widely used method of diesel particulate matter (PM) control. While the soluble organic fraction (SOF) in diesel particulate matter, generally liquid hydrocarbons adsorbed on the carbon soot particles, is effectively oxidized in the upstream diesel oxidation catalyst (DOC), the solid fraction of diesel PM is trapped in the DPF where it is destroyed via oxidation. Soot oxidation occurs by way of several mechanisms including thermal oxidation by O2 at temperatures above 600°C, indirect catalytic oxidation by NO2 at temperatures ~300-450°C (whereby platinum nanoparticles are utilized to oxidize NO to NO2) or direct catalytic oxidation by O2 (where dissociated oxygen atoms may spillover to soot particles in close proximity to the catalyst) [11]. Catalytic soot oxidation is often preferred due to the fact that additional energy is not required to heat the engine exhaust up to >600°C for non-catalytic oxidation. Many different catalysts have been proposed for the catalytic oxidation of soot; however noble metals, such as Pt, and rare earth metal oxides, such as CeO2, have been found to be especially active and are considered in this study. Catalytic oxidation of carbon has been monitored in this study by atomic force- (AFM) and scanning electron microscopy (SEM) and will be discussed later. While catalytic oxidation of soot has been studied extensively in literature, it has been shown in many studies [12-18] that the contact between the catalyst particles and soot significantly influences the oxidation rate. A study from Neeft et al showed that temperatures corresponding to the maximum carbon oxidation rates with some catalysts such as Co3O4, Fe2O3 and V2O5 can vary up to 200°C depending on the degree of contact with soot particles [13]. Large variations in reaction rates for carbon oxidation by V2O5 and MoO3 were 2 ACS Paragon Plus Environment
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explained by the degree of contact between carbon and the catalyst dictating the reaction mechanism where oxygen spillover only occurs in tight contact [14]. Another study [15] utilized environmental transmission electron microscopy to investigate the oxidation of carbon black with ceria catalysts where it was shown that catalytic oxidation only occurred when carbon particles were in close proximity, no more than a few nm, of ceria particles. In addition, [10] studied hydrogen spillover on both a reducible (TiO2) and a non-reducible (Al2O3) support materials to show that the spillover reaction occurs approximately 10 orders of magnitude slower on a non-reducible support in comparison to a reducible support. The objective of the present study is to investigate the influence of the degree of contact between Pt and CeO2 and carbon on the catalytic oxidation of carbon by oxygen. Nanostructured C/Pt, Pt/C, Pt/Al2O3/C, Pt/CeO2/C, Al2O3/C and CeO2/C samples are prepared using hole-mask colloidal lithography and the oxidation of carbon is monitored by atomic force microscopy and scanning electron microscopy. Experimental Hole-mask colloidal lithography Common fabrication techniques such as thin film deposition, wet etching and the selfassembly of charged colloidal spheres are used in hole-mask colloidal lithography to create nanometer-scale particles (henceforth referred to as nanoparticles) over large areas (up to 5 cm2) on various surfaces [19]. As the name implies, hole-mask colloidal lithography utilizes a patterned mask layer for the desired nanoparticle shape and lateral spacing. The mask is separated from the substrate by a sacrificial layer (such as Au, Cr or a polymer film), which can be wet-etched away to remove the mask after nanoparticle deposition. The pattern and shape of the hole-mask, and the surface coverage of nanoparticles (typically 10-50%) are a result of self-assembling, electrostatically charged nanospheres (Polystyrene beads) of a given radius that are deposited onto an oppositely charged surface [20]. After the self-assembled nanospheres are in place on the surface, the sacrificial layer, followed by the mask layer, is deposited. The nanospheres are then stripped from the surface leaving holes in the mask. From this point, many types of nanoparticle architectures can be fabricated such as discs, rings, cones and crescent-shaped particles, depending on deposition techniques [19,21]. Furthermore, the nanoparticle sizes are directly related to the polystyrene nanospheres, which range from 20 nm up to several hundred nm and generally have a very narrow size distribution (a maximum 5-10% deviation in particle diameter). The samples prepared in this study utilize hole-mask colloidal lithography in order to investigate the specific behavior of individual catalyst particles active in carbon oxidation. The large number of laterally ordered catalyst nanoparticles creates an increasingly controlled representation of carbon oxidation relevant to the mitigation of diesel particulate matter through the catalytic oxidation of PM trapped within a diesel particulate filter. Sample fabrication and oxidation A brief description of the catalyst nanoparticle fabrication process used for the sample preparation in this study follows an 11 step process, as discussed in [19], and is shown in the 3 ACS Paragon Plus Environment
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appendix. The samples used in this study are described schematically in figure 1, all of which include particles which are approximately 190 nm wide with varying heights as described below.
Figure 1. Schematic diagrams of fabricated samples used in this study where (a) shows samples used in experiment 1, with 35 nm carbon nanoparticles deposited on 40 nm platinum nanoparticles, (b) shows samples used in experiment 2 with 40 nm platinum nanoparticles on a carbon layer, (c) shows samples used in experiment 3 of this study where 40 nm platinum nanoparticles are separated from a carbon layer either by Al2O3 or CeO2 spacer layers which are 10 or 40 nm thick, and (d) shows samples used also in experiment 3 with 10 and 40 nm Al2O3 or CeO2 nanoparticles on a carbon layer. The purpose of the first two experiments of this study was to investigate carbon oxidation when platinum catalyst particles are in direct contact with carbon, as shown in figures 1(a) and 1(b), respectively. The purpose of experiment 3 was to investigate the catalytic carbon oxidation mechanism when the platinum catalysts are separated from the carbon layer as shown in figure 1(c). The catalytic activity of the spacer layers was also investigated in experiment 3 as described by figure 1(d). The experiments are summarized in table 1. Table 1. Samples and reaction conditions of the oxidation experiments in 10% O2 in Ar. Experiment Sample description # 1 35 nm C disks on 40 nm Pt disks (C/Pt) 2 40 nm Pt disks on a carbon layer (Pt/C) 3.1.1 40 nm Pt disks on 40 nm Al2O3 disks on a carbon layer (Pt/Al2O3/C)
# of samples
Oxidation
1
1, 1, 4, 8, 8 min. at 377°C
5
1, 2, 6, 14, 22 min. at 377°C
5
6, 14, 22 min., 2, 4 hr. at 377°C
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3.1.3
Supporting information
40 nm Al2O3 disks on a carbon layer (Al2O3/C) 40 nm Pt disks on 40 nm CeO2 disks on a carbon layer (Pt/CeO2/C) 40 nm CeO2 disks on a carbon layer (CeO2/C) 40 nm Pt disks on a carbon layer (Pt/C)
Supporting information 3.1.2 Supporting information 3.2.2
40 nm Pt disks on 10 nm Al2O3 disks on a carbon layer (Pt/Al2O3/C) 40 nm Pt disks on 10 nm CeO2 disks on a carbon layer (Pt/CeO2/C)
3.2.1
3.2.3
5 3
6, 14, 22 min., 2, 4 hr. at 377°C 6, 14, 22 min. at 377°C
3
6, 14, 22 min. at 377°C
5
14 min. at 327, 352, 377, 402, 427°C and 30 min. at 427°C 6, 14, 22 min., 2, 4 hr. at 377°C
5
2
6, 22 min. at 377°C
Due to the fact that the fabricated samples are quite small (1-4 cm2), either with 100 nm carbon layers or with 35 nm high carbon nanoparticles, carbon oxidation produces very small amounts of CO and CO2. For this reason, the extent of carbon oxidation was determined by SEM and AFM analysis. By measuring the height of the platinum nanoparticles in respect to the height of the surrounding carbon layer (>100 nm away from the nanoparticles) and assuming that the nanoparticles retain the same diameters throughout the oxidation process (validated by both AFM and SEM), the amount of carbon combusted and thus reaction rate can be calculated. The previous statement assumes that the reactive conditions, namely the relatively low temperature of 377°C, only very slowly oxidize the carbon that is not in direct contact with the platinum nanoparticles in comparison to the carbon that is in direct contact with platinum [11]. Non-catalytic oxidation of carbon by O2 generally occurs at temperatures above 600°C. Apparatus Gas Flow Reactor A gas-flow reactor was used to expose the samples to a reactive environment, where the reaction chamber consisted of an insulated, horizontal 25 mm diameter quartz tube heated by a Kanthal coil. Mass flow controllers regulated the supply of argon and oxygen while 2 thermocouples recorded the sample temperature. The sample was held vertically and parallel to the flow by an aluminum holder. Most samples oxidized in this study were done so at 377°C, and this temperature was reached from room temperature by a 60°C/min ramp in 100% Ar. A 150 ml/min flow rate was used to oxidize the particles with a mixture of 10% O2 and 90% Ar at 1 atm. Evaporation deposition The nanoparticles (Pt, C, Al2O3, CeO2), hole-masks (Au), sacrificial layers (Cr) and substrate layers (amorphous C) were all deposited by an electron-beam evaporation system, AVAC 5 ACS Paragon Plus Environment
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HVC600. This apparatus consisted of a 4 kW e-gun, 2 resistive evaporation sources and 4 source pockets with a CTI 8’’ cryo-pump giving a base pressure of ~10-6 Torr. Oxygen plasma etching The carbon film surfaces were activated prior to colloid deposition for 5 seconds with oxygen plasma using a PlasmaTherm Reactive Ion Etcher (RIE). SEM Both a JEOL JSM-6301F and a LEO Ultra scanning electron microscopes were used to characterize the samples before and after oxidation. AFM Atomic Force Microscopy (AFM) was used before and after oxidation to quantify height changes in the nanoparticles. A Veeco Dimension 3000 Scanning Probe Microscope was used in tapping mode in air. Results and discussion Experiment 1: Analysis of Pt/C interface To demonstrate the basic concept with using nanolithography and microscopy to study reaction mechanisms in general and spill-over in particular, Pt nanodisc samples were prepared. In experiment 1 carbon nanoparticles were deposited on top of the platinum discs supported on silicon wafers (C/Pt), as described in figure 1(a). By measuring the height of the discs before and after exposure to an oxidizing environment for different durations, it is possible to determine how fast the carbon discs are combusted. This sample configuration also offers excellent opportunities for visualization of the Pt-C interface by SEM.
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Figure 2. SEM images (a,b,c,e) of carbon nanoparticles stacked on platinum nanoparticles on a silicon wafer (C/Pt), (a) before oxidation, (b) after 2 minutes and (c) after 22 minutes oxidation. AFM tapping mode data (d) showing phase information. (e) shows the nanoparticles after 6 minutes oxidation at a higher resolution where a small ring of unknown material surrounds the particles. The plot (f) shows AFM data for changes in height of carbon nanoparticles stacked on platinum nanoparticles on a silicon wafer during oxidation at 377⁰C. Image (a) is taken at 60⁰ and images (b-e) are taken at 90⁰. The AFM data displayed in figure 2(f) shows that the 35 nm thick layer of carbon is gradually combusted during the first few minutes of the experiment and completely gone after 6 minutes at 377°C. However, a small height increase (+8 nm from 6 minutes to 22 minutes) was observed. This demonstrates how AFM can be used to follow the rate of carbon combustion. Although the height increase observed towards the end of the experiment are within the error bars, there are several possible explanations for such a small increase in height including slight carbon solubility in platinum, which could organize into layers on top of the platinum nanoparticles or platinum restructuring, causing the height to increase. Figures S1(a-f), in the supporting information, show the increasing surface roughness with temperature on the platinum nanoparticles’ surfaces which appears at temperatures 402°C and above. While there is little observed platinum restructuring at 377°C this effect may account for some of the observed height increase shown in figure 2(f). It should also be noted that the sample was exposed to the atmosphere outside of the reactor between oxidations in order to obtain AFM and SEM data, where perhaps adsorbed species from the air could have influenced the platinum nanoparticles behavior at elevated temperature. 7 ACS Paragon Plus Environment
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It has been shown in literature, however, that carbon is soluble in platinum, but only in very small amounts, where carbon solubility in platinum at 1700°C has been measured at
