Structure of Platinum Catalysts under CO, Hydrogen, and Oxygen

May 23, 2014 - By means of in situ time-resolved high-energy-resolution fluorescence X-ray spectroscopy and infrared spectroscopy, the structure of pl...
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Structure of Platinum Catalysts under CO, Hydrogen, and Oxygen; Anomalous Behavior of Pt on Ceria after Cyanide Leaching Jeroen A. van Bokhoven,*,†,‡ Cristina Paun,† and Jagdeep Singh† †

ETH Zurich, Institute for Chemical and Bioengineering, 8093 Zurich, Switzerland Paul Scherrer Institute, Laboratory for Catalysis and Sustainable Chemistry, 5232 Villigen, Aargau, Switzerland

‡

ABSTRACT: By means of in situ time-resolved high-energy-resolution fluorescence X-ray spectroscopy and infrared spectroscopy, the structure of platinum on alumina and ceria catalysts in a reaction mixture of 75% H2, 1% CO, and 1% O2 in He was determined as a function of temperature. After reduction, all the observed platinum in Pt/Al2O3 and Pt/Ce(La)Ox was present as Pt(0) nanoparticles covered with CO. The amount of CO depended on the temperature and CO concentration. Cyanide leaching of ceria-supported platinum catalyst yielded a high concentration of cationic platinum. Reduction occurred in the absence of oxygen; extremely small Pt clusters (≤0.5−0.7 nm) formed. Unlike nanoparticles on alumina and ceria, which chemisorb CO in the presence of CO and oxygen, these clusters are bonded to oxygen. This contrasting behavior paralleled the different selectivity for preferential oxidation of CO (24% selectivity for nonleached sample (331 K) versus 54% selectivity for leached sample (340 K)). Even without the knowledge of the exact structure of the platinum−oxygen species, this in situ study identifies the roles of reduced nanoparticles and cationic metal in generating activity and selectivity.

1. INTRODUCTION The removal of CO from a stream of hydrogen has attracted significant attention because proton-exchange membrane fuel cells (PEMFC) require CO-free hydrogen.1 The CO content must be decreased to levels below 1−100 ppm, which can be achieved by various methods: the preferential oxidation of CO (PROX),2 the water gas shift (WGS) reaction,3,4 and methanation.1,5,6 The WGS reaction is generally the first reaction to be applied to lower the CO content in the stream of hydrogen that may form by steam, auto thermal reforming, or partial natural gas oxidation. The WGS reaction reduces the CO concentration from 5−15 vol % to about 0.5−1 vol %, which is still too high for the fuel proton exchange membrane fuel cells. The PROX reaction is the subsequent step that decreases the CO concentration to acceptable concentrations (10 ppm). Materials such as ceria are commonly used for PROX because of their capacity to store oxygen.7−9 The addition of a metal is required to achieve highly active catalysts. Many different metals, such as Pt, Rh, Au, and multimetallic systems,10−16 have been proposed for this purpose. For all of the above reactions, the oxidation state of these metals, which is responsible for the catalytic activity, is still a topic of debate. Nonmetallic cationic Au and Pt on or in ceria have been proposed as being active.12,17−21 Recent theoretical results suggested that in doped ceria, the bond between the oxygen atoms and the oxide is weakened in the presence of the dopant.22 Experimental data show that the nanoscale Pd/ CeZrO4 interface plays a role in the reducibility of the oxide.23 The ceria doped with metal cations was thus suggested to be an active catalyst in the oxidation of CO. Cationic metals have been suggested to be active in PROX, WGS, oxidation, and even in hydrogenation reactions.24 On the other hand, also the © XXXX American Chemical Society

reduced particles were proposed to be the catalytically active species, with the reaction taking place either on the particle or at the metal−support interface.25,26 The role of metallic gold has been established by means of various techniques like in situ X-ray absorption spectroscopy, infrared spectroscopy, and Xray photoelectron spectroscopy.27−29 These proposals stem, in part, from the fact that in many of the studies, the structure of the catalyst was not determined in situ and that the authors relied on ex situ measurements or in situ conditions but with different samples and gas mixtures. However, in addition to the oxidation state of the metal, another factor adds to the complexity of the analysis. The catalytic performance of supported metals often shows a strong particle size effect.26,30 A metal that is deposited as very large particles does not exhibit the same catalytic performance as a (sub) nanometer-sized particle or cluster. Thus, relating catalytic activity to reduced or oxidic species is not straightforward. For example, in lowtemperature CO oxidation on Au nanoclusters on FeOx, (sub) nanometer-sized particles or clusters (0.5 nm) were suggested to be the active species.31 The structure (oxidation state, shape, size) of a metal on a ceria-based catalyst changes depending on the preparation methods, on the reaction conditions,32−34 and even as a function of the position in a single reactor.32,35 For example, in an oxidizing atmosphere, the surface of ceria stabilizes cationic platinum,12,22,36−40 or ceria-supported metal particles (2−6 nm) redisperse upon exposure to oxygen and enhanced temperatures (673−1073 K).38−41 The metal thus reversibly Received: March 24, 2014 Revised: May 23, 2014

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among the most suitable methods for determining the relationship between the electronic and the geometric structure and performance.

changes its structure from a reduced metal particle to platinumoxide and vice versa on the surface of ceria, depending on the reducing and oxidizing nature of the atmosphere, respectively.41 Nagai et al.38,41−43 explained this behavior for a Pt/Ce−Zr−Y catalyst at a temperature higher than 673 K: in an oxygen atmosphere, oxygen is adsorbed on the surface of large Pt particles (4−6 nm), and mobile Pt oxide species form, migrate, and are trapped on the support surface through a PtO−support interaction. However, in an hydrogen atmosphere, the PtO−Ce bond breaks, and Pt oxides are reduced to platinum particles. Substitution of different ions in Ce4+ sites in a CeO2 matrix has been described on samples prepared by different techniques.44 For example, Bara et al.13,14 reported that a single-phase oxide doped with Pt (Ce1−xPtxO2−δ) can form in a combustion-synthesized Pt/CeO2 catalyst, which is highly active in CO oxidation and WGS reactions at low temperature. Fu et al.12 prepared Pt and Au ceria catalysts by impregnation and by leaching of metal excess with NaCN and studied, ex situ, the surface of the material by X-ray photoelectron spectroscopy (XPS). In the case of the leached samples, the XPS data revealed the presence of Au ions and a greater amount of surface oxygen, indicating that lattice substitution took place. These species have also been considered to be the active sites in the WGS reaction, the addition of oxygen to a reactant WGS mixture being necessary to stabilize the cationic structure of the metal.21,45 Zhu et al.46 prepared ceria doped with Pt, Pd, and Rh by a solution-based hydrothermal process and examined these materials by ex situ transmission electron microscopy (TEM) and in situ ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) in the methane partial oxidation in the production of syngas. The catalysts showed a different catalytic behavior and surface chemistry. For example, in the case of Rh catalysts (as-prepared and reduced) after catalysis, the TEM images suggest that there are no Rh or RhOx nanoparticles on or in the CeO2. In situ AP-XPS studies (623−873 K) showed that Rh was in an ionic state before, during, and after catalysis, rationalized by the strong Rh−O bonds. On the contrary, Ptdoped catalysts showed different structures before, during, and after the reaction. On the surface of the catalysts there were initially three oxidation states in different ratios (Pt0 (Pt−Ce bond called “pseudometallic state”), Pt2+, Pt4+, and no Pt nanoparticles (NPs) observed). During catalysis, these ratios changed; reduction of the ionic state to a pseudometallic state occurred. However, the active sites for partial oxidation of methane are the Pt ions doped in the surface lattice of CeO2. Here, we determine the structure of platinum supported on alumina and La-doped ceria under a mixture of CO, H2, and O2 at different temperatures. Regardless of the catalysts, PROX and CO methanation occurred. We used infrared spectroscopy and high-energy-resolution fluorescence detected X-ray absorption spectroscopy (HERFD XAS) at the Pt L3 edge;47−49 the in situ infrared spectroscopy provides information about the adsorbed species on the metal surfaces during the reaction conditions. HERFD XAS is a very sensitive technique, which reveals more spectral detail than conventional spectra. It is thus easier to determine the local metal (platinum) structure, oxidation state, and the presence of adsorbates on the metal surface.50−52 It is well-established that adsorbates on the surface of a metal particle are detectable and that the adsorption mode (atop, bridged, or 3-fold sites) can also be distinguished. XAS also enables the estimation of the size of the metal particles on the support.53 XAS measures the metal structure of a functioning catalyst within a catalytic reactor and is thus

2. EXPERIMENTAL METHODS 2.1. Catalyst Preparation. Pt/Al2O3 (1.90 wt %), hereafter referred to as Pt/Al2O3, was synthesized by incipient-wetness impregnation. Pt(NH3)4(NO3)2 (1 g) in 14 mL of deionized water was added to 25 g of γ-alumina (surface area, 180 m2/g). Saturated NH4OH (2 mL) was then added to the mixture. The sample was dried overnight at 398 K and at 498 K for 3 h and was reduced at 523 K in a flow of pure hydrogen (100 mL/ min) at atmospheric pressure. This reduction ensured a defined structure of platinum and thus a good reference. The platinum elemental composition was determined by the inductively coupled plasma method. The catalyst was dried at 498 K. Ladoped ceria was synthesized by the coprecipitation. (NH4)2Ce(NO3)2 (13.7 g) was mixed with La(NO3)3·H2O (0.9 g) in 200 mL of deionized water together with 24 g of urea. The solution was heated to 373 K under constant stirring. After coprecipitation, the obtained gel was heated for about 8 h at 373 K, filtered, washed with hot water, and dried at 373 K in a vacuum oven. The obtained powder was then calcined at 923 K for 8 h in air, resulting in the final support material for supporting platinum. Pt/Ce(La)Ox (4.4 wt %), hereafter referred to as Pt/Ce(La)Ox, was synthesized by incipientwetness impregnation. H2PtCl6 (0.44 g) was dissolved in deionized water, the volume of which was equal to the pore volume of lanthanum-doped ceria. The powder was then dried at 383 K for 10 h under vacuum and then calcined in air for 10 h at 673 K. Afterward, the leaching of the Pt/Ce(La)Ox was done using a 2% aqueous solution of NaCN at room temperature for 24 h. Leaching resulted in 2 wt % Pt/ Ce(La)Ox; the platinum content was determined by atomic absorption spectrospcopy. The sample is referred to hereafter as L-Pt/Ce(La)Ox. 2.2. Electron Microscopy. The catalysts were characterized by scanning transmission electron microscopy (STEM). The sample was mounted on carbon foil after the evaporation of ethanol, and a copper grid was used to support the sample. The measurements were performed using a Tecnai F30 microscope operating with a field-emission cathode at 300 kV and a high-angle annular dark-field (HAADF) detector. Aberration-corrected STEM micrographs were taken to characterize Pt/Ce(La)Ox and L-Pt/Ce(La)Ox because of the small difference in the scattering potential of platinum and cerium. No chlorine was detected in the calcined samples. 2.3. Kinetic Measurements. The experiments were conducted using the flow scheme, according to which reactant gases were mixed before reaching the reactor54,55 using computer-controlled mass-flow controllers (MFCs). The reactor was operated in down flow, and simultaneous fluorescence HERFD measurements were recorded. The temperature of the reactor was controlled by a thermocouple embedded in the reactor housing. The reactor was filled to capacity with the catalyst, which was sieved to a fraction of 63− 125 μm. Two aluminum windows covered the catalyst particles. Pt/Al2O3 was pretreated in situ in 5% H2 in He at 473 K before switching to the reaction mixture of 1% CO, 1% O2, and 75% H2 in He. Pt/Ce(La)Ox and L-Pt/Ce(La)Ox catalysts were not pretreated before the reaction. The outlet from the reactor was connected to a GSD Omnistar mass spectrometer (Pfeiffer Vacuum) to monitor the gases. The measurements were B

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performed at a constant total flow of 30 mL/min through the reactor, corresponding to a space velocity of about 64 000 h−1. The selectivity toward CO oxidation (S) was calculated according to the following formula:

(

S = pCO / pCO + pH O 2

2

2

)

where pCO2 is the partial pressure of CO2 and pH2O the partial pressure of H2O. For a first heating trajectory in Pt/Ce(La)Ox, pCO2 was calculated from the mass spectrometer signal of CO, assuming that at the start of the reaction there is no conversion in CO and at 673 K the mass spectrometer signal corresponds to 100% conversion in CO. pH2O was calculated from the H2O signal assuming that H2O production started at around 460 K and at around 550 K, maximum H2O production occurred from H2 oxidation. The selectivity was calculated for the first heating trajectory in L-Pt/Ce(La)Ox. For all the other heating and cooling trajectories, the selectivity was calculated from the signal of CO2 in the respective trajectory relative to the signal of CO2 in the first heating trajectory. 2.4. High-Energy-Resolution Fluorescence Detected X-ray Absorption Spectroscopy. Experiments were carried out at beamline ID 26 at the European Synchrotron Radiation Facility (ESRF), Grenoble, France. The details of various instrumental parameters are reported elsewhere.54 The X-ray beam measured 0.3 mm horizontal and 1 mm vertical and traversed the reactor 1 mm from the top of the catalyst bed. The incident energy was selected by means of a pair of Si(111) crystals with an energy bandwidth of 1.5 eV at the Pt L3-edge. An X-ray emission spectrometer, based on perfect crystal Bragg optics, recorded the high-energy-resolution fluorescence detected X-ray absorption scans with an avalanche photodiode (APD) as the photon-counting detector. The instrumental bandwidth was 1.5 eV, which is smaller than the core hole lifetime broadening. A Canberra silicon photodiode was mounted to measure the total fluorescence at the same time as the HERFD XAS. The spectra were taken at a time resolution of 125 s during the heating and cooling trajectories at temperatures from 323 to 673 at 2 K/min. 2.5. Infrared Spectroscopy. In situ infrared measurements were carried out with a Bio-Rad spectrometer. The detector was a cryogenic mercury cadmium telluride (MCT) with a maximum beam divergence of 1.3 milliradians. The measurements were done on pellets prepared by pressing catalyst powder mixed with KBr in equal weight ratio. The pellets of Pt/Ce(La)Ox and L-Pt/Ce(La)Ox weighed approximately 6 mg. The infrared beam was transmitted through the sample. The background spectrum was collected with the KBr pellet at 303 K in a flow of He. The catalyst was then exposed to the reaction mixture. The measurements were performed, similar to the XAS measurements, at a constant total flux of 30 mL/min through the reactor, corresponding to a space velocity of about 64 000 h−1. Unlike the XAS measurements, there was only one heating and cooling cycle during the reaction over the Pt/ Ce(La)Ox and L-Pt/Ce(La)Ox catalysts. The pellet was heated at a rate of 5 K/min. The spectra were collected in continuous mode with a time resolution of 1 min.

Figure 1. STEM micrograph of (a) Pt/Al2O3 fresh; (b) particle size distribution of platinum particles for Pt/Al2O3 fresh; and STEM micrographs of (c) Pt/Ce(La)Ox fresh, (d) Pt/Ce(La)Ox used, (e) LPt/Ce(La)Ox fresh, and (f) L-Pt/Ce(La)Ox used; open circles indicate Pt NPs.

Ce(La)Ox (fresh and used) catalysts. Pt/Al2O3 (Figure 1a) has a narrow size distribution of platinum nanoparticles (about 1 nm size) (Figure 1b). The small bright spots were confirmed by EDX to be platinum particles supported on alumina, which was reflected as the less bright area in the STEM micrographs. Figure 2a,b shows the mass spectrometer traces of CO, O2, CO2, H2O, and CH4 in the gas flow from the reactor, which was packed with Pt/Al2O3, as a function of temperature. The catalyst was prereduced at 473 K prior to reaction in a flowing mixture of 5% H2 in He. The gas consisted of a mixture of 1% CO, 1% O2, 75% H2, and the rest He. In the heating trajectory (Figure 2a), at about 315 K, the O2 and CO signals began to decrease, indicating conversion to CO2. At around 420 K, all the O2 was consumed, and at approximately the same temperature, the H2O signal reached a maximum. At around 500 K, CO and H2O reached their minimal concentration and CO2 its maximum. Methanation started at around 550 K and continued until all the CO was converted to methane. These data are very similar to those previously reported,10 which indicates that CO is preferentially adsorbed on the metal surface and is thus preferentially oxidized. Increasing the temperature lowered the CO coverage and thus the selectivity to CO2. The cooling trajectory (Figure 2b) was essentially the same as the heating trajectory. Panels c and d of Figure 2 show

3. RESULTS 3.1. Pt/Al2O3. Figure 1 presents the STEM micrographs of Pt/Al2O3 (fresh), Pt/Ce(La)Ox (fresh and used), and L-Pt/ C

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Figure 2. Plots for 1.9 wt % Pt/Al2O3: mass spectrometer traces of CO (green), O2 (red), CO2 (orange), H2O (pink), and CH4 (blue) during (a) heating and (b) cooling in 1% CO, 1% O2, 75% H2, and the rest He; measured Pt L3 edge HERFD XANES reaction in 1% CO, 1% O2, 75% H2, and the rest He during (c) heating and (d) cooling.

by STEM (Figure 1c,e) was more challenging because of a rather small difference in the scattering potential of platinum and the support. In the case of the fresh samples, EDX confirmed the presence of platinum in the form of very small highly dispersed particles on roughly spherical ceria support; however, we were unable to determine the size of the platinum particles. After the two ceria catalysts (Pt/Ce(La)Ox and L-Pt/ Ce(La)Ox) had been used for two consecutive runs in the reactant gas mixture and at temperatures between 305 and 671 K, Pt particles of