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Efficient ceria-platinum inverse catalyst for partial oxidation of methanol Anna Sergeevna Ostroverkh, Viktor Johánek, Peter Kúš, Romana Šedivá, and Vladimír Matolín Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b01316 • Publication Date (Web): 02 Jun 2016 Downloaded from http://pubs.acs.org on June 9, 2016
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Efficient ceria-platinum inverse catalyst for partial oxidation of methanol Anna Ostroverkh, Viktor Johánek,* Peter Kúš, Romana Šedivá, and Vladimír Matolín
Department of Surface and Plasma Science, Charles University in Prague, V Holesovickach 2, 180 00 Prague 8, Czech Republic
Keywords: cerium oxide, platinum, inverse catalyst, methanol oxidation, hydrogen, magnetron sputtering
Abstract Ceria-platinum based bi-layered thin films deposited by magnetron sputtering were developed and tested in regard to its catalytic activity for methanol oxidation by employing temperature-programmed reaction (TPR) technique. The composition and structure of the samples were characterized by X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). Both conventional (oxide supported metal nanoparticles) and inverse configurations (metal with oxide overlayer) were analyzed in order to uncover structural dependence of activity and selectivity of these catalysts with respect to different pathways of methanol oxidation. We clearly demonstrate that the amount of cerium oxide (ceria) loading has a profound influence on methanol oxidation reaction characteristics. Adding a non-continuous adlayer of ceria greatly enhances the catalytic performance of platinum in favor of partial oxidation of methanol, gaining an order of magnitude in the absolute yield of hydrogen. Moreover, the undesired by-production of carbon monoxide is strongly suppressed making the ceria-platinum inverse catalyst a great candidate for clean hydrogen production. It is suggested that methanol oxidation process is facilitated by the synergistic effect between both components of the inverse catalyst (involving oxygen from ceria and providing a reaction site on adjacent platinum surface), as well as by the fact that the ability of ceria to exchange oxygen (i.e., to alter the oxidation state of Ce between 3+ and 4+) during the reaction is inversely proportional to its thickness. The icreased redox capability of discontinuous ceria adlayer shifts preferred reaction pathway from dehydrogenation of hydroxymethyl intermediate to CO in favor of its oxidation to formate.
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Corresponding author; e-mail:
[email protected]; Tel.: +420- 22191-2333, Fax: +420-22191-2297 Page 1 of 15
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Introduction Hydrogen generation from methanol is an attractive means of H2 production for, e.g., proton exchange membrane (PEM) fuel cells or hydrogen-based combustion engines because methanol is an abundant and environmentally friendly liquid fuel that can easily be transported and stored. In addition, methanol has high atomic ratio of H/C compared to other common fuels. Catalytic partial oxidation of methanol (POM, CH3OH + ½ O2 →2H2 + CO2) is considered the best way to convert methanol to hydrogen since it offers the advantage of an exothermic reaction, 100% hydrogen yield, and release of carbon in the form of harmless CO2. Amongst commonly used platinum-group catalysts, Pt is recognized to be the best 1, 2 monometallic catalyst for methanol oxidation. However, apart from POM, methanol on platinum also converts to side-products such as HCHO, HCOOH, H2O, or CO. Adsorbed CO is found to be the most stable surface adsorbate among all other methanolic fragments, blocking the adsorption sites on the Pt surface. The CO oxidation reaction is restricted to the boundaries between competing domains of adsorbed CO and oxygen 3, leading to reaction hysteresis or even deactivation of the catalyst. The same problem of surface blocking has been dealt with in methanol 4-11 electrochemistry studies and methanol fuel cell applications. 12, 13 14, or metal-oxide systems Bimetallic alloys (with Rh, Ru, Ir etc.) 15 have therefore been implemented widely in order to overcome this issue. Particularly cerium oxide (ceria), used as a support or promotor of a platinum catalyst, has received considerable attention for its unique ability to act as an efficient oxygen buffer 4, 16 because it can store and release oxygen reversibly . The low energetic cost to form an oxygen vacancy makes ceria a great candidate for an active catalyst in all Mars−van Krevelen-type oxidation−reducƟon reacƟons. 17 A typical (conventional) catalyst consists of a dispersed metal supported on an oxide substrate. The promoting effect of ceria is commonly attributed to enhanced O2 dissociation rate at oxide or metal-oxide interface; the cooperation between oxidic and metalic component of the catalyst is then mediated by spillover of 18-22 adsorbed reactants and reaction intermediates. Another important aspect turns out to be a direct mutual interaction 22, 23 between oxide and metal, which can lead to chemical or structural changes, e.g., increased redox activity or enhancement of the metal dispersion and its stabilization against thermal 24, 25 sintering. It is often observed, however, that under realistic reaction conditions of elevated temperatures and presence of reactants the nanostructured platinum becomes inevitably 5, 23, 24, 26 encapsulated by ceria. A different approach is so-called inverse catalyst 20, 26-31 where oxide nanoparticles are supported on a (typically continuous) metallic substrate. Such concept may seem counterintuitive since the design of the conventional catalyst is based on maximizing the effective surface area of the metal component and, eventually, taking advantage of size-related phenomena. 14, 15 However, contrary to many cases of standard oxide-supported metal
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nanoparticles, the participation of the oxide in the surface reaction can be active. Unlike for ideal (flat and stoichiomeric) bulk oxides, the properties of oxide nanoparticles may be influenced by defects such as oxide vacancies 32-34, less-coordinated sites at edges and 35 corners, by strain , or by enhanced electronic interactions with the supporting metal 27, 29, 36. It has been demonstrated that oxygen 16, 37, 38 vacancies present in ceria can provide distinct reactivity or, in turn, that the vacancies can be created by the interaction of 33 oxide surface with adsorbed species. Moreover, in the inverse configuration the reactive defect sites present in the oxide overlayer are not covered by metal particles as in the case of a 15 traditional metal/oxide catalyst. Several reactions have been examined on ceria nanoparticles 16 supported on a number of (111) faces of fcc metals , including 20, 21, 39, 40 Pt(111) . Investigations of ceria/metal model inverse catalysts have primarily focused on CO oxidation reaction, 20, 21, 28, 29, 40 30, 31, 41 water–gas shift reaction (WGS), and methanol 42 studied both under reaction synthesis from carbon dioxide, conditions as well as for adsorption properties of some of the individual species such as CO, O2, and H2O. Typical role of ceria on metals for oxidative reactions such as CO oxidation, WGS, or POM is to adsorb and dissociate either O2 or H2O, especially in the cases where this stage of the reaction pathway does not proceed (or 16, 23, 28, does too slowly) on the bare surface of the respective metal 29, 31 . The insight into elemental processes of interactions of these simple molecules with ceria/metal systems is usefull in understanding more complex reactions such as catalytic reforming or oxidation of hydrocarbons, alcohols, carboxylic acids etc. However, to the best of our knowledge no experimental study directly addressing methanol oxidation on ceria/platinum has been published so far. In this work we present temperatureprogrammed reaction (TPR) investigation of methanol oxidation reaction on thin film Pt/ceria and ceria/Pt catalysts prepared by magnetron sputtering deposition technique. The reactivity measurements are correlated with complementary chemical and structural information obtained by X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM).
Experimental Sample preparation The catalyst thin films were deposited onto a naturally oxidized Si(100) wafer (0.3 mm thick, P-type, 6-12 Ω·cm resistance) held at room temperature by means of magnetron sputtering of 2’’diameter CeO2 and Pt targets mounted to independent magnetrons in the same preparation chamber. Sputtering was carried out in Ar base atmosphere at constant partial pressure of 0.5 Pa. Radio frequency (RF) magnetron (100 W RF power) was used to deposit cerium oxide 43, 44, while platinum was sputtered by DC magnetron (20 W DC power). The growth rate of thin films under these conditions was approximately 0.5 nm/min (measured by a quartz crystal microbalance).
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Analytical methods Morphology of the deposited layers was examined by means of scanning electron microscopy (SEM) using the MIRA 3 (Tescan) microscope operating at primary energy of electron beam 30 keV. X-ray photoelectron spectroscopy (XPS) was used to investigate the chemical composition of the samples. It was performed in an ultrahigh vacuum chamber operating at base pressure lower than -7 10 Pa equipped with SPECS Phoibos MDC 9 energy analyzer. Aluminum X-ray source was used (total energy resolution ∆E = 1 eV; photon energy hν = 1486.4 eV) for all measurements. Catalytic activity measurements were characterized by temperature-programmed reaction (TPR) method conducted in a custom-built laboratory micro-reactor system. The actual reactor comprised of the analyzed sample (13×13 mm) sandwiched between a PID regulated resistive heater at the bottom and a silicate glass with two feeding holes and a network of channels for effective gas distribution over the sample surface at the top, forming a small micro-reactor cell volume. Silicon rubber sealing (100 µm thick) was placed around the perimeter of the sample to prevent leakage. The reactor cell was enclosed in a relatively massive metal block with integrated thermocouple (K-type) to assure good thermal stability and spatial homogeneity. All experiments were done at total gas pressure of 1 bar and the sample temperature ranging between 360 and 600 K; the heating rate of the temperature ramps was set to 2 K/min. The stream of reactants containing O2 + CH3OH with mutual molar ratio 1:2 was produced by mixing pure oxygen (Linde Gas, 5.0, 1.06 sccm flow) with methanol vapor generated by bubbling helium buffer gas (Linde Gas, 4.6, 10 sccm) through a heated saturator (303 K) filled with liquid methanol (Penta, 99.8% purity). The flows were adjusted by Alicat Scientific mass flow controllers. All stainless-steel tubing between the saturator and the reaction cell was heated to about 360 K to prevent condensation of methanol vapor before reaching the sample. The product stream was sampled through a manual metering valve and continuously monitored by a quadrupole mass spectrometer (QMS, Preiffer Prisma 200) connected to a vacuum chamber with 480 K) or Pt thin film (400 K (the QMS signals are too
Fig. 11: Temperature dependence of the estimated selectivity of all ceria/Pt catalysts involving 4 proposed competing reaction channels (see the discussion for details). Ceria thickness (dCeO2) is indicated at the top of each plot. (double-column width)
There is a common qualitative characteristics of the reaction pattern for all ceria/Pt samples which can be rendered as follows: The two main reaction channels are POM (1) competing with conversion to formic acid (4) or, eventually, its methyl ester via sequential binding to another methanol molecule (in fact, we found no evidence for methyl formate in the QMS spectra although it is one of the few organic molecules which should not freeze in the cold trap). The excess of CO2 observed at the beginning of reaction for ≤1 nm ceria has been attributed to COM (6), presumably originating from direct oxidation on uncovered platinum sites. Regarding POM we will only consider an overall balance since our experimental data do not allow us to distinguish quantitatively whether it proceeds via methoxy or hydroxymethyl intermediate branch. On the lower temperature side (up to approx. 430-440 K) the POM selectivity is at its maximum (it is actually the preferred pathway for ceria thickness
