Article pubs.acs.org/JPCC
Surface and Subsurface Dynamics of the Intermetallic Compound ZnNi in Methanol Steam Reforming Matthias Friedrich,† Detre Teschner,‡ Axel Knop-Gericke,‡ and Marc Armbrüster†,* †
Max-Planck-Institut für Chemische Physik fester Stoffe, Nöthnitzer Straße 40, 01187 Dresden, Germany Fritz-Haber-Institute of the Max-Planck-Society, Faradayweg 4-6, 14195 Berlin, Germany
‡
S Supporting Information *
ABSTRACT: The intermetallic compound ZnNi has been tested as unsupported powder catalyst in methanol steam reforming, showing severe performance changes during the process. Results from in situ XPS experiments proved the decomposition of the surface of ZnNi by Zn oxidation and complete desegregation of Ni. The decomposition of ZnNi even extends toward the bulk, as proven by in situ DTA/TG and XRD. On the basis of these findings, the catalytic properties initially ascribed to ZnNi can not be attributed to the intermetallic compound but have to be assigned to a mixture of the decomposition products.
1. INTRODUCTION Looking at the changing energy supply in today’s world, methanol steam reforming (MSR, CH3OH + H2O → 3H2 + CO2) has high potential to become an important reaction to generate clean hydrogen for fuel cell applications.1,2 Commonly used catalysts for this reaction are based on CuO/ZnO/Al2O3, but their performance is limited resulting in at least 1,100 ppm CO in the product gas due to side reactions, while only a concentration of less than 20 ppm CO is tolerated by fuel cell catalysts.3−6 Nobel metal-based systems like Pd/ZnO and Pt/ ZnO have also been tested, and the formation of the intermetallic compounds ZnPd and ZnPt, respectively, was observed on these catalysts, strongly enhancing the selectivity toward CO2.7−11 In this context, the intermetallic compound ZnNi may be an interesting alternative, because it is isostructural to ZnPd and ZnPt (CuAu type of crystal structure, space group P4/mmm) with nickel being much cheaper than palladium and platinum, thus having potential toward industrial application. In addition, quantum chemical calculations predict a high barrier against segregation for ZnNi,12 hence, the intermetallic compound is likely to be stable under reaction conditions. Iwasa et al. already investigated Ni/ZnO in MSR, but observed a very low selectivity to CO2 (1.1−3.0%) and no formation of Ni−Zn intermetallic compounds.7,8 In 2004 Tsai et al. tested the unsupported intermetallic compounds ZnM (M = Ni, Pd, Pt) in MSR, observing a rather poor selectivity to CO2 for ZnNi (8−35%), but higher compared to Iwasa’s results.13,14 The trends in selectivity observed on ZnPd, ZnPt, and ZnNi were correlated to the distance of their respective dband centers to the Fermi energy. A simple criterion like this to predict selectivity trends of intermetallic catalysts in MSR would be a very valuable tool to select promising compounds. Nevertheless, until now all experimental investigations on ZnNi in MSR were done without performing in situ or postreaction characterization. For a reliable knowledge-based approach it is © 2012 American Chemical Society
inevitable to verify the in situ stability of the intermetallic compound ZnNi in MSR and prove it to be present as catalytically active surface during the catalytic reaction. Contradictory to their publication in 2004, Tsai et al. recently reported on the catalytic properties of ZnNi prepared by reaction of Raney nickel with ZnCl2, showing almost 0% CO2 selectivity.15 Intermetallic compounds in catalytic reactions have drawn a lot of attention recently.16 Most of them were observed after pretreatments or after catalytic reactions on catalysts supported on MxOy (M = Zn, Ga, In). To identify their role in the respective catalytic system and to gain knowledge on their intrinsic catalytic properties the intermetallic compounds have been tested as unsupported catalysts in several chemical reactions.17,18 We report on the synthesis of the unsupported intermetallic compound ZnNi and its performance in methanol steam reforming. For characterization, XRD, ICP-OES, in situ DTATG, and in situ XPS are applied. The latter is used in particular to investigate the surface changes of the intermetallic compound under methanol steam reforming conditions.
2. EXPERIMENTAL SECTION For the synthesis of the intermetallic compound ZnNi, elemental nickel (99.99%, ChemPur, powder) and zinc (99.999%, ChemPur, granules) were used inside an argonfilled glovebox (H2O and O2 below 0.1 ppm). A physical mixture of the powders with a molar ratio of 1:1 was filled into a quartz glass ampule, which then was evacuated and sealed. The ampule was placed inside a furnace, heated to 673 K with 10 K/h, to 693 K (4 K/h), to 723 K (10 K/h) and finally to Received: April 3, 2012 Revised: June 14, 2012 Published: June 23, 2012 14930
dx.doi.org/10.1021/jp303174h | J. Phys. Chem. C 2012, 116, 14930−14935
The Journal of Physical Chemistry C
Article
dilute the catalyst to prevent the formation of cold or hot spots inside the reactor. Graphite was checked for catalytic activity and proven to be inactive for methanol steam reforming as well as the concurring water gas shift reaction (CO + H2O → CO2 + H2) at 350 °C. XPS measurements using synchrotron radiation were performed at the beamline ISISS-PGM at the Helmholtz Zentrum Berlin für Materialien and Energie, Electron storage ring BESSY II. A detailed description of the setup can be found elsewhere.19 A 200 mg sample of ZnNi was ground and pressed under argon to a pill of 8 mm in diameter and 0.5−1 mm in thickness using stainless steel pressing tools. The sample was transferred under argon to the XPS chamber. No surface treatments like argon sputtering or reduction in hydrogen were conducted prior to the measurements. Investigation of the surface of the as-prepared catalyst is carried out at room temperature in UHV. In situ experiments were carried out at 420 °C and 0.2 mbar, using a molar MeOH:H2O ratio of 1:2. Since only low catalytic conversion could be realized within the XPS cell, the formation of hydrogenmonitored by a mass spectrometer (Pfeiffer Prisma)was taken as a qualitative indicator of catalytic activity. Depth profiling was done for the UHV and in situ measurements using three different excitation energies for the respective core levels (Zn 3d, Ni 3p, O 1s) and the valence band. The Fermi edge was recorded at the respective excitation energy and used for energy calibration. Qualitative and quantitative analysis of the XPS data was done using the software Casa XPS.20 To calculate Zn:Ni ratios, the respective peak areas were corrected considering ring current, photon flux and tabulated cross sections.21 Determination of the information depth was based on the calculation of the inelastic mean free path (IMFP) using the NIST Standard Reference Database.22,23 It turned out that the IMFPs for elemental Zn and Ni do not differ much, thus, the IMFP of elemental Ni was used for intermetallic ZnNi. The information depth is three times the IMFP, thus 95% of all excited electrons originate from the respective depth.24 Differential thermal analysis in combination with thermogravimetric measurements (DTA/TG) under methanol steam reforming conditions were performed using a Netzsch STA 449 Jupiter F3. The ZnNi sample was weighed in air and placed inside a corundum crucible. The temperature was raised with 2 K/min to 600 °C, kept there for 30 min and cooled to room temperature with the same rate. Methanol (purity >99%) and deionized water were mixed in equimolar ratio and dosed with 0.3 g/h using a liquid flow controller (Bronkhorst). Subsequently, the liquids were evaporated in a heated 1/16 in. capillary (220 °C), mixed with 85% helium (99.999%, Praxair) and finally brought to the DTA/TG equipment using a heated transfer line (120 °C). To prevent condensation, liquids were only fed at temperatures ≥75 °C in the DTA/TG apparatus.
873 K (75 K/h) and kept at this temperature for ten days. Afterward, the ampule was quenched in water before being opened inside the glovebox. Powders of the sample of a certain size were achieved by grinding and subsequent sieving in the glovebox. The synthesized powders as well as the used catalyst samples were analyzed by transmission X-ray powder diffraction (XRD) performed on an image plate Guinier camera (G670, Huber, Cu Kα1 radiation, λ = 1.5405929(5) Å, Ge(111) monochromator, 3° ≤ 2θ ≤ 100°, internal standard LaB6, a = 4.15692 Å). To prevent contact with air, the powders were mounted between two Kapton foils in the glovebox. The composition of the sample was analyzed using inductively coupled plasma−optical emission spectrometry (ICP−OES) by dissolving 50 mg of the sample in aqua regia before injection into the spectrometer (Vista, Varian). Metal contents were quantified by matrix-matched calibration using multi element standards. All analyses were repeated three times. Catalytic tests were carried out in a plug flow reactor (i.d. 7.9 mm, silica coated stainless steel) which is built inside a hot box (120 °C) to prevent condensation of liquids (MicroActivity, PID Eng&Tech). For the tests, 100 mg of ground ZnNi of a sieve fraction of 20−32 μm was used. The feed consisted of 0.01 mL/min liquid (50 mol % MeOH, (purity >99%) and 50 mol % deionized water), 13.2 mL/min N2 (99.999%, Praxair) and 1.6 mL/min He (99.999%, Praxair). N2 was used as a carrier gas while He was used as inert tracer gas to calibrate the volumes of the gaseous products since remaining MeOH and H2O are not determined in the effluent. The conversion of MeOH is calculated as C=
n(COx ) ·100 n(MeOH feed)
wherein n(COx) is the sum of the amounts of substance of CO and CO2 per time unit in the product gas and n(MeOHfeed) is the initial amount of substance of MeOH fed to the reactor. The activity is defined as
A=
n(H2) mCat .t
wherein n(H2) is the amount of substance of hydrogen produced, mCat. is the mass of catalyst and t is the time. The product fractions were calculated by dividing the concentration of one product component by the sum of all products, disregarding the inert gases N2 and He. The selectivity to CO2 is defined as S(CO2 ) =
c(CO2 ) ·100 c(CO2 ) + c(CO)
wherein c(CO2) and c(CO) are the concentrations of CO2 and CO in the product gas. The gas composition in the product stream is determined by a gas chromatograph (Varian Micro GC CP4900), allowing quantitative determination of CO down to 20 ppm. Furthermore, all gaseous reactants and products were monitored by a mass spectrometer (Pfeiffer, Omnistar 300). Unconverted MeOH and H2O are not determined in both GC and MS because the product gas is dried by cooling trap and a subsequent Nafion membrane before the gas analysis. Before being filled into the reactor, the catalyst powder is mixed with 200 mg of graphite (99.9+%, ChemPur) to improve the flow characteristics in the reactor tube and to
3. RESULTS According to the method described above, the intermetallic compound ZnNi was synthesized as single-phase material, as proven by XRD, which shows solely the tetragonal crystal structure (P4/mmm) of the low-temperature modification of ZnNi with no signs of other Ni−Zn phases or oxides (Figure 1).24 The lattice parameters were determined as a = 2.7488(2) Å and c = 3.1952(4) Å. Chemical analysis by ICP-OES yielded a chemical composition of Zn50.5(6)Ni49.5(6) which is in 14931
dx.doi.org/10.1021/jp303174h | J. Phys. Chem. C 2012, 116, 14930−14935
The Journal of Physical Chemistry C
Article
Two species can be identified in all Zn 3d spectra with their ratio depending on the photon energy. The signal at 9.8 eV is assigned to Zn atoms in the intermetallic compound ZnNi, hereafter denoted as ZnIMC. The second signal at 10.6 is identified as oxidized Zn (Znox).18,29 This is in agreement with the assumption that the surface of the compound is partially oxidized, hence the intensity of the species at higher binding energy decreases with increasing information depth. This shows that even handling under argon with traces of O2 and H2O (both
