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Background: In a routine handling of a catalyst material, exposure to air can usually not be avoided. For noble metal catalysts that are resistant to oxidation, this is not an issue, but becomes important for intermetallic catalysts composed of two or more non-noble chemical elements that possess much different standard enthalpies of the oxide formation. The element with higher affinity to oxygen concentrates on the surface in the oxide form, whereas the element with lower affinity sinks into the subsurface region. This changes the number of active sites and the catalytic performance of the catalyst. We have investigated the instability of the surface composition to oxidation of the Ga Ni 3 2 noble metal-free intermetallic compound, a new catalyst for the CO reduction to CO, CH and methanol. 2 4 Methods: The instability of the oxidized Ga Ni surface composition to different heating–annealing conditions was 3 2 studied by X-ray photoelectron spectroscopy (XPS), used to determine the elemental composition and the chemical bonding in the near-surface region. The dispersion of active sites available for the chemisorption of H and CO on the Ga Ni catalyst surface was determined by H and CO temperature-programmed desorption. CO conversion 3 2 2 2 experiments were performed by using the catalyst material reduced in hydrogen at temperatures of 300 and 600 °C. Results: XPS study of the Ga Ni surface subjected to different heating–annealing conditions has revealed that the 3 2 concentration of Ga at the oxidized surface is strongly enhanced and the concentration of Ni is strongly depleted with respect to the values in the bulk. By annealing the surface at 600 °C in ultra-high vacuum, the oxides have evaporated and thermal diffusion of atoms near the surface has partially reconstructed the surface composition towards the energetically more favorable bulk value, whereas annealing at a lower temperature of 300 °C was ineffective to change the surface composition. Catalytic tests were in agreement with the XPS results, where an increased CO conversion for the catalyst reduced with hydrogen at a higher temperature followed an increased Ni/Ga surface concentration ratio. Conclusions: The instability of the active surface chemical composition to oxidation in air must be taken into account when considering noble metal-free intermetallic catalysts as alternatives to the conventional catalysts based on noble metals. Ga Ni and other Ga–Ni intermetallic compounds are good examples of binary intermetallic catalysts, whose 3 2 catalytic performance is strongly affected by exposure to the air. Keywords: Ga Ni intermetallic catalyst, Surface instability to oxidation, CO catalytic conversion 3 2 2 * Correspondence: firstname.lastname@example.org Jožef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia Faculty of Mathematics and Physics, University of Ljubljana, Jadranska 19, 1000 Ljubljana, Slovenia Full list of author information is available at the end of the article © The Author(s). 2018 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Wencka et al. Journal of Analytical Science and Technology (2018) 9:12 Page 2 of 10 Background energetically more favorable bulk value, but the degree Intermetallic compounds were proven to be attractive of restoration depends on the temperature and the time alternatives to pure and alloyed metals as catalyst mate- spent at high temperature. The number of active sites, rials in heterogeneous catalysis because of their in- their geometric distribution on the surface, and the creased selectivity to specific reactions and better consequent catalytic activity and selectivity will thus de- long-term stability (Armbrüster et al. 2014). Intermetal- pend on the heating–annealing conditions. lic compounds can be reproducibly prepared in a con- In this work, the instability of the active surface trolled manner, and their ordered crystal structures chemical composition to oxidation in air was studied for imply that the number of catalytically active sites and the Ga Ni intermetallic catalyst, belonging to the 3 2 the distances between them are well defined and nickel-poor side of the Ga–Ni phase diagram (Okamoto uniformly repeat over the crystal. By selecting interme- 2010) that has not been investigated previously. Ga Ni 3 2 tallic compounds with suitable crystal structures and is the only compound in the Ga–Ni system that is in chemical elements, the active sites can be tailored to the equilibrium with the binary Ga–Ni melt, so growth of needs of a particular chemical reaction (Armbrüster et single crystals is possible. Though single crystals are not al. 2011). Fixed crystal structures do not allow clustering used in catalytic applications, they are advantageous to of the catalytically active atoms during operation of the prepare well-defined crystalline surfaces along specific catalyst, which is a decisive difference to alloys. crystallographic planes for fundamental surface studies Palladium-based intermetallic compounds M Pd with related to catalysis. In addition, phase purity of a m n M = Ga or In (including GaPd, GaPd ,Ga Pd , and single-crystalline sample is usually superior to that of a 2 7 3 InPd) were proven to be highly selective and stable cata- polygrain material, so crushing a single crystal into pow- lyst materials for the selective hydrogenation of alkynes der provides phase-pure catalyst material. Our Ga Ni 3 2 and the methanol steam reforming reaction (Armbrüster powder catalyst material was prepared by crushing a sin- et al. 2010, 2011, 2014; Kovnir et al. 2007; Osswald et al. gle crystal grown by the Czochralski technique (Wencka 2008; Klanjšek et al. 2012; Wencka et al. 2014, 2015). et al. 2016). We have tested the effect of oxidation on For the industrial heterogeneous catalysis processes, it is the catalytic performance of Ga Ni in hydrogenation re- 3 2 advantageous to replace precious metals by less expen- actions that reduce CO to CO, CH , and methanol, by 2 4 sive alternatives. In such an attempt, the intermetallic varying heating–annealing conditions that alter the Al Fe was found to be an excellent replacement for surface chemical composition and the consequent cata- 13 4 palladium in the heterogeneous hydrogenation of al- lytic performance. Oxidation has happened during kynes (Armbrüster et al. 2012). Another precious metal crushing the Ga Ni single crystal into powder under 3 2 (and noble metal)-free series of intermetallic catalysts ambient conditions prior to the catalytic testing. The were discovered in the Ga–Ni system (GaNi, GaNi , foundation hydrogenation reactions are given below: Ga Ni ), which reduce CO to methanol at ambient H +CO ↔ CO + H O (reverse water–gas shift reac- 3 5 2 2 2 2 pressure (Studt et al. 2014; Sharafutdinov et al. 2014). tion (rWGS)) In a routine handling of the catalyst material, exposure 4H +CO ↔ CH +2H O (methanation) 2 2 4 2 to air can usually not be avoided. For noble metal cata- 3H +CO ↔ CH OH + H O (methanol synthesis) 2 2 3 2 lysts (Pt, Pd, Au, Ag, Cu, …) that are resistant to corro- On the microscopic level, the instability of the oxi- sion and oxidation in moist air, exposure to air is not an dized Ga Ni surface composition to different heating– 3 2 important issue, but it becomes essential for noble annealing conditions was studied by X-ray photoelectron metal-free intermetallic catalysts composed of two or spectroscopy (XPS), used to determine the elemental more chemical elements that possess much different composition and the chemical bonding in the standard enthalpies of the oxide formation. The element near-surface region. The XPS depth profiling technique with higher affinity to oxygen (more negative standard was applied to the oxidized monocrystalline surface at enthalpy of formation) will concentrate on the surface in temperatures between 25 and 600 °C in ultra-high vac- the oxide form, whereas the element with lower affinity uum (UHV) and in the presence of a diluted H will sink into the subsurface region. The surface chem- atmosphere. ical composition may thus be strongly altered with re- spect to the bulk one, which affects the number of active Methods sites. Surface reduction with hydrogen gas just prior to Ga Ni crystallizes in the Al Ni -type structure (Hellner the catalytic runs cleans the oxides, but the oxide-free 3 2 3 2 surface composition remains altered. Upon heating the and Laves 1947), trigonal space group P3m1, and lattice clean catalyst in an atmosphere that does not contain parameters a = 0.405 nm and c = 0.489 nm (Wencka et oxygen, thermal diffusion of atoms near the surface al. 2016). The trigonal unit cell is shown in Fig. 1.The tends to restore the surface stoichiometry towards the X-ray diffraction (XRD) pattern of the employed Ga Ni 3 2 Wencka et al. Journal of Analytical Science and Technology (2018) 9:12 Page 3 of 10 Prior to the reduction, the catalyst was pre-treated by −1 heating in a stream of argon (30 mL min ) at 400 °C for 30 min in order to remove physisorbed gases on the surface. Thereafter, 4.9 mol% hydrogen in argon was used as the reducing agent at the flow rate of −1 30 mL min and the temperature was raised from 25 to −1 600 °C using the ramp rate of 10 °C min . After the re- duction, the temperature was decreased to 80 °C. In the subsequent TPD experiments, an appropriate gaseous mixture was passed over the catalyst (4.9 mol% H in Ar for H –TPD and 10 mol% CO in He for CO–TPD) at −1 the flow rate of 30 mL min . The excess gas was re- moved by purging with helium for 30 min. The temperature was consequently gradually raised to 650 °C Fig. 1 The Ga Ni trigonal unit cell (Wencka et al. 2016; Hellner and 3 2 −1 by ramping at 10 K min under the flow of helium, and Laves 1947) the desorption data of H and CO were recorded. The signals from the thermal conductivity detector (TCD) single crystal grown by the Czochralski technique is were calibrated using various gas mixtures of H and shown in Wencka et al. 2016, demonstrating high degree CO. The above described H –TPD and CO–TPD exper- of structural order and phase purity. Its decomposition iments were repeated also for the catalyst reduced at a temperature was determined by differential thermal ana- lower temperature of 300 °C. lysis (DTA) (Wencka et al. 2016). Upon heating, there is Catalytic conversion runs were carried out in a vertical an onset of a pre-peak at 934 °C and the main endother- fixed-bed U-shaped quartz reactor (100 cm length and mic peak at 948 °C, whereas no thermal effects were 1.5 cm internal diameter). Electric furnace fitted with a observed at lower temperatures. The main peak corre- temperature-programmed controller heated the reactor, sponds well to the phase diagram (Okamoto 2010), and the temperature of the reactor was monitored using which shows the peritectic temperature of Ga Ni at a type K thermocouple. The flow rates of the gases were 3 2 950 °C. The onset of a step at temperatures lower than measured and controlled by Brooks mass flow meters. that of a peritectic decomposition (here at 934 °C) is The Ga Ni catalyst (0.1 g) was placed in the intermedi- 3 2 typical of solution-grown crystals that have been grown ate section of the reactor. The reaction mixture con- at tunable temperatures below the peritectic one. Upon sisted of CO :H molar ratio 1:1, and the gas hourly 2 2 DTA cooling, the peritectic back-reaction starts at the space velocity (GHSV) of the reaction was maintained at −1 same temperature of 948 °C and there is almost no 60,000 h . undercooling effect. The Ga Ni crystal is thus stable up XPS surface analysis was performed by using a TFA 3 2 to these high temperatures. XPS instrument (Physical Electronics Inc.) equipped The distribution of particles’ cross dimensions in the with an Al-monochromatic X-ray source (E = 1486 eV). metallic powder obtained by crushing a piece of the sin- The XPS-analyzed volume was 0.4 mm in diameter and gle crystal was determined from SEM backscattered 3–5 nm in depth. The XPS carbon C 1s signal at electron images (not shown), where 95% of the particles 284.8 eV measured at the surface was used to calibrate were in the range 50 nm–5 μm. The specific surface area the binding energy scale of the XPS spectra due to pos- of the catalyst determined by the single-point Brunauer– sible shift of the spectra related to sample charging. Emmett–Teller (BET) method using nitrogen as the probe molecule was found to amount to 0.8 m /g. This Results relatively small specific surface is a consequence of the Catalyst characterization small surface–to–volume ratio of the powder obtained The results of the TPD analysis are summarized in by crushing a crystal. Table 1, where the amounts of released H and CO per The dispersion of active sites available for the chemi- gram of the Ga Ni catalyst reduced at two different 3 2 sorption of H and CO on the Ga Ni catalyst surface temperatures of 300 or 600 °C, respectively, are given. 2 3 2 was studied by H and CO desorption experiments. The catalyst reduced at a higher temperature of 600 °C Temperature-programmed desorption (TPD) runs were showed a factor ~ 1.8 larger hydrogen coverage of the carried out using the Micrometrics 2920 Autochem II surface as compared to the catalyst reduced at a lower Chemisorption Analyzer. In a TPD analysis, the catalyst temperature of 300 °C. Higher reduction temperature is first reduced by hydrogen to activate the surface (the has also increased the CO adsorption by about the same same procedure is applied before the catalytic tests). factor (~ 1.7). The H –TPD data were used to calculate 2 Wencka et al. Journal of Analytical Science and Technology (2018) 9:12 Page 4 of 10 Table 1 Metal dispersion D, the amounts of released H and CO per gram of the Ga Ni catalyst determined from TPD experiments, 2 3 2 the reaction rate r , and the CO turnover frequency (TOF) at 500 °C of the catalyst reduced at two different temperatures of 300 CO 2 or 600 °C, respectively −1 −1 −1 −1 −1 ReductionTD (%) H –TPD (μmol H g )CO–TPD (μmol CO g ) r at 500 °C (μmol CO g s ) TOF at 500 °C (s ) 2 2 CO 2 300 °C 0.06 ± 0.01 1.8 ± 0.2 0.07 ± 0.01 1.4 ± 0.15 0.42 ± 0.04 600 °C 0.11 ± 0.01 3.2 ± 0.2 0.12 ± 0.01 2.2 ± 0.15 0.39 ± 0.04 the dispersion of active sites D (reported here as metal reduced catalyst to D= 0.11% for the 600 °C reduction, dispersion) by assuming Ni to be the catalytic species. an increase by a factor 1.8. Metal dispersion was defined as The H –TPD and CO–TPD profiles are shown in Fig. 2. The H –TPD profile (Fig. 2a) of the catalyst re- duced at 600 °C shows a steep increase of the desorbed 2 ðÞ no: H molecules chemisorbed D ¼ ; ð1Þ hydrogen at temperatures above 450 °C with the peak at Total no: of Ni atoms about 600 °C and a sharp drop at still higher tempera- tures. The catalyst reduced at 300 °C shows similar peak, where the Ni:H ratio in the chemisorption was taken as but of much lower intensity due to smaller hydrogen 1. Metal dispersion was found to be rather small for coverage of the surface. both reduction temperatures, because of the low The CO–TPD profile (Fig. 2b) of the catalyst reduced surface-to-volume ratio of the employed powder mater- at 600 °C shows two distinct peaks, a low-temperature ial. It has changed from D= 0.06% for the 300 °C- peak centered at about 120 °C and a high-temperature peak centered at about 550 °C. In the CO–TPD profile of the catalyst reduced at 300 °C, these peaks are absent (or there may still be a trace of the low-temperature peak hidden in the noise). Catalytic testing Prior to the activity tests, the catalyst was reduced under H for 2 h at 300 or 600 °C, respectively. The CO con- 2 2 version experiment was conducted in the temperature range between room temperature and 500 °C, and the CO conversion factor X was calculated from the 2 CO expression ðÞ moles CO −ðÞ moles CO 2 2 in out X ¼ : ð2Þ CO ðÞ moles CO in The CO conversion data over the Ga Ni catalyst re- 2 3 2 duced at 300 or 600 °C, respectively, are shown in Fig. 3a. For both reduction temperatures, the onset temperature for the CO conversion was from 100 °C and the maximum conversion was obtained at 500 °C. The conversion was below the equilibrium conversion of the rWGS reaction (black solid curve) (Kim et al. 2012, 2013). The conversion over the catalyst reduced at 600 °C was considerably higher than the conversion over the catalyst reduced at 300 °C, which correlates to the larger metal dispersion of the former that chemisorbs more CO at the surface. Upon comple- tion of the reaction, measurements were taken also on cooling and the results were within the same Fig. 2 a H –TPD and b CO–TPD profiles of the Ga Ni catalyst 2 3 2 range as those on heating. We have tested also the reduced at 300 or 600 °C, respectively. Vertical axis shows the signal bare nickel oxide NiO, which did not show any activ- from the thermal conductivity detector (TCD) ity within the temperature range tested (Fig. 3a). We Wencka et al. Journal of Analytical Science and Technology (2018) 9:12 Page 5 of 10 Fig. 4 a Reaction rate r as a function of temperature for the CO Fig. 3 a CO conversion data over the Ga Ni catalyst reduced at 2 3 2 −1 Ga Ni catalyst reduced at 300 or 600 °C, respectively. b The CO 3 2 2 300 or 600 °C, respectively (CO :H ratio of 1:1, GHSV = 60,000 h ). 2 2 turnover frequency (TOF) as a function of temperature The equilibrium conversion of the rWGS reaction is shown by the black solid curve (Kim et al. 2012, 2013). Conversion over the nickel oxide NiO and the unreduced Ga Ni was also tested, showing no 3 2 −1 −1 500 °C value r ¼ 2.2 μmol g s of the catalyst re- CO activity within the investigated temperature range. b CO selectivity duced at 600 °C is by a factor ~ 1.6 larger than r ¼ CO data over the Ga Ni catalyst reduced at 300 or 600 °C, respectively 3 2 −1 −1 1.4 μmol g s of the catalyst reduced at 300 °C. The reaction rate expressed in number of CO molecules also tested the unreduced Ga Ni catalyst for 3 2 converted per single metal (Ni) atom per second (the comparison, which showed no conversion at all. −1 CO turnover frequency (TOF), in units s ) was deter- Reaction rates were measured in separate experiments, 2 mined from the expression in which the conversion of the reactants was maintained below 100% such that differential reaction conditions X F CO CO 2 2 TOF ¼ ; ð4Þ and negligible heat and mass transfer effects could be as- no: Ni atoms exposed −1 −1 sumed. The reaction rate r (in units mol CO g s ) CO 2 2 was calculated from the expression where the CO flow rate is recalculated in number of 17 −1 CO molecules per second ( F ¼ 6.2 × 10 s ) and CO 2 2 X F no. Ni atoms exposed = D × Total no. Ni atoms. The CO CO 2 2 r ¼ ; ð3Þ CO TOF values at 500 °C of the catalyst reduced at 300 °C −1 −1 (0.42 ± 0.04 s ) or 600 °C (0.39 ± 0.04 s ) are identical −1 where F is the CO flow rate (1.35 mL min ) and m within the experimental uncertainty (the values are also CO 2 2 (0.1 g) is the mass of the catalyst. The temperature- given in Table 1). This is a plausible result, indicating dependent reaction rate of the Ga Ni catalyst reduced that the increased activity of the Ga Ni catalyst reduced 3 2 3 2 at 300 or 600 °C, respectively, is shown in Fig. 4a.The at 600 °C, as compared to the 300 °C-reduced one, origi- r values at the highest investigated temperature of nates from an increased number of active sites (since the CO 500 °C are also given in Table 1 for comparison. The metal dispersion D and the reaction rate r have both CO 2 Wencka et al. Journal of Analytical Science and Technology (2018) 9:12 Page 6 of 10 increased by about the same factor of ~ 1.7), whereas the type of active sites (Ni) remains the same (equal TOFs). The temperature-dependent TOF curve (an average of the TOF curves of the catalysts reduced at 300 and 600 °C, respectively) is shown in Fig. 4b. CO and CH were the only carbon products detected in the outlet gas stream, whereas no methanol has formed. The CO selectivity defined as moles CO CO selectivity ¼ ð5Þ moles CO þ moles CH is shown in Fig. 3b. For both reduction temperatures, only CO was produced at low temperatures. For the catalyst reduced at 300 °C, the 100% CO selectivity wasobtained attemperaturesbelow 200°C, where CO conversion is low, reaching a maximum of 5% only. At higher temperatures, CO formed via the rWGS reaction is converted to methane. The selectiv- ity towards CO consequently decreases with increas- ing temperature, reaching 50% at the highest investigated temperature of 500 °C. For the catalyst reduced at 600 °C, the range of 100% CO selectivity is extended up to 350 °C, where the CO conversion is already significant, reaching about 13%. At 500 °C, where the CO conversion reaches its maximum value of 22%, the CO selectivity drops to 75%. XPS surface analysis Fig. 5 a XPS elemental concentration depth profile (in at. %) at 25 °C XPS gives information on the elemental composition of the Ga Ni single crystal that was oxidized due to contact with air. 3 2 within a thin surface layer of the material (3–5 nm depth b The corresponding depth profile of the Ni/Ga concentration ratio in our case) and on the chemical bonding of the ele- ments (oxide, metal). Ar-ion sputtering (4 keV) was used in the XPS depth profiling to remove material with a is excluded from the analysis. The Ni/Ga concentra- rate of 2 nm/min, which was calibrated on a flat Ni/Cr tion ratio at the oxidized surface amounted to multilayer structure of known thickness. Composition surf R ¼ 0:12 0:02. With increasing depth, the Ni=Ga depth profile was measured for 25 min, reaching the oxygen concentration rapidly dropped, whereas the depth of 50 nm. concentrations of Ga and Ni increased and did not The XPS surface analysis was performed on a change much at depths larger than about 10 nm, bulky piece of the Ga Ni single crystal that was 3 2 where bulk values for the specific crystallographic oxidized due to contact with air. The surface orientation bulk orientation of the surface were reached (R ¼ 0: in the Ga Ni trigonal crystal system was ð210Þ, i.e.,  Ni=Ga 3 2 direction was normal to the surface. Elemental com- 83 0:04). The depth profile of the Ni/Ga concen- position was determined at temperatures between tration ratio is shown in Fig. 5b. Considering the 25 and 600 °C in either UHV or a diluted H Ga and Ni concentrations within the oxidized sur- atmosphere. The experiment included the following face layer, the surface was strongly enhanced in Ga steps: and depleted in Ni relative to the concentrations in the bulk. 1. The first experiment was performed at 25 °C in The nonzero oxygen concentration at large depths −9 UHV of 2 × 10 mbar on the as-grown oxidized of the concentration profile shown in Fig. 5a crystal. The resulting XPS elemental concentration (corresponding to the oxygen signal detected after a depth profile (in at. %) is shown in Fig. 5a. The ele- long sputtering time) is artificial. During the XPS ments detected were Ga, Ni, O, and C. Carbon con- depth profiling, a relatively large area of the sample tamination was mainly confined to the surface and (about 0.4 mm in diameter) is analyzed, where the Wencka et al. Journal of Analytical Science and Technology (2018) 9:12 Page 7 of 10 surface material is removed layer by layer by the Ar-ion sputtering process. Due to surface rough- ness, some regions are sputtered less effectively, yielding oxygen signal from the surface even after longer sputtering times. Some surface oxygen comes also from the borders of the sputtered re- gion. Since the sputtering time on the abscissa of Fig. 5a was converted into the sputtering depth (with the conversion factor 2 nm/min), these two effects yield an artificial nonzero oxygen concentra- tion at deeper regions of the concentration profile. 2. In the second step, the temperature was raised to 130 °C and the material was kept there for 14 h in UHV for surface degassing. The temperature was then raised to 300 °C and the concentrations of O, Ga, and Ni were measured in the uppermost surface layer (3–5 nm depth) as a function of time for 4 h. For the first 2 h, the sample was kept in UHV, then a diluted H atmosphere was introduced −7 into the XPS chamber (pressure of 4 × 10 mbar) and the concentrations were monitored for another 2 h. No change of the oxygen concentration with time was detected, and the Ni/Ga concentration ratio at the surface also remained constant at the surf value R ¼ 0:12 0:02. The time dependence of Ni=Ga surf R during annealing at 300 °C is shown in Ni=Ga Fig. 6a. The surface remained strongly enhanced in Ga and depleted in Ni relative to the bulk concentrations, and the annealing at 300 °C in either UHV or diluted H atmosphere did not change the surface composition. 3. In the third step, the temperature was raised from 300 to 600 °C in 2 h. The concentrations of O, Ga, and Ni were then measured in the surface layer as a function of time for 2.5 h after the 600 °C temperature was set. A diluted H atmosphere was introduced into the XPS chamber 12 min after the concentration measurements have started. The O, Ga, and Ni concentrations as a function of the annealing time are shown in Fig. 6b, whereas the time dependence of the Ni/Ga surface surf Fig. 6 a Time dependence of the Ni/Ga concentration ratio in the concentration ratio R is presented in Fig. 6c. Ni=Ga surface layer (3–5 nm depth) during annealing at 300 °C. b Time The oxygen concentration has dropped by a factor dependence of the O, Ga and Ni concentrations in the surface layer of 10 already within the short time period of during annealing at 600 °C. c Time dependence of the Ni/Ga 12 min before the H introduction (when the concentration ratio in the surface layer during annealing at 600 °C sample was still in UHV) and then did not change significantly anymore in the presence of H . Within time in the presence of H . These results indicate surf the same time period, R increased strongly that (i) the oxides were removed from the surface Ni=Ga surf at 600 °C in the time of about 12 min already in from the initial value R ¼ 0:23 0:02 to the Ni=Ga UHV, whereas the introduction of a diluted H surf value R ¼ 0:43 0:03 that was reached at the Ni=Ga atmosphere did not result in a further reduction of moment of the H introduction and then did not the residual oxygen concentration upon annealing, change significantly anymore with the annealing (ii) a significant increase of the Ni/Ga surface Wencka et al. Journal of Analytical Science and Technology (2018) 9:12 Page 8 of 10 concentration ratio has happened during the same annealing at 600 °C are shown in Fig. 7b. All Ga is me- initial period in UHV, so that the drop of the tallic at any depth, including at the surface. This oxygen concentration is directly related to the confirms that annealing at 600 °C has effectively cleaned the surface oxide. increase of the nickel concentration at the surface, surf The results for the chemical bonding of Ni obtained (iii) the starting R ¼ 0:23 value at 600 °C was a Ni=Ga from high-resolution Ni 2p XPS spectra are analo- 3/2 factor of about 2 larger than the corresponding gous. No signal of metallic Ni could be observed within surf value at 300 °C (and also at 25 °C) R ¼ 0:12, so Ni=Ga the surface layer of the oxidized material prior to the that some enhancement of the nickel concentration annealing (Fig. 7c), whereas metallic Ni is present at the at the surface has happened already during heating surface after annealing at 600 °C (Fig. 7d). the material from 300 to 600 °C in UHV (that was accomplished in the time of 2 h). Discussion 4. In the fourth step, the crystal was cooled back to The XPS study of a monocrystalline surface has shown room temperature in UHV. The measurement of that the elemental composition of the Ga Ni oxidized the elemental concentrations at 25 °C has shown 3 2 surface is strongly enhanced in Ga and depleted in Ni ( that the Ni/Ga surface concentration ratio surf surf remained enhanced to the value R ¼ 0:43. R ¼ 0:12 ) relative to the concentrations of these Ni=Ga Ni=Ga bulk elements in the bulk ( R ¼ 0:83 ) for the specific Ni=Ga The type of chemical bonding of Ga at different depths crystallographic orientation of the surface. Annealing at in the concentration profile was determined by decom- 600 °C has resulted in the oxide removal and partial posing Ga XPS signal into the Ga-oxide and Ga-metallic reconstruction of the surface towards higher Ni concen- signals. The high-resolution Ga 2p XPS spectra at sev- 3/2 surf tration (R ¼ 0:43), whereas the annealing at 300 °C eral depths between the surface and 30 nm of the oxi- Ni=Ga dized material at 25 °C (prior to the annealing did not. The surface reconstruction at 600 °C has hap- experiments, corresponding to the depth profile of pened already in UHV within the time of about 12 min, Fig. 5a) is shown in Fig. 7a. At the surface, Ga is fully whereas the subsequent introduction of a diluted H re- oxidic, whereas at the depths larger than about of ducing atmosphere and further annealing for 2 h did not 10 nm, Ga is in the metallic form. The thickness of the change the surface concentrations of Ni and Ga any- surface gallium-oxide layer was consequently estimated more. This indicates that the oxide removal and the sur- as 6 ± 2 nm. The spectra at 25 °C after the completed face reconstruction process were already completed a c Fig. 7 Ga 2p XPS high-resolution spectra at several depths between the surface and 30 nm at 25 °C of a the Ga Ni oxidized monocrystalline 3/2 3 2 surface (prior to the annealing experiments) and b after annealing at 600 °C. The corresponding Ni 2p spectra of c the oxidized surface and 3/2 d after annealing at 600 °C. The spectra are not normalized Wencka et al. Journal of Analytical Science and Technology (2018) 9:12 Page 9 of 10 during annealing in UHV, so that the effectiveness of the analysis, confirm that nickel is the catalytically active static diluted H reducing atmosphere could not be species of the Ga Ni catalyst. Microscopic identification 2 3 2 assessed. After cooling back to room temperature in of the active sites (coordination of Ni by Ga atoms and/ UHV, the Ni/Ga surface concentration ratio remained or by other Ni atoms) cannot be inferred from our surf experiments. enhanced to the value R ¼ 0:43. Ni=Ga The above results can be understood by the following Conclusions picture. Upon contact of the Ga Ni material to the air, 3 2 In a routine handling of the catalyst material, expos- surface gallium oxide (very likely Ga O ) and nickel 2 3 ure to air can usually not be avoided. For noble metal oxide (NiO) are formed almost instantaneously. The catalysts that are resistant to oxidation, this is not an standard enthalpy of formation of Ga O (ΔH = – 2 3 f issue, but becomes important for intermetallic cata- 1089.1 kJ/mol) (Haynes 2011) is a factor of 4.5 more lysts composed of two or more non-noble chemical negative than the one of NiO (ΔH = – 240.0 kJ/mol), so that higher affinity of Ga to oxygen pushes Ga to the elements that possess much different standard enthal- surface, whereas Ni consequently sinks into the subsur- pies of the oxide formation. The element with higher face region. As a result, the concentration of Ga at the affinity to oxygen (more negative standard enthalpy of oxidized surface is strongly enhanced and the concentra- formation) concentrates on the surface in the oxide tion of Ni is strongly depleted with respect to the values form, whereas the element with lower affinity sinks in the bulk. By annealing the surface at 600 °C in UHV, into the subsurface region. The surface stoichiometry the oxides evaporate (Ga O decomposes into volatile 2 3 maythusbestrongly altered with respecttothe bulk lower oxides and oxygen (Stepanov et al. 2016)) and one, which affects the number of active sites at the thermal diffusion of atoms near the surface partially re- surface and the catalytic performance. We have inves- constructs the surface composition towards the energet- tigated the instability of the active surface chemical ically more favorable bulk value. Since the standard composition to oxidation for the Ga Ni intermetallic 3 2 enthalpy of formation of Ga Ni is ΔH = – 45 kJ/mol 3 2 f catalyst, a new catalyst for the CO reduction from (de Boer et al. 1988), more nickel at the surface reduces the Ga–Ni phase diagram that has not been investi- the energy. By annealing the surface at a lower gated previously. XPS study of the surface subjected temperature of 300 °C, thermal energy of the atoms is to different heating–annealing conditions has shown too small to remove the oxides in UHV, whereas the H that the concentration of Ga at the oxidized surface reducing atmosphere appears to be too diluted to be ef- is strongly enhanced and the concentration of Ni is fective in cleaning the oxides (the H gas could be filled strongly depleted with respect to the values in the into the UHV chamber to a maximum pressure of −7 10 mbar only, in order that the XPS instrument was bulk, because of a 4.5 times more negative standard still operating). enthalpy of formation of Ga O relative to NiO. By 2 3 In the CO catalytic conversion experiments, the ex- annealing the surface at 600 °C in UHV, the oxides perimental conditions were different from those in the have evaporated and thermal diffusion of atoms near XPS. There was no UHV but the pressure was close to the surface has partially reconstructed the surface atmospheric, whereas the density of the H reducing at- composition towards the energetically more favorable mosphere was high. Reduction with H flowing gas has bulk value. By annealing the surface at a lower effectively cleaned the surface oxides at both 300 and temperature of 300 °C, thermal energy of the atoms 600 °C. The subsequent reconstruction of the oxide-free was too small to remove the oxides in UHV and the surface by atomic diffusion towards higher Ni concen- surface reconstruction did not take place. tration has proceeded in the same way as in the XPS In the CO conversion experiments, the reduction experiments. The Ni/Ga surface concentration ratio with H gas prior to the catalytic tests has effectively depended on the temperature and on the time spent at cleaned the surface oxides for both, 300 or 600 °C re- that temperature. The increased CO conversion of the duction temperatures. The subsequent reconstruction catalyst reduced at 600 °C with respect to the 300 °C-re- of the oxide-free surface by atomic diffusion towards duced one (Fig. 3a) is a consequence of the increased Ni higher Ni concentration has proceeded in the same concentration at the surface. This is supported by the way as in the XPS experiments. The Ni/Ga surface fact that the amounts of released H and CO in the TPD concentration ratio depended on the temperature and experiments, the metal dispersion D and the reaction the time spent at that temperature. A significantly rate r for the 600 °C-reduced catalyst all increase by increased CO conversion (by a factor 1.7) of the CO 2 2 the same factor of about 1.7 with respect to the 300 °C catalyst reduced at 600 °C, with respect to the reduction, whereas the TOF (representing the CO con- 300 °C-reduced one was detected, as a consequence version per single active site) is the same for both reduc- of the increased Ni concentration at the surface by tion temperatures. These results, together with the XPS thesamefactor. Wencka et al. Journal of Analytical Science and Technology (2018) 9:12 Page 10 of 10 The instability of the active surface chemical compos- Hellner E, Laves F. Crystal chemistry of indium and gallium in alloys with some transition elements (Ni, Pd, Pt, Cu, Ag, and Au). Z Naturforschung. 1947;2a: ition to oxidation in air must be taken into account 177–83. when considering noble metal-free intermetallic catalysts Kim SS, Lee HH, Hong SC. The effect of the morphological characteristics of TiO as alternatives to the conventional catalysts based on supports on the reverse water–gas shift reaction over Pt/TiO catalysts. Appl Catal B Environ. 2012;119–120:100–8. noble metals. Ga Ni and other Ga–Ni intermetallic 3 2 Kim SS, Park KH, Hong SC. A study of the selectivity of the reverse water–gas- compounds are good examples of binary intermetallic shift reaction over Pt/TiO catalysts. Fuel Process Technol. 2013;108:47–54. catalysts, whose catalytic performance is strongly af- Klanjšek M, Gradišek A, Kocjan A, Bobnar M, Jeglič P, Wencka M, Jagličić Z, Popčević P, Ivkov J, Smontara A, Gille P, Armbrüster M, Grin Y, Dolinšek J. fected by exposure to the air. PdGa intermetallic hydrogenation catalyst: an NMR and physical property study. J Phys Condens Matter. 2012;24:085703. Acknowledgements Kovnir K, Armbrüster M, Teschner D, Venkov TV, Jentoft FC, Knop–Gericke A, Yu We thank Prof. Marc Armbrüster from TU Chemnitz for fruitful discussions. G, Schlögl R. A new approach to well-defined, stable and site-isolated catalysts. Sci Technol Adv Mater. 2007;8:420–7. Funding Okamoto H. Ga-Ni (Gallium-Nickel). J Phase Equil Diffus. 2010;31:575–6. Slovenian authors acknowledge the financial support from the Slovenian Research Osswald J, Giedigkeit R, Jentoft RE, Armbrüster M, Girgsdies F, Kovnir K, Grin Y, Agency (research core funding No. P1–0125). MW is grateful for funding by the Schlögl R, Ressler T. Palladium–gallium intermetallic compounds for the German Academic Exchange Service (DAAD), Grant no. A/14/02942. selective hydrogenation of acetylene: part I: preparation and structural investigation under reaction conditions. J Catal. 2008;258:210–8. Availability of data and materials Sharafutdinov I, Elkjær CF, de Carvalho HWP, Gardini D, Chiarello GL, Damsgaard The data used in this study is presented in the main paper. Request for CD, Wagner JB, Grunwaldt J–D, Dahl S, Chorkendorff I. Intermetallic material to JD. compounds of Ni and Ga as catalysts for the synthesis of methanol. J Catal. 2014;320:77–88. Stepanov SI, Nikolaev VI, Bougrov VE, Romanov AE. Gallium oxide: properties and Authors’ contributions applications—a review. Rev Adv Mater Sci. 2016;44:63–86. MW and PG synthesized the catalysts material. AJ and SV performed Studt F, Sharafutdinov I, Abild-Pedersen F, Elkjær CF, Hummelshøj JS, Dahl S, characterization of the material. VDBCD and BL conducted catalytic testing. Chorkendorff I, Nørskov JK. Discovery of a Ni-Ga catalyst for carbon dioxide JK performed surface analysis by XPS. HJK contributed analysis of catalytic reduction to methanol. Nat Chem. 2014;6:320–42. results. JD has coordinated the work and wrote the paper. All authors read Wencka M, Hahne M, Kocjan A, Vrtnik S, Koželj P, Korže D, Jagličić Z, Sorić M, and approved the final manuscript. Popčević P, Ivkov J, Smontara A, Gille P, Jurga S, Tomeš P, Paschen S, Ormeci A, Armbrüster M, Grin Y, Dolinšek J. Physical properties of the InPd Competing interests intermetallic catalyst. Intermetallics. 2014;55:56–65. The authors declare that they have no competing interests. Wencka M, Pillaca M, Gille P. Single crystal growth of Ga Ni by the Czochralski 3 2 method. J Cryst Growth. 2016;449:114–8. Wencka M, Schwerin J, Klanjšek M, Krnel M, Vrtnik M, Koželj P, Jelen A, Kapun G, Publisher’sNote Jagličić Z, Sharafutdinov I, Chorkendorff I, Gille P, Dolinšek J. Physical Springer Nature remains neutral with regard to jurisdictional claims in properties of the GaPd intermetallic catalyst in bulk and nanoparticle published maps and institutional affiliations. morphology. Intermetallics. 2015;67:35–46. Author details Institute of Molecular Physics, Polish Academy of Sciences, Smoluchowskiego 17, 60-179 Poznań, Poland. Jožef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia. Department of Catalysis and Chemical Reaction Engineering, National Institute of Chemistry, Hajdrihova 19, 1000 Ljubljana, Slovenia. Department of Earth and Environmental Sciences, Crystallography Section, Ludwig-Maximilians-Universität München, Theresienstraße 41, 80333 Munich, Germany. Division of Material Science Research, Korea Basic Science Institute, Daejeon 305-333, Republic of Korea. Faculty of Mathematics and Physics, University of Ljubljana, Jadranska 19, 1000 Ljubljana, Slovenia. Received: 25 April 2018 Accepted: 31 May 2018 References Armbrüster M, Kovnir K, Behrens M, Teschner D, Grin Y, Schlögl R. Pd−Ga intermetallic compounds as highly selective semihydrogenation catalysts. J Am Chem Soc. 2010;132:14745–7. Armbrüster M, Kovnir K, Friedrich M, Teschner D, Wowsnick G, Hahne M, Gille P, Szentmiklósi L, Feuerbacher M, Heggen M, Girgsdies F, Rosenthal D, Schlögl R, Yu G. Al Fe as a low-cost alternative for palladium in heterogeneous 13 4 hydrogenation. Nat Mater. 2012;11:690–3. Armbrüster M, Kovnir K, Grin Y, Schlögl R. Complex metallic phases in catalysis. In: Dubois JM, Belin-Ferré E, editors. Complex metallic alloys: Fundamentals and Applications. Weinheim: Wiley–VCH; 2011. p. 385–99. Armbrüster M, Schlögl R, Yu G. Intermetallic compounds in heterogeneous catalysis—a quickly developing field. Sci Technol Adv Mater. 2014;15:034803. de Boer FR, Boom R, Mattens WCM, Miedema AR, Niessen AK. Cohesion in metals: transition metal alloys (cohesion and structure). Amsterdam: North Holland; 1988. Haynes WM. CRC Handbook of chemistry and physics. 92nd ed. Boca Raton: CRC Press; 2011.
"Journal of Analytical Science and Technology" – Springer Journals
Published: Dec 1, 2018
Keywords: Analytical Chemistry; Characterization and Evaluation of Materials; Monitoring/Environmental Analysis
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