Methanol oxidation on pure and platinum-doped tungsten oxide supported by activated carbon

Accepted Manuscript Methanol oxidation on pure and platinum-doped tungsten oxide supported by activated carbon Jan Polášek, Viktor Johánek, Anna Ostroverkh, Karel Mašek PII:

S0254-0584(19)30140-3

DOI:

https://doi.org/10.1016/j.matchemphys.2019.02.042

Reference:

MAC 21385

To appear in:

Materials Chemistry and Physics

Received Date: 18 September 2018 Revised Date:

2 February 2019

Accepted Date: 14 February 2019

Please cite this article as: J. Polášek, V. Johánek, A. Ostroverkh, K. Mašek, Methanol oxidation on pure and platinum-doped tungsten oxide supported by activated carbon, Materials Chemistry and Physics (2019), doi: https://doi.org/10.1016/j.matchemphys.2019.02.042. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Methanol oxidation on pure and platinum-doped tungsten oxide supported by activated carbon Jan Polášek,*, Viktor Johánek, Anna Ostroverkh, Karel Mašek

*e-mail: [email protected]

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Abstract

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Dept. of Surface and Plasma Science, Faculty of Mathematics and Physic, Charles University, V Holešovičkách 2, Prague 8, CZ-18000, Czech Republic

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Thin films of pristine and platinum-doped tungsten oxide were examined as potential catalysts for efficient partial oxidation of methanol. The tungsten oxide layers with different thicknesses were deposited by the means of reactive magnetron sputtering on oxidized silicon wafer and silicon coated with carbon activated via ion etching. Morphology and chemical analysis were checked before and after the reaction by X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). It was found that while pure tungsten oxide layers on silicon exhibit relatively small hydrogen production rates and high ratio of carbon oxides, both etched carbon interlayer (as long as the oxide thickness is low enough) and platinum doping can lead to significant improvement of reactivity, increasing hydrogen yield and decreasing fraction of undesired carbon monoxide in the product mix. It is suggested that it is due to the strong electronic interaction between WOx and Pt or active C, increasing the acidity of the tungsten oxide reaction sites and thus altering its reactivity preference from methanol dehydration towards dehydrogenation. If both species are present in a very thin oxide, however, the competing electronic interactions lead to partial reduction of their positive effect. As prepared sputtered thin films are chemically and morphologically unstable under reducing environment of methanol oxidation reaction, but the presence of dispersed Pt in the layer substantially stabilizes its physical structure.

Keywords: tungsten oxide, platinum, activated carbon, methanol oxidation, magnetron sputtering Declarations of interest: none

ACCEPTED MANUSCRIPT 1. Introduction

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Despite all disadvantages associated with their use, fossil fuels are still main source for heat and energy generation. Strive for more effective and environmentally friendly alternative led to an extensive research of possibilities to replace them with a more suitable resource. So far, hydrogen with its high energy density and clean characteristics seems to be one of the best possibilities. It can be burnt in traditional heat engines or, even more efficiently, utilized in a fuel cell (FC).

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There are several types of fuel cells, usually categorized by an electrolyte used. All the FC types have been studied extensively, but among them, proton exchange membrane (PEM) fuel cells seems to be the most promising candidate for replacing internal combustion engines in cars, as well as batteries in various smaller devices. It is due to their high efficiency [1] and low operating temperature [2]. These properties of PEM fuel cells rendered them good enough to be used commercially already in cars manufactured by Toyota, Hyundai and Daimler. Although fuel cell powered electric cars offers some great advantages over their battery powered counterparts, their share of the electric car market is still fairly low [3].

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One of the greatest obstacles for the widespread use of hydrogen fuel cells in cars or other devices represents the hydrogen itself. Its storage and manipulation are relatively complicated and risky, it is associated with more safety issues than traditional fuels. An elegant way to overcome these obstacles would be to generate hydrogen onboard from some a safe and and easy-to-manipulate liquid. The optimal process for onboard hydrogen generation should be simple, ideally exothermic (not putting additional energy demand), and without substantial amount of carbon monoxide among its by-products, as it tends to poison noble metal electrodes of the common fuel cells, significantly reducing its performance [4].

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Methanol is often discussed as a good source of hydrogen for onboard generation thanks to its good availability, transportability, and high hydrogen-to-carbon ratio. Production of hydrogen from methanol can be achieved via several processes, such as methanol decomposition [5–7], steam reforming [8–12], oxidative steam reforming [13–15] and partial oxidation (POM). Among them, only POM meets all the requirements for on board generation process laid down in the previous paragraph. It was therefore widely studied using catalysts based on Pt [16–18], Pd [19–21], Cu [22–26], and Au [27–31]. Tungsten oxide is known for its use in electrochromic devices [32], gas sensors [33], and as an efficient catalyst for various reactions [34]. In recent years, it was investigated as a catalyst for oxygen reduction reaction in PEM fuel cells [35] and for methanol oxidation in direct methanol fuel cells [36–38]. In this work, we explore catalytic activity and sensitivity towards methanol oxidation of pure and Pt-doped tungsten thin-film oxides supported by activated carbon and, as a reference, by a relatively inert substrate of oxidized silicon.

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2. Experimental

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Thin layers of pure and platinum doped tungsten oxide were prepared by reactive RF magnetron sputtering from 1'' WO3 target (Kurt J. Lesker, 99.9% pure). In case of doped samples, 2 mm platinum wire was placed onto the target. Working atmosphere was composed of Ar and O2 mixed with ratio 19:1. Discharge power was set to 60 W. Deposition rate under these conditions was 2.3 nm/min as determined by step height measurements of reference masked samples by AFM. Thin films were deposited onto two kinds of support: 1) Si(100) wafers passivated by oxidation in the atmosphere at room temperature and 2) Si(100) wafer coated with a continuous layer of activated amorphous carbon (a-C). Carbon layers were prepared in a commercial coating system MED020 (Baltec) by DC magnetron sputtering from 2'' carbon target (Goodfellow, 99.997% purity) in pure argon atmosphere followed by subsequent etching in oxygen plasma. Details on the preparation procedure and properties of the activated carbon can be found in [39].

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For the determination of morphology of the prepared samples, we used Tescan MIRA3 FEG scanning electron microscope (SEM) with primary electron beam energy 25 keV. Chemical state of the samples was investigated by x-ray photoelectron spectroscopy (XPS) using Specs XR50 x-ray source with Al anode and the VSW HA100 hemispherical analyser with multichannel detection located in the vacuum chamber with ultimate pressure under 1×10−7 Pa. Data fitting was performed in KolXPD software.

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Temperature programmed reaction (TPR) data were obtained at atmospheric pressure using home-made microreactor system, where a flat sample is placed on a PID regulated resistive heater and is capped by a quartz glass cover with two gas feeding holes and series of channels to ensure better distribution of gas over the sample. Space between the sample and the glass cover is retained by 0.1 mm thick silicone rubber sealing placed around edges of the sample. Methanol (MeOH) was fed to the sample from the gas bubbler using helium as a buffer gas bubbling through the condenser with liquid methanol kept at the temperature of 303 K. Streams of 1.06 sccm O2 and 10 sccm MeOH + He were admitted through digital mass flow controllers into the reaction chamber to form MeOH + O2 mixture with 2:1 molar ratio. Resulting gas composition was measured by Pfeiffer Prisma 200 quadrupole mass spectrometer (QMS). Before reaching the QMS, gas was sampled by manual leak valve and let through the cold trap to clear it from methanol and its higher fractions. QMS current signals of major reactants and products (H2, CO, CO2, O2) were converted to effective production rates using sensitivity factors obtained experimentally by system calibrations with pure gases.

3. Results and Discussion 3.1. Catalysts after preparation Morphology and chemical state of all the samples were checked before and after the reactivity measurements in the reactor. For these experiments, 35 nm and 0.8 nm thick layers were prepared on two types of substrate: oxidized silicon wafer and etched-carbon-coated silicon. The above particular thicknesses were chosen to represent the cases of a thick bulk-like layer

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Composition

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and a thin discontinuous deposit, respectively. XPS measurements showed that tungsten to platinum ratio was 5:1 for all Pt-doped samples. Platinum loading was determined to be 6.6 µm/cm2 for thick and 0.2 µm/cm2 for thin samples, respectively. Table 1 lists all types of samples used in this work. Each type has a designated label that will be used through the rest of the following text.

C/SiO2/Si

2.2C

Table 1: Parameters of all samples used in this work.

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Fig. 1 shows SEM images of the samples after deposition of tungsten oxide with thickness 35 nm on Si wafer (1a, sample 1.1S), and on etched carbon (1b, sample 1.1C), and 0.8 nm thin layer on etched carbon (c, sample 1.2C). Corresponding SEM images obtained from the samples doped with platinum are not presented as they have not revealed any discernible differences in morphology from their pristine WOx counterparts. We can see a smooth and continuous surface of the layer deposited onto SiO2/Si, while the deposits on the activated carbon exhibit rough granular structure with high specific surface area. The thick oxide overlayer forms larger grains while the thin one essentially reproduces the granularity of the substrate.

Fig. 1: SEM images of the as prepared samples - a) 35 nm thick layer of tungsten oxide on SiO2/Si (sample 1.1S), b) 35 nm thick layer of tungsten oxide on etched carbon substrate (sample 1.1C), and c) 0.8 thick layer of tungsten oxide on etched carbon substrate (sample 1.2C). Fig. 2 presents photoelectron spectra of the W 4f line measured after preparation of thick and thin layers of (a) pure tungsten oxide and (b) tungsten oxide with platinum on activated carbon. For the line-fitting procedure we used W 4f5/2–W 4f7/2 doublets with spin–orbit splitting of 2.14 eV and intensity ratio of 0.75. The peaks were fitted using Voigt line shapes

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W

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(convolution of Gaussian and Lorentzian functions) with the subtraction of Shirley type background. Two doublets can be identified in the W 4f spectra of pure tungsten oxide manifest the presence of two different oxidation states. The first one marked W6+ at the binding energy 35.3 eV was attributed to tungsten atoms bonded via six oxygen atoms in WO3 structure while the significantly smaller W4+ state at the binding energy 33.5 eV corresponds to defects (oxygen vacancies) [40]. There is no W4+ state present in the spectra of the thinner oxides. This is probably the result of the oxidation in air during the transfer from magnetron to the XPS chamber. W 4f spectra from the sample with platinum exhibit the same features as the spectra from the sample with pure tungsten oxide. Only difference was found in a 0.4 eV shift towards the lower binding energies. This shift is caused by a strong interaction between platinum and tungsten oxide, as it was observed previously for the PtWOx model systems [41,42]. Pt 4f spectra (Fig. 3) reveal that after the sample preparation, platinum is fully oxidized and present in Pt2+ and Pt4+ oxidation states while the metallic state is completely missing. We can see that while the intensity of Pt4+ doublet is proportional to the thickness of the sputtered material, the intensity of Pt2+ doublet is almost equal for both thicknesses. This suggests that Pt2+ state is most likely localized at the surface or at the platinum-tungsten interface, whereas Pt4+ can be primarily attributed to Pt dispersed in the PtWOx deposit. Regardless the oxide thickness, a presence of carbon interface had barely noticeable effect on the appearance of W 4f or Pt 4f spectra of as prepared samples, as can be seen in Figs. 2 and 3. Tungsten to platinum total intensity ratio corrected by relative sensitivity factors was approx. 5:1 in all the samples.

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Fig. 2: XP spectra of W 4f line of a) pure (samples 1.1C and 1.2C) and b) platinum-doped tungsten oxide (samples 2.1C and 2.2C) after the deposition on the etched carbon substrate.

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Fig. 3: XP spectrum of Pt 4f line of platinum-doped tungsten oxide after the deposition on the etched carbon substrate (samples 2.1C and 2.2C).

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In the case of thick samples, no Si was detected by XPS. A small amount of C in the spectra can be attributed to the surface contamination from air exposure during sample transfer. For the other samples, the presence of photoelectrons from the substrate material observed in the spectra indicates low thickness and/or possible discontinuity of the layers exposing the uncovered substrate surface. Typical C 1s emission line from the samples with thin oxide layer is shown in Fig. 4. Along with main carbon peak at binding energy of 284.3 eV, we can see two other peaks at higher binding energies (285.7 eV and 288.0 eV) that can be attributed to carbon bound to oxygen, probably originating from oxygen plasma treatment and reactive sputtering. C-C

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Fig. 4: XP spectrum of C 1s emission line of 0.8 nm thick layer of pure tungsten oxide after the deposition on the etched carbon substrate (sample 1.2C).

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3.2. Reaction on pure tungsten oxide In the first part of the experiments, samples with pure tungsten oxide deposited on silicon oxide and activated carbon substrates were subjected to two cycles of methanol oxidation. In the first cycle, the surface temperature was changed in a step-wise fashion to allow the system to achieve (near) equilibrium state at each temperature, which typically occurred in about 2030 minutes. This stabilisation process is a consequence of temperature induced morphological changes of the layers which will be described later in this chapter. Four temperatures between 475 and 600 K were chosen to cover the range in which all the samples exhibited wellmeasurable methanol oxidation reactivity. In the second cycle, sample was gradually heated with the constant rate of 2 K/min up to 600 K. Due to the stabilisation in the first cycle, samples morphology and chemical composition remained stable during the second cycle (as observed by XPS and SEM).

CH3OH + ½ O2 → 2H2 + CO2

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Generally, alcohol dehydration reactions yield alkenes and/or ethers while dehydrogenation yields carbonyls and carboxylates, ultimately leading to CO or CO2, respectively. The molecular ratio of H2 to total carbon carried within CO or CO2 can provide a measure of the preference towards either of the two classes of reactions. In the ideal scenario with respect to generation of CO-free hydrogen, methanol oxidative dehydrogenation follows an overall reaction for partial oxidation of methanol (POM): (1, POM)

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with C/H2 ratio of 0.5 and no CO present. On contrary, dehydration reactions release hydrogen within water molecule, thus C/H2 ratio smaller than 0.5 would indicate participation of a dehydration reaction branch. The C/H2 ratio calculated from CO and CO2 signals can also become smaller than 0.5 in the case the decomposition of methanol molecule is incomplete and proceeds via formation of formic acid and hydrogen molecule CH3OH + ½ O2 → H2 + HCOOH

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The same methanol dehydration reaction branch can also proceed with involvement of more oxygen atoms per methanol molecule yielding CO or CO2 via methanol combustion

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Although formic acid (product of the reaction (2)) is among the species which could not be detected directly (due to the cold trap), the distinction between different reaction pathways can be made using the balance of the measurable product concentrations and the oxygen consumption (derived from 32 amu signal variation). This allows us to estimate relative selectivities for particular reaction pathways at each experimental point of TPR [43].

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Fig. 5: Arrhenius plots of the main products (H2, CO2, CO) of methanol oxidation over the 35 nm thick layer of tungsten oxide supported by a) silicon (sample 1.1S) and b) carbon coated silicon (sample 1.1C).

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Fig. 5 shows ambient pressure reactivity of the thickest (35 nm) tungsten oxide layers on different supports (samples 1.1S and 1.1C) in the form of Arrhenius plots. If we compare both samples we can see that the overall activity expressed in terms of hydrogen production is higher on the sample with carbon support (1.1C). It can be attributed to its higher surface area given by the rough carbon substrate, as can be seen in Fig. 1b. In both cases, the ratio of released CO2 to H2 is at all temperatures higher than the theoretical ratio for partial methanol oxidation (0.5; see eq. 1), gradually decreasing with temperature to the value of approx. 0.65 at 600 K. This observation along with the occurrence of CO within the reaction products suggests that methanol oxidation proceeds simultaneously via several different reaction channels with temperature variable branching ratio. Supported tungsten oxide catalysts are known for their acidic character [34] and preference for alcohol dehydration over alcohol dehydrogenation [44]. It was shown previously that the result of methanol dehydration over supported tungsten oxide is almost exclusively dimethyl ether [45,46], which can produce formaldehyde, methylformate and COx species upon oxidation [47]. Formaldehyde as a direct product of methanol dehydration was already observed during catalytic oxidation of methanol over tungsten oxide [48], therefore we can expect it to be generated in our reaction as well. However, we could not detect the concentration of this species directly due to its freezing in the cold trap. Temperature dependence of H2 and CO2 yields are similar for both samples. The apparent activation energy for hydrogen production calculated from the slope of the hydrogen signal is slightly lower for the sample with carbon interlayer, giving a value of 0.47±0.01 eV as compared to 0.52±0.01 eV for the sample 1.1C. The only significant difference is the increased CO production rate on the silicon supported sample at temperatures above 490 K. Explanation for this behaviour is unclear to us at the moment. We can conclude that the carbon support does not have a significant chemical influence on the thick layers, its major role seems to be an increase of the effective surface area of the tungsten oxide overlayer.

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Fig. 6: Arrhenius plots of the main products (H2, CO2, CO) of methanol oxidation over the 0.8 nm thick layer of tungsten oxide supported by a) silicon (sample 1.2S) and b) carbon coated silicon (sample 1.2C).

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When we go from thick to thin samples (1.2S and 1.2C), the situation becomes quite different (see Figure 6). Temperature dependence and activation energy of the hydrogen production for the thin layer supported by silicon (0.58±0.02 eV) is similar to its thick counterpart, only difference is a higher relative fraction of the detected carbonaceous by-products (CO, CO2) of the reaction (see Fig. 8a), indicating even higher preference of this catalyst for dehydration.

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When the WOx thin film is supported by etched carbon, several differences can be noticed. Hydrogen production rate is significantly higher and there is significantly lower fraction of carbon monoxide in the products (Fig. 6b). Unlike the previous cases, CO2/H2 ratio is fairly constant (0.55±0.02) within the whole reactive temperature range and very close to the theoretical value of POM (0.5). Activation energy for the hydrogen production is lower (0.36±0.01 eV, Fig. 6b). These differences suggest that the reaction path has changed completely, preferring methanol dehydrogenation over its dehydration. This can be explained by an electronic interaction between tungsten oxide and carbon support. It is known that the activated carbon has basic character [49], as its surface contains electron-rich sites that can act as Lewis bases, donating electrons to the interacting material. This charge transfer alters the tungsten oxide acid sites, lowering their acidic strength [46,49]. It was already shown for another catalytic system of zeolite-supported copper, that specimens with higher acidity favoured methanol dehydration, while those with lower acidity favoured methanol dehydrogenation [50]. We therefore suggest that the same relationship is valid also for tungsten oxide and that the interaction between oxide and activated thus carbon leads to the higher selectivity towards hydrogen production owing to the lower acidity of the carbon supported tungsten oxide thin film.

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Fig. 7: Arrhenius plots comparing hydrogen production via methanol oxidation on different pristine WOx layers supported by silicon (blue) and carbon coated silicon (black).

In Fig. 7 the comparison of all pristine WOx samples is provided for hydrogen production rate, making a clear overview of the positive role of carbon interlayer on H2 yield and its stronger influence in the case of the thinner tungsten oxide.

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As was already mentioned above, the oxide layers underwent substantial morphological changes during methanol oxidation. Fig. 8 shows SEM images of the thick layer of tungsten oxide on silicon wafer (a) and on etched carbon substrate (b) after two complete TPR cycles. By comparing to the corresponding images obtained after the preparation (shown previously in Figs. 1a and 1b), we can see a strong sintering effect took place during the reaction.

Fig. 8: SEM images of 35 nm thick tungsten oxide layer deposited on a) silicon wafer (sample 1.1S), b) etched carbon layer (sample 1.1C) after two cycles of methanol oxidation.

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These morphological changes are also reflected in XPS spectra of W 4f emission lines presented in Fig. 9a. In all cases, intensity of the W 4f doublet dropped substantially while the intensity from the substrate material (Si or C) strongly increased with respect to the asprepared samples. Qualitatively the same observation was made for the thinner tungsten oxide as can be seen in Fig. 9b. These significant changes are the consequence of the combined effect of oxide sintering and of covering the surface by carbonaceous leftovers from the reaction, seen as an increase of C 1s line intensity in XPS (not shown). On the flat silica substrate, the decrease of W 4f photoelectron signal is in a very good agreement with the relative area of the bright islands seen in SEM (approx. 7%), supporting the interpretation of these islands as being WOx aggregates. By comparing W 4f intensity changes on different substrates (1.1S vs. 1.1C, 1.2S vs 1.2C) we can see that the signal decrease is about 2-3 times lower for the layers deposited on the carbon substrate. Along with the much lower abundance of the bright features observed by SEM on the sample 1.1C (Fig. 8b) it suggests that the presence of carbon interface somewhat limits the sintering effect and thus partially preserves the catalyst structure.

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It should also be noted that for the carbon supported layers the W 4f line was slightly shifted toward higher binding energy to 35.9 eV, presumably due to the electronic size effect of tungsten oxide grains forming the layer. Sintering or change of chemical state of tungsten oxide was not observed after heating in air or in vacuum within the temperature range used in this work, therefore we can deduce that it takes place only in an environment containing a reductive media such as methanol vapor as in our case. It has already been reported that the presence of hydrogen or hydrocarbons leads to a slight reduction of tungsten trioxide [34,51].

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Fig. 9: XP spectra of W 4f line of a) 35 nm thick (samples 1.1S and 1.1C) and b) 0.8 nm thick (samples 1.2S and 1.2C) layer of pure tungsten oxide as prepared (top) and after the second TPR cycle (bottom).

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3.3. Reaction on tungsten oxide doped with platinum

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Fig. 10: Arrhenius plots of the main products (H2, CO2, CO) of methanol oxidation over the 35 nm thick layer of tungsten oxide doped with platinum supported by a) silicon (sample 2.1S) and b) carbon coated silicon (sample 2.1C). The reactivity within the 2nd TPR cycles (solid lines) are compared to the 1st TPR cycle (solid symbols); the dashed lines represent linear fits of the experimental data.

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Another set of tungsten oxide samples doped by platinum was subjected to the same procedure of two cycles of ambient pressure methanol oxidation as in the previous case. Fig. 10 shows reactivity of 35 nm thick layers of Pt-WOx deposited directly on silicon wafer (a), and on the etched carbon interlayer (b). As one can expect for the thick layers, there is almost no difference discernible between the two types of support. The measured hydrogen production rates can be compared to the reference value of ~23×1014 cm-2s-1 obtained at 575 K with a continuous pure Pt layer on silicon prepared by magnetron sputtering [18,52]. At temperature around 500 K hydrogen production rate reaches its maximum and does not further increase with temperature, indicating that some rate limiting phenomena comes into play on this surface. It means that the reaction efficiency is driven by either desorption rate from the active sites on the surface or by adsorbate diffusion rate instead of intrinsic reaction rate of the catalyst. If we compare the second cycle of the reaction with the first one (solid symbols in Fig. 10) we can see that this effect does not take place during the initial TPR where the rates of products follow an Arrhenian dependence. Upon repeating the TPR for the second time the reaction onset becomes much steeper and the maximal rates of H2, CO, and CO2 production that were achieved at the end of the initial TPR (600 K) are reached already at 500 K. This dramatic change in behaviour between the two consecutive TPRs is likely a

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consequence of morphology changes of the oxide thin film as will be discussed later in this chapter.

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Carbon to hydrogen ratios in the reaction gas mixture (Fig. 10) are much higher than the theoretical stoichiometry of methanol oxidation suggesting that substantial fraction of hydrogen leaves the sample in the form of water or hydrocarbon species produced by uncomplete decomposition of methanol. This observation is in accordance with the above discussed preference of WOx surfaces toward methanol dehydration. There is however a general difference between the pristine and Pt-doped 35 nm layers in both total reactivity and selectivity. The addition of Pt leads to the enhancement of hydrogen yield (up to 50-times between approx. 450-500 K) and shifts the reaction mechanism more towards complete oxidation of methanol, presumably due to the high activity of platinum for CO oxidation [52– 55], as can be seen from the much higher mutual ratio of CO2 and CO in TPRs.

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Fig. 11: Arrhenius plots of the main products (H2, CO2, CO) of methanol oxidation over the 0.8 nm thick layer of tungsten oxide doped with platinum supported by a) silicon (sample 2.2S) and b) carbon coated silicon (sample 2.2C).

Unlike the thicker Pt-WOx layers, the 0.8 nm thin films (samples 2.2S and 2.2C) presented in Fig. 11 exhibit almost Arrhenian character for all major products (with only a slight bend) and significantly higher fraction of hydrogen in the product stream. Reaction patterns (relative rates of the products and their temperature dependence) of 0.8 nm pristine and Pt-doped WOx thin films on etched carbon interlayer are very similar (compare Figs. 6b and 11b), except that the presence of Pt brings an absolute increase of the reaction yields by factor of approx. 5. The analysis of relative production rates of the individual reaction products reveals that in both cases the reaction proceeds mostly via POM pathway (reaction selectivity 79% and 70% for samples 2.1C and 2.2C, respectively). Without the carbon interlayer the absolute hydrogen yield is even higher at all temperatures (about 2.4-times at 600 K), which will be discussed later.

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Apparent activation energies for the hydrogen production evaluated from the higher temperature sections of the Arrhenius plots (above 530 K) are close to that of the pristine WOx on carbon-coated substrate, 0.34±0.02 eV and 0.42±0.02 eV for Pt-WOx on Si and C/Si, respectively. This can lead to a conclusion that the presence of platinum alters the catalytic properties of tungsten oxide in a similar way as etched carbon. It was shown in previous work on Pt/WO3 model systems that the interaction between platinum and tungsten oxide is based on the electron transfer towards tungsten atom [41]. Similar mechanism plays the key role also in the interaction between tungsten oxide and etched carbon as described earlier. We can therefore expect a similar impact on the catalytic properties of tungsten oxide.

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This can result in a competition effect when both species are present. The qualitative resemblance of the TPR patterns obtained for both 0.8 nm thick Pt-WOx samples (Fig. 11) suggests that the interaction between tungsten oxide and activated carbon is considerably weakened by the strong interaction between tungsten oxide and platinum, as observed also by XPS. This is in a sharp contrast to the completely different picture given in Fig. 6. As was already mentioned earlier it is likely due to the similar character of both interactions (Pt— WOx, C—WOx), so that the competition between Pt and carbon results in lower reaction rates obtained on Pt-WOx/C (sample 2.2S) as compared to Pt-WOx catalyst deposited directly on Si (sample 2.2S). In turn, the CO oxidation capability of Pt in the presence of C interface is suppressed, favouring incomplete dehydrogenation or dehydration pathways (reactions (2) and (3)) over COM (reaction (4)) – note the higher fraction of CO and lower fraction of CO2 produced by the sample 2.2C in Fig. 11.

Fig. 12: Arrhenius plot comparing hydrogen production via methanol oxidation on different Pt-doped WOx layers supported by silicon (blue) and carbon coated silicon (black).

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Fig. 13: SEM images of 35 nm thick layer of tungsten oxide doped with platinum deposited on a) silicon wafer (sample 2.1S), b) etched carbon layer (sample 2.1C) after two TPR cycles.

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It turns out that platinum doping has determining influence on the character of morphology changes during the methanol oxidation reaction. SEM image of the 35 nm thick layer of platinum-doped tungsten oxide (Fig. 13a) shows no sign of a strong sintering, such as observed for the pure oxide layer (Fig. 8). Instead, a relatively homogeneous surface with frequent pits can be seen. Since no Si signal was observed by XPS, we can deduce that the substrate remains completely covered by the deposit even at the bottom of these pits, hence the Pt-WOx layer remains continuous. Dispersed platinum therefore seems to serve as a stabilizing element preventing the significant morphological changes to which the pure tungsten oxide is prone in a reducing environment, as was shown previously. This is also in accord with our observation of essentially identical reactivity of both Si-supported thick oxides (Figs. 10 and 12), in contrast to the corresponding pristine oxides (Figs. 5 and 7). For the Pt-doped layers supported on activated carbon no apparent alteration of morphology due to TPRs was found at all (Fig. 13b) within the resolution provided by our SEM (a possible minute structural changes might be obscured by the granular character of this type of surface).

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Fig. 14: XP spectra of Pt 4f line of a) 35nm and b) 0.8 nm thick layer of tungsten oxide doped with platinum (samples 2.1S, 2.1C, 2.2S and 2.2C) as prepared (top) and after the second cycle of methanol oxidation (bottom). 6+

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Fig. 15: XP spectra of W 4f line of a) 35nm and b) 0.8 nm thick layer of tungsten oxide doped with platinum (samples 2.1S, 2.1C, 2.2S and 2.2C) as prepared (top) and after the second cycle of methanol oxidation (bottom).

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In Figs. 14 and 15, Pt 4f and W 4f photoelectron spectra are presented for the Pt-doped thick (a) and thin (b) layers on both substrates. Observed changes are qualitatively similar regardless the oxide thickness – the reaction leads to the reduction of ionic platinum to mostly metallic with a small amount of Pt2+ residuum and, for the layers supported by etched carbon, also some Pt4+ (Fig. 14). Our previous experiments showed that when the same samples are annealed under similar conditions in vacuum or in air, platinum is reduced only partially from Pt4+ to Pt2+ state, but not completely to metallic Pt0 [56]. The reductive environment is therefore necessary for a full reduction of platinum at temperatures below 600 K.

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As can be seen in Fig. 15, methanol oxidation reaction induced significant changes also in the W 4f spectra. After the reaction, there are two states present: one at the position of the original W6+ state before the reaction (34.9 eV) and another one at the higher binding energy (35.4 eV for thick and 36.0 eV for thin layer). The binding energy of the latter spectral feature for the thick layer corresponds to the binding energy of pure tungsten oxide layers. Based on this data, we propose a model where platinum is dispersed homogenously throughout the layer after the preparation, but segregates during the first (stabilizing) cycle of the methanol oxidation. This process stabilizes the thin film, preventing its morphological changes observed on the pure tungsten oxide, and forms areas where tungsten oxide is in contact with platinum and therefore chemically altered (corresponding BE 34.9 eV), and areas where the tungsten oxide is unaltered (corresponding to higher BE). Difference in the position of the latter state for thick and thin layers can be explained by the electronic size effect as it was in the case of pure tungsten oxide layers.

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Intensity of the W 4f line decreased by a factor of 10 after the reaction for both thick and thin layers, hence less substantially than what was observed for pure tungsten oxide samples (Fig. 9). Similar drop in intensity was observed for the Pt 4f line of the thick layers. However, in the case of the thinner layers, the intensity of the Pt 4f line decreased only by a factor of two, approximately, increasing the observed Pt/W ratio from 0.2 to 1.0. This suggests that the 0.8 nm thick samples contained significantly higher fraction of platinum near the surface. This observation is in agreement with the W 4f spectra where the thin layers exhibit higher relative intensity of the state corresponding to oxide interaction with platinum. It should be noted in this context that according to the previous research on model systems [57] we expect platinum to be encapsulated by tungsten oxide after the first cycle of methanol oxidation. However, the encapsulating layer is probably too thin to cause noticeable intensity changes in the XPS spectra in this case. The above described thickness-dependent behaviour may explain why the reaction rate limitation is observed only for the thick layers (Fig. 12b). Based on the TPR data, we can suggest that the active sites for POM are in the vicinity of the platinum species where the tungsten oxide is chemically altered. Lower availability of these sites on the surface (such as on the thicker Pt-WOx layers) leads to their easier saturation restricting the overall reaction rate above certain surface temperature.

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Fig. 16: Reaction selectivity of all a) 35nm and b) 0.8 nm thick layers of tungsten oxide (pure and Pt-doped) at the end of the second cycle of methanol oxidation (i.e. at 600 K). POM, FA, MC, and COM denotes reactions pathways (1)-(4).

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In Fig. 16 an overview of reaction selectivity for the 0.8 nm and 35 nm thick catalyst layers in all four examined configurations is summarized for the final TPR temperature of 600 K. It clearly shows the strong positive role of the active carbon interlayer and of platinum in the catalytic ability of the thinner sample for POM and, on contrary, the (near) irrelevance of the active carbon for the reaction selectivity on the thicker layers. Yet on the thicker oxide film the presence of Pt, despite its partial encapsulation, strongly shifts the preference from methanol combustion (MC, reaction (3)) to complete methanol oxidation (COM, reaction (4)) pathways, presumably due to the high ability of Pt to oxidize CO to CO2.

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3.4.Overall Discussion

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The catalytic activity of tungsten oxide in methanol oxidation is influenced by both etched carbon support and platinum doping. In the case of pure tungsten oxide, usage of carbon substrate leads to changes in both the overall reactivity and ratios of reaction products. The impact of activated carbon interlayer on catalytic properties is much more significant for the thinner oxide – the hydrogen production rate increases by a factor of about 3-10 (0.8 nm thickness) or 2-3 (35 nm), respectively, depending on surface temperature. This enhancement is due to the combination of increased roughness (being major contribution in the case of 35 nm layer) and a change in reaction mechanism. The reaction selectivity shifts from dehydration towards dehydrogenation pathway, making POM the dominant reaction route (~80%) on the thinner oxide, while the contribution of carbon on the selectivity of thicker WOx is only minor. It is probably a consequence of the basicity of activated carbon substrate containing electron-rich sites capable to donate electrons to the interacting material, therefore acting as a Lewis base. This in turn affects the acidity of the overlaying tungsten oxide, altering its reactivity preference towards methanol dehydrogenation Significant morphological changes of the oxide layers were observed during the initial TPR cycles. SEM micrographs revealed a strong sintering effect that took place on both (SiO2/Si and C/SiO2/Si) substrates as a consequence of heating in the reductive environment. No changes of tungsten oxidation state were observed by XPS after the reaction cycles.

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Doping tungsten oxide by platinum has several impacts. Presence of platinum stabilizes morphology of the system and hinders the oxide sintering observed for pure WOx. Heating in reductive environment results in the reduction of the originally oxidized platinum (involving mainly the Pt4+ oxidation state) to metallic state (Pt0) already during the first TPR run, indicating coalescence of highly dispersed platinum to larger nanoparticles. These particles are subject to encapsulation by tungsten oxide, which appears to be much more effective in the thick layer.

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Regarding the reactivity, the Pt dopant positively impacts the hydrogen yield in all the types of WOx structures. The catalytic behaviour is, however, strongly thickness dependent. On the thicker (35 nm) layers the reactivity of Pt-WOx is higher than of pristine oxide and essentially insensitive of the presence of carbon interface due to its structural stability owing to the presence of Pt. The reaction selectivity is strongly shifted in favour of complete oxidation of methanol, yielding CO2 and H2O. Unlike for undoped oxides the reaction rate exhibits saturation above ~500 K, which is presumably a result of limited Pt availability at or near surface due to its partial encapsulation by tungsten oxide. Although there is a considerable amount of water among the reaction products in this case, we don’t expect it to have significant influence on the reaction. Water does not stay adsorbed or dissociate on the tungsten oxide surface at the temperatures applied in our experiments [58–60] and is thus promptly carried away with the rest of the reaction products and unreacted gases.

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With the thinner layers, on the other hand, the presence of activated carbon plays a key role. Addition of dispersed Pt leads to substantial enhancement of hydrogen production rate, almost 2 orders of magnitude for the 0.8 nm Pt-WOx/SiO2/Si system. With C interlayer, however, this enhancement becomes much smaller (no more than 5-times at 600 K) and is observed only above 470 K. It is explained by the essential similarity of the interaction mechanisms of both species (activated C and Pt) with tungsten oxide, which involves electron density transfer toward the tungsten atom. Nevertheless, despite the lower total yields, the coexistence of Pt and activated carbon distinctively affects the reaction selectivity of the WOx catalyst toward dehydrogenation. This observation is consistent with other reports showing that decomposition of alcohols on platinum at moderate conditions favours dehydrogenation pathway via the initial rapid scission of the O-H bond [61–63], also demonstrated on oxidesupported Pt nanoparticles [64]. The positive role of platinum for the overall reaction turnover rates, reported in this work, is probably further enhanced due to the involvement of surface Pt in the activation of C-H bond [65], which is often a rate-limiting step in the decomposition of oxygenated hydrocarbons.

4. Conclusions In this work, tungsten oxide was examined as a catalyst for hydrogen production via partial methanol oxidation. Tungsten oxide, both pure and doped with platinum, was prepared by reactive magnetron sputtering on silicon and activated carbon substrates. Two thicknesses of the oxide were chosen to discriminate morphological and chemical effect of the support.

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Activated carbon substrate was prepared by magnetron sputtering and subsequent etching in oxygen plasma. TPR on pure tungsten oxide showed that the presence of activated carbon as a substrate leads to increased activity towards hydrogen production and lower relative concentrations of undesired carbon oxides in the product mixture. Both effects were significantly more pronounced on the thin layers.

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Doping the above thin films by platinum leads to increased hydrogen yield in all cases. However, impact of the Pt addition depends heavily on the thickness of the oxide layers. Reaction on the thick Pt-doped layers is almost independent on the type of substrate and exhibits high preference for complete oxidation of methanol (production of water and carbon dioxide). Presence of platinum in the thin layers results in a substantial increase of the hydrogen production for both types of support, however, the activated carbon interlayer partially reduces the benefit of Pt due to the competing electronic interactions of WOx with both compounds. On the other hand, the activated carbon plays a positive role in enhancing reaction selectivity towards complete dehydrogenation pathways.

Acknowledgements

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It is demonstrated that the Pt-WOx system represents versatile material for catalytic oxidation of methanol (and possibly also of other alcohols) which can be relatively easily tailored through the variation of its thickness, platinum content and a presence of an activated carbon interlayer.

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Figure Captions:

Fig. 1: SEM images of the as prepared samples - a) 35 nm thick layer of tungsten oxide on SiO2/Si (sample 1.1S), b) 35 nm thick layer of tungsten oxide on etched carbon substrate (sample 1.1C), and c) 0.8 thick layer of tungsten oxide on etched carbon substrate (sample 1.2C).

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Fig. 2: XP spectra of W 4f line of a) pure (samples 1.1C and 1.2C) and b) platinum-doped tungsten oxide (samples 2.1C and 2.2C) after the deposition on the etched carbon substrate. Fig. 3: XP spectrum of Pt 4f line of platinum-doped tungsten oxide after the deposition on the etched carbon substrate (samples 2.1C and 2.2C).

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Fig. 4: XP spectrum of C 1s emission line of 0.8 nm thick layer of pure tungsten oxide after the deposition on the etched carbon substrate (sample 1.2C).

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Fig. 5: Arrhenius plots of the main products (H2, CO2, CO) of methanol oxidation over the 35 nm thick layer of tungsten oxide supported by a) silicon (sample 1.1S) and b) carbon coated silicon (sample 1.1C). Fig. 6: Arrhenius plots of the main products (H2, CO2, CO) of methanol oxidation over the 0.8 nm thick layer of tungsten oxide supported by a) silicon (sample 1.2S) and b) carbon coated silicon (sample 1.2C). Fig. 7: Arrhenius plots comparing hydrogen production via methanol oxidation on different pristine WOx layers supported by silicon (blue) and carbon coated silicon (black). Fig. 8: SEM images of 35 nm thick tungsten oxide layer deposited on a) silicon wafer (sample 1.1S), b) etched carbon layer (sample 1.1C) after two cycles of methanol oxidation. Fig. 9: XP spectra of W 4f line of a) 35 nm thick (samples 1.1S and 1.1C) and b) 0.8 nm thick (samples 1.2S and 1.2C) layer of pure tungsten oxide as prepared (top) and after the second TPR cycle (bottom).

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Fig. 10: Arrhenius plots of the main products (H2, CO2, CO) of methanol oxidation over the 35 nm thick layer of tungsten oxide doped with platinum supported by a) silicon (sample 2.1S) and b) carbon coated silicon (sample 2.1C). The reactivity within the 2nd TPR cycles (solid lines) are compared to the 1st TPR cycle (solid symbols); the dashed lines represent linear fits of the experimental data. Fig. 11: Arrhenius plots of the main products (H2, CO2, CO) of methanol oxidation over the 0.8 nm thick layer of tungsten oxide doped with platinum supported by a) silicon (sample 2.2S) and b) carbon coated silicon (sample 2.2C).

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Fig. 12: Arrhenius plot comparing hydrogen production via methanol oxidation on different Pt-doped WOx layers supported by silicon (blue) and carbon coated silicon (black). Fig. 13: SEM images of 35 nm thick layer of tungsten oxide doped with platinum deposited on a) silicon wafer (sample 2.1S), b) etched carbon layer (sample 2.1C) after two TPR cycles.

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Fig. 14: XP spectra of Pt 4f line of a) 35nm and b) 0.8 nm thick layer of tungsten oxide doped with platinum (samples 2.1S, 2.1C, 2.2S and 2.2C) as prepared (top) and after the second cycle of methanol oxidation (bottom). Fig. 15: XP spectra of W 4f line of a) 35nm and b) 0.8 nm thick layer of tungsten oxide doped with platinum (samples 2.1S, 2.1C, 2.2S and 2.2C) as prepared (top) and after the second cycle of methanol oxidation (bottom).

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Fig. 16: Reaction selectivity of all a) 35nm and b) 0.8 nm thick layers of tungsten oxide (pure and Pt-doped) at the end of the second cycle of methanol oxidation (i.e. at 600 K). POM, FA, MC, and COM denotes reactions pathways (1)-(4).

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Table 1: Parameters of all samples used in this work.

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Highlights:

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Tungsten oxide based catalysts prepared by magnetron sputtering were examined for their performance on hydrogen generation via partial oxidation of methanol. Chemical alteration of tungsten oxide by activated carbon interlayer or a small amount of highly dispersed platinum significantly improved both hydrogen generation rate and reaction selectivity. Dispersed platinum and activated carbon modify tungsten oxide in the similar way: by strong electronic interaction increasing the acidity of the tungsten oxide reaction sites.

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