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CO Poisoning and CO Hydrogenation on the Surface of Pd Hydrogen Separation Membranes Casey P O'Brien, and Ivan C Lee J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b05046 • Publication Date (Web): 05 Jul 2017 Downloaded from http://pubs.acs.org on July 18, 2017

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The Journal of Physical Chemistry

CO Poisoning and CO Hydrogenation on the Surface of Pd Hydrogen Separation Membranes Casey P. O’Brien, Ivan C. Lee* U.S. Army Research Laboratory, Sensors and Electron Devices Directorate, 2800 Powder Mill Road, Adelphi, MD 20783, USA *Corresponding author. E-mail address: [email protected]

Abstract To understand how CO inhibits hydrogen transport across Pd membranes, a 25 μm-thick Pd foil membrane was monitored by infrared-reflection absorption spectroscopy (IRAS) during exposure to H2/CO gas mixtures while the rate of hydrogen permeation across the membrane was measured simultaneously in the 373-533 K temperature range. As the coverage of CO on the membrane surface increases with increasing CO concentration and decreasing temperature, the rate of hydrogen permeation across the membrane decreases. However, in addition to adsorbing on the membrane surface, CO reacts with H2 to form surface-adsorbed methylene (CH2) species and methane in the gas phase. The coverage of methylene increases with decreasing temperature, and therefore, the strong poisoning effect of CO at low temperatures may be due to both CO and methylene species blocking H2 dissociation sites on the membrane surface. The activity and selectivity of the Pd membrane for CO methanation is much higher than expected from previous studies and the high activation barrier to CO dissociation on Pd. It is possible that the high concentration of defect sites on the polycrystalline Pd foil surface and hydrogen facilitate dissociation of CO at low temperatures. This work demonstrates that spectroscopic 1 ACS Paragon Plus Environment


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observation of membranes under realistic permeation conditions is critical for understanding surface poisoning mechanisms and for rational design of membranes that are resistant to poisoning.

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1. Introduction Palladium-based membranes have received significant attention for hydrogen separation applications, including hydrogen purification and membrane reactors, due in part to their nearinfinite selectivity to hydrogen separation and their high permeability to hydrogen. In both hydrogen purification and membrane reactor applications, hydrogen must be separated from other gases—typically CO, CO2, and H2O—which can decrease the rate of hydrogen permeation across the membrane.1-8 Understanding how these gases inhibit hydrogen transport is critical for designing efficient hydrogen separation processes. While it is generally believed that CO inhibits hydrogen transport across Pd-based membranes by adsorbing on the membrane surface and blocking H2 dissociation sites, it has been shown that CO can inhibit hydrogen transport by other mechanisms such as carbon deposition and formation of bulk Pd1-xCx phases.7 The inhibiting effect of CO on Pd-based membranes can also depend on the conditions of the membrane pre-treatment, which is not well understood.6 To better understand the mechanism by which CO inhibits hydrogen transport, and how the structure of the membrane influences its interaction with CO, it is necessary to observe that interaction insitu under realistic permeation conditions. To this end, we have recently developed a spectroscopic membrane permeation cell that enables the observation of Pd membranes surfaces using infrared-reflection absorption spectroscopy (IRAS) with simultaneous measurement of hydrogen permeation rates.9 We demonstrated that this device is (1) capable of measuring hydrogen permeation rates across Pd membranes accurately, (2) detecting sub-monolayer coverages of CO on the membrane surface at elevated temperature, and (3) simultaneously detecting CO adsorbed on the membrane surface while measuring the rate of hydrogen permeation across the membrane. By correlating microscopic surface processes observed by 3 ACS Paragon Plus Environment


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IRAS to macroscopic permeation rates with this device, it is possible to understand the mechanisms by which gases such as CO inhibit hydrogen transport across Pd. In this work, the interaction of CO with a 25 μm-thick Pd foil membrane is investigated by IRAS, both in the absence of hydrogen and during hydrogen permeation, to understand how CO inhibits hydrogen transport across Pd. In Sections 3.1 and 3.2, CO is used as a probe molecule with IRAS to investigate the effects of oxidative pre-treatment on the surface adsorption properties of the Pd membrane (Section 3.1) and the structure of the Pd membrane surface (Section 3.2). In Section 3.3, the interaction of CO and hydrogen with the Pd membrane surface is monitored by IRAS while the rate of hydrogen permeation across the membrane is measured simultaneously in the 373-533 K temperature range. We show that the rate of hydrogen permeation across the membrane decreases as the CO coverage on the membrane surface increases, as expected. However, we also show that CO reacts with hydrogen to form methylene (CH2) species on the membrane surface and methane in the gas-phase. The strong poisoning effect of CO at low temperatures may be due to both methylene and CO adsorbates blocking H2 dissociation sites on the membrane surface. The polycrystalline Pd foil membrane also displays unusually high activity and selectivity towards CO methanation, which warrants further investigation as a potential CO or CO2 methanation catalyst. 2. Experimental All experiments in this work were performed with a spectroscopic membrane permeation cell that was designed and fabricated at the U.S. Army Research Laboratory. A detailed description of its design is given in a previous publication.9 Hydrogen permeation and surface spectroscopy measurements were performed using Pd foil membranes, which were cut from Pd foil (Alfa Aesar, 99.9% metals purity). The membranes were washed with acetone and installed 4 ACS Paragon Plus Environment

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in the spectroscopic membrane permeation cell with graphite gaskets. The effective membrane surface area was 2.85 cm2. The assembled permeation cell was then installed in a Harrick RefractorTM Reactor Grazing Angle FTIR Accessory. A thermocouple was fixed to the surface of the permeation cell using the reactor’s sample holders. Mass Flow controllers (MKS Instruments 1179A Mass-Flo®) were used to regulate the flow rates of all gases to the reactor: Ar (Airgas, 99.999%), N2 (Airgas, 99.999%), O2 (Airgas, 99.999%), 10% CO in balance Ar (Airgas), and hydrogen. Hydrogen gas was generated by electrolytic dissociation of water in a Parker 60H Hydrogen Gas Generator, which has a purity rating of 99.999%. The CO/Ar feed gas was purified using a Pall Mini Gaskleen® Gas Purifier to remove Ni(CO)4 impurities. Without the CO gas purifier, we observed a slow decrease in the hydrogen flux across the membrane (transient deactivation) from accumulation of nickel on the membrane surface (see Section S1 in the Supporting Information). Installing the CO gas purifier eliminated the accumulation of nickel deposits and, as a result, transient deactivation was not observed with the CO gas purifier. The ports of the Harrick reactor were used to feed the gases to the retentate surface of the membrane while an Ar sweep gas flowed over the permeate surface of the membrane. The concentration of hydrogen in the sweep gas exiting the reactor was analyzed with an Agilent 3000A Micro GC. Prior to all hydrogen permeation and IRAS experiments, the membranes were pre-treated in the Harrick reactor by flowing 20 mL/min O2 and 80 mL/min N2 over the retentate surface of the membrane, and 100 mL/min Ar over the permeate surface of the membrane, for 60 minutes while the permeation cell was held at 573 K. The O2 in the reactor was then purged by flowing 100 mL/min N2 through the reactor for 5 min before exposing the retentate surface of the membrane to a gas mixture composed of 50% H2/N2 at a flow rate of 200 mL/min for 30 min at 5 ACS Paragon Plus Environment


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573 K. After 30 minutes of H2 exposure, the membrane surfaces were activated for hydrogen permeation and IRAS measurements. The influence of the activation procedure on the adsorption and permeation properties of the Pd membrane will be discussed in more detail in Section 3.1. The interaction of CO with the Pd membrane in the absence of H2 was investigated by IRAS of the membrane surface during exposure to a retentate gas mixture containing 0.1% CO, 0.9% Ar, and 99% N2 at atmospheric pressure. IRAS spectra were collected with a Bruker Vertex 70 Fourier transform infrared (FTIR) spectrometer equipped with a liquid nitrogen cooled MCT detector. All spectra were collected with a spectral resolution of 2 cm-1 and 128 scans-perspectrum. After reflection off the membrane surface and after exiting the reactor, the infrared beam passed through a Specac GS12501 wire grid polarizer. Background spectra were collected with both p- and s-polarized light while flowing 100 mL/min N2 over the retentate surface of the membrane. Following background collection, 1 mL/min of the 10% CO in Ar gas mixture was introduced to the retentate feed gas along with 100 mL/min N2. IR spectra were collected continuously with p-polarized light during exposure of the membranes to CO until the CO concentration in the reactor reached a steady-state after ~15 minutes. A spectrum was then collected with s-polarized light while the steady-state concentration of CO was flowing through the reactor. The interaction of CO with the Pd membrane was investigated starting from a high temperature of 533 K and decreasing the temperature in 40 K increments down to 333 K. The influence of CO on hydrogen permeation across the Pd membrane was investigated by IRAS during exposure to H2 and CO in the 373-533 K temperature range while simultaneously measuring the rate of hydrogen permeation across the membrane. Baseline H2 fluxes were established in the absence of CO while flowing 100 mL/min H2 and 100 mL/min N2 6 ACS Paragon Plus Environment

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across the retentate surface of the membrane and 100 mL/min Ar in the sweep gas. Background IR spectra were collected with both p- and s-polarized light during baseline H2 permeation measurements. Once a baseline H2 flux was established and the background IR spectra were collected, CO was introduced to the 50% H2/N2 feed gas by flowing 1 mL/min of the 10% CO in Ar gas giving a CO concentration of 0.05 %. IRAS spectra were collected with p-polarized light continuously during CO exposure until the CO concentration reached a steady-state after about 15 minutes, then an IR spectrum was collected with s-polarized light. After 20 minutes of exposure to 0.05% CO, the concentration of CO in the feed gas was increased to 0.5% by increasing the flow rate of the 10% CO in Ar feed gas to 10 mL/min and decreasing the N2 flow rate in the feed gas to 90 mL/min. The CO concentration in the feed gas was increased every ~20 minutes by increasing the flow rate of the CO/Ar feed gas and decreasing the N2 flow rate while holding the H2 flow rate (100 mL/min) and the total flow rate (200 mL/min) constant. IRAS spectra were collected with both p- and s-polarized light and H2 fluxes were measured at each CO concentration of 0.05%, 0.5%, 1%, 2%, 3%, 4%, and 5%. 3. Results and Discussion 3.1 Activation of Pd surface Many studies6,10-17 have shown that pre-exposure of Pd-based membranes to air or other oxidizing atmospheres at elevated temperatures increases their H2 permeability substantially. It is generally believed that the oxidative pre-treatment changes the structure or composition of the membrane surface, either by removing carbonaceous deposits,14 surface roughening,13 or formation of pores, defects, and palladium-oxides.13 However, it is not well understood how these structural and compositional changes enhance the permeability of the membrane. In this section, the influence of the oxidative pre-treatment on the surface adsorption properties of Pd 7 ACS Paragon Plus Environment


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membranes is investigated by IRAS of adsorbed CO on the fresh untreated membrane and after the oxidative pre-treatment. To demonstrate the effect of the oxidative pre-treatment on the H2 flux across a 25 μmthick Pd membrane, Figure 1(a) shows a comparison of the H2 fluxes across a fresh untreated Pd membrane, and across the same membrane following the oxidative pre-treatment described in Section 2, versus the difference in the square-root of the hydrogen partial pressure across the /

membrane (∆ ) at 473 K. As expected, the H2 flux across the pre-treated membrane is substantially higher than that of the untreated membrane over the entire H2 partial pressure /

range. Furthermore, the H2 flux across the pre-treated membrane is proportional to ∆ , which indicates that the rate of hydrogen transport is limited by the rate of atomic hydrogen diffusion through the Pd lattice,18 whereas the H2 flux across the fresh membrane is proportional to ∆ with n equal to 0.81. This deviation from square-root dependence on the hydrogen partial pressure suggests that the rate of hydrogen transport across the fresh membrane is not limited by the rate of atomic hydrogen diffusion through the Pd lattice, and surface effects may influence the permeation rate.19,20


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Figure 1. (a) H2 flux across a 25 μm-thick Pd membrane at 473 K before (“fresh,” black data points) and after the oxidative pre-treatment (red data points) versus the difference in the square-root of the partial pressure across the membrane. (b) IRAS spectra collected during exposure of the fresh (black) and pre-treated (red) Pd membrane to a flowing gas of 0.1% CO in inert gases at 373 K.

To understand how the oxidative pre-treatment enhances the rate of hydrogen transport across the Pd membrane, the surface adsorption properties of the fresh and pre-treated membrane were investigated by IRAS of the membrane during exposure to the probe molecule CO. Figure 1(b) shows the IRAS spectra of the fresh and pre-treated membrane during exposure to a flow gas containing 0.1% CO in inert gas (PCO2 = 101 Pa) at 373 K. There are no bands associated with surface-adsorbed CO in the spectrum of the fresh membrane exposed to CO. In the spectrum of the pre-treated membrane exposed to CO, on the other hand, there is a sharp band centered at ~1980 cm-1 which is associated with CO adsorbed on bridging sites on the Pd membrane surface.9,21-26 This result clearly shows that CO adsorption is hindered on the fresh Pd membrane, and the oxidative pre-treatment enhances CO adsorption on the membrane surface. It is not clear from our results how exactly the oxidative pre-treatment enhances CO adsorption, but these results unambiguously show that the oxidative pre-treatment of Pd membranes enhances its surface adsorption properties, which results in higher rates of hydrogen permeation across the membrane relative to the untreated membrane. All results in Sections 3.2 and 3.3 were obtained following oxidative pre-treatment of the membrane. 3.2 Adsorption of CO on Pd in the absence of H2 In a previous publication,9 we investigated the adsorption of CO on a Pd membrane surface by IRAS during exposure of the membrane to a flowing gas of 0.1% CO in inert gases (PCO2 = 101 Pa) in the 333-533 K temperature range. We observed a band associated with CO adsorbed on bridging sites on the Pd surface in the ~1920-1990 cm-1 region of the spectra. 9 ACS Paragon Plus Environment


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However, we could not eliminate the possibility that CO was also adsorbed linearly on Pd because the spectra were obscured by gas-phase CO absorption bands in the same region that linear-bound CO bands are expected (~2060-2110 cm-1).22,23,27-29 In this work, IRAS spectra were recorded with both p-polarized light, which interacts with both surface-adsorbed and gasphase species, and with s-polarized light, which interacts with only gas-phase species. Gasphase absorption bands were subtracted from the spectra by subtracting the spectra collected with s-polarization from that collected with p-polarization (p-s). Figure 2 shows the IRAS spectra (p-s) collected during exposure of a pre-treated 25 μm-thick Pd membrane to a flowing gas of 0.1% CO in inert gases (PCO2 = 101 Pa) in the 333-533 K temperature range.

Figure 2. IRAS of a 25-μm-thick Pd foil membrane during exposure to a gas mixture composed of 0.1% CO in inert gases in the 333 to 533 K temperature range.

There are no bands observed in the linear-bound CO region of the spectra (~2060-2110 cm-1) displayed in Figure 2, and CO is adsorbed on bridging sites in the entire 333-533 K temperature range. At 533 K, the interaction of CO with Pd is very weak and bands associated with surface-adsorbed CO are not observed in the IRAS spectrum. As the temperature is reduced to 493 K, a broad band at ~1950 cm-1 appears that becomes sharper and shifts to higher


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wavenumber as the CO coverage increases with decreasing temperature. At 333 K, this band is centered at ~1992 cm-1 with a broad shoulder extending below 1900 cm-1. The position of the bridge-bonded CO bands in Figure 2 are similar to that previously reported for CO adsorbed on Pd surfaces with high concentrations of low-coordination and defect sites: Pd(210),25 Pd(100),22 ion-bombarded Pd(111),23,26 and Pd nanoparticles.23,24,29 This suggests that the Pd membrane surface is highly defective, which is expected for polycrystalline foil. 3.3 Influence of CO on H2 permeation across Pd The influence of CO on the rate of H2 permeation across a pre-treated 25 μm-thick Pd membrane was investigated in the 373-533 K temperature range by measuring the H2 flux across the membrane during exposure to 50% H2 with varying concentrations of CO up to 5%. It should be mentioned here that the β-hydride phase, which should generally be avoided, is formed at temperatures below ~433 K (see Supporting Information in our previous work9). The influence of CO on H2 permeation across Pd was investigated at temperatures below 433 K to determine if the interaction of CO with Pd is influenced significantly by β-hydride formation. Figure 3(a) shows the H2 flux across the membrane versus time during exposure to increasing concentrations of CO at five different temperatures in the 373-533 K range. At 533 K, the H2 flux decreases with increasing CO concentration in the feed gas, from ~0.030 mol/m2/s in the absence of CO to ~0.028 mol/m2/s in the presence of 5% CO (~7% decrease). Following exposure to 5% CO, the CO was removed from the feed gas and the H2 flux returned to its baseline value.



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Figure 3. (a) H2 flux across a 25-μm-thick Pd foil membrane during exposure to a gas mixture with 50% H2 and 0%, 0.05%, 0.5%, 1%, 2%, 3%, 4%, and 5% CO at 533 (K) black, 493 K (red), 453 K (blue), 413 K (magenta), and 373 K (olive). (b) Normalized H2 flux across the 25-μm-thick Pd foil membrane versus CO concentration in the feed gas in the 373-533 K temperature range.

The behavior displayed in Figure 3(a) is significantly different than the behavior that we have reported in a previous publication under nearly identical conditions.9 The only difference in the conditions of this work and our previous work is that the CO gas was purified in this work to remove Ni(CO)4 impurities. In our previous work, we showed that for CO concentrations higher than 1% at 533 K, the H2 flux across the Pd membrane decreases irreversibly with increasing CO exposure time (transient deactivation). After exposure to 5% CO for 15 minutes, the H2 flux was reduced by 20% relative to its baseline value, compared to a reduction of only 7% in this work. Furthermore, the H2 flux continued to decrease after the CO was removed from the feed gas in our previous work, whereas the original baseline H2 flux was restored after CO was removed from the feed gas in this work. The irreversible deactivation of the Pd membrane observed in our previous work was most likely due to the build-up of nickel deposits on the membrane surface from Ni(CO)4 impurities in the CO gas. Section S.1 in the Supporting Information shows that, without purifying the CO gas, the H2 flux across Pd decreases


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continuously over the course of hundreds of minutes and x-ray photoelectron spectroscopic (XPS) analysis of the membrane surface following the reaction shows a significant amount of nickel on the surface. With a CO gas purifier installed, no transient deactivation was observed and no nickel was detected on the membrane surface by XPS analysis. The importance of purifying CO has been discussed in other publications.29 These observations demonstrate that Ni-carbonyl impurities commonly present in CO gas bottles can cause significant deactivation of Pd membranes, and the CO gas must be purified to eliminate contamination from these impurities. In the entire 373-533 K temperature range, there was no transient deactivation observed during CO exposure, and the H2 fluxes were restored to their original baseline values after CO was removed from the feed gas (Figure 3(a)). To demonstrate the influence of CO on the H2 flux across Pd more clearly, Figure 3(b) shows the H2 fluxes, normalized to their baseline values in the absence of CO, versus CO concentration in the feed gas. The H2 flux decreases with increasing CO concentration at all temperatures and the poisoning effect of CO is more severe at lower temperatures. At 373 K, the H2 flux decreases by 80% when only 0.05% CO is added to the 50% H2 feed gas, and continues to decrease with increasing CO concentration until the H2 flux is only 1% of the baseline value with 5% CO in the feed gas. This type of behavior, specifically the greater influence of CO on the H2 permeation flux at lower temperatures, has been observed before,1,3,7 and has been attributed to blocking of H2 dissociation sites on the membrane surface by chemisorbed CO. As the CO concentration in the feed gas increases, and the temperature decreases, the coverage of CO on the membrane surface increases and the number of available H2 adsorption sites decreases.



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To understand whether CO poisons the Pd membrane by adsorbing on its surface and blocking H2 adsorption sites, or by some other mechanism, IRAS spectra were collected during exposure of the membrane to increasing concentrations of CO in 50% H2. To eliminate absorption bands associated with gas-phase species, spectra collected with s-polarization were subtracted from spectra collected with p-polarization (surface spectra). Figure 4(a) shows the series of surface (p-s) IRAS spectra that were collected during exposure of the Pd membrane to 0.05-5% CO in 50% H2 at 453 K. For comparison, the IRAS spectra in Figure 4 were collected while the H2 fluxes at 453 K in Figure 3 were measured simultaneously. IRAS spectra collected during exposure of Pd to H2/CO at the other four temperatures investigated in the 373-533 K range are shown in Figure S2 in the Supporting Information.

Figure 4. (a) Surface and (b) gas-phase IRAS spectra recorded during exposure of a 25-μm-thick Pd foil membrane to 50% H2 and 0.05% CO (black), 0.5% CO (red), 1% CO (blue), 3% CO (magenta), and 5% CO (olive) at 453 K. The gas-phase spectra in (b) were collected with s-polarized light and the surface spectra in (a) were obtained by subtracting the spectra recorded with s-polarized light from the spectra collected with s-polarized light. IRAS spectra were recorded while the H2 fluxes in Figure 3 (453 K) were measured simultaneously.

With 0.05% CO in the feed gas, there is broad band centered at ~1880 cm-1 in Figure 4(a) that is associated with CO adsorbed either in three-fold hollow or bridging sites at low coverage on the Pd membrane surface.22,28 As the CO concentration in the feed gas increases, the surface14 ACS Paragon Plus Environment

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bound CO band increases in intensity and shifts to higher wavenumber (~1950 cm-1 with 5% CO in the feed gas), indicating an increase in the CO coverage. The H2 flux across the membrane (Figure 3) decreases as the CO coverage on the Pd surface increases with increasing CO concentration in the feed gas. These results are consistent with the hypothesis that CO inhibits hydrogen permeation across Pd by adsorbing on the membrane surface and blocking H2 adsorption sites. However, there are additional peaks at ~2917 cm-1 and ~2849 cm-1 in Figure 4(a) that suggest more complex surface chemistry than simple adsorption of CO. The positions of these bands are consistent with the symmetric (2917 cm-1) and asymmetric (2849 cm-1) C-H stretching vibrations in methylene (-CH2) adspecies.30-34 It is possible that the bands at 2917 cm-1 and 2849 cm-1 in Figure 4(a) are artifacts associated with changes in the optical properties of palladium with increasing CO concentration in the feed gas.9 In the supporting information of a previous publication,9 we showed that features resembling absorption bands at ~2920, ~2850, ~1080 and ~950 cm-1 increase in intensity while exposing a Pd membrane to increasing partial pressures of H2 (0-63 kPa) in N2 at 533 K. We demonstrated that these bands were most likely artifacts associated with the infrared source and wavelength-dependent changes in the reflectance of Pd as the concentration of hydrogen dissolved in its bulk increased. It is possible that the bands observed in this work at 2917 cm-1 and 2849 cm-1 could also arise from changes in the reflectance of Pd as the hydrogen flux across the membrane, and the amount of hydrogen dissolved in its bulk, decreases with increasing CO concentration in the feed gas. However, we did not observed bands at ~1080 cm-1 and ~950 cm-1 in this work, and these bands should be orders-of-magnitude more intense than the bands at 2917 and 2849 cm-1 if these bands are artifacts associated with optical effects. Therefore, it is likely



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that the bands at 2917 cm-1 and 2849 cm-1 are associated with methylene species adsorbed on the Pd membrane surface, and not artifacts. The observation of methylene adspecies indicates that CO reacts with H2, and C-O dissociates, on the Pd membrane surface. To demonstrate whether any gas-phase products result from the reaction of CO and H2, Figure 4(b) shows the series of gas-phase IRAS spectra collected with s-polarized light under identical conditions as the spectra displayed in Figure 4(a). There is a sharp band centered at 3017 cm-1 with many other sharp but less intense bands at lower and higher wavenumber, that all increase in intensity with increasing CO concentration. These bands are unambiguously assigned to gas-phase methane.34 No other gas-phase products were detected. These results clearly show that CO is reacting with H2 to form methylene species on the Pd surface, which are hydrogenated further to form methane in the gas-phase. The influence of temperature on the surface-bound species and gas-phase products is demonstrated in Figure 5, which shows the (a) surface and (b) gas-phase IRAS spectra collected during exposure of the Pd membrane to 1% CO and 50% H2 at 373, 413, 453, 493, and 533 K. For reference, the spectra displayed in Figure 5 were collected while the H2 fluxes (with 1% CO) in Figure 3 were measured. At 533 K, there are only weak, poorly-defined bands in the surface spectrum (Figure 5(a)) indicating that the coverages of CO and methylene species are very low. Due to the low coverages of CO and methylene species at 533 K, the H2 flux across the Pd membrane with 1% CO in the feed gas (Figure 3) is only reduced by ~3% relative to the baseline flux in the absence of CO. At 493 K, a broad band associated with triply-bonded CO at ~1850 cm-1 becomes visible, and methylene species appear with bands at ~2917 and 2849 cm-1. As the temperature is reduced further to 453 K, the intensity of the surface-adsorbed CO band increases significantly and shifts to ~1940 cm-1, indicating a change in the adsorption site from three-fold 16 ACS Paragon Plus Environment

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hollow at 493 K to bridging at 453 K. In the 373-453 K temperature range, CO is bridge-bonded to Pd with a band that shifts slightly to ~1960 cm-1 as its coverage increases with decreasing temperature. It is not clear from our results if β-hydride formation at temperatures below ~433 K influences the interaction of CO with the Pd surface significantly. The intensity of the methylene bands at ~2917 and 2849 cm-1 increase monotonically and do not change position with decreasing temperature. The increase in the coverage of methylene species with decreasing temperature is demonstrated more clearly in Figure 5(c), which shows the integral area in the methylene region (2800-2980 cm-1) of the surface IRAS spectra versus temperature. These results show that, not only does the coverage of CO increase, but also the coverage of methylene increases with decreasing temperature.

Figure 5. (a) Surface and (b) gas-phase IRAS spectra collected during exposure of at 25 μm-thick Pd membrane to 1% CO / 50% H2 / balance inert gases at 373, 413, 453, 493, and 533 K. (c) Integral area of the methylene region (2800-2980 cm-1) in the surface IRAS spectra (black data points, left axis) and gas-phase methane production rate (red data points, right axis) versus temperature. The surface spectra in (a) were obtained from subtracting the spectra collected with s-polarization from that collected with p-polarization, and the gas-phase spectra were collected with p-polarized light.

While the coverages of both CO and methylene adspecies decrease with increasing temperature (Figure 5(a)), the intensity of the bands associated with gas-phase methane in Figure 5(b) increase. The rate of methane production was estimated from the intensity of the band at



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3017 cm-1 in Figure 5(b), which was calibrated relative to the intensity of this band while flowing a 10% methane in Ar gas mixture through the reaction chamber. The methane production rate (mol/m2/s) during exposure of the Pd membrane to 1% CO and 50% H2 is plotted versus reaction temperature in Figure 5(c). The methane production rate increases sharply from zero at 373 K up to 1.3·10-4 mol/m2/s at 533 K. Our results indicate that the interaction of CO with the Pd membrane is more complex than simple adsorption of CO, and the severe poisoning effect of CO on the H2 permeation flux at low temperatures (Figure 3) is likely due to both CO and methylene adspecies blocking H2 adsorption sites. The observation that the coverage of methylene species increases as the methane production rate decreases with decreasing temperature (Figure 5(c)) suggests that hydrogenation of the methylene species is a relatively slow step in the overall CO methanation reaction. The significant coverage of methylene species at low temperatures also implies that the C-O bond in CO is broken at temperatures as low as 373 K, which is surprisingly low considering that the activation barrier for direct CO dissociation on clean Pd is very high (~260414 kJ/mol).35-37 Others38,39 have shown that CO dissociation on defect-rich Pd surfaces (Pd foil and Pd(111)) is negligible at temperatures below 400 K. However, Rupprechter et al.26 showed that hydrogen facilitates CO dissociation on defect-rich Pd(111) at 300 K, but not on smooth Pd(111). IRAS of CO adsorbed on the Pd membrane surface (Figure 2) indicate that the membrane surface is highly defective. Therefore, it is likely that hydrogen facilitates dissociation of CO on the defect-rich surface of the membrane at low temperature, resulting in poisoning of the membrane surface by both CO and methylene species. The high selectivity and activity of the Pd membrane for CO methanation is also surprising because several studies35,40-42 of CO hydrogenation over Pd have shown low 18 ACS Paragon Plus Environment

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selectivity towards methane with much higher selectivity towards methanol. The reasoning suggested for such low selectivity towards methane, relative to methanol, is the high activation barrier to direct CO dissociation on Pd.36 However, the observation of methylene species at low temperatures (373 K) in this work suggests that the barrier to C-O dissociation on the Pd membrane surface is not prohibitive, perhaps due to hydrogen facilitating CO dissociation on the defect-rich membrane surface. This hypothesis is supported by a recent theoretical study43 which shows that hydrogen facilitates CO dissociation on step sites of Pd(211), resulting in high selectivity towards methane production. In addition to the high selectivity towards CO methanation, the rate of methane production over the Pd membrane is also remarkably high. Assuming the areal density of Pd atoms (number of surface Pd atoms per square meter) on the surface of the membrane is equal to that of the (111) surface (1.53·1019 m-2), the calculated turnover frequency of methane is equal to 5 s-1 at 533 K with 1% CO in the 50% H2 feed gas. For comparison, the methane turnover frequency measured in this work over the Pd foil membrane is 1-2 orders-of-magnitude higher than the most active CO methanation catalysts (Ru, Rh, and Ni) at similar temperatures.31,44,45 However, the reactor and the method (IRAS) used in this work for measuring methane production rates are not ideal for catalytic activity measurements due to the strong likelihood of concentration gradients in the gas-phase. For example, the infrared beam samples the gas-phase where the methane concentration is highest in the reactor—near the membrane surface—which could result in significant error in the calculation of the methane production rate. The kinetics of CO methanation over Pd foil membranes should be analyzed more carefully in a tube reactor for comparison to literature. 4. Conclusions 19 ACS Paragon Plus Environment


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The interaction of CO with a 25 μm-thick Pd foil membrane, and the influence of CO on the rate of hydrogen permeation across the membrane, was investigated in this work by infraredreflection absorption spectroscopy (IRAS). The hydrogen flux across the membrane decreases with increasing CO concentration at all temperature in the 373-533 K temperature range, and the poisoning effect of CO is more severe at lower temperatures. IRAS of the membrane during hydrogen permeation measurements shows that the coverage of CO increases with increasing CO concentration in the feed gas and with decreasing temperature, as expected. However, the interaction of CO with the membrane surface is more complex than simple adsorption; CO reacts with hydrogen to form methylene (CH2) species on the membrane surface and methane in the gas phase. The coverage of methylene species increases with decreasing temperature, and therefore, the strong poisoning effect of CO at low temperatures may be due to both CO and methylene species blocking H2 dissociation sites. The high selectivity of the Pd membrane towards methane production with high turnover frequency was also unexpected and warrants further investigation to evaluate the performance of Pd membranes for methanation reactions.

Acknowledgments Research was sponsored by the U.S. Army Research Laboratory and was accomplished under Cooperative Agreement Number W911NF-12-2-0019. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressed or implied, of the U.S. Army Research Laboratory or the U.S. Government. The U.S. Government is authorized to reproduce and distribute reprints for Government purposes notwithstanding any copyright notation herein.


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Supporting Information Deactivation of Pd from Ni(CO)4 impurites in CO gas bottle (Figure S1), IRAS spectra collected during exposure of Pd to H2/CO at 533, 493, 413, and 373 K (Figure S2).

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