Zeolite Composites Demonstrate that Support

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Modular Pd/Zeolite Composites Demonstrate that Support Hydrophobic/ Hydrophilic Character is Key in Methane Catalytic Combustion Pit Losch, Weixin Huang, Olena Vozniuk, Emmett D. Goodman, Wolfgang Schmidt, and Matteo Cargnello ACS Catal., Just Accepted Manuscript • Publication Date (Web): 11 Apr 2019 Downloaded from http://pubs.acs.org on April 11, 2019

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Modular Pd/Zeolite Composites Demonstrate that Support Hydrophobic/Hydrophilic Character is Key in Methane Catalytic Combustion Pit Losch,a,b Weixin Huang,a Olena Vozniuk,b Emmett Goodman,a Wolfgang Schmidt,b Matteo Cargnelloa,* a Department of Chemical Engineering and SUNCAT Center for Interface Science and Catalysis, Stanford University, Stanford, California 94304, United States b Department of Heterogeneous Catalysis, Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, Mülheim an der Ruhr, Germany, 45470 ABSTRACT: Complete catalytic oxidation of methane in the presence of steam at low temperatures (T 300).30 Our standard procedure was to dealuminate the respective zeolite by dispersing 500 mg samples in 50 ml of a solution of 8 N nitric acid followed by stirring at 80 °C for 8 h. Resulting solids were washed with deionized water, oven-dried at 70 °C overnight and treated at 550 °C for 1 h (using 5 °C min-1 heating ramp).

The present study revolves around the investigation of the effect of zeolite supports, and in particular their hydrophobic/hydrophilic character, on Pd-catalyzed methane complete combustion via the controlled deposition of colloidally-synthesized monodisperse palladium nanoparticles into uniformly mesoporous zeolites (Scheme 1). This method allows a systematic comparison between catalysts where the only variable is the type of zeolite, given that the Pd phase is the same for all of the materials.26 We maintained the acidic character of the zeolites, thus also confirming the role played by acid sites in the reactivity and stability of the materials.

2..2.3. Mesoporous alumina and silica Mesoporous alumina was synthesized as described by Yuan et al.31 dissolving 1 g Pluronic P123 in 20 mL ethanol, 1.4 mL (12 N) HCl and 0.5 g citric acid at room temperature and eventually adding 2.04 g aluminum isopropoxide. The mixture was stirred at room temperature for 5 h, put into a drying oven at 60 °C to evaporate the

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solvent. After 2 days, a white solid was obtained. Calcination was performed in two steps, first at 400 °C for 6 h (1 °C min-1 heating ramp) then at 600 °C for two more hours (10 °C min-1 heating ramp) led to the final light grey mesoporous alumina.

with a position-sensitive detector (PSD) fabricated by Stoe. Samples were measured in 0.7 mm glass capillaries (wall thickness 0.01 mm). Micropore and mesopore volumes, as well as apparent surface areas (SBET) were measured by means of N2 physisorption on a Micromeritics 3Flex instrument. First, the samples were degassed for 8 h at 623 K using a Micromeritics Smart VacPrep unit. Sorption isotherms were collected at 77 K using a dosing and static volumetric method. Data evaluation was done by t-plot analysis and confirmed by DFT pore size analysis using the model for cylindrical mesopores in metal oxides in the 3Flex software package.

The original synthesis for KIT-6 reported by Kleitz et al. was employed herein.32 with 6 g Pluronic P123 dissolved in 217 mL distilled water, 12 mL (12 N) HCl and 7.4 mL 1-butanol at 35 °C. After 1 h stirring a clear solution was obtained to which 13.7 mL tetraethylorthosilicate (TEOS) were slowly added at 35 °C. The mixture was left under stirring for 24 h at 40 ± 5 °C, and subsequently kept for 24 h at 40 ± 5 °C under static conditions in a closed Erlenmeyer bottle. The solid product was filtered and the template was removed by extraction with an ethanol–HCl mixture and calcination at 550 °C (1°C min-1 heating ramp).

The 27Al MAS NMR spectra were recorded on a Bruker Avance III HD 500WB spectrometer using a double-bearing MAS probe (DVT BL4) at a resonance frequency of 130.3 MHz. The spectra were measured by applying single π/12-pulses (0.6 μs) with a recycle delay of 0.5 s (16,000 scans) at a spinning rate of 13 kHz. The spectra were referenced relative to an external 1 M aqueous solution of aluminum nitrate. The same solution was also used for calibrating the flip angle.

2.2.4. Pd nanoparticles Stock solutions of 3.2 nm Pd nanocrystals6 and 8 nm Pd nanocrystals were synthesized using previously published procedures.33 In short, a typical synthesis was as follows; 305 mg Palladium(II) acetylacetonate (Pd(acac)2) 1 eq., is dissolved in 30 mL 1-octadecene (ODE), to which 3.9 mL 1-oleylamine (OLAM) (10 eq.) for 3.2 nm particles , and 4.7 mL oleic acid (OLAC) (15 eq.) for 8 nm particles, were mixed in a three-necked flask and evacuated at room temperature for 15 min with stirring. 5 equivalents of trioctylphosphine (TOP) were added, and the mixture was heated to 50 °C for 30 min remaining under vacuum to remove all water and other impurities. At this point, the reaction mixture was a transparent yellow-orange solution. The reaction flask was then flushed with dry nitrogen and rapidly heated (40 °C min-1) to 280 °C. The solution turned black when reaching a temperature range of 180 – 200 °C. After 15 min of reaction at 280 °C and with magnetic stirring, the solution was quickly cooled to room temperature by removing the heating source and adding a water bath at temperature below 150°C. The particles were washed and precipitated by consecutively mixing hexanes, isopropanol and ethanol and separated by centrifugation (8000 rpm, 3 min) three times. Finally, the particles were dissolved in hexanes, producing a deep black stock solution. For deposition tests, two batches of ruthenium nanoparticles, of respective 2.6 and 4.4 nm in diameter were synthesized following a reported procedure.34

Transmission electron microscopy (TEM) was performed on a FEI Tecnai operating at 200 kV. Samples were prepared by drop-casting dilute nanoparticle solutions in hexane or isopropanol dispersions of powder catalysts directly onto carbon-coated Cu grids. X-ray photoelectron spectroscopy (XPS) was measured with the PHI Versaprobe 3. The samples were supported on carbon tape. The charge compensation of the samples was controlled by referencing to the C 1s line at 284.8 eV.19 Full range XPS data were used to confirm Si/Al ratios of prepared supports as well as in order to confirm the standard 0.1 wt. % loading in Pd.

2.2.5. Catalyst Preparation For impregnation of a desired loading of Pd nanoparticles onto the respective supports, metal concentrations of synthesized colloidal nanoparticle solutions were determined by thermogravimetric analysis (TGA) as we reported elsewhere.34,15 An appropriate amount of nanocrystals (to give a loading from 0.05 to 2.0 wt.-% Pd) dissolved in hexanes was added to a dispersion of the mesoporous supports in hexanes. The same nanocrystal batch was used for all supports to ensure identical metal loadings and nanocrystal size/morphology across supports. Complete adsorption on the mesoporous solids was guaranteed by treating the slurries for 3 h under sonication at room temperature as it was reported by Rioux et al.35 Solids were recovered by centrifugation (8000 rpm, 1 min) and dried at 60 °C overnight. Prior to catalytic tests, all samples were sieved below 180 μm grain size and treated in a furnace at 700 °C for 30 s to remove ligands as previously described,36 and sieved again below 180 μm grain size. 2.3. Characterization For the systematic study of the crystalline micro mesoporous supports a D8-Venture diffractometer from Bruker was used with CuKα1 (1.54060 Å) radiation in transmission mode (Debye Scherrer geometry). The samples were prepared in 0.5 mm Kapton HN-type polyimide capillaries. Raw data were treated with the Highscore software package. The post catalysis XRPD analyses were acquired on a Stoe STADI P transmission diffractometer in Debye–Scherrer geometry. The instrument was equipped with a bent primary germanium monochromator allowing measurements with monochromatic Cu Kα1 radiation. Diffracted intensities were recorded

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Figure 1. A, D) TEM micrograph and DFT pore size distributions, respectively, of ordered meso-structured zeolite m-USY(15). B, E) TEM micrograph and particle size distribution of colloidal monodisperse palladium nanoparticles. C, F) TEM micrograph and particle size distribution of the final catalyst in this study consisting of Pd nanoparticles deposited into the zeolite mesopores. Water temperature programmed desorption was performed as recently described.15 Samples (between 100 – 200 mg) were dried at 575 °C (10 °C min-1 heating ramp), under 20 mL min-1 5% O2 in Ar before isothermal water uptake at 100 °C. After adsorption, the samples were flushed for 30 min with dry Ar at 100 °C to remove physisorbed water. Then temperature programmed desorption (TPD) was measured under Ar flow of 20 mL min-1 up to 575 °C (10 °C min1 heating ramp). Breakthrough curves with CO and H O as probe 2 2 molecules were measured on the same setup. In this case the materials were also dried at 575 °C and then kept at 350 °C, while being flushed with 20 mL min-1 0.4 vol. % CO2 or 4.2 vol. % H2O respectively and Ar as a carrier gas. The blank measurement of an empty reactor tube was performed to calibrate the recorded breakthrough curves. The breakthrough of molecules was followed by mass spectrometry (Hiden HPR-20).

balanced with Ar, corresponding to a GHSV of 69,000 mL (g-1 h)-1. Effluent gases were monitored by online mass spectrometry (Hiden HPR-20), with a temporal resolution of ~12 seconds. Kinetic data was acquired on the same setup, by ramping to certain temperatures and acquiring steady state data for at least 10 min. Turnover frequencies (TOF) are given in mol(converted CH4) per mol(accessible Pd) per second. Carbon mass balance was verified.

3. Results and discussion 3.1. Synthesis of the materials library and structural characterization Colloidal nanocrystals are useful precursors to prepare materials with well-defined and uniform active phases and to fairly compare supports with different properties.26 Unfortunately, colloidal nanoparticles that are impregnated on microporous zeolite would end up on their external surface, leading to sintering and loss of the uniformity of the starting building blocks.37,38 Our strategy to prepare materials where we can systematically investigate the effect of zeolite support on the activity and stability of Pd catalysts for methane complete combustion therefore relied on a modular approach. First, inspired by the seminal work from Garcia-Martinez and collaborators,27 we introduced mesopores in commercially available zeolite materials such that we could introduce Pd nanocrystals in the mesopores. After the successful reproduction of results of m-USY with a Si/Al ratio of 15 (Figure 1A and D) we adapted the synthesis conditions in order to produce a library of mesoporous zeolites. Nineteen different mesoporous zeolites were prepared in total, using four zeolite frameworks (ZSM-5; MOR; Beta; USY) with different micropore sizes and different Si/Al ratios within each set: m-ZSM-5 (12.5; 25; 40; and >300), m-MOR (10), m-Beta (13; 19; 40; and >300) and m-(US)Y (2.5; 6; 15; 40; and >300). These supports were chosen to span a wide range of zeotype morphologies and structures.

Operando infrared spectra were collected in diffuse reflectance mode using 5 - 10 mg sample and a Harrick DRIFTS cell with KBr windows. Spectra were measured with a 4 cm-1 resolution averaging 250 scans on a Nicolet Magna-IR 560 spectrometer equipped with a MCT detector. Samples were dried at 523 K under flowing dry oxygen (5 vol. %) in nitrogen (10 mL min-1) for 30 min. Then a CH4 : O2 : Ar mixture (0.5, 4.5, 95 vol. %, respectively) was flown over the catalyst (5 mL min-1) for 20 min while spectra were collected. A gas hourly space velocity (GHSV) of 20,000 – 30,000 mL (g-1 h)-1 was assumed; though gas flow dynamics probably differ from a perfect plug flow reactor. 2.4. Catalysis To minimize hot-spots, catalysts were diluted with γ-alumina previously calcined at 900 °C. Each catalyst (20 mg) was thus mixed with 180 mg γ-alumina. furnace (Micromeritics) and the temperature of the catalyst was measured with a K-type thermocouple inserted inside the reactor and touching the catalyst. All experiments were conducted at atmospheric pressure. Ignition/extinction reactions were performed at a temperature ramp rate of 10 °C min-1. The reactant mixture composition was controlled by varying the flow rates of CH4(5 vol. %)/Ar, O2(5 vol. %)/Ar, H2(5 vol. %)/Ar and Ar (all certified mixtures with purity >99.999% from Airgas). In all experiments, 23.5 mL min-1 total gas reaction mixture was flown over the bed, consisting of 0.5 vol. % CH4, 4.0 vol. % O2, 4.2 vol. % H2O

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ACS Catalysis drawn from TEM characterization that the 4.4 nm Ru nanoparticles did not fully, and the 8 nm Pd nanoparticles did not enter the mesopore network (Figure S5-S7). In contrast, the 2.6 nm Ru nanoparticles were deposited in the mesopores just like the 3.2 nm Pd nanoparticles, because in both cases they could not be found on the surface of the mesoporous zeolite. Further evidence was provided by energy-dispersive xray spectroscopy (EDX) line scan analysis: in the case of 2.6 nm Ru-loaded sample, both Si and Ru signals overlapped; but for the 4.4 nm Ru-loaded sample, Ru signal was detected outside that of Si Figure S5). Similar results were found for all the m-zeolites prepared in this study. From these tests we conclude that our strategy to use monomodal mesopores in zeolites and matching monodisperse metal nanoparticles successfully produces modular metal/m-zeolite catalysts. In contrary, mismatching particle and pore sizes lead to particles deposited on the external surface of the zeolite crystals. Given that identical Pd nanoparticles were used for all the mesoporous zeolite supports, and that they were all present within the mesoporous framework, we ensured that we could individually vary the following parameters while keeping everything else constant: zeolite morphology, Si/Al ratio (and therefore the hydrophobic/hydrophilic character), and the Pd loading. 3.2. Catalytic performance in methane complete combustion in the presence of water

The synthesis of highly silicon-rich samples (Si/Al ratio >300) was carried out following a well-established nitric acid leaching treatment to remove the Al sites.30 It is noteworthy that the mesostructuring procedure for different zeolites needed adaptation as we observed rates of zeolite dissolution exceeding those of recrystallization processes for Si-rich samples, while Al-rich samples dissolved much more slowly. Nitrogen physisorption isotherms and DFT pore size distributions demonstrate the successful introduction of mesopores (~4.5 nm in size) in m-USY(15) and m-Beta(40) (Figure S1-S2). X-ray powder diffraction (XRPD) patterns demonstrated that the crystalline network is maintained for all the prepared mesostructured supports (Figure S3). TEM characterization of silicon-rich m-USY(40) provided evidence of ordered mesoporosity as well as preserved lattice fringes, corresponding to USY zeolite (Figure S4). It should be noted that it cannot be completely excluded that in some cases part of the silica dissolved during the restructuring mechanism may form layers of amorphous mesoporous ordered material on top or around the zeolite crystallites. In order to compare the activity of these materials with conventional supports, mesoporous alumina and mesoporous silica (KIT-6), synthesized following reported procedures, were also characterized and used31,32 showing similar textural properties (Table S1). In a second step, we used traditional colloidal synthesis to produce monodisperse Pd nanocrystals of average size of 3.2 nm. TEM characterization confirms their uniform nature (Figure 1B and E).6 We deposited the pre-formed Pd nanocrystals within the created mesoporous zeolite cavities. After screening several techniques for the deposition, we noticed that a prolonged sonication treatment, originally developed by Rioux et al. for the deposition of Pt nanoparticles in the mesoporous silica SBA-15,35 was required to fill the mesopores with the nanocrystals. In contrast, evaporation of solutions containing the nanocrystals and dispersed supports, as done for other support materials, was not successful.6 The sonication treatment resulted in a homogeneously colored material and a transparent solution indicating the quantitative deposition of the nanoparticles. The uniform deposition was confirmed by TEM that showed how the nanocrystals are positioned within the zeolite mesopores at a few nm from the support surface, with a maintained particle size (Figure 1C and F). The fact that no part-

Catalytic tests for methane complete combustion were performed in excess oxygen and in the presence of a large concentration of steam (4.2 vol. %) to mimic realistic conditions in which these catalysts have to operate. The performance for all the prepared catalysts is presented in Figure 2 in terms of methane conversion as a function of temperature and separated into four groups according to zeolite topology. A 0.1 % weight loading of Pd was chosen to ascertain a complete deposition on each support and to better appreciate differences in the catalysts light-off behavior. In all graphs, Pd/γ-Al2O3 is plotted in dark green as a reference catalyst. Intriguingly, we found two distinct trends. First, Pd/m-ZSM-5 and Pd/m-MOR samples, which feature small micropores (ø 300 °C). The results also prove that zeolite m-ZSM-5(40) adsorbs significantly more water and retains it stronger than zeolites with larger pores. In addition, breakthrough curves of both combustion products H2O and CO2 were measured for these four materials at 350 °C (Figure S14). CO2 was detected within 50 seconds on stream for the three mesostructured zeolites, while it was retained for longer on γ-Al2O3 where it could only be detected after 70 s. More

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Figure 4. A) Eyring plot for three different Pd/m-zeolites at same Si/Al-ratio and Pd/γ-Al2O3 with inset table activation enthalpies (ΔHact) and entropies (ΔSact) for the different materials; B) H2O TPD for three hydrophobic zeolites and γ-alumina.

Figure 5. A) ΔGact and B) TOF at 623 K as a function of log10(water uptake). Error bars on ΔGact and TOF were calculated with the standard deviation of the respective linear fits. Water uptake error bars represent experimental error. Plotted curves are guides to the eye.

observed beam damage (Figure S7). From Fig. 7 it is evident that whereas the m-ZSM-5(40) zeolite, with 3 H2O nm-3 uptake highlighting its more hydrophilic character, led to minor sintering of the Pd nanoparticles, the two more hydrophobic m-Beta(40), with 0.6 H2O nm-3 uptake and mUSY(40), with 0.4 H2O nm-3 uptake, confined Pd particles in a very stable manner, as no changes in particle size were observed. We also performed bulk analyses on spent catalyst beds to understand the stability of the zeolite structure to reaction conditions. Solid-state 27Al magic angle spinning nuclear magnetic resonance (MAS NMR) data was collected for a series of Beta zeolites: the microporous commercial sample, the Pd-loaded materials and the sample after catalysis (Figure 8A). The majority of Al remained in tetrahedrally coordinated positions (resonance line at 58 ppm) throughout the treatments and only negligible amounts of hexa- and penta-coordinated Al (resonance line between 20 - 0 ppm) formed under reaction conditions, indicative of the stability of the zeolite framework.

Figure 6. Stability test for Pd/m-USY(40) in the presence of steam.

distributions before and after catalysis for three different Pd/zeolite catalysts (Figure 7). Generally, TEM analyses of these silicon-rich zeolite samples were difficult and we often

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Figure 7. TEM micrographs of fresh and spent catalysts: A, D) Pd particles on m-ZSM-5(40); B, E) on m-Beta(40) and C, F) on mUSY(40). G, H and I) are the respective Pd particle size distributions for m-ZSM-5(40), m-Beta(40) and m-USY(40) before (black) and after (blue) catalysis.

Figure 8. A) 27Al MAS NMR, B and C) X-ray powder diffraction patterns of micro-Beta(40), meso-Beta(40), 0.1 wt.-% Pd(3.2 nm)/mBeta(40) and 0.1 wt.-% Pd(3.2 nm)/m-Beta(40) after reaction from bottom to top. The catalytic test has been performed on a bed composed of 20 mg 0.1 wt.-% Pd(3.2 nm)/m-Beta(40) in 180 mg m-Beta(40). XRD data are shifted by 1000 counts (top) and 500 counts (bottom) for clarity and theoretical pattern of zeolite Beta polymorph A is added in blue.

X-ray diffraction (XRD) data further confirmed the stability of the zeolite Beta structure. A wide-range ordering with

signals at low 2θ corresponding to d-spacings of 3.92, 2.26 and 1.96 nm, agreeing well with the observed mesopore sizes

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Figure 9. Operando DRIFTS study for A) Pd/γ-Al2O3 at different times on stream and 250 °C and B) Mechanistic sketch of the hydrophilic surface of γ-alumina saturating with water and forming aluminum hydroxides; C) Pd/m-Beta(40) system; D) Sketch of molecular transport of water via adsorption desorption processes on the Brønsted acidic zeolite surface.

Diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) was used as a further tool to characterize the state of the catalysts while running the reaction in the absence of water at 250 °C. Spectra of two samples, Pd/γ-Al2O3 and Pd/m-Beta(40), were acquired. Figure 9 shows plots in difference mode while methane and oxygen were admitted into the reaction chamber at a temperature at which the materials were active after 2, 10 and 20 minutes of ongoing reaction. Difference spectra were constructed by subtracting operando spectra (i.e. a spectrum of the active sample) from a spectrum of the activated sample (i.e. a spectrum of the sample before reaction but after required pre-treatments, Figure S16). The spectra of the Pd/γAl2O3 catalyst very closely resemble those reported by the group of Pfefferle et al. (Figure 9A): initially, the sample is very active (there is a strong CO2 and a weak CH4 signal indicative of high conversion), transiently O-CH3 was also observed as an intermediate, but the catalyst is gradually inhibited by H2O produced by the reaction (signal for gaseous water observed at 3500 – 3800 cm-1).40 Water continuously adsorbs on the surface of the support and saturates the γAl2O3, and aluminum hydroxide species are formed as demonstrated by the OH stretching modes also observed between 3500 and 4000 cm-1. In contrast, the Pd/m-Beta(40) system based on a support with higher hydrophobic character did not exhibit any altered activity even after 20 min on stream, thus supporting the hypothesis that water, even if formed during the catalytic reaction, did not strongly bind to the support or the PdO surface and the reactivity was maintained (Figure 9C). In the

was observed (Figure 8B). This ordering is partially smeared after Pd deposition, thus further corroborating the fact that the Pd nanocrystals reside within the mesoporous network. It can be concluded that none of the steps taken to prepare and test the samples (introduction of the mesopores, loading of Pd, catalytic tests) significantly affected the short range or wide range structure of the zeolite. It should still be noted that some more intense peaks appeared after reaction on the metastable zeolite Beta which was somewhat healed, or recrystallized under reaction conditions in presence of steam and heat. No new or other zeolite phases matched the sharpened reflections. We checked the Beta zeolite sample specifically as it is the hydrothermally least stable zeolite amongst the herein tested ones. The other most promising catalyst, the Pd/m-USY(40) sample on the other hand, proved very stable against steam even at high temperatures (> 650 °C) as it was described above (Fig. 6 and Fig. 7). In addition to this data, N2 physisorption isotherms for a realistic bed, composed of the catalyst mixed with γ-Al2O3 before and after reaction, were measured. The data shows that micro- and mesoporosity inherent to the synthesized catalyst are not significantly altered (Figure S15). 3.5. Reaction mechanism by operando DRIFTS

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O-H stretching region of 3200-3800 cm-1 we surprisingly observed a negative signal suggesting that silanols and Brønsted acid sites present both in the internal and external pore surface actively participated in the water transport process, likely via adsorption/desorption processes. This mechanistic hypothesis was further supported by the observation that zeolites with extremely low Al contents (Si/Al >300) were less efficient in catalysis. Acidic protons may directly participate in the diffusion of water via a vehicular pathway, since we recently revealed that such transport mechanisms are favorable over Si-rich zeolites at elevated temperatures.41 Considering these operando data (Figure 9A and 9C) as well as the inverse volcano trend of activation energies as a function of hydrophilicity (Figure 5A), we suggest that water either formed during the reaction or present in the gas stream interacts differently with the different supports. In the case of hydrophilic supports, water strongly adsorbs on the support surface, forming surface hydroxyl species that reside close to the PdO surface (Figure 9B). In the case of more hydrophobic zeolites, however, water seems to be efficiently transported off of the catalyst surface by rapid adsorption/desorption processes on silanol and Bronsted acid sites (Figure 9D). This information could be revealed through the use of preformed Pd particles allowing a fair comparison between different zeolite supports and sheds new light on the use of these valuable materials in catalysis. We are confident that the low cost and availability of Si-rich zeolites can trigger the development of large scale syntheses of methane combustion catalysts able to withstand realistic reaction conditions. 4. Conclusion We reported a method to investigate the role played by zeolite supports on the activity of Pd phases for methane complete combustion. By introducing mesopores into commercial zeolites and introducing pre-formed colloidal Pd nanocrystals from the same batch into the as-formed pores, we systematically studied a number of parameters on catalytic methane combustion. Zeolites with different structure, Si/Al ratio, and Pd weight loadings were investigated and could be fairly compared and the role of the zeolites could be isolated. Nineteen different (micro)-mesoporous supports, with a wide range of hydrophilic and hydrophobic character, were prepared. We found, through kinetic studies, a significant improvement in the water resistance of Pd catalysts for methane combustion when the zeolite hydrophobic/hydrophilic character is optimized. A very competitive steam-resistant catalyst, 0.5 wt. % Pd(3.2 nm)/mBeta(40), with a T50 of 355 °C, emerged from this study as an active and stable catalyst in the presence of water. We verified that the designed model catalysts were hydrothermally stable and an operando DRIFTS study confirmed that H2O is efficiently removed from the active PdO surface via adsorption/desorption processes at the level of zeolitic acid sites.

AUTHOR INFORMATION Corresponding Author * [email protected]

Funding Sources Fulbright Scholar Program , A.v. Humboldt foundation, Max Planck society, National Science Foundation, Department of Energy Conflict of Interest The authors state the absence of any conflict of interest.

ACKNOWLEDGMENT P. L. acknowledges financial support from the Fulbright Scholar Program and the A.v. Humboldt foundation. The work at Stanford University was supported by the U.S. Department of Energy, Chemical Sciences, Geosciences, and Biosciences (CSGB) Division of the Office of Basic Energy Sciences, via Grant DEAC02-76SF00515 to the SUNCAT Center for Interface Science and Catalysis. P. L., O.V. and W. S. are grateful to the Max Planck society for the support. The authors thank B. Zibrowius for discussions and his help with MAS NMR . E.D.G. acknowledges support from the National Science Foundation Graduate Research Fellowship under Grant DGE-1656518. M. C. acknowledges support from the School of Engineering at Stanford University and a Terman Faculty Fellowship.

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ASSOCIATED CONTENT

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Supporting Information details further information regarding exhaustive characterization of different mesoporous supports, further microscopy, N2-physisorption, XRD, XPS, breakthrough (9)

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