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Investigation of the effect of alumina binder addition to Pd/SO42--ZrO2 catalysts during sol-gel synthesis Sreshtha Sinha Majumdar, Gokhan Celik, and Umit S. Ozkan Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b03011 • Publication Date (Web): 14 Oct 2016 Downloaded from http://pubs.acs.org on October 25, 2016

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Industrial & Engineering Chemistry Research

Investigation of the effect of alumina binder addition to Pd/SO42--ZrO2 catalysts during sol-gel synthesis

Sreshtha Sinha Majumdar Gokhan Celik Umit S. Ozkan* William G. Lowrie Department of Chemical Engineering The Ohio State University Columbus, Ohio -43210, U.S.A.

*To whom correspondence should be addressed E-mail: [email protected] Phone: 614-292-6623

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Abstract We have incorporated alumina into palladium catalysts supported on sulfated zirconia (Pd/SZ) during sol-gel synthesis for application in a wash-coat for NOx emission control in natural gas-fired lean-burn engines. In situ incorporation of the adhesivity-enhancing “binder”, alumina, during the sol-gel synthesis of the Pd/SZ catalyst demonstrated significantly improved NOx reduction activity of the previously reported dual-catalyst bed when compared to the conventional ex-situ mode of binder addition in a slurry. The effect of alumina addition during sol-gel synthesis of Pd/SZ on the structural, textural and chemical properties of the resulting catalyst has been examined in this study. The evolution of the crystal phase of zirconia during calcination of the sol-gel binder incorporated Pd/SZ was studied using in-situ XRD during calcination under air. Comparison of the crystal phase composition of the binder-free and binderincorporated samples calcined at 700oC or 900oC demonstrated stabilizing effect on the tetragonal zirconia in the presence of alumina or boehmite dopants. N2 adsorption experiments at 77 K indicated the presence of slit-shaped multimodal hierarchical pores as a result of sol-gel alumina addition. EPR spectra indicated that sol-gel alumina incorporation affected the nature of the Pd species on the resulting catalyst.

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Al MAS NMR revealed strong interaction of the

alumina binder with the zirconia support. The possible formation of a zirconia-alumina solid solution has been suggested and is further examined by using laser Raman spectroscopy. The influence of alumina dopant in the zirconia matrix on the chemical states of Zr, S, O and Al was investigated using XPS. Ar+ ion gun sputtering provided valuable insights into the nature of the alumina- support interaction due to in-situ alumina incorporation during sol-gel synthesis of Pd/SZ catalysts.

Key words Sulfated zirconia, alumina, sol-gel, XPS, 27Al MAS NMR, in situ XRD, lean NOx reduction


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1. Introduction Natural gas-fired lean-burn engines are used in distributed power generation and other stationary applications 1. Although lean-burn engines are very efficient, one of the drawbacks of operating under lean conditions is the trade-off between engine efficiency and higher emission levels of NOx. As emission regulations are becoming more rigorous, the development of an effective aftertreatment system for reducing NOx emissions under lean burn conditions is essential

2-3

. Selective catalytic reduction of NOx emissions using the unburned hydrocarbons

such as methane present in the exhaust stream of lean-burn engines as reducing agents is an economical and attractive approach with wide applicability in large-scale as well as small scale systems 4-5. Studies have reported that palladium supported zeolites such as ZSM-5 and mordenite have demonstrated good activity for NOx reduction with methane under lean-burn conditions with moderate tolerance to water vapor

6-7

. However, poor hydrothermal stability due to

dealumination of the support and consequent formation of PdO aggregates favoring methane combustion has been reported for these zeolites 8. Metal oxide supports such as sulfated zirconia, sulfated titania, and alumina, on the other hand, have shown promising results in terms of durability in presence of water vapor at high temperatures4,9 (and references within). Particularly, sulfated zirconia, due to its acidic properties coupled with its chemical and thermal stability, it has gained attention as a suitable support for palladium for NOx reduction in presence of excess oxygen. A dual-catalyst aftertreatment system comprising a physical mixture of a reduction catalyst (palladium over sulfated zirconia- Pd/SZ) to oxidation catalyst (cobalt oxide over ceriaCo/CeO2) for NOx reduction under lean-burn conditions has been reported earlier by us

10

. The

functions of the dual catalyst bed include oxidation of NO to NO2, oxidation of the hydrocarbons and carbon monoxide in the exhaust stream unused for NOx reduction and reduction of NO2 to N2 11-12. In order to make this dual-catalyst system viable as an industrial aftertreatment unit, we have synthesized a catalytically active washcoat with superior adhesive properties for loading the powdered catalyst onto cordierite monolith cores 13. As the functionality of a catalyst can be specifically tailored at a molecular level according to its application using the sol-gel preparation method 14, in the initial stages of washcoat development, we incorporated adhesivity-enhancing materials called “binders” such as


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alumina into the reduction catalyst in situ during sol-gel synthesis. Studies have shown that the properties of sulfated zirconia and consequently the activity of catalysts with sulfated zirconia as the support is strongly dependent on parameters such as the method of preparation, the type of precursor used, the mode of sulfation and the calcination temperature of the sulfated zirconium hydroxide

15-17

. Important factors affecting the catalytic properties of the material such as the

surface area, amount of sulfates on the surface, acidity and the crystal phase of support can be engineered by modifying the sol-gel technique during catalyst synthesis

18-20

. Thus, the addition

of alumina into the sol-gel medium during catalyst synthesis can be expected to affect the structural, textural, chemical properties and therefore the activity for DeNOx catalysis of the resulting catalyst. Previously, we have investigated the effect of addition of alumina on the type of acid sites using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) with pyridine as a probe molecule, the state of the cationic Pd species using electron paramagnetic resonance (EPR) and have also tested the adhesive properties of the resulting Pd/SZ catalyst

13

. In the

current contribution, studies on evolution of the crystal phase of the support using in situ XRD experiments during calcination under air, surface area and pore-size distribution analysis and results from steady-state activity tests have been presented. Examination of the nature of alumina interaction with the support using

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Al Magic Angle Spinning Nuclear Magnetic Resonance,

Raman spectroscopy, X-ray photoelectron spectroscopy with Ar+ ion bombardment have also been discussed.

2. Experimental 2.1 Catalyst synthesis The reduction catalyst, 0.3 % by weight palladium supported on sulfated zirconia (Pd/SZ), is prepared using the “one-pot” sol-gel technique described by Mirkelamoglu et al. 10, 21. Palladium acetate and zirconium propoxide are used as the precursors, with sulfuric acid as the sulfating agent and acetic acid as the hydrolyzing agent. Sol-gel medium was stirred until gelation, then dried at 110oC and calcined in an oxidizing environment. The abovementioned technique was modified to incorporate “binders” such as alumina, boehmite or silica into the reduction catalyst by addition in situ during sol-gel synthesis, prior to gelation of the sol-gel medium

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. The conventional mode of incorporating binders to the reduction catalyst was also


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adopted for reference wherein the binder was added to the catalyst slurry ex-situ followed by the drying and calcination steps. The oxidation catalyst, 2% by weight Co/CeO2, is synthesized via the wet impregnation method detailed elsewhere

11, 22

, wherein the ceria support is prepared using the precipitation

technique and cobalt is impregnated on the ceria support using cobalt nitrate hexahydrate salt.

2.2 Catalyst Characterization 2.2.1 X-ray diffraction The X-ray diffraction patterns of the binder-free and binder-incorporated Pd/SZ catalyst samples were acquired in situ during calcination in air using the Bruker D8 advanced X-ray diffractometer equipped with a Cu Kα source and a HTK 1200 sample holder. The temperature was increased from 400oC to 900oC in 50oC increments. At each temperature, after a dwell time of 10 minutes to ensure that the temperature was stable, the data were collected between a 2θ range of 20o to 45o. Once calcination was completed, scans were also taken after cooling the samples down to 40oC for comparison. The phases were identified by using the international center for diffraction data (ICDD) database. ICDD# 81-1546 and ICDD# 37-1484 were used to identify the tetragonal and the monoclinic phases of zirconia respectively. Crystal phase analysis was done on the samples using the method described by Toraya et al. for monoclinic-tetragonal zirconia systems 23. The effect of the addition of the binder on the fraction of the tetragonal phase of zirconia in the samples was studied using the phase analysis. Scherrer equation was used to calculate the crystal size of the tetragonal phase of zirconia.

2.2.2 Surface area and pore volume analysis The surface area and the pore volume of the reference alumina sample, the binder-free Pd/SZ, ex-situ alumina-incorporated and sol-gel alumina-incorporated Pd/SZ samples were determined using the Micromeritics accelerated surface area and porosimetry instrument (ASAP 2020) instrument. Prior to analysis, the samples were degassed at 140oC under a vacuum of 2µmHg overnight. Sample analysis was conducted at 77 K and the N2 isotherms were used to estimate the surface area and pore volume using Brunauer-Emmett-Teller (BET) and BarrettJoyner-Halenda (BJH) method respectively.


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2.2.3 Electron Paramagnetic Resonance (EPR) EPR spectroscopy on the sol-gel alumina incorporated Pd/SZ and ex-situ alumina incorporated Pd/SZ was conducted using the Bruker EMXplus spectrometer. EPR spectra for the samples were collected at 300 K followed by cooling to 100 K using liquid nitrogen. For both samples, the spectrometer was operated at 9.43 GHz with field modulation of 100 kHz and modulation amplitude of 10 G. The spectral peaks for the samples were identified on the basis of their calculated g-factors. 2.2.4 27Al Magic Angle Spinning (MAS) Nuclear Magnetic Resonance (NMR) 27

Al MAS NMR was conducted on the Pd/SZ samples in which the alumina binder was

incorporated either in situ during the sol-gel synthesis or ex situ. The study was conducted using a simple one pulse experiment without proton decoupling on the Bruker DPX 300 MHz NMR spectrometer at a resonance frequency of 78.2 MHz for

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Al. The samples were loaded onto

zirconia rotors and spun at 10 kHz. The pulse width was 2.25 µs and the recycle delay was 0.5 s. 1024 scans were collected for each sample and Al(NO3)3 was used as the reference at 0 ppm. The free induction decay (FID) file for each sample has been processed with line broadening of 100 Hz using SpinWorks 4.2.3.0 ©2016.

2.2.5 Raman Spectroscopy The binder-free Pd/SZ, sol-gel alumina incorporated Pd/SZ and ex-situ alumina incorporated Pd/SZ samples were investigated using Raman spectroscopy. Sample spectra were collected between 100-2000 cm-1 at room temperature using the Renishaw-Smiths Detection Combined Raman-IR Microprobe equipped with a Leica microscope and a CCD detector (400 X 576). A helium-neon laser at 633 nm wavelength was focused on the powder sample. The samples were photo-bleached for 3 minutes each before spectra was collected to reduce background luminescence.

2.2.6 X-ray Photoelectron Spectroscopy (XPS) The surface oxidation state and elemental composition of the binder-free and sol-gel alumina incorporated Pd/SZ samples were investigated using the Kratos AXIS Ultra X-ray photoelectron spectrometer. Spectra were collected for the O 1s, S 2p, Zr 3d and Al 2p regions.


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The samples were ground and loaded onto double-sided carbon tape on a sample bar which was introduced into the spectrometer and pumped down overnight. The XP spectra were collected using Al Kα X-ray source at 13 kV and 10 mA. A charge neutralizer operating at 2 A current and filament bias of 1.3 V was used on the samples. For investigation of depth profile of the samples, Ar+ ion gun at 3kV was used to sputter the top layers of the surface of the samples. The sample spectra were collected before Ar+ ion bombardment and after each sputtering cycle. The data was corrected to C 1s binding energy of 284.5 eV and analyzed using CasaXPS 2.3.16. For quantitative analysis, the peak areas of the XPS spectra were corrected using their respective atomic sensitivity factors.

2.3 Steady-state activity Tests A ¼” stainless steel fixed-bed reactor was used to conduct the steady-state activity tests on the mixed beds containing a physical mixture of either the binder-free or the binderincorporated 0.3 % Pd/SZ and the oxidation catalyst (2% Co/CeO2) under simulated engineexhaust conditions. The dual-catalyst bed, referred to in terms of its reduction catalyst component unless mentioned otherwise, is supported between two quartz wool plugs and tested using a resistively-heated furnace fabricated in our laboratory. The reaction gas mixture is composed of 180 ppm NO2, 1737 ppm CH4, 208 ppm C2H6, 104 ppm C3H8, 10% O2, 0 or10% H2O, 650 ppm CO and 6.5% CO2. The total flow rate is maintained at 40 cc/min using balance helium and the gas hourly space velocity is kept constant at 32000 hr-1. The product stream is analyzed using a Thermoscientific 42i–HL chemiluminescence NO-NO2-NOx analyzer and an Agilent 3000A micro-gas chromatograph equipped with a thermal conductivity detector.

Results and discussion 3.1 Effect of the type of binder incorporated to Pd/SZ during sol-gel synthesis 3.1.1 X-ray Diffraction (XRD) The effect of addition of a binder to Pd/SZ during the sol-gel process on the evolution of the crystal phases of the catalyst was investigated by using in situ XRD during calcination of the catalyst under flowing air. High temperature calcination provides further insights into the effect of the type of binder added to the catalyst. Figure 1 displays the diffractograms of the binder-free and the different binder-incorporated Pd/SZ catalysts during in situ calcination until 900oC in the


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presence of air along with the spectra after cooling the samples down to 40oC following calcination at 900oC (Final 40oC). We observe the unique effect that each type of binder has on the crystal phase of the samples from the Final 40oC spectra for each sample in Figure 1. In the case of alumina-incorporated Pd/SZ, the quantity of alumina added to the sol-gel also significantly influences the crystal phase of the resulting catalyst (not shown). Figure 1 reveals the presence of both the monoclinic as well as the metastable tetragonal phases of zirconia at 40oC although the latter is known to be thermodynamically stable at temperatures above 1150oC 10

. This has been attributed to the stabilizing effect of the dopants on the tetragonal phase of

zirconia 10, 24. From the in situ calcination data shown in Figure 1, we can observe that for the binderfree Pd/SZ, the onset temperature for crystallization of zirconia is 450oC while for the aluminaincorporated Pd/SZ, it is 500oC. Addition of boehmite or silica to the Pd/SZ samples further delays the onset temperature of crystallization to 550oC. Studies on Al2O3-ZrO2 mixed oxide catalytic supports have reported that addition of alumina to zirconia retards the crystallization of the amorphous zirconia phases

25

. Thus, addition of binder to the Pd/SZ samples might be

instrumental in delaying the crystallization of zirconia in the support. In the binder-free as well as the binder-incorporated Pd/SZ samples, at the inception of crystallization, the tetragonal phase of zirconia is observed first. This may be due to the structural similarities between the amorphous zirconia and tetragonal zirconia phases or topotactic crystallization of the nuclei formed on amorphous zirconia

26-27

. Studies have also shown that when the zirconia crystallites

are less than 100Å in size, tetragonal zirconia is more likely to form

24, 28

. Thus, for each of the

samples, at lower calcination temperatures, we can observe only tetragonal zirconia. With increase in calcination temperature, in each catalyst sample, the peak corresponding to the tetragonal phase of zirconia becomes more prominent. For the binder-free Pd/SZ catalyst, the presence of the sulfate dopant has a stabilizing effect on the tetragonal phase of zirconia while for the binder-incorporated samples, the combined effect of the sulfate and the respective binder dopants stabilizes the tetragonal phase of zirconia. Typically, the sulfate or the binder dopants are known to retard crystallization and inhibit the growth of the tetragonal grains 24-25, 29. Table 1 shows that the tetragonal crystallite size for all the catalyst samples at 700oC is less than 30 nm which has been denoted by researchers as the critical crystal size beyond which the formation of the monoclinic phase of zirconia is favored

30

. As the binder-free catalyst samples


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are cooled down, particularly for the binder-free and the silica-incorporated samples, due to lower lattice strains as a result of loss of sulfates during calcination and/or Si4+ ions being much smaller than Zr4+ ions, the monoclinic phase of zirconia is formed. The unique effect of each type of binder on the crystal phase composition of zirconia was further demonstrated by comparing the samples calcined at 700oC with those calcined at 900oC. Crystal phase analysis of these samples has been conducted using the method described by Toraya et al.

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and are presented in Figure 2. For the binder-free Pd/SZ sample, while the

tetragonal phase of zirconia was more dominant in the sample calcined at 700oC, the monoclinic phase of zirconia dominated the sample calcined at 900oC. This change is likely due to the loss of sulfates from the catalyst when treated at high temperatures. This further indicates that, for the binder-free catalyst, the presence of sulfates have a stabilizing effect on the metastable tetragonal phase of zirconia. In the case of the alumina or boehmite-incorporated Pd/SZ samples, both samples show the tetragonal phase of zirconia to be retained as the dominant phase irrespective of the temperature at which these samples are calcined at. The distribution of the monoclinic to tetragonal phase of zirconia in the boehmite-incorporated sample remains largely unchanged and is possibly due to retardation of the growth of the tetragonal crystal grains with increase in temperature as a result of the presence of boehmite dopant. For both alumina and boehmiteincorporated samples, however, the stabilizing effect of the addition of binder on the tetragonal phase of zirconia is evident from these results. Furthermore, increasing the amount of the alumina-binder added to the sol-gel medium from 10% to 20% (not shown) further stabilizes the metastable tetragonal phase of zirconia at room temperature for samples calcined at 900oC. As the amount of alumina is increased, the tetragonal to monoclinic zirconia transition is likely inhibited due to delay in agglomeration of the tetragonal zirconia crystallites. These results indicate that not only is the addition of binder during the sol-gel process affecting the crystal phase composition of the catalyst, but the type of binder and the quantity added to the sol-gel medium also plays a significant role in the resulting crystal phase composition in these catalyst samples. In contrast, in the silica-incorporated Pd/SZ catalyst samples, the monoclinic phase was found to be the more dominant phase of zirconia when calcined at 700oC or at 900oC. As the calcination temperature was increased from 700oC to 900oC, the percentage of the monoclinic


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phase of zirconia in the catalyst also increased. Addition of silica to Pd/SZ does not constrain the growth of the tetragonal zirconia crystal grains like we observed in the case of the boehmite and alumina incorporated catalysts. Thus, on cooling the samples after calcination, the tetragonal grains coalesce and form monoclinic zirconia. It was also observed that none of the binders present themselves in the diffractograms. This may suggest that either the crystallites are too small for detection by this technique or the binders form a solid solution with zirconia.

3.1.2 Steady-state activity measurements The catalytic activity of the binder-free Pd/SZ, alumina-incorporated Pd/SZ, boehmiteincorporated Pd/SZ and silica-incorporated Pd/SZ were tested in terms of CO, NOx and CH4 conversions as part of the dual-catalyst bed under simulated engine-exhaust conditions. The Pd loading in all Pd/SZ catalysts was 0.3%. For the binder-incorporated Pd/SZ catalysts, the binder loading was 10% by weight. The steady-state reactions were conducted under wet conditions (10% H2O) at 450oC for 5 hours and the results are presented in Figure 3. In terms of CO conversions, whether binder-free or binder-incorporated, the dual-catalyst aftertreatment system is able to completely oxidize any CO present in the feed. Hydrocarbons present in the feed such as C2H6 and C3H8 are also completely oxidized to form CO2 and water vapor. These results are significant demonstrating the oxidizing capability of the dual-catalyst aftertreatment system. For the mixed bed containing the binder-free Pd/SZ shown in Figure 3, NOx conversions are ca. 50% while the CH4 conversions were close to 20%. The NOx and CH4 conversions for the alumina-incorporated Pd/SZ, boehmite-incorporated Pd/SZ and silica-incorporated Pd/SZ are also shown in Figure 3. The NO2 conversions for all three binders-incorporated samples are ca. 80% (not shown). Amongst the binder-incorporated Pd/SZ samples, in terms of activity for NOx, the alumina-incorporated Pd/SZ containing mixed bed demonstrates the highest conversions at 68% while the boehmite and silica-incorporated Pd/SZ mixed beds reduced 58% and 31% of the NOx respectively. The CH4 conversions were ca. 15% and 10% for the alumina and boehmite incorporated Pd/SZ samples and 51% for the silica-incorporated sample. A possible explanation for these observations can be attributed to presence of cationic Pd species in the aluminaincorporated Pd/SZ catalyst that we have reported in our earlier studies

13

which is known to

favor NOx reduction under excess oxygen conditions. On the other hand, the silica-incorporated Pd/SZ catalyst may have PdO clusters, similar to that reported by Ali and co-workers in the case


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of Pd supported on silica, which favors methane combustion 31. Furthermore, from our XRD data we observed that in the alumina-incorporated Pd/SZ catalyst the zirconia crystal phase was a combination of both monoclinic and tetragonal with the latter being more dominant while that for silica-incorporated Pd/SZ was primarily monoclinic 13. The tetragonal phase has been found to promote DeNOx catalysis under lean conditions while the monoclinic phase favors methane oxidation 32.

3.2 Effect of the mode of addition of alumina binder to Pd/SZ Figure 4 displays the NOx and CH4 conversions at 450oC under dry conditions for binderfree Pd/SZ, sol-gel alumina-incorporated Pd/SZ and ex-situ alumina incorporated-Pd/SZ in which the binder is added externally to Pd/SZ. While the binder-free and the sol-gel aluminaincorporated Pd/SZ mixed-beds demonstrate ca. 85% NOx conversions, the ex-situ alumina incorporated-Pd/SZ demonstrates poor NOx conversions at only 20% although the methane conversions between the two alumina-incorporated samples are comparable. This is a very significant result emphasizing the importance of the mode of addition of alumina-binder to Pd/SZ catalyst. Addition of the binder in the conventional ex-situ method severely affects the NOx reduction activity whereas sol-gel incorporation of alumina to Pd/SZ maintains the catalytic activity of the original binder-free Pd/SZ catalyst.

3.3 Comparison of performance of alumina-incorporated Pd/SZ with binder-free Pd/SZ Alumina-incorporated Pd/SZ exhibited the best performance in terms of NOx reduction compared to the other binder-incorporated Pd/SZ catalysts in the dual-catalyst bed. Thus, the mixed bed containing alumina-incorporated Pd/SZ as the reduction catalyst component was further tested under steady-state conditions and its activity was compared to the binder-free Pd/SZ reduction catalyst at 450oC and 500oC under dry as well as wet conditions. In Figure 5 we observe that under dry conditions, as temperature increases, NOx conversion for both mixed beds decreases whereas methane conversion increases. The alumina-incorporated Pd/SZ demonstrates higher methane conversions than binder-free Pd/SZ. A possible explanation for this observation may be that binder-free and binder-incorporated Pd/SZ beds contain the same amount of Pd/SZ and the increase in the methane conversions can be due to the addition of 10% binder to the bed. It was reported that metal oxides, even when used only as a support, show catalytic activity for


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the oxidation of methane 33. Our previous results also show that methane conversion is always higher in a dual catalyst bed than over the Pd/SZ catalyst alone, indicating that, once CH4 is activated over the reduction catalyst, presence of the oxidation catalyst promotes complete oxidation of methane 10-11. Under wet conditions, for the binder-free mixed bed, both NOx conversion and methane conversion increase with temperature. The effect of temperature on the catalytic activity under wet conditions can be explained by lower adsorption of water on the catalyst surface at higher temperatures thus making the active sites available for NOx reduction as well as methane oxidation 12. In the presence of water vapor in the feed, for the alumina-incorporated Pd/SZ, the catalytic activity for NOx reduction is comparable to the binder-free mixed bed. We have also observed in our earlier findings the presence of Lewis acid sites on both the binder-free Pd/SZ catalyst and the alumina-incorporated Pd/SZ catalyst acidity on zirconia in sulfated zirconia supports into Brønsted acid sites on hydration

34, 36

34-35

13

. Sulfates are known to induce Lewis

. A fraction of these Lewis sites transform

. These Brønsted acid sites decrease the electron

density on the metal, leading to the formation of cationic Pd species (37 and references therein) which have been found to be active for NOx reduction 31, 38. This provides a possible explanation for the observed catalytic activity under dry and wet conditions. The most important observation, from these experiments is that addition of alumina during the sol-gel synthesis has no negative effect on the activity of the catalyst for NOx and CH4 conversion under wet conditions. In fact, it appears to enhance the activity for NOx reduction at 450°C and maintain this activity during the 20 hours it was kept online as can be seen from the inset of the figure.

3.4 Surface area and pore volume analysis N2 adsorption experiments at 77 K were conducted on alumina binder, binder-free Pd/SZ, alumina-incorporated to Pd/SZ during sol-gel synthesis and that added to Pd/SZ ex-situ. The surface areas of these samples were calculated from the adsorption branch of the N2 isotherm using the Brunauer–Emmett–Teller (BET) method and are reported in Table 2. For all four samples, the adsorption-desorption isotherms in Figures 6 (a), (b), (c) and its inset display the Type IV isotherm characteristic of mesoporous materials. The observed hysteresis in the isotherms is attributed to capillary condensation of the adsorptive molecule in the mesopores.


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The structure of these mesopores can be inferred from the shape of hysteresis based on the classification of types of hysteresis

39

for Type IV isotherms. The isotherms of all the Pd/SZ

samples demonstrate Type H3 hysteresis which is associated with slit-shaped mesopores 40-41. Desorption of N2 from the mesopores of various sizes contributes to the slopes of hysteresis in the respective isotherms. The pore-size distribution for the samples in Figure 7 is analyzed based on the desorption branch in the N2 isotherm. The pore-volume is calculated based on the method proposed by Barrett-Joyner-Halenda which assumes cylindrical pores. A slight discrepancy in the actual pore volume is possible since the hysteresis shape of these samples indicate slit-shaped pores rather than cylindrical ones. However, by using this method, some valuable insights about the effect of alumina addition to Pd/SZ during the sol-gel synthesis can be gained. Figure 7 (a) shows that the reference alumina binder has pores in the range of 5-20 nm while the binder-free Pd/SZ sample in Figure 7 (b) shows the presence of pores ranging from 540 nm. In the case of alumina reference, the type H1 hysteresis indicates a narrower pore-size distribution than that for the binder-free Pd/SZ sample which shows the type H3 hysteresis. This variety of pore sizes has an effect on the smooth hysteresis shape observed for the latter sample as with decrease in the relative pressure of N2, evaporation of the condensed N2 in the pores based on their sizes is expected. The pore-size distribution for alumina added ex-situ to Pd/SZ in Figure 7 (c) shows a combination of pores present in alumina and binder-free Pd/SZ ranging from 5-40 nm. However, when the alumina is incorporated into Pd/SZ during the sol-gel synthesis, in addition to the pores ranging from 5-40 nm similar to those observed in the binderfree Pd/SZ sample, new type of pores in the 50-70 nm range, neither observed in the distributions for alumina binder nor in Pd/SZ, are formed as seen in Figure 7 (d). Klimova and co-workers have also reported substantial changes in the pore-size distribution of their Al2O3-ZrO2 mixed oxide due to addition of alumina to their zirconia support 25. This type of multimodal hierarchical porous system is highly suited for catalytic processes as it might make the active sites more accessible to the gaseous reactants

42

.

Thus, sol-gel addition of alumina to Pd/SZ has a

significant effect on the textural properties of the resulting catalyst.

3.5 Electron Paramagnetic Resonance (EPR) Electron paramagnetic resonance (EPR) studies were conducted on the sol-gel aluminaincorporated and ex-situ alumina-incorporated Pd/SZ samples to gain insights into the nature of


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the paramagnetic species in these samples. Previous studies on the reduction catalysts have revealed the presence of paramagnetic Pd+, Pd3+ and Zr3+ species in the reduction catalyst samples 13. Presence of the cationic Pd species on the acidic support favors NOx reduction under lean conditions. For both modes of alumina incorporation into Pd/SZ, the sample spectra collected at 300 K in Figure 8 (a) and (b), show the presence of Pd+ species indicated by the g∥ factor of 2.3 and g⊥ -factor of 2.13 as well as the g∥ -factors at 2.85 and 2.68. These species are likely formed due to partial transformation of Pd2+ species due to interaction with Pdo

43-44

. Zr3+

species are also observed in the samples from the g⊥-factor at 1.975 and g∥ -factor at 1.96 45. The EPR spectra of the ex-situ alumina-incorporated Pd/SZ sample collected at 100 K in Figure 8 (c) demonstrates similarities to the alumina-free Pd/SZ sample spectra reported earlier. The distribution of Pd3+ species shown by the presence of giso at 2.23 while being similar to the alumina-free Pd/SZ sample, is distinctly different from that observed in the sol-gel alumina incorporated Pd/SZ catalyst as seen in Figure 8 (d). This further indicates that the route of incorporation of alumina into the Pd/SZ catalyst significantly affects the state of the Pd species in the samples. From these results we can also infer that sol-gel incorporation of alumina likely results in interaction with the Pd/SZ catalyst at an atomic level such that it is able to influence the nature of the Pd species stabilized on the resulting catalyst.

3.6 27Al Magic Angle Spinning (MAS) Nuclear Magnetic Resonance (NMR) In order to study the effect of method of incorporation of alumina binder to the Pd/SZ catalyst on the interaction of Al with its local environment, 27Al MAS NMR was performed on the Pd/SZ catalysts with either sol-gel incorporated alumina or ex-situ incorporated alumina. In general, solid-state NMR is susceptible to peak broadening due to the orientation of the particles leading to chemical shift anisotropy; thus, displacing the chemical shift from the true chemical shift (isotropic)

46-47

. In addition, the nuclear spin (I) of

27

Al being >1/2, results in quadrupolar

interactions between the nucleus and the electric field gradient around the nucleus

48-49

. As a

result of these interactions, the NMR data may display broadening and distortion of peaks

48

.

This also results in wide ranges for chemical shifts corresponding to the different types of coordination environments for the

27

Al nuclei. MAS NMR technique is typically successful in

reducing the peak broadening effect and first order quadrupolar interactions; however, second order quadrupolar interactions still significantly distort the peaks


48

. Thus, valuable information 13

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about the local environment of Al can be gleaned from the chemical shifts as well as the peak shapes in the NMR spectra. 27

The inset in Figure 9 shows the

Al MAS NMR data for pristine ϒ-alumina. The

chemical shifts at ~9 ppm and ~70 ppm correspond to octahedral co-ordination of Al with O (AlO6) and tetrahedral co-ordination of Al with O (AlO4) respectively

50-52

. Due to the broad

peaks, the presence of AlO5 species is not discernable. The composition of the type of Al site was calculated by measuring the peak intensities as deconvolution of the peaks is difficult due to broadening and overlapping of the peaks

53

. The ratio of AlO6 to AlO4 species was calculated

using the correlation given by Balmer et al.

54

. The ratio was found to be 0.67: 0.33 for ϒ-

alumina which agrees well with the composition reported in literature 50. The low intensity broad side components of the peaks reflect regions with low symmetry or may indicate the presence of smaller crystallites 53, 55. A distinct difference in the line-shape of the NMR spectra for the sol-gel incorporated alumina to Pd/SZ when compared to the ex-situ incorporated alumina to Pd/SZ as well as the pristine ϒ-alumina is observed from the

27

Al MAS NMR spectra shown in Figure 9. While the

NMR spectra of the ex-situ incorporated alumina closely resembles that of the pristine ϒalumina, there is a slight shift upfield along with an increase in the relative percentage of the AlO6 coordination from 67% to 92% due to interaction of ϒ–alumina with water

52, 56

. On the

other hand, the sol-gel incorporated alumina demonstrates strong second order quadrupolar interactions due to a highly distorted local environment as can be evidenced by the presence of shoulders on the peaks corresponding to AlO6 and AlO4 environments. Unlike the ex-situ incorporated alumina to Pd/SZ, the upfield and downfield ends of the spectra in Figure 9 (a) does not display broad underlying components. Furthermore, the presence of any AlO5 species, often associated with low crystallinity, are not apparent in the spectra for the sol-gel incorporated alumina to Pd/SZ. 27Al MAS NMR spectra can reveal the effect of next-nearest-neighbor due the difference in bond valence of the next-nearest-neighbor. However, due to the bond valence of both Zr4+ (co-ordination of 8) and Al3+ (co-ordination of 6) being 0.5, no distinctive difference in the chemical shifts are expected in the spectra based on whether Zr4+ is next to a Al site or not 54. Sol-gel incorporation of alumina to Pd/SZ prior to the gelation step may result in molecular-level interaction of the binder with the zirconia matrix. Due to the strong interaction between Al3+ and Zr4+ as a result of the intimate mixing, formation of metal-oxygen-metal


14


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linkages have been reported by researchers

57-58

Page 16 of 43

. Addition of Al3+ to zirconia can result in the

formation of a binary ZrO2-Al2O3 system or a solid solution of the form Zr(1-x)AlxO(2-x/2) 54. In the latter case, the stability of metastable tetragonal zirconia is enhanced with improved solubility of Al3+ in the zirconia lattice. From the XRD data, we have observed earlier that with increase in the amount of alumina incorporated in the Pd/SZ catalyst during the sol-gel process, the metastable tetragonal zirconia phase becomes more dominant in the sample. The Al3+ cations most likely randomly replace some of the Zr4+ cations in the zirconia lattice leading to the formation of oxygen vacancies for maintaining the overall charge balance

57

. Thus, the method

adopted for incorporation of alumina binder to Pd/SZ has a significant effect on the interaction between the binder and the support. The

27

Al MAS NMR study reveals that the sol-gel

incorporated alumina to Pd/SZ has a stronger interaction between the alumina binder and zirconia than the ex-situ incorporated alumina route which resembles the spectral features of the pristine ϒ-alumina.

3.7 Raman Spectroscopy The effect of the mode of incorporation of alumina to Pd/SZ was compared using Raman spectroscopy with the 633 nm laser. The Raman spectra of these samples were also compared to the binder-free Pd/SZ sample for reference. As can be observed from Figure 10, the spectra of the binder-free Pd/SZ sample, the sol-gel incorporated alumina sample and the ex-situ incorporated alumina samples are primarily dominated by the peaks corresponding to zirconia. Although, Raman spectroscopy is less sensitive to the monoclinic phase of zirconia, it can be used to differentiate between the phases of zirconia 59. The bands corresponding to the tetragonal phase of zirconia at 146, 265, 311, 452, 643 cm-1 are observed in all three samples while those corresponding to Zr-Zr vibrations in the monoclinic phase of zirconia at 177 and 190 cm-1 are observed in the binder-free Pd/SZ and the Pd/SZ sample with ex situ alumina incorporation, but are not seen in the sample with the sol-gel incorporated alumina

59-62

. The band at 560 cm-1

observed in Figure 10 (a) is attributed to O-O vibrations. In all three samples, the S-O at close to 1015 cm-1 and S=O at 1390 cm-1 stretch vibrations are associated with the sulfate groups on the sulfated zirconia support (not shown) 63. In Figure 10 (a), the resolution of the band at ~364 cm-1 is ambiguous and can be attributed either to the monoclinic phase of zirconia or to the octahedral coordination of Al ions


15

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with oxygen atoms. The Raman shift at 494 cm-1 also corresponds to octahedral Al coordination. Furthermore, the bands observed at 461 cm-1 and 521 cm-1 indicate tetrahedral Al-O coordination in alumina

64-65

. These Al species are not apparent in the sol-gel incorporated alumina to Pd/SZ

species shown in Figure 10 (b) although both samples contain equal amounts of alumina. As no characteristic Raman bands associated with Al are observed when alumina is incorporated into Pd/SZ during the sol-gel process, it provides further evidence for the formation of a zirconiaalumina solid solution or a binary mixture.

3.8 X-ray Photoelectron Spectroscopy (XPS) The oxidation state of the catalyst plays an important role in redox catalysis. The surface oxidation state of the binder-free and the sol-gel alumina-incorporated Pd/SZ catalysts has been investigated using XPS. Due to the low Pd loading on the samples and the overlapping Pd 3d and Zr 3p regions, the analysis of the Pd region was inconclusive and is not included in this section. However, valuable insights into the effect of sol-gel alumina addition on the chemical states of O, S, Al and Zr have been gained using XPS. The O1s envelope of the binder-free Pd/SZ catalyst is shown in Figure 11 and the spectra was deconvoluted using peak-fitting into a main peak at binding energy (B.E.) value of 529.9 eV with a shoulder composed of a peak at 531.2 eV and another at 532.2 eV. The peak at 529.9 eV corresponds to lattice oxygen (O2-) species in zirconia 66

while the peak at 531.2 eV is attributed to oxygen species in sulfate groups and the at peak at

532.2 eV is attributed to chemisorbed oxygen or -OH groups 67. The O/S ratio for the O1s peak at 531.2 eV is close to 4 and is consistent with the stoichiometry of sulfate groups. Figure 11 shows the O1s spectra for the alumina-incorporated Pd/SZ sample in which the main peak at 529.8 eV corresponds to O2- in zirconia and alumina. The individual contribution of zirconia and alumina to this peak is difficult to ascertain as a result of overlapping due to their close B.E. values. A broadened shoulder to the main O1s peak is observed and is likely due to the effect of the presence of dopants such as sulfates and alumina in the support. Using peak-fitting, the shoulder can be deconvoluted into photopeaks at 531.1 eV and 532.3 eV attributed to O2- in an oxygen deficient environment and chemisorbed oxygen species respectively. The O/S ratio for the peak at 531.1 is greater than 4 and may indicate that the contribution to the intensity of this peak is not solely due to O2- associated with sulfate groups. The peak at 531.1 eV indicates


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Page 18 of 43

oxygen vacancies which, in the case of alumina-incorporated Pd/SZ, may be additionally influenced by the presence of the alumina dopant in the zirconia matrix. The S 2p spectra for the binder-free and alumina-incorporated Pd/SZ samples are also shown in Figure 11. The S species indicated by the main peak at 168.8 eV corresponds to sulfate groups in which S in the +6 oxidation state in both samples

68-69

. The alumina-incorporated

Pd/SZ sample demonstrated higher sulfate retention at 35% when compared to 20% for the binder-free sample as determined by the S/Zr atomic ratio on zirconia. Alumina incorporation in the catalyst improves sulfate retention on high temperature calcination. The Al 2p spectra in Figure 11 shows a main peak at 74.2 eV corresponding to Al3+ in aluminum oxide 70. A smaller peak at lower binding energy 69.5 eV is also observed and is attributed either to metallic Al or to Al ions associated with oxygen vacancies

71

indicating strong Al-support interaction and

suggesting the formation of an ZrO2- Al2O3 binary mixture. Al3+ ions may substitute some of the Zr4+ ions in the zirconia lattice leading to the formation of oxygen vacancies for overall charge balance. Such an effect of alumina addition to Pd/SZ is also reflected by the intensity of the peak (531.1 eV) indicating the presence of oxygen vacancies in the O1s spectra for this sample. The Zr 3d doublets for the binder-free and the alumina-incorporated Pd/SZ samples are also shown in Figure 11. While peak-fitting, the intensity of the ratio of the splitting peaks was maintained at a ratio of 2:3 for d-orbitals with the spin-orbit separation at 2.43 eV. Peak-fitting revealed two types of Zr species in both samples with Zr 3d5/2 B.E. values at 181.9 eV and at 182.9 eV. The Zr 3d5/2 peak at 181.9 eV is attributed to Zr3+ species and that at 182.9 eV to Zr4+ species

72-73

. The ratio of Zr3+ to Zr4+ in the binder-free sample was ~2:1 whereas that for the

alumina-incorporated sample was ~4:1. The increased proportion of Zr3+ species in the aluminaincorporated sample is possibly due to the effect of alumina addition into the catalyst. Zr3+ cations are stabilized by the presence of oxygen vacancies which in-turn favor the formation of metastable tetragonal phase of zirconia observed in the case of the alumina-incorporated Pd/SZ sample treated at high temperatures. The depth profile of the binder-free and the alumina-incorporated Pd/SZ samples was investigated by sputtering using an Ar+ ion gun. To minimize change in the surface oxidation state between sputtering cycles, the dwell time and the number of sweeps for each XPS scan was reduced resulting in noisier data compared to the XP spectra shown in Figure 11. Figure 12 displays the O1s spectra of the binder-free and the alumina-incorporated Pd/SZ samples. On


17

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bombarding the surface with Ar+ ions, the broad shoulder of the main O 1s peak at 529.9 eV reduces significantly in intensity. In the corresponding S 2p spectra for each sputtering cycle for the binder-free and alumina-incorporated Pd/SZ catalyst samples shown in Figure 12, we observe that the intensity of the peak corresponding to the sulfate groups, 168.8 eV, decreases with each cycle. When the S 2p peak disappears, the shoulder of the respective O 1s spectra significantly reduces in intensity. The sulfate groups are on the top layers of the catalyst and are likely to have been expelled to the surface of the catalyst during the crystallization of the zirconia support while the samples were being calcined. The Zr 3d doublet for the binder-free and the alumina-incorporated Pd/SZ samples are shown in Figure 13. In the binder-free sample, as the sulfates are removed from the surface, the Zr3+ doublet reduces in intensity while that corresponding to Zr4+ grows. With further sputtering cycles we also observe a low intensity Zr0 peak at 179.7 eV. On the other hand, for the aluminaincorporated Pd/SZ sample, as the sulfates are removed from the surface, there is no significant change in the intensity of the Zr3+ doublet. We observe the corresponding Al 2p peak at ~69.2 eV attributed to Al0 or Al3+ in an oxygen deficient environment in Figure 14. The Ar+ ion bombardment seems to have a surface-cleaning effect as the peaks associated with the Al species intensify with each cycle. Presence of Al has a stabilizing effect on the Zr3+ ions which can be explained by the substitution of some of the Zr atoms in the zirconia lattice with Al atoms leading to the formation of oxygen vacancies which in turn have a stabilizing effect on Zr3+ ions. The intensity of the peaks corresponding to Al3+ and Al0 species do not decrease with depth demonstrating that Al is not just on the top layers of the catalyst but appears to be in the bulk of the catalyst as well. The XRD results discussed earlier does not show the presence of alumina crystallites although XPS experiments demonstrate that Al species exist with depth from the surface of the catalyst. This is a significant result indicating the formation of a zirconia-alumina binary mixture when alumina is incorporated into Pd/SZ during sol-gel synthesis.

Conclusions Initial stages of wash-coat development for the dual-catalyst aftertreatment system for NOx reduction under lean-burn conditions involved the addition of adhesivity-enhancing materials called “binders” such as alumina into the reduction catalyst to prevent irreversible loss of catalytic activity due to physical separation of the wash-coat from the walls of the cordierite


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Page 20 of 43

monolith core. The best performing binder in terms of NOx reduction activity, alumina, was incorporated into Pd/SZ catalyst in situ during sol-gel synthesis. Properties of the catalyst, and thus the performance of the catalyst for NOx reduction in the presence of excess oxygen is strongly influenced by the synthesis parameters during the sol-gel catalyst preparation. Therefore, it is important to understand the effect of addition of alumina to the reduction catalyst in situ during the sol-gel synthesis on the structural, textural, chemical properties and its consequent effect on activity for NOx reduction under lean-burn conditions. Steady-state activity tests showed that the performance of the alumina-binder incorporated Pd/SZ was the most promising amongst the other binders tested. Mode of addition of alumina-binder to Pd/SZ, whether in situ during sol-gel synthesis or ex-situ in catalyst slurry, strongly influences the catalytic activity for NOx reduction. In-situ mode of incorporation of alumina during sol-gel preparation of Pd/SZ demonstrated significantly higher NOx conversions when compared to the ex-situ mode. In situ XRD experiments during calcination under air highlights the differences in the evolution of the crystal phases of zirconia depending on the type of binder incorporated to Pd/SZ during sol-gel synthesis. Incorporation of alumina or boehmite binder to Pd/SZ during the preparation step resulted in stabilization of the tetragonal phase of zirconia. Although addition of silica delayed the onset of crystallization of zirconia, after high temperature calcination, the monoclinic zirconia phase was dominant in the resulting catalyst. The stabilizing effect of alumina on the metastable tetragonal phase of zirconia was further evidenced when the crystal phase composition of the sample at 700oC and 900oC was compared to that of the binder-free Pd/SZ samples. While in the binder-free Pd/SZ sample, on increasing the calcination temperature, the dominant crystal phase of zirconia changed from the tetragonal phase at 700oC to monoclinic phase at 900oC. In the alumina-incorporated sample, irrespective of the calcination temperature, the tetragonal phase of zirconia remained dominant in the sample. EPR revealed the presence of cationic Pd species and paramagnetic Zr3+ species in both the ex-situ incorporated alumina and the sol-gel incorporated alumina to Pd/SZ samples. The exsitu alumina sample spectra at 100 K was similar to that of the binder-free Pd/SZ spectra whereas sol-gel incorporated Pd/SZ sample indicated a change in the distribution of the Pd species most likely due to atomic level interaction of alumina with Pd/SZ.


19

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27

Al MAS NMR spectroscopy revealed distinct differences in the interaction of the

alumina binder with the zirconia support between the sol-gel incorporated alumina and the exsitu incorporated alumina to Pd/SZ. The possible formation of a zirconia-alumina solid solution is suggested and further supported by Raman spectroscopy using which the presence of Al species in the ex-situ sample is apparent but no such bands corresponding to Al was observed in the sol-gel alumina incorporated Pd/SZ sample. As a result of addition of alumina dopant and its strong interaction with the support, its influence on oxygen vacancies, sulfate groups and chemical state of zirconia was demonstrated using XPS. Alumina incorporation has a stabilizing effect on the sulfate groups, the oxygen vacancies and the proportion of Zr3+ species favoring the formation of metastable tetragonal phase of zirconia. Ar+ ion sputtering gave insights into the change of oxidation state of the species with depth. The sulfate groups were found to be in the top layers of the catalyst. The presence of reduced Al and Zr species along with oxygen vacancies suggested the possible formation of a zirconia-alumina binary mixture when alumina is incorporated in situ during solgel synthesis into the Pd/SZ catalyst. In summary, in situ addition of alumina into Pd/SZ during sol-gel synthesis likely forms a binary zirconia-alumina mixture with unique structural, textural and chemical properties which consequently affect the catalytic properties of the resulting catalyst for NOx reduction under lean conditions.

Acknowledgements The financial support provided by the U.S. Department of Energy and Caterpillar Inc. is gratefully acknowledged. The authors would like to thank Dr. Gordon Renkes, Dr. Elizabeth Hommel and Dr. Philippe Nadaud for assistance with acquiring the Raman, XPS and NMR spectra.

References


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(1) Gerber, G. R.; Devine, M. A., Efficiency, emissions advances keep recip engines in the DG mix. Power 2003, 147, 24-29. (2) Raber, L. R., EPA's Air Standards Pushing too far, too fast? Chem. Eng. News 1997, 75, 10-18. (3) Heck, R. M.; Farrauto, R. J., Automobile exhaust catalysts. Appl. Catal., A 2001, 221, 443-457. (4) Iwamoto, M.; Yahiro, H., Novel catalytic decomposition and reduction of NO. Catal. Today 1994, 22, 5-18. (5) Amiridis, M. D.; Zhang, T.; Farrauto, R. J., Selective cataytic reduction of nitric oxide by hydrocarbons. Appl. Catal., B 1996, 10, 203-227. (6) Nishizaka, Y.; Misono, M., Catalytic reduction of nitrogen monoxide by methane over palladium-loaded zeolites in the presence of oxygen. Chem. Lett. 1993, 22, 1295-8. (7) Iwamoto, M.; Hamada, H., Removal of nitrogen monoxide from exhaust gases through novel catalytic processes. Catal. Today 1991, 10, 57-71. (8) Ozkan, U. S.; Kumthekar, M. W.; Karakas, G., Self-sustained oscillatory behavior of NO+CH4+O2 reaction over titania-supported Pd catalysts. J. Catal. 1997, 171, 67-76. (9) Witoon, T.; Permsirivanich, T.; Kanjanasoontorn, N.; Akkaraphataworn, C.; Seubsai, A.; Faungnawakij, K.; Warakulwit, C.; Chareonpanich, M.; Limtrakul, J., Direct synthesis of dimethyl ether from CO2 hydrogenation over Cu–ZnO–ZrO2/SO42−–ZrO2 hybrid catalysts: effects of sulfur-to-zirconia ratios. Catal. Sci. Technol. 2015, 5, 2347-2357. (10) Mirkelamoglu, B.; Liu, M.; Ozkan, U. S., Dual-catalyst aftertreatment of lean-burn engine exhaust. Catal. Today 2010, 151, 386-394. (11) Gawade, P.; Alexander, A.-M. C.; Clark, R.; Ozkan, U. S., The role of oxidation catalyst in dual-catalyst bed for after-treatment of lean burn natural gas exhaust. Catal. Today 2012, 197, 127-136. (12) Gawade, P.; Alexander, A.-M. C.; Silver, R.; Ozkan, U. S., Effect of Engine Exhaust Parameters on the Hydrothermal Stability of Hydrocarbon-Selective Catalytic Reduction (SCR) Catalysts for Lean- Burn Systems. Energy Fuels 2012, 26, 7084–7091. (13) Majumdar, S. S.; Celik, G.; Alexander, A.-M.; Gawade, P.; Ozkan, U. S., In-situ incorporation of binder during sol-gel preparation of Pd-based sulfated zirconia for reduction of nitrogen oxides under lean-burn conditions: Effect on activity and wash-coating characteristics. Appl. Catal., B 2017, 202, 134-146. (14) Hench, L. L.; West, J. K., The sol-gel process. Chem. Rev. 1990, 90, 33-72. (15) Bianchi, C. L.; Ardizzone, S.; Cappalletti, G., Surface state of sulfated zirconia: the role of the sol-gel reaction parameters. Surf. Interface Anal. 2004, 36, 745-748. (16) Song, X.; Sayari, A., Sulfated zirconia-based strong solid-acid catalysts: recent progress. Cat. Rev. - Sci. Eng. 1996, 38, 329-412. (17) Ardizzone, S.; Bianchi, C. L., Acidity, sulphur coverage and XPS analyzes of ZrO2-SO4 powders by different procedures. Appl. Surf. Sci. 1999, 152, 63-69. (18) Hamouda, L. B.; Ghorbel, A., Control preparation of sulfated zirconia by sol-gel process: impact on catalytic performances during n-hexane isomerization. J. Sol-Gel Sci. Technol. 2000, 19, 413-416. (19) Armendariz, H.; Coq, B.; Tichit, D.; Dutartre, R.; Figueras, F., Influences of some synthesis parameters and activation procedures on the one-step sol-gel synthesis of sulfatedzirconia catalysts, followed by TG-DSC and mass spectrometry. J. Catal. 1998, 173, 345-354.


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(20) Liu, Y.; Sun, J. C. Y., Preparation of tailored pore size mesoporous zirconia with ehnaced thermal stability via controlled sol-gel process. Stud. Surf. Sci. Catal. 2005, 156, 249-256. (21) Mirkelamoglu, B.; Ozkan, U. S., Effect of water vapor on the activity and stability of Pd/SZ and Co/ZrO2 in dual-catalyst treatment of simulated exhaust from lean-burn natural gas engines. Appl. Catal., B 2010, 96, 421-433. (22) Woods, M. P.; Gawade, P.; Tan, B.; Ozkan, U. S., Preferential oxidation of carbon monoxide on Co/CeO2 nanoparticles. Appl. Catal., B 2010, 97, 28-35. (23) Toraya, H.; Yoshimura, M.; Somiya, S., Calibration curve for quantitative analysis of the monoclinic-tetragonal ZrO2 system by X-ray diffraction. Comm. Am. Ceram. Soc. 1984, 67, C119-C121. (24) Zalewski, D. J.; Alerasool, S.; Doolin, P. K., Characterization of catalytically active sulfated zirconia. Catal. Today 1999, 53, 419-432. (25) Klimova, T.; Rojas, M. L.; Castillo, P.; Cuevas, R., Characterization of Al2O3-ZrO2 mixed oxide catalytic supports prepared by the sol-gel method. Microporous Mesoporous Mater. 1998, 20, 293 -306. (26) Livage, J.; Doi, K.; Mazieres, C., Nature and Thermal Evolution of Amorphous Hydrated Zirconium Oxide. J. Am. Ceram. Soc. 1968, 51, 349–353. (27) Tani, E.; Yoshimura, M.; Somiya, S., Formation of Ultrafine Tetragonal ZrO2 powder under Hydrothermal Conditions. J. Am. Ceram. Soc. 1983, 66, 11-14. (28) Mercera, P. D. L.; Ommen, J. G. V.; Doesburg, E. B. M.; Burggraaf, A. J.; Ross, J. R. H., Zirconia as a support for catalysts- Evolution of the texture and structure on calcination in air. Appl. Catal. 1990, 57, 127-148 (29) Lin, J.-D.; Duh, J.-G., Crystallite size and microstrain of thermally aged low-ceria- and low-yttria-doped zirconia. J. Am. Ceram. Soc. 1998, 81, 853–860. (30) Garvie, R. C., The occurrence of metastable tetragonal zirconia as a crystallite size effect. J. Phys. Chem. 1965, 69, 1238-1243. (31) Ali, A.; Alvarez, W.; Loughran, C. J.; Resasco, D. E., State of Pd on H-ZSM-5 and other acidic supports during the selective reduction of NO by CH4 studied by EXAFS/XANES. Appl. Catal., B 1997, 14, 13-22. (32) Bahamonde, A.; Mohino, F.; Rebollar, M.; Yates, M.; Avila, P.; Mendioroz, S., Pillared clay and zirconia-based monolithic catalysts for selective catalytic reduction of nitric oxide by methane. Catal. Today 2001, 69, 233-239. (33) Epling, W. S.; Hoflund, G. B., Catalytic Oxidation of Methane over ZrO2-Supported Pd Catalysts. Journal of Catalysis 1999, 182, 5-12. (34) Parera, J. M., Promotion of zirconia acidity by addition of sulfate ion. Catal. Today 1992, 15, 481-90. (35) White, R. L.; Sikabwe, E. C.; Coelho, M. A.; Resasco, D. E., Potential role of pentacoordinated sulfur in the acid site structure of sulfated zirconia. J. Catal. 1995, 157, 755-8. (36) Morterra, C.; Cerrato, G.; Bolis, V., Lewis and Bronsted acidity at the surface of sulfatedoped ZrO2 catalysts. Catal. Today 1993, 17, 505-515. (37) Stakheev, A. Y.; Sachtler, W. M. H., Determination by X-ray photoelectron spectroscopy of the electronic state of Pd clusters in Y zeolite. J. Chem. Soc., Faraday Trans. 1991, 87, 37033708. (38) Chin, Y.-H.; Pisanu, A.; Serventi, L.; Alvarez, W.; Resasco, D. E., NO reduction by CH4 in the presence of excess O2 over Pd/sulfated zirconia catalysts. Catal. Today 1999, 54, 419-429.


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 43

(39) Sing, K. S. W.; Williams, R. T., Physisorption Hysteresis Loops and the Characterization of Nanoporous. Adsorpt. Sci. Technol. 2004, 22, 773-782. (40) Río, D. A. d. H.-D.; Aguilera-Alvarado, A. F.; Cano-Aguilera, I.; Martínez-Rosales, M.; Holmes, S., Synthesis and Characterization of Mesoporous Aluminosilicates for Copper Removal from Aqueous Medium. Mater. Sci. Appl. 2012, 03, 485-491. (41) Kaneko, K., Determination of pore size and pore size distribution: 1. Adsorbents and catalysts. J. Membr. Sci. 1994, 96, 59-89. (42) Sun, M. H.; Huang, S. Z.; Chen, L. H.; Li, Y.; Yang, X. Y.; Yuan, Z. Y.; Su, B. L., Applications of hierarchically structured porous materials from energy storage and conversion, catalysis, photocatalysis, adsorption, separation, and sensing to biomedicine. Chem. Soc. Rev. 2016, 45, 3479-563. (43) Michalik, J.; Narayana, M.; Kevan, L., Studies of the Interaction of Pd3+ and Pd+ with Organic Adsorbates, Water, and Molecular Oxygen in Pd-Ca-X Zeolite by Electron Spin Resonance and Electron Spin-Echo Modulation Spectroscopy. J. Phys. Chem. 1985, 89, 45534560. (44) Kevan, L., Catalytically important metal ion intermediates on zeolites and silica surfaces. Rev. Chem. Intermed. 1987, 8, 53-85. (45) Wyrwalski, F.; Lamonier, J. F.; Siffert, S.; Zhilinskaya, E. A.; Gengembre, L.; Aboukais, A., Bulk and surface structures of iron doped zirconium oxide systems- Influence of preparation method. J. Mater. Sci. 2005, 40, 933– 942. (46) Ashbrook, S. E., Recent advances in solid-state NMR spectroscopy of quadrupolar nuclei. PCCP 2009, 11, 6892–6905. (47) Smith, M. E., Application of 27Al NMR Techniques to Structure determination in solids. Appl. Magn. Reson. 1993, 4, 1-64. (48) MacKenzie; Smith, 27 Al NMR. In Multinuclear Solid-State Nuclear Magnetic Resonance of Inorganic Materials, 1 ed.; Pergamon: 2002; pp 271-330. (49) Haouas, M.; Taulelle, F.; Martineau, C., Recent advances in application of 27Al NMR spectroscopy to materials science. Prog. Nucl. Magn. Reson. Spectrosc. 2016, 94-95, 11-36. (50) Samain, L.; Jaworski, A.; Edén, M.; Ladd, D. M.; Seo, D.-K.; Javier Garcia-Garcia, F.; Häussermann, U., Structural analysis of highly porous γ-Al2O3. J. Solid State Chem. 2014, 217, 1-8. (51) Mishra, R. S. J., V.; Majumdar, B.; Lesher, C. E.; and Mukherjee, A. K., Preparation of a ZrO2 –Al2O3 nanocomposite by high-pressure sintering of spray-pyrolyzed powders. Faculty Research & Creative Works 1999. (52) Koichumanova, K.; Gupta, K. B. S. S.; Lefferts, L.; Mojet, B. L.; Seshan, K., An in situ ATR-IR spectroscopy study of aluminas under aqueous phase reforming conditions. Phys. Chem. Chem. Phys. 2015, 17, 23795--23804. (53) Slade, R. C. T.; Southern, J. C.; Thompson, I. M., 27 Al nuclear magnetic resonance spectroscopy investigation of thermal transformation sequences of alumina hydrates. Part 2.— Boehmite, γ-AlOOH. J. Mater. Chem. 1991, 1, 875-879. (54) Balmer, M. L.; Eckert, H.; Das, N.; Lange, F. F., 27Al nuclear magnetic resonance of glassy and crystalline Zr(1-x)AlxO(2-x2) materials prepared from solution precursors. Journal of American Ceramic Society 1996, 79, 321-326. (55) Jung, W.-S.; Ahn, S.-K.; Kim, D.-C., 27Al Magic-angle spinning nuclear magnetic resonance spectroscopic study of the conversion of basic dicarboxylate aluminium(III) complexes to alumina and aluminium nitride. J. Mater. Chem. 1998, 8, 1869–1873.


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Page 25 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(56) Fitzgerald, J. J.; Piedra, G.; Dec, S. F.; Seger, M.; Maciel, G. E., Dehydration Studies of a High-Surface-Area Alumina (Pseudo-boehmite) Using Solid-State 1H and 27Al NMR. Journal of American Chemical Society 1997, 119, 7832-7842. (57) Srdić, V. V.; Rakić, S.; Cvejić, Ž., Aluminum doped zirconia nanopowders: Wetchemical synthesis and structural analysis by Rietveld refinement. Mater. Res. Bull. 2008, 43, 2727-2735. (58) Srdic, V.; Radonjic, L., Interactions in the Sol-Gel Processing of Alumina-Zirconia Composites. J. Eur. Ceram. Soc. 1994, 14, 237-244. (59) Djurado, E.; Boulc’h, F.; Dessemond, L.; Rosman, N.; Mermoux, M., Study on Aging of Tetragonal Zirconia by Coupling Impedance and Raman Spectroscopies in Water Vapor Atmosphere. J. Electrochem. Soc. 2004, 151, A774. (60) Holmgreen, E. M.; Yung, M. M.; Ozkan, U. S., Pd-based sulfated zirconia prepared by a single step sol-gel procedure for lean NOx reduction. J. Mol. Catal. A: Chem. 2007, 270, 101111. (61) Li, M.; Feng, Z.; Ying, P.; Xin, Q.; Li, C., Phase transformation in the surface region of zirconia and doped zirconia detected by UV Raman spectroscopy. PCCP 2003, 5, 5326. (62) Cheema, T. A.; Garnweitner, G., Phase-controlled synthesis of ZrO2nanoparticles for highly transparent dielectric thin films. CrystEngComm 2014, 16, 3366-3375. (63) Li, C.; Stair, P. C., Ultraviolet Raman spectroscopy characterization of sulfated zirconia catalysts- fresh, deactivated and regenerated. Catal. Lett. 1996, 36, 119-123. (64) Assih, T.; Ayral, A.; Abenoza, M.; Phalippou, J., Raman study of alumina gels. J. Mater. Sci. 1988, 23, 326-3331. (65) Reyes-López, S. Y.; Acuña, R. S.; López-Juárez, R.; Rodríguez, J. S., Analysis of the phase transformation of aluminum formate Al(O2CH)3 to a-alumina by raman and infrared spectroscopy. Journal of Ceramic Processing Research 2013, 14, 627~631. (66) Ardizzone, S.; Cattania, M. G.; Lugo, P., Interfacial electrostatic behaviour of oxidescorrelations with structural and surface parameters of the phase. Electrochim. Acta 1994, 39, 1509-1517. (67) Li, Y. S.; Wong, P. C.; Mitchell, K. A. R., XPS investigations of the interactions of hydrogen with thin films of zirconium oxide II. Effects of heating a 26 A thick film after treatment with a hydrogen plasma. Appl. Surf. Sci. 1995, 89, 263-269. (68) Ebitani, K.; Konno, H.; Tanaka, T.; Hattori, H., In-situ XPS study of zirconium oxide promoted by platinum and sulfate ion. J. Catal. 1992, 135, 60-67. (69) Paal, Z.; Matusek, K.; Muhler, M., Sulfur adsorbed on Pt catalyst- its chemical state and effect on catalytic properties as studied by electron spectroscopy and n-hexane test reactions. Applied Catalysis A: General 1997, 149, 113-132. (70) Bardi, U.; Atrei, A.; Rovida, G., Initial stages of oxidation of the Ni3AI alloy: structure and composition of the aluminum oxide overlayer studied by XPS, LEIS and LEED. Surf. Sci. 1992, 268, 87-97. (71) Chen, M.; Wang, X.; Yu, Y. H.; Pei, Z. L.; Bai, X. D.; Sun, C.; Huang, R. F.; Wen, L. S., X-ray photoelectron spectroscopy and auger electron spectroscopy studies of Al-doped ZnO films. Appl. Surf. Sci. 2000, 158, 134–140. (72) Nishino, Y.; Krauss, A. R.; Lin, Y.; Gruen, D. M., Initial oxidation of zirconium and Zircaloy-2 with oxygen and water vapor at room temperature. J. Nucl. Mater. 1996, 228, 346353.


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(73) Manoli, J.-M.; Potvin, C.; Muhler, M.; Wild, U.; Resofszki, G.; Buchholz, T.; Paal, Z., Evolution of the Catalytic Activity in Pt Sulfated Zirconia Catalysts- Structure, Composition, and Catalytic Properties of the Catalyst Precursor and the Calcined Catalyst. J. Catal. 1998, 178, 338–351.


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TOC


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Table 1: Crystal size analysis at 700oC of the binder-free and binder-incorporated samples

Samples

(1 0 1) t-ZrO2 nm

Pd/SZ

13.4

Pd/SZ (alumina-incorporated)

11.3

Pd/SZ (boehmite-incorporated)

9.7

Pd/SZ (silica-incorporated)

11.3


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Table 2: BET surface area using N2 physisorption at 77 K

Samples

BET surface area (m2/g)

Alumina reference

174

Pd/SZ

47

Pd/SZ (ex-situ alumina)

77

Pd/SZ (sol-gel alumina)

68


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Figure 1: XRD pattern of (a) Binder-free Pd/SZ, (b) 10% Alumina-incorporated Pd/SZ, (c) 10% Boehmite-incorporated Pd/SZ and (d) 10% Silica-incorporated Pd/SZ during insitu calcination under air showing tetragonal and monoclinic phases of zirconia.


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Figure 2: Crystal phase compositions of binder-free and binder-incorporated Pd/SZ catalysts: Comparison between calcination at 700oC and at 900oC



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Figure 3: Comparison of binder-free and binder-incorporated mixed beds in terms of NOx, and CH4 conversions at 450oC. Feed conditions: 180 ppm NO2, 1737 ppm CH4, 208ppm C2H6, 104 ppm C3H8, 650 ppm CO, 6.5% CO2, 10% O2, 10% H2O, GHSV 32000 h-1.


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Figure 4: NOx and CH4 conversion comparison between binder-free Pd/SZ, sol-gel and ex-situ alumina incorporated Pd/SZ in the mixed bed at 450°C. Feed conditions: 180 ppm NO2, 1737 ppm CH4, 208 ppm C2H6, 104 ppm C3H8, 650 ppm CO, 6.5% CO2, 10% O2, GHSV 32000 h-1.



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Figure 5: Comparison of (a) NOx and (b) CH4 conversions of the mixed bed containing binder-free Pd/SZ or alumina binder-incorporated Pd/SZ as reduction catalysts components at 450°C and 500°C. Inset shows steady state activity data at 450oC under wet conditions. Feed conditions: 180 ppm NO2, 1737 ppm CH4, 208 ppm C2H6, 104 ppm C3H8, 650 ppm CO, 6.5% CO2, 10% O2, (0% or 10%) H2O, GHSV 32000 h-1.


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Figure 6: N2 isotherm at 77 K of (a) Pd/SZ, (b) ex-situ alumina incorporated Pd/SZ, (c) sol-gel alumina incorporated Pd/SZ. Inset shows the alumina reference.



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Figure 7: Pore-size distributions of (a) alumina reference, (b) Pd/SZ, (c) ex-situ alumina incorporated Pd/SZ and (d) sol-gel alumina incorporated Pd/SZ from N2 isotherm at 77 K.


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Figure 8: Electron paramagnetic resonance at 300 K on (a) ex-situ alumina incorporated 0.3%

Pd/SZ, (b) sol-gel alumina incorporated 0.3% Pd/SZ and at 100 K for (c) ex-situ alumina incorporated 0.3% Pd/SZ and (d) sol-gel alumina incorporated 0.3% Pd/SZ.



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Figure 9: 27Al MAS NMR on (a) 0.3% Pd/SZ sol-gel alumina incorporation and (b) 0.3% Pd/SZ ex-situ alumina incorporation


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Figure 10: Raman spectra of (a) ex-situ alumina incorporated Pd/SZ, (b) sol-gel alumina incorporated Pd/SZ, and (c) binder-free Pd/SZ using 633nm laser



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Figure 11: XPS spectra of the O1s, S 2p, Zr 3d regions of Pd/SZ and alumina-incorporated Pd/SZ.


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Figure 12: XPS spectra of the O1s and S 2p regions of Pd/SZ and alumina-incorporated Pd/SZ on Ar+ ion sputtering.



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Figure 13: XPS spectra of the Zr 3d region for Pd/SZ and alumina-incorporated Pd/SZ on Ar+ ion sputtering.


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Figure 14: XPS spectra of the Al 2p region for alumina-incorporated Pd/SZ on Ar+ ion sputtering.


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