Effects of Controlled Crystalline Surface of Hydroxyapatite on Methane

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Effects of Controlled Crystalline Surface of Hydroxyapatite on Methane Oxidation Reactions Su Cheun Oh, Jiayi Xu, Dat T. Tran, Bin Liu, and Dongxia Liu ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b04011 • Publication Date (Web): 16 Apr 2018 Downloaded from http://pubs.acs.org on April 16, 2018

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ACS Catalysis

Effects of Controlled Crystalline Surface of Hydroxyapatite on Methane Oxidation Reactions

Su Cheun Oh1, Jiayi Xu2, Dat T. Tran3, Bin Liu2 and Dongxia Liu1*

1. Department of Chemical and Biomolecular Engineering in University of Maryland at College Park, Maryland 20742 2. Department of Chemical Engineering, Kansas State University, Manhattan, KS 66506 3. U. S. Army Research Laboratory, RDRL-SED-E, 2800 Powder Mill Road, Adelphi, MD, 20783, United States

*Corresponding author: Prof. Dongxia Liu Email: [email protected] Phone: (+1) 301-405-3522 Fax: (+1) 301-405-0523

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Abstract: Hydroxyapatite (HAP, Ca10(PO4)6(OH)2) has a hexagonal prismatic structure that exposes two crystalline surfaces: prism-faceted a- and basal-faceted c-surfaces. In this work, the predominant exposure of c-surface was controlled and its influences in methane oxidation reactions (combustion and oxidative coupling over HAP and lead-substituted HAP (Pb-HAP), respectively) were studied. The c-surface exposure was realized by crystal orientation in HAPbased catalyst film, which was created by an electrochemical deposition of HAP seeds on a titanium substrate, followed by hydrothermal crystallization and peeling off of the crystalline films from the substrate. In comparison to a-surface that is prevalently exposed in unoriented HAP-based catalysts, the c-surface (i.e., (002) crystalline plane) of HAP-based catalysts exhibited up to 47-fold enhancement in areal rate in both reactions. The distinct catalytic activity between these two crystalline surfaces is attributed to the preferential formation of oxide ions and vacancies on c-surfaces. The oxide ions and vacancies in turns function as actives sites for promoting methane activation and complete oxidation into CO2. Density functional theory calculations confirmed the close relationship between different catalytic activities in c-surface of oriented and a-surface of unoriented HAP through the tendency of vacancy formation. Without the presence of vacancies, the methyl or methylene group after methane activation forms ethane or ethylene via coupling. The present study explored the effects of HAP crystal orientation in methane oxidation reactions, which revealed distinct catalytic behaviors of crystal surfaces in HAP-based materials.

Keywords: Hydroxyapatite; crystalline plane; methane conversion; density functional theory, oxygen vacancy.


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1. Introduction Hydroxyapatite (HAP), as represented by chemical formula of Ca10(PO4)6(OH)2, is widely studied as biomedical materials for bones or teeth substitutions1-4, proton conductors for electrolyte membranes in fuel cells5-8, and acidic-basic catalysts in heterogeneous catalysis9-13. HAP has a hexagonal crystalline structure, in which the calcium (Ca2+) cations as well as the phosphate (PO43-) and hydroxide (OH-) anions can be substituted by the corresponding cations or anions to vary its physicochemical properties for these applications. For example, the lead (Pb2+) substitution of calcium ions enables the Pb-HAP material to be used as catalyst for C2 (ethylene and ethane) synthesis via oxidative coupling of methane (OCM) reactions14-17. The fluorine (F-) substitution of OH- anion enhances the corrosion resistance and mechanical strength of HAP in biomedical materials18. The substitution of PO43- anions by carbonate (CO32-) leads to oxide ion conductivity in HAP which can be used as electrolyte membranes in fuel cells19. An alternative approach to modify the physicochemical properties of HAP is to manipulate the crystal orientation of the developed materials. As a hexagonal crystal, HAP can be developed into prism-like structure, in which the growth is along the c-axis direction and with (002) plane (or c-surface) as the end face20. Similarly, HAP can be emerged into plate-like morphology, in which the growth along the a-axis direction is facilitated by exposing (102) and (201) faces21. The alignment of the HAP crystals along the c- or a-axis direction and selective control of the exposed crystal planes enable new properties for desired applications. For example, HAP crystals with highly preferred orientation to the a, b-axis for biomaterials application has shown to have lower cell attachment efficiency22. In contrast, the c-axis-oriented HAP forms biogenic hierarchical structures with collagen fibrils, and shows excellent mechanical property in


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biomedical applications23. High temperature electrochemical investigations have shown that HAP is proton conductive in which protons migrate along hydroxyl groups lining the c-axis of the crystals. The alignment of crystals along the c-axis direction in HAP electrolyte membranes enhanced proton conductivity for almost four orders of magnitude compared to the membrane with random crystal orientation20. Although the effects of orientation of HAP crystals on their physicochemical properties and the consequent applications in biomedical materials and electrolyte membranes have been explored, the effects of HAP orientation and controlled crystalline plane exposure in catalyst application have not been investigated. Herein, we report the influences of crystal orientation of HAP-based materials on catalytic reactions and take the opportunity to scrutinize the catalytic behaviors of individual HAP surfaces. Two catalyst materials, HAP and Pb-HAP, with c-axis orientation and controlled exposure of (002) plane (i.e., c-surface), were studied in two catalytic reactions: methane combustion and OCM reactions, respectively. The synthesis of c-axis oriented HAP or Pb-HAP catalyst materials was conducted by an electrochemical deposition of HAP seeds on a titanium substrate, the hydrothermal crystallization to develop crystalline films with c-axis normal to the substrate, and peeling off of the HAP-based crystalline films from the substrate in sequence. The as-obtained samples have (002) plane as the prevalent exposed surface. For comparison purpose, the c-axis oriented HAP-based catalyst films were ground into fine powders to expose other prominent crystalline surfaces, and they are denoted as a-surface. The morphology, surface composition and surface areas of the c-axis oriented catalyst films and unoriented catalyst powders were characterized using scanning electron microscopy (SEM), X-ray powder diffraction (XRD), energy dispersive X-ray spectroscopy (EDS), and nitrogen (N2) adsorptiondesorption isotherms. The performance of c-axis oriented and unoriented HAP and Pb-HAP


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catalysts in methane combustion and oxidative coupling of methane reactions were evaluated and compared. DFT calculations were conducted to understand the consequences of exposed crystalline surfaces in methane activation reactions. The present study for the first time rigorously explored the catalytic performance of individual crystalline surfaces in HAP-based catalyst structures by selective control of crystal orientation in methane oxidation reactions.

2. Experimental 2.1 Materials Calcium chloride dihydrate (CaCl2·2H2O, ≥99.0% purity) and ammonia hydroxide solution (NH4OH, 28-30%) were supplied from Sigma-Aldrich. Lead nitrate (Pb(NO3)2, ACS reagent) and hydrochloric acid (HCl, reagent grade, 37 wt%) were purchased from J.T. Baker while calcium nitrate tetrahydrate (Ca(NO3)2 ﹒ 4H2O, 99.0-103.0%), ammonium phosphate dibasic

((NH4)2HPO4,

≥99.0%),

disodium

ethylenediaminetetraacetate

dihydrate

(Na2EDTA·2H2O) (ACS reagent, 99.0–101.0% purity), (1-Hexadecyl)trimethylammonium bromide (CTAB, 98%), titanium (Ti) and platinum (Pt) plates (2.54 cm×2.54 cm×0.032 mm) were purchased from Alfa-Aesar. Sodium chloride (NaCl, ≥99.0% purity) was obtained from EMD.

Potassium

phosphate

dibasic

anhydrous

(K2HPO4,

99.99%

purity)

and

tris(hydroxymethyl)-aminomethane (99.8+% purity) were supplied from AMRESCO.

2.2 HAP-based catalysts preparation The synthesis of c-axis oriented HAP-based catalyst was carried out by electrochemical deposition of HAP seeds followed by seeded hydrothermal growth method, as reported elsewhere24. Firstly, the Ti and Pt metal plates which were used as cathode and anode, respectively, were polished with sand paper (1500 grit). After being washed with industrial soap 5 ACS Paragon Plus Environment

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solution, the metal plates were submersed in ethanol/acetone (50/50, volume ratio) solvent and sonicated in an ultrasonic cleaner for 30 min. The sonicated metal plates were then rinsed with deionized (DI) water. Electrolyte solution consisting of 1.67 mM K2HPO4, 2.5 mM CaCl2 and 138 mM NaCl in DI water was prepared for electrochemical reaction, as reported in literature24-25. The pH of the electrolyte solution was adjusted to 7.2 by using buffer solution of tris(hydroxymethyl)- aminomethane and 37% hydrochloric acid. The two metal plates were immersed in the electrolyte solution in which they were held parallel to each other with a fixed distance of separation of 10 mm. Both the anode and cathode were connected to a direct power supply (Extech, Model no. 382200) during the electrodeposition reaction. Electrodeposition of HAP seed layer was carried out at 368 K under 1 A constant current conditions for 15 min. After the electrochemical growth, the seeded metal substrate was taken out from the electrolyte solution, rinsed with DI water and dried in air for secondary and tertiary hydrothermal synthesis use. The HAP seed layer on the surface of the titanium plate obtained by electrochemical deposition method was used to promote hydrothermal growth of HAP coatings. In the secondary hydrothermal synthesis of HAP catalyst, 15 mL of 0.20 M of Ca(NO3)2 and 0.20 M of Na2EDTA solution and 15 mL of 0.12 M of (NH4)2HPO4 were prepared in two flasks separately. NH4OH was added to each source solution to adjust the pH to ~10. After that, (NH4)2HPO4 solution was added to the Ca(NO3)2/Na2EDTA solution and the mixture was stirred under magnetic stirring for 20 min at room temperature. The resulting mixture solution was then transferred to a Teflonlined stainless steel autoclave with HAP-seeded substrate submersed ~45° relative to the bottom of the Teflon liner, followed by hydrothermal synthesis for 15 hours in an oven heated at 473 K.


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After the synthesis, the substrate was rinsed with DI water several times and dried at room temperature. In the tertiary hydrothermal synthesis, 15 mL of 0.10 M of Ca(NO3)2, 0.10 M of Na2EDTA and 0.01 M of CTAB solution and 15 mL of 0.06 M of (NH4)2HPO4 solution were prepared in two flasks separately. After the pHs of both solutions were adjusted to ~10 using NH4OH solution, (NH4)2HPO4 solution was added to the Ca(NO3)2/ Na2EDTA/ CTAB solution. The mixture was stirred under magnetic stirring at room temperature for 20 min and then transferred to Teflon-lined stainless steel autoclave. The substrate obtained from secondary hydrothermal growth was placed inside the Teflon liner with angle tilted to ~45° relative to the bottom of the Teflon liner. The autoclave was placed in an oven heated at 473 K for 15 hour. After the synthesis, the sample was collected and washed with DI water several time. The resulted wet substrate was dried in oven overnight at 343 K. As for the synthesis of c-axis oriented Pb-HAP catalyst, the same secondary and tertiary hydrothermal synthesis procedures as those for HAP were applied except that Ca(NO3)2 was replaced with a mixture of 0.12 M Ca(NO3)2 and 0.08 M of Pb(NO3)2 in the secondary growth and 0.08 M Ca(NO3)2 and 0.02 M of Pb(NO3)2 in the tertiary growth. The remaining procedures were the same as those for HAP catalyst synthesis. All the HAP-based catalyst films on the Ti substrate were peeled off and were treated in flowing air (150 mL min-1, ultrapure, Airgas) at 973 K for 5 hours at a ramp rate of 17.5 K min-1 from room temperature. For comparison purpose, unoriented HAP-based catalysts were prepared by grinding the c-axis oriented HAP and Pb-HAP into fine powder. The films and fine powder were subsequently used for characterization and catalysis experiments discussed below.

2.3 Catalyst characterization 7 ACS Paragon Plus Environment

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The morphologies of the oriented and unoriented HAP-based catalysts were visualized using scanning electron microscopy (SEM) on a Hitachi SU-70 electron microscope. The surface compositions of the oriented and unoriented HAP-based catalysts were quantified using energy dispersive X-ray spectroscopy (EDS) on the same electron microscope. N2 adsorption-desorption isotherms of the HAP-based samples were measured using an Autosorb-iQ analyzer (Quantachrome Instruments) at 77 K. The samples were outgassed at 523 K for 8 hours and 1 mm Hg prior to measurements. The specific surface areas of the samples were determined using Brunauer, Emmett and Teller (BET) method. The crystalline phases were examined using powder X-ray diffraction (XRD) and obtained on Bruker D8 Advance Lynx Powder Diffractometer (LynxEye PSD detector, sealed tube, Cu Kα radiation with Ni β-filter). The temperature programmed desorption of ammonia (NH3-TPD) and carbon dioxide (CO2-TPD) was evaluated using an AutosorbiQ unit (Quantachrome, ASIQM0000-4) equipped with a thermal conductivity detector (TCD). Typically, 35 mg catalyst sample was loaded into a quartz reactor and pretreated at 973 K for 2 h under He (40 mL min-1, ultrapure, Airgas) at a heating rate of 10 K min-1 from ambient temperature. After being cooled to 343 K under He stream, the catalyst was exposed to NH3 stream (28 mL min-1, ultrapure, Airgas) for 0.5 h. The physisorbed NH3 was then removed by flowing He gas (40 mL min-1) for 2 h. Afterwards, the catalyst sample was ramped to 1100 K at a ramp rate of 10 K min-1 and the NH3-TPD profile was recorded during this step. The CO2-TPD profiles of these catalysts were measured using the same procedure as NH3-TPD except that NH3 was switched to CO2 in the measurement

2.4 Catalytic methane oxidation reaction


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2.4.1 Methane combustion catalytic test. The methane combustion reaction was performed in a U-shape tubular quartz reactor (10 mm inner diameter) under atmospheric pressure and at 973 K. Typically, 35 mg of catalyst sample in thin-filmed or powdered form diluted with inert quartz particle was loaded in the quartz reactor in which the reactor was placed inside a temperature controlled furnace (National Electric Furnace FA120 type). The temperature of the furnace was held constant by a Watlow Controller (96 series). The catalyst temperature was monitored by a K-type thermocouple attaching to the outer wall of the reactor. Prior to the reaction, the catalyst was pretreated in He and O2 atmosphere (33 mL min-1, volume ratio: 91% He, 9% O2) at 973 K for 5 hours. CH4 (3.5 mL min-1, 99.999% purity, Airgas) and O2 (35 mL min-1, 99.9993% purity, Airgas) diluted in N2 (as internal standard) (15 mL min-1, 99.95% purity, Airgas) were fed to the catalyst via heated transfer lines hold at 343 K to the reactor to avoid moisture condensation. The measurements were performed after the samples were heated to the desired temperature. The reactant and product gases were analyzed using gas chromatograph (Agilent Technologies, 6890N) equipped with ShinCarbon ST packed column connected to a thermal conductivity detector (TCD). 2.4.2 Oxidative coupling of methane (OCM) catalytic test. The OCM catalytic reaction was performed using the same reactor setup as that for combustion reaction. The c-axis oriented and unoriented HAP-based catalysts were tested at 973 K and 101 kPa pressure. The kinetics of OCM reactions on these catalysts were measured by varying partial pressure of methane (P ) or partial pressure of oxygen (P ). In the experiment, the flow rate of CH4 or O2 was varied while keeping the total flow rate of the gas stream constant at 46 mL min-1 by varying the helium flow rate accordingly. The space velocity of the reactions was kept at 184000 mL gcat-1 hr-1. All


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the kinetics data were measured under differential conditions in which the methane conversion was maintained below 5%.

2.5 Density functional theory (DFT) calculation. Periodic DFT calculations were performed using the Vienna ab initio simulation package (VASP)26-27. The GGA-PBE functional was used to account for the electron exchange-correlation effects28. The projector augmented wave (PAW) method was used to treat the ion-electron interactions29. K-point meshes of 2×2×4 and 2×2×1 based on the Monkhorst-Pack scheme30 were used for bulk and slab calculations, respectively. A cut off energy of 400 eV for the plane wave basis set was used for all calculations. The break condition for self-consistent iteration was 1×10-6. Ionic relaxation was stopped when the forces on all atoms are smaller than -0.02 eV/Å. Figure S1 schematically illustrates the optimized hexagonal primitive cell for HAP. The optimized lattice constants are listed in Table 1, and are in good agreement with both theoretical and experimental data31-35. The (002) facet from the c-axis, and (112), and (211) facets associated with a-axis are also illustrated in Figure S1 in the Supporting Information. A fivelayer slab was used to represent the HAP surface. The bottom two layer of the slab was fixed to the optimized bulk value. Total energies of gas phase species were calculated by placing the molecule in a box with dimensions of 20 × 20 × 25Å. Gamma-point k point is used for these calculations. Gaussian smearing was used to assist convergence. In this study, the hydroxyl vacancy is produced by removing one OH and one H (according to the stoichiometry of H2O). The energy of vacancy formation ( ∆E ) was calculated according to Eq. (1):

∆E = E + E − E

(1)


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where E , E , and E are total energies of HAP facet with a hydroxyl vacancy, gas phase H2O, and the corresponding stoichiometric facet, respectively. The binding energy (∆E ) was defined by Eq. (2): ∆E = E − E − E

(2)

where E , E , and E represent total energies of adsorbed species, clean surface, and gas phase molecule, respectively.

3. Results and discussion 3.1 Chemical structure and property of HAP crystal Figure 1 shows the schematic structure of HAP-based catalyst, viewed normal to (Figure 1(A)) and along with c-axis (Figure 1(B)), respectively. The structure of HAP crystal grown into a hexagonal prism along c-axis direction is shown in Figure 1(C). HAP belongs to the hexagonal space group of P63/m.36-38 The framework is skeletally constructed by PO43- tetrahedrons, in which P5+ ions are located in the center of the tetrahedrons. Within the unit cell of HAP crystal, as shown in Figure 1(B), six PO43- tetrahedrons divide equally into two layers with heights of 1/4 and 3/4, respectively. The PO43- tetrahedrons delimit two types of unconnected channels along the c-axis, which are surrounded by two sets of non-equivalent Ca2+ ions (denoted by Ca[1] and Ca[2], respectively) (Figure 1(A)). The first type of channel has a diameter of 2.5Å and is surrounded by four Ca[1] ions. Each Ca[1] is coordinated to six oxygen atoms belonging to different PO43- tetrahedrons and also to three oxygen atoms at a larger distance. There are two such channels in one unit cell, each of which contains two Ca[1] ions at heights 0 and 1/2. The second type of channel has a larger diameter (~3-4.5Å) than the first one, containing six other Ca[2] ions located at the periphery of the channel. The Ca[2] is coordinated to seven oxygen atoms, 11 ACS Paragon Plus Environment

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six of them belonging to PO43- tetrahedrons and one contributed from hydroxyl anion (OH-). The six Ca[2] ions form two equilateral triangles rotated 60 degrees relative to each other, at the heights of 1/4 and 3/4, respectively, of the unit cell. The OH- ions are located at the center of large channel and perpendicular to the unit cell face. The structure of HAP offers unique properties for this material. It plays important roles in catalysis39-45, adsorption46-49 and ion-transport50-52 applications. First of all, the existence of two different sets of Ca2+ ions in HAP is of special interest for catalysis because the material properties can be tuned by specific substitution of these sites with other metal ions43, 53-57 to endow different catalytic capabilities for targeted reactions. Secondly, HAP crystal has two sets of surfaces, prism-faceted a-surface (a- and b-surfaces are equivalent) and basal-faceted csurface, as shown in Figure 1(C). On the basal c-surface (i.e., (001) plane of HAP crystal), calcium ions is rich, and thus, this surface is charged positively. In contrast, the prismatic asurface (for example, (100) plane of HAP crystal) is rich in phosphate and hydroxide ions and hence negatively charged. The different charges on these two crystal planes render HAP surfaces with anisotropic characteristic, which in turns promote anisotropic adsorption capabilities for different biomolecules on the surfaces58-59. Thirdly, the large channel of HAP confers certain mobility to hydroxyl ions and allows proton migration along the channels in the c-axis direction. As a result, HAP has proton conductivity at elevated temperatures60-61. Additionally, upon heating treatment, HAP material undergoes gradual decomposition via dehydroxylation to produce oxyapatite and decomposition of oxyapatite to form calcium phosphates in sequence62-64. Both stages of the decomposition of HAP have been reported to be reversible under controlled heating and cooling conditions63. Particularly, the dehydroxylation of HAP involves debonding of OH- groups from HAP lattice and dehydroxylation reaction of OH- to form H2O, vacancies


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and O2- ions65. The presence of these ionic species has been verified by electron spin resonance after exposure of heat treated HAP to oxygen gas66-69.

3.2 SEM and XRD characterizations for oriented and unoriented HAP-based catalysts Morphologies of the c-axis oriented and unoriented HAP-based catalysts were examined by SEM observations, and the images are shown in Figure 2. The top view of the oriented HAP catalyst structure (Figure 2(A)) indicates that the prism-shaped HAP crystals were oriented perpendicularly to the surface of the metal substrate in the synthesis process. The crystal domains grew together and exhibited a uniform and homogeneous dense layer consisting of welldefined hexagonal platelet-like structure with width between 2 μm to 3 μm. The side view of the c-axis oriented HAP catalyst structure in Figure 2(B) shows that the HAP prism was ~10 μm in length. Substitution of Pb2+ into HAP to form Pb-HAP (Figure 2(C)) did not change the morphology, hexagonal crystal size and orientation significantly, as similar behavior of Pb-HAP crystals can still be observed. Similar to c-axis oriented HAP, the length of the hexagonal prism structure was ~10 μm after Pb2+ substitution (Figure 2(D)). The SEM observations suggest the successful synthesis of c-axis oriented HAP-based catalysts. On the contrary, the unoriented HAP and Pb-HAP were also purposely prepared by crushing and grinding the oriented HAP and Pb-HAP samples, respectively. As shown in Figure 2(E) and (F), both HAP and Pb-HAP samples contain irregular particle agglomerates, exposing HAP-based structures with different lengths and different crystalline planes compared to oriented HAP and Pb-HAP. The preparation of unoriented HAP and Pb-HAP from the oriented ones guarantees the samples to have same compositions but differently exposed crystal faces. This offers us a direct comparison of their performance in catalytic reactions that will be discussed below.


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XRD patterns was used to determine the crystallinity and crystalline plane exposure of the oriented and unoriented HAP-based catalysts. The XRD patterns for c-axis oriented and unoriented HAP as well as c-axis oriented and unoriented Pb-HAP in Figure 3 identify the crystals as HAP structure (PDF card no. 01-075-9526). A strong intensification in the (002) reflection peak relative to other peaks in oriented HAP and Pb-HAP samples suggests that c-axis HAP crystals preferentially oriented perpendicular to the metal substrate in the synthesis steps, consistent with the SEM observation. The exposure of (002) plane suggests that the HAP prisms have basal-faceted c-surface exposed in the synthesized crystal films. For the ground HAP and Pb-HAP samples, the XRD pattern is similar to the powdered sample prepared by coprecipitation method, in which the (002) diffraction peak intensity was much smaller compared to that of (211), (112) and (300) planes. This suggests that HAP crystals are oriented randomly in the ground powder generated from HAP films. Apparently, the prism-faceted a-surface is the dominant surface in the unoriented HAP-based catalyst samples.

3.3 Composition and surface area of oriented and unoriented HAP-based catalysts The compositions of the c-axis oriented and unoriented HAP-based catalysts were determined using EDS analysis, and the results are listed in Table 2. The (Ca+Pb)/P molar ratio on the surface of c-axis oriented HAP, c-axis oriented Pb-HAP and randomly oriented Pb-HAP are 1.55, 1.57 and 1.58 respectively, smaller than the ratio of (Ca+Pb)/P = 1.67 in catalyst synthesis. For unoriented HAP sample, the surface (Ca+Pb)/P molar ratios is 1.71, which is slightly higher than the starting ratio in the synthesis. The deviation of the cation/anion ratio in the HAP-based catalysts from the stoichiometric ratio might be caused by surface calcium enrichment or surface phosphorus impoverishment during synthesis process70. The surface


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concentration of Pb2+ in c-axis oriented Pb-HAP catalyst is slightly smaller than unoriented PbHAP, suggesting that the surface of unoriented Pb-HAP is more enriched with Pb2+ species. N2 adsorption-desorption isotherms were used to reveal the surface areas of the HAP-based catalysts and the results are also listed in Table 2. The results show that the surface areas of the unoriented HAP and Pb-HAP catalysts are much higher than the c-axis oriented HAP and Pb-HAP samples. The higher surface area of the unoriented HAP-based catalysts suggests that more crystalline faces are exposed in these samples, contributing to more surface areas. It should be noted that the surface area of oriented HAP is mainly contributed by the c-surface of HAP films. Since unoriented HAP is generated by grinding of oriented HAP films, the measured surface area of this sample is comprised of contributions from both c- and other side surfaces. The difference between surface areas of both c-axis oriented and randomly oriented HAP samples can be attributed to the a-surfaces since grinding of HAP films mainly led to segregation of HAP prismatic bundles into individual particles, as viewed by SEM observations.

3.4 Effects of controlled crystalline plane of HAP on methane oxidation reactions Methane, a main constituent of natural gas, is abundant in nature and mainly employed as an inexpensive and clean-burning fuel70-71. The diminishing reserves of petroleum oil have shifted the market attention to making more efficient use of methane. The methane combustion reaction to carbon dioxide (CO2) and ethane (C2H6))

6

72

and oxidative coupling of methane to C2 (ethylene (C2H4)

are promising approaches for efficient methane uses. The resultant heat

produced from methane combustion reaction is promising for energy production in gas turbine plants73; while the C2 products from OCM reactions can be used as feedstocks for chemical industry such as synthesis of polyethylene74. HAP-based materials have been explored as


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catalysts for OCM reactions since 1990s75-79. The substitution of calcium by other metal ions showed marked effects on shifting the product selectivity. For example, the lead substituted HAP (Pb-HAP) catalyzes selective oxidative coupling reactions for C2 production76, 80. In the present study, we aim to explore the influence of crystalline plane orientation of HAP catalysts on the OCM reactions and use it as a new approach to modifying the performances of HAP-based catalysts in catalysis. The methane combustion reaction using HAP material as catalyst, with and without crystal alignment, respectively, has been firstly explored.

3.4.1 Performance of oriented HAP in methane combustion reaction. The methane combustion reaction was carried out by flowing a mixture of CH4 and O2 gases (molar ratio of O2/CH4 = 10) over the catalyst at 973 K and 101 kPa pressure conditions, and the results are shown in Figure 4. For comparison purpose, the reaction in the absence of any catalyst was also examined. Without any catalyst, methane conversion was ~ 4% in the blank reactor. The CO and CO2 are the only carbon-based products with selectivity of 3% and 97%, respectively, as shown in Figure 4(A). The measured CH4 conversion at reaction temperature of 973 K over the catalysts was much higher (>21 times) than that in the blank reactor (Figure 4(A)). This indicates that HAP material is catalytically active for methane combustion reaction. A comparison across the HAP catalysts with and without crystal orientation showed that unoriented HAP enabled slightly higher methane conversion than c-axis oriented HAP at 973 K. In both cases, CO and CO2 were the only carbon-containing products detected under the investigated reaction conditions over the catalysts. Oriented HAP demonstrated higher CO2 selectivity than unoriented HAP (Figure 4(A)), suggesting that c-surface of HAP crystals favor CO2 formation than a-surface. The stability of the oriented and unoriented HAP catalysts in methane combustion reaction was tested by running the reaction at 973 K for 10 hours and the result is shown in Figure S2 in the Supporting 16 ACS Paragon Plus Environment

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ACS Catalysis

Information. The stability result exhibited no obvious deactivation during the test, in which CH4 conversion remained at ~83 % and ~98 % for oriented HAP and unoriented HAP, respectively. The differences in methane conversion and product selectivity in the methane combustion reaction could result from the differences in surface areas, surface composition and exposed crystal planes between the c-axis oriented and randomly orientated HAP catalysts. As shown in Table 2, the surface area of unoriented HAP is 15 times higher than that of c-axis oriented HAP. The methane conversion over the unoriented HAP is only 1.2 times higher than c-axis oriented HAP catalyst. The non-consistency in enhancements between the surface area and catalytic activity of HAP catalyst hints that surface area does not contribute to the performance difference directly. It should be noted that the unoriented HAP was obtained by grinding of the c-axis oriented HAP. The increase in surface area mainly arose from the exposure of the side crystal surface (i.e., a-surface) of the HAP material. The non-linear increase in surface area with the increase in catalytic activity, therefore, suggests that the crystal planes of the HAP material could have different activity in the methane combustion reaction. Table 2 also compares the surface composition (Ca/P ratio) of the oriented and unoriented HAP materials. It shows that the unoriented HAP has higher Ca/P ratio than the c-axis oriented one. In order to verify the effect of different Ca/P ratio of HAP material on the methane combustion reaction, we purposely synthesized the unoriented HAP sample with Ca/P ratio of 1.57 and tested its performance under the same reaction condition as what we have studied above. The results showed that methane conversion of 94% and CO2 selectivity of 62% was obtained, which is comparable to that of 98% methane conversion and 67% CO2 selectivity over the unoriented HAP with Ca/P ratio of 1.71.


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Therefore, the crystal plane of HAP catalyst should contribute to the different catalytic activity and selectivity in the methane combustion reaction. To distinguish the differences in catalytic behaviors of crystal planes in HAP, we further analyzed the data presented in Figure 4(A) by evaluating the areal reaction rate and selectivity on both a- and c-surfaces of HAP catalysts. Particularly, the (002) plane, which is the major exposed surface of oriented HAP catalyst film, is defined as c-surface of HAP catalyst. Other planes of HAP catalyst is denoted as a-surface, whose area was determined by deduction of surface area of oriented HAP from that of randomly oriented HAP catalyst in Table 2. Apparently, the c-surface of HAP catalyst highly favors CO2 formation compared to a-surface that promotes CO formation (Figure 4(B)). The areal rate of methane combustion over c-surface was 47 times higher than that of a-surface of HAP catalyst. The tremendous differences in areal rate and selectivity analysis indicates that the c-surface of HAP is more active in methane activation and favors complete combustion of methane to CO2 compared to other planes in HAP.

3.4.2 Performance of oriented HAP and Pb-HAP in OCM reactions. The catalytic activity and product selectivity of the c-axis oriented and unoriented HAP as well as Pb-HAP catalysts in OCM reactions were tested at 973 K, 101 kPa pressure, and at molar ratio of CH4/O2 ratio of 4. The stability of the oriented and unoriented HAP-based catalysts was first tested by running the reaction at 973 K for 10 hour and the results is shown in Figure S3 of the Supporting Information. Similar to methane combustion reaction, all the catalysts maintained their activity and stability in the OCM reaction conditions. Figure 5(A) shows the methane conversion and product selectivity in OCM reactions over these catalysts at time-on-stream of 6 hours. When the catalysts were ground to form irregular orientations, methane conversion was increased to ~11%


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ACS Catalysis

for both catalysts. Similar to that in the methane combustion reaction, unoriented HAP-based catalysts exhibited higher catalytic activity than c-axis oriented ones. Apparently, the enhancement in methane conversion is not linearly proportional to the increase in surface areas of these catalysts. This result confirms that the surface area is not the direct cause of the different catalytic performances between oriented and unoriented HAP catalysts. Table 2 shows that both oriented Pb-HAP and unoriented Pb-HAP catalysts have quite similar (Ca+Pb)/P ratios, which hints that the surface composition is not directly responsible for the different performances of both types of catalysts, either. Exclusively, it seems that the different crystal planes exposed on the HAP-based catalysts should again, contribute to different methane activation and product selectivity in OCM reactions. Figure 5(A) shows the product selectivity of the HAP-based catalysts at time-on-stream of 6 hours in OCM reactions. Substitution of Pb into the HAP structure improved C2 selectivity in both oriented and unoriented Pb-HAP samples78-80. The product selectivity data shows that the c-axis oriented HAP exhibited higher CO2 selectivity but lower CO, C2H4 and C2H6 selectivity than the unoriented HAP catalyst. Similarly, the c-axis oriented Pb-HAP demonstrated higher CO2 selectivity than the unoriented Pb-HAP catalyst. Consistent with those results obtained from methane combustion reaction, the c-surface of HAP crystals favors CO2 formation than the a-surface. To access the catalytic behavior of c- and a-surface of HAP-based catalyst, we further analyzed the areal rates and selectivity for OCM reactions over these catalysts, and the results are shown in Figure 5(B). Clearly, the c-surface of Pb-HAP showed the highest methane conversion rate in OCM reaction, followed by c-surface HAP. Methane conversion rate on c-surface was ~13 times higher than a-surface of Pb-HAP. On the other hand, methane conversion rate on csurface of HAP in OCM reaction was ~47 times higher than a-surface of HAP. The product


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selectivity analysis shows that c-surface of HAP and Pb-HAP catalysts favors CO2 formation while a-surface promotes CO production. The elevated methane conversion rates in both c-axis oriented HAP and Pb-HAP catalysts again suggest that (002) plane of the HAP-based catalysts promotes methane activation and complete oxidation to CO2 product. The stability, including crystallinity and structures, of the spent catalysts after methane combustion and OCM reactions were also studied using XRD, FT-IR and SEM measurements, and the results are shown in Section S3 of the Supporting Information. The measurements suggest that both oriented and unoriented HAP-based catalysts did not undergo significant phase change or morphological change after the reactions. 3.4.3 Functionality of crystal plane of HAP-based catalysts. As discussed in Section 3.1, the hexagonal HAP has prism-faceted a-surface and basal-faceted c-surface, which render it with anisotropic adsorption capabilities81-82. In the methane combustion and OCM reactions, the higher selectivity towards CO2 on the c-surface compared to a-surface of HAP-based catalysts indicates their anisotropic catalytic characteristics. The consistency between anisotropic adsorption and catalysis suggests that different intrinsic surface characteristics of HAP-based materials could be one of the causes for the anisotropic catalytic behaviors. The higher CO2 selectivity on the c-surface of HAP-based catalysts could be due to the preferential adsorption of O2 onto this surface, which subsequently promoted complete methane oxidation in both methane combustion and OCM reactions. Previous study83 has also shown that CO adsorbs stronger onto the c-surface than a-surface of HAP crystals due to different exposed surface termination groups in different surface facets of the HAP crystals. As an intermediate in methane oxidation to CO2, the adsorbed CO could be easily oxidized into CO2, which leads to higher selectivity to CO2 on the c-surface of HAP catalyst.


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ACS Catalysis

As a unique structural character, the hydroxyl ions of HAP are lined along the c-axis direction in the large channel of HAP materials. The anisotropic migration of OH- anions followed by dehydroxylation upon heating treatment could occur, as indicated previously by Wang et al. and Liu et al.

84-85

. The oxide ion and vacancies would be formed during the

dehydroxylation process, which can in turns function as active sites to accept O2 and activate O2 via the interface, i.e., c-surface, of HAP materials. Therefore, the existence and migration of these vacancy and oxide ion species along large channel of HAP and its c-surface could directly influence its performance in methane oxidation reactions. As a result, the c-surface has much higher activity that the a-surface of HAP, as well as higher CO2 selectivity. In contrast, the side planes of HAP-based materials (for example, the a-surface), disfavor oxygen migration and activation, and thus showing lower activity and making less CO2. A rigorous analysis on the kinetic behaviors of the c- and a-surfaces of the HAP-based catalysts was further carried out by employing the oriented and unoriented HAP and Pb-HAP catalysts in OCM reactions, as discussed below.

We further understand the acidity and basicity of the a- and c-surfaces of HAP and PbHAP from NH3-TPD and CO2-TPD measurements. As shown in Figure 6(A), both unoriented HAP and Pb-HAP have obvious NH3 desorption peaks in the temperature range 350 - 600 K, while the oriented HAP and Pb-HAP show very weak NH3 desorption peak in this temperature range. This data suggests the a-surfaces of HAP and Pb-HAP have stronger acidity than their csurfaces. The comparison between NH3-TPD profiles of unoriented HAP and Pb-HAP indicates that the former has a higher NH3 desorption temperature, which means a-surface of HAP has higher acidity than a-surface of Pb-HAP. The Pb-substitution in HAP decreased its acidity, consistent with previous studies78, 86. CO2-TPD profiles in Figure 6(B) illustrates that these four 21 ACS Paragon Plus Environment

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HAP-based catalysts have two CO2 desorption peaks: the low temperature peak in the temperature range 350-600 K and the high temperature peak > 600 K. The unoriented HAP and Pb-HAP have similar low temperature CO2 desorption peaks that are stronger than those of oriented HAP and Pb-HAP. This data hints that a-surfaces of HAP and Pb-HAP have more weak basic sites than their c-surfaces. The co-existence of acid and basic sites in HAP has been reported previously87-88. On the contrary, oriented HAP and Pb-HAP have stronger high temperature desorption peak than the unoriented counterparts, which suggests that c-surfaces of HAP and Pb-HAP have more strong basic sites. The slightly higher intensity in CO2-desorption peak at temperature range 600-800 K in the CO2-TPD profile of Pb-HAP than that of HAP corresponds to the higher basicity due to Pb substitution. In summary, the TPD measurements indicate a-surfaces of HAP and Pb-HAP have weak acid-base pairs, but c-surfaces of HAP and Pb-HAP have strong basic sites. The Pb-substitution leads to a decrease in acidity and increase in basicity in both a-surface and c-surface of HAP-based catalysts. It is reported that phosphate (PO43-) groups in HAP are responsible for the acidity of the catalyst, whereas the calcium ions (Ca2+) are responsible for the basicity89. The CO2 evolution peak in the temperature range 600-800 K in the CO2-TPD profiles can predominantly be linked to the reaction of CO2 with basic OH- groups (CO2 + 2OH- → CO32- + H2O)87. As discussed in Section 3.1 in the manuscript, the basal c-surface of HAP-based materials is rich in calcium ions, while the prismatic a-surface is rich in phosphate ions groups. The large channel of HAP confers mobility to hydroxyl ions and allows proton migration along the channels in the c-axis direction, especially under high temperature conditions. The higher acidity in a-surfaces of HAP and PbHAP can be attributed to the high density of phosphate groups. The higher basicity at elevated temperatures in c-surfaces of HAP and Pb-HAP, on the other hand, is due to the rich calcium 22 ACS Paragon Plus Environment

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ACS Catalysis

ions and basic OH- groups. The TPD data suggest that the basic sites of higher strength in oriented HAP and Pb-HAP might contribute to higher methane conversion rate of these two catalysts in OCM reaction while the strong acid sites in unoriented HAP and Pb-HAP might lead to low methane conversion rate in the same reaction. The selectivity to CO2 products in both methane combustion and OCM reactions can be linked to the high temperature basic sites as well, since higher basicity favors complete oxidation of CO into CO290.

3.4.4 Kinetics of c- and a-surfaces of Pb-HAP in OCM reaction. In this study, the kinetics of OCM reactions over oriented and unoriented Pb-HAP catalysts were rigorously analyzed by measuring methane consumption rates (r ) under various partial pressures of methane (P ) or oxygen (P ), respectively, at 973 K. The kinetic data for c-surface of Pb-HAP were determined from the oriented film catalyst directly; while the kinetics on the a-surface were evaluated from deduction of kinetic data of oriented Pb-HAP from those of unoriented catalysts. Eley-Rideal mechanism involving reaction between gaseous methane and adsorbed diatomic O2 species that was developed in our previous work91 is employed to accommodate the OCM kinetics over the c-surface and a-surface of Pb-HAP catalyst. The reaction steps involve quasi"#

equilibrated associative oxygen adsorption on surface site (∗) to form O!∗ species (O! +∗ ↔ O!∗ ). ()

This is followed by the rate limiting step (RLS,CH' + O∗! → CH+ ∙ +HO! ∙ + ∗) involving H abstraction from CH4 by O!∗ to form HO! ∙ and CH+ ∙ radicals. Under pseudo-steady state 12

assumption for O!∗ species, the rate law for methane consumption (r , μmol m1! s 12 ) is shown in Eq. (3): 45 #

2

= k + P 7K2 P 92:"

# ;<

=>

(3)


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where k3 (μmol [m]-2 s-1 kPa-1) is methane activation rate constant, K1 (kPa-1) is the equilibrium constant for the adsorption of oxygen, P (kPa) and P (kPa) are the partial pressures of methane and oxygen in the reaction, n2 (m2) is the surface area of the catalyst for O2 adsorption. When the CH4 pressure is fixed in the OCM reactions, the rate law for CH4 consumption in Eq. (3) can be linearized as, P n2 1 1 = + (4) k + K2 P k + r

where the slope is (

2

) "#

2

and the intercept is ( . The methane activation rate constant (k3) and )

adsorption equilibrium constant for oxygen (K1) were calculated from the intercept and slope in Eq. (4), respectively. Figure 7 shows that methane consumption rates depend on both P and P on either csurface or a-surface of the Pb-HAP catalysts. The rate of methane conversion increased with increasing P ' and P ! . A comparison across the c-surface and a-surface Pb-HAP catalysts showed that c-surface enabled much higher methane conversion than a-surface of Pb-HAP catalyst at the same P ' or P ! . Figure 8 shows the linearization treatment of Eq. (3) for extraction of kinetic parameters of the OCM reaction over c- and a-surfaces of Pb-HAP catalysts. Figure 8(A) shows the linearized correlations of

n1 ;45

45

versus

2

;<

. The linear fitting with

coefficient of determination (R2) >0.93, positive slope and intercept in each set of kinetic data support the Eley-Rideal mechanism for OCM reaction over the c- and a-surfaces of Pb-HAP catalysts. Figure 8(B) demonstrates the correlation between in which the slope is k + [K2 P (

2

2:"# ;

Effects of Controlled Crystalline Surface of Hydroxyapatite on Methane

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