Hydroxyapatite Foam as a Catalyst for Formaldehyde Combustion at. Room Temperature

Transcription

1 Supporting Information: Hydroxyapatite Foam as a Catalyst for Formaldehyde Combustion at Room Temperature Jing Xu, a Tim White, b Ping Li, a Chongheng He, a and Yi-Fan Han*,a a) State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai, , People s Republic of China; b) Centre for Advanced Microcopy, Australian National University, Canberra ACT * yifanhan@ecust.edu.cn. Tel: Fax: HAp structure Figure S1. Structure drawings of P63/m [Ca I 4Ca II 6](PO 4 ) 6 (OH) 2 hydroxyapatite. HAp foam with a Ca/P ratio of 1.67 and a surface area of 8.1 m 2 /g was prepared according to previous studies 6,S1 and comprehensively characterized using X-ray diffraction (XRD, Figure S2)). A clinographic projection of apatite in Figure S1 illustrates the spatial relationship between the framework 1

2 polyhedra and channel ions. The zeolitic [Ca I 4(PO 4 ) 6 ] 10- framework is constructed from face-sharing Ca I O 6 metaprisms (green polyhedra) and PO 4 tetrahedra (yellow polyhedra). The metaprism is a solid figure intermediate between an octahedron and trigonal prism. The one-dimensional channels are occupied by Ca II (grey cation) and OH (blue oxyanion) that combine as [Ca II 6(OH) 2 ] 10+ counterions. The framework is flexible, and during substitution of Ca, P, or OH by other species, the metaprisms twist cooperatively to dilate or constrict the channel according to the size and occupation factors of the substituents. 2. Characterization Powder X-ray diffraction (XRD): diffraction patterns was obtained using a Shimadzu diffractometer with CuKα radiation from specimens prepared by packing powder into a glass holder. The diffracted intensity was measured by step scanning over the 2θ range of 5 to 90. TOPAS refinement program was used to confirm the phase, composition and cell parameters of samples Intensity a.u Theta Figure S2. XRD pattern for HAP Surface area (BET): Specific surface area and nitrogen adsorption-desorption were determined 2

3 using an ASAP 2020 surface area and porosity analyzer (Micromeritics) via the Brunauer-Emmett-Teller (BET) method. A gaseous mixture of nitrogen and helium was allowed to flow through the analyzer at a constant rate of 30 ML/min. Nitrogen gas was used to calibrate the analyzer and also used as the adsorbate at liquid nitrogen temperature. The samples were thoroughly outgassed for 5 h at 423 K, prior to exposure to the adsorbent gas. Scanning Electronic Microscopy (SEM). SEM was performed on a JEOL JSM-6700F Field Emission SEM. Transmission Electron Microscopy (TEM). The catalyst powders were lightly ground, ultrasonically dispersed in ethanol, and several drops of the suspension loaded on holey carbon films supported by copper grids. Bright-field images were collected at 200 kev using a JEOL 2010 microscope fitted with an ultra-high resolution pole piece. 3. Calculations Absolute mass-specific reaction rates (eq.1) were calculated for the average concentration of each component, c& i, at the in- and outlet of the reactor. The conversion of HCHO was based on the CO 2 formation, the carbon balance is ca. 97%: r HCHO c& HCHO, in X HCHO V & gas = [moles s -1 g -1 Cat. ] (1) m Cat. where m cat. : mass of catalysts in the reactor bed, V & : total molar flow rate, X HCHO : conversion of HCHO based on CO 2 formation, c& HCHO : concentration of HCHO in gas mixture, equal to p i / p 0, p i : partial pressure of reactants, p 0 : total pressure in the system. 3

4 Sequentially, the reaction turnover rate for HCHO combustion ( r TOF, s -1 ) was obtained by eq. 2 r TOF r M D HCHO Pd / HAP = [s -1 ] (2) Pd / OH where M Pd : atomic weight of Pd or the weight of unit cell of Ca 10 (PO 4 ) 6 (OH) 2, D Pd : dispersion of Pd atoms exposure to reactants evaluated on the basis of the model of semi-hemispheric ball, about 20% dispersion of Pd was calculated according to an average size of 8.0 nm for Pd particles, which was measured by transmission electron microscopy (TEM, Tecnai TF 20 S-twin with Lorentz Lens); D OH : dispersion of OH in HAP = 2OH/(10Ca+6P+26O+2H) on the surface. Table S1. The mass-specific and TOF rates for both catalysts at 298 K. (0.14 kpa HCHO, 2.6 kpa H 2 O and air, GHSV: h -1 ). Catalysts Dispersion (%) Mass-Specific Rate TOF (s -1 ) (mol s -1 g -1 Pd/HAP) HAP 3.36 (OH groups) 2.92* *10-3 Pd(3wt%)/Al 2 O 3 20 (Pd particles) 1.35* * Experimental Section The hydroxyapatite (HAp) precursor was prepared via the addition of 3.6g Ca(NO 3 ) 2 (99.0%, Merck) and 1.2g (NH 4 ) 2 HPO 4 (99.0% purity, Aldrich) in the presence of 34 ml NH 4 OH and 145 ml H 2 O as described elsewhere. S2 Briefly, the mixture was magnetically stirred and heated to 363 K for 90 min then 373 K for 120 min. The precipitated HAp precursor was separated using vacuum filtration and washed several times prior to overnight drying at 373 K. A ceramic foam 4

5 of HAp was prepared by stirring 1g of methyl cellulose (Alfa Aesar) in 45 ml of deionized water with a stainless steel blade agitator at 20,000 rpm for 5 min. A mixture of 5 ml H 2 O with g HAp was added into polymer solution and stirred for 10 min. The product was calcined with a ramp rate of 0.7 K min -1 to 523 K, holding 30 min then increased to 923 K with a ramp rate of 3.3 K min -1 and held at this temperature for 300 min. The surface area of the produced HAP was 8.1 m 2 /g with pore volume of cm 3 /g. Palladium was introduced to the system as Pd (3 wt %) supported on a γ-al 2 O 3 powder (200 mesh) having a BET surface area of ca. 237 m 2 g -1. The catalyst was prepared by an incipient wetness method using an aqueous solution of Pd(NO 3 ) 2. The precursors were dried in an oven at 393 K overnight. HCHO combustion was carried out in a vertical fixed bed micro reactor (quartz reactor with 20 cm long and 0.4 cm in diameter) under atmospheric pressure. In a typical experiment, the catalyst is packed between two layers of quartz wool in the reactor. Ahead of the experiment, HAp was heated in an air flow of 20 ml/min at 673 K for 30 min; for the Pd catalyst, the precursor was pretreated in air (20 ml/min) at 673 K for 30min followed by a reduction in a H 2 flow (20 ml/min) at 573 K for 30 min. The reaction temperature was monitored by a thermocouple with its tip located in the catalyst bed and connected by a PID-type temperature indicator-controller. Catalytic tests were performed with a mixture of HCHO (1400 ppm) and air, with the former introduced by bubbling a HCHO (37 wt%, Fisher Scientific (A.P.)) water solution at 298 K. The gaseous products were analysed by online gas chromatography (Agilent 6890) equipped with a flame ionization detector (FID) using HP-5 capillary column, and a thermal conductivity detector, using 5

6 a Hayesep D column. For the detection of CO and CO 2, a methanizer was equipped. The methods for calculating the conversion of HCHO and the reaction rates are given in the section 3 in SI. The TPSR and TPD experiments were performed in the same reactor as used for activity tests, which was connected with a GC-QMS (HPR-20, Hiden Analytical Ltd.); masses (m/e: 2(H 2 ), 15(CH 4 ), 18(H 2 O), 25(C 2 H 2 ), 27(C 2 H 4 ), 28(CO), 29(HCHO), 30 (C 2 H 6 ), 31(CH 3 OH), 32(O 2 ), 44(CO 2 ), 45(HCOOH)) were monitored. After pretreatment, both catalysts were cooled down to room temperature in a He flow, and then purged with a O 2 flow (20 ml/min) for 2 h followed by a He (50 ml/min) for 5 min to expel residue oxygen from the pipelines. The TPSR experiment was carried out in a mixture of 0.14 kpa HCHO, 2.6 kpa H 2 O and He balance, with the total flow rate of 50 ml/min and a ramping rate of 20 K/min. HCHO was introduced into the system by the same method as described in activity tests. The temperature was ramped from 298 to 550 K at a rate of 20 K/min. CO x counts (a.u.) CO 2 CO Temperature (K) Figure S3. TPSR spectroscopy of HCHO on the reduced Pd surface. 6

7 CO 2 formation (a.u.) 376 A B Temperature (K) Figure S4. TPSR spectra of CO 2 formation obtained from the reaction between O 2 and the residues over the spent catalysts (A: HAP, B: Pd (3wt%)/γ-Al 2 O 3 ) from Figure 2b. The TPSR experiment was carried out in a mixture of 5.0 kpa O 2 and He balance, with the total flow rate of 50 ml/min and a ramping rate of 20 K/min. A gradual loss of activity was observed for both catalysts after running for 10 h under ambient conditions (Figure 2b). In a stream of oxygen, the spent HAp catalyst showed progressively generation of CO 2 beyond 425 K, while for Pd an intense CO 2 peak appeared at 376 K, rather than 384/424 K, in the pristine catalyst (Figure S4). Clearly, carbon-containing residues accumulate on the catalyst surfaces over time and are primarily responsible for deactivation by blocking the 7

8 active sites from adsorbing reactants. For the Pd catalyst the proximity of the CO 2 peaks in experiments in Figure 3 and S4 is consistent with self-poisoning resulting from adsorption of HCHO or its dissociative species. This phenomenon is often observed when oxidizing CO on precious metal catalysts at low temperatures. 11 In contrast, HAp, as a low-temperature combustion catalyst, will not encounter such problems in application; which is advantageous compared to Pd catalysts. It is also possible to almost fully restore both catalysts by 2 h burning in air (673 K). Reference: [S1] Kamitakahara, M.; Takahashi, A.; Tanihara, M.; Kawachi, G.; Ohtsuki, C. J. Cer. Soc. Jpn., 2007, 115, [S2] Nopphawan, P.; Zhong, Z.; Guo, J.; Han, Y.-F.; White, T. J. Gold Bulletin 2008, 41(1)