Interfacial bonding stabilizes rhodium and rhodium oxide nanoparticles on layered Nb- and Ta-oxide supports

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1 Interfacial bonding stabilizes rhodium and rhodium oxide nanoparticles on layered Nb- and Ta-oxide supports Megan E. Strayer, 1 Jason M. Binz, 2 Mihaela Tanase, 3,4 Seyed Mehdi Kamali Shahri, 2 Renu Sharma, 3 Robert M. Rioux, 2* and Thomas E. Mallouk 1* 1 Department of Chemistry and 2 Department of Chemical Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802; 3 Center for Nanoscale Science and Technology, National Institute of Science and Technology, Gaithersburg, Maryland 20899; 4 Institute for Research in Electronics and Applied Physics, University of Maryland, College Park, MD Supporting Information (6 pp.) Determination of enthalpy changes in the Born-Haber cycle from ITC experiments. Isothermal titration calorimetry (ITC) is a solution-based technique that is capable of measuring enthalpy changes in the sub mj range. When all species are in equilibrium, accurate ΔS and ΔG values can be obtained because the equilibrium constant, K, can be determined from the binding isotherm. In the experiments reported in this paper, only ΔΗ could be determined accurately because in most experiments, only the products or reactants were present in appreciable concentrations at equilibrium. In a typical ITC experiment, aliquots of a titrant solution are repeatedly injected into a suspension of basic nanosheets and a thermogram of heat flow (q) versus injection number can be plotted. By integrating the area under each peak in the thermogram, the differential heat production (kj mol -1 ) for each injection can be determined. For most of the conditions sampled, ΔH is coverage-independent, and therefore the peak areas of the injections are relatively constant at the beginning of the reaction and each one gives an independent value for ΔH of the overall reaction. The titrant used in these experiments is an aqueous rhodium trichloride hydrate solution. This solution hydrolyzes rapidly to yield the products shown in Reaction 0 and is allowed to thermally equilibrate before the ITC experiment begins. 1 RhCl 3 (H 2 O) 3 + H 2 O RhCl 2 (OH)(H 2 O) 3 + HCl (0) During the deposition of Rh(OH) 3 onto the suspended nanosheets, there are several reactions that occur in each injection, each with its own associated enthalpy change. The enthalpy change associated with the interaction of the rhodium hydroxide nanoparticles with the oxide nanosheets (ΔΗ 3 ) is not measured directly but is inferred from the measured heat of the overall reaction (ΔΗ 4 ) and the heats of hydrolysis of RhCl 2 (OH)(H 2 O) 3 (ΔΗ 1 ) and strong acidbase neutralization (ΔΗ 2 ). From Reaction 0, it is evident that RhCl 2 (OH)(H 2 O) 3 and HCl are injected into the nanosheet solution in equimolar amounts, where they react with excess TBA + OH -. RhCl 2 (OH)(H 2 O) 3(aq) + 2TBA + OH - (aq) Rh(OH) 3(s) +2TBA + Cl - + xh 2 O (1) HCl (aq) + TBA + OH - (aq) TBA + Cl - (aq) + H 2 O (l) (2) Rh(OH) 3(s) + TBA + / sheets - (s) Rh(OH) 3 /sheets - (s) + TBA + (aq) (3) RhCl 2 (OH)(H 2 O) 3(aq) + 3TBA + OH - (aq) +HCl (aq) + TBA + / sheets - (s) Rh(OH) 3 /sheets - (s) + 3 TBA + Cl - (aq) + xh 2 O (l) (4) ΔΗ 2 was measured by ITC titration of aqueous HCl into TBA + OH - solution. The value for ΔH 2 determined by ITC ((-58 ± 2) kj mol -1 ) matches well with the literature value (-57.3 kj mol -1 ) for the neutralization reaction at 25 C. 2 1

2 The value of ΔΗ 1 was found by ITC titration of 15 mmol L -1 RhCl 3 (H 2 O) 3 (= 15 mmol L -1 RhCl 2 (OH)(H 2 O) mmol L -1 HCl, according to Reaction 0) into a 3 mmol L -1 TBA + OH - solution to give ΔH 1 + ΔH 2 = (-85 ± 5) kj mol -1. Subtracting the value of ΔH 2, the heat of the alkaline RhCl 2 (OH)(H 2 O) 3 hydrolysis reaction (ΔH 1 ) was found to be (-27 ± 5) kj mol -1. Figure S1. Powder x-ray diffraction patterns of the proton-exchanged layered oxides that were exfoliated prior to Rh(OH) 3 deposition: HCa 2 Nb 3 O H 2 O, 3 K 1.1 H 2.9 Nb 6 O 17 nh 2 O 4 and Rb 0.1 H 0.9 TaO 3 1.3H 2 O. 5 HCa 2 Nb 3 O H 2 O and K 1.1 H 2.9 Nb 6 O 17 nh 2 O were determined to be phase pure, and Rb 0.1 H 0.9 TaO 3 1.3H 2 O has impurity peaks, marked with (*). These impurities were removed during the exfoliation process before deposition of Rh(OH) 3 by centrifugation of unexfoliated material. 2

3 Figure S2. Powder x-ray diffraction patterns of HCa 2 Nb 3 O H 2 O (black) and Rh(OH) 3 mass fraction of 0.01 (red) and 0.05 (blue) deposited on KCa 2 Nb 3 O 10. Figure S3. TEM images of a Rh(OH) 3 mass fraction of 0.05 deposited on (Left) γ-al 2 O 3 and (Right) SiO 2. 3

4 Figure S4. TEM image of a Rh(OH)3 mass fraction of 0.05 on Na-TSM after heating to 440 C in vacuum. Figure S5. (A) TEM image of a rhodium oxide nanoparticle on K4Nb6O17 at 750 oc under vacuum and (B) the resulting FFT which can be indexed to Rh2O3 [2 3 1]. The streaking in the FFT pattern is attributed to termination of the crystal periodicity at the particle edges. The FFT is in good agreement with the simulated electron diffraction pattern in (C). 4

5 Figure S6. TPRS for CO oxidation over Rh 2 O 3 /KCa 2 Nb 3 O 10 that was reduced at 600 C in hydrogen prior to the reaction. The light off curve was measured in a plug-flow reactor using a feed stream with a composition of ~7 kpa CO, 40 kpa Torr O 2 (balance He). The reaction was initiated at ambient temperature wth heating at 10 C/min to 400 C. Data are only plotted up to 350 C since 100% conversion of CO was observed by that temperature. Table S1. Number of measurements used to determine one standard deviation of the mean for each value represented in Figure 3A. Temperature ( C) Support KCa 2 Nb 3 O 10 N/A K 4 Nb 6 O Na-TSM 61 N/A N/A N/A N/A N/A N/A N/A N/A Table S2. Number of measurements used to determine one standard deviation of the mean for each value represented in Figure 4A. Support KCa 2 Nb 3 O 10 N/A K 4 Nb 6 O

6 Table S3. Tracking Rh 2 O 3 reduction to Rh(0) as a function of temperature using XANES analysis. Samples were heated ex situ in hydrogen (volume fraction of 0.035, balance nitrogen) at 100 kpa for 30 min prior to analysis at ambient conditions. The reported values correspond to percent of Rh(III) remaining on each oxide support at the indicated temperature. Temperature KCa 2 Nb 3 O 10 Na-TSM ± ± ± 2 18 ± ± 2 13 ± ± 2 14 ± ± 2 13 ± 2 References: 1. Cotton, F. A., Advanced inorganic chemistry. 6th ed.; Wiley: New York, 1999; p Pitzer, K. S. J. Am. Chem. Soc. 1937, 59, Chen, Y.; Zhao, X.; Ma, H.; Ma, S.; Huang, G.; Makita, Y.; Bai, X.; Yang, X. J. Solid State Chem. 2008, 181, Ma, R.; Kobayashi, Y.; Youngblood, W. J.; Mallouk, T. E. J. Mater. Chem. 2008, 18, Fukuda, K.; Nakai, I.; Ebina, Y.; Ma, R.; Sasaki, T. Inorg. Chem. 2007, 46,