SUPPLEMENTARY INFORMATION

Transcription

1 SUPPLEMENTARY INFORMATION Using first principles to predict bimetallic catalysts for the ammonia decomposition reaction Danielle A. Hansgen, Dionisios G. Vlachos, Jingguang G. Chen SUPPLEMENTARY INFORMATION. ADSORBATE CONFIGURATIONS a b c d e Supplementary Figure S. DFT adsorbate configurations. The configurations are at /9 (a), 2/9 (b), /3 (c), 2/3 (d), and (e) monolayer coverages. 2. HEATS OF CHEMISORPTION AS A FUNCTION OF COVERAGE Average binding energies were used in this study, although differential binding energies could also have been used for adsorption and desorption processes. The difference between the average and differential binding energies is small and only significant at high adsorbate coverages, accessed on the strong binding side of the volcano curve. Thus, the difference between the two binding energy definitions should not affect the volcano curve maximum. The nitrogen and hydrogen average binding energies were calculated as a fuction of coverage (Equation 3 of the main text), from /9 to monolayer. The binding energies at each coverage, with respect to the zero coverage binding energy, are shown in Figures S2 and S3, respectively. A nearly linear correlation was found for both nitrogen and hydrogen adsorbates. Although not completely linear, approximating the data with a linear function is adequate for trends and screening studies as performed here. The slopes of the lines and the binding energy extrapolated back to the zero coverage limit are shown in Table of the main article. The nearly linear correlation has also been observed for other surfaces, such as Au(), Pd(), and Pt() and other adsorbates including carbon, nitrogen and oxygen., 2 For these systems, Miller and Kitchin studied the electronic effects of the adsorbates and found that there is a broadening of the metal surface d-band causing the d-band center to shift and the binding energy to decrease. Due to the weak H-H interactions (Figure S3), the binary N-H interactions are also failry weak 3 and affect overall microkinetic model predictions only slightly (not shown). nature chemistry

2 0 BE N - BE No (kcal/mol) Co Ni Pd Pt Re Mo Nitrogen Coverage Supplementary Figure S2. Change in nitrogen binding energy as a function of coverage. The nitrogen binding energies were calculated at coverages ranging from /9 to ML and are compared to the binding energy extrapolated to zero coverage. The binding energies show a nearly linear dependence on coverage. 0.5 BE H - BE Ho (kcal/mol) Co Ni Pd Pt Hydrogen Coverage Supplementary Figure S3. Change in hydrogen binding energy as a function of coverage. The hydrogen binding energies were calculated at coverages ranging from /9 to ML and are compared to the binding energy extrapolated to zero coverage. The binding energies show a nearly linear dependence on coverage. 2 nature chemistry

3 supplementary information 3. MICROKINETIC MODEL SURFACE COVERAGES FOR EACH METAL AND SENSITIVITY ANALYSIS Pt Pd Ni Co Re Mo 0.8 Coverage (ML) N* H* QN / (kcal/mol) - Supplementary Figure S4. Surface coverages for each metal. Simulated surface coverages at the outlet of the reactor at 850 K obtained from the microkinetic models. Conversions in the reactor were the same as those from Figure from the main article. The coverages of NHx surface intermediates are less than ML for all surfaces. Note that the nitrogen coverage under (working) laboratory conditions increases with increasing conversion above the values indicated here (e.g., longer residence times) due to higher partial pressures of the products. Comment on sensitivity analysis As expected intuitively, it is straightforward to show that the Campbell degree of rate control, 4 defined as the normalized sensitivity coefficient of the reaction rate with respect to the rate constant, d ln r / d ln k, is mathematically equivalent to our definition of the normalized sensitivity coefficient based on perturbation of the rate with respect to the pre-exponential A, i.e., dlnr/dlnk = dlnr/dlna. An overall material balance indicates that the conversion in a tubular reactor is proportional to the reaction rate under differential conditions and to the average reaction rate under high conversions. Sensitivity analysis for different conversions (reactor lengths) indicate that the dominant chemistry does not significantly change with conversion for realistic reactor lengths. Thus, one can extend the conventional sensitivity analysis criterion from the reaction rate to the conversion. nature chemistry 3 3

4 4. OTHER VOLCANO CURVE DESCRIPTORS Other descriptors were also explored for this reaction. The experimental turn over frequencies and mincrokinetic model conversions were plotted against the NHx intermediate binding energies. Figures S5 show that alternative descriptors could be used for a reaction. This is an expected result because for ammonia, the binding energies of the intermediates are related to the nitrogen binding energy (they are dependent rather than independent variables). This relation can be rationalized through the bond-order conservation method 5 or linear correlations developed recently via DFT 6 where the nitrogen binding energy is used to calculate the binding energies for the NHx species. 00 Pt Pd Ni Co Re Mo 00 Pt Pd Ni Co Re Mo 00 Pt Pd Ni Co Re Mo Conversion (%) TOF (S - ) QNH3 (kcal/mol) QNH2 (kcal/mol) QNH (kcal/mol) Supplementary Figure S5. Additional descriptors for the ammonia decomposition reaction. The binding energies of NH 3, NH 2, and NH were also explored as descriptors for this reaction. 5. TPD RESULTS OF AMMONIA DESORPTION Ammonia was dosed at low temperatures to determine the binding energy of the ammonia molecule and to examine how the binding energy is modified by the different bimetallic surfaces. Figures S6(a) and (b) show the TPD spectra after dosing 0.3 L ammonia on each surface at 5 K. The m/z=6 amu was used for molecular ammonia desorption to reduce the overlap with the contribution from residual background water desorption, which has a strong signal in m/z=7 amu. The low temperature peak at 97 K on each surface is due to the desorption of multilayer NH 3. The Pt() surface has the highest desorption peak temperature at 32 K, followed by Ni-Pt-Pt at 305 K, and the thick Ni() film at 270 K. The Pt-Ni-Pt surface has the lowest desorption peak temperature, 4 nature chemistry

5 although the actual peak temperature is difficult to discern since there is significant overlap with the multilayer peak. To separate the first and multilayer peaks, a curve fitting program was used to fit the two peaks, resulting in an estimated desorption peak of 260 K. The Redhead equation was used to calculate the binding energies from the peak desoption temperatures, assuming unactivated adsorption and a desorption pre-exponential of 8.x0 s -, which was the literature value 3 used in the microkinetic models. The calculated binding energies were 7.7 kcal/mol for Pt(), 7.3 kcal/mol for Ni-Pt-Pt, 5.3 kcal/mol for thick Ni() and 4.8 kcal/mol for Pt-Ni-Pt. The binding energies of ammonia for the four surfaces were also calculated through DFT at /9 ML and compare well to the TPD results. The binding energies were calculated to be 2. kcal/mol for Pt(), 25.4 kcal/mol for Ni-Pt-Pt, 6.7 kcal/mol for Ni(), and 5.2 kcal/mol for Pt-Ni-Pt. Both the DFT binding energies and TPD peak desorption values are consistent with Pt() > Ni() > Pt-Ni- Pt. Based on DFT, the Ni-Pt-Pt surface configuration is expected to have the highest desorption peak temperature (highest binding energy), although this is not seen in Figure S6(a) due to the decomposition of strongly-adsorbed NH 3 to produce atomic nitrogen as shown in Figure S6(b). 97K 270K NH3 (6 amu) N2 (4 amu) 305K Ni() Film Ni() Film Intensity (arb. units) 32K Ni-Pt-Pt Pt-Ni-Pt Intensity (arb. units) Ni-Pt-Pt Pt-Ni-Pt 629K Pt() Pt() Temperature (K) Temperature (K) Supplementary Figure S6. Ammonia desorption spectra for Pt(), Pt-Ni-Pt, Ni-Pt-Pt, and a thick Ni() film. Spectra (a) shows 6 amu, representing molecular ammonia desorption and spectra (b) shows the desorption of molecular nitrogen which is evidence of ammonia decomposition. nature chemistry 5

6 6. VIBRATIONAL STUDIES OF AMMONIA DECOMPOSITION HREELS experiments were performed to further characterize the decomposition of ammonia on the Ni-Pt-Pt surface. As shown in Figure S7(a), an HREEL spectrum was recorded after dosing 3L of NH 3 at 68 K and then heating to 236 K to remove multilayer ammonia. This surface was further heated to 350 K, Figure S7(b). Additionally, 3L of ammonia was dosed on Ni-Pt-Pt at 350 K, Figure S7(c), which was a temperature where significant decomposition was observed in the TPD experiments. Due to the very reactive nature of the Ni-Pt-Pt surface, the vibrational spectra are complicated by the adsorption of CO from the UHV background (v(ni-c) at 440 cm - and v(co) at 759, 975 and 2029 cm - ), as well as the surface hydroxyl group from the reaction with the background H 2 O (v(oh) at 3450 and 3585 cm - ). 7 The adsorbed CO and OH species both have a strong dynamic dipole moment, resulting in intense vibrational features as compared to those of NH 3. Nevertheless, the HREELS results in Figure S7 provide useful complementary information regarding the decomposition of NH 3 on the Ni-Pt-Pt surface. At 236 K, vibrational features characteristic of ammonia are seen, including the symmetric (δ s (NH 3 )) and asymmetric (δ a (NH 3 )) umbrella deformation modes at 238 cm - and 576 cm -, respectively, as well as the symmetric stretching mode (υ s (NH 3 )) at 3233 cm - and the asymmetric stretching mode (υ a (NH 3 )) at 3342 cm -. The peak at 629 cm - was assigned to the ammonia rocking mode (ρ(nh 3 )) and the peak at 440 cm - is likely a combination of the M-C and M-N vibrations from CO and NH 3, respectively. The peaks at 2922 cm - and 434 cm - have been attributed to the stretching and deformation mode of background CHx species from the UHV background, based on previous studies on Ni-Pt-Pt. 7 Table S summarizes the assignments of NH 3 and additional possible intermediates. Upon heating, most of the ammonia has desorbed by 350 K (from TPD, Figure S6(a)) and this is evident in the HREEL spectrum, Figure S7(b), with a significant decrease in the ammonia umbrella deformation and stretching modes (238 cm -, 576 cm -, 3233 cm -, 3342 cm - ). The NH 3 rocking mode at 629 cm - decreases and becomes a shoulder on a peak at 704 cm -. The lack of the characteristic NH rocking mode at ~350 cm - indicates an absence of the NH intermediate at this temperature. The 704 cm - peak is therefore assigned to the r (NH 2 ) mode. The presence of NH 2 intermediate is consistent with NH 2 dehydrogenation being a kinetically significant step. When dosing 3L NH 3 on a clean Ni-Pt-Pt surface at 350 K, the resulting vibrational features are very similar to those obtained by dosing at low temperatures followed by heating to 350 K, indicating that the same intermediates are formed. Based on the TPD experiments where the dosing temperature is changed (Figure 4), it would be expected that the amount of atomic nitrogen or the NHx intermediate would increase, although this is difficult to conclude from the HREEL spectra due to the less quantitative nature of the HREELS measurements. The most important observation from Figure S7 is 6 nature chemistry

7 supplementary information that, in addition to adsorbed NH 3, partially decomposed intermediates including NH 2 and atomic nitrogen, are produced on Ni-Pt-Pt at 350 K. The presence of the latter is suggested based on the detection of the υ(m-n) mode of atomic nitrogen at 494 cm -, as summarized in Table S. These results provide complementary information supporting the TPD detection of the N 2 gas-phase product from the decomposition of ammonia on the Ni-Pt-Pt surface. ( ) Intensity (arb. units) x80 x (c) (b) x80 (a) Energy Loss (cm - ) Supplementary Figure S7. HREELS spectra of ammonia decomposition. Obtained after of 3L ammonia was dosed at 68 K on the Ni-Pt-Pt surface and heated to (a) 236 K and then (b) 350 K and (c) 3L dosed at 350 K. nature chemistry 7

8 Supplementary Table S. Vibrational assignments for adsorbed NH x species on various surfaces. Ammonia HREELS assignments, frequency (cm - ) Mode Ni-Pt-Pt (000) (-2) 8 Pt() 9 NH 3 υ(m-nh 3 ) ρ (M-NH 3 ) δs(nh 3 ) δa(nh 3 ) υs(n-h) υa(n-h) ΝΗ 2 υ(m-nh 2 ) ρ r (NH 2 ) δw(nh 2 ) 392 δsc(nh 2 ) υs(n-h) ^ υa(n-h) ^ 3379 NH υ(m-nh) 570 / , 78 ρ (N-H) 360 / υ (N-H) 3360 / N υ(m-n) , 468, 492, 556, 605 backgound CO 2029 (bridge) 759 (linear) (M-C) OH ^ unresolved symmetric and asymmetric modes nature chemistry

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