Bottom-Up Synthesis of Platinum Dual-Atom Catalysts on Cerium Oxide

We present here the synthesis and performance of dual-atom catalysts (DACs), analogous to well-known single-atom catalysts (SACs). DACs feature sites containing pairs of metal atoms and can outperform SACs due to their additional binding possibilities. Yet quantifying the improved catalytic activity in terms of proximity effects remains difficult, as it requires both high-resolution kinetic data and an understanding of the reaction pathways. Here, we use an automated bubble counter setup for comparing the catalytic performance of ceria-supported platinum SACs and DACs in ammonia borane hydrolysis. The catalysts were synthesized by wet impregnation and characterized using SEM, HAADF-STEM, XRD, XPS, and CO-DRIFTS. High-precision kinetic studies of ammonia borane hydrolysis in the presence of SACs show two temperature-dependent regions, with a transition point at 43 °C. Conversely, the DACs show only one regime. We show that this is because DACs preorganize both ammonia borane and water at the dual-atom active site. The additional proximal Pt atom improves the reaction rate 3-fold and enables faster reactions at lower temperatures. We suggest that the DACs enable the activation of the water–O–H bond as well as increase the hydrogen spillover effect due to the adjacent Pt site. Interestingly, using ammonia borane hydrolysis as a benchmark reaction gives further insight into hydrogen spillover mechanisms, above what is known from the CO oxidation studies.


characterization
The calculation of the interatomic distance is done according to the following formula: - () =    ( 2 /) ℎ  (%) 100 (%) * 1   (/) *   Where D M-M is the distance of two platinum metals in the SAC, S BET is the BET surface area, MW Pt is the weight of one mol of platinum (197 g/mol) and N AV is the Avogadro constant.Figure S8 -XP survey spectrum of the SAC (above) and DAC (below) and zoom in at the M-Cl and M-I peaks at the corresponding binding energies.No deposition of both Cl -and I -was observed in the SAC and DAC, respectively, by comparing the reference binding energies of metal iodides 2 and metal chlorides 3 .The peak at 207 eV is the electron in the Ce 4p 3/2 orbital.
Figure S9 -Fitted XAS spectra of polycrystalline CeO 2 (black) and cubic CeO 2 (red).The inserts are the fits of the two components in each peak of the respective peak.In Figure S10 we show the presence of the Ce 4+ -O 2--Ce 3+ site at a g-value of 1.96. 4  , which is not present on the CeO 2 particles before the reaction (c.f. Figure S1).

Calculation of occupation of SAC and DAC sites
Given the experimental conditions mentioned in the experimental details, 8. Then calculating the ammonia borane occupation was 1/580 * 100% = 0.17% and the adsorption of ammonia borane at DAC pair is 0.17%*2 = 0.34%.

Figure S6 -
Figure S6 -Fitted CO-DRIFTS spectra of the Pt SAC (above) and DAC (below).Both are fitted with a single component.

Figure
Figure S7 -HAADF-STEM images of the as-synthesized Pt 1 and Pt 2 CeO 2 cubes.No clustering or isolated atoms were observed.

Figure S10 -
Figure S10 -EPR measurements on the cerium oxide support.

Figure
Figure S11 -AC-HR-TEM images of the SAC (a) and DAC (b) without the rectangles as presented in the main text.

Figure
Figure S15 -(a) IR-spectra of NH 3 BH 3 , the spent catalyst powder and NH 4 BO 2 showing deposition of NH 4 BO 2 on the catalysts.(b) CO-DRIFTS spectra after the reaction showing no detectable Pt-species.(c) SEM-image of the spent catalyst showing that the CeO 2 cube retained its structure during catalysis.Notice the surface deposition around the particle (see zoom-in in (d)), which is not present on the CeO 2 particles before the reaction (c.f.FigureS1).

Figure S16 -
Figure S16 -Fittings of the ammonia borane hydrolysis experiments indicating the slope and intercept of the SAC (a) and the DAC (b).Note that the slope of the fit of the double-atom catalysed reaction is slightly lower and the pre-exponential factor is 4 times higher (after taking the exponential as the intercept is the natural logarithm of the pre-exponential factor).

Figure S17 -
Figure S17 -Experiments of the added salt to either the SAC (a) or the DAC (b) in a 1:2 Pt:X ratio.Note the very similar performance for the SAC.For the DAC a water bubble from the solution entered the detection cell due to evaporation at T = 47 °C, which affected the bubble formation.

Figure S18 -
Figure S18 -Optimized structure of the Pt x -CeO 2 models (a,b; top view -e,f; side view) and models where CO (yellow carbon atom and red oxygen atom) is adsorbed on the platinum (purple) site (c,d; top view -g,h; side view) of the SAC and DAC, respectively.