Oxidative dehydrogenation of cyclohexene on atomically precise subnanometer Cu 4-n Pd n (0 ≤ n ≤ 4) tetramer clusters: The effect of cluster composition and support on performance.

The pronounced effects of the composition of four-atom monometallic Cu and Pd and bimetallic CuPd clusters and the support on the catalytic activity and selectivity in the oxidative dehydrogenation of cyclohexene are reported. The ultra-nanocrystalline diamond supported clusters are highly active and dominantly produce benzene; some of the mixed clusters also produce cyclohexadiene, which are all clusters with a much suppressed combustion channel. The also highly active TiO2-supported tetramers solely produce benzene, without any combustion to CO2. The selectivity of the zirconia-supported mixed CuPd clusters and the monometallic Cu cluster is entirely different; though they are less active in comparison to clusters with other supports, these clusters produce significant fractions of cyclohexadiene, with their selectivity towards cyclohexadiene gradually increasing with the increasing number of copper atoms in the cluster, reaching about 50% for Cu3Pd1. The zirconia-supported copper tetramer stands out from among all the other tetramers in this reaction, with a selectivity towards cyclohexadiene of 70%, which far exceeds those of all the other cluster-support combinations. The findings from this study indicate a positive effect of copper on the stability of the mixed tetramers and potential new ways of fine-tuning catalyst performance by controlling the composition of the active site and via cluster-support interactions in complex oxidative reactions under the suppression of the undesired combustion of the feed.


Figure S1
Mass spectra with output of the cluster apparatus optimized for Cu4 + production

Figure S2
Mass spectra of CumPdn + clusters

Figure S3
Mass spectra with output of the cluster apparatus optimized for Pd4 + production

Figure S4
Temperature ramp used in catalyst testing and typical reactivity data shown on the example of Pd4/ZrO2

Figure S5
Mass spectra of effluent gas from the reactor and of calibration gases

Figure S6
Comparison of measured mass spectra for Pd4/ZrO2 and ZrO2 blank

Table S1
Summary of total benzene production rates for tetramers  Figure S1. Mass spectra of the molecular beam of pure Cu + clusters produced by cluster source 1 and optimized to obtain the highest flux of clusters Cu4 clusters for deposition recorded: (a) at higher resolution with the insert showing the resolved distribution of isotopes of Cu4 (black line) compared with the distribution of isotopes calculated by isotope calculator using natural abundance of copper isotopes (red line, https://www.sisweb.com/mstools/isotope.htm). (b) at lowered resolution for deposition to increase the flux of deposited clusters. With Ar atom(s) tagged Cun + clusters which also form in the magnetron source are assigned as well. Figure S2. Mass spectra of CumPdn + clusters produced by cluster source 2 and recorded: (a) at higher resolution with the insert showing resolved distribution of isotopes (black line) compared with the distribution of isotopes calculated by isotope calculator (red line) using natural abundance of copper and palladium isotopes. (b) at lowered resolution for deposition to increase the flux of deposited clusters. Figure S3. Mass spectra of pure Pd + clusters produced by cluster source 2 and optimized for the deposition of Pd4, recorded: (a) at higher resolution with the insert showing resolved distribution of isotopes (black line) compared with the distribution of isotopes calculated by isotope calculator (red line) using natural abundance of copper isotopes. (b) at lowered resolution for deposition to increase flux of deposited clusters. The unlabeled peaks in the mass spectrum corresponds to Arm + ions; the peaks corresponding to Ar atom(s) tagged Pdn + clusters are labeled in the plot as well. Figure S4. Top: temperature ramp sequence consisting of two temperature ramps 1 st -"short" (all temperature steps with dwell time of 20 min) and 2 nd "long" (with dwell time 120 min at 400 °C) is shown in upper part of figure. For 0.5 hour, a constant flow of helium (5 sccm) was maintained through the reaction cell at 800 Torr at 25 °C. After 0.5 hour of flushing with pure helium, the gas was switched to the reactant mixture (maintaining the 800 Torr pressure and 17.5 sccm flowrate) and sampling of gas to the mass spectrometer started. The flow reactant mixture was kept for 6 hours to stabilize the background in the mass spectrometer before the start of the temperature ramp. The reactant mixture consisted of 0.29 % cyclohexene and 0.29 % oxygen in helium (i.e., a 1:1 cyclohexene to oxygen molar ratio), obtained by mixing 12.50 sccm of 4000 ppm cyclohexene in helium with 5.00 sccm of 1.0 % oxygen diluted in helium. Bottom: Typical evolution of the massspec signal of reactants and identified products during the double temperature ramp showed on the example of zirconia supported Pd4 clusters (water, oxygen, carbon dioxide, cyclohexene, cyclohexadiene and benzene, with their corresponding mass to charge ratio m/z showed in the plot). Figure S5. Electron impact ionization mass spectra of the effluent gas from the reactor acquired during TPR of Pd4/UNCD sample (a) at 25 ˚C before start of the temperature ramp, (b) at 400 ˚C during the 2 nd temperature ramp. Mass spectra of (c) cyclohexene, (d) benzene and (e) cyclohexadiene (mean of 1,3-and 1,4-cyclohexadiene measurements) calibration gases. Mass peaks considered in the model for fitting the concentrations of cyclohexene reactant and benzene and cyclohexadiene products (m/z 67, 77, 78, 79 and 80) are marked by blue lines. Figure S6. (a) Temperature ramp. Comparison of processed mass spectra of Pd4/ZrO2 and blank reference. Normalized concentration of (b) cyclohexene, (c) benzene, (d)cyclohexadiene, (e) CO2 and (f) sum of the concentrations obtained from fitting of mass spectra acquired during the first (short) ramp in TPR proves that no product is missing. The two peaks at the first two temperature increments are caused by temperature desorption of cyclohexene, while the two peaks with negative values at the end of the ramp are caused by adsorption or the reactant. Values are normalized for the concentration of cyclohexene at 25 °C. Figure S7. AFM images of UNCD layer on Si substrate measured in tapping mode. Comparison of images of UNCD layer before reaction a) topography (1 x 1 µm 2 ) with mean roughness Ra = 7.5 nm, b) topography (0.5 x 0.5 µm 2 ) and c) phase image (0.5 x 0.5 µm 2 ) with Cu2Pd2 on UNCD after reaction d) topography (1 x 1 µm 2 ) with mean roughness Ra = 6.8 nm, e) topography (0.5 x 0.5 µm 2 ) and f) phase image (0.5 x 0.5 µm 2 ). The phase imaging highlights the details of their microstructures, including nanograin boundaries, which are not well resolved by topographic imaging.    Figure S12. Per total atom carbon-based rate of Pdx on ZrO2 of (a) benzene -short, (b) benzenelong, (c) cyclohexadiene -short, (d) cyclohexadiene -long, (e) CO2 -short, (f) CO2 -long; Carbonbased selectivity for benzene, cyclohexadiene and CO2 at 400 °C during (g) short ramp and (h) long ramp. Figure S13. XPS spectra of the Pd 3d region for Pd1 on ZrO2 before (top) and after (bottom) reaction are shown. The Pd 3d region strongly overlaps with Zr 3p, which complicates the deconvolution of the spectra as well as the quantitative assignment of Pd oxidation states. Between the Zr 3p peaks the Pd 3d5/2 and Pd 3d3/2 peaks can be found. For fresh sample Pd 3d5/2 reveals a binding energy of 338.0 eV. After TPR, two Pd doublets can be found with Pd 3d5/2 binding energies of 337.1 and 338.5 eV. (e-f): benzene formation during the second double ramp. (g): "short" and "long" temperature ramps, overlapped; (h): benzene rates from both double ramps, overlapped. Between the two double ramps, the sample was not exposed to air, it was kept it in the reactor under the stream of reactant mixture for 6 hours. Table S1. Total benzene production rates for tetramers Table S2. Total benzene production rates for Pd1 and Pd2