Selective hydrogenation of 1,3-butadiene on platinum–copper alloys at the single-atom limit

Platinum is ubiquitous in the production sectors of chemicals and fuels; however, its scarcity in nature and high price will limit future proliferation of platinum-catalysed reactions. One promising approach to conserve platinum involves understanding the smallest number of platinum atoms needed to catalyse a reaction, then designing catalysts with the minimal platinum ensembles. Here we design and test a new generation of platinum–copper nanoparticle catalysts for the selective hydrogenation of 1,3-butadiene,, an industrially important reaction. Isolated platinum atom geometries enable hydrogen activation and spillover but are incapable of C–C bond scission that leads to loss of selectivity and catalyst deactivation. γ-Alumina-supported single-atom alloy nanoparticle catalysts with <1 platinum atom per 100 copper atoms are found to exhibit high activity and selectivity for butadiene hydrogenation to butenes under mild conditions, demonstrating transferability from the model study to the catalytic reaction under practical conditions.


Supplementary Notes Supplementary Note 1: Carbon monoxide desorption from Pt/Cu single atom alloys
Supplementary Figure 1 shows TPD traces of CO from 0.02 ML Pt/Cu(111) surface saturated with CO (10 L). The 4 peaks <250 K correspond to the 3 packing structures of CO on Cu(111) and CO desorption from CO steps. [4][5][6] After deposition of Pt, an additional peak is seen for CO desorbing from the Pt sites at 350 K. The desorption of CO from isolated Pt sites is ~100 K lower than the desorption observed from Pt(111) (450 K) demonstrating the reduced binding strength of CO to single atom alloys. Pt atoms do not induce decomposition of butadiene or self-hydrogenation since we do not observe any desorption features for H 2 after the adsorption of butadiene on 0.02 ML Pt/Cu(111). The small amount of butenes desorbing at 230 K arises from a minor impurity in the butadiene sample.
In order to further investigate the ability of Pt/Cu(111) to hydrogenate butenes, the 0.02 ML Pt/Cu(111) surface was exposed to 50 L H 2 and 0.1 L 1-butene (Supplementary Figure 5). No hydrogenation products were observed.

Supplementary Note 3: Nanoparticle catalyst characterization
As described in Boucher et al., 1 the Cu NPs were prepared without a support. The average particle size of colloidal Cu NPs is 4.24 ± 1.00 nm with a narrow particle size distribution (Supplementary  11 The Cu NPs were shortly exposed to the air when transferred from the reduction furnace (100 % H 2 , 300 °C) to GR solution. The surface CuO layer that would otherwise impede the GR process was removed by the acid in the solution.
XRD patterns of selected samples are shown in Supplementary Figure 9. This suggests the addition of small amounts of Pt did not change the lattice structure of Cu, which is consistent with the lattice spacing determined by STEM ( Figure 5 Table 2). We conclude that the Pt/Cu bimetallic NPs were formed without creating Pt monometallic NPs, which is consistent with the exchange of Pt and Cu atoms in the GR reaction.
The surface composition of Pt/Cu alloys is affected by the gaseous atmosphere, as expected on the basis of the total free energy minimization. 12 Pt is preferentially located at the surface layer of Pt/Cu alloy in H 2 atmospheres, which agrees with the reports that annealing Cu coated Pt(111) in H 2 /Ar atmosphere at 400 °C for 2 min led to segregation of Pt that the surface was fully covered by Pt. 13 Since the Pt/Cu catalysts in this work were reduced in 100% H 2 at high temperatures prior to the hydrogenation reaction in a hydrogen-rich gas mixture, we expect the Pt species to be distributed on the surface of the NPs.

Supplementary Note 4: Selective hydrogenation of butadiene
The selective hydrogenation activity of the catalysts was studied at near-ambient temperature using lower space velocity, as shown in Figure 6. Under the same reaction conditions, the Pt/Cu SAAs samples were tested in consecutive ascending and descending temperature cycles. Supplementary Figure 17 shows the conversion of butadiene in a cycle over Pt 0.1 Cu 14 /Al 2 O 3 . The comparison of conversion at 150 °C and 120 °C between the increasing and decreasing temperature ramps shows that the hydrogenation activity was recovered during the decreasing temperature ramp. But at near ambient temperatures, the activity in the cooling part of the cycle is lower. The same was true with all of the Pt/Cu alloyed samples. The results suggest the hydrogenation activity was reduced at lower temperatures but recovered at higher temperatures.
Considering that oligomer formation adversely affects the metal-based catalysts in the hydrogenation of hydrocarbons at mild temperatures, 18 we hypothesize the oligomers partially blocked the surface sites at lower temperature and desorbed with increasing temperature.
To check this, after the first temperature heating-cooling cycle (Supplementary Figure 17A), the catalyst was treated at 350 °C in H 2 to fully desorb the weakly adsorbed hydrocarbons on the surface.
We found the selective hydrogenation activity was fully recovered after the high temperature treatment  Table S1. Extended X-ray absorption fine structure (EXAFS) was measured in H 2 . The spectra were recorded in the fluorescence mode at room temperature.
XAS data were processed and analyzed using Athena and Artemis. 2 The XANES spectra were background corrected and normalized. And the EXAFS data were fitted in r-space with the models based on metallic Pt, PtCu 3, and PtO. 3 The hydrogenation activity of the catalysts was tested in a packed-bed flow microreactor (L=22 inch, O.D.=1/2 inch) with 100 mg of catalyst diluted by 0.5 g of quartz particles. The samples were prereduced in 100% H 2 at 25 ml/min (300 °C for Cu 15  The activity tests with added propylene were conducted in a similar fashion with a gas mixture of 2 % 1,3-butadiene, 20 % propylene, 16 % H 2 and balance He. The total flow rate was 50 ml/min, and the catalyst load was 100mg (GHSV=12,000 h -1 ). The steady-state stability tests were conducted isothermally at 160 °C and 145 °C, for 12 h at each temperature. The exit gas stream composition was determined by GC as described above.
Cyclic hydrogenation tests were performed to probe the stability of the catalysts. The first cycle was performed as described for the activity test. Then the sample was treated in 100 % H 2 at 350 °C for 1 h. The reaction gas mixture (2 % 1,3-butadiene, 20 % H 2 and balance He. GHSV=12,000 h -1 ) was introduced at 170 °C followed by cooling down to other temperatures. GC injections were done at specified temperatures after the temperature was held stable for at least 10 min.
Temperature-programmed oxidation (TPO) was conducted in a Micromeritics AutoChem II 2920 instrument equipped with a mass spectrometer. The spent catalyst (33 mg) used in the long-time stability testing with or without addition of propylene was loaded to the microreactor for the TPO test.
The sample was degassed with He at 10 °C/min up to 100 °C, and cooled to 25 °C in He. Next, 20 % O 2 in He was introduced. The sample was stabilized at 25 °C for 5 min followed by raising the temperature to 800 °C at 3 °C/min. The CO 2 in the gas stream was analyzed online by a mass spectrometer (Pfeiffer, TCP270). Temperature programmed desorption in He (He-TPD) was performed with a similar setup. The spent catalyst (33 mg) from the long-time stability testing with or without addition of propylene was loaded to the micro reactor for the He-TPD test. After degassing in pure He flow (50 ml/min) for 20 min at ambient temperature, the sample was heated in the He flow (50 ml/min) to 800 °C at 3 °C/min. The exit gas stream was analyzed online by a mass spectrometer in histogram-spectrum mode scanning mass-to-charge ratio (m/z) between 1 and 120.