Supporting Information Phase coexistence of multiple copper oxides on AgCu catalysts during ethylene epoxidation Mark T. Greiner,*1,2 Jing Cao,1 Travis E. Jones,1 Sebastian Beeg,2 Katarzyna Skorupska,1 Emilia A. Carbonio,1,3 Hikmet Sezen,3,4 Matteo Amati,4 Luca Gregoratti,4 Marc-George Willinger,1 Axel KnopGericke,1 Robert Schlögl1,2 *
[email protected] 1
Fritz-Haber Institute of the Max-Planck Society, Department of Inorganic Chemistry, Faradayweg 46, 14195, Berlin, Germany 2
Max-Planck Institute for Chemical Energy Conversion, Department of Heterogeneous Reactions, Stiftstrasse 34-36, 45413, Mülheim an der Ruhr, Germany 3
Helmholz-Zentrum Berlin für Materialien und Energie, Albert-Einstein-Strasse 15, 12489, Berlin, Germany 4
Elettra-Sincrotrone Trieste, Strada Statale 14, 34149, Basovizza, Italy
Text from Video1: This video shows ESEM images of an AgCu foam containing a dilute (ca. 1%) concentration of Cu, while being heated in 0.3 mbar of 1:1 O2 and C2H4 at 350°C. Each frame represents 53 seconds of realtime.
1
Figure S1. EDX Maps of the diluted AgCu alloy after 5 hours under epoxidation conditions (0.3 mbar of 1:1 O 2 and C2H4 at 350°C)
Text from Video2: This video shows ESEM images of an AgCu foam containing a high (ca. 7%) concentration of Cu, while being heated in 0.3 mbar of 1:1 O2 and C2H4 at 350°C. Each frame represents 53 seconds of realtime.
Figure S2. EDX Maps of the concentrated AgCu alloy after 7 hours under epoxidation conditions (0.3 mbar of 1:1 O2 and C2H4 at 350°C)
2
Figure S3. Photoemission spectra measured with NAP-XPS of AgCu (0.5% Cu) under epoxidation conditions (0.3 mbar, 1:1 O2:C2H4, 250°C). a) O1s spectra measured using a photon energy of 700 eV, b) valence spectra measured using a photon energy of 150 eV, c) valence spectra measured using a photon energy of 100 eV, d) Ag3d5/2 spectra measured using a photon energy of 520 eV, and e) Cu2p 3/2 spectra measured using a photon energy of 1486.7 eV. The time off-set between spectra in a stack is 45 minutes.
3
Figure S4. Photoemission spectra measured with NAP-XPS of AgCu (0.5% Cu) under epoxidation conditions (0.3 mbar, 250°C) and dilute O2 concentrations or 0, 0.2, 0.4, 0.8 1.6, 3.2 and 12 %. a) Cu2p3/2 spectra measured using a photon energy of 1053 eV, b) O1s spectra measured using a photon energy of 700 eV, c) valence spectra measured using a photon energy of 150 eV, d) valence spectra measured using a photon energy of 100 eV and e) valence spectra measured using a photon energy of 700 eV. From the Cu2p3/2 spectrum, one can see that no Cu2+ satellite is present. From the O1s spectrum one finds a binding energy of 529.5 eV (similar to the O1s binding energy of O species on O-terminated Cu). The valence spectra at 100 eV and 150 eV photon energy show an increase in the O2p and Cu3d signals with increasing O2 concentration. The O2p and Cu3d signals are hardly discernible in the valence band spectrum measured at 700 eV.
4
Figure S5. a) in-situ SEM image of AgCu foil (0.5% Cu) reduced in H2 at 500 C. b) in-situ ESEM image of the same spot as in a) but after several hours in epoxidation conditions (0.3 mbar, 1:1 O2:C2H4, 300 C). c) Secondary electron image of the area used for the EDX maps. EDX maps of the d) Ag L, e) O K and f) Cu L emission lines
5
Figure S6. XPS and NEXAFS spectra of AgCu foil These spectra are from a homogeneous AgCu (2%) foil under epoxidation conditions (0.3 mbar, 250 C, 1:1 O 2:C2H4). The spectra show that the spectroscopic features seen on the powders measured in this study are also representative of homogeneous foils. This finding implies that the particle size (within the range used for this study, i.e. 1-5 micrometers diameter) makes no apparent difference to the spectra within the error sensitivity range possible for these measurements. Note: Spectrum c) is the Cu L3 edge after background subtraction. The raw spectrum is shown in f) and the background in g). The background spectrum was measured from a pure Ag foil by scanning the photon energy across the Cu L3-edge energy.
Figure S7. a) This series of spectra of a 2% AgCu foil, measured in 0.3 mbar of 1:1 O 2:C2H4 at various temperatures, also shows that the time evolution of the Cu species on the surface is similar to that observed on AgCu powders, namely, that the Cu intensity increases with time, starting with the formation of a Cu 1+ species, followed by the later formation of a Cu2+ species (evident from the satellite structure) and a mixture of species at steady state. b) These C1s spectra , measured concurrently with the Cu2p3/2 spectra here, show that the carbon species are removed at temperatures above 100°C. At 100°C, one can distinguish several species, including a CH/C-C bond, CO and C=O bonds. Such spectra are typical for samples measured in gas atmospheres containing hydrocarbons and oxygen.
6
Figure S8. In these Ag3d5/3 spectra, one can see that the 3d line of the AgCu alloys forms a low-BE shoulder when exposed to epoxidation conditions (0.3 mbar, 250°C, 1:1 O2:C2H4). This shoulder is not present on the reduced metal. The component is presumably Ag δ+. There is a slight difference between the 0.5%-Cu sample (green) and the 2%-Cu sample (blue-purple). The multiple spectra for each condition show how the spectra change with time, with both sample being relatively time independent.
Figure S9. a) Ag3d5/2: To see if the low-BE peak is also present without Cu, we measured a pure Ag sample under the same epoxidation conditions. This figure shows a comparison of pure Ag in epoxidation (0.3 mbar 250 °C, 1:1 O2:C2H4) with AgCu alloys of various compositions in epoxidation. One can see that they all have the lowBE shoulder. b) Valence band spectra of Ag in epoxidation: These spectra, measured at two different photon energies (100 eV and 150 eV) show the contribution to the DOS from O2p states (i.e. the shoulder at 2-3 eV).
Figure S10. This figure shows a comparison of O1s spectra measured from AgCu alloys and pure Ag. In general, there are several O1s species present and they cannot be completely resolved. Furthermore, several different species are known to have similar binding energies. Therefore, unambiguous interpretaion is not possible. A few point however can be noted. All spectra have a major component at ca. 530.5 eV. The Ag sample forms another peak at ca. 529 eV under epoxiation conditions. This peak is well known in the literature as the O α2 species. The Cu containing samples also form low-BE peaks. In the case of AgCu with 0.5% Cu, the low-BE peak is similar to on Ag, at ca. 529 eV, while on the AgCu with 2% Cu, the low-BE peak is at ca. 531 eV, which is consistent with the formation of Cu2O. Note, the O1s binding energy of CuO is 529.2 eV, so it would not be resolvable in these spectra. The O1s spectrum of an AgCu alloy (0.5 % Cu) in epoxidation conditions with dilute O 2 (0.2 %), showed the signature of the unique CuxOy phase in the valence band. This spectrum exhibited a main peak at 529.8 eV,
7
attributed to the CuxOy phase because its intensity increased as the Cu signal increased, and a high-BE shoulder at ca. 531 eV, which is attributed to O species on Ag.
Figure S11. Principle Component Analysis of Valence Spectra: a) Is a stack of valence spectra (hv = 150 eV) measured as a time series during epoxidation (0.3 mbar, 1:1 O 2:C2H4 at 250°C). b) A principle component analysis of the spectrum set was performed. Here we see that two abstract factors are needed to describe the data set. A possible third component is present, but at near negligible amounts. c) Difference spectra generated from the top-most and bottom-most spectra in a) were constructed. The factors were chosen for the difference spectra were such that the CuxOx spectrum resembles Cu2O as closely as possible, and the intensity of the AgCu spectrum does not drop below the interpolated intensity of the Ag4s band at 4 eV. Note the narrow Cu3d band in the blue spectrum and the states at 1-2 eV (probably from adsorbed O on Ag). d) A comparison of a reference Cu2O valence spectrum with the difference spectrum generated from c). One can see a distinct difference in the Cu3d-O2p band shape and width.
8
Figure S12. Principle Component Analysis of NEXAFS Spectra: a) Shows a time series stack of Cu L3 spectra of a 5% AgCu pellet heated epoxidation conditions (0.3 mbar, 1:1 O 2:C2H4, 250 C). The Cu2+ and Cu1+ components representing CuO and Cu2O are visually obvious, but to determine the number of components needed to describe the data set, we ran a principle component analysis (PCA) algorithm in the CasaXPS 2.3.19 software. The abstract factors determined by the PCA are shown in b). Here we can see that at least three components are needed to describe the data set. Presumably, one component is CuO, one is Cu2O and the other is CuxOy.
Figure S13. Here Figure a) shows a Cu L3 time series during the initial stages of oxidation of an AgCu foil. The PCA shows that only two factors are needed to construct the data set. The difference spectra of the top-most and bottom-most spectra in a) were used to construct vectors (i.e. reference spectra) in c). The spectra here show one vector that resembles CuO and the other resembles the Cu xOy species, and is distinctly different from a Cu2O spectrum. This spectrum also indicated that the CuxOy species might have a Cu2+ component (or at least a feature at nearly the same position of the Cu2+ absorption edge).
9
Figure S14. The GC data used to make figure 1 of the main text is shown here. Each sample was a pressed powder, using 50 mg each, of particle size 1-5 um. The set-up used for these measurements is described below. The GC’s sensitivity to EO and CO2 was calibrated using purchased gas mixtures, containing 5000 ppm EO in Ar (for EO calibration) and 2% CO2 in N2 (for CO2 calibration). The concentrations used for the calibration encompassed the range of intensities observed in the activity measurements. The calibration mixtures were comixed with reactant gases (O2 and C2H4) using the same flow rates and pressures used in the experiments (i.e. 3mln/min O2, 3mln/min C2H4, P = 0.3 mbar).
10
Figure S15. Experimental Set-up for GC measurements: GC measurements during NAP-XPS were accomplished as follows: Gas flows into a ca. 5L vacuum chamber at a rate of 6 mln/min.The gas is simultaneously pumped by a turbo-molecular pump with a throttle valve in between to regulate pumping speed and maintain the chamber pressure at 0.3 mbar. The turbo pump is backed by a leak diaphragm pump, whose exhaust is fed through a leak-tight stainless steel line into the injection nozzle of a Micro GC. The diaphragm pump compresses the exhaust from the turbo pump to 1 bar, which is needed for the inlet pressure of the Micro GC. The sample is a small, 8 mm diameter, 0.5 mm thick pressed powder pellet, positioned in the center of the vacuum chamber and 1 mm away from the nozzle to the electron energy analyzer. The sample is heated from the back side using an infrared laser. Small (0.05 mm diameter) type K thermocouple wires were pressed to the surface of the sample using a stainless steel clip.
11
Figure S16. Cu2p3/2 spectra measured using a focused X-ray spot (150 nm), showing that the Cu2p line-shape was stabe with time. Each spectrum took one minute to measure.
Figure S17. Cu LMM maps (blue regions indicate high Cu content), measured using a focused X-ray source (150 nm) and rastering the sample, show that the Cu surface concentration after oxidation was not uniform.
12
