S1

Supporting Information

Copper-Zinc Alloy-Free Synthesis of Methanol from Carbon Dioxide over Cu/ZnO/Faujasite Maxim Zabilskiy,a,* Vitaly L. Sushkevich,a Mark A. Newtonb and Jeroen A. van Bokhovena,b,* aLaboratory for Catalysis and Sustainable Chemistry, Paul Scherrer Institute, 5232 Villigen PSI (Switzerland) bInstitute for Chemistry and Bioengineering, ETH Zurich, Vladimir-Prelog-Weg 1, 8093 Zürich (Switzerland) ABSTRACT: The mechanism of carbon dioxide hydrogenation to methanol over Cu/ZnO materials has been explored for decades, however, the question of the active site still remains open to discussion. We used operando time resolved XAS and time-resolved isotope labeling experiments coupled with FTIR spectroscopy and MS analysis to elucidate the reaction mechanism and study the active sites and intermediates over a Cu/ZnO catalyst in the course of carbon dioxide conversion to methanol. No reduction of the zinc oxide, or formation of copper-zinc alloy were observed even under highly reducing conditions (15 bar of hydrogen, 260 °C), which leads to the conclusion that a copper zinc alloy phase is not required for high methanol yields and selectivity to be obtained. We attribute the reactive superiority of copper-zinc based system to the interplay between copper and zinc oxide phases. Our synthesis protocol provides a way to produce this copper zinc oxide interface, without having to go through an alloy phase.

KEYWORDS: CO2 hydrogenation, methanol, CuZn alloy, operando XAS, Cu/ZnO catalyst, isotope-labelling experiment.

*Corresponding Authors

Maxim Zabilskiy - Laboratory for Catalysis and sustainable Chemistry, Paul Scherrer Institute, 5232 Villigen PSI, Switzerland. E-mail: maxim.zabilskiy@psi.ch Jeroen A. van Bokhoven - Institute for Chemistry and Bioengineering, ETH Zurich, Vladimir-Prelog-Weg 1, 8093Zürich and Laboratory for Catalysis and sustainable Chemistry, Paul Scherrer Institute, 5232 Villigen PSI, Switzerland. E-mail: jeroen.vanbokhoven@chem.ethz.ch

S2

Table of Contents 1. Materials and Methods ............................................................................................................... S2

1.1. Synthesis of Cu/ZnO/FAU sample ............................................................................................ S2

1.2. BET measurements ..................................................................................................................... S3

1.3. TEM microscopy .......................................................................................................................... S3

1.4. Catalytic experiments ................................................................................................................. S4

1.5. Operando XAS experiment ........................................................................................................ S4

1.6. Analysis of XANES and EXAFS data .......................................................................................... S5

1.7. Operando steady-state 12CO2/13CO2 isotope transient experiment coupled with infrared spectroscopy ........................................................................................................................................... S5

2. TEM investigations ...................................................................................................................... S7

3. Additional IR observations ...................................................................................................... S10

4. Additional Cu K-edge XANES and EXAFS ............................................................................. S13

5. Additional Zn K-edge XANES and EXAFS observations .................................................... S18

6. Is formation of copper zinc alloy a prerequisite for catalyst activity in methanol synthesis reaction? ............................................................................................................................ S25

7. Supplementary References .......................................................................................................... S27

1. Materials and Methods

1.1. Synthesis of Cu/ZnO/FAU sample

Cu/ZnO based catalyst was prepared through consequent ion-exchange of FAU zeolite

(SiO2/Al2O3=12) followed by the precipitation of basic carbonates (firstly copper carbonate, and

after that zinc carbonate) in the zeolite pores using 0.1 M sodium carbonate. By using zeolite as a

career, we succeeded to separate both copper and zinc oxide and prevent possible copper-zinc

alloy formation during reduction and catalytic carbon dioxide hydrogenation. FAU zeolite

structure was selected due to its stability under methanol synthesis conditions (presence of water

vapor, high temperature and pressure) and its large pore size preventing diffusion limitation.

Commercially available faujasite (ammonium form, SiO2/Al2O3=12, CBV712, Zeolyst) was ion

exchanged with copper. 5 g of above mentioned zeolite was dispersed in 500 ml of 0.1 M solution

of copper nitrate (99%, Sigma-Aldrich) and stirred overnight at 50 °C. The suspension was then

filtered, washed with 2500 ml of pre-heated deionized water (50 °C) and finally dried at 393 K for

1 h. The resulting sample was re-dispersed in 2500 ml of 0.01 M solution of sodium carbonate (99%,

Sigma-Aldrich) and stirred at 50 °C for 5 hours in order to complete precipitation of copper

S3

hydroxyl-carbonate in zeolite structure. The resulting sample was filtered, thoroughly washed

with 2500 ml of pre-heated deionized water (50 °C), dried at 120 °C overnight, and finally calcined

in a flow of dry synthetic air at 350 °C for 4 h. Post-calcined material, containing copper oxide

nanoparticles encapsulated inside sodium form of zeolite faujasite, was then subjected to similar

procedure of zinc ion-exchange and deposition. The material was stirred in 500 ml of 0.1 M

solution of zinc nitrate (99%, Sigma-Aldrich) overnight at 50 °C. Then suspension was filtered at

ambient temperature, rinsed with 2500 ml of deionized water (pre-heated to 50 °C), and dried at

120 °C for 1 h. The solid was then re-dispersed in 2500 ml of 0.01 M solution of sodium carbonate

(99%, Sigma-Aldrich), and stirred for 5 hours at 50 °C. Finally, sample was filtered and thoroughly

washed with 2500 ml of deionized water until pH = 7. After overnight drying at 120 °C material

was calcined at 350 °C for 4 h in a flow of dry synthetic air. The prepared material was marked as

Cu/ZnO/FAU and accordingly to AAS analysis contains 2.6 wt. % of Cu and 2.4 wt. % of Zn.

1.2. BET measurements

Analysis of BET specific surface area, total pore volume, and pore size distribution was performed

at -196 °C using 3Flex Surface Characterization Analyzer (Micromeritics). Prior to measurement,

the catalyst sample was degassed according to the following protocol. Materials were evacuated

at 110 °C for 1 h, followed by 3 h soaking at 300 °C (heating ramp 2 °C/min) using the Micromeritics

SmartPrep degasser. The Brunauer-Emmett-Teller (BET) method was applied in order to calculate

specific surface area. The pore size distribution was derived from the desorption branch of the

isotherms employing the Barrett-Joyner-Halenda (BJH) method. The total pore volume was

estimated at a relative pressure of 0.99.

1.3. TEM microscopy

Particle size and morphology were studied by means of scanning transmission electron

microscopy with a high-angle annular dark field detector (HAADF-STEM) and high-resolution

transmission electron microscopy (HRTEM). Samples were transferred to TEM under ambient air

conditions. HAADF-STEM imaging and elemental mapping based on energy-dispersive X-ray

spectroscopy (EDXS) was performed on a probe-corrected HD2700CS (Hitachi) at 200 kV. For

HRTEM, a double aberration-corrected JEM-ARM300CF (JEOL) working at 300 kV was employed.

S4

1.4. Catalytic experiments

The Cu/ZnO/FAU was tested for catalytic carbon dioxide hydrogenation using a fixed-bed,

stainless-steel, reactor operating at up to 30 bar pressure. 50 mg of catalyst (fraction 50-100 μm)

diluted with 150 mg of SiC was fixed between two quartz wool beads and positioned inside

stainless-steel tube (O.D.=6 mm and I.D.=4 mm). The reactor was mounted inside a single-zone

furnace (Carbolite) equipped with Eurotherm 3508 controller. Temperature control was achieved

using a K-type thermocouple positioned inside the catalyst-bed. Before the catalytic experiment,

the sample was pre-treated in-situ under an ambient pressure in argon (5.0 quality, 50 mL/min)

at 260 °C (heating rate 5 °C/min) for 2 h. After that, the material was reduced in hydrogen flow

(50 mL/min) at 260 °C and ambient pressure for 1 hour. Finally, the total pressure was raised to 15

bar (controlled by Bronkhorst, EL-press series back pressure regulator). Catalytic carbon dioxide

hydrogenation to methanol was performed at 260 °C and 15 bar total pressure. A feed gas mixture

(Messer) containing 24 vol. % of carbon dioxide, 72 vol. % of hydrogen and 4 vol. % of argon (used

as tracer and internal standard) was then applied at a flowrate of 50 mL/min (controlled by

Bronkhorst mass flow-controller). Outlet gases were analyzed by gas chromatography using a

3000 Micro GC gas analyzer (Inficon) equipped with 10 m Molsieve and 8 m PlotU columns and

TCD detectors. During transient experiments, switches were performed using a remote controlled

6 port 2-position valve (VICI, Valco Instruments), while transient responses were monitored by

means of Omnistar GSD 300 O2 (Pfeiffer Vacuum) mass spectrometer.

1.5. Operando XAS experiment

In situ XAS measurements were performed on the DUBBLE (BM26A) and Swiss-Norwegian

(BM31) beamlines at European Synchrotron Radiation Facilities (ESRF), (Grenoble, France). Time

resolved operando XAS experiments were performed at the SuperXAS beamline, Swiss Light

Source, Paul Scherrer Institute (Villigen, Switzerland). About 5 mg of catalyst (fraction 50-100 μm)

was positioned inside the 1 mm, (wall thickness 0.01 mm) quartz capillary reactor and fixed

between two quartz wool plugs. A modified version of the plug-flow reactor system developed by

Chupas and co-workers for total X-ray scattering measurements was used during these

experiments.1 A 0.3 mm a K- type thermocouple was inserted into the sample bed in order to

control temperature. Gas flows (2-10 ml/min) were controlled by mass flow controllers

(Bronkhorst). The total pressure was controlled by Bronkhorst EL-Press digital back-pressure

S5

regulator. H2, Ar, He gases of grade 6.0 and CO2 of grade 4.8 were used. Transient switches were

again achieved using a remote controlled 6-port 2-position valve (VICI, Valco Instruments).

At SuperXAS, XAFS data was collected in a conventional transmission geometry using fast,

gridded ion chambers, and a quick scanning channel-cut Si(111) monochromator (oscillation

frequency of 10 Hz).2 Spectra of copper and zinc foils were collected simultaneously for internal

energy calibration. Transient responses of reaction mixture and products during gas switches

were monitored by means of Omnistar GSD 300 O2 (Pfeiffer Vacuum) mass spectrometer.

1.6. Analysis of XANES and EXAFS data

Cu K- and Zn K-edge XANES and EXAFS data were background subtracted and normalized using

either, PAXAS,3 Athena,4 or Prestopronto.5 Principal component analysis (PCA) was made using

the ITFA software due to Rossberg et al.6 Fitting of the EXAFS was made using EXCURV (v. 9.3).7

1.7. Operando steady-state 12CO2/13CO2 isotope transient experiment coupled with infrared

spectroscopy

Time-resolved isotope labeling experiments coupled with FTIR were performed using a standard

flow reactor configuration with “sandwich”-type transmittance IR cell.8 Prior to the

measurements, the sample (15 mg) was pressed into a self-supporting discs, activated in argon

(5.0 quality, 25 mL/min) at 260 °C (heating rate 10 °C/min) for 2 h. The sample was then reduced

in hydrogen (25 mL/min) at 260 °C and ambient pressure for 1 hour. The gas flow was then

switched from hydrogen to CO2/H2/Ar (24 vol. %, 72 vol. % amd 4 vol. % correspondingly, Messer)

mixture. Finally, the total pressure was raised to 15 bar (controlled by Bronkhorst, EL-press series

back pressure regulator). After steady-state methanol conversion was achieved (ca. 1 h on stream),

isotopic switching was performed using a two-position valve (VICI), from unlabeled 12CO2/H2

(Messer; 99.5%) to 13CO2/H2 labeled mixture (Cambridge Isotopes Laboratories, Inc.; 99% 13C). An

inert tracer (Argon, 4 vol. % of the total flow rate) was used to correct for the gas-phase hold-up

in the reactor. Isotope labeling experiments were carried out at constant total gas flow rates of 25

cm3/min. The 12C/13C switch was achieved without perturbing the steady-state of the reaction by

maintaining the reaction temperature at 260 °C, the total system pressure at 15 bar and carbon

dioxide conversion at ≈2%. On-line mass spectrometry analysis was performed using a quadrupole

mass analyzer (Balzers). The surface species on the catalyst were followed using IR spectroscopy

S6

(Thermo Nicolet iS50 equipped with MCT detector). The spectral resolution was 4 cm-1, and

spectra were acquired every 6 secs (summation of 4 scans per spectrum).

In the MS, the ion signals for m/z = 40 (Ar tracer), 44, 45, 28, 29, 31 and 33 were continuously

monitored to determine the isotope content of the original gas sample. Due to the overlapping of

MS signals of CO and CO2, the 12CO and 13CO responses were separately analyzed by passing the

outlet through a trap filled with the solid sodium hydroxide which absorbed all CO2.

Transient responses were normalized by the difference between the initial and final ion signals.

The argon decay curve was used to determine the gas-phase holdup of the reactor system, since

we assumed that the inert gas did not adsorb on the surface of the catalyst.

S7

2. TEM investigations

The fresh Cu/ZnO/FAU material, as well as this material after catalytic carbon dioxide

hydrogenation, were investigated by transmission electron microscopy. Results of this analysis

are presented on Figures S1-S3.

Figure S1. Secondary electron (a) and transmission electron (b) microscopy images of fresh

Cu/ZnO/FAU catalyst.

Figure S2. Secondary electron (a) and transmission electron (b) microscopy images of fresh

Cu/ZnO/FAU catalyst.

S8

Figure S3. DF-STEM micrograph of fresh Cu/ZnO/FAU catalyst as well as extracted EDX spectra

for different region marked on the STEM.

Figure S4. DF-STEM micrograph of spent Cu/ZnO/FAU catalyst (260 °C, 30 bar, 24 hours on

stream) as well as extracted EDX spectra for different region marked on the STEM.

S9

Figure S5. DF-STEM micrograph of spent Cu/ZnO/FAU catalyst (260 °C, 30 bar, 24 hours on

stream) as well as extracted EDX spectra of the region marked on the STEM.

Results of TEM investigation of fresh Cu/ZnO/FAU sample, prepared by consecutive ion-

exchange, followed by precipitation of basic copper and zinc carbonates in zeolite pores, are

presented in Figures S1-S3. By using this preparation technique, we were able to encapsulate

copper and zinc oxide nanoparticles inside zeolite structure (Figures S2-S3). By comparison of

secondary electron (Figure S1a) and transmission electron (Figure S1b) microscopy images of fresh

Cu/ZnO/FAU material we can clearly see the absence of deposition of copper and zinc oxides

phases on outer surface of zeolite. Analysis of EDX spectra extracted from two different regions

marked on the STEM micrograph (Figure S3) suggests formation of small nanoparticles of copper

and zinc oxides inside zeolite structure.

Figures S4-S5 show the Cu/ZnO/FAU catalyst after catalytic carbon dioxide hydrogenation. In

contrast to the fresh material, where highly dispersed copper and zinc oxides phases were

observed, in the spent Cu/ZnO/FAU catalyst we found the regions where copper and zinc oxides

were deposited on outer zeolite surface. Segregation of copper was found to be more prominent

(Figure S5), compared to zinc phase. At the same time, both copper and zinc phases were also

observed inside zeolite structure (Figure S4, second marked region). However, in this case, bulk

zeolite is enriched with zinc phase, while copper is present in trace amounts. These results

confirm that during Cu/ZnO/FAU catalyst reduction, and subsequent catalytic carbon dioxide

hydrogenation, the copper phase is extruded, to a significant degree, to the outer surface of the

zeolite crystal. At the same time, zinc oxide is more resistant to this process and only a relatively

small fraction of zinc undergoes extrusion to the zeolite surface.

S10

3. Additional IR observations

Cu/ZnO/FAU catalyst was activated in argon at 260 °C for 2 h, followed by reduction in hydrogen

260 °C and ambient pressure for 1 hour. After that the gas flow was switched from hydrogen to

the CO2/H2/Ar (24 vol. %, 72 vol. % amd 4 vol. % correspondingly, Messer) catalytic mixture, and

the total pressure in the reaction cell was gradually raised to 15 bar. Introduction of CO2/H2

reaction mixture leads to the appearance of several sets of bands at 1661, 1608 and 1383 cm-1 (Figure

S6). While increasing the pressure in reaction cell, we observed the development of the band at

1470 cm-1 that can be attributed to the bending vibrations in methoxy species. This indicates that

hydrogenation of formate species to methanol occurs at high pressure, while under ambient

pressure hydrogenation of formates is significantly hindered. It should be noted, that we did not

observe any spectroscopic evidence for the formation of carbonate species during catalytic carbon

dioxide hydrogenation to methanol, suggesting that carbonates are not involved in the catalytic

cycle for methanol synthesis. This was further confirmed by transient response during 12CO2/13CO2

isotope switch, where 12CO2 response matches precisely the response of inert gas tracer (Figure

2b). This indicates the fast and irreversible carbon dioxide hydrogenation to formate, which

proceeds without the formation of carbonate intermediates.

Figure S6. In situ FTIR spectra collected over Cu/ZnO/FAU catalyst during increasing CO2/H2

total pressure from 1 bar to 15 bar.

1800 1700 1600 1500 1400 1300

δ H2O1634

δs12CH3O

1470

δasH12COO1608

Wavenumbers (cm-1)

δsH12COO1383

Pres

sure

δasH12COO 1661

S11

Figure S7. Normalized IR transient responses of formate (black) and methoxy (red) species

following hydrogen to CO2/H2 reaction mixture switch over Cu/ZnO/FAU catalyst at 260 ºC and

15 bar. Inset: in situ FTIR spectra evolution during abovementioned switch over Cu/ZnO/FAU

catalyst.

Figure S7 shows the evolution of in situ FTIR spectra collected over Cu/ZnO/FAU catalyst at 260

ºC and 15 bar during transient switch from hydrogen to CO2/H2 reaction mixture. Normalized IR

transient responses of formate (black) and methoxy (red) species, subsequent to the switch from

hydrogen to a CO2/H2 reaction mixture, clearly show that formate is the primary intermediate

formed on the catalyst surface. The evolution of the surface methoxy species is significantly slower,

suggesting that methoxy groups are secondary species that are formed as a result of a number of

steps subsequent to the initial activation of carbon dioxide and formation of formate species in

this catalytic reaction.

To access the reactivity of formate and methoxy intermediates, we followed the dynamics of these

surface species during the 12CO2/13CO2isotope switch. After the switch, the intensity of all bands

at 1661, 1608 and 1383 cm-1, which correspond to unlabeled formate species, decreases. At the same

time, bands at 1608, 1558 and 1298 cm-1 appear and increase their intensity (Figure S8). These new

1800 1700 1600 1500 1400 1300

0 1 2 3 4 5 6 7 80.0

0.2

0.4

0.6

0.8

1.0

Wavenumbers (cm-1)

δ H2O1634

δs12CH3O

1470

δasH12COO1608

δasH12COO 1661

δsH12COO1383

time

Formate and water Methoxy

Nor

mal

ized

resp

onse

, F(t)

Time (min)

S12

bands correspond to the 13C-labeled formate species formed on the catalyst surface from 13CO2 of

the reaction feed. Since the position of 13C-labeled monodentate formate coincides with the

frequency of 12C bidentate formate, the difference spectra (Figure 2a) contain only 3 visible bands.

However, comparison of the intensities of the bands at 1608 and 1558 cm-1 provides an evidence

of superposition of 13C-labeled monodentate formate and 12C bidentate formate. The increase in

adsorption at 1558 cm-1 is almost as twice that of the observed decrease in intensity of the band

1608 cm-1, which is not possible for simple isotope exchange of (just one band) – of a single species.

Figure S8. Time resolved in situ FTIR spectra evolution over Cu/ZnO/FAU catalyst during

transient switch from 12CO2/H2 to 13CO2/H2 reaction mixture at 260 °C and 15 bar.

1800 1700 1600 1500 1400 1300

1298

1383

1558

16081661

Wavenumbers (cm-1)

time after switch

S13

4. Additional Cu K-edge XANES and EXAFS

Cu K-edge XANES spectra of Cu/ZnO/FAU catalyst during pretreatment steps are shown in Figure

S9. In the fresh Cu/ZnO/FAU material copper is present in the Cu2+ oxidation state, however after

heating to 260 °C in flowing helium undergoes partial reduction to Cu2O oxide. Switching gas to

hydrogen results in full reduction of oxide phase and formation of well developed fcc metallic

copper phase.

Figure S9. Cu K-edge XANES spectra evolution during pre-treatment steps.

Examples of in situ Cu K-edge EXAFS derived during these measurements are given in Figure S10

for three instances: (a) the starting, calcined catalyst, (b) the same catalyst after being heated to

260 °C in flowing helium; and, (c) after reduction in flowing hydrogen at 260 °C. The results of

analysis of the EXAFS in each case are given in Table S1. In the case of the reduced copper catalysts

EXAFS analysis takes both the temperature (260 °C) and the coefficient of linear expansion of

copper (17.1 K-1) into account. This approach, within the model of an anharmonic oscillator, having

been demonstrated to be appropriate for Cu K-edge EXAFS at least in respect to the first fcc

coordination shell 9. The B1 value (B1=0.006) used for EXAFS analysis of reduced copper phase is

the value of the third cumulant of the disorder term calculated using this approach.

8940 8960 8980 9000 9020 9040 9060 9080 91000.0

0.2

0.4

0.6

0.8

1.0

1.2

Nor

mal

ized

abs

ortp

ion

(a.u

.)

Energy (eV)

Cu/ZnO/FAU in He at RT Cu/ZnO/FAU in He at 260 °C Cu/ZnO/FAU in H2 at 260 °C

S14

Figure S10. Left Panel: k3-weighted Cu K-edge EXAFS derived from Cu/Zn/FAU samples at three

point their activation prior to catalysis; (a) as loaded sample under helium; (b) under helium after

heating to 260 °C (10 °C·min-1); and (c) at 260 °C under flowing hydrogen. The right hand panel

shows the corresponding Fourier transforms of the k3-weighted data. In spectrum (a) the arrows

indicate those scattering features that, most likely, arise from multiple scattering (MS) effects due

to the symmetry of the predominant Cu(II) environment. In spectrum (b) the arrow highlights

the appearance of a new, low r(Å) feature in the FT. In all cases the analysis the fits to the data,

derived using EXCURV 7, are shown in red.

S15

Table S1. Structural and statistical parameters derived from the fitting of k3-weighted copper K-

edge EXAFS for the Cu/ZnO/FAU catalyst under different conditions using EXCURV [S9]. Kmin=3,

Kmax=11.5, R=1-3 Å, AFAC=0.9

Sample Shell CN R (Å) DW (2σ2) Fitting

parameters

a) Cu/ZnO/FAU

in He at RT

Cu-O 4.3 1.91 0.007 EF = -5.7

R% = 29.3 Cu-Cu 1.1 3.11 0.014

Cu-O 1.2 3.35 0.023

b) Cu/ZnO/FAU

in He at 260 °C

Cu-O 3 1.91 0.0019 EF = -5.7

R% = 27.3 Cu-Cu 0.7 2.88 0.023

Cu-O 2.8 3.35 0.045

c) Cu/ZnO/FAU

in H2 at 260 °C Cu-Cu 8.8 2.53 0.028

EF = -5.7

R% = 28.7

Kmin and Kmax delineate the range in k (Å-1) used for the fitting of the EXAFS

AFAC: related to the proportion of electrons undergoing scattering post absorption that

contribute to the EXAFS.

CN: Coordination number (± 10 % of stated value)

DW: Debye – Waller factor where σ is the root mean square inter-nuclear separation (Å)

EF: Edge position relative to Vacuum zero (Fermi energy, eV)

R(%) (fit agreement factor): �∫�cT−cE�k3dk

[cE]k3dk�× 100 , where cT and cE are the theoretical and

experimental EXAFS and k is the photoelectron wave vector.

Taking the fresh sample (a) first. EXAFS analysis is consistent with copper being present as Cu(II),

though not entirely as monomeric species; the fitting of the pronounced second shell suggests at

least a proportion of dimeric or more extended Cu(II) species being present. This may represent

the partitioning of copper between intra-pore (within the zeolite), and extra pore (adsorbed at

the surface of the zeolite), Cu(II) species.

Moreover, it can be observed, that this single scattering analysis leads to a fit to the k3-weighted

data that is deficient in certain respects. This most likely arises as a result of a neglect of multiple

S16

scattering (MS) terms in the analysis. Evidence for MS can be observed in the Fourier transform

in the clear existence of apparent scattering shells at a R ≥ 4 Å from the emitting atom, that are

approximately found to be present at twice the R value of the fitted shells. This would further

suggest that in this sample the Cu (II) predominantly exists in a high symmetry environment

(square planar or pseudo - octahedral).

Heating of the sample to 260 °C under He results in a similar fitting, in many ways, to that

achieved for the as made sample. Equally however, differences are apparent, most specifically in

the FT. Firstly, whilst Cu-O and Cu-Cu coordination are retained the refined Cu-O and

coordination drops to a value of 3. This behavior has been reported in a number of studies of

copper hosted within zeolites [S11, S12] and is most likely resolved as a response the copper

(specifically that part retained within the zeolite pores) to thermally induced dehydration. At the

same time, we should expect a Cu-Al coordination to be manifested. However, the limitations of

the data, likely disorder (both thermal and static), and the distinct possibility that the second

shell component, modelled using copper alone, may actually be comprised of overlapping Cu-Cu

and Cu-Al components cannot be ruled out.

In addition, a new, low r(Å) feature appears in the FT. This can be fitted using O or C though the

returned bond distance is short (< 1.7Å). The inclusion of this shell to the fit of the EXAFS data

(not shown) has, however, a very significant effect on the returned R factors (27.3 to 17.4) whilst

also having a beneficial effect on other elements of the fitted parameters: i.e. a significant

reduction in the DW factor for the second Cu-O shell (from 0.045 to 0.036); and a considerable

reduction in EF (from -7 to 0.45 eV). The source of this new feature cannot at present be securely

established, but in the context of the current communication has no bearing upon the conclusions

arrived at.

Reduction in hydrogen leads to the transformation of the Cu K-edge EXAFS into that which is

clearly indicative of the presence of fcc copper nanoparticles of significant extension. The first Cu-

Cu scattering shell at a distance of 2.53 Å is indicative of reduced copper nanoparticles, and the

prominent higher fcc shell contributions clearly present in the FT suggest the development of

well-formed and three dimensional particles. Given the microporous nature of the support the

EXAFS points to a reduction of the copper that is accompanied by very extensive extrusion of the

copper present within the zeolite to yield these copper nanoparticles that then exist at the

external surface of the support. This indeed is in a good agreement with TEM results, where we

also observed segregation of copper phase after catalysis.

S17

In order to investigate involvement of copper phase in catalytic cycle of carbon dioxide

hydrogenation transient experiments, analogous to those already performed during FTIR

experiments, were made. By using operando time-resolved XAS, we have monitored possible

changes in Cu K-edge XANES upon cycling feed gas composition between CO2/H2 mixture and

hydrogen. Figure S11 shows spectra of Cu/ZnO/FAU catalyst collected under pure hydrogen

(black) and after switch to CO2/H2 mixture (red). Detailed analysis (see difference spectrum)

yields no evidence for oxidation of the copper or formation of formates species associable with

copper during methanol synthesis reaction. Furthermore, numerous works including isotope

experiments made by Campbell group,10,11 have shown that even when surface formate species

present on the copper surface during steady state methanol synthesis conditions, such a species

cannot be converted to methanol by reaction with hydrogen or with hydrogen plus added water.

Figure S11. Cu K edge XANES spectra of Cu/ZnO/FAU catalyst under different conditions: i) after

pretreatment in hydrogen at 260 °C and 15 bar (black) and ii) 30 mins after switch to CO2/H2

mixture at 260 °C and 15 bar (red). Difference spectra (blue) indicates absence of copper oxidation

and copper formats formation.

8950 9000 9050 9100

0.0

0.2

0.4

0.6

0.8

1.0

1.2 Cu/ZnO/FAU in H2 at 260 °C Cu/ZnO/FAU in CO2/H2 mixture at 260 °C Difference spectrum

Energy (eV)

Nor

mal

ized

abs

orpt

ion

(a.u

.)

0.00

0.05

0.10

0.15

0.20

0.25

0.30

Diff

eren

ce s

pect

rum

(a.u

.)

S18

5. Additional Zn K-edge XANES and EXAFS observations

The normalized Zn K-edge XANES spectra of the different zinc standard compounds are shown

in Figure S12. From difference spectra (Figure 2a) is clearly visible that main changes that occur

during the operando switch from hydrogen to CO2/H2 mixture are to be observed in the region

with maximum at 9666.3 eV. At the same time, the main peak of zinc formate standard is located

at ca. 9666.8 eV (Figure S12). This single point comparison of energies therefore may indicate

formation of zinc formate species during carbon dioxide hydrogenation reaction (Figure S13).

However, this method cannot be used to reliably identify the zinc phases presented in the system.

Therefore,. data collected during transient switches from hydrogen to CO2/H2 mixture and back

were submitted to PCA analysis in order to establish the spectroscopic character and evolution of

the Zn species participated in catalytic cycle of carbon dioxide hydrogenation.

Figure S12. Zn K edge XANES of standards: copper-zinc alloy (black), zinc oxide wurtzite (red),

zinc hydroxycarbonate (blue), zinc formate (green) and zeolite Y containing Zn2+ ions in cation

position (violet).

9630 9640 9650 9660 9670 9680 9690

9659.5

Nor

mal

ized

abs

orpt

ion

(a.u

.)

Energy (eV)

CuZn-foil ZnO Zn(OH)2·ZnCO3

Zn(HCOO)2

Zn2+/FAU

0.5

9666.8

S19

Figure S13. Difference spectrum obtained by subtraction of zinc oxide wurtzite from zinc formate

reference spectra (black), as well as difference spectrum obtained during transient switch from

CO2/H2 reaction mixture to hydrogen as shown on Figure 3a (red).

9640 9660 9680-0.4

-0.2

0.0

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0.6

0.8

Energy (eV)

Diff

eren

ce (a

.u.)

-0.02

-0.01

0.00

0.01

0.02

0.03

Diff

eren

ce (a

.u.)

S20

Figure S14. Comparison of reproduced zinc components found to be present during a switch from

hydrogen to CO2/H2 mixture at 260 °C and 15 bar: a) comparison of component 1 derived from

PCA analysis with Zn2+/FAU standard, b) comparison of component 2 derived from PCA analysis

with zinc hydroxycarbonate and zinc formate standards and c) comparison of component 3

derived from PCA analysis with zinc oxide wurtzite standard.

Figure S14 compares the reproduced components derived from PCA analysis with the Zn-

containing bulk standards. As can be seen, components 2 and 3 can be unambiguously assigned

to zinc formate and zinc oxide wurtzite phases. The spectral characteristics of component 1 are

very close to Zn2+/FAU standard which was prepared by ion exchange of zeolite Y (ammonium

form, SiO2/Al2O3=12, CBV712, Zeolyst) with zinc nitrate solution. EXAFS analysis further confirms

this assignment identifying the main zinc phase as highly dispersed zinc species encapsulated

within the zeolite matrix.

9640 9660 9680 97000.0

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0.4

0.6

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1.2

1.4

1.6

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1.4

1.6

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2.2

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1.2

1.4

1.6

Nor

mal

ized

abs

orpt

ion

(a.u

.)

Energy (eV)

Component 1 Zn2+/FAU standard

c)b)

Nor

mal

ized

abs

orpt

ion

(a.u

.)

Energy (eV)

Component 2 Zn(OH)2·ZnCO3 standard Zn(HCOO)2 standard

a)

Nor

mal

ized

abs

orpt

ion

(a.u

.)Energy (eV)

Component 3 ZnO standard

S21

Figure S15. An examples of the quality of spectral reproduction obtained from PCA analysis

during a switch from hydrogen to CO2/H2 mixture over Cu/ZnO/FAU catalyst at 260 °C and 15

bar: a) at time 1 min (before the switch) and b) at time 20 min (after the switch).

Figure S15 shows the quality of spectral reproduction achieved from PCA analysis of Zn K-edge

XANES spectra of Cu/ZnO/FAU catalyst during the switch from hydrogen to CO2/H2 reaction

mixture at two different times: a) 0 min and b) 20 mins. Comparison of the experimental and

reproduced spectra provide evidence that the PCA analysis reproduces spectra well and that the

generated data can be used to identify the time resolved evolution of zinc phases during transient

switch with a high degree of precision (Figure 3).

The PCA analysis, and the speciation that is derived from it (namely formation and hydrogenation

of zinc formate species), is entirely consistent with the isotope labeling experiments data (Figure

S2). By combining these two techniques, our study therefore confirms that it is formates

associated with the zinc that are the reactive intermediates responsible for the hydrogenation of

carbon dioxide to methanol. Figure 3B shows that only a small fraction of zinc, in this case zinc

oxide wurtzite, in Cu/ZnO/FAU is active in carbon dioxide hydrogenation to methanol. The

zeolite acts as a reservoir that permits the extrusion of both copper and zinc to the surface of the

9645 9660 9675 9690 97050.0

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1.6

9645 9660 9675 9690 97050.0

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1.2

1.4

1.6

Nor

mal

ized

abs

orpt

ion

(a.u

.)

Energy (eV)

experimental spectrum reproduced spectrum residual

b)a)N

orm

aliz

ed a

bsor

ptio

n (a

.u.)

Energy (eV)

experimental spectrum reproduced spectrum residual

S22

catalyst during reduction, and therefore attain the required intimacy of interaction to generate

the active sites required for the selective conversion of carbon dioxide. In this material copper is

extruded, reduced, and then sintered to yield sizeable copper nanoparticles, (Figures S5 and S9)

on the outer zeolite surface. At the same time, according to Zn K-edge EXAFS, as well as PCA

XANES analysis of activated Cu/ZnO/FAU sample indicate that the zinc component is more

resistant to extrusion and/or sintering, and only a small fraction of zinc is transformed into a

wurtzite-like zinc oxide located at the external zeolite surface. This phase, in intimate contact

with the metallic copper, is responsible for activation of carbon dioxide yielding zinc formate

species that are then hydrogenated to methanol.

Figure S16 gives EXAFS derived at the Zn K edge for the Cu/ZnO/FAU sample during the switching

from flowing hydrogen to CO2/H2 reaction mixture at 260 °C and 15 bar corresponding to two

points at the beginning (t = 0 mins) and end of the transient ( t = 20 mins) switch shown in the

main paper (Figure 3b). Table S2 summarizes the result of the fitting process.

The Zn K–edge EXAFS is consistent with the predominant Zn component present under the

applied treatments as remaining very highly dispersed and essentially unperturbed by the change

in conditions made during the transient switch from hydrogen to the catalytic feedstock: visual

inspection of the k3 weighted data is not suggestive of any significant change between the two

spectra. The majority of the Zn is therefore suggested to have remained bound within the zeolite

structure and is a spectator to the catalysis that is observed to occur. This viewpoint is, again,

entirely consistent with the results of the PCA analysis of the XANES shown in the main paper

(Figure 3b) wherein the vast majority of the Zn (ca. 96%) is shown to be inactive and unchanging

during the transient experiment. The catalytically active zinc phase, on the other hand is shown

to be a minority zinc oxide wurtzite-type phase (Figure S14c) present at a level of only ca. 4 % of

the total Zn content of the sample, which we do not expect EXAFS to be sensitive to. As the zinc

oxide is an extended phase, this form of ZnO is only present at the surface of the zeolites. It is

therefore able to achieve the required contact with the copper extruded from the support by

catalyst reduction at 260 °C and to yield the zinc formates that the Zn K –edge XANES show to be

reactive elements in this system.

S23

Figure S16. Zn K edge EXAFS derived from two points in the transient switching experiment

shown in Figure 3b of the paper: (a) at t = 0 mins i.e. commensurate with the switch from hydrogen

to the reaction mixture; and (b) at t = 20 mins i.e. just before the feed is returned to hydrogen

from the CO2/H2reaction mixture. The fits derived from analysis in EXCURV are show in red.

Table S2. Structural and statistical parameters for Zn K-edge EXAFS shown in Figure S16 at two

points during the transient switch from H2 to the H2/CO2 reaction mixture derived from analysis

using EXCURV [S9]. Kmin = 3 Kmax, 12.25, R=1-3 Å, AFAC = 0.9

Sample Shell CN R (Å) DW Fitting parameters

a) Cu/ZnO/FAU

in H2 at 260 °C

O 4.3 1.95 0.019 EF = -2.6

R% = 16.9 Al 1.1 3.11 0.018

Zn 1.2 3.38 0.025

b) Cu/ZnO/FAU

in CO2/H2 at 260 °C

O 4.3 1.91 0.019 EF = -2.87

R% = 15.4 Al 0.9 2.88 0.016

Zn 1.3 3.35 0.027

Kmin and Kmax delineate the range in k (Å-1) used for the fitting of the EXAFS

AFAC: related to the proportion of electrons undergoing scattering post absorption that

contribute to the EXAFS.

CN: Coordination number (± 10 % of stated value)

DW: Debye – Waller factor where σ is the root mean square inter-nuclear separation (Å)

EF: Edge position relative to Vacuum zero (Fermi energy, eV)

S24

R(%) (fit agreement factor): �∫�cT−cE�k3dk

[cE]k3dk�× 100 , where cT and cE are the theoretical and

experimental EXAFS and k is the photoelectron wave vector.

S25

6. Is formation of copper zinc alloy a prerequisite for catalyst activity in methanol

synthesis reaction?

The role of copper zinc alloy in carbon dioxide hydrogenation over copper zinc based catalyst

remains the subject of intensive discussion 12–15. While previous works have unambiguously

attributed a key role in this process to copper zinc alloy, recent publications have cast doubt on

involvement of reduced zinc in catalytic mechanism 12,14,16. For our study, we have observed that

Cu/ZnO/FAU catalyst did not form the copper zinc alloy during pre-treatment or catalytic steps

(Figure 3a). At the same time industrial Cu/ZnO/Al2O3 catalyst (purchased from Alfa Aesar)

containing 63.5 wt. % of copper oxide and 24.7 wt. % of zinc oxide and 10.1 wt. % of alumina indeed

forms copper zinc alloy during pre-treatment in hydrogen (Figure S17). The principle

physicochemical properties and catalytic activity of both samples are summarized in Table S3.

Despite the different preparation methods, nature of supports, formation or absence of copper

zinc alloy during pre-treatment and carbon dioxide hydrogenation steps, these two materials

show similar normalized catalytic activity as shown in Table S3. Therefore, the absence or

presence of the CuZn alloy phase does not appear to be a prerequisite for the attainment of

significant activity, or indeed selectivity, in the hydrogenation of carbon dioxide to methanol.

Table S3. Main physicochemical properties and results of catalytic investigations for investigated

copper zinc based catalysts.

Sample

SBET

(m2/g

)

Vpore

(cm3/g

)

wCu

(wt. %

)

wZn

(wt. %

)

AMeOH[a]

(μmol·gcat-1·min-

1)

SMeO

H

(%)

AMeOH[b]

(mmol·molC

u-1·min-1)

Cu/ZnO/FAU 735 0.49 2.6 2.4 11±1 40±2 27±3

Cu/ZnO/Al2

O3 102 0.21 50.7 19.8 280±20 48±2 35±3

[a] Methanol production per catalyst mass during catalytic carbon dioxide hydrogenation at

260 °C and 15 bar total pressure. [b] Methanol production normalized per copper loading at 260 °C

and 15 bar total pressure

S26

Figure S17. Evolution of Zn K edge XANES spectra during industrial Cu/ZnO/Al2O3 catalyst pre-

treatment in hydrogen at 15 bar. Spectra were collected during heating from ambient temperature

to 260 °C with heating ramp (10 °C/min).

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9659

Cu/ZnO/Al2O3 catalystheating in 15 bar H2

Temperature

S27

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(2) Müller, O.; Nachtegaal, M.; Just, J.; Lützenkirchen-Hecht, D.; Frahm, R. Quick-EXAFS Setup

at the SuperXAS Beamline for in Situ X-Ray Absorption Spectroscopy with 10ms Time Resolution.

J. Synchrotron Radiat. 2016, 23, 260–266.

(3) Binsted, N. PAXAS: Program for the Analysis of X-Ray Absorption Spectra. 1988.

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Absorption Spectroscopy Using IFEFFIT. J. Synchrotron Radiat. 2005, 12, 537–541.

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(6) Rossberg, A.; Ulrich, K.-U.; Weiss, S.; Tsushima, S.; Hiemstra, T.; Scheinost, A. C.

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(10) Yang, Y.; Mims, C. A.; Disselkamp, R. S.; Kwak, J. H.; Peden, C. H. F.; Campbell, C. T.

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S28

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