Core–shell silica@CuxZnAl LDH catalysts for efficient CO2 hydrogenation to methanol

The efficient production of methanol by reduction of CO2 using green hydrogen is a promising strategy from both a green chemistry and a carbon net zero perspective. Herein, we report the synthesis of well-dispersed core–shell catalyst precursors using silica@CuxZnAl-LDHs that can convert CO2 to methanol. The catalyst precursors can be formed using either a commercially available silica (ES757) or a mesoporous silica (e.g. MCM-48). These hybrid materials show significantly enhanced catalytic performance compared to the equivalent unsupported CuxZnAl LDH precursor. Space-time yields of up to 0.7 gMeOH gcat−1 h−1 under mild operating conditions were observed.

The resulting mixture was aged for one hour under stirring (stirring speed 500 rpm) after the completion of addition.The product was collected by Buchner filtration and washed with water till the pH of filtrate was equal to 7 (approximately 1.5 L DI water was used) before being dried under vacuum at 30 °C overnight.Core-shell mSiO 2 @Cu 1.3 ZnAl LDH was prepared using a co-precipitation method.The mSiO 2 (ES757, SBA-16 or MCM-48, 0.80 g) was mixed with de-ionised (DI) water (24.00 mL) in beaker (150 mL or 200 mL tall beaker) by thirty-minute sonication.Na 2 CO 3 (0.68 g, 6.41 mmol) was then added to the dispersion which was stirred for five minutes (stirring speed 500 rpm).The achieved mixture was named as solution A. The pH value of solution A was adjusted to 9 with 10 wt% nitric acid.An aqueous solution (64 mL) of Cu(NO 3 ) 2 •3H 2 O (1.22 g, 5.05 mmol), Zn(NO 3 ) 2 •6H 2 O (1.16 g, 3.89 mmol) and Al(NO 3 ) 3 •9H 2 O (1.46 g, 3.89 mmol) was added to solution A within eighty minutes (dropping rate of metal salt solution is 49 mL/min).The pH value of the reacting mixture was kept at 9 using 4 M NaOH aqueous solution.The resulting mixture was aged for one hour under stirring (stirring speed 500 rpm) after the completion of addition.The product was collected by Buchner filtration and washed with water till the pH of filtrate was equal to 7 (approximately 1.5 L DI water was used) before being dried under vacuum at 30 °C overnight.mSiO 2 @Cu 1.3 ZnAl LDO was prepared by the calcination of mSiO 2 @Cu 1.3 ZnAl LDHs at 330 °C for 4 hours at a 2 °C/min ramp rate under constant air flow at 50 mL n /min.During the process, the sample was held at 80 °C and 150 °C for one hour dwell time respectively.mSiO 2 @Cu 1.3 was obtained from the reduction of mSiO 2 @Cu 1.3 ZnAl LDO at 290 °C for 2 hours at 2 °C/min ramp rate under constant gas flow (5% H 2 /N 2 ) at 50 stp mL/min (stp = standard temperature and pressure: P = 101.3kPa, T = 298 K).

Synthesis procedure of ES757@Cu x ZnAl LDH
For preparing core-shells with different copper loading, only the usage of metal salt and the usage of sodium carbonate are changed, the rest procedure remains the same as the synthesis of ES757@Cu 1.3 ZnAl LDH.The details are listed in the table below.

Amount of metal nitrate (g) Sample name
Cu These composites were activated (calcined and reduced) follow the same procedure as that was used for ES757@Cu 1.3 ZnAl LDH to produce core-shell ES757@Cu x catalysts.
Rietveld refinement were performed adopting TOPAS-Academic V6. 1 For lab XRD data, refining crystallite size and microstrain broadening at the same time may lead to an over-parameterisation and end up with meaningless numbers.Therefore, only crystallite size is refined, which is calculated from the integral breadth of peaks.Its value is an underestimation of the average crystallite size; a change of the value indicates a change in the phase crystallinity.
High resolution SEM (HR-SEM) images and EDX was achieved using a Zeiss Merlin high-resolution scanning electron microscope at an accelerating voltage of 5 kV, at DCCEM within the Department of Materials, University of Oxford.
Transmission electron microscopy (TEM) images and EDX were obtained from a JEOL-2100 electron microscope with an accelerating voltage of 200 kV using a single tilt specimen at DCCEM within the Department of Materials, University of Oxford.
Fourier transform Infrared spectroscopy (FTIR) was conducted utilizing a ThermoScientific Nicolet iS5 spectrometer fitted with an iD3 attenuated total reflection stage.Signals between 4000-700 cm -1 wavelengths were recorded at 4 cm -1 resolution.
Thermogravimetric analysis (TGA) was carried out under a nitrogen atmosphere utilizing Mettler Toledo TGA/DSC 1 system or PerkinElmer TGA 8000.The weight change between 30-800 °C was collected with 5 °C/min ramp rate.The pore structure is investigated by the N 2 adsorption and desorption isotherm at 77 K. Specific surface area is estimated using the Brauner-Emmett-Teller (BET) method. 2 Pore size distribution within the micropore (< 2 nm) and mesopore (2-50 nm) is calculated by applying an appropriate DFT kernel to the adsorption isotherm.For silica and core-shell composite, the adsorption of N 2 at 77 K on metal oxide with cylindrical pore kernel was selected. 3For LDH, the adsorption of N 2 at 77 K with slitpore kernel was adopted. 4Macropore (> 50 nm) size distribution is determined utilising Barrett-Joyner-Halenda (BJH) method. 2

Hydrogen temperature programmed reduction (H 2 -TPR)
H 2 -TPR curves were recorded on a Micromeritics AutoChem II 2920 Chemisorption Analyser equipped with a thermal conductivity detector (TCD).Around 120 mg of calcined core-shell was loaded in a quartz U-tube and purged with He (10 cm 3 /min) for 5 minutes at room temperature.This He flow was maintained while the sample was heated to 150 °C (10 °C/min) and hold for 5 minutes to remove moisture and impurities.After cooling to 40 °C, the reduction step started.The sample was heated from 40 to 600 °C at a rate of 5 °C/min under a H 2 flow (10% H 2 in N 2 at 50 cm 3 /min) with a sampling rate of 1 measurement per second.

Nitrous oxide (N 2 O) reduction of ES757@Cu x ZnAl catalysts
The dispersion (D Cu ) and exposed surface area (S Cu ) of Cu were determined by dissociative N  Total pore volume (cm 3 /g) E S 7 5 7 The BET specific surface area and b) the total pore volume of ES757, ES757@Cu 1.3 ZnAl LDH, ES757@Cu 1.3 ZnAl LDO, ES757@Cu 1.3 and Cu 1.3 ZnAl LDH.

Supporting tables
Cu 1.3 ZnAl LDO was obtained from the calcination of to afford Cu 1.3 ZnAl LDH at 330 °C for 4 hours at a 2 °C/min ramp rate under constant air flow at 50 mLn/min.During the process, the sample was held at 80 °C and 150 °C for one hour dwell time respectively.Cu 1.3 catalyst was prepared by reduction of Cu 1.3 ZnAl LDO at 290 °C for 2 hours at 2 °C/min ramp rate under constant gas flow (5% H 2 /N 2 ) at 50 stp mL/min (stp = standard temperature and pressure: P = 101.3kPa, T = 298 K).1.1.2Synthesis of mSiO 2 @Cu 1.3 ZnAl LDH, mSiO 2 @Cu 1.3 ZnAl LDO and mSiO 2 @Cu 1.3 Elemental C, H, N analysis (EA) and inductively coupled plasma atomic emission spectroscopy (ICP-OES) were conducted by Dr Nigel Howard at Microanalysis, University of Cambridge.N 2 adsorption/desorption isotherms of mesoporous (as well as macro-porous) and microporous materials were collected employing Micrometric 3Flex respectively at 77 K.All sample were ex-situ degassed on Micromeritics VacPrep overnight before the measurement.ES757@Cu x ZnAl-CO 3 -LDH core-shells were ex-situ degassed overnight at 110 °C and in-situ degassed at 110 °C for 1 hour.ES757@Cu x ZnAlLDOs and ES757@Cu x ZnAl catalysts was ex-situ degassed at 35 °C and in-situ degassed at 100 °C for 1 hour.

Table S1 .
The experimental molar ratio of elements in Cu 1.3 ZnAl LDH, ES757@Cu 1.3 ZnAl LDH, ES757@Cu 1.3 , physical mixture of ES757 and Cu 1.3 ZnAl LDH and the catalyst derived from physical mixture.

Table S2 .
The Cu loading, Cu dispersion and Cu surface area of ES757@Cu x catalysts.Determined by the ICP-OES.b:Obtained from the Rietveld refinement of the XRD pattern.c:Dispersion and specific surface area of metallic Cu is obtained from N 2 O chemisorption results.[43][44][45]

Table S4 .
Summary of some best catalysts in the literature including the reaction conditions, CO 2 conversion, MeOH selectivity and STY of methanol.A mixture of H 2 /CO 2 (3:1 by molar) gas feed were used in all cases.
a Reduced at 270 o C.