Supporting information

Photoreduction of CO2 on ZIF-8/TiO2 nanocomposites in a gaseous photoreactor under pressure swing

Ehsan Pipelzadeh,1 Victor Rudolph,1 Graeme Hanson,2,# Christopher Noble,1 Lianzhou

Wang1,*

Photocatalyst preparation 

Titanium dioxide (P25)

Titanium dioxide (P25) was purchased and used as received.

ZIF-8 synthesis

0.117 g of zinc nitrate (Zn(NO3)2. 6H2O was dissolved in 80 ml of milli-Q water. 2.27 g of 2-

methyl imidazole was also dissolved in 80 ml of milli-Q water. The solutions were mixed,

spontaneously forming a milky white suspension. The mixture was stirred for 5 minutes

followed by centrifuging at 15000 rpm for 10 minutes. The resultant sample was washed

using MilliQ water for a number of times and finally vacuum dried at 80 °C overnight to

obtain the ZIF-8 samples.

ZIF-8/TiO2 Synthesis:

TiO2/ZIF-8 core-shell nanocomposites were prepared in a sequential step-by-step process, consisting

of the following steps;

(1) TiO2 (P25) was dispersed in zinc nitrate (100 mmol) solution under ultrasonic treatment for 5

minutes.

(2) The suspension was centrifuged at 4500 rpm and washed with MilliQ water twice.

(3) Subsequently 2-methyl-immidazole (100 mmol) was added to the re-dispersed TiO2

nanoparticle suspension to form ZIF-8 coating layer on the surface of TiO2, and the suspension

was also ultrasonically treated or 5 minutes. The bath temperature was controlled at 50 °C.

(4) The suspension was centrifuged at 4500 rpm and washed as in step (2). This cycle was repeated

for 10 consecutive times to obtain the ZIF-8/TiO2 core-shell type nanocomposites.

ZIF-8/TiO2 calcination

ZIF-8/TiO2 composites were calcined under argon controlled atmosphere at rate 5 °C/min up

to 300 °C and 400 °C respectively. The nanocomposite was kept at maximum temperature for

3 hours before the temperature was dropped to room temperature.

Photocatalytic reactor design

Process flow diagram for designed and used photoreactor system

Reactor design

Figure (S1) illustrates the process flow diagram (PDF) of designed photoreduction unit. In principle

the photoreactor operates in batch mode. The process consists of two main compartments namely; (i)

humidifier and (ii) photoreactor which are connected to each other utilizing a pump.

Humidifier

The humidifier can accommodate as much as 100 ml of water and is equipped with a 200 micrometer

forged sparger that assures full humidification of circulation gas and stripping water soluble products

from the circulation gas.

Photoreactor

The photoreactor is a window type reactor equipped with a quartz window. It consists of two ports for

gas to enter and leave the photoreactor chamber. The ports are designed to indirectly flow over the

catalysts which are positioned in the middle of the photoreactor using a brass holder. This will provide

adequate flow and prohibit blowing the low density catalyst from the holder.

Operational pressure is governed by the thickness and quality of quartz window. Operational pressure

was set at 5 bar(g), using a safety factor of 1.5 as compared to hydro-test pressure of 7.5 bar. A safety

valve is used and set at 7 bar to prohibit over pressurizing the reactor. A water bath is used to keep the

photoreactor constant during the photo-irradiation period. The bath temperature was monitored during

the test using a methanol thermometer.

Circulation pump

A syringe type pump (Teledyne ISCO D260) is used for circulation of the gas in the photoreactor.

This pump features a high pressure seal which would provide a leak free circulation. It consists of a

cylinder of 250 ml capacity. The pump is operated in (i) constant pressure and (ii) swing pressure

mode.

Constant pressure mode is provided by limiting the span of pump capacity while circulation flow rate

is governed by the pump charge discharge cycle. On the other hand; the use of full cylinder capacity

would increase the photoreactor set up volume, which subsequently reduces the pressure from 5 bar to

3 bar, providing the pressure swing mode accordingly. The rate of charge and discharge of gas in the

pump is limited to 100 ml/min, thus; each cycle for charge and discharge would take 5 min.

Catalyst loading configuration

A brass holder is used to place 0.1 g of catalyst in the photoreactor. Sample was vacuum dried at 80

°C before the test. The holder is placed parallel to the window and perpendicular to the light source.

Light source

For the photoreduction test, simulated sunlight 1.5 AM was used with light intensity 100 mW/cm2.

Photoreduction test configuration

In a typical photocatalytic test, 0.1 (g) of active material was placed in the sample holder in a flat and

uniform form and placed in the allocated space in the photoreactor chamber. The reactor air was

removed by a series of (at least 5) vacuum and CO2 (g) purge cycles. The chamber was subjected to a

final vacuum followed by injection of 60 ml water into the humidifier chamber. This water was

previously stripped of soluble oxygen by bubbling argon through it.

CO2 (g) of 99.996% purity was injected into the reactor up to 5 bar (g). The circulation pump was

programmed according to the desired test conditions and gas was circulated in dark condition for 30

min to allow the reactant-catalyst reach a steady condition. Subsequently; the catalyst was exposed to

light irradiation for the test duration. Samples were taken from the circulation circuit through a port

equipped with septum, using a 1 ml gas tight syringe. Samples were injected in 3 ml vacuum vials and

analysed using a calibrated GC equipped with FID and TCD detectors. Furthermore, water from the

reactor was collected from the humidifier at the end of the reaction and tested for any possible

dissolved hydrocarbons using a calibrated HPLC.

Figure S1 Process flow diagram for designed photocatalytic reactor consisting of (i) humidifier, (ii) window type reactor and (iii) circulation pump (syringe).

 Material Characterization

Evidence of catalyst deactivation:

Electro paramagnetic resonance spectroscopy (EPR) spectra were recorded using a Bruker ELEXSYS

cw EPR spectrometer using a shq cavity and an ER4131VT liquid nitrogen cooling system. Samples

were tested in dry form. Weight of the samples was measured for normalization purposes.

Figure S2 illustrates the paramagnetic signal corresponding to TiO2 (P25) before and after

photocatalytic test. As it can be seen two distinctive peaks are evident in the EPR spectroscopy which

corresponds to anatase and rutile at 1.981 g and 1.970 g respectively. Further characterization of TiO2

after the test (AT) reveals a decrease in population of Ti3+ active sites per gram of TiO2 which

approve initial uncertainties on intermediates and products accumulation overtime on the photocatalyst

surface and thus poison the active sites. Cupper ion was detected in the photocatalyst (TiO2) after the

test which was found to originate from the brass sample holder.

Figure S2 EPR signal corresponding to TiO2 (P25) before and after test (AT).

Raman spectroscopy analysis

Figure S3 Raman shift spectra of TiO2 (P25), ZIF-8 and ZIF-8/TiO2.

UV-Vis spectroscopy analysis

The absorption spectra were measured using a Shimadzu 2200 UV-Vis spectrometer. Spectra was

collected in powder form.

Figure S4 UV-Vis absorption of ZIF-8, TiO2 (P25) and ZIF-8/TiO2 (P25)

Electro-paramagnetic resonance spectroscopy (EPR) study ZIF-8/TiO2

Figure S5 EPR signal for ZIF-8/TiO2 nanocomposite collected at 130 K.

Adsorption analysis

Samples were degassed at 473 K and at pressure of 50 torr for 24 hr prior to CO 2 and N2

sorption measurements. Specific surface areas (SBET) were calculated by the Brunauer-

Emmett-Teller method at the relative pressures in the range of P/P0 = 0.05 - 0.30; and

volumes of micropores were calculated from both the 77 K N2 and 273 K CO2 isotherms

using the Dubinin-Astakhov (D-A) equation using Micromeritics Tristar 3020 system. The

micropore surface area was calculated using Dubinin-Radushkevich (D-R) equation on the

CO2 adsorption data at 273 K.

Figure S6 N2 adsorption-desorption isotherms for TiO2 (P25), uncalcined ZIF-8/TiO2 nanocomposite, the nanocomposite calcined at 300 °C and 400 °C respectively.

Figure S7 CO2 adsorption-desorption isotherms for TiO2 (P25), uncalcined ZIF-8/TiO2 nanocomposite, the nanocomposite calcined at 300 °C and 400 °C respectively.

Figure S8 Photos of ZIF-8/TiO2 nanocomposite before and after photocatalytic reduction test.

Crystallographic study

The X-ray diffraction (XRD) measurements were conducted on a Rigaku Miniflex XRD

instrument with Co Kα (λ=1.78897 Å) radiation. Plot is Cu corrected. Samples were prepared and

tested in powder form.

Figure S9 XRD patterns of ZIF-8/TiO2 nanocomposite, (AT) denotes sample after photocatalytic test.

TGA-DSC studies

Thermal gravimetric analysis (TGA) was performed using alumina crucibles in a TGA/DSC STARe

System, Mettler Toledo. The test was conducted in air and argon controlled environment. The heating

rate was set at 1 °C/min. Samples were dried and stored under room conditions.

Argon controlled atmosphere

Figure S10 illustrates the TGA-DSC curves of ZIF-8 and ZIF-8/TiO2 nanocomposites under argon

controlled environment. In argon controlled environment the TGA analysis illustrates a 10 percent

weight loss in the synthesized ZIF-8 at 150 °C which has a corresponding endothermic peak in the

DSC data suggesting to be due evaporation of absorbed humidity. No significant weight change is

evident up to 550 °C under argon atmosphere with a relative DSC matching data suggesting no

exothermic reaction to occur with ZIF-8 material until 550 °C where it initiates thermal

decomposition. An exothermic peak is observable at 620 °C in the DSC where it encounters a steady

but significant weight loss up to 800 °C leaving a grey carburized residue. Further TGA studies were

conducted using synthesized ZIF-8/TiO2 nanocomposites under argon controlled atmospheres. A

sharp but limited drop in the weight is observed in the nanocomposite weight at 250 °C followed by

an endothermic peak at 280 °C suggesting some degree of structural changes. The nanocomposite is

found to be stable with no sign of weight loss up to 400 °C, however; the nanocomposite weight

gradually decreases with some degree of exothermic reactions up to 800 °C leaving a dark blackish

residue. This shift in the decomposition temperature is thought to be due to the catalytic interaction of

TiO2 with the ZIF-8 matrix readily decomposing its structure at elevated temperatures.

Figure S10 TGA-DSC curves of ZIF-8 and ZIF-8/TiO2 nanocomposites under argon controlled environment.

Air controlled atmosphere

Figure S11 illustrates TGA-DSC curves of ZIF-8 and ZIF-8/TiO2 nanocomposite under air controlled

environment. In air controlled atmosphere the TGA data for ZIF-8 and ZIF-8/TiO2 nanocomposites

clearly suggests that the choice of calcination atmosphere is clearly significant. The calcined ZIF-8

material under air controlled atmosphere has also revealed a drop in its initial weight at 150 °C with a

corresponding endothermic peak in the DSC signal which may correspond to loss of humidity.

Further significant weight loss is not observed up to 400 °C where two stage sharp weight losses are

recorded resulting in almost 50% weight loss up to 550 °C when it becomes stable up to 800 °C

leaving a white aggregated residue behind. Further DSC studies have revealed that the ZIF-8

undergoes an exothermic reaction from 300 °C followed by two distinctive exothermic peaks at 420

°C and 530 °C under air controlled atmosphere suggesting ZIF-8 decomposition and phase

transformation.

Further exposure of nanocomposite to air controlled atmosphere reveals significance of calcination

environment. Two distinctive exothermic peaks are evident at 456 °C and 544 °C which correlate to

violent exothermic reactions involving the ZIF-8 and TiO2. This result clearly shows air atmosphere

is not a suitable environment thus it was subsequently eliminated from experimental design.

Figure S11 TGA-DSC curves of ZIF-8 and ZIF-8/TiO2 nanocomposite under air controlled environment.

Figure S12 UV-Vis spectra of calcined nanocomposite at 300 °C before and after photoreduction test (AT).

Electro-paramagnetic resonance spectroscopy (EPR) study calcined ZIF-8/TiO2

The calcined nanocomposite was further examined for superficial reactive sites using EPR

spectroscopy as results are illustrated in Figure S13. The results clearly illustrate that the signals

corresponding to Ti3+ centres in anatase and rutile phase were not observed. However; a significant

single resonance at g~2.0034 and 2.003 was observed for calcined nanocomposite at 300 °C and 400

°C respectively, which is correlated to carbon centred radicals. The carbon active sites are thought to

contribute towards electron-hole separation by acting as electron sinks. Since these carbon centres

species originate from the organic cross linker in the framework, this could potentially affect the

reaction path from TiO2 surface in to the framework active sites. This property can be used to justify

the superior photoreduction activity, product stability and catalyst reusability.

Figure S13 EPR spectra of samples ZIF-8/TiO2 samples calcined at 300 °C and 400 °C.

Morphological of the samples

Figure S14 SEM micrograph of ZIF-8/TiO2 nanocomposite calcined at (a) 300 °C and (b) 400 °C.

Crystallographic studies

Crystallographic analysis was conducted on the calcined nanocomposite and results are illustrated in

Figure S15. As it can be seen the calcined nanocomposite is crystalline and its crystal structure is

effectively unchanged after the photoreduction test. This highlights calcination process at 300 °C is

(a) (b)

effective towards stability of the nanocomposite. Crystal structure is lost at 400 °C, this shows the

significance of calcination temperature.

Figure S15 XRD patterns of calcined ZIF-8/TiO2 nanocomposite at 300 °C before and after photoreduction test (AT) and at 400 °C.

Figure S16 X-ray patterns of ZIF-8/TiO2 (P25) before and after photoreduction test (AT).

Photocatalytic CO2 reduction

Gas Chromatography (GC)

Angilent Technology 7820 GC System and HayeSep Q column was used for CO and CH 4

characterization. Gas tight syringe (100 ml) was used for sample injection. Calibration curves were

prepared using calibrated gases and gas mixtures prepared using known glass volumes. Varian 3900

equipped with Resteck Rt®-Q-BOND was used to confirm the methane results.

Background tests

A number of preliminary tests were conducted in order to illuminate (i) carbon sourced other than the

gas phase CO2 and (ii) the possibility of catalytic activity in the absence of light.

The catalysts were exposed light irradiation in the presence of humidified air and then dried under

vacuum conditions. FTIR spectra of the TiO2 samples to monitor the possible carbon that might

contaminate the material: no such carbon was detected. The TiO2 (P25) catalyst was then subjected to

further photo-irradiation in humidified N2 (99.99%) for a period of 10 hours, after which samples

were taken from the reactor and tested for any CO and CH4. The GC results failed to indicate any

presence carbon containing species. Therefore it was concluded that carbon containing species were

not present within detectable limits and consequently any carbon products detected in actual test

conditions could confidently be assigned to originate from CO2 photocatalytic reduction.

TiO2 (P25) was treated with humidified CO2 under dark conditions for 10 hours. Samples were taken

every hour and analysed using a GC. There was no sign of any carbon containing products:

consequently, products obtained during light irradiation time are assigned to photocatalytic reduction

activity.

ZIF-8

The as-prepared sample ZIF-8 was subjected to the same background test conditions described for

TiO2 (25).

The resultant ZIF-8 was dried at 100 °C under vacuum conditions to remove any solvents namely

(water) from the synthesis media. The ZIF-8 was subjected to photo-irradiation for 10 hours in

presence of N2 to account for any potential photo-degradation of its structure which might result in

gaseous products. Samples were taken every hour and tested using GC. No gas products were found

and no apparent change was observed in the ZIF-8 material.

The ZIF-8 sample was then exposed to humidified CO2 under dark conditions for 10 hours. Samples

were taken every hour and tested using GC. The results showed no sign of catalytic activity in the

dark.

ZIF-8 was then treated under humidified CO2 in presence of light irradiation. The results did not

indicate any photocatalytic activity from the ZIF-8 material. Thus ZIF-8 was considered to be a

dispersion matrix with no intrinsic catalytic and photocatalytic activity in presence of humidified CO2.

ZIF-8/TiO2 nanocomposite

Background tests on synthesized ZIF-8/TiO2 nanocomposite to account for catalytic activity and

photo-degradation activity were conducted following the same procedure as discussed below and no

apparent catalytic activity in the absence of light or photo-decomposition was detected during the test.