Cu/M:ZnO (M = Mg, Al, Cu) colloidal nanocatalysts for the solution hydrogenation of carbon dioxide to methanol

Article abstract of DOI:10.1039/d0ta00509f

Doped-ZnO nanoparticles, capped with dioctylphosphinate ligands, are synthesised by the controlled hydrolysis of a mixture of organometallic precursors. Substitutional doping of the wurtzite ZnO nanoparticles with 5 mol% Mg(ii), Al(iii) and Cu(i) is achieved by the addition of sub-stoichiometric amounts of the appropriate dopant [(n-butyl)(sec-butyl)magnesium, triethylaluminium or mesitylcopper] to diethylzinc in the precursor mixture. After hydrolysis, the resulting colloidal nanoparticles (sizes of 2-3 nm) are characterised by powder X-ray crystallography, transmission electron microscopy, inductively-coupled plasma optical emission spectrometry and X-ray photoelectron spectroscopy. A solution of the doped-ZnO nanoparticles and colloidal Cu(0) nanoparticles [M:ZnO : Cu = 1 : 1] are applied as catalysts for the hydrogenation of CO₂to methanol in a liquid-phase continuous flow stirred tank reactor [210 °C, 50 bar, CO₂ : H₂= 1 : 3, 150 mL min^?1, mesitylene, 20 h]. All the catalyst systems display higher rates of methanol production and better stability than a benchmark heterogeneous catalyst, Cu-ZnO-Al₂O₃[480 μmol mmol_metal^?1h^?1], with approximately twice the activity for the Al(iii)-doped nanocatalyst. Despite outperforming the benchmark catalyst, Mg(ii) doping is detrimental towards methanol production in comparison to undoped ZnO. X-Ray photoelectron spectroscopy and transmission electron microscopy analysis of the most active post-catalysis samples implicate the migration of Al(iii) to the catalyst surface, and this surface enrichment is proposed to facilitate stabilisation of the catalytic ZnO/Cu interfaces.

Full text of DOI:10.1039/d0ta00509f

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Cu/M:ZnO (M ¼ Mg, Al, Cu) colloidal nanocatalysts
for the solution hydrogenation of carbon dioxide to
methanol†
Cite this: DOI: 10.1039/d0ta00509f
a
Alice H. M. Leung,a Andres Garcıa-Trenco,a Andreas Phanopoulos,
´
´
b
a
de
Manfred E. Schuster,c Sebastian D. Pike,
Milo S. P. Shaffer
*
Anna Regoutz,
a
*
and Charlotte K. Williams
Doped-ZnO nanoparticles, capped with dioctylphosphinate ligands, are synthesised by the controlled
hydrolysis of a mixture of organometallic precursors. Substitutional doping of the wurtzite ZnO
nanoparticles with 5 mol% Mg(II), Al(III) and Cu(I) is achieved by the addition of sub-stoichiometric
amounts of the appropriate dopant [(n-butyl)(sec-butyl)magnesium, triethylaluminium or mesitylcopper]
to diethylzinc in the precursor mixture. After hydrolysis, the resulting colloidal nanoparticles (sizes of 2–3
nm) are characterised by powder X-ray crystallography, transmission electron microscopy, inductively-
coupled plasma optical emission spectrometry and X-ray photoelectron spectroscopy. A solution of the
doped-ZnO nanoparticles and colloidal Cu(0) nanoparticles [M:ZnO : Cu ¼ 1 : 1] are applied as catalysts
for the hydrogenation of CO2 to methanol in a liquid-phase continuous flow stirred tank reactor [210 ꢀC,
50 bar, CO2 : H2 ¼ 1 : 3, 150 mL minꢁ1, mesitylene, 20 h]. All the catalyst systems display higher rates of
methanol production and better stability than a benchmark heterogeneous catalyst, Cu–ZnO–Al2O3
h
ꢁ1], with approximately twice the activity for the Al(III)-doped nanocatalyst.
ꢁ1
[480 mmol mmolmetal
Despite outperforming the benchmark catalyst, Mg(II) doping is detrimental towards methanol
production in comparison to undoped ZnO. X-Ray photoelectron spectroscopy and transmission
electron microscopy analysis of the most active post-catalysis samples implicate the migration of Al(III) to
the catalyst surface, and this surface enrichment is proposed to facilitate stabilisation of the catalytic
ZnO/Cu interfaces.
Received 13th January 2020
Accepted 15th May 2020
DOI: 10.1039/d0ta00509f
rsc.li/materials-a
have found use as solution-processible electronic inks, and as
high surface area catalysts.8–10 Relevant to this work are elegant
Introduction
examples of property enhancement through heterovalent
substitutional doping of ZnO.3,11–14 Depending on the dopant
either shallow energy level donor (n-type) or acceptor (p-type)
states are incorporated, adjusting the properties.3 To synthe-
sise these materials, a judicious choice of reactants and
conditions must be employed to balance the reactivity of the
host and dopant i.e. to prevent phase separation.1,4 n-Type ZnO
doping is more easily achieved and is most oen accomplished
through incorporation of Group 13 elements at interstitial
sites.3 For instance, aluminium doping enhanced the optical
and electronic properties of ZnO, affording a cheaper and less
toxic competitor to the ubiquitous transparent conducting
oxide, indium tin oxide (ITO).11 Alternatively, doping with Mg(II)
afforded colloidal solutions that were better reductants than
ZnO, as demonstrated by the rapid and spontaneous electron
transfer from Zn0.75Mg0.25O to ZnO.14 On the other hand, p-type
doping oen suffers from compensating mechanisms during
synthesis, such as the presence of low-energy native defects
(interstitial zinc atoms or oxygen vacancies) or background
impurities.3 Nevertheless, ZnO doping with Group 1 elements15
The doping of semiconductors is critical for manipulating
carrier density and band-gap in (opto)electronics, and in catal-
ysis to control activity, selectivity and lifetime.1,2 Zinc oxide,
a wide band-gap (3.3 eV), n-type semiconductor, is an important
material in both contexts, providing useful properties such as
transparency and accessible redox chemistry, while using only
abundant elements.3–7 Consequently, ZnO nanoparticles (NPs)
aChemistry Research Laboratory, University of Oxford, 12 Manseld Road, Oxford,
OX1 3TA, UK. E-mail: charlotte.williams@chem.ox.ac.uk
bDepartment of Chemistry, University College London, 20 Gordon Street, London
WC1H 0AJ, UK
cJohnson Matthey Technology Centre, Blount's Court, Sonning Common, RG4 9NH, UK
dDepartment of Chemistry, Imperial College London, London, SW7 2AZ, UK. E-mail: m.
shaffer@imperial.ac.uk
eDepartment of Materials, Imperial College London, London, SW7 2AZ, UK
† Electronic supplementary information (ESI) available: Tables outlining amount
of dopant precursor used and mass analysis by EA and TGA. Additional gures
showing nanocatalyst characterisation, methanol production rates and
selectivity as well as post-catalysts characterisation. Rationale for catalytic
reaction conditions used. See DOI: 10.1039/d0ta00509f
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or Cu(I)16 has been achieved. For instance, the thermal treat- high activity and selectivity over the reverse water–gas shi
ment of heterometallic cubane ([Me3Zn3M(THF)OtBu4], M ¼ Li, (rWGS) reaction.28
Na, K), at 750 ꢀC for 3 h, produced p-type ZnO nanoparticles
To make these colloidal catalysts, we have developed
(20–70 nm) with dopant levels up to 10 mol%.15,17 Ultimately, a synthesis using the hydrolysis of organozinc reagents. The
the successful and reproducible, large-scale fabrication of both method generates small (2–4 nm) and monodispersed ZnO
n- and p-type doped-ZnO could allow for production of ZnO p–n nanoparticles, which are soluble in a range of organic
junctions for use in thin lm, large area, potential transparent solvents.48–50 The hydrolysis of organozinc compounds (e.g.
semiconducting electronic devices.18–20
ZnPh2, ZnCy2, ZnEt2), in the presence of an aqueous stable,
Beyond semiconducting applications, ZnO is also one anionic coordinating ligand (e.g. carboxylate or phosphinate),
component of the industrial heterogeneous three-component readily generates soluble ZnO nanoparticles,48,50–57 with a high
catalyst Cu–ZnO–Al2O3, which is used to produce methanol on density of surface defects, proposed to be benecial for catal-
a 50 million-tonne scale in conjunction with a syn-gas feed- ysis.9,44,58 Hydrolytic syntheses provide straightforward control
stock.21,22 Methanol is a clean burning liquid fuel that can be of ligand coverage and kinetic control over the growth of
blended with petrol providing an attractive ‘drop-in’ substitute particles, allowing access to small ZnO nanoparticles (2–4
with existing fuels, and moreover can be generated from nm).4,50,54,57 The methodology is potentially attractive for dopant
renewable raw materials (i.e. CO2/H2).23–26 Doping the ZnO incorporation since a second organometallic reagent can simply
component can enhance the catalytic performance using either be added to the one-pot hydrolysis reaction. The synthesis of
syn-gas or CO2/H2 gas mixtures.27–29 The range of dopants doped-ZnO nanoparticles via co-precipitation methods has
includes alkaline metals (e.g. Cs(I) or Mg(II)),30 transition (e.g. allowed doping levels >10% (e.g. 18% Mg-,14 13% Al-29 or 12%
Zr(II) or Mn(II)/(III))27,31 or main group metals (e.g. Al(III) or Cu-doping59) with a range of metals (e.g. Group 1,15,17 Group 2,14
Ga(III)).32,33 These dopants are proposed to increase activity by Group 13,28,29 and Group 11 16,59ꢁ61); however, there are only
acting as electronic and/or structural promoters, i.e. by altering a few examples of successful doping using organometallic
the electronic structure through addition of energy levels to routes.4,62,63 Chaudret and co-workers demonstrated that the
reduce activation energy, or by modifying the catalyst nano- addition of Li-amides during the hydrolysis of ZnCy2, in the
structure to increase the number of active sites, respectively.28,34 presence of octylamine, led to the formation of spherical Li-
In particular, there is an enhancement in activity when n-type doped-ZnO nanoparticles (2.4–4.3 nm, Li content 1–10 mol%).
dopants, such as Al(III) and Ga(III) are incorporated into Cu/ The nanoparticle growth was controlled by the localisation of
ZnO catalysts.28,29 It is proposed that dopants facilitate forma- LiOH on the surface of ZnO.62 Alternatively, our group prepared
tion of Cu/ZnO interfaces by labilising the ZnOx (x < 1) Mg-doped ZnO nanoparticles by mixing ZnEt2 with MgBu2
component.35 These interfaces, oen observed at step-defects in before hydrolysis, in the presence of dioctylphosphinic acid
the Cu phase,36 are the putative catalytic active sites.29,37 (DOPA–H, (C8H17)2PO2H). The resultant particles showed up to
Conversely, by lowering the density of donor states, e.g. by 10% Mg incorporation into the wurtzite ZnO structure.63
doping Cu/ZnO with Mg(II), an increase in optical band-gap and
This work aims to investigate the preparation of other doped
reduction potential were observed.28 The lower efficiency for ZnO materials, e.g. with Al(III), Mg(II) and Cu(I). These colloidal
methanol production was attributed to the converse effects nanoparticles will be tested as components in liquid phase
limiting the formation of critical Cu/ZnOx interfaces.28,29
Most methanol production occurs using syn-gas in xed bed
reactor congurations but slurry phase methods, where the
catalyst is suspended in solution, are attractive alternatives.
This slurry process can improve reaction kinetics and may
catalysts for the reduction of CO2 to methanol.
Experimental
Materials and methods
increase the intrinsic activity. As long as high enough pressures The syntheses of all the ZnO nanoparticles were carried out in
and dilute conditions are implemented, the mass transport a dry nitrogen-lled glovebox and using Schlenk line tech-
effects can be managed, though for productivity reasons, reac- niques. All solvents were purchased from VWR, UK; all chem-
tors may, in fact, be operated near the mass transport limit.38 icals from Sigma Aldrich and were used as supplied, unless
Additionally, liquid phase synthesis may mitigate local hot otherwise stated. Toluene was dried using an MBraun SPS-800
spots, accelerate catalyst activation and facilitate continuous solvent purication system and degassed by sparging with N2
product removal. These benets have led to the development of for 1 h to remove oxygen. Diethyl zinc, triethyl aluminium and
a
commercial liquid-phase methanol synthesis process (n-butyl)(sec-butyl)magnesium are highly pyrophoric and vola-
(LPMeOH™).39 One method to optimize liquid phase processes tile: extreme caution must be taken during usage. HPLC-grade
is to develop colloidal (quasi-homogeneous) catalysts. Our water was used for the controlled hydrolysis experiments.
group,8,9,40–43 and others,44–46 have previously outlined some of
Elemental analyses were performed by Mr Stephen Boyer at
the advantages of such species including easy control over London Metropolitan University and inductively-coupled
catalyst composition (Cu : Zn ratio), access to small, well- plasma optical emission spectrometry (ICP-OES) was performed
dened, high surface area nanoparticles, and control over by Mr Alan Dickerson at the University of Cambridge.
surface chemistry. These colloidal catalysts have been applied
Powder X-ray diffraction (XRD) experiments were performed
either to the synthesis of methanol from syn-gas21,47 or by using a PANalytical X'pert PRO diffractometer in the theta/theta
carbon dioxide hydrogenation (CO2/H2).9,40,41 They can show reection mode, tted with a nickel lter, 0.04 rad Soller slit,
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10 mm mask, 1/4ꢀ xed divergence slit, and 1/2ꢀ xed anti- acetone, the mixture went from a colourless solution, via a white
scatter slit. An operating voltage of 40 kV was used with a step gel-phase, to a white suspension and the mixture was stirred for
size of 0.033ꢀ 2q, scan step time of 70 s and scan range of 10–80ꢀ 2 h. The white precipitate was isolated by centrifugation
2q. Air sensitive samples were prepared in a glovebox and sealed (3900 rpm, 20 min), re-dissolved in toluene (3 mL), re-
with polyimide tape. The diffraction patterns were analysed precipitated with acetone (12 mL), and nally washed with
using Fityk (version 0.9.0; Marcin Wojdyr, 2010). The peaks were acetone only; with centrifugation being applied aer each
tted to a SplitPearson7 function, and the crystallite size was washing-precipitation step. The white precipitate was le to air-
calculated from the full-width half-maximum of the tted curve, dry overnight, yielding ZnO@DOPA as a white powder.
using the Scherrer equation, to the most intense and not over-
Synthesis of M:ZnO@DOPA (M
¼
Mg, Al, Cu). Dio-
lapped reections; Cu: 43.5ꢀ (hkl ¼ 111), ZnO: 47.5ꢀ (hkl ¼ 102), ctylphosphinic acid (200 mg, 0.69 mmol) was added to
57.2ꢀ (hkl ¼ 110). For the detailed XRD study of the (101) ZnO a Schlenk ask, with a stirrer bar, and the ask was subjected to
nanoparticle peak, the diffraction scans were performed using vacuum for 2 h. Toluene (23 mL) was added to the ask.
the same diffractometer with a step size of 0.0167ꢀ 2q, scan step Diethylzinc (404 mg, 3.27 mmol) was added dropwise to the
time of 120 s and scan range of 32–38ꢀ 2q. The expansion/ solution, followed by the addition of the dopant precursor
contraction of the ZnO lattice was determined, using Bragg's Mg(nBu)(sBu), AlEt3 or CuMes (0.172 mmol, amounts of doping
equation, from the position of the (101) peak which was deter- reagent added are found in Table S1†). The mixture was stirred
mined by Gaussian t of the diffraction peak. UV-Vis absorption overnight under N2. A 0.4 M solution of HPLC-grade water (123
spectra of ZnO nanoparticles were recorded using a Perki- mg) in acetone (17.2 mL) was added to the reaction mixture
nElmer Lambda 950 spectrometer, in transmission mode, from dropwise over a period of 8 min. Upon the addition of water in
300–500 nm with a scan speed of 1 nm sꢁ1. Fourier transform acetone, the mixture went from a colourless solution, via a white
infrared (FT-IR) spectra were recorded using a PerkinElmer gel-phase, to a white (or green for Cu-doping) suspension and
Spectrum 100 spectrometer tted with an ATR attachment and the mixture was stirred for 2 h. The separation of the precipitate
each spectrum was collected with 32 scans, at a resolution of was achieved through centrifugation (3900 rpm, 20 min) and
4 cmꢁ1. Thermogravimetric analysis (TGA) was carried out then the precipitate was re-dissolved in toluene (3 mL) and re-
using a Mettler Toledo TGA/DSC 1, under a ow of dry N2 at 60 precipitated with acetone (12 mL), and nally washed with
ꢁ1
mL minꢁ1, from 100–700 C, at a heating rate of 5 ꢀC min
.
acetone only. Centrifugation was applied aer each washing-
ꢀ
Bright eld (BF) TEM and HAADF-STEM images were precipitation step. The precipitate was le to air-dry over-
acquired with Johnson Matthey's probe corrected ARM200F, at night, yielding Mg:ZnO@DOPA (0.426 g, 87% based on Zn) and
the ePSIC facility (Diamond Light Source), with an acceleration Al:ZnO@DOPA (0.440 g, 89% based on Zn) as white powders
voltage of 200 keV. Measurement conditions were a CL aperture and a dark green powder for Cu:ZnO@DOPA (0.438 g, 92%
of 30 mm, convergence semiangle of 24.3 mrad, beam current of based on Zn).
12 pA, and scattering angles of 0–10 and 35–110 mrad, for BF
Synthesis of Cu@DOPA.64 Mesitylcopper (73.3 mg, 0.40
and HAADF-STEM respectively.
mmol) and dioctylphosphinic acid (11.7 mg, 0.04 mmol) were
XPS was performed on a Thermo Scientic Ka+ X-ray dissolved in mesitylene (11 mL) in a 50 mL Young's ampoule
photoelectron spectrometer system operating at 2 ꢂ 10ꢁ9 and the overall [CuMes] was 0.036 M. The mixture was stirred
mbar base pressure. This system incorporates
a mono- for 10 min before two freeze–pump–thaw cycles were per-
chromated, microfocused Al Ka X-ray source (hn ¼ 1486.6 eV) formed. H2 gas was charged into the ask whilst the ask was
and a 180ꢀ double focusing hemispherical analyser with a 2D under vacuum and submerged in liquid nitrogen (at room
detector. The X-ray source was operated at 6 mA emission temperature, this loading equates to 2 bar of H2). The reaction
current and 12 kV anode bias and a ood gun was used to mixture was thawed ꢀand warmed to room temperature before
minimize sample charging. Samples were mounted using being heated to 110 C for 2.5 h, yielding a deep red solution.
conductive carbon tape and transferred to the spectrometer, The solution was cooled to room temperature and the H2 gas
using a special glovebox module, which ensured that samples was removed by 4 cycles of short vacuum/N2 (care must be taken
were transferred without exposure to air. Data were collected at not to remove solvent during this degassing process). The deep
20 eV pass energy for core levels and Auger lines. Data were red Cu@DOPA solution was then used in the catalytic hydro-
collected over six measurement positions using an X-ray spot genation of CO2 to methanol, alongside ZnO@DOPA or
size of 400 mm to avoid beam damage affecting the results. Data M:ZnO@DOPA.
were then averaged across the different measurement positions.
All data were analysed using the Avantage soware package.
Catalytic CO2 hydrogenation to methanol. The methanol
synthesis from CO2 experiments were conducted in the same
Synthesis of ZnO@DOPA.9,40 Dioctylphosphinic acid manner as previously described.40 A 300 mL continuous ow
(200 mg, 0.69 mmol) was added to a Schlenk ask, with a stirrer stirred-tank reactor (CSTR, Parr) was used, with stirring speed of
bar, and the ask was subjected to vacuum for 2 h. Toluene (23 1500 rpm and vertical baffles to ensure homogeneous mixing of
mL) was added to the ask and diethylzinc (425 mg, 3.45 mmol) the liquid and gas phase. Air-stable ZnO or M : ZnO nano-
was added dropwise to the solution. The mixture was stirred particles were added directly to the reactor vessel together with
overnight under N2. A 0.4 M solution of HPLC-grade water (123 mesitylene (89 mL). The solution was stirred and degassed by
mg) in acetone (17.2 mL) was added to the reaction mixture a ow of N2 (600 mL minꢁ1) for 30 min. The air-sensitive
dropwise over a period of 8 min. Upon the addition of H2O in Cu@DOPA colloid was introduced into the reactor using
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a syringe under a ow of N2, giving a total reactor volume of 100 metal : ligand ratio was maintained at 5 : 1 as these conditions
mL. A Zn : Cu ratio of 1 : 1 (0.4 mmol each) was added in all yielded particles fully saturated with ligand. Upon reaction with
cases. The reactor was then pressurised to 50 bar using a gas water, the organometallic reagents produce only inert hydro-
mixture of 96 vol% of H2 : CO2 ¼ 3 : 1 and 4 vol% Ar (used as carbons as byproducts (i.e. ethane, butane and/or mesitylene,
ꢀ
internal standard for GC analysis), and heated to 210 C. The respectively) which are easily removed from the nal product in
experiments were performed at a ow rate of 150 mL minꢁ1 for vacuo. The post-hydrolysis mixture, consisting of a colloidal
20 h. The stability study with Al:ZnO@DOPA was performed at suspension of ZnO nanoparticles, was puried by reducing the
a ow rate of 150 mL minꢁ1 for 40 h.
solvent volume and via centrifugation, yielding Mg:ZnO@DOPA
The commercial CuO–ZnO–Al2O3 catalyst was obtained from and Al:ZnO@DOPA as white powders and Cu:ZnO@DOPA as
Alfa Aesar (45 776, pellet size 5.4 ꢂ 3.6 mm, mass composition a dark green powder.
CuO, 63.5%; ZnO, 25.1%; Al2O3, 10.1%; MgO, 1.3%). It was
Analysis of the as-synthesised particles by X-ray diffraction
ground to a ne powder and tested as a reference material. The (XRD) showed the formation of small crystalline particles (2–3
choice of catalyst loading was dictated by the normalisation nm) with sizing determined via peak analysis using the Scherrer
considerations, making sure that the total molar concentration equation (Table 1). In each case, only the wurtzite ZnO phase
of Cu and Zn was the same as in the Cu/ZnO colloids. The was observed (Fig. 1a–d), suggesting incorporation of any
catalyst is pre-activated using a diluted H2 stream (5 vol% H2/ dopants into the crystalline structure rather than co-
N2) at 4.5 bar and 240 ꢀC (ramp 2 ꢀC minꢁ1) for 3 h, according to crystallisation of other phases. High-angle angular dark-eld
a standard activation protocol for the ternary Cu–ZnO–Al2O3 scanning tunnelling electron microscopy (HAADF-STEM)
methanol synthesis catalyst in a slurry reactor.65 The catalytic images (Fig. 1e–g) revealed spherical nanoparticles with
runs were performed under the conditions stated above.
The reaction products and unreacted material were moni- amorphous phases were observed by electron microscopy.
tored by an online gas chromatograph (Bruker 450 GC), Incorporation of the second metal was assessed further, by
narrow size distributions (2.6–2.8 nm, Fig. S1†) and no obvious
equipped with a thermal conductivity detector (TCD) and considering changes to the lattice parameters, evidenced in the
a ame ionisation detector (FID). TCD was used to quantify CO, strongest (101) reection (Table 1, Fig. S2†). A signicant contrac-
CO2 and Ar, whilst FID was used to quantify methanol and other tion was observed in Al:ZnO@DOPA, indicative of successful
˚
oxygenates or hydrocarbons. The lines from the reactor to the substitution of tetrahedral Zn(II) sites (rion[Zn(II)] ¼ 0.74 A) with Al(III)
GC were heated to 180 ꢀC to avoid the condensation of any (rion[Al(III)] ¼ 0.53 A).76,77 No clear lattice changes were observed for
˚
products. All experiments were conducted under differential Mg:ZnO@DOPA and Cu:ZnO@DOPA due to the similar ionic radii
˚
˚
conditions, with an overall CO2 conversion <2%, avoiding mass of the metals (rion[Mg(II)] ¼ 0.71 A and rion[Cu(I)] ¼ 0.74 A) and the
transport limitations (see ESI† for details). Selectivity values are peak breadth resulting from the small particle sizes.14,78
given on a carbon basis, where the only two products were
methanol and CO.
Bulk compositional analysis by ICP-OES conrmed and
quantied the presence of the dopants. For Al:ZnO@DOPA and
Cu:ZnO@DOPA, the heteroatom concentration of 5 mol% is in
line with starting reagent stoichiometry, but the value for
Mg:ZnO@DOPA is slightly lower than expected (Table 2). The
solid-state27 Al-MAS-NMR spectrum of Al:ZnO@DOPA (Fig. S3†)
Results and discussion
Synthesis and characterisation of M:ZnO@DOPA
Small (2–3 nm) colloidal ZnO nanoparticles (ZnO@DOPA) were conrms the replacement of tetrahedral Zn(II) with Al(III), indi-
previously prepared by the controlled hydrolysis of diethylzinc cated by the sharp signal at d 55 ppm which corresponds to
and sub-stoichiometric amounts (vs. ZnEt2) of DOPA–H, in tetrahedral Al(III) environments. In addition, pentahedral (dAl:
a molar ratio of Zn : DOPA–H ¼ 5 : 1.9,40 The [DOPA]ꢁ ligand
stabilises the nanoparticles and results in high solubility in
organic solvents.9,40,57 Here, this general hydrolysis procedure
Table 1 Table showing the sizes and (101) peak information for
ZnO@DOPA and M:ZnO@DOPA
was applied to doped-ZnO nanoparticles by the reaction of
ZnEt2 with the desired dopant organometallic precursors (MRx)
prior to hydrolysis. Three dopants were targeted as they either
showed precedent for acting as structural and/or electronic
Size (nm)
(101) reectionc
ꢁ1
ꢁ1
promoters (Mg(II) and Al(III))28,29,32,66,67 or are required for the M : ZnO samples XRDa TEMb
2qꢀ
Dd A
˚
d A
˚
highly investigated Cu–ZnO synergy (Cu(I)),24,28,29,36,37,68–75 which
is proposed to be critical in methanol synthesis.
ZnO@DOPA
2.3
2.0
2.3
2.2
2.8 ꢃ 0.02 (0.6) 34.62 2.59
2.7 ꢃ 0.04 (0.5) 34.31 2.61
2.7 ꢃ 0.06 (0.9) 35.75 2.51
2.6 ꢃ 0.03 (0.4) 34.82 2.58
—
Mg:ZnO@DOPA
Al:ZnO@DOPA
Cu:ZnO@DOPA
+0.02
ꢁ0.08
ꢁ0.01
In each case, a pre-hydrolysis mixture was obtained by
reacting 0.2 equiv. of DOPA–H with commercially available
diethylzinc (0.95 equiv.) and 0.05 equiv. of commercially avail-
able (n-butyl)(sec-butyl)magnesium, triethylaluminium or
mesitylcopper (CuMes) in toluene. These precursors were
selected not only due to their availability, but also high solu-
bility in organic solvents and rapid reactivity with water to deter
phase separation between zinc and dopant. The overall
a
Average particle size determined by application of the Scherrer
b
equation to XRD (102) and (110) reections (Fig. 1a–d). Average
particle size ꢃ standard error (standard deviation) determined by size
analysis of TEM images (Fig. 1e–f). The standard error of the mean is
c
dened as standard deviation/(no. of measurements). The positions
of (101) peaks were determined using Gaussian ts to the XRD
reection in the range of 30–35ꢀ 2q (Fig. S2).
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Fig. 1 Powder XRD patterns and HAADF-STEM images of (a) ZnO@DOPA, (b) & (e) Mg:ZnO@DOPA, (c) & (f) Al:ZnO@DOPA and (d) & (g)
Cu:ZnO@DOPA. Patterns are indexed against ZnO as grey vertical bars (JCPDS 01-085-1326). All scale bars in the STEM images are 3 nm.
Table 2 Table showing the dopant quantities, obtained by ICP-OES
and XPS, and optical band-gaps of ZnO@DOPA, Mg:ZnO@DOPA,
Al:ZnO@DOPA and Cu:ZnO@DOPA, obtained from Tauc plots (Fig. S4)
of ZnO, with two features at 987.9 eV and 991.1 eV kinetic energy
(KE). The P 2p and C 1s lines show BEs expected for the DOPA
ligand. The O 1s core level has two contributions, one from ZnO
at 530.4 eV and one at 531.5 eV from DOPA.80 The Mg 1s and Al
Optical band-gap
(eV)
Dopant quantity (rel.
at%)
2p core level appear at BEs typical for Mg(II) and Al(III), 1304.2 eV
and 73.9 eV, respectively. The Cu 2p3/2 core level is at a BE of
932.3 eV, which can either stem from Cu(0) or Cu(I).81 Usually,
the Cu LMM Auger lines can be used to distinguish between
these two oxidation states, but in these samples, the lines over-
lap with the much stronger Zn LMM lines, preventing further
analysis. The core level also exhibits a small shoulder towards
higher BE, at 933.7 eV, indicating a small amount of Cu(II). From
peak t analysis of the Zn 2p3/2 and the metal core levels, relative
atomic ratios were determined (Table 2). Comparing the dopant
quantities obtained by XPS and ICP-OES highlights the likely
surface speciation of Mg, compared to Al which appears to be
distributed throughout the particles.
M:ZnO@DOPA
ICP-OES
XPS
UV-Visc
ZnO@DOPA
—
—
3.71
3.84
3.65
3.67
Mg:ZnO@DOPA
Al:ZnO@DOPA
Cu:ZnO@DOPA
3.77 ꢃ 0.05
4.96 ꢃ 0.07
5.05 ꢃ 0.07
4.5a
5.0b
3.3a
a
Derived from comparison of the peak area from peak t analysis of Zn
b
2p3/2 with Cu 2p3/2 and Mg 1s (Fig. 2). Error of ꢃ0.5%. Derived from
comparison of the peak area from peak t analysis of Zn 3s with Al 2p
c
(Fig. 2). Error of ꢃ0.5%. Error of ꢃ0.04 eV, derived from uncertainty
of linear ts (coefficient of determination, R2 > 0.99) (Fig. S4).28
The optical band-gaps were determined via Tauc plots from
the on-set absorption in solid-state UV-Vis spectra (Table 2,
Fig. S4†). The Al-doped sample showed a smaller band-gap than
ZnO, presumably due to the introduction of Al(III) additional
donor levels.11,82–84 In contrast, Mg(II) doping widens the band-
gap, as observed by others.14,28,78,85–87 A small decrease in the
band-gap of Cu:ZnO@DOPA (3.67 eV) suggests partial incor-
poration of Cu(I)/Cu(II) into the lattice, where the dopant
provides additional shallow acceptor levels,88,89 as a larger
decrease would be expected for 5% doping.90 It is possible that
47 ppm) and octahedral (dAl: ꢁ17 and 16 ppm) Al environments
are also present and are tentatively assigned as surface
sites.28,66,67
The chemical environments and oxidation states for all
samples were analysed using XPS (Fig. 2). All samples show
a binding energy (BE) for Zn 2p3/2 at 1021.7 eV, however from the
Zn core level alone, it is not possible to determine its oxidation
state, as the BEs of Zn metal and ZnO are identical.79 However,
the Zn L3M4,5M4,5 Auger lines clearly show the typical structure
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Fig. 2 XPS core levels and Auger spectra for ZnO@DOPA and M:ZnO@DOPA samples, including (a) Zn 2p3/2, (b) Zn L3M4,5M4,5 (c) P 2p/Zn 3s, (d)
C 1s, (e) O 1s, and (f) Mg 1s of Mg:ZnO@DOPA, (g) Al 2p of Al:ZnO@DOPA, and (h) Cu 2p3/2 of Cu:ZnO@DOPA.
some of the CuMes is also reduced to Cu(0), rather than doping hydrogenation of CO2 to methanol (Fig. 3), under differential
the ZnO. It was previously reported that CuMes reacts with conditions (CO2 conversion <2%). A short residence time
diethylzinc to produce Cu(0) nanoparticles.41 In this work, allowed accurate calculation of reaction rates, avoiding
however, there was no direct evidence for such small Cu(0) complications due to competing side reactions such as the
nanoparticles by TEM or XRD.
rWGS reaction. Cu@DOPA nanoparticles were synthesised by
Analysis of all the new nanoparticles by TGA showed a mass the hydrogenation of CuMes in the presence of DOPA–H and
loss of 25–30% between 150–500 ꢀC for all M:ZnO@DOPA, detailed characterisation data are consistent with previous
assigned to the thermal decomposition of the [DOPA]ꢁ ligand reports (Cu : DOPA–H ¼ 10 : 1).64,91 For each catalytic experi-
(Table S2, Fig. S5†). Along with elemental analysis (Table S2†), ment, the colloidal catalysts (0.4 mmol Cu and 0.4 mmol
ratios close to M:ZnO : [DOPA]ꢁ ¼ 6 : 1 were found for M:Zn@DOPA) were introduced into the continuous ow stirred
M:ZnO@DOPA. The ratio differs slightly to the starting reagent tank reactor with mesitylene as solvent (total of 100 mL), under
stoichiometry and it is proposed that low quantities of soluble a dynamic ow of nitrogen. Mesitylene was selected on the basis
molecular clusters e.g. [Zn4O(DOPA)6], are removed during nano- of its non-coordinating nature and high boiling point (165 ꢀC),
particle purication and account for the lower ligand loading.57
From IR spectroscopy (Fig. S6†), the presence of C–H stretches
(2800–3000 cmꢁ1), along with phosphinate, PO2ꢁ, stretches
(ꢄ1047 cmꢁ1, asymm; ꢄ1126 cmꢁ1, symm) associated with the
ligand were detected. The separation of 79–81 cmꢁ1 between the
asymmetric and symmetric phosphinate stretches indicates k2-
ligand coordination (see Fig. S6† inset). The presence of O–H
stretches centred at 3380 cmꢁ1, is indicative of hydroxyl and
water coordinated to the nanoparticle surface. Overall, all char-
acterisation data strongly suggest the successful incorporation of
Mg(II) and Al(III) into the lattice of ZnO nanoparticles, whilst Cu(I)
appears to have been partially incorporated.
Catalytic CO2 hydrogenation to methanol
Fig. 3 Overview of M:ZnO@DOPA syntheses and the subsequent
The M:ZnO@DOPA nanoparticles were combined with colloidal
combination with Cu@DOPA for the formation of colloidal Cu/ZnO
Cu@DOPA nanoparticles and tested as catalysts for the nanocatalysts in hydrogenation of CO2 to methanol.
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while being sufficiently volatile to allow for its complete removal
All post-catalysis colloids were characterised using air-
under vacuum for analysis of spent catalysts. The reactor was sensitive techniques to prevent the oxidation of Cu(0). By
pressurised to 50 bar with a gas feed of a CO2 : H2 mixture XRD, all samples displayed typical diffraction peaks attributed
(molar ratio of 1 : 3), at 210 ꢀC, following previous protocols.39,70 to crystalline ZnO and metallic Cu(0), with no additional crys-
Established colloidal Cu/ZnO@DOPA and the commercial talline phases observed (Fig. S9–S11†). Some ripening of the
heterogeneous catalyst Cu–ZnO–Al2O3 were also tested as original M:ZnO@DOPA and Cu(0) nanoparticles was indicated
benchmarks for this study. The activation of heterogeneous Cu– by XRD and TEM (Table 3).
ZnO–Al2O3 was conducted following a standard procedure.65
The difference in activity between the highest (Al-doped) and
All catalysts reached peak methanol production rate aer 3– lowest (Mg-doped) samples is tentatively attributed to the
4 h time-on-stream and remained stable for over 15 h (Fig. S7 quantity of Cu/ZnO interfaces. These interfaces, formed under
and S8†). High methanol selectivity was observed in all cases the reductive catalytic conditions, are commonly suggested to
and CO was the only by-product (from the rWGS reaction). Both be the active site for methanol synthesis. They can be identied
doped and undoped colloidal catalysts displayed higher activi- using electron microscopy, through analysis of the degree of
ties than the commercial heterogeneous catalyst (Fig. 4). Mg- contact between different identiable crystalline pha-
doping was somewhat detrimental to activity in comparison ses.36,37,46,68,69 To this end, the nanoscale structures of post-
with undoped ZnO@DOPA; Cu/Mg:ZnO@DOPA displays 30– catalysis samples of Cu/Mg:ZnO@DOPA and Cu/Al:ZnO@-
40% lower activity than the other three colloidal catalysts. Cu/ DOPA were investigated using BF-TEM, and were found to differ
Al:ZnO@DOPA showed a small but noticeable improvement signicantly.
when compared to Cu/ZnO@DOPA (+10%), but no clear
difference was observed for the Cu-doped sample.
Cu/Al:ZnO@DOPA showed contact between Cu and Al:ZnO
nanoparticles (Fig. 5a & S13†). The nanocrystalline particles
were additionally surrounded by an amorphous network that is
rich in Al(III) (identied by EDX) and likely to be alumina as
suggested by the emergence of a second, higher BE species (76.8
eV) in the Al 2p core line by XPS (Fig. S12 & S13†). The formation
of surface alumina under reducing conditions has also been
found in the heterogeneous Cu–ZnO–Al2O3 catalyst. In this case
it appears the Al(III) that remains within the lattice acts as an
electronic promoter, while the in situ formed alumina matrix
acts as a structural promoter.28,29,67 It is proposed that Al-doping
of ZnO increases the density of donor states (n-type) and
contributes to the formation of oxygen vacancies which accel-
erates migration of Zn onto Cu(0) and hence increases the
formation of Cu/ZnO interfaces.71–73
More isolated nanoparticles and fewer Cu/ZnO interfaces were
observed for Cu/Mg:ZnO@DOPA (Fig. 5b & S14†). Additionally,
the Mg-rich surface of Mg:ZnO@DOPA (as suggested by XPS)
might hinder ZnO and Cu(0) interface formation. EDX of the post-
catalysis sample shows that some Mg is distributed in between
Cu(0) and ZnO nanoparticles (Fig. S14†). It appears Mg(II) is
detrimental to activity as both a structural and electronic dopant.
Fig. 4 Peak methanol rates and selectivity for the colloidal Cu/
ZnO@DOPA, Cu/M:ZnO@DOPA catalysts and reference catalyst Cu–
ZnO–Al2O3 in the hydrogenation of CO2 to methanol. Reaction
conditions: 210 ꢀC, 50 bar, CO2 : H2 ¼ 1 : 3, 150 mL minꢁ1
.
Table 3 Cu(0) and ZnO nanoparticles size data from XRD and TEM for pre-catalysis and post-catalysis colloids (20 h time-on-stream, unless
otherwise specified)
ZnO (nm)
Pre-catalysis
XRDa
Cuf (nm)
Post-catalysis
XRDa
Post-catalysis
XRDa
Catalyst
TEMb
TEMb
TEMb
Cu/ZnO@DOPAc
Cu/Mg:ZnO@DOPA
Cu/Al:ZnO@DOPAd
Cu/Cu:ZnO@DOPA
2.3
2.0
2.3
2.2
2.8 ꢃ 0.02 (0.6)
2.7 ꢃ 0.04 (0.5)
2.7 ꢃ 0.06 (0.9)
2.6 ꢃ 0.03 (0.4)
3.2
2.3
4.9 ꢃ 0.1 (1.1)
3.5 ꢃ 0.04 (0.4)
4.5 ꢃ 0.07 (1.3)
—
6.4
4.4
7.2 ꢃ 0.2 (1.9)
4.3 ꢃ 0.06 (0.6)
4.6 ꢃ 0.02 (1.1)
—
3.6 (3.1)e
3.3
3.0 (2.4)e
2.0
a
b
Average particle size determined by application of the Scherrer equation to XRD (102) and (110) reections (Fig. 1a–d and S9–S11). Average
particle size ꢃ standard error (standard deviation) determined by size analysis of TEM images (Fig. 1e–f and S14, S15). The standard error of
c
the mean is dened as standard deviation/(no. of measurements). The sizing measurement is taken from previously published system.40
d
e
f
Measurement taken from 40 h time-on-stream study. Crystallite size of catalyst aer 20 h time-on-stream. For pre-catalysis Cu@DOPA, sizes
of ꢄ2 nm were reported using TEM.64
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increase from 2.4 to 3.0 nm and the ZnO from 3.1 to 3.6 nm
(Table 3 & Fig. S10†). Whilst the solvated, phosphinate ligand
stabilised particles ripen more slowly than the heterogeneous
catalyst, the presence of the alumina network may help to
decrease the sintering further through structural promotion.
CO2 conversion could likely be increased with longer residence
times (lower supplied gas space velocities), but at the expense of
rate and methanol selectivity.38,43 A detailed process study would
be needed to optimise any one specic catalyst.
Conclusions
Fig. 5 BF-TEM images of the post-catalysis sample of (a) Cu/Al:Z-
nO@DOPA and (b) Cu/Mg:ZnO@DOPA.
Doped-ZnO nanoparticles (2–3 nm), with ꢄ5 mol% dopant
(Mg(II), Al(III) and Cu(I)), and phosphinate capping ligands, were
synthesised by the room temperature hydrolysis of a mixture of
organometallic reagents. The particle analysis suggests the
localisation of Mg(II) on the surface of the ZnO nanoparticles,
whilst Al(III) is distributed throughout the structure. For Cu-
doped ZnO, incomplete incorporation of Cu(I) is suggested by
the optical band-gap and XPS, probably due to partial Cu(0)
formation. UV-Vis spectroscopy showed a smaller band-gap for
Al-doping but a larger band-gap for Mg-doped particles
compared to ZnO alone.
The doped ZnO nanoparticles were combined with Cu(0)
nanoparticles (<2 nm), capped by phosphinate, to form a series
of active colloidal nanocatalysts for tꢀhe hydrogenation of CO2 to
methanol in the liquid phase (210 C, 50 bar, H2 : CO2 molar
ratio of 3 : 1, 150 mL minꢁ1). All catalysts showed methanol
production rates higher than a benchmark heterogeneous
catalyst Cu–ZnO–Al2O3, although the Mg-doped catalyst showed
ꢄ40% lower activity than the other colloidal catalysts. It is
proposed that the Mg rich surface layer prevents formation of
catalytically important Cu/ZnO interfaces. In contrast, Al-
doping improved activity compared to ZnO nanoparticles. An
amorphous surface alumina network is indicated in these post-
catalysis samples and may hinder nanoparticle ripening and
improve catalyst stability over a 40 h time-on-stream.
This is in line with a previous study of Mg(II) doping into the
heterogeneous Cu–ZnO–Al2O3 methanol catalysts from syn-gas
(230 ꢀC, 30 bar, CO : CO2 : H2: inert gas ¼ 6 : 8 : 59 : 27).28
Stability of the colloidal catalyst
During the 20 h time-on-stream (210 ꢀC, 50 bar, mesitylene), the
colloidal catalysts all displayed good short-term stability. In
contrast, the commercial heterogeneous catalyst Cu–ZnO–Al2O3
was signicantly deactivated, by ꢄ15%, over this period. This
reduction in activity may be attributed to sintering and
agglomeration of Cu(0) under the operating conditions.92,93 In
this sense, CO2/H2 is a considerably more demanding feed-gas
than syn-gas. In order to explore colloidal catalyst stability over
a longer period, the Cu/Al:ZnO@DOPA sample was tested
against the heterogeneous catalyst over 40 h (210 ꢀC, 50 bar, 150
mL minꢁ1). Only a small reduction in activity was observed for
Cu/Al:ZnO@DOPA (ꢄ7%), whereas the Cu–ZnO–Al2O3 catalyst
experienced ꢄ30% deactivation (Fig. 6 & S15†). With both
catalysts, methanol selectivity remained high throughout
(Fig. S16†). The better stability of the colloidal catalyst is
tentatively attributed to the high particle stability, as post
catalysis analysis revealed that agglomeration was signicantly
prevented. Specically, the Cu(0) nanoparticles sizes only
The organometallic synthesis of doped colloidal ZnO nano-
particles is a useful means to tune properties and appears to be
somewhat generalizable. Future studies should target mixed
metal oxides and dopant levels. In particular, transition metal
and Group 13 n-type ZnO dopants should be explored, as these
materials show promising performances in syn-gas liquid phase
methanol synthesis.26
Conflicts of interest
There are no conicts to declare.
Acknowledgements
The Engineering and Physical Sciences Research Council are
acknowledged for research funding (EP/H046380/1, EP/K035274/
1; DTA studentships to AL). AR acknowledges the support from
Imperial College London for her Imperial College Research
Fellowship and from the Analytical Chemistry Trust Fund for her
CAMS-UK Fellowship. Johnson Matthey PLC is acknowledged for
Fig.
6 Methanol rates of Cu/Al:ZnO@DOPA and commercial
heterogeneous Cu–ZnO–Al2O3, over 40 h time-on-stream.
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