Shape selective plate-form Ga2O3 with strong metal-support interaction to overlying Pd for hydrogenation of CO2 to CH3OH

Article abstract of DOI:10.1039/c3cc38455a

A stronger metal-support interaction between Pd and plate-form Ga ₂O₃ nanocrystals covered with the predominant 002 surface than other Ga₂O₃ surfaces is found, which gives higher methanol yield in catalytic CO₂ hydrogenation. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3cc38455a

ChemComm
View Article Online
View Journal | View Issue
COMMUNICATION
Shape selective plate-form Ga O with strong
2
3
metal–support interaction to overlying Pd for
Cite this: Chem. Commun., 2013,
4
9, 1747
hydrogenation of CO to CH OH†
2
3
Received 24th November 2012,
Accepted 11th January 2013
a
a
a
a
a
b
Xiwen Zhou, Jin Qu, Feng Xu, Jingping Hu, John S. Foord, Ziyan Zeng,
b
a
Xinlin Hong and Shik Chi Edman Tsang*
DOI: 10.1039/c3cc38455a
www.rsc.org/chemcomm
A stronger metal–support interaction between Pd and plate-form
Ga nanocrystals covered with the predominant 002 surface of b-Ga O where oxygen and gallium ions terminated (002)
than other Ga
yield in catalytic CO
Here, we report a remarkable activity dependence on the shape
2
O
3
2
3
2
O
3
surfaces is found, which gives higher methanol surfaces of plate nanocrystal exert a stronger electronic interaction
hydrogenation. with Pd than other surfaces, giving a higher yield for methanol
production from CO hydrogenation. This result has echoes of the
Global warming caused by increasing atmospheric carbon dioxide previously reported shape effect of ZnO in Cu/ZnO for the same
CO ) concentration and the depletion of fossil fuels are becoming reaction where the electron rich (002) face, as alternative oxygen
the focus of worldwide attention. On site catalytic conversion of and zinc ions layers in plate form a ZnO crystal, also shows a
2
2
(
2
1,2
14
CO to liquid fuels or chemicals, given a steady supply of the CO
stronger material synergy with Cu than other ZnO facets. DFT
2
2
substrate and excess energy, is a promising route that may offer a calculations and characterization clearly suggest that these high
3,4
solution to these important environmental and energy issues. In energetic oxide surfaces interact more strongly with overlying metal
particular, installation of chemical reactors for the hydrogenation particles for beneficial SMSI for this important reaction.
of CO
power stations, iron refineries and incinerators, which not only first synthesized, followed by deposition of Pd for each sample (see
offers the abatement of CO on site but also provides transportable ESI†). X-ray powder diffraction (XRD), transmission electron micro-
2
to methanol could be an attractive supplement to coal fired
2 3
Two forms of crystalline Ga O nano-crystals (rod and plate) were
2
fuel from this reaction. The convenient production of hydrogen on scopy (TEM) and electron diffraction (ED) data shown in Fig. 1
a large scale from renewable sources (solar energy, hydropower, confirm both rod and plate-shaped particles from the observed
biomass, or excess chemical heat etc.) close to the point of use for morphologies, comprising a highly crystalline monoclinic b-Ga O3
2
5,6
this application may support such a green integrated process.
structure with a = 12.225 Å, b = 3.039 Å, c = 5.801 Å (JCPDS 43-1012).
Majority of current research on catalytic CO hydrogenation Only extremely broad Pd XRD peaks were detected in both samples
2
has been using modified industrial Cu/ZnO based catalysts from when Pd was incorporated, indicative of very high metal dispersion,
synthesis gas hydrogenation technology, which are operated at and it was therefore not possible to derive the particle size within
relatively slow gas flow and elevated temperature and pressure acceptable accuracy. Analysis near the edges of the rod Ga
due to slow kinetics. For the conversion of CO
2
O
3
nano-
2
to fuels it would crystals by TEM and ED suggested that the majority of their surfaces
7–9
be economic and technically more viable if the CO fixation can be were terminated with either (111) or (110) faces, with the rod growth
2
carried out more readily under milder conditions. There are some extending along the [110] growth direction; such stable low index
combinations of metal and metal oxide catalysts, which have been terminations are expected in high temperature synthesis and the rod
claimed to display higher turnover frequency towards methanol shape reflects the monoclinic crystal structure. In contrast, for plate
1
0
than Cu/ZnO catalysts. For example, the higher activity of Pd/ form Ga
Ga but with slightly lower selectivity than Cu/ZnO has been energy (002) face was generally observed for the dominant plate faces,
noted.
metal–support interaction (SMSI) of these catalysts, which plays (002) XRD peak intensity (Fig. 1b) also reflects the dominance of this
2 3
O , the same analysis showed that the more open, higher
2 3
O
11,12
We have been interested in understanding the strong suggesting a high preference for this surface termination. A higher
13,14
an important role in this area.
plane. Note that the single crystal nature of the Ga O nanoplate with
2 3
15
the (002) surface prepared by the same method was also identified.
Catalytic results of impregnated 5 wt% Pd on rod and plate
for methanol production from CO hydrogenation are shown
a
Wolfson Catalysis Centre, Department of Chemistry, University of Oxford, Oxford,
OX1 3QR, UK. E-mail: Edman.Tsang@chem.ox.ac.uk
Ga
2
O
3
2
b
College of Chemistry and Molecular Sciences, Wuhan University, Wuhan, 430072,
P. R. China
in Table 1 (testing conditions, see ESI†). CO was the only side
product detected in this reaction. As seen from Table 1, Pd on plate
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c3cc38455a
Ga
2
O
3
clearly exhibits higher activity than on the corresponding rod
This journal is c The Royal Society of Chemistry 2013
Chem. Commun., 2013, 49, 1747--1749 1747

View Article Online
Communication
ChemComm
Fig. 2 (a) Cyclic Voltammetry (CV) of Pd/plate and Pd/rod Ga
2
O
3
, scanned from
À1
0.0 V to 1.2 V (vs. SHE) at 50 mV s , the stable 20th cycle was recorded; (b) CO
2 3
stripping of Pd on rod and plate Ga O (ESI†).
careful morphology controls may give an exciting way to optimize
catalytic performances by this mean.
Fig. 1 (a) XRD of rod Ga
2
O
3
; (b) XRD of plate Ga
2
O
3
; TEM, lattice fringes,
In order to assess the possible electronic interaction between metal
and semiconducting oxide, Pd nanoparticles attached to the plate and
electron diffraction at edges of (c) rod Ga
2
O
3
; (d) plate Ga O .
2 3
2 3
rod forms of the Ga O nanocrystals, dispersed on glassy carbon
form, despite the fact that the same metal loading was used and the electrodes, were characterized by cyclic voltammetry and CO stripping
BET surface areas for the two forms were low but comparable to voltammetry (details in ESI†) and the data are shown in Fig. 2. It is
2
À1
each other (B15–20 m
g
2 3 2 3
). One possibility is that a higher noted from Fig. 2a that both Pd/plate Ga O and Pd/rod Ga O are
dispersion of Pd was obtained on the plate oxide due to enhanced clearly electrochemically active but the former shows a considerably
interaction of the Pd precursor with this support. It is noted that the higher CV signal, derived from H adsorption/desorption below 0.2 V,
plate oxide also gives higher selectivity towards methanol than the and Pd oxidation/reduction around 0.7 V. Also, the absence of a
rod form. As a result, the methanol yield of 5% Pd on plate Ga
nearly double that seen using the rod support, indicative of the (001) Pd deposition (epitaxyl deposition) instead of mixed Pd phases
remarkable activity dependence on the morphology of the support deposition in Pd/rod Ga . There are no electrochemical signals for
oxide used. Our earlier report on a similar trend from catalysts the two pure Ga supports, indicating that the activity is only
prepared by the physical mixing of plate or rod ZnO nanocrystals associated with the presence of Pd and interfaces with the oxides.
2 3
O
2 3
is charactersitic hump at 0.3 V in Pd/plate Ga O may suggest a primary
2 3
O
O
2 3
1
4
with preformed Cu nanoparticles is included for comparison.
It is noted from Table 1 that Pd/Ga
catalysts indeed give the pre-adsorption of CO on the supported Pd (Fig 2b). The calculated
higher activity than Cu/ZnO catalysts but with poorer selectivity in electrical active surface (EASco) from the charge passed reveals a
Electrochemical oxidative CO stripping was also conducted following
2 3
O
11,12
this work.
are structurally different oxides, but when using higher energy being 0.42 and 0.07 m gPd for 5% Pd/plate Ga
002) surfaces both can interact favourably with metals to give Ga , respectively (6 times), despite the comparable surface area of
2 3
Interestingly, Ga O
(monoclinic) and ZnO (wurtzite) surprisingly large difference between the two Pd supported samples,
2
À1
2 3
O and 5% Pd/rod
(
2 3
O
higher activity to methanol. Note that the cation and anion their supports. The electrochemical results thus reflect the stronger
arrangements over the exposed (002) surfaces of these two oxides interaction of the Pd with the plate than rod oxide, leading to
are unbalanced that they give an overall non-zero polarity in top significantly higher electrochemically active sites on the former mate-
16,17
layers.
Depending on the degree of covalency, the coloumbic rial. However, it is noted that the nature of electrochemically metal
repulsion between ions of the same charge tends to give rise to a active sites may not necessarily correspond to the same catalytically
higher surface energy than those of other exposed surfaces. active sites for methanol synthesis as it is known that synergetic sites
Energetic oxide surfaces have not been well studied as metal created at the metal–metal oxide interface may be required for both
13,14
catalyst supports, but the high methanol yields noted in this work CO
2
and H
2
activations and catalysis.
On the other hand, the onset
clearly suggest that they could be of considerable catalytic interest. potential for the stripping process was lower for the plate form, being
We believe that such energetic polar oxide surfaces interact 0.77 Æ 0.006 V, and for rod 0.83 Æ 0.008 V with the peakmax shifting
strongly with deposited metals and precursors, thereby imparting 15 Æ 3 mV to lower voltage in the plate sample (ESI†). This earlier
superior catalytic performance as demonstrated in the formation onset for the plate form indeed shows more chemically active form of
of methanol on the plate forms of Ga O
2 3
and ZnO. Thus, the surface Pd sites in synergy with this support material, compared to the
recent tailored synthesis of metal and oxide nanoparticles for rod form. In addition, for most supported Pd samples, adsorbed CO
on Pd is totally stripped off during the first anodic scan. Interestingly,
we noted here that CO stripping signals were still observed for both
samples on the second and subsequent scans (higher CO stripping
signals for each scan for the plate form, ESI†). Thus, the results suggest
2 3
Table 1 Summary of catalytic performance of 5% Pd on rod and plate b-Ga O
crystals in comparison to Cu on rod and plate ZnO
Methanol
select. (%)
Methanol
yield (%)
that COad can be inter-transferred between Pd and Ga
2 3
O sites
Catalyst
2
CO conv. (%)
presumably via surface carbonate/bicarbonate, resembling the reversi-
5
5
% Pd/rod Ga
% Pd/plate Ga
2
O
3
11.0
17.3
15.8
15.5
41.3
51.6
41.0
64.5
4.5
8.9
6.5
18
2 3
ble spillover phenomenon with strong MSI. Clearly, Ga O and its
2
O
3
1
4
energetic (002) surface with the plate morphology have an extensive
influence on catalytic chemistry of Pd phase.
Cu/rod ZnO/Al
Cu/plate ZnO/Al
2
O
3
O
1
4
2
3
10.0
1
748 Chem. Commun., 2013, 49, 1747--1749
This journal is c The Royal Society of Chemistry 2013

View Article Online
ChemComm
Communication
1
6
(
2.076 Â 10 spins per mg). The presence and quantity of spin reflect
the degree of structural defects present in the sample. The g value of
2
.30 (plate) and 2.20 (rod) can be attributed to deep trapped electrons
À
from oxygen defect sites near the valence band (O2 acts as an
electron acceptor when an O molecule from air adsorbs on oxygen
2
13,14
vacancy).
2 3
Interestingly, only plate Ga O gave a distinctive signal
with a g value of 1.96, which is attributed to shallow trapped donor
defects near the conduction band. The presence of such donor defects
clearly reflects the occupancy of excited electrons in the conduction
band. Similarly, plate ZnO also gives higher EPR signals for the deep
trapped acceptors and unique shallow donors as compared to rod
1
3,14
ZnO.
In addition, Yan et al. reported a higher degree of surface
15
defects for plate Ga O than powder form by photoluminescence.
2
3
It is therefore evident from our work that it is the instability of
the (002) Ga surface and its high propensity for electron transfer
with oxygen termina- that facilitate the interaction with Pd (as a way of stabilization) at
; DOS of (002) their Schottky–Mott interface (the oxide support has a higher
2 3
O
Fig. 3 (a) Atomistic model showing the (002) surface of Ga
tion (O red, Ga grey); (b) EPR spectra of rod Ga and plate Ga
surfaces in comparison to bulk Ga surface; (c) O termination; (d) Ga termination.
2 3
O
2
O
3
2 3
O
2 3
O
conduction band energy than the Fermi-level of the overlying
11–14
metal).
This can lead to higher metal dispersion and its
We then carried out DFT calculations to probe the electronic orientation and possible (Pd–Ga) alloy formation at the materials
19
properties of the energetic (002) surface, compared with bulk Ga
ESI†). As stated, the (002) polar surface for plate Ga has two tion than those of low energy surfaces.
possible terminations, namely Ga and O terminated surfaces,
an atomistic model is shown in Fig. 3a. The calculations for the density growth of nano-size Ga
of states (DOS) associated with these surfaces clearly reveal some them to host Pd. We have found a stronger MSI between Pd and
unusual electronic properties. On both terminations, a strong mod- polar (002) surface of plate form Ga than other non-polar
ification of the DOS with respect to bulk Ga O takes place. The surfaces with facilitated electron transfer, giving a higher activity
2
O
3
interface, giving higher activity and selectivity in methanol produc-
(
2 3
O
16,17
and
In conclusion, we have adopted a new approach by controlled
crystals of different shapes and used
2 3
O
2 3
O
2
3
principal effects consist of an important strong exchange splitting and for methanol production from CO hydrogenation.
2
distortion of the DOS towards higher energy at the top of the valence
We thank Dr X. Gong of East China University of Technology
band (up-shift) due to electrostatic repulsion on the O termination (the for calculations and facilities, Dr T. Li of Oxford Materials and
line at 0 eV represents the Fermi level) and a downward shift of the Dr Q. Lu of US Naval Research Laboratory for TEM and EPR.
surface conduction band on the Ga termination. As a result, the band
gap becomes narrower and electrons can be promoted more easily to Notes and references
higher surface bands. It is noted that similar calculations over (002)
1
K. M. K. Yu, I. Curcic, J. Gabriel and S. C. E. Tsang, ChemSusChem,
ZnO and other polar metal oxide surfaces reported the narrowing of
band gaps and in some cases, metallation (total disappearance of band
2008, 1, 893.
2 J. Hansen, L. Nazarenko, R. Ruedy, M. Sato, J. Willis, A. Del Genio,
D. Koch, A. Lacis, K. Lo, S. Menon, T. Novakov, J. Perlwitz,
G. Russell, G. A. Schmidt and N. Tausnev, Science, 2005, 308, 1431.
16,17
gap) was suggested.
energetic, non-polar surface shows a similar DOS to bulk Ga
In contrast, we found that a typical (110) non-
(ESI†).
2 3
O
3
P. G. Jessop, in Handbook of Homogeneous Hydrogenation, ed. H. de
Vries and K. Elsevier, Wiley-VCH, Weinheim, 2007, vol. 1, pp. 489–511.
T. Sakakura, J.-C. Choi and H. Yasuda, Chem. Rev., 2007, 107, 2365.
G. A. Olah, Angew. Chem., Int. Ed., 2005, 44, 2636.
J. Turner, G. Sverdrup, M. K. Mann, P. C. Maness, B. Kroposki,
M. Ghirardi, R. J. Evans and D. Blake, Int. J. Energy Res., 2008, 32, 379.
J. G. Wu, M. Saito, M. Takeuchi and T. Watanabe, Appl. Catal., A,
It is known that electrostatic repulsion of the same charged species
within the layer on a high energy oxide surface can render instability
of its structure. However, the high energetic surface may tend to seek
various mechanisms for stabilization, such as selective adsorption of
counter-ions, surface reconstruction and excitation of localized elec-
4
5
6
7
8
9
2001, 218, 235.
16,17
trons to delocalized bands as our calculations presently indicate.
If
G. C. Chinchen, P. J. Denny, J. R. Jennings, M. S. Spencer and
K. C. Waugh, Appl. Catal., 1988, 36, 1.
M. Saito, Catal. Surv. Jpn., 1998, 2, 175.
the latter takes place, the excitation of electrons to higher conduction
bands from localized oxygen anions would facilitate its oxygen release
to create anion vacancies. We have therefore calculated the minimum
energies required for the formation of an oxygen vacancy from (002)
polar O terminated and (110) non-polar surfaces with active oxygen
sites at the lowest coordination. Results were À0.78 eV for the polar
1
0 E. Vesselli, L. De Rogatis, X. Ding, A. Baraldi, L. Savio, L. Vattuone,
M. Rocca, P. Fornasiero, M. Peressi, A. Baldereschi, R. Rosei and
G. Comelli, J. Am. Chem. Soc., 2008, 130, 11417.
1
1
1 T. Fujitani and I. Nakamura, Bull. Chem. Soc. Jpn., 2002, 75, 1393.
2 T. Fujitani, M. Saito, Y. Kanai, T. Watanabe, J. Nakamura and
T. Uchijima, Appl. Catal., A, 1995, 125, L199.
surface and +1.74 eV for the non-polar surface, indicating that the 13 F. L. Liao, Z. Zeng, C. Eley, Q. Lu, X. Hong and S. C. E. Tsang, Angew.
Chem., Int. Ed., 2012, 51, 5832.
unstable high energy oxide (002) surface is readily prone to produce
oxygen vacancies and excited electrons. To verify this mechanism,
14 F. L. Liao, Y. Q. Huang, J. W. Ge, W. R. Zheng, K. Tedsree, P. Collier,
X. L. Hong and S. C. Tsang, Angew. Chem., Int. Ed., 2011, 123, 2210.
electron paramagnetic resonance (EPR) studies on rod and plate 15 S. Yan, L. Wan, Z. Li and Z. Zou, Chem. Commun., 2010, 46, 6388.
1
1
1
6 C. Noguera, J. Phys.: Condens. Matter, 2000, 12, R367.
7 J. Goniakowski, F. Finocchi and C. Noguera, Rep. Prog. Phys., 2008, 71, 1.
8 W. C. Conner and J. L. Falconer, Chem. Rev., 1995, 95, 759.
Ga
2 3
O were thus performed. EPR profiles of plate and rod samples
(Fig. 3b) clearly reveal that the plate sample gives a higher signal of
16
unpaired electrons (4.728 Â 10 spins per mg) than the rod sample 19 C. D. Wagner, Faraday Discuss. Chem. Soc., 1975, 60, 291.
This journal is c The Royal Society of Chemistry 2013
Chem. Commun., 2013, 49, 1747--1749 1749