The photo-induced vapor-phase decomposition of dimethyl ether was investigated on Pt metals deposited on pure and N-doped TiO2. Infrared spectroscopic measurements revealed that adsorption of dimethyl ether on TiO2 samples underwent partial dissociation to methoxy species. Illumination of the (CH3)2O-TiO2 and (CH3)2O-M/TiO2 systems led to the conversion of methoxy into adsorbed formate. In the case of metal-promoted TiO2 catalysts, CO bonded to the metals was also detected. Pure titania exhibited a very little photoactivity. Deposition of Pt metals on TiO2 markedly enhanced the extent of photocatalytic decomposition of dimethyl ether to give H2 and CO2 as the major products. A small amount of CO and methyl formate was also identified in the products. The most active metal was the Rh followed by Pd, Ir, Pt and Ru. When the bandgap of TiO2 was lowered by N-doping, the photocatalytic activity of metal/TiO2 catalysts appreciably increased. The effect of metals was explained by a better separation of charge carriers induced by illumination and by enhanced electronic interaction between metal nanoparticles and TiO2.
The production of H2, with a small amount of CO, is an important project for heterogeneous catalysis. In principle, the decomposition of methane and hydrocarbons seems to be a suitable process, but the carbon formed poisons the catalyst in early phase of the reaction [
DME is emerging as a replacement for diesel fuel due to its low NOx emission, and near-zero smoke compared with traditional diesel fuels [
Photocatalytic reaction was followed in the same way as described in our previous papers [
For FTIR studies a mobile IR cell housed in a metal chamber was used [
TiO2 of different origins was used: Hombikat, UV 100 (pure anatase, 300 m2/g), TiO2 nanowire (60 m2/g) and nanotube (40 m2/g). The synthesis of the last two compounds is described elsewhere [
The primary aim of IR study is to ascertain the development of adsorbed complexes formed on the effect of illumination on TiO2, and to establish the influence of metal deposition on these features. Exposing pure TiO2 to DME at 300 K resulted in a development of intense absorption bands in the C-H stretching region at 2950, 2921, 2878, 2842 and ~2830 cm−1 (
The photocataytic decomposition of DME has been investigated on different TiO2 samples. Whereas DME does not decompose at 300 K on pure TiO2, illumination induced the occurrence of the reaction to give H2 and CO2.
Effects of illumination time on the FTIR spectra of adsorbed dimethyl ether on TiO2 (Hombikat)
Effects of illumination time on the FTIR spectra of adsorbed dimethyl ether on Rh/TiO2 (a), Pt/TiO2 (b), Pd/TiO2 (c) and Ru/TiO2 (d) at 300 K
. Characteristic absorption bands (cm−1) following the adsorption of dimethyl ether, methanol and formic acid on various solids
Vibrational mode | DME(g) [33] [34] | DME(a) on Al2O3 at 150 K [34] | DME(a) on CeO2 at 300 K [28] | CH3O(a) on TiO2 at 300 K [35] | HCOO(a) on TiO2 at 300 K [14] | DME on TiO2 at 300 K [present study] | DME on Rh/TiO2 at 300 K [present study] |
---|---|---|---|---|---|---|---|
υa(CH3) | 2996 2925 | 2984 2922 | 2953 | 2965 2930 | 2958 | 2950 2921 | 2952 2906 |
υs(CH3) | 2817 | 2821 | 2841 | 2830 | 2886 | 2878 | 2879 |
2δ(CH3) | 2887 | 2890 | 2884 | 2842 | 2838 | ||
υa(OCO) | 1552 | ||||||
υs(OCO) | 1377 | ||||||
δas(CH3) | 1470 | 1477 | 1436 | 1462 | 1459 | 1458 | |
δs(CH3) | 1456 | 1459 | 1436 | ||||
γ(CH3) | 1244 1179 | 1252 1116 | 1229 1159 | 1151 | 1253 1159 | 1253 1153 | |
υas(CO) | 1102 | 1092 | 1066 | 1125 | 1277 | 1063 | 1052 |
(g) gaseous; (a) adsorbed.
However even on the most effective TiO2 (Hombikat) the extent of the decomposition was very low, about ~2% - 3%, in 210 min. The photocatalytic effect of TiO2 nanowire and nanotubes was also tested: we obtained a similar low photoactivity.
The deposition of Pt metals on TiO2 (Hombikat) markedly enhanced its photoactivity. In
In order to judge the contribution of thermal effect for the photoreaction, we also examined the thermal reaction on selected catalysts. A measurable reaction on Pt/TiO2 and Rh/TiO2 was observed only at 523 K. Attaching a thin thermocouple in the catalyst layer indicated only a temperature rise of only a few degrees during illumination. The results of these control experiments lead us to exclude the contribution of thermal effects to the photodecomposition of DME induced by illumination.
Effects of different Pt metals deposited on TiO2 (Hombikat) on the photocatalytic decomposition of dimethyl ether. Conversion (a), formation of H2 (b), CO2 (c) and CO (d)
. Some characteristic data for the photolysis of DME on metal-promoted TiO2
Samples | Dispersion (%) | Conversion (%, 210 min) | Methyl formate (nmol, 210 min) | CO/H2 (210 min) | MF/H2 (210 min) | |
---|---|---|---|---|---|---|
2% Rh/TiO2 | 16 | 22.5 | 0.063 | 5.9 | 0.023 | 0.052 |
2% Pd/TiO2 | 26 | 21.0 | 0.042 | 4.2 | 0.039 | 0.042 |
2% Ir/TiO2 | 54 | 12.5 | 0.024 | 1.6 | 0.044 | 0.028 |
2% Pt/TiO2 | 13 | 12.0 | 0.096 | 0.4 | 0.039 | 0.007 |
2% Ru/TiO2 | 6 | 6.5 | 0.023 | 2.0 | 0 | 0.142 |
Effects of illumination time on the formation of methyl formate (a) and on the methyl formate/H2 ratio (b) on TiO2-supported Pt metals
The effect of illumination on the reforming of DME was also investigated. Water exerted a positive influence on the conversion of DME and it appreciably increased the amount of H2 formed. As the hydrolysis of DME to CH3OH occurs more easily on the acidic centers of Al2O3, some experiments have been performed in the presence of Al2O3. Adding Al2O3 to Pd/TiO2 catalyst greatly enhanced the conversion of DME and the formation of H2.
Some experiments have been performed in visible light. These measurements were carried out with TiO2 (SX), which possessed better performance compared to other N-doped TiO2 [
. Effects of H2O and Al2O3 on the product distribution of photocatalytic decomposition of DME on Pd/TiO2 samples. Data refer to reaction time of 210 min
Samples | Conversion (%) | H2 (nmol) | H2 formed related to the amount of Pd/TiO2 (nmol/g) | CO/H2 ratio |
---|---|---|---|---|
Pd/TiO2 | 22 | 110 | 1.69 | 0.039 |
Pd/TiO2, DME:H2O (1:1) | 20 | 155 | 2.15 | 0.030 |
Pd/TiO2, DME:H2O (1:3) | 16 | 180 | 2.52 | 0.026 |
Pd/TiO2 + Al2O3 (1:1), DME:H2O (1:3) | 23 | 180 | 7.20 | 0.019 |
Photocatalytic decomposition of dimethyl ether on pure and N-doped TiO2 samples (SX) in visible light
Adsorption of DME on TiO2 at 300 K produced several intense absorption bands in the IR spectra. Taking into account the results of previous studies their possible assignment is presented in
The formation of methoxy species suggests the breakage of one of the C-O bonds in the adsorbed DME
As there it is no indication of the IR bands of adsorbed CH3 radical [
Illumination of adsorbed layer resulted in a slow attenuation of methoxy bands and the appearance of absorption features due to formate species. Its formation is described by the following elementary steps
DME proved to be very resistant towards illumination on TiO2. Deposition of Pt metals on the TiO2, however, enhanced its photoactivity, but the low reactivity of DME appeared on these catalysts, too. The effect of illumination can be explained by the donation of photoelectrons formed in the photo-excitation process
to the CH3O species:
producing a more reactive negatively charged species, which is converted into adsorbed CH2O and HCOO (Equations (3) and (4)). However, even the photo-induced reaction occurred to only a very limited extent on pure TiO2, a finding which can be attributed to the fast recombination of the electrons and holes formed in the photo-excitation process (Equation (5)). The formation of H2, CO2 and CO suggests the occurrence of the reactions
An interesting and somewhat surprising result of the photocatalytic decomposition of DME is the formation of methyl formate. This compound has been considered as a precursor in the preparation of several materials [
or by the reaction of CH2O with a further CH3O species:
An appreciable increase in the extent of photolysis of DME was observed in the presence of H2O, which can be attributed to the occurrence of the hydrolysis of DME,
e.g. to the formation of more reactive CH3OH. The addition of H2O to DME also lowered the extent of CO formation, very likely due to the occurrence of the water-gas shift reaction promoted by illumination. This was confirmed by a separate experiment. Mixing Pd/TiO2 with Al2O3 further enhanced the formation of H2, which can be also ascribed to the promotion of the hydrolysis of DME to methanol.
The deposition of metals onto TiO2 greatly improved the photocatalytic effect of the TiO2. We assume that the CH3O species formed at the metal/TiO2 interface is much more reactive than that located on TiO2. The promoting effect of Pt metals deposited on TiO2 is generally explained by the better charge carrier separation induced by illumination [
An important finding of this work is that the incorporation of N into TiO2 support enhanced the photoactivity of TiO2 (
As we studied the photocatalytic decomposition of several organic compounds on the same catalysts under exactly identical experimental conditions, this gives us a possibility to make a comparison. Some data are presented in
Effects of N-doping of TiO2 (SX) on the photocatalytic decomposition of dimethyl ether on 2% Rh/TiO2 (a), 2% Pt/TiO2 (b), and 2% Pd/TiO2 (c) in visible light. ○ ☆ TiO2, ● « TiO2 + N, ☆ « conversion, ○ ● H2 formation
. Comparison of the results obtained in the photocatalytic decomposition of various compounds
Compounds | TiO2 | 2% Rh/TiO2 | ||||
---|---|---|---|---|---|---|
conversion (%) | yield for H2 | conversion (%) | yield for H2 | |||
HOOOH | 32.1 | 90.4 | 29.0 | 100 | 99.9 | 99.9 |
CH3OH | 8.0 | 7.6 | 0.6 | 42 | 61.6 | 25.8 |
C2H5OH | 3.8 | - | - | 92.5 | 44.2 | 40.8 |
DME | 3.5 | - | - | 22 | 87.5 | 19.3 |
• IR spectroscopic study revealed that a fraction of adsorbed DME underwent the dissociation to methoxy species on TiO2 at 300 K.
• Illumination of adsorbed DME leads to the generation of formate species, and to the formation of CO bonded to Pt metals.
• Photocatalytic decomposition of DME on TiO2 is a very limited process.
• Deposition of Pt metals on TiO2 markedly enhanced the extent of photocatalytic reaction.
• Lowering the bandgap of TiO2 by N doping appreciably increased the photocatalytic activity of metal/TiO2 catalysts.
This work was supported by the grant OTKA under contract number K 81517 and TÁMOP under contract number 4.2.2.A-11/1/KONV-2012-0047.