Guido Mul

Artificial Photosynthesis over Crystalline TiO2-Based Catalysts: Fact or Fiction?

The mechanism of photocatalytic conversion of CO(2) and H(2)O over copper oxide promoted titania, Cu(I)/TiO(2), was investigated by means of in situ DRIFT spectroscopy in combination with isotopically labeled (13)CO(2). In addition to small amounts of (13)CO, (12)CO was demonstrated to be the primary product of the reaction by the 2115 cm(-1) Cu(I)-CO signature, indicating that carbon residues on the catalyst surface are involved in reactions with predominantly photocatalytically activated surface adsorbed water. This was confirmed by prolonged exposure of the catalyst to light and water vapor, which significantly reduced the amount of CO formed in a subsequent experiment in the DRIFT cell. In addition, formation of carboxylates and (bi)carbonates was observed by exposure of the Cu(I)/TiO(2) surface to CO(2) in the dark. These carboxylates and (bi)carbonates decompose upon light irradiation, yielding predominantly CO(2). At the same time a novel carbonate species is produced (having a main absorption at approximately 1395 cm(-1)) by adsorption of photocatalytically produced CO on the Cu(I)/TiO(2) surface, most likely through a reverse Boudouard reaction of photocatalytically activated CO(2) with carbon residues. The finding that carbon residues are involved in photocatalytic water activation and CO(2) reduction might have important implications for the rates of artificial photosynthesis reported in many studies in the literature, in particular those using photoactive materials synthesized with carbon containing precursors.

The six-flow reactor technology. A review on fast catalyst screening and kinetic studies

Catalyst testing in laboratory reactors requires careful experimentation and data interpretation. Current methods of catalyst development tend to be slow, laborious, and incapable of addressing most of the complex challenges, of multi-component chemical systems. In order to speed up this process in an efficient way, the six-flow parallel reactor technology is proposed. This enables parallel catalyst testing, which enhances the number of catalysts tested significantly and reduces the time for kinetic studies. Thus, operation costs are lowered and the success rate for important breakthroughs is increased. The six-flow set-up allows a proper catalyst testing, under more realistic and accurate conditions than in conventional combinatorial techniques, especially when the catalyst development stage is advanced and quantitative data are required. The application of this assessed technology is reviewed and combined with criteria for ideal behavior in reactor models and transport phenomena, crucial in order to achieve intrinsic catalyst performance data.

Isoreticular MOFs as Efficient Photocatalysts with Tunable Band Gap: An Operando FTIR Study of the Photoinduced Oxidation of Propylene

Photo frame(work): The first spectroscopic evidence of metal-organic frameworks (MOFs) acting as photocatalysts has been obtained. Isoreticular MOFs act as efficient photocatalysts in the photooxidation of propylene. The band gap energy can be tuned by changing the organic linker. Among the MOFs tested, the 2,6-naphthalenedicarboxylic acid based IRMOF was the most active, showing a higher activity than ZnO.

Soot oxidation catalyzed by a Cu/K/Mo/Cl catalyst: evaluation of the chemistry and performance of the catalyst

Several non-supported oxidic compounds potentially present in a Cu/K/Mo/Cl catalyst (copper molybdates, potassium molybdates, and a mixed copper-potassium molybdate (K2Cu2(MoO4)3)) have been tested individually on their activity in the oxidation of a model soot (Printex-U, which non-catalytically oxidizes at 875 K). These oxidic compounds are active between 665 and 720 K, but only after establishment of ‘tight contact’ between the catalyst and soot in a ball mill. Without the ball mill procedure (‘loose contact’) these oxides are less active (the soot oxidation temperature is shifted to about 790 K), while a ZrO2 supported Cu/K/Mo/Cl catalyst still shows a high activity around 670 K. Hence, the ‘loose contact’ activity of the supported Cu/K/Mo/Cl catalyst is not explained by the presence of an active oxidic compound. DRIFT and XRD analyses have shown that addition of KCl to CuMoO4 (two compounds present within the Cu/K/Mo/Cl catalysts) followed by calcination at 950 K in air, eventually results in the formation of a mixed potassium-copper molybdate. Simultaneously several volatile copper, potassium and chlorine containing compounds (e.g. K2CuCl4) are formed. These copper and chlorine containing compounds possess a high ‘loose contact’ soot oxidation activity between 600 and 690 K. A catalytic cycle, involving Cu2OCl2, is proposed to explain the high ‘loose contact’ activity of copper chlorides and supported Cu/K/Mo/Cl catalysts. The activity of the latter catalyst will be maintained as long as Cu2OCl2 can be reformed by reaction of copper molybdates with KCl, which serves as a chlorine supplier.

Catalytic oxidation of model soot by metal chlorides

Several metal chlorides were screened for their catalytic activity in the oxidation of model soot (Printex-U) in ‘loose contact’ by means of TGA/DSC. HgCl2, CaCl2, BaCl2, CoCl2, and NiCl2 show little activity. Hydrated BiCl3 and FeCl3 are converted in air into BiOCl and FeOCl, which have a moderate soot oxidation activity. MoCl5 is converted into the corresponding metal oxide and also shows a moderate ‘loose contact’ activity. PbCl2, CuCl2 and CuCl are very active catalysts; the soot oxidation temperature is lowered by 200–275 K. The activity of metal chlorides is thought to be induced by in situ formation of intimate contact between the soot and the metal chloride via ‘wetting’ and/or gas phase transport. A correlation between the melting point and the catalytic activity was found. Furthermore, a catalytic cycle is proposed involving activation of oxygen on the surface of the (oxy)chloride, followed by transfer of activated oxygen to the soot surface. DRIFT analyses showed that this results in the formation of carbon surface oxygen complexes. Decomposition of those complexes yields CO and CO2. Practical application of metal chlorides for the removal of soot from diesel exhaust is not recommended, because they suffer from instability or high vapour pressures.

The formation of carbon surface oxygen complexes by oxygen and ozone. The effect of transition metal oxides

Abstract Various surface oxygen complexes (SOCs) have been identified by DRIFT spectroscopy on the surface of carbon black (Printex-U) after partial non-catalytic conversion in 10% O2 in Ar and ozone. An in situ DRIFT analysis of the oxidation of fullerene C60 showed the formation of similar functionalities and validated the use of C60 as a carbon black model compound for DRIFT spectroscopic studies, although C60 is more reactive towards oxygen than carbon black. Ex situ DRIFT analyses of partially converted catalyst/carbon black mixtures and in situ analyses of catalytic fullerene C60 oxidation, revealed that several transition metal oxides (Cr2O3, MoO3, V2O5 and CuO) promote the formation of SOCs. Fe2O3 and Co3O4 do not enhance the formation of SOCs in 10% O2 in Ar and prevent the formation of SOCs on carbon black samples in ozone. Reaction of carbon black with oxygen associated with metal oxides (carbothermic reduction) does not yield SOCs. Apparently, lattice oxygen is not directly involved in the catalytic formation of these complexes. Indications for chemical interactions between metal oxides and either carbon black or C60, such as M–O–C bonds, have not been found. Spill-over of activated oxygen from the Cr2O3, MoO3, V2O5 and CuO surfaces onto the carbon black surface is likely to explain the catalytic formation of SOCs.

NO Adsorption on Ex-Framework [Fe,X]MFI Catalysts: Novel IR Bands and Evaluation of Assignments

IR spectra of NO adsorbed on isomorphously substituted [Fe,Al]MFI, [Fe,Ga]MFI and [Fe]MFI after steaming at 873 K in 30 vol% H2O are presented. On ex-[Fe,Al]MFI, NO adsorption leads to bands at 2133 cm-1 and a doublet at 1886 and 1874 cm-1. The 2133 cm-1 band is assigned to NO+ occupying cationic positions in the zeolite structure. Of the doublet, the 1874 cm-1 band is much more susceptible to reaction with O2 than the 1886 cm-1 band, yielding adsorbed NO2 with an absorption frequency of 1635 cm-1. After evaluation of the constitution of the catalyst and (sometimes contradictory) literature assignments, the 1886 cm-1 band is assigned to NO adsorbed on Fe ions located in isolated positions, and/or (FeO)n clusters inside the zeolite channels, whereas the 1874 cm-1 band is proposed to be induced by 2 nm FeAlOx nano-particles. The ex-[Fe,Ga]MFI catalyst showed a similar absorption pattern (doublet), which is shifted to lower wavenumbers (1881 and 1867 cm-1), suggesting that both frequencies are affected by the vicinity of Ga (or Al) to the Fe site involved. The absence of bands at 1765 and 1835 cm-1 suggests that the isolated sites causing these absorptions are in the FeIII state in ex-[Fe,Al]MFI and ex-[Fe,Ga]MFI. For the ex-[Fe]MFI sample, which did not contain any 2 nm FeOx nano-particles, an NO absorption band at 1854 cm-1 is assigned to mono-nitrosyl on extra-framework oligonuclear (FeIIO)n species in the zeolite channels.

Selective catalytic reduction of NO with NH3 over Fe-ZSM-5 catalysts prepared by sublimation of FeCl3 at different temperatures

Fe-ZSM-5 catalysts were prepared by subliming FeCl3 into H-ZSM-5. The method used allowed Fe-ZSM-5 catalyst preparation by FeCl3 exchange at a desired sublimation temperature and was found to be more precise. The sublimation of FeCl3 into H-ZSM-5 was carried out at 320 and 700 °C. Fe-ZSM-5 prepared by sublimation of FeCl3 at 320 °C followed by rapid heating to 700 °C and the catalyst prepared by subliming FeCl3 at 700 °C were found to be more active for NO reduction with NH3 in the presence of simulated exhaust gases containing water vapor than catalysts prepared by subliming FeCl3 at 320 °C. To determine the active sites, the catalysts were characterized by H2-TPR, in situ DRIFTS of NO adsorption, NH3-TPD, XRD and chemical analysis methods. The observed NO conversion differences in selective catalytic reduction using NH3 could be correlated to the iron cation species present at different locations determined from diffuse reflectance infrared spectroscopy. Enhanced NO reduction activity was obtained when γ positions in Fe-ZSM-5, corresponding to Fe2+(NO) band at 1877 cm-1 in DRIFTS, were preferentially occupied.

Feasibility study towards a Cu/K/Mo/(Cl) soot oxidation catalyst for application in diesel exhaust gases

Abstract The activity of a supported catalyst containing copper, potassium and molybdenum was studied. Model experiments revealed that chlorine plays an essential role in the activity of this catalyst. It was shown that the active species of the catalyst probably consist of copper chloride compounds. Deactivation of the catalyst was studied, and was found to be more pronounced for a poor contact between soot and catalyst compared with a tight contact. Deactivation rates were found to be low, which was tentatively suggested to be caused by loss of active species that are formed by solid-solid reactions (e.g., KCl + CuMoO4 → K-molybdates + Cu-(oxy)chlorides) which are slow as a result of low solid-state diffusion rates. A Cu/K/Mo-catalyst as a coating on small segments of a wall flow monolith downstream of a diesel engine showed a relatively low activity. Besides, the catalyst was found to deactivate rather fast, which corroborated the outcome of the above mentioned model study. As a result, the feasibility of this Cu/K/Mo-catalyst for use in practical applications is low due to a progressive loss of catalytic material by high vapour pressures of active components formed by solid-solid reactions of less volatile compounds present in the catalyst.

NO-Assisted N2O Decomposition over ex-Framework FeZSM-5: Mechanistic Aspects

The decomposition of N2O over an ex-framework FeZSM-5 catalyst is strongly promoted by NO. Activity data show that the promoting effect of NO is catalytic, and that besides NO2, O2 is formed much more extensively in the presence, than in the absence of NO. Transient in situ FT-IR/MS measurements indicate that NO is strongly adsorbed on the catalyst surface up to at least 650 K, showing absorption frequencies at 1884 and 1876 cm−1. A change in gas phase composition from NO to N2O results in the formation of adsorbed NO2, identified by a sharp IR band at 1635 cm−1. Switching back to the original NO gas phase induces a rapid desorption of NO2, restoring the original NO absorption frequencies. During the IR measurements, bands typical of nitro- or nitrate groups were not observed. Multi-Track (a TAP-like technique) experiments show that the presence of NO or NO2 on the catalyst surface significantly enhances the rate of oxygen desorption at the time of N2O exposure to the catalyst. The spectral changes and transient experiments are discussed and catalytic cycles are proposed, to explain the formation of NO2 and the (enhanced) formation of oxygen. The latter can be either explained by an indirect effect (electronic, steric) of NO adsorbed on sites neighboring the active sites, or by a direct effect involving reaction of adsorbed NO2 groups with neighboring oxidized sites yielding O2.