B.H. Sakakini

Adsorption and reaction induced morphological changes of the copper surface of a methanol synthesis catalyst

The morphology (surface structure) of the copper component of an industrial Cu/ZnO/Al2O3 methanol synthesis catalyst has been studied by carbon monoxide temperature programmed desorption (CO TPD). The initial state morphology produced by hydrogen reduction at 513 K showed evidence of the existence of Cu(111), Cu(110) and Cu(211) surfaces. Surface oxidation of the copper by CO2 decomposition at 213 K followed by CO reduction at 473 K did not reproduce the initial state morphology, most of the Cu(110) surface being lost; at the same time there was a six-fold increase in the surface population of the (211) face. This new surface produced by CO2 decomposition at 213 K and CO reduction at 473 K was considerably less active in its ability to decompose CO2 at 213 K. Treatment of it with hydrogen at 513 K for 16 h caused the surface to reconstruct almost completely to its original state, with the Cu(110) face reappearing and the Cu(211) face being reduced in population to roughly its original value. The ability of the copper to decompose CO2 was proportionately restored. It is evident that in the synthesis of methanol using CO/CO2/H2 mixtures over Cu/ZnO/Al2O3 catalysts, the morphology of the surface of the copper will be in a continuous state of restructuring, which, depending on the conditions, has the potential to result in chaotic behaviour.

The detailed kinetics of the desorption of hydrogen from polycrystalline copper catalysts

The kinetics of desorption of hydrogen from the copper component of an alumina-supported polycrystalline copper catalyst has been studied in detail by temperature-programmed desorption (TPD). Line-shape analysis of the hydrogen TPD spectra shows: (i) that the desorption is second order, (ii) that the desorption activation energy is in the range 64–68 kJ mol−1 in the coverage range 7–44% of a monolayer, and (iii) that the desorption pre-exponential term has a value ∼10−5 cm2 s−1 atom−1 consistent with the desorption being second order, involving mobile adsorbates and a mobile desorption transition state.

Structural changes of the Cu surface of a Cu/ZnO/Al2O3 catalyst, resulting from oxidation and reduction, probed by CO infrared spectroscopy

Abstract The morphology of the Cu surface of a Cu/ZnO/Al2O3 (60:30:10) catalyst, produced by reduction of the copper oxide component of the catalyst either with H2 or CO, has been probed by measuring the IR spectra of CO adsorbed on these surfaces. Pre-reduction in H2 produces a Cu surface, which contains the (110) and (211) faces and which is active in CO2 decomposition. Re-reduction of the CO2-oxidised Cu surface in CO results in a twofold increase in the Cu(211) surface. Re-reduction of the CO2-oxidised Cu surface in H2 results in the restoration of the original Cu morphology, i.e. in a decrease in the Cu(211) surface population. Initial reduction of the copper oxide component of the Cu/ZnO/Al2O3 catalyst by CO produces a surface, which is inactive in the decomposition of CO2.

The detailed kinetics of the adsorption of hydrogen on polycrystalline copper studied by reactive frontal chromatography

The coverage-dependent sticking probabilities of molecular hydrogen on polycrystalline copper supported on alumina have been determined in the temperature range 213–273 K by analysis of the hydrogen frontal adsorption line shape. In this temperature range the initial sticking probabilities increase from 8× 10−13 (213 K) to 1.3× 10−10 (273 K). The overall activation energy to adsorption has been found to be 42 kJ mol−1. The application of reactive frontal chromatography for the measurement of hydrogen sticking probabilities on copper is a novel variant of the N2O reactive frontal chromatographic method, developed for the measurement of copper surface areas. Its use here shows that reactive frontal chromatography may be applied generally to any adsorbate/adsorbent system involving activated adsorption and low sticking probabilities.

Investigation of the nature of the oxidant (selective and unselective) in/on a vanadyl pyrophosphate catalyst

The anaerobic oxidation of CO by a (VO)2P2O7 catalyst has been used to investigate the nature of the oxidant (selective and unselective) in/on that material. Three peaks were observed in the rate of production of CO2 - at 993, 1073 and 1093 K. The temperature of the maximum in the rate of production of the first CO2 peak and the amount of oxygen associated with it are the same as that observed in the selective anaerobic oxidation of n-butane to butene and butadiene, but-1-ene to butadiene and furan and but-1,3-diene to dihydrofuran, furan and maleic anhydride. The interaction of CO with the (VO)2P2O7 catalyst forming CO2 at 993 K is therefore concluded to be with the selective oxygen. The total amount of oxygen removed by the CO from the (VO)2P2O7 lattice (>5 monolayers) is about six times greater than that of the selective oxygen. The higher activation energies for the removal of the unselective oxygen accounts for the high selectivities (~80%) encountered commercially for the anaerobic oxidation of n-butane to maleic anhydride. Re-oxidation of the CO reduced (VO)2P2O7 by N2O quantitatively replaces all of the lattice oxygen removed by the formation of CO2, but does not restore the original morphology.

On the mechanism of the selective oxidation of n-butane, but-1-ene and but-1,3-diene to maleic anhydride over a vanadyl pyrophosphate catalyst

The mechanism of the selective partial oxidation of n-butane, but-1-ene and but-1,3-diene over a vanadyl phosphate catalyst has been investigated by temperature-programmed desorption (TPD) and by anaerobic temperature-programmed oxidation (TPO). TPD showed lattice oxygen to be desorbed in two states at 998 and 1023 K. The anaerobic TPO of n-butane produced butene and butadiene at 1020 K; anaerobic TPO of but-1-ene produced butadiene and furan at 990 K and dehydrofuran at 965 K, while anaerobic TPO of but-1,3-diene produced dehydrofuran at 970 K, furan at 1002 K and maleic anhydride at 1148 K. The total amount of oxygen removed from the lattice in these anaerobic selective partial oxidations was the same as that evolved from the vanadyl phosphate catalyst by TPD. This, and the fact that the selective oxidation reactions occurred at the same temperature at which the oxygen evolves from the lattice, suggests that the lattice oxygen is uniquely selective when it appears at the surface of the catalyst. (Under identical conditions of flow rate, weight of catalyst, heating rate etc., the reaction of n-butane or of but-1,3-diene in air produced only CO2 and H2O.)

Catalytic properties of SrSn1-xSbxO3 in methanol oxidation

Ceramic model catalyst materials of general formula SrSn1−xSbxO3 are synthesised by solid state reaction. X-ray diffraction shows the materials to be single phase over the composition range 0 ⩽x⩽0.085, with a second phase, identified as SrSb2O5, being produced at higher antimony doping levels. XPS, Auger electron spectroscopy and valence band photoemission show strong segregation of Sb to the surface of the material even within the single phase regime. The catalytic properties of the materials are examined using methanol oxidation as a test reaction. The activity and selectivity to formaldehyde production shown by these materials is found to be strongly correlated with the attainment of the bulk solubility limit of Sb in the SrSnO3 perovskite host.

The Partial Oxidation of CH3OH to CO2 and H2 Over a Cu/ZnO/Al2O3 Catalyst

The partial oxidation of CH3OH to CO2 and H2 over a Cu/ZnO/Al2O3 catalyst has been studied by temperature-programmed oxidation (TPO) using N2O and O2 as the oxidant. Post-reaction analysis of the adsorbate composition of the surface of the catalyst was determined by temperature-programmed desorption (TPD). The temperature dependence of the composition of the mixture of products formed by TPO was shown to depend critically on the partial pressure of the oxidant, with the highest partial pressure of oxygen used (10% O2 in He, 101 kPa—the CH3OH partial pressure was 17% throughout), producing marked non-Arrhenius fluctuations on temperature programming. Unsurprisingly, therefore, the adsorbate composition of the catalyst revealed by post-reaction TPD was also found to be determined by the partial pressure of the oxidant. Using high partial pressures of oxidant (5% and 10% O2 in He, 101 kPa), the only adsorbate detected was the bidentate formate species adsorbed on Cu. Lowering the oxygen partial pressure to 2% in He (101 kPa) revealed a catalyst surface on which the bidentate formate on Cu was the dominant intermediate with the formate on Al2O3 also being present. A further lowering of the partial pressure of the oxidant, obtained by using N2O as the oxidant (2% N2O in He, 101 kPa), resulted in a surface on which the formate adsorbed on ZnO was the dominant adsorbate with only a small coverage of the Cu by the bidentate formate.

Determination of the Kinetics of the Reaction of CCl4 with Prefluorided Cr2O3 and Evaluation of the Active Site Area

The reaction of CCl4 with prefluorided Cr2O3 has been studied by temperature-programmed and isothermal methods. Temperature-programmed reaction showed that CCl3F and CCl2F2 were produced simultaneously with activation energies of 64.5 and 62.5 kJmol−1, respectively, for the surface exchange reaction. It also allowed evaluation of the surface fluoride radius, a value of 1.86 Å being obtained which is slightly higher than the literature value of 1.33 Å. Isothermal reaction at 523 and 673 K of CCl4 produced simultaneous and instantaneous sharp peaks of CCl3F and CCl2F2. The fluoride ion radius derived from the 523 K experiment was 2.8 Å, suggesting that integration of the peak was terminated prematurely while the value at 673 K was 2.6 Å, suggesting involvement of the subsurface fluorine ions.