Stephen Daniells

The low temperature activity of Au/Fe2O3 catalysts towards CO oxidation was examined with respect to the temperature of pre-treatment and presence of water. The activity of all the prepared catalysts decreased as a result of a high temperature treatment (HTT) at 400 °C. The inclusion of water in the gas stream significantly enhanced the oxidation of CO at room temperature. When tested under water gas shift reaction (WGSR) conditions, significantly higher temperatures were required to convert CO… Show more

The low temperature activity of Au/Fe2O3 catalysts towards CO oxidation was examined with respect to the temperature of pre-treatment and presence of water. The activity of all the prepared catalysts decreased as a result of a high temperature treatment (HTT) at 400 °C. The inclusion of water in the gas stream significantly enhanced the oxidation of CO at room temperature. When tested under water gas shift reaction (WGSR) conditions, significantly higher temperatures were required to convert CO to CO2, thereby excluding the possibility of the WGSR during CO oxidation in the presence of H2O at room temperature. The loss of activity for CO oxidation is attributed to the loss of hydroxyl groups and reduction of Au3+ to metallic gold during HTT. The observations are consistent with the model for hydroxyl promotion of the decomposition of a carbonate intermediate by transformation to less stable bicarbonate. Show less

The gold-catalysed oxidation of carbon monoxide was studied by Mössbauer spectroscopy, in situ FTIR, and multiple time-resolved analysis of catalytic kinetics (MultiTRACK), an advanced TAP reactor system. The active catalyst studied was 3.4% Au/Fe2O3, which was used without drying and/or pretreatment. Mössbauer spectroscopy analysis of this sample showed that the fresh/as-received catalyst contained mostly Au3+ in the form of AuOOHdot operatorxH2O. Based on earlier studies, the support was… Show more

The gold-catalysed oxidation of carbon monoxide was studied by Mössbauer spectroscopy, in situ FTIR, and multiple time-resolved analysis of catalytic kinetics (MultiTRACK), an advanced TAP reactor system. The active catalyst studied was 3.4% Au/Fe2O3, which was used without drying and/or pretreatment. Mössbauer spectroscopy analysis of this sample showed that the fresh/as-received catalyst contained mostly Au3+ in the form of AuOOHdot operatorxH2O. Based on earlier studies, the support was proposed to be predominantly ferrihydrite, Fe5HO8 dot operator 4H2O. In situ FTIR in the presence of CO and CO + O2 showed an initial increase in the bicarbonate regions, a decrease in carbonates, and a signal at 1640 cm−1, attributed to consumption of OH/H2O during the reaction. MultiTRACK analysis showed that with pulsing of CO onto a fresh catalyst sample, initially only a limited, irreversible amount of CO adsorbed. Adsorption of CO increased with increasing number of pulses, and CO2 production and, to a lesser extent, H2O were observed after significant surface coverage by CO. A mechanism is proposed that involves a carbonate/bicarbonate intermediate and enhancement of the rate with the presence of surface OH. The activity of the sample seems to be a function of the presence of single bondOH species on the support, gold, or interface sites, the rate of desorption of CO2, or decomposition of surface carbonates. Show less

Catalytic formation of N2O via a (NO)2 intermediate was studied employing density functional theory with generalized gradient approximations. Dimer formation was not favored on Pt(111), in agreement with previous reports. On Pt(211) a variety of dimer structures were studied, including trans-(NO)2 and cis-(NO)2 configurations. A possible pathway involving (NO)2 formation at the terrace near to a Pt step is identified as the possible mechanism for low-temperature N2O formation. The dimer is… Show more

Catalytic formation of N2O via a (NO)2 intermediate was studied employing density functional theory with generalized gradient approximations. Dimer formation was not favored on Pt(111), in agreement with previous reports. On Pt(211) a variety of dimer structures were studied, including trans-(NO)2 and cis-(NO)2 configurations. A possible pathway involving (NO)2 formation at the terrace near to a Pt step is identified as the possible mechanism for low-temperature N2O formation. The dimer is stabilized by bond formation between one O atom of the dimer and two Pt step atoms. The overall mechanism has a low barrier of approximately 0.32 eV. The mechanism is also put into the context of the overall NO+H2 reaction. A consideration of the step-wise hydrogenation of O(ads) from the step is also presented. Removal of O(ads) from the step is significantly different from O(ads) hydrogenation on Pt(111). The energetically favored structure at the transition state for OH(ads) formation has an activation energy of 0.63 eV. Further hydrogenation of OH(ads) has an activation energy of 0.80 eV. Show less

The energetics of the low-temperature adsorption and decomposition of nitrous oxide, N2O, on flat and stepped platinum surfaces were calculated using density-functional theory (DFT). The results show that the preferred adsorption site for N2O is an atop site, bound upright via the terminal nitrogen. The molecule is only weakly chemisorbed to the platinum surface. The decomposition barriers on flat (111) surfaces and stepped (211) surfaces are similar. While the barrier for N2O dissociation is… Show more

The energetics of the low-temperature adsorption and decomposition of nitrous oxide, N2O, on flat and stepped platinum surfaces were calculated using density-functional theory (DFT). The results show that the preferred adsorption site for N2O is an atop site, bound upright via the terminal nitrogen. The molecule is only weakly chemisorbed to the platinum surface. The decomposition barriers on flat (111) surfaces and stepped (211) surfaces are similar. While the barrier for N2O dissociation is relatively small, the surface rapidly becomes poisoned by adsorbed oxygen. These findings are supported by experimental results of pulsed N2O decomposition with 5% Pt/SiO2 and bismuth-modified Pt/C catalysts. At low temperature, decomposition occurs but self-poisoning by O(ads) prevents further decomposition. At higher temperatures some desorption of O2 is observed, allowing continued catalytic activity. The study with bismuth-modified Pt/C catalysts showed that, although the activation barriers calculated for both terraces and steps were similar, the actual rate was different for the two surfaces. Steps were found experimentally to be more active than terraces and this is attributed to differences in the preexponential term. Show less

The decomposition of N2O was studied using a silica-supported Pt catalyst. The catalyst was found to exhibit short-lived activity at low temperatures to yield N2 and O(ads), the latter remained adsorbed on the surface and poisoned the active sites. Creation of hot-O(ads) atoms during N2O decomposition is proposed to allow O2 desorption at intermediate temperatures. Inclusion of H2 as a reducing agent greatly enhanced the activity and suppressed low temperature deactivation. Simultaneous and… Show more

The decomposition of N2O was studied using a silica-supported Pt catalyst. The catalyst was found to exhibit short-lived activity at low temperatures to yield N2 and O(ads), the latter remained adsorbed on the surface and poisoned the active sites. Creation of hot-O(ads) atoms during N2O decomposition is proposed to allow O2 desorption at intermediate temperatures. Inclusion of H2 as a reducing agent greatly enhanced the activity and suppressed low temperature deactivation. Simultaneous and sequential pulsing of N2O and H2 showed that H2 inclusion with the N2O gas stream produced the greatest activity. A mechanism involving H(ads) addition to ldquohotrdquo oxygen atoms for H2O formation is proposed. Show less

Classification of the active surface sites of platinum catalysts responsible for low temperature N2O decomposition, in terms of steps, kinks and terraces, has been achieved by controlled addition of bismuth to as-received platinum/graphite catalysts.

Catalytic formation of N2O and NO2 were studied employing density functional theory with generalized gradient approximations, in order to investigate the microscopic reaction pathways of these catalytic processes on a Pt(111) surface. Transition states and reaction barriers for the addition of chemisorbed N or chemisorbed O to NO(ads) producing N2O and NO2, respectively, were calculated. The N2O transition state involves bond formation across the hcp hollow site with an associated reaction… Show more

Catalytic formation of N2O and NO2 were studied employing density functional theory with generalized gradient approximations, in order to investigate the microscopic reaction pathways of these catalytic processes on a Pt(111) surface. Transition states and reaction barriers for the addition of chemisorbed N or chemisorbed O to NO(ads) producing N2O and NO2, respectively, were calculated. The N2O transition state involves bond formation across the hcp hollow site with an associated reaction barrier of 1.78 eV. NO2 formation favors a fcc hollow site transition state with a barrier of 1.52 eV. The mechanisms for both reactions are compared to CO oxidation on the same surface. The activation of the chemisorbed NO and the chemisorbed N or O from the energetically stable initial state to the transition state are both significant contributors to the overall reaction barrier Ea, in contrast to CO oxidation in which the activation of the O(ads) is much greater than CO(ads) activation. Show less