Experimental studies of the chemistry of metal clusters

The procedures for studying chemical reactions of metal clusters in a continuous-flow reactor are described, and examples of such studies are given. Experiments to be discussed include kinetics and thermodynamics measurements, and determination of the composition of clusters saturated with various adsorbate reagents. Specific systems to be covered include the reaction of iron clusters with ammonia and with hydrogen, the reaction of nickel clusters with hydrogen and with ammonia, and the reaction of platinum clusters with ethylene. The last two reactions are characterized by complex, multi-step processes that lead to adsorbate decomposition and hydrogen desorption from the clusters. Methods for probing these processes will be discussed. 26 refs., 8 figs

The structures of bare and deuterated Co{sub 19}

The structures of bare Co{sub 19} and deuterated Co{sub 19}D{sub m} clusters are examined by the chemical probe method, and earlier assignments of bare Co{sub 19} as an fcc octahedron are reconsidered. New experimental measurements of the reactivity of Co{sub 19} with ammonia, nitrogen, and deuterium are presented, and together with earlier measurements of the reactivity with water suggest that bare Co{sub 19} has an hcp structure (D{sub 3h} symmetry). The adsorption of deuterium on Co{sub 19} is found to proceed in steps, leading to successive saturation levels at Co{sub 19}D{sub 4}, Co{sub 19}D{sub 14}, and Co{sub 19}D{sub 18}. Using binding rules derived from earlier studies of larger cobalt and nickel clusters, possible D-atom binding sites on Co{sub 19}D{sub m} (both fcc and hcp) are proposed

Towards Improving the Utilisation of University Teaching Space

There is a perception that teaching space in universities is a rather scarce resource. However, some studies have revealed that in many institutions it is actually chronically under-used. Often, rooms are occupied only half the time, and even when in use they are often only half full. This is usually measured by the ‘utilization' which is defined as the percentage of available ‘seat-hours' that are employed. Within real institutions, studies have shown that this utilization can often take values as low as 20-40%. One consequence of such a low level of utilization is that space managers are under pressure to make more efficient use of the available teaching space. However, better management is hampered because there does not appear to be a good understanding within space management (near-term planning) of why this happens. This is accompanied, within space planning (long-term planning) by a lack of expertise on how best to accommodate the expected low utilizations. This motivates our two main goals: (i) To understand the factors that drive down utilizations, (ii) To set up methods to provide better space planning. Here, we provide quantitative evidence that constraints arising from timetabling and location requirements easily have the potential to explain the low utilizations seen in reality. Furthermore, on considering the decision question ‘Can this given set of courses all be allocated in the available teaching space?' we find that the answer depends on the associated utilization in a way that exhibits threshold behaviour: There is a sharp division between regions in which the answer is ‘almost always yes' and those of ‘almost always no'. Through analysis and understanding of the space of potential solutions, our work suggests that better use of space within universities will come about through an understanding of the effects of timetabling constraints and when it is statistically likely that it will be possible for a set of courses to be allocated to a particular space. The results presented here provide a firm foundation for university managers to take decisions on how space should be managed and planned for more effectively. Our multicriteria approach and new methodology together provide new insight into the interaction between the course timetabling problem and the crucial issue of space planning

Physisorption of deuterium on deuterated nickel clusters

Physisorption of deuterium molecules on small nickel clusters already saturated with chemisorbed deuterium atoms is identified. The principal evidence for physisorption is the appearance at reaction temperatures below 200 K of product species having more deuterium bound to them than the chemisorption maximum, and a lowering of cluster ionization potentials (IPs) for these species. It is argued that the IP lowering is a consequence of molecular physisorption, and that identification of the number of physisorption sites on cluster surfaces can be used to infer structural information

Mass spectrometric probes of metal cluster distributions and metastable ion decay

The study of metal clusters has provided both an opportunity and a challenge to the application of mass spectrometry. These days the most often-used technique for cluster generation - laser vaporization - leads to extensive distributions of cluster sizes, from one to perhaps thousands of atoms, and most studies reported to date use excimer laser ionization and time-of-flight mass spectrometry for cluster detection. Our apparatus is a simple one-stage TOF design employing Wiley-McLauren spatial focusing and a one-meter drift tube. In a second apparatus employing a pulsed valve in the cluster source, we see asymmetric broadening of niobium cluster mass peaks under multiphoton ionization conditions, indicating metastable decay of parent cluster ions. Other studies of niobium clusters have shown no such asymmetric peaks. 2 figs

The reactions of neutral iron clusters with D/sub 2/O: Deconvolution of equilibrium constants from multiphoton processes

The chemical reactions of neutral iron clusters with D/sub 2/O are studied in a continuous flow tube reactor by molecular beam sampling and time-of-flight mass spectrometry with laser photoionization. Product distributions are invariant to a four-fold change in reaction time demonstrating that equilibrium is attained between free and adsorbed D/sub 2/O. The observed negative temperature dependence is consistent with an exothermic, molecular addition reaction at equilibrium. Under our experimental conditions, there is significant photodesorption of D/sub 2/O (Fe/sub n/(D/sub 2/O)/sub m/ + h..nu.. ..-->.. Fe/sub n/ + m D/sub 2/O) along with ionization due to absorption of multiple photons from the ionizing laser. Using a simple model based on a rate equation analysis, we are able to quantitatively deconvolute this desorption process from the equilibrium constants. 8 refs., 1 fig

Generation of beams of refractory-metal clusters

Interest in the physical and chemical properties of small metal clusters has recently stimulated the development of sources for the generation of molecular beams of metal clusters, since the collision-free environment of a beam has the advantage of permitting in-flight study of isolated species free of interference from surroundings. For example, spectroscopic studies utilizing tunable lasers may be performed in the molecular beam environment. The objectives of our research program are the elucidation of the physical and chemical properties of clusters of refractory metal atoms, in particular those of the catalytically active transition metals. For these purposes we have built and tested two sources suitable for generation of cluster beams of refractory metals, one for continuous beams and the other for pulsed beams