Model catalysts surface analysis

Nelson, A.E. and Schulz, K.H. Surface chemistry and microstructural analysis of CexZii x02-y model catalyst surfaces. Appl Surf. Set. 2003, 210, 206-221. 

Fig. 2. Models of the primary particle (a) and polymer grain (b) for the analysis of the role of monomer diffusion to tbe catalyst surface, (a) 1—catalyst 2—polymer film, (b) 1—micrograin 2—macropore.

The development of modern surface characterization techniques has provided means to study the relationship between the chemical activity and the physical or structural properties of a catalyst surface. Experimental work to understand this reactivity/structure relationship has been of two types fundamental studies on model catalyst systems (1,2) and postmortem analyses of catalysts which have been removed from reactors (3,4). Experimental apparatus for these studies have Involved small volume reactors mounted within (1) or appended to (5) vacuum chambers containing analysis Instrumentation. Alternately, catalyst samples have been removed from remote reactors via transferable sample mounts (6) or an Inert gas glove box (3,4). 

To rationally govern the selectivity of a catalytic process, the elementary reaction steps on real catalyst surfaces must be identified. The use of well-defined organometallic compounds (possible intermediates in surface reactions) can be very useful in the determination of these steps. The use of kinetic modelling techniques combined with statistical analysis of kinetic... 

In the near future probably computer modelling, allowing the analysis of adsorption and elementary reactions at surfaces, will become increasingly helpful in catalyst selection. On the experimental side the field is changing drastically. Parallel testing equipment is now the state of the art. This field is often referred to as Combinatorial Chemistry . It is expected to have a large impact already in the near future. In fact, at present already companies have been formed in this field. 

Realistic model systems. Some techniques become much more informative if suitable model systems are used. Examples are the thin-film oxides used as conducting model supports, which offer much better opportunities for surface analysis than do technical catalysts. Another example is provided by the non-porous, spherical supports that have successfully been employed in electron microscopy. It is important that the model systems exhibit the same chemistry as the catalyst they represent. 

Recently, the steady-state reaction kinetics of CO oxidation at high pressure over Ru , Rh " , Pt, Pd, and Ir single crystals have been studied in our laboratory. These studies have convincingly demonstrated the applicability and advantages of model single crystal studies, which combine UHV surface analysis techniques with high pressure kinetic measurements, in the elucidation of reaction mechanisms over supported catalysts. 

The ammoxidation of isobutene has not received much attention. The only contribution in this field is by Onsan and Trimm [2.44] for a rather unusual catalyst, a mixture of the oxides of Sn, V and P (ratio 1/9/3) supported on silica. At 520 C, a maximum selectivity to methacrylonitrile + methacrolein of 80% was reached with a Sn—V—P oxide catalyst (ratio 1/9/3), an isobutene/ammonia/oxygen ratio of 1/1.2/2.5 and a contact time of 120 g sec l ]. The kinetics are very similar to those for the pro-pene ammoxidation. Again, the data are initially analysed by means of (parallel) power rate equations, for which the parameters were calculated, while a more detailed analysis proves that a Langmuir—Hinshelwood model with surface reaction as the rate-controlling step provides the best fit with regard to the two main products. At 520° C, the equation which applies for the production of methacrolein plus methacrylonitrile is... 

Diffusional mass transfer processes can be essential in complex catalytic reactions. The role of diffusion inside a porous catalyst pellet, its effect on the observed reaction rate, activation energy, etc. (see, for example, ref. 123 and the fundamental work of Aris [124]) have been studied in detail, but so far several studies report only on models accounting for the diffusion of material on the catalyst surface and the surface-to-bulk material exchange. We will describe only some macroscopic models accounting for diffusion (without claiming a thorough analysis of every such model described in the available literature). 

Thus if the multiplicity of steady states for the catalyst surface manifesting itself in the multiplicity of steady-state catalytic reaction rates has been found experimentally and for its interpretation a three-step adsorption mechanism of type (4) and a hypothesis about the ideal adsorbed layer are used, the number of concrete admissible models is limited (there are four). It can be claimed that some types of adsorption mechanism have "feedbacks , but for the appearance of the multiplicity of steady states these "feedbacks must possess sufficient "strength . The analysis of these cases (mechanisms 4-7 in Table 2) shows that, to achieve multiplicity, the reaction conditions must "help the non-linear step. 

Models of type (4) have been formulated [151-153] and used for the analysis of some concrete processes [see, for example, ref. 154 where the kinetic dependence P(60) was represented by a linear function]. Taking into account oxygen diffusion into the catalyst volume by using model (14) does not change the steady states of the catalyst surface compared with model (2)-(3). But the relaxation properties of these models are essentially different. The numerical algorithm developed by Makhotkin was used for the calculations. Discretization of the spatial variable was applied to go from the model in partial derivatives to the system of ordinary differential equations. For details of this algorithm, see ref. 155. 

Analysis of structure formation processes by using Monte Carlo methods. Monte Carlo methods will he used extensively for the calculation of processes during which new phases are formed. In particular, these are adsorption-desorption, diffusion, and reactions on the surfaces of solids. The results of this modelling will be used to decode structures formed on catalyst surfaces. 

Theoretical models based on first principles, such as Langmuir s adsorption model, help us understand what is happening at the catalyst surface. However, there is (still) no substitute for empirical evidence, and most of the papers published on heterogeneous catalysis include a characterization of surfaces and surface-bound species. Chemists are faced with a plethora of characterization methods, from micrometer-scale particle size measurement, all the way to angstrom-scale atomic force microscopy [77]. Some methods require UHV conditions and room temperature, while others work at 200 bar and 750 °C. Some methods use real industrial catalysts, while others require very clean single-crystal model catalysts. In this book, I will focus on four main areas classic surface characterization methods, temperature-programmed techniques, spectroscopy and microscopy, and analysis of macroscopic properties. For more details on the specific methods see the references in each section, as well as the books by Niemantsverdriet [78] and Thomas [79]. 

In any gas/solid catalytic system, the reactant must first be adsorbed on the catalyst surface. This is why surface characterization is so important. Studying the adsorption of various molecules under controlled conditions yields information regarding the catalyst surface area, pore volume, and pore size distribution [80]. The key factor here is accessibility. Sophisticated spectroscopic analysis of single-crystal models can tell us a lot about what goes on at the active site, but the molecules must get there first. 

The desire to formulate reaction schemes in terms of molecular processes taking place on a catalyst surface must be balanced with the need to express the reaction scheme in terms of kinetic parameters that are accessible to experimental measurement or theoretical prediction. This compromise between mechanistic detail and kinetic parameter estimation plays an important role in the use of reaction kinetics analysis to describe the reaction chemistry for a catalytic process. Here, we discuss four case studies in which different compromises are made to develop an adequate kinetic model that describes the available observations determined experimentally and/or theoretically. For convenience, we selected these examples from our work in this field however, this selection is arbitrary, and many other examples could have been chosen from the literature. 

In order to check various model assumptions a first step in the analysis requires to perform mass balance calculations involving the amount of gas oil injected and the amount of hydrocarbons obtained by pressure difference under vacuum conditions, with the key hypothesis that no hydrocarbons remain on catalyst surface at 2 psia. Given mass balance closure was within typically 5-7% error for all experiments conducted, the following was validated a) the total mass of hydrocarbons including various lumps during the reaction... 

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