There is wide interest in developing accurate theories for predicting rates of chemical reactions that occur at metal surfaces, especially for applications in industrial catalysis. Conventional methods contain many approximations that lack experimental validation. In practice, there are few reactions where sufficiently accurate experimental data exist to even allow meaningful comparisons to theory. Here, we present experimentally derived thermal rate constants for hydrogen atom recombination on platinum single-crystal surfaces, which are accurate enough to test established theoretical approximations. A quantum rate model is also presented, making possible a direct evaluation of the accuracy of commonly used approximations to adsorbate entropy. We find that neglecting the wave nature of adsorbed hydrogen atoms and their electronic spin degeneracy leads to a 10× to 1000× overestimation of the rate constant for temperatures relevant to heterogeneous catalysis. These quantum effects are also found to be important for nanoparticle catalysts.
We report accurate
time-resolved measurements of NH
3
desorption from Pt(111)
and Pt(332) and use these results to determine
elementary rate constants for desorption from steps, from (111) terrace
sites and for diffusion on (111) terraces. Modeling the extracted
rate constants with transition state theory, we find that conventional
models for partition functions, which rely on uncoupled degrees of
freedom (DOFs), are not able to reproduce the experimental observations.
The results can be reproduced using a more sophisticated partition
function, which couples DOFs that are most sensitive to NH
3
translation parallel to the surface; this approach yields accurate
values for the NH
3
binding energy to Pt(111) (1.13 ±
0.02 eV) and the diffusion barrier (0.71 ± 0.04 eV). In addition,
we determine NH
3
’s binding energy preference for
steps over terraces on Pt (0.23 ± 0.03 eV). The ratio of the
diffusion barrier to desorption energy is ∼0.65, in violation
of the so-called 12% rule. Using our derived diffusion/desorption
rates, we explain why established rate models of the Ostwald process
incorrectly predict low selectivity and yields of NO under typical
reactor operating conditions. Our results suggest that mean-field
kinetics models have limited applicability for modeling the Ostwald
process.
A new type of automobile catalyst based on an active Pd/Rh-layer is presently being introduced into the European market. Accordingly, in order to establish baseline information, this work attempts to assess the magnitude of Pd emissions by automobile catalysts to date. Thus, a survey is presented on Pd and Pt levels detected in polluted environmental matrices (grass, soil, dust, sludges) as available from the literature. Additionally, Pd data measured by selective preconcentration/GF-AAS-detection in polluted grass, in a time series of sewage sludge ashes back to 1972, in dust, as well as in urban rain are presented. Since the data basis from the literature is partly inconsistent, the possible impact of different analytical attempts on the data is discussed. The relationship between Pt and Pd concentration from most of the results is between 5 and 10 (Pt/Pd). However, relations down to 0.04 are also reported, thus implying anthropogenic Pd input from additional sources.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.