Figure 4: Hydrogen concentration profiles at different times and positions.

The concentration of hydrogen is plotted as the ratio of hydrogen to metal atoms over (a) distance and (b) time. Data is shown for different times and distances from the catalytic layer. The red lines represent least squares fits to the data using equation (8), as described in the Methods. Notice the sensitivity to the low amount of hydrogen with this method, where concentrations below 0.001 in H/V can easily be distinguished.