The reactions of titanium with oxygen and nitrogen are very energetic, among the strongest for metals.
They don't happen at moderate temperature because once a thin layer of oxide has formed, it is impervious enough to halt the reactions, like for aluminium, silicon, chromium, tantalum and others. But at heat, this doesn't suffice. For instance hot chips produced when turning or milling titanium catch fire easily, then they destroy the tool swiftly, and possibly set the workshop alight if cutting oil or plastic chips are present.
In the case of titanium welding, the big worry is that traces of nitrogen or oxygen embrittle titanium alloys. Welding titanium needs a much better protection against air than for aluminium. As well, molten titanium is much hotter and, as a bad heat conductor, titanium stays hot longer, well after the arc and its argon jet passed.
TiO2 catalyst: I don't know. There must be literature about that.